An integrated opto-mechanical measurement system for in-process defect measurement on a roll-to-roll process

This paper reports on the recent work carried out to develop and implement a high precision on-line optical measurement system with the aim of providing defect detection and characterisation for ALD coated vapour barrier films produced by a roll-to-roll process. This proof-of-concept system is designed to detect and measure pre-existing defects on the film and define their size, location, form and density. The aim is to be able to detect defects in a thin film Al2O3 layer that are critical to vapour barrier performance, and eventually provide valuable process control information. Such an inspection system must be fast in order to evaluate large areas involved (500 mm width foil) at high magnifications. In addition the flexibility of the foil introduces challenges in terms of dealing with surface deviation away from an ideal plane and vibrations. Our solution is a wavelength scanning interferometer (WSI) combined with two kinematic stages, vertical (for auto-focus) and a traverse stage to provide full coverage of the foil. A porous air-bearing conveyor system is used to hold the foil at a fixed height and improve the flatness of the film relative to the measurement plane. This paper describes the principle and design of the inspection system.