Integration of PZT on SOI Wafers: Increasing Piezoelectric Film Thickness for Providing a Wide Range of Ultrasonic MEMS
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0969-W05-09
Integration of PZT on SOI Wafers: Increasing Piezoelectric Film Thickness for Providing a Wide Range of Ultrasonic MEMS Applications Brahim Belgacem, Florian Calame, and Paul Muralt Ceramics Laboratory, Swiss Federal Institute of Technology in Lausanne, STI-IMX-LC, Station 12, Lausanne, 1015, Switzerland ABSTRACT Piezoelectric micromachined ultrasonic transducers comprising a 10 µm thick Si device layer and a 1-4 µm 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 with PZT thickness, as expected. 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%.
INTRODUCTION Piezoelectricity is one of the best principles for transduction in Micro-Electro–MechanicalSystem (MEMS). Indeed, piezoelectricity can be used in sensors, actuators, acoustic and ultrasonic devices [1, 2, 3]. Micromachining techniques allow for much higher precision than the reticulation techniques used for the fabrication of bulk ceramic elements. For MEMS application, the transverse piezoelectric coefficient (e31,f) is exploited in combination with the deflective structure. The control of the sol-gel process allows depositing PZT films with good properties in a thicknesses range from 0.25 to 5 µm. In order to identify the key parameters that control the electromechanical properties of pMUT devices, models based on clamped plate theory were adapted to calculate the electromechanical coupling coefficient of the structures. CE
γ
PZT Wet oxide (≈1µm)
Si device layer (≈10µm) 200 µm
Figure 1.
RSE
γr
Schematic view of the pMUT devices with two different top electrode shapes.
This study addresses the question of the impact of the film thicknesses, the shape and the dimension of the electrodes. As shown in figure 1, the electrode size impacts also on the PZT coverage of the plate. The fabricated devices consist of circular plates with radius a=100 µm. Two device designs have been realized. The first one has a centered electrode (CE) of area γπa2, the
second one a ring shaped electrode (RSE) with an area of γrπa2 where γ and γr are relative electrode area compared to the entire membrane surface. The determination of the coupling coefficient will briefly be presented. In the following, the process is described, and the experimental results are presented and discussed.
ELECTROMECHANICAL COUPLING COEFFICIENT DETERMINATION (K) The calculation of the theoretical electromechanical coupling coefficient as a function of the materials proprieties and device characteristics is possible. The detail of this calculation has already been presented in a previous proceeding (see [4]). The coupling coefficient is proportional to the effective transverse piezoelectr
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