Controlling the surface tension of alkaline etching solutions

Alkaline anisotropic etching with potassium hydroxide (KOH) and the additive 2-propanol (IPA) is a widely used process to texture monocrystalline silicon wafers industrially in order to reduce their reflection. Using IPA as additive in hot KOH (aq) the alcohol evaporates easily during the texturisation and the bath composition changes. Therefore IPA has to be redosed either during the texturisation or afterwards. The IPA concentration strongly influences the air to liquid surface tension of the etching solution. Adding IPA to an alkaline etching solution reduces its surface tension because IPA is surface active, resulting in a reciprocal proportionality. Online measurement of the surface tension allows dosing IPA to the bath solution using the surface tension as a control parameter. A proportional-integral-derivative controller can keep the surface tension and IPA concentration constant. This means that a texturisation can be processed at well defined conditions.