Non-monotonic roughening at early stages of isotropic silicon etching

Isotropic etching using a mixture of HF, HNO3 and CH3COOH was carried out for single crystalline Si surfaces for different times and the resulting morphologies were investigated using atomic force microscopy. The acquired data were analyzed using dynamic scaling theory. It was found that for each surface, there exists two roughness exponents which correspond to two different length scales. Moreover, the local roughness properties undergo a reversal between these two length scales before and after an etching time of 120 s. The power spectra density (PSD) curves of the analyzed images also show a reversal in the overall trend before and after 120 s. It is further noted that the PSD spectra of the surfaces resembles more that of a superstructured surface which is a distinct departure from the self affine nature of the surfaces investigated. The deviation is by far the largest for the 120 s etched surface. The morphology evolution in the present scenario does not follow the dynamical model of progressive hardening of the solid surface. Hillock flooding analyses of the AFM images exhibit the percolation nature of the process.