Enhanced lateral resolution in interference microscopy with surface slope and part tilt

Abstract

Lateral resolution is a key performance characteristic in microscopy, defining the ability to distinguish closely spaced features and setting fundamental limits on quantitative imaging and surface metrology. Here we show that surface slope and part tilt in interference microscopy enable detection of spatial-frequency components beyond the conventional Abbe limit, for both imaging and topography measurement. A scalar Fourier-optics model shows that tilt introduces a spatial-frequency offset in the optical interference transfer function, allowing otherwise rejected diffraction components to contribute to measured phase and interference signal strength. Unlike conventional bright-field microscopes, interferometers can leverage this phenomenon even with incoherent Köhler illumination. The result is enhanced sensitivity to high-spatial-frequency topography and reflectivity. Experiments with a commercial coherence scanning interference microscope confirm the predictions using reflectivity gratings, paired topographical trenches, and a binary pseudo-random array artifact. The measured response demonstrates increased spatial-frequency bandwidth and improved resolvability in the direction of tilt, with practical gains limited by signal loss, aberrations, nonlinearity, and cross-coupling between reflectivity and topography. These results clarify tilt-dependent instrument behavior with steep or freeform surfaces and suggest new strategies for detecting fine surface structure.