a thin film piezoelectric transformer for silicon integration
TRANSCRIPT
A Thin Film Piezoelectric Transformer
for Silicon Integration
by
Timothy Russell Olding
A thesis submitted to the Department of Physics in conformity with the requirements for the degree of Master of Science (Engineering)
Queen's University
Kingston, Ontario. Canada
June, 1999
O Timothy Russell Olding
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Abstract
Many applications in the electronics industry now require small, low profile
components with a high efficiency of operation. Electromagnetic transformers,
which consist of wire turns around a magnetic core, are unsuitable for integration
as mid-scale microelectronic components. A thin film piezoelectric transformer has
promise as a possible alternative. Radial mode thin film piezoelectric transformers
with a diameter of 1-2 mm and piezoelectric layer thickness of 1-2 pm have a
predicted operating range of 0.5-1 -0 MHz with vo&age gains of 0.1 -7 0. depending
on the dimensions and quality of the film. The device has been modelled using
Mason's model for piezoelectric transformers. The piezoelectric layers of the
transformer have been produced using an acetic acid based lead zirconate titanate
(PZT) solgel process. A stable solution chemistry and consistent thermal
processing route have been developed for producing multi-layer fully crystallized
PZT coatings of high electrical quality and thicknesses up to 5 pm, which is suitable
for transformer production. The coatings are piezoelectrically active and have been
characterized. One and two layer thin film transformers have been produced using
a process suitable for the manufacturing environment which employs standard
photo-lithographic and wet chemical etching techniques. A one layer thin film
transformer with a PZT layer thickness of 2 pm and a diameter of 5.1 mm yields a
voltage gain of 2.25 at 400 kHz. the resonant frequency of the device. The voltage
gain can easily be altered by changing the dimensions of the device or the driving
and output electrode patterns of the transformer.
Acknowledgements
I would like to thank my supervisor Dr. Michael Sayer for giving both his
guidance and a great degree of freedom for exploring new ideas. His refreshing
optimism and excitement about the project helped me greatly.
I would also like to thank the people at the Materials & Metallurgical
Department at Queen's University. Gennum Corporation, Datec Corporation, Royal
Military College (RMC), Canadian Microelectronics Corporation (CMC) and
Photonics Research Ontario (PRO) for their technical expertise and use of
equipment: Joyce Cooley, Charlie Cooney, Michael Watt, David Barrow, Ted
Petroff, Stewart Sherrit, Yasser Jamanni and Jeff Kablfleisch. My work was made
significantly easier because of your contributions. I would particularly like to
acknowledge and thank Stewart Sherrit for his assistance in transformer modelling.
Wthout him, I would likely still be working on the model. I would also like to thank
Gennum Corporation and Materials and Manufacturing Ontario for their financial
assistance and BM HiTech for supplying me with PZT disks.
My gratitude goes to the members of the Applied Solid State group for their
friendship, help and being available to listen to my successes and frustrations: Katia
Dyrda, Sarah Langstaff, Brian Leclerc, Marc Lukacs, Guofeng Pang and Lichun
Zou. I would particularly like to thank Marc Lukacs, Brian Leclerc, and Lichun Zou
for the many times they have helped me in my work.
Last, but not least, I would like to thank my wife Joy for loving and
supporting me in all my "mixing, baking and cookie cuttingn, and for making me
leave my work at work. And most importantly, I thank God for giving me
perspective and purpose in my work. His promise is true: "But seek first His
kingdom and His righteousness, and all these things will be given to you as well-"
For "what good will it be for a man who gains the whole worid, yet forfeits his
soul?".
4 . SolCel Processing ........................................... 54 4.1 Sol-Gel Routes ........................................ 55 4.2 Acetic Acid-Based PZT Sol-Gel ........................... 57 4.3 Solution Preparation .................................... 62
......................... 4.3. 1 Metal Alkoxide Precursor 64 ................................. 4.3.2 Lead Precursor 67
............................... 4.3.3 Choice of Solvent 68 4.3.4 Mixing Order ................................... 69
4.4 Thermal Processing .................................... 70 4.4.1 Simultaneous TGAlDTA Analysis ................... 71
............................. 4.4.2 Furnace Processing 73 ........................ 4.4.3 Rapid Thermal Processing 75
........................... 4.5 Glancing Angle X-ray Diffraction 75 4-51 Choice of Alkoxide Precursor ...................... 76 4.5.2 Choice of Solvent ............................... 78
............................ 4.6 Scanning Electron Microscopy 79 4.7 Electrical Characterization ............................... 81 4.8 Piezoelectric Characterization ............................. 83 4.9Summary ............................................. 87
References ......................................... 88
5 . Transformer Production ....................................... 90 5.1 Practical Design ........................................ 91 5.2 Production Process ..................................... 94
........................... 5.2.1 Two Layer Transformer 94 5.2.2 One Layer Alternative Transformer .................. 96
5.3 Transformer Response .................................. 97 5.3.1 Two Layer Thin Film Transformer ................... 98 5.3.2 Two Layer Bulk Ceramic Transformer ............... 100 5.3.3 One Layer Thin Film Transformer .................. 101
5.4 Applications .......................................... 102 References ........................................ 103
6.Conclusions ............................................... 104 ........................................ References 108
Appendix I : Thin Film Transformer Analysis ....................... 109
Appendix 2: JCPDS Files ....................................... 115
Appendix 3: Modified PZT Sol-Gel Recipe ......................... 116
Appendix 4: Bulk Ceramic Transformer Analysis ................... 117
List of Figures
FIGURE 1 .1.1 : AB03 perovskite crystal structure ........................ 4 ... FIGURE 1 -1 -2: PZT (a) tetragonal and (b) rhombohedra1 crystal structures 5
FIGURE 1.1 -3: PZT phase diagram .................................. 6 FIGURE 1 .3.1 : Operating principle of a piezoelectric transformer ........... 7 FIGURE 2.1.1 : Transformer vibration modes .......................... 14 FIGURE 2.2.1 : Equivalent circuit for a radial resonator .................. 22 FIGURE 2.2.2. Effect of the substrate on the radial resonator ............. 23
...... FIGURE 2.2.3. Equivalent circuit for a radial resonator on a substrate 23 FIGURE 2.3.1 : Free standing radial mode transformer .................. 24 FIGURE 2.3.2. Transformer circuit # 1 ............................... 25 FIGURE 2.3.3. Transformer circuit # 2 ............................... 26 FIGURE 2.3.4. Transformer circuit # 3 ............................... 27 FIGURE 2.4.1 : Predicted response of a thin film transformer ............. 29 FIGURE 3.1 -1 : Sol-gel deposition ................................... 32 FlGURE3.2.1. Schematicview ofan RTA ............................ 34 FIGURE 3.3.1 : TGNDTA experimental apparatus ...................... 35 FIGURE 3.4.1 : GA-XRD setup ..................................... 36 FIGURE 3.4.2. Location of soller slits ................................ 38 FIGURE 3.6.1 : DC magnetron sputtering process ...................... 40 FIGURE 3.7.1 : Mask dimensions ................................... 42 FIGURE 3.8.1 : Poling apparatus ................................... 43 FIGURE 3.1 0.1 : Schematic of the impedance measurements ............. 46 FIGURE 3.10.2. Schematic of the impedance measurement apparatus ..... 48 FIGURE 3.1 2.1 : Schematic of a scanning electron microscope ............ 51 FIGURE 4.2.1. Acetate process .................................... 61 FlGURE4.2.2. IMO process ....................................... 61 FIGURE 4.3.1. (a) Modified acetate and (b) Modified IMO processes ....... 63 FIGURE 4.3.1 . 1 : Examples of possible coordinate linkages .............. 66 FIGURE 4.4.1 -1 : TGAIDTA analysis of a butoxide-based IMO PZT sol-gel . . . 71 FIGURE 4.4.1 -2: TGAIDTA results for a P A sol-gel dried at I5O0C ........ 72 FIGURE 4.5.1 -1: Variation in crystallization with choice of alkoxide precursor:
(a) Zr butoxidefri butoxide, (b) Zr butoxidem isopropoxide and (c) Zr butoxidem isopropoxide precursors ........... 77
FIGURE 4.5.2.1 : Effect of solvent on crystallization of a PZT film ........... 78 FIGURE 4.6.1 : (a) Zr propoxidell7 isopropoxide 2.8 pm thick film (b) Zr
butoxidemi isopropoxide 3.5 prn thick film (c) Zr butoxide /Ti butoxide 4.4 pm thick film . . . . . . . . . . . . . . . . . . . . . . . 80
FIGURE 4.7.1 : Frequency response of (a) Zr butoxidemi butoxide ....... based film(b) Zr butoxidemi isopropoxide-based film 82
FIGURE 4.8.1. Impedance response of a PZT film with Cr-Au electrodes .... 84 ....... FIGURE 4.8.2. Impedance response of a PZTfilm with Pt electrodes 85
......... FIGURE 5.1 -1: Reducing substrate clamping via substrate etching 91
........ FIGURE 5.1.2. Flip-chip technique for reducing substrate clamping 92 ..... FIGURE 5.1 -3: (a)-@) Two layer and (d) one layertransformer designs 93
....................... FIGURE 5.2.1 : Transformer production process 94 ......................... FIGURE 5 - 2 2 Modified transformer structure 96
....................... FIGURE 5.2.3. Masks for one layer transformer 97 ................. FIGURE 5.3.1 : Two layer thin film transformer response 98
............ FIGURE 5.3.2. Two layer bulk ceramic transformer response 100 ................ FIGURE 5.3.2. One layer thin film transformer response 101
FIGURE 5.4.1. Schematic of a thin film transformer voltage converter ..... 103
vii
List of Tables
TABLE 3.1 -1 : Deposition parameters .............................. 33 TABLE 3.6.1 : Sputtering conditions ............................... 41 TABLE 3.1 0 .I : HP41 94A specifications [9] .......................... 47 TABLE3.11.1. PZTetch ......................................... 50 TABLE 4.3.1 : Chemicals for acetic acid-based PZT sol-gel ............. 64 TABLE 4.3.1 -1 : Reaction between alkoxide precursors ................. 65 TABLE 4.3.3.1 : Effect of solvent on solution stability ................... 69 TABLE 4.4.3.1 : RTA processing schedule ........................... 75 TABLE 4.8.1 : Material parameters for a PZT film on a Si (I 1 1) substrate . . 87
List of Symbols and Abbreviations
a A B C CRT CSD CVD Cii d D d, DMM DRAM DTA DTG E EEPROM e i j E f FeRAM F r G GA-XRD IMO I kP 4 MOD n N NMR P PCT Qm r R RTA RTP Sij S
disk radius area susceptance capacitance, Curie constant cathode ray tube chemical solution deposition chemical vapour deposition elastic stiffness lattice spacing dielectric displacement piezoelectric charge coefficient digital multimeter dynamic random access memory differential thermal analysis derivative therrnog ravimetric analysis electric field electrically erasable programmable read-only memory piezoeiectric coefficient relative dielectric constant frequency ferroelectric random access memory radial force conductance glancing angle x-ray diffraction inverse mixing order current radial mode electromechanical coupling constant thickness mode electromechanical coupling constant metallorganic deposition diffraction order term transformer turns ratio nuclear magnetic resonance density piezoelectric ceramic transformer mechanical quality factor radius resistance rapid thermal annealer rapid thermal processing elastic compliance strain
s,, SEM
T C
TGA
complex scattering parameter scanning electron microscope thickness stress, temperature Curie temperature therrnog ravimetric analysis radial displacement speed of sound voltage radial velocity admittance impedance angular frequency wavelength AC conductivity Poisson's ratio diffraction angle
1. Introduction
Polycrystalline ceramics have long been recognized as having unique
structural and electronic properties that could be extended to a wide variety of
applications. Structural ceramics are finding use as mechanical components in
engines and other machinery. Electronic ceramics have a wide range of use, from
passive oxides such as silica (SiOJ and alumina (A1,03) which are used as
substrates and insulators in electronic circuits [I], to active oxides such as zirconia
(Zr0.J which is used to manufacture the high temperature oxygen sensors used in
automobile engines [2]. Some of the ceramics with a more complex crystal
structure, such as the perovskite barium titanate (BaTiO,) and lead zirconate
titanate PbZr03-PbTiO, (PZT) compounds which have useful ferroelectric andlor
piezoelectric properties are used in capacitors, mechanical resonators, ultrasonic
transducers, pressure sensors, optical switches and other devices 131.
The development of thin film technology has allowed for the integration of
electronic ceramics with semiconductor technology, raising the possibility of a new
range of electronic devices. There is currently a great deal of interest in employing
1
the ferroelectric properties of some of the electronic ceramics in high density
capacitors for dynamic random access memory (DRAM) [4] and nonvolatile
ferroelectric memory (FeRAM) (51. Integrated couplingldecoupling capacitors and
filter capacitors to replace off-chip ceramic capacitors are also being developed [6]-
The piezoelectric properties of thin film electronic ceramics such as PZT are also
being employed in a variety of applications, including ultrasonic sensors and
focussed high frequency ultrasound sources for medical applications, mechanical
strain and pressure sensors in a range of applications including automobile pre-
ignition and timing sensors and diaphragm actuators for fluid flow valves, and
positioning sensors and actuators m. The major difference in piezoelectric device
development as opposed to ferroelectric memory and integrated capacitor
development is the requirement for thicker films which leads to different deposition
techniques.
A wide variety of thin film deposition techniques exist that can be
summarized in two broad categories [8]. Physical techniques include thermal and
electron beam evaporation; dc, radio frequency, and ion beam sputtering; and laser
ablation. Chemical techniques include spray pyrolysis, chemical vapour deposition
(CVD), and chemical solution deposition (CSD) which includes metal organic
decomposition (MOD) and sol-gel processes. The sol-gel process is particularly
suitable for the production of thick (greater than 1 pm) piezoelectric films in terms
of ease, time of processing, and low capital cost. The sol-gel process is the thin film
deposition technique employed in this work.
I 1 Piezoelectricity and Ferroelectricity
Piezoelectrics and ferroelectrics are closely associated in that they both are
part of the class of polar dielectrics. A polar dielectric is characterized by having
internal dipoles that may be aligned under an applied electric field to produce a net
dipole moment or polarization. The polarization is enhanced beyond that occurring
in any dielectric because of the asymmetry in the crystallographic structure of the
unit cell. In general, the centre of charge of the unit cell does not coincide with its
centre of mass, leading to a localized electric dipole moment within the unit cell.
Polar dielectrics are found in thirty of the crystal classes and within twenty of these
classes is the sub-group of piezoelectrics [9]. Piezoelectrics are distinguished by
the phenomenon of producing a surface charge when subjected to an external force
(termed the piezoelectricdirect effect) and conversely, a mechanical distortion when
subjected to an external electric field (termed the piezoelectric converse effect). Of
the twenty classes in which piezoelectrics are found, ten contain pyroelectrics,
which are characterized as having a spontaneous polarization in the absence of an
external electric field or mechanical stress. Within the pyroelectric subgroup are the
ferroelectrics, in which the direction of spontaneous polarization may be changed
by changing the direction of an externally applied electric field [I 01.
One of the important features of a ferroelectric is its Curie temperature. This
is the temperature above which the spontaneous polarization in the material
disappears. In this state the material is termed to be paraelectric. Since the
permittivity (E) of the material is directly dependent on the polarization, it also varies
with temperature. The relationship between the permittivity and temperature is
described by the Curie-Weiss law [I I]:
where C is the Curie constant. T is the temperature, and T, is the Curie
temperature. At temperatures near to Tc the permittivity of the material will be very
large. depending on the crystallographic direction in which the measurement is
made-
An important material with superior properties for piezoelectric applications
is PZT, a solid solution of PbZr0,-PbTiO, which is both ferroelectric and
piezoelectric. P A , along with many ferroelectric materials, crystallizes in the ABO,
perovskite crystal structure shown in Fig 1 .I .l . The large (A) cations are similar in
size to the oxygen ions and occupy the comers of a cubic unit cell. The small cation
"A" site
"6" site
Owgen
FIGURE 1.1 .I : ABO, perovskite crystal structure
(B) sits in the body centre of the cube and the oxygen ions are situated on the cube
faces. For P A , the large cations are lead (Pb) and the small cations are either
zirconium (Zr) or titanium (Ti). Above its Curie temperature, PZT is in the cubic
perovskite structure and is paraelectric. Below its Curie temperature. PZT becomes
ferroelectric, and depending on its composition, is in either a tetragonal or
rhombohedra1 phase (shown in Figure 1.1 -2). In the tetragonal phase, the cell is
* 0 6 Pbsite
e----e Q
0 zrm site
a Oxygen
FIGURE 1 .I -2: P A (a) tetragonal and (b) rhombohedral crystal structures
stretched along one side and has its direction of polarization parallel to the longest
side. In the rhombohedral phase. the cell is stretched along the body diagonal with
its polarization along the diagonal.
The Ti/Zr ratio in PZT determines whether the material will be in the
tetragonal or rhombohedral phase. The phase diagram for PZT(Figure 1 .I .3) shows
the regions where the various phases exist, with an important feature being the
% PbrlO 3 PbZr0: P b E 0 3
FIGURE 1 -1 -3: PZT phase diagram [I 01
morphotropic phase boundary between the tetragonal and rhombohedra1 phases.
PZT with a composition near this boundary has been shown to have excellent
ferroelectric and piezoelectric properties [lo], suitable for the production of
piezoelectric devices.
'1.2 Piezoelectric Transformers
With the onset of miniaturization, many applications in the electronics
industry now require small, low profile components with a high efficiency of
operation. Electromagnetic transformers, which consist of wire turns around a
magnetic core, are among the most bulky devices on a circuit board and are not
compatible with the requirements for most mid-scale microelectronic applications.
A significant amount of effort has been spent on the difficult task of producing
electromagnetic micro-transformers using thin film technology, but with limited
success [I 2,131
Piezoelectric ceramic transformers (PCTs) made from PZT bulk ceramic
have recently received considerable attention, particularly as voltage sources in
inverters used to drive the backlights of liquid crystal displays [14). Other
applications currently being studied include the use of PCTs as voltage sources for
air cleaners using a light load and voltage sources for computers with a medium or
heavy load 1141. PCTs were originally developed as high voltage generating
transformers for a high impedance load, having the advantage of a high step-up
ratio, relatively small size and low cost PCTs have been reported to have
operational efficiencies in excess of 90% [I 51.
The basic construction of a piezoelectric transformer involves two
mechanically coupled electrically insulated resonators (Fig.1.3.1). When an AC
signal with a frequency near the frequency of mechanical resonance is applied to
the input resonator, strong mechanical vibrations occur due to the piezoelectric
converse effect. This vibration is transferred to the output resonator, inducing a
charge on its electrodes due to the piezoelectric direct effect. Either a step-up or
step-down in voltage can be observed, depending on the dimensions of the
resonators and the electrical and mechanical qualities of the material used. This
is due to the impedance transformation of the load by the mechanical impedance
of the transformer.
0
vo Output Resonator
- Input Resonator
FIGURE 1.3.1: Operating principle of a piezoelectric transformer
Until recently piezoelectric transformers could only produced with quality in
bulk ceramic form, limiting its potential for silicon integration. It is now possible to
manufacture piezoelectric films of sufficient thickness and quality to make thin fiim
piezoelectric transformers via a sol-gel process. Producing piezoelectric
transformers using thin films allows for the possibility of integration with
conventional silicon technology, simple manufacturability of devices and access to
frequency ranges which are inaccessible using bulk ceramic manufacturing
techniques.
To be viable, several features are desirable in a thin film piezoelectric
transformer. It should operate at an efficiency comparable to that of
electromagnetic transformers and have an operational frequency range compatible
with available signal sources. The frequency range of use will likely be greater than
100 kHz and less than 10 MHz. The useful voltage gain range would be from 0.1
to 10 for most applications. The device must be small and of low profile in order to
achieve silicon integration and should be easily manufacturable using thin film
techniques.
1.3 Sol-Gel Science
Sol-gel technology was first developed as a new approach to the preparation
of glasses and ceramics as coatings on different substrates. There are basically
two different kinds of sol-gel technology 1161. The first kind of sol-gel starts with a
colloidal suspension of particles in a liquid forming a sol which is then destabilized
to form a gel. The second kind involves the polymerization of organometallic
precursors such as metal alkoxides to produce a gel network. In this thesis, only
sol-gel processing based on organometallic precursors will be discussed.
The organometallic precursor-based sol-gel process is conceptually quite
simple. A solution is made with appropriate molecular precursors containing the
elements of the desired compound in an organic solvent. The solution is
polymerized to form a gel, then the gel is dried and fired to remove the organic
components and form an inorganic oxide.
The most common precursors for making sol-gel solutions are alkoxides of
composition M(0-R)n, where R is an alkyl radical (methyl, ethyl, etc.). Their
properties are key to the preparation process. Additional oxides can be introduced
to multi-component systems as inorganic or organic salts. For PZT, zirconium and
titanium are usually introduced in the form of alkoxide precursors, whereas lead is
introduced as a salt. One important feature of the zirconium and titanium alkoxides
is that they are highly reactive toward water. This presents a problem with
premature gelation of these components during solution preparation, but can be
overcome by adding a chelating organic ligand to the solution to control the
hydrolysis rates of the alkoxides.
The sol-gel process has several advantages compared to conventional glass
and ceramic processing routes- Homogeneous multi-component systems of correct
composition can easily be obtained by mixing the corresponding molecular
precursor. The temperatures required for materials processing can be significantly
lowered due to the fact that the materials are mixed at the molecular level in solution
and diffusion distances are small. A variety of techniques such as spray, dip, paint
and spin coating exist for film fabrication and the viscosity, surface tension, and
solution concentration can be easily adjusted to meet specific requirements.
1.4 Thesis Objective
The objective of this thesis was to design and produce a thin film PZT
piezoelectric transformer with suitable operating parameters for device applications.
The device was modelled using Mason's model for piezoelectric transformers [I 71-
Different thin film sol-gel processes were evaluated for producing piezoelectric
resonator layers of suitable thickness with appropriate electrical and piezoelectric
properties. A stable solution chemistry and consistent thermal processing route
were developed for a multi-layer process capable of producing greater than 5 pm
thick PZT films. The film properties were characterized using glancing angle x-ray
diffraction (GA-XRD), dielectric analysis and high frequency impedance analysis.
The transformer was produced via the sol-gel process using standard photo-
lithographic and wet chemical etching techniques.
The outline of this thesis is as follows: Chapter 2 contains the model for the
thin film piezoelectric transformer. In Chapter 3, the tools and methods used forthe
production and characterization of the PZT films are outlined. In Chapter 4, the sol-
gel process used for the fabrication of the thick PZT films is described, as well as
the results from the characterization of the PZT films. The subject of Chapter 5 is
the practical implementation of the thin film piezoelectric transformer and potential
applications, Chapter 6 contains a discussion of the results and conclusions from
this work.
References
R. R. Tummala, Ceramic Bulletin, 67, 752 (1988).
G. Fisher, Ceramic Bulletin, 65, 622 (1 986).
M. Sayer and K. Sreenivas, Science. 247, 1056, (1990).
L. ti. Parker and A. F. Tasch, IEEE Circuits and Devices. 17 Jan (1990).
J. F. Scott and C. A. Araujo, Science, 246. 1400 (1989).
V. Chivukula, J. Ilowski, I. Emesh, D. McDonald, P. Leung and M. Sayer,
Integrated Ferroelectrics, 10 (4), 247 (1 995).
M. Sayer. D. A. Barrow, R. Noteboom, E. Griswold and 2. Wu, Science and
Technology of Electroceramic Thin Films, Kluwer Academic Publisher,
Netherlands (1 995).
L. I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology,
McGraw-Hill, New York (1 970)-
E. C. Henry, Electronic Ceramics, Doubleday & Company, Inc., New York
(1 969).
B. Jaffe. W. R. Cooke and H. JaRe, Piezoelectrics, Academic Press, New
York (1 991).
J. C. Burfoot and G. W. Taylor, Polar Dielectrics and fheir Applications,
University of California Press. Los Angeles (1 979).
T. Yachi et al., lEEE Power Electronics Specialists Conference, 20 (1 991).
C. R. Sullivan and S. R. Sanders, IEEE Power Electronics Specialists
Conference, 33 (1 993).
K. Nagata, J. Thongrueng, K. Kato, Japanese Journal of Applied Physics:
Part 1, 36 (9). 61 03 (1 997).
K. Sakurai eta/., Japanese Journal of Applied Physics, 37. Part 1,56,2896,
(1 998).
G. Yi and M. Sayer, Ceramic Bulletin, 70 (7). 1 173 (1 991)
W. P. Mason. Electromechanical Transducers and Wave Filters, D. Van
Nostrand Co., 205, (1948).
2. Transformer Desian
Many piezoelectric transformer designs have been introduced since C. A.
Rosen proposed a piezoelectric transformer operation in length extensional mode
[I-31. The basic principle of operation is to apply an AC voltage to one resonator
at the frequency of mechanical resonance causing it to strongly vibrate. This
resonator is bonded to another resonator, which also begins to vibrate and
produces an output voltage due to the piezoelectric effect. The voltage gain of the
transformer is a function of the input and output impedances of the device and is
limited by how well vibration transfer occurs between the two resonators. These
transformers have been manufactured using bulk piezoelectric ceramic and most
of them operate in the 200-300 kHz range. It has been difficult to achieve a
piezoelectric transformer with a power density as high as that found in conventional
electromagnetic transformers. One solution has been to use a new structure for a
piezoelectric transformer operating in the second thickness extensional vibration
mode [3], and this seen a considerable amount of success. Another way to
approach the design of a piezoelectric transformer is to transfer the device to the
realm of thin film technology. Moving to thin film technology drastically changes the
device dimensions and the consequent frequency range of operation, opening a
new range for the design and use of piezoelectric transformers. The structures that
can be produced using thin film technology are also significantly different. This
chapter focuses on the design of a piezoelectric transformer for production via thin
film technology.
2.1 Mode of Operation
Several vibration modes could be used for piezoelectric transformer
operation. The five most common modes used for piezoelectric ceramic materials
analysis [4] are shown in Figure 2.1.1 along with the recommended geometrical
aspect ratio and the direction of poling marked by an arrow. Some of these can be
dismissed immediately from consideration. Length extension mode transformers
have previously been investigated and found to be unsuitable for power conversion,
as they have a large internal impedance leading to undesirable power loss. The
resonator structure also cannot be produced easily using thin film technology. The
thickness extensional and thickness shear modes have fundamental mechanical
resonance frequencies f,, and f, respectively of:
where t is the resonator thickness, c, is the elastic stiffness of the material under
constant dielectric displacement D (in reduced form) and p is the density of the
piezoelectric material. For thin film thicknesses of 1-1 0 pm, the resonant frequency
of these modes is in the hundreds of megahertz to gigahertz range, which is
unsuitab!e for most applications. Finally, the length thickness mode has the
weakest coupling constant of all five modes for the conversion of mechanical to
electrical energy and vice versa. This leaves the radial mode structure as the most
promising candidate for use in a thin film piezoelectric transformer. Other modes
such as the bending mode of a bar and the torsional mode of a rod are too
complicated to produce using thin film technology.
(a) Radial &tension (D > 20t)
(c) Thickness Shear (w , w > 201)
(e) Length Extension (L > 5w . Sw,)
* )
FIGURE 2-1 .I : Transformer vibration modes
2.2 Radial Mode Resonator
The analysis of piezoelectric resonators can be approached on a continuum
basis for simple resonator structures using the wave equation and the linear
piezoelectric constitutive equations. However, it is often more convenient to use an
equivalent circuit approach where both the electrical and mechanical portions of the
resonator are represented by electrical equivalents. The equivalent circuit approach
has a distinct advantage overthe direct wave equation approach in that the versatile
methods of network theory can be employed. To the extent that the original
assumptions and boundary conditions used to obtain the equivalent circuit are valid.
the equivalent circuit can be considered an exact representation of the piezoelectric
resonator.
Much of the original work on equivalent circuits for piezoelectric resonators
was completed by Mason [5]. His work on equivalent electromechanical circuits for
piezoelectric resonators was then extended to piezoelectric transformers [I ,2.6] by
drawing an analogy between piezoelectric and conventional electromagnetic
transformers. A standard electromagnetic transformer is made of two coils coupled
by a magnetic field. An AC voltage applied to one coil generates a magnetic field
which passes to the other coil and induces a voltage. The coupling can be
increased by guiding the magnetic flux through a iron or ferrite core. The
transformer ratio is equal to the ratio of coil winding turns. In a piezoelectric
transformer, an inductor is analogous to a mass, a capacitor to a spring, and a coil
with distributed inductance and capacitance to a section of acoustic line (i-e. a
mechanical resonator). Thus, the two coils of an electromagnetic transformer are
analogous to two mechanically coupled, electrically insulated mechanical
resonators.
To obtain an equivalent electromechanical circuit for a radial resonator, one
may note that a derivation of the radial mode resonance impedance equation has
previously been developed for radial mode vibrations in a thin disc by Meitzler.
O'Bryan and Tiersten [7]. However, a review of the basic derivation with the
assumptions made and boundary conditions used is useful toward understanding
the limitations of the equivalent circuit model. The piezoelectric equations
governing the radial mode resonance are:
where T, S, E and D are the stress, strain, electric field and dielectric displacement,
and rand u, are the radius and radial displacement. These equations have been are
derived from the linear piezoelectric constitutive equations and are transformed to
a cylindrical coordinate system for convenience of calculation. The radial material
constants are defined in terms of standard cartesian constants as:
The radial material constants q3', c+', and e,: are the permittivity, elastic stiffness.
and piezoelectric constant respectively. The standard cartesian material constants
SF, and d,, are the free permittivity, elastic compliance under a constant
electric field. and the piezoelectric charge coefficient respectively. When a
sinusoidal voltage is applied to the piezoelectric resonator, the time dependence of
the fields TI S, E and D can be taken into account by an exponential eqwt term:
The wave equation for a thin disc is:
The general solution of the wave equation for a steady state vibration is of the form:
where A is a constant, J,(x) is a first order Bessel function and vP is the speed of
sound in the piezoelectric material, equal to:
The derivative of u, with respect to r can be evaluated using the relation
Then
This eqt ration (2.2.15) and equation 2-2-12 may then be s
(2.2-1 5)
)ubstituted into the first
piezoelectric equation (2.2.1) and this, combined with the boundary condition T, =
0 at r = a (the disk radius) gives:
With some rearranging,
A =
Then ur equals
Now, using the second piezoelectric equation (2.2.2), the current I in the
piezoelectric resonator is:
Substituting for the value of A, the current is then
The voltage is found by integrating E3 to get the voltage V = -E3 t, where t is the disk
thickness. The admittance is then
The admittance equation may be simplified using the following equations:
2 (~13')Z
(k') = ~ 3 3 ~ c11P
to obtain:
This equation may be written as:
where C, is the radially clamped capacitance, N is the turns ratio of the transformer,
and Z, is the specific acoustic impedance of the acoustic line, equal to:
*aZ cJJP Cp = (2.2.29)
t
These equations form the basis of an equivalent electromechanical circuit for a thin
disk radial mode resonator:
Electric Acoustic Port CP Port
FIGURE 2.2. I : Equivalent circuit for a radial resonator
where F, and v, are the radial force and velocity at the acoustic port of the resonator.
To confirm the validity of this equivalent circuit, the open and short circuit
impedances on the acoustic ports were checked, giving the right results. The
displacement and radial velocity were also checked and they are consistent with
each other.
For a free resonator, the acoustic port is shorted and the electrical
impedance is:
The effect of a substrate (Figure 2.2.1 -2) on which the piezoelectric layer is
deposited is to lower the voltage drop across the acoustic port of the resonator. It
is assumed that the radial strain is the same in both the piezoelectric layer and the
- . - Piezoelectric ,-- -
t s
v Substrate
FIGURE 2-2.2: Effect of the substrate on the radial resonator
substrate, which means that their acoustic elements have to be in series. The
specific acoustic impedance (ZJ of the substrate is:
and the equivalent circuit for the radial resonator modified by the substrate is:
Electric Port
Acoustic Port
FIGURE 2.2.3: Equivalent circuit for a radial resonator on a substrate
The electrical impedance of the resonator is then:
2.3 Radial Mode Transformer
In the radial mode transformer there are two piezoelectric disks of different
material properties and geometries (Figure 2-33). To understand the operational
dependence of the transformer on the material and dimensional parameters of the
device, the disks are assumed to be substrate-free. A voltage is applied to one
disk, and the voltage across the other disk is measured. An important quantity to
be
Electrodes -& Piezoelectric # 2 \ a-
v 4
Piezoelectric # 1
FIGURE 2.3.1 : Free standing radial mode transformer
determined is the input-to-output voltage ratio of the transformer overthe frequency
range of operation. The basic assumption is that the radial strain is the same in
each disk for radial mode vibrations. There is a common electrical terminal between
the two resonators, as the transformer is a three terminal device with a common
middle electrode- There is also a common acoustic terminal, as the resonators are
physically bonded. The equivalent transformer circuit is then [8]:
Electric Port 1 Cp2 -
Acoustic Port
(Fn1 v 4 7 - -
bl - Za2 -- - - - -
Electric Acoustic Port 1 Cp1 - Port
(Vl, 11) (Frl, V~I)
FIGURE 2-32 Transformer circuit # 1
where the circuit components were found from the derivation for the radial mode
resonator to be:
For a free resonator, the acoustic ports are shorted. The transformer equivalent
circuit can then be simplified by assuming an ideal transformer in which power is
conserved.
-- - Zal- Za2 -: - -
- -
Electric - - port 21 - -
(Vl, 11) - - -
FIGURE 2-3-3: Transformer circuit # 2
The power across the impedance element (Nd2Z, is conserved and is equal to
0/2)21& or (V,- Vl)2/q in terms of relative voltage. The current through this element
needs to be determined as a function of Vl in order to find the voltage ratio. The
total electrical impedance can be found as the parallel combination of 2, and &,
where Z, is the combination of the total impedance of the acoustic line combined
with the electrical input impedance of the second transformer, then transformed
over to the primary side of the first transformer. The total electrical impedance is:
The total input current and power are:
and the current I, is split between 2, and &. The current going to the primary of
first transformer (13 is then:
The current is divided by N, through the primary of the first transformer, so the total
current into the acoustic port is IJN, and the voltage at the front of the acoustic port
is N,V,. The circuit can then be simplified further as:
FIGURE 2.3.4: Transformer circuit # 3
The voltage drop across the parallel combination of f , and (NJ~Z, is:
and the current through the impedance (Nd2Z, is then:
The power to (Nd2Z, is I& which is equal to (V2)21Z, (or (V, - Vl)2& in terms of
relative voltage. The relative voltage ratio is then:
Voltage ratio = JI, v, 2, + v,
'4
The real and imaginary power flows into each of the various branches of the
transformer circuit for this mode are consistent, with no net loss for an ideal -
transformer.
2.4 Predicted Transformer Operation
The voltage ratio was calculated for a two layer transformer with a resonator
thickness of 2 ym and a resonator diameter of 13/32 (1 -032 cm), shown in Figure
2.4.1. Typical P A material property values were used in the analysis, which is
shown in Appendix I. The fundamental resonance occurs between 250-300 kHz,
yielding a peak voltage ratio of -4, which is suitable for a number of applications.
By varying the dimensional and material parameters, it was determined that the
voltage ratio of the transformer is relatively insensitive to the radius and thickness
of the resonators as long as the thickness of the output resonator is equal to or no
greater than twice the thickness of the input resonator. However, the voltage ratio
is highly dependent on the mechanical quality (Q,) and the dielectric loss (tan6).
According to the model, the thin film could be of a lesser thickness, but at lower
thicknesses (< 1 pm) the film is heavily clamped by the substrate and the
piezoelectric response is reduced. Producing films of a greater thickness is time
consuming and while possible, is unnecessary.
o 2.10 4a105 6-10 8-to5 Frequency (Hz)
FIGURE 2.4.1 : Predicted response of a thin film piezoelectric transformer
References
C. A. Rosen, "Ceramic Transformers and Filters" Proceedinas of the
Electronic Com~onents Svm~osium. 205 (1 956).
H. W. Katz. Solid State Magnetic and Dielectric Devices John Wiley & Sons,
New York (1 959)-
0. Ohnishi, Y. Sasaki, T. Zaitsu, H- Kishie and T. Inoue, lEiCE Transactions
on the Fundamentals of Electronics, E77-A (1 2). 2098 (1 994).
S. Sherrit, Ph.D Thesis, Royal Military College, Kingston, Canada (1 997).
W. P. Mason, Electromechanical Transducers and Wave Filters, 2" ed.,
Princeton, New Jersey, Van Nostrand Company (q 948).
W. P . Mason, Piezoelectric Crystals and Their Applications to Ultrasonics,
Znd ed., Princeton, New Jersey, Van Nostrand Company (1948).
A. H. Meitzler, H. M. OrBryan and H. F. Tiersten, IEEE Transactions on
Sonics and Ultrasonics, SU-20.233 (1973).
S. Sherrit, private communication.
3. Ex~erimental Techniaues
This chapter contains a description of the methods used for PZT film
fabrication and transformer production. The analytical techniques used to
characterize the PZT films and the piezoelectric transformer are also outlined.
3.1 PZT Film Deposition
Metallorganic processing of PZT thin films involves the deposition, drying and
firing of a metallorganic gel coated on a surface, followed by a high temperature
anneal to crystallize the film layer. The standard means of depositing thin films by
chemical solution deposition include spray. dip, paint and spin coating. While spray.
dip, and painting processes allow the coating of complex shapes, spin coating has
the advantage of producing films of uniform thickness and is the deposition process
most commonly used.
In spin coating, the solution is deposited onto a substrate which is held on a
spinner by a low level vacuum (Fig 3.1 .I). The spinner is then activated. Most of
the solution is expelled early in the spin cycle, but evaporation of solvent from the
Substrate RuOz or Pt /
FIGURE 3.1 -1 : Sol-gel deposition [I]
solution increases its viscosity and induces gelation, leaving a relatively thin
uniformly thick gel layer on the substrate [2]. Another advantage of this technique
is that a high level of control over the thickness of the film layer can be obtained by
setting the spin speed and spin cycle time. The P A films in this thesis were
deposited using a Headway Research Inc. photoresist spinner, and unless
otherwise stated, with a spin speed of 3000 rpm and a spin cycle time of 40
seconds. The film thickness also depends on the viscosity of the PZT solution.
Typical film thicknesses are shown in Table 3.1 -1 for the sol-gel recipes used in this
thesis.
TABLE 3.1.1 : Deposition parameters
3.2 Thermal Processing
Once the P A sol is spun onto a substrate, a thermal process is required to
promote further gelation, the pyrolysis and removal of organic components, and the
crystallization of the film layer. Two hot plates set at 150-250% and 400°C were
used for drying and firing the film layers. The high temperature crystallization was
then performed using either a conventional box furnace or a AG Associates Heat
Pulse 410 rapid thermal annealer (RTA) with an oxygen atmosphere, both at a
temperature of 650°C.
Rapid thermal processing (RTP) is an approach which is commonly used in
the microelectronics industry. Applications have included the improvement of the
crystalline quality of silicon and gallium arsenide, annealing of metal-semiconductor
contacts and the formation and annealing of dielectrics [3]. RTP has also been
used for the crystallization of PZT thin films [4,5]. An RTA essentially consists of
two sets of halogen quartz lamps fixed in place above and below a silicon wafer
(Figure 3.2.1). A sample is placed on the wafer, which is then heated rapidly by the
absorption of light from the halogen lamps in the silicon wafer. The atmosphere in
the processing chamber can be controlled, and in this thesis was chosen to be an
oxygen atmosphere. Since only the wafer is heated, high temperatures and
Spin Speed (rprn)
2000
3000
4000
Layer Thickness (pm)
0.23
0.20
0.1 7
Quartz Chamber Silicon Wafer & Thermocoup[e
Halogen Lamps Quartz Tray
FIGURE 3.2-1 : Schematic view of an RTA
temperature ramp rates up to 200°Cls are attainable. This feature is attractive for
processing PZT films with a reduced thermal budget (temperature - time product)
such that a minimum of thermal damage is caused. Conventional furnace
annealing requires long process times which may cause unwanted diffusion or
reaction processes between the film and substrate and large grain sizes which
impose limitations on memory cells in integrated high density devices. One
important item of note is that in contrast to conventional furnaces which heat the
whole sample uniformly, an RTA heats from the bottom of the sample. Thus, the
processing conditions for PZT film fabrication will be slightly different between the
two methods.
3.3 Thermogravimetric & Differential Thermal Analysis
The thermal processing schedule for the PZTsol-gel was investigated using
a Netzsch STA429 simultaneous thermogravimetric (TGA) and differential thermal
(DTA) analyzer. This apparatus shown schematically in Figure 3.3.1. In the TGA
analysis, weight loss is measured against temperature for a constant temperature
ramp rate up to some set temperature. In the DTA analysis, the temperature of the
sample is measured relative to that of a reference material heated at the same rate.
This allows the observation of endothermic or exothermic reactions as negative or
positive peaks respectively in the DTA data. The TGA and DTA results may be
combined to obtain a derivative therrnogravimetric analysis (DTG) which shows rate
of weight loss versus temperature. The TGA and DTA analyses are useful toward
identifying temperature regimes where the various organic components are
released from the film and where thermal stresses may potentially cause cracking
in the film.
Tube Furnace --- &
Sample Holder with .-
Thermocouple
Reference Thermocouple w -A
FIGURE 3.3.1 : TGAIDTA experimental apparatus
3.4 Glancing Angle X-ray Diffraction
Glancing angle x-ray diffraction (GA-XRD) was used to obtain an estimate
of the degree of crystallization and grain size of the PZTfilms. This method uses
a low angle of incidence to increase the diffracted volume of the film relative to the
substrate or to eliminate the intensity from the substrate altogether. The scattering
geometry of GA-XRD is illustrated in Figure 3.4.1 (a). The x-ray beam meets the
normal of the :diicacenormal a lattice planes
[ x-R:; ; Thin Film ' dl
FIGURE 3.4.1 : GA-XRD setup [5]
sample surface at an incident angle a, which is typically set to some low value in the
range of 1-So. In order to maintain this low angle of incidence, a is fixed and the
detector rotates through the 20 values.
In GA-XRD, the surface normal does not coincide with the normal of the
reflecting lattice planes (Figure 3.4.1 b). That is, the reflecting lattice planes are not
parallel to both the substrate and sample surfaces. The location of the detector
must be adjusted to satisfy the Bragg dimaction conditions of different
crystallographic planes in the sample. The Bragg diffraction condition is:
nh = 2d sine
where n is the order of diffraction, A is the wavelength of the incident x-rays, d is the
crystallographic lattice plane spacing and 8 is the angle between the plane of
incidence and the incident beam. Since GA-XRD is not a focussing. but rather a
parallel beam geometry, soller slits are placed in the detector circle (Figure 3.4.2)
to ensure that only a parallel reflected beam reaches the detector. Soller slits are
plates that are placed at right angles to the detector circle in order to eliminate
divergent beams and to collimate the parallel beams for enhanced resolution at the
detector
FIGURE 3.4.2: Location of soller slits [6]
A Rigaku Rotaflex rotating anode DMAX diffractorneter with a chromium
source, a graphite crystal monochromator and a thin film mounting accessory was
used to measure the diffracted intensity from the PZTfilms with a glancing angle of
5" over a 28 range of 30-90".
3.5 Substrates and Electrodes
3.5.1 Substrate
The substrates used in thi. s the. sis 1 were Si(l1 I) wafers coated with a 50 A
titanium adhesion layer and a 2000 A platinum layer forming the bottom electrode
for the PZT films. These substrates were provided courtesy of Gennum
Corporation, Burlington, ON. This substrate was chosen in keeping with the desire
to produce a piezoelectric transformer suitable for silicon integration. A possible
38
alternative would be to use a polished alumina (AI,O,) substrate with a platinum top
layer. Alumina substrates have been used for "flip-chip" applications in conjunction
with silicon-based technology and would also be suitable for producing the
transfonner-
3-52 Electrodes
Two different types of top electrode were used in this thesis. An Edwards
306A vacuum evaporator was used to deposit chromelgold (Cr-Au) electrodes on
the samples to be characterized using dielectric methods- The chrome layer, which
promotes the adhesion of gold on P A , was approximately 40 A thick, and the gold
layer was approximately 250 A thick. For the piezoelectric transformer however, the
choice of electrode material is limited by the requirement of being able to withstand
a processing temperature of at least 650°C. The electrodes should be relatively inert
so that they don't interact with the PZT film. The electrodes should also have low
fatigue, low leakage current and low dielectric loss. The two primary electrode
materials that meet these specifications are platinum and ruthenium oxide (RuOJ.
There are advantages and disadvantages with either type of electrode. RuO,
electrodes have been shown to have improved fatigue characteristicsm, but have
a high dc leakage current [8]. Platinum electrodes, while being more susceptible
to fatigue, were chosen for use in the transformer for the reason that they are widely
used in industry and are relatively insensitive to etching effects in the production of
the transformer. Platinum electrodes were deposited via reactive dc magnetron
sputtering.
3.6 Reactive OC Magnetron Sputtering
The process of sputtering involves a physical transfer of atoms from a target
to a substrate caused by bombardment of the target by highsnergy ions (Figure
r i i - Cooling Water
Platinum Target
Platinum Si(111) substrate
(heated)
FIGURE 3.6.1: DC magnetron sputtering process
In magnetron sputtering, a shaped magnetic field is used to trap and concentrate
electrons produced in the discharge in a ring above the target surface. The
sputtering gas is ionized when it encounters the cloud of electrons, and the positive
ions are then attracted to the target surface by a negative dc bias maintained on the
target. Atoms displaced from the target by the collision of the incident ions diffuse
through the plasma and are deposited on the substrate. DC bias is used for the
deposition of metals and an RF bias is used for insulating materials.
The middle electrodes for the piezoelectric transformer were deposited by
reactive dc magnetron sputtering using a VACTEC dc magnetron sputtering system.
The system was pumped by a Diffstak diffusion pump backed by a Varian
mechanical pump to a pressure less than 5 x lo6 Torr before sputtering under
argon. The processing conditions for the deposition of the platinum outlined in
Table 3.6.1 were chosen bascd on previous studies on platinum deposition by
reactive dc magnetron sputtering.
TABLE 3.6.1 : Sputtering conditions
1) Substrate-Target Separation ( 6-7 cm
Sputtering Parameter
Power
11 Substrate Temperature 1 50-350°C
Setting
150 W
11 Gas I 100% argon
I Chamber Pressure I 40 mTorr
11 Base Pressure I < 5 x 10" Torr
1 Sputtering Time I 2-5 minutes
The electrodes for the transformer were deposited by sputtering through a
photoresist mask layer patterned using conventional photo-lithographic techniques
or through a metal shadow mask.
3.7 Photo-lithographic Techniques
A mask with the dimensions shown in Figure 3.7.1 was manufactured from
a thin sheet of brass. A thick film A24620 positive photoresist was spun on the
PZT coating starting at 1500 rpm for 15 seconds then increasing the speed to 7000
rpm for 40 seconds. The sample was spun at the slower speed first to retain a
41
(a) mask (b) electrode dimensions
FIGURE 3.7.1 : Mask dimensions
significant thickness of photoresist, then at the faster speed to promote coating
uniformity. The layer was then soft-baked at a temperature of 100°C for 30 minutes
on a hot plate. The photoresist layer was exposed through the mask under an
ultraviolet lamp at a distance of 15 cm from the sample for 3 minutes. The exposed
regions of the photoresist layer were then removed using an AZ400K developer.
The platinum electrodes were sputtered onto the PZT coating through the
photoresist mask and the photoresist was then removed by immersing the sample
in ethanol until no visible photoresist remained. The sample was then immersed in
acetone for a short period of time to ensure complete removal of the photoresist.
3.8 Poling
Since PZT thin films have randomly oriented domains after thermal
processing with zero net dipole moment, they must be poled. The poling process
involves the reversal of 180" domains and the rotation of 90" domains within the
sample by an applied electric field and elevated temperature. The 180" domain
switch is the change in domain orientation from antiparallel to parallel to the
direction of applied field and the 90" domain switch is the change in domain
orientation from perpendicular to parallel to the direction of applied field. The 90'
domain switching usually occurs by the shortening of the axis of the unit cell which
is perpendicular to the applied field and elongating the axis which is parallel to the
applied field, inducing a strain in the sample. Samples in this thesis were poled at
a temperature of 160-1 80°C and an applied field of 6-8 Vlpm for 15 minutes using
the poling apparatus shown in Figure 3.8.1.
Ammeter Power Supply 0 . . . --
Computer I Sample
r- - Temperature -.
Controller Hot Plate
FIGURE 3.8.1 : Poling apparatus
A Fluke 4058 DC power supply was used to pole the samples. The
temperature of the hot plate was set using an Omega CN-2011 programable
temperature controller and a Keithley 197A autoranging microvolt DMM (digital
multimeter) was used to monitor the leakage current across the piezoelectric. The
leakage current and temperature were monitored at twenty second intervals via a
La bview-based program.
3.9 Electrical Characterization
PZT thin films can be characterized by their ferroelectric, dielectric and
piezoelectric properties, all of which are inter-related. In dielectric analysis, a
sinusoidal excitation
is applied to the film, where w = 2nf and f is the exciting frequency. The ideal
capacitance (C) of the PZT film under the electrode area (A) is equal to:
where E, and e are the free space and relative permittivity respectively, and d is the
film thickness. The AC impedance of the film is
with a phase shift of 90' between the current and the voltage. The impedance of
a real capacitor includes both capacitive (energy storage) and resistive (energy
dissipative) components. The relative permittivity is then written as a complex
quantity E* = E' + jen and the dielectric response is then described by two parameters
- the relative permittivity E and the dissipation factor D = tan U = ~"k ' , where b is the
deviation of the phase angle between the current and the voltage from 90". The ac
conductivity is then
Values for the relative permittivity of PZT thin films produced by the solgel method
are typically in the range of 300-1400, and tan 6 values are in the range of 0.005-
0.1 , depending on the solution chemistry. film composition, film processing, etc.
To measure these dielectric properties. the capacitor is modelled as a
complex admittance Y(o) which may be expressed as
where G(w) and B(o) are the conductance and susceptance. The device can be
further described as a capacitance C, and resistance Rp in parallel, and from G(o)
and B(o), C, and R, can be obtained as
Then the relative permittivity E, and the dissipation factor D can be found
for some area A and thickness d of the capacitor. The measurements were made
using a Hewlett Packard HP4284A LCR meter at 1 kHz and a voltage of 100 rnV.
45
3.1 0 Piezoelectric Characterization
3.10.1 lm~edance Analvzer
One way of determining the piezoelectric properties of PZT thin films is to
analyze the complex impedance response of the films as a function of frequency
using an impedance bridge. The most common measurement systems used forthis
purpose are the Hewlett Packard HP4192A and HP4194A impedance analyzers.
both of which are four terminal devices with the configuration shown in Figure
3.1 0.1 (a). The outer current probes are used to apply a current to the sample and
the inner probes measure the voltage drop across the sample. The input impedance
of the voltage probes is very large compared to the voltage drop due to the contact
Analyzer Analyzer z = VII z = VII R..., = 0 Z = Z ..-. + R ...c
Low l LowV HighV High I ' Low l LowV HighV High I
(a) Four terminal configuration (b) Two terminal configuration
FIGURE 3.1 0.1 : Schematic of the impedance measurements
resistance at the voltage probe, so an accurate measurement of the sample
impedance can be taken independent of the contact resistance. This arrangement
is useful for measuring the sheet resistance of metal, for instance. However, the
boundary conditions of a piezoelectric resonator are such that a two terminal
measurement with the high current and high voltage and the low current and low
voltage terminals connected together is necessary. Fortunately, the impedance of
a piezoelectric resonator is usually much larger than the contact resistance, and by
ensuring good contact with the resonator electrodes, the error can be minimized.
The phase magnitudes of the voltage and current are used to determine the
complex impedance.
Some of the measurements were taken using the HP4194A impedance
analyzer with a Z probe attachment. The Z probe is designed to operate at higher
frequencies with a larger DC bias voltage. There is a larger measurement error
associated with it, but the results from previous work were not found to change
substantially between the Z probe and the four terminal connections of the
HP4194A [9]. The apparatus for the measurements is shown in Figure 3.10.2 and
the specifications for the HP4194A impedance analyzer are tabulated in Table
3.1 0.1. Open and short circuit corrections were performed and the correction to the
data was automatically calculated in the impedance analyzer by linearly
interpolating the open and short circuit data to the measurement frequency.
TABLE 3.1 0.1 : HP4194A specifications [9]
Property
Frequency Range
11 DC bias I i 150 V external 11
HP4194A with Z probe
10 kHz - 1 MHz
11 AC test signal range (rms) I 1OmV- 1260mV
Accuracy for: z , = l k n f = l MHz
I integration t = medium samples average s 4
AZ=* 1-6%
I1 V,, = I V (rms) I A0 = * 0.91" 11
Computer with Kepco Bipolar lEEE488 - HP4194A Amplifier interface
Z Probe
Ground Plane Insulator - . 0
- Signal and Bias
FIGURE 3.10.2: Schematic of the impedance measurement apparatus [9]
3.10.2 Network Analvzer
For analysis in a frequency range above 100 MHz, an S-parameter HP8753D
network analyzer with a range of 30 kHz - 6 GHz was used. In this situation, the
complex scattering parameter S,, was measured. This quantity is the reflection
coefficient for a symmetrical two port network measured in terms of power. The
scattering parameter S,, is related to the impedance of the resonator as:
where Z,, is the complex impedance and Z, is the characteristic impedance of the
line (50 0 in this case). From this, the impedance of the resonator may be obtained
as:
Then the real and imaginary (resistance and reactance) components of the
impedance may be obtained:
This method was used in analyzing the thickness extensional mode of the sol-gel
P A films to obtain the electrical, mechanical and piezoelectric properties of the
films.
3.1 1 1 hickness Measurements
Thickness measurements of the PZT films were obtained in two different
ways. A Taylor-Hobson Talystep profilometer from Rank Precision Industries
Limited was the first method employed. The Talystep measures the force exerted
on a stylus (which is calibrated against thickness for a known standard) while
moving in a line across the film. The thickness of the film is obtained by etching an
edge of the film to the substrate and measuring the step from the film to the
substrate at that edge. Photoresist was used to cover the rest of the film up to the
edge to protect it against the etch. The etch used is shown in Table 3.1 1.1. A
scanning electron microscope was also used to take film measurements, using the
calibrated scale in the microscope to make a visual measurement of the thickness-
TABLE 3.1 1.1 : PZT etch
11 Hydrochloric acid (HCI) I 80 ml
Chemical
Hydrofluoric acid (HF)
I water I 20 r n ~
3.12 Scanning Electron Microscopy
Amount (ml)
4 ml
In addition to PZT film thickness, the surface roughness, porosity and the
integration of individual film layers of the PZT multilayer coatings were examined
using a JEOL840 scanning electron microscope (SEM). Since the samples were
non-conducting, a gold coating was sputtered on the samples priorto SEM analysis
to reduce sample charging in the electron beam. An SEM measures the secondary
and backscattered electrons produced by a steady source of electrons focussed on
the sample. For this work the sample was imaged via the secondary electrons. The
finely focussed beam is rastered across the sample surface by activating deflector
coils (Figure 3.12.1) and the secondary electrons are detected using a scintillator
photomultiplierwith a Faraday cage. The signal is amplified and the resultant image
is created on a cathode ray tube (CRT) with a 1:l correlation between beam
position on the sample and the image signal on the CRT. Since secondary
electrons are sensitive to surface topography and are weakly sensitive to atomic
number and crystal orientation, they act as contrast agent for obtaining an image.
A 10 kV accelerating voltage and working distances of 18-1 9 crn were used.
Condenser lens+
Beam deflector coib 7
Incident electron beam
Specimen
FIGURE 3.12.1 : Schematic of a scanning electron microscope [I 01
3.1 3 Transformer Measurements
The root mean square voltages of the input and output resonators of the bulk
and thin film transformer designs were measured as a function of frequency using
the apparatus shown in Figure 3.13.1, and the transformer voltage ratio was
calculated. A WAVETEK 1 I MHz model 22 stabilized sweep generator was used
as the driving source, and the driving signal and output signal were read using an
HP54501 100 MHz digitizing oscilloscope. The voltage divider was used to stabilize
the voltage to the transformer at high frequencies, where the impedance of the input
resonator drops to less than 50 C2, which is the impedance of the function generator.
I Oscilloscope '
Channel # 1 (Input Signal)
so n - a
Oscilloscope Channel # 2
(Output Signal)
Function - Generator
(50ohna20-W) -
Probe Thin Film Transformer
FIGURE 3.1 3.1 : Schematic of the transformer measurement apparatus
References
D. Mclntyre. MSc. Thesis, Queen's University (1995).
C. J. Brinker and G.W. Scherer, Sol-Gel Science: The Physics and Chemistry
of Sol-Gel Processing, Academic Press. New York (1990).
R. Singh, "Rapid Isothermal Processing" Journal of Applied Physics, 63, 8.
R59 (1 988).
C.V.R.V. Kumar, R. Pascual, and M. Sayer. "Crystallization of sputtered lead
zirconate titanate films by rapid thermal processing" Journal of Applied
Physics, 71, 2. 864 (1992).
E. Griswold, Ph.D Thesis, Queen's University (1 995).
B. D. Cullity , Elements of X-ray Diffraction, 2nd edition. Addison-Wesley
Publishing Go. Inc., London (1978).
D.S. Mclntyre, M. Sayer. L. Weaver, E.M. Griswold, V. Chivukula "Electrical
Characterization of P A on Rapid Thermally Annealed Ruthenium Oxide
Electrodes" l ntegrated Ferroelectrics 10,3O 1 (1 995).
8. D.P. Vijay, C.K. Kwok, W. Pan and S.B. Desu, Proceedings of the 8'"
International Symposium on the Applications of Ferroelectrics, 225 (1 992).
9. S. Sherrit, Ph.D Thesis. Queens University (1 997).
10. S. Langstaff, Ph.D Thesis, Queens University (1 998).
4. Sol-Gel Processing
P A thin films made from a sol-gel processing route have potential for use
in a variety of electronic and micro-electromechanical systems (MEMS). However,
a major factor limiting their use in MEMS applications is the maximum attainable
thickness of the films. Film thicknesses up to 5 pm have been achieved in a single
coat, but the process is difficult to reproduce [I]. Residual stress and trapping of
impurities seemed to be an inevitable consequence of the latter approach, which
included an undesirable processing step under vacuum. A multi-layering approach,
where the layer thickness is chosen such that adequate component diffusion can
occur during thermal processing, is preferable. This chapter focuses on the
development of a consistent acetic acid-based multi-layering sol-gel route for
producing high quality 5-1 0 prn thick PZT films. This work was based on the original
work completed by Yi et al. [2] in developing the acetic acid-based PZT sol-gel, and
the subsequent improvements to the processing of the solgel made by Schwartz
et al. 131.
4.1 Sol-Gel Routes
Most PZT sol-gel routes currently in use employ a lead salt, as well as
zirconium and titanium alkoxide precursors. These include the 2-methoxyethanol-
based solution created by Gurkovich and Blum [4] and developed by Budd and
Payne [5], and the acetic acid water-based solution created by Yi etaL [2], which
has been improved by Schwartz et al. by inverting the mixing order of solution [3].
A diol-based solution employing acetylacetonate modified zirconium and titanium
alkoxides and 1,3-propanediol as a solvent has also been shown to have potential
161 -
Most of the solution routes for producing PZT thin films reported in literature
have 2-methoxyethanol as a solvent The properties of the films produced by this
solution chemistry, particularly when extensive distillation is undertaken, are
probably the best and most consistent of those published in the literature. However,
two major difficulties have been identified. First, the carcinogenic nature of 2-
methoxyethanol is undesirable in any production process. Second, single layer PZT
films produced from Zmethoxyethanol based solutions usua!ly are limited to less
than 0.1 pm to avoid cracking during the firing process. Thus, multi-layering
becomes a labourious procedure. High boiling point additives such as polyethylene
glycol and glycerol, and drying agents such as formamide have been used to
increase the film thickness [7-91. However, these additives introduce new problems
due to residual carbon that degrades the film's electrical properties and the lengthy
firing time required to remove most of the carbon [I 01.
The acetic acid water-based PZT sol-gel is the other major solution chemistry
widely in use. When this chemistry was first introduced by Yi et a1.[2] the properties
of the films were quite poor, with an average permittivity of 350 and loss tangent of
5%. Changes introduced by Schwartz et al. [3] which included inverting the mixing
order and replacing zirconium propoxide (70 wt % in 1 propanol) with zirconium
butoxide (80 wt.% in 1-butanol) resulted in much improved film properties, though
still at a lower performance level than 2-rnethoxyethanol-based films. These films
can be deposited up to 0.3 pm thickness without modification of the sol gel by
additives. Having a water-based solution is advantageous, as such solutions are
chemically stable under conditions at room temperature. The solution is also far
less toxic.
The diol-based PZT sol-gel is a solution chemistry that has recently been
studied with an emphasis on increasing film thickness [I 1-13]. Single layer film
thicknesses up to 1 pm have been obtained using this approach. As opposed to the
previous two sol-gel chemistries, films deposited by the diol process do not undergo
a gelation process after deposition at room temperature. The diol-based films also
show a tendency to dewet the substrate. The first effect has been attributed to the
non-reactive nature of the species formed in the solution preparation phase and the
low volatility of the solvent, which generates no capillary pressure forcing oligomeric
reactions[l4]. The second effect is believed to result from the rearrangement of
precursor species following deposition. This is possible due to the low reactivity of
the precursors and is caused by the evaporation of solvent from the deposited film
layer [14]. This has caused problems with non-uniformity of the film layers and an
apparent increase in defect density in the film, leading to a potential drop in the
electrical quality. In addition, a lengthy distillation process is necessary in the
solution preparation phase, as the solution chemistry is sensitive to water. For
these reasons, the diol-based process has not seen wide usage.
One other sol-gel process technology worth mentioning is the hybrid sol-gel
composite route. Barrow and Petroff et al. have developed a composite coating
technology in which a sol-gel is used to bind together a powder which has a similar
composition [I 5,161. The coating is applied as a paint or by spray, spin, or dip
coating as a 3-5 pm thick layer and is subjected to a thermal process similar to that
employed for the pure sol-gel films. Coatings in the thickness range of 5-1 00 pm
can easily be produced by this method. The coating parameters can be adjusted
by varying the sol-gel composlion, the powder loading and the particle size
distribution. The surface topography is directly related to the granularity of the
powder, so for most applications the coating must be polished prior to top electrode
deposition and poling. Although increased thickness of the film layers is a definite
advantage, the sol-gel process is preferable for producing a piezoelectric
transformer, as the films: (1) are uniform, eliminating the need for an intermediate
polishing step, (2) avoid the use of powders which lead to quality control issues in
micro-fabrication, and (3) can be patterned using conventional photo-lithographic
techniques.
4.2 Acetic Acid-Based PZT Sol-Gel
The acetic acid-based PZT sol-gel process is based on the polymerization
of zirconium and titanium alkoxide precursors to form a gel network, with the lead
(Pb) component of PZT being introduced as a salt Two reactions that are integral
to the sol-gel process are the hydrolysis and condensation reactions, which govern
the behaviour of the metal alkoxide precursors. When the metal alkoxides are in
their preferred coordination state (in the absence of a catalyst), hydrolysis and
condensation both occur by nucleophilic substitution (S,) mechanisms involving
nucleophilic addition(AN). In the hydrolysis and condensation reactions, the
molecules that are electron-donating (H,O and MOH) are termed nucleophiles. This
reaction is followed by a proton transfer from an attacking molecule to an alkoxide
or hydroxo-ligand within the transition state, with the protonated species being
removed as either an alcohol (alcoxolation) or water (oxolation) [I 71:
M(OR), + H,O t, HO -M(OR),-, + ROH (hydrolysis)
(OR), - 1M - OR + HO - M(OR), - t, (OR), - ,M - 0 - M(OR), -, + ROH
(condensation - alcoxolation)
(OR), - 1M - OH + HO -M(OR), -, t, (OR), - 1M - 0 - M(OR), - + H20
(condensation - oxolation)
In these reactions R is the alkyl radical and M is titanium or zirconium. The
condensation reaction can proceed further to give linked M-0-M species. This
chain reaction ultimately leads to the polymerization of the alkoxide precursors.
To obtain a stable PZT sol, the metal alkoxide precursors need to be kept
stable in solution without inducing a condensation chain reaction. Since the
reactions are reversible, one might expect that the equilibrium point between the
forward and reverse reactions may be reached through addition of a suitable
amount of water andfor alcohol facilitating the reverse reaction. However,
electronegative alkoxo (OR) groups cause the metal atom in the metal alkoxide
precursor to be highly susceptible to hydrolysis. Simply adding water or alcohol
leads to the formation of hydrolyzed metal alkoxide precipitates in solution. The
rapid kinetics of the nucleophilic reactions make it difficult to establish any sort of
equilibrium.
Chemical modification of the metal alkoxide precursors is generally employed
to retard the hydrolysis and condensation reactions. In most cases. nucleophilic
XOH chelating agents such as acetic acid are added, reacting with the alkoxide to
give a new molecular precursor:
An exothermic reaction occurs when acetic acid is added to the titanium and
zirconium alkoxides, leading to a clear solution. It has been shown that titanium
isopropoxide and zirconium propoxide (70 wt.% in 1 -propanol) react in a 1 :1 molar
ratio with acetic acid to form titanium isopropoxide acetate and zirconium propoxide
acetate 1171. The other alkoxide molecules undergo a similar reaction. Titanium
butoxide, for example has the reaction:
where OAc is the acetate group, and titanium butoxide acetate and butanol are
formed. X-ray absorption experiments performed on titanium butoxide show that
the coordination number of Ti increases from 5 to 6 upon acetic acid addition [18].
Nuclear magnetic resonance (NMR) 13C and 'H experimental results demonstrate
that acetate groups bond to the titanium, while infrared spectra indicate that
CH,COO' behaves as a bidentate ligand, both chelating and bridging [I 81. Infrared
and NMRexperiments show that (BunOH) groups are removed first under conditions
of hydrolysis, while chelating acetates remain bonded much longer, slowing down
the condensation (gelation) process [18]. The acetic acid-based P A solgel is
dependent on this result. Acetic acid is used as the primary means for controlling
the hydrolysis and condensation reactions of the metal alkoxide precursors.
A variety of different recipes for the preparation of acetic acid-based PZT sol-
gel can be found in literature. However, these recipes essentially follow the basic
concept behind the original recipe introduced by Yi eta/. [2], which is referred to as
the acetate process (Figure 4.2.1). In this process, lead acetate tri-hydrate is
dissolved in acetic acid at 1 OO°C. Zirconium propoxide (70 wt.% in 1 -propanol) and
titanium isopropoxide are then added to the lead acetate/acetic acid solution and
the solution is mixed in an ultrasonic agitator. When the solution is well-mixed,
water and polyethylene glycol are added to adjust the viscosity, solution stability,
and the release of organics during thermal processing. The most significant
change to the original recipe was introduced by Schwartz et al. [3] who inverted the
mixing order of the chemicals (Figure 42.2) and substituted zirconium butoxide (80
wt.% in l -butanol) for zirconium propoxide (70 wt% in I -propanol). This is referred
to as the inverted mixing order (IMO) process. The improved results obtained
through use of this process have been attributed solely to the mixing order-
in Aceti& Acid at 100d= (mobr ratio Pb s acetic acid = 1:s)
Add Zr propoxide (mdar ratio PbZr = 1 r 0.535)
Add Ti isoprapoxide (mahr ratio Pb:Ti = 1 : 0-465)
for 5 minutes
-
Add Water (molar ratio of Pb-r = 1 25)
Add p o ~ y k t n s gfyd (Pb:polyahylene glycol = 911)
FIGURE 4.2.1 : Acetate process [2]
Add Ti isopro~oxide to Zr butoxide (molar ratio PbZr = 1 : 0.535. Pb:Ti = 1 : 0.465)
Add acetic acid and mix in ultrasonic mbuw for 5 minutes (mobr nth acetic acid : alkoxides = I:()
I *dd methanol I
Add lead acutate trihydrats and heat to 8 W to dissolve (molar ratio Pb: (Zr +Ti) = Y 3)
I I
Cod to 700C. then add additional ocatic add and methanol I
- - . - - - . I Coo, to 60oC. then add addi ia l acetic acid and watew I
FIGURE 4.2.2: IMO process [3]
61
Neither of these recipes for acetic acid-based PZT sol-gel are suitable for
producing thick PZTfilms via a multi-layering process. The acetate process yields
low electrical quality films and while an individual layer thickness of 0.2-0.3 pm is
attainable, it is difficult to avoid cracking of the film due to internal stresses. The
IMO process yields high electrical quality films, but has an individual layer thickness
of only 0.1-0.1 5 pm and also cracks, albel at greater thicknesses. To obtain a
viable multi-layering acetic acid-based PZT sol-gel process, a careful investigation
of the solution chemistry and thermal processing was required, with a principal
objective of reducing residual stress which leads to cracking of the multi-layer film.
4.3 Solution Preparation
Sol gel solutions of (535146.5) PZT were prepared from lead acetate
trihydrate (1 0-1 5% excess lead) and the following combinations of metal alkoxide
precursors: (1) zirconium propoxide (70 wt.% in 1-propanol) and titanium
isopropoxide, (2) zirconium butoxide (80 wt.% in 1-butanol) and titanium
isopropoxide, and (3) zirconium butoxide (80 wt% in I-butanol) and titanium
butoxide. Acetic acid, water, and methanol, ethanol, or butanol were also added to
the solutions. The solutions were prepared according to one of two mixing
methods, the first being a mixing order similar to that used by Yi et a/. [2] shown in
Figure 4.3.l(a) and the second method of preparation being a slightly modified
inverted mixing order shown in Figure 4.3.1 (b) similar to that employed by Schwa-
et al. [3]. The exact amounts used (unless otherwise stated) are listed in Table
4.3.1.
to I O50C to dissolve- Cool to room temperature
/ Lead acetate trihydrate / I Acetic add - I I Zr alkoxide
I Add Ti alkoxide to Lr a b i d e I I
Ti alkoxide I
- -
Add addtional acetic acid, alcohol and water
/ Wait thm drys bebre use 1
I Add Ti bpropoxide to Zr butoxide (molar ratio PbrLr* 1 : 0.535. PkTi = f : 0.45)
Add acetic acid and mix in uhronic mixer fur 5 minutes (molar ratio a& m-d : alkorides = 1 :4)
I Add alcohol 1
Add lead acetats trihydmte end heat to 85oC to dissolve (molar ratio Pb: (Zr + Ti) = 1:l)
Cool, then add addibional scaic acid. alcohol and water
Wait 3 days bobre use
FIGURE 4.3.1 : (a) Modified acetate and (b) Modified IMO processes
TABLE 4.3.1 : Chemicals for acetic acid-based PZT solgel
Choice of= Chemical Amount
Lead precursor lead acetate trih yd rate 3.980 g p0.27 g)
Alkoxide precursor Zirconium propoxide (70 wt.% in 1-propanol) 2.507 g Zirconium butoxide (80 wt% in I-butanol) 2.563 g
Titanium isopropoxide 1.390 g Titanium butoxide 1.597 g
Chelating agent acetic acid 2.407 g
Solvent rnethanoi 2.000 g ethanol 2.875 g propanol 3.750 g butanol 4.625 g water 1.125 g
Additional solvent methanol 1.000 g ethanol 1.438 g propanol 1-875 g butanol 2.31 3 g water 0.700 g
4.3.1 Metal Alkoxide Precursor
Two items are of particular significance in the solution preparation process
in regard to the alkoxide precursor. First, an exothermic reaction occurs when
titanium isopropoxide is added to either of the zirconium alkoxide precursors. In
contrast, when titanium butoxide is added to either of the zirconium alkoxide
precursors, no exothermic reaction occurs. This observation was quantified using
a bomb calorimeter with a thermocouple attached and the results are shown in
Table 4.3.1 -1. Second, both titanium isopropoxide and zirconium propoxide (70
64
wt.% in l-propanol) visibly react wlh trace amounts of water from the humidity in
the air during the preparation process-
TABLE 4.3.1 -1 : Reaction between alkoxide precursors
I Alkoxide Precursors
Zr propoxide (70 wt% in 1-propanol) Ti isopropoxide
Zr butoxide (80 wt.% in 1-butanol) Ti isopropoxide
Temperature Change
2-7 * 0.2 O C
Zr propoxide (70 wt.% in 1-propanol) Ti butoxide
butoxide (80 wt.% in 1-butan01) I
0-0 * 0.2 "C
I Ti butoxide I
The exothermic reaction can be explained in the following manner. Because
titanium and zirconium ions are electropositive and their alkoxides have highly polar
M-0 bonds, both metal ions can act as coordination centres. Any molecule or anion
with an unshared pair of electrons can donate them to a central titanium or
zirconium ion to form a coordinate covalent bond. Thus, it is possible that
coordinate linkages in titanium and zirconium alkoxides can occur throug h donation
of oxygen electrons if the steric conditions of the metal ions with respect to their
alkoxy groups are appropriate. Examples of possible coordinate linkages are
shown in Figure 4.3.1 -1. When titanium isopropoxide and zirconium propoxide (70
wt.% in I -propanol) are mixed, I -propano1 substitutes with the isopropoxy groups,
changing the steric conditions of the titanium ions leading to coordinate linkages
between them. This molecular association reduces the system energy, with a
consequent evolution of heat and an increase in viscosity of the solution [19]. A
similar reaction occurs when zirconium propoxide (70 wt-% in 1-propanol) is
replaced with zirconium butoxide (80 w t % in I-butanol). On the other hand, when
titanium butoxide and zirconium butoxide (80 wt-% in 1-butanol) are mixed, no
exothermic reaction occurs. This is because the formation of coordinate linkages
is dependent on the structure of the metal ion's alkoxy groups 1201. As the amount
of branching in the alkoxide groups increases, which is the case for the butoxide-
based precursors, coordinate linkages are less likely to occur. This is a favourable
result toward reducing the stress in the film layers during processing.
R - alkyl group M - metal ion 0 - oxygen
FIGURE 4.3.1 -1 : Examples of possible coordinate linkages
The visible reaction of the titanium isopropoxide and zirconium propoxide (70
wt.% in I-propanol) precursors with trace amounts of water in the air can be
explained by looking at metal alkoxide hydrolysis rates. It is known that titanium
butoxide and zirconium butoxide have a reduced reactivity to water as compared
to zirconium propoxide and titanium isopropoxide [18], and hence are not as
sensitive to trace amounts of water introduced in the solution preparation process.
For this reason, a visible reaction does not occur when titanium butoxide and
zirconium butoxide (80 wt.% in I-butanol) are used. Because of their reduced
hydrolysis rates, butoxide-based precursors are preferable to isopropoxide
Ipropoxide-based precursors toward promoting solution stability. It is probable that
there are more localized inhomogenelies formed during gelation and thermal
processing of the isopropoxidelpropoxide-based coatings-
4-32! Lead Precursor
The general practice in PZT sol-gel processing is to add an additional
amount of lead in the form of lead acetate trihydrate above the stoichiornetric
amount to account for lead loss in the form of lead oxide (PbO) during thermal
processing. Typically 5-20% excess lead is added to the solution. The control of
lead content in the PZT thin films is not especially critical when processing c 1 V r n
thick films, but becomes increasingly important in developing a multi-layering
process for films of 5-10 prn thickness, especially for furnace processing. For
processing in a box furnace, an optimum range of 10-1 5% excess lead has been
established. At lower lead contents, the films tend to crack sooner due to stress
caused by a lead deficiency and consequent lack of stoichiometry in the film. At
higher lead contents, the films tend to crack sooner due to stress caused by an
excess of lead in the films. For RTA processing, 540% excess lead seems to be
sufficient, as less lead is lost as lead oxide during RTA processing both due to the
s
heating mechanism and time of annealing. Lead acetate trihydrate is the obvious
choice for use as the lead precursor. Lead nitrate has been used, with the
objective of reduces the amount of organics in the sol-gel solution. However, it is
not very soluble in acetic acid, and is less soluble in water than lead acetate
trihydrate.
4.3.3 Choice of Solvent
A key factor affecting the stability of solution is the choice of solvent. P A
sol-gel solutions with the same molar quantity of water, methanol, ethanol, propanol,
and butanol were prepared, using zirconium butoxide (80 wt.% in I-butanol) and
titanium butoxide. It was found that methanol is the best solvent for promoting long-
term solution stability against the precipitation of re-hydrated lead acetate (Table
4.3.3.1). This is essentially due to the fact that methanol reacts more readily than
the other alcohols with excess acetic acid in solution to form esters [3], according
to the esterification reaction:
where R is the
ROH + CH3COOH +, CH3COOR + H20
alcohol group (methyl, ethyl, etc.) and the ester (when methanol is
used) is methyl acetate. This minimizes the problem of rehydrated lead acetate
precipitating in solution due to the common ion effect according to the solubility
constant:
and prevents consequent film defects from inhomogeneities in solution. No
observable difference in film thickness, uniformity, degree of crystallization or
electrical quality is observed when using different alcohols as a solvent, as later
results will show-
TABLE 4.3.3.1 : Effect of solvent on solution stability
I1 Solvent
II Pro pan01
11 Butanol
II Water
0.094 1 c 24 hours
Amount (mol)
0.016 (0.5 g) 0.031 (I -0 g) 0.047 (1 -5 g) 0.094 (3.0 g) 0.1 09 (4.0 g)
0.094 1 c 6 hours
Time of Solution Stability
< 2 hours c 24 hours c 2 weeks c 3 months 5 months +
0.094 1 e 1 hour
4.3.4 Mixina Order
Precipitates and gels are difFicult to avoid in the modified acetate process,
where lead acetate trihydrate dissolved in acetic acid is added to the metal
alkoxides. This is particularly true for solutions employing zirconium propoxide (70
wt.% in I-propanol) andlor titanium isopropoxide precursors. This is probably due
to the fact that when acetic acid is added first to the metal alkoxides, as in the IMO
method, the alkoxides can be sufficiently chelated before trace amounts of water
associated with lead acetate trihydrate are introduced. The modified acetate
method introduces water and acetic acid simultaneously to the metal alkoxides, with
the potential of causing localized hydrolysis of the metal alkoxides before they are
69
chelated. It is possible with careful solution preparation to avoid hydrolysis in both
methods though. if the lead acetate trihydratelacetic acid solution is dehydrated
before mixing with the metal alkoxides by heating it at a temperature slightly above
100°C. An indepth analysis has been completed to investigate the difference
between the acetate and IMO methods of chemical addition using 'H and 13C
nuclear magnetic resonance (NMR) spectroscopy [S]. Overall, the IMO solution
displayed less ester formation and less water is formed in solution. One way to
explain this result would be to note that the solution with a greater degree of
chelation would have less free acetic acid to react and form esters. This is
consistent with the proposal that metal alkoxides in solutions prepared using the
acetate method are more susceptible to hydration and consequently, local
inhomogeneities, by the simultaneous addition of the acetic acid and lead acetate
trihydrate together in solution-
4.4 Thermal Processing
The method of processing sol-gel derived P A thin films depends largely on
the thickness of the individual film layers. If the film is thin (< 0.1 pm), it can be fired
directly on a surface heated to 350-400°C until the organics have been removed
from the film, then annealed at a higher temperature [I]. For thick films in the range
of 5-1 0 pm prepared by a multi-layering process, it is preferable to have a larger
individual layer thickness. When these film layers (> 0.2 pm thick) are fired directly
after deposition at 350-40O0C, the buildup of internal stress due to removal of
organic components and the corresponding volume shrinkage usually causes
cracking. In light of this behaviour, it is desirable to know the temperatures at which
the various organic components are released from the film and the percentage
weight loss at any given temperature. From this, a thermal processing schedule can
be established-
4-4.1 Simultaneous TGAfDTA Analvsis
The results from a TGCVDfA analysis of a zirconium butoxide (80 wt.% in 1-
butanol)/titanium butoxide-based sol-gel prepared by the modified I MO method are
shown in Figure 4.4.1 .I. The solution was heated at a rate of 1 O°C/min. The TGA
TGA
Temperature ("C)
FIGURE 4.4.1 -1 : TGNDTA analysis of a butoxidabased IMO PZT sot-gel [21]
results show weight loss versus temperature. Most of the weight loss occurs below
a temperature of approximately 180°C. The derivative therrnogravimetric results
(DTG) combine the results from the TGA and DTA analyses to show rate of weight
loss versus temperature. Important temperatures in these results are 100°C and
285OC, at which the rate of weight loss peaks and the potential for failure of the film
layer increases. The peak in weight loss at 1 OO°C is due to the removal of free
acetic acid, water and alcohol and peak at 285°C corresponds to the decomposition
temperature for lead acetate [I 71.
The TGNDTA results shown in Figure 4.4.1 -2 are for the same PZT solgel
solution dried at a temperature of 150°C prior to TGAlDTA analysis. This increases
d
f n - fn 0 -
-
FIGURE 4.4.1.2: TGAiDTA results for a PZT sol-gel dried at 150°C
I
- I 1 I
- I I t
U) - I
UI DTG 1 1
c. - r *
cn - ; : - - 0 - a . u (D -
c I I 4 c 6 I I I I I 1 I I I I I I I
z - 1
- I I
7 -- I-
- - - I I
I I
0 200 400 6d0 800 1000
Temperature ("C)
the sensitivity to changes occurring at higher temperatures. In the TGA results. it
is important to note that weight loss is still occurring up to a temperature of -600°C-
Thus, one might expect that PZT films prepared using this solution will not
crystallize well when annealed at temperatures below 600°C. In fact. since the
temperature ramp rate in a furnace or an RTA is much greater than the ramp rate
of 1O0C/min and the hold time is not especially long, a temperature greater than
600% will be needed to sufficiently crystallize the film layer. This has been
confirmed in other work completed at Queen's University [21]. The DTG results in
this second case show another broad relatively flat peak in rate of weight loss
centered at approximately 400°C which was not obvious in the previous analysis
(Figure 4.4.1.1). This peak is due to the pyrolysis of the remaining organic
components in the film. One further point of interest is that TGAIDTA analyses
completed on solutions with zirconium propoxide (70 wt% in 1-propanol)/titanium
butoxide and zirconium propoxide (70 wt.% in 1 -propanol)/titanium isopropoxide
precursor combinations yield similar results.
4.4.2 Furnace Processing
Based on the above TGNDTA analysis, a processing schedule was
established for thermal processing. Individual layers were dried on a hot plate at
250 OC for 30 seconds, fired at on a second hot plate at 400 O C for 15-60 seconds
and annealed in a box furnace at 650 OC for 2 minutes. A final anneal of the
multilayer films for 15-30 minutes at 650 OC in a box furnace was performed. It was
important to perform the full processing schedule for each film layer. Although
attempts have been made to anneal up to four dried and fired film layers at a time,
these film layers were much thinner and diffusion distances in the film were
comparable. Without full processing of each film layer, residual organics tend to get
trapped in the film (as the diffusion distances are too large). One way to
compensate for this is to process for long periods of time. However, grain sizes in
the film then become unduly large and the film's dielectric, fernelectric and
piezoelectric properties suffer. The chosen schedule allows most of the free
organics to be released prior to the decomposition and removal of lead acetate,
which reduces the possibility of film cracking. An intrinsic stress relief mechanism
inherent in PZT processing is the shrinkage of the film as the lead acetate
decomposes to the more stable lead carbonate and acetic acid is evolved. Since
most stress relief processes are thermally activated, the stress relaxation time of the
film can be reduced by increasing the temperature. This prevents the buildup of
high internal stress in the film. Thus, firing the film at a temperature of 400°C (which
is significantly higher than 285OC) reduces the possibility of film cracking. This
intermediate temperature also facilitates the release of more organics, but not an
excessive amount as to promote film cracking. For this reason, firing at much
higher temperatures usually yields deleterious results. The films are fired at 650%
to properly crystallize the layer when past the point at which weight loss still occurs.
The final post-anneal for an extended period of time results in a slight improvement
in the dielectric properties of the film.
4.4.3 Ra~id Thermal Processinq
The RTA schedule used was very similar to that for furnace processing.
Individual film layers were dried on a hot plate at 250°C for 30 seconds, fired on a
hot plate at 400°C for 15 seconds, then processed in the RTA using the processing
schedule outlined in Table 4.4.3.1. One advantage of using RTA processing is that
the anneal times are cut in half and no final anneal is necessary. This minimizes
the damage to layers under the PZTfilm. This method of processing is suitable for
manufacturing environments.
TABLE 4.4.3.1 : RTA processing schedule
Act ion
Delay
11 Hold I 60 s I 650
- -
Hold
Ramp
Hold
Cool Down I 60 s I < 300
Hold Time I Ramp Rate
15 s
4.5 Glancing Angle X-ray Diffraction
The ABO, perovskite phase is the preferred crystal structure for PZT thin
films as it is the phase with strong ferroelectric and piezoelectric properties.
However, depending on the processing conditions, PZT thin films may crystallize in
Temperature PC)
< 200
- --
I 0 s
100 "CIS
15 s
--
285
400
400
an undesirable phase. In the literature on P A thin films, reference is often made
to a pyrochlore type structure that is observed as an intermediate phase during
fabrication. In general terms, pyrochlores are ternary metallic oxide with properties
isostructural with the mineral pyrochlore [(NaCa)(NbTa)O,fl and the general
formula &6,06X [22]. In oxide pyrochlores such as PZT, X is an additional oxygen
atom which is loosely attached to the lattice and often escapes to leave a non-
stoichiometric composition. Most pyrochlores have Curie temperatures at sub-zero
temperatures, and therefore do not exhibit ferroelectric or piezoelectric properties
at room temperature. In PZTfilm fabrication, a cubic intermediate phase has been
reported and referred to as pyrochlore. This is not strictly true, however, as no
verification has been provided on the precise crystal structure of the intermediate
phase. In this thesis the term pyrochlore phase is used for consistency with the
general sense used in literature; the phase observed has no ferroelectric or
piezoelectric properties and is not the perovskite s?ructure.
The JCPDS (Joint Committee on Powder Diffraction Spectra) file for P A is
included in Appendix 2. The peaks in the GA-XRD results for the PZT films were
identified using this standard.
4.5.1 Choice of Alkoxide Precursor
GA-XRD results for coatings processed from solutions based on the IMO
method consistently indicate a significant amount of the non-piezoelectric
pyroc h lore phase in p ropoxidelp ropoxide-based coatings, little or no p yroc hlo re
phase in the butoxidelpropoxide-based coatings and no pyrochlore phase in the
butoxidelbutoxide-based coatings, as shown in Figure 4.5.1.1. The pyrochlore
phase peaks are identified by (Py) and the perovskite phase peaks are identified by
their (hkl) crystallographic orientation. Variations in the perovskiie peak heights in
the results shown are due to texturing in the films. The samples shown were
processed with methanol as the solvent and had (a) no cracking on the 15'" layer,
(b) cracking on the 81 layer, and (c) cracking on the 3d layer. These films cracked
at a lower thickness compared to most of the samples that were due to a slower
spin speed of 1500 rpm leading to thicker individual layers. The fact that the
FIGURE 4.5.1 .l: Variation in crystallization with choice of alkoxide precursor: (a) Zr butoxidemi butoxide, (b) Zr butoxidem isopropoxide and (c) Zr butoxideni isopropoxide precursors.
zirconium butoxide (80 wt.% in 1-butanol) I titanium butoxide-based films do not
contain any measurable amount of pyrochlore phase indicate that they probably will
have the best dielectric and ferroelectric properties, which would make them the
preferred choice for alkoxide precursor.
4.5.2 Choice of Solvent
Films processed using the modified IMO method and zirconium butoxide (80
wt.% in l-butanol) and titanium butoxide as alkoxide precursors do not show an
appreciable difference in crystallization when different solvents are used. The films
for which the GA-XRD results are shown (Figure 4.6.2.1) are all in the range of 1.8 -
2.0 pm thick. No pyrochlore phase is observed, regardless of the solvent used.
FIGURE 4.5.2.1 : Effect of solvent on crystallization of the P A film. The solvent used in the films were (a) ethanol (b) methanol (c) water and (d) butanol.
4.6 Scanning Electron Microscopy
Films processed by the modified IMO method, with the various combinations
of aikoxide precursors and methanol as a solvent are shown in Figure 4.6.1. Films
deposited from a sol-gel solution with zirconium propoxide and titanium
isopropoxide (Figure 4.6.1 a) tend to crack at the fewest layers, with an approximate
upper limit of 3 pm at 0.2 pmllayer. The next films to crack were those containing
zirconium butoxide and titanium isopropoxide (Figure 4.6.1 b), with an approximate
upper limit of 4 pm at 0.2 pmllayer. An upper limit has not been established for the
zirconium butoxide and titanium butoxide-based coatings, as crack-free coatings of
greater than 5 pm thickness have been produced (Figure 4.6.1~). The pictures of
the coatings shown in Figure 4.6.1 were taken using a scanning electron
microscope and have a 10000~ magnification.
FIGURE 4.6.1 : (a) Zr propoxideni isopropoxide 2.8 pm thick film (b) Zr butoxidemi isopropoxide 3.5 pm thick film (c) Zr butoxide Ki butoxide 4.4 pm thick film
All of the films appear dense and individual film layers are not observable, which are
both desirable results. It is difficult to say whether the columnar structure seen in
the SEM images is a crystallographic result, or a result of the cleaving of the
samples to obtain an edge for examination. As expected, much thicker films were
produced using the butoxide/butoxide-based P A solgel, due to greater solution
stability and although not confirmed, probably fewer local inhomogeneities.
SEM images for Alms processed with different solvents exhibit no appreciable
difference in density, appearance or thickness. The choice of solvent does not
affect the maximum attainable film thickness-
4.7 Electrical Characterization
Samples processed using the IMO method consistently have a permittivity
of 11 00-1 300 and a loss tangent of 1-2% for the zirconium butoxide/titanium
butoxide and zirconium butoxide (80 wt.% in 1-butanol) I titanium isopropoxide-
based coatings, depending on the thickness of the coatings. The dielectric
response for the films shown in Figure 4.7.1 are reasonably consistent over the
range of 1 kHz to 1 MHz. The zirconium propoxide (70 wt.% in I -propanol)/titanium
isopropoxide coatings were of poorer quality, with dielectric constants of 600-900
and loss tangents of 3.5%. Up to 2 pm thick films wlh dielectric constants of 1000-
I 100 and loss tangents of 2-3% have also been produced using the modified
acetate method of chemical addition when zirconium butoxide (70 wt.% in 1-
butanol) and titanium butoxide are used as alkoxide precursors. However, it is
difficult to obtain consistent results using this method. When either titanium
0 200 400 600 800 1 000
Frequency (kHz)
FIGURE 4.7.1 : Frequency response of (a) Zr butoxidemi butoxide based film (b) Zr butoxidemi isopropoxide-based film (c) Zr propoxidem isopropoxide-based film
isopropoxide or zirconium propoxide are used, it becomes difficult to avoid the
formation of transient gels in solution during the solution preparation process, with
a corresponding decrease in film quality. This is consistent with the proposal that
metal alkoxides in solutions prepared using the acetate method are more
susceptible to hydration and consequently, to local inhomogeneities formed by the
simultaneous addition of the acetic acid and lead acetate trihydrate together in
solution. The IMO process is a preferable method of chemical addition.
4.8 Piezoelectric Characterization
The impedance response for the thickness mode resonance of the PZT films
was anaIyzed using an HP8753D network analyzer over the range of 150-700 MHz.
The results for resistance and reactance normalized against frequency are shown
in Figures 4.8.1 and 4.8.2 for a 5 pm thick zirconium butoxide (80 wt.% in 1-
butanol)ltitanium butoxide-based film. The film was produced on a 500 yrn thick
platinized Si(1ll) wafer from a sol-gel solution prepared using the modified IMO
method. Electrodes were formed by sputtering a top layer of platinum on one of the
PZT films and evaporating chrome-gold electrodes the other. Two hundred micron
square electrodes were cut out by laser machining using a Kr excimer laser (231.
The films was poled at 180 O C and 6 Vlum for 20 minutes prior to laser machining.
The effect of the substrate on the piezoelectric response of the P A film is
to significantly broaden the thickness mode resonance envelope and to constrain
the response to the frequencies of thickness mode resonance in the substrate. The
result is the "diffraction grating-liken features in the impedance response. A fit to the
impedance response of the films may be obtained from a one-dimensional model
of thickness mode vibrations for a piezoelectric film on a substrate developed by
Lukacs et al. [24]. The fits from this model, which uses Newton's force laws and the
linear constitutive piezoelectric equations, are shown in Figures 4.8.1 and 4.8.2.
Five complex material parameters may be obtained from the fit to the data, which
yields a total of ten material constants. One important feature to note is the
dispersion in the material parameters over the frequency range (i.e. the material
constants have a slight frequency dependence). Because the resonance is quite
1e+5 2e+5 3e+5 4e+5 5e+5 6e+5 7e+5 8e+5
Frequency (kHz)
FIGURE 4.8.1: Impedance response of a 5 urn thick P A film with Cr-Au electrodes
1e+5 2e+5 3e+5 4e+5 5e+5 6e+5 7e+5 8e+5
Frequency (kHz)
FIGURE 4.8.2: Impedance response of a 5 urn thick PZT film with Pt electrodes
broad, dispersion has a significant effect. This subject has been examined
extensively in other work [25]. In this work, the five peaks in the center of the
resonance envelope were fit. with the results summarized in Table 4.8.1. The
effects of dispersion can be seen away from the resonance, where the peaks in the
fit are shifted with respect to the peaks in the data. The material parameters are
assumed to be frequency-independent in the model, and cannot compensate forthe
effects of dispersion.
The results from the fit for the ten material constants are summarized in
Table 4.8.1. The electromechanical coupling constant (kJ is comparable to that
obtained from the solgel composite coatings which yield k, values in the range of
0.25-0.35 [23]. However, the thin film &value is less than the bulkceramic coupling
coefficient, which typically has a magnitude of 0.6-0.72 [26]. This is primarily due
to mismatch between the P A film and the substrate which leads to strain and
constraint of motion in the film by the substrate. A preferred crystallization direction
can be induced in the P A in keeping with the crystallographic orientation of the
substrate. In thicker films (s 1 pm), this effect of substrate clamping on the film is
reduced, but the strain induced by the substrate still has the effect of lowering the
magnitude of k, 1271. Further improvements could be made in the poling process,
which has not been optimized. However, the parameters obtained from the analysis . of the piezoelectric response of the PZT thin films demonstrate the adequacy of
PZT sol-gel films for use in piezoelectric devices, and in particular, a thin film
piezoelectric transformer.
TABLE 4.8.1 : Material parameters for a PZT film on a Si (1 11) substrate
Dasfic stift.lass (Pm (Wm3 (2289 & 0.004) xlb + i(252 i 0.04) xl d (1.841 k 0.003) x l b + i(1.08 f 0.05) xld a a s t i c ~ ( ~ e ) 4'' (Nfd) (1.789 k0.001) x?O1' + i(6.06 k0.17) xl0' (1.783 e0.001) x10" + i(5.86 k0.21) xlff S(ray Impedance c (Q) (220 k 0.09) + i(O.23 r 0.1 0) (4.30 20.07) - i(0.036 k 0.082)
4.9 Summary
A stable solution chemistry and a consistent thermal processing route have
been developed to produce multi-layer solgel lead zirconate titanate (PZT)
piezoelectric coatings of high electrical quality. Up to 5 pm thick fully perovskite and
piezoelectrically active crack-free coatings with permittivity of 1100-1 300 and loss
tangents of 1-2% can be produced with careful attention to the choice of titanium
and zirconium alkoxide precursors, the choice of solvent, the method of solution
preparation, and the thermal processing schedule. The optimum sol-gel recipe
used is found in Appendix 3. The films were deposited on platinized silicon,
spinning for 30 seconds at 3000 rpm. Individual layers were dried on a hot plate at
250 OC for 30 seconds, fired on a second hot plate at 400 OC for 15-60 seconds and *
annealed in either a box furnace at 650°C for 2 minutes or an RTA at 650°C for 1
minute. The film properties meet the design specifications for the production of a
thin film piezoelectric transformer.
References
G. Yi and M. Sayer, Proceedings of the 8" International Conference On the
Applications of Ferroelectrics. (1 992).
G. Yi, Z. Wu and M. Sayer, Journal Applied Physics. 64,2717 (1988).
R.W. Schwartz, R.A. Assink and T.J. Headley, MRS Symposium
Proceedings, 243, 245 (1 992)-
S-RGurkovich and J.B. Blum, Ferroelectrics, 62, 189 (1 985).
K.D. Budd. S.K. Dey and D.A. Payne, Proceedings of the British Ceramic
Society, 36. 107 (1 985).
Y.L. Tu and S.J. Milne, Journal of Materials Science, 30, 2507 (1995).
G. Yi and M. Sayer, Ceramic Bulletin, 70. 1 173 (1 990).
L.E. Sanchez, S-Y. Wu and I.K. Naik, Applied Physics Letters, 56, 2399
(1 990).
H. Sakai and T. Tuchiya, Journal of the Ceramic Society of Japan, 99,614
(1991).
M. Sayer, L. Zou, B. Leclerc, M. Lukacs, T. Olding and J.H. Schloss, MRS
Symposium Proceedings, 493, 391 (1 997).
M. Lourdes-Calzada, R. Sirera, F. Carmona and B. J. Jimenez, Journal of
the American Ceramic Society, 78, (7), 1802 (1995).
Y. L. Tu and S. J. Milne, Journal of Materials Research, 10, (12), 3222
(1 995).
Y. L. Tu, M. L. Caldaza, N. J. Phillips, and S. J. Milne, Journal of the
American Ceramic Society, 78, (2), 441 (1 996).
R. W. Schwark, T. L. Reichert, P. G. Clem, D. Dimos and D. Liu, Integrated
88
Ferroelectrics. 18, 275 (1997).
D. A. Barrow, T. E. Petroff and M. Sayer. Surfaces and Coatings
Technology. 76, 1 13 (1 995).
D. A. Barrow, T. E. Petroff. R. Tandon and M. Sayer. Journal of Applied
Physics, 81. 878 (1 997).
G. Yi. Ph.D Thesis, Queen's University (1993).
J. Livage, M. Henry and C. Sanchez, Progress in Solid State Chemistry, 18,
259 (1 988).
G. Yi and M. Sayer. Journal of SoCGel Science and Technology, 6, 65
(1 996).
D. C. Bradley, R. C. Mehotra and D. P. Gaur, Metal Alkoxides. Academic
Press, New York (1 978).
B. Leclerc, MSc. Thesis, Queen's University (1 999).
M. Sayer and C. V- R. Chivukula, "Ferrroelectric and Piezoelectric Thin
Films" Chapter in Handbook of Thin Film and Processing Technology
Boston, IOP Publishing (1 995)
M. Lukacs. PhD. Thesis (1 999). in press.
M. Lukacs, T. Olding, M. Sayer, R. Tasker and S. Sherrit, Journal of Applied
Physics, 85, 5 (1999).
S. Sherrit, Ph.0 Thesis. Queen's University, Kingston. Canada (1 997).
H.D. Chen et al., Journal of the American Ceramic Society, 79, 8. 2189,
(1 996).
Piezoelectric Ceramics: Product Catalogue & Application Notes, Sensor
Technology Limited (BM Hi-Tech Division), Collingwood, Canada (1 991).
5. Transformer Production
Several key issues of materials selection for transformer production have
already been addressed. First, platinum electrodes and a platinized silicon
substrate have been chosen in order to withstand the thermal processing required
by the firing, annealing and poling steps. The fact that the transformer is processed
on a platinized silicon substrate ensures compatibility of the device with
conventional silicon-based technology. Second, the PZTsol-gel has been modified
to minimize the effects of residual stress resulting from the thermal mismatch
between the PZT layers of the transformer and the substrate on which it is
produced, while obtaining the required thickness of the PZT resonator layers. Third,
using a sol-gel process ensures that the deposited P A layers have low surface
roughness to promote good electrical contact of the electrodes. In this chapter, the
issues related to the practical design and production of the transformer are
addressed. The response of a radial mode piezoelectric transformer is investigated,
and possible applications are discussed.
5.1 Practical Design
A major factor which influences the production of a thin film piezoelectric
transformer is the issue of substrate clamping (acoustic loss to the substrate).
Processing sequences for producing this device with the piezoelectric layers free
from the substrate have been examined and found to be relatively complicated, due
to the fragile film layers. Also, to remain focussed on the original intent of
producing the device for use in microelectmnic applications, the transformer must
be mounted in some fashion on a substrate. Some means of reducing substrate
clamping while still having the transformer mounted on a substrate must be found.
Two different approaches could be used to reduce substrate clamping. The
first approach would be to produce the transformer on a silicon substrate with no
prior modifications. Then, the substrate could be altered in some way to minimize
the effects of substrate clamping, while the transformer is suitably protected in some
way. A possible scenario is shown in Figure 5.1 -1. Wet chemical etching, reactive
Platinum Piezoelectric electrode Transformer
Etched Region
FIGURE 5.1 .I : Reducing substrate clamping via substrate etching
ion etching (RIE), or laser machining techniques could be used to accomplish this.
The other approach would be to produce the transformer as before, then employ
"flip-chip" techniques by which the transformer is bonded to a second substrate on
a structure designed to reduce substrate clamping (Figure 5.1 -2). The transformer
Alumina
Platinum Supporting electrode
Platinum Pieroelectric Piezoelectric
Platinum Transformer electrode Transformer electrode
-4 4 -k -
Si(ll1) Alumina
FIGURE 5.1 -2: Flip-chip technique for reducing substrate clamping
would then be released from the first substrate in some way. A proven technique
already exists for releasing a PZT thin film from a platinized silicon substrate. A
simple structure on which to flip the transformer for the purpose of minimizing
substrate clamping would be a post. The result would be the "mushroom"
transformer shown in Figure 5.1.2(b).
The problem with both of these methods is that electrode connections are
difficult to make, and the fact that the methods are technically difficult complicates
any further understanding of transformer operation. A simple transformer design
with no attention given to minimizing the effects of substrate clamping would be
more practical as a first step towards a viable transformer. Three possible
92
variations of the two layer transformer analyzed in Chapter 2 are shown in Figure
5.1.3. The main difference in these designs involves the manner in which the
; + Pt Electrodes 4. -- - C
\
Output Electrode Driving Electrode
Common Ground
r Electrode
FIGURE 5.1 -3: (a)-(c) Two layer and (d) one layer transformer designs
middle transformer electrode connection is made. A one layer alternative
transformer design is also shown. In this transformer, piezoelectric vibrations are
initiated at the driving electrodes and, assuming electrical isolation, an electrical
signal is obtained at the output electrodes. This type of design has not been
modelled, but is similar in its operating principle to a bulk piezoelectric transformer
93
operating in width shear mode studied previously [I]. It is conceivable that all these
designs could be produced via conventional photo-lithographic and wet chemical
etching techniques.
5.2 Production Process
5.2.1 Two Laver Transformer
The process first used to produce the two layer transformer structures
illustrated in Figure 5.1.3(a)-(c) is shown in Figure 5.2.1. The process is specific to
the design in Figure 5.1.3(a), but can be used for the other two designs as well, with
some minor changes. However, some major difficulties have been encountered in
this process with: (1) poor platinum adhesion and (2) undercutting of the electrodes
during the etching process. This has lead to some adaptations to the process.
- Deposit PZT - Spin on photoresist layer - Expose through mask & develop - Sputter Pt electrode - Remove photoresist
- Deposit PZT layer - Spin on photoresist layer - Spin on photoresist layer - Expose through mask& develop - Expose through mask & develop - Sputter Pt electrode -Wet chemical etch - Remove photoresist - Remove photoresist
FIGURE 5.2.1 : Transformer Production Process
The first major difficulty with the production process outlined in Figure 5.2.1
is poor adhesion of the middle platinum electrode. When a photoresist mask is
used, it is difficult to fully remove all of the photoresist in the developer and this
causes problems with platinum adhesion. One way to promote adhesion would be
to heat the substrate before sputtering. However, the substrate cannot be heated
past 1 1 0-120°C, as this would hard-bake the photoresist to the point where it could
not be removed. The best solution would be to perform an oxygen sputter-etch on
the sample prior to platinum deposition. As the sputter-etch process was not
operational on the VacTec sputtering unit, a plasma etch was attempted using the
Edwards 306A evaporator. The plasma produced in this unit was too weak to have
a significant effect. To bypass this difficulty until such time as a oxygen sputter-etch
process is available, a metal (shadow) mask was used directly in the sputtering
process, avoiding the photoresist mask option altogether. This results in reduced
edge definition of the electrode, but solves the problem of platinum adhesion. The
substrate temperature is not limited to low temperatures, as photoresist is not used
and can be raised to 200-300°C, removing residual organics from the sample
surface and promoting platinum adhesion.
A new difficulty was encountered when a shadow mask was used. Bronze
was initially used as the material for the shadow mask. However, it was found that
a significant amount of debris was left behind after sputtering at the edge of the
mask which caused problems with the adhesion of the platinum electrode and the
PZT film quality when depositing the second PZT layer. When a new mask made
of tantalum was used instead, the amount of debris was significantly reduced and
95
fewer problems were encountered in depositing the second PZTlayer. It is possible
that the edges of the bronze mask were being sputtered during the process,
whereas the tantalum mask edges were not.
The second major problem in the production process outlined in Figure 5.2.1
is related to the etching process. It was found that etching to the edge of the middle
and top electrodes was deleterious in that the etch undercut the electrode which
then YIoppedn, shorting to the electrode below. To correct for this problem, the
transformer structures were etched to a reasonable distance outside the middle
electrode edge and a small hole was etched to the middle electrode. The modified
structure for Figure 5.1.3(b), for example, is illustrated in Figure 5.2.2.
PZT Layers - --_ Pt Electrodes
-, \, '-.
\, a '..
FIGURE 5.2.2: Modified transformer structure
5.2.2 One Laver Alternative Transformer
The process for producing the one layer transformer in Figure 5.1.3(d) is
relatively simple. First, a PZT layer of an appropriate thickness is deposited. Since
the top electrode is deposited after all high temperature processing has been
96
completed, a chrome-gold electrode may be used instead, which has better
adhesion than the platinum electrode. A photoresist layer is deposited according
to the usual procedure, and exposed using a mask such as the one shown in Figure
5.2.3(a). Cr-Au electrodes are then deposited using the Edwards 306A evaporator,
and the photoresist layer is removed. A second photoresist layer is then deposited
on the film and patterned using the mask shown in Figure 5.2.3(b), with the
photoresist dots on top of the electrodes. The structure is then etched.
FIGURE 5.2.3: Mask:
(b)
for one layer transformer
The transformers produced by this method were poled at 6 Vlpm and 160°C for 15
minutes. No difficulties have been encountered in using this process.
5.3 Transformer Response
The input to output voltage ratio was determined as a function of frequency
for the one layer and two layer thin film transformer designs, as well as for a two
layer bulk ceramic transformer. The results were then compared to model
predictions.
5-3.1 Two Laver Thin Film Transformer
The two layer thin film transformers were the most difficult to produce with
consistent results. It was a challenging task to produce any of the three designs
shown in Figure 5.1.3 wlhout shorts between the resonators. While the shorts
were removed during the initial stages of poling due to local blowouts, this reduced
the electrical quality of the transformers. The response shown in Figure 5.3.1 is
from a transformer structure of the type shown in Figure 5.1.3(c), with an outer
radius of 5.2 * 0.2 mm and individual PZT layer thickness of 2.0 * 0.1 pm. The
inner radius of the output resonator was 0.8 * 0.2 mm. The transformer structures
shown in Figure 5.3.1 (a) and (b) yielded a similar response.
Frequency (kHz)
FIGURE 5.3.1 : Two layer thin film transformer response
According to the model predictions in Chapter 2, a peak in the voltage ratio was
expected in the frequency range of 200-300 kHz, which was not seen. This was
primarily due to the high dielectric loss of the piezoelectric layers, which was
determined to be greater than 10% in the 200-300 kHz frequency range. The model
developed in Chapter 2 demonstrates that the voltage ratio is highly dependent on
both the dielectric loss and mechanical quality of the resonators. The reason for the
high dielectric loss is most likely that the sputtered platinum middle electrode was
of poor quality. There are significant stresses exerted on the electrode during film
processing which degrade the electrode. In comparison, a sputtered platinum
electrode without a PZT layer on top of l yields a reasonable dielectric loss of 1-2%.
The peak in voltage ratio centred at 4.6 & 0.1 MHz was not predicted in the
model of a two layer free standing transformer. When the 500 t 10 pm silicon
substrate is included as part of the transformer structure, it is seen that this
resonance corresponds to the first thickness mode resonance in the substrate. The
resonant frequency for a thickness mode resonator is:
The elastic stiffness G~~ of SiO, is 47 GPa and the density p of SiO, is 2330 kg/m3
[2]. This yields a resonant frequency of 4.5 MHz, depending on the value of the
elastic stiffness. The peak in the voltage ratio is centred at a slightly higher
frequency, which can be accounted for by the loading of the substrate by the
piezoelectric layers. The voltage ratio continues to decrease above 7 MHz, as
predicted. The significant voltage gain due to the thickness mode resonance may
be usefully employed in some applications.
5.3.2 Two Laver Bulk Ceramic Transformer
A two layer bulk ceramic transformer was produced using two PLT disks
supplied from BM HiTech. both of which were 3.23 & 0.01 mm thick and 4.90 * 0.05
cm in diameter. The disks were characterized using the Piezoelectric Resonance
Analysis Program developed by Tasker [3]. The results from that analysis are
shown in Appendix 4. The disks were then bonded together using silver epoxy with
a thin silver foil sandwiched between the disks providing access to the middle
electrode. The transformer response shown in Figure 5.3.2 is relatively consistent
with the model developed in Chapter 2.
0.0
0 20 40 60 80 100 120 140 160 180
Frequency (kHz)
FIGURE 5.3.2: Two layer bulk ceramic transformer response
The two peaks in the voltage ratio at 51 * 0.5 kHz and 123 * 2 kHz are slightly
shifted down in frequency from the first and third harmonic resonant frequencies of
the disks at approximately 53.3 kHz and 130 kHz respectively. This is due to the
loading of one disk by the other disk. The voltage gain of the transformer peaks at
approximately 1.8 in the first resonance, which is an order of magnitude less than
100
the model prediction. An explanation for the discrepancy between the two results
is that the silver epoxy bonded middle electrode introduces some dielectric and
mechanical loss which degrades the response. An additional factor contributing to
the discrepancy is that the control of frequency on the function generator is
relatively coarse and the resonance of the device is quite narrow in frequency. It
is likely that the data taken did not adequately describe the full resonance envelope.
5.3.3 One Laver Thin Film Transformer
The one layer transformer structure shown in Figure 5.1 -3 (d) was produced
with a driving electrode radius of 1.3 i 0.1 mm, and inner and outer radii of the
output electrode of 1 -6 i 0.1 mm and 2.6 & 0.1 mm respectively. The PZTlayerwas
2.0 * 0.1 pm thick and the silicon substrate was 500 * 10 pm. as in the case of the
two layer thin film transformer. The baseline of the transformer response is quite
0 1000 2000 3000 4000 5000 6000 7000 8000
Frequency (kHz)
FIGURE 5.3.3: One layer thin film transformer response
similar to that of the two layer thin film transformer in the range of 1-7 MHz, with the
thickness mode resonance at the frequency of 4.6 i 0.1 MHz. The peak voltage
101
gain of 12.9 is greater than that observed in the two layer thin film transformer
response, due to the lower dielectric loss of I-2%. A peak in the voltage ratio of
2.27 was observed at approximately 400 kHz. corresponding to the fundamental
radial resonance in the PZT layer. This transformer has not been modelled, but is
similar in concept to the two layer transformer, with a significant difference in how
mechanical vibrations are transferred from the input to output resonator. This
voltage gain and load characteristics can readily be manipulated by changing the
electrode area of the input and output resonators and the thickness of the P A
layer. The performance of the one layer thin film transformer is promising.
5.4 Applications
Bulk piezoelectric transformers are currently used in high voltage power
supplies for LCD displays of notebook computers and in othemigh voltage, low
current power source applications, such as EEPROM (electrically erasable
programmable read-only memory) and electrostatic devices [4]. Thin film
piezoelectric transformers are of interest as part of integrated dc-to-dc voltage
converters (Figure 5.4.1), both in step up and step down applications. The
operating voltage of many integrated circuits is projected to decrease below
conventional battery voltages of 1.3 V and step down voltage converters will be
required to avoid costly adaptations to circuit design and architecture. On the other
hand, it is often convenient to design circuits to operate at voltages greater than 1 -3
V and step up circuits would be useful. The efficiency and effectiveness of such
circuits will depend not only on the characteristics ofthe transformer, but also on the
efficiency and real estate necessary for the driving and rectifying circuits. It is
estimated that a voltage ratio of 2:1 and 1 2 would be required for the step down
and step up applications, which is quite attainable.
Power 1 - Oscillator - Transformer - Rectifier --- Power 2
FIGURE 5.4.1 : Schematic of a thin film transformer voltage converter
Other applications could be found in the step up or step down in the voltage
of computer clock pulses, which are of an appropriate frequency for use with the
thin film piezoelectric transformer. There has been some interest in using the
transformer in inkjet printers, although the design specifications are not known.
References
1. N. Wakatsuki, M. Ueda and M. Satoh, Japanese Journal of Applied Physics
Part I, 32, 5B (1993).
2. G.W.C. Kaye and T.H. Laby, Tables of physical and chemical constants,
Longman, London, UK, 15th edition, 1993.
3. Tasi Technical Software, Piezoelectric Resonance Analysis Program,
http:/Ewww.canlink.cor;l/tasi/tasi.html, Kingston, Ontario, Canada (1 998).
4. K. Uchino and B. Koc, Piezoelectric Materials: Advances in Science,
Technology and Applications, ed. by C. Galassi and M. Dinescu, Kluwer
Academic Publisher, Netherlands, in press.
6. Conclusions
The objective of this thesis was to design and produce a thin film
piezoelectric transformer for silicon integration with suitable operating parameters
for device applications. A specific interest was to produce a transformer with an
output to input voltage ratio and operational efficiency suitable for use in a dc-to-dc
voltage converter which would step up and step down the voltage of a 1.3 volt
battery.
Model predictions for a two layer thin film piezoelectric transformer indicated
that a process for depositing PZT films with thicknesses greater than 2 pm was
necessary to attain a suitable level of transformer operation. Different sol-gel
processes capable of producing the piezoelectric resonator layers were evaluated
based on their thickness limitations, electrical properties and piezoelectric
properties. The acetate process originally developed by Yi et el. [I] and improved
by Schwartz et al. [2] was chosen for production of the PZT resonator layers.
However, the maximum attainable thickness of PZT films produced using the
acetate process was limited by film cracking due to residual stress and local
inhomogeneities. To obtain a viable multi-layering acetic acid-based PZT soLgel
process, a careful investigation of the solution chemistry and thermal processing
was required, with a principal objective of reducing residual stress which leads to
cracking of the multi-layer film.
An important result achieved in this thesis was the development of an acetic
acid-based PZT sol-gel suitable for depositing high electrical quality thin films of
thickness greater than 5 pm. This result not only enables the production of a thin
film piezoelectric transformer. but also introduces a new level of perforrnance to
related applications in the field of ferroelectrics. PZT thin films have a variety of
applications as piezoelectrics, but the thickness of the films has to date been a
limiting factor. The thickness range of 5-1 0 pm has been difficult to achieve via thin
film processing and is outside the practical range of bulk ceramics.
Important adaptions which improve the performance of the aceticacid-based
PZT sol-gel beyond that reported by Yi [ l ] and Schwartz [2] are: (1) the use of
titanium butoxide instead of titanium isopropoxide as the alkoxide precursor, (2) an
extended wait time of three days before use and, (3) the annealing of individual film
layers with no final anneal of the film. The improvements made by Schwartz eta/-
[2], which included (I) changing the mixing order of the chemicals, (2) using
methanol as a solvent, and (3) substituting zirconium butoxide (80 wt.% in 1-
butanol) for zirconium propoxide (70 wt.% in 1-propanol) were also verified.
Schwartz attributed the improved perforrnance of the sol-gel exclusively to the
change in mixing order, so the separation of individual factors listed above
constitutes an advancement in understanding of the acetic acid-based PZT sol-gel
process. A similar conclusion in regard to these contributing factors has also
reached by B. Leclerc 131.
The development of a one-dimensional model of the impedance response
of a piezoelectric film on a substrate vibrating in thickness extensional mode by
Lukacs ef al. [4] has been an important advancement in the piezoelectric
characterization of thin films. Direct measurement methods currently used for
piezoelectric characterization usually determine a single material parameter over a
limited frequency range [5,6], whereas the impedance measurement approach
allows the determination of 5 complex parameters over an extended frequency
range. In this thesis, the characterization of PZT thin films by this latter method has
been accomplished for the first time, and is important not only in terms of
establishing the performance level of the thin film transformer, but also in
demonstrating the versatility of the method itself.
The production of a thin film piezoelectric transformer for silicon integration
is the most significant advancement reported in this thesis. The device is a novel
inductive component for mid scale microelectronic applications and is unique in its
capabilities as an on-chip transformer. The size of the transformer can be altered
to accommodate dimensional requirements and the transformer gain can be
manipulated, although repeatability and accuracy limitations have not been
established. The peak in voltage gain due to the thickness resonance in the silicon
substrate is an interesting feature which bears further investigation. It is
conceivable that the gain characteristics of this resonance could be manipulated by
changing the substrate thickness.
Further studies that are recommended are as follows:
1. A more extensive characterization of the piezoelectric response of the
butoxide-based PZT thin films should be completed with greater attention to
dispersion in the material parameters as a function of frequency.
2. The optimum poling condlions of the piezoelectric layers of the transformer
should be established in order to achieve a maximum response from the
transformer-
3. A model should be developed for the response of the one layer thin film
piezoelectric transformer to further understanding of its operation.
4, Transformers of different radius, electrode dimensions, and resonator
thickness should be produced and characterized in order to improve the level
of knowledge in transformer design.
5. A study of the operational efficiency of the one and two layer thin film
piezoelectric transformers with respect to the load characteristics of the
device should be completed.
In conclusion, the objectives set forth of designing and producing a thin film
piezoelectric transformer for silicon integration with operating parameters suitable
for device applications, have been achieved.
References
1. G. Yi and M. Sayer, Proceedings of the 8m International Conference On the
Applications of Ferroelectrics, (1 992).
2. G. Yi, L Wu and M. Sayer, Journal Applied Physics, 64,2717 (1988).
3. B. Leclerc, MSc. Thesis, Queen's University (1999).
4. M. Lukacs, T. Olding, M. Sayer, R. Tasker and S. Sherrit, Journal of Applied
Physics, 85, 5 (1 999).
5. K. Uchino, S. Nishida and S. Nomera, Japanese Journal of Applied Physics
Part 1, 21 (I 982).
6 J.G. Smits, W. Choi and A. Balloto, IEEE Transactions on Ultrasonics,
Ferroelectrics and Frequency Control, 44 (1 997).
Appendix 1 : Thin Film Transformer Analysis
Material Parameters
dens 1 : = 7600
(diameter)
(radius)
(diameter)
(radius)
(density)
(density)
(thickness)
(thickness)
(elastic compliance)
(elastic compliance)
(elastic compliance)
(elastic compliance)
(piezoelectric charge coefficient)
(piezoelectric charge coefficient)
(permittivity - constant stress)
(nermittivitv - constant stress)
elp33 :=elt33- 2- dl3 lL
slel I + sie12
klp := e13 l2
elp33-clpl l
-sle12 pois 1 :=-
slell
Defining frequency ranae
(elastic stiffness)
(elastic stiffness)
(permittivity - constant polarization)
(permittivity - constant polarization)
(permittivity - constant polarization)
(permittivity - constant polarization)
(radial mode electromechanical coupling coefficient)
(radial mode electromechanical w upling coefficient)
(Poisson's ratio)
(Poisson's ratio)
Calculating Bessel's hrnctions
- c l p l l 2
dens l
Calculatina Equivalent Circuit Parameters
. - Zlm .- thikl
i-wm -elp33-1r a1 2
Total Electrical Impedance of the Two Layer Device
Voltage applied at input of electrical port 1
v1:= 1
Current at electrical port 1
Current into transformer
Voltage across parallel combo of Za2 and NZA2Z2
Current through N2A2Z2
Power to N2A2Z2
Ppm := vpm -Ipm
Voltage V2 relative to V l
Voltage ratio
Appendix 3: Modified PZT Sol-Gel Recipe
Put 2.563 g of zirconium butoxide (80 wt.% in 1-butanol) in a bottle.
Add 1.597 g of titanium butoxide (drop-bydrop, not all at once). Do not
shake the bottle to mix the two alkoxides.
Add 2.407 g of acetic acid dropby-drop, shaking after every 0.4 g added
until solution is clear-
Mix in the ultrasonic mixer for five minutes.
Add 2.0 g methanol (can be added quickly) and shake until clear.
Add 3.98 g of lead acetate trihydrate. Heat on a hot plate at a temperature
of 85-95OC until lead crystals have been dissolved. It is important to heat
the solution at this relatively low temperature so that the lead acetate is
dissolved slowly in solution (i.e. it should take half an hour to an hour).
Add 0.5 g of acetic acid and shake.
Add 1.0 g of methanol and shake.
Add 0.7 g of water and shake.
Let solution sit for approximately a day.
Add 0.5 g of methanol and shake.
Add 0.25 g of acetic acid and shake.
Add 0.200 g of lead acetate trihydrate. Heat until lead crystals are
dissolved, as before-
Let solution sit at least two more days before use. This is important, as
greater film quality is observed when the solution is left for a period of time
before use.
Appendix 4: Bulk Ceramic Transformer Analysis
The piezoelectric response of one of the BM HiTech PZT disks is shown
below, with the resistance and reactance normalized against frequency. A fit to
the data with the diameter and thickness of the resonator set at 4.9 cm and 3.23
rnm respectively yields the results shown below.
These values of the material parameters of the disk were used in the two
layer transformer, with the following response:
s.104 1.1 1.5~10~ Frequency (Hz)
It is important to note that the true response of the transformer will include the
effects of bonding the two disks together using silver epoxy, with a silver foil
between the disks. This will introduce a significant dielectric loss to the device,
which will lower the voltage gain at resonance.