Effects of mechanical agitation and surfactant additive on silicon anisotropic etching in alkaline KOH solution

Abstract

Agitation is a key method that significantly affects silicon wet anisotropic etching quality including the etching rate and surface roughness. This study introduced the ultrasonic agitation to improve the roughness quality of etched (1 0 0) silicon plane in 30 wt.% KOH solution. These etching characteristics have been compared with those using magnetic stirring and no agitation. In ultrasonic agitation condition, the etching rate increases linearly with agitating power, but the surface roughness worsens. Although the etching rate always exceeds 1.2 μm/min and the average roughness, R a is below 15 nm, the membrane microstructures are damaged easily by ultrasonic agitation. Moreover, the KOH solution with added anionic surfactant, dihexyl ester of sodium sulfosuccinic acid, is also used to evaluate the etching properties of (1 0 0) silicon under without agitation. Owing to increasing hydrophilic ability between the hydrogen bubble and silicon surface and the effect of silicon wettability, the etching properties are promoted drastically. Experimental results show that the average roughness, R a can be reduced to 7.5 nm in aqueous KOH with anionic surfactant, which is about eight times better than achieved by the pure KOH solution without agitation for etching temperature of 100 °C. Meanwhile, the etching rate can be enhanced to 4.9 μm/min, which is 1.44 times better than that is obtainable in a pure KOH solution with ultrasonic agitation. The etching rate of the KOH solution with added surfactant is about twice that in the KOH solution with isopropyl alcohol (IPA) for etching temperature of 80 °C. Furthermore, this study also illustrates the reflectivity of etched surface for visible wavelength and the fabrication of thin film microstructures.