The thermal deprotection process in an e‐beam exposed phenolic‐based polymer

In our current work, we have examined the e‐beam sensitivity and thermal processing behavior of pure PBOCST [poly(tert‐butoxycarbonyloxystyrene)]. An alkali soluble latent image can be generated in the PBOCST by e‐beam exposure followed by a post‐exposure bake (PEB) at relatively high temperatures (140 to 150°C). Pattern wet development operates on the same basis as a PBOCST/acid generator two component system. Deprotection, accelerated by the exposure, yields alkali soluble areas, while unexposed areas remain insoluble. Sensitivities of 5 to 10°C/cm 2 and contrasts of 6 to 10 have been measured. Complete TBOC deprotection results in a film thickness loss of 35 to 40% as CO 2 and isobutene are evolved. A laser interferometer/hotplate setup was used to track film thickness with time during the thermal deprotection process. IR analysis confirmed the correlation between conversion and thickness. It was found that a slow initial conversion rate was followed an exponentially steep rise in rate after 50% conversion. Activation energies were on the order of 30 kcal/mol. It was concluded that thermolysis in both exposed and unexposed resist occurred during the PEB, but that the difference in extent of conversion on exposure was sufficient to generate a pattern.