and instantly placed on a hot plate at a temperature of 400 C.
After one hour at this temperature, the device is again instantly
removed from the hot plate and cooled down to room temperature,
mounted on the PCB, and tested inside the environmental
chamber, and the entire process is repeated. Fig. 10 shows test
results for this sensor both before and after two exposures to the
high temperature environment. As evident, the response is quite
linear and consistent. The shift in capacitance (offset) is perhaps
due to the fact that the polyimide film has been further dried and
depleted of any residual moisture which may have been trapped
in it during processing. After the second exposure to 400 C,
we see very little change in the sensor sensitivity or offset. Out
of the devices that we tested from the 660 and 1200 Å batch,
all of them survived during the high temperature cycling step.
The yield from the 300-Å devices was about 70%. This loss is
most likely due to aluminum diffusing through the pinholes in
the polyimide at this high temperature. The failed 300-Å devices
as tested display very low resistance (few Ohms) indicative of
a short between the two electrodes. As suggested by the manufacturer
(Dupont) the polyimide film has pinholes and cannot
be reliably manufactured for thicknesses less than 300 Å.