Abstract:
There is proved a variable capacitor that includes a substrate, a signal line disposed on a surface of the substrate for feeding a signal, a ground electrode disposed on the surface, and a movable electrode opposed the signal line and the ground electrode, the movable electrode operable to move toward and away from the signal line and the ground electrode. The movable electrode can be displaced by an electrostatic attraction between the movable electrode and the signal line and between the movable electrode and the signal line. An amount of displacement of the movable electrode varies according to an amount of the voltage which generates the electrostatic attraction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-038808, filed on Feb. 20, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a variable capacitance element (variable capacitor) and, more particularly, to a variable capacitor that is fabricated with a Micro Electro Mechanical System (MEMS) technology, a matching circuit element that uses the variable capacitor, and a mobile terminal apparatus that uses the variable capacitor or the matching circuit element. 
     2. Description of the Related Art 
     A variable capacitor is an important component in an electric circuit, such as a variable frequency oscillator, a tuned amplifier, a phase shifter, and an impedance matching circuit. In recent years, the number of mobile terminal apparatuses in which a variable capacitor is mounted has been increasing. In the technical field of the mobile terminal apparatus, with an increase in the number of components mounted, and the like, for high performance, the need for miniaturization of components used is growing. To respond to the need, miniaturization of a variable capacitor is pursued utilizing a MEMS technology. The variable capacitor produced through the MEMS technology is advantageous in that a Q value may be increased because of a small loss as compared with a varactor diode that is mainly used currently, and development of the variable capacitor has been promoted. 
     A variable capacitor that is produced through the MEMS technology is, for example, described in Japanese Laid-open Patent Publication 2007-273932, in which the variable capacitor varies its capacitance by changing the distance between the opposite two electrodes. 
       FIG. 28  and  FIG. 29  are cross-sectional views of an existing typical variable capacitor F 1 . The variable capacitor F 1  includes a substrate  101 , a fixed electrode  102 , a movable electrode  104 , a dielectric layer  105 , and a pair of supports  106 . The fixed electrode  102  is provided on the upper face (a face on the upper side in  FIG. 28 ) of the substrate  101 . The movable electrode  104  is provided above the fixed electrode  102  so as to be bridged with the pair of supports  106 . The movable electrode  104  has a portion that faces the fixed electrode  102 . The dielectric layer  105  is provided on the upper face of the fixed electrode  102  in order to prevent short circuit due to contact of the fixed electrode  102  with the movable electrode  104 . The substrate  101  is made of a silicon material, and the fixed electrode  102  and the movable electrode  104  are respectively made of predetermined metal materials. 
     In the variable capacitor F 1 , a voltage applied between the fixed electrode  102  and the movable electrode  104 , generates electrostatic attraction between them. Due to the electrostatic attraction, the movable electrode  104  is attracted toward the fixed electrode  102  to change the distance between the electrodes  102  and  104 . This change in the distance varies a capacitance between the electrodes  102  and  104 . Thus, by changing a voltage applied between the fixed electrode  102  and the movable electrode  104 , it is possible to vary the capacitance. 
     Because of an extremely thin thickness of the dielectric layer  105 , the capacitance of the variable capacitor F 1  has a characteristic such that it is substantially inversely proportional to a distance d. The characteristic, however, differs in a region in which an influence on the capacitance value of the dielectric layer due to the distance d between the movable electrode  104  and the fixed electrode  102  cannot be ignored (for example, a region in which the movable electrode  104  is located in proximity to the fixed electrode  102 ). 
     Thus, the capacitance of the variable capacitor F 1  is minimal in a state where the fixed electrode  102  is separated from the movable electrode  104  (a state where the distance d between the electrodes is maximal, see  FIG. 28 ). On the contrary, the capacitance is maximal in a state where the fixed electrode  102  is in contact with the movable electrode  104  via the dielectric layer  105  (a state where the distance d between the electrodes is minimal, see  FIG. 29 ). 
       FIG. 30  shows the variation of a capacitance of the variable capacitor F 1  according to a driving voltage applied to the variable capacitor F 1 , which applied between the fixed electrode  102  and the movable electrode  104 . The abscissa axis represents a driving voltage, and the ordinate axis represents a capacitance. As the driving voltage is increased, the capacitance abruptly increases and then attains a constant value (maximum capacitance) (see point P 1 ). On the other hand, as the driving voltage is decreased, the capacitance abruptly decreases and then attains a constant value (minimum capacitance) (see point P 2 ). 
     The characteristic of the capacitance in  FIG. 30  varies such that it is inversely proportional to the distance d between the electrodes as described above. The point P 1  is a point at which the fixed electrode  102  contacts the movable electrode  104  via the dielectric layer  105 . The point P 2  is a point at which electrostatic attraction between the fixed electrode  102  and the movable electrode  104  is lost. Assuming Von for the driving voltage at the point P 1  and Voff for the driving voltage at the point P 2 , the variable capacitor F 1  may be used as a capacitive switch that switches a capacitance between at the driving voltage Voff and at the driving voltage Von. 
     However, when the variable capacitor F 1  is actually used as a capacitive switch, the driving voltage (direct current voltage) of the movable electrode  104  should be only applied to that movable electrode  104  and not applied to other circuits connected to the variable capacitor F 1 . Therefore, it is necessary to provide a circuit (hereinafter, the circuit is referred to as “DC block”) for blocking the driving voltage of the movable electrode  104 . 
       FIG. 31  shows an equivalent circuit diagram of the variable capacitor F 1  used as a capacitive switch connected in parallel with a signal line for an unbalanced alternating current signal. 
     As shown in  FIG. 31 , the fixed electrode  102  and the movable electrode  104  are respectively connected to a ground and a signal line  110 , for example, through which an RF signal (alternating current signal) flows, and a driving voltage is applied from a DC power  113  to the movable electrode  104 . A capacitor  111  as a DC block is provided between the signal line  110  and the variable capacitor F 1  in order not to apply a driving voltage from the DC power  113  to the signal line  110 . In addition, an inductor  114  as a circuit (hereinafter, the circuit is referred to as “RF block”) for blocking an RF signal, is provided between the DC power  113  and the movable electrode  104 . The inductor can prevent an RF signal flowing through the signal line  110  from being bypassed to a ground in a path between the capacitor  111  and the DC power  113 . 
     The capacitance of the capacitor  111  must be sufficiently large with respect to the variable capacitor F 1  not to influence the characteristic of the variable capacitor F 1 . Therefore the size of a capacitor  111  is large for a large capacitance. Thus, there has been a limit on miniaturization of a device that employs the variable capacitor F 1 . 
     In addition, the driving voltage Voff needs to be a voltage larger than a voltage induced between the fixed electrode  102  and the movable electrode  104  by an RF signal flowing through the signal line  110 . Therefore, it is necessary to set the driving voltage Voff to be large when a large RF signal flows in the signal line  110 . As is apparent from the characteristic shown in  FIG. 30 , the driving voltage Von is set to be larger than the driving voltage Voff. Consequently the driving voltage Von is so large during the maximum capacitance that an electrification phenomenon of the dielectric layer  105  is more likely to occur. 
     SUMMARY 
     An object of the invention to provide a variable capacitor that allows a driving voltage to be set small and that enables miniaturization of a device used therein. 
     According to an aspect of the present invention, a variable capacitor includes a substrate, a signal line that is provided on a surface of the substrate and through which a signal flows, a ground electrode that is provided on the surface of the substrate, and a movable electrode that faces the signal line and the ground electrode and that is provided displaceably in a direction to approach or leave the signal line and the ground electrode. 
     In a preferred embodiment of the invention, the movable electrode, when a driving voltage is applied to the movable electrode, is displaced by electrostatic attraction generated between the signal line and the ground electrode, and the amount of the displacement varies depending on the magnitude of the driving voltage. 
     According to the above configuration, it is configured so that a variable capacitor formed of the signal line and the movable electrode is connected in series with a variable capacitor formed of the ground electrode and the movable electrode. By so doing, a voltage applied to the signal line is divided by the two variable capacitors. Thus, it is possible to reduce a driving voltage necessary for applying electrostatic attraction between the signal line and the movable electrode and between the ground electrode and the movable electrode. In addition, because a driving voltage applied to the movable electrode is blocked by the two variable capacitors, it is not necessary to provide another capacitor, which serves as a DC block. Thus, it is possible to reduce the size of a device that employs the variable capacitor. 
     In a preferred embodiment, the movable electrode is fixed by supports to the surface of the substrate at both ends of the movable electrode and at a predetermined portion of the movable electrode between both ends. According to the above configuration, it is possible to suppress warpage of the movable electrode due to residual stress, or the like, in a manufacturing process. 
     In a preferred embodiment, the signal line and/or the ground electrode are provided movably in a vertical direction with respect to the surface of the substrate. According to the above configuration, because it is possible to reduce electrostatic attraction necessary for attracting the movable electrode toward the signal line and/or the ground electrode, it is possible to suppress a driving voltage applied to the movable electrode to a lesser magnitude. 
     In a preferred embodiment, the signal line and the ground electrode are formed in a Coplanar Waveguide (CPW) structure. According to the above configuration, it is possible to easily control the impedance of the signal line. 
     In a preferred embodiment, a dielectric layer is provided between the signal line and/or the ground electrode and the movable electrode. According to the above configuration, it is possible to prevent short circuit due to contact of the signal line or ground electrode with the movable electrode. 
     According to a second aspect of the invention, a variable capacitor includes a substrate, an input electrode that is provided on a surface of the substrate and to which a signal is input, an output electrode that is provided on the surface of the substrate, that is not electrically connected to the input electrode and that outputs a signal, and a movable electrode that faces the input electrode and the output electrode and that is provided displaceably in a direction to approach or leave the signal line and the ground electrode. 
     In a preferred embodiment, the movable electrode, when a driving voltage is applied to the movable electrode, is displaced by electrostatic attraction generated between the input electrode and the output electrode, and the amount of the displacement varies depending on the magnitude of the driving voltage. 
     According to the above configuration, it is configured so that a variable capacitor formed of the input electrode and the movable electrode is connected in series with a variable capacitor formed of the output electrode and the movable electrode. By so doing, because a driving voltage applied to the movable electrode is blocked by the two variable capacitors, it is not necessary to provide another capacitor, which serves as a DC block. Thus, it is possible to reduce the size of a device that employs the variable capacitor. 
     In a preferred embodiment, the movable electrode is fixed by supports to the surface of the substrate at both ends of the movable electrode and at a predetermined portion of the movable electrode between both ends. According to the above configuration, it is possible to suppress warpage of the movable electrode due to residual stress, or the like, in a manufacturing process. 
     In a preferred embodiment, the input electrode and/or the output electrode are provided movably in a vertical direction with respect to the surface of the substrate. According to the above configuration, because it is possible to reduce electrostatic attraction necessary for attracting the movable electrode toward the input electrode and/or the output electrode, it is possible to suppress a driving voltage applied to the movable electrode to a lesser magnitude. 
     In a preferred embodiment, a ground electrode that forms a CPW structure with the input electrode and the output electrode is further provided. According to the above configuration, it is possible to easily control the impedance of the input electrode and the impedance of the output electrode. 
     In a preferred embodiment, a dielectric layer is provided between the input electrode and/or the output electrode and the movable electrode. According to the above configuration, it is possible to prevent short circuit due to contact of the input electrode or output electrode with the movable electrode. 
     According to a third aspect of the invention, a matching circuit employs the variable capacitor provided according to the first aspect or the second aspect. Thus, it is possible to reduce the driving voltage, and it is possible to keep reliability. In addition, it is possible to achieve miniaturization. 
     According to a fourth aspect of the invention, a mobile terminal apparatus employs the variable capacitor provided according to the first aspect or the second aspect or employs the matching circuit element provided according to the third aspect. Thus, it is possible to reduce the driving voltage, and it is possible to keep reliability. In addition, it is possible to achieve miniaturization. 
     Further features of the invention will become apparent through the following detailed description made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a plan view of a variable capacitor according to the first embodiment; 
         FIG. 2  illustrates is a cross-sectional view taken along the line II-II in  FIG. 1 ; 
         FIG. 3  illustrates an equivalent circuit diagram of the variable capacitor connected to a DC power; 
         FIG. 4  illustrates a cross-sectional view of a first alternative example of the variable capacitor according to the first embodiment; 
         FIG. 5  illustrates a plan view of a second alternative example of the variable capacitor according to the first embodiment; 
         FIG. 6  illustrates a plan view of a third alternative example of the variable capacitor according to the first embodiment; 
         FIG. 7  illustrates a cross-sectional view of the third alternative example of the variable capacitor taken along the line VII-VII in  FIG. 6 ; 
         FIG. 8  illustrates a cross-sectional view of a fourth alternative example of the variable capacitor according to the first embodiment; 
         FIG. 9  illustrates a plan view of a variable capacitor according to a second embodiment; 
         FIG. 10  illustrates an equivalent circuit diagram of the variable capacitor shown in  FIG. 9  and a DC power; 
         FIG. 11  illustrates a plan view of a variable capacitor according to a third embodiment; 
         FIG. 12  illustrates a cross-sectional view of the variable capacitor according to the third embodiment taken along the line XII-XII in  FIG. 11 ; 
         FIG. 13  illustrates a cross-sectional view of the variable capacitor according to the third embodiment taken along the line XIII-XIII in  FIG. 11 ; 
         FIG. 14  illustrates a cross-sectional view of a first alternative example of the variable capacitor according to the third embodiment; 
         FIG. 15  illustrates a cross-sectional view of a second alternative example of the variable capacitor according to the third embodiment; 
         FIG. 16  illustrates a plan view of a variable capacitor according to a fourth embodiment; 
         FIG. 17  illustrates a plan view of a Γ matching circuit element E 1  using the variable capacitor shown in  FIG. 5 ;  FIG. 19  is a plan view that shows a Π matching circuit element E 2  in which a movable electrode  4  is further added to the variable capacitor A 3  shown 
         FIG. 18  illustrates an equivalent circuit diagram of the Γ matching circuit element shown in  FIG. 17  and a DC power connected thereto; 
         FIG. 19  illustrates a plan view of a Π matching circuit element using the variable capacitor shown in  FIG. 5 ; 
         FIG. 20  illustrates an equivalent circuit diagram of the Π matching circuit element shown in  FIG. 19  and a DC power connected thereto; 
         FIG. 21  illustrates a plan view of a Γ matching circuit element using the variable capacitor shown in  FIG. 16 ; 
         FIG. 22  illustrates a plan view of a Π matching circuit element using the variable capacitor shown in  FIG. 16 ; 
         FIG. 23  illustrates a plan view of a Γ matching circuit element using the variable capacitor shown in  FIG. 14 ; 
         FIG. 24  illustrates an equivalent circuit diagram of the Γ matching circuit element shown in  FIG. 23  and a DC power connected thereto; 
         FIG. 25  illustrates a plan view of a Π matching circuit element using the variable capacitor shown in  FIG. 14 ; 
         FIG. 26  illustrates an equivalent circuit diagram of the Π matching circuit element shown in  FIG. 25  and a DC power connected thereto; 
         FIG. 27  illustrates a block diagram of a front end portion of a mobile terminal apparatus in which the variable capacitors and matching circuit elements according to the embodiments are used as electronic components; 
         FIG. 28  illustrates a cross-sectional view of an existing typical variable capacitor in a state of the existing typical variable capacitor giving a minimal capacitance; 
         FIG. 29  illustrates a cross-sectional view of an existing typical variable capacitor in a state of the existing typical variable capacitor giving a maximum capacitance; 
         FIG. 30  shows a graph illustrating the variation of a capacitance of the variable capacitor according to a driving voltage applied to between a fixed electrode and a movable electrode in the variable capacitor; and 
         FIG. 31  illustrates an equivalent circuit diagram of the existing typical variable capacitor connected in parallel with a signal. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the invention will be specifically described with reference to the drawings. 
       FIG. 1  and  FIG. 2  are views for illustrating a first embodiment of a variable capacitor.  FIG. 1  is a plan view of the variable capacitor A 1 .  FIG. 2  is a cross-sectional view taken along the line II-II in  FIG. 1 . 
     The variable capacitor A 1  includes a substrate  1 , a signal line  2 , a ground electrode  3 , a movable electrode  4 , dielectric layers  5 , and a pair of supports  6 . When the variable capacitor A 1  is, for example, used as a capacitive switch, the variable capacitor A 1  is mounted on a printed circuit, and the signal line  2  and the ground electrode  3  are respectively connected to a signal line and ground of an electric circuit formed on the printed circuit. Note that actually the width of each dielectric layer  5  in the right-left direction in  FIG. 1  is substantially equal to the width of the movable electrode  4 , and the dielectric layers  5  are hidden behind the movable electrode  4  in plan view. Thus, in  FIG. 1 , to recognize the presence of the dielectric layers  5 , the width of each dielectric layer  5  in the right-left direction is drawn wider than the width of the movable electrode  4 . In addition, the width of each dielectric layer  5  in the top-bottom direction in  FIG. 1  is substantially equal to the width of the signal line  2  or the width of the ground electrode  3 ; however, to explicitly show the presence of the dielectric layers  5 , the width of each dielectric layer  5  in the top-bottom direction is also drawn wider than that is. The same applies to the plan views of the following variable capacitors. 
     The substrate  1  is a rectangular plate in plan view and made of a silicon material. The long side and short side of the substrate  1  each have a length of, for example, about 1 to 2 mm, and the substrate  1  has a thickness of, for example, about 300 μm. 
     A radiofrequency (RF) signal flows through the signal line  2 . The signal line  2  is provided on an upper face (a face on the upper side in  FIG. 2 ) of the substrate  1  so as to extend parallel to the long side of the substrate  1  to near both ends of the substrate  1  in the long side direction (lateral direction in  FIG. 1 ). The ground electrode  3  is an electrode that is grounded. The ground electrode  3  is provided on the upper face of the substrate  1  so as to extend parallel to the signal line  2  to near both ends of the substrate  1  in the long side direction. The signal line  2  and the ground electrode  3  are made of a conductive material, such as aluminum (Al) or copper (Cu). 
     The movable electrode  4  is provided parallel to the short side of the substrate  1  at the center in the long side direction thereof. The movable electrode  4  extends to near both ends in the short side direction (longitudinal direction in  FIG. 1 ) and the pair of supports  6 , which will be described later, supports the movable electrode  4  at the both thereof. The movable electrode  4  is movable in a vertical direction (top-bottom direction in  FIG. 2 ) with respect to the upper face of the substrate  1 . The movable electrode  4  is provided so as to be perpendicular to the signal line  2  and the ground electrode  3  and has portions that face the signal line  2  and the ground electrode  3 . The area of these faced portions is, for example, about 40000 (200×200) to 90000 (300×300) μm 2 . The thickness T 1  of the movable electrode  4  is, for example, about 1 to 2 μm. The distance L 1  between the movable electrode  4  and the signal line  2  or the ground electrode  3  is about 0.5 to 2 μm (see  FIG. 2 ). The movable electrode  4  is made of a conductive material, such as aluminum or copper. 
     The dielectric layers  5  are provided on the upper faces of the portions of the signal line  2  and ground electrode  3 , facing the movable electrode  4 . The dielectric layers  5  prevent short circuit due to contact of the signal line  2  or the ground electrode  3  with the movable electrode  4 , and the thickness of each dielectric layer  5  is, for example, about 0.1 to 0.5 μm. The dielectric layers  5  are, for example, formed of a dielectric material, such as alumina (Al 2 O 3 ), silicon oxide (SiO 2 ), or silicon nitride (SiNx). 
     The supports  6  are members that support the movable electrode  4  and are made of the same conductive material as that of the movable electrode  4 . The two supports  6  respectively support the ends of the movable electrode  4  in the long side direction. A predetermined wiring pattern (not shown) that is electrically connected to the movable electrode  4  through the supports  6 , the signal line  2  and the ground electrode  3  is provided on the upper face of the substrate  1 . 
     Note that the size, shape, material, arrangement, and the like, of the substrate  1 , signal line  2 , ground electrode  3 , movable electrode  4 , dielectric layers  5  and supports  6  are not limited to the above. 
     The variable capacitor A 1  is manufactured by means of a so-called MEMS technology. That is, first, the signal line  2 , the ground electrode  3  and the dielectric layers  5  are laminated on the substrate  1 . Next, a sacrificial film is formed and then the movable electrode  4  is formed. Lastly, the sacrificial film is removed through wet etching. By these processes, a movable space for the movable electrode  4  is formed. The distance L 1  between the movable electrode  4  and the signal line  2  or the ground electrode  3  is adjusted by adjusting the thickness of the sacrificial film. Note that the manufacturing method for the variable capacitor A 1  is not limited to the above. 
       FIG. 3  shows an equivalent circuit diagram of the variable capacitor A 1  which is connected to a DC power that supplies a driving voltage to the variable capacitor A 1 . In the drawing, the variable capacitor A 1  is represented by two variable capacitors  11  and  12  that are connected in series. The signal line  2  and the movable electrode  4  form the variable capacitor  11 . The ground electrode  3  and the movable electrode  4  form the variable capacitor  12 . The movable electrode  4  of the variable capacitor A 1  is connected to a DC power  13  through an inductor  14 , which serves as an RF block. 
     The movable electrode  4  is applied with a driving voltage from the DC power  13 , and the variable capacitor A 1  varies its capacitance according to the characteristic shown in  FIG. 30 . When the driving voltage is Voff, the movable electrode  4  is apart from the signal line  2  and the ground electrode  3 , because the electrostatic attraction is small. Thus, the capacitance of the variable capacitor A 1  is minimal. On the other hand, when the driving voltage is Von, the movable electrode  4  is in contact with the signal line  2  and the ground electrode  3  via the dielectric layers  5 , because the electrostatic attraction is large. Thus, the capacitance of the variable capacitor A 1  is maximal. Consequently, the variable capacitor A 1  operates as a capacitive switch in such a manner that the driving voltage applied from the DC power  13  is switched between Voff and Von. 
     As shown in  FIG. 3 , the variable capacitor A 1  is equivalently configured so that the two variable capacitors  11  and  12  are connected in series. Thus, a voltage generated on the signal line  2  by an RF signal is divided by the variable capacitors  11  and  12 . Hence, voltages applied to both ends of each of the variable capacitors  11  and  12  (a voltage between the movable electrode  4  and the signal line  2  and a voltage between the movable electrode  4  and the ground electrode  3 ) are about half the generated voltage when the capacitances of the variable capacitors  11  and  12  are equal. Thus, it is possible to set the driving voltage Voff of the variable capacitor A 1  to be smaller than the driving voltage Voff of the existing variable capacitor F 1  (see  FIG. 28  and  FIG. 29 ). In this case, because the driving voltage Von may also be reduced, it is possible to suppress occurrence of electrification phenomenon of the dielectric layers  5 . In addition, because only about half the voltage generated on the signal line  2  by an RF signal is applied to both ends of each of the variable capacitors  11  and  12 , smaller fluctuations in voltage of the signal line  2  reduce a load applied on the variable capacitors  11  and  12 . Therefore, it is possible to suppress a variation over time of the elastic constant of the movable electrode  4  to a lesser degree. Thus, it is possible to keep reliability of the variable capacitor A 1 . 
     In addition, since the variable capacitor  11  can block a driving voltage applied from the DC power  13 , it is possible to prevent other electric circuits formed on the substrate from being applied with the driving voltage through the signal line  2 . That is, because the variable capacitor  11  functions as a DC block, it is not necessary to provide a capacitor, which serves as a DC block, that is necessary when the existing variable capacitor F 1  is used. Thus, it is possible to reduce the size of a device that employs the variable capacitor A 1 . 
       FIG. 4  is a cross-sectional view of a first alternative example of the variable capacitor A 1 . The plan view of the first alternative example is the same as that of  FIG. 1 , and, therefore, is omitted, and only the cross-sectional view corresponds to  FIG. 2  is shown. A variable capacitor A 2  shown in  FIG. 4  differs from the variable capacitor A 1  in that a support  6 ′ is provided between the signal line  2  and the ground electrode  3 . Because the movable electrode  4  of the variable capacitor A 2  is supported by the support  6 ′ around the center in the long side direction, it is possible to suppress warpage due to residual stress, or the like, in a manufacturing process. Thus, fraction defective of the variable capacitor A 2  due to warpage of the movable electrode  4  is reduced as compared with the variable capacitor A 1 . Thus, it is possible to improve yields. 
       FIG. 5  is a plan view that shows a second alternative example of the variable capacitor A 1 . Note that a cross-sectional view that corresponds to  FIG. 2  of the variable capacitor A 1  is omitted. A variable capacitor A 3  shown in  FIG. 5  differs from the variable capacitor A 1  in that a ground electrode  3 ′ is provided on a side (upper side in  FIG. 5 ) opposite to the side of the ground electrode  3  with respect to the signal line  2 . The signal line  2  and ground electrodes  3  and  3 ′ of the variable capacitor A 3  form a coplanar waveguide (CPW) structure. Thus, in the variable capacitor A 3 , it is possible to easily adjust the designed impedance of the signal line  2 . 
       FIG. 6  and  FIG. 7  are views that show a third alternative example of the variable capacitor A 1 .  FIG. 6  is a plan view of a variable capacitor A 4 .  FIG. 7  is a cross-sectional view taken along the line VII-VII in  FIG. 6 . The variable capacitor A 4  shown in  FIG. 6  and  FIG. 7  differs from the variable capacitor A 1  in that a recess Ia is provided on the upper face of the substrate  1 . The recess Ia is provided below portions of the signal line  2  and ground electrode  3 , facing the movable electrode  4 . 
     Owing to the recess  1   a , the portions of the signal line  2  and ground electrode  3 , facing the movable electrode  4 , are movable in a vertical direction (top-bottom direction in  FIG. 7 ) with respect to the upper face of the substrate  1 . Because the movable electrode  4 , the signal line  2 , and the ground electrode  3  are movable, it is possible to reduce electrostatic attraction necessary to attract the movable electrode  4  and the signal line  2  and ground electrode  3  to each other. Thus, it is possible to suppress a driving voltage applied to the movable electrode  4  to a lesser magnitude. In addition, since the recess  1   a  decreases the area in which the signal line  2  and ground electrode  3  contact the substrate  1 , electric charge that leaks from the signal line  2  or the ground electrode  3  to the substrate  1  may be suppressed. Consequently, it is possible to improve the Q value of the variable capacitor A 4 . 
     Note that in the third alternative example, the recess  1   a  is formed as a single large recess, but it is not limited to it. For example, a recess  1   a  may be provided separately below a portion of the signal line  2 , facing the movable electrode  4 , and below a portion of the ground electrode  3 , facing the movable electrode  4 . In addition, even when no recess  1   a  is provided, it is possible to obtain the advantageous effect that a driving voltage applied to the movable electrode  4  may be suppressed to a lesser magnitude, if the portions of the signal line  2  and ground electrode  3 , facing the movable electrode  4 , are configured to be movable in the vertical direction. At this time, it is applicable that only one of them is movable and the other one is fixed. 
       FIG. 8  is a cross-sectional view that shows a fourth alternative example of the variable capacitor A 1 . Note that the plan view of the fourth alternative example is the same as that of  FIG. 6 , and, therefore, is omitted, and only the cross-sectional view corresponds to  FIG. 7  is shown. A variable capacitor A 5  shown in  FIG. 8  differs from the variable capacitor A 1  in that the support  6 ′ is provided between the signal line  2  and the ground electrode  3  and the recesses  1   a  are provided on the upper face of the substrate  1 . The recesses  1   a  are respectively provided below portions of the signal line  2  and ground electrode  3 , facing the movable electrode  4 . The variable capacitor A 5  has the advantageous effects of both the first alternative example and the third alternative example. 
     The dielectric layers  5  are provided on the upper faces of the signal line  2  and ground electrode  3  in the above described first embodiment and its alternative examples. However, the portion of forming the dielectric layer is not limited to on the upper faces of the signal line  2  and ground electrode  3 . The dielectric layers  5  may also be provided on the lower face of the movable electrode  4 . Further, the dielectric layer  5  may be provided only on the lower face of the movable electrode  4  without providing on the upper faces of the signal line  2  and ground electrode  3 . In addition, when it is configured so that the movable electrode  4  do not contact the signal line  2  or the ground electrode  3  even when a driving voltage is maximal, the dielectric layers  5  need not be provided. In addition, it is applicable that the dielectric layer  5  is provided for only one of the variable capacitors  11  and  12  (see  FIG. 3 ) when the capacitance values of the variable capacitors  11  and  12  are differentiated. 
     In the above described first embodiment, it is described that the variable capacitor is used in parallel connection; instead, a variable capacitor used in series connection will be described below. 
       FIG. 9  is a plan view for illustrating a second embodiment of a variable capacitor and shows a variable capacitor B 1  used in series connection. In the drawing, the same or similar components to those of the first embodiment are assigned with the same references. The variable capacitor B 1  differs from the variable capacitor A 1  in that a signal line  2 ′ is provided in place of the ground electrode  3  and in the shape of the signal lines  2  and  2 ′. 
     As shown in  FIG. 9 , in the variable capacitor B 1 , the signal line  2  extends from a vicinity of one end (left side in  FIG. 9 ) of the substrate  1  in the long side direction to a portion that faces the movable electrode  4 . The signal line  2 ′ extends from a vicinity of the other end (right side in  FIG. 9 ) of the substrate  1  in the long side direction to a portion that faces the movable electrode  4 . The above configuration is to not produce the parasitic capacitance, while parallel extended portions between signal lines  2  and  2 ′ as shown in  FIG. 1  produce a parasitic capacitance. 
       FIG. 10  shows an equivalent circuit diagram of the variable capacitor B 1  which is connected to a DC power for driving the variable capacitor B 1 . The variable capacitor B 1  is represented by two variable capacitors  11 ′ and  12 ′ connected in series. The signal line  2  and the movable electrode  4  form the variable capacitor  11 ′. The signal line  2 ′ and the movable electrode  4  form the variable capacitor  12 ′. The movable electrode  4  is connected to the DC power  13  through the inductor  14 , which serves as an RF block. 
     The variable capacitor B 1  differs from the variable capacitor A 1  in that it is used in series connection with a signal line for an RF signal flowing from the signal line  2  to the signal line  2 ′. However, the variable capacitor B 1  has a commonality with the variable capacitor A 1  in that it operates as a capacitive switch by switching a driving voltage applied from the DC power  13  between Voff and Von. 
     In addition, as shown in  FIG. 10 , the variable capacitor B 1  is configured so that the two variable capacitors  11 ′ and  12 ′ are connected in series. Thus, a driving voltage applied from the DC power  13  is blocked by the variable capacitors  11 ′ and  12 ′. Thus It is possible to prevent other electric circuits formed on the substrate from being applied with the driving voltage through the signal lines  2  and  2 ′. Since the variable capacitors  11 ′ and  12 ′ function as a DC block, it is not necessary to provide a capacitor as a DC block, which is necessary when the existing variable capacitor F 1  is used. Thus, it is possible to reduce the size of a device that employs the variable capacitor B 1 . 
     In the second embodiment as well, by providing the support  6 ′ between the signal line  2  and the signal line  2 ′ (see  FIG. 4 ), it is possible to obtain the same advantageous effect as that of the first alternative example of the first embodiment. In addition, by providing the recess  1   a  on the upper face of the substrate  1  (see  FIGS. 6 and 7 ), it is possible to obtain the same advantageous effect as that of the third alternative example of the first embodiment. Furthermore, by providing the support  6 ′ between the signal line  2  and the signal line  2 ′ and providing the recess  1   a  on the upper face of the substrate  1  (see  FIG. 8 ), it is possible to obtain the same advantageous effect as that of the fourth alternative example of the first embodiment. 
     In the above described second embodiment as well, the dielectric layers  5  may also be provided on the lower face of the movable electrode  4 , or may be provided only on the lower face of the movable electrode  4  instead of being provided on the both upper faces of the signal lines  2  and  2 ′. In addition, the dielectric layers  5  need not be provided when the variable capacitor B 1  is configured so that the movable electrode  4  do not contact the signal line  2  or  2 ′ at an application of a maximal driving voltage. In addition, it is applicable that the dielectric layer  5  is provided for only one of the variable capacitors  11 ′ and  12 ′ (see  FIG. 10 ) when the capacitance values of the variable capacitors  11 ′ and  12 ′ are differentiated, or the like. 
     Note that the shape of the signal lines  2  and  2 ′ is not limited to the one described in the second embodiment. If a parasitic capacitance may be ignored, the signal lines  2  and  2 ′ may be extended to near both ends of the substrate  1  in the long side direction. In addition, the shape of the movable electrode  4  is also not limited to the one described in the second embodiment. 
       FIGS. 11 ,  12  and  13  are views for illustrating a third embodiment of a variable capacitor.  FIG. 11  is a plan view of a variable capacitor C 1 ;  FIG. 12  a cross-sectional view taken along the line XII-XII in  FIG. 11 ;  FIG. 13  a cross-sectional view taken along the line XIII-XIII in  FIG. 11 . In the figures, the same or similar components to those of the second embodiment are assigned with the same references. The variable capacitor C 1  differs from the variable capacitor B 1  in the shape of the signal lines  2  and  2 ′ and the shape of a movable electrode  4 ′. 
     The signal line  2  is provided parallel to the long side of the substrate  1  at the center in the short side direction so as to extend from a vicinity of one end in the long side direction to a vicinity back from the center. The signal line  2 ′ is provided parallel to the long side of the substrate  1  at the center in the short side direction so as to extend from a vicinity of the other end in the long side direction to a vicinity back from the center. A predetermined gap is provided between the signal line  2  and the signal line  2 ′ to prevent electrical connection and development of parasitic capacitance. The movable electrode  4 ′ forms a substantially rectangle or square shape in plan view and is provided around the center of the substrate  1 . Further the movable electrode  4 ′ is bridged with the pair of supports  6  provided near both ends in the short side direction, and is movable in a vertical direction with respect to the upper face of the substrate  1 . The movable electrode  4 ′ has portions facing the signal lines  2  and  2 ′. The dielectric layers  5  are provided on the upper faces of the portions of the signal lines  2  and  2 ′, facing the movable electrode  4 ′. 
     Since the variable capacitor C 1  is formed so that only the shape of the signal lines  2 ,  2 ′ and movable electrode  4 ′ of the variable capacitor B 1  are modified, the equivalent circuit diagram connected to a DC power for supplying a driving voltage is the same as that of  FIG. 10 . Thus, the variable capacitor C 1  also does not need a capacitor, which serves as a DC block. Accordingly the variable capacitor C 1  has the same advantageous effect as that of the variable capacitor B 1  That is, the size of a device using the variable capacitor C 1  may be reduced. 
       FIG. 14  is a plan view that shows a first alternative example of the variable capacitor C 1 . Note that cross-sectional views that correspond to  FIGS. 12 and 13  of the variable capacitor C 1  are omitted. A variable capacitor C 2  differs from the variable capacitor C 1  in that a pair of ground electrodes  3 ′ are formed so that the signal lines  2  and  2 ′ are placed between the pair of ground electrodes  3 ′. The signal lines  2 ,  2 ′ and the pair of ground electrodes  3 ′ of the variable capacitor C 2  form a CPW structure. Thus, it is possible to easily adjust the designed impedance of the signal lines  2  and  2 ′ in the variable capacitor C 2  as well. 
       FIG. 15  is a cross-sectional view that shows a second alternative example of the variable capacitor C 1 . Note that the plan view that corresponds to  FIG. 11  of the variable capacitor C 1  and the cross-sectional view that corresponds to  FIG. 12  of the variable capacitor C 1  are omitted. A variable capacitor C 3  shown in  FIG. 15  differs from the variable capacitor C 1  in that the recess  1   a  is provided on the upper face of the substrate  1 . The recess  1   a  is provided below portions of the signal lines  2  and  2 ′, facing the movable electrode  4 . 
     Owing to the recess  1   a , the portions of the signal lines  2  and  2 ′, facing the movable electrode  4 , are movable in a vertical direction (top-bottom direction in  FIG. 15 ) with respect to the upper face of the substrate  1 . It is possible to suppress a driving voltage applied to the movable electrode  4 ′ to a lesser magnitude, because the movable electrode  4 , the signal lines  2  and  2 ′ are movable, as in the case of the variable capacitor A 4  shown in  FIGS. 6 and 7 . In addition, the recess  1   a  decreases the area in which the signal lines  2  and  2 ′ contact the substrate  1 . Accordingly, electric charge leaking from the signal line  2  or  2 ′ to the substrate  1  may be suppressed. Consequently, it is possible to improve the Q value of the variable capacitor C 3 . 
     Note that in the second alternative example, the recess  1   a  is formed as a single large recess, but it is not limited to it. For example, a recess  1   a  may be provided separately below a portion of the signal line  2 , facing the movable electrode  4 ′, and below a portion of the signal line  2 ′, facing the movable electrode  4 ′. In addition, even when no recess  1   a  is provided, it is possible to obtain the advantageous effect that a driving voltage applied to the movable electrode  4 ′ may be suppressed to a lesser magnitude by the configuration in which portions of the signal lines  2  and  2 ′, facing the movable electrode  4 ′, are movable in the vertical direction. At this configuration, it is applicable that only one of the portions is movable and the other one is fixed. 
     In the above described third embodiment and its alternative examples as well, the dielectric layers  5  may also be provided on the lower face of the movable electrode  4 ′, or may be not provided on the upper faces of the signal lines  2  and  2 ′ but provided only on the lower face of the movable electrode  4 ′. In addition, when it is configured so that the movable electrode  4 ′ do not contact the signal line  2  or  2 ′ even at a maximal driving voltage, the dielectric layers  5  need not be provided. In addition, when the capacitance values of the variable capacitor  11 ′ and variable capacitor  12 ′ (see  FIG. 10 ) are differentiated, or the like, it is applicable that the dielectric layer  5  is provided for only one of them and no dielectric layer  5  is provided for the other one. 
     In addition, in the variable capacitor used in series connection, the shape and arrangement of each electrode may be the same as those of the third embodiment. 
       FIG. 16  is a view for illustrating a fourth embodiment of a variable capacitor according to the invention and is a plan view that shows a variable capacitor D 1  used in parallel connection. In  FIG. 16 , the same or similar components to those of the third embodiment are assigned with the same references. The variable capacitor D 1  differs from the variable capacitor C 1  in that the ground electrode  3  is provided in place of the signal line  2 ′ and in the shapes of the signal line  2  and the ground electrode  3 . 
     As shown in  FIG. 16 , the signal line  2  and the ground electrode  3  each have a T-shape in plan view; instead, they may be formed in the same shape as the signal lines  2  and  2 ′ of the variable capacitor C 1 . 
     The variable capacitor D 1  is formed so that only the shapes of the signal line  2 , ground electrode  3  and movable electrode  4  of the variable capacitor A 1  are modified. Accordingly, the equivalent circuit diagram connected to a DC power that supplies a driving voltage is the same as that of  FIG. 3 . Thus, since a voltage generated on the signal line  2  by an RF signal is divided by the variable capacitors  11  and  12 , voltages applied to both ends of each of the variable capacitors  11  and  12  (a voltage between the movable electrode  4  and the signal line  2 ; and a voltage between the movable electrode  4  and the ground electrode  3 ) are about half the generated voltage. Thus, the variable capacitor D 1  also has the same advantageous effect as that of the variable capacitor A 1 . 
     In the fourth embodiment as well, by providing the recess  1   a  on the upper face of the substrate  1  (see  FIG. 15 ), it is possible to obtain the same advantageous effect as that of the second alternative example of the third embodiment. 
     In the above described fourth embodiment as well, the dielectric layers  5  may also be provided on the lower face of the movable electrode  4 ′, or may be not provided on the upper faces of the signal line  2  and ground electrode  3  but provided only on the lower face of the movable electrode  4 ′. In addition, when it is configured so that the movable electrode  4 ′ do not contact the signal line  2  or the ground electrode  3  even when a driving voltage is maximal, the dielectric layers  5  need not be provided. In addition, when the capacitance values of the variable capacitor  11  and variable capacitor  12  (see  FIG. 3 ) are differentiated, it is applicable that the dielectric layer  5  is provided for only one of them. 
     Note that in the above described first to fourth embodiments, the case in which the variable capacitor is used as a capacitive switch is described, but it is not limited to it. When it is configured so that the capacitance smoothly varies in accordance with a change in a driving voltage applied to the movable electrode  4  or  4 ′, it may be used as a variable capacitor that controls the capacitance using the driving voltage. 
     Next, the matching circuit element that employs a variable capacitor will be described. 
       FIG. 17  is a plan view that shows a Γ matching circuit element E 1  in which an inductor  2   a  is provided in the signal line  2  of the variable capacitor A 3  shown in  FIG. 5 .  FIG. 18  shows an equivalent circuit diagram which shows the Γ matching circuit element E 1  and a DC power for supplying a driving voltage to the Γ matching circuit element E 1 .  FIG. 19  is a plan view that shows a Π matching circuit element E 2  in which a movable electrode  4  is further added to the variable capacitor A 3  shown in  FIG. 5  and an inductor  2   a  is provided in the signal line  2  at a position between the two movable electrodes  4 .  FIG. 20  shows an equivalent circuit diagram which shows a DC power for supplying a driving voltage and the Π matching circuit element E 2  connected to the DC power. 
       FIG. 21  is a plan view that shows a Γ matching circuit element E 3  in which an inductor  2   a  is provided in the signal line  2  of the variable capacitor D 1  shown in  FIG. 16 . The equivalent circuit diagram which shows a DC power for supplying a driving voltage to the Γ matching circuit element E 3  and the Γ matching circuit element E 3  connected to the DC power is the same as that of  FIG. 18 .  FIG. 22  is a plan view that shows a Π matching circuit element E 4  in which a movable electrode  4 ′ is further added to the variable capacitor D 1  shown in  FIG. 16  and an inductor  2   a  is provided in the signal line  2  at a position between the two movable electrodes  4 ′. The equivalent circuit diagram when a DC power that supplies a driving voltage to the Π matching circuit element E 4  is the same as that of  FIG. 20 . 
     These matching circuit elements employ the variable capacitor according to the above described first embodiment or fourth embodiment. Accordingly, it is possible to set a driving voltage to a lesser magnitude as compared with the existing matching circuit element, and it is possible to keep reliability. In addition, these matching circuit elements may be reduced in size as compared with the existing matching circuit element. 
       FIG. 23  is a plan view that shows a Γ matching circuit element F 1  in which an inductor  2   a  is provided between the signal line  2  and ground electrode  3 ′ of the variable capacitor C 2  shown in  FIG. 14 .  FIG. 24  shows an equivalent circuit diagram which includes a DC power for supplying a driving voltage to the Γ matching circuit element F 1  and the Γ matching circuit element F 1  connected to the DC power.  FIG. 25  is a plan view that shows a Π matching circuit element F 2  in which an inductor  2   a  and an inductor  2 ′ a  are respectively provided between the signal line  2  and ground electrode  3 ′ and between the signal line  2 ′ and ground electrode  3 ′ of the variable capacitor C 2  shown in  FIG. 14 .  FIG. 26  shows an equivalent circuit diagram which includes a DC power for supplying a driving voltage to the Π matching circuit element F 2  and the Π matching circuit element F 2  connected to the DC power. 
     Because these matching circuit elements employ the variable capacitor according to the above described third embodiment, the size may be reduced as compared with the existing matching circuit element. 
     The variable capacitor and matching circuit element according to the embodiments may be used as an electronic component of a mobile terminal apparatus.  FIG. 27  is a block diagram that shows a front end portion of a mobile terminal apparatus in which the variable capacitors and matching circuit elements according to the embodiments are used as electronic components. 
     The front end portion G 1  of the mobile terminal apparatus includes an adaptive array antenna G 101  that uses a phaser, a tunable filter G 102 , a duplexer G 104 , impedance matching circuits G 103 , G 105  and G 107 , amplifiers G 106  and G 108  each having a matching circuit, and a variable frequency oscillator G 109  formed of a Voltage Controlled Oscillator (VCO). 
     In the front end portion G 1 , an RF signal (receiving signal) of a specific receiving band is extracted by the tunable filter G 102  from an RF signal received by the adaptive array antenna G 101 , and is input to the amplifier G 108  by the duplexer G 104 . The receiving signal is amplified in the amplifier G 108 , then mixed with a local signal output from the variable frequency oscillator G 109  and converted to a predetermined intermediate frequency. Then, the converted signal is output to a receiving portion (not shown). In addition, an RF signal (transmitting signal) input from a transmitting portion (not shown) is amplified by the amplifier G 106  and then input to the tunable filter G 102  through the duplexer G 104 . The transmitting signal is radiated from the adaptive array antenna G 101  after frequencies outside a specific transmitting band is removed by the tunable filter G 102 . The configurations and functions of the components G 101  to G 109  are typical, and therefore description thereof is omitted. 
     The variable capacitor according to the embodiments is used as an element for varying the frequency of the tunable filter G 102  and the frequency of the variable frequency oscillator G 109  in the front end portion G 1  of the mobile terminal apparatus shown in  FIG. 27 . The matching circuit element according to the embodiments is used as the phaser of the adaptive array antenna G 101 , the impedance matching circuits G 103 , G 105  and G 107  and the matching circuits of the amplifiers G 106  and G 108 . Thus, because it is possible to design the front end portion G 1  to be smaller than that using the existing variable capacitor and matching circuit element, it is possible to reduce the size of a mobile terminal apparatus. In addition, because it is possible to set the driving voltage of each variable capacitor and each matching circuit element to a lesser magnitude, it is possible to keep reliability. 
     Note that it is not necessary to use the variable capacitor and the matching circuit element according to the invention in all the electronic components shown in  FIG. 27 ; a mobile terminal apparatus that partially employs the variable capacitor and the matching circuit element according to the invention may also have the same advantageous effect. 
     The variable capacitor, matching circuit element and mobile terminal apparatus according to the invention are not limited to the above described embodiments. The specific configuration of each component of the variable capacitor, matching circuit element and mobile terminal apparatus may be variously changed in design.