极紫外投影光刻中若干关键技术研究
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摘要
极紫外投影光刻(EUVL，Extreme Ultraviolet Lithography)技术作为下一代光刻技术中最佳候选技术，建立于可见/紫外光学光刻的诸多关键单元技术基础之上，工作波长为11～14nm，适用于制造特征尺寸为65～35nm的数代超大规模集成电路，预计在2006年将成为主流光刻技术。
应用光学国家重点实验室自上世纪90年代初起一直关注着国际上的EUVL研究进展。出于进行EUVL原理及相关技术研究的目的，笔者采用微缩投影的计算机辅助装调技术研制了国内第一套由激光等离子体光源、掠入射椭球激光镜、透射掩模、微缩投影物镜及相应真空系统组成的EUVL原理装置。
EUVL微缩投影物镜为了同时实现大的曝光视场和0.1μm以下的成像分辨率，微缩投影光学系统需采用面形精度达亚纳米量级的非球面，但我们现阶段的光学加工和检测技术距此要求尚有一定的差距。作为研究的第一步，出于进行EUVL原理及系统设计、光学元件的制备、系统集成、掩模曝光实验等相关技术研究目的，采用球面Schwarzschild微缩投影物镜是比较切实可行的技术方案。
基于上述考虑，笔者设计了微缩倍率为10倍的球面Schwarzschild物镜作为EUVL原理装置的微缩投影物镜。经优化设计后，0.1mm像方视场内的成像分辨率优于0.1μm；近正入射条件下，Schwarzschild微缩物镜主、次镜Mo/Si多层膜的实测反射率为45％。
利用Zygo Mark干涉仪检测的Schwarzschild微缩投影物镜主、次镜面形精度表明，对可见光工作波段已具有足够高的面形精度，均为5nm(rms)，但在EUV(Extreme Ultraviolet)波段，将给Schwarzschild微缩投影物镜带来严重的波面误差。为此，在采用光学系统的计算机辅助装调技术进行Schwarzschild微缩投影物镜的光学装调时，笔者首次引入主、次镜的实测面形误差，利用ZAMEX光学设计软件模拟了理想装调下的波面误差，并用以计算物镜的失调量，将失调量引起的波面误差与主、次镜面形误差引起波面误差分离，使失调量的计算更加准确、装调过程更快收敛。计算机辅助装调后的Schwarzschild微缩投影物镜波面误差为18nm(rms)，与模拟的结果相符。
在集成后的EUVL原理装置上，采用ZEP520(Nippon Zeon Co.Ltd)正性抗蚀剂及掩模调焦方案，初步进行了透射掩模的曝光复制实验，国内首次获得了0.75μm线宽的EUVL复制图形，完成了EUVL的原理性贯通。目前，利用更细线宽透射掩模的曝光复制实验正在进行中，以获得最小的复制线宽及Schwarzschild微缩投影物镜的最佳焦面。
极紫外投影光刻系统中，多层膜反射元件的非镜面散射将严重影响系统的
摘 要
一
EUV传输效率和成像对比度。根据多层膜结构生长的理论模型，多层膜每一界
面的粗糙度由两部分构成。一是薄膜沉积过程中固有的粗糙度；二是继承下一层
界面的粗糙度。在空间频率小于20Urn”’的区间内，各界面的粗糙度具有良好的
相关性，是多层膜基底粗糙度的再现，多层膜的沉积不会改变此空间频率内的粗
糙度。因此，我们可以通过检测多层膜反射镜基底的粗糙度来表征多层膜反射镜
非镜面散射对光学系统性能的影响，亦即通过检测多层膜反射镜基底的粗糙度调
整抛光工艺参数，获得低散射的多层膜反射镜。
基于上述理论，笔者通过采用WYKO光学轮廓仪和原子力显微镜（AFM）
检测主、次镜不同空间频率段的功率谱密度函数，讨论了它们对Schwarzschild
微缩投影物镜传输效率和成像对比度的影响。结果表明，中频段的0．73urn）sm，
WYKO检测结果）波纹度和高频段的0．56urn（ms，AFM捡测结果＊将严重影响
Schwarzschild微缩投影物镜的成像对比度和传输效率。
为了适应EUVL技术对EUV多层膜空间分布均匀性的要求；本文的另一项
研究内容是＊＊V多层膜膜厚空间分布控制技术。我们采用了基于速度调整技术
的膜厚分布控制方法；并进行了相应多层膜装置的研制。它是通过改变基片转入
转出溅射区域时不同空间位置经过溅射靶材的公转速度，控制其在溅射镀膜区的
停留时间，获得均匀或梯形多层膜（Graded MultU盯er CoatingsL 速度调整曲
线是基片相对于磁控源位置的函数。例如，如果基片以恒定的公转速度扫过磁控
源时在基片上镀制的多层膜厚度空问分布为边缘薄、中心厚；则可通过减小基片
边缘进入和离开溅射区域的速度，获得更为均匀的多层膜。采用此方法，笔者在
pl 50mm完成了均匀 EUV多层膜的制备，膜厚空间分布非均匀性由恒定公转速
度的 7％减小到 1％，达到了 EUVL的要求。
通过利用EUVL原理装置进行的掩模初步曝光复制实验，笔者发现固体靶
LLP光源产生的“碎削”（Debris）会严重污染周围的光学元件，甚至会轻微污
染掩模。这?
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-coated
wafer,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
应用光学国家重点实验室自上世纪90年代初起一直关注着国际上的EUVL研究进展。出于进行EUVL原理及相关技术研究的目的，笔者采用微缩投影的计算机辅助装调技术研制了国内第一套由激光等离子体光源、掠入射椭球激光镜、透射掩模、微缩投影物镜及相应真空系统组成的EUVL原理装置。
EUVL微缩投影物镜为了同时实现大的曝光视场和0.1μm以下的成像分辨率，微缩投影光学系统需采用面形精度达亚纳米量级的非球面，但我们现阶段的光学加工和检测技术距此要求尚有一定的差距。作为研究的第一步，出于进行EUVL原理及系统设计、光学元件的制备、系统集成、掩模曝光实验等相关技术研究目的，采用球面Schwarzschild微缩投影物镜是比较切实可行的技术方案。
基于上述考虑，笔者设计了微缩倍率为10倍的球面Schwarzschild物镜作为EUVL原理装置的微缩投影物镜。经优化设计后，0.1mm像方视场内的成像分辨率优于0.1μm；近正入射条件下，Schwarzschild微缩物镜主、次镜Mo/Si多层膜的实测反射率为45％。
利用Zygo Mark干涉仪检测的Schwarzschild微缩投影物镜主、次镜面形精度表明，对可见光工作波段已具有足够高的面形精度，均为5nm(rms)，但在EUV(Extreme Ultraviolet)波段，将给Schwarzschild微缩投影物镜带来严重的波面误差。为此，在采用光学系统的计算机辅助装调技术进行Schwarzschild微缩投影物镜的光学装调时，笔者首次引入主、次镜的实测面形误差，利用ZAMEX光学设计软件模拟了理想装调下的波面误差，并用以计算物镜的失调量，将失调量引起的波面误差与主、次镜面形误差引起波面误差分离，使失调量的计算更加准确、装调过程更快收敛。计算机辅助装调后的Schwarzschild微缩投影物镜波面误差为18nm(rms)，与模拟的结果相符。
在集成后的EUVL原理装置上，采用ZEP520(Nippon Zeon Co.Ltd)正性抗蚀剂及掩模调焦方案，初步进行了透射掩模的曝光复制实验，国内首次获得了0.75μm线宽的EUVL复制图形，完成了EUVL的原理性贯通。目前，利用更细线宽透射掩模的曝光复制实验正在进行中，以获得最小的复制线宽及Schwarzschild微缩投影物镜的最佳焦面。
极紫外投影光刻系统中，多层膜反射元件的非镜面散射将严重影响系统的
摘 要
一
EUV传输效率和成像对比度。根据多层膜结构生长的理论模型，多层膜每一界
面的粗糙度由两部分构成。一是薄膜沉积过程中固有的粗糙度；二是继承下一层
界面的粗糙度。在空间频率小于20Urn”’的区间内，各界面的粗糙度具有良好的
相关性，是多层膜基底粗糙度的再现，多层膜的沉积不会改变此空间频率内的粗
糙度。因此，我们可以通过检测多层膜反射镜基底的粗糙度来表征多层膜反射镜
非镜面散射对光学系统性能的影响，亦即通过检测多层膜反射镜基底的粗糙度调
整抛光工艺参数，获得低散射的多层膜反射镜。
基于上述理论，笔者通过采用WYKO光学轮廓仪和原子力显微镜（AFM）
检测主、次镜不同空间频率段的功率谱密度函数，讨论了它们对Schwarzschild
微缩投影物镜传输效率和成像对比度的影响。结果表明，中频段的0．73urn）sm，
WYKO检测结果）波纹度和高频段的0．56urn（ms，AFM捡测结果＊将严重影响
Schwarzschild微缩投影物镜的成像对比度和传输效率。
为了适应EUVL技术对EUV多层膜空间分布均匀性的要求；本文的另一项
研究内容是＊＊V多层膜膜厚空间分布控制技术。我们采用了基于速度调整技术
的膜厚分布控制方法；并进行了相应多层膜装置的研制。它是通过改变基片转入
转出溅射区域时不同空间位置经过溅射靶材的公转速度，控制其在溅射镀膜区的
停留时间，获得均匀或梯形多层膜（Graded MultU盯er CoatingsL 速度调整曲
线是基片相对于磁控源位置的函数。例如，如果基片以恒定的公转速度扫过磁控
源时在基片上镀制的多层膜厚度空问分布为边缘薄、中心厚；则可通过减小基片
边缘进入和离开溅射区域的速度，获得更为均匀的多层膜。采用此方法，笔者在
pl 50mm完成了均匀 EUV多层膜的制备，膜厚空间分布非均匀性由恒定公转速
度的 7％减小到 1％，达到了 EUVL的要求。
通过利用EUVL原理装置进行的掩模初步曝光复制实验，笔者发现固体靶
LLP光源产生的“碎削”（Debris）会严重污染周围的光学元件，甚至会轻微污
染掩模。这?
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-coated
wafer,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
引文
1. Bruning et.al, "Optical lithography-Thirty year and three orders of magnitude,"SPIE Press,Vol.3049,1994.
2. M.Bom and E.Wolf, "Principles of Optics," 5th Ed, Pergamon P.,1975.
3. KKinoshita, K.Kurihara. YIshii and Y. Torii, "Soft X-ray reduction lithography,"J.Vac.Sci.Technol.,B7, 1648-1651,1989.
4． 德山巍，“超微细加工技术”，社，27-56，1997．
5. YNakayama,Y. Sohda, N.Saitou and H.Itoh, "Electron-Beam,X-ray, and Ion-Beam Submicromcter Lithographies for Manufacturing,"SPIE Press,Vol.1924,1993.
6. Fleming et.al, "Prospects for X-ray Lithography,"J.Vac.Sci.Technol,B10(6),1992.
7. Stearns D G, Rosen R S, Vernon S P, "Multilayer mirror technology for soft X-ray projection lithography," Appl Opt,32(34),6952-6960,1993.
8. Skulina K M, Alford C S, Bionta R M, et al, "Molybdenum/beryllium multilayer mirrors for normal incidence in the extreme ultraviolet," Appl Opt,34(19) 3727-3730, 1995.
9. H.Kinoshita, "SR Lithography for Manufacturing Integrated Circuit beyond 100 nm," SRI,11,2,1998.
10. MJtoh, S.Katagiri, H.Yamanashi, E.Seya, T. Ogawa, H.Oizumi and T. Terasawa, "Optical Technology for EUV Lithography,"OSA Trends in Optics and Photonics 4,9,1996.
11．木下，金子，武井，竹内，石原，“X线缩小投影露光检讨”，第47回应用物理学会学术讲演会予稿集，1986．
12. Kinoshita H, Kurihara K, Ishii Y and torii Y, "Soft X -ray reduction lithography using multilayer mirrors,"J .Vac. Sci. Technol, B7(6), 1648-1651,1989.
13．木下博雄，栗原健二…，“X线缩小投影露光技术”，NTT R＆D，43(11)，1221～1228，1994．
14．木下博雄，“極端紫外线用光学系开発评(?)”，Oplus E，Vol．22，No．5，567-572，2000年5月号．
15. Bjorkholm J E, Bokor J, Eichner L, Freeman R R, Gregus J, Jewell T E, et-al,"Reduction imaging at 14nm using multilayer-coated optics:Printing of features smaller than 0.1 μm," J.Vac.Sci.Technol,B8(6),1509-1513 1990.
16. "The National Technology Roadmap for Semiconductors," SIA Semiconductor Industry Association,1994.
17. http://www.sandia.gov/Main.html
2. M.Bom and E.Wolf, "Principles of Optics," 5th Ed, Pergamon P.,1975.
3. KKinoshita, K.Kurihara. YIshii and Y. Torii, "Soft X-ray reduction lithography,"J.Vac.Sci.Technol.,B7, 1648-1651,1989.
4． 德山巍，“超微细加工技术”，社，27-56，1997．
5. YNakayama,Y. Sohda, N.Saitou and H.Itoh, "Electron-Beam,X-ray, and Ion-Beam Submicromcter Lithographies for Manufacturing,"SPIE Press,Vol.1924,1993.
6. Fleming et.al, "Prospects for X-ray Lithography,"J.Vac.Sci.Technol,B10(6),1992.
7. Stearns D G, Rosen R S, Vernon S P, "Multilayer mirror technology for soft X-ray projection lithography," Appl Opt,32(34),6952-6960,1993.
8. Skulina K M, Alford C S, Bionta R M, et al, "Molybdenum/beryllium multilayer mirrors for normal incidence in the extreme ultraviolet," Appl Opt,34(19) 3727-3730, 1995.
9. H.Kinoshita, "SR Lithography for Manufacturing Integrated Circuit beyond 100 nm," SRI,11,2,1998.
10. MJtoh, S.Katagiri, H.Yamanashi, E.Seya, T. Ogawa, H.Oizumi and T. Terasawa, "Optical Technology for EUV Lithography,"OSA Trends in Optics and Photonics 4,9,1996.
11．木下，金子，武井，竹内，石原，“X线缩小投影露光检讨”，第47回应用物理学会学术讲演会予稿集，1986．
12. Kinoshita H, Kurihara K, Ishii Y and torii Y, "Soft X -ray reduction lithography using multilayer mirrors,"J .Vac. Sci. Technol, B7(6), 1648-1651,1989.
13．木下博雄，栗原健二…，“X线缩小投影露光技术”，NTT R＆D，43(11)，1221～1228，1994．
14．木下博雄，“極端紫外线用光学系开発评(?)”，Oplus E，Vol．22，No．5，567-572，2000年5月号．
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