Note: Descriptions are shown in the official language in which they were submitted.
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Capacitive Ceramic Relative-Pressure Sensor
FIELD OF THE INVENTION
This invention relates to capacitive relative-pressure sen-
sors. With such sensors, pressures of media, such as li-
quids, gases, or vapors, can be measured, the measurement
being made in relation to the current atmospheric or am-
bient pressure, which thus serves as a reference pressure.
BACKGROUND OF THE INVENTION
U.S. Patent 5,079,953 discloses, as one of three variants,
a capacitive ceramic relative-pressure sensor comprising
- a diaphragm
-- having a surface on which a first electrode is
deposited, and
- a substrate
-- having a bore for guiding reference air from a first
surface to an opposite, second surface,
--- on which at least a second electrode is deposited,
- said substrate and said diaphragm being soldered or
35
brazed together around the periphery by means of a spacer
to form a chamber.
In such relative-pressure sensors, referred to herein as
"pressure sensors" for simplicity, the so-called reference
air from the atmospheric air, referred to herein as "ambi-
ent air", which, of course, is always more or less humid,
flows through the bore in the substrate into the chamber.
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By a suitable design of the housing for the pressure sen-
sor, if the ambient air is saturated with humidity, the
ambient air, before entering the chamber, can be caused to
flow through or over a point having the temperature of its
dew point, so that thereafter the temperature of the refe-
rence air will not pass below the dew pointy this is accom-
plished by forcing the ambient air to pass a point of the
housing whose temperature is equal to or less than that of
the chamber in the sensor, so that the dew point of the
ambient air is reached, i.e., condensation occurs, already
at that point, and only reference air not saturated or
oversaturated with humidity will reach the chamber. The
relative humidity of the reference air may therefore be
high, e.g., up to 95%.
Such a high relative humidity in the chamber of the pres-
sure sensor causes the following problems, which are asso-
ciated with the way in which capacitive ceramic pressure
sensors are commonly manufactured.
Blanks for the diaphragm and substrate are so-called green
compacts, which are preformed from a powdered ceramic
starting material and a binder and subsequently sin-tered.
The starting ceramic material may be alumina, for example.
The sintered alumina compacts of pressure sensors, i.e.,
the respective substrates and diaphragms of the sensors,
generally have a purity of 96 wt.%. For special applica-
tions, however, the purity may be up to 99.9 wt.%. The
sintered compacts have not only rough surfaces but also
microcracks extending from these surfaces into the sintered
compacts.
To produce a pressure sensor from a sintered diaphragm com-
pact and a sintered substrate compact, the substrate and
the diaphragm are soldered or brazed together along the
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periphery with the interposition of a spacer, so that the
aforementioned chamber is formed. The solder or active
brazing solder used represents the spacer. The solder is,
for example, a glass frit, and the active brazing solder
is, for example, an NiTiZr alloy in which the NiZr content
is approximately equal to the NiZr eutectic, see U.S. Pa-
tent 5,334,344. The soldering or brazing is also referred
to as "joining".
Prior to the joining of the diaphragm and the substrate,
electrodes are deposited on the surfaces that will face
each other in the chamber after the joining. These elec-
trodes are made of tantalum, for example, see U.S. Patent
5,050,034, and are commonly deposited by sputtering. If the
diaphragm and the substrate are joined using active brazing
solder, the electrode or electrodes on the substrate must
be located at a distance, i.e., be electrically isolated,
from the joining material, since in the finished pressure
sensor, the electrode of the diaphragm is electrically con-
nected to the joining material.
The electrode or electrodes of the substrate must therefore
be sputtered through a mask that is fixed directly on the
surface of the substrate and covers that portion of the
surface which has to be kept free of electrode material.
During the sputtering it is unavoidable, however, that due
to so-called undersputtering, minute islands of electrode
material, which are not in electrical contact with each
other, are formed on the surface to be kept free of elec-
trode material. Furthermore, as a result of the undersput-
tering, electrode material can penetrate into the above-
mentioned microcracks.
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Since water molecules of the reference air can both be ad-
sorbed by the rough surface and penetrate into the micro-
cracks, electrical connections are formed between the indi-
vidual islands of electrode material, so that the area of
the substrate electrode increases. This, however, results
in a zero offset of the pressure sensor. This effect in-
creases with increasing relative humidity of the reference
air.
The problems described so far have hitherto been controlled
relatively well by applying to the finished electrodes a
spin-on glass layer of silicon dioxide, which actually
serves to eliminate another problem, see U.S. Patent
5,400,489. In this way, however, only relative humidities
up to about 80% are controllable. Thus, at a relative humi-
dity of 80%, related to 400 °C, such pressure sensors have
a zero offset of up to 1%. For high-accuracy measurements,
this is not acceptable.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide rela
tive-pressure sensors that have virtually no zero offset at
relative humidities up to near the saturation limit.
To attain this object, the invention provides a capacitive
ceramic relative-pressure sensor comprising:
- a diaphragm
-- having a surface on which a first electrode is
deposited and
- a substrate
-- having a bore for guiding reference air from a first
surface to an opposite, second surface,
-- the first surface of the substrate being polished and
provided with at least a second electrode,
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- which substrate and which diaphragm are soldered or
brazed together along the periphery by means of a spacer
to form a chamber,
- which chamber is covered from inside with a thin layer of
5 hydrophobic material that is introduced through the bore
after the soldering or brazing.
In a preferred embodiment of the invention, the hydrophobic
material is a silicone oil, a paraffin oil, or a silicone
resin based on fluorinated siloxanes or on methyl polysilo-
xanes.
In another preferred embodiment of the invention, the sub-
strate and the diaphragm are made of alumina ceramic.
One advantage of the relative-pressure sensors according to
the invention is that they have a zero offset less than
0.2% even at a relative humidity of 95%, related to 400 °C.
The invention will become more apparent by reference to the
following description of an embodiment taken in conjunction
with the accompanying schematic drawings, which are not to
scale to permit a better representation of details, and in
which:
Fig. 1 is a top view of a pressure sensor;
Fig. 2 is a section taken along line A-B of the pressure
sensor of Fig. 1: and
Fig. 3 shows schematically a coating facility in which the
chambers of the pressure sensors are made hydro-
phobic.
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DETATLED DESCRTPTION OF EXEMPLARY E_MRnnrM~,r1'TS
Figs. 1 and 2 show a capacitive ceramic pressure sensor 10.
The ceramic material is, for example, the above-mentioned
alumina of 96 wt.% to 99.9 wt.% purity. The pressure sensor
has a diaphragm 11 in the form of a circular plate with
parallel surfaces. Furthermore, the pressure sensor 10 com-
prises a substrate 12 which has the same shape as, but is
thicker than, the diaphragm 11.
Diaphragm 11 and substrate 12 are brazed together along the
periphery with the interposition of a spacer 20 holding
them at a distance d from each other, as explained above.
This is done in a high vacuum at a temperature of about
900 °C. Because of the distance d, diaphragm 11 and sub-
strate 12 form a chamber 13. The diaphragm 11 is thin and
elastic, so that it can deflect and, thus, move back and
forth when pressure is applied to it.
Facing surfaces of diaphragm 11 and substrate 12 are pro-
vided with electrodes 14 and 15, respectively, which are
preferably made of tantalum and whose facing surfaces are
covered by protective tantalum dioxide layers 21 and 22,
respectively, as is described in the above-mentioned
U.S. Patent 5,050,034. Instead of a single electrode 15,
two or more electrodes may be provided on the substrate 12.
Prior to the deposition of the electrode 15, the surface of
the substrate 12, in addition to being ground or lapped as
usual, was polished in order to further reduce the rough-
ness still present after the grinding or lapping; this
polishing is a first step toward the attainment of the ob-
ject of the invention, preferably, the surface is polished
to a roughness less than 0.05 Vim. Through this polishing,
the above-mentioned disadvantageous adsorptive capacity of
the surface is greatly reduced.
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The electrode 14 covers the diaphragm 11 completely and
thus combines with the spacer 20 of active brazing solder
during the joining process. By contrast, the electrode 15
of the substrate 11 is deposited in such a way as not to be
electrically connected with the spacer 20. The electrodes
14, 15 are deposited by the above-mentioned sputtering: in
the case of the electrode 15 of the substrate 12, the sput-
tering is performed through the above-explained mask.
The substrate 12 is provided with a bore 23, which was
formed already in the green-compact phase, for example. The
bore extends through the electrode 15, since the chamber-
side opening is not closed by the sputtering the electrode
15. Thus, the chamber 13 is not closed but communicates
with the outside, which is a basic requirement of a rela-
tive-pressure sensor.
As the above-mentioned undersputtering cannot be avoided
even if the surface of the substrate 12 was polished, the
chamber 13 of each joined pressure sensor 10 is covered
from inside with a thin layer 24 of hydrophobic material,
which is introduced through the bore 23. For the sake of
clarity, the layer 24 is shown in Fig. 2 only at the sur-
face of the substrate 12 not covered by the electrode 15.
The layer 24 covers all inner surfaces of the chamber 13.
Through the hydrophobic layer 24, not only the islands of
electrode material resulting from the undersputtering as
explained above, but also the surface portions of the sub-
strate 12 between these islands, i.e., the portions with
the roughness left after the polishing and with the micro-
cracks, are passivated against any adsorption of water.
Thus, the above-mentioned increase in the area of the sub-
strate electrode is virtually impossible, as is shown by
the measured values given above in connection with the
advantages.
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Fig. 3 shows schematically a coating facility 9 in which
the chambers 13 of the pressure sensors 10 are made hydro-
phobic. Prior to this step, cleaning is not necessary,
since the surfaces of the pressure sensors are extremely
clean following the above-mentioned high-vacuum sintering
and joining process; this is an ideal precondition for good
adhesion of the hydrophobic layer 24.
The clean pressure sensors 10 are put into the coating
facility 9. The latter comprises a liquid container 91
which is open to the atmosphere and has a valve 92 at its
bottom side. The atmospheric pressure p can thus act on the
liquid 93 in the container 91.
The liquid 93 is highly wetting and contains an impregnant,
such as silicone oil, paraffin oil, or a fluorinated-silo-
xane-based or methyl-polysiloxane-based silicone resin,
highly diluted with a solvent having a high vapor pressure.
Preferably, the impregnant and the solvent are mixed in a
ratio between 1:20 and 1:100. A 1% solution of a methyl-
polysiloxane resin in hexane has proved particularly ef-
f ective .
Below the valve 92, the clean pressur sensors 10 are
contained in a pressure-sensor-receiving space 94' of a
tube 94 above a flow bottleneck 95 for the liquid 93. The
tube 94 ends in a collecting tank 96 for the liquid 93,
which is connected via a second valve 97 to a vacuum pump
(indicated by an arrow).
After the pressure sensors have been filled in, the valve
92 is closed, the valve 97 is opened, and the vacuum pump
is switched on, so that the air in the chambers 13 of the
pressure sensors 10 is removed. After this evacuation, the
valve 97 is closed and the valve 92 opened.
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As a result, under the action of the atmospheric pres-
sure p, the liquid 93, i.e., impregnant plus solvent, flows
into the pressure-sensor-receiving space 94' and penetrates
into the chambers 13 of the pressure sensors 10. After the
entire amount of impregnant plus solvent filled into the
liquid container 91 has run into the pressure-sensor-re-
ceiving space 94', the valve 97 is opened. The resulting
atmospheric pressure in the pressure-sensor-receiving space
94' presses the chambers 13 of the pressure sensors 10 full
of impregnant plus solvent.
The impregnated pressure sensors 10 are then removed and
left at room temperature, i.e., at about 20 °C, preferably
for about two hours, or placed in a second vacuum chamber,
preferably for about 5 min. After that, the solvent has
evaporated. The pressure sensors are then heated, prefer-
ably to about 200 °C, whereby the impregnant is burnt into
the surfaces bounding the chambers, so that these surfaces
become hydrophobic.
While the invention has been illustrated and described in
detail in the drawing and foregoing description, such il-
lustration and description is to be considered as exemplary
and not restrictive in character: it being understood that
only exemplary embodiments have been shown and described
and all changes and modifications that come within the
spirit of the invention are desired to be protected.
