Deformation control of microbridges for flow sensors

To obtain good performance from a microbridge flow sensor, we have investigated the buckling mechanism of microbridges by fabricating various shapes of bridges. We have achieved deformation control for both the longitudinal axis and the transverse axis of Pt/SiNx double-layered microbridges by appropriate selection of the residual bridge stress of Pt/SiNx structures and of the stress gradient between the Pt film and the SiNx film. The deformation directions of the longitudinal axis of bridges for the beam bridge (Type I) are all the same as that of cantilevers and may be predicted via the stress gradient between the Pt and SiNx films of the bridges. The deflection of the transverse axis of the table bridge supported by four beams (Type II) changes linearly with the residual bridge stress of the Pt/SiNx structure and the deformation changes for the transverse axis are the same as that of completely free films, as predicted from the stress gradient between the Pt film and the SiNx film. The interesting result is that the deformation direction for the longitudinal axis of the Type II bridge is opposite to that of the Type I with the same film structure. We discuss the reason for this opposition via differences in the progress of the anisotropic etching. We consider that this result will expand the range of manufacturable shapes and film structures of microbridges.