Robert J. Lade - PhD Thesis, November 1999

'Pulsed Ultraviolet Laser Ablation of Carbon Containing Targets'

Pulsed UV (193 nm) laser ablation of poly(methyl methacrylate) (PMMA), graphite and CVD diamond targets have been investigated using optical emission spectroscopy (wavelength dispersed spectra together with spatially and temporally resolved measurements), ion probe measurements and quadrupole mass spectrometric plume diagnostics. The relative merits and limitations of each measurement are considered.

The OES measurements, which have been conducted as a function of angle of incidence and laser fluence, chamber pressure, viewing region and observation geometry, all reveal subtleties, details of which are needed before useful inter-laboratory comparisons can be made. Wavelength dispersed emission spectra reveal that the predominant emitting species are atomic neutrals and ions. Time of flight emission transients and CCD imaging have provided estimates of the propagation and expansion velocities of specific emitting species. An anisotropic emission distribution has been observed and identified as being the result of a geometric enhancement of laser-plasma energy coupling, occurring at the leading edge of the expanding plume.

Electron-ion recombination at the early stages of the plasma development is considered to be the most likely mechanism by which electronically excited species are observed in their emitting states far from the target. Similarities in the estimates of the propagation velocities of the emitting and charged species and their more highly charged precursors provides further support for this proposal.

Preliminary mass spectroscopic studies of the plume accompanying the ablation of graphite have identified C+ as the major ionic component of the plasma ions, but failed to reveal any molecular ions or neutral species with certainty, at the laser fluences investigated. Ion probes have provided estimates of the KE distributions of the ionic species within the plasma and can give qualitative agreement with measured by the electrostatic quadrupole plasma probe.

Ablation of the 3 targets investigated appear to produce films that are typical of PLD. LRS of the a-C films suggests that the sp3 bonding content is >60%, by comparison with prior LRS and EELS studies. a-C:H films deposited from the pulsed 193 nm laser ablation of PMMA have been shown to be deficient in H and O compared with the original PMMA target material, as determined by micro-combustion analysis and FTIR. Preferential ablation of the polymer side groups and the different relative displacement energies of the constituent atoms within the deposited films are both believed to contribute to the different stoichiometries of the target and film.