Volume viscosity sensor of a gaseous substance

Abstract

Volume viscosity sensor of a gaseous substance The present description relates to a sensor (200) of volume viscosity of a gaseous substance, the sensor comprising: – a MEMS transducer (201) comprising a mobile element (203) suspended above a cavity (205) and at least one vent (209) allowing gas exchanges between the cavity and an external environment; and – a control circuit of the MEMS transducer configured to: a) vibrate the mobile element around its resonance frequency; b) measure a signal representative of a resonance frequency and a resonance bandwidth of the MEMS transducer; and c) compare said signal with a reference signal and deduce therefrom information representative of the volume viscosity of the gaseous substance present in the cavity. Figure for abstract: Fig. 2B

Description

Title of the invention: Volume viscosity sensor of a gaseous substance Technical field

[0001] The present description relates generally to electronic devices, and in particular to gas sensors or detectors. Prior art

[0002] Gas sensors or detectors are electronic devices generally used to measure or detect the presence or concentration of various gases present in a gaseous substance, for example a gas mixture such as air. These devices are implemented in many applications, such as air quality monitoring applications, ventilation control, leak detection, etc. This leads to improvements, particularly in terms of safety and efficiency, in many technical fields.

[0003] Existing gas sensors can be classified, among others, according to the following two criteria: - according to a type of gas detected, for example carbon monoxide, methane, dihydrogen, ammonia, dioxygen, etc.; or - according to a detection technique implemented by the sensor, leading to the distinction, for example, between infrared sensors, photoionization sensors, electrochemical sensors, thermal conductivity sensors, metal oxide-based sensors, catalytic sensors, etc.

[0004] However, existing gas sensors suffer from various drawbacks. In particular, existing gas sensors are bulky, power-hungry, unstable, and expensive. Summary of the invention

[0005] There is a need to overcome all or part of the disadvantages of existing gas sensors. In particular, it would be desirable to have compact, energy-saving, stable and inexpensive gas sensors.

[0006] For this, one embodiment provides a volume viscosity sensor for a gaseous substance, the sensor comprising: - a MEMS transducer comprising a mobile element suspended above a cavity and at least one vent allowing gas exchanges between the cavity and an external environment; and - a MEMS transducer control circuit configured to: a) vibrate the moving element around its resonance frequency; b) measuring a signal representative of a resonant frequency and a resonant bandwidth of the MEMS transducer; and c) comparing said signal with a reference signal and deducing therefrom information representative of the volume viscosity of the gaseous substance present in the cavity.

[0007] According to one embodiment, the cavity is formed in a substrate made of a semiconductor material, preferably silicon, the mobile element being a membrane made of the semiconductor material.

[0008] According to one embodiment, the MEMS transducer is of the PMUT type.

[0009] According to one embodiment, the MEMS transducer comprises a structure excitation and detection comprising: - a first electrode, located on one side of the mobile element opposite the cavity; - a layer of piezoelectric material, located on one side of the first electrode opposite the mobile element; and - a second electrode, located on one side of the piezoelectric material layer opposite the first electrode.

[0010] According to one embodiment, the MEMS transducer comprises: - an excitation structure comprising a first electrode, located on one side of the mobile element opposite the cavity, a first layer of piezoelectric material, located on one side of the first electrode opposite the mobile element, and a second electrode, located on one side of the first layer of piezoelectric material opposite the first electrode; and - a detection structure distinct from the excitation structure and comprising a third electrode, located on one side of the mobile element opposite the cavity, a second layer of piezoelectric material, located on one side of the third electrode opposite the mobile element, and a fourth electrode, located on one side of the layer of piezoelectric material opposite the third electrode.

[0011] According to one embodiment, the MEMS transducer is of the CMUT type.

[0012] According to one embodiment, the MEMS transducer comprises a structure comprising a first electrode covering the bottom of the cavity, and a second electrode located on the movable element directly above the first electrode.

[0013] According to one embodiment, the MEMS transducer comprises: - an excitation structure comprising a first electrode, located on and in contact with the bottom of the cavity, and a second electrode, located on the mobile element directly above the first electrode; and - a detection structure distinct from the excitation structure and comprising a third electrode, located on and in contact with the bottom of the cavity, and a fourth electrode, located on the mobile element directly above the third electrode.

[0014] According to one embodiment, the control circuit is configured to: - during an excitation phase, applying an excitation voltage between the first and second electrodes of the transducer; and - during a detection phase, subsequent to the excitation phase, read a voltage produced between the first and second electrodes of the transducer under the action of a deformation of the moving element.

[0015] According to one embodiment, the control circuit is configured to apply an alternating excitation voltage between the first and second electrodes so as to cause an alternation of compression and expansion phases of the gaseous substance present inside the cavity, and to simultaneously read a voltage produced between the third and fourth electrodes under the action of the vibration of the mobile element.

[0016] One embodiment provides an air quality measuring or leak detection device comprising a sensor as described.

[0017] One embodiment provides a method of controlling a volume viscosity sensor of a gaseous substance, the sensor comprising: - a MEMS transducer comprising a mobile element suspended above a cavity and at least one vent allowing gas exchanges between the cavity and an external environment; and - a MEMS transducer control circuit, the method comprising the following steps, implemented by the control circuit: a) vibrate the moving element around its resonance frequency; b) measuring a signal representative of a resonant frequency and a resonant bandwidth of the MEMS transducer; and c) comparing said signal with a reference signal and deducing therefrom information representative of the volume viscosity of the gaseous substance present in the cavity. Brief description of the drawings

[0018] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

[0019] [Fig.l] is a very schematic and partial side view of a device for measuring the volume viscosity of a gaseous substance according to one embodiment;

[0020] [Fig.2A] and [Fig.2B] are respectively a top view and a side view and in section along plane BB of [Fig.2A], schematic and partial, of a volume viscosity sensor of a gaseous substance according to one embodiment;

[0021] [Fig. 3] is a schematic and partial side and sectional view of a volume viscosity sensor for a gaseous substance according to one embodiment;

[0022] [Fig.4] is a schematic and partial side and sectional view of a volume viscosity sensor for a gaseous substance according to one embodiment;

[0023] [Fig.5] is a schematic and partial side and sectional view of a volume viscosity sensor for a gaseous substance according to one embodiment;

[0024] [Fig. 6] is a schematic and partial side and sectional view of a volume viscosity sensor of a gaseous substance according to one embodiment; and

[0025] [Fig.7] is a schematic and partial side and sectional view of an electronic device integrating a volume viscosity sensor of a gaseous substance according to one embodiment. Description of the embodiments

[0026] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0027] For the sake of clarity, only the steps and elements useful for understanding the embodiments described have been shown and are detailed. In particular, the applications of the sensors of the present description have not been detailed, the embodiments described being compatible with all or most of the applications likely to benefit from a volume viscosity sensor of a gaseous substance, possibly subject to adaptations within the scope of the person skilled in the art upon reading the present description.

[0028] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be connected by means of one or more other elements.

[0029] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0030] Unless otherwise specified, the expressions “about”, “approximately”, “substantially”, and “of the order of” mean to within 10%, preferably to within 5%.

[0031] In the following description, the terms “insulator” and “conductor” mean respectively, unless otherwise specified, electrically insulating and electrically conductive.

[0032] [Fig.l] is a very schematic and partial side view of a device 100 for measuring the volume viscosity of a gaseous substance according to one embodiment.

[0033] In the example shown, the device 100 comprises a piston 101 of mass m and thickness t. The piston 101 has, for example, in top view, a periphery of substantially circular shape and radius a. In the example illustrated, the piston 101 is suspended, by a spring 103 of stiffness constant ks, from a fixed anchor 105T located above the piston 101. In the orientation of [Fig.l], the piston 101 is located above a horizontal wall 105B fixed relative to the anchor 105T. In this example, the piston 101 is suspended above the horizontal wall 105B by the spring 103. In the orientation of [Fig.l], the upper face of the horizontal wall 105B has, for example, an area greater than or equal to that of the lower face of the piston 101.

[0034] In the orientation of [Fig.l], the lower face of the piston 101 and the upper face of the horizontal wall 105B vertically delimit a region 107. The region 107 extends for example laterally under the entire lower face of the piston 101, and is interrupted substantially in line with the side wall of the piston 101. The region 107 contains for example a gaseous substance, for example a pure gas or a gaseous mixture. For example, the region 107 is filled with air.

[0035] In the example shown in [Fig.l], the device 100 is devoid of side walls laterally delimiting, or bordering, the region 107. In this example, the region 107 communicates with an external environment, or ambient environment. The gaseous substance present between the piston 101 and the horizontal wall 105B is thus free to circulate between the interior of the region 107 and the external environment.

[0036] The piston 101 is for example driven by a vibratory or oscillatory movement along a vertical axis, in the orientation of [Fig.l]. [Fig.l] illustrates more precisely the case of a compression phase, during which the piston 101 has moved, along a vertical axis, by a distance x relative to an equilibrium position, or rest position, symbolized by a dotted rectangle. When the piston 101 is in its equilibrium position, the region 107 has a height, or thickness, g0. In this example, the lower face of the piston 101 is separated from the upper face of the horizontal wall 105B by a distance g0 - x.

[0037] The movement imparted to the piston 101, for example by means of an electromechanical system not detailed in [Fig.l], is for example a periodic vibratory movement of pulsation co. The behavior of the gaseous substance located in the region 107 is for example characterized by a solution to the linearized Reynolds equation for a compressible gas. This solution takes the form of two forces which each depend on a dimensionless parameter o: a viscous damping force fd(o) and an elastic force fe(o). The parameter o, called the number of com- pressure ("squeeze number" in English), is defined by the following relation:

[0038] [Math.l] 12uœa2 gp-......y Paxg,y

[0039] In the preceding relationship, q represents the dynamic viscosity of the gaseous substance and Pa the ambient pressure. The ambient pressure Pa corresponds, for example, to the pressure prevailing in the external environment, or to the pressure of the gaseous substance contained in the region 107 when the piston 101 is in its equilibrium position.

[0040] In a case where the piston 101 oscillates at high frequency, for which the parameter o has a very large value, for example much greater than 1, for example greater than 100 or greater than 1000, the gaseous substance then does not have time to escape from the region 107 and is sometimes compressed, sometimes expanded under the action of the piston 101 which, by its oscillatory movement, alternately decreases and increases the volume of the region 107. Consequently, the gaseous substance present in the region 107 undergoes an alternation of compression phases and expansion phases. This corresponds for example to a mode of operation of the device 100 in which the piston 101 is mainly subjected to an elastic restoring force. The gaseous substance contained in the region 107 then behaves like a spring of stiffness constant ksg.

[0041] The stiffness constant ksg is defined by the following relation:

[0042] [Math.2] v _ï>s &Q

[0043] In the above relationship, S corresponds to the area of ​​the lower face of the piston 101 (S = ira2 in the case where the piston 101 is circular in shape and has a radius a). For example, the area S of the lower face of the piston 101 is of the order of a few thousandths of a square micrometer to a few square millimeters.

[0044] In this mode of operation, the piston 101 has a resonance frequency fr given by the relationship below:

[0045] [Math.3] n _ 1 / ks+ksg

[0046] The oscillatory movement of the piston 101 is further characterized by a resonance bandwidth BW. The resonance bandwidth of a resonator corresponds to a frequency band for which the oscillation amplitude is greater than a maximum oscillation amplitude divided by the root of two, i.e. an attenuation value equal to -3 dB. The resonance bandwidth BW is mainly a function of radiative losses in the ambient medium and of a force viscous damping caused by the presence of the gaseous substance in region 107.

[0047] Furthermore, because the gaseous substance located in the region 107 is subjected to compression and expansion phenomena under the action of the piston 101, the movement of the piston 101 is further determined by a physical parameter characteristic of the gaseous substance located in the region 107, called bulk viscosity or volume viscosity.

[0048] Like other parameters, such as density, dynamic viscosity and thermal conductivity, volume viscosity is a physical parameter characteristic of each fluid or gaseous substance. The volume viscosity of a liquid or gaseous substance reflects an irreversible resistance, greater than a reversible resistance due to an isentropic compression modulus of the substance, to a compression or to an expansion of the substance. The higher the volume viscosity, the higher the losses linked to the movement of the piston 101 and the greater, or wider, the resonance bandwidth BW. In addition to its nature, the volume viscosity of a substance depends on the temperature and pressure of the substance.

[0049] In a case of a mixture of two gaseous substances having respectively volume viscosities qB0 and qB[, for example in a case where a gas of volume viscosity qB[ much higher, for example at least one hundred or one thousand times higher than the volume viscosity qB0, is added to the gas or gas mixture of volume viscosity qB0 initially present in the region 107, for example in a case where carbon dioxide (qB[ about four thousand times higher than the volume viscosity qB0 of air at room temperature) is added to the air, the gaseous substance then contained in the region 107 after introduction of the gas of volume viscosity qB1 has a volume viscosity qB given by the following relation:

[0050] [Math.4] j+(

[0051] In the above relationship, the quantity Ci corresponds to a concentration, expressed as a molar fraction, of the gas of volume viscosity qB1 in the gaseous substance contained in the region 107. The volume viscosity qB of the gaseous substance containing the gas of volume viscosity qB1 is thus greater than its volume viscosity qB0 before the introduction of this gas. This increases the resonance bandwidth BW of the piston 101. For a concentration Ci much lower than 1, for example of the order of a few percent, for example lower than 10% or lower than 5%, the increase in the resonance bandwidth BW is substantially proportional to the concentration Ci. Even for higher concentrations than those described above, the function is close to a linear function or, at least, to a monotonic function. By measuring the increase in the bandwidth BW, it is thus possible to trace the concentration Ci of the gas of volume viscosity pBi in the gaseous substance contained in the region 107. By comparing the resonance frequency and the resonance bandwidth BW to a threshold, it is for example also possible to detect an introduction of the gas of volume viscosity pBi into the gaseous substance located in the region 107, and to measure its concentration. For this measurement, the movement of the piston is said to be "under-damped", that is to say that it is located in a range for which the ratio fr / BW is strictly greater than 1 / 2.

[0052] [Fig.l] illustrates, by way of example, the case of a device comprising an oscillating mobile element comprising a piston suspended, by a spring, above a fixed flat surface. However, this example is not limiting and the person skilled in the art is, as a variant, able to provide, from the indications of the present description, mobile elements different from that of the device 100 of [Fig.l]. Other examples of devices having an operation similar to that of the device 100 of [Fig.l] are set out below.

[0053] [Fig.2A] and [Fig.2B] are respectively a top view and a side view and in section along plane BB of [Fig.2A], schematic and partial, of a sensor 200 of volume viscosity of a gaseous substance according to one embodiment.

[0054] In the example shown, the sensor 200 comprises a transducer 201, for example a MEMS (microelectromechanical system) type transducer. The transducer 201 comprises a movable element 203 suspended above a cavity 205 formed in a substrate 207. For example, the substrate 207 is a piece of wafer made of a semiconductor material, for example silicon. In the example shown, the movable element 203 is a deformable plate or membrane making it possible to compress or expand a gaseous substance located in the cavity 205. For example, the movable element 203 is a membrane made of the material of the substrate 207, for example a silicon membrane, and has for example a thickness of the order of one tenth of a micrometer to several micrometers, for example equal to approximately 2 μm.For example, the cavity 205 has a height of between one tenth of a micrometer and several micrometers, for example equal to approximately 0.8 μm.

[0055] In the example shown, the movable element 203 and the cavity 205 each have, in top view, a substantially circular shape. In this example, the cavity 205 has more precisely a cylindrical shape of circular section. In the example illustrated, the movable element 203 has lateral dimensions substantially equal to those of the cavity 205. This example is however not limiting, the movable element 203 and the cavity 205 being able, as a variant, to have any shapes and different lateral dimensions.

[0056] The transducer 201 further comprises, for example, at least one vent 209 (several vents 209, in the example shown), or ventilation orifice, allowing gas exchanges between the cavity 205 and an external environment. In the example shown, the movable element 203 is intended to compress and expand the gaseous substance contained in the cavity 207 in a manner similar to that described previously in relation to [Fig.l] for the piston 101 and the cavity 107, the movable element 203 of the sensor 200 being similar to an assembly comprising the piston 101 suspended by the spring 103 of the device 100.

[0057] The vents 209 are for example through openings formed in the membrane, directly above the cavity 205, and can each have, in top view, any shape, for example polygonal, rectangular, square, circular, oval, etc. In the example shown, the vents 209 are located at the periphery of the resonant element 203. The membrane being intended to undergo maximum deformation in the vicinity of its central part, the fact of providing that the vents 209 are located at the periphery of the membrane has the effect of maximizing the efficiency of compression and expansion of the gaseous substance by the membrane. This has the advantage of maximizing the sensitivity of the sensor 200. Furthermore, the fact of providing the vents 209 on the periphery of the mobile element 203, that is to say on the periphery of the cavity 205, in the example shown, does not significantly modify the elastic and viscous forces exerted inside the cavity 205.

[0058] In the example shown, the sensor 200 further comprises a heating element 211 located on the movable element 203. The heating element 211 comprises, in the example illustrated in FIGS. 2A and 2B, a resistive element 213, for example a thin conductive layer made of a metal or a metal alloy, the ends of which are connected respectively to contact recovery elements 215, for example conductive pads made of a metal or a metal alloy. The resistive element is for example located on and in contact with a face of the movable element 203 opposite the cavity 205 (the upper face of the movable element 203, in the orientation of [Fig. 2B]). Furthermore, the contact recovery elements 215 are for example located on and in contact with the upper face of the substrate 207, for example out of the vertical plane of the movable element 203.The contact recovery elements 215 are for example diametrically opposed relative to the circle formed, in top view, by the cavity 205. The heating element 211 is intended to increase the temperature of the movable element 203. This makes it possible, for example, to evacuate the humidity likely to be found inside the cavity 205. For example, in a case where the movable element 203 is a deformable membrane, the membrane may stick, or adhere, to the bottom of the cavity 205 due to the presence of humidity. In this case, the heating element 211 advantageously makes it possible to eliminate at least partially the condensed water present in the cavity 205 and to detach the membrane from the bottom of the cavity. 205. Alternatively, the heating element 211 may be omitted.

[0059] In the illustrated example, pads 217, symbolized by triangles in [Fig.2B], are provided on the lower face of the mobile element 203 and / or on the bottom of the cavity 205 in order to prevent the mobile element 203 from sticking to the bottom of the cavity 205. The pads 217 may have any shape, for example pyramidal, parallelepiped, cubic, etc. As a variant, the pads 217 may be omitted.

[0060] Although this has not been detailed in figures 2A and 2B, the lower face of the mobile element 203 and / or the lateral face and the bottom of the cavity 205 can be coated with a hydrophobic layer.

[0061] The sensor 200 is for example free of lead-based materials. Furthermore, the sensor 200 has for example characteristics compatible with production by implementing so-called CMOS (Complementary Metal-Oxide-Semiconductor) processes.

[0062] In a manner similar to what has been explained previously in relation to [Fig.l], the mobile element 203 of the sensor 200 is intended to be subjected to a vibratory, or oscillatory movement, causing it to deform along a vertical axis, in the orientation of [Fig.2B], so as to cause an alternation of compression phases and expansion phases of the gaseous substance contained in the cavity 205. [Fig.2B] illustrates more precisely a case in which the mobile element 203 is in a rest position. Furthermore, in [Fig.2B], a dotted shape symbolizes the deformation of the mobile element 203 during one of the compression phases.

[0063] Although this has not been detailed in figures 2A and 2B, different types of actuation and detection devices can equip the sensor 200. The following description sets out some examples, without limitation.

[0064] [Fig. 3] is a schematic and partial side and sectional view of a sensor 300 for the volume viscosity of a gaseous substance according to one embodiment.

[0065] The sensor 300 of [Fig. 3] comprises elements in common with the sensor 200 of FIGS. 2A and 2B. These common elements will not be detailed again below. The sensor 300 differs from the sensor 200 in that it comprises a transducer 301 of the PMUT type (from the English “Piezoelectric Micromachined Ultrasonic Transducer”). The PMUT transducer 301 is for example more precisely with successive, or alternating, excitation and detection.

[0066] In the example shown, the PMUT transducer 301 comprises, in addition to the mobile element 203 and the cavity 205 formed in the substrate 207, an excitation and detection structure 302 comprising a first electrode 303, located on one side of the mobile element 203 opposite the cavity 205, a layer 305 made of a piezoelectric material, located on one side of the first electrode 303 opposite the mobile element 203, and a second electrode 307, located on one side of the layer of piezoelectric material 305 opposite the first electrode 303. When an appropriate excitation voltage is applied between the electrodes 303 and 307 of the transducer 301, the movable element 203 begins to vibrate. Conversely, when the movable element 203 vibrates, a voltage appears between the electrodes 303 and 307 of the transducer 301.

[0067] The PMUT transducer 301 is coupled to an electronic control circuit 351 configured to, during an excitation phase, apply an excitation voltage between the first and second electrodes 303 and 307 of the transducer 301, for example so as to cause compression or expansion of the gaseous substance present inside the cavity 205 under the action of a deformation movement imposed on the mobile element 203, and, during a measurement or detection phase subsequent to the excitation phase, read the voltage produced between the first and second electrodes 303 and 307 of the transducer 301 under the action of the deformation of the mobile element 203, which is conditioned by the volume viscosity of the gaseous substance located in the cavity 205.The electronic control circuit 351 is for example configured to vibrate the mobile element 203 around its resonance frequency fr, and to measure a signal representative of the resonance frequency and the resonance bandwidth BW of the mobile element 203 of the PMUT transducer 301.

[0068] In the example shown, the control circuit 351 comprises, associated in series between the electrodes 303 and 307, a voltage source 353 and a switch 355. Furthermore, in this example, the control circuit 351 further comprises an amplifier 357, a first input terminal of which is connected to the electrode 303 and a second input terminal of which is connected, by another switch 359, to the electrode 307. The switches 355 and 359, shown in the open state in [Fig. 3], are for example intended to be controlled in phase opposition.

[0069] More precisely, during the excitation phase, the switch 355 is in the closed state, so as to impose an excitation signal between the electrodes 303 and 307, while the switch 359 is in the open state so that no signal is transmitted to an output terminal of the amplifier 357. Conversely, during the detection phase, the switch 355 is in the open state, so that the excitation signal is not applied to the electrodes 303 and 307, while the switch 359 is in the closed state to allow the provision of a measurement or detection signal at the output of the amplifier 357.

[0070] By way of example, the excitation signal applied during the excitation phase of the sensor 300 is a peak or a voltage step. During the subsequent detection phase, the resonance frequency fr and the resonance bandwidth BW of the mobile element are for example measured from a transient response of the mobile element 203 in the time and frequency domains, for example by means of a so-called time-frequency domain analysis. English). More generally, the amplifier 357 provides at output a signal representative of the resonance frequency fr and the resonance bandwidth BW of the mobile element 203. This signal is for example then compared to a reference signal, in order to deduce, as explained previously, information representative of the volume viscosity of the gaseous substance present in the cavity 205.

[0071] [Fig.4] is a schematic and partial side and sectional view of a sensor 400 for the volume viscosity of a gaseous substance according to one embodiment.

[0072] The sensor 400 of [Fig.4] comprises elements in common with the sensor 300 of [Fig.3]. These common elements will not be detailed again below. The sensor 400 differs from the sensor 300 in that it comprises a transducer 401 of the PMUT type with simultaneous excitation and detection.

[0073] In the example shown, the PMUT transducer 401 comprises an excitation structure 402E comprising a first electrode 403E, located on one side of the mobile element 203 opposite the cavity 205, a layer 405E made of a piezoelectric material, located on one side of the first electrode 403E opposite the mobile element 203, and a second electrode 407E, located on one side of the layer of piezoelectric material 405E opposite the first electrode 403E. Similarly, the PMUT transducer 401 further comprises a detection structure 402D comprising a first electrode 403D, located on one side of the movable element 203 opposite the cavity 205, a layer 405D made of a piezoelectric material, located on one side of the first electrode 403D opposite the movable element 203, and a second electrode 407D, located on one side of the layer of piezoelectric material 405D opposite the first electrode 403D. For example, the layers 405E and 405D are made of the same material.When an appropriate excitation voltage is applied between the electrodes 403E and 407E of the excitation structure 402E of the transducer 401, the movable element 203 vibrates. The vibration of the movable element 203 causes a voltage to appear between the electrodes 403D and 407D of the detection structure 402D of the transducer 401.

[0074] The PMUT transducer 401 is coupled to an electronic control circuit 451 configured to apply an alternating excitation voltage between the first and second electrodes 403E and 407E of the excitation structure 402E of the transducer 401, so as to cause an alternation of compression and expansion phases of the gaseous substance present inside the cavity 205 under the action of the oscillatory deformation movement imposed on the mobile element 203, and to simultaneously read the voltage produced between the first and second electrodes 403D and 407D of the detection structure 402D of the transducer 401 under the action of the vibration of the mobile element 203 conditioned by the volume viscosity of the gaseous substance located in the cavity 205.

[0075] In the example shown, the control circuit 451 comprises the voltage source 353 connecting the electrode 403E to the electrode 407E of the excitation structure 402E. In this example, the control circuit 451 further comprises the amplifier 357 whose first input terminal is connected to the electrode 403D of the detection structure 402D and whose second input terminal is connected to the electrode 407D of the detection structure 402D.

[0076] By way of example, the resonant frequency fr and the resonant bandwidth BW of the mobile element 203 are measured by scanning a frequency domain using the excitation signal applied by the source 353, and the response of the mobile element 203 is measured, by the detection structure 402D, in the frequency domain, for example by means of a so-called frequency domain analysis.

[0077] [Fig. 5] is a schematic and partial side and sectional view of a sensor 500 for the volume viscosity of a gaseous substance according to one embodiment.

[0078] The sensor 500 of [Fig. 5] comprises elements in common with the sensor 300 of [Fig. 3]. These common elements will not be detailed again below. The sensor 500 differs from the sensor 300 in that it comprises a transducer 501 of the CMUT type with successive, or alternating, excitation and detection.

[0079] In the example shown, the CMUT transducer 501 comprises, in addition to the mobile element 203 and the cavity 205 formed in the substrate 207, a structure 502 comprising a first electrode 503, called the lower electrode, covering the bottom of the cavity 205, and a second electrode 507, called the upper electrode, located on the mobile element 203 directly above the electrode 503. The electrode 503 is for example more precisely located on and in contact with the bottom of the cavity 205 formed in the substrate 207. The electrode 507 is for example located on and in contact with the upper face of the mobile element 203. The electrode 507 has for example lateral dimensions less than or equal to those of the electrode 503. By way of example, the mobile element 203 is made of an insulating material and the electrodes 503 and 507 are made of a conductive material, for example a metal or a metal alloy. Alternatively, the electrode 507 may be made of graphene.This advantageously makes it possible to minimize the impact, on the deformations of the mobile element 203, linked to the presence of the electrode 507. As a variant, the electrode 507 and the mobile element 203 can be produced by the same element, for example a thin layer, for example a graphene layer.

[0080] When an appropriate excitation voltage is applied between the electrodes 503 and 507 of the transducer 501, the movable element 203 begins to vibrate. Conversely, when the movable element 203 vibrates, an alternating voltage appears between the electrodes 503 and 507 of the transducer 501.

[0081] In a similar manner to the PMUT transducer 301 of the sensor 300, the CMUT transducer 501 of the sensor 500 is coupled to an electronic control circuit 551 configured to, during an excitation phase, apply an excitation voltage between the first and second electrodes 503 and 507 of the transducer 501, for example so as to cause compression or expansion of the gaseous substance present inside the cavity 205 under the action of a deformation movement imposed on the mobile element 203, and, during a measurement or detection phase subsequent to the excitation phase, read the voltage produced between the first and second electrodes 503 and 507 of the transducer 501 under the action of the deformation of the mobile element 203, which is conditioned by the volume viscosity of the gaseous substance located in the cavity 205.

[0082] In the example shown, the control circuit 551 comprises, associated in series between the electrode 507 and a node for applying a reference potential, for example ground, the switch 355 and the voltage source 353. In this example, the control circuit 551 further comprises a DC voltage source 553 connecting the electrode 507 to the node for applying the reference potential. The DC voltage source 553 makes it possible to apply an electromagnetic force which brings the mobile element 203 closer to the horizontal wall 105B, thus increasing the capacity and the electromechanical coupling coefficient of the system. By way of example, the DC voltage 553 is adjusted so that the CMUT transducer 501 operates in the vicinity of a so-called collapse voltage, corresponding to a voltage value from which the spring constant no longer makes it possible to balance the electrostatic force. This advantageously makes it possible to maximize the electromechanical coupling.Furthermore, in the example illustrated, the control circuit 551 further comprises the amplifier 357 whose first input terminal is connected to the electrode 503 and whose second input terminal is connected, by the switch 359, to the electrode 507. The switches 355 and 359, shown in the open state in [Fig.5], are for example intended to be controlled in phase opposition.

[0083] The operation of the sensor 500, in particular the control of the switches 355 and 359, is analogous to what was previously described for the sensor 300 in relation to [Fig. 3] and will therefore not be described again below. The person skilled in the art is able to deduce the operation of the sensor 500 from the indications of the present description, in particular from the functional description of the sensor 300.

[0084] [Fig.6] is a schematic and partial side and sectional view of a sensor 600 for the volume viscosity of a gaseous substance according to one embodiment.

[0085] The sensor 600 differs from the sensor 500 in that it comprises a transducer 601 of the CMUT type with simultaneous excitation and detection.

[0086] In the example shown, the CMUT transducer 601 comprises an excitation structure 602E comprising a first electrode 603E, located on and in contact with the bottom of the cavity 205, and a second electrode 607E, located on the movable element 203 directly above the electrode 603E. Similarly, the CMUT transducer 601 further comprises a detection structure 602D comprising a first electrode 603D, located on and in contact with the bottom of the cavity 205, and a second electrode 607D, located on the movable element 203 directly above the electrode 603D. When an appropriate excitation voltage is applied between the electrodes 603E and 607E of the excitation structure 602E of the transducer 601, the movable element 203 begins to vibrate. The vibration of the movable element 203 causes an alternating voltage to appear between the electrodes 603D and 607D of the detection structure 602D of the transducer 601.

[0087] The CMUT transducer 601 is coupled to an electronic control circuit 651 configured to apply an alternating excitation voltage between the first and second electrodes 603E and 607E of the excitation structure 602E of the transducer 601, so as to cause an alternation of compression and expansion phases of the gaseous substance present inside the cavity 205 under the action of the oscillatory deformation movement imposed on the mobile element 203, and to simultaneously read the voltage produced between the first and second electrodes 603D and 607D of the detection structure 602D of the transducer 601 under the action of the vibration of the mobile element 203 conditioned by the volume viscosity of the gaseous substance located in the cavity 205.

[0088] In the example shown, the control circuit 651 comprises the voltage sources 353 and 553 each connecting the electrode 607E to the node for applying the reference potential. In this example, the control circuit 651 further comprises the amplifier 357 whose first input terminal is connected to the electrode 603D of the detection structure 602D and whose second input terminal is connected to the electrode 607D of the detection structure 602D.

[0089] The operation of the sensor 600 is analogous to what was previously described for the sensor 400 in relation to [Fig. 4] and will therefore not be described again below. The person skilled in the art is able to deduce the operation of the sensor 600 from the indications of the present description, in particular from the functional description of the sensor 400.

[0090] Although this has not been shown in Figures 3 to 6 in order not to overload the drawing, the transducers 301, 401, 501 and 601 may comprise the heating element 211 and / or the pads 217 of the transducer 201 of the sensor 200 of Figures 2A and 2B.

[0091] For the sake of clarity, the control circuits 351, 451, 551 and 651 have been represented in a simplified manner.

[0092] The sensors 200, 300, 400, 500 and 600 previously described have the advantages, compared to existing gas sensors, of being compact, energy-saving, stable and inexpensive.

[0093] [Fig.7] is a schematic and partial side and sectional view of an electronic device 700 integrating a volume viscosity sensor of a gaseous substance, for example the sensor 300 of [Fig.3], according to one embodiment. In order not to overload the drawing, the excitation and detection structure 302 and the control circuit 351 of the sensor 300 have not been shown in [Fig.7].

[0094] In the example shown, the electronic device 700 comprises a support and interconnection substrate 701 on which the sensor 300 is located. By way of example, the support and interconnection substrate 701 is a printed circuit board or a ceramic package.

[0095] In the illustrated example, the electronic device 700 further comprises an integrated circuit chip 703, for example an ASIC type chip (from the English "Application-Specific Integrated Circuit" - application-specific integrated circuit). In the example shown, the integrated circuit chip 703 is located on the support and interconnection substrate 701. Although this has not been detailed in [Fig. 7] for the sake of readability of the drawing, the control circuit 351 of the sensor 300 is for example formed at least partially in the integrated circuit chip 703. In the example shown, the sensor 300 is connected to the integrated circuit chip 703 by wire bonding, the sensor 300 and the integrated circuit chip 703 each comprising for example one or more contact recovery elements 705, for example conductive pads, interconnected by one or more conductive wires 707.Similarly, at least one of the contact recovery elements 705 of the integrated circuit chip 703 is for example connected to a contact recovery element 709, for example a conductive pad formed on the support and interconnection substrate 701, by a conductive wire 711. By way of example, the conductive wire 711 connecting one of the contact recovery elements 705 of the integrated circuit chip 703 to one of the contact recovery elements 709 of the support and interconnection substrate 701 makes it possible to transmit a power supply signal to the chip 703 from the substrate 701.

[0096] In the example illustrated in [Fig.7], the electronic device 700 further comprises an enclosure 713, or cover, located on the support and interconnection substrate 701 and inside which the sensor 300 and the integrated circuit chip 703 are located. A filter 715 is for example arranged opposite an opening 717 formed in the upper part of the enclosure 713. The filter 715 is for example intended to allow gas exchanges between the inside and the outside of the enclosure 713 while preventing or limiting the penetration of foreign bodies, for example dust, inside enclosure 713.

[0097] In the example shown, the electronic device 700 further comprises an ultrasonic absorber 719 placed against the internal wall of the enclosure 713 and opposite the sensor 300. This makes it possible, for example, to avoid or limit a disturbance of the movements of the mobile element 203 of the sensor 300 caused by reflections of ultrasonic waves against the upper part of the enclosure 713. The fact of providing the ultrasonic absorber 719 directly above the sensor 300 makes it possible in particular to avoid or limit the formation of standing waves inside the enclosure 713.

[0098] Although [Fig. 7] illustrates an example in which the filter 715 and the ultrasonic absorber 719 are two separate elements, this example is not limiting, the filtering and ultrasonic absorption functions being able, as a variant, to be carried out by means of the same element. Furthermore, although this has not been detailed in [Fig. 7], the electronic device 700 may further comprise a temperature sensor and / or a humidity sensor inside the enclosure 713. The fact of providing a temperature sensor and / or a humidity sensor inside the enclosure 713 makes it possible, for example, to correct the influence of variations in ambient temperature and / or humidity on the measurements made by the sensor 300. Furthermore, variations in ambient pressure are, for example, detected by taking advantage of the fact that this causes a modification of the resonant frequency of the mobile element 203 of the sensor 300.This advantageously makes it possible, for example, to avoid using a separate pressure sensor.

[0099] The electronic device 700 is, for example, more precisely an air quality measuring device. For example, the electronic device 700 makes it possible to detect a change in the carbon dioxide content in the air. As a variant, the electronic device 700 may be a gas leak detector having a high volume viscosity. The integrated circuit 703 is, for example, configured to estimate the carbon dioxide concentration Ci in the external environment and to compare the concentration Ci with a threshold value and to provide an alarm signal, indicating a deterioration in the air quality or the presence of a leak, in the event that this threshold value is crossed.

[0100] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art. In particular, although examples of sensors with piezoelectric or capacitive actuation have been detailed above, the embodiments are not limited to these cases but apply more generally to any type of actuation that may be used in a MEMS device. Alternatively, those skilled in the art are able, from the indications of the present description, to provide an ac- electrostatic, electrothermal, electromagnetic, etc. type operation. Similarly, although examples of sensors with piezoelectric or capacitive detection have been detailed above, the embodiments are not limited to these cases but apply more generally to any type of detection capable of being used in a MEMS device. As a variant, the person skilled in the art is able, from the indications of the present description, to provide detection of the piezoresistive, optical, etc. type.

[0101] Finally, the practical implementation of the described embodiments and variants is within the reach of those skilled in the art from the functional indications given above. In particular, the described embodiments are not limited to the particular examples of materials and dimensions mentioned in the present description.

[0102] Furthermore, the person skilled in the art is able to substitute the sensor 300 with the sensor 200, 400, 500 or 600 in the electronic device 700.

Claims

Claims

1. Sensor (200; 300; 400; 500; 600) of volume viscosity of a gaseous substance, the sensor comprising: - a MEMS transducer (201; 301; 401; 501; 601) comprising a mobile element (203) suspended above a cavity (205) and at least one vent (209) allowing gas exchanges between the cavity and an external environment; and - a control circuit (351; 451; 551; 651) of the MEMS transducer configured to: a) vibrate the mobile element around its resonant frequency; b) measure a signal representative of a resonant frequency and a resonant bandwidth of the MEMS transducer; and c) compare said signal with a reference signal and deduce therefrom information representative of the volume viscosity of the gaseous substance present in the cavity.

2. Sensor (200; 300; 400; 500; 600) according to claim 1, wherein the cavity (205) is formed in a substrate (207) made of a semiconductor material, preferably silicon, the movable element (203) being a membrane made of the semiconductor material.

3. Sensor (300; 400) according to claim 1 or 2, wherein the MEMS transducer (301; 401) is of the PMUT type.

4. Sensor (300) according to claim 3, wherein the MEMS transducer (301) comprises an excitation and detection structure comprising: - a first electrode (303), located on one side of the movable element (203) opposite the cavity (205); - a layer (305) of a piezoelectric material, located on one side of the first electrode opposite the movable element; and - a second electrode (307), located on one side of the layer of piezoelectric material opposite the first electrode.

5. Sensor (400) according to claim 3, in which the MEMS transducer (401) comprises: - an excitation structure (402E) comprising a first electrode (403E), located on one side of the movable element (203) opposite the cavity (205), a first layer (405E) of a piezoelectric material, located on one side of the first electrode opposite the movable element, and a second electrode (407E), located on one side of the first layer in piezoelectric material opposite the first electrode; and - a detection structure (402D) distinct from the excitation structure and comprising a third electrode (403D), located on one side of the mobile element opposite the cavity, a second layer (405D) in a piezoelectric material, located on one side of the third electrode opposite the mobile element, and a fourth electrode (407D), located on one side of the layer in piezoelectric material opposite the third electrode.

6. Sensor (500; 600) according to claim 1 or 2, wherein the MEMS transducer (501; 601) is of the CMUT type.

7. Sensor (500) according to claim 6, in which the MEMS transducer (501) comprises a structure (502) comprising a first electrode (503) covering the bottom of the cavity (205), and a second electrode (507) located on the movable element (203) directly above the first electrode.

8. Sensor (600) according to claim 6, wherein the MEMS transducer (601) comprises: - an excitation structure (602E) comprising a first electrode (603E), located on and in contact with the bottom of the cavity (205), and a second electrode (607E), located on the movable element (203) in line with the first electrode; and - a detection structure (602D) distinct from the excitation structure and comprising a third electrode (603D), located on and in contact with the bottom of the cavity, and a fourth electrode (607D), located on the movable element in line with the third electrode.

9. Sensor (300; 500) according to claim 4 or 7, wherein the control circuit (351; 551) is configured to: - during an excitation phase, apply an excitation voltage between the first and second electrodes (303, 307; 503, 507) of the transducer (301; 501); and - during a detection phase, subsequent to the excitation phase, read a voltage produced between the first and second electrodes of the transducer under the action of a deformation of the mobile element (203).

10. Sensor (400; 600) according to claim 5 or 8, wherein the control circuit (451; 651) is configured to apply an alternating excitation voltage between the first and second electrodes (403E, 407E; 603E, 607E) so as to cause an alternation of compression and expansion phases of the gaseous substance present inside of the cavity (205), and to simultaneously read a voltage produced between the third and fourth electrodes (403D, 407D; 603D, 607D) under the action of the vibration of the movable element (203).

11. Device (700) for measuring air quality or detecting leaks comprising a sensor (200; 300; 400; 500; 600) according to any one of claims 1 to 10.

12. Method for controlling a sensor (200; 300; 400; 500; 600) of volume viscosity of a gaseous substance, the sensor comprising: - a MEMS transducer (201; 301; 401; 501; 601) comprising a movable element (203) suspended above a cavity (205) and at least one vent (209) allowing gas exchanges between the cavity and an external environment; and - a control circuit (351; 451; 551; 651) of the MEMS transducer, the method comprising the following steps, implemented by the control circuit: a) vibrating the mobile element around its resonance frequency; b) measuring a signal representative of a resonance frequency and a resonance bandwidth of the MEMS transducer; and c) comparing said signal with a reference signal and deducing therefrom information representative of the volume viscosity of the gaseous substance present in the cavity.

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