Mechanical stability and handling-induced failure of micromachined wafers for RF applications

Micromachining has been identified as the enabling technology for future RF silicon ICs (Katchi et al, 2001). In this paper, a study of mechanical stability and handling-induced mechanical failure of micromachined RF silicon wafers, considering excessive bending and fracture as the primary failure modes, is presented. Selected mechanical characteristics of [100]-silicon wafers, with arrays of rectangular recesses and circular through-wafer vias, have been determined theoretically using FEM simulations and experimentally by performing 3- and 4-point bending measurements. Recommendations for layout and geometry of the micromachined structures are derived. It is shown that selective substrate removal by forming recesses on up to 40% of the initial surface area has only a moderate influence on the wafer stiffness and therefore processing using conventional equipment is not jeopardized. The handling-related reliability can be improved by minimizing the maximum stress levels within the micromachined wafers. This can be achieved by minimizing the characteristic recess dimensions, maximizing the separations between them, and smoothing of any sharp features like rough surfaces, edges and corners.