Molecular contamination mitigation in EUVL by environmental control

EUVL tools operate under vacuum conditions to avoid absorption losses. Under these conditions, the MoSi multilayer mirrors are contaminated, resulting in reduced reflection and thus throughput. We report on experiments on MoSi mirrors exposed to EUV radiation from a synchrotron. To mimic the effects of EUV radiation we also exposed samples using an electron gun. The oxidation rate was found to be similar to0.016 nm/h per mW/mm(2) of EUV radiation under conditions expected for a high throughput EUVL system. This oxidation can to a large extent be suppressed by using smart gas blend strategies during exposure, e.g. using ethanol. A carbon growth rate of 0.25 nm/h was found for a hydrocarbon pressure of 10(-9) mbar Fomblin. We demonstrate that carbonisation can be suppressed by admitting oxygen during electron gun exposure. (C) 2002 Elsevier Science B.V. All rights reserved.