Patent Description:
Ultrasound imaging, in particular 3D ultrasound imaging, is a powerful means of imaging and is particularly useful in applications such as sonar, gesture recognition, finger print sensors, medical imaging and non-destructive testing (NDT). One way of acquiring a 3D image is to use a <NUM>-dimensional array of ultrasonic transducers, which eliminates the need for a mechanical motor or manual movement of a one-dimensional array of transducers.

One of the main difficulties in fabricating a 2D array of transducers is the wiring to access the individual transducers. The individual transducers in a 2D array can be accessed by bonding or integrating a full diced piezoelectric transducer or capacitive transducer matrix directly with front-end electronics, for example on top of a CMOS chip (i.e. CMOS integration). In this way, each piezoelectric or capacitive element has access to its own front-end and can be controlled individually. This requires the size of the CMOS chip to be the same as that of the transducer array device matrix, which makes the entire device extremely expensive, which is a barrier to making very large 2D arrays. Moreover, this technology cannot be used in flexible ultrasound transducers, such as those fabricated on flexible substrates, like a silicone elastomer.

For high resolution imaging applications, micro-machined ultrasound transducers (MUT) can be used. Micro-machined ultrasonic transducer (MUT) technology is based on semiconductor materials and lithography techniques, and are becoming the main alternative to bulk PZT-based transducers in different applications. MUTs have a relatively simple fabrication process and can be miniaturized for a better image resolution.

In general, MUTs operate in two different mechanisms: capacitive force (referred to as "capacitive MUT" or "cMUT") or piezoelectric (also referred to as "piezoelectric MUT" or "pMUT") sensing-actuation. Hence, cMUT and pMUT are the commonly used types of MUTs.

CMUT is based on two parallel membranes with a very small vacuum gap (around <NUM>) in between the membranes. The upper membrane is attracted to the bottom membrane by an electrostatic force which is induced by an applied voltage across the membranes. CMUTs have a number of limitations, among which the requirement of a high DC bias voltage, which is a particular disadvantage in applications wherein the transducer is part of a handheld device; failure or drift in performance, due to the accumulated charge during the required high voltage operation; and difficulty with generating a sufficiently high acoustic pressure.

A pMUT includes a thin membrane, which vibrates due to an applied force generated by a thin piezoelectric layer. The piezoelectric layer is deposited on top of the membrane and is driven by an electric signal. PMUTs suffer from a low bandwidth, and low output pressure due to a low electromechanical coupling factor, which are two important factors for ultrasound imaging in both medical and NDT applications, among others. The low bandwidth results in difficulty achieving the short pulse response required for high spatial resolution. The low output pressure results in a low amplitude of the emitted ultrasound, which can cause a low signal-to-noise ratio. There is therefore still a need for ultrasonic transducers which can be easily miniaturised and have a sufficiently high output pressure and bandwidth.

MUTs have, in comparison with conventional transducers, a simpler and cheaper fabrication process. Moreover, MUTs can be miniaturized for a better image resolution. For example, for small size matrices, e.g. a 16x16 matrix arrangement of MUTs, individual wire-bonding is still possible in order to have access to all transducers in the matrix individually. Wire-bonding is a cheap process, which allows the CMOS chip to be designed and fabricated independent form the dimension of the matrix. However, for larger matrices the MUT technology, for both cMUT and pMUT, has the same problem as the conventional technology in making 2D arrays. Due to the density of the elements in the transducer matrix, addressing by wire-bonded contacts becomes very difficult.

"<NPL>) describes a row-column addressing arrangement for reducing the number of contacts needed in a 2D array of cMUTs. This reduces the number of electrical contacts needed for an n × n array of transducers from n<NUM> to 2n.

However, row-column addressing in arrays of MUTs suffers from cross-coupling: when a particular transducer is addressed by applying a signal to a row-column pair, the signal can be capacitively coupled into adjacent rows and columns, therefore causing other transducers to be activated when this is not desired. Thus, row-column addressing alone is an inefficient solution for 2D arrays of MUT arrays.

In addition to the need for ultrasonic transducers which can be easily miniaturised and have a sufficiently high output pressure and bandwidth, there is thus still a need in the art for ultrasonic transducers and associated arrays which address at least some of the issues outlined above. Relevant prior art is disclosed in <CIT>, <CIT>, <CIT> and <CIT>.

In one aspect, the invention relates to an ultrasonic transducer multilayer structure according to claim <NUM>.

In another aspect, the invention relates to a method of manufacturing an ultrasonic multilayer structure according to claim <NUM>.

The invention also relates to a use of an ultrasonic transducer multilayer structure according to claim <NUM>. Advantageous modifications are disclosed in the dependent claims.

Referring to <FIG>, an ultrasonic transducer array device <NUM>, hereinafter also referred to as transducer array device <NUM>, according to embodiments of the present invention is shown. The configuration as depicted in <FIG> corresponds with an array configuration of m rows and n columns located in a supporting surface, according to embodiments of the present invention. This supporting surface may be substantially flat or convex. Other array configuration may also be possible, like for example, a linear array, a symmetrical matrix formed array, an asymmetrical matrix formed array, a curved array, or an annular array, or a combination thereof, without being limited thereto.

The transducer array device <NUM> comprises a plurality of ultrasonic transducers <NUM> (indicated in <FIG>, <FIG>, <FIG> and <FIG> by a capacitor symbol) arranged in an array configuration, for example an array configuration of m rows and n columns. Each individual transducer <NUM>mn within this configuration can be uniquely identified by its row m and column n location.

The transducer array device <NUM> comprises at least one first electrode <NUM> for connecting ultrasonic transducers <NUM> along a first direction, and at least one second electrode <NUM> for connecting ultrasonic transducers <NUM> along a second direction.

For example, in an array configuration of m rows and n columns, the first electrode <NUM> may correspond with a row electrodes <NUM>m and the second electrode <NUM> may correspond with a column electrode <NUM>n. Each row electrode <NUM>m electrically connects ultrasonic transducers <NUM>mn along row m. Each column electrode <NUM>n electrically connects ultrasonic transducers <NUM>mn along column n.

For example, for the arrangement shown in <FIG>, the row electrodes <NUM>m connect, when in use, the anodes of the ultrasonic transducer <NUM>mn and the column electrodes <NUM>n connect the cathodes of the ultrasonic transducers <NUM>mn, wherein the high potential side of the ultrasonic transducers <NUM>mn is connected with the row electrodes <NUM>m and the low potential side of the ultrasonic transducers <NUM>mn is connected with the column electrodes <NUM>n.

Each ultrasonic transducer <NUM>mn in <FIG> is connected in series with a respective diode <NUM>mn, wherein the respective first or row electrode <NUM>m is connected to a respective second or column electrode <NUM>n via the respective diode <NUM>mn in series with the respective ultrasonic transducer <NUM>mn, wherein, when in use, a direction from an anode to a cathode of the respective diode <NUM>mn coincides with a direction from a high potential side to a low potential side of the respective ultrasonic transducer <NUM>mn, and wherein the low potential side of the respective ultrasonic transducer <NUM>mn is connected with the anode of the respective diode <NUM>mn. The first or row electrodes <NUM>m may instead connect cathodes of the ultrasonic transducers <NUM>mn and the second or column electrodes <NUM>n may connect anodes of the ultrasonic transducers <NUM>mn, provided that the diode directions as shown in <FIG> are reversed.

By connecting the ultrasonic transducers <NUM>mn in such a m-n row-column manner, the number of electrical connections (m + n) required in order to achieve individual addressing of each ultrasonic transducer <NUM>mn is greatly reduced as compared to an arrangement wherein each ultrasonic transducer <NUM>mn requires a unique, individual (e.g. wire-bonded) electrical contact (m × n connections required).

According to embodiments of the present invention, each first or row electrode <NUM>m is connected to each second or column electrode <NUM>n via a respective diode <NUM>mn. Thus, for example, in an ultrasonic transducer array <NUM> comprising four first or row electrodes <NUM><NUM>, <NUM><NUM>, <NUM><NUM>, <NUM><NUM>, and four second or column electrodes <NUM><NUM>, <NUM><NUM>, <NUM><NUM>, <NUM><NUM>, as shown in <FIG>, the row electrode <NUM><NUM> is connected to the column electrode <NUM><NUM> via diode <NUM><NUM>. The same row electrode <NUM><NUM> is connected to the column electrode <NUM><NUM> via diode <NUM><NUM>. The same row electrode <NUM><NUM> is connected to the column electrode <NUM><NUM> via diode <NUM><NUM>, and is connected to the column electrode <NUM><NUM> via diode <NUM><NUM>. However, it will be understood that the present invention is not limited to four (<NUM>) first or row electrodes <NUM>m and four (<NUM>) second or column electrodes <NUM>n, and other numbers of row electrodes and column electrodes are possible.

Each first or row electrode <NUM>m has a voltage input Vin, m at a first end of the row electrode <NUM>m. Each second or column electrode <NUM>n has a voltage output Vg, n at a second end of the column electrode <NUM>n. The diodes <NUM>mn are arranged so as to allow current to flow from the first end Vin, m of a row electrode <NUM>m to the second end Vg, n of a column electrode <NUM>n through the diode <NUM>mn and through the corresponding transducer <NUM>mn, provided that an appropriate voltage difference is established between the first end Vin, m and the second end Vg, n. In other words, when an ultrasonic transducer array device according to embodiments of the present invention is in use, a direction from an anode to a cathode of the diode <NUM>mn coincides with a direction from a high potential side to a low potential side of the respective ultrasonic transducer <NUM>mn, and wherein the low potential side of the respective ultrasonic transducer <NUM>mn is connected with the anode of the diode <NUM>mn.

For example, when a voltage is applied to the first end Vin, <NUM> of the first or row electrode <NUM><NUM> such that a positive voltage difference exists between the first end Vin, <NUM> of the first or row electrode <NUM><NUM> and the second end Vg, <NUM> of the second or column electrode <NUM><NUM>, current will flow between the first end Vin, <NUM> and the second end Vg, <NUM> via the diode <NUM><NUM> and via the transducer <NUM><NUM>.

Hence, it is an advantage of an ultrasonic transducer array device <NUM> according to embodiments of the present invention that by integrating diodes <NUM>mn according to embodiments of the present invention, activation of untargeted ultrasonic transducers <NUM>mn by capacitive coupling can be avoided.

For example, referring to <FIG>, a transducer array device <NUM> according to embodiments of the present invention is shown. This transducer array device <NUM> does not include the diodes <NUM>mn like presented in <FIG>. In order to activate ultrasonic transducer <NUM><NUM> a voltage is applied to the first end Vin,<NUM> of the first or row electrode <NUM><NUM> such that a voltage difference exists between the first end Vin,<NUM> of the first or row electrode <NUM><NUM> and the second end Vg,<NUM> of the second or column electrode <NUM><NUM>. This causes current to flow along a first path P1 along the first or row electrode <NUM><NUM> through the transducer <NUM><NUM> and then through the second or column electrode <NUM><NUM> to its second end Vg,<NUM>. However, due to capacitive coupling, current is also induced to flow along a second path P2 which activates, in order, ultrasonic transducers <NUM><NUM>, <NUM><NUM>, and <NUM><NUM>. Additionally, current is induced to flow along a third path P3 which activates, in order, ultrasonic transducers <NUM><NUM>, <NUM><NUM>, and <NUM><NUM>.

Referring to <FIG>, in a transducer array device <NUM> according to embodiments of the present invention, the second path P2 is terminated by diode <NUM><NUM> and so there is no closed circuit and current cannot flow through transducers <NUM><NUM> leaving it inactivated. The third path P3 is terminated by diode <NUM><NUM> and so there is no closed circuit and current cannot flow through ultrasonic transducers <NUM><NUM>, and <NUM><NUM>, leaving them inactivated. The first path P1 is unchanged. Thus, only the desired transducer <NUM><NUM> is activated.

Although the preceding embodiments according to the present invention are described with or without reference to an ultrasonic transducer array device <NUM> wherein each ultrasonic transducer <NUM>mn has or has not a corresponding diode <NUM>mn, embodiments of the present invention also encompass an array configuration wherein not every ultrasonic transducer <NUM>mn has a corresponding diode <NUM>mn. The advantages as described herein are still present in an ultrasonic transducer array device <NUM> wherein at least one diode <NUM>mn in series with a respective ultrasonic transducer <NUM>mn is provided as, for example, diode <NUM><NUM> in <FIG> can still prevent activation of three (<NUM>) ultrasonic transducers while ultrasonic transducer <NUM><NUM> is targeted. Thus even an ultrasonic transducer array device <NUM> of multiple ultrasonic transducers <NUM>mn comprising only one diode <NUM>mn in series with one of the respective ultrasonic transducers <NUM>mn can still provide an improvement in the selectivity of addressing of ultrasonic transducers <NUM>mn in the array configuration.

According to embodiments of the present invention, the plurality of ultrasonic transducers <NUM>mn (hereinafter also referred to with reference number <NUM>) comprises at least one micro-machined ultrasonic transducer (also referred to as "MUT"), wherein the at least one MUT is a piezoelectric micro-machined ultrasonic transducer (also referred to as "pMUT") or a capacitive micro-machined ultrasonic transducer (also referred to as "cMUT").

According to embodiments of the present invention, the ultrasonic transducer <NUM> and respective diode <NUM>mn (hereinafter also referred to with reference number <NUM>) in series is provided as a multilayer structure. For example, a diode <NUM> may be provided at least in part by a semiconductor layer stack and an electrically conductive layer disposed over the semiconductor layer stack so as to form a Schottky diode or a p-n diode, depending on the dopant conductivity type and concentration of the semiconductor layer in direct contact with the electrically conductive layer.

The electrically conductive layer, e.g. metal layer, and the semiconductor layer are part of the same multilayer structure as the ultrasonic transducer <NUM>, wherein the ultrasonic transducer <NUM> may be defined by a MUT layer stack disposed at least partly on the diode.

Referring to <FIG>, a schematic cross-sectional side view of a first multilayer structure <NUM> defined by a diode in series with a respective ultrasonic transducer according to embodiments of the present is illustrated. The first multilayer structure <NUM> comprises a substrate structure comprising a substrate <NUM> and an insulating layer <NUM>. The substrate <NUM> may be a silicon substrate. The insulating layer <NUM> may be a silicon dioxide layer disposed over the substrate <NUM>. A semiconductor layer stack <NUM>, defining a diode <NUM>, may be disposed over the substrate structure <NUM>, <NUM>. In some embodiments of the present invention, the silicon dioxide layer <NUM> may be replaced with a silicon nitride layer. Preferably, the silicon substrate <NUM> and silicon dioxide layer <NUM> are formed from a silicon-on-insulator ("SOI") wafer preferably having a (<NUM>) device layer crystal orientation. In some embodiments of the present invention, there can be no insulating layer.

The semiconductor layer stack <NUM>, defining the diode <NUM> according to embodiments described above, comprises a first semiconductor layer <NUM> which is disposed over and in contact with the insulating layer <NUM>, preferably made out of a silicon dioxide layer. The first semiconductor layer <NUM> is heavily doped with a dopant of a first conductivity type. According to embodiments of the present invention, the first semiconductor layer <NUM> is heavily n-doped. The first semiconductor layer <NUM> has a first thickness t<NUM>, measured in the z-direction in a first central region R1 of the first multilayer structure <NUM>, and a second thickness t<NUM>, smaller than the first thickness t1, elsewhere. The first central region R1 is measured in the x-direction.

A second layer <NUM> may be disposed over the first semiconductor layer <NUM> outside the first region R1, i.e. at locations in the x-y plane where the thickness of the first semiconductor layer <NUM> is t<NUM>. The second layer <NUM> may be a semiconductor layer substrate doped with a dopant of the first conductivity type. Substrate doped means that it has a very low doping (or no doping) concentration which is about <NUM><NUM> to <NUM><NUM> times less than the doping concentration of the third semiconductor layer <NUM> (see below). According to other embodiments layer <NUM> is an isolation layer, for example a polymer layer.

The semiconductor layer stack <NUM> comprises a third semiconductor layer <NUM> which is disposed over the first semiconductor layer <NUM> in the first central region R1, i.e. at locations in the x-y plane where the thickness of the first semiconductor layer <NUM> is t<NUM>. The third semiconductor layer <NUM> of the semiconductor layer stack <NUM> is weakly doped with a dopant of the first conductivity type. According to embodiments of the present invention, the third semiconductor layer <NUM> is n- doped and has a thickness preferably between <NUM> nanometres (nm) and <NUM> nanometres (nm).

The first multilayer structure <NUM> further comprises a first electrically conductive layer <NUM> preferably comprising platinum, but other materials may be used such as aluminium, molybdenum, chromium, gold, and/or silver.

The first electrically conductive layer <NUM> is at least partly disposed over the third semiconductor layer <NUM> and the second semiconductor layer <NUM> of the semiconductor layer stack <NUM>, wherein the first electrically conductive layer <NUM> defines electrically conductive layer gaps G, also referred to as gaps G, such that the part of the first electrically conductive layer <NUM> disposed over the third semiconductor layer <NUM> is not connected to the part of the first electrically conductive layer <NUM> disposed over the second semiconductor layer <NUM>. In other words, a first portion of the first electrically conductive layer <NUM> is sandwiched between the third semiconductor layer <NUM> and at least a portion of a piezoelectric layer <NUM> over the first central region R1 within the first region X<NUM>, and a second portion of the first electrically conductive layer is <NUM> disposed on at least a part of the second semiconductor layer <NUM> such that the first portion and the second portion of the first electrically conductive layer <NUM> define the first electrically conductive layer gap G.

According to embodiments of the present invention, the piezoelectric layer <NUM> may be, for example, of lead zirconate titanate (PZT), aluminum nitride (AIN), scandium-doped aluminium nitride (Sc-AIN), PMN (chemical formula: Pb(Mg1/3Nb2/<NUM>)O3), PZN (chemical formula: Pb(Zn1/3Nb2/<NUM>)O3), PMN-PT, PZT-PMN, ZnO, ZnO2, or any other piezoelectric material that can be fabricated as a thin film. The piezoelectric layer <NUM> may be disposed over the electrically conductive layer <NUM> and may fill the first electrically conductive layer gap or gaps G. The first multilayer structure <NUM> may further comprise a top electrode <NUM> disposed on the piezoelectric layer <NUM>. This top electrode <NUM> may be a first or a second electrode as previously described in embodiments of the present invention. The first multilayer structure <NUM> further comprises a recess P1 which may extend through the piezoelectric layer <NUM>, the electrically conductive layer <NUM>, and the second semiconductor layer <NUM>. A second electrode <NUM> may be provided in the recess P1 on the first semiconductor layer <NUM>.

According to embodiments of the present invention, a multilayer structure <NUM> may comprise the layers and features as described above related to <FIG>, wherein the layers on top of the semiconductor layer stack <NUM> may have more than one piezoelectric layer and corresponding electrodes, plus some insulating and protecting layer in between the piezoelectric and/or electrically conductive layer and/or on top of the whole device. Insulating layers can be polymers such as polyimide or ceramics, such as Al2O3, SiO2 or SiN, or a combination thereof without being limited thereto.

A Schottky junction is formed at the interface between the electrically conductive layer <NUM> and a doped, preferably low doped, semiconductor layer. When reference is made to this interface, reference is made to metal/n- doped interface or anode. The other side is referred to as metal/n+ doped interface or cathode.

A cavity C1 is provided through the silicon substrate <NUM> and the silicon dioxide layer <NUM>, such that a first portion of the first multilayer structure comprising the semiconductor layer stack <NUM>, the electrically conductive layer <NUM>, and the piezoelectric layer <NUM> is suspended over or overlying the cavity C1 over a first region, wherein the first region X<NUM> extends in the x-direction over a distance substantially corresponding with the width of the cavity C1 measured in the x-direction. Note the cavity is also extended in the y-direction, which is not shown in the figure.

This first portion extending over the first region X<NUM> defines an ultrasound source comprising at least one membrane and at least one means for causing the at least one membrane to vibrate so as to receive and emit ultrasonic waves. The at least one membrane, extending over the region first X<NUM>, may comprise a first portion of the semiconductor layer stack <NUM> and the electrically conductive layer <NUM>, whereas the means for causing the membrane to vibrate may comprise a portion of the piezoelectric layer <NUM>. Hence, the means for causing the membrane to vibrate may comprise, for example, a layer of piezoelectric material for forming a piezoelectric MUT ("pMUT") or a multilayer structure together with a vacuum cavity for forming a capacitive MUT ("cMUT"). Each MUT can transmit and receive acoustic waves. Acoustic waves are generated when a voltage difference is applied across a first electrode and a second electrode in contact with the means for causing the membrane to vibrate. Conversely, arriving or detected acoustic waves creates motion in the MUT, producing an electric signal detected by the means.

The first multilayer structure <NUM> along with the electrodes <NUM> and <NUM>, provide an ultrasonic transducer <NUM>. However, as will be described in more detail hereinafter, in some embodiments the ultrasonic transducer <NUM> is a cMUT defining a different cavity than the cavity C1 illustrated in <FIG>.

The piezoelectric layer <NUM> is capable of expanding and contracting upon application of a voltage difference between the high and low potential electrodes <NUM>, <NUM>. This causes vibration of the suspended portion over the first region X<NUM> of the piezoelectric layer <NUM> and the electrically conductive layer <NUM> and semiconductor layer stack <NUM> in the z-direction (being perpendicular to the plane of the multilayer structure <NUM>), which causes ultrasound waves to be emitted. Conversely, arriving or detected acoustic waves create motion of the at least one membrane in the pMUT, producing an electric signal detected by the electrodes <NUM>,<NUM>.

The first electrode <NUM>, for example high potential electrode, can be connected to, or form part of, a row electrode <NUM> of an ultrasonic transducer array device <NUM> as described hereinbefore. The second electrode <NUM>, for example low potential electrode, can be connected to, or form part of, a column electrode <NUM> of an ultrasonic transducer array device <NUM> as described hereinbefore.

Referring to <FIG>, a modified first multilayer structure <NUM>' is the same as the first multilayer structure <NUM> of <FIG> except that the modified first multilayer structure <NUM>' also comprises a metal layer <NUM>, to reduce the resistivity of the ground conductor, between the semiconductor layer stack <NUM> and the insulating layer <NUM>, wherein the insulating layer <NUM> may be a silicon dioxide (SiO2) layer <NUM>. Embodiments as shown in <FIG> and <FIG> may have a protective isolation layer on top.

In <FIG> a top view is presented of the multilayer structure of <FIG>. The first <NUM> and second <NUM> electrode can be seen, as well as the metal layer <NUM>. The region X1 defining the width of the cavity C1 is also indicated. Note that layers <NUM>, <NUM>, <NUM> are not shown in the figure in order to make layer <NUM> visible.

Referring to <FIG>, a second multilayer structure <NUM>, which may be comprised in an ultrasonic transducer array device <NUM> according to embodiments of the present invention, is shown. Similar to the first multilayer structure <NUM> as depicted in <FIG>, the second multilayer structure <NUM> comprises a second substrate structure, wherein the second substrate structure may comprise, for example, a second silicon substrate <NUM>, and a second silicon dioxide layer <NUM> disposed over the silicon substrate <NUM>. The second multilayer structure <NUM> further comprises a second semiconductor layer stack <NUM> disposed over the silicon dioxide layer <NUM>, a second electrically conductive layer <NUM>, e.g. a metal layer, disposed over at least a part of the second semiconductor layer stack <NUM>, and a second piezoelectric layer <NUM> disposed over the second electrically conductive layer <NUM>.

The second semiconductor layer stack <NUM> of the second multilayer structure <NUM> may comprise a fourth semiconductor layer <NUM> disposed over and in contact with the second substrate structure, in particular the silicon dioxide layer <NUM>. The fourth semiconductor layer <NUM> may be heavily doped with a dopant of the first conductivity type. Indeed, according to embodiments of the present invention, the fourth semiconductor layer <NUM> may be (a silicon) n+ doped, having a relative concentration between <NUM><NUM> and <NUM><NUM> per cm<NUM>.

A layer <NUM> may be disposed over the fourth semiconductor layer <NUM> except in a second central region R<NUM>. The second central region R2 comprises at least a portion of the second multilayer structure <NUM> and extends in the x-direction in which a second top electrode <NUM> is connected with the at least portion of the second multilayer structure <NUM>. The layer <NUM> may be a fifth semiconductor layer with a very low doping concentration of a dopant of the first conductivity type or even no doping at all. According to some embodiments of the present invention, the fifth layer <NUM> is substrate doped with a dopant of the first conductivity type, preferably at least <NUM><NUM> times less than the dopant concentration of a sixth semiconductor layer <NUM> (see next paragraph). According to other embodiments layer <NUM> is an isolation layer, for example a polymer layer.

The second semiconductor layer stack <NUM> may also comprise a sixth semiconductor layer <NUM> which is disposed over the fourth semiconductor layer <NUM> in the second central region R<NUM>, and a seventh semiconductor layer <NUM> which is disposed over the sixth semiconductor layer <NUM> in the second central region R<NUM>. According to embodiments of the present invention, the sixth semiconductor layer <NUM> is n- doped and the seventh semiconductor layer <NUM> is p+ doped. The combined thickness of the sixth semiconductor layer <NUM> and the seventh semiconductor layer <NUM> may be approximately one (<NUM>) to five (<NUM>) micrometres (µm), without being limited thereto.

A substrate doped region generally has a doping concentration that is less than that of an n- (or p-) doped region by a factor of approximately <NUM><NUM> to <NUM><NUM>. An n- (or p-) doped region generally has a doping concentration that is less than that of an n+ (or p+) region by a factor of approximately <NUM><NUM> to <NUM><NUM>.

The second multilayer structure <NUM> further comprises a second piezoelectric layer <NUM> which is disposed over the second electrically conductive layer <NUM> and fills the second electrically conductive layer gaps G. The second multilayer structure <NUM> further comprises a second top electrode <NUM> disposed on the second piezoelectric layer <NUM> within the second central region R<NUM>. The second multilayer structure <NUM> also has a second recess P2 which extends through the second piezoelectric layer <NUM>, the second electrically conductive layer <NUM>, and partially through the second semiconductor layer stack <NUM>. A second electrode <NUM> is provided in the recess P2 on the fourth semiconductor layer <NUM>. A p-n junction diode is formed at the boundary between the sixth and seventh semiconductor layers <NUM>, <NUM>. When a positive voltage difference is applied between the electrodes <NUM>, <NUM>, a depletion layer is formed at the p-n junction and current can flow between the electrodes <NUM>, <NUM>. In specific embodiments of the present invention wherein the capacitor between electrode <NUM> and <NUM> is not fully charged, a depletion layer is formed at the p-n junction and current can flow between the electrodes when a positive voltage difference is applied between the electrodes <NUM>, <NUM>.

A second cavity C<NUM> is provided through the second substrate, in particular the silicon substrate <NUM> and the silicon dioxide layer <NUM>, such that a portion X<NUM> of the layer structure comprising a portion of the semiconductor layer stack <NUM>, the second electrically conductive layer <NUM>, and the piezoelectric layer <NUM> is suspended over the second cavity C<NUM>. This portion X<NUM>, along with the electrodes <NUM> and <NUM>, provide an ultrasonic transducer <NUM> capable of emitting and receiving ultrasound waves as described hereinbefore in relation to <FIG>. The portion X<NUM> of the second multilayer structure <NUM> overlying or suspending the second cavity C<NUM> may define an ultrasound source and receiver comprising a membrane and a means for causing the membrane to vibrate so as to emit ultrasonic waves. The membrane may be defined by a portion of the semiconductor layer stack <NUM> and the second electrically conductive layer <NUM> extending within the second region X<NUM>, whereas the means for causing the membrane to vibrate may be defined by at least a portion of the piezoelectric layer <NUM> and the top electrode <NUM> within the second region X<NUM>.

The first, top or positive electrode <NUM> can be connected to, or form part of, a row electrode <NUM> of a transducer array as described hereinbefore. The second, bottom or negative electrode <NUM> can be connected to, or form part of, a column electrode <NUM> of a transducer array as described hereinbefore.

The embodiment of a second multilayer structure <NUM> as shown in <FIG> comprises a diode structure referred to as an p-n junction diode. This diode has a p+/n- junction. By connecting a electrically conductive region or layer to a p+ region or layer, the anode may be accessed. The n+ region or layer works as the ground signal connector and is accessed by electrically conductive layer <NUM> as cathode. The advantage of the very low doped or no doped region is the reduction of all parasitic components.

The multilayer structure as depicted in <FIG> is similar to the second multilayer structure as in <FIG> and discussed before, except for a metal layer <NUM> underneath the semiconductor layer stack <NUM>. This is because n+ has a high resistivity to work as the ground signal and is attached to a metal layer.

Further referring to <FIG>, a third multilayer structure <NUM>, which may be comprised in an ultrasonic transducer array device <NUM> according to embodiments of the present invention, is shown. The third multilayer structure <NUM> is similar to the second multilayer structure <NUM> and comprises a third substrate structure comprising a third substrate <NUM> and a third insulating layer, in particular a silicon dioxide layer <NUM> disposed over the third substrate <NUM>. The third semiconductor layer structure <NUM> may further comprise a third semiconductor layer stack <NUM> disposed over the third substrate structure <NUM>,<NUM>, a third electrically conductive layer <NUM> disposed over at least a portion of the third semiconductor layer stack <NUM>, and a third piezoelectric layer <NUM> of which at least a portion is disposed over the third electrically conductive layer <NUM>.

The third multilayer structure <NUM> additionally comprises a fourth electrically conductive layer <NUM> disposed between the third insulating layer <NUM>, in particular the silicon dioxide layer, and the third semiconductor layer stack <NUM>.

The third semiconductor layer stack <NUM> may be composed as follows. A layer <NUM> may be disposed over the fourth metal layer <NUM> except in a third central region R<NUM> between the top electrode <NUM> and the third substrate structure <NUM>, <NUM> in the z-direction. The layer <NUM> may be an eighth semiconductor layer <NUM> with a very low (or no doping) substrate doping concentration in some embodiments. According to other embodiments layer <NUM> is an isolation layer, for example a polymer layer.

The third semiconductor layer stack <NUM> may further comprise a ninth, tenth, and eleventh semiconductor layers <NUM>, <NUM>, <NUM> respectively, disposed over the second metal layer <NUM> in that order in the third central region R<NUM>. That is, the ninth semiconductor layer <NUM> is adjacent to the fourth metal layer <NUM>, the eleventh semiconductor layer <NUM> is adjacent to the third electrically conductive layer <NUM>, and the tenth semiconductor layer <NUM> is between the ninth and the eleventh semiconductor layers <NUM>, <NUM>. The combined thickness of the tenth and eleventh semiconductor layers <NUM>, <NUM> is approximately one (<NUM>) to five (<NUM>) micrometres (µm).

According to embodiments of the present invention, the ninth semiconductor layer <NUM> may be n+ doped. The tenth semiconductor layer <NUM> may be n- doped. The eleventh semiconductor layer <NUM> may be p+ doped. The functioning of these features is the same as explained for corresponding layers in <FIG>.

A third piezoelectric layer <NUM> is disposed over the third metal layer <NUM> and fills the gaps G. The third multilayer structure <NUM> further comprises a top electrode <NUM> disposed on the piezoelectric layer <NUM>. The third multilayer structure <NUM> also has a third recess P<NUM> which extends through the third piezoelectric layer <NUM>, the third metal layer <NUM>, and through the third semiconductor layer <NUM>. A negative voltage electrode <NUM> is provided in the third recess P<NUM> on the second metal layer <NUM>.

A third cavity C<NUM> is defined by the third substrate structure, in particular a silicon substrate <NUM> and the silicon dioxide layer <NUM>, such that a portion X<NUM> of the third multilayer structure comprising the fourth metal layer <NUM>, the third semiconductor layer stack <NUM>, the third metal layer <NUM>, and the third piezoelectric layer <NUM>, is suspended over the cavity C3. This third portion X3 may define at least one membrane and at least one means for causing the membrane to vibrate so as to emit ultrasonic waves. Indeed, this portion X<NUM>, along with the electrodes <NUM> and <NUM>, provide an ultrasonic transducer <NUM> capable of emitting ultrasound waves as described hereinbefore in relation to <FIG>.

The electrode <NUM> can be connected to, or form part of, a row electrode <NUM> of an ultrasonic transducer array device <NUM> as described hereinbefore. The electrode <NUM> can be connected to, or form part of, a column electrode <NUM> of a transducer array as described hereinbefore.

A diode comprised in an array device according to embodiments of the present invention may be provided by a Schottky diode or a p-n diode as described hereinbefore. However, other possibilities for providing the diode are within the scope of the present invention. For example, a field-effect transistor can be used as a diode so as to provide directional current flow.

Referring to <FIG>, another embodiment of a multilayer structure according to the invention is presented. The embodiment of <FIG> comprises a mechanical layer <NUM> which may be another semiconductor layer (e.g. Si or SiC) or a polymer or SiO2 or SiN layer. If it is a semiconductor layer, the layer may be doped, e.g. substrate doped with a dopant of the first conductivity type, or not doped at all. In some embodiments the layer <NUM> may not be present. The MUT layer comprises a piezoelectric layer <NUM> and a first electrically conductive layer <NUM>. The first electrically conductive layer is disposed on the mechanical layer <NUM>. In the particular embodiment shown in <FIG> a first portion of the electrically conductive layer <NUM> is sandwiched between at least a portion of the piezoelectric layer <NUM> and the mechanical layer <NUM>. The position of the heavily doped semiconductor layer <NUM> and the weakly doped semiconductor layer <NUM> is reversed compared to the embodiments of <FIG> : the heavily doped semiconductor layer is now farther away from the cavity than the weakly doped semiconductor layer. On top of the structure of <FIG> a top electrode electrically conductive track <NUM> (e.g. a metal track) is shown that covers part of an isolation layer <NUM> and the semiconductor layer stack defining the diode. This semiconductor layer stack is in this embodiment placed on an electrically conductive layer <NUM>. The semiconductor stack has in <FIG> a thickness t8. Note that this semiconductor layer stack can be implemented as any of the semiconductor layer stacks shown in other embodiments provided that the different layers of the stack are placed in reverse order in the z-direction. In some embodiments the electrically conductive layer <NUM> may comprise a plurality of layers, so forming a multi-layer structure. The isolation layer <NUM> protects the semiconductor layers e.g. from neighbouring diodes in the array. The semiconductor layer stack may in some embodiments comprise a semiconductor layer heavily doped with a dopant of a second conductivity type sandwiched between the first semiconductor layer and the first portion of the first electrically conductive layer within region R1.

Embodiments of the multilayer structure of the invention as described previously can be combined in various ways yielding for example multilayer structures with two or even more cavities and/or with additional semiconductor layer stacks and/or a stack of electrically conductive tracks, e.g. metal tracks, together with corresponding isolation layers. For example, <FIG> is the same as the <FIG> but with pMUTs in regions X1 and X2 and two semiconductor multilayer stacks in regions R1 and R2. The first electrically conductive track <NUM> is also disposed partially over the second semiconductor multilayer in region R2. The second electrically conductive track <NUM> disposed on top of the first electrically conductive track <NUM> on the second region R2. The first electrically conductive track <NUM> in region R1 and the isolation layer <NUM> are covered with an isolation layer <NUM>. A third electrically conductive track <NUM> (e.g. a metal track) is shown that covers part of an isolation layer <NUM> and the semiconductor layer stack defining the diode in region R2. The third electrically conductive layer <NUM> acts as the top electrode for the second ultrasonic transducer in region X2. Similarly, more electrically conductive tracks can disposed on top of each other by means of intermediate isolation layers to avoid short circuit.

Although the example multilayer structures described hereinbefore comprise a piezoelectric layer for forming a pMUT, the principles are equally applicable for providing a cMUT. For example, referring to <FIG>, a fourth multilayer structure <NUM> which can be included in an ultrasonic transducer array device <NUM> according to embodiments of the present invention is shown. The fourth multilayer structure <NUM> comprises a fourth substrate structure <NUM>, preferably made of silicon, and a fourth semiconductor layer stack <NUM> disposed over the silicon substrate <NUM>. The fourth semiconductor layer stack <NUM> in this cMUT-diode configuration may be any of the semiconductor layer stacks <NUM>, <NUM>, <NUM> described hereinbefore. A fifth electrically conductive layer <NUM>, e.g. a metal layer, is disposed over the fourth semiconductor layer stack <NUM>. A first non-conductive layer <NUM> is disposed over at least a portion of the fifth electrically conductive layer <NUM> except in fourth central region R<NUM>. A second non-conductive layer <NUM> is disposed over the first non-conductive layer <NUM> so as to form a cMUT cavity <NUM> in the fourth central region R<NUM>. The cMUT cavity <NUM> may be a vacuum cavity. An electrode <NUM> is disposed over the second non-conductive layer <NUM> in the fourth central region R<NUM>. A fourth recess P4 is provided through the second non-conductive layer <NUM>, the first non-conductive layer <NUM>, and the fifth electrically conductive layer <NUM>. An electrode <NUM> is disposed in the fourth recess P<NUM> on the semiconductor layer <NUM>. When an alternating voltage is applied between the electrode <NUM> and the electrode <NUM>, the portion of the second non-conductive layer which is in the fourth central region R<NUM>, i.e. is suspended over the cMUT cavity <NUM>, is caused to vibrate due to alternating attraction and repulsion between the electrode <NUM> and the electrically conductive layer <NUM>. This causes ultrasound waves to be emitted. The vibrations may also exist because of an alternative electrostatic force inside the cMUT cavity <NUM>, which works as a capacitor, and between the two electrodes <NUM> and <NUM>.

The non-conductive layers <NUM>, <NUM> may comprise e.g. poly Si, SiN, SiO2 or other polymers.

Embodiments of the ultrasonic transducer multilayer structure as shown in any of <FIG> may have a protective isolation layer on top.

In <FIG> another embodiment with a cMUT is illustrated. A cMUT cavity is created as explained with respect to <FIG> between electrically conductive layer <NUM> and non-conductive layers <NUM> and <NUM>. The fourth semiconductor layer stack <NUM> (which, as already mentioned, may be any of the semiconductor layer stacks <NUM>, <NUM>, <NUM> described previously) is now positioned on top of the electrode <NUM> but in reverse order in the z-direction. Just as in <FIG>, a top electrode electrically conductive track <NUM> (e.g. a metal track) is provided that at least partially covers the semiconductor layer stack <NUM>. Electrically conductive layer <NUM> is provided with gaps G in order to avoid the bottom electrode of the various MUTs being connected to each other.

According to embodiments of the present invention, the ultrasonic transducer array device further comprises at least one power source adapted to provide power to the ultrasonic transducer to keep the diode on and activate the receiving function for ultrasonic signals. This power source may be a DC power source arranged in series with the ultrasonic transducer and corresponding diode.

According to another aspect of the invention, a method of manufacturing an ultrasonic transducer array device according to any of the preceding embodiments is presented below. The method comprising the steps of: providing a first wafer according to an array configuration, wherein the first wafer defines a semiconductor layer stack; providing a second wafer, preferably a silicon-based wafer comprising a SiO2 layer or a silicon nitride layer; bonding the first wafer to the second wafer, defining a silicon-on-insulator (SOI) wafer wherein the second wafer defines a buried oxide (BOX) layer of the SOI wafer; processing the first wafer to a predetermined thickness of an ultrasonic transducer array device.

Embodiments of the present invention further comprises the present invention for the manufacturing of an ultrasonic transducer array device comprise the steps of providing a base layer comprising an array of diodes, and fabricating the array of ultrasonic transducers on the base layer.

In particular, embodiments of the method for the manufacturing of an ultrasonic transducer array device having a pMUT layer stack according to the present invention comprise the steps of: Providing a first wafer according to a predetermined array configuration, wherein the first wafer defines a semiconductor layer stack according to at least one embodiment of the invention described before; Providing a second wafer, preferably a silicon-based wafer comprising a SiO2 layer or a silicon nitride layer; Bonding the first wafer to the second wafer, defining a silicon-on-insulator (SOI) wafer wherein the second wafer defines the a buried oxide (BOX) layer of the SOI wafer; and Processing the first wafer to a predetermined thickness of the transducer array device layer.

The processing of the first wafer comprises a thinning down and polishing step.

Embodiments of the method according to the present invention may provide the first wafer which is ion-implemented. The ion-implementation may also be performed after the SOI wafer is formed.

Embodiments of the method according to the present invention may comprise a step adding an electrically conductive layer, having a predetermined pattern corresponding to the predetermined array configuration, on the first wafer before providing the second wafer using a deposition technique like physical vapour deposition (PVD) or chemical vapour deposition (CVD), without being limited thereto.

The method further comprises the steps of.

According to embodiments of the present invention, the first electrode, or top electrode, may be smaller than each membrane.

Referring to <FIG>, a micro-machined ultrasonic transducer (MUT), according to embodiments of the present invention is shown. Both capacitive MUTs, hereinafter also referred to as "cMUT" or "cMUTs", and piezoelectric MUTs, hereinafter also referred to as "pMUT" or "pMUTs", are micro-electro-mechanical systems (MEMs) devices manufactured using semiconductor batch fabrication.

The transducer <NUM> comprises an ultrasound source <NUM> and a cavity <NUM> acoustically coupled to the at least one ultrasound source <NUM>. The cavity <NUM> may be a pMUT or cMUT cavity as described in relation to the present invention above.

The ultrasound source <NUM> comprises at least one vibratable membrane <NUM>, having a membrane thickness defined along a first direction (z-direction), and at least one means <NUM> for causing the at least one vibratable membrane <NUM> to vibrate so as to emit ultrasonic waves and/or for detecting a vibration of the at least one vibratable membrane <NUM> in order to receive ultrasonic waves. The at least one vibratable membrane <NUM> may be at least a portion of the semiconductor layer stack defining a diode and a first electrically conductive layer disposed on the semiconductor layer as described in relation to embodiments of the present invention above. The at least one means <NUM> for causing the at least one vibratable membrane <NUM> to vibrate so as to emit ultrasonic waves and/or for detecting a vibration of the at least one vibratable membrane <NUM> in order to receive ultrasonic waves, may be a portion of the MUT layer stack disposed on the first electrically conductive layer and an electrode disposed on top of the MUT layer stack.

As used herein, and unless otherwise specified, when reference is made to the at least one vibratable membrane, reference may also be made to membrane <NUM>.

As used herein, and unless otherwise specified, when reference is made to at least one means for causing the at least one vibratable membrane to vibrate so as to emit ultrasonic waves and/or for detecting a vibration of the at least one vibratable membrane in order to receive ultrasonic waves, reference may also be made to means for causing the membrane to vibrate, or means <NUM>.

The cavity <NUM> has a first end <NUM> and a second end <NUM>, wherein the second end <NUM> is opposed to the first end <NUM> along the first direction (z-direction). The cavity <NUM> is bounded by a side wall <NUM>, the membrane <NUM> at the first end <NUM>, and an end wall <NUM> (hereinafter also referred to as second end wall <NUM>) at the second end <NUM>. The membrane <NUM> is located at the first end <NUM> of the cavity and closes the first end <NUM> of the cavity. For example, the membrane may be bonded to the side wall <NUM> at the first end of the cavity. Preferably, the side wall <NUM> and the second end wall <NUM> are monolithic. This can help to simplify fabrication of a transducer <NUM>. In some embodiments, the side wall <NUM> may be provided separately to the second end wall <NUM> and may be fixed to the second end wall <NUM> by, for example, applying an adhesive or using a bonding process.

As will be described in more detail hereinafter, the at least one means <NUM> for causing the membrane to vibrate may comprise, for example, a layer of piezoelectric material for forming a pMUT or a multilayer structure together with a vacuum cavity for forming a cMUT. Each MUT can transmit and receive acoustic waves. Acoustic waves are generated when a voltage difference is applied across a first electrode and a second electrode in contact with the means <NUM>. Conversely, arriving or detected acoustic waves creates motion in the MUT, producing an electric signal detected by the means <NUM>.

Referring in particular to <FIG>, depicting a schematic cross-sectional view in the z-x plane of a transducer in an "at-rest" configuration or state according to embodiments of the invention, wherein the membrane <NUM> is undeformed.

Referring to <FIG> and <FIG>, depicting the same schematic cross-sectional view as in <FIG> of a transducer in an "activated" configuration according to embodiments of the present invention, wherein the membrane <NUM> is caused to vibrate by the means <NUM>, the membrane <NUM> is periodically displaced in the z direction and moves between a maximum displacement in the z direction away from the cavity (<FIG>), and a maximum displacement in the z direction within the cavity (<FIG>).

The side wall <NUM> may be a single continuous side wall, for example if the cavity <NUM> has a cylindrical shape and thus a circular cross-section as taken in a plane parallel to the membrane <NUM> at rest, i.e. the x-y plane where the y axis is perpendicular to the x and z axes shown in the Figures. The side wall <NUM> may be composed of several adjoining side walls, for example if the cavity has a cross-section in the x-y plane which is polygonal. For example, the cavity <NUM> may have a cuboid shape and the cross-section may be square shaped.

Providing the membrane <NUM> such that the first end <NUM> of the cavity is closed allows the transducer to be used while in contact with a liquid or gel without the risk that the liquid or gel will leak into the cavity, which could change the acoustic properties of the transducer and damage the transducer.

The membrane may comprise a ceramic such as SiO<NUM>, SiC, or Al<NUM>O<NUM>; a semiconductor such as silicon; a polymer; a carbon based material such as a diamond thin film; a glass or quartz; or other suitable thin film.

The cavity <NUM> is acoustically coupled to the membrane <NUM> through at least one medium or material inside the cavity and is capable of supporting standing waves generated by the membrane <NUM>. The at least one medium or material inside the cavity <NUM> is at least partly connected to the membrane <NUM> and is a gas material, solid material or liquid material, including, without being limited thereto, air, helium, silicone oil, castor oil, gel, polyurethane, polyester, epoxy resin, foamed plastics, foamed metal, soft rubber, silicone rubber, sound absorption rubber, butyl rubber, glass wool, glass fibre, felt, silk, cloth and micro-perforated panel.

The periodic oscillation of the membrane <NUM> causes corresponding periodic changes in the pressure in the cavity <NUM>; provided that the frequency f of the oscillation of the membrane <NUM> satisfies the relationship as presented in equation (<NUM>): <MAT> where n is a positive integer, v is the speed of sound in the cavity <NUM>, and L is the length of the cavity. A standing wave can be set up in the cavity <NUM> with a node at the second end <NUM> and an antinode at the membrane <NUM>. The length L of the cavity <NUM> is measured between the first end <NUM> and the second end <NUM> of the cavity <NUM> when the membrane <NUM> is undeformed. The resonance frequency of the membrane <NUM> depends on its constructional characteristics like geometry, thickness, diameter, etc., as well as its mechanical properties as well as the mechanical properties of the other layers on top of the membrane and their mutual effect on each other, such as the effect of the residual stress of one layer to the other. If the source of the ultrasound is a cMUT, which means that that membrane <NUM> is the membrane of a cMUT floating on a vacuum cavity embedded in the means <NUM>, then the resonance frequency is also dependent to the DC bias voltage of the cMUT.

In order to support a standing wave, the cavity <NUM> does not contain a vacuum and is filled with at least one acoustically suitable medium such as a gas, solid or liquid medium, including, without being limited thereto, air, helium, silicone oil, castor oil, gel, polyurethane, polyester, epoxy resin, foamed plastics, foamed metal, soft rubber, silicone rubber, sound absorption rubber, butyl rubber, glass wool, glass fibre, felt, silk, cloth and micro-perforated panel. The at least one acoustically suitable medium allows a standing wave to have a specific wavelength for a given length L of the cavity <NUM> and frequency in which the standing wave originates.

According to preferred embodiments of the invention, the acoustically suitable material may lead to damping of the resonance and widening of the bandwidth of the MUT.

A cavity <NUM> which contains a vacuum is not capable of supporting a standing wave. An acoustically suitable material is one which is capable of compression and expansion when subjected to a force due to the movement of the membrane <NUM>, such that the membrane <NUM> does not lose contact with the material during vibration. If the cavity <NUM> is filled with a non-gas medium, the stiffness and rigidity of the cavity filling material should be significantly less than the stiffness and rigidity of the membrane <NUM>, to avoid that the cavity filling material prevents or restricts the membrane <NUM> from vibration and changes the mechanical properties of the whole device undesirably. In some embodiments, for example if the walls <NUM>, <NUM> of the cavity <NUM> comprise a conductive material, the material of the walls may be chosen as a material with a relatively small electrical conductivity so as not to interfere with a fabrication process.

Preferably, the acoustic impedance of the medium or material contained in the cavity <NUM> is substantially different from the acoustic impedance of the second end wall <NUM> at the second end <NUM> of the cavity <NUM>. This has the advantage that a substantial proportion of the ultrasound wave in the cavity <NUM> can be reflected back by the second end wall <NUM> at the second end <NUM> and contribute to a strong standing wave. The acoustic impedance of the medium contained in the cavity may be different than the acoustic impedance of the second end wall <NUM> at the second end <NUM>. According to embodiments of the invention, the acoustic impedance may be at least fifty (<NUM>) times smaller or greater than the acoustic impedance of the second end wall <NUM> at the second end <NUM>, more preferably at least one hundred (<NUM>) times greater or smaller, and still more preferably at least five hundred (<NUM>) or at least one thousand (<NUM>) times greater or smaller.

The membrane <NUM> exhibits an increased amplitude of vibration when caused to vibrate at a frequency which is among the resonance frequencies of the cavity <NUM>, as compared to when the membrane <NUM> is caused to vibrate at a frequency which is not a resonance frequency of the cavity <NUM>. When the cavity length L is an odd integer multiple of λ/<NUM>, where λ is the wavelength of the transmitted ultrasound wave in the cavity <NUM>, maximum constructive interference of the transmitted wave from the first end <NUM> and the reflected wave from the opposite second end <NUM> occurs at the interface between the cavity <NUM> and the membrane <NUM>. This constructive interference provides an additional driving force for the vibration of the membrane <NUM>, increasing the amplitude of the vibrations compared to situations where constructive interference does not occur. Thus, by choosing an appropriate cavity length L, the output pressure of the transducer can be increased. The output ultrasound waves are emitted in a direction R which is generally in the z direction as shown in <FIG>.

The cavity length L does not need to be precisely equal to λ/<NUM> in order to obtain an increased output pressure of the transducer. Indeed, the length L of the cavity can be any odd product of λ/<NUM> with an error margin of ± λ/<NUM>. For example, a cavity length L of five (<NUM>) times λ/<NUM> or seven (<NUM>) times λ/<NUM>, where λ is constant, is sufficient to support a standing wave with a resonance frequency of f=c/λ, wherein c is the speed of light in vacuum. Any variation in the length L between ± λ/<NUM> may cause an error in the standing wave resonance frequency. The amount of tolerance may depend on the application. For example, for imaging application, a membrane with a resonance frequency of one (<NUM>) to ten (<NUM>) Megahertz (MHz) underwater or gel, has a bandwidth of about <NUM>-<NUM>% the centre frequency f<NUM> (which depends on the application). Therefore, the frequency of the standing wave can be in the range of f<NUM> ∓ <NUM>*f<NUM>. As a result, the length L of the cavity can be between <NUM>×L < L < <NUM>×L, where L=n×λ/<NUM> and n is an odd number.

A further advantage of providing a cavity <NUM> capable of supporting standing waves is that this allows tailoring of the bandwidth of the emitted ultrasonic waves. The bandwidth can be increased due to the frequencies of the cavity <NUM> combining with the frequency of the membrane <NUM>, thus providing a transducer <NUM> with multiple resonance frequencies and therefore a broader bandwidth than a transducer which does not comprise a cavity capable of supporting standing waves.

Furthermore, the at least one acoustically suitable medium in the cavity <NUM> can help to damp vibration of the membrane <NUM>, which helps to prevent ringing without requiring damping layer(s) to be provided on the membrane <NUM>. Air as a cavity filling material provides some damping, and more damping can be provided if the cavity filling material comprises for example a permanent liquid such as an oil or gel. The cavity of a transducer <NUM> according to the present invention may also be filled with solid material, for example, solid resins.

A cavity filling material which provides substantial damping can reduce the displacement of the membrane and thus the output pressure, resulting in a lower signal-to-noise ratio, which can be compensated for example by use of a low noise analogue amplifier. Preferably, the solid material has a small amount of elasticity, which is higher than the membrane of the ultrasound source. Therefore, since the membrane is attached completely to the solid material according to embodiments of the present invention, it will be damped by the elasticity of the solid material.

Preferably, the flexural rigidity of the second end wall <NUM> is larger than the flexural rigidity of the membrane <NUM>, which can be implemented by choosing the thickness t809 of the second end wall <NUM> in the z direction to be substantially greater than the thickness t804 of the membrane <NUM>. This can help to reduce or avoid acoustic excitation of the second end wall <NUM> which would interfere with the standing wave in the cavity <NUM> and may add undesired mode of vibration in the transducer as a whole. Suitable thicknesses depend on the mechanical properties of the material of the membrane <NUM> and of the second end wall <NUM> and can be determined, for example, through experimentation by varying thicknesses t1 and/or t2 and measuring the output modes of the transducer. For example, in some embodiments the membrane <NUM> comprises silicon and has a thickness five (<NUM>) micrometres (µm) and the second end wall <NUM> comprises stainless steel and has a thickness of <NUM> millimetres (mm).

For similar reasons, i.e. preventing or reducing acoustic excitation of the side wall <NUM>, preferably the acoustic impedance of the side wall <NUM> is substantially larger than the acoustic impedance of the material in the cavity <NUM>.

Referring to <FIG>, a cross-sectional view of a first modified transducer <NUM> according to embodiments of the present invention is illustrated. The first modified transducer <NUM> is similar to the transducer <NUM> described in relation to <FIG>. In addition to the features of the transducer <NUM>, the first modified transducer <NUM> comprises a first micro-channel <NUM> through the second end wall <NUM>, having an opening at the second end <NUM> of the cavity <NUM>. The first micro-channel <NUM> connects the cavity <NUM> to the external environment of the first modified transducer <NUM>.

Referring to <FIG>, a second modified transducer <NUM> according to embodiments of the present invention. The second modified transducer <NUM> is similar to the transducer <NUM> described in relation to <FIG>. In addition to the features of the transducer <NUM>, the second modified transducer <NUM> comprises a second micro-channel <NUM> through the side wall <NUM>. The second micro-channel <NUM> connects the cavity <NUM> to the external environment of the second modified transducer <NUM>. In some embodiments, a transducer can include both the first micro-channel <NUM> and the second micro-channel <NUM>.

The micro-channels <NUM>, <NUM> can provide a means for regulating the pressure of the medium in the cavity <NUM>, <NUM>, such that preferably the pressure within the cavity <NUM>,<NUM> is constant. The micro-channels <NUM>, <NUM> can provide an inlet for replenishing the medium in the cavity, for example if gradual leakage occurs. The micro-channels <NUM>, <NUM> can help to provide reliable performance of the transducer <NUM>, <NUM> at varying temperatures, by providing an escape path for the medium if the temperature of the medium has increased, causing it to expand and the pressure in the cavity <NUM>, <NUM> to increase.

For example, if the medium is air, the micro-channels <NUM>, <NUM> can provide an escape path to the environment surrounding the transducer without needing further components. In some embodiments, for example if the medium comprises a gas which is not present in the environment surrounding the transducer <NUM>, <NUM> or if the medium comprises a fluid, an overflow compartment (not shown) may be provided adjacent to the transducer <NUM>, <NUM> which is connected to the microchannel <NUM>, <NUM>, for example via microfluidics, so as to allow exchange of the medium between the overflow compartment and the cavity <NUM>, <NUM>.

Preferably, the cavity <NUM>, <NUM> has a cross-sectional dimension which is substantially the same as a cross-sectional dimension of the membrane <NUM>. The cross-sectional dimension is measured in the x-y plane. This can help to prevent or reduce divergence of the acoustic wave generated by the transducer within the cavity, in the direction in which the cross-sectional dimension is measured. By reducing divergence, the amplitude of the standing wave can be increased.

More preferably, the cross-section of the cavity <NUM> in the x-y plane is substantially the same as the cross-section of the membrane <NUM> in the x-y plane. This can allow divergence to be reduced in more than one direction.

In embodiments wherein the length L of the cavity is comparable to the near field of the transducer (as defined below), the cavity cross-sectional dimension or cross section does not necessarily need to be substantially the same as the cross-sectional dimension or cross section of the membrane, as the length L of the cavity is small enough that far field behaviour is not reached at the second end of the cavity.

The ultrasound source <NUM> has a near field distance Dnf = A/πλ, where A is the area of the membrane <NUM>, also referred to as the membrane area, and λ is the wavelength of ultrasonic waves in the cavity. The membrane area is measured in a plane in or parallel to the x-y plane. For example, for a circular membrane of radius a the near field distance is a<NUM>/λ, where a is the radius of the membrane.

In some embodiments, the cavity length L is less than the near field distance Dnf. Such a configuration allows for high amplitude of the standing wave without requiring the dimensions of the membrane and the cavity in the x-y plane to be similar, as the distance over which the ultrasound wave propagates within the cavity before reflection is less than the near field distance and so no significant divergence occurs. A suitable cavity length L for such a configuration depends on the frequency of the standing wave, which depends on material properties of the membrane and the cavity filling material as described hereinbefore, and can be determined for example by simulations or by experiment.

In some embodiments, the cavity length L is greater than the near field distance Dnf. This allows for easier fabrication of the transducer. Another advantage of such an arrangement is that specific frequencies of the ultrasound wave can be achieved which require cavity lengths L longer than the near field distance Dnf. A suitable cavity length L for such a configuration depends on the frequency of the standing wave, which depends on material properties of the membrane and the cavity filling material as described hereinbefore, and can be determined for example by simulations or by experiment.

Referring to <FIG>, the means <NUM> for causing membrane <NUM> to vibrate preferably comprises a piezoelectric layer <NUM> disposed over the membrane <NUM> and first and second electrodes <NUM>, <NUM> respectively.

Suitable materials for forming the piezoelectric layer <NUM> include lead zirconate titanate (PZT), aluminium nitride (AIN), lead magnesium niobate (PMN), PMN-PZT, polyvinylidene fluoride (PVDF), zinc oxide (ZnO), among others. The first electrode <NUM> is disposed over the piezoelectric layer and the second electrode <NUM> is disposed between the piezoelectric layer <NUM> and the membrane <NUM>. By applying an AC voltage to the piezoelectric layer <NUM> via the first and second electrodes <NUM>, <NUM>, the piezoelectric layer <NUM> can be caused to expand and contract at the frequency of the AC voltage. As the piezoelectric layer is attached to the membrane <NUM> by its lateral interface, its lateral expansion and contraction causes the membrane to vibrate.

In some embodiments, the piezoelectric layer <NUM> is comprised in the membrane <NUM>. For example, the membrane <NUM> may comprise a multilayer stack comprising electrically conductive layer(s) and piezoelectric layer(s).

Referring to <FIG>, in some embodiments, the means <NUM> comprises a first electrically conductive layer <NUM>, e.g. a metal layer, disposed over the membrane <NUM> which, in combination with a second electrically conductive layer <NUM>, e.g. a metal layer, arranged such that the membrane <NUM> is between the first electrically conductive layer <NUM> and the second electrically conductive layer <NUM> in the z direction, forms a pair of electrodes which can serve as a capacitor. By applying an AC voltage across the electrodes, the membrane is caused to vibrate at the frequency of the AC voltage. The second electrically conductive layer <NUM> can be provided by, for example, a substrate <NUM> which seals the second end <NUM> of the cavity <NUM> and on which the side walls <NUM> are supported. In some embodiments, the substrate <NUM> comprises a multilayer stack and the second electrically conductive layer <NUM> can be provided by a layer in the multilayer stack, for example a titanium/platinum layer. In some embodiments, the second electrically conductive layer <NUM> may be provided by a buffer layer in a multilayer stack, for example in a SrRuO3 (SRO)//La0.5Sr0.5CoO3(LSCO)//CeO2//yttria-stabilized zirconia (YSZ) configuration.

The first electrically conductive layer <NUM> may comprise, for example, aluminium, silver, platinum, molybdenum, titanium, chromium, or other suitable metals.

Referring to <FIG>, an alternative transducer <NUM> according to embodiments of the present invention is shown. The transducer <NUM> comprises the ultrasound source <NUM> and cavity <NUM>. The cavity <NUM> is bounded by the membrane <NUM> at a first end <NUM>, by a second end wall <NUM> at a second end <NUM>, and by a side wall <NUM>. The cavity <NUM> is formed by bonding the second end wall <NUM> to the side wall <NUM> and is capable of supporting a standing wave as described hereinbefore. The direction R of ultrasound emission is away from the membrane <NUM> from the side of the membrane <NUM> which is furthest from the second end wall <NUM>. The membrane <NUM> is supported at its edges by a second side wall <NUM> which extends in a direction away from the membrane <NUM> in the z-direction and on the side of the membrane <NUM> which is furthest from the second end wall <NUM>. The second side wall <NUM> and the membrane <NUM> form boundaries for an open cavity <NUM>, the open cavity <NUM> being closed by the membrane <NUM> at a first end <NUM> and being open at a second end <NUM> opposite the first end <NUM> in the z direction. By providing the second side wall <NUM> and by providing the cavity <NUM> by bonding the second end wall <NUM> over the membrane <NUM> and means <NUM>, the membrane <NUM> and means <NUM>, which may comprise fragile components, can be protected from damage. Furthermore, such a configuration allows for greater freedom in choosing the cavity dimensions during fabrication.

Referring to <FIG>, in an alternative transducer <NUM> according to embodiments of the present invention, the at least one membrane <NUM> comprises two adjacent membranes <NUM><NUM>, <NUM><NUM>, and corresponding means <NUM><NUM>, <NUM><NUM> for causing the corresponding membrane to vibrate. The membranes <NUM><NUM>, <NUM><NUM> are capable of emitting ultrasound waves into the same cavity <NUM>. The membranes <NUM><NUM>, <NUM><NUM> are adjacent to each other in the x-y plane and may have the same or different coordinate in the z direction, that is, the cavity length L may be the same or different at each membrane, and at least two corresponding means for causing a corresponding membrane to vibrate. The cavity <NUM> is capable of supporting a standing wave which is generated by at least one single membrane or by both membranes <NUM><NUM>, <NUM><NUM>. The generated standing waves by each membrane are independent from each other and can have different resonance frequencies, which is dependent to the resonance frequency of the corresponding membrane. The side wall <NUM> of the alternative transducer <NUM> provides first and second open cavities <NUM><NUM>, <NUM><NUM> which help to protect the ultrasound source <NUM> from damage as described hereinbefore.

In some embodiments, three or more membranes which provide ultrasound waves into the same cavity may be comprised in a transducer. The plurality of membranes may be arranged, for example, in an array configuration.

Although in <FIG> the multiple membrane configuration is shown in a transducer arrangement which includes an open cavity, embodiments of the present invention include a multiple membrane configuration without also providing the open cavity, for example an arrangement similar to that shown in <FIG> wherein multiple membranes and means are provided for the same cavity.

Embodiments of the present invention provide an array made up of a plurality of transducers as described hereinbefore. Such an array may be a one-dimensional array (i.e. a line) of transducers, which is suitable for obtaining a two-dimensional ultrasound image. Such an array may be two-dimensional array of transducers, which is suitable for obtaining a three-dimensional ultrasound image.

Use of an ultrasonic transducer or transducer array as described herein may comprise placing the at least one membrane in contact with a liquid or gel as a transmission medium for ultrasonic waves generated by the transducer or transducer array.

An ultrasonic transducer as described hereinbefore in relation to <FIG> may be fabricated according to the following method. First, a substrate is provided. Then, the cavity is formed in the front side of the substrate, for example by deep reactive ion etching or wet etching on top side of the substrate. Before or after realizing the cavity, the electrically conductive layers and piezo layer are deposited on the membrane, forming the ultrasound source having a closed cavity.

An ultrasonic transducer as described hereinbefore in relation to <FIG> may be fabricated according to the following method. First, a substrate is provided. Then, the cavity is formed in the back side of the substrate, for example by deep reactive ion etching or wet etching on back side of the substrate. Before or after realizing the cavity, the electrically conductive layers and piezo layer are deposited on the membrane on top side of the substrate, forming the ultrasound source. Then, a second layer or wafer or substrate is bonded or attached by means of epoxies on the second end of the cavity forming the closed cavity.

An ultrasonic transducer as described hereinbefore in relation to <FIG> and <FIG> may be fabricated according to the following method. First, a substrate or wafer is provided. Then, the open cavity/cavities are formed in the back side of the wafer, for example by deep reactive ion etching, which is preferable for silicon or silicon-on-insulator substrates/wafers, or wet etching. Before or after realizing the cavity, the electrically conductive layers and piezo layer are deposited on the membrane on the top side of the membrane, forming the ultrasound source <NUM>. Next, the side wall of the closed cavity is provided on another wafer or substrate, for example by depositing a thick electrically conductive layer, glass layer, and/or silicon layer. Or the cavity and sidewalls can all be formed by bonding a second wafer (substrate) to the first one, while the cavity is already formed in the second substrate, e.g. by wet etching or deep reactive ion etching to a glass or silicon wafer. Finally, the cavity may be closed by bonding the second end wall to the side wall of the cavity or in which the side walls are already formed by etching the second substrate.

Referring to <FIG> (cross-sectional side view of transducer) and <FIG> (top view of the transducer in <FIG>), a method of fabricating a transducer <NUM> according to embodiments of the present invention comprising a microchannel M in a side wall <NUM> of the transducer next to the membrane <NUM> comprises the step of, before providing the membrane <NUM> on the substrate or wafer, performing an additional etching step to form the microchannel M on the front side of the substrate or wafer, the microchannel M extending outwards <NUM> from the cavity. The membrane <NUM> is then provided by bonding a second wafer to the top of the substrate and optionally thinning the second wafer to an appropriate thickness. Finally, a further etching step is performed in the second wafer to form an opening for the channel.

The cavity <NUM> may alternatively be provided by an etching process on the backside of the substrate wafer. The cavity <NUM> is realized by etching through the backside of a SOI wafer. The cavity <NUM> is etched to a depth such that the remaining layer of wafer at the closed end of the cavity has a thickness suitable for providing a membrane <NUM>. Then the second end of the cavity is sealed by bonding a layer to the second end of the cavity <NUM>. The microchannel is then provided on the front side of the wafer by an etching process.

The cavity <NUM> can be realized by backside DRIE process on a SOI wafer. Then the cavity <NUM> should be sealed by a second layer. The micro channel should be realized on the front side of the SOI wafer by a DRIE process through membrane/BOX/and if needed the handle layer of the SOI wafer (BOX is the buried oxide layer and can be removed by another method than DRIE). Then the microchannel M, except the opening <NUM>, should be sealed by some surface micro-machining techniques.

Referring to <FIG> (cross-sectional view of transducer) and <FIG> (top view), a method of fabricating a transducer <NUM> according to embodiments of the present invention comprising a microchannel M in a side wall <NUM> of the transducer and away from the membrane <NUM> in the z direction comprises the steps of providing a wafer; forming the cavity in the backside of the wafer by an etching process; forming the microchannel in the backside of the wafer by an etching process; and closing the second end of the cavity by bonding a layer at the second end of the cavity.

Referring to <FIG>, a method of fabricating a transducer <NUM> according to embodiments of the present invention comprising a microchannel M in the second end wall <NUM> of the transducer may comprise the following steps: first, a wafer is provided; then, the cavity <NUM> is formed by an etching process on the front side of the wafer; next, a second wafer is bonded to the front side of the wafer for providing the membrane and optionally is thinned to an appropriate thickness. The microchannel M is then formed on the backside of the wafer by an etching process.

Referring to <FIG>, an method of fabricating a transducer <NUM> according to embodiments of the present invention comprising a microchannel M in the second end wall of the transducer may comprise the following steps: first, a wafer is provided; then the cavity is formed by an etching process at the backside of the wafer; next, the second end of the cavity is sealed by bonding or adhering a layer on the backside of the wafer; then, the microchannel is formed in the sealing layer <NUM>, for example by an etching process e.g. if the sealing layer <NUM> comprises silicon, or by laser cutting e.g. if the sealing layer comprises e.g. steel or ceramic, or by any other suitable cutting method.

Referring to <FIG>, there is demonstrated a cavity <NUM> according to embodiments of the present invention, wherein the cavity <NUM> has a cross-sectional dimension, measured in the x-y plane, which varies in the z-direction. According to a preferred embodiment of the invention, the cross-sectional dimension of the cavity <NUM> may not have more than <NUM>% variation in comparison with a reference cross-sectional dimension, wherein the reference cross-sectional dimension corresponds with the cross-sectional dimension, measured in the x-y plane, of the membrane <NUM>. The Dashed lines refer to the fabrication method as Figs. 11a or <FIG>, which show that the bottom part can be unified with the whole body or can be connected later as a separate layer.

It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope and technical teachings of this invention. For example, any formulas given above are merely representative of procedures that may be used.

Claim 1:
An ultrasonic transducer multilayer structure (<NUM>,<NUM>',<NUM>,<NUM>,<NUM>), comprising :
- a semiconductor layer stack (<NUM>,<NUM>,<NUM>) defining a diode (<NUM>),
- a micro-machined ultrasonic transducer, MUT, layer stack being electrically in series with said diode and disposed at least partly on said semiconductor layer stack, characterized in said MUT layer stack comprising an electrically conductive layer sandwiched between said semiconductor layer stack and the MUT layer stack to provide electrical contact with said diode and the MUT layer stack,
- a cavity (C1,C2,C3,<NUM>) extending over a region (R1,R2,R3,R4) comprising at least a portion of said semiconductor layer stack and said electrically conductive layer,
wherein said MUT layer stack comprises a membrane extending at least partly over said region.