A NOVEL METHOD FOR STABILIZING THE PHOTOELASTIC MODULATOR USED IN A POLARIZATION MODULATED SPECTROSCOPIC ELLIPSOMETER

An achromatic optical feedback loop was developed for controlling strictly the retardation of the photoelastic modulator (PEM) used in a polarization modulated spectroscopic ellipsometer (PMSE). Reference signals for lock-in amplifiers were also produced from the optical signal that monitored the retardation of the PEM. The principles of the measurement are formulated in terms of the Mueller matrix formulation. The following results were obtained: (1) the warm-up time was reduced to nearly zero; (2) the stability was improved to within 0.1% of the full scale; (3) deviation from a reference value of the refractive index of synthesized fused quartz was within 0.01 in the wavelength range 240-640 nm.