Polymer Journal (2010) 42, 86–94 & The Society of Polymer Science, Japan (SPSJ) All rights reserved 0032-3896/10 $32.00 www.nature.com/pj ORIGINAL ARTICLE Developmentofchemicallyamplifiedreaction development patterning Xu Cheng, Akio Takahashi and Toshiyuki Oyama Various carboxylate and sulfonate esters were synthesized as acid amplifiers by reacting carboxylic acid chlorides and sulfonic acid chlorides with alcohols. The synthesized acid amplifiers were introduced into a photosensitive polyetherimide (PEI) system on the basis of reaction development patterning (RDP) composed of PEI (Ultem) and a diazonaphthoquinone compound (PC-5) as a photosensitive agent, and the availability of the resulting chemically amplified RDP (CARDP) system was examined. It was found that the amount of photosensitive agent and the exposure dose required for the formation of fine patterns were closely related to the structure of acid amplifiers. Consequently, the use of 2SCE with two sulfonate groups in a molecule enabled the formation of clear patterns with 5 wt% of the photosensitive agent and a 300-mJ/cm2 exposure dose, which achieved a sharp reduction of 30 wt% and 2000 mJ/cm2, respectively, in the amount of the photosensitive agent and exposure dose used in conventional RDP. Good thermal properties of the patterns prepared by CARDP were also indicated. Polymer Journal (2010) 42, 86–94; doi:10.1038/pj.2009.308 Keywords: acid amplifier; chemically amplified reaction development patterning; photoresist; photosensitive agent; polyetherimide INTRODUCTION plastics containing a photosensitive agent are irradiated and developed Photosensitive polymers, known as photoresists, have been widely by a hydrophilic nucleophilic developer. An acid generated from the used in the micropatterning of electronics such as semiconductor photosensitive agent by irradiation reacts with amine in the developer devices, printed wiring boards and color filters for liquid crystal to form salt, and the salt gives hydrophilicity to the polymer film. displays.1,2 High sensitivity, high resolution and good resistance to Therefore, at the exposed areas, infiltration of the hydrophilic devel- dry-etching are needed for photoresists that are used for microlitho- oper into the polymer occurs more rapidly. Thereafter, reaction of the graphy in semiconductor technology. However, long-term properties infiltrated amine in the developer with carboxylic acid derivatives are not very important for these photoresists because they are (-C(O)-X-) in the main chain of engineering plastics leads to removed after dry-etching.3 By contrast, there is another type of degradation of the polymer, and dissolution of the resulting photoresist that is used for producing printed wiring boards and for low-molecular-weight compounds yields positive-tone patterns. We protecting integrated circuits. The resolution needed for such photo- examined the applications of RDP to engineering plastics with -C(O)- resists is not very high (10–100 mm), but resist films require good X- linkages in the main chain such as PI7 and commercially available mechanical, thermal and insulating properties because of the perma- polyetherimide (PEI),8 polycarbonate (PC)9 and polyarylate.10,11 Clear nence of films in products. Thus, photosensitive polyimides (PIs), or patterns were formed with all polymers by RDP. However, a large polyamic acids as their precursors, have been used as photosensitive quantity of photosensitive agent (30 wt% for the polymer) is needed polymers for such purposes.4,5 However, all these polymers require the for selective infiltration of the developer into exposed areas. In introduction of functional groups to polymer frameworks to achieve addition, a clear pattern can be obtained only with a large ultraviolet photosensitivity and, as a result, they have the disadvantages of a exposure dose (2000 mJ cmÀ2) because of its low sensitivity. reduction in the excellent properties of PI frameworks and of a high Chemically amplified photoresist systems based on photoacid cost in preparing polymers. Thermal imidization processes are also generators (PAGs) and functionalized polymers, such as polystyrene needed for poly(amic acid) resists. containing a tert-butoxycarbonyl group, have been studied to reduce We have reported a new patterning process, reaction development the amount of photosensitive agent required and to achieve high patterning (RDP),6 which is fundamentally different from previously sensitivity in comparison with traditional photoresists.12 Acid groups developed processes. In RDP, the introduction of any specific func- (for example, a carboxyl group) protected with groups deprotectable tional groups or photosensitive groups to polymers is not necessary. by acid from PAGs are used as functional groups in polymers. At the In the process of RDP, films of commercially available engineering exposed areas only, the deprotection reaction by the acid from PAGs Faculty of Engineering, Department of Advanced Materials Chemistry, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Japan Correspondence: Dr T Oyama, Faculty of Engineering, Department of Advanced Materials Chemistry, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan. E-mail: [email protected] Received 9 July 2009; revised 4 September 2009; accepted 12 September 2009 Chemically amplified RDP XChenget al 87 occurs through post-exposure baking (PEB), and the following devel- Synthesis of acid amplifiers opment with an alkaline developer results in clear patterns. Synthesis of tert-butyl 2-naphthoate (NTB). 2-Naphthoyl chloride (5.00 g, In this study, we apply the principle of chemical amplification to RDP 26.2 mmol), tert-butanol (5.83 g, 78.6 mmol) and anhydrous pyridine (6.22 g, by adding a small quantity of photosensitive agent as a PAG and acid 78.6 mmol) were added to a 30-ml two-necked flask. tert-Butanol was used as amplifier instead of a large quantity of photosensitive agent as used in the solvent, and pyridine was added as an acid scavenger and solvent. The mixture was stirred at 30 1C for 24 h under a nitrogen atmosphere. The white conventional RDP. Acid amplifiers are protected low-molecular-weight suspension that formed during the reaction was poured into a large amount of acid molecules. A small amount of acid generated from the photo- pure water. After stirring for 1 h at room temperature, a white precipitate was sensitive agent by irradiation is expected to act as a catalyst for collected by filtration and washed with pure water. This process was repeated deprotection of the acid amplifier. The deprotection of the acid amplifier five times to completely remove salt, unreacted reactants and pyridine. There- at PEB after irradiation will increase the amount of acid only at exposed after, the obtained white precipitate was dried under reduced pressure at 60 1C areas. The produced acid can form salt with the developer, and the salt for 5 h to remove water. The resulting product was obtained with an 86% yield, 1 can promote preferential infiltration of the developer into exposed areas melting point 85.6–88.2 1C, H NMR (in dimethylsulfoxide (DMSO)-d6) (Figure 1). When sulfonic acid esters are used as acid amplifiers, the (d, p.p.m.) 1.60 (-CH3, s, 9H), 7.57–8.59 (Ar-H, m, 7H). generated sulfonic acid during PEB can also act as a catalyst for further 15,16 deprotection reactions, and acid can be proliferated as reported by Synthesis of isopropyl 2-naphthalenesulfonate (SIP). In a 100-ml two- necked flask, 2-naphthalenesulfonyl chloride (5.0 g, 22.0 mmol), 2-propanol Ichimura et al.13,14 Thus, the application of chemical amplification to (9.26 g, 154.0 mmol), pyridine (12.20 g, 154.0 mmol) and toluene (41.7 ml) RDP is expected to reduce the amount of photosensitive agent required were added at 0 1C. The mixture was stirred for 60 h at room temperature and to achieve a higher sensitivity than that obtained with conventional under a nitrogen atmosphere. The white precipitate that formed during the RDP. In this study, we examined the effect of the structure of acid reaction was filtered, and the resulting filtrate was extracted with 5 wt% amplifiers on pattern-forming properties in chemically amplified RDP NaOHaq. The organic layer was dried with MgSO4, filtered and evaporated to (CARDP) and explored the most suitable conditions for pattern forma- yield a white solid. The crude ester was further purified by recrystallization with tion. The sensitivity and amount of photosensitive agent required for hexane–ether. The pure product was obtained after drying under reduced pattern formation were also examined. In addition, the thermal proper- pressure for 5 h at room temperature at a 48% yield, melting point 54.8–55.8 1C 1 1 ties of patterns obtained by CARDP were investigated. (lit. 51%, 55.0–56.0 C), HNMR(inDMSO-d6)(d, p.p.m.) 1.20 (-CH3,d, 6H), 4.74 (-CH-, m, 1H), 7.70–8.64 (Ar-H, m, 7H). EXPERIMENTAL PROCEDURE Synthesis of 2-phenylethyl 2-naphthalenesulfonate (SPE). 2-Naphthalenesulfonyl Materials chloride (5.0 g, 22.0 mmol) and 2-phenylethanol (21.10 g, 154.0 mmol) were Commercially available PEI (Ultem) (Scheme 1a) was kindly provided by added to pyridine (12.20 g, 154.0 mmol) and toluene (41.70 ml) at room Sabic Innovative Plastics Japan (Tokyo, Japan). 1,2-Naphthoqiunonediazide-5- temperature, and the mixture was stirred for 68 h. The resulting suspension sulfonic acid p-cresol ester (PC-5) (Scheme 1b) was purchased from Toyo Gosei was filtered, and the filtrate was extracted with 5 wt% NaOHaq and 2 mol lÀ1 Kogyo (Ichikawa, Chiba, Japan). Other reagents and solvents were purchased HClaq. Thereafter, the organic layer was distilled under high vacuum to remove from Sigma-Aldrich Japan (Tokyo, Japan), TCI (Tokyo, Japan) and Wako unreacted alcohol. Ether was added to the obtained oil-like solid, and the ether- Chemicals (Osaka, Japan). soluble part was collected and evaporated. The product was further purified by 1.