Patent Publication Number: US-2021181158-A1

Title: Photoacoustic detector unit, photoacoustic sensor and associated production methods

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 102019134267.8 filed on Dec. 13, 2019, the content of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to photoacoustic detector units, photoacoustic sensors and associated production methods. 
     BACKGROUND 
     Photoacoustic sensors can detect specific gas species in the ambient air, for example. In particular, harmful or hazardous components in the ambient air can be detected in this case. The correct functioning of such sensors can thus be of extremely high importance in many applications, particularly if the sensors are used for the safety of work personnel. Photoacoustic sensors can be constructed from a plurality of components and generally consist of an emitter unit and a detector unit. 
     BRIEF SUMMARY 
     Implementations described herein may provide photoacoustic detector units configured to effectively detect different gas species in the ambient air. Furthermore, implementations described herein may provide cost-effective methods for producing such photoacoustic detector units. A first aspect relates to a photoacoustic detector unit. The photoacoustic detector unit comprises a housing having an opening. The photoacoustic detector unit furthermore comprises a photoacoustic transducer designed to convert optical radiation into at least one from a pressure signal or a heat signal, wherein the photoacoustic transducer covers the opening of the housing, such that the photoacoustic transducer and the housing form an acoustically tight cavity. The photoacoustic detector unit furthermore comprises a pressure pick-up arranged in the acoustically tight cavity. 
     A second aspect relates to a photoacoustic sensor. The photoacoustic sensor comprises an optical emitter and a photoacoustic detector unit in accordance with the first aspect. 
     A third aspect relates to a method. The method comprises bonding a first wafer composed of a first material to a second wafer composed of a second material in a reference gas atmosphere, wherein a plurality of hermetically sealed cavities are formed, which enclose the reference gas of the reference gas atmosphere. The method furthermore comprises singulating the bonded wafers into a plurality of photoacoustic transducers for a photoacoustic detector unit, wherein each of the photoacoustic transducers comprises one of the hermetically sealed cavities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Photoacoustic detector units, photoacoustic sensors and associated production methods in accordance with the disclosure are explained in greater detail below with reference to drawings. The elements shown in the drawings are not necessarily rendered in a manner true to scale relative to one another. Identical reference signs may designate identical components. 
         FIG. 1  shows a schematic view of a photoacoustic sensor  100  in accordance with the disclosure. 
         FIG. 2  shows a schematic view of a photoacoustic sensor  200  in accordance with the disclosure. 
         FIG. 3  schematically illustrates a cross-sectional side view of a photoacoustic detector unit  300  in accordance with the disclosure. 
         FIG. 4  schematically illustrates a cross-sectional side view of a photoacoustic detector unit  400  in accordance with the disclosure. 
         FIG. 5  schematically illustrates a cross-sectional side view of a photoacoustic detector unit  500  in accordance with the disclosure. 
         FIG. 6  schematically illustrates a cross-sectional side view of a photoacoustic detector unit  600  in accordance with the disclosure. 
         FIG. 7  schematically illustrates a cross-sectional side view of a photoacoustic detector unit  700  in accordance with the disclosure. 
         FIG. 8  schematically illustrates a cross-sectional side view of a photoacoustic detector unit  800  in accordance with the disclosure. 
         FIG. 9  schematically illustrates a cross-sectional side view of a photoacoustic detector unit  900  in accordance with the disclosure. 
         FIG. 10  schematically illustrates a cross-sectional side view of a photoacoustic sensor  1000  in accordance with the disclosure. 
         FIG. 11  illustrates a flow diagram of a method in accordance with the disclosure. 
         FIGS. 12A to 12E  schematically illustrate a cross-sectional side view of a method for producing a photoacoustic transducer  1200  for a photoacoustic detector unit in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The figures described below show photoacoustic detector units, photoacoustic sensors and associated production methods in accordance with the disclosure. In this case, the described devices and methods may be illustrated in a general way in order to describe aspects of the disclosure qualitatively. The devices and methods described may have further aspects that may not be illustrated in the respective figure for the sake of simplicity. However, the respective example can be extended by aspects described in association with other examples in accordance with the disclosure. Consequently, explanations concerning a specific figure may equally apply to examples of other figures. 
     The photoacoustic sensor or photoacoustic gas sensor  100  in  FIG. 1  can comprise a photoacoustic emitter unit  2  and a photoacoustic detector unit  4 . The photoacoustic detector unit  4  can comprise a photoacoustic transducer  6  and a housing  8 . The photoacoustic transducer  6  can be a cell having a hermetically sealed cavity  10 , in which a reference gas  12  can be enclosed. The photoacoustic transducer  6  can have an optically transparent window  18  on a first side and a membrane  20  on a second side situated opposite the first side. The photoacoustic transducer  6  can cover an opening of the housing  8 , such that the photoacoustic transducer  6  and the housing  8  can form an acoustically tight cavity  14 . A pressure pick-up  16  can be arranged in the acoustically tight cavity  14 . 
     Furthermore, a protective gas can optionally be enclosed in the acoustically tight cavity  14 . The protective gases specified in this description can be, for example, nitrogen or a noble gas, such as e.g. argon, xenon, krypton. Furthermore, depending on the application, one or more alternative or additional components can be arranged in the acoustically tight cavity  14 , for example one or more from a pressure pick-up ASIC, a photodetector, a photodiode, a temperature sensor, an optical emitter. 
     The photoacoustic emitter unit  2  can be a broadband emitter, which can be designed to emit optical radiation over a wide frequency range. In other words, the radiation emitted by the broadband emitter can comprise not just predetermined frequencies or predetermined frequency bands. The term “optical radiation” used in this description can generally refer to a partial range of the electromagnetic spectrum having wavelengths of between approximately 100 nm and approximately 100 μm. That is to say that the optical radiation can comprise, in particular, at least one from the following: ultraviolet radiation having a wavelength of approximately 100 nm to approximately 380 nm, infrared radiation having a wavelength of approximately 780 nm to approximately 100 μm, or radiation having a wavelength of approximately 780 nm to approximately 5 μm, e.g. near-infrared radiation and portions of mid-infrared radiation. The last-mentioned range can comprise, inter alia, the absorption lines/bands of carbon dioxide at 4.26 μm and of further gas species. Even more specifically, the optical radiation can have a wavelength of approximately 300 nm to approximately 20 μm. 
     The photoacoustic emitter unit  2  can be designed to emit optical pulses having a predetermined repetition frequency and one or more predetermined wavelengths. In this case, a predetermined wavelength can comprise an absorption band of a gas to be detected or of the reference gas  12 . The repetition frequency of the optical pulses can be within a low-frequency range or within a frequency range of approximately 1 Hz to approximately 10 kHz, in particular of approximately 1 Hz to approximately 1 kHz. Even more specifically, a typical frequency range can be between approximately 1 Hz and approximately 100 Hz, corresponding to a pulse duration range of approximately 0.01 s to approximately 1 s. 
     A manner of functioning of the photoacoustic sensor  100  is described below. The optical pulses emitted by the emitter unit  2  can pass through an interspace  22  situated between the emitter unit  2  and the detector unit  4 . By way of example, the interspace  22  can be filled with ambient air. During propagation through the interspace  22 , the optical pulses can be at least partly absorbed by portions of a gas to be detected if such a gas is present in the interspace  22  (e.g. in the ambient air). The absorption can be specific to the gas to be detected, e.g. characteristic rotation or vibration modes of atoms or molecules of the gas to be detected. 
     The optical pulses can pass through the material of the optically transparent window  18  and impinge on atoms or molecules of the reference gas  12  in the hermetically sealed cavity  10 . The reference gas  12  can correspond to the gas to be detected. The reference gases mentioned in this description can be, for example, carbon dioxide, nitrogen oxide, methane, ammonia. The optical pulses can at least partly be absorbed by the reference gas  12  and bring about local pressure increases in the reference gas  12 . The pressure increases can be passed on to the membrane  20  and through the latter into the acoustically tight cavity  14 . In other words, the photoacoustic transducer  6  can be designed to convert optical radiation in the form of e.g. optical pulses into pressure signals. The photoacoustic transducer  6  is acoustically coupled to the acoustically tight cavity  14 . 
     As an alternative or in addition to the pressure signals described, the photoacoustic transducer  6  can convert the optical radiation into heat signals. In this context, the photoacoustic transducer  6  can also be referred to as a photothermal transducer. In this case, the membrane  20  can be heated by absorption of the optical pulses, in particular with the predetermined repetition frequency of the optical pulses. As a result of the periodic heating and cooling of the membrane  20 , pressure changes can be produced in the downstream acoustically tight cavity  14 , which pressure changes can be detected by the pressure pick-up  16 . 
     Generally, the photoacoustic transducers in accordance with the disclosure as described herein can accordingly convert optical radiation into at least one from a pressure signal or a heat signal. In this case, the type of signal generated can be dependent on the respective configuration of the photoacoustic transducer. A conversion into a pressure signal can be provided in particular by way of an deflection or mechanical bending of the membrane, while a conversion into a heat signal can be provided in particular by heating and cooling of the membrane. Depending on the configuration of the respective photoacoustic transducer, a conversion into a pressure signal and/or a heat signal can take place. Pressure signals and heat signals generated can both be detected by a downstream acoustically tight cavity with pressure pick-up. Furthermore, the pressure signals can also be detected in the membrane itself, for example by one or more piezo-elements integrated into the membrane. 
     The expression “acoustically tight” used for the cavity  14 , for example, need not necessarily mean in this description that the cavity  14  is hermetically or completely sealed. Rather, the walls forming the cavity  14  can be designed to provide, during operation of the photoacoustic sensor  100 , pressure equalization with the surroundings such that the pressure pick-up  16  can be operated properly. In this case, it cannot be excluded, for example, that the walls of the cavity  14  have one or more small openings which do not influence, or which influence only negligibly, the pressure equalization for proper operation. The term “acoustically tight” can optionally be replaced by the term “semi-hermetic”. 
     The pressure signals and/or heat signals passed on by the membrane  20  can be detected by the pressure pick-up  16  in the acoustically tight cavity  14 . The pressure pick-ups specified in this description can be, for example, microphones or any other type of pressure sensors or pressure-sensitive sensors. The signals detected by the pressure pick-up  16  can be processed logically by one or more circuits. By way of example, such signal processing can be carried out by an ASIC. 
     If no portions of a gas to be detected are present in the interspace  22  or in the ambient air, the optical pulses emitted by the emitter unit  2  are merely absorbed by the reference gas  12  and the pressure pick-up  16  will detect a periodic measurement signal with the repetition frequency of the optical pulses and a first amplitude. If, in contrast thereto, portions of a gas to be detected are present in the interspace  22 , the optical radiation can additionally be absorbed by the portions. The pressure pick-up  16  will then output a periodic measurement signal having a second amplitude, which can be smaller than the first amplitude. A presence and/or a concentration of the gas to be detected in the ambient air can be determined on the basis of the magnitudes and profiles of the first and second amplitudes. If the concentration of the gas to be detected exceeds a predetermined threshold value, for example a signal, in particular a warning signal, can be output by the photoacoustic sensor  100  or a device connected thereto. 
     Using a broadband emitter  2  and a photoacoustic transducer  6  containing the species of a gas to be detected in its cavity  10 , any gas species whose absorption bands lie in the spectrum of a black body radiator can be detected by the photoacoustic sensor  100  in  FIG. 1 . 
     In conventional photoacoustic sensors, the pressure pick-up and the reference gas can be arranged in a common hermetically sealed cavity. Sealing the cavity and simultaneously filling the cavity with the reference gas can be demanding in terms of process engineering. In contrast thereto, the reference gas  12  in accordance with the present disclosure can be arranged in the cell of the photoacoustic transducer  6 . As a result, during the production of the photoacoustic sensor  100 , the process steps mentioned can be decoupled from mounting the pressure pick-up  16  in the cavity  14 . 
     In the case of the conventional photoacoustic sensors, the photoacoustic conversion can be provided in particular in the common cavity in which the reference gas and the pressure pick-up are arranged. In contrast thereto, in the case of the photoacoustic sensor  100  in accordance with the disclosure, the photoacoustic conversion can be provided in a separate hermetically sealed cavity  10  disposed upstream of the acoustically tight cavity  14  with the pressure pick-up  16  arranged therein. In accordance with the disclosure, the cavities  10  and  14  with reference gas  12  and pressure pick-up  16 , respectively, can be decoupled from one another. 
     In the case of the described construction of the photoacoustic sensor  100 , the pressure pick-up  16  can have an extremely high sensitivity, as a result of which an extremely high sensitivity of the photoacoustic sensor  100  can be provided. As a result, it is possible to achieve a reduced energy consumption during operation of the photoacoustic sensor  100 . 
     It is evident from the method in  FIGS. 12A-E  described further below that the photoacoustic sensor  100  or the photoacoustic transducer  6  can be produced on the basis of cost-effective method steps at the wafer level. 
     The photoacoustic sensor or photoacoustic gas sensor  200  in  FIG. 2  can comprise a photoacoustic emitter unit  2  and a photoacoustic detector unit  4 . The units  2  and  4  can be spaced apart from one another by one or more spacers  24 , as a result of which an interspace  22  arranged between the units  2  and  4  can be formed. An optical filter  40  can be arranged between the photoacoustic emitter unit  2  and the photoacoustic detector unit  4 . The photoacoustic emitter unit  2  can comprise a housing  26  having an cavity  28 , in which an emitter  30  and a protective gas  32 A can be arranged. The photoacoustic detector unit  4  can comprise a housing  34  having an cavity  36 . A photoacoustic transducer  6  in the form of a membrane  70 , a pressure pick-up  16  and a pressure pick-up ASIC  38  can be arranged in the acoustically tight cavity  36 . The components of the photoacoustic sensor  200  can be similar to corresponding components of the photoacoustic sensor  100  in  FIG. 1 , such that explanations concerning  FIG. 1  can also apply to  FIG. 2 . 
     A manner of functioning of the photoacoustic sensor  200  is described below. The emitter  30  can emit optical radiation, in particular in the form of optical pulses. In this case, the emitter  30  can be for example a broadband emitter that emits optical radiation over a wide frequency range. The (broadband) radiation emitted by the emitter  30  can firstly pass through the protective gas  32 A and the housing  26 . In this case, the housing  26  can be fabricated from a material that is transparent to the optical radiation, for example from IR-transparent silicon. The emitted radiation can be filtered by the optical filter  40  and pass through the interspace  22 . In this case, the optical filter  40  can be or comprise an optical bandpass filter, in particular. The optical bandpass filter  40  can be transmissive to optical radiation having a wavelength which can comprise an absorption band of a gas to be detected. Upon passing through the interspace  22  or the ambient air, the filtered optical radiation can impinge on portions of a gas to be detected if the ambient air contains such portions. 
     The optical radiation can pass through the upper part of the housing  34  and enter the cavity  36 . In this case, at least the upper part of the housing  34  can be fabricated from a material that is transparent to the optical radiation, for example from IR-transparent silicon. In the cavity  36 , the optical radiation can impinge on the membrane  70 , which can have a low thermal mass, in particular. The membrane  70  can absorb the optical radiation and thereby produce pressure changes in the cavity  36  lying below the membrane  70 . The pressure changes can be detected by the pressure pick-up  16 . The signals detected by the pressure pick-up  16  can be processed logically by the pressure pick-up ASIC  38 . 
     As already described in association with  FIG. 1 , the signals detected by the pressure pick-up  16  can depend on whether or not portions of the gas to be detected are present in the interspace  22  or the ambient air. A presence and/or a concentration of the gas to be detected in the ambient air can be determined on the basis of the signals detected. 
     The photoacoustic sensor  200  can be operated without the use of a reference gas. With the use of a broadband emitter  30  and a suitable optical filter  40 , it is possible to detect any gas species in the spectrum of a black body radiator using the photoacoustic sensor  200 . In this case, the gas selectivity need not necessarily be provided by the choice of a reference gas, but rather can be provided by the optical filter property of the photoacoustic emitter unit  2  and/or of the optical filter  40 . 
     In the case of the described construction of the photoacoustic sensor  200 , the pressure pick-up  16  can have an extremely high sensitivity, as a result of which an extremely high sensitivity of the photoacoustic sensor  200  can be provided. As a result, it is possible to achieve a reduced energy consumption during operation of the photoacoustic sensor  200 . 
     The photoacoustic sensor  200  can be produced on the basis of cost-effective method steps at the wafer level. 
     The photoacoustic detector unit  300  in  FIG. 3  can for example be used in the photoacoustic sensor  100  in  FIG. 1  and comprise similar components. With regard to operation of the photoacoustic detector unit  300 , reference is made to corresponding explanations concerning  FIG. 1 . 
     The photoacoustic detector unit  300  can comprise a photoacoustic transducer  6 , which can comprise an optically transparent window  18  and a membrane  20 . The optically transparent window  18  and the membrane  20  can form a hermetically sealed cavity  10 , which can enclose a reference gas  12 . In one example, the optically transparent window  18  can be fabricated from IR-transparent silicon. The membrane  20  can be fabricated from a glass material, for example from a borosilicate. The membrane  20  can be designed to absorb optical radiation such as e.g. IR radiation. As a result of the absorption, the membrane  20  can be heated and generate a heat signal. In other words, the optical radiation can be converted into a heat signal by the membrane  20 . On account of the periodic heating and cooling of the membrane  20 , pressure changes can be produced in an acoustically tight cavity  14  arranged below the membrane  20 . The pressure changes can be detected by a pressure pick-up  16 . 
     The optically transparent window  18  and the membrane  20  can be secured to one another by way of an anodic bond connection  42 . It is evident from the method in  FIGS. 12A-E  as described further below that anodic bonding of the optically transparent window  18  and the membrane  20  can be carried out at the wafer level. An antireflection coating  44  can be arranged on the top side of the window  18 , and can be designed to suppress reflection of optical radiation that can be provided by a photoacoustic emitter unit (not illustrated). The transmission of the optically transparent window  18  can be increased by the antireflection coating  44 . 
     The photoacoustic detector unit  300  can furthermore comprise a housing  8 , which can form the shape of a shell or a trough. In one example, the housing  8  can be fabricated from a mold compound. The mold compound can include at least one from an epoxy, a filled epoxy, a glass-fiber-filled epoxy, an imide, a thermoplastic, a thermosetting polymer, a polymer mixture. The photoacoustic transducer  6  can cover an opening on the top side of the housing  8 , wherein the housing  8  and the photoacoustic transducer  6  can form the acoustically tight cavity  14 . In  FIG. 3 , the photoacoustic transducer  6  and the housing  8  can be connected to one another by an adhesive  46 , for example. A protective gas can optionally be enclosed in the acoustically tight cavity  14 . 
     The pressure pick-up  16  can be arranged on the bottom surface of the housing  8 . The pressure pick-up can be a microphone chip, for example, which can comprise one or more MEMS structures and/or movable structures. Furthermore, the microphone chip of pressure pick-up  16  can include an ASIC for logically processing the signals detected by the MEMS structures. The microphone chip of pressure pick-up  16  can be electrically connected to one or more connecting conductors  50  by way of one or more electrical connection elements  48 . In the example in  FIG. 3 , the electrical connection element  48  is illustrated as a bond wire, for example. The connecting conductors  50  can extend through the housing  8  and provide an electrical connection between the microphone chip of pressure pick-up  16  and further components (not illustrated) arranged outside the housing  8 . 
     The photoacoustic detector unit  400  in  FIG. 4  can be used for example in the photoacoustic sensor  100  in  FIG. 1 . Furthermore, the photoacoustic detector unit  400  can at least partly be similar to the photoacoustic detector unit  300  in  FIG. 3  and comprise identical components. 
     In contrast to  FIG. 3 , the pressure pick-up  16  in  FIG. 4  can be embodied or arranged in a different way. In this case, the pressure pick-up  16  or its MEMS structures can be arranged in particular such that they lie outside a course of the optical radiation provided by a photoacoustic emitter unit (not illustrated). Signals detected by the pressure pick-up  16  can be corrupted by optical radiation impinging on the MEMS structures of the pressure pick-up  16 . On account of the arrangement of the pressure pick-up  16  outside the optical path as shown in  FIG. 4 , such corruption can be avoided or at least reduced. 
     The photoacoustic detector unit  400  can comprise a pressure pick-up device  52 . The pressure pick-up device  52  can comprise a circuit board or a substrate  54  with a pressure pick-up  16  and pressure pick-up ASIC  38  arranged on the underside of the circuit board or the substrate  54 . The pressure pick-up  16  and the pressure pick-up ASIC  38  can be electrically connected to one another by way of one or more bond wires  56 , for example. Furthermore, the pressure pick-up  16  and the pressure pick-up ASIC  38  can be electrically coupled to the connecting conductors  50  by way of one or more bond wires  58 , by way of a wiring layer  60  within the circuit board or the substrate  54  and by way of the electrical connecting elements  48 . The pressure pick-up device  52  can comprise a cover  62  having an opening  64 , the cover being arranged over the pressure pick-up  16  and over the pressure pick-up ASIC  38 . 
     The photoacoustic detector unit  500  in  FIG. 5  can be used for example in the photoacoustic sensor  100  in  FIG. 1 . Furthermore, the photoacoustic detector unit  500  can at least partly be similar to the photoacoustic detector unit  300  in  FIG. 3  and comprise identical components. 
     In contrast to  FIG. 3 , the photoacoustic detector unit  500  can comprise one or more metal layers and/or metal alloy layers  66 , which can be arranged on the membrane  20 . In the example in  FIG. 5 , a metal layer  66  can be arranged in each case on the top side and on the underside of the membrane  20 . In further examples, a metal layer can be arranged only on the top side or only on the underside of the membrane  20 . In the example in  FIG. 5 , only one metal layer  66  in each case is arranged on the top side and underside. In further examples, a layer stack having a plurality of metal layers stacked one above another can be arranged on the respective side of the membrane  20 . In the example in  FIG. 5 , the respective metal layer  66  can cover substantially the entire exposed surface of the membrane  20 . In further examples, the respective metal layer  66  can cover only selected parts of the membrane surfaces. A metal layer  66  arranged on the membrane  20  can have a lower heat capacity in comparison with the membrane  20 . 
     The photoacoustic detector unit  600  in  FIG. 6  can for example be used in the photoacoustic sensor  200  in  FIG. 2  and comprise similar components. With regard to operation of the photoacoustic detector unit  200 , reference is made to corresponding explanations concerning  FIG. 2 . 
     The photoacoustic detector unit  600  can comprise a housing  34  with a pressure pick-up  16  arranged therein. A photoacoustic transducer  6  in the form of a membrane  70  can cover an upper opening of the housing  34  and form with the latter an acoustically tight cavity  36 . The membrane  70  can have an elastic inner region  72  and a thicker edge region  74 . The edge region  74  can have the shape of a frame. The inner region  72  can be suspended from or secured to the edge region  74  and be designed to oscillate in the y-direction. As viewed in the y-direction, the inner region  72  can have a circular shape, for example. In the example in  FIG. 6 , the membrane  70  can be fabricated from a glass material, for example from a borosilicate. The inner region  72  of the membrane  70  can have at its outer regions one or more ventilation holes  68 , which can result from a structured suspension of the inner region  72  from the edge region  74  of the membrane  70 . In the example in  FIG. 6 , a metal layer  66  can be arranged on the underside of the membrane  70 . In a further example, a further metal layer can be arranged on the top side of the membrane  70 . 
     The photoacoustic detector unit  700  in  FIG. 7  can be used for example in the photoacoustic sensor  200  in  FIG. 2 . Furthermore, the photoacoustic detector unit  700  can for example at least partly be similar to the photoacoustic detector unit  600  in  FIG. 6  and comprise identical components. 
     The photoacoustic detector unit  700  can comprise a photoacoustic transducer  6  in the form of a membrane  70 . In contrast to  FIG. 6 , the membrane  70  can be fabricated from a doped semiconductor material. On account of the doping of the semiconductor material, the membrane  70  can be designed to absorb optical radiation and to convert it into at least one from a pressure signal or a heat signal. In one example, the membrane  70  can be fabricated from silicon and be doped with at least one from boron, phosphorus, aluminum, indium, arsenic, antimony. 
     In a further contrast to  FIG. 6 , the photoacoustic transducer  6  can furthermore comprise an optically transparent cover  76 , which can be connected to the membrane  70  and can form with the latter an (in particular hermetically sealed) cavity  10 . The cover  76  can be fabricated from silicon, for example. If the cover  76  and the membrane  70  are fabricated from silicon, they can be secured to one another by way of a eutectic silicon-silicon bond connection, for example. As evident from the method in  FIGS. 12A-E , it is possible to carry out eutectic bonding using an intermediate layer at the wafer level. The intermediate layer can be fabricated from gold, for example. 
     In yet another contrast to  FIG. 6 , the photoacoustic transducer  6  can optionally comprise an optical filter layer  78 , which can be arranged for example on the top side of the cover  76 . The optical filter layer  78  can be transmissive to electromagnetic radiation in a predetermined wavelength range. The wavelength range can comprise, in particular, an absorption band of a gas to be detected. 
     A reference gas can optionally be enclosed in the hermetically sealed cavity  10 . In this case, the photoacoustic detector unit  700  in  FIG. 7  can be used for example in the photoacoustic sensor  100  in  FIG. 1 . 
     The photoacoustic detector unit  800  in  FIG. 8  can at least partly be similar to the photoacoustic detector unit  700  in  FIG. 7  and comprise identical components, such that explanations concerning  FIG. 7  can also apply to the photoacoustic detector unit  800 . 
     In contrast to  FIG. 7 , the photoacoustic detector unit  800  can additionally comprise one or more piezo-elements  80  integrated into the membrane  70 . In this case, the piezo-elements  80  can be arranged for example at the edge region of the membrane  70  or at a suspension of the inner region of the membrane  70 . The piezo-elements  80  can be designed to provide an electrical signal designed as a reference signal for a measurement signal provided by the pressure pick-up  16 . By way of example, undesired acoustic influences that can occur during operation of the photoacoustic detector unit  800  can be averaged out on the basis of a comparison of the measurement signal with the reference signal. The membrane  70  can have electrical contact pads  82  on its underside, by way of which reference signals generated by the piezo-elements  80  can be provided. 
     In a further contrast to  FIG. 7 , the housing  34  can be embodied in a different way. The housing  34  in  FIG. 8  can be produced from a ceramic material, for example. In the cross-sectional side view in  FIG. 8 , the housing  34  can have a stepped shape. The pressure pick-up  16  can be arranged on the bottom surface of the housing  34 . Electrical contact pads  84  can be arranged on the top sides of the steps, and can be electrically connected to via connections  86  extending perpendicularly through the housing  34 . Further contact pads  88  can be arranged on the underside of the housing  34 . Reference signals detected by the piezo-elements  80  can be forwarded to one or more of the contact pads  88  by way of the contact pads  82  and by way of the via connections  86 . In a similar way, measurement signals of the pressure pick-up  16  can be forwarded to one or more of the contact pads  88  by way of the contact pads  84  and the via connections  86 . 
     The photoacoustic detector unit  900  in  FIG. 9  can be at least partly similar to the photoacoustic detector unit  700  in  FIG. 7 . In contrast to  FIG. 7 , the photoacoustic transducer  6  can comprise an additional intermediate layer  90 , which can be arranged between the membrane  70  and the cover  76 . The intermediate layer  90  can be designed to simplify connection of the membrane  70  to the cover  76  in terms of process engineering. By way of example, the membrane  70  can be fabricated from doped silicon and the cover  76  can be fabricated from silicon. In such a case, the intermediate layer  90  can be fabricated from a glass material, in particular a borosilicate. As a result, the membrane  70  and the cover  76  can be connected to the intermediate layer  90  in each case by anodic bonding. Furthermore, a use of the intermediate layer  90  makes it possible to adapt or increase the structural height of the photoacoustic transducer  6  in the y-direction in a simple manner. 
     The photoacoustic sensor  1000  in  FIG. 10  can be similar to one of the photoacoustic sensors  100  and  200  in  FIGS. 1 and 2 . In particular, the construction shown in  FIG. 10  can be used for a realization of the photoacoustic sensors in FIGS. 1  and  2 . 
     The photoacoustic sensor  1000  in  FIG. 10  can comprise a photoacoustic emitter unit  2  and a photoacoustic detector unit  4 . A spatial separation of the units  2  and  4  is indicated qualitatively in  FIG. 10  by a perpendicularly extending dashed line. In the example in  FIG. 10 , the photoacoustic detector unit  4  can for example be similar to the photoacoustic detector unit  700  in 
       FIG. 7 , such that in this regard reference can be made to explanations concerning  FIG. 7 . 
     The photoacoustic sensor  1000  can comprise a housing  8 , which can be separated into a left and right part by a separating structure  92 . In this case, the right part of the housing  8  can correspond to the housing  34  in  FIG. 7 . An optical emitter  30  can be arranged on the left part of the housing  8 . Depending on the implementation of the photoacoustic detector unit  4 , the optical emitter  30  can be a broadband emitter with or without a downstream optical bandpass filter. The photoacoustic sensor  1000  can furthermore comprise a cover  94  having an optically reflective inner surface, the cover being arranged above the units  2  and  4 . 
     During operation of the photoacoustic sensor  1000 , the emitter  30  can emit optical radiation that can propagate along an optical path represented by three arrows in  FIG. 10 . The emitter  30  can emit optical radiation in the direction of the cover  94 . The emitted radiation can be reflected at the inner surface of the cover  94 . In order to be able to provide the reflection course illustrated qualitatively in  FIG. 10 , the inner surface of the cover  94  can be shaped in a suitable manner. The optical radiation reflected from the inner surface of the cover  94  can impinge on the photoacoustic detector unit  4 . 
       FIGS. 12A-E  illustrate a flow diagram of a method in accordance with the disclosure. By way of example, one or more photoacoustic transducers for a photoacoustic detector unit in accordance with the disclosure can be produced with the aid of the method. 
     At  96  a first wafer composed of a first material is bonded to a second wafer composed of a second material in a reference gas atmosphere. In this case, a plurality of hermetically sealed cavities are formed, which enclose the reference gas of the reference gas atmosphere. At  98  the bonded wafer is singulated into a plurality of photoacoustic transducers for a photoacoustic detector unit. In this case, each of the photoacoustic transducers comprises one of the hermetically sealed cavities. 
     The method in  FIGS. 12A-E  can be regarded as a more detailed implementation of the method in  FIG. 11 . In  FIG. 12A , a first wafer  102  composed of a first material can be provided. The first wafer  102  can have a multiplicity of depressions  104 . In this case, the number of depressions  104  can correspond, in particular, to a number of photoacoustic transducers to be produced by the method in  FIGS. 12A-E . In the cross-sectional side view in  FIG. 12A , the depressions  104  can have a rounded shape. In further examples, the shape of the depressions  104  can be chosen differently, for example square, rectangular, polygonal, etc. The first wafer  102  can be fabricated from a glass material or a doped semiconductor material, for example. 
     In  FIG. 12B , a second wafer  106  composed of a second material can be provided. The second wafer  106  can have a multiplicity of depressions  108 . The number of depressions  108  can correspond in particular to the number of depressions  104  of the first wafer  102 . In the example in  FIG. 12B , the depressions  108  can have a rounded shape. In further examples, the shape of the depressions  108  can be chosen differently, for example square, rectangular, polygonal, etc. The second wafer  106  can be fabricated from a semiconductor material, for example. 
     In  FIG. 12C , the first wafer  102  can be bonded to the second wafer  106  in a reference gas atmosphere. For the bonding process, the first wafer  102  and the second wafer  106  can be arranged in a bonding chamber (not illustrated) designed to provide the reference gas atmosphere. During the bonding of the wafers  102  and  106 , a plurality of hermetically sealed cavities  110  can be formed, which enclose the reference gas of the reference gas atmosphere. 
     The bonding process employed in  FIG. 12C  can be dependent in particular on the materials of the wafers  102  and  106 . In a first example, the material of the first wafer  102  can comprise a glass material (e.g. a borosilicate) and the material of the second wafer  106  can comprise a semiconductor material (e.g. silicon). In this case, the bonding process can comprise anodic bonding. In a second example, the material of the first wafer  102  can comprise a doped semiconductor material (e.g. doped silicon) and the material of the second wafer  106  can comprise a semiconductor material (e.g. silicon). In this case, the bonding process can comprise eutectic bonding using an intermediate layer. The intermediate layer can be fabricated from gold, for example. 
     In  FIG. 12D , the arrangement from  FIG. 12C  can be singulated into a plurality of arrangements along perpendicular dashed lines. The singulating process can include for example an etching process, a plasma dicing process, a mechanical ultrasonic dicing process, a laser dicing process, or a combination thereof. 
       FIG. 12E  shows one of the photoacoustic transducers  1200  obtained as a result of the singulation, which photoacoustic transducer can comprise an optically transparent window  18  and a membrane  20 . In this case, the membrane  20  can be fabricated from the material of the first wafer  102  and the optically transparent window  18  can be fabricated from the material of the second wafer  106 . 
     The method in  FIGS. 12A-E  can comprise further steps, which are not explicitly illustrated and discussed for the sake of simplicity. The further steps can be carried out here in particular at the wafer level. By way of example, the method can be extended by a step in which an antireflection coating can be applied on the second wafer  106 , such that the photoacoustic transducers  1200  produced can each have an antireflection coating on the optically transparent window  18 . 
     EXAMPLES 
     Photoacoustic detector units, photoacoustic sensors and associated production methods are explained below on the basis of examples. 
     Example 1 is a photoacoustic detector unit, comprising: a housing having an opening; a photoacoustic transducer designed to convert optical radiation into at least one from a pressure signal or a heat signal, wherein the photoacoustic transducer covers the opening of the housing, such that the photoacoustic transducer and the housing form an acoustically tight cavity; and a pressure pick-up arranged in the acoustically tight cavity. 
     Example 2 is a photoacoustic detector unit according to example 1, wherein the photoacoustic transducer is designed to convert at least one from infrared radiation or ultraviolet radiation into at least one from a pressure signal or a heat signal. 
     Example 3 is a photoacoustic detector unit according to example 1 or 2, wherein the photoacoustic transducer comprises: a cell having a hermetically sealed cavity; and a reference gas enclosed in the hermetically sealed cavity, wherein the reference gas is designed to absorb the optical radiation. 
     Example 4 is a photoacoustic detector unit according to example 3, wherein the cell comprises: an optically transparent window on a first side of the cell; and a membrane on a second side of the cell, the second side being situated opposite the first side. 
     Example 5 is a photoacoustic detector unit according to example 4, wherein the optically transparent window is fabricated from silicon. 
     Example 6 is a photoacoustic detector unit according to example 4 or 5, wherein the membrane is fabricated from a glass material. 
     Example 7 is a photoacoustic detector unit according to any of examples 4 to 6, wherein the membrane is fabricated from doped silicon. 
     Example 8 is a photoacoustic detector unit according to any of examples 4 to 7, wherein the optically transparent window and the membrane form the hermetically sealed cavity. 
     Example 9 is a photoacoustic detector unit according to any of examples 4 to 8, wherein the optically transparent window and the membrane are wafer-bonded. 
     Example 10 is a photoacoustic detector unit according to any of examples 4 to 9, furthermore comprising: an antireflection coating arranged on the optically transparent window. 
     Example 11 is a photoacoustic detector unit according to any of examples 4 to 10, furthermore comprising: a metal layer arranged on the membrane. 
     Example 12 is a photoacoustic detector unit according to any of the preceding examples, furthermore comprising: a protective gas enclosed in the acoustically tight cavity. 
     Example 13 is a photoacoustic detector unit according to any of the preceding examples, wherein the housing is fabricated from a mold compound. 
     Example 14 is a photoacoustic detector unit according to example 1, wherein the photoacoustic transducer comprises: a membrane designed to absorb the optical radiation. 
     Example 15 is a photoacoustic detector unit according to example 14, wherein the membrane is fabricated from at least one from glass material or doped silicon. 
     Example 16 is a photoacoustic detector unit according to example 14 or 15, furthermore comprising: a piezo-element integrated into the membrane and designed to provide an electrical signal designed as a reference signal for a measurement signal provided by the pressure pick-up. 
     Example 17 is a photoacoustic detector unit according to any of examples 14 to 16, furthermore comprising: an optical filter layer, which is transmissive to electromagnetic radiation of a predetermined wavelength, wherein the optical filter layer is applied on at least one from the membrane or a cover arranged above the membrane. 
     Example 18 is a photoacoustic sensor, comprising: an optical emitter; and a photoacoustic detector unit according to any of the preceding examples. 
     Example 19 is a photoacoustic sensor according to example 18, wherein the optical emitter comprises an optical broadband emitter. 
     Example 20 is a photoacoustic sensor according to example 19, furthermore comprising: an optical bandpass filter disposed downstream of the optical broadband emitter, the optical bandpass filter being transmissive to electromagnetic radiation of a predetermined wavelength. 
     Example 21 is a method, comprising: bonding a first wafer composed of a first material to a second wafer composed of a second material in a reference gas atmosphere, wherein a plurality of hermetically sealed cavities are formed, which enclose the reference gas of the reference gas atmosphere; and singulating the bonded wafers into a plurality of photoacoustic transducers for a photoacoustic detector unit, wherein each of the photoacoustic transducers comprises one of the hermetically sealed cavities. 
     Example 22 is a method according to example 21, wherein: the first material comprises a glass material, the second material comprises a semiconductor material, and the bonding comprises anodic bonding. 
     Example 23 is a method according to example 21, wherein: the first material comprises a doped semiconductor material, the second material comprises a semiconductor material, and the bonding comprises eutectic bonding using an intermediate layer. 
     Although specific implementations have been illustrated and described herein, it is obvious to the person of average skill in the art that a multiplicity of alternative and/or equivalent implementations can replace the specific implementations shown and described, without departing from the scope of the present disclosure. This application is intended to cover all adaptations or variations of the specific implementations discussed herein. Therefore, the intention is for this disclosure to be restricted only by the claims and the equivalents thereof.