CHEMICAL VAPOR DEPOSITION OF THIN FILM MATERIALS FOR ...

J r J A -D n b-n 0
δ n
or for the fraction depletion of reactant at the surface,
f 0 n b-n 0
n b
J r
Dn b δ n
There are two limiting cases, diffusion control and reaction control, which can be derived from
Eq.(1.4). That is when f0 is very small and there is almost no difference of concentration of
species A from the bulk gas phase to the substrate surface, the thin film growth is in reaction
control mode and the thin film growth rate is dependent on the surface reaction rate only. The
other case is when f0 is approaching 1. In this case, the surface concentration of species A is very
low due to the much faster surface reaction rate than the diffusion rate and the thin film growth is
in diffusion control mode. The intermediate case is more difficult to analyze and control
practically, so it should be avoided in CVD process. Appropriate film growth mode can be
selected/controlled to achieve the best film uniformity in different CVD reactors. For example, in
the axisymmetric reactor, where the substrate temperature uniformity is difficult to achieve, the
thin film growth in diffusion control mode can help get uniform films.[12]
14
(1.3)
(1.4)
In the processes of both convection and diffusion, a unique phenomenon of CVD is
homogeneous reaction. Depending on the thermal stability/reactivity of the precursors and the
temperature profile in the reactor, gas phase homogeneous reaction can start from certain points.
Excessive gas phase reaction can generate particles on the grown thin film and affect its physical
properties. To minimize gas phase interaction, an effective way is to reduce the system pressure
and thus increase the molecular mean free path. Elimination of gas phase reaction can be
achieved by ALD thin film growth, which separates the injection of different reactants by