Stress characterization of MEMS microbridges by micro-Raman spectroscopy

In this research, micro-Raman spectroscopy is employed to examine, and characterize the residual stress in MUMPs polysilicon, micro-electro-mechanical systems (MEMS) microbridge structures. Currently, few techniques are available to measure the residual stress in MEMS devices. The residual stresses from the deposition processes can have a profound effect on the functionality of the fabricated MEMS structures. Typically, material properties of thin films used in surface micromachining are not controlled during deposition. The residual stress, for example, tends to vary significantly for different deposition methods. Several post-fabrication processes are available to reduce the inherent residual stress from these deposition methods. In an attempt to reduce the residual stress in MEMS microbridges, a phosphorous diffusion and accompanying anneals were performed. Residual stress profiles obtained through micro-Raman spectroscopy are presented, illustrating stress reduction is possible through these post-processing techniques. The stress profiles presented demonstrate the variations between the MUMPs structural layers (Poly1 and Poly2) for different microbridge widths. The improved stress levels could significantly increase device performance, reliability, and yield.