Processing and Characterization of Piezoelectric Materials into MicroElectroMechanical Systems

Weiqiang Wang

Outline

Introduction

Lost silicon (Si) mold process for PZT ceramic

microstructures

PZT based piezoelectric micromachined switch

Conclusion

Introduction

Piezoelectric effect

direct piezoelectric effect converse piezoelectric effect

Exploiting the piezoelectric materials into microelectromechanical systems (MEMS) Lead zirconate titanate (Pb(ZrxTi1-x)O3, PZT) microstructures

Piezoelectric-based MEMS are generally attractive due to their high sensitivity and low electrical noise in sensing applications and high-force output in actuation applications

MicroElectroMechanical Systems (MEMS) denote systems that include one or more small microstructures (sub- μm to mm) that often are fabricated using a technology μnamed micromachining, and materials,that originates partly from the semiconductor industry’s processes and partly from precision mechanics.

Lost silicon (Si) mold process for PZT ceramic microstructures

Fig. 1. Lost Si mold process

Results

The PZT rods were 7 μm square in cross section with a 12 μm period. The resulting aspect ratio was more than 12.

Fig. 2. SEM images of PZT microrods

Advantages• Fine scale and high aspect ratio. (rods 95 μm in diameter

and 400 μm in height using lost plastic mold method. )

• Advantages of Si– high melting point (1440°C) and high strength

– Si mold can be used as a part of the device

– Si micromachining techniques have been well developed

in-mold sintering

Plastic mold: LIGA (lithography, galvanoforming, plastic molding)

• HIP of PZT was successfully conducted at a temperature as low as 800°C

Problems

X-ray diffraction (XRD) analyses showsthat perovskite PZT was the major phase, at the same time, certain amounts of undesired pyrochlore-type PZT phase was also observed.

Oxygen deficiency

But it is believed producing the same dense PZT rods at lower temperatures by increasing the HIPing pressures may suppress the formation of undesired phases.

Application

1-3 PZT/polymer composite

piezoelectric transducers for medical imaging

PZT based piezoelectric micromachined switch

Fig. 3. Schematic view of cantilever

Fig. 4 Fabrication procedure

Fig. 5 SEM of PZT deposited on substrate

Principle of operation

Fig. 6 Schematic illustration of switch Fig. 7 SEM image of cantilever switch with transmission lines

Operation tests

Low frequency test High frequency test

Fig. 8 Switching response to a 1 Hz 20 V square wave

Fig. 9 Switching response to (a) 30-V and (b) 50-V 2-μs pulse.

Preliminary results using a gain-phase analyzer (HP4194A) have demonstrated that signals up to 100 MHz can be switched.

DiscussionA one-degree-of-freedom dynamic model

2

2

d z dzF m b kz

dt dt

0 0( ) cos( )nz t z z t

1

0 0

1cos (1 )

2on f z

The time required to close the gap(δ) between the contact and the transmission lines can be obtained from

(1) k: spring constant; b=0

(2)

(3)

n : natural frequency; 02n f

The calculated τon=3.3μs for this device, which is comparable to the

measured value.

Conclusion

Piezoelectric materials have been successfully applied in a variety of MEMS applications. The development of fabrication methods such as PZT structural micro-machining, low-stress silicon nitride deposition, and solution deposition of piezoelectric thin films has been essential. The MEMS applications described here compare favorably with other MEMS approaches based on commonly used electrostatic actuation. The continued promise for piezoelectric MEMS is attractive.

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