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Novel Photoimaging System Based on Photoinduced Electron Transfer in Polymers Containing Pendant Benzophenone-Borate Salts
Scheme 1. Photoliberation of the Polymeric Amine
Alexander Mejiritski, Alexander Y. Polykarpov, Ananda M. Sarker, and Douglas C. Neckers* Center for Photochemical Sciences1 Bowling Green State University Bowling Green, Ohio 43403 Received February 6, 1996 Revised Manuscript Received May 10, 1996 Photogenerated acids and bases are important in many industrial applications including cation-initiated photopolymerization,2 photo-cross-linking of polymers,3 and microlithography.4 Crivello5 initially reported methods for photogeneration of acids from iodonium salts which were subsequently used by Frechet in the first design of chemically amplified photoresists6,7 employing acid-sensitive polymers. Hitherto numerous modifications of both the acid precursors8 and of the polymeric matrixes have been developed. Several reviews highlighting the structural development of acid-catalyzed chemically amplified resists have been published recently.9,10 On the other hand, relatively few base photoprecursors have been obtained. Those which have been studied are established upon release of free amino groups within polymer films upon UV irradiation.11-13 After the first work by Willson and Kutal,12 who employed inorganic transition-metal salts, Frechet and co-workers focused attention on carbamates in which latent amino groups are shielded by photolabile R,Rdimethyl-[(3,5-dimethoxybenzyl)oxy]carbonyl (Ddz)11 or o-nitrobenzyl groups.13 Both methods were successfully used to generate bases which catalyzed cross-linking of polymers with pendant epoxy groups,14 in image-tone reversal of known negative-tone photoresists,14 in polyimidization15-17 and in several other reactions (1) Contribution No. 269 from the Center for Photochemical Sciences. (2) Crivello, J. V.; Lam, J. H. W. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 977. (3) Schlesinger, S. J. Polym. Eng. Sci. 1974, 14, 513. (4) Thompson, L. F.; Willson, C. G.; Bowden, M. J. Introduction to Microlithography, 2nd ed.; ACS Professional Reference Book; American Chemical Society: Washington, DC, 1994. (5) Crivello, J. V.; Lam, J. H. Macromolecules 1977, 10, 1307. (6) Frechet, J. M. J.; Eichler, E.; Ito, H.; Willson, C. G. Polymer 1983, 24, 995. (7) Ito, H.; Willson C. G. Polymers in Electronics; ACS Symp. Ser. 242; American Chemical Society: Washington, DC, 1984; p 11. (8) Crivello, J. V.; Lam, J. H. W. J. Polym. Sci., Polym. Chem. Ed. 1978, 16, 2441. (9) MacDonald, S. A.; Willson, C. G.; Frechet, J. M. J. Acc. Chem. Res. 1994, 27, 151. (10) Reichmanis, E.; Houlihan, F. M.; Nalamasu, O.; Neenan, T. X. Chem. Mater. 1991, 3, 394. (11) Cameron, J. F.; Frechet, J. M. J. J. Org. Chem. 1990, 55, 5919. (12) Kutal, C.; Willson, C. G. J. Electrochem. Soc. 1987, 134, 2280. (13) Cameron, J. F.; Frechet, J. M. J. J. Am. Chem. Soc. 1991, 113, 4303. (14) Frechet, J. M. J. Pure Appl. Chem. 1992, 64, 1239. (15) McKean, D. R.; Briffaud, T.; Volksen, W.; Hacker, N. P.; Labadie, J. W. Polym. Prepr. 1994, 35, 387. (16) Frechet, J. M. J.; Cameron, J. F.; Chung, C. M.; Haque, S. A.; Willson, C. G. Polym. Bull. 1993, 30, 369.
S0897-4756(96)00103-2 CCC: $12.00
catalyzed by bases.18 Attachment of the o-nitrobenzyl protected carbamate groups to the polymer chains in the form of photolabile pendants led to the photogeneration of polymeric amines successfully utilized in the UV-induced cross-linking of epoxy-containing resins.19 High-resolution, high-sensitivity positive-tone photoresists have been developed based on the irradiation of copolymers of O-acryloylacetophenone oxime and methyl methacrylate followed by development with aqueous HCl.20,21 In recent studies we have found that the benzophenone-like triplet state of N,N,N-trialkyl-N-(p-benzoyl)benzylammonium triphenylbutylborate22 is rapidly quenched (τ ≈ 300 ps in benzene/acetonitrile (1%)) presumably by single-electron transfer (SET) from the borate anion to carbonyl portion of the molecule followed by methylene carbon-nitrogen bond cleavage and reduction of the formed radical cation of the amine by the enolate of benzophenone that leads to the formation of the free amine. We postulated that application of the same process to suitably constituted polymeric quarternary ammonium borates would lead to photochemical generation of polymeric amine which results in development of a new class of positive-tone photoimageable polymers which, after irradiation, could be developed by dissolution in aqueous acid. Herein we wish to report such a new system based on the generation of polymeric amine in poly(2-[N,N-dimethyl-N-(p-benzoyl)benzylammonium (triphenyl-n-butylborate)] ethyl methacrylate) (polymer 1). In analogous fashion to N,N,N-trialkyl-N-(p-benzoyl)benzylammonium triphenylbutylborate, upon UV irradiation of polymeric quarternary ammonium borate 1 with 365 nm light, rapid quenching of the excited state via SET from the triphenyl-n-butylborate anion and subsequent reactions lead to the formation of polymeric amine 2. The liberation of the target polymer with its pendant dimethylamino functionalities is accompanied by the formation of biphenyl (from triphenylboron) and products of the coupling of p-benzoylbenzyl radical 3 (with itself and with n-butyl radical). The total reaction is presented in Scheme 1. (17) McKean, D. R.; Wallraff, G. M.; Volksen, V.; Hacker, N. P.; Sanchez, M. I.; Labadie, J. W. Polymers for Microelectronics: Resists and Dielectrics; ACS Symp. Ser. 537; American Chemical Society: Washington, DC, 1994; p 417. (18) Uranker, E. J.; Frechet, J. M. J. Polym. Prepr. 1994, 35, 933. (19) Beecher, J. E.; Cameron, J. F.; Frechet, J. M. J. J. Mater. Chem. 1992, 2, 811. (20) Song, K.-H.; Urano, A.; Tsunooka, M.; Tanaka, M. J. Polym. Sci., Polym. Lett. 1987, 25, 417. (21) Song, K.-H.; Tonogai, S.; Tsunooka, M.; Tanaka, M. J. Photochem. Photobiol. A: Chem. 1989, 49, 269. (22) Hassoon, S.; Sarker, A.; Rodgers, M. A. J.; Neckers, D. C. J. Am. Chem. Soc. 1995, 117, 11369.
© 1996 American Chemical Society
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To qualify as a photoresist, a material must produce three-dimensional relief images in that the lightexposed and the unexposed areas evidence differential solubility in a developing solvent. If exposed areas are rendered more soluble than their unexposed counterparts, a positive-tone image is produced.4 Having recognized that the process outlined in Scheme 1 leads to the generation of polymeric amine we assumed that the solubility of the target polymer 2 in aqueous acid would be different from that possessed by the starting polymer 1. Indeed, when irradiated through the mask by the 365 nm light the irradiated areas of the polymer film were selectively removed by immersion in aqueous HCl (∼1.0 M) followed by additional washing with distilled water and ethyl alcohol thus producing a positive-tone relief image of the mask. All three development stages proved to be necessary. Of particular interest is the fact that after aqueous HCl and water treatment the bulk of the polymer film has been removed. However, an ethanol wash was required to completely clear the surface from residual scum. To provide prefatory characterization23 of our new system we have carried out preliminary investigation of thermal stability of polymer 1, its etch resistance as well as image resolution and sensitivity of the process leading to image formation. Polymer 1 was synthesized according to Scheme 2 with 70% polymerization yield. Solutions (20 wt %) of polymer 1 in N,N-dimethylformamide where used to yield uniform, almost defect free, glassy films of ca. 0.5 µm thickness on silicon wafers (13 × 13 mm; non-dustfree conditions) by spin-coating at 6000 rpm for 30 s with a Headway Research, Inc. spin-coater. The films were subsequently prebaked in an oven at 80 °C for 30 min in order to remove residual solvent. Film thickness was determined by measuring the intensity of fluorescence at 500 nm (λex ) 350 nm) from a probe, 5-(N,N-dimethylamino)naphthalene-1-(N′,N′di-n-butyl)sulfonamide, which had been incorporated in the films.24 According to Beer’s law the intensity of the fluorescence will be linearly proportional to film thickness provided that concentration of a probe remains constant throughout the film. To ensure uniformity of dispersion the probe was added to the original polymer solution at 0.5-1% level based on the overall polymer weight. Additionally, several substrates with films of varying thickness (all containing the fluorescent probe) were broken, and the resulting film cross sections investigated by scanning electron microscopy (SEM).
Using film thickness determined by SEM, we established a linear calibration graph (probe fluorescence intensity vs measured thickness), thus ensuring that the fluorescence probe technique provides a quick and nondestructive method for measuring film thickness. A number of substrates bearing films of the same thickness (0.60 ( 0.03 µm) were produced for the resolution and sensitivity measurements. To measure the sensitivity, films were exposed at room temperature to different doses of defocused UV irradiation from a 200 W high-pressure mercury arc lamp filtered through a 365 nm filter (bandwidth ∼ 40 nm). The distance from film to lamp was fixed at 11 cm. Flux was measured by a PMT power meter appropriately placed in an identical geometrical arrangement. Irradiated films were developed by immersing the substrates sequentially in aqueous HCl (∼1.0 M) for 15 s, in distilled water for 30 s, and in 100% ethanol for 15 s at room temperature. Developed films were air dried. Areas exposed to the largest doses were completely cleared of polymer film upon development. Since an acrylate backbone is the carrier for the pendant dimethylamino functionalities in polymer 2, we assumed that disappearance of a spectroscopic feature that could be unequivocally attributed to the acrylate network could be utilized as a measure of a relief image formation. Hence, film thinning was evaluated by obtaining the ratio between integrated areas of FT-IR peaks corresponding to the methacrylate carbonyl (1730-1734 cm-1) measured for the given film before irradiation and after development. The system shows rather high contrast of γp ) 1.82.25 The sensitivity value, Dc, the minimum exposure dose required to completely remove the film of positively functioning polymer of given thickness under the specified processing and development conditions, for the aforementioned system is estimated at 4.5 J/cm2 with the D0 value, the minimum exposure dose required to start observing changes in film thickness as a result of photochemically induced process with subsequent development under the specified processing and development conditions, being close to 1.3 J/cm2. This sensitivity lies significantly below the 25-200 mJ/cm2 region of sensitivities for currently employed photoresists.25,26 We would not expect sensitivity to equal that obtained with systems in which the base liberated by irradiation serves as a catalyst for film destruction or cross-linking providing chemical amplification of the photochemical reaction. However, materials such as polymer 1 may be useful in a variety of photoimaging processes requiring less speed than microlithography. We are also exploring methods for amplifying images formed by polymeric amines. To increase absorption by a chromophore in the film a number of them are being investigated as a replacement for benzophenone. For testing of the attainable resolution TEM grids G200-Cu and T2000-Cu modeling the soft contact type masks with the smallest detail size of 38 and 3 µm, respectively, were employed. Substrates received the
(23) Reichmanis, E.; Galvin, M. E.; Uhrich, K. E.; Mirau, P.; Heffner, S. A. Polymeric Materials for Microelectronic Applications: Science and Technology; ACS Symp. Ser. 579; American Chemical Society: Washington, DC, 1994; p 52. (24) SGL/Oriel Rapid Scan Fluorimeter available from Oriel Corp. was used for the fluorescence intensity measurements.
(25) Sensitivity and contrast values have been determined from the “sensitivity curve” (normalized calculated ratio for film thinning vs measured exposure dose) as in: Reichmanis, E.; Thompson, L. F. Chem. Rev. 1989, 89, 1273. (26) Ito, H.; Willson, C. G.; Frechet, J. M. J. U.S. Patent 4,491,628, 1985; Chem. Abstr. 1984, 101, 81662w.
Scheme 2. Synthesis of the Polymer with Pendant Benzophenone-Borate Salt 1
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Figure 1. SEM micrograph with 3 µm resolution pattern.
exposure dose of 5 J/cm2 and were developed in the manner described above. We obtained a pattern with resolution on the order of 3 µm (Figure 1). The slight “footing” of the detail profile is the result of an imperfect exposure setup. However, our measurements suggest that resolution of less than 3 µm is achievable. Investigations with smaller masks using collimated light are currently under way to better determine the resolution limit.
Communications
Polymer 1 has a melting endotherm Tm at 115 °C as determined by differential scanning calorimetry (DSC) at a heating rate of 20 °C/min. To determine the thermal stability Td the polymer was subjected to thermogravimetric analysis (TGA) in nitrogen. A loss of 5 wt % was recorded at 235 °C when sample was heated with a heating rate of 20 °C/min. Although the original polymer 1 is soluble in concentrated acids (48% HF and 70% HNO3), it is insoluble and stable upon exposure to aqueous bases (25% tetramethylammonium hydroxide). This allows for pattern transfer onto the underlying substrates which are destroyed by wet etching with basic solutions. In conclusion, photoimaging of a polymer containing pendant benzophenone-borate salts that produces polymeric amine removable with aqueous acid has been demonstrated. Acknowledgment. We express our sincere gratitude to the McMaster Endowment, the National Science Foundation (NSF DMR-9013109) and the Office of Naval Research (Navy N00014-93-1-0772) for financial support and to Professors George S. Hammond and Thomas H. Kinstle for valuable discussions. CM960103+
