Mechanistic studies on the CD degradation of 193 nm resists during SEM inspection

Abstract

Pattern deformation during SEM inspection has been investigated for different types of 193 nm positive resists. The mechanism of the CD changes is discussed based on the slimming rate analyses as well as on the e-beam penetration depth. As a general tendency, the methacrylate resists exhibit faster line width reduction than the cycloolefin-maleic anhydride (COMA) systems; however, other resist components as well as CD SEM settings play an important role. Based on the exposure time vs. CD loss, the line width slimming (LWS) is found to proceed in three steps, which are assigned as: (1) chemical change of outer resist layer, (2) evaporation of volatiles and (3) bulk chain scission or deprotection. Countermeasures for CD degradation are proposed from both the formulation and process sides. A calculation of e-beam penetration depth suggests that deprotection, chain scission and other reactions occur in the first 20-40 nm, and these reaction rates combined with thermal effects determine LWS. The CD SEM measurement method has been improved to minimize e-beam exposure and to spread out the thermal load over a larger period of time. An optimized formulation exhibits less than 0.2% LWS per measurement with the improved CD measurement program.