Abstract
We demonstrate extreme ultraviolet (EUV) and soft x-ray sources in the 2- to 7 -nm spectral region related to the beyond extreme ultraviolet (BEUV) question at 6.x nm and a water window source based on laser-produced high-Z plasmas. Strong emissions from multiply charged ions merge to produce intense unresolved transition array (UTA) toward extending below the carbon K-edge (4.37 nm). An outline of a microscope design for single-shot live- cell imaging is proposed based on a high-Z UTA plasma source, coupled to x-ray optics. We will discuss the progress and Z-scaling of UTA emission spectra to achieve lab-scale table-top, efficient, high-brightness high-Z plasma EUV-soft x-ray sources for in vivo bio-imaging applications.
Keywords
- High-Z
- unresolved transition array (UTA)
- extreme ultraviolet (EUV)
- soft x-ray
- water window
1. Introduction
Laboratory- scale source development of shorter- wavelength spectral regions in the extreme ultraviolet (EUV) and soft x-ray has been motivated by their applications in a number of high- profile areas of science and technology. One such topic is the challenge of three-dimensional imaging and single-shot flash photography of microscopic biological structures, such as macromolecules and cells,
High-power EUV sources with high efficiency for semiconductor lithography at 13.5 [5] and 6.7 nm [6–8] based on laser-produced plasmas (LPP) have been demonstrated in high-volume manufacturing of integrated circuits (IC) having node sizes of 22 nm or less [9, 10]. The EUV emission at the relevant wavelength may be coupled with La/B4C or Mo/B4C multilayer mirror with a reflectivity of 40% to provide a source at 6.5−6.7 nm. Recently, a reflection coefficient of about 60−70% was shown to be feasible in a theoretical study [11]. Consequently, the development of a new wavelength EUV source for the next- generation semiconductor lithography, which can be coupled with an efficient B4C multilayer mirror, is particularly timely.
High-
Plasmas of the rare earth elements gadolinium (Gd) and terbium (Tb) produce strong resonant emission due to the presence of an intense UTA around 6.5−6.7 nm in the spectra of their ions [6]. In tin (Sn), the presence of the corresponding feature at 13.5 nm prompted its selection as the optimum source material at that wavelength. The UTA emission scales to shorter wavelength with increasing atomic number,
In this chapter, we show the efficient EUV and soft x-ray sources in the 2- to 7- nm spectral region related to the beyond extreme ultraviolet (BEUV) question at 6.
2. Characteristics of the Gd plasmas for BEUV source applications
In order to increase the energy CE from the incident laser energy to the interested wavelength emission energy with the defined bandwidth, it is important to suppress not only the reabsorption by assurance of the plasma is optically thin but also plasma hydrodynamic expansion loss, while maintaining a plasma electron temperature of
A Nd:glass laser system, GEKKO-XII at the Institute of Laser Engineering (ILE) in Osaka University was used to produce the 1D expanding uniform plasma [19]. The GEKKO-XII laser facility consists of 12 laser beams each at a wavelength of 1.053 μm and a constant 1 J pulse energy, irradiating a total energy of 12 J, with a temporal Gaussian- shaped pulse width of 1.3 ns [full width at half maximum (FWHM)]. The 12 laser beams were located at 12 faces of a regular dodecahedron to irradiate spherical targets uniformly. A thick metallic layer of 2 μm was coated onto spherical polystyrene balls for providing targets. The laser power imbalance was monitored to be within ± 6.3% of the average. Then, the laser beams were uniformly irradiated onto the target, to provide a 1D plasma expansion with low expansion loss.
Figure 1 shows the temporal history of the in-band emission around 6.7 nm with the bandwidth of 0.6% from Gd plasmas observed by the x-ray streak camera to provide 1D time-resolved imaging. The red and blue lines are the EUV emission at the optimum intensity of 1 × 1012 W/cm2 and the maximum intensity of 3 × 1013 W/cm2, respectively. Under optimum irradiation conditions with the highest CE, the temporal profile of the EUV emission was similar of that of the laser pulse shown by the dashed line and reached a maximum a little later. On the other hand, the behavior of the EUV emission profile at 3 × 1013 W/cm2 initially rose faster, but the peak was delayed by comparison with that obtained under optimum conditions. The initial steep rise indicates that the electron temperature quickly reaches a value necessary for the in-band EUV emission. The final electron temperature is expected to be higher than optimum, so that higher charge state ions higher than q = 28 are produced, which predominantly emit shorter- wavelength out-of-band emission around 2−4 nm. After the maximum electron temperature is attained, plasma recombination proceeds accompanied by adiabatic expansion, resulting in cooling. The in-band emission from ionic charge states of
![](http://cdnintech.com/media/chapter/50348/1512345123/media/fig1.png)
Figure 1.
Time-resolved spectral images at two different laser intensities of (a) 1 × 1012 W/cm2 and (b) 3 × 1013 W/cm2, respectively. (c) Temporal histories of the EUV emission at 6.7 nm from Gd plasmas at two different laser intensities of 1 × 1012 W/cm2 (red) and 3 × 1013 W/cm2 (blue), together with a temporal profile of the laser pulse (dashed). At an optimum laser intensity of 1 × 1012 W/cm2, the temporal behavior of the in-band emission is essentially the same as that of the laser pulse. It should be noted that intensities are normalized for timing comparison [
The in-band EUV CEs were evaluated at
In addition, it is important to understand the physics of the EUV emission and transport in laser-produced dense high-
![](http://cdnintech.com/media/chapter/50348/1512345123/media/image2_w.jpg)
Figure 2.
Schematic diagram of the experimental setup. Interferograms were produced by a Mach-Zehnder interferometer by the use of a Nd:YAG laser at a wavelength of 532 nm with a pulse duration of 6 ns (FWHM) [
![](http://cdnintech.com/media/chapter/50348/1512345123/media/image3_w.jpg)
Figure 3.
Profiles of the radial electron density (solid line) and radial EUV emission (dashed line) at the time of three different peak laser intensities of (a) 1 × 1012 W/cm2, (b) 7 × 1012 W/cm2, and (c) 1 × 1014 W/cm2, corresponding to laser focal spot and target diameters of (a) 500 μm, (b) 200 μm, and (c) 50 μm [
The production of low-density plasma by the use of CO2 LPPs has been proposed, because the critical electron density
We characterize the EUV emission from CO2 laser–produced plasmas (CO2-LPPs) of the rare earth element of Gd. The energy CE and the spectral purity in the CO2-LPPs were higher than that for solid-state LPPs at 1.06 μm, because the plasma produced is optically thin due to the lower critical density, resulting in a maximum CE of 0.7% at 6.76 nm with 0.6% bandwidth in the solid angle of 2π sr. The peak wavelength was fixed at 6.76 nm for all laser intensities. The plasma parameters at a CO2 laser intensity of 1.3 × 1011 W/cm2 was also evaluated using the hydrodynamic simulation code to produce the EUV emission at 6.76 nm.
Figure 4(a) shows time-integrated EUV emission spectra from the Nd:YAG-LPPs at different laser intensities ranging from 9.7 × 1011 to 6.6 × 1012 W/cm2. The peak wavelength shifts from 6.7 to 6.8 nm and is mainly due to
In the case of CO2-LPPs, the main spectral behaviors near 6.7 nm, on the other hand, are narrower than for Nd:YAG laser irradiating plasma, as shown in Figure 4(b). The CO2 laser intensity was varied from 5.5 × 1010 to 1.2 × 1011 W/cm2. The spectral structure was dramatically different to that from the Nd:YAG-LPPs. The peak wavelength of 6.76 nm remains constant with the increase of the laser intensity. Moreover, the emission intensity of the peak at 6.76 nm increases more rapidly with laser intensity than the emission in the ranges of
![](http://cdnintech.com/media/chapter/50348/1512345123/media/image4_w.jpg)
Figure 4.
(a) Time-integrated EUV emission spectra from the Nd:YAG LPPs at different laser intensities of 9.7 × 1011, 2.2 × 1012, and 6.6 × 1012 W/cm2, respectively. The peak wavelength shifts from 6.7 to 6.8 nm with increasing the laser intensity. (b) Time-integrated EUV emission spectra from the CO2 LPPs at different laser intensities of 5.5 × 1010, 8 × 1010, 9.8 × 1010, and 1.3 × 1011 W/cm2, respectively. The peak wavelength of 6.76 nm remains constant with increasing the laser intensity [
In order to infer the laser parameters that maximize 6.
The profile of the intense emission at 6.
![](http://cdnintech.com/media/chapter/50348/1512345123/media/image5_w.jpg)
Figure 5.
(a) The wavelength of the emission peaks near 6.
To verify the above explanation, charge-defined emission spectra were measured with the EBITs for different highest charge states. EUV emission spectra from EBIT experiments are shown in Figure 5(b), and calculated
3. Quasi-Moseley’s law for the UTA emission
In this section, we show that the strong resonance UTAs of Nd:YAG LPPs for elements with
Figures 6(a) −6(k) show LPP emission spectra from high-
![](http://cdnintech.com/media/chapter/50348/1512345123/media/image6.png)
Figure 6.
Time-integrated EUV emission spectra of the Nd:YAG LPPs for (a) 83Bi, (b) 82Pb, (c) 79Au, (d) 78Pt, (e) 75Re, (f) 74W, (g) 73Ta, (h) 68Er, (i) 65Tb, (j) 64Gd, and (k) 60Nd targets with 150-ps laser (red, solid line) and 10-ns laser (blue, dotted line), respectively. Typical laser power densities were 2.5 × 1014 W/cm2 for ps-laser illumination and 5.6 × 1012 W/cm2 for 10-ns laser irradiation. The measured LHD spectra (green, solid) for (l) Bi, (m) Pb, (n) Au, (o) W, (p) Gd, and (q) Nd targets, respectively. An emission line at 3.4 nm is from impurity carbon ions. Intensities were normalized at each maximum of the
As a result, we have not observed significant emission of the type 4
Figure 7 shows the atomic number dependence of the observed peak wavelength of
We propose here a pathway to produce feasible laboratory-scale high-
![](http://cdnintech.com/media/chapter/50348/1512345123/media/fig7.png)
Figure 7.
Atomic number dependence of the peak wavelength of
4. Water window soft x-ray source by high-Z ions
4.1. Spectroscopy of low electron temperature in lab-scale laser-produced ions
According to the quasi-Moseley’s law in Figure 7, the elements from 79Au to 83Bi are one of the candidates for high-flux UTA source in water window soft x-ray sources for single-shot (flash) bio-imaging in the laboratory size microscope, because the UTA emission is essentially high- power emission due to much resonant lines around the specific wavelength (photon energy). The UTA peak wavelengths of 79Au, 82Pb, and 83Bi reach the water window soft x-ray spectral region.
Figures 8(a) −8(c) show time-integrated spectra from Au, Pb, and Bi plasmas at a laser intensity of the order of 1014 W/cm2 with a pulse duration of 150 ps (FWHM). The time-integrated soft x-ray spectra between 1 and 6 nm from each element display strong broadband emission around 4 nm, which is mainly attributed to the
We compared the results of numerical calculation for some different experimental temperatures with the observed spectra as shown in Figure 9(a). Four regions corresponding to emission peaks were identified. The emission in the region of “1” results primarily from the 4
![](http://cdnintech.com/media/chapter/50348/1512345123/media/image8_w.jpg)
Figure 8.
Time-integrated spectra from the picosecond-laser–produced high-
In Figure 9(b), evaluated spectra at different electron temperatures higher than 900 eV were shown. Numerical calculations show that high-
![](http://cdnintech.com/media/chapter/50348/1512345123/media/image9_w.jpg)
Figure 9.
(a) The comparison between the observed spectrum with numerical calculation under assuming steady-state electron temperatures of 190 and 700 eV. (b) Calculated spectra for electron temperatures higher than 900 eV. [
4.2. Toward the laboratory water window soft x-ray microscope
Because of the broadband features of the emission, the zone plate components cannot be used, so one of the possible solutions would be to use a transmission planar x-ray nano-waveguide to image the sample. In order to achieve high resolution in the recorded image, we should also replace the recording device from the x-ray CCD camera to the sensitive EUV resist to overcome the resolution limitation of the CCD pixel size, coupling with the Schwarzschild optics, consisting of Sc/Cr multilayer mirrors. Although our proposal is based on a simple microscope construction, the key component is the UTA emitted from a hot dense Bi plasma point source, combined with Sc/Cr MLMs and sensitive EUV resists based on the photochemical reaction [26].
5. Summary
We have shown EUV and soft x-ray sources in the 2- to 7- nm spectral region related to the BEUV question at 6.
References
- 1.
Gorniak T, Heine R, Mancuso A. P, Staier F, Christophis C, Pettitt M. E, Sakdinawat A, Treusch R, Guerassimova N, Feldhaus J, Gutt C, Grübel G, Eisebitt S, Beyer A, Gölzhäuser A, Weckert E, Grunze M, Vartanyants I. A, Rosenhahn A: X-ray holographic microscopy with zone plates applied to biological samples in the water window using 3rd harmonic radiation from the free-electron laser FLASH. Optics Express. 2011;19:11059–11070. DOI: 10.1364/OE.19.011059] - 2.
Jansson P. A. C, Vogt U, Hertz H. M: Liquid nitrogen jet laser plasma source for compact soft x-ray microscopy. Review of Scientific Instruments. 2005;76:043503. DOI: 10.1063/1.1884186 - 3.
Takman P. A. C, Stollberg H, Johansson G. A, Holmberg A, Lindblom M, Hertz H. M: High-resolution compact X-ray microscopy. Journal of Microscopy. 2007;226:175–181. DOI: 10.1111/j.1365-2818.2007.01765.x - 4.
Skoglund P, Lundström U, Vogt U, Hertz H. M: High-brightness water-window electron-impact liquid-jet microfocus source. Applied Physics Letters. 2010;96:084103. DOI: 10.1063/1.3310281 - 5.
Bakshi V, editor. EUV Sources for Lithography. Bellingham: SPIE Press Book; 2006. 1094 p. DOI: 10.1117/3.613774 - 6.
Otsuka T, Kilbane D, White J, Higashiguchi T, Yugami N, Yatagai T, Jiang W, Endo A, Dunne P, O’Sullivan G: Rare-earth plasma extreme ultraviolet sources at 6.5–6.7 nm. Applied Physics Letters. 2010;97:111503. DOI: 10.1063/1.3490704; and references therein. - 7.
Otsuka T, Kilbane D, Higashiguchi T, Yugami N, Yatagai T, Jiang W, Endo A, Dunne P, O’Sullivan G: Systematic investigation of self-absorption and conversion efficiency of 6.7 nm extreme ultraviolet sources. Applied Physics Letters. 2010;97:231503. DOI: 10.1063/1.3526383 - 8.
Higashiguchi T, Otsuka T, Yugami N, Jiang W, Endo A, Li B, Kilbane D, Dunne P, O’Sullivan G: Extreme ultraviolet source at 6.7 nm based on a low-density plasma. Applied Physics Letters. 2011;99:191502. DOI: 10.1063/1.3660275 - 9.
Meiling H, Boeij W, Bornebroek F, Harned N, Jong I, Meijer H, Ockwell D, Peeters R, Setten E, Stoeldraijer J, Wagner C, Young S, Kool R, Kürz P, Lowisch M: From performance validation to volume introduction of ASML’s NXE platform. Proceedings of SPIE. 2012;8322:83221G. DOI: 10.1117/12.916971 - 10.
Wagner C, Harned N: EUV lithography: Lithography gets extreme. Nature Photonics. 2010;4:24–26. DOI: 10.1038/nphoton.2009.251 - 11.
Platonov Y, Rodriguez J, Kries M, Louis E, Feigl T, Yulin S. Status of Multilayer Coatings for EUV Lithography. In: Proceedings of 2011 International Workshop on EUV Lithography; 13-17 June 2012; Maui (Hawaii). Austin: EUV Litho, Inc.; 2012. p. 49 (P25). - 12.
O’Sullivan G, Carroll P. K: 4 d −4f emission resonances in laser-produced plasmas. Journal of the Optical Society of America. 1981;71:227–230. DOI: 10.1364/JOSA.71.000227 - 13.
Carroll P. K, O’Sullivan G: Ground-state configurations of ionic species I through XVI for Z = 57−74 and the interpretation of 4d −4f emission resonances in laser-produced plasmas. Physical Review A. 1982;25:275–286. DOI: 10.1103/PhysRevA.25.275 - 14.
Higashiguchi T, Li B, Suzuki Y, Kawasaki M, Ohashi H, Torii S, Nakamura D, Takahashi A, Okada T, Jiang W, Miura T, Endo A, Dunne P, O’Sullivan G, Makimura T: Characteristics of extreme ultraviolet emission from mid-infrared laser-produced rare-earth Gd plasmas. Optics Express. 2013;21:31837–31845. DOI: 10.1364/OE.21.031837; and references therein. - 15.
Okuno T, Fujioka S, Nishimura H, Tao Y, Nagai K, Gu Q, Ueda N, Ando T, Nishihara K, Norimatsu T, Miyanaga N, Izawa Y, Mima K: Low-density tin targets for efficient. Applied Physics Letters. 2006;88:161501. DOI: 10.1063/1.2195693 - 16.
Higashiguchi T, Dojyo N, Hamada M, Sasaki W, Kuobdera S: Low-debris, efficient laser-produced plasma extreme ultraviolet source by use of a regenerative liquid microjet target containing tin dioxide (SnO2) nanoparticles. Applied Physics Letters. 2006;88:201503. DOI: 10.1063/1.2206131 - 17.
Cummins T, Otsuka T, Yugami N, Jiang W, Endo A, Li B, O’Gorman C, Dunne P, Sokell E, O’Sullivan G, Higashiguchi T: Optimizing conversion efficiency and reducing ion energy in a laser-produced Gd plasma. Applied Physics Letters. 2012;100:061118. DOI: 10.1063/1.3684242 - 18.
Yamanaka C, Kato Y, Izawa Y, Yoshida K, Yamanaka T, Sasaki T, Nakatsuka M, Mochizuki T, Kuroda J, Nakai S: Nd-doped phosphate glass laser systems for laser-fusion research. IEEE Journal of Quantum Electronics. 1981;17:1639–1649. DOI: 10.1109/JQE.1981.1071341 - 19.
Shimada Y, Nishimura H, Nakai M, Hashimoto K, Yamaura M, Tao Y, Shigemori K, Okuno T, Nishihara K, Kawamura T, Sunahara A, Nishikawa T, Sasaki A, Nagai K, Norimatsu T, Fujioka S, Uchida S, Miyanaga N, Izawa Y, Yamanaka C: Characterization of extreme ultraviolet emission from laser-produced spherical tin plasma generated with multiple laser beams. Applied Physics Letters. 2005;86:051501. DOI: 10.1063/1.1856697 - 20.
Yoshida K, Fujioka S, Higashiguchi T, Ugomori T, Tanaka N, Ohashi H, Kawasaki M, Suzuki Y, Suzuki C, Tomita K, Hirose R, Ejima T, Nishikino M, Sunahara A, Scally E, Li B, Yanagida T, Nishimura H, Azechi H, O’Sullivan G: Efficient extreme ultraviolet emission from one-dimensional spherical plasmas produced by multiple lasers. Applied Physics Express. 2014;7:086202. DOI: 10.7567/APEX.7.086202 - 21.
Masnavi M, Szilagyi J, Parchamy H, Richardson M. C: Laser-plasma source parameters for Kr, Gd, and Tb ions at 6.6 nm. Applied Physics Letters. 2013;102:164102. DOI: 10.1063/1.4802789 - 22.
Yoshida K, Fujioka S, Higashiguchi T, Ugomori T, Tanaka N, Kawasaki M, Suzuki Y, Suzuki C, Tomita K, Hirose R, Ejima T, Ohashi H, Nishikino M, Sunahara A, Li B, Dunne P, O’Sullivan G, Yanagida T, Azechi H, Nishimura H: Density and x-ray emission profile relationships in highly ionized high- Z laser-produced plasmas. Applied Physics Letters. 2015;106:121109. DOI: 10.1063/1.4916395 - 23.
Ohashi H, Higashiguchi T, Li B, Suzuki Y, Kawasaki M, Kanehara T, Aida Y, Torii S, Makimura T, Jiang W, Dunne P, O’Sullivan G, Nakamura N: Tuning extreme ultraviolet emission for optimum coupling with multilayer mirrors for future lithography through control of ionic charge states. Journal of Applied Physics. 2014;115:033302. DOI: 10.1063/1.4862441; and references therein. - 24.
Ohashi H, Higashiguchi T, Suzuki Y, Arai G, Otani Y, Yatagai T, Li B, Dunne P, O’Sullivan G, Jiang W, Endo A, Sakaue H. A, Kato D, Murakami I, Tamura N, Sudo S, Koike F, Suzuki C: Quasi-Moseley's law for strong narrow bandwidth soft x-ray sources containing higher charge-state ions. Applied Physics Letters. 2014;104:234107. DOI: 10.1063/1.4883475 - 25.
Colombant D, Tonon G. F: X‐ray emission in laser‐produced plasmas. Journal of Applied Physics.1973;44:3524-3537. DOI: 10.1063/1.1662796 - 26.
Higashiguchi T, Otsuka T, Yugami N, Jiang W, Endo A, Li B, Dunne P, O’Sullivan G: Feasibility study of broadband efficient “water window” source. Applied Physics Letters. 2012;100:014103. DOI: 10.1063/1.3673912; and references therein.