Abstract:
A micromachined ultrasound transducer including a plurality of ultrasound transducer cells each of which includes a substrate on which a bottom electrode is formed, and a membrane provided apart from the substrate and having an upper electrode formed thereon, said ultrasound transducer generating ultrasound with the membrane vibrating when voltage is applied between the bottom electrode and the upper electrode, comprises: a vibration propagation suppression unit suppressing propagation of vibration of said each ultrasound transducer cell to an adjacent ultrasound transducer cell.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a Continuation Application of PCT Application No. PCT/JP2007/000063, filed Feb. 7, 2007, which was not published under PCT Article 21(2) in English. 
         [0002]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-057121, filed Mar. 3, 3006, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to a MUT (Micromachined Ultrasound Transducer). 
         [0005]    2. Description of the Related Art 
         [0006]    A commonly employed ultrasound diagnosis method is one in which ultrasound is transmitted to the walls of a body cavity and the state of the body cavity is converted into images on the basis of the echo signals thereof for the purpose of making diagnoses. An ultrasound endoscope is one of the devices used for this ultrasound diagnosis method. 
         [0007]    The ultrasound endoscope has an ultrasound probe at the distal end of an insertion tube to be inserted into body cavities, and this ultrasound probe converts electric signals into ultrasound in order to transmit the ultrasound to body cavities and also receives ultrasound reflected in the body cavities in order to convert the received ultrasound into electric signals. 
         [0008]    Conventionally, ceramic piezoelectric material PZT (lead zirconate titanate) has been used for piezoelectric elements that convert electric signals into ultrasound. However, capacitive micromachined ultrasound transducers (hereinafter referred to as cMUTs) have attracted attention. cMUTs are among a category of devices that are referred to as micro electro-mechanical systems (MEMS). 
         [0009]      FIG. 1A  is a cross-sectional view of a part of a conventional cMUT array. In  FIG. 1A , the cMUT array includes two transducer elements  210  having a trench  209  between them. The transducer element is the smallest unit for inputting and outputting driving control signals. This transducer element is made of a plurality of transducer cells  208 . 
         [0010]    The transducer element  210  includes a silicon substrate  201 , a bottom electrode  202 , a membrane supporting member  203 , a cavity  204 , and a membrane  206 . A unit that includes one cavity  204  is referred to as a transducer cell  208 . 
         [0011]      FIG. 1B  is an enlarged view showing the transducer cell  208 . In  FIG. 1B , the bottom electrode  202  is formed on the silicon substrate  201 , and the membrane  206  is supported by the membrane supporting member  203 . An upper electrode  205  is formed on the membrane  206 . The periphery of the membrane  206  is supported by the membrane supporting member  203 . 
         [0012]    When voltage is applied between the upper electrode  205  and the bottom electrode  202  of the transducer element  210 , the membranes  206  of the respective transducer cells  208  simultaneously start the bending vibrations in phase. Thereby, ultrasound is transmitted. 
         [0013]    By providing the trench  209 , crosstalk between the transducer elements consisting of a plurality of the transducer cells connected with each other is suppressed. Also, an ultrasound damping material may be formed in the trench  209 . Also, after removing sacrifice layers used for making the cavities, sacrifice layer removal holes are sealed. 
         [0014]    Also, in recent years, capsule endoscopes that are swallowed into body cavities in order to obtain images of the body cavities have been realized (for example, Patent Document 8). Also, a system in which a capsule having a cMUT for making ultrasound diagnoses on body tissues has been disclosed by the applicant of the present application (for example, Patent Documents 9 and 10). By using this ultrasound diagnosis medical capsule endoscope, it is possible to make ultrasound diagnoses at sites at which it was previously difficult to make diagnoses. 
         [0015]    Also, the techniques relating to the present invention are disclosed in, for example, patent documents 1 through 10 and the non-patent document 1 below. 
         [0016]      FIG. 2  shows the bending vibration in a conventional cMUT. In a conventional cMUT, the membranes  206  of the respective transducer cells  208  are continuously arranged in the plane direction (two-dimensional direction). When the membranes  206  are caused to vibrate by driving this type of a cMUT, bending vibrations are caused that can be expressed by a maximum bending deformation  214  caused immediately after the wave of condensation and by a maximum bending deformation  215  caused immediately after the wave of rarefaction. 
         [0017]    In this case, the membranes of the adjacent cells are connected to each other and accordingly the bending vibrations of one membrane  206  propagate to another membrane of the adjacent cell so that a portion of the ultrasound vibrations dissipate (expressed as leaky waves  212  in  FIG. 2 ), and thereby the output sound pressure decreases. 
         [0018]    Also, the membrane  206  is supported and fixed by the membrane supporting member  203  at the membrane periphery. This configuration causes a portion of the bending vibrations of the membrane  206  to be converted into longitudinal waves, and the longitudinal waves propagate via the membrane supporting member  203  to a semiconductor substrate  201  as longitudinal waves so that a portion of the ultrasound vibrations dissipate (expressed as supporting member propagation leaky longitudinal waves  213  in  FIG. 2 ), and thereby the output sound pressure decreases. 
       Patent Document 1: 
     Japanese Patent Application Publication No. 7-274287 
     Patent Document 2: 
     Japanese Patent Application Publication No. 8-274573 
     Patent Document 3: 
     Japanese Patent Application Publication No. 2004-274756 
     Patent Document 4: 
       [0019]    U.S. Pat. No. 6,262,946 
       Patent Document 5: 
       [0020]    U.S. Pat. No. 6,328,696 
       Patent Document 6: 
       [0021]    U.S. Pat. No. 6,328,697 
       Patent Document 7: 
       [0022]    Publication in Japan of translation of PCT International Patent Application No. 2004-503313 (WO 2001/097562) 
       Patent Document 8: 
     Japanese Patent Application Publication No. 2004-350705 
     Patent Document 9: 
     Japanese Patent Application Publication No. 2004-350704 
     Non-Patent Document 1: 
       [0023]    Pages 149 through 152 in the Twelfth version of Volume 1 of “Onkyo Kogaku Genron” written by Tsuyoshi Itoh and published by CORONA PUBLISHING CO., LTD, on Dec. 10, 1980 
       SUMMARY OF THE INVENTION 
       [0024]    The ultrasound transducer according to the present invention is a micromachined ultrasound transducer including a plurality of ultrasound transducer cells that each include a substrate on which a bottom electrode is formed and a membrane provided apart from the substrate and having an upper electrode formed thereon, said ultrasound transducer generating ultrasound with the membrane vibrating when voltage is applied between the bottom electrode and the upper electrode, comprising: 
         [0025]    a vibration propagation suppression unit suppressing propagation of vibration of said each ultrasound transducer cell to an adjacent ultrasound transducer cell. 
         [0026]    The method of producing an ultrasound transducer according to the present invention is a method of producing a micromachined ultrasound transducer including a plurality of ultrasound transducer cells each of which includes a substrate on which a bottom electrode is formed, and a membrane provided apart from the substrate and having an upper electrode formed thereon, said ultrasound transducer generating ultrasound with the membrane vibrating when voltage is applied between the bottom electrode and the upper electrode, comprising: 
         [0027]    providing a supporting member for supporting the membrane along nodes of a fundamental vibration caused when the membrane vibrates freely. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1A  is a cross-sectional view of a part of a conventional cMUT array; 
           [0029]      FIG. 1B  is a cross-sectional view of a conventional cMUT cell; 
           [0030]      FIG. 2  shows the bending vibration in a conventional cMUT; 
           [0031]      FIG. 3  show the concept of the cMUT cell according to the present invention; 
           [0032]      FIG. 4  is a cross-sectional view of a cMUT according to the first embodiment; 
           [0033]      FIG. 5  is across-sectional view of a cMUT according to the second embodiment; 
           [0034]      FIG. 6  is a top view of a cMUT  11  shown in  FIG. 5 ; 
           [0035]      FIG. 7  is a cross-sectional view along the line Aa-Ab of  FIG. 6 ; 
           [0036]      FIG. 8  is a cross-sectional view along the line Ba-Bb of  FIG. 6 ; 
           [0037]      FIG. 9  shows a variation example of the cMUT according to the second embodiment; 
           [0038]      FIG. 10  is a cross-sectional view of a cMUT according to the third embodiment; 
           [0039]      FIG. 11  is a cross-sectional view of a cMUT according to the fourth embodiment; 
           [0040]      FIG. 12  shows a variation example of the fourth embodiment; 
           [0041]      FIG. 13  is a cross-sectional view of a cMUT according to the fifth embodiment; 
           [0042]      FIG. 14  shows a variation example of the fifth embodiment; 
           [0043]      FIG. 15  is a cross-sectional view of a cMUT according to the sixth embodiment; 
           [0044]      FIG. 16A  is a top view of a bottom electrode according to the sixth embodiment; 
           [0045]      FIG. 16B  shows deformation of a membrane  15  vibrating when voltage is applied to a bottom electrode  80  in the sixth embodiment; 
           [0046]      FIG. 17A  is a top view of a conventional bottom electrode  90  shown for the comparison with the bottom electrode in the sixth embodiment; 
           [0047]      FIG. 17B  is a view showing deformation of the upper electrode  15  vibrating when voltage is applied to a conventional bottom electrode  90  shown for the comparison with the bottom electrode according to the sixth embodiment; 
           [0048]      FIG. 18A  shows a first production process for the cMUT according to a seventh embodiment; and 
           [0049]      FIG. 18B  shows a second production process for the cMUT according to the seventh embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0050]    As was explained in  FIG. 2 , the leaky waves  212  and  213  that have dissipated affect, as crosstalk, the ultrasound transmitted and received through the adjacent transducer cells. The crosstalk between the transducer cells constituting the transducer elements causes variations in the source sound pressure and in the phases of the vibration among the respective transducer elements, and accordingly when the phased array scanning is performed, noise is caused in ultrasound diagnosis images and thereby the ultrasound diagnosis images can be deteriorated. 
         [0051]    As described above, because the membrane  206  is supported by the membrane supporting member  203  at the periphery of the membrane, vibrations dissipate so that the mechanical quality factor Q is lowered. This lowered level of the mechanical quality factor Q causes a lowered output of ultrasound and lowered wide band characteristics. 
         [0052]    A state in which there is no vibration loss and vibration continues efficiently in a particular range (time axis) corresponds to a high mechanical quality factor Q. In other words, if the dissipation of vibration is made smaller, Q increases. The vibration amplitude at this resonance frequency is Q times the vibration amplitude at a non-resonance frequency. The vibration loss of the membrane is reduced and Q is made greater so that the vibration amplitude is made greater; as a result, great ultrasound transmission sound pressure is obtained. 
         [0053]    The present invention provides an ultrasound transducer in which the efficiency of the membrane vibration is increased and the ultrasound transmission efficiency is increased. 
         [0054]    A cMUT according to the present invention is formed on a semiconductor substrate. In the present invention, a unit in which a bottom electrode for applying driving control signals and a membrane including an upper electrode serving as a contact electrode are opposed to each other and have a cavity between them is referred to as a cMUT cell. A group of arrays that is the minimum unit made by arranging a large number of the cMUT cells for inputting and outputting the driving control signals is referred to as a cMUT element. A thing in which a large number of the cMUT elements are arrayed is referred to as an array type transducer. 
         [0055]    The membrane used in this cMUT cell is divided over the entirety of the periphery between the adjacent cMUT cells, and is independent from the membranes in the adjacent cells. Further, the supporting positions of the membrane supporting members are provided along positions that serve as nodes when the membranes cause the free vibrations. 
         [0056]      FIG. 3  shows the concept of a cMUT cell according to the present invention.  FIG. 3(A)  is a cross-sectional view, and  FIG. 3(B)  is a perspective view. In  FIG. 3 , the cMUT cell  1  includes the membrane  3  and the membrane supporting member  2 . 
         [0057]    The diameter (transducer maximum length) of the membrane  3  is represented by L 1 , and the distance (nodal circle diameter dimension  8 ) between the membrane supporting members is represented by L 2 . Also, the periphery of the membrane  3  is an area in the shape of a doughnut in which the circle defined by the membrane supporting members (diameter  8 ) is removed from the membrane  3  (referred to as a membrane node outside area  7 ). 
         [0058]    The membrane supporting member  2  supports the membrane in a configuration of concentric circles that is smaller than the diameter of the membrane  3  (a circle whose diameter is the diameter  8 ). The supporting position does not suppress the bending vibration of the membrane  3 , and when the fundamental vibration is caused only in the membrane  3 , in other words, when free vibration is caused without fixing the ends of the membrane, the membrane  3  causes a bending vibration that is expressed by a maximum bending deformation  5  caused immediately after the wave of condensation is caused and a maximum bending deformation  6  caused immediately after the wave of rarefaction is caused, and nodes  4  that do not vibrate at any time are caused. 
         [0059]    These nodes  4  are parts that do not deform due to the fundamental vibration of the membrane  3 . Along the positions at which the nodes  4  are caused, the membrane supporting members  2  are provided. When the radius of the membrane  3  is expressed by “a”, in the case of the fundamental vibration, the membrane supporting members  2  are provided at points that are 0.678a from the center of the membrane  3  (non patent document 1). 
         [0060]    According to non-Patent Document 1, a circular plate whose periphery is free causes to exist a diameter node line that is arranged at intervals equal to those of the node line of the concentric circle when vibrating. This is especially the case when the shape is symmetric, which POISSON solved in 1829, and the minimum vibration thereof causes the node line at 0.678a (a is the radius of the circular plate), and the next behavior is caused at 0.392a and 0.842a. 
         [0061]    As described above, because the membrane of each cell is not connected with the membrane of the next cell, the dissipation of the vibration toward the planar direction (adjacent membrane direction) is avoided. Also, the membrane is supported at the positions corresponding to nodes  4  at which deformation due to the fundamental vibration is not caused, and accordingly it is possible to prevent the vibration from leaking to the semiconductor substrate as the longitudinal wave. 
         [0062]    Hereinbelow, the embodiments of the present invention will be explained. 
       First Embodiment 
       [0063]      FIG. 4  is a cross-sectional view of a cMUT according to the present embodiment. A cMUT  11  includes a surface-oxidized silicon (Si) substrate  12 , membrane supporting member  13 , bottom electrodes  14 , upper electrodes  15 , membranes  16 , cavities  17 , a ground wire  18 , a through hole  19 , a through hole wire  20 , a signal wire electrode pad  21 , and a contact electrode pad  22 . 
         [0064]    In the present embodiment, each cMUT cell consists of a unit enclosed by a dashed line  25 . The cMUT  11  consists of a plurality of cMUT cells  25 . Additionally, the dashed line  25  in this unit also includes the ground wire  18 , the through hole  19 , the through hole wire  20 , and the signal wire electrode pad  21 , although they are not shown in the figure. 
         [0065]    As shown in  FIG. 3 , the membrane  16  is fixed by the membrane supporting members  13  at the positions at which the nodes  4  are caused. The supporting positions are provided at points that are 0.678a from the center of the membrane  16  when the radius of the membrane  16  is expressed by “a” (a is an arbitrary value). The upper electrode  15  is provided on the upper surface of the membrane  16 . The membrane supporting members  13  are provided on the upper surface of the silicon substrate  12 . 
         [0066]    The bottom electrodes  14  are arranged on the surfaces between the membrane supporting members  13  on the surface-oxidized silicon substrate  12 . The through hole  19  is provided through the silicon substrate  12 , and the through hole wire  20  whose inner wall has undergone the insulation process is provided. The through hole wire  20  and the bottom electrode  14  are electrically continuous to each other. The other end of the through hole wire  20  is electrically continuous to the signal wire electrode pad  21  provided on the silicon substrate  12  whose surface is covered by an insulation film. Thereby, the signal wire electrode pad  21  serves as a terminal on the bottom surface of the silicon substrate  12  with respect to the bottom electrode  14 . 
         [0067]    The contact electrode pad  22  is an electrode pad for connecting the upper electrode  15  to the GND, and also for causing the upper electrode  15  to be electrically continuous to the bottom surface of the membrane  16 . The cavity  17  refers to a space enclosed by the membrane  16  (including the membrane supporting members  13 ) and the silicon substrate  12  (bottom electrode  14 ). 
         [0068]    By providing diffusion layers (ohmic contact)  26  and  261 , it is possible to reduce the resistance between the upper electrode  15  and the contact electrode pad  22  to the minimum level. 
         [0069]    Also, the membranes  16  do not contact each other between the adjacent cells  25 , and gaps are provided between them. Also, there are spaces (adjacent-cell cavities  24 ) enclosed by the membrane supporting members  13  and edges of the membranes (membrane node outside area  7 ) at the lower portions in the spaces between the adjacent cells  25 . 
         [0070]    The operations of a cMUT  1  will be explained. When voltage is applied between a pair of electrodes of the upper electrode  15  and the bottom electrode  14 , the electrodes attract each other, and when the voltage becomes zero, they stop attracting each other. Ultrasound waves are transmitted in the upward direction of the upper electrode  15  when this voltage application is performed at a high speed, and vibrations are activated in the membrane, and thereby ultrasound waves are generated in the membrane  16  on the basis of the vibrations. 
         [0071]    According to the present embodiment, the membranes do not contact each other between the adjacent cells, and accordingly the dissipation of the vibrations in the planar direction (direction toward the adjacent membranes) can be avoided. Also, each membrane is supported at the positions of the nodes  4  at which the deformation is not caused by the fundamental vibrations, and accordingly it is possible to prevent the vibrations from leaking to the semiconductor substrate as longitudinal waves. 
       Second Embodiment 
       [0072]    In the first embodiment, the membranes do not contact each other between the adjacent cells and there are spaces between them. However, in the present embodiment, a cMUT in which a plurality of trenches are provided in place of the spaces will be explained. Additionally, in the explanations below, the same constituent elements as in the first embodiment are denoted by the same numerals, and the explanations thereof are omitted. 
         [0073]      FIG. 5  is a cross-sectional view of the cMUT according to the present embodiment. The configuration shown in  FIG. 5  is different from that shown in  FIG. 4  in that gaps are not provided between the membranes in the respective cells, it is configured of one membrane  16 , and a plurality of trenches (trench array  30 ) are provided at the portions corresponding to the gaps in  FIG. 4 . In  FIG. 5 , the ground wire  18 , the through hole  19 , the through hole wire  20 , the signal wire electrode pad  21 , and the contact electrode pad  22  are not shown. 
         [0074]    In the present embodiment, similarly to the first embodiment, the membrane  16  is supported at points that are 0.678a from the center of the membrane  16  when the radius of the membrane  3  is expressed by “a”. However, the positions of nodes may change depending on the number and the depth of trenches. In such a case, it is necessary to determine the positions of the nodes on the basis of experiments. 
         [0075]      FIG. 6  is a top view of a cMUT  11  shown in  FIG. 5 . The cMUT  11  has a plurality of membranes  16  defined by the trench arrays  30  provided in a circular shape (zonal trench group). 
         [0076]    A portion denoted by numeral  31  is a trench array of a portion in which it is the closest to the adjacent cell (hereinafter referred to as a trench array area  31  in which nodes of adjacent-cells are closest). A portion denoted by numeral  32  is an area that is among three adjacent cells. As will be explained in  FIG. 7 , in this area, the thickness of the membrane changes noncontinuously so that the acoustic impedance becomes noncontinuous. Hereinafter, this area is referred to as an acoustic-impedance noncontinuous area  32  among three adjacent cells. 
         [0077]    The circles  34  depicted by the dashed lines are portions corresponding to the portions supported by the membrane supporting members  13 , and inherently cannot be seen from above because they are below the membranes. 
         [0078]      FIG. 7  is a cross-sectional view along the line Aa-Ab of  FIG. 6 . A cavity that is under the membrane  16  and between the adjacent cells is referred to as a lower cavity  41  of acoustic-impedance noncontinuous-region between adjacent cells. The trench array area  31  that is corresponding to the lower cavity  41  in the membrane  16  is configured of a combination of a plane and a plurality of trench arrays  30 , and accordingly the thickness of the membrane changes noncontinuously. Thus, the membrane  16  corresponding to this portion causes the noncontinuous acoustic impedance. 
         [0079]      FIG. 8  is a cross-sectional view along the line Ba-Bb of  FIG. 6 . In  FIG. 8 , there is the acoustic-impedance noncontinuous area  32  between the adjacent trench arrays  30 . The portion between the trench arrays  30  has a thickness smaller than that of the other portions (a thin layer area  33 ). 
         [0080]    Also, in the silicon substrate  12 , the portion corresponding to the thin layer area  33  has a through hole  42 . The through hole  42  is made for the purpose of air ventilation. If a through hole  42  is not present, the lower cavity  41  is sealed, and the dumping effect is caused so that the mechanical quality factor Q decreases. 
         [0081]    Also, if the lower cavity  41  is sealed, dumping against the vibrations in the thin layer area  33  is caused because the thin layer area  33  vibrates while synchronizing the vibrations of the membrane  16 . This dumping decreases the vibration efficiency of the membrane. 
         [0082]    Thus, air ventilation is realized in the lower cavity  41  via the through hole  42  in order to increase the vibration efficiency of the vibrations in the membrane and to increase the ultrasound wave transmission efficiency so that the lower cavity  41  is prevented from being sealed, and the membrane  16  is caused to vibrate highly efficiently. 
         [0083]      FIG. 9  shows a variation example of  FIG. 6 .  FIG. 9  shows a configuration in which the shape formed by the trench array  30  is changed from a circle to a hexagon, and holes  43  that lead to the middle of the membrane in the thickness direction are made at the corners. The holes  43  are made in order to suppress the concentration of forces. The shape of the trench is not limited to a hexagon, and can be other polygons. 
         [0084]    According to the present embodiment, a zonal trench group is provided as an acoustic isolation unit between the ultrasound transducer cells, and thereby the bend (plate waves) in the membrane is enclosed in the cell and does not easily propagate to the membranes of the adjacent cells so that it is possible to suppress the dissipation of the vibration in the plane direction. Also, it is possible to suppress the variation in the ultrasound output caused by the liquid invading through the ultrasound transmission/reception plane because there is no gap between the membranes. 
       Third Embodiment 
       [0085]      FIG. 10  is a cross-sectional view of a cMUT according to the present embodiment.  FIG. 10  shows a configuration in which the upper surfaces of the membrane  16  and the upper electrode  15  of the cMUT shown in  FIG. 4  are coated with a protective layer  50 . 
         [0086]    Examples of the materials that can be used for the protective layer  50  are parylene, polymide, Teflon (registered trademark), Cytop, and the like. 
         [0087]    This protective film  50  may be a film made of nanometer-sized particles (nano-coating film). The components of the protective film  50  (product name: x-protect DS 3010; manufacturer name: NANO-X) are silicon (Si), zirconium (Zr), titanium (Ti), oxygen (O), and other organic constituents (polymer compound), and are in a mesh-configuration. This structure is obtained by hydrolyzing metallic alkoxide compound such as silicon (Si), zirconium (Zr), titanium (Ti) or the like. Also, the organic constituents of the base material are present along the mesh all over the film. This structure has a mesh configuration over its entirety; however, a portion in which the organic constituents are not along the mesh of the non-organic constituent can be generated. Therefore, nanoparticles that are free from the mesh can exist. Accordingly, it is possible to select a nanoparticle (also in a mesh-configuration) or a mesh-configuration over the entirety of which there are organic constituents by selecting the production conditions such as the heating condition, the manner of hydrolyzation, the PH adjustment, or the like. Also, the nanometer-sized nonorganic constituent may be one or a plurality of silicon, titanium, and zirconium. Also, the nanometer-sized nonorganic constituent may be one or a plurality of the oxidative products of silicon, the oxidative products of titanium, and the oxidative products of zirconium. The protective films made of these non-organic chemical compounds have excellent corrosion resistance and humidity resistance. 
         [0088]    According to an embodiment of the present invention, it is possible to prevent liquid from invading because the gaps between the membranes are sealed by the protective film  50 . Also, the present embodiment can be applied in combination with any of the first and second embodiments. 
       Fourth Embodiment 
       [0089]      FIG. 11  is a cross-sectional view of a cMUT according to the present embodiment. The configuration shown in  FIG. 11  is different from the configuration shown in  FIG. 4  in that the bottom electrode  14  is formed on the upper surface of the silicon substrate  12  (SiO 2 /Si) whose surface has undergone the insulation process and an insulation film  61  is formed thereon in the configuration shown in  FIG. 11 . Also, the adjacent-cell cavity  24  and a gap  23  are filled with filler agent  60  so that the entire membrane in the cMUT  11  is flat. 
         [0090]    For the insulation film  61 , materials having a high dielectric constant such as SrTiO 3 , barium titanate BaTiO 3 , barium strontium titanate, tantalum pentoxide, niobium pentoxide stabilized tantalum pentoxide, oxidized aluminum, oxidized titanium TiO 2 , or the like can be used. 
         [0091]    For the filler agent  60 , materials that are flexible to decrease a vibration loss and that have an acoustic impedance greatly different from that of the membrane material are used in order to increase the S/N ratio of the vibration in the membrane  16 . For the filler agent  60 , resin foam is used. It is desirable that the resin foam have an acoustic impedance greatly different from that of the membrane material. 
         [0092]    For the resin foam, such substances as obtained by making SiO 2  or a SiOF film porous can be used. The resin foam is used for the Low-k (insulation film between low dielectric layers) film material. Also, when an organic film is used as the resin foam, plasma CVD films made of polyimide, parylene, or teflon can be used. Also, the coefficient of elasticity becomes one tenth or lower. 
         [0093]    Also, for the filler agent  60 , porous silicon can be used. Porous silicon is a thing in which countless nano-scale micro pipes are formed in the plate thickness direction of silicon. The acoustic impedance is very small because there is air in the micro capillary tube. 
         [0094]      FIG. 12  shows a variation example of the configuration shown in  FIG. 11 . In  FIG. 12 , only the adjacent-cell cavity  24  is filled with the filler agent  60  and the gap  23  is not filled. Thereby, it is possible to increase sensitivity in the ultrasound reception because the membrane  16  is not restrained. 
         [0095]    Also, it is desirable that the acoustic impedance of the filler agent  60  filling the gap  23  be equal to or lower than twenty percent of the acoustic impedance of the membrane  16 , and more desirable that it be equal to or lower than ten percent. This is because if it exceeds ten percent, the vibration efficiency of the membrane decreases by 3 dB, and this affects the image sensitivity (brightness) and increases the crosstalk by 3 dB so that the image contrast is affected. 
         [0096]    According to the present embodiment, by filling in the gap around the periphery of the membrane with a filling agent, it is possible to suppress the variation of the ultrasound output that would otherwise be caused by the invasion of liquid. Also, the present invention can be applied in combination with any of the first through third embodiments. 
       Fifth Embodiment 
       [0097]    In the present embodiment, a cMUT in which the integration density of the ultrasound cells is increased will be explained. 
         [0098]      FIG. 13  is a cross-sectional view of a cMUT according to the present embodiment.  FIG. 13  shows a thing in which weight members  70  are added to the peripheries (membrane node outside area  7 ) of the membranes  16  shown in  FIG. 4 . The nodes are shifted to the side of the weight members  70  because the weight members  70  are arranged on the membrane node outside area  7 . As a result of this, the width of each membrane node outside area  7  is reduced so that the diameter  8  becomes greater. 
         [0099]      FIG. 14  shows a variation example of the configuration in  FIG. 13 .  FIG. 14  shows a thing in which the weight members  70  are provided on the membrane node outside area  7  shown in  FIG. 5 . 
         [0100]    According to the present embodiment, the nodes shift to the weight members  70  because the weight members  70  are arranged on the membrane node outside area  7 , and accordingly the width of each membrane node outside area  7  is reduced and the length of the diameter  8  becomes greater. As a result of this, the ratio of the area of the membrane node outside area  7  (the doughnut shape) to the area of the inner area (a circle defined by the diameter  8 ) becomes greater. 
         [0101]    Accordingly, the area of the circle defined by the diameter  8  becomes greater, and thus it is possible to increase the capacitance that directly relates to the sound pressure generated. Also, as the area of the membrane node outside area  7  is reduced, the intervals between the adjacent cells can be reduced, and thereby it is possible to increase the integration density of the cMUT cells. Thereby, the sensitivity of an ultrasound image increases and S/N also increases due to the reduction of the sidelobe. Also, the present embodiment can be applied in combination with any of the first through fourth embodiments. 
       Sixth Embodiment 
       [0102]    In the present embodiment, an ultrasound transducer will be explained in which the shape of the bottom electrode is changed in order to keep the shape of the vibrating membrane as flat as possible. 
         [0103]      FIG. 15  is a cross-sectional view showing a cMUT according to the present embodiment. In  FIG. 15 , bottom electrodes  80  are formed on the upper surface of the silicon substrate (SiO 2 /Si), and an insulation film  61  is formed thereon. 
         [0104]      FIG. 16A  is a top view of the bottom electrode  80  in the present embodiment. When the bottom electrode  80  is viewed from above, it is in the shape of a doughnut, and wires  81  are provided at the four points in order to be connected to the adjacent electrodes. 
         [0105]    Numeral  82  in  FIG. 16B  denotes the manner of the deformation of the upper electrode  15  vibrating when voltage is applied to the bottom electrode  80 . It is sufficient to make one of the upper and the bottom electrodes be in the shape of a doughnut; accordingly, the bottom electrode is in the shape of a doughnut and the upper electrode  15  is circular. Also, the upper electrode on the GND side has the effect of reducing the noise arriving from outside circumstances, and accordingly it is desirable that the bottom electrode on the signal side be in the shape of a doughnut. 
         [0106]    As shown in  FIG. 16B , no electrostatic force is applied between the central portion of the bottom electrode  80  at which there is no electrode and the corresponding central portion of the upper electrode  15 , and accordingly that portion of the upper electrode  15  does not deform so that it can be substantially flat. 
         [0107]    A conventional bottom electrode will be explained by referring to  FIGS. 17A and 17B  in order to explain the present embodiment more clearly. When viewed from above, a conventional bottom electrode  90  is circular, and wires  91  are provided at the four points in order to be connected to the adjacent electrodes ( FIG. 17A ). As described above, the conventional bottom electrode  90  is an electrode in its entirety. 
         [0108]    As shown in  FIG. 17B , when voltage is applied to the bottom electrode  14 , the electrostatic force is applied also to the central portion of the upper electrode  15 , and accordingly the central portion of the upper electrode  15  cannot remain flat, and thus deforms to a non-linear shape as denoted by numeral  92 . As described above, the non-linear vibrations affect operations such as the harmonic imaging in which non-linear characteristics are utilized. 
         [0109]    The present embodiment can be applied in combination with any of the first and fifth embodiments. 
         [0110]    According to the present embodiment, the central portion of the upper electrode corresponding to the portion in which there is no bottom electrode is kept substantially flat and is not deformed, and accordingly it is possible to apply a greater voltage and generate greater power. 
       Seventh Embodiment 
       [0111]    In the present embodiment, a method of producing a cMUT will be explained. There are two methods (the bulk machining method and the surface micromachining method) of producing a cMUT, and in each method the supporting member is provided further inward than the periphery and the supporting member is sealed in an air-tight manner so as to increase the deformation amount of the membrane. In the present embodiment, the bulk machining method will be explained. 
         [0112]      FIGS. 18A and 18B  show the steps for producing a cMUT according to the present embodiment. 
         [0113]    First, the surface of an N-type silicon substrate  101  (with a thickness between about 100μ and a bout 500μ) is masked with an oxide film (SiO 2 )  102 . An oxide film with a thickness between about 3000 angstroms and about 4000 angstroms is formed by using the wet oxidization method. Then, the patterning is performed in order to form through-hole electrodes  104  for bottom electrodes by using the photolithography process, and the oxide film that has undergone the patterning is removed in the etching step. 
         [0114]    Next, holes  103  are made by performing ICP-RIE (Inductively Coupled Plasma Reactive Ion Etching) at portions that are not masked. 
         [0115]    Next, an electrode film (Pt/Ti) is formed on the upper and lower surfaces of the silicon substrate  101  and the inner wall surface of the holes  103 . Bottom electrodes  105 , electrodes-for-wiring  106 , and electrode pads  107  are formed (step  1 ; a step is referred to as “S” hereinafter) after performing the patterning step and the etching step on the electrode film. The material of the electrodes is not limited to Pt/Ti, and Au/Cr, Mo, W, phosphor bronze, Al, and the like may be used. The structure made in S 1  is referred to as wafer A. 
         [0116]    Next, another Si substrate  111  is prepared, and an oxide film SiO 2    112  is formed by oxidizing one of upper and lower surfaces of the substrate  111 . An etching process (etching process using, for example, the CVD method) is performed on a part of the oxide film  112  so that it is tapered by the ICP-RIE (S 2 ) The portions formed through this etching process are referred to as concaves  113 . 
         [0117]    Next, membrane supporting members  114  are formed. Similarly to S 2 , the etching process is performed on the portions other than the membrane supporting members on the oxide film  112  by using ICP-RIE (S 3 ). This etching process is performed in such a manner that an oxide film  112  of the thickness of the membrane is not etched. Also, the portions between the two concaves  113  on the oxide film  112  will serve as the membrane, and accordingly the etching process is performed in such a manner that the membrane supporting members  114  are formed along points that are 0.678a from the center of each membrane (a is the radius of the membrane). 
         [0118]    Next, an electrode film  115  is formed on the surface of the oxide film (S 4 ). Thereafter, a patterning process is performed in order to remove the portions other than the portions denoted by numeral  116  on the electrode film (S 5 ). Thereby, electrodes  117  for wires are formed. The structure produced in this S 4  is referred to as wafer B. 
         [0119]    Next, wafer A and wafer B are jointed to each other (S 6 ). Thereafter, a silicon substrate  111  is removed by performing the etching process so that the oxide film  112  is exposed (S 7 ). This etching process can be performed by using, for example, TMAH (tetramethylammonium hydroxide) or the like. 
         [0120]    Next, an electrode film  118  is formed on the exposed oxide film  112 , and a protective film (SiN)  119  is formed thereon (S 8 ). 
         [0121]    Thereby, it is possible to produce an ultrasound transducer in which membrane supporting members are provided at the nodes that are caused when membranes vibrate freely. Also, in the present embodiment, a method of producing a cMUT according to the first embodiment has been explained as an example. However, cMUTs according to the other embodiments can be produced similarly by using the micromachining process. 
         [0122]    Also, the scope of the present invention is not limited to any of the above described embodiments, and it is possible to employ various configurations or shapes without departing from the spirit of the present invention. The first through seventh embodiments can be combined in various combinations. 
         [0123]    Also, the ultrasound transducer according to the present invention can be included in an endoscopic ultrasound diagnosis system as an ultrasound endoscope, a miniature ultrasound probe, an in-vessel ultrasound probe, or an ultrasound capsule endoscope. 
         [0124]    According to the present invention, the respective membranes are independent from the membranes of the adjacent cells so that it is possible to avoid dissipation of the vibrations in the plane direction. 
         [0125]    Also, according to the present invention, the membranes are supported at the nodes, which do not vibrate inherently, so that the vibrations of the membranes do not leak via the supporting members (in other words, vibration loss of the membranes does not occur) Accordingly, a high mechanical quality factor Q is obtained. As a result of this, the ultrasound transmission efficiency increases. 
         [0126]    Also, according to the present invention, the vibrations of the membranes do not leak via the supporting members, and accordingly crosstalk can be avoided, whereby vibrations that would leak via supporting members become the longitudinal waves, the longitudinal waves are reflected by the back plane of the silicon substrate, and the reflected waves are converted into membrane vibrations of the adjacent membrane cells through the supporting members of the adjacent cells.