Abstract:
These capacitive pressure sensor cells have joints between substrates and diaphragms being both pressure and/or tension-proof and high-vacuum-tight and long-term-stable. The sensor cell comprises a ceramic substrate ( 1 ) having a cylindrical surface ( 11 ), a major surfaces ( 12, 13 ). The major surface ( 12 ) includes a concave central area ( 121 ) merging, in the direction of and up to said cylindrical surface ( 11 ), into a convex surface ( 124 ) having a vertex line ( 125 ) and forming a planar ring surface ( 126 ) in its area. An electrode ( 122 ) is located in the concave area ( 121 ). An electrical connection ( 123 ) extends from electrode ( 122 ) through the substrate ( 1 ) to surface ( 13 ). A ceramic diaphragm ( 5 ) has a planar inner surface ( 51 ) on which an electrode ( 52 ) is located and which rests on the ring surface ( 126 ) of the substrate ( 1 ). The diaphragm ( 5 ) is joined to the substrate by an active brazing solder forming a circumferential wedge zone ( 91 ) in the area of the substrate between the ring surface ( 126 ) and the cylindrical surface ( 11 ). An electrical connection to the electrode ( 52 ) is made through the wedge zone ( 91 ). Respective differential pressure sensors can comprise a central substrate ( 2 ) and two outer diaphragms ( 61, 71 ) or a central diaphragm ( 8 ) and two outer substrates ( 3, 4 ).

Description:
This application claims the benefit of Provisional Application 60/129,086 filed Apr. 13, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to capacitive ceramic pressure sensor cells or differential pressure sensor cells and to methods for manufacturing the same. 
     BACKGROUND OF THE INVENTION 
     A capacitive ceramic pressure sensor cell commonly comprises a ceramic substrate and a ceramic diaphragm which covers the substrate and is spaced from the latter to form a sensing chamber between the diaphragm and a surface of the substrate facing the diaphragm. The facing surfaces of the substrate and the diaphragm are provided with electrodes which together form a capacitor that provides an electric signal which corresponds to a pressure of a process medium acting on and deforming the diaphragm. Under overload conditions, the substrate serves as a limiter for the movement of the diaphragm. 
     To measure a difference between two pressures (differential pressure), two sensing chambers are commonly used, one for each pressure, the sensing chambers being spatially and mechanically connected with one another and being provided with one sensing capacitor each. In this manner it is possible to produce an electric signal which corresponds to the difference between a pressure acting on one of the sensing chambers and a pressure acting on the other sensing chamber. 
     A particular problem encountered with ceramic pressure sensor cells is to fasten and join the diaphragm in its edge area to the substrate in such a manner that the joint is gasu and liquid-tight and can withstand high tensile and compressive loads. In addition, the joint is to be long-term-stable and free of relaxation effects. 
     Glass-frit joints used in conventional ceramic pressure sensor cells do not fully meet the above requirements. Therefore, a joint produced by means of an active brazing solder has been used. 
     U.S. Pat. No. 5,050,034, for example, discloses a capacitive pressure sensor cell comprising 
     a ceramic substrate having 
     a cylindrical surface and, 
     at a first major surface, a central area which 
     is provided with a first electrode and 
     has an electrical connection from the first electrode through the substrate to a second major surface, and 
     a ceramic diaphragm 
     which is joined to the substrate using a plane-parallel ring of active brazing solder to form a high-vacuum-tight sensing chamber, 
     with a second electrode being provided on a planar inner surface of the diaphragm facing the substrate. 
     The joint produced by means of active brazing solder meets the above-mentioned requirements for high stability, but in certain cases where the diaphragm is subjected to overpressure, it has turned out that the diaphragm cannot be supported on the substrate in a satisfactory manner. Because of the “angular” shape of the ring of active brazing solder, which serves as a spacer between the substrate and the diaphragm, tensile stresses may occur, particularly in the edge region of the diaphragm, which result in a failure of the diaphragm. 
     U.S. Pat. No. 4,329,826 discloses a capacitive differential pressure sensor cell comprising: 
     a substrate having 
     an edge area and, 
     at a first major surface, a concave first central area which 
     is provided with a first electrode, 
     has a first electrical connection to the first electrode, and, 
     in the direction of the edge area, merges into a convex first surface 
     which has a first vertex line intersecting the edge area and 
     forms a first planar ring surface in the area of the first vertex line, 
     said substrate further having, at a second major surface opposite the first major surface, a concave second central area 
     which is provided with a second electrode, 
     has a second electrical connection to the second electrode, and, 
     in the direction of the edge area, merges into a convex second surface which 
     has a second vertex line intersecting the edge area and 
     forms a second planar ring surface in the area of the second vertex line, 
     the substrate being provided with a connecting channel between the first central area and the second central area; 
     a first ceramic diaphragm 
     which rests on and is fixed to the first ring surface of the substrate, 
     with a third electrode being provided on a planar inner surface of the first diaphragm facing the substrate; and 
     a second ceramic diaphragm 
     which rests on and is fixed to the second ring surface of the substrate, 
     with a fourth electrode being provided on a planar inner surface of the second diaphragm facing the substrate. 
     In the case of this prior-art differential pressure sensor cell, the ring surfaces, which serve exclusively to join the respective diaphragms to the substrate, extend up to the cylindrical surface of the substrate. The way the joint is produced is not explained. 
     It has turned out that the joint produced solely by means of the ring surfaces is insufficient, particularly if great axially parallel forces act on these surfaces. In addition, such a joint is not high-vacuum-tight and not long-term-stable. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide capacitive pressure sensor cells or capacitive differential pressure sensor cells in which the joint between the substrate and the diaphragms is both pressure- and/or tension-proof and high-vacuum-tight and long-term-stable. 
     To attain this object, a first variant of the invention provides a capacitive pressure sensor cell comprising: 
     a ceramic substrate having 
     a cylindrical surface, 
     a first major surface and 
     a second major surface, 
     said second major surface being opposite said first major surface, 
     said first major surface including a concave central area which, in the direction of and up to said cylindrical surface, merges into a convex surface having a vertex line, 
     said convex surface forming a planar ring surface in the area of said vertex line, 
     a first electrode located in said concave central area of said first major surface, and 
     an electrical connection extending from said first electrode through said substrate to said second major surface; and 
     a ceramic diaphragm having a planar inner surface, 
     a second electrode located on said planar inner surface of said diaphragm, 
     said planar inner surface of said diaphragm resting on said planar ring surface of said first major surface of said substrate, 
     said diaphragm being joined to said substrate by an active brazing solder 
     which forms a circumferential wedge zone between said diaphragm and said substrate in the area of said substrate between said planar ring surface and said cylindrical surface, 
     a high-vacuum-tight sensing chamber being formed between said planar inner surface of said diaphragm and said first major surface of said substrate, and 
     electrical connection to said second electrode being made through said circumferential wedge zone. 
     To attain the above object, a second variant of the invention provides a capacitive differential pressure sensor cell comprising: 
     a ceramic substrate having 
     a cylindrical surface and, 
     at a first major surface, a concave first central area which 
     is provided with a first electrode, 
     has a first electrical connection from the first electrode through the substrate to a second major surface, and, 
     in the direction of and up to the cylindrical surface, merges into a convex first surface having a first vertex line, 
     said convex first surface forming a first planar ring surface in the area of the first vertex line, 
     which substrate further has, at a second major surface opposite the first major surface, a concave second central area which 
     is provided with a second electrode, 
     has a second electrical connection from the second electrode through the substrate to the cylindrical surface, and, 
     in the direction of and up to the cylindrical surface, merges into a convex second surface having a second vertex line, 
     said convex second surface forming a second planar ring surface in the area of the second vertex line, 
     said substrate further having a connecting channel between the first central area and the second central area; 
     a first ceramic diaphragm 
     which rests on the first ring surface of the substrate, and 
     which is joined to the substrate on the first ring surface and between the cylindrical surface and the first ring surface by means of active brazing solder forming a first circumferential wedge zone, to form a first high-vacuum-tight sensing chamber, 
     with a third electrode being provided on a planar inner surface of the first diaphragm facing the substrate, 
     to which third electrode contact is made through the first wedge zone; and 
     a second ceramic diaphragm 
     which rests on the second ring surface of the substrate, and 
     which is joined to the substrate on the second ring surface and between the cylindrical surface and the second ring surface by means of active brazing solder forming a second circumferential wedge zone, to form a second high-vacuum-tight sensing chamber, 
     with a fourth electrode being provided on a planar inner surface of the second diaphragm facing the substrate, 
     to which fourth electrode contact is made through the second wedge zone. 
     To attain the above object, a third variant of the invention provides a capacitive differential pressure sensor cell comprising: 
     a first ceramic substrate having 
     a first cylindrical surface and, 
     at a first major surface, a concave first central area which 
     is provided with a first electrode, 
     has an electrical connection from the first electrode through the first ceramic substrate to a second major surface opposite the first major surface, and, 
     in the direction of and up to the cylindrical surface, merges into a convex first surface having a first vertex line, 
     said convex first surface forming a first planar ring surface in the area of the first vertex line; 
     a second ceramic substrate having 
     second cylindrical surface and, 
     at a first major surface, a concave second central area which 
     is provided with a second electrode, 
     has a second electrical connection from the second electrode through the second ceramic substrate to a second major surface opposite the first major surface, and, 
     in the direction of and up to the second cylindrical surface, merges into a convex second surface having a second vertex line, 
     said convex second surface forming a second planar ring surface in the area of the second vertex line; and 
     a ceramic substrate which 
     rests with a first surface on the first ring surface of the first substrate, 
     is joined to the first substrate on the first ring surface and between the first cylindrical surface and the first ring surface of the first substrate by means of active brazing solder forming a first circumferential wedge zone, to form a first high-vacuum-tight sensing chamber, 
     rests with a second surface on the second ring surface of the second substrate, 
     is joined to the second substrate on the second ring surface and between the second cylindrical surface and the second ring surface of the second substrate by means of active brazing solder forming a second circumferential wedge zone, to form a second high-vacuum-tight sensing chamber, 
     the first surface of the diaphragm being provided with a third electrode 
     to which contact is made through the first wedge zone, and 
     the second surface being provided with a fourth electrode 
     to which contact is made through the second wedge zone. 
     To attain the above object, a fourth variant of the invention provides a method for manufacturing a capacitive pressure sensor cell comprising the steps of: 
     providing a ceramic substrate having 
     a cylindrical surface, 
     a first major surface and 
     a second major surface, 
     said second major surface being opposite said first major surface, 
     said first major surface including a concave central area which, in the direction of and up to said cylindrical surface, merges into a convex surface having a vertex line, 
     said convex surface being formed into a planar ring surface in the area of said vertex line; 
     depositing a first electrode on said concave central area; 
     forming an electrical connection from said first electrode through said substrate to said second major surface; 
     providing a ceramic diaphragm having a planar inner surface; 
     depositing a second electrode on the central portion of said planar inner surface of said diaphragm such that, when said diaphragm is placed on said substrate, said second electrode extends up to said planar ring surface of said substrate; 
     applying an active brazing solder to said convex portion of said substrate between said cylindrical surface and said planar ring surface; 
     placing said diaphragm on said planar ring surface of said substrate such that said second electrode of said diaphragm extends up to said planar ring surface, and said second electrode faces said first electrode; 
     heating said substraic and said diaphragm in a vacuum or inert-gas atmosphere until the active brazing solder has melted; and 
     allowing said substrate and said diaphragm to cool down. 
     To attain the above object, a fifth variant of the invention provides a method for manufacturing a capacitive differential pressure sensor cell comprising the steps of: 
     providing a ceramic substrate, at the first major surface thereof, with a concave first central area 
     which, in the direction of and up to a cylindrical surface, merges into a convex first surface having a first vertex line, 
     said convex first surface being formed as a first planar ring surface in the area of the first vertex line; 
     depositing a first electrode on the first central area and providing a electrical connection from the first electrode through the substrate to the cylindrical surface of the substrate; 
     providing the substrate, at a second major surface opposite the first major surface, with a concave second central area 
     which, in the direction of and up to a cylindrical surface of the substrate, merges into a convex second surface having a second vertex line, 
     said convex second surface being formed as a second planar ring surface in the area of the second vertex line; 
     depositing a second electrode on the second central area and providing an electrical connection from the second electrode through the substrate to the cylindrical surface of the substrate; 
     providing a first ceramic diaphragm congruent with the first major surface of the substrate, on a planar inner surface thereof, with a third electrode dimensioned so 
     that, after the first diaphragm has been placed on the first ring surface of the substrate, the third electrode extends up to said first ring surface; 
     providing a second ceramic diaphragm congruent with the second major surface of the substrate, on a planar inner surface thereof, with a fourth electrode dimensioned so 
     that, after the second diaphragm has been placed on the second ring surface of the substrate, said fourth electrode extends up to said second ring surface; 
     applying respective quantities of active brazing solder sufficient to braze the first and second diaphragms to the substrate to portions of the convex first surface of the substrate located between the first ring surface and the cylindrical surface and to portions of the convex second surface of the substrate located between the second ring surface and the cylindrical surface; 
     placing the surface of the first diaphragm provided with the third electrode on the first ring surface of the substrate; 
     placing the surface of the second diaphragm provided with the fourth electrode on the second ring surface of the substrate; and 
     heating the substrate and the diaphragm in a vacuum or inert-gas atmosphere until the active brazing solder has melted, and then allowing them to cool down. 
     To attain the above object, a sixth variant of the invention provides a method for manufacturing a capacitive differential pressure sensor cell comprising the steps of: 
     providing a first ceramic substrate, at a first major surface thereof, with a concave first central area 
     which, in the direction of and up to a first cylindrical surface, merges into a convex first surface having a first vertex line, said convex first surface being formed as a first planar ring surface in the area of the first vertex line; 
     depositing a first electrode on the first central area and providing an electrical connection from the first electrode through the first substrate to a second major surface of the substrate opposite the first major surface; 
     providing a second ceramic substrate, at a first major surface thereof, with a concave second central area 
     which, in the direction of and up to the second cylindrical surface of the second substrate, merges into a convex second surface having a second vertex line, 
     said convex second surface being formed as a second planar ring surface in the area of the second vertex line; 
     depositing a second electrode on the second central area and providing an electrical connection from the second electrode through the second substrate to a second major surface of the second substrate opposite the first major surface; 
     providing a ceramic diaphragm congruent with the first major surface of the first substrate, on a planar first surface thereof, with a third electrode dimensioned so 
     that, after the diaphragm has been placed on the first ring surface of the first substrate, said third electrode extends up to said first ring surface; 
     providing a planar second surface of the diaphragm opposite the first surface with a fourth electrode dimensioned so 
     that, after the diaphragm has been placed on the second ring surface of the second substrate, said fourth electrode extends up to said second ring surface; 
     applying respective quantities of active brazing solder sufficient to braze the first and second diaphragms to the substrate to portions of the convex first surface of the substrate located between the first ring surface and the cylindrical surface and to portions of the convex second surface of the substrate located between the second ring surface and the cylindrical surface; 
     placing the first surface of the diaphragm, provided with the third electrode, on the first ring surface of the substrate; 
     placing the second surface of the diaphragm, provided with the fourth electrode, on the second ring surface of the substrate; and 
     heating the substrate and diaphragm in a vacuum or inert-gas atmosphere until the active brazing solder has melted, and then allowing them to cool down. 
     In respective preferred embodiments of the first to third variants of the invention, the substrate or substrates and the diaphragm or diaphragms are made of alumina ceramic, and the active brazing solder is a Zr—Fe—Ti—Be alloy or a Zr—Ni—Ti alloy. 
     In respective further preferred embodiments of the first to third variants of the invention, at least the electrode of the diaphragm or the electrodes of the diaphragms are covered, at least in a respective edge region, with a solder resist layer. 
     In preferred embodiments of the fourth and fifth variants of the invention, at least the electrode of the diaphragm or the electrodes of the diaphragms are covered, at least in a respective edge region, with a solder resist layer. 
     The basic idea of the invention is, instead of holding the diaphragm and the substrate at a distance from each other at a joint by a quantity of active brazing solder as has been done so far, to braze the diaphragm and the substrate outside the area where they rest on each other by means of such a quantity of active brazing solder that long-term stability and high-vacuum tightness are ensured. 
     One advantage of the invention is that in contrast to the prior art, the volume of the sensing chamber does not depend on the thickness of the ring of active brazing solder and on manufacturing tolerances resulting after the brazing. 
     Another advantage of the invention is that an expensive formed part of active brazing solder is no longer necessary. Such formed parts have to be produced by the complex and costly melt-spinning process. 
     The invention is particularly suited for automated application of a active brazing solder paste by means of a dispenser. The special shape and arrangement of the joint between diaphragm and substrate ensures that solvents commonly contained in an active brazing solder paste will escape from the region of the joint residue-free. 
     A further advantage of the invention is that the special shape of the substrate or substrates in the region of the joint or joints ensures that no active brazing solder will penetrate into the sensing chamber or sensing chambers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more apparent from the following description of embodiments when taken in conjunction with the accompanying drawings. Like parts are designated by like reference characters throughout the FIGS.; to simplify the illustration, reference characters that have already been used are not repeated in subsequent FIGS. In the drawings: 
     FIG. 1 a  is a schematic perspective view of a substrate of a pressure sensor cell of a first variant of the invention; 
     FIG. 1 b  is a schematic perspective view of a diaphragm of the pressure sensor cell of FIG. 1 a;    
     FIG. 2 is a schematic vertical section of the first variant of the invention; 
     FIG. 3 a  shows an enlarged view of a section III of the pressure sensor cell of FIG. 2 before the diaphragm is joined to the substrate; 
     FIG. 3 b  shows the section III of the pressure sensor cell of FIG. 3 a  after the diaphragm has been joined to the substrate; 
     FIG. 4 is a schematic vertical section of a second variant of the invention; and 
     FIG. 5 is a schematic vertical section of a third variant of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 a,  there is shown a perspective view of a ceramic substrate  1  of a capacitive pressure sensor cell which has a cylindrical surface  11  and, at a first major surface  12 , a concave central area  121  provided with a first electrode  122 . In the direction of and up to cylindrical surface  11 , central area  121  merges into a convex area  124 , which has a vertex line  125  and forms a planar ring surface  126  in the area of vertex line  125  (see FIG.  2 ). 
     FIG. 1 b  is a perspective view of a ceramic diaphragm  5  whose planar inner surface  51 , which will face the substrate  1  of FIG. 1 a  after it has been fixed to the latter, is provided with an electrode  52 , which is a second electrode  52  the pressure sensor cell. 
     FIG. 2 shows a vertical section through a pressure sensor cell according to the first variant of the invention. The ceramic diaphragm  5  depicted in FIG. 1 b  rests on ring surface  126  of substrate  1 . Diaphragm  5  and the portion of convex surface  124  extending between ring surface  126  and cylindrical surface  11  form a circumferential wedge zone  91 . By means of active brazing solder  10  in wedge zone  91 , diaphragm  5  is joined to substrate  1  to form a high-vacuum-tight sensing chamber  9 . 
     Contact is made to electrode  52  on diaphragm  5  through wedge zone  91 . An electrical connection  123  is provided from electrode  122  through substrate  1  to a second major surface  13  of substrate  1 . Electrode  122  and electrode  52  together form a capacitor which provides a signal corresponding to the pressure acting on diaphragm  5 . 
     FIG. 2 clearly shows how central area  121  merges into convex surface  124  in the direction of and up to cylindrical surface  11 . In the area of vertex line  125  bounded in FIG. 2 by dashed lines  1261  and  1262 , convex surface  124  forms a planar ring surface  126 . A central line CL illustrates that the pressure sensor cell is preferably rotationally symmetrical. 
     FIG. 3 a  shows an enlarged view of section III of the pressure sensor cell of FIG. 2 prior to the joining of diaphragm  5  to substrate  1 . It shows clearly how central area  121  merges into convex surface  124  in the direction of and up to cylindrical surface  11 . In the area of vertex line  125  bounded by dashed lines  1261  and  1262  (see FIG. 2) convex surface  124  forms a planar ring surface  126 . Electrode  52  of diaphragm  5  extends up to the portion of inner surface  51  of diaphragm  5  which will subsequently rest on substrate  1 . By contrast, electrode  122  is confined to concave central area  121  of substrate  1  and does not extend up to convex surface  124 . 
     Before diaphragm  5  is placed on substrate  1 , active brazing solder  10  is applied to the portion of convex surface  12  (see FIG. 1) of substrate  1  which extends between cylindrical surface  11  and ring surface  126 . Preferably, use is made of an active brazing solder paste which is applied by means of a suitable dispenser and in a quantity sufficient to join diaphragm  5  and substrate  1 . However, other processes by which active brazing solder  10  can be applied to substrate  1  are also conceivable. 
     After diaphragm  5  has been joined to substrate  1 , the solidified active brazing solder  10  fills the wedge zone  91  formed between diaphragm  5  and substrate  1 , as shown in FIG. 3 b.  Although diaphragm  5  rests firmly on planar ring surface  126 , because of the ceramic material used for the substrate and diaphragm microscopically small pores with diameters of the order of their grain sizes are formed between the substrate and diaphragm. 
     Because of its wetting properties, the active brazing solder  10  migrates in the pores between diaphragm  5  and ring surface  126  of substrate  1  from wedge zone  91  to an inner edge of ring surface  126 , and thus to electrode  52 . The latter is wetted by active brazing solder  10 , so that an electrical connection is provided between wedge zone  91  and electrode  52 . 
     FIG. 4 shows a schematic vertical section of a capacitive differential pressure sensor cell according to a second variant of the invention which comprises a ceramic substrate  2  having a cylindrical surface  21  and, at a first major surface  22 , a concave first central area  221  which is provided with a first electrode  222 . A first electrical connection  223  is provided from first electrode  222  through substrate  2  to cylindrical surface  21 . 
     In the direction of and up to cylindrical surface  21 , central area  21  merges into a convex first surface  224 , which has a first vertex line  225  and which forms a first planar ring surface  226  in the area of vertex line  225 . 
     At a second major surface  23  opposite major surface  22 , substrate  2  has a concave second central area  231 , which is provided with a second electrode  232 . A second electrical connection  233  is provided from second electrode  232  through substrate  2  to cylindrical surface  21 . 
     In the direction of and up to cylindrical surface  21 , the second central area  231  merges into a convex second surface  234 , which has a second vertex line  235  and which forms a second planar ring surface  236  in the area of vertex line  235 . The first and second central areas  221 ,  231  are connected with one another through a connecting channel  239 . 
     A first ceramic diaphragm  6  rests on ring surface  226  of substrate  2 . This diaphragm  6  and the portion of convex surface  224  which extends between ring surface  226  and cylindrical surface  21  form a first circumferential wedge zone  92 . By means of active brazing solder  10  in wedge zone  92 , diaphragm  6  is joined to substrate  2  to form a first high-vacuum-tight sensing chamber M 1 . 
     On a planar first inner surface  61  of diaphragm  6  facing substrate  2 , a third electrode  62  is provided, to which contact is made through first wedge zone  92 . Third electrode  62  and electrode  222  on substrate  2  form a first capacitor. 
     A second ceramic diaphragm  7  rests on ring surface  236  of substrate  2 . This diaphragm  7  and the portion of convex surface  234  which extends between ring surface  236  and cylindrical surface  21  form a second circumferential wedge zone  93 . By means of active brazing solder  10  in wedge zone  93 , diaphragm  7  is joined to substrate  2  to form a second high-vacuum-tight sensing chamber M 2 . 
     On a planar inner surface  71  of diaphragm  7  facing the substrate  2 , a fourth electrode  72  is provided, to which contact is made through the second wedge zone  93 . Electrode  72  and electrode  232  together form a second capacitor. 
     FIG. 5 shows a vertical section of a capacitive differential pressure sensor cell according to a third variant of the invention. This differential pressure sensor cell comprises a first ceramic substrate  3 , which has a first cylindrical surface  31  and, at a first major surface  32 , a concave first central area  321 . The latter is provided with a first electrode  322 , and a first electrical connection  323  is provided from electrode  322  through substrate  3  to a second major surface  33  opposite first major surface  32 . In the direction of and up to cylindrical surface  31 , central area  321  merges into a convex first surface  324 , which has a first vertex line  325  and which forms a first planar ring surface  326  in the area of vertex line  325 . 
     The differential pressure sensor cell further comprises a second ceramic substrate  4 , which has a second cylindrical surface  41  and, at a first major surface  42 , a concave second central area  421 . The latter is provided with a second electrode  422 , and a second electrical connection  423  is provided from electrode  422  through substrate  4  to a second major surface  43  opposite first major surface  42 . In the direction of and up to cylindrical surface  41 , central area  421  merges into a convex second surface  424 , which has a second vertex line  425 . In the area of the second vertex line  425 , convex surface  424  forms a second planar ring surface  426 . 
     A ceramic diaphragm  8  rests with a first planar inner surface  81  on ring surface  326  of first substrate  3 . Diaphragm  8  and the portion of convex surface  324  which extends between ring surface  326  and cylindrical surface  31  of substrate  3  form a first circumferential wedge zone  95 . By means of active brazing solder  10  in wedge zone  95 , the diaphragm  8  is joined to substrate  3  to form a first high-vacuum-tight sensing chamber M 1 ′. 
     Diaphragm  8  rests with a second planar inner surface  85  on ring surface  426  of second substrate  4 . The diaphragm  8  and the portion of convex surface  424  which extends between ring surface  426  and cylindrical surface  41  form a second circumferential wedge zone  96 . By means of active brazing solder  10  in wedge zone  96 , diaphragm  8  is joined to substrate  4  to form a second high-vacuum-tight sensing chamber M 2 ′. 
     The inner surface  81  of diaphragm  8  is provided with a third electrode  82 , to which contact is made through wedge zone  95 . The inner surface  85  of diaphragm  8  is provided with a fourth electrode  86 , to which contact is made through wedge zone  96 . The first electrode  322  on substrate  3  and the third electrode  82  form a first capacitor, and the second electrode  422  on substrate  4  and the fourth electrode  86  form a second capacitor. 
     All of the substrates and diaphragms mentioned above and shown in FIGS. 1 to  5 , i.e., substrates  1 ,  2 ,  3 ,  4  and diaphragms  5 ,  6 ,  7 ,  8 , are preferably of alumina ceramic, particularly of a 96% alumina ceramic. In all cases, an active brazing solder  10  of a Zr—Fe—Ti—Be alloy (cf. EP-A 835 716) or a Zr—Ni—Ti alloy (cf. U.S. Pat. No. 5,334,344) has proved particularly advantageous since such an active brazing solder has an excellent wetting ability, high strength, and a coefficient of thermal expansion corresponding to that of the ceramic material of diaphragm  5  and sub-strate  1 . For the electrodes, tantalum can be used (cf. U.S. Pat. No. 5,050,034). 
     To make sure that no active brazing solder paste can penetrate into the sensing chambers M, M 1 , M 2 , M 1 ′, and M 2 ′, it has proved advantageous to cover electrode  52  on diaphragm  5 , electrodes  82  and  86  on diaphragm  8 , and electrodes  62  and  72  on diaphragms  6  and  7 , respectively, with a solder resist layer. If tantalum is used for electrodes  52 ,  62 ,  72 ,  82 ,  86 , such a solder resist cover can be implemented in a particularly simple manner with a tantalum-oxide layer. 
     The electrical connections through the substrates can be produced as described in U.S. Pat. No. 5,154,697 or U.S. Pat. No. 5,050,035, for example. 
     The pressure sensor cell of FIG. 2 is manufactured as follows. The ceramic substrate  1  is provided, at its first major surface  12 , with the concave central area  121 , which is shaped so as to merge into the convex surface  124  in the direction of and up to the cylindrical surface  11 . The convex surface  124  is flattened in the area of its vertex line  125  to form the planar ring surface  126 . This is achieved by grinding the substrate  1  in a suitable manner. 
     Electrode  122  is deposited on central area  121 , and an electrical connection is made from this electrode through substrate  1  to the second major surface  13  of the substrate in the usual manner. 
     On a planar inner surface  51 , ceramic diaphragm  5  is provided with the second electrode  52 , which is so dimensioned that, after diaphragm  5  has been placed on ring surface  126  of substrate  1 , this electrode extends only up to ring surface  126 . 
     A sufficient quantity of active brazing solder  10  is applied to the portions of convex surface  124  between ring surface  126  and cylindrical surface  11 . Preferably, use is made of an active brazing solder paste which is applied to substrate  1  by means of a suitable dispenser. 
     Next, the inner surface  51  of diaphragm  5 , provided with the second electrode  52 , is placed on ring surface  126  of substrate  1 , and substrate  1  and diaphragm  5  are heated in a vacuum or inert-gas atmosphere until the active brazing solder  10  has melted. After substrate  1  and diaphragm  5  have cooled down, the pressure sensor cell shown in FIG. 2 is complete. 
     The differential pressure sensor cell of FIG. 4 is manufactured as follows. The first major surface  22  of the ceramic substrate  2  is provided with the concave central area  221 , which merges into the convex surface  224  in the direction of and up to the cylindrical surface  21 . In the area of the vertex line  225  of the convex area  224 , the latter is formed into the planar ring surface  226 . Electrode  222  is deposited on central area  221 , and an electrical connection is provided from electrode  222  through substrate  2  to the cylindrical surface  21  of the substrate. 
     At its second major surface  23 , substrate  2  is provided with the concave central area  231 , which merges into the convex area  234  in the direction of and up to the cylindrical surface  21  of substrate  2 . Convex area  234  is also flattened in the area of its vertex line  235  to form the planar ring surface  236 . Electrode  232  is deposited on central area  231 , and an electrical connection is provided from electrode  232  through substrate  2  to cylindrical surface  21 . 
     The planar inner surface  61  of diaphragm  6  is provided with electrode  62 , which is dimensioned so as to extend up to, and only up to, ring surface  226  after diaphragm  6  has been placed on the ring surface  226 . The planar inner surface  71  of diaphragm  7  is provided with electrode  72 , which is dimensioned so as to extend up to ring surface  236  after diaphragm  7  has been placed on the ring surface  236 . 
     Next, quantities of active brazing solder  10  sufficient to braze diaphragms  6  and  7  to substrate  2  are applied to those portions of the convex surfaces  224 ,  234  of substrate  2  which are located between the respective ring surfaces  226 ,  236  and the cylindrical surface  21 . For this, too, an active brazing solder paste is preferably used, which is applied by means of a suitable dispenser. 
     After that, diaphragm  6 , provided with electrode  62  on its inner surface  61 , is placed on ring surface  226  of substrate  2 , and diaphragm  7 , provided with electrode  72  on its inner surface  71 , is placed on ring surface  236 . Then, substrate  2  and diaphragms  6 ,  7  are heated in a vacuum or inert-gas atmosphere until the active brazing solder  10  has melted, and subsequently allowed to cool down. 
     The differential pressure sensor cell of FIG. 5 is made as follows. The major surface  32  of the ceramic substrate  3  is provided with the concave central area  321 , which merges into convex surface  324  in the direction of and up to cylindrical surface  31  of substrate  3 . In the area of its vertex line  325 , convex surface  324  is formed as a planar ring surface  326 . Electrode  322  is deposited on central area  321 , and an electrical connection is made from electrode  322  through substrate  3  to surface  33  of substrate  3 . 
     The major surface  42  of substrate  4  is provided with the concave central area  421 , which merges into the convex surface  424  in the direction of and up to cylindrical surface  41 . In the area of its vertex line  425 , the convex surface  424  is formed as a planar ring surface  426 . Electrode  422  is deposited on central area  421 , and an electrical connection is made from electrode  422  through substrate  4  to surface  43  of substrate  4 . 
     The planar inner surface  81  of the ceramic diaphragm  8  is provided with the electrode  82 , which is dimensioned so as to extend up to the ring surface  326  of the first substrate  3  after diaphragm  8  has been placed on the ring surface  326 . On its other planar inner surface  85 , diaphragm  8  is provided with electrode  86 , which is dimensioned so as to extend up to ring surface  426  of substrate  4  after diaphragm  8  has been placed on the ring surface  426 . 
     Quantities of active brazing solder  10  sufficient to braze diaphragm  8  to substrates  3  and  4  are applied to the portion of convex surface  324  of substrate  3  located between ring surface  326  and cylindrical surface  31  and to the portion of convex surface  424  of substrate  4  located between ring surface  426  and cylindrical surface  41 . In this case, too, an active brazing solder paste is preferably used, which is applied by means of a suitable dispenser. 
     Next, the inner surface  81  of diaphragm  8 , provided with electrode  82 , is placed on ring surface  326  of substrate  3 , and the inner surface  85  of diaphragm  8 , provided with electrode  86 , is placed on ring surface  426  of substrate  4 . 
     Thereafter, substrates  3 ,  4  and diaphragm  8  are heated in a vacuum or inert-gas atmosphere until the active brazing solder  10  has melted, and allowed to cool down.