Chemistry Reference
In-Depth Information
calculated using binary collision models like the TRIM code. Chemical sputtering
[97] is dependent on
•
The energy and mass of the incident particle
•
The target temperature
•
The incident flux of particles
No threshold kinetic energy is needed for chemical sputtering to occur, and yields
even for sub-eV hydrogen ion bombardment are not negligible [103]. The strong
dependence on the threshold temperature is one of the key signatures of chemical
sputtering [104].
The energy deposited by the plasma on the surface determines the surface tem-
perature, which is an important parameter in determining the diffusion of the various
species and the molecule formation on the surface. At a very high incident energy
flux and/or insufficient cooling of the target, the surface temperature can increase
and the surface atoms can thermally evaporate. Furthermore, implanted gas atoms
can accumulate in the surface layers to form bubbles. This leads to blister and flake
formation and breaking of the surface layer [105].
The bombardment of energetic atoms creates damage sites (traps) and intersti-
tials within their range of penetration in the target. This process competes with the
annealing of the lattice defects by recombination of interstitials and vacancies. If the
recombination rate is not high enough to anneal all the damage, a net production of
damage and amorphization of the target surface arises. The porous structure of the
graphite offers a large internal surface area on which the incident hydrogen atoms
can diffuse and penetrate far beyond the implantation range of the ions [106].
9.3.1.1 Plasma-Wall Contact with an Electron Emitting Wall
If the wall material is emitting electrons (or ions), the plasma-wall contact is modified
[107]: The potential drop is reduced, since to reach zero net current with an emitting
wall with an electron emission coefficient γ, more electrons from the plasma have
to reach the wall. This causes a modification of the potential at the wall [108]. Nev-
ertheless, there is a limit for very strong emission, because with a reduced potential
drop, the electric field at the wall also drops and reaches zero for a critical emission
coefficient. A higher emission leads to field reversal and the creation of an electro-
static double layer at the wall, at which emitted electrons are partly reflected back
to the wall and are directly re-adsorbed (space-charge limit of the emission current).
Now the different emission processes will be shortly discussed. More details can be
found in [109].
•
Photoelectric emission:
When a photon (with the energy
h
ν) hits a solid
surface, an electron is released with the energy
h
ν
−
W
, where
W
is the
work function of the material.
•
Thermionic emission:
Electrons in a solid occupy energy levels according
to the Fermi-Dirac distribution, where the Fermi-energy
F
is the highest
occupied energy level (at zero temperature).
