Chemistry Reference
In-Depth Information
involved mechanisms is difficult. Depending on the impinging particle fluxes from
the gas/plasma phase, on the energy distribution of the incoming species, and on
the thermal conditions as well as on the electrical potentials in front of the substrate
surface, different mechanisms are dominating. For example, in case of physical sput-
tering (PS) one has to investigate mainly interactions between directed, energetic ions
and solids. Even in this case, an exact description is quite difficult, but it becomes
nearly impossible for surface film reactions that proceed via intermediate molecular
states. Examples for such processes are ion beam assisted etching (IBAE), plasma-
enhanced chemical vapor deposition (PECVD), and plasma polymerization (PP). The
mentioned intermediate states may include adsorption, surface diffusion, activation,
chemical reaction, and desorption of different species.
5.1 ADSORPTION AND DESORPTION
Particles originating from the gas/plasma phase, which collide with a solid surface,
can undergo several interactions [1]. They can be simply reflected from the surface or
they can be adsorbed by forming a (weak) bond to the surface. After adsorption, the
particles may diffuse along the surface or into the solid (bulk diffusion). Furthermore,
they can react during their residence time at the surface with other adsorbed species or
with surface atoms of the solid in order to form product molecules. Finally, depending
on the binding energy of the particles, they may either desorb back into the gas phase
or stick by forming a growing film.
A particle originating from the plasma may lose sufficient energy upon impact at
the surface to be trapped in a binding potential in front of the surface: A new bond is
formed. Two classes of bonds can be distinguished: physisorption and chemisorption.
In the case of physisorption, bonds are mediated by electrostatic forces as dipole-
dipole interaction or dispersion forces, respectively. The released adsorption heat is
in the order of
E
ads
<
0.5 eV. Typical examples for physisorption are the adsorption of
noble gases on metals at very low temperatures by van der Waals interaction (example
Ar on Zr:
E
ads
=
25 kJ/mol) or the adsorption of nitrogen molecules on
most surfaces. The induced dipole-dipole interaction may be calculated, for exam-
ple, by the Lenard-Jones potential or by the image force for metallic (e.g., very
conductive) substrates.
In the case of chemisorption, bindings are caused by valence forces of the
exchanging electronic orbitals of adsorbed particles and substrate atoms. The
adsorbed particles form a stronger chemical bond to the surface (
E
ads
>
0.24eV
∼−
0.5 eV) as,
for example, CO on metals (example CO on Pd:
E
ads
=
147 kJ/mol) or
oxygen on most surfaces. The incoming molecules might break apart upon impact
and the dissociation products are chemisorbed separately. This process of dissocia-
tive chemisorption occurs, for example, during passivation of metals or silicon by
hydrogen molecules.
The potential curve for adsorption above a real substrate surface consists usually
of a physisorbed precursor and a chemisorbed state. A qualitative potential diagram
for the adsorption of a diatomic molecule
A
2
is shown in Figure 5.1.
One can distinguish two potential curves above the surface for the molecule
A
2
(a) and for the atom
A
(b), respectively. The difference in the potential energies for
1.43 eV to
−
