Characterization of the dynamics of encapsulated silicon MEMS devices using low-coherence heterodyne LDV technology

Microscope-based Laser Doppler vibrometers (LDV) are optical instruments using laser Doppler interferometry to measure the motion of vibrating structures. As laser vibrometers measure without contact, they are also widely used for the characterization of the vibrational dynamics of silicon based micro-electro-mechanical systems (MEMS). Because silicon is opaque for visible light, MEMS-devices must be prepared without encapsulation to enable vibration measurements with standard laser vibrometers. However, the encapsulation itself is a critical process step during MEMS fabrication, and the reopening of the encapsulation bears the risk of damaging the device or altering its characteristics. Due to the high refractive index of silicon, vibrometry using infrared light is compromised by the inevitable influence of interfering reflections from encapsulation and device boundaries on the measurement results.A novel low-coherent measurement technique is presented allowing to effectively suppress spurious interferences. This way, highly accurate vibration measurements and thus reliable analysis of the device dynamics of encapsulated MEMS are possible.