Abstract:
A method for manufacturing piezoelectric components involves using a plurality of divided hard masks to form a highly accurate electrode pattern without degradation of electrical characteristics while increasing mask strength. The method is for forming vibration electrodes, terminal electrodes, and lead electrodes for connecting the vibration and terminal electrodes on a piezoelectric substrate and includes the step of preparing a first hard mask having openings corresponding to the vibration electrode and lead electrodes and a second hard mask having openings corresponding to the terminal and lead electrodes. By arranging the first hard mask on the piezoelectric substrate, the vibration electrodes and lead electrodes are formed by a thin-film depositing technique, and then, by arranging the second hard mask on the piezoelectric substrate, the terminal electrodes and lead electrodes are formed by the thin-film depositing technique. The first and second hard masks are arranged such that the openings of the first and second hard masks are partially overlapped, and the overlapped portion is located at a position that is spaced from the vibration electrode by at least about three times the thickness of the piezoelectric substrate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method for manufacturing piezoelectric components in which electrodes are formed on a surface of a piezoelectric substrate by a thin-film deposition technique such as a sputtering and a vacuum evaporation technique.  
           [0003]    2. Description of the Related Art  
           [0004]    When manufacturing piezoelectric components such as a dual-mode filter utilizing a thickness-longitudinal-vibration mode, which has complex-shaped electrodes, such as vibration electrodes, input-output-terminal electrodes, lead electrodes, and relay-capacitance electrodes, the electrodes are formed on a surface of a piezoelectric substrate by first applying an electrode material on the entire surface of the substrate by sputtering or vacuum evaporation, and then etching the electrode material via photolithography to form the electrodes. However, a large number of steps are required to complete the etching using photolithography, and thus, the manufacturing costs increase.  
           [0005]    A method for applying an electrode material on the substrate using a hard mask requires fewer steps than the etching process and is also less expensive. However, when forming an electrode with a complex shape, the shape of the opening in the hard mask also is complex which reduces the strength of the hard mask. Therefore, the hard mask deforms due to the heat produced during adhesion of the electrode material such that a space is formed between the hard mask and the substrate which allows the electrode material to flow into the space, and thus the accuracy of the electrode pattern is greatly reduced.  
           [0006]    As a solution to this problem, the use of divided hard masks, in which an electrode shape is divided into two or more patterns, has been proposed in Japanese Unexamined Patent Application Publication No. 11-297471.  
           [0007]    However, for the piezoelectric components, the electrode-forming method using the divided hard masks has not been utilized for the following reasons.  
           [0008]    At the divided portion of the electrode pattern, the electrode materials are built up for preventing wire breaking of the electrode. However, in the case of the piezoelectric components, electrical characteristics of the piezoelectric component deteriorate due to the position of the built up portion on the substrate, i.e., the electrode portion with a thickness larger than others.  
         SUMMARY OF THE INVENTION  
         [0009]    In order to overcome the above-described problems, preferred embodiments of the present invention provide a method for manufacturing piezoelectric components utilizing a plurality of divided hard masks to form an electrode pattern with high accuracies while improving the strength of the hard mask and not degrading electric characteristics of the piezoelectric component.  
           [0010]    According to a preferred embodiment of the present invention, a method for manufacturing piezoelectric components, in which a vibration electrode, terminal electrodes, and lead electrodes for connecting the vibration and terminal electrodes are formed on a piezoelectric substrate by a thin-film deposition technique, the method including the steps of preparing a first hard mask having openings corresponding to the vibration electrode and lead electrodes, preparing a second hard mask having openings corresponding to at least the terminal electrodes, continuously forming the vibration electrode and lead electrodes by the thin-film deposition technique while arranging the first hard mask on the piezoelectric substrate, and forming at least the terminal electrodes by the thin-film deposition technique while arranging the second hard mask on the piezoelectric substrate, wherein the first and second hard masks are arranged such that the openings of the first and second hard masks are partially overlapped, and wherein the overlapped portion of the first and second hard masks is located at a position on one of the lead electrode and the terminal electrode, the position being spaced from the vibration electrode by at least about three times the thickness of the piezoelectric substrate.  
           [0011]    First, the first hard mask having openings corresponding to the vibration electrode and the lead electrodes and the second hard mask having openings corresponding at least to the terminal electrodes are prepared. The openings formed on the first hard mask extend continuously from the vibration electrode toward the entire or a portion of the lead electrodes. The openings formed on the second hard mask correspond at least to the terminal electrodes and may continuously extend along a portion of the lead electrodes. The openings formed on the divided hard masks have a simple or small area configuration to prevent a reduction in the strength of the hard mask.  
           [0012]    Next, the first hard mask is arranged on the piezoelectric substrate, on which no electrode is formed, so as to continuously form the vibration electrode and the lead electrodes thereon by the thin-film depositing technique, such as a sputtering and vacuum evaporation technique. During the thin-film depositing, heat is applied to the hard mask, however, since the hard mask has increased strength as mentioned above, a highly accurate electrode pattern is formed.  
           [0013]    Then, the second hard mask is arranged on the piezoelectric substrate, on which the vibration electrode and the lead electrodes are formed, so as to form at least the terminal electrodes by the thin-film depositing technique in the same manner. At this time, to prevent wire breakage, the lead electrodes formed by the first hard mask are partially overlapped on the terminal electrodes or a portion of the lead electrodes formed by the second hard mask so as to increase the thickness of the electrodes. Since the overlapped portion, however, is located at a position spaced from the vibration electrode preferably by about three times or more than the thickness of the piezoelectric substrate, the piezoelectric vibration produced by the vibration electrode is not adversely affected by the overlapped portion, thus, preventing characteristic degradation caused by unnecessary vibration or reflection waves.  
           [0014]    Adhesion of an electrode material to a portion of a piezoelectric component which does not require it directly produces deterioration of electrical characteristics, such that accuracy in electrode configuration is demanded. When using the divided hard masks, the second or later-used hard mask is spaced from the substrate due to the thickness of the previously applied electrode material so as to produce a clearance between the hard mask and the substrate. When the electrode material extends into the clearance, blurring or deformation in shape of a finished electrode is produced which causes characteristic degradation.  
           [0015]    Preferably, a contact surface between the surface of the piezoelectric substrate and the second hard mask includes a concave portion formed to accommodate the vibration and lead electrodes formed using the first hard mask.  
           [0016]    That is, when the second hard mask is arranged on the piezoelectric substrate, the vibration and lead electrodes are located within the concave portion formed on the second hard mask to prevent the second hard mask from being spaced from the piezoelectric substrate. Accordingly, when forming the terminal electrodes and lead electrodes, the blurring or deformation in shape of the electrodes is prevented from occurring, and thus characteristic degradation is prevented.  
           [0017]    In addition, the order of forming electrodes on the piezoelectric substrate is not limited to the above-described procedure. Alternatively, the terminal electrodes and the lead electrodes may be formed using the second hard mask before the vibration electrodes are formed using the first hard mask.  
           [0018]    Then, preferably, a contact surface between the surface of the piezoelectric substrate and the first hard mask is provided with a concave portion formed to accommodate the terminal and lead electrodes formed using the second hard mask.  
           [0019]    When the lead electrode includes a curved portion at an intermediate portion thereof, the divided portion is the curved portion of the lead electrode, such that the opening shape is simplified. That is, preferably, the openings of the first hard mask extend from the vibration electrode to a position corresponding to the curved portion of the lead electrode, and the openings of the second hard mask extend from the terminal electrode to a position corresponding to the curved portion of the lead electrode.  
           [0020]    When the lead electrode has a straight shape, preferably, the openings of the first hard mask extend from the vibration electrode to positions corresponding to the connection portions of the lead electrodes to the terminal electrodes, and the openings of the second hard mask correspond only to the terminal electrodes. In this case, the terminal end of the lead electrode is overlapped on the terminal electrode.  
           [0021]    The divided portion is preferably located on the lead electrode when using divided hard masks for the following reasons. When the divided portion is located on the terminal electrode, an opening extending from the vibration electrode to a portion of the terminal electrode via the lead electrode on the piezoelectric substrate would be required, such that the opening configuration of the first hard mask is complicated, resulting in a substantial reduction in the strength of the first hard mask. Similarly, when the divided portion is located on the vibration electrode, an opening extending from the terminal electrode to a portion of the vibration electrode via the lead electrode on the piezoelectric substrate would be required, such that the opening configuration of the second hard mask is complicated, resulting in a substantial reduction in the of the second hard mask. Such a problem becomes noticeable especially when a curved portion is provided on the lead electrode. Furthermore, when the divided portion is located on the vibration electrode, the vibration electrode is partially changed in thickness which adversely influences the vibration characteristics. Therefore, it is preferable that the divided portion be located on the lead electrode.  
           [0022]    In addition, the width of the lead electrode is small, approximately 0.2 mm, for example. However, since accuracies in the size of the openings of the hard mask and in the positioning of the hard mask are greatly improved in preferred embodiments of the present invention, there are no problems with the accuracy of the lead electrodes.  
           [0023]    As described above, according to preferred embodiments of the present invention, since an electrode pattern is formed on the piezoelectric substrate using a plurality of separate hard masks, the configuration of openings formed on each hard mask is greatly simplified so as to form a high-accuracy electrode pattern by preventing the reduction in strength of the hard mask.  
           [0024]    Also, after forming the vibration electrodes and lead electrodes (or the terminal electrodes and lead electrodes) on the piezoelectric substrate using the first (or second) hard mask, the terminal electrodes and lead electrodes (or the vibration electrodes and lead electrodes) are continuously formed on the piezoelectric substrate using the second (or first) hard mask, while the terminal electrode or a portion of the lead electrode formed by the second (or first) hard mask is overlapped on the lead electrode formed by the first (or second) hard mask, such that wire breakage does not occur in a lead electrode portion so as to connect the electrodes securely.  
           [0025]    Furthermore, because the overlapped portion of the electrodes is located at a position that is spaced from the vibration electrode by about three times or more than the thickness of the piezoelectric substrate, the piezoelectric vibration caused by the vibration electrodes is not adversely affected by the overlapped portion, thereby preventing characteristic degradation due to unnecessary vibration or reflection waves.  
           [0026]    Other features, elements, characteristics and advantages of the present invention will become more apparent from the following description of preferred embodiments thereof with reference to the attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is a drawing of a piezoelectric component example manufactured by a method according to a preferred embodiment of the present invention, the drawing including a top surface view and back surface view.  
         [0028]    [0028]FIG. 2 is a circuit diagram of the piezoelectric component shown in FIG. 1.  
         [0029]    [0029]FIG. 3 includes a plan view and sectional view at the line of B-B of a first hard mask for forming topside electrodes of the piezoelectric component shown in FIG. 1.  
         [0030]    [0030]FIG. 4 includes a plan view, sectional view at the line of B-B, and sectional view at the line of C-C of a second hard mask for forming topside electrodes of the piezoelectric component shown in FIG. 1.  
         [0031]    [0031]FIG. 5 is a perspective view of an actual first hard mask and piezoelectric substrate (motherboard).  
         [0032]    [0032]FIG. 6 is a perspective view of an actual second hard mask and piezoelectric substrate (motherboard).  
         [0033]    [0033]FIG. 7 is a filter-waveform diagram of a piezoelectric component, in which an overlapped portion of lead electrodes is located at a position that is spaced about 0.6 mm from a vibration electrode.  
         [0034]    [0034]FIG. 8 is a filter-waveform diagram of a piezoelectric component, in which the overlapped portion of the lead electrodes is located at a position that is spaced about 0.4 mm from the vibration electrode.  
         [0035]    [0035]FIG. 9 is a surface view of another piezoelectric component manufactured by the method according to another preferred embodiment of the present invention.  
         [0036]    [0036]FIG. 10 is a plan view of a first hard mask for forming topside electrodes of the piezoelectric component shown in FIG. 9.  
         [0037]    [0037]FIG. 11 is a plan view of a second hard mask for forming topside electrodes of the piezoelectric component shown in FIG. 9. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0038]    [0038]FIG. 1 shows an example of piezoelectric components manufactured by a manufacturing method according to a preferred embodiment of the present invention, wherein an energy-trap type piezoelectric filter utilizing a thickness-longitudinal-vibration mode is exemplified. However, the present invention is not limited thereto.  
         [0039]    The piezoelectric filter preferably includes a substantially rectangular piezoelectric substrate  1  made of piezoelectric ceramics or piezoelectric monocrystal or other suitable material, in which the top major surface of the substrate is provided with two pairs of divided electrodes (vibration electrodes)  2 ,  3 ,  4 , and  5 , both pairs being arranged thereon at a desired distance from each other, while the back major surface is provided with common electrodes (vibration electrodes)  6  and  7  provided thereon to correspond to the pairs of divided electrodes  2 ,  3 ,  4 , and  5 , respectively. One dual-mode filter element F 1  is defined by these vibration electrodes  2 ,  3 , and  6 , while another dual-mode filter element F 2  is defined by the vibration electrodes  4 ,  5 , and  7 .  
         [0040]    On the top major surface of the piezoelectric substrate  1 , the divided electrode  2  is connected via a lead electrode  9  to an input terminal electrode  8  provided at one corner of the piezoelectric substrate  1 , while the divided electrode  4  is connected via a lead electrode  11  to an output terminal electrode  10  provided at another corner of the piezoelectric substrate  1 . Both lead electrodes  9  and  11  include curved portions  9   a  and  11   a  provided at intermediate portions thereof. Furthermore, the divided electrodes  3  and  5  are connected together via a lead electrode  12 . Also, the central portion of the piezoelectric substrate  1  is provided with a relay-capacitance electrode  13  provided thereon, which is connected to the central portion of the lead electrode  12  via a lead electrode  14 .  
         [0041]    On the back major surface of the piezoelectric substrate  1 , the common electrodes  6  and  7  are connected to a terminal electrode  17  for grounding and also defining a capacitance electrode via respective lead electrodes  15  and  16 . The grounding terminal electrode  17  opposes the capacitance electrode  13  with the piezoelectric substrate  1  interposed therebetween so as to form a relay capacitance C.  
         [0042]    The above-mentioned lead electrodes  9 ,  11 ,  12 ,  14 ,  15 , and  16  preferably have a width that is less than the other electrodes.  
         [0043]    [0043]FIG. 2 is a circuit diagram of the piezoelectric filter.  
         [0044]    [0044]FIG. 3 shows a first hard mask  20  used to form the electrodes on the top major surface of the piezoelectric filter, and FIG. 4 shows a second hard mask  30 .  
         [0045]    In addition, the hard masks shown in FIGS. 3 and 4 are for one piezoelectric filter, however, each of the actual hard masks  20  and  30 , as shown in FIGS. 5 and 6, include a plurality of openings so as to simultaneously form a plurality of electrodes on a motherboard.  
         [0046]    The first hard mask  20  is preferably made of a heat-resistant metallic or resin plate and includes openings  21 ,  22 ,  23 , and  24  for forming the divided electrodes  2 ,  3 ,  4 , and  5 , openings  25  and  26  for forming a portion of the lead electrode  9  and a portion of the lead electrode  11 , and an opening  27  for forming the lead electrode  12 . The openings  25  and  26  extend from positions corresponding to the divided electrodes  2  and  4  to positions corresponding to the curved parts  9   a  and  11   a  of the lead electrodes  9  and  11 .  
         [0047]    In addition, at edges of these openings  21  to  27 , tapered extended portions are provided to allow metallic particles to pass evenly through the openings.  
         [0048]    Also, the second hard mask  30 , in the same manner as in the first hard mask  20 , is preferably made of a heat-resistant metallic or resin plate, and includes openings  31 ,  32 , and  33  for forming the terminal electrodes  8  and  10  and the capacitance electrode  13  and small-width openings  34 ,  35 , and  36  for forming lead electrodes extending from the openings  31 ,  32 , and  33 . The openings  34  and  35  extend from positions corresponding to the terminal electrodes  8  and  10  to positions corresponding to the curved portions  9   a  and  11   a  of the lead electrodes  9  and  11 . The back surface of the hard mask  30 , i.e., a contact surface to the piezoelectric substrate  1 , is provided with a concave portion  37  provided therein to accommodate the divided electrodes  2 ,  3 ,  4 , and  5 , portions of the lead electrodes  9  and  10 , and the lead electrode  12 . The depth of the concave portion  37  is at least equal to the thickness of the electrodes formed by using the first hard mask  20 , and the shape of the concave portion  37  corresponds to the above-mentioned electrodes.  
         [0049]    In addition, at edges of these openings  31  to  36 , tapered extended portions are also provided.  
         [0050]    In the second hard mask  30 , respective distances L1 and L2 between edges of the openings  34  and  35  corresponding to portions of the lead electrodes  9  and  10  and the divided electrodes  2 ,  3 ,  4 , and  5  formed by using the first hard mask  20  are at least about three times the thickness t of the piezoelectric substrate  1 , such that the following relationships are satisfied:  
         [0051]    L1≧3t  
         [0052]    L2≧3t  
         [0053]    A method for forming electrodes on the top surface of the piezoelectric substrate  1  using the hard masks  20  and  30  will be described with reference to FIGS. 5 and 6.  
         [0054]    First, as shown in FIG. 5, the first hard mask  20  is provided on the surface of the piezoelectric substrate  1 , on which no electrodes are formed, so as to apply an electrode material (silver, for example) on the piezoelectric substrate  1  by using an evaporation or sputtering technique. Thereby, the electrode material passing through the openings  21  to  27  adheres on the piezoelectric substrate  1  so as to form the vibration electrodes  2  to  5  and the lead electrodes  9 ,  11 , and  12 , as shown in FIG. 6.  
         [0055]    Next, the second hard mask  30  is provided on this piezoelectric substrate  1  so as to apply electrode material on the piezoelectric substrate  1  by using the evaporation or sputtering technique in the same manner. At this time, the electrodes formed by the first evaporation or sputtering technique are accommodated within the concave portion  37  provided on the back surface of the second hard mask  30  so as to prevent the second hard mask  30  from being spaced from the piezoelectric substrate  1 . By the second evaporation or sputtering, the terminal electrodes  8  and  10 , the capacitance electrode  13 , and the balance of the lead electrodes  9  and  11  are formed. The electrodes formed by the second evaporation or sputtering are incorporated with the electrodes formed by the first evaporation or sputtering so as to form the piezoelectric component with the electrode configuration as shown in FIG. 1.  
         [0056]    In addition, although the specific description is omitted, the electrodes  6 ,  7 , and  15  to  17  formed on the back surface of the piezoelectric substrate  1  may be formed by using the divided hard masks in the same manner as described above or the electrodes may be simultaneously formed by a single hard mask.  
         [0057]    During the second evaporation or sputtering, the lead electrodes  9  and  11  formed using the second hard mask are partially overlapped on the lead electrodes  9  and  11  formed using the first hard mask, and the electrode material in the overlapped portions  9   a  and  11   a  has a thickness that is greater than that in other portions (see FIG. 1). The overlapped portions  9   a  and  11   a  are located at the curved portions of the lead electrodes  9  and  11 . When the overlapped portions  9   a  and  11   a  exist within the vibration range, thickness-longitudinal vibration is obstructed, thereby degrading characteristics. However, since the respective distances L1 and L2 between edges of the openings  34  and  35  of the second hard mask  30  corresponding to portions of the lead electrodes  9  and  10  and the divided electrodes  2 ,  3 ,  4 , and  5  are at least three times the thickness t of the piezoelectric substrate  1 , the overlapped portions (curved portions)  9   a  and  11   a  are also spaced from the divided electrodes  2 ,  3 ,  4 , and  5  by at least three times the thickness t of the piezoelectric substrate  1 . Therefore, as shown in FIGS. 7 and 8, electrical characteristics are thereby not adversely affected as a piezoelectric filter.  
         [0058]    In addition, an overlapped portion  12   a  is also produced between the lead electrodes  12  and  14  which does not adversely effect the electrical characteristics of a piezoelectric filter because the overlapped portion  12   a  is also spaced from the divided electrodes  2 ,  3 ,  4 , and  5  by at least about three times the thickness t of the piezoelectric substrate  1 .  
         [0059]    [0059]FIG. 7 shows the characteristics when L=0.6 mm, in which L is the distance between the divided electrodes  2 ,  3 ,  4 , and  5  and the overlapped portions  9   a ,  11   a , and  12   a , while FIG. 8 shows those when L=0.4 mm, wherein the piezoelectric substrate thickness t=200 μm (10.7 MHz), such that L≧3t in the former and L&lt;3t in the latter.  
         [0060]    As shown in FIGS. 7 and 8, the loss at 10.7 MHz is 3.6 dB in the former and 4.0 dB in the latter which is an increase of approximately 25%. Also, it is understood that the filter waveform in FIG. 7 is substantially laterally symmetrical while in FIG. 8, the filter waveform is laterally asymmetrical. Also, the bandwidth at 20 dB is 450 kHz in the former while it is 471 kHz in the latter such that the selectivity is degraded by approximately 5%.  
         [0061]    In addition, when electrical characteristics are measured for L=0.8 mm in the same manner, the same results as for L=0.6 mm are obtained.  
         [0062]    As is understood from the results mentioned above, when L≧3t, greatly improved electrical characteristics are obtained.  
         [0063]    [0063]FIG. 9 shows another piezoelectric component manufactured by the method according to another preferred embodiment of the present invention. The piezoelectric component is a ceramic discriminator utilizing thickness-longitudinal vibration.  
         [0064]    The discriminator preferably includes a substantially rectangular piezoelectric substrate  40  made of piezoelectric ceramics or piezoelectric monocrystal or other suitable material, in which the top major surface of the substrate is provided with a substantially circular vibration electrode  41  provided thereon, while the back major surface is provided with a vibration electrode  42  arranged to oppose the vibration electrode  41 . On the top major surface of the substrate  40 , the vibration electrode  41  is connected to an input terminal electrode  43  provided at one end of the piezoelectric substrate  40  via a thin lead electrode  44 . On the back major surface of the substrate  40 , the vibration electrode  42  is connected to an input terminal electrode  45  provided at the other end of the piezoelectric substrate  40  via a thin lead electrode  46 . Both the lead electrodes  44  and  46  have a substantially straight configuration.  
         [0065]    A connection portion between the terminal electrode  43  and the lead electrode  44  is provided with an overlapped portion  44   a  of the electrodes provided thereon, while similarly, a connection portion between the terminal electrode  45  and the lead electrode  46  is also provided with an overlapped portion (not shown) of the electrodes provided thereon. The respective distances L3 between these overlapped portions  44   a  and the vibration electrodes  41  and  42  are at least three times the thickness t of the piezoelectric substrate  40 .  
         [0066]    [0066]FIG. 10 shows a first hard mask  50  for forming topside electrodes of the piezoelectric component shown in FIG. 9, and FIG. 11 shows a second hard mask  60 . These hard masks  50  and  60  shown are used to produce one discriminator, however, each of the actual hard masks  50  and  60  includes a plurality of openings so as to simultaneously form a plurality of electrodes on a motherboard.  
         [0067]    The first hard mask  50  is preferably made of a heat-resistant metallic or resin plate and includes an opening  51  for forming the vibration electrode  41  and an opening  52  for forming the lead electrode  44 . In addition, at edges of these openings  51  and  52 , tapered extended portions are provided in the same manner as shown in FIGS. 3 and 4.  
         [0068]    Also, the second hard mask  60 , in the same manner as in the first hard mask  50 , is preferably made of a heat-resistant metallic or resin plate, and includes an opening  61  for forming the terminal electrode  43 . Also, the back surface of the hard mask  60 , i.e., a contact surface for contacting the piezoelectric substrate  40 , is provided with a concave portion  62  provided therein to accommodate the vibration electrode  41  and the lead electrode  44 . The depth of the concave portion  62  is at least equal to the thickness of the electrodes formed by using the first hard mask  50 , and the shape of the concave part  62  accommodates the above-mentioned electrodes  41  and  44 .  
         [0069]    In addition, at an edge of the opening  61 , a tapered extended portion is also provided.  
         [0070]    The distance L3 between the opening  61  of the second hard mask  60  and the vibration electrode  41  formed with the first hard mask  50  is preferably at least about three times the thickness t of the piezoelectric substrate  40 , such that the following relationship is satisfied.  
         [0071]    L3≧3t  
         [0072]    Also, according to this preferred embodiment, after forming the vibration electrode  41  and the lead electrode  44  on the piezoelectric substrate  40  with the first hard mask  50 , the terminal electrode  43  is formed using the second hard mask  60 . During the latter electrode-formation, the lead electrode  44  and the terminal electrode  43  are partially overlapped so as to produce the overlapped portion  44   a . Since the overlapped portion  44   a  is spaced from the vibration electrode  41  by the distance of at least about three times the thickness t of the piezoelectric substrate  40 , the vibration caused by the vibration electrodes  41  and  42  is not disrupted by the overlapped portion  44   a , and is also not adversely affected by reflection, thereby obtaining a discriminator with outstanding characteristics.  
         [0073]    In addition, the configuration of the electrodes on the back major surface of the piezoelectric substrate  40  is preferably substantially the same as that of the electrodes on the top major surface, such that the electrodes on the back major surface may be formed in the same manner by using the hard masks  50  and  60  used for forming the electrodes on the top major surface.  
         [0074]    According to the first preferred embodiment shown in FIGS.  1  to  6 , the lead electrodes  9  and  11  are provided with the curved portions  9   a  and  11   a  provided at intermediate portions thereof so as to produce the overlapped portions in the curved portions, whereas according to the preferred embodiment shown in FIGS.  9  to  11 , the lead electrodes  44  and  46  are formed in a substantially straight shape without the curved portion being formed on the lead electrodes. In this case, since the curved portion or changed-width portion is formed in the respective boundaries between the lead electrodes  44  and  46  and the terminal electrodes  43  and  45 , by providing an overlapped portion in this curved portion, the shape of the hard-mask opening is greatly simplified, which prevents the hard mask strength from being reduced.  
         [0075]    According to the preferred embodiments described above, two hard masks are used as the divided hard masks, however, three or more hard masks may be used.  
         [0076]    Also, the order of use of the first hard mask and second hard mask may be reversed. That is, after the terminal electrodes (and the lead electrode) are formed using the second hard mask, the vibration electrode and the lead electrode may be formed using the first hard mask. In this case, the concave portion for accommodating the electrodes may be preferably provided on the back surface of the first hard mask.  
         [0077]    While preferred embodiments of the invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.