Includes bibliographical references (page 36).We measured an emission of 6 mJ/pulse at 13.5 nm produced by the Li2+ Lyman-α transition excited by a fast capillary discharge, using a lithium hydride capillary. 75% of the energy emanated from a spot size of 0.6 mm. The emission is narrow band and would thus be useful in extreme-ultraviolet lithography imaging systems that use Mo:Si multilayer mirrors. The output within the bandwidth of Mo:Si mirrors was comparable with that of a laser-produced plasma (LPP), and the wall plug efficiency of 0.1% was nearly an order of magnitude better than that of a LPP

Bender, H. A.

Klosner, M. A.

Rocca, Jorge J.

Silfvast, W. T.

English

We measured an emission of 6 mJ/pulse at 13.5 nm produced by the Li2+ Lyman-α transition excited by a fast capillary discharge, using a lithium hydride capillary. 75% of the energy emanated from a spot size of 0.6 mm. The emission is narrow band and would thus be useful in extreme-ultraviolet lithography imaging systems that use Mo:Si multilayer mirrors. The output within the bandwidth of Mo:Si mirrors was comparable with that of a laser-produced plasma (LPP), and the wall plug efficiency of 0.1% was nearly an order of magnitude better than that of a LPP

Klosner, M. A., author

Silfvast, W. T., author

Bender, H. A., author

Rocca, Jorge J., author

Optical Society of America, publisher

Mountain Scholar (Digital Collections of Colorado and Wyoming)

34 OPTICS LETTERS / Vol. 22, No. 1 / January 1, 1997Intense plasma discharge source at 13.5 nm forextreme-ultraviolet lithographyM. A. Klosner, H. A. Bender, and W. T. SilfvastCenter for Research and Education in Optics and Lasers, University of Central Florida, Orlando, Florida 32816-2700J. J. RoccaDepartment of Electrical Engineering, Colorado State University, Fort Collins, Colorado 80523Received August 16, 1996We measured an emission of 6 mJypulse at 13.5 nm produced by the Li21 Lyman-a transition excited by a fastcapillary discharge, using a lithium hydride capillary. 75% of the energy emanated from a spot size of 0.6 mm.The emission is narrow band and would thus be useful in extreme-ultraviolet lithography imaging systems thatuse Mo:Si multilayer mirrors. The output within the bandwidth of Mo:Si mirrors was comparable with thatof a laser-produced plasma (LPP), and the wallplug efficiency of 0.1% was nearly an order of magnitude betterthan that of a LPP.  1997 Optical Society of AmericaThe development of eff icient normal-incidence multi-layer ref lective coatings in the 13–14-nm wavelengthregion has led to many new optical applications.One of the most demanding applications is extreme-ultraviolet lithography (EUVL). This lithographywould use reduction imaging to print microchipfeatures smaller than 0.1 mm. With the use of all-ref lective optics (ref lectivities up to 65% per surfacewith Mo:Si multilayer coatings) it should be possibleto operate a lithographic stepper in the 13–14-nmwavelength range with the throughput required by acommercial microchip manufacturing system.1 – 3 Thepresent sources under consideration for such a systeminclude synchrotrons and laser-produced plasmas(LPP’s).4 – 7 Although both of these sources have thecapability of producing the necessary f lux for EUVL,synchrotrons are large and expensive and LPP’s areinefficient and costly and have significant debrisproblems.8It was proposed that doubly ionized lithium would bean efficient source at 13.5 nm because the ratio of ex-citation energy to radiated energy is 53% and becauseonly two electrons per atom need to be removed to pro-duce that state.9 The f lux emitted at 13.5 nm by alithium plasma within the bandwidth of multilayer op-tics was calculated to be optimized at an electron den-sity of 1018–1019 cm23 and an electron temperatureof 15–20 eV.10 Low-inductance capillary dischargesof compact design were shown to be capable of pro-ducing these plasma conditions while maintaining asmall source size.11 Thus it appears that a lithiumplasma created in a capillary discharge may be a suit-able source for EUVL and would be far more efficientand compact and significantly less costly than a LPPsource.Emission from the hydrogenlike lithium 3–2 tran-sition at 78.9 nm radiated from plasmas excited incapillary discharges was previously characterizedin studies of recombination x-ray lasers. In thoseinvestigations time-resolved lithium emission oflithium hydride (LiH) capillaries was reported in thewavelength region from 50 to 110 nm.12 However,0146-9592/97/010034-03$10.00/0in no previous studies of which we are aware weremeasurements made of the spatial and output en-ergy characteristics of these discharges at shorterwavelengths. The measurements taken at 13.5 nmthat are reported herein show that the hydrogenlikelithium capillary discharge has significant potentialfor use as a compact and efficient 13.5-nm source ofsmall spot size for EUVL.Constructing and evaluating a lithium-vapor lampto test our calculations would require developing acomplex structure that must be compatible with thelithium vapor and operate at temperatures up to900 ±C. A simpler capillary discharge plasma devicesimilar to the one described in Ref. 13 was thereforeconstructed to carry out the initial evaluations. LiHwas used as the capillary material because it iselectrically nonconducting and because it is a sourceof lithium atoms that easily become vaporized bythe heat and radiation produced in the discharge.Furthermore, in the absence of contamination the onlyother species produced by the discharge is hydrogen,which has no transitions in the spectral region ofinterest and has only one possible ionization stage,thereby causing minimal loss of eff iciency.A 2-mm-diameter 6-mm-long LiH capillary wasfabricated in a fashion similar to that described inRef. 12. Forty-eight 3300-pF capacitors were ar-ranged on a circular plate connected to the capillary,as shown in Fig. 1. The plate was charged to 15 kV,and a triggered spark gap initiated a discharge alongthe inner surface of the capillary so that the ablatedmaterial became heated, creating a highly ionizedplasma. The geometry of the discharge circuit waschosen to minimize the inductance. The capillarywas integrated into the spark gap to further lowerthe inductance, which was measured to be 84 nH. Acalibrated Rogowski coil monitored the current pulse.The system was evacuated to a pressure of less than1024 Torr by a turbomolecular pump.The radiation was viewed axially through a hollowelectrode. It was analyzed with a f lat-field spectrom-eter and an x-ray diode. The spectral data, which 1997 Optical Society of AmericaJanuary 1, 1997 / Vol. 22, No. 1 / OPTICS LETTERS 35Fig. 1. Schematic diagram of the LiH capillary dischargesetup.were recorded on Kodak 101 film, were obtained witha spectrometer that covers a wavelength range of5–20 nm, operating at a 3± angle of grazing incidence.It is based on a design described by Kita et al.,14which has a variably spaced grating with a nomi-nal 1200 groovesymm. The diode, which has an alu-minum photocathode, detected radiation f iltered by a1-mm-thick beryllium (Be) foil. The Be filter trans-missivity peaks at 11 nm (transmissivity ,30%) wherethe K edge occurs and decreases to ,2% at 20 nm, sothe wavelength range in which the Be-filtered diode issensitive lies entirely within the range of the grating.Figure 2 shows the signal of the Rogowski coil andthe x-ray diode recorded during the discharge pulse.A peak current of 25 kA and a FWHM of 250 ns wereobtained. Nearly all the extreme-UV emission wasobserved to occur during the first half-cycle of thecurrent pulse. Figure 3 shows the spectral emissionover the bandwidth of the grating. In addition toemission lines from doubly ionized lithium, we alsoidentified lines from five-times-ionized oxygen. LiH ishygroscopic, and because no special care was taken inhandling the LiH the capillaries became contaminatedwith adsorbed water vapor during their construction.Thus we observed oxygen lines as well as lithium linesin the discharge spectrum.To calculate the total f lux over the wavelength rangeof 11–20 nm we integrated the diode signal weightedby both the diode responsivity and the Be filter trans-missivity to determine the total number of photonsdetected by the diode. We then determined the rela-tive contribution in the 11–20-nm bandwidth fromeach spectral line by direct measurement of the filmgrain density (Fig. 3) of each line weighted by the grat-ing efficiency. We measured the total f lux incidentupon the diode, and, assuming that the capillary was apoint source, we extrapolated 6 mJyshot radiated into ahemisphere. The total conversion eff iciency from elec-trical energy to energy radiated at 13.5 nm is nearly0.05%. We estimate that half of the electrical energyis dissipated in the spark gap, so the effective conver-sion eff iciency from energy deposited in LiH to 13.5-nmradiation is ,0.1%. It should be noted that this effi-ciency is obtained even in the presence of significantoxygen contamination.We obtained soft-x-ray pinhole photographs of unitymagnification, using a 100-mm pinhole 40 cm fromthe capillary, by inserting a 1-mm Be filter in frontof the f ilm plane. The images indicate that 75% ofthe energy was radiated from a spot size of 0.6 mm,corresponding to 4.5 mJ of energy emitted from acylindrical volume of approximately 2 mm3.The radiation from this source was not optimized.The contamination of the LiH by water vapor resultedin undesired radiation from highly ionized oxygen.Fig. 2. Current pulse (top) and time-resolved soft-x-rayemission filtered by a 1-mm-thick Be foil (bottom).Fig. 3. Time-integrated axial emission soft-x-ray spec-trum. Transitions from both lithium and oxygen areindicated.36 OPTICS LETTERS / Vol. 22, No. 1 / January 1, 1997One can significantly reduce this contamination byhandling the LiH in either a dry-nitrogen or a noble-gas atmosphere. The presence of f ive-times-ionizedoxygen indicated that an electron temperature of 20–30 eV was reached,15 a temperature higher than therequired 15–20 eV. It appears that more energy andefficiency could be obtained from the source if the LiHwere not contaminated. In addition, shaping the cur-rent pulse such that the required electron temperatureis attained would further increase the energy output.However, because this particular experiment was car-ried out only for the purpose of proof of principle, theseoptimizations were not subsequently implemented.By examination of Fig. 3 it might be interpretedthat oxygen could be an equal or better emitter inthe extreme-UV spectral region, depending on the de-sired bandwidth. This, however, is an incorrect con-clusion. For equivalent input energies we expect thata pure lithium plasma will be much more eff icient thana pure oxygen plasma, especially within the desiredwavelength range of 13–14 nm. There are four rea-sons for this: (1) Oxygen requires a higher plasmatemperature than lithium to reach the ionization stagewhere strong extreme-UV emission can occur.15 (2)The radiation from oxygen occurs in the fifth ioniza-tion stage, whereas in lithium it occurs in the second.Thus removing and heating the additional electronsfrom oxygen requires significantly more energy. (3)The energy required for populating the radiating statein lithium (from the neutral ground state) is 173 eV,whereas in oxygen it is 402 eV. (4) The lithium tran-sition involves a resonance transition originating fromthe first excited state of Li21 with a very high oscillatorstrength of 0.416, whereas each oxygen transition hasa lower oscillator strength. The energy radiated fromoxygen is divided among several transitions at a vari-ety of wavelengths, as shown in Fig. 3.The hydrogenlike lithium spectrum is narrow band(,0.1 nm) over the bandwidth of the Mo:Si multi-layer mirrors. In contrast, most LPP sources beingconsidered for EUVL use high-Z targets, and hencetheir emission is broadband. The emission outside thebandwidth of the mirrors not only is wasted but is alsodetrimental to the f irst collecting mirror in a lithog-raphy system because it is absorbed by the multilayercoatings, resulting in unwanted heating. In contrast,the lithium discharge source would cause almost no off-band heating. A lithium LPP was recently shown tobe a highly efficient line radiator at 13.5 nm, but it isnot a practical source for extreme-UV lithography be-cause of signif icant debris problems.10This experiment has successfully demonstrated thata LiH plasma is an effective source at 13.5 nm becauseit has produced a f luxypulse comparable with that ofthe most effective LPP (,6 mJ) within the bandwidthof a Mo:Si multilayer mirror. It is also compact andrelatively inexpensive to construct and thus would beuseful for single- or multiple-shot experiments such asthose in extreme-UV microscopy. However, owing toits limited capillary lifetime (25–50 shots), it wouldrequire modifications to be useful as a source for highaverage power applications. Consequently a lithiumsource for EUVL will most likely consist of an oxygen-free high-temperature lithium-vapor discharge lamp.We thank John Hunter and Glenn Kubiak of San-dia National Laboratories and Mohammed Sabre of theUniversity of Cairo for their assistance with variousaspects of this project. This research was supportedby Lawrence Livermore National Laboratories, SandiaNational Laboratories, and the U.S. Department of En-ergy. J. Rocca acknowledges the support of NationalScience Foundation ECS-94019S2.References1. D. A. Tichenor, G. D. Kubiak, M. E. Malinowski, R. H.Stulen, S. J. Haney, K. W. Berger, L. A. Brown, W. C.Sweatt, J. E. Bjorkholm, R. R. Freeman, M. D. Himel,A. A. MacDowell, D. M. Tennant, O. R. Wood II, J.Bokor, T. E. Jewell, W. M. Mansf ield, W. K. Waskiewicz,D. L.White, andD. L. Windt, Appl. Opt. 32,7068 (1993).2. A. A. MacDowell, J. E. Bjorkholm, K. Early, R. R.Freeman, M. D. Himel, P. P. Mulgrew, L. H. Szeto,D. W. Taylor, D. M. Tennant, O. R. Wood II, J. Bokor,J. Eichner, T. E. Jewell, W. K. Waskiewicz, D. L. White,D. L. Windt, R. M. D’Souza, W. T. Silfvast, andF. Zernike, Appl. Opt. 32, 7072 (1993).3. B. La Fontaine, D. P. Gaines, D. R. Kania, and G. E.Sommargren in Extreme Ultraviolet Lithography,Vol. 4 of Trends in Optics and Photonics Series (Op-tical Society of America, Washington, D.C., 1996),pp. 153–156.4. L. F. Thompson, C. G. Wilson, and M. J. Bowden, eds.,Introduction to Microlithography, 2nd ed. (AmericanChemical Society, Washington, D.C., 1994).5. J. B. Murphy, D. L. White, A. A. MacDowell, and O. R.Wood II, Appl. Opt. 32, 6920 (1993).6. P. D. Rockett, J. A. Hunter, G. D. Kubiak, K. Krenz,H. Shields, and M. Powers, in Extreme UltravioletLithography, F. Zernike and D. T. Attwood, eds., Vol. 23of OSA Proceedings Series (Optical Society of America,Washington, D.C., 1994), pp. 255–259.7. D. Torres, F. Jin, M. Richardson, and C. DePriest,in Extreme Ultraviolet Lighography, F. Zernicke andD. T. Attwood, eds., Vol. 23 of OSA Proceedings Series(Optical Society of America, Washington, D.C., 1994),pp. 110–113.8. H. A. Bender, D. O’Connell, and W. T. Silfvast, Appl.Opt. 34, 6513 (1995).9. W. T. Silfvast, ‘‘Eff icient narrow spectral width softx-ray discharge source,’’ U.S. patent 5,499,282(March 12, 1996).10. D. J. O’Connell, ‘‘Characterization of a lithium laserproduced plasma at 13.5 nm for extreme ultravioletprojection lithography,’’ master’s degree thesis (Univer-sity of Central Florida, Orlando, Fla., 1994).11. P. Bogen, H. Conrads, G. Gatti, and W. Kohlhaas,J. Opt. Soc. Am. 58, 203 (1968).12. M. C. Marconi and J. J. Rocca, Appl. Phys. 54, 2180(1989).13. J. J. Rocca, M. C. Marconi, and F. G. Tomasel, IEEEJ. Quantum Electron. 29, 182 (1993).14. T. Kita, T. Harada, N. Nakado, and H. Kuroda, Appl.Opt. 22, 512 (1983).15. F. Jin, ‘‘Advanced laser plasma EUV source,’’ Ph.D.dissertation (University of Central Florida, Orlando,Fla., 1995).

Intense plasma discharge source at 13.5 nm for extreme-ultraviolet lithography

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Intense plasma discharge source at 13.5 nm for extreme-ultraviolet lithography

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