Recent progress in EUV source development at GREMI

Microelectronic Engineering 61–62 (2002) 179–185www.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 DussartGREMI-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 fastcapillary discharge EUV source that is under investigation at GREMI. A current of a few kA having a fast risetime was applied across a xenon-filled alumina capillary to produce radiation, mostly in the EUV region (10–16nm). The EUV radiation from the capillary was characterized by employing an EUV spectrometer and an EUVpinhole camera. Time-integrated and time-resolved spectra provided information about the radiative processesfrom the xenon plasma together with the wall of the capillary. A superfast pinching effect is inferred from thepinhole images. At present, this EUV source is capable of operating at a high repetition rate (350 Hz in burstmode and 100 Hz in continuous mode) without causing too much damage to the wall of the capillary. This EUVsource may be a strong contender as the source of the Next Generation Lithography (NGL) assembly. 2002Elsevier 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-freeEUV source. Many studies have been carried out in various laboratories using different schemes aswell as different EUV radiation processes. Prominent front runners for EUV sources are a laser-produced plasma and a gas-filled capillary discharge plasma. It is premature to say which one will bethe novel EUV source for NGL. No doubt, gas-filled capillary discharge plasma sources have thepotential advantages that they are simpler in design, compact and cost-effective. Therefore, thesesources 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 thepioneering 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 2002 Elsevier Science B.V. All rights reserved.PI I : S0167-9317( 02 )00572-5

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 inthe EUV and soft X-ray region [1–5]. Very few studies have been made on FCD plasma as a suitableEUV incoherent radiation source [6–8]. We therefore feel strongly that a broad study on plasmacomposition and plasma dynamics inside a FCD should be made in order to understand the behaviorof 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 (hotand dense plasma) for the generation of EUV radiation. Grazing incidence spectrography wasemployed to investigate spectral emission from the plasma, whereas an EUV pinhole camera was usedto study the dynamics of the plasma inside the narrow capillary. In addition, other simple tools such asan XUV photodiode and a current probe were employed to detect the EUV photon output anddischarge current, respectively. The following section describes the experimental setup used in thisstudy 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, alongwith 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 capacitorbanks 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 twospecially designed electrodes that allow gas flow inside the capillary as well as facilitating viewing of

25the radiation at both end-on positions. The capillary was evacuated to below 10 mbar using aturbo-molecular pump (Leybold Cone Line). The pressure inside the capillary was monitored by acalibrated Alcatel Baratron pressure transducer (type 623A). The gas flow inside the capillary wascontrolled by a mass flow meter (Brooks 5850).

The time history of EUV emission from the FCD plasma was observed by employing an AXUV-12˚photodiode (IRD Inc.) in combination with a 1700 A thick Zr filter at the end-on position. Spectral

emission from the FCD plasma in the axial direction was monitored using a flat field glancing angleincidence spectrometer (PGS 500 Jobin-Yvon) provided with a 800 g/mm platinum-coated diffractiongrating with a maximum efficiency range of 15–35 nm. Details regarding the spectrometer arereported elsewhere [10]. The radiation dispersed by the grating was recorded by a detector consistingof a two-stage windowless micro-channel plate (MCP: Galileo 3040FM Chevron), a phosphorusscreen and an intensified charge coupled device (ICCD) array detector (Princeton Instruments). Thedynamics of the plasma column inside the FCD plasma system was investigated by capturing asequence of end-on pinhole images with the help of an EUV pinhole camera. A laser-drilled pinholeof diameter 50 mm was maintained at a distance of 100 mm from the capillary. The pinhole imageacquisition, in our case, was a combination of MCP and an ICCD detector, which was 700 mm fromthe pinhole.

3. Results and discussion

Several experiments were conducted to determine the maximum flux of photons in the 13.5 nmradiation 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 , Ne, Kr and Xe), energy of the capacitor2

bank (0.8–7.2 J), etc. It was observed that only the Xe-filled FCD system emits a strong band at 13.5nm [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 voltagesat 0.46 mbar Xe filling. It is interesting to note that a slow damped sinusoidal discharge current isobserved only at higher charging voltages, whereas only one discharge current pulse is observed atlower charging voltage. In order to monitor the EUV photon flux over the wavelength range ofapproximately 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. Photodiodesignals were recorded for a series of 60 continuous discharges to determine the shot-to-shot fluctuationin the photon flux per pulse. It was estimated that the shot-to-shot fluctuation was around 4%. Theconversion efficiency from electrical energy to EUV energy (10–16 nm) under optimum operatingconditions is estimated to be 2.0%.

Time-integrated axial emission spectra of the FCD system were recorded by employing an EUVspectrometer at one end-on position. Fig. 2 shows a typical time-integrated spectrum obtained at 0.46mbar filling pressure in the spectral region 10 to 26 nm as a function of charging voltage. One canclearly see from Fig. 2 three to four broad band peaks within the 10–16 nm range. In addition to thesebroad band peaks, a number of lines from oxygen impurities at 17.2, 18.3, 19.2 and 20.3 nm also

61 51appear. These lines (O and O ) are attributed to emission from the wall of the capillary. The mostdistinguished broad band peaks are centered at 11, 13.5 and 15 nm, having a bandwidth ofapproximately 1–1.5 nm. These broad band peaks are ascribed to the various ionization states of

111 101 91 111xenon (Xe , Xe and Xe ). The observation of an emission from Xe is indicative that theelectron temperature in the plasma column may have reached as high as 45 eV [7]. The spectrumreported by Klosner and Silfvast [7] is very similar to that obtained here. The time-integratedspectrum was utilized to estimate the radiated energy at 13.5 nm in a bandwidth of 0.9 nm. Takinginto account all experimental error factors it is estimated that our source emits 0.5 to 20 mJ/sr perpulse at 13.560.45 nm when the input electrical energy is around 7 J [9]. A sequence of time-resolvedspectra typically taken at a charging voltage of 24 kV and 0.46 mbar pressure is depicted in Fig. 3. Itshould 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 gaspressure.


Fig. 2. Time-integrated EUV spectra of the radiation emanating from a Xe capillary discharge in the 10–26 nm spectralregion 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 maximumcompression of the plasma (around 50 ns). The maximum compression time was inferred from pinholeimages. Then, at the instant of maximum current, the intensity of the 13.5 nm broad band is prominentamong 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 spectralregion. The time at which each spectrum was acquired with respect to the commencement of the current pulse is indicated.


current. The spectra recorded after the first half-cycle of the discharge current show only emissionsfrom oxygen impurities.

The dynamics of the plasma column inside the Xe-filled FCD plasma was investigated usingtime-resolved pinhole imaging. The pinhole images undoubtedly initially exhibit a rapid contraction ofthe 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 ofpinhole images recorded at 24 kV charging voltage and 1.33 mbar gas pressure. Beam profiles of thecorresponding 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 evidentfrom pinhole image 1. This image also reveals that most of the radiation during the early phaseoriginates 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 gaspressure is 1.33 mbar and charging voltage is 24 kV. The timing with respect to the beginning of the current pulse isindicated. (b) Beam profiles of the corresponding pinhole images. (c) Temporal evolution of the beam intensity and beamsize as inferred from the set of pinhole images. The current waveform at which the images are captured is also portrayed.


quite prominent in the case of capillary discharge schemes reported earlier [11]. After the initiation ofdischarge in the capillary, one can observe a superfast pinching phenomenon within a fewnanoseconds of the early phase, as shown in pinhole image 2. This pinching effect is observed 50 nsafter 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, capturedafter 10 ns of maximum compression, portrays expansion of the plasma column with fairlyhomogeneous radiation in the central zone of the capillary. A second faint compression is observed ataround the maximum of the discharge current waveform, as shown in pinhole image 4. The beamintensity profile clearly changes from a single peak pattern to an annular profile, as portrayed in trace3 of Fig. 4b. This annular beam profile structure is only observed at higher filling pressures ( . 0.9mbar). In the case of a lower filling pressure, evolution of the plasma column inside the capillary issimilar to that at higher filling pressures, except for the minor second compression and the annularbeam profile structure. The annular beam profile at higher filling pressure may be the result ofincreased 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 furtherexpansion of the plasma column. The expanded plasma column hits the wall of the capillary and thusproduces emission from the wall material. Fig. 4c portrays the time history of the beam intensity andspot size, together with the discharge current. A strong spike in the beam intensity curve and a strongdip in the spot size curve can be observed in Fig. 4c. A series of pinhole images were recorded at5262 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. Thesequences of pinhole images obtained in our investigation clearly illustrate that plasma compression inour 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 haveperformed experiments to achieve a repetition rate of up to 350 Hz (to date) in burst mode ofoperation without causing too much damage to the capillary or the electrode. Investigations arecurrently underway to achieve a high repetition rate of up to 1 kHz in continuous mode using a newdesign for the capillary system.

4. Conclusions

We have investigated EUV radiation emanating from a Xe-filled capillary in detail by makingtime-integrated and time-resolved measurements. Time-integrated spectra illustrate that our sourceproduces an intense flux of photons (with energy up to 20 mJ/sr per pulse) at 13.560.45 nm. Thedeviation of the images from the mean position is less than 45 mm. The shot-to-shot fluctuation in theEUV photon flux is less than 4%. Nevertheless, the source size, cost and operational simplicity makeit feasible to employ this source for NGL.

Acknowledgements

` `This work was supported by the Secretariat d’Etat a l’Industrie (Ministere de l’Economie, desFinances et de l’Industrie) through the French national federative program PREUVE. S.R. Mohanty is


grateful to MRT, France, for providing financial support to pursue this work at GREMI and he alsothanks the Director, Centre of Plasma Physics, Guwahati, India, for allowing him to carry out researchat GREMI.

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Recent progress in EUV source development at GREMI