J. MATER. CHEM., 1994, 4( 12), 1769-1773 Photoresists based on a Novel Photorearrangement of o-Nitrobenzyl ic Polymers A. Ajayaghosh,*ta M.V. George* and Tsuguo Yamaokab a Photochemistry Research Unit, Regional Research Laboratory (CSIR), Trivandrum 695 079, India Department of Image Science, Faculty of Engineering, Yayoi-Cho, 7-33, Inage-ku, 263 Japan A new photosensitive dicarboxylic acid, isophthalimido bis(a-methylamino-2-nitro-4-toluic acid) (4) was synthesized and characterized. Copolymerization of the corresponding acid chloride with diamines gave polyamides having photo- sensitive o-nitrobenzylic chromophores at symmetrical positions of every repeating unit. Photolysis of the new poly- amides resulted an interesting photorearrangement leading to the formation of azo polymers with carboxylic acid groups at the ortho positions. The polarity difference induced by the photorearrangement brings about a solubility difference between the irradiated and unirradiated polymers, which renders them useful as positive photoresist materials.Ciamician and Silberl as early as 1901 had observed that o-nitrobenzaldehyde underwent an interesting phototransform- ation leading to o-nitrozobenzoic acid. Since then, the photo- transformations of several o-nitrobenzylic systems have been examined by several groups of workers. Amit and Patchornick2 used the photochemistry of o-nitrobenzylic and related systems for temporary protection of functional groups, work which was extended later to many areas such as polymer- supported peptide synthesis and nucleotide synthesis.2p7 The same photorearrangement can induce large changes in the solubility of polymers and has led to the design of several deep-UV photoresists for microelectronic application^.',^ For example, Burzynski and Saenger" have used the Patchornick reaction to bring about solubility differences in poly(acry1ic acid) by photochemically deprotecting o-nitrobenzylic groups from their side-chain carboxylic esters.Another imaging system based on the photochemistry of o-nitrobenzyl esters of cholic acid was reported by Reichmanis et d." in which the solubility difference is achieved by a difference in dissolu- tion inhibition properties of o-nitrobenzyl cholic ester before and after exposure.The photochemistry of o-nitrobenzylic chromophores has also been used to design photodegradable polymer^.'^-'^ The backbone fragmentation in such polymers results in the reduction of the molecular weight of the polymer which eventually causes a change in solubility, sufficient for it to be useful as deep-UV positive photoresists. Several authors have also employed the photorearrangement of o-nitrobenzylic systems to generate p-toluene sulfonic and related acids from their esters, which catalyse the deprotection of poly(tert- butoxycarbonyl styrene^).'^.'^ Taking advantage of the photo- chemistry of o-nitrobenzylic systems, we previously designed a new photodegradable polyamide and a model compound which were subjected to detailed photochemical studies includ- ing nanosecond and picosecond laser flash photolysis experi- ments.18 Recently, we have reported the synthesis and phototransformations of polymers containing the o-nitro-benzylic chromophore in the main chain." The present paper describes the novel approach of inducing a solubility difference between the irradiated and unirradiated polymers through a polymer backbone rearrangement rather than the usual poly- mer chain degradation in designing positive resists and takes advantage of the phototransformation of the o-nitrobenzylic chromophore.t INSA-JSPS Exchange Fellow at the Department of Image Science, Chiba University August-December, 1993. $Also at the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 01 2, India.Experimental Materials and Methods All solvents used in this study were dried and freshly distilled before use. p-Toluic acid and isophthaloyl dichloride were obtained from Aldrich and used without further purification. 4-a-Methylamino-3-nitro-p-toluic acid was prepared by the reported procedure." All melting points were determined on a Buchi 530 melting point apparatus and are uncorrected. IR spectra were recorded on a Perkin-Elmer 880 IR spectrometer. The electronic spectra were recorded using a Shimadzu 2100 spectrophotometer. 'H and 13C NMR spectra were recorded on a JEOL GSC-400 spectrometer, using TMS as internal standard. Mass spectra were recorded on a Finnigan MAT model 8430 or a JEOL JMS-HX505 HA mass spectrometer.Elemental analyses were carried out by the Midwest Microlab (Indianapolis, USA). Viscosity measurements were carried out at 32 "C using an Ubbelohde viscometer. All solution-phase photoreactions were carried out in Pyrex vessels using a Srinivasan-Griffin-Rayonet photochemical reactor (RPR) equipped with a 300 nm light source. Synthesis of Isophthalimido Bis(a-methylamino-2-nitro-4-toluic Acid) (3) To a solution of 4-r-methylamino-3-nitro-p-toluic acid (1) (4.2g, 20mmol) in aqueous sodium hydroxide (2 mol l-', 10 ml) was added isophthaloyl dichloride (2.30 g, 10 mmol) in chloroform (10 ml), drop wise with stirring. The reaction mixture was kept alkaline by the slow addition of aqueous NaOH (1 mol 1-l). After 2 h, chloroform was removed under reduced pressure and the remaining mixture was poured over crushed ice, followed by acidification with hydrochloric acid (1 mol 1-l).The precipitated solid was filtered, washed with water and dried under vacuum to give 5.2g (95%) of the dicarboxylic acid 3, mp 23 1-232 "C, after recrystallization from methanol. IR vmaX (KBr)/cm-l: 3200-2800 (broad, C02H), 1724 (C=O, acid), 1622 (C=O, amide), 1540 and 1355 (NO,). 'H NMR (C2H6]acetone) 6: 3.1 (s, 6 H, NCH3), 5.1 (s, 4 H, CH,), 7.5-8.7 (m, 10 H, aromatic). MS (FAB)m/z: 551 (MH'). Calcd. for C26H22N4010: C, 56.73; H, 4.03; N, 10.17%. Found: C, 56.59; H, 3.89; N, 10.01%. Preparation of the Diacid Chloride 4 The dicarboxylic acid 3 (5.5 g, 10 mmol) was refluxed with thionyl chloride (12 ml) under a nitrogen atmosphere for 2 h.The excess of thionyl chloride was distilled off and the acid chloride obtained was used without further purification, 1770 Preparation of the Polyamide 5a To a solution of the diacid chloride 4 (2.94 g, 5 mmol) in chloroform (20 ml), a mixture of ethylenediamine (0.34 ml, 5 mmol) and triethylamine (0.74 ml, 10 mmol) in chloroform (5ml) was added at 30 "C. The reaction mixture was stirred vigorously for 3 h and the solvent was removed under reduced pressure. The polymer obtained was dissolved in DMF (10 ml) and poured into an excess of water. The precipitated polymer was filtered off, washed with water and dried in a vacuum oven at 60 "C for 24 h to give 2.65 g (92%) of the polyamide 5a.qred: 0.19 dl g-'. IR v,,, (neat film)/cm-': 1656 (C=O, amide), 1535 and 1357 (NO,). 'H NMR (C2H6]DMSO) 6:1.4 (CH2 -CH,), 2.5 (NCH,), 5.0 (CH,), 7.6-8.9 (aromatic). Preparation of the Polyamide 5b The polyamide 5b was prepared by the condensation of the diacid chloride 4 (2.94 g, 5mmol) and p-phenelenediamine (0.54 g, 5 mmol) in the presence of triethylamine (0.74 ml, 10 mmol) in DMF (20 ml) at 32 "C for 6 h. The reaction mixture was poured into an excess of water and the precipi- tated polymer was purified after two reprecipitations followed by drying in a vacuum oven at 100°C for 12 h to give 2.8 g (85%) of 5b. qred: 0.21 dl g-'. IR v,,, (neat film)/cmP1: 1655 (C=O, amide), 1535 and 1357 (NO,). 'H NMR (C2H6]DMSO) 6 :2.5 (NCH,), 5.0 (CH,), 7.5-8.9 (aromatic).Steady-state Photolysis of the Polyamide 5b The polyamide 5b (500 mg) was dissolved in dry DMF (100 ml) and the solution was deaerated by purging it with dry argon for 15 min. It was then irradiated in an RPR (3000 A) photoreactor for 18 h and the reaction mixture was concentrated under vacuum and poured into an excess of methanol. The precipitated red-brown solid was redissolved in DMF and reprecipitated from methanol and then repeat- edly washed with methanol. The solid product obtained was dried at 60 "Cin a vacuum oven for 6 h to give 352 mg (70%) of 8. IR vmax (neat film)/cm-': 3200-2800 (broad, CO,H), 1655 (C=O, amide), 1570 (weak, N=N). 'H NMR (C2H6]DMSO) 6 :7.5-8.9 (aromatic).Lithographic Evaluation Photoresist solutions were prepared by dissolving the poly- amide 5a or 5b (10 wt.%) in N-methyl-2-pyrrolidone. The polymer solutions were then filtered through a 0.5 pm PTFE filter and subsequently spin-coated on silicon wafers to yield films ca. 1 pm thick. These polymer films were then subjected to prebaking at 100°C for 1h and were then contact exposed with a pre-calibrated Kodak step tablet, using an unfiltered low-pressure mercury lamp. The exposed films were developed in an aqueous solution of tetramethylamonium hydroxide (10%) for 1 min and rinsed with distilled water. Characteristic exposure curves were obtained by plotting the normalized film thickness remaining against the exposure dose. Results and Discussion The reaction pathways employed for the synthesis of the polyamides 5a and 5b is shown in Scheme 1.The IR spectra of the polyamides 5a and 5b indicated two strong absorption peaks at 1350 and 1530crn-' due to the nitro groups. The 'H and 13C NMR spectra of both the polymers were in good agreement with their respective structures. We could not determine the molecular weight of these polymers owing to their poor solubility in THF. However, the reduced viscosities J. MATER. CHEM., 1994, VOL. 4 CI I 1 2 t 3 reflux SOCI,2h r 1 L Jn 5a X= -CH2-CH2-5b X= +-Scheme 1 (qred) of both the polymers were determined in DMF at 30 "C and indicated that they were low-molecular-weight polymers. The thermal properties of these polymers were obtained from their TG curves and indicated that both the polymers are stable up to 200°C and that they began to decompose above this temperature.Phototransformationsof the Polyamides 5a and 5b Irradiation of the polyamides 5a or 5b in DMF at 300nm resulted in the formation of a deep red-brown solution, which on evaporation gave a low-molecular-weight product and a red-brown polymeric product. The low-molecular-weight product was identified as N-methylisophthalimide (6). Comparison of the viscosities of the starting polymers with their respective photoproducts revealed an increase in vis- cosity, in each case indicating that polymeric products were formed on irradiation. The IR spectrum of the polymeric photoproduct showed the absence of the characteristic absorp- tion peaks due to the nitro group. Considerable reduction of the absorption peak due to the amide group was observed. The IR spectrum also indicated substantial growth of the J. MATEK.CHEM., 1994, VOL. 4 broad absorption peaks at 3200-2800 cm-' which are indica- tive of the formation of carboxylic acid groups in the photop- roduct. A weak absorption at 1570 cm-I indicated the presence of an azo chromophore in the photoproduct. The two new absorption bands at 290 and 320 nm also indicated the formation of an azo chromophore. The 'H NMR spectrum of the polymeric photoproducts, obtained from 5a and 5b indicated the absence of the o-nitrobenzylic protons at 5ppm and the NCH3 protons at 3.15 ppm. Considerable reduction of the aromatic peaks at 6.8-8.8 ppm was also noted. The I3C NMR spectrum of the photoproduct revealed substantial reduction in the absorption peaks at 162.7 ppm due to the CONCH, group, at 148 ppm due to the aromatic carbon attached to the nitro group and at 41.4 ppm due to the o-nitrobenzylic carbon.Considerable reduction in the number of the aromatic peaks and the formation of a new carbonyl peak at 165.4ppm was also noted. All these observations led to the conclusion that the polymeric photoproduct (from 5a and 5b) could be an azo dicarboxylic acid polymer having the structure 8 which could arise through the pathways shown in Scheme 2. However, for the polyamide 9 complete degradation of the polymer back- bone occurred during photolysis.Comparison of these two observations indicates that even though the basic photochem- istry involved in both cases is the same, the subsequent reaction pathways are different. The mechanism of photo- transformation of 5a and 5b could involve the pathways indicated in Scheme2. In this case the initial photocleavage leads to the formation of the intermediate 7, having two u-nitrosobenzaldehyde moeities, which subsequently undergoes thermal dimerization resulting in the formation of the azo polymer 8. However, in for polymer 9, the initial photocleav- age generates the intermediate 10, which has only one o-nitrosobenzaldehyde group and gives the dimerized product 11 (Scheme 3). Photoresist Evaluation of Polymers 5a and 5b The phototransformations of the polyamides 5a and 5b indi-cate the presence of carboxylic acid groups in the photoprod- uct which brings about a considerable polarity difference between the unirradiated and irradiated polymers.This photo- induced polarity difference could be sufficient to recog iiise the irradiated and unirradiated portions of the polymer film by a suitable base developer, which prompted us to carry out detailed photoresist evaluation of the polyamides 5a and 5b. In order to gain a better understanding of the photochemical properties of the polymer films before carrying out the resist evaluation, thin films of the polymers 5a and 5b were prepared and irradiated for varying periods of time. Fig.1 shows the photochemical changes of the polyamide 5b as a function of irradiation time, on irradiation at 300 nm. The build-up of two new absorption bands at ca. 290 and 320 nm through the formation of an isobestic point at 273 nm is observed. The Jn Sb insoluble in aqueous base lhv 6 7 8 soluble in aqueous base, positive pattern Scheme 2 9 Scheme 3 J. MATER. CHEM., 1994. VOL. 4 h cn c.-c3 240 300 360 420 wavelengt hlnm Fig. 1 Photochemical changes of the polyamide film 5b after irradiation at 300 nm for (a)30 s, (b)60 s, (c) 2 min, (d) 4min, (e)8 min new absorption bands could be attributed to the formation of the azo chromophore, as indicated in Scheme2. The IR spectral changes of the film of 5b on irradiation, as a function of time are shown in Fig.2. The absorption bands at 1530 and 1350 cm-l, corresponding to the nitro group, disappear whereas there is an increase in the broad absorption at 3200-2800cm-' due to the formation of carboxylic acid group. In order to evaluate the photosensitivity of 5a and 5b,films of ca. 1 pm thickness were prepared on silicon wafers by spin- coating a solution of the corresponding polymers in N-methyl- 2-pyrrolidone. The characteristic photosensitivity curves of 5a and 5b are shown in Fig. 3, which indicates that the photosen- sitivities of these polymers are comparatively low under irradiation using an unfiltered mercury lamp. One of the reasons for such a low sensitivity could be the strong absorp- tion of the phototransformed azopolymer at the wavelength of exposure (> 300 nm).Thus, the photoproduct formed over the top of the polymer film could act as an internal light filter, protecting the inner part of the film and slowing down the overall phototransformation of the starting polymer film. 1336 1 10 100 1000 exposure energy1mJ cm-2 Fig. 3 Characteristic photosensitivity curves of the polyamides 5a(a) and 5b (b) However, these polymers are expected to be ideal positive- type photoresists for excimer-laser lithographic applications which will be the subject of further studies. Currently we are trying to synthesize several polyamides and polyesters with improved solubility and photochemical properties through structural modifications.Conclusions A novel phototransformation of polyamides containing o-nitrobenzylic chromophores on symmetrical positions of each repeating unit leading to azo polymers bearing carboxylic acid group ortho to the azo group is reported. Symmetrical incorporation of chromophores on the polymer backbone causes polymerization of the intermediate photoproduct, instead of the usual photodegradation as in the case of polymer 9, which contains only one o-nitrobenzylic chromo- phore per repeat unit. This study further illustrates that the overall reaction pathways during photolysis can be controlled by proper tailoring of chromophores in a polymer backbone. The photorearrangement induces a considerable polarity difference to the irradiated and unirradiated polymer due to the formation of carboxylic acid groups after photoirradiation.Changing the dissolution properties of polymers through polymer backbone rearrangement is a different approach from the conventional polymer-chain degradation or pendant func- tional group deprotection strategies in designing positive-type photoresist. The authors thank CSIR, Government of India and the Regional Research Laboratory, Trivandrum for financial sup- port of this work. A. A. thanks the Indian National Science Academy, New Delhi and the Japan Society for Promotion of Science, Tokyo, for the award of an exchange fellowship. This is document No. RRLT-PRU-45 from the Photochemistry Research Unit. References 1 G. Ciamician and P.Silber, Berichte, 1901,34,2040.\ 2 B. Amit and A. Patchornik, Tetrahedron Lett.. 1973,2205.LL3 V. N. R. Pillai, Synthesis, 1980, 1. 4000 3200 2400 1700 1100 500 4 D. H. 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