Chemistry Reference
In-Depth Information
increases. However, the initial sticking coefficient also (e.g., at zero coverage) is
affected by several surface properties:
1. With increasing depth of the potential well, the coefficient increases due to
the trapping potential. For example, the initial sticking coefficient of
N
2
on
various faces of tungsten crystal differs remarkably from γ
=
0.1 for (111)
0.6 for (100).
2. Thecoefficientdecreaseswithincreasingenergyofincomingparticles,since
the kinetic energy has to be accommodated by the surface collision.
3. Forlargemassdifferencesofthecollidingpartners(e.g.,gas/plasmaparticles
and substrate atoms), the sticking coefficientes are small due to the low
coupling of the incoming particle to the phonon spectrum of the solid.
4. It decreases with increasing substrate temperature due to the increasing
velocity of the substrate atoms.
to γ
=
The surface loss probability β describes the loss of a particle upon impact, which
includes sticking at the surface, but also surface reactions in which a reactive particle
recombines at the surface to form a nonreactive volatile compound (example SiH
3
+
H
surface
−→
SiH
4
). In many plasma experiments, as described in the following text,
only the surface loss probability can be measured. The direct measurement of the
sticking coefficient requires absolutely quantified sources of the species of interest,
which is difficult to realize for the mixed conditions in plasmas. The distinction
between the sticking coefficient and surface loss probabilities is often overlooked in
the literature.
The rate of adsorption
R
ads
of a species
A
on a surface
B
is determined by
the particle flux
j
A
, the sticking coefficient γ
A
of
A
on
B
, and the state of the
surface
f
()
:
dn
ads
dt
=
R
ads
=
γ
A
j
A
f
()
.
(5.1)
The sticking coefficient is dominated by the ability of the incoming species to
transfer sufficient kinetic energy to the surface atoms to be trapped in the poten-
tial well. This transferred energy is then dissipated by the atom of the solid via
phonon excitation. The energy being transferred from the impinging species to the
surface atom can be roughly estimated on the basis of energy and momentum con-
servation in the binary collisions approximation. The maximum transferable energy
T
max
yields
4
m
1
m
2
T
max
=
E
0
2
.
(5.2)
(
m
1
+
m
2
)
E
0
denotes the kinetic energy of the incoming species upon collision with the surface
atom,
m
1
the mass of the projectile, and
m
2
the mass of the target atom. Equation 5.2
shows that for a light projectile impinging on a heavy target, the transferred energy
is small. Thereby, the incoming species may be unable to lose sufficient kinetic
energy to become chemisorbed, which leads to a small sticking coefficient. A typical
