Abstract:
An electronic device having a variable capacitance element, includes a support substrate providing physical support, a pair of anchors formed on the support substrate, and having support portions in a direction perpendicular to a surface of the substrate, a movable electrode supported by the support portions of the pair of anchors, having opposing first and second side surfaces constituting electrode surfaces, and at least partially capable of elastic deformation, a first fixed electrode supported above the support substrate, and having a first electrode surface opposing to the first side surface of the movable electrode, and a second fixed electrode supported above the support substrate, and having a second electrode surface opposing to the second side surface of the movable electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Patent Application PCT/JP2011/001542, filed on Mar. 16, 2011. 
    
    
     FIELD 
     Embodiments of this invention relate to electronic devices having variable capacitance element and manufacture methods thereof. 
     BACKGROUND 
     A variable capacitance element generally has such a structure in which a stationary or fixed electrode and a movable electrode are disposed facing each other, and the capacitance is varied by displacing the movable electrode. The movable electrode can be displaced by piezoelectric drive, electrostatic drive, etc. In mobile electronic devices, miniaturization and reduction of weight are required, and variable capacitance elements using MEMS (micro electro-mechanical system) are being developed. 
     Such a structure is known wherein a stationary or fixed electrode is formed on a support substrate, a movable electrode is supported by a flexible beam, etc. above the stationary electrode, and the capacitance is varied by controlling the distance between the electrodes (for example, see JP-A 2006-147995). 
       FIG. 7A  illustrates a structural example of such variable capacitance element. A variable capacitance element is formed by a variable capacitor with parallel plate structure one electrode of which is made movable, and a container structure sealing this variable capacitor. 
     A stationary electrode  103  and anchors  106  are formed on a semiconductor substrate  101  of such as silicon, via an insulating layer  102 . The anchors  106  support a plate-shaped movable electrode  104  above the stationary electrode  103  via U-shaped flexible beams  105 . The container including sidewall  110  and ceiling  111  is formed to surround outer periphery of the variable capacitor. By the existence of this container, it becomes possible to seal the variable capacitor in an inert gas atmosphere such as rare gas, or in a reduced pressure atmosphere. When the container is made of metal material, electric shield also becomes possible. 
     When voltage V is applied between the stationary electrode  103  and the movable electrode  104 , the movable electrode  104  is attracted toward the statinary electrode  103  by the electrostatic force. When the movable electrode  104  is displaced toward the stationary electrode  103 , the flexible beams  105  are bent. Restoring force proportional to the amount of displacement works to return the movable electrode  104  back to the original position. The movable electrode  104  is displaced up to the balanced position where the electrostatic force and the restoring force balance each other, and is held at the balanced position as long as the voltage V is applied. 
     When the voltage V is reduced to zero, the movable electrode  104  returns to the original position. Therefore, the capacitance element constituted of the stationary electrode  103  and the movable electrode  104 , works as a variable capacitance element the static capacitance of which can be controlled by the applied voltage V. 
       FIG. 7B  is a cross-section of another structural example of variable capacitance element. On a semiconductor substrate  101  of such as silicon, a stationary or fixed electrode  103  is formed through an insulating layer  102 , and another insulating layer  112  is formed on the insulating layer  102  to cover the stationary electrode  103 . Anchors  106  are formed on the insulating layer  112 . The anchors  106  support a plate shaped movable electrode  104  via flexible beams  105  above the stationary electrode  103  via the insulating layer  112 . A container including sidewall  110  and ceiling  111  is formed to surround the outer periphery of the variable capacitor. Since the surface of the stationary electrode  103  is covered with the insulating layer  112 , short-circuit and sticking between the electrodes can be suppressed. 
     In a digital type variable capacitor element, capacitance formed in a state where the movable electrode is separated from the fixed electrode, is the minimum value (off state), and capacitance formed in a state where the movable electrode touches the fixed electrode through a dielectric film, is the maximum value (on state). These two states are used as a variable capacitance. 
     The electrode of a capacitor can be formed not only parallel to the substrate surface, but also be formed perpendicular or vertical to the substrate surface (for example, see JP-A 2001-304868)). For example, a variable capacitor having electrodes perpendicular to the substrate surface can be formed using an SOI (silicon-on-insulator) substrate in which a single crystal silicon layer is provided above upper surface of a single crystal silicon substrate via a silicon oxide film serving as a binding layer. 
     Impurity atoms such as phosphor and boron are doped in the single crystal silicon layer to reduce the resistance of the single crystal silicon layer. A resist mask is formed on the single crystal silicon layer, and the single crystal silicon layer is etched by reactive ion etching, etc. leaving anchors, various comb shaped electrodes, and various pad portions on the silicon oxide film. The comb shaped electrodes are coupled in inter digital shape to form a capacitor. The respective electrodes are shaped perpendicular to the silicon substrate surface. 
     The silicon oxide film can be removed by selective etching by frolic acid aqueous solution, etc. to separate the active silicon layer from the support Si substrate, to give freedom of displacement. Such structures as vibrators, beams, and comb shaped electrodes can be formed. Conductor such as aluminum is vapor deposited on various pad portions to form electrode pads. Such a structure is obtained in which respective portions formed above the substrate are constituted of low resistivity layers insulated from the substrate, and vibrators, beams, comb shaped electrodes etc. are positioned floating above the substrate by a predetermined distance, and are supported by the substrate to be capable of vibration via the anchors. 
     Patent document 1: JP-A 2006-147995, 
     Patent document 2: LP-A 2001-304868 
     SUMMARY 
     According to one aspect of this invention, there is provided an electronic device having a variable capacitance element, including: 
     a support substrate providing physical support, 
     a pair of anchors formed on the support substrate, and having support portions in a direction perpendicular to a surface of the substrate, 
     a movable electrode supported by the support portions of the pair of anchors, having opposing first and second side surfaces constituting electrode surfaces, and at least partially capable of elastic deformation, 
     a first fixed electrode supported above the support substrate, and having a first electrode surface opposing to the first side surface of the movable electrode, and 
     a second fixed electrode supported above the support substrate, and having a second electrode surface opposing to the second side surface of the movable electrode. 
     According to another aspect of this invention, there is provided a method for manufacturing an electronic device having a variable capacitance, including: 
     preparing a substrate including a sacrificial layer on a support substrate, forming a first mask having apertures of shapes of opposing fixed electrodes on a surface of the substrate, 
     etching the sacrificial layer exposed in the apertures of the first mask, to form trenches for accommodating fixed electrodes, 
     forming a second mask having a slit shaped aperture of shape of a movable electrode disposed between the fixed electrodes, on the surface of the substrate, 
     etching the sacrificial layer exposed in the slit shaped aperture, to form a slit for accommodating a movable electrode, and forming conductive members in the trenches and in the slit. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are a plan view and a cross section schematically illustrating structure of a variable capacitance element according to a first embodiment, and  FIGS. 1C and 1D  are plan views illustrating operations of actions. 
         FIGS. 2A, 2B, and 2C  are a perspective view schematically illustrating structure of an electronic device having a variable capacitance element according to a second embodiment, and plan views illustrating two states of the variable capacitance element. 
         FIGS. 3A-3L  are cross sections illustrating manufacturing processes of an electronic device having a variable capacitance element according to the second embodiment. 
         FIGS. 4A-4E  are plan views of the states of  FIGS. 3C, 3D, 3E, 33, and 3K . 
         FIGS. 5A and 5B  are equivalent circuit diagrams illustrating two examples of applied circuits of an electronic device having a variable capacitance element. 
         FIGS. 6A-6D  are schematic plan views illustrating an electronic device having a variable capacitance element provided with stoppers. 
         FIGS. 7A and 7B  are cross sections illustrating structural examples of the variable capacitance element according to prior art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Even in the case wherein a fixed or stationary electrode is covered with a dielectric film, the dielectric film may be charged up along with repeated on/off operations, and sticking phenomenon may occur in which the movable electrode cannot be separated from the dielectric film even when the external power source is turned off. Countermeasure by the drive waveform is being discussed, but does not reach a solution. 
     In the case wherein the envelope of high frequency signal is modulated by a signal waveform, and is applied to a movable electrode, there is a phenomenon called self actuation in which the movable electrode moves by the voltage difference based on the signal waveform. There is a method of increasing the drive voltage in response to electric power of the thrown signal, for preventing self actuation. When the drive voltage is increased, sticking can more easily occur. Also, a booster circuit may become necessary for securing a higher voltage. 
     The present inventors thought of a structure wherein first and second fixed or stationary electrodes are located on both sides of a movable electrode, the movable electrode is attracted to the first fixed electrode through an insulating film in “on” state, and the movable electrode is attracted to the second fixed electrode through an insulating film in “off” state. In either of “on” and “off” states, the movable electrode is attracted to one of the two fixed electrodes, and the capacitance will not change. 
     Both the transition from “off” state to “on” state and the transition from “on” state to “off” state, can be positively performed by electrostatic attractive force by the voltage applied between the movable electrode and the first or the second fixed electrode. Even in the case wherein sticking phenomenon occurs in which the movable electrode is attracted to one fixed electrode, and cannot be separated therefrom, it becomes easier to separate the movable electrode by utilizing electrostatic attractive force by applying a voltage between the other fixed electrode and the movable electrode. Sticking can be suppressed. Since the movable electrode does not displace except the transient state, basically self actuation can also be suppressed. improvement in drive reliability and reduction in drive voltage can be expected. 
     One of the first and the second fixed electrodes may be a dummy electrode which does not function as electric circuit. Of course, the first and the second fixed electrodes may be positively utilized as two variable capacitors which have symmetric “on”/“off” states. 
     When the first and the second fixed electrodes are disposed in parallel, and the movable electrode is disposed near the first fixed electrode at one end and near the second fixed electrode at the other end, further effect will be obtained. In case the movable electrode is attracted to the first fixed electrode, when a voltage is applied between the second fixed electrode and the movable electrode, this voltage generates a strong electrostatic attraction force, reversely proportional to the distance, in a region where the movable electrode and the second fixed electrode are near (at the other end of the movable electrode). Therefore, it becomes easier to separate the movable electrode from the other end. When the movable electrode is attracted to the second fixed electrode, it becomes easy to separate the movable electrode from one end by applying a voltage between the first fixed electrode and the movable electrode, by similar reason. 
     In case of forming a plate shaped electrode on a surface of a semiconductor substrate, it would be not easy to form a movable electrode in oblique relationship to a surface of a fixed electrode. In case of using an SOI substrate and forming electrodes in substantially vertical or perpendicular direction relative to a surface of a semiconductor substrate, an oblique electrode between parallel electrodes would also be realized only by changing the pattern configulation. 
     Variable capacitance elements according to the embodiments will be described hereinafter referring to the drawings. 
       FIGS. 1A and 1B  are a schematic plan view and a schematic cross section illustrating basic structure of a variable capacitance element according to an embodiment. Between anchors ANC 1  and ANC 2 , a movable electrode ME having at least partially flexible portion is supported. As illustrated in  FIG. 1B , the anchors ANC 1  and ANC 2  are supported on a support substrate SS, and are formed of conducting material. The movable electrode ME is formed of, for example, a metal sheet capable of elastic deformation and oriented perpendicular to the surface of the support substrate SS, and is supported by the anchors ANC 1  and ANC 2 . Gap is formed between the lower edge of the movable electrode ME and the support substrate SS, allowing the movable electrode to be displaceable. As illustrated in  FIG. 1A , on both sides of the movable electrode ME, fixed electrodes FE 1  and FE 2  having side surfaces facing to the two electrode surfaces of the movable electrode, and having insulating films IF 1  and IF 2  on the opposing side surfaces, are disposed, and supported on the support substrate SS. Between the movable electrode ME and the fixed electrodes FE 1  and FE 2 , cavity (free space) denoted by CV in  FIG. 1A  is formed to secure a space in which the movable electrode is capable of displacement. 
     Structure of this kind can be manufactured, for example, by forming fixed electrodes by performing plating in a space defined by a resist mask on a support substrate, forming an insulating film, and forming anchors and a movable electrode by performing plating in a space defined by a resist mask on a support substrate again. Here, the metal sheet may include laminated metal layers, and may include an alloy layer. 
     As illustrated in  FIG. 1C , when a dc voltage is applied between the fixed electrode FE 1  and the movable electrode ME, electrostatic attraction is generated between the fixed electrode FE 1  and the movable electrode ME to pull the movable electrode ME toward the fixed electrode FE 1 . By forming the movable electrode ME in sufficiently deformable configulation, most area of the movable electrode ME facing the fixed electrode FE 1  adheres to the fixed electrode FE 1  through the insulating film IF 1 . 
     As illustrated in  FIG. 1D , the bias power source between the fixed electrode FE 1  and the movable electrode ME is turned off, and a dc voltage is applied between the fixed electrode FE 2  and the movable electrode ME. Electrostatic attraction between the movable electrode ME and the fixed electrode FE 1  disappears, and attraction between the movable electrode ME and the fixed electrode FE 2  is newly generated. The movable electrode ME is separated from the fixed electrode FE 1 , and is attracted toward the fixed electrode FE 2 , and adheres thereto. 
     Compared with the conventional case of separating a movable electrode from a fixed electrode only by elastic restoring force of the movable electrode, reliability of operation will be improved since the movable electrode is enforcedly pulled away from the fixed electrode by elastic restoring force and electrostatic attraction. 
     In the state of  FIG. 1C , the movable electrode ME is at a position separated from the second fixed electrode FE 2 , and in the state of  FIG. 1D , the movable electrode ME is at a position separated from the first fixed electrode FE 1 . when the distance is increased, electrostatic attraction decreases. If a portion of the movable electrode is restricted to a position near the first fixed electrode, and another portion of the movable electrode is restricted to a position near the second fixed electrode, positions can be secured where static attraction surely works. 
       FIG. 2A  is a schematic perspective view of a variable capacitance element according to a second embodiment. An SOI (silicon on insulator) substrate wherein an active Si layer  53  is bonded to a support Si substrate  51  with a bonding silicon oxide film  52 , is used. For example, the support Si substrate  51  has a thickness of 300 μm-500 μm, and the bonding silicon oxide film  52  has a thickness of 2 μm-7 μm. The active Si layer is a high resistivity single crystal Si layer of 500 Ωcm or higher, and has a thickness of 20 μm-30 μm. 
     Fixed electrodes  11  and  12  are formed, embedding trenches which penetrate the total thickness of the active Si layer  53 , with opposing side surfaces in parallel, sandwiching a movable electrode  10 . The heights of the opposing side surfaces of the movable electrode  10 , and the fixed electrodes  11  and  12  are 20 μm-30 μm, which is the same as the thickness of the active Si layer  53 . The fixed electrodes  11  and  12  have a length of, for example, 500 μm, and are opposedly disposed with a distance of 20 μm. The active Si layer  53  between the fixed electrodes  11  and  12  is removed, and the movable electrode  10  is disposed in the removed space. The bonding silicon oxide film  52  under the space for accommodating the movable electrode  10  is removed, to secure the freedom of the movable electrode  10 . The movable electrode  10  has, for example, a thickness of 2 μm-5 μm, and has a length longer than those of the fixed electrodes  11  and  12 . 
     The movable electrode  10  is supported by the anchors  16  and  17  at the two ends. The movable electrode  10 , the fixed electrodes  11  and  12 , and anchors  16  and  17 , include, for example Au or Cu as the main composition, and are made in the same plating process. The variable capacitor is constituted of the fixed electrodes  11  and  12 , the movable electrode  10 , and the anchors  16  and  17  supporting the movable electrode. On the upper surface (side surface as a capacitor) of one fixed electrode  11 , not facing the movable electrode, a dielectric film  18  of a thickness of 0.2 μm-0.5 μm, made for example of silicon oxide, silicon nitride, alumina, etc., is formed and an electrode  19  mainly composed of Au, Al, etc. is formed thereon, to constitute a fixed capacitor. Further, resistor elements  21  and  22  of Si—Cr alloy film extending from the upper surface of the fixed electrodes  11  and  12  to the exterior, are formed, and electrodes  23  and  24  are connected to the other ends of the resistor elements. The anchor  16  is connected to an electrode  25  between high frequency signal lines  31  and  32 . 
       FIG. 2B  will be referred to. An insulating film  13  is formed on the surface of the fixed electrodes for avoiding short-circuit between the electrodes. The surfaces of the fixed electrodes  11  and  12  are covered, for example, with an insulating film  13  of silicon nitride so that short-circuit between the movable electrode  10  and the fixed electrodes  11  and  12  is prevented. In this embodiment, the insulating film  13  is also formed on the surfaces of the anchors  16  and  17 , to enhance insulation between the electrodes of the variable capacitor and the surrounding active Si layer  53 . Any insulating film is formed on the surface of the movable electrode  10 , to secure elasticity of the movable electrode, and avoid peel-off of the insulating film. 
     The movable electrode  10  is disposed asymmetrically between side surfaces of the fixed electrodes  11  and  12  disposed in parallel, lower on the left side and higher on the right side in the figure. The movable electrode is so formed that it extends from the lower end of the anchor  16  in the figure to the upper end of the anchor  17  in the figure. Namely, left part of the movable electrode  10  near the anchor  16  is disposed nearer to the fixed electrode  12  than to the fixed electrode  11 , and right part of the movable electrode  10  near the anchor  17  is disposed nearer to the fixed electrode  11  than to the fixed electrode  12 . 
     When a voltage is applied between the movable electrode  10  and the fixed electrode  12 , the movable electrode  10  is attracted toward the fixed electrode  12  by the electrostatic attraction. At left part of the movable electrode  10  where the distance from the fixed electrode is short, the movable electrode  10  is attracted toward the fixed electrode  12 , and gradually more right part of the movable electrode  10  is attracted to the fixed electrode  12 . Since the right end of the movable electrode  10  is positioned near to the fixed electrode  11  than to the fixed electrode  12 , it is separated from the fixed electrode  12 . 
       FIG. 2C  will be referred to. When a voltage is applied between the movable electrode  10  and the fixed electrode  11 , the movable electrode  10  is attracted toward the fixed electrode  11  by the electrostatic attraction. Since the right end of the movable electrode  10  is positioned nearer to the fixed electrode  11  than to the fixed electrode  12 , it is swiftly attracted to the fixed electrode  11 , and gradually more left part of the movable electrode  10  is attracted to the fixed electrode  11 . 
     Thus, since the movable electrode is disposed obliquely between the directionally disposed fixed electrodes  11  and  12 , near the fixed electrode  11  on one side and near the fixed electrode  12  on the other side, there is a portion where attractive force easily acts in either case of being attracted to either fixed electrode, and the changing action can be swiftly performed. 
     Hereinafter, major processes of a method for manufacturing a semiconductor device including a variable capacitance element illustrated in  FIG. 52A  will be described referring to  FIGS. 3A-3L . 
     As illustrated in  FIG. 3A , an SOI substrate in which an active Si layer  53  of a thickness 25 μm having a high resistivity of 500 Ωcm or more is coupled with an Si substrate  51  of, for example a thickness of 300 μm-500 μm, via for example, a bonding silicon oxide film  52  of a thickness of about 5 μm, is prepared. 
     As illustrated in  FIG. 3B , a resist pattern PR 1  having apertures for defining trenches TR 1  and TR 2  for accommodating fixed electrodes is formed on the active Si layer  53 . The resist pattern PR 1  also has apertures for defining anchors. The active Si layer  53  is etched its full thickness using the resist pattern as a mask, for example by deep RIE. The deep RIE uses CF 4  (+O 2 ), SF 6  (+O 2 , or +H 2 ) as Si etching gas. Thereafter, the resist pattern PR 1  is removed. 
     As illustrated in  FIG. 4A , for example, the trenches TR 1  and TR 2  have parallel side surfaces of a length of 500 μm opposing each other at a distance of 20 μm. The reason of wider right trench TR 2  is to form a fixed capacitor thereon. Trenches TR 3  and TR 4  for anchors have structure for supporting the movable electrode between the fixed electrode. An electrode is connected to the upper anchor. 
     As illustrated in  FIG. 3C , a silicon nitride film  54  having a thickness 0.1 μm-0.5 μm is deposited on the substrate surface by CVD or low pressure (LP) CVD using silane series gas such as monosilane, disilane, etc. and ammonia gas. Exposed surfaces of the active Si layer  53  and the bonding silicon oxide film  52  are covered with the silicon nitride film  54 . This silicon nitride film  54  serves as an insulating film covering the surfaces of the fixed electrodes.  FIG. 4A  illustrates the silicon nitride film  54  deposited on the trench surfaces in penetrated viewing manner. 
     As illustrated in  FIG. 3D , a resist pattern PR 2  having an aperture defining a movable electrode is formed on the silicon nitride film  54 , and the active Si layer  53  exposed in the aperture is etched by its full thickness by deep RIE.  FIG. 4B  illustrates plan shape of the aperture. The insulating film is not formed on the surface of the movable electrode, by forming the slit for the movable electrode after deposition of the silicon nitride film  54 . Electric conduction between the movable electrode and the anchors is secured by overlapping the slit with part of the sidewall of the trenches for forming anchors. 
     The resist pattern PR 2  is removed to realize a state as illustrated in  FIGS. 3E and 4C . The active Si layer  53  is removed by its full thickness in the trenches TR 1 -TR 4 , and the silicon nitride film  54  is deposited on the inner surfaces of the trenches. The slit S does not have the silicon nitride film  54 , and penetrates the full thickness of the active Si layer  53  with a constant width, for example about 2 μm. 
     As illustrated in  FIG. 3F , on the surface of the substrate, for example a Ti layer is deposited to a thickness of the order of 50 nm, and an Au layer is deposited thereon to a thickness of the order of 500 nm to form a seed layer. In place of the Ti layer, a Cr layer with a thickness of the order of 50 nm can be used. The seed layer  55  serves as a current supplying layer in electrolytic plating. 
     As illustrated in  FIG. 3G , a resist pattern PR 3  is formed in the regions where plating is not necessary covering the seed layer  55 , an Au layer  56  is deposited by electrolytic plating, to embed the trenches TR, and the slit S. Here, in place of Au, Cu may be electrolytically plated. Thereafter, the resist pattern PR 3  is removed, and exposed seed layer  55  is removed by etching or milling. 
     As illustrated in  FIG. 3H , a resist pattern PR 4  for patterning a dielectric film is formed on the substrate formed with electrodes, a dielectric film  18  of silicon oxide film, silicon nitride film, or aluminum oxide film etc. is deposited by sputtering to a thickness of 0.2 μm-0.5 μm, and the dielectric film deposited on the resist pattern PR 4  is lifted off together with the resist pattern PR 4 . 
     As illustrated in  FIG. 3I , a resist pattern PR 5  for patterning resistance elements is formed on the substrate from which the resist pattern PR 4  has been removed, a Cr—Si alloy film is deposited by sputtering, and the Cr—Si alloy film on the resist pattern PR 5  is removed by lift-off. For example, using a sputtering target of Si(70-90):Cr(30-10), a Si—Cr alloy film of a thickness of the order of 0.2 μm (sheet resistance 300-600Ω) is formed, and resistance elements  21  and  22  are formed. Here, the resistance film may be formed before the formation of the dielectric film. 
     As illustrated in  FIG. 3J , a resist pattern PR 6  for patterning electrodes is formed on the substrate from which the resist pattern PR 5  has been removed, electrode film of Ti/Au lamination or Ti/Al lamination is deposited by sputtering to a thickness of the order of 1 μm, and the electrode film on the resist pattern is removed by lift-off. Those electrodes  19 ,  23 ,  24 , and  25  as illustrated in  FIG. 4D  are thus formed. 
     As illustrated in  FIG. 3K , a resist pattern PR 7  having an aperture in the region between the fixed electrodes is formed on the substrate, and the silicon nitride film  54  is etched by dry etching using CHF 3  gas, and the exposed silicon layer is removed by deep RIE using SF 6  gas and CF 4  gas. In  FIG. 4E , the hatched area is the region to be etched. 
     As illustrated in  FIG. 3L , the exposed silicon oxide film  52  is removed by dry etching using CF 4  gas. Here, the silicon oxide film may also be etched by wet etching using buffered fluoric acid or vapor etching using vaporized fluoric acid. When the silicon oxide film is removed by etching having isotropic nature, the silicon oxide film is also etched in the region intruded from the exposed region into the surroundings by side etching. Thus, a semiconductor device having a variable capacitor as illustrated in  FIG. 2A  can be manufactured. 
     The variable capacitor exhibits capacitance change between “on” state and “off” state, for example of the order of 0.9 pF (“off” state)-5.6 pF (“on” state). 
       FIG. 5A  is an equivalent circuit diagram illustrating one example of application circuit of variable capacitor thus formed. A movable electrode  10  is connected to a node in the electrode  25  of a high frequency line  31 - 25 - 32 , and forms variable capacitors  33  and  34  with the fixed electrodes  11  and  12 . The fixed electrode  11  is grounded via the fixed capacitor  35 , and connected to a terminal  24  of a switch SW. The other fixed electrode  12  is connected to the other terminal  23  of the switch SW via the resistance element  22 , to constitute a variable capacitance circuit  39 . A series connection of a dc power source  36  and an inductor  37  is connected between the change-over terminal of the switch and the high frequency line. For substantially preventing leakage, the resistance elements  21  and  22  are 10 kΩ or more, and the inductor is about 100 nH or more. 
     Variable capacitors  33  and  34  are connected to the high frequency signal line  31 - 25 - 32 , the fixed capacitor  35  is connected between the variable capacitor  33  and the ground, and the resistance elements  21  and  22  are connected between the variable capacitors  33  and  34  and the external power source  36 . The inductor  37  is connected between the other pole of the external power source  36  and the high frequency signal line  31 - 25 - 32  to cut high frequencies. Leakage of signal flowing in the high frequency signal line  31 - 25 - 32  to the external power source  36  is prevented by the resistance elements  21  and  22 . Short circuit between the external power source and the ground is prevented by the fixed capacitor  35 . One of the two digital states is selected whether the movable electrode  10  is attracted to the fixed electrode  11 , or to the fixed electrode  12 . 
       FIG. 5B  is an equivalent circuit diagram illustrating another structural example of application circuit. A plurality of variable capacitance circuits  39 - 1 , . . .  39 - i  are connected to a high frequency signal line  31 - 25 - 32 , and are connected to a common inductor  37  via external power sources  36 - 1 , . . . ,  36 - i . The plurality of variable capacitance circuits have differences in the capacitance value, corresponding to plural bits. This structure is fitted for multi-bit circuit, 
     In the second embodiment, the movable electrode is disposed obliquely in the space between the opposing fixed electrodes by the connection position of the anchors and the movable electrode, to enhance position change of the movable electrode. Further, stoppers may be provided for restricting the movable area of the movable electrode. 
     As illustrated in  FIG. 6A , stoppers  41   a  and  41   b  are disposed adjacent to or in line with the anchors  16  and  17 . The stopper  41   a  serves together with tha anchor  16  to dispose the movable electrode  10  in a space near the fixed electrode  12 . The stopper  41   b  serves together with the anchor  17 , to dispose the movable electrode  10  in a space near the fixed electrode  11 . In other words, in a right end portion where the movable electrode  10  is near the fixed electrode  11 , the stopper  41   b  is disposed between the movable electrode  10  and the fixed electrode  12 , to prevent the movable electrode  10  approaching the fixed electrode  12 . In left end portion where the movable electrode  10  is near the fixed electrode  12 , the stopper  41   a  is disposed between the movable electrode  10  and the fixed electrode  11 , to prevent the movable electrode  10  approaching the fixed electrode  11 . 
     By securing a certain width to the regions disposed near the fixed electrodes, a drive force will be surely applied to the movable electrode. The stopper  41  can be made by changing the pattern upon etching as illustrated in  FIG. 3B . An insulating film will be formed on the surface of the stopper in the steps illustrated in  FIGS. 3C and 4A . When the stopper is shaped in a square pole shape, the corner part may apply excessive stress to the movable electrode, and there may be a possibility of deforming the movable electrode. 
     The stopper may be so shaped that a corner or corners are rounded as illustrated in  FIG. 6B , that the stopper has round column shape as illustrated in  FIG. 6C , or that the stopper has a polygonal column shape as illustrated in  FIG. 6D , to migrate the stress which may be applied to the movable electrode. 
     Although the present invention has been described above along the embodiments, this invention is not limited thereto. Materials and numerical values given as examples are not limitative. For example, in place of an SOI substrate, a laminated substrate which has two sacrificial layers which are different in etching characteristics on a support substrate, can be used, to perform processes as illustrated in  FIGS. 3A-3L , to manufacture a structure as illustrated in  FIG. 1A  or in  FIG. 2A . In such case, the bonding silicon oxide film  52  and the active silicon layer  53  illustrated in  FIG. 2A  will be replaced with sacrificial layers of different etching characteristics. A laminated substrate having a single sacrificial layer on a support substrate can also be used by employing control etching, etc. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.