Development of an illumination module for stroboscopic phase-shift interferometry on MEMS devices

The proper functionality of microelectromechanical systems (MEMS) is governed by characterisation methods reliant on optical metrology. Static characterisation methods like interferometry are well established. The standard dynamic characterisation method is Laser Doppler Vibrometry (LDV), which can be used for areal measurements by merging data from pointwise measurements. However, the increasing complexity of MEMS structures is raising the demand for more efficient spatial dynamic analyses. Phase-shifting interferometry (PSI) is state of the art in high-resolution topography acquisition techniques. By equipping interferometers with a stroboscopic illumination source, it is possible to perform dynamic analyses of periodically oscillating samples by synchronising the illumination flashes to the sample motion. Although this is a known approach, interferometers with this technology embedded are scarce and restricted to lower bandwidths. This paper introduces an external standalone stroboscopic illumination module, which can be installed onto the interferometer objective through optical fibres. Firstly, fast-switching power-LEDs were selected as illumination sources. Optical pulse length as short as 15 ns was realized, leading to a bandwidth of some tens of MHz. Micromirror arrays (MMAs) developed at the Fraunhofer IPMS were used as test samples to prove the concept. For MMAs switched at 2 kHz, overshoot and oscillation frequency were determined. The main advantage of the stroboscopic interferometric method is in recording thousands of points simultaneously, in contrast to the LDV. This is equivalent to a performance increase in the order of tens or even hundreds of times.