Pressure measuring cell and methods for its manufacture

Abstract

Pressure measuring cell (1) comprising: a ceramic measuring membrane (2) and a ceramic counterbody (4), wherein the measuring membrane (2) is pressure-tightly joined to the counterbody (4) by means of an active brazing alloy (6) to form a pressure chamber between the measuring membrane (2) and the counterbody (4), wherein the pressure measuring cell (1) further comprises a brazing stop on a surface of the measuring membrane (2) and / or the counterbody (4), which prevents the active brazing alloy (6) from extending radially inwards beyond the brazing stop into the pressure chamber, wherein the brazing stop comprises a coating (26) comprising silicon carbide.

Description

The present invention relates to a ceramic pressure measuring cell and a method for producing the same.Ceramic pressure measuring cells of the aforementioned type have a ceramic measuring membrane and a ceramic counterbody, wherein the measuring membrane is connected to the counterbody in a pressure-tight manner along a circumferential joint which has an active brazing solder, wherein a pressure chamber is formed between the measuring membrane and the counterbody, wherein the equilibrium position of the measuring membrane results from the difference between a pressure prevailing in the pressure chamber and a pressure acting on the outer side of the measuring membrane facing away from the pressure chamber. Pressure measuring cells of the generic type also comprise a transducer for converting the pressure-dependent deformation of the measuring membrane into an electrical or optical signal. In the following, particular attention is paid to pressure measuring cells with a capacitive transducer, which the invention can equally relate to pressure measuring cells with other transducers.In particular, aluminum oxide ceramics are used as material for the counter body and the measuring membrane, which are suitable for producing pressure measuring cells because of their elastic properties and their media resistance. The ceramic components mentioned are joined in particular with an active brazing filler metal, which is preferably an active brazing filler metal containing Zr-Ni-Ti. The production of such an active brazing solder is disclosed, for example, in European laid-open specification EP 0 490 807 A2. According to the method described in the laid-open specification, rings can be produced in particular from the active brazing material, which rings are to be positioned between the measuring membrane and the counterbody in order to braze these together. However, rings of this type have a minimum thickness of a few 10 μm, as a result of which the distance between the measuring membrane and the mating body is predefined if it is not possible to allow the solder to flow off in a controlled manner. However, this desire is initially counteracted by the balance of the free surface energy of the ceramic components and the active brazing filler metal, because if the filler metal is brought into a sufficiently low-viscosity state by increasing it beyond the melting temperature, it only knows a flow direction: radially inward! Therefore, further measures are required to change the solder or to allow it to flow to where it is intended to be intended. For this purpose, for example, German Patent Application DE 100 36 433 A1 discloses a capacitive pressure measuring cell which likewise has a joint with an active brazing filler metal, wherein a annularly encircling groove is formed on the joint root, that is to say the inner radius of the joint, which groove on the one hand prevents the locating of notch stresses at the joint and on the other hand defines a reliable brazing filler metal stop via which the active brazing filler metal cannot flow further radially inward.Furthermore, the laid-open specification DE 10 2009 046 844 A1 discloses a capacitive ceramic pressure measuring cell having a joint which has an active hard solder, wherein a groove is formed in the counterbody in the radial portion which is covered by the joint, into which groove superfluous active hard solder is intended to flow in order to reduce the thickness of the joint and thus the distance between the measuring membrane and the counterbody. However, the preparation of such depressions in the ceramic mating body is expensive and associated with further disadvantages, since the filling of a groove under the joint with superfluous active brazing solder can be achieved reproducibly only in a very narrow range of process parameters, and a groove at the joint root leads, merely on account of the minimum width of such a structural element, to a reduction in the usable area for electrodes of the capacitive transducer.One practiced method for limiting the active brazing solder flowing radially inward consists in surface oxidizing a membrane-side electrode which comprises tantalum and which is intended to come into galvanic contact with the active brazing solder. At comparably very low soldering temperatures, it is thus possible to prevent the active hard solder from running into the pressure chamber with an acceptable yield. However, if the soldering temperature is increased, in order to promote, for example, the active hard solder flowing out of the joining layer to the outside, this solder stop no longer acts reliably, and the solder flows into the pressure chamber via the edge of the tantalum electrodes. Further approaches for preparing joints with an active brazing filler with the aim of being able to produce thinner joints are disclosed in the patent applications DE 10 2010 043 119 A1 and DE 10 2010 063 065 A1, wherein the methods described there each comprise the vapor deposition of the active brazing filler or at least of components of the active brazing filler. Although thin joints can be prepared in this way, such methods are however associated with comparatively high costs.Regardless of the possibility of allowing some of the solder to flow off, a higher temperature brings about a greater activity of the solder and can thus possibly contribute to a higher stability of the joint. For this reason alone, it is expedient to improve a pressure measuring cell to the effect that its components can be joined at temperatures at which the active brazing solder has a high activity.It is therefore the object of the present invention to provide a pressure measuring cell and a method for its production, wherein the pressure measuring cell has a reliable and simple solder stop, which in particular also acts at elevated temperature, and wherein the preparation of this solder stop is implemented in the production method.Pressure measuring cells and methods for producing these and further components, in particular using a solder resist, have become known, inter alia, from EP 1 010 973 A1, DE 10 2008 044 641 A1, US 2010 / 0 132 982 A1, EP 0 997,935 A1 and DE 10 2010 030 156 A1.The object is achieved according to the invention by the pressure measuring cell according to independent claim 1 and the method according to independent claim 7.The pressure measuring cell according to the invention comprises a ceramic measuring membrane and a ceramic counterbody, wherein the measuring membrane is joined pressure-tightly to the counterbody by means of an active brazing filler metal, forming a pressure chamber between the measuring membrane and the counterbody, wherein the pressure measuring cell further comprises a brazing filler metal on a surface of the measuring membrane and / or of the counterbody, which prevents the active brazing filler metal from extending radially inward into the pressure chamber beyond the brazing filler metal, wherein the brazing filler metal comprises a coating according to the invention which comprises silicon carbide.In a further development of the invention, the solder stop comprises a coating which has a width of not more than 0.5 mm, in particular not more than 0.2 mm, and in particular not more than 0.1 mm.In a development of the invention, the solder resist has a coating with a thickness of not more than 0.2 μm, in particular not more than 0.1 μm and in particular not more than about 50 nm.In a development of the invention, the coating comprises a sputtered layer. In another aspect of the invention, the coating comprises a line drawn with a pin.According to a further development of the invention, the ceramic material of the measuring membrane and of the counter body comprises an aluminum oxide ceramic, in particular a highly pure aluminum oxide ceramic, as described, for example, in German patent DE 10 2008 036 381 B3. The purities described there relate in particular to the ceramic of the measuring membrane, but for the mating body, such a high-purity ceramic is not absolutely necessary.According to a further development of the invention, the active brazing filler comprises a zirconium-nickel-titanium-containing active brazing filler, as described, for example, in European patent application EP 0 490 807 A2.The method according to the invention for connecting two ceramic components by means of an active brazing filler comprises:Providing the two ceramic components, coating at least one surface of at least one ceramic component with a solder resist which separates a portion of the surface to be wetted by the solder resist from a portion to be kept free of the solder, wherein the solder resist has a coating which comprises silicon carbide; providing the active hard solder between the first and the second ceramic component in the surface regions of the first and the second component to be wetted by the active hard solder; and heating the ceramic components and the active hard solder under vacuum to a temperature at which the active hard solder melts and reacts with the ceramic components; wherein the heating is followed by allowing the ceramic components to cool.In a development of the invention, the provision of the coating takes place by means of sputtering and / or by means of vapor deposition of the components of the coating.In another development of the invention, the coating is effected by pulling a line with a pin or lead.In another development of the invention, the coating is effected by means of screen printing.In a further development of the invention, after the coating has been applied, the coating is annealed under vacuum, in particular at a pressure below 10 -4 mbar, preferably below 10 -5 mbar and in particular at not more than 10 -6 mbar, wherein the temperature during the annealing is not less than 800° C., preferably not less than 900° C. and preferably not less than 950° C., in particular 1000° C.In a further development of the invention, the joining of the ceramic components with the active brazing filler is carried out at a temperature of not less than 82° C., in particular not less than 830° C.In a development of the invention, the solder stop separates a convex surface section, in particular a planar convex surface section, which is not to be wetted by the active hard solder, from a surface section which is to be wetted by the active hard solder.The term convex means that a straight connecting line between any two points of the convex surface portion does not extend outside the surface portion.In a further development of the invention, the method relates to a method for producing a pressure sensor, wherein the first ceramic component comprises a counterbody and the second ceramic component comprises a measuring membrane, wherein the counterbody is connected to the measuring membrane in a pressure-tight manner by means of the joint.According to a further development of the invention, the joint is prepared by providing an annular shaped active brazing material part between the counterbody and the measuring membrane, wherein the active brazing material has a greater material thickness in the axial direction than the desired distance between the measuring membrane and the counterbody in the region of the joint, wherein the vacuum brazing process detects the heating to a temperature above 80° C., in particular above 830° C., at which the active brazing material is of such low viscosity that it can flow off in a controlled proportion from the gap between the counterbody and the measuring membrane.The invention is now disclosed on the basis of the exemplary embodiment illustrated in the drawings.It shows: FIG. 1 : a longitudinal section through a pressure measuring cell according to the invention; FIGS. 2a-2d show a sequence of preparation steps for producing a pressure measuring cell according to the invention.The pressure measuring cell 1 in FIG. 1 comprises a measuring membrane 2 which has high purity (>99%) aluminum oxide and a counterbody 4 which likewise has aluminum oxide. The mating body 4 can have the same purity as the measuring membrane, wherein this is not absolutely necessary since the mating body is not in contact with the medium, on the one hand, so that the requirements for corrosion resistance are lower and is not exposed to the mechanical stresses as the measuring membrane 2, on the other hand. The measuring membrane is connected pressure-tightly to the counterbody along a circumferential joint 6, which has an active solder containing Zr-Ni-Ti, forming a pressure chamber between the measuring membrane 2 and the counterbody 4.For detecting a pressure-dependent deformation of the measuring membrane, the pressure measuring cell 1 comprises a differential capacitor which is formed by a membrane electrode 8 arranged on the measuring membrane 2, a central measuring electrode 10 in the form of a circular disk on the opposing body and a reference electrode 12 surrounding the measuring electrode. In the ideal case, the capacitances between the measuring electrode 10 and the membrane electrode 8 are equal to the capacitances between the reference electrode 12 and the membrane electrode 8 when the measuring membrane 2 is in the rest position. The electrodes mentioned comprise in particular tantalum, wherein the surfaces 14, 16, 18 of the electrodes are thermally oxidized in order to chemically stabilize them. The membrane electrode 8 is electrically contacted via the joint 6 and an electrical lead-through 20 which extends through the mating body in the radial region of the joint 6. The measuring electrode 10 and the reference electrode 12 are contacted directly via electrical feedthroughs 22, 24 through the mating body 4. The electrical feedthroughs 20, 22, 24 comprise tantalum pins, which are soldered pressure-tightly into holes through the mating body 4 by means of an active brazing solder.The invention relates to the solder stop, with which the active brazing solder is prevented from flowing radially inward from the edge region into the pressure chamber during the brazing of the counterbody 4 and the measuring membrane 2. The solder resist comprises a carbon layer, in particular. Graphite layer 26 which surrounds the reference electrode 12 in a ring shape and is galvanically separated from it. The outer radius of the carbon layer 26 defines the boundary to which the molten active brazing solder can flow radially inward. In the solidified state, the root of the joint 6 is then located at this radius.With reference to FIGS. 2a to 2d, the manufacturing steps for producing the pressure measuring cell according to the invention will now be briefly explained.First, as shown in FIG. 2 a, the surfaces of the membrane electrode 8, the measuring electrode 10 and the reference electrode 12 are deposited on the counter body 4 and the measuring membrane 2 by sputtering tantalum. Thermal oxidation of the electrode surfaces 14, 16, 18 follows in order to chemically stabilize the latter.Subsequently, as shown in FIG. 2 b, an annular carbon layer 26 is prepared around the reference electrode 12, the outer radius of which defines the boundary for the flow of the active brazing solder. The carbon layer can be realized by sputtering graphite, by drawing an annular line with a graphite lead or by screen printing, wherein at least after the screen printing method or the drawing of a line with a lead the counter-body has to be heated to a few 100° C. in order to remove foreign components from the carbon layer. The carbon layer has a thickness of about 50 nm, for example.To prepare the joining of the measuring membrane 2 to the mating body 4, the two joining partners are stacked coaxially therebetween with an annular shaped solder part 5, as is illustrated in FIG. 2 c. The ring-shaped shaped solder molding 5 has a height of approximately 30 μm to 50 μm, for example.Finally, the components are soldered in a high vacuum soldering process at temperatures above 800° C., wherein the melted active brazing solder reacts with the ceramic surfaces of the measuring membrane 2 and the counter body 4, but it cannot wet the carbon layer 26, so that it flows radially inward only as far as the outer radius of the carbon layer 26. The outer radius of the carbon layer is positioned such that galvanic contact nevertheless occurs between the joint 6 and the membrane electrode 8.If the temperature of the vacuum soldering process is selected to be sufficiently high, a portion of the low-viscosity, molten solder can flow off outwards in a controlled manner in order to reduce the distance between the measuring membrane 2 and the mating body 4.As a result, this leads to the measurement point shown in FIG. 2 d. Of course, this also includes the electrical feedthroughs described in connection with FIG. 1, which, however, have been omitted in FIGS. 2 ato 2 dfor the sake of clarity. The feedthroughs are likewise soldered in in the high-vacuum soldering process.In particular, in the case of the height ratios, scale scatter has been dispensed with in the drawings in order in particular to be able to represent the layers at all. The radial size of the pressure measuring cell is a few 10 mm. The height or axial thickness of the counter body is, for example, 3 to 15 mm. The thickness of the measuring membrane is, for example, not less than 100 μm and, for example, not more than 2000 μm. The layer thickness of the electrodes is about 100 nm. The dimensional data are given for illustrative purposes only and are not to be taken as a definition or a restrictive interpretation of the invention.

Claims

Pressure measuring cell (1) comprising: a ceramic measuring membrane (2) and a ceramic counterbody (4), wherein the measuring membrane (2) is joined to the counterbody (4) in a pressure-tight manner by means of an active brazing filler (6) to form a pressure chamber between the measuring membrane (2) and the counterbody (4), wherein the pressure measuring cell (1) further has a brazing filler on a surface of the measuring membrane (2) and / or of the counterbody (4) which prevents the active brazing filler (6) from extending beyond the brazing filler radially inward into the pressure chamber, wherein the brazing filler has a coating (26) comprising silicon carbide.Pressure measuring cell (1) according to claim 1, wherein the solder stop comprises a coating (26) having a width of not more than 0.5 mm, in particular not more than 0.2 mm, and in particular not more than 0.1 mm.Pressure measuring cell (1) according to claim 1 or 2, wherein the solder resist has a coating (26) in a thickness of not more than 0.2 μm, in particular not more than 0.1 μm and in particular not more than about 50 nm.Pressure measuring cell (1) according to one of claims 1 - 3, wherein the coating (26) is applied by sputtering, by screen printing or by means of a pin.Pressure measuring cell (1) according to one of the preceding claims, wherein the ceramic material of the measuring membrane (2) and of the counter body (4) comprises an aluminium oxide ceramic.Pressure measuring cell (1) according to one of the preceding claims, wherein the active hard solder (6) comprises an active hard solder containing Zr-Ni-Ti.A method for joining two ceramic components by means of an active brazing filler metal (6), comprising: providing the two ceramic components; coating at least one surface of at least one ceramic component with a brazing filler metal which separates a portion of the surface to be wetted by the brazing filler metal from a portion to be kept free of the brazing filler metal, wherein the brazing filler metal has a coating (26) comprising silicon carbide; providing the active brazing filler metal (6) between the first and the second ceramic component in the surface regions of the first and the second component to be wetted by the active brazing filler metal (6); heating the ceramic components and the active brazing filler metal (6) under vacuum to a temperature at which the active brazing filler metal (6) melts and reacts with the ceramic components; and allowing the ceramic components to cool.Method according to claim 7, wherein the coating is effected by means of sputtering and / or by means of vapor deposition of components of the coating (26).The method of claim 7, wherein the coating is performed by drawing a line with a pencil.The method of claim 7, wherein the coating is performed by screen printing.Method according to one of Claims 7 to 10, wherein, after the coating, the coating (26) is annealed under vacuum, in particular at a pressure below 10 -4 mbar, preferably below 10 -5 mbar and in particular at not more than 10 -6 mbar.The method according to claim 11, wherein the annealing temperature is not less than 800°C, preferably not less than 900°C, and preferably not less than 950°C, in particular 1000°C.Method according to one of Claims 7 to 12, wherein the joining of the ceramic components with the active brazing filler (6) takes place at a temperature of not less than 800°C, in particular not less than 82°C.Method according to one of claims 7 to 13, wherein the solder stop separates a convex surface section which is not to be wetted by the active hard solder (6) from a surface section which is to be wetted by the active hard solder (6).Method according to one of Claims 7 to 14 for producing a pressure sensor, wherein the first ceramic component comprises a mating body (4) and the second ceramic component comprises a measuring membrane (2), wherein the mating body (4) is connected to the measuring membrane (2) in a pressure-tight manner by means of a joint.Method according to claim 15, wherein the joint is prepared by providing an annular shaped active brazing material part (5) between the counterbody (4) and the measuring membrane (2), wherein the active brazing material (6) has a greater material thickness in the axial direction than the desired distance between the measuring membrane (2) and the counterbody (4) in the region of the joint (6), wherein the vacuum brazing process detects the heating to a temperature above 82°C, in particular above 830°C, at which the active brazing material (6) is of such low viscosity that it flows off in a controlled proportion from the gap between the counterbody (4) and the measuring membrane (2).

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