Gas type compensation for MEMS devices

Abstract

A control / processing unit (40) for a gauge system (1) includes at least a first sensor (10) comprising a membrane (12), a suspension (13) for 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 gauge system (1) includes a vibration generator unit (28) configured to vibrate at least the membrane (12). The control / processing unit (40) is configured to drive the vibration generator unit (28) in a first vibration mode and a second vibration mode, measure a first damping response while applying the first vibration mode, measure a second damping response while applying the second vibration mode, jointly process the first damping response and the second damping response, and derive a value for at least pressure and / or a molecular parameter based on the joint processing.

Description

[Technical Field]

[0001] METHOD OF MEASUREMENT USING MICROELECTROMECHANICAL SYSTEMS (MEMS) SENSORS FIELD OF THE INVENTION The present invention relates to devices comprising a vibrating element such as a resonator membrane or other moving element. [Background technology]

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

[0003] Pressure gauges are a key element in the operation and control of modern vacuum systems. Such systems are of paramount importance in various industries, such as the coating, automotive, optical and semiconductor industries, as well as in the manufacture of solar cells and medical devices. In all these industries, reliable measurement of the pressure in a vacuum chamber is essential to monitor or control process steps that must be carried out under reduced pressure.

[0004] Today, a wide variety of pressure gauges are available, differing in their basic measurement principle, the pressure ranges they address, their handling, and their reliability. In particular, MEMS pressure gauges are available, such as gauges with a suspended resonator in combination with a friction / squeeze film, hereafter referred to as "squeeze film".

[0005] The performance and reliability of MEMS devices with miniaturized components depend strongly on size variations in the micrometer regime. This places high demands on the device layout and fabrication process. In squeeze-film pressure gauges, the interaction between the sensing element and the gas molecules, especially those forming the squeeze film, must be optimized. This can be achieved by having a resonating membrane at a well-defined distance from the substrate within a few micrometers and extending over a large area parallel to the substrate, i.e., by forming a cavity with an aspect ratio of more than 100 between the membrane's extension parallel to the substrate and its distance to the substrate.

[0006] Furthermore, the membrane needs to exhibit a suitable resonant mode that can be achieved by loading the membrane with a suitable mass. Preferably, this is done in a process step that allows for easy adaptation of the mass, making it easy to integrate into the overall fabrication process of the pressure gauge or its pressure transducer (sensor).

[0007] For example, a respective friction / squeeze film gauge type sensor element is known from US Pat. No. 1,118,991. Here, the measurement method is based on the influence of ambient pressure on the system properties (resonance frequency, Q factor, etc.) of a microresonator. In this case, a properly designed cavity provides the necessary space for the vibration of the resonator or resonator element, which comprises a membrane that may be loaded by a mass, and the vibration properties of the resonator or resonator element indicate the pressure. The cavity, in particular in the form of a thin gas film cavity, forms a gap near the resonator, causing a pressure-dependent squeeze film damping of the resonator, so that the measured system properties indicate the pressure.

[0008] In particular, state-of-the-art MEMS sensors may be calibrated for use in defined process atmospheres with known gas species, and therefore provide accurate measurements.

[0009] However, as mentioned above, the interaction of gas molecules with the sensing element affects the damping behavior of the system. As a result, such 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 within the cavity, but such pressure measurements may also depend on the gas species present.

[0010] Such effects become even more relevant when using MEMS sensors with varying or undefined gas species, which can result in pressure readings that differ from the actual pressure value due to the presence of unknown or undefined molecules or molecular compositions. Summary of the Invention [Problem to be solved by the invention]

[0011] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved sensor that overcomes the above-mentioned drawbacks.

[0012] It is a further object of the present invention to provide an improved gauge that provides more reliable and accurate results.

[0013] It is a further object of the present invention to provide an improved sensor that provides additional information related to the gas species present. [Means for solving the problem]

[0014] These objects are achieved by implementing the characterizing features of the independent claims. Features which further develop the invention in alternative or advantageous ways can be gathered from the dependent claims.

[0015] The present invention is based on the observation that MEMS sensors for measuring pressure, in particular pressure transducers based on squeeze films, offer different sensitivities for different excitation frequencies. The sensitivity of such a sensor is here to be understood as the relationship between the pressure of the fluid and the damping effect caused on the vibrating element of the sensor by the fluid in its cavity.

[0016] A "fluid" in the context of the present invention should be understood to be 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 gauge system, particularly a manometer system, comprising a control / processing unit and a first sensor, particularly a squeeze film-based pressure transducer. The first sensor comprises a membrane, a membrane suspension, which allows the membrane to vibrate, and a bottom wafer, which is connected to the membrane suspension. The first sensor further comprises a cavity, which is formed between the membrane and at least a portion of the bottom wafer. In particular, the transducer comprises an inlet connecting the cavity to the surroundings. In particular, a structured top wafer provides the membrane and the membrane suspension.

[0018] The gauge system comprises a vibration generator unit configured to vibrate at least the membrane.

[0019] According to the invention, the control / processing unit comprises: driving the vibration generator unit in a first vibration mode and a second vibration mode; measuring a first damping response while applying a first vibration mode, the first damping response being dependent on a molecular parameter associated with the fluid and a pressure parameter associated with the fluid; measuring a second damping response while applying a second vibration mode, the second damping response being dependent on a molecular parameter associated with the fluid and a pressure parameter associated with the fluid; processing the first decay response and the second decay response together; deriving a value for at least the pressure parameter and / or a value for the molecular parameter based on the joint processing; The device is configured to:

[0020] By carrying out the above steps by the control / processing unit, a compensated measurement of the pressure of the fluid can be made.

[0021] In one embodiment, the sensor may comprise a resonator (membrane) with a large surface area suspended over a small air gap (cavity), both with an aspect ratio of >10:1, preferably >100:1, preferably >1000:1. The suspension may be asymmetric or symmetric, preferably asymmetric.

[0022] The resonator may be excited via electrostatic forces or may be mechanically (externally) excited.

[0023] The sensor readout (decay response) can be performed by capacitive readout methods. In one embodiment, readout can also be by optical, piezoelectric, or piezoresistive means. The electrode layout for excitation and / or capacitive readout can be symmetrical or asymmetrical, preferably asymmetrical.

[0024] The sensor may be temperature stabilized or non-stabilized, preferably temperature stabilized.

[0025] The bottom wafer should be understood to be any suitable type of base or substrate according to the state of the art, but not necessarily a crystalline substrate.

[0026] In one embodiment, processing the first and second decay responses together can include taking into account different sensitivities of one or more sensors, each of which also depends on the type of fluid (e.g., gas species) present in the cavity.

[0027] By driving the vibration generator unit in a first vibration mode and a second vibration mode, different sensitivities can be provided.

[0028] The difference in gas species sensitivity can be achieved using a single resonant element that is excited (simultaneously) in two or more vibration modes. Additionally or alternatively, two or more resonant elements (membranes) can be excited (simultaneously) in one or more vibration modes. These two elements can be mounted on a single MEMS chip or on several physically separated MEMS chips. The elements can be electrically connected in series or parallel to one or more dedicated readout electronic units. The two transducers (sensors) can use the same or different vibration modes.

[0029] Such a combination of pressure signals sensitive to two gas species can provide compensation for gas type dependency and can make the pressure signal independent of at least molecular weight.

[0030] Additionally or alternatively, determination of the gas species is made available by deriving molecular parameters (eg, average molecular weight).

[0031] The sensor, i.e., the membrane, can be driven to vibrate in a desired vibration mode (e.g., with a defined frequency, amplitude, vibration direction, etc.). This provides a specific sensitivity of the sensor. Such sensitivity is preferably determined before the measurement. In other words, the sensor and one or more specific excitations are calibrated. Calibration can be performed by measuring the sensitivity by measuring one or more actuation voltages (damping responses) at one or more pressure points for several different fluids (e.g., gases). Based on this, a relationship between sensitivity and molecular parameters can be fitted (see Figures 2a and 2b).

[0032] In one embodiment, the gauge system comprises one or more additional sensors, the one or more additional sensors being configured according to the first sensor described above, and the vibration generator unit comprises a first vibration generator configured to vibrate the membrane of the first sensor and a second or more vibration generators configured to vibrate the membranes of the one or more additional sensors.

[0033] In particular, a first vibration mode is provided by driving a first vibration generator and a second vibration mode is provided by driving a second or more vibration generators.

[0034] By providing at least two sensors (pressure transducers), two sensitivities can be provided simultaneously by driving each of the two sensors in a specific vibration mode, which provides robust and precise measurements due to the initial avoidance of interactions between the vibration modes.

[0035] According to one embodiment, a gauge system is capable of measuring the pressure of the fluid within the cavity, where gauge system refers to a pressure gauge system.

[0036] According to one embodiment, the gauge system is capable of measuring the molecular weight, in particular the average molecular weight, of the fluid in the cavity, which may allow to derive a specific type of fluid, in particular a gas species, where the gauge system represents a gas detection unit.

[0037] According to one embodiment, the gauge system is capable of measuring the molecular weight and pressure of the fluid within the cavity.

[0038] The present invention also relates to a control / processing unit for a gauge system, in particular a pressure gauge system. The gauge system comprises at least a first sensor, in particular a pressure transducer, comprising a membrane, a membrane suspension, the suspension enabling the membrane to vibrate, and a bottom wafer, the membrane suspension being connected to the bottom wafer (directly or by a connector). The sensor also comprises a cavity, the cavity being formed between the membrane and at least a portion of the bottom wafer. In particular, the transducer comprises an inlet connecting the cavity to the surroundings. In particular, a structured top wafer provides the membrane and the membrane suspension. The gauge system further comprises a vibration generator unit configured to vibrate at least the membrane.

[0039] The control / processing unit driving the vibration generator unit in a first vibration mode and a second vibration mode; measuring a first damping response while applying a first vibration mode, the first damping response depending on molecular parameters associated with the fluid being measured (particularly present in the cavity) and pressure parameters associated with the fluid; measuring a second damping response while applying a second vibration mode, the second damping response being dependent on a molecular parameter associated with the fluid and a pressure parameter associated with the fluid; processing the first decay response and the second decay response together; deriving a value for at least the pressure parameter and / or a value for the molecular parameter based on the joint processing; The device is configured to:

[0040] The control / processing unit may provide a compensated measurement of a pressure parameter (e.g., pressure) of the fluid by controlling the sensors respectively. Alternatively or additionally, the control / processing unit may provide a compensated measurement of a molecular parameter of the fluid by controlling the sensors respectively.

[0041] In particular, the control / processing unit according to the invention uses a gauge system as a pressure measuring system and / or gas detection system.

[0042] The step of driving the vibration generator unit in the first vibration mode and / or the second vibration mode may be performed by applying an actuation signal (e.g. a varying actuation voltage) to the vibration generator unit, in particular to an actuation electrode of the vibration generator unit.

[0043] It should be understood that the control / processing unit of the gauge system may be embodied in accordance with the control and processing as described below.

[0044] In one embodiment, the control / processing unit is configured to derive values ​​for the molecular parameters based on the joint processing, which also allows further information related to the fluid to be derived.

[0045] 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, having information about the molecular weight allows for the identification of specific gas species.

[0046] According to one embodiment, the pressure parameter represents the pressure of the fluid, which can be derived in this way, as already mentioned above.

[0047] In one embodiment, driving the vibration generator unit in the first vibration mode and / or the second vibration mode can be performed by applying or varying an actuation voltage for driving the vibration generator unit, the actuation voltage being applied or varied by use of a feedback loop, which can set and maintain the vibration mode constant and provide the desired vibration of the membrane.

[0048] The gauge system, in particular the sensor, may comprise actuation electrodes for exciting the vibration modes. Respective actuation signals (actuation voltages) may be applied to the actuation electrodes. The actuation electrodes may be provided by a vibration generating unit.

[0049] In particular, the actuation signal may be controlled by a feedback loop, whereby the phase relationship between the actuation signal and the mechanical vibration of the membrane is kept constant and / or the amplitude of the mechanical vibration of the membrane is kept constant.

[0050] In particular, the first damping response and / or the second damping response can be measured by determining an actuation signal, in particular by means of a feedback loop.

[0051] In one embodiment, the first vibration mode provides vibration of the membrane at a first amplitude and a first frequency, and the second vibration mode provides vibration of the membrane at 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.

[0052] In one embodiment, the control / processing unit is configured to provide vibration of the membrane in a first vibration direction in a first vibration mode and vibration of the membrane in a second vibration direction in a second vibration mode, the first vibration direction being different from the second vibration direction.

[0053] The vibration direction can be, for example, an out-of-plane vibration of the membrane, i.e., a positional variation of the membrane in a direction perpendicular to the extension of the membrane. Another vibration direction can be a tilt of the membrane around an axis (minor axis) that lies in the plane of the membrane.

[0054] In one embodiment, the first decay response and / or the second decay response can be measured by capacitance measurement, and a capacitor for providing the capacitance measurement is provided by the membrane and at least a portion of the bottom wafer.

[0055] The gauge system, in particular the sensor, may comprise capacitive electrodes for measuring the damping response of the vibration modes, the respective capacitances being measurable by the capacitive electrodes.

[0056] According to one embodiment, the sensor may comprise a first capacitor for measuring the amplitude and / or frequency of the vibration of the membrane relative to the bottom of the cavity, said first capacitor having, for example, the following electrodes: a bottom wafer including a membrane (particularly the surface of the membrane facing the cavity) and a first capacitor formed by a portion of the membrane and / or a portion of the bottom wafer; a first type of electrode and a membrane, the first type of electrode being disposed at the bottom of the cavity; a first type of electrode and a bottom wafer or a portion thereof, the first type of electrode being disposed on the membrane; or at least two electrodes of a first type, at least one electrode of the first type being arranged on the membrane and at least one electrode of the first type being arranged at the bottom of the cavity; is formed by

[0057] The first capacitor may further be connected to a control loop in which the amplitude and / or frequency of vibration of the membrane relative to the bottom of the cavity is measured.

[0058] The first type of electrodes are also referred to herein as capacitive electrodes.

[0059] In one embodiment, the sensor comprises a second capacitor for actuating the membrane, the second capacitor having, for example, the following electrodes: a bottom wafer including a membrane (particularly the surface of the membrane facing the cavity) and a second capacitor formed by a portion of the membrane and / or a portion of the bottom wafer; a second type of electrode and membrane, the second type of electrode being disposed at the bottom of the cavity; a second type of electrode and a bottom wafer or a portion thereof, the second type of electrode being disposed on the membrane; or At least two electrodes of a second type, at least one electrode of the second type being arranged on the membrane and at least one electrode of the second type being arranged at the bottom of the cavity. is formed by

[0060] The second capacitor can be part of a vibration generator unit that can be provided mounted to excite vibrations of the membrane by applying an excitation voltage to at least one of the electrodes forming the capacitor, the frequency of the excitation voltage possibly being adapted to the pressure-dependent resonance frequency of a suitable resonance mode of the membrane to be loaded.

[0061] The second type of electrode is also referred to herein as the working electrode.

[0062] Excitation and resonant frequencies in the range of 0.1-1000 kHz, particularly in the range of 1-100 kHz, are used, and it is therefore the pressure / squeeze film effect that results in such a frequency range.

[0063] The electrodes are vacuum compatible and in particular at least up to 10 -7 The electrodes can be made of any conductive material that is non-outgassing up to mbar and exhibits stable pressure-independent properties over time. For example, the electrodes may be made of doped silicon, Al, Ti, W, Au, Pt, Pd, Cr, Ta, Zr, or alloys thereof.

[0064] The sensor may include at least one electrode of a first type and at least one electrode of a second type, for example, the first type electrode and the second type electrode can share a common electrode that is disposed on the membrane, disposed on the bottom wafer, or is the bottom wafer itself.

[0065] The present invention also relates to a method for measuring a fluid by a gauge system, the gauge system comprising at least a first sensor (e.g., a pressure transducer) and a vibration generator unit. The first sensor comprises a membrane, a membrane suspension, the suspension enabling vibration of the membrane, a bottom wafer, the membrane suspension being connected to the bottom wafer, and a cavity, the cavity being formed between the membrane and at least a portion of the bottom wafer. In particular, the transducer comprises an inlet connecting the cavity to the surroundings. The vibration generator unit is configured to provide vibration of at least the membrane.

[0066] The method is: driving the vibration generator unit in a first vibration mode and a second vibration mode; measuring a first damping response while applying a first vibration mode, the first damping response being dependent on a molecular parameter associated with the fluid and a pressure parameter associated with the fluid; measuring a second damping response while applying a second vibration mode, the second damping response being dependent on a molecular parameter associated with the fluid and a pressure parameter associated with the fluid; processing the first decay response and the second decay response together; deriving a value for at least the pressure parameter based on the joint processing; Includes.

[0067] The step of driving the vibration generator unit in the first vibration mode and / or the second vibration mode can be performed by applying a respective actuation signal (e.g., a varying actuation voltage) to the vibration generator unit, in particular to one or more actuation electrodes of the vibration generator unit.

[0068] In one embodiment, the gauge system may include one or more additional sensors (e.g., pressure transducers), where the one or more additional sensors are configured according to any of the sensors described above. The vibration generator unit may include a first vibration generator configured to vibrate the membrane of the first sensor and a second or more vibration generators configured to vibrate the membranes of the one or more additional sensors.

[0069] In particular, a first vibration mode can be provided by driving a first vibration generator, and a second vibration mode can be provided by driving a second or more vibration generators.

[0070] In one embodiment, the method includes specific method steps that provide steps that can be performed according to the configuration of the control / processing unit.

[0071] The present invention also relates to a computer program product comprising program code stored on a machine-readable medium or embodied by electromagnetic waves containing program code segments, and having computer-executable instructions for implementing and / or controlling the above-mentioned method when executed in particular on the above-mentioned control / processing unit.

[0072] Thus, a computer program product can be implemented that, when executed by a control / processing unit, causes the steps of the method described above to be carried out automatically.

[0073] The device and method according to the invention will be described or explained in more detail below, purely by way of example, with reference to embodiments shown diagrammatically in the drawings, in particular: [Brief explanation of the drawings]

[0074] [Figure 1] 1 shows, in a schematic cross-sectional view, an embodiment of a gauging system according to the invention; [Figure 2a] FIG. 1 illustrates a specific relationship between sensor sensitivity and gas species (molecular weight). [Figure 2b] FIG. 1 illustrates a specific relationship between sensor sensitivity and gas species (molecular weight). [Figure 3] FIG. 1 illustrates one embodiment of a gauge system according to the present invention. [Figure 4] 4 is a flow chart depicting steps for performing gas-compensated pressure measurements in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0075] FIG. 1 shows in a schematic cross-sectional view an embodiment of a gauging system 1 according to the invention.

[0076] The gauge system 1 comprises a sensor 10 having a structured upper wafer 11 with a membrane 12, a suspension 13 and an inlet 14. The sensor 10 can be embodied as a pressure transducer. The suspension 13 provides mobility for the membrane 12, in particular vibration of the membrane 12. In the illustrated embodiment, the membrane 12 is loaded by a mass 15. However, according to alternative embodiments, the membrane may be provided as the exclusive vibration element, in particular without an additional mass, etc.

[0077] Sensor 10 includes a bottom wafer 21 structured to include a device recess 22 that forms the bottom and sidewalls of cavity 30. The depth of device recess 22 can be selected so that a friction / squeeze film can be established. The terms "top" and "bottom" wafers should not be understood to necessarily require that the transducer be aligned such that the top wafer is vertically above the bottom wafer, but rather should be understood as respective terms simply to name the respective components.

[0078] The top wafer 11 and bottom wafer 21 are bonded together such that all portions of the membrane 12 and all portions of the suspension 13 are positioned over the device recess 22. A cavity 30 is between the membrane 12 and a portion of the bottom wafer 21.

[0079] To fabricate the cavity according to the function of the sensor, i.e., so that the membrane and its suspension are not in direct contact with the bottom wafer, the top wafer can be positioned relative to the bottom wafer before bonding so that all parts of the suspension and its membrane are located on top of the device recess, which also means that it is the membrane that forms the top of the cavity.

[0080] In one embodiment, the extension of the cavity 30 along an axis perpendicular to the larger surface of the membrane (the "perpendicular" extension) may be small, so that the cavity 30 has only a small gap. For example, this extension may 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.

[0081] The top wafer 11 also includes an inlet 14 that provides a connection between the cavity 30 and the periphery of the transducer 10 .

[0082] Inlet 14 connects cavity 30 to the ambient atmosphere and ensures pressure equalization between the ambient region and cavity 30. In one embodiment of sensor 10, inlet 14 is an opening through the entire top wafer, the opening separating the membrane (and mass, if present) from the rest of the top wafer. This means that inlet 14 is specifically a gap surrounding membrane 12 except for the region of suspension 13. In other embodiments, inlet 14 may be formed by a channel in the top wafer. In yet other embodiments, inlet 14 may also be formed by a corresponding channel in the bottom wafer.

[0083] According to an alternative embodiment, the elements of the sensor 10, in particular the membrane 12, the suspension 13 and the cavity 30, may be provided in a different manner by known fabrication processes of the state of the art (e.g. additive structuring), i.e. not necessarily by means of top and bottom wafers.

[0084] The membrane 12 can be set in a vibrating state and thereby forms part of a resonator. In the illustrated embodiment, the membrane 12 has a round shape and is therefore disk-shaped. The diameter of the disk-shaped membrane 12 may be 100 μm to 10 mm, in particular 200 μm to 5 mm, and 500 μm to 5 mm.

[0085] However, according to alternative embodiments, membranes of any shape are possible, as well as membranes of, for example, rectangular or oval shape.

[0086] In this example, the vibration generating unit is provided by an actuation electrode 28 at the bottom of the device recess 22. The actuation electrode 28 serves to actuate the membrane 12 by applying a respective actuation signal thereto.

[0087] The sensor 10 also comprises a capacitive electrode 29 which constitutes one electrode of a capacitor for measuring the amplitude and frequency of vibration of the membrane 12. The other electrode of the capacitor is provided by the membrane 12. Respective contact pads 25 are embedded in the bottom wafer 21.

[0088] In one embodiment, the sensor 10 may comprise 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 vibration 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, for example in the form of a conductive area, or the bottom wafer 21 or a part thereof is used as the at least one electrode.

[0089] 1, the bottom wafer 21 is a Si wafer on which a thermal oxide is grown, particularly after etching of the device recess 22, in order to insulate the conductor lines, electrodes 28, 29, and contact pads 25 from one another. Alternatively, the top and / or bottom wafers are, in particular, crystalline or polycrystalline Si wafers, SOI wafers, or CMOS wafers. However, they can also be made of materials different from Si, such as glass.

[0090] The gauge system 1 also comprises a control / processing unit 40. The control / processing unit 40 is connected to the sensor 10.

[0091] The control / processing unit 40 is configured to provide desired vibrations of the membrane 12 by applying specific excitation signals to the vibration-generating unit, i.e., the actuation electrodes 28. This allows different vibration modes to be applied to the membrane 12. For example, the membrane can be vibrated at different frequencies and / or amplitudes and / or provide vibrations according to different types of membrane motion. The membrane 12 can be moved out-of-plane, i.e., vibrated in a direction perpendicular to the membrane's extension, or tilted about an in-plane axis, i.e., tilted vibration.

[0092] In one embodiment, at least two vibration modes can be applied simultaneously, thereby providing a superimposed vibration.

[0093] Pressure measurements based on squeeze film damping of a resonating membrane (resonator) are also sensitive to gas species, particularly the properties of the sensor. Gas species sensitivity is preferably related to molecular weight. Relevant sensor parameters can be related to geometric aspects such as gap size, resonator area and shape, as well as resonant frequency and vibration mode.

[0094] Gas species sensitivity can be measured by measuring the actuation voltage(s) at the pressure point(s) for several different gases.

[0095] 2a and 2b show exemplary relationships between sensitivity and gas species (molecular weight). The sensitivity is different for different frequencies and different sensor modes. Here, the relationship between sensitivity and molecular weight is fitted for each vibration mode. A nonlinear three-parameter physics-based model can be applied for the fitting. However, the fitting may be performed in alternative ways according to principles known in the art.

[0096] Figure 2a shows two exemplary fitted sensitivity curves for molecular weight. The two sensitivities are associated with two different sensors used in the respective measurements. Each sensor was excited at a different excitation frequency.

[0097] Figure 2b also shows two exemplary fitted sensitivity curves to molecular weight. The two sensitivity curves relate to different vibrational modes applied to the same sensor. The sensor was excited once in the out-of-plane mode and again in the tilted vibrational mode. Both vibrations were performed at different frequencies.

[0098] Each vibration mode has two (initially unknown) inputs: the fluid pressure and the molecular weight of the fluid. In addition, there is one output that can be measured: voltage. As a result, when measuring (and only measuring) one damping response (voltage), the system remains underdetermined.

[0099] According to solutions known in the art, such sensors (preferably pressure transducers) are used in a known and / or well-defined atmosphere regime, which allows the gas species (and therefore molecular weight) present in the cavity to be defined or selected. Knowing the gas type (or a small number of gas types that may be present) makes it possible to directly determine the pressure. However, this is unreliable or even impossible when using sensors in an undefined atmosphere without information about the gas type or composition of the gas type.

[0100] Herein, the relationship between the damping response (specifically corresponding to the actuation voltage measured by one of the capacitors) and pressure is referred to as the sensitivity. As mentioned above, this sensitivity also depends on the gas species.

[0101] In one embodiment, the technique according to the present invention takes into account the dependence of sensitivity on specific gas species (molecular weight) and compensates the respective pressure measurements.

[0102] The control / processing unit 40 is configured to drive the vibration generator unit 28 in a first vibration mode and in a second vibration mode. While applying the first vibration mode, a first damping response is measured. Such measurement can be performed in particular by means of the capacitive electrodes 29. Thus, the control / processing unit 40 is configured to measure a second damping response while applying the second vibration mode.

[0103] The first and second damping responses each depend on a molecular parameter associated with the fluid in the cavity and a pressure parameter associated with the fluid in the cavity. As a result, when measuring only one damping response, there are still two undetermined parameters (molecular and pressure parameters), one of which cannot be accurately determined. The molecular parameter may be the molecular weight, and the pressure parameter may be the pressure of the fluid.

[0104] Here, two (or more) vibrational modes (with different characteristics) and their respective damping responses can be processed to obtain a determined (or even over-determined) system. This makes it possible to extract (calculate) pressure and molecular parameters. Using three or more vibrational modes and damping responses can provide even more robust and accurate results, especially since the relationship between sensitivity and molecular weight is nonlinear.

[0105] Thus, the control / processing unit 40 is configured to perform a compensated measurement of the pressure of the fluid and / or the molecular weight of the fluid by processing the first damping response and the second damping response together and deriving values ​​for at least the pressure parameter and / or the molecular weight parameter based on the joint processing.

[0106] The respective sensitivities may be derived and known in advance by performing a pre-calibration step for the applied measurement excitation frequency.

[0107] 3 shows an embodiment of a gauge system 2 according to the invention. The gauge system 2 comprises a first sensor 50, one additional sensor 60 and a control / processing unit 40.

[0108] The first sensor 50 comprises a membrane 52 and a suspension 53 for the membrane, which provides mobility (vibration) of the membrane 52. Furthermore, the transducer 50 comprises a base substrate 51 (bottom wafer) having a recess. The membrane 52 is attached to the base substrate 51 such that a cavity 31 is provided between the membrane 52 and the bottom of the recess in the base substrate 51.

[0109] The additional sensor 60 comprises a membrane 62 and a suspension 63 for the membrane 62, providing mobility (vibration) of the membrane 62. Furthermore, the transducer 60 comprises a base substrate 61 (bottom wafer) having a recess. The membrane 62 is attached to the base substrate 61 such that the cavity 32 is provided between the membrane 62 and the bottom of the recess in the base substrate 61.

[0110] The gauge system 2 also comprises actuation electrodes 58 and 68 and capacitive electrodes 59 and 69. The electrodes 58, 59, 68, 69 are connected to the control / processing unit 40.

[0111] In one embodiment, actuation electrodes 58 and 68 may provide a vibration generator unit, particularly together with control / processing unit 40. In one embodiment, the vibration generator unit may be implemented with control / processing unit 40, and actuation electrodes 58 and 68 may be considered part of the sensor. Both of these embodiments are within the scope of the present invention.

[0112] The control / processing unit 40 is configured to provide vibration of the membranes 52 and 62 by applying respective actuation signals by the actuation electrodes 58 and 68 (in particular by the membranes 52 and 62), such that the membrane 52 can be excited to provide a first vibration mode and the membrane 62 can be excited to provide a second vibration mode.

[0113] The vibration modes may provide different sensitivities due to, for example, different excitation frequencies and / or different structural properties of the transducers 50,60.

[0114] The damping of the membrane vibrations is caused by the particular fluid present in cavities 31 and 32. In this embodiment, both transducers 50 and 60 are located in a common ambient environment, i.e. the fluids in cavities 31 and 32 are identical at least in terms of chemical composition and / or pressure.

[0115] The damping effect (damping response) can be measured by the control / processing unit 40 either by the actuation electrodes 58,68 and / or by the capacitive electrodes 59,69.

[0116] When measuring damping via the actuation electrodes 58, 68, the feedback signal of the feedback loop that allows the oscillation to be kept stable is processed, and the damping effect can be derived according to the amount of energy necessarily input into the system to provide a stable oscillation and thus overcome the damping effect.

[0117] When measuring damping via the capacitive electrodes 59, 69, the combination of each membrane 52, 62 and capacitive electrode 59, 69 provides a respective capacitor, and the damping effect can be derived by measuring the capacitance change across the capacitor.

[0118] Therefore, in one embodiment of the present invention, at least one sensor comprises only an actuation electrode, and the respective capacitive electrode may be omitted.

[0119] FIG. 4 shows a flow chart depicting steps for using a sensor to perform gas compensated pressure measurements in accordance with the present invention.

[0120] First, at least one sensor can be provided in the atmosphere whose pressure is to be determined: a fluid in the atmosphere (a particular gas species or composition of gas species) enters the cavity of the sensor through an inlet.

[0121] The starting point 101 for performing the respective measurements is the above-mentioned relationship between the pressure p of the fluid, the (average) molecular weight u of the fluid, and the respective damping effect caused by the fluid. The damping can be measured by a sensor, for example by measuring the voltage V by means of a feedback loop.

[0122] The sensor, in particular the sensor's membrane, or several membranes if more than one sensor is used, is excited to vibrate in two specific vibration modes (102a and 102b) with different frequencies f1 and f2. This means that two such relationships between pressure p, molecular weight u, and voltage V are considered, resulting in six initially unknown parameters.

[0123] In steps 103a and 103b for each of the excitations, the respective decay responses represented by voltages V1 and V2 can be measured, which determines two of the six parameters.

[0124] As a result, the effect of molecular weights u1 and u2 on pressures p1 and p2 has yet to be determined, as illustrated in steps 104a and 104b. The fluid is necessarily the same for both vibrational modes, so the pressures and molecular weights must be identical.

[0125] This therefore results in a mathematical system that can be determined and solved, so that in step 105 a value for the pressure p and also a value for the molecular weight u can be derived.

[0126] As a result, the pressure of the fluid can be derived by taking advantage of the different sensitivities of the sensor when applying different vibration modes, e.g., different frequencies and / or different vibration directions (out-of-plane or tilted).

[0127] Additionally or alternatively, the molecular weight of the fluid can be derived by exploiting the different sensitivity of the sensor when applying different vibration modes, e.g., different frequencies and / or different vibration directions (out-of-plane or oblique). The respective information on the molecular weight allows for the determination of the specific type of fluid present in the cavity.

[0128] Although the present invention has been described above in part with reference to some particular embodiments, it will be understood that numerous modifications and combinations of different features of the embodiments can be made and that different features can be combined with each other or with vacuum applications known from the prior art.

Claims

1. A control / processing unit (40) for a gauge system (1, 2), said gauge system (1, 2) comprising: at least a first sensor (10, 50, 60); a vibration generator unit (28, 58, 68); Equipped with The at least first sensor (10, 50, 60) comprises: a membrane (12, 52, 62); a suspension (13, 53, 63) for said membrane (12, 52, 62) that allows said membrane (12, 52, 62) to vibrate; 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); a cavity (30, 31, 32) formed between the membrane (12, 52, 62) and at least a portion of the bottom wafer (21, 51, 61); Equipped with In particular, a structured upper wafer provides the membrane and the suspension of the membrane, the vibration generator unit (28, 58, 68) is configured to vibrate at least the membrane (12, 52, 62); The control / processing unit (40) driving the vibration generator unit (28, 58, 68) in a first vibration mode and a second vibration mode; measuring a first damping response while applying the first vibration mode, the first damping response being dependent on a molecular parameter associated with the fluid being measured and a pressure parameter associated with the fluid; measuring a second damping response while applying the second vibration mode, the second damping response being dependent on the molecular parameter associated with the fluid and the pressure parameter associated with the fluid; processing the first decay response and the second decay response together; deriving a value for the pressure parameter and / or a value for the molecular parameter based on said joint processing; configured to: A control / processing unit (40).

2. the molecular parameter represents the molecular weight of the fluid; The control / processing unit (40) of claim 1.

3. the pressure parameter represents a pressure of the fluid; A control / processing unit (40) according to any one of claims 1 to 2.

4. the step of driving the vibration generator units (28, 58, 68) in the first vibration mode and / or the second vibration mode is performed by applying or varying an actuation signal for driving the vibration generator units (28, 58, 68), and an actuation voltage h is applied or varied by use of a feedback loop; A control / processing unit (40) according to any one of claims 1 to 3.

5. The actuation voltage is controlled by the feedback loop, whereby the phase relationship between the actuation signal and the mechanical vibration of the membrane (12, 52, 62) is kept constant; and / or The mechanical vibration amplitude of the membrane (12, 52, 62) is kept constant; A control / processing unit (40) according to claim 4.

6. the first damping response and / or the second damping response are measured by determining the actuation signal, in particular by the feedback loop. A control / processing unit (40) according to claim 4 or 5.

7. the first vibration mode provides vibration of the membrane (12, 52, 62) at a first amplitude and a first frequency, and the second vibration mode provides vibration of the membrane (12, 52, 62) at 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; A control / processing unit (40) according to any one of claims 1 to 6.

8. the control / processing unit (40) is configured to provide the vibration of the membrane (12, 52, 62) in a first vibration direction in the first vibration mode and the vibration of the membrane (12, 52, 62) in a second vibration direction in the second vibration mode, the first vibration direction being different from the second vibration direction; A control / processing unit (40) according to any one of claims 1 to 7.

9. the first attenuation response and / or the second attenuation response are measured by capacitance measurement, and a capacitor for providing the capacitance measurement is provided by the membrane (12, 52, 62) and at least a portion of the bottom wafer (21, 51, 61); A control / processing unit (40) according to any one of claims 1 to 8.

10. at least a first sensor (10, 50, 60); a vibration generator unit (28, 58, 68); A gauge system (1, 2) comprising: The first sensor (10, 50, 60) a membrane (12, 52, 62); a suspension (13, 53, 63) for said membrane (12, 52, 62) that allows said membrane (12, 52, 62) to vibrate; 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); a cavity (30, 31, 32) formed between the membrane (12, 52, 62) and at least a portion of the bottom wafer (21, 51, 61); Equipped with In particular, a structured upper wafer provides said membrane (12, 52, 62) and said suspension (13, 53, 63) of said membrane (12, 52, 62), the vibration generator unit (28, 58, 68) is configured to vibrate at least the membrane (12, 52, 62); The gauging system (1) comprises a control / processing unit (40) according to any one of claims 1 to 9. Gauge system (1, 2).

11. The gauge 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, The vibration generator unit comprises: a first vibration generator (58) configured to vibrate the membrane (52) of the first sensor (50); a second or more vibration generators (68) configured to vibrate the membranes (62) of the one or more additional sensors (60); Equipped with Gauge system (1, 2) according to claim 10.

12. the first vibration mode is provided by driving the first vibration generator (58); the second vibration mode is provided by driving the second or more vibration generators (68); The gauging system of claim 11.

13. A method for measuring a fluid by a gauge system (1, 2), said gauge system (1, 2) comprising at least a first sensor (10, 50, 60) and a vibration generator unit (28, 58, 68), said first sensor (10, 50, 60) comprising: a membrane (12, 52, 62); a suspension (13, 53, 63) for said membrane (12, 52, 62) that allows said membrane (12, 52, 62) to vibrate; a bottom wafer, the suspension (13, 53, 63) of the membrane (12, 52, 62) being connected to the bottom wafer; a cavity (30, 31, 32) formed between the membrane (12, 52, 62) and at least a portion of the bottom wafer; Equipped with the vibration generator unit (28, 58, 68) is configured to provide vibration of at least the membrane (12, 52, 62); The method comprises: driving the vibration generator unit in a first vibration mode and a second vibration mode; measuring a first damping response while applying the first vibration mode, the first damping response being dependent on a molecular parameter associated with a fluid and a pressure parameter associated with the fluid; measuring a second damping response while applying the second vibration mode, the second damping response being dependent on the molecular parameter associated with the fluid and the pressure parameter associated with the fluid; processing the first decay response and the second decay response together; deriving a value for at least the pressure parameter and / or a value for the molecular parameter based on said joint processing; A method comprising:

14. The gauging system (2) comprises one or more additional sensors (50, 60), the one or more additional sensors (50, 60) being configured in accordance with a sensor according to any one of claims 10 to 12, The vibration generator unit (28, 58, 68) a first vibration generator (58) configured to vibrate the membrane (52) of the first sensor (50); a second or more vibration generators (68) configured to vibrate the membranes (62) of the one or more additional sensors (60); Equipped with the first vibration mode is provided by driving the first vibration generator; The second vibration mode is provided by driving the second or more vibration generators. The method of claim 13.

15. A computer program product comprising program code stored on a machine-readable medium or embodied by electromagnetic waves comprising program code segments, and having computer-executable instructions for performing and / or controlling the method of any one of claims 13 to 14, in particular when executed on a control / processing unit of any one of claims 1 to 9.

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