Patent Publication Number: US-2022228974-A1

Title: Detector Cell for a Photoacoustic Gas Sensor and Photoacoustic Gas Sensor

Description:
This application is a continuation of U.S. patent application Ser. No. 16/935,726, filed Jul. 22, 2020, which application claims the benefit of European Patent Application No. 19193158, filed on Aug. 2, 2019, which applications are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure is related to a detector cell for a photoacoustic gas sensor, to photoacoustic gas sensors and to methods for fabricating a detector cell and a photoacoustic gas sensor. The present disclosure further relates to a wafer level bonded photoacoustic detector cell. 
     BACKGROUND 
     Photoacoustic gas sensors may be used to measure environmental conditions, for example, portions of a fluid, in particular a gas. 
     There is a request for a detector cell and for photoacoustic gas sensors having a high durability and being robust over a lifetime. There is further a request for methods for fabricating a detector cell and photoacoustic gas sensors. 
     SUMMARY 
     Embodiments provide for a detector cell for a photoacoustic gas sensor. The detector cell comprises a first layer structure, a second layer structure arranged at the first layer structure and comprising a membrane structure and comprises a third layer structure arranged at the second layer structure. The first layer structure and the third layer structure hermetically enclose a cavity, wherein the membrane structure is arranged in the cavity. By enclosing a cavity between the first and the third layer structure, the sealing to hermetically enclose the cavity may have a high durability and a high robustness. 
     An embodiment provides for a photoacoustic gas sensor comprising such a detector cell and comprising an electromagnetic source configured for emitting an electromagnetic radiation so as to excite a movement of the membrane structure based on an asymmetric energy absorption of the electromagnetic radiation in different sub-cavities of the cavity, the different sub-cavities arranged on different sides of the membrane structure. 
     Embodiments provide for a chip-scaled packaged photoacoustic gas sensors comprising a detector cell having a membrane structure inside a detector cell cavity, having a first sub-cavity of the cavity at a first side of the membrane structure and having a second sub-cavity of the cavity at a second, opposing side of the membrane structure. The chip-scaled packaged photoacoustic gas sensor comprises an electromagnetic source configured for emitting an electromagnetic radiation so as to excite a movement of the membrane structure based on a asymmetric energy absorption of the electromagnetic radiation in the first sub-cavity and the second sub-cavity. 
     An embodiment provides for a method for manufacturing a detector cell. The method comprises 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 carried out such that the first layer structure and the third layer structure hermetically enclose a cavity and such that the membrane structure is arranged in the cavity. 
     An embodiment provides for a method for manufacturing a photoacoustic gas sensor. The method comprises providing a detector cell having a membrane structure inside a detector cell 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 a second, opposing side of the membrane structure. The method comprises arranging an electromagnetic source configured for emitting an electromagnetic radiation so as to excite a movement of the membrane structure based on an asymmetric energy absorption of the electromagnetic radiation in the first sub-cavity and the second sub-cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further embodiments are described in the dependent claims. 
       Embodiments will be described in the following while making reference to the accompanying drawings in which: 
         FIG. 1  is a schematic side view of a detector cell according to an embodiment; 
         FIG. 2 a    is a schematic side view of a further detector cell according to an embodiment; 
         FIG. 2 b    is a schematic perspective exploded diagram of the detector cell of  FIG. 2   a;    
         FIG. 3  is a schematic side view of a detector cell according to an embodiment having a coating layer; 
         FIGS. 4 a -4 k    are example processing steps for manufacturing a detector cell according to an embodiment; 
         FIG. 5  is a schematic block diagram of a photoacoustic gas sensor according to an embodiment; 
         FIG. 6  is a schematic block diagram of a chip-scaled packaged photoacoustic gas sensor according to an embodiment; 
         FIG. 7  is a schematic side view of a chip-scaled packaged photoacoustic gas sensor according to an embodiment. 
         FIG. 8  is a schematic side view of a chip-scaled packaged photoacoustic gas sensor according to an embodiment, having a lid; and 
         FIG. 9  is a schematic side of a chip-scaled packaged photoacoustic gas sensor according to an embodiment comprising a stacked configuration. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Equal or equivalent elements or elements with equal or equivalent functionality are denoted in the following description by equal or equivalent reference numerals even if occurring in different figures. 
     In the following description, a plurality of details is 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 in order to avoid obscuring embodiments of the present invention. In addition, features of the different embodiments described hereinafter may be combined with each other, unless specifically noted otherwise. 
     Embodiments described herein relate to photoacoustic gas sensors and to detector cells that may be used in such photoacoustic gas sensors. Such a photoacoustic gas sensor may comprise a detector cell in which a target gas, i.e., molecules or same or different type, are enclosed. That is, a single gas or a combination of gases or fluids may be enclosed. Such a detector cell may be arranged in a housing of a photoacoustic gas sensor, the photoacoustic gas sensor comprising a source of electromagnetic radiation. Further details in view of a working principle of a photoacoustic gas sensor are described in connection with disclosed embodiments. 
     Embodiments are related to a detector cell being a microelectromechanical structure (MEMS). A MEMS structure may comprise one or more semiconductor materials, for example, an at least partially doped or undoped semiconductor material such as silicon, gallium, arsenide or the like. Materials derived therefrom such as a silicon nitride (SiN, Si 3 N 4 , respectively), silicon oxide (SiO 2 ) or the like may be arranged alternatively or in addition. Alternatively or in addition, other materials such as a metal material, e.g., copper, gold, silver, platinum or the like, may be part of a MEMS structure. 
     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 being larger than a lateral extension perpendicular hereto), but may also be a planar or two-dimensional structure in which lateral extensions perpendicular to each other are equal with respect to each other within a tolerance range. An example for such a structure may be a circular structure, e.g., a round or circular membrane, or a quadratic membrane structure. Such a membrane structure may be formed, for example, similar to a membrane structure being used in MEMS microphones or MEMS loudspeakers. 
       FIG. 1  is a schematic side view of a detector cell  10  according to an embodiment. The detector cell  10  may be usable or integrateable into a photoacoustic gas sensor. That is, the detector cell  10  may form a component of a photoacoustic gas sensor but may be implemented separately or individually. 
     The detector cell  10  may comprise 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 arranged at the layer structure  12 . The layer structure  16  may be arranged at the layer structure  14 . The layer structure  12  may comprise one or more layers. For example, the layer structure  12  may comprise two layers  12   1  and  12   2 , wherein the layers  12   1  and  12   2  may comprise same or different materials. 
     Alternatively or in addition, the layer structure  14  may comprise one or more layers. For example, the layer structure  14  may comprise layers  14   1  and  14   2  having same or different materials. Alternatively or in addition, the layer structure  16  may comprise one or more layers. For example, the layer structure  16  may comprise layers  16   1  and  16   2  having same or different materials. 
     A number of layers of the layer structure  12 ,  14  and/or  16  may be implemented individually and equal or different with regard to a number of layers of other layer structures. A number of layers of each layer structure  12 ,  14  and  16  may be, for example, one, two, three, four, five or higher, e.g., seven or ten. 
     The layer structure  14  may comprise a membrane structure  18 . The membrane structure  18  may comprise one or more layers, for example, a semiconductor layer a semiconductor layer and a conductive layer, e.g., a doped semiconductor material or a metal material, covering at least parts of one or two sides of the membrane structure  18 . The membrane structure  18  may be arranged such that sub-cavities  22   a  and  22   b  of a cavity  22  are arranged 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  so as to form sub-cavity  22   a . Alternatively or addition, a recess may be formed in the layer structure  14  and/or the layer structure  16  so as to form sub-cavity  22   b.  That is, embodiments relate to a structure having only one of sub-cavities  22   a  and  22   b,  wherein further embodiments relate to structures having sub-cavity  22   a  and sub-cavity  22   b . Sub-cavity  22   a  may be fluidically connected to sub-cavity  22   b  or may be sealed from sub-cavity  22   b.    
     Layer structure  12  and layer structure  16  thus hermetically enclose cavity  22 . The membrane structure  18  is arranged in the cavity. To hermetically enclose the cavity  22 , the layer structure  12  and the layer structure  18  may be connected to each other so as to form a hermetically tight mechanical connection. Further, layer structures  14  and  16  may be connected to each other so as to form a hermetically tight mechanical connection with respect to each other. This is different when compared to a cavity in which a structure is arranged which itself hosts a cavity in which a membrane is arranged. According to embodiments, it is enabled to generate the cavity  22  directly via mechanically connecting layer structures to each other. The layer structure  14  may form at least a part of a sidewall  10 A of the detector cell  10 . 
     The cavity  22  may comprise or host a fluid, for example, a gas being a target gas for a later photoacoustic gas sensor. 
       FIG. 2 a    shows a schematic side view of a detector cell  20  according to an embodiment. The layer structures  12  and  16  may comprise, for example, semiconductor materials, conductive materials and/or insulating materials. For example, the layer structure  12  may comprise a glass material or a ceramic material as an insulating material. As a semiconductor material, for example, a silicon material or a gallium-arsenide material may be used. As a conductive material, for example, a metal material such as gold, silver, aluminum, copper or the like, including allays, may be used. Alternatively or in addition, a doped semiconductor material may be used. For example, the layer structure  12  may be obtained from a glass wafer or a silicon wafer. In connection with the described embodiments, the layer structure  12  may be referred to as a bottom sealing wafer. The layer structure  16  may, in contrast, be referred to as a top sealing cap wafer and may comprise, for example, a semiconductor material or an insulating material. A semiconductor material such as silicon may allow for generating or obtaining sub-cavity  22   b  as a recess in the layer structure  16 , whilst a glass material does not exclude such a configuration but may provide for an increased hardness of the material. Sub-cavity  22   a  may be formed at least partially as a recess in the layer structure  14 . 
     The layer structure  14  may comprise a structure that corresponds, essentially, to a Si-microphone structure. For example, the membrane structure  18  may be a multi-layer structure. 
     Layer structures  12  and  14  may be bonded to each other, for example, during a wafer bonding process. For example, between the layer structures  12  and  14 , a boundary layer or an interface  24  may be arranged. The interface  24  may be a result of the wafer bonding process. For example, a material arranged at the layer structure  12  and a material of the layer structure  14  may each form a part of the interface  24 . 
     For example, the layer structure  12  may comprise a coating layer  26  and a substrate layer  28 . The substrate layer  28  may comprise, for example, a conductive, insulating or semiconductor material such as a silicon material. At least in a region of a later mechanical connection to the layer structure  14 , the coating layer  26  may be arranged, for example, comprising a metal material, e.g., a gold material, wherein, alternatively, other materials such as aluminum or other reflective metallic or non-metallic materials or structures. For example, gold (Au) and aluminum (Al) may be used for implementing a eutectic bond. Such a material may, at a same time, provide for reflective properties. This does not exclude to use different materials for bonding and for the reflective surface. 
     Further, embodiments are not limited hereto. For example, a glass frit may be used for bonding. Any reflective structure or material may be used as coating layer  26 . For example, Au may be inert and optically stable. Alternatively or in addition, a Bragg mirror structure may be used. For example, such a structure may be obtained from Si/SiO 2  material for the present embodiments. That is, the coating layer  26  may form a surface being reflective for electromagnetic radiation and may comprise at least one of a reflective material and a reflective structure. 
     During the wafer bonding process, the material of the coating layer  26  and the material of the layer structure  14  may form the interface  24 , thereby providing a tight mechanical connection and thus a part of the hermetic sealing. 
     The coating layer  26  may, optionally, be arranged in a region of the cavity, the sub-cavity  22   a  and/or the sub-cavity  22   b.  This may allow for a reflective surface, e.g., to reflect thermal radiation or other electromagnetic radiation. 
     The coating layer  26  may provide for a surface reflective for electromagnetic radiation. The coating layer  26  may be arranged at surface of the layer structure  12  so as to face the membrane structure  18 . Alternatively or in addition, the coating layer  26  may be arranged at the layer structure  16  so as to face the membrane structure  18 . The coating layer  26  may allow to prevent an entry of electromagnetic radiation into the shielded sub-cavity, e.g., from a bottom side of  FIG. 2 a   . Alternatively or in addition, the coating layer  26  may allow reflection of electromagnetic radiation  54  that has already entered the cavity so as to prevent an escape of the radiation. 
     In a same or a different manner, between the layer structures  14  and  16  a coating structure or coating layer  32  may be arranged, for example, comprising a gold material, an aluminum material or the like For example, a combination of materials may be arranged, e.g., gold/tin (AuSn). By way of wafer level bonding, layer structures  14  and  16  may be combined or connected to each other as described for the layer structures  12  and  14 . 
     Embodiments relate to host a target medium such as a fluid, e.g., a gas as illustrated by example molecules  34   1  to  34   i  in the cavity. A target medium may be, for example, CO 2 , CO, NO 2  or any other suitable fluid such as CH 4  (Methane) and SO 2 . For example, the membrane structure  18  may comprise connections between sub-cavity  22   a  and sub-cavity  22   b,  for example, implemented by ventilation holes  36   1  and  36   2 , wherein a number of ventilation holes may be different, for example, 0, 1, 3 or more, 5 or more, 10 or more, 20 or more, or even higher numbers. This may allow obtaining a connection between the layer structures  12  and  14  and/or between the layer structures  14  and  16  differently. For example, the layer structures  12  and  14  and/or the layer structures  14  and  16  may be formed as a common layer structure out of which a respective sub-cavity  22   a  or  22   b  is formed, for example, using an etching process. This may allow avoiding a wafer level bonding as a target gas may reach the respective sub-cavity  22   a  or  22   b  by use of the ventilation. 
     Nevertheless, a wafer level bonding process may allow for a precise and hermetically tight connection between layer structures. The coating layer  32  may be used as, for example, a seal ring and may have, for example, a ring-like structure corresponding to a structure of a protruding  38  of the layer structure  16 . Optionally, conductive structures  42 , e.g., bond pads or the like for connecting one or more conductive layers, e.g., of the membrane structure  18  and/or a backplate structure, may be arranged. The conductive structure  42  may be formed, at least in parts, by same materials when compared to the coating layer  32  which allows for simple processes. For example, the coating structure  32  may easily be formed in addition to the conductive structures  42  without severely changing manufacturing processes. 
     Using wafer level bonding processes may allow to fabricate or generate or manufacture a plurality of detector cells in parallel and to separate them afterwards easily, for example, using a dicing process. 
     The layer structures  12 ,  14  and/or  16  may have some or different extensions  44   1 ,  44   2 ,  44   3  respectively along a thickness direction  46 . The thickness direction  46  may be parallel to a surface normal N 1  of layer structure  12 , to a surface normal N 2  of layer structure  14  and/or to a surface normal N 3  of layer structure  16 . The surface normals N 1 , N 2  and/or N 3  may be perpendicular to N-plane directions along which a wafer that forms or has previously formed one or more layers of layer structures  12 ,  14 ,  16 , respectively mainly or basically and extends. For example, layer structure  12  may form a substrate. For example, a maximum extension  44   1  or layer structure  12  may be arbitrary, wherein a thin layer structure  12  may be desirable whilst maintaining a certain stability. Within these boundaries, example extensions  44   1  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  44   2  may have any value, for example, 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  44   2  may be implemented such that it is a summarized value of a thickness  48  of membrane structure  18  along the thickness direction  46  and of a thickness or height  52   1  of sub-cavity  22   a.  For example, thickness  48  may be in a range of at least 1 μm and at most 10 μm, of at least 2 μm at most 7 μm or at least 3 μm and at most 5 μm, e.g., 4 μm. For example, the height  52   1  may be in a range of at least 100 μm and at most 990 μm, of at least 150 μm and at most 700 μm or at least 200 μm and at most 500 μm, e.g., in a range between 246 μm and 396 μm. Alternatively, the height  52   1  may be a result of using or further processing a starting structure of layer structure  14  that has the extension  44   2 . After forming the membrane structure  18  by generating a recess, the sub-cavity  22   a,  the height  52   1  maybe a result of the desired thickness  48 . Other values and sequences may be implemented. Alternatively or in addition, the extension  44   3  may have any suitable value, for example, 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  44   3  exceeds a thickness or height  52   2  of sub-cavity  22   b  which may be, for example, at least 1 μm and at most 500 μm, at least 2 μm and at most 400 or at least 5 μm and at most 300  82  m, e.g., in a range of at least 10 μm and at most 200  82  m. The extension  44   3  may allow for a robust enclosure of sub-cavity  22   b,  i.e., it may comprise a larger extension  44   3  when compared to the height  52   2 . 
     Alternatively or in addition, a combination of gases may be arranged. The molecules  34   1 , . . . ,  34   i  shown relate, by non-limiting example only, to CO 2 . 
     The cavity  22 , sub-cavity  22   a  and/or  22   b  respectively may be acoustically isolated. That is, the membrane  18  is vibrateable with respect to acoustic sound at an exterior of the respective sub-cavity  22   a  and/or  22   b  only to a negligible effect or is insensitive to acoustic sound. 
     The layer structure  12 , the layer structure  14  and/or the layer structure  16  may at least in parts be transparent for an electromagnetic radiation  54 . This may allow the electromagnetic radiation  54  to travel into the cavity  22 , sub-cavity  22   a  and/or  22   b  respectively, so as to excite membrane  18  to vibrate. For example, the layer structure  14  is transparent for the electromagnetic radiation  54 . The layer structures  12 ,  14 , and/or  16  may be transparent for a wavelength of an emitter to be combined with the detector cell. For example, the layer structures  12 ,  14  and/or  16  may be transparent for an infrared spectrum, in particular, a mid-wavelength infrared spectrum. Whilst the infrared spectrum may comprise wavelengths of at least 760 nm to at most 1 mm, the mid-wavelength infrared spectrum may comprise wavelengths of at least 1 μm and at most 100 μm, of at least 2 μm and at most 70 μm or at least 3 μm and at most 50 μm. 
     The detector cell  20  may be formed such that the detector cell  20  is asymmetric with regard to a sensitivity to the electromagnetic radiation in the sub-cavity  22   a  and in the sub-cavity  22   b.  Such an asymmetry may be understood as having different forces in view of magnitude, frequency or time offset with regard to a generation of the electromagnetic radiation  54  so as to prevent equal forces acting on the membrane structure  18  in both sub-cavity  22   a  and  22   b  which might cancel out the vibration of the membrane  18 . By implementing the asymmetry, the electromagnetic radiation  54  may comprise a high sensitivity to the electromagnetic radiation  54 . As will be described later in more detail, the asymmetry may be generated alternatively or in addition to having different heights  52   1  and  52   2  by other means. That is, an asymmetry may be obtained at least partially by implementing extensions  52   1  and  52   2  so as to be different, for example, 1:1.1, 1:1.2 or 1:1.5 or higher numbers. 
     Alternatively or in addition, the sub-cavities  22   a  and  22   b  may be shielded different, shielding one sub-cavity whilst not shielding the other or shielding to a different extent such that the electromagnetic radiation  54  penetrates or pierces the sub-cavities  22   a  and  22   b  differently. Alternatively or in addition, different pressures of the target gas  34  may be implemented, for example, in structures having sub-cavities being sealed from each other. 
     Alternatively or in addition, for obtaining the asymmetry, the sub-cavities  22   a  and  22   b  may be sealed from each other and may comprise different gases or gas concentrations. By using different gases, the detector cell may be implemented so as to be sensitive for two gases. For example, the absorption characteristic of both gases may be disjoined in the wavelength-range or frequency-range such that the excitation of the membrane structure  18  may clearly be distinguishable when evaluating the vibration of the membrane structure  18 . 
       FIG. 2 b    shows a schematic perspective exploded diagram of the detector cell  20  to illustrate, for example, the circumferential course of the coating structure  32 , i.e., the seal ring. The membrane structure  18  may be formed, for example, as a round or circular structure. Although four ventilation holes  36   1  to  36   4  are illustrated, a different number, e.g., 0 or more, 1 or more, 2 or more, 3 or more, 5 or more or a higher number may be implemented. That is, the membrane structure may comprise at least one ventilation hole. 
     Insert to bottom sealing wafer: Terms like bottom, top, left, right and the like are used to facilitate the understanding of the present disclosure. It is clear that based on a varying orientation of the structure the appropriate terms may vary without changing the scope of the embodiments. 
     In other words, the Si-microphone wafer with top sealing wafer and bottom sealing wafer is shown. A dedicated gas atmosphere such as any concentration of more than 0% and at most 100% of the target gas, e.g., CO 2  may be enclosed during a bonding step, for example a last bonding step. A concentration of 100% may provide for a high sensitivity wherein a lower concentration may allow for combination of gases and thus for multiple sensitivities. A pressure of the target gas, may be higher or lower when compared to an ambient pressure of the later device. For example, a pressure may be of at least 10 mbar and at most 5 bar or any other suitable value, e.g., to enhance or reduce the absorption of electromagnetic radiation. 
     The steps may be implemented so as to first provide for a backside sealing (Au/Si eutectic bond) and to then seal under a CO 2  atmosphere (AuSn soldering of cap structure to metal ring on Si-MEMS topside). Those steps may be performed in different order. The bond pads of the microphone may remain accessible after the WLB processes. The whole step of silicon wafers may be transparent for the mid-wavelength infrared spectrum, which may be used for optical excitation in gas sensing. 
       FIG. 3  shows a schematic side view of a detector cell  30  according to an embodiment. The detector cell  30  may be formed similar to the detector cell  20 . When compared to the detector cell  20 , beside a coating layer  26   1  which may be the coating layer  26  of detector cell  20 , another coating layer  26   2  may be arranged at the layer structure  16  or as a part thereof, for example, so as to face the membrane structure  18 . Although the coating layer  26   1  and the coating layer  26   2  both are optional, the configuration of detector cell  20  and of detector cell  30  allows that a part of the cavity is sealed by a reflective coating from light or electromagnetic radiation adapted to excite the target medium  34  in the cavity. 
     The membrane structure  18  described in connection with detector cell  10 ,  20  and/or  30  may be evaluated for a vibration thereof. The detector cell  10 ,  20  and/or  30  may comprise a circuitry being configured for evaluating the vibration. Alternatively or in addition, the detector cell  10 ,  20  and/or  30  may be connectable to a suited circuitry, for example, using conductive structures  42 . The membrane structure  18  may be arranged, for example, in a single-backplate configuration or a dual-backplate configuration. A single-backplate configuration may refer to a configuration according to which a vibration of the membrane having a conductive surface is evaluated with regard to one counter electrode arranged adjacent to the membrane. In a dual-backplate configuration, for example, the vibrateable membrane may be sandwiched between two counter-electrodes. That is, the layer structure  14  may comprise a single backplate configuration or a dual-backplate configuration for the membrane structure  18 . Alternatively or in addition, the detector cell  10 ,  20  and/or  30  may comprise a piezoelectric or a piezo-resistive element so as to determine a deformation or vibration of the membrane structure  18 . 
     Whilst making reference to  FIGS. 4 a  to 4 k    example processing steps for manufacturing a detector cell  10 ,  20  and/or  30  are described in the following. It is noted that the figures neither limit such a manufacturing process to a specific sequence or order of steps nor are all of those steps necessary to manufacture for a detector cell in accordance of embodiments nor are further steps precluded. 
       FIG. 4 a    shows a schematic side view of the layer structure  14  having the microphone structure  18 , the seal ring  32  and conductive structure  42 . The conductive structure  42  may be, for example, a metallization using a metal material such as gold, silver, aluminum, copper or the like. The membrane structure  18  may be a single backplate structure or a dual backplate structure. In the figures of the present disclosure, the membrane structure and counter electrodes are displayed as a single block so as to facilitate the understanding of embodiments. The layer structure  14  may be similar to a silicon based microphone structure. A native insulating layer, for example, SiO 2  on a substrate backside may be removed, for example, using a HF (hydrogen fluoride) dip. 
       FIG. 4 b    shows a schematic side view of a configuration of the layer structure  12 , for example, comprising the substrate layer  28  at the present stage. The substrate layer  28  may be, for example, at least a part of a silicon wafer but may also comprise other materials. By way of example, the substrate layer  12  may be a wafer to be diced or separated later. 
       FIG. 4 c    shows a schematic side view of the layer structure  12 . When compared to  FIG. 4 b   , the coating layer  26  has been arranged, for example, over the complete wafer or at least large structures thereof. The deposition of the coating layer  26  may comprise a deposition of a metal material such as gold or the like on the silicon wafer. The deposition of the coating layer  26  may include a deposition of an adhesion layer, for example, tin (Ti). The coating layer  26  may serve for multiple purposes. For example, it may serve for forming an alloy with the layer structure  14  when performing a wafer level bonding (WLB). Further, it may serve as a reflection plane for optical radiation, e.g., electromagnetic radiation  54 . 
       FIG. 4 d    shows a schematic side view of a configuration of layer structures  12  of  FIG. 4 c    and of layer structure  14  of  FIG. 4 a    prior to a step of combining both layer structures whilst  FIG. 4 e    shows a schematic side view of layer structures  12  and  14  after the wafer level bonding. Based on the wafer level bonding, the interface  24  may be obtained allowing for a tight connection of layer structures  12  and  14 . The interface  24  may comprise the alloy comprising material of the coating layer  26  and of the semiconductor material of layer structure  14 , e.g., silicon material. The described eutectic Au/Si bond may be performed, for example, under a vacuum atmosphere or any other suitable atmosphere as the target gas may be included later when the membrane structure  18  comprises ventilation holes. Alternatively, a sealed sub-cavity may be bonded under the target atmosphere. 
       FIG. 4 f    shows a schematic side view of layer structure  16 , the layer structure  16  may comprise a topographic structure. In parts thereof, an interface forming material  56 , e.g., a gold material, an aluminum material, a tin material or a silver material or the like, including materials forming an alloy, e.g., gold/tin may be arranged as described for the conductive layer  26 . Recesses  58   1 ,  58   2 , and/or  58   3  may be arranged. Recess  58   2  may later provide at least partially for the sub-cavity  22   b  whilst recesses  58   1  and  58   3  may allow for facilitating a later dicing. For example, material may be removed, e.g., based on etching or grinding, until a level indicated by a line L is reached. The recesses  58   1 ,  58   2  and/or  58   3  may be optional. For example, the sub-cavity  22   b  may also be formed as a recess in the layer structure  14 , e.g., when arranging the membrane structure  18  in a center of the layer structure  14  with regard to the extension  44   2  illustrated in  FIG. 2   a.    
     In other words, a Si-cap may be built with a gold-Sn solder  56  at contact position. By having two or more cavities, the singulation of the final dies may be done by grinding. This may allow for preventing cracks in the structure. 
     For obtaining a structure illustrated in  FIG. 4   f,  a silicon wafer may be used into which the recesses  58   1  and  58   3  may be structured, for example, using an action process. The recesses  58   1  and  58   3  may be a same recess, for example, having a rectangular, elliptic or circular course. That is, a structuring of a first cavity  58   1 ,  58   3  may be performed into a silicon wafer  62 . 
     Prior or after generating the recess  58   1  and/or  58   3 , the recess  58   2  may be generated, for example, using an etching process. Etching may be performed as wet etching or dry etching or other concepts to remove material. That is, a structuring of a second cavity  58   2  may be performed into the silicon wafer  62 . 
     As shown in  FIG. 4   i,  the interface forming material  56  may be arranged at contact positions or contact regions  64  of the wafer  62 . That is, in regions where layer structures  14  and  16  are deemed to contact each other, the interface forming material  56  may be arranged at least partially. Alternatively or in addition, the interface forming material  56  may also be arranged at the layer structure  14 . In other words, a deposition of AuSn is performed at contact position. Optionally, the coating layer  26   2  may be arranged in recess  58   2  prior or afterwards or simultaneously. 
     The structure illustrated in  FIG. 4 e    and the structure illustrated in  FIG. 4 i    may both be arranged into a processing chamber that may comprise the target medium  34 . It is to be noted that the wafer level bonding being described in connection with  FIG. 4 e    may also be performed in an atmosphere having the target medium  34 . Alternatively, the wafer level bonding described in  FIG. 4 e    may be performed under a different atmosphere when compared to the wafer level bonding of  FIG. 4   j.  This may allow for hosting different media, pressures, or concentrations of gases in different sub-cavities being sealed from each other. One of such sealed cavities may also comprise a low pressure or vacuum, that is, the processing chamber may be evacuated when performing the wafer level bonding. Based on the coating layer  32  and the interface forming material  56  and by performing a wafer level bonding, layer structures  14  and  16  may mechanically be connected to each other. It is noted that wafer level bonding of layer structures  12  and  14  may be performed simultaneously or after having bonded layer structures  14  and  16 . 
     In other words, a wafer bonding of the top sealing wafer  16  may be performed on a metallization of the microphone (here: AuSn—Au bond). Other bonding techniques, i.e., other materials are possible. The process may be done under a target atmosphere, e.g., CO 2 . Depending on the target gas to be detected, also one or more different atmospheres may be chosen. 
     After having performed the wafer level bonding, the single detector cells may be separated from each other by removing a part of the layer structure  16 , for example, starting from a side  16 A, e.g., a top side, until the line L such that a configuration similar to  FIG. 4 k    may be obtained. Layer structures  12  and/or  16  may be diced as those structures are mechanically robust. 
     In other words, the final device may comprise a Si-microphone with a top and a bottom sealing wafer  12  and  16 . A target media (CO 2 ) is enclosed within the Si-microphone back volume as well as the cavity between the Si-cap and Si-microphone top side. The design of the Si-cap can be adjusted, e.g., the height of the cavity. Also, the overall shape of the resulting cap after singulation can be adjusted as shown, for example, in  FIG. 2 , e.g., with more DRIE (deep reactive ion etching processes) during structuring before the WLB process. A dual backplate Si-microphone may be used, wherein also different SiMiC (silicon microphone) technology may be used. A bottom sealing wafer, which may allow for an easy handling. However, this does not preclude a wafer having a topography from being handled. For example, the Si wafer may be coated with Au which may include a Ti adhesion layer. A HF dip may be used to remove native SiO 2  on a MEMS backside. An Au/Si eutectic bond may be performed, for example, using approximately 360° C. The top wafer may be processed by processing a Si cap wafer, which can be done on a carrier wafer. An AuSn/Au diffusion bond may be performed, e.g., by applying a temperature of approximately 320° C. Then, a release may be performed. 
       FIG. 5  shows a schematic block diagram of a photoacoustic gas sensor according to an embodiment. The photoacoustic gas sensor  50  may comprise the detector cell  10 , wherein alternatively or in addition one or more different detector cells may be arranged, for example, detector cell  20  and/or  30 . The photoacoustic gas sensor may comprise an electromagnetic source  66  configured for emitting the electromagnetic radiation  54  so as to excite a movement of the membrane structure  18  based on an asymmetric energy absorption of the electromagnetic radiation in sub-cavities  22   a  and  22   b  of the cavity of the detector cell. 
     The photoacoustic gas sensor  50  may comprise a control unit  68  configured for evaluating the vibration of the membrane structure  18  and/or for controlling the electromagnetic source  66 . That is, the control unit  68  may be in communication with the detector cell  10  and/the electromagnetic source  66 . The control unit  68  may comprise, for example, a processor, a microcontroller, a field programmable gate array (FPGA) and/or an application specific integrated circuit (ASIC). 
     The detector cell  10 , the detector cell  20  and/or the detector cell  30  may be obtained by processing on a wafer level. Embodiments relate to a chip-scaled packaging of a photoacoustic gas sensor, i.e., to chip-scaled packaged photoacoustic gas sensors. 
       FIG. 6  shows a schematic block diagram of a chip-scaled packaged photoacoustic gas sensor  60  according to an embodiment. The chip-scaled packaged photoacoustic gas sensor  60  may comprise a detector cell  65 . The detector cell  65  may have a membrane structure, e.g., the membrane structure  18  inside a detector cell cavity, e.g., cavity  22 . Sub-cavities  22   a  and  22   b  of cavity  22  may be arranged on different sides of the membrane structure  18 . The chip-scaled packaged photoacoustic gas sensor  60  may comprise the electromagnetic source or emitter  66  which may comprise a spacing  74  and a casing  76  and an emitting element E that may generate the electromagnetic radiation  54 , for example, based on a heating. That is, the element E may be a heater. Alternatively, the element E may be a black body or the like. 
     The electromagnetic source  66  may be configured for emitting the electromagnetic radiation  54  so as to excite a movement of the membrane structure  18  based on the described asymmetric energy absorption of the electromagnetic radiation  54  in sub-cavity  22   a  and sub-cavity  22   b.  The chip-scaled package photoacoustic gas sensor may be implemented such that the sub-cavities  22   a  and  22   b  have different sizes and/or different surface ratios so as to at least partially obtain the asymmetric energy absorption as described for the detector cells. The electromagnetic source  66  may be implemented to provide for a pulsed excitation of the electromagnetic radiation  54 , e.g., based on a respective control signal. A frequency of the pulsing and/or a wavelength of the signal may be adapted to the target gas and/or the resonance frequency of the membrane structure. 
     The electromagnetic radiation  54  may be referred to as light even if comprising harshly or completely invisible wavelengths when compared to human abilities. For example, the detector cell  65  may be implemented as described for the detector cell  10 ,  20  and/or  30 . Alternatively, a configuration may be implemented in which sub-cavities  22   a  and  22   b  are sealed from each other. The target medium  34  may be arranged in at least one sub-cavity  22   a  and/or  22   b.  The possible other sub-cavity may comprise a different target medium or no target medium, i.e., it may be evacuated. 
     As will be described, the asymmetric energy absorption may be based on an asymmetric energy input into the sub-cavity  22   a  and the sub-cavity  22   b  from the electromagnetic radiation  54 . Alternatively or in addition, the asymmetric energy absorption may be based on an asymmetric energy loss from sub-cavity  22   a  and sub-cavity  22   b.  Such an energy loss may be obtained, for example, by having different sizes of wall structures surrounding the cavities and/or different thermal conductivity. The energy loss may thus be based on an energy input of the electromagnetic energy or electromagnetic radiation into the sub-cavities  22   a  and  22   b.  The energy loss may thus be related to a thermal loss path that may lead to a reduction of resulting pressure in the target medium  34 , e.g., by cooling due to the energy loss. 
     The chip-scaled packaged photoacoustic gas sensor  60  may comprise a substrate  72  on which the detector cell  65 , the electromagnetic source  66  and/or the control unit  68  may be arranged. The substrate  72  may comprise a semiconductor material or a glass material or a ceramic material or a combination thereof. So as to allow for a low thermal loss, the electromagnetic source  66  may be spaced from the substrate  72  by a spacing structure  74  and/or encapsulated by a casing  76 . The electromagnetic source  66  may form an emitter together with the casing  76 . The emitter may include a filter for filtering a wavelength to be emitted towards the detector cell  65 , for example, so as to avoid ambiguities in measurement results obtained by the control unit  68 . For example, the fluid in the cavity, e.g., the target medium  34 , may comprise a target frequency at which the fluid is resonant. The chip-scaled packaged photoacoustic gas sensor may be implemented so as to comprise a filter structure, e.g., as part of the housing  76  and/or of the spacing structure  74  or arranged between the emitting element E and the detector cell  65 . The filter structure may be arranged for filtering the electromagnetic radiation  54  so as to attenuate a wavelength not corresponding to the target frequency in a larger amount, i.e. at least 20%, at least 30%, at least 50% or more, when compared to a wavelength corresponding to the target frequency. For example, the filter structure is incorporated in the casing  76  or the filter structure implements the casing  76 . 
     The chip-scaled packaged photoacoustic gas sensor  60  may comprise a housing  78  forming an enclosure for at least the electromagnetic source  66  and the detector cell  65 , wherein additional components may be arranged, for example, the control unit  68 . That is, the chip-scaled package photoacoustic gas sensor  60  may comprise a lid  78  at least partially forming a cavity  86  of the chip-scaled packaged photoacoustic gas sensor. The cavity  86  may host at least the detector cell  65  and the electromagnetic source  66 . The lid  78  may be reflective for the electromagnetic radiation. The enclosure may comprise a ventilation or opening  82  to allow environmental medium  84 , e.g., air or a different medium, to travel into an interior  86  of the enclosure. That is, the chip-scaled packaged photoacoustic gas sensor may comprise an inlet so as to let pass a target medium, i.e., the environmental medium  84 . The environmental medium  84  may thus be subjected to the electromagnetic radiation and may absorb energy therefrom at least in some specific wavelength ranges. In knowledge of a behavior of the membrane structure  18  in absence thereof, i.e., based on a calibration, a content of the environmental medium  84  may be determined. That is, at least a presence or concentration of the target medium  34  may be determined in the environmental medium  84 . 
     In other words, a gas sensor cell including the WLB detector unit is disclosed. An infrared emitter may be packaged within the same housing next to a detector unit with corresponding ASIC for the read-out of the detector unit. 
     A distance  88  between the source  66  and side  78 A may be small, for example, a preferably non-zero value of at most 1 mm, 500 μm or 100 μm. Such a small distance  88  may allow the electromagnetic radiation  54  to essentially arrive at the detector cell from a lateral side to excite the target medium  34 . This may allow for a same or comparable energy input into the sub-cavities  22   a  and  22   b.    
     Optionally, a shielding  92  may be arranged between the electromagnetic source  66  and the detector cell  65 . The shielding  92  may be configured for partially shielding the detector cell  65  from the electromagnetic radiation  54  so as to at least partially obtain the asymmetric energy absorption. The shielding  92  may at least partially shield sub-cavity  22   a  and/or at least partially shield sub-cavity  22   b.  For example, only one of both sub-cavities is shielded or the sub-cavities are shielded by a different extent. 
       FIG. 7  shows a schematic side view of a chip-scaled packaged photoacoustic gas sensor according to an embodiment. The distance  88  may be larger when compared to the chip-scaled packaged photoacoustic gas sensor  60 , for example, having a distance larger than described in connection with  FIG. 6 . An example value that does not limit the described embodiments may 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  may be measured between a main side  78 A which is spaced from the emitter and the detector cell  65  by a circumferential side  78 B of the lid  78 . The large distance  88  may allow scattering of the electromagnetic radiation  54  towards the detector cell  65  at the main side  78 A. In contrast, the small distance shown in  FIG. 6  may prevent scattering of the electromagnetic radiation  54  towards the detector cell  65  at the main side  78 A such that the electromagnetic radiation  54  laterally travels towards the detector cell  65 . 
     In other words, a gas sensor unit according to an embodiment may include the WLB detector unit. An infrared emitter may be packaged within the same housing next to the detector unit with corresponding ASIC for the read-out of the detector unit. The distance from the detector top side to the lid of the sensor unit may be big enough in order to have optical access to the top side of the detector unit. The light may be scattered and reflected within the main optical shielding (module package) making it hard to determine a main angle of incidence. 
     The control unit  68 , i.e. the circuit, may be covered with a material  94  being intransparent for the electromagnetic radiation  54 . Such an arrangement is optional. Alternatively or in addition but also optionally, the control unit or circuit  68  may be insensitive for the electromagnetic radiation  54  such that in both cases, the electromagnetic radiation  54  does not harm an operation of the control unit  68 . 
       FIG. 8  shows a schematic side view of a chip-scaled packaged photoacoustic gas sensor  80  according to an embodiment. The lid  78  may be formed as described in connection with the chip-scaled packaged photoacoustic gas sensor  70  but may also be formed as described for the chip-scaled photoacoustic gas sensor  60 . When compared to the detector cell  65 , a detector cell  85  of the chip-scaled photoacoustic gas sensor  85  comprises a reflective coating  26   2  completely or at least to an amount of more than 50%, more than 70% or more than 90% covering or shielding one of the sub-cavities  22   a  or  22   b,  e.g. sub-cavity  22   b.  Such a reflective coating  26   2  may be applied, for example, to an electrode  96  of the single backplate configuration or dual-backplate configuration of the microphone chip to prevent the light from shining through the bottom interface of the top volume, i.e. to prevent the electromagnetic radiation  54  to travel through the sub-cavity  22   b  to the sub-cavity  22   a.    
     In other words, the WLB may include a reflective coating of the inner or outer surfaces of the top sealing cap wafer, i.e. layer structure  16 . Thus, direct optical access into the upper gas volume  22   b  can be avoided. 
       FIG. 9  shows a schematic side of a chip-scaled packaged photoacoustic gas sensor  90  according to an embodiment. When compared to the chip-scaled packaged photoacoustic gas sensor  60 ,  70  or  80 , the chip-scaled packaged photoacoustic gas sensor  90  may comprise a stacked configuration. Different sub-packages  98   1  and  98   2  may be stacked with regard to each other and may thus extend to a different perpendicular extension when compared to the chip-scaled packaged photoacoustic gas sensors  60 ,  70  and  80 . Whilst reducing a required surface with the stacked arrangement, a height may be increased. Sub-package  98   1  may comprise the electromagnetic source  66 , for example including a filter. Along the thickness direction  48  sub-package  98   2  may be spaced with a spacing structure or spacer or thermally decoupling element  102  being arranged between a substrate  72   1  and a substrate  72   2  of the sub-packages  98   1  and  98   2 . The thermally decoupling element  102  may comprise a low thermal conductance. For example, a polymer material or the like may be used. 
     Sub-package  98   2  may comprise the detector cell  65 . The control unit  68  may be arranged in sub-package  98   1  or  98   2 . The electromagnetic radiation  54  may travel from the sub-package  98   1  to the sub-package  98   2 . For example, substrate  72   2  may comprise an opening  104  or an area of low thermal conductance. 
     In other words, embodiments relate to a closed photoacoustic gas sensing cell comprising 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 a hermetically sealed package under a defined atmosphere of the gas of interest (e.g., a specific percentage CO 2 , a target gas in a target concentration). The package of the detector unit may be hermetically sealed over lifetime, e.g., at least 5 years and possible ranging to 15 years. This requirement may be addressed with embodiments described herein that provide for a packaging process and corresponding structures. A wafer level bonding (WLB) processes under a desired atmosphere may decrease the packaging cost per unit since the whole packaging process may be performed on all devices still on wafer level. That is, may be prevented to fill single devices individually. WLB processes may furthermore decrease the form factor of the gas sensing detector unit in comparison to a standard packaging method and therefore enables further possibilities for integration into small-scale PCBs (printed circuit board), e.g. mobile phone applications. 
     A hermetically sealed gas detector unit can be formed by wafer level processes, thereby the unit may comprise a microphone wafer, a top sealing wafer acting as a cap above the membrane area as well as a bottom sealing wafer. The bottom and top sealing wafer can be equipped with reflective coating for optical shielding of the upper (above the microphone membrane) or lower (below the microphone membrane) gas volume from the outside. With this packaging, a very small enclosed gas volume can be realized, depending only or at least essentially on the thickness of the microphone wafer and the cavity in the top sealing wafer above the microphone front side. Values stated for single thicknesses do not limit the process limitations. Pulsed excitation with an infrared light source may lead to a pressure difference between above and below the microphone membrane within the enclosed gas volume and may thus lead to an acoustic signal dependent on the intensity of the infrared light. 
     The ASIC may be covered with light non-transparent material, e.g., globe top, or may be robust against broadband light. A gas exchange may be provided through openings in the optical shielding, depending on the optical path the ventilation can be adjusted in order to enhance the gas exchange diffusion time. With more light absorbed through the optical path outside of the detector unit (higher ambient CO 2  concentration), the photoacoustic pressure within the detector cell may get smaller, i.e., an inverse signal may be obtained at the ASIC. The WLB photoacoustic detector unit can be included in a photoacoustic sensor comprising, for example, a chopped MEMS infrared emitter, electromagnetic source, an optical filter for wavelength selective heating of a gas, the hermetically sealed MEMS microphone using WLB processes and a housing. The system may be operated by an internal ASIC which provides the input power of the infrared emitter as well as the acoustical read-out of the WLB detector unit. 
     Embodiments are based on produced hermetically sealed MEMS microphones under a dedicated gas atmosphere using wafer level bonding processes. The small hermetically enclosed gas volume may be beneficial for creating a photoacoustic pressure. It is mentioned that the ration between the two volumes (above and below the microphone membrane, the sub-cavities) may be important for the response of the detector unit to chopped infrared light. Optical shielding of one of the volumes (e.g. by metal coating of the inner part of the top volume) may enhance the detector sensitivity. In general, the height of the WLB PAS (photoacoustic sensor) detector cell may be defined or at least influenced by the thickness of the three wafers (layer structures) as well as the height of the cavity above the MEMS microphone. Thus, this may form a chip-sized solution to design a hermetically sealed WLB PAS detector cell with a height range which is possibly exclusively defined by process windows for the respective three wafers. This may allow providing small WLB PAS detector cells. 
     As pollution is a health effect and as health concerns due to air pollution are growing, embodiments allow to decrease the form factor as well as the production cost for a hermetically closed photoacoustic detector unit. Detector cells may be a stand-alone product but may also be included into photoacoustic gas sensors. This may provide for advantages compared to NDIR (non-dispersive infrared sensor) detectors. Embodiments relate to an infrared source that is integrated into the WLB process of the detector unit, e.g. as top or bottom wafer. The optical filter wafer may be used as top or bottom sealing wafer. That is, a process for manufacturing the electromagnetic source  66  may be similar to producing a MEMS microphone. The structure  74  and/or  76  may thus include filtering properties. 
     Although some aspects have been described in the context of an apparatus, 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. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. 
     The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein.