Optical inspection system with tunable exposure unit for micro-crack detection in solar wafers

Micro-cracks are a major cause of wafer breakage in the solar wafer manufacturing process. Furthermore, the existence of micro-cracks may lead to electrical failure in the post-fabrication inspection of solar cells and solar modules. Thus, the reliable detection of micro-cracks is an important concern in the photovoltaic industry. Accordingly, the present study proposes a novel micro-crack inspection system comprising a near infrared light source, a CCD camera and a tunable exposure unit. In the proposed system, the intensity of the light transmitted through the wafer is sensed by a photodetector and the exposure time of the CCD camera is tuned accordingly in order to maximize the contrast of the CCD image; thereby improving the performance of the crack detection process. The experimental results show that the proposed system enables the reliable detection of micro-cracks in solar wafers with a thickness of up to 240 μm within 1 s. In other words, the system provides an ideal solution for on-line micro-crack inspection applications in the photovoltaic industry.