Since the initial sealing of
Fig. 4. Measured capacitance versus relative humidity for a 300-Å device at
37 C.
Fig. 5. Measured capacitance versus relative humidity plot from a 660-Å
device at 37 C.
TABLE I
A COMPARISON OF DIFFERENT POLYIMIDE
DESIGNS
the package is performed in a low humidity environment, the
polyimide sensor is expected to perform satisfactorily. Furthermore,
as the device is to be used for long term low to moderate
humidity levels (indicative of the environment inside the anodically
sealed packages) we have decided to test the sensor at
37 C with 50% RH constant humidity for longer durations (10
days). Fig. 7 shows the behavior of the device from this test.
Even though the drift obtained after 10 days of testing is relatively
low ( 3%RH), the authors note that polyimides in general
are known to have long term stability problems and hence
additional longer term tests need to be conducted prior to the
actual field use of these sensors.
200