The influcence of the pulse duration on the emission characteristics of nearly debris-free laser-induced plasmas in the soft x-ray region (λ ≈1-5 nm) was investigated, using six different target gases from a pulsed jet. Compared to ns pulses of the same energy, a ps laser generates a smaller, more strongly ionized plasma, being about 10 times brighter than the ns laser plasma. Moreover, the spectra are considerably shifted towards shorter wavelengths. Electron temperatures and densities of the plasma are obtained by comparing the spectra with model calculations using a magneto-hydrodynamic code.
©2013 Optical Society of America
In recent years, large efforts have been undertaken to increase the brilliance of laser-produced plasma sources emitting in the extreme ultraviolet (EUV) and soft x-ray spectral region, as they allow for innovative applications such as EUV nano-lithography [1–4], surface analysis [5, 6], x-ray absorption spectroscopy [7, 8], and soft x-ray microscopy in the water window range [9–11]. Depending on the application, various types of laser targets are employed, i.e. solids (including tapes and cryo-targets), liquid jets or droplets, and gases, each target type showing different advantages and drawbacks. Comparatively bright and small plasmas, with spatial extensions of the order of several tens of µm [12–14], are generated using solid or mass-limited liquid targets. However, their main disadvantage is the unavoidable production of debris, which can severely damage optical elements employed in the beam path, especially condenser mirrors in relatively close vicinity to the plasma. Furthermore, the setup of liquid targets requires considerable experimental and technical effort, including large pumping powers. These problems are overcome by the use of gaseous targets, enabling the construction of rather clean, compact and long-term stable EUV/soft x-ray sources [15, 16]. On the other hand, due to the reduced particle density, the photon yield and peak brilliance are definitely smaller, as the plasma size increases to several hundreds of µm . Progress in the optimization of gaseous plasma sources has been achieved by using cluster beam targets , double-stream gas puff targets , or by forming a supersonic jet in combination with the so-called barrel shock , all leading to smaller and brighter plasmas due to increased particle densities. These studies were mainly conducted with solid-state Nd:YAG lasers having pulse durations of several ns. However, Vogt et al.  and Higashiguchi et al.  report that for a liquid water jet or a solid Bi target, respectively, the use of a Nd:YAG laser with sub-ns pulse duration increases the plasma brightness and leads to a shift towards shorter wavelengths, as the average electron temperature, and thus the degree of ionization, of the ps laser plasma exceeds that of the ns plasma.
In the work presented here, the spectral emission properties of both ns and ps laser-induced plasmas from six different target gases are investigated. For the ps laser the higher power density attainable due to the shorter pulse (at comparable pulse energy) results in a smaller, more strongly ionized plasma with higher average electron temperature. Thus, the emission spectra are brighter and shifted towards shorter wavelengths. The experimental results are compared with model calculations using a magneto-hydrodynamic code , from which the achieved effective electron temperatures and densities can be determined. Altogether, by employing a ps Nd:YAG laser in combination with a debris-less gaseous target, the brightness and peak brilliance of the table-top laser-induced soft x-ray source can be significantly increased.
2. Experimental setup
Due to the high absorption of soft x-rays in matter, the experiments were performed in a vacuum system with a base pressure of 5*10−5 mbar. The laser-induced plasma source is based on a gas-puff target and two Nd:YAG laser systems (Innolas, wavelength 1064 nm, pulse energy 450 mJ, pulse duration 7 ns, repetition rate 2 Hz and EKSPLA, wavelength 1064 nm, pulse energy 380 mJ, pulse duration 170 ps, repetition rate 2 Hz). The pulsed gas jet used as laser target is created by a conical nozzle with diameters d1 ≈400 µm, d2 ≈200 µm and a cone half angle of 6° behind a fast valve. The latter is based on the Proch-Trickl setup , consisting of a piezo disk translator (Physik-Instrumente, P-286.72) to generate short gas pulses (topen = 900 µs), allowing for a background pressure of about 5*10−3 mbar during operation (gas pressure p = 10-20 bar). For the ns laser measurements the maximum pulse energy of the laser was applied, achieving a power density of 2.2*1012 W/cm2 in the focal spot. For the ps laser the power density was set to values between 1.1*1014 W/cm2 and 1.6*1014 W/cm2 by varying the pulse energy, assuring that all spectra were recorded below the saturation level of the detector. The focal spot size was calculated from the given laser parameters and the focal length of the lens.
The radiation emitted by the plasma is filtered by a titanium (Ti) or aluminum (Al) filter (thickness of 200 nm or 400 nm) to block out-of-band radiation, such as visible light or scattered laser radiation. Plasma size and emission characteristics were monitored with a soft x-ray pinhole camera  and a spectrometer. The latter consists of a 100 µm entrance slit, an aberration-corrected concave flat-field grating (Hitachi, 2400 grooves per mm, wavelength range 1 nm – 5 nm), and a back-illuminated CCD camera (Roper Scientific, 13 µm*13 µm pixel size, 2048*512 pixels). The quantum efficiency of the CCD camera was taken into account in order to evaluate the spectral emission yield of the plasma.
3. Results and discussion
3.1 Spectral investigations
The measurements were conducted with both lasers using six different target gases, i.e. nitrogen, oxygen, neon, argon, krypton and xenon. The plasma emission spectra of all investigated gases are compiled in Fig. 2, indicating that the spectral characteristics of the soft x-ray source are strongly influenced by both the choice of the operating gas and the pulse duration (or power density) of the employed laser. Elements with a lower atomic number Z (nitrogen, oxygen and neon) emit a discrete line spectrum due to a low number of electronic transitions. With increasing Z the number of possible transitions grows rapidly, leading to quasi-continuous spectra for the elements argon, krypton and xenon. These three elements show complex line features which can be attributed to the combination and overlap of multiple transitions in numerous degrees of ionization. Assigning all features in the spectra is impossible due to a lack of detailed spectral data in the literature. Thus, only a range of ionization stages is given (cf. Figure 2).
The differences between ns and ps spectra are evident for all six investigated target gases. In addition to the strongly increased overall emission intensity of the ps laser plasma, the ps spectra are clearly shifted towards shorter wavelengths. Obviously, the higher power density results in a higher degree of ionization, as seen e.g. from the excitation of H-like nitrogen ions (N VII)  and He-like neon ions (Ne IX) [25, 26].
The brightness of specific spectral lines and regions was measured with a calibrated XUV photo-diode (IRD AXUV 100); the results of these measurements are compiled in Table 1.
Characteristic quantities describing a plasma are its electron temperature and the electron density, indicating the maximum achievable degree of ionization in the plasma. In order to estimate these parameters, emission spectra of argon were calculated using the software PrismSPECT, being a magneto-hydrodynamic, collisional-radiative spectral analysis code . To calculate the properties of the argon plasma, detailed configuration accounting (DCA) is employed, with the atomic levels being modeled explicitly using the “Emission Visible/UV/EUV Spectroscopy” atomic model . In this case the spectral properties were computed for local thermodynamic equilibrium plasmas in a planar geometry and steady-state atomic level populations, using the Saha equation and Boltzmann statistics .
Due to the strong temporal and spatial non-uniformities of pulsed laser-induced plasmas it is obviously not valid to assume thermodynamic equilibrium, and the recorded emission spectra clearly represent a mixture of spectra originating from regions with different electron temperature and electron density. Therefore, various plasma spectra were computed for a matrix of electron temperatures and densities. To reach common accord with the measured data, several of these computed spectra are superimposed, particularly taking into account the ratio of peak intensities stemming from transitions of different degrees of ionization. Figure 3 shows a comparison of measured and calculated argon emission spectra. For both ns and ps plasmas the correspondence is evident. Almost all features of the complex spectra are reproduced, proving the validity of the used parameters.
The relative contribution of different electron temperatures and electron densities used for the calculations of the overall argon spectra are given in Table 2.
Note that the indicated relative contributions of different electron temperatures and densities are clearly not unique and only represent an estimation to achieve a good correspondence between measured and calculated spectra. On the other hand, the accordance accomplished even in small details of the complex Ar spectra demonstrates that a good approximation has been found.
Furthermore, the electron temperature and electron density chosen for the calculations are in good agreement with values published elsewhere [29–34]. Fiedorowicz et al.  report electron temperatures between 20 eV and 200 eV for a range of power densities between 1012 W/cm2 and 1014 W/cm2 for a nitrogen plasma using a Nd:YAG laser system producing up to 700 mJ of either 10 ns or 0.9 ns time duration. Furthermore, they refer to a strong dependence of the coupling of the laser energy to the plasma on the gas density. A reduced gas density, e.g. by a slightly increased distance between nozzle and laser focus, results in a significantly lower electron temperature. The values of the electron temperature obtained in the present study are not as high as 200 eV due to the reduced pulse energy of the used laser system and probably to a reduced gas density.
A more detailed analysis of the plasma emission was performed for nitrogen and krypton, since these gases are of special interest in the water window regime (λ = 2.3 nm to λ = 4.4 nm): nitrogen represents a quasi-monochromatic line source at λ = 2.8787 nm when filtered with titanium (cf. Fig. 7), e.g. for soft x-ray microscopy, and krypton, being strong and continuous over the whole water window region, is an ideal target for spectroscopic measurements at the oxygen (λ = 2.3 nm), nitrogen (λ = 3.0 nm) or carbon K-absorption edge (λ = 4.4 nm). Thus, the influence of different pulse energies and pulse lengths on the emission spectra of nitrogen and krypton was investigated. As seen from Fig. 4, considering the same pulse energy for ns and ps laser, the higher emission intensity of the ps plasma and the shift to shorter wavelengths is clearly visible for both gases, which can be attributed to the higher electron density and higher electron temperature, respectively.
However, although the power density of the ps laser is at least two orders of magnitude higher than that of the ns laser, the shift to shorter wavelengths occurs only if the pulse energy is high enough to sufficiently heat up the plasma. Thus, the optimum excitation of the gas atoms in nitrogen and krypton is achieved only for both high pulse energies and short pulse lengths. Li et al.  observed a similar shift to shorter wavelengths with increasing pulse energy using a 150 ps Nd:YAG laser for plasmas induced on a solid state zirconium target. On the other hand, Li et al. note higher total emission intensities for ns laser-induced plasmas of Zr, Mo, C and N from solid targets compared to ps laser plasmas.
In order to gain further insight into the mechanisms of ns and ps laser plasma excitation in nitrogen and krypton, the fraction of the laser pulse energy absorbed by the target gases was measured, monitoring the amount of energy transmitted through and reflected from the plasma with a pyroelectric pulse energy detector.
The fraction of reflected laser radiation was measured to be smaller than 2%. This is compatible with electron densities of ≈1019 - 1020 e/cm3 calculated for the ps spectra (cf. Figure 3), since the critical density leading to a reflection of the incoming laser radiation is considerably higher (≈1021 e/cm3 ).
Figure 5 displays the coupling efficiency of laser energy into the target gases: Whereas for krypton a fraction of more than 90% of the ps laser energy is absorbed by the gas jet, this fraction reduces to nearly 60% for the highest ns laser energy. Obviously, the ps laser radiation is coupled in much more effectively, even at low pulse energies. The absorbed energy fraction in nitrogen is not as high as in krypton, but it shows the similar trend: 65% of the ps laser pulse is absorbed at an energy of 340mJ, about twice as much as for the same ns laser energy.
One possible explanation for the strongly different coupling could be related to high local density clusters, formed due to the sudden cooling during expansion of the high-pressure gas into vacuum; this results in enhanced collisional inverse bremsstrahlung absorption of the electrons [37, 38]. As clusters rapidly dissociate, the ns laser mainly interacts with a gaseous medium instead of the high density clusters, leading to a reduced absorption, and thus to a lower electron temperature, ionization level and soft x-ray emission. On the other hand, cluster decomposition is known to take place on the lower ps time-scale , which might be too short regarding our pulse duration of 170 ps. Nevertheless, we may still benefit from the higher local particle densities of the expanding clusters.
Taking into account the different absorbed energy fractions for the ns and ps laser from Fig. 5, krypton spectra recorded at comparable pulse energies Qabs coupled into the plasma are compiled in Fig. 6. Interestingly, as seen from Fig. 6 (left), nearly identical ns and ps spectra are obtained for similar absorbed energies (125mJ and 102mJ, respectively), although the power densities differ by more than two orders of magnitude. This emphasizes the importance of the coupling process.
However, for higher absorbed pulse energies (Qabs 200mJ) the ps spectra are effectively shifted to shorter wavelengths, and the emission intensity of the ps plasma increases more rapidly than that of the ns plasma (Fig. 6, right). Although the absorbed pulse energy is almost the same, the excitation of the krypton atoms is stronger at the shorter pulse length, leading to a higher electron temperature and a stronger emission of photons at shorter wavelengths.
3.2 Brilliance measurement
The brightness of the quasi-monochromatic titanium-filtered nitrogen plasma at λ = 2.8787 nm (cf. Fig. 7) was determined using a calibrated XUV photo-diode (IRD AXUV 100). The gas pressure was set to p = 20 bar, and the maximum laser pulse energy was chosen to assure power densities of 2.2*1012 W/cm2 and 1.7*1014 W/cm2 in the focal plane for the ns and ps laser, respectively. The emission characteristics achieved for the ns and ps laser-induced soft x-ray sources are listed in Table 3, indicating that the brightness of the ps laser plasma is increased by a factor of 16 compared to the ns laser plasma. The higher power density of the ps laser clearly leads to a considerably smaller plasma (ns: 470*190 µm2, ps: 310*150 µm2), which can be explained by the shorter time available for plasma expansion . In order to determine peak brilliances the soft x-ray pulse duration has to be considered. For the ns laser-induced nitrogen plasma this was measured with a fast photodiode to be ≈5 ns , from which a peak brilliance of ≈1.4*1015 photons/(s*mrad2*mm2) is computed. For the ps laser plasma the pulse duration is obtained from a MHD simulation as ≈500 ps, resulting in a peak brilliance of ≈1.3*1018 photons/(s*mrad2*mm2).
In this paper we have studied the spectral emission properties of ns and ps laser-induced plasmas using nearly debris-free gaseous targets of a table-top soft x-ray source. For all investigated target gases, i.e. N, O, Ne, Ar, Kr and Xe, the ps laser-induced plasmas are considerably smaller. Their brightness is strongly enhanced, for some emission lines by more than an order of magnitude. Moreover, a pronounced spectral shift to shorter wavelengths is observed for all gases, indicating a higher degree of ionization. For nitrogen and krypton it has been demonstrated that the ps laser is absorbed much more efficiently by the gas target. However, for an optimum excitation of gas atoms both high pulse energies and short pulse lengths are necessary. The experimental data were compared with computed spectra using a magneto-hydrodynamic code. From the good accordance achieved for the ns and ps laser-induced argon plasmas effective electron temperatures and electron densities could be determined.
Summarizing we can state that the employment of a ps laser for a pulsed gas jet-based soft x-ray source provides considerable improvements regarding brightness and peak brilliance, thereby maintaining the compactness and inherent cleanliness of the source.
The financial support by the “Deutsche Forschungsgemeinschaft” within the “Sonderforschungsbereich 755” “Nanoscale Photonic Imaging” Project C4 is gratefully acknowledged. Furthermore, we like to thank M. Vrbova and her group members for fruitful discussions on this topic.
References and links
1. I. Fomenkov, N. Böwering, D. Brandt, D. Brown, A. Bykanov, A. Ershov, B. La Fontaine, M. Lercel, and D. Myers, “Light sources for EUV lithography at the 22-nm node and beyond,” Proc. SPIE 8322, 83222N, 83222N-9 (2012). [CrossRef]
2. B. Wu and A. Kumar, “Extreme ultraviolet lithography: A review,” J. Vac. Sci. Technol. B 25(6), 1743–1761 (2007). [CrossRef]
3. I. Turcu, C. Mann, S. Moon, R. Allott, N. Lisi, B. J. Maddison, S. E. Huq, and N. S. Kim, “Deep, three dimensional lithography with a laser-plasma x-ray source at 1nm wavelength,” Microelectron. Eng. 35(1-4), 541–544 (1997). [CrossRef]
4. V. Banine, K. Koshelev, and G. Swinkels, “Physical processes in EUV sources for microlithography,” J. Phys. D Appl. Phys. 44(25), 253001 (2011). [CrossRef]
5. M. Banyay and L. Juschkin, “Table-top reflectometer in the extreme ultraviolet for surface sensitive analysis,” Appl. Phys. Lett. 94(6), 063507 (2009). [CrossRef]
6. A. Bayer, F. Barkusky, S. Döring, P. Großmann, and K. Mann, “Applications of compact laser-driven EUV/XUV plasma sources,” X-Ray Opt. Instrum. 2010, 1–9 (2010). [CrossRef]
7. C. Peth, F. Barkusky, and K. Mann, “Near-edge x-ray absorption fine structure measurements using a laboratory-scale XUV source,” J. Phys. D Appl. Phys. 41(10), 105202 (2008). [CrossRef]
8. R. K. Hocking, S. DeBeer George, K. N. Raymond, K. O. Hodgson, B. Hedman, and E. I. Solomon, “Fe L-edge x-ray absorption spectroscopy determination of differential orbital covalency of siderophore model compounds: electronic structure contributions to high stability constants,” J. Am. Chem. Soc. 132(11), 4006–4015 (2010). [CrossRef] [PubMed]
9. W. Chao, B. D. Harteneck, J. A. Liddle, E. H. Anderson, and D. T. Attwood, “Soft x-ray microscopy at a spatial resolution better than 15 nm,” Nature 435(7046), 1210–1213 (2005). [CrossRef] [PubMed]
12. U. Vogt, R. Frueke, T. Wilhein, H. Stollberg, P. Jansson, and H. Hertz, “High-resolution spatial characterization of laser produced plasmas at soft x-ray wavelengths,” Appl. Phys. B 78(1), 53–58 (2004). [CrossRef]
14. T. Higashiguchi, T. Otsuka, N. Yugami, W. Jiang, A. Endo, B. Li, P. Dunne, and G. O’Sullivan, “Feasibility study of broadband efficient “water window” source,” Appl. Phys. Lett. 100(1), 014103 (2012). [CrossRef]
15. H. Fiedorowicz, A. Bartnik, Z. Patron, and P. Parys, “X-ray emission from laser-irradiated gas puff targets,” Appl. Phys. Lett. 62(22), 2778–2780 (1993). [CrossRef]
16. S. Kranzusch and K. Mann, “Spectral characterization of EUV radiation emitted from a laser-irradiated gas puff target,” Opt. Commun. 200(1-6), 223–230 (2001). [CrossRef]
17. G. Kubiak and M. Richardson, US Patent 5,577,092 (1996).
18. H. Fiedorowicz, A. Bartnik, H. Daido, I. Choi, M. Suzuki, and S. Yamagami, “Strong extreme ultraviolet emission from a double-stream xenon/helium gas puff target irradiated with a Nd:YAG laser,” Opt. Commun. 184(1-4), 161–167 (2000). [CrossRef]
19. T. Mey, M. Rein, P. Großmann, and K. Mann, “Brilliance improvement of laser-produced soft x-ray plasma by a barrel shock,” New J. Phys. 14(7), 073045 (2012). [CrossRef]
20. U. Vogt, H. Stiel, I. Will, P. Nickles, W. Sandner, M. Wieland, and T. Wilhein, “Influence of laser intensity and pulse duration on the extreme ultraviolet yield from a water jet target laser plasma,” Appl. Phys. Lett. 79(15), 2336–2338 (2001). [CrossRef]
21. J. MacFarlane, C. Rettig, P. Wang, I. Golovkin, and P. Woodruff, “Radiation-hydrodynamics, spectral, and atomic physics modeling of laser-produced plasma EUVL light sources,” Proc. SPIE 5751, 588–600 (2005). [CrossRef]
22. S. Kranzusch, C. Peth, and K. Mann, “Spatial characterization of extreme ultraviolet plasmas generated by laser excitation of xenon gas targets,” Rev. Sci. Instrum. 74(2), 969–974 (2003). [CrossRef]
23. D. Proch and T. Trickl, “A high-intensity multi-purpose piezoelectric pulsed molecular beam source,” Rev. Sci. Instrum. 60(4), 713–716 (1989). [CrossRef]
24. Y. Ralchenko, A. E. Kramida, J. Reader, and N. A. Team, NIST Atomic Spectra Database 8 (ver. 4.1.0). http://physics.nist.gov/asd [accessed April 2012].
25. D. Verner, E. Verner, and G. Ferland, “Atomic Data for Permitted Resonance Lines of Atoms and Ions from H to Si, and S, Ar, Ca, and Fe,” At. Data Nucl. Data Tables 64, 1–180 (1996). [CrossRef]
26. L. Podobedova, J. Fuhr, J. Reader, and W. Wiese“Atomic spectral tables for the Chandra x-ray observatory. Part IV. Ne V – Ne VIII,” J. Phys. Chem. Ref. Data 33(2), 525–540 (2004).
27. J. MacFarlane, I. Golovkin, P. Wang, P. Woodruff, and N. Pereyra, “SPECT3D – A multi-dimensional collisional-radiative code for generating diagnostic signatures based on hydrodynamics and PIC simulation output,” H. Ener. Dens. Phys. 3(1-2), 181–190 (2007). [CrossRef]
28. D. McQuarri, Statistical Mechanics (Harper & Row, 1976).
29. D. Colombant and G. Tonon, “X-ray emission in laser-produced plasmas,” J. Appl. Phys. 44(8), 3524–3537 (1973). [CrossRef]
30. T. Otsuka, D. Kilbane, J. White, T. Higashiguchi, N. Yugami, T. Yatagai, W. Jiang, A. Endo, P. Dunne, and G. O’Sullivan, “Rare-earth plasma extreme ultraviolet sources at 6.5-6.7 nm,” Appl. Phys. Lett. 97(11), 111503 (2010). [CrossRef]
31. E. Gabl, B. Failor, G. Busch, R. Schroeder, D. Ress, and L. Suter, “Plasma evolution from laser-driven gold disks. I. Experiments and results,” Phys. Fluids B 2(10), 2437–2447 (1990). [CrossRef]
32. H. Fiedorowicz, A. Bartnik, M. Szczurek, H. Daido, N. Sakaya, V. Kmetik, Y. Kato, M. Suzuki, M. Matsumura, J. Tajima, T. Nakayama, and T. Wilhein, “Investigation of soft x-ray emission from a gas puff target irradiated with a Nd:YAG laser,” Opt. Commun. 163(1-3), 103–114 (1999). [CrossRef]
33. Y. Li and R. Fedosejevs, “Density measurements of a high-density pulsed gas jet for laser-plasma interaction studies,” Meas. Sci. Technol. 5(10), 1197–1201 (1994). [CrossRef]
34. R. Rakowski, A. Bartnik, H. Fiedorowicz, F. Gaufridy de Dortan, R. Jarocki, J. Kostecki, J. Mikołajczyk, L. Ryć, M. Szczurek, and P. Wachulak, “Characterization and optimization of the laser-produced plasma EUV source at 13.5 nm based on a double-stream Xe/He gas puff target,” Appl. Phys. B 101(4), 773–789 (2010). [CrossRef]
35. B. Li, T. Higashiguchi, T. Otsuka, W. Jiang, A. Endo, P. Dunne, and G. O’Sullivan, “‘Water window’ sources: Selection based on the interplay of spectral properties and multilayer reflection bandwidth,” Appl. Phys. Lett. 102(4), 041117 (2013). [CrossRef]
36. W. Kruer, The physics of Laser Plasma Interaction (Westview Press, 2001).
37. O. F. Hagena, “Nucleation and growth of clusters in expanding nozzle flows,” Surf. Sci. 106(1-3), 101–116 (1981). [CrossRef]
38. T. Ditmire, R. Smith, R. Marjoribanks, G. Kulcsár, and M. Hutchinson, “X-ray yields from Xe clusters heated by short pulse high intensity lasers,” Appl. Phys. Lett. 71(2), 166–168 (1997). [CrossRef]
39. T. Ditmire, T. Donnelly, R. W. Falcone, and M. D. Perry, “Strong x-ray emission from high-temperature plasmas produced by intense irradiation of clusters,” Phys. Rev. Lett. 75(17), 3122–3125 (1995). [CrossRef] [PubMed]
40. P. Vrba, M. Vrbová, P. Brůža, D. Pánek, F. Krejčí, M. Kroupa, and J. Jakůbek, “XUV radiation from gaseous nitrogen and argon target laser plasmas,” J. Phys. Conf. Ser. 370, 012049 (2012). [CrossRef]
41. B. Henke, E. Gullikson, and J. Davis, “X-Ray Interactions: Photoabsorption, Scattering, Transmission, and Reflection at E = 50-30,000 eV, Z = 1-92,” At. Data Nucl. Data Tables 54, 181–342 (1993). [CrossRef]