Anisotropically Etched Silicon Mirrors for Optical Sensor Applications

Optical devices in silicon often require mirror-quality surfaces. Membranes etched in KOH using the electrochemically controlled etch (ECE) stop have been found to have sidewalls less smooth than expected from the {111} etch-bordering planes. Furthermore, the membrane surfaces were also not as smooth as could be expected from the wafer surface quality before epitaxial growth, despite the use of the ECE stop. The influence of the concentration and temperature of the KOH solution on the roughness of both membrane and sidewalls has been investigated. The concentration of the KOH solution was varied between 5 and 50 weight percent, and the temperature between 30 and 100 o C. It was found that the membrane surface quality is highly dependent on the initial surface condition and the solution conditions, while the sidewall surface quality is primarily dependent on the oxygen content of the wafer and the thermal budget due to the integration of active bipolar devices in the epilayer prior to etching in KOH. A number of measures to be taken before, during, and after the etching for improving the membrane as well as the sidewall surface smoothness are presented. Under the best conditions, the etched surfaces had a roughness of less than 0.1 μm peak to peak