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Recent progress in EUV source development at GREMI
Microelectronic Engineering 61–62 (2002) 179–185 www.elsevier.com / locate / mee
Recent progress in EUV source development at GREMI Smruti Ranjan Mohanty*, Christophe Cachoncinlle, Claude Fleurier, Eric Robert, Jean-Michel Pouvesle, Raymond Viladrosa, Remi Dussart GREMI-ESPEO, CNRS UMR 6606, Universite d’ Orleans, 45067 Orleans Cedex 2, France
Abstract The objective of the present paper is to provide current information on a novel and truly compact fast capillary discharge EUV source that is under investigation at GREMI. A current of a few kA having a fast rise time was applied across a xenon-filled alumina capillary to produce radiation, mostly in the EUV region (10–16 nm). The EUV radiation from the capillary was characterized by employing an EUV spectrometer and an EUV pinhole camera. Time-integrated and time-resolved spectra provided information about the radiative processes from the xenon plasma together with the wall of the capillary. A superfast pinching effect is inferred from the pinhole images. At present, this EUV source is capable of operating at a high repetition rate (350 Hz in burst mode and 100 Hz in continuous mode) without causing too much damage to the wall of the capillary. This EUV source may be a strong contender as the source of the Next Generation Lithography (NGL) assembly. 2002 Elsevier Science B.V. All rights reserved. Keywords: EUV source; Capillary discharge; EUV lithography; EUV spectroscopy
1. Introduction The critical issue in EUV lithography is the search for an efficient, compact, stable and debris-free EUV source. Many studies have been carried out in various laboratories using different schemes as well as different EUV radiation processes. Prominent front runners for EUV sources are a laserproduced plasma and a gas-filled capillary discharge plasma. It is premature to say which one will be the novel EUV source for NGL. No doubt, gas-filled capillary discharge plasma sources have the potential advantages that they are simpler in design, compact and cost-effective. Therefore, these sources have drawn much attention in the last decade [1–8]. Much progress has been made in gas-filled capillary discharge studies in recent years after the pioneering experiments on soft X-ray amplification in a fast capillary discharge (FCD) plasma [2,3]. * Corresponding author. Tel.: 1 33-2-3849-4515; fax: 1 33-2-3841-7154. E-mail address: [email protected] (S.R. Mohanty). 0167-9317 / 02 / $ – see front matter PII: S0167-9317( 02 )00572-5
2002 Elsevier Science B.V. All rights reserved.
180
S.R. Mohanty et al. / Microelectronic Engineering 61 – 62 (2002) 179 – 185
Most studies on FCD plasma have concentrated only on how to achieve bright coherent radiation in the EUV and soft X-ray region [1–5]. Very few studies have been made on FCD plasma as a suitable EUV incoherent radiation source [6–8]. We therefore feel strongly that a broad study on plasma composition and plasma dynamics inside a FCD should be made in order to understand the behavior of the plasma inside the capillary channel and thus use FCD as an EUV incoherent radiation source. Pulsed power axial discharge across a xenon (Xe)-filled capillary was used to create active media (hot and dense plasma) for the generation of EUV radiation. Grazing incidence spectrography was employed to investigate spectral emission from the plasma, whereas an EUV pinhole camera was used to study the dynamics of the plasma inside the narrow capillary. In addition, other simple tools such as an XUV photodiode and a current probe were employed to detect the EUV photon output and discharge current, respectively. The following section describes the experimental setup used in this study together with a brief overview of the diagnostics. Section 3 presents the experimental results, namely the EUV spectra (time-integrated and time-resolved) and time-resolved pinhole imaging, along with a discussion of the results. The last section presents the concluding remarks.
2. Experimental setup The electrical energy to initiate the discharge inside the capillary was provided by two capacitor banks configured in Blumlein fashion as described earlier [9]. The capillary channel, made of alumina, was mounted with the help of an electrical insulating holder. The holder was placed between two specially designed electrodes that allow gas flow inside the capillary as well as facilitating viewing of the radiation at both end-on positions. The capillary was evacuated to below 10 25 mbar using a turbo-molecular pump (Leybold Cone Line). The pressure inside the capillary was monitored by a calibrated Alcatel Baratron pressure transducer (type 623A). The gas flow inside the capillary was controlled by a mass flow meter (Brooks 5850). The time history of EUV emission from the FCD plasma was observed by employing an AXUV-12 ˚ thick Zr filter at the end-on position. Spectral photodiode (IRD Inc.) in combination with a 1700 A emission from the FCD plasma in the axial direction was monitored using a flat field glancing angle incidence spectrometer (PGS 500 Jobin-Yvon) provided with a 800 g / mm platinum-coated diffraction grating with a maximum efficiency range of 15–35 nm. Details regarding the spectrometer are reported elsewhere [10]. The radiation dispersed by the grating was recorded by a detector consisting of a two-stage windowless micro-channel plate (MCP: Galileo 3040FM Chevron), a phosphorus screen and an intensified charge coupled device (ICCD) array detector (Princeton Instruments). The dynamics of the plasma column inside the FCD plasma system was investigated by capturing a sequence of end-on pinhole images with the help of an EUV pinhole camera. A laser-drilled pinhole of diameter 50 mm was maintained at a distance of 100 mm from the capillary. The pinhole image acquisition, in our case, was a combination of MCP and an ICCD detector, which was 700 mm from the pinhole.
3. Results and discussion Several experiments were conducted to determine the maximum flux of photons in the 13.5 nm radiation band by varying the experimental conditions, such as capillary material, capillary length and
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diameter, gas pressure (0.13–1.5 mbar), filling gases (Ar, O 2 , Ne, Kr and Xe), energy of the capacitor bank (0.8–7.2 J), etc. It was observed that only the Xe-filled FCD system emits a strong band at 13.5 nm [9]. All measurements discussed here were conducted in an alumina capillary filled with pure Xe. Fig. 1 shows signals of the discharge current and the photodiode output for different charging voltages at 0.46 mbar Xe filling. It is interesting to note that a slow damped sinusoidal discharge current is observed only at higher charging voltages, whereas only one discharge current pulse is observed at lower charging voltage. In order to monitor the EUV photon flux over the wavelength range of approximately 10–16 nm, the photodiode signals were recorded by employing the end-on position. For a higher charging voltage, the EUV photons are observed to emit in multiple pulses (shown in Fig. 1) for each discharge, whereas at lower charging voltage, only one pulse is observed. Photodiode signals were recorded for a series of 60 continuous discharges to determine the shot-to-shot fluctuation in the photon flux per pulse. It was estimated that the shot-to-shot fluctuation was around 4%. The conversion efficiency from electrical energy to EUV energy (10–16 nm) under optimum operating conditions is estimated to be 2.0%. Time-integrated axial emission spectra of the FCD system were recorded by employing an EUV spectrometer at one end-on position. Fig. 2 shows a typical time-integrated spectrum obtained at 0.46 mbar filling pressure in the spectral region 10 to 26 nm as a function of charging voltage. One can clearly see from Fig. 2 three to four broad band peaks within the 10–16 nm range. In addition to these broad band peaks, a number of lines from oxygen impurities at 17.2, 18.3, 19.2 and 20.3 nm also appear. These lines (O 61 and O 51 ) are attributed to emission from the wall of the capillary. The most distinguished broad band peaks are centered at 11, 13.5 and 15 nm, having a bandwidth of approximately 1–1.5 nm. These broad band peaks are ascribed to the various ionization states of xenon (Xe 111 , Xe 101 and Xe 91 ). The observation of an emission from Xe 111 is indicative that the electron temperature in the plasma column may have reached as high as 45 eV [7]. The spectrum reported by Klosner and Silfvast [7] is very similar to that obtained here. The time-integrated spectrum was utilized to estimate the radiated energy at 13.5 nm in a bandwidth of 0.9 nm. Taking into account all experimental error factors it is estimated that our source emits 0.5 to 20 mJ / sr per pulse at 13.560.45 nm when the input electrical energy is around 7 J [9]. A sequence of time-resolved spectra typically taken at a charging voltage of 24 kV and 0.46 mbar pressure is depicted in Fig. 3. It should be noted from these figures that the 11 nm broad band peak appears first (around 30 ns) after
Fig. 1. Measured discharge current waveform and photodiode signal for two different charging voltages at 0.46 mbar gas pressure.
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Fig. 2. Time-integrated EUV spectra of the radiation emanating from a Xe capillary discharge in the 10–26 nm spectral region for different charging voltages at 0.46 mbar gas pressure.
the initiation of the discharge inside the capillary and it reaches a maximum at the time of maximum compression of the plasma (around 50 ns). The maximum compression time was inferred from pinhole images. Then, at the instant of maximum current, the intensity of the 13.5 nm broad band is prominent among all the broad bands. The broad bands appear only in the first half-cycle of the discharge
Fig. 3. Sequence of time-resolved EUV spectra of the radiation emanated Xe capillary discharge in the 10–26 nm spectral region. The time at which each spectrum was acquired with respect to the commencement of the current pulse is indicated.
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current. The spectra recorded after the first half-cycle of the discharge current show only emissions from oxygen impurities. The dynamics of the plasma column inside the Xe-filled FCD plasma was investigated using time-resolved pinhole imaging. The pinhole images undoubtedly initially exhibit a rapid contraction of the plasma inside the capillary that is followed by heating of the plasma to a high temperature and, finally, a slow expansion of the plasma irrespective of the gas pressure. Fig. 4a depicts a series of pinhole images recorded at 24 kV charging voltage and 1.33 mbar gas pressure. Beam profiles of the corresponding pinhole images are shown in Fig. 4b. During the early part of the current rise (up to , 50 ns) the current density distribution inside the capillary is quite inhomogeneous, which is evident from pinhole image 1. This image also reveals that most of the radiation during the early phase originates from the central region of the capillary, but not from the wall. Radiation from the wall is
Fig. 4. (a) Sequence of time-resolved pinhole images showing evolution of the plasma inside the capillary channel. The gas pressure is 1.33 mbar and charging voltage is 24 kV. The timing with respect to the beginning of the current pulse is indicated. (b) Beam profiles of the corresponding pinhole images. (c) Temporal evolution of the beam intensity and beam size as inferred from the set of pinhole images. The current waveform at which the images are captured is also portrayed.
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quite prominent in the case of capillary discharge schemes reported earlier [11]. After the initiation of discharge in the capillary, one can observe a superfast pinching phenomenon within a few nanoseconds of the early phase, as shown in pinhole image 2. This pinching effect is observed 50 ns after the start of the discharge current and it lasts for less than 5 ns. The subsequent pinhole images [Fig. 4a (images 3–5)] show expansion and cooling of the plasma column. Pinhole image 3, captured after 10 ns of maximum compression, portrays expansion of the plasma column with fairly homogeneous radiation in the central zone of the capillary. A second faint compression is observed at around the maximum of the discharge current waveform, as shown in pinhole image 4. The beam intensity profile clearly changes from a single peak pattern to an annular profile, as portrayed in trace 3 of Fig. 4b. This annular beam profile structure is only observed at higher filling pressures ( . 0.9 mbar). In the case of a lower filling pressure, evolution of the plasma column inside the capillary is similar to that at higher filling pressures, except for the minor second compression and the annular beam profile structure. The annular beam profile at higher filling pressure may be the result of increased refraction of the EUV beam caused by a larger density gradient in the plasma column [12]. The last pinhole image (image 5) depicts the smearing of the hot annular ring as well as further expansion of the plasma column. The expanded plasma column hits the wall of the capillary and thus produces emission from the wall material. Fig. 4c portrays the time history of the beam intensity and spot size, together with the discharge current. A strong spike in the beam intensity curve and a strong dip in the spot size curve can be observed in Fig. 4c. A series of pinhole images were recorded at 5262 ns with respect to onset of the discharge current to observe the positional stability of the source. It is estimated that the deviation of the images from the mean position is less than 45 mm. The sequences of pinhole images obtained in our investigation clearly illustrate that plasma compression in our FCD has good axial symmetry. As the repetition rate of the source is a key factor for the practical use of any EUV source, we have performed experiments to achieve a repetition rate of up to 350 Hz (to date) in burst mode of operation without causing too much damage to the capillary or the electrode. Investigations are currently underway to achieve a high repetition rate of up to 1 kHz in continuous mode using a new design for the capillary system.
4. Conclusions We have investigated EUV radiation emanating from a Xe-filled capillary in detail by making time-integrated and time-resolved measurements. Time-integrated spectra illustrate that our source produces an intense flux of photons (with energy up to 20 mJ / sr per pulse) at 13.560.45 nm. The deviation of the images from the mean position is less than 45 mm. The shot-to-shot fluctuation in the EUV photon flux is less than 4%. Nevertheless, the source size, cost and operational simplicity make it feasible to employ this source for NGL.
Acknowledgements ` de l’Economie, des This work was supported by the Secretariat d’Etat a` l’Industrie (Ministere Finances et de l’Industrie) through the French national federative program PREUVE. S.R. Mohanty is
S.R. Mohanty et al. / Microelectronic Engineering 61 – 62 (2002) 179 – 185
185
grateful to MRT, France, for providing financial support to pursue this work at GREMI and he also thanks the Director, Centre of Plasma Physics, Guwahati, India, for allowing him to carry out research at GREMI.
References [1] C. Steden, H.-J. Kunze, Phys. Lett. A 151 (1990) 534–537. [2] J.J. Rocca, M.C. Marconi, F.G. Tomasel, IEEE J. Quantum Electron. 29 (1993) 182–191. ´ [3] J.J. Rocca, V.N. Shlyaptsev, F.G. Tomasel, O.D. Cortazar, D. Hartshorn, J.L.A. Chilla, Phys. Rev. Lett. 73 (1994) 2192–2195. [4] H.J. Shin, D.E. Kim, T.N. Lee, Phys. Rev. E 50 (1994) 1367–1382. [5] M. Hebenstreit, R. Fertner, T. Neger, M. Pockl, F. Aumayr, J. Phys. D: Appl. Phys. 29 (1996) 1933–1940. [6] M.A. Klosner, H.A. Bender, W.T. Silfvast, J.J. Rocca, Opt. Lett. 22 (1997) 34–36. [7] M.A. Klosner, W.T. Silfvast, Opt. Lett. 23 (1998) 1609–1611. [8] R. Lebert, K. Bergmann, G. Schriever, W. Neff, Microelectron. Eng. 46 (1999) 449–452. [9] E. Robert, B. Blagojevic, R. Dussart, S.R. Mohanty, M.M. Idrissi, D. Hong, R. Viladrosa, J.-M. Pouvesle, C. Fleurier, C. Cachoncinlle, Proc. SPIE 4343 (2001) 566–575. ¨ [10] D. Hong, R. Dussart, C. Cachoncinlle, W.E.S. Rosenfeld, S. Gotze, J. Pons, R. Viladrosa, C. Fleurier, J.M. Pouvesle, Rev. Sci. Instr. 71 (2000) 15–19. ¨ [11] R. Dussart, D. Hong, S. Gotze, W.E.S. Rosenfeld, J. Pons, R. Viladrosa, C. Cachoncinlle, C. Fleurier, J.-M. Pouvesle, J. Phys. D: Appl. Phys. 33 (2000) 1837–1842. [12] C.H. Moreno, M.C. Marconi, V.N. Shlyaptsev, B.R. Benware, C.D. Macchietto, J.L.A. Chilla, J.J. Rocca, Phys. Rev. A 58 (1998) 1509–1514.
Recent progress in EUV source development at GREMI Smruti Ranjan Mohanty*, Christophe Cachoncinlle, Claude Fleurier, Eric Robert, Jean-Michel Pouvesle, Raymond Viladrosa, Remi Dussart GREMI-ESPEO, CNRS UMR 6606, Universite d’ Orleans, 45067 Orleans Cedex 2, France
Abstract The objective of the present paper is to provide current information on a novel and truly compact fast capillary discharge EUV source that is under investigation at GREMI. A current of a few kA having a fast rise time was applied across a xenon-filled alumina capillary to produce radiation, mostly in the EUV region (10–16 nm). The EUV radiation from the capillary was characterized by employing an EUV spectrometer and an EUV pinhole camera. Time-integrated and time-resolved spectra provided information about the radiative processes from the xenon plasma together with the wall of the capillary. A superfast pinching effect is inferred from the pinhole images. At present, this EUV source is capable of operating at a high repetition rate (350 Hz in burst mode and 100 Hz in continuous mode) without causing too much damage to the wall of the capillary. This EUV source may be a strong contender as the source of the Next Generation Lithography (NGL) assembly. 2002 Elsevier Science B.V. All rights reserved. Keywords: EUV source; Capillary discharge; EUV lithography; EUV spectroscopy
1. Introduction The critical issue in EUV lithography is the search for an efficient, compact, stable and debris-free EUV source. Many studies have been carried out in various laboratories using different schemes as well as different EUV radiation processes. Prominent front runners for EUV sources are a laserproduced plasma and a gas-filled capillary discharge plasma. It is premature to say which one will be the novel EUV source for NGL. No doubt, gas-filled capillary discharge plasma sources have the potential advantages that they are simpler in design, compact and cost-effective. Therefore, these sources have drawn much attention in the last decade [1–8]. Much progress has been made in gas-filled capillary discharge studies in recent years after the pioneering experiments on soft X-ray amplification in a fast capillary discharge (FCD) plasma [2,3]. * Corresponding author. Tel.: 1 33-2-3849-4515; fax: 1 33-2-3841-7154. E-mail address: [email protected] (S.R. Mohanty). 0167-9317 / 02 / $ – see front matter PII: S0167-9317( 02 )00572-5
2002 Elsevier Science B.V. All rights reserved.
180
S.R. Mohanty et al. / Microelectronic Engineering 61 – 62 (2002) 179 – 185
Most studies on FCD plasma have concentrated only on how to achieve bright coherent radiation in the EUV and soft X-ray region [1–5]. Very few studies have been made on FCD plasma as a suitable EUV incoherent radiation source [6–8]. We therefore feel strongly that a broad study on plasma composition and plasma dynamics inside a FCD should be made in order to understand the behavior of the plasma inside the capillary channel and thus use FCD as an EUV incoherent radiation source. Pulsed power axial discharge across a xenon (Xe)-filled capillary was used to create active media (hot and dense plasma) for the generation of EUV radiation. Grazing incidence spectrography was employed to investigate spectral emission from the plasma, whereas an EUV pinhole camera was used to study the dynamics of the plasma inside the narrow capillary. In addition, other simple tools such as an XUV photodiode and a current probe were employed to detect the EUV photon output and discharge current, respectively. The following section describes the experimental setup used in this study together with a brief overview of the diagnostics. Section 3 presents the experimental results, namely the EUV spectra (time-integrated and time-resolved) and time-resolved pinhole imaging, along with a discussion of the results. The last section presents the concluding remarks.
2. Experimental setup The electrical energy to initiate the discharge inside the capillary was provided by two capacitor banks configured in Blumlein fashion as described earlier [9]. The capillary channel, made of alumina, was mounted with the help of an electrical insulating holder. The holder was placed between two specially designed electrodes that allow gas flow inside the capillary as well as facilitating viewing of the radiation at both end-on positions. The capillary was evacuated to below 10 25 mbar using a turbo-molecular pump (Leybold Cone Line). The pressure inside the capillary was monitored by a calibrated Alcatel Baratron pressure transducer (type 623A). The gas flow inside the capillary was controlled by a mass flow meter (Brooks 5850). The time history of EUV emission from the FCD plasma was observed by employing an AXUV-12 ˚ thick Zr filter at the end-on position. Spectral photodiode (IRD Inc.) in combination with a 1700 A emission from the FCD plasma in the axial direction was monitored using a flat field glancing angle incidence spectrometer (PGS 500 Jobin-Yvon) provided with a 800 g / mm platinum-coated diffraction grating with a maximum efficiency range of 15–35 nm. Details regarding the spectrometer are reported elsewhere [10]. The radiation dispersed by the grating was recorded by a detector consisting of a two-stage windowless micro-channel plate (MCP: Galileo 3040FM Chevron), a phosphorus screen and an intensified charge coupled device (ICCD) array detector (Princeton Instruments). The dynamics of the plasma column inside the FCD plasma system was investigated by capturing a sequence of end-on pinhole images with the help of an EUV pinhole camera. A laser-drilled pinhole of diameter 50 mm was maintained at a distance of 100 mm from the capillary. The pinhole image acquisition, in our case, was a combination of MCP and an ICCD detector, which was 700 mm from the pinhole.
3. Results and discussion Several experiments were conducted to determine the maximum flux of photons in the 13.5 nm radiation band by varying the experimental conditions, such as capillary material, capillary length and
S.R. Mohanty et al. / Microelectronic Engineering 61 – 62 (2002) 179 – 185
181
diameter, gas pressure (0.13–1.5 mbar), filling gases (Ar, O 2 , Ne, Kr and Xe), energy of the capacitor bank (0.8–7.2 J), etc. It was observed that only the Xe-filled FCD system emits a strong band at 13.5 nm [9]. All measurements discussed here were conducted in an alumina capillary filled with pure Xe. Fig. 1 shows signals of the discharge current and the photodiode output for different charging voltages at 0.46 mbar Xe filling. It is interesting to note that a slow damped sinusoidal discharge current is observed only at higher charging voltages, whereas only one discharge current pulse is observed at lower charging voltage. In order to monitor the EUV photon flux over the wavelength range of approximately 10–16 nm, the photodiode signals were recorded by employing the end-on position. For a higher charging voltage, the EUV photons are observed to emit in multiple pulses (shown in Fig. 1) for each discharge, whereas at lower charging voltage, only one pulse is observed. Photodiode signals were recorded for a series of 60 continuous discharges to determine the shot-to-shot fluctuation in the photon flux per pulse. It was estimated that the shot-to-shot fluctuation was around 4%. The conversion efficiency from electrical energy to EUV energy (10–16 nm) under optimum operating conditions is estimated to be 2.0%. Time-integrated axial emission spectra of the FCD system were recorded by employing an EUV spectrometer at one end-on position. Fig. 2 shows a typical time-integrated spectrum obtained at 0.46 mbar filling pressure in the spectral region 10 to 26 nm as a function of charging voltage. One can clearly see from Fig. 2 three to four broad band peaks within the 10–16 nm range. In addition to these broad band peaks, a number of lines from oxygen impurities at 17.2, 18.3, 19.2 and 20.3 nm also appear. These lines (O 61 and O 51 ) are attributed to emission from the wall of the capillary. The most distinguished broad band peaks are centered at 11, 13.5 and 15 nm, having a bandwidth of approximately 1–1.5 nm. These broad band peaks are ascribed to the various ionization states of xenon (Xe 111 , Xe 101 and Xe 91 ). The observation of an emission from Xe 111 is indicative that the electron temperature in the plasma column may have reached as high as 45 eV [7]. The spectrum reported by Klosner and Silfvast [7] is very similar to that obtained here. The time-integrated spectrum was utilized to estimate the radiated energy at 13.5 nm in a bandwidth of 0.9 nm. Taking into account all experimental error factors it is estimated that our source emits 0.5 to 20 mJ / sr per pulse at 13.560.45 nm when the input electrical energy is around 7 J [9]. A sequence of time-resolved spectra typically taken at a charging voltage of 24 kV and 0.46 mbar pressure is depicted in Fig. 3. It should be noted from these figures that the 11 nm broad band peak appears first (around 30 ns) after
Fig. 1. Measured discharge current waveform and photodiode signal for two different charging voltages at 0.46 mbar gas pressure.
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S.R. Mohanty et al. / Microelectronic Engineering 61 – 62 (2002) 179 – 185
Fig. 2. Time-integrated EUV spectra of the radiation emanating from a Xe capillary discharge in the 10–26 nm spectral region for different charging voltages at 0.46 mbar gas pressure.
the initiation of the discharge inside the capillary and it reaches a maximum at the time of maximum compression of the plasma (around 50 ns). The maximum compression time was inferred from pinhole images. Then, at the instant of maximum current, the intensity of the 13.5 nm broad band is prominent among all the broad bands. The broad bands appear only in the first half-cycle of the discharge
Fig. 3. Sequence of time-resolved EUV spectra of the radiation emanated Xe capillary discharge in the 10–26 nm spectral region. The time at which each spectrum was acquired with respect to the commencement of the current pulse is indicated.
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183
current. The spectra recorded after the first half-cycle of the discharge current show only emissions from oxygen impurities. The dynamics of the plasma column inside the Xe-filled FCD plasma was investigated using time-resolved pinhole imaging. The pinhole images undoubtedly initially exhibit a rapid contraction of the plasma inside the capillary that is followed by heating of the plasma to a high temperature and, finally, a slow expansion of the plasma irrespective of the gas pressure. Fig. 4a depicts a series of pinhole images recorded at 24 kV charging voltage and 1.33 mbar gas pressure. Beam profiles of the corresponding pinhole images are shown in Fig. 4b. During the early part of the current rise (up to , 50 ns) the current density distribution inside the capillary is quite inhomogeneous, which is evident from pinhole image 1. This image also reveals that most of the radiation during the early phase originates from the central region of the capillary, but not from the wall. Radiation from the wall is
Fig. 4. (a) Sequence of time-resolved pinhole images showing evolution of the plasma inside the capillary channel. The gas pressure is 1.33 mbar and charging voltage is 24 kV. The timing with respect to the beginning of the current pulse is indicated. (b) Beam profiles of the corresponding pinhole images. (c) Temporal evolution of the beam intensity and beam size as inferred from the set of pinhole images. The current waveform at which the images are captured is also portrayed.
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S.R. Mohanty et al. / Microelectronic Engineering 61 – 62 (2002) 179 – 185
quite prominent in the case of capillary discharge schemes reported earlier [11]. After the initiation of discharge in the capillary, one can observe a superfast pinching phenomenon within a few nanoseconds of the early phase, as shown in pinhole image 2. This pinching effect is observed 50 ns after the start of the discharge current and it lasts for less than 5 ns. The subsequent pinhole images [Fig. 4a (images 3–5)] show expansion and cooling of the plasma column. Pinhole image 3, captured after 10 ns of maximum compression, portrays expansion of the plasma column with fairly homogeneous radiation in the central zone of the capillary. A second faint compression is observed at around the maximum of the discharge current waveform, as shown in pinhole image 4. The beam intensity profile clearly changes from a single peak pattern to an annular profile, as portrayed in trace 3 of Fig. 4b. This annular beam profile structure is only observed at higher filling pressures ( . 0.9 mbar). In the case of a lower filling pressure, evolution of the plasma column inside the capillary is similar to that at higher filling pressures, except for the minor second compression and the annular beam profile structure. The annular beam profile at higher filling pressure may be the result of increased refraction of the EUV beam caused by a larger density gradient in the plasma column [12]. The last pinhole image (image 5) depicts the smearing of the hot annular ring as well as further expansion of the plasma column. The expanded plasma column hits the wall of the capillary and thus produces emission from the wall material. Fig. 4c portrays the time history of the beam intensity and spot size, together with the discharge current. A strong spike in the beam intensity curve and a strong dip in the spot size curve can be observed in Fig. 4c. A series of pinhole images were recorded at 5262 ns with respect to onset of the discharge current to observe the positional stability of the source. It is estimated that the deviation of the images from the mean position is less than 45 mm. The sequences of pinhole images obtained in our investigation clearly illustrate that plasma compression in our FCD has good axial symmetry. As the repetition rate of the source is a key factor for the practical use of any EUV source, we have performed experiments to achieve a repetition rate of up to 350 Hz (to date) in burst mode of operation without causing too much damage to the capillary or the electrode. Investigations are currently underway to achieve a high repetition rate of up to 1 kHz in continuous mode using a new design for the capillary system.
4. Conclusions We have investigated EUV radiation emanating from a Xe-filled capillary in detail by making time-integrated and time-resolved measurements. Time-integrated spectra illustrate that our source produces an intense flux of photons (with energy up to 20 mJ / sr per pulse) at 13.560.45 nm. The deviation of the images from the mean position is less than 45 mm. The shot-to-shot fluctuation in the EUV photon flux is less than 4%. Nevertheless, the source size, cost and operational simplicity make it feasible to employ this source for NGL.
Acknowledgements ` de l’Economie, des This work was supported by the Secretariat d’Etat a` l’Industrie (Ministere Finances et de l’Industrie) through the French national federative program PREUVE. S.R. Mohanty is
S.R. Mohanty et al. / Microelectronic Engineering 61 – 62 (2002) 179 – 185
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grateful to MRT, France, for providing financial support to pursue this work at GREMI and he also thanks the Director, Centre of Plasma Physics, Guwahati, India, for allowing him to carry out research at GREMI.
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