Super-resolved 3D optical profiling for surface metrology using structured illumination

Abstract

The lateral and axial resolution of optical techniques is bounded by diffraction, making the acquisition of surface topographies of samples with nanometric structures impossible. Super-resolution techniques, such as Structured Illumination Microscopy (SIM), have been developed to overcome this limit, enabling an increase in resolution up to a factor of two and allowing to resolve structures too small for conventional optical microscopes. SIM relies on the projection of structured illumination as periodic fringes with equally spaced phase shifts to recover high-frequency information. A Digital-Micromirror-Device (DMD) can be used to generate structured illumination, providing accurate control and stability of the fringe frequency and phase shifts. Additionally, optical sectioning of the scanned surface is provided, since the projected patterns are only well contrasted within the in focus regions of the sample. To reconstruct a 3D surface, an optical profiler exploits this optical sectioning capability to localise the maximum signal through the axial scan at each point. Whilst SIM based on laser interference has been used to super-resolve the axial dimension, this is not possible with a DMD approach. We explore how DMD-based SIM can be used to enhance the profiler’s ability to super-resolve structures within surface metrology. We modify a DMD-based optical profiler to enable lateral super-resolution of the image stack and explore how the quality of the 3D surface reconstruction can be improved. For this, we combine the super-resolved images with different optical sectioning techniques and assess the lateral resolution of the topographic detail via the characterization of the instrument transfer function (ITF).