Wet Etching of Pillar-Covered Silicon Surfaces: Formation of Crystallographically Defined Macropores

Abstract

Silicon substrates exposed to laser ablation in a chemically reactive environment such as or HCl can experience spontaneous formation of conical pillars. We use these pillars as a template to define the dimensions and order of macropores produced by etching such substrates in aqueous solutions of KOH or tetramethylammonium hydroxide. The pillars anchor the sidewalls of the pores during etching, and the interpillar spacing controls the width of the pores. The macropores have crystallographically defined shapes for which we develop an explanation based on the kinetics of etching. Si(001) macropores can be etched such that they are rectangular with straight walls and an inverted pyramidal bottom. They have been etched as through holes, which is of interest for optical applications. On Si(111), there is a transition from hexagonal to triangular macropores that are all aligned in one direction. The Si(111) pores exhibit an optimum etch time before they begin to disappear. The behavior of the macropores is quite similar regardless of whether the pillars are produced by nanosecond or femtosecond lasers. Differences observed relate to the different initial structures (spacing and regularity) of these two different types of pillar-covered surfaces.