Abstract:
A MEMS device includes: a first actuator having a first fixed end, including a stacked structure of a first lower electrode, a first piezoelectric film, and a first upper electrode, and being able to be operated by applying voltages to the first lower electrode and the first upper electrode; a second actuator having a second fixed end, being disposed in parallel with the first actuator, including a stacked structure of a second lower electrode, a second piezoelectric film, and a second upper electrode, and being able to be operated by applying voltages to the second lower electrode and the second upper electrode; and an electric circuit element having a first action part connected to the first actuator and a second action part connected to the second actuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-98556 filed on Mar. 31, 2006 in Japan, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a piezoelectric driven MEMS (Micro-electro-mechanical System) actuator. 
   2. Related Art 
   Recently, actuators fabricated by using the MEMS technique are drawing the attention. The actuators are driven by electrostatic force, thermal stress, electromagnetic force, piezoelectric force, and so on, and bent and displaced. An actuator using a piezoelectric thin film to rotate a shaft attached to a free end is known (see, for example, JP-A 2003-181800 (KOKAI)). 
   Furthermore, MEMS devices such as variable capacitance type capacitors and switches using actuators are being studied. The variable capacitance type capacitors and switches using the MEMS technique has, for example, a configuration in which an actuator supported at its first end by a strut provided on a substrate is provided, a movable electrode serving as an action part is provided at a second end of the actuator, a fixed electrode is provided on the substrate surface so as to be opposed to the second end of the actuator, and the distance between the movable electrode and the fixed electrode is changed by the actuator. In other words, the actuator serves as a movable beam. 
   Especially in the variable capacitance type capacitor including a piezoelectric driven actuator which uses an inverse piezoelectric effect or an electrostrictive effect as drive force for the movable beam, the spacing between the movable electrode and the fixed electrode can be changed sharply and continuously, and consequently the capacitance change rate can be made large. Furthermore, since air or gas can be used as a dielectric between the movable electrode and the fixed electrode, a very large Q factor is obtained. Thus, the variable capacitance type capacitor has a large number of advantages. 
   Furthermore, it is also possible to cause the variable capacitance type capacitor structure to function as a switch by bringing the movable electrode into contact with the fixed electrode via an extremely thin dielectric film (capacitive type) or bringing the movable electrode into contact with the fixed electrode directly (contact type). Such a switch fabricated by using the MEMS technique has both low on-resistance and high off-condition isolation characteristics as compared with the semiconductor switch, and it is also drawing keen attention. 
   However, the piezoelectric driven actuator is supported in the air, and has a long thin beam structure including a piezoelectric film interposed between upper and lower electrodes. Therefore, there is a very serious problem that the beam is warped upward or downward by slight residual stress in the material of the piezoelectric film. As a result, it is very difficult to adjust the capacitance value of the variable capacitance type capacitor obtained before and after applying the voltage in conformity with the design and make the drive voltage of the switch a constant value. 
   For example, in the action part connected to the piezoelectric driven actuator, a bendable displacement quantity D caused by the inverse piezoelectric effect is represented by the following expression.
 
D˜E·d 31 ·L 2 /t   (1)
 
Here, E is an electric field applied to the piezoelectric film, d 31  is a piezoelectric strain constant, L is the length of the actuator, and t is the thickness.
 
   On the other hand, denoting residual strain by S r , warp D w  of the piezoelectric driven actuator caused by residual strain which is generated in the piezoelectric film when forming the film is approximated by the following expression.
 
D W ˜S r ·L 2 /t   (2)
 
   As appreciated by comparing the expression (2) with the expression (1), the displacement quantity D and the warp D W  have similar relations to the length L and the thickness t of the piezoelectric driven actuator. The displacement quantity D and the warp D W  are in proportion to the square of the length L, and are inverse proportion to the thickness t. For example, if the length L of the actuator is increased or the thickness t is decreased in order to widen the drive range of the piezoelectric driven actuator, the quantity of the warp D W  also increases accordingly. In making the piezoelectric driven range D greater than the warp D W , therefore, geometric contrivance concerning the actuator brings about little effect. There are no ways other than making the absolute value of the residual strain S r  small as compared with the absolute value of piezoelectric strain (E·d 31 ) caused by the inverse piezoelectric effect. 
   For obtaining a high quality film as regards lead zirconate titanate (PZT) known as a piezoelectric film having a great inverse piezoelectric effect, it is necessary to form a film at the room temperature and then conduct annealing at approximately 600° C. Since the volume contraction is caused by the annealing, the residual strain of the piezoelectric film formed of PZT increases inevitably. On the other hand, aluminum nitride (AlN) and zinc oxide (ZnO) used as the material of the piezoelectric film, which can be formed near at the room temperature and can be controlled in film forming residual stress comparatively precisely by using the film forming condition, are smaller in inverse piezoelectric effect by at least one digit than PZT. 
   If a piezoelectric material that is great in inverse piezoelectric effect is used as the piezoelectric film of the piezoelectric driven actuator in order to increase the piezoelectric strain, it becomes difficult to control the residual strain and the warp cannot be controlled, as described above. If a piezoelectric material that is comparatively easy in control of residual strain in the piezoelectric film is used, then the inverse piezoelectric effect is small and the piezoelectric driven range cannot be made sufficiently large as compared with the warp of the actuator. 
   Industrial application of the piezoelectric driven actuator is obstructed by such problems. Because of a great problem in the structure of the piezoelectric driven actuator, i.e., its structure which is thin in thickness and long, the piezoelectric driven actuator is warped largely by slight residual stress. Therefore, it is difficult to fabricate variable capacitance type capacitors so as to yield a constant capacitance or keep drive voltages of switches constant. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of these circumstances, and an object thereof is to provide a piezoelectric driven MEMS device in which the influence of the warp caused by residual strain in the piezoelectric film or the like is held down and the displacement quantity of the piezoelectric drive can be controlled with high reproducibility and high precision. 
   A piezoelectric driven MEMS device according to a first aspect of the present invention includes: a substrate; a first actuator which has a first fixed end fixed at least at one end to the substrate, which includes a first lower electrode, a first piezoelectric film formed on the first lower electrode, and a first upper electrode formed on the first piezoelectric film, and which can be operated by applying voltages to the first lower electrode and the first upper electrode; a second actuator which has a second fixed end fixed at least at one end to the substrate, which is disposed in parallel with the first actuator, which includes a second lower electrode becoming the same layer as the first lower electrode, a second piezoelectric film formed on the second lower electrode so as to become the same layer as the first piezoelectric film, and a second upper electrode formed on the second piezoelectric film so as to become the same layer as the first upper electrode, and which can be operated by applying voltages to the second lower electrode and the second upper electrode; and an electric circuit element having a first action part connected to the first actuator and a second action part connected to the second actuator. 
   A piezoelectric driven MEMS device according to a second aspect of the present invention includes: a substrate; a first actuator which has a first fixed end fixed at least at one end to the substrate, which includes a first lower electrode, a first piezoelectric film formed on the first lower electrode, and a first upper electrode formed on the first piezoelectric film, and which can be operated by applying voltages to the first lower electrode and the first upper electrode; a second actuator which has a second fixed end fixed at least at one end to the substrate, which is disposed in parallel with the first actuator, which includes a second lower electrode becoming the same layer as the first lower electrode, a second piezoelectric film formed on the second lower electrode so as to become the same layer as the first piezoelectric film, and a second upper electrode formed on the second piezoelectric film so as to become the same layer as the first upper electrode, and which can be operated by applying voltages to the second lower electrode and the second upper electrode; a third actuator which has a third fixed end fixed at least at one end to the substrate, which is disposed across the first actuator from the second actuator and in parallel with the first actuator, which includes a third lower electrode becoming the same layer as the first lower electrode, a third piezoelectric film formed on the third lower electrode so as to become the same layer as the first piezoelectric film, and a third upper electrode formed on the third piezoelectric film so as to become the same layer as the first upper electrode, and which can be operated by applying voltages to the third lower electrode and the third upper electrode; and an electric circuit element having a first action part connected to the first actuator, a second action part connected to the second actuator, and a third action part connected to the third actuator. 
   A piezoelectric driven MEMS device according to a third aspect of the present invention includes: a substrate; a first actuator which has a first fixed end fixed at least at one end to the substrate, which includes a first lower electrode, a first lower piezoelectric film formed on the first lower electrode, a first intermediate electrode formed on the first lower piezoelectric film, a first intermediate electrode formed on the first lower piezoelectric film, a first upper piezoelectric film formed on the first intermediate electrode, and a first upper electrode formed on the first upper piezoelectric film, and which can be operated by applying voltages to the first intermediate electrode and at least one of the first lower electrode and the first upper electrode; a second actuator which has a second fixed end fixed at least at one end to the substrate, which is disposed in parallel with the first actuator, which includes a second lower electrode becoming the same layer as the first lower electrode, a second lower piezoelectric film formed on the second lower electrode so as to become the same layer as the first lower piezoelectric film, a second intermediate electrode formed on the second lower piezoelectric film so as to become the same layer as the first intermediate electrode, a second upper piezoelectric film formed on the second intermediate electrode so as to become the same layer as the first upper piezoelectric film, and a second upper electrode formed on the second upper piezoelectric film so as to become the same layer as the first upper electrode, and which can be operated by applying voltages to the second intermediate electrode and at least one of the second lower electrode and the second upper electrode; and an electric circuit element having a first action part connected to the first actuator and a second action part connected to the second actuator. 
   A piezoelectric driven MEMS device according to a fourth aspect of the present invention includes: a substrate; a first actuator which has a first fixed end fixed at least at one end to the substrate, which includes a first lower electrode, a first lower piezoelectric film formed on the first lower electrode, a first intermediate electrode formed on the first lower piezoelectric film, a first intermediate electrode formed on the first lower piezoelectric film, a first upper piezoelectric film formed on the first intermediate electrode, and a first upper electrode formed on the first upper piezoelectric film, and which can be operated by applying voltages to the first intermediate electrode and at least one of the first lower electrode and the first upper electrode; a second actuator which has a second fixed end fixed at least at one end to the substrate, which is disposed in parallel with the first actuator, which includes a second lower electrode becoming the same layer as the first lower electrode, a second lower piezoelectric film formed on the second lower electrode so as to become the same layer as the first lower piezoelectric film, a second intermediate electrode formed on the second lower piezoelectric film so as to become the same layer as the first intermediate electrode, a second upper piezoelectric film formed on the second intermediate electrode so as to become the same layer as the first upper piezoelectric film, and a second upper electrode formed on the second upper piezoelectric film so as to become the same layer as the first upper electrode, and which can be operated by applying voltages to the second intermediate electrode and at least one of the second lower electrode and the second upper electrode; a third actuator which has a third fixed end fixed at least at one end to the substrate, which is disposed across the first actuator from the second actuator and in parallel with the first actuator, which includes a third lower electrode becoming the same layer as the first lower electrode, a third lower piezoelectric film formed on the third lower electrode so as to become the same layer as the first lower piezoelectric film, a third intermediate electrode formed on the third lower piezoelectric film so as to become the same layer as the first intermediate electrode, a third upper piezoelectric film formed on the third intermediate electrode so as to become the same layer as the first upper piezoelectric film, and a third upper electrode formed on the third upper piezoelectric film so as to become the same layer as the first upper electrode, and which can be operated by applying voltages to the third intermediate electrode and at least one of the third lower electrode and the third upper electrode; and an electric circuit element having a first action part connected to the first actuator, a second action part connected to the second actuator, and a third action part connected to the third actuator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view showing a piezoelectric driven MEMS device according to a first embodiment; 
       FIG. 2  is a diagram showing a result obtained by observing a warp of the piezoelectric driven MEMS device according to the first embodiment; 
       FIG. 3  is a diagram schematically showing a warp of a piezoelectric driven actuator; 
       FIG. 4  is a diagram schematically showing warps obtained when two neighboring actuators are fabricated under the same condition; 
       FIG. 5  is a perspective view showing a piezoelectric driven MEMS device according to a second embodiment; 
       FIG. 6  is a perspective view showing a piezoelectric driven MEMS device according to a third embodiment; 
       FIG. 7  is a perspective view showing a piezoelectric driven MEMS device according to a fourth embodiment; 
       FIG. 8  is a perspective view showing a piezoelectric driven MEMS device according to a fifth embodiment; 
       FIG. 9  is a perspective view showing a piezoelectric driven MEMS device according to a sixth embodiment; 
       FIG. 10  is an equivalent circuit diagram of variable capacitor parts of a piezoelectric driven MEMS device according to the sixth embodiment; 
       FIG. 11  is a circuit diagram showing a frequency band selection circuit in a mobile communication device according to a seventh embodiment; 
       FIGS. 12A to 12C  are waveform diagrams for explaining operation of the frequency selection circuit in the seventh embodiment; 
       FIG. 13A  is a circuit diagram of a tunable filter according to an eighth embodiment, and  FIG. 13B  is a diagram showing pass-bands of the tunable filter; and 
       FIGS. 14A and 14B  are diagrams showing amplifier matching circuits according to a ninth embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First, before describing the embodiments of the present invention, the course of events for achieving the present invention will be described below. 
   The present inventors have repeated considerations concerning the movable range in the piezoelectric driven MEMS device and magnitude of the warp which is caused by imbalance of residual stress and which exerts the greatest influence upon the operation. On the basis of its result, a new idea for overcoming the warp occurred to the present inventors. 
   As for the problem to be solved, the warp quantity of the piezoelectric driven MEMS device is proportionate to square of the length of the actuator as already described in detail. Three-dimensionally, a piezoelectric driven MEMS device  11  warps so as to take the shape of a radial face  50  around an end  11   a  fixed to a substrate  1  as shown in  FIG. 3 . 
   Therefore, the present inventors have thought of fabricating first and second piezoelectric driven MEMS devices  11  and  21  under the same condition and disposing them side by side as schematically shown in  FIG. 4 . By using such a configuration, warp quantities of the actuators  11  and  21  become nearly the same along the radial face  50 . And by providing action parts  11   b  and  21   b  at the same distance respectively from fixed ends  11   a  and  21   a , displacement quantities caused at the action parts  11   b  and  21   b  by warps become equal to each other and the influence of the warps is held down. 
   Hereafter, embodiments of the present invention will be described in detail with reference to the drawings. In the ensuing description of the drawings, the same or similar parts are denoted by like or similar reference numerals. 
   First Embodiment 
   A piezoelectric driven MEMS device according to a first embodiment of the present invention is shown in  FIG. 1 . The piezoelectric driven MEMS device according to the present embodiment includes a first actuator  11  and a second actuator  21 . A first end of the first actuator is a fixed end, and it is fixed to a substrate  1  through an anchor  18 . The first actuator  11  includes a lower electrode  13 , a piezoelectric film  14 , an intermediate electrode  15 , a piezoelectric film  16  and an upper electrode  17 . The first actuator  11  is a piezoelectric driven actuator having the so-called bimorph structure. An action part  12  is connected to a second part of the first actuator  11 . 
   The second actuator  21  is formed with the same stacked structure as that of the first actuator  11 , under the same fabrication condition as that of the first actuator  11 , or concurrently with the first actuator  11 . A first end of the first actuator is a fixed end, and it is fixed to the substrate  1  through an anchor  28 . The second actuator  21  includes a lower electrode  23 , a piezoelectric film  24 , an intermediate electrode  25 , a piezoelectric film  26  and an upper electrode  27 . The lower electrode  23 , the piezoelectric film  24 , the intermediate electrode  25 , the piezoelectric film  26  and the upper electrode  27  in the second actuator  21  are formed so as to become the same layers respectively as the lower electrode  13 , the piezoelectric film  14 , the intermediate electrode  15 , the piezoelectric film  16  and the upper electrode  17  do. 
   An action part  22  is connected to a second part of the second actuator  21 . A fixed end of the second actuator  21  is positioned on a straight line that passes through the fixed end of the first actuator  11  and that is perpendicular to a direction in which the first actuator  11  extends. The distance between the fixed end of the first actuator  11  and the position where the action part  12  is connected is substantially equal to the distance between the fixed end of the second actuator  21  and the position where the action part  12  is connected. 
   Even if residual strain that is vertically asymmetrical arises in piezoelectric films or electrodes included in the first actuator  11  and the second actuator  21 , therefore, the warp quantities in the first actuator  11  and the second actuator  21  become substantially equal to each other. As a result, it becomes possible to hold down the difference in displacement caused between the action part  12  and the action part  22  by the warps to a very small value. 
     FIG. 2  shows a result obtained by observing the warp in the second actuator  21  which is formed with the same stacked structure and under the same fabrication condition as the first actuator  11 , by using a confocal laser microscope. The abscissa in  FIG. 2  indicates a length between the anchor of each actuator and an arbitrary point in the action part direction, and the ordinate indicates the warp quantity at that point. As apparent from  FIG. 2 , the warp quantity in the first actuator  11  is substantially equal to that in the second actuator  21 . It is appreciated that the difference in displacement caused between the action part  12  connected to the first actuator  11  and the action part  22  connected to the second actuator  21  by the warps is held down to a very small value. 
   If electrodes or contacts are provided in the action parts  12  and  22  in the present embodiment to form capacitor structures or contact structures, for example, as shown in  FIGS. 6 and 7  which will be described later, then it is possible to obtain a variable capacitance type capacitor or a switch in which the displacement quantity can be controlled with high reproducibility and at high precision. 
   As heretofore described, it is possible according to the present embodiment to obtain a piezoelectric driven MEMS device in which the displacement quantity of the piezoelectric drive can be controlled with high reproducibility and at high precision by holding down the difference in displacement caused by the warp which is in turn caused by residual strain in the piezoelectric film or the like. 
   Second Embodiment 
   A piezoelectric driven MEMS device according to a second embodiment of the present invention is shown in  FIG. 5 . The piezoelectric driven MEMS device according to the present embodiment has a configuration in which the action part  12  in the first embodiment is formed of the lower electrode  13 , the piezoelectric film  14 , the intermediate electrode  15 , the piezoelectric film  16  and the upper electrode  17  in the same way as the first actuator  11  and the action part  22  is formed of the lower electrode  23 , the piezoelectric film  24 , the intermediate electrode  25 , the piezoelectric film  26  and the upper electrode  27  in the same way as the second actuator  21 . 
   Fabricating the action parts  12  and  22  by using the materials used to form the actuators brings about an advantage that it becomes easy to hold down the displacements due to residual strains caused in the action parts, between the action parts and the actuators and the action parts can be formed simultaneously. 
   It is a matter of course that each action part may be formed of all of the piezoelectric film, the upper electrode and the lower electrode, which are included in the actuator, or each action part may be formed of a part of them. 
   It is a matter of course that the action part  12  may be formed of all of the materials forming the second actuator  21  or a part of them and the action part  22  may be formed of all of the materials forming the first actuator  11  or a part of them. 
   In the present embodiment as well, it is possible to obtain a piezoelectric driven MEMS device in which the displacement quantity of the piezoelectric drive can be controlled with high reproducibility and at high precision by holding down the difference in displacement caused by the warp which is in turn caused by residual strain, in the same way as the first embodiment. 
   Third Embodiment 
   A piezoelectric driven MEMS device according to a third embodiment of the present invention is shown in  FIG. 6 . The piezoelectric driven MEMS device according to the present embodiment has a configuration in which the action part  12  and the action part  22  in the first embodiment form a parallel plate capacitor to fabricate a variable capacitor. 
   The action part  12  is formed of the lower electrode  13  and the piezoelectric film  14 , which are included in the first actuator  11 . The action part  22  includes a main body part  22   a , and an extension part  22   b  which is connected to the main body part  22   a  and which extends in a direction substantially perpendicular to the extension direction of the second actuator  21 . In the same way as the second actuator  21 , the main body part  22   a  includes the lower electrode  23 , the piezoelectric film  24 , the intermediate electrode  25 , the piezoelectric film  26  and the upper electrode  27 . The extension part  22   b  includes the intermediate electrode  25 , the piezoelectric film  26  and the upper electrode  27 , which are included in the second actuator  21 . A tip part  22   b   1  of the extension part  22   b  is formed so as to overlap at least a part of the action part  12  when seen from the above and have a gap  2  between the action part  12  and the tip part  22   b   1 . Therefore, the tip part  22   b   1  is formed so as to be raised as compared with a part of the extension part  22   b  except the tip part  22   b   1  and kept away from the substrate  1 . In this way, a parallel plate capacitor is formed by providing the gap  2  between the tip part  22   b   1  and the action part  12 . The gap  2  can be formed by depositing a sacrifice layer which is not illustrated on the piezoelectric film  14  of the action part  12 , subsequently forming the action part  22 , and removing the sacrifice layer. 
   The spacing of the gap  2  is changed by driving one or both of the first actuator  11  and the second actuator  21 . As a result, the capacitance of the parallel plate capacitor is changed. It thus becomes possible to make the parallel plate capacitor function as a variable capacitor. By forming such a parallel plate capacitor, a capacitor having a comparatively large capacitance can be fabricated. 
   If a voltage in the range of 0 V to 3 V is applied between the lower electrode  13  and the intermediate electrode  15  and between the upper electrode  17  and the intermediate electrode  15  in the first actuator and a voltage in the range of 0 V to 3 V is applied between the intermediate electrode  25  and the lower electrode  23  and between the intermediate electrode  25  and the upper electrode  27  in the second actuator, then the first actuator  11  displaces upward as compared with the substrate  1  and the second actuator  21  displaces downward as the applied voltage increases. Finally, at the applied voltage of 3 V, the action part  11  comes in contact with the tip part  22   b   1  of the action part  22 . Namely, the lower electrode  13  in the action part  12  comes in contact with the intermediate electrode  25  of the tip part  22   b   1  in the action part  22  via the piezoelectric film  14  in the action part  12 . As a result, the parallel plate capacitor formed of the action part  12  and the tip part  22   b   1  of the action part  22   b  changes very largely in capacitance from 0.11 pF to 5.33 pF. 
   The above-described parallel plate capacitor is fabricated by using materials used for the first actuator  11  and the second actuator  21 . Even if the parallel plate capacitor is fabricated by using other materials, however, no problems are posed. 
   By using a configuration similar to the above-described configuration, impedance for an AC signal becomes high when the capacitance of the parallel plate capacitor is small, whereas the impedance becomes low when the capacitance is large. It is also possible to form a capacitive switch which functions as a switch for an AC signal, by utilizing such a property. 
   What is to be attended to when using the piezoelectric driven MEMS device according to the present embodiment will now be described. In each of the first actuator  11  and the second actuator  21 , a signal which passes through the variable capacitor is also present besides a drive signal for driving the actuator. Therefore, it is necessary to separate the signal which passes through the variable capacitor, by using some means such as a bias T. 
   Fourth Embodiment 
   A piezoelectric driven MEMS device according to a fourth embodiment of the present invention is shown in  FIG. 7 . The piezoelectric driven MEMS device according to the present embodiment has a configuration obtained from the second embodiment shown in  FIG. 5  by providing the action part  12  on the side part of the first actuator  11  so as to direct the action part  12  toward the second actuator  21  and providing the action part  22  on the side part of the second actuator  21  so as to direct the action part  22  toward the first actuator  11 . Each of the action part  12  and the action part  22  takes the shape of teeth of a comb when seen from the above. The action part  12  and the action part  22  are arranged so as to cause comb teeth of the action part  12  to alternate with those of the action part  22 . In the present embodiment, therefore, the action part  12  and the action part  22  form a variable capacitor which takes the shape of comb teeth. Capacitances of the capacitor are generated between the lower electrode and the intermediate electrode and between the intermediate electrode and the upper electrode in each action part. 
   In the present embodiment, the action part  12  is made by using absolutely the same material as that of the first actuator  11 , and the action part  22  is made by using absolutely the same material as that of the second actuator  21 . 
   The overlap quantity between the lower electrode and the intermediate electrode and the overlap quantity between the intermediate electrode and the upper electrode in each of the action part  12  and the action part  22  are changed by driving one or both of the first actuator  11  and the second actuator  21 . As a result, the capacitance of the capacitor taking the shape of comb teeth is changed. It thus becomes possible to make the first actuator  11  and the second actuator  21  function as a variable capacitor. It becomes unnecessary to fabricate capacitor forming electrodes at right angles to the substrate by forming such a comb tooth capacitor. As compared with the parallel plate capacitor, therefore, there is an advantage that the fabrication is highly facilitated. 
   If a voltage in the range of 0 V to 3 V is applied between the lower electrode  13  and the intermediate electrode  15  and between the upper electrode  17  and the intermediate electrode  15  in the first actuator and a voltage in the range of 0 V to 3 V is applied between the intermediate electrode  25  and the lower electrode  23  and between the intermediate electrode  25  and the upper electrode  27  in the second actuator, then the first actuator  11  displaces upward as compared with the substrate  1  and the second actuator  21  displaces downward as the applied voltage increases. As a result, the comb tooth capacitor formed of the action part  12  and the action part  22  changes in capacitance from 0.32 pF to 0.08 pF. 
   The above-described comb tooth capacitor is fabricated by using absolutely the same materials used for the first actuator  11  and the second actuator  21 . Even if the comb tooth capacitor is fabricated by using the same material only for a part, however, no problems are posed. Even if the comb tooth capacitor is fabricated by using materials other than the materials forming the first actuator  11  and the second actuator  21 , there are no problems at all. 
   It is also possible to form a capacitive switch which functions as a switch for an AC signal, by using a configuration similar to the above-described configuration. 
   What is to be attended to when using the piezoelectric driven MEMS device according to the present embodiment will now be described. In each of the first actuator  11  and the second actuator  21 , a signal which passes through the variable capacitor is also present besides a drive signal for driving the actuator. Therefore, it is necessary to separate the signal which passes through the variable capacitor, by using some means such as a bias T. 
   Fifth Embodiment 
   A piezoelectric driven MEMS device according to a fifth embodiment of the present invention is shown in  FIG. 8 . In the piezoelectric driven MEMS device according to the present embodiment, an electric contact is formed by using the action part  12  and the action part  22  in the first embodiment to form a switch. 
   The action part  12  includes a main body part  12   a  and an extension part  12   b  connected to the main body part  12   a . The main body part  12   a  is provided on the side part of the first actuator  11  adjacent to the second actuator  21 , along the first actuator  11 . The main body part  12   a  is fixed at a first end to the substrate  1  via an anchor  18 . The extension part  12   b  is provided at a second end of the main body part  12   a  so as to extend toward the second actuator  21  at substantially right angles to the main part  12   a . The main body part  12   a  has absolutely the same configuration as the first actuator  11  does. In other words, the main body part  12   a  includes the lower electrode  13 , the piezoelectric film  14 , the intermediate electrode  15 , the piezoelectric film  16  and the upper electrode  17 . The extension part  12   b  is formed of the lower electrode  13 . The action part  12  is connected to the first actuator  11  by a plurality of connection parts  41  which are the same layer as the piezoelectric film  16  and which are formed of the same material as that of the piezoelectric film  16 . 
   The action part  22  includes a main body part  22   a  and an extension part  22   b  connected to the main body part  22   a . The main body part  22   a  is provided on the side part of the second actuator  21  adjacent to the first actuator  11 , along the second actuator  21 . The main body part  22   a  is fixed at a first end to the substrate  1  via an anchor  28 . The extension part  22   b  is provided at a second end of the main body part  22   a  so as to extend toward the first actuator  11  at substantially right angles to the main part  22   a . The main body part  22   a  has absolutely the same configuration as the second actuator  21  does. In other words, the main body part  22   a  includes the lower electrode  23 , the piezoelectric film  24 , the intermediate electrode  25 , the piezoelectric film  26  and the upper electrode  27 . The extension part  22   b  is formed of the upper electrode  27 . The action part  22  is connected to the second actuator  21  by a plurality of connection parts  43  which are the same layer as the piezoelectric film  26  and which are formed of the same material as that of the piezoelectric film  26 . 
   The extension part  12   b  of the action part  12  and the extension part  22   b  of the action part  22  are configured so as to partially overlap via a gap  3 . In the same way as the third embodiment, the gap  3  is formed by depositing a sacrifice layer which is not illustrated on the extension part  12   b  of the action part  12 , subsequently forming the extension part  22   b  of the action part  22 , and then removing the sacrifice layer. The first actuator  11  is electrically insulated from the action part  12 , and the second actuator  21  is electrically insulated from the action part  22 . 
   The extension part  12   b  formed of the lower electrode  13  and the extension part  22   b  formed of the upper electrode  27  are formed so as to overlap each other via the gap  3 . This overlap part forms an electric contact. If the size of the gap  3 , i.e., the distance between the extension part  12   b  and the extension part  22   b  is not zero, then the extension part  12   b  of the action part  12  is electrically insulated from the extension part  22   b  of the action part  22 , resulting in an off-state of the switch. If one or both of the first actuator  11  and the second actuator  21  is driven and the size of the gap  3  has finally become zero, then electric conduction between the extension part  12   b  of the action part  12  and the extension part  22   b  of the action part  22  is achieved, resulting in an on-state of the switch. 
   It is possible to make the area of electrodes forming the contact comparatively large by fabricating such an electric contact. As a result, it is advantageous to reduction of the contact resistance. 
   If a voltage in the range of 0 V to 5 V is applied between the lower electrode  13  and the intermediate electrode  15  and between the upper electrode  17  and the intermediate electrode  15  in the first actuator  11  and a voltage in the range of 0 V to 5 V is applied between the intermediate electrode  25  and the lower electrode  23  and between the intermediate electrode  25  and the upper electrode  27  in the second actuator  21 , then the first actuator  11  displaces upward as compared with the substrate  1  and the second actuator  21  displaces downward as the applied voltage increases. When the applied voltage is in the range of 0 to 3.3 V, the switch between the lower electrode  13  in the first action part  12  and the upper electrode  27  in the second action part  22  comes in the off-state. When the applied voltage is in the range of 3.3 V to 5 V, the switch between the lower electrode  13  in the first action part  12  and the upper electrode  27  in the second action part  22  comes in the on-state. In the present embodiment, the drive voltage of the switch can be kept constant. 
   Although the switch is fabricated by using the materials which form the first actuator  11  and the second actuator  21 , there are no problems at all even if other materials are used. 
   Sixth Embodiment 
   A piezoelectric driven MEMS device according to a sixth embodiment of the present invention is shown in  FIG. 9 . The piezoelectric driven MEMS device according to the present embodiment has a configuration obtained from the third embodiment shown in  FIG. 6  by providing a third actuator  31  across the first actuator  11  from the second actuator  21 . In other words, the piezoelectric driven MEMS device according to the third embodiment is a variable capacitor having two actuators  11  and  21 , whereas in the present embodiment three actuators  11 ,  21  and  31  form a variable capacitor. 
   The third actuator  31  is fixed at its first end to an anchor  38  provided on the substrate  1 . An action part  32  is connected to a second end of the third actuator  31 . The third actuator  31  includes a lower electrode  33 , a piezoelectric film  34 , an intermediate electrode  35 , a piezoelectric film  36  and an upper electrode  37 . The third actuator  31  has the so-called bimorph structure. The first to third actuators  11 ,  21  and  31  are formed with the same stacked structure, under the same fabrication condition, or concurrently. 
   In the same way as the action part  22  described with reference to the second embodiment, the action part  32  includes a main body part  32   a , and an extension part  32   b  which is connected to the main body part  32   a  and which extends in a direction substantially perpendicular to the extension direction of the third actuator  31 . In the same way as the third actuator  31 , the main body part  32   a  includes the lower electrode  33 , the piezoelectric film  34 , the intermediate electrode  35 , the piezoelectric film  36  and the upper electrode  37 . The extension part  32   b  includes the intermediate electrode  35 , the piezoelectric film  36  and the upper electrode  37 , which are included in the third actuator  31 . A tip part  32   b   1  of the extension part  32   b  is formed so as to overlap a part of the action part  12  when seen from the above and have a gap  2   a  between the tip part  32   b   1  and the action part  12 . Therefore, the tip part  32   b   1  is formed so as to be raised as compared with a part of the extension part  32   b  except the tip part  32   b   1 , and kept away from the substrate  1 . In this way, parallel plate capacitors are formed by providing the gap  2  between the tip part  22   b   1  of the action part  22  and the action part  12  and providing the gap  2   a  between the tip part  32   b   1  of the action part  32  and the action part  12 . The gap  2   a  can be formed in the same way as in the description of the third embodiment. 
   The spacing of the gap  2  or  2   a  is changed by driving at least one of the first to third actuators  11 ,  21  and  31 . As a result, capacitances of the parallel plate capacitors are changed. In the same way as the third embodiment, the parallel plate capacitors function as variable capacitors. As for a signal passing through these parallel plate capacitors, however, the signal is passed through, for example, a parallel plate capacitor formed of the action part  22  and the action part  12 , and then passed through a parallel plate capacitor formed of the action part  12  and the action part  32 , again. In other words, the equivalent circuit is shown in  FIG. 10 . In  FIG. 10 , for example, a variable capacitor  100   a  is a parallel plate capacitor formed of the action part  22  and the action part  12 , and a variable capacitor  100   b  is a parallel plate capacitor formed of the action part  12  and the action part  32 . A line  110   a  corresponds to the intermediate electrode  25  in the action part  22 , a line  110   b  corresponds to the intermediate electrode  35  in the action part  32 , and a line  110   c  corresponds to the lower electrode  13  in the action part  12 . 
   In the case where only the first actuator  11  is driven for a capacitance change and a drive voltage is not applied to the second actuator  21  and the third actuator  31 , only the signal that has been passed through the capacitor flows through the actuators  21  and  31 . This brings about an advantage that it is not necessary to use means that separates the drive signal for driving the actuator from the signal passing through the variable capacitor. 
   In the same way as the fifth embodiment, the MEMS device can be used as a switch by forming the action part  12  of only the lower electrode  13 , forming the extension part  22   b  in the action part  22  of only the upper electrode  27 , and forming the extension part  32   b  in the action part  32  of only the upper electrode  37 . 
   In the piezoelectric driven MEMS device in the present embodiment shown in  FIG. 9 , a composite capacitance of two parallel plate capacitors connected in series has changed from 0.04 pF to 2.4 pF by driving the first actuator  11  with a voltage in the range of 0 V to 3 V. 
   The piezoelectric driven MEMS device in the present embodiment can also be used as a capacitive switch which functions as a switch for an AC signal. 
   Although the parallel plate capacitors are fabricated by using materials used to form the first actuator  11  and the second actuator  12 , there are no problems even if they are fabricated by using other materials. 
   As for the operation of the piezoelectric driven MEMS devices according to the first to sixth embodiments, gaps between actuators may be changed by, for example, applying a voltage between the upper electrode and the lower electrode and applying a voltage opposite to that of the first actuator or a voltage different from that of the first actuator. Or there is an operation method in which only the first actuator is driven and the second actuator is not driven, or an operation method in which only the second actuator is driven and the first actuator is not driven. 
   As for each of the piezoelectric driven actuators, not only a uniform structure sandwiched between the upper and lower electrodes, but also a piezoelectric driven actuator of the so-called S mode in which the polarity of the upper and lower electrodes is inverted in the middle of the actuator. 
   As for each of the piezoelectric driven actuators, not only the so-called cantilever structure fixed at one end, but also the so-called doubly-clamped-beam structure fixed at both ends can be used. 
   As for the movable actuator mechanism having the piezoelectric driven mechanism, it is possible to use a structure called unimorph structure or asymmetric bimorph structure and obtained by laminating a piezoelectric film and a support film sandwiched between an upper electrode and a lower electrode, or a structure called bimorph structure and obtained by piling up two layers of piezoelectric film sandwiched between the upper electrode and the lower electrode. 
   As for the material of the piezoelectric film used in the piezoelectric driven part, wurtzite crystal such as aluminum nitride (AlN) or zinc oxide (ZnO) is stable and desirable. It is also possible to use a perovskite ferroelectric material such as lead zirconate titanate (PZT) or barium titanate (BTO). 
   As for the material of the upper electrode and the lower electrode used in the piezoelectric driven part, it is desirable to use a low-resistance metal such as aluminum (Al), gold (Au), platinum (Pt), copper (Cu), iridium (Ir), tungsten (W), or molybdenum (Mo) because of the resistivity and easiness in fabricating a thin film. 
   As for the material used in the action part, it is desirable to use the same material as that of the piezoelectric film, the upper electrode or the lower electrode, or a material typically used in the semiconductor process. 
   Seventh Embodiment 
   A mobile communication device according to a seventh embodiment of the present invention will now be described with reference to  FIG. 11  to  FIG. 12C . 
   In general, the mobile communication device includes a frequency selection circuit which selects a frequency band, because the mobile communication device handles a plurality of frequency bands.  FIG. 11  is a circuit diagram showing a frequency band selection circuit in a mobile communication device according to the present embodiment. The frequency band selection circuit includes a switch  120 , band pass filters  131 ,  132  and  133 , a control circuit  140 , and a switch  150 . As the switches  120  and  150 , the piezoelectric driven MEMS device described with reference to the embodiments is used. As shown in  FIG. 12A , the band pass filters  131 ,  132  and  133  pass frequencies in bands having center frequencies F 1 , F 2  and F 3 , respectively. 
   The switch  120  selects one of the band pass filters  131 ,  132  and  133  on the basis of a command given by the control circuit  140 , and sends an input signal S in  shown in  FIG. 12B  to the selected band pass filter, for example, the band pass filter  131 . Thereupon, the signal passed through the band pass filter  131  is output via the switch  150  activated on the basis of a command given by the control circuit  140 . The output signal is shown in  FIG. 12C . 
   Eighth Embodiment 
   A tunable filter according to an eighth embodiment of the present invention is shown in  FIG. 13A . The tunable filter according to the present embodiment includes a circuit obtained by connecting circuits  160  in a ladder form. Each of the circuits  160  includes a resonator  161 , a variable capacitor  162  connected in parallel with the resonator  161 , and a variable capacitor  163  connected in series with the resonator  161 . As shown in  FIG. 13B , the passband can be varied. The circuit  160  can change the resonance frequency and the antiresonance frequency of the resonator  161 . As the variable capacitors  162  and  163 , the piezoelectric driven MEMS device described with reference to the embodiments is used. 
   In the present embodiment, the circuits  160  each including the resonator  161  and the variable capacitors  162  and  163  connected respectively in parallel with and connected in series with the resonator  161  are connected in a ladder form. Even if the circuits  160  are connected in a lattice form, similar effects can be obtained. 
   Ninth Embodiment 
   An amplifier matching circuit according to a ninth embodiment of the present invention is shown in  FIG. 14A . 
   In general, when connecting a circuit to an amplifier, it is desirable to match an impedance of a circuit connected in a stage preceding the amplifier with an input impedance of the amplifier and match an impedance of a circuit connected in a stage subsequent to the amplifier with an output impedance of the amplifier in order to prevent reflection from being caused by mismatching. When amplifying different frequency signals, it is necessary to achieve matching for respective frequencies. It becomes possible to achieve matching for respective frequencies by varying values of capacitors and inductors included in the matching circuit, i.e., capacitances and inductor inductances. 
   In the present embodiment, the piezoelectric driven MEMS device described with reference to the embodiments is used as the capacitors included in the matching circuit. As shown in  FIG. 14A , the matching circuit according to the present embodiment includes variable capacitors  171  and  172 , variable inductors  174 ,  175  and  176 , and transistors  177  and  178 . 
   A first end of the variable capacitor  171  is connected to an input end “in.” A second end of the variable capacitor  171  is connected to the transistor  178  at its gate. A first end of the variable inductor  174  is connected to the input end “in.” A second end of the variable inductor  174  is connected to ground. A first end of the variable inductor  175  is connected to a power supply. A second end of the variable inductor  175  is connected to the transistor  177  at its source and connected to a first end of the variable capacitor  172 . The transistor  177  and the transistor  178  are connected in series. Drains of the transistor  177  and the transistor  178  are connected together. The transistor  178  is connected at its source to the ground via the variable inductor  176 . The transistor  177  is connected at its gate to the power supply. A second end of the variable capacitor  172  is connected to an output end “out.” 
   In the present embodiment, each of the variable inductors  174 ,  175  and  176  is implemented as a configuration in which a plurality of fixed inductors  180   a ,  180   b  and  180   c  connected in series can be changed over by switches  181  and  182 , as shown in  FIG. 14B . Each of the variable inductors may have a configuration in which a plurality of fixed inductors connected in parallel are changed over by switches. 
   Even if a piezoelectric driven actuator is warped by residual stress of a material included therein, the difference in displacement between the action parts connected to the piezoelectric driven actuator is held down and kept constant. By using the piezoelectric driven MEMS device structures according to the embodiments of the present invention as heretofore described in detail, it becomes possible to provide a MEMS vari-caps and MEMS switches having a control mechanism which is excellent in reproducibility and reliability. Thus, the industrial value of the present invention is extremely high.