Research On MEMS Surface Topography Measurement Technology Based On Phase Shift Interference Of Arbitrary Steps

In the process of micro-electro mechanical systems (MEMS) structure parameters measurement and MEMS reliability testing, it often require measure a three-dimensional structure surface topography, roughness, small displacement and deformation precisely. Currently, the microscopic interferometry with its high accuracy, high vertical resolution, quick and easy measurements, non-contact, etc., has become one of the most widely used of measurement means, including phase-shifting interferometry, white light interferometry, heterodyne interferometry and so on. Among them, the phase-shifting interferometry also plays an important role. The piezoelectric transducer (PZT) is often necessary to induce precise phase-shifts in phase-shifting interferometry to reconstruct three-dimensional topography, and high-precision PZT system is expensive, so it can not achieve widespread application. How common equipment in the process of MEMS can be used easily with a sufficient precision in three-dimensional topography measurement is a very meaningful work.Due to the MEMS process of oxidation, etching, depositing and sputtering techniques, usually there are many planes (such as a substrate) on the MEMS devices, and these planes are very close to the ideal planes, then based on the theory of light interference, we can get that interference fringes of the ideal plane is the cluster of many parallel straight lines. Besides, the light intensity distributions are cosine functions with the same period. Based on this characteristic, this paper presents a practical algorithm to calculate phase-shifts by fast Fourier transformation (FFT) from gathered interference fringes of the substrate, then micro surface topography can be reconstructed according to the obtained phase shifts. Obviously, an expensive and high-precision PZT is unnecessary in the presented method, and simply rotating the microscope focus knob can move up and down the stage to record multiple interferograms for reconstructing MEMS topography.In this paper, we also studied a SU-8 MEMS structure fabrication process based on sacrificial layer technology, and successfully fabricated SU-8 films of cantilever as our test MEMS structures. By means of the presented method in this paper, we reconstruct the test MEMS surface topography. Finally, the accuracy and feasibility of the method have been verified by experiments. Experiments indicated that the presented method can satisfy the needs of in-situ MEMS topography measurement and is very simple.