Thymine-Based Photoresist Polymers: Synthesis and Molecular Structure

F. GARCÍA; D. MARTINO; D. ESTENOZ; G. MEIRA; J. WARNER

Toulouse, France

Congreso; Green Processes Engineering; 2007

Styrene-based biopolymers containing ionic and thymine groups chemically attached to the polystyrene structure were experimental- and theoretically investigated. The ionic groups make the polymer water-soluble, thereby eliminating the environmental and health concerns associated with the use and clean up of volatile organic solvents. The material is called a photoresist, given that after illumination with UV light a photocyclization reaction between adjacent thymines takes place, rendering it water-insoluble. The microelectronics industry is based on photoresist technology to manufacture printed circuit boards, IC chips, and other components. Conventional photoresist processes are not environmentally-friendly. These materials are based on organic solvent dependent monomers, and undergo both polymerization and crosslinking simultaneously upon irradiation. These monomers are often toxic, are dumped in the organic wash stage, and require a strict monitoring of waste and solvent evaporation. The advantages of thymine-based polymers can be summarized as: a) they are water soluble and non-toxic; and b) the material is already polymerized. In the photo-initiated crosslinking mechanism, neighboring chains are "tied" together leading to an insoluble state. The synthesis conditions determine the molecular weights distribution and chemical composition of the copolymers, influencing the solubility and the photoreactive behavior. The curing process determines the final thermal and mechanical properties of the photoresist. The relationships between the synthesis conditions, the original molecular structure before crosslinking, the curing conditions, and the final mechanical and thermal properties are a challenging issue that is at present only beginning to be understood. In this work, the syntheses of water-soluble copolymers based on vinylbenzylthymine (VBT) and ionically charged monomers (vinylbenzyl triethylammonium chloride and vinylphenylsulfonic acid sodium salt) are investigated. The study focused on the effects of the monomer type, comonomer molar ratio, monomer-to-solvent ratio, reaction temperature, and reaction time, on the molecular structure, solubility, and photoreactivity properties. Samples were taken along the reactions to determine: a) monomer conversion and solubility by gravimetric analysis, and b) molecular weight distribution (MWD) by size exclusion chromatography (SEC). The obtained VBT-based copolymers were cast onto a compatible substrate (polyethylene terephthalate), and cured by UV irradiation at room temperature. The cured material was characterized by Atomic Force Microscopy (AFM), Fourier Transform Infrared (FTIR), and UV-Vis Spectroscopy. A mathematical model was developed that simulates the synthesis of these biocompatible polymers. The model assumes the standard low temperature styrene kinetics, and calculates monomer conversion, copolymer composition, and MWD. It was validated with experimental data. The theoretical predictions are in good agreement with measurements.