Fig. 1. Schematic representation of the microsphere assisted Linnik interferometer comprising two high NA microscope objective lenses (MO) and a beam splitter (BS). The illumination (blue, representing Koehler illumination) and imaging (red) beam paths are shown. The illumination beam path appearing as a plane wave below the objective lens is omitted for better visibility. The figure is not drawn to scale to enhance the comprehensibility.

Fig. 2. Measurement result of a 230 nm grating (linewidth/pitch standard). For phase analysis of the interference signals an evaluation wavelength of 650 nm was used. The dependency of the evaluation wavelength on the phase evaluation algorithm used for the surface reconstruction is elaborated in [1].

Fig. 3. Representation of the Ewald limiting sphere for monochromatic light of wave number k₀ in q – space. The q – values belonging to specular reflection (q_x = 0) and backscattering are marked. The most outlying edges at q_x = ±2k₀NA correspond to the highest resolvable spatial frequencies for the maximum incident angle θ_in,max. k_r is representing the reflected wave vector. As an example, the first diffraction orders for a periodic structure with a period length Λ were inserted.

Fig. 4. Interferometric measurement data obtained from a rectangular grating (SiMETRICS RS-N, Λ = 300 nm) using royalblue (a) TM and (b) TE polarized light through a microsphere (SiO₂, 5–9 μm diam.), simulated data sets of a similar grating with (c) TM and (d) TE polarized light assuming a microcylinder of 5 μm diameter. The corresponding 3D spatial frequency representations are depicted in (e) – (h). The results shown in (a) and (e) are also analyzed in [11].

Fig. 5. Simulated interferometric data set with a rectangular grating structure similar to the SiMETRICS RS-N grating as a specimen (Λ = 300 nm) for the (a) TM and (b) TE polarization case.