Analysis and performance of electrochemically synthesized barium titanate films and electrolytic capacitors

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Title:
Analysis and performance of electrochemically synthesized barium titanate films and electrolytic capacitors
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xviii, 229 leaves : ill. ; 29 cm.
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English
Creator:
Venigalla, Sridhar, 1964-
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Materials Science and Engineering thesis, Ph. D
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Thesis:
Thesis (Ph. D.)--University of Florida, 1995.
Bibliography:
Includes bibliographical references (leaves 220-228).
Statement of Responsibility:
by Sridhar Venigalla.
General Note:
Typescript.
General Note:
Vita.

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University of Florida
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ANALYSIS AND PERFORMANCE OF ELECTROCHEMICALLY SYNTHESIZED
BARIUM TITANATE FILMS AND ELECTROLYTIC CAPACITORS












By
SRIDHAR VENIGALLA


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE
UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY



























Dedicated to:

my wife
Pramila Rani

and son
Srimanth

in appreciation of their patience, support and encouragement














ACKNOWLEDGMENTS

I would like to thank a number of people for their support, suggestions and


contributions towards this study.


First, I would like to express the utmost and most


sincere thanks to my advisor and committee chair, Dr. James H. Adair, for his support,


guidance, and constructive criticism in performing this study.


I would also like to thank


Drs. Ellis D.


Verink, Jr.,


John R. Ambrose, Rajiv K. Singh, all of the Department of


Materials Science and Engineering, and Dr. Daniel R.


Talham of the Department of


Chemistry, for useful comments, encouragement, advice, and assistance.


Special thanks to


Stephen A.


Costantino,


Cabot Performance


Materials, Boyertown, PA, for his


suggestions and guidance in the processing and characterization of electrolytic capacitors.


I would like to thank all of my present and former colleagues in Dr. Adair's

research group for their collaboration and team participation in learning experimental


techniques and performing research. Spec

Takayuki Tsukada for research assistance.


:ial thanks to Rob Chodelka, Jeff Opalko, and

I am grateful to Richard Linhart and Henrik


Krarup for their insights into computer calculations and analyzing thermodynamic data. I


sincerely acknowledge the help from Jooho


Moon and Jeff Kerchner in


gathering


information for the preparation of this manuscript I also thank Dr. Augusto Morrone, Eric


Lambers, Richard Crockett, and Wayne


Acree,


the staff


of the


Major


Analytical


Instrumentation Center at the University of Florida, for their technical help and analytical

support in the characterization of specimens. I take this opportunity to thank Pam Howell

for her administrative support and assistance all along my research.















TABLE OF CONTENTS


page


ACKNOWLEDGMENTS

LIST OF TABLES .

LIST OF FIGURES .

ABSTRACT .


CHAPTERS


INTRODUCTION . .

LITERATURE REVIEW ... ... .


Introduction


. Dielectric Polarization and Ferroelectricity .
2.2.1. Theory of Charge Storage and Capacitance .
2.2.2. Mechanisms of Dielectric Polarization .
2.2.3. Ferroelectricity . . .
2.2.4. Ferroelectric Materials . .
2.2.5 Thin Film Ferroelectrics and Their Applications
. Methods for Synthesizing BaTiO3 Thin Films .
2.3.1. Vapor Deposition Methods . .
2.3.2. Sol-Gel Synthesis . . .
2.3.3. Hydrothermal and Electrochemical Synthesis
. Characterization of Thin Films . .
2.4.1. Structural Characterization . .
2.4.2. Chemical Characterization . .
2.4.3. Dielectric Characterization . .
. Dielectric Properties of BaTiO3 Thin Films .
2.5.1. Dielectric Constant and Dissipation Factor .
2.5.2. Hysteresis Behavior . . .
2.5.3. Resistivity and Leakage Current . .
I Capacitors as Charge Storage Devices . .
2.6.1. Capacitor Characteristics .
2.6.1.1. Volumetric Efficiency . .
2.6.1.2. Equivalent Series and Parallel Resistance
2.6.1.3. Dielectric Strength . .


. 6


. .
. .
. .


* .
* .


. S .
. .
. .
. .


. .











-ags


2.7.3.
2.7


Ceramic Capacitors
.3.1. Ceramic Disc and Tube Capacitors


2.7.3.2.
2.7.3.3.
Feasibility
Summary


Multilayer Ceramic Capacitors .
Barrier Layer Capacitors .
of BaTiO3 Electrolytic Capacitors


. . 4
. *


LOW TEMPERATURE ELECTROCHEMICAL SYNTHESIS OF
BaTiO3 THIN FILMS ON Ti SUBSTRATES UNDER
GALVANOSTATIC CONDITIONS . .


3.2.
3.3.
3
3
3
3.4.


Introduction


Background and Approach
Materials and Methods .
.3.1. Electrochemical App<
.3.2. Electrochemical Trea
.3.3. Characterization
Results and Discussion .


3.4.1.
3.4.2.


S. 75


. .t .
. . .
4 5 4 p p 4 4 5 p


aratus
tment


* . 4 .
. . *.


" 0 .
S . . .


Phase Stability of the BaTiO3 Films .
Thickness, Grain Size, and Porosity of The BaTiO3 Films


3.4.3. Discussion of Film Formation Mechanisms
Conclusions . .


. .
. 5 .


ELECTROCHEMICAL PREPARATION AND DIELECTRIC
CHARA TION OF BaTiO3 FILMS ON Ti SUBSTRATES
UNDER CONSTANT VOLTAGE CONDITIONS USING NON-
ALKALIELECTROLYTES . . . .


. 103


Introduction


S. . 103


Materials and Methods . .
4.2.1. Electrochemical Synthesis .
4.2.1.1. Materials . .
4.2.1.2. Electrolysis Method .
4.2.1.3. Film Thickness Calculation


4.2.


.2. Characterization


4.2.2.1.


* p .5
* .


Phase, Chemistry, and Microstructure


4.2.2.2. Dielectric Properties .
Results and Discussion .
4.3.1. Effect of Processing Parameti
4.3.1.1. Electrolyte pH .
4.3.1.2. Synthesis Temperature
4.3.1.3. Applied Voltage .
4.3.1.4. Atmosphere .
4.3.1.5. Reaction Time


ers on Synthesis of BaTiO3 Films


* . P .
* . p p
. 4 p . P
* P p .
S . . .










pa-e


4.3.3. Effect of Heat Treatment on Crystal Structure
4.3.3.1. Crystal Structure of BaTiO3 .
4.3.3.2. Phase Transition After Heat Treatment
4.3.4. Dielectric Properties of BaTiO3 Films .


4.3.4.1.
4.3.4.2.
4.3.4.3.
4.3.4.4.


. . .
. .


Structural Dependence of Dielectric Properties
As-Prepared Films . . .
Heat Treated Films . .
Ferroelectric Hysteresis . .


4.4 Conclusions


PREPARATION AND CHARACRIZATION OF BaTiO3
ELECTROLYTIC CAPACITORS FROM POROUS Ti BODIES


Introduction
Materials and Methods.
5.2.1. Preparation of Sintered Porous
5.2.2. Electrochemical Deposition of
5.2.2.1. Vacuum Impregnation of
5.2.2.2. Electrolysis .


.. 130


Ti Anodes .
BaTiO3 . .
the Electrolyte Solution


Microstructural Characterization ..
Fabrication of BaTiO3 Electrolytic Capacitors


5.2.5. Dielectric Characi
Results and Discussion
5.3.1. Early Attempts .
5.3.2. Sintered Porous Ti
5.3.3. Electrochemical I


* a .
. . .


terization


i Anodes from Mixtures of Ti and Polystyrene
formationn of BaTiO3 . .


5.3.3.1. Effect of Electrolyte Salt and Synthesis Temperature
5.3.3.2. Effect Applied Cell Voltage . .
5.3.3.3. Effect of Treatment Time . . .
5.3.4. Capacitor Fabrication and Dielectric Properties .


5.3.4.1.
5.3.4.2.
5.3.4.3.
5.3.4.4.
5.3.4.5.
5.3.4.6.


Conclusions


First Generation Capacitors . .
New Generation BaTiO3 Electrolytic Capacitors


Effect of Purity of Ti Powder .
Effect of Heat Treatment .
Volumetric Efficiency ..
Penetration of Colloidal Carbon


. .
. . .
. . .


.144
147
147
152
158
158
160
160
160
163
164
166
166


THEORETICAL MODELING AND EXPERIMENTAL VERIFICATION
OF ELECTROCHEMICAL EQUILIBRIA IN Ba-Ti-C-H20 SYSTEM .


Introduction .
Thermodynamic Data


6.2.1.


. .


Sources of Thermodynamic Data .


5.2.3.
5.2.4.


. .


.










ias


6.3.1.2. Reactions Involving the Transfer of H+ Ions Only
6.3.1.3 Reactions Involving the Transfer of Both
Electrons and H+ Ions . .
6.3.2. Stability of Water . . .
6.3.3. Representation of Equilibria .


6.3.4.
Limi
6.4.1.
6.4.2.
6.4.3.
6.4.4.


HSC Chemistry for Wind
stations of E1-pH Diagrams
Thermodynamic Data .
Species Not Considered
Thermodynamics versus K
Activities versus Concentr


ows 2.0


. .
. .


inetics
nations


Discussion on Electrochemical Equilibria


6.5.1.
6.5.2.
6.5.3.
6.5.4.


Ba-H20 System .
Ti-H20 System .
Ba-Ti-H20 System .
Ba-Ti-C-H20 System


Experimental Verification


6.6.1.
6.6.2.
6.6.3.
6.6.4.


Synthesis of BaTiO3 at Low Temperatures
Effect of pH and Ba2+ Concentration
Effect of Applied Potential .
Effect of Carbon Contamination


* . .


Conclusions


APPENDICES


REACTIONS AND EQUILIBRIUM FORMULAE FOR THE
Ba-H20 SYSTEM AT 25, 55, AND 100C AND
1 ATMOSPHERE PRESSURE . .


REACTIONS AND EQUILIBRIUM FORMULAE FOR THE
Ti-H20 SYSTEM AT 25, 55, AND 100C AND
1 ATMOSPHERE PRESSURE . .


REACTIONS AND EQUILIBRIUM FORMULAE FOR THE
Ba-Ti-H20 SYSTEM AT 25, 55, AND 1000C AND
1 ATMOSPHERE PRESSURE . .


REACTIONS AND EQUILIBRIUM FORMULAE FOR THE
Ba-Ti-C-H20 SYSTEM AT 25, 55, AND 1000C AND
1 ATMOSPHERE PRESSURE . .


REFERENCES


nmn Tflfl n ny nflAI nrrwnu'














LIST OF TABLES


Tabe


page


Dielectric properties of ceramics and glasses


. . 1


Properties and applications of ferroelectric thin film materials


Thin film deposition techniques


. 24


S. 26


Typical values of the volumetric efficiency for various types of
capacitors . . .


Dielectric properties of ceramic dielectric materials


. 67


Electrochemical synthesis conditions and characterization data for
selected BaTiO3 thin films . . .


Difraction data calculated from the electron diffraction pattern
obtained in the TEM analysis of the BaTiO3 film (Sample BT-55-
24), compared with standard JCPDS data . .


Electrochemical synthesis conditions and characterization data for
BaTiO3 films . .


Electrolysis time dependent characteristics of BaTiO3 Films


Chemical and physical characteristics of SM Ti and
JM Ti powders . . .

Electrochemical processing conditions for depositing BaTIO3 on
various sintered porous Ti anodes . .

Free energy of formation data for the solid species in Ba-Ti-C-H20
system at 25, 55, and 100C and 1 atmosphere pressure .


Free energy of formation data for the dissolved species in Ba-Ti-C-
H20 system at 25, 55, and 1000C and 1 atmosphere pressure

Experimental conditions and synthesis products described in the
published literature on hydrothermal/electrochemical synthesis of
BaTiOC thin films and nowders . .


55














LIST OF FIGURES


Figum


pagc


Charge storage between two parallel plate conductors: (a) in free
space, (b) with a dielectric material . .


Schematic illustration of dielectric constant (k) and dielectric loss
(tan 6) as a function of applied frequency in a hypothetical
material . .

Schematic description of various mechanisms of dielectric
polarization: (a) electronic, (b) atomic or ionic, (c) high-frequency
oscillatory dipole, (d) low frequency orientation dipole, (e)
interfacial space charge, and (f) interfacial polarization at
heterogeneities . .

Octahedral coordination of Ti4+ ion in the perovskite unit cell of
BaTiO 3 . . .


Crystallographic transformations in BaTiO3


2-6.




2-7.

2-8.


2-9.



2-10.



2-11.


. 18


Phase transformation-induced temperature dependence of
polarization characteristics in BaTi03: (a) changes in unit cell
dimensions, and (b) variation of dielectric constant along
crystallographic axes . .

Typical polarization hysteresis loop of a ferroelectric material.

Applications of ferrelectric thin films in micro-electronic
devices. . 5

Schematic diagram of the apparatus for hydrothermal-
electrochemical synthesis of perovskite titanate thin films on Ti
substrates . . .


Effect of Ba2+ concentration in the electrolyte on formation of
BaTiO3 on Ti substrate under hydrothermavelectrochemical
conditions . . . .


Seeman-Bohlin grazing angle X-ray diffraction geometry for the
nnnini, ,t 1, f#ll, tF.










Figure


2-13.


Schematic illustration of a combined AES and XPS
instrumentation . .


page


2-14.


2-15.



2-16.


2-17.


2-18.


2-19.


2-20.


Typical AES compositional depth profile analysis of a BaTiO3
thin film on Si substrate . . .

Schematic circuit diagrams for dielectric characterization of
ferroelectric thin films: (a) auto-balancing bridge for impedance
analysis, and (b) modified Sawer-Tower bridge in virtual ground
mode for ferroelectric hysteresis measurement .


Dependence of dielectric constant and dissipation factor on film
thickness for sol-gel derived BaTiO3 films . .

Frequency and capacitance ranges for applications of various
types of capacitors . . . .


Schematic cross section of a solid tantalum porous sintered anode
electrolytic capacitor . . .


Variation of (a) dissipation factor and (b) capacitance with
frequency for typical solid tantalum porous sintered anode
electrolytic capacitors . . .
Typical impedance Vs frequency characteristics of solid Ta


electrolytic capacitors


2-21.


2-22.


2-23.


3-1.


3-2.


Most common types of ceramic capacitors: (a) ceramic chip
capacitor, (b) ceramic tube capacitor, and (c) ceramic multilayer
capacitor . . .

Schematic process diagram of the twet' and 'dry' fabrication
routes for ceramic multilayer capacitors . .

Variation of (a) capacitance and (b) dissipation factor with
frequency for capacitors made from various classes of dielectric
ceramics . . .

Calculated phase stability diagram for Ba-Tl system at 250C in a
sealed vessel containing 20 v/o atmospheric air .


Schematic diagram of the electrochemical apparatus for the low
temperature synthesis of BaTiO thin films in open vessels.


62










Figure


page


Comparison of X-ray diffraction data for the thin films
synthesized at (A) 55 C (Sample BT-55-24), and (B)100C
(Sample BT-100-24) . .


3-5.


3-6.



3-7.


3-8.







3-9.



3-10.


4-1.


X-ray diffraction data for the thin film (Sample BT-98-48)
synthesized at pH 12.25. Titanium oxides are the predominant
phases . .


Scanning electron micrographs of the surfaces of the thin films
synthesized at (A) 1000C (Sample BT-100-4) and (B) 55C
(Sample BT-55-24). Additional details for the synthesis are
provided in Table 3-1 . .

SEM photomicrograph of Sample BT-29-24 prepared at 290C
showing large, discrete particles . .

Transmission electron microscopy data of the BaTiO3 thin
film prepared at 55C (Sample BT-55-24): (A) Bright field
image showing the fine crystallites of BaTiO3 embedded
in a poorly crystalline TiO2 film and surrounded by the fully
grown BaTiO3 grains seen as large, dark areas, and (B)
selected area diffraction pattern collected from the center
region of the above image. Indexing information for the
SAD pattern is provided in Table 3-2 . .

Cell voltage as a function of time for samples prepared at (A)
100C (Sample BT-100-24), (B) 550C (Sample BT-55-24), and
(C) 29C (Sample BT-29-24). Electrothermal conditions for all
samples were similar other than the temperature .

SEM photomicrographs of large deposits on Sample BT-29-24
prepared at 29C: (A) before argon milling and (B) after argon
milling showing the texture in the hemispherical deposits .

Schematic diagram illustrating the experimental apparatus for
electrochemical deposition of BaTiO3 under constant applied
voltage conditions . . . .


Scanning electron micrographs of electrochemically prepared
BaTiO3 films on as-received Ti foils, showing the development
of microstructure with reaction time: (A) Sample BT-55-4-3
- treated for 4 hours, (B) Sample BT-56-8-3 treated for 8 hours,
'(r\ amnil RT-.7I-1A thmateA fnr 1K hnnur and (Tn wamnlp










Figurm


page


Scanning electron micrographs of electrochemically prepared
BaTiO3 films on polished Ti coupons, showing the
microstructure at short reaction times: (A) Sample BT-56-2-3 -
treated for 2 hours and (B) Sample BT-56-4-3 treated for 4
hours . .

Auger electron spectroscopy data for BaTiO3 film formed on as-
received Ti foil (Sample BT-55-24-3). Top: Compositional
survey on as-prepared film surface showing no presence of
carbon, and Bottom: Compositional depth profile obtained by Ar
ion sputtering (500A/min) . . . .

Scanning electron micrographs of BaTlO3 film (Sample BT-55-
24-3) showing a thickness of 2-3 pm as revealed by a scratch

X-ray diffraction data for BaTiO3 films formed on as-received Ti
foils showing the peak intensities as a function of reaction time: 8
hours (Sample BT-56-8-3), 16 hours (Sample BT-57-16-3), and
24 hours (Sample BT-55-24-3) .

Relation between calculated film thickness and X-ray diffraction
peak intensity and the total electric charge passed through the
electrolytic cell for BaTIO3 films prepared at various reaction
times . . . .


Comparison between calculated film thickness, film thickness
estimated from AES depth profile analysis, and the total electric
charge passed through the electrolytic cell for BaTiO3 films
prepared at various reaction times .


4-9.



4-10.


4-11.


Typical strip chart data recorded during the electrochemical
synthesis of BaTIO3 films: Ag/AgCl reference electrode potential
(top) and electrolysis current (bottom) for a 24 h reaction (Sample
BT -54-24-3) . .

X-ray diffraction data of as-prepared and heat treated BaTiO3 film
(Sample BT-55-24-3) showing the peak shifts toward tetragonal
structure after heat treatment . . .

Dependence of relative dielectric constant and dissipation factor
on applied frequency for the as-prepared and heat treated BaTiO3
films prepared with TEAOH containing electrolyte (Sample BT-
55-24-3) and NaOH containing electrolyte (Sample BT-55-24-










Eigum


page


Processing diagram for the preparation of sintered porous Ti
anodes . . .


Schematic cross section of the modified die assembly for uniaxial
pressing of Ti and polystyrene powder mixtures with solid Ti
wire embedded in the compact . .


Processing scheme for the preparation and characterization of
BaTiO3 electrolytic capacitors from sintered porous Ti anodes .

Schematic illustration of the electrochemical apparatus for
depositing BaTiO3 on the surface of sintered porous Ti anode.

Scanning electron micrographs of the cross section of JM-NH-
BA-99-12, taken from different regions revealing only a partial
infiltration of the electrolyte. (A) Low magnification image
showing most of the cross section of the Ti anode, (B) high
magnification image from the region close to the surface,
showing the presence of BaCO3, (C) high magnification image
from the region about 200 pmn away from surface, showing the
presence of BaTi03 coating on Ti particles, and (D) high
magnification image from a deeper region showing no presence
of coating on Ti surfaces . . .

Thermal analysis of polystyrene spheres heated at 10/min, in Ar
atmosphere. TG: Thermal Gravimetry, DTA: Differential
Thermal Analysis, and DTG: Differential Thermal Gravimetry

Scanning electron micrographs of green and sintered Ti bodies
made from mixtures of polystyrene and two different grades of
Ti A: green body, 20 v/o polystyrene, JM Ti, B: sintered body,
8000C, JM Ti, C: green body, 40 v/o polystyrene, SM Ti, D:
sintered body, 9000C, SM Ti . .

Scanning electron micrograph of the cross section of a sintered
porous Ti anode showing the presence of Ti wire electrode that
was embedded during pressing .


5-10.


Scanning electron micrographs of the cross section of an
electrochemically treated sintered Ti anode (SM-TH-BH-97-8):
(A) at low magnification, revealing complete infiltration of the
electrolyte, and (B) at high magnification, showing the presence
of a uniform, dense laver of BaTiO . . .










Ergumre


5-11.


5-12.


5-13.


5-14.


5-15.


5-16.


5-17.

5-18.


paC


Scanning electron micrographs of the cross sections of
electrochemically treated sintered Ti anodes showing the
formation of BaTiO3 in various electrolytes at about 55C and
1000C: A: barium acetate at 56C, JM-TH-BA-56-4, (B) barium
acetate at 980C, JM-TH-BA-98-4, (C) barium nitrate at 55C, JM-
TH-BN-55-4, (D) barium nitrate at 970C, JM-TH-BN-97-4, (E)
barium hydroxide at 55C, JM-TH-BH-55-4, and (F) barium
hydroxide at 95C, JM-TH-BH-95-4 . .


X-ray diffraction data for the electrochemically treated sintered Ti
anode (JM-TH-BH-95-4) shown with JCPDS standard reference
patterns for cubic BaTiO3 and Ti . . .


Low magnification scanning electron micrographs of the cross
sections of electrochemically treated sintered Ti anodes, revealing
the extent of contamination formed on the surfaces of Ti particles
with various electrolyte solutions: (A) barium acetate, JM-TH-
BA-98-4, (B) barium nitrate, JM-TH-BN-97-4, and (C) barium
hydroxide, JM-TH-BH-95-4 . . .


Scanning electron micrographs of the cross sections of
electrochemically treated sintered Ti anodes showing the
formation of as function of applied cell voltage: (A) 3V,
SM-TH-BH-96-12, (B) 6V, SM-TH-BH-99-12, (C) 9V,
SM-TH-BH-97-12, and (D) 12V, SM-TH-BH-95-12 .


Scanning electron micrographs of the BaTiO3 film formed on Ti
at 12V cell voltage (SM-TH-BH-95-12), showing (A) film
detachment due to cracking, and (B) the rough film surface and
cracks formed during dielectric breakdown . .


Scanning electron micrographs of the cross sections of
electrochemically treated sintered Ti anodes showing the
formation of BaTiO3 as a function of treatment time: (A) 2h, SM-
TH-BH-97-2, (B) 4h, SM-TH-BH-96-4, (C) 8h,
SM-TH-BH-97-8, and (D) 12h, SM-TH-BH-99-12 .


Capacitance as a function of frequency for BaTiO3 electrolytic
capacitors compared with oxidized Ti anodes .
Dissipation factor (tan 8) as a function of frequency for BaTiO3
electrolytic capacitors compared with oxidized Ti anodes .










Figure


5-20.


Scanning electron micrographs of the cross section of the BaTiO3
electrolytic capacitor (SM-TH-BH-96-8) showing (A) the depth of
colloidal carbon (external electrode) penetration into the anode,
and (B) the saturation of porosity with colloidal carbon near to the
anode surface . .. .


pae


6-1(a).




6-1(b).




6-2(a).




6-2(b).


6-3(a).




6-3(b).




6-4(a).


Eh-pH diagram for Ba-H20 system at 25C and 1 atmosphere
pressure, considering the formation of BaH2. Total activity of
dissolved Ba species is varied from 1 m to 10-3 m. See Appendix
A for more information on reactions and equilibrium formulae.


Eh-pH diagram for Ba-H20 system at 25 (*), 55 (A), and 100C
(*), and 1 atmosphere pressure, considering the formation of
BaH2. Total activity of dissolved Ba species is kept constant at 1
m. See Appendix A for more information on reactions and
equilibrium formulae . . .


Eh-pH diagram for Ba-H20 system at 250C and 1 atmosphere
pressure, not considering the formation of BaH2. Total activity of
dissolved Ba species is varied from 1 m to 10-3 m. See Appendix
A for more information on reactions and equilibrium formulae.


Eh-pH diagram for Ba-H20 system at 25 (*), 55 (A), and 100C
(.), and 1 atmosphere pressure, not considering the formation of
BaH2. Total activity of dissolved Ba species is kept constant at 1
m. See Appendix A for more information on reactions and
equilibrium formulae . . . .
Eh-pH diagram for Ti-H20 system at 25C and 1 atmosphere
pressure, considering the formation of TiTH2. Total activity of
dissolved Ti species is varied from 1 m to 10-3 m. See Appendix
B for more information on reactions and equilibrium formulae .


Eh-pH diagram for Ti-H20O system at 25 (.), 55 (A), and 100(C
(4), and 1 atmosphere pressure, considering the formation of
rTlH2. Total activity of dissolved Ti species is kept constant at 1
m. See Appendix B for more information on reactions and
equilibrium formulae . .

Eh-pH diagram for Til-H20 system at 25C and 1 atmosphere
nassuem. not considering the formation of Tib. Total activity of










Figum


6-4(b).


6-5(a).




6-5(b).




6-6(a).


page


Eh-pH diagram for Ti-H20 system at 25 (*), 55 (A), and 100C
(*), and 1 atmosphere pressure, not considering the formation of
I1H2. Total activity of dissolved Ti species is kept constant at 1
m. See Appendix B for more information on reactions and
equilibrium formulae . . .


Eh-pH diagram for Ti-H20 system at 250C and 1 atmosphere
pressure, for extended range of pH, not considering the
formation of T'H2. Total activity of dissolved Ti species is varied
from 1 m to 10"3 m. See Appendix B for more information on
reactions and equilibrium formulae . . .

Eh-pH diagram for Ti-H20 system at 25 (*), 55 (A), and 100'C
(.), and 1 atmosphere pressure, for extended range of pH, not
considering the formation of TiH2. Total activity of dissolved Ti
species kept constant at 1 m. See Appendix B for more
information on reactions and equilibrium formulae

Eh-pH diagram for Ba-Ti-H20 system at 25C and 1 atmosphere
pressure. Total activities of dissolved species are independently
varied for Ba and Ti, in the range 1 m to 103 m. See Appendix
C for more information on reactions and equilibrium formulae .


6-6(b).




6-7(a).


6-7(b).


Eh-pH diagram for Ba-Ti-H20 system at 25
100( (*), and 1 atmosphere pressure. T<
dissolved species of Ba and Ti are kept coi
Appendix C for more information on react
formulae . . .


(*), 55 (A), and
total activities of
instant at 1 m each. See
ions and equilibrium
.


Eh-pH diagram for Ba-Ti-C-H20 system at 250C and 1
atmosphere pressure. Total activities of dissolved species are
independently varied for Ba, Ti and C, in the range 1 m to 10-6
m. See Appendix D for more information on reactions and
equilibrium formulae . . .


Eh-pH diagram for Ba-Ti-C-H20 system at 25 (.), 55 (A), and
1000C (*), and 1 atmosphere pressure. Total activities of
dissolved species of Ba, Ti, and C are kept constant at 1 m each.
See Appendix D for more information on reactions and
equilibrium formulae . . .













Abstract of Dissertation Presented to the Graduate School of the University of Florida
in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

ANALYSIS AND PERFORMANCE OF ELECTROCHEMICALLY SYNTHESIZED
BARIUM TITANATE FILMS AND ELECIROLYTIC CAPACITORS

By
SRIDHAR VENIGALLA


December 1995

Chairperson: Dr. James H. Adair
Major Department: Materials Science and Engineering


Polycrystalline BaTiO3 thin films of approximately


1 jim thickness have been


synthesized on Ti substrates by an electrochemical process, at temperatures as low as 550C.

The effect of various processing parameters, such as solution chemistry, atmosphere,

quality of substrate surface, applied voltage, current density, temperature and reaction time

have been discussed. Formation of BaTiO3 is found to be favored only in highly alkaline

(pH-14) solutions. Film thickness and uniformity increase with reaction time up to 24


hours.


The quantity of total electric charge passed through the electrolytic cell is found to


govern the film thickness and uniformity.


Use of tetraethylammonium hydroxide, a non-


alkali base reagent to adjust solution pH, has resulted in films having improved dielectric


properties as opposed to the films prepared with alkali metal bases such as NaOH.


Heat


treatment at 200C further improved the dielectric properties through a phase transition to

the thermodynamically stable, ferroelectric tetragonal BaTiO3.


Current work also involves the


fabrication and characterization


of BaTiO3


electrolytic capacitors. Effects of electrochemical processing parameters on the formation








electrolyte solution and an uniform coating of BaTiO3 was achieved over the entire surface


of Ti using high porosity (35-40% theoretical density) sintered Ti anodes.


Samples treated


for 8h in 0.5M Ba(OH)2h8H20 electrolyte solutions at 1000C with 12V applied cell voltage

showed the formation of dense, uniform BaTiO3 coating on the surface of Ti anode.


Higher purity (Sumitomo, 99.96%


Ti), chloride free


powder provided smaller


dissipation factors at low frequencies. Heat treatment at 4000C has significantly increased


the capacitance at all frequencies,


frequencies.


while it lowered the dissipation factors at low


Calculated volumetric efficiencies are comparable to those typically obtained


for Ta solid electrolytic capacitors. Penetration of colloidal carbon (external) electrode was


found to be limited to a depth of 300Xpm,


which might have limited the volumetric


efficiencies.

Finally, the electrochemical equilibria in Ba-H20, Ti-H20, Ba-Ti-H20 and Ba-Ti-


C-H20 systems are represented in the form of Et-pH diagrams.


The diagrams are


constructed based on the most recent thermochemical data available, at temperatures of 25


55, and 1000C.


The effect of total activities of dissolved species of Ba, Ti, and C on


thermodynamic equilibria has been represented in the diagrams. BaTiO3 is found to be the


stable phase at high pH and moderate potentials.


The presence of even small amounts of


carbon, usually in the form of dissolved atmospheric CO2, significantly restricts the


stability of BaTiO3.


The validity Eh-pH equilibrium diagrams obtained for Ba-Ti-H20 and


Ti-H20 systems is verified


using the data published in


literature for the


hydrothermal and electrochemical synthesis of BaTiO3.













CHAPTER 1
INTRODUCTION


Barium titanate (BaTiO3) is an important engineering material for electronic


applications due to its high dielectric permittivity and ferroelectric properties.


It is widely


used in multilayer ceramic capacitors, ceramic chip capacitors, pyroelectric sensors, and the


piezoelectric devices [1-4].


In the world of miniaturization, BaTiO3 thin films are


promising for the future development of hybrid microelectronic devices such as high charge


storage capacitors


(used as


Dynamic


Random


Access


Memories) and nonvolatile


ferroelectric memories [5,

barium titanate thin films.


Many techniques have been investigated for preparing


The physical vapor deposition (PVD) techniques are vacuum


intensive and require high temperatures (above 5000C) to deposit crystalline films.


amorphous films prepared at low temperatures require a post-deposition annealing, which

may result in selective evaporation of chemical constituents or undesirable reactions with


atmosphere


substrate.


Chemical


methods


such


as sol-gel,


metal-organic


decomposition, and chemical vapor deposition (CVD) also require either a post-deposition

annealing or a high temperature substrate to prepare phase-pure, crystalline BaTiO3 [7].

Use of high temperatures to deposit or heat treat the films not only causes compatibility

problems with integrated silicon technology, but also induces thermal stresses in the films.

Therefore, an inexpensive process to directly prepare crystalline BaTiO3 films at low

temperatures is highly desirable.

Electrochemical methods have potential for the near room temperature production of









metal surface to form the resistive metal oxide coating.


Composition of the coating


depends upon the composition of the metal and the solution in which the anodization is

performed, with cationic and anionic species incorporated into the structure of the oxide. It

is critical to ensure that the polarization induced reactions proceed with sufficient singularity

to assure that the desired reaction product is formed and that parallel or side reactions that

result in formation of alternative compositions are avoided.

Several investigations were recently reported on the synthesis of perovskite (ABO3)


complex oxides such as BaTiOs,


SrTiO3,


and CaTiO3 and their solid solutions on Ti


substrates using hydrothermal-electrochemical methods.


This process involves the


anodization of a Ti substrate in an electrolyte containing A ion (Ba2+


elevated temperatures (150-8000C) and pressures [12-22].


uniformity,


Sr2+, and Ca2+) at


The thickness, microstructural


and crystallinity of these films depend on various processing parameters such


as temperature, reaction time, electrolyte chemistry, atmosphere, applied potential and


current density.


To fully exploit the advantages of electrochemical methods and make them


commercially viable, it is critical to lower the synthesis temperatures to below the boiling

point of water, thereby avoiding the use of sealed reaction vessels that are capital intensive,


low in productivity and industrial safety.


By understanding the fundamentals of BaTiO3


phase stability and the mechanisms of electrochemical corrosion and anodization of metals


in aqueous solutions,


the early part of this dissertation attempts to synthesize BaTiO3 thin


films on


Ti substrates at low temperatures, employing open reaction vessels.


Several


processing conditions such as electrolyte chemistry and pH, atmosphere, cell voltage,

current density, reaction temperature and time were studied to optimize the film thickness


and microstructure.

electron microscoov


Several techniques (scanning electron microscopy, transmission

Aueer electron spectroscoov. and X-ray diffraction) were utilized to


ft







3
Electrolytic capacitors are very popular in applications where a large capacitance


(charge storage ability) is required [2]. Typical
flash equipment and charge blockage devices.


applications include motor starters, photo

Electrolytic capacitors provide a large


capacitance per unit volume of the device volumetricc efficiency), owing to the large surface


area of the dielectric.


The dielectric material in an electrolytic capacitor consists of an


anodically formed oxide of the anode material, which serves as the positive electrode of the


capacitor.


The metals most employed are aluminum and tantalum, and the anodic oxides


are y-Al203 and Ta205 respectively [23].

is 8.4 and that of tantalum oxide is 28.


The effective dielectric constant of pure y-A1203

In certain commercial grade capacitors, the


dielectric constant can be slightly lower for aluminum oxide and appreciably lower for

tantalum oxide due to the presence of impurities and the amorphous nature of the as-formed


film [24].


Most commonly used among the several kinds of electrolytic capacitors is the


'porous sintered anode solid tantalum'


electrolytic capacitor.


They are fabricated from


lightly sintered porous (about 60 percent density) Ta anodes that contain internally attached


solid Ta wire (which acts as the internal electrode).


The anodes are electrochemically


treated in a bath containing borates or phosphates to form Ta205 coating on the entire


surface of tantalum.


The porous nature of the anode and the fine particle size of Ta powder


used to prepare the anode provide a large surface area of the dielectric per unit volume.

Subsequent electroding and cathode encasement provide a capacitor with a very large

capacitance [24-26].

Development of BaTiO3 electrolytic capacitors, utilizing the electrochemical process

developed for the synthesis of BaTiO3 thin films on Ti substrates and the technology

described above for the solid tantalum electrolytic capacitors will have significant impact on

the apolicabilitv of these capacitors. Higher dielectric constant of BaTiOq (-3000 for single







4
to electrochemically synthesize BaTiO3 thin films on Ti substrates at low temperatures


make it feasible to economically prepare BaTiO3 electrolytic capacitors.


The later part of


investigation in this dissertation focuses on the optimization of electrochemical processing

parameters to prepare BaTiO3 electrolytic capacitors from porous, sintered Ti anodes.
Dielectric characterization to evaluate the electrical properties of these capacitors has also

been performed and comparisons are made with the conventional Ta solid electrolytics.

Understanding phase stability relationships is critical to succeed in synthesizing

novel materials, controlling their properties and predicting their environmental stability.

Chemical synthesis methods for the preparation of structural and electronic ceramic


materials has seen significant advances in recent years.


However, very few studies were


performed to construct phase stability diagrams in the relevant systems [27, 28].


advent of electrochemical methods to prepare perovskite titanate thin films on Ti substrates


further warrants the construction of phase equilibria in aqueous electrolytes.


Therefore,


representation of electrochemical equilibria in Ba-Ti-C-H20 system in the form of En-pH

diagrams (where Eh is the equilibrium electrode potential versus the standard hydrogen

electrode) will be beneficial to optimize the processing conditions in the electrochemical


synthesis of BaTiO3.


The final segment of this dissertation involves the construction of


Eh-pH diagrams for Ba-H20, Ti-H20, Ba-Ti-H20, and Ba-Ti-C-H20 systems, utilizing


the most recent thermodynamic data available for various species in each system.


diagrams were constructed at 25, 55, and 1000C, and at various activities of the dissolved


species to study the phase equilibria under various conditions.


The resulting phase


equilibria are verified with the synthesis data obtained in the current work as well as the

data reported in the literature.


The dissertation is divided into five maior chapters.


Chapter 2 provides a detailed







5

the various kinds of capacitors, their fabrication methods, device characteristics and


applications are discussed,


with a special emphasis on Ta solid electrolytic capacitors.


Chapters 3 and 4 present the study on low temperature electrochemical synthesis of BaTiO3


films


on Ti


substrates,


under


galvanostatic and constant voltage conditions,


respectively. Film formation mechanisms under electrochemical conditions are discussed

in Chapter 3, while the use of non-alkali electrolyte solutions to improve dielectric


properties of the prepared films is the focus of Chapter 4.


Preparation of BaTiO3


electrolytic capacitors from lightly sintered porous Ti bodies is described in Chapter 5,


including


dielectric


characterization


data.


Eh-pH


diagrams


describing


electrochemical


equilibria in


Ba-Ti-C-H20 system are presented in Chapter 6.


Experimental verification of these diagrams is provided, based on the available literature as

well as the data obtained in the present study.













CHAPTER 2
LITERATURE REVIEW


Introduction


Barium titanate, the first ceramic material in which ferroelectric behavior was


observed, is extremely popular in electrical and electronic applications,


ferroelectric and piezoelectric properties are exploited.


where its


Used in various forms ranging from


single crystals to thin films, BaTiO3 is frequently chosen to fabricate a wide variety of

devices, such as high frequency capacitors, transducers, memory devices and pyroelectric


sensors.


Since ferroelectricity is the single most important characteristic behind the


prominence of BaTiOs, the early part of this chapter provides a detailed description on the

theory of dielectric polarization and ferroelectricity, ferroelectric materials and their


properties.


In the later sections, a review of literature on methods to fabricate BaTiO3 thin


films is provided.


To emphasize the importance of high dielectric constant for capacitor


applications, a summary on


various types of


capacitors, their characteristics, and


fabrication methods is also provided.


This chapter concludes with a discussion on the


feasibility of developing BaTiO3 electrolytic capacitors, based on the concept of solid Ta

electrolytic capacitors and the electrochemical methods to prepare BaTiO3 thin films on Ti

substrates.


2.2. Dielectric Polarization and Ferroelectricitv


2.2.1


Theory of Charae Storaee and Canacitance


When an electric field is applied to an ideal dielectric material there is no long-range






7




















(a)






-- -T- C -




(b)

Figure 2-1. Charge storage between two parallel plate conductors: (a) in free
space, (b) with a dielectric material [1].






8
capacitance is present between any two adjacent conductors that are separated by non-
conducting medium. A conventional capacitor consists of two parallel metal plates (termed


electrodes) separated by a dielectric material.


When a voltage is applied across the plates,


the capacitor will become charged, and the amount of charge primarily depends on the


source voltage and the polarizability of the dielectric material.


The capacitance (C) of a


capacitor is defined as the ratio of the charge (0) acquired to the applied voltage (V):


C(farads)


Q(coulombs)


V(volts)


[2.1]


A capacitor is said to possess 1 farad of capacitance if its potential is raised 1 volt when it


receives a charge of 1 Coulomb.


This unit of capacitance is inconveniently large and in


practice only sub multiples like mF (103 F), 4F (10-F), nF (10 F), and pF (10-12 F)

are used to specify the capacitances of most commercial capacitors.
Capacitance, as defined above, determines the amount of charge that can be stored


by a capacitor at any given applied voltage.


Therefore, capacitance is a device characteristic


that depends on geometry of the capacitor and permittivity of the dielectric, as described by
the following relation for a parallel plate capacitor:


[2.2]


d


where e is the dielectric permittivity, A is the area of one plate in square meters, and d is


the distance between the plates in meters.


The higher the permittivity of the dielectric


between the plates, the higher the capacitance. Dielectric polarizability of a material is often
represented by the relative dielectric constant (k), defined as the ratio of permittivity (e) of


the material to the permittivity of free space (E,


= 8.854


x 10-12F m).


k = [2.31








Dissipation factor is a measure of loss in a capacitor.


alternating current will lead the voltage by 90


In an ideal capacitor, the


. In practice, the current leads the voltage


by some lesser phase angle 6 owing to the series resistance R


the complement of this


angle (90 -


6) is called the loss angle 8.


The dissipation factor (also called loss tangent)


is given by tan 8.


Variation of dissipation factor with frequency of applied alternating


voltage implies the presence of polarization mechanism in the dielectric material. Molecular

interference in the material limit the frequency at which the alignment (polarization) can


occur.


If the applied frequency is less than the limiting frequency, full alignment can


occur, then the capacitance is high and the dissipation factor is low. If the applied

frequency is comparable to the limiting frequency, the losses become high. However, as

explained in the following sections, there can be more than one polarization mechanism in a

dielectric material and each mechanism can exhibit a frequency dispersion, owing to

variations in microstructure and chemical homogeneity. As a result, most dielectrics show


a frequency dependence of dielectric constant and dissipation factors.


migration can


In addition, ionic


cause space charge polarization at low frequencies, resulting high


capacitances, but also high loss factors.

2.2.2. Mechanisms of Dielectric Polarization


Dielectric processes in materials are very important for their applications in


electronic and electrical components.


These processes determine the suitability of a material


either as a capacitive device (high dielectric polarization) or as an insulating device (low


dielectric polarization).


There are several ways in which a dielectric material responds to


neutralize the external applied electric field,


which are called dielectric polarization


mechanisms, as described below:

There are four major polarization mechanisms in dielectric materials, involving a






10

have a frequency limit below which they can follow an applied alternating electric field.

Above the limiting frequency, the structural features in the material restrict the ability of the

charge centers to align with the applied field dielectricc relaxation) and the polarization is


lost.


Therefore,


when a material has more than one polarization mechanism, the


contribution from all mechanisms is present at low frequencies, and as the frequency is

increased, each mechanism loses its contribution at the relaxation frequency, until all the


polarization is lost at extremely high (> 10s5 Hz) frequencies.


Figure 2-2 schematically


illustrates the frequency dependence of the polarization mechanisms in dielectric materials.

The dielectric losses (represented as dielectric loss factor, tan 8) that occur due to the
resonance between the relaxation frequency and the applied frequency are also shown in

Figure 2-2.

The schematic description of the four major kinds of polarization mechanisms is
provided in Figure 2-3. Electronic polarization (P,) arises from the shift of the negatively

charged valence electron cloud of the ions within the material with respect to the positively


charged nucleus.


This very short range charge displacement occurs in the optical regime


(1015 Hz) of the electromagnetic spectrum and contributes to the refractive index of the


material.


At lower frequencies, in the infrared range (1012


- 10o' Hz), atomic or ionic


polarization occurs (P,) by the displacement of oppositely charged ions with respect to


each other.


The exact frequency at which the relaxation occurs is characteristic of the bond


strength between the ions.


In the sub-infrared range of frequency,


dipole polarization


occurs, which involves the perturbation of the thermal motion of ionic and molecular


dipoles producing a net dipolar orientation in the direction of applied field.


general types of dipolar polarization.


There are two


First, the molecules containing a permanent dipole


moment rotate against an elastic restoring force and align in the direction of applied field













Interfacial Polarization


8


4


0 -
10-3


Dipole Polarization
(low freq.) (high frequency)


Pl2 Or Pd1


Atomic (or ionic)


Polarization


T= 300K


- -


Electronic
Polarization


LOG [FREQUENCY (Hz)]


-10-1


-'105-
10-3


(b) LOG [FREQUENCY (Hz)]
(b)


Figure


Schematic illustration of dielectric constant (k) and dielectric loss (tan 8) as a


function of applied frequency in a hypothetical material [1].


10-1 10 103 105 107 109 1011 1013 1015


10-1 10 103 105 107 109 1011 1013 1015


















Shift in
Electron
Orbitals


atom M


atom N


I,
\-m


"L1
1a-


Na+
r(la


a+


.Na+
Na+
t^-'


7imirr..r Qrb,, v-ijw A100c, -tn nC\f ,,rn'c; rnonhnnf crnc afc- AnIjrmv-r


Na+
Na+
Na+
Na+
Na+


E-


wws^L-


E----









high, in the range of 1011


In the second type of dipolar polarization, the molecules


rotate between two equivalent equilibrium positions (Pr).


It is the spontaneous alignment


of dipoles in one of the equilibrium positions which gives rise to the large, nonlinear


polarization in ferroelectric materials, as described later.


Since an appreciable distance of


charge migration is involved between the equilibrium positions, the relaxation occurs at a


frequency range of 103


- 1O Hz.


The last mechanism of polarization is the interfacial or space charge polarization
which occurs when mobile charge carriers are impeded by a physical barrier that


restricts charge migration, the charges accumulate at the barriers producing a localized

polarization in the material. Since the charge migration is relatively long ranged, this


occurs only at a sufficiently low frequency (<


1 Hz).


However, if the barriers are an


internal structural feature, or the density of charges contributing to the interfacial


polarization is sufficiently large, the frequency range may extend to


103 Hz range (P,2).


Table 2-1 lists the frequencies and dielectric constants for several ceramics and glasses.


2.2.3.


Ferroelectricitv


Ferroelectricity is special characteristic of a group of dielectric materials (termed as

non-linear dielectrics or ferroelectrics) leading to the presence of spontaneous polarization

(polarization without the external applied electric field), and the ability to switch the


direction of the internal polarization with an externally applied electric field.


spontaneous alignment of dipoles which occurs at the onset of ferroelectricity is often

associated with a crystallographic phase change from a centrosymmetric, non polar lattice


to a non-centro symmetric polar lattice.


Of the 32 crystal classes or point groups, 11 are


centrosymmetric and therefore can not possess polar properties or spontaneous polarization
Til ('n nf tha ra mon n '71 n,-nnnnntrnnmnmnn ah-,n) nnntn rn hr (ihn AT)"\ he e,,c,',,., tr
















Table


Dielectric properties of ceramics and glasses.


Dielectric Material Frequency (Hz) Dielectric Constant Ref.

SiO2 102-1010 3.78 1

A203 de 8.60 1
Pyrex 102-106 5.02 4.84 1
Diamond dc 6.6 1
a-SiC d 9.7 1

LiP dc 9.0 1

KBr cd 4.9 1
MgO dc 9.6 1
Mg2SiO4 cd 6.2 1
BaTi03 102-10o0 1400 2
PbTiO3 102-1010 210 2
SrTiO3 102-101o 320 6
LiNbO3 102-1010 68 2

PZT 102-1010 1800 6


Note: 'dc' refers to frequency independent behavior, which indicates that electronic polarization is the only
dielectric mechanism in the material.







15

Piezoelectricity is the property of a crystal to exhibit electric polarity when subjected

to a stress, and conversely, if an electric field is applied, a piezoelectric material will expand


or compress depending on the orientation of the applied field.


Of the 20 piezoelectric


classes, 10 have unique polar axis, an axis which shows properties at one end different


than the other.


Crystals in these classes are called polar crystals because they are


spontaneously polarized and exhibit ferroelectricity. Spontaneous polarization is a function

of temperature, and therefore, if a change in temperature is imposed on a polar crystal,


electric charge is induced on the opposite
ferroelectric crystals are also pyroelectric.


1 faces. This effect is called pyroelectricity.
However, ferroelectric crystals are only those


crystals for which the spontaneous polarization can be reversed by applying an electric

field.

Barium titanate (BaTiO3) is an excellent example to illustrate the structural changes

that occur when a crystal changes from nonferroelectric (paraelectric) to a ferroelectric state.

The Ti4+ ions of BaTiO3 are surrounded by six oxygen ions in an octahedral configuration,
as shown in Figure 2-4. Since a regular Ti06 octahedron has a center of symmetry, the six


Ti-O dipole moments cancel each other in antiparallel pairs.


A net permanent dipole


moment of the octahedron can result only by a unilateral displacement of the positively

charged Ti4+ ion against its negatively charged 02 surroundings. Ferroelectricity requires


the coupling


such


displacements and


the dipole moments associated


with


displacements. In the ABO3 or BaTlO3 (perovskite-like) structure, named after the CaTiO3


perovskite mineral, each oxygen has to be coupled to only two Ti ions.


As a result, the


TiO6 octahedra in BaTiO3 can be placed in identical orientations, joined at their comers,


and fixed in position by Ba ions.


Thus, in BaTiOs, the Ba and O ions form a fcc lattice,


with Tl ions fitting into octahedral (body center) interstices.


The characteristic feature of













































Figure 2-4.
BaTiO3 [2].


Octahedral coordination of Ti4+ ion in the perovskite unit cell of








energy positions for the


Ti atom which are off-center and can therefore give rise to


permanent electric dipoles.


At a temperature higher than T,


' (curie temperature) the thermal energy is sufficient


to allow the Ti atoms to move randomly from one position to another, so there is no fixed


asymmetry.


The open octahedral site still allows the Ti atom to develop a large dipole


moment when an external field is applied, but there is no spontaneous polarization.


material is called paraelectric in this symmetric configuration.


When temperature is lowered


below T0, the position of the Ti ion and the octahedral site changes from cubic to tetragonal

symmetry, with the Ti ion in an off-centered position corresponding to a permanent dipole.

Those dipoles are ordered, giving a domain structure with a net spontaneous polarization


within the domains.


1200C.


The cubic to tetragonal phase transition in BaTiO3 occurs around


It also undergoes two other phase transitions (Figure 2-5) at lower temperatures: a


tetragonal to orthorhombic transition at 50C, and an orthorhombic to rhombohedral


transition at -900C [1].


The crystallographic dimensions of the BaTiO3 lattice change with


temperature, and at the phase transitions, as shown in Figure 2-6 (a), mainly due to the

distortion in the TiO6 octahedra as the temperature is lowered from the high temperature


cubic phase.


Because the distorted octahedra are coupled together, there is a very large


spontaneous polarization, giving rise to a large dielectric constant (-3000 at 250C), and a

strong dependence of dielectric constant on temperature [2], as shown in Figure 2-6 (b). In

regard to capacitors, it is extremely important that the dielectric constant is high over a wide


range of temperature.


The presence of the two lower ferroelectric transitions ensures that


the dielectric constant remains high below the Curie temperature.


As seen in Figure 2-6


(b), the dielectric constant along the a axis is larger than that along the polar c axis.


This is


attributed to the instability of the structure that makes it easy to tilt the spontaneous

















>1200C


Cubic


5C

PS


I
I
I
/ v
I,


Tetragonal


a-ax


-900C

Orthorhombic


parallel with


12 equivalent


<110>


<110>


b-axis


orP


directions


T<-90C


Rhombohedral


parallel


with


<111


CL


*r *












E 404
Q-


402



400



398


-100


-50 0 50 100


TEMPERATURE (C)


(a)


10 r


-160


-120 -80 -40 0 40 80


TEMPERATURE (C)








structure (Figure 2-6(a)).


Even though this mistake has been realized for a long time, no


attempts were made to obtain corrected data due to several problems associated with low

temperature (-90C) dielectric measurements while applying mechanical load to prevent
twinning [29].

An essential consequence of the presence of spontaneous polarization in the


ferroelectric materials is their hysteresis behavior.


The hysteresis loop (Figure 2-7) is due


to the presence of crystallographic domains within which there is complete alignment of


electric dipoles.


At low field strengths in unpolarized (also called virgin) ferroelectric


material, the polarization (P

applied field (E, V/cm). At


, pC/cm2) is initially reversible and is nearly linear with the
higher field strengths, the polarization increases considerably


as a result of the switching of the ferroelectric domains.


The polarization switches so as to


align with the applied field by means of domain boundaries moving through the crystal. At

high enough field strengths, the change in polarization is small due to polarization


saturation (PsT),


, all the domains of like orientation are aligned with the field.


Extrapolation of the saturation curve


back to


zero


field


gives


the saturation


polarization, corresponding to the spontaneous polarization with all the dipoles aligned in


parallel, with no applied field.


When the applied field continues to increase beyond the


value required for saturation polarization, the polarization also continues to increase, albeit

slowly and linearly with the field. Even though all the domains are aligned parallel to each

other, the individual TiO6 polarizable units can continue to be distorted increasing the unit


polarization.


This is an important contrast to ferromagnetic materials, where application of


magnetic field greater than required for saturation magnetization (M,) does not increase the


net magnetic moment of the material.


When the applied field is withdrawn, a finite


polarization remains in the material, called remnant polarization, P,


,. This is due to the






















PSAT


P.7


Figure 2-7.


Typical polarization hysteresis loop of a ferroelectric material [4].







22
characteristic of a ferroelectric material, as it determines the ability of the material to switch

polarization direction.


Materials


Ferroelectric materials include titanates, zirconates, and niobates with oxygen


octahedral structure types.

BaTiO3, PbTiO3, PbZrO3,


Some of the prominent materials are perovskite compounds

Pb(Mg,Nb)O3 (PMN), KNbO3, and their solid solutions,


tungsten-bronze compounds such as (Sr,Ba)Nb206, and the ilmenite compound LiNbO3.

Among the solid solutions of perovskites, perhaps the most important thin film material for

memory applications is Pb(Zr,Ti)03 (PZT), the solid solution of PbTiO3 and PbZrO3.

PbTiO3 and PZT are suitable for pyroelectric sensing applications due to their large


pyroelectric


coefficients,


whereas


BaTiO3 and PMN


are considered for capacitor


applications due to their high dielectric constants [5].

Since barium titanate is by far the most popular ferroelectric material, used as the

dielectric in multilayer ceramic capacitors, there have been many studies of its solid


solutions [3-7, 30, 31].


Substitutions for Ba2+ or Ti4+ are used to raise the permittivity,


decrease the temperature dependence of dielectric constant, and lower the dielectric losses.

Substituting a divalent cation for barium in BaTiO3 modifies the transition temperatures.


The three most commonly used "Curie point shifters" are Pb2+


Sr2+


and Ca2+


. Modest


amounts of Pb2+ raise T,, and Ca2+ has little effect. Divalent Pb is one of the very few

additions which increases the transition temperature. This is because the tetragonal

pyramidal coordination favored by Pb2+ stabilizes the tetragonal phase with respect to the


adjacent cubic and orthorhombic phases.


All three curie point shifters destabilize the


orthorhombic and rhombohedral phases of BaTiO3 as the lower two transition temperatures


drop when increasing amounts of Pb2+


, Sr2+,


r v


or Ca2+ are added.


The opposite effect is


F~erroelectric


2.2.4.






23
electrooptic, nonlinear optic, photorefractive, pyroelectric, bulkwave, and surface acoustic


wave (SAW) devices [5].


These niobates include Ba2NaNb5015 (BNN), (Sr,Ba)Nb206


(SBN), (PaBa) Nb206 (PBN), K3Li2Nb5015 (KLN), and Pb2KNb5015 (PKN).
2.2.5 Thin Film Ferroelectrics and Their Applications

The quest for miniaturization and high performance in the electronic device
manufacturing, and the necessity to integrate the bulky, discrete electronic components has


led to the development of thin film materials for many applications.


Thin films have the


added advantages of small volume, large geometrical flexibility, and convenient integration


with semiconductor or optical integrated circuits.


Prominent among them are the


ferroelectric thin films, owing to the wide variety of properties and high charge storage


capacities.


Although there are many applications for ferroelectric thin films such as in


dielectric barrier layers, ultrasonic transducers, and electrooptic modulators/switches, the

most significant application is in the case of computer and microprocessor memories where
more capacitance is required for reliability as the density of the dynamic random access
memory (DRAM) increases beyond several megabits, and a permanent memory is desirable
to prevent loss of information during power failure. Nonvolatile memories also have other

potential advantages such as high access speed, high density, radiation hardness, and low


operating voltage.


Table 2-2 summarizes the properties and applications of several thin


film ferroelectric materials [5].


The schematic illustrations of various device applications


for ferroelectric thin films are presented in Figure 2-8.


2.3. Methods for Synthesizing BaTIO Thin _Flms
The various techniques available to prepare ferroelectric thin films are listed in Table
2-3. In general, there are two major categories of deposition techniques for thin films, i.e.,
















Table


Properties and applications of ferroelectric thin film materials [5].


Thin Film Material Phenomena Applications


Perovsklte
BaTiO3
(Ba,Sr)TiO3


Titanates


Dielectric


Capacitors, sensors, phase shifters


Pyroelectric
PTCR


PbTiO3 (PT)


Pyroelectric
Piezoelectric


Pyrodetectors
Thermistors


Pyrodetectors
Acoustic transducer


Pb(Zr,Ti)03 (PZT)


(Pb,La)(Zr,ri)O3 (PLZT)


Dielectric


Pyroelectric
Piezoelectric
Electrooptic

Pyroelectric
Electrooptic


Nonvolatile memory
Pyrodetectors
Waveguide devices
SAW substrates


Pyrodetectors
Wave guide devices, optical memory, displays, SHG


Nlobates


Pb(Mgl3, Nb23)03 (PMN)
PMN/PT


LiNb03 (LN)
LiTaO3 (LT)


KNbO (KN)


K(TaNb)03 (KTN)


Tungsten-bronze
(Sr,Ba)Nb206 (SBN)


Dielectric


Electrooptic


Piezoelectric
Electrooptic


Electrooptic


Pyroelectric
Electrooptic


Dielectric


Capacitors, memory
Waveguide devices

Pyrodetectors
Waveguide devices, optical modulators, SAW, SHG

Wave guide devices, frequency doubler, holographic storage

Pyrodetectors
Waveguide devices


Memory


SrTi03







































E

U.
uC
I-
Q-
a-


e
*-


-J.0)
b so
*4 Al
-* wP


CA
NJ


,,8







26





Table 2-3. Thin film deposition techniques [31].

Physical vapor deposition (PVD)

Sputtering (de, ion beam, rf magnetron)
Evaporation (e-beam, resistance, molecular beam epitaxy (MBE))
Laser ablation

Chemical vapor deposition (CVD)

Metalorganic (MOCVD)
Plasma enhanced (PECVD)
Low pressure (LPCVD)

Chemical solvent deposition

Sol-gel (solution-gelation)
Metalo-organic decomposition (MOD)

Melt solution deposition

Liquid phase epitaxy (LPE)

Hydrothermal/electrochemical methods

Hydrothermal oxidation
Anodic oxidation







27
identify these two categories as vacuum and nonvacuum techniques, respectively. In both


these cases, the fabricated films are most commonly polycrystalline.


However, in some


cases, it is desirable to produce epitaxial growth on a substrate of the proper crystalline


orientation and lattice matching.


Recent reports show that epitaxial film growth is more


readily achieved with vacuum techniques, whereas chemical deposition methods have

proved to be most successful in regard to their ease of preparation and low equipment costs


Some of these methods are discussed in more detail in the following sections.


2.3.1. Vapor Deposition Methods
Vacuum deposition processes to synthesize solid, thin films are mainly divided into
two types: physical vapor deposition (PVD) and chemical vapor deposition (CVD)

processes. In PVD, solid precursors (targets) to the desired film material are vaporized and

deposited on a substrate by a variety of sputtering techniques such as radio-frequency (rf)


magnetron, ion beam, laser ablation and electron beam evaporation.


Sputtering has been


one of the most common deposition methods and has the advantage of producing thin films


of very high quality [32-40].


Radio-frequency sputtering has been the most widely used


technique for the deposition of BaTiO3 thin films on silicon substrates.


Substrate


temperatures for depositing crystalline films varied between 100-8000C, and deposition


rates of upto 20 A/min were achieved [32-34].


Films deposited at lower substrate


temperatures were amorphous in nature and required a post deposition annealing between


400-5000C to crystallize.


Amorphous BaTiO3 films, however, possess interesting


dielectric properties such as large charge storage capacity, and have been studied in detail


[35, 36].


Ion beam assisted evaporation and ion beam sputtering are continuing to become


more popular in order to increase film uniformity and deposition rates, and have been used
to deposit several ferroelectric compositions including BaTiO3, PbTiO3, PZT, PLZT, and







28

oxygen partial pressure, and laser energy density are the typical processing parameters that


effect the composition and microstructure of the deposited films [39, 40].


Molecular beam


epitaxy (MBE) technique has been used to grow epitaxial BaTiO3 on Si substrates, with


BaO as the intermediate layer to facilitate lattice matching [41].


Physical vapor deposition


techniques described above offer a wide choice of parameters to control film composition,


quality,


deposition rate and crystallinity.


However, PVD processes have several


shortcomings: deposition rates are slow, high energy atoms and particles can damage the


substrate, and the stoichiometry of the films formed can vary from that of the source.


additional limitation of sputtering is that it requires expensive and complicated equipment

In chemical vapor deposition (CVD), volatile chemical precursors are vaporized and


transported to a reaction chamber and deposited on heated substrates.


the substrate to form the desired thin-film material.


Reactions occur at


Advantages include high deposition


rates,


superior


microstructure


(pin-hole


free),


good


stoichiometry


control.


Metalorganic chemical vapor deposition (MOCVD) has been the most widely used CVD


technique to prepare BaTiO3 thin films [42-44].


The potential advantages of MOCVD


include the ability to deposit high quality, ultrathin layers on three dimensional complex

geometries and the amenability to large scale processing. MOCVD is typically carried out

at low pressures (4-5 Torr) in a horizontal quartz reactor with a cylindrical configuration

and equipped with a gas inlet, substrate mounting, and a gas outlet, all are mounted coaxial


with the walls of the reactor.


The substrate is held facing down, and the metalorganic


precursors are introduced through separate manifolds and allowed to react at the heated

substrate before they escape through the outlet Precursor manifolds and reactor walls are

heated to avoid condensation of the reacting gases. BaTiO3 films are grown on Si, SiTiO3,


NdGaO3, and La A103 substrates at temperatures ranging from 600-10000C.


The success








precursor is less obvious.


Ba(hfa)2(tetraglyme), Ba(fod)2


and Ba(dpm)2 are some of the


examples.


The precursors are transported to the reaction zone by an argon carrier gas.


Growth rates of 100-150 A/min are typically achieved.


By carefully controlling the


substrate


surface quality,


deposition rate,


substrate


temperature,


it has


been


demonstrated that epitaxial BaTiO3 films can be grown using MOCVD process.


Chemical


vapor deposition methods have excellent potential as a production method for device quality


ferroelectric thin films, especially when MOCVD is used.


The CVD process can be


extremely reproducible once conditions are established to produce a film with a particular


composition and crystal structure.


An additional advantage is that scale-up of a CVD


process from the laboratory to production is typically not as difficult as it is for PVD


(sputtering).


Unfortunately, the CVD processes require high temperatures, which is a


limitation when device integration and metallization with non-refractory electrode materials


such as doped aluminum or tungsten are required.


The CVD deposition temperatures have


significantly decreased in recent years, through the use of plasma enhanced CVD (PECVD)

methods [45].


2.3.2.


Sol-Gel Synthesis


Sol-gel processing is currently one of the most actively studied processing


techniques for metal oxide based ferroelectric materials [7].


In its simplest form, the sol-


gel process first involves the preparation of a homogeneous solution containing the


precursors, usually as metal alkoxides.


The deposition of a thin film on the substrate is


performed while simultaneous hydrolysis and polycondensation occur, thus producing an


amorphous thin film.


The final step involves heating (500-800'C) of this amorphous thin


film, until it crystallizes to give the ferroelectric crystalline thin film.


The sol-gel process is


similar to the MOCVD process exDlained above, in the sense that both Drocesses use







30
room temperature hydrolysis and condensation to produce an amorphous film in sol-gel

process. In addition, sol-gel doesn't employ vacuum and therefore low in capital costs.


Metal alkoxides have a general formula: M(OCH2n,1),

an integer, and x is the valence state of the metal. All met


where M is a metal, n is


als form alkoxides, and by


varying the alkyl group, liquids or solids, soluble or insoluble, volatile or non-volatile


materials can be obtained.


It is convenient for sol-gel process to choose liquid materials


that are soluble in organic solvents and volatile and therefore readily purified by distillation.
Most metal alkoxides are readily hydrolyzed to the corresponding oxide or hydroxide [46]:


M(OR), + xH2


t M(OH), + xROH


2M(OH), a M,0 + xH,O


[2.5]
[2.6]


where R represents CH2n+1.


Sol-gel methods begin with the preparation of liquid precursors.

materials can be metalorganics such as alkoxides described above, ac


The starting


;etates, acetyl


acetonates, or inorganics such as metal hydroxide or nitrates. Recently, much attention has

been directed towards the effect of the correlation between the solution structure and the

final crystal structure of the sol-gel derived systems. It was found that a close resemblance

of the local atomic arrangement of the alkoxide complexes in the solution can play a critical


role during the crystallization processes [47]. The 'sol' is prepared by dispersing the

starting materials in an organic solvent (usually alcohols). Water and catalysts are added to


the sol solution to initiate a series of hydrolysis and polycondensation reactions which


produces a viscoelastic oxide gel network.


The conditions of partial hydrolysis (i.e.,


alkoxide concentration, relative amount of water, additive) have important effect on the


deposition characteristics, quality


of final


microstructure,


microstructure


ApMallnmsnt in cthh awr aftoir heat roaflnant h


Thnaa lrlalll hurtltlmela ornk ahra rito tn








used to prepare many ferroelectric materials including BaTlO3, PZT, PLZT


and LiNbO3


Preparation of BaTiO3 films using sol-gel techniques have been reported [46, 48-


Titanium isopropoxide is generally used as the Tl precursor, and several variations of


Ba metalorganics such as barium isopropoxide [Ba(OC3H7)2],


[Ba(C7H15COO)2],


precursors.


barium 2-ethyl hexanoate


and barium neodecanoate [Ba(C9H19COO)2] are used as the


Sol-gel films can be deposited on arbitrary surfaces, as in the case for BaTiO3:


stainless steel, fused silica, platinum,


and platinized silicon.


Usually,


the films are


prepared directly on electrode surfaces to facilitate characterization or device fabrication.

Annealing densificationn, crystallization) temperatures for sol-gel thin films of BaTiO3


typically varied between 700-8000C.


Sol-gel processing is a relatively new method for the


fabrication of ferroelectric thin films and is one of the most promising techniques.


advantages of sol-gel processing are good homogeneity, ease of composition control, low
sintering temperature, large area thin films, possibility of epitaxy and lower capital cost

than other techniques. Main disadvantages of this technique include high cost of precursor

materials, difficulty of handling atmosphere-sensitive alkoxides, and lack of control in the

crystallization of thicker films.

An alternative chemical process, called metal organic decomposition (MOD), is
similar to sol-gel processing, except that no hydrolysis step is performed prior to


annealing.


MOD methods often use carboxylate precursors with large organic groups,


such as barium neodecanoate [Ba(C9H19COO)2] and titanium dimethoxy dineodecanoate


[(CH30)2Ti(C9Hl9COO)2] to yield viscous precursor solutions,


thick precursor films after spin coating [52].


to facilitate formation of


Cracking can be a problem because of the


high organic content of the films.








and their solid solutions [such as (Ba,Sr)TiO3] on substrates of B metal [12-22].


process involves a hydrothermal treatment of polished metal (Ti or Zr) substrates in


aqueous solutions containing A cation.


The film growth is accelerated by combining


electrochemical treatment with hydrothermal treatment.


substrate by applying an external


Anodization of the metallic


dc voltage enhances the dissolution (corrosion) of


metallic species into solution which react with A type (Ba2+


, Sr2


, andCa2+) cations and


form the complex oxide ABO3


The process has several advantages over the conventional


deposition techniques. Low synthesis temperature and no necessity for a crystallization

treatment eliminate the substrate-film interactions, and facilitate device integration.

Deposition on complex geometries and large area substrates is another advantage of this

solution deposition technique. Less toxicity of inorganic precursors and aqueous solutions

help keep the capital and processing costs low.

A schematic diagram describing the hydrothermal-electrochemical apparatus to


deposit BaTiO3 thin film on Ti substrate is shown in Figure 2-9.


A mechanically polished


and degreased Ti substrate was coupled to the positive terminal of an external galvanostat

and a platinum foil was placed opposite to the Ti anode and connected to the negative


terminal.


Platinum wires were used as the electrical leads.


The electrode assembly was


immersed in a Teflon beaker containing the aqueous solution of Ba(OH)2.8H20, which in


turn is placed in an electrolytic autoclave.


Sufficient amount of redistilled water is placed


around the beaker, to avoid the over heating of the Teflon vessel, and also to control the


pressure inside the autoclave.


The autoclave was equipped with a stirrer and pressure tight


electrical feedthroughs for the electrode leads.


Early investigations in this technique


involved the use of high temperatures (400-6000C) under supercritical conditions of
aueous solutions. to deposit tetragonal BaTiOh films with referred crvstalloeranhic















Galvanostat


Redistilled Water




-Stainless Spacer

- 0.05 to 0.5N


Room Temperature to 2000C
Saturation Vapor Pressure to 1.8 MPA









Figure 2-10 [5]. With increasing concentration of Ba2+, formation of BaTi03 is favored at

lower temperatures. Similar attempts have been successfully made to synthesize CaTiO3


and SrTi03 thin films on Ti substrates.


Formation of titanate films on Ti substrates under


purely hydrothermal conditions has also been reported [17-19, 21],


verifying that the


anodization of the Ti substrate is not necessary to form the titanate film, but only accelerates


the reaction through enhanced corrosion of Ti.


The process also has been adopted to


deposit high quality titanate films on arbitrary surfaces by sputtering Ti on to glass, silicon,


and polymer films.


Major limitations of the technique include slow deposition rates,


contamination from carbonates, difficulties in growing thick films, and microstructural

defects caused by the electrode reactions (gas evolution) and electrolytic breakdown of the

films.


Characterization of Thin Films


With the explosive growth of thin film utilization in microelectronics and the

interdisciplinary nature of applications, there is a compelling urge for film characterization


and intrinsic property measurements.


As a result, several techniques are employed to


evaluate structural, chemical and dielectric characteristics of thin films.


Some of these


techniques provide unprecedented structural resolution and chemical analysis capabilities


over small lateral and depth dimensions. Some techniques only sense and provide
information on the first few atomic layers of the surface. These diverse, yet complimentary


characterization techniques are described in the following sections.


2.4.1.


Structural Charac


teat


Structural


features


such


as grain


size,


thickness,


uniformity, surface


tonoeranhv. mnorhnlnhol of film cnn tintents_ voids and nther defect are of sicmificant





















Ba(OH)2 Solution


200


150


100


50
R.T.


CONCENTRATION (N)




Figure 2-10. Effect of Ba2+ concentration in the electrolyte on formation of
BaTiO3 on Ti substrate under hydrothermal/electrochemical conditions [5].







36
The most common imaging mode in SEM is the secondary electron imaging, where low

energy electron emitted from the sample surface by the interaction of the electron beam are
detected. Sloping surfaces produce a greater secondary electron yield because the portion

of the interaction volume projected on the emission region is larger than on a flat surface.


Similarly,


edges will appear even brighter.


Compositional information in SEM can be


obtained using backscattered imaging mode.


Backscattered electrons are the high energy


electrons that are elastically scattered and essentially possess the same energy as the
incident electrons. The probability of backscattering increases with the atomic number Z of

the sample material. Useful contrast can develop between regions of the specimen that


differ widely in Z.

Transmission electron microscopy (TEM) is used to obtain internal structural


information from specimens that are thin enough to transmit electrons.


Thin films are,


therefore, ideal for study, but they must be removed from electron-impenetrable substrates


prior to insertion in the TEM.


This problem is usually solved by making a cross sectional


sample, from a thin slice cut from the sample.

gun are accelerated by a high voltage bias (100

specimen by means of a condenser lens system.


Electrons thermionically emitted from the


800 kV) and first projected on to the


The scattering processes experienced by


electrons during their passage through the specimen determine the kind of information


obtained.


Elastic scattering, involving no energy loss when electrons interact with the


potential field of the ion cores, gives rise to diffraction patterns.


Inelastic interactions


between


beam


and matrix electrons at heterogeneities such


as grain boundaries,


dislocations, second phase particles, defects, density variations, etc.,


cause complex


absorption and scattering effects, leading to a spatial variation in the intensity of the


transmitted beam.


This effect is used to imaee the internal microstructure of the secimen.








orientation of polycrystals, defects, stresses, etc.


Extension of X-ray diffraction methods


to thin film analysis poses a limitation: the great penetrating power of X-rays means that

with typical incident angles, their path length through films is too short to produce
diffracted beams of sufficient intensity. Under such conditions, the substrate, rather than
the film, dominates the scattered X-ray signal. To overcome this problem, Seeman-Bohlin

diffraction geometry (Figure 2-11) is employed [53]. This is done by using a grazing angle

of incidence y, which increases the effective thickness of the film by several times. The


focal point of the X-ray source, film specimen, and detector slit are all located on the


circumference of one great circle.


Each of the diffracted peaks are sequentially swept


through as the X-ray detector moves along the circumference.


Several spectroscopic techniques are employed to characterize the chemistry of thin

films. Most popular among these are X-ray energy dispersive spectroscopy (EDS), Auger

electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), secondary ion


mass spectroscopy (SIMS), and Rutherford backscattering (RBS).


Principle of detection


and the specific applications of each of these techniques are briefly described here.
Most EDS systems are interfaced with an SEM, where the electron beam serves to

excite characteristic X-rays from the area of the specimen being probed. A Si(Li) detector


is aimed to efficiently intercept emitted X-rays.


An incoming X-ray


generates a


photoelectron in Li doped Si, that eventually dissipates its energy by creating electron-hole


pairs.


The incident photon energy is linearly proportional to the number of pairs produced


or equivalently proportional to the amplitude of the voltage pulse they generate when


separated.


The pulses are amplified and then sorted according to voltage amplitude by a


multichannel analyzer, which also counts and stores the number of pulses within given


. Chemical Characterization


2.4.2


































Cu ka 20-Y 28


Sample




Figure 2-11. Seeman-Bohlin grazing angle X-ray diffraction geometry for the
analysis of thin films [53].







39
energies in the EDS spectrum with the characteristic X-ray energies for all the elements in


the periodic table.


Si(Li) detectors typically have a resolution of about 150 eV, so overlap


of peaks occurs when they are not separated in energy by more than this amount.


Overlap


usually occurs in multicomponent samples or when neighboring elements in the periodic


table are present.


A variation of EDS is the wavelength-dispersive spectroscopy (WDS),


where wavelength rather than energy is dispersed, a factor of 20 or so improvement in X-


ray line width resolution is possible.


In this case, emitted X-rays, rather than entering a


Si(Li) detector, are diffracted from single crystals with known interplanar spacings.


From


Bragg's


law,


each


characteristic


wavelength


reflects


constructively


at different


corresponding angles, which can be measured with very high precision.


The electron


microprobe (EMP) is an instrument specially designed to perform WDS analysis.

The Auger process (Figure 2-12), which is the basis of AES, first involves an


electron transition from an outer level (e.g.,


1) to a hole in the inner shell (K).


resulting excess energy is not channeled into the creation of a photon but is expended in


ejecting an electron from yet a third level (e.g., L2).


The atom finally contains two electron


holes after starting with a single hole created by the incident electron beam.


The electron


that leaves the atom by the Auger process is known as an Auger electron, and it possesses

an energy given by [53]:


-EL2


-EL,


[2.7]


The last equality indicates KLL and KL2. transitions are indistinguishable in terms of


energy and consequently denoted by


denoted by LMM and MNN


KLL.


Since the K


Similarly, other transitions observed are


, L, and M energy levels in a given atom are


unique, the Auger spectral lines are characteristic of the element in question.


Therefore, by


measuring the energy of the Auger electrons emitted by the sample, it is possible to identify


=EK


= Eg



















Vacuum


//-/,//77777
.Valance Band:
/i/////////I


//


3s etc.


M

L2,3


Initial State


Electron Ejected


=EK


(d) EKLL


=EK


-EL2


X-Ray Emission


Figure 2-12.


Auger Electron
Emission


Schematic of electron energy transitions: (a) initial state, (b) incident


X-rayv or P.elelrn p.ipet IK hpll lctrtmn fr\ Y-rav pmiccinn whPn


Alprtrnn flie


1L ppI-

rp.


~rrr~rr


.:I I._I II~. 1~1


IItI IKm


.1 ._I


- EL1


- EL1








helium is not possible by AES technique.


The extremely low energy of Auger electrons


limits the analysis depth to the top few atomic layers of the sample, making AES a true

surface analytical technique.

A typical AES spectrometer schematically shown in Figure 2-13, is housed within a


ultrahigh vacuum chamber maintained at


= 10-10 torr.


This level of cleanliness is required


to prevent surface coverage by contaminants (e.g., C, O) in the system.


Two very useful


capabilities of AES for thin film analysis are depth profiling and lateral scanning.


The first


is accomplished with incorporated ion guns that enable the specimen surface to be
continuously sputtered away while Auger electrons are detected. Multielement composition

depth profiles can thus be determined over total film thicknesses of several thousand

angstroms by sequentially sampling and analyzing arbitrarily thin layers. Although depth

resolution is extremely high, the frequently unknown sputter rates for various materials


makes precise depth determinations difficult

BaTiO3 thin film is shown in Figure 2-14.


A typical AES depth profile analysis for a


Through raster or line scanning the electron


beam, the AES is converted into an SEM and images of the surface topography can be


obtained.


By modulating the imaging beam with the Auger electron signal, a lateral


composition mapping of the surface can be obtained.

The basis for XPS lies in the same atomic core electron scheme that is considered


for X-ray and Auger electron emission.


Rather than incident electrons in the case of EDS


and AES, relatively low-energy X-rays (Mg or Al K,) impinge on the specimen in this


technique.


The absorption of the photon results in the ejection of electrons via photoelectric


effect. Energetics of this process are governed by the equation [53]:


= hv


-E


[2.8]


where E,, hv


, and E, are the energies of the ejected electron, incident photon, and the





































a-a)
Z "-
Q0 '2
O0
C)


-.
*0 0
c
- >-
0


c
O


O^ -
O-

a 2


C)
QT 3
0
X W


I--1A


e-

o
tU
-


\
\
I


0)


o


0 0


0-o

cna(
W<





*o*
0
S




>- 0
oe
Ca




0,
3<
C



0)





*
As
0-Q
an


0 -
N O
C
C O
' o












































6 12 18 24


SPUTTER TIME (min)


Figure


Typical AES compositional depth profile analysis of a BaTiO3


thin film on


Substrate [42].







44
rays are less prone to damage surfaces than are electrons. For example, electron beams can
reduce hydrocarbon contaminants to carbon, destroying the sought-after evidence. For this

reason, XPS tends to be preferred in assessing the cleanliness of semiconductor films.

RBS is a popular thin film characterization technique which relies on the use of very


high energy (MeV) beams of low mass (usually 4He+) ions.


These ions can penetrate


thousands of angstroms or even microns deep into films or film-substrate combinations.


However, the high energy ions cause negligible sputtering of the surface atoms. Instead,
they lose their energy through electronic excitation and ionization of target atoms. These


'electronic collisions'

continuous with depth.


are so numerous that the energy loss can be considered to be

Sometimes the fast moving light ions penetrate the electron cloud


and undergo elastic collisions with the nuclei of the much heavier stationary target atoms.

The resulting scattering from the Coulomb repulsion between ion and nucleus is known as


Rutherford backscattering.


Energy of the backscattered ion precisely depends on the mass


of the target atom, and the incident energy of the He ion at the point of collision.


Since the


incident energy of the He ion is a function of the distance traveled into the film, it is

possible to estimate the depth of collision and the mass of the colliding target atom from the


measured energy of the backscattered ion.


Thus, by measuring the number and energy of


backscattered


ions,


information


on the


nature of the elements


present,


their


concentration, and depth distribution can all be simultaneously determined without

appreciably damaging the specimen. All elements and their isotopes including Li and those

above it in the periodic table are, in principle, detectable with 4He+ ions. It may be difficult

to resolve neighboring elements, and the limit depends on the resolution of the detector.


High-Z


elements


produce


a stronger


backscattered


signal


than


low-Z elements.


Consideration of these factors sunxests that specimens for RBS analysis should ideally








of about 1 at%.


The technique is unmatched in determining the stoichiometry of thin film


binary compounds.

A critical need to measure thermally diffused and ion-implanted depth profiles of


dopants in semiconductor devices spurred the development of SIMS.


In typical devices,


peak dopant levels are about 10r/cmS


, while background levels are 1016/cm3


These


correspond to atomic concentrations in Si of 0.2% to 2 *10 %, respectively [53].


None


of the analytical techniques considered thus far has the capability of detecting such low
concentration levels. In SIMS, a source of ions bombards the surface and sputters neutral

atoms, for the most part, but also positive and negative ions from the outer most film


layers.


Once in the gas phase, the ions are mass analyzed in order to identify the species


present as well as determine their abundance.


are Ar+

energy.


The primary ions most frequently employed


, 2", and Cs+, and these are focused into a beam ranging from 2 to 15 keV in
One of the unique features of SIMS is the mass discrimination, which allows to


distinguish the isotopes.


2.4.3.


Dielectric Characterization


Dielectric properties of ferroelectric thin films that are frequently determined include

dielectric constant and dissipation factor capacitivee characteristics), ac / dc resistivities

and leakage current (impedance characteristics), and ferroelectric polarization hysteresis
behavior.
Since dielectric polarization response of a material depends on the frequency regime

of the applied field and the measurement temperature, the capacitive and impedance

characteristics are usually determined as a function of frequency and temperature. An LCR

meter or an impedance analyzer equipped with an auto-balancing bridge is employed to


measure these properties [54, 55].


The material is stimulated with an ac source and the













Fixture


V1 R2
V2


Vdrive
from
DAC


Mv


Sample


-j


Integrating
Capacitor
(0.1%, polystyrene)
HF-i


V to
out to
ADC


Current
Integrator


Current
Amplifier


C
Virtual Ground

Cl Cparasitic


V1
Z--
12







47
parameters are derived by knowing the dimensions of the material and by measuring its


capacitance and dissipation factor.


The auto-balancing bridge technique provides highly


accurate capacitance and dissipation factor measurements between 5 Hz and 40 MHz.


A ferroelectric hysteresis loop (also termed P


-E loop,


where


stands for


polarization and E is the applied electric field) is the most important criterion for verifying


the existence of ferroelectricity in materials.


AP


-E loop is usually measured by a


modified Sawer-Tower bridge [56, 57],


shown schematically in Figure 2-15 (b). Although


earlier methods utilized a dynamic (60 Hz) measurement with a oscilloscope readout, the


more recent techniques essentially involve a

plotter or computerized calculations [30].


1/10 Hz) measurement using an X-Y


Hysteresis loops are usually run on virgin


(thermally depoled), electroded samples, by varying the electric field E across the sample


while monitoring the charge collected on a large (approximately


1000 times larger in


capacitance than the sample), low-loss capacitor (also called sensing capacitor) in series


with the sample.


familiar Q


This charge (actually measured as voltage, which relates to charge by the


= CV) on the low-loss capacitor is proportional to the charge on the sample,


and thus one obtains a continuous plot of polarization as a function of the electric field. In

a strict sense, dielectric displacement rather than polarization is being measured, however,

in the case of high dielectric constant materials such as ferroelectrics, the two quantities are


very nearly equal and the term 'polarization'


is usually preferred.


For some particular


cases,


-E loop may be observed in non-ferroelectric materials, especially if the


specimen has a high electrical conductivity while the compensation of signal phase in


Sawer-Tower bridge is not suitable.


However, this kind of loop should be distinguishable


from a 'real'


ferroelectric loop.


When the applied field increases to a sufficiently high


value, the value of the polarization increases continuously in the 'false' loop, but nearly








2.5. Dieletric Pronerties ofBaTiOh Thin Films


The dielectric and ferroelectric properties of BaTiO3 thin films vary significantly

from those of bulk BaTiO3 single crystal or polycrystalline ceramic, and the extent of

deviation is dependent on film thickness, thermal history, microstructure and synthesis

technique. In the following sections, these irregularities are discussed in more detail.


2.5.1.


Dielectric Constant and Dissination Factor


BaTiO3 films prepared by e-beam evaporation technique showed a strong


dependency of dielectric behavior on film thickness [58].


Films having thicknesses in the


range of 0.1 jrm were cubic at room temperature and possessed very small dielectric

constants with no dielectric anomaly in the neighborhood of Curie temperature for BaTiO3


(near 125C).


Thicker films (about 1.0 pmn), however, showed essentially bulk dielectric


constant at room temperature, and dielectric anomaly as Tc is approached.


In a separate


study [59], the sputtered BaTiO3 films showed a sharp, well defined dielectric anomalies


near 1200C, down to film thicknesses of about 0.023 jim.


At a film thickness of about


0.01 pm, this anomaly disappeared, even though the films exhibited the anomaly associated


with the tetragonal to orthorhombic transition at 0C.


These discrepancies are mainly


attributed to the variation of the defect structures in the BaTiO3 films prepared by various


techniques.


Evaporation of ternary compounds such as BaTiO3 are complicated by the


different volatilities of the constituents, so that the defect structure in the film depends on


several processing parameters.


BaTiO3 thin films deposited by MOD technique showed


room temperature spontaneous polarization of 3.1 pC/cm2 and bias field and frequency

dependence of dielectric constant (k-1000 at 20kV/cm field, and 1kHz frequency) at a rain


size up of 0.2 p.m,


which are similar to those obtained for bulk BaTiO3 [52].


spontaneous polarization decreased with decreasing grain size, and films with an average
















1400



1200




1000




800




600


0.08


0.07


0.06


0.05


0.04


0.03


0.02


0 2000 4000 6000
THICKNESS (A)




Figure 2-16. Dependence of dielectric constant and dissipation factor on film
thickness for sol-gel derived BaTiO3 films [7].








remnant polarization of 0.65 iaC/cm2, indicating the presence of ferroelectricity [42].


room temperature dielectric constant reached a high value of 1300, with a low dissipation


factor of 0.03, beyond a film thickness of about 0.3 pmn.


The dielectric properties of sol-


gel derived BaTiO3 films were also reported to be dependent on film thickness, substrate


material, and relative humidity of the ambient atmosphere [51].


Films deposited on


stainless steel substrates showed increasing dielectric constant with increasing film


thickness.


This was attributed to the presence of a electrode barrier capacitance. Ni and Fe


present in stainless steel might have diffused into BaTiO3, increasing the film resistance


(barrier layer) in the vicinity of the electrode.


The effect of barrier layer capacitance on the


measured dielectric constant diminishes with increasing film thickness. Dielectric losses in

these films were very high at room temperature and showed a substantial decrease at higher

temperatures. In addition, dielectric losses measured in ambient air were higher than those


obtained in dry air.


These observations were attributed to the presence of porosity as well


as uncondensed hydroxyl groups in the sol-gel derived films,

susceptible to surface conduction due to adsorbed moisture. It has a


which makes them


iso been reported that


the electrical properties of BaTiO3 can be affected by moisture and this property has been

related to the microstructure of the material and that samples having smaller grain size were


more affected by moisture [19, 51].


BaTiO3 thin films prepared on Ti substrates using


electrochemical methods exhibited similar dependence of dielectric properties on relative


humidity.


Capacitance and dielectric loss factor measured at 70% relative humidity were


extremely large at low frequencies, owing to surface conduction.


Dielectric constants


reported for these films range between 100-200.


This


'size effect,' i.e.,


the anomalous


change in electrical properties associated with film thickness, is also complicated by the
additional influences of comnositinnal and structural disorders and residual stress 71 m






51
inhibited by an inhomogenous distribution of stresses and electric fields resulting from the

elastic clamping of the material near the grain boundaries by the neighboring grains. A near

total clamping effect was observed in a film with 34 nm grain size, as evidenced by an


extremely narrow hysteresis loop, similar to that of a paraelectric material.


With increasing


grain size, the hysteresis loop gradually exhibited ferroelectric behavior with a remnant


polarization of about 3.1 pC/cm2 for a 0.2 pm grain size film.


Another characteristic


behavior of ferroelectric thin films is the presence of a nested, non-saturating hysteresis


loop, as reported for BaTiO3 thin films deposited by MOCVD process [42].


While the


contamination of the film material was one of the proposed reasons for this behavior, a
strong domain fixing effect of the intermediate layer between the film and substrate was

reported to be the reason in high purity films.


2.5.3.


Resistivity and Leakane Current


The electrical resistivity and associated leakage currents in BaTiO3 thin films were


found to depend on dielectric constant, temperature, and frequency [37, 42, 51, 60].


Films


with lower dielectric constants exhibited higher resistivities and smaller leakage currents.
In the films that possessed higher dielectric constants, the leakage current increased

substantially when the Curie temperature for tetragonal to cubic transformation was


approached [60].


For BaTiO3 films deposited by rf magnetron sputtering, the dielectric


constant was increased from 16 to 330 as the substrate temperature during deposition was


raised from room temperature to 7000C.

1.2x1010 to 3.7x104 n.cm. This was


The corresponding dc resistivity decreased from


attributed to the amorphous and polycrystalline


natures of the deposited films formed at low temperatures and high temperatures,


respectively.


A sandwich of amorphous/polycrystal structure was found to give a


combination of high dielectric constant (k=210) and high electrical resistivity (1010 acm).








high power circuits.


In a separate study, BaTiO3 films deposited by sol-gel method


showed a room temperature dc resistivity of about 1015 Q.cm, and breakdown strength of

20 MV/m [51].


2.6. Capacitors as Charee Storaee Devices

Capacitors can perform several functions in electrical circuits including blocking,


coupling and decoupling, ac -e separation, filtering and energy storage.


They can


block direct currents but pass alternating currents, and therefore can couple alternating


currents from one part of a circuit to another while decoupling dc voltages.


A capacitor,


when suitably designed can separate direct from alternating currents and also discriminate


between different frequencies.


The charge storage capability of capacitors is more


commonly exploited in the electronic devices, for example, the photoflash unit of a camera.

The device characteristics of a capacitor, and the several kinds of capacitors and their

applications are discussed in the following sections.

2.6.1. Capacitor Characteristics


2.6.1.1.


Volumetric


Efficiency.


Volumetric efficiency is a measure of the


capacitance that can be accommodated in a given size of capacitor. In the case of a parallel


plate capacitor of area A and electrode separation d,


_ke0
-2


the volume efficiency can be written


[2.9]


where C is the capacitance and V is the volume of the capacitor.


It is clear from this


relation that the volumetric efficiency is directly proportional to dielectric constant and


inversely proportional to the square of the dielectric thickness.


However, for a given


. t nn nt n S .ni nt ~:1~A 1 4kfA1 a1 J1Y al s..L. -~ I- Aire anl a n- aa .aa -aa SsC.j a








= Ed


where q is the limiting factor.


[2.10]


The maximum permissible energy density in a capacitor can


be calculated from the above two relations:


CU keE,2f
2V 2 q2


[2.11]


The term KE~ is a figure of merit for a dielectric that is to be considered for use in high


field strengths.


However, from a practical standpoint, a useful figure of merit is the


Cu
product of volumetric efficiency and the maximum recommended working voltage ( V ),

which represents the charge storage efficiency of the capacitor, but often quoted as the

volmetric efficiency.


2.6.1.2.


Equivalent Series and Parallel Resistance.


When an ac voltage is applied


to an ideal capacitor, no energy is dissipated, since there is no current flowing through the


capacitor.


However, a real capacitor dissipates energy due to the lead and electrode


resistance, do leakage current, and more importantly, the dielectric losses.


As explained


earlier, the dielectric losses contribute to the dissipation factor, or loss tangent, tan 8.


the purpose of characterizing the losses in capacitors, it is convenient to consider a real
capacitor as an ideal capacitor shunted with a parallel resistance R, or and series with a

resistance R,. All dielectric materials used in manufacture of capacitors will allow certain


amounts of


ct flow.


This current, known as do leakage current, resulting from an


applied voltage, can be expressed in terms of resistance in capacitor at a specified


measurement voltage and temperature. Also termed 'insulation resistance'


it is measured


across the terminals of a capacitor and consists primarily of the parallel resistance Rp.


nAA n fh ann .nW i n a *l. a nan a at A al -- na aL a


SU,









range.


In the case of electrolytic capacitors, where the R, is low owing to the very high


capacitance and thin dielectric, dc


leakage current is normally specified rather than


resistance.


2.6.1.3.


The ability of a capacitor dielectric to withstand an


applied dc voltage without breakdown is called dielectric strength and is normally specified


as volts per meter at a specified temperature.


Breakdown in a capacitor results in the


replacement of a reactive insulating component by either a low-resistance short circuit or an

open circuit, usually with disastrous consequences as far as the overall circuit function is


concerned.


Certain types of capacitors, e.g. metallized polyester film and electrolytic


capacitors have self-healing properties, as described later, such that a breakdown cases a


brief short circuit followed by a rapid restoration of normal reactive behavior.


So far, no


method of making a ceramic unit self healing has been devised, and therefore recommended

working voltages are usually about a factor of 5 below the minimum breakdown voltage.



2.7. Types of Capacitors

Capacitors are classified into different categories, primarily based on the type of

dielectric material used to construct them. Since the polarization characteristics of dielectric

materials strongly depend on field strength, frequency, and temperature, an understanding

of the interrelationship between the design and construction of the capacitors with their

nominal characteristics is generally required for proper selection. Depending on the type of

dielectric material used and the design of construction, capacitors offer a wide range of


volumetric efficiencies, as listed in Table 2-4.


Figure 2-17 shows the frequency ranges for


the application of various capacitor types, and their ranges of capacitance.


Capacitors can


k A A4nafom t una fr nn^nr lvnA o. can~nnenn'dn anti n n n fa a tIe 4AC


Dielectric Strength.


Fnnn~cnur AAnrrln.rr~Al CAwr












Table 2-4.


Typical values of the volumetric efficiency for various types of capacitors [2].


Maximum


Capacitor


Type


C
-(pF mm
V


2-.U (pC.mm4
V


Permissible


) Energy


Density


(nJ mm4)


Electrolytics
Wet aluminum
Wet tantalum
Solid tantalum


15000
35000


4.105

4-106
1.3.106


10000
33000
24000


Polymer


Paper foil/metal foil
Paper foil/metallized
Polystyrene/metal foil

Polyester/metallized


Polymer chip


5000
15000


1000


11000
50000


1300


Single


crystal


2000


Single-plate


ceramic


2500


Barrier layer


Ceramic


multilayer


10000
30000


15000
3.5-105


6000

17000


= capacitance. V


= volume, Uw


= working voltage, pF


= pieco farad, pC


= pieco Coulomb, nJ = nano joule


NPO, X7R, and ZSU are the Electronics Industries Association (EIA) codes for Class II dielectrics possessing

















PaDer and various


meric


High permittivity ceramics


Low-loss ceramic




Ta electrolytic


log (frequency, Hz)


Mica


Multilayer ceramic


Single layer ceramic


r


Solid and foil tantalum


Al electrolytic


Paper and various polymeric


log (Capacitance, p.F)








factors, and operational voltages.


In the following sections, each of these capacitor types


are described in more detail, with regard to their construction, properties and applications.


2.7.1.


Non-Ceramic CaDacitors


2.7.1.1.


Polymer Film Canacitors.


Polymer film capacitors essentially comprise


dielectric films (polymer or paper or both together) interleaved with aluminum electrodes.

The electrodes can be either an aluminum foil or more commonly, in the form of a layer


evaporated directly on the dielectric, and rolled together.


They are sealed in an aluminum


can or in epoxy resin. Because the dielectric films and the electrode layers can be as thin as

0.025 pun, the volumetric efficiencies can be high, despite the low dielectric constants of


this class of materials.


The dielectric films are typically polystyrene, polypropylene,


polyester, polycarbonate or paper.


insulating liquid.


Paper dielectrics are always impregnated with an


Polystyrene capacitors have exceptionally low dissipation factors (less


than 10-3), making them well suited in frequency selective circuits in telecommunications.

Other polymer and paper units are widely used in power-factor correction circuits for

fluorescent lighting units, start/run circuitry for medium type electric motors, and filter

circuits to suppress radio frequencies transmitted along main leads.


2.7.1.2.


Mica Canacitors.


Mica used in the construction of capacitors is obtained


from mineral muscovite (KAl2(Si3Al)O1(OH)2) which can be cleaved into single crystal


plates between 0.25 and 50 mn thick.


Capacitors are constructed from mica plates carrying


fired-on silver electrodes stacked and clamped together to form a set of capacitors


connected in parallel.


The assembly is encapsulated in a thermosetting resin to provide


protection against the moisture. Special features of mica capacitors are long-term stability,

low temperature coefficient of capacitance, and a low dissipation factor.

Owing to their unique design, construction and properties, electrolytic capacitors








2.7.2. Electrolytic Capacitors


Electrolytic capacitors are the first to

capacitance are needed in electrical circuits.


be considered when large blocks of

No other capacitors offer such large


capacitance per unit volume at a low cost. Electrolytic capacitors are deservedly popular in

by-pass, blocking, and power supply filter applications and for motor starting purposes.

The dielectric material in an electrolytic capacitor consists of an anodically formed oxide of


the anode material, which serves as the positive electrode of the capacitor.


The metals most


employed are aluminum and tantalum, and the anodic oxides are y-A1203 and Ta205


respectively [23].


The effective dielectric constant of pure y-A1203 is 8.4 and that of


tantalum oxide is 28. In certain commercial grade capacitors, the dielectric constant can be

slightly lower for aluminum oxide and appreciably lower for tantalum oxide due to the

impurities.


2.7.2.1.


Aluminum


Foil Electrolytic Capacitors.


Electrolytic capacitors are


produced in two basic styles.


The sintered anode or pellet style with either wet or dry


electrolyte is used only for tantalum electrolytic capacitors [24-26].


The foil style, which is


primarily used for aluminum electrolytics, employs a wet, dry, or paste electrolyte. The

aluminum electrolytic capacitor comprises two high-purity (99.99% Al) aluminum foil

electrodes, approximately 50 pmn thick, interleaved with porous paper and wound into the


form of a cylinder [2].


One electrode


- the anode


- carries an anodically formed alumina


layer approximately 0.1 pm thick.


The completed winding is placed into an aluminum can


where the porous paper is vacuum impregnated with an electrolyte (adipic acid or


ammonium penta borate).


The formed aluminum foil is the anode, the A1203 layer is the


dielectric and the electrolyte together with the unformed Al foil is the cathode.


After the


capacitors have been sealed in the can, they are reformed by subjecting them to a do







59
Aluminum electrolytic capacitors are exploited in a range of applications and their relatively

low cost makes them attractive for printed circuits for car radios, stereo equipment, pocket

calculators, digital clocks etc. Also the very high value capacitors are used in large photo-
flash equipment and for voltage smoothing.


2.7.2.2.


Tantalum Electrolytic Canacitors.


electrolytic capacitors: 'wet' and 'solid'. Both var

sintering tantalum powder at 18000C in vacuum.


There are two types of tantalum


ieties consist of a porous anode made by

In the wet type, the porous structure is


impregnated with sulfuric acid, anodized to form a thin layer of Ta205 and encapsulated in
a tantalum container that also serves as the cathode. The use of sulfuric acid gives a lower
resistance and increases the temperature range of operation. The sintered anode tantalum

electrolytic capacitor is fundamentally different from any other electrolytic in that it contains

no liquids, but it consists solely of stable, inorganic, non-volatile materials. This results in

important advantages, including small volume, absence of the necessity for an hermetic

seal, flexibility as to shape, superior temperature characteristics, relatively low dielectric


loss factor (0.1-0.5), and indefinitely long shelf-life.


However, the main disadvantage of


this kind of electrolytic capacitors is their low maximum recommended dc operating


potential (5-100 V) and poor high freuency performance.


This limitation, however, has not


proved serious in most applications, especially in transistor circuits where the principal

consumption of electrolytic capacitors is [24].

A schematic cross section of a sintered anode solid Ta electrolytic capacitor is
shown in Figure 2-18. Fabrication of these capacitors involves attaching a dense tantalum
wire to the porous tantalum body (about 60 percent density) either by embedding it in the


porous block during pressing or by subsequent welding.


A layer of dielectric tantalum


oxide is formed on the tantalum surfaces electrochemically by making the tantalum the

















<-- Anode Leac

















--- Cathode Le








-Cathode Lea


Tantalum
Ta20s
MnO2


Carbon

Metal Outer


Layer & Cathode


Figure 2-18.


Schematic cross section of a solid tantalum porous sintered


anode electrolytic capacitor [24].











used for this purpose and is applied in the following way.


The pellet is saturated with an


aqueous solution of manganous nitrate of 50 percent or greater concentration [24]. It is

then placed in a furnace until the water evaporates and the decomposition of manganous


nitrate is complete as evidenced by cessation of evolution of nitrogen oxides.


of Mn02 are applied this way.


graphite dispersion.


Several coats


The unit is then coated with a layer of carbon from a


This coating makes intimate electrical contact with the MnO2 and


guards against excessive thermal and mechanical shock of the underlying layers in

subsequent operations. At this point, the unit is ready for application of a metallic cathode

encasement Because solid tantalum capacitors are very stable with respect to temperature

and time and have high reliability, they are widely used, particularly in large main-frame


computers,


military


systems


telecommunications.


Currently


they


command


approximately 30% of the capacitor market [2].


Dielectric properties of sintered anode


solid Ta electrolytic capacitors depend on the formation voltage and the capacitance range.

Variation of dissipation factor with applied frequency for typical Ta solid electrolytics is


shown in Figure 2-19 (a).


19 (b).


Change in capacitance with applied frequency is seen Figure 2-


The high dissipation factors of Ta electrolytic capacitors make them unsatisfactory


for most high frequency applications such as tuning circuits and also generally limit the use


ac circuits, bypass, coupling, and filter applications.


Typical impedance versus


frequency characteristics for several solid Ta electrolytics of varying capacitance are shown


in Figure 2-20.


High capacitance is generally associated with a low impedance value and


also a flatter frequency dependance compared to the ones with lower capacitance.


High


dissipation factors possessed by the high capacitance electrolytics make the impedance


relatively insensitive to applied frequency.


As the applied frequency increases, the


capacitor eventually passes through a self-resonance and becomes inductive with gradually













28
24

20
16
12
8



0100


FREQUENCY (Hz)


10k


(b) FREQUENCY (Hz)




Figure 2-19. Variation of (a) dissipation factor and (b) capacitance with frequency
for typical solid tantalum porous sintered anode electrolytic capacitors [23].


0.9

0.7

0.5

0.3

0.1
1(


Reference 1.0 at
+250C and 120 Hz


+850C
S+250C
-20oC


-550C





S1 1 1111 I I I111 I I 11 I1111


2














































10k 0OOk IM 10M


100M


tOOk 1M


100 M


10k 100k IM t10M


100 M


1OO k


100M


Frequency,Hz


- ** r -i


Extended range ratings
o39y F, 50 V

S,68xF, 35V

220pF, 20V



330F. 15V
5600F, 10V
-1O00p.F, 6V
I I l ll slii I III l 1 11i le i iiti i 1 I I ilI


1








electrolytic capacitor is particularly interesting.


The positive electrode of a solid Ta


electrolytic capacitor consists only of tantalum metal core, and therefore does not present


any complications.


materials.


However the cathode is a complex structure of heterogeneous


As a result, heavy charge or discharge currents could produce localized hot


spots, rapidly raising small sites to temperatures far above ambient.


Normally amorphous


Ta205 will crystallize at high temperatures, losing much of the dielectric strength.


severely heated site in the dielectric could then fail catastrophically under an electric field


well


below


rated


value,


exposing


bare


metal.


However,


reduction


semiconducting manganese dioxide can provide a counter action (by re-forming Ta205 to

heal the hot spot) to isolate the fault and prevent catastrophic failure.


2.7.3.


Ceramic Canacitors


Ceramic materials used for capacitor applications are broadly divided into three


categories [2].


Class I dielectrics, usually include low and medium permittivity ceramics,


with dissipation factors less than 0.003. Medium permittivity covers a dielectric constant


range of 15-500 with stable temperature coefficients.


Class II dielectrics consist of high-


permittivity ferroelectric materials, with dielectric constant ranging between 2000-20000

and properties that are more dependent on temperature, field strength, and frequency than


Class I dielectrics.


Dissipation factors for Class II ceramics are generally below 0.03 but


may exceed this level in some temperature ranges and in many cases become much higher


when high ac fields are applied.
volumetric efficiency. Class ID


The main advantage of Class II ceramics is their high

: dielectrics contain a conductive phase that relatively


reduces the thickness of the dielectric in capacitors by atleast an order of magnitude.

Properties are generally similar to those of Class II, but their working voltages are only


between 2 and 25V.


Advantage of Class III ceramics is that the simple structures, such as




















Electrodes


Leads


Dielectric Tube


Silver Electrodes


Leads


Ceramic Disc Capacitor


Ceramic
S"Electrodes


Ceramic Tube Capacitor


Ceramic Multilayer Capacitor







66

electrical and electronic devices. dielectric properties of some of the ceramic dielectrics are

presented in Table 2-5.


2.7.3.1


Ceramic Disc and Tube Canacitors.


Discs can be formed by dry pressing


calcined and milled powders, typically containing some organic binder (5-10 vol %).


Alternatively, they can be cut from an extruded ribbon or a cast-tape.


The pieces are fired


in small stacks and when suitably formulated, do not adhere strongly to one another during

sintering. After firing, silver paint is applied to the major surfaces and the discs are briefly


retired in a single layer at 600-8000C.


Tubes are formed by extrusion.


They have the


advantage of being less fragile than flat pieces and are more suitable in some types of circuit


assembly.


After sintering, the tubes are completely coated with silver.


Silver from one


end-face is removed by automatic grinding machines, and a ring of silver from the outer


surface of the other end is cut (Figure 2-21(b)).


The leads are looped around each end and


soldered in place by immersing in molten solder. Disc and tubular shapes are used for all


classes of dielectric since they are lowest in cost.


Using Class I dielectrics they cover 0.1-


1000 pF capacitance range,


with Class II they cover


-100 nF


and with Class III


dielectrics they cover 0.1-2.0 pF


Except for Class HI, the safe working voltages are


usually at least 100V although in electronic circuits they are not likely to encounter voltages


more than 10V


Thicknesses of dielectric layers vary in the range 50 pm to 2 mm, thicker


units designed to withstand higher voltages, such as mains supply.


Multilaver Ceramic Caoacitors.


2.7.3.2.


Multilayer ceramic capacitors usually


made from Class II ferroelectric materials (mostly BaTiO3) provide large volumetric


efficiencies.


The multilayer structure enables the maximum capacitance available from a


thin dielectric to be packed into the minimum space in mechanically robust form.


interelectrode distance is typically about 20 pm, and the overall dimensions range from lpF
















Table 2


Dielectric properties of ceramic dielectric materials.


Dielectric Material Dielectric Constant tan 8 (104) Ref.
(at 250C) (at 1 MHz)


Class

A1203


MgO
MgTiO3

BaT409

Ti02

CaTrO3

SrTi03


Class
BaTiO3

PbTiO3


dlelectrics


.5-2.0

1-2


1-3

2-4

10-20

10-20


II dielectrics


70-100

220-250


30-50


Class


III dielectrics


SrTiO3-based

BaTiO3-based


10000-20000

20000-50000


2500-3200

3000-4000






















'Wet'


Prepare slip


Screen print alternate
layers of dielectric ink
and electrode ink


I

Cak tnpe


Screen print electrodes


Stack printed electrodes
* I -


Bum out binder


Sinter


Terminate


Attach leads


Encapsulate


Starting Materials:
derived from either mixed oxide
or wet chemical routes


Mix starting powders,
solvent, binders,
and additives








two kinds of fabrication schemes.


In the dry process, the dielectric powder is formed into


a slip with organic solvents and a polymeric binder, and is tape cast to form a continuous


strip.


The cast tape is cut into sheets, typically 15 cm square, on to which the electrode


pattern is printed.


about 700C.


The electroded sheets are stacked, and consolidated under pressure at


The consolidated stack is diced along lines in such a fashion that the electrodes


of successive layers are exposed at the opposite end-faces of the stack.


The polymer


binder, which may comprise upto 35 v/o of the green body, is next removed by heating in
air without any disruption of the multilayer structure. Following removal of the polymer,

the chips are fired to the full sintering temperature (1200-1300C) during which process the


electrodes must remain solid and in place. T1

contains Ag-Pd alloy as submicron particles.


he ink used for screen printing the electrodes

Palladium (m.p. 15500C), and silver (m.p.


960C) form solid solutions with melting points in proportion to the content of the end


members.


The sintered chips are then 'terminated', which involves coating the end faces


with a paint typically made of Ag-Pd powder, glass frit, and an organic carrier.


terminations are fired at about 8000C.


The terminations make contact with the alternating


electrodes that are exposed on that end face, connecting up the stack of plate capacitors in


parallel.


In the 'wet' process, a slip carrying the ceramic powder is laid down, usually by


screen printing, on to a suitable temporary carrier, such as a glass substrate.


The process


can be repeated to build up the required thickness of the dielectric, on to which the


electrodes are screen printed.


repeated.


The next dielectric layer is then laid down and the process is


The multilayer structure is then diced for subsequent stages of fabrication as


described for 'dry' process.


2.7.3.3. Barrier Layer Capacitors.


Most titanate based Class II dielectrics,


whether as a single phase or in combination with other oxides, become conductive on firing








capacitors are based on the limited reoxidation of a reduced composition.


This results, in


the simplest case, a surface layer of high resistivity and a central portion of conductive

material (Class Ill dielectrics), so that the effective dielectric thickness is twice the thickness
dc of a single reoxidized layer and there is an apparent gain in permittivity over that of a


fully oxidized unit by a factor of ddb


where


d is the overall dielectric thickness.


Alternatively, each conductive grain may be surrounded by an insulating barrier layer so
that the dielectric property is dispersed throughout the ceramic.
Barrier layer capacitors depending on a reoxidized surface layer are generally made

by firing BaTIO3 or SiTiO3 discs, approximately 0.5 mm thick, under reducing conditions.

A silver electrode paint is applied to the surfaces of the disc and fired on at about 8000C.

the silver paint typically contains a PbO-Bi203-B203 glass frit to which is added a small


amount (-1%) of acceptor ions, e.g. Cu.


This leads to the formation of a thin (-10 pmn)


insulating layer separating the electrodes from the semiconducting titanate and to an


associated very high capacitance.


Because the major part of the applied voltage is dropped


across the two thin dielectric layers, the working voltage is low, typically about 10 V.
Barrier layer capacitors based on SrTiO3 are more stable with respect to field and


temperature than those based on BaTiO3.


Their capacitance is only reduced by 5% at the


maximum dc field and their variation with temperature can be kept within 20% over a -20


to +85C range.


Their effective dielectric constant is 10000-20000.


BaTiO3 units have


effective permittivities of upto 50000.


Barrier layer capacitors are less expensive to


manufacture than multilayer units and compete in the low-voltage applications.

Dielectric characteristics of ceramic capacitors significantly depend on the type

(Class) of ceramic used as the dielectric. Figure 2-23 (a) shows the variation of capacitance
with frequency for ceramic capacitors made from various types of ceramics and the





























10 k 100 k 1 M 10 M 100 M


FREQUENCY (Hz)


100 k


10M


100M


FREQUENCY (Hz)


Fionire 7-73


Varintinn nf (2~ cannitancPe ndrl (h diccinntinn fartnr with frpnnpncpv


Class III, 12V


__/ ___C lass II, k 1100
/ Class III 25 (with Bismuth)


Class II, 18V

MI Class II, k 6000








2.8. Feasibilitv of BaTiO Electrolvtic Canacitors


Tantalum solid electrolytic capacitors, as described earlier in this chapter, offer a

variety of advantages such as large volumetric efficiency, high operating voltages, long


shelf life, temperature stability,


and high reliability.


However, the reliability and


volumetric efficiency of solid Ta anode electrolytic capacitors are best for those designed


for use at high operating voltages.


For low voltage, high capacitance applications, a thin


dielectric film is desired to obtain the maximum volumetric efficiency.


tailor the dielectric thickness in solid Ta electrolytic capacitors, since the thickness of

dielectric film (anodically formed Ta205) depends on the forming voltage. However, thin


Ta205 films formed at low voltages (below


10V) tend to be defective, possess high


It is possible to


dissipation factors, crystallize at the hot spots developed during charging and fail


prematurely during service [23-26].


The typical maximum safe operating (rated) voltage


for a Ta solid electrolytic capacitor is about a third of its forming voltage.


However, the


anodes for low voltage applications are formed at considerably higher (5-10 times the rated

value) voltages, to avoid the operational and reliability limitations imposed by the thinner
films. As a result, the volumetric efficiencies for low rated voltage Ta solid electrolytics are


significantly lower than their high voltage counterparts.


The low dielectric constant of


Ta205, further sacrifices the volumetric efficiency, particularly at low operating voltages.
Therefore, use of an alternative dielectric system that possesses a higher dielectric constant

and can perform satisfactorily at small operating voltages employing thin dielectric layers is


highly desirable.


successful


Attempts to utilize other anodic oxides such as Ti02 on Ti, were not


, for lack of acceptable structural and electrical properties in such films [61].


However, the advent of electrochemical methods to prepare BaTiO3 thin films on Ti
substrates provides an onDortunitv to conceive and develop BaTiOt electrolvtic camcitors








dielectric constant of BaTiO3 (-


1400 for polycrystalline ceramic, and -300 for thin films) is


advantageous for achieving high volumetric efficiencies.


Summary


Barium titanate exhibits spontaneous polarization below the Curie temperature (Tc


-125C), in its ferroelectric, tetragonal state.


Polarization is induced by the asymmetric


alignment of Ti4+ ion in the Ti06 octahedral coordination of the perovskite unit cell of


BaTiO3, resulting in a large dipole moment.
constant (k 3000 for a single crystal and


* As a result, it possesses a large dielectric
- 1400 for a polycrystalline ceramic), and


exhibits non-linear ferroelectric, piezoelectric, and pyroelectric properties.


Most popular


among the family of ferroelectric ceramics, BaTiO3 finds a variety of applications in


electrical, electronic, and sensing devices.


Used in various forms ranging from single


crystal to thin films, BaTiO3 is used to fabricate multilayer capacitors, transducers,
pyroelectric sensors, and microelectronic devices.
Several methods are available to synthesize BaTi03 thin films. Most conventional

methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD)

involve high temperature (above 5000C) processing (a heated substrate during deposition,


or a post-deposition annealing) to obtain crystalline films.


While providing advantages


such as uniform, dense microstructure, high deposition rates and chemical purity, vapor
deposition methods are capital intensive and not suitable for device integration in Si-based


microelectronics.


Sol-gel methods promise a low-cost alternative and flexibility in


substrate size and nature, combined with high deposition rates. However, these techniques

also require a high temperature processing step to either crystallize or pyrolize the precursor

film. Recently developed electrochemical methods to prepare perovskite type titanate film

on Ti substrates offer an unique opportunity to directly prepare crystalline BaTiO3 thin







74
the process makes it feasible to lower the synthesis temperature by understanding the phase
stability of BaTiO3 and mechanisms of electrochemical processes.

Electrochemical method to prepare BaTi0O3 films on Ti substrates also offers an


opportunity to develop BaTiO3 electrolytic capacitors.


Solid tantalum sintered anode


electrolytic capacitors, possess a large capacitance per unit volume volumetricc efficiency),

owing to the porous nature of the Ta anode, which provides a large surface area of the


dielectric, Ta205, electrochemically formed on the surface of tantalum.


has a very small dielectric constant (k


efficiency that can be achieved in solid Ta electrolytics.


Ta205, however


- 20) and thus limits the maximum volumetric


The use of an alternative dielectric


system, such as BaTiO3fli has potential to improve the volumetric efficiency of electrolytic


capacitors, due to the large dielectric constant of BaTiO3.


process to synthesize BaTiO3 films on


Based on the electrochemical


Ti substrates, electrolytic capacitors can be


fabricated by anodizing porous, lightly sintered Ti bodies, in a Ba2+ containing electrolyte,


to form a layer of BaTiO3 on the entire surface of Ti.


The proposed BaTiO3 electrolytics


have potential applications in high capacitance, low voltage circuits.













CHAPTER 3
LOW TEMPERATURE ELECTROCHEMICAL SYNTHESIS OF BaTiO3 THIN FILMS
ON Ti SUBSTRATES UNDER GALVANOSTATIC CONDITIONS


Introduction


Based on the recent investigations reported on the electrochemical synthesis of

perovskite titanate thin films such as CaTiO3, SrTiO3 and BaTiO3 on titanium substrates

[12-16], the present work primarily focuses on lowering the synthesis temperature of this


novel, inexpensive route to fabricate thin films of complex oxides.


To achieve this


objective, with specific emphasis on synthesis of BaTlO3 on Ti substrates, the investigation

reported in this chapter involves understanding the basic concepts of electrochemical

corrosion and anodic oxidation of metals in aqueous solutions [8-11] and the theoretical

phase stability of BaTiO3 at low temperatures [27, 28].

Preliminary experiments in the current work confirm the electrochemical synthesis


of BaTiO3.


More significantly, BaTiO3 thin films have been synthesized in aqueous


solutions at temperatures as low as 550C, thus eliminating the use of pressurized vessels.

Attempts to form BaTiO3 near 250C have produced isolated deposits of an unknown


material.


In this chapter, the conditions and procedures required to achieve approximately


1pm thick films at 550C are presented followed by an experimentally verifiable hypothesis

for the formation mechanism.


3.2. Background and Approach








boiling of the solution.


temperatures.


Continuous processing is more efficient with open vessels at lower


Thus, lower synthesis temperatures make the overall process more feasible.


Theoretical phase stability for BaTiO3 reported in the literature provides the

fundamental understanding of the solution chemistry of the Ba-Ti-C02-H20 system to

develop procedures to produce BaCO3-free BaTiO3 thin films at low temperatures [27, 28].

As shown in the phase stability diagram in Figure 3-1 for this system, BaTiO3 is predicted

to be the stable phase only under highly alkaline conditions (pH above 13) even with a


small amount of CO2 present.


With increased CQ2 concentration, BaCO3 becomes stable


over the entire range of practical pH values.


Therefore care must be exercised to exclude


CQ2 if the low temperature synthesis of BaTiO3 is to be achieved as predicted.

Experimental work on hydrothermal synthesis and dissolution studies in solution

verify that the phase stability theoretically predicted for BaTiO3 at 250C is valid [63-75].
The phase stability of a material in a particular solution is best determined by both

precipitation and dissolution of the relevant material because reaction kinetics may be slow


particularly for the former phenomenon.


Several studies have demonstrated that the


dissolution behavior of BaTi03 at 250C is consistent with the phase stability diagram in


Figure 3-1.


BaTiO3 measurably decomposes in aqueous solutions below pH 11 to either


BaCO3 or Ba2+ and a TiO2-rich layer depending on the CO2 concentration in solution [27,


28, 63-65].


In contrast, BaTiO3 particles have been synthesized in aqueous solutions at


temperatures as low as 600C [68, 73].


The reaction rates, however, were sluggish for the


hydrothermal synthesis of BaTiO3 at low temperatures and only nanometer size particles

are produced. In general, it has been demonstrated that BaTiO3 can be synthesized over a

large range of temperatures with reaction rate increasing with increasing temperature [64-

75].
























BaCOs3() BaC03(s) + BaTiO3(s)








BaTi03(s)


Ba2+







BaOH+

I I I I


Figure 3-1


Calculated phase stability diagram for Ba-Ti system at 250C in a


rnflo ,, ora irrr-ntnoinn ")NA ,.- nnnnht.-arr- onr FIQi







78
studies support the theoretical prediction that BaTiO3 is the stable phase in alkaline aqueous


solution at 25C [27].


The sluggish reaction rates and the inability to form particles from


aqueous solution at temperatures below 600C supports the contention that the required


levels of supersaturation of


cationic species, particularly


Ti4+


are not achieved in


conventional precipitation.


Thus, a key feature of the electrochemical process is that a


heterogeneous surface with a high local chemical potential of the cationic species is


provided for the formation of BaTiO3.


This observation may have implications in the


formation of electrothermal films in other systems as well


3.3. Materials and Methods


A schematic diagram of the experimental setup is shown in Figure 3-2.


Preliminary


experiments were performed in a borosilicate vessel, but the highly alkaline solution pH


values required for BaTiO3 formation lead to etching of the glass.


teflon vessel was used in all reported experiments.


Therefore, a one liter


All surfaces in contact with the


electrolyte solution were either teflon or platinum except the anode.


The cathode and anode


were a platinum plate and a titanium corrosion coupon, respectively.


The titanium


corrosion coupons* (1.5 cm x 1.5 cm x 0.1 cm) were mechanically polished with alumina


to 0.05 upm and degreased in ethanol# prior to anodic deposition.


The electrodes were


suspended from platinum wires and placed in a teflon beaker containing the electrolyte. All

experiments were performed under galvanostatic conditions (i.e., constant current) with the


current density varied from 0 to 2.5 mA-cm-2 with a commercial power sources


. Changes


in potential as a function of reaction time were collected on a strip chart recorder for


selected experiments.


Since all experiments were conducted


under galvanostatic


3.3.1. Electrochemical Atpparatus












































1. Heated Sand Bath
2. Teflon Beaker
3. Gas Inlet
4. Pt Cathode


6. Ti Anode
7. Thermocouple
8. Teflon Lid
9. Electrolyte Bath


5. Condenser





Figure 3-2. Schematic diagram of the electrochemical apparatus for the low
temperature synthesis of BaTiO3 thin films in open vessels.







80
atmosphere in the vessel was controlled by purging the solution with suitable gas (02 or


N2) using a gas dispersion tube made of teflon.


The flow rate for the gas through the


solution was maintained between 1 and 10 cm3-min1 for all experiments.

3.3.2. Electrochemical Treatment


Details for each of the experiments are outlined in Table 3-1.


Preliminary


experiments indicated that the formation of BaTiO3 required solution pH values greater than


pH 12.5.


Ba2+ in 0.5 M concentration as either Ba(OH)2-8H20 or Ba(CH3COO)2, the


acetate salt, were used to assess the effect of the source of the electrolyte on the formation


of BaTiO3 films.


NaOH as a 2 M solution was used to achieve the desired pH.


Atmospheres of ambient air, N2, and 02 were also evaluated.


Electrolyte solutions were


prepared by preheating the sodium hydroxide to remove dissolved C02 and then adding the


Ba2+ salt.


Prior to anodic deposition, electrolyte solutions were purged for 24 hours with


the desired atmosphere.

After each experiment, the coupons were washed in water adjusted to pH 9-10 with


NH40OH, rinsed in isopropanol or ethanol, and air dried prior to characterization.


were stored in covered containers in a desiccator prior to characterization.


to expose the samples to the ambient environment for as


Samples


Care was taken


short a time as possible during


handling to minimize BaCO3 formation.


3.3.3.


Characterization


X-ray diffraction (XRD)S


, scanning electron microscopy (SEM)5


transmission


electron microscopy (TEM)t


and Auger electron spectroscopy (AES)% were used to


characterize the structural, compositional and topographical features of the BaTiO3 thin

films. XRD was performed on the washed and dried samples using Cu-Ka radiation and a


scanning rate of 2.4 degrees per minute from


10 to


70 degrees two-theta.


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photomicrographs were obtained on uncoated samples.


During SEM analysis, energy


dispersive analysis for X-rays was performed with an ultra-thin window for light elements


(UTW)o to verify the presence of Ba, Ti, and 0 in each of the samples.


Samples for TEM


analysis were prepared by mechanically polishing the treated coupons on one side to about

75 micron thickness and subsequently cut into 3 mm diameter discs using an ultrasonic disc


cutter.


The disc samples were then dimpled on the polished side to obtain a central


thickness of about 15 microns, and finally, back-thinned employing Ar-ion milling at room
temperature to remove Ti substrate at the center of the sample. The milled samples were

coated with carbon to improve conductivity. Bright field imaging and selected area

diffraction (SAD) studies were performed on the central thin regions of the samples using a
200 kV accelerating voltage (electron beam wave length =0.0251A) and 820 mm camera


length (L).


The SAD patterns were indexed by measuring the distance (R) between


individual spots and rings using a magnifier lens with an accuracy of 0.1 mm.


The d-


spacings for various sets of crystallographic planes were calculated using the formula RdM/A
=Li.


AES was performed using a 10 kV electron beam to determine the near surface


composition of the films.


Argon ions accelerated at 3 kV on a 3 mm by 3 mm area were


used to remove layers of material to obtain a compositional profile of the film as a function


of depth into the film.


The sputter rate was estimated to be 10 nm per minute.


The films


were incrementally sputtered at 30 second intervals (-5 nm depths) and AES performed

until the titanium substrate was reached to obtain composition as a function of depth


through the film.


All films in which BaTiO3 is present also have an underlying layer of


TiO2 or TiO indicated by XRD and AES analyses.


Therefore, BaTiO3 film thickness was


defined as the midpoint between the sputter times at which Ba and Ti crossover and Ti and