Surface slope measurement of steep silicon V-grooves using high NA Linnik interferometry

Optical topography measurements are of high interest in a lot of industrial and academic fields. One of the most common associated measurement methods is coherence scanning interferometry, but even though it provides sub-nanometer axial resolution, its lateral resolution is diffraction limited. Not only the feature size is a limiting factor for optical measurements, but also steep surface slopes may lead to problems, since the acceptance angle of the objective lens limits the maximum surface slope angles that can be measured. Here we use a Linnik-type interferometer with objective lenses of numerical apertures of 0.95 in order to maximize the measurable surface slope angle. We demonstrate that silicon V-groove structures with a slope angle of 54.74° can be measured. We compare the directly measured surface slope angle with an angle calculated from light that is reflected two times by the V-grooves. To verify our measurement we compare the measurement results to rigorous FEM simulations.