Method for sealing a reference gas inside a MEMS cell

Abstract

In a first aspect, the present invention relates to a method for manufacturing a hermetically sealed gas-filled reference chamber, whereby a gas for filling the reference chamber is introduced through an opening into a separate coating chamber only after bonding of the wafers forming the reference chamber, the reference chamber preferably containing a MEMS device. In another aspect, the invention relates to a photoacoustic gas sensor comprising such a reference chamber in which a MEMS sensor resides.

Description

[Technical Field]

[0001] In a first aspect, the present invention relates to a method for fabricating a sealed, gas-filled reference chamber, whereby, after bonding wafers forming the reference chamber, a gas for filling the reference chamber is introduced through an opening in a separate coating chamber, and a microelectromechanical system (MEMS) device is preferably mounted within the reference chamber.

[0002] In another aspect, the invention relates to a photoacoustic gas sensor comprising such a reference chamber in which a MEMS sensor resides. [Background technology]

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

[0004] One application of PAS is the detection of very minute concentrations of gases. Here, intensity-modulated infrared radiation is used according to the frequency of the absorption spectrum of a 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 time scale reflects the modulation frequency of the radiation. The heating and cooling processes lead to expansion and contraction of the gas, which induces sound waves at the modulation frequency. These can then be measured by a sound detector, such as a microphone or a flow sensor.

[0005] One example is the detection of CO2, which plays a role in research and air conditioning technology. However, it is also relevant in applications where not only toxic but also explosive or corrosive gases, such as ammonia NH3, must be detected.

[0006] In particular for ammonia (NH3), there are already numerous applications in industry and nature.

[0007] For example, ammonia is used as a refrigerant in cold storage warehouses, breweries, and slaughterhouses. It can also be used in large refrigeration plants. In the middle of the 20th century, ammonia was often replaced by chlorofluorocarbons (CFCs), which are now banned. Nevertheless, ammonia has always been able to maintain its important role in industrial refrigeration due to its favorable thermodynamic properties.

[0008] Ammonia is often referred to as a natural refrigerant, despite the fact that it is synthetically produced for refrigeration processes. NH3 is mostly produced during the decomposition of organic nitrogenous materials. Today, it is used in large refrigeration systems for cold storage and as a refrigerant for air conditioning in airports, office buildings, production halls, or sports facilities.

[0009] However, ammonia is corrosive, especially to copper materials. Therefore, the piping in systems using ammonia as a refrigerant must be made of steel. Ammonia is also toxic and somewhat flammable, requiring special safety regulations for the construction, operation, and maintenance of these systems.

[0010] At high concentrations, ammonia vapors can cause eye and respiratory irritation, and at higher concentrations, mucous membranes and lungs can be damaged, potentially leading to death. Furthermore, ammonia is classified as toxic to water. Because it dissolves easily in water, seepage into soil and the associated damage to groundwater must be avoided at all costs.

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

[0012] For the reasons mentioned above, it is important to detect possible ammonia leaks at an early stage. Due to their high sensitivity, PAS offers excellent possibilities for continuously monitoring the ammonia concentration. Several devices are known from the prior art for this purpose.

[0013] In 2016, Peng et al. disclosed a sensor capable of detecting ammonia in high-temperature environments. It uses a quantum cascade laser (QCL) to irradiate a cylindrical measurement chamber approximately 1.8 m long. The measurement chamber itself is heated, and a flow is generated within it by supplying air from the environment. In addition, other compounds, such as CH4 (methane) and 1% NH3 / Ar (ammonia / argon), are introduced to control the flow within the chamber. A detector measuring the signal strength of the QCL is located at the end of the resonator. If a higher percentage of ammonia is present in the air introduced into the measurement chamber, it absorbs the beam from the QCL. Therefore, a weaker signal is also registered by the detector. Because the measurement chamber is approximately 1.8 m long, the device is designed for industrial applications and cannot be used flexibly. Additionally, other components, such as a thermocouple to measure the temperature distribution, a BaF2 (barium fluoride)-coated window located at the end of the resonator, or a thermal jacket for the resonator, are added. This makes the device expensive and complex to construct.

[0014] In Schilt et al. (2004), a CO2 laser illuminates a photoacoustic measurement cell. It comprises a cylindrical resonator and two buffer volumes, which act as acoustic filters. A microphone is located at the end of the resonator. Additionally, a semiconductor detector measures the intensity of the laser beam. The measurement principle is identical to that of Peng et al. (2016). If ammonia molecules or molecules are present in the beam path of the CO2 laser, part of the laser radiation is absorbed. In the absence of ammonia in the resonator, the measured pressure signal is maximized. A disadvantage arises from the openings located in the two buffer volumes. Through these openings, other gases could, in principle, diffuse into the resonator, distorting the measurement signal.

[0015] Bonilla-Manrique et al. (2019) also described a resonant gas cell containing two buffers and a 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 and act as an acoustic detector. These are placed in the center of the resonator, with the microphone brought through the resonator and the diaphragm attached to the opposite side from the outside. The photoacoustic effect also vibrates the diaphragm. In the experimental setup, both the gas inlet embodied by one of the two buffers and the diaphragm are illuminated by a laser beam. In this process, the measurement cell is already filled with 5000 ppm NH3. Therefore, the device described by Bonilla-Manrique et al. (2019) also has dimensions visible to the naked eye. In addition, it is not guaranteed that the measurement signal will remain undistorted by the input of other molecules that may absorb the laser beam, such as CO2 (carbon dioxide) and H2O (water).

[0016] Therefore, in view of the prior art, there is interest in alternative devices and / or methods for reliably sealing potentially toxic, corrosive and / or explosive gases in a chamber or measurement cell, which have wider application due to miniaturization.

[0017] U.S. Patent No. 6,124,145 discloses a method for filling two or more wafers with gas, particularly CO2. In this process, a first wafer, into which a cavity is introduced, is placed in a bonding chamber, which is filled with the gas that is to be placed inside the wafer. A second wafer is then bonded to the first wafer in the bonding chamber, creating a chamber or cell consisting of the two wafers containing the gas. However, this method is not readily suitable for filling corrosive or explosive gases, such as ammonia.

[0018] On the one hand, high temperatures are required for bonding. This means that gases that are flammable or even explosive at the corresponding bonding temperatures, such as ammonia, cannot be enclosed in the chamber. Otherwise, they will damage the wafer or even the bonding chamber itself, and in the worst case, destroy it. In addition, this method does not easily allow for forming a chamber from a wafer with electronic circuits or MEMS devices, such as sensors, and then filling the chamber with a corrosive gas, such as ammonia. This is because the corrosive properties of ammonia would damage the corresponding electronic circuits or MEMS devices. In addition, the bonding process itself takes a certain amount of time, and the conditions (such as temperature) are favorable for the reactivity of the corrosive gas.

[0019] Further methods and devices that pursue the goal of transferring gases into compact systems are known in the prior art.

[0020] For example, U.S. Patent Application Publication No. 2018 / 0339900(A1) discloses a method for fabricating a MEMS device containing at least two sensors. The first sensor is preferably a rotational speed sensor, and the second sensor is preferably an acceleration sensor. The two sensors are formed in separate regions within a wafer stack. U.S. Patent Application Publication No. 2018 / 0339900(A1) aims 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 elevated temperatures. For example, H2 may diffuse from an accelerometer to the rotational speed sensor. To solve this problem, the fabrication process first provides a MEMS wafer and a cap wafer, forms MEMS structures for the two sensors in the MEMS wafer, and then seals the MEMS wafer with the cap wafer. After sealing the two wafers, a first access hole is formed, then a first pressure is transferred to the cavity of the first sensor, and finally, the access hole is sealed. A similar method is used for the second sensor, which allows two different internal pressures to exist in the cavity. In particular, after bonding, H2 is removed from the cavity of the second sensor (accelerometer), and it is also conceivable to introduce, for example, oxygen, ozone, and / or a defined plasma. The formation of the access hole is performed by a laser. Sealing the access hole is also performed using a laser.

[0021] U.S. Patent Application Publication No. 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 in a wafer stack. The gas injected into the cavity is a noble gas. The second substrate has an area permeable to the noble gas. A layer impermeable to the noble gas introduced into the cavity is coated on the second substrate, and an opening is then formed thereon. Another layer impermeable to the injected helium is applied to seal the noble gas. The method does not disclose a method for introducing a gas more reactive than the noble gas described.

[0022] U.S. Patent Application Publication No. 2020 / 0057031(A1) discloses a detector module for a photoacoustic gas sensor. The detector module is constructed so that a first substrate and a second substrate can be bonded together, and the recess can be filled with a reference gas in an airtight manner. The reference gas can be introduced into the recess in a reference gas environment by forming a through-hole in the first or second substrate during or after bonding, which is then sealed. The reference gas is selected from the group consisting of CO2, NOx, H2O, O2, N2, CH4, or alcohol. Filling the recess with gas in a coating chamber is not described.

[0023] U.S. Patent Application Publication No. 2021 / 0055207(A1) also discloses a sensing cell for a photoacoustic gas sensor, where a gas atmosphere of a gas to be enclosed can be fabricated and enclosed within the cavity during bonding.

[0024] US Patent Application Publication No. 2019 / 0353157(A1) discloses a miniature transport device that can be used for fluid control and / or pressurization. The miniature transport device and miniature valve device can be assembled, and gas can be introduced through an inlet. Through a piezoelectric actuator, gas can flow through multiple pressure chambers and can flow continuously in the transport direction. The gas can be released by a user-defined amount of gas or when ambient pressure increases.

[0025] U.S. Patent Application Publication No. 2007 / 0295456(A1) describes a material for bonding wafers. The bonding material is characterized by containing conductive particles in addition to insulating adhesive capabilities. It is further disclosed that air can be replaced with a gas encapsulated in the bonding chamber, particularly for the operation of MEMS devices. The gas disclosed herein is non-explosive and inert.

[0026] U.S. Patent Application Publication No. 2003 / 0183916(A1) discloses a method for packaging a MEMS device. In one embodiment, the sealing process may be performed in a controlled environment such that the cavity contains a desired ambient gas at a desired pressure. To this end, an opening is described as being located sufficiently far from the MEMS device to avoid damaging the MEMS device. A cover or sealant (patch) for sealing the opening is also disclosed.

[0027] U.S. Patent Application Publication No. 2020 / 0198964(A1) discusses an encapsulation process for mounting MEMS devices within a wafer stack, where openings can be sealed by a hole sealing layer, which can be coated on itself.

[0028] There is no known safe and reliable method for introducing and sealing a corrosive and / or explosive gas into a MEMS cell without damaging the MEMS device or the electronic circuitry within the MEMS cell. In particular, a reliable and safe introduction of a corrosive and / or explosive gas during the fabrication of a photoacoustic gas sensor is not apparent from the prior art. Therefore, there is a need to make the introduction of gas into a MEMS cell more efficient and safer for users. [Prior art documents] [Patent documents]

[0029] [Patent Document 1] U.S. Patent No. 6,124,145 [Patent Document 2] U.S. Patent Application Publication No. 2018 / 0339900(A1) [Patent Document 3] U.S. Patent Application Publication No. 2014 / 0038364(A1) [Patent Document 4] U.S. Patent Application Publication No. 2020 / 0057031(A1) [Patent Document 5] U.S. Patent Application Publication No. 2021 / 0055207(A1) [Patent Document 6] U.S. Patent Application Publication No. 2019 / 0353157(A1) [Patent Document 7] U.S. Patent Application Publication No. 2007 / 0295456(A1) [Patent Document 8] U.S. Patent Application Publication No. 2003 / 0183916(A1) [Patent Document 9] U.S. Patent Application Publication No. 2020 / 0198964(A1) [Patent Document 10] U.S. Patent Application Publication No. 2014 / 0028264(A1) [Non-patent literature]

[0030] [Non-Patent Document 1] 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 [Non-patent document 2] Peng WY et al. “High-sensitivity in situ QCLAS-based ammonia concentration sensor for high-temperature applications” Applied Physics B 122.7(2016):188 [Non-patent document 3] Schilt Stephane 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 Summary of the Invention [Problem to be solved by the invention]

[0031] Object of the invention The object of the present invention is to provide an apparatus and a method for manufacturing the same that overcomes the disadvantages of the prior art. In particular, one object of the present invention is to enable PAS (photoacoustic spectroscopy) of corrosive and / or explosive gases by an apparatus that can be reliably and safely manufactured, which apparatus is characterized by a compact design and hermetic sealing of corrosive and / or explosive gases. The manufacturing method should also be simple, inexpensive, suitable for mass production, and usable for applications in a wide range of fields. [Means for solving the problem]

[0032] The object of the invention is solved by the features of the independent claims. Preferred embodiments of the invention are set forth in the dependent claims.

[0033] In a first aspect, the present invention relates to a method for manufacturing a gas-filled reference chamber in which a MEMS device and / or an electronic circuit resides, the method comprising the following steps: a) providing a first and a second wafer, at least one of the first and second wafers having a cavity, and MEMS devices and / or electronic circuits present on the first and / or second wafers; b) bonding a first wafer to a second wafer in a bonding chamber to form a volume fillable with a reference gas, wherein after bonding an opening remains in the area where the two wafers are in contact, or an opening is made in the first and / or second wafer before or after bonding; c) filling a reference chamber with a reference gas through an opening in the coating system; d) sealing the opening of the reference chamber within the coating system; Includes:

[0034] The method according to the present invention differs from U.S. Pat. No. 6,124,145 in that the gas introduced into the reference chamber is not present in the bonding chamber during the bonding process. This conventional method significantly limits the choice of gas to be included in the reference chamber because of the high temperatures generated during the bonding process. Depending on the bonding method used, temperatures ranging from 250°C to 1000°C are possible. Gases that are flammable or even explosive at these temperatures and corresponding pressure conditions cannot be introduced using the method of U.S. Pat. No. 6,124,145. If this were to occur, the entire experimental setup (especially the bonding chamber) could be damaged or even destroyed. In particular, the method according to the present invention allows the introduction of ammonia, which has an ignition temperature of 630°C, into the reference chamber.

[0035] The method according to the invention also allows a corrosive gas to be introduced and sealed in the reference chamber. This is particularly important if a MEMS device and / or electronic circuitry is present in the reference chamber. Using the method known from U.S. Pat. No. 6,124,145, it is not possible to seal a corrosive gas in the reference chamber without damaging any MEMS device and / or electronic circuitry that may be present. Using the method known from U.S. Pat. No. 6,124,145, the entire bonding chamber is filled with corrosive gas, and the MEMS device and / or electronic circuitry are then surrounded by a large amount of corrosive gas.

[0036] In particular, in U.S. Patent No. 6,124,145, the bonding process occurs after the bonding chamber is filled with the enclosed gas. Because the bonding process takes a certain amount of time, the MEMS device and / or electronic circuitry are surrounded by corrosive gas for the time required for bonding, and oxygen present in the bonding chamber for the same time can also promote the reaction. Therefore, the MEMS device and / or electronic circuitry can be damaged by the large amount of corrosive gas and by the exposure to the corrosive gas and the residence time required for bonding.

[0037] The method according to the present invention also differs from U.S. Patent Application Publication No. 2014 / 0038364 in that the reference gas is introduced inside the coating chamber. In contrast, in U.S. Patent Application Publication No. 2014 / 0028264, the gas is introduced into the cavity in the bonding chamber. However, since temperatures exist during bonding at which undesirable reactions, such as explosions, can occur with explosive and / or reactive gases, this significantly limits the choice of gas to be introduced. For this reason, in U.S. Patent Application Publication No. 2014 / 0028264, only inert noble gases are used as gases for introduction into the cavity of the MEMS cell. However, according to the present invention, the introduction of the reference gas is carried out in the coating chamber. Advantageously, this also allows the introduction into the bonding chamber of gases that, unlike helium, are flammable or even explosive at the corresponding bonding temperatures, such as ammonia. Thus, the method according to the invention represents a significant improvement over the prior art, since explosive and / or corrosive gases can be safely introduced into the reference chamber and hermetically sealed.

[0038] Furthermore, in the method according to the present invention, the reference gas can be preferably directly filled into the volume of the reference chamber, whereby the reference gas is filled into the coating chamber and then immediately sealed therein. Therefore, the reference gas can preferably enter the volume of the reference chamber by diffusion, a naturally occurring physical process due to Brownian molecular motion. In contrast, U.S. Patent Application Publication No. 2018 / 0339900 discloses an exchange process for filling the volume in which one of the two sensors described herein is located. U.S. Patent Application Publication No. 2018 / 033990 discloses, for example, that H2 is removed from the cavity of the second sensor and then filled with oxygen, ozone, and / or a defined plasma, resulting in gas exchange. In particular, the gases described in U.S. Patent Application Publication No. 2018 / 033990 may at least partially penetrate the surface of the MEMS element and react with hydrogen or, if absorbed by the surface, may reduce the release energy of hydrogen that dissolves in the solid. This type of bonding reaction or exchange is avoided by the method according to the invention.

[0039] Furthermore, in U.S. Patent Application Publication No. 2018 / 033990, the filling of the sensor cavity does not occur within the coating system, so the coating system cannot be used to seal the reference chamber opening. Instead, the opening is sealed by a laser.

[0040] The disadvantages of the prior art are avoided or eliminated by the method according to the present invention. In the method according to the present invention, a MEMS cell having an opening is provided, which is then filled with a gas to be enclosed. In this method, a reference chamber is first formed using two wafers. This is performed by a bonding process in a bonding chamber. The opening into the volume of the reference chamber can remain after bonding. However, the opening can also be made in one of the two wafers before or after bonding. After the bonding process, the MEMS cell is thereby formed, which is transferred to a coating system.

[0041] The coating system is then filled with the gas to be sealed in the MEMS cell, so that the gas diffuses through the opening into the volume of the MEMS cell. In a next step, the opening is sealed and sealed. Ammonia, which has particularly explosive and corrosive properties, can be sealed in the MEMS cell by the method according to the invention. However, other gases with such deteriorating properties can also be sealed in the MEMS cell by this method.

[0042] Advantageously, the method according to the present invention can be used to encapsulate corrosive and / or explosive gases in the MEMS cell without damaging or destroying the MEMS device and / or the electronic circuits present therein. This is due to the fact that the introduction of gas into the MEMS cell is more controlled, because, on the one hand, the filling of the coating system occurs downstream of the bonding process, and, on the other hand, the openings can be designed with smaller dimensions or additional sealing mechanisms can be provided. Thus, the gas can be introduced into the MEMS cell in a highly controlled manner with respect to timing, concentration, and duration.

[0043] Additionally, inside the gas-filled coating system there are less reactive conditions, especially lower temperatures, than inside the bonding chamber.

[0044] For the purposes of the present invention, a MEMS cell preferably refers to a device comprising two or more wafers and in which a MEMS device is present. In this context, the term MEMS cell may be interpreted as a generic term. A MEMS cell may include one or more openings that may be sealed. A MEMS cell may 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.

[0045] For the purposes of the present invention, a 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, the two or more wafers form a volume, and all of the wafers and the resulting volume are preferably comprised by the reference chamber. Preferably, the reference chamber is gas-fillable. This may also mean that the reference chamber has one or more openings present for filling the volume of the reference chamber. Preferably, the openings are configured to be resealable in order to keep or seal the gas in the reference chamber. Advantageously, gas exchange with the environment is thereby not possible. The gas introduced or already introduced into the reference chamber is also referred to as a reference gas.

[0046] Preferably, the MEMS device and / or electronic circuit is located in the reference chamber. For the purposes of the present invention, a MEMS device is a part or component based on MEMS technology. MEMS stands for the English term microelectromechanical system, i.e., microsystem, whereby compact (micrometer range) designs are achieved with increasingly low manufacturing costs and at the same time excellent functionality. The MEMS device can be, for example, a MEMS sensor or even a MEMS actuator. Many MEMS devices are known in the prior art. Advantageously, the method according to the present invention allows a wide variety of MEMS devices to be placed in the reference chamber and filled with corrosive and / or explosive gases without damaging them.

[0047] Within the meaning of the present invention, an electronic circuit means a combination of individual electrical or electromechanical elements to form a functional arrangement, preferably allowing data or electrical signals to be transmitted, received and / or processed.

[0048] MEMS devices are often arranged on a substrate together with electronic circuits for control and / or evaluation, and are contacted to the electronic circuits via electrical connections provided, for example, by wire bonds and / or conductor tracks arranged on the substrate. The substrate may in particular act as a carrier and may also perform electrical functions, for example, providing electrical connections for the individual components.

[0049] Suitable electronic circuits include, but are not limited to, integrated circuits (ICs), application specific integrated circuits (ASICs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), microprocessors, microcomputers, programmable logic controllers, and / or other electronic circuits that are preferably programmable.

[0050] In a preferred embodiment, first, two wafers are provided, at least the first wafer and / or the second wafer having a cavity, and the MEMS device and / or electronic circuitry is present on the first and / or second wafer.

[0051] For the purposes of the present invention, a cavity preferably means a depression or indentation in the wafer. Advantageously, the presence of one or more cavities on the first and / or second wafer may result in an appropriate volume in the reference chamber by bonding the two wafers. Preferably, the cavities in the first and / or second wafer form the reference chamber after bonding both wafers.

[0052] Wafer can refer to, for example, a circular or rectangular disk having a thickness in the millimeter or sub-millimeter range. Wafers are typically made from monocrystalline or polycrystalline (semiconductor) blanks known as ingots and usually serve as substrates for, for example, coatings or components, particularly MEMS devices and / or electronic circuits. The use of the term substrate for wafer is also known in the art, and substrate preferably refers to the material being processed. For the purposes of the present invention, the terms wafer and substrate may be used synonymously.

[0053] In a preferred embodiment, the two wafers comprise a material selected from the group consisting of monocrystalline silicon, polycrystalline silicon, silicon dioxide, silicon carbide, silicon germanium, silicon nitride, nitride, germanium, carbon, gallium arsenide, gallium nitride, indium phosphide, and / or glass.

[0054] These materials are particularly easy and cheap to process in semiconductor and / or microsystem technology and are also very suitable for mass production. Likewise, these materials are particularly suitable for doping and / or coating to achieve desired electrical, thermal and / or optical properties for specific applications. The above-mentioned materials offer various advantages due to the availability of standardized manufacturing techniques, which also make them particularly suitable for the integration of further components, such as electronic circuits.

[0055] Preferably, the MEMS devices and / or electronic circuits are found in cavities in one of the two wafers.

[0056] In a preferred embodiment, the two wafers are formed in a reference chamber by bonding. Preferably, the bonding occurs in a bonding chamber. In the context of the present invention, a bonding chamber means a device in which wafers are placed to be bonded together.

[0057] Wafer bonding describes a process step in semiconductor and microsystem technology in which two wafers or slices, preferably made from, for example, silicon, quartz, glass and / or suitable substrate materials mentioned above, are bonded together.

[0058] Preferably, various processes can be used for bonding, which are also referred to as bonding processes or bonding methods within the meaning of the present invention. Suitable bonding processes include direct bonding, anodic bonding, bonding with an interlayer, glass frit bonding, adhesive bonding and / or selective bonding.

[0059] In direct bonding, particularly of silicon wafers, the hydrophilic and hydrophobic surfaces of the wafers are preferably brought into contact at elevated temperatures. Preferably, one wafer is pressed against the other in the center, advantageously creating a first contact point. This mechanical connection of the contact areas is preferably based on hydrogen bonding and / or van der Waals interactions. Therefore, the contact areas to be connected are preferably extended to the remaining wafer surface by successively removing the spacer that is present between these surfaces. For this purpose, the process temperature is preferably between 1000°C and 1200°C, and pressure is applied to the wafers, for example, on the order of 10 megapascals (MPa) to 25 MPa. Direct bonding can preferably be used to bond two silicon wafers and / or silicon dioxide wafers.

[0060] In anodic bonding, glass with an increased Na+ ion concentration (preferably positively charged sodium ions) is used, which glass is preferably in contact with a silicon wafer. In this process, an electrical voltage is applied, which is particularly configured to generate a negative polarity in the glass. Therefore, preferably, and in particular using high process temperatures, sodium ions (Na +) diffuses to the electrode, whereby a space charge zone preferably forms at the interface, causing an increase in the electric field and generating Si-O-Si bonds. These bonds preferably extend continuously across the entire interconnection surface between the glass and silicon. In this way, glass and silicon wafers can be particularly bonded together. With appropriate adaptation of the process, two silicon layers and / or silicon-metal layers can also be bonded to glass. Anodic bonding can preferably occur at a temperature of about 400°C, and equally preferably at a "low" temperature of about 180°C, preferably preserving the materials being bonded. Preferably, the various materials mentioned above can also be bonded.

[0061] Preferably, 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 based on bonding through a eutectic alloy, preferably as an intermediate layer, such as Si-Au (silicon gold) or Ge-Al (germanium aluminum). Eutectic alloys are preferably alloys in which the components are mixed in such a ratio that the entire alloy is liquid or solid at a certain temperature. Eutectic bonding can be used, for example, to bond two silicon wafers. However, other materials mentioned above can also be preferably bonded.

[0062] Glass frit bonding is also preferably based on the use of an intermediate layer between the wafers to be bonded, with the bond formation being carried out in particular by melting a glass solder / glass frit. The glass solder preferably comprises glass with a low softening temperature, for example around 400°C. The glass frit preferably comprises a superficially melting glass powder, the glass particles of which are preferably baked or at least partially sintered together. This type of bonding can preferably bond silicon and / or silicon dioxide wafers together, but also preferably the other aforementioned materials.

[0063] Adhesive bonding preferably describes a bonded structure with an intermediate layer comprising an adhesive. By adhesive bonding preferably the various above-mentioned materials can be bonded together.

[0064] Preferably, selective bonding can be performed by photolithography, etching and / or lift-off processes.

[0065] Preferably, the reference chamber can be easily manufactured by bonding two wafers that are pre-processed to have a cavity therein.

[0066] Bonding of structures from pre-processed wafers allows for the simple fabrication of complex structures that could only be fabricated at great expense from a single wafer, thereby allowing the fabrication of a reference chamber without the need to laboriously cut through the wafer stock to create a volume within the reference chamber.

[0067] In a preferred embodiment, the wafers have contact surfaces that are used in bonding two wafers together. The contact surfaces preferably comprise areas of the wafers that are provided or coated with the material required for bonding. The materials on the contact surfaces of the two wafers are preferably bonded or connected to create a reference chamber. This type of material may also be referred to as bonding material in the context of the present invention.

[0068] In a preferred embodiment, both wafers are bonded such that openings remain on the contact surfaces of the two wafers after bonding. The openings preferably result from the fact that no bonding occurs on the corresponding contact surfaces. Preferably, the openings are located in lateral regions of the reference chamber. Advantageously, the reference gas can be efficiently introduced into the reference chamber through suitable openings in the lateral regions of the reference chamber.

[0069] Within the meaning of the present invention, an opening refers to an entrance into the volume of the reference chamber. Advantageously, the reference gas can be introduced into the volume of the reference chamber through or via the opening, which is preferably sealed therein. In particular, the introduced reference gas can diffuse directly into the volume of the reference chamber through the opening. Diffusion, in this case, is known as the passive movement of particles of the introduced reference gas along the concentration gradient that exists between the coating chamber and the reference chamber volume at the beginning of the step of filling the coating chamber with the reference gas. Diffusion leads to concentration equalization based on Brownian molecular motion. Advantageously, since diffusion is a passive transport process, no additional energy is required. In particular, in contrast to U.S. Patent Application Publication No. 2018 / 0339900, no chemical reaction or exchange is required to introduce the gas into the reference chamber. This advantageously enables a particularly process-efficient introduction of the reference gas into the reference chamber within the coating system.

[0070] In another preferred embodiment, the opening is already present before bonding. Preferably, this is achieved by an etching process starting from the outside of the first or second wafer. Advantageously, this allows all contact surfaces to be used for bonding, so that the opening placed before the bonding process remains on the first or second wafer and leads into the volume of the reference chamber.

[0071] In another 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 can be advantageously used for bonding. Preferably, the opening results after bonding via an etching process.

[0072] In a preferred embodiment, after the bonding process the resulting reference chamber is removed from the bonding chamber and placed into a coating system.

[0073] Within the meaning of the present invention, a coating system preferably means a device for carrying out the production and processing of thin layers of different materials. Within the meaning of the present invention, a thin layer or film preferably means a layer of a solid material in the micrometer or nanometer range.

[0074] In a preferred embodiment, the coating system is used to fill the reference chamber with a reference gas. For purposes of the present invention, reference gas preferably means a gas that is introduced into the volume of the reference chamber, and is preferably sealed or enclosed therein.

[0075] Preferably, the coating system is first filled with a reference gas, which enters the volume of the reference chamber through an opening. Advantageously, the coating system does not have the temperature range absolutely necessary in the bonding chamber for bonding. Therefore, the reference gas, which would otherwise be a flammable or explosive gas at the high temperatures present in the bonding chamber, is introduced into the volume of the reference chamber. This expands the possibilities for gases that can be introduced into the reference chamber.

[0076] Advantageously, the reference chamber can also be filled with a particularly corrosive gas, which the MEMS device and / or electronic circuitry can contact in a controlled manner through the opening. Damage to the MEMS device and / or electronic circuitry from the corrosive gas can be avoided, since the next step, sealing, can occur immediately within the coating system.

[0077] In a preferred embodiment, after the process step of introducing a gas into the reference chamber, the reference chamber is sealed in the coating system. Advantageously, after the sealing process, there is no longer any exchange of the reference chamber with the environment. Thus, the introduced reference gas is present in the chamber and can no longer leave it or enter the environment.

[0078] For the purposes of the present invention, the environment preferably means the entirety outside the reference chamber, preferably including the coating system and the reference gas introduced therein, which was filled with the reference gas in a previous process step.

[0079] Advantageously, after the reference chamber is sealed, the reference gas can no longer leak out of the reference chamber and reference gas from the environment cannot enter the reference chamber, so the method can be used to ensure that the correct amount of reference gas is present within the volume of the reference chamber.

[0080] Preferably, the reference chamber is sealed in the coating chamber via a coating process, in particular the opening. Preferably, at least the opening can be coated with a thin film for this purpose. A coating extending at least along the opening, preferably substantially along the opening, has proven advantageous in that a particularly precise seal is applied. Furthermore, the coating material for sealing the opening is saved, ensuring a high degree of economy through efficient material utilization.

[0081] However, to ensure a tight seal, it may be preferable to coat substantially the entire reference chamber with a thin film. Advantageously, a coating extending substantially along or around the entire reference chamber provides a particularly reliable seal. Thus, on the one hand, a coating around the entire reference chamber ensures a special seal, and on the other hand, it provides particularly strong protection from possible damage from the outside. This is particularly relevant for applications in which the gas is explosive and / or corrosive and therefore must meet the highest safety requirements. Furthermore, by minimizing the risk that the introduced gas may leak, particularly accurate and therefore reliable measurements are possible, for example, in the context of photoacoustic spectroscopy.

[0082] For the purposes of the present invention, hermetic sealing preferably means a tight seal that prevents exchange of substances with the environment of the reference chamber. Hermetic sealing is preferred to prevent even the slightest contamination of the volume of the reference chamber. A device such as a reference chamber is preferably absolutely sealed against substance or substance exchange by hermetic sealing.

[0083] Within the meaning of the present invention, contamination means an undesirable amount of a substance or mixture that can get into the volume of the reference chamber. Preferably, contamination is prevented by sealing the reference chamber.

[0084] In a preferred embodiment, the reference gas comprises a corrosive and / or explosive gas, preferably methane, propane, propylene, silane, chlorosilane, aluminum triisopropanolate, hydrogen and / or oxygen, particularly preferably ammonia.

[0085] Advantageously, not only toxic but also explosive and / or corrosive gases can be introduced into the volume of the reference chamber by the method according to the invention. Thus, advantageously, the possibilities for reference gases are expanded. This represents a significant improvement over the prior art, since the choice of gas is not limited or fixed to inert gases (e.g., noble gases as in U.S. Patent Application Publication No. 2014 / 0038364).

[0086] In the method known from U.S. Pat. No. 6,124,145, a gas is introduced into the bonding chamber and sealed inside the MEMS cell. However, high temperatures are generated during various bonding processes. Therefore, gases that are flammable or explosive at those temperatures cannot be sealed in the reference chamber using known methods. Otherwise, dangers such as an explosion of the bonding chamber could result. This is also disadvantageous for users, who may be harmed in the event of an explosion. Advantageously, with the method according to the present invention, flammable and / or explosive gases can also be sealed in the reference chamber. Also, in U.S. Patent Application Publication No. 2014 / 0038364, a gas (such as a noble gas) is introduced into a cavity in the bonding chamber. However, in a departure from the prior art, according to the present invention, the gas is preferably introduced into the MEMS cell in the coating chamber, in particular by filling the coating chamber with the reference gas, for example, by passive diffusion.

[0087] Additionally, for various applications, it is desirable to include a MEMS device and / or electronic circuit in a reference chamber and fill the reference chamber with a reference gas, for example, for PAS (photoacoustic spectroscopy). However, in the method known from U.S. Pat. No. 6,124,145, it is not possible to seal a corrosive gas in the reference chamber in this case, since the bonding chamber is filled with the sealed gas. Therefore, the MEMS device and / or electronic circuit are initially exposed to a large amount of corrosive gas, which can result in damage. The time during which the MEMS device and / or electronic circuit are exposed to the corrosive gas also plays a role in this regard. The wafer bonding process itself also takes time. The MEMS device and / or electronic circuit are exposed to the corrosive gas until the bonding process is completed. Therefore, during the bonding process, the corrosive gas can damage or even destroy the MEMS device and / or electronic circuit.

[0088] With the method according to the invention, the disadvantages of the prior art are avoided or eliminated. Advantageously, therefore, with the method according to the invention, explosive and / or corrosive gases can be introduced into the reference chamber and sealed.

[0089] For the purposes of the present invention, flammable or pyrophoric preferably refers to the property of a substance or mixture having a low ignition point. Preferably, the ignition point of a substance refers to the lowest temperature at which an ignition mixture can occur on the substance. Preferably, a gas or gas mixture is considered to be flammable if it has the explosion range of air at 20°C and a standard pressure of 101.3 kPa (kilopascals).

[0090] For the purposes of this invention, explosive gas means a gas or mixture that, when sufficiently energized, for example by high temperature, undergoes a specific chemical reaction that can produce a high percentage of thermal energy. This results in a violent, expanding effect that can cause considerable destruction. If explosive substances or gases are handled improperly, a danger to life exists.

[0091] For the purposes of the present invention, a corrosive gas preferably refers to a gas that undergoes a chemical reaction with the reference chamber and / or its components, such as a MEMS device and / or electronic circuitry, resulting in significant changes. This process is preferably referred to as corrosion for the purposes of the present invention. Corrosion can result in a deterioration of the functionality of the reference chamber or components of the reference chamber.

[0092] Advantageously, explosive and / or corrosive reference gases can preferably be introduced into the reference chamber by the method according to the invention, but any other reference gas can also be introduced by the method according to the invention.

[0093] In a preferred embodiment, an inert gas, preferably nitrogen, is additionally introduced into the reference chamber through an opening to establish a partial pressure of the reference gas within the reference chamber.

[0094] For purposes of the present invention, partial pressure is the partial pressure of a single component of a gas mixture. Partial pressure corresponds to the pressure that an individual gas component would exert if it were present alone in the relevant volume. Total pressure is additively composed of partial pressures, i.e., the sum of all partial pressures equals the total pressure.

[0095] Advantageously, the amount or concentration of the reference gas in the reference chamber can be precisely determined by adjusting the partial pressure of the reference gas. This is advantageous for certain applications of the reference chamber, such as a sensor for PAS. In the application, the desired reference concentration or sensitivity can be predetermined by adjusting the concentration or amount of the reference gas. Thus, the amount of gas introduced can be optimized in a targeted and precise manner.

[0096] For the present invention, the inert gas is preferably a gas that is inert to the reaction. The gas does not or only slightly participates in the chemical reaction. For PAS applications, the inert gas must also have different absorption characteristics in the relevant excitation region. 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.

[0097] The above-mentioned advantages of the present invention are based on the fact that openings remain on the contact surfaces of the two wafers after bonding, or openings are made in the first and / or second wafer before or after bonding.

[0098] In a preferred embodiment, the first wafer and the second wafer have contact areas for bonding the first wafer to the second wafer, and regions of the contact areas are not bonded to form openings.

[0099] Preferably, this forms an opening in the lateral region of the reference chamber, which advantageously allows good penetration of the reference gas into the reference chamber within the coating system.

[0100] Advantageously, with this embodiment, no further process steps are required to form an opening in the reference chamber. The opening is preferably already produced during the bonding process. Also, the fact that no further process steps are required eliminates the need for additional materials and structuring. Manufacturing costs and time are reduced. Advantageously, this results in higher process efficiency.

[0101] In a preferred embodiment, an opening remains on the contact surface of the two wafers after bonding, and the opening is 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.

[0102] Advantageously, in this preferred embodiment, the dimensions of the opening result in a diffusion-hindering effect. In this context, diffusion within the meaning of the present invention means a phenomenon in which the equalization of concentration differences occurs without external influence. Initially, when the coating system is filled with a reference gas, there is little or no gas in 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 filling process can be enhanced by introducing a reference gas into the coating system at high pressure and / or concentration.

[0103] The preferred dimensioning of the substantially elongated aperture results in substantially one-dimensional diffusion dynamics in which the reference gas diffuses through the aperture into the volume and then does not exit therefrom or only over a longer time constant.

[0104] The diffusion-hindering effect can be used to overcome, for example, intermediate processing times. For example, it may be preferable to first fill a reference chamber with a reference gas in the coating system, then exchange the reference gas with a process gas in the chamber, and seal the opening during the coating process. Due to the diffusion-hindering effect, the reference gas does not leak out during the gas exchange or coating process.

[0105] By dimensioning the opening accordingly, it is advantageously possible to set the period during which the reference gas can diffuse into the reference chamber and the period during which diffusion outwards is reliably prevented: the preferred dimensions mentioned above represent a good compromise in this respect.

[0106] In a preferred embodiment, before or after bonding the first wafer to the second wafer, the openings are formed in the first or second wafer starting from the outside and working inwards, preferably by an etching process.

[0107] Advantageously, the position and / or dimensioning of the opening can be precisely selected by inserting the opening before or after. For example, the positioning can be selected depending on the MEMS device and / or electrical circuitry inside the reference chamber to avoid interference. For example, it may be preferable to position the opening in the center outside the first or second wafer, starting from the outside of the reference chamber and working its way inward.

[0108] In the context of the present invention, the etching process preferably comprises dry etching, wet chemical etching and / or plasma etching, in particular reactive ion etching, deep reactive ion etching (Bosch process).

[0109] The etching process preferably refers to the removal of material from a surface. The removal can take the form of a depression that leaves a cavity on the wafer.

[0110] In semiconductor technology and microsystems engineering, dry etching is a group of erosive microstructural processes that are not based on wet chemical reactions (e.g., wet chemical etching, chemical mechanical polishing). Material is removed by accelerated particles or using plasma-excited gases. Chemical and physical influences are used depending on the process.

[0111] Dry etching processes can be divided into three groups: physical dry etching processes, which are based on material removal by particle bombardment, and chemical dry etching processes, which are based largely on chemical reactions of plasma-excited gases. The third group, physical-chemical dry etching processes, combines processes using both mechanisms of action and thus minimizes the disadvantages of the first two groups.

[0112] In wet chemical etching, an etch-resist mask is transferred to the wafer through a chemical ablation process.

[0113] Plasma etching is a material removal, plasma-assisted dry etching process. In plasma etching, a distinction is made between etching removal due to a chemical reaction and physical removal of the surface due to ion bombardment.

[0114] In chemical plasma etching, material removal occurs through a chemical reaction. Therefore, it is generally isotropic due to its chemical nature and highly material-selective. Physical plasma etching, also known as plasma-assisted ion etching, is a physical process. In this process, a specific preferred direction of etching attack can occur, so the process may exhibit anisotropy in material removal. In physical plasma etching, non-reactive ions are generated in a plasma. An applied electric field accelerates these ions onto the surface, removing portions of the surface. This process is typically used to remove native oxide on silicon wafers.

[0115] Reactive ion etching (RIE) is an ion-assisted reactive etching process. Due to the good controllability of the etching behavior, RIE is a process for the fabrication of local structures for micro- and nanosystems technologies. The process allows isotropic and anisotropic (direction-independent) etching by chemical-physical ablation. Etching is performed by charged particles (ions) generated in a gas plasma. Appropriate masking of the surface (e.g., produced by photolithography) gives the shape of the structure.

[0116] Deep reactive ion etching (DRIE) is a further development of reactive ion etching (RIE) and is a highly anisotropic dry etching process for the fabrication of microstructures in wafers with aspect ratios (depth to width) of up to 50:1, making structure depths of several hundred micrometers achievable. The DRIE process is a two-step process consisting of a dry etching process alternating with an etching and passivation step. The goal is to etch as anisotropically as possible, i.e., directionally perpendicular to the wafer surface. In this way, for example, very narrow trenches can be etched.

[0117] The above-mentioned etching processes are known to those skilled in the art. Depending on the desired opening and / or wafer being provided, a suitable process can be selected to ensure effective performance.

[0118] In a preferred embodiment, the valve is present at the edge of the opening in the first or second wafer, and the valve is preferably located within the volume of the reference chamber at the edge of the opening that starts from the outside of the first or second wafer after bonding the first wafer to the second wafer. The edge of the opening preferably marks the transition from the opening in the first or second wafer to the (reference) volume of the reference chamber.

[0119] With the valve in this position, the diffusion of the reference gas into the volume of the reference chamber can be controlled and the coating chamber is filled with the reference gas.

[0120] By providing a valve, it is advantageously possible to automatically seal off the reference gas, in particular to prevent leakage of the reference gas already dispersed in the volume, without limiting the filling dynamics.

[0121] In contrast to a diffusion-hindering effect, for example through an elongated opening with substantially one-dimensional diffusion dynamics, a valve can be used to impede diffusion from the reference chamber in a more targeted manner, while at the same time allowing diffusion into the reference chamber to occur rapidly and unhindered.

[0122] Thus, the valve allows for efficient filling, whereby, for example, a reference gas can be exchanged with a process gas in the chamber, and the opening can be sealed by a coating process, preventing the reference gas from leaking out due to the provision of the valve.

[0123] Additionally, the provision of a valve advantageously allows for more precise regulation of the amount and / or concentration of the reference gas introduced into the reference chamber.

[0124] Preferably, a valve refers to a component used for directional control or blocking of a fluid flow. Preferably, the valve can be formed, for example, by a flexible shutter (for example based on a thin film structure), which allows unidirectional diffusion of a reference gas into the reaction chamber during filling and prevents diffusion from the diffusion chamber after filling.

[0125] Preferably, before bonding and before forming the opening in the first or second wafer, a material layer can be applied to the opposite side of the wafer where the opening will be introduced (e.g., etched) (preferably to form a flexible seal). Preferably, the material layer can be structured as a valve in a further process. In this case, the opening in the first or second wafer can be formed first, and then the material layer can be structured to form the valve. The opposite order is also possible.

[0126] In a preferred embodiment, the material layers that will form the valve are structured prior to bonding. The structures formed in this process can include contact pads, conductive paths, alignment features, corners, edges, depressions, depressions and / or holes.

[0127] Preferably, the material layer can be initially applied to the entire surface of one side of the wafer, in particular the structuring can confine the material layer to the area around the opening, so that the existing area of ​​material layer can act as a valve at the edge of the opening.

[0128] Preferably, the openings in the wafer are formed on the opposite side of the material layer to be structured as valves by an etching process. A person skilled in the art can select an appropriate etching process for this purpose and ensure that the valves are not damaged in the process.

[0129] Preferably, the first wafer with the opening with the valve is then bonded to the second wafer, such that the valve is located at the edge of the opening and within the volume of the reference chamber, thereby achieving the described functionality.

[0130] In a preferred embodiment, after the first wafer is bonded to the second wafer, a reference chamber in the coating system is filled with a reference gas, and the gas enters the volume of the reference chamber through an opening and via a valve.

[0131] In a preferred embodiment, the valve (or the material layer to be structured) comprises a soft metal. Preferably, for the purposes of the present invention, a soft metal can be defined as a metal having a hardness lower than that of iron (e.g., in the Brinell hardness test according to DIN EN ISO 6506-1 to 4). Suitable soft metals are non-ferrous metals selected from the group consisting of lead, gold, indium, copper, platinum, silver, zinc, tin or compounds thereof, particularly preferably aluminum or compounds thereof.

[0132] Advantageously, soft metals, on the one hand, can be precisely structured by simple means, so that the process can be configured in a time-saving and cost-effective manner, and, on the other hand, due to their flexibility, soft metals are suitable as valves for use according to the invention.

[0133] In addition to material selection, the dimensioning of the material layers also plays a role in this regard.

[0134] Preferably, the material layer or valve structured as a valve is designed as a thin film structure, which preferably means a layer thickness of less than 100 μm, preferably less than 10 μm.

[0135] In particular, with a thin film structure preferably comprising a soft metal, good flexibility of the valve ensures that the diffusion process can be adjusted accordingly.

[0136] The reference gas enters the valve through an opening in the first or second wafer and exerts pressure against the valve. Depending on the flexibility of the valve, at some point the exerted pressure will be high enough to open the valve toward the reference chamber volume and allow the gas to pass through.

[0137] In a preferred embodiment, after filling the reference chamber with the reference gas, the solder is melted to seal the opening.

[0138] For this reason, the solder is preferably placed near the opening of the reference chamber, preferably before bonding.

[0139] The application of solder in microsystem technology is a well-known procedure and does not pose any difficulties to the person skilled in the art.

[0140] Preferably, before the reference gas is filled into the coating system, the solder is placed near the opening of the reference chamber and the reference gas diffuses into the volume of the reference chamber through the opening.

[0141] Advantageously, in this embodiment, the reference gas can diffuse particularly efficiently into the volume of the reference chamber before the solder melts, without the need to provide a valve or diffusion-limiting effect of an elongated opening that may have a residual (albeit small) resistance to diffusion.

[0142] Preferably, the solder melt seals the opening of the reference chamber. The solder melting is a process step well known to those skilled in the art, whereby the melting temperature is selected depending on the material to be melted.

[0143] Preferably, as the solder melts, part of the material comprising the solder enters the opening and seals the cross section. The solder or melting material can fill the entire opening or only a portion of it, 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 to use the entire melting material to seal the opening or only a portion of it.

[0144] Placing the solder close to the openings preferably means positioning the openings in spatial proximity to ensure that the molten solder can also flow into the openings, which can mean, for example, a distance of less than 100 μm, preferably less than 10 μm.

[0145] In a preferred embodiment, the solder comprises a fusible material selected from the group comprising lead, tin, zinc, silver, copper, alloys thereof and / or compounds thereof.

[0146] Preferably, the solder reaches its melting temperature during melting, which is the temperature at which the solder is transformed from a solid to a liquid state. Preferably, the solder has a melting temperature at which the reference gas does not react flammably and / or explosively, so that any sparks that may occur do not pose a hazard.

[0147] Advantageously, the above-mentioned suitable materials for the solder have a melting temperature below the temperature at which the suitable reference gas reacts flammably and / or explosively.

[0148] In a preferred embodiment, the openings are sealed by a coating process in a coating system, preferably by spray coating, mist coating and / or vapor coating.

[0149] The coating process preferably seals the reference chamber well. Advantageously, this means that the reference gas can no longer leave the reference chamber, ensuring a permanent seal.

[0150] When the valve is at the edge of the opening, the melting of the solder already causes a first sealing of the opening. The additional application of a cover layer stabilizes the seal over time and ensures that the reference chamber remains sealed for the entire life of the reference chamber.

[0151] A variety of coating processes can be used to apply the cover coating.

[0152] Spray coating refers specifically to the two-dimensional application of the cover layer, whereby the cover layer is preferably pressurized (e.g., higher than the prevailing ambient pressure) before spraying, so that fine particles / aerosols and / or foams of the cover layer are formed. In this way, particularly fine coatings can be achieved that cover the entire spray area, even in cases where the surface is at an angle unfavorable to the spray direction. Thus, even surfaces / areas that are at an angle to one another can preferably be directly covered.

[0153] Preferably, the liquid cover layer is atomized and applied to the surface under greater than ambient pressure.

[0154] The spray coating is preferably a spray lacquer. The spray coating can also be a vapor deposition.

[0155] Mist coating preferably comprises coating with fine droplets of the mask layer that are finely dispersed in an environment, preferably a gas, whereas vapor coating preferably comprises coating with a cover layer that is applied in a vapor or gaseous state.

[0156] In a preferred embodiment, the coating system comprises a physical coating system or a chemical coating system, preferably a plasma-enhanced chemical coating system, a low pressure chemical and / or epitaxial coating system.

[0157] Physical coating systems preferably refer to coating systems that perform coatings by physical vapor deposition (PVD). Physical vapor deposition, or sometimes referred to as physical vapor deposition, refers to a group of vacuum-based coating processes or thin-film technologies. Unlike chemical vapor deposition processes, physical vapor deposition uses a starting material to be converted into a gas phase. The gaseous material is then applied to the wafer to be coated and condenses to form the target layer.

[0158] Arc evaporation, or arc PVD, is a coating process from the group of physical vapor deposition methods. In this process, an arc burns between the chamber in which the process occurs and a target at a negative potential. This arc melts and vaporizes the target material, which is then deposited on a workpiece (wafer), for example. The target acts as a cathode, a vacuum chamber wall, or a designated electrode, such as an anode. In this process, most of the evaporated material (up to 90%) is ionized. The material vapor (target material) radiates from the target, similar to thermal evaporation. A negative potential is also applied to the wafer, further accelerating the ionized material vapor toward the substrate. The material vapor condenses on the wafer surface.

[0159] An epitaxial coating system is preferably a system in which an epitaxial process, preferably molecular beam epitaxy, is used. Molecular beam epitaxy (MBE) is a physical vapor deposition (PVD) process for producing crystalline thin films (or film systems). Epitaxy means that the crystalline structure of the growing layer adapts to the structure of the substrate, as long as the physical properties of the two materials do not differ too much.

[0160] MBE requires an ultra-high vacuum to avoid contamination by residual gas atoms. However, during the growth process, the pressure rises into the high vacuum range due to outflows. The material from which the layer is to be formed is heated in an evaporation crucible and reaches the wafer as a guided molecular beam (without colliding with background gases). This too is heated, so the layer can grow in an orderly manner.

[0161] Sputtering, also known as cathode sputtering, is a physical process in which atoms are ejected from a solid (target) and transferred to the gas phase by bombardment with high-energy ions (mainly noble gas ions).

[0162] Chemical coating systems preferably refer to coating systems that perform coating via chemical vapor deposition (CVD). In chemical vapor deposition, solid components are deposited on the heated surface of a wafer as a result of a chemical reaction from the gas phase. A prerequisite for this is the presence of a volatile mixture of the layer's constituents, which deposits a solid layer at a specific reaction temperature. Chemical vapor deposition processes are characterized by at least one reaction at the surface of the workpiece to be coated. This reaction must involve at least one gaseous starting compound (reactant) and at least two reaction products, at least one of which must be in the solid phase. To promote these reactions at the surface over competing gas-phase reactions and thus avoid the formation of solid particles, chemical vapor deposition processes usually operate at reduced pressures (typically 1 to 1000 Pa). A special feature of the process is conformal coating deposition, which allows, for example, even the finest recesses in the wafer to be uniformly coated.

[0163] Chemical vapor deposition also includes atomic layer deposition (ALD). Atomic layer deposition is a highly modified chemical vapor deposition (CVD) process that relies on two or more cyclically performed self-limiting surface reactions. The materials to be deposited are chemically bonded to one or more carrier gases, called precursors. These precursors are alternately supplied to a reaction chamber where they react with the wafer, and the material bonded in the gases is deposited on the substrate material. The resulting layer typically has a polycrystalline or amorphous structure.

[0164] Plasma-Enhanced Chemical Vapor Deposition (PECVD) preferably refers to a system that uses a plasma-enhanced or plasma-assisted chemical vapor deposition process. Plasma-Enhanced Chemical Vapor Deposition is a special form of Chemical Vapor Deposition (CVD) in which the chemical deposition is assisted by a plasma. The plasma can be fired directly at the wafer to be coated (direct plasma method) or in a separate chamber (remote plasma method).

[0165] In CVD, dissociation of gas molecules occurs as a result of an external heat supply and the energy released in the subsequent chemical reaction, whereas in PECVD, this task is performed by accelerated electrons within the plasma. In addition to the radicals formed in this way, ions are also generated in the plasma and, together with the radicals, cause layer deposition on the wafer. The gas temperature in the plasma typically increases by only a few hundred degrees Celsius, meaning that, in contrast to CVD, more temperature-sensitive materials can also be coated. In direct plasma processes, a strong electric field is applied between the wafer to be coated and a counter electrode that ignites the plasma. In remote plasma processes, the plasma is positioned so that it does not come into direct contact with the substrate. This offers the advantage of selective excitation of individual components of the process gas mixture and reduces the possibility of plasma damage to the wafer surface by ions.

[0166] Low Pressure Chemical Vapor Deposition (LPCVD) is a process commonly used in semiconductor technology for the deposition of silicon oxide, silicon nitride and polycrystalline silicon, as well as metals.

[0167] In a preferred embodiment, a cover layer is applied in a coating system at least over the area of ​​the opening and preferably around the entire reference chamber, the material for the cover layer being preferably a nitride, preferably silicon nitride, silicon carbonitride, silicon oxynitride, titanium nitride and / or tantalum nitride, an oxide, preferably silicon oxide, aluminum oxide, silicon dioxide, titanium dioxide or tantalum oxide, or a metal, preferably aluminum and / or a noble metal, preferably gold, platinum, iridium, palladium, osmium, silver, rhodium and / or ruthenium.

[0168] The cover layer over the entire reference chamber advantageously provides a particularly good sealing of the reference chamber, which means that the reference gas remains particularly well contained within the volume of the reference chamber, which ensures with a particularly high degree of reliability that gas from the volume of the reference chamber cannot escape therefrom after sealing, especially if the cover layer is coated substantially along or around the reference chamber.

[0169] Advantageously, the reference chamber is also additionally protected from the outside, i.e. from its environment, in particular according to the manufacturing method, by a cover layer, which also minimizes the risk of external damage, for example through fluid contamination and through mechanical influences.

[0170] On the other hand, the above-mentioned materials are easily processable and can therefore be used to apply a cover layer, and in addition, due to their impeding properties, the materials represent a reliable and long-term stable barrier against the leakage of the reference gas and / or the ingress of external gases.

[0171] In a preferred embodiment, a process gas is introduced into the coating system to seal the opening and form a cover layer, and the process gas is introduced into a reference chamber after filling it with a reference gas, or the material for forming the cover layer is selected so that the reference gas can simultaneously function as a process gas.

[0172] Within the meaning of the present invention, a process gas preferably refers to a gas used to ensure that a coating is achieved by a cover layer in a subsequent process step. The application or adhesion of the cover layer to the reference chamber occurs via a physical and / or chemical reaction. Preferably, in this physical and / or chemical reaction, the process gas assists or enables the cover layer to be coated on the reference chamber.

[0173] Diffusion-restricting or diffusion-impeding openings are particularly desirable when additional process gas is introduced into the coating system along with the reference gas. This is advantageously achieved by the method according to the present invention, in which diffusion-impeding solder or valves or diffusion-restricting openings are attached to the lateral regions of the reference chamber with corresponding dimensions during the bonding process. Advantageously, the reference gas can first enter the volume of the reference chamber, preferably through these openings. After the introduction of the process gas, gas exchange within the coating system can then follow, as the coating process occurs. During the (at least partial) gas exchange from the reference gas to the process gas, the diffusion-restricting or diffusion-impeding openings ensure that the reference gas does not leak out. For certain applications, it may be preferable not to introduce additional process gas into the coating system, but for the reference gas to correspond to the process gas. This is preferably the case for ammonia as the reference gas, which can serve as a process gas, especially for cover layers containing nitride compounds.

[0174] Advantageously, in this case, there is no need for gas exchange within the coating system. Rather, the coating step can continue seamlessly after the reference chamber is filled with the reference gas. In a preferred embodiment, ammonia is the process gas that also serves as the reference gas, i.e., the gas for filling the reference chamber. Advantageously, by using ammonia as the process gas, no other gas needs to be introduced into the coating chamber as a process gas for bonding between the cover layer and the reference chamber, and the cover layer containing the nitride compound can serve as a particularly efficient seal for the reference gas. Regarding the retention of the reference gas within the coating system, the desired concentration of the reference gas in the chamber can be set particularly precisely. Unwanted diffusion of process gases different from the reference gas into the chamber is essentially prevented by targeted selection of process parameters.

[0175] In other embodiments, the MEMS device is a MEMS sensor and / or a MEMS actuator, and / or the electronic circuit comprises a processor, a switch, a transistor, and / or a transducer.

[0176] MEMS sensors or MEMS actuators refer in particular to sensors or actuators in the form of microsystems (microelectromechanical systems, abbreviated MEMS). Microsystems are in particular miniature devices, assemblies and / or components whose dimensions are in the micrometer range (1 μm to 1000 μm) or less and which interact as a system. MEMS sensors are, for example, sound detectors.

[0177] In another embodiment, the MEMS device comprises a sound pressure detector, preferably comprising a piezoelectric, piezo-resistive and / or magnetic bar, which may be capacitive or optical, and / or a capacitive, piezoelectric, piezo-resistive and / or optical microphone.

[0178] The embodiment is particularly suited to the use of a reference chamber within the PAS, where sound pressure waves can be detected directly within the reference chamber by a sound pressure detector.

[0179] The piezoelectric beam is preferably a vibrating structure, in particular in the form of a bending beam, and comprises, for example, piezoelectric material in the form of an actuator.

[0180] It may be preferable for the bending beams to be passive, which preferably means that they are vibrated by acoustic pressure waves. These in turn generate a voltage through the deformation of the piezoelectric material based on the piezoelectric effect. The (direct) piezoelectric effect preferably describes the generation of an electrical voltage and / or a change in impedance when a solid body made of the corresponding material undergoes elastic deformation. The voltage can be tapped, for example, by suitable contacts and read out by a corresponding electronic circuit.

[0181] It may also be preferred that the bending beam is active, which means that it is vibrated due to the inverse piezoelectric effect. The piezoelectric effect preferably describes the deformation of a material when an electric voltage and / or electric field is applied, thereby allowing a force to be exerted by the material in particular. Acoustic pressure waves can preferably cause variations in the damping of the vibrating beam, which can be measured, for example, by a change in the resonant frequency of the vibrating beam.

[0182] A bar that vibrates passively due to acoustic pressure waves can also preferably be readable, for example by capacitive, magnetic and / or piezoresistive methods. The idea is also that electrically readable changes are preferably generated by the vibrations, for example by a changing capacitance between the vibrating pole and a fixed electrode based on a changing magnetic flux through a resonant magnet, and / or by a changing electrical resistance of a piezoresistive material.

[0183] The microphone preferably comprises a vibrationally mounted diaphragm that is stimulated to vibrate by sound pressure waves, which can then be read out electrically, similar to the beam described above. Capacitive, piezoelectric and / or piezoresistive measurement methods of the vibration design can also be used.

[0184] Preferably, an optical microphone can also be used, whereby these vibrations can be preferably converted into an optical signal, for example by reflection of a laser beam on the membrane, which optical signal is read out, for example in an interferometer arrangement.

[0185] In a further aspect, the invention relates to a reference chamber producible by the method according to the invention.

[0186] The present invention therefore preferably relates to a reference chamber producible by a method, the method comprising: a) providing first and second wafers, wherein at least one of the first wafer and the second wafer includes a cavity, and wherein MEMS devices and / or electronic circuits are present on at least one of the first wafer and the second wafer; b) bonding a first wafer to a second wafer in a bonding chamber to form a volume that can be filled with a reference gas, wherein openings remain on the contact surfaces of the two wafers after bonding, or openings are made in the first and / or second wafers before or after bonding; c) filling a reference chamber with a reference gas through an opening in the coating system; d) closing the opening of the reference chamber in the coating system; Includes:

[0187] Those of ordinary skill in the art will recognize that the technical features, definitions and advantages of the preferred embodiments that apply to the method for manufacturing a reference chamber according to the present invention apply equally to the manufacturable reference chamber, and vice versa.

[0188] In a preferred embodiment of the invention, the reference chamber has a height of 10 μm to 2 mm, preferably 50 μm to 1 mm, more preferably 100 μm to 500 μm.

[0189] In this way, a flat and compact design, in particular a low overall height of the reference chamber, can be advantageously achieved.

[0190] In other preferred embodiments, the reference chamber has a length and / or width of 100 μm to 5 mm, preferably 200 μm to 3 mm, more preferably 500 μm to 2 mm.

[0191] These dimensions advantageously allow for the introduction of a sufficient amount of reference gas into the volume of the reference chamber, while also advantageously allowing for the attachment of a MEMS device, preferably a MEMS sensor, particularly preferably an acoustic pressure detector, preferably for use in a PAS.

[0192] In another aspect, the present invention provides a method for manufacturing a photoacoustic gas sensor, comprising the steps of: - manufacturing a reference chamber filled with a reference gas by a manufacturing method according to the present invention or a preferred embodiment thereof, wherein the MEMS sensor is present in the reference chamber as a MEMS device; providing a modulatable emitter; - providing a reference chamber filled with a reference gas and a modulatable emitter; Including, The reference chamber is in the beam path of the emitter, such that the emitter excites a reference gas in the reference chamber with modulatable, emissive radiation to form acoustic pressure waves that are detectable by the MEMS sensor.

[0193] In another aspect, the present invention provides a photoacoustic gas sensor, comprising: a modulatable emitter; a reference chamber filled with a reference gas, the MEMS sensor being present in the reference chamber; The present invention relates to a photoacoustic gas sensor comprising: a reference chamber located in a beam path of an emitter, such that the emitter can excite a reference gas in the reference chamber with modulatable and emissive radiation to form an acoustic pressure wave that is detectable by the MEMS sensor; and the reference chamber filled with the reference gas can be manufactured according to the method described above.

[0194] A person of ordinary skill in the art will recognize that the technical features, definitions and advantages of the preferred embodiments disclosed for the method for manufacturing a reference chamber according to the present invention or the reference chamber producible thereby apply equally to a photoacoustic gas sensor or gas sensor manufacturing method comprising such a reference chamber, and vice versa.

[0195] In particular, the present invention allows the provision of a compact photoacoustic gas sensor that can safely seal even corrosive or explosive gases (e.g., ammonia) as a reference gas, thus enabling the monitoring of toxic gas emissions in ambient air. The method according to the present invention ensures that sensitive components of the photoacoustic gas sensor (such as a MEMS sensor) are not attacked during the filling of the reference gas. Furthermore, the leakage of potentially toxic gases from the reference chamber is prevented by the sealing, and the compact configuration also ensures a high degree of safety from potential damage.

[0196] In a preferred embodiment, the reference chamber of the photoacoustic gas sensor forms a sealed system that is filled with a reference gas, and the gas to be analyzed, preferably ambient air, is present in the optical 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 an acoustic pressure wave in the reference chamber.

[0197] The basic features and essential components of a photoacoustic gas sensor are known to those skilled in the art: a modulatable emitter generates electromagnetic radiation, preferably infrared, positioned and configured such that the radiation emitted by the emitter substantially or at least partially influences the gas in the reference chamber.

[0198] When modulated irradiation occurs at a wavelength corresponding to the absorption spectrum of the molecules of the gas component present in the gas mixture, modulated absorption occurs, leading to heating and cooling processes whose time scale reflects the modulation frequency of the radiation. According to the photoacoustic effect, the heating and cooling processes lead to the expansion and contraction of the gas component, forming an acoustic pressure wave with a substantially modulated frequency. The acoustic pressure wave is also referred to as a PAS signal and can be measured by a sensor, such as a sound detector. The power of the acoustic wave is preferably directly proportional to the concentration of the absorbing gas component.

[0199] The term gas component is preferably understood to mean the ratio of chemically (and spectroscopically) identical gas molecules (e.g. ammonia) in a gas mixture, and a gas mixture means the totality or mixture of several (preferably different) gas components (e.g. air).

[0200] Various emitters are preferably considered as radiation sources for the above applications, for example narrowband laser sources can be used, which advantageously allow high radiation intensities and can be modulated preferably at high frequencies by standard components for photoacoustic spectroscopy.

[0201] Preferably, broadband emitters can also be used, which advantageously have a wide spectrum that can be further selected, for example, by means of (tunable) filters.

[0202] In a preferred embodiment of the invention, the modulatable emitter is a thermal emitter and comprises a heating element, the heating element comprising a substrate at least partially deposited with a heatable layer of conductive material on which are present contacts for a current source and / or a voltage source.

[0203] In this context, the heating element comprises a heatable layer made of an electrically conductive material that generates Joule heat when an electric current flows through it. In particular, the heating element comprises a substrate on which the heatable layer resides. The substrate preferably forms the base of the heating element. In this context, the substrate may also at least partially comprise other elements of the IR emitter, such as the base element and / or the housing element. Advantageously, the substrate can be optimally formed by process steps established in semiconductor and / or microsystem manufacturing. The aforementioned materials are particularly easy and inexpensive to process in semiconductor and / or microsystem manufacturing and are also highly suitable for mass production. Likewise, these materials are particularly suitable for doping and / or coating to achieve desired electrical, thermal, and / or radiative properties in specific applications.

[0204] The substrate may preferably be selected from the group comprising silicon, monocrystalline silicon, polycrystalline silicon, silicon dioxide, silicon carbide, silicon germanium, silicon nitride, nitride, germanium, carbon, gallium arsenide, gallium nitride and / or indium phosphide.

[0205] The electrically conductive material used to form the heatable layer can preferably be selected from the group comprising platinum, tungsten, (doped) tin oxide, monocrystalline silicon, polycrystalline silicon, molybdenum, titanium, tantalum, titanium-tungsten alloys, metal silicides, aluminum, graphite and / or copper, which on the one hand exhibit the desired thermal, electrical, mechanical and / or radiative properties and, on the other hand, are particularly easy and cheap to process.

[0206] The (micro)heating element is preferably at least partially self-supporting, allowing for thermal expansion as a result of strong temperature changes and translational movements, e.g., within the IR emitter. Partially self-supporting means that it is not at least partially rigidly and / or frictionally coupled to other elements of the emitter at an interface and therefore has freedom of movement in a direction substantially perpendicular to the interface.

[0207] The emitter is modulatable, meaning that the intensity of the emitted radiation, preferably the intensity of the beam, can be varied in a controllable manner over time. The modulation preferably causes a temporal change in intensity as a measurable variable. This means, for example, that there is an intensity difference over time between the weakest intensity measured within a measurement period and the strongest intensity measured within the same period, and this difference is typically greater than the sensitivity of the equipment used to measure or determine the intensity for the radiation spectrum and application. Preferably, the difference between the strongest and weakest adjustable intensities is significantly greater than a factor of 2, more preferably significantly greater than a factor of 4, 6, or 8. Particularly preferably, the intensity of the modulated beam is modulated for one or more predetermined resonant wavelengths.

[0208] Preferably, direct modulation can be performed by varying the current source. In the case of thermal emitters, this type of modulation is usually limited to a certain range of the modulation spectrum, for example in the order of magnitude of up to 100 Hz, due to the thermal time constant. In the case of lasers or LEDs, for example, much higher modulation rates are preferably possible, for example in the kHz range and above.

[0209] Modulation of the infrared emitter can preferably be achieved by external modulation as well, for example using a rotating chopper wheel and / or an electro-optical modulator.

[0210] A modulatable emitter preferably refers to a device that emits electromagnetic radiation in a wavelength range within a particular spectrum. In particular, the spectrum is selected to correspond to a preferred application field of the emitter, namely photoacoustic spectroscopy. In particular, excitation of vibrations of gas molecules to be investigated and / or detected in spectroscopy is preferred, which, depending on the gas molecule, corresponds to a preferred spectral range.

[0211] The emission of the IR emitter is preferably in the form of a linearly and preferably oriented beam. The term beam is intended to describe a preferably focused portion of radiation emitted by the emitter along the preferred beam direction of the emitter, with an area of ​​maximum intensity along this direction defining the beam in particular. The intensity is preferably defined as area power density, preferably in W / m 2 It has units of watts per square meter, abbreviated as watts per square meter.

[0212] Additional components, such as lenses, may be incorporated into the emitter or attached externally to provide beam focusing or collimation. Those skilled in the art know how to shape the emission profile of a radiation source by designing the emitter and using additional components to result in a desired beam profile and a desired beam direction. Preferably, the modulatable emitter can dispense with additional lenses or can be a system comprising a radiation source and at least one lens for collimating the beam.

[0213] Preferably, the reference chamber is located in the beam path of the emitter. This preferably means that the intensity of the beam is substantially or at least partially incident 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 area of ​​maximum intensity of the beam impinges on the detection chamber. Preferably, it means that the beam is focused and / or collimated so that a substantial part of the intensity impinges on the side facing the emitter. A suitable example is a Gaussian beam, in particular one having a cross-sectional profile according to a Gaussian curve. Along the beam, the z-axis is preferably defined by the distance with maximum intensity. The beam radius w at the "height" z of the beam is thereby preferably such that the intensity is 1 / e 2is defined as the distance to the z-axis reduced by approximately 13.5% (preferably by about 13.5%). Following this definition, "the reference chamber is in the path of the emitter" preferably means that substantially all of the beam radius is incident on the side of the reference chamber facing the emitter.

[0214] Preferably, the side of the reference chamber facing the emitter is transparent to the emitted radiation, so that the radiation substantially reaches the interior of the gas-fillable chamber. Preferably, in particular the side of the reference chamber facing the emitter is also referred to as the illumination surface.

[0215] The fact that the reference chamber is in the beam path of the infrared emitter means in particular that the emitter can excite the gas in the detection chamber with modulatable and emissive radiation to form an acoustic pressure wave, since this is at least partially irradiated (preferably at least 40%, more preferably at least 50%, in particular at least 60%), and in particular a substantial part of the radiation reaches the gas-fillable volume inside the detection chamber, where substantial part means in particular at least 80%, more preferably 90%, in particular 95%.

[0216] Terms such as "substantially," "approximately," "about," etc. preferably describe a tolerance range of less than ±20%, preferably less than ±10%, more preferably less than ±5%, and especially less than ±1%. The indications "substantially," "approximately," "about," etc., always" also disclose and include the exact value described. The reference chamber contains a reference gas that matches the IR emitter so that modulated infrared radiation transmitted through the reference chamber uses gas molecules of the reference gas therein to enable PAS. If the measurement path between the infrared emitter and the reference chamber also includes a portion of the reference gas in the air being analyzed (especially ambient air), and the reference gas absorbs infrared radiation, PAS occurs. This reduces the strength of absorption in the reference chamber.

[0217] Depending on the magnitude of the decrease, conclusions can be drawn about the concentration of the reference gas in the measurement path. Preferably, the more reference gas is present in the beam path outside the detection chamber, the smaller the formation of the acoustic pressure wave in the reference chamber, since absorption and excitation then already occur there. Preferably, a narrowband IR emitter is used here, so that, if possible, only the reference gas can be excited.

[0218] This measurement principle allows even the smallest concentrations of molecules of the reference gas to be detected in the ambient air, and therefore the monitoring of toxic, corrosive or explosive gases, such as ammonia, is particularly safe and reliable.

[0219] Advantageously, the measurement principle also inherently ensures no false alarms or errors. Only when the gas sensor operates correctly can the PAS signal, which exhibits the maximum amplitude expected in the absence of significant ammonia concentrations in the ambient air, be reliably detected. Limit values ​​can be defined for the monitoring range, corresponding to the permitted ammonia concentrations.

[0220] If the sound detector, emitter, or any other component is defective, this will be 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 possibly defective functioning of the gas sensor and / or unacceptable limits.

[0221] The warning does not have to be issued by a monitoring function or the like, but is inherent in the measurement principle. No warning and potentially dangerous increases in concentration going undetected are effectively avoided.

[0222] Preferably, the gas sensor further comprises a control unit configured to control the modulatable emitter and / or the MEMS sensor, to receive data from the modulatable emitter and / or the MEMS sensor and, if necessary, to evaluate the data.

[0223] The control unit may preferably comprise the above-mentioned electronic circuitry, which is located within the reference chamber and connected to the MEMS sensor. Furthermore, the control unit preferably comprises at least one (external) data processing unit (e.g., an integrated circuit (IC), an application specific integrated circuit (ASIC), a programmable logic circuit (PLD), a field programmable gate array (FPGA), a microprocessor, a microcomputer, a programmable logic controller and / or other electronic components, preferably a programmable circuit), which is located outside the reference chamber and connected to the electronic circuitry within the reference chamber.

[0224] For example, it may be preferable for the control unit, on the one hand, to output electrical control signals by an external data processing unit and to control the emitters and MEMS sensors, which electrical control signals can be modulated. On the other hand, preferably by the external data processing unit, an evaluation of the measurement data recorded by the MEMS sensors (in particular the measurement data on the PAS signal) can be performed. The internal electronic circuitry can also already perform a (pre-) evaluation of the measurement data of the MEMS sensors. However, it may also be preferable for the internal electronic circuitry to subsequently transfer the raw measurement data to the external data processing unit for further processing and / or evaluation.

[0225] In the following, the invention is explained in more detail by means of examples, without however being limited thereto. [Brief explanation of the drawings]

[0226] [Figure 1A] 1 is a schematic diagram of a first preferred embodiment of a method according to the invention for forming a reference chamber filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 1B] 1 is a schematic diagram of a first preferred embodiment of a method according to the invention for forming a reference chamber filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2A]1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2B] 1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2C] 1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2D] 1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2E] 1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2F] 1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2G] 1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2H] 1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2I]1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2J] 1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 2K] 1 is a schematic diagram of preferred process steps of a first preferred embodiment of a method for manufacturing a reference chamber that is filled with a reference gas, in which an opening remains above the contact surface of the wafer after bonding. [Figure 3A] 4 is a schematic diagram of a second preferred embodiment of a method according to the invention for forming a reference chamber filled with a reference gas, showing the valves; FIG. [Figure 3B] 4 is a schematic diagram of a second preferred embodiment of a method according to the invention for forming a reference chamber filled with a reference gas, showing the valves; FIG. [Figure 4A] 3A-3C are schematic illustrations of preferred method steps of a second preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, showing valves; [Figure 4B] 3A-3C are schematic illustrations of preferred method steps of a second preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, showing valves; [Figure 5A] 3 is a schematic diagram of a third preferred embodiment of a method according to the invention for forming a reference chamber filled with a reference gas, the opening of which is sealed by a hot solder; [Figure 5B] 3 is a schematic diagram of a third preferred embodiment of a method according to the invention for forming a reference chamber filled with a reference gas, the opening of which is sealed by a hot solder; [Figure 6A] 3 shows a schematic illustration of a preferred method step of a third preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, the opening of which is sealed by means of a hot solder; [Figure 6B]3 shows a schematic illustration of a preferred method step of a third preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, the opening of which is sealed by means of a hot solder; [Figure 6C] 3 shows a schematic illustration of a preferred method step of a third preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, the opening of which is sealed by means of a hot solder; [Figure 6D] 3 shows a schematic illustration of a preferred method step of a third preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, the opening of which is sealed by means of a hot solder; [Figure 6E] 3 shows a schematic illustration of a preferred method step of a third preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, the opening of which is sealed by means of a hot solder; [Figure 6F] 3 shows a schematic illustration of a preferred method step of a third preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, the opening of which is sealed by means of a hot solder; [Figure 6G] 3 shows a schematic illustration of a preferred method step of a third preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, the opening of which is sealed by means of a hot solder; [Figure 6H] 3 shows a schematic illustration of a preferred method step of a third preferred embodiment of a method according to the invention for manufacturing a reference chamber filled with a reference gas, the opening of which is sealed by means of a hot solder; DETAILED DESCRIPTION OF THE INVENTION

[0227] 1A to 1B show a schematic diagram of a first variant of the manufacturing method according to the invention.

[0228] A first wafer 1 and a second wafer 2 are provided, whereby the first (top) wafer 1 and the second (bottom) wafer 2 each have a cavity 6. MEMS devices and / or electronic circuits are present on the first wafer 1 and / or the second wafer 2 (e.g., within the cavity, not shown).

[0229] Bonding of the first wafer 1 to the second wafer 2 occurs in a bonding chamber and forms a volume 7 that can be filled with a reference gas 11. An opening 9 remains on the contact surfaces 3 of the two wafers after bonding. The first wafer 1 and the second wafer 2 preferably also have contact surfaces 3 for this purpose, and contact surfaces 3 are used to bond the first wafer 1 to the second wafer 2, with an area on contact surfaces 3 not being bonded to form the opening 9.

[0230] Figure 1A shows on the right side an unbonded lateral region of the reference chamber, which provides an opening 9. Filling the reference chamber with a reference gas 11 through the opening 9 can be performed in the coating system as described. In Figure 1A, the filling step is indicated by an arrow.

[0231] In a preferred embodiment, sealing the reference chamber opening 9 in a coating system is performed by a coating process that applies a cover layer 12 at least over the area of ​​the opening and preferably around the entire reference chamber, as shown in Figure 1B. The cover layer 12 may preferably be a nitride that ensures a tight seal around the entire reference chamber.

[0232] 2A to 2K show preferred method steps of a first variant of the method according to the invention for manufacturing a reference chamber filled with a reference gas 11, in which an opening remains above the contact surface 3 of the wafer after bonding.

[0233] As shown in Figure 2A, in a first process step, a first wafer 1 can be sputtered on its rear surface with a first bonding material 4. The first bonding material 4 can preferably be gold. Here, bonding material means a material that is preferably suitable for bonding.

[0234] 2B shows a preferred structuring of the first bonding material 4. The left side shows a central cross section through the reference chamber to be formed. The right side additionally shows a plan view. The structured first bonding material 4 forms a nearly closed boundary, but has an interruption on the left side. The interruption is not bonded and serves to form an opening into the reference chamber.

[0235] As shown in Figures 2C to 2F, the structuring of the first wafer 1 is carried out by means of a photoresist 8 and etching process.

[0236] In FIG. 2C, photoresist 8 is applied to the front side of the first wafer 1, leaving certain areas for further processing. As shown in FIG. 2D, this area of ​​the first wafer 1 is etched via an etching process, preferably deep reactive ion etching (DRIE). Next, a region of the first wafer 1 is etched in the etching process. However, the first wafer is preferably not entirely etched. In FIG. 2E, photoresist 8 is applied to the back side of the first wafer 1. This can be the same photoresist 8 as in the previous step, or it can be different. Starting from the back side, an etching process (preferably DRIE) is performed again at the relevant location (see FIG. 2F) to cut the first wafer 1 in the area previously etched on the front side. Additionally, the backside etching process creates a cavity 6 in the first wafer 1, which can be used to form a volume 7 in the reaction chamber.

[0237] In Figure 2G, a first wafer 1 is bonded to a second wafer 2, on which a second bonding material 5 is located. The second wafer 2 also has cavities 6 that are complementary to the cavities 6 of the first wafer 1. The second wafer 2 can be provided by a similar etching process.

[0238] The second bonding material 5 may preferably be aluminum, copper and / or gold. The second bonding material 5 may, but need not be, present in a structured form. It is sufficient for the first bonding material 4 of the first wafer 1 to have an interruption on the left side to avoid bonding at the opening 9 that is formed. Preferably, the first wafer 1 and the second wafer 2 are bonded together via thermo-compression bonding (TC, short for bonding). However, the left side region is not bonded at the contact surface 3.

[0239] In this preferred embodiment of the process, the unbonded contact surface area is used as an opening 9 for filling the reference chamber with a reference gas 11, e.g., ammonia. Thus, a reference chamber is produced that includes a volume 7 and has an opening 9. This provides for the fact that the opening 9 is located in a lateral region of the reference chamber, and that the bonding process does not occur in this region. On the left of FIG. 2G, the opening 9 can be seen in the lateral region of the reference chamber. In the illustrative example on the right of the reference chamber, a top view of the first bonding material 4, preferably gold, is shown. The bonding process occurs within the bonding chamber, but this is not shown.

[0240] In FIG. 2H, the reference chamber is no longer located in the bonding chamber, but in a coating system, preferably a PECVD (Plasma Enhanced Chemical Vapor Deposition) system. In this case, any gas at room pressure can first be located in the volume 7 of the reference chamber, which is then pumped into the coating system in FIG. 2I, and then a vacuum is created in the volume 7 of the reference chamber. Those skilled in the art know that in practice, a vacuum is never absolute and is characterized by a pressure significantly lower than normal atmospheric pressure.

[0241] In FIG. 2J, the coating system is filled with a reference gas 11, so that the reference gas 11, e.g., ammonia, enters the volume 7 of the reference chamber via the opening 9. In FIG. 2K, the reference chamber is sealed in the coating system via a cover layer 12, in particular by depositing nitride, so that the opening 9 is reliably sealed and therefore the reference gas 11 can no longer leak out of the reference chamber.

[0242] 3A to 3B show a schematic diagram of a second variant of the method according to the invention for manufacturing a reference chamber filled with a reference gas 11. A first wafer 1 and a second wafer 2 are provided, both wafers 1 and 2 also showing a cavity 6. MEMS devices and / or electronic circuits on the first wafer 1 and / or the second wafer 2 are, for example, present in the cavity 6. However, these are not shown.

[0243] After bonding the two wafers 1 and 2, an opening 9 is etched in the first wafer 1 or the second wafer 2. A valve 14 is located at the edge of the opening 9. The two wafers 1 and 2 are bonded together so that the valve 14 is located at the edge of the opening 9 in the volume 7 of the reference chamber. Preferably, the valve 14 is attached to the first wafer 1 or the second wafer 2 before the bonding process. In contrast, during the bonding process itself, the first wafer 1 is bonded to the second wafer 2 entirely using the contact surfaces 3. After bonding the first wafer 1 to the second wafer 2, the reference chamber is removed from the bonding chamber and placed in a coating system, which is filled with a reference gas 11, which enters the volume 7 of the reference chamber via the opening 9 and the valve 14.

[0244] Finally, a cover layer 12, preferably having a nitride deposit, is used to seal the reference chamber, preferably over the entire area of ​​the reference chamber.

[0245] This manufacturing process is particularly preferred if a process gas different from the reference gas 11 is used to apply the cover layer 12. Advantageously, the valve 14 already seals the reference gas 11 in the reference chamber before coating with the cover layer 12, so that any gas exchange (from reference gas to process gas) cannot lead to contamination.

[0246] 4A to 4I show preferred method steps of a second variant of the method according to the invention for manufacturing a reference chamber filled with a reference gas 11. FIG.

[0247] 4A, the first wafer 1 is coated on its rear surface with a first bonding material 4. Preferably, the first bonding material 4 is gold, which is coated on the rear surface of the first wafer 1 via a sputtering process.

[0248] In FIG. 4B, a first bonding material 4 is patterned on the rear surface of the first wafer 1. A top view of the first bonding material 4 is shown on the right. In FIG. 4C, a photoresist 8 is applied to the front surface of the first wafer 1. In FIG. 4D, a material layer 13 is applied to the rear surface of the first wafer 1. Preferably, a soft metal, particularly preferably aluminum, is applied as a thin film to the rear surface of the first wafer 1 via a sputtering process. In FIG. 4E, starting from the front surface of the first wafer 1, an opening 9 is formed via an etching process, preferably a dry etching process. In this process, the opening 9 is etched down to the material layer 13. Then, in FIG. 4F, the material layer 13 is structured to form a flexible valve 14.

[0249] In FIG. 4G, a first wafer 1 is bonded to a second wafer 2, preferably via a TC bond (see above). The second bonding material 5 on the contact surface 3 of the second wafer 2 can preferably be gold, copper, and / or aluminum. In FIG. 4H, a reference chamber in a coating system, preferably a PECVD system, is filled with a reference gas 11 through an opening 9; a valve 14 opens when the gas is introduced. In FIG. 4I, the reference chamber is sealed via a cover layer 12, preferably nitride.

[0250] 5A to 5B show a schematic representation of a third variant of the manufacturing method according to the invention. In this third variant too, the two wafers 1 and 2 have cavities 6 into which MEMS devices and / or electronic circuits can be inserted. The MEMS devices and / or electronic circuits are not shown. The two wafers 1 and 2 are preferably bonded to one another on all contact surfaces 3 in a bonding chamber.

[0251] Preferably, the opening 9 is formed via an etching process, preferably dry etching, before bonding. After bonding both wafers 1 and 2, solder 15 is placed near the opening 9. After introducing a reference gas 11, e.g., ammonia, into the volume 7 of the reference chamber in the coating system, the solder 15 melts so that it flows into the opening 9. As a result of the solder melting and flowing into the opening 9, the opening 9 is sealed. The reference chamber is sealed via a cover layer 12, preferably nitride.

[0252] 6A to 6H show preferred method steps of a third variant of the method according to the invention for manufacturing a gas-filled reference chamber. In FIG. 6A, a first wafer 1 is provided and coated on its rear side with a first bonding material 4. Preferably, the first bonding material 4 is gold and is applied to the rear side of the first wafer 1 via a sputtering process. In FIG. 6B, the first bonding material 4 is structured on the rear side of the first wafer 1.

[0253] A plan view of the structuring of the first bonding material 4 is shown on the right. In FIG. 6C, a photoresist 8 is applied to the front side of the first wafer 1. In FIG. 6D, starting from the front side, an opening 9 is formed in the first wafer 1 via an etching process, preferably a dry etching process, particularly preferably deep reactive ion etching. In FIG. 6E, a second wafer 2 is bonded to the first wafer 1 in the contact area 3. The second wafer is provided with a second bonding material 5, preferably gold, copper, and / or aluminum, on the contact surface 3. Preferably, the two wafers 1 and 2 are bonded together via TC bonding. After the bonding process, in FIG. 6F, a thermal solder 15 is applied near the opening 9. Next, in FIG. 6G, a reference chamber in the coating system is filled with a reference gas 11, e.g., ammonia. Finally, in FIG. 6H, the thermal solder 15 melts and flows into the opening 9, sealing it.

[0254] Furthermore, since the reference chamber is coated with a cover layer 12, preferably of nitride, the reference chamber is particularly well sealed. [Explanation of symbols]

[0255] 1. First wafer 2. Second wafer 3 Contact Surface 4. First bonding material 5 Second bonding material 6 cavities 7 Volume 8 Photoresist 9 aperture 11 Reference gas 12 Cover Layer 13 Material layers for structuring as valves 14 valves 15 Solder

[0256] Bibliography 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 WY et al. “High-sensitivity in situ QCLAS-based ammonia concentration sensor for high-temperature applications” Applied Physics B 122.7(2016):188 Schilt Stephane 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, September 26, 2000

Claims

1. 1. A method of fabricating a photoacoustic gas sensor including a gas-filled reference chamber in which a MEMS device and additional electronic circuitry reside, comprising: a) providing a first wafer (1) and a second wafer (2), wherein at least the first wafer (1) and / or the second wafer (2) has a cavity (6), and the MEMS device is present on the first wafer (1) and / or the second wafer (2), and the MEMS device is a MEMS sensor; b) bonding the first wafer (1) to the second wafer (2) in a bonding chamber to form a reference chamber having a volume (7) that can be filled with a reference gas (11), wherein openings (9) remain on the contact surfaces (3) of the two wafers after bonding, or openings (9) are made in the first wafer (1) and / or the second wafer (2) before or after bonding; c) after bonding, transferring the reference chamber from the bonding chamber to a coating system; d) filling the reference chamber with a reference gas (11) through the opening (9) in the coating system; e) sealing the opening (9) of the reference chamber in the coating system, the opening (9) being sealed by a coating extending around the entire reference chamber; f) providing a modulatable emitter; g) arranging the reference chamber filled with the reference gas and the modulatable emitter, the reference chamber being in a beam path of the emitter, the emitter exciting the reference gas (11) in the reference chamber with modulatable radiation to form an acoustic pressure wave detectable by the MEMS sensor; A method of manufacturing comprising:

2. 2. The method according to claim 1, characterized in that the reference gas (11) comprises a corrosive and / or explosive gas, preferably methane, propane, propylene, silane, chlorosilane, hydrogen and / or oxygen, particularly preferably ammonia.

3. 3. The method according to claim 1, wherein an inert gas, preferably nitrogen, is additionally introduced into the reference chamber via the opening (9) in order to set the partial pressure of the reference gas (11) in the reference chamber.

4. the first wafer (1) and the second wafer (2) have contact surfaces (3) used to bond the first wafer (1) to the second wafer (2), and areas on the contact surfaces (3) are not bonded to form the openings (9); and / or An opening (9) remains on the contact surface (3) of the two wafers after bonding, said opening (9) having a diameter 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.

5. 5. The method according to one or more of claims 1 to 4, characterized in that before or after bonding the first wafer (1) to the second wafer (2), the openings (9) are formed in the first wafer (1) or the second wafer (2) starting from the outside and working inwards, preferably by an etching process.

6. a valve (14) is present at the edge of the opening (9) of the first wafer (1) or the second wafer (2), and after bonding the first wafer (1) to the second wafer (2), the valve (14) is preferably located at the edge of the opening (9) starting from outside the first wafer (1) or the second wafer (2) and within the volume (7) of the reference chamber; and / or 6. The method according to claim 1, 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. 7. The method according to claim 1, wherein after bonding the first wafer (1) to the second wafer (2), the reference chamber in the coating system is filled with the reference gas (11), which enters the volume (7) of the reference chamber via the opening (9) and the valve (14).

8. 8. The method according to claim 1, wherein after filling the reference chamber with the reference gas (11), a solder (15) is melted to seal the opening (9).

9. 9. The method according to one or more of the preceding claims, characterized in that the solder (15) comprises a fusible material selected from the group comprising lead, tin, zinc, silver, copper, alloys thereof and / or compounds thereof.

10. 10. The method according to one or more of the preceding claims, characterized in that the openings (9) are sealed by spray coating, mist coating and / or vapor coating.

11. 11. The method according to one or more of the preceding claims, characterized in that the coating system is a physical or chemical coating system, a low-pressure chemical coating system and / or an epitaxial coating system.

12. 12. The method according to one or more of the preceding claims, wherein a cover layer (12) is applied at least over the area of ​​the opening (9) in order to seal the opening (9) in the coating system.

13. A process gas is introduced into the coating system to seal the opening (9) and to form a cover layer (12), 13. The method according to claim 1, wherein the process gas is introduced into the reference chamber after being filled with a reference gas.

14. the material for forming the cover layer (12) is selected so that the reference gas can simultaneously function as the process gas; The method of claim 12.

15. The method of claim 14, wherein the cover layer (12) is made of a nitride material and the process gas is ammonia.

16. 16. The method according to one or more of the preceding claims, characterized in that the electronic circuitry comprises a processor, a switch, a transistor and / or a transducer.

17. 17. The method according to one or more of the preceding claims, characterized in that the MEMS device is a sound pressure detector, the sound pressure detector preferably comprising a capacitively or optically readable piezoelectric, piezoresistive and / or magnetic bar, and / or a capacitive, piezoelectric, piezoresistive and / or optical microphone.

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