Application of a Dual-spectral-range, Divergent-beam Spectroscopic Ellipsometer for High-Speed Mapping of Large-area, Laterally-inhomogeneous, Photovoltaic Multilayers

Abstract

We have developed a prototype spectroscopic ellipsometer for imaging/mapping purposes requiring only one measurement cycle (one rotation period of a polarizer or analyzer) for the acquisition of a two-dimensional array of data points. Our new measurement technique serves as a novel form of imaging ellipsometry, using a divergent (uncollimated, diffuse) source system and a detection system consisting of an angle-of-incidence-sensitive pinhole camera. By incorporating broad-band sources and wavelength dispersion optics, the instrument provides continuous high-resolution spectra along a line image of the sample surface. As a result, information on multilayer photovoltaics stacks can be obtained over large areas (several dm²) at high speed. The technique can be expanded to even larger areas by scaling-up the optical geometry. The spatial resolution of the line image is limited by the minimum resolved-angle as determined by the detection system. Small-aperture polarizers (25 mm diameter) are incorporated into the instrument, which reduces its cost. Demonstration mapping measurements have been performed ex situ on a multilayer sample deposited on a polymer substrate, including an intentionally graded 80–350 nm thick hydrogenated amorphous silicon (a-Si:H) layer and an intended uniform 400–500 nm thick transparent conducting ZnO:Al layer, both on opaque silver. Alternative commercial instruments for ex situ SE mapping must translate the sample in two dimensions. Even a 15 x 15 cm² sample requires > 200 measurements with cm-resolution and at least 15 min. By collecting ex situ data in parallel along one dimension through imaging, the divergent-beam system can measure with similar spatial resolution in < 2 min. In situ measurements on both roll-to-roll polymer and rigid glass will be possible in the future.