Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications

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Handbook of dielectric, piezoelectric and ferroelectric materials720

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24.1 Introduction

Piezoelectric thick films integrated on silicon substrate have attracted

considerable attention in recent years for potential applications in micro-

electromechanical systems (MEMS) devices, such as sensors and actuators

used in liquid, micro-pumps for micro-fluidic devices, ultrasonic transducers

for high-frequency medical imaging, and so on. Although much attention

has been paid to prepare thick piezoelectric films on silicon substrate

1–6

the past decade, integration of high-performance piezoelectric thick films

with a uniform thickness of up to 10µm on a wafer scale, either by deposition

or patterning, is still a main challenge in the micromachining of piezoelectric

MEMS. This is because piezoelectric films of such thickness are unfortunately

located within a technological gap between the processing capabilities of

thin film deposition techniques and the machining of bulk ceramics. In

addition, the dry etching of Pb(Zr,Ti)O

(PZT) film is not well developed

because there is no common halogenous gas that can form volatile compounds

with all three elements (Pb, Zr, Ti) to guarantee the residue-free removal of

film.

A composite thick film deposition technique is capable of producing thick

films at low temperatures

5–10

compatible with integrated unit (IC) technology.

The process is featured at producing composite slurry consisting of a PZT

ceramic powder and a PZT-producing sol. The composite slurry is then

deposited onto an electroded silicon substrate using a spin-coating process.

The added PZT powder can reduce the stress generated within the film

during processing and therefore a thick film can be achieved. A final heat

treatment as low as 650°C is sufficient to develop the piezoelectric films. In

this chapter, we will first introduce the novel composite route

7–9

to integrate

thick PZT films with uniform microstructure on silicon substrate and the

corresponding properties of the films; the design concept of piezoelectric

micromachined ultrasonic transducer (pMUT) based on thus-prepared thick

composite PZT films; and some key issues of the microfabrication process

Piezoelectric thick films for MEMS

application

Z WANG, W ZHU and J MIAO, Nanyang

Technological University, Singapore

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Piezoelectric thick films for MEMS application 725

of pMUTs. Finally, the performances of the fabricated pMUT devices have

been presented as an application example of piezoelectric MEMS.

24.2 A composite coating process for preparing

thick film on silicon wafer

The key technique of the composite coating process is how to prepare uniform

slurry by dispersing PZT powder (APC 850 from APC International Ltd) of

the desired size into PZT sol–gel precursor solution (Zr/Ti = 53/47). In this

section, the slurry preparation and film deposition will be described in detail.

24.2.1 Preparation of PZT xerogel and precursor solution

A chelate method

9,11,12

was used to prepare PZT xerogel. This ‘chelate’

method is essentially a molecularly modified alkoxide precursors approach

to create moisture tolerant metal–organic precursors by changing the molecular

structure of the precursor. The moisture-tolerant precursors created in this

way can be used to prepare stable sol and slurry. The raw chemicals used in

this process are listed in Table 24.1. Acetylacetone was used as a chelating

agent for titanium isopropoxide. The xerogel preparation procedure is

schematically shown in Fig. 24.1.

Firstly, the titanium isopropoxide is chelated with acetylacetone in a molar

ratio of 1:1 by heating and stirring the mixture at 80°C for 10min. Before

heating, titanium isopropoxide was slowly dripped into the acetylacetone

with a transfer to avoid the effect of moisture. This reaction is an exothermic

reaction so that white fumes were observed during stirring. The hydrolysis

speed of the modified titanium isopropoxide was greatly reduced after one

isopropoxide group was replaced with acetylacetone group. This step was

the key process for the xerogel preparation.

Secondly, the lead acetate trihydrate with 10 wt% excess was added into

the modified titanium isopropoxide solution. The Ti-Pb mixture was obtained

Table 24.1

Chemicals and solvents used for preparing PZT xerogel and

sol

Chemical Formula Molar mass Purity

Tetraisopropyl [(CH

)

CHO]

Ti 284.22 97%

titanate

Zirconium [CH

COCH==C(O–)CH

]

Zr 487.66 98%

acetylacetonate

Lead acetate (CH

COO)

Pb·3H

O 379.33 >99.5%

Acetylacetone CH

COCH

100.12 >99.0%

2-methoxyethanol CH

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by stirring at 120°C for 10mins. After lead acetate trihydrate was added, oil-

like coacervation was observed.

Thirdly, the zirconium acetylacetonate was dissolved into the Ti-Pb mixture.

By stirring at 200°C for 30 min, a yellow clear sticky solution was obtained.

After that, the sticky solution was placed into a vacuum drying oven to

remove volatile products under reduced pressure at 100°C for more than

4h. The xerogel, a light yellow coloured powder, can thus be obtained. The

xerogel is insensitive to moisture and can be steadily stored for very long

time. By dissolving 10–40 wt% xerogel into 2-methoxyethanol solvent, PZT

precursor solution can be obtained.

24.2.2 Preparation of uniform slurry

High-energy ball milling is effective in getting well-dispersed slurry.

7–9

The

preparation procedure is summarized in Fig. 24.2. First, commercially available

PZT powders (APC 850) were high-energy ball milled to get the desired

particle size. Secondly, a selected dispersant was added to the milled powders

to get the surface-modified powders. The smaller the powder, the more

important this procedure. Afterwards, PZT precursor solution was added to

these surface-modified powders and mixed by further ball milling. Finally,

the resultant uniform slurry was ready for further processing, such as spin

coating, tape casting, screen printing and molding. The recipe for the slurry,

including the concentration of xerogel solution and powder to solution mass

ratio, depends on the further processing method employed. For our convenience,

the recipes for the slurry were given four numbers with regard to the above

Mixing Ti-isopropoxide with

acetylacetone at 80 °C for 10 min

Dissolving Pb acetate into the above

solution at 120 °C for 10 min

Mixing Zr-Acac into Pt-Ti mixture at

200°C for 30 min

Removing the volatile contents in

vacuum at 100 °C for 4 h

24.1

Flowchart of PZT 53/47 xerogel preparation process using

chelating method.

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Piezoelectric thick films for MEMS application 727

two important parameters. For example, in 3025, the first two numbers

represent the concentration of the xerogel solution

in weight percent, i.e. 30

wt%, and the last two numbers represent the mass ratio of the added PZT

powder to xerogel solution, namely 2 to 5.

24.2.3 The features of the composite process

Figure 24.3 outlines the remarkable features of this route. All the features

result from the basic characteristic of this technique, i.e. the combination of

the sol–gel precursor and powder. If the calcinated sol–gel precursor has the

same composition as the powder, we call the mixture a homogeneous matrix;

therefore, the resulting material will have single composition. On the other

hand, if we mix the precursor solution and powder with different compositions,

a multiphase composite will be produced from the heterogeneous matrix.

With this method, a uniformly distributed multi-phase composite can be

obtained easily.

Moreover, if nanosized powder is used, the matrix should exhibit all the

merits of the nanopowder and sol–gel precursor. We can find different

applications of this route by utilizing its respective merits. For example, by

using the uniformly dispersed nanopowders in the sol–gel solution we could

Commercial PZT

powders

Powders with desired

size distribution

Surface dispersant or

organic vehicle

Surface-modified

powders

PZT sol–gel solution

High-energy ball milling

Ball milling

Uniform slurry

Spin coating

Spray coating

Screen printing

Tape casting

Slip casting

……

24.2

Flowchart for preparing uniform slurry.

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Handbook of dielectric, piezoelectric and ferroelectric materials728

obtain homogeneous thick films by spin coating. Also, by using the sol–gel

precursor as a binder, the density of the green compact can be increased and

the sintering temperature of nano-scaled powders can be further reduced.

Thus, we can expect low-temperature sintering of the screen-printing films.

This unique feature is particularly useful to low-temperature co-firing of a

mutilayer device because cheap internal electrode paste can be used if this

idea can be realized. The simplicity of this approach should make it useful

to most ceramic processes. However, in this chapter, we will only report the

results of our attempts on spin coating thick films.

24.2.4 Thick film deposition by spin coating

Figure 24.4 shows the flowchart for processing the slurry by spin coating.

The pure sol–gel thin films and slurry-derived thick films are deposited

alternately. The first layer pure thin film is deposited on the bottom electrode

as a buffer layer to enhance the adhesion between the thick film and the

substrate. After depositing one or more slurry layers, another pure thin film

is deposited subsequently. The intermediate pure sol–gel layers can infiltrate

the pore in the slurry layers to increase the film density.

The processing parameters of the composite route include the properties

of the powder, the recipe of the slurry and the deposition conditions. The

optimized processing parameters have been summarized below.

Single

composition

materials

Two features of this process:

1. Low crystalline temperature

of the sol–gel precursor

2. Nano-sized particles

Homogeneous

matrix

A new process based on

uniformly mixed suspension

of sol–gel precursor and

nano-sized powders

Multi-phase

composites

Heterogeneous

matrix

Uniformly dispersing the

nanosized powders into

sol–gel precusor solution,

thick films can be

achieved by spin coating

Using the sol–gel precursor

as a binder to form a paste,

low-temperature sintering

of multilayer devices can

be realized

Using the sol–gel

precursor as a sintering

additive or grain growth

inhibitor, dense

nanocrystalline ceramic?

24.3

Illustration of the features of the nanocomposite process.

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• Powder properties. The commercial available powder should be presintered

at 1200–1300 °C to improve its crystallinity before being mixed with

precursor solution and the powder size should be in the range of 100–

300nm.

• Slurry recipe. Concentration of the sol–gel solution: 30 to 40 wt%;

powder/solution mass ratio: 2/5 to 2/3; vehicle content: 5wt%.

• Deposition procedure. Deposited at 3000 RPM, preheated at 250°C and

500 °C on a hot plate, and finally annealed at 650–750°C for 30min in

a tube furnace.

24.3 Characterization of spin-coated thick films

24.3.1 Microstructure of the thick film

Since the resultant films are composite and the added powder occupies the

majority of the volume fraction, the microstructure of the resultant film is

mainly determined by the size of the added powder. Figure 24.5 shows two

typical microstructures of the composite films using nano-sized (10–30nm)

and submicrometer-sized (150–300nm) PZT powder, respectively. The surfaces

of both films are sufficiently smooth for device fabrication. Nanocomposite

film has relatively dense microstructure in comparison with that of the film

prepared with submicrometer-sized powder. Because the aggregation of the

nano-sized powder in slurry has been successfully eliminated by using

dispersant, the obtained microstructure is quite uniform and dense. From the

cross-section image of the film prepared by using nano-sized powders shown

Thick film

Sintering at 600–850°C for 30–120min

Spin coating one or more slurry layer

pyrolyzing at 250°C and 500°C each

for 2 min

Spin coating pure PZT buffer layer

pyrolyzing at 250°C and 500°C

each for 2min

PZT sol–gel solution

Uniform PZT slurry

Multiple

coating

24.4

Flowchart for depositing thick film on silicon wafer by spin

coating.

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