Gas-based spectral filter for mitigating 10.6 μm radiation in CO 2 laser produced plasma extreme ultraviolet sources

Abstract

Next generation high volume manufacturing lithography tools will likely use CO₂ laser produced plasma sources to generate extreme ultraviolet (EUV) radiation needed for resist exposures. Existing mitigation techniques for out-of-band radiation from these sources result in reduced EUV (13.5 nm) transmission to the resist plane which decreases desired throughput. New methods to suppress the 10.6 μm radiation, which dominates the out-of-band spectrum at the intermediate focus (IF), need to be examined. A spectral filter design that uses an infrared absorbing gas to target the mitigation of 10.6 μm in these EUV tools may provide another alternative to suppress the unwanted radiation. This work explores infrared absorption of gases at 10.6 μm while focusing on gaseous sulfur hexafluoride (SF₆) whose υ₃ infrared active mode is vibrationally excited by 10.6 μm photons. A compact tunable CO₂ laser is used to measure the room temperature, low fluence absorption of SF₆ in the range of 10.53-10.65 μm. In addition, the EUV transmission of SF₆ as a function of pressure is estimated based on the absorption cross section measured for wavelengths between 11-17 nm. Design considerations such as the EUV transmission vs. infrared absorption tradeoff are discussed.