Radiation hardness of molybdenum silicon multilayers designed for use in a soft-x-ray projection lithography system

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Abstract
A molybdenum silicon multilayer is irradiated with 13.4-nm radiation to investigate changes in multilayer performance under simulated soft-x-ray projection lithography (SXPL) conditions. The wiggler–undulator at the Berlin electron storage ring BESSY is used as a quasi-monochromatic source of calculable spectral radiant intensity and is configured to simulate an incident SXPL x-ray spectrum. The test multilayer receives a radiant exposure of 240 J/mm2 in an exposure lasting 8.9 h. The corresponding average incident power density is 7.5 mW/mm2. The absorbed dose of 7.8 × 1010 J/kg (7.8 × 1012 rad) is equivalent to 1.2 times the dose that would be absorbed by a multilayer coating on the first imaging optic in a hypothetical SXPL system during 1 year of operation. Surface temperature increases do not exceed 2 °C during the exposure. Normal-incidence reflectance measurements at λ0 = 13.4 nm performed before radiation exposure are in agreement with measurements performed after the exposure, indicating that no significant damage had occurred.