Method of predicting resist sensitivity for 6.x nm extreme ultraviolet lithography

Potential extension of 13.5nm extreme ultraviolet lithography (EUVL) to the soft x-ray region in the 6.x nm range (6.6-6.8nm) has been discussed recently in academia and the semiconductor industry in terms of the light source, optics, and resist performance. In this study, the authors investigated the precise sensitivities of several resists (both nonchemically amplified and chemically amplified resists) using highly monochromatized soft x rays from synchrotron radiation with accurate dosimetry. The selected wavelengths included 6.7nm, which is one of the candidates for 6.x nm EUVL. Each obtained "dose/sensitivity, E (mJ cm^-2)" was converted into the "absorbed dose, D (absorbed energy per unit mass, Gray; Gy=J kg ^-1)" in terms of radiation and nuclear science. Although the absorbed dose in a resist film depends on the distance from the top surface of the resist, the required absorbed doses (D₀ or D₅₀) corresponding to the dose/sensitivities (E₀ or E₅₀) were almost constant for each resist, regardless of the exposure wavelength from 3.1 to 6.7nm. This would be applicable in the EUV/soft x-ray region, where nearly the same chemical reactions are induced. According to the obtained results, the resist sensitivities for any exposure wavelength in the EUV/soft x-ray region can be predicted easily by using the sensitivity that is measured at a certain wavelength, the resist's thickness, and the linear absorption coefficients that can be calculated using the chemical composition and density of a resist. The resist sensitivity at 6.x nm can be predicted by evaluating the sensitivity using a conventional 13.5nm EUV exposure tool. Moreover, this prediction method can be used for dose calibration of a simplified EUV exposure tool by utilizing a resist sensitivity that is obtained for a calibrated exposure source.