Implementation of wavelength scanning interferometry for R2R flexible PV barrier films

Roll to Roll manufacture of nano-scale thin film layers faces the challenge of micro/nano-scale defects appearing in the films. Atomic Layer Deposition (ALD) coatings of aluminium oxide, Al₂O₃ are used as barrier layers for photovoltaic (PV) solar modules where the primary function of the barrier layer is to prevent the water vapour ingress to the PV cells. Barrier layer defects have been shown to have negative impact on the performance of the barrier layers. Poor barriers cause module degradation resulting in reduced PV efficiency and lifespan. In order to ensure the quality of manufacture of the barriers, defects should be detected during the barrier production process and the information used to optimise the production process. This paper introduces, as part of EU funded NanoMend project, a full solution for inspection of entire surface regions of Al₂O₃ barrier films across large area substrates. The solution principle is based on implementing an opto-mechanical in-process inspection system to measure the significant defects using a wavelength scanning interferometer (WSI) embedded within the film-rewinder stage and integrated with the substrate translation and kinematic stages. The opto-mechanical system allows full surface measurement over full substrate widths of approximately 0.5m. The system provides an auto-focus for the WSI with an accuracy and repeatability better than 6 μm at optimum optical alignment conditions. The system is combined with a porous air-bearing conveyor used to hold the film web at fixed height within the focal depth of WSI objective lens and with height variation of < 5 μm under optimum vacuum pressure. The paper also outlines a computerised data handling process that can be used to assess hundreds/thousands of measurement files automatically by extracting and monitoring the areal root mean square roughness parameter (Sq) and defect statistics. Measurement results for functionally significant defects with lateral size > 3 μm is also presented as a case study to highlight the system capability.