Investigation On Extreme Ultraviolet Lithography

Extreme ultraviolet lithography (EUVL) represents one of the promising technologies for supporting integrated circuit (1C) industry's lithography needs during the first decade of the 21st Century. This technology builds on conventional optical lithography experience and infrastructure,uses 11- to 14-nm photon illumination,and is expected to support multiple technology generation from 65 nm to 35 nm.SKLAO (State Key Laboratory of Applied Optics) gazed at the progress in EUVL from the beginning of 1990's. For the first step investigation of EUVL,the author have developed an experimental EUVL system,which includes a laser produced plasma (LPP) source,an ellipsoidal condenser,a transmission mask,a reduced projection optics,and vacuum system.The projection optics with aspherical mirror of sub-nanometer accuracy are required to get a resolution of less than 0.1 M m and wide exposure area simultaneously. The precision of polishing and testing for such ashperical surface is fairly high and it has not been achieved yet in our current state. It might preferable,however,to use shperical mirror for the first step studies of EUVL in a series investigation of system design,component fabrication,assembly technique and experimental process.Along this thought,the author designed a Schwarzschild optics using spherical mirrors with 10:1 reduction projection optics. The optical system is optimized to achieve 0.1 u m resolution over a O.lmm diameter image field of view and the mirrors of the objective were coated with Mo/Si multilayer to provide 45% reflectance at near-normal incidence angle for 13.0nm radiation.The primary and secondary mirrors of the Schwarzschild optics were fabricated in our institute and measured using Zygo Mark IV interferometer. Figure errors were observed in both primary and secondary mirror of 5nm (rms). These magnitudes are very small at visible wavelength but sufficient to cause significant degradation in the wave-front quality of the Schwarzschild optics in extreme ultraviolet (EUV) wavelength. Using computer-aided alignment method,the Schwarzschild optics was assembled with wave-front error of 18nm in rms value which is a good match to the simulation wave-front error by introducing figure errors of primary and secondary mirrors using ZEMAX optical software.Positive resist of ZEP520 (Nippon Zeon Co. Ltd) was employed in exposure experiments. In initial result,line width of 0.75 u m was replicated on resist-coatedwafer,and using more narrow mask imaging experiments are being carried out to find the imaging ability and the best focus position of the optics.For a EUVL imaging system,a nonspecular scattering of multiplayer coated mirror will reduce the throughput and the contrast of the image. In order to produce low scatter optics,it is essential that the manufacturer have a way to measure the roughness and feed this information back into the polishing process. Based on the growth model of a multiplayer structure,the roughness of each boundary has two components,an intrinsic roughness of the individual film and roughness completely replicated from the roughness of the substrate for spatial frequencies below 20 u m"1. Therefore,in principle the scattering may be predicted from measurements of the surface profile.In this paper the author also discussed nonspecular scattering for Mo/Si multlayer coated primary and secondary mirrors of the measured Schwarzschlid optics based on power spectral density of these mirrors measured by both optical profilometer (WYKO) and atomic force microscopy (AFM). The roughness of WYKO and AFM measuring are 0.73nm and 0.56nm in rms value respectively,in both primary and secondary mirrors,and will severity reduce the Schwarzschlid optics throughput and the contrast of the image.According to EUVL requirement,this paper also presents a multiplayer thickness distribution control method by use of a platter velocity profiling technique in which the platter revolution speed is varied as a function of its position relative to the sputtering source. The optim...