The mechanism of stain formation in the chemical etching reaction of silicon has been investigated in HF–oxidizing agent–H2O solutions. The chemical formula of the stain formed during the silicon etching reaction is K2SiF6. The concentration of holes on silicon surface increases with the increase of redox potential and the concentration of oxidizing agent used in manufacturing the etching solution. The increase in the hole concentration accelerates not only the etch rate but also the formation rate of K2SiF6. The etched silicon surfaces are covered with a K2SiF6layer when redox potential and concentration of oxidizing agent are great at low HF concentrations. This happens because the formation rate of K2SiF6is much greater than its dissolution rate by HF. Sufficiently high HF concentration in the etching solution is apparently essential to increase the etch rate without the formation ofK2SiF6. ©1997 American Institute of Physics.