Chemistry Reference
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growth occurs apparently via insertion of SiH
3
radicals into strained Si-Si surface
bonds.
This example shows that although the macroscopic growth rate can eas-
ily be explained by the hot precursor model, the microscopic mechanisms can
be different and require the knowledge of the detailed surface structure and
morphology.
5.7 ION BOMBARDMENT IN PLASMA PROCESSING
The influence of ion bombardment on chemical reactions in a process plasma will
be considered here. Ion bombardment during thin film growth from low-temperature
plasmas plays a dominant role for many applications:
1. Physical sputtering by ion bombardment is used in deposition systems where
sputtering of solid targets in an argon plasma is used to produce volatile
growth precursors. These sputtered precursors can then produce thin films
on a substrate that is placed in front of the sputter target. A typical example
is the preparation of thin metallic films. In reactive magnetron sputtering
(RMS), a reactive gas is added to the argon discharge. As an example,
titanium-nitride films (TiN
x
) are prepared from a titanium sputter target by
adding N
2
to the argon discharge. For details see Refs. [37-39].
2. In most growth processes, sputtering of the growing film surface itself is
not desired, since it reduces the effective growth rate or may deteriorate the
quality of the material due to the formation of defects at the surface or in the
bulk. For example, the production of device quality a-Si:H films requires
the prevention of any bombardment with high energy particles, since the
ion-induced defects act otherwise as recombination centers for the electron
transport in the material. Consequently, the substrates for a-Si:H film growth
are placed on the grounded electrode of an RF discharge reactor [23].
3. For the preparation of amorphous hydrogenated carbon films, however, ion
bombardmentisessentialtoproducedensefilmswithsuperiormaterialqual-
ities. This is explained by the subplantation model [40,41]. Ions with kinetic
energies above
90 eV have a penetration range of a few angstrom, leading
to their subplantation beneath the first few monolayers. Due to the incorpora-
tion of carbon atoms at interstitial network sites, compressive stress evolves
that favors the formation of a dense sp
3
-hybridized network. By depositing
films from monoenergetic carbon ions, amorphous sp
3
-coordinated car-
bon films can be prepared with a density and hardness close to that of
a diamond.
4. Ion bombardment creates defects at the growing film surface due to sput-
tering or displacement of surface atoms. These open bonds can act as
adsorption sites for incoming radicals. This leads to ion-radical synergism,
since the effective sticking coefficient for radicals is enhanced by the ion
bombardment. This ion radical for the adsorption of CH
3
radicals on amor-
phous hydrogenated carbon film surfaces has been proposed in a number of
growth models for a-C:H film growth [42-44]. A nice confirmation of this
∼
