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Delineation of Subsurface Damage in Sapphire Substrate by Low Temperature Wet Etching Technique

Permanent Link: http://ufdc.ufl.edu/UFE0044316/00001

Material Information

Title: Delineation of Subsurface Damage in Sapphire Substrate by Low Temperature Wet Etching Technique
Physical Description: 1 online resource (43 p.)
Language: english
Creator: Lee, Gwangwon
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: cmp -- damage -- etching -- polishing -- sapphire -- subsurface
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: Sapphire wafer is the most common substrate for growth of GaN epi-layer. It is used in light emitting diodes, high power and high temperature AlGaN based transistors. Sapphire wafer is prepared by sawing, grinding, and lapping of crystalline sapphire followed by chemical mechanical polishing to produce epi ready wafers. Epi-ready sapphire wafers should be oriented, ultra-planar, and sub-surface damage-free. These surfaces are required to achieve good quality epitaxial films. Removal of sub-surface damage is very critical for reducing the defect density in epitaxial film, which adversely affects the device properties. Typically, technique like, TEM, XRD, Raman-Spectroscopy is used to measure subsurface damage. These techniques are expensive and require special skills or needs of cumbersome sample preparation. In this study we have demonstrated a non-destructive, simple, low cost, large area technique based on low temperature wet etching to reveal sub-surface damage in sapphire. Sapphire subsurface damage in epi-ready sapphire wafer was induced by lapping with 0.25µm and 1µm diamond particles. Thereafter CMP was used to polish the wafer. Low temperature wet etching in different acids were performed to reveal the sub-surface damage. Sub-surface damage manifested as scratches in the AFM images. Weight loss measurement was done to calculate the removal rate and etch rate. Sub-surface damage could be revealed by etching in 3:1 volume mixture of H2SO4 and H3PO4 at temperatures 75°C-100°C and also in H2SO4 at a temperature of 125°C. At these temperatures, there was no bulk etching of sapphire. The lapping induced damaged layer can be clarified into two layers; heavily damaged layer and lightly damaged layer. Heavily damaged layer showed scratches during CMP. No etching was required. In lightly damaged layer, etching was required to reveal damage in form of scratches in AFM image. Heavily damaged layer of 1.67 µm ±0.1 light damage layer of 2.24 µm ±0.15 were induced by 1 µm diamond particles.
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 Gwangwon Lee.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Singh, Rajiv K.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044316:00001

Permanent Link: http://ufdc.ufl.edu/UFE0044316/00001

Material Information

Title: Delineation of Subsurface Damage in Sapphire Substrate by Low Temperature Wet Etching Technique
Physical Description: 1 online resource (43 p.)
Language: english
Creator: Lee, Gwangwon
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: cmp -- damage -- etching -- polishing -- sapphire -- subsurface
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: Sapphire wafer is the most common substrate for growth of GaN epi-layer. It is used in light emitting diodes, high power and high temperature AlGaN based transistors. Sapphire wafer is prepared by sawing, grinding, and lapping of crystalline sapphire followed by chemical mechanical polishing to produce epi ready wafers. Epi-ready sapphire wafers should be oriented, ultra-planar, and sub-surface damage-free. These surfaces are required to achieve good quality epitaxial films. Removal of sub-surface damage is very critical for reducing the defect density in epitaxial film, which adversely affects the device properties. Typically, technique like, TEM, XRD, Raman-Spectroscopy is used to measure subsurface damage. These techniques are expensive and require special skills or needs of cumbersome sample preparation. In this study we have demonstrated a non-destructive, simple, low cost, large area technique based on low temperature wet etching to reveal sub-surface damage in sapphire. Sapphire subsurface damage in epi-ready sapphire wafer was induced by lapping with 0.25µm and 1µm diamond particles. Thereafter CMP was used to polish the wafer. Low temperature wet etching in different acids were performed to reveal the sub-surface damage. Sub-surface damage manifested as scratches in the AFM images. Weight loss measurement was done to calculate the removal rate and etch rate. Sub-surface damage could be revealed by etching in 3:1 volume mixture of H2SO4 and H3PO4 at temperatures 75°C-100°C and also in H2SO4 at a temperature of 125°C. At these temperatures, there was no bulk etching of sapphire. The lapping induced damaged layer can be clarified into two layers; heavily damaged layer and lightly damaged layer. Heavily damaged layer showed scratches during CMP. No etching was required. In lightly damaged layer, etching was required to reveal damage in form of scratches in AFM image. Heavily damaged layer of 1.67 µm ±0.1 light damage layer of 2.24 µm ±0.15 were induced by 1 µm diamond particles.
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 Gwangwon Lee.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Singh, Rajiv K.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044316:00001


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1 DELINEATION OF SUBSURFACE DAMAGE IN SAPPHIRE SU BSTRATE BY LOW TEMPERATURE WET ETCHING TECHNIQUE By GWANGWON LEE A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREM ENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

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2 2012 G wangwon L ee

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3 To Mom and Dad

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4 ACKNOWLEDGMENTS I appreciat e my advisor, family members, colleagues, and friends whose support made my m aster degree in the U nited S tate s At this time, I would like to thank Dr. Rajiv K. Singh for his guidance as an advisor. To my committee members I thank Dr. Stephen Pearton and Dr. Brij M. Moudgil for their endless support In addition, I would like to mention the faculty and staff s at the p article e ngineering r esearch center and the major analytical instrumentation center especially Dr. Kevin Powers Dr. Kerry Siebein, and Eric Lambers for the training chances of equipments I am deeply honor ed to my family for all their help which have truly made it possible to reach the goal of life Especially, I would like to thank my grandmother, Doori Lee, for her lessons and encouragement s which gave me inspiration. I also would like to thank my grandmother and grandfather on my mother s side. F rom bottom of my heart, I sincerely thank my mom and dad Jungsook Lim and Sangh o Lee, for their great love. I also thank my brother s family Kwangmin Lee and Y ousu n Kim, for their affection g ave me everything to overcome hardships I could complete this study with Dr. Purushottam Kumar for his core idea, Jungbae Lee for his valuable training Jinhyung Lee for his research leadership, Shaoyu Chang for his generous discussions Minfei Xu for her activeness and Jongcheol Kim for his passion and sprits. Moreover, I thank to my seniors and friends, Kyeongwon Kim as a swimming partner Sunhoo Kim for the S tarbucks coffee Sungwook Min for the brightness Sangjun Lee for the seaweed birthday soup, Wooram Youn for his priceless support at the beginning in Gaines ville Hyuksu Han for his free dom Eunhee Youn for the soybean paste soup, EunJ u Cho for the BBQ Changwoo Jee for the warm ing heart

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5 and encouragement Especially, I thank Jaehan Jung who gave me ultra sense of kinship and superior grand thank Aekyung Jung who is always on my side, upgrade my soul to the perfection. I could not go further without her affection and intimacy. I want to express very special thank Minki and Sanki s mom and dad. Sunjoo Park helped m e at my first time in Brookfield. My roommate Vishal Vahia, funny counselor living next door in my house, gave me humanis tic viewpoint of life Finally, I thank Dr. Antural Dr. Nongmoon Hwang, Dr. Cheolsung Hwang, Dr. Youngae Jeon who support ed my application as a recommendation provider.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES ............................................................................................................ 8 LIST OF FIGURES .......................................................................................................... 9 ABSTRACT ................................................................................................................... 10 CHAPTER 1 INTRODUCTION .................................................................................................... 12 Motivation ............................................................................................................... 12 O bjectives ............................................................................................................... 13 2 BACKGROUND ...................................................................................................... 14 Properties of Sapphire ............................................................................................ 14 Physical Properties ........................................................................................... 14 Chemical Properties ......................................................................................... 14 Subsurface Damage ............................................................................................... 15 Chemical Mechanical Planarization ........................................................................ 16 3 LOW TEMPERATURE WET ETCHING OF SAPPHIRE TO REVEAL SUBSURFACE DAMAGE LAYER .......................................................................... 19 Introduction to Subsurface Damage ....................................................................... 19 Methods .................................................................................................................. 20 Results and Discussion ........................................................................................... 20 Summary of Subsurface Damage ........................................................................... 23 4 EFFECT OF LAPPING PARTICLE SIZE ON THE THICKNESS OF SUBSURFACE DAMAGED LAYER IN SAPPHIRE ....................................................... 31 Introduction to Thickness of Subsurface Damaged Layer ....................................... 31 Methods .................................................................................................................. 32 Sample Preparation .......................................................................................... 32 Thickness Measurement .................................................................................. 32 Results and Discussion ........................................................................................... 33 Summary of Thickness Measurement ..................................................................... 34 5 CONCLUSIONS ..................................................................................................... 39 LIST OF REFERENCES ............................................................................................... 40

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7 BIOGRAPHICAL SKETCH ............................................................................................ 43

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8 LIST OF TABLES Table page 3 1 Lists of AFM evaluation on various etchants and temperatures ......................... 30 4 1 The detailed lapping and CMP conditions .......................................................... 38

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9 LIS T OF FIGURES Figure page 2 1 Aluminum oxide (Al2O3) structure. ...................................................................... 17 2 2 Schematic diagram of CMP ................................................................................ 18 3 1 25 3 2 Polished with Silica 30% 80nm with RMS roughness 1.3nm .............................. 26 3 3 Results of material removal rate on various etchants ......................................... 27 3 4 Scratch lines revealed after etching in 3:1 volume mixture of 3H2SO4:1H3PO4 at 75 100C ....................................................................................................... 28 3 5 Scratch lines revealed after etching in H2SO4 at 125C ...................................... 29 4 1 Schematic of damaged layers in sapphire .......................................................... 35 4 2 RMS roughness during process ......................................................................... 36 4 3 ....................... 37

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10 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 DE LINEATION OF SUBSURFACE DAMAGE IN SAPPHIRE SUBSTRATE BY LOW TEMPERATURE W ET ETCHING TECHNIQUE By Gwangwon Lee M ay 2012 Chair: Rajiv K. Singh Major: Materials Science and Engineering Sap phire wafer is the most common substrate for growth of GaN epi layer. It is used in light emitting diodes, high power and high temperature tr ansistors. Sapphire wafer is prepared by sawing, grinding, and lapping of crystalline sapphire followed by chemical mechanical polishing to produce epi ready wafers. Epi ready sapphire wafers should be oriented, ultraplanar, and subsurface damagefree to achi eve good quality epitaxial film Removal of sub surface damage, which adversely affects the device properties is very critical for reducing the defect density in epitaxial film Typically, techniques like, TEM, XRD, Raman Spectroscopy have been used to measure subsurface damage. Thes e techniques are expensive and require special skills or needs of cumbersome sample preparation. In this study we have demonstrated new technique which has benefits of nondestructive, simple process low cost, large area coverage based on low temperature wet etching S apphire subsurface damage in epi ready sapphire wafer was induced by lapping Low temperature wet etching in different acids were performed to reveal the sub surface

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11 damage. Sub surface damage manifested as scratches in the AFM images. Weight loss measurement was done to calculate the removal rate and etch rate. Sub surface damage could be revealed by etching in 3:1 volume mixture of H2SO4 and H3PO4 at temperatures 75C 100 C and also in H2S O4 at a temperature of 125 C. At critical temperature, there was no significant bulk etching of sapphire The lapping in duced damaged layer can be observed into two layers; heavily damaged layer and lightly damaged layer. Heavily damaged layer accompany surface scratches during CMP. N o etching was required to distinguish scratches In lightly damaged layer e tching was required to reveal damage in form of scratches in AFM image. Heavily damaged layer of 1.67 m 0. 1 light ly damage layer of 2 .24 m 0. 15 were induced by 1 m diamond particles.

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12 CHAPTER 1 INTRODUCTION Motivation Sapphire, which is a s alumina is an important ceramic material used widely in a range of applications such as optics, electronics, and temperature sensing. In the first step of fabrication, sawing, grinding, lapping and fi nally CMP processing are used. In many of these steps, stri ct surface quality requirements are ne ed ed. Especially, subsurface damage is the important role in quali ty of sapphir e Fine surface machining and polishing of sapphire for optoelectronic applications may have major fraction in the total cost The use of chemically assisted polishing techniques such as chemical mechanical polishing (CMP) may produce high quality surface finishes at low cost and with fast material removal rates [1] Subsurface damage is generated in manufacturing processes directly influences performances of optical devices To simple, nonexpensive detecting method is needed. Fur thermore, s ize of wafers has been increasing to response the demand of mass production. In the end, simple, nonexpensive, and large area inspection techniques are required. Speaking generally, evaluations of qualities of wafer fall into two categories: destructive and nondestructive evaluation [2] The destructive methods can measure subsurface damage precisely and quantitatively and can provide useful information on the specimens being tested such as TEM [3] but they may be timeconsuming and inevitably alter or even destroy the finished surface of the samples. Consequently, the samples may be unusable and the cost of production may be raised. Another draw b ack to destructive methods is statically meaningful data and it is unlikely to inspect every

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13 sub area of the finished optics. Thus the subsurface damage of measured subarea may not fully reflect the characters of the whole sample. Therefore, nondestructi ve means were put forward to examine optical components quickly without damaging samples. Many practical methods were introduced over the last several decades; these techniques such as XRD [4], Raman spectroscopy [4] work well for specific materials and fabrication processes. Nonetheless, the non destructive measurements have obvious limitations: they are generally quite operator dependent; they sometimes provide only qualitative data. The measurements are usually expensive and the mechanisms are rather complicated. In this paper, we developed advanced non destructive method which is not expensive, simple, and suitable for large area. T o reveal subsurface damage wit h CMP and low temperature wet etching on sapphire substrate, w e investigated at different etchants and effective temperatures to reveal the s ubsurface scratch lines. H2SO4, H3PO4, and a 3:1 volume mixture of H2SO4:H3PO4 as a function of temperature was us ed. O bjectives The objectives in this thesis are as follows : To reveal sub surface damaged area by low temperature wet et ching technique To calculate the thickness of sub surface damage layer by weight loss The first objective was achieved by low temperature wet etching w hich we developed It was achieved by H.C. Starck Levasil 100K V1/30% 80nm silica slurries and TEGRA POL5 polisher w ith atomic force microscopy Dimension3100 and a software WsxM [5] for characterization. The second objective was achieved by weight loss measurement after CMP.

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14 CHAPTER 2 BACKGROUND P roperties of Sapphire Physical P roperties Sapphire, al uminum oxide (Al2O3), is an electrical insulator but has a relatively high thermal conductivity for a ceramic material. It is most commonly occurring aluminum oxide. Metallic aluminum is very reactive with atmospheric oxygen, and a t hin passivation layer of aluminum oxide (4 nm thickness) forms in about 100 picoseconds on any exposed aluminum surface [6] This layer protects the metal from further oxidation. Figure 21 shows that crystal structure of sapphire. It has three distinctive planes which have various application to grow epi layers such as GaN Si, and superconducting materials. C hemical P ropertie s The oxygen ions nearly form a hexagonal closepacked structure with aluminum ions An understanding of ionic structure of sapphire is essential for aqueous corrosion in sapphire. CMP and etching are related with the chemical re action of its environment. During CMP, sapphire reacts with water and f o r ms the hydrates of sapphire. T he hardness of such hydrates is lower than silica slurry and sapphire. This is the reason why silica, whic h is softer than sapphire, could remove the sapphire. Chemical reactions were founded in literature review [7]. Below is the possible surface reaction on sapphire without s ilica Al2O3 + H2O 2AlO(OH) (2 1) Al2O3 + 3H2O 2AlO(OH)3 (2 2) Al2O3 + OH2AlO + H2O (2 3)

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15 Al(OH)3 + OHAlO+ 2H2O (2 4) Below is the well known surface reaction with silica [8]. Al2O3 + SiO2 Al2SiO5 (2 5) As usual, higher pH value is beneficial to chemical reaction. With pH value elevating, the silica abrasive will begin to accumulate. The accumulates can action pad jamming, whic h interfere the slurry trans f er and result in chemical reaction to be slower, removal rate to dec rease and surface level to be down [7]. In etching reaction, etch rate is the highest with 3:1 volume mixture of H2SO4 and H3PO4. The mechanism w as suggested by literature review [9]. At first period of reaction, Al2O3 dissolved efficiently during 075% volume fraction increasing It is caused by attacking of sulfate ions of H2SO4 to Al O bonding. However, dur ing 75100 % volume fraction of H2SO4 decreased the dissolve reaction because insoluble products dominant result in limiting the etching rate of sapphire. H2SO4 Al2(SO4)3 17H2O (insoluble in water) (2 6 ) Phosphoric acid helped the dissolve reaction because phosphate anion is more electrophilic than sulfate anion and forms the soluble products easily H3PO4 Al(H2PO4)3 and AlPO4 (soluble in H3PO4) (2 7 ) Subsurface Damage The relatively larg er abrasive grinding of materials usually causes two damaged layers one is cracked layer on the top layer and other is subsurface damage which usually takes the form of micro cracks underneath the surface. Sub surface damage could weaken the strength of material. The electric field inside the cracks serve as a reservoir could be enhance d and adversely influence the materials [2].

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16 Chemical Mechanical P lanarization Figure 22 shows schematic diagram of CMP [10] CMP has become one of the integrated operations of semiconductor manufacturing and has lead to the development of next generation nanoscale devices. Mechanical grinding alone may theoretically achieve planarization but the surface damage is very high as compared to CMP. Chemistry alone, on the other hand, cannot attain planarization because most chemical reactions are isotropic. Combination of mechanical and chemical have been yielded better results all the time CMP process produces both global and local p lanarization by combining chemical and mechanical interactions using slurry which is com posed of chemicals and nanop articles

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17 F igure 21 Aluminum oxide (Al2O3) structure.

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18 Figure 22 Schematic diagram of CMP

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19 CHAPTER 3 LOW TEMPERATURE WET ETCHING OF SAPPHIRE TO REVEAL SUBSURFACE DAMAGE LAYER Introduction to Subsurface Damage Sapphire is the most c ommon substrate for growth of GaN based epitaxial films for light emitting diodes. The potential of LED based solid state lighting source to provide an efficient alternative to t ypical lamp used for general lighting has led to wa fer research and commercial activity in recent years. The size of sapphire wafer also increased from 2 inch diameter to 10 inch wafer. U ltra planar, goodq uality sapphire wafers are obtained by sawing single crystal sapphire boule followed by i ts grinding lapping and chemical mechanical polishing. In the surface preparation by grinding and lapping progressively small size abras ives are used to remove the damage from previous step and also reduce the thickness of damaged layer in the current step. Chemical mechanical polishing is used as a final surface preparation step to remove the surface and sub surface damage. After final lapping process, che mical mechanical polishing used to be finishing by using softer a bras ive particle and chem ical effect to polish the wafers without inducing damage of its own. T hus, determination of CMP time and thicknes s of damaged layer result from previous process is important role in optimization finishing proc ess of m aterials Several techniques have been used to characterize the surface damage of sapphire wafers. AFM is us ed for surface scratches and roughness measurement, TEM [3], XRD [4], Raman [4] has been used to measure damage in the sub surface layer These equipment s apart from being expensive, and require skillful operation, cumb ers ome sample preparation, and small area techniques.

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20 In this chapter, we have investigated a low temperature wet etching based process to reveal sub surface damage in sapphire substrates. We have studied different etchants both acidic and basic to reveal damage d la yer Most of the studies in literature using these solvents have been done to determine the bulk etch rate of sapphire at high temperature. L ow temperature wet etching for subsurface damage revealing h as been dem onstrated as a developed technique in this study. Methods An e pi ready C plane (0001), sapphire wafer was lapped with 0. 25 m diamond particle for 1hr using a Tegra Force5 polisher at a pressure of 60psi, linear velocity of 1m/s, and lapping with diamond par ticle induced both surface and sub surface damage. There after the wafer was polished with 80nm silica slurry The surface was characterized using a tomic force microscopy (Dimension 3100). The removal rate during lapping, polishing and etching was determined using weight loss measurements a fter removal of the surface scratches the wafers were etched in different etchants (H2S O4, H3PO4, 3:1 volume mixture of 3H2SO4:1H3PO4, HCl, HNO3, KOH solution) at different temperatures. The etchant was heated in a quartz beaker using a hot plate. The temperature was measured using a thermometer. Every experiment was repeated at least three times. After etching, the sapphire wafers were cleaned using acetone, methanol and DI w ater by sonicat ion for 20mins respectively Pad was changed after lapping process In etching experiments, two sides of wafer were ex posed by etchants during the process One side was not protected by coated film Results and Discussion Figure 3 1 shows a typical AFM image of sapphire surface along with a line profile after lapping with 0.25 m diamond particles. I t shows abrasive induced scratch running

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21 across the image. The line profile shows the depth of these scratches. 0.25 m particle was observed to induce surface scratch with depth of < 5 n m. The RMS roughness of sapphire after lap ping was 3 nm. The wafer was polished using 30% 80nm silica particles Even t hough the surface scratches had a depth of < 5 nm, the wafer was polished to r emove deeper than the la yer to clear the surface scratches. During CMP it produces dimples on the sc ratch es Figure 32 shows the AFM image of sapphire wafer after polishing with surface roughness 1. 3 nm. The continuous surface scratches were observed to be removed. The comparison between scratches in Figure 3 1 and Figure 32 indicates to a roughness drop from heavily damaged to lightly damaged layer Since the lightly defe ctive region underneath the surface scratch had a small roughness change it continuously appeared until defect reveal ing completed, where it could no longer be differentiated using CMP. Even t hough CMP shows a planar scratch free surface, it is typically very well known that the damage layer runs much deeper. This has normally been referred t o sub surface damaged layer. The damaged layer is typically reported to be 35 time s the particle s ize used during lapping [2]. Thin sample which has all surface scratches removed was etched in different etchants to reveal subsurface damage. Even t hough the reactivity of the damage region has reduced, it can still be expected to have higher reactivity than pristine regions. Table 3 1 shows the etchants that were used in this study. Sapphire wafer has been known to etch in H2SO4, H3PO4 and 3:1 volume mixture of H2SO4 and H3PO4 solutions at high tem peratures. W e used low temperature range to reveal subsurface damage.

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22 Figure 33 S hows the etch rate of sapphire in traditional etchants at different temperatures. It can be seen from the figure that there is negligible etching rate below 100C. The etching experiments were done for 1hr and removal determ ined by weight loss measurement The weight of the sample was formed to be same when etched at temperature lo wer than 100C ev en after 1hr of etching. S apphire wafer with sub surface damage was etched at a temperature of 25C (Room Temperature), 50C, 75 C and 100C In this experiment, etching rate was not accurate for determ ining etching rate exactly because one side of wafer was not protected by coated film in t he experiment. Even though it cannot show the exact etching rate, it could show that negligible etching rate was observed at below 100C. Figure 34 shows the image and line profile of AFM result of the surface after etching. The line profile does not show any distinct dimple at scratch lines. The depth of the scratch line has same amplitude as the adjoining areas. The scratch lines were visible in the AFM image At this temperature, the etch rate of sapphire has been formed to be negligible. The AFM image shows that 3:1 volume mixture of H2SO4 and H3PO4 in the temperature range below 100 C does not even etch defect areas apart from the top atomic layers. Etching for longer dissolution of time for 30mins did not change the surface. S o etching loss does not need to be considerable for weight loss measurement Figure 35 shows the sapphire wafer etched at minimum temperature for detecting subsurface damage at 125C in H2SO4 revealed t he scratches. H2SO4 needed higher temperature than 3:1 volume mixture of H2SO4 and H3PO4.

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23 The mechanism of etching of sapphire [9] has been suggested to be formation of Al2(SO4)3 which is in soluble but substituted to the chemicals which could been dissolved in the presence of H3PO4. The continuous formation and substitution of Al2(SO4)3 in 3:1 volume mixture help th e dissolution in etching solution. Phosphoric acid helped the dissolve reaction because phosphate anion is more electrophilic than sulfate anion and forms the soluble products. H3PO4 Al(H2PO4)3 and AlPO4 ( soluble in H3PO4) (2 7) In H2SO4 solution, the etching rate has been reported to be lower. Formation of Al2(SO4)3 crystalline on the surface of sapphire has also been reported which hinders further etching of sapphire H2SO4 Al2(SO4)3 17H2O (insoluble in water) (26) The sub surface revelation of 125C using H2SO4 is similar to that using 3:1 volume mixture of H2SO4 and H3PO4 at 75 100 C Summary of Subsurface D amage In this study, we have investigated low temperature etching using different acids to reveal sub sur face damage in sapphire. Sub surface damage in sapphire was created by lapping with diamond particles. The surface scratches were removed by chemical mechanical polishing using silica particles. The 3:1 v olume mixture of 3 H2SO4: 1 H3PO4 and H2SO4 showed subsur face damage at etching temperature of 75100 C and above 125 C respectively. The sub surface damage reveal as scratch lines in AFM image. These scratch lines were very shallow < 5 nm in depth. Etching at these temperatures remove d the damage lines of atomi c layers and gain negligible etch ing rate. Low temperature etching to reveal

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24 sub surface damage is an easy way. I t can be alternative conventional techniques such as XRD, Raman, and TEM

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25 Figure 31 Lapped (0. 25 m) RMS roughness 3nm w ith maximum indentation depth 5nm

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26 Figure 32 Polished with Silica 30% 80nm with RMS roughness 1.3nm

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27 Figure 33 Results of material removal rate on various etchants

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28 Figure 34 Scratch lines revealed after etching in 3:1 volume mixture of 3H2SO4:1H3PO4 at 75 100C

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29 Figure 35 Scratch lines revealed after etching in H2SO4 at 125C

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30 Table 31 Lists of AFM e valuation on various etchants and temperatures Etchant Temperature AFM Evaluation H 3 PO 4 RT 100 C No Scratches Revealing H 2 SO 4 RT 75 C No Scratches Revealing H 2 SO 4 125 C Reveal Scratches 3H 2 SO 4 :1H 3 PO 4 RT 50 C No Scratches Revealing 3H 2 SO 4 :1H 3 PO 4 75 100 C Reveal Scratches HNO 3 RT 100 C No Scratches Revealing HCl RT 100 C No Scratches Revealing

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31 CHAPTER 4 EFFECT OF LAPPING PARTICLE SIZE ON THE T HICKNESS OF SUB SURFACE DAMAGED LAYER IN SAPPHIRE Introduction to Thickness of Subsurface Damaged Layer Sapphire wafer preparation process involves sawing of single crystal grinding, lapping and chemical mechanical polishing of wafer surface. Grinding used large, ultra strong abrasive particles which induce heavy damage to the surface of wafer s, which are very deep beneath the surface. The damaged layer is removed during subsequent lap ping step which are progressively smaller abrasive sizes than grinding The size of ultra strong abrasi ve (e.g. diamond) during grinding and lapping in optimized to reduce the overall cost and time required to remove both surface and sub surface damage layer the wafer. Table 41 shows the optimized conditions of lapping and polishing conditions in this study. Figure 41 shows the schematic of the cross section of a wafer with different sub surface damage layers. The surface and sub surface damage ca n be divided into 3 laye rs: 1) surface crack 2) heavily damaged layer in which the damaged region show higher polish rate during CMP 3) sub surface damaged layer which is not delineated during CMP but requires etching using concentrated acids at elevated temperatures as discussed in chapter 3. These damaged layers are harmful to the defect free growth of epitaxial layer It is difficult to determine the thickness of these damaged layers. So research interests have been focused on measurements and treat ments of subsurface damage [11 24] Cross sectioned TEM [3] can be used, to a certain extent to determine the damaged layer. Other techniques e.g. cross sectional XRD [4], Raman [4] at give o nly qualitative results.

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32 In this chapter, we have used CMP, along with etching to determine the thickness of sub surface damaged la yer. The damage region showed scratch lines which were characteriz ed by using AFM. The thickness of various damaged layer s wa s determined by weight loss of the samples lapped by 1 m diamond particles. Methods Sample P reparation Epi ready sapphire wafer s were lapped for 1hr using 1 m diamond particles lapping was done using Tegra Force 5 polisher at a pressure of 6psi and 1m/s li near velocity. AFM Dimension 3100 was used to characterize the surface. Removal rate was calculated using weight loss measurement. The density of sapphire was taken at 3.97g/cm3. 2 inch wafer was polished for these experiments The flat cut of the 2 inch wafe r was ignored for area calculation. Weight loss measurement and AFM characterization was done after every 30mins of polishing. Polishing was done using 30% 80nm Silica particle s. Politex pad were used polishing. The slurry was flowed at 10ml/min. The slurr y pH was 11 0 1 The removal rate of sapphire was 0.7 m/hr. In each polishing 0.35 m of material was removed. Etching to reveal defects was done after CMP T hickness M easurement By using sapphire t heoretical d en sity 3.9 7 g/c m3 = Mass/Volume = w [g] / (Area [cm2] X Thickness [cm]) (4 1) Thickness = w [g] / (Area [cm2] X [g/cm3]) (4 2) By definition of Material Removal Rate, Removal Rate [cm/s] = Thickness [cm] / t [s] (4 3) Removal Rate [cm/s] = w [g] / (Area [cm2] X [g/cm3] X t [s]) (4 4)

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33 Removal Rate [cm/s] = w [g] / (Area [cm2] X 3.97 [g/cm3] X t [s]) (4 5) Results and Discussion Figure 42 show s the decrease in the RMS roughness as a function of polishing time also after etching. The RMS roughness is 3nm after lapping with 1 m diamond particles. The RMS roughness decreased steep ly as the sample was polished with silica particle. Silica particles are softer than sapphire in hardness and does not cause any damage duri ng polishing. It has been generally suggested that during polishing silica partic le react with sapphire to form a lumina silicate layer which is then mechanically removed [8] This phenomenon is different from conventional CMP concept when the chemical in the slurry reacts with the surface to form a modified layer Al2O3 + SiO2 Al2SiO5 (2 5) The removal rate of sapphire observed in this slurry is similar to that reported in literature [8] The depth of the scratch line after lapping was 5 nm, where the roughness was 3nm I nitially the roughness decreased steeply to 1. 5 nm. The RMS roughness using in AFM also depends on the scan size. In this study, the typical scan area was 100 m 100 m. The depth of t he scratches also reduced from 0. 5 nm to 5 nm. The scratches that were delineated because of higher removal rate at defect si tes during CMP had a depth of 0.5 nm. Since the scra tch lines only a very small fra ction of the total area, it has the very lim ited effect on the overall roughness values. In addition, the scratches are also very shallow in nature ( 0.5 nm) after CMP. The change in RMS roughness after the initial CMP is very small. The RMS roughness of 5 5 m2 area is the range of 0.1 0.2 nm.

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34 Fi gure 4. 3 describes the thickness of heavily damaged sub surface damaged layer. The material removal after lapping till the surface stopped showing scratches in the AFM after CMP constituted the heavily damag ed region 1.6 0.1 m Thereafter etching at 3:1 vol ume mixture of H2SO4 and H3PO4 was done to reveal scratches. The thickness of the sapphire removed until the removal of these scratch lines after etching constituted sub surface damage 2.2 0.15 m S apphire was r emoved to get a smooth, scratch free surface. The t otal removal after lapping was 3. 8 0.25 m which is 4 times the size of diamond particle, 1 m, which we used during lapping. It is typically known that the damage layer is 35 times larger than the particle size [2]. The sub surface damaged layer was greater than the heavily damage layer Summary of Thickness M easurement The thickness of surface and sub surface damaged layer in sapphire was calculated when lapping was done using 1 m diamond p article. The determination was done using a combination of CMP, etching and AFM techniques. The heavily damaged layer and lightly damaged layer which we called sub surface damaged la yer w ere found to be 1.6 0.1 m and 2.2 0.15 m respectively. The total damage layer thickness was approximately 4 times of the lapping particle size.

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35 Figure 4 1 Schematic of damaged layers in sapphire

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36 Figure 42 RMS roughness during process

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37 Figure 43 T hickness of damaged layers lapped by 1 m diamond particl es

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38 Table 41 The detailed lapping and CMP conditions Process Conditions Lapping Abrasives BUEHLER Diamond Suspension, 0.25 m and 1 m Abrasives feeding was 5 streams per every 5 mins Holder and grinder rotational direction was same, 1m/s, 6psi Po lishing pad EMINESS politex reg2 12 Polishing Abrasives 15% 80nm SiO 2 Particles Holder and grinder rotational direction was same, 1m/s, 6psi Slurry feeding 10ml/min Polishing pad EMINESS politex reg2 12

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39 CHAPTER 5 CONCLUSIONS This thesis has inves tigated the subsurface damage revealing by etching CMP on sapphire after lapping processes in order to realize the simple accurate, and large area measurement of subsurface damage. C onclusions are summarized as follows: First, 3:1 volume mixture of H2SO4:H3PO4 at 75 100 C and H2SO4 at 125 C reveal the sub surface damage layer in sapphire. Second, The heavily damage, sub surface da maged layer was found to be 1.6 0.1 m and 2.2 0.15 m respectively.

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40 LIST OF REFERENCES [1] American Ceramic Society Annual Meeting 101st Indianapolis I, Finishing of Advanced Ceramics and Glasses Symposium Indianapolis I, Greenhut VA, Pantano CG, Sabia R. Finishing of advanced ceramics and glasses / edited by Robert Sabia, Victor A. Greenhut, Carlo G. Pantano. Westerville, Oh: Am erican Ceramic Society, 1999. [2] Wang JA, Li YG, Han JH, Xu QA, Guo YB. Evaluating subsurface damage in optical glasses. Journal of the European Optical Society Rapid Publications 2011;6:16. [3] Hockey BJ. Observations on Mechanically Abraded Aluminum Oxi de Crystals by Transmission Electron ?Microscopy. Symposium on the Science of Ceramic Machining and Surface, Finishing; Schneider, Samuel J.; Rice, R. W.; United States. National Bureau of, Standards; American Ceramic, Society; United States. Office of Nav al, Research. Washington: U.S. Dept. of Commerce; for sale by the Supt. of Docs., U.S. Govt. Print. Off. [4] Wang YZ, Liu SL, Peng GL, Zhou SM, Xu J. Effects of surface treatment on sapphire substrates. Journal of Crystal Growth 2005;274:241. [5] Horcas I, Fernandez R, Gomez Rodriguez JM, Colchero J, Gomez Herrero J, Baro AM. WSXM: A software for scanning probe microscopy and a tool for nanotechnology. Review of Scientific Instruments 2007;78:8. [6] Campbell T, Kalia RK, Nakano A, Vashishta P, Ogata S, Rodgers S. Dynamics of oxidation of aluminum nanoclusters using variable charge molecular dynamics simulations on parallel computers. Physical Review Letters 1999;82:4866. [7] Niu XH, Liu YL, Tan BM, Han LY, Zhang JX. Method of surface treatment o n sapphire substrate. Transactions of Nonferrous Metals Society of China 2006;16:S732. [8] Zhu H, Tessaroto LA, Sabia R, Greenhut VA, Smith M, Niesz DE. Chemical mechanical polishing (CMP) anisotropy in sapphire. Applied Surface Science 2004;236:120. [9] W uu DS, Wang WK, Wen KS, Huang SC, Lin SH, Horng RH, Yu YS, Pan MH. Fabrication of Pyramidal Patterned Sapphire Substrates for HighEfficiency InGaN Based Light Emitting Diodes. Journal of The Electrochemical Society 2006;153:G765. [10] Chemical mechanical planarization Wikipedia, the free encyclopedia. 2012.

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41 [11] Kamimura T, (Japan) KNCoT, Akamatsu S, (Japan) KNCoT, Yamamoto M, (Japan) KNCoT, Yamato I, (Japan) KNCoT, Shiba H, (Japan) KNCoT, Motokoshi S, (Japan) OU, Sakamoto T, (Japan) OU, Jitsuno T, (Japan) OU, Okamoto T, (Japan) OOC, Yoshida K, (Japan) OIoT. Enhancement of surfacedamage resistance by removing subsurface damage in fused silica. Proceedings of SPIE, vol. 5273: COPYRIGHT SPIE -The International Society for Optical Engineering. Downloading o f the abstract is permitted for personal use only., 2012. p.244. [12] Wu HZ, Roberts SG, Mbus G, Inkson BJ. Subsurface damage analysis by TEM and 3D FIB crack mapping in alumina and alumina/5vol.%SiC nanocomposites. Acta Materialia 2003;51:149. [13] Park H, Chan HM. A novel process for the generation of pristine sapphire surfaces. Thin Solid Films 2002;422:135. [14] Pollicove F, Golini D. Deterministic manufacturing processes for precision optical surfaces. Advances in Abrasive Technology V 2003;2382:53. [15] Fhnle OW, Wons T, Koch E, Debruyne S, Meeder M, Booij SM, Braat JJM. iTIRM as a Tool for Qualifying Polishing Processes. Applied Optics 2012;41:4036. [16] Lodha GS, Yamashita K, Kunieda H, Tawara Y, Yu J, Namba Y, Bennett JM. Effect of surface roughness and subsurface damage on grazing incidence x ray scattering and specular reflectance. Applied Optics 1998;37:5239. [17] Arrasmith SR, (USA) UoR, Jacobs SD, (USA) UoR, Lambropoulos JC, (USA) UoR, Maltsev A, (USA) UoR, Golini D, QED Technologies IU, Kord onski WI, QED Technologies IU. Use of magnetorheological finishing (MRF) to relieve residual stress and subsurface damage on lapped semiconductor silicon wafers. Proceedings of SPIE, vol. 4451: COPYRIGHT SPIE -The International Society for Optical Engineer ing. Downloading of the abstract is permitted for personal use only., 2012. p.286. [18] Lucca DA, Wetteland CJ, Misra A, Klopfstein MJ, Nastasi M, Maggiore CJ, Tesmer JR. Assessment of subsurface damage in polished II VI semiconductors by ion channeling. N uclear Instruments & Methods in Physics Research Section B Beam Interactions with Materials and Atoms 2004;219:611. [19] Steinert J, (Germany) FIfAOaPE, Gliech S, (Germany) FIfAOaPE, Wuttig A, (Germany) FIfAOaPE, Duparre A, (Germany) FIfAOaPE, Truckenbrodt H, (Germany) ITU. Advanced methods for surface and subsurface defect characterization of optical components. Proceedings of SPIE, vol. 4099: COPYRIGHT SPIE --The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only., 2012. p.290.

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42 [20] Chen LQ, Zhang X, Zhang T Y, Lin HY, Lee S, Chen LQ, Zhang X, Zhang T Y, Lin HY, Lee S. MicroRaman Spectral Analysis of the Subsurface Damage Layer in Machined Silicon Wafers. Journal of Materials Research 2012;15:144 1. [21] Qu W, Wang K, Miller MH, Huang Y, Chandra A. Using vibrationassisted grinding to reduce subsurface damage. Precision Engineering 2000;24:329. [22] Liew T, Wu SW, Chow SK, Lim CT. Surface and subsurface damages and magnetic recording pattern degradation induced by indentation and scratching. Tribology International 2000;33:611. [23] Sankaranarayanan K, Sumathi RR, Udhayasankar M, Jayavel P, Kumar J. A new etchant to reveal the subsurface damage on polished gallium arsenide substrates. Journal of Cry stal Growth 1997;178:229. [24] Li Y, Danyluk S. Dynamic measurements of damage generation in single crystal silicon due to sliding contact with a spherical diamond. Wear 1996;200:238.

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43 BIOGRAPHICAL SKETCH Lee, Gwan gwon was born in 1984, in S uwon, South Korea. In August 2010, he graduated with a bachelor s degree in materials science and e ngineering from Seoul National University in South Korea. He started his graduate study at University of Florida in August of 201 0, where he pursued a m aster s degree in the department of materials science and e ngineering under the advisory of Dr. Rajiv K. Singh. He was pursuing his master s degree that was focused on delineation of s ub surface d amage i n s apphire substrate with wet etching technique. He graduated fro m Univ ersity of Florida with a master s degree from the Department of Materials Science and E ngineering in May 2012.