Effects of doping on release stiction for polysilicon MEMS applications

The effects of doping the bottom and structural polysilicon layers for MEMS cantilever beam structures with respect to release stiction has been investigated. Test structures consisting of beams ranging in length from 60 to 400 microns were doped with either phosphorus or boron at two extreme concentrations, as well as un-doped control wafers, and analyzed via optical interferometry for release stiction. Results indicate large differences in release stiction with the highly doped phosphorus samples exhibiting more than a fifteen fold increase in release stiction compared to boron and control samples. The main effect of highly doped phosphorus samples appears to be in changing the surface from hydrophobic to hydrophilic, allowing more water to remain on the surface. Results indicate no difference between doping the bottom poly silicon layer or the top structural layer, with both showing large degrees of release stiction.