Applications of laser-Doppler vibrometry during MEMS device qualification

Since a laser Doppler vibrometer (LDV) allows one to directly measure mechanical quantities, this technique is particularly well suited for characterising micro-electromechanical systems (MEMS) and for performing long-term reliability tests on such devices. In the work presented here, a microscope-mounted system is used at different stages of the MEMS device qualification process. This is illustrated by a wide range of applications. Mounted on a semi-automatic probestation, the system is used for mapping the within-wafer uniformity of several parameters of MEMS devices, such as pull-in voltage, out-of-plane displacement or resonance frequency. In turn, these quantities can be translated into material properties such as Young's modulus, residual stress and stress gradient. The same LDV system was also used on a manual prober for reliability testing of MEMS devices. Since scanning LDV measurements are performed by successively pointing a laser beam at a predefined set of locations, it is of the utmost importance to have no movement of the MEMS device relative to the laser source. A compensation algorithm was developed in order to compensate for the inevitable drift that occurs over extended periods of time. Electrostatic discharge (ESD) testing is another application to which the LDV system was adapted For this purpose, a Human Body Model (HBM) ESD tester was homemade. The tester is small enough to fit inside a vacuum probe station, while the LDV system is mounted above a measurement window. This setup allows electrical as well as mechanical measurements to be performed during ESD events on MEMS devices. As LDV measurements can be performed through translucent media, measurements can even be performed on MEMS devices after packaging, provided that transparent capping materials are used Hermeticity of wafer-level packaged MEMS devices was evaluated in this manner.