High frequency piezocomposite with triangular cross-sectional shaped pillars

A transducer with triangular cross-sectional shaped pillars is described for suppressing lateral modes within a composite, and a method for producing the same. According to one aspect of the present application, a plurality of triangular cross-sectional shaped pillars extends outwardly from a substrate and form an array of pillars. The resulting array of pillars is configured to suppress the lateral modes of the transducer at higher operating frequencies, such as, at or above 15 MHz, at or above 20 MHz, or at or above 30 MHz.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to piezoelectric composites, and more particularly to piezoelectric composites for high-frequency ultrasound applications and methods of manufacturing such composites.

2. Background Art

Typically, high quality medical imaging uses ultrasonic transducers or transducer arrays that posses the properties of good sensitivity and wide frequency bandwidth. Conventional transducers utilizing monolithic piezoelectric material such as, for example, lead zirconate titanate (“PZT”), typically exhibit a large acoustic impedance mismatch between the transducer and the medium under test, such as, for example, water, human tissue, and the like. To overcome this problem, piezoelectric composites that are made of individual small piezoelectric elements, which can be surrounded and isolated by a polymer matrix, such as, for example, epoxy, have been proposed and implemented at low frequencies. These small piezoelectric elements play an increasingly important role in the development of ultrasonic transducers for medical imaging. One commonly used structure of piezoelectric composite consists of small rectangular or square pillars of PZT that are embedded in a host matrix of polymer material. In one example, the height of the pillars is normally about one half of one wavelength at the operating frequency if the backing material is lower in acoustic impedance.

Unfortunately, developing a high-frequency (>15 Hz) ultrasound transducer is also very challenging due to the extremely small pillar dimensions required in order to avoid significant lateral resonances in the piezoelectric composite. Conventionally, the design of piezo-composites is limited by the blade size limit of micro-dicing saws or other conventional apparatuses that are used to cut the bulk piezoelectric into composite pillars. It is very difficult using conventional dice and fill techniques to sufficiently reduce the size/spacing of the composite pillars enough to push the lateral resonances outside the operating bandwidth of a transponder that is configured to operate at high frequencies. For example, to push the first “lamb mode” frequency to about 80 MHz, while still maintaining a volume fraction of piezoelectric above 25%, a kerf width of approximately 6 μm is required (assuming a typical piezoelectric and epoxy filler). What is needed is a high-frequency ultrasound transducer that operatively suppresses these lateral modes within the piezoelectric composite.

In a further aspect, a lens is typically used to passively focus high-frequency ultrasound transducers. Developing a suitable acoustic lens, however, can be very challenging because the lens materials commonly used for lower frequency transducers are far too attenuating at frequencies at higher frequencies. Alternatively, the need for an acoustic lens can be avoided by geometrically curving the transducer. This can be accomplished by using a flexible piezo-composite material as the transducer substrate.

SUMMARY

In one aspect, the present application provides a transducer with triangular cross-sectional shaped pillars for suppressing lateral modes within a piezoelectric composite, and a method for producing the same.

A substrate having a longitudinal axis is provided. According to one aspect, a plurality of pillars is formed that extend outwardly from the substrate. In this aspect, the plurality of pillars can be positioned in adjacent rows that extend substantially parallel to the longitudinal axis of the substrate, forming an array of upright pillars. In one embodiment, each pillar can have a triangular cross-sectional shape formed from a pair of side walls and a base.

In one aspect, the array of pillars can comprise a plurality of paired pillars. In this aspect, each of the paired pillars comprises a first pillar positioned adjacent to a second pillar such that a base of the first pillar is spaced from and substantially opposes a base of the second pillar. Further, each row of the array of pillars can comprise a plurality of paired pillars that are positioned adjacent each other such that one side wall of the first pillar is spaced from and substantially opposes one side wall of the second pillar. In this embodiment, the triangular pillars and arrangement thereof are configured to operatively suppress the lateral modes of the transducer at higher operating frequencies, such as, at or above 15 MHz, at or above 20 MHz, or at or above 30 MHz.

In another aspect, a method of producing an ultrasonic wave emission pattern at higher operating frequencies, such as, at or above 15 MHz, at or above 20 MHz, or at or above 30 MHz, is provided. In this aspect, an electric signal can be applied to a piezoelectric substrate of a transducer, which has a plurality of triangular cross-sectional shaped pillars extending outwardly therefrom the substrate. The plurality of pillars can be positioned in rows substantially parallel to a longitudinal axis of the substrate to form an array of pillars. In one aspect, the array of pillars can comprised a plurality of paired pillars in which each of the paired pillars comprises a first pillar positioned adjacent to a second pillar such that a base of the first pillar is spaced from and substantially opposes a base of the second pillar. In a further aspect, it is contemplated that each row of the array of pillars can comprise a plurality of paired pillars that are positioned adjacent each other such that a side wall of the first pillar is spaced from and substantially opposes a side wall of the second pillar.

Additional advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be understood more readily by reference to the following detailed description, examples, drawing, and claims, and their previous and following description. However, before the present devices, systems, and/or methods are disclosed and described, it is to be understood that this invention is not limited to the specific devices, systems, and/or methods disclosed unless otherwise specified. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.

As used throughout, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a pillar” can include two or more such pillars unless the context indicates otherwise.

As illustrated inFIGS. 1 and 2A, in one embodiment, a transducer10of the present application can comprise a substrate20and a plurality of pillars30extending outwardly from the substrate. The substrate has an upper surface21and a longitudinal axis. In one aspect, the substrate can be rigid. In another aspect, however, the substrate can be flexible. In yet another aspect, at least a portion of the substrate of the transducer can have a substantially planar cross-sectional shape. Optionally, it is contemplated that at least a portion of the transducer can have a curved or arcuate cross-sectional shape.

It is contemplated that the substrate can be formed from any desired material having the appropriate electrical and acoustical properties, as commonly known in the art. In one aspect, the substrate20can be formed from an electrostrictive material. In another aspect, the substrate can be formed from a piezoelectric material such as, for example and without limitation, lead zirconate titanate. In still another aspect and not meant to be limiting, the substrate can be formed from a single-crystal piezoelectric.

Referring toFIG. 2B, one skilled in the art will appreciate that it is contemplated that the solid thin layer of monolithic substrate20can be conventionally lapped or ground off in a final fabrication step. Thus, in this aspect, when this thin layer of solid substrate20is removed, the pillars extend substantially completely through the formed composite from top to bottom. In this example, the composite becomes a conventional 1-3 composite.

Optionally and referring toFIG. 2A, if the thin strip of substrate20is retained at the base of the pillars allows for ease in defining the array electrodes with precision, which allows for ease in operably connecting to the electrodes. Thus, in this aspect, if the thin strip of substrate20is retained, the composite formed in a 3-2 composite. One skilled in the art will appreciate that the exemplary 3-2 composite allows for precision definition of array electrodes due to the smooth and continuous bottom surface of the substrate20, for example and without limitation, by using conventional photolithography techniques and also for ease of connectivity to the electrodes, for example and without limitation, by using conventional wire-bonding techniques. In another aspect, the exemplary substrate can be configured to be flexible such that the composite structure can be curved without fracturing the monolithic layer. Optionally, the monolithic layer can have a thickness that is configured to be thin enough in order to be able to curve the composite without fracturing the monolithic layer. In various examples, it is contemplated that the thickness of the monolithic layer (labelled as “20” inFIG. 2A) can be less than about 15 μm; alternatively less than about 10 μm; and optionally less than about 7 μm.

The pillars extend outwardly from the substrate20. In one aspect, and as illustrated inFIGS. 1 and 3A, each pillar can have a triangular cross-sectional shape that has an apex32, an opposed base34having opposed edges, and a pair of side walls36that extend from the apex to the respective edges of the base. In one exemplary aspect, each of the triangular pillars has an isosceles shape. Alternatively, each of the triangular pillars can have a right angle shape. Optionally, it is also contemplated that the pillars can have any generally triangular shape.

The plurality of pillars can be formed from any desired material having the appropriate electrical and acoustical properties, as commonly known in the art. In one aspect, the pillars30can be formed from an electrostrictive material. In another aspect, the pillars can be formed from a piezoelectric material such as, for example and without limitation, lead zirconate titanate. In still another aspect, the pillars can be formed from a single-crystal piezoelectric. In one exemplary aspect, the plurality of pillars can be formed from the same material as the substrate20. Optionally the substrate and the pillars can be formed from a single-crystal piezoelectric.

In a further aspect, it is contemplated that each pillar extends substantially the same height (h) from the upper surface21of the substrate20. Alternatively, it is contemplated that the pillars may vary in height. In various examples, it is contemplated that the height can range from between about 5 μm to 150 μm; alternatively from between about 20 μm to 70 μm; and optionally from between about 40 μm to 50 μm.

In a further aspect, it is contemplated that each pillar can have a width (w) from the base to the apex of the triangular shaped pillar that ranges from between about 40 μm to 50 μm; alternatively from between about 20 μm to 60 μm; and optionally from between about 5 μm to 70 μm.

According to one aspect, the plurality of pillars30can form an array of pillars positioned in adjacent rows40, as illustrated inFIG. 1. In one aspect, each row of the array of pillars can extend substantially parallel to the longitudinal axis of the substrate20. Referring toFIG. 3A, in another aspect the array of pillars can comprise a plurality of paired pillars. In this exemplary aspect, each of the paired pillars can comprise a first pillar42positioned adjacent to a second pillar44such that the base34of the first pillar is spaced from and substantially opposes the base of the second pillar. Further, in this exemplary aspect, each row40of the array of pillars can comprise a plurality of paired pillars that are positioned adjacent each other such that one side wall36of the first pillar42is spaced from and substantially opposes one side wall of the second pillar44.

As exemplarily illustrated inFIGS. 1 and 3A, in one aspect, the array of pillars can define a plurality of first troughs or kerfs50that extend substantially parallel to the longitudinal axis of the substrate20and a plurality of second troughs or kerfs52that extend substantially transverse to the longitudinal axis of the substrate. Further, the array of pillars can define a plurality of third troughs or kerfs54that extend at an acute angle θ relative to the longitudinal axis of the substrate. In a further aspect, it is contemplated that acute angle θ can range from between about 20° to 70°; alternatively from between about 30° to 60°; and optionally from between about 40° to 50°. In another aspect, the width of the first trough can be substantially the same as the width of the second trough, and the width of the second trough52can be substantially the same as the width of the third trough. Of course, it is also contemplated that the widths of the respective troughs or kerfs can vary in width dimension.

In a further aspect, it is contemplated that each trough or kerf has a width that ranges from between about 1 μm to 20 μm; alternatively from between about Sum to 17 μm; and optionally from between about 10 μm to 15 μm. In yet another aspect, it is contemplated that the transponder can be configured such the PZTH5H or single crystal volume fraction ranges from between about 10% to 75%; alternatively from between about 15% to 50%; and optionally from between about 20% to 30.

In yet another aspect, at least a portion of the respective first, second, and third troughs can be at least partially filled with a fill material22. The fill material can comprise, for example and without limitation, a polymeric material, such as loaded epoxy, polymer micro-spheres, crystal bond, photoresist material and the like, as is customary and standard practice in the manufacture of composite transducers. In one exemplary aspect, the fill material may be SU8 photoresist. Optionally, fill material can comprise a PZT powder. Alternatively, in one aspect, the respective first, second, and third troughs can be left, at least in part, unfilled. One will appreciate that the troughs may not be completely filled or that they may only be filled temporarily as some or the entire trough filling material can removed using conventional methods.

With reference toFIGS. 1-3Aand3C, a transducer can be fabricated to comprise any or all of the features as described above. In one aspect, the substrate20can be diced with a dicing saw. A first cutting operation can be performed into the substrate substantially parallel to the longitudinal axis of the substrate so that the plurality of first troughs50is defined in the planar upper surface21of the substrate. A second cutting operation can be performed into the substrate substantially transverse to the longitudinal axis of the substrate so that the plurality of second troughs52is also defined in the planar upper surface of the substrate20. A third cutting operation can be performed onto the upper surface of the substrate at the acute angle θ relative to the longitudinal axis of the substrate so that the plurality of third troughs50are defined in the planar upper surface21of the substrate. The cutting operations can be performed so that a plurality of pillars30extend outwardly from the substrate20as described above, forming an array of pillars.

According to various aspects, the first, second, and third troughs can then be at least partially filled with a fill material, for example and without limitation, comprising SU8 photoresist PZT powder, and the like. Next, the fill material can be cured, as is known in the arts. The substrate20, plurality of pillars and/or fill material can be ground, lapped, or otherwise removed until the desired thickness is achieved. In one exemplary aspect, the thickness of the substrate20, plurality of pillars and/or fill material can be between 10 and 100 μm. In one exemplary aspect, the thickness of the substrate, plurality of pillars30and/or fill material can be about 40-50 μm. In another aspect, the composite structure can have a fill material volume fraction of between 10% and 50%. In one exemplary aspect and in consideration of the maintenance of a desirable electromechanical coupling coefficient, the fill material volume fraction can be about 20% to 30%.

According to other aspects, at least a portion of the substrate20, plurality of pillars and/or fill material can be formed into a spherical geometry having a radius of curvature between 1 and 50 mm. In another aspect, the substrate, plurality of pillars30and/or fill material can be formed into a spherical geometry having a radius of curvature of about 9 mm. The substrate, plurality of pillars and/or fill material can then be mounted into an SMA connector using a conductive backing epoxy, as commonly known in the arts. Finally, the transducer10can be machined down to a desired diameter and a ground electrode can be evaporated onto a front face of the transducer. The desired diameter of the transducer, in one exemplary aspect, can be between 1 and 10 mm. In a further exemplary aspect, the desired diameter of the transducer can be about 3 mm.

In other embodiments and as shown inFIG. 3C, the transducer10of the current application can comprise a plurality of pillars30for emitting energy in response to an input signal. The plurality of pillars can be positioned in adjacent, substantially parallel rows, wherein each pillar has a substantially triangular cross-sectional shape for suppressing the lateral modes at the operating frequencies. Each pillar can have an apex32, an opposed base34having opposed edges, and a pair of side walls36that extend from the apex to the respective edges of the base. In this aspect, the plurality of pillars can comprise of a plurality of paired pillars, wherein each of the paired pillars comprises a first pillar42positioned adjacent to a second pillar44such that the base of the first pillar is spaced from and substantially opposes the base of the second pillar. Further, each row of the array of pillars can comprise a plurality of paired pillars that are positioned adjacent each other such that one side wall of the first pillar is spaced from and substantially opposes one side wall of the second pillar.

In this exemplary embodiment, the transducer can further comprise a substrate20that is at least partially curved. It is contemplated that the substrate and the plurality of pillars can form an electrically monolithic structure, which is configured for emitting energy in response to the input signal. The array of pillars can define a plurality of first troughs50extending substantially parallel to a longitudinal axis of the substrate, a plurality of second troughs52extending substantially transverse to the longitudinal axis of the substrate, and a plurality of third troughs54extending at an acute angle θ relative to the longitudinal axis of the substrate. In one aspect, the width of the respective first, second, and third troughs can be substantially equal. In another aspect, at least a portion of the respective first, second, and third troughs can be at least partially filled with a fill material.

In another embodiment, the transducer10of the current application can comprise a substrate20having a longitudinal axis, a plurality of pillars30extending outwardly therefrom the substrate, and a means for suppressing the lateral modes of the transducer at higher operating frequencies, such as, at or above 15 MHz, at or above 20 MHz, or at or above 30 MHz. In one aspect, at least a portion of the substrate20can be flexible. In another aspect, at least a portion of the substrate can be curved in cross-section such that the transducer is geometrically curved. In yet another aspect, the substrate and the plurality of pillars30can be formed from a single-crystal piezoelectric or from a piezoelectric material such as, for example lead zirconate titanate.

In yet another aspect, the plurality of pillars can form an array of pillars positioned in adjacent rows40, wherein each row of the array of pillars extends substantially parallel to the longitudinal axis of the substrate. In yet another aspect, the means for suppressing the lateral modes can comprise each pillar30having a triangular cross-sectional shape that has an apex32, an opposed base34having opposed edges, and a pair of side walls36that extend from the apex to the respective edges of the base. In one example, the array of pillars can define a plurality of first troughs50extending substantially parallel to the longitudinal axis of the substrate, a plurality of second troughs52extending substantially transverse to the longitudinal axis of the substrate, a plurality of third troughs54extending at about an acute angle θ relative to the longitudinal axis of the substrate, such as, for example and not meant to be limiting, a 45° angle. Optionally, a plurality of fourth troughs56can be formed that each extends substantially transverse to the third troughs.

In this embodiment, in one aspect, the width of the first trough can be substantially the same as the width of the second trough. In another aspect, the width of the second trough52can be substantially the same as the width of the third trough. In a further aspect, the width of the third trough can be substantially the same as the width of the fourth trough. In yet another aspect, at least a portion of the respective first, second, third, and fourth troughs can be at least partially filled with a fill material22. The fill material can comprise, for example, a polymeric material, such as, for example, loaded epoxy, polymer micro-spheres, crystal bond, photoresist material and the like, as is customary and standard practice in the manufacture of composite transducers, or they may be left, at least in part, unfilled. In one exemplary aspect, the fill material can be, for example and without limitation, SU8 photoresist, PZT powder, and the like.

In yet another embodiment, a method of producing an ultrasonic wave emission pattern at higher operating frequencies, such as, at or above 15 MHz, at or above 20 MHz, or at or above 30 MHz, is provided. In one aspect, the method can comprise applying an electric signal to a piezoelectric substrate of a transducer10having a plurality of pillars30extending outwardly therefrom the substrate20. The lateral modes of the transducer can be suppressed by providing each pillar30with a triangular cross-sectional shape that has an apex32, an opposed base34having opposed edges, and a pair of side walls36that extend from the apex to the respective edges of the base. In one aspect, the array of pillars can comprise a plurality of paired pillars, wherein each of the paired pillars comprises a first pillar42positioned adjacent to a second pillar44such that the base of the first pillar is spaced from and substantially opposes the base34of the second pillar. In another aspect, each row40of the array of pillars can comprise a plurality of paired pillars that are positioned adjacent each other such that one side wall of the first pillar is spaced from and substantially opposes one side wall36of the second pillar.

In this embodiment, the array of pillars can define a plurality of first troughs50extending substantially parallel to a longitudinal axis of the substrate, a plurality of second troughs52extending substantially transverse to the longitudinal axis of the substrate, and a plurality of third troughs54extending at an acute angle θ relative to the longitudinal axis of the substrate. In one aspect, the width of the respective first, second and third troughs can be substantially equal. In another aspect, at least a portion of the respective first, second, and third troughs can be at least partially filled with a fill material. In yet another aspect, the substrate can be flexible.

In use, in one aspect, the transducer10is configured, at higher operating frequencies, such as, at or above 15 MHz, at or above 20 MHz, or at or above 30 MHz, can spread the lateral energy out over a broad spectrum of frequencies. At these higher operating frequencies, the triangular cross-sectional shape of the plurality of pillars30can remove or suppress virtually all of the lateral modes within the individual pillars and can break up the pillar-to-pillar periodicities that can cause spurious modes. Further, the spread in lateral energy can also help to rapidly dampen secondary ringing in the thickness mode.

EXAMPLE

In order to demonstrate the efficacy of the transducer described herein, two sample transducers were prepared and tested, as described below. Example 1 was a transducer having a plurality of pillars having a triangular cross-sectional shape, as described herein. The triangular pillars were isosceles in shape and arranged in the pattern shown inFIG. 3A. Example 2 was a transducer having a plurality of pillars having a square cross-sectional shape, as is known in the art, and as shown inFIG. 3B. Each composite structure had 15 μm troughs, a 42 μm pillar height, and a fill material volume fraction of approximately 25%.FIGS. 3A and 3Bshow the arrangement of both composite structures as well as identifying different points on the exemplary transducers that were closely analyzed.

For each point marked on the respectiveFIGS. 3A and 3B, the displacements were simulated in all three dimensions when excited with a monocycle excitation pulse in the thickness dimension. To analyze the lateral modes, the displacements100for each point were summed in the frequency domain.FIGS. 4A and 4Bshow the displacements100summed together for the x direction. As can be seen inFIGS. 4A and 4B, the square-pillar composite design (FIG. 4B) possesses lateral displacements in much narrower bands than the triangular-pillar composite (FIG. 4A).

To evaluate the performance of both transducers, the following characteristics were measured: electrical impedance, pulse-echo response, and one-way radiation pattern. The electrical impedance magnitude at 40 MHz was measured to be 9 ohms for the square-pillar composite and 11 ohms for the triangular cross-sectional shaped pillar composite. The −6 dB pulse echo bandwidths were measured to be 20% for the square-pillar composite and 55% for triangular cross-sectional shaped pillar composite. The pulses are shown in the form of oscilloscope screen captures inFIGS. 5A and 5B. The pulse echoes were generated by situating a quartz flat in front of the transducers at the geometric focus. The pulse amplitude for the square-pillar composite was measured to be 5.5 dB more sensitive than the triangular cross-sectional shaped pillar composite. The peak insertion losses of the two composites were measured to be approximately −25 dB for the square-pillar and −31 dB for the triangular cross-sectional shaped pillar composite. The one-way radiation pattern for the triangular cross-sectional shaped pillar composite was then measured by scanning a needle hydrophone in front of the transducer in all three dimensions.FIG. 7shows the resulting radiation pattern in two perpendicular planes (x, y planes). The average −3 dB beamwidth was measured to be 120 μm and the −3 dB depth-of-field was measured to be 2.5 mm.