Environmental Engineering Reference
In-Depth Information
where e ¼ C
p
, and K ¼
qx
2
3l
.
When we consider the evaporation of the solvent during the coating process, the
final film thickness can be expressed as follows:
h
f
¼ xh
w
and finally [
6
],
1
=
2
l
qx
h
f
/
x
ð
2
Þ
Therefore, the film thickness can be precisely controlled by the parameters in
Eq. (
2
). However, very small amounts of the solution remain on the substrate to
form the film during the spinning process. The rest of the solution is totally wasted.
As a result, material utilization in the spin-coating process is commonly too low,
sometimes less than 5 % of the solution volume. Therefore, spin coating is not
considered as a practical manufacturing process.
1.2.2 Blade (Knife-Edge) Coating
The blade-coating method is commonly used in various coating industries and
generally referred as knife-edge coating. The amount of solution (metered or not
metered) is dispensed ahead of the blade, and the blade moves with a given speed
as shown in Fig.
2
. Contrary to spin coating where the film thickness is inversely
proportion to the spin rate, the film thickness in the blade coating increases with
the blade speed (V
blade
). The film thickness in blade coating is also related to
viscosity of the solution l, surface tension of the solution, and curvature radius of
the downstream meniscus R [
7
].
2
=
3
lV
blade
r
h ¼ 1
:
34
R
ð
3
Þ
The downstream meniscus can be expressed with pressure difference between
P
1
and P
2
.
2
=
3
lV
blade
r
h ¼ 1
:
34
ð
P
2
P
1
Þ
ð
4
Þ
Thus, the film thickness in the blade coating increases with blade speed,
because as the blade moves faster, it has less chance to remove the solution on the
substrate with the other parameters fixed. The volume of the moving bead will be
smaller in the high-speed blade coating. Therefore, a large amount of solution will
remain on the substrate after the blade and result in a thicker film. In the same
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