Laser-induced liquid tin discharge plasma and its EUV spectra

Laser-induced discharge plasmas(LDPs) have the potential to be inspection and metrology sources in extreme ultraviolet(EUV) lithography. An LDP EUV source was developed to avoid tin electrode erosion in which a tin pool was used as a cathode. A CO_2 pulse laser was focused on the liquid tin target surface, and then a breakdown occurred in a very short time. The voltage-current characteristics of the discharge oscillated, lasting for several microseconds, and an RLC fitting model was used to obtain the inductance and resistance. An intensified chargecoupled device(ICCD) camera was used to investigate the dynamics of LDP, which can explain the formation of a discharge channel. The EUV spectra of laser-induced liquid tin discharge plasma were detected by a grazing incident ultraviolet spectrometer, compared with a laser-produced tin droplet plasma EUV spectrum. To explain the EUV spectrum difference of laser-induced liquid tin discharge plasma and laser-produced tin droplet plasma,the collision radiation(CR) model combined with COWAN code was used to fit the experimental EUV spectrum, which can estimate the electron temperature and density of the plasma.     

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.     

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。     

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。     

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。     

Effect of focal spot size on in-band 13.5 nm extreme ultraviolet emission from laser-produced Sn plasma

The effect of focal spot size on in-band 13.5 nm extreme ultraviolet (EUV) emission from laser-produced Sn plasmas was investigated for an EUV lithography light source. Almost constant in-band conversion efficiency from laser to 13.5 nm EUV light was noted with focal spot sizes from 60 to 500 microm. This effect may be explained by the opacity of Sn plasmas. Optical interferometry showed that the EUV emission must pass through a longer plasma with higher density when the focal spot is large, and strong reabsorption of EUV light was confirmed by a dip located at 13.5 nm in the spectrum.     

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.     

Xe介质极紫外光源时间特性及最佳条件研究

理论和实验上研究了Xe介质毛细管放电极紫外光源等离子体时间特性和最佳条件. 从理论上建立了Xe介质一维辐射磁流体力学模型,模拟了不同气压和电流条件下等离子体压缩和辐射特性;实验上测量了放电电流30 kA时不同气压条件下13.5 nm （2%带宽）动态特性. 理论和实验结果表明：不同放电电流条件下,存在最佳气压值,最佳气压随着电流的增加而增加;同时,电流增加时,13.5 nm （2%带宽）辐射光强峰值时刻减小. 关键词： 极紫外光刻光源 毛细管放电 磁流体力学 Xe等离子体     

基于双程放大的毛细管放电69.8 nm激光增益特性

建立了类氖氩C线69.8 nm激光的双程放大实验后反射腔结构,利用45 cm长毛细管作为放电负载得到了其双程放大输出.在相同初始实验条件下,分别测量了单程放大输出与双程放大输出的激光脉冲光强、脉冲宽度以及激光束散角.通过对比单程与双程输出实验结果,利用双程放大激光光强的计算公式,分析得到了增益持续时间大于4 ns,以及增益在毛细管径向上的分布特点.以上结果为建立谐振腔进行毛细管放电类氖氩激光的多程放大实验奠定了基础.     

Observations of extreme ultraviolet emission from plasma produced by capillary discharges

This study reports the results of a pilot experiment concerning observations of extreme ultraviolet emission from plasma produced by the capillary discharges. A few kA current was applied across the gas-filled alumina capillary (1 mm diameter and 8 mm long) to generate radiation in the EUV region (12–63 nm). Spectroscopic studies were carried out by means of a XEUV spectrometer which was upgraded for special lithography purposes. The results obtained from the EUV spectroscopic measurements provided information about the radiation processes from xenon and argon plasma and testifies that given capillary is an effective source of EUV emission. Additionally we showed a simulation which describes plasma dynamics parameters and dynamics of various ionization stages in capillary discharge. Our computer simulation confirmed the presence of ions, which spectra was registered in the experiment.     

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.     

Pinch Plasma Radiation Sources for the Extreme Ultraviolet

Extreme ultraviolet lithography (EUVL) is under discussion to be implemented in the production of chips as early as 2005 to 2007 for reducing structures in semiconductor devices to below 70 nm. The challenging task of developing optical components and radiation sources within this short period of time is pushing technology. As discharge produced and laser produced plasmas are the main candidates for EUV‐sources, plasma technology is forced to leap forward significantly. Progress in EUV‐sources is expected to open EUV‐technology for other applications in science and technology with increased need for spatial resolution, elemental contrast or sensitivity. Various technical concepts for realising high power sources for EUV lithography are under investigation world‐wide. Laser produced and discharge produced plasmas are the most promising schemes. Discharge produced pinch plasmas in general are of special interest, because their prospected costs (esp. cost of ownership) for the demanded throughput is expected to be much lower than with laser produced plasmas. However, the discharge plasmas are of high risk, because many crucial tasks have to be solved before reaching the required power levels. Beside the optimisation of the EUV‐generation ‐ which is the key to build the most reliable device with demanded EUV power ‐ the success will depend on the individual technical aspects of each source concept. Currently investigated pinch plasma concepts are evaluated based on their potential to be upgraded to fulfil the challenging demands of EUV‐lithography.     

Pump–probe studies of EUV and X-ray emission dynamics of laser-irradiated noble gas droplets

The interaction of high intensity 100-ps laser pulses with micron-sized noble gas (argon and krypton) droplets is experimentally investigated via a series of pump–probe experiments monitoring the delay-dependent X-ray and extreme ultraviolet (EUV) emission, and by imaging frequency-doubled probe light scattered from the interaction region. An understanding of the time scales for this interaction is important for optimization of EUV sources for next-generation lithography that utilizes laser-produced plasmas (LPP). Depending on the spectral region of interest, the type of emission, and the droplet characteristics, the effective emission lifetime was found to extend from a few hundred picoseconds to as long as several nanoseconds, in agreement with the expected plasma expansion, EUV excitation, and recombination emission time scales. Received: 22 August 2002 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-301/3149-363, E-mail: riq@wam.umd.edu     

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。     

Gd靶激光等离子体6.7nm光源的实验研究

本文对Gd靶激光等离子体极紫外光源进行了实验研究, 在 6.7 nm附近获得了较强的辐射, 并研究了6.7 nm 附近光辐射随打靶激光功率密度变化的规律以及收集角度对极紫外辐射的影响. 同时, 对平面Gd靶激光等离子光源的离子碎屑角分布进行了测量, 发现从靶面的法线到沿着靶面平行方向上Gd离子数量依次减少. 进一步研究结果表明采用0.9 T外加磁场的条件下可取得较好的Gd 离子碎屑阻挡效果.     

Extreme ultraviolet interferometry of warm dense matter in laser plasmas

We demonstrate that interferometric probing with extreme ultraviolet (EUV) laser light enables determination of the degree of ionization of the "warm dense matter" produced between the critical and ablation surfaces of laser plasmas. Interferometry has been utilized to measure both transmission and phase information for an EUV laser beam at the photon energy of 58.5 eV, probing longitudinally through laser-irradiated plastic (parylene-N) targets (thickness 350 nm) irradiated by a 300 ps duration pulse of wavelength 438 nm and peak irradiance 10(12) W cm(-2). The transmission of the EUV probe beam provides a measure of the rate of target ablation, as ablated plasma becomes close to transparent when the photon energy is less than the ionization energy of the predominant ion species. We show that refractive indices η below the solid parylene N (η(solid) = 0.946) and expected plasma values are produced in the warm dense plasma created by laser irradiation due to bound-free absorption in C(+).     

等离子体密度对放电等离子体极紫外光源影响研究

等离子体状态是决定极紫外光源功率和转换效率的最重要因素之一,理论和实验研究上Xe气流量对放电等离子体极紫外光源辐射谱和等离子体状态的影响,对于优化光源工作条件具有重要的意义。理论上,采用碰撞-辐射模型,模拟了非局部热力学平衡条件下,不同电离度的离子丰度分布随电子温度和离子密度的变化。推导了Xe8+~Xe11+离子4d-5p跃迁谱线强度随电子温度的变化趋势。实验上,采用毛细管放电机制,利用罗兰圆谱仪测量和分析了不同等离子体密度条件下,放电等离子体极紫外光谱的变化,分析了Xe气流量对等离子体状态的影响。理论和实验结果表明： 相同的电流条件下,等离子体箍缩时的平均电子温度随着Xe气流量的增加而降低。对于4d-5p跃迁,低电离度离子与高电离度离子谱线强度的比值随着温度的增加而减少。电流28 kA、Xe气流量0.4 sccm(cm3·min-1)时,等离子体Z 箍缩平均电子温度位于29 eV附近。Xe气流量增加时,受离子密度和最佳电子温度的影响,实现Xe10+离子4d-5p跃迁13.5 nm(2%带宽)辐射谱线强度最优化的Xe气流量位于0.3~0.4 sccm之间。     

EUV micropatterning for biocompatibility control of PET

We have investigated the influence of oriented microstructures at modified polyethylene terephthalate (PET) on the adhesion and alignment of Chinese hamster ovary (CHO) cells. For surface modification, the PET foils were exposed to the radiation of a laser-plasma extreme ultraviolet (EUV) source based on a double-stream gas-puff target. The emission of the plasma was focused onto the samples by means of a gold-plated ellipsoidal collector. The spectrum of the focused radiation covered the wavelength range from 9 to 70 nm. The PET samples were irradiated with the EUV pulses at a repetition rate of 10 Hz in a high vacuum. For control experiments, PET samples were also irradiated in air with the light of a 193 nm ArF-excimer laser. Different kinds of surface microstructures were obtained depending on the EUV or laser fluence and pulse number, including oriented wall- and ripple-type structures with lateral structure periods of a few μm. The surface morphology of polymer samples after the irradiation was investigated using a scanning electron microscope (SEM). Changes in chemical surface structure of the irradiated samples were investigated using X-ray photoelectron spectroscopy (XPS). We demonstrated that the cells show good adhesion and align along oriented wall- and ripple-type microstructures on PET surfaces produced by the EUV irradiation.     

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.     

Spectral and ion emission features of laser-produced Sn and SnO_2 plasmas

We have made a detailed comparison of the atomic and ionic debris, as well as the emission features of Sn and SnO_2 plasmas under identical experimental conditions. Planar slabs of pure metal Sn and ceramic SnO_2 are irradiated with1.06 μm, 8 ns Nd:YAG laser pulses. Fast photography employing an intensified charge coupled device(ICCD), optical emission spectroscopy(OES), and optical time of flight emission spectroscopy are used as diagnostic tools. Our results show that the Sn plasma provides a higher extreme ultraviolet(EUV) conversion efficiency(CE) than the Sn O2 plasma.However, the kinetic energies of Sn ions are relatively low compared with those of SnO_2. OES studies show that the Sn plasma parameters(electron temperature and density) are lower compared to those of the SnO_2 plasma. Furthermore, we also give the effects of the vacuum degree and the laser pulse energy on the plasma parameters.