Study of electrochemical etch-stop for high-precision thickness control of silicon membranes

A method is described to control the thickness of single-crystal silicon membranes, fabricated by wet anisotropic etching. The technique of an electrochemical etch-stop on an epitaxial layer is used to yield better thickness control over the silicon membranes (±0.2 μm s.d.) and hence improve the reproducibility of piezoresistive pressure sensors. The output characteristics of such sensors are compared with previously fabricated pressure sensors not utilizing accurate control over membrane thickness. The benefits of the etch-stop approach become apparent when reductions in the pressure-sensitivity variations are considered. Without etch-stop, the sensitivity on one wafer varied by a factor of two from one sensor to the other. With etch-stop, the pressure sensitivity of devices fabricated on the same wafer can be controlled to within ±4% s.d