Si(100) Etching in Aqueous Fluoride Solutions: Parallel Etching Reactions Lead to pH-Dependent Nanohillock Formation or Atomically Flat Surfaces

A dramatic, pH-dependent change in the steady-state chemical and morphological structure of Si(100) surfaces etched in aqueous fluoride solutions is observed with infrared spectroscopy and scanning tunneling microscopy. Low pH solutions (5 ≤ pH ≤ 7), such as the technologically important buffered oxide etchant (buffered HF), produce rough surfaces covered with nanoscale Si{110}-faceted hillocks. In contrast, higher pH solutions (7.8 ≤ pH ≤ 10), including 40% NH4F (aq.), produce atomically smooth surfaces. The etched surfaces are terminated by a monolayer of H atoms irrespective of pH. The pH-dependent transition is attributed to two competing multistep reaction pathways. At higher pH, the base-catalyzed formation of a surface silanone leads to the production of smooth surfaces. This reaction channel is suppressed at low pH, leading to the formation of {110}-faceted hillocks by a second reaction. The morphological transition is not affected by dissolved O2 in the etchant.