Ultimate efficiency of extreme ultraviolet radiation from a laser-produced plasma

An analytical formula for maximizing radiation efficiency from a laser-produced plasma is derived. The maximum efficiency is achieved when the plasma expansion distance during laser heating is equal to the laser absorption length. The dependence of the radiation efficiency on the plasma density is confirmed by experiments using a particle-cluster target. By creating a relatively uniform density plasma with a 300 microm diameter by dispersing SnO(2) particles coated on a Si wafer, the conversion efficiency at 14 nm, as high as 4 times as that for a Sn plate target, is achieved.     

Adsorption studies by ultraviolet photoelectron spectroscopy

The application of ultraviolet photoelectron spectroscopy (UPS) for adsorption studies is illustrated using three examples. The observation of difference curves for various gases adsorbed on the same substrate allows those features due to the substrate or the adsorbate to be distinguished. The observation of the coverage dependence of UPS difference spectra allows one to relate the observed features to results from other techniques, as shown for oxygen adsorbed on the (100) face of tungsten. Finally, the measurement of difference spectra of the same species adsorbed on different faces of the same crystal gives information on the substate-adsorbate bonding as illustrated for the case of hydrogen on the (100) and (110) faces of tungsten.     

An extreme ultraviolet radiation source based on plasma heated by millimeter range radiation

An extreme ultraviolet (EUV) radiation source based on a low-pressure discharge sustained in a magnetic trap by high-power electromagnetic radiation at a frequency of 75 GHz is discussed. The experimental radiation power in the wavelength range of 13.5 nm ±1% was as high as 50 W. The conversion of microwave radiation into EUV radiation was about 1%.     

Multiphoton double ionization of Ar in intense extreme ultraviolet laser fields studied by shot-by-shot photoelectron spectroscopy

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ～ 21 eV, ～ 5 TW/cm2), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e? and Ar2(+) + 2hν→Ar(+) + e?. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).     

Classical ultraviolet photoelectron spectroscopy of polymers

Although X-ray photoelectron spectroscopy of polymers was well established by Clark and coworkers in the 1970s, ultraviolet photoelectron spectroscopy of polymer films, was developed later. Previous to the 1970s, the first attempts to use ultraviolet light on polymer films took the form of appearance potential (valence band edge) measurements. Only some years later could the full valence band region of thin polymer films, including insulating polymers, semiconducting polymers and electrically conducting polymers. The development of what might be termed “classical ultraviolet photoelectron spectroscopy” of polymer films may be loosely based upon a variety of issues, including adapting thin polymer film technology to ultra high vacuum studies, the widespread use of helium resonance lamps for studies of solid surfaces, the combined advent of practical and sufficient theoretical–computational methods. The advent of, and the use of, easily available synchrotron radiation for multi-photon spectroscopies, nominally in the area of the near UV, is not included in the term “classical”. At the same time, electrically conducting polymers were discovered, leading to applications of the corresponding semiconducting polymers, which added technologically driven emphasis to this development of ultraviolet photoelectron spectroscopy for polymer materials. This paper traces a limited number of highlights in the evolution of ultraviolet photoelectron spectroscopy of polymers, from the 1970s through to 2008. Also, since this issue is dedicated to Prof. Kazuhiko Seki, who has been a friend and competitor for over two decades, the author relies on some of Prof. Seki's earlier research, unpublished, on who-did-what-first. Prof. Seki's own contributions to the field, however, are discussed in other articles in this issue.     

Investigating ultraviolet photoelectron spectroscopy of ice

Photoelectron energy distribution curves (EDCs) from ice excited by HeI (21.2 eV) and NeI (16.8 eV) radiation are presented. The strict connection between valence density of states and EDCs forces to rule out the previous suggestion by Shibaguchi et al. that conduction band density of states is of paramount importance in determining the EDCs excited by ultraviolet light. The results also allow a discussion of band calculations published till now; the need for a theoretical investigation on photoemission from ice is put forward.     

Vacuum ultraviolet resonance line radiation source from rare gas atoms and ions for UHV photoelectron spectroscopy

A source and differential pumping system for producing high intensity resonance line radiation from rare gas atoms and ions for ultrahigh vacuum (UHV) photoelectron spectroscopy has been developed. Photoelectron count rates from a gold sample, as measured with a double-pass cylindrical mirror analyzer at pass energy 15 eV and 0.10 eV resolution, are ～ 300,000 c s^?1 for the He(I) (21.22 eV) line and ～30000 c s^?1 for the He(II) (40.81 eV) line. The source design is based on the principle of the electrostatic charged particle oscillator and is capable of sustaining discharges over the pressure range 1 to ～ 10^?6 torr. The discharge segment consists of a cylindrical cold cathode surrounding two tungsten rod anodes which are held at high positive potential. Three stages of differential pumping are employed in order that the vacuum in the main spectrometer chamber can be maintained at 2 × 10^?10 torr during operation. The calculated helium flow reaching the main chamber under these conditions is < 101 s^?1. Details of the construction and operating characteristics of the source are presented.     

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.     

Vacuum ultraviolet photoelectron spectroscopy of atoms and molecules

For a complete study of the photoionization of atoms and molecules it is essential to make use of the technique of Photoelectron Spectroscopy and the continuum characteristics of synchrotron radiation. A brief review is given of the application of the above techniques in measuring partial photoionization cross sections and the angular distribution assymetry parameter β. Selected results are given, which are compared to theoretical values.     

Highly-angle-resolved ultraviolet photoelectron spectroscopy of C8Cs

Highly-angle-resolved ultraviolet photoelectron spectroscopy was carried out for a C₈Cs single crystal to study the electronic charge transfer in alkali metal graphite intercalation compounds. The dispersive π¹-band at the K̃ point in the Brillouin zone was observed for the first time. The electron occupation in the π¹-band was estimated to be 0.45±0.05 unit electronic charge. This strongly suggests that a substantial part of an interlayer band exists below the Fermi level at the γ point, forming a spherical Fermi surface on the center of the Brillouin zone.     

A helium 30.4 nm photon source for photoelectron spectroscopy

A new type of photon source of helium 30.4 nm (HeII) radiation is described. The cold cathode discharge is operated at pressures ～10^?4-torr of     

Electron temperature measurements in low-density plasmas by helium spectroscopy

Methods of measuring the electron temperature in low-density plasmas by He spectroscopy are examined. These utilize either the relative intensities of singlet and triplet lines or the absolute intensities of single lines. Calculations from measured and theoretical data show that both methods are seriously influenced by secondary processes, the most important of which are excitation from the metastable levels 2¹S and 2³S, and excitation transfer in electron-atom collisions combined with imprisonment of resonance radiation. The calculations give parameter limits for the validity of different methods and combinations of lines. Due to the secondary processes, the determination of T_e from relative line intensities is restricted to low-density, short-duration plasmas (typically n_e < 2 × 10¹⁶ m^-3, t_ex < 5 × 10^-6 s) or to even lower densities that depend on the apparatus dimensions (typically n_e < 3 × 10¹⁵ m^-3, L ≈ 0.1 m). For the determination of T_e from absolute line intensities, the situation is more favourable and, with a suitable choice of lines, typical restrictions on n_e and t_ex are n_e < 5 × 10¹⁷ m^-3, t_ex < 10^-5 s or n_e < 10¹⁷ m^-3, L ≈ 0.1 m for electron temperatures above 10 eV. For temperatures below 10 eV and degrees of imprisonment below 7% measurements are possible for electron densities up to 10¹⁹–10²⁰ m^-3, without any limits on t_ex or L.     

Method for high-resolution frequency measurements in the extreme ultraviolet regime: random-sampling Ramsey spectroscopy

Ramsey-like schemes have been recently introduced in combination with high-order laser harmonic sources for high-resolution spectroscopic studies in the extreme ultraviolet (XUV). Here we demonstrate a novel method, combining measurements only in a limited subset of randomly chosen time-sampling intervals, which lead us to perform the first high-resolution XUV spectroscopy of atomic argon with a simple split-pulse setup. Providing an experimentally simple and convenient solution to the problem of performing high-resolution absolute frequency measurements in the XUV, our approach will help paving new roads into this challenging spectral territory.     

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.     

Surface electronic state on stepped Cu(755) studied by angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation

The surface electronic state on the stepped surface of Cu(755) has been investigated by means of angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation(SR-ARUPS). We have observed a free-electron-like surface state below the Fermi level. In spite of the anisotropy of the atomic arrangement due to steps, the surface state is shown to be isotropic since the dispersion profile and the peak shape are almost identical in the directions parallel and perpendicular to the steps. This result makes a clear contrast with the previous SR-ARUPS results on Ni(755) surfaces which have the surface structure similar to Cu(755). Those experimental evidences are discussed based upon the electron configurations of both metal substrates.     

双激光脉冲打靶形成Gd等离子体的极紫外光谱辐射

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Gd¹⁸⁺—Gd²⁷⁺离子最外层4d壳层的4p-4d和4d-4f能级之间跃迁、以及Gd¹⁴⁺—Gd¹⁷⁺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...