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Synthesis and Characterization of Core/Shell Silica Nanoparticles for Chemical Mechanical Planarization of Low-K dielect...

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Title: Synthesis and Characterization of Core/Shell Silica Nanoparticles for Chemical Mechanical Planarization of Low-K dielectric and Copper Wafers
Physical Description: 1 online resource (48 p.)
Language: english
Creator: Balasundaram, Kannan
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: checmical, cmp, core, ctab
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Monodispersed core/shell silica particles with a hard core and microporous shell have been synthesized by surfactant template method. Ca. 75nm silica particles were used as core and Cetyltrimethylammonium bromide (C_16_TAB) as template for generating microporous shell. Concentration of the surfactant was varied and the growth of porous shell analyzed using high-resolution transmission electron microscopy and nitrogen adsorption. TGA and FTIR were used to confirm the surfactant removal after heat treatment. The synthesized particles were monodispersed and had a hard core and highly microporous shell with pore size in the range of 1.3-2.2nm and total pore volume in the range of 0.57-0.77cm^3/g. The Chemical mechanical Planarization (CMP) performance of core/shell silica particles were analyzed and compared with that of core silica particles. Polishing was done on copper wafers and low-k dielectric material such as black diamond. The core/shell silica particles produced higher removal rates and better surface finish for both the wafers. Spectral reflectance technique and atomic force microscopy (AFM) were used as analyzing tool.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Kannan Balasundaram.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Singh, Rajiv K.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-02-28

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042215:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042215/00001

Material Information

Title: Synthesis and Characterization of Core/Shell Silica Nanoparticles for Chemical Mechanical Planarization of Low-K dielectric and Copper Wafers
Physical Description: 1 online resource (48 p.)
Language: english
Creator: Balasundaram, Kannan
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: checmical, cmp, core, ctab
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Monodispersed core/shell silica particles with a hard core and microporous shell have been synthesized by surfactant template method. Ca. 75nm silica particles were used as core and Cetyltrimethylammonium bromide (C_16_TAB) as template for generating microporous shell. Concentration of the surfactant was varied and the growth of porous shell analyzed using high-resolution transmission electron microscopy and nitrogen adsorption. TGA and FTIR were used to confirm the surfactant removal after heat treatment. The synthesized particles were monodispersed and had a hard core and highly microporous shell with pore size in the range of 1.3-2.2nm and total pore volume in the range of 0.57-0.77cm^3/g. The Chemical mechanical Planarization (CMP) performance of core/shell silica particles were analyzed and compared with that of core silica particles. Polishing was done on copper wafers and low-k dielectric material such as black diamond. The core/shell silica particles produced higher removal rates and better surface finish for both the wafers. Spectral reflectance technique and atomic force microscopy (AFM) were used as analyzing tool.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Kannan Balasundaram.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Singh, Rajiv K.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-02-28

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042215:00001


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SYNTHESIS AND CHARACTERIZATION OF CORE/SHELL SILICA NANOPARTICLES
FOR CHEMICAL MECHANICAL PLANARIZATION OF LOW-K DIELECTRIC AND
COPPER WAFERS

















By

KANNAN BALASUNDARAM


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2010
































2010 Kannan Balasundaram






























This work is dedicated to my parents.









ACKNOWLEDGMENTS

I would like to express my sincere thanks to my advisor, Dr. Rajiv K. Singh, for

giving me an opportunity to work under his guidance. His encouragement and support

during the course of the study was outstanding. I am also grateful to Dr. Hassan El-

Shall, Dr. Stephen J. Pearton for serving as committee members and supervising my

study.

I would like to thank Dr. Kevin Powers of Particle Science and Technology for

sharing his valuable knowledge with me during my research. I would also like to

acknowledge my co-researches Dr. Purushottam Kumar, Mr. Sushant Gupta, Mr.

Myoung-Oh for their valuable suggestions and support while carrying out my

experimental work.

I would like to recognize the help of the staff, Ms. Kerry Siebein, in MAIC (Major

Analytical Instrumentation Center) and Gill Brubaker PERC (Particle Engineering

Research Center) for their help in training me using the equipment and characterizing

my samples.

Finally, I would like to thank my parents for their financial support as well as moral

support all through my life in US. I also owe sincere thanks to all my friends who have

been supportive in my life.









TABLE OF CONTENTS

page

ACKNOWLEDGMENTS................... ................ .. ............................ 4

L IS T O F T A B LE S ...................... ................ ............... .................................. 7

L IS T O F F IG U R E S ...................... ................ .............. .. ................................ 8

A B S T R A C T ........... .. ......... .. .............. .. ...................................................... 9

CHAPTER

1 INTRODUCTION ............. ................................ .............. ........... 10

M o tiv a tio n ............. ......... .. .............. .. ...................................................... 1 1
O b je c tiv e ............. ......... .. .............. .. ...................................................... 1 2

2 BA C KG RO U N D ......... ....... ................................... ......................... ... 13

Introduction to Chemical Mechanical Planarization......................... ....... 13
CMP Slurry Preparation and Characteristics ............................. ......... .... 17
Introduction to Nanoporous Materials .................................... ........ 20
Micro/Mesoporous Silica Particles ....... ................................... ............ 20
Advantage of Nanoporous Silica in CMP ...................................................... 22

3 SYNTHESIS OF CORE/SHELL MICROPOROUS SILICA PARTICLES ................24

In tro d u c tio n ....................... .. .............. .. .....................................................2 4
E x p e rim e n ta l ............. ......... .. .............. .. ................................................... 2 5
M materials ............... ................. ......................................25
Synthesis of Mesoporous Shell Silica Particles ........ ...... ...............26
Characterization ............................ ..................26
Result and Discussion ...... ............................ ........ 27
Synthesis Method...... ................... .2... ............27
CTAB Adsorption on SiO2 Nanoparticles ...................... ......... ...................27
Mechanism of CTAB Molecules Arrangement on Silica Particles ................ 29
Surface Morphology of Porous Shell Silica and Pore Characterization ............. 30
Summary ................ ....... ........... .............................. 34

4 CHEMICAL MECHANICAL PLANARIZATION USING CORE/SHELL SILICA........35

In tro d u c tio n ..................................................................................................... 3 5
E x p e rim e n ta l .......................................................................................................... 3 6
M a te ria ls .......................................................................................................... 3 6
S lu rry P re p a ra tio n ................................................................................ ..... ........ 3 6
C M P P o lis h in g S e tu p .......................................................................................... 3 7









R results and D discussion ..................... ............. .................... ............... 38
Properties of Core/Shell Silica Particles...................................... ................ 38
Polishing Rate and Surface Roughness ..........................................................41
S u m m a ry ......................................................................... ................ 4 3

5 C O N C L U S IO N ...................................... .............................................. 4 4

LIST OF REFERENCES .................. ................................. ....... ............ 45

B IO G R A P H IC A L S K E T C H ................................................................................................ 4 8









































6









LIST OF TABLES


Table page

3-1 N2 sorption measurement of Core/shell silica particles. ................. .............. .. 32

4-1 Polish rates and surface roughness of black diamond and copper wafers.........41









LIST OF FIGURES


Figure page

2-1 Chemical Mechanical Polishing set up.......................................................... 14

2-2 Different wafer structures polished by CMP process and low-k dielectric
CM P, Tungsten m etal CM P ................................................_.. .... ............. ....... 16

2-3 SEM Images of abrasive particles used in CMP Alumina coated silica and
Ceria coated silica abrasives ....... ...... .......... .................. ...... .......... 18

2-4 Material removal rate and friction force of silica as a function of solids loading
of 0.5[tm silica abrasives ............ ............................................................19

3-1 Schematic of Core/Shell silica particles preparation................ ................ 29

3-2 FESEM Images of calcined sample-C core/shell silica particles under
different m agnification ................. ............ ....................... .......30

3-3 TEM micrographs of core/shell SiO2 particles as increase in surfactant
concentration from (Al to D1). ........ .... ....... .. .. ........................... .......... 31

3-4 TGA of core/shell silica particle- sam ple C ..................................... ................... 33

3-5 FTIR of core/shell silica particle- sam ple C ..................................... .................. 33

4-1 Schematic of different morphology of silica nanoparticles................................ 38

4-2 Comparison of TEM images of core and core/shell Silica................................39

4-3 Particle size distribution of abrasives in slurry A and B ..................... ........ 39

4-4 Nitrogen sorption isotherm of silica core and core/shell silica particles...............40









Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

SYNTHESIS AND CHARACTERIZATION OF CORE/SHELL SILICA NANOPARTICLES
FOR CHEMICAL MECHANICAL PLANARIZATION OF LOW-K DIELECTRIC AND
COPPER WAFERS

By

Kannan Balasundaram

August 2010

Chair: Rajiv K. Singh
Major: Materials Science and Engineering

Monodispersed core/shell silica particles with a hard core and microporous shell

have been synthesized by surfactant template method. Ca. 75nm silica particles were

used as core and Cetyltrimethylammonium bromide (C16TAB) as template for

generating microporous shell. Concentration of the surfactant was varied and the

growth of porous shell analyzed using high-resolution transmission electron microscopy

and nitrogen adsorption. TGA and FTIR were used to confirm the surfactant removal

after heat treatment. The synthesized particles were monodispersed and had a hard

core and highly microporous shell with pore size in the range of 1.3-2.2nm and total

pore volume in the range of 0.57-0.77cm3/g. The Chemical mechanical Planarization

(CMP) performance of core/shell silica particles were analyzed and compared with that

of core silica particles. Polishing was done on copper wafers and low-k dielectric

material such as black diamond. The core/shell silica particles produced higher removal

rates and better surface finish for both the wafers. Spectral reflectance technique and

atomic force microscopy (AFM) were used as analyzing tool.









CHAPTER 1
INTRODUCTION

The demand for increased circuit density, functionality and versatility has lead to

tremendous advancement in the front end of the chip manufacturing line. One such

development in semiconductor industry is Chemical mechanical planarization (CMP)

process. The ever growing CMP technology has made possible more intricate designs

with decreased feature size and multi -level interconnects for next generation

nanoscale devices [1]. The science of CMP is quite different from conventional

semiconductor manufacturing processes like ion implantation, photo lithography,

thermal annealing and so on. These traditional processes are well established and

understand by both academia and industry. However, in the case of CMP process, the

whole idea and technology was developed and put into use by industry itself. This made

it difficult for researchers in academia to fully understand the science and theory behind

CMP process. As time progressed, a new knowledge base and entire skills was

developed involving CMP process variables such as particle technology, tribology, wet

and surface chemistry, fluid flow, properties of polymers and so on. CMP slurries were

given more importance for the abrasive particles and chemical additives used and it has

become a potential market by its own. The abrasive particles generally in nanometer

scale are one of the largest uses of present nanotechnology. The development of CMP

slurries took place simultaneously with development of synthesis techniques for various

nanoparticles.

A whole range of nanoparticles was developed in short period of time and particles

were also modified and functionalized for specific targeted applications. Nanoparticles

like iron, copper, gold, silver, silica, alumina, ceria etc, have become common these









days. Functionalized nanoparticles such as core/shell silica coated gold [2], alumina

coated Titania [3], silver coated magnetite [4], ceria coated silica particles [5] are being

widely researched now a days and few of these materials have already found potential

applications. Those synthesis methods which yield large quantities of nanoparticles and

possibility of bulk production are always well adopted by industry. Silica nanoparticles is

one such material which has versatile application and can be synthesized in large scale.

Ability to synthesize in wide size ranges, easy to functionalize and modify the surface

has made silica nanoparticles ideal candidate for CMP slurries.

Motivation

This research focuses on synthesizing hard core-porous shell silica nanoparticles

for CMP of copper and low-k dielectric material such as black diamond. Conventional

non-porous silica particles and fully porous silica particles has few disadvantages in

CMP performance of low-k dielectric materials. The non-porous silica particles have

high young's modulus and are harder abrasives resulting in high penetration depth on

polishing surface. This produces poor surface finish and more number of scratches

during CMP process. In case of conventional fully porous silica particles, due to pore

structure running throughout, the particles have very low density reducing the hardness

of the nanoparticles. This affects one of the key outputs of CMP i.e. removal rate.

Porous silica particles produces superior surface finishes with very low surface

defectivity on the polishes wafer with a compromise on removal rate. There has always

been challenge to use functionalized nanoparticles in CMP slurries which could not only

yield higher removal rates but also delivers wafers with low surface defectivity. This was

the key motivating factor for this research work. By functionalizing the silica particles to









be porous at the surface while still maintaining a hard core, the overall performance of

low-K dielectric and copper CMP can be improved.

Objective

The objectives in this thesis are as follows:

* Synthesize of Core/Shell silica particles with hard core and highly porous shell.

* To study the effect of surfactant concentration on porous shell formation and
explain the mechanism.

* Perform CMP process on two different wafers such as copper and black diamond
using core/shell particles and non-porous silica particles.

* Compare the results and explain the behavior of core/shell nanoparticles on
different wafers materials.

The first objective was achieved by surfactant templated synthesis method. A

suitable cationic surfactant such as cetyl trimethyl ammonium bromide was chosen to

act as structure directing agent. The porous coated samples were characterized using

transmission electron microscopy and Autosorb-1 instruments. Following the synthesis

of nanoparticles, suitable slurry was prepared for performing CMP polishing. Polishing

was achieved on CMP STRUERS TEGRA POL-35 equipment and results analyzed

using atomic force microscopy (AFM) and spectral analysis technique as Filmetrics.









CHAPTER 2
BACKGROUND

Introduction to Chemical Mechanical Planarization

CMP has become one of the integrated operations of semiconductor

manufacturing and has lead to the development of next-generation nanoscale devices.

CMP not only eases the design and production of high density Integrated Circuits (IC)

by eliminating several photolithographic and film issues generated by severe

topography but also enables greater flexibility with process complexity and associated

designs. With the development of process technologies and automation in a very fast

pace, the use of CMP process has expanded greatly. CMP was just used to remove

topography from silicon oxide and few other surfaces earlier, but now it has been

successfully used to planarize Shallow trench Isolation (STI) layers, trenched metal Cu

interconnections, tungsten plugs and low resistivity metals. In spite of all the advantages

and developments, CMP challenges both academia and industries due to large number

of input and output variables which is making it difficult to optimize the process and is

being addressed individually.

Mechanical grinding alone may theoretically achieve palanarization but the surface

damage is high as compared to CMP. Chemistry alone, on the other hand, cannot attain

planarization because most chemical reactions are isotropic. Combination of both has

always yielded better results. CMP processes produce both global and local

planarization by combining chemical and mechanical interactions using slurry

composed of chemicals and submicron-sized particles. The process consists of moving

a sample surfaces against a pad and to feed the slurry between the sample surfaces

and pad to achieve palanarization. Figure 2-1 shows a schematic of chemical









mechanical polishing setup. The abrasives particles in the slurry induces mechanical

damage to the surface, loosening the material for enhanced chemical attack or

fracturing off the pieces of the surface and easy removal thereafter. Out of all the

parameters involved in obtaining best results from CMP, there are three main

components which must be given careful attention. They include, the surface to be

polished, the pad and the slurry.

Down Force




Wafer Carrier 6 Slurry

f Polish Pad
Carrier Film I




Wafer
Platen




Figure 2-1. Chemical Mechanical Polishing set up.

The surface to be polished can be classified based on metals, dielectrics and

special materials. CMP of metals includes polishing surfaces of Polysilicon, Al and

alloys, Cu and alloys, Ta, W, Ti and alloys such as TiN and TiNxCy. Increasingly metal

CMP is being used for the formation of studs and interconnections. There are several

advantages to using CMP to remove metal overburden. First, metal CMP yields a high

degree of local planarity. The high degree of planarity allows vias to be stacked directly

on top of each other. Stacked vias result in considerable reduction in circuit area over









staggered vias. Another group of surfaces polished using CMP includes dielectrics such

as silicon dioxide, phosphosilicate glasses (PSG), borophosphosilicate glasses (BPSG),

Si3N4 and SiOxNy. Figure 2-2 illustrates CMP polishing of various substrates used in

semiconductor industry. Though many of the oxide CMP remain proprietary, many

studies have been undertaken to understand the mechanisms of material removal in

these types of surfaces. There are many factors which influences the performance of

oxide CMP such as abrasive materials used, slurry pH, solid loading, etc. Some of the

benefits of oxide CMP includes improved bulk material removal, lithographic capability

and reduced defect densities. Some of the special surfaces polished using CMP method

includes aerogels, high K dielectrics, high Tc superconductors, optoelectronic materials,

plastics and ceramics. These materials are planarized to be used in high end

applications such as flat panel, packaging, advanced devices and circuits.

Another key component for better CMP results is characteristics of polishing pad.

The role of pad and its mechanical properties such as surface roughness and surface

porosity play a key role in determining polishing rate and planarization ability of the

CMP process. Pad porosity is indicated by specific gravity; the lower the specific gravity,

the higher the porosity. Pad porosity aids in slurry transportation, removal of reaction

products from polishing site. Pad hardness and compressibility have been found to

influence planarity. The harder and more noncompressible the pad, the less it will bend

and conform to the wafer surface to remove material at lower regions. The pad

materials are generally composed of polyurethane foam matrix with diamond or other

filler materials. Pads are often tailored to required application and expectations.

Continuous use of pad for various runs leads to degrading surface properties and poor








CMP results; hence it is necessary to condition the pad frequently between trials for

longer life and better outcome.


Drain Source



S102
(B) /



Si


(C)



SE


11.
CMP'


CMP i



MP


GCMP


Gate Si Q2

Drain Source
Si



St3N4 S1O2
/ _/







w



Sf


Figure 2-2. Different wafer structures polished by CMP process (A) and (B) Low-k
dielectric CMP, (C) Tungsten metal CMP
A separate section has been dedicated to slurry preparation and characteristics.

With many of advantages discussed, some of the disadvantages of CMP and

challenges it faces are explained here. The main disadvantage of CMP is its

optimization. An entire new tool set including metrology and process control tools is

required to make CMP more robust. Added to this, high circuit density and advanced

level of pattern geometry effects result in narrow design, increasing the overall cost of

the circuit design. Some of the other problems include defects arising from CMP









process such as scratching from the abrasive materials used in slurry, residual abrasive

particles and corrosive attack of chemicals used in slurry, delamination at weaker

interfaces, stress cracking and variation in oxide layer thickness. Post CMP cleaning

has always posed problems in the entire cleaning process, which is being addressed by

industry now a days. The main challenge that CMP faces is the integration into

semiconductor manufacturing line. Since most of the procedures and key notes are

proprietary, it is difficult to bring one single methodology for CMP process and optimize

the system. Also a detailed understanding of material removal and surface planarization

during CMP is lacking. With market demands increasing day by day, these critical

issues must be addressed in an effective manner.

CMP Slurry Preparation and Characteristics

Slurries provide both the chemical action through the solution chemistry and the

mechanical action through the abrasives. High polishing rates, planarity, selectivity,

uniformity, post-CMP ease of cleaning including environmental health and safety issues,

shelf life, and dispersion stability are the factors considered to optimize the slurry

performance. Chemical reagents in the CMP slurry react with the wafer surface being

polished forming a chemically modified top layer with desirable properties compared to

the initial wafer surface. Etch rate is dependent on slurry composition. Any commercial

CMP slurry will have the chemical agents such as oxidizers, buffering agents, slurry

stabilizers and completing agents. Oxidizers are generally added to metal CMP slurries

due to the fact that, they react with metal surfaces to raise the oxidation state of the

metal, resulting in either dissolution of the metal or the formation of surface film on the

metal. On the other hand completing agents are added to increase the solubility of the

film being polished. Buffering agents are added to keep the slurry pH constant









throughout the volume and over time .The overall concentration of all these chemicals

added are to be monitored carefully since they increase the reaction rates at the

polishing surface.

Abrasives in the slurry play the very important role of providing mechanical action

during polishing. Commonly used CMP abrasives are Si02, A1203 and Ce02 particles.

Various multifunctional and tunable particles such as ceria coated silica [6], alumina

coated silica particles [7] (shown in Figure 2-3) are becoming popular. The chemically

modified surface layer of the wafer is abraded continuously with the submicron size

slurry abrasives resulting in material removal.


















Figure 2-3. SEM Images of abrasive particles used in CMP (a) Alumina coated silica
[Ref.7] and (b) Ceria coated silica abrasives [Ref .5]

To achieve an optimal polishing performance with minimal deformations and good

planarity, it is necessary to optimize, the rates of chemical modification and mechanical

abrasion. The intensity of mechanical abrasion also varies with the slurry particle size

and concentration, as these factors determine the load applied per particle. Furthermore

the frequency of abrasion depends on the number of slurry abrasives in contact with the









wafer surface. Therefore, abrasive particle size and concentrations as well as the

particle size distribution are very important parameters while designing the slurry. This

is illustrated in Figure 2-4, which shows material removal rate and frictional force of

silica as function of solid loading of 0.5pm silica abrasives. The effect of particle size

distribution in form of agglomerates has been reported [8]. A small variation in any one

of the above parameters may result in major changes in the particle-substrate

interactions and material removal rates vary resulting in poor process control. Hardness

of abrasive particles in slurry plays important role in achieving higher removal rates,

however care must be taken to minimize surface damage.


C =Material Removal Rate (A/min)
S IIln-situ Friction Force (N)



2000 2 2
1




0 0
0 5 10 15 20 25 30
Slurry Solids Concentration C wt)

Figure 2-4. Material removal rate and friction force of silica as a function of solids
loading of 0.5[tm silica abrasives (Ref. [9])

Some of the other key parameters to be taken into account while preparing CMP

slurry are the viscosity of the slurry, isoelectric point (pH), slurry flow rate and stability of

the abrasives. Viscosity affects slurry transport across the wafer and lubrication of the

wafer-pad interface. The more viscous the slurry is, poor is the transport of reactants

and products to and from the wafer surface. Hence optimal viscosity is to be









maintained. The isoelectric points (IEP) is the pH at which abrasive surface is charge

neutral. The charge on abrasive particles determines the mechanism of material

removal on various surfaces. The importance of this factor is currently being researched

and very few reports available. Slurry flow rate normally having units of I/min or ml/min

is the amount of abrasives delivered to the pad while polishing. It significantly affects the

removal rate and lubrication properties of the system.

Introduction to Nanoporous Materials

IUPAC has classified porous materials based on their pore sizes. Materials with

pore diameter of less than 2 nm are considered as microporous, those with pore

diameters greater than 50 nm are referred to as macroporous and those that fall in

between 2 to 50 nm are mesoporous materials. Some of the commonly synthesized

nanoporous materials are silica and alumina. Other oxides of titanium, zirconium,

cerium, tin porous materials are also being widely researched. The main reason why

these porous materials were able to find wide range of applications in industry was due

to the fact that the pore size, pore integrity and the ordered and disordered nature of the

pores can be controlled precisely. There are various methods of synthesis of porous

materials such as self-assembly, templated self-assembly, Sol-gel processing and spray

drying methods. However the most common method which is widely used is the

surfactant templated synthesis. Some of the potential applications of micro/mesoporous

materials include industrial catalysis, separation technology, environmental protection,

electrochemistry, membranes, sensors, optical devices and polishing.

Micro/Mesoporous Silica Particles

Porous silica particles are inorganic materials first developed by researchers in

Japan a decade back. It was further industrially developed by Mobil Corporation









laboratories and nomenclatured as Mobil crystalline of Materials (MCM-41, MCM-48,

MCM-50) with hexagonal, cubic and lamellar mesostructures respectively and different

morphologies. Their pore size and wall thickness do not go beyond 4.0 nm and 2.0 nm,

respectively. Other popular mesoporous particles are SBA type materials with different

mesostructures and porous characteristics somewhat similar to the MCM-X type

materials.SBA-15and SBA-16 silica (SBA: Santa Barbara University) with large pore

sizes and thicker wall were prepared. Mesoporous silica particles have been

synthesized by various methods. One of the most commonly synthesized ways is in the

presence of surfactants as templates for the poly condensation of silica species,

originating from different sources of silica such as sodium silicate, alkoxydes-TEOS

tetraethyll orthosilicate) and TMOS (tetramethyl orthosilicate). Synthesis conditions

such as source of silica, type of surfactant, ionic strength, pH and composition of the

reaction mixture, temperature and duration of the synthesis effect the surfactant micellar

conformation, the silica-surfactant interactions and the degree of silica poly

condensation. These conditions determine the characteristics of the porous structure.

Several other synthesis methods have been reported.

Nanoporous silica particles are interesting materials for high performance liquid

chromatography application (HPLC) due to their high surface area and their organized

porous structure. Additionally their content of silanol groups as well as their chemical

and mechanical stability under chromatographic conditions makes it well suited for the

application. Different nanostructures are used as supports for immobilization of

bioactive enzymes and drugs. The most common immobilization methods are

adsorption, covalent bonding, cross-linking and entrapment. Since silicates are









biocompatible, nanoporous silica particles are well suited as support for controlled drug

delivery systems or for immobilizing enzymes, which are used as biosensors and in

bioconversion processes. The immobilization process is very efficient if the support

exhibits high surface area and size of the pores is similar or slightly higher than the

diameter of the biomolecule. The aluminosilicates and mesoporous silicas are widely

used in heterogeneous catalysis as catalysts or as support for the catalyst. The

nanoporous particles are very promising candidates for this application due to their high

surface area and pore volume, besides of the possibility of surface modification and

pore distribution control. The adequate diffusion of molecules through the catalyst pores

allows the direct interaction with the acidic sites on the wall surface, promoting the

conversions. Macropores formed between these particles allow a fast mass transfer to

the surface of the primary particles. Incase of bio-imaging mesoporous silica particles

are considered as highly efficient MRI contrast agents and its usefulness is being

researched in bio-engineering field extensively. Other significant applications of

nanoporous silica particles are in CMP polishing though most of the reports are either

not published or in proprietary with industries.

Advantage of Nanoporous Silica in CMP

Nanoporous silica particles have reduced density due to high porosity; as a result

the Hamaker constant is very low for these particles, which implies less adhesion on the

surface that is polished. Due to lower hardness of these particles compared to

conventional non porous silica particles, impregnation of these particles on the wafer

surface can be prevented. Moreover the porous silica particles have low refractive index

and dielectric constant due to surface porosity of these particles. This reduces the van

der waals attractive force while polishing and one can expect minimal indentation and









very less surface damage on the wafer surface. Another main advantage of these

nanoporous silica particles is reduction in scratches in the polished surface. The

penetration depth of the scratches varies linearly with the particle size and inversely

with the young's modulus of the impacting abrasive particles. Hence in order to reduce

the depth of the particle indent and resulting scratches, the particle size as well as the

Young's modulus of the particle should be reduced. By using nanoporous silica

particles, this can be achieved which would result in significantly reducing the scratches.

The low normal stress on these nanoporous silica particles compared to that of non-

porous particles reduces the film delamination effect considerably. Most of the CMP

polishing of low-k dielectric materials is done at alkaline pH. The replacement of

conventional silica particles with the porous particles will not alter the slurry chemistry

and hence by modifying the surface morphology with high porosity and surface area

particles will not only improve the removal rates but also high quality surface finish can

be achieved. Thus the synthesis of wide range of tailored nanoporous silica particles,

which are highly monodispersed are potential candidates for CMP.









CHAPTER 3
SYNTHESIS OF CORE/SHELL MICROPOROUS SILICA PARTICLES

Introduction

Silica has become one of the easily synthesized nanoparticles, ever since Stober

et al. [10] introduced sol-gel process for growing particles of sizes ranging from nm to

ptm in diameter, simply by varying the catalyst and precursor concentrations. This has

lead to various developments in synthesis of multi-functional silica particles such as

microporous & mesoporous silica and core-shell particles with different components in

the core and shell layers [11]. Porous Silica spheres have attracted many researchers

for its wide range of applications such as chromatography, catalysis, drug delivery,

waste water treatment, etc. [12,13,14]. Another potential application of these highly

porous silica particles are in Chemical mechanical planarization (CMP) as abrasives for

polishing dielectric layers on Si wafers [15]. The particles can be nearly tuned

functionally to get desired results in such applications.

Various methods have been employed to synthesis porous silica particles with

controlled pore systems [16,17,18]. However the most common method is the surfactant

templated synthesis (STS). Here a cationic surfactant such as n-alkyl trimethyl

ammonium bromides (CnTAB) is mixed in the water-alcohol mixture, followed by

polymerization of alkoxide precursor such as tetraethyl orthosilicate (TEOS) and finally

removing the template by calcination [19] or other methods [20,21]. Nanoparticles

synthesized by such methods are porous throughout. A modification in the conventional

porous silica particles is synthesis of hard core and porous shell silica particles. Jungo

Ho [22] and M. Mesa [23] et al. synthesized such particles in the size range of 400-









500nm with hard core and porous shell of ca. 50nm in thickness used for HPLC (high

performance liquid chromatography) application.

Here, we report a modified and simple method for synthesizing highly microporous

shell with hard core silica particles by using C16TAB as structure directing agents on

75nm silica particles. Concentration of surfactant was varied and effect on porous shell

formation and increase surface area were analyzed. As the particles were highly porous

at surface with a hard core and also highly monodispersed, they should be of great

interest for CMP slurries. The main advantage of core/shell silica particles is to obtain

less defective polished surface after CMP while still maintaining optimum removal rates.

The high surface porosity influences the refractive index and dielectric constant of the

silica particles, which decreases van der Waals attractive forces [24]. As a result, one

can expect lower adhesion force between porous abrasive slurry particles and wafer

surface resulting in reduced indentation and minimal scratches on the surface. Such

surface defectivity is always a concern in case of conventional non-porous silica

particles depending on their sizes [25]. Advantage of having a hard core is to maintain

higher removal rates of the dielectric layers, which are otherwise difficult to obtain,

incase of fully porous silica particles. Overall, the core/shell silica particles are useful in

improving the performance of low-k dielectric CMP process.

Experimental

Materials

All the solutions were prepared using analytical grade reagents. Silica colloid

EM7530A (with average particle size of 75nm, having a 30% solid concentration in a

H20/NH4OH solution) was provided by Silco International, Inc. Tetraethyl orthosilicate

(TEOS, 98 wt %), Cetyl trimethylammonium bromide-C16TAB (99+ %) surfactant were









purchased from Sigma-Aldrich. Ethanol and Ammonium hydroxide (29 wt% NH3 in

water) were purchased from Fisher scientific. Water was deionized to 18.2MQ cm-1

using an E-pure Barnstead model D4641 instrument.

Synthesis of Mesoporous Shell Silica Particles

25 ml of commercial EM75 silica colloid was diluted by adding 85ml of H20 and 10

ml ethanol. The solution was ultra-sonicated for 1 hr and then magnetically stirred for 1

hr to disperse the silica particles more uniformly. The pH of this solution was maintained

at 10 by adding ammonium hydroxide. A separate mixture of H20 (15 ml), ethanol (10

ml) and C16TAB (with varying concentration) was prepared and added to the above

solution under vigorous magnetic stirring. A relaxation time of 3 hours was allowed for

the surfactant to adsorb on the silica surface and then a solution of 1.25 ml TEOS and

3.5 ml ethanol was added slowly to the above mixture. The reaction was allowed to

proceed for 6 hours after which particles were centrifuged and washed with ethanol

three times to remove most of the surfactant present in solution. Particles were calcined

at 4500C for 6 hours to completely remove surfactant. Concentration of C16TAB used

was 0.78 mM, 1.56 mM, 3.1 mM and 6.2 mM.The particles were designated as Sample-

A, B, C and D respectively.

Characterization

The morphology of the core/shell particles was examined using High Resolution

Transmission Electron Microscope (HRTEM). Nitrogen sorption measurements were

performed on Quantachrome instrument. Samples were degassed at 2500C for 6 hours

prior to analysis. The surface areas were calculated by BET (Brunauer-Emmett-Teller)

method and the pore size distribution curves were obtained from the adsorption branch









by using BJH method. The total pore volumes were estimated from the adsorption

branch of the isotherm at P/Po=0.98 assuming complete pore saturation. The

monodispersity of the core/shell particles were shown using Field Emission Scanning

Electron Microscopy (FESEM) images. TGA-DTA and Fourier Transform Infrared

Spectroscopy (FTIR) measurements were done on Sample-C to show the removal of

surfactant after calcination.

Result and Discussion

Synthesis Method

The main aim of the work is to synthesis silica particles, with higher surface porosity in

one-step reaction for application such as chemical mechanical planarization.

Commercially available silica particles were used as seeds for nucleating porous silica

shell. Number of seeds was chosen carefully such that, a uniform and monodispersed

core/shell particles are synthesized. Various trials with different concentration of core

silica particles were carried out first, keeping the basic synthesis method same. Among

the 2.5 wt%, 5 wt%, 10wt%, 15wt% and 20 wt%, best results were obtained for seed

concentration of 5wt% and below. As the concentration of seed particles increased

severe coagulation, with poor particle dispersibility and non-uniform porous shell coating

was formed. Hence, hereafter all the particles were synthesized using 5wt%

concentration of seed.

CTAB Adsorption on SiO2 Nanoparticles

The study of CTAB adsorption on silica particles was critical in forming the porous shell.

Extensive work has gone into the study of cetyltrimethylammonium ion (CTA+) on SiO2

surfaces. Wei Wang [26], et.al reported that at lower surface coverage (less than a

monolayer), CTA' molecules were strongly bound to the SiO2 surface via their trimethyl









ammonium head groups. A bilayer sorption of the CTA was observed at higher surface

coverage and the sorption was attributed to the hydrophobic interactions between

aliphatic tails of CTA ions. Adsorption of CTAB on silica is mainly attributed to

electrostatic interaction between CTA+ ions and hydroxyl groups at silica surface. In our

work, the pH of the solution was maintained at 10 before CTAB was added. This

provided more negative charge sites on the surface of silica as a result of ionization of a

greater proportion of surface hydroxyl groups and as a result increased the electrostatic

attraction between CTA ions and silica surface. A 5- to 10-fold increase in CTAB

adsorption onto silica gel at pH 10 (compared with pH 5.6) at concentration range from

0.05 to 0.4 mM was reported by Fleming [27]. He also reported that most of the

adsorption of CTA+ molecules was observed in the first 10 s, followed by a slow rise (2

to 3h) after which the equilibrium excess was reached. In our synthesis method, a

relaxation time of hours after addition of CTAB was given for most of the CTA+

molecules to adsorb on the particles before TEOS was added to form the porous shell.

A 15% of ethanol in water was maintained in our reaction. The percentage of ethanol in

water affects the micellization of CTAB surfactant. The volume ratio of water/ethanol

affected the way in which CTAB molecules arrange onto the silica surface. Nazir [28]

et.al reported that critical micelle concentration (CMC) of CTAB in ethanol-water media

increases upto 10% ethanol and decreases on further addition of ethanol. This helps in

reducing the overall CTAB concentration used as a template in the synthesis of

core/shell particles. Micelle formation of CTAB in the bulk solution and on the silica

surfaces is considered better for formation of porous shell. The critical micelle

concentration (CMC) of CTAB surfactant is 0.92 -1 mM. A well defined porous shell was









formed on particles synthesized above this concentration of surfactant such as (C) and

(D).

Mechanism of CTAB Molecules Arrangement on Silica Particles






-J \ Vx






(1) Adsorption of C16TAB on negatively charged silica particles
(2) Addition of TEOS to form SiO2 Shell followed by calcination to remove
C1,TAB

Figure 3-1. Schematic of Core/Shell silica particles preparation

The critical micelle concentration (CMC) of CTAB surfactant is 0.92 -1 mM. The

mechanism of CTA+ ions arrangement on negatively charged silica surface with

increase in surfactant concentration is shown by a simple schematic in Figure 3-1. At

concentration below cmc, the surface of silica is covered by a single layer of monomers

and as the concentration of CTAB increases, more number of monomers tends to crowd

the surface forming bilayers and finally at higher concentrations well above CMC, the

surfactant aggregates as micelles. The samples A and B were prepared with

concentration of CTAB below CMC and samples C and D were prepared above CMC in

increasing order.









Surface Morphology of Porous Shell Silica and Pore Characterization

Figure 3-2 presents the FESEM micrographs of calcined sample-C core/shell

particles that shows particles are spherical, uniform and highly monodispersed. A closer

observation at the surface of these particles was done using high-resolution

transmission electron microscopy (shown in Figure 3-3). Sample-A which was prepared

using lowest CTAB concentration, showed a less rough surface morphology and the

porous shell was not visibly seen in the picture. As the concentration of CTAB increased

the thickness of the porous shell increased and the distinction between core and the

shell can be seen clearly. Sample-C showed a thicker porous shell compared to

sample-B (as marked using arrows in the figures). Sample-D showed a well defined

porous shell, with the thickness of the shell in the range of 10-12 nm and a much rough

surface compared to other samples.

















Figure 3-2. FESEM Images of calcined sample-C core/shell silica particles under
different magnification.

The BET surface area, total pore volume and BJH pore size for the porous shell

coated silica particles are listed in Table 3-1. Particles which were prepared with high

CTAB concentration (D) exhibited, highest BET surface area of 72.25m2/g compared to











































, .. .
."-. ,
i'~. =_.



- ,. 3r. 4


Figure 3-3. TEM micrographs of core/shell Si02 particles as increase in surfactant

concentration from (Al to D1).


:-
I' ., ... ;

I'' :
*-,~-~,a:~~~a~"~--::2~









that of sample (A) which resulted in a surface area of 47.17m2/g. The total pore volume

also increased linearly from particles A to D suggesting that, a prominent porous shell is

formed on particles with higher concentration of CTAB. The BJH pore size of the

particles was in the range of 1.38-2.198 nm. As reported in [16,17], particles with pore

size in this range were highly microporous in nature.

Table 3-1. N2 sorption measurement of Core/shell silica particles.
Sample [CTAB] BET surface Total Pore volume BJH pore
Name mM area (m2/g) (cm3/g) size (nm)
A 0.78 47.17 0.5722 1.38
B 1.56 49.79 0.6167 1.386
C 3.1 52.65 0.6784 1.46
D 6.2 72.25 0.776 2.198

TGA and FTIR Analysis

For TGA and FTIR analysis, sample-C, prepared using highest surfactant

concentration was used. The amount of template in as-synthesized and heat treated

core/shell silica particles were tested using thermo gravimetric analysis first. Most of the

surfactant in the as-synthesized sample was removed while centrifugation and washing

with ethanol for three times. The remaining surfactant was removed in the temperature

range of 150-3000C. This is confirmed from the TGA graph of as-synthesized sample-C

particles shown in Figure 3-4, which showed a weight loss of approximately 2.5% in this

temperature range, mainly due to the decomposition of C16TAB surfactant (the melting

and decomposition temperature of C16TAB is 2300C [29]). The sample which was

calcined at 4000C for 6 hours, showed almost a constant weight loss in the temperature

range of 200-8000C, indicating negligible concentration of surfactant left over after heat

treatment. This was further confirmed by FTIR analysis of sample-C.












100.0

99.5

99.0

98.5

98.0

97.5

97.0

96.5

96.0


100 200 300 400 500

Temperature (C)

Figure 3-4. TGA of core/shell silica particle- sample C


3500


600 700 800


3000 2500 2000

WVavenu mber( cm )


1500


Figure 3-5. FTIR of core/shell silica particle- sample C









The FTIR spectrum in Figure 3-5 shows two distinct bands in the 2950-2850 cm-1

region, which are due to the CH2 units of the C16TAB aggregates asymmetric and

symmetric vibrations. After heat treatment, the intensity of those two CH2-stretching

vibration bands significantly reduced indicating most of the surfactant removed after

heat treatment [30]. The other bands in the spectra such as 1621 cm-1, 1878 cm-1 and

3314cm-1 are attributed to the bending vibration of the associated water due to O-H

stretching frequency. The 1130 cm-1, and 800cm-1 band shown in the insert of Figure 3-

5 are attributed to the asymmetric stretching vibrational mode of Si-O-Si and symmetric

stretching of bulk Si-O-Si respectively.

Summary

Monodispersed core/shell silica particles, with hard core and microporous shell

silica particles have been prepared by simple method of surfactant adsorption on

optimum concentration of silica seed particles, followed by hydrolysis of TEOS and

finally removing the surfactant by heat treatment at 4000C for 6 hours. The morphology

of the porous shell was altered by changing the concentration of surfactant and

difference in morphology observed using TEM. The specific surface area, total pore

volume and pore size, obviously increased as the concentration of the surfactant

increased. Following this investigation, we are now able to tailor the surface porosity of

the silica particles of ca. 75 nm sizes just by varying the concentration of surfactant. The

thickness and porosity of the porous shell formed can be further manipulated by altering

TEOS concentration, which was not performed in our study. This possible tailoring of

silica particles is of great interest for chemical mechanical polishing of low-k dielectrics

in semiconductor industries to obtain less defective wafers.









CHAPTER 4
CHEMICAL MECHANICAL PLANARIZATION USING CORE/SHELL SILICA

Introduction

CMP slurries are designed to avoid surface defectivity on wafers during polishing

process. Common defectivity issues include surface scratches, indentations, surface

roughness, particle adhesion and corrosion. Controlling the size of the abrasives in the

slurry and size distribution of the particles will help to control micro-scratching on the

wafers. As reported [25], concentration of the particles also pay important role in surface

finish. The data presented clearly indicates that even slight increase in concentration of

large particles will degrade the quality of surface to greater extent. Other factors such as

time-dependent aggregation of particles, pH drift and long term stability of the slurry

also dominate the surface defectivity issue during CMP. Aggregation of particles as time

progresses increases the overall particle sizes and the wafers are subjected to higher

contact stresses, thereby increasing the surface defectivity. Likewise unstable slurries

result in particles settling onto the wafer surface and causes particle adhesion which is

difficult to remove during post-CMP cleaning.

Removal rates are important output variable in any CMP process. A high removal

rates are expected from well designed and perfect CMP slurry. In case of shallow trench

isolation CMP, the removal rates are typically 2000 A/min and that of metal CMP such

as copper, tungsten, it can be as high as 6000 A/min. The removal rates are governed

by various factors and incase of metal CMP, a thin oxide layer forms first, which is

subsequently removed by abrasive particles in the slurry. For better removal rates, the

time for oxide layer formation must be rapid since the particle interaction time is

relatively very fast. In case of Silica CMP, the surface is made softer by penetration of









water and forming a gel-like layer, followed by abrasive action of particles to remove the

surface. Chemicals in slurry play key role in achieving higher removal rates. The

chemical in the slurry react and form passivation layer with the wafer surface at much

higher rate than the abrasive particles present in the slurry. Additives such as oxidizers

are used to control the reaction rate and surface passivation.

In this work, I report CMP performance of low-k dielectric and Copper films using

slurry prepared by core/shell silica particles (synthesized by method explained in

Chapter 3). The particles used in slurry were microporous in nature and have higher

surface area. Another slurry was prepared using non-porous silica particles of

approximately 75nm size. Removal rates and surface roughness of each material (Black

diamond and Copper) were compared for non-porous silica and core/shell silica slurries.

Experimental

Materials

Silica colloid of approx.75nm particles were purchased from Silco Inc. Core/shell silica

particles prepared in lab. For CMP polishing, black diamond wafers were purchased

from Applied Materials Inc. and copper wafers from Wafer Net Inc. All wafers were cut

into 1" squares and edges smoothed by grinding operation followed by cleaning in

acetone and deionized water in ultrasonicator.

Slurry Preparation

Two slurries were prepared using core/shell silica particles and non-porous silica

particles (commercial silica colloid) for CMP polishing. The particles were mixed in

deionized water without adding any additives and the pH of the solution was adjusted to

9 by adding NH40H. The particles were dispersed by using ultrasonification in bath

sonicators, until all the aggregates were broken down. To find the effect of dispersion









after sonification, particle size analysis was done using laser diffraction particle size

analyzer (LS 13 320 Coulter Instrument) and FESEM used to confirm the particle size.

CMP Polishing Setup

Chemical mechanical polishing was performed in lab using a STRUERS TEGRA

POL-35 polisher along with a flow pump for slurry feed and Struers rotoforce polishing

head for conducting all the polishing experiments. The polishing unit has a 12 inch

diameter platen on which polishing pad is mounted. IC 1000/Suba IV stacked pads were

used for polishing. Downward pressure was applied pneumatically and the polishing

time and pad rotation speed set to desired values. The sample holder is a 2.25 inch

diameter stainless steel cylinder, with a height of 1.13 inches. A flat square recess is

machined in the center of one of the flat surfaces. A backing material is mounted inside

the recess. It brings the wafer slightly (0.05 inch) above the flat sample holder surface

and is made wet before the sample is put on it, in order to hold the wafer using capillary

forces. The experimental conditions were set as follows: the sample and pad rotation

speed was set at 100 rpm; the downward force was set at 6 Psi, and the sample was

offset by 3.5 inches from the centre of the pad. Slurry feed rate was set at 80ml/min and

time for polishing was 1 minute. After polishing the samples were ultrasonicated in

alkaline water to dislodge the particles adhering to the surface. Incase of black diamond

wafers, the polishing rate was determined by measuring the thickness of the films in

various marked regions of the wafer using FILMETRICS, a spectral reflectance

technique for thickness measurement, before and after CMP. Incase of copper wafers,

a Four-Point Probe was used to measure the resistivity of the copper, before and after

CMP. From resistivity, the thickness of wafers was calculated. The surface roughness

was characterized using atomic force microscopy (AFM).









Results and Discussion

Properties of Core/Shell Silica Particles

Hard core microlmeso pores Porous
Shell






Hard core

(1) (2) (3)
Non Porous Silica Fully Porous Silica Core/Shell Silica


Figure 4-1. Schematic of different morphology of silica nanoparticles

For better understanding the silica particles, three schematics are shown in Figure

4-1. The first one represents non-porous silica, second one fully porous silica and third

one, a non-porous core/porous shell silica particles. The figure is self-explanatory and

one can understand that the density and hardness of silica particles are very much

affected by pores present in the particles. Figure 4-2 shows TEM images of (a) non

porous silica particles and (b) core/shell silica particles. From the images, one can

easily differentiate between a hard core and porous shell silica particles. The particle

sizing was performed on LSS coulter instrument for both the slurries prepared. In case

of core silica particles (slurry A), the average size of particles was 75-80 nm and incase

of core/shell silica particles (slurry B), the average size of particles was 98 nm. Overall

the particle distribution was uniform and normal for both the slurries (as shown in Figure

4-3). A nitrogen sorption test was performed on core silica particles and core/shell silica

particles using Autosorb-1 instrument.


























Figure 4-2. Comparison of TEM images of (A) core and (B) core/shell Silica


Particle Diameter (pm)

Figure 4-3. Particle size distribution of abrasives in slurry A and B









The pores in core/shell silica particles were microporous in nature with increase in

total pore volume compared core silica particles. The adsorption/desorption curves of

core and core/shell silica particles are shown in Figure 4-4. In fully porous silica

particles, pores run through out the particle and have very low hardness, such particles

are used in CMP for producing better surface finish on low-K dielectric materials. They

are less useful in polishing metal films due to their poor hardness. In case of non-porous

silica particles, the hardness is very high and mostly utilized in application were higher

removal rates are required. A new approach in designing CMP slurry is to choose

particles with optimum hardness and porosity level so that surface finish is better and

removal rates are also not compromised.


0.4 0.6
Relative Pressure (P/Po)


Figure 4-4. Nitrogen sorption isotherm of silica core and core/shell silica particles.









Polishing Rate and Surface Roughness

As mentioned, CMP polishing of copper and low-k dielectric material such as

black diamond were conducted in two different slurry system, one with non-porous silica

particles [Slurry A] and other with core/shell silica particles [Slurry B]. The concentration

of solid loading and pH of the slurry was fixed as 5wt% and 9 respectively for both slurry

systems A and B. Table [4-1] shows the comparison of polishing rates among two

wafers for both the slurries.

Table 4-1. Polish rates and surface roughness of black diamond and copper wafers.
Material Removal Rates(A/min) Surface Roughness RMS (nm)

Slurry A Slurry B Slurry A Slurry B
Black Diamond 388 + 15 720 + 15 0.57 0.56
Copper 430 +15 882 +15 1.85 1.73

Black diamond is a low-K dielectric material developed by Applied Materials Inc. It

is a carbon doped SiO2 film deposited by PECVD (Plasma Enhanced Chemical Vapor

Deposition) technique. Due to the carbon doping, dielectric constant is lowered below 3

and is used for 590nm copper/low k interconnects. The hardness of black diamond film

is roughly in the range of 3-4.5 GPa [31]. This value is higher than copper films and

lower than that of normal Si02 dielectric films. Hence removal rates using any abrasives

particles are supposed to be in the decreasing order of copper, black diamond and Si02

films. However due to various other mechanisms involved during CMP, this order may

not be true. For example as reported by KS CHoi [24] even though hardness of black

diamond is lower than Si02, the removal rates are higher for Si02 film due to more of

chemical activity of slurry than mechanical action of the abrasives. In our work, the

removal rate of copper was higher than that of black diamond for both the slurry

systems. This is due to the fact that copper wafers are less hard compared to black









diamond. Slurry properties such as abrasives, pH plays a less important role

considering the hardness of the wafer. As observed from the table [4-1], the removal

rates are higher for Slurry-B for both the wafers. Slurry-B was prepared using core/shell

silica particles and Slurry-A using non-porous silica particles. One would expect the

removal rates of Slurry-A to be higher since the particles are nonporous in nature. But

the opposite is observed. This behavior of core/shell particles can be explained on the

basis of hardness of core and high surface area of shell.

The hard core of the core/shell silica particles helps in retaining the overall

hardness of the particles when compared to fully porous silica particles. This hardness

coupled with high surface area caused by micro-pores present in the shell, increases

the material removal rate. In case of carbon-doped SiO2 (i.e.) black diamond wafer, the

removal mechanism is explained by formation of gel-type layer on the wafer surface due

to dissolution of silica film by chemical reaction, followed by mechanical polishing.

Incase of copper wafers, the material removal mechanism is due to the formation of

oxide passivation layer such as Cu20 on the surface of copper, in reaction with the

chemicals present in slurry, suitably transported by abrasive particles. This is followed

by removal of the passivation layer along with base copper layer due to abrasive action

of particles in slurry. In general, the chemical activity at the wafer surface increases

when the abrasive particles have higher surface area. This is the case observed in

slurry B and the reason for higher removal rates in case of both the material.

Slurry B (core/shell silica) yielded better surface roughness on the polished wafers

for both copper and black diamond materials. This can be explained by the surface

morphology of the core/shell particles. The microporous nature of the shell make the









surface less dense, thereby reducing the surface hardness of the particles compared to

the core. This induces very less scratches on the surface. Due to surface porosity of

core/shell silica particles, the dielectric constant is lowered. As a result Hamaker

constant is very low for these particles. [Since Hamaker constant is linearly proportional

to van der Waals force between two surfaces.] This implies the core/shell silica particles

have less adhesion on the surface that is polished due to very low van der Waals

forces. From the table [4.1], we can see that the surface roughness of copper is very

much high compared to that of black diamond for both the slurry systems. This is partly

due to the fact that the initial copper wafers (before polishing) had high surface

roughness than black diamond and partly due to hardness of base material, which is

lower for copper, prone to more surface scratches. Apart from some the reasons

mentioned here, various other factors such as solid loading, pH of the slurry, pad

characteristics play an important role.

Summary

Core/shell silica particles slurry was successfully prepared and used for chemical

mechanical planarization of copper and low-k dielectric material, black diamond. Non-

porous silica particles slurry was prepared to compare the behavior of wafers in two

different slurry systems.. CMP of the hard core/microporous shell particles slurry

produced higher removal rates and better surface finish compared to non-porous silica

particles. Filmetrics and Atomic force microscopy was used to characterize wafers. This

research work is an attempt to develop functionalized abrasives for targeted CMP

application.









CHAPTER 5
CONCLUSION

The synthesis of finely tuned nanoparticles for various CMP applications has lead

to advanced circuit designs and multi level chip manufacturing. As copper interconnects

are replacing aluminium in a fast pace these days and different low-k dielectric materials

developed for multilevel designs, critical understanding on CMP polishing of these

materials is necessary. The whole semiconductor industry has benefited from the

development of CMP process and its integration in main stream semiconductor

processing in a big way. CMP consumables have become an independent market now-

a-days. Abrasives particles are nearly fined tuned and synthesized in large quantity for

targeted application. One such attempt has been made in this thesis work to synthesis

finely tuned silica particles for CMP of Copper films and low-k dielectric materials.

According to experiments and discussions in previous chapters, the following

conclusion for the thesis could be summarized:

* A simple and one-pot synthesis technique developed to synthesis Core/Shell silica
particles with hard core and microporous shell.

* The specific surface area and total pore volume of the particles ranged 37-72 m2/g
and 0.233-0.776 cm3/g respectively as measured by N2 adsorption/desorption
technique.

* The surface morphology of the particles studied well using high-resolution
transmission electron microscopy (HR-TEM) and clear distinction between core
and shell observed.

* Chemical Mechanical Polishing successfully performed on copper wafer and low-k
dielectric material such as black diamond using core/shell silica particles and
commercial non-porous silica particles

* The core/shell silica slurry produced higher removal rates and better surface finish
in CMP polishing of Cu and black diamond wafers, compared to that of non-porous
silica slurry









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BIOGRAPHICAL SKETCH

Kannan Balasundaram was born in Coimbatore, an industrial city in southern part

of India. Coimbatore, hailed as the Manchester of South India, is an important foundry

cluster in India which produces more than 25000 to 40000 tonnes of castings monthly

catering to various domains such as automobiles, oil and gas industry and domestic

applications. Coming from such a backdrop it was natural for him to choose

Metallurgical Engineering as mainstream after schooling. In 2001, he joined one of the

premier engineering colleges, PSG College of Technology in Tamil Nadu, India. His

interest in metals and materials was well harnessed during four years of undergraduate

study. After a short stint at ESSAR OIL & REFINERY as graduate engineer trainee, he

joined EMERSON Process management as application engineer.

The work was to design and develop control valves for various upstream and

downstream industries. He had to take up more challenging jobs and think globally

during his deputation to Emerson Asia-Pacific headquarters at Singapore. After 3 years

of industrial exposure, he decided to quit work and pursue higher studies. He joined the

master's program in the Materials Science and Engineering Department at the

University of Florida, USA. Until now, he has been working under the guidance of Dr.

Rajiv Singh. During this period, he worked on semiconductor materials and developing

abrasive nanoparticles for Chemical Mechanical Polishing. He has worked closely with

MAIC and PERC during this period. Under the able guidance of his advisor and

committee members he was able to complete the research work successfully.





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1 SYNTHESIS AND CHARACTERIZATION OF CORE/SHELL SILICA NANOPARTI C LES FOR CHEMICAL MECHANICAL PLANARIZATION OF LOW -K DIELECTRIC AND COPPER WAFERS By KANNAN BALASUNDARAM A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DE GREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2010

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2 2010 Kannan Balasundaram

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3 This work is dedicated to my parents

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4 ACKNOWLEDGMENTS I would like to express my sincere thanks to my advisor, Dr. Rajiv K. Singh, for giving me an opportunity to work under his guidance. His encouragement and support during the course of the study was outstanding. I am also grateful to Dr. Hassan El Shall, D r. Stephen J. Pearton for serving as committee members and supervising my study I would like to thank Dr. Kevin Powers of Particle Science and Technology for sharing his valuable knowledge with me during my research. I would also like to acknowledge my co -researches Dr. Purushottam Kumar Mr. S ushant Gupta, Mr. Myoung -Oh for their valuable suggestions and support while carrying out my experimental work. I would like to re cognize the help of the staff, Ms. Kerry Siebein, in MAIC (Major Analytical Instrumentation Center) and Gill Brubaker PERC (Particle Engineering Research Center) for their help in training me using the equipments and characterizing my samples. Finally, I would like to thank my parents for their financial support as well as moral support all through my life in US. I also owe sincere thanks to all my friends who have been supportive in my life.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...................................................................................................... 4 LIST OF TABLES ................................................................................................................ 7 LIST OF FIGURES .............................................................................................................. 8 ABSTRACT .......................................................................................................................... 9 CHAPTER 1 INTRODUCTION ........................................................................................................ 10 Motivation .................................................................................................................... 11 Objective ..................................................................................................................... 12 2 BACKGROUND .......................................................................................................... 13 Introduction to Chemical Mechanical Planarization ................................................... 13 CMP Slurry Preparation and Characteristics ............................................................. 17 Introduction to Nanoporous Materials ........................................................................ 20 Micro/Mesoporous Silica Particles ............................................................................. 20 Advantage of Nanoporous Silica in CMP ................................................................... 22 3 SYNTHESIS OF CORE/SHELL MICROPOROUS SILICA PARTICLES ................. 24 Introduction ................................................................................................................. 24 Experimental ............................................................................................................... 25 Materials ............................................................................................................... 25 Synthesis of Mesoporous Shell Silica Particles .................................................. 26 Characterization ................................................................................................... 26 Result and Discussion ................................................................................................ 27 Synthesis Method................................................................................................. 27 CTAB Adsorption on SiO2 Nanoparticles ............................................................ 27 Mechanism of CTAB Molecules Arrangement on Silica Particles ...................... 29 Surface Morphology of Porous Shell Silica and Pore Characterization ............. 30 Summary ..................................................................................................................... 34 4 CHEMICAL MECHANICAL PLANARIZATION USING CORE/SHELL SILICA ........ 35 Introduction ................................................................................................................. 35 Experimental ............................................................................................................... 36 Materials ............................................................................................................... 36 Slurry Preparation ................................................................................................ 36 CMP Polishing Setup ........................................................................................... 37

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6 Results and Dis cussion .............................................................................................. 38 Properties of Core/Shell Silica Particles .............................................................. 38 Polishing Rate and Surface Roughness ............................................................. 41 Summary ..................................................................................................................... 43 5 CONCLUSION ............................................................................................................ 44 LIST OF REFERENCES ................................................................................................... 45 BIOGRAPHICAL SKETCH ................................................................................................ 48

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7 LIST OF TABLES Table page 3 -1 N2 sorption measurement of Core/shell silica particles. ....................................... 32 4 -1 Polish rates and surface roughness of black diamond and copper wafers. ......... 41

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8 LIST OF FIGURES Figure page 2 -1 Chemical Mechanical Polishing set up. ................................................................. 14 2 -2 Different wafer structures polished by CMP process and low k dielectric CMP, Tungsten metal CMP ................................................................................... 16 2 -3 SEM Images of abrasive particles used in CMP Alumina coated silica and Ceria coated silica abrasives ................................................................................. 18 2 -4 Material removal rate and friction force of silica as a function of solids loading of 0.5 m silica abrasives ........................................................................................ 19 3 -1 Schematic of Core/Shell silica particles preparation ............................................. 29 3 -2 FESEM Images of calcined sample-C core/shell silica particles under different magnification. ........................................................................................... 30 3 -3 TEM micrographs of core/shell SiO2 particles as increase in surfactant concentration from (A1 to D 1). ............................................................................... 31 3 -4 TGA of core/shell silica particle sample C ........................................................... 33 3 -5 FTIR of core/shell silica particle sample C ........................................................... 33 4 -1 Schematic of different morphology of silica nanoparticles .................................... 38 4 -2 Comparison of TEM images of core and core/shell Silica ................................... 39 4 -3 Particle size distribution of abrasives in slurry A and B ........................................ 39 4 -4 Nitrogen sorption isotherm of silica core and core/shell silica particles. .............. 40

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9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science SYNTHESIS AND CHARACTERIZATION OF CORE/SHELL SILICA NANOPARTICLES FOR CHEMICAL MECHANICAL PLANARIZATION OF LOW -K DIELECTRIC AND COPPER WAFERS By Kannan Balasundaram August 2010 Chair: Rajiv K. Singh Major: Material s Science and Engineering Monodispersed core/shell silica particles with a hard c ore and microporous shell have been synthesized by surfactant template method. Ca. 75nm silica particles were used as core and Cetyltrimethylammonium bromide (C16TAB) as template for generating microporous shell. Concentration of the surfactant was varied and the growth of porous shell analyzed using high-resolution transmission electron microscopy and nitrogen adsorption. TGA and FTIR were used to confirm the surfactant removal after heat treatment. The synthesized particles were monodispersed and had a hard core and highly microporous shell with pore size in the range of 1.3 -2.2nm and total pore volume in the range of 0.570.77cm3/g. The Chemical mechanical Planarization (CMP) performance of core/shell silica particles were analyzed and compared with that of core silica particles. Polishing was done on copper wafers and low -k dielectric material such as black diamond. The core/shell silica particles produced higher removal rates and better surface finish for both the wafers. Spectral reflectance technique and atomic force microscopy (AFM) were used as analyzing tool.

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10 CHAPTER 1 INTRODUCTION The demand for increased circuit density, functionality and versatility has lead to tremendous advancement in the front end of the chip manufacturing line. One such de velopment in semiconductor industry is Chemical mechanical planarization (CMP) process. The ever growing CMP technology has made possible more intricate designs with decreased feature size and multi level interconnects for next generation nanoscale device s [1]. The science of CMP is quite different from conventional semiconductor manufacturing processes like ion implantation, photo lithography, thermal annealing and so on. These traditional processes are well established and understand by both academia and industry. However, in the case of CMP process, the whole idea and technology was developed and put into use by industry itself. This made it difficult for researchers in academia to fully understand the science and theory behind CMP process. As time progr essed, a new knowledge base and entire skills was developed involving CMP process variables such as particle technology, tribology, wet and surface chemistry, fluid flow, properties of polymers and so on. CMP slurries were given more importance for the abr asive particles and chemical additives used and it has become a potential market by its own. The abrasive particles generally in nanometer scale are one of the largest uses of present nanotechnology. The development of CMP slurries took place simultaneousl y with development of synthesis techniques for various nanoparticles. A whole range of nanoparticles was developed in short period of time and particles were also modified and functionalized for specific targeted applications. Nanoparticles like iron, cop per, gold, silver, silica, alumina, ceria etc, have become common these

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11 days. Functionalized nanoparticles such as core/shell silica coated gold [2] alumina coated Titania [3], silver coated magnetite [4] ceria coated silica particles [5] are being widely researched now a days and few of these materials have already found potential applications. Those synthesis methods which yield large quantities of nanoparticles and possibility of bulk production are always well adopted by in dustry. Silica nanoparticles is one such material which has versatile application and can be synthesized in large scale. Ability to synthesize in wide size ranges, easy to functionalize and modify the surface has made silica nanoparticles ideal candidate for CMP slurries. Motivation This research focuses on synthesizing hard core porous shell silica nanoparticles for CMP of copper and lo w -k dielectric material such as black diamond. Conventional non-porous silica particles and fully porous silica particles has few disadvantages in CMP performance of low k dielectric materials. The non -porous silica particles have high youngs modulus and are harder abrasives resulting in high penetration depth on polishing surface. This produces poor surface finish and more number of scratches during C MP process. In case of conventional fully porous silica particles, due to pore structure running throughout, the particles have very low density reducing the hardness of the nanoparti c l es. This affects one of the key outputs of CMP i.e. removal rate. Porou s silica particles produces superior surface finishes with very low surface defectivity on the polishes wafer with a compromise on removal rate. There has always been challenge to use functionalized nanoparticles in CMP slurries which could not only yield higher removal rates but also delivers wafers with low surface defectivity This was the key motivating factor for this research work. By functionalizing the silica particles to

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12 be porous at the surface while still maintaining a hard core, the overall perf ormance of low -K dielectric and copper CMP can be improved. Objective The objectives in this thesis are as follows: Synthesize of Core/Shell silica particles with hard core and highly porous shell. To study the effect of surfactant concentration on porous shell formation and explain the mechanism. Perform CMP process on t wo different wafers such as copper and black diamond using core/shell particles and non-porous silica particles. Compare the results and explain the behavior of core/shell nanoparticles on different wafers materials. The first objective was achieved by surfactant templated synthesis method. A suitable cationic surfactant such as cetyl trimethyl ammonium bromide was chosen to act as structure directing agent. The porous coated samples wer e characterized using transmission electron microscopy and Autosorb1 instruments. Following the synthesis of nanoparticles, suitable slurry was prepared for performing CMP polishing. Polishing was achieved on CMP STRUERS TEGRA POL 35 equipment and results analyzed using atomic f orce microscopy (AFM) and spectral analysis technique as Filmetrics.

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13 CHAPTER 2 BACKGROUND Introduction to Chemical M echanical P lanarization CMP has become one of the integrated operations of semiconductor manufacturing and has lead to the development of next generation nanoscale devices. CMP not only eases the design and production of high density Integrated Circuits (IC) by eliminating several photolithographic and film issues generated by severe topography but also enables greater flexibility with process complexity and associated designs. With the development of process technologies and automation in a very fast pace, the use of CMP process has expanded greatly CMP was just used to remove topography from silicon oxide and few oth er surfaces earlier, but now it has been successfully used to planarize Shallow trench Isolation (STI) layers, trenched metal Cu interconnections, tungsten plugs and low resistivity metals. In spite of all the advantages and developments, CMP challenges bo th academia and industries due to large number of input and output variables which is making it difficult to optimize the process and is being addressed individually. Mechanical grinding alone may theoretically achieve palanarization but the surface damage is high as compared to CMP. Chemistry alone, on the other hand, cannot attain planarization because most chemical reactions are isotropic. Combination of both has always yielded better results. CMP processes produce both global and local planarization b y combining chemical and mechanical interactions using slurry composed of chemicals and submicron-sized particles. The process consists of moving a sample surfaces against a pad and to feed the slurry between the sample surfaces and pad to achiev e palanari zation. Figure 2 1 shows a schematic of chemical

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14 mechanical polishing setup. The abrasives particles in the slurry induces mechanical damage to the surface, loosening the material for enhanced chemical attack or fracturing off the pieces of the surface and easy removal thereafter. Out of all the parameters involved in obtaining best results from CMP, there are three main components which must be given careful attention. They include, the surface to be polished, the pad and the slurry. Figure 2 1 Chemic al Mechanical P olishing set up. The surface to be polished can be classified based on metals, dielectrics and special materials. CMP of metals includes polishing surfaces of Polysilicon, Al and alloys, Cu and alloys, Ta, W, Ti and alloys such as TiN and Ti NxCy. Increasingly metal CMP is being used for the formation of studs and interconnections. There are several advantages to using CMP to remove metal overburden. First, metal CMP yields a high degree of local planarity. The high degree of planarity allows vias to be stacked directly on top of each other. Stacked vias result in considerable reduction in circuit area over

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15 staggered vias. Another group of surfaces polished using CMP includes dielectrics such as silicon dioxide, phosphosilicate glasses (PSG), borophosphosilicate glasses ( BPSG ), Si3N4 and SiOxNy. Figure 2 2 illustrates CMP polishing of various substrates used in semiconductor industry. Though many of the oxide CMP remain proprietary, many studies have been undertaken to understand the mechanisms of material removal in these types of surfaces. There are many factors which influences the performance of oxide CMP such as abrasive materials used, slurry pH, solid loading, etc. Some of the benefits of oxide CMP includes improved bulk material removal, lithographic capability and reduced defect densities. Some of the special surfaces poli shed using CMP method includes aerogels, high K dielectrics, h igh Tc superconductors, optoelectronic materials, plastics and ceramics. These materials are planarized to be used in high end applications such as flat panel, packaging, advanced devices and circuits. Another key component for better CMP results is characteristics of polishing pad. The role of pad and its mechanical properties such as surface roughness and sur face porosity play a key role in determining polishing rate and planarization ability of the CMP process. P ad porosity is indicated by s pecific gravity ; the lower the specific gravity, the higher the porosity. Pad porosity aids in slurry transportation, removal of reaction products from polishing site. Pad hardness and compressibility have been found to influence planarity. The harder and more noncompressible the pad, the less it will bend and conform to the wafer surface to remove material at lower regions The pad materials are generally composed of polyurethane foam matrix with diamond or other filler materials. P ads are often tailored to required application and expectations. Continuous use of pad for various runs leads to degrading surface properties and poor

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16 CMP results; hence it is necessary to condition the pad frequently between trials for longer life and better outcome. Figure 2 2 Different wafer structures polished by CMP process (A) and (B) Low k dielectric CMP, (C) Tungsten metal CMP A sepa rate section has been dedicated to slurry preparation and characteristics. With many of advantages discussed, some of the disadvantages of CMP and challenges it faces are explained here. The main disadvantage of CMP is its optimization. An entire new tool set including metrology and process control tools is required to make CMP more robust. Added to this, high circuit density and advanced level of pattern geometry effects result in narrow design, increasing the overall cost of the circuit design. Some of th e other problems include defects arising from CMP

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17 process such as scratching from the abrasive materials used in slurry, residual abrasive particles and corrosive attack of chemicals used in slurry, delamination at weaker interfaces, stress cracking and variation in oxide layer thickness. Post CMP cleaning has always posed problems in the entire cleaning process, which is being addressed by industry now a days. The main challenge that CMP faces is the integration into semi conductor manufacturing line. Since most of the procedures and key notes are proprietary, it is difficult to bring one single methodology for CMP process and optimize the system. Also a detailed understanding of material removal and surface plan arization during CMP is lacking. W ith market d emands increasing day by day, these critical issues must be addressed in an effective manner. CMP Slurry P reparation and C haracteristics Slurries provide both the chemical action through the solution chemistry and the mechanical action through the abrasives. High polishing rates, planarity, selectivity, uniformity, post -CMP ease of cleaning including environmental health and safety issues, shelf life, and dispersion stability are the factors considered to optimize the slurry performance. Chemical reagents i n the CMP slurry react with the wafer surface being polished forming a chemically modified top layer with desirable properties compared to the initial wafer surface. Etch rate is dependent on slurry composition. Any commercial CMP slurry will have the chem ical agents such as oxidizers, buffering agents, slurry stabilizers and complexing agents. Oxidizers are generally added to metal CMP slurries due to the fact that, they react with metal surfaces to raise the oxidation state of the metal, resulting in eith er dissolution of the metal or the formation of surface film on the metal. On the other hand complexing agents are added to increase the solubility of the film being polished. Buffering agents are added to keep the slurry pH constant

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18 throughout the volume and over time .The overall concentration of all these chemicals added are to be monitored carefully since they increase the reaction rates at the polishing surface. Abrasives in the slurry play the very important role of providing mechanical action during polishing. Commonly used CMP abrasives are SiO2, Al2O3 and CeO2 particles. Various multifunctional and tunable particles such as ceria coated silica [6], alumina coated silica particles [7] (shown in Figure 2 -3 ) are becoming popular The chemically modifi ed surface layer of the wafer is abraded continuously with the submicron size slurry abrasives resulting in material removal. Figure 2 3 SEM Images of abrasive particles used in CMP (a) Alumina coated silica [Ref.7] and (b) Ceria coated silica abrasi ves [Ref .5] To achieve an optimal polishing performance with minimal deformations and good planarity, it is necessary to optimize, the rates of chemical modification and mechanical abrasion. The intensity of mechanical abrasion also varies with the slurry particle size and concentration, as these factors determine the load applied per particle. Furthermore the frequency of abrasion depends on the number of slurry abrasives in contact with the

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19 wafer surface. Therefore, abrasive particle size and concentrati ons as well as the particle size distribution are very important parameters while designing the slurry. This is illustrated in F igure 2 -4 which shows material removal rate and frictional force of silica as function of solid loading of 0.5m silica abrasiv es. The effect of particle size distribution in form of agglomerates has been reported [ 8 ]. A small variation in any one of the above parameters may result in major changes in the particle-substrate interactions and material removal rates vary resulting in poor process control. Hardness of a brasive particles in slurry plays important role in achieving higher removal rates, however care must be taken to minimize surface damage. Figure 2 4 Material removal rate and friction force of silica as a function o f solids loading of 0.5 m silica abrasives (Ref [ 9 ]) Some of the other key parameters to be taken into account while preparing CMP slurry are the viscosity of the slurry, isoelectric point (pH), slurry flow rate and stability of the abrasive s Viscosity a ffects slurry transport across the wafer and lubrication of the wafer -pad interface. The more viscous the slurry is, poor is the transport of reactants and products to and from the wafer surface. Hence optimal viscosity is to be

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20 maintained. The isoelectric points (IEP) is the pH at which abrasi ve surface is charge neutral. The charge on abrasive particles determines the mechanism of material removal on various surfaces. The importance of this factor is currently being researched and very few reports available. Slurry flow rate normally having units of l/min or ml/min is the amount of abrasives delivered to the pad while polishing. It significantly affects the removal rate and lubrication properties of the system. Introduction to Nanop orous M aterials IUPAC h as classified porous materials based on their pore sizes. Materials with pore diameter of less than 2 nm are considered as microporous, those with pore diameters greater than 50 nm are referred to as macroporous and those that fall in between 2 to 50 nm ar e mesoporous materials. Some of the commonly synthesized nano porous materials are silica and alumina. Other oxides of titanium, zirconium, cerium, tin porous materials are also being widely researched. The main reason why these porous materials were able t o find wide range of applications in industry was due to the fact that the pore size, pore integrity and the ordered and disordered nature of the pores can be controlled precisely. There are vari ous methods of synthesis of porous materials such as self ass embly, templated self assembly, Sol -gel processing and spray drying methods. However the most common method which is widely used is the surfactant templated synthesis. Some of the potential applications of micro/ mesoporous materials include industrial catalysis, separation technology, environmental protection, electrochemistry, membranes, sensors, optical devices and polishing. Micro/Mesoporous Silica P articles P orous silica particles are inorganic materials first developed by researchers in Japan a decade back. It was further industrially developed by Mobil Corporation

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21 laboratories and nomenclatured as Mobil crystalline of Materials (MCM -41, MCM48, MCM-50) with hexagonal, cubic and lamellar mesostructures respectively and different morphologies. Their por e size and wall thickness do not go beyond 4.0 nm and 2.0 nm, respectively. Other popular mesoporous particles are SBA type materials with different mesostructures and porous characteristics somewhat similar to the MCM X type materials.SBA 15and SBA 16 sil ica (SBA: Santa Barbara University) with large pore s izes and thicker wall were prepared. Mesoporous silica particles have been synthesized by various methods O ne of the most commonly synthesized ways is in the presence of surfactants as templates for the poly condensation of silic a species, originating from different sources of silica such as sodium silicate, alkoxydes -TEOS (tetraethyl orthosilicate) and TMOS (tetramethyl orthosilicate) Synthesis conditions such as source of silica, type of surfactant, i onic strength, pH and composition of the reaction mixture, temperature and duration of the synthesis effect the surfactant micellar conformation, the silica -surfactant interactions and the degree of silica poly condensation. These conditions determine the characteristics of the porous structure. Several other synthesis methods have been reported. Nano porous silica particles are interesting materials for high performance liquid chromatography application (HPLC) due to their high surface area and their organized porous structure. Additionally their content of silanol groups as well as their chemical and mechanical stability under chromatographic conditions makes it well suited for the application. Different nanostructures are used as supports for immobilizat ion of bioactive enzymes and drugs. The most common immobilization methods are adsorption, covalent bonding, cross -linking and entrapment. Since s ilicates are

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22 biocompatible, nanoporous silica particles are well suited as support for controlled drug deliver y systems or for immobilizing enzymes, which are used as biosensors and in bioconversion processes. The immobilization process is very efficient if the support exhibits high surface area and size of the pores is similar or slightly higher than the diameter of the biomolecule. The aluminosilicates and mesoporous silicas are widely used in heterogeneous catalysis as catalysts or as su pport for the catalyst The nano porous particles are very promising candidates for this application due to their high surface a rea and pore volume, besides of the possibility of surface modification and pore distribution control. The adequate diffusion of molecules through the catalyst pores allows the direct interaction with the acidic sites on the wall surface, promoting the con versions. Macropores formed between these particles allow a fast mass transfer to the surface of the primary particles. Incase of bio -imaging mesoporous silica particles are considered as highly efficient MRI contrast agents and its usefulness is being res earched in bioengineering field extensively. Other significant applications of nano porous silica particles are in CMP polishing though most of the reports are either not published or in proprietary with industries. Advantage of N ano porous Silica in CMP Na noporous silica particles have reduced density due to high porosity; as a result the H amaker constant is very low for these particles, which implies less adhesion on the surface that is polished. Due to lower hardness of these particles compared to convent ional non porous silica particles, impregnation of these particles on the wafer surface can be prevented. Moreover the porous silica particles have low refractive index and dielectric constant due to surface porosity of these particles. This reduces the van der waals attractive force while polishing and one can expect minimal indentation and

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23 very less surface damage on the wafer surface. Another main advantage of these nanoporous silica particles is reduction in scratches in the polished surface. The penetr ation depth of the scratches varies linearly with the particle size and inversely with the youngs modulus of the impacting abrasive particles. Hence in order to reduce the depth of the particle indent and resulting scratches, the particle size as well as the Youngs modulus of the particle should be reduced. By using nanoporous silica particles, this can be achieved which would result in significantly reducing the scratches. The low normal stress on these nanoporous silica particles compared to that of nonporous particles reduces the film delamination effect considerably. Most of the CMP polishing of low k dielectric materials is done at alkaline pH. The replacement of conventional silica particles with the porous particles will not alter the slurry chemis try and hence by modifying the surface morphology with high porosity and surface area particles will not only improve the removal rates but also high quality surface finish can be achieved. Thus the synthesis of wide range of tailored nanoporous silica par ticles, which are highly monodispersed are potential candidates for CMP

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24 CHAPTER 3 SYNTHESIS OF CORE/SHELL MICROPOROUS SILICA PARTICLES I ntroduction Silica has become one of the easily synthesized nanoparticles, ever since Stober et al. [1 0 ] introduced s ol gel process for growing particles of sizes ranging from nm to m in diameter, simply by varying the catalyst and precursor concentrations. This has lead to various developments in synthesis of multi -functional silica particles such as microporous & mesoporous silica and core-shell particles with different components in the core and shell layers [ 11 ]. Porous Silica spheres have attracted many researchers for its wide range of applications such as chromatography, catalysis, drug delivery, waste water treat ment, etc. [12 13 14 ]. Another potential application of these highly porous silica particles are in Chemical mechanical planarization (CMP) as abrasives for polishing dielectric layers on Si wafers [ 15 ]. The particles can be nearly tuned functionally to ge t desired results in such applications. Various methods have been employed to synthesis porous silica particles with controlled pore systems [ 16,17 18]. However the most common method is the surfactant templated synthesis (STS). Here a cationic surfactant such as nalkyl trimethyl ammonium bromides (CnTAB) is mixed in the water alcohol mixture, followed by polymerization of alkoxide precursor such as tetraethyl orthosilicate (T EO S) and finally removing the template by calcination [19 ] or other methods [ 20, 2 1 ]. Nanoparticles synthesized by such methods are porous throughout. A modification in the conventional porous silica particles is synthesis of hard core and porous shell silica particles. Jungo Ho [22] and M. Mesa [ 23 ] et al. synthesized such particles in the size range of 400 -

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25 500nm with hard core and porous shell of ca. 50nm in thickness used for HPLC (high performance liquid chromatography) application. Here, we report a modified and simple method for synthesizing highly microporous shell with hard core silica particles by using C16TAB as structure directing agents on 75nm silica particles. Concentration of surfactant was varied and effect on porous shell formation and increase surface area were analyzed. As the particles were highly porous at surface wit h a hard core and also highly monodispersed, they should be of great interest for CMP slurries. The main advantage of core/shell silica particles is to obtain less defective polished surface after CMP while still maintaining optimum removal rates. The high surface porosity influences the refractive index and dielectric constant of the silica particles, which decreases van der Waals attractive forces [ 24 ]. As a result, one can expect lower adhesion force between porous abrasive slurry particles and wafer sur face resulting in reduced indentation and minimal scratches on the surface. Such surface defectivity is always a concern in case of conventional nonporous silica particles depending on their sizes [ 25 ]. Advantage of having a hard core is to maintain highe r removal rates of the dielectric layers, which are otherwise difficult to obtain, incase of fully porous silica particles. Overall, the core/shell silica particles are useful in improving the performance of low k dielectric CMP process Experimental Mater ials All the solutions were prepared using analytical grade reagents. Silica colloid EM7530A (with average particle size of 75nm, having a 30% solid concentration in a H20/NH4OH solution) was provided by Silco International, Inc. Tetraethyl orthosilicate (TEOS, 98 wt %), Cetyl trimethylammonium bromide -C16TAB (99+ %) surfactant were

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26 purchased from Sigma Aldrich. Ethanol and Ammonium hydroxide (29 wt% NH3 in water) were purchased from Fisher scientific. Water was deionized to 18.2M cm1 using an E -pure Barnstead model D4641 instrument. Synthesis of M esoporous Shell Silica Particles 25 ml of commercial EM75 silica colloid was diluted by adding 85ml of H2O and 10 ml e thanol. The solution was ultra-sonicated for 1 hr and then magnetically stirred for 1 hr to disperse the silica particles more uniformly. The pH of this solution was maintained at 10 by adding ammonium hydroxide. A separate mixture of H2O (15 ml), ethanol (10 ml) and C16TAB (with varying concentration) was prepared and added to the above solution under vigorous magnetic stirring. A relaxation time of 3 hours was allowed for the surfactant to adsorb on the silica surface and then a solution of 1.25 ml T EOS and 3.5 ml ethanol was added slowly to the above mixture. The reaction was allowed to proceed for 6 hours after which particles were centrifuged and washed with ethanol three times to remove most of the surfactant present in solution. Par ticles were calcined at 45 0 C for 6 hours to completely remove surfactant. Concentration of C16TAB used was 0.78 mM, 1.56 mM, 3.1 mM and 6.2 mM.The particles were designated as SampleA, B, C and D respectively. Characterizatio n The morphology of the core/shell particles was examined usin g High Resolution Transmission Electron M icroscope (HRTEM). Nitrogen sorption measurements were performed on Quantachrome instrument. Samples were degas sed at 250 C for 6 hours prior to analysis The surfa ce areas were calculated by BET (Brunauer -Emmett -Teller) method and the pore size distribution curves were obtained from the adsorption branch

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27 by using BJH method. The total pore volumes were estimated from the adsorption branch of the isotherm at P/Po=0.98 assuming complete pore saturation. The monodispersity of the core/shell particles were shown using Field Emission Scanning Electron Microscopy (FE SEM ) images. TGA -DTA and Fourier Transform Infrared Spectroscopy ( FTIR ) me asurements were done on Sample-C to show the removal of surfactant after calcination. Result and D iscussion Synthesis M ethod The main aim of the work is to synthesis silica particles, with higher surface porosity in one-step reaction for application such as chemical mechanical planarization. Commercially available silica particles were used as seeds for nucleating porous silica shell. Number of seeds was chosen carefully such that, a uniform and monodispersed core/shell particles are synthesized. Various trials with different concentration of core silica particles were carried out first, keeping the basic synthesis method same. Among the 2.5 wt%, 5 wt%, 10wt%, 15wt% and 20 wt%, best results were obtained for seed concentration of 5wt% and below. As the concentration of seed partic les increased severe coagulation, with poor particle dispersibility and non-uniform porous shell coating was formed. Hence, hereafter all the particles were synthesized using 5wt% concentration of seed. CTAB A dsorption on SiO2 N anoparticles The study of CT AB adsorption on silica particles was critical in forming the porous shell. Extensive work has gone into the study of cetyltrimethylammonium ion (CTA+) on SiO2 surfaces. Wei Wang [ 26 ], et.al reported that at lower surface coverage (less than a monolayer), CTA+ molecules were strongly bound to the SiO2 surface via their trimethyl

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28 ammonium head groups. A bilayer sorption of the CTA+ was observed at higher surface coverage and the sorption was attributed to the hydrophobic interactions between aliphatic tails of CTA+ ions Adsorption of CTAB on silica is mainly attributed to electrostatic interaction between CTA+ ions and hydroxyl groups at silica surface. In our work, the pH of the solution was maintained at 10 before CTAB was added. This provided more negati ve charge sites on the surface of silica as a result of ionization of a greater proportion of surface hydroxyl groups and as a result increased the electrostatic attraction between CTA+ ions and silica surface. A 5to 10-fold increase in CTAB adsorption onto silica gel at pH 10 (compared with pH 5.6) at concentration range from 0.05 to 0.4 mM was reported by Fleming [27] He also reported that most of the adsorption of CTA+ molecules was observed in the first 10 s, followed by a slow rise (2 to 3h) after w hich the equilibrium excess was reached. In our synthesis method, a relaxation time of 2hours after addition of CTAB was given for most of the CTA+ molecules to a dsorb on the particles before T EO S was added to form the porous shell. A 15% of ethanol in wat er was maintained in our reaction. The percentage of ethanol in water affects the micellization of CTAB surfactant. The volume ratio of water/ethanol affected the way in which CTAB molecules arrange on to the silica surface. Nazir [28 ] et.al reported that c ritical micelle concentration (CMC) of CTAB in ethanol water media increases upto 10% ethanol and decreases on further addition of ethanol. This helps in reducing the overall CTAB concentration used as a template in the synthesis of core/shell particles. Micelle formation of CTAB in the bulk solution and on the silica surfaces is considered better for formation of porous shell. The critical micelle concentration (CMC) of CTAB surfactant is 0.92 1 mM. A well defined porous shell was

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29 formed on particles synt hesized above this concentration of surfactant such as (C) and (D). Mechanism of CTAB M olecules A rrangement on Silica P articles Figure 3 1 Schematic of Core/Shell silica particles preparation The critical micelle concentration (CMC) of CTAB surfactant is 0.92 1 mM. The mechanism of CTA+ ions arrangement on negatively charged silica surface with increase in surfactant concentration is shown by a simple schematic in Figure 3 1 At concentration below cmc, the surface of silica is covered by a single layer of monomers and as the concentration of CTAB increases, more number of monomers tends to crowd the surface forming bilayers and finally at higher concentrations well above CMC, the sur factant aggregates as micelles. The samples A and B were prepared with concentration of CTAB below CMC and sample s C and D were prepared above CMC in increasing order.

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30 Surface M orphology of Porous Shell S ilica and Pore C haracterization Figure 32 presents the FE SEM micrographs of calcined sample-C core/shell particles that s hows particles are spherical, uniform and highly monodispersed. A closer observation at the surface of these particles was done using high-resolution transmission electron microscopy (shown in Figure 3 3) Sample A which was prepared using lowest CTAB conc entration, showed a less rough surface morphology and the porous shell was not visibly seen in the picture. As the concentration of CTAB increased the thickness of the porous shell increased and the distinction between core and the shell can be seen clearl y. Sample-C showed a thicker porous shell compared to sample -B (as marked using arrows in the figures). Sample-D showed a well defined porous shell, with the thickness of the shell in the range of 10 12 nm and a much rough surface compared to other samples Figure 3 2 FESEM Images of calcined sample -C core/shell silica particles under different magnification. The BET surface area, total pore volume and BJH pore size for the porous shell coated silica particles are listed in Table 3 1. Particles which w ere prepared with high CTAB concentration (D) exhibited, highest BET surface area of 72.25m2/g compared to

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31 Figure 3 3 TEM micrographs of core/shell SiO2 particles as increase in surfactant concentration from (A1 to D1)

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32 that of sample (A) which result ed in a surface area of 47.17m2/g. The total pore volume also increased linearly from particles A to D suggesting that, a prominent porous shell is formed on particles with higher concentration of CTAB. The BJH pore size of the particles was in the range o f 1.382.198 nm. As reported in [16,17], particles with pore size in this range were highly microporous in nature. Table 3 1 N2 sorption measurement of Core/shell silica particles. Sample Name [CTAB] mM BET surface area (m2/g) Total Pore volume (cm3/g) BJH pore size (nm) A 0.78 47.17 0.5722 1.38 B 1.56 49.79 0.6167 1.386 C 3.1 52.65 0.6784 1.46 D 6.2 72.25 0.776 2.198 TGA and FTIR Analysis For TGA and FTIR analysis, sample-C prepared using highest surfactant concentration was used. The amount of template in as -synthesized and heat treated core/shell silica particles were tested using thermo gravimetric analysis first. Most of the surfactant in the as -synthesized sample was removed while centrifugation and washing with ethanol for three times. The remaining surfactant was removed in the temperature range of 150 300 C. This is confirmed from the TGA graph of as -synthesized sample-C particles shown in Figure 3 -4 which showed a weight loss of approximately 2.5% in this temperature range, mainly due to the decomposition of C16TAB surfactant (the melting and decomposition temperature of C16TAB is 230 C [29]). The sample which was calcined at 400 C for 6 hours showed almost a constant weight loss in the temperature range of 200 800 C, indicati ng negligible concentration of surfactant left over after heat treatment. This was further confir med by FTIR analysis of sample -C

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33 F igure 3 4 TGA of core/shell silica particle sample C Figure 3 5 FTIR of core/shell silica particle sample C

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34 The FTIR spectrum in Figure 3-5 shows two distinct bands in the 2950-2850 cm1 region, which are due to the CH2 units of the C16TAB aggregates asymmetric and symmetric vibrations. After heat treatment, the intensity of those two CH2-stretching vibration bands significantly reduced indicating most of the surfactant removed after heat treatment [30]. The other bands in the spectra such as 1621 cm1, 1878 cm1 and 3314cm1 are attributed to the bending vibration of the associated water due to O -H stretching freque ncy The 1130 cm1, and 800cm1 band shown in the insert of F igure 35 are attributed to the asymmetric stretching vibrational mode of Si -O -Si and symmetric stretching of bulk Si -O -Si respectively. Summary Monodispersed core/shell silica particles, with ha rd core and microporous shell silica particles have been prepared by simple method of surfactant adsorption on optimum concentration of silica seed particl es, followed by hydrolysis of T EO S and finally removing the surfactant by heat treatment at 400 C for 6 hours The morphology of the porous shell was altered by changing the concentration of surfactant and difference in morphology observed using TEM. The specific surface area, total pore volume and pore size, obviously increased as the concentration of th e surfactant increased. Following this investigation, we are now able to tailor the surface porosity of the silica particles of ca. 75 nm sizes just by varying the concentration of surfactant. The thickness and porosity of the porous shell formed can be further manipulated by altering TEOS concentration, which was not performed in our study. This possible tailoring of silica particles is of great interest for chemical mechanical polishing of low -k dielectrics in semiconductor industries to obtain less defec tive wafers.

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35 CHAPTER 4 CHEMICAL MECHANICAL PLANARIZATION USING CORE/SHELL SILICA Introduction CMP slurries are desig ned to avoid surface defectivity on wafers during polishing process. Common defectivity issues include surface scratches, indentations, su rface roughness, particle adhesion and corrosion. Controlling the size of the abrasives in the slurry and size distribution of the particles will help to control micro -scratching on the wafers. As reported [ 25 ] concentration of the particles also pay important role in surface finish. The data presented clearly indicates that even slight increase in concentration of large particles will degrade the quality of surface to greater extent. Other factors such as time dependent aggregation of particles, pH drift and long term stability of the slurry also dominate the surface defectivity issue during CMP. Aggregation of particles as time progresses increases the overall particle sizes and the wafers are subjected to higher contact stresses, thereby increasing the surface defectivity. Likewise unstable slurries result in particles settling onto the wafer surface and causes particle adhesion which is difficult to remove during post -CMP cleaning. Removal rates are important output variable in any CMP process. A high removal rates are expected from well designed and perfect CMP slurry. In case of shallow trench isolation CMP, the removal rates are typically 2000 /min and that of metal CMP such as copper, tungsten, it can be as high as 6000 /min. The removal rates ar e governed by various factors and incase of metal CMP, a thin oxide layer forms first, which is subsequently removed by abrasive particles in the slurry. For better removal rates, the time for oxide layer formation must be rapid since the particle interact ion time is relatively very fast. In case of Silica CMP, the surface is made softer by penetration of

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36 water and forming a gel like layer, followed by abrasive action of particles to remove the surface. Chemicals in slurry play key role in achieving higher removal rates. The chemical in the slurry react and form passivation layer with the wafer surface at much higher rate than the abrasive particles present in the slurry. Additives such as oxidizers are used to control the reaction rate and surface passivati on. In this work, I report CMP performance of low -k dielectric and Copper films using slurry prepared by core/shell silica particles ( synthesized by metho d explained in C hapter 3 ) The particles used in slurry were microporous in nature and have higher s urface area. Another slurry was prepared using nonporous silica particles of approximately 75nm size. Removal rates and surface roughness of each material ( Black diamond and Copper) were compared for nonporous silica and core/shell silica slurr ies. Exper imental Materials Silica colloid of approx.75nm particles were purchased from Silco Inc. Core/shell silica particles prepared in lab. For CMP polishing b lack diamond wafers were purchased from Applied Materials Inc. and copper wafers from Wafer Net Inc. A ll wafers were cut into 1 squares and edges smoothed by grinding operation followed by cleaning in acetone and deionized water in ultrasonicator Slurry P reparation Two slurries were prep ared using core/shell silica particles and nonporous silica partic les (commercial silica colloid) for CMP polishing. The particles were mixed in deionized water without adding any additives and the pH of the solution was adjusted to 9 by adding NH4OH. The particles were dispersed by using ultrasoni fication in b ath sonicators, until all the aggregates were broken down. To find the effect of dispersion

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37 after sonification, particle size analysis was done using laser diffraction particle size analyzer (LS 13 320 Coulter Instrument) and FE SEM used to confirm the particle size. CMP Polishing Set up Chemical mechanical polishing was performed in lab using a STRUERS TEGRA POL -35 polisher along with a flow pump for slurry feed and Struers ro toforce polishing head for conducting all the polishing experiments. The polis h ing unit has a 12 inch diameter platen on which polishing pad is mounted. IC 1000/Suba IV stacked pads were used for polishing. Downward pressure was applied pneumatically and the polishing time and pad rotation speed set to desired values. The sample holder is a 2.25 i nch diameter stainless steel cylinder, with a height of 1.13 inches. A flat square recess is machined in the center of one of the flat surfaces. A backing material is mounted inside the recess. It brings the wafer slightly (0.05 inch) above the flat sample holder surface and is made wet before the sample is put on it, in order to hold the wafer using capillary forces. The experimental conditions were set as follows: the sample and pad rotation speed was set at 100 rpm; the downward force was set at 6 Psi, a nd the sample was offset by 3.5 inches from the centre of the pad. Slurry feed rate was set at 80ml/min and time for polishing was 1 minute. After polishing the samples were ultrasonicated in alkaline water to dislodge the particles adhering to the surface. Incase of black diamond wafers, t he polishing rate was determined by measuring the thickness of the films in various marked regions of the wafer using FILMETRICS, a spectral reflectance tech nique for thickness measurement, before and after CMP. Incase of copper wafers, a Four Point Probe was used to measure the resistivi ty of the copper, before and after CMP. From resistivity, the thickness of wafers was calculated. The surface roughness was characterized using atomic force microscopy ( AFM).

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38 Results and D iscussion Properties of C ore/ S hell Silica Particles Figure 4 1 Schematic of different morphology of silica nanoparticles For better understanding the silica particles, three schematics are shown in F ig ure 4 -1 The first one represents non -porous silica second one fully porous silica and third one, a non -porous core/porous shell silica particles. The figure is self explanatory and one can understand that the density and hardness of silica particles are very much affected by pores present in the particle s. Fig ure 4 2 shows TEM images of ( a) non porous silica particles and (b) core/shell s ilica particles. From the images one can easily differentiate between a hard core and porous shell silica particles. The particle sizing was performed on LSS coulter ins trument for both the slurries prepared. In case of core silica particles (slurry A), the average size of particles was 7580 nm and incase of core/shell silica particles (slurry B), the average size of particles was 98 nm. Overall the particle distribution was uniform a nd normal for both the slurries (as shown in Figure 4 -3 ). A nitrogen sorption test was performed on core silica particles and core/shell silica particles using Autosorb-1 instrument.

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39 Figure 4 2 C ompar ison of TEM images of (A) core and (B) core/shell Silica Figure 43 Particle size distribution of abrasives in slurry A and B

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40 The pores in core/shell silica particles were microporous in nature with increase in total pore volume compared core silica pa rticles Th e adsorption/desorption curves of core and core/shell silic a particles are shown in Figure 4 4 In fully porous silica particles, pores run through out the particle and have very low hardness, such particles are used in CMP for producing better surface finish on low -K dielectric materials. They are less useful in polishing metal films due to their poor hardness. In case of nonporous silica particles, the hardness is very high and mostly utilized in application were higher removal rates are requi red. A new approach in designing CMP slurry is to choose particles with optimum hardness and porosity level so that surface finish is better and removal rates are also not compromised. Figure 44 Nitrogen sorption isotherm of silica core and core/shell silica particles.

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41 Polishing Rate and Surface R oughness As mentioned, CMP polishing of copper and low k dielectric material such as black diamond were con ducted in two different slurry system, one with non -porous silica particles [Slurry A] and other with core/shell silica particles [Slurry B]. The concentration of solid loading and pH of the slurry was fixed as 5wt% and 9 respectively for both slurry systems A and B. Table [ 4 1 ] shows the comparison of polishing rates among two wafers for both the slurrie s. Table 4 1 Polish rates and surface roughness of black diamond and copper wafers. Material Removal Rates (/min) Surface Roughness RMS (nm) Slurry A Slurry B Slurry A Slurry B Black Diamond 388 15 720 15 0.57 0.56 Copper 430 15 882 15 1 .85 1.73 Black diamond is a low -K dielectric material developed by Applied Materials Inc. It is a carbon doped SiO2 film deposited by PECVD (Plasma Enhanced Chemical Vapor D eposition) technique. Due to the carbon doping, dielectric constant is lowered below 3 and is used for interconnects. The hardness of b lack diamond film is roughly in the range of 3-4 .5 GPa [31]. This value is higher than copper films and lower than that of normal SiO2 dielectric films. Hence removal rates using any abrasives particles are supposed to be in the decreasing order of copper, b lack d iamond and SiO2 films However due to various other mechanisms involved during CMP, this order may not be true. For example as reported by KS CHoi [24] even though h ardness of black diamond is lower than SiO2, the removal rates are higher for SiO2 film due to more of chemical activity of slurry than mechanical action of the abrasives. In our work, the removal rate of copper was higher than that of black diamon d for both the slurry systems. This is due to the fact that copper wafers are less hard compared to black

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42 diamond. Slurry properties such as abrasives, pH plays a less im portant role considering the hardness of the wafer. As observed from the table [41], the removal rates are higher for Slur ry -B for both the wafers. Slurry -B was prepared using core/she ll silica particles and Slurry -A using non-porous silica particles. One would expect the removal rates of Slurry A to be higher since the particles are nonporous in nature. Bu t the opposite is observed. This behavior of core/shell particles can be explained on the basis of hardness of core and high surface area of shell. The hard core of the core/shell silica particles helps in retaining the overall hardness of the particles w hen compared to fully porous silica particles. This hardness coupled with high surface area caused by micro pores present in the shell, increases the material removal rate. In case of carbon -doped SiO2 (i.e.) b lack diamond wafer, the removal mechanism is e xplained by formation of gel -type layer on the wafer surface due to dissolution of silica film by chemical r eaction, followed by mechan ical polishing. Incase of copper wafers, the material removal mechanism is due to the formation of oxide passivation laye r such as Cu2O on the surface of copper, in reaction with the chemicals present in slurry, suitably transported by abrasive particles. This is followed by removal of the passivation layer along with base copper layer due to abrasive action of particles in slurry. In general, t he chemical activity at the wafer surface increases when the abrasive particles have higher surface area. This is the case observed in slurry B and the reason for higher removal rates in case of both the material. Slurry B (core/shel l silica) yielded better surface rou ghness on the polished wafers for both copper and black diamond materials This can be explained by the surface morphology of the core/shell particles. T he microporous nature of the shell make the

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43 surface less dense, thereby reducing the surface hardness of the particles compared to the core. This induces very less scratches on the surface. Due to surface porosity of core/shell silica particles, the dielectric constant is lowered. As a result Hamaker constant is very low for these particles. [ Since H amaker constant is linearly proportional to van der Waals force between two surfaces .] This implies the core/shell silica particles have less adhesion on the surface that is polished due to very low van der W aals forces From the table [4.1] we can see that the surface roughness of copper is very much high compared to that of black diamond for both the slurry systems. This is partly due to the fact that the initial copper wafer s (before polishing) had high surface roughness th an black diamond and partly due to hardness of base material, which is lower for copper, prone to more surface scratches. Apart from some the reasons mentioned here, v arious other factors such as solid loading, pH of the slurry, pad characteristics play an important role. Summary Core/shell silica particles slurry was successfully prepared and used for chemical mechanical planarization of coppe r and low k dielectric material, black diamond. Non porous silica particles slurry was prepared to compare the behavior of wafers in two different slurry systems.. CMP of t he hard core/microporous shell particles slurry produced higher removal rates and better surface finish compared to non-porous silica particles. Filmetrics and Atomic force microscopy was used to cha racterize wafers. This research work is an attempt to develop functionalized abrasives for targeted CMP application.

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44 CHAPTER 5 CONCLUSION The synthesis of finely tuned nanoparticles for various CMP applications has lead to advanced circuit designs and mult i level chip manufacturing. As copper interconnects are replacing aluminium in a fast pace these days and different low -k dielectric materials developed for multilevel designs, critical understanding on CMP polishing of these materials is necessary. The wh ole semiconductor industry has benefited from the development of CMP process and its integration in main stream semiconductor processing in a big way CMP consumables have become an independent market now a -days Abrasives particles are nearly fined tuned and synthesized in large quantity for targeted application. One such attempt has been made in this thesis work to synt hesis finely tuned silica particles for CMP of Copper films and l ow k dielectric materials According to experiments and discussions in pr evious chapters, the following conclusion for the thesis could be summarized: A simple and one-pot synthesis technique developed to synthesis Core/Shell silica particles with hard core and microporous shell. The specific surface area and total pore volume of the particles ranged 37 72 m2/g and 0.2330.776 cm3/g respectively as measured by N2 adsorption/desorption technique. The surface morphology of the particles studied well using high-resolution transmission electron microscopy (HR -TEM) and clear distinc tion between core and shell observed. Chemical Mechanical Polishing successfully performed on copper wafer and low k dielectric material such as black diamond using core/shell silica particles and commercial nonporous silica particles The c ore/shell silic a slurry produced higher removal rates and better surface finish in CMP polishing of Cu and black diamond wafers, compared to that of non-porous silica slurry

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45 LIST OF REFERENCES [1] J. J. Sniegowski, Chemical mechanical polishing: Enhancing the manufacturability of MEMS, Proc. SPIE, 2879 (1996) 104-150. [2] S. Liu, Z. Zhang, Y. Wang, M.Y. Han, Surface-functionalized silica-coated gold nanoparticles and their bioapplication, Nanosci. & Nanotech, 67 (2005) 456-461. [3] J. P. Biosvert, J. Persello, J. C. Casta ing, B. Cabane, Dispersion of alumina -coated TiO2 particles by adsorption of sodium polyacrylate, Colloids & Surfaces A, 178 (2001) 187198. [4] E. I. Silva, J. Rivas, L. M. Leon Isidro, M. A. L. Qunitela, Synthesis of silver -coated magnetite nanoparticles J. Non-Crysltalline solids, 353 (2007) 829831. [5] M. H. Oh, J. S. Lee, S. Gupta, F. C. Chang, R. K. Singh, Preparation of monodispersed silica particles coated with ceria and control of coating thickness using sol -type precursor, Colloids & Surfaces A 355 (2010) 16. [6] S. H. Lee, Z. Lu, S. V. Babu, E. Matijevic, Chemical mechanical polishing of thermal oxide films using silica particles coated with ceria, J. Mater. Res. 17 (10) (2002) 2744 2749. [7] H. Lei and P.Z. Zhang, Preparation of alumina/sil ica core -shell abrasives and their CMP behavior. Appl. Surf. Sci. 253 (2007) 87548761. [8] G. B. Basim, B. M. Moudgil, Effect of Soft Agglomerates on CMP Slurry Performance, J. Colloild & Interface Sci. 256 (2002) 137142. [9] G. B. Basim, B. M. Moudgil, Slurry design for Chemical Mechanical Polishing, KONA Power Technol.Jpn.21 (2003) 178184. [10] W.Stber, A.Fink, E.Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid. Interface Sci. 26 (1968) 6269. [11] A.G Martinez, J. P. Juste, L.M. L. Marzan, Recent progress on silica coating of nanoparticles and related nanomaterials, Adv. Mater. 22 (2010) 11822-1195. [12] L. F. Giraldo, B. L. Lpez, L. Prez, S. Urr ego, L. Sierra, M. Mesa, Mesoporous silica applications, Macromol.Symp.258 (2007), 129141. [13] I. I. Slowing, B. G. Trewyn, S. Giri, V. S. Y. Lin, Mesoporous silica nanoparticles for drug delivery and biosensing applications Adv.Functional Mtls.17 (2007) 1225 1236. [14] A. Sayari, S. Hamoudi, Y. Yang, Application of pore expanded mesoporous silica.1.Removal of heavy metal cations and organic pollutants from wastewater, Chem. Mater.17 (2005) 212 -216.

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46 [15] K. S. Choi, R. Vacassy, N. Bassim, R.K. Singh, Eng ineered Porous and coated silica particulates for CMP applications S.V. Babu, Kenneth C. Cadien, James G. Ryan, Hiroyuki Yano, Editors, M5.8. Mater. Res. Soc. Symp. Proc. Vol. 671, MRS Pittsburgh, PA ( 2001 ). [16] Y Murakami, K Tanaka, Y Takechi, S Takah ashi, Y. Nakano, T. Matsumoto, W. Sugimoto, Y. Takasu Microporous silica particles prepared by the salt -catalytic sol -gel process with extremely low content of water, J.Sol -Gel.29 (2004), 19-24. [17] Q. Huo, J. Feng, F. Schuth, GD. Stucky, Preparation of hard mesoporous silica spheres Chem.Mater.9 (1997), 14 -17. [18] Y. Lu, H. Fan, A. Stump, T. L. Ward, T. Rieker, C. J. Brinker, Aerosol assisted self assembly of mesostructured spherical nanoparticles, Nature.398 (1999), 223226. [19] K. Yano, Y. Fukushima, Particle size control of mono-dispersed super microporous silica spheres, J.Mater.Chem.13 (2003), 25772581. [20] H. Ji, Y. Fan, W. Jin, C. Chen, N. Xu, Synthesis of Si MCM-48 membrane by solvent extraction of the surfactant template, J.Non-Crystalline S olids.354 (2008), 20102016. [21] Z. Huang, L. Huang, S.C. Shen, C.C. Poh, K. Hidajat, S. Kawi and S.C. Ng, High quality mesoporous materials prepared by supercritical fluid extraction: effect of curing treatment on their structural stability, Micropor.Mes opor.Mater .80 (2005), 157. [22] JH Kim, SB Yoon, JY Kim, YB Chae, JS Yu, Synthesis of monodisperse silica spheres with solid core and mesoporous shell: Morphologic al control of mesopores Colloids and Surfaces A..313 (2008), 7781. [23] M. Mesa, J. L. Guth, L. Sierra, Micron -sized spherical core -shell particles of mesoporous silica suitable for HPLC applications, S.Surface Sci. & Cat.158 (2005), 20652072. [24] K. S. Choi, Synthesis and characterization of nanoporous silicon dioxide particulate for low defectivity in low k dielectric chemical mechanical polishing, PHD dissertation, University of Florida, Gainesville, FL (2002), 73 -81 [25] R. K. Singh, S. M. Lee, K. S. Choi,G .B Basim, W. Choi, Z. Chen, B. M. Moudgil, Fundamentals of slurry design for CMP of metal and dielectric materials, MRS.Bull.27 (2002) 752-760. [26] W. Wang, B. Gu, L. Liang, W. A. Hamilton, Adsorption and structural arrangement of cetyltrimethyl ammonium cations at the silica nanoparticle water interface, J.Phys.Chem. B.108 (2004), 17477-17483.

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47 [27] B. D. Fleming, S. Biggs, E. J. Wanless, Slow Organization of Cationic Surfactant Adsorbed to Silica from Solutions Far below the CMC J.Phys.Chem. B.105 (2001), 95379540. [28] N. Nazir, M. S. Ahanger, A. Akbar, Micellization of Cationic surfactant cetyltrimethylammonium bromide in mixed water alcohol media, J.Dispersion.Sci.Tech.30 (2009), 51 -55. [29] Yang, G. Wang, Zhenzhong Yang, Synthesis of hollow spheres with mesoporous silica nanoparticles shell Mater.Phy.Chem.111 (2008), 5 -8. [30] J.M. Berquier, L. Teyssedre, C. Jacquiod, Synthesis of Transparent Mesoporous and Mesostructured Thin Silica Films J.Sol -Gel.13 (1998), 739742 [31] N.Chandrasekaran, S. Ramarajan, W. Lee, G.M.Sabde, S. Meikle, Effect of CMP process conditions on Defect gerenation in Low k materials. J. Electro chem. society. 1 51 (2004), G882-G 8 89

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48 BIOGRAPHICAL SKETCH Kannan Balasundaram was born in Coimbatore, an industrial city i n southern part of India. Coimbatore, hailed as the Manchester of South India, is an important foundry cluster in India which produces more than 25000 to 40000 tonnes of castings monthly catering to various domains such as automobiles, oil and gas industry and domestic applications. Coming from such a backdrop it was natural for him to choose Metallurgical Engineering as mainstream after schooling. In 2001, he joined one of the premier engineering colleges, PSG College of Technology in Tamil Nadu, India. Hi s interest in metals and materials was well harnessed during four years of undergraduate study. After a short stint at ESSAR OIL & REFINERY as graduate engineer trainee, he joined EME RSON Process management as application engineer. The work was to design and develop control valves for various upstream and downstream industries. He had to take up more challenging jobs and think globally during his deputation to Emerson Asia Pacific headquarters at Singapore. After 3 years of i ndustrial exposure, he decided to quit work and pursue higher studies. He joined the m aster s program in the Material s Science and Engineering Department at the University of Florida, USA. Until now, he has been working under the guidance of Dr. Rajiv Singh. During this period, he worke d on semiconductor materials and developing abrasive nanoparticles for Chemical Mechanical Polishing. He has worked closely with MAIC and PERC during this period. Under the able guidance of his advisor and committee members he was able to complete the research work successfully.