Sensor element for ascertaining at least one physical or chemical measurement variable, and sensor arrangement

Abstract

The invention relates to a first variant of a sensor element for ascertaining at least one physical or chemical measurement variable, comprising: - a planar substrate (1) having a first surface and a second surface opposite the first side; - one or more sensor structures (2), which are applied to the first surface of the substrate (1) or on an insulation layer applied to the first surface of the substrate; - a passivation layer (3) that at least partially covers the sensor structure (2) or the sensor structures (2); at least two electrical contact surfaces (4) that are each connected to the sensor structure (2); - a spacer layer (5) applied to one or more first portions of the second surface of the substrate (1); and - a sinterable and / or solderable metal layer (6) applied to one or more second portions of the second surface of the substrate (1) and / or on the space layer (5), wherein a layer thickness of the spacer layer (5) is greater than or equal to a layer thickness of the sinterable and / or solderable metal layer (6). The invention also relates to a second variant of a sensor element and to a sensor arrangement which comprises at least one sensor element of this kind.

Description

[0001] Sensor element for detecting at least one physical or chemical measured variable and sensor arrangement

[0002] The invention relates to a first and second variant of a sensor element for detecting at least one physical or chemical measurement variable. Furthermore, the invention relates to a sensor arrangement comprising one or more sensor elements according to the invention.

[0003] Sensor elements are used in a wide variety of applications and, depending on their design, are used to measure temperature, flow, gas concentration or composition, humidity, pH, and / or biological variables, but can also be used as heating elements, for example. For each specific application, the sensor elements often need to be connected, either directly to other sensor elements or to surfaces, for example, directly to the surface to be measured when using a sensor element as a surface probe. For today's state-of-the-art sensor elements, various processes are used for this purpose, such as soldering or gluing (using a polymer- or ceramic-based adhesive).

[0004] However, these processes used so far have several disadvantages:

[0005] For example, during soldering, mechanical stresses can arise when the solder solidifies. These stresses depend on the difference in the thermal expansion coefficient of the joining partners, the difference between the joining partner and the solder, as well as the expansion differences of intermetallic phases in the solder structure. These stresses can influence the temperature coefficient of resistance (TCR), the resistance offset, and thus the accuracy of the element. Furthermore, flux can poison the sensor element. Furthermore, temperature resistance is limited to application areas typically below 280 °C. A major disadvantage is the hysteresis behavior of solder joints, which makes it even more difficult to reproduce small deviations in the sensor signal. Soldering processes are particularly unsuitable for applications that require high accuracy with a fast response time.

[0006] For applications in critical atmospheric environments (e.g., exposure to humidity, extreme temperature changes, and elevated temperatures), it is necessary to clean the circuit board and solder joints of flux residue, as this could lead to failures later in the application. This process usually requires chemicals and is an additional step that is associated with costs.

[0007] When using the adhesive mentioned above, the phase transition or shrinkage during curing can induce stress in the sensor element and reduce accuracy. Adhesive bonds also cause higher contact resistances, which change over time and thus exhibit drift in long-term performance. The adhesive bond is also brittle, and adhesion is limited depending on the substrate (this only applies to bonding with glass and ceramic adhesives).

[0008] In addition, both during soldering and bonding, different expansion coefficients between the adhesive or solder can induce stresses when temperatures change, thus altering the TCR of the sensor element. Adjusting the TCR is not always possible and is very complex. Furthermore, long-term stability at higher temperatures is very limited.

[0009] Silver sintering processes are used to reduce or even completely prevent these disadvantages. Targeted temperature measurement is particularly necessary in printed circuit board technology for the production of electrical assemblies in the high-temperature range with active and passive components (including integrated semiconductor modules). However, a reproducible and, above all, drift-free connection is also a prerequisite for connecting sensor elements to metallic surfaces, such as stainless steel tubes with resistance elements for flow measurement. Methods are known from the prior art, for example from WO 2020 / 057859 A1 and DE 10 2010 050 315 C5, which allow the silver sintering of a passive or active component (including resistance elements or sensor elements) onto a carrier element.However, these components have a flat underside, which means that the component and carrier are sintered onto the substrate at an undefined and imprecise distance. This can also lead to the component being mounted at an angle. This has, among other things, a direct impact on the thermal bond and, above all, a very significant influence on the shear strength of the joint. This results in the major disadvantage (which also applies to soldering and gluing) that the manufacturing process is not reproducible. This means that the thermal transition is always different and can sometimes be better or worse. The same applies to the induced voltage, which causes an offset during the joining process. However, these disadvantages are extremely important for manufacturing, so that a sufficient yield can be achieved and the components can be manufactured at all.

[0010] Based on the problems listed, the object of the invention is to provide a sensor element which allows a firm and thermally stable thermal contact with a carrier element.

[0011] The object is achieved by a sensor element according to claim 1, a sensor element according to claim 2, and by a sensor arrangement according to claim 11.

[0012] With regard to a first sensor element according to the invention, it is provided that this serves to detect at least one physical or chemical measurement variable, wherein the sensor element comprises:

[0013] - a planar substrate having a first surface and a second surface opposite the first side;

[0014] - one or more sensor structures applied to the first surface of the substrate or to an insulation layer applied to the first surface of the substrate;

[0015] - a passivation layer at least partially covering the sensor structure(s); - at least two electrical contact surfaces each connected to the sensor structure;

[0016] - a spacer layer applied to one or more first partial regions of the second surface of the substrate; and

[0017] - a sinterable and / or solderable metallic layer applied to one or more second partial regions of the second surface of the substrate and / or to the spacer layer, wherein a layer thickness of the spacer layer is greater than or equal to a layer thickness of the sinterable and / or solderable metallic layer.

[0018] With regard to a second sensor element according to the invention, it is provided that this serves to detect at least one physical or chemical measurement variable, wherein the sensor element comprises:

[0019] - a planar substrate having a first surface and a second surface opposite the first side;

[0020] - one or more sensor structures applied to the first surface of the substrate or to an insulation layer applied to the first surface of the substrate;

[0021] - a passivation layer at least partially covering the sensor structure or structures;

[0022] - at least two electrical contact surfaces each connected to the sensor structure;

[0023] - a sinterable and / or solderable metallic layer applied to one or more second partial regions of the second surface of the substrate or to the entire second surface of the substrate; and

[0024] - a spacer layer applied to one or more first partial regions of the second surface of the substrate and / or at least partially to the sinterable and / or solderable metallic layer, wherein the layer thicknesses of the spacer layer and the sinterable and / or solderable metallic layer are selected such that a distance between the surface of the spacer layer and the second surface of the substrate is greater than or equal to a distance between the surface of the sinterable and / or solderable metallic layer and the second surface of the substrate. The sensor elements according to the invention thus each have structures for silver sintering or soldering, which allow the stress-free, thermal, high-temperature-resistant, simple, fast, and on-site connection of the substrate to a suitable surface.The spacer layer on the second surface of the substrate can define the volume of silver sintering or soldering paste in the subsequent silver sintering or soldering process. Furthermore, if designed appropriately (e.g., when using two or more first partial areas), the spacer layer can also prevent the sensor element from tilting during soldering or silver sintering.

[0025] The spacer layer has, for example, a height of 5 to 400 pm, preferably 5 to 150 pm.

[0026] The two variants of the sensor element according to the invention differ in their layer structure. While in the first variant, the spacer layer is applied first, followed by the sinterable and / or solderable metallic layer, this is reversed in the second variant. Here, the sinterable and / or solderable metallic layer is applied first, followed by the spacer layer. This means that the layer thicknesses of both layers must be selected differently for each variant to ensure a defined distance or space for the solder or paste.

[0027] According to an advantageous embodiment of the sensor element according to the invention, the sinterable and / or solderable metallic layer consists of gold, platinum, copper, nickel, chromium, glass, ceramic, titanium, palladium, or a combination of the aforementioned materials. Combinations of the aforementioned materials can be, for example, NiAu, NiCrNiAu, NiPdAu, CrPtAu, TiPtAu, AgPd, AgPdPt, or AuPd.

[0028] According to an advantageous embodiment of the sensor element according to the invention, the spacer layer is made of metal, polymer, glass, ceramic, or a combination of the aforementioned materials. According to an advantageous embodiment of the sensor element according to the invention, the spacer layer is three-dimensionally structured. This means that the individual parts of the spacer layer, applied according to the number of first subregions, have a structure within themselves. This structure can be columnar, for example.

[0029] According to an advantageous embodiment of the sensor element according to the invention, it is provided that one or more third partial regions of the second surface are provided which are free of the sinterable and / or solderable metallic layer and the spacer layer.

[0030] According to an advantageous embodiment of the sensor element according to the invention, the sensor structure or structures are designed such that the sensor element can be used as a temperature sensor, a flow sensor, a gas sensor, a humidity sensor, a heating element, a pH sensor, and / or a biosensor. Other sensor applications not listed here are also conceivable.

[0031] According to an advantageous embodiment of the sensor element according to the invention, the one or more sensor structures are made of a metallic material, in particular platinum, and are applied to the first surface of the substrate, or to the insulation layer, using a thin-film or thick-film process. For example, a screen printing process can be used as the thick-film process. A PVD or CVD process can be selected as the thin-film process, for example.

[0032] According to an advantageous embodiment of the sensor element according to the invention, it is provided that the spacer layer is applied by means of a thick-film process.

[0033] According to an advantageous embodiment of the sensor element according to the invention, the sinterable and / or solderable metallic layer is applied using a thick-film or thin-film process. With regard to the sensor arrangement, it is provided that it comprises one or more sensor elements according to the invention and a carrier element with a metallic surface, wherein the sensor element(s) are connected to the carrier element by silver sintering or soldering, wherein a silver sintered or soldered layer is arranged between the metallic surface of the carrier element and the sinterable and / or solderable metallic layer of the sensor element(s).

[0034] According to an advantageous embodiment of the sensor arrangement according to the invention, the carrier element is a printed circuit board, wherein the metallic surface is formed by a metallization applied to the printed circuit board, wherein the metallization consists of one or more metallic materials. Instead of a printed circuit board, a carrier element made of a ceramic material can also be used, which also has a metallic surface formed by such a metallization.

[0035] According to an advantageous embodiment of the sensor arrangement according to the invention, the carrier element is a tube or plate made of a metallic material. In this case, the carrier element directly has one or more suitable metallic surfaces.

[0036] The invention is explained in more detail with reference to the following figures.

[0037] Fig. 1: an embodiment of a sensor arrangement according to the invention, in which a sensor element is applied to a carrier element.

[0038] The sensor element consists of a planar substrate 1, which is made, for example, from a ceramic material, a metallic material, or a semiconductor material. A sensor structure 2 is applied to a first surface of the substrate 1. If the substrate 1 is made of a metallic material, an insulating layer is applied between the first surface and the sensor structure 2 to electrically insulate the sensor structure 2 from the metallic substrate 1. To protect it from environmental influences, e.g., mechanical and / or chemical stresses, the sensor structure 2 is at least partially covered with a passivation layer 3.

[0039] Sensor structure 2 is a resistance structure or an electrode structure. The sensor element can therefore be operated as a temperature sensor or a heating element. However, depending on the number and design of the sensor structures 2, the sensor element can be of various other types, including a temperature sensor, a flow or current sensor, a gas sensor, a humidity sensor, a pH sensor, a biosensor, etc.

[0040] The sensor structure 2 is conductively connected to at least two electrical contact surfaces 4. The sensor structure can be controlled or electrically operated via the conductive surfaces 4, for example, with an external control and evaluation unit.

[0041] For surface mounting on any surface, e.g. any of a conductor track or another carrier element 8, the sensor element has a special structuring on a second surface of its substrate 1:

[0042] On the second surface of the substrate, at least a portion of this surface, or even several portions, are covered with a sinterable and / or solderable metallic layer 6. This layer is applied, for example, using a thin-film process and serves as an anchor point for the paste or solder during the subsequent silver sintering or soldering process. The sinterable and / or solderable metallic layer 6 can, for example, consist of one or more materials from the group: NiAu, NiCrNiAu, NiPdAu, CrPtAu, TiPtAu, AgPd, AgPdPt, AuPd, Au, or Cu.

[0043] In addition, spacer layers 5 are applied to one or more second partial regions of the second surface of the substrate 1. These spacer layers may consist of metal, polymer, glass, ceramic, or combinations thereof and are applied by means of a thick-film or thin-film process. Such a spacer layer 5 is thicker than the sinterable and / or solderable metallic layer 6. The volume of the solder or paste can be determined by the height difference between these two layers (sinterable and / or solderable metallic layer 6 and spacer layer). Furthermore, the spacer layer 5, or several of these spacer layers

[0044] 5 ensures that the sensor element can be positioned parallel to the surface being sintered or soldered during soldering or silver sintering. The material selection of the spacer layer 5 also determines whether the spacer layer 5 bonds with the solder or paste, or whether no additional mechanical adhesion is generated. For example, ventilation channels can be specifically structured, allowing volatile reaction components to escape during the process and resulting in a better-quality connection. Furthermore, mechanical stress distributions can be specifically relocated, decoupled, or even significantly reduced.

[0045] The sensor element can be connected to a carrier element 8, for example on metallic materials but also on printed circuit boards, as shown in Fig. 1, by means of soldering or silver sintering 8. On printed circuit boards, the silver sintering takes place on a metallization as a metallic surface, which consists of one or more metals.

[0046] It can also be provided that the sinterable and / or solderable metallic layer

[0047] 6 the spacer layer 5 is completely or partially covered, for example by completely coating the entire back of the substrate 1 including the spacer layer 5 with a thin layer.

[0048] It can also be provided that the sinterable and / or solderable metallic layer 6 is applied first, followed by the spacer layer 5. If the spacer layer 5 covers the sinterable and / or solderable metallic layer 6, the spacer layer 5 does not have to be thicker than the sinterable and / or solderable metallic layer 6. However, care must be taken to ensure that the distance between the surface of the spacer layer 5 and the second surface of the substrate 1 is greater than or equal to the distance between the surface of the sinterable and / or solderable metallic layer 6 and the second surface of the substrate 1, so that a volume for the solder or paste is created.

[0049] Some advantages of the inventive solution are described below:

[0050] - Lower drift and hysteresis behavior (compared to a soldering process);

[0051] - a mechanical decoupling to avoid stresses due to temperature gradients and different expansion coefficients between the sensor element and the carrier element 8;

[0052] - Good thermal transition when using a silver paste;

[0053] - Temperature resistance up to min. 400°C;

[0054] - No aggressive substances or other elements are required that can poison the sensor;

[0055] - Can be carried out on-site at the customer's premises - for silver sintering applications, there are now process systems that are also suitable for mass production.

[0056] - Greater process reliability and reproducibility: Not only a defined distance, but also defined volumes can be established. This also allows for more precise definition of the heat capacity and heat transfer, which can be advantageous in flow applications, for example.

[0057] List of reference symbols

[0058] 1 substrate

[0059] 2 sensor structure(s)

[0060] 3 Passivation layer

[0061] 4 Electrical contact surfaces

[0062] 5 spacer layer

[0063] 6 sinterable and / or solderable metallic layer

[0064] 7 metallic surface 8 carrier element

Claims

Patent claims Sensor element for detecting at least one physical or chemical measurement variable, comprising: - a planar substrate (1) having a first surface and a second surface opposite the first surface; - one or more sensor structures (2) which are applied to the first surface of the substrate (1) or to an insulation layer applied to the first surface of the substrate (1); - a passivation layer (3) at least partially covering the sensor structure (2) or the sensor structures (2); - at least two electrical contact surfaces (4) each connected to the sensor structure (2); - a spacer layer (5) applied to one or more first partial regions of the second surface of the substrate (1); and - a sinterable and / or solderable metallic layer (6) applied to one or more second partial regions of the second surface of the substrate (1) and / or to the spacer layer (5), wherein a layer thickness of the spacer layer (5) is greater than or equal to a layer thickness of the sinterable and / or solderable metallic layer (6). A sensor element for detecting at least one physical or chemical measurement variable, comprising: - a planar substrate (1) having a first surface and a second surface opposite the first surface; - one or more sensor structures (2) which are applied to the first surface of the substrate (1) or to an insulation layer applied to the first surface of the substrate (1); - a passivation layer (3) at least partially covering the sensor structure (2) or the sensor structures (2); - at least two electrical contact surfaces (4) each connected to the sensor structure (2); - a sinterable and / or solderable metallic layer (6) applied to one or more second partial regions of the second surface of the substrate (1) or to the entire second surface of the substrate; and - a spacer layer (5) applied to one or more first partial regions of the second surface of the substrate (1) and / or at least partially to the sinterable and / or solderable metallic layer (6), wherein the layer thicknesses of the spacer layer and of the sinterable and / or solderable metallic layer (6) are selected such that a distance between the surface of the spacer layer (5) and the second surface of the substrate (1) is greater than or equal to a distance between the surface of the sinterable and / or solderable metallic layer (6) and the second surface of the substrate (1).

3. Sensor element according to claim 1 or 2, wherein the sinterable and / or solderable metallic layer (6) consists of gold, platinum, copper, nickel, chromium, glass, ceramic, titanium, palladium or a combination of the aforementioned materials.

4. Sensor element according to one of the preceding claims, wherein the spacer layer (5) consists of metal, polymer, glass, ceramic or a combination of the aforementioned materials.

5. Sensor element according to one or more of the preceding claims, wherein the spacer layer (5) is structured three-dimensionally, in particular columnar.

6. Sensor element according to one or more of the preceding claims, wherein one or more third partial regions of the second surface are provided which are free of the sinterable and / or solderable metallic layer (6) and the spacer layer (5).

7. Sensor element according to one or more of the preceding claims, wherein the sensor structure (2) or the sensor structures (2) are designed such that the sensor element can be used as a temperature sensor, a flow sensor, a gas sensor, a humidity sensor, a heating element, a pH sensor and / or a biosensor.

8. Sensor element according to one or more of the preceding claims, wherein the one or more sensor structures (2) consist of a metallic material, in particular platinum, and are applied to the first surface of the substrate (1) or to the insulation layer by means of a thin-film or thick-film process.

9. Sensor element according to one or more of the preceding claims, wherein the spacer layer (5) is applied by means of a thick-film process.

10. Sensor element according to one or more of claims 1 to 8, wherein the sinterable and / or solderable metallic layer (6) is applied by means of a thick-film or thin-film process.

11. Sensor arrangement, comprising one or more sensor elements according to one or more of the preceding claims, and a carrier element (8) with a metallic surface (7), wherein the sensor element, or the sensor elements, is connected to the carrier element (8) by means of silver sintering or soldering, wherein a silver sintered or soldering layer is arranged between the metallic surface (7) of the carrier element (8) and the sinterable and / or solderable metallic layer (6) of the sensor element, or the sensor elements.

12. Sensor arrangement according to claim 11, wherein the carrier element (8) is a printed circuit board, wherein the metallic surface (7) is formed by a metallization applied to the printed circuit board, wherein the metallization consists of one or more metallic materials.

13. Sensor arrangement according to claim 11, wherein the carrier element (8) is a tube or plate consisting of a metallic material.

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