Deprotection Kinetics of Alicyclic Polymer Resist Systems Designed

Chapter 14

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Deprotection Kinetics of Alicyclic Polymer Resist Systems Designed for ArF (193 nm) Lithography 1,3

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Uzodinma Okoroanyanwu , Jeffrey D. Byers , Ti Cao , Stephen E. Webber , and C. Grant Willson 1

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Department of Chemistry, University of Texas, Austin, TX 78712 SEMATECH, 2706 Montopolis Drive, Austin, TX 78741 2

The deprotection kinetics of alicyclic polymer resist systems designed for 193 nm lithography was examined using IR and fluorescence spectroscopic techniques. A kinetic model was developed that simulates the deprotection of the resists fairly well. A new, simple, and reliable method for monitoring photoinduced acid generation in polymer films and in solutions of the kind used in 193 nm and deepUV lithography was developed. This technique could find application in the study of diffusional processes in thin polymer films.

The need to understand and monitor the photoacid generation process i n chemically amplified resists cannot be over-stated. First, with the minimum feature size expected to reach the 0.1 p m mark by 2007 (7), preserving the integrity of the latent image has now become a major concern i n microlithography. Diffusion of the photoacid during the time between exposure and development can cause significant contrast loss and ultimately lead to loss o f the latent image, especially i n chemically amplified photoresists that must require a post-exposure baking step, which facilitates the diffusion o f the acid due to the high temperature normally used. Thus, there is no question that the progress of microlithography within the deep-UV and vacuum-UV regimes w i l l depend significantly on how well the acid generation and diffusion processes i n photoresists are understood and controlled. Infra-red spectroscopy and fluorescence spectroscopy provide convenient methods for studying these processes. Using F T I R , we investigated the deprotection kinetics o f some of our alicyclic polymer resist systems (2,3) (containing triphenylsulfonium hexafluoroantimonate) that were exposed to 248 and 193 nm 'Current address: Advanced Micro Devices, One A M D Place, P.O. Box 3453, MS 78, Sunnyvale, C A 94088-3453.

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© 1998 American Chemical Society

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radiations. W i t h fluorescence spectroscopy we monitored and quantified the photogeneration of acid in some of the alicyclic polymer resist films. The photoresist polymers studied included (1) poly(methylpropyl bicyclo[2.2.1]hept-5-ene-2carboxylate-co-bicyclo[2.2.1]hept-5-ene-2-carboxylic acid) (trivial name: poly(carbof-butoxynorbornene-co-norbornene carboxylic acid) [ p o l y ( C B N - c o - N B C A ) ] and (2) poly(methylpropyl bicyclo[2.2.1 ]hept-5-ene-2-carboxylate-co-maleic anhydride) (trivial name: poly(carbo-t-butoxynorbornene-co-maleic anhydride) [poly(CBN-a/fM A H ) ] (See C h a r t 1 below).


(m = n)

Poly(CBN- 430 and X > 4 7 0 - » 4 6 0 nm and an increase in the intensity of these peaks relative to the spectrum of the sample with no T F A . Since similar fluorescence spectral changes have been observed in other monoazines upon protonation i n aqueous solution (6,7), we conclude that the changes observed are due to protonation of A C R A M by T F A as shown in the Scheme 3 2

Scheme 3

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Figure 8 shows fluorescence intensity (normalized to the pure A C R A M spectrum) of T F A / T H F solution. The two straight lines on the graph are linear fits to

Wavelength (nm) Figure 7. Fluorescence spectra of 27.1 n M A C R A M in T H F with and without T F A / T H F solution. (1) + 0 u L of T F A / T H F solution (2) + 6 p L of T F A solution. (3) + 10 p L of T F A / T H F solution. (4) + 40 p L of T F A / T H F solution The excitation wavelength was A« = 406 nm. x

0

I 0

•

i

•

10

i 20

.

i 30

•

i 40

50

Figure 8. Fluorescence intensity (normalized to the pure A C R A M spectrum around X : 440 -> 430) of T F A / T H F solution. The excitation wavelength was A, = 406 nm. {

ex

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the initial and final parts of the intensity profile. The intersection of these two straight lines occurs at the point of maximum protonation limit ( M P L ) , where the protonation site of the acid sensor (probe) is fully protonated. A t M P L , the number of moles of acid is exactly equal to the number of moles of A C R A M in solution. Beyond the M P L , any further addition of the titrant does not significantly change the observed spectral properties of the probe. Multiplying the volume of the added acid corresponding to the M P L (14.2 p L ) by the concentration of the stock solution of T F A ( 1 . 8 9 x l 0 M ) afforded the number of moles of T F A (needed to fully protonate A C R A M . This procedure was employed in the determination of the M P P s for all the other titration cases investigated in this study. Micro- and Nanopatterning Polymers Downloaded from pubs.acs.org by FUDAN UNIV on 03/09/17. For personal use only.

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Figure spectrum) of p o l y C B N has (Figure 9 and

9 shows fluorescence intensity (normalized to the pure A C R A M T F A / T H F and T F A / p o l y N B C A / T H F solutions. The addition of an insignificant effect on observed fluorescence spectra of A C R A M Table I).

" TFA •

TFA+polyNBCA

Volume of titrant added

Figure 9. Fluorescence intensity (normalized to the pure A C R A M spectrum) of T F A / T H F and T F A / p o l y N B C A / T H F solutions around X\: 440 -> 430). The excitation wavelength was X = 406 nm. tx

From Figures 8 and 9, it can be seen that beyond the M P L , any further addition of the titrant does not significantly change the observed spectral properties of the probe. This implies that most of the A C R A M is fully protonated beyond this point, which occurs essentially at a 1:1 molar ratio between the two reactants. This observation is consistent with the fact that A C R A M has only one protonation site, namely at the nitrogen i n the ring. The amide carbonyl oxygen or even the amide nitrogen does not get protonated under the conditions we studied. Even with the addition of p o l y N B C A , the molar ratio between the titrant and the titrand remained essentially the same, indicating that the titrant (TFA) is by far a stronger acid than p o l y N B C A (Figure 9).

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Table I. Summary o f the Results of the Titration of T F A / T H F and T F A / P o l y N B C A / T H F Solutions against A C R A M

Moles A C R A M (nM) Moles T F A (nM) atMPE atMPE


Titrant TFA TFA/polyNBCA (1 : 9 M Z M )

27.1 28.3

26.8 26.6

TFA/ACRAM

0.99 0.94

Therefore, in principle, it is possible to monitor the photogeneration of acid in photoresist films by fluorimetric titration of solutions of the exposed resist with A C R A M . T o check whether this inference is correct, we irradiated solutions of T P S H F A at 193 nm with dose ranges of 500 to 2000 mJ. Figure 10 shows typical fluorescence spectra obtained for a solution of 27.1 n M A C R A M in T H F . Again, there is a clear hypsochromic shift in the peaks around X\: 440 -> 430 and 4 7 0 - » 4 6 0 nm and an increase in the intensity of these peaks relative to A C R A M samples without added solutions of the irradiated T P S H F A . This increase in fluorescence intensity is consistent with photogeneration of acid and subsequent protonation of A C R A M .

4

450

500

550

600

Wavelength (nm)

Figure 10. Fluorescence spectra of 27.1 n M A C R A M in T H F with and without T P S H F A solution (2.82 x l O " M ) in T H F . (1) + 0 p L of T P S H F A solution. (2) + 75 p L of T P S H F A / T H F solution. (3) + 125 p L of T P S H F A / T H F solution (4) + 275 p L of T P S H F A / T H F solution. 2


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Figure 11 shows the amount of photoacid generated from solutions of T P S H F A i n T H F at different irradiation doses. It is noteworthy to mention that the high doses shown in this Figure that were employed i n this experiment is a reflection of the difficulty of finding suitable solvents that have enough transparency to 193 nm radiation to permit this type of experiment. Our choice of T H F as the solvent for the experiment stems from the fact that of all the organic solvents that are able to dissolve the resist polymers, T H F has the least optical density, ca. 1.5 for a 2 m L solution in a curvette with 1 c m optical path length, which implies that only 3.2 % of the radiation gets to the P A G , with most of the radiation being actually absorbed by T H F . B y irradiating with such high doses, we sought to increase the amount of radiation that the P A G molecules get. A t doses much lower than 500 mJ, no significant photoacid was produced. Experiments in Films. Given that lithography is done on polymer films, not solutions, it was necessary to determine whether the results obtained in solution could be reproduced in polymer films. The effect of photogenerated acid on the fluorescence spectra of A C R A M was investigated by dissolving the 193 nm-exposed resist films in T H F and titrating the resulting solution against the former. Figure 12 shows the change of fluorescence intensity (normalized to the pure A C R A M spectrum) with the addition of photoresist solution formulated from poly(CBN-aZf-MAH) and T P S H F A (4.41x 10" M ) . These spectra show hypsochromic spectral shifts of the peaks around 7

5.0 10-4

6000 193 nm Irradiated dose (mJ)

Figure 11. Photoacid generation from solutions of T P S H F A in T H F as a function of irradiation dose at 193 nm. Concentration of T P S H F A = 2.82 x 10" M 2

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k\: 440 —» 430 and Xi. 470->460 nm, as well as an increase in the intensity of these peaks, which are identical to those observed i n the solution experiments. This indicates the protonation of A C R A M / T H F solution by the photogenerated acid.


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450

500

550

600

Wavelength (nm)

F i g u r e 12. Fluorescence spectra of 27.1 n M A C R A M in T H F with and without photoresist solution formulated from p o l y ( C B N - a / f - M A H ) and T P S H F A (4.41x 10" M ) . (1) + 0 p L of photoresist/THF. (2) + 35 pLphotoresist/THF. (3) + 60 p L of photoresist/THF (4) + 120 p L of photoresist/THF. 7

T a b l e II. Photoacid generation at 193 nm from photoresist formulated with p o l y ( C B N - a / f - M A H ) and different T P S H F A concentrations. The exposure dose was with 50 m J / c m 2

PAG TPSHFA TPSHFA

2

Dose (mJ/cm ) Cone, of T P S H F A ( M ) 50 50

4.41 x l O " 6.61 x l O

7

- 8

Cone, of photoacid(M) 1.04 x l O " 4.13 x l O

7

- 8

T a b l e I I summarizes the amount of photoacid generated at 193 nm from photoresists from different P A G s with 50 mJ/cm . Using the values of Table II, the quantum yield for acid production was determined from the number of acid molecules produced per photon absorbed by the film. In this way, a quantum yield of 1.1 x l O ' was determined for the P A G ( T P S H F A ) in p o l y ( C B N - a l t - M A H ) resist system that was exposed to 193 nm radiation. Such low quantum yield is indicative of the fact that most of the 193 nm radiation that is absorbed by the resist is not utilized in the production of photoacid from the P A G ( T P S H F A ) . The value of kc used i n the modeling was determined as the product of the quantum yield and the resist absorption coefficient. 2

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S u m m a r y . The deprotection kinetics of alicylic polymer resist systems designed for 193 nm lithography was examined. A kinetic model was developed that simulates the deprotection of the resists fairly well. The activation energies of 6.7 and 9.4 Kcal/mol were determined for resists formulated with p o l y ( C B N - c o - N B C A ) made by Pd(II)-

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catalyzed addition and free radical polymerization techniques, and exposed to 248 n m radiation, while an activation energy o f 18.3 Kcal/mol was determined for resist formulated with p o l y ( C B N - a l t - M A H ) that was exposed to 193 n m radiation. A quantum yield o f 1.1 X l O " at 193 n m was determined for the P A G ( T P S H F A ) i n the resist system formulated with p o l y ( C B N - a l t - M A H ) . A new simple and reliable method for monitoring photoinduced acid generation i n polymer films and i n solutions o f the kind used in 193 n m and deep-UV lithography was developed. B y using N-(9-acridinyl)acetamide, a fluorescent acidsensitive sensor, we have been able to study the effects o f trifluoroacetic acid and photoacids generated from triphenylsulfonium hexafluoroantimonate on the spectral properties o f the acid sensor i n T H F solution and i n alicyclic polymer resist films exposed at 193 nm. In both cases a hypsochromic spectral shift and an increase i n fluorescence intensity were observed upon protonation. This technique could find application in the study of diffiisional processes in thin polymer films.


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Acknowledgments: Financial support o f this study by S E M A T E C H Semiconductor Research Corporation is gratefully acknowledged.

and

Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13 14. 15.

16. 17.

SIA The National Technology Roadmap for Semiconductors, Semiconductor Industry Association (SIA), San Jose, CA, 1994, pp. 81-93. Okoroanyanwu, U.; Shimokawa, T.; Medeiros, D.; Willson, C.G.; Byers, J.; Allen, R.D. Proc. SPIE. 1997, 3049, 92. Okoroanyanwu, U. Ph.D. Thesis, University of Texas at Austin, 1997. Lackowicz, J.R. Principles of Fluorescence Spectroscopy. Plenum Press: New York, 1986, Chps. 1 & 2. Pohlers, G.; Virdee, S.; Sciano, J.C.; Sinta, R. Chem. Mater. 1996, 8, 2654. Weller, A.Z. Elektrochem. 1957, 61, 956. Kasama, K.; Kikuchi, K; Yamamoto, S.; Uji-ie, K.; Nishida, Y.; Kokubun, H. J Phys. Chem. 1981, 85, 1291. Crivello, J.V.; Lam, J.H.W. J. Org. Chem. 1978, 43, 3055. Shields, C.J.; Falvey, D.E.; Schuster, G.B.; Burchardt, O.; Nielsen, P.E. J Org. Chem. 1987, 53, 3501. Tomoska, H.; Omata, S.; Anzai, K. Biosci. Biotech. Biochem. 1994, 58, 1420. Dectar, J.L.; Hacker, N.P. J. Chem. Soc., Chem. Commun. 1987, 1591. Knapzyck, J.W.; McEwen, W.E. J. Org. Chem., 1970, 35, 2539. Crivello, J.V. ; Lam, J.H.W. J. Polym. Sci. Polym. Chem. Ed., 1979, 17, 977. Davidson, R.S.; Goodin, J.W. Eur. Polym. J. 1982, 18, 487. McKean, D.R. Schaedeli, U.; MacDonald, S.A. In Polymers in Microlithography Materials and Processes, Reichmanis, E.; Iwayanagi, T., Eds., ACS Symposium Series 412, American Chemical Society, Washington, DC, 1990, pp. 26-38. McKean, D.R. Schaedeli, U.; MacDonald, S.A. J. Polym.Sci.,Part A: Polym. Chem. 1989, 27, 3927. Carey, W.P.; Jorgensen, B.S. Appl. Spectrosc., 1991, 45, 834.

Deprotection Kinetics of Alicyclic Polymer Resist Systems Designed

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