Observations of soft X ray emission and plasma dynamics of a compact capillary discharge operated in xenon
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(2) PHYSICS OF PLASMAS 20, 093113 (2013). Observations of soft X-ray emission and plasma dynamics of a compact capillary discharge operated in xenon J. C. Valenzuela,a),b) E. S. Wyndham,c) M. Favre, and H. Chuaquid) Facultad de Fısica, Pontificia Universidad Cat olica de Chile, Av. Vicu~ na Mackenna 4860, Macul, Santiago, Chile. (Received 26 April 2013; accepted 23 August 2013; published online 27 September 2013) We report observations of a low stored energy, low inductance compact capillary discharge operated in xenon. Even though the stored electrical energy is less than 1 J, significant output in the optical windows at 110 and 135 Å is measured. The soft X-ray emission is time-resolved and the conversion energy of the source is obtained. A lower bound to the conversion efficiency at 110 Å 6 2% and 135 Å 6 1% of 3.6% and 1.6% is obtained, respectively. The use of moire-schlieren optical diagnostic allows the evolution of the line electron density. In particular, we observe a significant degree of compression in a tight on axis pinch as well as radial compression waves. The temporal evolution of the X-ray emission, which occurs during the current reversal and later, is discussed in relation to work in argon discharges and in relation to model C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4823706] calculations. V. I. INTRODUCTION. Capillary discharges with a large aspect ratio have been researched in a wide variety of implementations both for its interesting physics as well as its applications as an intense extreme ultra-violet (EUV) or soft X-ray source, initially incoherent1,2 and more recently coherent.3 Temperatures of order of tens of eV are easily obtained with quite modest stored electrical energy with accompanying electron densities in excess of 1 1017 cm3. While earlier work relied on material ablated from the walls,2 a gas filling is usually preferred, allowing good spectral purity. In this context, it was soon recognized that an axial electron beam generated a volume initiated discharge4,5 having important advantages in the dynamic and the radiated spectra. The small size of the emitting source is compatible with soft X-ray microscopy in the water window band (23–44 Å), of great interest to the biological sciences6 and also for soft X-ray metrology and microscopy at 135 Å.7 The experimental results of this work were obtained in a low stored energy, low inductance discharge that exploits the Transient Hollow Cathode Mechanism (THCM)5,8 in order to obtain a volume initiated discharge, as opposed to a wall initiated discharge where the plasma lifts off from the wall. This results in a spectrum mostly of the filling gas, as it has been demonstrated in discharges in nitrogen9 and in argon,8 although presence of wall material is more evident in the case of nitrogen. The THCM capillary discharges are selftriggered as the hollow cathode generated axial electron beam causes a returning ionization wave leading to a). This work was performed when J. C. Valenzuela was with the Instituto de Fısca, Pontificia Universidad Cat olica de Chile, Chile. b) Present address: Center for Energy Research, University of California at San Diego, La Jolla, California 93093, USA. c) Author to whom correspondence should be addressed. Electronic mail: [email protected] d) Deceased. 1070-664X/2013/20(9)/093113/7/$30.00. breakdown.10 In operation, the gas pressure is adjusted so that discharge occurs at the peak of the charging voltage, which is derived from the external pulsed power charger.8 Both the volume pre-ionization and minimal inductance geometry have the effect of limiting the wall plasma interaction. The low stored energy (<1 J) may be compensated for by fast repetition rate operation.8,11 In this work, we present an extensive series of experimental results obtained from a fast capillary discharge exploiting the THCM,8 operated with xenon. A particular motivation is to study conditions of this implementation of the capillary discharge as a possible radiation source whereby the requirements7 for EUV lithography may be satisfied. While a precise measurement of the soft X-ray output was not feasible, the lower bound of output at two wavelengths of particular interest, 110 and 135 Å, is obtained with sufficient accuracy for a preliminary measurement. Xenon is preferred for such an EUV source. Nevertheless, only Xe XI emits in the 135 Å 6 1% spectral band resulting in low conversion efficiency (CE). Consequently, high energy input is required to meet the requirements7 of high volume manufacturing. The highest CE measured for Xe in discharge produced plasmas (DPPs) and laser produced plasmas (LPPs) so far is approximately 1%,12 whereas theoretical models predict a limit of 2%–4%.13 For tin, in contrast, multiple ionization stages contribute to emissions around 135 Å, namely Sn IX–XIII, thus a higher CE can be obtained. Theoretical models predict as much as 7.5% of CE for tin.13 So far, a 2% and 2.1% has been reported in tin for DPP and LPP, respectively.14 Nevertheless, debris mitigation remains a serious technical problem in Sn EUV sources. The initial industry requirement was for 115 W of light at the intermediate focus (IF),7 but recent requirements are even more severe;15 hence, even a marginal difference in source efficiency is significant. In consequence, a detailed understanding of the physical processes of this compact, fast capillary discharge is relevant to achieve greater CE. Experimental observations. 20, 093113-1. C 2013 AIP Publishing LLC V. 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(3) 093113-2. Valenzuela et al.. are valuable as benchmarks for theoretical models. To this end, we present time resolved observations of both the soft X-ray spectra and the electron line density using moireschlieren deflectometry. Together, these diagnostics allow some important points of the plasma dynamics to be resolved. Peak soft X-ray emission occurs in the second quarter period of the current cycle, extending well into the current reversal period. This observation differs from the results of the closest theoretical model16 to our geometry. The moire-schlieren diagnostic17 reveals an axial volume with extreme density gradients during the emission. Within the inherent limits of electron line density profiles, we obtain evidence of incoming radial compression waves which may be associated with an important degree of compression on axis. By associating the scale length of the electron line density structure with the scale length of the X-ray source plasma, an estimate of the local electron density may be obtained. II. EXPERIMENTAL DETAILS. The capillary discharge of this report is optimized for minimal inductance and high repetition rate operation by using deionized water as dielectric and coolant.8 In Fig. 1, we show the capillary and X-ray diagnostics. The capillary tube of 1.6 mm inner diameter and 21 mm long is mounted integrally between the two capacitor plates, whose capacitance is 1.6 nF. The capacitor is directly charged in 1 ls to 20–30 kV. The current is monitored using a single groove Rogowsky (coil) placed as close to the anode as possible, which is at ground potential, see Fig. 1 of Ref. 8. For a given capillary geometry, the HC inlet pressure is adjusted so that breakdown occurs at peak voltage. The whole assembly is mounted on NW50 vacuum fittings. Gas is fed in at the hollow cathode end, which is also the live electrode. The assembly has open ends to allow gas flow through the capillary tube as well as visual access to the plasma. Both electrodes are perforated. The gas flow produces a pressure gradient along the tube. This effect also promotes the THC mechanism. To avoid the interaction of the intense electron beams (a characteristic of the hollow cathode effect) with the diagnostics, a static magnetic field is placed in the path of the e-beam (inside the NW50 tube). The magnetic field is established by a group of 8 permanent magnets that produce strength of approximately 50 mT at the center of the NW50 port. Fluorescence from the NW50 port’s wall, generated by the e-beam, is blocked out with the help of two collimators placed downstream of the magnets. The diagnostics are placed beyond the collimators. They consist of a EUV spectrometer, filtered diodes, and Faraday cups. The EUV spectrometer may be used to obtain time integrated or time-resolved spectra with a temporal. FIG. 1. Experimental arrangement (not to scale) showing the capillary discharge and Rowland circle grazing incidence spectrometer.. Phys. Plasmas 20, 093113 (2013). resolution of 2 ns, but at the cost of finesse (k / Dk 50–80). Axial moire deflectometry refractive diagnostic17 is implemented using a 12 ps, 532 nm Q-switched laser. This allows the electron line number density to be followed without fringe blurring due to fast dynamics. The sensitivity of the system is such that useful images are obtained from peak current onwards. The X-ray output is measured using the AXUV series of silicon photodiodes from IRD.18 These diodes have been well characterized both in sensitivity and reproducibility as they are extensively used.19 While an absolute measurement of sensitivity was not possible here, the manufacturer’s specification is assumed and this coincides well with Ref. 19. Fig. 5 of Ref. 19 shows that the sensitivity is wavelength dependent. At 135 Å, a value of 0.25 6 0.01 A/W is measured, while at 108 Å a value of 0.22 6 0.03 A/W is noted. The diodes are very stable to long term use with simple precautions and are not subject to significant aging effects within the limits of precision of this work.19,20 Another source of error that must be accounted for is the shot-to-shot discharge stability, corresponding to running the discharge for the period of time (1 h) required to build the spectrum. We have estimated an error of 15% in the diode signals. This error in the measurements of the conversion efficiency is the most significant. Sources of this error are ascribed to variations of the inlet gas pressure and to variations in the high voltage power supply to the IGBT (Insulated Gate Bipolar Transistor) pulser, but not the aging of the capillary itself. III. RESULTS A. Time integrated spectra. The discharge is operated in a self-breakdown mode. In all the experimental results presented in this work, a 21 mm long, 1.6 mm internal diameter (ID) capillary was used. By varying the pressure conditions, breakdown is adjusted to occur at peak charging voltage, where the soft X-ray output is greatest over the whole range observed. The peak charging voltage always increases on decreasing the pressure, but eventually all soft X-ray ceases abruptly when the discharge reverts from a volume discharge to a sliding spark wall initiated discharge.8 This coincides with an extremely rapid destruction of the capillary wall. The pressure at the anode is maintained at 5–12 mTorr, principally to limit self-absorption at 135 Å. The HC inlet pressure affects the source brightness and spectrum in a very significant way. In Fig. 2(a), we present time integrated spectra from a lower limit of 100 mTorr, determined by obtaining reliable breakdown, up to an upper limit of 350 mTorr, where the peak charging voltage is insufficient to allow useful output. On trying a range of cathode apertures, 0.8 mm was found to be the optimum diameter for soft X-ray output over the whole range monitored. In Fig. 2(b), we show the relevant line transitions from NIST,21 which are for Xe VIII to XI. It is also worth noting that there is no further information for higher ionization states in the literature. At the highest pressure shown in Fig. 2(a), 350 mTorr, we see that the dominant species is Xe IX and higher ionization stages are absent. On lowering the pressure to 295 mTorr, it is clear that three prominent structures around 110,. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:34:55.
(4) 093113-3. Valenzuela et al.. FIG. 2. (a) Upper: The observed time-integrated emission of Xe as a function of pressure together with ((b): lower) transitions from the NIST database for Xe VIII to XI. The bands with available optics for soft X-ray lithography are also indicated.. 135, and 150 Å become noticeable. At lower pressure, where a higher charging voltage is obtained, the lines of the unresolved transition array (UTA) near 110 Å shift towards the shorter wavelength of 108 Å. The shift to shorter wavelengths is ascribed to Xe XII and higher ionic states,22–24 which is to be expected with the increased initial stored energy at higher charging voltages. The optimum pressure for emission at 135 Å is seen to occur between 170 and 200 mTorr. Above 250 mTorr, the output falls off, as may be seen from the trace at 295 mTorr. Even though the emission at 135 Å does become significant, the predominant emission always corresponds to the UTA. If the 4% window at 110 Å, corresponding to Be/Mo optics, for lithography were considered viable by the lithographic community, operation at or below 200 mTorr is preferred. It has also been reported that the peak at 110 Å becomes dominant when a buffer gas is added to the plasma.25 While only Xe XI is important at 135 Å, many higher stages of xenon strongly emit in the UTA.. Phys. Plasmas 20, 093113 (2013). some 4 ns longer than the value at 250 mTorr. Both effects are indicative of the increase of the discharge resistivity with lowered pressure. An additional effect, which is particularly obvious at 80 mTorr, is that the current does not follow a symmetrical exponential decay about the zero value. It appears that the forward resistance is lower than the reverse resistance. We bring to the attention of the reader a number of features relevant to the plasma dynamics that may be observed in Fig. 3. First, X-ray emission starts at the beginning of the second quarter cycle and peaks as current inverts. The fiducial line at current reversal in the figure helps observe this point. Second, for pressures above 130 mTorr a second peak occurs during the fourth quarter cycle. The amplitude of this peak becomes the maximum output at a filling pressure of 250 mTorr. Further information of the X-ray emission is obtained by following the temporal evolution of the spectrum. This is achieved by operating the spectrometer as a monochromator and using semi-automatic data acquisition to build up a spectrum from diode’s signal, during approximately 1 h of operation. The spectrum is synthesized in 0.4 Å bins, where each bin is the average of 8 discharges. The temporal resolution of the system is 2 ns, given by the time response of the diode and bandwidth of the recording system. In Fig. 4, we show the temporal evolution for a pressure in the HC inlet of. B. Time resolved soft X-ray emission. The variation of the soft X-ray emission and the behavior of the current as a function of both the HC inlet pressure and the time is the subject of Fig. 3. In Fig. 3(a), we show the soft X-ray emission using a Si/Zr filtered diode18 sensitive over the range of 122 to 180 Å (with some residual sensitivity to below 100 Å). Other experimental conditions are as follows: the anode exit pressure is maintained below 10 mTorr, the cathode aperture is 0.8 mm and the anode aperture is 1.6 mm (also the ID of the capillary). The minimum value of the pressure shown, 80 mTorr, is determined by the onset of severe jitter. Maximum output occurs for the pressure range of 130 to 170 mTorr. In Fig. 3(b), we show the discharge current for three pressures, 80, 150, and 250 mTorr. Even though the breakdown voltage increases with diminishing pressure, from 20 kV at 300 mTorr to 28 kV at 80 mTorr, the current at 80 mTorr is well below that at 250 mTorr in proportion to the increased breakdown voltage. The first quarter period is also. FIG. 3. (a) Upper: The temporal evolution of a Si/Zr filtered diode signal as a function of the hollow cathode inlet pressure, together with ((b): lower) the discharge current at three of the seven pressures. The time of current reversal is emphasized for discharges at 250 mTorr.. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:34:55.
(5) 093113-4. Valenzuela et al.. FIG. 4. Temporal evolution of the discharge emission at a pressure of 200 mTorr. The monochromator diode signal is shown as contours without infilling.. 200 mTorr and 8 mTorr at the anode exit. The current as a function of time may be obtained by referring to the previous figure (Fig. 3(b)). The greater part of the emission is from the UTA centered on 108 Å (it was noted in respect of Fig. 3(a) that the Si/Zr filtered diode has reduced sensitivity here). This pulse lasts 5 ns at full width half maximum (yellow contours). This emission is concurrent with two other significant groups of transitions centered at 134 and 148 Å. The maxima of three other groups of lines at 90, 97, and 122 Å occur approximately 2 ns later. In the fourth quarter cycle, at approximately 33 ns, a second significant output is observed centered on 135 Å, together with less prominent groups centered on 111, 147, and 154 Å. On referring to Fig. 2, the transitions at this later time are predominantly from Xe X and Xe XI. It is important to verify the agreement between the timeintegrated spectrum taken with the CCD detector and the time-integrated spectrum obtained by integrating diode’s signals using the spectrometer in monochromator mode. The sensitivity of diode detector as a function of wavelength is known, but the CCD response is not well specified. The comparison of the results from the two methods is shown in Fig. 5(a). Here, we compare two spectra taken at 200 mTorr, one with the CCD and the other with the scanning system. It is clear that the agreement is very good. It should be noted that the finesse of the spectrometer in the monochromator mode is substantially lower, with the consequent loss of detailed spectral transition information. In both observation modes of the spectrometer presented in Fig. 5(a), the same significant broad band background signal is observed. This broadband soft X-ray background is highly significant for all the. FIG. 5. (a) Upper: Comparison between the time-integrated spectrum obtained using the CCD detector and the time-integrated signal obtained using the spectrometer as a monochromator. (b) Lower: The effect of absorption between the source and the detector at the operating pressure of Xe gas.. Phys. Plasmas 20, 093113 (2013). operating pressures below 350 mTorr (see Fig. 2, upper). Various tests were performed to ensure that this background is real. Within the limitations of the calibration of the IRD photodiode discussed in Sec. II, the monochromator-based spectrum allows an accurate measurement of the energy radiated into 2p sr. In Fig. 5(b), we also show the background gas absorption produced along the path of the light (50 cm), from the capillary tube to the diode. This is seen to be significant, especially in the region of 135 Å. Considering this, the spectral energy integrated from 90 Å to 180 Å is 135 6 24 mJ emitted in 2p sr, which is approximately 23% 6 4% of the total initial stored energy with the capillary energy storage capacitor charged to 27 kV. It is worth noting here that this amount of radiated energy is an order of magnitude greater for the same discharge operated in argon. We ascribe this at least in part to the much larger number of energy levels and transitions of xenon in this spectral range as compared to argon. From Fig. 5(a), it is also possible to measure the emission in the spectral bands of potential applications. The values obtained are 9.0 6 1.4 mJ at 135 Å 6 1% and 21 6 4 mJ at 110 Å 6 2%, which corresponds to a CE of 1.6% 6 0.3% and 3.6% 6 0.7%, respectively. These spectral windows are also shown in Fig. 2 to illustrate the ionization stages that fall within these two windows. Here, we have assumed, in the absence of external measurements or calibrations, 100% reflectivity into the first order of diffraction, so the values of CE must be considered an approximate lower bound. These values include the effect of absorption of Xe gas at the corresponding pressure, shown in Fig. 5(b), along the line of sight. deflectometry C. Moire. The line electron density was observed over the whole capillary volume by means of increasing the cathode diameter to that of the capillary internal diameter (1.6 mm). This change lowers the self-breakdown voltage to 20 kV, which is obtained with a HC inlet pressure of 150 mTorr. The use of higher pressures, with correspondingly lower breakdown voltages, did not allow sufficient fringe shifts in the moire pattern to be observed. The advantage of obtaining a full axial view leads to two effects that may cause the plasma dynamics to vary. First, the current is lower due to the lower. FIG. 6. The times at which the moire-schlieren images were taken with respect to the current and the Si/Zr filtered diode signal.. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:34:55.
(6) 093113-5. Valenzuela et al.. FIG. 7. Moire-schlieren images taken at the times indicated in Fig. 6. The line density derived from the moire effect is shown in yellow. Segmented lines indicate that an estimate of the line density is made.. self-breakdown voltage, with a maximum value of 4.8 kA (see Fig. 6) as compared with that shown in Fig 3(b) (9 kA). Second, the pressure gradient within the capillary is changed. We address the problem of the extent of possible variation to the plasma dynamics in two ways: first, in Fig. 6, we show the signal from the same Si/Zr filtered diode as shown in Fig. 3(a) together with the current. On comparing the trace at 150 mTorr of the two figures, both the timing and form are remarkably similar; only the amplitude is reduced by a half. Second, moire deflectograms were taken over a limited field of view but maintaining the pressure gradient using a cathode aperture of 0.8 mm. One such example will be presented below in Fig. 8. In Fig. 7, we show both the moire patterns over the whole capillary diameter and the line density as an overlay. The times of the images are indicated by circles on the current trace of Fig. 6. When it was not possible to follow the fringe shift with certainty, the line density is drawn as a segmented line. The asymmetry observed in the line density (at. Phys. Plasmas 20, 093113 (2013). later times) is presumed due to the propagation of error of the values obtained in the central region. The effect of schlieren,17 inherent in the optical system, reveals the presence of a growing volume on axis with severe density gradients, such that the light is refracted outside the optical collection at the output of the capillary. The axial line density full width at half maximum at 21.5 ns, which corresponds to the maximum in the Si/Zr filtered diode signal, is approximately 0.3 mm. From 25.5 ns, which corresponds to full current reversal, and for the following 10 ns we observe from the schlieren effect an inwards propagating radial feature with a mean velocity of 2.5 6 0.8 106 cm/s. This is reflected in an increased on axis line density reaching a maximum measurable value of 1.4 1018 cm2 at 32.5 ns. Between 30 and 37 ns, we observe a second maximum in the X-ray diode signal and as has been mentioned above a substantial axial volume of strong density gradients. If an average ionization stage of 10 is taken, a mean value of compression on axis of approximately a factor of six is obtained at 21.5 ns, the time of peak X-ray output. As mentioned earlier in this section, the effect of opening the cathode aperture on the plasma dynamics must be addressed. To this end, we show in Fig. 8 a moire schlieren deflectogram using a 0.8 mm cathode aperture and a HC inlet pressure of 200 mTorr. As it is not possible to integrate the deflection starting from the capillary wall to obtain the line density, we integrate from the central peak, where the value is taken to be zero. Hence, the negative values of the line density shown in Fig. 8 correspond to a distribution, which decreases radially. A further difficulty in interpretation is caused by diffraction at the edges. The time of the image is at 18.7 ns, which allows comparison with the first image of Fig. 7. On comparing the two deflectograms, we observe that the general cone hat-like shape is reproduced in both images. However, the full width half maximum is 0.3 mm in the former case and 0.2 mm in the latter. The line density in the latter case of the central conical hat-like feature is approximately twice the former case. Images taken at later times (20 ns) using the smaller aperture do not allow integration with any degree of confidence. The results of this comparison at 18.7 ns are easily explained by the increased current of the discharge obtained at the 0.8 mm diameter cathode aperture. IV. DISCUSSION. FIG. 8. Moire-schlieren image taken at 18.7 ns using a cathode aperture of 0.8 mm. The conditions correspond to those of Fig. 3. The electron line density, referenced to the axial maximum, is shown in yellow.. Large aspect ratio capillaries are difficult to access, rendering local axial measurements of plasma parameters essentially intractable to observation. In this work, we present measurements of the soft X-ray emission and the integrated electron density along the line of sight. On the other hand, it is a straightforward matter to measure the EUV/soft X-ray emission using filtered diodes and a scanning monochromator. Recently, the line electron density has been measured in argon discharges using moire-schlieren.17 Comparisons with the argon discharges indicate important characteristics of the plasma dynamics. First, the diode signals occur well after peak current, at 12 ns for argon and at 21 ns for xenon discharges. Taking into account the filling mass of the two gases and their atomic weights, the time to emission is. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:34:55.
(7) 093113-6. Valenzuela et al.. roughly proportional if an equal radial compression force (J B) is assumed for both cases. This is reasonable given that the current evolution is nearly identical in both cases. For Ar, we have reported a mean (averaging over the discharge length of 2.1 cm) on-axis electron density of 4 1017 cm3 (see Table 1 in Ref. 17), which is close to the value observed here of 3 1017 cm3. The mean ionization stage from the time resolved X-ray spectra is 11 for Xe and 8 for Ar at the time of peak emission. While an appreciable degree of compression at peak emission of approximately 4–5 has been reported for Ar, here the compression is rather greater at 6–8, while the filling number density is 40% of the argon discharges. In both gases, the line electron density continues to increase after peak emission, while an accompanying but diminished X-ray emission continues well into the third current quarter cycle in both cases. It is here that the limitation of only integrated axial data is particularly obvious, as there is a strong internal initial filling pressure gradient. However, a simple argument, supposing an approximately constant temporal behavior of the radial force along the axis, suggests that the conversion of radial kinetic energy to hot axial plasma will occur at ever increasing times on moving from the anode towards the cathode. In this respect, we observe increasing values of the line electron density well into the fourth current quarter cycle. The radial diameter at these later times reaches 0.5 mm, but the plasma still has a second emission peak at this time. At the start of the fourth current quarter cycle (26 ns), another compression wave appears from the border of the capillary with velocities of the order of 2.5 6 0.8 106 cm/s collapsing with supersonic radial velocity. This gives rise to the second peak of EUV emission at 32 ns. It might be argued that this compressional wave is also, at least in part, an ionization wave. However, due to important energy sink in the UTA, which is very significant for Xe XI/XII, we suspect that the average ionization stage does not change by more than 20% across the observed wave. If that is so, the wave is largely compressional. The size of the emitting source of the discharge has been measured from time-integrated filtered pinhole images for discharges in nitrogen.9 For emission in He-like nitrogen, a diameter of approximately 200 lm was obtained. Unpublished measurements by the same authors in argon discharges give similar values. This value is remarkably close to the characteristic width of the moire-schlieren images of Fig. 7. It is reasonable to expect a similar value here. The diameter of the Xray emitting volume, obtained from pinhole images,26 has been reported for similar discharges to this work. The dimensions, pressure gradient, and peak current of this experiment are comparable. This report covers both low and high dI/dt pulse operation of the discharge giving peak currents at 500 and 150 ns, respectively. These values are much slower than this work. Interestingly, they observe a notable decrease in the diameter of the X-ray emitting plasma in going from the long to the short current pulse, from 1 to 0.55 mm, respectively. In our work, we observe that the source size is further reduced by moving to a half period here of 20 ns. A detailed theoretical computational model exists for capillary discharges in noble gases with geometrical and electrical characteristics similar to those used here.16 The. Phys. Plasmas 20, 093113 (2013). effect of the axial pressure gradient is discussed where the combined axial and radial effects produce a 3D plasma compression. For argon discharges, a local maximum of the electron number density of 7 1019 cm3 is reported. While isopycnic contours for the argon discharges show the principal temporal behavior up to 12 ns, there are no results for the behavior of the electron density in Xe, nor is there information of the temporal behavior of the emission. Whereas the model calculations performed in a Kr:He mix find conditions where there is substantial emission during the first quarter cycle, we do not observe emission until well into the second current quarter cycle. Transient hollow cathode dynamics are well known to emit intense bursts of high energy electrons (of the order of the charging voltage) at the pre-ionization stage of the plasma.8,27 These electrons beams play a very important role in the emission process, they shift the average ionization stage to higher values, making possible to obtain emission from highly ionized states that are only possible to obtain with Maxwellian plasmas at much higher temperatures. For example, emission in argon discharges has been found from ionization states as high as Ar XI.8 Such emission is characteristic for Maxwellian plasmas with temperatures above 50 eV. However, the same ionization stages may be obtained in a thermal plasma of 15–20 eV with a small percentage (2%) of a fast electron distribution of order 200 eV.8,16 Therefore, a complete understanding of the emission may well involve these high energy low density electron beam at later times. For example, any necking or m ¼ 0 instability could be such a mechanism. The effect of schlieren, inherent in the moire-schlieren diagnostic, where all the light from the axis is refracted out of the cone of the collecting optics certainly indicates very significant radial density gradients during times of soft X-ray emission. From the results of Fig. 4, the capillary discharge operated in Xe emits approximately 23% of the initially stored energy into EUV radiation over the range of 80 to 180 Å. It should be noted that we have assumed that the grating is 100% efficient, as no manufacturer details are available. From the same figure, we obtain a conversion efficiency of 1.6% 6 0.3% at 135 Å (61%) and 3.6% 6 0.7% at 110 Å (62%). These values are the highest observed for dense pulsed plasma and are only comparable with the values obtained in tin.14 These values may be compared to those obtained from pseudospark gas discharges.28 Nevertheless, the values reported here are still impractical for EUV lithography, which requires 1 kW into 2p sr. One approach to the problem involves multiplexing capillaries where many beams are optically combined to reach the desired power at the intermediate focus.29 V. CONCLUSIONS. We have reported the soft X-ray emission characteristics of a low stored energy compact pulsed capillary discharge. The particular interest here is operation in Xe as this has industrial potential in lithography and metrology. However, the physics is particularly interesting given the 3D nature of the discharge caused by the strong filling gradient necessary. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:34:55.
(8) 093113-7. Valenzuela et al.. for soft X-ray output. The evolution of the plasma dynamics has been achieved using moire-schlieren technique as an optical diagnostic. These results may be taken in comparison with previously published results in argon. It is hoped that the difficulties of interpreting the line integrated values of the electron density and the X-ray spectrum may be resolved with future theoretical calculations. We have been able to set a lower bound to the conversion efficiency in both optical windows at 110 Å and at 135 Å. There are clear advantages of operating at 110 Å. ACKNOWLEDGMENTS. The authors of this work would like to recognize the scientific trajectory of the founder of the experimental plasma physics group in Chile and contributing author, Professor Hernan Chuaqui, who passed away on July 27, 2012. This work has been funded by the Chilean research Grant No. FONDECYT 1120816. 1. P. Bogen, H. Conrads, G. Gatti, and W. Kohlhaas, J. Opt. Soc. Am. 58, 203–206 (1968). 2 R. McCorkle and H. Vollmer, Rev. Sci. Instrum. 48, 1055–1063 (1977). 3 J. J. Rocca, V. Shlyaptsev, F. Tomasel, O. Cortazar, D. Hartshorn, and J. Chilla, Phys. Rev. Lett. 73, 2192–2195 (1994). 4 P. Choi, H. Chuaqui, and M. Favre, IEEE Plasma Sci. 15, 428–433 (1987). 5 P. Choi and M. Favre, Rev. Sci. Instrum. 69, 3118 (1998). 6 J.-F. Adam, J.-P. Moy, and J. Susini, Rev. Sci. Instrum. 76, 091301 (2005). 7 V. Banine and R. Moors, J. Phys. D: Appl. Phys. 37, 3207–3212 (2004). 8 E. S. Wyndham, M. Favre, M. P. Valdivia, J. C. Valenzuela, H. Chuaqui, and H. Bhuyan, Rev. Sci. Instrum. 81, 093502 (2010).. Phys. Plasmas 20, 093113 (2013) 9. M. P. Valdivia, E. S. Wyndham, M. Favre, J. C. Valenzuela, H. Chuaqui, and H. Bhuyan, Plasma Sources Sci. Technol. 21, 025011 (2012). M. Favre, P. Choi, H. Chuaqui, I. Mitchell, E. Wyndham, and A. M. Le~ nero, Plasma Sources Sci. Technol. 12, 78–84 (2003). 11 K. Bergmann, O. Rosier, W. Neff, and R. Lebert, Appl. Opt. 39, 3833–3837 (2000). 12 U. Stamm, J. Kleinschmidt, and K. Gabel, in EUV Source Workshop, 2005. 13 V. Bakshi, J. Gillaspy, and B. Rice, in EUV Source Modeling Workshop (Antwerp, Belgium, 2003). 14 S. George, K. Hou, and K. Takenoshita, Opt. Express 15, 16348–16356 (2007). 15 V. Bakshi, EUV Lithography (SPIE Press, 2009). 16 S. V. Zakharov, V. S. Zakharov, V. G. Novikov, M. Mond, and P. Choi, Plasma Sources Sci. Technol. 17, 024017 (2008). 17 J. C. Valenzuela, E. S. Wyndham, H. Chuaqui, D. S. Cortes, M. Favre, and H. Bhuyan, J. Appl. Phys. 111, 103301 (2012). 18 See www.ird-inc.com for specifications of the AXUV HS5 detector. 19 J. W. Keister, Proc. SPIE 6689, 66890U-12 (2007). 20 F. Scholze, R. Klein, and T. Bock, Appl. Opt. 42, 5621–5626 (2003). 21 V. Bakshi, EUV Sources for Lithography (SPIE Press, 2006). 22 M. A. Klosner and W. T. Silfvast, J. Opt. Soc. Am. B 17, 1279–1290 (2000). 23 K. Fahy, P. Dunne, L. McKinney, G. O’Sullivan, E. Sokell, J. White, A. Aguilar, J. M. Pomeroy, J. N. Tan, B. Blagojević, E.-O. LeBigot, and J. D. Gillaspy, J. Phys. D: Appl. Phys. 37, 3225–3232 (2004). 24 P. Zuppella, A. Reale, A. Ritucci, P. Tucceri, S. Prezioso, F. Flora, L. Mezi, and P. Dunne, Plasma Sources Sci. Technol. 18, 025014 (2009). 25 K. Bergmann, S. V. Danylyuk, and L. Juschkin, J. Appl. Phys. 106, 073309 (2009). 26 I. Song, K. Iwata, Y. Homma, S. R. Mohanty, M. Watanabe, T. Kawamura, A. Okino, K. Yasuoka, K. Horioka, and E. Hotta, Plasma Sources Sci. Technol. 15, 322–327 (2006). 27 G. Avaria, F. Guzman, M. Ruiz, M. Favre, E. Wyndham, H. Bhuyan, and H. Chuaqui, Plasma Sources Sci. Technol. 18, 045014 (2009). 28 K. Bergmann, F. Ku€ upper, and M. Benk, J. Appl. Phys. 103, 123304 (2008). 29 P. Choi, S. V. Zakharov, R. Aliaga-Rossel, A. Bakouboula, O. Benali, P. Bove, M. Cau, G. Duffy, B. Lebert, C. Sarroukh, O. Wyndham, E. Zaepffel, and V. Zakharov, in International EUV Symposium, Prague, 2009. 10. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 146.155.94.33 On: Tue, 24 May 2016 16:34:55.
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