The hot plate is heated
with two Kapton KH flexible heaters made by Omegalux.
One Kapton heater is 12'' x 10'' and the other is 10''
x 10''. The smaller heater was purchased only because
of the limited availability of the larger heater. The
combined heat output of the two heaters is 2200 Watts
(10 Watts/ in^2). The Kapton heaters are placed
between two 12'' x 12'' x 1/16'' copper plates.
Omegatherm 201 thermal paste was used to provide a
conductive heat transfer pathway between the heaters
and the conductive surface. The heaters have a high
heat output so a good heat transfer pathway is needed
to prevent the Kapton from melting and damaging the
heat source.
The copper plates containing the heating source
were placed on a 12'' x 12'' x 1/2'' aluminum base.
The aluminum base provides support for the heating
source and contains channels for a cooling fluid. A
circulating fluid (water) is used to cool the hot
plate after each post-application bake cycle. The
cooling channel has a spiral trajectory with a 1/4'' x
1/4'' cross sectional area. The cooling fluid from a
tap water facet enters the middle of the bottom
surface of the aluminum base through a 1/4 '' ID
flexible rubber tubing. The cooling water exits at the
end of spiral channel through the same size of tubing
and drains in a local sink. After the hot plate is
cooled with water, air is used to remove the water
from the channel inside of the aluminum channel. The
water needs to be removed from the chill plate before
the plate can be heated for the next run.
Temperature Process Control
The hot plate temperature control scheme is
designed to model the temperature trajectory of the
post-application bake temperature profile used by UL
Coat for photoresist baking. The control scheme uses a
technique called gain scheduling that follows a series
of multiple temperature set points on a trajectory.
The PID (proportional/ integral/ derivative)
parameters at each set point are adjusted to control
the heater to heat aggressively during high
temperature ramping periods and to heat slowly during
the plateauing temperature regions. This control
scheme should be capable of following Hoya's
temperature trajectory within 1 șC. However, gain
scheduling requires that the PID parameters be
readjusted for new trajectories or external
temperature changes.
