Mechanisms of ablation-rate decrease in multiple-pulse laser ablation

We present two sets of experimental results on the ablation-rate decrease with increase of the number of consecutive laser pulses hitting the same spot on the target surface. We have studied laser ablation of a carbon target with nanosecond pulses in two different interaction regimes: one with a XeCl laser (λ=308 nm) and the other with a Nd:YAG laser (λ=1064 nm), in both cases at the intensity ∼5×10⁸ W/cm² Two different mechanisms were found to be responsible for the ablation-rate decrease; they are directly related to the two different laser–matter interaction regimes. The UV-laser interaction is in the regime of transparent vapour (surface absorption). The increase of the neutral vapour density in the crater produced by the preceding laser pulses is the main reason for the decrease of ablation rate. With the IR laser each single laser pulse interacts with a partially ionised plume. With increase of the number of pulses hitting the same spot on the target surface, the laser–matter interaction regime gradually changes from the near-surface absorption to the volume absorption, resulting in the decrease in absorption in the target and thus in the decrease in the ablation rate. The change in the evaporation rate was considered for both vacuum and reactive-gas environments. Received: 21 February 2001 / Accepted: 26 February 2001 / Published online: 23 May 2001     

Optimization of EUV radiation yield from laser-produced plasma

We optimize the conversion of laser energy into extreme ultraviolet (EUV) radiation by tailoring the laser parameters for a laser-produced plasma generated from 20 μm diameter water droplets. It is shown that mass-limited targets require careful adaption of laser-pulse energy and laser-pulse duration separately, rather than laser intensity, which seems to be adequate for bulk targets. The optimal pulse duration scales with the droplet radius, and the optimal pulse energy with the droplet volume. With optimized parameters, we obtain a conversion efficiency of 0.23% in 4π and 2.5% bandwidth for 13 nm radiation, the future EUV lithography light. Received: 16 July 2001 / Revised version: 25 September 2001 / Published online: 7 November 2001     

Effects of a prepulse on laser-induced EUV radiation conversion efficiency

The influence of prepulses on the conversion efficiency (CE) of laser radiation into 13 nm extreme ultraviolet (EUV) radiation, resulting from lithium-like oxygen ions from O20 μm water droplets, was investigated. The laser pulse durations in the experiment ranged from 200 fs to 120 ps. Applying prepulses preceding the main pulse at various delays of up to 11 ns, it was shown that the CE increases differently for each measured pulse duration. The strongest dependence on the introduction of a prepulse was observed for 2 ps laser pulses with a 20 mJ laser pulse energy. The EUV CE was improved by a factor of 15 by the introduction of a prepulse. Calculations on the atomic physics of oxygen ions and simulations of the laser–plasma interaction revealed the influence of the prepulse on the EUV yield. Received: 25 October 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-3641/947-202, E-mail: duesterer@ioq.uni-jena.de     

Short-laser-pulse-driven emission of energetic ions into a solid target from a surface layer spalled by a laser prepulse

An efficient emission of picosecond bunches of energetic protons and carbon ions from a thin layer spalled from a organic solid by a laser prepulse is demonstrated numerically. We combine the molecular dynamics technique and multi-component collisional particle-in-cell method with plasma ionization to simulate the laser spallation and ejection of a thin (∼20–30 nm) solid layer from an organic target and its further interaction with an intense femtosecond laser pulse. In spite of its small thickness, a layer produced by laser spallation efficiently absorbs ultrashort laser pulses with the generation of hot electrons that convert their energy to ion energy. The efficiency of the conversion of the laser energy to ions can be as high as 20%, and 10% to MeV ions. A transient electrostatic field created between the layer and surface of the target is up to 10 GV/cm. Received: 13 March 2001 / Accepted: 20 March 2001 / Published online: 20 June 2001     

Study of the plasmas produced during the deposition of TiC/SiC thin films in a hybrid magnetron-laser system

Time- and spatially-resolved optical emission spectroscopy was performed to characterize the plasma produced in a hybrid magnetron-sputtering-laser deposition system, which is used for TiC or SiC thin films preparation. A graphite target was ablated by a KrF excimer laser (λ=248 nm,τ=20 ns) and either Ti or Si targets were used for DC magnetron sputtering in argon ambient. Spectra were measured in the range 250–850 nm. The evolution of the spectra with varying magnetron powers (0–100 W) and argon pressures (0.3–10 Pa) was studied. Spectra of the plasmas produced by a) the magnetron alone, b) the ablation laser alone, and c) the magnetron and the ablation laser together, were recorded. Spectra (a) were dominated by Ar atoms and Ar⁺ ions. Emission lines of Ti and Si were detected, when Ti target and Si target was used, respectively. Spectra (b) revealed emission of C, C⁺, C₂, Ar, Ar⁺. Spectra (c) showed presence of all previously mentioned species and further of Ti⁺ ions emission was detected. The research was supported by Grant Agency of the Czech Republic No. 202/06/02161, GA ASCR project number A1010110/01 and Institutional Research Plan AV CR No. AV0Z 10100522.     

Laser acceleration of light ions from high-intensity laser-target interactions

A systematic theoretical study of laser-irradiated targets made of material with increasing atomic number has been performed. The formation of energetic light ions resulting from the interaction of an intense ultrashort pulse laser with thin planar targets is investigated theoretically with a two-dimensional relativistic electromagnetic particle-in-cell model. A common parameter, the areal electron density of the foil, can be used to describe qualitatively targets made of different material. By varying either the laser intensity or the target thickness we observe a gradual transition of various ion acceleration mechanisms from one into another. Light ions, such as H⁺, Li³⁺, C⁶⁺, and Al¹³⁺, can be accelerated to GeV energies with existing laser systems at a laser fluence of 10–20 J/μm².     

Optical absorption in early stage low temperature laser produced plasma

We describe a new technique to measure the UV/visible absorption spectrum of the ablated material during the laser pulse. The technique utilizes the continuum emission from one laser produced plasma as a light source to measure the absorption properties of a second laser produced plasma which is formed on a semi-transparent target with an array of 40 μm holes. A 6 ns, 1064 nm laser was used to ablate a Ag target and the plasma absorption was measured in the range 450–625 nm for a laser fluence of 1 J cm⁻². The total absorption cross-section is (0.5–1.5)×10⁻¹⁷ cm² in the range 450–540 nm. By comparing the measured absorption with a calculation using the plasma spectroscopy code FLYCHK it can be concluded that, in the wavelength region examined here, the absorption is mainly due to bound-bound transitions.     

High-efficiency clean EUV plasma source at 10–30 nm, driven by a long-pulse-width excimer laser

A long-pulse-width high-output energy (120 ns FWHM, 7 J) XeCl laser has been focused on thin tape targets (Cu and Ta) to generate more than 100-ns-long (FWHM) EUV pulses in the 10–30 nm spectral region, suitable for projection microlithography. The conversion efficiency was more than 20% over a 2π solid angle. We observed debris emission using a gated CCD camera, and measured the debris speed for different irradiation conditions. We found irradiation conditions such that the measured velocities were low enough that simple mechanical devices combined with krypton at low-pressure could efficiently stop both ionic debris and cluster debris. Our results show that a suitable combination of driving-laser characteristics, target material and thickness, environment gas and mechanical choppers can make clean and increase the power of EUV solid-target laser-plasma sources. Received: 21 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +39-06/9400-5334, E-mail: bollanti@frascati.enea.it     

Strong green luminescence of Ni2+-doped ZnS nanocrystals

ZnS nanoparticles doped with Ni²⁺ have been obtained by chemical co-precipitation from homogeneous solutions of zinc and nickel salt compounds, with S^2- as precipitating anion, formed by decomposition of thioacetamide (TAA). The average size of particles doped with different mole ratios, estimated from the Debye–Scherrer formula, is about 2–2.5 nm. The nanoparticles could be doped with nickel during synthesis without altering the X-ray diffraction pattern. A Hitachi M-850 fluorescence spectrophotometer reveals the emission spectra of samples. The absorption spectra show that the excitation spectra of Ni-doped ZnS nanocrystallites are almost the same as those of pure ZnS nanocrystallites (λ_ex=308–310 nm). Because a Ni²⁺ luminescent center is formed in ZnS nanocrystallites, the photoluminescence intensity increases with the amount of ZnS nanoparticles doped with Ni²⁺. Stronger and stable green-light emission (520 nm) (its intensity is about two times that of pure ZnS nanoparticles) has been observed from ZnS nanoparticles doped with Ni²⁺. Received: 18 December 2000 / Accepted: 17 March 2001 / Published online: 20 June 2001     

Hot-electron-induced plasma formation on the rear surface of a foil

The plasma jet formed on the rear surface of a foil in laser–solid interaction is investigated by laser probing. The rear plasma jet, which is in line with the laser, formed a few picoseconds after the incidence of the focused laser, is due to a beam of fast electrons propagating through the target and is collimated by a strong magnetic field in the plasma. Received: 14 January 2003 / Revised version: 2 April 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531 E-mail: jzhang@aphy.iphy.ac.cn