Influence of Sodium Impurities on ArF Excimer-Laser-Induced Absorption in CaF_2 Crystals

Influence of sodium impurities on ArFexcimer-laser-induced absorption in CaF2 crystals

Norio Komine, Shigeru Sakuma, Masaki Shiozawa, Tsutomu Mizugaki, and Eiji Sato

The formation of color centers induced by irradiation with ArF excimer lasers in CaF2 crystals was foundto depend strongly on the sodium impurity concentration. Sodium-related color centers were generatedby two-photon absorption because the slope of the induced absorption coefficient just after irradiationstarted was proportional to the square of the laser fluence. The saturation absorption also depended onlaser fluence, and a photobleaching induced absorption phenomenon was observed. We concluded thatthe saturation absorption level was determined by the equilibrium between two-photon excitation andone-photon reverse reaction. © 2000 Optical Society of America

OCIS codes: 160.4670, 140.3330, 140.2180, 220.3740, 300.1030.

t

p

1. Introduction

Crystalline calcium fluoride is a promising materialfor use in optical lenses in photolithography systemswith ArF excimer lasers or F2 gas lasers as illumina-tion sources.1,2 CaF2 has high transmission at thewavelengths of those lasers, is optically isotropic, andis chemically stable. However, even in pure CaF2crystals, when they are irradiated by excimer lasers,color centers appear and cause a considerable de-crease in transmission. It is important to clarify themechanism of color-center formation induced by ir-radiation to improve resistance to VUV lasers or VUVlight. There have been many reports3–5 on the in-fluence of impurities on color-center formation inCaF2 crystals induced by irradiation with high-energy radiation such as x rays. Recently, Mizugu-chi et al.6–8 pointed out that the color-centerformation induced by ArF excimer-laser irradiation isstrongly influenced by yttrium and trivalent rare-earth ion impurities in CaF2 crystals. However, wehave evaluated the durability against ArF excimer-laser irradiation of many CaF2 crystals with yttriumimpurities of less than 1 part in 106 ~ppm!; even thenthe degree of the durability varied widely. So it is

The authors are with the Nikon Corporation, 10-1, Asamizodai1-chome, Sagamihara, Kanagawa 228-0828, Japan. N. Komine’se-mail address is [email protected].

Received 21 January 2000; revised manuscript received 3 May2000.

0003-6935y00y220001-06$15.00y0© 2000 Optical Society of America

necessary to find other factors that influence durabil-ity.

Many studies of color centers in sodium-dopedCaF2 crystals induced by x-ray irradiation at roomtemperature have been made.9–11 However, weknow of no report of the influence of sodium impuri-ties on ArF excimer-laser-induced absorption in CaF2crystals. Thus we initiated an investigation of al-kali metal impurities, particularly those of sodium.

We surveyed the dependence of ArF excimer-laser-induced absorption on the sodium impurity concen-tration in pure and sodium-doped CaF2 crystals andinvestigated the behavior of ArF excimer-laser-induced absorption relative to various laser fluences.

2. Experiment

We prepared 20 samples of pure CaF2 crystals pur-chased from four different manufacturers for ArFexcimer-laser exposure tests. The sodium concen-tration in the samples was measured by radio acti-vation analysis by neutron irradiation with adetection limit of 1 part in 109 ~ppb!. A concentra-ion of a few parts in 109 to a few ppm was detected

in these samples. The amounts of yttrium and otherrare-earth metal impurities in all samples were lessthan 1 ppm. The amounts of alkali-earth metal im-purities that we detected were as follows: Mg, ,5

pm; Sr, 20–250 ppm; Ba, ,5 ppm. The amounts ofmetal impurities, except for sodium, were measuredby inductively coupled plasma atomic emission spec-trometry. We also prepared a sodium-doped CaF2crystal. This crystal was grown by the standardBridgman method. Sodium was added to the start-ing material as 0.05-mol. % NaF. As a result, the

1 August 2000 y Vol. 39, No. 22 y APPLIED OPTICS 3925

Mv~

fla~tF

vv

ss~tbw1

3

sodium content of the sample was 16 ppm as mea-sured by radio activation analysis by neutron irradi-ation.

The samples were cut into plates 30 mm in diam-eter and 10 mm thick. The circular surfaces of eachsample were polished and were in the ~111! plane.

easurements of absorption spectra in the UV–isible region were made with a spectrophotometerVarian, Cary5E!. The absorption coefficient ~a! in-

duced by ArF excimer-laser irradiation at room tem-perature was as follows:

Fig. 1. Experimental setup for in situ transmission measure-ments during ArF excimer-laser irradiation.

tdDstmcAtdo;

togtre

where the theoretical transmission is the transmis-sion with no internal loss but only with Fresnel re-flection on the sample surfaces. For example, thetheoretical transmission at a wavelength of 193 nm is92.29%. Before ArF excimer-laser irradiation, theabsorption coefficients of all samples were less than0.001 cm21.

A schematic diagram of the experimental setupor in situ transmission measurements at a wave-ength of 193 nm during ArF excimer-laser irradi-tion is shown in Fig. 1. The ArF excimer laserLambda-Physik, LPX140! was operated at a repe-ition rate of 100 Hz with a pulse duration of ;10 nsWHM, or 17 ns in integral square-pulse width.12

The dimensions of the laser beam were approxi-mately 8 mm 3 25 mm at the full width of 1ye2

maximum intensity. Then the diameter of the la-ser beam was reduced to 5 mm by an aperturelocated at the center of beam and in front of beamsplitter No. 1 ~Fig. 1!. Thus the beam at the sam-ple position was 5 mm in diameter and the intensityprofile of the laser beam had a nearly top-hat dis-tribution. The differences in intensity of thebeams at 80% energy were not more than 10% ~rmsalue!. The intensities of the two laser pulses di-ided by two beam splitters ~1-mm-thick fused-

926 APPLIED OPTICS y Vol. 39, No. 22 y 1 August 2000

ilica plates! located in front of and behind theample were detected by two silicon photodiodesHamamatu Photonics, S2281-01!. Degradation inransmission of the beam splitters was neglectedecause the transmission change with no samplesas less than 0.1% after irradiation with more than06 pulses. Output voltages from those photo-

Fig. 2. Dependence of the induced absorption coefficient after ArFexcimer-laser irradiation on the concentration of sodium in pureCaF2 crystals. The induced absorption coefficient was completelysaturated in all samples under the irradiation conditions shown.

diodes were proportional to the total pulse energiesand were amplified by amplifiers ~Hamamatu Pho-onics, C2719!, converted into digital signals by aigitizing oscilloscope ~Yokogawa Electric,L3110B!, and then sent to a computer ~Seiko Ep-

on, PC486SR!. Transmission of a sample was ob-ained as the ratio of these two voltages. Weaintained the two voltages at adequate levels by

hanging the number of diffusers. The path of therF excimer laser lay inside an aluminum chamber

hat had been purged with nitrogen gas to preventegradation of the laser intensity by absorption ofxygen in the air. The volume of the chamber was0.05 m3. The concentration of oxygen gas in the

chamber was monitored by an oxygen-gas analyzer~Toray Engineering, LC-750L!, and was always lesshan 20 ppm by volume at a nitrogen-gas flow ratef 0.03 m3ymin. This concentration of oxygen wasood enough to allow us to measure the in situransmission. The average power during ArF ir-adiation was monitored by with a powermeter ~Sci-ntech, AC25UV!.

3. Results and Discussion

Figure 2 shows the dependence of the induced ab-sorption after ArF excimer-laser irradiation at the

a 5 2ln~transmission after irradiationytheoretical transmission!

sample thickness, (1)

A

TS~SbyM

193-nm wavelength on the sodium concentration inpure CaF2 crystals when the average fluence of the

rF excimer laser was 100 ~mJycm2!ypulse, the rep-etition rate was 100 Hz, and the total number ofirradiation pulses was 1 3 104. Under these irradi-ation conditions the induced absorption coefficientwas completely saturated in all samples. Obviously,the induced absorption by ArF excimer-laser irradi-ation depended on the sodium impurity concentra-tion. The strontium impurity concentration had norelation to the induced absorption.

Figure 3 shows a comparison of ArF-induced ab-sorption spectra in the sodium-doped CaF2 crystal~Sample A! and in a typical pure-CaF2 crystal ~Sam-ple B; the concentration of sodium is 0.86 ppm!.

he peak wavelengths of ArF-induced absorption inample B were in good agreement with those

three-band structure: 600, 384, and 332 nm! inample A and were not consistent with those ~four-and structure: 580, 400, 335, and 225 nm! ofttrium-associated F centers ~YFC’s! reported byizuguchi et al.6–8 The absorption bands induced

by ArF irradiation in the sodium-doped CaF2agreed with those ~three-band structure: 600, 385,and 325 nm! of the F2A or ~F2

1!A color centers in-duced by x-ray irradiation.9–11 This indicates thatthe structure of the sodium-related color center in-duced by ArF irradiation in our study is almost thesame as that induced by x-ray irradiation. There-fore the formation of color centers in pure CaF2crystals was found to depend strongly on the so-dium impurity concentration.

Figures 4~a! and 4~b! show the behavior of in situabsorption induced by ArF irradiation at various flu-ences in the Sample A ~16 ppm of sodium! and Sam-ple B ~0.86 ppm of sodium!, respectively. At allfluences, the ArF-induced absorption showed a mono-tonic increase from zero to plateau ~saturation ab-sorption!. We confirmed that the level of thesaturation absorption did not change when the sam-ples were irradiated over 5 3 107 pulses at the samefluence. The solid curves in Fig. 4 were the best-fitting curves of a single exponential function defined

Fig. 3. Comparison of ArF-induced absorption spectra in thesodium-doped CaF2 crystal ~Sample A! and in a typical pure-CaF2

crystal ~Sample B!: p, pulse. We show the results at differentfluences to make clear the different fluence values needed for theabsorption coefficients to appear roughly the same.

by the following equation with measured data:

a~193 nm! 5 asatF1 2 expS2NTcDG , (2)

where asat is the absorption coefficient when the in-duced absorption reaches saturation, N is the num-ber of ArF laser pulses, and TC is a time constant thatis the number of pulses when the induced absorptionequals 0.632 3 asat and relates to the pulses withwhich the induced absorption coefficient reaches thesaturation point. This parameter is significant inhelping us to deduce the mechanism for formationand reverse reaction of color centers. The changesin induced absorption at all fluences clearly followeda single exponential function of the laser pulses.This result indicates that the mechanism of sodium-related color-center formation is first-order kinetics.

If color-center formation from precursors follows asimple model of first-order reaction kinetics with re-verse reaction, the changes in concentration of theprecursors and the color centers will be as follows:

d@P#

dN5 2k2@P# 1 k1@F#, (3)

d@F#

dN5 1k2@P# 2 k1@F#, (4)

@P# 1 @F# 5 @P0# 5 const., (5)

Fig. 4. Behavior of in situ absorption induced by ArF irradiationat various fluences in Samples A and B: p, pulse. At all thefluences the ArF-induced absorption showed a monotonic increasefrom zero to plateau ~saturation absorption!. Solid curves, bestfitting curves of a single exponential function defined as Eq. ~1!with measured data.


p

e

r~a

Esto

ectw~

evwt

d

t

m

3

where @F# is the concentration of the color centers thatcorrespond to ~F2

1!A centers, @P# is the concentration ofthe precursors, @P0# indicates the initial concentrationof the precursors, N is the number of ArF laser pulses,k2 is a rate constant of the formation process from therecursor to the color center, and k1 is a rate constant

of the reverse reaction. If the reaction is assumed tooccur only between @P# and @F#, the sum of @P# and @F#is constant all the time, as described in Eq. ~5!, and thesum is equal to the initial concentration of the precur-sors. From Eqs. ~3!–~5! the concentration change ofthe color centers will be as follows:

d@F#

dN5 k2@P0# 2 ~k2 1 k1!@F#. (6)

Integration of Eq. ~6! leads to a single exponentialfunction of the following form:

@F# 5k2@P0#

k2 1 k1$1 2 exp@2~k2 1 k1!N#%. (7)

Inasmuch as the induced absorption coefficient ~a! atthe wavelength of 193 nm is assumed to be propor-tional to @F#, we can describe a as follows:

a 5k2a0

k2 1 k1$1 2 exp@2~k2 1 k1!N#%, (8)

where a0 is the absorption coefficient when @F# isqual to @P0#. Here we discuss the induced absorp-

tion coefficient in the region just after irradiationwith the ArF excimer laser starts ~astart!. In thisegion, because ~k1 1 k2!N ,, 1, we can develop Eq.4! and neglect second- and higher-order items. As

result, we get following the approximation:

astart 5 k2a0 N. (9)

quation ~9! clearly indicates that the induced ab-orption coefficient in this region just after irradia-ion starts increases linearly relative to the numberf pulses and is dependent only on k2 and indepen-

dent of k1. The value of k2 is associated with theprocess of color-center formation. Therefore the de-pendence on laser fluence of the slope of the inducedabsorption coefficient just after irradiation startsgives us information on the photon process of color-center formation.

Figure 5~a! shows the dependence on laser flu-nce of the slope of the induced absorption coeffi-ient just after irradiation starts. The values ofhe slopes were obtained from Fig. 4. The slopesere proportional to the square of the laser fluence

k2 } I2; I is the laser fluence! for both Samples Aand B. Therefore we conclude that the sodium-related color-center formation process is a two-photon absorption process.

Figure 5~b! shows the dependence on laser flu-nce of the saturation absorption coefficient. Thealues of the saturation absorption coefficientsere obtained from Fig. 4. The saturation absorp-

ion coefficients depended on the laser fluence for


both Samples A and B. From the Eq. ~8! we canescribe the saturation absorption as follows:

asat 5k2

k2 1 k1a0. (10)

Fig. 5. Laser fluence dependence of ~a! the slope of the inducedabsorption coefficient just after irradiation starts ~astart!, ~b! thesaturation absorption coefficient ~asat!, and ~c! the time constant~TC!. The slopes of astart were proportional to the square of thelaser fluence, and the saturation absorption coefficients dependedon the laser fluence for both Samples A and B. The value of TC forhe YFC formation was ;5 3 105 pulses at a laser fluence of 2

~mJycm2!ypulse ~from Fig. 2 of Ref. 6! and larger by an order ofagnitude than that ~5 3 104 pulses! of sodium-related color-

center formation.

3

t~apw~cs

s

ppptfla5c

rm

a

F

c

i

This indicates that the saturation absorption de-pends on both k2 and k1. If there were no reversereaction k1 5 0, then asat 5 a0, so we could predictthat the saturation absorption would be independentof the laser fluence. However, the saturation ab-sorption actually depended on the laser fluence, so wededuce that there is a reverse reaction. Because wealso confirmed that there was no change in the in-duced absorption at room temperature within tens ofhours after irradiation was stopped, we can neglectthermal relaxation in the reverse reaction process.Therefore we suspect that the reverse reaction isdominated by laser-induced relaxation, i.e., that k1depends on the laser fluence ~k1 } IM; M 5 1, 2,, . . . !.Figure 6 shows the change in the absorption spec-

rum during ArF irradiation with a fluence of 0.8mJycm2!ypulse at the same position on Sample After irradiation with a high fluence of 10 ~mJycm2!yulse. As expected, a large amount of absorptionas induced by irradiation with a high fluence of 10

mJycm2!ypulse. However, the absorption de-reased and returned to the level of saturation ab-orption at a low fluence of 0.8 ~mJycm2!ypulse when

the low-fluence irradiation impinged upon the sampleat the same position. That is, the photobleachingphenomenon of induced absorption was observed.The same phenomenon was observed in pure CaF2crystals. The reverse reaction must be a laser-induced relaxation. If k1 is proportional to thequare of the laser fluence ~k1 } I2!, i.e., if the reverse

reaction process is a two-photon process, then, fromEq. ~10!, the saturation absorption should be inde-

endent of the laser fluence. If the reverse reactionrocess is a multiphoton rather than a two-photonrocess, then, from Eq. ~10!, the saturation absorp-ion coefficient will decrease with increasing laseruence. However, the saturation absorption actu-lly increased with increasing laser fluence @Fig.~b!#. So we can neglect two-or-more-photon pro-esses in the reverse reaction and conclude that

Fig. 6. Photobleaching of ArF excimer-laser-induced absorptionin a sodium-doped CaF2 crystal: p, pulse. The large absorptionnduced by irradiation with a high fluence of 10 ~mJycm2!ypulse

decreased and returned to the level of saturation absorption at thelow fluence of 0.8 ~mJycm2!ypulse when the low-fluence irradiationimpinged upon the sample at the same position as the high-fluenceirradiation. The same phenomenon was observed in pure CaF2

crystals.

everse reaction of sodium-related color-center for-ation is a one-photon process ~k1 } I!. We also

confirmed that the induced absorption reached thesaturation absorption coefficients at similar fluenceswhen the sample was irradiated with an ArF excimerlaser at alternately high and low fluences.

Therefore we conclude that the saturation absorp-tion level of a sodium-related color center induced byArF excimer-laser irradiation is determined by theequilibrium between the processes of two-photon ex-citation and one-photon reverse reaction.

Figure 5~c! shows the laser fluence dependence ofTC. The values of TC were obtained from Fig. 4. Atthe same laser fluence, the TC value of Sample B wasin agreement with that of Sample A. This fact indi-cates that the mechanism of color-center formation inpure CaF2 crystals is intrinsically the same as that inthe sodium-doped CaF2. The TC value of the YFCformation was ;5 3 105 pulses at a laser fluence of 2~mJycm2!ypulse ~from Fig. 2 of Ref. 6!. These datare plotted in Fig. 5. From Fig. 5, the TC value of the

sodium-related color-center formation was ;5 3 104

pulses at the same laser fluence of 2 ~mJycm2!ypulseas that of the YFC formation. The TC value of theYFC formation was an order of magnitude largerthan that of the sodium-related color-center forma-tion. Mizuguchi et al. conlcuded that the YFC for-mation process is a one-photon process with noreverse reaction. Therefore the mechanism of for-mation of a sodium-related color center is intrinsi-cally different from that of the YFC.

The time constants in CaF2 crystals under ArFexcimer-laser irradiation with a fluence of the orderof a millijoule per square centimeter at room temper-ature were found to be of the order of 104–105 pulses.The time constants are estimated to be 1024 to 1023

s, i.e., of the order of a millisecond, because the pulseduration of our excimer laser was 10 ns FWHM.Thus 1yk2 is inferred to be of the same order, and thesodium-related color-center formation process atroom temperature is expected to be on a millisecondtime scale.

In alkali halide crystals, the time constants of theF-center formation process were reported to be on apicosecond scale.13 F- or FA-center formation inalkali-earth halide crystals is expected to occur on thesame time scale. But for CaF2 crystals it is wellknown that F and FA centers do not appear at roomtemperature. It has been reported that FA centersdisappear as a result of the thermal conversion pro-cess from FA to ~F2

1!A in sodium-doped CaF2 crys-tals,9 and that only ~F2

1!A centers exist at roomtemperature.

For describing the sodium-related color-centerformation process, a three-level model @precursor3

A 3 ~F21!A# is strictly preferable; however, we

applied a two-level model @the precursor and the~F2

1!A center#. As suggested in Ref. 9, the thermalonversion process from FA to ~F2

1!A requires dif-fusion and the addition of an anion vacancy at theFA site. The rate constant of the process from FA to~F2

1!A is expected to decrease with decreasing FA or


stf

3. J. H. Schulman and W. D. Compton, Color Centers in Solids

3

sodium concentration; thus the slope of the satura-tion absorption coefficient against laser fluence isalso expected to increase with decreasing sodiumconcentration. In fact, as shown in Fig. 5~b!, thelope increased with decreasing sodium concentra-ion. Thus we suggest that thermal conversionrom FA to ~F2

1!A is involved in ~F21!A formation by

means of laser interaction. The process of thermalconversion is also expected to be much slower thantwo-photon excitation from the precursor to the FAcenter. Therefore we infer that thermal conver-sion is a rate-determining step in the sodium-related color-center formation process. In fact, thebehavior of the induced absorption by an ArF exci-mer laser has been simply described well by thetwo-level model. More experiments at low temper-ature will be needed to confirm details of the pro-cess.

4. Summary

We have found that the sodium impurity concentrationhas a strong influence on ArF excimer-laser-inducedabsorption in pure CaF2 crystals. Sodium-relatedcolor centers were generated by two-photon absorp-tion, and saturation absorption was determined by theequilibrium between the processes of two-photon exci-tation and one-photon reverse reaction.

References1. D. R. Cope, “UV lasers require tough transmissive materials,”

Laser Focus World 28~1!, 107–111 ~1992!.2. T. M. Bloomstein, M. Rothschild, R. R. Kunz, D. E. Hardy, R. B.

Goodman, and S. T. Palmacci, “Critical issues in 157 nm li-thography,” J. Vac. Sci. Technol. B 16, 3154–3157 ~1998!.


~Pergamon, Oxford, 1962!, Chap. X.4. D. L. Staebler and S. E. Schnatterly, “Optical studies of a

photochromic color center in rare-earth-doped CaF2,” Phys.Rev. B 3, 516–526 ~1971!.

5. J. R. O’Connor and J. H. Chen, “Color centers in alkaline earthfluorides,” Phys. Rev. 130, 1790–1901 ~1963!.

6. M. Mizuguchi, H. Hosono, H. Kawazoe, and T. Ogawa, “Gen-eration of optical absorption bands in CaF2 single crystals byArF excimer laser irradiation: effect of yttrium impurity,” J.Vac. Sci. Technol. A 16, 3052–3057 ~1998!.

7. M. Mizuguchi, H. Hosono, H. Kawazoe, and T. Ogawa, “Colorcenter formation and time-resolved photoluminescence by ArFexcimer laser irradiation in CaF2 single crystals,” in InorganicOptical Materials, J. A. Marker, ed., Proc. SPIE 3424, 60–70~1998!.

8. M. Mizuguchi, H. Hosono, H. Kawazoe, and T. Ogawa, “Time-resolved photoluminescence for diagnosis of resistance to ArFexcimer laser damage to CaF2 single crystals,” J. Opt. Soc. Am.B 16, 1153–1159 ~1999!.

9. J. M. G. Tijero and F. Jaque, “Thermal and optical propertiesof the FA and ~F2

1!A centers in Na-doped CaF2 crystals,” Phys.Rev. B 41, 3832–3836 ~1990!.

10. J. M. G. Tijero and J. Casas-Gonzalez, “On the coloration ofCaF2:Na,” J. Phys. Lett. 46, L861–L868 ~1985!.

11. J. L. Doualan, J. Margerie, F. Martin-Brunetiere, and E.Rzepka, “Magnetic circular dichroism of color centers inCaF2:Na,” J. Phys. Lett. 44, L375–L385 ~1985!.

12. V. Liberman, M. Rothschild, J. H. C. Sedlacek, R. S. Uttaro,and A. Grenville, “Excimer-laser-induced densification of fusedsilica: laser-fluence and material-grade effects on the scalinglaw,” J. Non-Cryst. Solids 244, 159–171 ~1999!.

13. E. D. Thoma, H. M. Yochum, P. A. Sheldon, and R. T. Williams,“Subpicosecond absorption spectroscopy of band-gap excitationand defect formation in alkali halide crystals,” Nucl. Instrum.Methods Phys. Res. B 141, 552–554 ~1998!.

Documents

Influence of Sodium Impurities on ArF Excimer-Laser-Induced Absorption in CaF_2 Crystals