FUNDAMENTALS OF 2-STEP ETCHING TECHNIQUES FOR IDEAL SILICON-HYDROGEN TERMINATION OF SILICON(111)

White deposits are seen to form in highly basic etching solutions whic h contain NH4F such as 40% NH4F or pH-enhanced buffered HF (BHF) (a mi xture of BHF and NH4OH). Surface infrared spectroscopy, x-ray photoele ctron spectroscopy, and Auger electron spectroscopy are used to charac terize a hydrogen terminated Si(111) surface and the chemistry of thes e 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 tunneli ng microscopy, and atomic force microscopy. A two-step etching, a 7 mi n dipping in 1% HF followed by a short dipping (approximately 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 se cond etching step converts a microscopically rough surface with all de fects on Si(111) into an atomically smooth, ideally monohydride-termin ated surface within 3 s. Similar results are obtained with a 3 s dippi ng 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 w hite surface deposits. Possible ways of inhibiting the formation of wh ite surface deposits from cleaning solutions are found to be the use o f purer chemicals and shorter last-step cleaning times. For example, a n increase of the last-step cleaning duration from 10 to 20 min is obs erved to increase the substrate area covered by the white deposits fro m about 1% to 5% for purer 40% NH4F, and from about 20% to 30% for les s pure 40% NH4F solutions.