Wide-spectral-range, Expanded-beam Spectroscopic Ellipsometer and its Application for Imaging/Mapping of Graded Nanocrys
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Wide-spectral-range, Expanded-beam Spectroscopic Ellipsometer and its Application for Imaging/Mapping of Graded Nanocrystalline Si:H Films A. Nemeth1,2, D. Attygalle1, L. R. Dahal1, P. Aryal1, Z. Huang1, C. Salupo1, P. Petrik2, G. Juhasz2, C. Major2, O. Polgar2, M. Fried2, B. Pecz2, R. W. Collins1 1 Center for Photovoltaics Innovation & Commercialization University of Toledo, Toledo, OH, USA 2 Photonics, MFA, Budapest, Hungary ABSTRACT A prototype expanded-beam spectroscopic ellipsometer has been developed that uses uncollimated (non-parallel, diffuse) illumination with a detection system consisting of an angleof-incidence-sensitive pinhole camera for high-speed, large-area imaging/mapping applications. The performance of this novel instrument is being tested for imaging/mapping of mixed-phase hydrogenated silicon films having graded amorphous (a-Si:H) and nanocrystalline (nc-Si:H) components throughout the film depth. The speed of the measurement system makes the instrument suitable for use on production lines. The precision enables detection of subnanometer thicknesses, and refractive index and extinction coefficient changes of 0.01. Angle-of-incidence and mirror calibrations are made via well-known sample structures. Alternative commercial instrumentation for mapping by spectroscopic ellipsometry must translate the sample or ellipsometer in two dimensions. For this instrumentation, even a 15 x 15 cm2 sample with cm2 resolution requires > 200 measurements and at least 15 min. By imaging along one dimension in parallel, the expanded-beam system can measure with similar resolution in < 2 min. The focus of recent instrumentation efforts is on improving the overall system spectral range and its performance. INTRODUCTION The highest efficiencies measured on laboratory-sized thin film solar cells are generally well ahead of the best production module efficiencies. Thus, in photovoltaic (PV) module production for commercialization, many key problems are related to scale-up. The difference is primarily caused by fluctuations in area uniformity of properties caused by each individual processing step. In order to learn more about uniformity, several mapping measurements and evaluation methods are available [e.g., single-spot reflectance and spectroscopic ellipsometry (SE), Kelvin probe, laser beam induced currents (LBIC), etc.]. The primary problem with many probes is their reduced utility due to the long measurement time that makes in-line mapping a challenge. With long measurement times, the ability to detect non-uniformities is limited, which in turn limits feedback in process monitoring and control for optimization. The aim of our study is to develop and demonstrate a high-speed and high-resolution measurement method to monitor the thin film PV process in-line over large areas.
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EXPERIMENTAL DETAILS Hydrogenated amorphous silicon (a-Si:H), nanocrystalline silicon (nc-Si:H), and mixed phase layers were deposited by very high frequency plasma enhanced chemical vapor deposition (vhf PECVD) onto ~ 15 x 15 cm2 soda lim
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