Application of the footing effect in the micromachining of self-aligned, free-standing, complimentary metal–oxide–semiconductor compatible structures

The footing or notching effect is observed when silicon or polysilicon layers on buried dielectric films are overetched during dry processing. This effect is considered an undesirable feature for most applications, even though it is usually negligible in conventional reactive ion etching of submicron features due to the low current fluxes and small dimensions. However, the etching of wide trenches (several microns or more) using newer high density plasma etchers, can exhibit a notching effect that extends laterally a few microns. The minimization and suppression of notching depends on achieving a balance between the deposition of passivating films and silicon etching. Therefore, we review the dependence of the footing effect on etching conditions in a time multiplexed deep etcher. The microfabrication of cantilevered structures using the footing effect is demonstrated by the micromachining of self-aligned, released electrostatic actuators. Silicon etching, structure release and sidewall passivation or dielectric isolation deposition were done in situ using very large scale integrated complimentary metal–oxide–semiconductor (CMOS) compatible plasma chemistries only. Thus, the low-temperature, soft-mask scheme presented here can be easily integrated in the microfabrication of intelligent sensors and actuators. The measured pull-in voltage for a cantilevered beam 1000 $\mu$ m long and 3.2 $\mu$ m thick, of the order of 80 V, agrees with predicted values. The electrostatic actuators prepared in this fashion, subsequently underwent ${10}^5$ pull-in cycles of wear testing of up to 100 V without failure, confirming the robustness of this approach in the microfabrication of CMOS-compatible actuators and suspended structures.