The same etchant produces both near-atomically flat and microfaceted Si(100) surfaces: The effects of gas evolution on etch morphology

The effects of H2 gas evolution during the etching of silicon surfaces by aqueous ammonium fluoride (NH4F) solutions were investigated by scanning tunneling microscopy, atomic force microscopy, optical microscopy, and noncontact profilometry. If H2 bubbles, a reaction product, were removed from the etching surface or if their coalescence was suppressed, near-atomically flat surfaces were produced. Otherwise, the etched surface developed significant roughening on many length scales with several characteristic morphological features, including nested, nearly-concentric circular etch pillars, circular etch pits, and faceted micropits. Mechanisms for the production of all three types of features are proposed. Chemical and physical means of suppressing bubble-induced surface roughening are presented. These results explain the conventional wisdom that aqueous fluoride etchants roughen Si(100) surfaces, in part by promoting the formation of Si{111} microfacets. Although some conditions promote the formation of a...