Patent Publication Number: US-9426579-B2

Title: Electro-acoustic transducer

Description:
RELATED APPLICATIONS 
     This application claims priority from Korean Patent Application No. 10-2014-0011738, filed on Jan. 29, 2014, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Exemplary embodiments relate to an electro-acoustic transducer, and more particularly, to a micromachined electro-acoustic transducer. 
     2. Description of the Related Art 
     Electro-acoustic transducers are devices that convert electric energy to acoustic energy or vice versa and may include ultrasonic transducers and microphones. Micromachined electro-acoustic transducers are transducers that use a micro-electro-mechanical system (MEMS). A typical example of a micromachined electro-acoustic transducer is a micromachined ultrasonic transducer (MUT), which is a device that converts an electric signal to an ultrasonic signal or vice versa. An MUT may be classified into a piezoelectric MUT (pMUT), a capacitive MUT (cMUT), and a magnetic MUT (mMUT), based on its converting method. 
     A pMUT has been mainly used in the past. Recently, the cMUT is increasingly under development because of its merits, such as a capability of transmitting/receiving a broadband signal, a conduciveness to mass production using a semiconductor process, and a capability of integration with an electric circuit. Accordingly, a cMUT is widely used in medical image diagnosis devices or sensors. 
     Recently, as a demand for various types of ultrasound signal acquisition methods and resulting images such as a B-mode image, a Doppler image, a harmonic image, and a photoacoustic image, which are obtainable for use in an ultrasound diagnosis, increases, ultrasound equipment having broadband characteristics is increasingly under development. Further, on order to cover diagnosis of various organs having different sizes and depths such as the abdomen, the heart, and the thyroid gland, the development of ultrasound equipment having a broadband characteristic is essential. Compared to a general piezoelectric ultrasonic transducer, although a cMUT is capable of transceiving broadband signals, it has a limit in receiving the overall frequency band. Accordingly, methods of embodying broadband by combining cells having different resonant frequencies are being developed. 
     SUMMARY 
     Provided is a micromachined electro-acoustic transducer. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented exemplary embodiments. 
     According to an aspect of one or more exemplary embodiments, an electro-acoustic transducer includes a plurality of elements, in which each of the plurality of elements includes a plurality of cells of which at least one of the plurality of cells includes a trench that is formed in a membrane. 
     Each of the plurality of elements may include a first frequency band that is wider than a respective frequency band of each of the plurality of cells constituting the respective element. 
     For each of the plurality of elements, a frequency characteristic of the at least one of the plurality of cells that includes the trench may vary based on at least one from among a number, a shape, a size, and a position of the trench. 
     For each of the plurality of elements, at least two cells of the plurality of cells may include different numbers of trenches. 
     For each of the plurality of elements, a plane shape of the trench may include at least one from among a circle and a polygon. 
     For each of the plurality of elements, a sectional shape of the trench may include at least one from among a rectangle, a triangle, and a semicircle. 
     For each of the plurality of elements, the membrane may include silicon. 
     Each of the plurality of elements and each of the pluralities of cells may be arranged in a respective two-dimensional arrangement. 
     For each of the plurality of elements, each of plurality of cells may have a same size. 
     Each of the plurality of cells may include a substrate, a support provided on the substrate and comprising a cavity, the membrane configured to cover the cavity, and an electrode provided on an upper surface of the membrane. 
     According to another aspect of one or more exemplary embodiments, an element of an electro-acoustic transducer includes a plurality of cells comprising a first cell and a second cell, wherein each of the first cell and the second cell has a same size and a frequency characteristic of the first cell is different from a frequency characteristic of the second cell. 
     Each of the first cell and the second cell may include a respective membrane, and at least one from among the first cell and the second cell may include a trench that is formed in at least one from among an upper surface and a lower surface of the corresponding membrane. 
     According to another aspect of one or more exemplary embodiments, an electro-acoustic transducer includes a plurality of elements, in which each of the plurality of elements includes a plurality of cells, wherein for each of the plurality of elements, each of the plurality of cells includes a substrate, a support provided on the substrate and comprising a cavity, a membrane configured to cover the cavity, and an electrode provided on an upper surface of the membrane, and wherein, for each of the plurality of elements, at least one of the plurality of cells includes a trench that is formed in the membrane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a plan view of a transducer chip of an electro-acoustic transducer, according to an exemplary embodiment; 
         FIG. 2  is a plan view of an element illustrated in  FIG. 1 ; 
         FIG. 3A  is a cross-sectional view taken along line A-A′ of  FIG. 2 ; 
         FIG. 3B  is a cross-sectional view taken along line B-B′ of  FIG. 2 ; 
         FIG. 3C  is a cross-sectional view taken along line C-C′ of  FIG. 2 ; 
         FIG. 3D  is a cross-sectional view taken along line D-D′ of  FIG. 2 ; 
         FIG. 4  is a graph which illustrates a result of a simulation of resonant frequencies which are calculated based on a number of trenches formed in a membrane of a cMUT; 
         FIG. 5  is a graph which illustrates a frequency characteristic of the element embodied by combining cells having different resonant frequencies illustrated in  FIG. 2 ; 
         FIGS. 6A and 6B  are sectional views which illustrate modified sectional shapes of the trench formed in the membrane; 
         FIGS. 7A and 7B  are plan views which illustrate modified plane shapes of the trench formed in the membrane; 
         FIG. 8  is a cross-sectional view of a cell of an electro-acoustic transducer, according to another exemplary embodiment; 
         FIG. 9  is a cross-sectional view of a cell of an electro-acoustic transducer, according to another exemplary embodiment; 
         FIG. 10  is a plan view of an element of an electro-acoustic transducer, according to another exemplary embodiment; 
         FIG. 11A  is a plan view of an element of an electro-acoustic transducer, according to another exemplary embodiment; and 
         FIG. 11B  is a plan view of an element of an electro-acoustic transducer, according to another exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the exemplary embodiments are merely described below, by referring to the figures, to explain aspects of the present disclosure. Also, the thickness or size of each layer illustrated in the drawings may be exaggerated for convenience of explanation and clarity. In the following description, when a layer is described to exist on another layer, the layer may exist directly on the other layer or a third layer may be interposed therebetween. A material forming each layer in the following exemplary embodiments is merely exemplary and thus other material may be used therefor. 
     As used herein, expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
       FIG. 1  is a plan view of a transducer chip  100  of an electro-acoustic transducer, according to an exemplary embodiment. The electro-acoustic transducer may include a plurality of transducer chips  100 .  FIG. 1  illustrates one of the transducers chips  100  which constitutes an electro-acoustic transducer. The electro-acoustic transducer may be a capacitive micromachined electro-acoustic transducer, such as, for example, a capacitive micromachined ultrasonic transducer (cMUT). Referring to  FIG. 1 , the transducer chip  100  of the electro-acoustic transducer may include a plurality of elements  110  that are arranged in a two-dimensional arrangement. The elements  110  may be independently driven. Each of the elements  110  includes a plurality of cells  111  that are arranged in a respective two-dimensional arrangement, as described below. 
       FIG. 2  is a plan view of one of the elements  110  illustrated in  FIG. 1 . Referring to  FIG. 2 , the element  110  includes the cells  111  that are arranged in a two-dimensional arrangement. In detail, the cells  111  may include four cells which are arranged in a rectangular shape, that is, first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d .  FIG. 2  illustrates an example in which the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  are arranged in a clockwise order. In addition, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  may be arranged in any one or more of a variety of shapes. The first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  may have a same size. That is, when each of the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  has a circular structure, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  may have a same outer diameter (OD). Membranes  115  of  FIGS. 3A, 3B, 3C, and 3D  forming the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  may have a same OD and a same thickness t. However, the present exemplary embodiment is not limited thereto. The first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  may have different frequency characteristics, that is, different resonant frequencies. As described below, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  may have different numbers of trenches, so that the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  may have different resonant frequencies. 
       FIGS. 3A, 3B, 3C, and 3D  are cross-sectional views of the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , which constitute the element  110 . In detail,  FIG. 3A  is a cross-sectional view taken along line A-A′ of  FIG. 2 , illustrating the first cell  111   a .  FIG. 3B  is a cross-sectional view taken along line B-B′ of  FIG. 2 , illustrating the second cell  111   b .  FIG. 3C  is a cross-sectional view taken along line C-C′ of  FIG. 2 , illustrating the third cell  111   c .  FIG. 3D  is a cross-sectional view taken along line D-D′ of  FIG. 2 , illustrating the fourth cell  111   d.    
     Referring to  FIGS. 3A, 3B, 3C, and 3D , each of the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  includes a substrate  112 , a support  114  provided on the substrate  112 , the membrane  115  provided on the support  114 , and an electrode  116  provided on the membrane  115 . The substrate  112  may function as a lower electrode. To this end, the substrate  112  may include a conductive material. For example, although the substrate  112  may include low-resistance silicon, the present exemplary embodiment is not limited thereto. An insulation layer  113  formed of, for example, silicon oxide, may be further formed on an upper surface of the substrate  112 . The support  114  is provided on the insulation layer  113 , and a cavity  120  is formed therein. Although the support  114  may include, for example, silicon oxide, the present exemplary embodiment is not limited thereto. The membrane  115  is provided on the support  114  to cover the cavity  120 . The membrane  115  may include, though the present exemplary embodiment is not limited thereto, for example, silicon. The electrode  116  is provided on an upper surface of the membrane  115 . The electrode  116  functions as an upper electrode and may include, though the present exemplary embodiment is not limited thereto, for example, a metal. 
     The first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  which constitute the element  110  may include different numbers of trenches. In detail, referring to  FIGS. 2 and 3A , in the first cell  111   a  of the cells  111  constituting the element  110 , no trench is formed in the membrane  115 . Referring to  FIGS. 2 and 3B , among the cells  111  constituting the element  110 , in the second cell  111   b , one trench  131  is formed in an upper surface of the membrane  115 . The trench  131  may be formed, for example, circularly in the upper surface of the membrane  115  (as illustrated in  FIG. 2 ), and a sectional shape of the trench  131  may be rectangular (as illustrated in  FIG. 3B ). The plane shape and the sectional shape of the trench  131  may be variously modified. 
     Referring to  FIGS. 2 and 3C , in the third cell  111   c  of the cells  111  constituting the element  110 , two trenches, that is, first and second trenches  131 ′ and  132 ′, are formed in the upper surface of the membrane  115 . The first and second trenches  131 ′ and  132 ′ may be formed, for example, circularly in the upper surface of the membrane  115 , and separate from each other (as illustrated in  FIG. 2 ). The sectional shape of each of the first and second trenches  131 ′ and  132 ′ may be rectangular (as illustrated in  FIG. 3C ). The plane shape and the sectional shape of each of the first and second trenches  131 ′ and  132 ′ may be variously modified. Referring to  FIGS. 2 and 3D , in the fourth cell  111   d  of the cells  111  constituting the element  110 , three trenches, that is, first, second, and third trenches  131 ″,  132 ″, and  133 ″, are formed in the upper surface of the membrane  115 . The first, second, and third trenches  131 ″,  132 ″, and  133 ″ may be formed, for example, circularly in the upper surface of the membrane  115 , and separate from one another (as illustrated in  FIG. 2 ). The sectional shape of each of the first, second, and third trenches  131 ″,  132 ″, and  133 ″ may be rectangular (as illustrated in  FIG. 3D ). The plane shape and the sectional shape of each of the first, second, and third trenches  131 ″,  132 ″, and  133 ″ may be variously modified. Conversely, the respective intervals between the first, second, and third trenches  131 ″,  132 ″, and  133 ″ may be constant or irregular. The sectional shapes of the first, second, and third trenches  131 ″,  132 ″, and  133 ″ may be identical to or different from one another. 
     The first cell  111   a  has no trenches. The second cell  111   b  includes one trench  131  formed in the membrane  115 . The third cell  111   c  includes two trenches, that is, the first and second trenches  131 ′ and  132 ′, formed in the membrane  115 . The fourth cell  111   d  includes three trenches, that is, the first, second, and third trenches  131 ″,  132 ″, and  133 ″, formed in the membrane  115 . As such, because the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  constituting the element  110  include different numbers of trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d  may have different frequency characteristics, in detail, different resonant frequencies. Because one element is manufactured by combining the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , having different resonant frequencies, a frequency band which is wider than a respective frequency band of each of the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , may be embodied. 
     In general, a resonant frequency f r  of a cell in a cMUT is expressible by Equation 1. 
     
       
         
           
             
               
                 
                   
                     f 
                     r 
                   
                   = 
                   
                     
                       
                         1 
                         
                           2 
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                           k 
                           
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                             e 
                           
                         
                       
                     
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                           2 
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                             t 
                             m 
                           
                         
                         
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                             E 
                             + 
                             T 
                           
                           
                             1.8 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ρ 
                               ⁡ 
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     v 
                                     2 
                                   
                                 
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                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
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                     1 
                   
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     In Equation 1, “k” and “m e ” denote a strength of a membrane and a mass of the membrane, respectively, and “t m ” and “a” denote a thickness of the membrane and a radius of the membrane, respectively. The radius “a” signifies one-half of the OD. “T”, “E”, “v”, and “ρ” denote an internal stress, a Young&#39;s modulus, a Poisson ratio, and a density of the membrane, respectively. 
     Referring to Equation 1, it may be seen that a resonant frequency of a cell may be changed by varying the thickness “t m ” or the radius “a” of the membrane. Accordingly, one element which has broadband characteristics may be manufactured by combining cells having different resonant frequencies that are manufactured by varying the thickness or radius of the membrane. However, in this case, it may be difficult to make various thicknesses of the membrane and, when cells have different sizes (i.e., different outer diameters), it may be difficult to arrange cells densely or in two dimensions. In the present exemplary embodiment, by varying the number of trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ formed in the membrane  115 , the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , which have different respective resonant frequencies are manufactured. By combining the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , the element  110  having a broadband frequency characteristic may be embodied. In particular, when different numbers of trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ are formed in the membrane  115 , the strength “k” and the mass “m e ” of the membrane  115  in Equation 1 are changed. Accordingly, the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , having different resonant frequencies may be manufactured. 
       FIG. 4  is a graph which illustrates a result of a simulation of resonant frequencies calculated based on the number of trenches formed in a membrane of a cMUT. In  FIG. 4 , a silicon membrane having a radius, that is, one-half of the OD, of about 21 μm and a thickness of about 0.9 μm is used as the membrane. The trench is formed to a depth of about 0.5 μm and a width of about 1 μm in the upper surface of the membrane. Referring to  FIG. 4 , the resonant frequency of a cell that does not include a trench is approximately equal to 8 MHz. It may be seen that, as the number of trenches formed in the membrane increases, the resonant frequency decreases to about 6.5 MHz. 
       FIG. 5  is a graph which illustrates a frequency characteristic of the element  110  embodied by combining the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , which have different respective resonant frequencies and are illustrated in  FIG. 2 . Referring to  FIG. 5 , among the cells  111  constituting the element  110 , the first cell  111   a  having no trenches has the highest resonant frequency, compared to the other cells, namely, the second, third, and fourth cells  111   b ,  111   c , and  111   d . The resonant frequencies of the cells, namely, the second, third, and fourth cells  111   b ,  111   c , and  111   d , which have the trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″, decrease as the number of trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ increases. In particular, it may be seen that, among the cells  111  constituting the element  110 , the resonant frequency of the fourth cell  111   d  that has the largest number of trenches, namely, the first, second, and third trenches  131 ″,  132 ″, and  133 ″, is the lowest resonant frequency as compared to the resonant frequencies of the other cells, namely, the first, second, and third cells  111   a ,  111   b , and  111   c , which have different respective resonant frequencies. As such, when one element  110  is manufactured by combining the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , having different resonant frequencies, the ranges of frequencies output from the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , overlap one another, and thus, the element  110  may have a broadband frequency characteristic that is wider than the individual frequency band which is output from each of the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d . In a detailed example, when the first cell  111   a  has a resonant frequency of about 8.0 MHz and a bandwidth of about 5˜11 MHz, the second cell  111   b  has a resonant frequency of about 7.5 MHz and a bandwidth of about 4.5˜10.5 MHz, the third cell  111   c  has a resonant frequency of about 7.0 MHz and a bandwidth of about 4˜10 MHz, and the fourth cell  111   d  has a resonant frequency of about 6.5 MHz and a bandwidth of about 3.5˜9.5 MHz, the element  110  manufactured by combing the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , may have a broadband frequency characteristic, that is, a bandwidth of about 3.5˜11 MHz. 
     In the above exemplary embodiment, all the cells  111  constituting the element  110  are described to include different numbers of trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″. However, the present exemplary embodiment is not limited thereto, and some of the cells  111  may not include a trench, or may include a same number of trenches as others of the cells  111 . In particular, at least one of the cells  111  may include a trench. In this case, at least two cells of the cells  111  may include different numbers of trenches. Further, in the above description, the cells  111  are described to have different frequency characteristics based on the respective number of trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ formed in the membrane  111 . However, the frequency characteristics of the cells  111  may vary not only based on the number of trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ but also based on any one or more of the shape, the size, and/or the position of the trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″. In detail, the cells  111  may have different frequency characteristics based on at least one of the number, shape, size, and position of the trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ formed in the membrane  115 . 
       FIGS. 3B, 3C, and 3D  illustrate that each of the trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ formed in the membrane  115  has a rectangular sectional shape. However, the present exemplary embodiment is not limited thereto, and the trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ may have any one or more of various sectional shapes. The frequency characteristic may vary based on the sectional shape of the trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″.  FIGS. 6A and 6B  illustrate modified sectional shapes of trenches  134  and  135  formed in the membrane  115 . In detail,  FIG. 6A  illustrates that the trench  134  formed in the membrane  115  has a triangular sectional shape, and  FIG. 6B  illustrates that the trench  135  formed in the membrane  115  has a semicircular sectional shape. The sectional shape of a trench is not limited thereto, and the trench may have any of a variety of sectional shapes. 
       FIG. 2  illustrates that each of the trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ formed in the membrane  115  has a circular plane shape. However, the present exemplary embodiment is not limited thereto, and the trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″ may have any one or more of a variety of plane shapes. The frequency characteristic of a cell may vary based on the plane shape of the trenches  131 ,  131 ′,  132 ′,  131 ″,  132 ″, and  133 ″.  FIGS. 7A and 7B  are plan views illustrating modified plane shapes of trenches formed in the membrane  115 . In detail,  FIG. 7A  illustrates that two trenches  136  and  137  are formed, and that each of trenches  136  and  137  has a rectangular plane shape. The number of trenches  136  and  137  may be variously modified. Further, the position and/or interval (i.e., relative spacing) of the trenches  136  and  137  may be variously modified.  FIG. 7B  illustrates that each of trenches  138  and  139  formed in the membrane  115  has a hexagonal plane shape.  FIG. 7B  illustrates that the trenches  138  and  139  are formed. The number of trenches  138  and  139  may be variously modified. Further, the position and/or the interval (i.e., relative spacing) of the trenches  138  and  139  may be variously modified. In addition, a trench having a different polygonal sectional shape or a different plane shape may be formed. Further, a trench may be formed at a center portion of the membrane  115 . As described above, the cells  111  having different frequency characteristics may be manufactured by varying any one or more of the sectional shape, the plane shape, and/or the position of the trench formed in the membrane  115 . In addition, the one element  110  having a broadband characteristic may be embodied by combining the cells  111  that are manufactured as above. 
       FIG. 8  is a cross-sectional view of a cell  211  of an electro-acoustic transducer, according another exemplary embodiment.  FIG. 8  illustrates an example of only one cell  211  of cells  211  constituting one element for convenience of explanation. Referring to  FIG. 8 , the cell  211  includes a substrate  212 , a support  214  provided on the substrate  212  and having a cavity  220  formed therein, a membrane  215  provided on the support  214  to cover the cavity  220 , and an electrode  216  provided on an upper surface of the membrane  215 . The substrate  212  may be formed of, for example, a conductive material such as low resistance silicon. An insulation layer  213  that is formed of, for example, silicon oxide, may be further formed on an upper surface of the substrate  212 . 
     At least one of the cells  211  constituting the element of an electro-acoustic transducer according to the present exemplary embodiment includes a trench  231  formed in the membrane  215 . In this case, at least two cells  211  of the cells  211  may include different numbers of trenches  231  as described above. Unlike the above-described exemplary embodiment, the trench  231  may be formed in a lower surface of the membrane  215 . Although  FIG. 8  illustrates that the trench  231  formed in the lower surface of the membrane  215  has a rectangular sectional shape, the trench  231  may have any one or more of a variety of sectional shapes, and any one or more of the number, the position, and the size of the trench  231  may be variously modified. As such, at least one of the cells  211  constituting the element may have a frequency characteristic which is different from those of the other cells  211  by varying at least one of the number, shape, size, and position of the trench  231  formed in the lower surface of the membrane  215 . Accordingly, an element having a broadband frequency characteristic may be embodied by combining the cells  211 . 
       FIG. 9  is a cross-sectional view of a cell  311  of an electro-acoustic transducer, according another exemplary embodiment.  FIG. 9  illustrates an example of only one cell  311  of the cells  311  constituting one element for convenience of explanation. Referring to  FIG. 9 , the cell  311  includes a substrate  312 , a support  314  provided on the substrate  312  and having a cavity  320  formed therein, a membrane  315  provided on the support  314  to cover the cavity  320 , and an electrode  316  provided on an upper surface of the membrane  315 . The substrate  312  may be formed of, for example, a conductive material such as low resistance silicon. An insulation layer  313  that is formed of, for example, silicon oxide, may be further formed on an upper surface of the substrate  312 . 
     At least one of the cells  311  constituting the element of an electro-acoustic transducer according to the present exemplary embodiment includes trenches  331  and  332  formed in the membrane  315 . In this case, at least two cells  311  of the cells  311  may include different numbers of trenches as described above. Unlike the above-described exemplary embodiments, the trenches, for example, first and second trenches  331  and  332 , are formed in lower and upper surfaces of the membrane  315 , respectively. In detail, the first trench  331  is formed in the lower surface of the membrane  315 , and the second trench  332  is formed in the upper surface of the membrane  315 . Although  FIG. 9  illustrates that each of the first and second trenches  331  and  332  has a rectangular sectional shape, the first and second trenches  331  and  332  may have any one or more of a variety of sectional shapes, and any one or more of the number, the position, and the size of the first and second trenches  331  and  332  may be variously modified. As such, at least one of the cells  311  constituting the element may have a frequency characteristic that is different from those of the other cells  311  by varying at least one of the number, shape, size, and position of the first and second trenches  331  and  332  formed in the lower and upper surfaces of the membrane  315 , respectively. Accordingly, an element having a broadband frequency characteristic may be embodied by combining the cells  311 . 
     Although the four cells, namely, the first, second, third, and fourth cells  111   a ,  111   b ,  111   c , and  111   d , constitute the element  110  according to the exemplary embodiment illustrated in  FIG. 2 , the number of cells constituting one element of an electro-acoustic transducer may be variously modified.  FIG. 10  is a plan view of an element  410  of an electro-acoustic transducer, according another exemplary embodiment. Referring to  FIG. 10 , 16 cells  411  constituting one element  410  are arranged in a two-dimensional array. As described above, at least one of the cells  411  includes a trench  430  in order to embody the element  410  having a broadband frequency characteristic. In this case, at least two cells  411  of the 16 cells  411  may include different respective numbers of trenches  430 . The positions of the cells  411  having different frequency characteristics may be variously modified. Although the cells  411  may have the same size, the present exemplary embodiment is not limited thereto. Although  FIG. 10  illustrates that the 16 cells  411  are arranged in a square-shaped array, the number and arrangement of the cells  411  may be variously modified. 
       FIG. 11A  is a plan view of an element  510  of an electro-acoustic transducer, according another exemplary embodiment. Referring to  FIG. 11A , a plurality of cells  511  constituting one element  510  are arranged in a two-dimensional array, and the cells  511  may be arranged hexagonally. As described above, in order to embody the element  510  having a broadband frequency characteristic, at least one of the cells  511  includes a trench  530 . In this case, at least two cells  511  of the plurality of cells  511  may include different respective numbers of trenches  530 . The positions of the cells  511  having different frequency characteristics may be variously modified. Although the cells  511  may have the same size, the present exemplary embodiment is not limited thereto. 
       FIG. 11B  is a plan view of an element  610  of an electro-acoustic transducer, according another exemplary embodiment. Referring to  FIG. 11B , a plurality of cells  611  constituting one element  610  are arranged in a two-dimensional array, and the cells  611  may be arranged hexagonally in a different manner than the hexagonal arrangement of  FIG. 11A . In order to embody the element  610  having a broadband frequency characteristic, at least one of the cells  611  includes a trench  630 . In this case, at least two cells  611  of the plurality of cells  611  may include different respective numbers of trenches  630 . The positions of the cells  611  having different frequency characteristics may be variously modified. Although the cells  611  may have the same size, the present exemplary embodiment is not limited thereto. Although, in the above-described exemplary embodiments, the cells are arranged in a square-shaped array or in a hexagonally-shaped array, the cells may be arranged in any one or more of a variety of shapes. 
     As described above, in the electro-acoustic transducer according to the above exemplary embodiments, at least one of the cells constituting one element may include a trench which is formed in the membrane. The cells having different frequency characteristics may be manufactured by varying any one or more of the number, the size, the shape, and the position of the trenches formed in the membrane. Accordingly, an element having a broadband frequency characteristic may be embodied by combining the cells manufactured as above. The electro-acoustic transducer which includes the element having a broadband frequency characteristic may be applied to ultrasonic equipment that is configured for executing any one or more of various types of ultrasound signal acquisition methods which correspond to various types of images, such as a B-mode image, a Doppler image, a harmonic image, and a photoacoustic image, or to an ultrasonic equipment field which covers diagnoses of various organs having different sizes and depths, such as, for example, the abdomen, the heart, and the thyroid gland. 
     In the above descriptions, although the electro-acoustic transducer is described as an example of a capacitive micromachined electro-acoustic transducer, the electro-acoustic transducer may be applied to all types of electro-acoustic transducers in which a plurality of cells constitute one element and at least one of the cells includes a trench that is formed in a membrane. 
     It should be understood that the exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each exemplary embodiment should typically be considered as available for other similar features or aspects in other embodiments. 
     While one or more exemplary embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present inventive concept as defined by the following claims.