New insights into acidic wet chemical silicon etching by HF/H sub(2)O-NOHSO sub(4)-H sub(2)SO sub(4) solutions

Acidic wet chemical etching of crystalline silicon has been examined by utilization of HF-NOHSO sub(4)-H sub(2)SO sub(4) 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/H sub(2)SO sub(4) solutions is determined by NO super(+)-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 super(+) as limiting for the overall etching process. Direct participation in the rate-limiting step was confirmed by calculation of activation energies. Increasing NO super(+)-ion contents cause transition from reaction (E sub(A)=55 kJ mol super(-1)) to diffusion controlled (E sub(A)=10 kJ mol super(-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 super(19)F-NMR and Raman spectroscopy identified SiF sub(5) super(-)/HF sub(2) super(-) complexes as well as elementary hydrogen (H sub(2)) as hitherto unknown products of silicon dissolution reactions in HF-NOHSO sub(4)-H sub(2)SO sub(4) mixtures. Based on our findings a strategy for fundamental investigations of relevant reaction pathways is presented and discussed with regard to reported mechanistic concepts.