PHET, an electrodeless photovoltaic electrochemical etchstop technique

A novel etchstop technique for the fabrication of micromechanical components in single-crystal silicon is presented. The photovoltaic electrochemical etchstop technique (PHET) was characterized for anisotropic etching in 7M KOH in the 60° to 80°C temperature range, but can in principle be extended towards other anisotropic etchants. PHET is based on the photovoltage generated across a p/n junction illuminated during etching. In contrast with the established two-, three- or four-electrode electrochemical etchstop on n-epi, the technique does not require any external electrodes or connections to be made to the wafer, hence alleviating the need for a mechanical etchholder that protects one wafer side from the etchant. These etch-fielders are known to introduce stress and to substantially reduce yield in the production of the more fragile microstructures. Moreover, PHET is suited for the realization of diaphragm type structures in p-epi as well as for undercutting boron-diffused beam or bridge type of structures. In contrast with the established high boron dose etchstop, with its known adverse mechanical and electrical side effects, PHET does not require excessive boron concentrations. PHET therefore merges the fabrication possibilities offered by the conventional electrochemical etchstop and the conventional high boron dose etchstop, without being subject to the respective drawbacks of these techniques. The only technology required in addition to those used in the conventional etchstops is platinum sputtering. The possibilities that are offered for postprocessing standard p-well or twin-tub CMOS substrates are considered to be a major asset of the presented photovoltaic etchstop