Gas sensor for measuring the concentration of an analysis gas

Abstract

Gas sensor (1) for measuring the concentration of an analysis gas based on a thermal conductivity principle, comprising at least one analysis heating element arranged on a first membrane (2) for heating the analysis gas, a reference heating element arranged on a second membrane (4) for heating a reference gas, and at least one evaluation electronics unit (28) for measuring a change in resistance of the analysis heating element caused by the analysis gas relative to an electrical resistance of the reference heating element, characterized in that the first membrane (2) and the second membrane (4) are arranged adjacent to each other in a sensor substrate (6), wherein a measuring volume (12) and a reference volume (16) can be formed between the first membrane (2) and the base substrate (8) by means of a base substrate (8) arranged on one side of the sensor substrate (6).wherein the reference volume (16) is fluidly connected to an adjacent environment via an opening (18) in the second membrane (4), wherein the analyzer gas can be introduced into the measuring volume (12) through at least one opening (14) in the base substrate (8), and wherein a cap substrate (20) is arranged on a side of the sensor substrate (6) opposite the base substrate (8), the cap substrate being configured such that a connecting opening is formed from the environment of the gas sensor (1) to the reference volume (16).

Description

[0001] The invention relates to a gas sensor for measuring the concentration of an analysis gas based on a thermal conductivity principle, comprising at least one analysis heating element arranged on a first membrane for heating the analysis gas, a reference heating element arranged on a second membrane for heating a reference gas, and at least one evaluation electronics unit for measuring a change in resistance of the analysis heating element caused by the analysis gas relative to an electrical resistance of the reference heating element. State of the art

[0002] In gas sensors that operate on resistive measurement principles, the gas or gas mixture being measured directly influences the conductivity of a gas-sensitive sensor element. This change in resistance serves as the measurement parameter for the concentration of the gas or gas mixture. The gas-sensitive sensor element can be a sensor layer or a heating element. For example, one or more heating elements in the form of platinum heaters can be arranged on a membrane. These heating elements can be operated with a constant current or constant power and can be warmer than the ambient temperature.

[0003] For example, the higher thermal conductivity of hydrogen (1810 µW / cmK) compared to air (260 µW / cmK) can be used to measure hydrogen concentration. If hydrogen is present in the vicinity of the heating element, its higher thermal conductivity and the resulting greater heat dissipation cause the element's temperature to decrease, thus reducing its resistance. This change in resistance, and therefore the additional heating power required to maintain the element at a constant temperature, is proportional to the hydrogen concentration. Since thermal conductivity depends on the ambient temperature, the ambient temperature can be measured using an additional temperature sensor.

[0004] Furthermore, it is known that the resistance of the heating element changes due to the temperature coefficient of the heating element material when the ambient temperature changes or due to different operating voltages. Using suitable evaluation algorithms, hydrogen can, for example, be distinguished from humidity.

[0005] For a gas sensor to reliably and accurately measure the concentration of a gas, changes in the electrical resistance of the heating element due to variations in ambient temperature, humidity, and operating voltage must be taken into account. This can result in complex evaluation electronics and a larger sensor size. Disclosure of the invention

[0006] The object underlying the invention can be seen as proposing a compact gas sensor which can perform precise concentration measurements of at least one gas independently of external conditions or aging effects.

[0007] This problem is solved by means of the respective subject matter of the independent claims. Advantageous embodiments of the invention are the subject matter of dependent claims.

[0008] According to one aspect of the invention, a gas sensor for measuring the concentration of an analysis gas is provided based on a thermal conductivity principle. The gas sensor has at least one analysis heating element arranged on a first membrane for heating the analysis gas. A reference heating element for heating a reference gas is arranged on a second membrane. Evaluation electronics serve to measure a change in the resistance of the analysis heating element caused by the analysis gas relative to an electrical resistance of the reference heating element. According to the invention, the first membrane and the second membrane are arranged adjacent to each other in a sensor substrate, wherein a measuring volume can be formed between the first membrane and the base substrate by means of a base substrate arranged on one side of the sensor substrate, and a reference volume can be formed between the second membrane and the base substrate.

[0009] To enable dimensioning of the gas sensor or a chip area of ​​the gas sensor, an additional wafer or a base substrate is attached to the underside of the sensor substrate to create a reference volume containing no or only a defined amount of hydrogen gas, water vapor, or a reference gas. By using the gas sensor as a dual-membrane chip with a concealed reference volume relative to the gas to be detected, the dimensions of the gas sensor can be significantly reduced. The underside of the base substrate can, for example, be used as a mounting surface or as a substrate for additional functionalities.

[0010] By using a reference volume, manufacturing variations in resistance can be compensated for, as these variations affect both the reference and measurement volumes equally. This relationship can result, in particular, from the simultaneous and almost concurrent manufacturing of both components of the gas sensor. Due to the direct proximity of the first membrane to the second membrane, the heating elements are also equally affected by these variations. Furthermore, temperature changes and changes in other parameters, such as humidity, can directly affect both heating elements in both volumes, meaning that deviations in the boundary conditions of a reference measurement and an analysis measurement do not need to be considered by the evaluation electronics. The sensor substrate can consist of a doped or undoped semiconductor, such as silicon, a glass, a plastic, or a ceramic.

[0011] According to one embodiment of the gas sensor, the measuring volume and / or the reference volume are formed at least partially within the sensor substrate and / or at least partially within the base substrate. For example, a reference volume and a measuring volume can be introduced into the sensor substrate by material removal and at least partially sealed with a base substrate. Alternatively or additionally, the reference volume and / or the measuring volume can extend at least partially into the base substrate.

[0012] In another embodiment of the gas sensor, the reference volume is either open or closed. For example, the reference volume can be fluid-conductingly connected to an adjacent environment via an opening in the second membrane. Alternatively, the reference volume can be sealed and contain a defined reference gas. This allows for more precise concentration measurements of an analytical gas.

[0013] According to another embodiment of the gas sensor, the analysis gas can be introduced into the measuring volume through at least one opening in the base substrate. In the simplest embodiment, the analysis gas can be introduced into the measuring volume via a bore. In addition, inlet and outlet openings can also be used for a continuous flow of the analysis gas. The openings can be created in the base substrate, for example, by etching, trenching, laser cutting, milling, or drilling. The at least one opening can be structured and shaped for optimized flow. In particular, the at least one opening can be designed to provide particle, moisture, and water protection for the measuring volume. For this purpose, for example, a water-repellent coating of the at least one opening can reduce the adhesion of ambient moisture.The at least one opening can be dimensioned and shaped such that gas flow through it is laminar. This allows for reproducible, defined boundary conditions for the heat conduction equation or heat sink of the gas sensor's heating elements. Furthermore, this increases the comparability and reproducibility of the measurement results.

[0014] In another embodiment of the gas sensor, at least one heating element can be applied to or embedded in the base wafer. This allows the base substrate to be further functionalized. For example, additional heating elements or heating traces can be used to heat the reference volume and / or the measurement volume. This allows the gas sensor to reach operating temperature more quickly or to defrost more rapidly. Furthermore, additional heating can prevent condensation of atmospheric humidity in the measurement volume. With a ceramic substrate, a sintered heating coil or a heating coil printed using a thick-film process can be used for high power output. Additional heating elements can also maintain the reference volume and the measurement volume at the same temperature, thus increasing comparability.For low heating power, a silicon or glass substrate with a platinum resistance heating element can be used. Alternatively, doped semiconductors such as silicon carbonate with a resistance heating element made of tungsten, silver, gold, copper, or aluminum can also be used.

[0015] In another embodiment of the gas sensor, at least one gas filter is arranged in the measuring volume. The measuring volume can be coated with a functional layer. The functional layer can, for example, be a getter material that can bind certain unwanted gases or particles, at least temporarily. This allows interfering or measurement-falsifying components in the analysis gas to be filtered out.

[0016] According to another embodiment of the gas sensor, at least one gas filter is arranged on one side of the base substrate. A specially functionalized layer can be applied to the base substrate to filter specific types of gas. This prevents unwanted gases from entering the measuring volume and thus reaching the first membrane through the at least one opening. For example, certain gases can react at a getter layer and be introduced into a solid structure. Alternatively or additionally, the gas filter can span or cover the at least one opening of the measuring volume to prevent, for example, the ingress of water vapor or other unwanted gases or particles. This ensures the separation of water and hydrogen.

[0017] In another embodiment of the gas sensor, the base substrate can be connected to the sensor substrate via an adhesive. The base substrate can be attached to the sensor substrate, for example, by a glass-frit bond, an anodic bond, a eutectic bond, a solder joint, or adhesive bonding. The base substrate can thus be connected to the sensor substrate using a variety of possible methods.

[0018] According to a further embodiment of the gas sensor, the base substrate has a bonding surface on one side for receiving an adhesive or sealant. Due to the reduced dimensions of the gas sensor and the high degree of planarity of the substrates, the underside of the base substrate can be used as a bonding surface. In particular, the base substrate can be bonded or sealed for effective protection against humidity and other environmental influences.

[0019] In a further embodiment of the gas sensor, a cap substrate is arranged on the side of the sensor substrate opposite the base substrate. To increase the mechanical stability of the gas sensor, an additional cap substrate can be arranged on the sensor substrate. In particular, the cap substrate can be pre-positioned on the sensor substrate to provide a stable base for further processing steps in the manufacture of the gas sensor. Preferably, the cap substrate has recesses in the area of ​​the first and second membranes. These recesses, like the reference volume and the measurement volume, can be created in the cap substrate by material removal.

[0020] According to a further embodiment of the gas sensor, at least part of the evaluation electronics is arranged on or in the cap substrate. The cap substrate can be used to accommodate electrical conductors and components. This allows the gas sensor to be designed in a particularly compact manner. In particular, this eliminates the need for external evaluation electronics. For example, silicon vias, wire bonds, and trenches can be incorporated into the cap substrate and the sensor substrate to form the evaluation electronics. Furthermore, an additional heating element can also be incorporated into or onto the cap substrate. Alternatively or additionally, evaluation electronics can be arranged at least partially in or on the base substrate.

[0021] In another embodiment of the gas sensor, the cap substrate has at least one connecting opening to the reference volume. This allows the reference volume to be open, enabling pressure equalization with the environment surrounding the gas sensor.

[0022] Preferred embodiments of the invention are explained in more detail below with reference to highly simplified schematic representations. These show Fig. 1 a schematic representation of a gas sensor according to a first embodiment, Fig. 2 a schematic representation of a gas sensor according to a second embodiment and Fig. 3 a schematic representation of a gas sensor according to a third embodiment.

[0023] In the figures, the same constructive elements each have the same reference numerals.

[0024] The Fig. Figure 1 shows a schematic representation of a gas sensor. 1 according to a first embodiment. The gas sensor 1 a first membrane 2 and a second membrane 4 on. On the first membrane 2 An analysis heating element is arranged on the second membrane. 4 A reference heating element is positioned. The first membrane 2 and the second membrane 4 are caused by material removal from the sensor substrate 6 shaped. On one underside of the sensor substrate 6 is a base substrate 8 via an adhesive 10 attached. Between the first membrane 2 the base substrate 8 A measurement volume will be created. 12 enclosed. Through an opening 14 Can an analysis gas be introduced into the measurement volume? 12 be guided and analyzed.

[0025] Between the second membrane 4 and the base substrate 8A reference volume will be used. 16 formed via an opening 18 in the second membrane 4 There is a fluid-conducting connection between the reference volume 16 and an environment of the gas sensor 1 This allows, for example, pressure equalization within the reference volume. 16 This is made possible. On one side of the sensor substrate. 6 is a cap substrate 20 arranged. The cap substrate 20 exhibits in the area of ​​the first membrane 2 and the second membrane 4 Recesses on one underside of the base substrate. 8 is a sealant 22 to seal the opening 14 in front of the gas sensor 1 applied. Into the base substrate 8 is an additional heating element 24 for heating the reference volume 16 and the measuring volume 12 integrated.

[0026] In the Fig. Figure 2 is a schematic representation of a gas sensor. 1 shown according to a second embodiment. In contrast to the gas sensor 1 According to the first embodiment, the underside of the base substrate 8 with a gas filter 26 coated. The gas filter 26 This conceals the supply line 14 of a gas to be analyzed into the measuring volume 12 This can, for example, prevent water vapor from entering the measuring volume. 12 be prevented. The supply line 14 According to the exemplary embodiment, it is designed in the form of four openings arranged parallel to each other.

[0027] The Fig. Figure 3 shows a schematic representation of a gas sensor. 1 according to a third embodiment. In contrast to the gas sensor 1 according to the first embodiment and the gas sensor 1According to the second embodiment, the gas sensor 1 according to the third embodiment, a sealed reference volume 16 on. In the reference volume 16 A reference gas is included, which is used for comparison when measuring an analytical gas in the measuring volume. 12 can be used. The reference volume 16 This is achieved through the cap substrate 20 sealed.

[0028] The cap substrate 20 According to the exemplary embodiment, it has a [feature] on the cap substrate 20 arranged electronic circuit 28 on. Part of the electronic circuit 28 is in the cap substrate 20 integrated. The electronic circuit 28 This includes an evaluation electronics unit. 28 The evaluation electronics 28 This affects the second membrane 4 arranged reference heating element and the one on the first membrane2 An arranged analysis heating element with a defined voltage and a defined current for setting a defined temperature.

[0029] By introducing an analysis gas into the measuring volume 12 For example, the heat from the analysis heating element can be dissipated more quickly, so that the evaluation electronics 28 The heating output must be increased to maintain the defined temperature. This is determined by comparing the required heating output of the reference volume. 16 and the heating power of the measuring volume 12 can determine the concentration of a gas to be measured in the measuring volume 12 to be determined.

Claims

[1] Gas sensor (1) for measuring the concentration of an analysis gas based on a thermal conductivity principle, comprising at least one analysis heating element arranged on a first membrane (2) for heating the analysis gas, a reference heating element arranged on a second membrane (4) for heating a reference gas, and at least one evaluation electronics unit (28) for measuring a change in resistance of the analysis heating element caused by the analysis gas relative to an electrical resistance of the reference heating element. characterized by , that the first membrane (2) and the second membrane (4) are arranged adjacent to each other in a sensor substrate (6), wherein a measuring volume (12) and a reference volume (16) can be formed between the first membrane (2) and the base substrate (8) by means of a base substrate (8) arranged on one side of the sensor substrate (6). [2] Gas sensor according to claim 1, wherein the measuring volume (12) and / or the reference volume (16) is formed at least partially within the sensor substrate (6) and / or at least partially within the base substrate (8). [3] Gas sensor according to claim 1 or 2, wherein the reference volume (16) is an open or a closed volume. [4] Gas sensor according to one of claims 1 to 3, wherein the analysis gas can be introduced into the measuring volume (12) through at least one opening (14) in the base substrate (8). [5] Gas sensor according to one of claims 1 to 4, wherein at least one heating element (24) can be applied in or onto the base substrate (8). [6] Gas sensor according to one of claims 1 to 5, wherein at least one gas filter (26) is arranged in the measuring volume (12). [7] Gas sensor according to one of claims 1 to 6, wherein at least one gas filter (26) is arranged on one side of the base substrate (8). [8] Gas sensor according to one of claims 1 to 7, wherein the base substrate (8) can be connected to the sensor substrate (6) via an adhesive (10). [9] Gas sensor according to any one of claims 1 to 8, wherein the base substrate (8) has on one side a joining surface for receiving an adhesive or a sealant (22). [10] Gas sensor according to one of claims 1 to 10, wherein a cap substrate (20) is arranged on a side of the sensor substrate (6) opposite the base substrate (8). [11] Gas sensor according to claim 10, wherein at least part of the evaluation electronics (28) is arranged on or in the cap substrate (20). [12] Gas sensor according to claim 10 or 11, wherein the cap substrate (20) has at least one connecting opening to the reference volume (18).

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