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

Specialized fabrication techniques created polycrystalline silicon (poly-Si) thin films that were permeable to a concentrated hydrofluoric acid (HF) solution, in order to support in situ vacuum encapsulation of surface microsensor devices on silicon wafers. 0.1-mu m-thick poly-Si thin films were made permeable through processes of amorphous silicon film formation by low pressure chemical vapor deposition, successive postannealing for crystallization, and excessive phosphorus diffusion by a phosphorus oxichloride predeposition. Permeability was obtained both on borophosphosilicate glass and nondoped silica glass sacrificial layers. Porous microstructures at the silicon grain boundaries, which were induced by heavy doping of phosphorus and consequent segregation of soluble precipitates, were first observed using a secondary electron microscope and a field emission secondary electron microscope. These observations confirmed that the submicron pores permitted passage of the fluid to dissolve the underlying sacrificial oxide films. Vacuum encapsulation of shell structures covered with the permeable poly-Si thin film was also carried out to demonstrate the vibration of a gyroscope structure in the sealed shell. (C) 2000 American Vacuum Society. [S0734-2101(00)16704-1].