ANISOTROPICALLY ETCHED SILICON MIRRORS FOR OPTICAL SENSOR APPLICATIONS

Optical devices in silicon often require mirror-quality surfaces. Memb ranes 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 we re also not as smooth as could be expected from the wafer surface qual ity before epitaxial growth, despite the use of the ECE stop. The infl uence of the concentration and temperature of the KOH solution on the roughness of both membrane and sidewalls has been investigated. The co ncentration of the KOH solution was varied between 5 and 50 weight per cent, and the temperature between 30 and 100 degrees C. It was found t hat the membrane surface quality is highly dependent on the initial su rface condition and the solution conditions, while the sidewall surfac e quality is primarily dependent on the oxygen content of the wafer an d 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 ta ken before, during, and after the etching for improving the membrane a s well as the sidewall surface smoothness are presented. Under the bes t conditions, the etched surfaces had a roughness of less than 0.1 mu m peak to peak.