Effects of metal impurities on the etch rate selectivity (110)/(111) in (110) Si anisotropic etching

We have investigated the effects of metal impurities and thermal oxidation process on (110) Si anisotropic KOH etching characteristics for fabricating microsensors and microactuators having precise deep narrow grooves or channels. The effect of metal impurities such as Al, Cr, Fe, Pb and Cu are evaluated using 30wt% KOH etchant at 60 degrees C. Al, Cr and Fe have no relation to the etching characteristics, however, Pb and Cu strongly affect the etching characteristics. The etching rate of (110) Si decreases with increasing Pb and Cu concentration. The etch rate selectivity (110)/(111), which is an anisotropy ratio defined by the etch rate ratio of (110) and (111) plane, is not changed by Pb impurity, but extremely decreases with increasing Cu impurity. It drops from 150 to 80 when the Cu concentration is over 50ppb, and saturated over the concentration. With respect to the surface roughness within our evaluated metal impurities, Cu impurity also strongly affects the roughness of the etched (110) surface. In addition to the effect of metal impurities, we have experienced the effect on the etching selectivity by the thermal oxidation process. The selectivity is decreased with repeating thermal oxidation process, and this tendency is more pronounced with higher oxygen concentration of evaluated Si wafers. As a result, controlling Cu impurity concentration and using a low oxygen concentration wafer are crucial considerations when fabricating sensors and actuators with narrow grooves or channels by (110) Si anisotropic KOH etching.