2004 ANNUAL REPORT
ANNUAL REPORT 2004
During the year 2004, the work at the Excimer Laser Laboratory was mainly focused on laser applications, on design and test of optics, and on applications of a laser-driven plasma emitting in the extreme ultraviolet (EUV). The Excimer Laboratory had also the honour of organising the annual meeting of the Project Eureka S! 2566 EULASNET (the EUropean Laser and optics ApplicationS NETwork) and the associated international workshop on "Laser material processing: from basic research to pre-competitive applications and devices". Twenty-seven, from twelve European countries, attended both workshop and meeting.
All the experimental activities summarised in the following have been done in the frame of international projects, cooperation agreements, and contracts with SME, research institutions and universities. In the following, we present a short summary of the results achieved in each line of activity.
1) We have continued the work of know-how transfer related to the transfocal homogeniser patented by ENEA to the SME Info&Tech, which signed a licensing agreement with ENEA for the commercial exploitation of this technology. In this frame, we have investigated the limits of this technology to transform the shape of very coherent (M2 < 2) laser beams into a flat-top spatial profile with steep edges.
2) In the frame of the Eureka project S! 2359 (Choclab II), we have experimentally tested the standard parameters defined by the I.S.O. (International Organisation for Standardisation) in the document ISO 13694 "Test methods for laser beam power (energy) density distribution". When measuring the laser beam intensity shape with a CCD camera, we found that the standard definition of both edge steepness and plateau uniformity may overestimate the importance of data coming from few dummy pixels, altering the value of the maximum energy density and thus giving unreliable values of these parameters. We have proposed a simple change in the standard definitions (namely, a new normalizing energy density value) that unambiguously allows overcoming this problem.
3) We continued the work in the frame of the Italian project FIRB-EUVL for the projection lithography in the Extreme Ultra-Violet (EUV), started in 2003. This project involves six main partners (ENEA, L’Aquila University, Padua University, the INFN Laboratories of Legnaro, El.En. S.p.A., Media Lario S.p.A.) and it aims to the growth of the Italian know-how in the key technical aspects of the EUV projection lithography, namely the development of high-efficiency EUV sources based on laser-plasmas, the realisation of multilayer mirrors in the EUV, the development of masks, the design of ultra-high-resolution (sub-100 nm) projection optics. The final product will be a prototype able to replicate test masks into silicon wafers and LiF with a 100-nm-resolution pattern. The activity carried on in 2004 was mainly devoted to the general architecture of the experimental setup of the prototype, including the design of the projection optical system and of the high-vacuum chamber that will contain the projection optics. Moreover, in collaboration with El.En. S.p.A. we have made the new electrical modulator of the discharge pumping circuit of Hercules, the excimer laser facility that drives the plasma source, in order to improve the reliability of the laser-plasma emission in repetition rate.
4) We started the work in the frame of a European Integrated Project on EUV-lithography, named "More Moore", funded by the 6th Framework Program and leaded by ASML (NL), the world leader manufacturer of steppers for lithography. The main task of the ENEA Excimer Laboratory is the development and optimisation of a system able to stop debris emitted by plasma EUV sources. This is a key topic, as every plasma source emits a large number of debris (ions, neutrals, clusters of particulate) that may damage the expensive optics, filters and diagnostics put close to the source. As a consequence, debris can render a plasma source virtually useless, and stopping debris is a basic pre-requisite for plasma sources development. In this frame, we made many measurements to characterise the spatial and temporal behaviour of debris by using a gated CCD camera, glass plates analysed with an optical microscope, a ballistic pendulum, a Faraday cup and an electrostatic analyzer. For ionic debris the most accurate characterization was obtained by using the Faraday cup and the electrostatic analyzer. Figure 2.1b shows the Faraday cup signal obtained irradiating a tantalum (Ta) tape-target. In this case the target was placed in the focal plane with an incidence angle of 45° in order to allow the ionic beam detection on the normal to the target surface as shown in Fig. 2.1a. Figure 2.1b shows that the kinetic energy of ions reaches values of few keV with a corresponding speed, in the case of Ta, ranging between 20 and 80 km/s. On the right side of Fig. 2.1b, the approximate value of ions flux is reported assuming an average ions charge state Z = 2 (this value is obtained from the Energy Analyser measurements as discussed later). Being the ions flux in the order of 1020 ions/sr/s emitted within 15 µs, we estimate a total amount of 2.1015 ions/sr.
|Figure 2.1. a) Experimental set-up for the ions
characterization by means of a Faraday-cup detector.
||b) Current measured by the Faraday cup with the corresponding velocity, energy and flux of ions.|
|Figure 2.2. a) Experimental set-up of the Energy Analyzer.||b) Charge state distribution of tantalum ions, obtained from the Energy Analyzer signals.|
By using the Energy Analyser (EA) schematised in Fig. 2.2a, we measured the charge state distribution and the ion kinetic energy of different target materials. We found that at 1.8 m from the target the charge state distribution is peaked around a charge Z = 2 as shown in Fig. 2.2b for the case of Ta target. Moreover, we found that for a given laser intensity, the kinetic energy of different ions is almost independent of the target material, as shown in Fig. 2.3 where the EA signals for Ta, Cu and Al (atomic number ranging from 13 to 73) are compared.
|Figure 2.3. a) EA signal for three different target materials
obtained from a plasma driven by a 1.5-J, 10-ns-pulsewidth XeCl
laser, selecting on the EA a Ek/Z ratio of 1.2 keV.
b) The same like in a) after converting the horizontal scale to kinetic energy.
|Figure 2.4. Calculated range of flight of ionic debris emitted by Ta- Sn- Li-plasma sources with a kinetic energy of 4 keV, travelling through argon or krypton gases.|
5) We continued the activity on irradiating LiF with EUV photons emitted by our laser-plasma source (see the Annual Report 2003). EUV photons are strongly absorbed in the first 20 nm of LiF films, with a corresponding large energy density for unit volume that is able to generate a huge amount of colour centres. After irradiation, illuminating the EUV-generated colour centres allows the emission of a luminescent pattern complementary to the shadow of the object put in front of the LiF, with sub-mm resolution. As an example, Fig. 2.5 shows a micro-radiography of a mosquito wing, imaged with different zoom factors.
|Fig. 2.5: Microradiography of a mosquito wing obtained on a LiF films with different zooms. The LiF was irradiated by 1000 EUV shots by the ENEA laser-plasma source.|
6) In collaboration with a Company leader in the market of bio-medical laser systems, we have characterised the emission properties of a commercial XeCl excimer lamp used for the UV-B photo-therapy of psoriasis and vitiligo. In particular, we carefully measured the spectrum and the spatial distribution of the radiation emitted by the excimer lamp, to check the uniformity of the dose delivered to the skin when changing either the distance from the lamp or the surface to be irradiated. This is a crucial issue, as only an accurate knowledge of the light intensity allows to set the correct irradiation time that optimizes the therapy and minimizes the probability of collateral effects. Furthermore, it is important to know the surface irradiated with a homogeneous light intensity, that is, the area where the intensity fluctuations do not exceed the safety range.
The results show that the uniformity of the UV intensity (and then, of the delivered dose) is guaranteed within 5% over a surface of 13 cm × 6 cm when the target is put at 15 cm from the lamp, as shown in Fig. 2.6.
|Fig. 2.6. Contour map of the UV intensity spatial distribution at 15 cm from the lamp. The intensity levels differ of 5%. The outline of a human hand is drawn for reference.|
Our experimental data are in good agreement with the analytical results achieved approximating the lamp by a uniform two-dimensional source. This simple analytical model allows one to accurately know the absolute intensity value of the UV light emitted by the excimer lamp in any point of the space.
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