Soft X-ray emission of laser-produced plasmas: Comparison for 30-ps and 20-ns laser pulses

JOURNAL

OF X-RAY SCIENCE AND TECHNOLOGY

I,12

l- 133 ( 1989)

Soft X-Ray Emission of Laser-Produced Plasmas: Comparison for 30-ps and 20-ns Laser Pulses H. VAN BRUG,' G. E. VAN DORSSEN, AND M. J. VAN DERWIEL Association Euratom-FOM, FOM Institutefor Plasma Physics ‘Rijnhuizen, ” Edisonbaan 14.3439 MN Nieuwegein, The Netherlands Received August 1,1988; revised October 12,1988 Soft x-ray emission spectra (250-875 eV) are presented for plasmas, produced by picosecond and nanosecond frequency-doubled NdYAG-glass laser pulses incident on 14 different target materials. The emitted spectra have been corrected for various apparatus functions which enables a direct comparison between plasmas produced by pica- and nanosecond laser pulses. The relative integrated emission intensity as a function of Z number, obtained from the corrected spectra, showsan oscillatory behavior, with distinct maxima for those elements exhibiting a dominant line emission in our photon energy window. We found for our two pulse lengths an approximately equal conversion e5ciency from laser light into x-ray photons. General suggestionsare given as to what target material should be used for different applications using the laser plasma as x-ray source in the energy range studied. 0 1989 AcademicPress, hc. 1. INTRODUCTION

Laser-produced plasmas (LPPs) are currently being used as x-ray sources in different types of experiments (1-8). The conversion efficiency of laser photons into x rays has been studied extensively, and models that can be used for the description of LPPs have been derived (9-13). However, very few articles show spectra of LPPs, notably in the soft x-ray region. Such spectra yield information on what sort of laser target material to use for which application. In this paper we present spectra, in the soft xray region, of LPPs produced both by pica- and nanosecond laser pulses. Why did we choose an LPP as soft x-ray source instead of a storage ring or rotating anode source? The main advantages of the LPP as x-ray source are the high momentary photon flux, the small source size, and the easeof energy calibration of the spectrometer. Furthermore, an LPP is a relatively simple system that can be built at low cost. These advantages make the LPP an ideal source for time-resolved and smallscale experiments. For the purpose of energy calibration it is useful to work with laser target materials that emit a line spectrum in the energy range of interest. Since the energies of these lines are known, this provides the user with a rapid and simple scheme for calibrating the equipment at several energies in the range under investigation. ’ Present address:DeM University of Technology, Department of Applied Physics, Lorentzweg 1,2628 CJ Delft, The Netherlands. 121

0895-3996189$3.00 Copyright 0 1989 by Academic Pms, Inc. All rights of reproduction in any form reserved.

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To determine what target material must be chosen for a given experiment, one should know in what energy range the experiments will take place and whether a continuum or sharp line structures are required. In x-ray absorption measurements, like EXAFS, it is common practice to remove spectral features inherent to the instrument and not to the sample, by measuring two spectra, one with and one without the sample in the x-ray beam. After dividing the spectrum without sample (the reference spectrum) by the one with the sample, one obtains the absorption spectrum of the sample. If background signal is present, the division yields a modified absorption spectrum from which the instrumental features will not be fully removed. If the instrumental structures have different frequencies than those in the EXAFS signal, these structures do not need to be a problem as long as only the bond distances are investigated. In practice, since the background function is generally not known, a smooth source spectrum is to be preferred. With regard to characteristic line emission from an LPP, in previous work (6) we demonstrated that it can be used for low-2 element analysis of the laser target material. In these experiments we determined the relation between the intensity of the lines originating from specific elements in the target material and the concentration of these elements in the target. It was found that low-2 elements could be detected quantitatively as long as the concentration was low (typically below 1%) and as long as the matrix did not consist of high-2 elements. This statement holds for our photon energy range (between 250 and 1000 eV) and our plasma conditions. Another type of application of an LPP is in x-ray microscopy and lithography. For these applications the spectral shape is not so very important, as long as the x-ray yield is high inside a desired energy window, which is determined by the photosensitive resist material and protective foils in front of the x-ray mask. The type of experiment for which the LPP is ideally suited is the “pumpandprobe” experiment where the spectra are recorded with one single laser shot, in order to obtain good time resolution. This schemewas used for the study of the laser anneal process of Si (8) and the laser-induced cluster formation of Si ( 7). The main part of the laser output is used to produce the plasma, while a small part is used to irradiate the sample. The obtained time resolution is equal to the length of the x-ray pulse if an integrating detector is used. In the case of 20-ns laser pulses we measured the xray pulse to be equally long. For the 30-ps laser pulses the x-ray pulse length was not measured, but in the literature (14) ps x-ray pulses are mentioned. We are in the processof extending our earlier work on nanosecond pulses for timeresolved absorption and reflection experiments ( 7, 8) to the picosecond regime. The goal of the present investigation is to determine the spectral shape and the x-ray flux of the laser plasmas produced by ps pulses for several target materials, and the differences with the plasmas produced by ns pulses. The main differences between these two types of spectra, as well as the relative conversion efficiency, will be discussed. 2. EXPERIMENTAL

The apparatus used consists of a frequency-doubled Nd:YAG-glass laser system (532 nm light after frequency doubling), a polychromator, and a multichannel x-ray detector. For a schematic outline of the apparatus seeFig. 1.

SOFT X-RAY EMISSION OF LASER-PRODUCED

PLASMAS

123

FIG. 1. Schematic drawing of the apparatus. ( 1) Nd:YAG/glass laser system, (2) second harmonic generator, (3) beam splitter, (4) lens of 13cm focal length, (5) laser target, (6) toroidal mirror, (7) source image position used as measuring position for source size determination and for absorption measurements, (8) concave grating, (9) multichannel detector, (10) vacuum system, = 10m6Pa.

The laser is a commercially available system (Quantel, NGdONP), which can be operated in both a ns and a ps mode. In the ns mode it produces 20-ns pulses with an energy of -20 J per pulse, by Q-switching. In the ps mode, which is achieved by mode locking the system, the output pulses of the laser have a 30-ps duration with a pulse energy of 0.6 J. Mode locking is done both passively and actively, using a dye cell and acoustic mode locker, respectively. From the pulse train coupled out of the oscillator, the strongest pulse is selected for further amplification. After frequency doubling, the output of the laser is focused on a target to a spot 65 pm in diameter by a lens of 13-cm focal length. We frequency-double the output pulse to prevent light, reflected by the plasma or target surface, from reentering the laser system and damaging the oscillator YAG rod. Another reason for frequency-doubling is that the coupling with the plasma is better for shorter wavelengths, resulting in higher electron temperatures in the plasma and thus in a higher conversion efficiency. The target materials used to record spectra are C, Mg, Al, Ti, Fe, Ni, Cu, MO, Ag, Sn, Ta, Au, Pb, and Bi. The power density at the focus was approximately 6 X lOI W/cm2 (4 J of 532 nm laser light) for the 20-ns pulses and 1 X lOI W/cm2 (0.1 J of 532 nm laser light) for the 30-ps pulses, which is sufficiently high to produce a plasma. This plasma yields photons from the visible region up to as high as 1000 eV and even above. The emission above 1000 eV could not be detected due to our inappropriate polychromator design; i.e., the throughput of the polychromator becomes too low. An image of the plasma is created outside the source chamber by means of a collecting, Au-coated, toroidal mirror. An angle of 2.39” is used to obtain a stigmatic image of the plasma source outside the source chamber. The size of the x-ray source can be determined by scanning a 50-pm slit through this intermediate image and recording the throughput. We found an x-ray spot size of 85 Mm. The x-ray beam is dispersed by a holographic, blazed concave Pt grating. A grazing angle of incidence of 4” is used in order to obtain a line focus for the dispersed spectrum at the image plane. This grating is optimized for the first order in the soft x-ray region, by blazing for a wave-

124

VAN BRUG, VAN DORSSEN, AND VAN DER WIEL

length of 23 A. Between the toroidal mirror and the grating a 1000-A carbon foil is inserted in the x-ray beam in order to reduce the amount of intense UV, which causes scattering at apertures and optical components, and a tail of the UV zero order of the grating that is superimposed on the high-energy side of our spectrum. The optimum of the x-ray throughput has been found to be at approximately 400 eV, which is the energy range where the C foil absorbs -40%. Therefore the insertion of the C foil suppressesthe scattered light contribution and the zero-order tail drastically. The dispersed emission spectrum from 250 to 1000 eV, with a resolution of 4 eV at 250 eV, increasing to 6 eV at 1000 eV, is recorded on a gold photoelectrode. Gold was chosen as conversion electrode material for its smooth emission curve as a function of energy in the photon range of interest. Registration of the photoelectron yield across the full electrode on a position-sensitive detector provides the possibility of single-shot spectral measurements, with a 20-m or 30-ps probe time. The electron yield is recorded by a multichannel plate-phosphorscreen assembly and an optical multichannel detector. The positional information is retained by a 0.12-T magnetic field, guiding the electrons from the photoelectrode to the position-sensitive detector. For more information on the polychromator design and the detector see Refs. (15, 16). 3. RESULTS AND DISCUSSION

3.1. Uncorrected Spectra In Fig. 2 emission spectra for three of the elements used as LPP target are shown, Al, Ti, and Ta, because of their characteristic spectral shape. Aluminum represents the low-Z, titanium the medium-Z, and tantalum the high-Z elements. The spectra on the left-hand side are produced by 20-ns laser pulses with a pulse energy of 4 J of 532 nm light and are summations over three laser shots. For the right-hand side spectra, five shots of approximately 100 mJ of 532 nm light per 30-ps laser pulse were accumulated. These spectra are uncorrected for system features, they have been converted only to energy scale. For the energy calibration, lines emitted by Al and Ti targets were used (Table 1). Knowing the energy of the emission lines (I 7) and using the formula for the dispersion of the grating, we can convert the channel number scale in the raw data to the photon energy scale. The first thing to be seen from these spectra is that a typical low-Z element shows sharp line structures that can be resolved by our resolution (about 4-6 eV in the energy range under investigation). An intermediate-Z element shows structures that consist of peaks which overlap at our resolution, and a high-Z element finally shows hardly any line structure at all. The reason for this lies in the fact that the low-2 elements in the plasma are stripped to an ionization state where very few electrons are present in the outermost shell. This results in a low number of electrons contributing to the bound-bound transitions. For low-Z elements the energy separation between many of these transitions is larger than our resolution, becausethe transition is between two low or a high and a low n-number energy level (where n is the principal quantum number). The intermediate and high-Z elements are stripped up to a level where there are still many electrons in the outer shell of the highly ionized atom, resulting in closely packed bound-bound transitions, i.e., between two high n-num-

SOFT X-RAY EMISSION OF LASER-PRODUCED 2.0

125

PLASMAS

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FIG. 2. The uncorrected emission spectra of both the ns- and ps-produced plasmas for three sample elements (Al, Ti, and Ta).

ber energy levels. For the higher energy levels the number of allowed transitions becomes larger. When the separation between allowed transitions becomes smaller than the intrinsic linewidths of these transitions one speaks of unresolved transition arrays (UTA). The line structures in the ps spectra at 367.5, 547, and 653.6 eV originate from contaminations on the surface of the target materials. The peak at 367.5 eV is present in the spectra of Ag, Al, Cu, Fe, Mg, and Ti and is due to carbon (C VI ls-2p transition). It is needlessto say that this peak is also present in the spectrum of carbon. The other two peaks are clearly visible only in the Al, Cu, and Mg spectra and are due to TABLE 1 Assignment and Energies of Line Structures Observed in the Plasma Emission Spectra of Al and Ti, Used for the Calibration of the Emission Spectra

Al XI

Ti XIII

Transition

Energy WI

2s3p 2p-4d 2s-4p 2p6-2~~3s 2p6-2p53d

256.5,256.7 316.5,317.2 338.1 459.9,465.4 523.2,530.9

126

VAN BRUG, VAN DGRSSEN, AND VAN DER WIEL

oxygen (653.6 eV, 0 VIII ls-2p transition) and nitrogen (-547 eV, probably N VII ls-2p, ls-3p transitions). From these measurements it becomes clear that the ps mode is more sensitive to surface contaminations than the ns mode. This is because in the ns mode more material is removed from the bulk of the target by each laser pulse, due to the higher energy deposition into the plasma. 3.2. Corrected Spectra In order to obtain the true spectral behavior of the plasma emission, it is necessary to correct for the instrumental throughput. Structures in the throughput originate from the toroidal mirror, the concave grating, and the 1000-A carbon foil. The reflectivity of the toroidal mirror, for the grazing angle of incidence of 2.39”, is shown in Fig. 3a. Calculations of the reflectivity were performtd by use of the scattering factors given by Henke (18). The transmission of the 1000-A carbon foil is shown in Fig. 3b (18). The reflectivity of Pt at an angle of incidence w.r.t. the surface of the grating of 6.5” is shown in Fig. 3c, calculated using Ref. (18) (4” w.r.t. the overall surface plus 2.5” becauseof the blaze angle). The product of the reflectivities and the transmission is shown in Fig. 3d, where the blaze influence has also been taken into account, by using a rule of thumb to calculate the efficiency of the grating: the efficiency drops to 50% for an energy of $ times the blaze energy (EO)and $ times the blaze energy (19). Using these values we constructed an efficiency curve as a function of the energy, consisting of cosine functions between the f and the $E,,points, with a maximum at EO (lOO%), and exponentially decreasing functions below and above this energy range. Note that we take the gold photon-to-electron conversion efficiency to be constant. Given the above set of assumptions, it is clear that the result of the throughput 801

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SOFT X-RAY EMISSION OF LASER-PRODUCED

127

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spectrum is only approximate. Nevertheless, it is obvious that sizable corrections are necessaryin order to obtain at least a qualitative idea of the true spectral behavior of the x-ray emission. To correct the emission spectra for the features inherent to the apparatus, the spectra are divided by the instrumental throughput function as shown in Fig. 3d, after background subtraction. The direct result of the measurement, X(E), can be defined as Y(E)*f(@ + B, where Y(E) is the x-ray yield as a function of energy, f(E) the relative throughput function of the apparatus, and B the background contribution. We assume the latter to be constant, for lack of a proper method for determining its actual behavior. This assumption is not unreasonable, since the main contribution of the background originates from scattering at the surface of the grating. This results in a more or lesshomogeneous distribution of scattered light over the gold conversion electrode. The background contribution was taken as the minimum in the uncorrected spectra, in all casesat 1000 eV. This can be understood sincef(E) is practically zero for this energy. After subtraction of the background the spectra were divided by f(E). The resulting x-ray yields are shown in Figs. 4a-4e, grouped according to the rows in the periodic table. The results in the 87% 1000 eV region are subject to very large uncertainties and are therefore suppressed. In these figures the left-hand side has been recorded with the ns and the right-hand side with the ps pulses. These spectra provide information on the relative contribution from line radiation and continuum. The spectra of Fig. 4 are all given on the same arbitrary scale, and therefore the relative intensity of the elements can be derived from these spectra. As regards the continuum contribution, in previous work (6) a value of 60 eV for the plasma temperature has been found for the plasma produced in the ns mode. The shape of the continuum can be calculated using x-ray yieldfreeebound = Ce-‘h”‘kTJ, which was obtained from Ref. (12). It is clear that all spectra exhibit a continuum emission similar to that given by this formula. Below it is discussed that 100 eV is about the plasma temperature in the ps mode. The error in the spectral shape due to uncertainties inf(E) and in the assumption on the background contribution makes it impossible to derive the plasma temperature by fitting the continuum emission function to the measured low-energy side of the spectrum. Nearly all spectra, in both ns and ps modes, show the same continuum contribution. pica

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4a. X-ray yield of an LPP as a function of energy, for a C target.

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In Fig. 4a the emission spectra of carbon are presented. From these spectra it can be seenthat the emission of carbon is mainly a very low continuum, with a sharp and very strong set of emission lines superimposed upon it. The line at 367.5 eV indicates that carbon is ionized to the C5+state ( 1s-2p transition). In the ps-produced plasma, the plasma temperature is higher due to the higher power density, which causesthe continuum to be relatively more intense. Figure 4b shows emission spectra of the third-row elements magnesium and aluminum. In the ps-pulse spectra the additional lines originating from pollution of the surface of the target material have been discussed in Section 3.1. The line structure at the low-energy side of the spectra is more intense for the ns-pulse spectra than for the ps-pulse spectra. For the aluminum spectrum the three peaks at about 350 eV can no longer be seenin the ps spectrum due to the carbon pollution and the reduced emission strength caused by the shifted optimum in the x-ray line emission, i.e., a shift in abundance from All’+ to Al”+ . This requires a plasma temperature shift from 60 up to about 100 eV, under the assumption that the collisional-radiative model of Ref. (I I) is valid for our plasmas. From Fig. 4c, giving the Ti, Fe, Ni, and Cu results, it can be seen that for the row-four elements the line emission of x rays increases for the higher energies. Also apparent from these spectra is that with increasing Z number the UTA (originating from L-shell transitions) shifts toward higher energies, indicating that the photons are created by the same set of transitions. The UTA for the spectrum of Ti can be seen to be broader for the ps spectrum than for the ns spectrum, again indicating a higher plasma temperature for the picosecond LPPs. The spectra of the row-five elements MO, Ag, and Sn are shown in Fig. 4d. For these elements it is also clear that the line emission is relatively more intense at the high-energy side of the spectrum for the ps emission spectra. Whereas in the ns spectrum of MO clear peaks can be seenat low energies, there is mainly a broad hump in the caseof the ps spectrum and the peaks at low energies are hardly visible. Both the

SOFT X-RAY EMISSION OF LASER-PRODUCED

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4c. X-ray yield of an LPP as a function of energy, for Ti, Fe, Ni, and Cu targets.

peaks and the hump are likely to be M-shell UTAs. The same holds for Ag and Sn: a decreaseat lower energies, less pronounced features, and an increase in yield at the high-energy side of the spectrum when going from ns to ps pulses. In Fig. 4e, the spectra of Ta, Au, Pb, and Bi are shown (row-six elements). In the ns casethese spectra are all very smooth with a small increase in yield toward higher energies. For the ps mode there is a very small hump at about 600-700 eV for all these spectra. The corresponding UTA is hardly visible in the ns mode.

3.3. Integrated Yield The x-ray yield, obtained by integrating the spectra of Fig. 4 from 300 to 875 eV, is shown in Fig. 5 as a function of the atomic number, for both the ns and the ps spectra on the same relative scale. This energy range was chosen because we wanted to exclude the sharp features around the C-K edge (=284 eV).

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4d. X-ray yield of an LPP as a function of energy, for MO, Ag, and Sn targets.

For determining the relative conversion efficiencies for the ns and ps modes several experimental differences must be corrected for. When recording the spectra in the ns mode the channel plate voltage was lower than that in the ps mode, which accounts for a factor of 2 difference in amplification. Also, the numbers of shots accumulated for the two modes were different. The voltage difference has been taken into account as an amplification factor, and the spectra are divided by the number of accumulated shots. We have normalized the yield on a per photon base. Doing this we found that for the two power densities used the number of x-ray photons per laser photon is constant within a factor of 3. The largest differences are observed for the low-2 elements (Al, Mg) and intermediate-Z (Ti, Fe, Ni, Cu), for which the ns plasma produces more efficiently x rays in our energy window than the ps plasma. For the high-Z elements Au, Pb, and Bi, the ps plasma is more efficient than the ns plasma. In Fig. 5a the integrated yield is given for the ns mode, in 5b for the ps mode. Our data allow the conclusion that oscillatory behavior occurs; this is consistent with earlier work (20-26) in other energy windows. In both modes it is found that the total yield is relatively high when line radiation is present in the spectrum and that the integrated yield of Ag is lower than can be expected from these figures, probably due to misalignment of the Ag target. In Fig. 5 the maxima are labeled with the letters K, L, M, and N indicating up to what shell the elements are stripped. For C the 1s electrons produce the line radiation, for Mg and Al mainly the 2s and 2p electrons. For the higher Z elements higher level electrons are responsible for the radiation in our

SOFT X-RAY EMISSION OF LASER-PRODUCED “Cl”0

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energy window. In Fig. 5b it is clear that the Npeak is shifted toward higher 2 numbers. The differences in the yield between the ns and the ps mode are caused by the higher power density in the ps mode. It can be seen from the height of the L peak that the elements Al, Ti, and Fe are stripped of their L electrons to a smaller extent in the ns mode than in the ps mode. Again this is consistent with a higher temperature in the ps mode. 4. CONCLUSIONS

Laser-produced plasmas are very powerful soft x-ray sources. They provide either intense line emission on top of a relatively low continuum or high-intensity continuum emission, depending on the choice of the target material. In general the low-Z elements show a high line emission level in the 250- 1000 eV range, while the high-Z elements (Z k 73) show dominantly a smooth emission. We have shown emission spectra for 14 elements after correction for the features inherent to the apparatus. From these spectra the total yield as a function of atomic number has been obtained.

132

VAN BRUG, VAN DORSSEN, AND VAN DER WIEL

0

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FIG. 5. The X-ray yield, integrated from 300 to 875 eV as a function of the atomic number for (a) ns spectra and (b) ps spectra.

In the caseof absorption measurements, where one must take quotients of spectra in order to cancel instrumental features it is best to use high-2 elements, e.g., Ta or Au, becauseof the smooth emission spectrum and high photon flux, allowing the use of a thicker foil, or a higher density in the absorption cell. For experiments where emission in a given energy window is required, like lithography, other materials may be better. For these experiments the throughput of protective foils, resist absorption characteristics, and x-ray mask determine the desired energy range. Since most x-ray masks are Si based, it is important to stay either below or well above the Si-K edge.With the power density achieved with a laser system like ours operated in the nanosecond mode, this restricts the experimentalist to elements like Fe, Ni, and Cu or perhaps to other row-four elements. If an LPP is used for the element analysis of the laser target itself, the lines emitted by the elements in the target should be sharp, and the lines should not be too closely packed. This restricts the technique to low-Z elements only. ACKNOWLEDGMENTS This work is part of the research program of the Association Euratom-FOM (Foundation for the Research on Matter) and was made possible by financial support from NW0 (the Netherlands organization for scientific research).

SOFT X-RAY EMISSION OF LASER-PRODUCED

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