Abstract:
An acoustic wave filter includes: a substrate; resonators that are arranged on the substrate and excite acoustic waves; a ground terminal on the substrate; interconnection lines interconnecting the resonators and connecting predetermined ones of the resonators to the ground terminal; and a shield electrode disposed so as to be close to and face the interconnection lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of PCT/JP2010/064773 filed Aug. 31, 2010 claiming the priority of Japanese Patent Application No. 2009-222661 filed Sep. 28, 2009, the contents of which are herein wholly incorporated by reference. 
     
    
     FIELD 
       [0002]    A certain aspect of the present invention relates to an acoustic wave filter, a communication module using such a filter, and a method for manufacturing an acoustic wave filter. 
       BACKGROUND 
       [0003]    Recently, advanced wireless communication devices, which are typically portable telephones, have a multiband system using multiple frequency bands used for communications and a systematization with various functions, and multiple wireless devices have been installed in one communication device. However, downsizing and thinning of the portable telephone have been continuously demanded, and downsizing and thinning of components installed therein have been strongly desired. 
         [0004]    Further, there is a strong demand for cost reduction of the devices. Therefore, in many cases, multiple components are incorporated into a module to thus achieve downsizing and cost reduction in mounting the components. 
         [0005]    Among the components installed in the wireless communication devices, an acoustic wave filter and a duplexer using such a filter are produced by a method quite different from that for producing semiconductor components. It is therefore difficult to integrate these components on the same substrate. Actually, the acoustic wave filter and the duplexer are mounted separately from the semiconductor components. However, multiband communications in one portable telephone use multiple acoustic wave filters and multiple duplexes suitable for the multiple frequency bands. For the purpose of downsizing and weight lighting, the multiple acoustic wave filters and duplexers should be incorporated into a single module, which is also to be downsized and thinned. 
         [0006]    Now, there is a considerable activity in the development of modules in which acoustic wave filters and duplexers are incorporated. In order to downsize the module, the individual acoustic wave filters and duplexers should be arranged on a module substrate as close to each other as possible, and interconnections lines for interconnecting the duplexers should be as close to each other as possible. Further, the acoustic wave filters and the duplexers should be downsized. Therefore, many cases have close arrangements of interconnection lines that interconnect vibration elements in the acoustic wave filter elements and those for interconnecting the acoustic wave filters. The above-mentioned arrangements may raise the following problems. An unwanted electromagnetic coupling may increase between the lines for interconnecting the acoustic wave filters. Another unwanted electromagnetic coupling may increase between the interconnection lines in the module substrate. These unwanted electromagnetic couplings may degrade the rejection characteristics of the acoustic wave filters, and may degrade isolation between the duplexers. 
         [0007]    As a problem raised by downsizing of the acoustic wave filters, it is known that a ripple occurs in the frequency characteristic of the pass band of the acoustic wave filter and increases the insertion loss. Japanese Patent Application Publication No. 2008-28826 (Document 1) proposes a structure directed to solving the problem. 
       SUMMARY 
       [0008]    According to an aspect of the present invention, there is provided an acoustic wave filter including: a substrate; resonators that are arranged on the substrate and excite acoustic waves; a ground terminal on the substrate; interconnection lines interconnecting the resonators and connecting predetermined ones of the resonators to the ground terminal; and a shield electrode disposed so as to be close to and face the interconnection lines. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagram of a circuit configuration of an acoustic wave filter in accordance with a first embodiment; 
           [0010]      FIG. 2  is a diagram of another circuit configuration of the acoustic wave filter in accordance with the first embodiment; 
           [0011]      FIG. 3  is a cross-sectional view taken along a line A-A illustrated in  FIG. 2 ; 
           [0012]      FIG. 4  is a perspective view of the acoustic wave filter in accordance with the first embodiment; 
           [0013]      FIG. 5  is another perspective view of the acoustic wave filter in accordance with the first embodiment; 
           [0014]      FIG. 6  is a perspective view of a shield electrode; 
           [0015]      FIG. 7  is a cross-sectional view taken along a line B-B in  FIG. 5 ; 
           [0016]      FIG. 8  is a diagram of characteristics of the acoustic wave filter of the first embodiment and an acoustic wave filter of a comparative example; 
           [0017]      FIGS. 9A through 9E  are diagrams of a method for manufacturing an acoustic wave device having a dielectric layer between an interconnection line and a shield electrode; 
           [0018]    FIGS.  10 A through  1 OF are diagrams of a method for manufacturing an acoustic wave device having an empty spacing between an interconnection line and a shield electrode; 
           [0019]      FIG. 11  is a block diagram of a duplexer having filters of the first embodiment in accordance with a second embodiment; and 
           [0020]      FIG. 12  is a block diagram of a communication module in accordance with a third embodiment, wherein the module has filters of the first embodiment and the duplexer of the second embodiment in accordance with the first embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In order to solve the above-described problem, the inventors modified a package of an acoustic wave filter and the layer structure of a module substrate on which the acoustic wave filter is mounted, so that unwanted electromagnetic coupling in the module can be suppressed. Further, the inventors found out that further downsizing needs to suppress electromagnetic coupling in the acoustic wave filter, and created a structure capable of suppressing such electromagnetic coupling. 
         [0022]    Embodiments are now described with reference to the accompanying drawings.  FIG. 1  is a circuit diagram of a circuit configuration of an acoustic wave filter  10  used in a first embodiment. The acoustic wave filter  10  is an exemplary bandpass filter composed of series resonators S 11  through S 15  connected in series with each other, and parallel resonators P 21  through P 23  connected in parallel. Each of the above resonators may be a surface acoustic wave (SAW) element or a film bulk acoustic resonator (FBAR), for example. The following description assumes that each resonator is FBAR. The series resonators S 11 ˜S 15  and the parallel resonators P 21 ˜P 23  are formed on a silicon substrate, and are electrically connected as illustrated in  FIG. 1  by interconnection lines  31   a  through  37   a  made of ruthenium (Ru), aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), iridium (Ir) or the like. Electrodes for applying an electric field to a piezoelectric film of FBAR include an upper electrode and a lower electrode, which are distinguished from the interconnection lines in the description. However, the upper and lower electrodes are preferably made of the same material as that of the interconnection lines, which may be formed by patterning. 
         [0023]      FIG. 2  is a diagram of a circuit configuration of an acoustic wave filter  20  in which shield electrodes  31   b  through  37   b  are arranged along the interconnection lines  31   a  through  37   a  between the resonators of the acoustic wave filter  10  so that the shield electrodes  31   b  through  37   b  are vertically spaced apart from the interconnection lines  31   a  through  37   a  by a given distance. The widths of the shield electrodes  31   b  through  37   b  are preferably larger than those of the corresponding interconnection lines  31   a  through  37   a.  The shield electrodes  31   b  through  37   b  are grounded.  FIG. 3  is a cross-sectional view taken along a line A-A in  FIG. 2 , and illustrates a positional relationship between the interconnection pattern and the shield electrode in the direction perpendicular to the drawing sheet. As illustrated in  FIG. 3 , the interconnection line  37   a  is formed on a substrate  40 , and the shield electrode  37   b  is vertically spaced apart from the interconnection line  37   a  by a given distance. 
         [0024]    The spacing between the interconnection line  37   a  and the shield electrode  37   b  may be empty or may be full of a dielectric material for mechanically stabilizing the shield electrode  37   b.  A method for forming the shield electrodes  31   b  through  37   b  will be described later. 
         [0025]      FIG. 4  illustrates the acoustic wave filter  10  illustrated in  FIG. 1 , and  FIG. 5  illustrates an arrangement of an shield electrode above the interconnection lines of the acoustic wave device.  FIG. 6  illustrates only the shield electrodes  31   b  through  37   b,  and  FIG. 7  is a cross-sectional view of the shield electrode  31   b,  the resonator and the substrate. In  FIGS. 5 and 6 , only one acoustic wave filter  10  (acoustic wave filter  20 ) is illustrated. However, multiple filters may be provided. 
         [0026]      FIG. 4  is a perspective view of the acoustic wave filter  10  not having the shield electrodes. As illustrated in  FIG. 4 , the interconnection lines  31   a  though  37   a  are integrally formed with the upper or lower electrodes of the series resonators S 11  through S 15  and the parallel resonators P 21  through P 23 . The input terminal  13  and the output terminal  12  illustrated in  FIG. 1  are integrally formed with the interconnection lines  31   a  and  36   a.    
         [0027]      FIG. 5  is a perspective view of a structure in which a shield electrode  30  is provided on the acoustic wave filter  10  illustrated in  FIG. 4 . Although the shield electrodes  31   b  through  37   b  are separated parts in  FIG. 2 , the shield electrodes  31   b  trough  37   b  are preferably formed into a single shield electrode  30 . The shield electrode  30  covers the interconnection lines  31   a  through  37   a  between the resonators, and has openings above the resonators. The integrated shield electrode  30  have four ground terminals G 1  through G 4  in  FIG. 5 . However, the number of ground terminals and the positions thereof are not limited to those in  FIG. 5 . 
         [0028]      FIG. 6  illustrates only flat portions of the shield electrode  30  in order to facilitate easy understanding of the shield electrode  30 . In  FIG. 6 , folded portions of the shield electrode  30  for forming the ground terminals are omitted. 
         [0029]    The shield electrode  30  may be made of the same material as that of the upper and lower electrodes. 
         [0030]      FIG. 7  is a cross-sectional view taken along a line B-B in  FIG. 5 . An upper electrode  31   c  of the resonator S 11  of FBAR type is integrally formed with the patterned interconnection line  31   a,  and a lower electrode  32   c  thereof is integrally formed with an interconnection line  32   a.  The substrate  40  has a cavity  42  below the lower electrode  32   c  in order to improve the Q value of the resonator S 11 . The cavity  42  is not limited to an opening formed in the substrate  40  but may be a portion having a small acoustic impedance. For example, the cavity  42  may be replaced by a curved or dome-shaped gap defined by curving the resonator S 11  so as to be away from the substrate  40 . The shield electrodes  31   b  and  32   b  are disposed above the interconnection lines  31   a  and  32   a  at given intervals. An opening  31   d  is located above the resonator S 11 . 
         [0031]    The shield electrodes are preferably wider than the patterned interconnection lines because such wider shield electrodes are capable of effectively suppressing the electromagnetic couplings between the interconnection lines. 
         [0032]    The spacings between the interconnection lines and the shield electrodes may include an empty spacing, a dielectric layer, or both of an empty spacing and a dielectric layer. 
         [0033]    The shield electrodes may be disposed so as to be close to and face at least one of the interconnection lines that is not grounded. 
         [0034]    The shield electrodes may be electrically connected to an electrode of at least one of the resonators that is connected to ground. 
         [0035]    A description is now given of characteristics of the acoustic wave filters of the present embodiment. The acoustic wave filter  10  does not have the shield electrode  30 . An acoustic wave filter  20  has the shield electrode  30 . The characteristics of the acoustic wave filters  10  and  20  are obtained by computer simulation under the following conditions. 
         [0036]    (1) The characteristic of each resonator of each filter is computed by the mode-mode coupling theory. 
         [0037]    (2) The characteristics of the entire filters including the interconnection lines and the shield electrodes are computed by three-dimensional electromagnetic field analyzing software. 
         [0038]    (3) The pass bands of the filters are those of a reception filter of BAND1 (a band of 2110 MHz to 2170 MNz). 
         [0039]    (4) The distance between the interconnection lines of the acoustic wave filter  20  and the shield electrodes is 15 μm. Since the acoustic wave filter  10  does not have the shield electrodes, the simulation assumes that the distance is 100 μm. 
         [0040]    The computation results of the pass characteristics of the acoustic wave filters  10  and  20  under the above conditions are illustrated in  FIG. 8 . The horizontal axis of  FIG. 8  denotes the frequency, and the vertical axis denotes the amount of attenuation. A solid line indicates the pass band characteristic of the acoustic wave filter  20  of the present embodiment, and a dotted line indicates that of the acoustic wave filter  10 , which is also referred to as a comparative example. It can be seen from  FIG. 8  that the acoustic wave filter  20  has an improved suppression ratio on both sides of the pass band. Particularly, there is a large attenuation peak at around 2070 MHz, and the suppression is greatly improved by about twenty and a few dB. 
         [0041]    A description is now given, with reference to  FIGS. 9A through 9E  and  10 A through  10 F, of a method for manufacturing the acoustic wave filter  20  of the present embodiment.  FIGS. 9A through 9E  illustrate a part of the acoustic wave filter  20  configured so that a dielectric layer is provided between the interconnection line and the shield electrode.  FIGS. 10A through 10F  illustrate a part of the acoustic wave filter  20  configured so that an empty spacing is provided between the interconnection pattern and the shield electrode. 
         [0042]    Referring to  FIGS. 9A through 9E , the shield electrode of the acoustic wave filter  20  is formed as follows. It is now assumed that the resonators and the interconnection lines of the acoustic wave filter  20  have been formed by a known process. 
         [0043]    Referring to  FIG. 9A , an interconnection line  50  formed by patterning has been formed on the substrate  40  by a known process. The interconnection line  50  is one of the interconnection lines  31   a  through  37   a.  Next, an insulative layer made of a dielectric material is formed on the interconnection line  50 . Since the thickness of the dielectric layer on the interconnection line  50  is preferably a few μm or more, the dielectric film is formed by depositing photosensitive polyimide  51  and sintering it after patterning, as illustrated in  FIG. 9B . Then, the shield electrode is formed by a liftoff process in which resist  52  is patterned so as to remove a portion in which the shield electrode is to be removed to expose the photosensitive polyimide  51 , as illustrated in  FIG. 9C . After that, as illustrated in  FIG. 9D , a metal layer is formed on the whole surface by a vapor deposition process. The metal layer on the resist  52  is indicated by a reference numeral  53 , and that in the opening is indicated by a reference numeral  54 . Finally, the resist is removed, so that the metal layer  54  remaining in the opening is the shield electrode. 
         [0044]    A description is now given, with reference to  FIGS. 10A through 10F , of the method for manufacturing the acoustic wave filter  20  in which an empty spacing is formed between the shield electrode and the interconnection line. It is now assumed that the resonators and the interconnection lines of the acoustic wave filter  20  have been formed by a known process. It is preferable that an empty spacing is formed between the interconnection line and the shield electrode, as compared with the use of the dielectric layer. This is because the capacitance between the interconnection line and the shield electrode facing each other through the empty spacing has a small capacitance, as compared with the capacitor in which the interconnection line and the shield electrode face each other through the dielectric layer. The use of the empty spacing needs a support post for holding the shield electrode. 
         [0045]      FIG. 10A  is a cross-sectional view of the substrate  40  on which the interconnection line  50  is formed. As illustrated in  FIG. 10B , a pattern of a support pattern  61  for holding a shield electrode is formed by patterning a resist  60 . The support pattern  61  may have a cylindrical shape or a circular truncated cone shape. Next, as illustrated in  FIG. 10C , a seed electrode  62  for plating is formed by sputtering or the like. Subsequently, as illustrated in  FIG. 10D , an opening is formed by resist  63  for forming a shield electrode  64 . After that, as illustrated in  FIG. 10E , the opening is filled with a metal formed into the shield electrode  64  by plating. Finally, as illustrated in  FIG. 10F , the resist  63  is removed, whereby the shield electrode  64  and the support post  65  are formed. The support post  65  is electrically conductive and is connected to the shield electrode  64 . Since the shield electrode  64  is an interconnection line in the air, the main portion of the shield electrode  64  is preferably plated with Cu taking stress into account. That is, the shield electrode  64  and the support post  65  have Cu as a main component. 
         [0046]      FIG. 11  is a block diagram of an exemplary communication module in accordance with a second embodiment having acoustic wave filters, each of which is configured in accordance with the first embodiment. Referring to  FIG. 11 , a duplexer  72  includes a reception filter  72   a  and a transmission filter  72   b.  Reception terminals  73   a  and  73   b  for balance output are connected to the reception filter  72   a.  The transmission filter  72   b  is coupled with a transmission terminal  75  via a power amplifier  74 . The reception filter  72   a  and the transmission filter  72   b  include the acoustic wave filters of the first embodiment. 
         [0047]    In reception operation, the reception filter  72   a  passes only signals in a predetermined frequency band among signals received via an antenna terminal  71 , and outputs these signals to the outside of the duplexer via the reception terminals  73   a  and  73   b.  In transmission operation, the transmission filter  72   b  passes only signals in a predetermined frequency band among signals applied via the transmission terminal  75  and amplified by the power amplifier  74 , and outputs these signals to the outside of the duplexer via the antenna terminal  71 . The communication module of the present invention is not limited to the structure illustrated in  FIG. 11  but may have another structure in which at least one acoustic wave device of the present invention is used. Such a communication module has effects similar to those of the communication module illustrated in  FIG. 11 . 
         [0048]      FIG. 12  is a block diagram of an RF block of a portable telephone as a communication device equipped with a communication module in accordance with a second embodiment. The structure illustrated in  FIG. 12  conforms to GSM (Global System for Mobile Communications) and W-CDMA (Sideband Code Division Multiple Access). The portable telephone has a microphone, a speaker and a liquid display in addition to the structure illustrated in  FIG. 12 . However, these components are not needed when the present embodiment is described, and are therefore omitted in  FIG. 12 . Reception filters  83   a,    87 ,  88 ,  89  and  90 , and a transmission filter  83   b  include the acoustic wave filters of the first embodiment or the duplexers of the second embodiment. 
         [0049]    A signal received via an antenna  81  is distributed to an appropriate LSI by an antenna switch circuit  82  on the basis of whether the received signal conforms to the W-CDMA or GSM. When the received signal conforms to W-CDMA, the switch  82  forms a path to output the received signal to the duplexer  83 . The received signal applied to the duplexer  83  is limited to the predetermined frequency band by the reception filter  83   a,  and is output to a low-noise amplifier (LNA)  84  in the form of balanced output. The LNA  84  amplifies the received signal, and outputs the amplified signal to an LSI  86 , which performs a demodulation process on the received signal for reproduction of the original audio signal, and controls various parts of the portable telephone. 
         [0050]    In transmission operation, the LSI  86  generates a transmission signal, which is amplified by a power amplifier  85  and is applied to the transmission filter  83   b.  The transmission filter  83   b  passes only signals in the predetermined frequency band among the signals received from the power amplifier  85 . The transmission signal from the transmission filter  83   b  is output to the outside of the communication device via the antenna switch circuit  82  and the antenna  81 . 
         [0051]    When the reception signal conforms to GSM, the antenna switch circuit  82  selects one of the reception filters  87 ˜ 90  in accordance with the frequency band used, and outputs the reception signal to the selected reception filter. The reception signal is limited to the frequency band realized by the selected one of the reception filters  87 ˜ 90 , and is applied to an LSI  93 . The LSI  93  performs a demodulation process on the received signal for reproduction of the original audio signal, and controls various parts of the portable telephone. When a signal is to be transmitted, the LSI  93  generates a transmission signal, which is then amplified by a power amplifier  91  or  92 , and is output to the outside of the communication device via the antenna switch circuit  82  and the antenna  81 . 
         [0052]    The present invention is not limited to the specifically described embodiments, but may include other embodiments and variations without departing from the scope of the present invention.