Accurate heterodyne interferometric ellipsometer

We describe a heterodyne interferometric ellipsometer which requires only a single reflection from the sample surface. A simple arrangement is described that enables a reference beam to be created in one arm of a modified Mach-Zehnder interferometer such that the p- and s-polarisations in this arm have a common phase and fixed relative amplitude, irrespective of the sample. When this beam is recombined interferometrically with the measurement beam, the spatially separated p and s fringes have an amplitude ratio and relative phase that are directly proportional to tanψ and Δ, respectively. Adjusting the azimuthal angle of the input linear polarisation allows both ψ-tracking to be implemented and error reduction through averaging. Measurements made with this instrument of a native oxide layer on a silicon substrate are in excellent agreement with those obtained using a commercial ellipsometer.