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.

The etch rate of silicon in solutions of various compositions selected from the system HF , HNO3 , H2O , and HC2H3O2 has been investigated over the temperature range 0° to 50 °C. The activation energy of the etching process has been found to be different in the different composition regions. In the high HNO3 region values of about 4 kcal/mole have been observed and interpreted as characteristic of a diffusion governed reaction. In compositions containing H2O or HC2H3O2 diluents the activation energy increases, and two values are found. In the high HF region two values are also observed, one in the range of 10–14 kcal/mole, and the other in the range of about 20 kcal/mole. The significance of the various values of the activation energy is discussed.

The kinetics of the reaction of silicon with solutions of hydrofluoric acid, nitric acid, and acetic acid are reported as a function of the composition of the etchant. The system qualitatively behaves in the same manner as the system hydrofluoric acid, nitric acid, and water, which has been reported previously. Quantitatively, the acetic acid diluted system shows a much higher tolerance for the diluent than does the water diluted system. The greater tolerance for acetic acid has been explained on the basis of the lesser ionization of nitric acid in acetic acid than in water. The reaction mechanisms postulated for the water diluted system have been found to apply equally well to the acetic acid diluted system.

The etching of silicon in HNO3‐HF based systems proceeds by a sequential oxidation‐followed‐by‐dissolution process. In those composition regions where the solution is very low in HNO3 and rich in HF , the rate‐limiting process is the oxidation step. Consequently, electron concentration, surface orientation, crystal defects, and catalysis by lower oxides of nitrogen play an important role. In those compositions where HF is in limited supply, dissolution of the formed oxide is the rate‐controlling step and diffusion of the complexing fluoride species is the important factor. Therefore, crystal orientation and conductivity type independence as well as hydrodynamic control are the consequences. In order to meaningfully select an etching composition to solve a specific processing problem, it is necessary to understand this composition‐mechanism interaction. Corollary with the mechanism understanding, sample geometry effects have been followed as a function of solution composition. The HF‐HNO3‐H2O solution composition plane has been characterized into various regions where the two basic mechanisms interact and specific procesing utilization is shown. Similar results are shown for the system HF‐HNO3‐HC2H3O2 . In addition, a number of particular etching problems are posed, and solutions offered, that make use of these composition characterizations, and show how one can use their information to solve other practical processing problems.

A study has been performed on the acid etching of germanium, and the results have been compared with those of a similar study on the acid etching of silicon. The conclusion is reached that subfluorinated compounds of germanium are formed as intermediates in the diffusion‐limited composition region. It has also been found that passivating films play an important role in the dissolution of germanium. In addition, the tolerance of the system to dilution with HC2H3O2 is much greater than it is to dilution with H2O .