Piezoelectric acoustic sensors and ultrasonic transducers based on textured PZT thin films

This work dealt with the establishment of a reproducible and industrially exploitable fabrication technology for piezoelectric MEMS based on PZT thin films. {100}-textured piezoelectric 1-4 μm thick films were found to be the most suited materials for MEMS applications where a strong transverse e31,f coefficient is required. Values of 11.5 to 12.5 C/m2 throughout the investigated thickness range have been obtained. PZT MEMS requires some special microfabrication processes. The patterning of platinum electrodes in ICP reactor with an Ar/Cl2 gas mixture gave satisfying results. For 1 μm PZT films, excellent results were achieved with dry etching in an ECR/RF low-pressure reactor. Above 1 μm thickness, wet etching technique was used. The chosen process flow was such that all layers up the top electrode were first deposited, and then sequentially patterned. In this way, the functional properties were completely maintained. Finally, the process flow related to PZT was successfully combined with deep silicon etching techniques to produce free standing structures. The applicability of the processes has been demonstrated by means of two piezoelectric micro-electro-mechanical devices. First, a cantilever acoustic sensor for low frequency applications (< 50 Hz) has been developed for integration in photoacoustic gas detectors. A high device sensitivity of up to 150 mV/Pa has been obtained with noise level corresponding to a few mPa, mainly due to the preamplifier. The second device was based on a silicon/PZT diaphragm to produce a vibrating structure in view of fabricating ultrasonic transducers for position sensors and non-destructive testing probes. The study of the basic properties of such structures has shown excellent results as coupling factors k2 up to 5% for PZT 2 μm / Si 4 μm SOI membranes held with four small bridges. Ultrasonic wave transmission between two devices has been achieved up to a distance of 200 mm. The microfabrication of piezoelectric MEMS was found to be a complex task where all aspects from materials, microfabrication, and device design to assessment of performance are closely interconnected. Piezoelectric MEMS can offer exciting opportunities to fabricate innovative and performing devices.