Boosting blister formation on EUV masks by hydrogen and water vapor

Abstract

Abstract: Blister formation on extreme ultraviolet (EUV) masks by EUV-induced plasma irradiation is a problem that causes patterning errors. We investigated the blister formation under EUV-induced plasma containing hydrogen and water vapor through an accelerated mask lifetime test. Background: Interaction between hydrogen gas and EUV radiation generates hydrogen ions and radicals which react with contaminants. Hydrogen gas is introduced in EUV scanners to remove contaminants, but EUV-induced hydrogen plasma forms blisters on EUV masks. Accelerated mask lifetime tests have reported on EUV-induced hydrogen plasma durability but the effect of water vapor on blister formation remains unclear. Aim: We evaluate the impact of water vapor on blister formation through EUV accelerated mask lifetime tests at NewSUBARU synchrotron radiation facility. Approach: The EUV intensity was in the range of approximately 20 to 47 W/cm², and hydrogen pressure was varied from 5 to 70 Pa with and without water vapor during 10^-3 to 10^-1 Pa. The hydrogen pressures were controlled, and the EUV dose was determined from the blister formation time in accelerated mask lifetime tests. The surface oxidation of the irradiated positions on the EUV masks was investigated by X-ray absorption spectroscopy (XAS), Rutherford backscattering spectrometry (RBS), and hydrogen forward scattering spectrometry (HFS). Results: The blister formation dose decreased from 500.0 to 229.6 kJ/cm² as the hydrogen pressure increased from 15 to 70 Pa. Adding water vapor at 10⁻³ Pa lowered the dose at every hydrogen pressure and triggered blister formation at 5 Pa. Hydrogen was found to penetrate through the absorber and the Ru capping layer into the Mo/Si multilayer at the irradiated positions. Conclusion: We clarify that the water vapor significantly affects the EUV dose and hydrogen pressure thresholds for blister formation. The result provides insight into plasma-induced oxidation and contributes to mask process development.