Method for confining reference gases in MEMS cells

Description

[0001] In a first aspect, the invention relates to a method for manufacturing a hermetically sealed, gas-filled reference chamber. The gas used to fill the reference chamber is introduced through an opening in a separate coating chamber only after the wafers forming the reference chamber have been bonded. MEMS components are installed in the reference chamber.

[0002] In a further aspect, the invention relates to a photoacoustic gas sensor comprising such a reference chamber within which a MEMS sensor is present. Background and state of the art

[0003] Photoacoustic spectroscopy (PAS) is a physical investigation method based on the photoacoustic effect and has a wide range of applications.

[0004] One application of PAS is the detection of extremely fine concentrations of gases. This involves intensity-modulated infrared radiation with frequencies in the absorption spectrum of the molecule to be detected in the gas. If this molecule is present in the beam path, modulated absorption occurs, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. These heating and cooling processes lead to expansion and contraction of the gas, causing sound waves with the modulation frequency. These can then be measured by sound detectors such as microphones or flow sensors.

[0005] One example is the detection of CO 2 , which plays a role in research and air conditioning technology. Also relevant are applications where not only toxic but also explosive or corrosive gases, such as ammonia NH 3 , must be detected.

[0006] Ammonia (NH 3 ) in particular already has numerous applications in industry and nature.

[0007] For example, ammonia is used as a refrigerant in cold storage facilities, breweries, and slaughterhouses. It can also be used in large-scale refrigeration systems. In the mid-20th century, ammonia was largely replaced by chlorofluorocarbons (CFCs), which are now banned. Nevertheless, ammonia has always maintained its important role in industrial refrigeration due to its good thermodynamic properties.

[0008] Despite the fact that ammonia is often synthetically produced for use in refrigeration processes, it is considered a natural refrigerant. NH3 is usually produced during the decomposition of organic, nitrogen-containing materials. Today, it is used as a refrigerant in large refrigeration systems for cold storage facilities and in the air conditioning of airports, office buildings, production halls, and sports facilities.

[0009] However, ammonia is corrosive, especially to copper materials. Pipelines in systems using ammonia as a refrigerant must therefore be made of steel. Since ammonia is also toxic and slightly flammable, special safety regulations are required for the construction, operation, and maintenance of these systems.

[0010] In concentrated form, ammonia vapors can cause eye and respiratory irritation; in higher concentrations, they can damage mucous membranes and lungs, and in the worst case, lead to death. Furthermore, ammonia is classified as a water pollutant. It is highly soluble in water, so penetration into the soil and the resulting damage to groundwater must be avoided at all costs.

[0011] In addition, ammonia is explosive (ignition temperature at approx. 630°C).

[0012] For the reasons above, it is important to detect a possible ammonia leak at an early stage.

[0013] Due to its high sensitivity, PAS offers an excellent opportunity for continuously monitoring ammonia concentrations. Several devices for this purpose are known from the state of the art.

[0014] Peng et al. (2016) discloses a sensor that can detect ammonia in high-temperature environments. It uses a quantum cascade laser that irradiates a cylindrical measurement chamber approximately 1.8 m long. Furthermore, the measurement chamber itself is heated, and a flow is generated within it by introducing air from the surrounding environment. Additional compounds such as CH4 (methane) and 1% NH3 / Ar (ammonia / argon) are introduced to control the flow within the chamber. At the end of the resonator is a detector that measures the signal strength of the quantum cascade laser. If a higher proportion of ammonia is present in the air introduced into the measurement chamber, it absorbs the quantum cascade laser beams. This also results in a weaker signal being registered at the detector. With a measurement chamber approximately 1.8 m long, the device is designed for industrial use and cannot be used flexibly.In addition, additional components are added, such as thermocouples that measure the temperature distribution, BaF2 (barium fluoride)-coated windows located at the ends of the resonator, and a heat jacket for the resonator. This makes the device more complex to build and complicates the design.

[0015] In Schilt et al. (2004), a CO2 laser (carbon dioxide laser) irradiates the photoacoustic measuring cell. This comprises a cylindrical resonator and two buffer volumes, with the two buffer volumes acting as acoustic filters. A microphone is located at the end of the resonator. There is also a semiconductor detector that measures the intensity of the laser beams. The measuring principle is the same as in Peng et al. (2016). If there are particles or molecules of ammonia in the beam path of the CO2 laser, part of the laser radiation is absorbed. If there is no ammonia in the resonator, the measured pressure signal is maximum. A disadvantage arises from the openings located at the two buffer volumes. This could in principle allow additional gases to diffuse into the resonator, which could distort the measurement signal.

[0016] Bonilla-Manrique et al. (2019) describes a resonant gas cell that also comprises two buffers and a cylindrical resonator, with the cylindrical resonator connecting the two buffers. The resonator is 88 mm long, and the two buffers are each 44 mm long, resulting in a total device dimension of 176 mm. A microphone and a thin diaphragm are placed on the resonator to act as acoustic detectors. These are mounted centrally on the resonator, with the microphone inserted through the resonator and the diaphragm attached externally on the opposite side. The photoacoustic effect also causes the diaphragm to vibrate. In the experimental setup, both the gas inlet, which is released through one of the two buffers, and the diaphragm are irradiated with laser beams. The measuring cell is already filled with 5000 ppm NH3.The device developed by Bonilla-Manrique et al. (2019) also has macroscopic dimensions. Furthermore, there is no guarantee that the measurement signal will remain unaffected by the entry of other molecules, such as CO2 (carbon dioxide) and H2O (water), which could absorb the laser beams.

[0017] In light of the state of the art, there is therefore an interest in alternative devices and / or methods to reliably hermetically seal potentially toxic, corrosive and / or explosive gases, which have further application possibilities due to miniaturization, in a chamber or a measuring cell.

[0018] US Pat. No. 6,124,145 discloses a method for filling two or more wafers with gas, particularly CO2. A first wafer, in which a cavity is created, is placed into a bonding chamber filled with the gas that is also to be contained within the wafer. The second wafer is then bonded to the first wafer within the bonding chamber, creating a chamber or cell consisting of two wafers containing gas. However, this method is not readily suitable for the confinement of corrosive or explosive gases, such as ammonia.

[0019] Firstly, bonding requires high temperatures. This prevents gases that are flammable or even explosive at the corresponding bonding temperatures, such as ammonia, from being enclosed in the chamber. Otherwise, they would damage the wafers or the bonding chamber itself, and in the worst case, destroy it. Furthermore, this process does not easily allow the creation of chambers from wafers containing electronic circuits or MEMS components, such as sensors, and then filling them with corrosive gases, such as ammonia. This is because the corrosive properties of ammonia would damage the corresponding electronic circuit or MEMS component. In addition, the bonding process itself requires a certain amount of time, and the conditions (temperatures, etc.) favor the reactivity of corrosive gases.

[0020] Further methods and devices are known in the state of the art which aim to transfer a gas into a miniaturized system.

[0021] For example, US 2018 / 0339900 A1 discloses a method for manufacturing a MEMS component containing at least two sensors. The first sensor is preferably a yaw rate sensor, while the second sensor is preferably an acceleration sensor. The two sensors are formed in a wafer stack, but in separate regions. The object of US 2018 / 0339900 A1 is to prevent gases such as H2 (hydrogen) or light noble gases, such as helium and neon, from diffusing through oxide layers and other layers at the temperatures encountered. For example, H2 can diffuse from the acceleration sensor into the yaw rate sensor. To solve this problem, the manufacturing process first provides a MEMS wafer and a cap wafer, forms MEMS structures in the MEMS wafer for the two sensors, and then hermetically seals the MEMS wafer with the cap wafer.After sealing the two wafers, a first access hole is formed, then an initial pressure is applied to the cavity of the first sensor, and the access hole is finally sealed. An analogous procedure is used for the second sensor. This is intended to enable two different internal pressures to exist in the cavities. In particular, it is also planned that after bonding, H2 is removed from the cavity of the second sensor (acceleration sensor), for example, to introduce oxygen, ozone, and / or a defined plasma. The access holes are formed using a laser. The access holes are also sealed using a laser.

[0022] US 2014 / 0038364 A1 discloses a method for encapsulating a microelectronic device. The microelectronic device is located on a first substrate and bonded to a second substrate in a bonding chamber. The second substrate has a cavity, so that after bonding, the microelectronic device is located within the wafer stack. The gases to be injected into the cavity are noble gases. The second substrate has a region that is permeable to the noble gases. A layer that is impermeable to the noble gas to be introduced into the cavity is coated onto the second substrate, and openings are subsequently formed thereon. To hermetically enclose the noble gas, a further layer is applied, which is impermeable to the injected helium. A possibility for introducing gases that are more reactive than the described noble gases is not disclosed.

[0023] US 2020 / 0057031 A1 discloses a detector module for a photoacoustic gas sensor. The detector module is constructed such that a first substrate and a second substrate are bonded together, and a recess can be hermetically filled with a reference gas. The reference gas can be introduced into the recess in a reference gas atmosphere during bonding or after bonding by forming a through-hole in the first or second substrate, which is then sealed. The reference gas is selected from a group comprising CO2, NOx, H2O, O2, N2, CH4, or alcohol. Filling the recess with a gas within a coating chamber is not described.

[0024] US 2021 / 0055207 A1 also discloses a detector cell for a photoacoustic gas sensor. A gas atmosphere of a gas to be enclosed is created and can be enclosed in a cavity during bonding.

[0025] US 2019 / 0353157 A1 discloses a miniature transport device that can be used as a fluid controller and / or for performing pressurization. The miniature transport device and a miniature valve device can be assembled, after which a gas can be introduced through an inlet. A piezoelectric actuator allows the gas to flow through a plurality of pressure chambers and continuously in a transport direction. The gas can be released by a user determining the amount of gas themselves or when the ambient pressure increases.

[0026] US 2007 / 0295456 A1 describes a material for bonding wafers. The bonding material is characterized by the fact that, in addition to its insulating adhesive properties, it also contains electrically conductive particles. Furthermore, it is explained that air in a bonding chamber can be exchanged for a gas to be encapsulated, particularly for the operation of a MEMS device. The gases disclosed therein are not explosive, but rather inert.

[0027] US 2003 / 0183916 A1 discloses a method for packaging a MEMS device. In one embodiment, a sealing process can be performed in a controlled environment so that the cavity contains the desired ambient gas at a desired pressure. To this end, it is described that the openings are located far enough from the MEMS device so as not to damage it. A covering or sealing component (patch) is also disclosed to close the openings.

[0028] US 2020 / 0198964 A1 deals with an encapsulation method for mounting a MEMS device within a wafer stack. Here, the openings can be sealed by a sealing layer (get seal layer) can be sealed, whereby this should be made possible by the coating itself.

[0029] A safe and reliable manufacturing process for introducing corrosive and / or explosive gases into a MEMS cell and hermetically sealing them without damaging MEMS components or electronic circuits within the cell is currently unknown. In particular, the state of the art does not provide a reliable and safe introduction of corrosive and / or explosive gases during the production of photoacoustic gas sensors. Therefore, there is a need to make the introduction of gases into MEMS cells more efficient and safer for users. Object of the invention

[0030] The object of the invention is to provide a device and a method for its production that eliminate the disadvantages of the prior art. In particular, one object of the invention is to enable PAS (photoacoustic spectroscopy) of corrosive and / or explosive gases using a reliable and safe device that is characterized by a compact design and hermetic containment of the corrosive and / or explosive gases. Furthermore, the production process should be simple, cost-effective, and suitable for mass production, enabling applications in a wide variety of fields. Summary of the invention

[0031] The object of the invention is achieved by the features of the independent claims. Preferred embodiments of the invention are described in the dependent claims.

[0032] In a first aspect, the invention relates to a manufacturing method for a photoacoustic gas sensor comprising a gas-filled reference chamber within which a MEMS component and optionally an electronic circuit are present, comprising the following steps: a) Providing a first and a second wafer, wherein at least the first wafer and / or the second wafer has a cavity, and wherein the MEMS component is present on the first and / or second wafer, wherein the MEMS component is a MEMS sensor, b) Bonding the first wafer to the second wafer within a bonding chamber to form a volume fillable with reference gas, wherein an opening remains at a contact surface of the two wafers after bonding, or an opening is introduced into the first and / or second wafer before or after bonding, c) Flooding a reference gas into the reference chamber via the opening within a coating system, d) Closing the opening of the reference chamber within the coating system, e) Providing a modulatable emitter, f) Arranging the reference chamber filled with the reference gas and the modulatable emitter, wherein the reference chamber is located in the beam path of the emitter,so that the emitter can excite the reference gas in the reference chamber by means of modulatably emitted radiation to form sound pressure waves, which can be detected by the MEMS sensor.

[0033] The method according to the invention differs from US Patent No. 6,124,145 in that the gas to be introduced into the reference chamber is not present in the bonding chamber during the bonding process. This prior art method significantly limits the choice of gases to be enclosed in the reference chamber. This is due to the high temperatures that occur during the bonding process. Depending on the bonding process used, a temperature range from 250°C to 1000°C is possible. Gases that are flammable or even explosive at these temperatures and the corresponding pressure conditions could not be introduced using the method of US Patent No. 6,124,145. If this were done, the entire test facility, in particular the bonding chamber, could be damaged or even destroyed.In particular, the method according to the invention can be used to introduce ammonia into the reference chamber, the ignition temperature of which is 630°C.

[0034] The method according to the invention also allows gases with a corrosive effect to be introduced into and enclosed in the reference chamber. This is particularly crucial if the reference chamber contains MEMS components and / or electronic circuits. With the method known from US Patent No. 6,124,145, it is not possible to enclose corrosive gases in the reference chamber without damaging any MEMS components and / or electronic circuits that may be present. If the method known from US Patent No. 6,124,145 were used, the MEMS components and / or electronic circuits would be surrounded by a large amount of corrosive gas, as the corrosive gas is flooded throughout the entire bonding chamber.

[0035] In particular, in US Patent No. 6,124,145, the bonding process only takes place after the bonding chamber is filled with the gas to be enclosed. Because the bonding process requires a certain amount of time, the MEMS component and / or electronic circuit would be surrounded by the corrosive gas during this time, which is required for bonding. At the same time, oxygen present in the bonding chamber could also promote the reaction. Thus, the MEMS component and / or electronic circuit could be damaged by the high amount of corrosive gas and the duration of exposure to the corrosive gas required for bonding.

[0036] The method according to the invention also differs from US 2014 / 0038364 A1, in particular through the introduction of the reference gas within a coating chamber. In contrast, in US 2014 / 0028264 A1 the gas is introduced into the cavity in the bonding chamber. However, this considerably limits the selection of gases to be introduced, since temperatures occur during bonding at which explosive and / or reactive gases can trigger an undesirable reaction, such as an explosion. For this reason, in US 2014 / 0028264 A1 only noble gases, which are inert, are used as gases for introduction into the cavity of the MEMS cell. According to the invention, however, the introduction of the reference gas is carried out in a coating chamber. This advantageously also makes it possible to introduce gases which - unlike helium, for example - are flammable or even explosive in a bonding chamber at the corresponding bonding temperatures, such as e.g. B. Ammonia.Consequently, the method according to the invention achieves a significant improvement over the prior art, since it enables explosive and / or corrosive gases to be safely introduced into the reference chamber and hermetically sealed.

[0037] Furthermore, in the method according to the invention, the reference gas can advantageously be filled directly into the volume of the reference chamber by flooding the reference gas in a coating chamber, in which the reference gas is then immediately sealed. Thus, the reference gas can preferably penetrate into the volume of the reference chamber by diffusion - a naturally occurring physical process due to Brownian molecular motion. In contrast, in US 2018 / 0339900 A1, an exchange process takes place for filling a volume in which one of the two sensors described therein is located. In US 2018 / 033990 A1, it is explained that, for example, H2 is removed from the cavity of the second sensor and then filled with oxygen, ozone and / or a defined plasma, whereby an exchange of gases takes place.In particular, the gases mentioned in US 2018 / 033990 A1 can at least partially penetrate the surface of the MEMS element to react with the hydrogen there or, if adsorbed by the surface, reduce the discharge energy of the hydrogen dissolved in the solid. Such a binding reaction or exchange is avoided in the process according to the invention.

[0038] Furthermore, in US 2018 / 033990 A1, the sensor cavity is not filled within a coating system, so the coating system could be used to seal the openings of a reference chamber. Instead, the opening is sealed using a laser.

[0039] The disadvantages of the prior art are circumvented or eliminated by the method according to the invention. In the method according to the invention, a MEMS cell is provided with an opening, which is then filled with the gas to be enclosed. First, the reference chamber is formed using two wafers. This is achieved through a bonding process within a bonding chamber. An opening in the volume of the reference chamber can remain after bonding. However, the opening can also be introduced into one of the two wafers before or after bonding. After the bonding process, which forms the MEMS cell, it is transferred to a coating system.

[0040] The coating system is then flooded with the gas to be enclosed in the MEMS cell, allowing it to diffuse into the MEMS cell through the opening. In the next step, the opening is closed and hermetically sealed. In particular, the process according to the invention can be used to enclose ammonia, which has explosive and corrosive properties, in the MEMS cell. However, other gases with similar aggravating properties can also be enclosed in the MEMS cell.

[0041] Advantageously, the method according to the invention allows, in particular, corrosive and / or explosive gases to be enclosed in the MEMS cell without damaging or destroying the MEMS components and / or electronic circuits present therein. This is due to the fact that the introduction of the gas into the MEMS cell is more controlled. Firstly, the flooding takes place downstream of the bonding process in a coating system, and secondly, the opening can be designed with a smaller dimension or additional sealing mechanisms can be provided. Thus, the gas can be introduced into a MEMS cell in a highly controlled manner with regard to the timing, concentration, and duration.

[0042] In addition, the coating system, which is flooded with the gas to be enclosed, has less reactive conditions, in particular a lower temperature, than the one inside the bonding chamber.

[0043] For the purposes of the invention, a MEMS cell preferably refers to a device comprising two or more wafers and within which a MEMS component is located. The term "MEMS cell" can be understood as a generic term. A MEMS cell can comprise one or more openings that can be closed. A MEMS cell can also be used as a reference chamber. The term "MEMS cell" frequently appears in connection with MEMS-based technologies and is familiar to those skilled in the art.

[0044] For the purposes of the invention, the reference chamber refers to a cavity formed by two or more wafers and comprising a volume formed by the two or more wafers. In other words, two or more wafers form a volume and the entirety of the wafers and the resulting volume is preferably encompassed by the reference chamber. The reference chamber has one or more openings which are provided for filling the volume of the reference chamber. In order to retain or enclose the gas within the reference chamber, the opening is designed to be resealable. Advantageously, this prevents gas exchange with the environment. The gas to be introduced into the reference chamber or which has already been introduced is also referred to as the reference gas.

[0045] A MEMS component and / or an electronic circuit is located within the reference chamber. For the purposes of the invention, a MEMS component refers to a component or part based on MEMS technology. MEMS stands for microelectromechanical system, i.e., a microsystem, which achieves a compact (micrometer-scale) design combined with outstanding functionality and ever-lower manufacturing costs. A MEMS component can, for example, be a MEMS sensor or a MEMS actuator. Many MEMS components are known in the prior art. Advantageously, the method according to the invention allows a wide variety of MEMS components to be introduced into a reference chamber and filled with corrosive and / or explosive gases without damaging them.

[0046] For the purposes of the invention, an electronic circuit refers to a combination of individual electrical or electromechanical elements in a functional arrangement. The electronic circuit preferably allows data or electrical signals to be sent, received, and / or processed.

[0047] The MEMS components are often arranged on a substrate together with an electronic circuit for control and / or evaluation, and are in contact with it via electrical connections, for example, through wire bonds and / or conductor tracks laid in the substrate. The substrate acts primarily as a carrier and can also implement electrical functions, e.g., providing electrical connections for the individual components.

[0048] Preferred electronic circuits include, without limitation, an integrated circuit (IC), an application-specific integrated circuit (ASIC), a programmable logic device (PLD), a field-programmable gate array (FPGA), a microprocessor, a microcomputer, a programmable logic controller, and / or another electronic, preferably programmable, circuit.

[0049] First, two wafers are provided, wherein at least the first wafer and / or the second wafer has a cavity and wherein a MEMS component and / or an electronic circuit is present on the first and / or the second wafer.

[0050] For the purposes of the invention, a cavity preferably refers to a notch or depression in a wafer. Advantageously, the presence of one or more cavities on the first and / or second wafer can result in a suitable volume within the reference chamber by bonding the two wafers. The reference chamber is preferably formed by the cavities of the first and / or second wafer after bonding both wafers.

[0051] A wafer can, for example, refer to a circular or square disc with a thickness in the millimeter or submillimeter range. Wafers are typically manufactured from monocrystalline or polycrystalline (semiconductor) blanks, so-called ingots, and generally serve as substrates for coatings or components, in particular MEMS components and / or electronic circuits. The use of the term "substrate" for a wafer is also known in the prior art, with the term "substrate" preferably referring to the material to be treated. For the purposes of the invention, the terms "wafer" and "substrate" can be used synonymously.

[0052] In a preferred embodiment, the two wafers comprise materials selected from the group consisting of monocrystalline silicon, polysilicon, silicon dioxide, silicon carbide, silicon germanium, silicon nitride, nitride, germanium, carbon, gallium arsenide, gallium nitride, indium phosphide and / or glass.

[0053] These materials are particularly easy and cost-effective to process in semiconductor and / or microsystem technology and are also well-suited for mass production. These materials are also particularly suitable for doping and / or coating to achieve the desired electrical, thermal, and / or optical properties in specific areas. The aforementioned materials offer a wide range of advantages due to the usability of standardized manufacturing techniques, which are also particularly suitable for the integration of additional components, such as electronic circuits.

[0054] Preferably, the MEMS component and / or the electronic circuit is located within the cavity of one of the two wafers.

[0055] According to the invention, the two wafers are bonded together to form a reference chamber. According to the invention, the bonding takes place within a bonding chamber. For the purposes of the invention, a bonding chamber refers to a device in which wafers are placed for bonding to one another.

[0056] Wafer bonding describes a process step in semiconductor and microsystem technology in which two wafers or discs, e.g. made of silicon, quartz, glass and / or the aforementioned preferred substrate materials, are bonded together.

[0057] Various methods can preferably be used for bonding. These are preferably also referred to as bonding processes or bonding methods within the meaning of the invention. Preferred bonding processes include direct bonding, anodic bonding, bonding methods with intermediate layers, glass frit bonding, adhesive bonding, and / or selective bonding.

[0058] In direct bonding, particularly of silicon wafers, hydrophilic and hydrophobic surfaces of the wafer are preferably brought into contact at high temperatures. Preferably, one wafer is pressed centrally against the other, advantageously creating a first contact point. This mechanical connection in the contact area is preferably based on hydrogen bonds and / or van der Waals interactions. The contact area thus connected is preferably extended to the remaining wafer surface(s) by successively removing any spacers initially present between these surfaces. Process temperatures are preferably between 1000°C and 1200°C, and a pressure of, for example, 10 megapascals (MPa) to 25 MPa is exerted on the wafers. Direct bonding can preferably be used to join two silicon wafers and / or silicon dioxide wafers.

[0059] In anodic bonding, a glass with an increased Na+ ion concentration (preferably positively charged sodium ions) is used, which is preferably brought into contact with a silicon wafer. An electrical voltage is applied, which is particularly configured to create a negative polarity on the glass. This preferably results, and in particular with the aid of an increased process temperature, in the sodium ions (Na+<) diffusing to the electrode, whereby a space charge zone preferably forms at the interface, which increases the electric field and creates Si-O-Si bonds. These bonds preferably expand successively to cover the entire connection area between glass and silicon. This allows glass and silicon wafers, in particular, to be bonded to one another. With appropriate adaptation of the process, bonding of two silicon layers and / or a silicon-metal layer with a glass is also possible.Anodic bonding can preferably take place at temperatures of approximately 400°C, but it can also preferably take place at "low temperature" temperatures of approximately 180°C, whereby the materials to be bonded are preferably protected. Preferably, various of the aforementioned materials can also be bonded.

[0060] Bonding processes with so-called intermediate layers between the wafers to be bonded can also be used, such as so-called eutectic bonding, which is preferably based on a connection using a eutectic alloy as an intermediate layer, e.g., Si-Au (silicon-gold) or Ge-Al (germanium-aluminum). A eutectic alloy is preferably an alloy whose components are mixed in such a ratio that the entire alloy becomes liquid or solid at a certain temperature. Eutectic bonding can be used, for example, to join two silicon wafers. However, other of the aforementioned materials can also preferably be joined.

[0061] Glass frit bonding is also preferably based on the use of an intermediate layer between the wafers to be joined, with the bond being formed in particular by melting glass solders / glass frits. Glass solder preferably comprises a glass with a low softening temperature, e.g., approximately 400°C. Glass frit preferably comprises superficially molten glass powder, the glass grains of which preferably at least partially cake or sinter together. This type of bonding can preferably bond silicon and / or silicon dioxide wafers, but also preferably other aforementioned materials.

[0062] Adhesive bonding preferably describes the formation of a connection through an intermediate layer comprising adhesive. Adhesive bonding can preferably be used to bond various of the aforementioned materials together.

[0063] Preferably, selective bonding can be carried out by photolithography, etching and / or lift-off processes.

[0064] The reference chamber can be easily manufactured by bonding the two wafers, which are preferably pre-processed to have cavities.

[0065] Bonding structures from pre-processed wafers allows for the simple fabrication of complex structures that would be difficult to produce from a single wafer. This allows the reference chamber to be manufactured without the need for laboriously removing the raw material from the wafer interior to create the volume within the reference chamber.

[0066] In a preferred embodiment, the wafers have contact surfaces that are used for bonding the two wafers. Contact surfaces preferably comprise regions of the wafer that are provided or coated with a material required for bonding. The materials at the contact surfaces of the two wafers are preferably connected or bonded to each other to create the reference chamber. Such a material can also be referred to as bonding material in the context of the invention.

[0067] In a preferred embodiment, both wafers are bonded in such a way that, after bonding, an opening remains at a contact surface of the two wafers. The opening preferably results from the fact that no bonding takes place at the corresponding contact surface. The opening is preferably located in a lateral region of the reference chamber. Advantageously, a reference gas can be efficiently introduced into the reference chamber through the preferred opening in the lateral region.

[0068] For the purposes of the invention, the opening designates an inlet into the volume of the reference chamber. According to the invention, the reference gas can be introduced into the volume of the reference chamber through or via the opening and can preferably be hermetically sealed therein. In particular, the reference gas to be introduced can diffuse directly into the volume of the reference chamber through the opening. Diffusion is known as the passive movement of particles of the reference gas to be introduced along a concentration gradient, which in this case exists between the coating chamber and the volume of the reference chamber at the beginning of the flooding of the reference gas into the coating chamber. Diffusion leads to an equalization of the concentrations, which is based on Brownian molecular motion. Since diffusion is a passive transport process, advantageously no additional energy is required.In particular, no chemical reaction or exchange is required to introduce the gas into the reference chamber—unlike in US 2018 / 0339900 A1. This advantageously enables a particularly process-efficient introduction of the reference gas into the reference chamber within a coating system.

[0069] In one embodiment, an opening is already present prior to bonding. This is preferably done via an etching process starting from an outer side of the first or second wafer. This allows all contact surfaces to be used for bonding, so that the opening remains on the first or second wafer, which was created prior to the bonding process and leads into the volume of the reference chamber.

[0070] In a further preferred embodiment, the opening is formed after bonding the two wafers, starting from the outside of the first or second wafer. In this embodiment, too, all contact surfaces could advantageously be used for bonding. The opening is preferably created after bonding via an etching process.

[0071] According to the invention, the reference chamber resulting after the bonding process is removed from the bonding chamber and placed within a coating system.

[0072] For the purposes of the invention, a coating system preferably refers to a device that produces and processes thin layers of different materials. For the purposes of the invention, a thin layer or thin film preferably refers to a layer of a solid material in the micrometer or nanometer range.

[0073] According to the invention, the coating system is used to fill the reference chamber with the reference gas. For the purposes of the invention, the reference gas refers to a gas that is introduced into the volume of the reference chamber and is preferably hermetically sealed or enclosed therein.

[0074] According to the invention, the coating system is first flooded with the reference gas. The reference gas enters the volume of the reference chamber through the opening. Advantageously, the temperature ranges required for bonding within the bonding chamber are not present within the coating system. Therefore, the reference gas can be a gas that would otherwise be flammable or explosive at the high temperatures present within the bonding chamber and introduced into the volume of the reference chamber. This expands the range of gases that can be introduced into the reference chamber.

[0075] Advantageously, the reference chamber can also be flooded with corrosive gases. The MEMS component and / or electronic circuit within the reference chamber is brought into contact with the corrosive gas in a controlled manner through the opening. Since the subsequent hermetic sealing step can be performed immediately afterward in the coating system, damage to the MEMS component and / or electronic circuit caused by the corrosive gas is avoided.

[0076] According to the invention, after the process step of introducing the gas into the reference chamber, the chamber is sealed within the coating system. According to the invention, after the sealing process, no further exchange with the environment of the reference chamber takes place. Thus, the introduced reference gas is contained within the chamber and can no longer leave it or enter the environment.

[0077] For the purposes of the invention, the environment preferably refers to everything located outside the reference chamber. This preferably includes the coating system and the reference gas introduced therein, which was flooded with it in the previous process step.

[0078] Advantageously, once the reference chamber is closed, the reference gas can no longer escape from the reference chamber, nor can reference gas from the environment enter the reference chamber. This method can thus ensure that a precisely defined amount of reference gas is present within the volume of the reference chamber.

[0079] The reference chamber is preferably sealed using a coating process within the coating chamber, with the opening in particular being sealed using a coating process. For this purpose, at least the opening can preferably be coated with a thin layer. A coating that extends at least along the opening, preferably substantially along the opening, has proven advantageous in that a particularly precise seal is applied. Furthermore, coating material for sealing the opening is saved, thus ensuring high cost-effectiveness through efficient material use.

[0080] However, it may be preferable to coat substantially the entire reference chamber with a thin film to ensure a hermetic seal. A coating that extends substantially along or around the entire reference chamber advantageously provides a particularly reliable hermetic seal. Thus, a coating around the entire reference chamber ensures a particularly hermetic seal on the one hand and, on the other hand, provides particularly robust protection against possible external damage. This is particularly relevant for applications in which the gas is explosive and / or corrosive and which therefore have to meet the highest safety requirements. By minimizing the risk of the introduced gas escaping, particularly precise and therefore reliable measurements are enabled, for example in photoacoustic spectroscopy.

[0081] For the purposes of the invention, a hermetic seal preferably refers to a tight seal that prevents any exchange of substances with the environment of the reference chamber. To prevent even the slightest contamination of the volume of the reference chamber, a hermetic seal is preferred. Devices such as the reference chamber are preferably made absolutely impermeable to any exchange of substances or matter by means of a hermetic seal.

[0082] For the purposes of the invention, contamination refers to an undesirable amount of substance components or mixtures that can enter the volume of the reference chamber. Contamination is preferably prevented by sealing the reference chamber.

[0083] In a preferred embodiment, the reference gas comprises corrosive and / or explosive gases, preferably methane, propane, propylene, silane, chlorosilane, aluminum triisopropoxide, hydrogen and / or oxygen, particularly preferably ammonia.

[0084] Advantageously, not only toxic, but also explosive and / or corrosive gases can be introduced into the volume of the reference chamber using the method according to the invention. This advantageously expands the range of reference gases. This represents a significant improvement over the prior art, as the selection of gases is not limited or specified to inert gases (e.g., noble gases as in US 2014 / 0038364 A1).

[0085] In the method known from US Patent No. 6,124,145, the gas to be enclosed within the MEMS cell is introduced into the bonding chamber. However, high temperatures arise during various bonding processes. Therefore, gases that are flammable or explosive at these temperatures cannot be enclosed in the reference chamber using the known method. Otherwise, hazards such as the explosion of the bonding chamber could result. This would also be disadvantageous for the user, who could be injured in the event of an explosion. The method according to the invention can also advantageously be used to enclose flammable and / or explosive gases in the reference chamber. In US 2014 / 0038364 A1, the gas (a noble gas) is also introduced into the cavity within the bonding chamber.However, in contrast to the prior art, according to the invention the gas is preferably introduced into the MEMS cell within the coating chamber, in particular by flooding the coating chamber with the reference gas, for example by passive diffusion.

[0086] Furthermore, it is desirable for various applications to install MEMS components and / or electronic circuits within the reference chamber and fill them with a reference gas, e.g., for PAS (photoacoustic spectroscopy). However, the process known from US Patent No. 6,124,145 cannot be used to enclose a corrosive gas in the reference chamber in this case. This is because the bonding chamber is flooded with the gas to be enclosed. Therefore, the MEMS component and / or electronic circuit would initially be exposed to a large amount of the corrosive gas, which could lead to damage. The time the MEMS component and / or electronic circuit would be exposed to the corrosive gas also plays a role. The bonding of the wafers itself also takes some time. Until the bonding process is complete, the MEMS component and / or electronic circuit would be exposed to the corrosive gas.During the bonding process, the corrosive gas could therefore damage or even destroy the MEMS component and / or the electronic circuit.

[0087] The method according to the invention circumvents or eliminates the disadvantages of the prior art. Thus, the method according to the invention advantageously allows explosive and / or corrosive gases to be introduced into the reference chamber and hermetically sealed.

[0088] For the purposes of the invention, flammable or combustible preferably refers to the property of a substance or mixture having a low flash point. The flash point of a substance preferably refers to the lowest temperature at which an ignitable mixture can form over a substance. Gases or mixtures thereof that have an explosion range in air at 20°C and a standard pressure of 101.3 kPa (kilopascals) are preferably considered flammable.

[0089] For the purposes of the invention, an explosive gas is a gas or mixture that, when exposed to sufficient energy, e.g., due to a high temperature, undergoes a specific chemical reaction, which can release a high proportion of thermal energy. This results in a violent expansion effect that can cause significant damage. Improper handling of explosive substances or gases poses a risk to life.

[0090] For the purposes of the invention, a corrosive gas preferably refers to a gas that enters into a chemical reaction with the reference chamber and / or its components, such as the MEMS component and / or the electronic circuit, thereby causing a measurable change. This process is preferably referred to as corrosion for the purposes of the invention. Corrosion can impair the function of the reference chamber or its components.

[0091] Advantageously, the method according to the invention preferably allows explosive and / or corrosive reference gases to be introduced into the reference chamber. However, any other reference gas can also be introduced using the method according to the invention.

[0092] In a preferred embodiment, in order to adjust a partial pressure of the reference gas within the reference chamber, an inert gas, preferably nitrogen, is additionally introduced into the reference chamber via the opening.

[0093] For the purposes of the invention, partial pressure refers to the partial pressure of an individual component of a gas mixture. The partial pressure corresponds to the pressure that the individual gas component would exert if present alone in the respective volume. The total pressure is the additive result of the partial pressures, i.e., the sum of all partial pressures equals the total pressure.

[0094] Advantageously, by regulating the partial pressure of the reference gas, the quantity or concentration of reference gas in the reference chamber can be precisely specified. This is advantageous for certain applications of the reference chamber, e.g., as a sensor for PAS. In these applications, the desired reference concentration or sensitivity can be specified by adjusting the concentration or quantity of the reference gas. This allows the quantity of introduced gas to be optimized in a targeted and precise manner.

[0095] For the purposes of the invention, an inert gas is preferably referred to as an unreactive gas. The gas participates only slightly or not at all in chemical reactions. With regard to PAS applications, the inert gas should also exhibit different absorption characteristics in the relevant excitation range. Examples of inert gases include gases such as nitrogen or noble gases such as helium, neon, argon, krypton, xenon, or gaseous molecular compounds such as sulfur hexafluoride.

[0096] The above-mentioned advantages of the invention are based on the fact that an opening remains on a contact surface of the two wafers after bonding, or an opening is introduced into the first and / or second wafer before or after bonding

[0097] In a preferred embodiment, the first and second wafers have contact surfaces that serve to bond the first to the second wafer, wherein a region of contact surfaces is not bonded to form the opening.

[0098] This preferably creates an opening in the lateral area of the reference chamber. This advantageously allows the reference gas to penetrate well into the reference chamber within the coating system.

[0099] Advantageously, with this embodiment, no further process steps are required to create an opening in the reference chamber. The opening is preferably created during the bonding process. Because no further process steps are required, additional material and structuring are also eliminated. Manufacturing costs and time are reduced. This advantageously results in greater process efficiency.

[0100] In a preferred embodiment, an opening remains after bonding at a contact surface of the two wafers, wherein the opening has a cross section of 1 µm 2< to 1000 µm 2< , preferably 1 µm 2< to 100 µm 2< and a length of 1 µm to 1000 µm, preferably 10 µm to 500 µm.

[0101] Advantageously, the dimensions of the opening in this preferred embodiment result in a diffusion-inhibiting effect. Diffusion, in the sense of the invention, refers to the phenomenon whereby concentration differences are equalized without external influence. Initially, when the coating system is flooded with the reference gas, there is little to no gas within the volume of the reference chamber. Over time, the reference gas diffuses through the opening into the volume of the reference chamber. This is a passive physical process due to Brownian molecular motion. The flooding process can be promoted by introducing the reference gas into the coating system at increased pressure and / or concentration.

[0102] The preferred dimensioning as a substantially elongated opening results in a substantially one-dimensional diffusion dynamics, in which after diffusing through the opening into the volume, the reference gas does not leave it or only leaves it over longer time constants.

[0103] The diffusion-inhibiting effect can be used, for example, to bridge intermediate processing times. For example, it may be preferable to first flood the reference chamber in the coating system with a reference gas, then exchange the reference gas for a process gas within the chamber, and close the opening using a coating process. Due to the diffusion-inhibiting effect, the reference gas does not escape during the gas exchange or the coating process.

[0104] By appropriately dimensioning the opening, it is possible to advantageously adjust the time period within which the reference gas can diffuse into the reference chamber and the time period during which diffusion out is reliably prevented. The above-mentioned preferred dimensions represent a good compromise in this regard.

[0105] In a preferred embodiment, before or after bonding the first wafer to the second wafer, the opening is formed from an outer side to an inner side of the first wafer or the second wafer, preferably by means of an etching process.

[0106] Advantageously, by introducing the opening before or after the process, the position and / or dimensions of the opening can be precisely selected. For example, the positioning can be determined depending on a MEMS component and / or an electrical circuit in the reference chamber to avoid interference. For example, it may be preferable for the opening to be structured centrally on the outside of the first or second wafer, starting from the outside and moving toward the inside of the reference chamber.

[0107] For the purposes of the invention, an etching process preferably comprises dry etching, wet chemical etching and / or plasma etching, in particular reactive ion etching, reactive ion deep etching (Bosch process).

[0108] An etching process typically refers to the removal of material from a surface. This removal can manifest itself in the form of depressions that leave cavities on the wafer.

[0109] In semiconductor technology and microsystems engineering, dry etching refers to a group of abrasive microstructuring processes that are not based on wet chemical reactions (such as wet chemical etching or chemical-mechanical polishing). Material removal occurs either through accelerated particles or with the aid of plasma-activated gases. Thus, depending on the process, both chemical and physical effects are utilized.

[0110] Dry etching processes can be divided into three groups. First, physical dry etching processes are based on material removal by bombardment with particles. Second, chemical dry etching processes are based on a chemical reaction of a gas, usually plasma-activated. The third group, physico-chemical dry etching processes, encompasses processes that utilize both mechanisms and are thus able to minimize the disadvantages of the first two groups.

[0111] In wet chemical etching, an etch-resistant mask is transferred into the wafer through a chemical removal process.

[0112] Plasma etching is a plasma-assisted dry etching process that removes material. Plasma etching distinguishes between etching removal due to a chemical reaction and physical removal of the surface due to ion bombardment.

[0113] In chemical plasma etching, material is removed through a chemical reaction. Therefore, it is generally isotropic and, due to its chemical nature, also very material-selective. Physical plasma etching, also called plasma-assisted ion etching, is a physical process. This process can result in a certain preferred direction in the etching attack, which is why the processes may exhibit anisotropy in material removal. In physical plasma etching, non-reactive ions are generated in the plasma. An applied electric field accelerates these ions toward a surface, thus removing portions of the surface. This process is typically used to remove the natural oxide on silicon wafers.

[0114] Reactive ion etching (English reactive ion etching,RIE is an ion-assisted reactive etching process. Due to the high controllability of the etching behavior, RIE is a method for producing topographic structures for micro- and nanosystem technology. The process allows for both isotropic (direction-independent) and anisotropic etching through chemical-physical removal. Etching is performed by charged particles (ions) generated in a gas plasma. Appropriate masking (e.g., created by photolithography) of the surface determines the shape of the structures.

[0115] Reactive ion deep etching (English deep reactive ion etching,DRIE is a further development of reactive ion etching (RIE) and a highly anisotropic dry etching process for the production of microstructures in wafers with an aspect ratio (ratio of depth to width) of up to 50:1, whereby structure depths of several hundred micrometers can be achieved. The DRIE process is a two-stage, alternating dry etching process in which etching and passivation steps alternate. The goal is to etch as anisotropically as possible, meaning directionally dependent, perpendicular to the wafer surface. This method can be used to etch very narrow trenches, for example.

[0116] The aforementioned etching processes are known to those skilled in the art. Depending on the desired opening and / or the wafer provided, advantageous methods can be selected to ensure efficient implementation.

[0117] In a preferred embodiment, a valve is provided at the end of the opening of the first or second wafer. The valve is preferably located in the volume of the reference chamber after bonding the first wafer to the second wafer at the end of the opening, starting from the outside of the first wafer or the second wafer. The end of the opening preferably designates the transition from the opening in the first or second wafer to the (reference) volume of the reference chamber.

[0118] A valve at the position can be used to regulate the diffusion of the reference gas, which floods the coating chamber, into the volume of the reference chamber.

[0119] The installation of a valve advantageously enables automatic closure of the reference gas. In particular, the escape of reference gas that has already diffused into the volume can be prevented without restricting the dynamics of the flooding.

[0120] In contrast to a diffusion-inhibiting effect, for example through an elongated opening with an essentially one-dimensional diffusion dynamic, diffusion from the reference chamber can be blocked more specifically by means of a valve, while diffusion into the reference chamber can occur quickly and unhindered.

[0121] A valve thus enables efficient flooding, allowing, for example, the reference gas within the chamber to be exchanged for a process gas and the opening to be closed using a coating process. The provision of a valve also prevents the reference gas from escaping during this time.

[0122] In addition, the provision of a valve can advantageously enable more precise regulation of the amount and / or concentration of the reference gas to be introduced into the reference chamber.

[0123] A valve preferably refers to a component that serves to control the direction or shut off a flow of fluids. A valve can preferably be formed, for example, by a flexible closure—for example, based on a thin-film structure—which allows unidirectional diffusion of the reference gas into the reaction chamber during flooding, while preventing diffusion out of the diffusion chamber after flooding.

[0124] Preferably, before bonding and before forming the opening in the first or second wafer, a material layer—preferably to form a flexible closure—can be applied to the opposite side of a wafer from which an opening is introduced (e.g., etched). Preferably, the material layer can be used to structure a valve in the subsequent process. In this case, the opening in the first or second wafer can be formed first, and then the material layer can be structured into a valve. A reversed sequence is also possible.

[0125] In a preferred embodiment, the material layer intended to form the valve is structured prior to bonding. Structures formed may include contact pads, conductive traces, alignments, corners, edges, recesses, depressions, and / or holes.

[0126] Preferably, the material layer can initially be applied to the entire surface of one side of the wafer. Structuring can, in particular, restrict the material layer to an area around the opening, so that the existing area of the material layer at the end of the opening can act as a valve.

[0127] Preferably, the opening in the wafer is formed on the opposite layer of material to the valve patterned layer using an etching process. A person skilled in the art can select suitable etching processes for this purpose to ensure that the valve is not damaged during the process.

[0128] This is preferably followed by bonding the first wafer, comprising an opening with a valve, to the second wafer. Bonding is performed such that the valve is located at the end of the opening and within the volume of the reference chamber, thus enabling the described functionality.

[0129] In a preferred embodiment, after bonding the first wafer to the second wafer, the reference chamber in the coating system is flooded with the reference gas, wherein the gas enters the volume of the reference chamber via the opening and via the valve.

[0130] In a preferred embodiment, the valve (or the material layer to be structured) comprises a soft metal. For the purposes of the invention, soft metals are preferably defined as metals that have a lower hardness than iron (e.g., in a Brinell hardness test according to DIN EN ISO 6506-1 to 4). Preferred soft metals are non-ferrous metals selected from the group consisting of lead, gold, indium, copper, platinum, silver, zinc, tin, or their compounds, particularly preferably aluminum or its compounds.

[0131] Advantageously, soft metal can be precisely structured using simple means, making the process time- and cost-effective. Furthermore, soft metals are suitable for the inventive use as a valve due to their flexibility.

[0132] In addition to the choice of material, the dimensioning of the material layer also plays a role.

[0133] Preferably, the material layer for structuring the valve, or the valve itself, is designed as a thin-film structure. Thin-film structure preferably means a layer thickness of less than 100 µm, preferably less than 10 µm.

[0134] In particular, by using a thin film structure, preferably comprising soft metal, good flexibility of the valve can be ensured in order to regulate the diffusion process accordingly.

[0135] The reference gas penetrates the opening in the first or second wafer to the valve, where it exerts pressure against the valve. Depending on the flexibility of the valve, at a certain point the pressure exerted is large enough to open the valve toward the volume of the reference chamber, allowing passage.

[0136] In a preferred embodiment, after the reference gas has been flooded into the reference chamber, the solder is melted to close the opening.

[0137] For this purpose, the solder is preferably applied near the opening of the reference chamber, preferably before bonding.

[0138] The application of a solder in microsystem technology is a well-known process and does not present any difficulties for the expert.

[0139] Preferably, the solder is placed near the opening of the reference chamber before the reference gas is flooded in the coating system and the reference gas diffuses into the volume of the reference chamber via the opening.

[0140] Advantageously, in this embodiment, the reference gas can diffuse particularly efficiently into the volume of the reference chamber before the solder melts. This embodiment eliminates the need for a diffusion-limiting effect of an elongated opening or a valve, which may exhibit a (albeit slight) residual resistance to diffusion.

[0141] Preferably, the opening of the reference chamber is closed by melting the solder. Melting the solder is a well-known process step for those skilled in the art, with the melting temperature being selected depending on the material to be melted.

[0142] When the solder melts, a portion of the solder material preferably enters the opening and seals its cross-section. The solder or molten material can fill the entire opening or only a portion, as long as the cross-section is tightly sealed. The size of the solder is adapted to the opening to be sealed. It may be preferable for the entire molten material to be used to close the opening or only a portion of it.

[0143] Applying the solder near the opening preferably refers to positioning it in a spatial proximity that ensures that the molten solder can also flow into the opening. "Nearby" can, for example, mean a distance of less than 100 µm, preferably less than 10 µm.

[0144] In a preferred embodiment, the solder comprises a fusible material selected from the group comprising lead, tin, zinc, silver, copper, their alloys and / or their compounds.

[0145] During melting, the solder is preferably brought to its melting point. The melting point is the temperature at which the solder transforms from a solid to a liquid state. The solder preferably has a melting point at which the reference gas does not react flammably and / or explosively, so that any sparks that may occur would not pose a hazard.

[0146] Advantageously, the above-mentioned preferred materials for the solder have melting temperatures below the temperature at which the preferred reference gases react flammably and / or explosively.

[0147] In a preferred embodiment, the opening is closed by a coating process within the coating system, preferably by spray coating, mist coating and / or vapor coating.

[0148] The coating process preferably ensures a well-sealed reference chamber. This advantageously prevents the reference gas from escaping the chamber, ensuring a permanent seal.

[0149] In the case of the valve at the end of the opening and in the case of the melted solder, the opening is already initially sealed. The additional application of a cover layer stabilizes the sealing for the long term and ensures a hermetic seal of the reference chamber for its entire service life.

[0150] Various coating processes can be used to apply a covering layer.

[0151] Spray coating refers, in particular, to the application of a covering layer over a large area. The covering layer is preferably pressurized prior to spraying (e.g., greater than the prevailing ambient pressure), creating fine particles / aerosols of the covering layer and / or a foam. This allows for a particularly fine coating that covers all sprayed areas, even if these areas, for example, are at an unfavorable angle to the spray direction. Even surfaces / areas that are at an angle to one another can preferably be directly coated in this way.

[0152] For this purpose, a liquid covering layer is preferably atomized under higher pressure than the ambient pressure and applied over the entire surface.

[0153] The spray coating is preferably a spray coating. The spray coating can also be a vapor deposition.

[0154] Mist coating preferably involves coating with fine droplets of the covering layer, which are finely dispersed in an atmosphere (preferably a gas). Vapor coating preferably involves applying a covering layer in vapor or gaseous form.

[0155] In a preferred embodiment, the coating system comprises a physical coating system or chemical coating system, preferably a plasma-assisted chemical coating system, a low-pressure chemical coating system and / or an epitaxial coating system.

[0156] A physical coating system preferably refers to a coating system that performs the coating by physical vapor deposition. Physical vapor deposition (English physical vapor deposition, short PVD ) ,Physical vapor deposition, also known as physical vapor deposition, refers to a group of vacuum-based coating processes or thin-film technologies. Unlike chemical vapor deposition, physical processes convert the starting material into the gas phase. The gaseous material is then directed to the wafer to be coated, where it condenses and forms the target layer.

[0157] Arc evaporation or Arc-PVD (also arc evaporation, from English arc,arc) is a coating process from the group of physical vapor deposition. In this process, an arc burns between the chamber in which the process takes place and the target, which is at a negative potential. This arc melts and evaporates the target material that is later applied to a workpiece (the wafer). The target acts as the cathode, the chamber wall of the vacuum chamber or a defined electrode as the anode. A large part (up to 90%) of the evaporated material is ionized. The material vapor (target material) spreads radially from the target, similar to thermal evaporation. Since a negative potential is also applied to the wafer, the ionized material vapor is additionally accelerated towards the substrate. The material vapor condenses on the wafer surface.

[0158] An epitaxial coating system preferably refers to a system in which an epitaxial process is applied, preferably molecular beam epitaxy. Molecular beam epitaxy (English molecular beam epitaxy, Physical vapor deposition (MBE) is a physical vapor deposition (PVD) process for producing crystalline thin films (or layer systems). Epitaxy means that the crystal structure of the growing film adapts to that of the substrate, as long as the physical properties of the two substances do not differ too significantly.

[0159] MBE requires an ultra-high vacuum to avoid contamination from residual gas atoms. During the growth process, however, the pressure rises into the high vacuum range due to effusion. The materials that will comprise the layer are heated in evaporation crucibles and reach the wafer as a directed molecular beam (without collisions with the background gas). The wafer is also heated, thus allowing for orderly growth of the layer.

[0160] Sputtering (from English to sputter = Sputtering), also called cathode sputtering, is a physical process in which atoms are released from a solid (target) by bombardment with high-energy ions (mainly noble gas ions) and pass into the gas phase.

[0161] A chemical coating system preferably refers to a coating system that applies coatings using chemical vapor deposition. In chemical vapor deposition (CVD), chemical vapor deposition,In chemical vapor deposition (CVD), a solid component is deposited from the gas phase onto the heated surface of a wafer as a result of a chemical reaction. This requires the existence of volatile compounds in the layer components that deposit the solid layer at a specific reaction temperature. The chemical vapor deposition process is characterized by at least one reaction on the surface of the workpiece to be coated. This reaction must involve at least one gaseous starting compound (educt) and at least two reaction products – at least one of which is in the solid phase. In order to promote surface reactions over competing gas-phase reactions and thus avoid the formation of solid particles, chemical vapor deposition processes are usually carried out at reduced pressure (typically 1-1000 Pa). A special feature of the process is the conformal layer deposition, which also enables, for example,finest recesses in wafers are evenly coated.

[0162] Chemical vapor deposition also includes atomic layer deposition. In atomic layer deposition (English atomic layer deposition, ALD is a significantly modified chemical vapor deposition (CVD) process with two or more cyclically performed self-limiting surface reactions. The material to be deposited is chemically bound to one or more carrier gases, the so-called precursors. These precursors are alternately fed into a reaction chamber and reacted with the wafer, whereupon the substance bound in the gas is deposited on the substrate material. The resulting layers typically have a polycrystalline or amorphous structure.

[0163] The plasma-assisted chemical coating system preferably refers to a system that uses the process of plasma-assisted or plasma-enhanced chemical vapor deposition. Plasma-assisted chemical vapor deposition (PDE) plasma-enhanced chemical vapor deposition, PECVD; also engl.plasma-assisted chemical vapor deposition, PACVD (also known as PACVD) is a special form of chemical vapor deposition (CVD) in which the chemical deposition is assisted by a plasma. The plasma can be applied directly to the wafer to be coated (direct plasma method) or in a separate chamber (remote plasma method).

[0164] While in CVD the dissociation of the gas molecules occurs through the external supply of heat and the energy released from the subsequent chemical reactions, in PECVD this task is carried out by accelerated electrons in the plasma. In addition to the radicals formed in this way, ions are also generated in the plasma, which together with the radicals cause the layer to be deposited on the wafer. The gas temperature in the plasma usually only increases by a few hundred degrees Celsius, which means that, in contrast to CVD, more temperature-sensitive materials can be coated. In the direct plasma method, a strong electric field is applied between the wafer to be coated and a counter electrode, which ignites a plasma. In the remote plasma method, the plasma is arranged so that it has no direct contact with the substrate. This offers advantages in terms ofselective excitation of individual components of a process gas mixture and reduces the possibility of plasma damage to the wafer surface by the ions.

[0165] Low-pressure CVD (LPCVD) is the process frequently used in semiconductor technology for the deposition of silicon oxide, silicon nitride and polysilicon, as well as metals.

[0166] In a preferred embodiment, a covering layer is applied within the coating system at least over a region of the opening, preferably around the entire reference chamber, wherein the material used for the covering layer is preferably a nitride, preferably a silicon nitride, silicon carbon nitride, silicon oxynitride, titanium nitride and / or tantalum nitride, an oxide, preferably a silicon oxide, aluminum oxide, silicon dioxide, titanium dioxide or tantalum oxide, or a metal, preferably an aluminum and / or a noble metal, preferably gold, platinum, iridium, palladium, osmium, silver, rhodium and / or ruthenium.

[0167] The cover layer covering the entire reference chamber advantageously provides a particularly good hermetic seal for the reference chamber. This ensures that the reference gas remains particularly well sealed within the reference chamber volume. This ensures with particularly high reliability that gas cannot escape from the reference chamber volume after sealing, especially if the cover layer was coated substantially along or around the reference chamber.

[0168] Advantageously, the reference chamber is also protected from the outside, i.e., from its surroundings, and especially after the manufacturing process, by the cover layer. This also minimizes the risk of external damage, e.g., due to fluid contamination and mechanical impact.

[0169] The above-mentioned materials are easy to process and can therefore be used to apply a covering layer. Furthermore, due to their blocking properties, the materials represent a reliable and long-term stable barrier against the escape of the reference gas and / or the ingress of foreign gases.

[0170] In a preferred embodiment, a process gas is introduced into the coating system to close the opening and to form a covering layer, wherein the process gas is introduced into the reference chamber after a reference gas has been flooded, or wherein a material for forming the covering layer is selected such that the reference gas can simultaneously serve as the process gas.

[0171] For the purposes of the invention, the process gas preferably refers to a gas that serves to ensure that the coating is applied through the cover layer in the subsequent process step. The application or adhesion of the cover layer to the reference chamber occurs via a physical and / or chemical reaction. Preferably, the process gas assists or enables the coating of the cover layer on the reference chamber during this physical and / or chemical reaction.

[0172] Particularly when flooding an additional process gas with the reference gas, a diffusion-limiting or diffusion-blocking opening is desirable. This is advantageously achieved by the method according to the invention by attaching a diffusion-blocking solder or valve or a diffusion-limiting opening in the lateral region of the reference chamber during the bonding process with the appropriate dimensions. Advantageously, the reference gas can initially preferably enter the volume of the reference chamber through these openings. This can then be followed by a gas exchange within the coating system, so that the coating process takes place after the introduction of a process gas. Both during the (at least partial) gas exchange from a reference gas to a process gas, the diffusion-limiting or diffusion-blocking opening ensures that the reference gas does not escape.For certain applications, it may also be preferable that no additional process gas is flooded into the coating system, but rather that the reference gas corresponds to the process gas. This is preferably the case for ammonia as a reference gas, which can serve as a process gas, particularly for a covering layer comprising nitride compounds.

[0173] Advantageously, in this case, no gas exchange is required in the coating system. Rather, the coating step can follow seamlessly on from flooding the reference chamber with reference gas. In preferred embodiments, ammonia is the process gas, which simultaneously functions as the reference gas, i.e. as the gas for filling the reference chamber. Advantageously, the use of ammonia as the process gas allows a covering layer comprising nitride compounds to be particularly efficient for hermetically enclosing the reference gas, since it is not necessary to introduce another gas into the coating chamber as a process gas for bonding between the covering layer and the reference chamber. With regard to maintaining the reference gas within the coating system, the desired concentration of the reference gas within the chamber can be set particularly precisely.An undesired diffusion of a process gas different from the reference gas into the chamber is inherently prevented by the skillful choice of the process parameters.

[0174] According to the invention, the MEMS component is a MEMS sensor and / or MEMS actuator and / or the electronic circuit comprises a processor, a switch, transistors, and / or transducers.

[0175] The MEMS sensor or MEMS actuator refers in particular to a sensor or actuator in the form of a microsystem (English: Micro-Electro-Mechanical System, A microsystem is, in particular, a miniaturized device, assembly, and / or component, where the components have dimensions in the micrometer range (1 µm to 1000 µm) or smaller and interact as a system. An MEMS sensor, for example, is a sound detector.

[0176] In a further embodiment, the MEMS component comprises a sound pressure detector, wherein the sound pressure detector preferably comprises a capacitively or optically readable, piezoelectric, piezoresistive and / or magnetic bar and / or a capacitive, piezoelectric, piezoresistive and / or optical microphone.

[0177] The embodiment is particularly suitable for the use of the reference chamber in the PAS, whereby the sound pressure waves can be detected by the sound pressure detector directly in the reference chamber.

[0178] A piezoelectric beam is preferably an oscillatable structure, in particular in the form of a bending beam, which comprises a piezoelectric material, e.g. in the form of an actuator.

[0179] It may be preferable for the bending beam to be passive, meaning that it is caused to vibrate by the sound pressure waves. These, in turn, generate a voltage due to the deformation of the piezoelectric material, which is based on the piezoelectric effect. The (direct) piezoelectric effect preferably describes the occurrence of an electrical voltage and / or a change in impedance in a solid body made of a corresponding material when it is elastically deformed. The voltage can be tapped, for example, through suitable contact and read out by a corresponding electronic circuit.

[0180] It may also be preferred for the bending beam to be active, meaning, in particular, that it is caused to oscillate due to the inverse piezoelectric effect. The piezoelectric effect preferably describes the deformation of a material upon application of an electrical voltage and / or an electric field, whereby a force can be exerted, in particular, by the material. The sound pressure waves can preferably cause a variation in the damping of the oscillating beam, which is measurable, e.g., by a change in the resonant frequency of the oscillating beam.

[0181] A beam passively vibrating due to sound pressure waves can preferably also be read out, e.g., using capacitive, magnetic, and / or piezoresistive methods. The idea here is also preferably that the vibration generates an electrically readable change, e.g., based on a changing magnetic flux through a resonating magnet, a changing capacitance between a vibrating and a stationary electrode, and / or a changing electrical resistance in a piezoresistive material.

[0182] A microphone preferably comprises a diaphragm mounted for vibration, which is excited into vibration by sound pressure waves, which in turn can be electrically read, similar to the beam described above. Capacitive, piezoelectric, and / or piezoresistive measurement methods for vibration design can also be used.

[0183] Preferably, an optical microphone can also be used, whereby these vibrations can be converted into an optical signal, preferably by reflection, e.g., of a laser beam on the membrane, which is read out, e.g., in an interferometric arrangement.

[0184] In a further aspect, the invention relates to a photoacoustic gas sensor as set out in the appended claims, producible by the method according to the invention.

[0185] In a preferred embodiment of the invention, the reference chamber has a height of 10 µm to 2 mm, preferably of 50 µm to 1 mm, particularly preferably of 100 µm to 500 µm.

[0186] In this way, a flat and compact design, in particular a low height of the reference chamber, can be advantageously achieved.

[0187] In a further preferred embodiment, the reference chamber has a length and / or width of 100 µm to 5 mm, preferably of 200 µm to 3 mm, particularly preferably of 500 µm to 2 mm.

[0188] These dimensions advantageously allow for a sufficient volume of reference gas to be introduced into the reference chamber. At the same time, these dimensions also advantageously allow for the installation of a MEMS component, preferably a MEMS sensor, particularly a sound pressure detector, preferably for use in PAS.

[0189] In a further aspect, the invention relates to a photoacoustic gas sensor as set out in the appended claims.

[0190] The average person skilled in the art recognizes that technical features, definitions and advantages of preferred embodiments which apply to the manufacturing method according to the invention equally apply to a photoacoustic gas sensor comprising such a reference chamber or to a manufacturing method of the gas sensor, and vice versa.

[0191] In particular, the invention enables the provision of a miniaturized photoacoustic gas sensor that safely and hermetically encloses corrosive or explosive gases (such as ammonia) as reference gases, thus enabling monitoring of the presence of such hazardous gases in the ambient air. The method according to the invention ensures that sensitive components of the photoacoustic gas sensor (such as the MEMS sensor) are not attacked during the enclosing of the reference gas. Hermetic enclosing also prevents potentially hazardous gases from escaping from the reference chamber, which, due to the compact arrangement, also ensures a high degree of safety against potential damage.

[0192] In a preferred embodiment, the reference chamber of the photoacoustic gas sensor forms a closed system which is filled with the reference gas and wherein a gas to be analyzed, preferably ambient air, is present in the beam path between the emitter and the reference chamber, so that the proportion of the reference gas in the gas to be analyzed can be measured based on the formation of sound pressure waves in the reference chamber.

[0193] A photoacoustic gas sensor is known to those skilled in the art in its basic principles and essential components. A modulatable emitter generates electromagnetic radiation and is preferably arranged and configured such that the radiation emitted by the infrared emitter essentially or at least partially impinges on the gas in the reference chamber.

[0194] If the modulated irradiation occurs at a wavelength that corresponds to the absorption spectrum of a molecule of a gas component present in the gas mixture, a modulated absorption occurs, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. According to the photoacoustic effect, the heating and cooling processes lead to expansion and contraction of the gas component, which stimulates it to form sound pressure waves with essentially the modulation frequency. The sound pressure waves are also called PAS signals and can be measured using a sensor, for example, a sound detector. The power of the sound waves is preferably directly proportional to the concentration of the absorbing gas component.

[0195] The term gas component preferably refers to the proportion of chemically (and spectroscopically) identical gas molecules (e.g. ammonia) in a gas mixture, while the gas mixture refers to the totality or mixture of several (preferably different) gas components (e.g. air).

[0196] Various emitters are preferred as radiation sources for the aforementioned applications. For example, narrowband laser sources can be used. These advantageously allow the use of high radiation intensities and can be modulated, preferably at high frequencies, using standard components for photoacoustic spectroscopy.

[0197] Broadband emitters can also be used. These advantageously have a broad spectrum, which can be further selected, for example, by using (tunable) filters.

[0198] In a preferred embodiment of the invention, the modulatable emitter is a thermal emitter and comprises a heating element, wherein the heating element comprises a substrate on which a heatable layer of a conductive material is at least partially applied, on which contacts for a current and / or voltage source are present.

[0199] The heating element comprises a heatable layer made of a conductive material that produces Joule heat when an electric current flows through it. The heating element particularly comprises a substrate on which the heatable layer is located. The substrate preferably forms the base of the heating element. The substrate can also at least partially comprise other elements of the IR emitter, such as the base element and / or housing elements. The substrate can advantageously be suitably shaped using established process steps, in particular from semiconductor and / or microsystem manufacturing. The aforementioned materials are particularly easy and cost-effective to process in semiconductor and / or microsystem manufacturing and are also well suited for mass production.These materials are also particularly suitable for doping and / or coating in order to achieve the desired electrical, thermal and / or radiation properties in specific areas.

[0200] The substrate may preferably be selected from a group comprising silicon, monocrystalline silicon, polysilicon, silicon dioxide, silicon carbide, silicon germanium, silicon nitride, nitride, germanium, carbon, gallium arsenide, gallium nitride and / or indium phosphide.

[0201] The conductive material for forming the heatable layer can preferably be selected from the group comprising platinum, tungsten, (doped) tin oxide, monocrystalline silicon, polysilicon, molybdenum, titanium, tantalum, titanium-tungsten alloy, metal silicide, aluminum, graphite, and / or copper. These materials exhibit the desired thermal, electrical, mechanical, and / or radiation properties and are also particularly easy and cost-effective to process.

[0202] The (micro) heating element is preferably at least partially freestanding and allows, for example, thermal expansion within the IR emitter due to significant temperature changes and translational movements. Partially freestanding means that it is at least partially not connected to other elements of the emitter in a force-locking and / or form-locking manner at the interfaces and therefore has a degree of freedom of movement in a direction essentially perpendicular to the interface.

[0203] The emitter is modulatable, which means that the intensity of the emitted radiation, preferably the intensity of the beam, can be controllably changed over time. The modulation should preferably cause a temporal change in the intensity as a measurable quantity. This means, for example, that the intensity over time between the weakest intensity measured within the measuring period and the strongest intensity measured within the same period exists a difference that is greater than the sensitivity of a device typically used for measuring or determining the intensity for the radiation spectrum and the application. Preferably, the difference is significantly greater than a factor of 2, more preferably 4, 6 or 8 between the strongest and the weakest adjustable intensity. Particularly preferably, the modulation of the intensity of the modulated beam occurs for one or more predetermined resonance wavelengths.

[0204] Preferably, direct modulation can be achieved by varying the current supply. In a thermal emitter, such modulation is usually limited to a specific range of a modulation spectrum due to thermal time constants, e.g., in the range of up to 100 Hz. For a laser or an LED, for example, significantly higher modulation rates, e.g., in the kHz range and beyond, are preferably possible.

[0205] Modulation of the infrared emitter can preferably also be achieved by external modulation, e.g., by using a rotating chopper wheel and / or an electro-optical modulator.

[0206] A modulatable emitter preferably refers to a device that emits electromagnetic radiation in a wavelength range within a specific spectrum. The spectrum is selected, in particular, to correspond to the emitter's preferred field of application, namely photoacoustic spectroscopy. In particular, the vibrational excitation of the gas molecules to be spectroscoped and / or detected is preferred, which, depending on the gas molecules, correspond to a preferred spectral range.

[0207] The IR emitter preferably emits a beam oriented in a preferred direction in the form of a ray. The term "beam" is intended to describe the preferably focused portion of the radiation emitted by the emitter along the preferred beam direction of the emitter, with the areas of greatest intensity along this direction defining the beam. Intensity is preferably defined as surface power density and preferably has the unit watts per square meter, or abbreviated as W / m².

[0208] Additional components such as lenses can be integrated into the emitter or attached externally to focus or collimate the beam. A person skilled in the art knows how to shape the emission profile of the radiation source through the design of the emitter and the use of additional components to achieve a desired beam profile and beam direction. The modulatable emitter can preferably operate without additional lenses or be implemented as a system comprising a radiation source and at least one lens for collimating the beam.

[0209] According to the invention, the reference chamber is located in the beam path of the emitter. This means that the intensity of the beam substantially or at least partially impinges on the side of the reference chamber facing the emitter. Partially preferably means at least 40%, preferably at least 50%, 60% or more. In particular, it means that the region of the maximum intensity of the beam impinges on the detection chamber. Preferably, it means that the beam is focused and / or collimated such that a substantial portion of the intensity impinges on the side facing the emitter. A preferred example is a Gaussian beam, which in particular has a transverse profile according to a Gaussian curve. The path along the beam is preferably defined by the path with the maximum intensity as the z-axis.The beam radius w at the "height" z of the beam is preferably defined as the distance from the z-axis at which the intensity has fallen to 1 / e 2< (preferably approximately 13.5%). Following this definition, it is preferred that "the reference chamber is located in the beam path of the emitter" means that essentially the entire beam radius impinges on the side of the reference chamber facing the emitter.

[0210] Preferably, the side of the reference chamber facing the emitter is transparent to the emitted radiation, so that the radiation essentially reaches the gas-fillable interior of the chamber. The side of the reference chamber, in particular, facing the emitter is preferably also referred to as the incident radiation surface.

[0211] The fact that the reference chamber is located in the beam path of the infrared emitter means that the emitter can excite gas in the detection chamber to generate sound pressure waves using modulatably emitted radiation, since this radiation is at least partially irradiated (preferably at least 40%, more preferably at least 50%, especially at least 60%) and, in particular, a substantial portion of the radiation reaches the gas-fillable volume inside the detection chamber. A substantial portion means, in particular, at least 80%, more preferably 90%, and especially 95%.

[0212] Terms such as substantially, approximately, about, approx. etc. preferably describe a tolerance range of less than ± 20%, preferably less than ± 10%, even more preferably less than ± 5% and in particular less than ± 1%. Details such as substantially, approximately, about, approx. etc. always disclose and include the exact value stated. The reference chamber contains a reference gas which is matched to the IR emitter in such a way that modulated IR radiation penetrating the reference chamber enables PAS with the help of the gas molecules of the reference gas there. If the air to be analyzed (especially ambient air) in the measuring section between the infrared emitter and the reference chamber also contains a proportion of reference gas which absorbs the IR radiation, PAS takes place. The strength of the absorption in the reference chamber is thereby reduced.

[0213] The magnitude of the reduction allows for inferences about the concentration of the reference gas in the measurement section. The formation of sound pressure waves in the reference chamber is preferably smaller the more reference gas is present in the beam path outside the detection chamber, since absorption and excitation then occur particularly there. A narrowband IR emitter is preferably used here so that only the reference gas can be excited.

[0214] This measurement principle allows the detection of even the smallest concentrations of molecules of a reference gas within the ambient air. This makes monitoring toxic, corrosive, or explosive gases, such as ammonia, particularly safe and reliable.

[0215] The measuring principle also inherently ensures error control and alarms. Only if the gas sensor is functioning correctly will a PAS signal be reliably detected, which, in the absence of any noticeable ammonia concentration in the ambient air, exhibits the expected maximum amplitudes. Limit values can be defined for the monitoring area, which correspond to permissible ammonia concentrations.

[0216] If the sound detector, emitter, or other component is faulty, this is detected as a change in the expected PAS signal. A warning can be issued automatically. Depending on the deviation from the PAS signal, the warning message may indicate a suspected malfunction of the gas sensor and / or impermissible limit values.

[0217] The warning therefore doesn't have to be issued by a watchdog function or similar device, but is inherent in the measuring principle. The lack of a warning and an undetected increase in potentially dangerous concentrations are effectively prevented.

[0218] Preferably, the gas sensor further comprises a control or regulating unit which is configured to control the modulatable emitter and / or the MEMS sensor and to receive and, if necessary, evaluate data from the modulatable emitter and / or the MEMS sensor.

[0219] The control unit can preferably comprise a described electronic circuit, which is located within the reference chamber and is connected to the MEMS sensor. Furthermore, it is preferred that the control unit comprises at least one (external) data processing unit (e.g., integrated circuit (IC), application-specific integrated circuit (ASIC), programmable logic device (PLD), field programmable gate array (FPGA), microprocessor, microcomputer, programmable logic controller, and / or other electronic, preferably programmable, circuit), which is located outside the reference chamber and is connected to the electronic circuit within the reference chamber.

[0220] For example, it may be preferred that the control unit, on the one hand, outputs electrical control signals via the external data processing unit, which control the modulatable emitter and the MEMS sensor. On the other hand, the external data processing unit can preferably be used to evaluate the measurement data recorded by the MEMS sensor (in particular, measurement data relating to the PAS signal). The internal electronic circuit can also perform a (pre-)evaluation of the measurement data from the MEMS sensor. However, it may also be preferred that the internal electronic circuit essentially forwards the measurement data unprocessed to the external data processing unit for further processing and / or evaluation. Detailed description

[0221] In the following, the invention will be explained in more detail using examples, without being limited to them. Short description of the figures

[0222] Fig. 1 AWAYSchematic overview of a first preferred embodiment of a method for forming a reference chamber filled with reference gas, in which an opening remains at the contact surfaces of the wafers after bonding. Fig. 2 A - K Schematic illustration of preferred method steps of the first preferred embodiment of the method for manufacturing a reference chamber filled with reference gas, in which an opening remains at the contact surfaces of the wafers after bonding. Fig. 3 AWAY Schematic overview of a second preferred embodiment of a method for forming a reference chamber filled with reference gas, which has a valve. Fig. 4 AWAY Schematic illustration of preferred method steps of the second preferred embodiment of a manufacturing method of a reference chamber filled with reference gas, which has a valve. Fig. 5 AWAYSchematic overview of a third preferred embodiment of a method for forming a reference chamber filled with reference gas, the opening of which is closed with a thermal solder. Fig. 6 UH Schematic illustration of preferred method steps of the third preferred embodiment of a manufacturing method of a reference chamber filled with reference gas, the opening of which is closed with a thermal solder. Detailed description of the illustrations

[0223] Fig. 1 AWAY shows a summary of a first variant of a manufacturing process.

[0224] Here, a first wafer 1 and a second wafer 2 provided, whereby in this case both the first (upper) wafer 1 as well as the second (lower) wafer 2 one cavity each 6 On the first 1 and / or second wafer 2a MEMS component and / or an electronic circuit is present (e.g. within the cavities, but not shown).

[0225] Bonding the first wafer 1 with the second wafer 2 takes place within a bonding chamber to form a reference gas 11 fillable volume 7, where after bonding at a contact surface 3 the two wafers an opening 9 remains. The first wafer 1 and the second wafer 2 For this purpose, they also prefer contact surfaces 3 which is used to bond the first wafer 1 with the second wafer 2 serve, whereby to form the opening 9 an area at the contact surfaces 3 is not bonded.

[0226] In Fig. 1 A The non-bonded lateral area of the reference chamber is illustrated on the right side, creating an opening 9can be provided. A flooding of a reference gas 11 into the reference chamber via the opening 9 can be carried out as described within a coating system. Fig. 1 A the flooding step is illustrated as an arrow.

[0227] Closing the opening 9 The reference chamber within the coating system is coated in the preferred embodiment by a coating process which forms a covering layer 12 at least over an area of the opening, as in Fig. 1B illustrated, preferably around the entire reference chamber. The cover layer 12 It can preferably be a nitride, which reliably seals the entire reference chamber hermetically.

[0228] Fig. 2 A - K shows preferred process steps of a first variant of a manufacturing process for forming a reference gas 11filled reference chamber, at the contact surfaces 3 an opening remains in the wafer after bonding.

[0229] As in Fig. 2 A As illustrated, in a first process step the first wafer 1 with a first bonding material 4 sputtered on the back. The first bonding material 4 can preferably be gold. Bonding material refers to a material that is particularly suitable for bonding.

[0230] In Fig. 2 B a preferred structuring of the first bond material 4 The left side shows a central cross-section through the reference chamber to be formed. The right side also shows a top view. The structured first bond material 4forms a nearly closed border, but has a recess on the left side. The recess is not bonded, but serves to form the opening into the reference chamber.

[0231] Structuring the first wafer 1 is done using a photoresist 8 and etching processes, as in the Fig. 2C to Fig. 2F illustrated.

[0232] In Fig. 2 C a photoresist 8 front side of the first wafer 1 applied, leaving an area free for further processing. Using an etching process, preferably deep reactive ion etching (DRIE), the area of the first wafer is 1 etched, as in Fig. 2 D However, the first wafer is preferably not etched continuously. In Fig. 2 E a photoresist 8 on the back of the first wafer 1The same photoresist can be 8 as in the previous steps, or another. Starting from the back, another etching process (preferably DRIE) takes place at the relevant location (see Fig. 2 F ), for the first wafer 1 at the previously etched area on the front side. In addition, the back etching process creates a cavity 6 in the first wafer 1 which contributes to the formation of the volume 7 in the reaction chamber.

[0233] In Fig. 2 G the first wafer 1 with the second wafer 2 bonded, on which a second bonding material 5 The second wafer 2 also has a cavity 6 which are complementary to the cavity 6 of the first wafer 1 The provision of the second wafer 2can be done using analog etching processes.

[0234] The second bond material 5 can preferably be aluminum, copper and / or gold. The second bonding material 5 can, but does not have to be structured. To ensure bonding at the opening to be formed 9 To avoid this, it is sufficient that the first bonding material 4 of the first wafer 1 has a recess on the left side. Preferably, the first wafer 1 and the second wafer 2 bonded together using thermocompression bonding (TC bonding). However, the left area is bonded to contact surfaces 3 not bonded.

[0235] In this preferred embodiment of the method, the area at non-bonded contact surfaces is designated as an opening 9 used to fill the reference chamber with a reference gas 11, e.g. ammonia. Thus, a reference chamber is created, which has a volume7 and an opening 9 The opening is 9 in a lateral area of the reference chamber and is provided by the fact that no bonding process takes place in this area.

[0236] In the Fig. 2 G is the opening 9 in the lateral area of the reference chamber on the left. To the right of the illustration of the reference chamber, a top view of the first bonding material 4, preferably gold. Bonding processes take place in a bonding chamber, which is not shown.

[0237] In Fig. 2 H The reference chamber is no longer located in the bonding chamber, but in a coating system, preferably a PECVD system (Plasma-Enhanced Chemical Vapour Deposition). Initially, any gas at room pressure can be in the volume 7 the reference chamber, which is located in the Fig. 2 Iis pumped out within the coating system, so that a vacuum is created in the volume 7 the reference chamber. Experts know that in reality, a vacuum is never absolute, but is characterized by a significantly lower pressure than atmospheric pressure under standard conditions.

[0238] In the Fig. 2 J the coating system is supplied with the reference gas 11 flooded, so that the reference gas flows through opening 9 11, e.g. B. ammonia, into the volume 7 the reference chamber. In the Fig. 2 K The reference chamber is covered by a cover layer 12, in particular by depositing a nitride, hermetically sealed within the coating system, whereby the opening 9 is also reliably sealed and therefore the reference gas 11 can no longer escape from the reference chamber.

[0239] Fig. 3 AWAYshows a summary of a second variant of a manufacturing process to produce a reference gas 11 filled reference chamber. The first wafer 1 and the second wafer 2 are provided, with both wafers 1 and 2 cavities again 6 There is a MEMS component and / or an electronic circuit on the first 1 and / or the second wafer 2, e.g. within the cavities 6. However, these are not shown.

[0240] After bonding the two wafers 1 and 2 an opening 9 in the first 1 or second wafer 2 etched. A valve 14 is located at the end of the opening 9. The two wafers 1 and 2 are bonded together in such a way that the valve 14 within the volume 7the reference chamber at the end of the opening 9 The preferred valve is 14 on the first 1 or the second wafer 2 before the bonding process. During the bonding process itself, however, the contact surfaces 3 continuous to bond the first wafer 1 with the second wafer 2 used. After bonding the first wafer 1 with the second wafer 2 The reference chamber is brought from the bonding chamber into the coating system and filled with the reference gas 11 flooded, whereby the reference gas 11 about the opening 9 and via the valve 14 into the volume 7 the reference chamber.

[0241] Finally, the covering layer 12, preferably with a nitride deposition, the reference chamber is hermetically sealed, preferably over the entire area of the reference chamber.

[0242] This manufacturing process is particularly preferred when applying the covering layer 12 a reference gas 11 different process gas is used. Advantageously, the valve closes 14 before coating with the covering layer 12 the reference gas 11 within the reference chamber so that any gas exchange (from reference gas to process gas) cannot lead to contamination.

[0243] Fig. 4 A - I show preferred process steps of the second variant of a manufacturing process of a reference gas 11 filled reference chamber.

[0244] In Fig. 4 A the first wafer 1 with a first bonding material 4 applied to the back. Preferably, the first bonding material 4 Gold and is sputtered onto the back of the first wafer 1 coated.

[0245] In Fig. 4 B becomes the first bond material 4 on the back of the first wafer 1 structured. On the right is a top view of the first bond material 4 illustrated. In Fig. 4 C a photoresist 8 on the front of the first wafer 1 applied. In Fig. 4 D a layer of material 13 on the back of the first wafer 1 The preferred material layer is 13 a soft metal, particularly preferably aluminum, as a thin layer via a sputtering process onto the back of the first wafer 1 applied. In Fig. 4 E Starting from the front side of the first wafer 1, an opening 9 formed by an etching process, preferably a dry etching process. The opening 9 up to the material layer 13 etched. Then, in Fig. 4 F the material layer 13to a flexible valve 14 structured.

[0246] In Fig. 4 G the first wafer 1 with a second wafer 2 bonded, preferably via TC bonding (see above). The second bonding material 5 on the contact surfaces 3 of the second wafer 2 can preferably be gold, copper and / or aluminum. In Fig. 4 H The reference chamber is opened within a coating system, preferably within a PECVD system, via the opening 9 with the reference gas 11 flooded, whereby when the gas is introduced the valve 14 opens. In Fig. 4 I The reference chamber is covered by a cover layer 12, preferably via a nitride, hermetically sealed.

[0247] Fig. 5 AWAY-shows a summary of a third variant of a manufacturing process. In this third variant, the two wafers also have 1 and 2 cavities 6 into which a MEMS component and / or an electronic circuit may be inserted. The MEMS component and / or the electronic circuit are not shown. The two wafers 1 and 2 are preferred on all contact surfaces 3 bonded together in a bond chamber.

[0248] Preferably, the opening 9 before bonding using an etching process, preferably dry etching. After bonding the two wafers 1 and 2 becomes a lot 15 near the opening 9 placed. After introducing the reference gas 11, e.g. B. ammonia, into the volume 7 the reference chamber within the coating system, the solder 15melted so that it fits into the opening 9 By melting and flowing in the solder, the opening is 9 sealed. A covering layer 12, preferably a nitride, the reference chamber is hermetically sealed.

[0249] Fig. 6 UH shows preferred process steps of the third variant of a manufacturing process for a gas-filled reference chamber. In Fig. 6 A the first wafer 1 provided and with the first bonding material 4 coated on its back. Preferably, the first bonding material 4 Gold and is applied to the back of the first wafer 1 applied using a sputtering process. In Fig. 6 B becomes the first bond material 4 on the back of the first wafer 1 structured.

[0250] On the right is a top view of the structuring of the first bond material 4 illustrated. In Fig. 6 C a photoresist 8 on the front of the first wafer 1 attached. In Fig. 6 D an opening is made starting from the front 9 via an etching process, preferably via a dry etching process, particularly preferably via reactive ion deep etching, into the first wafer 1 formed. In Fig. 6 E a second wafer 2 with the first wafer 1 on the contact surfaces 3 bonded. The second wafer comprises at the contact surfaces 3 a second bonding material 5, which is preferably gold, copper and / or aluminum.

[0251] Preferably, the two wafers 1 and 2 bonded together using TC bonding. After the bonding process, Fig. 6 F a thermal solder 15near the opening 9 Then, in Fig. 6 G the reference chamber within the coating system with the reference gas 11, e.g., ammonia. Finally, in Fig. 6 H the thermal solder 15 melted to enter the opening 9 to flow and close it.

[0252] Furthermore, the reference chamber is covered with a cover layer 12, preferably a nitride, coated so that the reference chamber is particularly well hermetically sealed. List of reference symbols

[0253] 1First wafer 2Second wafer 3Contact area 4First bonding material 5Second bonding material 6Cavity 7Volume 8Photoresist 9Opening 11Reference gas 12Cover layer 13Material layer for structuring as a valve 14Valve 15Solder BIBLIOGRAPHY

[0254] Bonilla-Manrique, Oscar E., et al. "Sub-ppm-Level Ammonia Detection Using Photoacoustic Spectroscopy with an Optical Microphone Based on a Phase Interferometer." Sensors 19.13 (2019): 2890. Peng, W. Y., et al. "High-sensitivity in situ QCLAS-based ammonia concentration sensor for hightemperature applications." Applied Physics B 122.7 (2016): 188. Schilt, Stéphane, et al. "Ammonia monitoring at trace level using photoacoustic spectroscopy in industrial and environmental applications."Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 60.14 (2004): 3259-3268. Stemme, Goran, and Edvard Kalvesten. "Micromachined gas-filled chambers and method of microfabrication." U.S. Patent No. 6,124,145. 26 Sep. 2000.

Claims

1. A method of producing a photoacoustic gas sensor comprising a gas-filled reference chamber within which a MEMS component and optionally an electronic circuit are present, comprising the following steps: a) providing a first (1) and a second wafer (2), wherein at least the first wafer (1) and / or the second wafer (2) have / has a cavity (6) and wherein the MEMS component is present on the first (1) and / or second wafer (2), wherein the MEMS component is a MEMS sensor, b) bonding the first wafer (1) to the second wafer (2) within a bonding chamber to form a volume (7) that can be filled with reference gas (11), wherein, after the bonding, an opening (9) remains on a contact surface (3) of the two wafers, or an opening (9) is made in the first (1) and / or second wafer (2) before or after the bonding, c) flooding a reference gas (11) into the reference chamber via the opening (9) within a coating system, d) sealing the opening (9) of the reference chamber within the coating system e) providing a modulable emitter, f) arranging the reference chamber filled with the reference gas and the modulable emitter, wherein the reference chamber is present in the beam path of the emitter, so that the emitter can excite the reference gas in the reference chamber by means of modulably emittable radiation to form sound pressure waves that can be detected by means of the MEMS sensor.

2. The production method according to any one or more of the preceding claims, characterized in that the reference gas (11) comprises corrosive and / or explosive gases, preferably methane, propane, propylene, silane, chlorosilane, hydrogen and / or oxygen, particularly preferably ammonia.

3. The production method according to any one or more of the preceding claims, characterized in that an inert gas, preferably nitrogen, is additionally introduced into the reference chamber via the opening (9) to set a partial pressure of the reference gas (11) within the reference chamber.

4. The production method according to any one or more of the preceding claims, characterized in that the first wafer (1) and the second wafer (2) have contact surfaces (3) which serve for bonding the first wafer (1) to the second wafer (2), wherein, to form the opening (9), an area of contact surfaces (3) is not bonded and / or, after bonding, an opening (9) remains on a contact surface (3) of the two wafers, wherein the opening (9) has a cross section of 1 µm2 to 1000 µm2, preferably 1 µm2 to 100 µm2 and a length of 1 µm to 1000 µm, preferably 10 µm to 500 µm.

5. The production method according to any one or more of the preceding claims, characterized in that before or after bonding the first wafer (1) to the second wafer (2), the opening (9) is formed starting from an outer side to an inner side of the first wafer (1) or the second wafer (2), preferably by means of an etching process.

6. The production method according to any one or more of the preceding claims, characterized in that a valve (14) is present at the end of the opening (9) of the first (1) or the second wafer (2), wherein, after bonding the first wafer (1) to the second wafer (2), the valve (14) is preferably located at the end of the opening (9) starting from the outside of the first wafer (1) or the second wafer (2) and in the volume (7) of the reference chamber, and / or wherein the valve (14) is preferably a soft metal, preferably a non-ferrous metal selected from the group comprising lead, gold, indium, copper, platinum, silver, zinc, tin and / or compounds thereof, particularly preferably aluminum and / or compounds thereof.

7. The production method according to any one or more of the preceding claims, characterized in that after bonding the first wafer (1) to the second wafer (2), the reference chamber in the coating system is flooded with the reference gas (11), wherein the gas enters the volume (7) of the reference chamber via the opening (9) and the valve (14).

8. The production method according to any one or more of the preceding claims, characterized in that after the reference gas (11) has been flooded into the reference chamber, a solder (15) is melted to seal the opening (9), wherein the solder (15) preferably comprises a fusible material selected from the group comprising lead, tin, zinc, silver, copper, alloys and / or compounds thereof.

9. The production method according to any one or more of the preceding claims, characterized in that the opening (9) is sealed by a coating method within the coating system, preferably by spray coating, mist coating and / or steam coating, and / or the coating system is a physical coating system or a chemical coating system, a low-pressure chemical coating system and / or an epitaxial coating system.

10. The production method according to any one or more of the preceding claims, characterized in that to seal the opening (9) within the coating system, a covering layer (12) is applied at least over a section of the opening (9), wherein a nitride, an oxide, a metal, and / or a noble metal is preferably used as the material for the covering layer (12).

11. The production method according to any one or more of the preceding claims, characterized in that to seal the opening (9) and to form a covering layer (12), a process gas is introduced into the coating system, wherein the process gas is introduced into the reference chamber after flooding with a reference gas, or wherein a material for forming the cover layer (12) is selected such that the reference gas can simultaneously serve as a process gas.

12. The production method according to any one or more of the preceding claims, characterized in that the electronic circuit comprises a processor, a switch, transistors and / or transducers.

13. The production method according to any one or more of the preceding claims, characterized in that the MEMS component is a sound pressure detector, wherein the sound pressure detector preferably comprises a capacitively or optically readable, piezoelectric, piezoresistive and / or magnetic bar and / or a capacitive, piezoelectric, piezoresistive and / or optical microphone.

14. A photoacoustic gas sensor, comprising a modulable emitter, a reference chamber filled with a reference gas (11), wherein a MEMS sensor is present within the reference chamber, wherein the reference chamber is present in the beam path of the emitter, so that the emitter can excite the reference gas (11) in the reference chamber by means of modulably emittable radiation to form sound pressure waves which can be detected by means of the MEMS sensor, characterized in that the photoacoustic gas sensor was manufactured by a method according to claims 1-15.

15. The photoacoustic gas sensor according to the preceding claim characterized in that the reference chamber forms a sealed system which is filled with the reference gas (11), and a gas to be analyzed, preferably ambient air, is present in the beam path between the emitter and the reference chamber, so that the proportion of the reference gas (11) in the gas to be analyzed can be measured by means of the formation of sound pressure waves in the reference chamber.