Detector unit for photoacoustic gas sensor and photoacoustic gas sensor

Abstract

Embodiments of the present disclosure relate to a detector unit for a photoacoustic gas sensor and a photoacoustic gas sensor. The detector unit for a photoacoustic gas sensor includes a first layer structure, a second layer structure disposed at the first layer structure and including a membrane structure, and a third layer structure disposed at the second layer structure. The first layer structure and the third layer structure hermetically enclose a cavity, wherein the membrane structure is disposed in the cavity.

Description

Technical Field

[0001] The present disclosure relates to a detector unit for a photoacoustic gas sensor, a photoacoustic gas sensor, and a method for manufacturing the detector unit and the photoacoustic gas sensor. The present disclosure also relates to a wafer-level bonded photoacoustic detector unit. Background Art

[0002] Photoacoustic gas sensors can be used to measure ambient conditions, e.g., components of a fluid, especially a gas.

[0003] There is a need for detector units that are highly durable and robust over their lifetime and for photoacoustic gas sensors that are highly durable and robust over their lifetime. There is also a need for methods for manufacturing detector units and photoacoustic gas sensors. Summary of the Invention

[0004] Embodiments provide a detector unit for a photoacoustic gas sensor. The detector unit includes a first layer structure, a second layer structure disposed at the first layer structure and including a membrane structure, and a third layer structure disposed at the second layer structure. The first layer structure and the third layer structure hermetically enclose a cavity, wherein the membrane structure is disposed in the cavity. By enclosing the cavity between the first layer structure and the third layer structure, the seal for hermetically enclosing the cavity can have high durability and high robustness.

[0005] Embodiments provide a photoacoustic gas sensor that includes such a detector unit and includes an electromagnetic source configured to emit electromagnetic radiation to excite movement of the membrane structure based on asymmetric energy absorption of the electromagnetic radiation in different sub-cavities of the cavity, with the different sub-cavities being disposed on different sides of the membrane structure.

[0006] Embodiments provide a chip-level packaged photoacoustic gas sensor including a detector unit having a membrane structure inside a detector unit cavity, a first sub-cavity of the cavity at a first side of the membrane structure, and a second sub-cavity of the cavity at an opposite second side of the membrane structure. The chip-level packaged photoacoustic gas sensor includes an electromagnetic source configured to emit electromagnetic radiation to excite movement of the membrane structure based on asymmetric energy absorption of the electromagnetic radiation in the first sub-cavity and the second sub-cavity.

[0007] Embodiments provide a method for manufacturing a detector unit. The method includes providing a first layer structure, attaching a second layer structure having a membrane structure at the first layer structure, and attaching a third layer structure at the second layer structure. The method is performed such that the first layer structure and the third layer structure hermetically enclose a cavity and such that the membrane structure is disposed in the cavity.

[0008] An embodiment provides a method for manufacturing a photoacoustic gas sensor. The method includes providing a detector unit having a membrane structure inside a detector unit cavity, a first sub-cavity of the cavity at a first side of the membrane structure, and a second sub-cavity of the cavity at an opposite second side of the membrane structure. The method includes arranging an electromagnetic source configured to emit electromagnetic radiation to excite movement of the membrane structure based on asymmetric energy absorption of the electromagnetic radiation in the first and second sub-cavities.

[0009] Further embodiments are described in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments will be described hereinafter with reference to the accompanying drawings, in which:

[0011] Figure 1 is a schematic side view of a detector unit according to an embodiment;

[0012] Figure 2a is a schematic side view of another detector unit according to an embodiment;

[0013] Figure 2b is Figure 2a a schematic perspective exploded view of the detector unit of

[0014] Figure 3 is a schematic side view of a detector unit according to an embodiment with a coating;

[0015] Figures 4a to 4k is an example processing step for manufacturing a detector unit according to an embodiment;

[0016] Figure 5 is a schematic block diagram of a photoacoustic gas sensor according to an embodiment;

[0017] Figure 6 is a schematic block diagram of a chip-scale packaged photoacoustic gas sensor according to an embodiment;

[0018] Figure 7 is a schematic side view of a chip-scale packaged photoacoustic gas sensor according to an embodiment.

[0019] Figure 8 is a schematic side view of a chip-scale packaged photoacoustic gas sensor with a cover according to an embodiment; and

[0020] Figure 9 is a schematic side view of a chip-scale packaged photoacoustic gas sensor according to an embodiment including a stacked configuration.

[0021] Equal or equivalent elements or elements having equal or equivalent functionality are denoted by equal or equivalent reference numerals in the following description, even if present in different figures. Detailed implementation mode

[0022] In the following description, a number of details are set forth to provide a more thorough explanation of embodiments of the present invention. However, it will be apparent to those skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form rather than in detail to avoid obscuring embodiments of the present invention. Additionally, features of different embodiments described below may be combined with each other unless otherwise specifically stated.

[0023] The embodiments described herein relate to photoacoustic gas sensors and detector units that can be used in such photoacoustic gas sensors. Such a photoacoustic gas sensor may include a detector unit in which a target gas (i.e., molecules or the same or different types) is enclosed. That is, a single gas or a combination of gases or fluids may be enclosed. Such a detector unit may be arranged in a housing of the photoacoustic gas sensor, and the photoacoustic gas sensor includes an electromagnetic radiation source. Additional details of the working principle of the photoacoustic gas sensor are described in connection with the disclosed embodiments.

[0024] The embodiments relate to a detector unit as a microelectromechanical structure (MEMS). The MEMS structure may include one or more semiconductor materials, for example, at least partially doped or undoped semiconductor materials such as silicon, gallium, gallium arsenide, etc. Optionally or additionally arranged materials derived therefrom, such as silicon nitride (SiN, Si3N4 respectively), silicon dioxide (SiO2), etc. Optionally or additionally, other materials such as metallic materials (e.g., copper, gold, silver, platinum, etc.) may be part of the MEMS structure.

[0025] The embodiments described herein may relate to a membrane structure. Such a membrane structure may be understood as a beam-like structure (having a longitudinal extension greater than the lateral extension perpendicular thereto), but may also be a planar or two-dimensional structure in which the lateral extensions perpendicular to each other are equal to each other within tolerances. Examples of such structures may be circular structures (e.g., circular or circular membranes, or secondary membrane structures). Such a membrane structure may be formed, for example, similar to the membrane structures used in MEMS microphones or MEMS loudspeakers.

[0026] Figure 1 is a schematic side view of a detector unit 10 according to an embodiment. The detector unit 10 may be available or may be integrated into a photoacoustic gas sensor. That is, the detector unit 10 may form a component of the photoacoustic gas sensor, but may be implemented separately or individually.

[0027] The detector unit 10 may include a first layer structure 12, a second layer structure 14, and a third layer structure 16 arranged as a stack of layer structures. That is, the layer structure 14 may be disposed at the layer structure 12. The layer structure 16 may be disposed at the layer structure 14. The layer structure 12 may include one or more layers. For example, the layer structure 12 may include layer 121 and layer 122, where layer 121 and layer 122 may include the same or different materials.

[0028] Alternatively or additionally, the layer structure 14 may include one or more layers. For example, the layer structure 14 may include layer 141 and layer 142 having the same or different materials. Alternatively or additionally, the layer structure 16 may include one or more layers. For example, the layer structure 16 may include layer 161 and layer 162 having the same or different materials.

[0029] A number of layers of the layer structure 12, the layer structure 14, and / or the layer structure 16 may be implemented separately and the number relative to the number of layers of other layer structures may be equal or different. The number of layers of each of the layer structure 12, the layer structure 14, and the layer structure 16 may be, for example, 1, 2, 3, 4, 5, or higher, 7, or 10.

[0030] The layer structure 14 may include a membrane structure 18. The membrane structure 18 may include one or more layers, such as a semiconductor layer, a semiconductor layer and a conductive layer (e.g., a doped semiconductor material or a metal material), covering at least some portions of one or both sides of the membrane structure 18. The membrane structure 18 may be arranged such that sub-chambers 22a and 22b of the chamber 22 are provided on different sides of the membrane structure 18. For example, a recess may be implemented in the layer structure 12 and / or the layer structure 14 to form the sub-chamber 22a. Alternatively or additionally, a recess may be formed in the layer structure 14 and / or the layer structure 16 to form the sub-chamber 22b. That is, embodiments relate to a structure having only one of the sub-chambers 22a and 22b, where additional embodiments relate to a structure having both the sub-chambers 22a and 22b. The sub-chamber 22a may be fluidly connected to the sub-chamber 22b or may be sealed from the sub-chamber 22b.

[0031] Accordingly, the layer structure 12 and the layer structure 16 hermetically enclose the chamber 22. The membrane structure 18 is disposed in the chamber. To hermetically enclose the chamber 22, the layer structure 12 and the layer structure 18 may be connected to each other to form an airtight and tight mechanical connection. Additionally, the layer structure 14 and the layer structure 16 may be connected to each other to form an airtight and tight mechanical connection relative to each other. This is different when compared to a chamber in which the structure is disposed, where the structure itself houses a chamber in which a membrane is disposed. According to an embodiment, the chamber 22 can be directly generated by mechanically connecting the layer structures to each other. The layer structure 14 may form at least a part of the sidewall 10A of the detector unit 10.

[0032] The cavity 22 can comprise or contain a fluid, e.g., a gas which is the target gas for a subsequent photoacoustic gas sensor.

[0033] Figure 2a A schematic side view of a detector unit 20 according to an embodiment is shown. The layer structures 12 and 16 can comprise, for example, semiconductor materials, conductive materials, and / or insulating materials. For example, the layer structure 12 can comprise a glass material or a ceramic material as the insulating material. As the semiconductor material, for example, a silicon material or a gallium arsenide material can be used. As the conductive material, for example, a metallic material such as gold, silver, aluminum, copper, etc. can be used. Alternatively or additionally, a doped semiconductor material can be used. For example, the layer structure 12 can be obtained from a glass wafer or a silicon wafer. In connection with the described embodiment, the layer structure 12 can be referred to as a bottom sealing wafer. In contrast, the layer structure 16 can be referred to as a top sealing cap wafer and can comprise, for example, a semiconductor material or an insulating material. A semiconductor material such as silicon can allow for the generation or obtaining of a sub-cavity 22b as a depression in the layer structure 16, while a glass material does not exclude such a configuration but can provide increased hardness of the material. The sub-cavity 22a can be at least partially formed as a depression in the layer structure 14.

[0034] The layer structure 14 can comprise a structure substantially corresponding to a silicon-microphone structure. For example, the membrane structure 18 can be a multi-layer structure.

[0035] The layer structures 12 and 14 can be joined to each other, for example, during a wafer bonding process. For example, a boundary layer or interface 24 can be arranged between the layer structure 12 and the layer structure 14. The interface 24 can be the result of a crystal bonding process. For example, the materials arranged at the layer structure 12 and the material of the layer structure 14 can each form a part of the interface 24.

[0036] For example, the layer structure 12 may include a coating 26 and a base layer 28. The base layer 28 may include, for example, a conductive material, an insulating material, or a semiconductor material, such as a silicon material. At least in the region of a subsequent mechanical connection with the layer structure 14, the coating 26 may be arranged to include, for example, a metallic material (e.g., a gold material), where, alternatively, other materials such as aluminum or other reflective metals or non-metallic materials or structures. For example, gold (Au) and aluminum (Al) may be used to implement a eutectic bond. Such materials may provide reflective properties simultaneously. This does not exclude the use of different materials for bonding and for the reflective surface. Additionally, the embodiments are not limited thereto. For example, a glass medium may be used for bonding. Any reflective structure or material may be used as the coating 26. For example, Au may be inert and optically stable. Alternatively or additionally, a Bragg mirror structure may be used. For example, for the present embodiment, such a structure may be obtained from Si / SiO2 materials. That is, the coating 26 may form a surface that reflects electromagnetic radiation and may include at least one of a reflective material and a reflective structure.

[0037] During the wafer bonding process, the material of the coating 26 and the material of the layer structure 14 may form an interface 24, thereby providing a tight mechanical connection and thus forming part of an airtight seal.

[0038] The coating 26 may optionally be arranged in the regions of the cavity, sub-cavity 22a, and / or sub-cavity 22b. This may allow a reflective surface, for example, to reflect thermal radiation or other electromagnetic radiation.

[0039] The coating 26 may provide surface reflection for electromagnetic radiation. The coating 26 may be arranged at the surface of the layer structure 12 so as to face the membrane structure 18. Alternatively or additionally, the coating 26 may be arranged at the layer structure 16 to face the membrane structure 18. The coating 26 may allow preventing electromagnetic radiation, for example, from Figure 2a entering the shielded sub-cavity from the bottom side. Alternatively or additionally, the coating 26 may allow reflecting the electromagnetic radiation 54 that has entered the cavity so as to prevent the radiation from escaping.

[0040] In the same or a different manner, a coating structure or coating 32 may be arranged between the layer structure 14 and the layer structure 16, for example, including a gold material, an aluminum material, etc. For example, a combination of materials [e.g., gold / tin (AuSn)] may be arranged. Through wafer-level bonding, the layer structure 14 and the layer structure 16 may be combined or connected to each other as described for the layer structure 12 and the layer structure 14.

[0041] The embodiments relate to accommodating a target medium, such as a fluid, e.g., a gas as shown by example molecules 341 to 34i, in a cavity. The target medium can be, for example, CO2, CO, NO2 or any other suitable fluid, e.g., CH4 (methane) and SO2. For example, the membrane structure 18 can include a connection between sub-cavities 22a and 22b implemented by, for example, ventilation holes 361 and 362, where the number of ventilation holes can be different, e.g., a number of 0, 1, 3 or more, 5 or more, 10 or more, 20 or more or even more. This can allow obtaining different connections between layer structure 12 and layer structure 14 and / or between layer structure 14 and layer structure 16. For example, layer structure 12 and layer structure 14 and / or layer structure 14 and layer structure 16 can be formed as a common layer structure that forms corresponding sub-cavities 22a or 22b, for example, using an etching process. This can allow avoiding wafer-level bonding because the target gas can reach the corresponding sub-cavities 22a or 22b by using ventilation.

[0042] However, the wafer-level bonding process can allow for an exact and airtight connection between layer structures. The coating 32 can be used as, for example, a sealing ring and can have, for example, an annular structure corresponding to the structure of the protrusion 38 of layer structure 16. Optionally, a conductive structure 42 (e.g., bonding pads, etc.) can be arranged for connecting one or more conductive layers of, for example, the membrane structure 18 and / or the backplane structure. When compared with the coating 32 that allows a simple process, the conductive structure 42 can be formed at least partially of the same material. For example, in addition to the conductive structure 42, the coating structure 32 can be easily formed without severely changing the manufacturing process.

[0043] Using the wafer-level bonding process can allow for the parallel assembly or generation or manufacture of multiple detector units and subsequent easy separation thereof, for example, using a cutting process.

[0044] The layer structures 12, 14, and / or 16 may have some or different extensions 441, 442, 443 respectively along the thickness direction 46. The thickness direction 46 may be parallel to the surface normal N1 of the layer structure 12, the surface normal N2 of the layer structure 14, and / or the surface normal N3 of the layer structure 16. The surface normals N1, N2, and / or N3 may be perpendicular to the N-plane direction, along which one or more layers of the wafer forming the layer structures 12, 14, 16 have been respectively mainly or substantially formed and extended. For example, the layer structure 12 may form a substrate. For example, the maximum extension 441 or the layer structure 12 may be arbitrary, where a thin layer structure 12 is desirable while maintaining a certain stability. Within these boundaries, the exemplary extension 441 may be at least 20 μm and at most 1 mm, at least 50 μm and at most 800 μm, or at least 70 μm and at most 500 μm. The extension 442 may have any value, such as at least 100 μm and at most 1 mm, at least 250 μm and at most 500 μm, or at least 250 μm and at most 400 μm. The extension 442 may be implemented such that it is a general value of the thickness 48 of the film structure 18 and the thickness or height 521 of the subcavity 22a along the thickness direction 46. For example, the thickness 48 may be in the range of at least 1 μm and at most 10 μm, at least 2 μm to at most 7 μm, or at least 3 μm and at most 5 μm, for example, 4 μm. For example, the height 521 may be in the range of at least 100 μm and at most 990 μm, at least 150 μm and at most 700 μm, or at least 200 μm and at most 500 μm, for example, in the range between 246 μm and 396 μm. Alternatively, the height 521 may be the result of using or further processing the starting structure of the layer structure 14 having the extension 442. After forming the film structure 18 by generating a depression, the subcavity 22a, the height 521 may be the result of the desired thickness 48. Other values and sequences may be implemented. Alternatively or additionally, the extension 443 may have any suitable value, such as at least 50 μm and at most 1 mm, at least 100 μm and at most 500 μm, or at least 150 μm and at most 300 μm. The extension 443 exceeds the thickness or height 522 of the subcavity 22b, which may be, for example, at least 1 μm and at most 500 μm, at least 2 μm and at most 400 μm, or at least 5 μm and at most 300 μm, for example, in the range of at least 10 μm and at most 200 μm. The extension 443 may allow a robust housing of the subcavity 22b, which may include a larger extension 443 when compared to the height 522.

[0045] Alternatively or additionally, a combination of gases may be arranged. The molecules 341, …, 34 i is only involved as a non-limiting example in CO2.

[0046] The cavity 22, sub-cavity 22a and / or sub-cavity 22b can be acoustically isolated respectively. That is to say, the membrane 18 can vibrate with only a negligible effect or be insensitive to the acoustic sound outside the corresponding sub-cavity 22a and / or sub-cavity 22b.

[0047] The layer structure 12, layer structure 14 and / or layer structure 16 can be at least partially transparent to the electromagnetic radiation 54. This can allow the electromagnetic radiation 54 to enter the cavity 22, sub-cavity 22a and / or sub-cavity 22b respectively to excite the vibration of the membrane 18. For example, the layer structure 14 is transparent to the electromagnetic radiation 54. The layer structure 12, 14 and / or 16 can be transparent to the wavelength of the emitter to be combined with the detector unit. For example, the layer structure 12, 14 and / or 16 can be transparent to the infrared spectrum, especially the mid-wavelength infrared spectrum. Although the infrared spectrum can include wavelengths from at least 760 nm to at most 1 mm, the mid-wavelength infrared spectrum can include wavelengths from at least 1 μm and at most 100 μm, from at least 2 μm and at most 70 μm or from at least 3 μm and at most 50 μm.

[0048] The detector unit 20 can be formed such that the detector unit 20 has an asymmetric sensitivity to the electromagnetic radiation in the sub-cavity 22a and in the sub-cavity 22b. Such an asymmetry can be understood as having different forces in terms of the amplitude, frequency or time offset generated with respect to the electromagnetic radiation 54, so as to prevent equal forces acting on the membrane structures 18 in the two sub-cavities 22a and 22b, which can cancel the vibration of the membrane 18. By implementing the asymmetry, the detector unit 20 can include a high sensitivity to the electromagnetic radiation 54. As will be described in more detail later, the asymmetry can be generated alternatively or additionally in other ways by having different heights 521 and 522. That is to say, the asymmetry can be obtained at least partially by implementing the extensions 521 and 522 to be different, for example, 1:1.1, 1:1.2 or 1:1.5 or a larger number.

[0049] Alternatively or additionally, the sub-cavity 22a and the sub-cavity 22b can be shielded differently, shielding one sub-cavity while not shielding the other or shielding the other to a different degree, such that the electromagnetic radiation 54 penetrates or pierces the sub-cavity 22a and the sub-cavity 22b differently. Alternatively or additionally, different pressures of the target gas 34 can be implemented, for example, in a structure with sub-cavities sealed to each other.

[0050] Alternatively or additionally, to obtain asymmetry, the sub-cavities 22a and 22b can be sealed from each other and can include different gases or gas concentrations. By using different gases, the detector unit can be implemented to be sensitive to two gases. For example, the absorption characteristics of the two gases can be separated within a wavelength range or a frequency range such that when evaluating the vibration of the membrane structure 18, the excitation of the membrane structure 18 can be clearly distinguished.

[0051] Figure 2b A schematic perspective exploded view of the detector unit 20 is shown to illustrate, for example, the circumferential path of the coating structure 32 (i.e., the sealing ring). The membrane structure 18 can be formed, for example, as a circular or round structure. Although four ventilation holes 361 to 364 are shown, a different number can be implemented, for example, 0 or more, 1 or more, 2 or more, 3 or more, 5 or more, or a greater number. That is, the membrane structure can include at least one ventilation hole.

[0052] Inserting the bottom sealing wafer: Terms such as "bottom", "top", "left", "right", etc. are used to facilitate the understanding of the present disclosure. Obviously, based on the orientation of the structural changes, the appropriate terms can vary without changing the scope of the embodiments.

[0053] In other words, a silicon-microphone wafer with a top sealing wafer and a bottom sealing wafer is shown. A dedicated gas atmosphere, such as a target gas (e.g., CO2) at any concentration greater than 0% and up to 100%, can be enclosed during a bonding step (e.g., the last bonding step). A concentration of 100% can provide high sensitivity, where lower concentrations can allow for a combination of gases and thus multiple sensitivities. The pressure of the target gas can be higher or lower when compared to the ambient pressure of the subsequent device. For example, the pressure can be at least 10 mbar and up to 5 bar or any other suitable value, for example, to enhance or reduce the absorption of electromagnetic radiation.

[0054] These steps can be implemented to first provide a dorsal seal (Au / Si eutectic bonding) and then seal under an atmosphere of CO2 (AuSn soldering of the cap structure on the Si-MEMS top side to the metal ring). These steps can be performed in a different order. The bonding pads of the microphone can remain accessible after the WLB process. The entire steps of the silicon wafer can be transparent to mid-wavelength infrared spectroscopy, which can be used for optical excitation in gas sensing.

[0055] Figure 3Shows a schematic side view of a detector unit 30 according to an embodiment. The detector unit 30 may be formed similarly to the detector unit 20. When compared with the detector unit 20, another coating 262 may be arranged at or as part of the layer structure 16 next to the coating 261 which may be the coating 26 of the detector unit 20, so as to face the membrane structure 18. Although both the coating 261 and the coating 262 are optional, the configurations of the detector unit 20 and the detector unit 30 allow sealing a part of the cavity by a reflective coating that is adapted to reflect light or electromagnetic radiation for exciting the target medium 34 in the cavity.

[0056] The membrane structure 18 described in connection with the detector unit 10, the detector unit 20, and / or the detector unit 30 may be evaluated for its vibration. The detector unit 10, the detector unit 20, and / or the detector unit 30 may include circuitry configured to evaluate the vibration. Alternatively or additionally, the detector unit 10, the detector unit 20, and / or the detector unit 30 may be connected to a suitable circuitry, for example, using the conductive structure 42. The membrane structure 18 may be arranged in a (for example) single-backplate configuration or a double-backplate configuration. The single-backplate configuration may refer to a configuration for evaluating the vibration of a membrane having a conductive surface relative to a pair of electrodes arranged near the membrane. In the double-backplate configuration, for example, the vibratable membrane may be sandwiched between two pairs of electrodes. That is, the layer structure 14 may include a single-backplate configuration or a double-backplate configuration for the membrane structure 18. Alternatively or additionally, the detector unit 10, the detector unit 20, and / or the detector unit 30 may include piezoelectric or piezoresistive elements to determine the deformation or vibration of the membrane structure 18.

[0057] Referring simultaneously to Figures 4a to 4k , the following describes example processing steps for manufacturing the detector units 10, 20, and / or 30. Note that the drawings neither limit such a manufacturing process to a specific order or sequence of steps, nor exclude all steps necessary for manufacturing a detector unit according to an embodiment, nor exclude additional steps.

[0058] Figure 4a Shows a schematic side view of a layer structure 14 having a microphone structure 18, a sealing ring 32, and a conductive structure 42. The conductive structure 42 may be, for example, metal spraying using a metal material such as gold, silver, aluminum, copper, etc. The membrane structure 18 may be a single-backplate structure or a double-backplate structure. In the drawings of the present disclosure, the membrane structure and the counter electrodes are shown as a single block for ease of understanding the embodiments. The layer structure 14 may be similar to a silicon-based microphone structure. A native insulating layer (for example, SiO2 on the back side of the substrate) may be removed by dipping, for example, using HF (hydrogen fluoride).

[0059] Figure 4bA schematic side view of a configuration of a layer structure 12 is shown, for example, including a base layer 28 at this stage. The base layer 28 can be at least a portion of a silicon wafer, for example, but can also contain other materials. For example, the base layer 12 can be a wafer to be cut or separated later.

[0060] Figure 4c A schematic side view of the layer structure 12 is shown. Figure 4b In comparison, coating 26 has been arranged, for example, on a complete wafer or at least a large structure thereof. The deposition of coating 26 may include the deposition of a metallic material such as gold on a silicon wafer. The deposition of coating 26 may include the deposition of an adhesion layer, such as zinc (Ti). Coating 26 may serve a number of purposes. For example, it may be used to form an alloy with layer structure 14 when performing wafer level bonding (WLB). In addition, it may be used as a reflective surface for optical radiation (e.g., electromagnetic radiation 54).

[0061] Figure 4d The two layers are shown before being combined. Figure 4c The layer structure 12 and Figure 4a A schematic side view of the configuration of the layer structure 14, while Figure 4e A schematic side view of the layer structure 12 and the layer structure 14 after wafer-level bonding is shown. Based on the wafer-level bonding, an interface 24 can be obtained, thereby allowing a close connection of the layer structure 12 and the layer structure 14. The interface 24 can include an alloy of the material comprising the coating 26 and the semiconductor material (e.g., silicon material) of the layer structure 14. The described eutectic Au / Si bonding can be performed, for example, in a vacuum atmosphere or any other suitable atmosphere, because when the membrane structure 18 includes ventilation holes, a target gas can be included later. Alternatively, the sealed subcavity can be bonded under the target atmosphere.

[0062] Figure 4f A schematic side view of a layer structure 16 is shown, which may include a topographic structure. In its portion, an interface forming material 56 (e.g., a gold material, an aluminum material, a tin material, or a silver material, etc.), including a material forming an alloy (e.g., gold / tin), as described for the conductive layer 26. Recesses 581, 582, and / or 583 may be arranged. Recess 582 may be used at least partially for subcavity 22b later, while recess 581 and recess 583 may allow for later cutting. For example, material may be removed based on etching or grinding until a level indicated by line L is reached. Recesses 581, 582, and / or 583 may be optional. For example, subcavity 22b may also be formed in layer structure 14 as layer structure 14, for example, for Figure 2a The extension 442 is shown when the membrane structure 18 is arranged in a central position of the layer structure 14 .

[0063] In other words, a silicon cap can be constructed with a gold-tin solder 56 at the contact location. By having two or more cavities, the final die separation can be accomplished by grinding. This can allow preventing cracks in the structure.

[0064] To obtain the structure shown in Figure 4f , a silicon wafer can be used. For example, the depressions 581 and 583 can be structured into the silicon wafer using an operation process. The depressions 581 and 583 can be the same depression, for example, having a rectangular, oval, or circular path. That is, the structuring of the first cavity 581 and the first cavity 583 can be performed into the silicon wafer 62.

[0065] Before or after generating the depression 581 and / or the depression 583, the depression 582 can be generated using, for example, an etching process. The etching can be performed as wet etching, dry etching, or other concepts to remove the material. That is, the structuring of the second cavity 582 can be performed into the silicon wafer 62.

[0066] As shown in Figure 4i , the interface forming material 56 can be disposed at the contact location or contact area 64 of the wafer 62. That is, in the area where the layer structure 14 and the layer structure 16 are considered to be in contact with each other, the interface forming material 56 can be at least partially disposed. Alternatively or additionally, the interface forming material 56 can also be disposed on the layer structure 14. In other words, the deposition of AuSn is performed at the contact location. Optionally, the coating 262 can be disposed in the depression 582 before, after, or simultaneously.

[0067] In Figure 4e the structure shown and in Figure 4i the structure shown can both be disposed into a processing chamber that can include a target medium 34. It should be noted that the wafer-level bonding described in connection with Figure 4e can also be performed in an atmosphere having the target medium 34. Alternatively, when compared with the wafer-level bonding of Figure 4j , the wafer-level bonding described in Figure 4e can be performed in a different atmosphere. This can allow different media, pressures, or gas concentrations to be accommodated in different sub-cavities that are sealed from each other. One of such sealed cavities can also include a low pressure or a vacuum, that is, when performing the wafer-level bonding, the processing chamber can be evacuated. Based on the coating 32 and the interface forming material 56 and by performing the wafer-level bonding, the layer structure 14 and the layer structure 16 can be mechanically connected to each other. It should be noted that the wafer-level bonding of the layer structure 12 and the layer structure 14 can be performed simultaneously or after bonding the layer structure 14 and the layer structure 16.

[0068] In other words, the wafer bonding of the top-sealed wafer 16 can be performed on the metallization of the microphone (here: AuSn-Au bonding). Other bonding techniques, i.e., other materials, are also possible. The method can be carried out in a target atmosphere (e.g., CO2). Depending on the target gas to be detected, one or more different atmospheres can also be selected.

[0069] After performing wafer-level bonding, the individual detector units can be separated from each other by removing a part of the layer structure 16 (e.g., starting from the side 16A, e.g., the top side) until the line L, such that a configuration similar to Figure 4k can be obtained. The layer structure 12 and / or the layer structure 16 can be cut, as these structures are mechanically robust.

[0070] In other words, the final device can include a silicon-microphone having a top 12 and a bottom-sealed wafer 16. The target medium (CO2) is enclosed in the post-volume of the silicon-microphone and in the cavity between the silicon-cap and the top side of the silicon-microphone. The design of the silicon-cap can be adjusted, e.g., the height of the cavity. Also, during structuring before the WLB process, the overall shape of the resulting cap after segmentation can be adjusted as shown, for example, in FIG. 2, e.g., with more DRIE (Deep Reactive Ion Etching process). A double-backplane silicon-microphone can be used, where different SiMiC (Silicon Microphone) technologies can also be used. The bottom-sealed wafer allows for easy handling. However, this does not exclude the handling of wafers with topography. For example, a silicon wafer can be coated with gold, which can include a titanium adhesion layer. HF dipping can be used to remove native SiO2 on the MEMS backside. Au / Si eutectic bonding can be performed, for example, using approximately 360 °C, and the top wafer can be processed by processing the silicon-cap wafer that can be done on a carrier wafer. For example, by applying a temperature of about 320 °C, Au-Sn / Au diffusion bonding can be performed. Then, release can be carried out.

[0071] Figure 5 A schematic block diagram of a photoacoustic gas sensor according to an embodiment is shown. The photoacoustic gas sensor 50 can include a detector unit 10, where alternatively or additionally, one or more different detector units can be arranged, such as detector units 20 and / or 30. The photoacoustic gas sensor can include an electromagnetic source 66, which is configured to emit electromagnetic radiation 54 in order to excite the movement of the membrane structure 18 based on the asymmetric energy absorption of the electromagnetic radiation in the sub-cavities 22a and 22b of the cavity of the detector unit.

[0072] The photoacoustic gas sensor 50 may include a control unit 68 configured to evaluate the vibration of the membrane structure 18 and / or to control the electromagnetic source 66. That is, the control unit 68 may communicate with the detector unit 10 and / or the electromagnetic source 66. The control unit 68 may include, for example, a processor, a microcontroller, a field programmable gate array (FPGA), and / or an application specific integrated circuit (ASIC).

[0073] The detector units 10, 20, and / or 30 may be obtained by processing at the wafer level. Embodiments relate to chip-level packaging of photoacoustic gas sensors, i.e., to chip-level packaging of photoacoustic gas sensors.

[0074] Figure 6 A schematic block diagram of a chip-level packaged photoacoustic gas sensor 60 according to an embodiment is shown. The chip-level packaged photoacoustic gas sensor 60 may include a detector unit 65. The detector unit 65 may have a membrane structure (e.g., the membrane structure 18 inside the detector unit cavity), e.g., the cavity 22. The sub-cavities 22a and 22b of the cavity 22 may be arranged on different sides of the membrane structure 18. The chip-level packaged photoacoustic gas sensor 60 may include an electromagnetic source or emitter 66, which may include a spacer 74 and a housing 76 and an emission element E that may generate electromagnetic radiation 54, for example, based on heating. That is, the element E may be a heater. Alternatively, the element E may be a black body or the like.

[0075] The electromagnetic source 66 may be configured to emit electromagnetic radiation 54 in order to excite the movement of the membrane structure 18 based on the asymmetric energy absorption of the electromagnetic radiation 54 in the described sub-cavities 22a and 22b. The chip-level packaged photoacoustic gas sensor may be implemented such that the sub-cavities 22a and 22b have different sizes and / or different surface ratios in order to at least partially obtain the asymmetric energy absorption as described for the detector unit. The electromagnetic source 66 may be implemented to provide pulsed excitation of the electromagnetic radiation 54, for example, based on a corresponding control signal. The frequency of the pulse and / or the wavelength of the signal may be adapted to the target gas and / or to the resonance frequency of the membrane structure.

[0076] The electromagnetic radiation 54 may be referred to as light, even if it includes wavelengths that are almost invisible or completely invisible to human capabilities. For example, the detector unit 65 may be implemented as described for the detector units 10, 20, and / or 30. Alternatively, a configuration may be implemented in which the sub-cavities 22a and 22b are sealed from each other. The target medium 34 may be arranged in at least one of the sub-cavities 22a and / or the sub-cavity 22b. Possible other sub-cavities may include different target media or no target medium, i.e., it may be evacuated.

[0077] As will be described, the asymmetric energy absorption can be based on an asymmetric energy input into sub - cavities 22a and 22b from electromagnetic radiation 54. Alternatively or additionally, the asymmetric energy absorption can be based on an asymmetric energy loss from sub - cavities 22a and 22b. For example, such an energy loss can be obtained by having wall structures with different dimensions around the cavity and / or different thermal conductivities. Thus, the energy loss can be based on the energy input of electromagnetic energy or electromagnetic radiation into sub - cavities 22a and 22b. Thus, the energy loss can be related to a heat loss path, which can lead to a reduced pressure in the target medium 34, for example, due to cooling from the energy loss.

[0078] The chip - level packaged photoacoustic gas sensor 60 can include a substrate 72 on which a detector unit 65, an electromagnetic source 66, and / or a control unit 68 can be arranged. The substrate 72 can include a semiconductor material or a glass material or a ceramic material or a combination thereof. To allow for low heat loss, the electromagnetic source 66 can be spaced apart from the substrate 72 by a spacer structure 74 and / or encapsulated by a housing 76. The electromagnetic source 66 can form an emitter together with the housing 76. The emitter can include a filter for wavelength - filtering the emission towards the detector unit 65, for example, to avoid ambiguity in the measurement results obtained by the control unit 68. For example, the fluid in the cavity, (e.g., the target medium 34) can include a target frequency for fluid resonance. The chip - level packaged photoacoustic gas sensor can be implemented to include a filter structure (e.g., as part of the housing 76 and / or part of the spacer structure 74 or arranged between the emitting element E and the detector unit 65). The filter structure can be arranged to filter the electromagnetic radiation 54 such that wavelengths not corresponding to the target frequency are attenuated by a greater amount (i.e., at least 20%, at least 30%, at least 50% or more) when compared to the wavelength corresponding to the target frequency. For example, the filter structure is incorporated in the housing 76, or the filter structure implements the housing 76.

[0079] The chip-scale packaged photoacoustic gas sensor 60 may include a housing 78 that forms a housing for at least the electromagnetic source 66 and the detector unit 65, where additional components, such as a control unit 68, may be arranged. That is, the chip-scale packaged photoacoustic gas sensor 60 may include a cover 78 that at least partially forms a cavity 86 of the chip-scale packaged photoacoustic gas sensor. The cavity 86 may accommodate at least the detector unit 65 and the electromagnetic source 66. The cover 78 may be reflective to electromagnetic radiation. The housing may include a vent or opening 82 to allow an ambient medium 84 (such as air or a different medium) to enter the interior 86 of the housing. That is, the chip-scale packaged photoacoustic gas sensor may include an inlet to allow the target medium (i.e., the ambient medium 84) to pass through. Thus, the ambient medium 84 may be subjected to electromagnetic radiation and may absorb energy from it at least within some specific wavelength ranges. In the absence of the behavior of the calibration-based membrane structure 18, the content of the ambient medium 84 may be determined. That is, the presence or concentration of the target medium 34 may be determined at least in the ambient medium 84.

[0080] In other words, a gas sensor unit including a WLB detector unit is disclosed. The infrared emitter may be encapsulated in the same housing adjacent to the detector unit having a corresponding ASIC for reading the detector unit.

[0081] The distance 88 between the source 66 and the side 78A may be small, for example, preferably a non-zero value up to 1 mm, 500 μm, or 100 μm. Such a small distance 88 may allow the electromagnetic radiation 54 to reach the detector unit substantially from the side to excite the target medium 34. This may allow the same or comparable energy input into the sub-cavities 22a and 22b.

[0082] Optionally, a shield 92 may be arranged between the electromagnetic source 66 and the detector unit 65. The shield 92 may be configured to partially shield the detector unit 65 from the electromagnetic radiation 54 to obtain at least partially asymmetric energy absorption. The shield 92 may at least partially shield the sub-cavity 22a and / or at least partially shield the sub-cavity 22b. For example, only one of the two sub-cavities is shielded, or the sub-cavities are shielded to different extents.

[0083] Figure 7 A schematic side view of a chip-scale packaged photoacoustic gas sensor according to an embodiment is shown. When compared with the chip-scale packaged photoacoustic gas sensor 60, the distance 88 may be greater, for example, having a ratio compared to Figure 6The described distance is a greater distance. The exemplary values of the described embodiments are not limited and can be between 0.5 mm and 5 mm, between 0.75 mm and 3 mm, or between 1 mm and 2.5 mm, such as 1.6 mm. The distance 88 can be measured between the main side 78A spaced from the emitter and the detector unit 65 through the circumferential side 78B of the cover 78. The large distance 88 can allow the electromagnetic radiation 54 to scatter toward the detector unit 65 at the main side 78A. In contrast, in Figure 6 the small distance shown in can prevent the electromagnetic radiation 54 from scattering toward the detector unit 65 at the main side 78A, such that the electromagnetic radiation 54 travels laterally toward the detector unit 65.

[0084] In other words, the gas sensor unit according to the embodiment can include a WLB detector unit. The infrared emitter can be encapsulated in the same housing adjacent to the detector unit having a corresponding ASIC for reading the detector unit. The distance from the top side of the detector to the cover of the sensor unit can be large enough to have an optical path to the top side of the detector unit. Light can be scattered and reflected within the main optical shield (module package) such that it is difficult to determine the main incident angle.

[0085] The control unit 68, (i.e., the circuit) can be covered with a material 94 opaque to the electromagnetic radiation 54. Such an arrangement is optional. Alternatively or additionally, but optionally, the control unit or circuit 68 can be insensitive to the electromagnetic radiation 54 such that in both cases, the electromagnetic radiation 54 does not impair the operation of the control unit 68.

[0086] Figure 8 A schematic side view of a chip-scale packaged photoacoustic gas sensor 80 according to an embodiment is shown. The cover 78 can be formed as described in connection with the chip-scale packaged photoacoustic gas sensor 70, but can also be formed as described for the chip-scale photoacoustic gas sensor 60. When compared with the detector unit 65, the detector unit 85 of the chip-scale photoacoustic gas sensor 85 includes a reflective coating 262 that covers or shields, completely or at least to an amount of 50%, greater than 70%, or greater than 90%, one of the sub-cavities 22a or 22b, such as sub-cavity 22b. Such a reflective coating 262 can be applied to the electrodes 96 of a single-backplane configuration or a double-backplane configuration of, for example, a microphone chip to prevent light from passing through the bottom interface of the top volume (i.e., to prevent the electromagnetic radiation 54 from passing through sub-cavity 22b to reach sub-cavity 22a).

[0087] In other words, the WLB can include a reflective coating on the inner or outer surface of the top sealing cap wafer, i.e., the layer structure 16. Thus, a direct optical path into the upper gas volume 22b can be avoided.

[0088] Figure 9Shows a schematic diagram of a chip - level packaged photoacoustic gas sensor 90 according to an embodiment. When compared with chip - level packaged photoacoustic gas sensors 60, 70, or 80, the chip - level packaged photoacoustic gas sensor 90 may include a stacked configuration. When compared with chip - level packaged photoacoustic gas sensors 60, 70, and 80, different sub - packages 981 and sub - packages 982 may be stacked on top of each other and may thus extend to different vertical extents. While reducing the required surface area with a stacked arrangement, the height can be increased. The sub - package 981 may include an electromagnetic source 66, for example including a filter. Along the thickness direction 48, the sub - package 982 may be spaced apart from a spacer structure or a spacer or a thermal decoupling element 102 disposed between the substrates 721 of the sub - package 981 and the substrate 722 of the sub - package 982. The thermal decoupling element 102 may include a low thermal conductivity. For example, a polymer material or the like may be used.

[0089] The sub - package 982 may include a detector unit 65. The control unit 68 may be disposed in the sub - package 981 or the sub - package 982. The electromagnetic radiation 54 may travel from the sub - package 981 to the sub - package 982. For example, the substrate 722 may include an opening 104 or a region of low thermal conductivity.

[0090] In other words, the embodiment relates to a closed photoacoustic gas sensing unit that includes an infrared emitter, an optical filter, and a detector unit (e.g., a Si - microphone) enclosed by a housing (package). The detector unit (microphone) may be enclosed in an airtight package under a defined atmosphere of the gas of interest (e.g., a specific percentage of CO2, a target gas at a target concentration). The package of the detector unit may be airtight sealed during its lifetime (e.g., in the range of at least 5 years and possibly 15 years). This requirement can be addressed by the embodiments that provide a packaging process and corresponding structures described herein. The wafer - level bonding (WLB) process under the required atmosphere can reduce the packaging cost per unit because the entire packaging process can be performed on all the devices while still at the wafer - level. That is, filling individual devices separately can be prevented. Compared with standard packaging methods, the WLB process can also reduce the form factor of the gas sensing detector unit and thus has the potential to be integrated into small - scale PCBs (printed circuit boards) (e.g., for telephone applications).

[0091] An airtight sealed gas detector unit can be formed by a wafer-level process, whereby the unit can include a microphone wafer, a top-sealing wafer serving as a cap above the membrane area, and a bottom-sealing wafer. The bottom and top-sealing wafers can be equipped with a reflective coating for optically shielding the upper (above the microphone membrane) or lower (below the microphone membrane) gas volume from the outside. Through this encapsulation, a very small enclosed gas volume can be achieved, which depends only on or at least substantially on the thickness of the microphone wafer and the cavity in the top-sealing wafer above the front side of the microphone. The values stated for a single thickness do not limit the process limitations. Pulse excitation with an infrared light source can cause a pressure difference above and below the microphone membrane within the enclosed gas volume, and thus can cause an acoustic signal depending on the intensity of the infrared light.

[0092] The ASIC can be covered by an optically opaque material, such as a spherical top, or can be robustly resistant to broadband light. Gas exchange can be provided through an opening in the optical shield, which depends on the optical path through which ventilation can be adjusted in order to enhance the gas exchange diffusion time. As more light is absorbed by the optical path outside the detector unit (higher ambient CO2 concentration), the photoacoustic pressure within the detector unit can become smaller, i.e., an inverse signal can be obtained at the ASIC. The WLB photoacoustic detector unit can be included in a photoacoustic sensor that includes, for example, a chopped MEMS infrared emitter, an electromagnetic source, an optical filter for wavelength-selective heating of the gas, an airtight sealed MEMS microphone using the WLB process, and a housing. The system can be operated by an internal ASIC that provides the input power for the infrared emitter and the acoustic readout of the WLB detector unit.

[0093] Embodiments are based on an airtight sealed MEMS microphone produced using a wafer-level bonding process in a dedicated gas atmosphere. A small airtight enclosed gas volume can be beneficial for generating photoacoustic pressure. It should be mentioned that the quantification between the two volumes (above and below the microphone membrane, sub-cavities) can be important for the response of the detector unit to chopped infrared light. The optical shielding of one of the volumes (e.g., by metal coating of the inner part of the top volume) can enhance the detector sensitivity. In general, the height of the WLB PAS (photoacoustic sensor) detector unit can be defined or at least affected by the thickness of three wafers (layer structure) and the height of the cavity above the MEMS microphone. Thus, this can form a chip-size solution for designing an airtight sealed WLB PAS detector unit with a height range that may be specifically defined by the process window of the corresponding three wafers. This can allow for the provision of small WLB PAS detector units.

[0094] Since pollution is an impact on health and since the health problems of air pollution are becoming increasingly serious, embodiments allow reducing the form factor and the production costs of a closed photoacoustic detector unit. The detector unit can be an independent product, but can also be included in a photoacoustic gas sensor. This can provide advantages compared to NDIR (non-dispersive infrared sensor) detectors. Embodiments relate to an infrared source integrated into the WLB process of the detector unit, for example as a top or bottom wafer. The filter wafer can be used as a top or bottom sealing wafer. That is, the process for manufacturing the electromagnetic source 66 can be similar to that for producing a MEMS microphone. Thus, the structure 74 and / or the structure 76 can include filtering characteristics.

[0095] Although some aspects have been described in the context of a device, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Similarly, aspects described in the context of method steps also represent a description of the corresponding block or item or feature of the corresponding device.

[0096] The above embodiments are merely illustrative of the principles of the present invention. It should be understood that modifications and variations of the arrangements and details described herein will be apparent to other technicians in the art. Therefore, it is intended to be limited only by the scope of the pending patent claims, rather than by the specific details presented by the description and explanation of the embodiments herein.

Claims

1. A detector unit for a photoacoustic gas sensor, comprising: A first layer structure (12); A second layer structure (14), arranged at the first layer structure and including a membrane structure (18); And A third layer structure (16), arranged at the second layer structure (14); Wherein the first layer structure (12) and the third layer structure (16) hermetically enclose a cavity (22), and wherein the membrane structure (18) is arranged in the cavity (22), Wherein a first sub-cavity (22a) of the cavity (22) at a first side of the membrane structure (18) and a second sub-cavity (22b) of the cavity (22) at a second side of the membrane structure (18) are sealed relative to each other and include different gases or gas concentrations.

2. The detector unit according to claim 1, wherein the second layer structure (14) forms at least a part of a side wall (10A) of the detector unit.

3. The detector unit according to claim 1, wherein the second layer structure (14) is transparent to electromagnetic radiation (54).

4. The detector unit according to claim 1, wherein the cavity (22) is acoustically isolated.

5. The detector unit according to claim 1, wherein the first layer structure (12) and the second layer structure (14) are attached to each other by wafer-level bonding and / or wherein the second layer structure (14) and the third layer structure (16) are attached to each other by wafer-level bonding.

6. The detector unit according to claim 1, wherein the detector unit is asymmetric with respect to the sensitivity to electromagnetic radiation (54) of the first sub-cavity (22a) of the cavity (22) between the first layer structure (12) and the second layer structure (14) and the second sub-cavity (22b) between the second layer structure (14) and the third layer structure (16).

7. The detector unit according to claim 1, wherein a reflective surface (26) for electromagnetic radiation (54) is arranged at a surface of the first layer structure (12) facing the membrane structure (18) or at a surface of the third layer structure (16) facing the membrane structure (18).

8. The detector unit according to claim 7, wherein the surface includes at least one of a reflective material and a reflective structure.

9. The detector unit according to claim 1, wherein a first extension (521) of the first sub-cavity (22a) between the first layer structure (12) and the second layer structure (14) and a second extension (522) between the second layer structure (14) and the third layer structure (16) along a direction (46) parallel to the surface normal (N2) of the main side of the membrane structure (18) are different.

10. The detector unit according to claim 1, wherein the detector unit has a target medium (34) located between the first layer structure and the third layer structure.

11. The detector unit according to claim 1, wherein at least one of the first layer structure (12), the second layer structure (14), and the third layer structure (16) is transparent to mid-wavelength infrared spectroscopy.

12. The detector unit according to claim 1, wherein in order to measure the movement of the membrane structure (18), the second layer structure (14) includes a single-backplane configuration or a double-backplane configuration for the membrane structure (18); or the detector unit includes a piezoelectric element or a piezoresistive element.

13. The detector unit according to claim 1, wherein the membrane structure includes at least one vent hole (36).

14. The detector unit according to claim 1, wherein a part of the cavity (22) is sealed by a reflective coating to prevent light suitable for exciting the fluid in the cavity (22).

15. A photoacoustic gas sensor, comprising: the detector unit (10; according to any one of the preceding claims; 20；30)； and an electromagnetic source (66) configured to emit electromagnetic radiation (54) to excite the movement of the membrane structure (18) based on the asymmetric energy absorption of the electromagnetic radiation (54) in different sub-cavities (22a-22b) of the cavity (22) arranged on different sides of the membrane structure (18).

16. A chip-level packaged photoacoustic gas sensor, comprising: a detector unit (65) having a membrane structure (18) inside a detector unit cavity (22), a first sub-cavity (22a) of the cavity (22) at a first side of the membrane structure (18); and a second sub-cavity (22b) of the cavity (22) at an opposite second side of the membrane structure (18); an electromagnetic source (66) configured to emit electromagnetic radiation (54) to excite the movement of the membrane structure (18) based on the asymmetric energy absorption of the electromagnetic radiation in the first sub-cavity (22a) and the second sub-cavity (22b), wherein the detector unit is the detector unit according to claim 1.

17. The chip-level packaged photoacoustic gas sensor according to claim 16, wherein the asymmetric energy absorption is based on the asymmetric energy input from the electromagnetic radiation (54) into the first sub-cavity (22a) and the second sub-cavity (22b); and / or based on the asymmetric energy loss from the first sub-cavity (22a) and the second sub-cavity (22b), the energy loss being based on the energy input of the electromagnetic energy into the sub-cavities.

18. The chip-level packaged photoacoustic gas sensor according to claim 16, including a shield between the electromagnetic source and the detector unit, the shield (92) being configured to partially shield the detector unit from the electromagnetic radiation (54) to at least partially obtain the asymmetric energy absorption.

19. The chip-level packaged photoacoustic gas sensor according to claim 16, wherein the first sub-cavity (22a) and the second sub-cavity (22b) have different dimensions (521, 522) and / or different surface ratios to obtain the asymmetric energy absorption at least in part.

20. The chip-level packaged photoacoustic gas sensor according to claim 16, having a cover (78) that at least partially forms a cavity (86) of the chip-level packaged photoacoustic gas sensor, the cavity (86) accommodating at least the detector unit and the electromagnetic source (66), wherein the cover (78) is reflective for the electromagnetic radiation (54) and includes an inlet (82) for the target medium (84) to pass through.

21. The chip-level packaged photoacoustic gas sensor according to claim 20, wherein the cover (78) includes a main side (78A), the main side (78A) being spaced apart from the electromagnetic source (66) by a circumferential side (78B) of the cover (78), wherein the distance (88) between the main side and the electromagnetic source (66) is implemented to allow scattering of the electromagnetic radiation (54) at the main side (78A) towards the detector unit; or wherein the distance (88) between the main side (78A) and the electromagnetic source (66) is implemented to prevent scattering of the electromagnetic radiation (54) at the main side (78A) towards the detector unit, such that the electromagnetic radiation (54) travels laterally towards the detector unit.

22. The chip-level packaged photoacoustic gas sensor according to claim 16, wherein a circuit (68) for evaluating movement of the membrane structure is covered by a material (94) that is opaque to the electromagnetic radiation (54) and / or insensitive to the electromagnetic radiation (54).

23. The chip-level packaged photoacoustic gas sensor according to claim 16, wherein the electromagnetic source (66) forms a first sub-package (981) of the chip-level packaged photoacoustic gas sensor; and wherein the detector unit forms a second sub-package (982) of the chip-level packaged photoacoustic gas sensor; wherein the chip-level packaged photoacoustic gas sensor includes a thermal decoupling element (102) between a substrate (721) of the first sub-package (981) and a substrate (722) of the second sub-package (982).

24. The chip-level packaged photoacoustic gas sensor according to claim 16, wherein the fluid in the cavity (22) includes a target frequency of the fluid resonance, wherein the chip-level packaged photoacoustic gas sensor includes a filter structure between the electromagnetic source (66) and the detector unit, the filter structure being configured to filter the electromagnetic radiation (54) to attenuate a greater amount of wavelengths that do not correspond to the target frequency when compared to the wavelength corresponding to the target frequency.

25. A method for manufacturing a detector unit, the method comprising: providing a first layer structure (12); Attach a second layer structure (14) having a membrane structure (18) at the first layer structure (12); and Attach a third layer structure (16) at the second layer structure (14); such that the first layer structure (12) and the third layer structure (16) hermetically enclose a cavity (22), and such that the membrane structure (18) is disposed within the cavity; wherein a first sub-cavity (22a) of the cavity (22) at a first side of the membrane structure (18) and a second sub-cavity (22b) of the cavity (22) at a second side of the membrane structure (18) are sealed relative to each other and include different gases or gas concentrations.

26. The method according to claim 25, wherein attaching the second layer structure (14) at the first layer structure (12) and / or attaching the third layer structure (16) at the second layer structure (14) includes wafer-level bonding.

27. A method for manufacturing a photoacoustic gas sensor, comprising: Providing a detector unit having a membrane structure (18) inside a detector unit cavity (22), a first sub-cavity (22a) of the cavity at a first side of the membrane structure (18); and a second sub-cavity (22b) of the cavity at an opposite second side of the membrane structure (18); and Arranging an electromagnetic source (66) configured to emit electromagnetic radiation (54) to excite movement of the membrane structure (18) based on asymmetric energy absorption of the electromagnetic radiation in the first sub-cavity (22a) and the second sub-cavity (22b), wherein a first sub-cavity (22a) of the cavity (22) at a first side of the membrane structure (18) and a second sub-cavity (22b) of the cavity (22) at a second side of the membrane structure (18) are sealed relative to each other and include different gases or gas concentrations.

Images