Line-scanning laser scattering system for fast defect inspection of a large aperture surface

Inspection of defects with micrometer level on large aperture surfaces with hundreds of millimeters is one of the challenges in surface quality evaluation. Various microscopic imaging methods have been applied to inspecting the surface defects, while they are time-consuming for the small field of view and the sub-aperture stitching. To tackle this problem, a high-speed line scanning system based on the dark-field laser scattering method is proposed. The laser beam is scanned by the rotating polygon mirror to a laser line for high throughput and then the telecentric F-theta lens converges each incoming laser beam to a focused spot that creates a high intensity to enhance the signal-to-noise ratio. The scattered light from surface defect is collected by the designed integrating sphere for low background noise and the scattering signal is detected for each focused spot at a proper acquisition rate by a photomultiplier (PMT) detector with extremely short response time. In the meanwhile, the tested surface is moving perpendicular to the laser line to realize high-speed large area inspection. The defect inspection system is confirmed experimentally with laser line length of 60 mm, minimum detectable size less than 0.5 μm, and figure of merit of 9.6  cm² s^-1 μm^-1. The work put forward an effective method for automatic discovery of surface defects such as scratches, digs, and contaminants on large aperture surfaces.