The identification of a complete three-dimensional (3D) photonic band gap in real crystals always employs theoretical or numerical models that invoke idealized crystal structures. Thus, this approach is prone to false positives (gap wrongly assigned) or false negatives (gap missed). Therefore, we propose a purely experimental probe of the 3D photonic band gap that pertains to many different classes of photonic materials. We study position and polarization-resolved reflectivity spectra of 3D inverse woodpile structures that consist of two perpendicular nanopore arrays etched in silicon. We observe intense reflectivity peaks (R > 90%) typical of high-quality crystals with broad stopbands. We track the stopband width versus pore radius, which agrees much better with the predicted 3D photonic band gap than with a directional stop gap on account of the large numerical aperture used. A parametric plot of s-polarized versus p-polarized stopband width agrees very well with the 3D band gap and is model-free. This practical probe provides fast feedback on the advanced nanofabrication needed for 3D photonic crystals and stimulates practical applications of band gaps in 3D silicon nanophotonics and photonic integrated circuits, photovoltaics, cavity QED, and quantum information processing. arXiv:1909.01899v2 [physics.optics]

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Measuring overlay between two layers of semiconductor devices is a crucial step during electronic chip fabrication. We present dark-field digital holographic microscopy that addresses various overlay metrology challenges that are encountered in the semiconductor industry. We present measurement results that show that the point-spread function of our microscope depends on the position in the field-of-view. We will show that this novel observation can be explained by a combination of the finite bandwidth of the light source and a wavelength-dependent focal length of the imaging lens. Moreover, we will also present additional experimental data that supports our theoretical understanding. Finally, we will propose solutions that reduce this effect to acceptable levels.

A dark-field Digital Holographic Microscope with a single lens for imaging is a potential candidate for future overlay metrology on semiconductor wafers. Aberrations caused by this single lens are computationally corrected allowing high-resolution imaging over a large wavelength range. However, the spatially-coherent imaging conditions in our microscope introduce coherent imaging artifacts that can limit the metrology performance. We present computational apodization of the optical field in the exit pupil of the lens as a potentially effective solution to mitigate these coherent imaging effects. A comparison of experimental data and simulations is presented that demonstrates the importance of this apodization in metrology applications. Moreover, our data also shows that exploiting the full potential of DHM requires an imaging lens with low optical scattering levels.