An interferometric auto-focusing method for on-line defect assessment on a roll-to-roll process using wavelength scanning interferometry

The quality and lifetime of flexible photovoltaic (PV) modules is dependant on having an effective vapour barrier layer which protects active elements from environmental degradation. An effective flexible barrier layer can be produced by employing a thin film of Al2O3 (40-100 nm thick) on a polymer substrate using atomic layer deposition (ALD). By measuring the surface of the barrier coating critical defects that will result in excess vapour transmission can be detected and minimised. Such, knowledge of defect size, type and distribution forms a crucial dataset for informing process control. This paper reports on a practical embedded inspection technique to measure the surface topography of the thin ALD deposited Al2O3 barrier film on a roll-to-roll manufacturing platform. The method combines wavelength scanning interferometry (WSI) along with an auto-focus method. Example measurements are given and discussed.