Automated contactless characterization of local thin film thickness and film stress with standard MEMS structures at wafer level

Abstract

The fabrication of microelectromechanical systems (MEMS) devices comprises many steps, each of which adds to the tolerance, resulting in device performances that may fall outside the defined limits in the design process. Hence, it is important to know local thin film properties most accurately, directly affecting the performance of the MEMS device. Furthermore, the capability of monitoring and mapping the thin film thickness and stress across a wafer enables device statistics and the strengthening of scientific statements. Within this study, we used standard MEMS structures consisting of a cantilever and a step profile to perform automated and contactless characterization of the local thin film thickness and stress across six 4-inch (100 mm) wafers. For this purpose, we constructed a measurement setup combining white light interferometry (WLI) to measure the static deflection of the cantilevered beams and plates and the thickness of the thin film through a step profile etched into the thin film. Even more, an XYZ-stage positions hundreds of devices below the objective lens of the WLI. This leads to precise maps of the local thin film thickness and to the extraction of a mean stress and a gradient stress from the static deflection of slender beams. The beams are oriented parallel and perpendicular to the wafer flat so that the measurement of orientation-dependent stress values is possible.