Gas type compensation for MEMS devices

Abstract

A control and processing unit (40) for a metering system (1) comprising at least a first sensor (10) comprising a membrane (12), a suspension (13) of the membrane (12), a bottom wafer (21) and a cavity (30) formed between the membrane (12) and at least a portion of the bottom wafer (21). The metering system (1) comprises an oscillation generator unit (28) configured to cause at least the membrane (12) to generate oscillations. The control and processing unit (40) is configured to drive the oscillation generator unit (28) in a first oscillation mode and a second oscillation mode, measure a first damping response during application of the first oscillation mode, measure a second damping response during application of the second oscillation mode, jointly process the first damping response and the second damping response, and control the oscillation generator unit (28). And deriving at least one value of the pressure and / or molecular parameter based on the joint processing.

Description

[0001] The present invention relates to a measurement method using a microelectromechanical system (MEMS) sensor. The present invention relates to a device comprising an oscillating or movable element (such as a resonator membrane).

[0002] Microelectromechanical systems are commonly used in many devices that combine mechanical and electronic functions on a scale ranging from a few micrometers to a few millimeters. Some examples of the broad application areas of MEMS devices are applications in sensors, actuators, oscillators, and microfluidics.

[0003] Pressure gauges are key elements for operating and controlling modern vacuum systems. Such systems are of utmost importance in various industries, such as the coating, automotive, optical, and semiconductor industries, or in the production of solar cells and medical devices. In all these industries, in order to monitor or control process steps that must be carried out under reduced pressure, the pressure in the vacuum chamber must be measured reliably.

[0004] Today, there are various pressure gauges available, which differ in their basic measurement principle, the pressure range they handle, their manipulation, and their reliability. In particular, MEMS pressure gauges are available, such as pressure gauges comprising a suspended resonator combined with a friction / squeeze film (hereinafter referred to as "squeeze film").

[0005] The characteristics and reliability of MEMS devices comprising small-sized components depend to a large extent on dimensional variations in the micrometer range. This leads to high requirements regarding the device layout and the manufacturing process. In the case of squeeze-film type pressure gauges, it is necessary to optimize the interaction of the sensing element with gas molecules, especially the gas molecules forming the squeeze film. This can be accomplished by having a resonantly oscillating membrane at a well-defined distance within a few micrometers from the substrate and extending over a large area parallel to the substrate, i.e., by forming a cavity having an aspect ratio greater than 100, for example, between the extension of the membrane parallel to the substrate and the distance of the membrane from the substrate.

[0006] Furthermore, the membrane needs to exhibit appropriate resonance modes, which can be achieved by loading the membrane with a suitable mass. Preferably, this is done in a process step that is easily integrated into the overall manufacturing process of the pressure gauge or its pressure transducer (sensor), and the process step allows for easy adjustment of the mass.

[0007] For example, the corresponding friction / squeeze film measurement sensor element is already known from US11 118 991 B2. Here, the measurement method is based on the influence of ambient pressure on the system characteristics (such as resonance frequency, Q factor) of a microresonator. In this case, a properly designed cavity gives the space required for the oscillation of a resonator or resonator element, which includes a membrane that may be loaded with mass, and the oscillation characteristics of the resonator or resonator element indicate pressure. The cavity (especially in the form of a thin gas film cavity) forms a gap near the resonator and causes pressure-dependent squeeze film damping of the resonator, such that the measured system characteristics indicate pressure.

[0008] In particular, MEMS sensors according to the prior art can be calibrated for use in a defined process atmosphere with a known gas species and thus provide accurate measurement values.

[0009] However, as described above, the interaction of gas molecules with the sensing element affects the damping behavior of the system. Thus, this damping depends not only on the amount of molecules but also on the type of molecules present in the cavity. Therefore, measuring the damping of a MEMS resonator can provide information about the pressure in the cavity, but such pressure measurement can also depend on the current gas species.

[0010] This effect becomes even more relevant when using MEMS sensors in the presence of varying or undefined gas species. Due to the presence of unknown or undefined molecules or molecular compositions, this can lead to pressure measurement values that differ from the actual pressure value.

[0011] Therefore, an object of the present invention is to provide an improved sensor that overcomes the above-mentioned drawbacks.

[0012] Another object of the present invention is to provide an improved measuring instrument that provides more reliable and accurate results.

[0013] Another object of the present invention is to provide an improved sensor that provides additional information related to the current gas species.

[0014] These objects are achieved by implementing the characterizing features of the independent claims. Features for further developing the present invention in an alternative or advantageous manner can be gathered from the dependent claims.

[0015] The present invention is based on the observation that MEMS sensors for measuring, for example, pressure, especially pressure transducers based on squeeze film for example, provide different sensitivities for different excitation frequencies. The sensitivity of such a sensor should be understood here as the relationship between the damping effect caused by the fluid in its cavity at the oscillation element of the sensor and the pressure of the fluid.

[0016] In the context of the present invention, "fluid" shall be understood as a gas, a specific gas species, a composition of different gas species, a precursor, a liquid, or a combination of at least two of these.

[0017] The present invention relates to a metering system, in particular a manometer system, which comprises a control and processing unit and a first sensor, in particular a squeeze-film based pressure sensor. The first sensor comprises: a membrane; a suspension of the membrane, which allows the membrane to oscillate; and a bottom wafer, wherein the suspension of the membrane is connected to the bottom wafer. The first sensor further comprises a cavity, which is formed between at least a part of the membrane and the bottom wafer. In particular, the transducer comprises an inlet connecting the cavity to the surrounding environment. In particular, a structured top wafer provides the membrane and the suspension of the membrane.

[0018] The measurement system comprises an oscillation generator unit, which is configured to at least cause the membrane to oscillate.

[0019] According to the present invention, the control and processing unit is configured to provide

[0020] · driving the oscillation generator unit in a first oscillation mode and a second oscillation mode,

[0021] · measuring a first damping response during application of the first oscillation mode, the first damping response depending on a molecular parameter related to the fluid and a pressure parameter related to the fluid,

[0022] · measuring a second damping response during application of the second oscillation mode, the second damping response depending on the molecular parameter related to the fluid and the pressure parameter related to the fluid,

[0023] · jointly processing the first damping response and the second damping response, and

[0024] · based on the joint processing, at least deriving the value of the pressure parameter and / or the value of the molecular parameter.

[0025] By performing the above steps by means of the control and processing unit, a compensated measurement of the pressure of the fluid can be provided.

[0026] In one embodiment, the sensor may comprise a resonator (membrane) with a large surface area, which is suspended over a small air gap (cavity) and has an aspect ratio > 10:1, preferably > 100:1, preferably > 1000:1. The suspension may be asymmetric or symmetric, preferably asymmetric.

[0027] The resonator may be excited via electrostatic force, or alternatively, it may also be mechanically (externally) excited.

[0028] The sensor readout (damping response) can be performed by a capacitive readout scheme. In one embodiment, it can also be read out by an optical, piezoelectric, or piezoresistive device. The electrode layout for excitation and / or capacitive readout can be symmetric or asymmetric, preferably asymmetric.

[0029] The sensor can be temperature-stable or non-stable, preferably temperature-stable.

[0030] The bottom wafer should be understood as any suitable kind of substrate or base according to the prior art, but not necessarily a crystal-based substrate.

[0031] In one embodiment, jointly processing the first damping response and the second damping response can include taking into account the different sensitivities of one or more sensors. Each of these sensitivities also depends on the type of fluid (e.g., gas species) present in the cavity.

[0032] Different sensitivities can be provided by driving the oscillation generator unit in a first oscillation mode and a second oscillation mode.

[0033] The difference in gas species sensitivity can be achieved using a single resonant member excited (simultaneously) in two or more oscillation modes. Additionally or alternatively, two or more resonant members (membranes) can be excited (simultaneously) in one or more oscillation modes. These two members can be placed on a single MEMS chip or on several physically separated MEMS chips. These members can be electrically connected in series or in parallel to one or more dedicated readout electronics units. Two transducers (sensors) can use the same or different oscillation modes.

[0034] By this combination of two pressure signals sensitive to gas species, compensation for gas species correlation can be provided, and a pressure signal that is at least independent of the molecular weight can be obtained.

[0035] Additionally or alternatively, the gas species can be determined by deriving molecular parameters (e.g., average molecular weight).

[0036] The sensor (i.e., the membrane) can be driven to oscillate in a desired oscillation mode (e.g., with a defined frequency, amplitude, oscillation direction, etc.). Thereby, a specific sensitivity of the sensor is provided. This sensitivity is preferably determined before the measurement. In other words, calibrate the sensor and one or more specific excitations. Calibration can be performed by measuring the sensitivity by measuring one (or more) actuation voltages (damping responses) of several different fluids (e.g., gases) at one (or more) pressure points. Based on this, the relationship between the sensitivity and the molecular parameters can be fitted (see Figure 2a and Figure 2b ).

[0037] In one embodiment, the metering system includes one or more additional sensors, which are configured according to the above first sensor. The oscillation generator unit includes a first oscillation generator and two or more oscillation generators. The first oscillation generator is configured to oscillate the membrane of the first sensor, and the two or more oscillation generators are configured to oscillate the membranes of the one or more additional sensors.

[0038] In particular, a first oscillation mode is provided by driving the first oscillation generator, and a second oscillation mode is provided by driving the two or more oscillation generators.

[0039] By providing at least two sensors (pressure transducers), two sensitivities can be provided simultaneously by driving each of the two sensors in a specific oscillation mode. Since any interaction between the oscillation modes is initially avoided, this provides robust and accurate measurements.

[0040] According to one embodiment, the metering system can be used to measure the pressure of the fluid in the cavity. Here, the metering system represents a pressure gauge system.

[0041] According to one embodiment, the metering system can be used to measure the molecular weight of the fluid in the cavity, particularly the average molecular weight. This can allow the derivation of a specific type of fluid, particularly a gas species. Here, the metering system represents a gas detection unit.

[0042] According to one embodiment, the metering system can be used to measure the molecular weight and pressure of the fluid in the cavity.

[0043] The present invention also relates to a control and processing unit for a metering system, particularly for a pressure gauge system. The metering system includes at least a first sensor, particularly a pressure transducer, which includes a membrane, a suspension of the membrane, and a bottom wafer. The suspension allows the oscillation of the membrane, and the suspension of the membrane is connected to the bottom wafer (directly or through a connector). The sensor further includes a cavity formed between at least a portion of the membrane and the bottom wafer. In particular, the transducer includes an inlet connecting the cavity to the surrounding environment. In particular, a structured top wafer provides the membrane and the suspension of the membrane. The metering system further includes an oscillation generator unit configured to oscillate at least the membrane.

[0044] The control and processing unit is configured to provide:

[0045] · driving the oscillation generator unit in a first oscillation mode and a second oscillation mode,

[0046] · Measure a first damping response during application of the first oscillation mode, the first damping response depending on molecular parameters associated with the fluid to be measured (in particular present in the cavity) and pressure parameters associated with the fluid.

[0047] · Measure a second damping response during application of the second oscillation mode, the second damping response depending on the molecular parameters associated with the fluid and the pressure parameters associated with the fluid.

[0048] · Jointly process the first damping response and the second damping response, and

[0049] · Based on the joint processing, at least derive the value of the pressure parameter and / or the value of the molecular parameter.

[0050] The control and processing unit can provide a compensated measurement of the pressure parameter (e.g., pressure) of the fluid by controlling the sensors respectively. Alternatively or additionally, the control and processing unit can provide a compensated measurement of the molecular parameter of the fluid by controlling the sensors respectively.

[0051] In particular, the control and processing unit according to the present invention provides for using the metering system as a pressure gauge system and / or a gas detection system.

[0052] By applying an actuation signal (e.g., a varying actuation voltage) to the oscillation generator unit, in particular to the actuation electrode of the oscillation generator unit, it is possible to drive the oscillation generator unit in the first oscillation mode and / or the second oscillation mode.

[0053] It should be understood that the control and processing unit of the metering system can be implemented according to the control and processing described below.

[0054] In one embodiment, the control and processing unit is configured to derive the value of the molecular parameter based on joint processing. This also allows for deriving further information associated with the fluid.

[0055] In one embodiment, the molecular parameter represents the molecular weight of the fluid. Thus, the average molecular weight of the fluid can be derived. In particular, specific gas species can be identified by having information about the molecular weight.

[0056] According to one embodiment, the pressure parameter represents the pressure of the fluid. As described above, the pressure of the fluid can be derived in this way.

[0057] In one embodiment, by applying or changing the actuation voltage for driving the oscillation generator unit, it is possible to provide driving the oscillation generator unit in the first oscillation mode and / or the second oscillation mode, wherein the actuation voltage is applied or changed by using a feedback loop. Thereby, the oscillation mode can be set and maintained in a constant manner, and the desired oscillation of the membrane can be provided.

[0058] The metrology system, in particular the sensor, may include an actuation electrode for exciting the oscillation mode. A corresponding actuation signal (actuation voltage) can be applied to the actuation electrode. The actuation electrode can be provided by the oscillation generator unit.

[0059] In particular, the actuation signal can be controlled by means of a feedback loop such that the phase relationship between the actuation signal and the mechanical oscillation of the membrane remains constant and / or such that the amplitude of the mechanical oscillation of the membrane remains constant.

[0060] In particular, the first damping response and / or the second damping response can be measured by determining the actuation signal, in particular by means of a feedback loop.

[0061] In one embodiment, the first oscillation mode provides an oscillation of the membrane with a first amplitude and a first frequency, and the second oscillation mode provides an oscillation of the membrane with a second amplitude and a second frequency, wherein at least the first frequency is different from the second frequency and / or the first amplitude is different from the second amplitude.

[0062] In one embodiment, the control and processing unit is configured to provide an oscillation of the membrane in the first oscillation mode in a first oscillation direction and an oscillation of the membrane in the second oscillation mode in a second oscillation direction, wherein the first oscillation direction is different from the second oscillation direction.

[0063] The oscillation direction can be, for example, an out-of-plane oscillation of the membrane, i.e., a change in the position of the membrane in a direction perpendicular to the extension of the membrane. Another oscillation direction can be an inclination of the membrane around an axis (short axis) located in the plane of the membrane.

[0064] In one embodiment, the first damping response and / or the second damping response can be measured by capacitance measurement, wherein the capacitor for providing the capacitance measurement is provided by at least a part of the membrane and the bottom wafer.

[0065] The metrology system, in particular the sensor, may include a capacitance electrode for measuring the damping response of the oscillation mode. The corresponding capacitance can be measured by means of the capacitance electrode.

[0066] According to one embodiment, the sensor may include a first capacitor for measuring the amplitude and / or frequency of the oscillation of the membrane relative to the bottom of the cavity. The first capacitor is formed, for example, by the following electrodes:

[0067] · The membrane (in particular, the surface of the membrane facing the cavity) and the bottom wafer, where the bottom wafer includes a first capacitor formed by a part of the membrane and / or a part of the bottom wafer;

[0068] · A first type of electrode and the membrane, where the first type of electrode is arranged at the bottom of the cavity;

[0069] · The first type of electrode and the bottom wafer or a part thereof, where the first type of electrode is arranged on the membrane; or

[0070] · At least two first type of electrodes, where at least one first type of electrode is arranged on the membrane and at least one first type of electrode is arranged at the bottom of the cavity.

[0071] The first capacitor may also be connected to a control circuit, and the control circuit is used to measure the amplitude and / or frequency of the oscillation of the membrane relative to the bottom of the cavity.

[0072] The first type of electrode is also referred to as a capacitive electrode herein.

[0073] In one embodiment, the sensor includes a second capacitor for actuating the membrane. The second capacitor is formed, for example, by the following electrodes:

[0074] · The membrane (in particular, the surface of the membrane facing the cavity) and the bottom wafer, where the bottom wafer includes a second capacitor formed by a part of the membrane and / or a part of the bottom wafer;

[0075] · A second type of electrode and the membrane, where the second type of electrode is arranged at the bottom of the cavity;

[0076] · The second type of electrode and the bottom wafer or a part thereof, where the second type of electrode is arranged on the membrane; or

[0077] · At least two second type of electrodes, where at least one second type of electrode is arranged on the membrane and at least one second type of electrode is arranged at the bottom of the cavity.

[0078] The second capacitor can be part of the oscillation generator unit or can provide an oscillation generator unit that is equipped to excite the oscillation of the membrane by applying an excitation voltage to at least one of the electrodes forming the capacitor. The frequency of the excitation voltage can be adapted to the pressure-related resonance frequency of a suitable resonance mode of the membrane that may be loaded.

[0079] The second type of electrode is also referred to herein as the actuating electrode.

[0080] Excitation and resonance frequencies in the range from 0.1 kHz to 1000 kHz, in particular in the range from 1 kHz to 100 kHz, are used, wherein such a frequency range is caused by the impact of pressure / squeezing the membrane.

[0081] The electrode can be made of any conductive material compatible with vacuum, in particular not outgassing at least up to 10 -7 mbar and showing stable and pressure-independent properties over a long period of time. For example, the electrode can be made of doped silicon, Al, Ti, W, Au, Pt, Pd, Cr, Ta, Zr or an alloy thereof.

[0082] The sensor can include at least one first type of electrode and at least one second type of electrode. For example, the first type of electrode and the second type of electrode can be arranged on the membrane and share a common electrode arranged on the bottom wafer or share the bottom wafer itself as the common electrode.

[0083] The invention also relates to a method for measuring a fluid by means of a metering system. The metering system includes at least a first sensor (e.g., a pressure transducer) and an oscillation generator unit. The first sensor includes a membrane, a suspension of the membrane, a suspension allowing the oscillation of the membrane, a bottom wafer, wherein the suspension of the membrane is connected to the bottom wafer, and a cavity formed between at least a part of the membrane and the bottom wafer. In particular, the transducer includes an inlet connecting the cavity to the surrounding environment. The oscillation generator unit is configured to provide at least the oscillation of the membrane.

[0084] The method includes the following steps:

[0085] · Driving the oscillation generator unit in a first oscillation mode and a second oscillation mode,

[0086] · Measuring a first damping response during the application of the first oscillation mode, the first damping response depending on molecular parameters related to the fluid and pressure parameters related to the fluid,

[0087] · Measuring a second damping response during the application of the second oscillation mode, the second damping response depending on the molecular parameters related to the fluid and the pressure parameters related to the fluid,

[0088] ·Combine the first damping response and the second damping response, and

[0089] ·Derive the value of at least the pressure parameter based on the combination process.

[0090] By applying a corresponding actuation signal (e.g., a varying actuation voltage) to the oscillation generator unit, particularly to one or more actuation electrodes of the oscillation generator unit, it is possible to drive the oscillation generator unit in the first oscillation mode and / or the second oscillation mode.

[0091] In one embodiment, the metering system may include one or more additional sensors (e.g., pressure sensors), which are configured according to any of the above sensors. The oscillation generator unit may include a first oscillation generator and a second or more oscillation generators, the first oscillation generator being configured to oscillate the membrane of the first sensor, and the second or more oscillation generators being configured to oscillate the membranes of one or more additional sensors.

[0092] In particular, the first oscillation mode can be provided by driving the first oscillation generator, and the second oscillation mode can be provided by driving the second or more oscillation generators.

[0093] In one embodiment, the method includes specific method steps, which can be performed according to the configuration of the control and processing unit.

[0094] The present invention also relates to a computer program product, which includes program code stored on a machine-readable medium or embodied by an electromagnetic wave including program code segments, and having computer-executable instructions for performing and / or controlling the above method, particularly when running on the above control and processing unit.

[0095] Therefore, the computer program product can be implemented such that when executed by the control and processing unit, the steps of the method according to the above are automatically performed.

[0096] The apparatus and method according to the present invention will be described or explained in more detail below by way of a working example schematically shown in the accompanying drawings, purely by way of example. Specifically,

[0097] Figure 1 An embodiment of the metering system according to the present invention is shown in a schematic cross-sectional view;

[0098] Figures 2a to 2b A specific relationship between sensor sensitivity and gas type (molecular weight) is shown;

[0099] Figure 3 shows an embodiment of a metering system according to the present invention; and

[0100] Figure 4 shows a flowchart representing steps for providing gas-compensated pressure measurement according to the present invention.

[0101] Figure 1 An embodiment of a metering system 1 according to the present invention is shown in a schematic cross-sectional view.

[0102] The metering system 1 includes a sensor 10 having a top wafer 11, the top wafer 11 being configured such that it includes a membrane 12, a suspension 13, and an inlet 14. The sensor 10 can be implemented as a pressure transducer. The suspension 13 provides mobility to the membrane 12, in particular oscillation of the membrane 12. In the illustrated embodiment, the membrane 12 is loaded with a mass block 15. However, according to an alternative embodiment, the membrane can be provided as a dedicated oscillating element, in particular without an additional mass block or the like.

[0103] The sensor 10 includes a bottom wafer 21, the bottom wafer 21 being configured such that it includes a device recess 22 forming the bottom wall and side walls of a cavity 30. The depth of the device recess 22 can be selected such that a friction / squeeze film can be established. The "top" and "bottom" wafers should not be understood to mean that the transducer must be aligned such that the top wafer is above the bottom wafer in the vertical direction, but rather as corresponding terms used simply to name the respective components.

[0104] The top wafer 11 and the bottom wafer 21 are bonded together such that all parts of the membrane 12 and all parts of the suspension 13 are positioned above the device recess 22. The cavity 30 is between the membrane 12 and a part of the bottom wafer 21.

[0105] In order to fabricate the cavity according to the function of the sensor (i.e., the membrane and its suspension do not directly contact the bottom wafer), before bonding, the top wafer can be positioned relative to the bottom wafer such that all parts of the suspension and its membrane are above the device recess. This also means that the membrane forms the top of the cavity.

[0106] In one embodiment, the extension of the cavity 30 along an axis perpendicular to the large surface of the membrane ("vertical" extension) can be small such that the cavity 30 is only a small gap. For example, this extension can be less than 20 μm, in particular less than 10 μm or at most 5 μm and at least 0.7 μm or at least 1 μm.

[0107] The top wafer 11 further includes an inlet 14 to provide connection of the cavity 30 with the surrounding environment of the transducer 10.

[0108] The inlet 14 connects the cavity 30 to the surrounding atmosphere and ensures pressure equalization between the surrounding area and the cavity 30. In an embodiment of the sensor 10, the inlet 14 is an opening through the entire top wafer, wherein the opening separates the membrane (and the mass block, if present) from the rest of the top wafer. This means that the inlet 14 is in particular a gap around the membrane 12 (except in the region of the suspension 13). In other embodiments, the inlet 14 can be formed by a channel in the top wafer. In other embodiments, the inlet can also be formed by a corresponding channel in the bottom wafer.

[0109] According to an alternative, the elements of the sensor 10 (in particular the membrane 12, the suspension 13 and the cavity 30) can be provided by different methods according to known manufacturing processes of the prior art (e.g., additive structuring), i.e., not necessarily by the top wafer and the bottom wafer.

[0110] Since it can be excited to generate oscillations, the membrane 12 forms part of a resonator. In the illustrated embodiment, the membrane 12 has a circular shape and is thus disc-shaped. The diameter of the disc-shaped membrane 12 can be between 100 μm and 10 mm, in particular between 200 μm and 5 mm and between 500 μm and 5 mm.

[0111] However, according to an alternative embodiment, membranes of any shape and, for example, rectangular or elliptical membranes are possible.

[0112] In this example, the oscillation generator unit is provided by actuating the electrode 28 at the bottom of the device recess 22. The actuating electrode 28 is used to actuate the membrane 12 by applying a corresponding actuating signal.

[0113] The sensor 10 further includes a capacitive electrode 29 that forms one electrode of a capacitor for measuring the amplitude and frequency of the oscillation of the membrane 12. The other electrode of the capacitor is provided by the membrane 12. The respective contact pads 25 are embedded in the bottom wafer 21.

[0114] In one embodiment, the sensor 10 can include at least one electrode, wherein the at least one electrode and the membrane 12 form at least one capacitor for actuating and measuring the amplitude and / or frequency of the oscillation of the membrane 12 relative to the bottom of the cavity 30. In particular, the at least one electrode is arranged at the bottom of the cavity 30 (e.g., in the form of a conductive region), or the bottom wafer 21 or a part thereof is used as the at least one electrode.

[0115] In Figure 1In the illustrated embodiment, the bottom wafer 21 is a Si wafer, where a thermal oxide is grown after etching (especially the etching of the device recess 22) in order to isolate the wires, electrodes 28, 29 and contact pads 25 from each other. Alternatively, the top wafer and / or the bottom wafer are especially single-crystalline or polycrystalline silicon Si wafers, SOI wafers or CMOS wafers. However, they can also be made of a material different from Si, such as glass.

[0116] The metering system 1 further includes a control and processing unit 40. The control and processing unit 40 is connected to the sensor 10.

[0117] The control and processing unit 40 is configured to provide a desired oscillation of the membrane 12 by applying a specific excitation signal to the oscillation generator unit (i.e., the actuating electrode 28). Thereby, different oscillation modes can be applied to the membrane 12. For example, the membrane can oscillate at different frequencies and / or amplitudes and / or can provide oscillations according to different types of membrane movement. The membrane 12 can move out of the plane (i.e., vibrate in a direction perpendicular to the extension of the membrane), or can tilt about an in-plane axis (i.e., tilt oscillation).

[0118] In one embodiment, at least two oscillation modes can be applied simultaneously. Thereby, a superimposed oscillation can be provided.

[0119] The (pressure) measurement based on the squeeze film damping of a resonant membrane (resonator) is also sensitive to the gas species, especially to the characteristics of the sensor. The gas species sensitivity is preferably related to the molecular weight. The relevant sensor parameters can relate to geometric aspects, such as the gap size, the resonator area and the resonator shape, as well as the resonant frequency and the oscillation mode.

[0120] The gas species sensitivity can be measured by measuring one (or more) actuation voltages of several different gases at one (or more) pressure points.

[0121] The relationship between the sensitivity and the gas species (molecular weight) is exemplarily shown by Figure 2a and Figure 2b shown. For different frequencies and different sensor modes, the sensitivity is different. Here, for each vibration mode, the relationship between the fitted sensitivity and the molecular weight is determined. A physical-based non-linear three-parameter model can be applied for fitting. However, the fitting can be performed in an alternative manner according to principles known in the art.

[0122] Figure 2a Exemplarily, two fitted sensitivity curves varying with the molecular weight are shown. The two sensitivities relate to two different sensors used for the corresponding measurements. Each sensor is excited with a different excitation frequency.

[0123] Figure 2bTwo fitted sensitivity curves are also shown as an example as a function of molecular weight. The two sensitivity curves are related to different oscillation modes applied to the same sensor. The sensor was once excited in the out-of-plane mode and additionally in the tilt vibration mode. Both oscillations are performed at different frequencies.

[0124] For each oscillation mode, there are two (initially unknown) inputs: the pressure of the fluid and the molecular weight of the fluid. In addition, there is one output that can be measured: the voltage. Therefore, when (only) one damped response (voltage) is measured, the system is still uncertain.

[0125] According to solutions known in the art, such sensors (preferably pressure transducers) are used for known and / or well-defined atmospheric environments, which allow to define or select the gas species (and therefore the molecular weight) present in the cavity. Knowing the gas type (or the small amount of gas type that may be present) allows to determine the pressure directly. However, this is unreliable or even impossible when using the sensor in an uncertain atmosphere without information about the gas type or the composition of the gas type.

[0126] In this context, the relationship between the damping response (in particular corresponding to the actuation voltage measured by means of one of the capacitors) and the pressure is referred to as sensitivity. As mentioned above, this sensitivity also depends on the gas species.

[0127] In one embodiment, the method according to the invention provides for taking into account the dependency of the sensitivity on the specific gas species (molecular weight) and for compensating the pressure measurement, respectively.

[0128] The control and processing unit 40 is configured to drive the oscillation generator unit 28 in a first oscillation mode and in a second oscillation mode. When applying the first oscillation mode, a first damping response is measured. Such a measurement can in particular be provided by the capacitive electrode 29. Therefore, the control and processing unit 40 is configured to measure a second damping response during application of the second oscillation mode.

[0129] The first damped response and the second damped response each depend on a molecular parameter associated with the fluid in the cavity and a pressure parameter associated with the fluid in the cavity. Thus, when only one damped response is measured, there are still two undetermined parameters (the molecular and the pressure parameters) that do not allow for precise determination of one of these parameters. The molecular parameter may be the molecular weight, and the pressure parameter may be the pressure of the fluid.

[0130] It is now possible to process two (or more) oscillation modes (with different characteristics) and the corresponding damping responses to obtain a determined (or even overdetermined) system. This allows for the extraction (calculation) of pressure parameters and molecular parameters. Using three or more oscillation modes and damping responses can provide even more robust and accurate results, especially since the relationship between sensitivity and molecular weight is non-linear.

[0131] Accordingly, the control and processing unit 40 is configured to provide a compensated measurement of the pressure of the fluid and / or the molecular weight of the fluid by jointly processing the first damping response and the second damping response and deriving at least one value of the pressure parameter and / or the molecular parameter based on the joint processing.

[0132] The corresponding sensitivities are derived and pre-known by performing a pre-calibration step on the applied measurement excitation frequencies.

[0133] Figure 3 An embodiment of the metering system 2 according to the present invention is shown. The metering system 2 includes a first sensor 50 and an additional sensor 60 as well as a control and processing unit 40.

[0134] The first sensor 50 includes a membrane 52 and a suspension 53 for the membrane, and the suspension 53 provides the mobility (oscillation) of the membrane 52. In addition, the transducer 50 includes a base substrate 51 (bottom wafer) having a groove. The membrane 52 is mounted to the base substrate 51 such that a cavity 31 is provided between the membrane 52 and the bottom of the groove of the base substrate 51.

[0135] The additional sensor 60 includes a membrane 62 and a suspension 63 for the membrane 62, and the suspension 63 provides the mobility (oscillation) of the membrane 62. In addition, the transducer 60 includes a base substrate 61 (bottom wafer) having a groove. The membrane 62 is mounted to the base substrate 61 such that a cavity 32 is provided between the membrane 62 and the bottom of the groove of the base substrate 61.

[0136] The metering system 2 further includes actuation electrodes 58 and 68 and capacitance electrodes 59 and 69. The electrodes 58, 59, 68, 69 are connected to the control and processing unit 40.

[0137] In one embodiment, the actuation electrodes 58 and 68 can provide an oscillation generator unit, especially together with the control and processing unit 40. In one embodiment, the oscillation generator unit can be implemented together with the control and processing unit 40, and the actuation electrodes 58 and 68 can be considered as part of the sensor. Both embodiments are within the scope of the present invention.

[0138] The control and processing unit 40 is configured to provide oscillations of the membranes 52 and 62 by applying corresponding actuation signals by means of the actuation electrodes 58 and 68 (and in particular by means of the membranes 52 and 62). Thereby, the membrane 52 can be excited to provide a first oscillation mode, and the membrane 62 can be excited to provide a second oscillation mode.

[0139] Due to, for example, different excitation frequencies and / or different structural characteristics of the transducers 50, 60, the oscillation modes can provide different sensitivities.

[0140] The damping of the membrane oscillations is caused by a specific fluid present in the cavities 31 and 32. In the present embodiment, the two transducers 50 and 60 are located in a common atmospheric environment, i.e., the fluid in the cavities 31 and 32 is at least the same in terms of chemical composition and / or pressure.

[0141] The damping effect (damping response) can be measured by the control and processing unit 40 by means of the actuation electrodes 58, 68 and / or by means of the capacitance electrodes 59, 69.

[0142] In the case of measuring the damping via the actuation electrodes 58, 68, the feedback signal of the feedback loop (which allows the oscillation to be kept stable) is processed, and the damping effect can be derived from the amount of energy that must be input into the system to provide a stable oscillation and thus overcome the damping effect.

[0143] In the case of measuring the damping via the capacitance electrodes 59, 69, the combination of the corresponding membranes 52, 62 and the capacitance electrodes 59, 69 provides corresponding capacitors, and the damping effect can be derived by measuring the change in capacitance of the capacitors.

[0144] Thus, in one embodiment of the present invention, at least one sensor includes only actuation electrodes. The capacitance electrodes can be omitted.

[0145] Figure 4 A flowchart showing the steps of providing gas-compensated pressure measurement using a sensor according to the present invention is shown.

[0146] Initially, at least one sensor can be arranged in the atmosphere for which the pressure is to be determined. The fluid (a specific gas species or a composition of gas species) in the atmosphere enters the cavity of the sensor through the inlet.

[0147] The starting point 101 for performing the corresponding measurement is the above-mentioned relationship between the pressure p of the fluid, the (average) molecular weight u of the fluid, and the corresponding damping effect caused by the fluid. The damping can be measured by the sensor by measuring the voltage V (for example, by means of a feedback loop).

[0148] The sensor (in particular the membrane of the sensor or, in the case of using more than one sensor, several membranes) is excited to oscillate in two specific oscillation modes (102a and 102b) having different frequencies f1 and f2. This means considering two such relationships between the pressure p, the molecular weight u, and the voltage V, resulting in six initially unknown parameters.

[0149] In steps 103a and 103b, for each excitation, the corresponding damping responses represented by the voltages V1 and V2 can be measured. Thereby, two of the six parameters are determined.

[0150] This results in the effects of the molecular weights u1 and u2 on the pressures p1 and p2 remaining undetermined, as shown in steps 104a and 104b. Since the fluid must be the same for the two oscillation modes, the pressure and the molecular weight must be the same.

[0151] Therefore, this results in a mathematical system that can be determined and solved. As a result, in step 105, the value of the pressure p and the value of the molecular weight u can be derived.

[0152] Thus, when applying different oscillation modes (e.g., different frequencies and / or different oscillation directions (out-of-plane or tilted)), the pressure of the fluid can be derived by exploiting the different sensitivities of the sensor.

[0153] Additionally or alternatively, when applying different oscillation modes (e.g., different frequencies and / or different oscillation directions (out-of-plane or tilted)), the molecular weight of the fluid can be derived by exploiting the different sensitivities of the sensor. The corresponding information regarding the molecular weight allows determination of the specific type of fluid present in the cavity.

[0154] Although the present invention has been illustrated above in part with reference to some specific embodiments, it must be understood that many modifications and combinations of the different features of the embodiments can be made, and the different features can be combined with each other or with vacuum applications known in the prior art.

Claims

1. A control and processing unit (40) of a metering system (1, 2), said metering system (1, 2) comprising: · At least a first sensor (10, 50, 60), the first sensor comprising □ a membrane (12, 52, 62), □ a suspension (13, 53, 63) of the membrane (12, 52, 62), the suspension (13, 53, 63) allowing oscillation of the membrane (12, 52, 62), □ a bottom wafer (21, 51, 61), wherein the suspension (13, 53, 63) of the membrane (12, 52, 62) is connected to the bottom wafer (21, 51, 61), and □ a cavity (30, 31, 32) formed between at least a part of the membrane (12, 52, 62) and the bottom wafer (21, 51, 61), In particular, wherein a structured top wafer provides the membrane and the suspension of the membrane, and · An oscillation generator unit (28, 58, 68), the oscillation generator unit being configured to cause at least the membrane (12, 52, 62) to oscillate, Characterized in that the control and processing unit (40) is configured to provide · Driving the oscillation generator unit (28, 58, 68) in a first oscillation mode and a second oscillation mode, · Measuring a first damping response during application of the first oscillation mode, the first damping response depending on molecular parameters related to the fluid to be measured and pressure parameters related to the fluid, · Measuring a second damping response during application of the second oscillation mode, the second damping response depending on the molecular parameters related to the fluid and the pressure parameters related to the fluid, · Jointly processing the first damping response and the second damping response, and · Based on the joint processing, deriving the value of the pressure parameter and / or the value of the molecular parameter.

2. The control and processing unit (40) according to claim 1, characterized in that The molecular parameter represents the molecular weight of the fluid.

3. The control and processing unit (40) according to any one of the preceding claims, characterized in that The pressure parameter represents the pressure of the fluid.

4. The control and processing unit (40) according to any one of the preceding claims, characterized in that Driving the oscillation generator unit (28, 58, 68) in the first oscillation mode and / or the second oscillation mode is provided by applying or changing an actuation signal for driving the oscillation generator unit (28, 58, 68), wherein the actuation voltage is applied or changed by using a feedback loop.

5. The control and processing unit (40) according to claim 4, characterized in that The actuation voltage is controlled by means of a feedback loop such that · The phase relationship between the actuation signal and the mechanical oscillation of the membrane (12, 52, 62) remains constant, and / or · The amplitude of the mechanical oscillation of the membrane (12, 52, 62) remains constant.

6. The control and processing unit (40) according to claim 4 or 5, characterized in that The first damping response and / or the second damping response is measured by determining the actuation signal, in particular by means of the feedback loop.

7. The control and processing unit (40) according to any one of the preceding claims, characterized in that The first oscillation mode provides an oscillation of the membrane (12, 52, 62) having a first amplitude and a first frequency, and the second oscillation mode provides an oscillation of the membrane (12, 52, 62) having a second amplitude and a second frequency, wherein at least the first frequency is different from the second frequency and / or the first amplitude is different from the second amplitude.

8. The control and processing unit (40) according to any one of the preceding claims, characterized in that The control and processing unit (40) is configured to provide oscillation of the membrane (12, 52, 62) in the first oscillation mode in a first oscillation direction and oscillation of the membrane (12, 52, 62) in the second oscillation mode in a second oscillation direction, wherein the first oscillation direction is different from the second oscillation direction.

9. The control and processing unit (40) according to any one of the preceding claims, characterized in that The first damping response and / or the second damping response is measured by capacitance measurement, wherein at least a part of the membrane (12, 52, 62) and the bottom wafer (21, 51, 61) provides a capacitor for providing the capacitance measurement.

10. A metering system (1, 2), the metering system comprising: · At least a first sensor (10, 50, 60), the first sensor (10, 50, 60) comprising □ A membrane (12, 52, 62), □ A suspension (13, 53, 63) of the membrane (12, 52, 62), the suspension (13, 53, 63) allowing oscillation of the membrane (12, 52, 62), □ A bottom wafer (21, 51, 61), wherein the suspension (13, 53, 63) of the membrane (12, 52, 62) is connected to the bottom wafer (21, 51, 61), and □ A cavity (30, 31, 32) formed between at least a part of the membrane (12, 52, 62) and the bottom wafer (21, 51, 61), In particular, wherein a structured top wafer provides the membrane (12, 52, 62) and the suspension (13, 53, 63) of the membrane (12, 52, 62), and · An oscillation generator unit (28, 58, 68), the oscillation generator unit being configured to cause at least the membrane (12, 52, 62) to oscillate, Characterized in that The metering system (1) comprises a control and processing unit (40) according to any one of the preceding claims.

11. The metering system (1, 2) according to claim 10, characterized in that · The metering system (1, 2) comprises one or more additional sensors (10, 50, 60), the one or more additional sensors (10, 50, 60) being configured according to the first sensor of claim 10, and · The oscillation generator unit comprises □ A first oscillation generator (58), the first oscillation generator being configured to cause the membrane (52) of the first sensor (50) to oscillate, and □ A second or more oscillation generators (68), the second or more oscillation generators (68) being configured to cause the membranes (62) of the one or more additional sensors (60) to oscillate.

12. The metering system according to claim 11, characterized in that · A first oscillation mode is provided by driving the first oscillation generator (58), and · A second oscillation mode is provided by driving the second or more oscillation generators (68).

13. A method for measuring a fluid by means of a metering system (1, 2), the metering system (1, 2) comprising at least a first sensor (10, 50, 60) and an oscillation generator unit (28, 58, 68), the first sensor (10, 50, 60) comprising · a membrane (12, 52, 62), · a suspension (13, 53, 63) of the membrane (12, 52, 62), the suspension (13, 53, 63) allowing oscillation of the membrane (12, 52, 62), · a bottom wafer, wherein, The suspension (13, 53, 63) of the membrane (12, 52, 62) is connected to the bottom wafer, and · A cavity (30, 31, 32) formed between at least a part of the membrane (12, 52, 62) and the bottom wafer, wherein the oscillation generator unit (28, 58, 68) is configured to provide at least oscillation of the membrane (12, 52, 62), and wherein the method comprises the steps of: · driving the oscillation generator unit in a first oscillation mode and a second oscillation mode, · measuring a first damping response during application of the first oscillation mode, the first damping response depending on a molecular parameter associated with the fluid and a pressure parameter associated with the fluid, · measuring a second damping response during application of the second oscillation mode, the second damping response depending on the molecular parameter associated with the fluid and the pressure parameter associated with the fluid, · jointly processing the first damping response and the second damping response, and · based on the joint processing, at least deriving a value of the pressure parameter and / or a value of the molecular parameter.

14. The method according to claim 13, characterized in that · the metering system (2) comprises one or more additional sensors (50, 60), the one or more additional sensors (50, 60) being configured according to the sensor of any one of claims 10 to 12, and · the oscillation generator unit (28, 58, 68) comprises □ a first oscillation generator (58) configured to oscillate the membrane (52) of the first sensor (50), and □ a second or more oscillation generators (68) configured to oscillate the membranes (62) of the one or more additional sensors (60), · the first oscillation mode is provided by driving the first oscillation generator, and · the second oscillation mode is provided by driving the second or more oscillation generators.

15. A computer program product, the computer program product comprising program code stored on a machine-readable medium or embodied by an electromagnetic wave including a program code segment and having computer-executable instructions for performing and / or controlling the method according to any one of claims 13 to 14, particularly when run on a control and processing unit according to any one of claims 1 to 9.

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