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

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Handbook of dielectric, piezoelectric and ferroelectric materials730

in Fig. 24.5(b), one can hardly identify the added powders from the sol–gel

derived crystallites. However, the added submicrometer-sized powder can

be clearly seen from Fig. 24.5(c) and (d). The powder is uniformly distributed

in the matrix nano-grains derived from the sol–gel solution. It is because the

thick films obviously have the two phases (the added powder and the sol–gel

nano-grains) in the microstructure that thick films are called composite films.

From Fig. 24.5(d) we can also see that, for the submicrometer-sized powder,

only the sol–gel phase was sufficiently processed or crystallized at the annealing

temperature above 600°C, no substantial grain growth of the added powders

occurred during annealing of the film at temperatures below 700°C. Thus

the most important approach to obtain good properties in resultant composite

films is presintering the added powder to ensure it has good crystallinity and

sufficient size, and thereby will exhibit good ferroelectricity.

A high-quality microstructure can be obtained within a relatively broad

range of the slurry recipe. This flexibility much benefits the preparation of

the slurry and the adjustment of the single layer thickness. By using this

composite route, we can conveniently prepare crack-free composite film in

(a) (b)

24.5

Cross-section images of two 10 µm-thick composite films: (a)

and (b) film 4012 prepared with nano-sized powder; (c) and (d) film

4023 prepared with submicrometre-sized powder.

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the thickness range of several micrometers to several ten micrometers within

ten spin-coating steps.

24.3.2 Ferroelectric and piezoelectric properties

Figure 24.6 compares P–E hysteresis curves and effective piezoelectric

coefficients of the above-mentioned two composite films. The P–E hysteresis

loops of the composite films were measured using Precision Pro Ferroelectric

Analyzer (Radiant Technology Inc.) with a high-voltage interface at 1kHz.

For films prepared with submicrometer-sized powder, the relations between

effective longitudinal piezoelectric coefficients (d

) of the films and applied

dc bias were measured with a home-developed scanning modulated

interferometer

13–15

A 12 V

p-p

alternating current driving signal was applied

on the film during measurement. For nanocomposite films, the d

were

measured with a PSV-300 scanning vibrometer.

16,17

A 4 V

p-p

alternating current

voltage was applied as a driving signal.

It can be seen from Fig. 24.6 that the average remanent polarization (P

–

(–P

))/2 of the films prepared with submicrometer-sized powder were 20.0,

18.8 and 15.0µC/cm

for the recipe of 4023, 3023 and 3025, respectively

(see Fig. 24.6c), much higher than that (around 7µC/cm

) of the films prepared

with nanocomposite process (see Fig. 24.6a).

Figure 24.6(d) shows the effective d

of the film at zero bias is around 75

pm/V, and the highest d

values are around 120pm/V without considering

the clamping effect of the substrate. Note that the films were not poled

before the measurement, which implies that the saturated d

will be higher

if the film is well poled. Again, the piezoelectric performances of these films

are quite good compared with those of the nanocomposite films shown in

Fig. 24.6(b). In that case, the d

at 0 bias ranged from 20 to 30 pm/V, and

the highest d

at certain bias was around 80pm/V for films sintered at

between 650 and 800°C.

These results reveal that the piezoelectric and ferroelectric properties of

the nanocomposite film are not as good as expected, although the use of the

nano-sized powder is of great benefit to the uniformity and density of the

PZT film. The film exhibits lower remanent polarization and thus lower

piezoelectric coefficient. This is reasonable because too small a grain size in

the nanometer range does not allow the grain to split into domains. Using

submicrometer-sized PZT powder is an effective way to improve ferroelectric

and piezoelectric performances of the composite films. Therefore, we have

to compromise between uniformity of the microstructure and the piezoelectric

performances of the thick film when selecting the powder size. Considering

the comprehensive properties of the films, the submicrometer-sized PZT

powder was employed to prepare the slurry for device fabrication.

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24.6

P-E hysteresis loops and effective piezoelectric coefficients of the composite films: (a) and (b) nanocomposite film

3025 prepared with nano-sized powder sintered at different temperature; (c) and (d) films with different recipe prepared

with submicrometer-sized powder sintered at 650 °C.

4023

3023

3025

650°C

750°C

800°C

3023 7µm

3025 5µm

4023 10µm

Fired 800

Fired 750

Fired 650

(µC/cm

)

–10

–20

–30

–400 –300 –200 –100 0 100 200 300 400 500

(kV/cm)

(a)

(µC/cm

)

–20

–40

–60

–600 –400 –200 0 200 400 600

(kV/cm)

(c)

–400 –300 –200 –100 0 100 200 300 400

(kV/cm)

(d)

(pm/V)

150

100

–50

–100

–150

(pm/V)

–200 –150 –100 –50 0 50 100 150 200

Bias field (kV/cm)

(b)

Unpoled

p–p

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24.3.3 Leakage current density and dielectric properties

The data presented in Fig. 24.7 are dependence of leakage current density on

applied field of the films measured with a Precision Pro Ferroelectric Analyzer.

The mean leakage current density of the films is around 2 × 10

–6

A/cm

and

the breakdown field is higher than 500 kV/cm for all the investigated recipes.

The data also indicate that the driving field of 200kV/cm, i.e. 20V/µm, is

definitely safe for the composite thick film. Besides, the measured relative

permittivity of the films ranges from 600 to 1000 at 1kHz, and the

nanocomposite film has relatively higher permittivity. The dielectric loss is

less than 0.02 within the investigated slurry recipe range. These dielectric

properties are also quite acceptable for device application.

24.4 Piezoelectric micromachined ultrasonic

transducer (pMUT) based on thick PZT film

Miniaturized MUTs represent a promising approach to ultrasound detection

and generation. They are of particular interest in high-frequency medical

imaging systems,

18,19

non-destructive evaluation, underwater exploration

and low-frequency airborne ultrasound applications,

21,22

such as vehicle control

and object detection for robots. In the diaphragm-type pMUTs, the individual

transducer element consists of a micromachined membrane embedded with

a PZT film. The PZT film, sandwiched between two electrode layers is poled

in the thickness direction and only the transverse mode of d

is utilized. The

laminated inert membrane, which forms the supporting structure layer of the

so-called bi-morph structure of PZT/silicon, is necessary to realize the

ultrasound reception or transmission. The schematic structure of the pMUT

Leakage current density (A/cm

)

1E-5

1E-6

1E-7

1E-8

1E-9

3023

4025

3025

4023

3012

–600 –400 –200 0 200 400 600

Electric field (kV/cm)

24.7

Dependence of leakage current density on electric field of the

composite films prepared with submicrometer-sized powder.

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and its cross-section is sketched in Fig. 24.8. A PZT thick film as the active

layer was deposited on Pt/Ti(TiO

)/LTO/Si

/SiO

/Si wafer. The uppermost

layer is Pt/Ti top electrode with an intermediate layer of polyimide as an

isolation layer to define the effective top electrode area of an individual

transducer element. The descriptions of each layer within the multilayer

diaphragm and relevant materials processing steps are listed in Table 24.2.

As shown in Fig. 24.8, a square diaphragm will be fabricated as the active

pMUT element when the substrate is anisotropically etched by KOH. For

pMUT array operating at high frequency, deep reactive ion etching (DRIE)

is preferred for silicon etching. In this case, the shape of the diaphragms is

no longer limited to square. Various element shapes, e.g. circular dots and

rectangular bars, can be achieved. To simplify electrical interconnection, the

top electrode in a pMUT array is designed as the common electrode for all

the elements. Only the overlapping part sandwiched between top electrode

24.8

Schematic drawings of a diaphragm-type pMUT: (a) perspective

view and (b) cross-section of multilayer structure.

Polyimide

Top electrode

contact pad

Driving

voltage

Bottom electrode

contact pad

LTO/Si

SiO

(a)

PZT

movements

Top electrode

Pt/Ti

Polyimide

Bottom

electrode Pt/Ti

PZT

LTO

SiO

Diaphragm deflection

(b)

Top electrode Pt

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Table 24.2

Multilayer structure descriptions and materials processing steps in pMUT fabrication

Layer type Typical Thickness Purpose of Fabrication methods

composition the layer and issues

Top electrode Pt 160 nm Electrical contact Sputtering, lift-off

Adhesion layer Ti 60 nm To improve adhesion of Pt to PZT Sputtering, lift-off

Isolation layer Polyimide 4 µm To isolate and reduce parasitic Spin-coating, open contact holes to

capacitance bottom electrode

Piezoelectric layer PZT53/47 2 ~ 20 µm Active layer Composite thick film process.

Patterning of dense thick PZT film,

Bottom electrode Pt 200 nm Electric contact Lift-off, precise patterning

Adhesion layer Ti or TiO

20nm or 100 nm To prevent delamination of the Oxidation control: to avoid Ti

electrode migration through grain of the

electrodes

LTO 350 nm To avoid crack of PZT films LPCVD

Support membrane Si

200 nm Etching mask, stress control LPCVD

SiO

1.8µm Mechanical support Dry and wet thermal oxidation

Si (100) 0–20 µm Thickness control, SOI wafer

LTO = Low temperature oxide; LPCVD = Low pressure chemical vapor deposition; SOI = silicon on insulator

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and patterned bottom electrode can be excited and thus becomes the active

part of the transducer element. Piezoelectric response and parasitic capacitance,

caused by the overlapping part between the top electrode and patterned

leading wire of the bottom electrode, were minimized by using a polyimide

layer.

A pMUT can operate either as ultrasonic transmitter or receiver. By applying

an ac electric field across the thickness of the PZT film, the induced lateral

deformation of the piezoelectric film causes structure vibration; thus generating

a pressure sound wave to propagate in the medium in contact. When it is

used as a receiver, the multilayer diaphragm turns an input sound pressure

into charge generation. Figure 24.9 shows the ANSYS finite element analysis

(FEA) result of flux density in response to a uniform pressure of 10 Pa. The

result indicates that the induced charge changes its sign at around 70% of its

side length of square diaphragm. Therefore, in the pMUT design, the active

electrode should not cover the whole area of the diaphragm. Otherwise, the

induced charges in the inner part and outer part will be partially neutralized

and thus reduce the receiving sensitivity of the pMUT.

MAY 19 2006

12: 45: 59

NODAL SOLUTION

STEP = 1

SUB = 6

FREQ = 41500

REAL ONLY

DZ (AVG)

RSYS = 0

DMX = 1.95

SMN = – .001982

SMX = .002079

–.001982 –0.001079 –.177E-03 .726E-03 .001628

–.001531 –.628E–03 .274E–03 .001177 .002079

24.9

FEA simulation result of flux density in response to a uniform

pressure of 10Pa, indicating that the induced charge changes its sign

at around 70% of its side length of the square diaphragm.

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24.5 Microfabrication of thick film pMUT

PMUTs were fabricated using standard bulk micromachining fabrication

techniques with some minor modifications needed for the PZT thick film

deposition.

The fabrication processes are depicted with reference to Fig.

24.8(b) and Table 24.2. A layer of 1.8µm-thick thermal silicon oxide was

first grown on a 4-inch (100mm) double side polished silicon or SOI wafer

as supporting structure layer. A LPCVD silicon nitride layer of 200 nm and

an LTO layer of 350nm were then deposited on both sides of the wafer. The

silicon nitride layer is used as the etching mask layer during the KOH wet

etching of backside silicon; this layer can be omitted if DRIE is used to etch

the silicon. A window on the backside is opened for silicon etching after wet

etching of LTO by buffered oxide etch (BOE), dry etching of Si

and SiO

by RIE using CF

+ O

. Backside silicon was anisotropically wet etched by

KOH first until the remaining thickness of silicon is reached at about 50 µm.

Ti/Pt or TiO

/Pt layers of 20 nm/200 nm as bottom electrode are sputtered

and patterned by a lift-off process. Thick PZT layer was deposited using the

composite thick film deposition technique described in Section 24.2. Wet

etching of PZT was performed in diluted HCl : HF solution with a photoresist

mask to open a window for the access to the bottom electrode pad. A polyimide

layer is spin-coated, patterned and cured as an insulation layer. Ti (20nm)/

Pt (200nm) as the top electrode were sputtered and patterned on the front

side by lift-off process. After that, the backside silicon was etched off by

KOH or DRIE until the required silicon thickness was reached.

Figure 24.10 shows the SEM cross-section image of the multilayer structure

on the front side of a fabricated pMUT, where the composite PZT layer is

Pt/Ti

Polyimide

LTO SiN

SiO

24.10

SEM cross-section image of a multilayer structure fabricated

on the front side of the silicon wafer. The dense polyimide layer of

around 4 µm is coated on the 5µm-thick composite PZT layer for the

protection purpose. The smooth surface of polyimide facilitates the

wiring of the top electrode.

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about 5µm in thickness. Some important process issues will be further

addressed in the following subsections.

24.5.1 Buffer layer of bottom electrode

After the multiple coating steps, the PZT film together with its underlayer

structure has to be sintered at relatively higher temperature. Pt/Ti/SiO

/Si is

usually adopted as the substrate of PZT films. Initially, we also used Ti as the

buffer layer of bottom electrode Pt. However, after pMUT fabrication, we

found that the Pt/Ti bilayer cannot withstand a high sintering temperature.

Blister occurs within the bottom electrode when the sintering temperature

exceeds 700°C.

Figure 24.11(a) and (b) shows the blisters within the Ti/Pt bottom electrode

and a pMUT element. The picture of the pMUT was taken through the

transparent SiO

and Si

layers from the backside. TEM observation of

the electrode interface shown in Fig. 24.12 reveals that the possible cause of

the blisters can be attributed to the diffusion of the Ti and oxygen into the Pt

layer and the formation of TiO

in the Pt grain boundaries during high-

(a)

(b)

(c)

(d)

Blisters

24.11

(a) A blister occurs within Pt/Ti bottom electrode due to high

annealing temperature at 700°C. (b) Backside view of a pMUT

element shows blisters within Pt/Ti bottom electrode after annealing

at 700°C. (c) Cross-section of a composite film on Pt/TiO

bottom

electrode sintered at 800°C. (d) Backside view of a pMUT element

with Pt/TiO

bottom electrode. No blister occurs after the final

annealing of PZT composite film at 750°C.

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temperature annealing in oxidizing atmosphere. High compressive stress

within the TiO

layer results in the occurrence of the blister. Therefore, we

attempted incorporating a well-reacted TiO

layer instead of a pure metallic

Ti layer as the adhesion layer to avoid the formation of the blister.

The TiO

buffered layer was deposited onto a thermal-oxidized silicon

wafer at 400°C using radio frequency (RF) magnetron sputtering. The 100

nm-thick TiO

layer is reactively sputtered from the titanium target at 500W

and 1.8 × 10

–3

mbar with an O

/Ar ratio of 1:9. The Pt layer is then sputtered

at 200W and 1.8 × 10

–3

mbar in argon. Figure 24.11(c) and (d) shows a film

cross-section and a square pMUT element with a Pt/TiO

bottom electrode

sintered at high temperature. The result reveals that the Pt/TiO

combination

significantly improves the heat resistance of the bottom electrode. By employing

the Pt/TiO

/SiO

/Si substrate, the composite thick films can be finally sintered

up to 800°C without the occurrence of the blister.

24.5.2 Polyimide layer

PI2542 wet etch polyimide and PI2771 photosensitive polyimide were used

in the pMUT fabrication to protect the PZT films. As mentioned before, the

top electrode of the pMUT array is a common electrode of all the elements

in the array. The effective top electrode of each element is actually defined

by an etch window patterned on polyimide layer. In the window area, the top

electrode is directly deposited on PZT film as shown in Fig. 24.13. Since the

dielectric constant of polyimide layer is much smaller than that of PZT layer,

the induced parasitic capacitance is very small compared with the total

capacitance, even if the electric interconnections of top and bottom electrode

overlapped each other.

Pb:Zr:Ti:O=14:3:12:17

TiO2

50 nm

24.12

TEM image shows the formation of TiO

within the Pt layers.

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