Capacitive pressure sensors or capacitive differential pressure sensors

In order to avoid short-circuits between the electrodes, the pressure sensor comprises: a substrate (11) made of ceramic material, on one surface of which an electrode (13) and a glass layer (14) formed from an original glass frit are disposed; a diaphragm (12) made of ceramic material, which is permanently joined and pressure-sealed to the substrate by means of the original glass frit acting as a spacer (17) holding the diaphragm and the substrate from each other to form a cavity (16), and which on the cavity side is provided with an electrode (15); and leads (18, 19) which extend inwards and/or through the diaphragm (12) to the electrodes (13, 15) in the region of the spacer (17). The differential pressure sensor is formed in a mirror-inverted arrangement in relation to the centre plane of the substrate, so that it carries two pressure sensors. These are pressure-linked by a hole (41) filled with oil and connecting the two cavities and leading through the substrate and outwards. In a further development, thin insulating layers (20; 40, 40') made of the material of the ceramic of the substrate or of a high-temperature glass can be fixed on the glass layer(s), respectively.

FIELD OF THE INVENTION
 The invention relates to capacitive pressure sensors and capacitive
 differential pressure sensors.
 BACKGROUND OF THE INVENTION
 U.S. Pat. No. 4,388,668 describes a capacitive pressure sensor comprising:
 a substrate made of ceramic material,
 on the one surface of which
 a first metallic electrode is disposed in direct contact with the
 substrate;
 a diaphragm made of ceramic material,
 which is permanently joined and pressure-sealed to the substrate by means
 of an original glass frit acting as a spacer holding the diaphragm and the
 substrate at a defined distance from each other to form a cavity, and
 which on the surface facing the substrate is provided with a second
 metallic electrode; and
 leads
 which extend inwards to the electrodes in the region of the spacer.
 Further, U.S. Pat. No. 4,531,415 describes a capacitive differential
 pressure sensor, comprising:
 a substrate made of ceramic material,
 whose surfaces facing away each other--the first and second
 surfaces--support
 a first metallic electrode and a third metallic electrode, respectively,
 each being in direct contact with the substrate;
 a first and second diaphragm made of ceramic material,
 which are permanently joined and pressure-sealed to the first and second
 surfaces of the substrate, respectively, by means of an original glass
 frit acting as a spacer holding the diaphragms and the substrate at
 defined distances from each other to form a first and second cavity,
 respectively, and
 which on each cavity side are provided with a second and a fourth metallic
 electrode, respectively; and
 a hole connecting the two cavities and leading through the substrate and
 outwards,
 the two cavities and the hole being filled with oil.
 With the two types of pressure sensors mentioned, the prevention of
 short-circuits between the electrodes facing each other is important. Such
 short-circuits may be caused by material particles coming between the
 electrodes during manufacture and not being discovered there, or during
 operation by pressures which are higher than the pressure admissible for a
 concrete type of pressure sensor and which thus lead to contact between
 the electrodes, or through parts of the electrodes detaching themselves
 from the diaphragm and/or substrate and contacting each other again.
 According to U.S. Pat. No. 4,531,415 already mentioned, an attempt was made
 to solve the short-circuit problem by fixing to the substrate some parts
 of insulating material protruding from the electrodes as projections, so
 that in case of an inadmissible high pressure the electrodes of the
 diaphragm can be intercepted by the projections and thus cannot touch the
 electrodes of the substrate.
 SUMMARY OF THE INVENTION
 This design, however, only eliminates the second cause of a short-circuit
 described above, while the two other causes are not precluded. The
 invention therefore serves for a general solution to the short-circuit
 problem.
 To this end, the invention consists on the one hand of a capacitive
 pressure sensor, comprising:
 a substrate made of ceramic material,
 on one surface of which
 a first metallic electrode is disposed and
 a glass layer formed from an original glass frit is disposed at least on
 the first metallic electrode;
 a diaphragm made of ceramic material,
 which is permanently joined and pressure-sealed to the substrate by means
 of the original glass frit acting as a spacer holding the diaphragm and
 the substrate at a defined distance from each other to form a cavity, and
 which on the cavity side is provided with a second metallic electrode; and
 leads
 which extend inwards and/or through the diaphragm to the electrodes in the
 region of the spacer.
 The invention consists on the other hand of a capacitive differential
 pressure sensor, comprising:
 a substrate made of ceramic material,
 whose surfaces facing away each other--the first and second
 surfaces--support
 a first metallic electrode and a third metallic electrode, respectively,
 a first glass layer formed from an original glass frit at least on the
 first metallic electrode,
 a second glass layer formed from the original glass frit at least on the
 third metallic electrode;
 a first and a second diaphragm made of ceramic material,
 which are permanently joined and pressure-sealed to the first and second
 surfaces of the substrate, respectively, by means of the original glass
 frit acting as a spacer holding the diaphragms and the substrate at
 defined distances from each other to form a first and a second cavity,
 respectively, and
 which on each cavity side are provided with a second and a fourth metallic
 electrode, respectively;
 leads,
 which extend inwards and/or through the first and the second diaphragm,
 respectively, or through both diaphragms to the respective electrodes; and
 a hole connecting the two cavities, leading at least through the substrate
 and outwards,
 the two cavities and the hole being filled with oil.
 In accordance with a development of the pressure sensor of the invention, a
 thin insulating layer made of the ceramic material of the substrate or of
 a high-temperature glass is disposed on the glass layer covering the first
 electrode, the diaphragm being permanently joined to the insulating layer
 via the spacer.
 In accordance with a development of the differential pressure sensor of the
 invention, thin insulating layers made of the ceramic material of the
 substrate or of a high-temperature glass are disposed on the glass layers
 covering the first and the third electrodes, respectively, the respective
 diaphragms being permanently joined to the respective insulating layers by
 means of the respective spacers.
 The glass layer disposed on the electrode of the substrate ensures that no
 short-circuits whatsoever can occur between the electrodes of the
 substrate and of the diaphragm, since one of the electrodes is completely
 covered with a coating of insulating material.
 Furthermore, the electrodes of the substrate are protected substantially
 completely against any moisture that can enter the cavity of the pressure
 sensor first referred to above when said sensor is not used as an absolute
 pressure sensor with evacuated cavity but as a reference pressure sensor
 with a cavity which is connected to the environment by means of an
 aperture, cf. U.S. Pat. No. 5,079,953, or which may be contained as a
 water content in the oil filling in the differential pressure sensor
 referred to above in second position. This moisture protection increases
 the long-term stability of the pressure sensor and/or the differential
 pressure sensor.

DETAILED DESCRIPTION OF THE DRAWINGS
 The capacitive pressure sensor of the embodiment shown in cross-section in
 FIG. 1 comprises a substrate 11 and a diaphragm 12, each of which consists
 of ceramic material, preferably the same ceramic material. Highly pure
 alumina is particularly suitable for this purpose. In the top view not
 shown the substrate and the diaphragm are preferably circular or
 rectangular. A first metallic electrode 13 and a glass layer 14 formed
 from an original glass frit are disposed on the one surface of the
 substrate 11.
 The substrate 11 and the diaphragm 12 form a cavity 16 since they are
 permanently joined and pressure-sealed to each other along the respective
 edges 21 at a defined distance of e.g. 20 .mu.m by means of the material
 of the original glass frit as a spacer in a manner equivalent to spacer 17
 in FIG. 2.
 The diaphragm 12 is provided with a second metallic electrode 15 on the
 cavity side. Therefore, the two electrodes 13, 15 form a capacitor whose
 dielectric consists partly of the glass layer 14 and partly of air or
 vacuum.
 Leads 18 and 19 extend to the electrodes in the direction of the cavity in
 the region of the glass layer 14. It is also possible that the electrode
 15 of the diaphragm 12 extends through it to the outside.
 During the manufacture of the capacitive pressure sensor the glass frit is
 disposed as a layer to the surface of the substrate 11 containing the
 electrode 13, preferably over the whole surface. At the edges 21 the
 thickness of the layer is increased in such a way that the spacer is
 formed after a heating step which serves to melt the glass frit.
 Therefore, at the edges 21 the spacer forms one piece with the glass layer
 14. The increase mentioned above can, for example, be achieved by
 attaching a glass frit ring to the glass frit layer and the inner side of
 the diaphragm, respectively.
 In the development of the capacitive pressure sensor according to FIG. 1
 shown in cross-section in FIG. 2, a thin insulating layer 20, consisting
 of the ceramic material of the substrate 11 or of such a high-temperature
 glass that an adequate bond with the spacer 17 which is subject to shear
 stress in case of overloading, is guaranteed, is fixed to the uniformly
 thin glass layer 14 which in turn was formed from a glass frit after said
 frit had been heated.
 In this case the glass layer 14 is not reinforced at the edge as in FIG. 1,
 but the spacer 17 is formed by a glass frit ring fixed to the edge of the
 insulating layer 20.
 The differential pressure sensor shown in FIG. 3, which shows it in
 cross-section, comprises a substrate 31 and a first diaphragm 32 as well
 as a second diaphragm 32', each of which consists of ceramic material,
 preferably the same material. Highly pure alumina is particularly suitable
 again for this purpose. Here too, the substrate and the diaphragms in the
 top view, which is not shown, are preferably circular or rectangular.
 On the surfaces of the substrate 31 facing away each other, structures are
 present in a substantially mirror-inverted arrangement with respect to the
 centre plane of the substrate. Thus a first metallic electrode 33 is
 located on a first surface and a third metallic electrode 33' on a second
 surface. A first and a second glass layer 34, 34', each of which was
 formed from an original glass frit, are disposed on the these electrodes,
 respectively.
 The substrate 31 with the diaphragm 32 forms a first cavity 36 and with the
 diaphragm 32' a second cavity 36' since the diaphragms 32, 32' are
 permanently joined and pressure-sealed to the first and second surfaces of
 the substrate, respectively, along the respective edges 45, 45' at a
 defined distance of e.g. 20 .mu.m by means of the material of the original
 glass frit as respective spacers in a manner equivalent to spacers 37, 37'
 in FIG. 4.
 The diaphragms 32, 32' are each provided with a second metallic and a
 fourth metallic electrode 35, 35' on the cavity side. Thus the pairs of
 electrodes 33, 35 and 33', 35' each form a first and a second capacitor,
 whose dielectric consists partly of the respective glass layers 34, 34'
 and partly of oil (see below) or air or vacuum.
 Leads 38, 38', 39, 39' extend to the electrodes in the direction of the
 respective cavity in the region of the glass layers 34, 34'. It is also
 possible that the electrodes 15, 15' of the diaphragms 32, 32' extend
 through them to the outside, respectively.
 What was said in explaining FIG. 1 concerning the disposal and heating of
 the glass frit applies by analogy to FIG. 3.
 In the development of the capacitive differential pressure sensor according
 to FIG. 3 shown in cross-section in FIG. 4, a first and a second thin
 insulating layer 40, 40' consisting of the ceramic material of the
 substrate 31 or of a high-temperature glass are fixed to the uniformly
 thin glass layers 34, 34' which in turn were formed from a glass frit
 after said frit had been heated.
 In this case too, the glass layers 34, 34' are not reinforced at the edges
 as in FIG. 3, but the spacers 37, 37' are formed by a glass frit ring
 fixed to each edge of the insulating layers 40, 40'.
 The two versions of the differential pressure sensor as shown in FIGS. 3
 and 4 also have a hole 41 connecting the two cavities 36, 36' and leading
 through the substrate 31 as well as through the layers arranged between it
 and the respective cavities as well as outwards. This hole and the
 cavities are filled with oil which has been poured in through a connection
 piece 42.
 The ceramic or high-temperature glass insulating layers have, for example,
 a thickness of approx. 150 .mu.m if they are attached as prefabricated
 parts. If required, even thinner insulating layers (down to 40 .mu.m) can
 be achieved, when the insulating layer is applied more thickly and then
 reduced by grinding.