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


Material Information

Analysis and performance of electrochemically synthesized barium titanate films and electrolytic capacitors
Physical Description:
xviii, 229 leaves : ill. ; 29 cm.
Venigalla, Sridhar, 1964-
Publication Date:


Subjects / Keywords:
Materials Science and Engineering thesis, Ph. D
Dissertations, Academic -- Materials Science and Engineering -- UF
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non-fiction   ( marcgt )


Thesis (Ph. D.)--University of Florida, 1995.
Includes bibliographical references (leaves 220-228).
Statement of Responsibility:
by Sridhar Venigalla.
General Note:
General Note:

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University of Florida
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All applicable rights reserved by the source institution and holding location.
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aleph - 002070306
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Dedicated to:

my wife
Pramila Rani

and son

in appreciation of their patience, support and encouragement


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.


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


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

Lambers, Richard Crockett, and Wayne


the staff

of the



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.











. 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 . Volumetric Efficiency . . Equivalent Series and Parallel Resistance Dielectric Strength . .

. 6

. .
. .
. .

* .
* .

. S .
. .
. .
. .

. .



Ceramic Capacitors
.3.1. Ceramic Disc and Tube Capacitors

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

. . 4
. *




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


S. 75

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


* . 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 .


. 103


S. . 103

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


.2. Characterization

* p .5
* .

Phase, Chemistry, and Microstructure Dielectric Properties .
Results and Discussion .
4.3.1. Effect of Processing Parameti Electrolyte pH . Synthesis Temperature Applied Voltage . Atmosphere . Reaction Time

ers on Synthesis of BaTiO3 Films

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


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

. . .
. .

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

4.4 Conclusions


Materials and Methods.
5.2.1. Preparation of Sintered Porous
5.2.2. Electrochemical Deposition of Vacuum Impregnation of 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 .
. . .


i Anodes from Mixtures of Ti and Polystyrene
formationn of BaTiO3 . . Effect of Electrolyte Salt and Synthesis Temperature Effect Applied Cell Voltage . . Effect of Treatment Time . . .
5.3.4. Capacitor Fabrication and Dielectric Properties .


First Generation Capacitors . .
New Generation BaTiO3 Electrolytic Capacitors

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

. .
. . .
. . .



Introduction .
Thermodynamic Data


. .

Sources of Thermodynamic Data .


. .


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


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

ows 2.0

. .
. .


Discussion on Electrochemical Equilibria


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

Experimental Verification


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

* . .



Ba-H20 SYSTEM AT 25, 55, AND 100C AND

Ti-H20 SYSTEM AT 25, 55, AND 100C AND

Ba-Ti-H20 SYSTEM AT 25, 55, AND 1000C AND

Ba-Ti-C-H20 SYSTEM AT 25, 55, AND 1000C AND


nmn Tflfl n ny nflAI nrrwnu'




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 . .





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







. 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.



Schematic illustration of a combined AES and XPS
instrumentation . .









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






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.




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) . .








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



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 .




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-



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 .


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 . . .











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 .



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 . .. .









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







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 .




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



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


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.


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,


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


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.


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




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






as sol-gel,


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,


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].


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.


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


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


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

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.


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


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


on Ti



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,









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.



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


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


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


2.2. Dielectric Polarization and Ferroelectricitv


Theory of Charae Storaee and Canacitance

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



-- -T- C -


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

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):





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:



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


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


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



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


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



0 -

Dipole Polarization
(low freq.) (high frequency)

Pl2 Or Pd1

Atomic (or ionic)


T= 300K

- -







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

atom M

atom N






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





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.



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


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.


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


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


displacements and

the dipole moments associated


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


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.


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





/ v





parallel with

12 equivalent












*r *

E 404





-50 0 50 100



10 r


-120 -80 -40 0 40 80


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




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


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



Figure 2-7.

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

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

polarization direction.


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




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+


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



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


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.,


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

Thin Film Material Phenomena Applications




Capacitors, sensors, phase shifters


PbTiO3 (PT)



Acoustic transducer

Pb(Zr,Ti)03 (PZT)

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




Nonvolatile memory
Waveguide devices
SAW substrates

Wave guide devices, optical memory, displays, SHG


Pb(Mgl3, Nb23)03 (PMN)

LiNb03 (LN)
LiTaO3 (LT)


K(TaNb)03 (KTN)

(Sr,Ba)Nb206 (SBN)







Capacitors, memory
Waveguide devices

Waveguide devices, optical modulators, SAW, SHG

Wave guide devices, frequency doubler, holographic storage

Waveguide devices






b so
*4 Al
-* wP




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

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.


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


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,


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









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


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


surface quality,

deposition rate,



it has


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


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


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].


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

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


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



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],



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.


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


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


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


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


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.


Structural Charac





as grain



uniformity, surface

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

Ba(OH)2 Solution






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

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.


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


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

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

Inelastic interactions



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


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


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


Cu ka 20-Y 28


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

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.


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.









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]:




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

energy and consequently denoted by

denoted by LMM and MNN


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


= Eg


.Valance Band:


3s etc.



Initial State

Electron Ejected


(d) EKLL



X-Ray Emission

Figure 2-12.

Auger Electron

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-



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


.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


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


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



where E,, hv

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

Z "-
Q0 '2

*0 0
- >-


O^ -

a 2

QT 3







0 0




>- 0




0 -
' o

6 12 18 24



Typical AES compositional depth profile analysis of a BaTiO3

thin film on

Substrate [42].

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




on the

nature of the elements



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.




a stronger




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


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


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


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+


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.


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
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


V1 R2





(0.1%, polystyrene)

V to
out to



Virtual Ground

Cl Cparasitic


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,


stands for

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

the existence of ferroelectricity in materials.


-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


-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.


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


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













0 2000 4000 6000

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


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


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.


'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

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.


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].


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 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,


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, 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



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,


the volume efficiency can be written


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.


The maximum permissible energy density in a capacitor can

be calculated from the above two relations:

CU keE,2f
2V 2 q2


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

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.

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


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



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


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


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].




-(pF mm

2-.U (pC.mm4


) Energy


(nJ mm4)

Wet aluminum
Wet tantalum
Solid tantalum






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


Polymer chip











Barrier layer







= 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


High permittivity ceramics

Low-loss ceramic

Ta electrolytic

log (frequency, Hz)


Multilayer ceramic

Single layer ceramic


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.


Non-Ceramic CaDacitors

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.

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



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

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.

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



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








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.


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







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






Reference 1.0 at
+250C and 120 Hz



S1 1 1111 I I I111 I I 11 I1111


10k 0OOk IM 10M


tOOk 1M

100 M

10k 100k IM t10M

100 M

1OO k



- ** 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


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.


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










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.


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



Dielectric Tube

Silver Electrodes


Ceramic Disc Capacitor


Ceramic Tube Capacitor

Ceramic Multilayer Capacitor


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

presented in Table 2-5.

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


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.

Multilayer ceramic capacitors usually

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


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)

















II dielectrics





III dielectrics








Prepare slip

Screen print alternate
layers of dielectric ink
and electrode ink


Cak tnpe

Screen print electrodes

Stack printed electrodes
* I -

Bum out binder



Attach leads


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


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


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


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.


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. 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


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


100 k




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.


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.


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


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

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.



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


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.


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-


BaCOs3() BaC03(s) + BaTiO3(s)





Figure 3-1

Calculated phase stability diagram for Ba-Ti system at 250C in a

rnflo ,, ora irrr-ntnoinn ")NA ,.- nnnnht.-arr- onr FIQi

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


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.


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.

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.


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


Care was taken

short a time as possible during

handling to minimize BaCO3 formation.



X-ray diffraction (XRD)S

, scanning electron microscopy (SEM)5


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.

















a 5


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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


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

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