Characterization of laser induced plasmas by optical emission spectroscopy: A review of experiments and methods

Nouveau Traité de Chimie Minérale.

We discuss the generation of high-density and high-temperature plasmas by focusing high peak power laser radiation onto a solid target. Emphasis will be put on the process of laser ablation and on its basic, physical mechanisms. A survey will be given of the main experimental techniques, namely optical emission and absorption spectroscopy, mass spectrometry, time-of-flight and charge collection measurements, devised to characterize laser-produced plasmas. The fundamental theoretical and numerical approaches developed to analyse laser-target interaction, plasma formation, as well as its expansion will also be reviewed, and their predictions compared with the experimental findings. Although the main emphasis of the review will be on metal target ablation, reference and comparison to results on multicomponent targets will also be frequently given.

https://iopscience.iop.org/article/10.1088/0953-4075/32/14/201/pdf

Characterization of laser-ablation plasmas

Laser nitriding of metals

Characterization of a laser-induced plasma by spatially resolved spectroscopy of neutral atom and ion emissions.. Comparison of local and spatially integrated measurements

Time‐ and space‐resolved emission and laser‐induced fluorescence spectroscopic measurements were performed to investigate vaporization and plasma formation resulting from excimer laser irradiation of titanium targets in a low‐pressure nitrogen atmosphere. Measurement series have been done by varying the laser intensity from the vaporization threshold at 25 MW cm−2 up to values of about 500 MW cm−2 typically applied in pulsed laser deposition processing of titanium nitride films. Thus, the transition from thermal evaporation to the high‐density plasma formation process, leading to the production of reactive species and high‐energy ions, was evidenced. An interesting result for the comprehension of the reactive deposition process was the observation of a quantity of dissociated and ionized nitrogen, which is transported with the plasma front in the direction of the substrate.

Plasma diagnostics in pulsed laser TiN layer deposition

The initial step of particulate growth in a dust forming low pressure radio‐frequency discharge has been studied in situ by laser induced particle explosive evaporation (LIPEE). With respect to the conventional light scattering, this method has been found much more efficient to observe small nanometer size particles, especially in the case of UV excimer laser radiation. Experimental results interpreted by a simple model of laser‐particle interaction show that the intensity of LIPEE continuum emission depends on the particle radius roughly as r4. This interaction is essentially different from Rayleigh scattering, as the latter varies as r6. A study of time evolution of powder formation by LIPEE emission reveals the initial formation of nanometer size crystallites and the coalescence process leading to larger scale particles. It could be demonstrated that the critical step of dust formation is the initial clustering process leading to nanometer scale crystallites.

Study of initial dust formation in an Ar‐SiH4 discharge by laser induced particle explosive evaporation

New experimental results are reported on plasma initiation in front of a titanium sample irradiated by ir (λ=10.6 μm) laser pulses in an ambient gas (He, Ar, and N2) at pressures ranging from several Torr up to the atmosphere. The plasma is studied by space‐ and time‐resolved emission spectroscopy, while sample vaporization is probed by laser‐induced fluorescence spectroscopy. Threshold laser intensities leading to the formation of a plasma in the vapor and in the ambient gases are determined. Experimental results support the model of a vaporization mechanism for the plasma initiation (vaporization‐initiated plasma breakdown). The plasma initiation is described by simple numerical criteria based on a two‐stage process. Theoretical predictions are found to be in a reasonable agreement with the experiment. This study provides also a clear explanation of the influence of the ambient gas on the laser beam‐metal surface energy transfer. Laser irradiation always causes an important vaporization when performed i...

Multistage plasma initiation process by pulsed CO2 laser irradiation of a Ti sample in an ambient gas (He, Ar, or N2)

A new laser method is proposed for the deposition of high purity, hard fcc TiN layers of unlimited thickness. The film thickness can be very finely controlled mainly through the intermediary of the number of applied laser pulses as the deposition rate is of only 0.02–0.05 nm/pulse. The ablation is promoted from a Ti target by high intensity multipulse excimer laser irradiation in a low pressure N2 ambient gas while the forming compound is collected on a Si single‐crystalline wafer. The best results have been obtained for an ambient pressure of p=10–30 mTorr and a distance between the target and support of d=10 mm. It is shown that the formation of a liquid phase within the irradiated zone, maintained even after the end of a laser pulse, is the most important requisite for TiN formation. TiN is then ablated as a stoichio‐ metric phase.

Deposition of high quality TiN films by excimer laser ablation in reactive gas

The formation and role of a plasma resulting from the interaction between a laser beam and a metal target in an ambient gas are reviewed in this paper. The plasma is initiated after the production of primary electrons by multiphoton absorption and thermionic photoemission mechanisms. Vaporization of the surface then occurs at a lower threshold than theoretically deduced, due to surface defects and impurities. Vapour ionization is first a thermal process, primary electrons gaining energy by inverse Bremsstrahlung than electron cascade growth occurs. The plasma propagates with absorption waves; laser-supported combustion or detonation waves depending on the laser irradiance regime. The ablation process is favoured with the shortest wavelengths, whereas ambient gas breakdown occurs with the largest wavelengths. The nature of the ambient gas must be adapted for the chosen application. Plasma development can be an inconvenience with target shielding effect when the ionization degree is too high and with laser-supported detonation wave generation occurring in this case. However, transparent plasmas provide optimum laser machining, as good surface coupling with plasma formation occurs which is useful for drilling or welding.

https://iopscience.iop.org/article/10.1088/0963-0252/2/3/013/pdf

B. Dubreuil

Papers

Plasma diagnostics in pulsed laser TiN layer deposition

Study of initial dust formation in an Ar‐SiH4 discharge by laser induced particle explosive evaporation

Multistage plasma initiation process by pulsed CO2 laser irradiation of a Ti sample in an ambient gas (He, Ar, or N2)

Deposition of high quality TiN films by excimer laser ablation in reactive gas

Plasma formation resulting from the interaction of a laser beam with a solid metal target in an ambient gas