A transmission ellipsometer with the configuration of modified Mach-Zehnder heterodyne interferometer is demonstrated. Two acoustooptical modulators are employed to generate a 20-kHz beat frequency. The scheme offers high resistance to environmental turbulence because of the interferometric components passing through the same path. A single layer of indium tin oxide on a glass substrate was measured, and an error up to several nanometers of the sample thickness is observed. The polarization mixing error is mainly due to the imperfection of polarizing beamsplitters PBSs and to the elliptical polarization and nonorthogonality of the light beams produced by the laser source and wave plates. The mechanism governing the error and its influence on measurement accuracy is analyzed with the Jones matrix method. In contrast with interferometric reflection ellipsometry using a Zeeman laser, the theoretical analysis indicates that only second-order error is introduced in this system. The elliptical polarization and nonorthogonality, occurring only before the light splitting, have little influence on measurement accuracy; the imperfection of PBSs is the major contributor to the
polarization mixing error