The variations of the intensity of the reflected light near null as a function of the angle of incidence are compared for three ellipsometric techniques. For films of either SiO2 or Si3N4 on silicon, the highest accuracy of the film thickness measurement is always obtained when the angle of incidence is either equal or nearly equal to the principal angle for all thicknesses of the film layer. Based on these results, it is shown that a variable angle of incidence spectroscopic ellipsometer operated at the principal angle of incidence and using a rotating analyzer combines the advantage of versatility with near maximum accuracy and sensitivity.
D. Chandler-Horowitz and G. A. Candela, "Principal angle spectroscopic ellipsometry utilizing a rotating analyzer," Appl. Opt. 21, 2972-2977 (1982)