Estimating the out-of-band radiation flare levels for extreme ultraviolet lithography

Abstract

For the commercialization of extreme ultraviolet lithography (EUVL), discharge or laser-produced, pulsed plasma light sources are being considered. These sources are known to emit into a broad range of wavelengths that are collectively referred to as out-of-band (OOB) radiation by lithographers. Multilayer EUV optics reflect OOB radiation emitted by the EUV sources onto the wafer plane, resulting in unwanted background exposure of the resist (flare) and reduced image contrast. The reflectivity of multilayer optics at the target wavelength of 13.5 nm is comparable to that of their reflectivity in the deep ultraviolet (DUV) and UV regions from 100 to 350 nm. The aromatic molecular backbones of many of the resists used for EUV are equally absorptive at specific DUV wavelengths as well. To study the effect of these wavelengths on imaging performance in a real system, we are in the process of integrating a DUV source into the Sematech Berkeley 0.3-NA microfield exposure tool (MET). We present the simulation-based imaging results predicting the potential impact of OOB based on known resist, mask, and multilayer conditions. It should be noted that because the projection optics work equally well as imaging optics at DUV wavelengths, OOB radiation cannot be treated simply as uniform background or DC flare.