Patent Application
20130116568
The present invention relates to a method for generating ultrasound using capacitive micromachined ultrasonic transducers (CMUTs). It also relates to a device for generating ultrasound using such a method. It relates finally to a method and a system for medical imaging using CMUTs.
The field of the invention is the field of the generation of ultrasound using CMUTs.
A CMUT transducer is formed from several hundred, or even a few thousand mechanically isolated “micro-membranes” capable of being actuated by electrostatic forces. These are called CMUTs for Capacitive Micromachined Ultrasonic Transducers. Each CMUT is constituted by a rear electrode formed by a semi-conductor material (generally doped polysilicon), a vacuum cavity having a height Hgap, a membrane made of microelectronics material overlaid by an electrode, the membrane/electrode unit constituting the “mobile” part of the device. The material used for the membrane is often silicon nitride but is highly dependent on the technology of fabrication of the device itself. Other materials such as doped polysilicon (in the “wafer bonding” method), a metal or a polymer could be used. CMUTs are now commonly used in the field of medical imaging to excite an organ or a tissue of a human or animal subject. The use of the capacitive micromachined ultrasonic transducers in ultrasound medical imaging is based on the same usage protocols as piezoelectric devices. Typically, the CMUT transducer is polarized with direct current voltage and the sending of a pressure wave is carried out by means of wideband excitation which covers the entire pass band of the transducer. The central frequency of these devices, i.e. the resonance frequency, is defined by the membrane/fluid pair which plays the role of a spring/mass system where the elasticity depends only on the properties of the membrane and the mass of the fluid. This mass effect is moreover dependent on the effects of mutual interactions between membranes the consequence of which is to create cut-off frequencies in the pass band of the transducer.
However, the generation of low-frequency ultrasound, for example ultrasound at frequencies less than or equal to 2 MHz requires membranes having a low mechanical rigidity that can be obtained either by increasing their width, or by reducing their thickness or using materials that have a low Young's modulus. The low resonance frequency devices generally have a low functional capability. In fact, as their mechanical rigidity is relatively low, the membranes are subjected to the pressure of the outside air and are thus deformed by several tens of nanometres or even around a hundred. The deformation can lead to the membrane becoming jammed at the base of the cavity, thus rendering the device unusable. In order to compensate for this deflection, the height of the cavity can be increased in order to retain a “free” space between the membrane and the rear of the cavity, but this leads to a significant increase in the supply voltages necessary to drive the CMUTs. The increase in the supply voltage reduces the possibilities for use, as a very high voltage of use (several hundred volts) requires specific voltage supply means. In order to avoid this deflection, a gas, the pressure of which is equal to average outside pressure, can be maintained in the cavity. However, the dynamic damping effects linked to the presence of this gas significantly change the resonance of the device and require an architecture of complex CMUTs intended to eliminate these effects (perforation of the rear cavity). These solutions are easy to implement for the very low-frequency devices (less than 100 kHz) but relatively costly and difficult to carry out for higher frequencies.
A purpose of the present invention is to remedy the above drawbacks. A another purpose of the present invention is to propose a method and a device for generating ultrasound with at least one CMUT transducer that is easier to fabricate, cheaper and operates with a supply voltage that is more accessible and acceptable for low-voltage supplies, while making it possible to obtain satisfactory useful pressure levels.
The invention proposes to achieve the above-mentioned purposes by a method for generating ultrasound in a given fluid using at least one capacitive micromachined ultrasonic transducer (CMUT) comprising a membrane and having a predetermined resonance frequency defined by the membrane-fluid pair, characterized in that said at least one transducer is supplied with an excitation signal having a frequency lower than said central frequency.
Of course the frequency f of the ultrasound wave generated is lower than the resonance frequency f0 and more particularly equal to the frequency of the excitation signal.
The invention relates to the transducers the membranes of which have the same architecture such that they all have the same and a single resonance frequency.
According to the invention the CMUT transducer comprises at least one capacitive micro-machined (CMUT) cell, also called “micro-membrane”, that is mechanically isolated and capable of actuation by electrostatic forces.
The inventors of the present invention surprisingly found, on the basis of experimental results obtained in air and in water, that a capacitive micromachined ultrasonic transducer is capable of producing high-amplitude displacements, well below its membrane-fluid interaction frequency. Unlike the piezoelectric systems which have a high mechanical stiffness, it is not necessary for the membrane of the CMUT transducer to resonate in order to produce displacements that are sufficiently large to generate pressure at significant levels.
Thus, the inventors propose an ultrasound generation based on the exploitation of the purely “elastic” behaviour mode of the membranes of the CMUT transducers, which are capable of producing the entire gap height as the amplitude of displacements. Moreover, the inventors also found that in the low-frequency range, each membrane behaves as an “ideal” pressure point source, which means that a single parameter sets the amplitude of the ultrasound pressure emitted: the number of CMUT membranes present in an array. In other words: for an equivalent surface area, this is the coverage rate and the average amplitude of the displacements which define the radiated ultrasound intensity.
Thus, when generating ultrasound from one or more CMUT transducers excited with an excitation signal below the central frequency of the transducer(s), it is not necessary to design acoustic transducers as complex, costly and difficult to use or to implement as if they were used at their resonance frequency. The invention therefore makes it possible to generate ultrasound in a simpler and less costly manner.
More particularly, the inventors found that the frequency of the excitation signal is advantageously at least 20% or even 50% lower than the central frequency of the at least one capacitive micromachined ultrasonic transducer.
Even more particularly the inventors found that the frequency f of the excitation signal f0 can be lower than one half of the resonance frequency, and more particularly 0.2 f0≦f<0.5 f0, and more particularly 0.3 f0≦f<0.5 f0, 0.4 f0≦f<0.5 f0.
The inventors have succeeded in generating ultrasound, with a CMUT transducer having a single resonance frequency f0, at frequencies well below f0, typically below f0/2. The property exploited for this method of generation, called “forced elastic regime”, is the ability of CMUT technologies to produce local displacements of several tens, or even around a hundred nanometres without requiring the membranes to resonate. This procedure then allows the generation of low-frequency ultrasound waves in a wide frequency band, independently of the geometry and topology of the diaphragm.
For example, with respect to a transducer the resonance frequency of which is 4 MHz, it is equally possible with this same device to emit an ultrasound wave at 1 MHz, or at 1.5 MHz without necessarily needing to design a device having several resonance frequencies.
In order to illustrate the pressure levels transmitted in water, the following parameters of the transducer are considered:
At 1 MHz the pressure transmitted at the focal point is 1 MPa and at 1.5 MHz it is 1.5 MPa.
Thus, in a particular embodiment, with a CMUT transducer having a central frequency of 4 MHz in water and 12 MHz in air, the inventors have carried out ultrasound generation at frequencies comprised between:
Advantageously, the at least one capacitive micromachined ultrasonic transducer can be designed so that its central frequency is greater than or equal to 4 MHz and with a gap height comprised between 100 nm and 300 nm, said at least one transducer being excited with an excitation signal having a frequency less than 2 MHz in order to generate ultrasound having frequencies comprised between 200 kHz and 2 MHz.
Moreover, according to the invention, the supply voltage of the at least one capacitive micromachined ultrasonic transducer can be comprised between 1 V and 150 V. These voltages are lower voltages than those used in the state of the art to supply CMUT transducers for generating low-frequency ultrasound, in particular for frequencies less than 2 MHz in water and 1 MHz in air.
The method according to the invention can be used for generating ultrasound having frequencies less than 1 MHz in a gaseous medium with an excitation signal comprised between 200 kHz and 1 MHz.
In this case the supply voltage can be comprised between 50 and 150 V with a gap height Hgap comprised between 100 and 300 nm.
The method according to the invention can also be used for generating ultrasound having frequencies less than 2 MHz in a liquid or aqueous medium with an excitation signal comprised between 200 kHz and 2 MHz.
In this case, the supply voltage can be comprised between
According to a particular implementation, the method according to the invention allows the generation of ultrasound:
According to another aspect of the invention, a method is proposed for the medical imaging of a tissue or an organ of a human or animal subject comprising the following steps:
According to another aspect of the invention, a device is proposed for generating ultrasound in a given fluid using at least one capacitive micromachined ultrasonic transducer (CMUT) comprising a membrane and having a predetermined resonance frequency defined by the membrane-fluid pair, characterized in that said transducer is supplied with an excitation signal having a frequency less than said central frequency, preferably at least 20% or even 50%.
Advantageously, the device according to the invention can comprise at least one capacitive micromachined ultrasonic transducer (CMUT) designed so that it has:
According to the invention, the transducer is supplied with a supply voltage comprised between 1V and 150 V delivered by supply means.
According to a particular example of the device according to the invention, when the device according to the invention is used for generating ultrasound in an aqueous or liquid medium, the capacitive micromachined ultrasonic transducer has:
According to another particular embodiment of the device according to the invention, when the device according to the invention is used for generating ultrasound in an aqueous or liquid medium, the capacitive micromachined ultrasonic transducer has:
According to yet another embodiment, when the device according to the invention is used for generating ultrasound in an aqueous or liquid medium, the capacitive micromachined ultrasonic transducer having:
According to another particular embodiment of the device according to the invention, when the device according to the invention is used for generating ultrasound in a gaseous medium, the capacitive micromachined ultrasonic transducer has:
According to yet another particular embodiment of the device according to the invention, when the device according to the invention is used for generating ultrasound in a gaseous medium, the capacitive micromachined ultrasonic transducer has:
According to yet another particular embodiment of the device according to the invention, when the device according to the invention is used for generating ultrasound in a gaseous medium, the capacitive micromachined ultrasonic transducer has:
According to a particularly advantageous embodiment, the device according to the invention can comprise:
According to another particularly advantageous embodiment, the device according to the invention can comprise:
According to yet another aspect of the invention an ultrasound medical imaging system is proposed comprising:
Other advantages and characteristics will become apparent on examination of the detailed description of an embodiment which is in no way imitative, and the attached diagrams, in which:
A CMUT transducer is formed by several hundred, even a few thousand mechanically isolated “micro-membranes” capable of being actuated by electrostatic forces. These are called CMUTs, for Capacitive Micromachined Ultrasonic Transducers. These membranes are simple capacitive microphones, the operating principle of which is similar to that of the devices used in audio for applications in air. There are however appreciable differences, as the cavities on which the membranes rest are at zero pressure and are isolated from the outside, thus also allowing use in a fluid medium.
The CMUT transducer 100 comprises, non-limitatively, 24 elementary cells 102, or micro-membranes, having a square geometry arranged in 6 rows of 4. The width of the transducer 100 is 0.165 mm.
The CMUT transducer also comprises supply lines 104 of each of the cells.
The elementary cell 102 comprises:
The material used for the membrane is for example silicon nitride but is highly dependent on the technique of fabrication of the device. Other materials such as doped polysilicon (in wafer bonding), a metal or a polymer could be used.
The mobile electrode 208 can be made of aluminium, or any other type of conductor material that is compatible with the use. Similarly, the materials used for producing the mobile electrode 208 are distinguished only by their Young's modulus.
Finally, it should be noted that the metallization on the front face on each membrane can be from 100% of the surface area to a few percent. It is often accepted that 50% metallized surface is a good compromise between stiffness/mass and effectiveness of the electrostatic forces. It is important to specify that, from a mechanical point of view, changing the thickness of the membranes or the Young's modulus of the materials or the metallization rate is defined by an overall parameter called flexural rigidity, which is the single useful mechanical parameter of these microsystems.
The two design parameters of these microsystems are:
The resonance frequency depends:
The collapse voltage depends:
The collapse voltage Vc increases if the flexural rigidity increases and/or if the surface area increases.
The present invention proposes, in the present example, compromises or compromise areas of interest, constituting “technical pathways” of interest for low-frequency work where the membrane of each of the CMUT cells is used in forced regime and not in “resonant” mode. In air, this corresponds to the capacity for generating significant amplitude displacements for frequencies less than 1 MHz while the resonance frequency is considerably greater. In water, the low frequency is situated below 2 MHz. This then corresponds to the ability to generate significant low-frequency displacements while the resonance is situated well above 2 MHz, typically above 4 MHz.
Thus, the invention proposes to produce transducers capable of generating low-frequency ultrasound in air and in water, relying on lower-cost production methods, less complex than the devices of the state of the art, in this case the techniques of surface micro-machining over very great widths or using particularly flexible materials.
In fact, the use of the “resonant” mode as low-frequency source in the state of the art imposes production methods that are much more costly, such as the “wafer bonding” type techniques. These methods offer compromises in terms of width (of the order of a millimetre) and membrane thickness (typically 50 μm) of interest for achieving a resonance frequency which is low, with however very high supply voltages (greater than 500 Volts).
Simulations carried out by the inventors make it possible to show and identify technology pathways allowing the generation of low-frequency ultrasound, i.e. less than 1 MHz in air and 2 MHz in water, using CMUT ultrasound transducers the central frequencies of which are well above the generated ultrasound frequencies.
These simulations make it possible to identify, as a function of the gap height Hgap, the Young's modulus, the membrane width, the membrane thickness and the central frequency of the CMUT transducers, the compromises obtained for a supply voltage less than or equal to 150 V while obtaining a useful pressure level for a radiation surface area equivalent to 100 mm2 at an excitation frequency of 500 kHz that is greater than or equal to:
Thus,
In
In each of these figures, the grey area marked (2) corresponds to the technical compromise values for generating ultrasound having a frequency less than or equal to 2 MHz with transducers having a central frequency greater than or equal to 4 MHz.
With respect to a gap height of Hgap=100 nm, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [10 μm, 100 nm], [10 μm, 400 nm], [30 μm, 600 nm], [30 μm, 1000 nm].
With respect to a gap height of Hgap=200 nm, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [10 μm, 200 nm], [15 μm, 200 nm], [25 μm, 400 nm], [35 μm, 1000 nm].
With respect to a gap height of Hgap=300 nm, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [15 μm, 300 nm], [25 μm, 300 nm], [30 μm, 600 nm], [30 μm, 800 nm].
In
In each of these figures, the grey area marked (2) corresponds to the technical compromise values for generating ultrasound having a frequency less than or equal to 1 MHz with transducers having a central frequency greater than or equal to 4 MHz.
With respect to a gap height of Hgap=100 nm, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [10 μm, 100 nm], [15 μm, 100 nm], [35 μm, 700 nm], [25 μm, 1000 nm].
With respect to a gap height of Hgap=200 nm, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [10 μm, 200 nm], [15 μm, 200 nm], [40 μm, 600 nm], [35 μm, 1000 nm].
With respect to a gap height of Hgap=300 nm, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [15 μm, 300 nm], [25 μm, 300 nm], [45 μm, 600 nm], [40 μm, 700 nm].
In
In each of these figures, the grey area marked (2) corresponds to the technical compromise values for generating ultrasound having a frequency less than or equal to 2 MHz with transducers having a central frequency greater than or equal to 4 MHz.
With respect to a Young's modulus Emb=50 GPa, the area marked (2) is bounded by the coordinate points: [membrane width, membrane thickness]: [10 μm, 200 nm], [15 μm, 200 nm], [30 μm, 1000 nm], [25 μm, 1000 nm].
With respect to a Young's modulus Emb=200 GPa, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [10 μm, 200 nm], [15 μm, 200 nm], [25 μm, 400 nm], [35 μm, 1000 nm].
With respect to a Young's modulus Emb=300 GPa, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [10 μm, 200 nm], [20 μm, 200 nm], [35 μm, 600 nm], [35 μm, 1000 nm].
In
In each of these figures, the grey area marked (2) corresponds to the technical compromise values for generating ultrasound having a frequency less than or equal to 1 MHz with transducers having a central frequency greater than or equal to 4 MHz.
With respect to a Young's modulus Emb=50 GPa, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [10 μm, 200 nm], [15 μm, 200 nm], [40 μm, 1000 nm], [25 μm, 1000 nm].
With respect to a Young's modulus Emb=200 GPa, the area marked (2) is bounded by the coordinate points [membrane width, membrane thickness]: [10 μm, 200 nm], [15 μm, 200 nm], [40 μm, 600 nm], [35 μm, 1000 nm].
With respect to a Young's modulus Emb=300 GPa, the area marked (2) is bounded by the coordinate points: [membrane width, membrane thickness]: [10 μm, 200 nm], [20 μm, 200 nm], [35 μm, 500 nm], [30 μm, 1000 nm].
The pressure field emitted accurately follows the excitation frequency initially applied to the CMUT transducer. The pressure values reached are comparable to the values required for operation of these devices in water.
The invention makes it possible to replace the conventional piezo-electric materials with silicon components on which are etched thousands of capacitive microcomponents capable of vibrating. This CMUT (Capacitive Micromachined Ultrasonic Transducers) technology has a remarkable property for these applications: at a low frequency, the CMUT membranes, more elastic than inertial, are capable of deformation over amplitudes of a few hundred nanometres for excitation voltages of less than 100 Volts.
Advantageously, the invention can be used to produce low-frequency sensors (100 kHz-2 MHz) based on CMUT technologies.
CMUTs are used under operating conditions that are different from those used in medical imaging where the emission is a wide band excitation (greater than 20 MHz), the amplitude of which is typically 150 Volt. The invention makes it possible to use them under quasi-static conditions (low band excitation <2 MHz) so as to impose high-amplitude displacements on the membranes, close to the cavity height. These technologies offer several advantages which make them particularly advantageous for low-frequency applications:
The device 1700 comprises an acoustic transducer 100 as shown in
As specified above, the invention also makes it possible to connect onto the same excitation device two different and complementary functions, namely:
In order to carry out the two functions mentioned above, a part 1806 of the acoustic transducers 1804 is used for generating a low-frequency ultrasonic beam, for example of 1 MHz, used in therapy. These transducers 1804 are therefore used in elastic mode, below their central frequency.
The other part 1808 of the acoustic transducers 1804 is used for generating a high-frequency ultrasonic beam, for example of 4 to 8 MHz, used in ultrasound imaging. The acoustic transducers 1808 are therefore excited at their central frequency or around this central frequency.
As the two functions using CMUT membranes have exactly the same topology, the design and fabrication of the double-function device are simplified as all the cells are exactly identical. Such a device has the advantage of being able to separate the low-frequency emission electronics for therapy from the electronics dedicated to conventional ultrasound imaging.
In fact, for the therapy part, the low-frequency signals make it possible to scan the entire height of the cavity in order to benefit from an adequate ultrasound pressure level. Consequently, in the elastic regime, a polarization voltage equal to the collapse voltage divided by two (Vc/2) and a dynamic amplitude corresponding to 100% of Vc is used. The acoustic transducers 1806 are therefore used in the elastic regime and are excited with an excitation signal having a frequency below their central frequency, supplied by a supply module 1810.
For the imaging part, the acoustic transducers 1808 are excited by an excitation signal of the wide band impulse type, centred on the central frequency of the CMUTs combined with a polarization voltage corresponding to 80% Vc and supplied by a supply module 1812 to the acoustic transducers 1808. This choice promotes reception sensitivity. The amplitudes of excitation used for the imaging transducers 1808 are lower than the amplitudes used for the therapy transducers 1806 as the transducers 1808 are used in “resonant” mode and as the pressure is proportional to the square of the frequency, it is higher on that basis.
To this end, the device 1900 comprises supply means 1902 and a set of identical ultrasound transducers 1904. Each ultrasound transducer 1904 is used both in therapy and in imaging/diagnostics and has the same central frequency.
The supply means 1902 comprise a first supply module 1906 supplying a low-frequency signal for therapy, for example 1 MHz, and a second supply module 1908 supplying a high-frequency signal for imaging/diagnostics, for example comprised between 4 MHz and 8 MHz. The supply means 1902 also comprise a selection module 1910 making it possible to select the source of supply of the transducers 1904 manually or automatically and optionally programmable.
Thus, when the device 1900 is used in therapy, the selection module 1910 chooses the supply module 1906. In the event that the device 1900 is used in imaging/diagnostics the selection module 1910 chooses the supply module 1908.
The advantage of the device 1900 is linked to the orientation of the high- and low-frequency beams, which with the device 1900 are accurately superimposed.
Of course the invention is not limited to the non-limitative embodiments described above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1056040 | Jul 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2011/051705 | 7/18/2011 | WO | 00 | 1/21/2013 |
