A Research On High Speed Focusing Using Line-scan CCD In Projection Lithography Tool

With the improving of lithograph resolution, the depth of focus(DOF) of lithographic projection objective is decreasing, generally in hundreds-nm level. In order to take full advantages of the restricted DOF, focusing and leveling becomes very popular. The focusing and leveling system must ensure the precision of focusing,and the projection lithography tool entering step-and-scan age and the increasing of the wafer size make it more and more important to take consideration to the performance of both focusing frequency and real-time.At present, the most frequently used focus method is based on four-quadrant detector or array CCD to grab light signal, which carries the defocusing amount information of the wafer, then process image on computer and send the defocusing amount to the motor driver throught RS232. This method makes closed loop throught upper computer, and cannot focusing and leveling when the stage is in step-move,causing it slow. Therefore, a focus detection method, which is based on line scan CCD for image grabbing and FPGA for image processing, closed loop in lower computer, is provided in this paper. Combining with sub-pixel position detection algorithm, this method has implemented real-time focusing and leveling, cut the cost and reduced power consumption, by utilizing the high-speed of line scan CCD and FPGA’s parallelism. While the precision is better than 1um and the range is more than 400 um,the frequency of focusing also has a great improvement to 1.3KHz.Detail design and experiment process of the line-scan CCD and lower computer closed looping focusing system will be presented in this paper. The first section introduces the research status and development trend of both lithography and focusing.The second section introduces the line-scan CCD and Camera Link specification in detail, and discusses the design and optimization of the overall solution for the system.In the third section, the design experience of the PCB hardware has been shared. The fourth section analyses the theory of image processing algorithm, and uses the Verilog HDL to implement the algorithms and behavior simulations. The experimental verification is presented in section 5, and the result indicates this system is faster than before and meets the design demand with the profermance of higher than 1um precision, 400um-range, and 1.3KHz focusing frequency. In the end, a conclusion has been made for this paper, and proposes my views for the next research work.