Patent Publication Number: US-2009231778-A1

Title: High frequency electrical element

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application 2008-63161 filed on Mar. 12, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high frequency electrical element, and particularly, to a high frequency electrical element adapted for implementation of a variable capacitor provided with a dielectric film, having a high Q value. 
     2. Description of Relevant Art 
     Recent years have observed emerging developments of high frequency MEMS (micro electro mechanical systems) being a miniature component fabricated as a high frequency electrical element by use of micro-fabrication techniques for semiconductors. Those high frequency MEMS are advantageous in that they have a reduced transmission loss even for high frequencies of transmission signals such as in a microwave band or millimeter waveband, and are adapted for high power transmission signals to have a small distortion of waveform at high frequencies. Accordingly, for the high frequency MEMS, there are expected applications such as to switches and variable capacitors for high frequency use. 
     The high frequency MEMS, being fabricated by use of semiconductor fabrication techniques, can be integrated on the same silicon substrate as conventional circuits using a silicon semiconductor, such as high frequency amplifier or power supply, permitting the components to be miniaturized in size or reduced in cost. 
     However, for a high frequency circuit formed on a silicon substrate, the high frequency characteristics may be degraded by an effect of the silicon substrate. To this point, Japanese Patent Publication No. 3,818,176 has proposed employing a high-resistance silicon substrate, and Japanese Patent Application Laid-Open Publication No. 2005-277,675 has disclosed a structure provided with an air gap between a downside of a high frequency MEMS portion and a silicon substrate. 
       FIG. 10A  is a plan of a general high frequency MEMS including a variable capacitor,  FIG. 10B , a section along line XB-XB of  FIG. 10A , and  FIG. 10C , a section along line XC-XC of  FIG. 10A .  FIG. 10B  illustrates a state of the high frequency MEMS with a lower electrode voltage turned off, and  FIG. 10C  illustrates a state of the high frequency MEMS with a lower electrode voltage turned on. 
     As illustrated in  FIG. 10B , the high frequency MEMS has a silicon substrate  100  insulated by an insulation film  101 . On the insulated silicon substrate  100 , as illustrated in  FIG. 10A , it has arranged a ground line  102 , an RF signal line  103  as a high frequency signal line, an upper electrode  104 , and a lower electrode  105 . The RF signal line  103  is given an RF signal S. The upper electrode is formed as a conductive beam  106  bridging corresponding parts of the ground line  102 . The RF signal line  103  crosses the beam  106  in a local region above the silicon substrate  100 , where a dielectric film  108  is interposed as an insulator in between. The dielectric film  108  is attached to the RF signal line  103 . The high frequency MEMS is thus configured to have, between the RF signal line  103  and corresponding parts of the ground line  102 , a variable capacitor  107  composed of the upper electrode  104  to be actuated by the lower electrode  105  in an electrostatic manner, the dielectric film  108 , and surrounding air. 
     The lower electrode  105  is adapted to have a voltage turned off as illustrated in  FIG. 10B , and turned on as illustrated in  FIG. 10C , whereby the beam  106  is moved up and down as illustrated by arrows  109 , changing capacitance of the variable capacitor  107  between ground line  102  and RF signal line  103 . The variable capacitor  107  has a small capacitance with the lower electrode  105  in a voltage-off state as in  FIG. 10B , and has an increased capacitance about several pF with the lower electrode  105  in a voltage-on state as in  FIG. 10C . 
     SUMMARY OF THE INVENTION 
     It however is difficult for techniques disclosed in Japanese Patent Publication No. 3,818,176 and Japanese Patent Application Laid-Open Publication No. 2005-277,675 to implement a variable capacitor with a high Q value for a circuit loss reduction. 
     For instance, the configuration of variable capacitor illustrated as a general high frequency MEMS in  FIG. 10A  has such an issue that follows:  FIG. 11  illustrates an equivalent circuit of, as superposed on a section of, a high frequency circuit with a ground line  102  and an RF signal line  103  formed on a silicon substrate  100  insulated by an insulation film  101 , and a developed equivalent circuit of the same. As illustrated in  FIG. 11 , the RF signal line  103  and associated parts of the ground line  102  have capacitances C and resistances through corresponding portions of the silicon substrate  100  in between, which cause circuit losses degrading the Q value. 
     Most variable capacitors as high frequency MEMS are connected between such a combination of RF signal line and ground line on an insulated silicon substrate.  FIG. 12  illustrates an equivalent circuit of, as superposed on a section of, a high frequency circuit having a variable capacitor added to the high frequency circuit of  FIG. 11 , and a developed equivalent circuit of the same. The variable capacitor is made up by a lower electrode  105  on the substrate  100 , an upper electrode as a beam  106  bridging associated parts of the ground line  102 , a dielectric film  108  on the RF signal line  103 , and surrounding air. The beam  106  has inductances and resistances depending on lengths of two arms thereof. The variable capacitor has a characteristic Q value defined by the real part “Re (Zin)” and the imaginary part “Im (Zin)” of an impedance Zin thereof, such that: 
         Q=|Im ( Zin )|/| Re ( Zin )| 
     As illustrated in  FIG. 12 , the RF signal line  103  and associated parts of the ground line  102  have capacitances C and resistances through corresponding portions of the silicon substrate  100  in between, whereby the variable capacitor as high frequency MEMS has a degraded Q value. 
     The present invention has been devised in view of such points. It therefore is an object of the present invention to provide a high frequency electrical element including a variable capacitor implemented with a high Q value, allowing for a reduced circuit loss. 
     To achieve the object described, according to a first aspect of the present invention, a silicon substrate wholly formed with an insulation film, a first signal line provided on the silicon substrate, a second signal line provided on the silicon substrate, the second signal line crossing the first signal line within a first region above the silicon substrate, and a dielectric film interposed between the first signal line and the second signal line, and provided on one of the first signal line and the second signal line, within the first region, the first signal line and the second signal line being relatively movable in directions for a contacting approach and a mutual spacing in between. 
     According to a second aspect of the present invention, in the high frequency electrical element according to the first aspect, a first portion as part of the first region, a second portion extending in a second region different from the first region, the second portion being connected to the first portion, and spaced from the silicon substrate more than the first portion, and a third portion connected to the second portion and a coplanar line formed to the silicon substrate for external connection. 
     According to a third aspect of the present invention, in the high frequency electrical element according to the first aspect, an electrode for electrostatic force for the second signal line to be movable above the silicon substrate is disposed at a lateral side of the second signal line, and the electrode for electrostatic force and the second signal line are linked to each other by a linking insulator. 
     According to a fourth aspect of the present invention, the high frequency electrical element according to the third aspect comprises a capacitor bank composed of a plurality of series-connected unit structures each respectively comprising an electrode for electrostatic force disposed at a lateral side of the second signal line, the electrode for electrostatic force being linked to the second signal line by a linking insulator, and the second signal line being connected to a metal electrode on the silicon substrate. 
     According to a fifth aspect of the present invention, the high frequency electrical element according to the third aspect comprises a capacitor bank composed of a plurality of series-connected unit structures each respectively comprising an electrode for electrostatic force disposed at a lateral side of the second signal line, the electrode for electrostatic force being linked to the second signal line by a linking insulator, and the second signal line floating relative to the silicon substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a plan of a high frequency electrical element according to a first embodiment of the present invention. 
         FIG. 1B  is a section along line IB-IB of  FIG. 1A . 
         FIG. 1C  is a section along line IC-IC of  FIG. 1A . 
         FIG. 2  is a graph of Q characteristic curves of the high frequency electrical element of  FIG. 1A . 
         FIG. 3  is an equivalent circuit diagram of the high frequency electrical element of  FIG. 1A . 
         FIG. 4A  is a plan of a high frequency electrical element according to a second embodiment of the present invention. 
         FIG. 4B  is a section along line IVB-IVB of  FIG. 4A . 
         FIG. 5  is a graph of Q characteristic curves of the high frequency electrical element of  FIG. 4A . 
         FIG. 6  is a plan of a high frequency electrical element according to a third embodiment of the present invention. 
         FIG. 7A  is a plan of a high frequency electrical element according to a fourth embodiment of the present invention. 
         FIG. 7B  is a detail of part VIIB of  FIG. 7A . 
         FIG. 7C  is a section along line VIIC-VIIC of  FIG. 7B . 
         FIG. 8A  is a plan of a high frequency electrical element according to a fifth embodiment of the present invention. 
         FIG. 8B  is a detail of part VIIIB of  FIG. 8A . 
         FIG. 8C  is a section along line VIIIC-VIIIC of  FIG. 8B . 
         FIG. 9A  is a circuit diagram including a high frequency electrical element according to an embodiment of the present invention. 
         FIG. 9B  is another circuit diagram including a high frequency electrical element according to an embodiment of the present invention. 
         FIG. 9C  is another circuit diagram including a high frequency electrical element according to an embodiment of the present invention. 
         FIG. 10A  is a plan of a general high frequency MEMS including a variable capacitor. 
         FIG. 10B  is a section along line XB-XB of  FIG. 10A . 
         FIG. 10C  is a section along line XC-XC of  FIG. 10A . 
         FIG. 11  is an equivalent circuit diagram of a fundamental portion of the high frequency MEMS of  FIG. 10A . 
         FIG. 12  is an equivalent circuit diagram of an essential portion including the variable capacitor of the high frequency MEMS of  FIG. 10A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There will be described the preferred embodiments of the present invention with reference to the drawings. 
     First Embodiment 
       FIG. 1A  is a plan of a high frequency MEMS  1  as a high frequency electrical element according to a first embodiment of the present invention,  FIG. 1B , a section along line IB-IB of  FIG. 1A , and  FIG. 1C , a section along line IC-IC of  FIG. 1A . 
     As illustrated in  FIG. 1A  and  FIG. 1B , the high frequency MEMS  1  has a silicon substrate  2  insulated by an insulation film  3 . On this insulated silicon substrate  2 , a first signal line  4  and a second signal line  5 . The first signal line  4  is a ground line (referred herein sometimes to “ground”)  4 , the second signal line  5  is an RF signal line  5  as a high frequency signal line. Further thereon, it has arranged an upper electrode  6  composed of a conductive beam  8  (as a ground portion ‘associated’ in a manner of) bridging corresponding ground portions of the ground line  4  for instance, a lower electrode  7  disposed on the silicon substrate  2  in a vertically opposing manner relative to the upper electrode  6 , and a dielectric film  9  as an insulation film interposed between the RF signal line  5  and the upper electrode  6 . 
     The RF signal line  5  has a portion connected to an external high frequency electric circuit as another high frequency electrical element, by a coplanar line formed to the insulated silicon substrate  2 . The RF signal line  5  has, in a local region above the insulated silicon substrate  2 , a corresponding portion thereof crossing the upper electrode  6 . 
     The upper electrode  6  and the lower electrode  7  have a control voltage applied across them for an electrostatic action to be described later. The upper electrode  6  is connected to the above-noted corresponding ground portions of the ground line  4 , and has a ground potential, and the lower electrode  7  has an electrostatic potential corresponding to the control voltage. The lower electrode  7  is configured as an electrode for producing electrostatic forces to actuate the beam  8  of the upper electrode  8  to move in a Z-axis direction. The upper electrode  6  and the lower electrode  7  extend in an X-axis direction, and the RF signal line  5  extends in a Y-axis direction. 
     In the embodiment illustrated in  FIG. 1A , the dielectric film  9  is formed as part of a variable capacitor  13  on an intermediate part of the RF signal line  5 . It is noted that a dielectric film or dielectric films  9  may well be formed on one or both of mutually opposing surfaces of the beam  8  and the corresponding portion of RF signal line  5  crossing each other in the above-noted local region. 
     As illustrated in  FIGS. 1A and 1C , in each of local regions (within enclosing broken lines G) different from the above-noted local region above the silicon substrate  2 , a corresponding portion of the RF signal line  5  is spaced, i.e., floated, off from the silicon substrate  2 , more than in other regions, thereby defining an air layer  12 . The portion within enclosing broken line G is formed as a rectangular convex portion  10  that is convex in the Z-axis direction. 
     Rectangular convex portions  10  of the RF signal line  5  are floated upward from the silicon substrate  2  insulated by the insulation film  3 . A central portion of the RF signal line  5  is associated with the variable capacitor  13 , and air layers  12  are provided at both sides of the variable capacitor  13  on the RF signal line  5 , by making the RF signal line  5  air-bridged. That is, the RF signal line  5  is air-bridged at locations outside the variable capacitor  13 . 
     Given a control voltage turned on, the lower electrode  7  causes the beam  8  to come downward, till it contacts the dielectric film  9 , increasing the capacitance. With the lower electrode  7  given a control voltage turned off, the beam  8  goes upward, so it is spaced off from the dielectric film  9 . Like this, there occurs a contacting approach or a spacing between the beam  8  as an associated portion of the ground line  4  and a corresponding portion of the RF signal line  5  provided with the dielectric film  9 , whereby the variable capacitor  13  between RF signal line  5  and ground line  4  has a varied capacitance. In other words, the variable capacitor  13  has a varied capacitance, as a contact or dynamic non-contact sate develops between the dielectric film  9  and either or both of the RF signal line  5  and (the beam  8  of upper electrode  6  of) the ground line  4 . As illustrated in  FIG. 1B , the provision of air layers  12  defined by two rectangular convex portions  10  of the RF signal line  5  implements a series connection of a pair of low capacitances C 1 , allowing for a reduced stray capacitance. 
       FIG. 2  shows, in a graph, improvements of Q values as results of simulations made of an example of high frequency MEMS  1  illustrated as a high frequency electrical element in FIGS.  1 Aa to  1 C. 
     In the graph of  FIG. 2 , the axis of ordinate represents Q values, and the axis of abscissas represents a frequency, to show variations of the Q values to the frequency.  FIG. 2  involves curves L 1  and L 3  showing variations of Q values in MEMS  1  as the example of the first embodiment illustrated in  FIGS. 1A to 1C , and curves L 2  and L 4  showing Q values in a high frequency MEMS as a comparative example configured without air layers (without being air-bridged). Among them, curves L 1  and L 2  each show values for a corresponding beam in motion toward a dielectric film with a prescribed control voltage applied to a lower electrode, and curves L 3  and L 4  each show values for the corresponding beam in motion away from the dielectric film with no control voltage applied to the lower electrode. 
     As will be seen from comparison of curve L 1  with curve L 2  in  FIG. 2 , the high frequency MEMS  1  as an example of the first embodiment has improved Q values in motion of beam toward dielectric film, relative to the high frequency MEMS as a comparative example configured without air layers. 
     As will be seen from comparison of curve L 3  with curve L 4 , the high frequency MEMS  1  as an example of the first embodiment has improved Q values in motion of beam away from dielectric film, as well, relative to the high frequency MEMS as a comparative example configured without air layers. 
     It can thus be caught that the high frequency MEMS  1  as an example of the first embodiment has improved Q values relative to the high frequency MEMS as a comparative example configured without air layers, whether on-voltage or off-voltage is applied as the control voltage between upper electrode and lower electrode. It is thus possible to implement a variable capacitor with high values of Q in the first embodiment illustrated in  FIGS. 1A to 1C , in which the RF signal line  5  is air-bridged at both outsides of the variable capacitor  13 . 
       FIG. 3  is an equivalent circuit diagram of the high frequency MEMS  1  as a high frequency electrical element illustrated in  FIGS. 1A to 1C . In comparison with the equivalent circuit diagram of general high frequency MEMS illustrated in  FIG. 12 , stray capacitances are eliminated at areas designated by reference character  19 . 
     Second Embodiment 
     Description is now made of a high frequency electrical element according to a second embodiment of the present invention, with reference to  FIGS. 4A and 4B . 
       FIG. 4A  is a plan of a high frequency MEMS  41  as a high frequency electrical element according to the second embodiment, and  FIG. 4B , a section along line IVB-IVB of  FIG. 4A . 
     The high frequency MEMS  1  as the first embodiment illustrated in  FIGS. 1A to 1C  has the RF signal line  5  air-bridged at both outsides of the variable capacitor  13 . In the high frequency MEMS  41  as the second embodiment illustrated in  FIGS. 4A and 4B , a beam  48  of an RF signal line  45  constituting a variable capacitor  53  is floated from an insulated silicon substrate  42 , whereby the RF signal line  45  is air-bridged. 
     As illustrated in  FIGS. 4A and 4B , the high frequency MEMS  41  has the silicon substrate  42  insulated by an insulation film  43 . And, on the insulated silicon substrate  42 , it has disposed a ground line  44 , the RF signal line  45 , and a pair of lower electrodes  47 . 
     As illustrated in  FIGS. 4A and 4B , the paired lower electrodes  47  are arranged in vertical opposition to a pair of upper electrodes  46 , and they are configured as electrodes for producing electrostatic forces to actuate the beam  48  in a Z-axis direction. Respective pairs of vertically opposing upper and electrodes  46  and  47  are disposed at both lateral sides of the RF signal line  45 , avoiding extending there below. The beam  48  is widthwise centered to the RF signal line  45 , and is linked with and held by the upper electrodes  46  through a pair of linking insulation films  43 C at both lateral sides thereof. 
     As illustrated in  FIG. 4B , below the beam  48 , there is a dielectric film  49  formed as insulation film on an associated ground portion of the ground line  44 . The ground line  44  has a pair of ground portions  44 B rising in the X-axis direction on the insulated silicon substrate  42  at both lateral sides of the above-noted associated ground portion, to support the paired upper electrodes  46 . 
     As illustrated in  FIG. 4A , the RF signal line  45  is formed in an X-axis direction as a coplanar line to the silicon substrate  42 , and receives an RF signal S from an external high frequency electric circuit. 
     As illustrated in  FIG. 4B , the beam  48  as a portion of the RF signal line  45  has an X-directional centerline thereof crossing at right angles in a top view with a Y-directional center-connecting line of the linking insulation films  43 C and the upper electrodes  46  as parts of the ground line  44 , in a local region above the insulated silicon substrate  42 . 
     Between the beam  48  as a portion of the RF signal line  45  and the associated ground portion, there is a variable capacitor  53  made up by the dielectric film  49  and surrounding air. The variable capacitor  53  has the beam  48  as a portion working as an electrode thereof and air-bridged to define an air layer associated therewith. 
     Depending on a control voltage applied to the lower electrode  47  being turned on or off, the beam  48  of the RF signal line  45  goes up and down in the Z-axis direction, making a contacting approach or spacing relative to the dielectric film  49 , whereby the variable capacitor  53  between RF signal line  45  and ground line  44  has a varied capacitance. In other words, the variable capacitor  53  has a varied capacitance, as a contact or dynamic non-contact sate develops between the dielectric film  49  and either or both of beam  48  as a portion of the RF signal line  45  and an associated ground portion of the ground line  44 . 
     In the second embodiment illustrated in  FIGS. 4A and 4B , electrostatic upper electrodes  46  and lower electrodes  47  are disposed in positions at laterally outer sides of the RF signal line  45 , i.e., offset in opposite Y 1  and Y 2  directions from the RF signal line  45  at a center in  FIG. 4A . Therefore, the lower electrodes  47  are not disposed anywhere under the beam  48  of the RF signal line  45 , allowing for an improved high frequency characteristic. That is, the lower electrodes  47  are not disposed anywhere under any high frequency signal line, but spaced there from, thus constituting no noise source to the high frequency signal line, allowing for an enhanced high frequency characteristic. 
       FIG. 5  shows, in a graph, improvements of Q values as results of simulations made of an example of high frequency MEMS  41  illustrated as a high frequency electrical element in  FIGS. 4A and 4B . 
     In the graph of  FIG. 5 , the axis of ordinate represents Q values, and the axis of abscissas represents a frequency, to show variations of the Q values to the frequency.  FIG. 5  involves curves L 5  and L 7  showing variations of Q values in MEMS  41  as the example of the second embodiment illustrated in  FIGS. 4A and 4B , and curves L 6  and L 8  showing Q values in a high frequency MEMS as a comparative example configured without air layers (without being air-bridged). Among them, curves L 5  and L 6  each show values for a corresponding beam in motion toward a dielectric film with a prescribed control voltage applied to lower electrodes, and curves L 7  and L 8  each show values for the corresponding beam in motion away from the dielectric film with no control voltage applied to the lower electrodes. 
     As will be seen from comparison of curve L 5  with curve L 6  in  FIG. 5 , the high frequency MEMS  41  as an example of the second embodiment has improved Q values in motion of beam toward dielectric film, relative to the high frequency MEMS as a comparative example configured without air layers. As will be seen from comparison of curve L 7  with curve L 8 , the high frequency MEMS  1  as an example of the second embodiment has improved Q values in motion of beam away from dielectric film, as well, relative to the high frequency MEMS as a comparative example configured without air layers. It can thus be caught that the high frequency MEMS  41  as an example of the second embodiment has improved Q values relative to the high frequency MEMS as a comparative example configured without air layers, whether on-voltage or off-voltage is applied as the control voltage between upper electrodes and lower electrodes. It is thus possible to implement a variable capacitor with high values of Q in the second embodiment, in which part of the RF signal line  45  constituting an electrode of the variable capacitor is air-bridged. 
     Third Embodiment 
     Description is now made of a high frequency electrical element according to a third embodiment of the present invention, with reference to  FIG. 6 . 
       FIG. 6  is a plan of a high frequency MEMS  61  as a high frequency electrical element according to the third embodiment. 
     The high frequency MEMS  61  as the third embodiment illustrated in  FIG. 6  is substantially identical to the high frequency MEMS  41  illustrated in  FIGS. 4A and 4B , except for a beam  48 B of a spring structure. 
     The beam  48 B is configured with a spring structure for facilitated movements of the upper electrodes  46  in a Z-axis direction. More specifically, the beam  48 B is configured with a rectangular electrode plate portion linked at both lateral sides thereof by linking insulation films  43  with upper electrodes  46 , and front and rear arm portions interconnecting front and rear sides of the rectangular electrode plate portion and corresponding portions of an RF signal line  45  extending on a silicon substrate insulated by an insulation film  43 , each arm portion being divided into two branches and bent to cooperatively define a cross-shaped central void, to thereby provide the spring structure with an increased flexibility. An increased pressing force the beam  48  has against a dielectric film  49  permits lower electrodes  47  for electrostatic actuation to be formed with an increased area, and disposed nearer to the RF signal line  45 . With a control voltage applied between upper electrodes  46  and lower electrodes  47 , the beam  48  of the spring structure is forced to contact the dielectric film  49 . With the voltage to the lower electrodes  47  turned off, the beam  48 B is allowed to disengage from the dielectric film  49  by spring forces. 
     In the third embodiment illustrated in  FIG. 6 , the lower electrodes  47  for electrostatic actuation can be disposed laterally outside of the RF signal line  45 , allowing for an enhanced high frequency characteristic. The electrodes  47  for electrostatic actuation can be offset off, not just under, the high frequency signal line  45 , thus constituting no noise source to the high frequency signal line  45 , allowing for the more enhanced high frequency characteristic. The spring structure of RF signal line  45  may be combined with a modified layout pattern of lower electrodes  47 , as well as of a ground line  44 , for still enhanced vertical movements of the beam  48  at a variable capacitor  53 . 
     Fourth Embodiment 
       FIG. 7A  is a plan of a high frequency MEMS  71  as a high frequency electrical element according to a fourth embodiment of the present invention,  FIG. 7B , a detail of part VIIB of  FIG. 7A , and  FIG. 7C , a section along line VIIC-VIIC of  FIG. 7B . 
     The high frequency MEMS  71  as the fourth embodiment illustrated in  FIG. 7A  is provided with a capacitor bank composed of a plurality of series-connected unit capacitors each respectively configured as a high frequency MEMS  61  according to the third embodiment illustrated in  FIG. 6 . In the high frequency MEMS  71  illustrated in  FIG. 7A , the capacitor bank is composed of four high frequency MEMS  61 , for instance. Those high frequency MEMS  61  have areas of their rectangular electrode plate portions changed in proportion to associated capacitances, in an binary order being 1:2:4:8 for instance, thereby permitting 2 4 =16 combinations of capacitances. 
       FIG. 7B  illustrates an interconnection structure  74  between signal lines as branches of neighboring beams  48 B between unit variable capacitors constituting the capacitor bank of the high frequency MEMS  71  illustrated in  FIG. 7A . Neighboring beams  48 B as air-bridged signal line portions are interconnected by an anchoring structure composed of a metal electrode  72  formed on an insulated silicon substrate  42 . Respective sets of signal lines of neighboring beams  48 B between unit variable capacitors are connected by anchoring the beams  48 B on the metal electrode  72 . 
     Fifth Embodiment 
       FIG. 8A  is a plan of a high frequency MEMS  71  as a high frequency electrical element according to a fifth embodiment of the present invention,  FIG. 8B , a detail of part VIIIB of  FIG. 8A , and  FIG. 8C , a section along line VIIIC-VIIIC of  FIG. 8B . 
     The high frequency MEMS  81  as the fifth embodiment illustrated in  FIG. 8A  is different from the high frequency MEMS  71  as the fourth embodiment illustrated in  FIG. 7A , in that the former has no anchoring structure for interconnection between unit variable capacitors.  FIG. 8B  illustrates an interconnection structure  84  between signal lines between unit variable capacitors. As illustrated in  FIG. 8B , the interconnection structure  84  between signal lines between unit variable capacitors is implemented by a direct connection between signal lines, so lengths of associated beams  48 B are air-bridged, cooperating with a silicon substrate  42  insulated by an insulation film  43  to define a connected spatial region  83  therebetween, whereby respective unit variable capacitors are connected to constitute a capacitor bank. Accordingly, the high frequency MEMS  81  can be free of influences of substrate losses at anchor portions, allowing for the more enhanced Q value. 
       FIGS. 9A to 9C  each illustrate an example of circuit diagram including a high frequency MEMS as a high frequency electrical element according to an embodiment of the present invention. In the example of  FIG. 9A , a high frequency MEMS  90  is implemented to a tunable antenna, allowing for a miniaturized antenna configuration for a wide-band terrestrial digital broadcasting. In the example of  FIG. 9B , high frequency MEMS  91  are implemented as matching circuits of an amplifier, allowing for a reduced number of amplifier types. In the example of  FIG. 9C , high frequency MEMS  93  are implemented to a power supply  94  on a silicon substrate using RF connection lines  96  for connection to an amplifier  95 . 
     According to an embodiment of the present invention, a high frequency electrical element comprises a silicon substrate wholly formed with an insulation film, a first signal line provided on the silicon substrate, a second signal line provided on the silicon substrate, the second signal line crossing the first signal line within a first region above the silicon substrate, and a dielectric film interposed between the first signal line and the second signal line, and provided on one of the first signal line and the second signal line, within the first region, the first signal line and the second signal line being relatively movable in directions for a contacting approach and a mutual spacing in between. Permitting a variable capacitor to be implemented with a high Q value, allowing for a reduced circuit loss. 
     The second signal line comprises a first portion as part of the first region, a second portion extending in a second region different from the first region, the second portion being connected to the first portion, and spaced from the silicon substrate more than the first portion, and a third portion connected to the second portion and a coplanar line formed to the silicon substrate for external connection. 
     An electrode for electrostatic force for the second signal line to be movable above the silicon substrate is disposed at a lateral side of the second signal line, and the electrode for electrostatic force and the second signal line are linked to each other by a linking insulator, so that the electrode for electrostatic force can be set off, not just under, the second signal line, thus constituting no noise source to the second signal line, allowing for an enhanced high frequency characteristic. 
     The high frequency electrical element comprises a capacitor bank composed of a plurality of series-connected unit structures each respectively comprising an electrode for electrostatic force disposed at a lateral side of the second signal line, the electrode for electrostatic force being linked to the second signal line by a linking insulator, and the second signal line being connected to a metal electrode on the silicon substrate, and the second signal line at the unit structure is connected to the metal electrode on the silicon substrate, whereas other portions are floated, allowing for an enhanced Q value. 
     The high frequency electrical element comprises a capacitor bank composed of a plurality of series-connected unit structures each respectively comprising an electrode for electrostatic force disposed at a lateral side of the second signal line, the electrode for electrostatic force being linked to the second signal line by a linking insulator, and the second signal line floating relative to the silicon substrate, and the second signal line at the unit structure is not connected any metal electrode on the silicon substrate, permitting losses at the silicon substrate to be reduced, allowing for an enhanced Q value. 
     According to an embodiment of the present invention, a second signal line outside a variable capacitor is air-bridged, or a second signal line inside a variable capacitor is air-bridged, permitting a variable capacitor to be implemented with a high Q value, allowing for a reduced circuit loss. 
     It is noted that according to the present invention, the foregoing embodiments are not restricted as they are, but may be implemented by modifying their components without departing from the spirit. 
     Components of the foregoing embodiments may be combined in an adequate manner to provide a variety of inventions. For instance, out of whole components of the embodiments, some components may be eliminated. Further, components of different embodiments may be adequately combined. 
     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes, and it is to be understood that changes and variations may be made without departing from the scope of the following claims.