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, particularly CO2, in a gas. [Background technology]

[0002] Photoacoustic gas sensors rely on the physical effect of, for example, infrared radiation being absorbed by molecules of a component of interest in a gas, e.g., CO2, which causes the molecules to enter an excited state. Subsequently, due to non-radiative decay of the excited state, heat is generated, e.g., by molecular collisions, which results in an increase in pressure. By modulating the absorbed infrared radiation with a modulation frequency, the pressure fluctuates at the modulation frequency. Such 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, the pressure transducer and the controller for controlling the emitted radiation can be located in the same volume in which the photoacoustic reaction takes place, i.e. the measurement volume of the photoacoustic measurement cell. This is beneficial in that the footprint of the gas sensor device is small. However, when switching the current to modulate the emission of radiation from the radiation source, for example the controller heats up and cools at the same frequency as the electromagnetic radiation source emits the modulated radiation. Such pulsed heating then amplifies the pressure fluctuations in the measurement cell and thus falsifies 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 a 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 arrangement according to claim 1.

[0006] Photoacoustic gas sensor devices (also abbreviated as PA gas sensor devices) allow for determining values ​​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 arranged 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 arranged on the first side of the substrate and not on the opposite side of the substrate, and possibly 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 arranged on the first side of the substrate in the measurement cell. The cap and the substrate define a cap volume. In one embodiment, the cap itself contributes largely or even completely to the cap volume, if the cap defines an interior space, for example by vertical walls, or by a concave interior, or by any other cavity in the cap. In a different embodiment, the substrate contributes largely or even completely to the cap volume by a recess therein, which recess 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, e.g., some 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 such that photoacoustic transduction within the measurement volume is not affected by electrical components present within the cap volume. That is, pressure variations 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 that the cap, substrate, and any potential mechanical contact surfaces between them are designed and / or configured to not allow pass-through pressure variations 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 at least comprise 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 the 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] The PA gas sensor device relies on the photoacoustic effect: molecules of the chemical component of interest, for example CO2 in the surrounding gas, for example ambient air, absorb electromagnetic radiation, in a preferred example radiation in the infrared band, leading to non-radiative decay, for example heat generation due to collisions between molecules of the chemical component and / or with different molecules of the chemical component, which then leads to an increase in pressure. Modulation of the pressure 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, for example 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, in particular as a function of the amplitude of the pressure fluctuations, since the amplitude can be assumed to be proportional to the amount of EM radiation absorbed by the chemical component and therefore proportional to the concentration of the chemical component in the gas, if all other factors, for example the average optical path length in the measurement volume, remain equal.

[0011] In addition to the EM radiation source and the pressure transducer, the PA gas sensor device comprises further electrical components including a controller configured to control the EM radiation source. The controller is arranged 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 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, for example 40 Hz. Preferably, a heater of the EM radiation source is switched 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 to determine a value indicative of the presence or concentration of a chemical component dependent on this electrical signal. In this respect, the controller preferably performs signal processing such as filtering and / or linearization and / or compensation and / or A / D conversion. In particular, the value is determined as a function of the amplitude of the electrical signal, for example the volume in 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 the 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 in their surroundings, and / or components that have 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 dedicated access to the surroundings of the photoacoustic gas sensor permitted.

[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 if the controller were placed in the measurement volume.

[0015] To avoid adding such controller-induced offsets to the pressure signal (the effect of adding offsets is also called 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 especially for electrical components that adversely affect the photoacoustic transduction.

[0016] In a highly preferred embodiment, the controller is embodied as an integrated circuit, in particular as 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 allows to 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 component preferably arranged in the cap volume may include a transistor, in particular a power transistor, and / or a component comprising such a transistor. Such a component may be a voltage controller. Thus, it is preferred that one or more transistors and / or a component having one or more transistors are arranged in 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 variations under the application of a switched current. It is therefore preferable to place 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 electrical components may include bulky components having a top surface exceeding 100 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 the EM radiation of the measurement volume, the reflectivity of the measurement volume is reduced. This may be a reason to position such electrical components within the cap volume.

[0020] The cap is attached to the 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 to the measurement volume. Thus, the cap eliminates or only makes negligible pressure fluctuations propagating from the cap volume into the measurement volume. Thus, the cap containing the substrate and the attachment of the cap onto 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 may also have a detrimental effect. However, considering that the heat emitted from the electrical components under the cap is isolated in and by the cap volume and the medium therein, the cap at the same time limits or avoids 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 even smaller.

[0022] The cap is preferably made of a material that prevents such gas exchange. This also includes that the cap has no holes or other openings that would allow gas exchange between the cap volume and the measurement volume. This also includes that the cap is mounted on the substrate in a sealing manner, preferably by glue or solder. For example, the cap is made of an airtight material.

[0023] Thus, crosstalk, in particular thermal and / or acoustic crosstalk, from the controller to the measurement volume and ultimately to the pressure transducer can be avoided: the pressure signal provided by the pressure transducer is not or can be negligibly subjected to a controller induced offset.

[0024] The cap may include or be made of a conductive material. Such a material selection may provide the additional advantage that electrical components within the cap volume, such as the controller, may 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 may 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 a good reflectivity of all surfaces that define the measurement volume of the emitted EM radiation. The inner surface of the measurement cell body and the first side of the substrate are such surfaces, but also the (outer) surface of the cap, i.e. the surface of the cap that faces the measurement volume and defines the measurement volume, is important for the overall reflectivity in the PA gas sensor device. Therefore, it is preferable that such a surface of the cap that faces the measurement volume has a reflectivity of more than 70%, preferably more than 80%, more preferably more than 90%. The inner surface of the measurement cell body that shows a reflectivity of more than 70%, preferably more than 80%, more preferably more than 90%, potentially in combination with a reflective coating on the substrate surface and / or electrical components that face the measurement cell, can improve the overall reflectivity of the surfaces that define the measurement volume.

[0026] In particular, the cap is made from metal. In a different embodiment, the cap is made from plastic coated with metal on its outer surface facing the measurement volume. In this embodiment, reflectivity and electromagnetic shielding can be achieved simultaneously. The same material choice is preferred for the measurement cell body. If the measurement cell body is made from plastic, its inner surface may be coated with a metal resulting in a reflective coating. Such a coating may be made, for example, from one of gold, aluminum, nickel, copper. Gold is also particularly convenient for plating substrates, for example PCBs, since gold is applied as corrosion protection to the surfaces of conductors during the formation of the PCB in any case. In another embodiment, the measurement cell body is made from sheet metal, for example by deep drawing. Sheet metal has the advantage that it is mechanically stable even at low thickness and shows a high reflectivity for electromagnetic radiation even without further 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 average optical path length of the EM radiation in the measurement volume. In this way, the average optical path length in the measurement volume can be increased to >1 cm, preferably >3 cm, more preferably >5 cm. This in turn makes it easier to make the measurement cell smaller, for example with dimensions of 2 x 2 x 2 cm. 3 Smaller, preferably 1x1x1cm 3 The entire photoacoustic gas sensor device is smaller than 1×1×0.7 cm. 3 , which is 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 comprises vent holes arranged to connect the cap volume to the surroundings of the photoacoustic gas sensor device. Thus, in the region of the substrate covered by the cap, through-channels are provided from the first side of the substrate to its second side, i.e. from its front side to its back side, in order to allow the exchange of gas between the cap volume and the surroundings. Such vent holes may be preferred, for example, to allow pressure equilibrium to be achieved during the manufacturing or assembly process, for example after heating of the cap volume. The diameter of such vent holes is preferably between 100 μm and 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 surroundings. This arrangement is advantageous in manufacturing. After the manufacture of the measuring cell, the vent membrane can be attached to the substrate in a final step. In a different embodiment, the vent membrane is attached during the preparation of the substrate, before the measuring cell body is attached to the substrate.

[0030] In a preferred embodiment, the electrical component includes an other sensor for sensing an environmental parameter. Assuming that the pressure transducer represents the sensor, this sensor is also referred to as the "other sensor". The other sensor may be configured to sense one or more of the following: humidity and / or pressure and / or chemical components in the gas. The other sensor may thus be embodied as one or more of a pressure sensor, an air pressure sensor, another pressure transducer, another gas sensor, for example of metal oxide type or 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 of the other sensor is used to improve the signal provided by the pressure transducer, for example by compensating for the influence of the measured environmental variables on the gas concentration in the measurement volume. Thus, the influence of the ambient conditions on the measurement can be reduced or eliminated. Such compensation makes the resulting concentration values ​​more accurate and more reliable, in other words, the gas sensor device can be adapted to various environmental conditions.

[0032] For both purposes, the other sensor can be placed either inside the cap volume in the measurement cell or outside the cap volume. When placed inside the cap volume, if there is a vent in the substrate, ambient gas can enter the cap volume and the desired variations in the environment 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 by the aperture and the desired variations in the environment can be sensed in the aperture.

[0033] In one embodiment of the invention, the EM radiation source is disposed within the cap volume, 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 to bandpass filter 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 preferably used is 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 sound waves that the component generates 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 an absorption peak of the chemical component of interest. A band is considered as a subrange of the EM spectrum, preferably symmetrical around an absorption peak, with a maximum / minimum band limit of + / - 15% of the absorption peak value. In the case of infrared, the corresponding band is advantageously selected to correspond to an 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 on 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 over a wide spectrum, for example 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, for example a dielectric filter, or a metamaterial filter, or a CMOS absorbing layer, so that only infrared radiation of the band is emitted into the measurement volume. In other embodiments, narrowband EM radiation sources such as lasers or LEDs, possibly including metasurface resonators, are used.

[0038] A pressure transducer is present within and / or acoustically coupled to the measurement volume to detect pressure variations occurring therein in response to PA binding in the presence of a 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), for example made from FR4 or a ceramic material that provides more mechanical stability. At least the pressure transducer, the electromagnetic radiation source, the controller, and the measurement cell body and the 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, such 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 allows for easier assembly with other components of the device by the customer. Other options for the contacts include DFN, QFN, or castellated holes.

[0040] In one embodiment, the cap may be of planar shape. The cap volume is then substantially defined by a recess in the substrate, i.e. a recess extending from a first side of the substrate. The substrate is thus thinner than usual in the thickness of the recess. Such 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 as a ceramic circuit board including conductive paths. The PCB is preferably a multi-layer PCB including a plurality of structured metallization layer conductive paths made from different levels of PCBs. Such a PCB can preferably be etched or ground or otherwise processed from its first side / front, thereby removing material and generating recesses in the substrate. Such processing can also include pre-structuring of the individual layers of the PCB, for example by cutting of a mold, before laminating such layers together. However, the substrate is not processed in such a way that through holes arise in the recesses. Instead, the substrate remains thinned in 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. The planar cap closes the recess of the substrate and together with the recess of the substrate establishes / defines an acoustically tight compartment, and thus a cap volume.

[0042] In particular, the planar cap is made from metal, taking into consideration that its surface facing the measurement cell and desirably has reflective properties, hi 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 on 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, completely covering and / or sealing the recess of the substrate. However, if the electromagnetic radiation source is disposed in the recess of the substrate, the cap preferably includes an opening covered by a transparent material if the cap is not transparent to the electromagnetic radiation emitted by the electromagnetic radiation source. Such an opening is preferably covered by an optical filter 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 different embodiments, the cap is made of silicon. Again, if only the controller and / or any components that do not require 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 of the substrate, and may even be opaque or coated with an opaque material. However, if an electromagnetic radiation source is placed in the recess of the substrate, the silicon cap is transparent to the electromagnetic radiation generated in the cap volume, or comprises an opening, or comprises a transparent part. Preferably, an optical filter, transparent to electromagnetic radiation, preferably only to electromagnetic radiation of a certain wavelength, is applied to the cap, either as an element separate 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. either facing the measurement cell or facing the substrate or facing both. In different embodiments, the optical filter is integrated into the cap and thus may represent part of the cap. Such part may have different optical properties than the other parts 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 for passing 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 called the first level surface, is at a first level of the vertical axis, if 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 normal surface level of the substrate. The first level surface may be a plane for placing one or more of the electrical components. This may mean, in one embodiment, 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, such that the first level surface is at a level somewhat above the level of this metallization layer in the PCB. The PCB may therefore be ground or etched or otherwise processed to such a first level to create a recess that allows the electrical components to be easily electrically contacted.

[0046] In another embodiment, the recess is shaped so that two different level surfaces along a vertical axis are generated, the second level surface being lower than the first level surface. In particular, the second level surface is arranged between the first level surfaces such that the first side of the substrate presents 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. Thus, 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 present a thickness that prevents the arrangement of components on the first level surface taking into account the cap.

[0047] In an embodiment with a recess in the substrate, the further component arranged outside the cap volume may in one embodiment be installed in a non-recessed part of the substrate, i.e. at its normal level. However, in a different embodiment, another recess may be provided in the substrate, on a first side of which a third level surface is generated for the installation or SMD mounting of such a component. Such other recess is preferably covered with a cover. The other recess in the substrate and the cover form another cap volume. This other cap volume is not connected with the cap volume. If the cover is reflective and / or the cover is coated with a reflective coating, at least on the side facing the measurement volume, the reflectivity in the measurement volume is enhanced and promoted compared to providing a component with a reflective surface. The level of the third surface in the other recess may be, but does not necessarily have to be, the same as the level of the first surface in the recess. Preferably, the pressure transducer is arranged on the substrate in another recess in the substrate. However, further components may be arranged in another recess in the substrate.

[0048] Most preferably, the cover of the other recess is represented by a cap of the recess. The cap may therefore be elongated and may cover the other recess. For the material of such a cover, see the material of the cap. Preferably, the cover is also coated with a reflective coating, in one example, which reflective coating faces the measurement volume. Preferably, the coating may be a continuous coating on the cap and the 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 molded such that a surface is generated at two different levels along the vertical axis, the fourth level surface being lower than the third level surface. In particular, the fourth level surface is arranged between the third level surfaces such that the first side of the substrate presents 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 that require a backside volume are preferably arranged on the third level surface resulting in the fourth level surface. Such electrical components are preferably pressure transducers. In a different scenario, one or more components may be arranged on the fourth level surface, which components present a thickness that prevents the placement of components on the third level surface in view of the cap. Preferably, the third level is equal to the first level. Preferably, the fourth level is equal to the second level.

[0051] In different embodiments 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 the cap structure, for example by a base of the cap structure, and sit on the substrate. The pressure transducer is now placed in the other cap volume, although one or more electrical components other than the pressure transducer are placed in the cap volume. The cap structure preferably exhibits an opening towards the measurement volume in order to acoustically couple the measurement volume to the pressure transducer.

[0052] In such an embodiment, all the electrical components may be present on the first side of the substrate. However, compared to other embodiments with a single cap, other electronic components that are located outside the cap, such as a pressure transducer, are also capped by the other cap. Preferably, both caps are manufactured 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 generate the cavities. In the case of a PCB, a process such as that disclosed for generating one or more recesses in a PCB substrate can be applied to generate the cavities. As a result, even other electrical components are not directly facing the measurement volume, but are placed in the other cap volume that is coupled with the measurement volume by one or more openings in the cap structure. Again, it is preferred that at least the surface of the cap structure that faces the measurement volume is reflective or coated with a reflective coating.

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

[0054] Preferably, the cap structure is attached to the substrate with the cavity facing the substrate, in particular by bonding if the cap structure is a silicon wafer, or by gluing or soldering if the cap structure is made from a PCB. If the cap structure has the same or similar footprint (+ / -10%) in the x / y direction as the substrate, the measurement cell body is preferably attached to the cap structure. 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. Still, the substrate shows a portion that is not covered by the measurement cell body or by the cap structure for implementing the circuit, but the measurement cell body can be attached to the cap structure.

[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 in particular be made of one or more of the following materials: sintered metal, ceramic, polymer. The membrane also advantageously acts as a decoupling element between the measurement volume and the surroundings of the measurement cell. Thus, it preferably damps the movement of gas molecules through the membrane, so that pressure fluctuations from the surroundings, for example sound waves, are damped when propagating into the measurement volume, and pressure fluctuations in the measurement volume are largely kept inside.

[0056] In case there are other recesses in the substrate and / or cap structures forming two caps as developed above, i.e. in case the other cap volume is implemented next to the cap volume, the aperture is preferably implemented as a through hole in the substrate terminating in the other cap volume. In that 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 surroundings and the measurement volume via the other cap volume. In a different embodiment, a further opening may be provided in the cover or cap structure serving for gas exchange between the surroundings and the measurement volume, this further opening being located closer to the aperture in the substrate than the first opening located closer to the pressure transducer. Thus, the (first) opening in the cover or cap structure may be mainly dedicated to acoustic coupling, taking into account 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 mainly dedicated to gas exchange, taking into account 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 sensor deployed above may also be disposed in the other cap volume, given the aperture terminating in the other cap volume, and such other sensor may thus sense one or more parameters of the surroundings, given access to the surroundings through the aperture.

[0059] However, in further embodiments, in combination with the substrate and / or cover or other recesses of the cap structure implementing the second cap volume, a further cap volume separated from the other cap volumes can be provided. The aperture terminates in the further cap volume. Thus, the gas exchange between the surroundings and the measurement volume is provided by the further cap volume, thereby not affecting the measurement by the pressure transducer in the other cap volume. And / or the path that the gas exchanges between the surroundings 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 recess. 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 sits 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 on the aperture in the substrate, but alternatively may be arranged on the cap / cover / cap structure covering the further opening or may be integrated in the cover / cap / cap structure, for example by etching.

[0062] In summary, for substrates providing apertures for gas exchange between the surroundings and the measurement volume, three scenarios are preferred: 1) the apertures terminate directly within the measurement volume, 2) the apertures terminate within another cap volume such that the gas reaches the measurement volume through an opening in the cover or cap structure via the other cap volume, and 3) the apertures terminate in a further cap volume such that the gas reaches the measurement volume from the surroundings through an opening in the cover or cap structure 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 invention, a method is provided for manufacturing an optoacoustic gas sensor device for determining a value indicative of the presence or concentration of a chemical component in a gas. A substrate is provided, preferably comprising conductor tracks for electrically interconnecting electrical components. The electrical components are mounted, preferably in an SMD process, on a first side of the substrate. The electrical components comprise at least an electromagnetic radiation source for emitting electromagnetic radiation, a pressure transducer for measuring sound waves generated by the chemical components in response to the chemical components absorbing 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 the 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 substrate form a measurement cell. The measurement cell body, substrate, and cap limit a measurement volume into which electromagnetic radiation is emitted. The measurement cell includes an aperture for gas to enter the measurement volume.

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

[0067] During 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 are later used to attach the components to the substrate.

[0068] Preferably, the attachment of the cap to the substrate comprises either gluing or soldering. In particular, soldering is preferred, most preferably with a solder alloy having a higher melting point than the solder alloys commonly used to withstand high temperatures, e.g. to allow the customer to solder the PA gas sensor device to a specified system PCB. In such a scenario, it is proposed to use a solder, in particular 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 apart from the substrate. The same is true when attaching the measuring cell body, i.e. gluing the measuring cell body to the substrate with an adhesive, where an adhesive with the above mentioned melting properties is preferred. When gluing both the cap and the measuring cell body 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 description of the drawings]

[0071] [Figure 1] FIG. 1 is a cut-away view of a photoacoustic gas sensor apparatus according to one embodiment of the present invention. [Diagram 2] FIG. 1 is a cutaway top view of a photoacoustic gas sensor apparatus according to one embodiment of the present invention. [Diagram 3] FIG. 1 is a cut-away view of a photoacoustic gas sensor apparatus according to one embodiment of the present invention. [Figure 4] FIG. 1 is a cut-away view of a photoacoustic gas sensor apparatus according to one embodiment of the present invention. [Diagram 5] FIG. 1 is a cut-away view of a photoacoustic gas sensor apparatus according to one embodiment of the present invention. [Figure 6] FIG. 1 is a cut-away view of a photoacoustic gas sensor apparatus according to one embodiment of the present invention. [Figure 7] FIG. 1 is a cut-away view of a photoacoustic gas sensor apparatus according to one embodiment of the present invention. [Figure 8] FIG. 1 is a cut-away view of a photoacoustic gas sensor apparatus according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

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

[0074] The substrate 1 has a first side 11 and a second side 12. The measuring cell body 2 is arranged 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, in this example an infrared source. Both electrical components are located inside the measurement volume 21.

[0076] The infrared source emits infrared radiation, indicated by reference 41. If the wavelength / band of the 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 chemical component of interest present in the measurement volume 21. The infrared source may be a broadband infrared emitter, emitting radiation over 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 gas component of interest. For the detection of CO2, the band is for example centered around 4.3 μm and has a typical bandwidth of 0.5 μm or less, for example 0.2 μm or 0.1 μm, so that the measurements are indeed selective for CO2.

[0077] The 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 the first side 11 of the 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 able to receive and process signals from the pressure transducer 3 and one or more other sensors, if available. The controller 8 is able to control the EM radiation source 4 and provide to the outside a PA gas sensor device signal, represented for example by a processed or at least pre-processed signal from the pressure transducer 3. For these reasons, the substrate 1 preferably comprises through vias (not shown) for connecting conductor tracks, provided for example 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 several layers of conductive tracks, such as FR4-PCB.

[0080] Thus, the controller 8 is configured to control the EM radiation source 4, for example by imposing an 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 may be up to 100 kHz, or may go down to 5 Hz. The controller 8 is further configured to receive measurements from the pressure transducer 3 and to determine a value of the chemical constituent concentration from those measurements, for example by using a predefined or resettable calibration function linking the measurements to a concentration value of the chemical constituent. The value of the chemical constituent concentration may be output via a digital interface, for example an I2C interface. When determining the value of the chemical constituent concentration, the controller 8 may also take into account one or more other measurements, if available, for example the value of temperature and / or humidity, and perform compensation as described above. In the case of CO2 as the relevant gas constituent, measurements in the range of 0 to 10'000 ppm, or 0 to 40'000 ppm, or 0 to 60'000 ppm of CO2 are possible.

[0081] The substrate 1 preferably has 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 for alignment with ambient pressure under overpressure or underpressure 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] Figure 2 shows a cutaway top view of a PA gas sensor arrangement according to an embodiment of the 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 quadrant-like parts. A pressure transducer 3 and an EM radiation source 4 are arranged in opposite quadrants. Other components 5 and a controller 8 are arranged 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 arrangement by air vents 6.

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

[0084] Furthermore, other sensors 9 are provided which are advantageously one or more of the following: temperature sensors, humidity sensors, combined temperature / humidity sensors, pressure sensors, in particular air pressure sensors, other pressure transducers, other gas sensors, for example of metal oxide type or electrochemical type. Via the temperature and / or humidity measurements, the gas concentration values ​​can be compensated, for example, for the influence of temperature and / or humidity. 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 the ambient conditions on the measurements 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 invention. In comparison with the embodiment shown in FIG. 3, not only is the controller 8 capped in the measurement cell, but also one or more other electrical components, such as the illustrated capacitor 51, as well as the electromagnetic radiation source 4. The electromagnetic radiation source 4 requires an optical window 72 in the cap 7. The optical window 72 allows the 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 located in the measurement volume 21 outside the cap volume 71. Thus, the pressure transducer 3 is enabled to detect pressure variations caused by the reaction of the gas molecules with the emitted radiation.

[0086] In this embodiment also, another sensor 9 is provided. In this example, the other sensor 9 is a humidity sensor. The humidity sensor can provide its signal representative of the humidity measured in the measurement cell to the controller 8, which can 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 mentioned above.

[0087] FIG. 5 shows a cutaway view of a PA gas sensor arrangement according to a further embodiment of the invention. Compared to the embodiment shown in FIG. 4, the other sensor 9 (again a humidity sensor) is now arranged under the cap 7, i.e. in the cap volume 71. The signal provided by the pressure transducer 3 can therefore again be compensated by humidity variations. 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 that is not arranged in the capped volume 71, or even the only component. Such an approach improves, i.e. increases, the average optical path length of the radiation 41 in 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 embodiment, the cap itself contributed significantly to the cap volume in that it provided a cavity and thus defined an interior space. In the embodiment of Fig. 6 and Fig. 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 comprising several 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 generate 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 the components are / are placed. The recess 15 is not to be understood as a through channel but as a depression, such that in the recess of the substrate 1 the substrate 1 is thinned compared to its normal thickness. In other words, the substrate 1 is a continuous substrate 1 across the recess.

[0090] The recess 15 of the substrate 1 leads along the z-axis to a surface of a first level L1 and a surface of a second level L2, which is lower than the first level L1. On the surface of this first level L1, here two components are arranged: an electromagnetic radiation source 4 and a controller 8. As can be seen in figures 6 and 7, these components 4, 8 are bond-wire connected to the surface of the first level L1 of the substrate 1, so that the first level L1 is meant to represent the level of the substrate 1 at which one of the conductive layers is structured to generate conductive paths and contact pads. The electromagnetic radiation source 4 bridges the surface of the second level L2.

[0091] The substrate 1 comprises a second recess 16. The pressure transducer 3 is arranged 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. An aperture 23 is now 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 Fig. 6, the recess 15 is closed by a planar cap 7. The other recess 16 is closed by a planar cover 6. The cap 7 supports a reflective coating 73 facing the measurement volume 21. The cover 6 supports a reflective coating 703 facing the measurement volume 21. The cap 7 and the recess in the substrate 1 define a cap volume 71. The cover 6 and the other recess in the substrate 1 define a further cap volume 701. Preferably, the cap 7 and the 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 close to the pressure transducer 3 and provides acoustic coupling between the measurement volume 21 and the pressure transducer 3. The opening 62 is close / above the aperture 23 and specifically supports gas exchange between the surroundings and the measurement volume 21.

[0094] In the embodiment of FIG. 7, the 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 a 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 a further cap volume 701. Alternative electrical connections between the components 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 invention. The configuration is similar to the embodiment shown in FIG. 6 and FIG. 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 comprises a first cavity 74 for forming the cap 7 and another cavity 704 for forming the other cap 70. The other cavity 704 is thus provided to contribute to another cap volume 701 in combination with the substrate 1 on which the cap structure 7 / 70 is placed. The cap volume 71 and the other cap volume 701 sit on the substrate 1 separated from each other by the cap structure 7 / 701, for example a base B in the cap structure 7 / 70. One or more electrical components other than the pressure transducer 3 are placed in the cap volume 71, whereas the pressure transducer 3 is placed 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 the electrical components may be present 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 contain a single cap to cap 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 from a reflective material or coated with a reflective material, such as a coating 703. Such a coating 703 can be achieved more easily 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 Fig. 8. The common cap structure 7 / 70 may be a silicon wafer with 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 located 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, providing a surface of a first and a third level L1 / L3, respectively, both at a level L1 / L3 lower than the normal level surface 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. Capacitor 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 bounded 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 disposed outside the cap volume; the controller is disposed within the cap volume; The photoacoustic gas sensor apparatus, wherein the controller is configured to process an electrical signal provided by the pressure transducer 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 of more than 70%, preferably more than 80%, more preferably more than 90%; and Preferably, the inner surface of the measurement cell body has a reflectivity of more than 70%, preferably more than 80%, more preferably more than 90%. The photoacoustic gas sensor device according to claim 1 .

3. At least one transistor, preferably a transistor of a voltage controller, is arranged within said 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 a 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 that is supplied to the other sensor via 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 disposed 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; Preferably, the cap is made from metal or metal coated plastic; Preferably, the measuring cell body is made of metal or metal-coated plastic, Preferably, the cap and the measuring cell body are made from the same material. 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 of 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; Preferably, the reflective coating is made from gold; Preferably, the controller is disposed on the substrate within the recess. 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 of the substrate and the planar cap; the cap is made of silicon; Preferably, the controller is disposed on the substrate within the recess. 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 supporting one or more of the electrical components and a second level surface 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 second 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 a further cap volume between the further 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 electromagnetic radiation source being disposed on the third level surface and bridging the fourth level surface; Preferably, 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, such that the cap volume and another cap volume are 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 fabricated from a silicon wafer bonded to the substrate; or The cap structure is made from a printed circuit board that is soldered or glued to the substrate. The photoacoustic gas sensor apparatus according to claim 14.

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

17. the aperture is disposed in the substrate; The aperture defines a periphery of the measurement volume, directly, via 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 apparatus according to claim 14.

18. 1. 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, comprising: Providing a substrate; Attaching an electrical component to a first side of the substrate, preferably by an SMD process, the electrical component comprising: an electromagnetic radiation source for emitting electromagnetic radiation; a pressure transducer for measuring sound 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 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; mounting a measurement cell body on the first side of the substrate to surround the cap and electrical components disposed outside the cap, the measurement cell body and the substrate forming a measurement cell, the measurement cell body, the substrate and the cap defining a measurement volume through which the electromagnetic radiation is emitted, the measurement cell comprising an aperture for the gas to enter the measurement volume; A method comprising:

19. creating a recess in the substrate prior to mounting the electrical component on the first side of the substrate; at least one of the electrical components other than the pressure transducer is mounted in a recess in the substrate on the first side of 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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