Chemical Etching of Silicon I. The System HF, HNO, and H20

The kinetics of the etching of silicon in the system HF, HNO~, and H~O was studied as a function of the composition of the etchant at 25 ~ A triaxial plot of the etch rate vs. composition of the etchant shows two extreme modes of be-havior. In the region of high nitric acid compositions, etch rates are functions only of the hydrofluoric acid concentration. In the region of high hydrofluoric acid compositions, nitric acid concentration determines the etch rates. The kinetic behavior in the latter region is complicated by autocatalysis n which the reduction products of nitric acid are involved. The reaction proceeds by an oxidation step followed by the dissolution of the oxide. In the high hydrofluoric acid region the oxidation step is rate l imit-ing. In the high nitric acid region the dissolution step is rate limiting. In both regions the flow of reagent to the surface by diffusion determines the etch rates. A plot of the etch rates as a function of the concentration of the rate-l imiting reagent indicates an exponential relationship between the etch rate