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 laye rs on buried dielectric films are overetched during dry processing. This ef fect is considered an undesirable feature for most applications; even thoug h it is usually negligible in conventional reactive ion etching of submicro n features due to the low current fluxes and small dimensions; However, the etching of wide trenches (several microns or more) using newer high densit y plasma etchers, can exhibit a notching effect that extends laterally a fe w microns. The minimization and suppression-of notching depends on achievin g a balance between the deposition of passivating films and silicon etching . Therefore, we review the dependence of the footing effect on etching cond itions in a time multiplexed deep etcher. The microfabrication of cantileve red structures using the footing effect is demonstrated by the micromachini ng of self-aligned, released electrostatic actuators. Silicon etching,;stru cture release and sidewall passivation or dielectric isolation deposition w ere done iri situ using very large scale integrated complimentary metal-oxi de-semiconductor (CMOS) compatible plasma chemistries only. Thus, the low-t emperature, soft-mask scheme presented here dan be easily integrated in the microfabrication of intelligent sensors and actuators; The measured pull-i n 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 p repared in this fashion, subsequently underwent 10(5) pull-in cycles. of we ar testing of up to 100 V without failure, confirming the robustness of thi s approach in the microfabrication of CMOS-compatible actuators and suspend ed structures. (C) 1999 American Vacuum Society. [S0734-2101(99)23304-0].