Automated Wafer-Level Testing of Critical MEMS Parameters Using Optical Vibration Measurement
Authors: Marco Meinig, Eric Lawrence, Steffen Kurth, Heinrich Steger Company: Fraunhofer ENAS and Polytec Date Published: 11/11/2014
IWLPC (Wafer-Level Packaging)
Abstract: Laser-Doppler-Vibrometry is a well-established measurement technique for the characterization of the dynamic properties of a MEMS device . In addition, for many MEMS devices, critical geometric parameters and material properties, not accessible by direct measurement, can be derived from the results of a vibration measurement. Examples are the determination of pressure sensor membrane thickness  or the depth of spring elements in MOEMS Fabry-Perot interferometers, which are used as tunable IR-filters . For quality control purposes this process can be completely automated: A specific device on the wafer is electrically excited on an automated probe station with a broadband signal. The electrostatic fringing field is created by means of a special probe card with a transparent indium-tinelectrode (ITO). The vibration that is generated in the membrane is measured non-invasively with a laser vibrometer without any direct mechanical or electrical contact. The frequency response function (FRF) can be determined from the measurement data and the excitation signal. The natural frequencies are determined from the FRF by curve fitting. Finally an analytical multivariate description of the MEMS natural frequencies as a function of geometric dimensions and material properties is adapted to the measured natural frequencies by an optimization algorithm. The result is a very precise determination of the geometric and material properties. These steps, usually completed in less than one second, are then repeated automatically for all devices on the wafer. The combination of laser Doppler interferometry for motion analysis, and electrical forces for excitation result in a non-contact, non-destructive and time efficient measurement technique perfectly suited for clean environments and 100% control during MEMS production.