Coherence effects in applications of frequency and time domain full field optical coherence tomography to optical metrology

The interplay of spatial versus temporal coherence in the context of full field optical coherence tomography (OCT) for optical metrology applications is investigated both theoretically and experimentally using the Linnik and the Mirau interference microscopes. It is shown that multiple interference effects have a little influence on the thin film thickness measurement when the coherence length is much smaller than the layers thickness. When high numerical apertures used it is possible to rely on the longitudinal spatial coherence and work with narrow spectral band thus improving the measurement accuracy. This is particularly important as it avoids taking the dispersion relations into account. For frequency domain (FD) operation we have shown that using objectives with low numerical aperture (NA) up to 0.3 it is still possible to use FD-OCT with high accuracy. For large NA objectives the use of narrow annular apertures is shown to be possible without sacrificing the axial resolution. In this case one can even use the inverse scattering approach in which a simplified model is built for the multilayered structure and fitting is performed with the measured spectrogram. This is shown to be more sensitive technique than polarized reflectometry.