Chemistry Reference
In-Depth Information
ion-radical synergism has been found by Shiratani et al. [20]. They measured
the decay time of CH
3
radicals in the afterglow of a methane rf-discharge
using ionization threshold mass spectrometry. It was observed that the decay
of CH
3
radicals after switching off the methane discharge is characterized
by two time constants corresponding to the two surface loss probabilities
of 10
−
2
and 10
−
3
. This was explained by the fact that immediately after
stopping the discharge, the ion bombardment disappears on the time scale
of the confinement time for the ions in the discharge (
ms). The radi-
cals, however, have a much longer confinement time, because they are not
accelerated by electric fields. Immediately after the disappearance of the
ions, only radicals are therefore present in the afterglow. The ion bom-
bardment during plasma exposure creates dangling bonds at the surface.
At these dangling bonds, the radicals present in the afterglow can adsorb
more easily than at an undisturbed surface leading to an enhanced sur-
face reaction probability of 10
−
2
. Since no ion bombardment is present
in the afterglow, these chemisorptions sites are consumed by the adsorp-
tion of the radicals. After the complete saturation of all dangling bonds, a
surface loss probability of 10
−
3
is measured, corresponding to the reactiv-
ity of CH
3
with a hydrogen saturated surface. A similar example for the
occurrence of ion-neutral synergism exists for the etching of silicon in CF
4
discharges [13].
∼
The influence of ion bombardment on thin film growth is summarized as follows:
1. An impinging ion leads to defects at the growing film surface due to dis-
placement or sputtering of target atoms. This displacement depends on the
energy transfer in a collision and, thereby, on the masses of projectile and tar-
get atom. A large displacement yield is obtained if the mass of the projectile
matches the mass of the target atoms.
2. The threshold for physical sputtering depends on the surface-binding energy
of the solid. A part of the momentum of the incoming ion has to be reversed
by collisions to transfer kinetic energy to a surface atom directed away
from the surface. This surface atom might then be able to overcome the
surface-binding energy and be released from the solid.
3. With increasing kinetic energy of the projectile, impinging species penetrate
deeper into the solid and the nuclear stopping becomes only dominant at the
end of range. As a consequence, the maximum of the energy transfer and
therefore the maximum amount of displacements occur further inside the
solid.
4. The ion-induced formation of defects at the film surface can create adsorp-
tion sites for incoming radicals. This leads to ion-radical synergism during
film growth.
These are only qualitative arguments and many of these effects can be quantified by
computer codes like TRIM, SRIM, or TRYDIN [45]. However, for low ion energies
or complex molecular ions, molecular dynamic simulation of ion-solid interactions
