Etching Mechanism Behind the High-Speed Etching of Silicon in NH₂OH-added Alkaline Solutions

Abstract

Anisotropic wet bulk micromachining is one of the main techniques used in microelectromechanical systems to fabricate microstructures that have applications in sensors and actuators. Potassium-hydroxide-based and tetramethylammonium-hydroxide-based solutions are the most commonly used wet anisotropic etchants. However, the etch rate attainable using these conventional etchants is limited and, hence, affects industrial productivity. Alkaline solutions with some additives show improved etching characteristics. The addition of NH₂OH to pure alkaline etchant solutions increases etch rate significantly. The etch depth attained per unit time is more in these modified solutions compared with pure alkaline solutions. This can be exploited to make microstructures with low fabrication time and high productivity. In this work, a simple etching mechanism is proposed to explain silicon wet anisotropic etching in NH₂OH-added alkaline solutions. The mechanism is based on a two-step etching process: (i) oxidation and (ii) removal of Si atoms by water molecules. The initial oxidation step has two possible pathways: chemical and electrochemical oxidation. In the presence of alkaline solutions, NH₂OH gives NH₂O^- ions and H₂O molecules. Thus, NH₂O^- ions, OH^- ions, and H₂O molecules are the reactive species in the modified etchant solution. The aim of this work is to elucidate the detailed reaction mechanism in NH₂OH-added alkaline solution and highlight the reason behind the enhanced etch rate.