Common-path digital holographic microscopy

Markus Finkeldey, Photonics and Terahertz Technology, Ruhr-Universität Bochum, Bochum, Germany
Nils C. Gerhardt, Photonics and Terahertz Technology, Ruhr-Universität Bochum, Bochum, Germany
Martin R. Hofmann, Photonics and Terahertz Technology, Ruhr-Universität Bochum, Bochum, Germany

We investigate digital holographic microscopy (DHM) in reflection geometry for non-destructive 3D imaging of semiconductor devices. This technique provides us with high resolution information on the inner structure of a sample while maintaining its integrity. The setup is based on a common-path off-axis interferometer which offers several advantages in comparison to standard Mach-Zehnder or Michelson geometries. In a common-path interferometer, both reference and image field travel through the same optics on a path very close to each other leading to a significantly increased phase stability. Furthermore, the implemented geometry is self-referencing using spatial filtering to extract a reference field from the image field. As a consequence, the common-path interferometer can easily be implemented in standard microscopes and camera ports.

Here, we use the common-path DHM to get phase and amplitude images of buried and non-buried semiconductor structures. While DHM easily offers high resolution phase imaging of surface structures in reflection geometry they are typically incapable to provide high quality phase images of buried structures due to the interference of reflected waves from different interfaces inside the structure. In order to investigate buried structures, we implement a numerical depth-filtering technique to suppress the surface reflections and reveal the phase topography of buried layers.

In general, our method provides nondestructive real time high resolution topographic imaging of material surfaces and even enables monitoring inner structures of partially transparent materials like semiconductors.

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