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。     

Ablation of polymers by focused EUV radiation from a table-top laser-produced plasma source

We have investigated ablation of polymers with radiation of 13.5 nm wavelength, using a table-top laser produced plasma source based on solid gold as target material. A Schwarzschild objective with Mo/Si multilayer coatings was adapted to the source, generating an EUV spot of 5 μm diameter with a maximum energy density of ∼1.3 J/cm². In combination with a Zirconium transmission filter, radiation of high spectral purity (2% bandwidth) can be provided on the irradiated spot. Ablation experiments were performed on PMMA, PTFE and PC. Ablation rates were determined for varying fluences using atomic force microscopy and white light interferometry. The slopes of these curves are discussed with respect to the chemical structure of the polymers. Additionally, the ablation behavior in terms of effective penetration depths, threshold fluences and incubation effects is compared to literature data for higher UV wavelength.     

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     

由强激光产生的烧蚀等离子体随时间演化的数值模拟

采用单流体模型描述由强激光产生的烧蚀等离子体,并用具有五阶精度的广义Godunov差分格式——加权本质无振荡格式对该模型进行离散化,考虑激光与等离子体相互作用和能量耦合,数值模拟强激光与固体靶相互作用时产生的烧蚀等离子体随时间演化的物理过程,给出数值模拟结果,并对其进行分析和讨论.数值模拟结果表明,激光能量在靶面等离子体中被强烈吸收,激光支持LSD(Laser Supported Detonation)波速度约为理想LSD波速度的一半.     

脉冲CO2激光烧蚀锡靶等离子体的数值研究

使用一维辐射流体力学程序MULTI模拟了脉冲CO2激光烧蚀平面锡靶的过程,研究了脉冲宽度、峰值功率密度、靶材初始密度对锡等离子体电子密度、电子温度的时空分布的影响,并结合统计分析得到最有利于产生13.5 nm 极紫外光的激光脉冲宽度。模拟结果表明,脉冲宽度为100~200 ns的长脉冲激光产生的等离子体有利于实现极紫外输出的最佳条件,通过分析等离子体的电子密度、电子温度的分布对这一结论进行了解释。临界电子密度区域有效吸收了脉冲能量,而低密度的羽辉对激光与极紫外辐射的吸收很少。采用长脉冲激光,使得辐射极紫外等离子体持续时间更长,是提高极紫外辐射效率的有效手段。同时模拟还发现,靶材初始密度对等离子体参数的影响不大。     

Plasma-based X-ray laser at 21 nm for multidisciplinary applications

An overview of recent advances in applications of currently the most energetic X-ray laser at 21 nm is given. The unique parameters of this half-cavity based X-ray laser such as record output energy of 10 mJ, highly symmetric beam, robustness and reproducibility, have made it possible to carry out a number of multidisciplinary scientific projects featuring novel applications of intense coherent X-ray radiation. Selected results obtained in these experiments are reviewed, including X-ray laser probing of dense plasmas, measurements of transmission of focused soft X-ray radiation at intensities of up to 10¹² W cm^-2, measurements of infrared laser ablation rates of thin foils, and ablative microstructuring of solids.     

基于高次谐波产生的极紫外偏振涡旋光

突破传统涡旋光场束缚,发展短波极紫外涡旋光场是实现阿秒脉冲偏振控制的有效途径.本研究利用自制的平场光栅光谱仪和超快时间保持的单色仪,以800 nm,35 fs高斯或具有偏振奇点的涡旋光脉冲驱动诱导氩原子产生高次谐波,分别获得相应的高次谐波光谱以及谐波谱单阶光源的分布.实验结果表明,基于高次谐波产生实现近红外波段的涡旋光束特性转移到极紫外波段,优化后的极紫外涡旋可以实现每秒108光子数输出.同时发现极紫外波段的涡旋场和高斯场高次谐波产生具有相似相位匹配机制.基于高次谐波产生的极紫外波段的偏振涡旋光为探究和操控原子分子量子态的含时演化动力学以及形成阿秒矢量光束提供了重要的方法和技术手段.     

激光烧蚀高纯Zn形成的微米金属球体对后续脉冲激光的耦合增强效应

脉冲激光束在低真空(约2 Pa)环境下聚焦到高纯Zn靶表面, 烧蚀区域不仅有中心深孔的宏观损伤, 而且还发现大量微米量级的类似足球形状的金属Zn球体结构附着生长在孔洞内侧表面. 实验过程中采用等离子体光谱诊断技术研究宏观和微观损伤对后续脉冲激光的影响程度. 与聚焦于金属Zn平滑表面相比, 宏观损伤可以使后续激光诱导的Zn原子334.5 nm谱线强度提高10.3%, 在此基础上大量Zn微米球体附着在内表面可以使谱线强度再提高34.3%. 因此, 推断这些金属Zn微球表面镶嵌着光洁的纳米量级六边形和五边形小平面, 可以对后续脉冲激光产生镜面反射, 使得激光能量汇聚并耦合增强, 提高烧蚀效率. 实验结果还表明, 这些微米球体的数目随着激光脉冲次数的增加而增多, 使得后续激光能够诱导产生更为致密高温的等离子体. 研究结果有望为激光-金属微孔技术提供新思路. 关键词： 脉冲激光烧蚀 微纳米结构 激光诱导等离子体     

Single-state measurement of electrical conductivity of warm dense gold

We report on a single-state measurement of electrical conductivity of warm dense gold in the solid to plasma transition regime. This is achieved using the idealized slab plasma approach of isochoric heating of ultrathin samples by a femtosecond laser, coupled with femtosecond probe measurements of reflectivity and transmission. The experiment also reveals the time scale associated with the disassembly of laser heated solid.     

Isochoric heating of solid-density matter with an ultrafast proton beam

A new technique is described for the isochoric heating (i.e., heating at constant volume) of matter to high energy-density plasma states (>10(5) J/g) on a picosecond time scale (10(-12)sec). An intense, collimated, ultrashort-pulse beam of protons--generated by a high-intensity laser pulse--is used to isochorically heat a solid density material to a temperature of several eV. The duration of heating is shorter than the time scale for significant hydrodynamic expansion to occur; hence the material is heated to a solid density warm dense plasma state. Using spherically shaped laser targets, a focused proton beam is produced and used to heat a smaller volume to over 20 eV. The technique described of ultrafast proton heating provides a unique method for creating isochorically heated high-energy density plasma states.     

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.     

铀原子多色三光子共振电离谱研究

利用Ｎｄ：ＹＡＧ二倍频激光同步泵浦的脉冲染料激光系统、原子束装置、飞行时间质谱和Ｂｏｘｃａｒ技术测量了＾２３８Ｕ的多色三光子共振电离谱。研究了电场对铀原子第一激发态的影响。解决了如何确定三角三光子共振电离谱图中所含的单色,双色和三色共振电离峰的问题。     

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。     

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.     

Dynamics of femtosecond laser-produced plasma ions

We investigated the ion laser-produced plasma plume generated during ultrafast laser ablation of copper and silicon targets in high vacuum. The ablation plasma was induced by ≈50 fs, 800 nm Ti:Sa laser pulses irradiating the target surface at an angle of 45°. An ion probe was used to investigate the time-of-flight profiles of the emitted ions in a laser fluence range from the ablation threshold up to ≈10 J/cm². The angular distribution of the ion flux and average velocity of the produced ions were studied by moving the ion probe on a circle around the ablation spot. The angular distribution of the ion flux is well described by an adiabatic and isentropic model of expansion of a plume produced by laser ablation of solid targets. The angular distribution of the ion flux narrows as the laser pulse fluence increases. Moreover, the ion average velocity reaches values of several tens of km/s, evidencing the presence of ions with kinetic energy of several hundred eV. Finally, the ion flux energy is confined in a narrow angular region around the target normal.     

Lattice Boltzmann model for multi-component mass transfer in a solid oxide fuel cell anode with heterogeneous internal reformation and electrochemistry

We have deposited SiC thin films using two different deposition techniques, Pulsed Electron Deposition (PED) and Pulsed Laser Deposition (PLD). The PED technique is a relatively new technique where a pulsed electron flux with high energy directly hits the target surface producing a plasma in a very similar way to PLD, where, instead, a pulsed laser beam is used. These two techniques can give very dense thin films with stoichiometry much closer to the target one’s compared to others physical vapour deposition techniques (sputtering, e-beam). The major drawback of PED is that the thin film surface is affected by the presence of particulate, due to the impact of the high energy electron beam with the target. In the PLD system we used a magnetic field to curve the plasma path in the vacuum and we placed the substrate at 90^○ with respect to the target allowing the deposition of particulate-free samples. The characterization of the films has been made by measuring the optical reflectance vs angle of incidence in the EUV region (from 121.6 nm down to 40.7 nm), taking measurements at different time from deposition. X-ray photoemission measurements have been also carried out to show stoichiometry and the presence of contaminants. Other measurements such as X-ray diffraction, atomic force microscopy and profiling were also carried out to check crystalline domains and surface roughness.     

Maximum material thickness for extreme ultra-violet and X-ray backlighter probing of dense plasma

Extreme ultra-violet (EUV) lasers, X-ray lasers and other backlighter sources can be used to probe high-energy density materials if their brightness can overcome self-emission from the material. We investigate the maximum plasma thickness of aluminum, silicon and iron that can be probed with EUV or X-ray photons of energy 89–1243 eV before self-emission from the plasma overwhelms the backlighter output. For a uniform plasma, backlighter transmission decreases exponentially with increasing thickness of the material following Beer's law at a rate dependent on the plasma opacity. We evaluate the plasma opacity with the Los Alamos TOPS opacity data. The self-emission is assumed to be either that of a black body to arise from a plasma in LTE or to only consist of free–free and free–bound emission. It is shown that at higher plasma temperature (?40 eV), EUV radiation (e.g. photon energy=89 eV) can probe a greater thickness of plasma than X-ray radiation (e.g. photon energy=1243 eV).     

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。     

Short-laser-pulse-driven emission of energetic ions into a solid target from a surface layer spalled by a laser prepulse

An efficient emission of picosecond bunches of energetic protons and carbon ions from a thin layer spalled from a organic solid by a laser prepulse is demonstrated numerically. We combine the molecular dynamics technique and multi-component collisional particle-in-cell method with plasma ionization to simulate the laser spallation and ejection of a thin (∼20–30 nm) solid layer from an organic target and its further interaction with an intense femtosecond laser pulse. In spite of its small thickness, a layer produced by laser spallation efficiently absorbs ultrashort laser pulses with the generation of hot electrons that convert their energy to ion energy. The efficiency of the conversion of the laser energy to ions can be as high as 20%, and 10% to MeV ions. A transient electrostatic field created between the layer and surface of the target is up to 10 GV/cm. Received: 13 March 2001 / Accepted: 20 March 2001 / Published online: 20 June 2001     

Neutrals' temperature in laser-generated plasma at LNS

A Nd–YAG laser operating at 532 nm with a maximum intensity of 10¹⁰ W/cm² was used to ablate aluminium and tantalum targets placed in vacuum.

A mass quadrupole spectrometer (MQS) at high sensitivity, operating in the range of 1–300 amu, with a resolution better than 1 amu, was used to analyse the atomic emission produced by the laser ablation. The neutrals' emission produced by laser-generated plasma at INFN-LNS was investigated in terms of temperature, ablation yield, angular distribution and velocity.

The neutrals' detection through the MQS permitted to measure the mass energy distribution. Results demonstrate that the maximum temperatures of the neutral species are of the order of 100 eV. The angular emission of neutrals is peaked along the normal to the target surface, as it was observed for the ions; the ablation yield increases suddenly at low laser intensity and decreases at high laser intensity, owing to the higher ionization processes; the flow velocity follows the adiabatic expansion of the plasma in vacuum and it is of the order of 10⁴ m/s.

Measurements will be presented and discussed, according to the available models.