Micro crack detection of multi-crystalline silicon solar wafer using machine vision techniques

Purpose - The detection of invisible micro cracks (mu-cracks) in multi-crystalline silicon (mc-si) solar wafers is difficult because of the wafers' heterogeneously textured backgrounds. The difficulty is twofold. First, invisible mu-cracks must be visualized to imaging devices. Second, an image processing sequence capable of extracting mu-cracks from the captured images must be developed. The purpose of this paper is to reveal invisible mu-cracks that lie beneath the surface of mc-si solar wafers. Design/methodology/approach - To solve the problems, the authors first set up a near infrared (NIR) imaging system to capture images of interior mu-cracks. After being able to see the invisible mu-cracks, a region-growing flaw detection algorithm was then developed to extract mu-cracks from the captured images. Findings - The experimental results showed that the proposed mu-cracks inspection system is effective in detecting mu-cracks. In addition, the system can also be used for the inspection of silicon solar wafers for stain, pinhole, inclusion and macro cracks. The overall accuracy of the defect detection system is 99.85 percent. Research limitations/implications - At present, the developed prototype system can detect mu-crack down to 13.4 mu m. The inspection resolution is high but the speed is low. However, the limitation on inspection speed can easily be lifted by choosing a higher resolution NIR camera. Practical implications - Generally, this paper is a great reference for researchers who are interested in developing automatic optical inspection systems for inspecting solar wafer for invisible mu-cracks. Originality/value - The research described in this paper makes a step toward developing an effective while low-cost approach for revealing invisible mu-crack of mc-si solar wafers. The advantages provided by the proposed system include excellent crack detection sensitivity, capability of detecting hidden subsurface mu-cracks, and low cost.