Abstract:
The present invention relates to an ultrasonic transducer that has a diaphragm in which an electrode layer is formed thereon. A fixed electrode having a plurality of asperities on the surface facing the diaphragm is provided. An alternating current signal is applied between the electrode layer formed on the diaphragm and the fixed electrode to generate ultrasonic waves. A groove is formed on the upper surface of the projections of the asperities of the fixed electrode to prevent the diaphragm from sticking to the fixed electrode.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority to Japanese Patent Application No. 2004-165784 filed Jun. 3, 2004 which is hereby expressly incorporated by reference herein in its entirety.  
       BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to an ultrasonic transducer and a method of manufacturing the ultrasonic transducer. In particular, it relates to an electrostatic ultrasonic transducer capable of increasing the efficiency of conversion between an electrical signal and a sound signal to increase an output sound pressure level and facilitating micromachining of a fixed electrode (a lower electrode) necessary therefor and a method of manufacturing the electrostatic ultrasonic transducer.  
         [0004]     2. Related Art  
         [0005]     Most related-art ultrasonic transducers are of a resonance type that use piezoelectric ceramics. A structural example of the related-art resonant ultrasonic transducers is shown in  FIG. 10 . The ultrasonic transducer shown in  FIG. 10  performs both conversion from an electrical signal into an ultrasonic wave and conversion from an ultrasonic wave into an electrical signal (transmission and reception of an ultrasonic wave) using piezoelectric ceramics as a vibration element.  
         [0006]     The bimorph ultrasonic transducer shown in  FIG. 10  includes two piezoelectric ceramics  61  and  62 , a cone  63 , a case  64 , leads  65  and  66 , and a screen  67 . The piezoelectric ceramics  61  and  62  are bonded together. The leads  65  and  66  are connected to the surfaces opposite to the bonded surfaces, respectively. The resonant ultrasonic transducer uses the resonance phenomenon of the piezoelectric ceramics, so that ultrasonic transmission- and reception-characteristics are in good condition in a relatively narrow frequency band around its resonant frequency.  
         [0007]     Unlike the resonant ultrasonic transducer shown in  FIG. 10 , electrostatic ultrasonic transducers have been known as broad-band-oscillating ultrasonic transducers capable of generating high sound pressure across a high frequency band. A concrete example of the broad-band-oscillating ultrasonic transducers is shown in  FIGS. 11A and 11B .  
         [0008]     The electrostatic ultrasonic transducer shown in  FIG. 11A  uses a dielectric (an insulator)  131  of the order of 3 to 10 μm thick, made of polyethylene terephthalate resin, as a diaphragm or vibrator. To the dielectric  131 , an upper electrode  132  made of metal foil such as aluminum is formed on the upper surface thereof by vapor deposition, and a fixed electrode (a lower electrode)  133  made of brass or the like is disposed below the lower surface of the dielectric  131 . The dielectric  131  and the upper electrode  132  (Bank of the upper electrode  132 ) contact each other by applying a DC bias voltage, and the dielectric  131  vibrates by applying an AC voltage.  
         [0009]     Random uneven microscopic asperities of the order of tens to several hundred μm are formed on the surface of the lower electrode  133  adjacent to the dielectric  131 . The asperities form a space between the lower electrode  133  and the dielectric  131 , so that the distribution of the capacitance between the upper electrode  132  and the lower electrode  133  varies slightly. The random microscopic asperities can be formed by roughening the surface of the lower electrode  133  manually. The electrostatic ultrasonic transducer has such asperities to form a large number of capacitors. Accordingly, the frequency response of an ultrasonic transducer  122  can be a broad band response as shown by curve Q 1  in  FIG. 11B . The asperities also offer the advantage of increasing the efficiency of conversion between an electrical signal and a sound signal (increasing the level of output sound pressure).  
         [0010]     The characteristics of the electrostatic ultrasonic transducer are thus improved by the asperities on the fixed electrode. However, when the surfaces (the upper surfaces) of the projections of the asperities are flat, a strong electrostatic force is applied to the space between it and the diaphragm to make the diaphragm stick to the flat surfaces of the projections, so that the diaphragm is sometimes restrained by the projections and so hardly vibrates, decreasing the efficiency of conversion between a sound signal and an electrical signal.  
         [0011]     Several inventions of an electrostatic ultrasonic transducer have been disclosed in which asperities (voids) are provided between the diaphragm and the fixed electrode (the lower electrode). For example, an invention in which voids are formed by a dielectric spacer is disclosed in JP-A-2000-50392 and an invention in which the fixed electrode is provided with communication holes connecting with the groove, thereby decreasing the resonant frequency and increasing the conversion efficiency between a sound signal and an electrical signal is disclosed in JP-A-58-46800.  
         [0012]     However, the related arts have not been able to solve the problem of the diaphragm sticking to the flat surfaces of the projections so that the diaphragm is restrained by the projections and hardly vibrates.  
       SUMMARY  
       [0013]     An advantage of the invention is to provide an electrostatic ultrasonic transducer that has asperities on the surface of a fixed electrode (lower electrode) to achieve broad band response and increase the efficiency of conversion between an electrical signal and a sound signal, thereby increasing the level of output sound pressure, in which the level of output sound pressure is further increased and the fixed electrode (lower electrode) therefor can easily be micromachined, and a method of manufacturing the electrostatic ultrasonic transducer.  
         [0014]     According to a first aspect of the invention, there is provided an ultrasonic transducer including a diaphragm in which an electrode layer is formed on an insulator, and a fixed electrode having a plurality of asperities on the surface facing the diaphragm, wherein an alternating current signal is applied between the electrode layer formed on the diaphragm and the fixed electrode to generate ultrasonic waves, wherein a groove is formed on the upper surface of the projections of the asperities of the fixed electrode.  
         [0015]     With such a structure, in the electrostatic ultrasonic. transducer, the surface of the fixed electrode (the lower electrode) facing the diaphragm having an upper electrode has an asperity structure and the surfaces of the projections of the fixed electrode (the lower electrode) have a groove (a continuous groove or recesses).  
         [0016]     This structure prevents the diaphragm from being adsorbed by (sticking to) the fixed electrode (the lower electrode), thereby improving the efficiency of converting an electrical signal to a sound signal to increase the level of output sound pressure. Also the capacitance between the upper electrode and the fixed electrode (the lower electrode) can be reduced, thereby decreasing the drive current of the ultrasonic transducer.  
         [0017]     It is preferable that the groove be formed by applying droplets onto the upper surface of the projections by an ink jet method to form banks.  
         [0018]     With such a structure, a droplet material is applied onto the surfaces of the projections of the fixed electrode (the lower electrode) of the ultrasonic transducer by an ink jet method to form the banks of a minute height, thereby forming a groove (a continuous groove or recesses) on the surfaces of the projections.  
         [0019]     Accordingly, the banks of a minute height and the groove can be formed extremely easily on the projections of the fixed electrode (the lower electrode). The ink jet method has high flexibility in the direction of application including rotation, so that the continuous groove or independent recesses can be easily formed on the circular projections. Since the groove or recesses of a minute depth generally needed to be formed by etching in the past, it required an etching mask and etchant, posing the problem of an increase in cost and environmental problem of waste disposal. However, by the ink jet method, material can be applied only to the required portion to be raised, and so it is advantageous in cost and environmental considerations.  
         [0020]     In one embodiment the groove formed by the banks is in the form of a continuous groove.  
         [0021]     With such a structure, the banks of a minute height are arranged in parallel on the projections of the fixed electrode (the lower electrode) to form a continuous groove.  
         [0022]     Thus, a groove can easily be formed on the projections of the fixed electrode (the lower electrode), thereby improving the characteristics of the ultrasonic transducer. Particularly, the use of the ink jet method facilitates the banks of a minute height to be formed on the projections.  
         [0023]     The groove formed by the banks can alternatively be in the form of independent recesses.  
         [0024]     With such a structure, the banks of a minute height are first disposed in parallel on the projections of the fixed electrode (the lower electrode) to form a continuous groove, then banks serving as partitions are formed in the continuous groove to make the groove into the form of holes (recesses).  
         [0025]     This increases the level of output sound pressure of the ultrasonic transducer and decreases the drive current. Particularly, the use of the ink jet method facilitates the banks of a minute height and the holes (recesses) to be formed on the projections.  
         [0026]     It is preferable that the droplets be made of an electrically conductive material.  
         [0027]     With such a structure, a conductive material is used as the droplet material to be applied to the projections of the fixed electrode (the lower electrode). In the case of using the conductive material, when the diaphragm is insulative, the diaphragm can be used as it is; when the diaphragm is noninsulative, it is necessary to form an insulating film on the surface of the diaphragm facing the fixed electrode (the lower electrode).  
         [0028]     This allows a conductive droplet material to be used for forming the banks, thus extending the range of choices for the droplet material to form the banks on the projections.  
         [0029]     Alternatively, the droplets can be made of an electrically nonconductive material.  
         [0030]     With such a structure, a nonconductive material is used as the droplet material to be applied to the projections of the fixed electrode (the lower electrode). This allows a nonconductive droplet material to be used for forming the banks, thus extending the range of choices for the droplet material to form the banks on the projections.  
         [0031]     According to a second aspect of the invention, there is provided a method of manufacturing an ultrasonic transducer including a diaphragm in which an electrode layer is formed on an insulator, and a fixed electrode having a plurality of asperities on the surface facing the diaphragm, wherein an alternating current signal is applied between the electrode layer formed on the diaphragm and the fixed electrode to generate ultrasonic waves, the method including forming the asperities having a plurality of projections and depressions on the surface of the fixed electrode facing the diaphragm, and forming a groove on the upper surfaces of the projections of the fixed electrode.  
         [0032]     Thus, in the electrostatic ultrasonic transducer, the surface of the fixed electrode (the lower electrode) facing the diaphragm having an upper electrode is provided with an asperity structure and the surfaces of the projections of the fixed electrode (the lower electrode) is provided with a groove (a continuous groove or recesses).  
         [0033]     This structure prevents the diaphragm from sticking to the fixed electrode (the lower electrode), thereby improving the efficiency of converting an electrical signal to a sound signal to increase the level of output sound pressure. Also the capacitance between the upper electrode and the fixed electrode (the lower electrode) can be reduced, thereby decreasing the drive current of the ultrasonic transducer.  
         [0034]     It is preferable that the groove be formed by applying droplets onto the upper surfaces of the projections by an ink jet method to form banks.  
         [0035]     Thus, a droplet material is applied onto the surfaces of the projections of the fixed electrode (the lower electrode) of the ultrasonic transducer by an ink jet method to form the banks of a minute height, thereby forming a groove (a continuous groove or recesses) on the surfaces of the projections.  
         [0036]     Accordingly, the banks of a minute height and the groove can be formed extremely easily on the projections of the fixed electrode (the lower electrode). The ink jet method has high flexibility in the direction of application including rotation, so that the continuous groove or independent recesses can be easily formed on the circular projections. Since the groove or recesses of a minute depth generally need to be formed by etching, it requires an etching mask and etchant, posing the problem of an increase in cost and environmental problem of waste disposal. However, by the ink jet method, droplets can be applied only to the necessary portion, and so it is advantageous in cost and environmental considerations.  
         [0037]     In one embodiment the groove formed by the banks is in the form of a continuous groove.  
         [0038]     Thus, the banks of a minute height are arranged in parallel on the projections of the fixed electrode (the lower electrode) to form a continuous groove.  
         [0039]     Accordingly, a groove can easily be formed on the projections of the fixed electrode (the lower electrode), thereby improving the characteristics of the ultrasonic transducer. Particularly, the use of the ink jet method facilitates the banks of a minute height to be formed on the projections.  
         [0040]     It is preferable that the groove formed by the banks be in the form of independent recesses.  
         [0041]     Thus, the banks of a minute height are first disposed in parallel on the projections of the fixed electrode (the lower electrode) to form a continuous groove, then banks serving as partitions are formed in the continuous groove to make the groove into the form of holes (recesses).  
         [0042]     This increases the level of output sound pressure of the ultrasonic transducer and decreases the drive current. Particularly, the use of the ink jet method facilitates the banks of a minute height and the holes (recesses) to be formed on the projections.  
         [0043]     The method of manufacturing an ultrasonic transducer according to an embodiment of the invention further includes a heating process after forming the banks by the ink jet method.  
         [0044]     Thus, after the droplet material has been applied onto the projections of the fixed electrode (the lower electrode), the solvent is evaporated by the heating process.  
         [0045]     This allows the droplet material to be firmly fixed to the projections in a short time.  
         [0046]     The droplets can be made of an electrically conductive material.  
         [0047]     Thus, a conductive material is used as the droplet material to be applied to the projections of the fixed electrode (the lower electrode). In the case of using the conductive material, when the diaphragm is insulative, the diaphragm can be used as it is; when the diaphragm is noninsulative, it is necessary to form an insulating film on the surface of the diaphragm facing the fixed electrode (the lower electrode).  
         [0048]     This allows a conductive droplet material to be used for forming banks, thus extending the range of choices for the droplet material to form the banks on the projections.  
         [0049]     Alternatively, the droplets may be made of an electrically nonconductive material.  
         [0050]     Thus, a nonconductive material is used as the droplet material to be applied to the projections of the fixed electrode (the lower electrode). This allows a nonconductive droplet material to be used for forming banks, thus extending the range of choices for the droplet material to form the banks on the projections.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]     The invention will be described with reference to the accompanying drawings, wherein like reference numbers refer to like elements, and wherein:  
         [0052]      FIG. 1  is a sectional view of an example of a fixed electrode (a lower electrode) of an ultrasonic transducer according to an embodiment of the invention;  
         [0053]      FIG. 2  is a plan view of the fixed electrode;  
         [0054]      FIG. 3A  is a plan view of a circular fixed electrode having a hole (recess)-like groove on a projection;  
         [0055]      FIG. 3B  is a plan view of an elliptical fixed electrode having a hole (recess)-like groove on the projection;  
         [0056]      FIGS. 4A  to  4 D are diagrams in which examples of application of droplets by an ink jet method are shown;  
         [0057]      FIG. 5  is a diagram showing an example of application of droplets such that a continuous groove is formed on the projection;  
         [0058]      FIG. 6A  is a plan view of another structural example of the fixed electrode;  
         [0059]      FIG. 6B  is a plan view of still another structural example of the fixed electrode;  
         [0060]      FIG. 7  is a schematic perspective view of a droplet ejector;  
         [0061]      FIG. 8A  is a perspective view of an ink jet head;  
         [0062]      FIG. 8B  is a side view of the ink jet head;  
         [0063]      FIG. 9A  is a diagram in which another method of forming banks is shown;  
         [0064]      FIG. 9B  is a diagram in which still another method of forming banks is shown;  
         [0065]      FIG. 10  is a diagram in which a prior art resonant ultrasonic transducer is shown;  
         [0066]      FIG. 11A  is a diagram in which a prior art electrostatic ultrasonic transducer is shown; and  
         [0067]      FIG. 11B  is a graph of the frequency response of the prior art electrostatic ultrasonic transducer. 
     
    
     DETAILED DESCRIPTION  
       [0068]     Preferred embodiments of the invention will be described hereinbelow with reference to the drawings.  
         [0069]      FIG. 1  is a sectional view of a fixed electrode (a lower electrode)  1  of an ultrasonic transducer according to an embodiment of the invention. Also shown in  FIG. 1  is a diaphragm  10  and second electrode  11 .  
         [0070]     In  FIG. 1 , the fixed electrode  1  has valleys or depressions  2  and hills or projections  3 . On the upper surfaces of the projections  3 , ridges or banks  4  are formed by applying droplets (an epoxy-based droplet material or the like) by an ink jet method. A groove (a continuous groove or holes (recesses))  5  is formed between the banks  4 . The banks  4  and the groove  5  formed on the surface of each projection  3  prevent the diaphragm  10  from sticking to or being adsorbed by the fixed electrode (the lower electrode)  1 , thereby improving the efficiency of converting an electrical signal to a sound signal to increase the level of output sound pressure for the transducer. The banks  4  and the groove  5  also reduce the capacitance between the upper electrode (not shown) and the fixed electrode  1 , thereby decreasing the drive current of the ultrasonic transducer.  
         [0071]     In the example of the fixed electrode  1  shown in  FIG. 1 , the depression  2  is 0.6 mm in depth and 0.3 mm in width. The projection  3  is 0.2 mm in width and 0.6 mm in height. The banks  4  on the upper surfaces of the projections  3  are arranged in parallel, with a spacing between each bank of 0.1 mm. Each bank is 50 μm in width and 10 μm in height, in this example.  
         [0072]     The groove  5  between the banks  4  is 0.1 mm in width, which can be set in the range from 0.05 mm to 0.15 mm by varying the position of the banks  4 . The height of the banks  4  can be set in the range from 5 μm to 20 μm.  
         [0073]     The material of the fixed electrode  1  can be, for example, nickel, SUS, a copper-zinc alloy or brass, copper, and aluminum. When the fixed electrode  1  is made of aluminum, adherence with a droplet material can be improved by, for example, applying chrome plating onto the upper surfaces of the projections  3 , or alternatively, by applying a liquid affinity treatment onto the upper surfaces of the projections  3 .  
         [0074]      FIG. 2  is a schematic plan view of the fixed electrode  1 , in which the banks  4  are formed on the upper surfaces of the projections  3 . In the example of  FIG. 2 , the banks  4  are formed on the projections  3  in parallel at intervals of 0.1 mm to form the groove  5 . The groove  5  can be a continuous groove or it may take the form of holes or recesses defined by crossing banks which form partitions as will be described in later examples. Although there are three projections  3  in the example of  FIG. 2 , there may be more than three projections  3  if required.  
         [0075]      FIGS. 3A and 3B  are plan views of the fixed electrode  1  having a partitioned groove on the projection  3 , in which the banks  4  and holes (recesses)  6  are shown on an enlarged scale for the sake of easy understanding.  FIG. 3A  shows an example of a circular fixed electrode (lower electrode) and  FIG. 3B  shows an example of an elliptical fixed electrode (lower electrode). As shown in  FIGS. 3A and 3B , a droplet material is applied onto the surfaces of the projections  3  by the ink jet method such that the banks  4  form the holes (recesses)  6 . For the droplet material, an adhesive epoxy-based material is used when insulating banks need to be formed.  
         [0076]      FIGS. 4A  to  4 D are diagrams in which examples of application of droplets by the ink jet method are shown.  FIG. 4A  shows an example in which the banks  4  are formed so as to form the holes (recesses)  6  on the projections  3 . As shown in  FIG. 4A , droplets  7  are emitted continuously onto the upper surfaces of the projections  3  by the ink jet method to form continuous banks of a minute height. Droplets are also applied across the continuous banks in a radially inwardly direction to form partitions that define holes or recesses  6  about 10 μm deep. The partitions are provided at intervals of 0.1 mm to 0.2 mm.  
         [0077]     After the banks  4  have been formed by applying the droplets  7 , a burning or heating process is executed to solidify the applied droplet material by evaporating the solvent. For example, the burning process is executed at temperatures from 100° C. to 200° C. Thus, after the droplet material has been applied onto the projections  3  of the fixed electrode (the lower electrode)  1 , the solvent is evaporated by the burning process, allowing the droplet material to be firmly fixed to the projections  3  in a short time.  
         [0078]     The droplet material to be applied to the projections  3  may be either a conductive material or a nonconductive material. In the case of using the conductive material, when the diaphragm is insulative, the diaphragm can be used as it is; when the diaphragm is noninsulative, it is necessary to form an insulating film on the surface of the diaphragm facing the fixed electrode (lower electrode)  1 .  
         [0079]     There are several methods for applying the droplets  7 . For example,  FIG. 4B  shows an application method in which the droplets  7  are applied in such a manner so as to overlap with each other.  FIG. 4C  shows a skip method in which after droplets “a” are applied at intervals and then droplets “b” are applied.  FIG. 4D  shows a droplet application method whereby the holes (recesses)  6  are formed, in which the droplets “a” are first applied to form one bank, then the droplets “b” are applied to form the other bank, and finally, droplets “c” are applied to form the partitions defining the holes (recesses)  6 .  
         [0080]      FIG. 5  is a diagram showing an example of application of the droplets  7  such that a continuous groove is formed on the projection  3  without forming the holes (recesses)  6 .  
         [0081]      FIGS. 6A and 6B  are plan views of other structural examples of the fixed electrode.  FIG. 6A  shows an example in which the banks  4  and the holes (recesses)  6  are formed on rectangular projections  3 , and  FIG. 6B  shows an example in which the projections  3  are arranged linearly, on which the banks  4  and the holes (recesses)  6  are formed. Thus, the fixed electrode (lower electrode)  1  and the projections  3  may have any shape. The projections  3  may have a continuous groove without the holes (recesses)  6 .  
         [0082]      FIG. 7  is a schematic perspective view of a droplet ejector  100  (hereinafter, also referred to as an ink jet unit  100 ). The ink jet unit  100  includes a base  31 , a board moving unit  32 , a head moving unit  33 , an ink jet head (head)  34 , an ink (liquid) supply unit  35 , and so on. The base  31  has the board moving unit  32  and the head moving unit  33  thereon.  
         [0083]     The board moving unit  32  is provided on the base  31  and has guide rails  36  along the Y-axis (in the main scanning direction). The board moving unit  32  moves a slider  37  along the guide rails  36  by, for example, a linear motor. The slider  37  has a θ-axis motor (not shown). The motor is, for example, a direct drive motor, whose rotor (not shown) is fixed to a table  39 . With such a structure, when the motor is energized, the rotor and the table  39  are rotated in the direction θ, so that the table  39  is rotated a specified angle θ with respect to the Y-axis and is fixed there.  
         [0084]     The table  39  is used to position a board S (corresponding to the fixed electrode  1  to be processed) and hold it. Specifically, the table  39  has a known adsorbing unit (not shown) and holds the board S on the table  39  by adsorption of the adsorbing unit. The board S is positioned properly and held in a specified position on the table  39  with a positioning pin (not shown) of the table  39 . The table  39  has a waste-ejection area  41  to which the ink jet head  34  emits ink (a liquid compound) by way of trial. The waste-ejection area  41  is provided along the X-axis (in the subscanning direction) and at the rear end of the table  39 .  
         [0085]     The head moving unit  33  includes a pair of stands  33   a  provided at the rear of the base  31  and a running path  33   b  provided on the stands  33   a . The running path  33   b  is arranged in the X-axis (in the subscanning direction), or in the direction perpendicular to the Y-axis (the main scanning direction) of the board moving unit  32 . The running path  33   b  includes a retaining plate  33   c  built between the stands  33   a  and a pair of guide rails  33   d  provided on the retaining plate  33   c , and movably holds a slider  42  that retains the ink jet head  34  along the length of the guide rails  33   d . The slider  42  runs on the guide rails  33   d  by the operation of a linear motor (not shown) or the like to move the ink jet head  34  in the direction of X-axis.  
         [0086]     To the ink jet head  34 , motors  43 ,  44 ,  45 , and  46  serving as movement positioning means are connected. When the motor  43  connected to the slider  42  and the ink jet head  34  is started, the ink jet head  34  moves vertically along the Z-axis, and so is positioned on the Z-axis. The Z-axis is orthogonal to the X-axis and the Y-axis (in the vertical direction). When the motor  44  is started, the ink jet head  34  moves in the β-direction in  FIG. 7 ; when the motor  45  is started, the ink jet head  34  moves in the γ-direction; and when the motor  46  is started, the ink jet head  34  moves in the α-direction. Thus, the ink jet head  34  is positioned.  
         [0087]     Thus, the ink jet head  34  moves linearly along the Z-axis on the slider  42  and also moves along the α-, β-, and γ-axes, thereby being positioned. Thus, the position of the ink-emitting surface of the ink jet head  34  with respect to the board S (the fixed electrode  1  to be processed) on the table  39  can be controlled properly.  
         [0088]     As shown in  FIG. 8A , the ink jet head  34  includes a nozzle plate  112  made of stainless steel or the like and a vibrating plate  113 , both of which are bonded together via a partition (reservoir plate)  114 . Between the nozzle plate  112  and the vibrating plate  113 , a plurality of spaces  115  and a reservoir  116  are formed by the partition  114 . The interior of the spaces  115  and the reservoir  116  are filled with ink (the compound of the projections) and communicate with each other through supply ports  117 . The nozzle plate  112  has a plurality of nozzle holes  118  in a row, for emitting a jet of ink (the compound of the projections) from the spaces  115 . The vibrating plate  113  has a hole  119  for supplying the ink (the compound of the projections) into the reservoir  116 .  
         [0089]     Referring to  FIG. 8B , a piezoelectric element  120  is joined to the surface of the vibrating plate  113  opposite to the surface facing the spaces  115 . The piezoelectric element  120  is located between a pair of electrodes  121  and, when energized, it is bent to project outward. The vibrating plate  113 , to which the piezoelectric element  120  is joined with such a structure, is bent outward together with the piezoelectric element  120  at the same time, thus increasing the capacity of the spaces  115 . Accordingly, ink (the compound of the projections) corresponding to the increased capacity flows from the reservoir  116  into the spaces  115  through the supply ports  117 . When the energization to the piezoelectric element  120  is cut off from this state, the piezoelectric element  120  and the vibrating plate  113  return to the original shape. Accordingly, also the spaces  115  resumes the original capacity, increasing the pressure of the ink (the compound of the projections) in the spaces  115 , so that ink droplets  122  are emitted from the nozzle holes  118  toward the board S. The ink jet method of the ink jet head  34  may be other than the piezo-jet type using the piezoelectric element  120 , such as a bubble-jet (registered trademark) method.  
         [0090]     Referring back to  FIG. 7 , the ink supply unit  35  includes a ink source  47  for supplying ink (the compound of the projections) to the ink jet head  34  and an ink supply tube  48  for feeding the ink (the compound of the projections) from the ink source  47  to the ink jet head  34 . In other words, the system adopts the method of storing ink (the compound of the projections) in the ink source  47 , or a stainless container, temporarily, and feeding the ink to the ink jet head  34  through the ink supply tube  48 .  
         [0091]     As has been described, a groove or holes (recesses) are formed in the projections of the fixed electrode with the ink jet unit  100 , so that banks of a minute height can be formed on the fixed electrode (lower electrode) extremely easily. The ink jet method has high flexibility in the direction of application including rotation, so that the continuous groove or independent recesses can be easily formed on the circular projection, as in the embodiment of the invention.  
         [0092]     Since a droplet material can be applied in sequence onto a plurality of works (fixed electrodes) placed on the stage, the invention offers high productivity. Furthermore, since there is no need to prepare the spacer of complex shape as in the prior art, the invention costs low. Also, the larger the electrostatic ultrasonic transducer is, the higher the degrees of making good use of the material and the flexibility of machinability are, increasing the advantage of the invention.  
         [0093]      FIGS. 9A and 9B  are diagrams in which other methods of forming the banks are shown.  FIG. 9A  shows a method of forming banks by etching, and  FIG. 9B  shows a method of forming banks by electroforming.  
         [0094]     In the case of forming the banks by etching, shown in  FIG. 9A , a resist  8  is first formed on the surface of each projection  3  to form a groove  6 a (step S 1 ). The upper surface of the fixed electrode  1  is subjected to etching with the resist  8  and etchant to form the groove  6   a  on the projection  3  (step S 2 ). Then the depressions  2  are formed by electrical discharge machining (step S 3 ). In the electroforming method shown in  FIG. 9B , the fixed electrode  1  is first subjected to nickel plating  9  by electroforming to form portions to be the banks  4  and the groove  6   a  (step S 11 ). Then the depressions  2  are formed by electrical discharge machining (step S 12 ).  
         [0095]     Since the conventional etching method requires to form the groove or recesses of a minute depth by etching, it needs an etching mask and etchant, posing the problem of an increase in cost and environmental problem of waste disposal. However, by the ink jet method according to the invention, only requirement can be applied only to a necessary portion, and so it is advantageous in cost and environment. The method by electroforming requires more expenses and labor than the ink jet method of the invention.  
         [0096]     As described above, the method of forming banks by the ink jet method of the invention is becoming more useful with an increase in the size of the electrostatic ultrasonic transducer as a high-decibel output speaker.  
         [0097]     While the invention has been described with reference to preferred embodiments, it is to be understood that the ultrasonic transducer and the method of manufacturing the ultrasonic transducer of the invention are not limited to the foregoing embodiments, and that various modifications can be made without departing from the sprit and scope of the invention.