An Integrated MEMS sensor for real time control of plasma etching

This paper explores a novel technique for monitoring film thickness in reactive ion etching that incorporates a micromachined sensor. The prototype sensor correlates film thickness with the change in resonant frequency that occurs in the micromachined platform during etching. The platform is suspended over a drive electrode on the surface of the substrate and electrically excited into resonance. As material is etched from the platform, its resonant vibrational frequency shifts by an amount proportional to the amount of material etched, allowing etched rate to be inferred. One challenge in this approach is the small current signals that must be sensed to locate the resonance condition. A possible means to circumvent this issue is to place the platform at the input of an autozeroing floating gate amplifier (AFGA). Use of the AFGA circuit will facilitate high sensitivity to the changes in capacitance during etching. In this paper, the micromachined sensor is simulated using ANSYS 7.0. ANSYS simulation shows a direct correlation between platform film thickness and resonant frequency, as well as between the platform thickness and its capacitance Modeling the sensor as a variable capacitor in the AFGA circuit using HSPICE reveals that the deflections of the platform are amplified as expected.