07-12-2012, 05:54 PM
Sputteing - a Review of Some Recent Experimental and Theoretical Aspects
[attachment=43007]
Abstract.
After a brief outline of the present sputtering theory for a random solid, recent
results of the sputtering yield S for polycrystalline targets are discussed, in particular in
view of the influence of the projectile mass and the bombarding angle. The angle dependence
of S at low bombarding energies, and results on the angular distribution of sputtered
particles for oblique ion incidence point out necessary modifications of present sputtering
theories with respect to the anisotropy of the collision cascades in the solid and the influence
of the target surface. The energy distribution of the neutral particles ejected along the target
normal is related to the theoretically predicted E- 2-distribution of low energy recoils in the
solid. Recent mass spectrometric studies of postionized sputtered neutrals are discussed
in view of the formation of sputtered molecules and the application of sputtered neutral
mass spectroscopy for surface analysis. Finally, the paper deals with ion-induced surface
effects on non-elementary sputtering targets, and the protracted removal of foreign atoms
from a matrix.
Sputtering Yield
Theory
The application of modern transport theory to atomic
collision cascades has proved to be the most promising
concept for the theoretical description of the sputtering
process for amorphous - or in a rather good approach
- for polycrystalline solids [21-23]. (For a
recent review see [24].) The most sophisticated
treatment of the binary collision cascade model in a
random target has been given by Sigmund [23].
Basically, distribution functions for the statistic quantities
describing the spatial and energetic properties
of the collision cascades in an infinite target are
derived from corresponding transport equations. For
the calculation of the sputtering yield S, the spatial
distribution of the energy deposited in the solid in the
motion of low energy cascade particles is the most
essential information. The corresponding stationary
transport equation may be obtained applying a
variation principle introduced by Lindhard et al. [25],
Sputtered Particles
Angular Distribution of Sputtered Particles
The anisotropic particle ejection from ion bombarded
single crystal surfaces, first reported by Wehner in
1955 [75], has attracted the highest interest in this
field for a long time. The generation of characteristic
emission patterns with "Wehner-spots" has been
primarily ascribed to focused atomic collision sequences
in the lattice [76, 77] although spots are found
at low bombarding energies, where the occurence of
focusing chains becomes questionable [-75, 78]. At
present, both from theoretical [79, 80] and from
numerous experimental arguments [81-84] spot formation
also must be regarded merely as a consequence
of the regular order of surface atoms.
[attachment=43007]
Abstract.
After a brief outline of the present sputtering theory for a random solid, recent
results of the sputtering yield S for polycrystalline targets are discussed, in particular in
view of the influence of the projectile mass and the bombarding angle. The angle dependence
of S at low bombarding energies, and results on the angular distribution of sputtered
particles for oblique ion incidence point out necessary modifications of present sputtering
theories with respect to the anisotropy of the collision cascades in the solid and the influence
of the target surface. The energy distribution of the neutral particles ejected along the target
normal is related to the theoretically predicted E- 2-distribution of low energy recoils in the
solid. Recent mass spectrometric studies of postionized sputtered neutrals are discussed
in view of the formation of sputtered molecules and the application of sputtered neutral
mass spectroscopy for surface analysis. Finally, the paper deals with ion-induced surface
effects on non-elementary sputtering targets, and the protracted removal of foreign atoms
from a matrix.
Sputtering Yield
Theory
The application of modern transport theory to atomic
collision cascades has proved to be the most promising
concept for the theoretical description of the sputtering
process for amorphous - or in a rather good approach
- for polycrystalline solids [21-23]. (For a
recent review see [24].) The most sophisticated
treatment of the binary collision cascade model in a
random target has been given by Sigmund [23].
Basically, distribution functions for the statistic quantities
describing the spatial and energetic properties
of the collision cascades in an infinite target are
derived from corresponding transport equations. For
the calculation of the sputtering yield S, the spatial
distribution of the energy deposited in the solid in the
motion of low energy cascade particles is the most
essential information. The corresponding stationary
transport equation may be obtained applying a
variation principle introduced by Lindhard et al. [25],
Sputtered Particles
Angular Distribution of Sputtered Particles
The anisotropic particle ejection from ion bombarded
single crystal surfaces, first reported by Wehner in
1955 [75], has attracted the highest interest in this
field for a long time. The generation of characteristic
emission patterns with "Wehner-spots" has been
primarily ascribed to focused atomic collision sequences
in the lattice [76, 77] although spots are found
at low bombarding energies, where the occurence of
focusing chains becomes questionable [-75, 78]. At
present, both from theoretical [79, 80] and from
numerous experimental arguments [81-84] spot formation
also must be regarded merely as a consequence
of the regular order of surface atoms.