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A Novel EUV Exposure Station for Nanotechnology Studies

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A Novel EUV Exposure Station for Nanotechnology Studies
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  Nuclear Instruments and Methods in Physics Research A 582 (2007) 254–257 A novel EUV exposure station for nanotechnology studies John Wallace a,  , Yang-Chun Cheng a , Artak Isoyan a , Quinn Leonard a , Mike Fisher b ,Mike Green b , Joseph Bisognano b , Paul Nealey a,c , Franco Cerrina a a Center for Nanotechnology, University of Wisconsin, Stoughton, WI 53589, USA b Synchrotron Radiation Center, Stoughton, WI 53589, USA c Nanoscale Science Engineering Center, University of Wisconsin, Madison, WI 53706, USA Available online 17 August 2007 Abstract The soft X-ray region around 13nm (Extreme Ultra Violet, EUV) has become very important for applications to nanopatterning.Typically, high-resolution patterning of less than 50nm pitch lines and spaces requires optical systems that are complex and expensive.An appealing alternative is provided by interferometric patterning, whereby an EUV beam of relatively high degree of coherence is usedto create a pattern by interfering two (or more) parts of the wavefront. Briefly, the radiation from a short undulator illuminates twogratings with pitch in the range of 40–100nm. The first orders overlap, and in this region an interference pattern is generated with pitchhalf the srcinal. This technique produces very high resolution 20–50nm pitch patterns. Thus, in a collaborative effort involving CNTechand the SRC we have designed, constructed and tested a new beamline dedicated to nanopatterning using the radiation from a newundulator on the Aladdin storage ring. The beamline provides 210mW/cm 2 average in-band power flux to the endstation. We note thatthe beamline is designed to provide a relatively large area exposure, with good wavefront quality in intensity and phase. The endstationsupports interferometric lithography techniques, with demonstrated patterning capability down to 40nm pitch lines and spaces. Inaddition to nanopattering, a branch-line experimental station is designed for studying the photo-desorption from materials duringexposure. The outgassing species are captured and transferred to a gas-chromatograph for off-line characterization. This paper describesthe undulator source, the beamline, and the exposure stations. r 2007 Elsevier B.V. All rights reserved. PACS:  81.05.  t; 81.16.Rf; 81.07.  b; 81.16.  c Keywords:  Planar undulator; EUV lithography; Interferometric lithography; Holographic lithography; Nanolithography; Nanotechnology 1. Introduction The UW-Center for Nanotechnology (CNTech) hasconstructed an Extreme Ultra Violet (EUV) beamline forstudying nanotechnology materials. EUV nanolithographycan provide large quantities of nanometer scale patternsneeded for scientific studies. These patterns can be used tocharacterize photo-resist material properties, or used asnanopatterned templates for self-assembling structures.Previously, our work was based on a time-shared beamline,with insufficient access time analysis showed that a ‘‘shortundulator’’, i.e. one that fits in the short sections betweenbending magnets on the Aladdin Storage ring, couldprovide sufficient power and coherence to conductnanotechnology research. The beamline could also bedirected into the existing CNTech Beamline Cleanroom,providing class 100 cleanroom experimental work space(Fig. 1). 2. The source The proposed short undulator was designed andmanufactured by Advanced Design Consulting (ADC)[1,2]. The device was installed in April 2006. It is a variablegap, planar, pure permanent magnet type. It has 40 half-period poles in a total 0.9m length (45cm wavelength). Theundulator is designed to provide 1st harmonic 92.5eV forboth 0.8MeV and 1.0GeV ring operation. The adjustable ARTICLE IN PRESS www.elsevier.com/locate/nima0168-9002/$-see front matter r 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.nima.2007.08.124  Corresponding author. E-mail address:  jwallace@nanotech.wisc.edu (J. Wallace).  undulator gap allows a range of   K   from 1.7 to 0.8, and thusproduces a useful energy range of 60–110eV. 3. Beamline and experimental stations The CNTech EUV beamline is designed to preserve bothpower and coherence. To obtain this, we have minimized thenumber of elements within the beam path. The beamlinecontains a cooled adjustable aperture system, two planarmolybdenum mirrors, and energy defining insertable filters.The two molybdenum mirrors are at 18 1  and 17 1  grazingincident, and provide a means to remove unwanted higherenergy harmonics generated by the undulator. These twomirrors are water cooled, and have a single molybdenumcoating, with a 20nm thin silicon protective topcoat. With noenergy matching optics, such as multilayer mirrors, we canextend our experimental energy range to that of the undulator.The natural divergence of the beam provides sufficientexposure area and uniformity at the experimental plane.To separate between ring UHV and experiment lowvacuum, a differential ion pump is used. This pump wasdesigned in house, and creates a separation between 10  6 and 10  9 Torr. The internal clear aperture diameter of thepump is  38 in. Use of this pump removes the need forvacuum separation windows.At the end of the beamline are two experimental branchlines. One branch is for step-and-repeat imaged exposures.This exposure system stage travel is 2.5in.  2.5in., andallows for up to a 15  15 exposure matrix. Stage motionand exposure control are all automated. The chambervolume is small, and with turbo pumping, sample loadingtime is less than 5min. ARTICLE IN PRESS Fig. 1. Photo of CNTech EUV experimental stations. Sample1 Diffraction-1 Diffraction Two or multiple beams interfere to yielddense lines and spaces, or arrays of dots EUVPattern period is half of grating periodTransmissionGrating Mask Fig. 2. Interferometric Lithography, 60nm pitch lines/spaces in PMMA. J. Wallace et al. / Nuclear Instruments and Methods in Physics Research A 582 (2007) 254–257   255  For high-resolution patterns, we use EUV interfero-metric lithography (EUV-IL) [3]. For EUV-IL, thecoherent radiation illuminates two gratings with pitch inthe range of 40–100nm. The first orders overlap, and inthis region an interference pattern is generated with half thepitch of the srcinal (Fig. 2). This technique is achromatic,and produces very high resolution 20–50nm pitch patterns.For complex, non-periodic structures, EUV Holographicimaging is used [4]. Using CNTech image modelingtoolsets, Computer Generated Hologram (CGH) masksare created for EUV exposure.As part of EUV photo-resist characterization, we have abranch line designed for exposing experimental EUV photoresists, and capturing outgassed materials during exposure.This system has the ability to expose an entire 200mmwafer. These outgassed materials are then analyzed off-lineusing GCMS. This information provides industry withcriteria for future exposure stations, and the resists that gointo them. 4. Experimental results In the EUV-IL system, the exposure area is4mm  4mm, with an average 210mW/cm 2 in-bandpower. This is enough power to create sub-second PMMAphoto-resist exposures. Currently, the power variation overthe field is 20%, which we hope to reduce with improvedoptics.In the previous work, CNTech has demonstrated EUV-IL-generated 40nm pitch lines and spaces. CNTech hasalso printed multidirectional EUV-IL arrays of 70nmdiameter holes. Due to limitations of the mask, currentEUV-IL exposures have only printed down to 60nm pitchlines and spaces (Fig. 2). CNTech has developed in-housemask fabrication, and is currently working on improvedmask designs. This imaging technique will be used forphoto-resist resolution and line-edge-roughness studies.The printed nanometer scale features can also be used astemplates for growth or alignment of other nanomaterials.Implementation of CGH masks for EUV exposure hasyielded patterns with features down to 900nm. Thepatterns are in good agreement with the modeling used tocreate the CGH, giving us encouragement for futureexperiments (Fig. 3).The resist outgassing station is currently in use, and isgenerating a great deal of interest in industry, includingIntel and Sematech. 5. Conclusions The Center for Nanotechnology has a new tool for use inits nanotechnology studies.A short undulator-based EUV beamline providesnanolithography capabilities. To image, EUV InterferenceLithography provides high-quality nanometer scale peri-odic structures, while EUV Holographic Lithography canbe used to create arbitrary shapes. The photo-resistoutgassing experimental system provides necessary litho-graphic process information to industry. Acknowledgments This work was supported by the National ScienceFoundation under Grant no. DMR-0425880 through theNanoscale Science and Engineering Center and DMR-0084402 through the Synchrotron Radiation Center, bothof the University of Wisconsin—Madison. ARTICLE IN PRESS Experimental Schemeto reconstruct the imagewith a an in-line hologram maskOptical MicroscopePhotoEUV HolographicMask Si Waferwith PMMA resistSRCEUV source λ = 13nm Fig. 3. Holographic lithography. J. Wallace et al. / Nuclear Instruments and Methods in Physics Research A 582 (2007) 254–257  256  References [1]  / www.adc9001.com S .[2] A. Deyhim, E. Johnson, J. Kulesza, A. Lyndaker, D. Waterman,D. Eisert, M.A. Green, G. Rogers, K. Ingvar Blomqvist, Develop-ment of a planar undulator, in: Proceedings of the Ninth Inter-national Conference on Synchrotron Radiation Instrumentation,2006.[3] H.H. Solak, C. David, J. Gobrecht, L. Wang, F. Cerrina, J. Vac. Sci.Technol. B 20 (2002) 2844.[4] Y.-C. Cheng, A. Isoyan, J. Wallace, M. Khan, F. Cerrina, Appl. Phys.Lett. 90 (2007) 023116. ARTICLE IN PRESS J. Wallace et al. / Nuclear Instruments and Methods in Physics Research A 582 (2007) 254–257   257
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