role of surface hydrides and fluorides in the Si chemical vapor deposition (CVD) process was studied by in situ Fourier transform infrared spectroscopy using a porous silicon film as the substrate, Hydrides protect the silicon surface from oxidation until they are desorbed at 350-400 degrees C. However, the hydrides are replaced almost completely by surface fluorides when SiH2F2 is used as a reactant in the CVD process at temperatures above 400 degrees C. The surface is then etched due to a reaction between the fluorides and the surface silicon, When the process temperature is lowered to 250 degrees C, the surface is covered by both hydrides and fluorides, and a silicon film is deposited.