Abstract：The metrological ultraviolet microscope is a traceable optical device.It uses 248 nm deep ultraviolet illumination light and a 100 × UV objective lens to up to 100 nm optical resolution.Because of the high resolution,it will be easily occur defocus phenomenon.In order to enhance the measurement accuracy, it is required the real-time auto-focus function when measuring lithography photomask.As a part of metrological UV microscope, the auto-focus system utilizes UV camera, UV lens, and lens actuator to obtain a series of optical slice images,then several key areas are set as the focus window,next the image sharpness evaluation function based on coif wavelet transform is used to find the best focus position.Finally, the mixed technology of VC and Matlab is used to achieve auto-focus for the optical measurement.After experimental verification, it is proved that the focusing accuracy of the system can achieve 0.5μm.
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2014, Vol. 35 
