Mass-limited Sn target irradiated by dual laser pulses for an extreme ultraviolet lithography source

A thin Sn film was investigated as a mass-limited target for an extreme ultraviolet (EUV) lithography source. It was found that those energetic ions that are intrinsic with the mass-limited Sn target could be efficiently mitigated by introducing a low-energy prepulse. High in-band conversion efficiency from a laser to 13.5 nm EUV light could be obtained using an Sn film with a thickness down to 30 nm when irradiated by dual laser pulses. It was shown that the combination of dual pulse and inert Ar gas could fully mitigate ions with a low ambient pressure nearly without the penalty of the absorption of the EUV light.     

基于脉冲CO2激光锡等离子体光刻光源的极紫外辐射光谱特性研究

研究了不同条件下脉冲放电CO₂激光烧蚀平板锡靶产生的等离子体极紫外辐射特性, 设计并建立了一套掠入射极紫外平焦场光栅光谱仪, 结合X射线CCD探测了光源在6.5～16.8 nm波段的时间积分辐射光谱,得到了极紫外光谱随激光脉宽, 入射脉冲能量及背景气压的变化规律。实验结果发现：入射激光脉冲能量在30～600 mJ变化时,极紫外辐射光谱的强度随辐照激光脉冲能量的增加而增加, 但并不是线性关系, 具有饱和效应, 且产生极紫外辐射的脉冲能量阈值约为30 mJ,当激光脉冲能量为425 mJ时具有最高的转换效率,此时中心波长13.5 nm处2%带宽内的转换效率约为1.2%。激光脉冲半高全宽在50～120 ns范围内变化时, 极紫外辐射光谱的峰值位置均位于13.5 nm,光谱形状几乎没有什么变化, 但是脉宽从120 ns变到52 ns后,由于激光功率密度的提高,极紫外辐射强度也随之增强了约1.6倍。极紫外光谱的强度随背景气压的增大而迅速下降, 当腔内空气气压为200 Pa时, 极紫外辐射光子几乎被全部吸收,而当缓冲氦气气压为7×104 Pa时,仍能够探测到微弱的极紫外辐射信号,计算表明100 Pa的空气对13.5 nm极紫外光的吸收系数为3.0 m-1,而100 Pa的He气的吸收系数为0.96 m-1。     

Nonclassical hydrodynamic behavior of Sn plasma irradiated with a long duration CO2 laser pulse

It was found that the electron density scale length of Sn plasma irradiated with a long duration CO₂ laser pulse is much shorter than that predicted by the classical isothermal model. The experimentally observed small dominant region of in-band (2% bandwidth) 13.5-nm extreme ultraviolet (EUV) emission coincides with this constrained hydrodynamic behavior. The lower hydrodynamic efficiency may come from the strongly inhibited ablation mass and makes a CO₂-laser-produced Sn plasma suitable as an EUV radiation source.     

极端远紫外光刻的等离子体光源及其光学性质研究进展

现代技术的飞速发展需要集成电路不断小型化，因而开发下一代光刻光源以满足小型化的要求成为当前的一项紧迫任务。目前工业界确定的下一代光刻光源是波长为13．5nm的极端远紫外（EUV）光源，它能够把光刻技术扩展到32nm以下的特征尺寸，氙和锑材料的等离子体光源被认为是这种光源的最佳候选者。文章在介绍EUV光刻原理和EUV光源基本概念的基础上，讨论了目前研究得最多、技术最成熟的激光产生的和气体放电产生的等离子体EUV光源，对EUV光源的初步应用进行了简单介绍，并着重对氙和锑材料产生的等离子体发射性质和吸收性质的实验与理论研究进展进行了详细介绍与讨论。目前的理论研究进展表明，统计物理模型还不能很好地预测氙和锑等离子体的发射与吸收光谱，因此迫切需要发展细致能级物理模型，以得到更为精确的等离子体光学性质参数，并用于指导实验设计。提高EUV转换效率。     

Parametric optimization of a narrow-band 13.5-nm emission from a Li-based liquid-jet target using dual nano-second laser pulses

We demonstrate the applicability of a Li-based liquid jet as a regenerative source of narrow-band extreme-ultraviolet (EUV) emission at 13.5 nm. It was found that a conventionally used single laser pulse did not produce optimum plasma conditions for a low-Z target, like Li. It was shown that deployment of dual nano-second laser pulses enhanced the in-band EUV conversion efficiency (CE) at 13.5 nm in 2 sr by three times its value using a single laser pulse. Dependence of the emission spectra and EUV CE on the delay time between dual laser pulses revealed that the emission at 13.5 nm from Li ions was preferably enhanced at a lower plasma temperature compared to that at 13.0 nm from oxygen ions.     

Opacity effect on extreme ultraviolet radiation from laser-produced tin plasmas

Opacity effects on extreme ultraviolet (EUV) emission from laser-produced tin (Sn) plasma have been experimentally investigated. An absorption spectrum of a uniform Sn plasma generated by thermal x rays has been measured in the EUV range (9-19 nm wavelength) for the first time. Experimental results indicate that control of the optical depth of the laser-produced Sn plasma is essential for obtaining high conversion to 13.5 nm-wavelength EUV radiation; 1.8% of the conversion efficiency was attained with the use of 2.2 ns laser pulses.     

Demonstration of fiber-laser-produced plasma source and application to efficient extreme UV light generation

Efficient generation of extreme UV (EUV) light at lambda = 13.5 nm from a bulk Sn target has been demonstrated by using a fiber laser. The conversion efficiency from the 1064 nm IR to the EUV was measured to be around 0.9% into 2pi steradians within a 2% bandwidth. To the best of our knowledge, this is the first time an all-fiber system was used to generate EUV or soft x rays.     

Characterization and optimization of the laser-produced plasma EUV source at 13.5 nm based on a double-stream Xe/He gas puff target

The paper describes a debris-free, efficient laser-produced plasma source emitting EUV radiation. The source is based on a double-stream Xe/He gas-puff. Its properties and spectroscopic signatures are characterized and discussed. The spatio-spectral features of the EUV emission are investigated. We show a large body of results related to the intensity and brightness of the EUV emission, its spatial, temporal, and angular behavior and the effect of the repetition rate as well. A conversion efficiency of laser energy into EUV in-band energy at 13.5 nm of 0.42% has been gained. The electron temperature and electron density of the source were estimated by means of a novel method using the FLY code. The experimental data and the Hullac code calculations are compared and discussed. The source is well suited for EUV metrology purposes. The potential of the source for application in EUV lithography was earlier demonstrated in the optical characterization of Mo/Si multi-layer mirrors and photo-etching of polymers.     

Extreme-ultraviolet spectral purity and magnetic ion debris mitigation by use of low-density tin targets

We investigate the extreme-ultraviolet (EUV) emission from targets that contain tin as an impurity and the advantages of using these targets for ion debris mitigation by use of a magnetic field. The EUV spectral features were characterized by a transmission grating spectrograph. The in-band EUV emission energy was measured with a calorimeter of absolute calibration. The ion flux coming from the plume was measured with a Faraday cup. Our studies indicate that 0.5% Sn density is necessary to obtain a conversion efficiency very close to that of full-density Sn. The use of Sn-doped low-Z targets provides a narrower unresolved transition array and facilitates better control of energetic ions in the presence of a moderate magnetic field of 0.64 T.     

脉冲激光辐照液滴锡靶等离子体极紫外辐射的实验研究

采用波长13.5 nm的极紫外光作为曝光光源的极紫外光刻技术是最有潜力的下一代光刻技术之一, 它是半导体制造实现10 nm及以下节点的关键技术. 获得极紫外辐射的方法中, 激光等离子体光源凭借转换效率高、收集角度大、碎屑产量低等优点而被认为是最有前途的极紫外光源. 本文开展了脉冲TEA-CO₂激光和Nd:YAG激光辐照液滴锡靶产生极紫外辐射的实验, 对极紫外辐射的谱线结构以及辐射的时空分布特性进行了研究.实验发现: 与TEA-CO₂激光相比, 较高功率密度的Nd:YAG激光激发的极紫外辐射谱存在明显的蓝移; 并且激光等离子体光源可以认为是点状光源, 其极紫外辐射强度随空间角度变化近似满足Lambertian分布.     

Characterization of a laser-plasma extreme-ultraviolet source using a rotating cryogenic Xe target

A laser-plasma source for extreme-ultraviolet (EUV) light that uses a rotating cryogenic solid-state Xe target has been characterized. We focused on parameters at the wavelength of 13.5 nm with 2% bandwidth required for an EUV lithography source and investigated improvements of the conversion efficiency (CE). With the drum rotating, there was an increase in CE and less fast ions compared with the case for the drum at rest. It is considered that the Xe gas on the target surface can produce optimal-scale plasma, and satellite emission lines in Xe plasma effectively increase the EUV intensity, and the ion number is decreased by the gas curtain effect. The dependence of CE on the laser wavelength, laser energy and intensity also studied. As a result, the maximum CE was 0.9% at 13.5 nm with 2% bandwidth under the optimal condition. By continuous irradiation of a Nd:YAG slab laser at a repetition rate of 320 Hz and an average power of 110 W, the target continuously generated EUV light with an average power of 1 W at 13.5 nm with 2% bandwidth. The achieved performances provide valuable information for the design of a future EUV lithography source.     

Micro-pinch formation and extreme ultraviolet emission of laser-induced discharge plasma

Extreme ultraviolet(EUV) source produced by laser-induced discharge plasma(LDP) is a potential technical means in inspection and metrology. A pulsed Nd:YAG laser is focused on a tin plate to produce an initial plasma thereby triggering a discharge between high-voltage electrodes in a vacuum system. The process of micro-pinch formation during the current rising is recorded by a time-resolved intensified charge couple device camera. The evolution of electron temperature and density of LDP are obtained by optical emission spectrometry. An extreme ultraviolet spectrometer is built up to investigate the EUV spectrum of Sn LDP at 13.5 nm. The laser and discharge parameters such as laser energy, voltage, gap distance,and anode shape can influence the EUV emission.     

Mitigation of energetic ion debris from Gd plasma using dual laser pulses and the combined effect with ambient gas

For the next-generation beyond extreme ultraviolet lithography(EUVL) sources, gadolinium(Gd) plasma with emission wavelength at 6.7 nm seems to be the leading candidate. Similar to the Sn target 13.5 nm light source, ion debris mitigation is one of the most important tasks in the laser-produced Gd plasma EUV source development. In this paper,a dual-laser-pulse scheme, which uses a low energy pulse to produce a pre-plasma and a main pulse after a time delay to shoot the pre-plasma, is employed to mitigate the energetic ion generation from the source. Optimal conditions(such as pre-pulse energy and wavelength, and the time delay between the pre-pulse and the main pulse for mitigating the ion energy) are experimentally obtained, and with the optimal conditions, the peak of the ion energy is found to be reduced to1/18 of that of a single laser pulse case. Moreover, the combined effect by applying ambient gas to the dual-pulse scheme for ion debris mitigation is demonstrated, and the result shows that the yield of Gd ions is further reduced to around 1/9 of the value for the case with dual laser pulses.     

Development of a target for laser-produced plasma EUV light source using Sn nano-particles

Nano-structured and tin-based targets have been fabricated by the pulsed-laser ablation method, in order to develop efficient and debris-free targets for the laser-produced plasma extreme ultraviolet (EUV) light source at 13.5 nm. Characteristic spectra that have the radiation peak around 13.5 nm were obtained from CO₂ laser produced plasma using the films as a target. A nano-structured target produced EUV light as intense as a bulk target and a narrower line spectrum at 13.5 nm than a bulk target. PACS 32.30.Rj; 52.38.-r; 52.38.Mf; 61.46.+w; 68.37.-d     

Bi靶激光等离子体极紫外光源以及碎屑辐射特性研究

激光等离子体极紫外光源具有体积小、稳定性高和输出波长可调节等优势,在极紫外光刻领域发挥着重要的作用。Bi靶激光等离子体极紫外光源在波长9～17 nm范围内具有较宽的光谱,可应用于制造极紫外光刻机过程中所需的极紫外计量学领域。利用平像场光谱仪和法拉第杯对Bi靶激光等离子体极紫外光源以及离子碎屑辐射特性进行了实验研究。在单脉冲激光打靶条件下,实验中观察到Bi靶激光等离子极紫外光谱在波长12.3 nm处出现了一个明显的凹陷,其对应着Si L-edge的吸收,是Bi元素光谱的固有属性。相应地在波长为11.8和12.5 nm位置处产生了两个宽带的辐射峰。研究了两波长光谱特性以及辐射强度随激光功率密度的变化。结果表明,在改变聚焦光斑大小实现不同激光功率密度（0.7×1010～3.1×1010 W·cm-2）过程中,当功率密度为2.0×1010 W·cm-2时两波长处的光辐射最强,其原因归结为Bi靶极紫外光辐射强度受激光能量用于支撑等离子膨胀的损失和极紫外光被等离子体再吸收之间的平衡制约所致。在改变激光能量实现不同激光功率密度过程中,由于烧蚀材料和产生两波长所需高阶离子随着功率密度的增加而增加,增强了两波长处的光辐射。进一步,研究了双脉冲激光对Bi靶极紫外光谱辐射特性影响,实验发现双脉冲打靶下原来在单脉冲打靶时出现在波长13～14 nm范围内的凹陷消失。最后,对单脉冲激光作用Bi靶产生极紫外光源碎屑角分布进行了测量。结果表明,当探测方向从靶面法线方向移动到沿着靶面方向上的过程中,探测到Bi离子动能依次减小,并且离子动能随激光脉冲能量降低而呈线性减小。此项研究有望为我国在研制极紫外光刻机过程所需的计量学领域提供技术支持和打下夯实的基础。     

Ion debris mitigation from tin plasma using ambient gas,magnetic field and combined effects

Mitigation of energetic ions from laser produced tin plasma is one of the principal issues in the development of an extreme ultraviolet lithographic light source. We explored the possibility of using an ambient gas and/or a magnetic field for controlling the energetic ions from tin plasma. Hydrogen, helium and argon gases provide good transmission to 13.5 nm and are found to be an effective stopper for tin ions. Absorption of in-band radiation limits the density of gas below levels needed to completely protect optics. Tin ion ranges in buffer gases were estimated using the Monte Carlo simulation code SRIM and compared with experimental results. The presence of a moderate transverse magnetic field of 0.64 T slowed the propagation of fast moving tin ions but failed to stop them. The synergistic effect of a combination of magnetic field and an ambient gas is found to be very promising for mitigating tin ions without exceeding EUV gas absorption limits. PACS 42.72.Bj; 52.50.Jm; 52.55.Jd; 52.70.kz     

CO2激光锡等离子体极端紫外及可见光光谱

利用CO2激光烧蚀锡靶产生等离子体,当入射到靶面的单个脉冲能量为400mJ,半峰全宽(FWHM)为75ns时,使用光谱仪和增强型电荷耦合器件(ICCD)采集了等离子体的时间分辨光谱。在局域热平衡假设下,利用谱线的斯塔克展宽和五条Sn II谱线的相对强度计算并得到了等离子体电子密度、电子温度和辐射谱线强度随时间的变化规律;利用掠入射极端紫外平场光栅光谱仪,结合X射线CCD同时探测了光源在6.5～16.8nm波段的时间积分极端紫外辐射光谱。实验结果表明:激光点燃等离子体早期的100ns内有很强的连续谱,此后才能分辨出明显的原子和离子线状谱。在延时0.1～2.0μs的时间区间内,等离子体中的电子温度和密度分别在2.3～0.5eV和7.6×1017～1.2×1016 cm-3范围内,均随时间经历了快速下降,然后再较缓慢下降的过程。激光锡等离子体极端紫外不可分辨辐射跃迁光谱峰值中心位于13.5nm,FWHM为1.1nm。     

提高极紫外光谱纯度的多层膜设计及制备

极紫外光刻是实现22nm技术节点的候选技术。极紫外光刻使用的是波长为13.5nm的极紫外光,但在160～240nm波段,极紫外光刻中的激光等离子体光源光谱强度、光刻胶敏感度以及多层膜的反射率均比较高,光刻胶在此波段的曝光会降低光刻系统的光刻质量。从理论和实验两方面验证了在传统Mo/Si多层膜上镀制SiC单层膜可对极紫外光刻中的带外波段进行有效抑制。通过使用X射线衍射仪、椭偏仪以及真空紫外(VUV)分光光度计来确定薄膜厚度、薄膜的光学常数以及多层膜的反射率,设计并制备了[Mo/Si]40SiC多层膜。结果表明,在极紫外波段的反射率减少5%的前提下,带外波段的反射率减少到原来的1/5。     

Effect of Nd:YAG Laser Energy on Multilayer Hollow Nanofiber Target's Extreme Ultraviolet Conversion Efficiency

Extreme ultraviolet (EUV), 13.5 nm, lithography techniques have attracted a great deal of attention because of the mass production of 50 nm critical dimensions as the future generation lithography. One of key issues to be clarified for the development of a 13.5 nm EUV light source is to improve the conversion efficiency (CE). In this paper, hollow multilayer nanofibers were fabricated through the combination of a layer-by-layer (LBL) technique with the electrospinning technique. The obtained hollow fibers were employed as EUV targets. In order to improve the CE, the irradiated laser energy was increased from 10¹⁰ to 10¹¹ W/cm² and the CE was correspondingly increased from 0.43 to 0.83%.     

极紫外平场光栅光谱仪的研制和性能测试

研制了一台高分辨率极紫外光谱仪,用于磁约束等离子体诊断。采用一块具有平场特性的全息球面变线距光栅作为分光元件,光栅公称线密度为1 200 lines·mm-1,掠入射角为3°。一台可深度制冷、背照式面阵CCD作为光谱探测器,用机械快门控制曝光时间。通过CCD在光谱聚焦面的移动,可以记录的光谱范围为5～50 nm。用Penning放电光源测试了光谱仪的性能; 利用光源的标准谱线,进行了波长标定,波长精度为0.003 nm,并计算出系统各参数的实际值;当入缝宽度设置为30 μm时,在20 nm附近,光谱分辨率达0.015 nm,达到设计指标。