1–22
Motorola Sensor Device Data
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Figure 5. Experimental results for the etching of (100)
silicon with approximately 5x10–5 cm–3 boron
doping density in a commercially available detergent
as a function of temperature and detergent
concentration (which is proportional to pH).
Etch Rate
Prediction
from Model
<= 0.10
<= 0.40
<= 0.20
> 0.40
<= 0.30
10
T
0
60
70
80
90
100
110
120
20
30
40
50
Contour Plot of Detergent Concentraion and
Temperature vs Etch Rate ( m/hr)
ULTRA TIDE CONC (g/l)
Polymer Swelling or Dissolution
Swelling or dissolution affects those polymers typically
employed to package the micromachined structure and
depending on the nature of the media, may have a degrading
effect on device performance. These two related phenomena
are caused by solvent diffusing into the material and
occupying free volume within the polymer. The solubility
parameter gives a quantitative measure of the potential for
swelling [21]: i.e., it provides a quantitative measure of “l(fā)ike
dissolves like” (Fig. 6). Both the polymer and the solution
contribute to this failure mechanism, while the media
(specifically, the solubility parameter), the temperature, and
the pressure can be used as acceleration factors.
Figure 6. Typical values of solubility parameter
(
δ
[cal/cm3]1/2) for solvents and polymers.
ISOOCTANE
TOLUENE
ETHANOL
METHANOL
WATER
22.5
20
17.5
15
12.5
10
7.5
Solvents
POLYSULFONE
POLYURETHANE
PET
NYLON
POLY (ACRYLONITRILE)
Polymers
PTFE
PMMA
δ
= [cal/cm3]1/2
Figure 7 shows a photograph of a device after exposure to
a harsh fuel containing corrosive water solution. This
corrosion and evidence of swelling of the gel demonstrates the
vital importance the package has on the reliability of the
pressure sensor device. Also, it has been observed that
corrosion occurs more readily following swelling of a
polymeric encapsulant.
Figure 7. Photograph of a pressure sensor device
after extended exposure to harsh fuel containing
corrosive water, followed by exposure to a strong
acid. Evidence of the gel swelling during the test,
and corresponding shrinkage after removal from the
test media can be seen by the gel retracting away
from the sidewall of the package.
INITIAL EDGE
OF GEL
GEL EDGE
AFTER EXPOSURE
TO GASOLINE
WITH ETHANOL
F
Freescale Semiconductor, Inc.
n
.