双激光脉冲打靶形成Gd等离子体的极紫外光谱辐射

Characteristics of extreme ultraviolet emission from Gd plasma produced by dual pulse laser

高端芯片制造所需要的极紫外光刻技术位于我国当前面临35项“卡脖子”关键核心技术之首.高转换效率的极紫外光源是极紫外光刻系统的重要组成部分.本文通过采用双激光脉冲打靶技术实现较强的6.7 nm极紫外光输出.首先,理论计算Gd^(18+)-Gd^(27+)离子最外层4d壳层的4p-4d和4d-4f能级之间跃迁、以及Gd^(14+)-Gd^(17+)离子最外层4f壳层的4d-4f能级之间跃迁对波长为6.7 nm附近极紫外光的贡献.其后开展实验研究,结果表明,随着双脉冲之间延时的逐渐增加,波长为6.7 nm附近的极紫外光辐射强度呈现先减弱、后增加、之后再减弱的变化趋势,在双脉冲延时为100 ns处产生的极紫外光辐射最强.并且,在延时为100 ns处产生的光谱效率最高,相比于单脉冲激光产生的光谱效率提升了33%.此外,发现双激光脉冲打靶技术可以有效地减弱等离子体的自吸收效应,获得的6.7 nm附近极紫外光谱宽度均小于单激光脉冲打靶的情形,且在脉冲延时为30 ns时刻所产生的光谱宽度最窄,约为单独主脉冲产生极紫外光谱宽度的1/3.同时,Gd极紫外光谱的变窄提高了波长为6.7 nm(0.6%带内)附近的光谱利用效率.

The extreme ultraviolet(EUV)lithography technology,which is required for high-end chip manufacturing,is the first of 35“neck stuck”key core technologies that China is facing currently.The EUV source with high conversion efficiency is an important part of EUV lithography system.The experiment on dual-pulse irradiated Gd target is carried out to realize the stronger 6.7 nm EUV emission output.Firstly,we compute the contribution of transition arrays of the form 4p-4d and 4d-4f from their open 4d subshell in charge states Gd^(18+)−Gd^(27+),and transition arrays of the form 4d-4f from their open 4d subshell in charge states Gd^(14+)−Gd^(17+) on the near 6.7 nm EUV source.Subsequently,the experimental results of the dual pulse laser irradiated Gd target show that the intensity of 6.7 nm peak EUV emission decreases first,then increases and drops again due to the plasma density decreasing gradually when the delay time between the pre-pulse and main-pulse increases from 0−500 ns.The strongest intensity of 6.7 nm peak EUV emission is generated when the delay time is 100 ns.At the same time,the spectrum efficiency is higher when the delay time is 100 ns,which is 33%higher than that of single pulse laser.In addition,the experimental results show that the half width of EUV spectrum produced by dual pulse in the delay between 10−500 ns is narrower than that of signal laser pulse due to the fact that the method of dual pulse can suppress the self-absorption effect.The half width is the narrowest when the delay is 30 ns,which is about 1/3 time of EUV spectrum width generated by a single pulse.At the same time,the narrowing of Gd EUV spectrum improves the spectral utilization efficiency near 6.7 nm wavelength(within 0.6%bandwidth).