DE102018130547A1 - Sensor element, method for its production and thermal flow sensor - Google Patents

Abstract

The invention comprises a sensor element (100) for determining a physical property, in particular for measuring the temperature, a measuring medium (2) and / or for at least temporarily heating or cooling the measuring medium (2), comprising a carrier element (110), at least one functional layer (120) applied to the carrier element (110) and structured after the application, which consists of a material with a defined temperature coefficient, and one passivation layer (130) applied to the functional layer (120), the functional layer ( 120) is structured in such a way that it is designed to detect the physical property of the measuring medium (2) and / or to heat or cool the measuring medium (2) by applying an electrical voltage or an electrical current, and a method for producing the same of such a sensor element according to the invention.

Description

The invention relates to a sensor element for determining a physical property, in particular for measuring the temperature, a measuring medium and / or for at least temporarily heating or cooling the measuring medium. The invention further relates to a thermal flow sensor, comprising at least one sensor element according to the invention. Furthermore, the invention relates to a manufacturing method for a sensor element according to the invention.

Structures of sensor elements, for example temperature sensors, heating elements, thermal conductivity sensors and heat capacity sensors, are described in detail in the prior art. Materials with a defined TCR ("Temperature Coefficient of Resistance"), often platinum, are selected as the material for the resistance, when the sensor element is configured as a heating element or as a temperature sensor. The types of sensor elements listed above, and others, are often placed directly in the measuring medium to be examined (e.g. in the form of membrane sensors). These sensor elements are very sensitive, but have the disadvantage that these sensor elements are directly exposed to the medium and can suffer damage from corrosion. In order to protect sensor elements from aggressive and corrosive media, sensor elements can be soldered onto a carrier element that is insensitive to the medium and serves as a protective layer between the sensor element and the measuring medium. The advantage now is that the sensor is protected against damage from corrosion, but has the disadvantage that the thermal contact is poorer compared to a directly used sensor element described above and is therefore less sensitive to the measuring medium and thus a poorer response time having. The thermal contact is worse because the distance between the sensor structure and the medium is comparatively large and the solder layer can contain cavities (voids) and other imperfections due to the soldering process (regardless of whether conventional or vacuum-soldered). It is possible that instead of soldering to the protective layer with the substrate layer, the passivation is to be soldered onto the protective layer (so-called “flip-chip” design). In this way, the thermal contact can be improved - but the imperfections of the soldering process remain.

Based on this problem, the object of the invention is to present a sensor element which can be manufactured reliably and which has a high sensitivity to a measuring medium.

The object is achieved by a sensor element for measuring the temperature of a measuring medium and / or for at least temporarily heating or cooling the measuring medium, comprising a carrier element, at least one functional layer applied to the carrier element and structured after application, which is made of a material with a defined temperature coefficient, and a passivation layer applied to the functional layer, the functional layer having at least one structured functional area which is used to record the temperature of the measuring medium and / or to heat or cool the measuring medium by applying an electrical voltage or an electric current.

The advantage of the sensor element according to the invention is that the sensitivity of the sensor element compared to the known solutions using a carrier element is increased. The thermal contact of the functional layer with the measuring medium, or the Biot number (this is a dimensionless index of thermodynamics and fluid mechanics and gives the ratio of the heat (conductive) resistance of the body to the heat transfer resistance of the surrounding one during heat transport through the surface of a body Medium on) is improved since the distance between the functional layer and the measuring medium is reduced. This is achieved by eliminating a soldering process. This eliminates a solder layer, which could contain imperfections.

The sensor element can serve as a temperature sensor or as a heating element or cooling element. Alternatively, the sensor element can also serve, for example, as a thermal conductivity sensor or as a heat capacity sensor.

The passivation layer consists in particular of a ceramic material and serves to protect the functional layer, in particular against corrosion.

According to an advantageous embodiment of the sensor element according to the invention, it is provided that the carrier element has a first surface facing away from the measuring medium and a second surface facing towards the measuring medium, the functional layer being applied to the first surface, the second surface being contactable with the measuring medium , and wherein the carrier element is in particular a tube or a plate.

According to a preferred development of the sensor element according to the invention, it is provided that the functional layer is structured by means of a laser ablation method. A Laser ablation process refers to a process in which material is removed by a laser. As with 3D lithography, this enables the creation of three-dimensional structures. For this purpose, the focal point of the laser at which the material is removed can be moved in all three spatial directions.

In principle, all structures can be produced with laser ablation, which can also be produced with a lithographic process. In contrast to 3D lithography, the material is not etched away, but is vaporized by the energy supplied by the laser. The application of photoresist and the subsequent etching processes (for example ion etching and / or wet etching) for structuring the functional layer are completely eliminated. A complex multi-step process is reduced to a one-step process. This also has advantages in terms of environmental and health aspects, since aggressive etching materials can be dispensed with. In addition, this method is usually also less expensive than a lithographic method, since only a laser system is required and the lithographic infrastructure (coating systems, exposure systems, etching systems) is eliminated.

Depending on the design of the laser, thick-film processing is also possible.

A preferred embodiment of the sensor element according to the invention provides that the first surface of the carrier element is curved, in particular convex. Accordingly, the functional layer applied to the first surface of the carrier element is curved, corresponding to the curvature of the first surface. In particular, the support element is a tube, and the first surface is accordingly formed by the outer tube wall. By means of the laser ablation method, this curved functional layer can be structured thanks to the focus point of the laser, which can be moved in all spatial directions.

The manufacturing process of the sensor element can hereby be further simplified. It is known from the prior art to mount sensor element on pipe walls, for example by soldering the sensor element onto the pipe wall. The tube wall must be processed before soldering. For example, the curvature of the tube wall must be straightened for the area to which the sensor element is soldered, since the sensor element usually has a flat soldering surface.

In a first variant of the sensor element according to the invention it is provided that the carrier element essentially consists of an electrically non-conductive material, in particular of a plastic or a ceramic material. In this variant, the functional layer is applied directly to the first surface of the carrier element. This minimizes the distance between the functional layer and the measuring medium.

It is advantageous for the sensitivity of the sensor element if the thickness of the carrier element is as small as possible and a material with a high thermal conductivity is used for the carrier element.

In a second variant of the sensor element according to the invention it is provided that the carrier element consists essentially of an electrically conductive material, in particular of a metallic material.

According to an advantageous embodiment of the second variant of the sensor element according to the invention, it is provided that an electrical insulation layer is applied to the carrier element, on which electrical insulation layer the functional layer is applied. The electrical insulation layer prevents a short circuit between the functional layer and the carrier element. The electrical insulation layer can be applied by means of a thin layer or thick layer method and ideally has a small thickness. It is intended to choose a small thickness for the electrical insulation layer in order to increase the distance between the functional layer and the measurement medium only slightly (in comparison to the first variant), so that the sensitivity of the sensor element is only influenced to a small extent.

According to an advantageous embodiment of the first variant of the sensor element according to the invention, it is provided that the electrical insulation layer essentially consists of a ceramic material, in particular aluminum oxide, an oxide layer or plastic. It is advantageous that the electrical insulation layer consists of a material with a high thermal conductivity in order to minimize the influence of the electrical insulation layer on the sensitivity of the sensor element.

According to a preferred embodiment of the sensor element according to the invention, it is provided that the functional layer consists of a metallic material, in particular platinum. It is essential here that the material has a defined TCR (“Temperature coefficient of resistance”) in the event that the sensor element is designed as a heating element or as a temperature sensor.

According to an advantageous embodiment of the sensor element according to the invention, that the structured functional layer has a resistance structure, in particular meandering, and / or a capacitive structure, in particular a comb or a finger structure. The resistance structure is used in a heating element or in a temperature sensor. The current temperature of the measuring medium is determined via the defined change in resistance depending on the temperature.

Alternatively, the functional layer can also be structured as a Peltier element, which is used to cool the measuring medium.

The object is further achieved by a thermal flow sensor for determining the flow velocity of a measurement medium, the thermal flow sensor having at least one sensor element according to the invention, the sensor element being used to detect the temperature of the measurement medium and / or the sensor element being designed to act upon a electrical voltage and / or an electrical current to heat or cool the measuring medium at least briefly.

A thermal flow sensor is used to determine a flow or the flow velocity of a measuring medium or a fluid, for example a gas or gas mixture, thermal flow sensors are known. These take advantage of the fact that a (flowing) measuring medium transports heat away from a heated surface. Thermal flow sensors typically consist of several functional elements, usually at least one low-resistance heating element and one high-resistance element, which serves as a temperature sensor. Alternatively, thermal flow sensors with several low-resistance heating elements are constructed as heaters and temperature sensors. Thermal flow sensors are also known from the prior art, which have only a single element in the form of a combination of a heating element / temperature sensor.

The flow sensor according to the invention is suitable for determining the flow velocity, the mass flow and / or the volume flow of the measuring medium.

• Kalorimetrische thermische Strömungssensoren bestimmen über eine Temperaturdifferenz zwischen zwei Temperatursensoren, welche flussabwärts (engl. „downstream“) und flussaufwärts (engl. „upstream“) von einem Heizelement angeordnet sind, den Durchfluss bzw. die Flussrate des Fluids in einem Kanal. Hierzu wird ausgenutzt, dass die Temperaturdifferenz bis zu einem gewissen Punkt linear zu dem Durchfluss bzw. der Flussrate ist. Dieses Verfahren bzw. die Methode ist in der einschlägigen Literatur ausgiebig beschrieben.

The flow sensor further comprises a voltage / current source, a control unit (for controlling the heat / cooling output) and an evaluation unit, the control unit and the evaluation unit often being combined in an electrical circuit. The flow sensor according to the invention can be operated in different operating modes:

• Calorimetric thermal flow sensors determine the flow or the flow rate of the fluid in a channel via a temperature difference between two temperature sensors, which are arranged downstream of a heating element and upstream. To this end, use is made of the fact that the temperature difference is linear to the flow or the flow rate up to a certain point. This method or method is described in detail in the relevant literature.

Anemometric thermal flow sensors consist of at least one heating element, which is heated during the measurement of the flow. Due to the flow around the heating element with the measuring medium, heat is transported into the measuring medium, which changes with the flow velocity. The flow velocity of the measuring medium can be inferred by measuring the electrical quantities of the heating element.

Flow sensors based on the so-called "time-of-flight" measuring principle have at least one heating element and one temperature sensor. The heating element emits a brief heat pulse into the measuring medium, which causes the measuring medium to heat up locally. The flowing measuring medium causes a movement of the local heating in accordance with the current flow. If the local heating comes close to the temperature sensor, this is detected by the temperature sensor. An evaluation unit determines the time difference between the induction of the heat pulse and the detection of local heating by the temperature sensor. The time difference represents a measure of the flow velocity of the measurement medium. The lower the time difference, the higher the flow velocity of the measurement medium, and vice versa.

Instead of a heating element, a suitable cooling element, for example a Peltier element, can also be used. The operating modes described above can also be carried out using a cooling element; a cooling pulse is induced in the measuring medium.

• - Aufbringen einer funktionalen Schicht auf einem Trägerelement;
• - Strukturieren der funktionalen Schicht derart, dass die die funktionale Schicht zum Erfassen der Temperatur des Messmediums dient; und
• - Aufbringen einer Passivierungsschicht auf der funktionalen Schicht.

Furthermore, the object by a method for producing a sensor element for measuring a physical property, in particular the temperature, and / or for heating a measuring medium, comprising:

• - Applying a functional layer on a carrier element;
• - Structuring the functional layer in such a way that the functional layer serves to record the temperature of the measuring medium; and
• - Application of a passivation layer on the functional layer.

The advantage of the method according to the invention over the production methods known from the prior art is that the functional layer is applied directly, or with an additional electrical insulating layer, to the carrier substrate. A soldering step, which could lead to error susceptibility due to imperfections of the solder layer, is omitted. The invention thus describes a reliable manufacturing method for a sensitive sensor element.

According to an advantageous embodiment of the method according to the invention, it is provided that the functional layer is applied to the carrier element by means of thin-film technology, in particular vapor deposition or sputtering. Alternatively, it can also be provided that the functional layer is applied to the carrier element by means of a suitable thick-film method, for example by means of a screen printing method.

According to a preferred embodiment of the method according to the invention, it is provided that in the event that the carrier element consists essentially of an electrically conductive material, an electrical insulation layer is applied to the carrier element before the functional layer is applied, and the functional layer the electrical insulation layer of the carrier element is applied. The electrical insulation layer is applied to the carrier element by means of a thin layer method, for example vapor deposition, sputtering, etc., or by means of a thick layer method. It is essential that the electrical insulation layer is made as thin as possible and consists of a thermally highly conductive material in order to ensure good heat transfer from the functional layer to the measuring medium, or vice versa.

According to an advantageous development of the method according to the invention, it is provided that the functional layer is structured by means of a laser ablation method, and the functional layer is structured in such a way that, after structuring, it has an electrical resistance structure and / or a capacitive structure. The wavelength and the pulse energy of the laser are chosen in particular in such a way that only the functional layer is removed, but not the underlying electrical insulation layer or the material of the carrier element, in order to avoid short circuits.

According to an advantageous embodiment of the method according to the invention, it is provided that an ultrashort pulse laser is used for the laser ablation method. The pulse width of an ultrashort pulse laser is in the picosecond to femtosecond range. The use of such a laser type minimizes the thermal influence on the functional layer by the structuring process. In addition, the physical properties (TCR, refractive index, etc.) of the functional layer are not changed.

According to a preferred embodiment of the method according to the invention, it is provided that a region of the carrier element on which the functional layer is applied is pre-structured by means of the laser ablation method before the functional layer is applied. For example, the thickness of the carrier element is reduced. In this way, the sensitivity of the sensor element can be increased further.

• 1: Ausführungsbeispiel von Sensorelementen, welche aus dem Stand der Technik bekannt sind; und
• 2: ein Ausführungsbeispiel eines erfindungsgemäßen Sensorelements.

The invention is explained in more detail with reference to the following figures. It shows

• 1 : Embodiment of sensor elements which are known from the prior art; and
• 2nd : An embodiment of a sensor element according to the invention.

1 shows exemplary embodiments for sensor elements 10th , 20th , as they are known from the prior art. With the sensor elements 10th , 20th in this example these are temperature sensors. However, it can be any other type of sensor, for example temperature sensors, heating elements, thermal conductivity sensors or heat capacity sensors, which have a fundamentally similar structure.

In 1a is a cross section of a first sensor element 10th pictured. On a substrate 14 , which consists of a ceramic material, is a functional layer 12 applied by means of a thin layer or thick layer process. The substrate has a thickness of 200 μm, for example. The functional layer 12 has a layer thickness of approx. 800 nm and has a resistance structure, by means of which the temperature of a measuring medium 2nd is recorded. On the surface of the substrate 14 and on the functional layer 12 a passivation layer is also applied, for example consisting of a ceramic material, for example Al ₂ O ₃ , with a layer thickness of 30 μm. For example, there is air above the passivation layer 3rd .

The functional layer is removed using a soldering process 12 opposite side of the substrate 14 on a carrier element 11 upset. The carrier element is one of the measuring medium 2nd flowable pipe; the carrier element 11 is applied to the outer side of the pipe wall. The carrier element 11 consists of a metallic material, for example stainless steel, and has a wall thickness of approx. 150 µm. The connection between the support element 11 and substrate 12 thus takes place by means of a solder layer 15 , which for example has a thickness of less than 10 microns. The substrate 14 acts as an electrical insulation layer between the carrier element 11 (or the solder layer 15 ) and the functional layer 12 .

• Die funktionale Schicht 12 weist einen relativ großen Abstand d₁ zwischen der funktionalen Schicht 14 und dem Messmedium 2 auf. Die Erfassung der Temperatur des Messmediums 2 erfolgt durch mehrere Schichten hindurch, und zwar durch das Substrat 12, durch die Lötschicht 15 und durch das Trägerelement 11. Die Materialen dieser Schichten sollten zwar eine hohe Wärmeleitfähigkeit aufweisen - trotzdem weist ein solches Sensorelement 10 eine vergleichsweise geringe Sensitivität und eine schlechte Ansprechzeit auf. Spezielle Techniken können nur äußerst erschwert angewandt werden, wie beispielsweise die „3-Omega-Methode“, welche der Ermittlung von thermischen Parametern eines Messmediums dient. Hierbei wird das Sensorelement 10 mit einem Treibersignal beaufschlagt und gleichzeitig der Spannungsabfall über dem Sensorelement 10 als Antwortsignal erfasst. Durch einen Vergleich der des Treibersignals mit dem Antwortsignal in der dritten harmonischen Oberschwingung kann auf die thermischen Parameter, beispielsweise die Wärmekapazität und/oder die Wärmeleitfähigkeit, des Messmediums geschlossen werden. Jede zwischen dem Messmedium 2 und der funktionalen Schicht 12 befindliche Schicht erschwert die Anwendung dieser Technik.

Such a sensor element 10th is disadvantageous for several reasons:

• The functional layer 12 has a relatively large distance d ₁ between the functional layer 14 and the measuring medium 2nd on. The detection of the temperature of the measuring medium 2nd takes place through several layers, namely through the substrate 12 through the solder layer 15 and through the support element 11 . The materials of these layers should have a high thermal conductivity - nevertheless, such a sensor element has 10th a comparatively low sensitivity and a poor response time. Special techniques can only be used with extreme difficulty, such as the “3 omega method”, which is used to determine the thermal parameters of a measuring medium. Here the sensor element 10th with a driver signal and at the same time the voltage drop across the sensor element 10th recorded as a response signal. By comparing the driver signal with the response signal in the third harmonic, the thermal parameters, for example the heat capacity and / or the thermal conductivity, of the measurement medium can be inferred. Any between the measuring medium 2nd and the functional layer 12 located layer complicates the application of this technique.

Furthermore, the solder layer tends 15 to imperfections and cavities, which can further reduce sensitivity and response time. Often the solder layer is 15 not reproducible for all batches, so that individual sensor elements 10th can sometimes make a big difference in terms of sensitivity and response times.

1b shows a second example of a sensor element 20th , which is known from the prior art. The sensor element 20th is similar in structure to the sensor element 10th out 1a . The carrier element 21st , the functional layer 22 , the passivation layer 23 and the substrate 24th correspond in the choice of the material and in the layer thicknesses to the equivalent structures of the sensor element 10th out 1a .

• Das Substrat 24 mit den darauf befindlichen Schichten 22, 23 wird gedreht, so dass die Passivierungsschicht 23 mit der Oberfläche des Trägerelements 21 verlötet wird.

The attachment on the support element 21st differs from 1a :

• The substrate 24th with the layers on it 22 , 23 is rotated so that the passivation layer 23 with the surface of the carrier element 21st is soldered.

This makes the distance d ₂ between the functional layer 22 and the measuring medium 2nd compared to the distance d ₁ , which increases the sensitivity somewhat and the response time somewhat. However, the solder layer has 25th same disadvantages as those of the first, in 1a shown example of a sensor element 10th , on.

Both sensor elements 10th and 20th What is common is that the surfaces to be soldered (substrate 14 and passivation layer 23 ) have a flat surface. The surface of the support element 11 , 21st would have to be processed, for example flattened, in particular if one is to be soldered onto a curved surface of a pipe wall.

2nd shows an embodiment of a sensor element according to the invention 100 in the form of a temperature sensor. 2a shows an isometric view of the sensor element 100 . 2 B shows a cross section orthogonal to a flow direction F of the measuring medium 2nd through the support element 110 of the sensor element 100 .

As a carrier element 110 a tube is used, which is from a measuring medium 2nd can be flowed through in a flow direction F. The carrier element 110 consists of a metallic material, in particular stainless steel, and has a wall thickness of 150 µm, for example.

On a first surface facing away from the measuring medium 111 the carrier element is an electrical insulation layer 140 applied by means of a thin layer process. With the electrical insulation layer 140 in this example it is a ceramic material, for example Al ₂ O ₃ , with a layer thickness of approx. 2 µm.

On the electrical insulation layer 140 then becomes a functional layer 120 applied from platinum. The functional layer is applied by means of a thin layer process, for example sputtering or vapor deposition, and has a layer thickness of approximately 800 nm. The electrical insulation layer serves for electrical insulation between the functional layer 120 and the carrier element. The electrical insulation layer 140 and the functional layer 120 have substantially the same convex curvature as the first surface 111 .

In a subsequent process step, the functional layer is structured using a laser ablation process. Here, a laser removes the material of the functional layer successively, so that a resistance structure is created. The resistance structure is used to determine the temperature of the measuring medium. To do this, the resistance value of the resistance structure can be measured. The measured resistance value is a directly proportional measure for the temperature applied to the resistance structure.

Another method for structuring the functional layer is a 3D lithography method. However, the advantage of using the laser ablation method compared to the 3D lithography method is less effort, resulting in lower costs, and increased environmental friendliness.

Then a passivation layer 130 , in particular consisting of a ceramic material, on the functional layer 120 and on the electrical insulation layer 140 applied. This has a layer thickness of approx. 30 µm and serves to protect the functional layer 120 and the electrical insulation layer 140 against environmental influences, such as corrosion.

• - Geringer Abstand d₃ zwischen der funktionalen Sicht 120 und dem Messmedium 2. Dadurch weist das Sensorelement eine vergleichsweise niedrige Ansprechzeit und eine entsprechend hohe Sensitivität auf. Der Wärmeübertrag zwischen der funktionalen Schicht 120 und dem Messmedium, bzw. vice versa, erfolgt lediglich durch das Trägerelement und ggf. durch die elektrische Isolationsschicht 140 hindurch.
• - Durch direktes Auftragen der funktionalen Schicht 140 auf dem Trägerelement kann auf eine Lötschicht, wie diese bei herkömmlichen Sensorelementen verwendet wird, verzichtet werden. Dadurch wird die Serienstreuung verringert, welche sich in einer variablen Ansprechzeit und einer variablen Sensitivität von Sensorelementen untereinander äußert.
• - Vorteilhaft beim Verwenden des Laserablationsverfahrens ist, dass, im Gegensatz zu Sensorelementen, wie diese aus dem Stand der Technik bekannt sind, muss die Oberfläche 111 nicht bearbeitet, beispielsweise begradigt werden. Der Fokuspunkt des Lasers ist dreidimensional verschiebbar.
• - Das Sensorelement 100 eignet sich für die Anwendung spezieller Techniken, wie beispielsweise die weiter oben beschriebene „3-Omega-Methode“.

The advantages of the sensor element according to the invention 100 can be described as follows:

• - Small distance d ₃ between the functional view 120 and the measuring medium 2nd . As a result, the sensor element has a comparatively low response time and a correspondingly high sensitivity. The heat transfer between the functional layer 120 and the measuring medium, or vice versa, takes place only through the carrier element and possibly through the electrical insulation layer 140 through.
• - By applying the functional layer directly 140 a solder layer, such as is used in conventional sensor elements, can be dispensed with on the carrier element. This reduces the series spread, which manifests itself in a variable response time and a variable sensitivity of sensor elements to one another.
• It is advantageous when using the laser ablation method that, in contrast to sensor elements as are known from the prior art, the surface must 111 not processed, e.g. straightened. The focus point of the laser can be shifted three-dimensionally.
• - The sensor element 100 is suitable for the application of special techniques, such as the "3-Omega method" described above.

An alternative embodiment of the sensor element 100 consists in using a non-conductive material for the carrier element 110 . In this case, the electrically insulating layer 140 be dispensed with, so that it is provided the functional layer 10th directly on the support element 110 to apply.

As an alternative to a temperature sensor, the sensor element 100 Train additional sensor types. These differ essentially in the structuring of the functional layer and the subsequent electronic activation of the sensor element. For example, heating elements, thermal conductivity sensors and heat capacity sensors can be formed.

At least one sensor element according to the invention can advantageously be used 100 be used in a thermal flow sensor to determine the flow velocity of the measuring medium 2nd . The sensor element serves as a heating or cooling element for introducing a quantity of heat or a quantity of cold into the measuring medium and / or as a temperature sensor. Provision can be made for two or more sensor elements, in particular arranged in the flow direction 100 use one of the sensor elements 100 is designed as a heating element and is arranged in the center, and in each case one of the sensor elements 100 are designed as temperature sensors and are arranged in the flow direction F upwards and in the flow direction F downwards from the heating element.

Such a thermal flow sensor can be operated in the conventional known operating modes "calorimetric flow measurement", "anemometric flow measurement" and "time-of-flight flow measurement" and has in addition to the sensor elements 100 a control unit, a voltage / current source, an evaluation unit and wiring.

Reference symbol list

10, 20, 100

Sensor element

11, 21, 110

Carrier element

111

first surface

112

second surface

12, 22, 120

functional layer

13, 23, 130

Passivation layer

14, 24, 140

electrical insulation layer

15, 25

Solder layer

2nd

Measuring medium

3rd

air

Claims (17)

Sensor element (100) for determining a physical property, in particular for measuring the temperature, a measuring medium (2) and / or for at least temporarily heating or cooling the measuring medium (2), comprising a carrier element (110), at least one on the Carrier element (110) applied and structured after the application functional layer (120), which consists of a material with a defined temperature coefficient, and a passivation layer (130) applied to the functional layer (120), the functional layer (120) structured in this way is that it is designed to detect the physical property of the measuring medium (2) and / or to heat or cool the measuring medium (2) by applying an electrical voltage or an electrical current. Sensor element (100) after Claim 1 , The carrier element (110) has a first surface (111) facing away from the measuring medium (2) and a second surface (112) facing the measuring medium (2), the functional layer (120) on the first surface (111) is applied, the second surface (112) being contactable with the measurement medium (2), and the carrier element (110) being in particular a tube or a plate. Sensor element (100) after Claim 2 , wherein the functional layer (120) is structured by means of a laser ablation method. Sensor element (100) after Claim 3 , The first surface (111) of the carrier element (110) being curved, in particular convex. Sensor element (100) according to at least one of the preceding claims, wherein the carrier element (110) consists essentially of an electrically non-conductive material, in particular of a plastic or a ceramic material. Sensor element (100) according to at least one of the Claims 1 to 4th , wherein the carrier element (110) consists essentially of an electrically conductive material, in particular a metallic material. Sensor element (100) after Claim 6 wherein an electrical insulation layer (140) is applied to the carrier element (110), on which electrical insulation layer (140) the functional layer (120) is applied. Sensor element (100) after Claim 7 , wherein the electrical insulation layer (140) consists essentially of a ceramic material, in particular aluminum oxide, an oxide layer or plastic. Sensor element (100) according to at least one of the preceding claims, wherein the functional layer (120) consists of a metallic material, in particular platinum. Sensor element (100) according to at least one of the preceding claims, wherein the structured functional layer (120) has a resistance structure, in particular meandering, and / or a capacitive structure, in particular a comb or a finger structure. Thermal flow sensor for determining the flow rate of a measuring medium (2), the thermal flow sensor comprising at least one sensor element (100) according to at least one of the Claims 1 to 10th The sensor element (100) is used to record the temperature of the measuring medium (2) and / or the sensor element (100) is designed to apply the measuring medium (2) at least briefly when an electrical voltage and / or an electrical current is applied warm or cool. Method for producing a sensor element (100) for measuring a physical property, in particular the temperature, and / or for heating a measuring medium (2), comprising: - Application of a functional layer (120) on a carrier element (110); - Structuring the functional layer (120) in such a way that the functional layer (120) is used to record the temperature of the measuring medium (2); and - Applying a passivation layer (120) on the functional layer (120). Procedure according to Claim 12 wherein the functional layer (120) is applied to the carrier element (110) by means of a thin layer technique, in particular vapor deposition or sputtering, or a thick layer technique. Procedure according to one of the Claims 12 or 13 In the event that the carrier element (110) consists essentially of an electrically conductive material, an electrical insulation layer (140) is applied to the carrier element (110) before the functional layer (120) is applied to the carrier element (110), and wherein the functional layer (120) is applied to the electrical insulation layer (140) of the carrier element (110). Method according to at least one of the Claims 12 to 14 , wherein the functional layer (120) is structured by means of a laser ablation method, and wherein the functional layer (120) is structured in such a way that it has an electrical resistance structure and / or a capacitive structure after structuring. Procedure according to Claim 15 , where an ultrashort pulse laser is used for the laser ablation process. Procedure according to Claim 15 or 16 A region of the carrier element (110) on which the functional layer (120) is applied is pre-structured by means of the laser ablation method before the application of the functional layer (120).

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