Study On Fine Alignment In Lithography

The denseness and performance index of Integrate Circuit (IC) have been enhanced rapidly in geometric series according to Moor's Law. Such extraordinary growth has made enormous contributions to the development of whole IT industry. Lithography is an indispensable technology to manufacture IC and is the key of modern information technology. Mask-Wafer Alignment is one of the three key technologies (projection lens, aligner, wafer stage) in lithography. With the development of lithography technology, alignment lithography systems with nanometer critical line-width dimension require higher alignment accuracy.In this thesis, the scheme of high accuracy alignment lithography is worked out and analyzed and several feasible methods of improving alignment accuracy, which can be used in high accuracy projection mask aligner, are proposed. The results are of great importance. The main contents and results are as following:1.The principal and scheme of high accuracy alignment lithography are introduced. The alignment method and alignment mark design in the system are analyzed. The flow chart of high accuracy reticle-wafe alignment design is mapped out. In order to realize the high accuracy alignment, the performance index of optics, mechanism and electricity must be improved. Every step of the technology scheme in the alignment system is very significant.2 In alignment system Wafer Processes such as chemical polishing (CMP) and spin coating can result in the asymmetric deformation of alignment marks. The alignment accuracy is beneficial in this process. An automatic alignment algorithm for decreasing the deformation of reticle-wafer was constructed, which has been applied in wafer, reticle and workpiece flat alignment model in co-axis & off-axis alignment system. The relative position of Machine Coordinator System(MCS), Reticle Coordinator System( RCS), Wafer Coordinator System(WCS) is determined by the automatic alignment algorithm. In this mixed alignment system, the deviations of offset, scaling, rotation and orthogonality which are caused by the reticle-wafer are well corrected.3.The modulation principal of off-axis alignment light source signal and the photodetection method of phase grating mark are analyzed. The method of restraining the stray light using electro-optical phase modulator is introduced for the alignment accuracy in the illumination light source. The alignment accuracies are analyzed and compared when the stray light exists or not. Results show that the alignment accuracy is obviously improved by the electro-optical modulation method. Thus, electro-optical modulation is one of the effective technologies to improve the alignment accuracy in projection photolithography.4.The high order diffraction alignment signal strength that is induced by phase grating mark is simulated and analyzed based on the scalar diffraction theory and interference principle in the off-axis alignment system. The result shows that the position of alignment mark is better determined by the high diffraction order which is induced by the enhanced subdivisional phase grating mark. The alignment accuracy is up to 10nm. Meanwhile, the specific higher order of diffraction can be chosen for measurement according to the alignment accuracy of different resolution. So this method is one of the effective technologies to enhance the flexibility and adaptability of the process and is credible and stable for the capture of mark signal.In general, this study is significant. The method of improving alignment accuracy, which is based on the domestic 100nm lithography alignment unittechnology, is studied according to the lithograph overlay accuracy in the aspects of mark design, the scheme of alignment, light source and extracting alignment signal. Several methods of improving alignment accuracy, which are feasible, are introduced...