Alignment and calibration of the MgF2 biplate compensator for applications in rotating-compensator multichannel ellipsometry.

Biplate compensators made from MgF2 are being used increasingly in rotating-element single-channel and multichannel ellipsometers. For the measurement of accurate ellipsometric spectra, the compensator must be carefully (i) aligned internally to ensure that the fast axes of the two plates are perpendicular and (ii) calibrated to determine the phase retardance delta versus photon energy E. We present alignment and calibration procedures for multichannel ellipsometer configurations with special attention directed to the precision, accuracy, and reproducibility in the determination of delta (E). Run-to-run variations in external compensator alignment, i.e., alignment with respect to the incident beam, can lead to irreproducibilities in delta of approximately 0.2 degrees . Errors in the ellipsometric measurement of a sample can be minimized by calibrating with an external compensator alignment that matches as closely as possible that used in the measurement.