MEMS-TAPPING-MODE-CANTILEVER AS ACOUSTIC NANO POWER SENSOR

Description

[0001] The invention relates to the technical field of MEMS microphones. In a first aspect, the invention relates to a MEMS microphone for detecting acoustic signals. The MEMS microphone has a vibrating microphone diaphragm that is excited to vibrate by sound waves entering through a sound inlet aperture. Furthermore, the MEMS microphone has a cantilever comprising a measuring tip. An actuator actively excites the cantilever to vibrate, so that the measuring tip is guided against the microphone diaphragm in a non-contact manner. A tunneling current flows between the microphone diaphragm and the measuring tip, which allows the vibration behavior of the microphone diaphragm, dependent on the sound waves, to be detected. An electronic circuit is configured to measure the tunneling current between the microphone diaphragm and the measuring tip.The MEMS microphone according to the invention makes it possible to detect particularly low sound pressure levels with high resolution.

[0002] In another aspect, the invention relates to methods for detecting acoustic signals using the MEMS microphone according to the invention. Background and state of the art

[0003] Microphones are electroacoustic transducers that convert a sound event, i.e., sound waves, into an electrical signal. The electrical signal corresponds to the acoustic input signal. Currently, there is a wide variety of microphone designs and operating principles, each fulfilling different applications and functions.

[0004] In particular, extremely compact microphones based on MEMS technology, so-called MEMS microphones, are known in the prior art. MEMS stands for microelectromechanical system and is characterized by a miniaturized and compact design, especially in the micrometer range, and offers excellent functionality with ever-decreasing manufacturing costs.

[0005] A MEMS microphone typically comprises a vibrating microphone diaphragm designed to detect pressure waves from a fluid. This fluid can be either gaseous or liquid, but preferably it is sound pressure waves. A MEMS microphone preferably converts these pressure waves into electrical signals, thus acting as a sound detector.

[0006] MEMS microphones offer a multitude of advantages. For example, their compact design makes them particularly easy to arrange in arrays, which is useful for sound measurements with a directional characteristic. Furthermore, they can be manufactured using common, largely automated semiconductor processes.

[0007] It is known from the prior art to combine MEMS microphones with other measuring sensors to measure multiple signals and thereby cover a wider range of measurement ranges and applications. For example, US Patent 2015 / 0158722 A1 discloses a MEMS device comprising a MEMS microphone and another MEMS sensor in the form of a motion sensor. In one embodiment described therein, the diaphragm of the MEMS microphone is positioned in front of a perforated backplate. The MEMS microphone is located on a substrate with an opening for the entry of sound waves. Furthermore, another sensor is located on the substrate, which could be, for example, a gyroscope, an accelerometer, or a pressure sensor.In addition to measuring quantities using the MEMS microphone and other sensors, further information can also be measured and / or processed, such as ultrasound waves, infrared light, temperature, humidity and / or a gas species of the environment of the MEMS device.

[0008] However, the majority of MEMS microphones are designed for audio applications, i.e., for telephones and / or hearing aids. These applications are typically characterized by a bandwidth of less than 20 kHz and a sound pressure level of less than approximately 120 dB.

[0009] However, current technology also focuses on measuring particularly low sound pressure levels, especially using microsystems technology. This is relevant, for example, in photoacoustic spectroscopy. Photoacoustic spectroscopy employs intensity-modulated radiation with frequencies in the absorption spectrum of a molecule to be detected in a gas. If this molecule is present in the beam path, modulated absorption occurs, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. These heating and cooling processes cause expansions and contractions of the gas, generating sound waves at the modulation frequency. These sound waves can be measured, for example, by acoustic detectors such as MEMS microphones. The resulting sound pressures are extremely low.

[0010] Photoacoustic gas sensors are well-known in the prior art. A variant of a photoacoustic gas sensor is disclosed in US 2011 / 0296900 A1. In this variant, all components necessary for operation are integrated into a single MEMS device. The photoacoustic gas sensor features an infrared source mounted on a substrate. An integrated microphone can be located on the substrate itself or on a second substrate. A filter for the infrared light is located on a third substrate.

[0011] Prior art also includes approaches for measuring low sound pressure levels, particularly for photoacoustic spectroscopy. In photoacoustic spectroscopy, measuring low sound pressure levels is crucial for obtaining reliable measurement results.

[0012] In Sievilä et al. (2013), a fabrication method for a cantilever and a sensor arrangement for measuring sound pressure levels using photoacoustic spectroscopy are disclosed. Modulated infrared light is emitted into a sample cell containing a gas that absorbs the wavelength of the infrared light. A cantilever is located at one end of the sample cell and is deflected by the absorption of the infrared light. A Michelson-Morley interferometer is located outside the sample cell. The beam path of the Michelson-Morley interferometer is designed such that it intersects the cantilever through a transparent window. A change in the absorption of the gas also affects the position of the cantilever and thus the beam path of the Michelson-Morley interferometer. This allows for the measurement of changes in sound pressure. Further applications are not described.The setup is particularly complex due to the use of a Michelson-Morley interferometer and is disadvantageous for a compact design of the measurement system.

[0013] With current MEMS microphones, it is only possible to a limited extent to precisely measure low sound pressure levels. In particular, most measurements with MEMS microphones are unsuitable for low frequencies and exhibit a low signal-to-noise ratio in this range.

[0014] US Patent 2005 / 0249041 A1 discloses a MEMS microphone that is said to possess increased sensitivity. The MEMS microphone of US Patent 2005 / 0249041 A1 is said to have, among other things, several advantages over a theoretical approach outlined in introductory paragraphs

[0007] and

[0008] of US Patent 2005 / 0249041 A1. According to the discussed approach, a tunneling current between a measuring tip and the diaphragm is kept constant by a closed control loop. This is achieved by adjusting the cantilever, to which the measuring tip is attached, when the diaphragm is set into vibration by incident sound waves. According to US Patent 2005 / 0249041 A1, such a method would, disadvantageously, exhibit high sensitivity to, for example, vibrations and be more expensive to manufacture.Furthermore, according to US 2005 / 0249041 A1, with such an approach, the resonant frequency of the cantilever may fall within the range of the frequencies to be detected, making control of the measuring tip difficult or even impossible. Therefore, according to the disclosure in US 2005 / 0249041 A1, such an approach is unsuitable for providing a MEMS microphone.

[0015] The MEMS microphone disclosed in US patent application 2005 / 0249041 A1 comprises a measuring tip located on a perforated support plate, directly behind the microphone diaphragm. Sound waves cause the microphone diaphragm to vibrate. A current flows between the measuring tip on the support plate and the microphone diaphragm, which is evaluated as a measurement signal. The measuring tip itself does not move, thus reducing the influence of vibrations and eliminating the need to control its movement. However, the openings in the support plate can cause sound loss, resulting in less than precise sound measurements. Furthermore, if high sound pressure levels occur, the microphone diaphragm can touch the measuring tip, as the tip is not designed to move. The measurement is therefore static.If the measuring tip and the microphone diaphragm touch, a tunnel current no longer flows, but an ohmic current, which leads to a change in the measurement signal and can impair reliable and continuous operation.

[0016] Therefore, there is a need for improvement regarding the provision of MEMS microphones for sensitive measurements, even at low sound pressure levels. Object of the invention

[0017] The object of the invention was to eliminate the disadvantages of the prior art and to provide a MEMS microphone with which low sound pressure levels can be measured with high resolution. Furthermore, the MEMS microphone should preferably be characterized by the possibility of particularly reliable and continuous measurements as well as a cost-effective and compact design. Summary of the invention

[0018] The problem solved by the invention is achieved by the features of the independent claims. Advantageous embodiments of the invention are described in the dependent claims.

[0019] In a preferred embodiment, the invention relates to a MEMS microphone for detecting acoustic signals comprising a sound inlet aperture, a vibrating microphone diaphragm, and an electronic circuit, wherein sound waves entering through the sound inlet aperture excite the vibrating microphone diaphragm to oscillations, characterized in that the MEMS microphone has a cantilever comprising a measuring tip and an actuator, wherein the electronic circuit is configured for measuring a tunneling current between the microphone diaphragm and the measuring tip as well as for actively exciting the cantilever to oscillate, wherein, for detecting acoustic signals, the measuring tip is guided contactlessly against the microphone diaphragm, while the measurable tunneling current allows detection of the vibration behavior of the microphone diaphragm dependent on the sound waves.

[0020] The MEMS microphone according to the invention has proven to be extremely advantageous in that it can precisely measure low sound pressure levels with particularly high resolution. Compared to known MEMS microphones of the prior art, the MEMS microphone exhibits a significantly higher sensitivity. This is evident, for example, in the fact that a particularly high signal-to-noise ratio can be achieved despite low sound pressure levels.

[0021] The high sensitivity at low sound pressure levels is based on the advantageous measurement principle utilizing a tunneling current. Preferably, the cantilever, encompassing the measuring tip, is guided by the actuator in a contactless oscillation towards the microphone diaphragm. The distance between the measuring tip and the microphone diaphragm is so small that a tunneling current flows between the microphone diaphragm and the measuring tip due to the quantum mechanical tunneling effect. Preferably, the tunneling current is induced by applying a voltage. The tunneling current depends exponentially on the distance between the microphone diaphragm and the measuring tip. Due to the highly sensitive dependence of the tunneling current on the distance, even the smallest changes in distance between the microphone diaphragm and the measuring tip can be detected by measuring the tunneling current.

[0022] By actively exciting the cantilever (especially the measuring tip) to vibrate, the distance between the measuring tip and the microphone diaphragm preferably changes periodically with the known periodicity of the exciting vibration. Thus, the tunneling current is preferably a periodic signal whose amplitude depends on the distance between the measuring tip and the microphone diaphragm during the vibration.

[0023] When a sound event occurs, i.e., sound waves strike the microphone diaphragm, the microphone diaphragm is deflected and set into vibration. This changes the distance between the microphone diaphragm and the vibrating probe tip. The measurable tunneling current therefore changes depending on the vibration behavior of the microphone diaphragm, which is dependent on the sound waves.

[0024] The device can be configured differently for the detection of acoustic signals by measuring the tunnel current (this will be explained in detail later).

[0025] For example, analogous to a constant height mode The detection is carried out in such a way that the deflection and center position of the oscillating measuring tip on the cantilever are not changed, so that a vibration of the microphone diaphragm changes the distance between it and the measuring tip and thus also the tunnel current.

[0026] Alternatively, the device could, for example, be designed so that the center position of the cantilever's oscillation is adjusted to maintain a constant amplitude of the tunnel current. This configuration would correspond to a constant current mode Measurement principle, whereby the tunnel current or its amplitude is used as a control variable for the center position of the oscillating measuring tip.

[0027] According to the invention, it is advantageously exploited in every case that the tunnel current signal is extremely sensitive even to the smallest deflections of the microphone diaphragm, so that extremely small changes in sound pressure level are reliably detected.

[0028] Advantageously, the device according to the invention can be used to measure particularly low sound pressure levels with high accuracy. For example, the MEMS microphone according to the invention can advantageously measure sound pressure levels that are particularly less than approximately 20 dB (decibels), preferably less than approximately 10 dB, most preferably less than approximately 5 dB, and even less than approximately 1 dB.

[0029] It is particularly advantageous that the MEMS microphone according to the invention can measure low sound pressure levels with a particularly high signal-to-noise ratio.

[0030] Terms such as "essentially", "approximately", etc. preferably describe a tolerance range of less than ± 40%, preferably less than ± 20%, particularly preferably less than ± 10%, even more preferably less than ± 5%, and particularly less than ± 1%, and especially include the exact value. "Partially" preferably describes at least 5%, particularly preferably at least 10%, and particularly at least 20%, and in some cases at least 40%.

[0031] Furthermore, the method according to the invention is advantageous in that the mobility of the measuring tip, in particular its oscillation, prevents contact with the microphone diaphragm. Such contact could occur, for example, as described in US 2005 / 0249041 A1, when the microphone diaphragm experiences strong deflections at high sound pressure levels. Advantageously, this can be avoided by the MEMS microphone according to the invention, since the measuring tip is guided to the microphone diaphragm by oscillation without contact. Thus, wear is avoided and the long-term functionality of the MEMS microphone according to the invention is ensured.

[0032] Furthermore, the dynamic measurement of the tunneling current using a contactless oscillating probe tip allows for highly sensitive scanning of the diaphragm's vibration behavior across virtually any frequency range. In particular, the modulation frequency of the oscillating probe tip can be freely selected, enabling measurements at very low (0 Hz) to very high frequencies (MHz) independent of the excitation frequency. This independence of the probe tip's modulation frequency from the excitation frequency advantageously allows for a high signal-to-noise ratio over a wide frequency range. This represents a significant advantage over conventional MEMS microphones, which exhibit increased noise, especially at lower frequencies.

[0033] In the context of the invention, it is preferably provided that the cantilever, encompassing the measuring tip, performs a periodic oscillation to which it is actively excited. The periodic oscillation of the cantilever preferably occurs in the sense of a tapping. This preferably means that the cantilever performs a continuous, periodic oscillation that is independent of any movement of the microphone diaphragm. Instead, the measuring tip is guided against the microphone diaphragm in a non-contact, oscillating manner, in order to enable the detection of any diaphragm vibrations, provided they are excited by sound waves. With such a non-contact oscillation of the cantilever, in the sense of a tapping The measuring tip therefore changes its distance to the vibrating membrane preferably periodically.

[0034] This also distinguishes the MEMS microphone according to the invention from the disadvantageous approach described in US 2005 / 0249041 A1 in paragraphs

[0007] and

[0008] , in which a measuring tip of a cantilever is moved to follow any vibrations of the diaphragm. In the described approach, there is no active excitation of a cantilever, which is thereby moved against the microphone diaphragm in a non-contact manner. Instead, the distance of the measuring tip to the diaphragm is kept constant.

[0035] The actively excited periodic oscillation of the cantilever in accordance with the invention tappingThis is therefore not comparable to a cantilever that simply follows vibrations of the microphone diaphragm, merely responding to any movement of the diaphragm caused by sound excitation. Instead, the actively excited periodic vibration of the cantilever occurs independently of vibrations of the microphone diaphragm and is maintained throughout the entire detection process.

[0036] This is also evident from the fact that the cantilever oscillates preferentially at a frequency many times higher (for example, by a factor of 2, 3, 4 or more) than the expected oscillations of the microphone diaphragm. In contrast to tracking the cantilever, this also avoids a detrimentally high sensitivity to vibrations.

[0037] Thus, by actively exciting the cantilever to periodic oscillations, the measurement of sound events with high sensitivity can be advantageously ensured even at extremely low sound pressure levels. The MEMS microphone according to the invention advantageously succeeds in reducing or eliminating the influence of interfering factors.

[0038] Furthermore, the MEMS microphone according to the invention is advantageous in that no flow losses occur in the fluid through which the sound propagates. Instead, the sound preferably strikes the microphone diaphragm directly, without distortion of the sound waves or energy losses due to other components. Thus, a particularly accurate and unadulterated signal can advantageously be measured. In this respect, it may also be advantageous for the MEMS microphone according to the invention to have no openings other than a sound inlet opening.

[0039] Furthermore, the MEMS microphone according to the invention can be manufactured particularly efficiently, as it can be produced using standardized processes in semiconductor and microsystems technology. In particular, proven, automated semiconductor processing methods can be used to achieve cost-effective mass production.

[0040] The MEMS microphone according to the invention utilizes a novel application of measuring tips known from scanning tunneling microscopy and can therefore also draw on known processes for manufacturing such measuring tips. However, the measuring tip according to the invention is not used for recording the structure of a material surface, but rather for acoustic purposes to detect sound events. Thus, the MEMS microphone according to the invention efficiently combines two different technical fields to enable a significant improvement in the detection of acoustic signals.

[0041] The MEMS microphone according to the invention represents an advantageous further development of conventional MEMS microphones, in which highly sensitive measurements of the vibration behavior of a microphone diaphragm are used by dynamically measuring a tunneling current, and the measuring range of the MEMS microphone with respect to sound pressure level can be extended significantly below 20 dB. As explained above, this method exploits the fact that the distance between the measuring tip and the microphone diaphragm depends exponentially on the distance. Alternative measurement methods, such as capacitive measurement methods, exhibit only a linear correlation between the (capacitive) measurement signal and the displacement of the microphone diaphragm.While a MEMS microphone based on a tunneling current is less interesting for classical audio applications due to its non-linearity, it has been recognized according to the invention that the non-linearity can be exploited for highly sensitive sound detections, for example for photoacoustic spectroscopy.

[0042] A MEMS microphone preferably refers to a microphone based on MEMS technology whose sound-receiving structures have at least partial dimensions in the micrometer range (approximately 1 µm to approximately 1000 µm). These sound-receiving structures are preferably referred to as the microphone diaphragm. Preferably, the microphone diaphragm can have dimensions of less than 1000 µm in width, height, and / or thickness.

[0043] Preferably, the microphone diaphragm is planar, meaning in particular that its dimensions in each of the two dimensions (height, width) of its surface are greater than in a dimension perpendicular to these (thickness). For example, size ratios of at least 5:1, preferably at least 10:1, 50:1 or more, are preferred.

[0044] In a preferred embodiment, the length or width of the microphone diaphragm is between 1 µm and 1000 µm, preferably between 10 µm and 500 µm. Intermediate ranges from the aforementioned ranges may also be preferred, such as 1 µm to 10 µm, 10 µm to 50 µm, 50 µm to 100 µm, 100 µm to 200 µm, 200 µm to 300 µm, 300 µm to 400 µm, 400 µm to 500 µm, 600 µm to 700 µm, 700 µm to 800 µm, 800 µm to 900 µm or even 900 µm to 1000 µm. A person skilled in the art will recognize that the aforementioned range limits can also be combined to obtain further preferred ranges, such as 10 µm to 200 µm, 50 µm to 300 µm or 100 µm to 600 µm.

[0045] In a preferred embodiment, the membrane thickness is between 100 nm and 10 µm, preferably between 500 nm and 5 µm. Intermediate ranges from the aforementioned ranges are also preferred, such as 100 nm to 500 nm, 500 nm to 1 µm, 1 µm to 1.5 µm, 1.5 µm to 2 µm, 2 µm to 3 µm, 3 µm to 4 µm, 4 µm to 5 µm, 5 µm to 6 µm, 6 µm to 7 µm, 7 µm to 8 µm, 8 µm to 9 µm, or even 9 µm to 10 µm. A person skilled in the art will recognize that the aforementioned range limits can also be combined to obtain further preferred ranges, such as 500 nm to 3 µm, 1 µm to 5 µm or 1500 nm to 6 µm.

[0046] During the development of MEMS microphones, various designs have become established. These can be categorized according to their sound input method. If the sound reaches the microphone diaphragm via the bottom of the housing, it is called a bottom-port MEMS microphone. Bottom-port MEMS microphones also require an opening in the substrate on which the MEMS microphone components are located (also called the support substrate), as this is the only way for sound waves to reach the microphone diaphragm. If the sound reaches the sensor via the top of the housing, it is called a top-port MEMS microphone. Whether a top-port or bottom-port MEMS microphone is preferred usually depends on factors such as the microphone's placement within the product and / or manufacturing considerations. A MEMS microphone can be either a top-port or a bottom-port MEMS microphone.

[0047] The microphone diaphragm is designed to receive pressure waves from the fluid. The fluid can be either gaseous or liquid, preferably sound pressure waves. A MEMS microphone thus preferably converts pressure waves into electrical signals. The microphone diaphragm is preferably sufficiently thin so that it deflects under the influence of the air pressure changes caused by the sound waves and begins to vibrate. While the microphone diaphragm vibrates, electrical quantities can change, such as, in particular, the current of the tunneling current between the microphone diaphragm and the measuring tip. The change in an electrical quantity can be measured, recorded, and / or evaluated by an electronic circuit, preferably integrated into the MEMS microphone, for example, an ASIC or a processing unit.The electronic circuit preferably measures changes in such electrical quantities that arise when the microphone diaphragm vibrates under the influence of sound waves.

[0048] The electronic circuit preferably converts the vibrations of the microphone diaphragm into electrical signals. The electronic circuit preferably includes electrical connections, for example, wires. Preferably, the electronic circuit is connected to the cantilever and / or the measuring tip, so that the tunneling current can preferably be read out.

[0049] Furthermore, the electronic circuit may include an ASIC (application-specific integrated circuit), a computing unit, an integrated circuit (IC), a programmable logic circuit (PLD), a field programmable gate array (FPGA), a microprocessor, a microcomputer, a programmable logic controller and / or other electronic circuit elements.

[0050] The electronic circuit is preferably configured, firstly, to perform a measurement of the tunneling current between the microphone diaphragm and the measuring tip. Secondly, the electronic circuit is preferably configured to actively excite the cantilever to oscillate.

[0051] The wording, according to which the electronic circuit is set up to perform a specific process step, such as measuring a tunnel current or actively exciting the cantilever to oscillate, preferably means that software or firmware is installed on the electronic circuit which includes commands to carry out the aforementioned process steps.

[0052] The electronic circuit is preferably also programmable, so that settings regarding the operation of the MEMS microphone can offer various operating options. For example, it may be preferred that an excitation mode for the measuring tip and / or the cantilever is selectable, for which various operating options can be installed in the software.

[0053] The sound inlet opening preferably refers to an opening in the MEMS microphone through which sound waves can pass and strike the microphone diaphragm. Preferably, the sound inlet opening is located in the direction of fluid flow, particularly air, in front of the microphone diaphragm.

[0054] An acoustic signal preferably refers to an electrical signal generated by sound. Sound, in this context, is preferably a mechanical deformation propagating as a wave in a medium. In a fluid, sound is always a longitudinal wave, especially in air. The terms "sound" and "sound wave" can therefore be used synonymously. In gases like air, sound can be described as a sound pressure wave superimposed on the static air pressure. In sound waves, the fluctuations of the state variables pressure and density are usually small compared to their resting values. When air is discussed below as the fluid of sound waves, the average person skilled in the art knows that the explanations are also applicable to other fluids.

[0055] A cantilever, or synonymously a bending beam, is preferably a spatially extended, particularly elongated, element which is oscillatively mounted along at least one side and is otherwise preferably freestanding. The oscillating side of the cantilever can also be referred to as the free end. A cantilever can, for example, have the form of a flat, elongated cuboid whose thickness is significantly smaller compared to its transverse and / or longitudinal dimensions, with the transverse dimension preferably being smaller than the longitudinal dimension. For example, it may be preferred that the cantilever has a thickness of 0.1 µm to 10 µm, preferably 0.5 µm to 5 µm, a length of 10 µm to 1000 µm, preferably 20 µm to 500 µm, and a width of 5 µm to 100 µm, preferably 10 µm to 50 µm. The thickness of the cantilever is particularly relevant, as a greater thickness is associated with a reduced ability to bend. Therefore, it is preferable that the thickness be many times greater, i.e.,The width is reduced by a factor of 2, 3, 5, 10 or more compared to the length and / or width. It is also preferred that the width is reduced compared to the length. However, a bending beam that is oscillatively mounted on both sides or on multiple sides may also be preferred. The cantilever can be of various designs, which are preferably relevant for the selection of the actuator.

[0056] The cantilever can preferably be a unimorphic or monomorphic cantilever, which preferably comprises an active layer and an inactive or passive layer. An active layer is preferably a piezoelectric layer in which a force or deformation is triggered by an applied electric field, in particular by applying an electrical control voltage (generable by the electronic circuit). This force or deformation preferably causes the beam to deflect and / or deform, which can preferably trigger an active oscillation by a periodic electrical control signal. The inactive layer preferably comprises a non-piezoelectric material.It is preferred that the active and inactive layers interact in such a way that a resultant force is generated due to the applied control voltage, which causes a deflection of the beam. With a periodicity of the electrical control signal, this deflection preferably causes an oscillation. It is equally preferred that the inactive layer also comprises a piezoelectric material, which, however, is not electrically contacted and / or controlled by a control signal. Advantageously, no electrical control signal is applied to the inactive layer, and in particular, it is not subject to an external electric field that would trigger an internal force and / or deformation due to the indirect piezoelectric effect of the inactive layer.

[0057] The cantilever can preferably be a bimorphic cantilever, which preferably comprises at least two active layers. An inactive layer may preferably be present between the at least two active layers. It is preferred that, upon application of an electrical voltage, one active layer contracts while the second active layer expands, thereby advantageously achieving a bending of the cantilever that is particularly pronounced compared to a unimorphic cantilever, i.e., exhibits a greater amplitude for the same applied voltage.

[0058] Preferably, the cantilever includes a measuring tip. The measuring tip (English tipThe probe tip is preferably located substantially at the free end of the cantilever. The electronic circuit is configured to vibrate the cantilever, and thus the probe tip, in such a way that the probe tip vibrates without contact with the microphone diaphragm, with the distance being so small that a tunnel current flows between the microphone diaphragm and the probe tip. Preferably, the probe tip and the microphone diaphragm are made of electrically conductive material. It is also preferred that the cantilever is vibrated in such a way that the probe tip vibrates essentially perpendicularly without contact with the microphone diaphragm. Preferably, the probe tip has only a few atomic layers at its pointed end. For example, the probe tip can have a cross-section of less than 50 nm² at its pointed end, preferably less than 20 nm², and particularly preferably less than 10 nm².

[0059] In preferred embodiments, the measuring tip can be formed as an additional component on the cantilever and connected to it. In further preferred embodiments, the cantilever and the measuring tip are a single, integrated component. Preferably, the measuring tip is characterized by its geometric shape in that its lateral lines meet at a common point, this common point being preferably the pointed end of the measuring tip.

[0060] A vibration is preferably defined as a repeated temporal fluctuation of the spatial displacement of the cantilever, and thus in particular also of the measuring tip, from a central position. In particular, the vibration is essentially or at least partially periodic, meaning primarily regular over time. The cantilever is actively excited to periodic vibrations by the actuator, whereby the measuring tip is guided against the microphone diaphragm in a non-contact manner to detect acoustic signals. The vibration of the cantilever or the measuring tip thus preferably occurs independently of the movements of the diaphragm, with the measuring tip, however, exhibiting periodicity in the sense of a tappingis brought close to the diaphragm without touching it. A contactless vibration therefore preferably means that the cantilever vibrates in such a way that the measuring tip does not touch the microphone diaphragm during the vibration. In particular, contact between the measuring tip and the microphone diaphragm is also avoided when the microphone diaphragm is set into vibration by the impact of sound waves. Specifically, contact between the microphone diaphragm and the cantilever is avoided to ensure the maintenance of a tunneling current and preferably to prevent a resistive current. Contact between the measuring tip and the microphone diaphragm in constant height mode This can be avoided, for example, by adjusting the amplitude of the cantilever, and thus the measuring tip, to the expected deflections of the microphone diaphragm. constant current modeFor example, contact between the measuring tip and the microphone diaphragm can be avoided by regulating their distance from each other through a closed control loop in order to maintain a constant tunneling current.

[0061] Periodic oscillations, particularly when considered over several periods, can preferably be described by the vibration mode. The vibration mode is preferably a way of describing certain time-steady properties of an oscillation. Different vibration modes differ, in particular, in the spatial distribution of the vibration intensity, with the shape of the vibration modes preferably being determined by boundary conditions under which the oscillation propagates. These boundary conditions can be, for example, the material, dimensions, and / or mounting of the cantilever, and preferably at least one force vector acting on the cantilever.The vibration capability of the cantilever means in particular that the cantilever can be excited to a mechanical vibration over a longer period of time by a suitable drive in the form of an actuator, without structural changes (damage) occurring.

[0062] In a vibrating cantilever, there can be, for example, several bending modes in which the cantilever deflects along a preferred direction, e.g., perpendicular to a plane of the cantilever's suspension. These modes can differ, in particular, in their vibration frequency, maximum vibration amplitude, and spatial distribution. This corresponds, in particular, to the vibration of the measuring tip. A bending mode is characterized, in particular, by the fact that the vibration describes a dynamic bending process in the direction essentially normal to a principal plane of the cantilever. Preferably, the measuring tip, which is preferably located at the free end, is periodically guided by the vibration without contact to the microphone diaphragm, resulting in a tunnel current between the microphone diaphragm and the measuring tip.

[0063] Preferably, both the cantilever and the actuator are suitable for exciting such oscillations. In particular, the electronic circuit is operatively connected to the actuator in such a way that the actuator fulfills the function of actively exciting the cantilever. Active excitation of the cantilever preferably refers to actively exciting the cantilever's oscillations. It is especially preferred that the actuator forces the cantilever to oscillate.

[0064] An actuator is a component that converts an electrical signal, preferably originating from an electronic circuit, into a mechanical movement and / or a change in a mechanical quantity. In particular, the actuator actively contributes to setting the cantilever into vibration. The actuator must be capable of transmitting a force it generates to the cantilever, for example, by being connected to the cantilever in a way that enables force transmission.

[0065] Preferably, the cantilever can also at least partially encompass the actuator. The force itself must be suitable for inducing the vibrations, which in particular means that the force is periodic and preferably has essentially the same frequency as the vibrations of the cantilever to be generated, and is suitable for setting the cantilever and thus also the measuring tip into vibration, preferably in one vibration mode.

[0066] Tunneling current preferably refers to an electric current that flows between the microphone diaphragm and the measuring tip, even though they are not in mechanical contact. In other words, tunneling current refers to an electric current that flows despite a barrier between the microphone diaphragm and the measuring tip, where the barrier is particularly a potential barrier resulting from the gap created by the lack of contact between the microphone diaphragm and the measuring tip. Preferably, in the context of the invention, the tunneling current is the measured quantity that allows the detection of the sound wave-dependent vibration behavior of the microphone diaphragm. Preferably, the tunneling current makes it possible to advantageously measure various sound wave quantities that excite the microphone diaphragm to vibration.Sound quantities such as sound displacement, sound pressure, sound pressure level, sound energy density, sound energy, sound flux, sound velocity, sound impedance, sound intensity, sound power, particle velocity, sound amplitude and / or sound radiation pressure can be determined by measuring the tunnel current.

[0067] The tunneling current is due to the tunneling effect, which is known from quantum mechanics or quantum physics; thus, it is a quantum mechanical or quantum physical effect and cannot be explained by the laws of classical physics. According to the laws of classical physics, a potential barrier, i.e., an energy barrier, exists between the measuring tip and the microphone diaphragm, which prevents the transfer of charge carriers, especially electrons. This potential barrier is specifically related to the work function.

[0068] From a quantum physical perspective, the temporal evolution of the non-relativistic system is described by the Schrödinger equation. "Non-relativistic" means, in particular, that effects of relativity can be disregarded. The Schrödinger equation is a partial differential equation whose solution is the wave function, which in turn describes the state of particles, especially their location. Even in the "forbidden" region, i.e., inside and / or beyond the potential barrier, the wave function is never zero, but rather decays exponentially with increasing penetration depth. Thus, even at the end of the forbidden region, its value is not zero. Since the square of the wave function's magnitude is interpreted as the probability density for the particle's location, there is a non-zero probability for the particle to appear on the other side of the potential barrier.Since this is a quantum mechanical effect, the tunneling effect is also referred to as the quantum physical or quantum mechanical tunneling effect.

[0069] To utilize the quantum mechanical tunneling effect for conserving the tunneling current, an electrical voltage is preferably applied, ranging from a few mV (millivolts) to a few V (volts). Preferably, the measuring tip is positioned at a distance in the angstrom range (10⁻¹⁰ m (meters)) from the microphone diaphragm during contactless oscillation. Due to the applied voltage and the small distance, a tunneling current flows between the microphone diaphragm and the measuring tip. The tunneling current is essentially dependent on the distance, the applied voltage, and the work function of the materials used.

[0070] The measurable tunneling current is generally associated with low current intensities, typically ranging from a few pA (picoamperes) to a few nA (nanoamperes) and sometimes even down to a few mA (milliamperes). Therefore, even small deviations in the tunneling current are particularly noticeable as a measurement signal, since the tunneling current depends exponentially on the distance, and the smallest changes in distance lead to a considerable change in the tunneling current as a measurement signal.

[0071] In a further preferred embodiment, the MEMS microphone is characterized in that the cantilever performs vibrations with a frequency of more than 20 kHz (kilohertz), preferably more than 50 kHz, particularly preferably more than 100 kHz.

[0072] The specified frequencies of the cantilever's vibration are preferably many times higher than the expected vibrations of the microphone diaphragm resulting from the impact of sound waves. The frequency of the cantilever's vibration is preferably at least 2, 3, 4, 5, 10, 15, 20, 50, 100, 200, 500, 1000 or more higher than possible frequencies of the microphone diaphragm.

[0073] Advantageously, the vibration behavior of the cantilever, due to its specified vibration frequencies, is independent of the vibration of the microphone diaphragm. This allows the microphone diaphragm to vibrate across a wide frequency range while still ensuring reliable detection of an acoustic signal via the tunneling current. Since it is preferred that the cantilever vibrates at a frequency significantly higher than that of the microphone diaphragm, the measurement of sound events is possible continuously, reliably, and with high accuracy. The specified frequencies have proven particularly efficient for easily and effectively generating and measuring a tunneling current between the microphone diaphragm and the measuring tip.Advantageously, the MEMS microphone according to the invention allows a large frequency range of the microphone diaphragm to be precisely detected, in particular from very low frequencies (<10Hz) up to high frequencies in the kHz or even MHz range.

[0074] In a further preferred embodiment, the MEMS microphone is characterized in that the electronic circuit is configured to keep the amplitude and center position of an oscillation of the measuring tip and / or the cantilever constant, wherein a change in the amplitude of the tunneling current between the microphone diaphragm and the measuring tip is measured, the amplitude of the tunneling current depending on the oscillation behavior of the microphone diaphragm.

[0075] The embodiment described in the last disclosed paragraph corresponds, by analogy to scanning tunneling microscopy, to the so-called constant height mode (English). constant height modeIn scanning tunneling microscopes, the measuring tip follows a predefined height profile without the need to readjust the distance between the sample being examined and the measuring tip of the scanning tunneling microscope.

[0076] The principle of constant height mode can be advantageously applied to the MEMS microphone for the detection of sound events according to the invention via a tunnel current. The electronic circuit is preferably configured such that the amplitude and the center position of the cantilever and / or the measuring tip remain constant. A center position of the cantilever and / or the measuring tip preferably refers to a spatial position that serves as a reference point for the amplitude of the vibration. The amplitude of the vibration is preferably the maximum displacement of the cantilever and / or the maximum displacement of the measuring tip around the center position. The amplitude, in particular the maximum displacement, can preferably be specified by a quantity with the dimension of a distance.Preferably, the amplitude is set in such a way that, with regard to expected deflections of the microphone diaphragm, contact between the microphone diaphragm and the measuring tip is avoided.

[0077] In preferred embodiments, the cantilever, and in particular the measuring tip, performs vibrations that exert deflections in the range of pm (picometers) to nm (nanometers) and µm (micrometers).

[0078] To illustrate the measurement principle, two cases are considered: a theoretical case in which the microphone diaphragm is not excited to vibrate by sound waves, but is essentially stationary, and the actual case of vibration excitation of the microphone diaphragm by sound waves.

[0079] In the theoretical case where the microphone diaphragm essentially does not vibrate, there is a fixed distance between the microphone diaphragm and the center position of the cantilever and / or the probe tip, provided that the cantilever, and therefore also the probe tip, vibrates with a constant amplitude around a fixed center position. Consequently, the tunneling current also has a uniform amplitude, since the distance between the microphone diaphragm and the probe tip only changes within the range of the cantilever's vibration.

[0080] Sound waves entering through the sound inlet and striking the microphone diaphragm cause it to vibrate. These vibrations mean there is no fixed distance between the center position of the cantilever and / or the probe tip and the microphone diaphragm. Specifically, the distance between the microphone diaphragm and the probe tip changes depending on the diaphragm's vibration behavior. The higher the sound pressure level, the greater the deflection of the microphone diaphragm. Conversely, the greater the deflection, the smaller the distance between the probe tip and the microphone diaphragm, resulting in a measurably increased tunneling current.

[0081] Conversely, the lower the sound pressure level of incident sound waves, the weaker the deflection of the microphone diaphragm and the lower the tunneling current.

[0082] The changes in the signal strength of the tunnel current thus directly reflect the vibration behavior of the microphone diaphragm, which depends on the incident sound waves.

[0083] Advantageously, the measuring principle of a constant height while maintaining a constant amplitude and center position of the vibration of the cantilever and / or the measuring tip is particularly easy to set up and allows for robust high-frequency vibration excitation of the cantilever.

[0084] In a further preferred embodiment, the MEMS microphone is characterized in that the electronic circuit is configured to keep the amplitude of the tunneling current between the microphone diaphragm and the measuring tip constant, whereby an oscillation of the cantilever and / or the measuring tip is regulated to keep a constant distance between the microphone diaphragm and a central position of the measuring tip.

[0085] The embodiment described in the last disclosed paragraph corresponds, by analogy to scanning tunneling microscopy, to the so-called constant tunneling current mode (English). constant current mode ) . The height of the probe tip is continuously adjusted to maintain a constant tunneling current. In scanning tunneling microscopes, this is achieved via a closed-loop control system that regulates the distance between the probe tip and the sample under investigation.

[0086] The principle of constant tunneling current mode from scanning tunneling microscopy can advantageously be transferred to a detection of sound events by tunneling current according to the invention. It is preferred that the electronic circuit is configured to keep the amplitude of the tunneling current between the microphone diaphragm and the measuring tip constant. For this purpose, the vibration of the cantilever and / or the measuring tip is regulated such that the distance between a microphone diaphragm and a central position of the measuring tip remains constant.

[0087] The center position of the measuring tip, in particular, defines the reference point for the amplitude of the measuring tip's vibration. The center position of the cantilever need not be identical to the center position of the measuring tip. The measuring tip is preferably located essentially at the free end of the cantilever and also has a certain spatial extent. Thus, the center position of the measuring tip refers to the position from which the amplitude of the measuring tip's vibration can be described. Since the cantilever encompasses and vibrates the measuring tip, the measuring tip exhibits the same vibration pattern as the cantilever; that is, the vibration trajectory is essentially the same.

[0088] It is preferred that the amplitude of the tunneling current remains constant by maintaining a constant distance between the center position of the measuring tip and the microphone diaphragm. This is preferably achieved by a closed-loop control system. Preferably, the closed-loop control system adjusts the tunneling current to a predetermined amplitude during oscillation, particularly displacement, of the microphone diaphragm. This is preferably accomplished by adjusting the distance between the center position of the measuring tip and the displacement of the microphone diaphragm. The closed-loop control system is a circuit that can be configured, for example, by the electronic circuitry of the MEMS microphone. The closed-loop control system performs the tasks of measuring, comparing, and adjusting.The closed-loop control system measures the tunneling current, compares the measured tunneling current's amplitude to a predefined value, and adjusts it accordingly if there is a deviation. Preferably, the tunneling current amplitude is adjusted by changing the center position of the measuring tip. For this purpose, it is preferred that the cantilever (comprising the measuring tip) performs a translational movement to adjust the distance between the measuring tip and the microphone diaphragm. Preferably, this translational movement of the cantilever is performed by an additional actuator. In preferred embodiments, the additional actuator for performing the translational movement of the cantilever can be a MEMS-based drive. For example, a MEMS drive can be a microactuator connected to the cantilever via coupling elements.

[0089] To illustrate the measurement principle, as above, two cases are considered: an idealized case in which the microphone diaphragm essentially does not vibrate, and an actual case in which the microphone diaphragm is excited to vibrate by sound waves.

[0090] In the theoretical case where the microphone diaphragm essentially does not vibrate, the distance between the center position of the measuring tip and the microphone diaphragm remains constant. The tunneling current has a constant amplitude. The closed-loop control system recognizes that it is unnecessary to adjust the distance between the center position of the measuring tip and the microphone diaphragm, since there is no deviation from a predetermined amplitude of the tunneling current.

[0091] When sound waves enter the microphone diaphragm through the sound inlet, the diaphragm vibrates. These vibrations initially change the distance between the diaphragm and the center position of the measuring tip. This also affects the tunneling current, specifically causing its amplitude to deviate from a predefined value. The closed-loop control system detects this change in the tunneling current amplitude. In particular, the closed-loop system allows for a comparison between the predefined and measured tunneling current, for example, by calculating a difference and / or a ratio. To then restore the tunneling current to the predefined value, the distance of the measuring tip to its center position is adjusted until the predefined tunneling current amplitude is restored.

[0092] The higher the sound pressure level, the greater the deflection of the microphone diaphragm. The greater the deflection of the microphone diaphragm, the greater the change in the distance between the center position of the measuring tip and the microphone diaphragm, which must be adjusted based on the control variable of a constant tunneling current.

[0093] Conversely, the lower the sound pressure level of the incident sound wave, the weaker the deflection of the microphone diaphragm. The weaker the deflection of the microphone diaphragm, the less the distance between the center position of the measuring tip and the microphone diaphragm changes, which must be regulated based on the measured tunneling current.

[0094] Controlling the distance based on the constant tunnel current allows for the advantageous measurement of sound signals with particularly high sensitivity and accuracy. Furthermore, a closed-loop control system enables very precise monitoring of the microphone diaphragm's vibration behavior, which allows conclusions to be drawn not only about the frequency and sound pressure level of the sound signals, but also about their entire temporal evolution.

[0095] In a further preferred embodiment, the MEMS microphone is characterized in that the electronic circuit is configured to apply a bias voltage to the microphone diaphragm so that a zero point position and / or oscillation capability of the microphone diaphragm can be regulated.

[0096] Advantageously, the sensitivity of the microphone diaphragm can be regulated and / or adjusted by applying a bias voltage to the microphone diaphragm. In the context of the invention, a bias voltage is defined as an electrical voltage applied such that the microphone diaphragm acquires a curvature that exceeds its neutral position. The neutral position of the microphone diaphragm preferably refers to the force-free resting position of the microphone diaphragm when no deflection occurs due to sound waves and no bias voltage is applied. Thus, by applying a bias voltage, the positioning of the microphone diaphragm can deviate from the neutral position and / or cause the microphone diaphragm to exhibit a curvature that deviates from the neutral position. It is preferred that the bias voltage can be regulated by the electronic circuitry.

[0097] Applying a bias voltage advantageously allows the sensitivity of the microphone diaphragm to be regulated. Applying a bias voltage affects the diaphragm's ability to vibrate. The higher the applied bias voltage, the lower the diaphragm's ability to vibrate. Conversely, the lower the applied bias voltage, the higher the diaphragm's ability to vibrate. This allows for precise adjustment of the microphone diaphragm's sensitivity.

[0098] The sensitivity of the microphone diaphragm preferably refers to the ability to experience a deflection, the deflection being dependent on the sound pressure levels of the sound waves entering through the sound inlet opening.

[0099] Regulating the vibration capability of the microphone diaphragm is advantageous in that, particularly with regard to the small distances involved in the occurrence of tunneling current, optimal adaptation of the MEMS microphone to different sound pressure levels can be ensured. Thus, the MEMS microphone according to the invention exhibits high dynamic range and is advantageously suited for a wide range of sound signals to be measured.

[0100] In a further preferred embodiment, the MEMS microphone is characterized in that the MEMS microphone has a sensitivity which allows it to measure sound pressure waves with a sound pressure level of less than 20 dB, particularly preferably less than 10 dB, less than 5 dB, less than 1 dB or less than 0 dB.

[0101] Advantageously, the MEMS microphone according to the invention can measure low sound pressure levels, as specified above, with extreme precision. The sound pressure level (SPL) is the base-10 logarithm of the squared ratio between the RMS value of the measured sound pressure and its reference value of 20 µPa (micropascals), which is commonly used in acoustics. This allows sound pressure levels with a high signal-to-noise ratio to be measured advantageously.

[0102] It is further advantageous that the compact MEMS microphone according to the invention enables the measurement of sound events with high resolution despite low sound pressure levels. Lateral resolution preferably refers to a resolution perpendicular to the path of a measurement using sound waves. The opposite of lateral resolution is axial resolution along the longitudinal path of the measurement, i.e., the path of the sound. In particular, lateral resolution is the distance between two adjacent objects, e.g., two sound sources, which can be imaged as two points. This advantageously makes it possible to create a very accurate image of a sound field, even at low sound pressure levels.

[0103] Low sound pressure levels are particularly relevant in photoacoustic spectroscopy, making the MEMS microphone according to the invention especially suitable for efficient use in photoacoustic measurements. Further advantageous applications of the MEMS microphone according to the invention are also possible. Low sound pressure levels can also be generated by machines, such as lighting fixtures. Therefore, the MEMS microphone according to the invention is advantageously suited for monitoring devices where low sound pressure levels are relevant. The MEMS microphone can be used optimally and efficiently in a wide variety of possible applications.

[0104] In a further preferred embodiment, the MEMS microphone is characterized in that the electronic circuit is configured such that the actuator regulates the vibration of the cantilever in such a way that the distance between the maximum deflections of the measuring tip and the vibrating microphone diaphragm is between 0.1 nm and 100 nm. Intermediate values, such as maximum distances between 1 nm and 50 nm or 0.5 nm and 10 nm, are also preferred.

[0105] The distance between the measuring tip and the microphone diaphragm is regulated, in particular, by the center position of the cantilever's vibration and / or the amplitude of the vibration excitation. This distance range has proven particularly advantageous for enabling high-resolution measurement of the diaphragm's vibration behavior using the tunneling current, while simultaneously effectively preventing contact between the measuring tip and the diaphragm. The MEMS microphone according to the invention is therefore characterized by long-term stable measurement capability.

[0106] In a further preferred embodiment, the MEMS microphone is characterized in that the actuator excites the cantilever comprising the measuring tip to vibrate, wherein the actuator is preferably selected from a group comprising a piezoelectric actuator, an electrostatic actuator, an electromagnetic actuator, a magnetostrictive actuator and / or a thermal actuator.

[0107] The actuators mentioned above are particularly well-suited for exciting a large number of rapid vibrations and, due to their compact design, exhibit low energy consumption. Furthermore, the bandwidth of achievable vibrations is advantageously high due to their compact design and low inertia.

[0108] Preferably, the actuator is a MEMS actuator. A MEMS actuator is preferably one that is manufactured using standard microsystems technology methods and has dimensions on the order of micrometers (µm). Such an actuator is particularly compact, robust, and requires little maintenance, and can be manufactured easily and cost-effectively. In particular, the cantilever, which is excited to vibration by the actuator, can also be a MEMS element. This means that preferably the cantilever and the actuator can be manufactured in a single step with the MEMS actuator and are compact. Ideally, the same substrate can be used for some of the manufacturing processes. This significantly simplifies and reduces the cost of production.

[0109] A piezoelectric actuator preferably refers to an actuator that utilizes the piezoelectric effect. In particular, a piezoelectric actuator can excite the cantilever to vibrate by utilizing the inverse piezoelectric effect. The piezoelectric effect comprises the direct piezoelectric effect, which describes the generation of an electrical voltage upon deformation of certain solids, especially piezoelectric crystals, and the inverse piezoelectric effect, in which deformation is induced by applying an electrical voltage. Preferably, a piezoelectric actuator comprises a piezoelectric crystal that deforms after the application of an electrical voltage. Depending on the piezoelectric crystal and its cross-section, it becomes longer, wider, and / or bends.

[0110] An electrostatic actuator preferably uses electrostatic fields to move components, particularly the cantilever. For this purpose, the cantilever preferably comprises a material that reacts to the applied electrostatic fields. An electromagnetic actuator converts electrical energy into mechanical energy. This preferably utilizes the effects of electromagnetism. In a thermal actuator, a heat source is preferably used to generate movement, particularly of the cantilever.

[0111] Magnetostrictive actuators are preferably based on the change in length of ferromagnetic materials. These actuators are preferably manufactured using sintering techniques and change their length under magnetic fields. Advantageously, these actuators can also be used under high pressures and temperatures as positioning elements with high positioning accuracy in the micrometer range.

[0112] In a further preferred embodiment, the MEMS microphone is characterized in that the vibrating microphone diaphragm comprises an electrically conductive material, wherein the electrically conductive material is preferably selected from a group comprising monosilicon, polysilicon, molybdenum, tantalum, aluminum, graphite, tungsten, titanium, platinum, gold, palladium, iron, copper, silver, brass, chromium, their compounds and / or alloys, wherein optionally the vibrating microphone diaphragm comprises an additional non-electrically conductive material, which is preferably selected from a group comprising silicon nitride and / or silicon dioxide.

[0113] These materials are easy and inexpensive to process in semiconductor and / or microsystem manufacturing and are suitable for large-scale production. The microphone diaphragm can be advantageously manufactured with flexibility due to the materials and / or manufacturing methods. In particular, it is preferable to manufacture the MEMS microphone, comprising the vibrating microphone diaphragm together with a support, in a single (semiconductor) process, preferably on a substrate. This further simplifies and reduces manufacturing costs, enabling the cost-effective provision of a compact and robust MEMS microphone.

[0114] Furthermore, the listed materials are advantageous in that they are characterized by particularly inert behavior. Due to their inertness, they do not react with the environment in which the MEMS microphone according to the invention is to be installed, thus advantageously providing a particularly robust and insensitive MEMS microphone.

[0115] Preferably, electrically conductive material is used in this process, in particular to ensure the maintenance of the tunnel current.

[0116] It is also preferred to provide the microphone diaphragm with semiconducting and / or dielectric materials. Dielectric materials preferably mean electrically non-conductive materials. In particular, dielectric materials are preferably introduced into the microphone diaphragm in such a way that they are embedded within it. Advantageously, the preferably used semiconducting and / or dielectric materials in the microphone diaphragm result in it being mechanically supported. Advantageously, the sensitivity and thus the vibration capability of the microphone diaphragm can also be adjusted in this way.

[0117] In a further preferred embodiment, the MEMS microphone is characterized in that the cantilever and / or the measuring tip comprises a material selected from a group including silicon, iridium, tungsten, platinum, palladium and / or gold.

[0118] These materials advantageously possess the desired electrical, mechanical, and / or thermal properties to efficiently facilitate the flow of tunnel current between the microphone diaphragm and the measuring tip. Furthermore, the materials are very easy and cost-effective to process in order to provide the cantilever and / or the measuring tip.

[0119] The tunneling current is particularly relevant with regard to the dimensioning of the measuring tip. Specifically, the measuring tip can be designed such that one atom forms the foremost tip of the measuring tip and / or is responsible for the majority of the tunneling current.

[0120] In a further preferred embodiment, the MEMS microphone is characterized in that the measuring tip has a radius of up to 15 nm, preferably up to 10 nm, particularly preferably up to 5 nm.

[0121] In another preferred embodiment, the MEMS microphone is characterized in that the cantilever has a length of up to 1000 µm, a width of up to 100 µm and a thickness of up to 10 µm.

[0122] The dimensions of the cantilever and / or the measuring tip listed above have proven advantageous in that they reliably enable a tunnel current between the microphone diaphragm and the measuring tip, and are optimally designed to oscillate, so that the guidance to the microphone diaphragm can be carried out particularly efficiently, for example by the actuator.

[0123] The average person skilled in the art recognizes that technical features, definitions and advantages of preferred embodiments which apply to the MEMS microphone according to the invention for the detection of acoustic signals also apply to a method for the detection of acoustic signals comprising the MEMS microphone according to the invention and vice versa.

[0124] In a further aspect, the invention relates to a method for detecting acoustic signals comprising a MEMS microphone comprising a sound inlet aperture, a vibrating microphone diaphragm and an electronic circuit, wherein sound waves entering through the sound inlet aperture excite the vibrating microphone diaphragm to vibrate, characterized in that the MEMS microphone has a cantilever comprising a measuring tip and an actuator, wherein the cantilever and / or the measuring tip is actively excited to vibrate by the actuator and is guided to vibrate contactlessly against the microphone diaphragm, so that a tunneling current between the measuring tip and the microphone diaphragm is measured and the tunneling current allows detection of the vibration behavior of the microphone diaphragm which depends on the sound waves.

[0125] The method for detecting acoustic signals comprising the MEMS microphone according to the invention advantageously allows for high-resolution measurement of low sound pressure levels. Thus, sound pressure levels can be measured that are particularly less than 20 dB (decibels), preferably less than 10 dB, more preferably less than 5 dB, and most preferably less than 1 dB. It is particularly advantageous that the low sound pressure levels can be measured with a particularly high signal-to-noise ratio.

[0126] The average person skilled in the art knows that the signal-to-noise ratio (SNR) describes the ratio of the actual signal component to the noise component. Expressing the SNR in dB allows for better quantification. Noise refers to any interference that can impair the signals. Thus, the SNR can be used to evaluate the reception quality of the recorded signals. The higher the SNR, the lower the noise component compared to the desired or measured signal, and the easier it is to filter out. Especially in the context of measuring low sound pressure levels, the SNR in the prior art was not high, as the influence of noise is weighted more heavily. Therefore, it is particularly advantageous that the MEMS microphone according to the invention can measure with a comparatively high SNR even at low sound pressure levels.

[0127] Measuring with a tunnel current allows for a highly sensitive measurement, as even the smallest deviations are clearly detectable, especially by using a small tunnel current.

[0128] At the same time, contact between the measuring tip and the microphone diaphragm can be advantageously avoided, thus enabling a reliable, long-term stable and safe measurement of the vibration behavior of the microphone diaphragm and thus of the incident sound waves.

[0129] Furthermore, the measurement can be advantageously carried out without flow losses of the fluid in which the sound propagates, thus enabling a particularly accurate picture of the sound field.

[0130] In another aspect, the invention relates to the use of the MEMS microphone according to the invention or preferred embodiments thereof for photoacoustic spectroscopy.

[0131] In another aspect, the invention relates to a photoacoustic gas sensor comprising a modulatable emitter, a gas-fillable analysis volume and a MEMS microphone according to the invention or a preferred embodiment thereof wherein the modulatable emitter and the MEMS microphone are arranged such that the emitter can excite gas in the analysis volume to form sound pressure waves by means of modulatably emittable radiation, which can be detected with the help of the MEMS sensor.

[0132] The average person skilled in the art recognizes that the technical features, definitions and advantages of preferred embodiments of the MEMS microphone according to the invention also apply to the method or use according to the invention as well as to a photoacoustic gas sensor comprising a described MEMS microphone and vice versa.

[0133] A photoacoustic gas sensor is known to those skilled in the art in its basic design and essential components. A modulatable emitter generates electromagnetic radiation and is preferably arranged and configured such that the radiation emitted by the infrared emitter essentially or at least partially strikes the gas in the analysis volume.

[0134] If the modulated irradiation occurs at a wavelength corresponding to the absorption spectrum of a molecule of a gas component within the gas mixture, modulated absorption takes place, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. According to the photoacoustic effect, these heating and cooling processes cause expansion and contraction of the gas component, exciting it to generate sound pressure waves with essentially the modulation frequency. These sound pressure waves are also known as PAS signals and can be measured with particular sensitivity using the MEMS microphone as described. The power of the sound waves is preferably directly proportional to the concentration of the absorbing gas component.Due to the possibility of measuring extremely low sound pressure levels using the MEMS microphone according to the invention, even the smallest proportions of gas components can be advantageously detected.

[0135] Various emitters are preferably suitable as radiation sources for the applications mentioned. For example, narrowband laser sources can be used. These advantageously allow the use of high radiation intensities and can be modulated, preferably at high frequencies, using standard components for photoacoustic spectroscopy. Broadband emitters can also be used preferentially. These advantageously have a broad spectrum, which can be further selected, for example, by using (tunable) filters. Preferably, the modulatable emitter is a modulatable infrared emitter.

[0136] In one embodiment, the modulatable emitter can be a thermal emitter comprising a heating element, wherein the heating element includes a substrate on which at least a heatable layer of a conductive material is applied, and on which contacts for a current and / or voltage source are located. The heating element comprises a heatable layer of a conductive material that produces Joule heating when an electric current flows through it. The heating element particularly includes a substrate on which the heatable layer is located. The substrate preferably forms the base of the heating element. The substrate can also at least partially comprise other elements of the IR emitter, such as a base element and / or housing elements.

[0137] The emitter is modulatable, meaning that the intensity of the emitted radiation, preferably the beam intensity, can be controlled and varied over time. The modulation should preferably produce a temporal change in intensity as a measurable quantity. This means, for example, that the difference in intensity over time between the weakest intensity measured within the measurement period and the strongest intensity measured within the same period is greater than the sensitivity of a device typically used for measuring or determining intensity for the radiation spectrum and application. Preferably, the difference is significantly greater than a factor of 2, more preferably 4, 6, or 8, between the strongest and weakest adjustable intensity. The modulation of the beam intensity is particularly preferably performed for one or more predetermined resonant wavelengths.

[0138] Preferably, direct modulation can be achieved by varying the current supply. With a thermal emitter, such modulation is usually limited to a specific range of a modulation spectrum due to thermal time constants, e.g., on the order of up to 100 Hz. With, for example, a laser or an LED, significantly higher modulation rates, e.g., in the kHz range and beyond, are preferably possible.

[0139] Modulation of the emitter can preferably also be achieved by external modulation, e.g. by using a rotating chopper wheel and / or an electro-optic modulator.

[0140] The gas to be analyzed is preferably located in a gas-fillable analysis volume. In a preferred embodiment, this is a volume (or chamber) that is at least partially enclosed or closable from the outside, into which the gas is located or can be introduced, e.g., through a closable opening in the form of a closure and / or valve and / or through a supply line. However, it can also be a completely enclosed or closable volume or chamber that has at least one, preferably two, closable openings for introducing and / or venting the gas to be analyzed. This allows the gas to be very precisely localized, particularly within a beam region of the emitter, for example, infrared radiation.

[0141] The analysis volume can preferably also be at least partially open. This allows, in particular, the measurement and analysis of the composition of a gas atmosphere surrounding the spectrometer, against which the analysis volume is at least partially open. This is especially interesting for applications in the field of pollutant measurement, but also, for example, for military applications or counterterrorism, e.g., against a chemical weapons attack.

[0142] In this case, it is advantageous that the analysis volume is well defined, so that the emitter, the analysis volume and the MEMS microphone are arranged in such a way that the radiation emitted by the emitter, which can be modulated, can excite the gas in the analysis volume to form sound pressure waves, which can be measured with the help of the MEMS microphone.

[0143] The analysis volume is preferably located in the beam path of the emitter. This preferably means that the intensity of the beam is substantially or at least partially directed onto the side of the analysis volume facing the emitter. Partially preferably means at least 40%, more preferably at least 50%, 60%, 70%, 80% or more.

[0144] An analysis volume can be formed by a single chamber. However, it may also be preferable for the analysis volume to comprise a sample chamber and a reference chamber, which are connected or connectable via a connecting channel.

[0145] In the case of an embodiment of an analysis volume which has a sample chamber and a reference chamber, it may be preferred to position a MEMS microphone in each chamber in order to measure separately in each chamber and thus be able to preferably remove sources of interference, e.g. external sound pressure waves, which do not originate from the radiation absorbed in the sample chamber, preferably after the measurement.

[0146] It is also preferable for the emitter to irradiate the sample chamber and not the reference chamber, with a connecting channel between the sample chamber and the reference chamber containing the MEMS microphone as an acoustic detector. The sample volume and the reference volume may contain the same gas. It is equally preferable for the sample volume and the reference volume to contain different gases, with the reference volume containing a gas with known properties and the sample volume containing a gas to be analyzed. This embodiment is characterized by particularly precise photoacoustic spectroscopy, since, for example, sound from unwanted sound sources is excluded or not measured during the measurement and / or evaluation of the measurement. Preferably, the sample volume and a reference volume can have essentially the same dimensions to implement a precise differential measurement method.

[0147] The MEMS microphone according to the invention will be explained in more detail below using examples, without being limited to these examples. FIGURES Brief description of the characters

[0148] Fig. 1 Schematic representation of a preferred MEMS microphone Detailed description of the figures

[0149] Fig. 1 Figure 1 shows a schematic representation of a preferred MEMS microphone 1. As explained above, the MEMS microphone 1 is particularly well suited for measuring low sound pressure levels with high resolution.

[0150] The MEMS microphone 1 includes a vibrating microphone diaphragm 3. When a sound event occurs, sound waves propagate 5 starting from a sound source in the direction of the MEMS microphone 1 through a sound inlet opening and strike the microphone diaphragm 3,which is then excited to vibrate. Furthermore, the MEMS microphone exhibits 1 a cantilever 7 including a measuring tip 9 on. Further components such as an electronic circuit and an actuator are in Fig. 1 not shown.

[0151] The electronic circuit is set up so that the cantilever 7 comprehensive the measuring tip 9 It is actively excited to vibrate, whereby the transmission for the execution of the vibration can be carried out by the actuator. The vibration of the cantilever 7 and thus the measuring tip 9 The dotted line and the curved arrow below the measuring tip should indicate 9 This will be made clear. The measuring tip 9 and the microphone diaphragm 3 They comprise electrically conductive material. The measuring tip 9 It vibrates so close to the microphone diaphragm without contact 3This resulted in a tunneling current (not shown) flowing, based on the quantum mechanical tunneling effect. The tunneling current reflects the oscillation of the measuring tip as a periodic signal. 9, because the distance between the measuring tip 9 and the microphone diaphragm changes periodically. The measurable tunneling current allows detection of the microphone diaphragm's vibration behavior. 3, the sound waves 5, especially depends on factors such as the sound pressure level and / or the frequency.

[0152] Advantageously, the MEMS microphone can be used 1 Low sound pressure levels can be measured with a simultaneously high signal-to-noise ratio, since the tunneling current, due to its exponential dependence on distance, allows for even the smallest deflections of the microphone diaphragm. 3 can measure with exceptional precision.

[0153] The MEMS microphone 1This is also advantageous in that it ensures particularly reliable contact between the measuring tip and the probe. 9 and the microphone diaphragm 3 This is avoided. Furthermore, there are advantageously no sound flow losses, allowing for a particularly precise sound image to be generated. Thus, the MEMS microphone can 1 Measure sound pressure levels with high precision and reliability and accurately detect even the smallest deviations.

[0154] The cantilever 7 and therefore also the measuring tip 9 They perform vibrations that are significantly faster than the expected vibrations of the vibrating microphone diaphragm. 3, so that the vibration of the cantilever is also advantageously affected 7 and / or the measuring tip 9 independent of the vibration of the microphone diaphragm 3 is. Therefore, the microphone diaphragm can 3oscillate in a wide frequency range while simultaneously offering reliable and particularly sensitive detection of sound waves through the tunnel current.

[0155] The MEMS microphone 1 It can have different operating modes.

[0156] The electronic circuit can be configured to measure the amplitude and center position of an oscillation of the measuring tip. 9 and / or the cantilever 7 The current is kept constant. A change in the amplitude of the tunneling current is measured, which provides information about the vibration behavior of the microphone diaphragm. 3 and with sound parameters such as the sound pressure level of incident sound waves 5 gives.

[0157] Furthermore, the electronic circuit can be configured such that an amplitude of the tunneling current between the microphone diaphragm 3 and the measuring tip 9is kept constant. This is achieved by maintaining a vibration of the cantilever. 7 and thus the measuring tip 9 is regulated in such a way that a distance between the microphone diaphragm 3 and a central position of the measuring tip 9 The distance between the microphone diaphragms is kept constant. This is achieved primarily through a closed control loop, which can be implemented, for example, by the electronic circuitry. In this case, the necessary distance adjustment between the microphone diaphragms is permitted. 3 and a central position of the measuring tip 9 direct conclusions can be drawn about the vibration behavior of the membrane and thus the incident sound waves 5. REFERENCE MARK LIST

[0158] 1 MEMS microphone 3 Vibrating microphone diaphragm 5 Sound waves 7 Cantilever 9 Measuring tip BIBLIOGRAPHY

[0159] Sievilä, Päivi, et al. "Sensitivity-improved silicon cantilever microphone for acousto-optical detection." Sensors and Actuators A: Physical 190 (2013): 90-95.

Claims

1. MEMS microphone (1) for detecting acoustic signals, comprising a sound inlet opening , a vibratable microphone membrane (3) and an electronic circuit, wherein sound waves (5) entering through the sound inlet opening induce the vibratable microphone membrane (3) into vibrations, characterized in that the MEMS microphone (1) exhibits a cantilever (7) comprising a measuring tip (9) and an actuator, the electronic circuit being configured for measuring a tunnel current between the microphone membrane (3) and the measuring tip (9) and for an active induction of the cantilever (7) into vibrations, wherein, for the detection of acoustic signals, the measuring tip (9) is guided to the microphone membrane (3) in a contactless vibrating manner, while the measurable tunnel current permits detection of the vibration behavior of the microphone membrane (3) which is dependent on the sound waves (5), wherein the cantilever (7) performs a periodic vibration due to the active induction in which a distance between the microphone membrane (3) and the measuring tip (9) changes with the periodicity of the active induction.

2. MEMS microphone (1) according to the previous claim characterized in that the cantilever (7) performs vibrations independently of the vibration behavior of the microphone membrane (3) due to the active induction, preferably with a frequency of more than 20 kHz, preferably more than 50 kHz, particularly preferably more than 100 kHz.

3. MEMS microphone (1) according to one or more of the preceding claims characterized in that the electronic circuit is arranged to keep an amplitude and a center position of a vibration of the measuring tip (9) and / or the cantilever (7) constant, wherein a change in an amplitude of the tunnel current between the microphone membrane (3) and the measuring tip (9) is measured, wherein the amplitude of the tunnel current depends on the vibration behavior of the microphone membrane (3).

4. MEMS microphone (1) according to one or more of the preceding claims characterized in that the electronic circuit is arranged to keep an amplitude of the tunnel current between the microphone membrane (3) and the measuring tip (9) constant, wherein a vibration of the cantilever (7) and / or the measuring tip (9) is regulated to keep a distance between the microphone membrane (3) and a center position of the measuring tip (9) constant.

5. MEMS microphone (1) according to one or more of the preceding claims characterized in that the electronic circuit is configured to apply a bias voltage to the microphone membrane (3) such that a zero point position and / or vibration capability of the microphone membrane (3) can be regulated.

6. MEMS microphone (1) according to one or more of the preceding claims characterized in that the MEMS microphone (1) exhibits a sensitivity which allows sound pressure waves (5) with a sound pressure level of less than 20 dB, particularly preferably less than 10 dB, to be measured.

7. MEMS microphone (1) according to one or more of the preceding claims characterized in that the electronic circuit is configured such that the actuator regulates the vibration of the cantilever (7) in such a way that there is a distance of between 0.1 nm and 100 nm between maximum deflections of the measuring tip (9) and the vibratable microphone membrane (3).

8. MEMS microphone (1) according to one or more of the preceding claims characterized in that the actuator induces the cantilever (3) comprising the measuring tip (9) into vibrations, wherein preferably the actuator is selected from a group comprising a piezoelectric actuator, an electrostatic actuator, an electromagnetic actuator and / or a thermal actuator.

9. MEMS microphone (1) according to one or more of the preceding claims characterized in that the vibratable microphone membrane (3) comprises an electrically conductive material, wherein preferably the electrically conductive material is selected from a group comprising monosilicon, polysilicon, molybdenum, tantalum, aluminum, graphite, tungsten, titanium, platinum, gold, palladium, iron, copper, silver, brass, chromium, their compounds and / or alloys, wherein optionally the vibratable microphone membrane (3) comprises an additional non-electrically conductive material, which is preferably selected from a group comprising silicon nitride and / or silicon dioxide.

10. MEMS microphone (1) according to one or more of the preceding claims characterized in that in that the cantilever (7) and / or the measuring tip (9) comprises a material selected from a group comprising silicon, iridium, tungsten, platinum, palladium and / or gold.

11. MEMS microphone (1) according to one or more of the preceding claims characterized in that the measuring tip (9) exhibits a radius of up to 15 nm, preferably up to 10 nm, particularly preferably up to 5 nm.

12. MEMS microphone (1) according to one or more of the preceding claims characterized in that the cantilever (7) exhibits a length of up to 1000 µm, a width of up to 100 µm and a thickness of up to 10 µm.

13. Method for detecting acoustic signals comprising a MEMS microphone (1) comprising a sound inlet opening, a vibratable microphone membrane (3) and an electronic circuit, wherein sound waves (5) entering through the sound inlet opening induce the vibratable microphone membrane (3) into vibrations, characterized in that the MEMS microphone (1) exhibits a cantilever (7) comprising a measuring tip (9) and an actuator, the cantilever (7) and / or the measuring tip (9) being actively induced into vibrations by the actuator and being guided to the microphone membrane in a contactless vibrating manner, such that a tunnel current between the measuring tip (9) and the microphone membrane (3) is measured and the tunnel current permits detection of the vibration behavior of the microphone membrane (3) dependent on the sound waves (5), wherein a distance between the microphone membrane (3) and the measuring tip (9) changes with the periodicity of the active induction.

14. Use of the MEMS microphone (1) according to one or more of claims 1 - 12 for photoacoustic spectroscopy and / or infrared spectroscopy.

15. Photoacoustic gas sensor comprising - a modulable emitter, - an analysis volume that can be filled with gas and - a MEMS microphone (1) according to any of the previous claims 1-12, wherein the modulable emitter and the MEMS microphone are arranged in such a way that the emitter can excite gas in the analysis volume by means of modulably emittable radiation to form sound pressure waves, which can be detected with the aid of the MEMS sensor.