PHET, AN ELECTRODELESS PHOTOVOLTAIC ELECTROCHEMICAL ETCHSTOP TECHNIQUE

A novel etchstop technique for the fabrication of micromechanical comp onents in single-crystal silicon is presented, The photovoltaic electr ochemical etchstop technique (PHET) was characterized for anisotropic etching in 7M KOH in the 60 degrees to 80 degrees 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 illu minated during etching, In constrast with the established two-, three- or four-electrode electrochemical etchstop on n-epi, the technique do es not require any external electrodes or connections to be made to th e wafer, hence alleviating the need for a mechanical etch-holder that protects one wafer side from the etchant, These etch-holders are known to introduce stress and to substantially reduce yield in the producti on of the more fragile microstructures, Moreover, PHET is suited for t he realization of diaphragm type structures in p-epi as well as for un dercutting boron-diffused beam or bridge type of structures, In contra st with the established high boron dose etchstop, with its known adver se mechanical and electrical side effects, PHET does not require exces sive boron concentrations, PHET therefore merges the fabrication possi bilities offered by the conventional electrochemical etchstop and the conventional high boron dose etchstop, without being subject to the re spective drawbacks of these techniques, The only technology required i n addition to those used in the conventional etchstops is platinum spu ttering, The possibilities that are offered for postprocessing standar d p-well or twin-tub CMOS substrates are considered to be a major asse t of the presented photovoltaic etchstop. [34]