The chemistry and application of nonchemically amplified (non-CA) chain-scission resists

The process of chemical amplification involves the diffusion of photolytically generated acid species and the catalytic deprotection of sufficient protecting groups to produce a change in the solubility of the resist polymer. The random walk nature of the diffusion of the acid species and hence stochastic variation in the deprotection reactions is thought to be a major contributor to the formation of rough features in patterned photoresists. On the other hand, it is well known that polymeric resists not reliant on chemical amplification, the nonchemically amplified (non-CA) resists can provide superior pattern fidelity and reduced line-edge roughness (LER). However, non-CA resists generally suffer from poor sensitivity compared with the chemically amplified variants. Very sensitive non-CA systems have been introduced; however, these materials tend to have inferior etch resistance making pattern transfer impossible. As with every aspect of this industry the selection of the most appropriate material involves a trade-off of properties. With an increasing recognition that LER may present a significant barrier to progression to anticipated lithographic nodes, the slow photospeed of non-CA resists is becoming less of an obstacle, and they are therefore being re-examined for sub-10-nm patterning. In this review, we summarize the history and development of chain-scission non-CA resists in particular and their performance at 193 nm and for extreme ultraviolet and electron beam lithography.