DE102020123199A1 - Photoacoustic sensor array - Google Patents

Abstract

In order to simplify the manufacture of photoacoustic sensor arrangements (2) for use in gas detectors (1), it is proposed that a reference gas chamber (3) to be filled with reference gas (3) be designed as a via (9) in a carrier substrate (7) and an optical window (5) and is closed off on the underside by means of an acoustic measurement value recorder (6), which can in particular be designed as an assembly. This approach enables the parallel manufacture of the sensor arrays (2) based on a wafer stack (13) which is produced by a full-wafer bond of said carrier substrate (7) with an IR-transparent substrate (8). As a result, the individual sensor arrangements (2) can be obtained as a compact stack (20) after being separated and can thus be manufactured inexpensively, in particular together with the gas detectors (1) (cf. FIG. 6).

Description

The invention relates to a photoacoustic sensor arrangement with a reference gas chamber that is closed off from the outside and has an optical window at at least one point, and with an acoustic measuring value sensor for detecting acoustic oscillations of a reference gas in the reference gas chamber. Such a photoacoustic sensor arrangement is particularly suitable for use in a photoacoustic gas detector.

The invention also relates to a method for the parallel production of photoacoustic sensor arrays. These sensor arrangements each have at least one reference gas chamber and at least one acoustic measurement value recorder.

Sensor arrangements as described above are known and are used, for example, in photoacoustic gas detectors using a subtractive measurement method for gas detection. For this purpose, an excitation light, which is typically in the infrared wavelength range, is guided through a measuring chamber of the gas detector and then through the optical window mentioned into the reference gas chamber of the sensor arrangement, where it interacts with a reference gas enclosed in the reference gas chamber on the basis of the photoacoustic effect. The molecules of the reference gas are excited to oscillate by the excitation light. If the excitation light is modulated in time, a detectable acoustic modulation (sound waves) is created in the reference gas. This modulation can be detected with the acoustic transducer.

A gas sample to be examined can now be introduced into the measuring chamber of the gas detector in order to measure the gas sample photoacoustically. In particular when this gas sample contains molecules that are the same as those in the reference gas, the excitation light is already absorbed in the measuring chamber, ie before it enters the reference gas chamber.

As a result, the excitation light that has already been absorbed can no longer generate acoustic excitations in the reference gas chamber, so that a drop in the acoustic signal in the reference gas chamber can be detected with the acoustic measurement value recorder. If, on the other hand, the measurement gas differs from the reference gas, the excitation light in the relevant wavelength ranges is transmitted unhindered through the measurement chamber, then enters the reference gas chamber and triggers the acoustic excitation of the reference gas there via light absorption. Accordingly, in this case no drop in the acoustic signal can be detected in the reference gas chamber.

In this way, when using very broadband light and a chemically very pure reference gas, it is possible to use the gas detector to detect gases in a targeted manner, ie with high selectivity, with the reference gas having to be the same as the target gas to be detected. Additional optical filters, on the other hand, are unnecessary and broadband light sources such as thermal radiators can be used.

Furthermore, numerous methods are known in the field of microsystems technology with which sensor arrangements can be produced in parallel on wafers. The wafers can be processed both in so-called batch processes (e.g. when cleaning or growing layers) and individually using single-wafer processes (such as full-wafer bonding, photolithography, etc.).

The object of the invention is to simplify a manufacturing process for a photoacoustic sensor arrangement as mentioned at the outset and at the same time to ensure high reproducibility of the metrologically relevant characteristics of the sensor arrangement and an associated gas detector.

To solve this problem, the features of claim 1 are provided according to the invention in a photoacoustic sensor arrangement. In particular, it is therefore proposed according to the invention to solve the problem in a photoacoustic sensor arrangement of the type mentioned at the outset that the measured value recorder mentioned at the outset seals off the reference gas chamber, in particular in a gas-tight manner.

The advantage here is that a very compact photoacoustic sensor arrangement can be obtained, with which a compact gas detector can be implemented. In addition, a high degree of reproducibility of the sensor arrangements can be achieved through parallel production, as will be explained in more detail below. A high gas sensitivity can also be achieved by using a reference gas.

In other words, the invention makes it unnecessary for the reference gas chamber to be closed before the measured value pickup is attached to the sensor arrangement, in order to enclose the reference gas. The measured value pick-up, which can be designed in particular as a microphone, can namely have a sealing surface provide, with which an outside opening of the reference gas chamber can be sealed gas-tight. For this purpose, it is advantageous if said opening of the reference gas chamber is designed to match the sealing surface, that is to say in particular in a suitable shape and/or smaller than the sealing surface, of the measured-value sensor. The aforesaid sealing or covering area of the measured value pickup can be a base area (footprint) of the measured value pickup, for example.

According to the invention, the object can also be achieved by further advantageous embodiments according to the dependent claims.

A preferred embodiment provides that the acoustic measuring value pickup is placed on a carrier substrate by means of surface-mount technology (SMT, surface-mounted technology), which also forms the reference gas chamber. To this end, it is preferred if the acoustic measurement value pick-up is designed as an electronic SMD (surface-mounted device) component. This approach has the advantage that the connection between the measured value recorder and the carrier substrate of the sensor arrangement can be established via soldered connections. In particular, electrical contacting of the acoustic measurement value recorder can be realized via these soldered connections.

The advantage here is that no further electrical contacting steps are required for the acoustic measurement value pickup in the production process, such as wire bonding, for example, which can typically only be carried out after the sensor arrangement has been separated. The use of SMT thus has the advantage that the entire sensor arrangement can be produced in parallel with other sensor arrangements in a wafer stack.

The connection of the acoustic measurement value pickup to the carrier substrate can be done, for example, in that the measurement value pickup has metallizations for the formation of soldered connections on its side facing the substrate. The tightness of the reference gas chamber can then be established, for example, by means of a circumferential soldered connection between the measured value sensor and the carrier substrate.

Provision can thus be made in particular for the acoustic measurement value pickup to seal the reference gas chamber in a gas-tight, preferably hermetic, manner. For this purpose, the seal can be realized by means of a circumferential soldered connection and/or by means of a gas-tight adhesive bond. Such a gas-tight adhesive can also be applied after the SMD assembly and electrical contacting.

A further configuration provides that the optical window is formed by full wafer bonding of an IR-transparent substrate, preferably a silicon wafer, to the carrier substrate. In this case, the carrier substrate can preferably be an IR-opaque substrate, particularly preferably an LTCC (low temperature cofired ceramics) substrate. In such configurations, it is preferable for a simplification of the production if the measured value pick-up is soldered onto the carrier substrate on the outside. Such a full-wafer-bonding approach enables cost-effective, parallel production of a larger number of sensor arrangements on a common substrate, in particular on the basis of a common wafer stack.

As has already been mentioned, it is of great advantage for a particularly simple manufacture if the acoustic measuring value pickup is electrically contacted, in particular exclusively, by soldered connections to metallizations which are provided on the carrier substrate. Such electrical contacting of the measured-value recorder can then preferably take place without bonding wires that are expensive to produce.

In order to enable the widest possible field of application for the sensor arrangement in a wide variety of gas detectors, it is also advantageous if the optical window for wavelengths in the infrared range (0.78 µm - 1 mm), in particular in the middle IR range (MIR = 2 µm - 25 µm), is transparent. For example, one of the main absorption bands of CO ₂ is around 4.3 µm wavelength.

For a robust measurement, it is also advantageous if the carrier substrate is non-transparent for wavelengths in the said infrared ranges, ie in particular in the MIR range. Because it is favorable here that light incident from the side, for example ambient light, can be prevented from entering the reference gas chamber, so that the photoacoustic measurement can take place free of optical interference.

A further preferred embodiment proposes that the acoustic measurement recorder has a MEMS microphone and/or is designed as a hybrid assembly. A MEMS (micro-electro-mechanical-system, microelectromechanical) microphone can be understood here to mean a microphone which is produced using microsystems technology and has components or structures with microscopic dimensions. A hybrid assembly can be understood here as an electronic assembly which, in addition to semiconductor components, also has passive components, in particular arranged on a printed circuit board.

The MEMS microphone can record the acoustic oscillations of the reference gas, for example piezoresistively or capacitively or optically.

In general, it can be provided that a sound opening of the acoustic measurement value pickup opens into the reference gas chamber. This enables a particularly simple assembly and production with good acoustic coupling of the measured value recorder at the same time.

In order to achieve the stated object, the features of the subordinate claim directed to a use are also provided according to the invention when a photoacoustic sensor arrangement is used in a photoacoustic gas detector. In particular, it is therefore proposed according to the invention that a photoacoustic sensor arrangement according to the invention, in particular as described above and/or according to one of the claims directed to a photoacoustic sensor arrangement, is used in a photoacoustic gas detector. In this case, it can be provided in particular that the optical window of the sensor arrangement described at the outset delimits a measuring chamber for receiving a gas sample to be measured photoacoustically.

In such a case, it is preferable for particularly simple production if the sensor arrangement, in particular the optical window, is connected to a substrate for this purpose by means of full-wafer bonding, which substrate also forms the measuring chamber. The substrate mentioned here, which also forms the measuring chamber, can be, for example, a further carrier substrate (carrier substrate II), in particular a further LTCC substrate. Said substrate can, however, also be a functional substrate which provides IR light sources for the gas detector (in particular if the carrier substrate II is dispensed with—cf. the description of a preferred exemplary embodiment further below). Furthermore, the connection can be direct, for example to the carrier substrate, or indirect, for example to the functional substrate, namely when a carrier substrate is arranged between the functional substrate and the optical window/sensor arrangement.

• - Ausbildung von Vias in einem Trägersubstrat;
• - Ausbildung von Metallisierungen auf dem Trägersubstrat;
• - Full-Wafer-Bonden des Trägersubstrats mit einem IR-transparenten Substrat zu einem Wafer-Stapel;
• - SMT-Löten von akustischen Messwertaufnehmern auf die Metallisierungen, insbesondere zum gasdichten Abschließen von jeweiligen Referenzgaskammern;
• - Vereinzeln der Sensoranordnungen aus dem Wafer-Stapel;

In order to achieve the stated object, the features of the independent method claim are also provided according to the invention. In particular, it is therefore proposed according to the invention to solve the task in a method of the type described above that the method comprises the following method steps:

• - formation of vias in a carrier substrate;
• - Formation of metallization on the carrier substrate;
• - Full-wafer bonding of the carrier substrate with an IR-transparent substrate to form a wafer stack;
• - SMT soldering of acoustic measurement sensors on the metallization, in particular for gas-tight sealing of the respective reference gas chambers;
• - Separating the sensor arrays from the wafer stack;

The individual process steps can, but do not necessarily have to, be processed in the order given above; in particular, the order of individual steps can vary. The last two method steps in particular can also be carried out in reverse order, depending on the design of the sensor arrangement or an associated gas detector.

In the case of the above-described parallel production of a multiplicity of sensor arrangements based on a bonded wafer stack, each of the photoacoustic sensor arrangements has a reference gas chamber and an acoustic measurement value recorder. The sensor arrangements can therefore be based on at least one common substrate. As a result, the sensor arrangements can initially be processed together and only separated and/or complemented at the end of production.

As will be described in more detail below, the sensor arrays can first be connected to other substrates as a wafer stack in order to produce complex gas detectors in parallel. These can then be separated, in which case each of the gas detectors can then have at least one of the sensor arrangements. The wafer stack from which the sensor arrangements are finally separated can also include other substrates.

The sensor arrangements can also be configured in particular as described above or according to one of the claims directed to a sensor arrangement.

An IR-transparent substrate can be understood here to mean a substrate which transmits wavelengths in the IR range. A via can be understood here in particular as a recess leading through the substrate, the geometric dimensions of which also determine the reference gas chamber. The vias can thus form the reference gas chambers of the individual sensor arrangements.

The necessary introduction of the reference gas into the individual reference gas chambers can take place before the reference gas chambers are sealed, for example before the SMD soldering of the acoustic measuring transducers or before the application of a gas-tight adhesive to seal off the reference gas chamber; Alternatively, the reference gas can also be introduced after the reference gas chambers have been produced and sealed, for example through (micro) gas channels created in the carrier substrate. In other words, preferably microstructured gas supply channels can be formed in the carrier substrate, which is connected to the IR-transparent substrate (8), for introducing a reference gas into a respective reference gas chamber in the manufacturing process. The target gas can therefore either be introduced into the reference gas chamber through special gas channels, in which case the gas channels can be formed in the carrier substrate, for example, and can be closed after filling. Alternatively, the installation of the acoustic measurement value recorder and thus the closing of the reference gas chambers can also take place in an atmosphere which is the same as the target gas, so that the reference gas chamber is automatically filled with the target gas.

Various technologies can be used to produce the bonded wafer stack, such as anodic bonding or adhesive bonding, which are particularly suitable when using LTCC substrates.

As previously mentioned, gas-tight seals can be formed between the support substrate and the acoustic transducer(s) respectively to seal off the reference gas chambers. This sealing can preferably be effected by means of soldered connections and/or by means of gas-tight adhesive bonds.

A gas-tight seal can be understood here in particular as a seal that meets typical industrial requirements, such as those required for hermetic encapsulations or seals of MEMS bolometers (in particular in a vacuum atmosphere), microresonators or absolute pressure sensors. Such requirements are typically classified by standards such as U.S. MIL-STD-883 Test Method 1014, which can also be used for the gas sensors discussed here.

A particularly preferred embodiment of the method provides that said wafer stack is connected to a further functional substrate, which can carry a functional layer, via full-wafer bonding, in order thereby to enable gas detectors to be manufactured in parallel. In this case, the functional substrate, more precisely the functional layer, can provide optical IR light sources, preferably in the form of electrically heated micromembranes. These IR light sources can then (respectively) serve to illuminate a respective reference gas chamber.

Furthermore, it is particularly advantageous for compact dimensions of the gas detectors if the functional substrate also forms a measurement chamber for receiving a gas sample to be photoacoustically measured with the sensor arrangement.

The micromembranes mentioned can preferably have a thickness of less than 100 μm, preferably less than 10 μm in the center, in order to achieve low thermal inertia and thus rapid modulation of the emitted IR radiation. Filigree structures of this type can be produced particularly easily and reproducibly at the wafer level, which shows further advantages of the approach according to the invention for the parallel production of gas detectors.

The invention will now be described in more detail using an exemplary embodiment, but is not limited to this exemplary embodiment.

Further exemplary embodiments result from combining the features of individual or multiple claims with one another and/or with individual or multiple features of the respective exemplary embodiment.

In particular, developments of the invention can thus be obtained from the following description of a preferred exemplary embodiment in conjunction with the general description, the claims and the drawings.

• 1 Komponenten einer erfindungsgemäßen Sensoranordnung, noch vor dem Zusammenbau,
• 2 eine Querschnittsansicht durch eine erfindungsgemäße Sensoranordnung nach dem Wafer-Bonden und Verschließen der Referenzgaskammer,
• 3 weitere Komponenten eines erfindungsgemäßen Gasdetektors auf Basis der Sensoranordnung,
• 4 die obere Hälfte eines Wafer-Stapels zur Ausbildung von Gasdetektoren vor Verbindung mit einem weiteren Wafer-Stapel, welcher Sensoranordnungen aufweist,
• 5 das Ergebnis eines full-wafer-bonds zwischen zwei Wafer-Stapeln zur Ausbildung von Gasdetektoren,
• 6 einen erfindungsgemäßen Gasdetektor mit einer erfindungsgemäßen Sensoranordnung nach dem Vereinzeln aus dem in 5 gezeigten Wafer-Stapel.

It shows:

• 1 Components of a sensor arrangement according to the invention, before assembly,
• 2 a cross-sectional view through a sensor arrangement according to the invention after wafer bonding and sealing of the reference gas chamber,
• 3 further components of a gas detector according to the invention based on the sensor arrangement,
• 4 the upper half of a wafer stack for the formation of gas detectors prior to connection to a further wafer stack which has sensor arrangements,
• 5 the result of a full wafer bond between two wafer stacks to form gas detectors,
• 6 a gas detector according to the invention with a sensor arrangement according to the invention after separation from the in 5 wafer stack shown.

the 1 shows a carrier substrate 7 in the form of an LTCC wafer, in which vias 9 are formed as continuous recesses and metallizations 10 have been applied to the underside of the carrier substrate 7 . The view of 1 1 shows only a section of the entire carrier substrate 7 (as indicated by the dashed lines).

To produce a photoacoustic sensor arrangement 2 according to the invention (cf. 6 ) as part of a gas detector 1, the carrier substrate 7 is first connected to the in 1 IR-transparent substrate 8 shown, which is present in the form of a silicon wafer, is assembled into a wafer stack 13 by means of full-wafer bonding. Acoustic measurement sensors 6, each in the form of a hybrid assembly, are then soldered onto the metallization 10 provided on the underside of the carrier substrate 7 in an SMT process, as shown in FIG 2 to see.

The measured value recorder 6 has its own metallizations 10 that match this. The SMT soldering takes place in a reference gas atmosphere, so that the in 1 to be recognized by the via 9 formed reference gas chamber 3 is filled with a reference gas 4 after soldering.

As an alternative to this, gas supply channels can also be formed, for example by microstructuring the upper side of the carrier substrate 7, in order to introduce the reference gas through these gas supply channels into the reference gas chamber 3 only after the SMT soldering. Then the gas supply channels (not shown in 2 ) to be closed.

The soldered connection 11a formed between the carrier substrate 7 and the measured-value sensor 6 closes off the reference gas chamber 3 in a gas-tight manner. At the top, the wafer bond formed between the IR-transparent substrate 8 and the carrier substrate 7 ensures the necessary encapsulation of the reference gas chamber 3.

Thus, in the course of the manufacturing process, gas-tight seals 12 (cf. 2 ) formed between the carrier substrate 7 and a plurality of acoustic transducers 6, preferably by means of respective soldered connections 11. Because the in 2 The sensor arrangement shown is repeated several times laterally in the wafer stack 13, so that the sensor arrangements 2 are manufactured in parallel on the carrier substrate 7.

In a final singulation step, with reference to 6 As will be explained in more detail below, the individual sensor arrangements 2 produced in parallel are finally separated from the wafer stack 13, for example by means of wafer saws.

In the exemplary manufacturing process shown in the figures, the wafer stack 13 is first attached to a further functional substrate 17, more precisely to a second wafer stack 13 (cf. 4 ), connected via full wafer bonding, with this functional substrate 17, as in 6 as can be seen, provides IR light sources 15 in the form of electrically heated hotplates designed as a micromembrane 21 . Each IR light source 15 serves to illuminate a respective reference gas chamber 3 defined by the carrier substrate 7.

The functional substrate 17 of the further wafer stack 13 (cf. 4 ), on the other hand, forms respective measuring chambers 14 for receiving a gas sample 16 to be measured photoacoustically with the sensor arrangement 2 . By appropriately aligning the two wafer stacks 13 ( 2 and 4 ) when bonding to one another, it must be ensured that the measuring chambers 14 and associated light sources 15 are aligned with a respective reference gas chamber 3, as is shown in 6 is easy to see. As a result, the entire gas detector 1 is realized as a stack 20, at least after it has been separated from the (third) wafer stack 13, which is formed by connecting the first and second wafer stack 13 (cf. 5 with 2 and 4 ).

Decisive for the compact design of the in 2 The photoacoustic sensor arrangement 2 shown is the fact that the measured value recorder 6 closes off the reference gas chamber 3 so that another substrate on the underside of the reference gas chamber 3 can be dispensed with. For this purpose, the lateral inner dimensions of the reference gas chamber 3 are designed to be smaller than an outer circumference of the measured-value sensor 6. In other words, the measured-value sensor 6 protrudes laterally beyond the via 9, so that there is sufficient overlapping area for forming the peripheral soldered joint 11. As an alternative to a soldered connection 11, it is also possible to implement the necessary sealing of the reference gas chamber 3 by means of a circumferential gas-tight adhesive bond.

The acoustic measurement sensor 6, which is placed on the first carrier substrate 7 by means of surface-mount technology (SMT), more precisely soldered on the outside, thus forms the reference gas chamber 3, since it forms its bottom (cf. 2 ).

The sensor 6 itself is designed as an electronic SMD component. To be more precise, it is a hybrid assembly that, in addition to a MEMS microphone, has an associated ASIC on a separate circuit board and other passive electrical components. The assembly itself is hermetically encapsulated by means of a metallic housing, but has a sound opening 22 on its underside, which opens into the reference gas chamber 3 . This allows the MEMS microphone to detect the acoustic vibrations with high sensitivity, which are generated by the in 6 IR light source 15 shown in the reference gas 4 are excited.

The design of the acoustic measurement value pickup 6 as an SMD component not only has the advantage that the reference gas chamber 3 is hermetically sealed by means of the soldered connection 11 . Rather, the SMT process can also be used to make electrical contact between the acoustic measurement value pickup 6 and the carrier substrate 7 very easily, which offers the metallic contact pads required for this. Elaborate wire bonding for contacting the measured value pickup 6 can thus be completely dispensed with. In other words, the acoustic measurement value pickup 6 is only possible through additional soldered connections 28 (cf. in 2 ) electrically contacted to corresponding metallizations 10 on the carrier substrate 7, with no bonding wires. These additional soldered connections 28 lie within the outer peripheral soldered connection 11 (cf. 1 and 2 ), which is formed between the housing of the sensor 6 and the carrier substrate 7 and ensures the gas-tight seal.

In the 2 and 6 It is also easy to see that an optical window 5, which is transparent to the IR radiation generated by the light source 15, is formed by full-wafer bonding of the IR-transparent substrate 8 to the carrier substrate 7. More precisely, all optical windows of the sensor arrangements 2 of the wafer stack 13 (cf. 2 ) produced simultaneously and in parallel by full-wafer bonding. The carrier substrate 7, on the other hand, is non-transparent for wavelengths in the infrared range, so that the reference gas 4 cannot be excited from the outside, for example by ambient light.

As the 6 shows, the sensor arrangement 2 described above is finally used in a photoacoustic gas detector 1 . In this case, the structure is chosen such that the optical window 5 of the sensor arrangement 2 delimits a measuring chamber 14 which is provided for receiving a gas sample 16 to be measured photoacoustically. This is achieved in that the entire wafer stack 13, more precisely the uppermost wafer 8, is connected to a substrate 17, which also forms the measuring chamber 14, by means of full-wafer bonding. As in 6 As can be seen, a further carrier substrate 7 can be provided for this purpose, which establishes the connection between the substrate 17 and the substrate 8 .

In summary, in order to simplify the production of photoacoustic sensor arrangements 2 for use in gas detectors 1, it is proposed that a reference gas chamber 3 to be filled with reference gas 3 is designed as a via 9 in a carrier substrate 7 and has an optical window 5 on the top and an acoustic measuring value pickup 6 on the bottom , which can be designed in particular as an assembly, is completed. This approach enables the parallel manufacture of such sensor arrangements 2 on the basis of a wafer stack 13 which is produced by a full-wafer bond of said carrier substrate 7 with an IR-transparent substrate 8. As a result, the individual sensor arrangements 2 can be obtained as a compact stack 20 after being separated and can thus be manufactured inexpensively, in particular together with the gas detectors 1 (cf. 6 ).

Complete photoacoustic gas detectors 1 (cf. 6 ) produce, which realize a complete integration of a monolithic light source 15, a measurement chamber 14, a reference gas chamber 3 and an acoustic transducer 6 in a wafer stack 20.

As can be seen from the schematically illustrated gas molecules 16 in 6 As can be seen, such a gas detector 1 has a measuring chamber 14 which is open to the outside, with a gas sample 16 to be measured flowing into the measuring chamber 14 via a gas supply line 25 . As indicated by the curved arrows, the gas sample 16 can be irradiated with a light source 15 of the gas detector 1 . To be more precise, the radiation emitted by the light source 15 in the direction of the gas sample 16 first passes through the measuring chamber 14 and then through an optical window 5 and thus reaches a reference gas chamber 3 of the gas detector 1 filled with a reference gas 4. Since the radiation is modulated in time, the reference gas 4 excited to acoustic vibrations, which can be detected by means of an acoustic transducer 6 of the gas detector 1.

the inside 6 The gas detector 1 shown is distinguished precisely by the fact that the light source 15 is formed in a functional substrate 17 and that this functional substrate 17 is connected to an IR-transparent substrate 8 by means of full-wafer bonding, which forms the optical window 5 of the reference gas chamber 3 . In other words, in the gas detector 1 the light source 15, the measurement chamber 14, the optical window 5, the reference gas chamber 3 and the acoustic measurement value recorder 6 are arranged in a stack. In this case, the functional substrate 17, the IR-transparent substrate 8 and a carrier substrate 7 forming the reference gas chamber 3 are connected to form a compact stack 20. The connections between the substrates 8 and 7, between 7 and 17, and between 8 and the further carrier substrate 7 located above them were each made via wafer bonding (thus the entire in 6 shown wafer stack 13 produced by means of a total of three wafer bonds).

In 6 it can also be seen that the light source 15 is formed monolithically in the functional substrate 17 . This is because the functional substrate 17 , a silicon wafer, has a grown silicon nitride layer as the functional layer 18 . This functional layer 18 is thus produced monolithically and forms the light source 15, which is implemented as an IR emitter.

As in 5 is indicated, the light source 15 is designed as a micromembrane 21 made of silicon nitride and is suspended on the functional substrate 17 in a self-supporting manner. On the upper side of the silicon nitride layer 18 in the area of the micromembrane 21, an electrically conductive platinum thin layer is applied in the form of a meander (cf. 3 ). By means of this meander, the micromembrane 21 can be heated electrically and thus used as a modulable hotplate with which IR radiation that can be modulated at high frequency can be generated. The advantage of using a micromembrane 21 is its low thermal capacity, which allows rapid modulation.

As the 3 disclosed, the functional substrate 17 has a cavity 27 which also forms the measurement chamber 14 . This cavity 27 has been introduced monolithically into the silicon substrate 17 together with other cavities 27 in the functional substrate 17 by means of a batch etching process. In this case, the silicon nitride layer 18 was used precisely as an etch stop level.

A comparison of 3 and 6 also shows that a further carrier substrate 7 is provided to form the measuring chamber 14 . This further carrier substrate 7 is arranged between the functional substrate 17 and the IR-transparent substrate 8, which can be formed by a silicon wafer, for example (cf. 5 ) and forms the measuring chamber 14 with the help of a via 9 . By using the additional carrier substrate 7, the absorption path of the IR radiation from the light source 15 when passing through the measuring chamber 14 is increased by the thickness of this carrier substrate 7, as a result of which a stronger measurement signal can be generated.

How based on 6 and the 4 As can be seen, the functional substrate 17, the IR-transparent substrate 8 and the carrier substrate 7 forming the reference gas chamber 3 are connected to form a stack 20. the inside 6 The gas detector 1 shown was produced in a batch process by separating it using wafer saws from the in 5 still completely shown wafer stack 13 produced.

This production results in the in 6 shown vertical stacking. This structure is characterized, among other things, by the fact that the light source 15, the measuring chamber 14, the optical window 5, the reference gas chamber 3 and the acoustic measuring value sensor 6 are arranged along an (imaginary) line which runs transversely to the plane of the functional substrate 17 and in 6 just the direction of the IR radiation 26 corresponds.

Based on 2 and 6 it is also easy to see that the acoustic measurement value recorder 6 closes off the reference gas chamber 3 . For this purpose, the sensor 6 has been soldered onto the carrier substrate 7 using an SMT process (cf. 1 and 2 ), with a circumferential soldered joint 11 ensuring gas-tight sealing of the lower opening of the reference gas chamber 3.

The reference gas chamber 3 is closed at the top by the substrate 8, which in turn is bonded to the carrier substrate 7 in a gas-tight manner (cf. 2 ). The SMT process can also be used to create additional electrical contacts (cf. the additional soldered connection 28 in 2 ) formed between the carrier substrate 7 and the sensor 6, so that the sensor 6 can be electrically controlled and read out via the LTCC carrier substrate 7.

In the 1 , 2 and 6 It can also be seen that the acoustic measurement value pickup 6 , which is designed as a complex hybrid assembly and is shown only schematically in the drawings, has a sound opening 22 on its underside, which opens into the reference gas chamber 3 .

Based on the figures can also be an inventive batch production of photo acoustic gas detectors 1 like that of 6 , which combines a monolithic light source 15, a measuring chamber 14, a reference gas chamber 3 and an acoustic measuring value pickup 6 in a compact stack 20, clearly and by way of example. the 1 until 5 show only a section of the respective substrate or the respective wafer stack 13, as indicated by the dashed lines.

In this method, for example, all gas detectors 1 of a respective batch share the four substrates 17, 7 (two carrier substrates) and 8 shown, but can be equipped with different measurement value sensors 6, or, for example, the respective reference gas chambers 3 can also be filled with different reference gases 4. For efficient production, however, it is preferable if all reference gas chambers 3 of a wafer stack 13 are filled with the same reference gas 4 at the same time.

Starting from the in 3 The functional substrate 17 shown as a silicon wafer, which is shown, is firstly grown on the wafer 17 with a functional layer 18 made of silicon nitride. The functional substrate 17 is then wet-chemically etched on the back in order to form a large number of cavities 27 which later form the measuring chambers 14 of the respective gas detectors 1 .

Next, a multiplicity of light sources 15 are formed in the functional substrate 17 . This is done first by applying and photolithographically structuring the in 3 indicated electrically conductive platinum thin layer 19 on top of the functional layer 18. The layer 19 is structured as a meander in the area of the micromembranes 21 created by the wet-chemical etching, with which the micromembranes 21 can be electrically heated. In addition, the layer 19 allows temperature sensors to be implemented in the direct vicinity of or on the micromembrane 21 .

Another photolithography then takes place on the upper side of the functional substrate 17, followed by a dry etching step from the front side. The self-supporting suspensions of the micromembranes 21 are defined by the dry etching step, where—as in 3 indicated - passage openings can also be formed through which the gas sample to be measured 16 can flow into the measuring chamber 14 from the outside and can also flow out again.

In order to increase the absorption length of the measuring chamber 14, the functional substrate 17 finished in this way is combined with the further carrier substrate 7 to form the 4 shown wafer stack 13 bonded. In this case, the substrates 17 and 7 are aligned with one another in such a way that the respective vias 9 formed in the carrier substrate 7 are aligned with the cavities 27 of the functional substrate 17 in order in this way to define the measurement chambers 14 .

Next, the in 2 shown wafer stack 13 is produced, which realizes a plurality of photoacoustic sensor arrays 2, each having a reference gas chamber 3. For this purpose, the carrier substrate 7, which has a large number of vias 9 for forming the reference gas chambers 3, is first bonded to the IR-transparent substrate 8 in a full-wafer bonding process. The IR-transparent substrate 8 then forms the optical windows 5 of the respective reference gas chambers 3 and closes them off at the top (cf. 5 ).

The reference gas chambers 3 can then already be filled with the reference gas 4 and sealed off in a gas-tight manner by soldering on a respective measuring value recorder 6 (to metal coatings 10 applied for this purpose on the underside of the carrier substrate 7). However, this step can preferably only be carried out after the bonding of the in 2 shown wafer stack 13 with the in 4 shown wafer stack 13 take place.

The third full wafer bond thus attaches the IR-transparent substrate 8 to the underside of the in 4 shown carrier substrate 7 bonded, whereby the in 5 shown wafer stack 13 arises. If the carrier substrate is 7 in 4 omitted (for example if the functional substrate 17 is sufficiently thick), the substrate 8 can also be bonded directly to the functional substrate 17, more precisely to its underside. In both cases, however, the respective reference gas chambers 3 are aligned with a respective measurement chamber 14 before bonding, as in FIG 5 illustrated to achieve the desired compact stack arrangement.

Eventually the in 5 The wafer stack 13 shown is separated into the individual gas detectors 1, which can be done using typical processes such as wafer sawing or laser cutting, with the delicate micromembranes 21 and the reference gas chambers 3 being able to be protected during separation by photoresist or UV adhesive films.

As already mentioned, preferably before separating the in 5 wafer stack 13 shown, one acoustic measurement value pickup 6 can be electrically and mechanically connected to the carrier substrate 7 by means of an SMT process, as in 1 and 2 illustrated. This is because the respective reference gas chamber 3 with the respective measured value recorder 6 can be sealed off in a gas-tight manner.

Thus, in addition to the cost-effective and reproducible production of miniaturized photoacoustic gas detectors 1, it is proposed to use a wafer stack 13 consisting of a functional substrate 17, which forms a light source 15 for emitting radiation, a substrate 8 that is transparent to the radiation of light source 15, and at least one carrier substrate 7 respective wafer bonding to form. In the wafer stack 13 produced in this way (cf. 6 ) the transparent substrate 8 separates a measurement chamber 14 for receiving a gas sample 16 to be transilluminated with the radiation from a reference gas chamber 3 which is filled with a reference gas 4 . Furthermore, a measured value sensor 6 is provided as part of the stack arrangement, with which acoustic oscillations of the reference gas 4, which can be generated with the radiation of the light source 15, can be read out ( 6 ) and which can be controlled and read out preferably by means of SMT connections to the carrier substrate 7 . In this case, the wafer stack 13 has a photoacoustic sensor arrangement 2 which, for its part, is designed as a stack of two substrates 7 and 8 .

Reference List

1

Photoacoustic gas detector

2

Photoacoustic sensor array

3

reference gas chamber

4

reference gas

5

optical window

6

acoustic transducer

7

carrier substrate

8th

IR transparent substrate

9

Via

10

metallization

11

solder connection

12

seal

13

wafer stack

14

measuring chamber

15

light source

16

gas sample (to be measured)

17

functional substrate

18

Functional layer (e.g. silicon nitride)

19

electrically conductive thin film (e.g. Pt)

20

stack

21

micromembrane

22

sound port

23

cover

24

reflector layer

25

gas supply

26

IR radiation

27

cavity

28

Additional solder connection

Claims (13)

Photoacoustic sensor arrangement (2) with a reference gas chamber (3) which is closed off from the outside and has an optical window (5) at at least one point, and with an acoustic measurement sensor (6) for detecting acoustic oscillations of a reference gas (4) in the reference gas chamber ( 3), characterized in that the measured value recorder (6) closes off the reference gas chamber (3). Photoacoustic sensor arrangement (2) after claim 1 , wherein the acoustic measurement value recorder (6) is placed on a carrier substrate (7) by means of surface-mount technology (SMT), which forms the reference gas chamber (3), preferably wherein the acoustic measurement value recorder (6) is designed as an electronic SMD component . Photoacoustic sensor arrangement (2) after claim 1 or 2 , wherein the acoustic measurement sensor (6) seals the reference gas chamber (3) gas-tight, preferably hermetically, in particular the sealing being realized by means of a circumferential soldered connection (11) and/or by means of a gas-tight adhesive bond. Photoacoustic sensor arrangement (2) according to one of the preceding claims, wherein the optical window (5) is formed by full-wafer bonding of an IR-transparent substrate (8), preferably a silicon wafer, to the carrier substrate (7), in particular wherein the carrier substrate (7) is an IR-opaque substrate, preferably an LTCC substrate, preferably with the sensor (6) being soldered to the outside of the carrier substrate (7). Photoacoustic sensor arrangement (2) according to one of the preceding claims, wherein the acoustic measurement value pickup (6) is electrically contacted, in particular exclusively, by soldered connections (11) to metallizations (10) on the carrier substrate (7), preferably without bonding wires. Photoacoustic sensor arrangement (2) according to one of the preceding claims, wherein the optical window (5) is transparent for wavelengths in the infrared range (0.78 µm - 1 mm), in particular in the MIR range (2 µm - 25 pm). Photoacoustic sensor arrangement (2) after claim 6 , wherein the carrier substrate (7) for wavelengths in the infrared range, in particular in the MIR range, is opaque. Photoacoustic sensor arrangement (2) according to one of the preceding claims, in which the acoustic measurement value pickup (6) has a MEMS microphone and/or is designed as a hybrid assembly, preferably in which a sound opening (22) of the acoustic measurement value pickup (6) extends into the reference gas chamber (3 ) ends. Use of a photoacoustic sensor arrangement (2) according to one of the preceding claims in a photoacoustic gas detector (1), characterized in that the optical window (5) of the sensor arrangement (2) has a measuring chamber (14) for receiving a gas sample (16) to be measured photoacoustically. limited, preferably wherein for this purpose the sensor arrangement (2), in particular the optical window (5), is connected by means of full-wafer bonding to a substrate (17, 7), which also forms the measuring chamber (14). Method for the parallel production of photoacoustic sensor arrangements (2), each having a reference gas chamber (3) and an acoustic measuring value sensor (6) and in particular according to one of Claims 1 until 8th can be formed, characterized by the following method steps: - formation of vias (9) in a carrier substrate (7); - formation of metallizations (10) on the carrier substrate (7); - Full-wafer bonding of the carrier substrate (7) with an IR-transparent substrate (8) to form a wafer stack (13); - SMT soldering of acoustic measurement sensors (6) on the metallization (10), in particular for gas-tight sealing of respective reference gas chambers (3); - Separating the sensor arrays (2) from the wafer stack (13); procedure according to claim 10 , wherein gas-tight seals (12) are each formed between the carrier substrate (7) and the acoustic measurement sensors (6), preferably by means of soldered connections (11) and/or by means of gas-tight adhesive bonds. procedure according to claim 10 or 11 , wherein in the carrier substrate (7), which is connected to the IR-transparent substrate (8), preferably microstructured, gas supply channels are formed for each introduction of a reference gas (4) in each of the reference gas chambers (3). Method according to one of Claims 10 until 12 , wherein the wafer stack (13) is connected to a further functional substrate (17) via full-wafer bonding, - in particular wherein the functional substrate (17) carries a functional layer which optical IR light sources (15), preferably in the form of electrically heated micromembranes (21), which are used to illuminate one of the reference gas chambers (3), - particularly preferably the functional substrate (17) has a measuring chamber (14) for receiving a gas sample (16 ) trains with.

Images