Polycrystalline silicon thin films with hydrofluoric acid permeability forunderlying oxide etching and vacuum encapsulation

Specialized fabrication techniques created polycrystalline silicon (poly-Si ) thin films that were permeable to a concentrated hydrofluoric acid (HF) s olution, in order to support in situ vacuum encapsulation of surface micros ensor devices on silicon wafers. 0.1-mu m-thick poly-Si thin films were mad e permeable through processes of amorphous silicon film formation by low pr essure chemical vapor deposition, successive postannealing for crystallizat ion, and excessive phosphorus diffusion by a phosphorus oxichloride predepo sition. Permeability was obtained both on borophosphosilicate glass and non doped silica glass sacrificial layers. Porous microstructures at the silico n grain boundaries, which were induced by heavy doping of phosphorus and co nsequent segregation of soluble precipitates, were first observed using a s econdary electron microscope and a field emission secondary electron micros cope. These observations confirmed that the submicron pores permitted passa ge of the fluid to dissolve the underlying sacrificial oxide films. Vacuum encapsulation of shell structures covered with the permeable poly-Si thin f ilm was also carried out to demonstrate the vibration of a gyroscope struct ure in the sealed shell. (C) 2000 American Vacuum Society. [S0734-2101(00)1 6704-1].