WO2003081194A1

WO2003081194A1 - Pressure sensor, especially for the capacitive determination of absolute pressure

Info

Publication number: WO2003081194A1
Application number: PCT/EP2003/002829
Authority: WO
Inventors: Roland Singpiel
Original assignee: Nord-Micro Ag & Co. Ohg
Priority date: 2002-03-22
Filing date: 2003-03-18
Publication date: 2003-10-02
Also published as: DE10212947C1; AU2003218788A1

Abstract

The invention relates to a pressure sensor which is especially used for the capacitive determination of absolute pressure and is provided with a base body (10) comprising at least one electrode (11a, 11b; 12a, 12b), and a membrane (20). Said membrane (20) is separated from the base body (10) by means of a spacer (30) and comprises at least one counter-electrode (21a, 21b; 22a, 22b) for generating an electrical field between the base body (10) and the membrane (20). Both the base body and the membrane (20) consist of a ceramic material. The membrane (20) is arranged on a carrier body consisting of a ceramic material and is connected to said carrier body (40) by means of a material connection which is free of filler materials. The inventive pressure sensor is characterised by a measuring cell (50) consisting of the base body (10), the membrane (20), the spacer (30) and the carrier body (40), and is arranged in a hermetic housing (60). An evaluation device (70) used to detect capacitive changes of the measuring cell (50) is arranged on the base body (10) and comprises a sensor for detecting the temperature of the measuring cell (50).

Description

Pressure sensor, in particular for the capacitive determination of the absolute pressure

The invention relates to a pressure sensor, in particular for the capacitive determination of the absolute pressure, which is provided with a base body which has at least one electrode and with a membrane. The membrane is separated from the base body by a spacer and has at least one counter electrode for generating an electric field between the base body and the membrane. The base body and the membrane are each made of a ceramic material. The membrane is arranged on a carrier body made of a ceramic material and is connected to the carrier body by a material connection that is free of additional materials.

Pressure sensors are generally used to measure the pressure of media, such as liquids, gases or vapors, absolutely or relative to a reference pressure, for example the ambient pressure. In the case of a capacitive pressure sensor, the pressure is determined on the basis of the change in a capacitor formed by two electrodes lying opposite one another. A capacitive pressure sensor is described, for example, in EP 1 039 284 A1.

The known pressure sensor has a base body made of a ceramic material, which is provided with an electrode. Spaced from the electrode is an elastically deformable membrane, which consists of a ceramic material and is provided with a counter electrode. A pressure medium is applied to the membrane on the side facing away from the base body, which causes the membrane to deform. Depending on the deformation of the membrane, the distance between the electrode and the counterelectrode changes and thus the capacitance of the through the electrode and counter electrode formed capacitor. An evaluation device that detects the change in capacitance delivers an electrical signal that is proportional to the pressure of the medium acting on the membrane.

A capacitive sensor in which the base body and the membrane are made of ceramic are also disclosed in DE 39 10 646 C2 and DE 40 23 420 A1. The known pressure sensors also have in common that a measuring cell is formed by the membrane and the base body, which is used as a connected unit. It has been found to be disadvantageous here that the fastening of the measuring cell in a component causes a stress state in the membrane, which can lead to hysteresis and thus to falsifications of the measured changes in capacitance.

In contrast, WO 99/34184 discloses a capacitive vacuum measuring cell which has a first housing body, a membrane and a second housing body. The housing body and the membrane are made of aluminum oxide. The membrane is arranged between the first housing body and the second housing body and is integrally connected to the latter. For this purpose, either finds a glass solder, which also serves the membrane of the housing bodies to space, or a se by ^{'beispielswei-} welding or diffusion-generated cohesive connection between the membrane and the housing bodies application. The material connection can be free of additional materials.

The invention has for its object to develop a pressure sensor of the type mentioned in such a way that a comparatively accurate measurement result can be achieved with a simple, inexpensive and robust design and a largely stress-free storage of the membrane.

To achieve this object, in accordance with claim 1, a measuring Cell is provided, which is composed of the base body, the membrane, the spacer and the support body and is arranged in a hermetic housing, wherein an evaluation device which detects capacitive changes in the measuring cell and which has a sensor which detects the temperature of the measuring cell is arranged on the base body ,

A pressure sensor designed in this way adopts the knowledge that by providing a support body for the membrane, a largely stress-free mounting of the membrane is achieved. The main reason for this is that, due to the support body, no forces act on the membrane which, as in the prior art, cause undesirable stress on the membrane, for example due to torsion. The cohesive connection of the membrane and the support body also contributes significantly to a largely stress-free mounting of the membrane. For example, the occurrence of undesirable thermal stresses in the joining zone of the membrane and carrier body is avoided by the fact that the material bond is formed without additional materials. In this way, the joining zone between the membrane and the carrier body remains free of any foreign materials which impair the measuring behavior of the pressure sensor, for example as a result of hysteresis.

Because of the support body, the measuring unit formed by the membrane and base body can be easily attached to a component. A robust design of the pressure sensor is thus ensured. The measuring cell, which is composed of the base body, the membrane, the spacer and the carrier body and is arranged in a hermetic housing, also contributes to this, the carrier body being attachable to the housing. The measuring cell comprising the carrier body is protected against environmental influences by the arrangement in the housing. This encapsulation of the measuring cell results in an extremely robust pressure sensor. The attachment of the measuring cell to the housing by means of the carrier body enables Free and therefore low-stress arrangement of the base body and membrane. This ensures a relatively high accuracy of the pressure measurement.

In addition, the direct arrangement of the evaluation device designed as an application-specific integrated circuit (ASIC) on the base body ensures an undistorted measurement of the change in capacitance of the measuring cell. The evaluation device is protected from environmental influences by the hermetic housing. In this way, the electrical lines required for connecting the evaluation device can be easily laid on the surface of the base body. This guarantees a simple and inexpensive construction.

It is also particularly advantageous that the evaluation device has a sensor that detects the temperature of the measuring cell. Because with the help of this temperature sensor, the temperature of the measuring cell can be recorded in order, for example, to be able to take into account the influence of the temperature when evaluating the measured change in capacitance of the measuring cell.

Advantageous embodiments of the pressure sensor according to the invention are the subject matter of claims 2 to 13.

With regard to a comparatively simple and inexpensive production, it is therefore advantageous to prefabricate the membrane and the support body and to connect them to one another by sintering, preferably co-sintering. The separate provision of membrane and support body enables a modular design that meets individual requirements. Since both the support body and the membrane consist of a ceramic material, they can be connected to one another by sintering or co-sintering. Co-sintem in the above sense means a connection method in which components are connected to one another in a cohesive manner without changing shape in the manner of diffusion welding. In contrast to the conventional sintering, in which the material closure by compressing powdery or fine-grained substances by means of pressure and temperature below the melting point is produced, which is inevitably associated with a change in shape, the Co-Sintem is characterized in that the components to be connected to one another, in the present case the membrane and the support body, experience no impairment in their shape.

It is also advantageous to provide the support body with a recess in which a pressure medium can be applied to the membrane. In this way, the gaseous or liquid medium, the pressure of which is to be measured, is directed specifically to the membrane. For this purpose, the carrier body preferably has an opening for feeding the medium into the depression.

With regard to a compact design of the pressure sensor, the housing preferably has a supply line for a pressure medium, which is connected to the opening of the carrier body.

Furthermore, in an advantageous development of the pressure sensor according to the invention, a protective gas surrounding the measuring cell in the interior of the housing is provided. The protective gas, for example nitrogen, prevents environmental influences, such as, for example, air humidity, on the measuring behavior of the measuring cell. An accurate measuring behavior of the measuring cell is thus permanently guaranteed.

A preferred design of the pressure sensor according to the invention provides that the base body is provided with at least one pressure electrode and at least one reference electrode and the membrane with at least one pressure counter electrode and at least one reference counter electrode. The provision of pressure and reference electrodes or counter-electrodes results in at least two capacities, the changes of which are used to measure the absolute pressure of the pressure medium acting on the membrane. A quotient of the determined changes in capacity leads to a dimensionless indicator that is proportional to the absolute pressure to be determined. The pressure electrode and pressure counter electrode are expediently in the center of the base body and Diaphragm arranged, whereas the reference and reference counterelectrode are located in the edge area of the membrane and base body. In this way, the different curvature of the membrane along the diameter can be taken into account due to its usually edge-side clamping. It has proven to be advantageous from a constructional point of view if the base body has two inner pressure electrodes, which are surrounded by two outer reference electrodes, and that the membrane has two inner pressure counter electrodes, which are surrounded by two outer reference counter electrodes. By doubling the number of pressure and reference electrodes or counter-electrodes, the accuracy of the determined changes in capacitance can be increased.

In order to ensure simple and economical manufacture, the electrodes and the counter electrodes are preferably made of platinum or gold and are preferably applied to the base body and the membrane by sputtering. Sputtering in the above sense means a process for atomizing solids in a high vacuum. In this way, electrodes with a thin layer thickness can be provided on the base body and membrane consisting of a ceramic material.

Furthermore, in a development of the pressure sensor according to the invention, it is proposed that the spacer be a solder made of metal or glass, which integrally connects the base body and the membrane to one another. Using the plumb bob, the membrane and the base body can be positioned precisely on top of one another. Precise positioning is crucial with regard to the changes in capacity to be determined.

It has proven to be particularly advantageous if the membrane consists of sapphire. Because sapphire, a ceramic material that has a single-crystalline structure, does not generate any hysteresis in mechanical and thermal terms, so that there is a relatively high reproducibility and measuring accuracy. In addition, sapphire has a relatively high dimensional stability at high temperatures, which contributes, for example, to that of Co-sintem of membrane and carrier body no undesirable change in the shape of the membrane occurs. The single-crystal structure of sapphire has no grain boundary, so that the membrane can be made almost as thin as desired. In contrast, the thickness of a membrane made of a conventional ceramic material is limited by the grain size and the statistical distribution of the grains. The consequence of this is that membranes made of conventional ceramic materials have a minimum thickness of a few grain layers, which both limit the measuring range of the pressure sensor and also prevent miniaturization of the measuring cell. Membranes made of conventional ceramic materials also have the disadvantage, compared to a membrane made of sapphire, that they tend to crack in very thin configurations, which, for example, occur during grinding and can lead to an undesirable long-term deviation of the pressure sensor.

Details and further advantages of the pressure sensor according to the invention result from the following description of a preferred exemplary embodiment. In the drawing schematically illustrating the exemplary embodiment, the following are illustrated in detail:

Fig. 1 is an exploded view of the measuring cell of the pressure sensor according to the invention and

FIG. 2 shows an illustration of the measuring cell according to FIG. 1 arranged inside a housing.

The measuring cell 50 shown in FIG. 1 essentially consists of a base body 10, a membrane 20, a spacer 30 arranged between the base body 10 and the membrane 20 and a support body 40 arranged on the side of the membrane 20 opposite the spacer 30 together. The base body 10, the membrane 20 and the carrier body 40 consist of an oxide ceramic, for example based on aluminum oxide. Alternatively, the membrane 20 can also consist of sapphire, a single-crystalline ceramic material, which has a relatively high shape. constant and has no mechanical or thermal hysteresis. The annular spacer 30, on the other hand, is a solder that consists of metal or glass and integrally connects the base body 10 and the membrane 20 in a manner known per se. The membrane 20 and the carrier body 40 are also integrally connected to one another.

Here, however, co-sintering is used, which produces a material bond that is free of additional materials.

As can also be seen in FIG. 1, the base body 10 has two essentially semicircular reference electrodes 11a, 11b, which enclose two essentially sickle-shaped pressure electrodes 12a, 12b. The membrane 20 is provided with reference counter electrodes 21a, 21b corresponding to the reference electrodes 11a, 11b and with pressure counter electrodes 22a, 22b which are congruent with the pressure electrodes 12a, 12b. In this way, the reference electrodes 11a, 11b and the reference counter electrodes 21a, 21b on the one hand and the printing electrodes 12a, 12b and the printing counter electrodes 22a, 22b on the other hand form capacitors, the capacitance of which depends on the respective distance between the electrodes 11a-12b and the counter electrode 21a-22b. The electrodes 11a-12b and the counter electrodes 21a-22b are formed as a few micrometers thick layer of gold or platinum on the base body 10 ^'and the membrane 20th To achieve this, the electrodes 11a-12b and the counter electrodes 21a-22b are applied to the base body 10 and the membrane 20 by means of sputtering, the respective shape of the electrodes 11a-12b and the counter electrodes 21a-22b being provided by appropriate masks for cathode sputtering is caused.

The carrier body 40 has a depression 41 below the membrane 20, which is connected to an opening 42. An edge 43 of the carrier body 40 which surrounds the recess 41 in a circular manner serves for the integral connection with the membrane 20. In this way, the membrane 20 in the recess 41 can be pressurized with a pressure medium, such as air or a liquid. As can be seen in particular in FIG. 2, the measuring cell 50 is arranged inside a housing 60. The carrier body 40 is fastened to a flange 61 on the side facing away from the membrane 20. The flange 61 has a feed line 62 which opens into the opening 42 of the carrier body 40. The pressure medium acting on the membrane can thus flow through the feed line 62 into the recess 41.

Furthermore, it can be seen in FIG. 2 that an evaluation device 70 is arranged on the base body 10, which includes capacitive changes in the measuring cell 50 and forwards them as digital signals via an electrical line 71 for further processing. The evaluation device 70 is also provided with a sensor which detects the temperature of the measuring cell 50 and which likewise transmits the measured temperature as a digital signal via the electrical line 71.

The electrical line 71 is led through an opening 63 of the housing 60, which is hermetically sealed. The measuring cell 50 is also hermetically sealed on the flange 61. In this way, it is possible to provide a protective gas 64 in the interior of the housing 60, which protects the evaluation device 70 against harmful environmental influences, such as moisture.

To determine the absolute pressure of the pressure medium, the change in the capacitance between the electrodes 11a-12b and the counter electrodes 21a-22b is measured by the measuring cell 50 due to a bending of the membrane 20, a vacuum prevailing between the membrane 20 and the base body 10. The averaged capacitance changes between the pressure electrodes 12a, 12b and the pressure counter-electrodes 22a, 22b are compared to the averaged capacitance changes between the outer reference electrodes 11a, 11b and the reference counter-electrodes 21a, 21b in order to obtain a dimensionless index that is proportional to the change of absolute pressure. The evaluation device 70 converts the measured capacitance change directly into a digital signal, for which a so-called sigma / delta modulator can be used, for example.

The pressure sensor described above is used to measure the absolute pressure of a gaseous or liquid pressure medium. The pressure sensor is characterized by a compact and robust design. This is helped by the fact that the measuring cell 50 is protected by the housing 60 and the protective gas 64 present in it from environmental influences which impair the measuring result. Furthermore, a comparatively high accuracy of the pressure sensor is achieved in that the measuring cell 50 is largely mechanically decoupled from the housing 60. This is made possible by the fact that only the carrier body 40 is fastened to the housing 60, whereas the base body 10 and the membrane 20 are arranged inside the housing 60 largely without tension. The fact that the membrane 20 and the carrier body 40 are integrally connected to one another by co-sintering essentially contributes to the largely stress-free arrangement of the membrane 20. Foreign filler materials, which inevitably cause thermal stresses in the joining zone between membrane 20 and carrier body 40, are avoided in this way. In addition, the carrier body 40 prevents the membrane 20 from being subjected to torsion. In this context, it is beneficial that the carrier body 40 is arranged centrally on the flange 61 of the housing 60, as can be seen in FIG. 2.

Due to the largely tension-free mounting of the membrane 20, hysteresis phenomena that falsify the measurement result are avoided. The base body 10, which is separated from the membrane 20 by the spacer 30, serves primarily to generate the electric field between the electrodes 11a-12b and the counter electrodes 21a-22b. Thus, almost no stresses occur in the base body 10. The same applies to the joining zone between the base body 10 and the membrane 20 formed by the spacer 30, which likewise does not impair the measurement result. The base body 10 also enables the evaluation device 70 directly in a simple manner to be arranged on the measuring cell 50. Measurement errors due to a longer signal transmission are therefore excluded. With regard to an accurate measurement result, it is also possible to use the temperature of the measuring cell 50, which is determined by the sensor of the evaluation device 70, to correct the measured absolute pressure. Due to the high thermal conductivity of the measuring cell 50, which mainly consists of an oxide ceramic, and the direct arrangement of the evaluation device 70 and thus of the temperature sensor on the measuring cell 50, the temperature can be detected exactly. Last but not least, the encapsulation of the measuring cell 50 and the evaluation device 70 through the housing 60 and the protective gas 64 present in the latter take into account electromagnetic compatibility (EMC).

LIST OF REFERENCE NUMBERS

Base body a reference electrode b reference electrode a printing electrode b printing electrode

Membrane a reference counter electrode b reference counter electrode a pressure counter electrode b pressure counter electrode

spacer

support body

deepening

opening

edge

measuring cell

casing

flange

Supply line hermetically sealed opening

protective gas

Evaluation device electrical line

Claims

claims

1. Pressure sensor, in particular for the capacitive determination of the absolute pressure, with a base body (10) which has at least one electrode (11a, 11b; 12a, 12b), and with a membrane (20) through a spacer (30) from the The base body (10) is separated and has at least one counter electrode (21a, 21b; 22a, 22b) for generating an electric field between the base body (10) and the membrane (20), the base body (10) and the membrane (20) each consisting of are made of a ceramic material and the membrane (20) is arranged on a carrier body (40) made of a ceramic material and is connected to the carrier body (40) by a material connection that is free of additional materials, characterized by a measuring cell (50 ), which is composed of the base body (10), the membrane (20), the spacer (30) and the support body (40) and is arranged in a hermetic housing (60), with a capacitive on the base body (10) An evaluation device (70) detecting changes in the measuring cell (50) is arranged, which has a sensor that detects the temperature of the measuring cell (50).

2. Pressure sensor according to claim 1, characterized in that the membrane (20) and the carrier body (30) are prefabricated and are interconnected by sintering, preferably co-sintering.

3. Pressure sensor according to claim 1 or 2, characterized in that the carrier body (40) is provided with a recess (41) in which the membrane (20) can be acted upon with a pressure medium.

4. Pressure sensor according to claim 3, characterized in that the carrier body (40) has an opening (42) for the supply of a pressure medium in the recess (41).

5. Pressure sensor according to one of claims 1 to 4, characterized in that the carrier body (40) is attached approximately centrally to a flange (61) of the housing (60).

6. Pressure sensor according to one of claims 1 to 5, characterized in that the housing (60) has a supply line (62) for one

Has pressure medium which is connected to the opening (42) of the carrier body (40).

7. Pressure sensor according to one of claims 1 to 6, characterized by a protective gas surrounding the measuring cell (50) inside the housing (60).

8. Pressure sensor according to one of claims 1 to 7, characterized in that the evaluation device (70) provides digital signals.

9. Pressure sensor according to one of claims 1 to 8, characterized in that the base body (10) with at least one pressure electrode (12a, 12b) and at least one reference electrode (11a, 11b) and the membrane (20) with at least one Pressure counter electrode (22a, 22b) and at least one reference counter electrode (21a, 21 b) are provided.

10. Pressure sensor according to claim 9, characterized in that the base body (10) has two inner pressure electrodes (12a, 12b) which are surrounded by two outer reference electrodes (11a, 11b), and that the membrane (20) has two inner pressure counter-electrodes ( 12a, 12b), which are enclosed by two outer reference counter electrodes (11a, 11b).

11. Pressure sensor according to claim 9 or 10, characterized in that the electrodes (11a, 11b; 12a, 12b) and the counter electrodes (21a, 21b; 22a, 22b) consist of platinum or gold and preferably are applied by sputtering to the base body (10) and the membrane (20).

12. Pressure sensor according to one of claims 1 to 11, characterized in that the spacer (30) is a metal or glass solder which connects the base body (10) and the membrane (20) integrally.

13. Pressure sensor according to one of claims 1 to 12, characterized in that the membrane (20) consists of sapphire.