The laser produced plasma of Extreme Ultraviolet (EUV) sources create energetic tin ions. Hydrogen buffer gas is used to slow down the ions through scattering. The scattering cross sections are not well known and are key to modeling the device. Beam attenuation experiments of tin ions in molecular hydrogen gas are underway in order to determine its effective cross section and intermolecular potential. This can be used to model tin ion transport inside of EUV source tools. Measurements are still underway and the exact values of the interatomic potential will be the topic of future publications. Once the potential is well characterized, it will be added to the open-source binary-collision-approximation code RustBCA.
During plasma excitation of CO2 molecules in drive lasers, up to 60% of the CO2 decomposes into CO. Typically, Au is used as a catalyst to preferentially recombine CO and O radicals into CO2. By adding a secondary, microwave driven plasma to the system at the Au catalyst, O atoms can be stripped away from contaminants created in the laser such as Ox and NOx compounds. It is hypothesized that this will decrease the CO:CO2 ratio, which increases overall laser efficiency. This work serves as a status update on the measurement of CO:CO2 ratios for 4 tests: 1) control, 2) with Au catalyst installed, 3) with secondary plasma active, and 4) with Au catalyst installed and secondary plasma active.
Tin contamination of the collector mirror surface remains one of the crucial issues of EUV (Extreme Ultraviolet) sources, directly impacting the availability of the tool. Hydrogen plasma-based tin removal processes employ hydrogen radicals and ions to interact with tin deposits to form gaseous tin hydride (SnH4), which can be removed through pumping. An annular surface wave plasma (SWP) source developed at the University of Illinois—Urbana Champaign is integrated into the cone and perimeter of the collection mirror for in situ tin removal. The SWP is characterized by high ion and radical densities, low electron temperature, and local generation where etching is needed. This method has the potential to significantly reduce downtime and increase mirror lifetime. Radical probe measurements show hydrogen radical densities in the order of 1019 m−3, while Langmuir probe measurements show electron temperatures of up to 6 eV and plasma densities on the order of 1017–18 m−3. The generated ions are essential to the tin cleaning and have sufficiently low energy to cause no damage to the collector capping layer. Tin etch rates of up to 270 nm/min were observed in a variety of experimental conditions, including various powers, pressures, flowrates, and temperatures. The high etch rates demonstrated in this study exceed the expected contamination rate of the EUV source.
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