Megapixel imaging of vibrations of microscopic structures and MEMS

22nd May 2018

Vibrations play a key role in the performance of micromechanical structures. In many cases the operating bandwidth of a device is directly defined by its resonance frequencies. Think of miniature microphones and loudspeakers in mobile telephones as well as acceleration sensors, micro-cantilevers etc. For other devices, such as automotive micro-electronics, excessive vibrations can lead to material fatigue and therefore complete system failure. Here, it is important to have methods to recognize undesired mechanical resonances, such that the design can be optimized towards a minimal response to the mechanical environment.

Numerical simulations are commonly used to predict the dynamic performance but practice shows often significant differences which are in part explained by production tolerances and mechanical boundary conditions (e.g. mounting the MEMS on a PCB). Therefore, actual measurements remain essential and laser interferometry based techniques have emerged as the method of choice for the contactless characterization of vibrations. To detect eigenmodes it is necessary to measure the vibrations at multiple positions. Although various solutions to scan the measurement laser over the sample are on the market, whole-frame imaging of vibrational modes at 1 Megapixel has not yet been possible. Furthermore, the increasing complexity of multi-layered or encapsulated MEMS designs makes it difficult to access such devices with conventional vibrometer designs.

To overcome these limitations SmarAct GmbH has developed the PICOSCAN Vibrometer. Here, a focused laser beam of a Michelson interferometer is raster-scanned over the sample to detect oscillations of up to 2.5 MHz. Scanning is performed by SmarAct’s established linear piezo positioners that provide nm accuracy in closed loop with a scan range of 20 mm. A confocal optical design of the interferometer ensures that only light reflected from the focus will be detected while all out-of-focus light is suppressed. It thus becomes possible to look through layers at underlying structures. The use of an infrared laser even allows to measure vibrations of a silicon structure through a silicon enclosure. Depending on the operating mode, the vibrations are automatically quantified by FFT analysis or with the integrated digital lock-in amplifier. The latter allows to record the amplitude and phase of oscillations while scanning the laser beam over the sample. It is thus possible to produce Megapixel images of vibrational modes. Vibrations can be excited directly, by actuating the MEMS with the built-in function generator signal or mechanically through the piezo-based shaker stage. Via single point measurements also extremely small spontaneous motions can be analyzed with single pm resolution. 

Another design feature that helps to make SmarAct’s PICOSCAN the most compact scanning vibrometer on the market is that the interferometer laser beam is used simultaneously to record a reflection image of the sample with a lateral resolution of 5 µm. This microscopy image is thus intrinsically aligned with the vibration measurements and a separate microscope imaging system is not required. The unique capabilities of the competitive priced PICOSCAN Vibrometer will enable designers, manufacturers and researchers to study MEMS dynamics at unprecedented detail.






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