In-line metrology for defect assessment on large area Roll 2 Roll substrates

This paper reports on the recent work carried out as part of the EU funded research and development project NanoMend. The project seeks to develop integrated process inspection, cleaning, repair for nano-scale thin films on large area substrates. Flexible photovoltaic (PV) films based on CIGS (Copper Indium Gallium Selenide CuInxGa(1-x)Se2) have been reported to have light energy conversion efficiencies as high as 19%. Their adoption has many advantages in terms of applications and in particular building integration. These CIGS based multi-layer flexible devices are fabricated on polymer film by the repeated deposition, and patterning, of thin layer materials using roll-to-roll processes (R2R), where the whole film is approximately 3μm thick prior to final encapsulation. The resultant films are lightweight and flexible however wide scale implementation is hampered by long term environmental degradation of efficiency due to water ingress to the CIGS modules through the polymer layers causing electrical shorts and corresponding efficiency drops. In order to prevent water ingress the PV modules are coated with a protective barrier layer of Al2O3 using atomic layer deposition (ALD) methods. Unfortunately defects in this layer have been shown to reduce efficiency over a period of time due to water vapour passage. The present work concentrates on defect detection and reports on the use of areal surface metrology to correlate defect morphology with water vapour transmission rate (WVTR) through the protective barrier coatings The use of advanced segmentation techniques is demonstrated where topographic information on functionally significant defects can be extracted and quantified. The work also reports on the deployment of new in line interferometric optical sensors designed to measure and catalogue the defect distribution and size where they are present in the 40nm thick barrier film. The sensors have built-in environmental vibration compensation and have been deployed on a demonstrator system at a Roll2Roll production facility.