Galvanic etching for sensor fabrication

This paper describes the application of a novel etch-stop technique, based on galvanic element formation, to the fabrication of micromechanical sensor s. The theory of operation in tetramethyl ammonium hydroxide (TMAH) and hyd rofluoric acid (HF) solutions is discussed, together with the main limitati ons. A number of devices are presented. These include a piezoresistive pres sure sensor made with a galvanic etch stop in a 25% TMAH solution at 80 deg reesC and the contactless fabrication in similar solutions of a vibration s ensor based on free-standing crystalline silicon beams. The thickness defin ition and uniformity were found to be poorer with the cantilever than with the membrane type sensors. Oxygen in the solution was used to provide the c ell current, which was therefore rather low. This resulted in a low package density of the sensor elements on the wafer. Free-standing thick polysilic on structures were also formed with a galvanic etch-stop technique in HF so lutions. The selectivity of this process, and the thickness definition and uniformity were good. In these solutions, the cell current could be easily increased by adding other strong oxidizing agents, such as hydrogen peroxid e. Therefore, high package densities could be achieved in these solutions.