Out-of-band radiation in extreme ultraviolet (EUV) exposure tools remains one of the critical issues that must be addressed before the implementation of this lithography technique for high-volume manufacturing. The out-of-band spectrum at the intermediate focus of EUV sources that use a CO2 laser-produced plasma is dominated by scattered radiation from the drive laser, which operates near 10.6-μm wavelength. To reduce the unwanted heating of optical components in the exposure tool caused by the infrared wavelength, a 10.6-μm wavelength-absorbing gas can be used to reduce the number of photons at this wavelength reaching the intermediate focus. Gaseous sulfur hexafluoride (SF6), whose υ3 infrared active mode is vibrationally excited by radiation around 10.6-μm wavelength, can be used to function as a molecular absorber and thus, mitigate part of the infrared radiation. In this work, the optical absorption of gaseous SF6 is experimentally investigated at the CO2 laser wavelengths close to 10.6 μm as well as the EUV wavelength. Various considerations affecting the use of a gas-based spectral filter are also discussed in this paper.
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.
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