Patent Publication Number: US-10770058-B2

Title: Acoustic lens for micromachined ultrasound transducers

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/353,720, allowed, which is a continuation of U.S. patent application Ser. No. 14/205,123, filed on Mar. 11, 2014, now U.S. Pat. No. 9,502,023, entitled “ACOUSTIC LENS FOR MICROMACHINED ULTRASOUND TRANSDUCERS,” which claims benefit of U.S. Provisional Patent Application Ser. No. 61/793,124, filed on Mar. 15, 2013, and entitled “ACOUSTIC LENS FOR MICROMACHINED ULTRASOUND TRANSDUCERS,” both of which are hereby incorporated herein in their entireties by reference. 
    
    
     TECHNICAL FIELD 
     The disclosed technology relates generally to ultrasound transducers, and more specifically matching layers for ultrasound transducers. 
     BACKGROUND 
     In ultrasound imaging devices, images of a subject are created by transmitting one or more acoustic pulses into the body from a transducer. Reflected echo signals that are created in response to the pulses are detected by the same or a different transducer. The echo signals cause the transducer elements to produce electronic signals that are analyzed by the ultrasound system in order to create a map of some characteristic of the echo signals such as their amplitude, power, phase or frequency shift etc. The map therefore can be displayed to a user as images. 
     One class of transducer is a Micromachined Ultrasound Transducer (MUT), which can be fabricated from, for example, silicon and configured to transmit and receive ultrasound energy. MUTs may include Capacitive Micromachined Ultrasound Transducer (CMUTs) and Piezoelectric Micromachined Ultrasound Transducer (PMUTs). MUTs can offer many advantages over other conventional transducers such as, for example, lower cost of production, decreased fabrication time, and/or wider frequency bandwidth. MUTs, however, can be fragile and are typically utilized in single-use internal ultrasound imaging applications. 
     The use of a transducer in an external probe generally involves bonding or otherwise attaching an acoustic lens to the transducer. The acoustic lens can protect the transducer from damage and/or may also provide acoustic focusing into a subject. In some low frequency applications, MUTs may be utilized in external probes having acoustic lenses made from, for example, an elastomer material. However, these elastomer lenses may not be suitable for high frequency ultrasound applications (e.g., greater than about 15 MHz) due to, among other reasons, increased acoustic attenuation of the materials at the higher frequencies. Accordingly, a need for a low-loss and durable acoustic lens exists for an external MUT probe suitable for use at higher frequencies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side schematic view of a prior art Capacitive Micromachined Ultrasound Transducer. 
         FIG. 2A  is an isometric front view of an ultrasound transducer stack configured in accordance with one or more embodiments of the disclosed technology. 
         FIG. 2B  is an enlarged side view of an ultrasound transducer stack configured in accordance with an embodiment of the disclosed technology. 
         FIGS. 3A and 3B  are side views of an ultrasound transducer stack configured in accordance with an embodiment of the disclosed technology. 
         FIGS. 4A and 4B  are side views of an ultrasound transducer stack configured in accordance with an embodiment of the disclosed technology. 
     
    
    
     DETAILED DESCRIPTION 
     The present technology is generally directed to matching layers configured for use with ultrasound transducers. It will be appreciated that several of the details set forth below are provided to describe the following embodiments in a manner sufficient to enable a person skilled in the relevant art to make and use the disclosed embodiments. Several of the details described below, however, may not be necessary to practice certain embodiments of the technology. Additionally, the technology can include other embodiments that are within the scope of the claims but are not described in detail with reference to  FIGS. 1-4B . 
     Capturing ultrasound data from a subject using an exemplary transducer stack generally includes generating ultrasound, transmitting ultrasound into the subject, and receiving ultrasound reflected by the subject. A wide range of frequencies of ultrasound may be used to capture ultrasound data, such as, for example, low frequency ultrasound (e.g., less than 15 MHz) and/or high frequency ultrasound (e.g., greater than or equal to 15 MHz) can be used. Those of ordinary skill in the art can readily determine which frequency range to use based on factors such as, for example, but not limited to, depth of imaging and/or desired resolution. 
     The disclosed transducers can be operatively connected to an ultrasound imaging system for the generation, transmission, receipt, and processing of ultrasound data. For example, ultrasound can be transmitted, received and processed using an ultrasonic scanning device that can supply an ultrasonic signal of any frequency. In some aspects of the present disclosure, an ultrasound system or device capable of operating at 15 MHz or above can be used, while in other aspects ultrasound systems or devices configured to operate below 15 MHz may also be used. While the transducers disclosed below may be used in ultrasonic medical measurement and/or imaging applications, they are not limited to such uses. In some embodiments, for example, the transducers below can be used in biometric applications, such as, for example, fingerprint scanners. 
       FIG. 1  is a side view of an ultrasound transducer  100  configured in accordance with an embodiment of the disclosed technology. The transducer  100  includes an electric power source  101  coupled to a top electrode  104  and a bottom electrode  102  deposited on a substrate  114  (e.g., a silicon substrate). The top electrode  104  is coupled to or otherwise adjacent to a membrane  108 . As explained in further detail below, the top electrode  104  can be configured to cause deflections in the membrane  108 , which can, for example, cause an ultrasound wave to propagate therefrom. A gap  110  allows the membrane  108  to deflect sufficiently downward (e.g., toward the bottom electrode  102 ) without coming into contact with the bottom electrode  102  and/or the substrate  114 . The membrane  108  may deflect in response to, for example, a change in voltage between the bottom and top electrodes  102  and  104  and/or acoustic energy (e.g. ultrasound waves) incident on the membrane  108 . 
     In the illustrated embodiment, the transducer  100  is configured as a Capacitive Micromachined Ultrasound Transducer (CMUT). As those of ordinary skill in the art will appreciate, a bias voltage may be applied by the power source  101  to the top electrode  104  and the bottom electrode  102 . The power source  101  can include an alternating current source and/or a direct current source (not shown). Acoustic energy (e.g., ultrasound waves) striking the transducer  100  can deflect the membrane  108 , causing variations in the voltage between the top and bottom electrodes  102  and  104  to generate an electric signal. Conversely, applying an alternating current signal between the top and bottom electrodes  102  and  104  can deflect the membrane  108  to generate an ultrasound signal that can propagate away from the transducer  100 . 
       FIG. 2A  is a isometric front view of an ultrasound transducer stack  200  configured in accordance with embodiments of the disclosed technology. The transducer stack  200  includes a transducer layer  201  below a first layer  220  and a second layer  224 . The transducer  201  may comprise, for example, a single array element, one-dimensional array of transducer elements, or a multi-dimensional array of transducer elements. Moreover, the transducer  201  may be made from any suitable transducer known in the art, such as, for example, piezoelectric transducers, CMUTs, piezoelectric micromachined ultrasound transducers (PMUTs), etc. A transducer top surface  203  underlies a first layer bottom surface  223 , and a first layer top surface  221  underlies a second layer bottom surface  225 . A second layer top surface  226  can be applied to or placed proximate to a subject (e.g., a human, an animal, etc.). 
       FIG. 2B  is a side view of the transducer stack  200  configured in accordance with an embodiment of the disclosure. In the illustrated embodiment, the transducer layer includes a CMUT (e.g., the transducer  100  of  FIG. 1 ). As those of ordinary skill in the art will appreciate, CMUT transducers (e.g., the transducer  201 ) can be fragile and may lack the durability of other types of transducers (e.g., PZT transducers). However, placing a low acoustic loss and durable stiff material directly onto a CMUT transducer can significantly reduce the efficiency of the array and may prevent the array from even functioning. Accordingly, as described in further detail below, assembling an ultrasound transducer stack with a relatively thin compliant layer (e.g., the first layer  220 ) between the lens (e.g. the second layer  224 ) and the transducer (e.g., transducer  201 ) allows the transducer to emit ultrasound efficiently while also bonding the transducer  201  to the second layer  224 . Furthermore, bonding a stiff outer layer can also maintain the flatness of the transducer, thereby improving transducer efficiency and accuracy. 
     Referring to  FIGS. 2A and 2B  together, the first layer  220  is made from a compliant material (e.g., a PDMS-type silicone) configured to join or otherwise couple the second layer  224  to the transducer  201 . In other embodiments, however, the first layer  220  can be made from, for example, any material suitable for matching layers known in the art, such as, for example, an elastomer, a gel, a polymerized material, etc. In further embodiments, the first layer  220  may be made from any suitable fluid such as, for example, water, an oil, etc. In some embodiments, for example, the first layer  220  may be a thin, low-stiffness layer with a low Young&#39;s modulus, (e.g., less than 100 MPa). 
     As shown in  FIGS. 2A and 2B , the second layer  224  can be configured as an acoustic window disposed on or near the first layer  220 . As those of ordinary skill in the art will appreciate, the second layer  224  can be a low acoustic loss, durable layer and can robustly protect the transducer  201  from impacts and/or exposure to contaminants while also protecting a subject (e.g., a human patient, a small animal, etc.) from a excessive heat and/or charge produced by the transducer  201 . The second layer  224  may be made from any lens material known in the art suitable for use with ultrasonic imaging such as, for example, a plastic, a plastic composite, a polymer, etc. In some embodiments, for example, the second layer  224  may be made from a thermoset cross-linked styrene copolymer (e.g., Rexolite) and/or polymethylpentene (e.g., TPX). 
     While the illustrated embodiment of  FIGS. 2A and 2B  is shown with only the first layer  220  and the second layer  224 , more than two layers may be alternatively utilized in accordance with the disclosed technology. In some embodiments, for example, at least a third layer (not shown) with an acoustic impedance approximately between a first acoustic impedance of the first layer and a second acoustic impedance of the second layer. In other embodiments, for example, a thin composite of layers (not shown) can be used or incorporated into the transducer  200  between the first layer  220  and second layer  224 . The layers within the composite of layers can include acoustic impedances that gradually change (e.g., increasing or decreasing) from the first acoustic impedance to the second acoustic impedance to improve the acoustic impedance matching between the first layer  220  and the second layer  224 . 
     In some embodiments, the first layer  220  may have a thickness less than ¼ wavelength of a ultrasound frequency range of interest. In other embodiments, however, the first layer  220  can have a thickness any suitable fraction (e.g., 1/1, ½, ¼, ⅛, etc.) of the wavelength of ultrasound frequency of interest. In some embodiments, for example, the thickness of the first layer  220  may be chosen to be suitably thin to reduce attenuation through the first layer  220  while having a suitable thickness to allow the movement of the membrane of the transducer  201  and without being inhibited by the second layer  224 . Moreover, in the illustrated embodiment, the second layer  224  is shown having a second thickness T 2  greater than a first thickness T 1  of the first layer  220 . In other embodiments, however, the first thickness T 1  may have a thickness equal to and/or greater than the second thickness T 2 . In some further embodiments, the first layer  220  can have varying thickness based upon, for example, performance characteristics (e.g., MUT membrane thickness, cell structural characteristics, etc) and/or frequency of ultrasound to be emitted from the transducer  201 . In some embodiments, for example, the second layer  224  can be configured to be removably attached to the first layer  220  such that a plurality of different layers  224  (not shown) can be attached to the transducer  201  and the first layer  220 . 
     As those of ordinary skill in the art will appreciate, directly bonding the second layer  224  to the transducer  201  may prevent or reduce the emission of ultrasound energy from the transducer  201 . For example, disposing the second layer  224  in direct or near contact with the transducer  201  could significantly impede the movement of the membrane  108  ( FIG. 2A ) in response to changes in the alternating current in the array. Accordingly, placing the first layer  220  (e.g., a layer made from a compliant material) between the transducer  201  and the second layer  224  can allow movement of the membrane  108  while also improving an acoustic impedance match therebetween. In some embodiments, the first layer  220  may also be configured to adhere, bond, or otherwise couple the second layer  224  to the transducer  201 . 
     In some other embodiments, for example, the transducer stack  200  can include additional layers. For example, an interstitial layer between the first layer  220  and the transducer  201  can include a thin material or a coated substance on the transducer  201  that can protect the transducer  201  from corrosion while being sufficiently thin to not significantly affect the performance of the transducer  201 . The first layer  220  can comprise, for example, water or any other suitable liquid having an acoustic impedance that is relatively similar to the acoustic impedance of the second layer  224 . 
     As those of ordinary skill in the art will appreciate, an acoustic impedance mismatch between the first layer  220  and the second layer  224  may cause reverberation and/or ring echoes. One way to reduce the impact of acoustic impedance mismatches is to create a textured interface between the first layer  220  and the second layer  224 , as shown in, for example,  FIG. 3A . Another way to reduce the impact of acoustic impedance mismatches is to configure and/or select the first layer  220  to have an acoustic impedance at least generally close to the acoustic impedance of the second layer  224  or vice versa. For example, one approach is to form the first layer  220  from a composite material that has similar acoustic impedance as the material from which the second layer  224  is formed. One of ordinary skill in the art will know that adding particles of a more dense material to the chosen first layer material can increase the density of the resulting composite and therefore the acoustic impedance as well. For example, in one embodiment, sub-micron particles can be doped into the first layer  220  to increase or decrease the mass or otherwise vary the density of the first layer  220  to be as matched as closely as possible to the second layer  224 . In some other embodiments, for example, the first layer  220  can be doped with a plurality of micron-sized particles and a plurality of nano-sized particles. For example, the first layer  220  can include a low stiffness compliant material, such as, for example, silicone, and a micron-sized powder can be added to the silicone. However, merely adding the micron-sized particles to the first layer  220  may cause the micron-sized powder to settle at the bottom of the first layer  220 . Accordingly, a second powder with nano-sized particles can be added to the first layer  220  to fill in the spaces between the various micron-sized particles. This is described in further detail below with reference to  FIGS. 4A and 4B . 
       FIG. 3A  is a side view of a transducer stack  300 , configured in accordance with an embodiment of the disclosure. A first layer  320  couples a second layer  324  to the transducer  201 . A textured surface  325  of the second layer  324  overlies and/or contacts a top surface  321  of the first layer  320 , and includes a plurality of grooves  322  and a plurality of ridges  323 . The ridges  323  are configured to extend into a portion of the thickness of the first layer  320 . In the illustrated embodiment, the grooves  322  and the ridges  323  extend longitudinally straight across the second layer  324 . In other embodiments, however, the grooves  322  and the ridges  323  may have other patterns, such as, for example, helical, diagonal, zig-zag, etc. 
     As those of ordinary skill in the art will appreciate, the textured surface  325  can reduce acoustic impedance mismatches in the transducer stack  300  by providing a graduated interface between the first layer  320  and the second layer  324 . The textured surface  325  may also, in some embodiments, improve adhesion between the first layer  320  and the second layer  324 . As described above with reference to the first layer  220 , the first layer  320  can be made from, for example, a compliant material, such as, for example, silicone or another suitable material with a low stiffness that can be bonded to the transducer  201 . 
       FIG. 3B  is a side view of a transducer stack  301 , configured in accordance with an embodiment of the disclosure. A first layer  340  couples a second layer  344  to the transducer  201 . A bottom surface  345  of the second layer  344  overlies and/or contacts a top surface  342  of the first layer  340 . The bottom surface  345  includes a plurality of peaks  346  and a plurality of troughs  347  with a plurality of grooves  348  formed therebetween. In the illustrated embodiment, the troughs  347  extend from the second layer  344  into the first layer  340  is approximately equal to the thickness of the first layer  340 . In other embodiments, however, the troughs  347  may only extend a portion of the thickness of the first layer  340 . 
       FIG. 4A  is a side view of a transducer stack  400  configured in accordance with an embodiment of the present disclosure.  FIG. 4B  is an enlarged view of a portion of  FIG. 4A . Referring to  FIGS. 4A and 4B  together, the transducer stack  400  includes a first layer  420  between the transducer  201  and the second layer  224 . As shown in  FIG. 4A , the first layer  420  can have top surface underlying a bottom surface of the second layer  224  (e.g., an acoustic lens or window). The first layer can also have a bottom surface overlying a top surface (e.g., the membrane  108  and/or the top electrode  104  of  FIG. 1 ) of the transducer  201 . The first layer  420  can be a matching layer configured for use with ultrasound having a matrix material  427  doped or filled with a first set of particles  428  (hereinafter “first particles”) and a second set of particles  429  (hereinafter “second particles”). For example, in some embodiments, the matrix material  427  can be made of a compliant material (e.g., a PDMS-type silicone, an elastomer, a fluid, and/or any suitable low-stiffness material having a relatively low Young&#39;s Modulus (e.g., less than 100 MPa)), and the transducer  201  can be configured as a CMUT transducer as described with reference to  FIG. 1 . In other embodiments, however, the matrix material  427  may be made of an epoxy or other suitable material and the transducer  201  may be configured as a PZT transducer configured for use with ultrasound. 
     As explained in more detail in the U.S. Pat. No. 8,343,289, incorporated herein by reference in its entirety, the first particles  428  and/or the second particles  429  can be separately selected based on desired operating parameters (acoustic impedance, acoustic attenuation, electrical conductivity, density etc.) of the first layer  420 . In some embodiments, for example, the first particles  428  may comprise micron-sized particles (e.g., greater than or equal to one micron) of a suitable first metal (e.g., tungsten, gold, platinum, alloys thereof, and/or a mixture thereof) and the second particles  429  may comprise nano-sized particles (e.g., less than one micron) of the first metal or a suitable second metal (e.g, tungsten, gold, platinum, etc.). In other embodiments, for example, the first particles  428  and the second particles  429  may be made from the same material. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or,” in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
     The above Detailed Description of examples of the disclosed technology is not intended to be exhaustive or to limit the disclosed technology to the precise form disclosed above. While specific examples for the disclosed technology are described above for illustrative purposes, various equivalent modifications are possible within the scope of the disclosed technology, as those skilled in the relevant art will recognize. For example, while processes or blocks are presented in a given order, alternative implementations may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative or subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed or implemented in parallel, or may be performed at different times. Further any specific numbers noted herein are only examples: alternative implementations may employ differing values or ranges. 
     The teachings of the disclosed technology provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various examples described above can be combined to provide further implementations of the disclosed technology. Some alternative implementations of the disclosed technology may include not only additional elements to those implementations noted above, but also may include fewer elements. 
     These and other changes can be made to the disclosed technology in light of the above Detailed Description. While the above description describes certain examples of the disclosed technology, and describes the best mode contemplated, no matter how detailed the above appears in text, the disclosed technology can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the disclosed technology disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the disclosed technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the disclosed technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the disclosed technology to the specific examples disclosed in the specification, unless the above Detailed Description section explicitly defines such terms.