Ceramic sensor with metallisation layers

Abstract

The present invention relates to a sensor comprising a ceramic substrate (1) and two metallisation layers (2) applied on opposite surfaces of the ceramic substrate (1), wherein the two metallisation layers (2) each comprise an adhesive layer (2A) which is applied directly onto the ceramic substrate (1) and each comprise a gold layer (2B) which is applied to the respective adhesive layer (2A). The invention further relates to a method for producing the sensor.

Description

[0001] Description

[0002] Ceramic sensor with metallization layers

[0003] The present invention relates to a sensor with a ceramic substrate and metallization layers applied thereon and to a method for producing the sensor.

[0004] The constantly increasing demands on temperature sensors with regard to higher operating temperatures combined with particularly high reliability and low manufacturing costs require the use of coordinated, innovative material combinations and manufacturing technologies.

[0005] Currently available sensors are limited in their operating temperature and can usually only be used up to 300 ° C.

[0006] According to the state of the art, temperatures for monitoring and control in a wide variety of applications are measured primarily with ceramic thermistor elements.

[0007] Metallic electrodes must be applied to electrically connect the ceramic. Thick-film metallization is the current state of the art. For example, silver pastes are applied using a screen-printing process followed by firing. Silver metallization is primarily used for soldered connections with connecting wires.

[0008] However, the operating temperature of soldered joints is limited by the melting point of the solder. High-lead solders have a melting point of approximately 300 °C, and most lead-free solders melt at temperatures below 230 °C. Soldered joints are not sufficiently reliable under frequent thermal cycling. Many solder materials are also prone to migration in humid or wet environments.

[0009] Welding wires to the ceramic electrode is not an alternative, as this would result in pre-damage to the electrode or the ceramic. The wires, which are thicker than the electrode layer, require a very high energy input to melt the wire and create the connection. This, in turn, leads to a complete detachment of the electrode layer from the ceramic, and the associated thermal shock can cause cracks in the ceramic or negatively affect the electrical properties.

[0010] An object of the present invention is to describe an improved sensor and a method for manufacturing the sensor .

[0011] The present invention relates to a sensor comprising a ceramic substrate. Metallization layers are applied or deposited on two opposite surfaces of the ceramic substrate. Opposite surfaces are understood here in particular to mean surfaces of the substrate that point in opposite or at least approximately opposite directions. Preferably, the two opposite surfaces are two outer surfaces of the ceramic substrate that are minimally spaced from one another. The two opposite surfaces can be, for example, an underside and a top side of the ceramic substrate or a front side and a back side of the ceramic substrate.

[0012] Each of the two metallization layers comprises an adhesion layer applied directly to the ceramic substrate and a gold layer applied to the respective adhesion layer. In one embodiment, the metallization layers or one of the metallization layers consists of the respective gold layer and the respective adhesion layer. The respective metallization layer then comprises no further layers besides the gold layer and the adhesion layer.

[0013] The fact that the adhesive layer is applied directly to the ceramic substrate means that the adhesive layer is in direct contact with the substrate and there are no further layers between the adhesive layer and the ceramic substrate.

[0014] The metallization layers can partially or completely cover the two opposing surfaces. Preferably, the adhesion layer has at least the surface area of ​​the gold layer, so that the gold layer is applied entirely to the adhesion layer and not directly to the ceramic substrate.

[0015] The gold layer is preferably applied directly to the adhesive layer.

[0016] In one embodiment, the metallization layers are circular. In other embodiments, the metallization layers are designed as finger electrodes. In one embodiment, the metallization layers cover the entire respective surfaces of the ceramic substrate and are thus adapted to the shape of the ceramic substrate. Other suitable geometries are also possible.

[0017] The adhesive layer can act as an adhesion promoter between the gold layer and the ceramic layer. The adhesive layer positioned between the gold layer and the ceramic substrate improves the adhesion of the gold layer to the ceramic substrate. The adhesive layer creates a stable bond with high peel forces between the ceramic substrate and the metallization layer. The sensor is therefore more stable against mechanical stress with the adhesive layer than without the adhesive layer. The longevity of the sensor is also increased compared to a sensor without an adhesive layer, because premature detachment of the gold layer from the ceramic substrate is prevented.

[0018] In one embodiment, the gold layer comprises at least 80 mass percent gold. Furthermore, the gold layer may comprise other metals or metal oxides such as nickel, aluminum, vanadium, chromium, or manganese, and the oxides of these metals. In further embodiments, the gold layer may comprise at least 90 mass percent gold or consist of gold.

[0019] The adhesive layer can preferably be a copper layer. In one embodiment, the copper layer comprises at least 80 mass percent copper. Furthermore, the adhesive layer can comprise other metals or metal oxides such as nickel, aluminum, vanadium, chromium, or manganese, and the oxides of the aforementioned metals. In further embodiments, the copper layer can comprise at least 90 mass percent copper or consist of copper.

[0020] Gold metallization is more stable against chemical reactions than silver metallization. Oxidation or ionization of the gold occurs comparatively rarely. In particular, the selected gold metallization is stable against migration effects, even at high temperatures, which can occur, for example, when using silver metallization. A short circuit between the two metallization layers can thus be avoided.

[0021] Furthermore, the described metallization layers can be used to achieve a constant electrical behavior of the sensor. The sensor's electrical parameters, such as electrical resistance at different temperatures, show very little variance in repeated measurements.

[0022] In particular, the metallization layers function as electrical contact layers, i.e., as external electrodes of the ceramic substrate. The ceramic substrate can be contacted via electrical contact elements such as wires at the metallization layers.

[0023] According to one embodiment, the ceramic substrate is cuboid-shaped. This is understood, in particular, to be a cuboid with rectangular base and side surfaces.

[0024] In this embodiment, the two metallization layers are applied to opposite surfaces of the cuboid-shaped ceramic substrate. A cuboid shape allows for simple, for example, modular, installation of the ceramic substrate in the sensor. A cuboid shape is also easy to manufacture and therefore cost-effective.

[0025] According to one embodiment, the ceramic substrate comprises a thermistor ceramic. The thermistor ceramic can be a thermistor ceramic with a positive temperature coefficient (PTC), i.e., a cold conductor, or a thermistor ceramic with a negative temperature coefficient (NTC), i.e., a hot conductor.

[0026] Alternatively, the ceramic substrate may comprise, for example, a varistor.

[0027] The thermistor ceramic is preferably an NTC ceramic.

[0028] The use of a thermistor ceramic, and in particular an NTC ceramic, enables the sensor to electrically measure an ambient temperature.

[0029] In particular, the use of an NTC ceramic is suitable for detecting even high temperatures with the sensor and converting them into corresponding electrical signals.

[0030] A sensor comprising an NTC ceramic as a ceramic substrate and the metallization layers as defined above, which include adhesion layers, for example, copper layers, and gold layers, is suitable for use at high temperatures. The sensor exhibits unrestricted functionality even at high temperatures. According to one embodiment, the ceramic substrate comprises a ceramic with a perovskite structure.

[0031] The ceramic with perovskite structure is suitable for adjusting the heat conducting properties of the ceramic substrate.

[0032] The ceramic with perovskite structure also has a high temperature resistance, which enables the use of the ceramic substrate even at high temperatures.

[0033] According to a further embodiment, the ceramic of the ceramic substrate can also have a different structure, for example, a spinel structure. A spinel structure can, in particular, also serve to provide a ceramic substrate with heat-conducting properties.

[0034] According to one embodiment, the metallization layers are sputtered on. In particular, the adhesion layers and the gold layers are each sputtered on. For this purpose, during the manufacture of the sensor, the adhesion layer is preferably sputtered on in a first step, and the gold layer is sputtered onto the adhesion layer in a second step.

[0035] A sputtered layer can be significantly thinner than one applied by (screen) printing, for example. This saves material and reduces the cost of sensor production. Furthermore, sensor production is more resource-efficient and environmentally friendly.

[0036] Furthermore, a sputtered layer exhibits increased surface wetting compared to a layer applied by screen printing, for example. This means that the sputtered metallization layers completely or almost completely cover the surface of the ceramic substrate, without holes appearing in the layer where the ceramic substrate is not covered by the metallization layer.

[0037] The advantageous structure of the sputter layer also leads to low variance in the sensor's electrical properties. For example, the electrical resistance of the metallization layer hardly changes at different temperatures and in repeated measurements.

[0038] In particular, for example, the resistance R25 at room temperature (25 °C), the resistance R100 at 100 °C, and the B value B are approximately constant. In particular, the coefficient of variation relative to the resistance R25 or the resistance R100 is a maximum of 1.0; preferably a maximum of 0.9.

[0039] The coefficient of variation related to the B value is preferably a maximum of 0.1.

[0040] The coefficient of variation is defined as the ratio of standard deviation to mean of a series of measurements.

[0041] The B value B of a thermistor is defined as follows: B = 1483.4

[0042] According to one embodiment, the adhesive layer has a

[0043] The layer thickness is preferably 200 nm or less. A layer thickness of this size can be achieved, in particular, by sputtering.

[0044] The adhesive layer is preferably formed as a thin layer.

[0045] The layer thickness mentioned is already sufficient to ensure sufficient adhesion of the metallization layer to the ceramic substrate.

[0046] The layer thickness mentioned is also sufficient to achieve complete surface wetting on the surface of the ceramic substrate , i.e. wetting without holes in the metallization layer .

[0047] By applying the layer thinly, material can be saved.

[0048] According to one embodiment, the gold layer has a maximum layer thickness of 1000 nm.

[0049] Preferably, the maximum layer thickness is 500 nm. Such a small layer thickness can be achieved, in particular, by sputtering.

[0050] The gold layer is preferably formed as a thin layer.

[0051] The stated layer thickness is already sufficient to provide functionality for the metallization layer as an electrical contact layer, i.e., as the sensor's outer electrode. The stated layer thickness is also sufficient to achieve complete surface wetting, i.e., wetting without holes in the metallization layer.

[0052] By applying the layer thinly, material can be saved.

[0053] According to one embodiment, the sensor is a temperature sensor. In this case, embodiments in which the ceramic substrate comprises or consists of a thermistor ceramic, in particular an NTC ceramic, are relevant.

[0054] According to one embodiment, the sensor exhibits unrestricted functionality at temperatures up to 600 °C. In particular, the sensor exhibits unrestricted electrical functionality at these temperatures. This means that the sensor's functionality is not impaired by the elevated temperatures.

[0055] For this purpose, in particular as described above, a suitable choice of the material for the ceramic substrate and for the metallization layers as well as a suitable structure of the ceramic material and the metallization layer, in particular by choosing a suitable application method of the metallization layer, is necessary.

[0056] By choosing a ceramic substrate with a spinel structure instead of a perovskite structure, the sensor exhibits constant functionality at least at temperatures up to 300 °C.

[0057] According to one embodiment, the sensor further comprises electrical contact elements, such as wires. The electrical contact elements, such as the wires, are applied directly to the metallization layers for electrical contacting. In particular, the electrical contact elements or the wires are directly connected to the surface of the metallization layers, in particular the gold layers. The metallization layer borders the wires or at least partially surrounds them.

[0058] According to one embodiment, the electrical contact elements or wires are sintered to the metallization layers. This means that a closed connection between the wires and the metallization layers is created by sintering the metallization layers and a metallization paste surrounding the ends of the wires in a single step.

[0059] For example, the wires are dipped in a gold paste containing gold metal. The paste-coated outer surfaces of the wires are then pressed onto the metallization layers. The sensor, with the pressed wires, is placed in a furnace and subjected to a thermal profile so that the metallization layer is sintered together with the wire.

[0060] The wires can be, in particular, high-temperature-resistant nickel alloy wires. For example, they can be wires made of nickel-chromium alloys, especially Inconel® wires, which are particularly temperature-resistant and thus enable the sensor to be used even at high temperatures. Alternatively, any other suitable materials, such as copper or nickel, can be used for the wires.

[0061] The wires contact the sensor with electronics, which can be used to control the ceramic substrate and / or to process and evaluate electrical signals from the sensor.

[0062] According to one embodiment, the sensor further comprises a glass encapsulation.

[0063] According to one embodiment, the ceramic substrate and the metallization layers are surrounded by the glass encapsulation.

[0064] Preferably, the ceramic substrate and the metallization layers are completely surrounded, i.e., enclosed, by the glass encapsulation. Preferably, the end pieces of the electrical contact elements, for example, the wires that are connected to the metallization layers, are also surrounded or enclosed by the glass encapsulation.

[0065] The glass encapsulation protects the sensor from environmental influences and thus prevents, for example, changes to the surface of the ceramic substrate, the metallization layers or the wire ends due to corrosion.

[0066] Furthermore, the glass encapsulation provides electrical insulation of the sensor from the environment.

[0067] In addition, the glass encapsulation is stable even at high temperatures up to 600 °C. The glass encapsulation can be provided, for example, by means of a preformed glass body, for example in the form of a glass cylinder, which is placed over the rest of the sensor and then deformed, or by immersing the rest of the sensor in a glass paste. The rest of the sensor can, in particular, comprise the ceramic substrate, the metallization layers, and the ends of the wires.

[0068] A glass encapsulation formed in this way can be subjected to a subsequent temperature treatment in order to achieve a hermetically sealed encapsulation.

[0069] The invention further relates to a method for producing a sensor. The sensor can, in particular, be designed as described above. All of the aforementioned features and embodiments can also apply to the method.

[0070] In the process, in a first step, an adhesive layer, e.g. a copper layer, is applied to two opposite surfaces of a ceramic substrate.

[0071] In a second step, two gold layers are applied to the respective bonding layers. Thus, one gold layer is applied to each of the bonding layers.

[0072] By applying the various metal layers, i.e., the bonding layer and the gold layer, in separate steps, the described layer sequence is ensured. For this purpose, the second step is preferably performed after the first step has been completed.

[0073] The gold layer is not in direct contact with the ceramic substrate , but the adhesion of the gold layer to the ceramic substrate is improved by an adhesive layer arranged in between as an adhesion promoter .

[0074] Both layers can be applied in the same production unit and, for example, by the same sputtering device.

[0075] According to one embodiment of the method, the layers are applied by sputtering.

[0076] The invention is described in more detail below with reference to exemplary embodiments and associated figures. The invention is not limited to the exemplary embodiments shown in the figures.

[0077] Figure 1 shows a first embodiment of the sensor in cross section.

[0078] Figure 2 shows an embodiment of the sensor, which further includes wires for electrical contact and a glass encapsulation. The glass encapsulation is shown transparent for illustrative purposes.

[0079] Figure 3 shows another embodiment of the sensor, including wires for electrical contact, during the manufacturing process. A glass cylinder is arranged around the remaining components of the sensor. Similar or apparently identical elements in the figures are provided with the same reference numerals. The figures and the proportions in the figures are necessarily not to scale.

[0080] Figure 1 shows a first embodiment of the sensor. The sensor comprises, in particular, a ceramic substrate 1

[0081] The ceramic substrate 1 preferably comprises a thermistor ceramic. The thermistor ceramic is, for example, an NTC ceramic with a negative temperature coefficient, i.e., a thermistor ceramic.

[0082] The ceramic substrate 1 preferably comprises a ceramic having a perovskite structure.

[0083] The ceramic substrate 1 comprises, for example, a drift-stable perovskite ceramic. In particular, the ceramic substrate 1 can be a ceramic material of the general formula [ SEi- x M II x] [Cri- y-z R y L z ] Oa, where RE stands for one or more rare earth metals, M 11stands for one or more metals of the oxidation state +II, L stands for Al and / or Ga, R stands for one or more metals selected from Fe, Zn, Ge, Sn, and wherein preferably: 0 < x < 1; 0 < y < 1; 0.5 < z < 1; y+z < 1; 0.1 < 1-yz < 0.2.

[0084] In the exemplary embodiment, the ceramic substrate 1 is cuboid-shaped. The cuboid-shaped ceramic substrate has six outer surfaces. The surfaces are rectangular in shape.

[0085] Two metallization layers 2 are applied to two opposite surfaces of the ceramic substrate 1, for example on a front side and a back side or on a bottom side and a top side. For example, a metallization layer 2 is applied on the

[0086] bottom and a metallization layer 2 on the

[0087] Top side of the substrate 1 .

[0088] For example, the metallization layer 2 covers the entire surface of the ceramic substrate 1 to which it is applied. Alternatively, the metallization layers 2 are, for example, circular.

[0089] Alternatively, the metallization layer 2 can also cover only a portion of the corresponding surface of the ceramic substrate 1.

[0090] Both metallization layers 2 each comprise an adhesion layer 2A, in particular a copper layer, and a gold layer 2B.

[0091] The adhesive layer 2A is applied directly to the surface of the ceramic substrate 1. Preferably, the adhesive layer 2A, e.g., the copper layer, is sputtered on. No additional layers are provided between the adhesive layer 2A and the ceramic substrate 1. The adhesive layer exhibits high adhesion to the ceramic substrate 1.

[0092] A gold layer 2B is applied to each of the surfaces of the adhesion layers 2A facing away from the ceramic substrate 1. The gold layer 2B is preferably applied directly to the adhesion layer 2A, preferably by sputtering.

[0093] The adhesive layer 2A acts as an adhesion promoter to ensure sufficient adhesion of the gold layer 2B to the ceramic substrate 1 in order to guarantee stable functionality and a long service life of the sensor.

[0094] Both layers 2A and 2B are preferably applied by sputtering.

[0095] By applying the layers 2 by sputtering, the layers 2 can be thin and made with little material.

[0096] For example, the adhesive layer 2A has a layer thickness of no more than 200 nanometers, preferably no more than 100 nanometers. The gold layer 2B, for example, has a layer thickness of a maximum of 1000 nanometers, preferably no more than 500 nanometers.

[0097] These specified layer thicknesses are sufficient to ensure complete wetting of the ceramic substrate 1 by the metallization layers 2. Complete wetting means, in particular, that there are no holes in the adhesive layer 2A or in the gold layer 2B that are free of the respective metal.

[0098] The metallization layers 2, and in particular the gold layers 2B, act as external electrodes, i.e., as electrical contact layers through which the ceramic substrate 1 can be electrically contacted from the outside. For this purpose, electrical contact elements such as wires 3 are brought into contact with the metallization layers 2.

[0099] Figure 2 shows an embodiment of the sensor, which further comprises wires 3 for electrical contact and a glass encapsulation 4. The sensor is otherwise constructed, for example, according to the first embodiment. The wires 3 are sintered to the surface of the gold layer 2B.

[0100] For this purpose, a contact paste containing gold and other paste components is provided. The other paste components are primarily organic additives that, for example, modify the plasticity of the paste.

[0101] Contact sections of the wires 3, for example their end sections, are immersed in the contact paste and thus wetted with the contact paste. Preferably, the entire outer surface of the corresponding wire sections is wetted with the paste. However, the edge length of the outer surface along the running direction of the wire, which is wetted with the paste, should be shorter than one dimension of the gold layer 2B in the corresponding direction.

[0102] Subsequently, the wires 3 with the wetted contact sections are pressed against the sensor using mechanical tension in a suitable mechanical device and then thermally processed. For this purpose, the mechanical device with the sensor is placed in a furnace and subjected to a thermal heating profile. The wires 3 are thus sintered onto the surfaces of the gold layers 2B.

[0103] Subsequently, as also shown in Figure 2, the sensor is enclosed in a glass encapsulation 4.

[0104] The glass encapsulation 4 can be produced, for example, according to a first method by immersing the remaining sensor with the sintered ends of the wires 3 into a container containing glass paste. The remaining sensor thus comprises, in particular, the ceramic substrate 1, the metallization layers 2, and the ends of the wires 3. The sensors are immersed at a defined speed until the sensor and the wire ends are completely enclosed by the glass paste.

[0105] To ensure bubble-free encapsulation, a defined lateral movement of the sensors within the glass paste occurs. The sensors are then withdrawn from the glass paste at a defined speed. The defined speed is preferably between 0.1 mm / min (millimeters per minute) and 5 mm / min. For example, the speed is 1 mm / min.

[0106] A defined, multi-stage temperature profile is then applied to the sensors coated with glass paste for drying and glazing.

[0107] The drying process must also be carried out with sufficient care to ensure that the glass encapsulation is free of bubbles and defects and that the additives required for the production of the glass paste can escape.

[0108] After drying, the glazing process is carried out, in which the glass encapsulation is heated to a temperature above the glass softening point using a defined temperature profile, thus melting it. The hermetically sealed glass encapsulation is formed during the glazing process. Figure 3 also shows an alternative procedure for producing the glass encapsulation.

[0109] According to the embodiment in Figure 3, a preformed glass cylinder 4A is positioned around the remaining sensor comprising the ceramic substrate 1, the metallization layers 2 and the ends of the wires 3 sintered thereto.

[0110] The sensor is then placed in a furnace with the surrounding glass cylinder 4A, and the glass cylinder 4A is melted in the furnace.

[0111] In case of using a non-pre-sintered glass cylinder, the temperature profile in the furnace may again include several stages to evaporate the organic additives that may be contained in the glass cylinder shape.

[0112] The molten glass can finally be brought into the desired shape in the glazing process and then cooled and hardened.

[0113] The last-described method has the particular advantage that it is particularly cost-effective and can be carried out more easily and quickly.

[0114] Reference symbol list

[0115] 1 ceramic substrate

[0116] 2 Metallization layer 2A Adhesive layer

[0117] 2B gold layer

[0118] 3 wires

[0119] 4 Glass encapsulation

[0120] 4A glass cylinder

Claims

Patent claims 1. Sensor comprising a ceramic substrate (1) and two metallization layers (2) applied to opposite surfaces of the ceramic substrate (1), wherein the two metallization layers (2) each comprise an adhesive layer (2A) applied directly to the ceramic substrate (1) and each comprise a gold layer (2B) applied to the respective adhesive layer (2A).

2. Sensor according to claim 1, wherein the adhesive layer (2A) is a copper layer (2A).

3. Sensor according to one of claims 1 to 2, wherein the ceramic substrate (1) comprises a thermistor ceramic, in particular an NTC ceramic.

4. Sensor according to one of claims 1 to 3, wherein the ceramic substrate (1) comprises a ceramic having a perovskite structure.

5. Sensor according to one of claims 1 to 4, wherein the metallization layers (2) are sputtered on.

6. Sensor according to one of claims 1 to 5, wherein the adhesive layer (2A) has a layer thickness of at most 200 nm, preferably at most 100 nm.

7. Sensor according to one of claims 1 to 6, wherein the gold layer (2B) has a layer thickness of at most 1000 nm, preferably at most 500 nm.

8. Sensor according to one of claims 1 to 7, wherein the sensor is a temperature sensor.

9. Sensor according to one of claims 1 to 8, wherein the sensor has unrestricted functionality at temperatures up to 600 °C.

10. Sensor according to one of claims 1 to 9, wherein wires (3) for electrical contacting are sintered directly with the gold layers (2B).

11. Sensor according to one of claims 1 to 10, wherein the ceramic substrate (1) and the metallization layers (2) are surrounded by a glass encapsulation (4).

12. A method for producing a sensor, wherein in a first step an adhesive layer (2A) is applied to two opposite surfaces of a ceramic substrate (1) and in a second step two gold layers (2B) are applied to the respective adhesive layers (2A).

13. The method according to claim 12, wherein the adhesion and gold layers (2A, 2B) are applied by sputtering.

14. The method according to any one of claims 12 or 13, wherein the second step is performed after the first step has been completed.

15. Method according to one of claims 12 to 14, wherein in a third step wires for electrical contacting are sintered directly with the gold layers (2B).

16. Method according to one of claims 12 to 15, wherein in a further step a glass encapsulation (4) is formed around the remaining sensor.

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