In-line Metrology for Defect Assessment on Large Area Roll to Roll Substrates

This paper reports on work carried out as part of the EU funded research project “Nanomend”. The project seeks to develop integrated process inspection, cleaning, repair for nano-scale thin films on large area substrates. In order to prevent water ingress into flexible PV modules they are coated with a protective barrier layer of Al2O3 using atomic layer deposition (ALD) technique. Unfortunately defects in this layer have been shown to reduce module efficiency over a period of time due to water vapour ingress. The present work concentrates on defect detection and reports on the use of areal surface metrology parameters 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 barrier film. The sensors have built-in environmental vibration compensation and are being deployed on a demonstrator system at a Roll2Roll production facility in the UK.