Wu Tao, Wang Xin-Bing, Wang Shao-Yi, Lu Pei-Xiang
Wuhan National Laboratory for Optoelectronics, College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.
Guang Pu Xue Yu Guang Pu Fen Xi. 2012 Jul;32(7):1729-33.
The extreme ultraviolet (EUV) emission characteristics from Sn plasma for lithography produced by a pulse discharge CO2 laser was investigated under different conditions. Extreme ultraviolet spectral measurements were made throughout the wavelength region of 6.5 nm to 16.8 nm using a grazing incidence flat-field spectrograph coupled with an X-ray charge-coupled device camera for detection of time-integrated spectra. The dependence of spectral properties of the EUV emission on pulse duration, incidence pulse energy, and buffer gas pressure was investigated. The results show that the peak of EUV spectra was located at 13.5 nm. The intensity of EUV emission increased with increasing laser energy ranging from 30 mJ to 600 mJ in a nonlinear manner with saturation effect. The critical energy of incident pulse laser for the generation of EUV emission is near 30 mJ in our experiment. The highest conversion efficiency of 1.2% in producing 13.5 nm EUV light with 0.27 nm bandwidth was achieved at pump energy of 425 mJ. The EUV spectra from a plate target produced by laser pulse with full width at half maximum range from 50 ns to 120 ns were recorded and negligible differences in their spectral features noticed even though higher spectral intensity was observed by shorter pulse duration. The 2% in-band EUV intensity with 52 ns pulse duration was 1.6 times higher than that with 120 ns pulse duration due to the increase in laser intensity. It was also found that the detected EUV spectral intensity rapidly decreased with increasing buffer air pressure, and the EUV emission could be totally absorbed at the pressure of 200 Pa, while weak EUV emission could be still detected at the buffer He gas pressure of 7 x 10(4) Pa. The experimental results showed that the absorption coefficient of 13.5 nm light at air buffer gas pressure of 100 Pa was 3.0 m(-1), while the absorption coefficient was 0.96 m(-1) at the same He buffer gas pressure.
研究了脉冲放电二氧化碳激光产生的用于光刻的锡等离子体在不同条件下的极紫外（EUV）发射特性。使用掠入射平场光谱仪结合X射线电荷耦合器件相机，在6.5纳米至16.8纳米的整个波长范围内进行极紫外光谱测量，以检测时间积分光谱。研究了EUV发射的光谱特性对脉冲持续时间、入射脉冲能量和缓冲气体压力的依赖性。结果表明，EUV光谱的峰值位于13.5纳米处。EUV发射强度随着激光能量从30毫焦增加到600毫焦以非线性方式增加，并具有饱和效应。在我们的实验中，产生EUV发射的入射脉冲激光的临界能量接近30毫焦。在泵浦能量为425毫焦时，产生带宽为0.27纳米的13.5纳米EUV光的最高转换效率达到1.2%。记录了半高宽范围从50纳秒到120纳秒的激光脉冲产生的平板靶的EUV光谱，尽管较短脉冲持续时间观察到更高的光谱强度，但它们的光谱特征差异可忽略不计。由于激光强度增加，脉冲持续时间为52纳秒时的2%带内EUV强度比脉冲持续时间为120纳秒时高1.6倍。还发现，检测到EUV光谱强度随着缓冲空气压力增加而迅速下降，在200帕的压力下EUV发射可被完全吸收，而在7×10⁴帕的缓冲氦气压力下仍可检测到微弱的EUV发射。实验结果表明，在100帕的空气缓冲气体压力下，13.5纳米光的吸收系数为3.0米⁻¹，而在相同的氦缓冲气体压力下吸收系数为0.96米⁻¹。
