Laser driver for soft-x-ray projection lithography

Lloyd A. Hackel, Raymond J. Beach, C. Brent Dane, and Luis E. Zapata

A design of a diode-pumped Nd:YAG laser for use as the driver for a soft-x-ray projection lithographysystem is described. This laser will output up to 1 J per pulse with a 2- to 5-ns pulse duration and a400-Hz pulse repetition rate. The design employs microchannel-cooled diode laser arrays, zigzag slabenergy storage, a regenerative amplifier cavity that uses phase conjugator beam correction fornear-diffraction-limited beam quality, and stimulated Brillouin scattering pulse compression to achievethe required pulse length.

Key words: Projection lithography, soft x rays, solid-state laser, pulse compression, diode-pumpedlaser.

Introduction

A laser-driven soft-x-ray projection lithography sys-tem of production quality requires (1) laser sourceswith peak power in the range of 10 MW to 1 GW, (2)per pulse energy of 0.5 to 1 J, and (3) repetitionrates of 400 Hz to permit high-volume throughput.'The required pulse duration is in the range of 1 to 5ns. In support of an integrated lithography systembeing developed at the Lawrence Livermore NationalLaboratory, a diode-pumped, solid-state laser systemhas been designed to meet these basic system require-ments.

Laser Design

The requirement of operating the laser system athigh average power dictates the use of crystalline hostmaterials such as YAG, GGG, or YLF. In addition,the use of diode laser pumping minimizes heating ofthe medium, thus maximizing the average poweravailable from a given gain volume. The narrowlinewidth emission of laser diodes matches well withNd3+ absorption in the crystalline hosts, allowing forefficient coupling of diode pump energy to the upperlaser level.

The proposed laser system design incorporates anoscillator and a regenerative amplifier. The oscilla-tor produces a single-frequency output pulse, whichis expanded to full spatial size before being injectedinto the amplifier. The amplifier extraction geom-

The authors are with the Lawrence Livermore National Labora-tory, P.O. Box 808, Livermore, California 94550.

Received 20 November 1992.0003-6935/93/346914-06$06.00/0.© 1993 Optical Society of America.

etry uses passive polarization rotation to switch intoand out of a four-pass ring. A stimulated Brillouinscattering (SBS) cell placed between the second andthird pass provides gain isolation and beam qualityrestoration. Figure 1 shows a schematic layout ofthe laser. A version of this concept is now success-fully operating in a high-peak-power (to 3 GW) glasslaser system with an average power of 100 W in ourlaboratory2 at Lawrence Livermore National Labora-tory.

A pulse duration of several nanoseconds or less canbe difficult to achieve with a standard Q-switchedoscillator. The high peak power involved can alsolead to optical damage through nonlinear self-focusing (B integral) of the beam within the amplifier.In order to accommodate this operational regime, thelaser is operated with a longer pulse (i.e., 20 ns),and an SBS pulse compressor shortens the pulse tothe required 1- to 5-ns duration.

Diode Pumping

The high average power and high repetition raterequired for this laser are attained by the use of alaser diode pump with a thermally efficient method ofpackaging in which the diode bars are mounted onmicrochannel coolers.3 These coolers minimize theimpedance typically associated with heat transportacross the cooling water boundary layers of high-performance heat sink systems. This modular diodepackaging concept permits the construction of largediode arrays with high-duty factor operation. Alaser that employs 160 array packages is currentlypumping a 2 cm x 8 cm x 0.4 cm Nd:YAG slab, withan average output power of 1000 W.4

Figures 2 are photographs of our present modulardiode package. One can assemble arbitrarily large

6914 APPLIED OPTICS / Vol. 32, No. 34 / 1 December 1993

Sigte frequenoscitttaor

i

HW ptate

AG stabAl ifer

Fig. 1. Schematic layout of a soft-x-ray projection lithographylaser system.

(a)

(b)

Fig. 2. (a) Modular microchannel-cooled laser diode package thataccepts 1.8 linear centimeters of diode array. The approximatepackage dimensions are 2 cm x 2 cm x 0.075 cm. (b) The diodebar is mounted at the edge of a coated surface, emitting light downand toward the right in this photograph.

two-dimensional (2-D) apertures by simply stackingmodules, using metal-impregnated (conductive) sili-cone rubber gaskets patterned with the same through-holes as the cooler module shown in Fig. 2(a). Themodules of the resulting stack are thus connectedelectrically in series. Two examples of such 2-Darrays fabricated by stacking 16 modules in one caseand 80 modules in the other are shown in Fig. 3.

The module shown in Fig. 2 consists of three layersin a silicon-glass-silicon sandwich configuration.The top layer contains etched-silicon channels, whichare used to supply water from the inlet ports to themicrochannels just below the location of the laserdiode. The central glass insert is slotted and hasthrough holes that match the silicon layers. Theslot permits the water in the microchannels to flow tothe bottom piece of silicon, which contains a manifoldthat delivers the water to the output drain ports.The thickness of the silicon-glass-silicon sandwichand silicone rubber gasket are approximately 0.75and 0.25 mm, respectively, yielding a stacking densityof 10 packages per centimeter.

The finlike structure of the microchannels and thelaminar flow of cooling water through it account forthe unique cooling capability of the package.

(a)

(b)

Fig. 3. (a) 2-D laser diode array constructed by stacking 16modular microchannel packages. (b) This 80-module stack wasfabricated for pumping a Nd 3 +:YAG high-average-power crystal-line slab laser.

1 December 1993 / Vol. 32, No. 34 / APPLIED OPTICS 6915

Oscillator

The oscillator is designed to produce 2 to 5 mJ perpulse and is configured to operate with a single-frequency output to optimize the performance of theSBS phase conjugator in the power amplifier. Thegain medium is a diode-pumped Nd:YAG rod. Fig-ure 4 shows a conceptual layout of the design.

The single-frequency output is achieved when thecavity is self-seeded by an electronic linewidth narrow-ing technique.5 6 The oscillator consists of a two-mirror resonator with a polarizer and a Pockels cell tohold off oscillation during the gain buildup. A waveplate is inserted in the cavity and is rotated a smallamount, causing the Q-switch to leak slightly. A30-mm talon in the cavity narrows the gain band-width but cannot alone hold off the buildup of mul-tiple frequencies after the Q-switch is opened. Asthe diodes are pulsed, the cavity gain increases, and,with the leakage properly adjusted, the lowest-orderlongitudinal cavity mode with a frequency near thepeak of the gain curve will come above threshold andoscillate. At this point, a photodiode, depicted inFig. 4, senses the small output leaking through thehigh reflector and triggers the electronics to drive thePockels cell fully open. The low-power oscillatorserves to seed the buildup of the high-powerQ-switched pulse, suppressing other output modesthat would otherwise be seeded from noise. Thusthe single-frequency mode builds most rapidly, deplet-ing the gain and preventing the emergence of otherlongitudinal modes. This method produces a high-quality, single-frequency output.2

Amplifier

In order to provide the necessary energy per pulse,the laser amplifier operates in a storage mode with asingle-pass gain of 20 (3 Np). To achieve this gain,two diode arrays, each comprising 150 diode bars, areassembled. The diodes are driven with a 200-iscurrent pulse of 140 A and output a total energy of 6J/pulse. For the lithography application the laser isrun at 400-Hz pulsed repetition frequency. At sucha high gain, the amplifier geometry must be carefullydesigned to prevent parasitic oscillation and to mini-mize losses caused by amplified spontaneous emission(ASE). In addition, the seeded, four-pass, phase-conjugated, regenerative amplifier architecture pro-posed depends on a spatially uniform gain profile tooperate successfully. An amplifier that fulfills allthese requirements at high average powere has never

been achieved. A set of computer codes, collectivelynamed TECATE/BREW, has been built and bench-marked against operational laser systems. Theseare fully 3-D theoretical models that solve for thethermal, stress, and deformation distributions for agiven pumping and cooling geometry. Using them,we can analyze the zigzag propagation through themedium and compute interferograms and birefrin-gence for the thermally loaded slab. Other codesaddress the more subtle issues of ASE and parasiticsthat have an impact on the specific facet-cut anglesfor the crystal amplifier. These codes have beenverified experimentally and can be utilized with confi-dence in the design of zigzag slab amplifiers such asthe one presented here.

Because of the high average power required, theamplifier design is based on a zigzag Nd:YAG slab.The nominal slab geometry selected is shown in Fig.5. The slab is cut at 400 wedge angles and is 2.5 cmhigh by 6 mm wide by 20 cm long. The ASE/parasiticperformance is sensitive to the indices of refraction atthe slab boundaries and to the relative facet angles.The angle of 400 at the sharp end is chosen tominimize ASE losses and parasitic oscillation.Several relevant slab geometry cases are shown inFig. 6. The curves reaching the highest gain levelswere obtained by invoking a perfectly matched indexof refraction at the edges of the slab and varying thesharp wedge angle. The lower set assumes a mis-matched index of 1.47 at the edges compared withthat of 1.82 for YAG. In practice, the higher gainmay be achieved if the edge surfaces are eitherdiffuse, closely index matched, or coated with anabsorbing layer. Figure 6 thus sets the possibleboundaries for the performance of the actual device.A single-pass gain of 30 (3.4 Np) should be possiblewhen a 40° wedge angle is used and if the edgesurfaces are made to be absorbing. We expect tooperate at a single-pass gain of 20 (3 Np).

We have computed the effects of thermal loadingand face cooling of this slab design using TECATE/BREW.Figure 7 shows examples of the temperature, stress,and deformation distributions generated when theexpected levels of pumping and cooling are modeled.The computed interferogram of a beam making 22

30 mm etalon withPIN photodiode 40% reflective coatings

\ High reflector Thin-filmradius - 5 m polarizer

\ EI 0 I

X/1A4 plateKD P Pocket cell I

To Pockelcell trigger

I -

AR 300 \\

Nd:YAG diodepumped laser rod

oModa-limitingaperture

Uncoated 5 mm etalonoutput coupler

Fig. 4. Schematic diagral of a single-frequency master oscillator.

4- coolant

Pump zone -- 6mmcoolant

Z Diode array I

-..................- -200 m m ---- ------------Fig. 5. Schematic diagram showing the overall dimensions of azigzag Nd:YAG slab and its pumping and cooling geometry. Thebeam makes 22 zigzag traverses through the slab. The gain isexpected to reach 3 Np (single pass Go = 20).

6916 APPLIED OPTICS / Vol. 32, No. 34 / 1 December 1993

^ - || | >~~~~~~~~~~~~~~~~~~~~~~~~~~... .. .. .. . ... .. -- - - -- - - -__ _- ---- ---

3.5

3.0

2.5

2.0S

a

1.0

0.5 /

I/

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Pumped gan

Fig. 6. Effective gain expected when parameterized against the

slab wedge angle and the edge refractive index. The top set of

curves assumes perfect index matching and the lower set assumesan index of 1.47 at the edges. The curves stop at the point wherethe codes find parasitic lasing for the wedge angles indicated. Thedashed line represents the gain expected if ASE effects are absent.

zigzag traverses through the slab is also shown forpropagation incident at 300 with respect to the nor-mal of the entry facet. These results are idealistic inthe sense that, in this instance, perfectly uniformpumping and cooling were invoked. Although it isdifficult to know what the uniformity of the realdevice will be, sensitivity runs and past experiencehave shown that gradients in pumping and cooling ofa few percent generate measurable wave-front errorsat the quarter-wave levels. Nevertheless, experiencehas indicated that the phase conjugator can correct

(a)

(b)

for up to several waves of residual distortions gener-ated by the real device.

It is an engineering challenge to achieve pumpingand face temperature uniformity of the slab to withina few percent. With respect to the diode laser pump,there is an ongoing effort to measure and maximizethe uniformity of the output from the array. Be-cause of the modular nature of the diode packages,the packing density, the slab area pumped, andpossibly the current to individual sections of diodescan be adjusted to achieve the required uniformity.The slab cooling is achieved by water flow across thelarge slab faces. The uniformity of the slab facetemperature is dominated by the surface heat trans-fer in the cooling channel. Ideally the heat transferis rapid enough that the slab face temperature uni-formly approaches that of the cooling water. Inorder to achieve this condition, the viscous boundarylayer in the flow channel must be minimized; that is,the flow should be fully turbulent. The water in thesupply plenum for the channel is laminar and musthave changed to highly turbulent before contactingthe slab faces. A key feature of the design, therefore,will be a sufficiently long transition section withappropriate flow trips to ensure that full turbulence isachieved as the flow first contacts the slab face. Thecombination of uniform pumping and face tempera-ture will provide an amplifier with uniform gain andminimal induced phase aberrations.

Optical Extraction Architecture

The 5-mJ oscillator output is amplified to 1 J in fourpasses of the amplifier. In this entire multipasspropagation the beam is image relayed with unitymagnification: from the single-frequency oscillator,to the input-output polarizer, to the slab, to theconjugator, and then in the reverse sequence back tothe input-output polarizer. The oscillator outputbeam is expanded to 7 mm x 1.3 cm for propagation

(C)

I x ~~~~~~~~~~~~~

(d)

Fig. 7. Representative TECATE/BREW code results for the Nd:YAG slab of our design. The scale factors in (a), (b), and (d) expand the thin

dimension for clarity: (a) Temperature distribution in a cross section perpendicular to the slab axis through the slab center. Isotherms

are 3 0C apart. (b) Stress contours in longitudinal cross section through the center of the slab. 6-MPa contours go from 50 MPa tension

on the surface to 25 MPa of compression in the midplane next to the pumped-unpumped transition. (c) Deformations magnified 3000

times. (d) Single-pass interferogram for the 22-bounce zigzag path of the extracting beam (one-wave contours).

1 December 1993 / Vol. 32, No. 34 / APPLIED OPTICS 6917

through the amplifier. The relay process is impor-tant in minimizing the potential for optics damagecaused by the propagation of an aberrated high-powerbeam and also in obtaining a high-fidelity return fromthe phase conjugator. A schematic layout of theoptical architecture is given in Fig. 1. The single-frequency output of the oscillator is anamorphicallyexpanded by the beam shaping optics to an input sizeof 7 mm x 13 mm to nearly fill the slab amplifierinput face. The beam is coupled into and out of thefour-pass amplifier by means of polarization switching.The S-polarized output of the oscillator reflects offthe input-output polarizer and is rotated to P by theFaraday rotator. The beam propagates through theslab amplifier, passes through the half-wave plate,where its polarization is rotated to S, and thenthrough the 320-mm relay telescope. (The telescopeis set up to relay the image of the slab back onto itselfexactly. In this way phase aberrations accumulatedin the thermally loaded slab do not propagate intointensity modulations.) The S-polarized beam re-flects off the internal polarizer and again propagatesthrough the slab, is rotated by the half-wave plate,and after relay propagates to the SBS phase conjuga-tor. At the conjugator the phase aberrations accumu-lated in the two passes through the ring are reversed,and the reflected beam retraces its steps through theamplifier. After accumulating four total slab passes,the beam propagates back out through the Faradayrotator and, because it does not rotate in this direc-tion, propagates to the pulse compressor.

Since the conjugator is a nonlinear thresholdeddevice, it acts as an isolator of amplified spontaneousemission between stages. The phase aberrationsaccumulated in the amplifier, as the beam propagatesto the conjugator, are reversed in the conjugator andare then canceled in propagating back out, producinga near-diffraction-limited output beam at high aver-age power.

This amplifier configuration has been extensivelytested by the use of a flashlamp-pumped 1 cm x 12cm x 40 cm Nd:glass slab. With a single-frequencyinput of 40 mJ from a master oscillator, an output of25 J per pulse at a 12-ns pulse length with near-diffraction-limited beam quality is routinely obtained.Limited by the fracture strength of the glass, thissystem currently has an average power capability of

100 W. With the increased thermal conductivityand strength of the crystalline YAG slab and with theincreased efficiency of diode pumping, this averagepower capability should be increased to the 1-kWrange.

Compression of the Output PulseThe 0.5- to 1-J laser output energy needs to bedelivered to the x-ray target in - 1 to 5 ns to generatethe desired plasma temperature and subsequent x-rayflux efficiently. However, if the amplifier is operatedat this high irradiance with its gain medium pathlength, catastrophic nonlinear self-focusing (B inte-gral) can result. A simple calculation of the B inte-

gral for this design yields 0.92 for a 3-ns pulse. A Bintegral above 1 has been determined to be beyondsafe limits for a reliable fielded system. A longer,lower-peak-power pulse is desired for operation withinthe amplifier.

To achieve the required output peak power whileminimizing the peak power within the laser, wepropose to operate the laser with a safe and efficient20-ns pulse length and then compress the outputusing an efficient SBS pulse compressor. Materialssuch as CC14 can be used to compress 20-ns pulses to

2 ns efficiently. However, SBS pulse compressionresults reported in the literature extend to inputenergies only up to 500 mJ.8' 6 Competing nonlin-ear processes such as Raman generation, self-focus-ing, and optical breakdown limit the input energy forfocused geometries of SBS generation. Schemes in-volving two cells (an oscillator and an amplifier withoptical attenuation in between) permit larger inputenergies but at the expense of efficiency.

In order to overcome this limitation, we havedeveloped a pulse compression concept that permitsefficient compression at our required energy andaverage power.'7 Figure 8 shows an optical configu-ration of this concept. The design includes an SBSgenerator cell, an amplifier cell, and optics to separateand control the beams by means of polarization. Inthe concept, a small (adjustable) portion of the long-pulse input is split off and directed to the SBSgenerator after transiting an optical delay. Thisinput is focused into the cell and generates a weakStokes return pulse. The delay is adjusted so thatthe Stokes return pulse is incident upon the back endof the SBS amplifier just as the main pulse arrives.The amplifier length is specified so that its one-waytransit time is equal to half the input pulse width.With a gain coefficient of 6 cm/GW in CC14, a1.5-cm2 cross-sectional area beam is used in theamplifier in order to keep the one-way gain below thethreshold of 30 Np.

We have recently demonstrated the performance ofthis pulse compression concept. When the setup,shown in Fig. 9 with a CC14 medium, was used, 2.5-J

Compressedoutput

Fig. 8. With this oscillator-amplifier configuration and an appro-priate choice of the SS medium, pulses in the MJ range could becomipressed with high-energy efficiency.

6918 APPLIED OPTICS / Vol. 32, No. 34 / 1 December 1993

References and Notes1. R. L. Kauffman and D. W. Phillion, "X-ray production effi-

ciency at 130 A from laser-produced plasmas," in Soft-X-RayProjection Lithography, J. Bokor, ed., Vol. 12 of OSA Proceed-ings Series (Optical Society of America, Washington, D.C.,

7 1991), pp. 68-71.

~ \ 1.7 ns 2. J. L. Miller, C. B. Dane, L. Zapata, L. Hackel, and J. Abate,

FWHM ~ "Neodymium:glass zigzag slab regenerative amplifier lasersystem for x-ray lithography," in Conference on Lasers and

15.8 ns Electro-Optics, Vol. 12 of 1992 OSA Technical Digest Series- ' ~ (Optical Society of America, Washington, D.C., 1992), p. 90.

3. R. Beach, W. J. Benett, B. L. Freitas, D. Mundinger, B. J.0 10 20 30 40 50 60 Comaskey, R. W. Solarz, and M. A. Emanuel, "Modular

Time (ns) microchannel cooled heatsinks for high average power laser

3ults of using the SBS to compress a 2.5-J pulse from a diode arrays," J. Quantum Electron. 28, 966-976 (1992).8 to 1.7 ns. The 80% efficiency resulted in a five-time 4. G. Albrecht, R. J. Beach, and B. Comaskey, "High-repetition-peak power. rate diode-pumped solid state lasers," in Energy and Technol-

ogy Review, UCRL-52000-92-6 (Lawrence Livermore NationalLaboratory, Livermore CA 94550, 1992).

5. D. C. Hanna, B. Luther-Davies, and R. C. Smith, "Singlere compressed from a width of 15.8 to 1.7 ns longitudinal mode selection of high power actively Q-switch

efficiency. The experiment was operated lasers," Opto-electronics 4, 249-256 (1972).Hz pulse repetition rate. Figure 9 shows 6. Y. K. Park and R. L. Byer, "Electronic linewidth narrowingassed and compressed pulses. The only real method for single axial mode operation of Q-switched Nd:YAGscaled operation at the 400-Hz repetition lasers," Optics Commun. 37,411-416(1981).

he thermal energy deposited at the focus. 7. See for example. A. E. Siegman, Lasers (University Science,

, a major reason for the choice of CC14 as the Mill Valley, Calif., 1986), pp. 385-386.[ium is its very low (10-6/cm) absorption 8. D. T. Hon, "Pulse compression by stimulated Brillouin scatter-t. Additionally, if required, the small spot ing," Opt. Lett. 5, 516-518 (1980).he SBS generator focal region (- 10 [um) 9. R. R. Buzyalis, A. S. Dement'ev, and E. K. Kosenko, "Forma-

rmits the beam to be translated in the SBS tion of subnanosecond pulses by stimulated Brillouin scatter-)etween laser pulses, minimizing the energy ing of radiation from a pulse-periodic YAG:Nd laser," Sov. J.i in any single small volume, thus permitting Quantum Electron. 15, 1335-1337 (1985).i rates up to 1 kHz. 10. E. Gaizhauskas, V. Krushas, N. Ya. Nedbaev, R. A. Petrenko,

A. Piskarskas, and V. Smil'gyavichyus, "Generation of picosec-ond pulses as a result of stimulated Brillouin scattering in

-average-power laser we are designing for liquids," Sov. J. Quantum Electron. 16, 854-855 (1986).Cray projection lithography system has re- 11. S. B. Papernyi, V. F. Petrov, and V. R. Startsev, "Observationits that surpass currently available laser of a quasisoliton interaction in stimulated Brillouin scatter-

Furthermore, this design incorporates con- ing," Sov. Tech. Phys. Lett. 7, 185-186 (1981).Furtherones whosesperf poaeehave 12. M. J. Damzen and M. H. R. Hutchinson, "High-efficiency

y componets w os performance w hav laser-pulse compression by stimulated Brillouin scattering,"ally demonstrated in our laboratory. This Opt. Lett. 8,313-315(1983).cludes (1) high-average-power output from a 13. V. A. Gorbunov, S. B. Papernyi, V. F. Petrov, and V. R.

mped crystal, (2) high beam quality and Startsev, "Time compression of pulses in the course of stimu-extraction with our master oscillator- lated Brillouin scattering in gases," Sov. J. Quantum Electron.

Live amplifier design, and (3) pulse compres- 13,900-905(1983).high peak power without the problems 14. D. N. G. Roy and D. V. G. L. N. Rao, "Optical pulse narrowing

d with nonlinear self-focusing. During the by backward, transient stimulated Brillouin scattering," J.r, we will construct and test the performance Appl. Phys. 59,332-335 (1986).iser subsystem and then integrate it into a 15. M. A. Davydov, K. F. Shipilov, and T. A. Shmaonov, "Forma-y projection lithography system. It is antici- tion of highly compressed stimulated Brillouin scatteringat the laser will meet all the requirements for pulses in liquids," Sov. J. Quantum Electron. 16, 1402-1403i source driver and help bring soft-x-ray (1986).a lithography into commercial production 16. R. Fedosejevs and A. A. Offenberger, "Subnanosecond pulsesturn of the century. from a KrF Laser pumped SF6 Brillouin amplifier," IEEE J.

Quantum Electron. 21, 1558-1562 (1985).ork was performed under the auspices of the 17. C. B. Dane, W. A. Neuman, and L. A. Hackel, "High energy,artment of Energy by Lawrence Livermore SBS pulse compression," submitted to J. Quantum Electron.Laboratory under contract W-7405-Eng-48. (1992).

1 December 1993 / Vol. 32, No. 34 / APPLIED OPTICS 6919

1000

800

600

400300-

200

Fig. 9. ReEwidth of 15.increase in

pulses wewith 80cat a 10-Iuncompriissue forrate is t]However,SBS meccoefficiensize of teasily peimedium 1depositedrepetitior

Summary

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