SAW pressure sensor on quartz membrane lapping

Abstract

National audienceThe fabrication of SAW quartz-based pressure sensors has received a strong interest for many years, yielding various development using either delay lines or resonators. However, most approaches have been developed exploiting quartz machining along standard chemical or mechanical etching, rarely compatible with batch processes as used for Micro-ElectroMechanical Systems (MEMS). In this work, we propose a temperature/pressure sensor fabricated on compound Quartz/Silicon substrates obtained by Au/Au bonding at room temperature and lapping/polishing of Quartz. This approach allows for a collective and accurate production of sensors, the sensor sensitivity being controlled by the membrane thickness and diameter. As the pressure is intricately connected with temperature, an objective estimation of this parameter requires accurate temperature measurements as well. As a consequence, the proposed sensor combines a reference resonator designed to be temperature compensated (AT,X cut of quartz) together with a resonator which propagation direction is chosen according the targeted temperature coefficient of frequency in order to give access to a linear differential temperature measurement. In addition, a third resonator with propagation axis along X is placed at the right center of a circular membrane. When the membrane is bent by pressure effects, the corresponding resonance frequency drifts linearly, allowing for another differential pressure measurement. Hence, with three resonators, one can easily demonstrate the unambiguous determination of temperature and pressure at once. Until now pressure sensor based on SAW with quartz material are unity processed. This process allows us a collective manufacturing of sensors. We start by seal the quartz wafer with the silicon substrate using the thin gold layer. This process yields an homogeneous and high quality bond. It is subsequently thinned and polished to an overall thickness of 100 microns. Aluminum electrodes are deposited on the quartz to achieve three SAW resonators. Process flow based on collective manufacturing is now developed. Electrical responses of SAW resonators are done. Results show the operability of the sensors and the responses are conformed to the design. Electrical test under pressure is currently under development