We investigated chlorine trifluoride (CIF3) etching of silicon with a patterned oxide mask layer prepared by e-beam lithography. The mask apertures varied from 0. 1 mu m to 300 mu m. This enables to adjust the flow rate of CIF3 molecules into the etched cavity leading to a strong variation of the CIF3 abundance at the silicon surface. A crucial dependence of the etch rate on the aperture area was observed revealing a maximum of the etch rate for a specific CIF3 abundance. A physical description of the etch process is developed in order to distinguish between different mechanisms within the etching process. At low CIF3 abundance the etch rate is limited due to a lack of CIF3. For high abundance the etch rate is assumed to be hampered by a diffusion like transport of CIF3 molecules through a fluorosilyl layer formed on the silicon surface. It can be shown that the etch rate of silicon with CIF3 is not limited by the chemical reaction at high CIF3 abundance. Additionally, we observed a change in etching behavior from isotropic to anisotropic with a strong correlation to the etching regime. (c) 2005 American Vacuum Society.