Resist polyphthaldehyde

Ito has also extended this type of photochemistry to the electron-beam-induced catalytic acidolysis of acid-labile main chain acetal linkages in polyphthaldehyde. These polymers, like the poly(2-methylpentene-l-suIfone) (PMPS) sensitizer in NPR resist described earlier have ceiling temperatures on the order of -40 °C. As normally used, the polyaldehydes are end-capped by acylation or alkylation and are thus quite stable. The main chain bonds are very sensitive to acid-catalyzed cleavage which in turn allows the whole chain to revert to monomer in an unzipping sequence similar to that occuring in irradiated PMPS. Irradiation of polyphthaldehyde containing 10% of a suitable sensitizer such as triphenylsulfonium hexafluoroarsenate with either deep UV... 

Faced with the shortcomings of the polyphthaldehyde resist (presented helow in chemical amplification resists based on depolymerization), the search for chemically amplified DUV resists resulted in a quick switch to more stable materials based on poly(p-hydroxystyrene), a phenolic polymer that Willson et al. were studying as a potential replacement for novolac. They observed that poly(p-tert-butoxycarbonyloxystyrene) (PBOCST), which is poly(vinyl phenol) protected with tert-butoxycarbonyl groups (t-BOC), is far more stable than the unprotected p-hydroxystyrene and could be purified and polymerized under controlled conditions. The resulting protected polymer could be easily deprotected thermally by heating it to 200°C or to a much lower temperature (100°C) by treatment with acid generated from the exposure of onium salts, just as in the poly(phthaldehyde)... 

The requirement of the presence of the polymerization catalyst in the depolymerization process stems from the principle of microscopic reversibility. If, for example, all free radicals are removed from the system by simply endcapping the polymer, the thermodynamic equilibrium dictates that depolymerization state cannot be reached and the system will be stable. This was the approach employed by Ito and Willson in stabilizing polyphthaldehyde resists. [See for example, C.G. Willson, H. Ito, J.M.J. Frechet, T.G. Tessier, F.M. Houlihan, Approaches toward the design of radiation sensitive polymeric imaging systems with improved sensitivity and resolution, J. Electro chem. Soc. 133, 181 (1986)]. 

Scheme 7.48 Endcapping of polyphthaldehyde resist polymers in order to prevent unwanted depolymerization.

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