APPARATUS FOR DETERMINING OPTIMUM POSITION OF FOCUS OF AN IMAGING SYSTEM

Fast on-line electro-optical detection of wafer defects by illuminating with a short light pulse from a repetitively pulsed laser, a section of the wafer while it is moved across the field of view of an imaging system, and imaging the moving wafer onto a focal plane assembly, optically forming a continuous surface of photo-detectors at the focal plane of the optical imaging system. The continuously moving wafer is illuminated by a laser pulse of duration significantly shorter than the pixel dwell time, such that there is effectively no image smear during the wafer motion. The laser pulse has sufficient energy and brightness to impart the necessary illumination to each sequentially inspected field of view required for creating an image of the inspected wafer die. A novel fiber optical illumination delivery system, which is effective in reducing the effects of source coherence is described. Other novel aspects of the system include a system for compensating for variations in the pulse energy of a Q-switched laser output, methods for autofocussing of the wafer imaging system, and novel methods for removal of repetitive features of the image by means of Fourier plane filtering, to enable easier detection of wafer defects.