PATENT ABSTRACT
An electric signal fed into a line generates electric field in response to its frequency, and a resonator placed closely to the line and in a substantially vacuum condition not higher than 100 pascal is excited by electrostatic force of the electric field and vibrates. Detecting means converts mechanical vibrations of the resonator into a signal in another form than the electric signal, then it detects the vibrations. The foregoing structure allows the resonator to be a micro-body and to process properly a high-frequency input signal of MHz or GHz band. A tight space between an input side and an output side does not permit an electric signal fed into the line to couple directly to the output side, and the resonator downsized to a micro-body is not subject to viscosity of air.

PATENT DESCRIPTION
FIELD OF THE INVENTION 
     The present invention relates to filters used in radio communication circuits. More particularly it relates to filters that convert mechanical vibrations into an output signal at frequencies of MHz band or GHz band, where resonators having a size of μm order are used. This micro-mechanical resonator is excited by an input signal having a frequency around the resonance frequency of the resonator, thereby producing the fine mechanical vibrations to be converted by this filter to the output signal. 
     BACKGROUND OF THE INVENTION 
     A conventional filter is disclosed in IEEE Journal of solid-state circuits, Vol. 35 No. 4, April 2000, issue. FIG. 11 shows a structure of a conventional filter that is formed on substrate  90 . This filter comprises input line  94 , output line  95 , two resonators  91 ,  92  of which both ends are fixed to substrate  90  slightly spaced from lines  94  and  95 , having an identical resonance frequency, and coupling beam  93  that couples the two resonators. 
     A signal fed into input line  94  generates electric field responsive to the frequency of the signal and applies electrostatic force to resonator  91 . At this time, when the frequency of the input signal generally agrees with the resonance frequency of resonator  91 , resonator  91  is excited to vibrate, and resonator  92  coupled to resonator  91  with beam  93  also vibrates. 
     As such, only a signal having a frequency generally agreeing with the resonance frequency of resonators  91 ,  92  is selectively converted from an electric signal to mechanical vibrations. Then the mechanical signal is converted again to an electric signal between resonator  92  and output line  95 . This is an inverse conversion to the conversion from the electric signal to the mechanical signal done between input line  94  and resonator  92 . 
     The foregoing structure can work as a filter such that among signals fed into the input line, only the signals having a frequency generally agreeing with the resonance frequency of resonator  91 ,  92  are allowed to pass through output line  95 . Resonance frequency “f 0 ” of resonator  91  is expressed with the equation below:          f   0     =       1     2                 π              k   m                                
     where resonance frequency f 0  is a function of mass “m” of resonator  91  and spring constant “k”. A similar equation is applicable to resonator  92 . 
     Another conventional filter is disclosed in Japanese Patent Application Non-examined Publication No. H05-327393. This filter receives an unprocessed signal at an excitation coil, and oscillatory-wave components, of which frequency generally agrees with the resonance frequency of the resonators, are extracted out of the oscillatory waves of the unprocessed signal. This extraction is carried out by launching light from a fixed scale to a variable scale disposed at an oscillator, and changes of the power of the reflected light is extracted. As a result, this filter allows only the frequency resonant with the oscillator to pass through. 
     In order to work the conventional filters discussed above at frequencies of MHz band or GHz band, the mass of the resonators should be micro-miniaturized, which naturally requires the filter per se to be downsized to a micro-body. 
     For instance, FIG. 12 shows relations between resonance frequencies and lengths of resonators in the case of scaling down resonators  91 ,  92  of the conventional filter shown in FIG.  11 . Resonators  91 ,  92  are actually 40 μm long and 3 μm wide, and those dimensions are scaled down with the same ratio. 
     In order to use this conventional filter as a device in the mobile communication field where a frequency band ranging of 1 GHz-5 GHz holds great promise to use this kind of filters, the length should be shortened to 0.04 μm from 0.2 μm. The relative distance between input line  94  and output line  95  placed via resonators  91 ,  92  is naturally required to be shorter. 
     As a result, in the conventional filter, input line  94  is placed closer to output line  95 , and they make a direct coupling between them, so that the isolation lowers and the filter does not work properly. 
     FIG. 13A shows isolation characteristics of a filter having no direct coupling between an input line and an output line. FIG. 13B shows isolation characteristics of a filter where a coupling of 0.1 μm space between an input signal and an output line is produced. In the case where the frequency is so low that a width between input line  94  and the output line  95  can be prepared wide enough to neglect a coupling between the two lines, the filter can work properly as shown in FIG.  13 A. However, as the available frequency becomes higher, the resonator becomes smaller, and when input line  94  is directly coupled to output line  95 , isolation in the frequencies higher than the resonance frequency greatly lowers as shown in FIG.  13 B. As a result, the filter cannot work properly. On the other hand, in the frequencies lower than the resonance frequency, a capacitance generated between input line  94  and output line  95  resonates with an inductance component of the resonator, thereby sometimes producing unnecessary notches. 
     The filter in which input line  94  is placed close to output line  95  can be downsized to a micro-body process-wise; in fact, a direct coupling between the two lines degrades the filter characteristics, and the filter thus cannot be used in high frequencies such as MHz band or GHz band. 
     A filter used in high frequencies such as MHz band or GHz band includes resonators of micro-body of μm order, so that its oscillators (resonators) are hard to oscillate properly due to the viscosity of air. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problems discussed above and aims to provide a filter free from characteristics degradation due to a direct coupling between an input line and an output line in high frequencies such as MHz band or GHz band. Further the filter of the present invention includes a resonator not influenced by the viscosity of air. 
     The filter of the present invention comprises the following elements: 
     a line through which an electric signal is input; 
     a resonator, disposed closely to the line and in vacuum, for resonating by applying electrostatic force of electric field generated responsive to a frequency of the electric signal; and 
     detecting means for detecting mechanical vibrations of the resonator. 
     The detecting means detects mechanical vibrations as a signal in another form than the electric signal. 
     Since the input electric signal is output in another form, this structure does not permit an input electric signal to be directly coupled to an output side. Even if an input side is placed immediately close to an output side because the resonator is downsized to a micro-body in high frequencies such as MHz band or GHz band. Further the resonator works in the vacuum, the resonator of a micro-body is not influenced by the viscosity of air, and micro-mechanical vibrations of the resonator can be converted into an appropriate signal before being detected. The vacuum referred in the present invention includes a true vacuum condition and a substantially vacuum condition not more than 100 pascal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a structure of a filter in accordance with a first exemplary embodiment of the present invention. 
     FIG. 2A shows a sectional view illustrating what is done in step  1  of processes forming a resonator in accordance with the first exemplary embodiment. 
     FIG. 2B shows a sectional view illustrating what is done in step  2  of processes forming the resonator in accordance with the first exemplary embodiment. 
     FIG. 2C shows a sectional view illustrating what is done in step  3  of processes forming the resonator in accordance with the first exemplary embodiment. 
     FIG. 2D shows a sectional view illustrating what is done in step  4  of processes forming the resonator in accordance with the first exemplary embodiment. 
     FIG. 2E shows a sectional view illustrating what is done in step  5  of processes forming the resonator in accordance with the first exemplary embodiment. 
     FIG. 2J shows a front view illustrating what is done in step  1  of processes forming the resonator in accordance with the first exemplary embodiment. 
     FIG. 2K shows a front view illustrating what is done in step  2  of processes forming the resonator in accordance with the first exemplary embodiment. 
     FIG. 2L shows a front view illustrating what is done in step  3  of processes forming the resonator in accordance with the first exemplary embodiment. 
     FIG. 2M shows a front view illustrating what is done in step  4  of processes forming the resonator in accordance with the first exemplary embodiment. 
     FIG. 2N shows a front view illustrating what is done in step  5  of processes forming the resonator in accordance with the first exemplary embodiment. 
     FIG. 3 shows a sectional view illustrating the resonator in accordance with the first exemplary embodiment housed in an airtight package. 
     FIG. 4 shows a schematic structure of a filter in accordance with a second exemplary embodiment. 
     FIG. 5 shows a schematic structure illustrating a multi-beam is used in a laser source of the filter in accordance with the second embodiment. 
     FIG. 6A shows a top view illustrating a schematic structure of a filter in accordance with a third exemplary embodiment. 
     FIG. 6B shows a sectional view illustrating a schematic structure of the filter in accordance with the third embodiment. 
     FIG. 7A shows a top view illustrating another schematic structure of a filter in accordance with the third embodiment. 
     FIG. 7B shows a sectional view illustrating another schematic structure of a filter in accordance with the third embodiment. 
     FIG. 8A shows a top view illustrating a placement of resonators in accordance with a fourth exemplary embodiment. 
     FIG. 8B shows a sectional view illustrating the placement of the resonators in accordance with the fourth embodiment. 
     FIG. 9A shows a random placement of resonators in accordance with the fourth embodiment. 
     FIG. 9B shows resonators placed like a diffraction grating in accordance with the fourth embodiment. 
     FIG. 10A shows a sectional view illustrating a case where no input signal is available in accordance with a fifth exemplary embodiment. 
     FIG. 10B shows a top view illustrating the case where no input signal is available in accordance with the fifth exemplary embodiment. 
     FIG. 10C shows a sectional view illustrating a case where an input signal is available in accordance with the fifth embodiment. 
     FIG. 10D shows a top view illustrating the case where an input signal is available in accordance with the fifth embodiment. 
     FIG. 11 shows a structure of a conventional filter. 
     FIG. 12 shows relations between sizes of conventional filters and resonance frequencies. 
     FIG. 13A shows isolation characteristics of a conventional filter where a coupling between input and output is negligible. 
     FIG. 13B shows isolation characteristics of a conventional filter where a coupling between input and output is available. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exemplary embodiments of the present invention are demonstrated hereinafter with reference to the accompanying drawings. The vacuum referred in the embodiments includes true vacuum and a substantially vacuum condition not more than 100 pascal. 
     Exemplary Embodiment 1 
     FIG. 1 shows a schematic structure of a filter in accordance with the first exemplary embodiment of the present invention. The filter of the present invention includes input line  2 , disposed on substrate  1 , for receiving an electric signal, and resonator  3  equipped with dielectric layer  7  closely above input line  2 . The atmospheric pressure is kept at vacuum or substantially vacuum condition not more than 100 pascal using a decompression package which is omitted in FIG.  1 . The structure including a decompression package will be described later. 
     Laser source  4  launches a beam of light onto the surface of resonator  3  via half mirror  10 , and photo detector  5  receives detected light  14  from the surface of resonator  3  via half-mirror  12 . Half mirror  10  splits the light beam from laser source  4  and permits some of the light beam as reference light  13  to travel to light detector  5  via mirror  11  and half mirror  12 . DC control unit  15  controls a direct potential between resonator  3  and input line  2  via inductor  17 , thereby changing the resonance frequency of resonator  3 . 
     Mechanical vibration detector  19  is formed of laser source  4 , half mirrors  10 ,  12 , mirror  11  and photo detector  5 . An electric signal fed into input line  2  includes a desirable signal and other unnecessary signals. When the electric signal fed into input line  2  substantially agrees with the resonance frequency of resonator  3 , resonator  3  is excited to vibrate. 
     The vibration of resonator  3  is detected by, e.g., an interference measuring method based on a laser heterodyne system using laser beam. To be more specific, a laser beam launched from laser source  4  is split into two, and one beam strikes resonator  3 , and the other strikes half mirror  12  via mirror  11  as reference light  13 . Detected light  14  reflected from resonator  3  interferes with reference light  13  at half mirror  12 , and the interfered light is received at light detector  5 . 
     A vibration of resonator  3  changes an optical path length of detected light  14 , so that an optical path difference between detected light  14  and reference light  14  changes. When respective optical path lengths are equal to each other or the optical path difference is an integral multiple of the wavelength, the amplitude of the interfered light becomes the maximum. When the optical path length equals to an odd multiple of the half wavelength, both the lights cancel with each other, so that the amplitude of the interfered light becomes the minimum. Then measurement of intensity of the signal received at photo detector  5  allows measuring a change of the optical path difference, namely, the vibrations of resonator  3 . 
     DC control unit  15  applies a direct potential between resonator  3  and input line  2 , then electrostatic force works between resonator  3  and line  2 , thereby bowing resonator  3 , and this bowing changes spring constant k of resonator  3 . Thus the resonance frequency f 0  changes as per the equation shown in the prior art. Therefore, control unit  15  regulates the direct potential applied, so that a center frequency of a pass-band is regulated. 
     A filter (not shown) formed by arranging plural resonators in parallel, having identical resonance frequency f 0 , can increase a coupling area between an input line and a resonator, thereby reinforcing the coupling strength, and on top of that, changing an input impedance of the input line. 
     Next, steps of manufacturing resonator  3  are demonstrated with reference to FIGS. 2A-2E (sectional views) and  2 J- 2 N (top views). 
     Step 1: Deposit nitride layer  22  on high-resistive Si substrate  21  with a thickness of 200 nm by a decompression chemical vapor deposition (CVD) method. 
     Step 2: Deposit aluminum on the entire face of silicon nitride layer  22  with a thickness of 1 μm by sputtering. Carry out patterning with photo resist such that the resist remains in the area where input line  2  is formed. Carry out aluminum dry etching with the photo resist as a mask, thereby forming input line  2 . 
     Step 3: Deposit silicon oxide layer  24  as a sacrificial layer by a decompression CVD method. The thickness of the sacrificial layer formed on input line  2  is later to be a gap between input line  2  and resonator  3 , thus deposit the silicon oxide layer  24  up to a desirable thickness on input line  2 . For instance, deposit the silicon oxide layer  24  by 100 nm, then mask a given area with the photo resist, and carry out etching on unnecessary area using a reactive ion etching method (RIE) for forming silicon oxide layer  24  only on the given area. 
     Step 4: Deposit poly-silicon by the decompression CVD method, and carry out etching on unnecessary area for leaving a given area, where resonator  3  is to be formed, using a reactive ion etching method (RIE). 
     Step 5: Finally, carry out wet-etching for removing silicon oxide layer  24 , so that resonator  3  becomes hollow. 
     In this first embodiment high-resistive Si substrate  21  is used; however, this does not limit the present invention, and an ordinary Si substrate, a compound semiconductor substrate, or a substrate made of insulating material can be used. 
     In this embodiment input line  2  made from aluminum is used; however, the input line can be made from other metals such as Mo, Ti, Au, Cu, or semiconductor materials such as amorphous silicon that includes impurity at a high density, or conductive high-polymer materials. 
     In this embodiment a sputtering method is used for forming a layer; however, a CVD method, or a plating method can be used instead. A surface of resonator  3  can coated with Au or Al having a high reflectance in order to increase reflection efficiency of light. 
     In this embodiment, interfered light is used for measuring a position of resonator  3 ; however, any method that can measure the vibration of the resonator can be used. For instance, place an electrode close to a resonator, and apply a voltage between the electrode and the resonator, then a tunnel current runs responsive to a gap therebetween. A method of measuring this tunnel current can be used. An interatomic microscope, which observes peaks and valleys on a surface, uses interatomic force. This interatomic force or intermolecular force can be used for measuring the vibration of the resonator. 
     Resonator  3  in accordance with the first embodiment works properly in high frequencies such as MHz band or GHz band. For this purpose, resonator  3  is downsized to a micro-body. In order to avoid degradation of factor Q due to viscosity of air, resonator  3  is used in vacuum or a substantially vacuum condition not more than 100 pascal. 
     A decompression package of highly air-tight is thus necessary for resonator  3  to work properly. However, since the vibration of resonator  3  is detected using interfered light, the decompression package should be made of the material that transmits the light irradiating resonator  3  or the light reflected from resonator  3 . The whole package is not necessarily made of light transmissible material, but windows that transmit the light can be prepared at only necessary parts of the package. 
     FIG. 3 shows a sectional view illustrating the resonator in accordance with this embodiment, and the resonator is accommodated in the decompression package of highly airtight. Resonator  3  is placed on silicon nitride layer  22  deposited on high-resistive Si substrate  21 . Resonator  3  is then housed in decompression package  26  that has windows  18  transmitting laser beam and maintains vacuum or substantially vacuum condition therein. Windows  18  can be made of, e.g., quartz. 
     From another view of point, the filter used in this embodiment works as an optical modulator which modulates the laser beam with an electric signal fed into the input line via the mechanical vibration of the resonator. Therefore, the filter in accordance with the first embodiment can be used as an optical modulator. 
     Exemplary Embodiment 2 
     FIG. 4 shows a schematic structure of a filter in accordance with the second exemplary embodiment. In FIG. 4, elements similar to those in FIG. 1 have the same reference marks, and the descriptions thereof are omitted here. A decompression package is omitted in FIG. 4; however, the filter can work in a similar way to that described in FIG.  3 . This second embodiment describes the filter having selectable three different resonance frequencies. 
     In the filter, resonator  35  having resonance frequency f 1 , resonator  36  having resonance frequency f 2 , and resonator  37  having resonance frequency f 3  are placed in parallel with respect to input line  2 . Respective resonators  35 ,  36  and  37  are formed of two sub-resonators by a coupling means. First DC control unit  31  controls resonator  35 , second DC control unit  32  controls resonator  36 , and third DC control unit  33  controls resonator  37 . 
     Laser source  4  irradiates resonators  35 ,  36  and  37  with laser beam via half mirror  10 , and the irradiating area is defined as single beam spot  39 . 
     An electric signal fed into input line  2  is supplied to three resonators  35 ,  36 ,  37  evenly, and when the input electric signal generally agrees with any one of the resonance frequencies f 1 , f 2  and f 3 , the resonator having the agreeing resonance frequency is excited to vibrate, thereby converting the electric signal to mechanical vibrations. 
     For detecting the mechanical vibrations, the same method as used in the first embodiment can be adopted. However, a use of material, which can directly detect an optical phase, as photo detector  35  allows irradiating directly resonators  35 ,  36  and  37  with laser beam, and photo detector  35  can receive the reflected light from the resonators. 
     For instance, vibration of resonator  35  changes an optical path length, so that a phase of the signal of the received light changes. Laser source  4 , having frequency f 4  and wavelength λ, launches signal Sin(f 4 ×t), and the light reflected from resonator  35  that includes signal Sin(f 4 ×t+Δφ) is received at photo detector  5 . Phase difference Δφ changes due to an occurrence of optical path difference Δy, i.e., displacement of resonance amplitude of resonator  35 , so that Δφ=Δy/λ is held. 
     The foregoing mechanism allows extracting only the signals that have passed through a desirable band, namely, frequency f 1  of resonator  35 . Resonators  36  and  37  work in a similar way. In this second embodiment, since photo detector  5  simultaneously receives vibrations generated by the respective resonance frequencies of the resonators, it is necessary not to vibrate resonators other than a desirable resonator, or not to detect vibrations other than desirable one. 
     In this embodiment, resonators  35 - 37  are irradiated evenly with laser beam, and photo detector  5  receives the reflected light. In this case, resonators  36  and  37 , other than resonator  35  having a desirable resonance frequency f 1 , are forced to stop vibrating. The forcible stop can be done by applying direct potential from DC control units  32 ,  33  so that electrostatic force is applied to resonators  36 ,  37 . As a result, resonators  36 ,  37  are brought into contact with input line  2 , and resonators  36 ,  37  do not vibrate. 
     At this time, a dielectric layer (not shown) between resonators  36 ,  37  and line  2  prevents a dc from running. Since desirable resonator  35  only vibrates, the signal received by photo detector  5  contains only the vibration information of resonator  35 . 
     As shown in FIG. 5, multi-beam is used in laser source  41  so that source  41  can be switched, and for instance, laser source  41  irradiates only desirable resonator  35  as multi-beam spot  43  with laser beam. Source  41  can also irradiate resonator  36 ,  37  by switching multi-beam spot  44 ,  45  with laser beam. A decompression package is omitted in FIG. 5; however, this embodiment can be carried out in the same structure as that used in FIG.  3 . 
     As discussed above, laser source  41  is switched to irradiate a desirable resonator with laser beam, so that the photo detector receives the signal only from the desirable filter. As a result, the signal of high SIN ratio is obtainable. In stead of laser source  41  using the multi-beam, a laser source using a single-beam (not shown) can be linked to a movable mirror, thereby irradiating resonators  35 - 37  with a switchable single beam. 
     In this second embodiment, half mirrors  10 ,  12  are not necessarily formed of plural mirrors respectively, and half mirrors  10 ,  12  may be movable so that the numbers of mirrors are reduced. Three resonators are used in this embodiment; however, the present invention is not limited to this number and any quantity of mirrors can be used. 
     Exemplary Embodiment 3 
     FIG. 6A is a top view illustrating a schematic structure of a filter in accordance with the third exemplary embodiment of the present invention. FIG. 6B shows a sectional view of the same item shown in FIG.  6 A. The sectional view is taken along alternate long and short dash line a 1 -a 2  in FIG.  6 A. In FIGS. 6A and 6B, elements similar to those in FIG. 1 have the same reference marks and the descriptions thereof are omitted here. A decompression package is omitted in FIGS. 6A and 6B; however, this embodiment can be carried out in the same structure as that used in FIG.  3 . In this embodiment, resonator  3  and input line  2 , both similar to those in the first embodiment, are formed on silicon nitride layer  22  deposited on high-resistive Si substrate  21 . 
     In high-resistive Si substrate  21 , input wave-guide  47  and output wave-guide  48  are formed for guiding laser beam that is used for measuring the vibration of resonator  3 . Just under resonator  3 , input wave-guide  47  and output wave-guide  48  are obliquely cut off, so that the laser beam traveling through input wave-guide  47  is refracted vertically with respect to substrate  21  and launched from substrate  21 . Then the laser beam strikes resonator  3  via half mirror  10 . 
     The laser beam incident onto resonator  3  is reflected as detected light, and strikes half mirror  11  together with reference light arrived on mirror  11 . The laser beam is thus interfered is returned to substrate  21  at a place, where output wave-guide  48  is placed. When resonator  3  resonates and vibrates, a phase of a signal received in the output wave-guide is changed, so that the vibration information of resonator  3 , namely, the information of the input high frequency signal can be obtained. 
     The wave-guide is not necessarily formed on substrate  21  under resonator  3 , and as shown in FIGS. 7A,  7 B, input wave-guide  51 , output wave-guide  52  can be prepared above resonator  3  at the place where the laser beam is irradiated. The sectional view is taken along alternate long and short dash line b 1 -b 2  of the top view. A decompression package is omitted in FIGS. 7A,  7 B; however, the filter can work in the same way as described in FIG.  3 . 
     Exemplary Embodiment 4 
     FIGS. 8A,  8 B,  9 A and  9 B show placements of resonators in accordance with the fourth exemplary embodiment. In those drawings, elements similar to those in FIG. 1 have the same reference marks and the descriptions thereof are omitted here. The sectional view is taken along long and short dash line c 1 -c 2  in the top view. A decompression package is omitted in FIGS. 8A,  8 B,  9 A and  9 B; however, the filter can work in a similar way to that described in FIG.  3 . 
     In this fourth embodiment, resonators  3  used in the first embodiment are basically arranged in an array on silicon nitride layer  22  deposited on high-resistive substrate  21 . Input lines  2  are not placed just under individual resonators  3 , but resonators  3  are placed on, e.g., micro-strip line  54 . In a similar structure to the optical system employed in the second embodiment, the laser source (not shown) launches laser beam, and the reflected laser beam from vibrating resonator  3  is received by a photo detector (not shown), so that changes of the reflected laser beam can be detected. 
     The foregoing placement can work in higher frequencies, and downsized resonator  3  can be advantageously used in this placement over other resonators. As shown in FIG. 9A, the resonators can be placed random, or as shown in FIG. 9B, the resonators can be placed in an optically significant shape. If positioning of the resonators cannot be controlled, the random placement as shown in FIG. 9A is used. 
     The individual resonators of micro-body are placed in stripes as shown in FIG. 9B, so that a diffraction grating can be formed, which allows diffracted light to radiate in a specific direction. In this case, the diffracted light can be radiated in a specific direction, thereby increasing an efficiency of collecting lights. 
     Exemplary Embodiment 5 
     FIGS. 10A-10D show sectional views and top views of filters in accordance with the fifth exemplary embodiment of the present invention. The sectional view is taken along long and short dash line d 1 -d 2  in the top view. At least two resonators are necessary for this embodiment, and four resonators are used for the description purpose. A decompression package is omitted in FIGS. 10A,  10 B,  10 C and  10 D; however, the filter can work in a similar way to that described in FIG.  3 . Four resonators  61 ,  62 ,  63 , and  64  are equidistantly placed from each other like bridges on fixed stands  66 ,  67  with their both ends rigidly mounted on stands  66 ,  67 . As shown in FIG. 10A, two input lines  56 ,  57  are placed between resonators  61  and  62  as well as between resonators  63  and  64  in parallel with respect to respective resonators  61 - 64 , which vibrate in horizontal direction. 
     When a signal is fed into input line  56 , electrostatic force of the signal running through input line  56  excites resonators  61 ,  62  horizontally. Since resonators  61 ,  62  are equidistant from input line  56  symmetrically, they vibrate in an identical amplitude; however, they vibrate in reversal phase to each other. Therefore, a relative displacement amount between resonators  61 ,  62  is twice as much as a vibration amount of a single resonator with the same input high-frequency signal. 
     Resonators  63 ,  64  vibrate with respect to input line  57  in the same way. A use of resonators  61 - 64  as a diffraction grating produces a displacement two times as much as the case where the same signal is used, so that the higher sensitive mechanical vibration can be detected. 
     As discussed above, the filter of the present invention can prevent a coupling between an input line and an output line in high frequencies such as MHz band or GHz band, and also can prevent the characteristics of the filter from being degraded by viscosity of air.