EXCIMER LASERS LABORATORY
1999 ANNUAL REPORT
      
 

Introduction  up

The term excimer laser covers a family of laser systems that uses a gaseous mixture excited by electrical discharges as active medium. Excimer means any diatomic-molecule such as xenon chloride (XeCl) that can exist only when excited: in fact, excimer molecules are bound only in the excited state and unbound in the ground state. Such excimers make good lasing materials because the population inversion is guaranteed as long as excited molecules are present. The high wall-plug efficiency and the improved reliability of commercial excimer lasers promoted their widespread use in many applications requiring intense ultraviolet light, e.g., photolithography, micro-electronics, photo-chemistry, material processing, micromachining, remote sensing, corneal-corrective-surgery. However, commercial excimer lasers are not suitable for important applications, such as large area material processing and laser beam propagation down optical fibres. This is due, respectively, to the limited (< 5 cm²) beam size (which in turn limits the output energy) and to the short (< 30 ns) laser pulsewidth typical of commercial excimer laser systems. These limitations were addressed at the ENEA Frascati back in the late seventies, when work started on designing and constructing of XeCl lasers with large active medium, high output energy and long pulsewidth.
During 1999, the work at Frascati was mainly focused on using the XeCl laser facility Hercules to anneal amorphous silicon panels and to drive a plasma X-ray source. These two activities were done in the frame, respectively, of the European Project FOTO (aimed at achieving high-mobility thin-film-transistors) and of an INTAS Project for the development of an high-charge, heavy-ions source driven by laser-plasmas. In addition, work was done on the optimisation of the X-ray laser-plasma source for microlithography applications, on the optimisation of an hybrid laser system emitting ultrashort UV laser pulses and on pictures restoration by selective laser paint removal.
 

The project FOTO  up

The European project FOTO aims at the realisation of a clean room for the laser annealing of amorphous silicon (a-Si) in order to achieve high-mobility thin-film-transistors (TFTs) and high-efficiency photovoltaic cells. This project involves many partners: among them, we had a close collaboration with the ERG FORI Division (ENEA Portici), CNR IESS (Rome) and the Company EL.EN. S.p.A. (Calenzano). In this frame, the main tasks of our Laboratory were:
 


Hercules L excimer laser
The new XeCl excimer laser, named “Hercules L”, is an improved version of the laser facility Hercules described in the 1998 Progress Report of the ENEA Applied Physics Division. Table 1 compares the performance of Hercules and Hercules L. The main improvements can be summarised as follows:

During 1999, Hercules L has been assembled and tested in collaboration with EL.EN. and it will be installed at ENEA Portici within the first half of the year 2000.

Table 1. Main features of the laser systems Hercules (Frascati) and Hercules L (Portici)


HERCULES (Frascati) HERCULES L (Portici)
Active medium XeCl   XeCl
Active medium pressure 3,5 bar 5 bar
Wavelength 308 nm 308 nm
Preionisation  X-rays (70 keV) X-rays (80 keV)
Gas flow Axial Axial
Discharge voltage 40 kV  50 kV
Laser pulse width  120-160 ns  100-150 ns
Max output energy 8 J  > 10 J
Peak power 50 – 70 MW > 100 MW
Maximum repetition rate 10 Hz  10 Hz
Average power 35 – 40 W > 100 W
Laser beam size 5 cm x 10 cm  5 cm x 10 cm

Laser beam homogeniser
The second task was the design and test of a laser beam homogeniser for improving the intrinsic spatial fluctuations of the laser beam energy. A good spatial uniformity of the laser beam incident onto the a-Si panel is a preliminary condition to ensure a good uniformity of the poly-Si grain size and of the electric properties of the TFTs. During 1998, a first prototype of beam homogeniser was designed, mounted and tested on Hercules, as detailed in the 1998 Progress Report of the ENEA Applied Physics Division. We used this homogeneiser to irradiate a-Si films, obtaining the results detailed in the following paragraph. Based on these results, a new beam homogeniser was designed to irradiate the a-Si at high (0.8 J/cm2) and low (0.4 J/cm2) energy density level. The new homogeniser was designed thanks to dedicated software we have developed for (see Fig. 2.1). The basic principle of the beam homogeniser is using cylindrical lenses to divide the incoming beam in a 2-D matrix of secondary beamlets that overlap in the focal plane of the condenser lens. In this way, local spatial fluctuations of the beam energy are averaged due to the overlap of different portions of the input beam. Figure 1 shows the calculated ray-tracing of the homogeniser acting along one direction, with four secondary beamlets.
As a further step toward a versatile and general-purpose beam homogeniser, we have studied a novel optical scheme that allows to continuously change the spot size of the homogenised beam, still maintaining the same degree of homogeneity and, within proper limits, the same length of the homogeniser.

Fig. 2 – 2-D schematic of the working principle of the laser beam homogeniser as drawn by the ENEA software

Optimum working point
The third task within the project FOTO was seeking the optimum irradiation conditions to achieve poly-Si grains uniformly distributed and having a size as large as possible. Irradiation experiments were carried out at Frascati by using Hercules and the first prototype homogeniser. Colleagues of the ERG-FORI Division at ENEA Portici Centre prepared the a-Si samples. Preliminarily, we obtained a curve grain-size vs. laser-energy-density. This measurement was done in single-shot on 50-nm-thick a-Si films deposited on glass by using the Hercules beam without the homogeniser. By a careful measurement of both intensity profile and grain size (the latter performed in collaboration with the CNR-IESS laboratories), we obtained the plot shown in Fig. 2.

Fig. 2 - Plot of the average grain size of poly-Si vs. the laser energy density, as obtained by irradiating a-Si films on glass with Hercules. The experimental uncertainty of the laser fluence values is less than 10%.

In the figure, we can discern three regions: the first and the third one are characterised by a grain size of about 100-200 nm and a behaviour almost independent of the laser energy density. In the second one, small variations of the energy density have dramatic consequences on the size of the silicon grains: we found a gradient (grain size / laser fluence) as large as 0.5 mm/mJ/cm2. This phenomenon is known as Super-Lateral-Growth (SLG) and it is characterised by a total melting of the a-Si layer. From Fig. 2 it is clear that this working point is hardly manageable, so that we decided to work away the SLG region.
Other measurements carried at Frascati and at the CNR-IESS laboratories have shown for the first time the advantage of using a long-duration laser pulse in order to increase the grain size on the poly-Si film. Fig. 3 shows the maximum size of the grains obtained in the SLG regime by using a short pulse (25 ns, emitted by a commercial laser system) and a long-pulse (150 ns, emitted by Hercules) XeCl laser.

Fig. 3 – Maximum lateral growth of the grains for films having different thickness,
respectively irradiated by Hercules (squares) and by a Lambda Physik XeCl laser (triangles).

An unexpected phenomenon we observed during the a-Si irradiation was a sort of fringe texture, occasionally visible on the surface of the poly-Si panels, as shown in Fig. 4. The results of accurate measurements showed that this pattern is due to the interference of the photons diffracted by the borders of the cylindrical lens arrays of the homogeniser. This interference appears only when approaching the highly non-linear SLG regime, because very small energy density variations just above and just below the SLG threshold are magnified by the non linear grain-size behaviour, see Fig. 2. An experimental method to eliminate this effect is in progress.

Fig. 4 - Photo of an irradiated poly-Si showing a texture-like pattern due to the interference
of the laser beam diffracted by the cylindrical lens array of the homogeneizer.
The dark regions are characterized by very large grains (size > 1 mm, due to the SLG),
while, in the clear zones, the grain size is about 0.2 mm.




Selective paint removal for old pictures restoration    up

In collaboration with the Istituto Centrale del Restauro (Rome) we have performed some experiments devoted to find the optimum laser irradiation conditions to selectively remove thin layers from ancient oil paintings. Experiments were done irradiating a number of pictures made by overlapped layers of oil painting, covered by a layer of garanza-lacquer. The raw-wood substrate was previously prepared according to the old recipe based on chalk of Bologna, linseed oil and rabbit glue. The proper Hercules energy density to remove the external layer of oil paint without damaging the underlying paint was 1 J/cm2. This energy density value is substantially larger than the 0.4 J/cm2 reported in literature by using excimer lasers having a pulsewidth six times shorter than the Hercules one. These results rise a question about the relative importance among energy density and power density in paint removal processes. We also observed that multiple shots remove a thicker paint layer than a single shot, for the same total energy density. This phenomenon is probably related to the screening effect of the plasma-plume.
 

The laser ion source up

Our laser-plasma X-ray source is driven by focusing the laser beam of Hercules on a tape target, as described in the 1998 Progress Report of the ENEA Applied Physics Division. It emits ions at a kinetic energy up to more than 10 keV and with a peak current density up to more than 10 mA/cm2 (measured at 1 meter from the source). Potentially, laser ion sources are promising for single-turn injection into a synchrotron. During 1998-1999 we were involved in an European-funded Project (INTAS open 97-2090) for the development of highly charged heavy ions source for the CERN hadrons collider. This project involves 6 participants (3 from Europe and 3 from the New Independent States of the former Soviet Union) and it has the purpose of developing and comparing different laser ion sources. In particular, the ENEA group is the co-ordinator of the project and has the responsibility of a laser ion source. The first analysis of the ions emitted by the laser plasma driven by Hercules showed a poor ionisation state (see the 1998 Progress Report). During 1999, a deeper investigation of the ionisation-state as a function of the flying length of ions has been carried on by a spectral analysis of the soft X-rays emitted by the ions. We discovered that close to the source (up to few mm of flight) the ionisation state is very high (+19 for a copper target and much larger for heavier targets like tantalum) but during the fly the ionisation state reduces due to the strong recombination. A special optical system allowing a coaxial irradiation scheme is going to be developed, in order to collect ions in the same direction of the laser beam. In this way, the recombination should be much less effective due to the higher temperature of the plasma during the early stage of expansion.
 

Microlithography applications of the laser-plasma source    up

Potentially, the Extreme Ultra-Violet projection Lithography (EUVL) allows a sub-100 nm spatial resolution for microchip fabrication. This is a paramount goal for the future microelectronics and nano-mechanics. Our laser-plasma source is very efficient in the EUV (up to 20% in the 40-70 eV spectral range) so that a moderate repetition rate of 100 Hz would be sufficient to achieve a good production yield of 60 wafer layers (having a 300-mm-diameter) per hour.
A main problem of solid-target plasma-sources is the emission of debris (small drops of melted target), which limits the lifetime of the first mirror that collects the EUV radiation from the source and reflects it to the following optics. During 1999 we continued to investigate the possibility to reduce the flux of the debris emitted by our laser-plasma source. We successfully damped the large-size debris (diameter F > 1mm) by a proper choice of the target material and by reducing the laser intensity on the target. In this way, in fact, the plasma temperature falls down and the spectral emission shifts to longer (EUV) wavelengths. The debris flux was reduced by more than two orders of magnitude, as shown in Fig. 5.
 


Fig. 5 – Images of debris flux in different experimental conditions. FL= focused laser beam diameter; IL= laser intensity.

9000 debris/sr/J/shot

300 debris/sr/J/shot 

30 debris/sr/J/shot
Yttrium target
450 shots in 1 bar He
FL=30 mm
IL=3*1012 W/cm² 
Tantalum target
600 shots in 1 bar He
FL=30 mm
IL=3*1012 W/cm² 
Tantalum target
500 shots in vacuum
FL=450 mm
IL=1010 W/cm²

Even the speed of the debris considerably decreased as a result of the lower plasma pressure, thus allowing the use of mechanical stoppers. Finally, we could reach a debris flux of only 6 debris/shot/sr/J, corresponding to a lifetime of the first mirror of about 107 shots. In the condition of low laser intensity, we also got a very good shot-to-shot stability (see Fig. 6), which is an important parameter for EUVL applications. We are investigating the interaction of small-size debris (F < 1mm) and of ions with different gases (see Fig. 7). The final goal is the development of a clean EUVL prototype demonstrating a sub-100 nm resolution. This project will involve different institutions (ENEA, Universities, private companies, etc.) and will be based both on the solid target laser-plasma source developed at ENEA and on the gas plasma source (pumped by a capillary electric discharge) developed in collaboration with the INFN laboratories of L'Aquila.

Fig. 6 - Burst of 28 X-ray pulses @ hn=40-70 eV, obtained from a Ta target at a laser intensity of 1010 W/cm² with a repetition rate of 0.5 Hz. The EUV pulse energy fluctuation is 1.1% rms.
To keep the figure compact, the sampling is stopped after each shot (laser beam: 120 ns - 4 J/shot).

Fig. 7 - The soft X-ray laser plasma source. Note the blue-light emitted by the plume, due to the interaction of the emitted ions with air at 0.7 mbar (a) or with a helium gas at 20 mbar (b). The laser beam, focused by the lens on the right hand of the photos is not visible. On the copper tape target is visible the chain of holes due to copper ablated by the former laser shots.
 
Fig 7a
 Fig 7b

The ultrashort UV laser source    up
In the frame of a project ENEA-MURST, we installed and tested a commercial laser system emitting ultrashort (< 10-12 s) pulses at 308 nm. This FAMP laser system uses an excimer laser to pump several dye cells in cascade, emitting 100 ps pulses within a broad spectrum in the visible. These pulses pump a distributed-feedback dye laser (DFDL) unit, which generates ultrashort pulses (< 1 ps) tunable to the desired wavelength, in our case 616 nm. The DFDL pulses are then amplified and frequency-doubled to 308 nm by a second harmonic generation crystal. The final goal is to use it as a seed pulse to be injected in a large XeCl laser systems for picosecond pulse amplification. Figure 8 shows a photo of this hybrid excimer-dye-DFDL laser source.
 
 

Fig. 8 - Photo of the picosecond laser operative in the excimer laboratory.
The plastic cables are coloured by the flowing dyes used as active media.




The measured energy and the time duration of the radiation emitted by this laser are, respectively, 10 mJ and 0.5 ps, see figure 9. According to the simulations (performed in collaboration with the Pecs University), a proper amplification of this ultrashort pulses with our Hercules or Ianus system should deliver a pulsed beam with up to 0.5 J in 1 ps at 10 Hz, corresponding to a peak power of 0.5 Terawatt.
 
 

Fig. 9 – Curve achieved by a Michelson-like, background-free autocorrelator.
The FWHM laser pulse duration estimated by this autocorrelation curve is 0.5x10-12 s.



PAPERS   up

REPORTS

T. Letardi, P. Di Lazzaro, G. Giordano, C.E. Zheng: "A note for X-ray preionizer design” RT/INN/99 (1999).
 

PUBLICATIONS

S. Bollanti, P. Di Lazzaro, F. Flora, G. Giordano, T. Letardi, D. Murra, G. Schina, C. E. Zheng: “Pulsed X-ray diode with a long-lifetime plasma cathode” Appl. Phys. B 68, 683 (1999)

 S. Bollanti, G. Clementi, P. Di Lazzaro, F. Flora, G. Giordano, T. Letardi, F. Muzzi, G. Schina, C.E. Zheng: "Excimer lamp pumped by a triggered discharge" IEEE Trans. Plasma Sci. 27, 211 (1999)

 F. Rosmej, A. Faenov, T. Pikuz, I. Skobolev, F. Flora, S. Bollanti, P. Di Lazzaro, T. Letardi, K. Vigli-Papadaki , A. Reale, L. Palladino, G. Tomassetti, A. Scafati, L. Reale, T. Auguste, P. Oliveira, S. Hulin, P. Monot, A. Zigler, M Frankel: “Radiation from autoionising levels correlated with single excited states of highly charged ions in dense cold plasmas” Physica Scripta 80, 547 (1999)

 S. Bollanti, F. Bonfigli, P. Di Lazzaro, F. Flora, T. Letardi and D. Murra: “Repetition-rate influence on the beam quality of a XeCl excimer laser” Opt. Commun. 167, 291 (1999)

M. Belli, A. Nottola, F. Flora, T. Jin: “Irradiation of cultured mammalian cells with ultrasoft X-rays: experimental set-up and dose calculation for non-monochromatic beams”, Radiation Physics and Chemistry, 54, 393 (1999)

S.V. Kukhlevsky, Cs Vér, J. Kaiser, L. Kozma, L. Palladino, A Reale, G. Tomassetti, F. Flora and G. Giordano “Generation of high density, pure metal vapour plasma by capillary discharge”, Appl. Phys. Lett., 74, 1 (1999)

S.V. Kukhlevsky, J. Kaiser, L. Palladino, A. Reale, G. Tommasetti, F. Flora, G. Giordano, L. Kozma, M. Lišca and O. Samek: "Physical processes in high-density ablation-controlled capillary plasmas", Physics Letters A 258, 335 (1999)

D. Della Sala, C. Privato, P. Di Lazzaro, G. Fortunato: “Microelettronica gigante” Energia, Ambiente Innovazione 4, 52 (1999)
 

CONFERENCES PROCEEDINGS

F. Flora, S. Bollanti, P. Di Lazzaro, G. Giordano, T. Letardi, A. Nottola, A. Marinai, K. Papadaki, G. Schina, P. Albertano, L. Palladino, A. Reale, A. Scafati, L. Reale, M. Belli, F. Ianzini, A. Tabocchini, A. Grilli, A. Faenov, T. Pikuz: “Applications of a soft X-ray plasma source pumped by a long pulse excimer laser", Proc. Int. Conf. on Lasers '98, V.J. Corcoran, T. Goldman Eds. (STS Press Mc Lean, VA 1999) pp. 454 – 461.

T.A. Pikuz, A. Ya. Faenov, M. Fraenkel, A. Zigler, F. Flora, S. Bollanti, P. Di Lazzaro, T: Letardi, A. Grilli, L. Palladino, G. Tomassetti, A. Reale, L. Reale, A. Scafati, T. Limongi: “Large-field high resolution X-ray monochromatic microscope, based on spherical crystal and high-repetition-rate laser-produced plasmas”, EUV, X-Ray, and Neutron Optics and Sources, C. McDonald, K. Goldberg, J. Maldonado, H. Chen-Mayer, S. Vernon Eds. SPIE vol. 3767, (1999) pp. 67 - 78.

S. Bollanti, P. Di Lazzaro, F. Flora, T. Letardi, A. Marinai, A. Nottola, K. Vigli-Papadaki, A. Vitali, F. Bonfigli, N. Lisi, L. Palladino, A. Reale, C.E. Zheng: “Toward a high average power and debris free soft X-ray source for microlithography, pumped by a long pulse excimer laser” EUV, X-Ray, and Neutron Optics and Sources, C. McDonald, K. Goldberg, J. Maldonado, H. Chen-Mayer, S. Vernon Eds. SPIE vol. 3767, (1999) pp. 33 - 44.
 

CONFERENCES PARTICIPATION

S. Bollanti, P. Di Lazzaro, D. Murra, X. Cheng: “Omogeneizzazione spaziale di fascio laser: disegno delle ottiche ed esperimenti” SIF 99, Congresso della Società Italiana di Fisica (Pavia, Settembre 1999)

P. Di Lazzaro, S. Bollanti, F. Bonfigli, F. Flora, T. Letardi, D. Murra: “Beam quality of an XeCl laser vs. the repetition rate” Int. Forum on AHPLA ’99 Advanced High-Power Lasers (Osaka, Giappone, Novembre 1999)

T. Letardi, A. Baldesi, S. Bollanti, F. Bonfigli, P. Di Lazzaro, F. Flora, G. Giordano, D. Murra, G. Schina, C.E. Zheng “ Industrial large aperture XeCl laser for surface processing” Int. Forum on AHPLA ’99 Advanced High-Power Lasers (Osaka, Giappone, Novembre 1999)

S. Bollanti, F. Bonfigli, D. Della Sala, P. Di Lazzaro, D. Murra “Experimental results on silicon annealing by a long pulse XeCl laser” Int. Conf. ALT ’99, Advanced Laser Technologies (Potenza-Lecce, Settembre 1999)

S. Bollanti, F. Bonfigli, E. Burattini, P. Di Lazzaro, F. Flora, A. Grilli, T. Letardi, N. Lisi, A. Marinai, A. Nottola, L. Palladino, A. Reale, K. Vigli-Papadaki, C.E. Zheng “High efficiency XUV plasma source at 10-30 nm for projection microlithography, pumped by a long pulse excimer laser” MNE ’99, Micro and Nano Engeneering (Roma, Settembre 1999)
 

SEMINARS

P. Di Lazzaro, D. Murra: “Omogeneizzazione di fascio laser e annealing di silicio amorfo: risultati sperimentali nell’ambito del progetto FOTO”