EP4670856A1

EP4670856A1 - POROUS AND STRUCTURED SILICON SUPPORT ELEMENT FOR ULTRASONIC DEVICES

Info

Publication number: EP4670856A1
Application number: EP24306008.4A
Authority: EP
Inventors: Arnaud CAPRI; Franck LEVASSORT; Samuel Calle; Gaël GAUTIER; Thomas DEFFORGE
Original assignee: Centre National de la Recherche Scientifique CNRS; Trophy SAS; Universite de Tours; Institut National des Sciences Appliquées Centre Val de Loire; Carestream Dental LLC
Current assignee: Centre National de la Recherche Scientifique CNRS; Trophy SAS; Universite de Tours; Institut National des Sciences Appliquées Centre Val de Loire; Carestream Dental LLC
Priority date: 2024-06-25
Filing date: 2024-06-25
Publication date: 2025-12-31
Also published as: WO2026006225A1

Abstract

The present disclosure provides a backing element (210) for an ultrasound transducer (100), the backing element comprising a front side (205) to face a piezoelectric element (105) and a back side (210) opposite the front side, the backing element comprising, on its back side, at least one set of cavities (215) and a porous layer formed from the cavity surface.

Description

FIELD OF THE DISCLOSURE

The present disclosure relates to the field of ultrasound measurements and/or imaging devices. Particularly, but not exclusively, the disclosure relates to compact ultrasound transducers and to porous and structured silicon backing elements, used in different industries including health care industry, dental industry, and non-destructive inspection industry.

BACKGROUND OF THE DISCLOSURE

The integration of ultrasound techniques into portable and very easy-to-use devices, in particular for medical imaging, offers numerous advantages, in particular because of the non-emission of ionizing radiation, ultrasound imaging is inherently safer than ionizing methods and also allows the repeatability of the examination if needed. Ultrasound imaging can be used as a substitute for, or a complement to, various types of radiography (cone beam computed tomography or CBCT, panoramic x-ray, or intraoral x-ray imaging), magnetic resonance imaging (MRI), or nuclear medicine.
An ultrasound imaging apparatus generally comprises one or several transducers that act as ultrasound emitters and/or ultrasound receivers to receive echoes from the emitted signals. An ultrasound transducer generally comprises a piezoelectric element, an acoustic element having attenuation capabilities, located on the back side of the piezoelectric element, also called a backing element, and a housing. It may also comprise an acoustic lens and matching layers on the front side of the piezoelectric element, to improve acoustic focus and acoustic energy transmission. The piezoelectric element typically comprises piezoelectric material arranged in between two electrodes that are connected to a pulse generator and receiver. The piezoelectric material may convert electrical pulses into an ultrasound signal, when the electrodes are stimulated electrically, and may convert an ultrasound signal into an electrical signal. By processing, in a computing device (e.g. a personal computer) in signal communication with the ultrasound transducer, emitted ultrasound signals and measured echoed ultrasound signals, it is possible to generate images characterizing the object reflecting the emitted ultrasound signal, for example gums and other intraoral soft tissues (and possibly tooth surfaces). The piezoelectric element may be, for example, a poly(vinylidene fluoride-trifluoro ethylene) (P(VDF-TrFE)) piezoelectric film.
The backing element aims at damping piezoelectric element (from a mechanical point of view) and at attenuating ultrasound signals generated from the back side of the piezoelectric element, in order to reduce parasitic echoes that may perturb the measurements performed on the front side of the transducer. To avoid any perturbation of the measurements performed on the front side of the transducer, the back side echoed signals should be received by the piezoelectric element after the front side echoed signals. In addition, the back side echoed signals should be fully absorbed or their amplitude should be reduced as much as possible.
For applications such as intra-oral imaging applications, the size of the ultrasound transducers must be minimized, it being observed that the largest internal element in this type of transducer is the backing element. It is therefore necessary to reduce the size of the backing element while maintaining its attenuation capabilities, which is tricky because the thickness of the material used for the backing element is inversely proportional to the effective attenuation in it.
The present disclosure has been devised to address one or more of the foregoing concerns.

SUMMARY OF THE DISCLOSURE

According to a first aspect of the disclosure there is provided a backing element for an ultrasound transducer, the backing element comprising a front side to face a piezoelectric element and a back side opposite the front side, the backing element comprising, on its back side, at least one set of cavities and a porous layer formed from a surface of the cavities.
Such a backing element may be compact and made it possible to achieve an efficient acoustic attenuation, for example above 25 dB at a frequency of about 30 MHz.
In an embodiment, the backing element is made from a silicon wafer, making it possible to integrate circuits.
Still in an embodiment, the cavities of the set of cavities are pyramid-shaped cavities, that can be made using standard process such as etching. Accordingly, in an embodiment, the backing element further comprises integrated circuits.
Still in an embodiment, the cavities have a square basis and the value of the sum of the length of the side of the base of a cavity and of the distance between two adjacent cavities is of an order between twice the value of a wavelength of an ultrasound signal to be attenuated in the backing element and a twentieth of the value of the wavelength.
Still in an embodiment, the cavities are uniformly distributed over at least a portion of the back side. All the cavities of the set of cavities may have a same shape and a same size.
Still in an embodiment, cavities of a first subset of the set of cavities are regularly distributed over a first portion of the back side and cavities of a second subset of the set of cavities are regularly distributed over a second portion of the back side, the distribution over the first portion being different from the distribution over the second portion. Accordingly, such a backing element may be used in conjunction with multi-element transducer.
Still in an embodiment, an internal surface of a porous layer formed next to a surface of a first cavity and an internal surface of a porous layer formed next to a surface of a second cavity, the first and second cavities being adjacent cavities, meet each other at a distance from the external surface of the backing element, between the first and the second cavities, greater than 5 µm.
Still in an embodiment, the porous layer has a porous layer volume and comprises pores having a pore volume, the pore volume representing between 30 % to 50 % of the porous layer volume.
A second aspect of the disclosure provides an ultrasound transducer comprising a backing element as described above and a piezoelectric element.
Such an ultrasound transducer may be compact and particularly efficient for applications like imaging, in particular dental imaging.
In an embodiment, the piezoelectric element is of a P(VDF-TrFE) type.
Still in an embodiment, the piezoelectric element is a multi-element transducer having at least a first and a second portions each operating at a given frequency, the backing element being the backing element described above with a first and a second subsets of cavities, the first subset facing the first portion and the second subset facing the second portion. Each subset of cavities may have a different shape and/or size.
A third aspect of the disclosure provides a method for manufacturing a backing element as described above, the method comprising the steps of:

forming a protective layer on a side of a silicon wafer, the protective layer comprising openings, each opening corresponding to a basis of each of cavities to be made,
applying a first etching process to make cavities in the silicon wafer at the locations of the openings, and
applying a second etching process, different from the first etching process, to make porous a surface of the cavities.

Such a method makes it possible to make compact backing elements achieving an efficient acoustic attenuation, for example above 25 dB at a frequency of about 30 MHz.
In an embodiment, the first etching process is a potassium hydroxide etching process.
Still in an embodiment, the second etching process is an electrochemical etching process or a metal assisted chemical etching process.
Still in an embodiment, the method further comprises integrating circuits on the silicon wafer, the integrating being carried out before or after the steps of forming a protective layer, applying a first etching process, and applying a second etching process.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure will now be described, by way of example only, and with reference to the following drawings in which:

Figure 1 schematically illustrates an example of an ultrasound transducer, according to some embodiment of the disclosure;
Figure 2 schematically illustrates a cross-sectional view of a backing element according to some embodiment of the disclosure, for example the backing element illustrated in Figure 1;
Figure 3 , comprising Figures 3a to 3d, is a top view of the back side of a backing element, schematically illustrating examples of spatial distributions of pyramid-shaped cavities of a structured pattern made in a silicon wafer, according to some embodiments of the disclosure;
Figure 4 schematically illustrates a perspective view of an example of the back side of a backing element, comprising a structured pattern formed with V-groove shaped cavities, according to some embodiment of the disclosure;
Figure 5 represents an example of attenuation of an ultrasound signal in a backing element, as a function of the aperture ratio of the structured pattern in the backing element, for two different types of backing elements;
Figure 6 illustrates a first example of steps for manufacturing a backing element, according to some embodiments of the disclosure;
Figure 7 illustrates a second example of steps for manufacturing a backing element, according to some embodiments of the disclosure;
Figure 8 schematically illustrates a cross-sectional view of an ultrasound transducer comprising a backing element according to some embodiments of the disclosure; and
Figure 9 schematically illustrates an arrangement for measuring the attenuation of a backing element that is not attached to a piezoelectric element.

DETAILED DESCRIPTION OF THE DISCLOSURE

The following is a detailed description of particular embodiments of the disclosure, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the figures.
In the drawings and text that follow, like components are designated with like reference numerals, and similar descriptions concerning components and an arrangement or interaction of components already described are omitted. Where they are used, the terms "first", "second", and so on, do not necessarily denote any ordinal or priority relation, but may simply be used to more clearly distinguish one element from another, unless specified otherwise.
In the context of the present disclosure, the phrase "in signal communication" indicates that two or more devices and/or components are capable of communicating with each other via signals that travel over some type of signal path. Signal communication may be wired or wireless. The signals may be communication, power, data, or energy signals. The signal paths may include physical, electrical, magnetic, electromagnetic, optical, wired, and/or wireless connections between the first device and/or component and second device and/or component. The signal paths may also include additional devices and/or components between the first device and/or component and second device and/or component.
According to some particular embodiments, the disclosure is directed to structuring materials such as silicon and to make them at least partially porous to make backing elements. Such backing elements provide improved characteristics in terms of compactness and of acoustic attenuation capabilities compared to known backing elements. It is observed that an advantage of using silicon over other materials is that it enables positioning of the electronic devices necessary for the proper operation of the acoustic component in the vicinity of the later (in agreement with the more than Moore concept). By extension, it reduces the chip dimensions, which may be particularly useful for some applications like intra-oral imaging for which the frequencies of the used ultrasound signals are high (i.e., they are comprised within the range of 10 MHz to 100 MHz, preferably between 20 MHz and 100 MHz) to obtain good spatial resolutions and the presence of connectivity (coaxial cable type) may have a detrimental influence (reduction of the bandwidth or sensitivity) on the device. In addition, the use of silicon makes it possible to integrate some control electronic elements close to the transducer or even directly into its internal structure, which may contribute to minimizing the size of the transducer and to improve its efficiency.
Still according to some embodiment, the rear (or back) side of a silicon wafer, for example a silicon wafer having a thickness of 500 µm, is structured with cavities before making porous the external layer of the back side. These two steps provide a porous and structured back surface that makes it possible to reach an efficient level of acoustic attenuation of an ultrasound signal (back and forth).
Figure 1 schematically illustrates an example of an ultrasound transducer, according to some embodiments of the disclosure.
According to the illustrated example, the ultrasound transducer 100 comprises a piezoelectric element 105, a backing element 110, and an electrical wire 115 to exchange signals and/or electrical power with other electronic devices. According to other embodiments, the ultrasound transducer communicates wirelessly with other electronic devices. For the sake of clarity, the housing of the ultrasound transducer is not represented.
Ultrasound transducer 100 may be a mono-element transducer or a multi-element transducer, for example an annular multi-element transducer or a 1D or 2D array transducer.
Piezoelectric element 105 has a front side to be used for emitting ultrasound signals and for receiving ultrasound signals to be measured and a back side. Backing element 110 has a front side facing the piezoelectric element back side, the backing element being made from a crystalline silicon wafer or a portion of a crystalline silicon wafer, referred to as a silicon wafer hereafter, the silicon wafer comprising one structured and porous transversal portion with respect to a transducer longitudinal axis, as described with reference to Figures 2 to 4. The structured portion is a structured pattern comprising cavities, for example inverted micro-pyramids, also denoted pyramid-shaped cavities, or other concave-shaped cavities suitable to deflect ultrasound waves.
Piezoelectric element 105 may be made of any piezoelectric material such as Zinc oxide (ZnO), Aluminium nitride (AIN), or other piezo electric film that thickness is preferably in the range of 10 to 30 µm. The piezoelectric thin film may be deposited directly on the silicon wafer forming the backing element, it being noted that the thin film may require an additional sintering step which must be compatible with the silicon wafer design.
According to particular embodiments, the piezoelectric element is a poly(vinylidene fluoride-trifluoro ethylene) (P(VDF-TrFE)) piezoelectric film that may be deposited or glued on the front side of the backing element. If the assembling is achieved by using an adhesive, the latter should have an acoustic impedance that is similar to the one of the P(VDF-TrFE). The adhesive should preferably not be too thick to prevent any perturbation with the ultrasound signal (which may make difficult the use of a thick adhesive for conforming the piezoelectric element to a desired curved shape). The adhesive thickness may be in the range of few microns (e.g., 2 to 3 µm). Still according to some particular embodiments, the back side of the piezoelectric element is a flat surface so as to make the assembling with the backing element easier and to provide a homogeneous contact (provided that the front side of the backing element is also a flat surface). According to other embodiments, the back side of the piezoelectric element is not flat.
According to some embodiments, backing element 110 is made of a silicon wafer comprising a structured and porous surface on its back face and a flat opposite surface intended to face the piezoelectric element. While the doping type of the silicon wafer may be either p+ or n+, a p+ doped silicon wafer may be advantageously used to optimize the manufacturing process. Indeed, since the manufacturing process of baking element 110 comprises an etching step for structuring the silicon wafer and since the etching speed of a p+ doped silicon wafer is faster than the one of a n+ doped silicon wafer, the use of a p+ doped silicon wafer may be advantageous. Regarding the structure of the silicon wafer, it may be, for example, of the (100), (110), or (111)-oriented type, it is being noted that the orientation impacts the structure angles and shapes of the cavities that are obtained after etching and that the orientation impacts the etching speed (e.g., the etching speed of a silicon wafer of the (111)-oriented type is very slow, about 9 nm per minute, which increases the manufacturing cost).
The thickness of the silicon wafer is preferably less than 1mm, it is for example around 500µm, an aspect of the disclosure being to provide a compact backing element to be used in a compact ultrasound transducer.
According to some embodiments, the back side of the backing element is protected by a resin (that is preferably applied before it is mounted on the piezoelectric element), as described by reference to Figure 8. It may be a standard epoxy resin which has been degassed beforehand. The resin preserves the backing from humidity and moisture.
Figure 2 schematically illustrates a cross-sectional view of a backing element according to some embodiment of the disclosure, for example backing element 110 in Figure 1.
As illustrated, the silicon wafer 200, for example having a structure of the (100)-oriented type, has a front side 205 facing the transducer (not represented) and a back side 210 comprising a set of cavities, generically referenced 215, forming a structured pattern in the silicon wafer. According to the illustrated example, each cavity has a pyramid shape with a square base, the sides of the square base having a length equal to a, and an angle denoted α of 54.74° between the base and each side of the pyramid. The height denoted h is determined by the angle α and the square dimension, as follows: $h = (a / 2) \times \tan (α)$ it being noted that the maximum size of the square base of the pyramid is determined as a function of the silicon wafer thickness denoted t, so that the height of the pyramid is smaller than the thickness of the silicon wafer. According to some embodiments, the silicon wafer comprises a thickness margin denoted m where the wafer is not structured (i.e., t = h + m). As illustrated with reference 220, the portion of the silicon wafer surrounding each cavity, comprising a portion of the margin thickness, is made porous.
For the sake of illustration, the length a of the square base side and the height h of the pyramid-shaped cavity may be chosen in the order of 350 and of 230 µm, respectively for a wafer thickness of 500 µm. The thickness of the porous layer may be in the range of 50-100µm. For example, it may be equal to 65 µm.
The distance between two adjacent pyramid-shaped cavities is denoted d and the distance d and the length a of the square base side are linked to the frequency of the ultrasound signal transmitted in the silicon wafer, for example so that the distance a + d is in the order of twice to twentieth the wavelength λ of the ultrasound signal transmitted in the silicon wafer and satisfies the following relation: $λ / 20 < a + d < 2 λ$ in order to maximize the acoustic wave interaction with the structured pattern and thus, to maximize ultrasound signal attenuation.
It is noted that there exist many possible spatial distributions of the pyramid-shaped cavities, it being observed that because of the crystalline network of the silicon wafer and of the standard manufacturing process for making the pyramid-shaped cavities, all the pyramid-shaped cavities are oriented in the same direction.
According to some embodiments, the pyramid-shaped cavities are regularly spaced and have the same size to ensure, during the porosity rendering process step, an easier control of the structure leading to a peak effect, as described hereafter.
Figure 3 , comprising Figures 3a to 3d, is a top view of the back side of a backing element, schematically illustrating examples of spatial distributions of pyramid-shaped cavities in the silicon wafer structured pattern, according to some embodiments of the disclosure. According to these examples, the silicon wafer has a structure of the (100)-oriented type.
According to the example illustrated in Figure 3a, all the pyramid-shaped cavities have the same size, with a square base of size length a. They are regularly spaced, with a distance d between two contiguous cavities, and distributed according to a row and column scheme.
The spatial distribution illustrated in Figure 3b is close to the one of Figure 3a: all the pyramid-shaped cavities have the same size, with a square base, and they are regularly spaced, but according to a staggered distribution.
The spatial distribution illustrated in Figure 3c is close to the one of Figure 3b: all the pyramid-shaped cavities have the same size, they are regularly spaced according to a staggered distribution (with a distance d between two contiguous cavities along the horizontal axis and a distance d' between two contiguous cavities along the vertical axis), but with a rectangular base of size lengths a and a', along the horizontal axis and the vertical axis, respectively.
According to other embodiments, the size of the pyramid-shaped cavities and the aperture ratio (i.e., the ratio of the base surfaces of the pyramid-shaped cavities to the structured pattern surface of the silicon wafer) vary, for example as a function of the location, to be dimensioned as a function of a facing multi-element annular piezoelectric transducer, each of the annular portions operating at a different frequency. For the sake of illustration, a backing element may be adapted to each ring of a dual frequency annular transducer, one ring emitting ultrasound signals having a 10 MHz frequency and another ring receiving ultrasound signals having a 20 MHz frequency, by setting the pyramid-shaped cavity size and the aperture ratio appropriately to optimize the acoustic attenuation i.e. different sets of cavities are used, where each set of cavities is adapted to and faces its corresponding frequency ring.
Figure 3d illustrates an example of such a spatial distribution of pyramid-shaped cavities of various sizes, with a central disc and two different rings. It is observed that the annular portions may not be adjacent (i.e., there may be some space between two adjacent rings). It is also observed that the same principle could be applied to a linear or an array multi-element piezoelectric transducer.
According to some particular embodiments, one or more first sets of cavities are made on a first side of the backing element and one or more second sets of cavities are made on a second side of the backing element, opposite the first side, the cavities of the one or more first sets being not aligned with the cavities of the one or more second sets according to a longitudinal axis of the backing element. A first and a second piezoelectric elements or multi-elements may be attached to the first and second sides of the backing element, respectively, such that the first piezoelectric elements or multi-elements are aligned with the one or more second sets of cavities and the second piezoelectric elements or multi-elements are aligned with the one or more first sets of cavities.
As described above, when considering a structured pattern having pyramid-shaped cavities with a square base and a regular space arrangement, the pyramid-shaped cavity arrangement on the back side surface may be characterized by an aperture ratio, denoted OR, for example using the following relation: $OR = a^{2} / D^{2}$ wherein D = a + d and wherein d is the space between two pyramid-shaped cavity bases. Again, the length a of the square base should be chosen as a function of the frequency of the ultrasound signal transmitted through the backing element, and the lengths a and a + d are in the order of twice to twentieth the wavelength λ of the ultrasound signal transmitted through the backing element and should satisfy the following relationship: $λ / 20 < a + d < 2 λ$ that makes it possible to maximize the interaction between the acoustic wave and the structured pattern.
The average value of the aperture ratio corresponds to the sum of the structured pattern areas divided by the back side surface area (it being noted that there may be small variations in the structured patterns).
According to some embodiments, the textured pattern covers the whole back side of the backing element.
While the examples provided in Figures 2 and 3 are directed to structured pattern formed with pyramid-shaped cavities, other shapes of cavities may be used, for example conical cavities, bowl-shaped cavities, ogival-shaped cavities, or V-groove shaped cavities as illustrated in Figure 4 . It is observed that when carrying out an isotropic edging step, flat hemispherical pattern can be obtained.
As described above, the external layer of the backing element back side, where the cavity based structured pattern is designed, may be made porous to decrease the acoustic impedance of the backing element. The pore size of the porous layer is very small compared to the wavelength. For the sake of illustration, the pore size may be less than a hundredth of the wavelength. Accordingly, from an acoustic wave perspective, the porous layer is a homogeneous material having an acoustic impedance lower than the one of the dense silicon. Still for the sake of illustration, the diameter of the pores may be chosen between 2 nm and 100 nm, preferably between 5 nm and 50 nm.
Regarding the thickness of the porous layer, the inventors have observed that its optimum depends on the space between two adjacent structures. The internal surface (e.g., 227-1 in Figure 2) of the porous layer (e.g., 225-1 in Figure 2) formed next to a first cavity surface (e.g., 222-1 in Figure 2) and the internal surface (e.g., 227-2 in Figure 2) of the porous layer formed next to the surface of the neighboring cavity should meet each other (e.g., 230 in Figure 2) close to the external surface between the cavities (e.g., 235 in Figure 2) to create a "peak effect", for example at a distance from the external surface (referenced p in Figure 2) greater than 5 µm. The point where the porous layer formed next to the first cavity surface meets the porous layer formed next to the surface of the neighboring cavity depends on the process used to form the porous layer. For pyramid-shaped cavities having a base size a of about 350 µm that are separated by about 100 µm, the distance p may be comprised between 5 and 15 µm, preferably between 8 and 12 µm. It may be approximately equal to 10 µm.
It is noted that "next to" is to be understood as comprising the case according to which the porous layer is directly formed from the surface of the cavity.
According to some embodiments, the thickness of the porous layer is greater than (D - a) / 2 (i.e., d / 2) such that the external surface between two cavities is entirely porous. If the porous layer is too thick, the peak effect is reduced due to the flattening effect resulting from the variation between the base and the apex of the cavity (e.g., the pyramid-shape cavity). In addition, the backing element is more fragile if the porous layer is too thick. On the contrary, if the thickness of the porous layer is too thin, it is less efficient.
It is noted here that the acoustic impedance of the dense silicon is about 19.10⁶ kg.m^-2.s^-1 or Rayl (Rayleigh), 19 MRayl, and the acoustic impedance of the porous silicon is about 10 to 12 MRayl if the porous silicon comprises 30 % to 50 % of empty space (corresponding to the volume of the pores).
It is observed here that a porosity gradient may be achieved along the thickness layer.
It is also observed that like the structured pattern, the porosity may be determined as a function of the elements facing the backing element and operating at different frequencies.
Figure 5 represents an example of attenuation of an ultrasound signal in a backing element, as a function of the aperture ratio of the structured pattern in the backing element, for two different types of backing elements.
For the sake of illustration, the first type of backing element used for the measurements is used in conjunction with a piezoelectric element operating at 28 MHz and is defined as follows: the structured pattern is formed with pyramid-shaped cavities having a square base of side length a equals to 350 µm and the distance d between the cavities varies, defining an aperture ratio of the structured pattern varying from 0 to 85%. The second type of backing element is similar to the first one but comprises a porous layer on its back side. The results obtained with the first type of backing element are represented with a solid line (the bottom curve on the graph) and the results of the second type of backing element are represented with a bold line (the top curve on the graph). For each configuration, three measures have been obtained and the average has been used to plot the curve. For example, the attenuation is equal to 16.3 dB for the first backing element and to 30.6 dB for the second backing element when the distance d between the cavities is equal to 150 µm, defining an aperture ratio of the structured pattern equals to 49%.
As illustrated, a plateau with attenuation values above 25 dB is obtained for a backing element comprising a structured pattern likes the one illustrated in Figure 3b, having an aperture ratio comprised between 25% and 70%.
Figure 6 illustrates a first example of steps for manufacturing a backing element, according to some embodiments of the disclosure.
For the sake of illustration, the backing element is based on a p-type silicon wafer of the (100)-oriented type, having a resistivity of 0.01 to 0.02 Ω.cm.
The left part of Figure 6 illustrates some of the steps of the method, the central part of Figure 6 illustrates a perspective view of a portion of the silicon wafer, illustrating the result of the corresponding step on a portion of the silicon wafer corresponding to a single cavity of the structured pattern, and the right part of Figure 6 illustrates a cross-section view of a portion of the silicon wafer, illustrating the result of the corresponding step on a portion of the silicon wafer corresponding to several cavities of the structured pattern.
As illustrated, after having obtained a silicon wafer 650, a first step (step 600) is directed to removing native silicon dioxide on both sides of the silicon wafer. This can be done, for example, by using dilute hydrofluoric acid (HF), for example by applying a 10% solution (by mass) during 5 minutes.
Next, a fluoropolymer (FP) thin film 655 is deposited, for example by plasma, on both side of the silicon wafer (step 605), to protect it. The deposit of the FP thin film may be carried out by using a standard process, for example as described in the document entitled "Optimized plasma-polymerized fluoropolymer mask for local porous silicon formation", Lu, B., Defforge, T., Fodor, B., Morillon, B., Alquier, D., & Gautier, G., Journal of Applied Physics, 119(21), 2016. According to other embodiments, the fluoropolymer (FP) thin film is deposited only on one face, the other face being protected by other means during the manufacturing steps directed to making the cavities and rendering the external layer porous.
Next, a mask 660 is deposited on the back side of the silicon wafer, making it possible to remove portions of the fluoropolymer layer where cavities of the backing element structured pattern are to be designed (step 610). For the sake of illustration, the photoresist material that is used to make the mask may be the one known under the AZ 5214 E reference. A hotplate annealing may be carried out, for example at a temperature of 110°C, before exposing the photoresist material, for example at 210mJ/cm², applying a temperature inversion at 120°C during 2 minutes, and flooding the surface, for example at 210mJ/cm².
Next, after having removed the portions of the fluoropolymer layer where cavities of the backing element structured pattern are to be designed, the remaining photoresist material is moved (step 615), for example by applying acetone during 5 minutes.
Next, a first etching step is carried out (step 620), for example using potassium hydroxide (KOH), for example by immerging the silicon wafer in a 20% aqueous potassium hydroxide solution (by mass), maintained at 80°C, for example during 3.5 hours.
Alkaline solutions, such as concentrated KOH, make it possible to produce an anisotropic etching of single-crystal silicon, it being noted that the (111)-oriented silicon is dissolved a hundred times more slowly than the other crystallographic planes the (100) or (110)-oriented silicon, for example as described in the document entitled "Anisotropic etching of crystalline silicon in alkaline solutions: I. Orientation dependence and behavior of passivation layers", Seidel, H., Csepregi, L., Heuberger, A., & Baumgärtel, H., 1990, Journal of the electrochemical society, 137(11), 3612 and in the document entitled "Orientation-resolved chemical kinetics: using microfabrication to unravel the complicated chemistry of KOH/Si etching", Wind, R. A., Jones, H., Little, M. J., & Hines, M. A., 2002, The Journal of Physical Chemistry B, 106(7), 1557-1569. This leads to the formation of square-based pyramid-shaped cavities where the portions of the fluoropolymer layer have been removed, as shown in Figure 6. The duration of KOH etching is adapted to the size of the apertures and, consequently, the size of the pyramid-shaped cavities. After KOH etching, the samples are thoroughly rinsed with deionized (DI) water.
Next, a second etching step is carried out (step 625). It may comprise, for example, an electrochemical etching (KOH etching). Such a step makes it possible to form a porous layer on the back side of the backing element. According to the example illustrated in Figure 6, the silicon wafer comprising the pyramid-shaped cavities is immersed in a HF-based solution to form a porous silicon layer on the cavities' sidewalls. Still for the sake of illustration, porous silicon-covered pyramid-shaped cavities may be obtained in a HF 30 % by mass - acetic acid 25 % by mass electrolyte using a current density of 30 mA/cm² that may be applied, for example, using a generator such as the SP-150 generator proposed by the Biologic company (Biologic is a trademark). The electrochemical etching duration is also adapted to the size of the pyramid-shaped cavities. After electrochemical etching, the silicon wafer is thoroughly rinsed with DI water.
It is observed here that according to the example of Figure 6, the front side of the backing element is protected with a fluoropolymer layer that is removed once the porous layer is formed (not represented). As disclosed above, other protections may be used.
Figure 7 illustrates a second example of steps for manufacturing a backing element, according to some embodiments of the disclosure.
Like Figure 6, the left part of Figure 7 illustrates some of the steps of the method, the central part of Figure 7 illustrates a perspective view of a portion of the silicon wafer, illustrating the result of the corresponding step on a portion of the silicon wafer corresponding to a single cavity of the structured pattern, and the right part of Figure 7 illustrates a cross-section view of a portion of the silicon wafer, illustrating the result of the corresponding step on a portion of the silicon wafer corresponding to several cavities of the structured pattern.
Steps 700 to 720 are similar to steps 600 to 620, respectively.
Contrary to the second etching step described in reference to Figure 6, the second etching step of the example illustrated in Figure 7 is a metal-assisted chemical etching (MaCE), for example using Ag nanoparticles as a catalyst. In such an example, the silicon wafer comprising the pyramid-based cavities is immersed in an aqueous HF - AgNOa solution (e.g., 4.8 M of HF and 0.02 M of AgNO₃), for 5 to 17 hours. After the second etching step, the silicon wafer is also rinsed with DI water.
The obtained silicon wafer may then be mounted on a piezo electric element as illustrated in Figure 8.
Figure 8 schematically illustrates a cross-sectional view of an ultrasound transducer comprising a backing element according to some embodiments of the disclosure.
The illustrated ultrasound transducer comprises a piezoelectric element 800 on which is glued a backing element 805, with glue layer 810. On its back side, backing element 805 is protected by resin layer 815.
For the sake of illustration, piezo element 800 may be of the P(VDF-TrFE) type. It may have a thickness of 18 µm and the thickness of the glue layer may be in the range of 2 to 3 µm. The backing element may be made of p-type silicon, comprising a structured pattern made of pyramid-shaped cavities, and comprising a porous portion. Still for the sake of illustration, the cavities may be staggered and have a depth equal to approximatively 350 µm. The porosity layer may have a thickness equal to 70 µm. The size of the pores may be in the range of 2 to 100 nm, with an aperture ratio of 60,5 %. With an operating frequency of 26 MHz and a bandwidth (-6 dB) of 22,5 MHz, the measured attenuation is equal to 24,4 dB between the first and the second echoes received by the piezoelectric element.
Such an ultrasound transducer is particularly adapted for being used within an intraoral tool, for example an intraoral image acquisition device.
For the sake of efficiency, the attenuation of the backing element may be measured without gluing the backing element on a piezoelectric element, for example using the arrangement illustrated in Figure 9.
Figure 9 schematically illustrates an arrangement for measuring the attenuation of a backing element that is not attached to a piezoelectric element.
As illustrated, the arrangement 900 comprises an ultrasound transducer 905, for example the plane ultrasound transducer proposed by the Olympus company under the reference PI75-1-R0.50 (Olympus is a trademark), having an aperture diameter of 3.1 mm and a center frequency set to 30 MHz, and a backing element 910 that is mounted in a housing 915, for example a polymer housing. For the sake of illustration, the backing element may be mounted in a recess of the housing and secured with a locking element 920. In order to make the upper part 930 of the backing element watertight, an O-ring seal 925 may be inserted between the backing element and the locking element. To carry out the measurements, the volume 932 above the upper part 930 may be fill with pure water, in which the ultrasound transducer is immersed, as illustrated. Accordingly, the front side of the backing element is in contact with the pure water and the back side remains in the air 935.
The transducer is precisely oriented at the normal incidence of the backing element, for example using a positioning system having six-axis degree-of-freedom, such as the one provided by the Staufen company under the reference OWIS dc 500 (Staufen is a trademark). Still for the sake of illustration, the ultrasound transducer may be positioned at a few millimeters of the top face of the backing element, for example at 3 mm.
Once positioned, the transducer may be excited using a broadband pulser, for example using the pulser provided by the Avtech company under the reference AVG-3B-C-PN (Avtech is a trademark). Then, waveforms may be acquired using an oscilloscope, for example the oscilloscope provided by the Tektronix company under the reference DPO 4054 (Tektronix is a trademark). The first received echo is the one reflected by the front side of the backing element interface, indicated with reference 940 in Figure 9, and the second echo is the one reflected by the back side of the backing element interface, indicated with reference 945 in Figure 9. The amplitude difference between these two echoes makes it possible to determine the acoustic attenuation of the backing element.
Although the present disclosure has been described hereinabove with reference to specific embodiments, the present disclosure is not limited to the specific embodiments, and modifications will be apparent to a skilled person in the art which lie within the scope of the present disclosure.
Many modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the scope of the disclosure, that being determined solely by the appended claims. In particular, the different features from different embodiments may be interchanged, where appropriate.
In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

Claims

A backing element (210) for an ultrasound transducer (100), the backing element comprising a front side (205) to face a piezoelectric element (105) and a back side (210) opposite the front side, the backing element comprising, on its back side, at least one set of cavities (215) and a porous layer formed from a surface of the cavities.
The backing element of claim 1, wherein the backing element is made from a silicon wafer.
The backing element of claim 1 or claim 2, wherein the cavities of the set of cavities are pyramid-shaped cavities.
The backing element of claim 3, wherein the cavities have a square basis and wherein the value of the sum of the length of the side of the base of a cavity and of the distance between two adjacent cavities is of an order between twice the value of a wavelength of an ultrasound signal to by attenuated in the backing element and a twentieth of the value of the wavelength.
The backing element of any one of claims 1 to 4, wherein the cavities are uniformly distributed over at least a portion of the back side.
The backing element of any one of claims 1 to 5, wherein all the cavities of the set of cavities have a same shape and a same size.
The backing element of any one of claims 1 to 5, wherein cavities of a first subset of the set of cavities are regularly distributed over a first portion of the back side and cavities of a second subset of the set of cavities are regularly distributed over a second portion of the back side, the distribution over the first portion being different from the distribution over the second portion.
The backing element of any one of claims 1 to 7, wherein an internal surface of a porous layer formed next to a surface of a first cavity and an internal surface of a porous layer formed next to a surface of a second cavity, the first and second cavities being adjacent cavities, meet each other at a distance from the external surface of the backing element, between the first and the second cavities, greater than 5 µm .
The backing element of any one of claims 1 to 8, wherein the porous layer has a porous layer volume and comprises pores having a pore volume, the pore volume representing between 30 % to 50 % of the porous layer volume.
The backing element of claim 2 or of any one of claims 3 to 9, depending directly or indirectly on claim 2, wherein the backing element further comprises integrated circuits.
An ultrasound transducer comprising a backing element according to any one of claims 1 to 10 and a piezoelectric element.
The ultrasound transducer of claim 11, wherein the piezoelectric element is of a P(VDF-TrFE) type.
The ultrasound transducer of claim 11 or claim 12, wherein the piezoelectric element is a multi-element transducer having at least a first and a second portions each operating at a given frequency, the backing element being the backing element of claim 7 or any one of claims 8 to 10 depending directly or indirectly on claim 7, the first subset facing the first portion and the second subset facing the second portion.
A method for manufacturing a backing element of any of claims 1 to 10, the method comprising the steps of:
forming (600, 605, 610, 615, 700, 705, 710, 715) a protective layer on a side of a silicon wafer, the protective layer comprising openings corresponding to a basis of each of cavities to be made,
applying (620, 720) a first etching process to make cavities in the silicon wafer at the locations of the openings, and

applying (625, 725) a second etching process, different from the first etching process, to make porous a surface of the cavities.
The method of claim 14, wherein the first etching process is a potassium hydroxide etching process.
The method of claim 14 or claim 15, wherein the second etching process is an electrochemical etching process or a metal assisted chemical etching process.
The method of any one of claims 14 to 16, further comprising integrating circuits on the silicon wafer, the integrating being carried out before or after the steps of forming a protective layer, applying a first etching process, and applying a second etching process.