A review of silicon bulk micromachining in NaOH solutions for MEMS fabrication

Silicon wet bulk micromachining using alkaline etchants such as KOH, TMAH, EDP, NaOH, and CsOH is a widely used technique for fabricating a variety of microstructures, including microcantilever beams, diaphragms, grooves, trenches, and microchannels. It is extensively employed in both research laboratories and the industrial manufacturing of microelectromechanical systems (MEMS). This technique offers several advantages, including low processing cost, crystallographic orientation-dependent etching, tunable etch rates through additives, the ability to fabricate three-dimensional structures with slanted sidewalls, controlled undercutting for suspended structures, and high selectivity toward masking materials. Among alkaline etchants, NaOH-based solutions have attracted considerable attention for silicon micromachining. The etching process involves the chemical dissolution of silicon, with the etch rate strongly influenced by parameters such as NaOH concentration, solution temperature, and the crystallographic orientation of the silicon substrate. Recent studies have focused on enhancing the etch rate and undercutting behavior through the incorporation of suitable additives. In this review, we comprehensively present the etching characteristics of NaOH solutions, including etch rate, undercutting, and etched surface morphology, both in the absence and presence of additives. Particular emphasis is placed on the influence of NH₂OH as an additive, which significantly modifies the etch rate and undercutting behavior. The etching performance of NH₂OH-modified NaOH solutions for Si{100}, Si{110}, and Si{111} wafers is systematically analyzed. This paper aims to serve as a consolidated reference for NaOH-based wet bulk micromachining in the fabrication of various MEMS components.