Abstract:
Sacrificial electrodes with fractal-shaped are formed on a SAW (surface acoustic wave) device. The sacrificial electrodes discharge electro-static charge in the SAW device for protecting the IDT (inter-digital transducer) from electrostatic break. Moreover, the sacrificial electrodes can control the path and the discharging degree of the electro-static discharge to avoid losing the electro-static discharge protection due to the sacrificial electrodes are broken.

Description:
RELATED APPLICATIONS 
   This application is a divisional of U.S. application Ser. No. 11/685,317, filed Mar. 13, 2007, now U.S. Pat. No. 7,361,964, which is a divisional of U.S. application Ser. No. 11/126,671, filed May 11, 2005 (now U.S. Pat. No. 7,227,293), said patent applications fully incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention is related to a surface acoustic wave (hereafter referred to SAW) device with ESD (electrostatic discharge) protection, especially to a SAW device having sacrificial electrodes. 
   BACKGROUND OF THE INVENTION 
   Resistors, capacitors and inductances required in conventional filters or sensors may be replaced since the SAW device is capable of processing signals with the design of surface electrodes. The SAW device has the advantages of high performance, small size, low cost, and repetitive manufacturing. Therefore, the SAW device is important both in electronic industry and communication systems, especially in cellular phones which require high performance and small size. 
   Take SAW filter as an example, as shown in  FIG. 1A , the piezoelectric substrate  10 , such as quartz, Lithium Niobate or Lithium Tantalate, is coated with designed inter-digital transducers (hereafter referred to IDTs)  4  and  5  having interval D. After receiving high frequency signals  1 , the IDT  4  will transform the received signals to surface acoustic waves  2 . Then, the surface acoustic waves  2  are passed to IDT  5  through the piezoelectric substrate  10  and outputted after transformed to high frequency signals  3 . In the aforementioned IDTs  4  and  5 , each interval space between fingers is D, but these intervals may be changed or modified with different dimension according to the various demands or application. Hence, the input high frequency signals having the same resonance frequency with IDT  4  can be transformed to surface acoustic waves  2 , and the surface acoustic waves  2  having the same resonance frequency with IDT  5  can be transformed to high frequency signals  3  efficiently. In above transformation, it is imperative to get rid of the unlimited signals and noises but let go signals with certain wavelengths (wavelength selection), to filter signals. 
   In the process of SAW device manufacturing, such as the procedure of forming IDTs, the SAW device has to bear some temperature rise and fall treatment such as the thermal cycle for curing photo-resistance. For the piezoelectric substrate is made of thermoelectric materials, the change in temperature would produce ionization electrons accumulated on the SAW device so as to generate electrostatic charges. The electrostatic discharge will damage the SAW device as well as alter their characteristics. In view of this, as shown in  FIG. 1B , the Japan Patent No. 6-224682 discloses that not only IDTs  14   a ,  15   a  and reflection portions  14   b ,  15   b , but also the sacrificial electrodes  16   a ,  17   a  for destroying the electrostatic charges are formed on the input electrode  12  and output electrode  13  of the piezoelectric substrate  11 . Because of the sacrificial electrodes, the electrostatic discharging would take place on the sacrificial electrodes rather than IDTs  14   a  and  15   a.    
   Although aforementioned structure makes the electrostatic discharging occur on the sacrificial electrodes, however, it is not easy to control the discharging process, sometimes excessive electrostatic discharge would still destroy the sacrificial electrodes, thereby leading the sacrificial electrodes fail to perform the protection function. Hence, if there are more electrostatic charges, they may discharge on the IDTs  14   a ,  15   a  and the characteristics and functions of them would be altered and destroyed. Concerning the excessive electrostatic discharge, the other disadvantage is that the sacrificial electrodes will melt to contact each other to cause short circuit. Accordingly, Japan Patent No. 8-321739 improves forgoing structure. Resistant films  16  and  17  are attached on the sacrificial electrodes  16   a  and  17   a  to increase their resistance against electrostatic discharge, as shown in  FIG. 1C . The IDTs can be reused to protect the SAW device from electrostatic discharge. However, the structures both require higher specific surface area to attain better resistance against electrostatic discharge. Besides, the path of electrostatic discharge is not controllable, and the sacrificial electrodes are those with smaller digital intervals. The oscillation thereof would influence the characteristics of the SAW device, so the sacrificial electrodes  16   a ,  17   a  must be disposed perpendicular to the IDTs  14   a ,  15   a , respectively. 
   Japan Patent No. 5-121993 discloses another SAW device protection structure, as shown in  FIG. 1D . Generally, the horizontal fingers  25 ,  27  are formed on the front end of the middle between the fingers  24   a ,  24   b  and  26   a ,  26   b  of the IDT  22  ( 23 ) to prevent the ESD break. This structure will increase the capacity of electrostatic charges and then effect the oscillation of the SAW device. Consequently, the horizontal fingers  25 ,  27  are moved afterward to avoid foregoing problem. Nevertheless, with this structure, it is easy to occur electrostatic discharging to damage SAW device in the position designated P between two digital transducers. Another improved structure is shown in  FIG. 1E , the horizontal fingers  25 ,  27  are extended to form horizontal fingers  28  and adjacent fingers are connected to avoid such a problem. In this way, the properties of frequency response will be altered. Besides, the scope of usage is limited since the energy loss increases during the filtering process. 
   U.S. Pat. No. 6,034,578 discloses a SAW device with discharge electrodes electrically independent from the IDTs. As shown in  FIG. 1F , IDTs  32 , input wire bonding pad  33 , output wire bonding pad  34 , ground wire bonding pads  35 , common electrodes  36 , and thin film electrodes  37   a ,  37   b ,  37   c  and  37   d  are provided on the piezoelectric substrate  31 . The thin film electrodes  37   a ,  37   b ,  37   c  and  37   d  are not connected to any aforementioned elements and electrically independent from them. In addition, the spaces S 1  between the thin film electrodes  37   a  and  37   b  as well as between  37   c  and  37   d  are smaller than the spaces S 2  and S 3  between IDTs  32 . The generation of electric charges due to spontaneous polarization is not uniform, and the quantity of charge generation is largest in the dicing margin portion. Since the space S 1  is set to be sufficiently narrow, the static electricity is discharged between the thin film electrodes  37   a  and  37   b  or between  37   c  and  37   d  selectively. Accordingly, there is no fear that the IDTs  2  are subject to ESD breaking. However, this structure requires additional regions to dispose thin film electrodes  37   a - 37   d  so that the size thereof increases. Besides, there exist electrostatic charges in the inner region of thin film electrodes  37   a - 37   d , too. These charges would attach to the elements such as IDTs  32 . Since IDTs  32  are electrically independent from thin film electrodes  37   a - 37   d , these electrostatic charges cannot be transferred to thin film electrodes  37   a - 37   d . Hence, IDTs may suffer electrostatic breaking. 
     FIG. 1G  shows a SAW device with the protection against electrostatic breaking according to U.S. Pat. No. 6,486,752. The digital transducers  41 ,  42 , and  43  forms first IDTs  61 , and the digital transducers  44 ,  45  and  46  forms second IDTs  62 . The IDTs  61  and reflectors  63 ,  64  and  65  are combined with the series connected resonator, and the IDTs  62  as well as reflectors  67 ,  68 ,  69  and  70  are combined with the parallel resonator. The digital transducers  42  and  45  respectively connect to dicing line  55  via connection patterns  91  and  92 . Digital transducers  41  and  44 ,  43 ,  46  connect to dicing line  55  via connection patterns  81 ,  84 ,  86 , respectively. Reflectors  65 ,  66 ,  67  and  68  connect to dicing line  55  via connection pattern  86 , and reflectors  63 ,  64 ,  69  and  70  connect to dicing line  55  via connection pattern  86 ,  82 ,  83 ,  85 , respectively. Accordingly, all devices connect with dicing line  55  to discharge electrostatic charges thereon to avoid electrostatic break. But, the dicing line will be cut off so that the electrostatic protection vanishes. It is unavoidable to suffer electrostatic break for package process. 
   SUMMARY OF THE INVENTION 
   In view of the aforementioned problems, one purpose of this invention is to provide a SAW device with ESD protection, so as to prevent the SAW device from electrostatic break. 
   Another purpose of this invention is to provide a sacrificial electrode with small specific surface area so that the required area of the sacrificial electrode would be smaller. 
   Yet another purpose of this invention is to provide a sacrificial electrode with fractal configurations, so as to control the discharging path and intensity. 
   Yet another purpose of this invention is to provide a sacrificial electrode with simple manufacturing process. 
   Still another purpose of this invention is to prevent the sacrificial electrode from melting to cause short circuit for electrostatic discharge. 
   Still another purpose of this invention is to make electrostatic charges difficult to accumulate, so as to moderate the intensity of each discharging. 
   Still further purpose of this invention is to provide a sacrificial electrode with sustaining effectiveness during the entire manufacturing and packaging process of the sacrificial electrode. 
   According to above purposes, this invention provides a SAW device which comprises a piezoelectric substrate, an input inter digital transducer, an output inter digital transducer, an input sacrificial electrode, and an output sacrificial electrode. The input inter digital transducer is formed on the piezoelectric substrate and comprises a first digital structure and a second digital structure, which are interlaced with and electrically independent from each other. The output inter digital transducer is formed on the piezoelectric substrate and comprises a third digital structure and a fourth digital structure, which are interlaced with and electrically independent from each other. The input sacrificial electrode is formed on the piezoelectric substrate and comprises a first electrode and a second electrode, which are electrically independent from each other. The first electrode is electrically connected to the first digital structure and the second electrode is electrically connected to the second digital structure. The output sacrificial electrode is formed on the piezoelectric substrate and comprises a third electrode and a fourth electrode, which are electrically independent from each other. The third electrode is electrically connected to the third digital structure and the fourth electrode is electrically connected to the fourth digital structure. The first, second, third, and fourth electrodes include fractal configurations. 
   This invention provides a SAW device comprising a piezoelectric substrate, an input inter digital transducer, an output inter digital transducer, an input wire bonding pad, an output wire bonding pad, and an electrode set. The input inter digital transducer is formed on the piezoelectric substrate and comprises a first digital structure and a second digital structure, and the first and second digital structures are interlaced with and electrically independent from each other. The output inter digital transducer is formed on the piezoelectric substrate and comprises a third digital structure and a fourth digital structure, and the third and fourth digital structures are interlaced with and electrically independent from each other. The input wire bonding pad is electrically connected to the first digital structure, and the output wire bonding pad is electrically connected to the fourth digital structure. The electrode set is formed on the piezoelectric substrate and comprises a first electrode and a second electrode, which are electrically independent from each other. The first electrode is electrically connected to one of the first, second, third, and fourth digital structures. The second electrode is electrically connected to either the input or the output wire bonding pad. The first and second electrodes include fractal configurations. 
   This invention also provides sacrificial electrodes of a surface acoustic wave device for electrostatic discharge protection, the sacrificial electrodes comprising: a first sacrificial electrode electrically connected to an input bonding pad of the surface acoustic wave device; and a second sacrificial electrode electrically connected to an output bonding of the surface acoustic wave device; wherein the first and second sacrificial electrodes are electrically independent from each other and have fractal configurations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  presents the structure of a conventional SAW device; 
       FIG. 1B-1G  present the structure of conventional SAW devices with ESD protection; 
       FIG. 2A  presents the structure of one SAW device in according with this invention; 
       FIG. 2B  presents the structure of another SAW device in according with this invention; 
       FIG. 2C  presents the structure of a sacrificial electrode in according with this invention; 
       FIG. 2D  presents the electric field distribution of partial sacrificial electrode; 
       FIG. 2E  presents the relationship between the electrostatic discharge and time; 
       FIGS. 2F and 2G  presents the electric field distribution of another sacrificial electrode; 
       FIG. 3A  presents the structure of original electrode without fractal configuration; 
       FIG. 3B  presents the structure of electrode with fractal configurations in first rank; 
       FIG. 3C  presents the structure of electrode with fractal configurations in second rank; 
       FIG. 3D  presents the electric field distribution of the original electrode as shown in  FIG. 3A ; 
       FIG. 3E  presents the electric field distribution of the electrode with fractal configurations in first rank as shown in  FIG. 3B ; 
       FIG. 3F  presents the electric field distribution of the electrode with fractal configurations in second rank as shown in  FIG. 3C ; 
       FIG. 4  presents the smoothed sharp angles of the sacrificial electrode; and 
       FIG. 5  presents the structure of the capacitor with fractal configurations according to this invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Embodiments of the present invention will be described below. Except these embodiments, this invention can also be applied in other various embodiments. However, the present invention is defined by the accompanying claims rather than any embodiment. 
   To provide clear description and understanding, some parts of the drawings are not made according to actual scale, and irrelevant details are omitted. 
   The invention utilizes the concept that a fractal has infinite circumferences within the same measure of area. Thus, more discharging paths can be provided in an extremely small area, and the discharging position and intensity may be controlled by applying the nature of point discharge. Hence, the ability of ESD protection would be raised. 
     FIG. 2A  presents one embodiment according to this invention. Input IDTs  101  and output IDTs  106  as well as input wire bonding pad  104  and output wire bonding pad  109  are formed on a piezoelectric substrate  100 . The digital structures  102  and  103  constitute the input IDTs  101 , wherein the digital structure  102  connect to input wire bonding pad  104  electrically. The digital structures  107  and  108  constitute the output IDTs  106 , wherein the digital structure  107  connect to output wire bonding pad  109  electrically. The input digital sacrificial electrode  111  is formed by two electrically independent digital structures just as the input IDTs  101 , and one of both electrically connects to digital structure  102  and the other to digital structure  103 . The electrical connection type between output digital sacrificial electrode  112  and output IDTs  106  is identical to that between input digital sacrificial electrode  111  and input IDTs  101 . As shown in  FIG. 2A , only one of electrical connection type between digital sacrificial electrodes  111 ,  112  and IDTs  101 ,  106  is presented, and the configurations of the digital sacrificial electrodes  111 ,  112  are omitted since the size of digital sacrificial electrodes  111 ,  112  are far smaller than that of IDTs  101 ,  106 . 
     FIG. 2B  presents another embodiment of this invention. The layout of digital sacrificial electrodes is different from that of  FIG. 2A . Except input IDTs  101  and output IDTs  106 , ordinary SAW device may also have input wire bonding pad  104 A,  104 B and output wire bonding pad  109 A,  109 B. The digital sacrificial electrodes  111 ,  112  may be one part of input wire bonding pads  104 A,  104 B and output wire bonding pads  109 A,  109 B, respectively. That is, digital sacrificial electrodes  111 ,  112  electrically connect to input wire bonding pads  104 A,  104 B and output wire bonding pads  109 A,  109 B. 
     FIG. 2C  is the partial enlargement of the region designated S in  FIG. 2A  and presents the structure of sacrificial electrodes. In this embodiment, triangles are used in the fractal of the digital structure as basic motif element. The shape of fractal is not regular in macro view, but when taking some part of the fractal, no matter how large the taken part is, a few common basic motif elements can be found (Self-duplication). Therefore, in  FIG. 2C , the peripheral portions of digital transducers constituting the sacrificial electrodes are formed by triangles. These fractals formed on the sacrificial electrodes may only have basic triangles (first rank) as shown in regions G 1 , G 3 , or have smaller basic elements (second rank) on each basic triangle as shown in region G 2 . Besides, there may be further smaller basic elements formed on the second rank figures (third rank) as shown in region G 5 . Higher rank figures can be applied in this invention. The number of smaller basic elements on each basic element, i.e. the number of the (X+1)th rank figures on each Xth rank figure, is unlimited, and the rank order herein merely presents the stack sequence not the relative size. That is the (X+1)th rank figures formed on certain Xth rank figure, and the former may be larger than the latter. However, the (X+1)th rank figures are preferably smaller than Xth rank figures. In the present invention, the fractals may be protruding, concave, or both. The region G 6  shows a concave triangle, and the region G 7  illustrates a triangle with two smaller concave triangles formed thereon. The region G 8  presents triangles with different heights. 
   Besides the triangles, other figures may be introduced in this invention to form the sacrificial electrodes. The introduced figures preferably have at least one sharp angle thereon. Two kinds of basic elements may be used to constitute one sacrificial electrode. Although all of the basic elements in  FIG. 2C  are not the same, the invention may adopt any combination of basic elements including the situation with only one kind of basic element. Moreover, the basic elements may not only be deposited on the long side of digital transducers but also on the short side, as shown in region G 6  of  FIG. 2C . 
   In  FIGS. 2A ,  2 B, and  2 C, the number and arrangement of transducers are presented for the purpose of illustration, not for limitation, and various number and arrangement may be applied in the invention according to different demands and purposes. 
   The electric field intensity of adjacent digital transducers in regions G 1 , G 2 , G 3 , and G 4  will be illustrated. Referring to  FIG. 2D , edge  114  has protruding fractals and edge  116  has concave configuration corresponding to edge  114 . It is obvious in electric field distribution curve  118  that the intensity is relatively higher at the tips of the triangles. Since the high electric field intensity regions tend to be formed at the tips of the triangles, the charges are mostly distributed thereon. Besides, the electric field intensity between tips is too low to let through the charges, so the charges are confined within these tips. Therefore, the electrostatic discharging path can be guided to these tips. After being discharged, the discharged tip will not receive any charge from the other tips, and the discharged tip will not be damaged due to excessive discharge. That is, discharging at each tip would cease right after releasing all of its charges, and therefore the process is quite mild. 
   In  FIG. 2E , a dotted line  130  illustrates the relationship between the energy and time in conventional discharging process, and a line  120  presents the situation as applying the invention. Generally, the discharge would concentrate on certain point with highest electric field intensity, thus the change level of electrostatic charge would be so huge that the discharging point will be destroyed. However, the discharging in the present processes step by step, as shown in the line  120  of  FIG. 2E . A plurality of tips would share all of the electrostatic discharge, so the process may be quite mild. Alternatively, the relationship between energy and time would be close to the dotted line  130  when the discharging at some tip triggers that at other tips. Since the discharges are distributed to a plurality of tips, the generated energy will not be so huge to damage any discharging point. Accordingly, in the invention, the discharging paths are controllable, the discharging process is mild, and the sacrificial electrodes would not melt to produce short circuit. 
   As mentioned above, the invention utilizes the tips to discharge. Regarding the edge with protruding figures, except the edge with concave figures as shown in region G 4 , G 6  of  FIG. 2C  and edge  116  of  FIG. 2D , the adjacent edges may have the configurations of straight lines or arcs.  FIG. 2F  is similar to  FIG. 2D  excluding the edge corresponding to the protruding edge  134  is modified to the edge of arc configuration. Besides, the electric filed distribution curve  138  of  FIG. 2F  is like the curve  118  of  FIG. 2C  that the electrostatic field intensities at tips of triangles would be relatively higher. Hence, the configurations of corresponding edges are unlimited in the invention. The difference of shortest distance between each tip and its corresponding edge is preferably quite little to avoid the concentration of excessive energy. The distances between the edges of adjacent digital transducers are preferably 0.1 to 10 micrometer. Consequently, the protruding edge  134  may have a corresponding protruding edge  140 , and the tips thereof may correspond to the concave of edge  134 . The electric field distribution is shown in a curve  142 . In this way, there are more discharging paths and the process would be much more moderate for the wide distribution of energy. 
   Additionally, more electrostatic discharging paths may be provided by utilizing the self-replication property of fractals. Hence, the other advantage of this invention is to reduce the required area of sacrificial electrodes significantly. 
   Other electric field distributions are presented on  FIG. 3 . As shown in  FIG. 3A , two electrodes  210 ,  220  are formed on a substrate  200 , wherein the electrode  210  without fractals is referred as original configuration. Electrode  211  has two fractal configurations in first rank  230 , as shown in  FIG. 3B . Electrode  212  has two fractal configurations in second rank  235 , as shown in  FIG. 3C .  FIG. 3D  presents the electric field distribution of electrode  210 , and the electric field concentrate on electrode  210  since electrode  210  has no fractal configuration.  FIG. 3D  is made according to  FIG. 3A . The electric field distribution of electrode  211  with fractal configurations in first rank  230  is illustrated in  FIG. 3E . Comparing  FIG. 3E  with  FIG. 3D , except the position of electrode  211 , two places around coordinates ( 20 ,  90 ) and ( 40 ,  90 ) in  FIG. 3E  also have higher electric field intensity and do correspond to the fractal configurations  230  in  FIG. 3B . The electric field distribution of electrode  212  with fractal configuration in second rank  235  is illustrated in  FIG. 3F . Two places around coordinates ( 20 ,  90 ) and ( 40 ,  90 ) have higher electric field intensity and do correspond to the fractal configurations  235  in  FIG. 3C . Comparing  FIG. 3F  with  FIG. 3E , the electric field intensity generated by fractals in second rank is higher and there are more places having higher intensity. The electric field intensity would be higher and the point discharge may be more obvious if the fractal configurations are more complicated. 
   The electrodes of this invention may be made of pyroelectric material or piezoelectric material, such as barium titanate piezoelectric ceramic, lead titanate piezoelectric ceramic, lead titanate-zirconate piezoelectric ceramic, triple system piezoelectric ceramic, polymer piezoelectric material, and compound piezoelectric material. The electric dipole of the piezoelectric material would not work until being heated. Therefore, during manufacturing process the electrostatic charges generated by shifting temperature will be discharged instantly instead of accumulating. Moreover, the electrostatic charges generated by electromagnetic waves of SAW devices would be released due to the thermal energy produced by the SAW devices. For example, as a component of a cellular phone, the SAW device would have a temperature when the cellular phone is used, so as to discharge the sacrificial electrode. That is, whenever the SAW device is tend to generate electrostatic charges, the sacrificial electrode is also likely to be discharged. Consequently, each electrostatic discharging would be so mild that the sacrificial electrode may not be broken. 
   Additionally, some defects are unavoidable in the manufacturing process of semiconductor devices (such as the edges of the configuration having some indentations or protrusions). However, the sacrificial electrodes of this invention could endure these defects well, and sometimes certain defects may work as electrostatic discharging paths. Therefore, the tolerance of the manufacturing process of the sacrificial electrodes is quite high and easy. The sharp angles of the sacrificial electrodes may melt to smooth corners as shown in  FIG. 4 , but their functions would not be influenced. Hence, the SAW devices are provided with ESD protection during the entire manufacturing and packaging process. 
   The sacrificial electrodes of this invention may be electrically independent from each other, and have fractal configurations as well as ESD protection. Moreover, the distance between two electrodes is not uniform for the irregularity of fractals. The sacrificial electrodes would neither affect the frequency of the SAW device nor have to be perpendicular to the IDTs. As shown in  FIG. 2A  or  FIG. 2B , the two electrodes of the sacrificial electrode may be electrically connected to two elements or wire bonding pads of the semiconductor device. Alternatively, one electrode of the sacrificial electrode is electrically connected to any one element of the semiconductor device and the other electrode is electrically connected to any input or output terminal, such as a wire bonding pad. 
   According to the characteristic of fractals, the size of opposite surfaces may be so large that the sacrificial electrode can be used as a capacitor. As a capacitor, the sacrificial electrode would rather discard the shape angles to avoid the point discharge. The edges of two electrodes are preferably parallel to each other, so the distance between entire edges is almost consistent. As shown in  FIG. 5 , the electrodes  150 ,  152  constitute a capacitor. 
   This invention provides a SAW device, which comprises a piezoelectric substrate, an input inter digital transducer, an output inter digital transducer, an input sacrificial electrode, and an output sacrificial electrode. The input inter digital transducer is formed on the piezoelectric substrate and comprises a first digital structure and a second digital structure, and the first and second digital structures are interlaced with and electrically independent from each other. The output inter digital transducer is formed on the piezoelectric substrate and comprises a third digital structure and a fourth digital structure, and the third and fourth digital structures are interlaced with and electrically independent from each other. The input sacrificial electrode is formed on the piezoelectric substrate and comprises a first electrode and a second electrode, which are electrically independent from each other. The first electrode is electrically connected to the first digital structure and the second electrode is electrically connected to the second digital structure. The output sacrificial electrode is formed on the piezoelectric substrate and comprises a third electrode and a fourth electrode, which are electrically independent from each other. The third electrode is electrically connected to the third digital structure and the fourth electrode is electrically connected to the fourth digital structure. The first, second, third, and fourth electrodes include fractal patterns with sharp angles, so as to electrostatically discharge thereat. 
   This invention provides a SAW device comprising a piezoelectric substrate, an input inter digital transducer, an output inter digital transducer, a input wire bonding pad, a output wire bonding pad, and an electrode set. The input inter digital transducer is formed on the piezoelectric substrate and comprises a first digital structure and a second digital structure, and the first and second digital structures are interlaced with and electrically independent from each other. The output inter digital transducer is formed on the piezoelectric substrate and comprises a third digital structure and a fourth digital structure, and the third and fourth digital structures are interlaced with and electrically independent from each other. The input wire bonding pad is electrically connected to the first digital structure, and the output wire bonding pad is electrically connected to the fourth digital structure. The electrode set is formed on the piezoelectric substrate and comprises a first electrode and a second electrode, which are electrically independent from each other. The first electrode is electrically connected to one of the first, second, third, and fourth digital structures. The second electrode is electrically connected to either the input or the output wire bond pad. The first and second electrodes include fractal configurations. 
   This invention also provides sacrificial electrodes for ESD protection of semiconductor device, which comprises an input sacrificial electrode and an output electrode. The input sacrificial electrode is electrically connected to an input terminal of the semiconductor device and the output terminal is electrically connected to an output terminal of the semiconductor device. The input and output sacrificial electrodes are electrically independent from each other. The configurations of the input and output sacrificial terminals are fractals with sharp angles, so as to discharge at the tips thereof. 
   Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.