Mitigating the Insufficient Etching Selectivity in the Wet Release Process of CMOS-MEMS Metal Resonators Via Diffusion Control

This letter presents an effective approach to mitigating the prolonged issue of the insufficient etching selectivity between the sacrificial oxide and structural metal Al during the final release step of the CMOS-MEMS metal resonators. Particularly, the approach purposely slows down the etching rate not only for the sacrificial oxide but also more effectively for the structural metal by covering the resonator devices with upper metal layers in the CMOS process that contain release holes to pinch the diffusion of the etchant. Doing so realizes a 10x extension in the allowable time window for over-etch, from a rather critical 5 min to over 50 min. This work further investigates the influence of the HF-based etchant temperature from 20°C to 40°C on the etch rate. The results show that the technique introduces the diffusion limiting effect at lower temperatures, leading to a less temperature sensitive etch rate compared to that of the reference counterpart. This method provides a promising and effective remedy for improving the yield during the time- and solution condition-sensitive oxide wet release step of CMOS-MEMS-based resonant devices. [2020-0299].