Molecular design of radiation resists for next-generation lithography

Because of their short wavelength, ionizing radiations such as X-rays and electron beams are expected as exposure sources for next-generation lithography. Development of radiation resists with high sensitivity and high spatial resolution is therefore one of challenging studies for radiation chemists. We report here three types of resist polymers that are developing in our laboratory. These are chemically-superamplified, evenly-scissile, and chain-fissile ones. The chemically-superamplified resist is composed of acetalized polyvinyl alcohol as a base polymer and diaryl iodonium salt as an acid generator. Irradiation of the resist generates a strong acid and a phenyl radical from the acid generator. A radical chain reaction then takes place between the acid generator and the residual secondary alcohol on the polymer skeleton to produce the strong acid repeatedly. The generated acid acts as a catalyst for changing the acetal group to water-soluble one. Since the acid itself is amplified by the irradiation, the sensitivity to ionizing radiation is in principle much higher than regular chemically-amplified resists. The evenly-scissile resist is composed of polymer blocks of the same length that are connected with benzyl esters. Secondary electrons generated by ionizing radiations are selectively captured by the benzyl esters to dissociate into benzyl radicals and carboxylate anions. Since the scission of the polymer skeletons exclusively takes place at the benzyl esters, the resultant products show the same molecular weight, which is advantageous for increasing the sensitivity and the resolution of the resist. The skeleton of the chain-fissile polymer is composed of p-benzyl phenyl ether monomer unit with an electron acceptor at the end of the skeleton. The acceptor captures a secondary electron generated by an ionizing radiation and then dissociates into a neutral acceptor radical and a phenoxide anion. The dissociation triggers the successive dissociation of the polymer anion into monomer units, so that one electron is enough for complete fragmentation of the polymer. Although this type of polymer is categorized into chemically-amplified resists, the main difference from regular ones is that the amplification is restricted within a molecule. Increase of the sensitivity is therefore not compensating the spatial resolution. The quantum-chemical aspect for the molecular design of such a polymer will be discussed. (authors)