1 January 1998

Ž .Optics Communications 145 1998 109–112

Mass-limited, debris-free laser-plasma EUV source

Martin Richardson, David Torres, Chris DePriest, Feng Jin, Gregory ShimkavegLaser Plasma Laboratory, CREOL, UniÕersity of Central Florida, 4000 Central Florida BlÕd., Orlando, FL 32816, USA

Received 5 May 1997; accepted 20 July 1997

Abstract

The development of a laser-plasma EUV line emission source based on frozen water droplet targets which is essentiallyŽ . Ž .debris-free and capable of continuous, high-repetition-rate )1 kHz operation is described. Created by modest -1 J laser

energies, this plasma produces copious emission at 13 and 11.6 nm, the preferred wavelengths for EUV projectionlithography, with negligible target operation costs. q 1998 Elsevier Science B.V.

Current designs for EUV projection lithography sys-Ž .tems required for high density )1 Gbit computer chip

production in the next century call for a modular, prefer-Ž .ably narrow-band F4% , stable, incoherent irradiation

Žsource of EUV emission at a wavelength of 13 nm or.perhaps 11.6 nm . It must have an average power of )7

W within a bandwidth of ;0.4 nm at these wavelengths,which corresponds to the peak reflectivity of the multilayer

w xoptical elements used in the imaging system 1–4 . Laser-plasmas offer many attractive advantages as a high-repe-tition-rate source for this lithography in that they arecompact, modular, have demonstrated the required mini-

w xmum efficiency 5,6 , and are therefore perhaps the sourceof choice at the present time. However, these advantagesnotwithstanding, laser-plasmas have yet to achieve the

Ž y6 .long term, low-cost ;$10 rshot , debris-free, high-rep-etition-rate continuous operation that would be required.Moreover, those created from high-Z materials radiatebroadband emission that leads by absorption to off-band

w xheating of the primary collection optics 7 . In this paper,we describe a laser plasma source that circumvents theseproblems and meets all of the criteria required for EUVprojection lithography. This source relies on the use of the

Ž . Ž .4d–2p 13 nm or the 4p–2s 11.6 nm line emission fromw xLi-like oxygen that is produced in a dense plasma 7–9 .

The emission at these wavelengths is narrower than thebandwidth of current multilayer mirrors. We have previ-

Ž .ously reported emission studies from solid water icew xtargets 8–10 . Laser conversion efficiencies within a 0.3

mn bandwidth of each of the above lines was ;0.6%,Ž .similar to the best achieved with metal targets ;0.85%

w x5 . The use of oxygen as a laser-plasma target has otheradvantages. Our initial studies of the requirements of

w xdebris-less laser-plasma X-ray sources 4 showed the needto use a target mass no larger than that necessary to

w xprovide the required number of ionized EUV radiators 4 ,typically ;10y6 g. This led to the adoption of liquiddroplet technology. Cryogenic gas targets were first sug-

w xgested as a laser-plasma source by Trail 11 , and Rymelland co-workers have recently used liquid alcohol droplets

w xas laser-plasma sources for microscopy 12,13 . We haveadapted this technology to provide a mass-limited waterdroplet system that presents a continuous stream of ice

w xdroplets into the laser focus 14,15 .Our current laser-plasma source incorporates this sys-

Ž y5 .tem in a vacuum chamber 10 Torr at the focus of acommercial 10 Hz, 400 mJ, 20 ns, 1064 nm Nd:YAG laserŽ .Fig. 1a . A nozzle diameter of 10 mm and a jet velocity of50 mrs results in a stream of droplets that have a frozenpellet diameter of ;20 mm with a nearest-neighbor sepa-ration of ;50 mm. The precision of the droplet’s lateraltrajectory can be seen from Fig. 1b, which shows an imageof the droplet stream illuminated with a cw frequency-dou-bled Nd:YAG laser, taken with a high resolution longdistance microscope. A liquid nitrogen ‘cold finger’ isused to collect the unused droplets which agglomerate assnow, permitting periodic mechanical removal of the un-used target material from the chamber.

0030-4018r98r$17.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0030-4018 97 00421-5

( )M. Richardson et al.rOptics Communications 145 1998 109–112110

The principal drawback of previously reported solidw xmetal or tape targets 3–6 is the creation of solid particu-

lates or liquid metal aerosols, accelerated from the targetsurface as high velocity projectiles that progressively dam-age or coat any optical elements in close proximity. The

Ž y6 .laser light interaction with a limited mass -10 g offrozen ice eliminates the generation of any high velocityprojectiles. Most of the target is ablated in the form of highvelocity ions, and any un-ionized matter that remains willbe vaporized. Two separate measurements were made tocheck this. First a Si witness plate, placed 4 cm from theplasma, was exposed to )105 shots and then examinedwith an atomic force microscope. No evidence of materialdeposition or surface damage through particle impact wasobservable. Second, the reflectivity of a MorSi multilayer

Ž .mirror Rs60% at 13 nm placed 4 cm from the target atan angle of 45 degrees was monitored with an X-rayphotodiode for a large number of shots. The change inreflectivity as a function of the number of laser shots isshown in Fig. 2. The reflectivity remains constant towithin 1% for )105 shots, a number limited only by thelaser repetition rate. With this minimal reduction in reflec-tivity, and with the expectation that the source-collectionmirror distance in an EUV lithography system would be;20 cm, it is predicted that this system could run formore than 108 shots without any major degradation of theEUV optics.

Since the spectral intensity of X-rays emitted from a

hot, dense laser-plasma depends strongly on the plasmaconditions we have used a combination of a one-dimen-sional hydrodynamic code and an atomic ionization codeto optimize the irradiation and target conditions. The one-

w xdimensional Lagrangian hydrodynamic code Medusa 16 ,is used to model the electron and ion density and tempera-ture as a function of time and space, in spherical geometry,dividing the plasma into 90 temporally evolving cells. Thecode takes into account laser-plasma coupling mechanismsŽ .principally inverse bremmstrahlung absorption radiationand electron transport and the flow of ions and electrons.The electron density and temperature predicted by thiscomputer code are then used as input to an atomic physics

w xcode RATION 17 , which calculates the ion populationsof each ionization stage, and the resulting emission spectrafrom H-like, He-like and Li-like ions, for given plasmaconditions, under the assumption of steady state. A thirdcode, SPIN, spatially and temporally integrates the contri-butions of each shell to the emission spectrum by couplingthe output of both Medusa and RATION to produce a

w xtime-integrated synthetic emission spectrum 18 . The re-sults of these calculations are shown in Figs. 3 and 4. Fig.3 shows the electron density and temperature, at the tem-poral peak of the laser pulse, as a function of the radius ofthe plasma, calculated for the laser and target conditionsused in these studies. The simulation predicts that half ofthe droplet radius has been ablated during the first half ofthe laser pulse. This confirms our belief that the entire

Ž .Fig. 1. The experimental facility. The ice droplet source delivers a highly uniform, flat trajectory stream right of 20 mm droplets , travelingŽ y5 .at 50 mrs through vacuum 10 Torr to the focus of a commercial Nd:YAG laser, where the laser-plasma is formed.

( )M. Richardson et al.rOptics Communications 145 1998 109–112 111

Fig. 2. MorSi multilayer mirror reflectivity versus number of laser shots, for the water droplet and planar tin laser-plasma EUV sources.

droplet is consumed during the laser–material interaction.Fig. 4 shows a synthetic emission spectrum from the icedroplet plasma. Strong emission from the 4d–2p and 4p–2stransitions of Li-like oxygen is predicted, as the meanelectron temperature of the plasma during the laser irradia-tion is ;26 eV, sufficient to produce an abundance ofcollisionally excited Li-like oxygen ions. This syntheticspectrum is compared to the measured emission spectrum,obtained with a high resolution flat-field X-ray spectro-

w xgraph 19 . This shows the oxygen line emission superim-posed on the broadband bremmstrahlung emission of theplasma. Identification of all the lines in this spectra con-

firms that the predominant emitting ion in the plasma isthe Li-like oxygen ion, with weaker contributions fromBe-like and He-like oxygen ions.

This source can relatively easily be extended in perfor-mance beyond that reported here. Since the droplet systemoperates at frequencies in excess of 100 kHz, it can readilybe used with high repetition-rate lasers, leading to a highaverage power short-wavelength radiation source. In com-bination with a state-of-the-art, 0.4 J, 1 kHz, solid-state

w xlaser 20 , it would provide a )7 W EUV narrow-bandsource at 13 or 11.6 nm, sufficient for a demonstration

w xprojection lithography stepper 1 . The droplet system

Ž .Fig. 3. Theoretically modeled plasma dynamical variables of n , and T , predicted electron density n and temperature at the peak of thee e eŽ . 12 2 Ž .laser pulse 19 ns FWHM, Gaussian . Peak laser intensity 1.26=10 Wrcm pulse 20 ns . N is the plasma critical electron density, andc

r is the initial radius of the ice droplet.drop

( )M. Richardson et al.rOptics Communications 145 1998 109–112112

Ž .Fig. 4. The measured and theoretical spherical geometry spectra. The Li-like oxygen transitions are labeled.

should work well with high average power Q-switchedŽlaser systems pulse energies ;10 mJ focused to power

12 2 .densities of ;10 Wrcm at frequencies ;20 kHz . Inaddition, we are adapting the water droplet system toprovide XUV and X-ray emission at other wavelengths.

w xSeeded with other liquids 12,13 , or with colloidal mix-tures of high-Z materials, it will provide a debris-less laserplasma source with emission in particular wavelength bandsextending into the keV range. Finally, in combination with

Ž 16 2.new high repetition-rate high intensity )10 Wrcmfemtosecond laser pulses, the droplet system should pro-vide a high power source of ultrashort keV continuumemission.

Acknowledgements

The authors acknowledge useful discussions with Drs.G. Kubiak, A. Hawryluk, F. Zernicke and W.T. Silfvast,and thank Dr. J. Underwood for multilayer mirror samples,Dr. D. Kania for an X-ray diode, Dr. C. Brown for X-rayfilm digitization, and J. Cormier, G. Luntz and M. Nguyenfor technical support. This work was supported by ARPA

Ž .through contract LLNL B3 13948, and the State ofFlorida.

References

w x Ž .1 A.M. Hawryluk, N.M. Ceglio, Appl. Optics 32 1993 7062.w x2 S.V. Haney, K.W. Berger, G.D. Kubiak, P.D. Rockett, J.

Ž .Hunter, Appl. Optics 32 1993 6934.w x3 W.T. Silfvast, M.C. Richardson, H. Bender, A. Hanzo, V.

Yanovsky, F. Jin, J. Thorpe, J. Vac. Sci. Technol. B 10Ž .1992 3126.

w x4 M. Richardson, W.T. Silfvast, H. Bender, A. Hanzo, V.P.Ž .Yanovsky, F. Jin, J. Thorpe, Appl. Optics 32 1993 6901.

w x5 R.L. Kauffmann, D.W. Phillion, R.C. Spitzer, Appl. OpticsŽ .32 1993 6897.

w x6 G.D. Kubiak, K.W. Berger, S.J. Haney, P.D. Rockett, J.A.Ž .Hunter, OSA Proc. on Soft X-ray Lithography 18 1993

127.w x7 F. Jin, Ph.D. Thesis, University of Central Florida, 1995.w x8 F. Jin, M. Richardson, Proc. OSA Top. Mtg. on EUV

Ž .Lithography 23 1995 260.w x Ž .9 F. Jin, M. Richardson, Appl. Optics 34 1995 5750.

w x10 M. Richardson, F. Jin, Proc. OSA Top. Mtg. on EUVŽ .Lithography 23 1995 65.

w x11 J. Trail, Ph.D. Thesis, Stanford University, 1989.w x Ž .12 L. Rymell, H.M. Hertz, Optics Comm. 103 1993 105.w x13 L. Malmquist, L. Rymell, M. Borglund, H.M. Hertz, Rev.

Ž .Sci. Instr. 67 1996 4150.w x14 F. Jin, M. Richardson, G. Shimkaveg, D. Torres, SPIE 2523

Ž .1995 81.w x15 F. Jin, M. Richardson, G. Shimkaveg, D. Torres, in: G.D.

Ž .Kubiak, D. Kania Eds. , TOPS on EUV Lithography, Opti-cal Society of America, Washington, DC, 1996, p. 89.

w x16 J.P. Christansen, D.E.T.F. Ashby, K.V. Roberts, Com. Phys.Ž .Commun. 7 1974 271.

w x17 R. Lee, User manual for RATION, University of Californiaand LLNL, 1990.

w x18 D. Torres, F. Jin, M. Richardson, C. DePriest, in: G.D.Ž .Kubiak, D. Kania Eds. , TOPS on Extreme Ultraviolet

Lithography, Optical Society of America, Washington, DC,1996, p. 75.

w x19 W. Schwanda, K. Eichmann, M. Richardson, J. X-Ray Sci.Ž .Technol. 4 1993 8.

w x20 M. Hermann, J. Honig, L. Hackel, Proc. OSA Top. Mtg. onŽ .EUV Lithography 23 1994 238.