Feasibility of unzipping polymer polyphthalaldehyde for extreme ultraviolet lithography

Background: Polyphthalaldehyde (PPA)-based systems can be interesting candidates for extreme ultraviolet (EUV) lithography as dry development resists with simple chemistry. Aim: We present EUV-induced mechanistic and contrast curve studies for the end-capped PPA. Approach: Fourier transform infrared (FTIR) spectroscopy and desorption studies were conducted to understand the EUV-induced mechanistic pathway. Although, contrast curve analysis was used to check its feasibility for EUV-patterning. Results: First, FTIR and desorption studies confirmed that the EUV-radiation is capable to remove the end-capping group and induce unzipping (direct depolymerization) reaction in the PPA polymer chain. Second, contrast curve analysis showed a gradual decrease in the film thickness with respect to the EUV dose, which is due to the low EUV sensitivity of the PPA polymer. A post exposure bake step is important to improve the contrast curve, as it helps to depolymerize any residual polymer remaining after exposure. Further, even though the polymer sublimates when exposed to the EUV radiation, eliminating the need to apply a wet development step, crosslinking at high doses causes deposition of residual film onto the wafer surface. Therefore, a thin film (below 20 nm) and a wet development process might be important to get clean patterns. Conclusion: This study confirms that the end-capped PPA, with a simple patterning mechanism, can be a useful system for the development of new dry development resists for EUV lithography.