Epoxy Methacrylate Resin as Binder Polymer for Black Negative-Tone Photoresists

Epoxy acrylate (EA) resin, which originates from epoxides, has long been served as a photocurable coating and adhesive material owing to its double bonds. Specifically, alkaline-developable EA resins can be used as a binder polymer in negative-tone photoresists. In this work, we synthesized a series of acidic polyester-type epoxy methacrylate resins, characterized the intermediates and products, and tested their performance as a binder polymer for the photolithographic micro-patterning of the pixel-defining layer on organic light-emitting diodes in comparison to a widely used commercial binder polymer. Copolymer-type binder polymer BP-2-2 was produced excellent patterning with no residue due to its high compatibility with the black mill base.


Introduction
Epoxy resins, commercialized in the late 1940s, have been used in surface coatings, adhesives, laminates, and other miscellaneous fields [1]. Epoxy acrylate (EA, including epoxy methacrylate) resins can be synthesized by the first reaction of diepoxide with acrylic/methacrylic acid to give double bonds and secondary alcoholic -OH groups, followed by polyesterification between dianhydride and diol units. They excel epoxy resins due to their much faster radiation/thermal curing rate compared to that between curing agents and epoxy resins [2] [3] [4]. Most commercial EA resins are based on epoxy novolacs and bisphenol diglycidyl ethers because of their low cost and easy functionalization.
Apart from the well-known coating and adhesive applications, EA oligomers How to cite this paper: Shi, G., Baek, K., Ahn, S.H., Bae, J., Kim with acidic phenolic -OH [5] [6] [7] (Figure 1(a)) or carboxyl (-COOH) groups [8] [9] [10] (Figure 1(b)) have found applications as alkaline-developable binder polymer for negative-tone photoresists. These photoresists have been used for patterning color filter and black matrix [11] of liquid crystal displays (LCDs) as well as the pixel-defining layer (PDL) of organic light-emitting diodes (OLEDs) [12] [13]. Since the acidity of phenolic -OH groups of EAs based on novolac resin is relatively weak, incomplete development may occur, and the introduction of double bonds on the novolac oligomer also requires sacrificial phenolic -OH groups. However, EA resins with polyester linkage, as shown in Figure 1(b), have carboxylic groups as the source of acidity and have better performance in the developmental stage than that with phenolic -OH groups. Moreover, in the case of black photoresists, carboxyl groups can also serve as anchoring groups for the black pigment, enhancing the compatibility among other photoresist components [14].
In this study, we used dicarboxylic acids as starting materials to obtain epoxy methacrylate resin-type binder polymers and evaluated their suitability in the photolithographic process to improve black PDL patterns of OLEDs.

Materials
The binder polymers were synthesized by first reacting dicarboxylic acid with glycidyl methacrylate (GMA) to form diol intermediates with methacrylate groups, followed by the reaction with dianhydrides (with/without external diol as comonomer) to obtain the final binder polymer. The chemical names, code names, and structures of diacids, dianhydrides, and diols used for the syntheses are shown in Table 1. All chemicals were reagent grades and reacted without   further purification. TBPB and MSMA were obtained from Alfa Aesar, PM6 was synthesized by us [10], and the other chemicals were from Tokyo Chemical Industry Co. Ltd. (TCI).

Synthesis of Homo Binder Polymers: BP-1-x Series
Under nitrogen atmosphere, 5 mmol of 6FDC diacid, 10 mmol of GMA, 0.025 mmol of polymerization inhibitor BHT, 0.05 mmol of catalyst TBPB, and certain amount of propylene glycol methyl ether acetate (PGMEA) solvent (40 wt% solid) were mixed and stirred at 110˚C for 3 h. After cooling, 5 mmol of 6FDA was added with extra PGMEA (40 wt% solid) and continued stirring at 110˚C for another 3 h. Up to this point, the reaction mixture appeared as a clear solution.
This solution was cooled down and directly used without further work-up (BP-1-1). Binder polymers synthesized with ITCA, ODC, MLDC, and 8FDC as diacids were obtained by the same procedure, and the products were given code names as BP-1-2~BP-1-5, respectively, and those synthesized using 6FDC and BPDA were given BP-1-6 ( Figure 2). Specifically, BP-1-3 was insoluble in PGMEA; therefore, it was not tested in the photolithographic evaluation.

Characterization
The liquid chromatography-mass spectrometry (LC-MS) was recorded using a Bruker 1200 Series & HCT Basic System. The monomer sample in the PGMEA solution was diluted with the eluent (MeOH-H 2 O system) and directly subjected to the column. The gel permeation chromatography (GPC) was recorded using an Agilent 1200 S/miniDAWN TREOS. The polymer sample in the PGMEA solution was diluted with THF and directly subjected to the column.
The Fourier-transform infrared spectroscopy (FT-IR) was recorded using a Varian 670 spectrometer. N-hexane was added to the polymer solution, and the precipitate was collected and washed 3 times with n-hexane using centrifugation. The resulting solid was dried under vacuum and subjected to the KBr pellet method.

Syntheses and Characterizations of Binder Polymers
Due to the low cost, high diversity, and much higher chemical stability of dicarboxylic acid species compared with diepoxies, we utilized various diacids as starting materials to react with monoepoxide GMA to obtain diepoxy methacrylate-type diol monomers. As shown in Figure 1

Photolithographic Evaluation of Binder Polymers
The photolithographic performance of the synthesized epoxy methacrylate binder polymers was evaluated in comparison to the commercial cardo binder polymer CBP, whose structure is shown in Figure 1(b). In a typical black photoresist formulation (Table 2)     The black photoresist with CBP binder polymer (PR-0) gave better pattern and residue level (Table 3)

Conclusion
In this work, a series of epoxy methacrylate-type binder polymers were synthesized using dicarboxylic acids as starting materials. The photo-initiator loading, UV dose, and DBD of binder polymers contributed to the photolithographic cross-linking reaction simultaneously; therefore, we tuned these parameters carefully to optimize cross-linking. Moreover, -CF 3 groups and bulky fluorene groups were found to improve the compatibility with the black mill base, resulting in less residue on the wafer after the development. The black photoresist PR-2-2 with the copolymer-type binder polymer BP-2-2 produced the optimal PDL pattern with the least residue in the photolithographic tests. This work can pave the way for the design of binder polymers for negative-tone black photoresist and the optimization of photolithographic process conditions.