Abstract:
High transfer sound pressure and high reception sensitivity are realized, and reliability is improved in terms of long term operation, in a capacitive detector-type ultrasonic transducer (CMUT). The ultrasonic transducer, which has a lower electrode ( 201 ), a hollow portion ( 202 ) that is formed on the lower electrode and surrounded by insulating films ( 209,208 ), an upper electrode ( 205 ) that is formed on the hollow portion, and a plurality of insulating film projections ( 204 ) that are formed in the hollow portion ( 202 ), comprises a plurality of rigid members ( 203 ) that are formed on the hollow portion, either the lower electrode ( 201 ) and/or the upper electrode ( 205 ) is disposed in a position that does not overlap with the insulating film projections ( 204 ) when viewed from the upper surface by carving out the portion that overlaps with the insulating film projections ( 204 ), and the respective rigid members ( 203 ) are disposed such that a region is present that overlaps with the insulating film projections ( 204 ) when viewed from the upper surface.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an ultrasonic transducer and an ultrasonic diagnostic equipment using the ultrasonic transducer. More particularly, the present invention is concerned with an ultrasonic transducer manufactured using a micro-electro-mechanical systems (MEMS) technology. 
       BACKGROUND ART 
       [0002]    Ultrasonic transducers that transmit or receive an ultrasonic wave are employed in diagnosis of a tumor or the like in a human body, nondestructive testing of a structure, sensing of the velocity of a fluid, or the like. 
         [0003]    Ultrasonic transducers that utilize vibrations of a piezoelectric entity have been used in the past. With recent advancement of the MEMS technology, a capacitive micromachined ultrasonic transducer (CMUT) having a vibration unit fabricated on a silicon substrate is being developed in earnest. 
         [0004]    For example, patent literature 1, patent literature 2, and patent literature 3 have disclosed improvement of reliability in activities of a CMUT. Patent literature 4 has disclosed a method of controlling a center frequency and a bandwidth offered by the CMUT. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: U.S. Patent Application Publication No. 2005/0228285 
         Patent Literature 2: U.S. Patent Application Publication No. 2009/0322181 
         Patent Literature 3: Japanese Patent Application Laid-Open No. 2007-74263 
         Patent Literature 4: International Publication No. 2007-046180 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    The patent literature 1 has disclosed a structure in which: projections of an insulating film jutting out to a hollow portion of a CMUT are formed; and even when a direct-current (dc) voltage or alternating-current (ac) voltage equal to or higher than a collapse voltage is applied, although the projection undersurfaces come into contact with the hollow portion bottom, a membrane undersurface does not come into contact with the hollow portion bottom. However, since the projections are sandwiched between upper and lower electrodes, charge injection into the insulating film of the projections cannot be avoided. 
         [0010]    In contrast, the patent literature 2 describes a structure in which portions of an electrode are pierced for fear fields in which a membrane undersurface comes into contact with a hollow portion bottom may be sandwiched between upper and lower electrodes. In the case of the structure, since the electric field strength in a membrane insulating film in the contact fields is decreased, charge injection can be avoided. However, the area of layered portions of the upper and lower electrodes diminishes accordingly. This invites a rise in a driving voltage for a CMUT or degradation in receiving sensitivity. 
         [0011]    In the patent literature 3, projections of an insulating film jutting out to a hollow portion are not sandwiched between upper and lower electrodes. Even in this case, by the same reason as the foregoing one, a rise in a driving voltage for a CMUT or degradation in receiving sensitivity is invited. 
         [0012]    In the patent literature 4, a membrane is provided with rigid members in order to adjust the rigidness of the entire membrane. Thus, the center frequency and bandwidth offered by a CMUT are controlled. However, introduction of the rigid members brings about a rise in a driving voltage. How to ensure reliability in device activities becomes an object to be accomplished. 
         [0013]    Accordingly, an object of the present invention is to provide an ultrasonic transducer which is a CMUT that has projections jutted out to a hollow portion and has at least one of an upper electrode and lower electrode located at a position at which the electrode is not layered over the projections when viewed from the upper surface, and an ultrasonic diagnostic equipment employing the CMUT. Herein, a voltage margin between the instant projection undersurfaces come into contact with a hollow portion bottom and the instant a membrane undersurface comes into contact with the hollow portion bottom is preserved, charge injection into a membrane insulating film is suppressed, and a high transmission sound pressure and a highly reliable structure are realized. 
       Solution to Problem 
       [0014]    In order to address the foregoing problem, an ultrasonic transducer of the present invention includes a lower electrode, a hollow portion formed over the lower electrode and enclosed with an insulating film, an upper electrode formed over the hollow portion, and plural projections of the insulating film formed in the hollow portion. The ultrasonic transducer has plural rigid members formed over the hollow portion. At least one of the lower electrode and upper electrode has portions thereof, which are layered over the projections of the insulating film, pierced, so that the electrode is located at a position at which the electrode is not layered over the projections of the insulating film when viewed from the upper surface. The rigid members are disposed so that they have fields which are layered over the respective projections of the insulating film when viewed from the upper surface. 
         [0015]    In the ultrasonic transducer of the present invention, the rigid members are beam members. A membrane including the insulating film, upper electrode, and beam members may have a portion thereof, in which the beam members are disposed, made thicker than a portion thereof, in which the beam members are not disposed, by the thickness of the beam members. 
         [0016]    The ultrasonic transducer of the present invention includes an upper insulating film formed to cover the upper electrode and hollow portion. The beam members may be disposed over the upper insulating film. 
         [0017]    The ultrasonic transducer of the present invention includes an upper insulating film formed to cover the upper electrode and hollow portion. The beam members may be embedded in the upper insulating film. 
         [0018]    In the ultrasonic transducer of the present invention, the rigid members may be members exhibiting a higher Young&#39;s modulus than the membrane, which includes the insulating film, upper electrode, and beam members, does. 
         [0019]    The ultrasonic transducer of the present invention includes an upper insulating film formed to cover the upper electrode and hollow portion. The members of the higher Young&#39;s modulus may be embedded in the upper insulating film. 
         [0020]    In the ultrasonic transducer of the present invention, the members of the higher Young&#39;s modulus may be formed by embedding a metallic material such as tungsten or a ceramic material such as alumina. 
         [0021]    In the ultrasonic transducer of the present invention, the members of the higher Young&#39;s modulus may be formed by reforming the upper insulating film through ion implantation. 
         [0022]    In the ultrasonic transducer of the present invention, the rigid members may be disposed so that the centers thereof coincide with the centers of the projections of the insulating film when viewed from the upper surface. 
         [0023]    In the ultrasonic transducer of the present invention, the rigid members may be disposed so that one rigid member is layered over plural projections of the insulating film. 
         [0024]    In the ultrasonic transducer of the present invention, the projections of the insulating film may be disposed over the top of the hollow portion. 
         [0025]    In the ultrasonic transducer of the present invention, the projections of the insulating film may be disposed over the bottom of the hollow portion. 
         [0026]    In the ultrasonic transducer of the present invention, the plural projections of the insulating film and the plural rigid members may be equidistantly disposed when viewed from the upper surface. 
         [0027]    In the ultrasonic transducer of the present invention, the hollow portion may be circular or polygonal when viewed from the upper surface. 
         [0028]    In the ultrasonic transducer of the present invention, the projections of the insulating film may be circular or polygonal when viewed from the upper surface. 
         [0029]    In the ultrasonic transducer of the present invention, the rigid members may be circular, cruciform, or polygonal when viewed from the upper surface. 
         [0030]    An ultrasonic diagnostic equipment of the present invention includes any of the foregoing ultrasonic transducers and a bias unit. 
       Advantageous Effects of Invention 
       [0031]    According to the present invention, a capacitive micromachined ultrasonic transducer (CMUT) can realize a high transmission sound pressure and high receiving sensitivity, and improve reliability in long-term driving. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0032]      FIG. 1  is a cross-sectional diagram of an ultrasonic transducer in accordance with an embodiment 1 of the present invention. 
           [0033]      FIG. 2  is a top view of the ultrasonic transducer in accordance with the embodiment 1 of the present invention; 
           [0034]      FIG. 3  is a cross-sectional diagram of a CMUT which the present inventor et al. have discussed. 
           [0035]      FIG. 4  is a cross-sectional diagram showing a state in which projections of the ultrasonic transducer which the present inventor et al. have discussed come into contact with a hollow portion bottom. 
           [0036]      FIG. 5  is a cross-sectional diagram showing a membrane of the ultrasonic transducer, which the present inventor et al. have discussed, that comes into contact with the hollow portion bottom. 
           [0037]      FIG. 6  is a cross-sectional diagram showing a state in which the projections come into contact with the hollow portion bottom. 
           [0038]      FIG. 7  is a graph plotted based on a distance from the center of a projection in order to express a deformation of the membrane occurring when a certain external force is applied to the membrane with the projections brought into contact with the hollow portion bottom. 
           [0039]      FIG. 8A  shows the positional relationship between a rigid member and a projection. 
           [0040]      FIG. 8B  is a graph expressing a relationship between a deviation quantity of a rigid member disposed in the membrane from the center of a projection and a deformation near the projection. 
           [0041]      FIG. 8C  is a graph expressing a relationship between a deviation quantity of a rigid member disposed in the membrane from the center of a projection and a membrane contact voltage. 
           [0042]      FIG. 9  is a top view of the ultrasonic transducer in accordance with the embodiment 1 of the present invention showing a case where the centers of the rigid members do not coincide with the centers of the projections. 
           [0043]      FIG. 10  is a top view of the ultrasonic transducer in accordance with the embodiment 1 of the present invention showing a case where one of the rigid members is layered over plural projections. 
           [0044]      FIG. 11  is a top view of the ultrasonic transducer in accordance with the embodiment 1 of the present invention showing a case where the rigid members are cruciform. 
           [0045]      FIG. 12  is a cross-sectional diagram of another ultrasonic transducer in accordance with the embodiment 1 of the present invention. 
           [0046]      FIG. 13  is a cross-sectional diagram of an ultrasonic transducer in accordance with an embodiment 2 of the present invention. 
           [0047]      FIG. 14  is a cross-sectional diagram of another ultrasonic transducer in accordance with the embodiment 2 of the present invention. 
           [0048]      FIG. 15  is a cross-sectional diagram of an ultrasonic transducer in accordance with an embodiment 3 of the present invention. 
           [0049]      FIG. 16  is a top view of the ultrasonic transducer in accordance with the embodiment 3 of the present invention. 
           [0050]      FIG. 17  is a cross-sectional diagram of an ultrasonic transducer in accordance with an embodiment 4 of the present invention. 
           [0051]      FIG. 18  is a top view of an ultrasonic transducer in accordance with an embodiment 5 of the present invention; 
           [0052]      FIG. 19  is a top view of an ultrasonic transducer in accordance with an embodiment 6 of the present invention. 
           [0053]      FIG. 20  is a block diagram of the configuration of an ultrasonic diagnostic equipment in accordance with an embodiment 7 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0054]    Referring to the diagrams, embodiments of the present invention will be described below. 
         [0055]    In the embodiments to be described below, for convenience&#39; sake, plural sections or embodiments will be dividedly described when needed. Unless explicitly mentioned, the sections or embodiments are not unrelated to one another, but one of the sections or embodiments is part of any other one or a variant of all of the others, or is cited in order to detail or supplement any other section or embodiment. In the embodiments to be described below, when the number of elements (the number of pieces, a numerical value, a quantity, a range, or the like) is stated, unless the number of elements is explicitly specified or is in principle apparently limited to a specific value, the number of elements is not limited to the specific value but may be equal to or larger than the specific value or may be equal to or smaller than the specific value. Further, in the embodiments to be described below, unless components (encompassing element steps) are explicitly specified or thought to be in principle apparently essential, the components are not always needed. Likewise, in the embodiments to be described below, when the shape of a component or the like or a positional relationship between components is stated, unless the shape or positional relationship is explicitly specified or is not in principle apparently thought to be correct, the shape encompasses a substantially approximate or analogous shape. The same applies to the numerical value or range. Even in plan views, hatching may be performed for a better understanding. 
         [0056]    In the embodiments to be described below, in a capacitive micromachined ultrasonic transducer (CMUT), an object of suppressing charge injection to a membrane insulating film or a decrease in a dielectric strength by expanding a voltage margin between a state in which the undersurfaces of projections jutting out to a hollow portion come into contact with the hollow portion bottom and a state in which a membrane undersurface comes into contact with the hollow portion bottom is accomplished with such a disposition that at least parts of rigid members or high-Young&#39;s modulus members which are included in the membrane are layered over the projections jutting out to the hollow portion when the CMUT is viewed from the upper surface. 
       Embodiment 1 
       [0057]    Referring to  FIG. 3 , a fundamental structure of a CMUT and activities thereof will be described below. A hollow portion  102  enclosed with an insulating film  103  is formed in an upper layer of a lower electrode  101 . An upper electrode  104  is disposed over the hollow portion  102  with the insulating film  103  between them. When a dc voltage and ac voltage are convoluted between the upper electrode  104  and lower electrode  101 , electrostatic force works between the upper electrode  104  and lower electrode  101 . A membrane  105  including the insulating film  103  and upper electrode  104  over the hollow portion  102  is vibrated at the frequency of the applied ac voltage, whereby an ultrasonic wave is originated. 
         [0058]    For receiving, when the dc voltage alone is applied between the upper electrode  104  and lower electrode  101 , the membrane  105  is vibrated due to the pressure of an ultrasonic wave having reached the surface of the membrane  105 . This causes the distance between the upper electrode  104  and lower electrode  101  to vary. Eventually, the ultrasonic wave can be detected as a change in a capacitance. 
         [0059]    One of the most significant features of an ultrasonic transducer is that a signal-to-noise ratio is high. For example, when it says that an ultrasonic diagnostic equipment or flaw inspection apparatus offers a high signal-to-noise ratio, it means that image quality is sharp. The high signal-to-noise ratio is significant in terms of the performance of the equipment or apparatus. In order to raise the signal-to-noise ratio to be offered by a CMUT, a capacitance change at a receiving time should be increased. The capacitance change may be increased by increasing the number of CMUTs or expanding the area of the CMUT. In this case, the size of the transducer gets larger. Therefore, it is necessary to increase the capacitance change per unit area. In other words, it is necessary to improve receiving sensitivity. Otherwise, by increasing a transmission sound pressure so as to raise a sound pressure of ultrasonic waves reflected from an object of inspection, the capacitance change at the receiving time can be increased. 
         [0060]    In order to improve receiving sensitivity, the spacing between the upper electrode  104  and lower electrode  101  should be narrowed as greatly as possible. Therefore, it is necessary to apply the highest possible dc voltage. However, if an equivalent in vacuum of a magnitude of a deformation of the membrane corresponds to a distance equal to or larger than a one-third of the distance between the electrodes, an electrostatic force between the electrodes gets larger than a spring resilience of the membrane. Eventually, a membrane undersurface  106  comes into contact with a hollow portion bottom  107 . The voltage is called a collapse voltage. For these reasons, a dc voltage that is slightly lower than the collapse voltage is applied at the receiving time. 
         [0061]    In order to improve a transmission sound pressure, the amplitude of the membrane should preferably be maximized. However, if the membrane undersurface  106  comes into contact with the hollow portion bottom  107 , charge injection occurs in the insulating film. This causes a transmitting/receiving characteristic to drift. Therefore, a driving condition for a CMUT has to be designated to such an extent that the membrane undersurface  106  does not come into contact with the hollow portion bottom  107 . In reality, since the height of the hollow portion  102  varies due to a fabrication variance, the driving condition is that the membrane should not come into contact with the bottom. In addition, the membrane is designed with a margin preserved, and the amplitude of the membrane is much smaller than the height of the hollow portion  102 . Accordingly, the upper limit of the transmission sound pressure is restricted. 
         [0062]    Projections of the insulating film jutting out to the hollow portion are formed, and the electrode is pieced in order to realize a structure in which the projections are not sandwiched between the upper and lower electrodes. Thus, the electric field strength at projection contact points is decreased, and charge injection can therefore be avoided. However, since the electrode is pierced, the area of layered portions of the upper and lower electrodes gets smaller. This invites a rise in a driving voltage for a CMUT or degradation in receiving sensitivity. 
         [0063]    In order to realize a high transmission sound pressure without causing a drift in the transmitting/receiving characteristic or inviting a rise in the driving voltage for the CMUT or degradation in receiving sensitivity, it is necessary to minimize the number of projections and an area by which the electrode is pierced. What should be noted will be described below in conjunction with  FIG. 4  and  FIG. 5 .  FIG. 4  is a cross-sectional diagram showing a state in which projections  108  formed on the lower side of a membrane  105  are in contact with the hollow portion bottom  107 . This state shall be called a projections-in-contact state. In order to obtain the high transmission sound pressure, the amplitude of the membrane should preferably be maximized. Therefore, a voltage equal to or higher than a voltage that brings about the projections-in-contact state is applied. When the voltage equal to or higher than the voltage bringing about the projections-in-contact state is applied, the membrane  105  between the projections  108  deforms to approach the hollow portion bottom  107 . The membrane  107  comes into contact with the hollow portion bottom  107  at a voltage equal to or higher than a certain voltage. This state shall be called a membrane-in-contact state, and is shown in  FIG. 5 . In the membrane-in-contact state, charge injection to the membrane  105  takes place. Therefore, a key to improvement of activity reliability is to expand a voltage margin between the membrane-in-contact state and projections-in-contact state so as to avoid the membrane-in-contact state. In order to expand the voltage margin, the rigidness of the entire membrane should be upgraded. However, since a driving voltage is also raised, it is unacceptable. If the rigidness of the entire membrane observed with the projection undersurfaces  106  in non-contact with the hollow portion bottom  107  is not upgraded but the rigidness of the membrane in the projections-in-contact state is upgraded, the margin voltage can be expanded. The transmission sound pressure can be improved without the drift in the transmitting/receiving characteristic. 
         [0064]    The cross-sectional structure of a CMUT in accordance with the present embodiment will be described below in conjunction with  FIG. 1 . The CMUT includes a parallelepiped lower electrode  201  disposed over a substrate  206 , a parallelepiped hollow portion  202  disposed over the lower electrode  201 , and a parallelepiped upper electrode  205  disposed over the hollow portion  202 . An insulating film  209  is formed to cover the lower electrode  201  between the lower electrode  201  and hollow portion  202 , and an insulating film  208  is formed to cover the hollow portion  202  and lower electrode  201  between the upper electrode  205  and hollow portion  202 . In the hollow portion  202 , at least one or more cylindrical projections  204  that jut out from the undersurface of the insulating film  208  to the hollow portion  202  are disposed. The insulating film  208  is coated with an insulating film  207 . The foregoing projections  204  and the insulating films  207 ,  208 , and  209  are made of a silicon oxide or silicon nitride. Over the top of the insulating film  207 , at least one or more parallelepiped beam members that are rigid members  203  are disposed. The beam members that are the rigid members  203  may be, similarly to the insulating films  207 ,  208 , and  209 , made of the silicon oxide, silicon nitride, or the like, or may be made of another material. A membrane  210  including the insulating film  207 , upper electrode  205 , and insulating film  208  is vibrated, whereby an ultrasonic wave is transmitted. For convenience&#39; sake, the surfaces of the projections  204  opposed to the insulating film  209  with the hollow portion between them shall be called projection undersurfaces  211 , and the surface of the insulating film  209  exposed to the hollow portion shall be called a hollow portion bottom  213 . The surface of the membrane  210  exposed to the hollow portion shall be called a membrane undersurface  212 . Further, an incident that the projection undersurfaces  211  come into contact with the hollow portion bottom  213  shall be called projection contact, and a voltage at that time shall be called a projection contact voltage. An incident that the membrane undersurface  212  comes into contact with the hollow portion bottom  213  shall be called membrane contact, and a voltage at that time shall be called a membrane contact voltage. A difference between the membrane contact voltage and projection contact voltage shall be called a margin voltage. 
         [0065]    A fabrication method for the CMUT shown in  FIG. 1  will be described below. The fundamental fabrication method is described in the patent literature 3. Herein, a formation method for a beam member that is the rigid member  203  will be described below. The beam member is formed according to a photography technique and dry etching technique after a thin film which is a material of the beam member, for example, a silicon oxide or silicon nitride is deposited over the insulating film  207  according to a plasma chemical vapor deposition (CVD) method. Since the thickness of the membrane  210  largely affects a device characteristic, dry etching should be carefully performed for fear the insulating film  207  may get thinner. For example, the beam member that is the rigid member  203  is formed as a laminated film having the silicon nitride and silicon oxide layered in that order when viewed from the upper surface, and the insulating film  207  is made of the silicon nitride. Thus, a shaved volume of the insulating film  207 , which serves as a bed, to be observed after etching the beam member that is the rigid member  203  is completed gets diminished, and a change in the thickness of the membrane  210  before and after the etching is performed can be suppressed. 
         [0066]    The top structure of the CMUT in accordance with the present embodiment will be described in conjunction with  FIG. 2 .  FIG. 2  is the top view of the structure shown in  FIG. 1 .  FIG. 1  is a cross-sectional diagram showing a section cut along a line A-A° in  FIG. 2 , In  FIG. 2 , for convenience&#39; sake, the projections  204  or the like are seen through the rigid members  203 . Hereinafter, for convenience&#39; sake, as for the hollow portion  202  and upper electrode  205 , they shall have a width in the lengthwise direction of the sheet of paper and a lengths thereof in the sideways direction thereof. As for the rigid members  203 , they shall have a length in the lengthwise direction of the sheet of paper and a width in the sideways direction thereof. To begin with, the shape of the upper electrode  205  will be described. For example, the upper electrode  205  has plural circular holes. When viewed from the upper surface, the respective projections  204  are disposed nearly in the center positions of the plural circular holes  214  of the upper electrode  205 . When viewed from the upper surface, since the projections  204  are disposed in the holes  214  of the upper electrode  205 , the projections are located at positions at which the upper electrode  205  is not layered over the projections. In short, the upper electrode  205  is absent from the vertical directions of the projections  204 . In  FIG. 2 , the upper electrode  205  is partly pierced, and is disposed so that the upper electrode  205  is not layered over the projections  204  when viewed from the upper surface. Alternatively, the electrode to be pierced may be the lower electrode  201 . What counts is that at least one of the upper electrode  205  and lower electrode  201  should be located at a position at which the electrode is not layered over the projections  204  when viewed from the upper surface. The reason why the disposition is adopted is to increase an inter-electrode distance by piercing the electrode and thus decrease electric field strength for fear a strong electric field may be applied to each of the projections  204  and insulating film  209  and charge injection may thus take place in case a dc voltage or ac voltage is applied across the electrodes and the projection undersurfaces  211  come into contact with the hollow portion bottom  213  accordingly. 
         [0067]    The positional relationship between the rigid members  203  and projections  204  will be described in conjunction with  FIG. 2 . At least one or more projections  204  are disposed in the center in the width direction of the hollow portion  202  when viewed from the upper surface. The rigid members  203  are disposed so that they are even partly layered over the holes  214  of the upper electrode  205  and the projections  204  alike when viewed from the upper surface. In  FIG. 1  and  FIG. 2 , the rigid members  203  and projections  204  are equidistantly disposed in the length direction of the hollow portion when viewed from the upper surface. Alternatively, the rigid members  203  and projections  204  may be disposed non-equidistantly. However, in this case, plural membrane contact voltages are present within one CMUT. This is not preferred in terms of design. Therefore, the rigid members and projections should be disposed equidistantly. The spacing between the rigid members  203  or between the projections  204  should be determined based on the magnitude relationship between the membrane contact voltage and projection contact voltage. For example, if the membrane contact voltage is lower than the projection contact voltage, the spacing between the rigid members  203  or projections  204  is narrowed in order to raise the membrane contact voltage. However, when the spacing between the projections  204  is narrowed, the area of the upper electrode  205  decreases. This invites degradation in sensitivity of the CMUT. Therefore, an inter-projection spacing that meets a margin voltage which is minimum necessary on a design stage should be adopted. 
         [0068]    A feature of the embodiment 1 is, as shown in  FIG. 1  and  FIG. 2 , that the CMUT has at least one of the upper electrode  205  and lower electrode  201  located at a position at which the electrode is not layered over the projections  204 , which jut out to the hollow portion  202 , when viewed from the upper surface. In the CMUT, the rigid members  203  included in the membrane  210  and the projections  204  are disposed to be even partly layered when viewed from the upper surface. Owing to the structure, the membrane immediately above the projections  204  gets thicker, rigidness is upgraded, and a margin voltage can be raised. In other words, even when the membrane  210  is vibrated to the greatest extent for the purpose of obtaining a high transmission sound pressure under a driving condition that the projection undersurfaces  211  come into contact with the hollow portion bottom  213 , membrane contact can be prevented. In an actual CMUT, the membrane contact voltage may be partly lower than a design value because of a fabrication variance. By adopting the foregoing structure, the membrane contact can be prevented, and charge injection to the insulating film  208  of the membrane  210  can be minimized. Eventually, the activity reliability of the CMUT can be improved. 
         [0069]    The reason why disposing the rigid member  203  and projection  204  so that the rigid members  203  and projections  204  are even partly layered when viewed from the upper surface is advantageous will be described in conjunction with  FIG. 6 . Noted herein is a point that the membrane  210  in the fields in which the membrane  210  is layered over the projections  204  when viewed from the upper surface is made thicker in order to locally upgrade rigidness. In  FIG. 6 , the insulating film  207 , upper electrode  205 , and insulating film  208  shown in  FIG. 1  are all shown as a membrane  310 .  FIG. 7  shows a result of simulation obtained by uniformly applying an external force of a certain magnitude to the entire membrane  310  in a projections-in-contact state shown in  FIG. 6 , and plotting the deformation of the membrane  310  in relation to a distance from the center of the projection  304 . The deformation takes on a maximum value near the projection  304 , and monotonously decreases as the projection gets farther. The result signifies that when the membrane  310  deforms in the projections-in-contact state, since the projection  304  serves as a fixed end, the deformation near the projection  304  is maximized. Therefore, if the deformation near the projection can be minimized, a margin voltage can be expanded.  FIG. 8A  to  FIG. 8C  include graphs expressing the relationship between the positional relationship between a rigid member and a projection and the magnitude of the deformation near the projection. As shown in  FIG. 8B , when a deviation quantity d between the rigid member and projection is small, the deformation near the projection is small. When the center of the rigid member coincides with the center of the projection, the deformation is a minimum. In contrast, when the rigid member and projection are not layered, the deformation near the projection is large.  FIG. 8C  shows the relationship between the deviation quantity d between the rigid member and projection and a membrane contact voltage. When the deviation quantity d is small, the membrane contact voltage is high. When the deviation quantity d gets larger, the membrane contact voltage decreases. Accordingly, wherever the rigid member is disposed largely affects the deformation near the projection. When the rigid member is disposed so that the center of the rigid member coincides with the center of the projection, the deformation near the projection is minimized. The deposition is therefore effective. However, similarly to a structure shown in  FIG. 9  that is a top view, when parallelepiped rigid members  303  and cylindrical projections  304  are even partly layered when viewed from the upper surface, an effect of minimizing the deformations near the respective projections is exerted. The shape and disposition of the rigid members  303  viewed from the upper surface should be appropriately determined depending on a desired frequency characteristic. As shown in  FIG. 10 , one parallelepiped rigid member  303  may be layered over at least parts of plural cylindrical projections  304  when viewed from the upper surface. Even in this structure, the plural projections  304  are disposed so that the upper electrode  305  is not layered over the plural projections  304 . The rigid members  303  are disposed to be at least partly layered over the plural projections  304  when viewed from the upper surface. The structure would prove useful as a means for raising the membrane contact voltage in case the width of a hollow portion  302  is large and the rigidness in the width direction of the hollow portion is low. However, when the number of projections  304  is increased, the area of the upper electrodes  305  is decreased and the sensitivity of the CMUT is degraded. The number of projections  304  should be a minimum necessary value. As shown in  FIG. 11 , the shape of the rigid members  303  may be cruciform. In this structure, at a position in a membrane between the projections  304  at which the rigid member  303  is located, the membrane gets thicker. Thus, the rigidness can be improved, and the contact voltage of the inter-projection membrane can be raised. By enlarging the width of the rectangular rigid members  203  shown in  FIG. 2 , the rigidness of the inter-projection membrane can be upgraded. However, the area of the rigid members  203  gets larger, the rigidness of the entire membrane gets higher, and a driving voltage gets higher. This is disadvantageous in terms of reliability. Namely, the cruciform rigid members  303  shown in  FIG. 11  make it possible to raise the contact voltage of the inter-projection membrane without a large change in the driving voltage. The structures having rigid members disposed over the surface of the membrane have been described so far. Alternatively, as shown in  FIG. 12 , the rigid members  303  may be embedded in the membrane  310 . The shape of the membrane  310  is preferably flat, but may dilate or dent due to residual stresses of films included. As shown in  FIG. 12 , by varying the positions at which the respective rigid members  303  are embedded, a residual stress distribution in the membrane  310  can be controlled. The dilated or dented shape of the membrane  310  can be controlled. 
       Embodiment 2 
       [0070]    The structure of a CMUT in accordance with an embodiment 2 will be described in conjunction with  FIG. 13 . The CMUT includes a parallelepiped lower electrode  501  disposed over a substrate  506 , a hollow portion  502  disposed over the lower electrode  501 , and a parallelepiped upper electrode  505  disposed over the hollow portion  502 . An insulating film  509  is formed to cover the lower electrode  501  between the lower electrode  501  and hollow portion  502 , and an insulating film  508  is formed to cover the hollow portion  502  and lower electrode  501  between the upper electrode  505  and hollow portion  502 . At least one or more cylindrical projections  504  jutting out from the top of the insulating film  509  to the hollow portion  502  are disposed in the hollow portion  502 . The projections  504  are formed with the insulating film. Over the top of the insulating film  507 , at least one or more parallelepiped rigid members  503  are disposed. Beam members that are the rigid members  503  may be, similarly to the insulating films  507 ,  508 , and  509 , made of a material such as a silicon oxide or silicon nitride, or may be made of another material. A membrane  510  including the insulating film  507 , upper electrode  505 , and insulating film  508  is vibrated in order to transmit an ultrasonic wave. Hereinafter, for convenience&#39; sake, the surfaces of the projections  504  opposed to the insulating film  508  with the hollow portion between them shall be called projection tops  514 , and the surface of the insulating film  509  exposed to the hollow portion shall be called a hollow portion bottom  513 . The surface of the membrane  510  exposed to the hollow portion shall be called a membrane undersurface  512 . An incident that the projection tops  514  come into contact with the membrane undersurface  512  shall be called projection contact, and a voltage at that time shall be called a projection contact voltage. An incident that the membrane undersurface  512  comes into contact with the hollow portion bottom  513  shall be called membrane contact, and a voltage at that time shall be called a membrane contact voltage. A difference between the membrane contact voltage and projection contact voltage shall be called a margin voltage. The spacing between the rigid members  503  or projections  504  should be, similarly to the contents described in relation to the embodiment 1, determined based on the magnitude relationship between the membrane contact voltage and projection contact voltage. For example, when the membrane contact voltage is lower than the projection contact voltage, the spacing between the rigid members  503  or between the projections  504  is narrowed in order to raise the membrane contact voltage. 
         [0071]    A difference from the aforesaid embodiment 1 lies in a point that the projections  504  jut out from the hollow portion bottom  513  to the hollow portion  502 . In this structure, if the membrane undersurface  512  comes into contact with the projection tops  514 , the structure becomes identical to the structure that is shown in  FIG. 6  and that has the projections  304  jutted out from the membrane undersurface  312  to the hollow portion  302 . Even in the structure having the projections  504  jutted out from the hollow portion bottom  513 , if the centers of the rigid members coincide with the centers of the projections when viewed from the upper surface, the difference between a projection contact voltage and membrane contact voltage can be increased. Therefore, the same advantage as that of the ultrasonic transducer of the embodiment 1 can be provided. 
         [0072]    Similarly to the ultrasonic transducer of the embodiment 1, the disposition in which the centers of the rigid members  503  coincide with the centers of the projections  504  when viewed from the upper surface provides the greatest advantage. If the rigid members are even partly layered over the projections, an advantage is provided. When this structure is adopted, since the projections are absent from the membrane side of a CMUT vibration unit, the frequency characteristic of the CMUT can be readily designed. The structure having the rigid members disposed over the surface of the membrane has been described so far. Alternatively, as shown in  FIG. 14 , the rigid members  503  may be embedded in the membrane  510 . This is advantageous as mentioned in relation to the embodiment 1 in that the shape of the membrane  510  can be readily controlled. 
       Embodiment 3 
       [0073]    The structure of a CMUT in accordance with an embodiment 3 will be described in conjunction with  FIG. 15  and  FIG. 16 .  FIG. 15  is a cross-sectional diagram showing one CMUT. A CMUT cell includes a parallelepiped lower electrode  701 , a hollow portion  702  disposed over the lower electrode  701 , a parallelepiped upper electrode  705  disposed over the hollow portion  702 , or the like. An insulating film  709  is formed to cover the lower electrode  701  between the lower electrode  701  and hollow portion  702 , and an insulating film  708  is formed to cover the hollow portion  702  and lower electrode  701  between the upper electrode  705  and hollow portion  702 . At least one or more cylindrical projections  704  that jut out from the undersurface of the insulating film  708  to the hollow portion  702  are disposed in the hollow portion  702 . The projections  704  are formed with the insulating film. The projections  704  and insulating films  707 ,  708 , and  709  are made of a silicon oxide, silicon nitride, or the like. Parallelepiped high-Young&#39;s modulus members  703  are embedded in a membrane  710 . In  FIG. 15 , the high-Young&#39;s modulus members  703  are embedded in the insulating film  707 . Alternatively, the high-Young&#39;s modulus members  703  may be embedded in the insulating film  708 , or may be embedded in both of the insulating film  707  and insulating film  708 . The high-Young&#39;s modulus members  703  are made of a material whose Young&#39;s modulus is higher than that of the surrounding insulating film  707  or insulating film  708 , for example, a metallic material such as tungsten or a ceramic material such as alumina, or formed by reforming the insulating film  707  or insulating film  708  through ion implantation so as to improve the Young&#39;s modulus. Hereinafter, for convenience&#39; sake, the surfaces of the projections  704  opposed to the insulating film  709  with the hollow portion between them shall be called projection undersurfaces  711 , and the surface of the insulating film  709  exposed to the hollow portion shall be called a hollow portion bottom  713 . The surface of the membrane  710  exposed to the hollow portion shall be called a membrane undersurface  712 . An incident that the projection undersurfaces  711  come into contact with the hollow portion bottom  713  shall be called projection contact, and a voltage at that time shall be called a projection contact voltage. An incident that the membrane undersurface  712  comes into contact with the hollow portion bottom  713  shall be called membrane contact, and a voltage at that time shall be called a membrane contact voltage. 
         [0074]    The positional relationship between the high-Young&#39;s modulus members  703  and projections  704  will be described below in conjunction with  FIG. 16 .  FIG. 15  is a cross-sectional diagram showing a section cut along a line A-A′ in  FIG. 16 . In  FIG. 16 , for convenience&#39; sake, the projections  704  and others are seen through the high-Young&#39;s modulus members  703 . Hereinafter, for convenience&#39; sake, as for the hollow portion  702  and upper electrode  705 , they shall have a width in the lengthwise direction of a sheet of paper and a length in the sideways direction thereof. As for the high-Young&#39;s modulus members  703 , they shall have a length in the lengthwise direction of the sheet of paper and a width in the sideways direction thereof. At least one or more projections  704  are disposed in the center in the width direction of the hollow portion  702  when viewed from the upper surface. The high-Young&#39;s modulus members  703  are disposed so that the centers thereof coincide with the centers of the projections  704  when viewed from the upper surface. At least one of the upper electrode  705  and lower electrode  701  is located at a position at which the electrode is not layered over the projections  704  when viewed from the upper surface. In  FIG. 16 , the upper electrode  705  is partly pierced and disposed so that the projections  704  and upper electrode  705  are not layered when viewed from the upper surface. Alternatively, the electrode to be pierced may be the lower electrode  701 . The reason why the disposition is adopted is to increase an inter-electrode distance by piercing the electrode and thus decrease electric field strength for fear a strong electric field may be applied to each of the projections  704  and insulating film  709  and charge injection may thus take place in case a dc voltage or ac voltage is applied across the electrodes and the projection undersurfaces  711  come into contact with the hollow portion bottom  713  accordingly. In  FIG. 15  and  FIG. 16 , the high-Young&#39;s modulus members  703  and projections  704  are equidistantly disposed in the length direction of the hollow portion. Alternatively, the high-Young&#39;s modulus members  703  and projections  704  may be non-equidistantly disposed. However, when the high-Young&#39;s modulus members  703  and projections  704  are non-equidistantly disposed, plural membrane contact voltages are present in one CMUT. This is not preferred in terms of design. Therefore, the high-Young&#39;s modulus members  703  and projections  704  should be equidistantly disposed. The spacing between the high-Young&#39;s modulus members  703  or between the projections  704  may be determined based on the magnitude relationship between the membrane contact voltage and projection contact voltage. For example, if the membrane contact voltage is lower than the projection contact voltage, the spacing between the high-Young&#39;s modulus members  703  or between the projections  704  is narrowed in order to improve the membrane contact voltage. 
         [0075]    The feature of the embodiment 3 is, as shown in  FIG. 15  and  FIG. 16 , that in the CMUT having at least one of the upper electrode  705  and lower electrode  701  disposed at a position at which the electrode is not layered over the projections  704 , which jut out to the hollow portion  702 , when viewed from the upper surface, the high-Young&#39;s modulus members  703  included in the membrane  710  and the projections  704  are disposed to be even partly layered when viewed from the upper surface. Noted herein is a point that the high-Young&#39;s modulus members are disposed within the membrane in fields, in which the membrane is layered over the projections  704  when viewed from the upper surface, in order to locally upgrade rigidness. Owing to the structure, the difference between the projection contact voltage and membrane contact voltage can be made larger. In other words, even when the membrane  710  is vibrated to the greatest extent in order to obtain a high transmission sound pressure under a driving condition that the projection undersurfaces  711  come into contact with the hollow portion bottom  713 , membrane contact can be prevented. In an actual CMUT, the membrane contact voltage may be partly lower than a design value because of a variance stemming from fabrication. When the foregoing structure is adopted, the membrane contact can be prevented, and charge injection into the insulating film  708  of the membrane  710  can be minimized. Therefore, the activity reliability of the CMUT can be improved. In addition, the CMUT surface can be evened. 
       Embodiment 4 
       [0076]    The structure of a CMUT in accordance with an embodiment 4 will be described in conjunction with  FIG. 17 . The CMUT includes a parallelepiped lower electrode  701  disposed over a substrate  706 , a hollow portion  702  disposed over the lower electrode  701 , and a parallelepiped upper electrode  705  disposed over the hollow portion  702 . An insulating film  709  is formed to cover the lower electrode  701  between the lower electrode  701  and hollow portion  702 , and an insulating film  708  is formed to cover the hollow portion  702  and lower electrode  701  between the upper electrode  705  and hollow portion  702 . At least one or more cylindrical projections  704  jutting out from the top of the insulating film  709  to the hollow portion  702  are disposed in the hollow portion  702 . The projections  704  are formed with the insulating film. Over the top of the insulating film  707 , at least one or more parallelepiped high-Young&#39;s modulus members  703  are embedded. For convenience&#39; sake, the surfaces of the projections  704  opposed to the insulating film  708  with the hollow portion between them shall be called projection tops  714 , and the surface of the insulating film  709  exposed to the hollow portion shall be called a hollow portion bottom  713 . The surface of a membrane  710  exposed to the hollow portion shall be called a membrane undersurface  712 . An incident that the projection tops  714  come into contact with the membrane undersurface  712  shall be called a projection contact, and a voltage at that time shall be called a projection contact voltage. An incident that the membrane undersurface  712  comes into contact with the hollow portion bottom  713  shall be called a membrane contact, and a voltage at that time shall be called a membrane contact voltage. The spacing between the high-Young&#39;s modulus members  703  or projections  704  should be determined based on the magnitude relationship between the membrane contact voltage and projection contact voltage. For example, when the membrane contact voltage is lower than the projection contact voltage, the spacing between the high-Young&#39;s modulus members  703  or between the projections  704  is narrowed in order to raise the membrane contact voltage. 
         [0077]    A difference from the aforesaid embodiment 3 lies in a point that the projections  704  jut out from the hollow portion bottom  713  to the hollow portion  702 . In this structure, if the membrane undersurface  712  comes into contact with the projection tops  714 , the same state as the projections-in-contact state of the structure shown in  FIG. 15  is attained. Therefore, even in the structure having the projections  704  jutted out from the hollow portion bottom  713 , if the centers of the high-Young&#39;s modulus members coincide with the centers of the projections when viewed from the upper surface, the difference between the projection contact voltage and membrane contact voltage can be made larger, and the activity reliability of the CMUT can be improved. Therefore, the same advantage as that of the ultrasonic transducer of the embodiment 3 can be provided. Similarly to the ultrasonic transducer of the embodiment 3, if the high-Young&#39;s modulus members and projections are even partly layered when viewed from the upper surface, an advantage is provided. When the structure is adopted, since the projections are absent from the membrane side of the CMUT vibration unit, the frequency characteristic of the CMUT can be readily designed. 
       Embodiment 5 
       [0078]      FIG. 18  is a top view showing an ultrasonic transducer of an embodiment 5. The ultrasonic transducer includes circular projections  804 , a circular membrane  810 , and circular rigid members  803  or high-Young&#39;s modulus members  805  disposed over the top of the membrane  810 . The rigid members  803  and projections  804  are disposed in the center of the membrane  801  and at intermediate positions between the center of the membrane and each of the upper, lower, left, and right membrane edges. The rigid members  803  and projections  804  are disposed so that they are partly layered when viewed from the upper surface. The circular projections  804  are an example. A triangular, pentagonal, heptagonal, or any other polygonal shape will do. Even for the rigid members  803  or high-Young&#39;s modulus members  805 , the circular shape is an example, but a triangular, pentagonal, heptagonal, or any other polygonal shape will do. The number of rigid members  803  or projections  804 , and the disposition thereof may be determined based on the magnitude relationship between a projection contact voltage and a membrane contact voltage. For example, if the membrane contact voltage is lower than the projection contact voltage in the structure shown in  FIG. 18 , the number of rigid members  803  or projections  804  should be increased. In that case, the dispositional positions should be points at which the membrane comes into contact with the projections with the membrane contact voltage. 
       Embodiment 6 
       [0079]      FIG. 19  is a top view showing an ultrasonic transducer of an embodiment 6. The ultrasonic transducer includes circular projections  904 , an octagonal membrane  910 , and circular rigid members  903  or high-Young&#39;s modulus members  905  disposed over the top of the membrane  910 . The rigid members  903  and projections  904  are disposed in the center of the membrane  910  and around the center thereof. The rigid members  903  and projections  904  are disposed so that they are partly layered when viewed from the upper surface. The octagonal membrane  901  is an example. A triangular, pentagonal, heptagonal, or any other polygonal shape will do. The circular projections  904  are an example. A triangular, pentagonal, heptagonal, or any other polygonal shape will do. Even for the rigid members  903  or high-Young&#39;s modulus members  905 , the circular shape is an example, but a triangular, pentagonal, heptagonal, or any other polygonal shape will do. The number of rigid members  903  or projections  904  and the disposition thereof may be determined based on the magnitude relationship between a projection contact voltage and membrane contact voltage. For example, if the membrane contact voltage is lower than the projection contact voltage in the structure shown in  FIG. 19 , the number of rigid members  903  or projections  904  should be increased. In that case, the dispositional positions should be points at which the membrane comes into contact with the projections with the membrane contact voltage. 
       Embodiment 7 
       [0080]    Referring to  FIG. 20 , the configuration of an ultrasonic diagnostic equipment including an ultrasonic transducer of the present invention, and the activities thereof will be described below. An ultrasonic diagnostic equipment  1001  includes an ultrasonic probe  1002 , a transmitting/receiving separation unit  1003 , a transmission unit  1004 , a bias unit  1006 , a receiving unit  1008 , a phasing and summation unit  1010 , an image processing unit  1012 , a display unit  1014 , a control unit  1016 , and an operation unit  1018 . 
         [0081]    The ultrasonic probe  1002  is a device that is brought into contact with a subject and transmits or receives ultrasonic waves to or from the subject. The ultrasonic waves are transmitted from the ultrasonic probe  1002  to the subject, and reflected echo signals sent from the subject are received by the ultrasonic probe  1002 . The ultrasonic transducer in accordance with any of the embodiments 1 to 6 is stored in the ultrasonic probe  1002 , and electrically coupled to the transmitting/receiving separation unit  1003  to be described later. The transmission unit  1004  and bias unit  1006  are devices that feed a driving signal to the ultrasonic probe  1002 . The receiving unit  1008  is a device that receives the reflected echo signals outputted from the ultrasonic probe  1002 . The receiving unit  1008  further performs such processing as analog-to-digital conversion on the received reflected echo signals. The transmitting/receiving separation unit  1003  switches or separates transmitting and receiving, so that the driving signal can be passed from the transmission unit  1004  to the ultrasonic probe  1002  during transmission, or a receiving signal can be passed from the ultrasonic probe  1002  to the receiving unit  1008  during receiving. The phasing and summation unit  1010  is a device that phases and summates received reflected echo signals. The image processing unit  1010  is a device that constructs a diagnostic image (for example, a tomographic image or bloodstream image) on the basis of the phased and summated reflected echo signals. The display unit  1014  is a display device that displays the diagnostic image which has undergone image processing. The control unit  1016  is a device that controls the foregoing components. The operation unit  1018  is a device to be used to give instructions to the control unit  1016 . The operation unit  1018  includes pieces of input equipment, for example, a trackball, a keyboard, and a mouse. 
       INDUSTRIAL APPLICABILITY 
       [0082]    The ultrasonic transducer of the present invention can be employed in an ultrasonic diagnostic equipment including an ultrasonic probe, a defect inspection apparatus for the interior of a structure, an object position sensing apparatus, a flow velocity measurement apparatus, or the like. A high transmission sound pressure and high receiving sensitivity can be realized. In addition, reliability in long-term driving can be improved. 
       REFERENCE SINGS LIST 
       [0000]    
       
         
           
               201 ,  301 ,  501 ,  701 ,  801 ,  901 : lower electrode 
               202 ,  302 ,  502 ,  702 : hollow portion 
               207 ,  208 ,  209 ,  309 ,  507 ,  508 ,  509 ,  707 ,  708 ,  709 : insulating film 
               205 ,  305 ,  505 ,  705 : upper electrode 
               210 ,  310 ,  510 ,  710 ,  810 ,  910 : membrane 
               213 ,  313 ,  513 ,  713 : hollow portion bottom 
               203 ,  303 ,  503 ,  803 ,  903 : rigid member 
               204 ,  304 ,  504 ,  704 ,  804 ,  904 : projection 
               206 ,  306 ,  506 ,  706 : substrate 
               211 ,  311 ,  711 : projection undersurface 
               212 ,  312 ,  512 ,  712 : membrane undersurface 
               514 ,  714 : projection top 
               703 ,  805 ,  905 : high-Young&#39;s modulus member 
               1001 : ultrasonic diagnostic equipment 
               1002 : ultrasonic probe 
               1003 : transmitting/receiving separation unit 
               1004 : transmission unit 
               1006 : bias unit 
               1008 : receiving unit 
               1010 : phasing and summation unit 
               1012 : image processing unit 
               1014 : display unit 
               1016 : control unit 
               1018 : operation unit