Molecular Glass Resists Based on 9,9′-Spirobifluorene Derivatives: Pendant Effect and Comprehensive Evaluation in Extreme Ultraviolet Lithography

A series of molecular glass compounds (SP-BOC, SP-AD, and SP-BU) based on 9,9′-spirobifluorene backbone with different kinds of pendant groups (t-butyloxycarbonyl, adamantyl ester, and t-butyl ester groups) were synthesized. The thermal analysis of the compounds indicated that no apparent glass transition temperature (T_g) was observed before the onset of the thermal decomposition temperatures (T_d) up to 150 °C. The good thermal resistance suggests that they can satisfy the lithography process and are candidates for photoresist materials. They were used as positive photoresists by mixing with minor components of photoacid generators and other additives, respectively. The amorphous films on the silicon substrate were obtained by spin-coating of the photoresists. The extreme ultraviolet (EUV) performance on the films was evaluated by using soft X-ray interference lithography. The SP-BOC resist achieves the most excellent patterning capability down to 22 nm lines with a line-edge roughness (LER) of 3.3 nm. The outgassing amounts (N_s) of SP-BOC, SP-AD, and SP-BU resists are 3.1 × 10¹⁵, 2.3 × 10¹⁴, and 2.0 × 10¹⁴ molecule/cm², respectively, at exposure dose of ∼20 mJ/cm². The results of etch resistance are in the order of SP-AD (0.5 nm/s) > SP-BU (0.8 nm/s) > PMMA (1.1 nm/s) > SP-BOC (1.6 nm/s) > Si₃N₄ (1.8 nm/s). All the results indicate a significant pendant effect on pattern ability, etching durability, and outgassing for SP resists. This study will help us to understand the relationship between the pendant structure and the EUV lithography and supply useful guidelines for designing molecular resists.