Fundamentals of two-step etching techniques for ideal silicon-hydrogen termination of silicon(111)

Abstract

White deposits are seen to form in highly basic etching solutions which contain NH4F such as 40% NH4F or pH-enhanced buffered HF (BHF) (a mixture of BHF and NH4OH). Surface infrared spectroscopy, x-ray photoelectron spectroscopy, and Auger electron spectroscopy are used to characterize a hydrogen terminated Si(111) surface and the chemistry of these white deposits; such analyses show significant amounts of nitrogen, nonbonded to silicon, and fluorine in the deposits. The morphology of the flat surface is examined with optical microscopy, scanning tunneling microscopy, and atomic force microscopy. A two-step etching, a 7 min dipping in 1% HF followed by a short dipping (∼3 s) in pH-enhanced 1% HF solution (a mixture of 1% HF and NH4OH, pH=9.25), is proposed to obtain an atomically smooth and impurity-free surface, based on studies with all spectroscopies and microscopies used. The second etching step converts a microscopically rough surface with all defects on Si(111) into an atomically smooth, ideally monohydride-terminated surface within 3 s. Similar results are obtained with a 3 s dipping in pH-enhanced BHF solution (a mixture of BHF and NH4OH, pH=9.25); however, studies with the spectroscopies and microscopies used show that longer treatments with this solution result in the formation of white surface deposits. Possible ways of inhibiting the formation of white surface deposits from cleaning solutions are found to be the use of purer chemicals and shorter last-step cleaning times. For example, an increase of the last-step cleaning duration from 10 to 20 min is observed to increase the substrate area covered by the white deposits from about 1% to 5% for purer 40% NH4F, and from about 20% to 30% for less pure 40% NH4F solutions.