Study On Overlay Alignment Method Of An Optical Projection Lithography System

With the rapid development of information industry, lithography technology has been a key element to hinder VLSI and micro optics devices fabrication. Optical lithography exposure technology had gone through contact lithography, proximity lithography, optical projection lithography and laser interference lithography technology progress. Due to the technical maturity and the equipment stability, up till now, projection exposure lithography technology is still widely applied in the market. Optical projection lithography system, which act as the lithography technology loading platform and the incubator of most modern semiconductor and eletronical industry, consists of exposure light source, photolithography projection objectives, mask-wafer aligning system and laser positioning stage. Based on the existing excimer laser projection lithography system in our laboratory, in this thesis, we carried out some related research about mask-wafer aligning system and exposure light source.In this paper, firstly we overview the research background, lithography system classificaton, analysis the research status both at home and abroad, the research purpose and significance and explain the contents and arrangement of the whole thesis in Chapter 1. In Chapter 2, more details related to the overlay technology theory, the aligning technology and classification and the key factors influencing lithography system performance are introduced. Also the advantages and disadvantages of major aligning methods are analyzed.Pointing at the present development status of overlay alignment technology, in Chapter 3, a novel algorithm for matching and aligning in high-precision projection overlay alignment system is studied and presented. The proposed algorithm ultilizes contour feature points matching types to find aligning parameters, which based on the affine transformation homography matrix after strict mathematical deduction. Its close relatives have not been reported at home and abroad. First of all, we use centroid method to find the mass center of the points set which comes from the related ROI after feature points extracting in the mask and wafer respectively, then we rearrange the points set according to the euclidean distance between each point and the centroid from small to large. In terms of the proposed matrix formula, high accuracy alignment parameters can be easily obtained by a single operation. Contrast with traditional methods which spends long time for convergent iterative computation, the proposed method can meet the real-time alignment demand. The mismatching results can be omitted by the distance threshold. Finally, the precious matching results can be obtained with the maximum probability of rotation angles. Simultaneously, substituting the parameters of corresponding matching points into the proposed matrix formulas, we can also obtain the translation parameters. The relevant software system is introduced briefly in Chapter 4, simultaneously, the demo algorithm is demonstrated and the experimental results are analyzed. Combinated the MTF analysis of the optical microscope objectives we used in this system and the results images analysis, the system precision and errors are evaluted in detail. The final experimental results show that the system is of good precision, strong robustness, and certain practical value.In Chapter 5, we introduce briefly how to get flat-top distribution based on the existing excimer laser. In theory, based on the combination of Gaussian-Schell model and partially coherent beam, a set of parameters could be used to generate flat-top distribution beam is obtained, which can be lay a solid foundation for the further study combining the proposed aligning system.Generally speaking, the main innovation of this paper is applying a simplified mathematical model of actual overlay alignment system-affine transformation without significant scaling. Based on this model, we can obtain a group of homography matrix formulas after strict mathematical deduction. High accuracy alignment parameters can be easily obtained by a single operation. This can avoid the time-consuming element because of iterative computation before we get the parameters to be determined. Combined with digital image processing, a realtime alignment prototype software is developed. The core task of overlay alignment technology is accomplished.