Aqueous solution of KOH or pottasium hydroxide is widely used in MEMS to define deep cavities with slanted walls like that of the pressure sensor cavity. KOH based anisotropic etching gives good accuracy and control over the etch profile with very limited lateral etching. Thermal oxide is often used to selectively mask the substrate during KOH etch due to the low etch rate of oxide in KOH.
This design interface can be used to predict the etch rate of <100> and <110> silicon substrate exposed to an aqueous solution of KOH of a given temperature and concentration. The temperature can be between 20°C and 70°C and concentration between 10% and 60%. It can be seen that the etch rate peaks at about 20%. Similarly the etch rate for thermally grown silicon dioxide can be found out. In this case the etch rate peaks at about 35% concentration.
The plot shows the variation of etch rate with etchant concentration for the given temperature and selected material. Using the crosshair, the etch rate for any concentration between 10% and 60% can be read out.