Defocusing detection based on asymmetric placement of dual-quadrant detector

In the field of laser photolithography, automatic focusing is a key technology. The focusing degree of the writing light on the sample surface determines the quality of photolithography. In addition to the mask manufacturing in semiconductor industry, autofocus is also widely used in optical imaging and optical information reading. With the increasing demand of semiconductor market, it is necessary to reduce the cost of autofocus as much as possible on the premise of ensuring the accuracy. Dual-quadrant detector(DQD) is a kind of detector combining two photodetectors, in which the common edge of two photodetectors is usually a straight line. In this work, a defocusing detection method using a dual-quadrant detector is proposed, the expression of focusing error signal(FES) is defined. The relationship between defocus amount and FES is analyzed and simulated, accordingly. A focusing error detection system is established to demonstrate the theoretical analysis. The tracking range is up to 20μm, and the tracking accuracy is approximately 50. The theoretical and experimental results indicate that the defocusing detection method with a dual-quadrant detector takes into account the tracking range and tracking accuracy, and has good results. This technology is expected to be used in maskless laser direct writing lithography and optical imaging.