Absorption-coefficient calculation of short-wavelength photoresist materials: From EUV to BEUV and water window X-ray.

In photolithography, shortening the exposure wavelength from ultraviolet to extreme ultraviolet (EUV, 13.5 nm) and soft X-ray region in terms of beyond EUV (BEUV, 6.X nm) and water window X-ray (WWX, 2.2-4.4 nm) is expected to further miniaturize the technology node down to sub-5 nm level. However, the absorption ability of molecules in these ranges, especially WWX region, is unknown, which should be very important for the utilization of energy. Herein, the molar absorption cross sections of different elements at 2.4 nm of WWX were firstly calculated and compared with the wavelengths of 13.5 nm and 6.7 nm. Based on the absorption cross sections in these ranges and density estimation results from the density-functional theory calculation, the linear absorption coefficients of typical resist materials, including metal-oxy clusters, organic small molecules, polymers, and photoacid generators (PAGs), are evaluated. The analysis suggests that the Zn cluster has higher absorption in BEUV, whereas the Sn cluster has higher absorption in WWX. Doping PAGs with high EUV absorption atoms improves chemically amplified photoresist (CAR) polymer absorption performance. However, for WWX, it is necessary to introduce an absorption layer containing high WWX absorption elements such as Zr, Sn, and Hf to increase the WWX absorption.