Patent Publication Number: US-11039255-B2

Title: Wide-passband capacitive vibrating-membrane ultrasonic transducer

Description:
The invention relates to acoustic transducers that operate in the ultrasound range and in particular to transducers of this type that are capacitive and comprise a vibrating membrane. Description 
     Ultrasonic waves are pressure waves the frequency range of which starts at 20 kHz and extends up to a few tens of MHz. Ultrasonic waves propagate at a speed that depends on the propagation medium: about 343 m/s in air and 1500 m/s in water. The waves undergo, as they propagate, absorption at a rate that increases with their frequency. Moreover, when the wave encounters a discontinuity in the propagation medium, some of the wave is transmitted and some is reflected. 
     Various applications use ultrasonic transducers with a view to:
         creating an absorption of ultrasonic waves, for example in order to heat material locally;   emitting and receiving waves, for example for an application to the transmission of information;   analysing the reflection of waves from obstacles, for example for an application to range finding.       

     In such applications, there is an increasing need for miniaturized electroacoustic transducers for propagating ultrasound through fluid media. In a fluid, an acoustic wave is generated by the movement of a movable surface. At the movable surface, the acoustic intensity of the source is equal to the impedance of the medium multiplied by the square of the speed of the movable surface. For a given frequency, the larger the amplitude of the movements of the movable surface of the transducer, the greater the intensity of the source. 
     Vibrating-membrane transducers are being developed for miniaturized applications. Such transducers include membranes that are suspended above cavities that are produced in a carrier or that are open. The diameter of circular cavities is generally comprised between a few tens of and a few hundred microns. The thickness of such membranes is generally larger than 50-100 nm and up to several microns. The resonant frequency of the complete device depends on the geometry of the cavity/membrane assembly and on the materials used. 
     One particular case of a vibrating-membrane transducer is the capacitive vibrating-membrane transducer. The membrane of an emitter is for example subjected to an electrostatic force by applying an alternating potential difference across this membrane and a conductive electrode housed at the bottom of the cavity. For a detector, the value of the capacitance formed between the membrane and the electrode housed at the bottom of the cavity is determined at any given time by the deformation of the membrane, and therefore by the instantaneous pressure incident on the membrane. Detection involves measuring the variations in this capacitance. 
     The movement of the membrane is maximum at the resonant frequency of this membrane. Emission intensity is therefore maximum at the resonant frequency of the membrane. The same goes for the detection of a wave: the sensitivity of the sensor is maximum at the resonant frequency of its membrane. 
     Ultrasonic transducers are therefore generally associated with an optimal operating frequency that is determined by the resonance of their membrane. The quality factor of the mechanical resonator including the membrane determines the passband of the transducer. A bandwidth is conventionally bounded by frequencies corresponding to a decrease of half in the acoustic intensity with respect to resonance, on either side of this resonant frequency. The widest passbands may be of the same order of magnitude as the resonant frequency: for example, a transducer of resonant frequency of 1 MHz with a bandwidth of 600 kHz, i.e. a passband extending from 700 kHz to 1300 kHz, is considered to be a wide-band transducer. Outside of the passband, the amplitudes of vibration may be lower by several orders of magnitude than the amplitudes at resonance. 
     This resonant operating mode implies that each application requires a specific transducer, because very different ultrasonic frequencies cannot be covered by one and the same transducer: the detection of obstacles is typically carried out at 40 kHz with a range of a few metres in air, the capture of gestures is carried out between 100 kHz and 400 kHz with a range of a few tens of centimetres in air, the detection of fingerprints is carried out between 1 MHz and 10 MHz with a millimetric range in a nonuniform medium, and ultrasonic medical imaging uses frequencies between 5 MHz and 50 MHz in aqueous-type media. 
     Document WO2012010786 describes a capacitive vibrating-membrane ultrasonic transducer. In this transducer, a cavity of a carrier is kept under vacuum under a membrane. The document suggests making the transducer operate at a frequency below the resonant frequency and considering a wider range of operating frequencies, with the performance level varying depending on the frequency used. 
     There is therefore a need for transducers having wider frequency bands of use to be designed. Moreover, in range-finding applications, there is a need to minimize the blind spot found in close proximity. 
     The invention aims to solve one or more of these drawbacks. The invention thus relates to a capacitive vibrating-membrane ultrasonic transducer such as defined in the appended claims. 
     The invention also relates to the variants in the dependent claims. Those skilled in the art will understand that each of the features of the dependent claims and of the description may be independently combined with the features of an independent claim, without however constituting an intermediate generalization. 
    
    
     
       Other features and advantages of the invention will become clear from the nonlimiting description that is given thereof below, by way of indication, with reference to the appended drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of a vertical plane of an example of a capacitive vibrating-membrane ultrasonic transducer according to one embodiment of the invention; 
         FIG. 2  is a schematic cross-sectional view of a vertical plane of an example of a capacitive vibrating-membrane ultrasonic transducer according to another embodiment of the invention; 
         FIG. 3  is a schematic cross-sectional view of a vertical plane of an example of a capacitive vibrating-membrane ultrasonic transducer according to another embodiment of the invention; 
         FIG. 4  is a schematic cross-sectional view of a vertical plane of an example of a capacitive vibrating-membrane ultrasonic transducer according to another embodiment of the invention; 
         FIG. 5  is a schematic cross-sectional view of a horizontal plane of a matrix array of ultrasonic transducers according to the embodiment of  FIG. 4 ; 
         FIG. 6  is a schematic cross-sectional view of a vertical plane of an example of a capacitive vibrating-membrane ultrasonic transducer according to another embodiment of the invention; 
         FIG. 7  is a graph illustrating the results of measurements of amplitudes of vibration in air of a membrane for various exciting voltages. 
     
    
    
       FIG. 1  is a schematic cross-sectional view of a vertical plane of an example of a capacitive vibrating-membrane ultrasonic transducer  1  according to a first embodiment of the invention. The transducer  1  comprises a carrier  13  in which a cavity  14  is produced. The cavity  14  is for example cylindrical. In the illustrated example, the carrier  13  notably includes a substrate  131  taking the form of a plate, and a dielectric layer  132  also taking the form of a plate. In the illustrated example, the carrier  13  also includes a conductive element  101 . The substrate  131  and the dielectric layer  132  are here fastened to the conductive element  101 . The dielectric layer  132  comprises a bore defining the sidewalls of the cavity  14 . The bore depth of the cavity  14  is smaller than the height of the layer  132  or of the two layers  132  and  102  together. The bottom  141  of the cavity  14  is thus advantageously delineated by a dielectric layer  15 . One portion of the conductive element  101  is thus housed under the cavity  14 , under the dielectric layer  15 . 
     A vibrating membrane  11  is fastened to the carrier  13  and covers the cavity  14 . The membrane  11  has an external upper face  113  and an internal lower face  114 . The membrane  11  is placed facing the conductive element  101 . The membrane  11  and the conductive element  101  are separated by the cavity  14  and the dielectric layer  15 . 
     In the illustrated example, the membrane  11  is fastened to the dielectric layer  132  of the carrier  13  by way of an electrode  102 . As detailed below, the electrode  102  is merely an optional component for exciting the membrane  11 . The electrode  102  here takes the form of a plate. The electrode  102  is here fastened to an upper face of the dielectric layer  132  and has a similar shape thereto given that it is passed through by the same bore. The electrode  102  makes electrical contact with the membrane  11  on the periphery of the cavity  14 . 
     The conductive element  101  forms an electrode of the transducer  1 . An exciting circuit  2  has its terminals connected on the one hand to the electrode  102  and on the other hand to the conductive element  101 . By applying an alternating potential across its terminals, the exciting circuit  2  allows an electric field to be created between the membrane  11  and the conductive element  101 , this subjecting the membrane  11  to an electrostatic force and causing it to bow. The transducer  1  is therefore capacitive. 
     In linear regime, the movement d of the centre of the membrane  11  in a direction normal to its plane at rest is proportional to the applied force F and to the shear modulus of the membrane: F=D*d. 
     For a membrane  11  forming a plate, in the absence of tension:
 
 D=E*h   3 /12*(1η 2 )
 
     with E Young&#39;s modulus and η the Poisson&#39;s coefficient of the material of the membrane  11  and h its thickness. 
     In the mechanics of vibrations, theory allows different vibratory behaviours to be distinguished between depending on the geometry and design of the vibrating membrane  11 . 
     To simplify, different one-dimensional objects, such as a beam and a rope, may firstly be analysed. A beam will have a behaviour and a resonant frequency that are mainly determined by its geometry (its length and its cross section) and the Young&#39;s modulus of the material from which it is made. The behaviour of a rope, for its part, will be essentially defined by its tension. The tauter the rope, the higher its resonant frequency. 
     Likewise, for two-dimensional objects, the following are both encountered:
         a behaviour of plate type, determined by the geometry of the object and its material. A resonant frequency fp is associated with this behaviour;   a behaviour of membrane type, mainly defined by the tension in the object.       

     Another resonant frequency fm is associated with this behaviour. 
     The resonant frequency of the object is the quadratic sum of the resonant frequencies due to each of these two behaviours. 
     For a circular object of radius R embedded on its periphery, the resonant frequency fr is defined by the relationship:
 
 fr ( R )=√( fm   2 ( R )+ fP   2 ( R ))
 
     The resonant frequency fm in membrane mode may notably be defined in this case by the following relationship, with T the tension of the object (in N/m) and s its density per unit area (in kg/m 2 ): 
     
       
         
           
             
               
                 
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     The resonant frequency fp in plate mode may notably be defined in this case by the following relationship, with p the density of the circular object (in kg/m 3 ): 
     
       
         
           
             
               
                 
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     Those skilled in the art will be able to define, empirically or analytically, the resonant frequencies fp and fm for other vibrating-membrane geometries. 
     According to one preferred aspect of the invention, the vibrating membrane  11  of the transducer  1  respects the following relationship: fm&gt;fp. Preferably, the vibrating membrane  11  of the transducer  1  respects the following relationship: fm&gt;1.5*fp, and even more preferably fm&gt;2.5*fp. The vibrating membrane  11  of the transducer  1  therefore has a membrane mode that is preponderant with respect to its plate mode. Advantageously, by satisfying this inequality, it is possible to place the membrane in a mode in which it exhibits significant movement far from resonance, i.e. in linear mode. 
     According to the invention, the exciting circuit  2  is configured to apply, across its terminals, a signal the frequency components of which are included in the frequency interval [0−fo], fo respecting the relationship f0&lt;fr, and preferably fo&lt;0.66*fr (i.e. fr&gt;1.5*fo). Therefore, it may be deduced therefrom that f0&lt;fr&lt;fm. Thus, the membrane  11  is excited at a frequency clearly below its resonant frequency in membrane mode: the movements of the membrane are not caused by a resonance effect, but by a forced-oscillation mechanism, this allowing a wide range of usable excitation frequencies, extending from very low frequencies (a few hertz) up to 0.66*fr, and in which the level of performance remains constant, to be obtained. The same transducer  1  may thus be used for many different applications. Contrary to resonant excitations, the use of forced oscillations also allows short pulses to be generated and, therefore, for range-finding applications for example, blind spot to be minimized. The use of forced oscillations also allows the exciting power to be increased at constant frequency. Advantageously, the exciting circuit  2  is configured to apply, across its terminals, an exciting signal with a maximum frequency fo respecting the relationship fr&gt;f0, and advantageously fr&gt;1.5*fo. 
     Advantageously, the exciting circuit  2  is configured to apply, across its terminals, a signal such that the ratio between the total electrical power applied across these terminals and the electrical power applied in a frequency range comprised between 0.9*fr and 1.1*fr is at least equal to 10. Thus, most of the exciting power is applied outside of the resonant range. 
     The graph of  FIG. 7  illustrates the results of measurement of amplitudes of vibration in air of a membrane for various exciting voltages. A membrane above a cavity of 2 μm was excited at fo=5 Hz with various exciting voltages. The fundamental resonance of this type of membrane is located at frequencies between 20 MHz and 30 MHz. The solid line corresponds to an exciting voltage of 16 V, the dashed line corresponds to an exciting voltage of 12 V, the dotted line corresponds to an exciting voltage of 8 V, and the dash-dotted line corresponds to an exciting voltage of 4 V. The amplitudes of deformation at 5 Hz are therefore in forced-oscillation mode, and these amplitudes increase with the exciting voltage, as this graph shows. Amplitudes of a few tens of nanometres were obtained for a cavity diameter of 2 μm. These very large deformations off resonance allow ultrasonic waves to be generated. 
     Ultrasonic waves were emitted experimentally between 20 kHz and 140 kHz by membranes of 15 nm thickness suspended above circular cavities of 10 μm diameter. 
     The components of the transducer  1  may have the following dimensions and compositions:
         the conductive element  101  may for example have a thickness comprised between 150 and 250 nm, of 200 nm for example. The conductive element  101  may for example be made of tungsten, of aluminum, of titanium, of copper, of gold or of a combination of these materials;   the dielectric layer  132  may for example have a thickness comprised between 0.8 and 1.25 μm, 1 μm for example. The dielectric layer  132  may for example be made of SiO 2 ;   the substrate  131  may for example be made of glass, of quartz, of alumina, of silicon covered with a dielectric layer or even of SiN;   the cavity  14  may have a diameter comprised between 5 and 50 μm, 10 μm for example (defining the suspended length of the membrane  11 );   the electrode  102  may for example have a thickness comprised between 80 and 150 nm, of 100 nm for example. The electrode  102  may for example be made of tungsten, of aluminum, of titanium, of copper, of gold, or of any other conductive material or alloy. The electrode  102  may be formed by depositing a conductive material on an insulating carrier;   the membrane  11  may for example have a thickness comprised between 5 and 25 nm, of 10 nm for example. The membrane  11  may for example comprise a layer of amorphous carbon. The membrane  11  may be fastened to the carrier  13  without tensile prestress.       

     Advantageously, the membrane  11  has a thickness at most equal to 100 nm. The membrane  11  may advantageously be intended to vibrate in the cavity  14  with an amplitude of at least 5% of the suspended length and lower than the depth of the cavity. 
     The diameter of the cavity  14  may be decreased in order to increase the resonant frequency of the membrane  11 . 
     A continuous or very low frequency electrostatic force may be applied by the exciting circuit  2  in order to impose an initial mechanical tension on the vibrating membrane  11 . The exciting circuit  2  will then apply, across the electrode  102  and the element  101 , a potential difference with a continuous or very-low-frequency component (for example of frequency at most equal to 50 Hz) so as, inter alia, to allow the sensitivity and dynamic range of the transducer  1  to be modulated. 
       FIG. 2  is a schematic cross-sectional view of a vertical plane of an example of a capacitive vibrating-membrane ultrasonic transducer  1  according to a second embodiment of the invention. The transducer  1  comprises a carrier  13 , a conductive element  101 , an electrode  102  and a membrane  11  that are identical to those of the first embodiment. In this example, a measuring circuit  3  is electrically connected to the membrane  11  and the conductive element  101  (by way of the connecting via  103 ). 
     The measuring circuit  3  measures the charge movements related to the instantaneous variation in the capacitance between the electrode  102  and the element  101 , which variation is induced by the vibrations of the membrane  11 . 
     The measuring circuit  3  will also possibly apply, across the electrode  102  and the element  101 , a potential difference with a continuous or very-low-frequency component (for example of frequency at most equal to 50 Hz) so as to be able to modulate the sensitivity and dynamic range of the transducer  1 . 
     It is also possible to envision connecting an exciting circuit  2  and a measuring circuit  3  such as described above to the membrane  11  and conductive element  101 . The exciting circuit  2  and the measuring circuit  3  may be connected selectively and independently by respective switches. It will then be possible to independently process the emission and reception of an acoustic signal, for example in order to implement a range-finding mode. 
       FIG. 3  is a schematic cross-sectional view of a vertical plane of an example of a capacitive vibrating-membrane ultrasonic transducer according to a third embodiment of the invention. The transducer  1  comprises an exciting circuit  2  (and may comprise in addition or instead a measuring circuit  3 ), a conductive element  101 , an electrode  102  and a membrane  11  that are identical to those of the first embodiment. In this example, the carrier  13  comprises at least one duct  104  placing in communication the interior of the cavity  14  and the exterior of the transducer. The ducts  104  allow the cavity  14  of the transducer  1  (and therefore the internal face  114  of the membrane  11 ) to be placed in communication with the external face  113  of the membrane  11 . It is thus possible to equilibrate the pressures on the faces  113  and  114  of the membrane  11  and to ensure that all the cavities are at the same pressure, in this case atmospheric pressure. The ducts  104  may also be replaced by grooves in the upper surface of the dielectric layer  132 . 
       FIG. 4  is a schematic cross-sectional view of a vertical plane of an example of a capacitive vibrating-membrane ultrasonic transducer according to a fourth embodiment of the invention. In this example, the membrane  11  includes a plurality of superposed layers made of different materials. The membrane  11  thus includes a superposition of a layer  111  and of a layer  112  made of different materials. The layer  111  is for example made of a conductive material such as titanium. The layer  111  may for example have a thickness comprised between 3 and 7 nm, and typically of 5 nm. The layer  112  is for example made of amorphous carbon. The layer  112  may for example have a thickness comprised between 8 and 12 nm, and typically of 10 nm. Such a configuration proves to be an optimal way, on the one hand, of promoting the capacitive effect via the use of the conductive layer  111 , and, on the other hand, of promoting the flexibility and durability of the membrane  11  via the use of the layer  112 . 
     In the various examples, if the membrane  11  includes a conductive layer, it is possible not to interpose an electrode  102  between this membrane  11  and the circuits to which it is connected. In particular, in the following embodiment a membrane  11  including a combination of a conductive layer and of a layer chosen for its mechanical properties is described: the electrode  102  may be omitted if a circuit is directly connected to the conductive layer. This corresponds to the example of  FIG. 6 , in which the membrane  11  described with reference to  FIG. 5  has been shown again. 
       FIG. 5  is a schematic cross-sectional view of a horizontal plane of a matrix array of ultrasonic transducers  1  according to the embodiment of  FIG. 4 . The transducers may be arranged in a matrix array of 500 by 500 transducers  1  along two perpendicular axes. A corresponding matrix array of cavities  14  is thus produced in a common dielectric layer  132 . The cavities  14  of a given column of transducers  1  are placed in communication by way of ducts  109 . At the ends of the matrix array, the transducers  1  are placed in communication with the exterior via ducts  104 . Thus, the cavity of each transducer  1  is placed in communication with the external face of its membrane by way of the ducts  104 , and  109  where appropriate. A given membrane  11  may be used to cover all of the cavities  14 . 
     The pressure in the cavities  14  may also be different from the surrounding pressure. A peripheral seal may thus be employed if the various cavities  14  of the matrix array are placed in communication with one another. 
     In order to be able to orient the emission or reception beam, an array of transducers  1  may comprise a plurality of conductive elements  101  (for example arranged in parallel) and/or a plurality of electrodes  102  (for example in parallel). A plurality of channels may for example be formed. Parallel conductive elements  101  may be positioned perpendicular to the parallel electrodes  102 . The elements of the array are thus defined by superposing an electrode  102  and a conductive element  101  and are individually addressable. The pitch between the conductive elements of  101  or  102  may be decreased to the pitch of the array of elementary transducers. A small elementary transducer diameter of 10 μm with a pitch of 15 μm for example allows beamforming to be carried out at up to more than 10 MHz in air.