Micromachined sensor for in-situ monitoring of wafer state in plasma etching

This paper presents a prototype etch rate sensor which correlates film thickness with the change in resonant frequency that occurs in a 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 remaining etched, allowing etch rate to be inferred. As a proof-of-concept experiment, a platform made of DuPont 2611 polyimide electrodes is fabricated. The sensor is driven into resonance electrostatically, and the shift in resonance is detected by monitoring the change in impedance between the drive electrode and platform as the drive frequency is swept. To enhance filtering of the sensor signal in the noisy plasma environment, the platform was designed such that the ratio of the plasma frequency to the fundamental mode of the sensor is approximately 400:1. The prototype was etched in a plasma therm RIE system in a CHF3/O2 plasma for 60 second intervals. After each etch, the thickness of the platform and its resonance frequency was measured, and it was confirmed that the resonant frequency decreased with decreasing film thickness. Ultimately, the sensor has the potential to allow in-situ process monitoring of both etch rate and uniformity at a nominal cost.