Abstract:
The present invention relates to an ultrasound transducer assembly ( 10 ), in particular for intra vascular ultrasound systems. The assembly ( 10 ) comprises a transducer array ( 12 ) including a plurality of transducer elements ( 14 ) for transmitting and receiving ultrasound waves. Two support elements ( 16, 18 ) are provided for supporting the transducer array ( 12 ) in a curved or polygonal shape. The support elements ( 16, 18 ) are connected via a flexible connection layer ( 20 ) to the transducer array ( 12 ) for flexibly connecting the support elements ( 16, 18 ) to the transducer array ( 12 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an ultrasound transducer assembly, in particular a capacitive micro-machined ultrasound transducer (CMUT) cell for intra vascular ultrasound transducer and a method of manufacturing the same. The present invention relates further to an ultrasound transducer. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the field of intra vascular ultrasound devices, it is generally known to mount an ultrasound transducer on the tip of a catheter to form a radial ultrasound image e.g. of a blood vessel or the surrounding tissue. The ultrasound transducer elements may be rotated in order to emit and receive ultrasound waves in a radial direction of the catheter tip. 
         [0003]    It is further known to replace the mechanically scanning ultrasound transducer elements in the intra vascular ultrasound devices by electronically scanning devices having an annular array of ultrasound transducer elements. The ultrasound transducer elements used for the electronically scanning intra vascular ultrasound systems are usually based on ceramic piezo-electric material, wherein the fabrication of these devices is expensive and complex and the transducer elements cannot be scaled down to reduce the size of the intra vascular transducer. 
         [0004]    Capacitive micro-machined ultrasound transducers (CMUT) are manufactured on the basis of a silicon wafer by means of IC process technologies and can be manufactured with low costs and can be scaled down to the dimensions of an intra vascular ultrasound transducer. For manufacturing an annular ultrasound transducer array, a semi-flexible ultrasound transducer array is formed on the basis of a silicon wafer substrate wrapped or bend around and attached to a cylindrically shaped submount structure in order to form the annular array and to transmit and receive ultrasound waves in the radial direction of a catheter. 
         [0005]    A bendable micro machine ultrasound transducer array which can be attached to a cylindrical submount structure to form the cylindrically shaped ultrasound transducer array is known from US 2005/0146247 A2. 
         [0006]    The submount structure for supporting the flexible ultrasound transducer array has to be fabricated with high precision and the mounting of the ultrasound transducer array on the submount structure is difficult and requires high precision alignment and complex handling equipment in order to produce the annular shaped ultrasound transducer array for intra vascular ultrasound systems. 
         [0007]    WO 2012/066430 A1 discloses an ultrasound imaging apparatus suitable for minimally invasive ultrasound diagnostic devices in cardiac ablation monitoring and in tumor ablation monitoring, wherein the transducer assembly and the transducer system is manufactured on a patterned flexible foil by embedding transducer patches in apertures of the foil surface. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an object of the present invention to provide an improved ultrasound transducer assembly, in particular for intra vascular ultrasound systems having a reduced size and which can be produced precisely with low technical effort. 
         [0009]    In a first aspect of the present invention, an ultrasound transducer assembly, in particular for intra vascular ultrasound systems is provided comprising: 
         [0010]    a transducer array including a plurality of transducer elements for transmitting and receiving ultrasound waves, 
         [0011]    two support elements flexibly connected to opposite sides of the transducer array for supporting the transducer array in a curved or polygonal shape, and 
         [0012]    a flexible connection layer connected to the transducer array and to the support elements for flexible connecting the support elements to the transducer array. 
         [0013]    In a further aspect of the present invention, a method for manufacturing an ultrasound transducer assembly, in particular for intra vascular ultrasound systems, is provided comprising the steps of: 
         [0014]    providing a transducer array connected to a transducer support portion of a substrate, 
         [0015]    connecting a flexible connection layer to the substrate, 
         [0016]    separating a portion of the substrate from the transducer support portion to form a support element for supporting the transducer portion so that the support element is connected to the transducer portion via a flexible layer, and 
         [0017]    bending the flexible connection layer to connect the support element to the transducer support portion. 
         [0018]    In a still further aspect of the present invention, an ultrasound transducer, in particular for intra vascular ultrasound systems is provided, comprising an elongated probe including a tip and an ultrasound transducer that comprises an ultrasound transducer assembly of this kind. 
         [0019]    Preferred embodiments of the invention are defined in the dependent claims. It should be understood that the claimed method has similar and/or identical preferred embodiments as the claimed device and as defined in the dependent claims. 
         [0020]    The present invention is based on the idea to manufacture the transducer elements and the support elements in the same micro-fabrication process to reduce the manufacturing effort and to connect the transducer array and the support element via a flexible connection layer in the same manufacturing process so that the support elements and the transducer elements can be assembled to each other precisely with low technical effort. The ultrasound transducer assembly can be assembled by bending the support element via the flexible connection layer to the transducer array for supporting the transducer array in the curved or polygonal shape. Hence, the manufacturing effort is reduced since the support element and the transducer array are preassembled by means of the flexible connection layer and the precision of the assembling is increased, since the flexible connection layer connects the support element and the transducer array in a predefined position. Further, the size of the ultrasound transducer assembly can be reduced, since the transducer array and the support element can be manufactured in a micro-fabrication process. 
         [0021]    In a preferred embodiment, the connection layer comprises electrical interconnects for electrically connecting the transducer elements to the support elements. This is a possibility to further reduce the manufacturing effort, since the electrical connection of the transducer elements is integrated in the transducer assembly. 
         [0022]    In a further preferred embodiment, the support element comprises at least two electrical contact portions for electrically connecting the transducer assembly to a driver device. This is a possibility to connect the ultrasound transducer assembly with low technical effort to a driver device e.g. by means of a connector or a plug. 
         [0023]    In a preferred embodiment, the support element comprises a support portion having a circular or polygonal shape. This is a possibility to define the outer form of the ultrasound transducer assembly and to support the transducer array in a predefined shape. 
         [0024]    It is further preferred that the support element comprises a connection portion having a larger diameter than the support portion, wherein the connection portion is connected to the connection layer. Hence, the transducer array can be supported in a radial direction by means of the support portion and in an axial direction by means of the connection portion which has a larger diameter than the support portion so that the assembled ultrasound transducer assembly has a solid main body. 
         [0025]    In a further preferred embodiment, the connection layer is connected to an end face of the support element. This is a possibility to further reduce the manufacturing effort, since the support element and the transducer array can be connected to a plane surface to the flexible connection layer. 
         [0026]    In a further preferred embodiment, the support element comprises a central recess for supporting the ultrasound transducer assembly. This is a possibility to mount the transducer assembly coaxial to a catheter tip or the like by means of a single connection bolt connectable to the recess. 
         [0027]    The ultrasound transducer assembly comprises two support elements flexible connected to opposite sides of the transducer array. This is a possibility to further improve the stability of the ultrasound transducer assembly since the transducer array can be supported at opposite sides. 
         [0028]    In a preferred embodiment, the flexible layer is a polyimide layer or a parylene layer. This is a simple possibility to provide the flexible connection between the support element and the transducer array with low technical effort. 
         [0029]    In a preferred embodiment, the assembly has a cylindrical or a multifaceted shape, wherein the transducer elements are disposed at a circumferential surface of the ultrasound transducer assembly in order to transmit and receive the ultrasound waves in a radial direction. This is a possibility to transmit and receive ultrasound waves in a radial direction and to combine the ultrasound transducer assembly with a catheter or the like to form an intra vascular ultrasound system. 
         [0030]    In a preferred embodiment of the method for manufacturing an ultrasound transducer assembly, the substrate is covered by an etch mask at a surface opposite to the flexible layer. This is a possibility to separate the support element from the transducer array with low technical effort while the support element and the transducer array are still connected via the flexible layer. 
         [0031]    In a preferred embodiment, the support element is separated by dry etching of the substrate. This is a simple solution to provide a support element having a precise and predefined shape with low technical effort. 
         [0032]    It is further preferred that the ultrasound transducer assembly comprising the transducer array and the support element is separated from the substrate by dry etching of the substrate and by mechanically separating the flexible layer. This is a simple possibility to define the transducer assembly within the substrate and to extract the transducer assembly from the substrate with low technical effort by breaking the elements defined by the dry etching process out of the substrate. 
         [0033]    As mentioned above, due to the flexible layer connecting the support element and the transducer array, the ultrasound transducer assembly can be assembled by bending the support element to the transducer array so that the transducer elements can be connected to the support element in a curved or polygonal shape. Since the transducer array and the support element are formed from a silicon substrate by means of a micro-fabrication process, the ultrasound transducer assembly can be scaled down and provided with small dimensions so that the ultrasound transducer assembly can be used for intra vascular ultrasound systems and can be manufactured with high precision and low technical effort. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings 
           [0035]      FIG. 1  shows a schematic drawing of an ultrasound transducer assembly in a plane view before assembling; 
           [0036]      FIG. 2  shows a schematic drawing of the transducer assembly of  FIG. 1  in a side view; 
           [0037]      FIG. 3 a, b    show perspective views of the ultrasound transducer assembly for illustrating the assembling thereof; 
           [0038]      FIG. 4  shows a schematic cross section of the assembled ultrasound transducer assembly in an axial view; and 
           [0039]      FIG. 5 a - k    show a sequence of manufacturing steps for manufacturing the ultrasound transducer assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]      FIG. 1  shows a schematic top view of an ultrasound transducer assembly generally denoted by  10 . The transducer assembly comprises a transducer array  12  including a plurality of transducer elements  14  for emitting and receiving ultrasound waves. The transducer assembly  10  comprises two support elements  16 ,  18  which serve as submount elements for the transducer array  12 . The support elements  16 ,  18  have a circular shape and are connected to the transducer array  12  by means of a flexible connection layer  20 . The transducer array  12  has an elongated shape in the direction of longitudinal axis  21 . 
         [0041]    The transducer elements  14  are formed as capacitive micro-machined ultrasound transducers (CMUT). The transducer elements  14  are flexibly connected to each other so that the transducer array  12  can be bent in order to form an annular, circular or polygonal transducer array  12  as described in the following. The transducer elements  14  can be flexibly connected to each other by means of a flexible layer, which may be connected in one piece to the connection layer  20 . 
         [0042]    The support elements  16 ,  18  each comprise a central recess  22 ,  24  or a central opening  22 ,  24  or a central hole  22 ,  24  in order to support the transducer assembly  10 . The flexible connection layer  20  flexibly connecting the support elements  16 ,  18  to the transducer array  12  comprises integrated electrical interconnects  26  for electrically connecting the transducer elements  14  to the support elements  16 ,  18 . Preferably, the electrical interconnects  26  are connected to electrical connection elements  28  or bond bads  28  formed at a surface of the support elements  16 ,  18  in order to electrically connect the transducer elements  14  to a driver device for driving the transducer array  12 . Each of the transducer elements  14  may be connected to one connection element  28  of the first support element  16  and to one of the connector elements of the second support element  18  in order to drive each of the transducer elements  14  separately. The electrical interconnects  26  may be integrated in the connection layer  20  or may be printed on the connection layer  20 . 
         [0043]    The transducer assembly  10  is made from a silicon wafer by means of a micro-fabrication process using integrated circuit processing technology to form the transducer array  12  and the support elements  16 ,  18 . The silicon wafer may be a blank wafer or may include preprocessed active devices or circuits like CMOS transistors. 
         [0044]    The transducer elements  14 ,  18  may consist of silicon islands containing CMUT transducers connected to each other by means of the flexible connection layer  20 . 
         [0045]    The support elements  16 ,  18  are connected to the transducer array  12  by means of the flexible connection layer  20  so that the support elements  16 ,  18  can be bended by 90° so that the flexible transducer array  12  can be wrapped around the support element  16 ,  18  in order to form the transducer assembly  10  in a cylindrical form as further described in the following. 
         [0046]    The embodiment shown in  FIG. 1  comprises a linear array of transducer elements  14 , however, any shape and any formation of transducer elements  14  is possible and can be used by the present invention, e.g. circular or polygonal transducer elements which are disposed in a one dimensional array or a two dimensional array including columns and rows of transducer elements  14  which may be alternatingly displaced. 
         [0047]      FIG. 2  shows a schematic diagram of the transducer assembly  10  in a side view along the longitudinal axis  21  of the transducer array  12 . The transducer array  12  comprises the transducer elements  14  and is attached to the flexible connection layer  20 , which is preferably formed as one piece. The support elements  16 ,  18  have a cylindrical shape including a support portion  30  and a connection portion  32 , wherein the connection portion  32  has a larger diameter than the support portion  30 . In other words, the support elements  16 ,  18  have a “champagne cork” shape. A base surface  34  or an end face  34  of each the connection portions  32  of the support elements  16 ,  18  are connected to the flexible connection layer  20 . The support elements  16 ,  18  and the transducer array  16  are separated by a distance from each other and connected to each other only by means of the flexible connection layer  20 . 
         [0048]    In order to assemble the transducer assembly  10 , the support elements  16 ,  18  are bended around the longitudinal axis  21  of the transducer array  12  as indicated by arrows  36 . 
         [0049]    In the fully assembled state, the circumferential surfaces of the support portions  30  are attached to a backside surface  38  of the transducer array  12  in order to support the transducer array  12  and to stabilize the form of the assembled transducer assembly  10 . The connection portion  32  of the support elements  16 ,  18  abut at a side surface  40  of the transducer array  12  in order to support the transducer array  12  in the axial direction of the assembled transducer assembly  10 . 
         [0050]      FIG. 3 a, b    show schematic perspective views of the transducer assembly  10  to explain the assembling of the transducer assembly  10 . Identical elements are denoted by identical reference numerals, wherein here merely the differences are explained in detail. 
         [0051]    In a first step, the support elements are bended around the longitudinal axis  21  of the transducer array  12  so that inner end faces  42 ,  44  of the support elements  16 ,  18  may be attached to each other, e.g. by an adhesive or by soldering. 
         [0052]    In  FIG. 3 b   , the support elements  16 ,  18  are attached to each other at their inner end faces  42 ,  44  to form a cylindrical element to support the transducer array  12 . The flexible transducer array  12  is bended as indicated by an arrow  46  and wrapped around the support portions  30  of the support elements  16 ,  18 . Hence, in the assembled form, the transducer array  12  is attached radially to the support portions  30  and supported in an axial direction by means of the connection portions  32 . 
         [0053]    The inner end faces  42 ,  44  are connected to each other by means of an adhesive or soldering or the transducer array  12  is connected to the support portions by means of an adhesive, silicon rubber like PDMS or a heat shrinking tube. 
         [0054]    The circumferential surface of the support portions  30  can have a circular or annular shape or may have a multi-faceted shape so that the assembled transducer assembly  10  and in particular the assembled transducer array  12  has a corresponding outer shape. 
         [0055]      FIG. 4  shows a schematic cross sectional view of the assembled transducer assembly  10  in an axial viewing direction. 
         [0056]    The transducer elements  14  are wrapped around the multi-faceted support portion  30  of the support element  18  and respectively attached to the different faces of the support portion  30 . The transducer elements  14  are flexibly connected to each other by means of the connection layer  20  which may be provided at the outside of the transducer array  12 . By means of the so assembled transducer assembly  10 , ultrasound waves can be transmitted and detected in a radial direction of the transducer assembly  10 . 
         [0057]      FIG. 5 a - k    show single method steps for manufacturing the transducer assembly  10 . It is in general manufactured on a silicon wafer base by means of a micro-fabrication process. 
         [0058]    First, pre-processed transducer elements  14  are attached to a silicon wafer substrate  50 . The substrate may be a fully processed wafer including active devices like CMOS transistors. The transducer elements  14  are attached to or formed at a front side  51  at the portion of the substrate  50  where the transducer array  12  will be formed as shown in  FIG. 5   a.    
         [0059]    Further, a hardmask  52  is formed at a backside  54  of the substrate  50  by deposition and patterning of an oxide layer, e.g. a PECVD (plasma-enhanced chemical vapor deposition) oxide as shown in  FIG. 5   b.    
         [0060]    In the following step, the flexible connection layer  20  is formed at the front side  51  of the substrate  50  on the transducer elements  14  as shown in  FIG. 5 c   . The flexible connection layer  20  is a polyimide flexible foil consisting of an aluminum re-routing layer sandwiched between two layers of polyimide of equal thickness. The flexible connection layer  20  may be a parylene layer. 
         [0061]    On the front side  51  of the substrate  50 , a thin aluminum layer is deposited and patterned in order to form a hardmask  56  to structure the flexible connection layer  20  at the end of the process as shown in  FIG. 5   d.    
         [0062]    As shown in  FIG. 5 e   , the backside  54  of the substrate  50  is etched by means of a dry-etching process to a depth of approximately 100 μm by using the hardmask  52 . Further, a timed oxide etch process is used on the backside  54  of the substrate  50  in order to remove thinner portions  58  of the hardmask  52  as shown in  FIG. 5   f.    
         [0063]    In a second dry etch process performed on the backside  54  of the substrate  50 , the substrate  50  is partially etched down to the flexible connection layer  20  in order to separate the support elements  16 ,  18  from the transducer portion attached to the transducer elements  14 . Due to the anisotropic dry etch process, the thickness of the transducer portion is reduced and a step is formed at the support elements  16 ,  18  forming the difference between the connection portion  32  and the support portion  30 . As shown in  FIG. 5 g   , after this etch process, the support elements  16 ,  18  are separated from the surrounding silicon substrate  50  and are merely connected to the surrounding substrate  50  by means of the flexible connection layer  20 . 
         [0064]    Using the aluminum layer  56  on the front side  51  of the substrate  50  as a hardmask, the flexible connection layer  20  is patterned by means of an oxygen plasma in order to form the recesses  22 ,  24 , openings  22 ,  24  or the central holes  22 ,  24  in the support elements  16 ,  18  as shown in  FIG. 5 h   . After the recesses or openings or holes  22 ,  24  are etched in the support element  16 ,  18 , the hardmask  56  is removed by means of a timed wet etch as shown in  FIG. 5 i   . At this stage of the process, the transducer assembly  10  is entirely structured and only supported within the substrate  50  by means of small tabs formed from flexible connection layer  20 . 
         [0065]    Finally, the transducer assembly  10  is removed from the substrate  50  by breaking the small flexible tabs  20  which is the only connection of the transducer assembly  10  to the substrate  50  as shown in  FIG. 5   k.    
         [0066]    The overall height of the assembled transducer assembly  10  is limited to two times of the thickness of the substrate  50 , however, by stacking a plurality of transducer assemblies  10  in a column, an ultrasound transducer system having an elongated shape can be formed having an arbitrary length. 
         [0067]    The recesses  22 ,  24  or the openings  22 ,  24  or the holes  22 ,  24  may have a circular shape, a square shape, a triangular shape or a polygonal shape in order to prevent the rotation of the transducer assembly  10  connected to an ultrasound transducer system. 
         [0068]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustrations and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
         [0069]    In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
         [0070]    Any reference signs in the claims should not be construed as limiting the scope.