Ultrasensitive chemically amplified resist systems

Microelectronic Engineering 13 (1991) 19-22 Elsevier

ULTRASENSITIVE

CHEMICALLY

19

AMPLIFIED

RESIST

SYSTEMS

II. Sachdev, W. Brunsvold, R. Kwong, W. Montgomery, W. Moreau, K. Welsh, R. Kvitek, W. Conley Microelectronic Materials Development IBM-E. Fishkitl llopewell Jct., N.Y. (USA)

I.

INTRODUCTION

Chemically amplified resists have become quite popular since first reported by Willson and Ito ~ . The use of aromatic compounds to photosensitize onium salts in the 300 to 450 nm region is well documented2,3 Recently, other acid generating compounds which are non-metallic have been developed for chemical amplification schemes. These non-metallic acid generators include onium triflates4 , oxime sulfonates 5 , dicarboximide sulfonates6 , triazine derivatives7 , and 2,6-dinitrobenzyl sulfonates8 . Unfortunately, acids generated from these photoactive compounds are not as strong as ttAsF 6 or ItSbF 6 and have a higher nucleophilicity~ . The result is an apparent loss of sensitivity in positive resists based on deprotection mechanisms and in negative resists which undergo crosslinking. One way to regain the loss in sensitivity is through the use of additives which can boost the efficiency of acid generation. Recently, dicarboximide sulfonates6 and N-acyloxyphthalimides9 have been sensitized to 300 - 450 nm light. The sensit~ers are usually polyaromatic such as substituted anthracene and pyrcne derivatives that have reasonable absorbance in the Mid and Near UV regions, llowever, they have large extinction coefficients at 248 mn so that the concentration of these polyaromatics in resist films must be 1% or less by weight to keep DUV absorbance at a useful level. For onium salts the photosensitization mechanism is believed to involve electron transfer from the excited singlet state of the sensitizer to the onium salt 3 . Electron transfer has also been postulated in the photoinduced N-O bond cleavage of N-acyloxyphthalimides to form carboxy radicals 9. Presumably, sulfonic acids are formed in the same manner from dicarboximide sulfonates like N-tosyloxyphthalimide (p'l'S)10. In this paper, we describe how hydroquinone and related phenols when used in conjunction with acid generators can increase the sensitivity of chemically amplified resists to Deep UV, X-ray, and e-beam exposures.

2.

EXPERIMENTAL

2. I

Materials

Additives !-_7 shown in Figure I were obtained from Aldrich Chemical Co. The polymer used in the sensitizer comparison study was poly(t-butyloxycarbonyloxystyrene) = I'BOCST that was prepared by free radical polymerization of the monomer using a known method tl to give a Mw = 80,000. PTS shown in Figure 2 was prepared by a known method 6. Triphenylsulfonium triflate (TPS) was obtained from Eastman Kodak Co. The Novolak resin used in the proprietary negative resist (EBX) was obtained from Shipley. In addition to a crosslinking agent, EBX contains an acid generator which is also used in XPR resisUL 2.2

Lithographic Studies

Resist solutions consisting of a additives i - 7 (17 mole % relative to PBOCST), PBOCST, and PTS in diglyme were coated on Si wafers to give 0.95 um films after a 90°C, 1 min. bake on a hot plate. Wafers were then exposed through a step wedge mask on a PE 500 exposure tool (UV-2 mode) followed by a post expose bake (PEB) at 90°C for 90 seconds. Film thickness change was measured by a Nanospec film thickness analyzer. Contrast was measured following development in isopropanol (IPA), 20% It20 in IPA, or with 5% anisole in IPA which gives positive tone images. Results are shown in "Fable I. 0167-9317/91/$3.50 © 1991 - Elsevier Science Publishers B.V.

20

H. Sachdev et al. / Chemically amplified resist systems

For imaging studies, our experimental Deep UV resist XPR was evaluated with three different acid generators: TPS, PTS, and a proprietary material, PAG-1, which is a non-ionic triflate. The acid formed during exposure catalytically increases the alkaline solubility of the resin to give positive tone images. X P R resist containing hydroquinone (8 weight% based on polymer) was exposed on a variety of tools including a Perkin Ehner 5(10 (UV-2 mode), an IBM F,I,-3 e-beam system (25KeV), and at the X-ray synchrotron at the Brookhaven National l,ab. In all cases, silicon wafers were coated with a 0.9 um film of X P R and received a PEB before immersion development in aqueous base. For our negative tone x-ray experiments, bis-phenol A was added to I~;BX at about 10% of the Novolak polymer 13. The resist was coated to give 1.2 um films which were baked at 90°C for 1 minute on a hotplate. Wafers were exposed at Brookhaven with doses ranging from 10 to 100 m.l/cm 2 and then received a PEB before development in 0.32 N T M A l l for 150 seconds and rinsing in 1)1 water for 10 seconds.

3.

RESULTS AND D I S C U S S I O N

3.1 Deep UV Sensitivi|y Comparison PBOCST was chosen as the polymer matrix for the sensitivity enhancement study because it does not contain any phenol groups that might contribute to the sensitivity increase. Therefore, any difference in sensitivity can be attributed to the additives. Also, a large film thickness change occurs during removal of the BOC groups and formation of CO 2 and isobutylene. When widely varying film thinning values following development make a sensitivity comparison difficult, film thickness changes (luring PEB can be used as a measurement of relative sensitivity. In a typical experiment, equimolar amounts of the various additives were added to PBOCST conlaining 15% PTS of solids and the resulting films were exposed with broad band Deep UV light (240-270 nm) on a PE 500. Following a PF.fl of 90°C for 90 see., film thickness readings indicated a higher amount of conversion when 1-_5 were present than when no additive was in the fthn. PBOCST fihns were developed in IPA based solvents to give positive tone images. An effort was made to keep film thinning the same. Additive _3 with two isolated phenol groups, that are both readily deprotonated, has higher than normal thinning and 6 which has no phenol groups has the lowest film loss. Dose to clear results in Table 1 show that hydroquinone, _1, 4-hydroxyacetophenone, 2, and Bisphenol A, .3, were most effective in increasing sensitivity while _4 had a smaller effect. 1,4-l)imethoxybenzene, -6, and 2,3,4-trihydroxybenzophenone, _7, did not influence dose to clear. That electron transfer for -6 is thermodynamically favorable bt, t no effect on sensitivity is observed points to availability of a proton as a necessary component for p-TsOlt production. On the other hand, the additive is not operating solely as a proton source. For example, _7 has available protons but does not influence sensitivity because it has a high El/2 o× value which makes electron transfer unfavorable.

3.2 l.ilhography Evaluation In a resist system that is base developable, some of the additives may not be useful because they cause excess fihn thinning. For example, with XPR, the addition of up to 8% by weight of _1 in the polymer resulted in less than a 20% increase in unexposed thinning while an additive with two easily ionizable phenol groups like _4 gave a several thousand angstrom film loss. 1lydroquinone was selected as the photosensitizer for the imaging studies in XPR because it afforded a high sensitivity increase, low thinning, and has high solubility in the casting solvent. Contrast remains constant as shown in Figure 3. A lithography evaluation on a Perkin Elmer 500 (UV-2 mode) shows thai the typical exposure dose of 6 m.I/cm 2 (Figure 4) can be cut in half by adding g% (by weight of polymer) of 1. There is no degradation of image profile. An XPR formulation where the photoacid generator was replaced with TPS (5% by weight of polymer) was coated on a 125 mm wafer, prebaked at 90°C for 1 minute and exposed on a Perkin Elmer 500 at 4 m.I/cm 2 (UV-2 mode) to give 1 um images. With the addition of_l (8% by weight of polymer) to tile film, the imaging dose decreased to about 2 mJ/cm 2 for 1 um features. When tile acid generator in XPR was PTS (9% by weight of polymer), the original imaging dose of 70 m.I/cm 2 was reduced to about 20 m.I/cm 2 after addition of R% hydroquinone.

H. Sachdev et al. / Chemically amplified resist systems

21

The same XPR film containing _1 described above was exposed on an IBM El,-3 e-beam tool at 25 KeV. The imaging dose decreased from 3 uC/cm z for XPR to 2.5 uC/cm 2 for XPR and 8% hydroquinone. X-ray imaging was carried out on the synchrotron at the Bmokhaven National l,ab. A typical imaging dose for XPR resist is about 50 mJ/cm 2 although the 0.5 um L/S features in Figure 5 are slightly overexposed at that dose. With 8% of I in the film, the required dose decreases significantly to 10 mJ/cm 2 to print the same features. This is particularly significant because it makes the point source a more attractive alternative for X-ray lithography. As with the positive tone system, definite cnhancemcnts were observed with the negative tone EBX with the addition of bis-phenol A as sensitizer. A clcar decrcase in dose required to retain a working film thickness has been achieved. The resist with bis-phcnol A required only 20 m,l/cm 2 with no additional thinning (Figure 6) compared to 90 mJ/cm 2 for F,BX with no additive.

4.

CONCLUSION

We have demonstrated that hydroxy substituted benzenes such as hydroquinone and bisphenol A can sensitize acid generators to Deep UV, e-beam, and X-ray exposures. The additives were effective in PBOCST as well as in base soluble resins. The gain in sensitivity was observed for both positive tone and negative tone resist Some of the phenol additives may not be usefid in positive resists because of excessive thinning in unexposed areas. Further studies are necessary to determine the impact that these sensitized systems will have on the development of soft x-ray tools and Dcep UV tools like Micrascan which require highly sensitive resists. ACKNOWLEDGEMENTS The authors would like to thank R. l)undatscheck and .1. tlorvat for e-beam exposures. We are also grateful to Bruce ltill, Bob Devenuto, L. C. Ilsia, and Jerry Silverman for helping with X-ray exposures. X-ray exposures were obtained at the National Synchrotron I,ight Source, Brookhaven National laboratory which is sponsored by the United States Department of Energy, Division of Material Sciences mid Division of Chemical Sciences under contract number DI~,-AC02-76CI|00016. REFERENCES

1. 11. lto, C.G. Willson, and J. Frechet, 1982 Symposinm o11 VSI,I Technology, Oiso, Japan, Sept., 1982. 2..I. Crivello, and J. lain, J. Poly. Sci., Poly. Chem. F,d., 16, 2441 (1978). 3. S. Pappas, .l. Imaging Tech., 11, 146(1985). 4. C. Osuch, K. Brahim, F. llopf, M. McFarland, A. Mooring, and C. Wu, Proc. SPI E, 631, 68(1986). 5. M.Shirai, S. Wakinaka, H. Ishida, M. Tsunooka, and M. Tanaka, J. Poly. Sci., Polymer l~et., 24, 119(1986). 6. (7. Renner, U.S. Patent 4,371,605 (1983) to l)uPont. 7. A. Bruns, II. l,uethje, F. Vollenbroek, and E. Spiertz, Microelectmnics Eng., 6, 467(1987). 8. T. Neenan, F. lloulihan, E. Reichmanis, .I. Kometani, B. Bachmau, and I,. Thompson, Proc. SPIE, 1086, 2(1989). 9. K. Okada, K. Okamoto, and M. Oda, J. Am. Chem. Soc., 110, 8736(19 88). 10. W. Brunsvold, R. Kwong, W. Montgomery, W. Moreau, II. Sachdev, K. Welsh, Proc. SPIE, 1262, 162(1990). 11. J. Frechet, F. Bouchard, E. Eichler, F. tloulihan, I. lazawa, B. l)ryczk, and C.G. Willson, Polymer .|., 19, 31(1987). 12. R. Wood, C. Lyons, R. Mueller, and .I. Conway, Proc. KTI Micmelectronics Seminar, Nov. 1988, San Diego, CA. 13. W. Conley et. al., to be published.

22

H. Sachdev et al. / Chemically amplified res&t systems

OH

OH

OH

OH

OH

OCHa

0

0

OH PhaS +>->0S02CF3

~N-

OTs

O OH

C

OCH3

TPS

PTS

OH k

Fig. 2 Acid Generators

Fig. 1 Additives

TABLE 1

• XPR + 8% Hydroquinone • XPR

DUV Sensitivity Comparison in 15% PTS/PBOCST Films Additive None -1 2 3 4 5 6 7 -

Dose to Clear mJ/cm 2 >120 12 8 12 44 15 > 120 > 120

Thinning 550A 620 910 2200 720 760 220 2300

Developer 5% Anisole/IPA 5% Anisole/IPA 5% Anisole/IPA IPA 5% Anisole/IPA IPA 5% Anisote/IPA 5% Anisole/IPA

E~ ~ ~ -oE N~~ ."~u~ N o -~ z

E~

0.8 0.6[ 0.4 0.2 i

017 4 Dose

1.5 2.53

(mJ/cm 2)

Fig. 3 XPR Contrast Curve

XPR Resist (50mJ/cm2) XPR Resist (6mJ/cm2)

XPR Resist + 8% h.ydroquinone (3mJ/cm")

Fig. 4 PE 500 UV-2 Exposures (1 um I,/S)

EBX Resist (90 mJ/cnl 2 )

XPR Resist + 9% Hydroquinone (10mJ/cm 2)

lqg. 5 X-Ray lixposures (0.5 um I,/S)

F,IIX + 10% BPA, ,3 (2[) mJ/cm 2 )

Fig. 6 X-Ray i~,xposures (EBX Resist)