KOH anisotropic silicon etching for MEMS accelerometer fabrication

This dissertation is intended as a guidebook for processing crystalline silicon by anisotropic potassium hydroxide (KOH) wet etching. In low cost bulk micromachining of silicon for MEMS (Micro Electro Mechanical Systems), anisotropic wet etching of single crystalline silicon in aqueous KOH solutions is a technically important process. Anisotropic etching of silicon is used to create mechanical microstructures in silicon. Differences in etch rates between the different crystal planes are utilized in the process. The present dissertation explores six different aspects of silicon anisotropic etching using an accelerometer as a test device. The first part of the thesis concentrates on liquid phase phenomena, and the latter part with the interactions of the silicon crystal with the etching process. It is shown that etch rates and anisotropy are extremely sensitive to the process conditions and equipment, for example rotational flow in the etch bath is shown to be superior to laminar flow. It is also shown that the etch rate and the resulting surface quality are sensitive to very small changes in Pb concentrations in the range of 200 to 300 ppb. The effects of wafer oxygen levels have been studied from etching point of view and were found to have a marked effect especially on etching the (111) crystal plane. A novel method for eliminating the effects of grinding damage on etching has been introduced. Ultra Poligrind wafers were used as substrate material for MEMS accelerometer structures. These wafers eliminate the need for double side polished wafers and are expected to reduce the cost of starting material considerably. Taken together, the thesis provides an extensive set of guidelines for silicon anisotropic KOH etching. By careful optimization of both the etchant composition and equipment, and matching the design to wafer specifications, it is shown that a very high degree of control can be achieved by KOH wet etching. These guidelines will be useful for further MEMS and NEMS (Nano Electro Mechanical Systems) development in cases where critical structures are defined by KOH etching.