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Research on focusing technique based on dual-phase lock-in amplifier in 193nm lithography system

With the integration of large scale integrated circuit devices improved, the requirement on resolution of Lithography has been enhanced day by day. The shortening of wavelength of light source and the augment of numerical aperture of projection objective make the effective depth of focus greatly reduced. The aggrandizement of wafer size and exist of focusing error make it clearly that traditional focusing methods cannot meet the harsh requirements of Nano- Lithography any more. Therefore, we need to develop new focusing technologies further. Aiming at the weak output signal of photodetector in nano-scale real-time focusing technique, this paper puts forward an innovative way in which we can make use of dual-phase lock-in amplifier to detect, amplify, filter and demodulate the signal. After a series of post-processing, we can get the defocusing amount of the wafer to make sure whether it is within the scope of effective depth of focus, for the purpose of assuring the exposure quality. We mainly use Simulink tools in Matlab to simulate the scheme model. From the simulation results we can see that dual-phase lock-in amplifier technique can extract the defocusing signal which we cares about excellently, and it does not rely on phase shifter. Compared with traditional methods, this method can obtain the defocusing signal more accurately. It helps to improve focusing precision significantly and lay a theoretical foundation for practicality in the near future.