Saw device having a plurality of electrodes and a fabrication method thereof

Abstract: A surface-acoustic-wave device comprises a substrate (51) of a piezoelectric material for providing a passage (CH) of surface acoustic waves at an upper major surface of the substrate, a plurality of interdigital electrodes (71, 72) provided on the piezoelectric substrate in a row in correspondence to the passage of surface acoustic waves such that the electrodes are aligned in the propagating direction of the surface acoustic waves, each of the plurality of interdigital electrodes having a first part (EL1, EL1′) connected to a bonding pad (78, 78′) on the substrate for external electric connection and having a plurality of finger electrodes (f₁ - f n ) extending parallel with each other in a direction crossing the propagating surface acoustic waves, and a second part (EL2, EL2′) separated from the first part and having a plurality of finger electrodes (g₁ - g n ) extending parallel with each other in a direction opposite to the direction of the finger electrodes of the first part. An interconnection pattern (54) is provided on the surface of the substrate in correspondence to a region offset from the passage of the surface acoustic waves for connecting the second parts of adjacent interdigital electrodes with each other.

Description: The present invention generally relates to the so-called surface-acoustic-wave devices having a plurality of interdigital electrodes, and in particular to the structure and fabrication process for eliminating the occurrence of sparking discharge between the fingers of the electrodes during the process of device fabrication. Recently, the demand for increased operational speed of information processing apparatuses and communication apparatuses has caused the shift of frequency used for the carriers or signals to higher frequency regions. In correspondence to such a shift of the frequency, filters capable of operating in such high frequency regions are required. For this purpose, the surface-acoustic-wave (abbreviated hereinafter as SAW) devices are used. In view of the expected developments in the future, particularly in the field of automobile telephones and portable telephones, efforts are made to develop the SAW devices having a sharp attenuation in the frequency region outside the pass-band while maintaining a uniform band-pass characteristic. By using the SAW device in place of the conventional dielectric filters, the size of the filters can be reduced to about 1/30 and the size of the telephone can be reduced accordingly. A typical SAW device such as the SAW filter uses a piezoelectric substrate having large electromechanical coupling coefficients and small temperature coefficient of frequency. For example, a single crystal of LiTaO₃ is used widely. The crystal of LiTaO₃ is cut in a predetermined orientation, and interdigital electrodes are provided on the substrate as the input and output electrodes. FIG. 1 shows the geometrical parameters characterising a typical interdigital electrode. Referring to FIG. 1, the electrode comprises a first part EL1 and a second part EL2 each having a number of fingers f l - f n or g l - g n , wherein each finger has a width W and separated from adjacent fingers by a separation S. Designating the wavelength of the surface acoustic wave as λ, the width W and the separation S are generally set to satisfy the relation W = S = λ/4. Thereby, the pitch defined in FIG. 1 as P is set to P = λ/2. Further, each finger in the electrode EL1 and each finger in the electrode EL2 are provided to form a uniform overlap as shown in FIG. 1. Such an electrode is called the uniform overlap electrode. EP 0 176 786 discloses a transducer for SAW filter constructed as group-type unidirectional transducer having structures arranged in one channel K and comprising finger electrodes connected to busbars including a meander-type busbar which is inserted between two of these busbars the finger electrodes enmeshing interdigitally in a manner such that first half-groups forming a first subtransducer for a first phase 1 and second half-groups forming a second subtransducer for the second phase are present, half-groups of one subtransducer being alternatingly interlaced with half-groups of the other subtransducer and being arranged with a phase shift of ± λ/4 between the subtransducers. In this example the first half-groups of one subtransducer and the second half-groups of the other subtransducer are split up into two or more channels arranged in parallel such that whenever the sequence of the half-groups of one subtransducer in one channel is interrupted by half-groups of the other different-phase substranducer, this one subtransducer is continued in a staggered manner in another channel so that, for all the channels of the transducer considered in combination, both first half-groups of one subtransducer and also second half-groups of the other substransducer extend over the length of the transducer. When forming a SAW filter having a central band pass frequency of 835 MHz, for example, the pitch P is set to 2.45 µm while the width W and the separation S are set to 1.23 m in correspondence to the velocity of 4090 m/sec of the surface acoustic wave in the X-direction. It should be noted that the foregoing velocity provides a wavelength λ of 4.9 µm for the surface acoustic wave of 835 MHz. Generally a pair of such electrodes EL1 and EL2 are provided. In the particular applications of SAW devices such as automobile telephones or portable telephones, on the other hand, devices having a small insertion loss, a wide pass-band and a large suppression for the frequency components outside the pass band, are required. For example, an insertion loss of 3- 5 dB or less, a pass band of 25 MHz or more and a side lobe suppression of 24 - 25 dB or more may be required for the SAW filter having a central frequency of 835 MHz. In order to satisfy these various requirements, the applicants of the present invention have proposed previously in the United States, European, Korean and Canadian patent applications entitled "SURFACE-ACOUSTIC-WAVE FILTER HAVING A PLURALITY OF ELECTRODES," based upon the Japanese patent applications 2-69121 and 2-86236, which are incorporated herein as reference. FIG.2 shows a wafer 1 on which a number of SAW devices 2 are formed. Typically, the SAW devices 2 are arranged in the rows and columns with dicing lines 3 formed between adjacent SAW devices 2. When the formation of the SAW devices 2 is completed, the wafer 1 is subjected to a dicing process wherein the wafer 1 is cut by a diamond saw along the dicing lines 3. Thereby, the SAW devices 2 are separated from each other. FIG.3 shows a typical conventional example of the SAW device 2 before the dicing process is carried out. Referring to FIG.3, the device 2 is surrounded by the dicing line 3, and a number of input electrodes 21 and a number of output electrodes 22 are formed alternately on the surface of the substrate 1 to form a row of electrodes. Each of the input electrodes 21 comprises a first part corresponding to the part EL1 of FIG.1 connected to a common input bonding pad 28 and a second part corresponding to the part EL2 of FIG.1 connected to a ground pad 27. The ground pad 27 is provided in correspondence to each interdigital electrode 21. Similarly, each output electrode 22 comprises a first part connected to a common output bonding pad 28' and a second part connected a ground pad 27'. Further, a pair of reflectors 25 of the open strip type are formed at both sides of the row of electrodes 21 and 22. The dicing line 3 is merely a hypothetical line for dicing the wafer into the individual devices. A similar device is disclosed in JP-A-58031608. An electrode pattern of an interdigital electrode and grating reflectors is etched, and lattice-shaped short-circuited fine wires 15 are formed to surround each surface acoustic wave resonator thus formed. A pair of comb-type electrodes forming each of the interdigital electrodes are connected with wires and short-circuited. After the etching process is finished and resist is removed, the short-circuited fine wires of a piezoelectric substrate are cut off by dicing to be split into individual surface acoustic wave resonators. In the process, since the pair of comb-type electrodes constituting the interdigital electrode can be separated electrically by taking a cutting width larger than the line width of the wires, correct operation can be attained. Meanwhile during the fabrication of the conventional SAW devices, there arises a problem in that, associated with various heating processes employed during the fabrication, the surface of the piezoelectric substrate is charged due to the pyroelectricity. It should be noted that, in the structure of FIG.3, the input electrodes 21 and the output electrodes 22 are isolated from each other. Further, in each input and output electrodes, the second part connected to the ground pad 27 or 27' is isolated from each other. In such a structure, the electric charges induced by the pyroelectric effect are accumulated and such accumulation of electric charges induces a sparking discharge between adjacent fingers of the interdigital electrodes. As the separation between the fingers is in the order of several microns in correspondence to the wavelength of the surface acoustic waves, small amount of electric charges are sufficient to cause such sparking. FIG.4 shows the electrode wherein such sparking has occurred. It should be noted that, such sparking discharge damages the finger of the interdigital electrodes and hence the SAW device. In FIG.4, the fingers 50a and 50b represent the fingers that have experienced the sparking discharge. Thereby, the yield of the device is inevitably deteriorated. Accordingly, it is a general object of the present invention to provide a novel and useful SAW device and a fabrication process thereof, wherein the foregoing problems are eliminated. Another and more specific object of the present invention is to provide a SAW device and a fabrication process thereof, wherein the sparking discharge between the fingers in the electrode is eliminated. Another object of the present invention is to provide a SAW device and a fabrication process thereof, wherein the sparking discharge between the fingers in the electrode is eliminated while not degrading the operational characteristics of the device. Another object of the present invention is to provide a SAW device and a fabrication process thereof, wherein a plurality of interdigital electrodes are formed on a piezoelectric substrate, and the interdigital electrodes are formed such that each finger of each interdigital electrode is connected with each other commonly prior to the dicing process. According to the present invention, the problem of sparking discharge between the finger electrodes during the fabrication process due to the pyroelectricity, is positively eliminated by neutralizing the electric potential induced in various conductor parts of the device. Another object of the present invention is to provide a SAW device and a fabrication process thereof, wherein a plurality of interdigital electrodes are formed on a piezoelectric substrate, the interdigital electrodes being formed such that each finger of each interdigital electrode is connected in common with each other by an interconnection pattern prior to the dicing process, in which the interconnection pattern is formed to avoid the path of the surface acoustic waves in the device. According to the present invention, undesirable deterioration of the device characteristics by the interconnection pattern disturbing the passage of the surface acoustic waves is avoided. In accordance with a first aspect of the invention there is provided a wafer containing a plurality of surface-acoustic-wave devices, comprising: a substrate of a piezoelectric material and having an upper major surface and a lower major surface, said substrate providing a passage of surface acoustic waves propagating in a predetermined direction at the upper major surface of the substrate, each surface acoustic wave device having; a plurality of interdigital electrodes provided on the upper major surface of the piezoelectric substrate to form a row in correspondence to the passage of surface acoustic waves such that the electrodes are aligned, as a row, in the propagating direction of the surface acoustic waves, each of said plurality of interdigital electrodes comprising a first part connected commonly to a bonding pad provided on the upper major surface of the substrate for external electric connection and having a plurality of finger electrodes extending parallel with each other in a direction that is included in the upper major surface of the substrate and crossing the passage of the surface acoustic waves, and a second part separated from the first part and having a plurality of finger electrodes extending parallel with each other in a direction opposite to the direction of the finger electrodes of the first part, said first part and said second part being so disposed, in each interdigital electrode, that said finger electrodes of the first part and said finger electrodes of the second part are repeated alternately in the propagating direction of the surface acoustic waves with an overlapping of the opposing finger electrodes when viewed in the propagating direction of the surface acoustic waves, said opposing finger electrodes defining the passage of the surface acoustic waves; an interconnection pattern on the upper major surface of the substrate in correspondence to a region offset from the passage of the surface acoustic waves for connecting the second parts of adjacent interdigital electrodes with each other; said row of interdigital electrodes including a plurality of input interdigital electrodes and a plurality of output interdigital electrodes disposed alternately along the passage of the surface acoustic waves, said bonding pad including an input bonding pad and an output bonding pad both provided on the upper major surface of the substrate with a separation therebetween, said first part of the input interdigital electrodes being connected commonly to the input bonding pad, and said first part of the output interdigital electrodes being connected commonly to the output bonding pad, the wafer being characterised in that a first conductor layer lies on the upper major surface around a plurality of device patterns such that a first conductor strip separates the substrate into a plurality of chips and is connected to the conductor layer and in that the lower major surface and a side wall are covered by a second conductor layer such that the first and second conductor layers are connected electrically to each other and in that the second conductor layer is left unpatterned. In accordance with a second aspect of the invention there is provided a method for fabricating a surface-acoustic-wave device, comprising the steps of: covering an upper major surface of a piezoelectric substrate wafer by a conductor layer; patterning the conductor layer to form a plurality of device patterns that are separated from each other by a first conductor strip, said first conductor strip being formed to extend along a dicing line defined on the upper major surface of the substrate for separating the substrate into a number of chips each corresponding to a surface-acoustic-device, said first conductor strip being connected with each other electrically, each of said device patterns including a plurality of interdigital electrodes provided on the upper major surface of the substrate to form a row in correspondence to a passage of surface acoustic waves such that the electrodes are aligned, as a row, in the propagating direction of the surface acoustic waves, each of said plurality of interdigital electrodes comprising a first part connected to a bonding pad provided on the upper major surface of the substrate for external electric connection and having a plurlaity of finger electrodes extending parallel with each other in a direction that is included in the upper major surface of the substrate and crossing the passage of the surface acoustic waves, and a second part separated from the first part and having a plurality of finger electrodes extending parallel with each other in a direction opposite to the direction of the finger electrodes of the first part, said first part and said second part being so disposed, in each interdigital electrode, that said finger electrodes of the first part and said finger electrodes of the second part are repeated alternately in the propagating direction of the surface acoustic waves with an overlapping of the opposing finger electrodes when viewed in the propagating direction of the surface acoustic waves, said opposing finger electrodes defining the passage of the surface acoustic waves, a second conductor strip provided on the upper major surface of the substrate in correspondence to a region offset from the passage of the surface acoustic waves for connecting the second parts of adjacent interdigital electrodes with each other, a third conductor strip provided on the upper major surface of the substrate in correspondence to a region offset from the passage of the surface acoustic waves for connecting the bonding pad to the first conductor strip electrically, and a fourth conductor strip provided on the upper major surface of the substrate in correspondence to a region offset from the passage of the surface acoustic waves for connecting the second part of the interdigital electrodes to the first conductor strip, separating the surface-acoustic-wave devices from each other by dicing along the dicing line such that the first conductor strip is removed from each surface-acoustic-wave device, said step of patterning the conductor layer being made such that the conductor layer is left on the upper major surface of the substrate to surround the plurality of device patterns, said patterning being made such that the first conductor strip is connected to the conductor layer left on the upper major surface of the substrate; characterised in that said step of covering the upper major surface of the substrate is carried out such that a side wall and a lower major surface of the substrate opposing the upper major surface of the substrate is covered by a conductor layer such that the conductor layer covering the side wall and the lower major surface of the substrate is connected electrically to the conductor layer left on the upper major surface of the substrate, and said step of patterning is achieved such that the conductor layer covering the side wall and the lower major surface of the substrate is left unpatterned. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which:- FIG. 1 is a diagram showing a typical interdigital electrode; FIG.2 is a diagram showing a wafer on which a number of SAW devices are formed; FIG.3 is a diagram showing the construction of a SAW device formed on the wafer of FIG.2; FIG.4 is a diagram showing the example of sparking discharge occurred in the SAW device of FIG.3 during the fabrication process thereof; FIG.5 is a diagram showing the wafer carrying the SAW devices according to a first embodiment of the present invention; FIG.6 is a diagram showing the details of the SAW device of the first embodiment of the present invention; FIG.7 is a diagram showing the SAW device according to a second embodiment of the present invention; FIG.8 is a diagram showing a SAW device presented as a reference for showing the effect of the present invention; FIG.9 is a diagram showing the frequency characteristics of the SAW device of FIG.7 in comparison with the characteristics of the device of FIG.8; and FIG.10 is a diagram showing a section through a SAW wafer according to the present invention. FIG.5 shows a wafer 51 on which a number of SAW devices 52 according to a first embodiment of the present invention are formed. The wafer 51 comprises a piezoelectric single crystal such as 36°Y-X LiTaO₃ and may have a diameter of 51 cm and a thickness of 0.5 mm. The SAW devices 52 are bounded by a number of scribe lines 53 that are formed of a conductor material that may be identical with the material forming the electrodes of the SAW devices. The surface of the substrate 51 is covered by a conductor film 50 except for those regions on which the SAW devices 52 are formed, and the scribe lines 53 are connected to the conductor film 50. Further, the electrode of each SAW device 52 is connected to the conductor film 50 via a number of conductor strips 55a - 55e. It should be noted that FIG.5 shows the wafer before the dicing is made. FIG.6 shows the construction of the SAW device according to the first embodiment. Referring to FIG.6, the SAW device 52 has a construction substantially identical with the SAW device of FIG.3 in that a number of input interdigital electrodes 71 corresponding to the input interdigital electrodes 21 and a number of output interdigital electrodes 72 corresponding to the output interdigital electrodes 22 are formed on the upper major surface of the piezoelectric wafer 51. The SAW device is bounded by the scribe line 53 as shown, and each interdigital electrode 21, comprising a first part ELI corresponding to the first part EL1 of FIG.1 and a second part EL2 corresponding to the second part EL2 of FIG.1, are connected commonly to an input bonding pad 78 on the upper major surface of the wafer 51. Thereby, the first part EL1 is connected to the pad 55a while the second part EL2 is connected to a ground pad 77 provided on the upper major surface of the wafer 51 in correspondence to each input interdigital electrode 71. Further, each interdigital output electrode 72 comprises a first part EL1' corresponding to the part EL1 of FIG.1 and a second part EL2' corresponding to the part EL2 of FIG.1. There, the first part EL1' is connected commonly to an output bonding pad 78' on the upper major surface of the wafer 51 while the second part EL2' is connected to a ground pad 77' provided also on the upper major surface of the wafer 51 in correspondence to each electrode 72. The electrodes 71 and 72 are provided alternately on the upper major surface of the wafer 51 to form a row extending in the propagation direction of the surface acoustic wave. Thereby, a passage or channel CH of the surface acoustic wave is defined in each SAW device to extend in the X-direction as a region wherein the fingers of the first part EL1 or EL1' make an overlapping with the fingers of the second part EL2 or EL2' when viewed in the propagating direction of the surface acoustic wave. Further, at both sides of the row of electrodes 71 and 72, a pair of reflectors 75 are provided in correspondence to the channel CH. In the structure of FIG.6, the input bonding pad 78 is electrically connected to the conductor scribe line 53 by the conductor strip 55a. Similarly, the output bonding pad 78' is connected electrically to the scribe line 53 by the conductor strip 55b. Further, the second part EL2 and the second part EL2' of the electrodes 71 and 72 are connected with each other by a pair of interconnection strips 54, and these second parts EL2 and EL2' are connected to the scribe line 53 via the conductor strip 55c at the two electrodes 71 located at the outer ends of the electrode row. Further, each reflector 75 is connected to the part EL2 of the adjacent electrodes 71 via the interconnection strip 54 and further to the dicing line 53 via the interconnection strip 55e. As already noted, the scribe lines 53 are formed from a conductor strip and connected to the conductor layer 50 covering the surface of the wafer 51. The conductor layer 50, the scribe lines 53, the interconnections 54 and 55a - 55e, the electrodes 71 and 72, the reflectors 75, and the bonding pads 78 and 78', are all formed from aluminum-copper alloy sputtered on the wafer 51 with a thickness of 200 nm and patterned subsequently by the photolithographic process. When the SAW device is designed to have the central frequency of 836 MHz, the pitch P is set to 2.4 µm while the width W and the separation S are set to 1.2 µm, as described previously. In the illustrated example, the number of input electrodes 71 is seven while the number of output electrodes 72 is six. In each input electrode 71, there are 19 pairs of opposing electrode fingers with a uniform overlap throughout the electrodes 71. In each output electrode 72, on the other hand, there are 30 pairs of opposing electrode fingers, with a uniform overlap throughout the electrodes 72. In the reflector 75, there are thirty pairs of fingers. It should be noted that the reflectors 75 have the open-strip construction wherein the first part EL1 and the second part EL2 are isolated from each other. The concept of electrode finger pair and the concept of overlap are defined in FIG.1. The channel CH is defined as the overlap of the electrode fingers. Here, it should be noted that the interconnection strips 54 connecting the second parts EL2 and EL2′ of adjacent electrodes 71 and 72 are provided outside the channel region CH to avoid the disturbance to the propagating surface acoustic waves. In FIG.6, the channel region CH is shown to be included in the region defined between the opposing interconnections 54 to avoid the overlapping of lines in the drawing. However, it should be noted that the upper and lower boundaries of the region CH may coincide with the interconnection strips 54 as long as the propagation of the surface acoustic wave is not disturbed. The effect of the interconnection strips 54 provided within the region CH will be presented later in comparison with the SAW device of the present invention. It should be noted that a single interconnection strip 54 may be used for connecting the part EL2 and the part EL2′, instead of using two such strips 54 as illustrated in FIG.6. The interconnections 55a - 55e are also provided such that the propagation of the surface acoustic waves is not disturbed. By forming the devices 52 as shown in FIGS.5 and 6 before the dicing process, any positive and negative electric charges that are induced on the surface of the piezoelectric wafer 51 by the pyroelectricity, are neutralized with each other by flowing through the scribe lines 53, the interconnections 54 and the interconnections 55a - 55e. After the SAW devices 52 are formed, the SAW devices 52 are separated from each other by dicing along the dicing lines 53 by a diamond saw fixture. Thereby, the devices 52 are mechanically as well as electrically separated from each other. FIG.7 shows a second embodiment of the SAW device of the present invention. This embodiment shows the construction of the SAW device in the more practical form. It should be noted, however, that the essential feature of the device of FIG.7 is substantially identical with the device of FIG.6. Thus, those parts corresponding to the parts described previously with reference to FIG.6 are designated by the same reference numerals and the description will be omitted. As shown in FIG.7, the scribe line 53 is formed with a width d that is typically about 50 µm. After the completion of formation of the SAW device on the wafer 51, the devices are separated with each other by the dicing process carried out along the dicing line 53. This dicing process is achieved by a diamond saw that removes the material of the wafer 51 with a width D that is larger than the width d. Typically, the width D has a value of 80 - 100 µm. Thereby, the interconnections 55a - 55e are separated completely from the ground and hence various parts including the . finger electrodes of the input and output electrodes 71 and 72 that are connected to the interconnections 55a - 55e. With this process, the SAW device becomes operational. As the finger electrodes are connected to the ground via the dicing line 53 and the interconnection strips 55a - 55e during the process of forming the SAW devices 52 on the wafer 51, the problem of accumulation of electric charges and associated sparking discharge during the heating process employed in the fabrication of the device is successfully avoided. According to the present invention, production of the defective devices due to the sparking discharge between the electrodes is eliminated almost completely. It should be noted that, in the conventional process, 50 - 80 % of the devices usually had some form of defects due to the sparking discharge. FIG.9 shows the frequency characteristics of the SAW device of FIG.7. This device has the central frequency set to 835 MHz and is designed to have a pass-band between 822.5 MHz and 850.5 MHz. Further, the device is designed to satisfy a specification requiring the side lobe rejection of -25 dB or more in the frequency range between 869 MHz and 894 MHz. In FIG.9, there is another characteristic curve shown by a broken line. This characteristic curve is for the device of FIG.8 that is substantially identical with the device of FIG.7 except that the region between opposing pair of interconnection strips 54 is filled by aluminum. In other words, the device of FIG.9 has the interconnection structure provided to intersect the propagating path CH of the surface acoustic wave. As can be seen in FIG.9, the degree of side lobe rejection at 869 MHz is smaller by more than 5 dB than the device of FIG.7. Thus, the SAW device of the present invention, having the interconnection strips 54 provided to avoid the channel region of the surface acoustic wave, is not only capable of eliminating the sparking discharge at the time of fabrication but also provides an excellent frequency characteristic. FIG.10 shows devices on a wafer of the present invention, wherein only the cross section is shown. It should be noted that the SAW devices of fig. 10 have the construction identical with the device of FIG.6 or FIG.7 in the plan view. Referring to FIG.10, the side wall and the lower major surface of the wafer 51 are completely covered by conductor layers 50a and 50b. Such conductor layers may be formed by depositing a titanium film with a thickness of about 50 nm and then an aluminum film with a thickness of about 300 nm by the vacuum deposition process such that the layers 50a and 50b make an electric connection to the conductor layer 50 on the upper major surface of the wafer 51. By covering the side wall and the lower major surface of the wafer 51 as such, the electric potential at the surface of the wafer 51 becomes identical throughout the entire surface and the problem of sparking discharge of the finger electrodes due to the pyroelectricity is completely eliminated. After the structure of FIG.10 is formed, the SAW devices 52 are formed by a series of photolithographic patterning processes that are well established in the art. Further, the present invention is not limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the appended claims.

Claims: A wafer (51) containing a plurality of surface-acoustic-wave devices (52), comprising: a substrate of a piezoelectric material and having an upper major surface and a lower major surface, said substrate providing a passage (CH) of surface acoustic waves propagating in a predetermined direction at the upper major surface of the substrate, each surface-acoustic-wave device having; a plurality of interdigital electrodes (71, 72) provided on the upper major surface of the piezoelectric substrate to form a row in correspondence to the passage of surface acoustic waves such that the electrodes are aligned, as a row, in the propagating direction of the surface acoustic waves, each of said plurality of interdigital electrodes comprising a first part (EL1, EL1') connected commonly to a bonding pad (78, 78') provided on the upper major surface of the substrate for external electric connection and having a plurality of finger electrodes (f₁ - f n or g₁ - g n ) extending parallel with each other in a direction that is included in the upper major surface of the substrate and crossing the passage of the surface acoustic waves, and a second part separated from the first part and having a plurality of finger electrodes extending parallel with each other in a direction opposite to the direction of the finger electrodes of the first part, said first part and said second part being so disposed, in each interdigital electrode, that said finger electrodes of the first part and said finger electrodes of the second part are repeated alternately in the propagating direction of the surface acoustic waves with an overlapping of the opposing finger electrodes when viewed in the propagating direction of the surface acoustic waves, said opposing finger electrodes defining the passage of the surface acoustic waves; an interconnection pattern (54) on the upper major surface of the substrate in correspondence to a region offset from the passage (CH) of the surface acoustic waves for connecting the second parts of adjacent interdigital electrodes with each other; said row of interdigital electrodes including a plurality of input interdigital electrodes (71) and a plurality of output interdigital electrodes (72) disposed alternately along the passage of the surface acoustic waves, said bonding pad (78, 78') including an input bonding pad (78) and an output bonding pad (78') both provided on the upper major surface of the substrate with a separation therebetween, said first part (EL1) of the input interdigital electrodes being connected commonly to the input bonding pad, and said first part (EL1') of the output interdigital electrodes being connected commonly to the output bonding pad; the wafer being characterised in that a first conductor layer (50) lies on the upper major surface (51) around a plurality of device patterns and a first conductor strip means (53) separates the substrate into a plurality of chips and is connected to the conductor layer (50); and in that the lower major surface and a side wall are covered by a second conductor layer (50a,50b) such that the first and second conductor layers are connected electrically to each other and in that the second conductor layer (50a,50b) is left unpatterned. A surface-acoustic-wave device formed on a wafer as claimed in claim 1, which is provided with a plurality of ground connection pads (77, 77') on the upper major surface of the substrate in correspondence to each interdigital electrode (71, 72), and said second part (EL2, EL2') of each interdigital electrode is connected to a corresponding ground connection pad. A surface-acoustic-wave device formed on a wafer as claimed in claim 1 or as claimed in claim 2 wherein said finger electrodes (f₁ - f n ) in the first part (EL1) of the input interdigital electrodes (71) extend in a first direction substantially perpendicular to the direction of propagation of the surface acoustic waves, said finger electrodes (g₁ - g n ) in the second part (EL2) of the input interdigital electrodes extending in a second direction opposite to the first direction, said finger electrodes (f₁ - f n ) in the first part (EL1') of the output interdigital electrodes (72) extending in the second direction, and said finger electrodes (g₁ - g n ) in the second part (EL2') of the output interdigital electrodes (72) extend in the first direction. A surface-acoustic-wave device as claimed in claim 3 wherein said interconnection pattern (54) is provided to connect the finger electrode (g₁ , g n ) of the second part (EL2) located at the outermost side, when viewed in the propagating direction of the surface acoustic waves, of the input interdigital electrode (71) to the opposing finger electrode (g₁, g n ) of the second part (EL2') located at the outermost side, when viewed in the propagating direction of the surface acoustic waves, of the adjacent output interdigital electrode (72). A surface-acoustic-wave device as claimed in claim 2, 3 or 4 wherein said interconnection pattern (54) is provided to extend parallel to the passage (CH) of the surface acoustic waves at both sides thereof. A surface-acoustic-wave device formed on a wafer according to claim 1, or as claimed in any other preceding claim wherein said surface-acoustic-wave device further comprises a pair of reflectors (75) on the upper major surface of the substrate (51) at both sides of the row of interdigital electrodes (71, 72), when viewed in the propagating direction of the surface acoustic waves, each of said reflectors comprising a first part (EL1) having a plurality of finger electrodes extending parallel with each other to intersect the passage of the surface acoustic waves, and a second part (EL2) having a plurality of finger electrodes extending parallel with each other in a direction opposite to the finger electrodes of the first part, to intersect the passage of the surface acoustic waves, said first part and said second part being so disposed that the fingers of the first part and the fingers of the second part are repeated alternately in the propagating direction of the surface acoustic waves, said first part being connected to the second part (EL2, EL2') of the interdigital electrodes (71, 72) via an interconnection pattern that is provided on the upper major surface of the substrate to avoid the passage (CH) of the surface acoustic waves. A method for fabricating a surface-acoustic-wave device, comprising the steps of: covering an upper major surface of a piezo-electric substrate wafer (51) by a conductor layer (50); patterning the conductor layer to form a plurality of device patterns (52) that are separated from each other by a first conductor strip (53), said first conductor strip being formed to extend along a dicing line defined on the upper major surface of the substrate for separating the substrate into a number of chips each corresponding to a surface-acoustic-device, said first conductor strip being connected with each other electrically, each of said device patterns including a plurality of interdigital electrodes (71, 72) provided on the upper major surface of the substrate to form a row in correspondence to a passage of surface acoustic waves such that the electrodes are aligned, as a row, in the propagating direction of the surface acoustic waves, each of said plurality of interdigital electrodes comprising a first part (EL1, EL1') connected to a bonding pad (78, 78') provided on the upper major surface of the substrate for external electric connection and having a plurality of finger electrodes (f₁ - f n ) extending parallel with each other in a direction that is included in the upper major surface of the substrate and crossing the passage of the surface acoustic waves, and a second part (EL2, EL2') separated from the first part and having a plurality of finger electrodes (g₁ - g n ) extending parallel with each other in a direction opposite to the direction of the finger electrodes of the first part, said first part and said second part being so disposed, in each interdigital electrode, that said finger electrodes of the first part and said finger electrodes of the second part are repeated alternately in the propagating direction of the surface acoustic waves with an overlapping of the opposing finger electrodes when viewed in the propagating direction of the surface acoustic waves, said opposing finger electrodes defining the passage (CH) of the surface acoustic waves, a second conductor strip (54) provided on the upper major surface of the substrate in correspondence to a region offset from the passage (CH) of the surface acoustic waves for connecting the second parts (EL2, EL2') of adjacent interdigital electrodes with each other, a third conductor strip (55a, 55b) provided on the upper major surface of the substrate in correspondence to a region offset from the passage of the surface acoustic waves for connecting the bonding pad (78, 78') to the first conductor strip (53) electrically, and a fourth conductor strip (55c) provided on the upper major surface of the substrate in correspondence to a region offset from the passage of the surface acoustic waves for connecting the second part (EL2, EL2') of the interdigital electrodes to the first conductor strip (53), separating the surface-acoustic-wave devices (52) from each other by dicing along the dicing line such that the first conductor strip (53) is removed from each surface-acoustic-wave device, said step of patterning the conductor layer (50) being made such that the conductor layer is left on the upper major surface of the substrate (51) to surround the plurality of device patterns (52), said patterning being made such that the first conductor strip (53) is connected to the conductor layer (50) left on the upper major surface of the substrate; characterised in that said step of covering the upper major surface of the substrate (51) is carried out such that a side wall and a lower major surface of the substrate opposing the upper major surface of the substrate is covered by a conductor layer (50a, 50b) such that the conductor layer covering the side wall and the lower major surface of the substrate is connected electrically to the conductor layer (50) left on the upper major surface of the substrate, and said step of patterning is achieved such that the conductor layer covering the side wall and the lower major surface of the substrate is left unpatterned. A method as claimed in claim 7 wherein said step of separating the surface-acoustic-wave devices (52) comprises a sawing process wherein the substrate (51) is cut along the dicing line by a saw with a width (D) substantially larger than a width (d) of the first conductor strip. A method as claimed in claim 7 or claim 8 wherein each of said plurality of device patterns further comprises a pair of reflector patterns (75) provided at both ends of the row of electrodes (71, 72) for reflecting the surface acoustic waves, wherein each of said reflector patterns is connected to the first conductor strip.

Inventors: MIYASHITA TSUTOMU, SATOH YOSHIO, MIYASHITA, TSUTOMU, SATOH, YOSHIO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: H03H9
IPC codes: H03H3, H03H9