19 February 2018 Digital holography for the investigation of buried structures with a common-path reflection microscope
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Abstract
Digital holographic microscopy (DHM) is an established technique for the investigation of biological samples and very promising for non-destructive testing. As DHM is an optical metrology technique it enables a non-contact, non-destructive and fast measurement which can be used for material characterization and quality control. DHM provides amplitude and phase information which originate from optical path differences and refractive index changes of the material, thus it is able to measure topographic structures. Especially the non-destructive inspection of buried or capped structures such as microelectromechanical systems (MEMS) is still challenging, due to the absorption in the covering layer and interference of light between the different layers. In this paper we present a common-path digital holographic setup for the investigation of reflective samples. By choosing a wavelength at which the covering layer is transparent, laser light can pass this surface. Furthermore, we employ a coherence gating effect utilizing a laser diode below threshold with reduced temporal coherence to diminish speckle patterns and to suppress interference between the layers. At the same time the spatial coherence remains high which improves the image quality. Using the angular spectrum method and a quality guided phase unwrapping algorithm, we successfully reconstruct 3D images of buried structures. In addition Zernike polynomials reduce the effect of wave distortions within the setup. Overall a lateral resolution of about 1.5 μm can be achieved.
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Lena Gö­ring, Lena Gö­ring, Markus Finkeldey, Markus Finkeldey, Martin R. Hofmann, Martin R. Hofmann, Nils C. Gerhardt, Nils C. Gerhardt, } "Digital holography for the investigation of buried structures with a common-path reflection microscope", Proc. SPIE 10558, Practical Holography XXXII: Displays, Materials, and Applications, 1055808 (19 February 2018); doi: 10.1117/12.2289524; https://doi.org/10.1117/12.2289524
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