Materials for Microlithography - American Chemical Society

9 Laser-Induced Polymerization

Downloaded by PURDUE UNIVERSITY on March 13, 2013 | http://pubs.acs.org Publication Date: July 1, 1985 | doi: 10.1021/bk-1984-0266.ch009

C. DECKER Laboratoire de Photochimie Générale associé au CNRS Ecole Nationale Supérieure de Chimie 3 rue A. Werner 68200 Mulhouse, France Ultra-violet radiation is being increasingly used to initiate polymerization reactions, mainly because of the high efficiency and the rapidity characteristic of this type of initiation. The limited penetration of UV light into organic materials has restricted the field of applications of this technique, mostly to surface treatment processes; the highly cross-linked photopolymer films obtained give very resistant protective coatings as well as the high-resolution relief images required in microprinting and microlithography (1,2). Such photoresist materials often consist of multifunctional systems that polymerize under UV light within a fraction of a second leading to a totally insoluble polymer network. In the continuing search for faster cure rates, one would naturally conclude that lasers should be the ultimate light source that would provide a quasi-instantaneous polymerization and allow operation at extremely high scanning speeds. Several investigations on laser-induced polymerization have been reported in the last few years (3-11) but the actual development of this technology has apparently not met the efficiency, reliability and economic requirements essential for industrial applications. Still laser-initiated radical production offers some remarkable advantages over conventional UV initiation that result mainly from the large power output available, the narrow bandwidth of the emission and the spatial coherence of the laser beam which can be finely focused. Rapid curing of thick sections (up to 2 inches) of polymers can be achieved by visible-laser irradiation, as shown recently by Castle (72). Full use of the high power of the laser beam can only be made if the irradiated system obeys the reciprocity law, i.e., if the product of the light intensity and the required exposure time is independent of the intensity. This was shown to be the case for several positive working photoresists that undergo degradation under laser exposure (73). This law was not expected to hold true for conventional photopolymerization reactions where the kinetic chain length and thus the quantum efficiency is known to decrease as the light-intensity is increased. Most of the work reported so far on laser-induced photopolymerization deals with near UV and visible radiation ranging from blue (—400 nm) (3,12) to red light (—700 nm) (4). We present here a kinetic investigation on the 0097-6156/84/0266-0207S06.00/0 © 1984 American Chemical Society

In Materials for Microlithography; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

MATERIALS FOR MICROLITHOGRAPHY

208

photopolymerization of multifunctional acrylate monomers initiated by U V laser irradiation. A pulsed U V laser has been used for large area processing whereas a continuous wave (C.W.) U V laser has been employed to perform spatially localized polymerizations, suitable for direct writing of micrometer-size structures. One of the advantages of light-induced reactions, from a pure scientific point of view, is that the rate of initiation can be easily varied over a large range by varying the light-intensity. From the relationship between the rate of polymerization (R ) and the rate of initiation O7), one can infer some basic information about the mechanism of this process, especially the termination step. By using the powerful U V lasers which are now on the market, the light-intensity range available can be greatly extended, primarily towards the very high values. The close to first-order dependence of R on the light-intensity that we observed in these multifunctional systems, over an 8 order of magnitude range, suggests that the growing polymer chains terminate mostly by a unimolecular process, probably because of the limited mobility of the reactive species in the rigid network. This means, that the reciprocity law is still obeyed so that the extent of the polymerization will not be restricted by the high rates of initiation provided by the laser beam, as would be the case for monofunctional systems. p


p

Background Laser-initiated Radical Production. Although there are different physical mechanisms involved in laser chemistry, we are concerned here with the photodissociation, i.e., the breaking of molecular bonds directly by U V photons. The laser emission is used to produce electronically excited molecules which split into reactive radicals, with the highest possible quantum yield. Since the substrate usually behaves as a poor photoinitiator, an additional molecule must be introduced in order to enhance the radical production, much in the same way as in conventional photoinitiated reactions. In this work, 2,2dimethoxy-2-phenylacetophenone ( D M P A ) was chosen as photoinitiator for two main reasons: (i) its absorption spectrum extends into the near U V region where both the C . W . and pulsed lasers used exhibit their emission lines (337.1 nm and 363.8 nm) and (ii) its quantum yield of radical production is high (14). The laser-initiation reaction can be written formally as: OCH3

©-H-® (A)

0

DMPA

OCH3

0CH3

_

hi/ LASER

I

J

0 OCH-3 Ί

250 2

2900

3

1700

6

5.8 x 10

4 x ΙΟ"

6

2 x 10

8 x ΙΟ"

4

1.2 x 10

5

600

1.2 x ΙΟ"

2

1.6 x 10

6

540

1.1

1 x 10

8

420

12

~~ 10

1.5 x ΙΟ"

C.W.-Ar Ion 363.8 Laser

Pulsed Nitrogen Laser

337.1 9

400

against 7 , in a logarithmic scale (Figure 5). The value of the slope of the straight line obtained, 0.85, reveals a close to first-order process for the polymerization of these multifunctional acrylate systems instead of the expected half-order relationship between R and / . The important consequences of this kinetic law on the cross-linking polymerization mechanism are discussed below. For an accurate evaluation of the efficiency of U V photons in initiating the polymerization of acrylate photoresists, it is necessary to determine the polymerization quantum yield, φ , that corresponds to the number of acrylate functions which have polymerized per photon absorbed. φ can be expressed as the ratio of the rate of polymerization to the absorbed light-intensity and calculated from the following expression, after introducing Equations 1 and 2: 0

p

0

ρ

ρ

=

p

^

e (A -A ) x

l

10 /? (t - t ) A

a

2

x

[M]

2

7

0

0

( 4 )

7 [1 - exp(-2.3€é?c)J 0

The high values of φ reported in Table I indicate that, despite the high rate of initiation provided by the laser irradiation, the propagation chain reaction still develops effectively in these multifunctional systems; each photon induces the polymerization of up to 1700 monomer units in air-saturated systems and up to 10,000 in the absence of oxygen. Such a large amplification factor is necessary in this system when one considers the high cost of laser photons (28). When using the pulsed N laser as excitation source, the polymerization quantum yield value was found to be in the order of 400, instead of less than 1 if the expected half-order rate equation (R ~ IQ) would have prevailed (Figure 6). ρ

2

p



218


Io

(einstein s- cm- ) 1

2

Figure 5. Dependence of the rate of polymerization (R ) on the light-intensity 0$) upon UV-laser irradiation of polyester- (O) and epoxy- (+) multiacrylate photoresists in the presence of air. p

By taking a quantum yield value of 0.1 (18,19) for the production of initiating radical in D M P A photolysis, it was then possible to evaluate from φ values the kinetic chain length (kcl) or the degree of polymerization. Over the whole range of light intensity investigated, kcl values range between 4000 and 32,000 acrylate functions polymerized or cross-links formed per initiating radical; it corresponds to a calculated average molecular weight of the polymer network of over 3 x 10 for the laser curing in air and up to 10 in N -saturated systems. It is important to note that φ values are of the same order of magnitude whether conventional U V or laser radiation were used (Table I). This result is in contrast with the recent work of Sadhir et al. (7) on the photopolymerization of maleic anhydride and styrene, induced by the 363.8 nm emission of an argon ion laser. That work showed to be the laser initiation 1000 times more energy efficient than the UV-induced polymerization. This discrepancy may arise from two factors: (i) the lower light-intensity used in the laser irradiation that should favor chain propagation and (ii) differences in the way of comparing the ρ

6

7

2

ρ



9.

DECKER

219

Laser-Induced Polymerization

1

2

I (einstein s~ cm- ) Figure 6. Dependence of the polymerization quantum yield (φ ) on the light intensity (IQ) in the laser-induced polymerization of epoxy-acrylate photoresists (---: expected variation of φ on I for a half-order kinetic law). 0

ρ

ρ

0

efficiency from the energy output (7) rather than from the energy actually absorbed by the polymer. Moreover, the two systems undergoing polymerization, multiacrylates in one case and vinyl monomers in the other, were quite different, as well as different initiation processes. Discussion Overall the aforementioned kinetic results clearly demonstrate that multiacrylate photoresists can be polymerized efficiently by a short exposure to near-UV laser radiation. Using this technique it becomes possible to achieve extremely fast polymerizations that are not possible with conventional U V light sources. One of the most remarkable features is that the huge concentration of initiating radicals generated by the intense continuous or pulsed laser irradiation does not prevent the chain propagation from developing extensively, as indicated by the surprisingly large values of the polymerization quantum yield. Although unexpected, this favorable result permits consideration of this laser technique for applications where extremely fast curing is required. Further, it has also direct implications which shed new light on the basic mechanism of the cross-linking polymerization of these photoresist materials. In consideration of the kinetic law obtained, R ~~ 7 over a 8 orders of magnitude range, one can conclude that the common polymerization mechanism, based on bimolecular termination reactions, is no longer valid for these multifunctional systems when irradiated in condensed phase. Indeed, for conventional radical-induced polymerizations, the termination step consists of the interaction of a growing polymer radical with another radical from the initiator (/£·), monomer ( M ) or polymer (P) through recombination or disproportionation reactions: 0 , 8 5

p

0


MATERIALS FOR MICROUTHOGRAPHY

220 Ρ

+

Ρ

>

Ρ - Ρ or ΡΗ + P - C H = C H - R '

Ρ

+

M

>

Ρ —M

Ρ

+

R

>

Ρ - R

Assuming steady-state conditions, i.e., equal rates of initiation and of termination (r = φ I = 2 k lP] ), it can be inferred that the polymerization rate must be half-order in light-intensity: 2

t

ί

a

t

R

= ^


p

l" a

[M]

(5)

where lM] is the concentration in monomer functions, k and k the rate constants of propagation and bimolecular termination reactions, respectively. Actually, it was found that increasing the light-intensity was accompanied by a more rapid increase of R than that which would be expected on the basis of Equation 5. In order to account for the close to first-order kinetic law observed, one has to consider the contribution of a unimolecular termination process that would involve only one reactive radical. Various reactions can be postulated: p

t

p

• a radical transfer from the growing polymer chain to an acceptor site that would yield inactive species. • the so-called wall-effect that consists in the deactivation or scavenging of polymer radicals by electron traps located on the support; if possible, such a reaction is likely to play some role in the present case because of the large surface to volume ratios involved: 1000 c m for a 10 μπι thick film, as compared to less than 0.1 c m for conventional photochemical reactors. - 1

- 1

• a degradative reaction between the propagating radicals and the monomer or oligomer molecules; such a reaction was already suggested to explain a similar first-order kinetic law observed in the photopolymerization of vinylimidazole (29); it seems difficult to consider in our acrylic systems. • the scavenging by oxygen of polymer radicals to yield peroxy radicals that would be inactive; the fact that the close to first-order rate equation was also observed in N -saturated systems eliminates this possibility. 2

• a radical occlusion phenomenon due to the formation of a tight network which strongly restricts the molecular mobility and thus prevents the monomer from diffusing towards the radical sites. This last explanation appears to be the most feasible since we did not observe deviations from the expected half-order kinetic law when those multiacrylate monomers were polymerized in dilute solution where no rigid network is formed (30). A further feature which corroborates this conclusion is


9.


DECKER

221


that neat monofunctional acrylic monomers do polymerize according to Equation 5 kinetics. Strong arguments in favor of the radical isolation hypothesis have been provided together by Tryson and Shultz (31) through their calorimetric study of the photopolymerization of multifunctional acrylates in condensed phase and by Julien (32) who detected long living polymer radicals by e.s.r. spectroscopy in U V cured multifunctional acrylate photoresists. Based on the assumption that termination of the growing polymer chains occurs by both mono- and bimolecular pathways, the overall rate of polymerization can then be expressed, at a first approximate, as the sum of two terms which depend on the first power and on the square root of the absorbed light-intensity, respectively: R

P

=

a

ΊΤ- ΐΜ]φ k t

(

I + (1 - a) a

2

2k

[Μ]φ/

/2

//

2

(6)

t

where k' is the rate constant of the unimolecular termination process, i.e., the reciprocal of the polymer radical lifetime, and a a coefficient that reflects the relative contribution of the unimolecular pathway in the termination step, i.e., the probability for a given polymer radical to become occluded. From our kinetic data, a was calculated to be close to 0.8 for both the epoxy and polyester multiacrylate photoresists investigated; it means that, under the given experimental conditions, one fifth of the polymer radicals are likely to terminate through bimolecular reactions, the remaining radicals becoming occluded in the polymer matrix. The value of a is yet assumed to strongly depend on the structure and cross-link density of the network; for instance, it is expected to be substantially lower in polyurethane based photoresists that are known to give more flexible UV-cured materials and thus exhibit a higher segmental mobility of the network chains (33). Moreover, it should be noticed that polymerization rates were determined from the maximum slope of the kinetic curves, namely at degrees of conversion between 20 and 40%. A t that time, the large increase in viscosity of the photoresist may already have reduced the chain mobility, thus favoring radical isolation and first-order termination. It is therefore very likely that the intensity exponent of the photopolymerization rate equation will be less than 0.85 in the early stages and that it increases with conversion to reach almost unity in the solid network. Such a kinetic behavior was indeed observed for the photopolymerization of neat hexanedioldiacrylate (31). Finally, it must be pointed out that the close to first-order kinetic law observed in this study is by no means specific to polymerizations induced by intense laser irradiation; a similar kinetic law was obtained by exposing these multiacrylic photoresists to conventional U V light sources that were operated at much lower light-intensities (27,34). This indicates that the unimolecular termination process does not depend so much on the rate and type of initiation used but rather on the monomer functionality and on the cross-link density which appear as the decisive factors. t



222


Conclusion As a result of this study, it should be apparent that both C.W. and pulsed laser beams are capable of very efficiently initiating the cross-linking polymerization of multifunctional acrylate photoresists, provided adequate initiators are used to absorb the laser photons and generate the reactive species. The main advantages of this technology arise from the specific properties of the laser emission: • Spatial and temporal control of the radical production. • A large power output allowing instantaneous and deep section polymerizations to be carried out. • The spatial coherence that permits to draw high-resolution images by means of sharply focused laser beams. • The narrow bandwidth that can be matched with the maximum absorption of the initiator and thus eliminates undesirable secondary photochemical reactions; non-linear processes may however become important at high power densities. The multifunctional acrylate systems investigated appear to be particularly appropriate for UV-laser initiation because kinetic chain length of the radicals is very long, giving DP's of up to 10 and thus making the economics attractive. Potential applications of laser curing are expected to concentrate mostly in microprinting and microelectronics for the production of polymer relief images by a lithographic process. While pulsed lasers will serve mainly as powerful projection light-sources for the processing of entire wafers, C.W. laser beams are more suited to the direct writing of small size patterns. Other possible uses of high-intensity UV laser beams include the curing of thick sections of polymers, the high-speed surface treatment of optical fibers by UV curable coatings and the direct etching of photodegradable polymers working then as positive photoresists. The economic prospects of this laser technology applied to polymer processing will primarily depend on further progress in the scientific and technological understanding and control of the basic phenomena involved, together with the development of reliable and low cost lasers, such as those used in this work, that can be safely operated in the chemical industry environment. Literature Cited 1. Pappas, S. P. "UV Curing Science and Technology" — Technology Marketing Corporation, Stamford, Connecticut 1978. 2. Green, G. E.; Stark, B. P.; Zahir, S. A. J. Macromol. Sci., Rev. Macro. Chem. 1982, C21, 187. 3. Parts, L. P.; Feairheller, W. R., Jr. U.S. Patent 3 477 932, 1969. 4. Frigerio, G. E.; Stefanini, A. Lett. Nuovo Cimento Soc. Ital. Fis. 1971, 2, 810. 5. Decker, C. Microcircuit Engineering 1982, 82, 299. Intern. Conf. Microlithography, Grenoble, France 1982. 4



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6. Williamson, M.A.; Smith, J. D. B.; Castle, P. M.; Kauffman, R. N. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 1875. 7. Sadhir, R. K.; Smith, J. D. B.; Castle, P. M. J. Polym. Sci., Polym. Chem. Ed. 1983 21, 1315. 8. Decker, C. Polym. Photochem. 1983, 3, 131. 9. Decker, C. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 2451. 10. Tsao, J. Y.; Ehrlich, D. J. Appl. Phys. Lett. 1983, 42, 997. 11. Hoyle, C. E.; Hensel, R. D.; Grubb, M. B.; Polym. Photochem., (in press). 12. Castle, P. M. IUPAC 28th Macromol. Symp. Amherst 1982 - Preprints p. 282. 13. Jain, K.; Willson, C. G.; Rice, S.; Pederson, L.; Lin, B. J. Proc. Int. Conf. Microcircuit Engineering, Grenoble, France, 1982, p. 69. 14. Kirchmayr, R.; Berner, G.; Rist, G. Farbe and Lack 1980, 86, 224. 15. Decker, C.; Fizet, M. Makromol Chem. Rapid. Commun. 1980,1,637. 16. Lieberman, R. A. Radiation Curing 1981, 8, 13. 17. Decker, C. SME Techn. Paper, FC 83-265 (1983); Int. Conf. Rad. Curing, Lausanne 1983. 18. Carlblom, L. H.; Pappas, P. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 381. 19. Merlin, Α.; Fouassier, J. P. J. Chim. Phys. 1981, 78, 267. 20. Wight, F. R. J. Polym. Sci., Polym. Lett. 1978, 16, 1121. 21. Decker, C. J. Appl. Polym. Sci. 1983, 28, 97. 22. Decker, C. J. Coat. Technol. (in press), Polym. Mat. Sci. Eng. 1983, 49 32. 23. Biedermann, K.; Holmgren, O. Appl. Opt. 1977, 16, 2014. 24. Becker, R. Α.; Sopori, B. L.; Chang, S. C. Appl. Opt. 1978, 17, 1069. 25. Loh, I. H.; Martin, G. C.; Kowel, S. T.; Kornreich, P. Polymer Preprints 1982, 23, 195. 26. Jain, K. Lasers and Applications September 1983, 49. 27. Decker,C.;Bendaikha, T. Europ. Polym. J., (in press). 28. Kaldor, Α.; Woodin, R. Proc. IEEE, 1982, 70, 565. 29. Bamford, C. H.; Schofield, E. Polymer 1981, 22, 1227. 30. Fizet, M., Ph.D. Thesis, University of Haute Alsace, Mulhouse 1981. 31. Tryson, G. R.; Shultz, A. R. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 2059. 32. Julien, private communication. 33. Roffey, C. G. "Photopolymerization of Surface Coatings"; J. Wiley Sons: New York, 1982. 34. Decker, C.; Bendaikha, T. GFP Conference on Tridimensional Polymer Systems, Strasbourg 1983, Preprints p. 121. RECEIVED October 3,

1984


Materials for Microlithography - American Chemical Society

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