Synthesis and characterization of photobase generators for pitch division photolithography
Pitch division lithography is a resolution enhancing technique that doubles the resolution of features in a photomask when recorded in photoresist without imposing costly additional steps or demanding manufacturing equipment. In conventional chemically amplified photolithography, acid is generated when the photoresist is exposed to ultraviolent radiation. The photogenerated acid then catalyzes chemical reaction that generates a solubility change in the photoresist polymer. Once a sufficient quantity of acid is produced, known as the threshold acid concentration, the resist becomes soluble in a developing solution. A pitch division resist generates acid at medium UV doses, but at high dose, the acid is neutralized. This results in effectively crossing the threshold acid concentration twice, and produces two features instead of one. Pitch division performance is accomplished by incorporating a photobase generator (PBG) into the resist. The amount of PBG is in excess of the photoacid generator, but the rate of base generation must be less than that of acid generation Pitch division lithography works well with typical photobase generators. However, the line edge roughness of the resulting features is unacceptable for industrial applications. One cause of unsatisfactory line edge roughness is a low chemical contrast at the threshold acid concentration. The acid concentration in a pitch division photoresist does not change as sharply as it does in a conventional resist due to partial quenching of acid in medium dose regions by the photo-generated base. This problem could be remedied by creating a photobase generator that has a built in delay in onset of base production and then accelerates as a function of increasing dose. To achieve this type of higher order kinetics, we propose a two-stage photobase generator that must convert to an active form before generating base. Various novel two-stage photobase generators have been synthesized and are currently undergoing kinetic and imaging characterization. This thesis will describe the characterization of first generation photobase generators as well as the synthesis and ongoing characterization efforts of second generation PBGs.