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 behavior. 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 in 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 limiting. 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 ratelimiting reagent indicates an exponential relationship between the etch rates and the concentration. This relationship has been explained qualitatively on the basis of a second, nonchemical autocatalytic factor, the heat of reaction.
This study was undertaken for the purpose of elucidating the physical and chemical processes that determine the behavior of an acid etching solution. The system HF, HNO~, H~O was chosen for study because it is the simplest, from the point of view of composition, of all the acid etching systems used on silicon. It was felt that an understanding of the simple system was a prerequisite for the understanding of the more complicated systems containing additives such as acetic acid, bromine, or heavy metal salts.