Mass and Electron Balance for the Oxidation of Silicon during the Wet Chemical Etching in HF/HNO3 Mixtures

The stoichiometry of the wet-chemical etching of silicon in concentrated HF/HNO3 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 HNO3 to yield 4 mol of NO could not be confirmed. In HNO3-rich HF/HNO3 mixtures, approximately 1.1 mol of HNO3 are required to oxidize 1 mol of Si. Excess HNO3 leads to massive accumulation of N(III) species in the etchants and massive formation of nitrous oxides due to incomplete reduction of the HNO3. An excess of HNO3 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 HNO3 are needed to oxidize 1 mol of Si yielding a lower accumulation of N(III) species and a higher utilization of the HNO3. Two parallel pathways contribute to the oxidation of silicon in such solutions: (i) via the oxidation by HNO3 and reactive intermediates generated by the reduction of HNO3 and (ii) via the formation of hydrogen. A comprehensive treatment covering alkaline etching, electrochemical etching in HF media, and etching in concentrated HF/HNO3 mixtures is proposed based on the reactivity of the hydrogen terminated silicon surface against the applied oxidizing agent.