An Impact of the Residual Stress on the Sacrificial Release of Microelectromechanical Membranes

Abstract

Residual stress in silicon nitride membranes, deposited by PECVD technique is studied. Substrate bending technique
with application of laser interferometry was used for the stress measurement. The controllability of tensile stress on the
functional film (from 26 MPa to 220 MPa) is demonstrated by alternating the monosilane to ammonia flows during the
deposition. The sacrificial PECVD polysilicon film was found to have dominating compressive stress in the range from
–34 MPa to –74 MPa. Thermal component of the stress was investigated too. An impact of the intrinsic stress (tensile
and compressive) on the sacrificial release process was demonstrated by several release experiments. The SiNx-Cu-
-Cr2O3 : H2O : H2SO4 combination of constructional-sacrificial-etch materials was found to work reliably, when releasing
ø30 μm membranes, with moderate tensile stress of 75 MPa. The SiNx-polySi-KOH combination of materials was found
to have too low of etching selectivity either unsatisfactory stress impact on the released membranes.