Freescale Semiconductor, Inc.
Polymers
Solvents
PTFE
Contour Plot of Detergent Concentraion and
Temperature vs Etch Rate ( m/hr)
7.5
ISOOCTANE
TOLUENE
POLYSULFONE
POLYURETHANE
PET
10
PMMA
120
110
NYLON
12.5
15
ETHANOL
METHANOL
POLY (ACRYLONITRILE)
100
90
17.5
20
80
70
60
22.5
WATER
3 1/2
δ = [cal/cm ]
0
10
20
30
40
50
Figure 6. Typical values of solubility parameter
(δ [cal/cm ] ) for solvents and polymers.
ULTRA TIDE CONC (g/l)
3 1/2
Etch Rate
Prediction
from Model
<= 0.10
<= 0.40
<= 0.20
> 0.40
<= 0.30
Figure 7 shows a photograph of a device after exposure to
a harsh fuel containing corrosive water solution. This
corrosionandevidenceofswellingofthegeldemonstratesthe
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 5. Experimental results for the etching of (100)
–5
–3
silicon with approximately 5x10 cm boron
doping density in a commercially available detergent
as a function of temperature and detergent
concentration (which is proportional to pH).
Polymer Swelling or Dissolution
INITIAL EDGE
OF GEL
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 “like
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.
GEL EDGE
AFTER EXPOSURE
TO GASOLINE
WITH ETHANOL
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.
1–22
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Motorola Sensor Device Data
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