Passivated to package as robust reference chamber for a photoacoustic gas sensor

Abstract

The invention relates to a reference chamber for a photoacoustic gas sensor. The reference chamber comprises a TO package, a MEMS sensor and / or an electronic circuit and a reference gas introduced in an interior region of the TO package. The TO package comprises a base and a cap each comprising a front side facing the interior region. The MEMS sensor and / or the electronic circuit are / is mounted on the front side of the base. Furthermore, at least one connection line is present in the base. In this case, the front side of the base and / or of the cap is coated at least in portions with a passivation layer. The invention furthermore relates to a method for producing the reference chamber according to the invention. Moreover, the invention is directed to a photoacoustic gas sensor and to a method for producing the photoacoustic gas sensor.

Description

[0001] PASSIVATED TO HOUSING AS A ROBUST REFERENCE CHAMBER FOR A PHOTOACOUSTIC GAS SENSOR

[0002] DESCRIPTION

[0003] The invention relates to a reference chamber for a photoacoustic gas sensor. The reference chamber comprises a TO housing, a MEMS sensor and / or an electronic circuit, and a reference gas that is introduced into an interior region of the TO housing. The TO housing comprises a base and a cap, each of which has a front side facing the interior region. The MEMS sensor and / or the electronic circuit is mounted on the front side of the base. Furthermore, at least one connecting line is present in the base. The reference chamber is characterized in that the front side of the base and / or the cap is coated, at least in sections, with a passivation layer.

[0004] Furthermore, the invention relates to a method for producing the reference chamber according to the invention, a photoacoustic gas sensor comprising a reference chamber and a method for producing the same.

[0005] Background and state of the art

[0006] Photoacoustic spectroscopy (PAS) is a physical investigation technique based on the photoacoustic effect and has a wide range of applications. One application of PAS is the detection of very fine concentrations of gases. This involves intensity-modulated radiation with frequencies in the absorption spectrum of the molecule to be detected in the gas. If this molecule is present in the beam path, modulated absorption occurs, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. These heating and cooling processes lead to expansions and contractions of the gas, causing sound waves with the modulation frequency. These can then be measured by sound detectors such as microphones, flow sensors, and / or resonant microstructures.

[0007] One example is the detection of CO2, which plays an important role in research and air conditioning technology. Also relevant are applications where not only toxic but also explosive and / or corrosive gases, such as ammonia (NH3), must be detected.

[0008] To enable the detection of gases using photoacoustic spectroscopy, it is common to use so-called photoacoustic gas sensors (also known as PAS sensors). These comprise a reference chamber located in the beam path of a modulatable emitter, within which the gas to be detected is present. Various designs and configurations of photoacoustic gas sensors are known in the prior art.

[0009] US 2020 / 0400553 A1, for example, describes a photoacoustic gas sensor. The photoacoustic gas sensor comprises a substrate to which a first membrane and a second membrane are attached. These membranes are spaced apart from each other, creating a detection volume between the membranes into which particles or gas can flow. Upon appropriate excitation with electromagnetic radiation within the spectrum of the particles in the detection volume, the photoacoustic effect causes vibrations of the membranes, which can be read, for example, using a capacitive principle. For this purpose, the membranes can comprise doped semiconductor materials or be coated with a metal layer.

[0010] US 2019 / 0360975 A1 relates to a photoacoustic gas sensor comprising a housing, a housing body, and a housing cover. The housing cover encloses a cavity of the housing body. The photoacoustic gas sensor is intended to reduce the influence of a thermoacoustic effect on the sound measurement. According to US 2019 / 0360975 A1, when the reference gas is excited, thermoacoustic sound waves can be generated by heating the walls of the reference chamber, which overlay the photoacoustic measurement signal. It is proposed to use a controllable heating source that specifically generates inverted thermoacoustic sound waves to reduce an (undesired) thermoacoustic component of the (measurement) sound signal.

[0011] CN 109 765 185 A discloses a photoacoustic gas sensor capable of detecting multiple gas components. An array of quantum cascade lasers is used for this purpose. A resistance wire heater is used to maintain a temperature of 50°C in the reference gas chamber.

[0012] In addition, approaches are known in the prior art in which a component of a photoacoustic gas sensor, for example a reference chamber, comprises a TO housing.

[0013] For example, Ananya et al. (2024) analyzes the temporal behavior of the effects of combined temperature and humidity fluctuations on a photoacoustic CO2 sensor. The experimental setup includes a photoacoustic sensor, which includes a TO housing for the reference chamber, in which CO2 is enclosed. The TO housing contains a PAS detector in the form of a MEMS microphone and an NTC temperature sensor.

[0014] DE 10 2020 200275 A1 also relates to a photoacoustic gas sensor. The photoacoustic gas sensor comprises a detector component with a pressure sensing element, e.g., a MEMS microphone, and a package / housing that defines a reference volume into which a reference gas is introduced. The sensitivity of the pressure sensing element is increased by a reference path structure that ensures that the length of a reference path (length of the light beam through the reference gas) is less than or equal to 0.5 mm. For this purpose, the reference path structure can comprise a filler material (e.g., epoxy, a polymer, or a semi-material block) that surrounds the detector component and thus reduces the reference path. Alternatively, it may also be preferable to apply a base layer (e.g., made of ceramic or a semiconductor material) under the detector component to reduce the distance between the detector component and the light inlet.In some embodiments, the package may be a metal package, preferably a TO-Can package.

[0015] CN 109 781 659 A relates to a reference chamber for a gas sensor and a method for manufacturing the same. The reference chamber can, in particular, comprise a TO housing with a cap, a base, and a TO pin. A reference gas is introduced into the reference chamber. A photodetector, which is installed on the base and is intended to detect a wavelength-specific absorption of a modulatable semiconductor laser, serves as the sensor element.

[0016] Methods for manufacturing reference chambers and photoacoustic gas sensors are also known in the prior art. In particular, it is necessary to introduce the gas to be detected into a reference chamber to create the photoacoustic gas sensor. However, the prior art offers only limited options for introducing gases into a reference chamber that would also be suitable for critical gases.

[0017] A common option in the state of the art is to introduce the gas to be encapsulated in a reference chamber using a friction welding system. The reference chamber is sealed within the atmosphere of the friction welding system, thus enclosing the system's atmospheric gas in the reference chamber. However, a friction welding system has a large volume, so a correspondingly large quantity of the reference gas to be enclosed must be available. Particularly with cost-intensive gases, introducing the gas into the reference chamber using a friction welding system can generate high costs. Friction welding systems are more suitable for enclosing non-critical and / or inexpensive gases.

[0018] WO 2022 / 184906 A1 discloses a method for producing a gas-filled reference chamber, which is particularly suitable for critical gases. For this purpose, a first and a second wafer are first provided, with an electronic circuit and / or a MEMS component present on at least one of the two wafers. The two wafers are bonded together. The bonding can be carried out in such a way that an opening remains at a contact surface for bonding and / or an opening is made before or after bonding. The gas can be introduced into the reference chamber through the opening. To seal the reference chamber, the reference chamber is sealed within a coating system.

[0019] By using wafers and process steps from semiconductor and / or microsystem technology, the resulting reference chamber is small in size. However, creating such a gas-filled reference chamber using process steps from semiconductor and microsystem technology can be technically complex. Therefore, there is a need for alternative methods for creating gas-filled reference chambers that utilize simple means while simultaneously ensuring the safe confinement of even critical gases.

[0020] Furthermore, it has been shown that, particularly when aggressive or corrosive gases are enclosed, the functionality of sensitive components within the reference chamber, such as a MEMS sensor and / or an electronic circuit, is impaired, at least over long periods of time. However, for long-term gas monitoring, for example, even in difficult-to-access locations within industrial plants, it is desirable for PAS sensors to have a service life of several years. Therefore, there is a need for optimization in the state of the art in this regard as well.

[0021] Object of the invention

[0022] The object of the invention is to eliminate the disadvantages of the prior art. In particular, one object of the invention is to provide a reference chamber for a photoacoustic gas sensor and a method for its production, which enables reliable confinement of gases, even critical gases, using simple means while simultaneously achieving particularly long operating times.

[0023] Summary of the invention

[0024] The object of the invention is achieved by the object of the invention. Advantageous embodiments of the invention are disclosed in the dependent claims. In one aspect, the invention relates to a reference chamber for a photoacoustic gas sensor comprising a TO housing, a MEMS sensor and / or an electronic circuit and a reference gas which is introduced into an interior region of the TO housing, wherein the TO housing comprises a base and a cap, each of which has a front side facing the interior region and wherein the MEMS sensor and / or the electronic circuit is attached to the front side of the base and furthermore at least one connecting line for electrical contact is present in the base, characterized in that the front side of the base and / or the cap is coated at least in sections with a passivation layer.

[0025] The reference chamber according to the invention is characterized in particular by its long service life. This advantageously allows reliable functionality for service lives of four years or more to be achieved, even when sensitive MEMS detectors for PAS applications are included. The reference chamber can thus be used as a PAS detector cell even in hard-to-reach locations in industrial plants for monitoring gases, including critical gases, without requiring replacement or maintenance over long periods of time. This particularly pronounced longevity is achieved in particular by coating at least some sections of the front of the base and / or cap with a passivation layer.

[0026] Advantageously, aggressive and / or corrosive reference gases can also be introduced into the reference chamber, with no or only a reduced risk of internal components being damaged by the aggressive and / or corrosive gas through unwanted chemical reactions. The advantageous longevity of the reference chamber therefore advantageously provides a particularly long-term stable reference chamber, enabling reliable gas monitoring by PAS even for critical gases. For example, corrosive gases such as ammonia NH3 can be reliably monitored and detected over long periods of time.

[0027] A further advantageous effect resulting from the reference chamber according to the invention is that the possible introduction of high concentrations of reference gases into the reference chamber enables particularly sensitive measurements. If, for example, a gas or gas component with particularly fine concentrations is to be detected, it is desirable to be able to provide high concentrations of the gas or reference gas to be detected in the reference chamber itself. The measuring principle is preferably based on the absorption of radiation, for example infrared radiation, of the gas to be detected outside the reference chamber, which can be detected as a decrease in a PAS signal within the reference chamber. The higher the concentrations of the reference gas provided in the reference chamber, the more sensitively the decrease in a PAS signal can be determined depending on the presence of the gas to be detected.

[0028] While the prior art did not allow high concentrations of aggressive and / or corrosive gases to be introduced, in particular, to avoid compromising functionality, this is prevented by the inventive introduction of a passivation layer. Potential damage to sensitive components within the reference chamber can be advantageously prevented even at high concentrations of the reference gas.

[0029] Another advantage is that the housing comprising a base with a connecting lead inherently offers an electrical contact option for the MEMS sensor and / or the electronic circuit. Thus, the reference chamber produced by the method can be electrically connected to a gas sensor system, for example a photoacoustic gas sensor, via the at least one connecting lead on the base. This enables direct, quick, and easy installation on sensor systems used for the detection of reference gases. To provide a housing with a cap and base with a connecting lead for electrical contact, TO housings can be advantageously used. These are sufficiently compact and can also be purchased in large quantities with desired dimensions and connection options.In addition, TO packages are usually sealed between the base and the cap in such a way that, as recognized by the inventors, they are also suitable for a reference chamber.

[0030] In this regard, it is also advantageous that the use of a TO housing for the reference chamber allows for a hermetic seal of the enclosed reference gas. This advantageously prevents at least a portion of the reference gas introduced into the reference chamber from leaving the reference chamber or portions of a gas from the environment from entering the reference chamber. The hermetic encapsulation can thus ensure a high-precision concentration of the reference gas within the chamber over a long period of time, which also has a beneficial effect on long-term and reliable operational capability.

[0031] Those skilled in the art will recognize that explanations regarding a hermetic encapsulation or closure of the reference gas within the reference chamber preferably refer to permeability. Permeability preferably refers to the property of allowing a fluid or portions of a fluid to pass through. The stronger or tighter the closure, the lower the permeability. With a substantially hermetic encapsulation of, for example, the reference gas within the reference chamber, there is preferably essentially no passage. Those skilled in the art will recognize that there are several options for quantifying permeability. This can be done, for example, using the unit Barrer, where 1 Barrer in SI units is approximately 7.5 * 10-18 m 3*s / kg. The (hermetic) seal can preferably also be quantified by a transmission rate, which can also depend on the enclosed reference gas. The transmission rate also depends in particular on the specific choice of the reference gas and / or the gases in the environment of the reference chamber and can preferably be expressed in the unit g / (m 2 *d) or cm 3 / (m 2 *d) are specified (g corresponds to gram, m corresponds to meter, cm corresponds to centimeter, d corresponds to day).

[0032] The hermeticity can preferably also be determined with regard to standardized gases, which do not necessarily correspond to the actual reference gas. In preferred embodiments, the reference chamber comprising a TO housing can be selected such that the transmission for oxygen is less than 0.01 cm 3 / (m 2 *d) and / or for water vapor less than 0.01 g / (m 2*d). Since electronic components are to be protected in particular from oxygen and / or water vapor, it is customary to specify a tight seal of a TO housing with regard to transmission rates of oxygen and / or water vapor. The preferred values ​​for the tightness against water vapor and / or oxygen preferably also show a tightness against preferred reference gases in the context of the invention, such as methane, propane, propylene, silane, chlorosilane, carbon monoxide, carbon dioxide, sulfur hexafluoride and / or ammonia. In preferred embodiments, the transmission for an enclosed reference gas (preferably one of the aforementioned reference gases) is less than 0.01 cm 3 / (m 2 *d), preferably less than 0.001 cm 3 / (m 2 *d).

[0033] Another preferred option for quantifying the sealability of the reference chamber is to consider the long-term stability of the pressure within the reference chamber. If the pressure within the reference chamber deviates only insignificantly (e.g., less than 20%, less than 10%, less than 5%, less than 1%, or less) from a measured initial pressure value after a certain time interval, e.g., 30 days, 6, 12, 24 months, or more, this indicates hermetic sealing of the reference chamber.

[0034] The passivation layer is preferably formed by applying a passivation material. Preferably, the passivation layer or the passivation material is non-reactive or only slightly reactive with the reference gas. Therefore, the passivation material is preferably chemically inert toward a reference gas introduced into the reference chamber.

[0035] Thus, it is preferred that a passivation layer be applied to the front side of the base and / or the cap in a targeted manner by means of a coating that is chemically inert to a reference gas introduced into the reference chamber. Thus, the passivation layer does not result from a chemical reaction or change in the material of the reference chambers after their provision, but rather is created through a targeted manufacturing step involving the application of a passivation material.

[0036] The provision of a reference chamber by applying metal layers which are not inert to the reference gas but only become partially inert to the reference gas over time, possibly due to oxidation or chemical reaction of the metal, does not therefore constitute a coating of a front side of the base and / or the cap, at least in sections, with a passivation layer comprising a passivation material. As a precaution, it should be pointed out that a person skilled in the art can also recognize from the finished chamber provided whether a passivation layer was obtained by applying a passivation material which is chemically inert to the reference gas or whether a partial passivation of a layer, for example a metal layer, has taken place which was not chemically inert to the reference gas but was oxidized by it.

[0037] The reference chamber preferably refers to a chamber comprising a volume into which the reference gas can be introduced. In the case of a gas-filled reference chamber, the reference gas is preferably also considered a component of the reference chamber. The reference chamber can particularly preferably be a component of a photoacoustic gas sensor, which has a reference gas to enable photoacoustic measurements. For this purpose, the reference chamber preferably contains a MEMS sensor and / or an electronic circuit with which the photoacoustic signals (PAS signals) can be detected.

[0038] The reference chamber can therefore preferably also be referred to as a detector or sensor cell. The detection of PAS signals is made possible in particular by a modulatable emitter, wherein the reference chamber is mounted in a beam path of the modulatable emitter. The beam path can, for example, be guided through a gas tube containing the gas to be analyzed before impinging on the reference chamber. In particular, the gas tube can comprise a tube body that encloses an analysis volume and within which a measuring section is located. The gas tube can also have openings through which a gas can diffuse and / or flow into the analysis volume. If the reference gas is present at least partially in the gas that enters the beam path between the emitter and the reference chamber, a PAS signal detectable within the reference chamber is attenuated.Thus, the proportion of reference gas in the measurement path upstream of the reference chamber leads to an absorption of the radiation, and the PAS signal in the reference chamber is correspondingly reduced. By measuring the reduction or attenuation, a direct conclusion can be drawn about the proportion of reference gas located in the beam path (e.g., in a gas tube) upstream of the reference chamber. Structurally, the reference chamber according to the invention comprises a reference gas, a MEMS sensor, and / or an electronic circuit and a TO housing for detecting the PAS signal.

[0039] TO (transistor outline) packages comprise a base and a cap and offer a advantageous design. Furthermore, one or more connecting leads are attached to the base, for example 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more, which enable electrical contact with the MEMS sensor and / or the electronic circuit. TO packages offer a wide range of electrical contact options, particularly due to the inclusion of at least one connecting lead. TO packages can advantageously be directly integrated into other sensor systems, such as photoacoustic gas sensors. They also enable fast and high data transmission rates through appropriate connecting lead(s). Furthermore, they are sufficiently compact, allowing for simple mechanical installation options.In addition, they can be purchased in large quantities with consistent dimensions and / or components, so that a correspondingly large number of reference chambers with essentially identical properties can be produced.

[0040] The base of the TO housing preferably forms a base for mounting components to be encapsulated, such as a MEMS sensor and / or an electronic circuit. The base is preferably formed essentially from a metal, in alternative preferred forms from a plastic. The base can preferably comprise a substantially elliptical or circular-cylindrical configuration. Elliptical or circular-cylindrical here preferably means a cylinder shape which has a corresponding elliptical or circular shape as its base area. Other shapes for the TO housing can also be preferred, such as triangular, quadrangular, in particular rectangular, pentagonal, hexagonal, or other polygonal cross-sections. The base of the TO housing preferably has a diameter of 2-70 mm, preferably 3-30 mm, preferably 5-20 mm, particularly preferably 8-15 mm.Furthermore, it is preferred that the base has a height of 3-20 mm, preferably 5-15 mm, particularly preferably between 8-13 mm.

[0041] The base of the TO housing preferably has a front side and a back side, with the MEMS sensor and / or the electronic circuit being located on the front side and the at least one connecting line for electrical contact being located on the back side. It may be preferred for a carrier component to be located on the front side of the base, to which the MEMS sensor and / or the electronic circuit is applied. When using a carrier component, the MEMS sensor and / or the electronic circuit is preferably not attached directly to the front side of the base. The carrier component can comprise a component or a group of components that can preferably be involved in the electrical functionality of the MEMS sensor and / or the electronic circuit. The carrier component can comprise, for example, a circuit board, bonding wires, solder material and / or contact points, without being limited to these examples.Furthermore, it is preferred that the carrier component and / or the MEMS sensor and / or the electronic circuit be connected to the base by a solder material, in particular by one or more solder balls. This advantageously achieves a firm connection to the base.

[0042] The cap of the TO housing serves in particular to protect the components located on the base, such as the MEMS sensor and / or the electronic circuit. Furthermore, it is advantageous for the cap of the TO housing to enable the transmission of optical signals by acting as an optical interface. For this purpose, for example, there may be a transparent section on the housing that is installed as a window. Preferably, the cap of the TO housing can have further optical components, such as one or more lenses. The cap of the TO housing can preferably be formed essentially from plastic and / or metal. Furthermore, it is preferred that the cap has a contact surface that enables mechanical contact, in particular via a permanent connection, with the base.The cap preferably has a diameter that essentially corresponds to the diameter of the base to ensure sufficient coverage of the MEMS sensor and / or the electronic circuit. The height of the cap is preferably 1-40 mm, preferably between 5-30 mm, and particularly preferably between 10-30 mm.

[0043] In a further preferred embodiment, the base and / or the cap comprises a material selected from a group comprising a metal and / or a transition metal, preferably from a group comprising copper, silver, gold, molybdenum, tungsten, aluminum, and / or combinations thereof. The aforementioned preferred materials for the base and / or the cap have proven advantageous for reliably preventing chemical corrosion, which could lead to damage and / or leaks in the reference chamber. Thus, the preferred materials are suitable for producing a particularly durable reference chamber and increasing the potential operating time. Furthermore, the listed preferred materials can be used in standardized processes to provide the reference chamber.

[0044] The TO housing as such comprises in particular the base and the cap, which are connected to one another. By firmly connecting the base to the cap, an inner region is formed in which the MEMS sensor and / or the electronic circuit as well as the reference gas are located. The inner region preferably refers to the volume present within the reference chamber. In other words, the inner region is preferably encased by the base and the cap, so that the base and the cap can also be referred to as an encasing of the TO housing. Thus, the base and a cap fulfill a covering or protective function as well as a supporting function and, through the connecting lines, also an electrical contacting function. The MEMS sensor and / or the electronic circuit is preferably attached to the base. The MEMS sensor and / or the electronic circuit is covered in particular by the cap.

[0045] The surroundings of the TO housing and thus also of the reference chamber are therefore preferably understood as the outer region, while the inner region is formed by a volume that is predetermined within the TO housing and comprises a base connected to a cap. Regions of the cap and / or the base that face the inner region of the reference chamber are preferably referred to as the front side in the context of the invention. The terms front side (of a component of the reference chamber) and inner side (of the component of the reference chamber) can therefore also be used synonymously in the present case. Thus, the front side (or inner side) of the base and / or the cap is exposed to the reference gas. Accordingly, the rear side of the cap and / or the base can be understood as the region that faces away from the inner region, preferably facing the surroundings of the reference chamber.The terms back (of a component of the reference chamber) and outside (of a component of a reference chamber) can therefore also be used synonymously in this case.

[0046] In a further preferred embodiment, the back of the base and / or the cap is coated at least in sections, preferably completely, with the passivation layer.

[0047] Coating at least sections of the base and / or cap is particularly advantageous when the environment of the TO package contains (high) proportions of a gas to be detected that has a corrosive effect. This embodiment advantageously provides protection not only for the front side of the base and / or cap, but also for the rear side of the base and / or cap. Thus, particularly comprehensive protection for the TO package is achieved when the front side, in combination with the rear side, of the base and / or cap is at least partially, preferably completely, coated with the passivation layer.

[0048] Preferred embodiments for the passivation layer on the front side can also apply to the passivation layer on the back side of the base and / or the cap. Thus, as explained in more detail below, it can be preferred that the passivation layer on the back side of the base and / or the cap is coated with the passivation layer to a proportion of at least 50%, preferably at least 60%, at least 70%, at least 80%, particularly preferably essentially completely. Furthermore, it can be preferred that the passivation layer on the back side has a thickness of 10 nm - 1 mm, preferably 100 nm - 800 μm, particularly preferably 1 μm - 700 μm, particularly preferably 100 μm - 500 μm. Materials or material compositions of the passivation layer on the front side can also be used for the passivation layer on the back side.

[0049] The above-mentioned preferred thickness values ​​and / or surface areas for the passivation layer have proven advantageous for particularly reliable corrosion protection against the reference gas. This contributes significantly to the service life of the reference chamber and the components contained therein. The preferred thickness values ​​and / or surface areas for the passivation layer advantageously prevent unwanted chemical reactions on the surface of the TO package that could impair the performance of a photoacoustic gas sensor in which the reference chamber can be used. Thus, precise measurements can be ensured using the reference chamber.

[0050] According to the invention, the base and / or the cap is / are coated on the front side at least in sections with a passivation layer. The passivation layer refers to a layer that protects components located inside the TO housing from at least one undesirable chemical and / or electrochemical reaction with the reference gas. Thus, the passivation layer provides at least partial corrosion protection. In the worst case, corrosion could render the reference chamber unusable. The passivation layer can therefore preferably be designed as a protective layer that reduces or prevents corrosion. The passivation layer preferably prevents direct contact between the reference gas and the section of the front side on which the passivation layer is coated.Any undesirable chemical reaction between the reference gas and areas of the base and / or cap and / or with components of the reference chamber, such as the MEMS sensor and / or the electronic circuitry, should these react with the reference gas, is thus effectively prevented by the passivation layer. The protective effect achieved by the passivation layer can also be referred to as passivation.

[0051] In a further preferred embodiment, the reference chamber is characterized in that the front side of the base and / or the cap is / are coated with the passivation layer to a proportion of at least 50%, preferably at least 60%, at least 70%, at least 80%, particularly preferably substantially completely.

[0052] The preferred proportions preferably relate to a total surface area of ​​the front, ie the surface area formed by the sum of the surface areas of the front of the cap and the base.

[0053] Accordingly, it may be preferable to perform the passivation of the reference chamber in a proportionate manner. This advantageously allows for a targeted protective effect within the reference chamber. This can be relevant, for example, if a specific area within the TO package is susceptible to the reference gas and requires separate protection. A substantially complete coating of the passivation layer is advantageous due to the comprehensive passivation, so that both the front side of the cap and the front side of the base are protected from the reference gas.

[0054] The passivation layer may preferably cover one or more regions and / or components of the reference chamber in order to achieve desired passivation at that location.

[0055] For example, it may be preferred that the passivation layer is coated at least partially on the front side of the base. It may also be preferred that the passivation layer is coated at least partially on the front side of the cap. Furthermore, the passivation layer can preferably be applied to the MEMS sensor and / or the electronic circuit. Furthermore, any possible combination of a section of the front side of the TO housing and / or a component of the reference chamber can preferably be passivated with the passivation layer. For example, the base and the cap can preferably be at least partially coated with the passivation layer. It may also be preferred that at least partially the base and the MEMS sensor and / or the electronic circuit are coated with the passivation layer.It is also preferred that components which serve for the electrical and / or mechanical contact within the reference chamber for the MEMS sensor and / or the electronic circuit are coated with the passivation layer at least in sections, for example conductor tracks and / or soldering materials.

[0056] The passivation layer can preferably be applied uniformly, i.e., with a substantially identical thickness, to the base, the cap, a MEMS sensor, and / or an electronic circuit. However, it may also be preferred for the passivation layer to be applied with a greater thickness (and thus a greater protective effect) to particularly sensitive components. For example, it may be preferred for the passivation layer to be applied with a greater thickness to the MEMS sensor and / or an electronic circuit than to a base and / or the cap, for example, by a factor of 1, 5, 2, 3, or more.If the MEMS sensor and / or an electronic circuit are already at least partially protected against a potential negative influence of the reference gas, for example, by appropriately selecting suitable inert materials, it may also be preferable to provide the passivation layer on the MEMS sensor and / or an electronic circuit with a smaller thickness than on sections of the base and / or the cap, for example by a factor of 1, 5, 2, 3, or more. The latter embodiment may be preferred, for example, to minimize any influences of the passivation layer on the functionality of the components, for example, a detector component of a MEMS sensor.

[0057] By providing a passivation layer with appropriately selected layer thicknesses, it is possible to advantageously ensure, on the one hand, a high level of protection and, on the other hand, excellent functionality of the components of the reference chamber.

[0058] In a further preferred embodiment, the reference chamber is characterized in that the passivation layer comprises a passivation material selected from a group comprising a conductor, a semiconductor and / or an insulating material.

[0059] A conductor preferably refers to an electrical conductor. An electrical conductor preferably means a material that has an electrical conductivity of > 10 4 S / cm (Siemens per centimeter). An insulating material preferably means an electrical insulating material that preferably has an electrical conductivity of <10' 8 S / cm. A semiconductor means a material which preferably has an electrical conductivity between the ranges mentioned for the electrical conductivity of electrical conductors and electrical insulating materials, ie between 10 4 S / cm and 10' 8S / cm. The average person skilled in the art knows that there may be overlaps in the boundary regions of the three materials mentioned. Against this background, it is preferable that semiconductors can also be characterized by their negative temperature coefficient of resistivity. The electrical conductivity of semiconductors increases with increasing temperature. This can be explained, without being limited to theory, among other things, by the band gap between the so-called conduction band and the valence band in the band model of semiconductors.

[0060] If a conductor is preferably used for the passivation layer, it is preferred that the conductor be applied in combination with a semiconductor and / or an insulating material. In particular, it is preferred that a semiconductor and / or an insulating material be applied to the conductor to ensure electrical insulation of the conductor as a component of the passivation layer. This eliminates or minimizes the risk of a short circuit and / or arcing.

[0061] In a further preferred embodiment, the reference chamber is characterized in that the passivation material is selected from a group comprising silicon, preferably as monosilicon or polysilicon, polyimide, aluminum nitride, novolak, polymethylglutarimide, tetraethylorthosilicate (TEOS), parylene, perfluorodecyltrichlorosilane and / or epoxy resin.

[0062] The aforementioned preferred materials for the passivation layer have proven advantageous for ensuring reliable passivation of the front side of the base and / or cap. Furthermore, the aforementioned preferred materials can be applied using processes known and proven in the art for providing the passivation layer. Furthermore, the aforementioned preferred materials are inexpensive to acquire.

[0063] A passivation material preferably refers to a material that does not react chemically, or only reacts to a limited extent, with the reference gas enclosed in the reference chamber. A reference gas introduced into the TO housing can act as a de facto reactant, so that a chemical reaction could occur with a portion of the front side of the base and / or the cap and / or a component of the reference chamber, such as the MEMS sensor and / or the electronic circuit. To avoid or minimize this, at least some of the front side is coated with at least one passivation layer comprising a passivation material.

[0064] The passivation of the passivation material, i.e. in particular the protective effect, preferably by avoiding or reducing the chemical reactivity of the front side of the reference chamber with the reference gas, which is achieved by the passivation material, can be measured and / or quantified by various methods. Preferably, the passivation can be determined in such a way that the reference gas, from which the front side of the reference chamber is to be protected, and / or ions of the reference gas are applied to the passivation material that is to form the passivation layer. Application can take place, for example, in a gaseous environment or in an aqueous environment. A gaseous environment can be provided, for example, by an atmosphere containing the reference gas. An aqueous environment can be provided, for example, by a suitable electrolyte.

[0065] In a liquid environment, a so-called potentiodynamic test can also be carried out. In a potentiodynamic test, ions of a reference gas are first provided in an electrolyte. The passivation material as a sample is anodically polarized in a potential loop starting from a resting potential. Resting potential is understood to be the mixed potential that arises at an electrode, relative to a reference electrode, in a specific electrolyte. In this state, no current is measurable because the anodic and cathodic currents are equal. The reversal point of the loop is determined by a maximum current density (so-called reversal current density). The result of such a measurement is the current behavior during polarization and is represented graphically in a so-called current density-potential curve.Depending on the position of the resting potential relative to the repassivation potential of the potential loop, i.e., from the area after the potential loop where the anodic and cathodic currents coincide, it can be determined whether the passivation material allows for solid passivation and is suitable for the reference gas. If the passivation material is unsuitable for passivation or not sufficiently reliable, the repassivation potential will be lower than the resting potential. If the passivation material is suitable for passivation, the repassivation potential will be higher than the resting potential.

[0066] In other words, the passivation material or passivation layer prevents a chemical reaction between the front side of the reference chamber and the reference gas introduced into the reference chamber from taking place, either at a significantly slowed or even completely. In particular, the application of the passivation layer to the front side of the reference chamber reduces the reactivity of the reference gas with the correspondingly coated components or sub-regions of the reference chamber. The average person skilled in the art will be aware that the effect of the passivation, which is achieved by the passivation layer, depends on various factors. In particular, the passivation can depend on the structure and / or chemical composition of the passivation layer, the enclosed reference gas itself and / or the temperature in the reference chamber.

[0067] Passivation materials such as silicon, especially monosilicon and polysilicon, and / or silicon dioxide are advantageous in that, in addition to reducing chemical reactivity, they can also eliminate dangling bonds and other surface defects that can impair the functionality of the MEMS sensor and / or electronic circuit. Since silicon is a semiconductor, it is preferred that silicon be used in combination with an electrically insulating material when used as a passivation material.

[0068] Polymeric materials as passivation materials, such as parylene, polyimide, and / or FDTS, are advantageous in that, in addition to their high passivation effect, they exhibit particularly low thermal expansion. This ensures that, when elevated temperatures are present within the reference chamber, the passivation layer does not adversely affect the functionality of the MEMS sensor and / or the electronic circuit due to the elevated temperature. Furthermore, it is advantageous that polymeric materials as passivation materials, such as parylene, polyimide, and / or FDTS, can be used to achieve a surface-conforming coating. A surface-conforming coating preferably means a coating that essentially adheres directly and tightly to the surface.Essentially direct and tightly fitting preferably means that the coating is mostly in direct contact, but in some areas includes volumes not filled by components, for example, in corner areas on the front of the reference chamber. The surface-conformal coating is preferably completely surface-conformal. This means, in particular, that the layer is perfectly tight-fitting or surface-conformal, and even the smallest structures can be coated tightly. The smallest structures are preferably structures with dimensions of the order of a maximum of 10 nanometers (nm), a maximum of 100 nm, a maximum of 1 micrometer (pm), a maximum of 10 pm, or a maximum of 100 pm.

[0069] If the passivation layer comprises a semiconductor material such as silicon, for example in the form of monosilicon and polysilicon, and / or silicon dioxide, special advantages for the reference chamber are also achieved. Semiconductor materials exhibit stable electronic properties. This contributes to increased performance and reliability. Furthermore, standardized processes can be used for coating with a semiconductor material as a passivation material, which simplifies the manufacturing process.

[0070] Parylene, polyimide, FDTS, and also silicon (in the form of silicon and / or monosilicon) are particularly advantageous for ammonia (NH3) when enclosed as a reference gas within the reference chamber. These materials have proven particularly advantageous for the confinement of ammonia. In particular, a particularly high passivation effect for ammonia can be achieved by using one or more of these materials.

[0071] Ammonia is particularly relevant in many industrial fields, for example in refrigeration technology as a coolant, as it has advantageous thermodynamic properties for use as a coolant. However, ammonia is corrosive to some materials that are also used in semiconductor technology. Furthermore, ammonia is toxic and explosive, with an ignition temperature of approximately 630°C. Against this background, reliable monitoring of ammonia is necessary to avoid dangerous situations that could arise, for example, as a result of the undesired spread of ammonia in an industrial plant. This can advantageously be achieved by the reference chamber according to the invention. With the preferred introduction of ammonia as a reference gas, reliable detection of ammonia can advantageously be ensured through the use of PAS.The protective effect of the passivation layer and the associated extension of the operational function can advantageously achieve particularly reliable monitoring of ammonia, for example in industrial plants, even over a long period of several years.

[0072] In a further preferred embodiment, the reference chamber is characterized in that the passivation layer has a thickness of 10 nm - 1 mm, preferably of 100 nm - 800 pm, particularly preferably of 1 pm - 700 pm, most particularly preferably of 100 pm - 500 pm.

[0073] The thickness of the passivation layer preferably refers to a dimension that is oriented substantially perpendicular to a surface section. Thus, the thickness of the passivation layer preferably refers to a dimension that extends along a solder located on the passivation layer. The thickness of the passivation layer can preferably also be interpreted as a height of the passivation layer.

[0074] Various parameters are preferred to enable quantification of passivation. A preferred parameter for quantifying the effect of passivation is the electric current or the electric current density. This is particularly preferred when the passivation layer comprises a dielectric material, for example a polymer. When a potential, i.e. an electric voltage, is applied, the electric current flowing through the passivation layer can be measured. For example, electric currents of -0.1 - 10 mA (milliamperes) can be determined at a potential difference of -10 - +10 V (volts). The exact value of the electric current also depends on the specific composition of the passivation layer and on other parameters, such as temperature. With a strong passivation effect, i.e. with a high degree of passivation, a reduced electric current is measured.If the degree of passivation is less pronounced, the electrical current can be higher. A thicker passivation layer increases the passivation effect. Accordingly, a thicker passivation layer can increase the passivation effect or the degree of passivation.

[0075] In preferred embodiments, the passivation effect or the degree of passivation of the passivation layer can be specified by a transmission rate, which can be used to indicate the volume that can diffuse through the passivation layer within a specific unit area and unit time (days, d). The transmission rate is preferably particularly low in order to achieve a high passivation effect. The thickness can advantageously influence the transmission rate and thus also the passivation of the passivation layer.

[0076] In preferred embodiments, the transmission for an enclosed reference gas (preferably one of the aforementioned reference gases) with respect to the passivation layer is less than 0.1 cm 3 / (m 2 *d), preferably less than 0.01 cm 3 / (m 2 *d), particularly preferably less than 0.001 cm 3 / (m 2 *d), most preferably less than 0.0001 cm 3 / (m 2 *d). A greater thickness of the passivation layer results in a lower transmission rate, which in turn can increase the passivation effect. The exact transmission rate depends not only on the thickness of the passivation layer but also on the specific composition of the passivation layer.

[0077] By adjusting the thickness of the passivation layer, a variable is advantageously available with which the degree of passivation can be adjusted. The degree of passivation preferably refers to the extent of the protective effect brought about by the passivation layer. In particular, the degree of passivation can characterize the strength of the protective function of the passivation layer. For example, it may be preferable for a greater thickness to bring about a higher degree of passivation than does a passivation layer with a lower thickness. The average person skilled in the art recognizes that, in addition to the thickness, other parameters can also determine the degree of passivation, for example, the material selection of the passivation layer and / or the geometric design of the reference chamber, without being limited to these examples.

[0078] It may be preferred for the passivation layer to have a substantially constant thickness along its length. It may also be preferred for the passivation layer to have varying thicknesses. Furthermore, it may be preferred for the passivation layer to have irregular or regular thickness configurations. Consequently, a different degree of passivation can advantageously be achieved in different regions of the TO housing by varying the thickness of the passivation layer. This can be particularly useful when certain regions and / or components of the reference chamber are more sensitive to the reference gas than other regions and / or components of the reference chamber.

[0079] For example, it may be preferred that an area for electrical contacting of the MEMS sensor and / or the electronic circuit, for example, conductor tracks and / or wire bonds, is more sensitive to a potentially reactive reference gas than a section of the front side of the base and / or the cap. Therefore, the area of ​​electrical contacting of the MEMS sensor and / or the electronic circuit, for example, conductor tracks and / or wire bonds, can preferably be coated with a passivation layer that has a greater thickness than the section of the passivation layer that was applied to the base and / or the cap. For example, if it is the case that all areas of the front side and / or components of the reference chamber have substantially the same sensitivity or reactivity with respect to the reference gas, it may be preferred that the passivation layer has a substantially constant thickness.

[0080] Preferred passivation layer thicknesses should, on the one hand, be sufficiently thin for the geometry of the reference chamber along the front side, thus not impairing, or only minimally impairing, one or more functions of the components within the reference chamber. At the same time, however, the desired passivation effect should also be achieved. These preferred thicknesses have proven to be an optimal range for achieving sufficient compactness, ensuring the functional suitability of the reference chamber components, and saving material.

[0081] In a further preferred embodiment, the reference chamber is characterized in that the reference chamber is operatively connected to a heating element, wherein the heating element is designed to heat the reference gas within the reference chamber, wherein preferably the reference gas is heated to no more than 100°C, preferably to a temperature of 20°C - 100°C, preferably of 30°C - 80°C, particularly preferably of 40°C - 60°C.

[0082] An operative connection of a heating element means that the transfer of the heating element's effect into or to the reference chamber is ensured. The preferred use of a heating element specifically refers to the heat transfer to the reference chamber through the operative connection between the heating element and the reference chamber.

[0083] Advantageously, the operative connection of the heating element to the reference chamber makes it possible to reduce or prevent adsorption and / or absorption of the reference gas on and / or in the passivation layer. As a result, the protective effect of the passivation layer and the associated extension of the operational function can be increased surprisingly significantly. Without wishing to be limited to one theory, this can be attributed, among other things, to increased kinetic energy of the gas molecules achieved by heating by means of the heating element. The increased kinetic energy of the gas molecules counteracts adsorption and / or absorption of the reference gas on and / or in the passivation layer. The average person skilled in the art would have assumed that an increase in temperature increases the reactivity of the passivation layer, so that the introduction of a heating element counteracts any protective effect.Instead, it was surprisingly found that this is not the case. Due to the predominant effect of reduced adsorption and / or absorption of a reference gas (such as NH3) on and / or in the passivation layer, the protective function is likely to be enhanced by heating.

[0084] This advantageously achieves long-lasting stability of the design of the passivation layer, in particular with regard to the geometric shape and / or the thickness.

[0085] The aforementioned preferred temperatures have proven advantageous in that they can be implemented for a wide spectrum of reference gases. Accordingly, with a particularly large number of reference gases, undesired adsorption and / or absorption of the reference gas on and / or in the passivation layer can advantageously be avoided or prevented. This particularly applies to ammonia, so that undesired absorption and / or adsorption of ammonia on the passivation layer can be reduced or prevented. The aforementioned preferred temperatures have proven particularly effective in counteracting undesired adsorption and / or absorption, particularly with regard to the aforementioned reference gases and passivation materials, and at the same time counteracting reduced reactivity of the reference gas with the passivation layer.Furthermore, the preferred temperatures are easy to achieve using a heating element, so that this does not require a lot of effort, but can be implemented directly by the heating element.

[0086] To advantageously avoid or reduce the adsorption and / or absorption of the reference gas on and / or in the passivation layer, several options are advantageously available to achieve a corresponding effective connection of the heating element to the reference chamber.

[0087] In a preferred embodiment, the reference chamber is characterized in that the heating element is located within the reference chamber, preferably on the front side of the base and / or the cap. Advantageously, the heating element is located in the interior of the reference chamber. This provides a high degree of compactness for the reference chamber, since the dimensions of the reference chamber are not increased by components that are attached to the rear side of the TO housing. A more compact reference chamber advantageously simplifies the installation of the reference chamber on other components and / or systems, for example, in the context of a photoacoustic gas sensor.

[0088] In a further preferred embodiment, the heating element is provided by at least one heating section as a section of the passivation layer, wherein the heating section preferably has a substantially annular configuration.

[0089] The described embodiment is particularly preferred when the heating element is located within the reference chamber. The heating section of the heating element designates a region of the passivation layer through which a heating effect can develop. This can be made possible, for example, by the heating section comprising a conductor through which the heating effect can develop within the reference chamber.

[0090] Advantageously, the introduction of the heating element as a heating section of the passivation layer is particularly simple within a process for producing the reference chamber. In particular, no separate component needs to be introduced to achieve the desired heat transfer to the reference gas. Instead, the applied passivation layer can be further processed in the same process sequence to provide the heating element. In this case, an electrically conductive conductor is preferably introduced into the passivation layer, which serves to heat the chamber due to the Joule heat generated when current flows through it.

[0091] It may be preferred, for example, that an electrically conductive material is applied to the front side of the base and / or the cap. It may also be preferred that the electrically conductive material for providing the heating element forms a component of the passivation layer and is applied in combination with a dielectric material. Thus, it may be preferred that the dielectric material at least partially encloses the electrically conductive material. An unenclosed region of the electrically conductive region enables reliable heat transfer, while the region of the electrically conductive region encased by the dielectric material is advantageously chemically protected and electrically insulated.

[0092] The annular configuration of the heating section preferably means a self-contained shape, which can preferably be elliptical, in particular circular. An annular configuration enables particularly efficient heat transfer within the reference chamber, thus achieving a substantially homogeneous heat distribution in the interior of the reference chamber. It may also be preferred for the heating element to have a helical shape. This ensures that the electric current has to travel a longer distance. The heat energy that can be generated by the electric current can advantageously be increased.

[0093] In preferred embodiments, the heating element can be incorporated as a separate component within the reference chamber. In a further preferred embodiment, the reference chamber is characterized in that the heating element is an electrical heating element, preferably a heating resistor. The heating element preferably refers to the component from which a heat field emanates by releasing heat energy. An electrical heating element preferably refers to a heating element capable of converting electrical energy into heat energy, which in the context of the invention is used as a heat field. In an electrical heating element, the heat energy or the heat field is preferably generated by using an electrically conductive material through which an electrical current flows and which is heated by Joule heat.A heating resistor has proven particularly suitable for this purpose, which in particular comprises metallic alloys that do not melt and / or oxidise even at the particularly high temperatures they can provide.

[0094] The heating element preferably comprises a material selected from a group comprising monosilicon, polysilicon, aluminum, copper, gold, platinum, nickel, silver and / or tungsten, wherein the material preferably has a doping as semiconductor material.

[0095] The preferred materials are particularly easy and cost-effective to process in semiconductor and / or microsystem technology and are also well-suited for mass production. These materials are also particularly suitable for doping and / or coating to achieve the desired electrical, thermal, and / or optical properties in specific areas. The aforementioned materials offer a variety of advantages due to the usability of standardized manufacturing techniques, which are also particularly suitable for the integration of additional components, such as electronic circuits.

[0096] In a further preferred embodiment, the reference chamber is characterized in that the heating element is located outside the reference chamber, wherein the heating element is positioned at a distance from or contacts a rear side of the cap and / or the base.

[0097] When contact is made with the back of the cap and / or the base, direct heat transfer can preferably occur from the heating element to the reference chamber and the reference gas enclosed therein. Contacting also preferably includes attaching the heating element to the cap and / or the base via a heat-conducting intermediate layer. However, a spaced-apart positioning can also be preferred, with no direct contact between the heating element and the reference chamber. The heating element and the reference chamber can, for example, have a distance of 0.1 - 30 cm, preferably 5 - 35 cm, particularly preferably 10 - 25 cm. When the heating element is positioned at an incorrect distance from the reference chamber, heat transfer preferably occurs through thermal radiation and / or convection.

[0098] Mounting the heating element outside the reference chamber is advantageous in that it provides greater flexibility regarding its positioning. Furthermore, it is advantageous in that maintenance and / or repairs on the heating element can be performed more easily. Furthermore, in addition to the temperature of the heating element, the heating effect on the reference gas can be adjusted by selecting heat-conducting intermediate layers or by adjusting the distance between the heating element and the reference chamber.

[0099] In a further preferred embodiment, the reference chamber is characterized in that the MEMS sensor comprises an inert material.

[0100] The inert material preferably refers to a material that is inert to a reference gas introduced into the reference chamber. In particular, the reference gas does not participate, or only participates to a very limited extent, in a chemical reaction with the reference gas. The inert material is particularly characterized by long-term stability in its composition, as it maintains essentially constant material properties, which in turn is due to its inertness.

[0101] The inert material is preferably an inert solid as a component of the MEMS sensor. The inert material preferably has a negative standard enthalpy of formation. The standard enthalpy of formation preferably refers to an enthalpy released during the formation of one mole of the inert material from the most allotropically stable form under standard conditions (100 kPa (kilopascals) and 25°C (Celcius)) (negative sign). One of ordinary skill in the art will recognize that the property of inertness, i.e., the avoidance or reduction of a chemical reaction with the reference gas, depends on several factors, for example, the composition of the inert material, the reference gas, and / or parameters such as the temperature within the reference chamber.

[0102] The inert material of the MEMS sensor preferably refers to an inert material that is comprised by a component of the MEMS sensor. In preferred embodiments, the MEMS sensor and / or the electronic circuit and / or a connection can comprise an inert material or can preferably be constructed from an inert material. Preferably, at least the sections of a MEMS sensor, the electronic circuit and / or a connection that are in contact with the reference gas can comprise an inert material. If an inert material is used, additional passivation by means of a passivation layer can preferably be dispensed with. It may also be possible to apply a further passivation layer to the inert material in order to ensure particularly long operational suitability.

[0103] In preferred embodiments, one or more (different) inert materials may be used.

[0104] In preferred embodiments, in particular, a detector component of the MEMS sensor can comprise an inert material. A detector component of the MEMS sensor preferably means a component that can experience a deflection and / or oscillation as a result of the effect of the reference gas. The deflection and / or oscillation of the detector component can preferably be used to detect a PAS signal, thereby making it possible to draw conclusions as to whether and / or which proportional composition of the reference gas is present between a beam path between an emitter and the reference chamber. The detector component can preferably be formed by a membrane or a cantilever. The inert material, which can preferably be comprised by a detector component, can advantageously ensure that the detector component is protected from the corrosive effects of the reference gas.

[0105] By forming a detector component of the MEMS sensor using an inert material, the application of a passivation layer for the detector component can be advantageously omitted without the risk of damage from the reference gas. This embodiment, particularly in conjunction with the application of a passivation layer to other components of the reference chamber or the MEMS sensor that are not directly involved in detection, can advantageously achieve high functionality and sensitivity as well as a long operating time.

[0106] It may also be preferred for an electrical connection to comprise an inert material. An electrical connection line can preferably be selected from a group comprising one or more bond wires, solder connections, conductor strips, conductor tracks, vias, and / or bumps. The inert material can, for example, be coated onto one or more of the connection connections, so that the connection connection is preferably encased in the inert material. This advantageously ensures a long-lasting electrical connection without being structurally and / or functionally impaired by the reference gas.

[0107] Thus, an inert material of the MEMS sensor enables, in addition to or as an alternative to the passivation layer, which provides a protective function for the casing of the reference chamber in an interior area, also dedicated protection for the MEMS sensor itself. In particular, the passivation layer can preferably be present in combination with an inert material for the MEMS sensor. For example, the passivation layer can preferably be coated onto the MEMS sensor and / or the electronic circuit and / or an electrical connection, and the MEMS sensor can additionally comprise an inert material, for example for its detector component. Advantageously, this can achieve particularly pronounced protection against a reference gas enclosed within the reference chamber. This applies in particular to particularly aggressive reference gases, for example ammonia.This provides long-lasting and permanent protection for the MEMS sensor, which can last for several years, even more than 4, 5, 6, 7, or 8 years. In a further preferred embodiment, the inert material is selected from a group comprising silver, platinum, gold, and / or copper.

[0108] The preferred materials mentioned have proven to be low susceptible to the preferred reference gases or inert and can be implemented easily using simple semiconductor and / or microsystem technology processes known in the art to provide a component of the MEMS sensor.

[0109] In a further preferred embodiment, the reference chamber is characterized in that the reference gas is a corrosive, toxic and / or explosive gas, wherein the reference gas is preferably selected from a group comprising methane, propane, propylene, silane, chlorosilane, carbon monoxide, carbon dioxide, sulfur hexafluoride and / or ammonia.

[0110] A corrosive gas is preferably a gas that can enter into a chemical reaction with the reference chamber and / or its components, such as the MEMS sensor and / or the electronic circuit, and thereby cause a measurable change. This process is referred to as corrosion. Corrosion can impair the function of the reference chamber and / or its components.

[0111] A toxic gas is preferably a gas that can cause health damage to a living being, particularly a human, by penetrating the organism. The health damage can preferably only occur above a certain concentration of the gas. As the amount of the toxic gas increases, the risk of health damage typically increases. An explosive gas is preferably a gas or mixture which, when exposed to sufficient energy, e.g., due to a high temperature, undergoes a certain chemical reaction in which a high proportion of heat energy can be released. This results in a strongly expanding effect that can cause destruction. Improper handling of explosive substances or gases can be life-threatening.

[0112] Advantageously, there are no restrictions regarding the reference gas that can be introduced into the reference chamber. The option to safely introduce toxic, explosive, and / or corrosive gases into the reference chamber as reference gas advantageously expands the range of reference gases available. This ensures a high level of tightness against unintentional leakage of the reference gas and allows for the precise introduction of extremely small quantities of the reference gas. Due to their aggressive, toxic, and / or explosive effects, monitoring the listed gas types, such as methane, propane, propylene, silane, chlorosilane, carbon monoxide, carbon dioxide, sulfur hexafluoride, ammonia, and / or their compounds, is particularly relevant, but only possible to a limited extent in the current state of the art.In the context of the invention, the reference chamber advantageously allows one or more of the aforementioned gases to be introduced into the interior, enabling reliable monitoring of environments in which such a gas occurs and / or is present, using a PAS. For this purpose, the reference chamber can be used, as described, as a component of a photoacoustic gas sensor.

[0113] Therefore, a photoacoustic gas sensor with such a reference chamber can also measure the concentration of toxic, explosive, and / or corrosive gases with exceptional reliability, which can be particularly relevant for safety applications. Toxic gases such as carbon monoxide (CO) or ammonia (NH3) can pose serious health risks at high concentrations. Explosive gases such as methane (CH4) or hydrogen (H2) can cause fires or explosions. The reference chamber enables these gases to be precisely detected, enabling early warning before concentrations exceed dangerous thresholds. This protects both personnel and equipment. Monitoring toxic or explosive gases in the reference chamber ensures that even the smallest concentrations of these dangerous gases can be reliably detected.

[0114] At the same time, the provision of a passivation layer ensures effective protection of the components of the reference chamber even against the aforementioned aggressive gases, so that a long service life of the reference chamber can be guaranteed.

[0115] In a further preferred embodiment, the reference chamber is characterized in that the at least one connecting line for electrically contacting the MEMS sensor and / or the electronic circuit is located on a rear side of the base, wherein preferably the at least one connecting line is guided through the base, wherein preferably the at least one connecting line is sealed with an insulating material and is electrically insulated from the base.

[0116] At least one connecting line is located within the base. Preferably, several connecting lines can also be located within the base, for example 2, 3, 4, 5, 6 or more connecting lines. A connecting line preferably means an electrical connection, such that an electrical connection to the MEMS sensor and / or to the electronic circuit is enabled via the connecting line. The average person skilled in the art will recognize that for this purpose there can preferably be a feedthrough on the base through which the at least one connecting line is routed. The feedthrough preferably means an opening into which the at least one connecting line can be introduced. If there are preferably several connecting lines, there are accordingly several feedthroughs on the base. It can also be preferred that if there are several connecting lines, e.g. two, three or more connecting lines, the several connecting lines are arranged in one feedthrough.

[0117] The at least one connecting line preferably runs essentially perpendicular to the top and bottom of the base, such that the at least one connecting line is guided essentially parallel to a main direction of extension of the feedthrough. The connecting line preferably forms a return line for the reference chamber through its electrical contact option for the MEMS sensor and / or the electronic circuit. This advantageously allows the reference chamber, in particular the MEMS sensor and / or the electronic circuit, to be connected to external components. This already provides a simple electrical contact option to external components, by means of which, for example, the MEMS sensor and / or the electronic circuit can be controlled and / or read out. Advantageously, the presence of the at least one connecting line on the base eliminates the need for measures to provide a connection option.TO packages are structurally advantageous in that they are already provided with at least one connecting lead on the base. Furthermore, a sealing edge between the base and cap, as well as on the respective connecting leads, is usually already so tightly formed, even in commercially available TO packages, that the inclusion of a reference gas is possible. Alternatively, it may also be possible to seal a commercially available TO package using additional measures and / or to make the connections, for example, between the base and cap, sufficiently tight through suitable process steps during the provision of a TO package.

[0118] The connecting cable is preferably sealed with an insulating material to ensure electrical insulation of the connecting cable from the base. The feedthrough itself preferably has a height that corresponds to the height of the base. The diameter of the feedthrough is preferably adapted to the diameter of the connecting cable. The diameter of the connecting cable can preferably have a value of 0.1 - 2 mm, preferably 0.1 - 1 mm, particularly preferably 0.2 - 0.5 mm. Furthermore, it is preferred that the insulating material is located in the feedthrough in which the at least one connecting cable is routed. In particular, an insulating material can preferably form a sheath within the feedthrough of the base, wherein the connecting cable is introduced through the sheath of the insulating material, thus preventing direct contact between the connecting cable and the base.Preferably, the insulating material is selected from a group comprising glass, glass ceramic, plastic, preferably epoxy resin.

[0119] In a further preferred embodiment, the reference chamber is characterized in that the MEMS sensor is a MEMS microphone, wherein the MEMS microphone is preferably read capacitively, optically, magnetically, piezoresistively and / or piezoelectrically.

[0120] A MEMS microphone preferably refers to a microphone that is manufactured using microsystem technology and / or semiconductor technology processes and / or comprises components from these technological fields. The MEMS microphone preferably comprises a membrane mounted in a vibration-capable manner. Vibration of the membrane can occur as a result of sound waves. This vibration can preferably be read capacitively, optically, magnetically, piezoresistively, and / or piezoelectrically.

[0121] In a capacitive readout, the capacitance is preferably measured as a result of the vibrations, whereby the membrane vibrates relative to a fixed electrode and changes in distance due to the vibrations lead to a change in the capacitance.

[0122] In an optical readout, the vibrations are preferably converted into an optical signal by reflections, e.g. of a laser beam, on the membrane, which can be read out, e.g. in an interferometric arrangement.

[0123] In a magnetic readout, a changing magnetic flux is preferably measured, which is based, for example, on a resonating magnetic component.

[0124] Preferably, the piezoresistive effect is utilized via a piezoresistive readout, so that the change in electrical resistance caused by the vibration is read out. The membrane preferably comprises a piezoresistive material.

[0125] In an analogous manner, the piezoelectric effect is preferably used in a piezoelectric readout, wherein the membrane preferably comprises a piezoelectric material so that a change in the electrical voltage due to the vibration of the membrane can be detected.

[0126] This preferred embodiment, in which the MEMS sensor is a MEMS microphone, is particularly suitable for use in the reference chamber of the PAS, whereby the sound pressure waves can be detected directly in the reference chamber by the MEMS microphone. The listed variants for designing the MEMS microphone have proven particularly useful for this purpose, enabling reliable detection of sound waves in the volume provided by the reference chamber.

[0127] The preferred electronic circuit can be mounted on the base alone or in combination with the MEMS sensor. The electronic circuit can be used, for example, to evaluate a signal, particularly a photoacoustic signal.

[0128] In a further preferred embodiment, the reference chamber is characterized in that the electronic circuit is selected from a group comprising an integrated circuit (IC), an application-specific integrated circuit (ASIC), a programmable logic circuit (PLD), a field programmable gate array (FPGA), a microprocessor, a microcomputer, a programmable logic controller and / or a programmable electronic circuit.

[0129] Preferably, the electronic circuit is configured to process data from the MEMS sensor and / or a modulated absorption of the reference gas. Processing data preferably means that the electronic circuit is configured to perform computational operations and / or computational steps, for example, with recorded measurement data. The electronic circuit can thus be present as evaluation, readout, and / or control electronics, which enables control of the heating element, readout of the MEMS sensor, and / or evaluation of the measurement data. If an evaluation is performed, the electronic circuit can also be referred to as a computing unit. The term

[0130] "Computing unit" preferably refers to any device or unit that can be configured to perform computing operations. The computing unit is preferably, for example, a processor, a processor chip, a microprocessor, and / or a microcontroller. The computing unit may also preferably be a programmable circuit board. The computing unit may also preferably comprise a computer-usable or computer-readable medium, such as a hard disk, random access memory (RAM), read-only memory (ROM), flash memory, etc.

[0131] Method steps for evaluating the modulated absorption of the reference gas are preferably performed by the electronic circuit or computing unit. The phrase "is configured to" preferably means that computer code and / or software is installed on the electronic circuit or computing unit to perform the method step, for example, to check the extent to which modulated absorption has been performed and / or what proportion of the reference gas is present within a beam path between an emitter and the reference chamber. The computer code and / or software can be written in any programming language or a model-based development environment, e.g., C / C++, C#, Objective-C, Java, Basic / VisualBasic, MATLAB, Simulink, StateFlow, Lab View, and / or Assembler.The computer code and / or software, which is preferably installed on the electronic circuit or processing unit, can be considered a technical feature because a direct physical effect is utilized for the operation of the reference chamber. Functional descriptions of the computer code and / or software can therefore be considered preferred and defining embodiments of the invention.

[0132] In a further aspect, the invention relates to a method for producing a reference chamber according to the above-described, comprising the following steps: a) providing a base and a cap for a TO housing, wherein a MEMS sensor and / or an electronic circuit is present on a front side of the base, b) coating the base and / or the cap with a passivation layer, c) filling the TO housing with a reference gas, d) closing the TO housing to encapsulate the reference gas.

[0133] The average person skilled in the art recognizes that technical features, definitions and advantages of embodiments that apply to the reference chamber according to the invention and to preferred embodiments of the reference chamber equally apply to the manufacturable reference chamber, and vice versa.

[0134] The process has proven to be a process-efficient method for producing the reference chamber. Advantageously, complex, time-consuming, and cost-sensitive process steps for preparing the reference chamber can be eliminated. Thus, rapid and cost-effective production of the reference chamber is possible.

[0135] Preferably, a TO housing is first provided which comprises a base and a cap. Preferably, the TO housing can be provided directly, i.e. the base and the cap are already connected to one another. It can also be preferred for the TO housing to be provided in such a way that a base and a cap are provided and then connected to one another. The base and / or the cap, in particular the front side thereof, is coated with the passivation layer at least in sections. This provides protection for the components in the interior of the reference chamber which could be impaired by a possible corrosive effect of the reference gas within the reference chamber. The filling of the reference gas preferably takes place after the passivation layer has been coated within the reference chamber.After filling the reference gas, the reference gas is sealed to prevent the reference gas from escaping from the reference chamber and / or to prevent unwanted entry of foreign gas into the reference chamber.

[0136] In a further preferred embodiment, the method is characterized in that the coating of the passivation layer is carried out by a method selected from a group comprising spray coating, mist coating, vapor coating and / or via at least one fluid line located on the base of the TO housing.

[0137] The aforementioned preferred methods for coating the passivation layer are known to those of ordinary skill in the art. They are simple to implement, cost-effective, and can be implemented in mass applications. Furthermore, they ensure a reliable, tight application of the passivation layer to the front side of the TO package, providing a protective effect for the TO package.

[0138] In particular, spray coating advantageously enables a close-fitting coating of a dielectric material for providing the passivation layer on the front side of the socket and / or the cap and / or on the MEMS sensor, the electronic circuit, and / or a connection. Applying a spray coating to polymeric materials, such as TEOS and / or parylene, achieves a particularly close-fitting coating. A secondary coating of, for example, TEOS and / or parylene advantageously achieves a completely surface-conformal coating for forming the passivation layer. A completely surface-conformal coating is also advantageously possible through vapor deposition, for example with regard to TEOS and / or parylene.

[0139] In a further preferred embodiment, the method is characterized in that the reference gas is flooded into a chamber, preferably a friction welding system, in which the base and the cap are provided and these are connected to each other within the chamber, so that the reference gas is enclosed within an interior region of the TO housing.

[0140] The reference gas is preferably enclosed within the TO housing by connecting the base and cap within the chamber. Preferably, the chamber is flooded with the reference gas beforehand, i.e., before the base and cap are connected. The reference gas spreads evenly within the chamber through diffusion. The chamber preferably refers to a closed volume into which the reference gas is introduced. By connecting the base to the cap after flooding the chamber with the reference gas, the reference gas is encapsulated within the TO housing.

[0141] In embodiments of the invention, the reference gas can be enclosed in the TO housing using a friction welding system. A friction welding system preferably refers to a system designed to perform friction welding, whereby the base and cap are joined together. Friction welding preferably refers to a welding process in which the base and cap are moved relative to one another and touch at contact surfaces. The friction created by this contact heats and plasticizes the material of the base and / or cap. At the end of the friction process, the base and / or cap is positioned as desired, and high pressure is applied. This advantageously results in a particularly strong connection between the base and cap, which enables hermetic enclosure of the reference gas in the interior of the reference chamber.

[0142] In a further preferred embodiment, the method is characterized in that the reference gas is introduced into the TO housing via at least one fluid line located on the base.

[0143] The at least one fluid line preferably has a hose- or tube-shaped configuration. The fluid line is preferably introduced into a passage in the base. The passage preferably refers to a bore through the base. The fluid line therefore provides a cavity through which the reference gas can be transferred into the housing. The at least one fluid line preferably has a first and a second open end. The second open end is preferably introduced into the base, in particular into a passage in the base, via which the reference gas is introduced into the housing. The preferably first end of the at least one fluid line is preferably positioned such that it protrudes from the base. This provides a contacting option for introducing the reference gas into the at least one fluid line and thus into the housing via a further fluid connection means.

[0144] When using a fluid line, a low pressure can preferably be provided by sucking an output gas from the TO housing before introducing the reference gas.

[0145] Low pressure preferably means a pressure that is lower than the ambient pressure of the reference chamber. At a standard ambient pressure of approximately 100,000 Pa (Pascal), the low pressure has a value of, for example, approximately 0 - 99,000 Pa, 0 - 90,000 Pa, 0 - 80,000 Pa, 0 - 70,000 Pa, 0 - 60,000 Pa, 0 - 50,000 Pa, 0 - 40,000 Pa, 0 - 30,000 Pa, 0 - 20,000, 0 - 10,000 Pa, 0 - 5,000 Pa, 0 - 2,000 Pa, 0 - 1,000 Pa, 0 - 500 Pa, 0 - 200 Pa, 0 - 100 Pa, 0 - 10,10 Pa, 0 - 5 Pa. In particular, the low pressure may be a vacuum. A person skilled in the art knows that a vacuum does not have to be absolute, i.e. it does not correspond to an ideal vacuum of 0 Pa, but rather concerns sufficiently low values, particularly compared to the ambient pressure of the reference chamber, e.g. a rough vacuum up to approximately 100 Pa, a fine vacuum with approximately 100 mPa - 100 Pa, a high vacuum with approximately 100 mPa - 1 pPa, an ultra-high vacuum with approximately 1 pPa - 1 nPa, an extremely high vacuum with a pressure of less than approximately 1 nPa.The absolute value of the low pressure depends on the absolute value of the ambient pressure, wherein the low pressure in the context of the invention is to be designed such that it is lower than the ambient pressure, preferably lower by a factor of at least 10, at least 100, at least 1000, at least 10,000, at least 100,000 or more.

[0146] The starting gas preferably means a gas that is located in the housing of the reference chamber before the reference gas is introduced into the housing. The starting gas can comprise a gas, a gas compound, or a gas mixture. The starting gas can comprise, for example, air or an inert gas (e.g., nitrogen), without being limited to this example. Other gases, gas compounds, or gas mixtures can also form the starting gas. Preferably, the starting gas is sucked out of the housing, and after sucking, the reference gas is introduced into the housing. Suctioning comprises at least partially removing the starting gas from the interior of the housing to the outside. For this purpose, it is preferred to fluidically connect a suction device to the housing via precisely one fluid line in order to suction out the starting gas.A fluidic connection preferably means a connection to enable, in particular, a gas exchange between the housing and the suction device. If a fluidic connection is present, fluid communication is possible, i.e. a transfer of one or more fluids can take place. In the sense of the invention, fluid means a liquid or a gas, preferably a gas. The person skilled in the art knows that, for example, a hose and / or pipe connection can be attached for a fluidic connection. The suction device can preferably comprise a pump, which allows reliable removal of the source gas. The person skilled in the art is able to select corresponding suction devices from the prior art.

[0147] The availability of low pressure depends on the amount of outlet gas remaining within the housing after extraction. The lower the amount of outlet gas in the housing, the lower the low pressure within the housing. A higher amount of outlet gas remaining in the housing after extraction corresponds to a higher pressure within the housing.

[0148] Preferably, after the low pressure in the housing has been established, the housing is filled with the reference gas. The reference gas can be introduced into the housing via a single fluid line. The amount of reference gas to be introduced depends on the amount of source gas extracted. The more source gas extracted, i.e., the less source gas remains inside the housing after extraction, the more reference gas can be introduced into the housing.

[0149] Conversely, if less source gas has been extracted, i.e., if correspondingly more source gas remains in the housing after extraction, less reference gas can be introduced into the housing. After the reference gas has been introduced via the single fluid line, this line is sealed to prevent the reference gas from escaping.

[0150] Alternatively, it may also be preferable to provide two or more fluid lines.

[0151] In a preferred embodiment, the method is characterized in that the reference gas is introduced into the TO housing via two fluid lines by purging.

[0152] When at least two fluid lines comprising the first fluid line and the second fluid line are attached to the base, the reference gas is preferably introduced into the housing by a type of purging. The preferred introduction of the reference gas via the first fluid line displaces the source gas located in the housing. As a result of the displacement of the source gas, which results from the introduction of the reference gas, the source gas can at least partially, preferably essentially entirely, leave the TO housing through the second fluid line. The amount of reference gas to be introduced can be used to determine how much source gas can still be present in the housing after filling.When introducing reference gas into the housing by attaching a first fluid line and a second fluid line, the two process steps, namely the introduction of the reference gas and the removal of the source gas, can therefore take place in one process sequence.In a further aspect, the invention relates to a method for producing a photoacoustic gas sensor comprising the following steps: a) providing a reference chamber as described above or producing a reference chamber filled with a reference gas as described above, b) providing a modulatable emitter, c) arranging the reference chamber filled with the reference gas and the modulatable emitter such that the reference chamber is present in a beam path of the modulatable emitter and the modulatable emitter can excite the reference gas in the reference chamber by means of modulatably emittable radiation to form sound pressure waves which can be detected with the aid of the MEMS sensor.

[0153] In a further aspect, the invention relates to a photoacoustic gas sensor comprising a modulatable emitter, a reference chamber filled with a reference gas as described above, wherein the reference chamber comprises a MEMS sensor and is present in a beam path of the emitter, so that the emitter can excite the reference gas in the reference chamber by means of modulatably emittable radiation to form sound pressure waves, which can be detected with the aid of the MEMS sensor.

[0154] The average person skilled in the art recognizes that technical features, definitions and advantages of embodiments disclosed for the method according to the invention for producing the reference chamber or the reference chamber that can be produced thereby equally apply to a photoacoustic gas sensor that comprises such a reference chamber or to a method for producing the photoacoustic gas sensor, and vice versa.

[0155] Advantageously, the invention makes it possible to obtain a miniaturized photoacoustic gas sensor that can safely and hermetically enclose corrosive, toxic, and / or explosive gases as reference gases, particularly at a selected concentration, and thus monitor the presence of such hazardous gases in the environment. The method according to the invention ensures that sensitive components of the photoacoustic gas sensor, in particular the MEMS sensor and / or the electronic circuit, are not attacked either during the introduction of the reference gas into the housing or over a potentially extended period of use. Furthermore, the hermetic enclosure prevents the potentially hazardous gases from escaping from the reference chamber, which, due to the compact arrangement, also ensures a high degree of safety against potential damage.

[0156] Advantageously, there are no restrictions regarding the selection of gases that can be monitored by the photoacoustic gas sensor using photoacoustic spectroscopy. Instead, a broad spectrum of possible gases can advantageously be introduced into the reference chamber and thus monitored using the photoacoustic gas sensor. In particular, it is advantageously possible to reliably introduce even corrosive, toxic, and / or explosive gases and monitor them over long periods of time using photoacoustic spectroscopy. Due to the presence of a passivation layer, no or only a reduced chemical reaction of the reference gas occurs with a direct portion of the front side of the base and / or cap and / or components of the reference chamber, for example with a MEMS sensor, an electronic circuit, and / or electrical connection.This advantageously ensures long-lasting functionality of the photoacoustic gas sensor, since the passivation layer prevents or reduces impairment of functionality due to a chemical reaction caused by the reference gas.

[0157] Thus, the photoacoustic gas sensor achieves a long service life by having the passivation layer within the reference chamber. In particular, reliable operational capability can be achieved over several years, for example, three, four, or more. Advantageously, it eliminates the need for complex maintenance and / or refilling of photoacoustic gas sensors within (short) time intervals. Instead, long-term and reliable monitoring is achieved by using a single photoacoustic gas sensor comprising a reference chamber coated with a passivation layer.

[0158] Another major advantage of the photoacoustic gas sensor is that, thanks to the passivation layer coating, a high concentration of an aggressive gas can be introduced into the reference chamber without a high risk that areas and / or components present in the reference chamber could be adversely affected by the aggressive reference gas. The fact that a high concentration can be introduced into the reference chamber enables the detection of particularly small amounts of the reference gas in the environment to be monitored by the photoacoustic gas sensor. Thus, the use of a passivation layer within the reference chamber also increases the sensitivity of the photoacoustic gas sensor.

[0159] In a particularly preferred embodiment, the reference gas comprises ammonia. Accordingly, the photoacoustic gas sensor is particularly well suited for monitoring ammonia. Reliable monitoring of other corrosive, aggressive, and / or toxic gases is also advantageously possible. Ammonia is highly industrially relevant, for example, in refrigeration technology. However, due to its chemical properties, an undesired release is associated with certain risks, so the prior art strives to enable reliable and long-lasting monitoring. This is advantageously achieved by the photoacoustic gas sensor according to the invention. Consequently, the photoacoustic gas sensor according to the invention represents a significant advance in industries requiring monitoring of toxic, corrosive, and / or explosive gases, such as ammonia.

[0160] In a further preferred embodiment, the photoacoustic gas sensor comprises a control and / or regulating unit which is configured to control the modulatable emitter and / or the MEMS sensor and / or the electronic circuit, to receive and / or evaluate data. The control and / or regulating unit can preferably be external (i.e., located outside the reference chamber) and have a data connection (preferably via the at least one connecting line) to the MEMS sensor and / or the electronic circuit which is located within the reference chamber. The control and / or regulating unit can, for example, be selected from a group comprising a data processing unit, e.g.integrated circuit (IC), an application-specific integrated circuit (ASIC), a programmable logic circuit (PLD), a field programmable gate array (FPGA), a microprocessor, a microcomputer, a programmable logic controller and / or another electronic, preferably programmable, circuit. For example, it may be preferred that the control and / or regulating unit, on the one hand, outputs control signals which control the modulatable emitter and the MEMS sensor. On the other hand, the control and / or regulating unit can preferably be used to evaluate the measurement data recorded by the MEMS sensor (in particular measurement data relating to the PAS signal). The MEMS sensor and / or the electronic circuit (internal electronic circuit) within the reference chamber can also preferably already perform a (pre-)evaluation of the measurement data.However, it may also be preferred that the internal electronic circuit essentially forwards the measurement data unprocessed to the control and / or regulation unit for further processing and / or evaluation, which is preferably located externally, i.e. outside the reference chamber.

[0161] In a further preferred embodiment, the photoacoustic gas sensor is characterized in that the reference chamber is a closed system into which the reference gas is introduced and a gas to be analyzed is present in the beam path between the emitter and the reference chamber, so that a proportion of the reference gas in the gas to be analyzed can be measured by the MEMS sensor based on the formation of sound pressure waves in the reference chamber.

[0162] A photoacoustic gas sensor is known to those skilled in the art in its basic principles and essential components. A modulatable emitter generates electromagnetic radiation and is preferably arranged and configured such that the radiation emitted by the emitter substantially or at least partially impinges on the reference gas in the reference chamber.

[0163] If the modulated irradiation occurs at a wavelength that corresponds to the absorption spectrum of a molecule of a gas component present in the gas mixture, a modulated absorption occurs, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. According to the photoacoustic effect, the heating and cooling processes lead to expansions and contractions of the gas component, which stimulates it to form sound pressure waves with essentially the modulation frequency. The sound pressure waves are also referred to as PAS signals and can be measured using a MEMS sensor, for example, a MEMS microphone, and / or an electronic circuit. The power of the sound waves is preferably directly proportional to the concentration of the absorbing gas component.

[0164] If a molecule corresponding to a molecule of the reference gas enters the beam path between the reference chamber and the modulatable emitter, an at least partially modulated absorption of the radiation occurs. This reduces the intensity of the radiation hitting the reference chamber for the wavelength range of the absorption spectrum of molecules of the reference gas. Accordingly, the sound pressure waves within the reference chamber, which result from excitation of the molecules of the reference gas, are also reduced. As a result, a reduced or attenuated PAS signal is detected by the MEMS sensor and / or the electronic circuit within the reference chamber. The extent of the reduction or attenuation allows a conclusion to be drawn about the proportion of the reference gas that is in the beam path or the measuring section.

[0165] The term gas component preferably refers to the proportion of chemically (and spectroscopically) identical gas molecules in a gas mixture, while the gas mixture refers to the totality or mixture of several (preferably different) gas components.

[0166] Various emitters are preferred as radiation sources for the aforementioned applications. For example, narrowband laser sources can be used. These advantageously allow the use of high radiation intensities and can be modulated, preferably at high frequencies, using standard PAS components.

[0167] Broadband emitters can also be used. These advantageously have a broad spectrum, which can be further selected, for example, by using (tunable) filters.

[0168] In a further preferred embodiment, the modulatable emitter is a thermal emitter and comprises a heating element, wherein the heating element comprises a substrate on which a heatable layer of a conductive material is at least partially applied, on which contacts for a current and / or voltage source are present.

[0169] The heating element preferably comprises a heatable layer made of a conductive material that produces Joule heat when an electric current flows through it. The heating element particularly comprises a substrate on which the heatable layer is located. The substrate preferably forms the base of the heating element. The substrate may also at least partially comprise other elements of the emitter, such as base elements and / or housing elements. The substrate may preferably be selected from a group comprising silicon, monocrystalline silicon, polysilicon, silicon dioxide, silicon carbide, silicon germanium, silicon nitride, nitride, germanium, carbon, gallium arsenide, gallium nitride, and / or indium phosphide.The conductive material for forming the heatable layer can preferably be selected from a group comprising platinum, tungsten, (doped) tin oxide, monocrystalline silicon, polysilicon, molybdenum, titanium, tantalum, titanium-tungsten alloy, metal silicide, aluminum, graphite, and / or copper. These materials exhibit the desired thermal, electrical, mechanical, and / or radiation properties and are also particularly easy and cost-effective to process.

[0170] The emitter is modulatable, which preferably means that the intensity of the emitted radiation, preferably the intensity of the beam, can be controllably changed over time. The modulation should preferably cause a temporal change in the intensity as a measurable quantity. This means, for example, that the intensity over time between the weakest intensity measured within the measuring period and the strongest intensity measured within the same period exists a difference that is greater than the sensitivity of a device typically used for measuring or determining the intensity for the radiation spectrum and the application. Preferably, the difference is significantly greater than a factor of 2, more preferably 4, 6 or 8 between the strongest and the weakest adjustable intensity. Particularly preferably, the modulation of the intensity of the modulated beam takes place for one or more predetermined resonance wavelengths.

[0171] Preferably, direct modulation can be achieved by varying the current supply. In a thermal emitter, such modulation is usually limited to a specific range of a modulation spectrum due to thermal time constants, e.g., in the range of up to 100 Hz. For a laser or an LED, for example, significantly higher modulation rates, e.g., in the kHz range and beyond, are preferably possible.

[0172] Modulation of the emitter can preferably also be achieved by external modulation, e.g., by using a rotating chopper wheel and / or an electro-optical modulator. A modulatable emitter preferably refers to a device that emits electromagnetic radiation in a wavelength range within a specific spectrum. The spectrum is particularly selected to correspond to the preferred field of application of the emitter, namely PAS. In particular, vibrational excitation of the gas molecules to be spectroscoped and / or detected in the reference gas is preferred, which, depending on the gas molecule, correspond to a preferred spectral range. The emitter can preferably be an IR emitter, i.e., an emitter designed to emit infrared radiation.

[0173] The emitter preferably emits a beam oriented in a straight line toward the reference chamber. Additional components, such as lenses, can be integrated into the emitter or attached externally to focus or collimate the beam. A person skilled in the art knows how to shape the emission profile of the radiation source through the design of the emitter and the use of additional components to achieve a desired beam profile and beam direction.

[0174] In a further preferred embodiment, the emitter is also housed within a TO housing, with the reference chamber preferably comprising another TO housing. Advantageously, a high-performance contacting option is thus offered for both a modulatable emitter and a reference chamber comprising a MEMS sensor and / or electronic components, enabling direct and simple integration of the components into a gas sensor (system). Preferably, both the TO housing of the modulatable emitter and the TO housing of the reference chamber each have a transparent region, which are arranged opposite one another in the gas sensor. An analysis volume, for example in the form of a gas tube, is arranged between the TO housings.

[0175] Preferably, the reference chamber is located in the beam path of the emitter. This preferably means that the intensity of the beam impinges substantially or at least partially on the side of the reference chamber facing the emitter. Partially preferably means at least 40%, preferably at least 50%, 60% or more.

[0176] Preferably, the side of the reference chamber facing the emitter is at least partially transparent to the emitted radiation, so that the radiation essentially reaches the gas-fillable interior of the reference chamber, the reference gas. The side of the reference chamber facing the emitter is preferably also referred to as the incident radiation surface.

[0177] The fact that the reference chamber is located in the beam path of the emitter means, in particular, that the emitter can excite gas in the detection chamber to generate sound pressure waves using modulatably emittable radiation, since this radiation is at least partially irradiated (preferably at least 40%, more preferably at least 50%, in particular at least 60%) and, in particular, a substantial portion of the radiation reaches the volume inside the reference chamber that can be filled with reference gas. A substantial portion means, in particular, at least 80%, more preferably 90%, and in particular 95%.

[0178] Terms such as substantially, approximately, about, approx. etc. preferably describe a tolerance range of less than ± 20%, preferably less than ± 10%, even more preferably less than ± 5% and in particular less than ± 1%. Specifications such as substantially, approximately, about, approx. etc. always disclose and include the exact value stated. The reference chamber contains the reference gas, to which the emitter is tuned in such a way that modulated radiation penetrating the reference chamber excites gas molecules of the reference gas there to PAS. As explained above, this takes advantage of the fact that if the measuring section between the emitter and the reference chamber also contains a proportion of reference gas, part of the emitter's radiation is absorbed by the reference gas present in the measuring section. The strength of the absorption in the reference chamber and thus the measured PAS signal in the reference chamber is thereby reduced.The measuring section can, for example, be located in a gas tube which has an open connection to the environment of the gas sensor.

[0179] The magnitude of the reduction allows conclusions to be drawn about the concentration of the reference gas in the measuring section (and thus, preferably, about the surroundings of the gas sensor). The formation of sound pressure waves in the reference chamber is preferably smaller the more of the reference gas (i.e., the gas to be detected) is present in the beam path outside the reference chamber, since absorption and excitation already occur there.

[0180] This measurement principle allows the detection of the smallest concentrations of molecules of a reference gas within the environment or within an analysis volume. This makes monitoring toxic, corrosive, and / or explosive gases particularly safe and reliable.

[0181] The measuring principle also advantageously ensures intrinsic error control and alarm. Only if the photoacoustic gas sensor is functioning correctly will a PAS signal be reliably detected, which exhibits the expected maximum amplitudes in the absence of an abnormal concentration of the reference gas in the environment or in the analysis volume. Limit values ​​can be defined for the monitoring range, which correspond to a permissible concentration of the reference gas.

[0182] If a component of the photoacoustic gas sensor, e.g., the MEMS sensor and / or the electronic circuit, is faulty, this is detected as a change in the expected PAS signal. A warning can be issued automatically. Depending on the deviation from the PAS signal, the warning message may indicate a suspected malfunction of the photoacoustic gas sensor and / or impermissible limit values. The warning therefore does not necessarily have to originate from a watchdog function or similar, but is inherent in the measurement principle. The absence of a warning and an undetected increase in potentially dangerous concentrations of the reference gas in the environment or in the analysis volume are effectively prevented.

[0183] In a further preferred embodiment, the photoacoustic gas sensor is characterized in that a gas tube is mounted within the beam path between the modulatable emitter and the reference chamber, which gas tube preferably has openings so that a gas to be analyzed can flow in and / or diffuse through the openings.

[0184] The gas tube preferably encloses the analysis volume in whose measuring section a portion or concentration of reference gas is detected. A gas tube can also be referred to as a gas pipe and is characterized by an elongated container that is longer than its height and / or width. The length dimension preferably refers to the extension of the gas tube along the measuring section or the distance from the modulatable emitter to the reference chamber.

[0185] In particular, the gas tube comprises a tube body enclosing an analysis volume. The radiation from the modulatable emitter passes through the analysis volume to the reference chamber. On the one hand, the gas tube advantageously allows the beam path between the modulatable emitter and the reference chamber to be defined. On the other hand, the preferred openings in the gas tube advantageously ensure the flow and / or diffusion of ambient air, for example, thus enabling continuous monitoring.

[0186] If one or more gas molecules pass through the opening and thus into the beam path between the modulatable emitter and the reference chamber, the PAS signal to be detected is attenuated, which serves as an indicator for the presence of reference gas components. Appropriate measures can advantageously be taken quickly.

[0187] The modulatable emitter and the reference chamber are permanently connected to the gas tube, allowing a fixed position to be specified, in which the emitter irradiates the reference gas with modulated radiation through the analysis volume of the gas tube. The modulatable emitter and / or the reference chamber can preferably be housed in a TO housing, enabling simultaneous electrical contact to operate the modulatable emitter and / or components located in the reference chamber, such as the MEMS sensor and / or the electronic circuit.

[0188] Preferably, the connection between the gas tube and the reference chamber and / or between the gas tube and the emitter is detachable to achieve a modular design for the photoacoustic gas sensor. The modular design, particularly the detachable connection of the housing to the gas tube, advantageously enables easy maintenance of the components, for example, when a component needs to be replaced. Cleaning work can also be carried out more easily. This ensures long-term usability for the photoacoustic gas sensor.

[0189] Furthermore, the modular design allows the photoacoustic gas sensor to be adapted depending on the application. For example, a corresponding set of reference chambers can be provided for the detection of different reference gases, each of which contains the respective reference gas. In preferred embodiments, the same modulatable emitter can be used to excite different reference gases. Alternatively, modular interchangeability of the modulatable emitter can also be provided. This type of modular design enables a particularly cost-effective provision of a gas sensor for monitoring a wide variety of reference gases.

[0190] In preferred embodiments, the gas sensor is characterized in that the gas sensor comprises two reference chambers, each of which encloses a first and a second reference gas. Depending on the desired application, the reference chambers can be detachably connected to a gas tube of the gas sensor. It is understood that only one reference chamber is connected to monitor a reference gas. Therefore, the preferred embodiment can also be considered a kit comprising a set of reference chambers. By selecting a reference chamber accordingly, a gas sensor configured for a desired application can be provided.

Claims

PATENT CLAIMS 1 . Reference chamber for a photoacoustic gas sensor comprising a TO housing, a MEMS sensor and / or an electronic circuit and a reference gas which is introduced into an interior region of the TO housing, wherein the TO housing comprises a base and a cap, each of which has a front side facing the interior region and wherein the MEMS sensor and / or the electronic circuit is mounted on the front side of the base and wherein at least one connecting line for electrical contact is further present in the base, characterized in that the front side of the base and / or the cap are coated at least in sections with a passivation layer.

2. Reference chamber according to the preceding claim, characterized in that the front side of the base and / or the cap is / are coated with the passivation layer to a proportion of at least 50%, preferably at least 60%, at least 70%, at least 80%, particularly preferably substantially completely, and / or the passivation layer has a thickness of 10 nm - 1 mm, preferably 100 nm - 800 pm, particularly preferably 1 pm - 700 pm, particularly preferably 100 pm - 500 pm.

3. Reference chamber according to one or more of the preceding claims, characterized in that the passivation layer comprises a passivation material selected from a group comprising an electrical conductor, a semiconductor and / or an insulating material, wherein the electrical conductor or the semiconductor is preferably present in combination with the insulating material for electrical insulation and / or wherein the passivation material is preferably selected from a group comprising silicon, preferably as monosilicon or polysilicon, polyimide, aluminum nitride, novolak, polymethylglutarimide, tetraethylorthosilicate (TEOS), parylene, perfluorodecyltrichlorosilane and / or epoxy resin.

4. Reference chamber according to one or more of the preceding claims, characterized in that a rear side of the base and / or the cap facing away from the interior area is also coated at least in sections with a passivation layer.

5. Reference chamber according to one or more of the preceding claims, characterized in that the reference chamber is operatively connected to a heating element, wherein the heating element is designed to heat the reference gas within the reference chamber, wherein the reference gas is preferably heated to no more than 100°C, preferably to a temperature of 20°C - 100°C, preferably of 30°C - 80°C, particularly preferably of 40°C - 60°C.

6. Reference chamber according to the preceding claim, characterized in that the heating element is present within the reference chamber, preferably on the front side of the base and / or the cap, wherein the heating element is preferably provided by at least one heating section as a section of the passivation layer, wherein the heating section preferably has a substantially annular configuration.

7. Reference chamber according to one or more of the preceding claims, characterized in that the heating element is located outside the reference chamber, wherein the heating element is positioned at a distance from or contacts a rear side of the cap and / or the base.

8. Reference chamber according to one or more of the preceding claims 5-7, characterized in that the heating element is an electrical heating element, preferably a heating resistor, wherein the heating element preferably comprises a material selected from a group comprising monosilicon, polysilicon, aluminum, copper, gold, platinum, nickel, silver and / or tungsten, wherein the material preferably has a doping as semiconductor material.

9. Reference chamber according to one or more of the preceding claims, characterized in that the MEMS sensor comprises an inert material, wherein the inert material is preferably selected from a group comprising silver, platinum, gold and / or copper.

10. Reference chamber according to one or more of the preceding claims, characterized in that the reference gas is a corrosive, toxic and / or explosive gas, wherein the reference gas is preferably selected from a group comprising methane, propane, propylene, silane, chlorosilane, carbon monoxide, carbon dioxide, sulfur hexafluoride and / or ammonia.

11. Reference chamber according to one or more of the preceding claims, characterized in that the at least one connecting line for electrically contacting the MEMS sensor and / or the electronic circuit is located on a rear side of the base, wherein the at least one connecting line is preferably guided through the base, wherein the at least one connecting line is preferably sealed with an insulating material and is electrically insulated from the base.

12. Reference chamber according to one or more of the preceding claims, characterized in that the MEMS sensor is a MEMS microphone, wherein the MEMS microphone is preferably read capacitively, optically, magnetically, piezoresistively and / or piezoelectrically.

13. A method for producing a reference chamber according to one or more of the preceding claims 1-12, comprising the following steps: a) providing a base and a cap for a TO housing, wherein a MEMS sensor and / or an electronic circuit is present on a front side of the base, b) coating the base and / or the cap with a passivation layer, c) filling the TO housing with a reference gas, d) closing the TO housing to encapsulate the reference gas.

14. Photoacoustic gas sensor comprising a modulatable emitter, a reference chamber filled with a reference gas according to one of the preceding claims 1-12, wherein the reference chamber comprises a MEMS sensor and is present in a beam path of the emitter, so that the emitter can excite the reference gas in the reference chamber by means of modulatably emittable radiation to form sound pressure waves which can be detected by means of the MEMS sensor.

15. A method for producing a photoacoustic gas sensor comprising the following steps: a) providing a reference chamber according to one or more of the preceding claims 1-12 or producing a reference chamber filled with a reference gas according to a method according to claim 13, b) providing a modulatable emitter, c) arranging the reference chamber filled with the reference gas and the modulatable emitter such that the reference chamber is present in a beam path of the modulatable emitter and the modulatable emitter can excite the reference gas in the reference chamber by means of modulatably emittable radiation to form sound pressure waves which can be detected with the aid of the MEMS sensor.