Comparison of the ablation plumes arising from ArF laser ablation of graphite, silicon, copper, and aluminum in vacuum

Abstract

The ablation plumes arising after irradiation of graphite, silicon, copper and aluminum with a pulsed nanosecond ArF (λ=193 nm) laser at fluences between 2 and 20 J cm−2 in vacuum are studied and compared. The neutral and ionic components in the ablation plume have been measured via quadrupole mass spectrometry and ion probes, respectively. Additional information about the degree of ionization and the velocities of singly and multiply charged ions in the plume have been deduced via optical emission spectrometry, and the electron velocity distributions have been measured with Langmuir probes. Probing the plasma properties with this range of techniques is shown to provide a rather detailed picture of the ablation characteristics. The velocity distributions of the neutral atoms are comparatively narrow (∼1 km s−1 full width at half maximum) and peaked at a center of mass velocity of ∼3–4 km s−1. Their general form is reminiscent of those of species expanding supersonically from a pulsed nozzle. The electron and ion velocity distributions are much broader, and centered at much higher velocities (and kinetic energies). The relative ion yield, and the overall degree of ionization, both increase with increasing fluence and scale inversely with the ionization potentials of the respective target materials. Both charged components are found to be accelerating at short distances from the target. Such effects have been rationalized by assuming that incident laser radiation ionizes (by multiphoton ionization) neutral species ablated from the target surface, and that these ions and electrons then act as “seeds” for subsequent plume heating, ionization and plasma formation by inverse bremsstrahlung. This absorption due to inverse bremsstrahlung ensures the incident laser intensity is highest at the outer edge of the expanding plume. The outer region thus receives preferential excitation and heating—traditionally pictured in terms of the so-called two electron temperature model. Some of the resulting “hot” electrons escape from this coronal region, leading to an overall charge imbalance within the plasma, which manifests itself as an acceleration (driven by Coulombic interactions) of the charged components within the plume.

Dynamics of ions produced by laser ablation of several metals at 193 nm

“…11) and this fraction increases as fluence increases. With some exceptions, 5,9,10,14 most of the studies only report mean kinetic energies of ions. Instead, processes at substrate level have been related to ions having kinetic energies higher than 200 eV.…”

“…Instead, processes at substrate level have been related to ions having kinetic energies higher than 200 eV. 11 In addition, mean kinetic energies generally vary little with fluence, 10 while distributions are known to expand significantly toward high kinetic energy values up to 1 keV as fluence is increased. [9][10][11]14 There are several publications reporting studies related to plasmas produced by laser ablation.…”

“…Ions are the species in the laser induced plasma having the higher kinetic energies, [9][10][11] and considered responsible for sputtering or implantation at substrate level during pulsed laser deposition. 4,6,7 The ionization fraction is determined by the fluence and wavelength of the laser used for ablation.…”

The origin of the metal enrichment of carbon nanostructures produced by laser ablation of a carbon–nickel target

“…4(a). The 426.7 nm feature is correlated with the carbon lines are usually detected on ablation of pyrolytic graphite [17,27,28] at equivalent laser fluence but evidence for these is not apparent in the mixed plume spectra. However, it is unlikely that one could observe these specific carbon emission lines due to the overwhelming intensity of Ni-related features in equivalent wavelength regions.…”

Lézerrel kezelt szilícium felületek, valamint impulzuslézeres rétegépítéssel előállított amorf szilícium és SixC vékonyrétegek ellipszometriai vizsgálata

“…Ha a céltárgy anyagában szilícium is van, az önmagában még nem kellene, hogy megváltoztassa a szén részecskék plazmabeli energiáját és így továbbra is gyémántszerű rétegeknek kellene épülniük. Azonban, ahogy azt Cleayssens és munkatársai kimutatták [137], az általam használt energiasűrűség tartományban (6-7 J/cm 2 ) a Si ionok plazmabeli energiája jóval nagyobb, mint a C ionoké. Következésképpen a Si ionok okozhatják a réteg felszínének melegedését, és így a réteg grafitizálódását.…”