Laboratory Spectroscopic Techniques
Laser Ablation
Viale Ferdinando Stagno d'Alcontres, n. 37, 98158 Messina, Italy.
direct line: +3909039762210
operator: +3909039762
fax: +390903974130
Pulsed Laser Deposition (PLD) is an experimentally simple and a very versatile method
for growing thin films of a wide range of materials with good crystalline quality. This
possibility has generated an effort in the study of the basic mechanism
of PLD and of the plasma expansion dynamics produced by laser ablation of different material
targets. The technique is based on the ablation process of a material induced by focusing
a high energy pulsed laser on its surface. In spite of the simplicity of the experimental
setup, the physical phenomena involved in the ablation process are very complex. The
involved mechanisms depend on the optical and structural properties of the target material
and on the characteristics of the incident radiation such as the wavelength and the energy
density. The stream of atoms, molecules and clusters that is ejected from the target surface
during the ablation process, commonly known as "plume", rapidly expands, in vacuum or
through a gas, towards the substrate surface. Although a lot of experimental and theoretical
work has been done, many effects of the interaction of the laser beam with different
targets in an ambient atmosphere are still not satisfactorily explained. It is known that,
being the plume composed of excited neutral and ionized species, it emits radiation that
can be appropriately analyzed to obtain information about its composition and dynamics.
In particular, by means of fast photography measurements, different regimes of the laserinduced
plasma expansion dynamics have been evidenced. This information can be strictly
correlated with the structural properties of the deposited thin films and can so give a very
useful contribution to the comprehension and to the control of the deposition process itself.
During PLD experiments the plasma generated from the target ablation is attenuated
and thermalized by the gas environment, varying the film growth parameters such as the
deposition rate and the kinetic energy distribution of the species. Furthermore some reactive
gases can aid the formation of some molecules inside the plasma allowing the addition
of the gaseous atomic species into the growing film. The raise of the background pressure
generally leads to the following effects that depart from the free expansion behaviour:
1)an increase of the collisions on the expansion front with an increase of the fluorescence
coming from all the species; 2) a sharpening of the plasma boundary that indicates the
presence of a shock wave front; 3) a spatial confinement of the plasma due to a decrease of
its velocity, determined by the repeated collisions with the gas molecules.
The plasma expansion in a gas environment has been described in the literature in terms
of different phenomenological models depending on the experimental regimes, affected by
the pressure range of the gas present during the deposition process, by the energy of the
plasma species and by the temporal stage of propagation.
In vacuum, the plasma will expand in a way similar to a supersonic expansion with
a free linear behaviour and a weak fluorescence will be visible, close to the target, due to
collisions between the plasma species occurring just after the termination of the laser pulse.
At low pressure and in the early times of the expansion, the plasma dynamics is in good
agreement with the drag-force model. In this model the ejected species are regarded
as an ensemble that experiences a viscous force proportional to its velocity V through the
background gas [1]:
↑ Time resolved images of a Si plasma expanding in a Ar/O2 atmosphere at different pressure values.
Experimentally the plasma generated in PLD experiments shows a behavior described by a mix of these two models: it starts following the drag model and then, when the viscous slowing of the plasma front edge coalesces to form the shock front, it will expand according to the blast wave model. In the last years, Arnold et al. [3] proposed an analytical approach to explain the complete dynamics of the laser generated plasma into ambient gas valid for any pressure value and for any spatial and temporal regime. This phenomenological model provides some differential equations for the characteristic radii describing the spherical plasma expansion and takes into account three different process stages: an early stage where the plasma expansion is characterized by a free expansion with a linear behaviour; an intermediate stage where there is the shock wave formation and the plasma dynamics behaves as R ~ t2/5; and a final stage where the plasma expansion stops. These regimes, that depend on the different process conditions, are also temporally unified so that a single analytical curve gives a complete description of the phenomenon. Experimentally, the chemical nature of the background gas, its pressure and the plasma energy can determine the occurrence of one or more of these regimes and a family of R-t curves can be obtained. If these R-t experimental data are expressed in terms of the following dimensionless variables: