Pulsed laser ablation (PLA) and deposition (PLD) provide a means of growing thin coatings of a wide range of materials on an equally wide range of substrates. However, despite the versatility and broad applicability of PLA and PLD, many details of the chemical physics underlying the ablation and deposition processes are still far from completely understood. We use optical emission spectroscopy (OES) and mass spectrometry to characterize the composition of the plume of material ejected from a surface by laser ablation.
Schematic diagram of the apparatus used for PLD of thin films.
The ablation event is often envisaged as a sequence of steps, beginning with the interaction of laser radiation with electrons at, and near to, the surface of the target. If the laser pulse persists for more than a fraction of a picosecond, additional energy absorbed in this way couples into the solid, causing intense localized heating, and thereby inducing a variety of thermophysical and physico-chemical effects. Even non-volatile target materials such as ceramics and refractory metals are subject to strong evaporation (and even explosive boiling) under these conditions. The composition and properties of the resulting ablation plume may further evolve during its expansion, both as a result of (Coulomb) collisions between particles in the plume and through laser–plume interactions at a range of energy scales. Finally the plume impinges on the substrate to be coated: incident material may be accommodated, rebound back into the gas phase, or induce surface modification via sputtering, compaction, sub-implantation, etc. However, although this description, with each process occurring stepwise and in isolation, has conceptual appeal, it is somewhat over-simplistic, especially for the common case of plumes produced by nanosecond pulsed lasers.
Laser–target interactions will be sensitively dependent both on the choice of target material and on the laser parameters (wavelength, irradiance, temporal profile, etc.). The detailed properties of the laser radiation are even more critical to subsequent laser–plume interactions, while the evolution and propagation of the plume will also be sensitive to collisions and thus to the identity and pressure of the ambient medium. Indeed, these effects are often intimately coupled via photochemical processes and effects such as electron–neutral inverse bremsstrahlung absorption. The ultimate composition and kinetic energy distribution (or distributions, in the case of a multi-component ablation plume) of the ejected material will generally be reflected in the detailed characteristics of any deposited film.
We use exciplex (ArF*) or Nd:YAG laser radiation to ablate a range of prototypical target materials, including metals (Mg, Al, Zn, Fe, Cr, etc.), and two-component systems (e.g. oxides such as Al2O3, ZnO, MoO3 and WO3), both under vacuum and in the presence of low pressures of either inert or reactive background gas. Fundamentals of the ablation process are investigated by spatially and temporally resolved OES.
Optical emission spectrum of a Zn ablation plume
The figure above shows a spatiospectrally-resolved image of optical emission following 355 nm PLA of a Zn target, in vacuo, with an irradiance of 6.9 GW/cm2 recorded in a 10 ns time window centred 80 ns after the ablation pulse impinges on the target. The vertical axis represents distance from the target, the horizontal axis shows the emission wavelength, and the false colours indicate relative intensity. The different emitting species (Zn, Zn+, Zn2+, …) propagate with very different velocities. Analysis of spatiospectral images recorded at different time delays and irradiances, and with different ablation wavelengths and ambient pressures, affords insight into the relative contributions from laser–target and laser–plume interactions in establishing the plume composition and propagation dynamics.
Representative Publications:
Quadrupole mass spectrometry and time-of-flight analysis of ions resulting from 532 nm pulsed laser ablation of Ni, Al and ZnO targets, R.S. Sage, U.B. Cappel, M.N.R. Ashfold and N.R. Walker, J. Appl. Phys. 103, 093301 (2008).
Studies of the plume emission during femtosecond and nanosecond ablation of graphite in nitrogen, G.M. Fuge, M.N.R. Ashfold and S.J. Henley, J. Appl. Phys. 99, 014309 (2006).
