Experimental studies on the mechanism of wet chemical etching of silicon in HF/HNO3 mixtures

A detailed kinetic study was performed to elucidate the mechanism of wet chemical etching of silicon in a HF-rich HF/HNO3 mixture. In contrast to earlier studies, the etch rates were determined by dissolution of only a few milligrams of silicon in carefully thermostatted acid mixtures in order to avoid a change in composition during the experiments and an uncontrolled warming of the etchant. All etch experiments were followed by chemical analytics. The etch rates were studied as a function of temperature, silicon content of the etchant (utilization), and stirring speeds. By choosing proper reaction conditions, intermediates of the reduction process of HNO3, such as N2O3, were stabilized and spectroscopically identified. Furthermore, it was found that the nitrite ion concentration, measured in diluted etchant solution by ion chromatography, acts as a parameter for the reactive N(III) species in the concentrated etchant. Two different etch regimes were identified. In the region of high nitrite concentrations, the etch rate is apparently independent on the nitrite concentration. At lower nitrite concentrations, the etch rate decreases linearly with the nitrite concentration. Kinetic examinations showed that the reaction mechanism remains unchanged in both regimes. Furthermore, the kinetic parameters of nitrite decays were determined. The obtained results provide the first explanation of why an etch mixture of constant HF-HNO3 ratio at given Si content can exhibit different etch behavior. A mechanistic model on the role of N(III) species and dissolved gases in the etching process including a suggestion of the rate-limiting step is presented. Consequences for technical applications are discussed.