Improving the conversion efficiency of EUV emission using CO₂ reheating of Nd:YAG pre-pulse produced Sn plasma

Full text: Current photolithography methods utilize a 193 nm light source in the microfabrication of electronic circuits. While this technique has served its purpose for the past three decades, an inherent restriction on the minimum detail prescribed to an etching process is imposed by the large photon wavelength limits its continued practice. In meeting this challenge, next-generation lithography technologies are being developed to reduce the limits of wafer etching. Extreme ultraviolet lithography (EUVL) is a leading lithography technique in next-generation lithography due to the small 13.5 nm photon emission from tin and the availability of 2%-BW Mo/Si multilayer mirrors with 70% reflectivity. In order to meet the throughput demand, a high conversion efficiency of EUV light is required. Additional techniques have shown that the conversion efficiency of 13.5 nm light can be improved by exercising pre-pulse LPP of Sn followed by CO₂ laser reheat. This study is to optimize the conversion efficiency (CE) of 13.5 nm Sn-based light by varying several parameters – spot size and wavelength of pre-pulse Nd:YAG laser, and time delay between pulses. Using HEIGHTS package, the effect of spot size on EUV emission and conversion efficiency was studied. The modeling results were then compared with the experimental findings to further benchmark and optimize the experiments. Spot sizes of the Nd:YAG laser were varied to identify the optimal focus for enhanced conversion efficiency. The experiment was conducted with both single-pulse Nd:YAG and double-pulse Nd:YAG with CO₂ laser reheat. Inter-pulse delay study was conducted to identify the behavior of CE across a range of time delays, making note of the optimal time delay producing the highest CE. This procedure was then repeated operating the Nd:YAG laser at its 266 nm fourth-harmonic. The experiment was conducted using a TEA CO₂ laser (τ_p = 35 ns, λ = 10.6 μm) and Nd:YAG laser (τ_p = 6 ns, λ = 1064 nm) capable of operating at fourth harmonic. An absolutely calibrated EUV Power Tool coupled with a 1 GHz digital oscilloscope was implemented to determine the CE, and the EUV emission spectra were acquired from an EUV transmission grating spectrograph (TGS). (author)