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Chapter 16

Single-Layer Resist for ArF Excimer Laser Exposure Containing Aromatic Compounds

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Tohru Ushirogouchi, Takuya Naito, Koji Asakawa, Naomi Shida, Makoto Nakase, and Tsukasa Tada Materials and Devices Research Laboratories, Toshiba Research and Development Center, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan

Abstract: Aromatic compounds have been considered as indispensable materials for resist, since the aromatic backbone has high thermal stability, high etching resistance to plasmas and high photo-efficiency. The aromatic phenolic moiety also has good solubility characteristics in alkaline developers. However, few papers have reported the application of aromatic compounds as resists for ArF excimer laser exposure, since the conventional aromatic compounds have strong absorption at 193 nm. Using MO calculation, the authors tried to find an effective modification method for obtaining aromatic compounds transparent to the ArF excimer laser. The calculated absorption maximum of series of aromatic compounds were found to be significantly red shifted upon conjugation of the aromatic ring, such as in polycyclic aromatic compounds. This expectation was confirmed with spectral experiments. We tried to prepare a novel resist for the ArF laser, consisting of aromatic compounds, and acceptably fine pattern with 0.17 µm size was obtained with up to 30 wt % of aromatic compounds. The aromatic phenolic moiety of the polymer in the resist was also found to effect the efficiency of photo-acid generation in the polymer film.

Continuing reduction of pattern geometries in order to increase the packing density of semiconductor devices has Ted to shorter wavelength light exposure systems, since the resolving power of an optical image is proportional to the wavelength of the light source. Since the exposure environment has strong absorption due to absorption by oxygen at wavelengths up to 180 nm light, the ArF excimer laser (193 nm wavelength) exposure system is regarded as the limit of photo-exposure systems suitable for mass production of semiconductor devices. However, in this wavelength range, conventional components used in resist materials, such as photosensitive components and polymers, usually have extremely low transparency. Thus, the development of ArF excimer laser resist materials is a key technology for 193 nm lithography. Some papers have reported novel dry-etch resistant acrylic co-polymers having aliphatic poly-rings in the side chains \ This idea extended the possibility of single-layer resist processing in 193 nm lithography; however, the alkaline solubility achieved with this kind of the polymer was relatively 1-5

0097-6156/95/0614-0239$12.00/0 © 1995 American Chemical Society

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lower than phenol-based polymer, therefore, organic solvents have been introduced in the developer. Few papers, however, have reported the application of aromatic compounds as resists for ArF excimer laser exposure. This is because of extremely strong absorption (about 30 /μπι) near 193 nm, which is due to the π - π * electronic transition of the aromatic ring. The authors believe that aromatic compounds are indispensable materials for resists for achieving high solubility in alkaline developers, high thermal stability, high etching resistance to plasmas, and high photo-efficiency, and by means of molecular orbital (MO) calculations , have tried to find an effective modification method for obtaining aromatic compounds transparent to ArF excimer laser irradiation. This paper reports a study of transparency of aromatic species using MO calculations, and aromatic materials (polymers, photo-acid generators or dissolution inhibitors) suitable for ArF laser exposure, and also demonstrates single-layer resist performance of a novel resist using aromatic components. The authors also discuss advantages of aromatic polymers for 193 nm resist by comparison with non-aromatic resists, then consider a modification method enhancing transparency of the aromatic compound to 193 nm wavelength UV.

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EXPERIMENTAL SECTION EMG-160MSC type ArF excimer laser (Lambda Physik) was used for the contact exposure. Exposure energy of the laser measured with a TPM-310 power meter (GENTECH), was 0.25-0.4 mJ/pulse. A prototype ArF excimer laser exposure system, equipped with a 0.55 numerical aperture (NA) projection lens and 0.7 σ illumination system (Nikon), was used for resist micro-patterning. The absorption spectra of resist materials in the vacuum ultraviolet (VUV) region were measured with a VTMS-502 VUV spectrophotometer system (Acton Research Co.). Polyvinyl (acetal) (70% acetal capped, Sekisui Kagaku Co.) was effectively used as a transparent binder polymer, and 1-mmoi/g sample (in the case of the polymer, the sample corresponding to 1-mmol of monomer unit) was incorporated in to it in the spectral measurement of VUV. Photo acid generators (PAGs) chemical species (triphenyl sulfonium triflate (TPSTF), NAT-105 [137867-61-9], TAZ-106 [69432-40-2], NAI-105[85342-62-7]) were obtained from Midori Co. Organic alkaline developer (eluted AD-10, Tama Kagaku Kogyo Co.: an aqueous solution of tetramethyl ammonium hydroxide ) was used as a developer. An epoxy resin with a naphthalene backbone (EP-1) was synthesized by the reaction of epichlorohydrin with 1,5-dihydroxy naphthalene using NaOH. A co-polymer of naphthyi (methacrylate), methacrylic acid, tert-butyl methacrylate and methyl methacrylate was synthesized according to the previously reported method . A l l the other chemicals were obtained from Aldrich and Poly Sciences Inc.. Photo-acid generation efficiency of PAG in polymers was measured as follows; 5 wt% of PAG was dissolved in a polymer solution, then spin-coated to a thickness of 1 μηι on Si wafer. After exposing the film, 2.5 cm of exposed area is dissolved to a solution of 0.036 mmol/1 tetrabromophenolblue (TBPB) indicator in cyclohexanone. Then, the absorption changes at 627 nm ( which corresponds to the amount of the acid generated) of the indicator solution was measured with a UV-310PC (Shimazu) UV spectral measurement system. Acid-catalyzed conversion of the poly (tert-butyl methacrylate) to poly (acrylic acid) was measured by IR absorption change at 2940 cm" ( corresponds to C - H vibration for tert-butyl groups) and 1715 cm" ( corresponds to C=0 vibration for carboxylic acid ) by employing microscopic FTIR measurement system (Nippon Bunko Co.). 9)

2

1

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RESULTS AND DISCUSSION Study of Absorbance of Aromatic Compounds at 193 nm light using MO calculation Configuration Interaction approaches (CI), which consist of modeling excited states as combinations of multiple substitutions of Hartree-Fock ground states, are qualitatively predictive methods for the excitation energy and its oscillation strength of the molecules. The excitation energies (corresponding to the maximum absorption wavelength of the molecules (X )) are calculated using the Q method in MOPAC? (semi-empirical MO calculation program, 8 orbitals near HOMO-LUMO boundary levels with all configurations of electrons taken into consideration). CNDO/S (semi-empirical spectra calculation program using the CI method with singly excited configurations) was used for excitation energy simulation including oscillation-strength evaluation. ZINDO * was also used for spectral simulation of the molecules. The absorption near 193 nm of benzene derivatives is the third absorption band from longer wavelength side of the UV region and is extremely strong compared with the other two forbidden transition absorptions. This absorption is considered to arise from degenerate, allowed 1st π - π * electron transitions. If the transition energy can be changed by substituent effects in the benzene ring, the transparency of the aromatic compound might be increased. Therefore, the excitation energies for the third absorption band of model aromatic compounds having different types of substituents were calculated using the CI method in MOPAC and CNDO/S. In each calculation method, the geometry applied to the CI calculation had been optimized with MOPAC using the AMI Hamiltonian. The AMI Hamiltonian was effectively used for calculation in MOPAC, since this approximation was found to result the best precision. Table 1 shows the results of calculated X . The tendencies of the shift in X were thought to be similar between two calculation methods, but the results obtained with MOPAC were slightly shorter than those for CNDO/S, in spite of the MOPAC method included the doubly-excited states, which usually interact with ground states and are therefore thought to lower the energy of the ground state. This may due to the difference of empirical parameters used in two calculation methods. MOPAC was suitable for calculation of absolute value of X , since this program was found to result better precision than CNDO/S. The introduction of typical functional groups (electron-withdrawing CI and N0 , or electron donating OMe) on the benzene ring had only a small effect, resulting in a small decrease of the π - π * transition energy (red shift of Xmax), whereas substituent groups having double bonds conjugated with the benzene ring indicated significant decrease of the transition energy implying decrease of absorption near 193 nm. The calculated transition energies were found to decrease with increasing length of conjugation of double bonds. Multi-membered aromatic rings, such as naphthalene, are effective in lowering the transition energy, since they have rigid molecular structures to align the double bonds in a plane. Figure 1 shows simulated spectra of typical aromatic backbones calculated by ZINDO. Benzene was simulated as having a very strong absorption at 193 nm, whereas the spectrum simulated for naphthalene clearly indicated a significant decrease of absorption at 193 nm. Anthracene also has a similar spectral window at 193 nm. From these results, the authors predicted that aromatic compounds transparent to 193 nm light will be based on polycyclic compounds. We named this novel technique of creating a spectral max

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}

7

m a x

m a x

m a x

2

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window at 193 nm with a polycyclic compound, as the " Absorption band shift method". Transparent resist materials with Aromatic Rings for ArF Excimer Laser Exposure In order to confirm the above expectations, VUV absorption spectra of the aromatic compounds have been measured. Figure 2 shows a typical example of the comparison of VUV spectra of polycyclic compounds in a transparent polymer. As shown in Figure 2, naphthalene and anthracene derivatives were more transparent than benzene derivatives. Anthracene derivatives, however, were found to have other small absorption bands near 193 nm, that are thought to be the vibronic allowed transition bands. We therefore tried to look for resist materials in naphthalene derivatives. The compounds with naphthalene backbones tested for resist materials were as follows; PAGs: NAT-105 (onium salt known as i-line PAG ), TAZ-106 (chlorine-substituted triazine for i-line PAG), NAI-105 (naphthalimide triflate, non-ionic PAG). Acid-sensitive materials; 2-Npt-tBoc (tert-butoxy carbonyloxy naphthalene), NCR-1 (2-naphthaldehyde) (acid catalyzed cross-linker with naphthol backbone). Polymers; NV-1 (2-naphthol novolak), PNMA (poly (naphthyl methacrylate) ), EP-1 (epoxy resin synthesized with 1,5-dihydroxy naphthalene and epichlorohydrin.). Figure 3 shows the chemical structures of the above components. Table 2 summarizes the calculated K employing the C.I. method in MOPAC (approximations with a one monomer unit model terminated by hydrogens were used in polymer calculations instead of calculating for the huge polymer molecule) and the experimentally obtained Xmax for the above components. The calculated values with MOPAC were found to be in fairly good agreement with the experimental values. Table 2 also includes experimental values of the absorbance of the aromatic derivatives at 193 nm (this was measured as a transparent polymer matrix containing 1 mmol/g of the aromatic derivatives, which corresponds to very high concentration of aromatic compounds, such as 0.389g/lg for species having Mw=389.). As shown in Table 2, naphthalene compounds with low molecular weight are characterized as materials very transparent to ArF excimer laser exposure. The naphthalene polymer species, however, were found to have relatively high absorption, revealed by the broadening of the absorption band. This broadening was thought to occur with absorbed energy distribution due to the non-uniformity of naphthalene configuration in the polymer (this will be discussed in later section). Example of the typical VUV absorption spectrum of NAT-105 (1.4 wt%) with comparative data are shown in Figure 4. 8)

9)

max

Novel Resists for ArF Excimer Laser Exposure Containing Aromatic Compounds. We have examined the above resist components and prepared some effective compositions. The following materials have been precisely examined as a prototype resists for ArF excimer laser exposure. PAGs: We have already reported the photo-efficiency of the PAGs mentioned above. The photo-efficiency of NAT-105 was estimated by acid titration, and found to have the highest efficiency compared with other aromatic PAGs. We therefore selected NAT-105 as the best PAG for ArF excimer laser exposure. TAZ-106 was also employed for negative resist composition (because of the difficulty of the titration method in the photo-efficiency measurement, no efficiency data for TAZ-106 was available except transparency data ) . Dissolution inhibitor: 2-Npt-Boc was used as a dissolution inhibitor. It works as a sensitizer rather than dissolution inhibitor in chemically amplified resist system . 4)

9)

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ArF Excimer Laser Exposure

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190.0

240.0 Wavelength / nm

290.0

Figure 2. Experimentally obtained UV spectra for typical polycyclic aromatic compounds.

NV4

BM

Figure 3. Structure of resist materials with naphthalene rings.

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Table 1. Calculated Xtmx for model aromatic compounds using CI methods in CNDO/S and MOPAC.

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Compounds

Benzene

Q

Chlorobenzene ®" ' Nitrobenzene Q- < Methoxy ~ H0

benzene \J~on»

Styrene P hteand yielne i^Y**! bu UJ) J Naphthalene iQQl

Length of Conjugation

Calculated Absorption maximum (nm) [CNDO/S]

Calculated Absorption maximum (nm) [MOPAC]

3 3 3 3 4 5 5

225 229 217 224 228 262 259

197 203 206 205 226,215 231,226 225

Table 2. Calculated Xmax using MOPAC and experimentally obtained X x of the naphthalene containing resist materials . ma

Materials

Calculated Λ max (nm)

Measured Λ max (ran)

Measured Abs. at 193 m

TPSP NAT-105 TAZ-106 N AI-105 2Npt-Boc NCR-1 NV-1 PNMA EP-1

205 251 232 224 222 223 226 219 230

197 251 229 232 222 228 210, 240 222 222, 248

9.83 1.64 2.9 5.0 0.52 0.97 3.2 2.5 1.17

*1: Concentration in the film was 1 m mol (monomer unit for the polymer) / Ig solid.

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180

200

220

240

260

280

300

Wavelength / nm

Figure 4. Typical VUV absorption spectra for naphthalene-containing resist materials.

Table 3. Resist composition and sensitivity of ALR-1 and ALR-2. Resist Name

Polymer

ALR-1 (positive)

NATM

40 wt% EP-1 ALR-2 in poly (negative) (vinylacetal)

PAG (wt%)

Inhibitor (wt%)

Sensitivity (mJ/cm 2)

NAT105 (2)

2-Npt-Boc

150

TAZ-106 (5)

(20)

A

210

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Although the solubility of acrylic co-polymers in the alkaline developer tends to decrease with increasing etching resistance, this kind of sensitizer ( material enhancing the dissolution rate in exposed area ) helps to increase the alkaline solubility effectively without loss of etching resistance. Acid sensitive polymers: EP-1 was insoluble in the alkaline developer, therefore, it was developed by organic solvents such as methyl (isobutyl) ketone. Acceptably high (about 2.0) absorbance were achieved with the polymer mixture of EP-1 homo-polymer and transparent polymer up to 30 wt% of EP-1. It was possible to use this polymer as an acid-curable, transparent polymer. The copolymer of naphthyl (methacrylate), methacrylic acid, tert-butyl methacrylate and methyl methacrylate (NTÂM) is used as an acid-sensitive co-polymer for positive tone 193 nm resist . Compositions of novel prototype single layer resists containing aromatic compounds are shown in Table 3. The total amount of naphthalene rings in both polymers was about 30 wt%. Table 3 also contains typical sensitivity data for these resists, and it should be noted that both resists have good sensitivity to ArF excimer laser exposure in spite of the 30 wt % of aromatic composition. Pattern formation with these resists was also carried out using the contact exposure method. Acceptable patterning was obtained in the positive tone resist, ALR-1,but A L R - 2 showed poor patterning performance, due to swelling of the fabricated patterns during development. By optimizing the co-polymerization ratio of the monomer and content of the components of ALR-1 (naphthyl (methacrylate) : methacrylic acid : tert-butyl methacrylate : methyl methacrylate = 5 : 20 : 50 : 25), patterns with 0.175-0.17 μπι feature size were successfully fabricated using an NA 0.55 prototype ArF excimer laser lens. The patterns are shown in Figure 5. We also found here a problem of absorption enhancement in the 193-nm region especially in this prototype polymer ( 10 moi% introduction of naphthyl(methacryiate) to copolymer caused Abs.=0.9y^m ), according to the increase in naphthalene content in the polymer. Therefore, sufficient etching rate could not be obtained ( about 1.73 times of Novolac etching rate for CF plasma) by the polymer with 5 mol % introduction of naphthyl (methacryrate). However, it was striking fact that acceptable fine patterns were obtained even in 30 wt % of total aromatic compounds of the resist composition. This means the aromatic component is no longer the minor part of the ArF excimer laser resist. We think, at least, aromatic PAGs and aromatic inhibitors are materials imperative for 193- nm resist.

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9)

9)10)

4

Properties of the acrylic-based 193-nm single layer resist. In this section, we would like to discuss the acrylic polymer species for 193-nm resist. Acrylic polymers have been commonly employed in published ArF prototype resists, as it is in our prototype resist. In developing the ALR-1 resist, we saw several problems for acrylic-based alkaline-soluble polymers. For example, the dissolution rate of acrylic-based polymers is very fast in conventional concentrations of developer. This may due to the relatively higher acidity of carboxylic acids in comparison with the phenolic moiety. Therefore, eluted developers must be used for acrylic-based resist. Figure 6 shows plots of the dissolution rate of poly (tert-butyl (methacrylate)) (PTBM) vs. deprotection % of tert-butyl group, which has measured with FTIR. Great increase in dissolution rate, corresponding to the increase in amount of deprotection were observed when the polymer was developed with conventional 2.38% tetramethyl (ammonium) hydroxide aqueous solution (TMAH). This step-function-like large dissolution rate change usually reveals the difficulty in controlling the synthesis of a polymer having appropriate dissolution rate or difficulty of micropatterning. In contrast, eluted developer, such as 0.436% TMAH soin.,

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Figure 5. The fabricated patterns of ALR-1 with the prototype ArF projection system.

1000 TMAH 0.486% • TMAH 2.38%

20 40 60 80 Conversion r a t i o of TBMA (%)

100

Figure 6. Relationship Between Dissolution Rate and Conversion Ratio of TBMA.

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seems to indicate better dissolution, similar to that of a phenol-based resist. In general, the eluted developer loses basicity easily, which therefore leads to low stability of developer and large change of pattern size, depending on the density of the patterns. A problem was also found in "adhesion" of the pattern in acrylic-based resist. Figure 7 shows the typical example of pattern collapse after development. This may occur due to peeling of the pattern by shrinkage of the polymer during PEB. Figure 8 is a plot of PTBM thickness vs. deprotection % of the tert-butyl group. This large shrinkage seems to be a big problem for micro-patterning. Another interesting phenomenon was difference in sensitivity of resists, depending on polymer matrix. Naphthalene-containing PAGs usually have sensitivity to i-iine exposure, therefore, if the polymers are transparent to i-iine, the photo-acid generation efficiency of the PAG can be estimated with removal of electron transfer from the polymer matrix to PAGs. Table 4 is the comparison of the sensitivity of the chemically amplified resist with the phenol moiety and the acrylic-based resist (co-polymer of tert-butyl (methacrylate) and methyl (methacrylate)). A sensitivity about a order of the magnitude lower was observed for the system without phenol, in spite of the same amount of PAG. In this case, the sensitivity of the resist is thought to be determined by a combination of the efficiency of photo acid generation, the mobility of the acid in the polymer and the rate of the deprotection reaction. We therefore directly measured the amount of acid generated in these polymers by titration to remove the combination due to mobility of acid and the deprotection reaction. Figure 9 shows the plot of difference in 627-nm absorption of the titration solution vs. absorbed i-line exposure dose. The slope of the curve at origin is directly indicates the efficiency of photo-acid generation. A 3-times-higher photo-efficiency was observed in the phenolic polymer. From these results, the phenolic-polymer is considered to be a very effective media for photo-acid generation in a chemically amplified resist. Disadvantages of acrylic polymers mentioned in this section can occur in all aliphatic 193-nm resists. We reported that the full substitution of naphthol containing polymers led to relatively strong absorption compared with acrylic-based aliphatic polymers. However, the authors believe that the partial substitution of acrylic-polymer by the naphthol moiety is the solution for the problems mentioned above, and also reproduces "phenol chemistry " in the 193-nm resist. How do we enhance the transparency at 193 nm for the naphthalene-containing polymer? We reported that the naphthalene-containing polymers have relatively high absorption revealed by the broadening of the absorption band. As mentioned in 3-2, the broadening of absorption occurs due to absorbed energy distribution. This is the "polymer effect", which has been commonly observed in polymer spectroscopy. Although the broadening of the absorption band in polymer species seems a natural phenomenon, our study has been extended to control this phenomenon with molecular structures. In this case, we also started from the simulations of polymer spectra. In order to evaluate the influence on X ax of the distance between naphthalene rings, a 2-naphthalenes-molecule system was also calculated. We used CNDO/S-CI for this calculation, since this method can treat huge molecular systems. Hie calculations were tested under conditions in which the two naphthalenes approach perpendicular to each other or parallel to each other. In both cases, the energy of the 2-naphthalenes system started to increace from a distance of 7Â . Figure 10 shows the calculated values of X according to the distance between the 2-naphthalenes molecules. A blue shift of about 5 nm was calculated at distance of 5Â , and 4Â distance resulted 10-nm shift, that strongly influenced the absorption minimum near m

max

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Figure 7. Typical example of the pattern collapse after development.

0

0.2

0.4

0.6

0.8

Film thickness Figure 8. Relationship betweenfilmthickness and de-protection % in TBMA. FTIR spectroscopy was used for de-protection % determination.

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A

Exposed energy (mJ/cm 2)

Figure 9. Difference between photo-acid generation in the polymers.

12

Γ-

Perpendicular approach 10

Para I l e i approach

8 Λ max Shift

6

(nm)

4 2 0 8

9

10

11

Distance (À) Figure 10. Influence on maximum absorption wavelength of the distance between two naphthalene molecules.

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0

CH,

NAT-105 (5wt%)

NAT-105 (5wt%)

PAG A

350

48

a phenol-based system and an acrylic-based system.

Table 4. Comparison of the i-line sensitivity of chemically amplified resist between

H

0

CH,

0

f

Ç-o/,

J+CHj-Ç

ç=o/À

j+CHa-Ç

ç=o/„\

CH1-C-CH3

V

4CH,-Ç

OH

Acrylic- based

OCHflOC(CH,),

{cHa-CH^CHa-CH)^

Phenol-based

Polymer

Sensitivity to i-line exposure (mJ/cm 2)

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193 nm. We think 6-7 Â is sufficiently far to result in negligible interaction of electrons between the two naphthalenes. Therefore, the three dimensional structure of the polymer appears to be an important factor in reducing 193 nm absorption of polymer materials. The relatively low absorption in the EP-1 polymer may arise from this structural effect in the polymer species, since the two long epoxy groups perform the role of separator of the naphthalene residues. Figure 11 is a comparison of the VUV spectra for EP-1 and NV-1. A difference of about 10 nm in maxima seems to be fatal for NV-1 in creating spectral window at 193 nm. Our study is continuing now and is at the phase of synthesizing a highly transparent naphthol polymer using the separation effect noted before. CONCLUSIONS A novel single layer resist containing aromatic compounds have been developed employing the "absorption band shift method", which creates spectral windows at 193 nm by extended conjugation with polycyclic aromatic rings. This was expected from the molecular orbital calculations. By developing ArF resist materials with containing naphthalene, the fine patterns with 0.17 μπι L/S could be obtained in spite of 30% content of naphthalene compounds. Polymer materials with naphthalene rings lead to relatively lower transparency, but molecular orbital calculations showed a method for obtaining transparent naphthalene polymers for the ArF excimer laser.

100

Ο c £

CO QL. Φ

c

CO ο

hr

ο

Ο) r

ο

τM

O

ο

io

CO (M

CM

o

hCM

O CM

°

8

τCO

CO CO

ο ιο CO

Wavelength (nm) Figure 11. Typical examples of VUV spectra for naphthalene-containing polymers. Solid line : EP-1, Doted line : NV-1.

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Acknowledgment The authors thank Nikon Co. for exposing resist on their prototype ArF excimer laser exposure equipment.

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Literature Cited 1) M . Takahashi et al., J. Photopolym. Sci. Technol. 1994,7(1),31. 2) Robert D. Allen et al., J. Photopolym. Sci. Technol. 1994,7(3),507. 3) Kaichiro Nakano et al., SPIE, 1994,2195,194. 4) T. Ushirogouch, N. Kihara, S. Saito, T. Naito, K. Asakawa, T. Tada and M . Nakase, SPIE, 1994,2195,205. 5) J. J. P. Stewart et al., Research Lab. U.S.Air Force Academy, MOPAC (QCPE#455): Colorado, 1988. 6) J. Del Bene and H. H. Jaffe, J. Chem. Phys. ,1968,48,1807. 7) Anderson W. P. ,Edwards W. D. and Zenner M. C. ,Inorg. Chem., 1986, 2728. 8) N. Hayashi et al. , Jpn. J. Appl. Phys. ,1990,29,2632. 9) N. Naito, K. Asakawa, N. Shida, T. Ushirogouchi and M. Nakase, Jap. J. Appl. Physics. , 1994,33,7028. 10) M.Nakase, T. Naito, K. Asakawa, A. Hongu, N. Shida and T. Ushirogouchi.,SPIE, 1995, in press. RECEIVED August 14,

1995

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