Stability of polarimetric grating characterization with beam spot larger than grating box

In this work we report the results of the optical characterization of a periodic grating carried out with a probe spot size larger than the sample grating "box". The measured depolarizing Mueller matrices resulting from incoherent superposition of the optical responses of the grating and substrate were filtered by using the eigenvalues decomposition method. The retrieved Mueller matrices of the grating alone were fitted using rigorous coupled-wave method and the standard trapezoidal model with three parameters: the middle line-width (CD), the grating depth and the side-wall angle. The results are shown for all measured azimuthal angles and compared with reference values taken on a similar grating on the same wafer, in the usual conditions (beam spot inside the grating). The observed stability of the parameters very closely coincides with the reference grating except for some azimuthal angles, where the grating contributed only 5% of the signal. The overall dispersion of the parameters is within the few nanometers from the statistical mean value, a performance comparable to that of standard grating characterizations with the probe beam illuminating only the grating.