A detailed kinetic study was performed to elucidate the mechanism of wet chemical etching of silicon in a HF-rich HF∕HNnormalO3 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 HNnormalO3 , such as normalN2normalO3 , 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–HNnormalO3 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.

The wet chemical etching of silicon using HNO 3 -rich HF/HNO 3 mixtures has been studied. The effect of different parameters on the etch rate of silicon, for example, the HF/HNO 3 mixing ratio, the silicon content of the etchant, temperature, and stirring speed in these solutions, has been examined and discussed in light of a previous study on etching in HF-rich HF/HNO 3 mixtures. Nitrogen(III) intermediates are generated owing to the dissolution of silicon and the decomposition if the solution is exposed to air. The nitrite ion concentration, measured in diluted etchant solution by ion chromatography, acts as a sum parameter for the reactive N(III) species in the concentrated etchant. The etch rate shows two different correlations to the nitrite concentration. In the region of high nitrite concentrations, the etch rate decreases slightly with decreasing nitrite concentration, whereas at lower nitrite concentrations, the etch rate increases linearly with further decreasing nitrite concentration. Stirring experiments and the determination of activation energies show that the etching of silicon in HNO 3 -rich etchants is controlled by diffusion. X-ray photoelectron spectroscopy measurements of the silicon surface after etching revealed a hydrogen termination independent of the concentration of reactive species and the content of HNO 3 in the etchant. Si-O containing surface species were not found. A combined electrochemical (injection of holes into the valence band of silicon) and chemical (Si-Si back-bond breaking by an attack of HF) reaction mechanism of silicon etching without generation of SiO 2 is proposed.

The role of intermediate species generated during wet chemical etching of silicon in a HF-rich HF/HNO3 mixture was studied by spectroscopic and analytical methods at 1 degrees C. The intermediate N2O3 was identified by its cobalt blue color and the characteristic features in its UV-vis and Raman spectra. Furthermore, a complex N(III) species (3NO+.NO3-) denoted as [N4O6(2+)] is observed in these solutions. The time-dependent decay of the N(III) intermediates, mainly by their oxidation at the liquid-air interface, serves as a precondition for the study of the etch rate as function of the intermediate concentration measured by Raman spectroscopy. From a linear relationship between etch rate and [N4O6(2+)] concentration, NO+ is considered to be a reactive species in the rate-limiting step. This step is attributed to the oxidation of permanent existing Si-H bonds at the silicon surface by the reactive NO+ species. N2O3 serves as a reservoir for the generation of NO+ leading to a complete coverage of the silicon surface with reactive species at high intermediate concentrations. As long as this condition is valid (plateau region), the etch rate is constant and yields a smooth silicon surface upon completion of the etching. If the N2O3 concentration is insufficient to ensure a coverage of the Si surface by NO+, the etch rate decreases linearly with the N2O3 concentration and results in a roughening of the etched silicon surface (slope region).

The reduction of nitric acid that occurs during wet chemical etching of silicon using HF/HNO3 mixtures had been studied under different reaction conditions. The proof of the generation of considerable amounts of ammonium shows that the reduction of nitric acid is more complex than assumed so far and that the current model of NO as final product of nitric acid reduction during silicon etching does not hold. A qualitative model of the nitric acid reduction during silicon etching in HF/HNO3 mixtures that bases on experimentally verified and also assumed nitrogen intermediates is presented and explains the formation of ammonium out of nitric acid by four successively proceeding two-electron reduction steps. The formed ammonium shows an unexpected temperature-dependent decomposition behavior in concentrated etch solutions that has to be discussed in light of a previous study on the generation and decay of N(III) intermediates, mainly dissolved N2O3 and NO+, during the etching. Below 8 °C, ammonium is found to be stable in concentrated etch solutions and in the presence of N(III) intermediates. The observed time-dependent decay of ammonium that occurs at temperatures ≥8 °C is directly related to the decay of N(III) intermediates. Consequently, the decay of ammonium was found to be finished immediately after all N(III) intermediates are completely decomposed. As one possible explanation, a model that links the decomposition of ammonium with the thermal and oxidative decomposition of the N(III) intermediates is suggested.

The stoichiometry of the wet-chemical etching of silicon in concentrated HF/HNO 3 mixtures has been studied. By quantifying the major reaction products in solution, the established model that 3 mol of Si are oxidized by 4 mol of HNO 3 to yield 4 mol of NO could not be confirmed. In HNO 3 -rich HF/HNO 3 mixtures, approximately 1.1 mol of HNO 3 are required to oxidize 1 mol of Si. Excess HNO 3 leads to massive accumulation of N(III) species in the etchants and massive formation of nitrous oxides due to incomplete reduction of the HNO 3 . An excess of HNO 3 leads to higher consumption and poorer utilization indicated by the massive accumulation of N(III) species in the etchants. In HF-rich mixtures, only 0.9 mol of HNO 3 are needed to oxidize 1 mol of Si yielding a lower accumulation of N(III) species and a higher utilization of the HNO 3 . Two parallel pathways contribute to the oxidation of silicon in such solutions: (i) via the oxidation by HNO 3 and reactive intermediates generated by the reduction of HNO 3 and (ii) via the formation of hydrogen. A comprehensive treatment covering alkaline etching, electrochemical etching in HF media, and etching in concentrated HF/HNO 3 mixtures is proposed based on the reactivity of the hydrogen terminated silicon surface against the applied oxidizing agent.