PHOTOACUSTIC GASS SENSOR AND METHOD FOR MANUFACTURING

Description

[0001] The invention relates to a photoacoustic gas sensor for a target gas.

[0002] Furthermore, the invention relates to a method for producing a photoacoustic gas sensor for a target gas.

[0003] A gas sensor is a sensor for detecting gaseous substances—in other words, for detecting the target gas. The proportion of the target gas in the gas under investigation, i.e., chemical information, is converted by the gas sensor into an electrical signal. Many methods for converting the chemical information into the electrical signal are known, which also result in advantages and disadvantages of different sensor types.

[0004] With the resistive principle, the target gas directly influences the conductivity of a gas-sensitive sensor layer, which is formed, for example, by an inorganic metal oxide semiconductor (MOX). The advantages of this measuring principle are the possibility of micromechanical manufacturing and high sensitivity. However, disadvantages include aging-related drift in the sensor signal and low selectivity.

[0005] Non-dispersive infrared spectroscopy (NDIR), on the other hand, offers a method with very high selectivity, long-term stability, and reproducibility. An NDIR setup essentially consists of a spectrally broadband light source, an optical path, and a photodetector whose spectral response is limited by suitable spectral filters. However, current NDIR setups do not allow for microsystem manufacturing, as the sensitivity is essentially determined by the optical path.

[0006] One way to downsize the design is photoacoustic spectroscopy (PAS), which allows gases to be detected very precisely and selectively. The basic measuring principle is as follows: If a pulsed light source is irradiated onto a gas sample in a measuring cell, the gas molecules absorb the light and the gas sample heats up. If the volume of the measuring cell remains constant, this creates acoustic waves whose frequency matches the modulation frequency of the light source. These acoustic waves, i.e. the photoacoustic signal, can be detected with sound-sensitive elements (e.g., microphones). The signal amplitude correlates with the strength of the absorption and thus provides information about the gas concentration in the measuring cell. Due to the design, no photodetector is required.

[0007] The publication Chem. Eng. Technol. 2018, 41, No. 4, 696-701 - "Real-Time Gas Quality Data for On-Demand Production of Biogas" - describes a gas sensor based on the photoacoustic measurement principle and suitable for the detection of both methane and carbon dioxide.

[0008] The manufacturing processes of known photoacoustic gas sensors are based on the use of multiple wafers that are connected to one another using welding, soldering, and / or anodic bonding processes. Due to the manufacturing-related connection points, the robustness and miniaturization of known photoacoustic gas sensors are limited. Furthermore, the manufacturing process is associated with increased complexity and costs. US Pat. No. 6,344,647 B1 describes such a photoacoustic gas sensor constructed from multiple wafers.

[0009] The object of the invention is therefore to provide a manufacturing method that simplifies the production process for photoacoustic gas sensors. In particular, a method is to be provided that does not require the bonding of multiple wafers. Furthermore, the object of the invention is to provide a robust photoacoustic gas sensor that also allows cost-effective production.

[0010] This object is achieved by the subject matter of the independent claims. Preferred developments of the invention are described in the subclaims.

[0011] According to the invention, a method according to claim 1 for producing a photoacoustic gas sensor for a target gas is thus provided.

[0012] According to the invention, a photoacoustic gas sensor according to claim 10 for a target gas is also provided.

[0013] The core idea of ​​the invention is that the photoacoustic gas sensor is manufactured starting from exactly one silicon wafer. The wafer, which is provided in the first step of the method, has the buried insulation layer. A buried insulation layer is a layer inside the silicon wafer that runs parallel to the front and back of the wafer and is not made of silicon. Instead, the buried insulation layer comprises a material that preferably acts as an etch stop in etching processes. In the method, starting from exactly one wafer, a cavity is created on the front and back of the wafer. This creates two cavities that are separated from each other by the buried insulation layer. The first cavity forms the measuring volume of the gas sensor and can be flowed through by the gas and / or gas mixture to be analyzed.The sound-sensitive element, which comprises the second cavity, serves as a detection device. The same gas species as the gas to be detected by the gas sensor – i.e., the target gas – is enclosed in the second cavity. Gas cannot flow through the second cavity. The gas sensor is therefore a sensor specific for the target gas. In other words, the target gas is the gas species whose presence and / or proportion in a gas and / or gas mixture to be analyzed is to be detected by the gas sensor. A gas sensor for a target gas, for example, means a gas sensor for CO2. This means, on the one hand, that the gas sensor can detect CO2 in a gas and / or gas mixture to be analyzed that flows through the first cavity and can preferably determine the proportion of CO2 in the gas and / or gas mixture. On the other hand, it also means that CO2 is enclosed in the second cavity of the gas sensor.

[0014] Furthermore, the target gas enclosed in the second cavity can also be a gas mixture, i.e., a gas mixture comprising more than one gas species. For example, the target gas can be a mixture of CO2 and methane. This provides a gas sensor that is specific to the multiple gas species present in the gas mixture—in this example, a gas sensor for CO2 and methane.

[0015] With regard to the buried insulation layer, it is particularly preferred that the buried insulation layer has a transmittance of at least 40% for electromagnetic radiation of a wavelength used to detect the target gas. Using the example of a gas sensor for CO2, this means the following: Since CO2, among other things, has a vibration band at approximately 2380 to 2320 cm-1 (approximately 4200 nm to 4300 nm), radiation of this wavelength can be used to detect CO2. Thus, for the example of the CO2 sensor, it is preferred that the transmittance of the buried insulation layer be 40% for radiation at approximately 2380 to 2320 cm-1.In a gas sensor that is specific for a plurality of gas species, i.e. the target gas is a gas mixture comprising a plurality of gas species, it is preferably provided that the buried insulation layer has a transmittance of at least 40% for electromagnetic radiation of those wavelengths that are used to detect the plurality of gas species of the target gas.

[0016] With regard to the manufacturing method according to the invention, no connection process takes place between several wafers. In particular, the method does not provide for connecting the first cavity and the second cavity to one another. Instead, starting from the one wafer that has the buried insulation layer in its interior, the first and second cavities are created by removing material. In particular, the buried insulation layer acts as an etch stop during the production of the first and second cavities by means of an etching process, so that after steps b) and c) have been carried out, the first and second cavities are only separated from one another by the buried insulation layer. The buried insulation layer therefore forms a separating structure between the first cavity and the second cavity in the gas sensor produced by the method.At the same time, the buried insulation layer in the gas sensor produced by the process forms an optical window between the first cavity - the measuring volume - and the second cavity - the detection device.

[0017] The method essentially comprises three steps. In the first step a), the wafer is provided. The wafer is a silicon wafer. It can be a monocrystalline or polycrystalline wafer. The wafer preferably has a disk shape and / or a flat front side and a parallel, flat back side. The wafer preferably has a thickness of between 0.5 and 2 mm. As already mentioned, the wafer has the buried insulation layer inside the silicon. In a section perpendicular to the plane, the wafer therefore has the following layer structure: silicon - buried insulation layer - silicon. Silicon as a wafer material has the advantage that it is optically transparent to electromagnetic radiation in the IR range of more than 1.1 µm.

[0018] In the second step b), the suspended thermal emitter comprising the first cavity is created on the back of the wafer. The thermal emitter is preferably suspended from webs and connected to the wafer via the webs. To create the webs and the thermal emitter, the silicon of the wafer is preferably removed by etching processes, which simultaneously creates the first cavity.

[0019] In the third step c), the sound-sensitive element is created on the front side of the wafer. The sound-sensitive element occupies the second cavity. The target gas is enclosed in the second cavity. For example, if a gas sensor for methane is being manufactured, methane is enclosed in the second cavity.

[0020] Steps b) and c) can also be performed in reverse order. This makes it possible to first create the sound-sensitive element on the front side of the wafer and then create the suspended thermal emitter on the back side of the wafer. Steps b) and c) are implemented using photolithographic and etching processes. These are processes commonly used in the manufacture of microsystem technology components.

[0021] Photolithography is a key process in semiconductor and microsystem technology for the production of integrated circuits and other components. An exposure process transfers an image from a photomask to a light-sensitive photoresist. The exposed areas of the photoresist are then dissolved. Alternatively, the unexposed areas of the photoresist can be dissolved when the photoresist cures under light. This creates a lithographic mask that enables further processing using chemical and physical processes, such as the introduction of material into the open windows or the etching of recesses beneath the open windows.

[0022] Chemical and physical etching processes, such as reactive ion etching, plasma etching or wet chemical processes such as etching silicon with potassium hydroxide, use physical and / or chemical processes that lead to the removal of material.

[0023] The method according to the invention thus allows the photoacoustic gas sensor to be manufactured with a monolithic structure starting from a single wafer, making the gas sensor extremely robust. Furthermore, the manufacturing process is greatly simplified because the bonding of multiple wafers is eliminated. Further advantages of the method include the possibility of miniaturizing the photoacoustic gas sensor and the possibility of further reducing manufacturing costs through mass production.

[0024] Preferably, the method can be used to produce gas sensors with dimensions of less than 2 mm. It is also preferably provided that the gas sensor has dimensions of less than 2 mm. Thus, it is preferably a manufacturing method for micromechanical gas sensors or a micromechanical gas sensor.

[0025] As already mentioned, this is a photoacoustic gas sensor. The measuring principle of the photoacoustic gas sensor is based on the fact that gas molecules only absorb electromagnetic radiation of very specific wavelengths. Which wavelengths these are depends on the gas species. In photoacoustic measurements, absorption is measured using a pressure transducer via the pressure increase resulting from the absorption. Furthermore, by providing the target gas in the second cavity, the gas species to be detected itself serves as a gas-selective detector. Therefore, no photodetector for electromagnetic radiation is necessary. To detect a specific gas species—the target gas—in a gas and / or gas mixture under investigation, electromagnetic radiation is used whose wavelength can be absorbed by the target gas.Preferably, the thermal emitter is configured to emit electromagnetic radiation at a wavelength that can be absorbed by the target gas. Furthermore, with regard to the emission spectrum of the thermal emitter, it can be adapted to the absorption spectrum of the target gas, for example, by photonic crystals.

[0026] The pressure increase caused by the absorption can be measured as a sound wave in the second cavity using the sound-sensitive element. If the target gas or gas mixture flowing through the first cavity also contains the target gas, a portion of the electromagnetic radiation emitted by the thermal emitter will be absorbed by the target gas in the gas or gas mixture, which affects the sound wave measured in the second cavity. For a target gas that comprises multiple gas species, the signals can be differentiated from one another, for example, by using different modulation frequencies of the thermal emitter for the different gas species in the target gas.

[0027] With regard to the manufacturing method and in connection with step a), according to a preferred embodiment of the invention, a silicon-on-insulator wafer and / or a {100}Si wafer is provided in step a). A silicon-on-insulator wafer (SOI wafer) is a wafer in which a thin silicon layer is separated by an electrically insulating layer (BOX, English buried-oxide ) from the silicon substrate of the wafer. The SOI wafer therefore has a buried insulation layer through the BOX layer.

[0028] A {100}Si wafer is a wafer in which the front and back sides of the wafer are formed by the crystal plane designated by the Miller indices {100}. The orientation of the crystal plane influences the shape of the structures created in the wafer by chemical etching. During chemical etching of a {100}Si wafer, trapezoidal trenches are initially formed. The sloped sidewalls of the trenches are formed by the {111}Si surfaces, which have a lower etch rate than the {100}Si surfaces. By providing the {100}Si wafer, the thermal emitter and the sound-sensitive element can be realized particularly easily.

[0029] According to a further development of the invention, it is particularly preferred that the wafer provided in step a) comprises a first insulation layer on the back side, a second insulation layer on the front side, and the buried insulation layer between the first insulation layer and the second insulation layer and within the silicon. In other words, in the first step, a wafer is particularly preferably provided whose front and back sides are provided with an insulation layer. This further simplifies the production of the thermal emitter and the sound-sensitive element. The wafer provided in step a) therefore preferably has the following structure in a section perpendicular to the plane, starting with the back side: first insulation layer - silicon - buried insulation layer - silicon - second insulation layer.

[0030] Alternatively, in this context, the method may include the steps of applying a first insulation layer to the backside of the wafer and / or applying a second insulation layer to the frontside of the wafer. Thus, starting from a wafer with the layer structure silicon - buried insulation layer - silicon, a wafer with the layer structure first insulation layer - silicon - buried insulation layer - silicon - second insulation layer is realized.

[0031] The buried insulation layer preferably has a thickness of 1–100 µm. The first insulation layer preferably has a thickness of 1–2 µm, and the second insulation layer preferably has a thickness of 1–10 µm. These insulation layer thicknesses enable the gas sensor to be manufactured in a particularly miniaturized design.

[0032] As already mentioned, with regard to the buried insulation layer, it is preferably provided that the buried insulation layer is made of a material that has a transmittance of at least 40% for the electromagnetic radiation used to detect the target gas. Further preferably, the buried insulation layer is made of a material that has a transmittance of at least 40% for the electromagnetic radiation emitted by the thermal emitter. In this context, according to a further preferred development of the invention, it is provided that the first insulation layer, the second insulation layer and / or the buried insulation layer comprise silicon dioxide, silicon carbide and / or sapphire. The buried insulation layer is preferably formed from silicon dioxide and / or sapphire. Silicon dioxide and sapphire have the advantage of being optically transparent to electromagnetic radiation in the IR range.Accordingly, the electromagnetic radiation emitted by the thermal emitter in the IR range can penetrate unhindered from the first cavity into the second cavity. The first insulation layer is preferably made of a material that has a high emissivity in a wavelength range of the electromagnetic spectrum used for detecting the target gas. This makes it particularly easy to implement an efficient thermal emitter on the backside of the wafer. The first insulation layer is preferably made of silicon carbide. This material has the advantage of having a high emissivity (spectral emissivity) in the IR range.

[0033] As already mentioned, the first cavity in the gas sensor forms the measurement volume through which the gas or gas mixture to be analyzed can flow. The size of this measurement volume, and in particular the distance between the thermal emitter and the second cavity—the detection chamber—influences the sensitivity of the gas sensor. In this context, according to a preferred embodiment of the invention, step a) comprises providing a wafer with a distance between the buried insulation layer and the back side of 500 µm to 2000 µm. Alternatively, step a) comprises providing a wafer with a distance between the buried insulation layer and the first insulation layer of 500 µm to 2000 µm.This results in a gas sensor manufactured using this method with a distance of between 500 µm and 2000 µm between the buried insulation layer, which forms the window to the detection chamber, and the thermal emitter. This enables a gas sensor that has sufficiently high sensitivity while being small enough for many applications.

[0034] With regard to step b)—the realization of the thermal emitter—according to a preferred development of the invention, step b) comprises the realization of a suspended beam source and / or a suspended micromechanical hotplate on the backside of the wafer. A micromechanical hotplate, also referred to as a micro-hotplate (MHP), is a micromechanical component that is used in many sensors as a heating device or beam source. The micromechanical hotplate preferably comprises a heating structure, which is preferably configured as a meandering wire on a support surface. The first cavity in the gas sensor is located between the support surface and the buried insulation layer. Further preferably, the micromechanical hotplate comprises webs via which the support surface of the heating device is connected to the wafer.

[0035] According to a further preferred development, in connection with the thermal emitter, step b) also includes the implementation of an IR emitter. This is advantageous since several target gases absorb in the IR range.

[0036] Also in connection with step b), according to a further preferred development of the invention, it is provided that the wafer comprises the first insulation layer on the back side and step b) comprises the following steps: Applying a heating structure to the first insulation layer by means of photolithographic processes, and creating the first cavity between the first insulation layer and the buried insulation layer in the region of the applied heating structure by means of chemical and / or physical etching processes.

[0037] In other words, the first insulation layer preferably forms the support surface and the webs for the heating structure, wherein the support surface and the webs are formed by removing silicon from the wafer when creating the first cavity. It is particularly preferred that the creation of the first cavity between the first insulation layer and the buried insulation layer in the region of the applied heating structure comprises a partial removal of the first insulation layer next to the applied heating structure, particularly preferably by reactive ion etching of the back side of the wafer, and a subsequent wet-chemical etching of the silicon of the wafer between the first insulation layer and the buried insulation layer in the region of the applied heating structure. As already mentioned, a particularly advantageous feature of the method is that the buried insulation layer acts as an etch stop during the subsequent wet-chemical etching.In this way, the first cavity is realized, which extends to the buried insulation layer.

[0038] Reactive ion etching (RIE) is an etching process that uses reactive ions generated in a gas discharge. RIE combines a physical component—removal by ejecting particles—with a chemical component—removal by chemical reaction.

[0039] In wet chemical etching, etching solutions such as potassium hydroxide (KOH) are used to achieve a material removal process through a chemical reaction. The etching process can be direction-dependent, allowing for the creation of different structures.

[0040] In connection with step c)—the realization of the sound-sensitive element—according to a further preferred embodiment of the invention, step c) comprises realizing a microphone, in which the target gas is enclosed, on the front side of the wafer. This is preferably a condenser microphone using microsystems technology (MEMS microphone), in which the micro-membrane that changes the electrical capacitance is realized directly on the wafer.

[0041] Also in connection with step c), according to a further preferred development of the invention, it is provided that the wafer comprises the second insulation layer on the front side and step c) comprises the following steps: Partial removal of the second insulation layer by means of chemical and / or physical etching processes, application of a cover plate on the front side of the wafer by means of photolithographic processes, wherein the cover plate has a perforated structure in the region of the removed second insulation layer, creation of the second cavity between the cover plate and the buried insulation layer in the region of the perforated structure by means of chemical etching processes, and sealing the second cavity under a target gas atmosphere by applying a sealing layer to the cover plate under a target gas atmosphere.

[0042] Particularly preferably, the second insulation layer is removed on the front side in an area corresponding to the applied heating structure and / or the suspended thermal emitter on the back side. In other words, the second insulation layer opposite the heating structure and / or opposite the thermal emitter is preferably removed. In this way, the sound-sensitive element is realized directly opposite the first cavity, thus ensuring that the first cavity and the second cavity are separated from one another only by the buried insulation layer. When realizing the sound-sensitive element, the second insulation layer and the partially exposed silicon of the wafer preferably initially serve as a carrier for the end plate, which has a perforated structure in the area of ​​the removed second insulation layer and thus preferably forms the microphone diaphragm.

[0043] The silicon of the wafer, which serves as the carrier, is then preferably etched away beneath the perforated structure, creating the second cavity. To introduce the target gas into the second cavity, the sealing layer is applied to the end plate, and thus also to the second cavity, preferably in the target gas atmosphere.

[0044] Particularly preferably, the buried insulation layer acts as an etch stop when creating the second cavity using chemical etching methods, in particular wet-chemical etching. This makes it easy to ensure that the second cavity is separated from the first cavity by the insulation layer.

[0045] As already mentioned, the invention also relates to the photoacoustic gas sensor. The photoacoustic gas sensor for the target gas comprises the thermal emitter, the measurement volume configured as a first cavity, and the sound-sensitive element. The measurement volume is permeable to the gas to be analyzed. The sound-sensitive element comprises the second cavity in which the target gas is enclosed. The first cavity and the second cavity are separated from each other only by the buried insulation layer of the silicon wafer used for production.

[0046] In contrast to gas sensors manufactured by bonding multiple wafers, the photoacoustic gas sensor according to the invention is a monolithic gas sensor. The photoacoustic gas sensor has only the buried insulation layer between the first cavity and the second cavity.

[0047] Furthermore, according to a preferred embodiment, no adhesive layer is present between the first cavity and the second cavity. Instead, the buried insulation layer forms the separating structure between the first cavity and the second cavity and, as an inherent component of the wafer, simultaneously ensures the monolithic structure of the photoacoustic gas sensor.

[0048] The invention will be described in more detail below using a preferred embodiment of the invention with reference to the drawings.

[0049] In the drawings Fig. 1 and 2 schematic representations of the steps of a method for producing a photoacoustic gas sensor according to a preferred embodiment of the invention.

[0050] In the Figures 1 and 2The steps for manufacturing a photoacoustic gas sensor 10 are shown schematically. In the present embodiment, it is a CO2 microsensor 10 with a monolithic structure.

[0051] In the first step of the process, a wafer 12 made of silicon is provided, which is Figure 1is shown at the very top. The wafer 12 has a front side 14 and a back side 16. Furthermore, the wafer 12 has a buried insulation layer 18 within the silicon. The wafer 12 provided in this preferred embodiment is an SOI wafer 12, which also has a further insulation layer 20, 22 on its surfaces, i.e., the front side 14 and the back side 16, respectively. In the present case, the provided wafer 12 has the first insulation layer 20 on the back side 16 and the second insulation layer 22 on the front side 14, as well as the buried insulation layer 18 between the first insulation layer 20 and the second insulation layer 22 and within the silicon. In addition, the wafer 12 provided in the present embodiment is a {100}Si wafer, which means that the front side 14 and back side 16 of the wafer 12 are formed by the crystal plane {100}.

[0052] The insulation layers 18, 20, and 22 of the wafer 12 are all made of silicon dioxide in this case. Furthermore, the distance 24 between the first insulation layer 20 and the buried insulation layer 22 is 1000 µm in this case. The first insulation layer 20 is 1.5 µm thick in this case, and the second insulation layer is 5 µm thick, while the buried insulation layer 18 has a thickness of 30 µm.

[0053] In the second step of the method, a suspended thermal emitter 26 comprising a first cavity 28 is realized on the backside 16 of the wafer 12. In the present case, the suspended thermal emitter 26 is designed as a micro-hotplate 26 that emits electromagnetic radiation in the IR range. The first cavity 28 extends from the thermal emitter 26 to the buried insulation layer 18. The thermal emitter 26 realized according to this method step is in Figure 1shown at the very bottom, while the figures in between schematically show the intermediate steps of how the thermal emitter 26 is realized by means of photolithographic processes and etching processes.

[0054] In the present preferred embodiment of the invention, the thermal emitter 26 and the first cavity 28 are realized by applying a heating structure 30 to the first insulation layer 20 by means of photolithographic methods. Thereafter, next to the applied heating structure 30, a reactive ion etching, which is symbolically indicated by the arrows 32 in Figure 1As shown, the first insulation layer 20 is partially removed. In this way, the silicon of the wafer 12 is exposed. The exposed silicon is then removed in the region of the applied heating structure 30 by wet-chemical etching using potassium hydroxide solution, whereby the first cavity 28 is created. The buried insulation layer 18 serves directly as an etch stop during the wet-chemical etching process. Due to the anisotropic etching process, the first cavity 18 has inclined sidewalls 34, which are formed by the crystal plane {111} of the silicon of the wafer 12. In addition, the first cavity 28 has at least two openings 36, namely at the locations where the first insulation layer 20 was partially removed. These openings 36 allow the gas and / or gas mixture to be examined to flow into the first cavity 28 in the gas sensor 10 produced by the method, so that the first cavity 28 in the gas sensor 10 can assume the function of the measuring volume.After the second process step, the thermal emitter 26 and the first cavity 28 are realized on the back side 16 of the wafer 12 (. Figure 1 at the bottom).

[0055] In the subsequent third process step, the front side 14 of the wafer 12 is processed, which Figure 2 The aim of the further steps is to realize a sound-sensitive element 46 comprising a second cavity 44 on the front side 14, wherein the target gas 48 - in this case CO 2 - is introduced into the second cavity 44 ( Figure 2 at the bottom).

[0056] To achieve this, in an area 40 on the front side 14 of the wafer 12, which essentially corresponds to the area of ​​the thermal emitter 26 suspended on the back side 16, the second insulation layer 22 is removed by means of reactive ion etching 32 ( Figure 2(at the very top). A cover plate 38 is then applied to the exposed silicon of the wafer 12, as well as to the remaining second insulation layer 22, by means of photolithographic processes. This cover plate has a perforated structure 42 in the region 40 of the removed second insulation layer 22.

[0057] Furthermore, the second cavity 44 is created in region 40 by wet-chemical etching using potassium hydroxide solution to remove the silicon of wafer 12 down to the buried insulation layer 18. Analogous to the wet-chemical etching during processing of the rear side 16 of wafer 12 in the second process step, the buried insulation layer 18 also serves as an etch stop during processing of the front side 14 of wafer 12. Accordingly, the first cavity 28 and the second cavity 44 are separated from each other only by the buried insulation layer 18.

[0058] To realize the sound-sensitive element 46, the second cavity 44 is finally sealed under an atmosphere of the target gas 48—in this case, CO2—by applying a sealing layer 50 to the end plate 38. Thus, the microsensor 10 with dimensions of 2x2 mm2 can be provided by the method in a monolithic design, i.e., starting from exactly one wafer 12 and without connecting several wafers together. List of reference symbols

[0059] 10Gas sensor, CO2 sensor 12Silicon wafer 14Front side 16Back side 18Buried insulation layer 20First insulation layer 22Second insulation layer 24Distance between first insulation layer and buried insulation layer 26Thermal emitter 28First cavity, measurement volume 30Heating structure 32Reactive ion etching, arrow 34Sidewall, crystal plane {111} 36Opening 38End plate 40Area corresponding to thermal emitter 42Holes in end plate, perforated structure 44Second cavity 46Sound-sensitive element 48Target gas, CO2 50Sealing layer

Claims

1. Method for producing a photoacoustic gas sensor (10) for a target gas (48) comprising the steps of a) providing a wafer (12) made of silicon and comprising a front side (14) and a rear side (16), wherein the wafer (12) comprises a buried insulation layer (18) within the silicon; b) realizing a suspended thermal emitter (26) comprising a first cavity (28) which is disposed at the rear side (16) of the wafer (12) and forms a measuring volume and through which the gas and / or gas mixture to be examined can flow; c) realizing a sound-sensitive element (46) at the front side (14) of the wafer (12), wherein the sound-sensitive element (46) comprises a second cavity (44) in which the target gas (48) is enclosed, wherein steps b) and c) are realized by means of photolithographic and etching processes such that the first cavity (28) and the second cavity (44) are separated from each other only by the buried insulation layer (18).

2. Method according to claim 1, wherein step a) comprises providing a silicon-on-insulator wafer (12) and / or providing a {100} Si wafer and / or wherein the wafer (12) provided in step a) comprises a first insulation layer (20) on the rear side (16), a second insulation layer (22) on the front side (14), and the buried insulation layer (18) within the silicon between the first insulation layer (20) and the second insulation layer (22).

3. Method according to any one of the preceding claims, wherein the first insulation layer (20), the second insulation layer (22) and / or the buried insulation layer (18) comprises silicon dioxide, silicon carbide and / or sapphire.

4. Method according to any one of the preceding claims, wherein step a) comprises providing a wafer (12) with a distance (24) between the buried insulation layer (18) and the rear side (16) of 500 µm to 2000 µm, or wherein step a) comprises providing a wafer (12) with a distance (24) between the buried insulation layer (18) and the first insulation layer (20) of 500 µm to 2000 µm.

5. Method according to any one of the preceding claims, wherein step b) comprises realizing a suspended beam source and / or a suspended micromechanical hotplate (26) at the rear side (16) of the wafer (12).

6. Method according to any one of the preceding claims, wherein step b) comprises realizing an IR emitter.

7. Method according to any one of the preceding claims, wherein the wafer (12) provided in step a) comprises a first insulation layer (20) at the rear side (16) and step b) comprises the following steps: applying a heating structure (30) on the first insulation layer (20) by means of photolithographic processes; and creating the first cavity (28) between the first insulation layer (20) and the buried insulation layer (18) in the area of the applied heating structure by means of chemical and / or physical etching processes.

8. Method according to any one of the preceding claims, wherein step c) comprises realizing a microphone in which the target gas (48) is enclosed at the front side (14) of the wafer (12).

9. Method according to any one of the preceding claims, wherein the wafer (12) provided in step a) comprises a second insulation layer (22) at the front side (14) and step c) comprises the following steps: partially removing the second insulation layer (22) by means of chemical and / or physical etching processes; applying an end plate (38) to the front side (14) of the wafer (12) by means of photolithographic processes, wherein the end plate (38) has a perforated structure (42) in the area (40) of the removed second insulation layer (22); creating the second cavity (44) between the end plate (38) and the buried insulation layer (18) in the area (40) of the perforated structure (42) by means of chemical etching processes; and sealing the second cavity (44) under a target gas atmosphere by applying a sealing layer (50) to the end plate (38) under the target gas atmosphere.

10. Photoacoustic gas sensor (10) for a target gas (48), comprising a thermal emitter (26), which comprises a measuring volume (28) configured as a first cavity (28), and a sound-sensitive element (46), wherein a gas and / or a gas mixture to be examined can flow through the measuring volume (28), and wherein the sound-sensitive element (46) comprises a second cavity (44), in which the target gas (48) is enclosed, and wherein the first cavity (28) and the second cavity (44) are separated from one another only by a buried insulation layer (18) of a wafer (12) of silicon used for the manufacture.

11. Photoacoustic gas sensor (10) according to claim 10, wherein there is no adhesive layer between the first cavity (28) and the second cavity (44).