F. Calame

Piezoelectric thin films: evaluation of electrical and electromechanical characteristics for MEMS devices

We present a new measurement method to characterize piezoelectric thin films utilizing a four-point bending setup. In combination with a single- or a double-beam laser interferometer, this setup allows the determination of the effective transverse and longitudinal piezoelectric coefficients e31,f and d33,f respectively. Additionally, the dielectric coefficient and the large signal electrical polarization are measured to add further important characteristics of the film. These data are essential for piezoelectric thin film process specification and the design and qualification of microelectromechanical systems devices.

Design, modeling and fabrication of piezoelectric micromachined ultrasonic transducers

An analytical model based on the clamped plate theory has been used to design the Bimorph pMUT (i.e. frequency). This model gives a good prediction of the experimental frequency of the 5-layers pMUT plate resonance. Our calculation show that central electrodes need to cover 58% and 33% of the membrane surface for a long rectangular and circular membrane respectively to achieve optimal coupling. The electromechanical coupling coefficient (k) and quality factor (Q) were measured as k=4.4% and Q=145 in air for a low frequency transducer (250 kHz). d. The 17 MHz version yielded values of Q=25 and k=3%..

5I-2 Thick PZT Sol-Gel Films for pMUT Transducers Performances Improvement

Piezoelectric micromachined ultrasonic transducers comprising a 10 mum thick Si device layer and a 1-4 mum thick piezoelectric PZT layer were investigated. The PZT films were deposited by a sol-gel technique. The transverse piezoelectric coefficient was measured as -14.9 C/m2. The electromechanical coupling increased stronger as expected with PZT thickness. The influence of both the shape and area of the top electrode on the device performance has been investigated. The electromechanical coupling coefficient (k) and quality factor (Q) have been measured in air and were fitted to an equivalent circuit model. The maximal k2 was obtained as 7.8%

Thick Sol-Gel PZT Film integrated in SQI-Based, Micromachined Ultrasonic Transducers

Piezoelectric micro machined ultrasonic transducers (pMUT) are piezoelectric laminated plates operating at flexural modes. The pMUTs fabricated in this work contained a 4 mum thick lead zirconate titanate (PZT) thin film deposited by a sol-gel technique, and exhibiting a permittivity of epsiv_r = 1200, and an effective transverse piezoelectric coefficient e_31,f of 12 C/m² . A further optimization of the sol-gel process yielded larger grain diameters and consequently improved properties: epsiv_r = 1600, e_31,f of 16 C/m² . A design and micromachining concpet for easy scaling in frequency has been developed. The electromechanical coupling coefficient (k² ) and the quality factor (Q) of rectangular clamped elements (with former PZT process) were measured as k² = 4.4% and Q = 145 in air for a low frequency transducer (@240 kHz). The 16.9 MHz transducer yielded values of Q = 25 and k² = 3% in air. The effect of DC bias voltage on frequency and k² has been studied.

Local growth of sol-gel films by means of microhotplates

PZT (PbZr/sub x/Ti/sub 1-x/O/sub 3/) thin films have been locally grown by means of sol-gel deposition and local anneals on microhotplate. The microhotplate was based on a tantalum silicide filament (Ta/sub 5/Si/sub 3/) formed on stress compensated SiN/SiO/sub 2/ membrane. On this filament, a passivation layer of SiO/sub 2/, a layer of Ti/TiO/sub 2/ and a bottom electrode of Pt were deposited. Due to the low heat conductivity of the membrane, crystallization of the PZT film occurs exactly on the resistor filament. The formation of crystalline, i.e. perovskite PZT was verified by means of X-ray diffraction. A random PZT texture has been observed.

LOCAL GROWTH OF SOL-GEL FILMS BY MEANS OF MICROHOTPLATES

PZT (PbZr/sub x/Ti/sub 1-x/O/sub 3/) thin films have been locally grown by means of sol-gel deposition and local anneals on microhotplate. The microhotplate was based on a tantalum silicide filament (Ta/sub 5/Si/sub 3/) formed on stress compensated SiN/SiO/sub 2/ membrane. On this filament, a passivation layer of SiO/sub 2/, a layer of Ti/TiO/sub 2/ and a bottom electrode of Pt were deposited. Due to the low heat conductivity of the membrane, crystallization of the PZT film occurs exactly on the resistor filament. The formation of crystalline, i.e. perovskite PZT was verified by means of X-ray diffraction. A random PZT texture has been observed.

Qualification and Applications of a Piezoelectric MEMS Process

An industrializable process for manufacturing of piezoelectric ultrasound transducer elements is demonstrated. The PZT film is deposited by chemical solution deposition on SOI wafers, and standard MEMS processes are utilized to make cantilevers, bridges and membranes. The PZT film of -50% of the devices exposed to thermal aging were found to fail at 85degC - 85%RH (relative humidity), while a real-life test of 8-1010 cycles at resonance only gives a slight change in Q-value and resonance frequency.