Acid–base reactions in a positive tone chemically amplified photoresist and their effect on imaging

The inclusion of base in chemically amplified resist formulations is known to improve imaging, yet the mechanism by which this occurs, and the effect of different chemical types of base is poorly understood. Previously we showed that tetrabutylammonium hydroxide, an ionic base, added to p-tert-butyloxycarbonyloxystyrene (PTBOCST)-t-butyl iodonium perfluorobutane sulfonate (TBI-PFBS) positive tone chemically amplified resist affects local acid concentrations in exposed photoresist through a proportional neutralization mechanism. We have now extended these studies to include a neutral base, 7-diethylamino-4-methylcoumarin, and show that it operates by a qualitatively different process, competitive neutralization. Using the rate coefficients obtained for both bases, we investigate the impact of the details of the neutralization kinetics on image formation in PTBOCST/TBI-PFBS resist as a function of pitch, dose, base concentration, and temperature. The results show that base dynamically reduces local acid concentrations in competition with acid-catalyzed deprotection, and does not significantly block acid diffusion as a primary process. Thus, the selection of base can be expected to play an important and complex role in the evolution of the acid and deprotected resist images during postexposure bake.