Photoacoustic gas sensor device

Abstract

A photoacoustic gas sensor device for determining a value indicative of the presence or concentration of a chemical component in a gas comprises a substrate (1) and a measurement cell body (2) arranged on a first side (11) of the substrate (1). The substrate (1) and the measurement cell body (2) define a measurement cell. A cap (7) is arranged on the first side (11) of the substrate (1) in the measurement cell. The cap (7) and the substrate (1) define a cap volume (71). The cap (7) and the substrate (1) acoustically seal the cap volume (71). A measurement volume (21) is limited by the measurement cell body (2), the substrate (1) and the cap (7). An aperture (23) is provided in the measurement cell for gas to enter the measurement volume (21). Electrical components are arranged on the first side (11) of the substrate (1) and within the measurement cell. The electrical components include at least an electromagnetic radiation source (4) for emitting electromagnetic radiation (41) into the measurement volume (21), a pressure transducer (3) for measuring sound waves generated by chemical components present in the measurement volume (21) in response to the chemical components absorbing the electromagnetic radiation (41), and a controller (8) configured to control the electromagnetic radiation source (4). The pressure transducer (3) is disposed outside the cap volume (71), and at least one of the electrical components other than the pressure transducer (3) is disposed within the cap volume (71).

Description

[Technical Field]

[0001] The present invention relates to a photoacoustic gas sensor device configured to determine a value indicative of the presence or concentration of a component in a gas, particularly CO2. [Background technology]

[0002] Photoacoustic gas sensors rely on the physical effect of infrared radiation being absorbed by molecules of a target component in a gas, e.g., CO2, thereby transitioning the molecules to an excited state. The subsequent non-radiative decay of the excited state generates heat, e.g., through molecular collisions, resulting in an increase in pressure. By modulating the absorbed infrared radiation with a modulation frequency, the pressure fluctuates at the modulation frequency. These pressure fluctuations can be measured by a pressure transducer. The concentration of the component is proportional to the amplitude of the pressure fluctuations.

[0003] The radiation emitter, pressure transducer, and controller for controlling the emitted radiation can be located within the same volume where the photoacoustic reaction occurs—the measurement volume of the photoacoustic measurement cell. This is beneficial in that it reduces the footprint of the gas sensor device. However, if the radiation emitted from the radiation source is modulated by switching an electric current, for example, the controller heats and cools at the same frequency as the electromagnetic radiation source emits the modulated radiation. Such pulsed heating then amplifies pressure fluctuations within the measurement cell, thus corrupting the signal provided by the pressure transducer by adding an offset.

[0004] It is therefore an object of the present invention to provide a photoacoustic gas sensor device, the measurement signal of which is less susceptible to sound and / or temperature fluctuations caused by the controller and / or other electrical components of the gas sensor device. Summary of the Invention

[0005] This object is achieved by a photoacoustic gas sensor device according to claim 1.

[0006] A photoacoustic gas sensor device (also referred to as a PA gas sensor device) allows for determining a value indicative of the presence or concentration of a chemical component in a gas. The PA gas sensor device comprises a substrate and a measurement cell body disposed on a first side of the substrate. This includes mounting the measurement cell body directly on the substrate, as long as the measurement cell body is disposed on the first side of the substrate rather than on the opposite side of the substrate, and, if possible, mounting the measurement cell body on any intermediate components between the substrate and the measurement cell body. The substrate and / or any intermediate components, as well as the measurement cell body, define a measurement cell. A cap is disposed on the first side of the substrate within the measurement cell. The cap and substrate define a cap volume. In one embodiment, the cap itself largely or even completely contributes to the cap volume, for example, if the cap defines an internal space by vertical walls, a concave interior, or any other cavity within the cap. In a different embodiment, the substrate largely or even completely contributes to the cap volume by a recess therein, which is covered by a planar cap. In a further embodiment, both the cap and the substrate contribute to the cap volume, with recesses in the substrate contributing partially to the cap volume and cavities in the cap contributing partially to the cap volume.

[0007] The measurement cell, the substrate, and the cap, for example, part or all of its outer surface, then define a measurement volume in which photoacoustic transduction takes place. The measurement cell includes an aperture for gas to enter the measurement volume.

[0008] The cap volume is acoustically sealed so that photoacoustic transduction within the measurement volume is not affected by electrical components present within the cap volume. That is, pressure fluctuations in the acoustic spectrum within the cap volume are prohibited or at least reduced from propagating into the measurement volume. Acoustic sealing properties can include designing and / or configuring the cap, substrate, and any potential mechanical contact surfaces therebetween to not allow pressure fluctuations to pass from the cap volume to the measurement volume.

[0009] The electrical components are disposed on the first side of the substrate and within the measurement cell. This does not exclude that additional electronic components may be disposed on the substrate outside the measurement cell if the substrate extends beyond the area covered by the measurement cell body. Preferably, the electrical components are interconnected by conductor tracks on or within the substrate. The electrical components disposed within the measurement cell include at least an electromagnetic radiation source (also abbreviated as EM radiation source) for emitting electromagnetic radiation into the measurement volume, a pressure transducer for measuring sound waves generated by chemical components present in the measurement volume in response to the chemical components absorbing the electromagnetic radiation, and a controller configured to control the electromagnetic radiation source. The pressure transducer is disposed outside the cap volume, and at least one of the electrical components other than the pressure transducer is disposed within the cap volume.

[0010] PA gas sensor devices rely on the photoacoustic effect: molecules of a chemical component of interest, such as CO2 in an ambient gas (e.g., ambient air), absorb electromagnetic radiation, preferably in the infrared range, resulting in non-radiative decay, e.g., heat generation due to collisions between molecules of the chemical component and / or with other molecules, which then leads to an increase in pressure. Pressure modulation can be achieved by modulating the intensity of the EM radiation at a modulation frequency, including on / off modulation of the EM radiation source. Such pressure modulation or pressure fluctuations, i.e., sound waves, can be measured by a pressure transducer, e.g., a microphone. A value indicative of the presence or concentration of the chemical component, i.e., the concentration of the chemical component, can then be determined as a function of the pressure fluctuations, particularly the amplitude of the pressure fluctuations, since the amplitude is proportional to the amount of EM radiation absorbed by the chemical component and can therefore be assumed to be proportional to the concentration of the chemical component in the gas, assuming all other factors, such as the average optical path length within the measurement volume, remain equal.

[0011] In addition to the EM radiation source and the pressure transducer, the PA gas sensor device includes further electrical components, including a controller configured to control the EM radiation source. The controller is located on the same side of the substrate as the other electrical components, i.e., on its first side and within the measurement cell. The controller is configured to control the intensity of the electromagnetic radiation to modulate it at a modulation frequency. The modulation frequency is between 1 Hz and 100 kHz, preferably between 10 Hz and 200 Hz, more preferably between 20 Hz and 60 Hz, e.g., 40 Hz. Preferably, a heater of the EM radiation source is switched on at the modulation frequency.

[0012] In a highly preferred embodiment, in addition to controlling the EM radiation source, the controller is configured to process the electrical signal provided by the pressure transducer and determine a value indicative of the presence or concentration of a chemical component dependent on the electrical signal. In this regard, the controller preferably performs signal processing such as filtering, linearization, compensation, and / or A / D conversion. In particular, the value is determined as a function of the amplitude of the electrical signal, e.g., the volume in the case of sound waves. Preferably, the measurement signal is band-pass filtered around the modulation frequency. This improves the robustness of the determination, since sound waves of other frequencies are not taken into account.

[0013] At least one electrical component other than the pressure transducer is disposed within the cap volume. Preferably, one or more electrical components disposed within the cap volume may affect photoacoustic transduction if disposed within the measurement volume instead of the cap volume. Such electrical components include components that are involved in pressure and / or thermal fluctuations around them, and / or components with non-reflective surfaces that adversely affect the reflectivity of the electromagnetic radiation emitted by the EM radiation source, and / or components that require the enhanced mechanical protection provided by the cap, and / or components that require authorized dedicated access to the surroundings of the photoacoustic gas sensor.

[0014] Assuming that modulating the EM radiation source is accomplished by modulating or switching a current in the controller, the controller heats and cools at the same frequency as the EM radiation source emits pulsed radiation. This affects the atmosphere in the measurement cell, inducing pressure fluctuations at the same frequency as the pressure fluctuations induced by chemical components interacting with the modulated EM radiation when the controller is placed in the measurement volume.

[0015] To avoid adding such a controller-induced offset to the pressure signal (the effect of adding an offset is also called a crosstalk scenario), the controller is preferably separated from the measurement volume by a cap. Thus, by introducing the cap, the measurement volume of the present PA gas sensor device is separated from the cap volume, which is defined by the measurement cell body, the substrate, and the cap, and which may be reserved for electrical components that may adversely affect photoacoustic transduction.

[0016] In a highly preferred embodiment, the controller is embodied as an integrated circuit, specifically an ASIC (Application Specific Integrated Circuit). This helps to miniaturize the PA gas sensor device. However, the integrated circuit representing the controller can even be embodied as a bare die in a chip-scale package, which can further reduce the footprint of the PA gas sensor device. On the other hand, such an embodiment of the controller may require additional mechanical protection, which is then provided by a cap.

[0017] The electrical components preferably disposed within the cap volume may include transistors, particularly power transistors, and / or components including such transistors. Such components may be voltage controllers. Therefore, it is preferred that one or more transistors and / or components having one or more transistors are disposed within the cap volume.

[0018] The same may be true for passive electrical components such as capacitors, resistors or coils that may affect their surroundings through pressure fluctuations under the application of a switched current. It is therefore preferable to locate one or more such passive components within the cap volume.

[0019] The electrical components located under the cap within the cap volume are 9mm 2 or more preferably 3 mm 2 The cap volume may include a bulky component having a top surface greater than 1000 nm. The top surface may be defined as the surface of the component excluding the portion facing the substrate. If such a top surface is not reflective or may not be coated to reflect EM radiation in the measurement volume, the reflectivity of the measurement volume will be reduced. This may be a reason to position such an electrical component within the cap volume.

[0020] The cap is attached to a first side of the substrate and preferably establishes an acoustic barrier and / or an acoustic seal and / or a thermal barrier and / or a thermal seal of the cap volume with respect to the measurement volume. Therefore, the cap prevents or only negligible pressure fluctuations from propagating from the cap volume into the measurement volume. Therefore, the cap containing the substrate and its attachment to the substrate preferably limit or avoid gas exchange, and thus medium migration, between the cap volume and the measurement volume.

[0021] Thermal fluctuations due to the operation of the electrical components can also have a detrimental effect. However, considering that the heat emitted from the electrical components below the cap is isolated within and by the cap volume and the medium therein, the cap simultaneously limits or prevents the transfer of thermal fluctuations from the cap volume to the measurement volume. If the cap volume is evacuated, the thermal impact on the measurement volume due to the cap volume can be further reduced.

[0022] The cap is preferably made from a material that prevents such gas exchange. This also includes the absence of holes or other openings in the cap that would allow gas exchange between the cap volume and the measurement volume. This also includes the cap being attached to the substrate in a sealing manner, preferably by adhesive or solder. For example, the cap is made from an airtight material.

[0023] Thus, crosstalk, particularly thermal and / or acoustic crosstalk, from the controller to the measurement volume and ultimately to the pressure transducer can be prevented: the pressure signal provided by the pressure transducer is not or negligibly affected by the controller-induced offset.

[0024] The cap may include or be made of a conductive material. Selecting such a material can provide the additional benefit of allowing electrical components within the cap volume, such as the controller, to be EM shielded by the cap. Considering that the controller is preferably embodied as an integrated circuit that is susceptible to electromagnetic radiation, while the EM radiation source also emits EM radiation that is potentially harmful to the integrated circuit, the cap can also protect the controller in this scenario, especially if the EM radiation source is located outside the cap on the substrate, i.e., outside the cap volume.

[0025] The PA effect in the measurement cell depends on good reflectivity of all surfaces defining the measurement volume of the emitted EM radiation. While the inner surface of the measurement cell body and the first side of the substrate are such surfaces, the (outer) surface of the cap, i.e., the surface of the cap facing the measurement volume and defining the measurement volume, is also important for the overall reflectivity of the PA gas sensor device. Therefore, it is preferable that such a surface of the cap facing the measurement volume has a reflectivity of more than 70%, preferably more than 80%, and more preferably more than 90%. The inner surface of the measurement cell body exhibiting a reflectivity of more than 70%, preferably more than 80%, and more preferably more than 90%, potentially in combination with a reflective coating on the substrate surface and / or electrical components facing the measurement cell, can improve the overall reflectivity of the surfaces defining the measurement volume.

[0026] In particular, the cap is made of metal. In a different embodiment, the cap is made of plastic with a metal coating on its outer surface facing the measurement volume. This embodiment allows for simultaneous reflectivity and electromagnetic shielding. The same material is preferred for the measurement cell body. If the measurement cell body is made of plastic, its inner surface may be coated with a metal that provides a reflective coating. Such a coating may be made of, for example, gold, aluminum, nickel, or copper. Gold is also particularly convenient for plating substrates, such as PCBs, because gold is applied to the surfaces of conductors as corrosion protection during PCB formation. In another embodiment, the measurement cell body is made of sheet metal, for example, by deep drawing. Sheet metal has the advantage of being mechanically stable even at low thicknesses and exhibiting high reflectivity for electromagnetic radiation even without additional coatings. Preferably, the measurement cell body and the substrate are connected in an airtight manner, for example, by gluing or soldering. Advantageously, the measurement cell is acoustically tight.

[0027] All these measures contribute to a longer mean optical path length of the EM radiation within the measurement volume. In this way, the mean optical path length within the measurement volume can be increased to >1 cm, preferably >3 cm, and more preferably >5 cm. This then facilitates making the measurement cell smaller, e.g., the dimensions of the measurement cell are 2 x 2 x 2 cm. 3 Smaller, preferably 1x1x1cm 3 The entire photoacoustic gas sensor device is smaller than the conventional one. 3 , which is particularly smaller than conventional photoacoustic gas sensors having a linear optical path length.

[0028] Thus, the present PA gas sensor device offers a small footprint but does not sacrifice signal performance.

[0029] Preferably, the substrate includes a vent hole arranged to connect the cap volume to the periphery of the photoacoustic gas sensor device. Therefore, in the region of the substrate covered by the cap, a through-channel is provided from the first side of the substrate to its second side, i.e., from its front side to its back side, to allow gas exchange between the cap volume and the periphery. Such a vent hole may be preferred, for example, after heating the cap volume, to allow pressure equilibrium to be achieved during the manufacturing or assembly process. The diameter of such a vent hole is preferably 100 μm to 300 μm. To protect the cap volume from particles or liquids, a vent membrane covering the vent hole may be provided, which is impermeable to liquids but permeable to gases. Preferably, the vent membrane is attached to the substrate, for example, to the first side of the substrate or to the second side of the substrate, i.e., the back side facing the periphery. This arrangement is advantageous for manufacturing. After the manufacturing of the measurement cell, the vent membrane can be attached to the substrate in a final step. In a different embodiment, the vent membrane is attached during substrate preparation, before the measurement cell body is attached to the substrate.

[0030] In a preferred embodiment, the electrical component includes an additional sensor for sensing an environmental parameter. Assuming that the pressure transducer represents the sensor, this sensor is also referred to as the "additional sensor." The additional sensor may be configured to sense one or more of humidity, pressure, and / or chemical components in the gas. Therefore, the additional sensor may be embodied as one or more of a pressure sensor, a barometric pressure sensor, an additional pressure transducer, or an additional gas sensor, e.g., of a metal oxide type or an electrochemical type.

[0031] In one embodiment, an electrical signal provided by another sensor is provided at the output of the PA gas sensor device, for example, to provide additional information about the measurement scenario. In a different embodiment, the signal from the other sensor is used to improve the signal provided by the pressure transducer, for example, by compensating for the effect of measured environmental variables on the gas concentration in the measurement volume. This can reduce or eliminate the effect of ambient conditions on the measurement. Such compensation makes the resulting concentration values ​​more accurate and reliable, which in turn makes the gas sensor device adaptable to a variety of environmental conditions.

[0032] For both purposes, the other sensor can be placed either inside or outside the cap volume in the measurement cell. When placed inside the cap volume, if the substrate has a vent, ambient gas can enter the cap volume and the desired ambient fluctuations can be sensed by the other sensor. In such a scenario, the vent membrane can exhibit different properties, for example, in terms of diffusion, selectivity, etc., and the membrane covers an aperture in the measurement cell body, which will be introduced in more detail later. When placed outside the cap volume, ambient gas can enter the measurement volume through the aperture and the desired ambient fluctuations can be sensed within the aperture.

[0033] In one embodiment of the present invention, the EM radiation source is disposed within the cap volume, and in this embodiment, the cap includes an optical window positioned and configured to allow radiation emitted by the EM radiation source to enter the measurement volume and / or an optical window that bandpass filters the radiation emitted by the EM radiation source before it enters the measurement volume.

[0034] In a different embodiment, when the EM radiation source is located outside the cap volume and configured to emit ultraviolet light, the cap is preferably made from a material that is opaque to ultraviolet light to protect the controller within the cap volume from the ultraviolet light.

[0035] The applied EM radiation can generally be any radiation in the spectrum from radio to ultraviolet, but the EM radiation used is preferably infrared. This means that the EM radiation source is an infrared source for emitting infrared radiation. Infrared radiation is preferably defined as radiation having a wavelength in the range of 700 nm to 1 mm. In this embodiment, the pressure transducer is configured to measure the acoustic waves generated by the component in response to the component absorbing the infrared radiation. In other embodiments, the electromagnetic radiation source is a source for emitting radiation with a wavelength in the range of 100 nm to 700 nm.

[0036] Preferably, the EM radiation emitted by the EM source is emitted only in a band that corresponds to the absorption peak of the chemical component of interest. A band is considered to be a subrange of the EM spectrum, preferably symmetrical around the absorption peak, with maximum and minimum band limits of + / - 15% of the absorption peak value. In the case of infrared radiation, the corresponding band is advantageously selected to correspond to the absorption peak of the chemical component of interest. In one embodiment, the photoacoustic gas sensor device is used as a CO2 sensor. In that case, the infrared band is centered around a wavelength of 4.3 μm. Preferably, the band has a full width at half maximum of less than 0.5 μm, which can be understood as a narrow band.

[0037] The EM radiation source may be a broadband radiation source, defined as emitting radiation across the entire infrared spectrum or a broad spectrum, e.g., from 0.8 μm to 10 μm. Such a broadband source may be a conventional infrared emitter with a heater. The broadband source is preferably covered by a bandpass filter, e.g., a dielectric filter, a metamaterial filter, or a CMOS absorption layer, so that only infrared radiation in that band is emitted into the measurement volume. In other embodiments, a narrowband EM radiation source, such as a laser or LED, possibly including a metasurface resonator, is used.

[0038] The pressure transducer resides within and / or is acoustically coupled to the measurement volume to detect pressure fluctuations occurring therein in response to PA binding in the presence of the chemical component desired to be detected. In embodiments, the pressure transducer may be either a general pressure transducer or a microphone that is specifically sensitive only to a specific range of frequencies around the modulation frequency. The pressure transducer may be a bottom-port microphone, with the bottom port facing the substrate.

[0039] In one embodiment, the substrate is a printed circuit board (PCB), made of, for example, FR4 or a ceramic material that provides greater mechanical stability. At least the pressure transducer, the electromagnetic radiation source, the controller, and the measurement cell body and cap are mounted on a common side, i.e., the first side, of the substrate. Preferably, all electronic components are mounted on the first side of the substrate. Preferably, all electronic components are surface-mounted on the first side of the substrate so that the photoacoustic gas sensor device is an SMD (surface-mounted device). Preferably, the second side of the substrate, i.e., the side opposite the first side, includes only contacts for electrically connecting the photoacoustic gas sensor device to a carrier. In one embodiment, the contacts include land grid array (LGA) pads arranged for SMD assembly and / or reflow soldering. This facilitates assembly by the customer with other components of the device. Other options for the contacts include DFN, QFN, or castellated holes.

[0040] In one embodiment, the cap may be planar. The cap volume is then substantially defined by a recess in the substrate, i.e., a recess extending from the first side of the substrate. Thus, the substrate has a thinner thickness in the recess than usual. The recess in the substrate is then covered by a planar cap. In this embodiment, the substrate is preferably embodied as a printed circuit board (PCB) or a ceramic circuit board including conductive paths. The PCB is preferably a multilayer PCB including multiple structured metallization layer conductive paths made from different levels of PCB. Such a PCB can preferably be etched, ground, or otherwise processed from its first side / front surface, thereby removing material and creating recesses in the substrate. Such processing can also include pre-structuring the individual layers of the PCB, for example, by cutting a mold, before laminating such layers together. However, the substrate is not processed to create through-holes in the recesses. Instead, the substrate remains thinned within its recesses. In other words, the substrate is a continuous substrate even in the area of ​​the recesses.

[0041] In this embodiment, at least one electrical component other than the pressure transducer is disposed in the recess of the substrate, and the planar cap closes the recess of the substrate and together with the recess of the substrate establishes / defines an acoustically tight compartment, thus the cap volume.

[0042] In particular, the planar cap is made from metal, taking into account that its surface facing the measurement cell and desirably has reflective properties. In another embodiment, the cap, preferably at least the side of the cap facing the measurement volume, is coated with a reflective coating such as gold.

[0043] Preferably, at least the controller is disposed in the recess of the substrate. If only the controller and / or any components not requiring access to the measurement volume are disposed in the cap volume, the planar cap can be a continuous sheet of a desired potentially opaque material that completely covers and / or seals the recess of the substrate. However, if the electromagnetic radiation source is disposed in the recess of the substrate, and the cap is not transparent to the electromagnetic radiation emitted by the electromagnetic radiation source, the cap preferably includes an opening covered by a transparent material. Such an opening is preferably covered by an optical filter that is transparent to the electromagnetic radiation to allow the electromagnetic radiation generated in the cap volume to pass through the optical filter and enter the measurement volume. The optical filter is preferably sealed in the cap.

[0044] In various embodiments, the cap is made of silicon. Again, if only the controller and / or any components not requiring access to the measurement volume are placed in the cap volume, the planar cap may be a continuous sheet of the desired material, completely covering and / or sealing the recess in the substrate, and may even be opaque or coated with an opaque material. However, if an electromagnetic radiation source is placed in the recess in the substrate, the silicon cap is transparent to the electromagnetic radiation generated in the cap volume, or has an opening or a transparent portion. Preferably, an optical filter transparent to electromagnetic radiation, preferably only to electromagnetic radiation of a specific wavelength, is applied to the cap as a separate element from the silicon cap or as an optical coating deposited on the silicon cap in the form of a layer, for example by PECVD, on one or both sides of the cap, i.e., facing the measurement cell, facing the substrate, or both. In various embodiments, the optical filter is integrated into the cap and thus can represent a part of the cap. Such a part may have different optical properties from the rest of the cap. In the alternative, the entire cap can be manufactured from a material having the desired optical properties, but only a portion is provided to pass electromagnetic radiation, taking into account that other portions of the cap may be optically impaired, for example, by an optically opaque coating on such portions.

[0045] The recess results in a thinned substrate, with the top surface representing the first side of the substrate. The top surface, also referred to as the first level surface, is at a first level on the vertical axis when the substrate has a thickness along the vertical axis and planar extensions along the x-axis and y-axis. The first level surface is lower than the substrate's normal surface level. The first level surface may be a plane for mounting one or more electrical components. In one embodiment, this may mean that the first level surface exposes contact pads for electrically contacting one or more electrical components. Such contact pads may be formed from one of the metallization layers in the PCB so that the first level surface is at a level somewhat above the level of this metallization layer in the PCB. The PCB can then be ground, etched, or otherwise processed to this first level to create recesses that allow electrical contact of electrical components easily.

[0046] In another embodiment, the recess is shaped to generate two different level surfaces along a vertical axis, the second level surface being lower than the first level surface. In particular, the second level surface is arranged between the first level surfaces so that the first side of the substrate exhibits the following profile: normal level, drop to the first level, drop to the second level, rise to the first level, rise to the normal level. Therefore, components requiring backside volume may be arranged on the first level surface bridging the second level surface. Such electrical components are preferably electromagnetic radiation sources. In a different scenario, components may be arranged on the second level surface, which exhibit a thickness that, considering the cap, prevents the placement of components on the first level surface.

[0047] In embodiments with a recess in the substrate, additional components located outside the cap volume may be installed in a non-recessed portion of the substrate, i.e., at its normal level. However, in different embodiments, another recess may be provided in the substrate, creating a third level surface on its first side for installing or SMD-mounting such components. Preferably, such another recess is covered with a cover. The other recess in the substrate and the cover form another cap volume. This other cap volume is not connected to the cap volume. If the cover is reflective and / or coated with a reflective coating, at least on the side facing the measurement volume, reflectivity within the measurement volume is improved and promoted compared to providing components with a reflective surface. The level of the third surface in the other recess may be, but is not necessarily, the same as the level of the first surface in the recess. Preferably, the pressure transducer is located on the substrate within another recess in the substrate. However, additional components may be located within another recess in the substrate.

[0048] Most preferably, the cover of the other recess is represented by a recess cap. The cap may therefore be elongated and may cover the other recess. For the material of such a cover, please refer to the cap material. Preferably, the cover is also coated with a reflective coating, in one example, which faces the measurement volume. Preferably, the coating may be a continuous coating on the cap and cover.

[0049] If the pressure transducer is located in the other recess, the cover preferably comprises an opening for acoustically coupling the measurement volume with a volume below the cover, also referred to as the other cap volume, the opening being dimensioned such that acoustic coupling between the measurement volume and the other cap volume allows sound waves generated in the measurement volume to be detected by the pressure transducer.

[0050] In another embodiment, the other recess is shaped to generate surfaces at two different levels along the vertical axis, with the fourth level surface being lower than the third level surface. In particular, the fourth level surface is positioned between the third level surfaces so that the first side of the substrate exhibits the following profile within the other recess: normal level, drop to the third level, drop to the fourth level, rise to the third level, rise to the normal level. Components requiring backside volume are preferably positioned on the third level surface, which provides the fourth level surface. Such electrical components are preferably pressure transducers. In a different scenario, one or more components may be positioned on the fourth level surface, which exhibits a thickness that prevents placement of components on the third level surface due to capping considerations. Preferably, the third level is equal to the first level. Preferably, the fourth level is equal to the second level.

[0051] In a different embodiment of the cap, a cap structure is provided that contains a first cavity for forming a cap and another cavity for forming another cap. The first cavity contributes partially or completely to the cap volume. The second cavity, in combination with the substrate, contributes partially or completely to the other cap volume. The cap volume and the other cap volume are separated from each other by a cap structure, e.g., a base of the cap structure, and are seated on the substrate. One or more electrical components other than the pressure transducer are located in the cap volume, but the pressure transducer is located in the other cap volume. The cap structure preferably exhibits an opening toward the measurement volume for acoustically coupling the measurement volume to the pressure transducer.

[0052] In such an embodiment, all electrical components may reside on the first side of the substrate. However, compared to other embodiments with a single cap, other electronic components, such as a pressure transducer, located outside the cap are also capped by the other cap. Preferably, both caps are fabricated from a common cap structure, which may be embodied as a silicon wafer or a printed circuit board (PCB) with two cavities forming the two caps. In the case of a silicon wafer, known processes such as etching can be applied to create the cavities. In the case of a PCB, processes such as those disclosed for creating one or more recesses in a PCB substrate can be applied to create the cavities. As a result, even other electrical components do not directly face the measurement volume, but are instead located in the other cap volume, which is coupled to the measurement volume by one or more openings in the cap structure. Again, at least the surface of the cap structure facing the measurement volume is preferably reflective or coated with a reflective coating.

[0053] Nevertheless, the substrate may also include a recess to provide a first level surface at a level lower than the substrate's normal level surface. Such a recess may be used, for example, as a back volume for an electromagnetic radiation source. The portion of the substrate capped by the other cap may also include another recess to provide a third level surface at a level lower than the substrate's normal level surface. Such another recess may be used, for example, as a back volume for a pressure transducer.

[0054] Preferably, the cap structure is attached to the substrate with the cavity facing the substrate, by bonding, particularly if the cap structure is a silicon wafer, or by gluing or soldering if the cap structure is made from a PCB. The measurement cell body is preferably attached to the cap structure if the cap structure has the same or similar footprint (+ / - 10%) in the x / y direction as the substrate. Thus, the cap structure acts as an intermediate element between the substrate and the measurement cell body. In other embodiments, the cap structure may have a smaller footprint than the substrate. The measurement cell body can still be attached to the cap structure, although the substrate still exhibits a portion that is not covered by either the measurement cell body or the cap structure for implementing the circuit.

[0055] Preferably, the aperture of the measurement cell allows gas exchange between the measurement volume and the surroundings of the measurement cell. The aperture may be provided in the measurement cell body or in the substrate. In one embodiment, a gas-permeable membrane covers the aperture. The membrane is permeable to gas exchange between the measurement volume and the surroundings of the measurement cell. The gas-permeable membrane may be made of one or more of the following materials, in particular: sintered metal, ceramic, polymer. The membrane also advantageously acts as a decoupling element between the measurement volume and the surroundings of the measurement cell. Therefore, the membrane preferably damps the movement of gas molecules through the membrane, so that pressure fluctuations from the surroundings, such as sound waves, are damped when propagating into the measurement volume, and pressure fluctuations in the measurement volume are largely maintained inside.

[0056] If there are other recesses in the substrate and / or cap structures forming two caps as described above, i.e., if another cap volume is implemented adjacent to the cap volume, the aperture is preferably implemented as a through-hole in the substrate terminating in the other cap volume. In one embodiment, the opening in the cover or cap structure provided for acoustic coupling of the pressure transducer may also be sufficient for the desired gas exchange between the ambient and measurement volume via the other cap volume. In a different embodiment, an additional opening may be provided in the cover or cap structure to facilitate gas exchange between the ambient and measurement volume, and this additional opening may be located closer to the aperture in the substrate than the first opening located near the pressure transducer. Thus, the (first) opening in the cover or cap structure may be primarily dedicated to acoustic coupling, given its location closer to the pressure transducer than the through-hole in the substrate. The other (second) opening in the cover or cap structure may be primarily dedicated to gas exchange, given its location closer to the opening in the substrate than the pressure transducer.

[0057] In either case, a membrane may be provided covering the aperture, the membrane preferably being disposed on a first side of the substrate.

[0058] In such a scenario, the other sensors deployed above may also be positioned within the other cap volume, given the aperture terminating within the other cap volume, and thus such other sensors may sense one or more parameters of the surroundings, given access to the surroundings through the aperture.

[0059] However, in a further embodiment, an additional cap volume can be provided in combination with the substrate and / or cover or other recesses in the cap structure that implement the second cap volume, separated from the other cap volumes. The aperture terminates within the additional cap volume. Thus, gas exchange between the ambient and the measurement volume is provided by the additional cap volume, thereby not affecting measurements by the pressure transducer in the other cap volume. And / or, the path for gas exchange between the ambient and the measurement volume may be shorter than through the other cap volume.

[0060] In the case of a planar cap / cover, the further cap volume is preferably constructed by a further recess in the substrate, separated from the other recesses by a portion of the substrate reaching its normal level of thickness. The cover of the further recess may in particular be represented by an elongated cap, which at the same time represents the cover of the other recesses. In the case of a cap structure, the further cap volume is preferably constructed by a further cavity in the cap structure, separated from the other cavities by a base, which rests on the substrate after mounting.

[0061] In such an embodiment, the cover / cap / cap structure comprises a further opening to connect the further cap volume with the measurement volume 21 and allow gas exchange with the surroundings. In such an embodiment, a membrane may be arranged over the aperture in the substrate, but may alternatively be arranged on the cap / cover / cap structure covering the further opening or may be integrated into the cover / cap / cap structure, for example by etching.

[0062] In summary, for substrates that provide apertures for gas exchange between the ambient and the measurement volume, three scenarios are preferred: 1) the aperture terminates directly within the measurement volume, 2) the aperture terminates within another cap volume such that gas reaches the measurement volume through an opening in a cover or cap structure and via the other cap volume, or 3) the aperture terminates in a further cap volume such that gas reaches the measurement volume from the ambient through an opening in a cover or cap structure and via the further cap volume.

[0063] The PA gas sensor device can be used in many different application areas due to its small dimensions / footprint: application areas may include, but are not limited to, tabletop or portable devices for measuring air quality, wall-mounted thermostats, probes in air ducts, etc.

[0064] According to a further aspect of the present invention, a method for manufacturing a photoacoustic gas sensor device for determining a value indicative of the presence or concentration of a chemical component in a gas is provided. A substrate is provided, preferably including conductor tracks for electrically interconnecting electrical components. The electrical components are mounted on a first side of the substrate, preferably using an SMD process. The electrical components include at least an electromagnetic radiation source for emitting electromagnetic radiation, a pressure transducer for measuring acoustic waves generated by the chemical components in response to their absorption of the electromagnetic radiation, and a controller configured to control the electromagnetic radiation source. A cap is mounted on the first side of the substrate. The cap and substrate acoustically seal a cap volume. The cap encloses at least one of the electrical components other than the pressure transducer.

[0065] A measurement cell body is attached to a first side of the substrate, thereby enclosing the cap and electrical components disposed outside the cap, thereby defining a measurement cell together with the substrate. The measurement cell body and the substrate form the measurement cell. The measurement cell body, the substrate, and the cap limit a measurement volume through which electromagnetic radiation is emitted. The measurement cell includes an aperture for allowing gas to enter the measurement volume.

[0066] In one embodiment, prior to mounting the electrical components on the first side of the substrate, recesses are created in the substrate on the first side of the substrate, for example by one of the processes disclosed above, and the electrical components are then mounted on the first side of the substrate within and outside the recesses of the substrate before applying the cap.

[0067] During the ongoing assembly of the PA gas sensor device, the already attached cap can further mechanically protect any underlying electrical components, in particular from mechanical shocks, from dust and / or liquids, and from adhesives and / or solders that may later be used to attach the components to the substrate.

[0068] Preferably, the cap is attached to the substrate by gluing or soldering. Soldering is particularly preferred, most preferably using a solder alloy with a higher melting point than commonly used solder alloys to withstand high temperatures, for example, to allow the customer to solder the PA gas sensor device to the specified system PCB. In such a scenario, we suggest using a glue / adhesive with a melting point above the melting point of the solder expected to be used to solder the PA gas sensor device to the system PCB. The customer is then free to use any attachment technique without risking the cap coming off the substrate. The same applies when attaching the measuring cell body, i.e., when using an adhesive to bond the measuring cell body to the substrate, and an adhesive with the above-mentioned melting properties is preferred. When both the cap and the measuring cell body are bonded to the substrate, both adhesives are annealed in the same step, i.e., simultaneously.

[0069] Further advantageous embodiments of the photodetector are set forth in the dependent claims and the following description.

[0070] Embodiments, aspects and advantages of the present invention will become apparent from the following detailed description, which refers to the accompanying drawings. [Brief explanation of the drawings]

[0071] [Figure 1] 1 is a cutaway view of a photoacoustic gas sensor device according to one embodiment of the present invention; [Figure 2] 1 is a cutaway top view of a photoacoustic gas sensor device according to one embodiment of the present invention; [Figure 3] 1 is a cutaway view of a photoacoustic gas sensor device according to one embodiment of the present invention; [Figure 4] 1 is a cutaway view of a photoacoustic gas sensor device according to one embodiment of the present invention; [Figure 5] 1 is a cutaway view of a photoacoustic gas sensor device according to one embodiment of the present invention; [Figure 6] 1 is a cutaway view of a photoacoustic gas sensor device according to one embodiment of the present invention; [Figure 7] 1 is a cutaway view of a photoacoustic gas sensor device according to one embodiment of the present invention; [Figure 8] 1 is a cutaway view of a photoacoustic gas sensor device according to one embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0072] The same elements are referred to by the same reference numbers throughout the figures.

[0073] FIG. 1 shows a cutaway view of a photoacoustic (PA) gas sensor device according to one embodiment of the present invention. The PA gas sensor device comprises a substrate 1, e.g., a printed circuit board (PCB), and a measurement cell body 2, which together form a measurement cell surrounding a measurement volume 21. The measurement cell has an aperture 23 that allows gas exchange between the measurement volume 21 and the surroundings of the PA gas sensor device. In FIG. 1, the aperture 23 is located within the measurement cell body 2. The aperture 23 is preferably covered by a membrane 22 that is gas-permeable to allow gas exchange such that the concentration of the chemical component of interest in the gas is similar to that of the surroundings.

[0074] The substrate 1 has a first side 11 and a second side 12. The measuring cell body 2 is disposed on the first side 11, as are the electrical components described below. The second side 12 of the substrate, opposite the first side 11, is provided with pads 13, e.g., land grid array (LGA) pads, for SMD assembly and reflow soldering by the customer. Other contacts, such as DFN, QFN pads, or castellated holes, are also possible.

[0075] The electrical components arranged on the first side 11 of the substrate 1 comprise a pressure transducer 3 and an electromagnetic radiation source 4, which in this example is an infrared source. Both electrical components are located inside the measurement volume 21.

[0076] The infrared source emits infrared radiation, designated by reference numeral 41. When the wavelength / band of radiation emitted by the infrared source is tuned to the gas molecules to be detected, the infrared radiation 41 is selectively absorbed by the molecules of the target chemical component present in the measurement volume 21. The infrared source may be a broadband infrared emitter, emitting radiation across the entire infrared spectrum, for example, covered by an optical bandpass filter. The optical bandpass filter passes only radiation in a band set according to the target gas component. For CO2 detection, the band is centered, for example, around 4.3 μm, with a typical bandwidth of 0.5 μm or less, e.g., 0.2 μm or 0.1 μm, so that the measurement is practically selective for CO2.

[0077] Pressure transducer 3 may be a MEMS microphone or any other pressure transducer. The sensitivity of the pressure transducer is not necessarily limited to the acoustic frequency band and may be configured to measure frequencies up to 100 kHz. One or more other electrical components may be disposed on first side 11 of substrate 1, collectively referenced by 5.

[0078] Furthermore, a cap 7 having a cavity 74 is disposed on the first side 11 of the substrate 1 within the measurement cell. The cap 7 separates the measurement volume 21 from a capped volume 71 defined by the substrate 1 and the cap 7. A controller 8 is disposed within the capped volume 71, i.e., the controller 8 is surrounded by the cap 7. The pressure transducer 3, the electromagnetic radiation source 4, the controller 8, and other components 5, if any, are electrically connected via conductor tracks in or on the substrate 1.

[0079] In a highly preferred embodiment, the controller 8 is embodied as an ASIC. In one embodiment, the controller 8 is capable of receiving and processing signals from the pressure transducer 3 and one or more other sensors, if available. The controller 8 is capable of controlling the EM radiation source 4 and providing an external PA gas sensor device signal, represented for example by a processed or at least preprocessed signal from the pressure transducer 3. For these reasons, the substrate 1 preferably comprises through vias (not shown), for example for connecting conductor tracks provided on the first side 11 of the substrate 1 to the aforementioned pads 13 on the second side 12 of the substrate 1. Preferably, the substrate 1 comprises multiple layers of conductive tracks, such as an FR4-PCB.

[0080] Thus, the controller 8 is configured to control the EM radiation source 4, for example, by imposing intensity modulation on the infrared radiation at a modulation frequency. The modulation frequency may be within the audible spectrum, for example, 20 Hz to 20 kHz, or up to 100 kHz, or even down to 5 Hz. The controller 8 is further configured to receive measurements from the pressure transducer 3 and determine chemical component concentration values ​​from those measurements, for example, by using a predetermined or resettable calibration function that links the measurements to chemical component concentration values. The chemical component concentration values ​​may be output via a digital interface, for example, an I2C interface. When determining the chemical component concentration values, the controller 8 may also take into account one or more other measurements, such as temperature and / or humidity values, if available, and perform compensation as described above. For CO2 as the relevant gas component, measurements in the range of 0 to 10,000 ppm, 0 to 40,000 ppm, or 0 to 60,000 ppm of CO2 are possible.

[0081] The substrate 1 preferably comprises a through channel in its capped region from its first side 11 to its second side 12, which through channel functions as a vent 6, for example to allow overpressure or underpressure to be aligned with ambient pressure and / or to allow gas in the cap volume 71 to escape and / or to allow ambient gas to enter the cap volume 71.

[0082] FIG. 2 shows a cutaway top view of a PA gas sensor device according to one embodiment of the present invention. In this embodiment, a substrate 1 has a square base area on which a measurement cell body 2 is mounted. Within the measurement cell body 2, electrical components are arranged, for example, in four quadrants. A pressure transducer 3 and an EM radiation source 4 are located in opposite quadrants. Other components 5 and a controller 8 are located in the remaining two opposite quadrants. The controller 8 is capped by a cap 7. The resulting cap volume 71 is connected to the periphery of the PA sensor device by a vent 6.

[0083] 3 shows a cutaway view of a PA gas sensor device according to one embodiment of the present invention. In this embodiment, an EM radiation source 4, a pressure transducer 3, and a capacitor 51 are arranged on a first side 11 of the substrate 1 facing a measurement volume 21 defined by the substrate 1 and a measurement cell body 1 arranged on the same first side 11 of the substrate 1. A controller 8 embodied as an ASIC is arranged on the same first side 11 of the substrate 1, but is capped by a cap 7. A vent hole 6 is provided through the substrate 1 to balance the pressure in the cap volume 71. On a second side 12 of the substrate 1, the vent hole 6 is covered by a vent membrane 14 that allows gas to pass from or to the cap volume 71.

[0084] Further, other sensors 9 are advantageously provided, which may be one or more of the following: temperature sensors, humidity sensors, combined temperature / humidity sensors, pressure sensors, particularly barometric pressure sensors, other pressure transducers, other gas sensors, for example, of the metal oxide or electrochemical type. The gas concentration values ​​can be compensated for, for example, the influence of temperature and / or humidity via the temperature and / or humidity measurements. In the presence of the other sensors 9, the controller 8 in the cap volume 71 is preferably configured to compensate the values ​​indicative of the presence or concentration of the chemical component for the influence of variables measured by the other sensors 9 and thus dependent on the measurements of the other sensors 9. Thus, the influence of ambient conditions on the measurement of the chemical component can be reduced or eliminated.

[0085] FIG. 4 shows a cutaway view of a PA gas sensor device according to a further embodiment of the present invention. Compared to the embodiment shown in FIG. 3, not only is the controller 8 capped within the measurement cell, but one or more other electrical components, such as the illustrated capacitor 51, are also capped, as is the electromagnetic radiation source 4. The electromagnetic radiation source 4 requires an optical window 72 within the cap 7. The optical window 72 allows radiation emitted by the EM radiation source 4 to enter the measurement volume 21 and interact with the gas molecules of interest. In this embodiment, the pressure transducer 3 remains positioned within the measurement volume 21, outside the cap volume 71. Thus, the pressure transducer 3 is able to detect pressure fluctuations caused by the reaction of the emitted radiation with the gas molecules.

[0086] In this embodiment, another sensor 9 is also provided. In this example, the other sensor 9 is a humidity sensor. The humidity sensor can provide its signal, representing the humidity measured in the measurement cell, to the controller 8, which can then use it to compensate the pressure transducer signal for fluctuations caused by humidity. Considering that there is no air vent here, the cap volume 71 is not connected to the outside world. However, in different embodiments, an air vent may actually be provided for the reasons discussed above.

[0087] FIG. 5 shows a cutaway view of a PA gas sensor device according to a further embodiment of the present invention. Compared to the embodiment shown in FIG. 4, the other sensor 9 (again, a humidity sensor) is now located below the cap 7, i.e., in the cap volume 71. Thus, the signal provided by the pressure transducer 3 can again be compensated for humidity fluctuations. In this example, a vent hole 6 and a vent membrane 14 are provided to allow the cap volume 71 to be vented. In this embodiment, the pressure transducer 3 is one of the components not located in the capped volume 71, or even the only component. This approach improves, i.e., increases, the mean optical path length of the radiation 41 within the measurement volume 21. This is achieved by one or more of the following options: The material of the measurement cell body 2 is selected to be reflective, such as sheet metal. Alternatively or additionally, the inner surface of the measurement cell body 2 is coated with a reflective coating. The reflective coating may not only be disposed on the inner surface of the measuring cell body 2, but also on one or more of the following: part of the first side 11 of the substrate 1, part of the pressure transducer 3, e.g. its top surface, the outer surface of the cap 7 facing the measuring volume 21. In this way, the overall reflectivity within the measuring cell is increased, leading to a more accurate measurement of the concentration of the component.

[0088] 6 and 7 show cutaway views of a photoacoustic (PA) gas sensor device according to a further embodiment of the present invention. In the previous embodiments, the cap itself contributed significantly to the cap volume in that it provided a cavity and thus defined the interior space. In the embodiment of FIGS. 6 and 7, the cap is embodied as a planar cover, while the cap volume is defined by a recess in the substrate.

[0089] In the embodiment of Figures 6 and 7, the substrate 1 is preferably a printed circuit board (PCB), preferably a multi-layer PCB including multiple conductive layers at different vertical levels of the PCB. The PCB is structured, for example by etching or grounding or other material removal techniques, to create at least one recess 15 in the PCB, resulting in a recess in the PCB. The recess 15 is located on a first side 11 of the PCB, i.e., the side 11 on which components are / will be placed. The recess 15 is understood as a depression, not a through channel, such that at the recess in the substrate 1, the substrate 1 is thinner compared to its normal thickness. In other words, the substrate 1 is a continuous substrate 1 across the recess.

[0090] The recess 15 in the substrate 1 defines a first level L1 and a lower second level L2 along the z-axis. Two components, namely, an electromagnetic radiation source 4 and a controller 8, are located on the first level L1. As can be seen in FIGS. 6 and 7, these components 4 and 8 are bonded to the first level L1 of the substrate 1. Therefore, the first level L1 represents the level of the substrate 1 where one of the conductive layers is structured to create conductive paths and contact pads. The electromagnetic radiation source 4 bridges the second level L2.

[0091] The substrate 1 comprises a second recess 16. The pressure transducer 3 is disposed in the second recess 16 on the surface of the third level L3 and bridges the face of the fourth level L4 of the second recess 16. The pressure transducer 3 is electrically connected to conductive pads / paths of a metallization layer in the substrate 1. Here, an aperture 23 is provided in the substrate 1 and is covered by a membrane 22 on the first side 11 of the substrate 1 and in the other recess of the substrate 1.

[0092] In the embodiment of Figure 6, recess 15 is closed by a planar cap 7. The other recess 16 is closed by a planar cover 6. Cap 7 supports a reflective coating 73 facing measurement volume 21. Cover 6 supports a reflective coating 703 facing measurement volume 21. Cap 7 and the recess in substrate 1 define cap volume 71. The other recess in cover 6 and substrate 1 define another cap volume 701. Preferably, cap 7 and cover 6 are one piece with uniform coatings 73, 703.

[0093] The cover 6 provides two openings 61 and 62. The opening 61 is located near the pressure transducer 3 and provides acoustic coupling between the measurement volume 21 and the pressure transducer 3. The opening 62 is located near / above the aperture 23 and specifically supports gas exchange between the surroundings and the measurement volume 21.

[0094] In the embodiment of Figure 7, recess 15 and the other recess 16 are closed by a one-piece planar cap 7 made from silicon. The other recess 16 is closed by the planar cap 7. The cap 7 itself is of a reflective material. The cap 7 provides an opening 61 for both acoustic coupling and gas exchange with the measurement volume 21. The cap 7 and the recess in the substrate 1 define a cap volume 71, and the cap 7 and the other recess in the substrate 1 define another cap volume 701. Alternative electrical connections between the component and the substrate can include solder balls, soldering, or conductive adhesives.

[0095] FIG. 8 shows a cutaway view of a photoacoustic gas sensor device according to another embodiment of the present invention. The configuration is similar to the embodiment shown in FIGS. 6 and 7. However, the cap 7 is no longer a planar cover but is formed by a cap structure 7 / 70 exhibiting at least one cavity. Here, the cap structure 7 / 70 includes a first cavity 74 for forming the cap 7 and another cavity 704 for forming the other cap 70. Thus, the other cavity 704 is provided to contribute to another cap volume 701 in combination with the substrate 1 on which the cap structure 7 / 70 is disposed. The cap volume 71 and the other cap volume 701 are separated from each other by the cap structure 7 / 701, e.g., a base B in the cap structure 7 / 70, and are seated on the substrate 1. One or more electrical components other than the pressure transducer 3 are disposed in the cap volume 71, while the pressure transducer 3 is disposed in the other cap volume 701. The cap structure 7 / 70 provides at least one opening 702 for acoustically coupling the measurement volume 21 with the pressure transducer 3. This opening 702 is close to the position of the pressure transducer 3. A second opening 705 is provided to allow gas exchange between the surroundings of the photoacoustic sensor and the measurement volume 21 via the other cap volume 701. For this purpose, the substrate 1 contains an aperture 23 close to the other opening 705 and a membrane 22.

[0096] In such an embodiment, all of the electrical components may reside on the first side 11 of the substrate 1, and the substrate 1 may not exhibit recesses or different surface levels. However, compared to other embodiments that include a single cap for capping several selected components, here all components are capped. As a result, components such as the pressure transducer 3 do not directly face the measurement volume 21, but instead face the cap structure 7 / 70. Considering that it is desirable for any surface facing the measurement volume 21 to be reflective, the cap structure 7 / 70 and at least its surface facing the measurement volume 21 are preferably made of a reflective material or coated with a reflective material, such as coating 703. Such a coating 703 may be easier to achieve than coating or housing the individual electrical components.

[0097] Preferably, the cap 7 and the other cap 70 are formed by a common cap structure 7 / 70 as shown in Figure 8. The common cap structure 7 / 70 may be a silicon wafer having two cavities 74, 704 etched therein to form the two caps 7 / 70. Such a silicon wafer is preferably bonded onto the first side 11 of the substrate 1. In this example, the measurement cell body 2 is attached to the cap structure 7 / 70 while still positioned on the first side 11 of the substrate 1.

[0098] Nevertheless, the substrate 1 is provided with one or more recesses, here two recesses 15, 16, to provide surfaces at first and third levels L1 / L3, respectively, both at levels L1 / L3 lower than the normal level surfaces of the substrate. Such recesses 15, 16 can be used, for example, as back volumes for the electromagnetic radiation source 4 and the pressure transducer 3.

[0099] While embodiments of the present invention have been shown and described above, it is to be understood that the invention is not limited thereto and may be variously embodied and carried out in other ways within the scope of the following claims. [Explanation of symbols]

[0100] 1 board 11 First Aspect 12 The Second Aspect 13 Pad 14 Vent membrane 15 recess 16 Other recesses 17 Ventilation holes L1 First level L2 Second Level 2. Measuring cell body 21 Measurement volume 22 membrane 23 Through-channel 24 Coating 3 Pressure Transducers 4 EM radiation source 41 Infrared 5. Capacitors 6 Cover 61 First Opening 62 Second Opening 7 Cap 71 Cap volume 72 Optical Filters 73 Coating 74 Cavity 70 other caps 701 Other Cap Volume 702 First Opening 703 Coating 704 Other Cavities 705 Second Opening 7 / 70 Cap structure 8 Controller 9 Other Sensors

Claims

1. 1. A photoacoustic gas sensor apparatus for determining a value indicative of the presence or concentration of a chemical component in a gas, comprising: A substrate; a measurement cell body disposed on a first side of the substrate, the substrate and the measurement cell body defining a measurement cell; a cap disposed on the first side of the substrate within the measurement cell, the cap and the substrate defining a cap volume, the cap and the substrate acoustically sealing the cap volume; and a measurement volume defined by the measurement cell body, the substrate, and the cap; an aperture in the measurement cell for allowing the gas to enter the measurement volume; an electrical component disposed on the first side of the substrate and within the measurement cell, the electrical component comprising at least: an electromagnetic radiation source for emitting electromagnetic radiation into the measurement volume; a pressure transducer for measuring acoustic waves generated by the chemical components present in the measurement volume in response to the chemical components absorbing the electromagnetic radiation; a controller configured to control the electromagnetic radiation source; an electrical component comprising: Equipped with the pressure transducer is located outside the cap volume; the controller is disposed within the cap volume; The photoacoustic gas sensor device, wherein the controller is configured to process an electrical signal provided by the pressure transducer that is indicative of the presence or the concentration of the chemical component in the gas.

2. the surface of the cap facing the measurement volume has a reflectivity greater than 70%; The photoacoustic gas sensor device according to claim 1 .

3. At least one transistor is disposed within the cap volume. The photoacoustic gas sensor device according to claim 1 .

4. the substrate includes a vent hole arranged to connect the cap volume to the periphery of the photoacoustic gas sensor device; at least one other sensor is disposed within the cap volume for sensing an environmental parameter in an ambient gas of the photoacoustic gas sensor device supplied to the other sensor through the vent; The photoacoustic gas sensor device according to claim 1 .

5. the controller is configured to determine the value indicative of the presence or concentration of the chemical component in the gas in dependence on the electrical signal provided by the pressure transducer and in dependence on the electrical signal provided by the other sensor. The photoacoustic gas sensor device according to claim 4 .

6. the electromagnetic radiation source is disposed within the cap volume; the cap includes an optical filter positioned and configured to allow the electromagnetic radiation emitted by the electromagnetic radiation source to enter the measurement volume and / or to band-pass filter the electromagnetic radiation emitted by the electromagnetic radiation source before it enters the measurement volume. The photoacoustic gas sensor device according to claim 1 .

7. the electromagnetic radiation source is configured to emit ultraviolet light; the electromagnetic radiation source is located outside the cap volume; the cap is made of a material that is opaque to ultraviolet light to protect the controller within the cap volume from ultraviolet light; The photoacoustic gas sensor device according to claim 1 .

8. the cap comprises a conductive material; The cap is made of metal or metal-coated plastic. The photoacoustic gas sensor device according to claim 1 .

9. a recess in a first side of the substrate; the substrate is a printed circuit board or is made of a ceramic material and includes conductive paths; at least one electrical component other than the pressure transducer is disposed on the substrate within the recess; the cap has a planar shape and closes the recess in the substrate; the cap volume is defined by at least the recess in the substrate and the planar cap; the cap is made of metal, and / or the cap is at least partially coated with a reflective coating; The photoacoustic gas sensor device according to claim 1 .

10. a recess in a first side of the substrate; the substrate is a printed circuit board or is made of a ceramic material and includes conductive paths; at least one electrical component other than the pressure transducer is disposed on the substrate within the recess; the cap has a planar shape and closes the recess in the substrate; the cap volume is defined by at least the recess in the substrate and the planar cap; The cap is made of silicon. The photoacoustic gas sensor device according to claim 1 .

11. a recess in a first side of the substrate; the substrate is a printed circuit board or is made of a ceramic material and includes conductive paths; at least one electrical component other than the pressure transducer is disposed on the substrate within the recess; the recess provides a first level surface of the substrate that supports one or more of the electrical components and a second level surface that is lower than the first level surface; the electromagnetic radiation source is disposed on the first level surface and bridges the second level surface; The photoacoustic gas sensor device according to claim 1 .

12. a recess in a first side of the substrate; the substrate is a printed circuit board or is made of a ceramic material and includes conductive paths; at least one electrical component other than the pressure transducer is disposed on the substrate within the recess; a recess on the first side of the substrate; the pressure transducer is disposed on the substrate within the other recess; The photoacoustic gas sensor device according to claim 1 .

13. a cover for covering the other recess in the substrate; an opening in the cover for acoustically coupling another cap volume between the other recess of the substrate and the cover to the measurement volume; the other recess provides a third level surface in the substrate supporting one or more of the electrical components and a fourth level surface lower than the third level surface, the pressure transducer being disposed on the third level surface and bridging the fourth level surface; The cover is represented by the planar cap. The photoacoustic gas sensor device according to claim 12 .

14. a cap structure including a first cavity forming the cap and including another cavity forming another cap; the cap structure is disposed on the first side of the substrate, resulting in the cap volume and another cap volume between the cap structure and the substrate; the pressure transducer is disposed within the other cap volume; the other cap includes at least one opening for acoustically coupling the measurement volume to the other cap volume. The photoacoustic gas sensor device according to claim 1 .

15. the cap structure is made from a silicon wafer bonded to the substrate; or the cap structure is made from a printed circuit board soldered or glued to the substrate; The photoacoustic gas sensor device according to claim 14.

16. the measurement cell body is attached to the cap structure; the cap structure has the same footprint as the substrate; The photoacoustic gas sensor device according to claim 14.

17. the aperture is disposed in the substrate; The aperture defines the measurement volume from the periphery, directly, through said other cap volume, or via a further cap volume between the further recess in the substrate and its cover and / or between the substrate and the cap structure. and the cover or cap structure, respectively, is permeable to allow the gas to enter the measurement volume. The photoacoustic gas sensor device according to claim 14.

18. 1. A method of manufacturing a photoacoustic gas sensor device for determining a value indicative of the presence or concentration of a chemical component in a gas, comprising: Providing a substrate; Mounting an electrical component on a first side of the substrate, the electrical component comprising: an electromagnetic radiation source for emitting electromagnetic radiation; a pressure transducer for measuring acoustic waves generated by the chemical component in response to the chemical component absorbing electromagnetic radiation; a controller configured to control the electromagnetic radiation source, the controller configured to process an electrical signal provided by the pressure transducer indicative of the presence or concentration of the chemical component in the gas; Mounting an electrical component, the electrical component comprising at least attaching a cap to the first side of the substrate, the cap and the substrate acoustically sealing a cap volume, the cap enclosing at least the controller but not the pressure transducer; attaching a measurement cell body to the first side of the substrate to surround the cap and electrical components disposed outside the cap, wherein the measurement cell body and the substrate form a measurement cell, the measurement cell body, the substrate, and the cap define a measurement volume through which the electromagnetic radiation is emitted, and the measurement cell includes an aperture for allowing the gas to enter the measurement volume; and A method comprising:

19. creating a recess in the substrate prior to attaching the electrical component to the first side of the substrate; at least one of the electrical components other than the pressure transducer is mounted on the first side of the substrate in a recess in the substrate; the pressure transducer is mounted on the first side of the substrate outside the recess; 20. The method of claim 18.

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