New insights into acidic wet chemical silicon etching by HF/H2O–NOHSO4–H2SO4 solutions

Acidic wet chemical etching of crystalline silicon has been examined by utilization of HF–NOHSO4–H2SO4 mixtures. In light of our previous studies the effects of nitrosyl ion concentrations on etching rates were studied time- and temperature resolved. The reactivity of crystalline silicon surfaces in HF/H2SO4 solutions is determined by NO+-ion concentrations at the silicon/electrolyte interface, measured by ion chromatography. Quantitative solution analysis proofed accumulation of ammonium ions and indicated the conversion of NO+ as limiting for the overall etching process. Direct participation in the rate-limiting step was confirmed by calculation of activation energies. Increasing NO+-ion contents cause transition from reaction (EA=55 kJ mol−1) to diffusion controlled (EA=10 kJ mol−1) etching procedures. In combination with time and concentration dependent studies of produced structures a convenient regime for selective texturing or polishing polycrystalline silicon surfaces is reported. Qualitative analysis by 19F-NMR and Raman spectroscopy identified SiF5−/HF2− complexes as well as elementary hydrogen (H2) as hitherto unknown products of silicon dissolution reactions in HF–NOHSO4–H2SO4 mixtures. Based on our findings a strategy for fundamental investigations of relevant reaction pathways is presented and discussed with regard to reported mechanistic concepts.