Working with KOH

KOH belongs to the family of anisotropic Si-etchants based on aqueous alkaline solutions. The anisotropy stems from the different etch rates in different crystal directions. The {111}-planes are almost inert whereas the etch rates of e.g. {100}- and {110}-planes are several orders of magnitude faster.

At NBI cleanroom, we use as standard a 28 wt% KOH solution. The etch rate and the selectivity towards a SiO2 mask depends on the temperature. We normally use a temperature of 80°C but it may reduce it to, e.g., 60 °C or 70°C for a higher-precision timed etch.

Si(001) temperature	Etch rate (μm/min)
60°C	0.4
70°C	0.7
80°C	1.3
Si(110) temperature	Etch rate (μm/min)
80°C	2.5
Thermal SiOx temperature	Etch rate (μm/min)
60°C	0.0012
80°C	0.0075