Abstract

An instrument has been developed that measures the magnitude of the Bidirectional Scattering Distribution Function (BSDF) employing coherent optical heterodyne detection at 633nm in a Mach-Zehnder interferometer configuration. In preliminary measurements, instrument performance agrees with theoretical predictions and is characterized by diffraction limited resolution, 15 decades of dynamic range and shot noise limited sensitivity. The scatterometer is constructed from readily available components and the instrument configuration can be adapted to measure scattering from objects in transmission or reflection. In the scatterometer, the beam from a commercially available stabilized 3mW He-Ne laser is split into two beams: a scattering beam and a local oscillator beam. The frequency of each beam is individually shifted by separate acousto-optic modulators operating at different drive frequencies. The scattering beam is directed to the target whose orientation is adjustable. The scattered light and the local oscillator beam are combined and directed onto a photodetector where the resulting heterodyne signal is the frequency difference between the two beams. The RF power of the heterodyne signal is proportional to the optical power that is scattered from the target and is polarization-and mode-matched to the local oscillator beam. The combination of shot-noise-limited sensitivity of a heterodyne detector and milliwatt saturation levels of typical photodetectors provides the large dynamic range necessary for accurate measurements of BSDF. Moreover, characteristics inherent to coherent detection systems permit measurements with diffraction limited angular resolution and yield information of the polarization content of the scattered light. Results of BSDF measurements of various surfaces and materials will be presented.

© 1988 Optical Society of America