In Situ Monitoring of Silicon Membrane Thickness during Wet Etching using a Surface Acoustic Wave Sensor

In this paper, we propose a novel method using the surface acoustic wave (SAW) sensor for monitoring in situ the thickness of a silicon membrane during wet etching. Similar to pressure sensors and accelerometers, some micro-electro-mechanical systems (MEMS) devices require the thickness of silicon membranes to be precisely known. Precisely controlling the thickness of a silicon membrane during wet etching is important, because it strongly influences post-processing and device performance. Furthermore, the proposed surface acoustic wave sensor enables the thickness of a silicon membrane, from 50 mum to 80 mum. to be monitored in situ. In summary, the proposed method for measuring the thickness of a silicon membrane in real time from 50 mum to 80 mum, is highly accurate, is simple to implement and can be used for mass production. The principles of the method, detailed process flows, the set up for measuring thickness and simulation and experimental results are all discussed. The theoretical and measured values differ by an error of less than 2 mum; thus the experimental and theoretical values correlate well with each other.