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 as a background gas 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 residual 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 45 W/cm², and hydrogen pressure was varied from 5 to 70 Pa with and without water vapor at around 10-3 Pa. The hydrogen pressures were controlled, and the EUV exposure 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 test revealed the dependence of blister formation on hydrogen pressure, both with and without water vapor. We observed enhanced blister formation and greater surface oxidation while applying water vapor, compared to introducing hydrogen alone on EUV masks. Conclusion: We clarify that the residual water vapor significantly affects the EUV dose and hydrogen pressure thresholds for blister formation under scanner-relevant conditions. The result provides insight into plasma-induced oxidation and contributes to mask process development.