Abstract:
A micro-electro-mechanical system (MEMS) includes a first electrode interposed between a first fixed end and a second fixed end, the first electrode being movable by an actuator element. The MEMS also includes a substrate on which the first and second fixed ends are located. The MEMS further includes a second electrode formed on the substrate to face the first electrode. A shape from the first electrode to the first fixed end and a shape from the first electrode to the second fixed end are asymmetrical, the first electrode to be lowered to the second electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-134011, filed May 12, 2006, 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 micro machine or MEMS (Micro-Electro-Mechanical Systems) device using an actuator. For example, the present invention relates to a MEMS device using a piezoelectric type or electrostatic type actuator. 
     2. Description of the Related Art 
     The following bridge structure is preferably used in order to reduce a warp resulting from residual stress when switch and variable capacitor are formed using a MEMS device using a piezoelectric type actuator. According to the bridge structure, both ends of a beam (elastic member) where an electrode is arranged are supported by two fixed ends. In the variable capacitor having the bridge structure, if tensile residual stress exists in the beam, there is a problem that piezoelectric displacement decreases. In order to solve the foregoing problem, it is effective to employ a spring structure (flexure structure) as part of the beam. 
     However, the following problem arises as seen from the description of the following document 1 if the spring structure is symmetrically given on both sides of the electrode of the variable capacitor. Specifically, a restoring force of the electrode becomes weak from a state that electrodes contact each other. As a result, a failure resulting from stiction is easy to occur. 
     Document 1: K. F. Harsh et al., “The realization and design considerations of a flip-flop integrated MEMS tunable capacitor”, Sensors and Actuators 80 (2000) 108-118. 
     Even if the spring structure is employed, a fixed electrode may be further provided above the electrode to sufficiently secure the restoring force as seen from the description of the following document 2. 
     Document 2: D. Peroulis et al., “Electromechanical Considerations in Developing Low-Voltage RF MEMS Switches”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL 51, No. 1, January 2003. 
     However, according to the structure that is described in document 2, a cavity must be formed double, and in addition, the fixed electrode must be formed thick. For this reason, the following problem arises. Specifically, the process becomes complicated, and as a result, the cost increases. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a micro-electro-mechanical system (MEMS) comprising: a first electrode interposed between a first fixed end and a second fixed end, the first electrode being movable by an actuator element. 
     A shape from the first electrode to the first fixed end and a shape from the first electrode to the second fixed end are asymmetrical. 
     According to a second aspect of the present invention, there is provided a micro-electro-mechanical system (MEMS) comprising: a first fixed end formed on a substrate; a second fixed end formed on the substrate separating from the first fixed end; a first actuator whose one end is connected to a first fixed end; a second actuator whose one end is connected to a second fixed end; a first electrode arranged between the other end of the first actuator and the other end of the second actuator; and a spring structure member arranged at one of (i) a position between the first fixed end and the first electrode and (ii) a position between the second fixed end and the first electrode. 
     According to a third aspect of the present invention, there is provided a micro-electro-mechanical system (MEMS) comprising: a first fixed end formed on a substrate; a second fixed end formed on the substrate separating from the first fixed end; a beam having one end fixed to the first fixed end and the other end fixed to the second fixed end so that a cavity is formed between the beam and the substrate; a first electrode formed on a surface of the beam facing the substrate; a first actuator arranged in the beam between the first fixed end and the first electrode; a second actuator arranged in the beam between the second fixed end and the first electrode; and a spring structure member arranged in the beam between the first electrode and the second actuator. 
     The beam is set so that a spring constant between the first actuator and the first electrode differs from a spring constant between the second actuator and the first electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a perspective view showing the structure of a MEMS device according to a first embodiment of the present invention; 
         FIG. 2A  is a perspective view showing the structure of the MEMS device according to the first embodiment; 
         FIG. 2B  is a view showing a spring constant between first and second fixed ends corresponding to  FIG. 2A ; 
         FIG. 3A  is a perspective view showing a MEMS device having a spring structure member symmetrically arranged; 
         FIG. 3B  is a view showing a spring constant between first and second fixed ends corresponding to  FIG. 3A ; 
         FIG. 4  is a graph showing a temperature dependency of the maximum displacement when a thermal type actuator is used in place of first and second piezoelectric type actuator in the MEMS device of the first embodiment; 
         FIG. 5A  is a perspective view showing the MEMS device of the first embodiment; 
         FIG. 5B  is a view showing a spring constant between first and second fixed ends corresponding to  FIG. 5A  in view of an electrode; 
         FIG. 6A  is a perspective view showing a MEMS device having a spring structure member symmetrically arranged; 
         FIG. 6B  is a view showing a spring constant between first and second fixed ends corresponding to  FIG. 6A  in view of an electrode; 
         FIG. 7  is a graph showing a force dependency of the displacement when a thermal type actuator is used in place of first and second piezoelectric type actuator in the MEMS device of the first embodiment; 
         FIG. 8  is a top plan view showing a variable capacitor using a MEMS device according to a second embodiment of the present invention; 
         FIG. 9  is a cross-sectional view taken along the line  9 - 9  of  FIG. 8 ; 
         FIG. 10  is a top plan view showing a variable capacitor using a MEMS device according to a third embodiment of the present invention; 
         FIG. 11  is a cross-sectional view taken along the line  11 - 11  of  FIG. 10 ; 
         FIG. 12  is a top plan view showing a variable capacitor using a MEMS device according to a fourth embodiment of the present invention; 
         FIG. 13  is a cross-sectional view taken along the line  13 - 13  of  FIG. 12 ; 
         FIG. 14  is a top plan view showing a capacitive switch using a MEMS device according to a fifth embodiment of the present invention; 
         FIG. 15  is a cross-sectional view taken along the line  15 S- 15 S of  FIG. 14 ; 
         FIG. 16  is a top plan view showing a switch using a MEMS device according to a sixth embodiment of the present invention; and 
         FIG. 17  is a cross-sectional view taken along the line  17 - 17  of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various embodiments of the present invention will be hereinafter described with reference to the accompanying drawings. In the following description, the same reference numerals are used to designate common portions over all drawings. 
     First Embodiment 
     A MEMS device according to a first embodiment of the invention will be hereinafter described. 
       FIG. 1  is a perspective view showing the structure of a MEMS device according to a first embodiment. 
     The MEMS device has a bridge structure given in a manner that a beam is supported by a first and a second fixed end  11  and  12 . As shown in  FIG. 1 , the first fixed end  11  is connected with one end of a first piezoelectric type actuator  13 - 1 . The second fixed end  12  is connected with one end of a second piezoelectric type actuator  13 - 2 . The other end of the first piezoelectric type actuator  13 - 1  is connected with one end of an electrode  14 . The other end of the electrode  14  is connected to the other end of the second piezoelectric type actuator  13 - 2  via spring structure members (flexure structure members)  15 - 1  and  15 - 2 . As described above, the MEMS device of the first embodiment has the following asymmetrical structure given on both sides of the electrode  14 . One is a structure from the electrode  14  to the first fixed end  11 . Another is a structure from the electrode  14  to the second fixed end  12 . In other words, a shape from the electrode  14  to the first fixed end differs from a shape from the electrode  14  to the second fixed end  12 . 
     The effect of the MEMS device having the structure shown in  FIG. 1  will be hereinafter explained as compared with the following MEMS device. The MEMS device has structures symmetrically given on the electrode  14  as illustrated in  FIG. 3A . 
       FIG. 3A  is a perspective view showing a MEMS device having a structure from the electrode  14  to the first fixed end  11  and a structure from the electrode  14  to the second fixed end  12 , which are symmetrically formed. Piezoelectric type actuators  13 - 1  and  13 - 2  are provided on both sides of the electrode  14  via spring structure members  15 - 1  and  15 - 2 . 
       FIG. 2A  shows the MEMS device of the first embodiment.  FIG. 2B  is a view showing a spring constant between first and second fixed ends corresponding to  FIG. 2A .  FIG. 3B  a view showing a spring constant between first and second fixed ends corresponding to  FIG. 3A . 
     As seen from  FIG. 2B  and  FIG. 3B , the spring constant between the first and second fixed ends  11  and  12  has no change, that is, K 1 =kK/(2 k+2K) even if spring structure members  15 - 1  and  15 - 2  are asymmetrically or symmetrically arranged. In this case, each spring constant of the first and second piezoelectric type actuators  13 - 1  and  13 - 2  is set as K. Each spring constant of the spring structure members  15 - 1  and  15 - 2  is set as k. In the MEMS device having the bridge structure, the spring structure members are arranged to relax tensile stress. As seen from the foregoing description, the effect does not depend on the inserted position of the spring structure members. Thus, even if the spring structure members are given on one side only of the electrode like the first embodiment, the same effect is obtained. Specifically, the tensile stress is relaxed like the case where the spring structure members are arranged on both sides of the electrode. 
     The foregoing effect is confirmable from the simulation result shown in  FIG. 4  expressing a temperature dependency of the maximum displacement of the electrode.  FIG. 4  is a graph showing a temperature dependency of the maximum displacement when a thermal type actuator is used in place of first and second piezoelectric type actuator in the MEMS device of the first embodiment. The maximum displacement of the electrode in the asymmetrical spring structure members is larger than that in the symmetrical spring structure members. This results from the following reason. Specifically, the asymmetrical spring structure member obtains the same relaxed effect of tensile stress; in addition, the distance between the electrode and the actuator is close. As a result, the force of the actuator is easy to be transmitted. Here, the simulation result when the thermal type actuator is used in place of first and second piezoelectric type actuator is given. The same simulation result is obtained even if piezoelectric type or electrostatic type, electromagnetic type or hybrid type actuators combining those are used. 
       FIG. 5A  shows the MEMS device of the first embodiment.  FIG. 5B  is a view showing a spring constant between first and second fixed ends corresponding to  FIG. 5A  in view of the electrode.  FIG. 6A  shows the MEMS device having spring structure members symmetrically arranged like  FIG. 3A .  FIG. 6B  is a view showing a spring constant between first and second fixed ends corresponding to  FIG. 6A  in view of the electrode. 
     According to the first embodiment, the spring structure members  15 - 1  and  15 - 2  are asymmetrically arranged. When K&gt;&gt;k, a spring constant is K 2 =K+k/(2+k/K)=O(K) in view of the electrode  14  between the first and second fixed ends  11  and  12  as seen from  FIG. 5B . In  FIG. 6A , the spring structure members  15 - 1  and  15 - 2  are symmetrically arranged. A spring constant between first and second fixed ends  11  and  12  is K 2 ′=2 k/(1+k/K)=O(k) as seen from  FIG. 6B . In this case, each spring constant of the first and second piezoelectric type actuators  13 - 1  and  13 - 2  is set as K. Each spring constant of the spring structure members  15 - 1  and  15 - 2  is set as k. Therefore, the following matter can be seen from the spring constant in view of the electrode. Namely, the spring constant in the case where the spring structure members are asymmetrically arranged is larger than that in the case where the spring structure members are symmetrically arranged. Thus, the MEMS device having spring structure members asymmetrically arranged has a strong force of restoring a top electrode. In other words, the restoring force acting on the electrode becomes large in the MEMS device having the following structure. Specifically, a MEMS device having the spring structure members on one side of the electrode has strong restoring force as compared with a MEMS device having spring structure members symmetrically arranged on both sides of the electrode. 
     The foregoing effect is confirmable from  FIG. 7 , that is, a simulation result of displacement when force is applied to the electrode.  FIG. 7  shows the case where a thermal type actuator is used in place of first and second piezoelectric type actuators in the MEMS device of the first embodiment. The displacement of the electrode in the asymmetrical spring structure members becomes smaller than that in the symmetrical spring structure members. This means that the restoring force of the asymmetrical spring structure members is larger than that of the symmetrical spring structure members. In other words, the MEMS device having the asymmetrical spring structure members has a strong force of restoring the top electrode as compared with the MEMS device having the symmetrical spring structure members. In this case, the simulation result when the thermal type actuator is used in place of first and second piezoelectric type actuators is given. The same simulation result is obtained even if piezoelectric type or electrostatic type, electromagnetic type or hybrid type actuators combining those are used. 
     Second Embodiment 
     A MEMS device according to a second embodiment of the invention will be described below. According to the second embodiment, a variable capacitor using a MEMS device having piezoelectric type and electrostatic type actuators will be described. 
       FIG. 8  is a top plan view showing a variable capacitor using a MEMS device of the second embodiment.  FIG. 9  is a cross-sectional view taken along the line  9 - 9  of  FIG. 8 . 
     The variable capacitor using a MEMS device includes variable capacitance member  21 , electrostatic type actuators  22 - 1 ,  22 - 2 , piezoelectric type actuators  23 - 1 ,  23 - 2  and spring structure member  15 . Anchors  24 - 1  and  24 - 2  are arranged on a semiconductor substrate  10  separate from each other. A beam, that is elastic member  16  is fixed to these anchors  24 - 1  and  24 - 2  at both its ends. In the structure in which both ends of the elastic member  16  are fixed to anchors  24 - 1  and  24 - 2  on the semiconductor substrate  10 , the following components are linearly arranged along one direction. The components are piezoelectric type actuator  23 - 1 , electrostatic type actuator  22 - 1 , variable capacitance member  21 , spring structure member  15 , electrostatic type actuator  22 - 2  and piezoelectric type actuator  23 - 2 . A cavity  25  is formed between the semiconductor substrate  10  and the elastic member  16 . Piezoelectric type actuators  23 - 1 ,  23 - 2 , electrostatic type actuators  22 - 1  and  22 - 2  are actuated, and thereby, the center portion of the elastic member  16  is deformed to close to the semiconductor substrate  10 . 
     The variable capacitance member  21  includes a top electrode  31  formed in the elastic member  16 , and bottom electrodes  32  and  33  formed on the semiconductor substrate  10 . The top electrode  31  becomes a floating state, and actuators  22 - 1 ,  22 - 2 ,  23 - 1  and  23 - 2  are actuated, and thereby, the distance between the top electrode  31  and bottom electrodes  32 ,  33  changes. The top electrode  31  of the variable capacitance member  21  is lowered to the bottom electrodes side by actuators  22 - 1 ,  22 - 2 ,  23 - 1  and  23 - 2 , and thereby, the top electrode  31  closes to bottom electrodes  32  and  33 . By doing so, bottom electrodes  32  and  33  are combined in its capacitance. In a state that the top electrode  31  is upwardly positioned, a gap of 1.5 μm is formed between the top electrode  31  and an insulating film  44 . Therefore, in this state, the capacitance between the bottom electrodes  32  and  33  becomes so small as possible to disregard. As described above, the top electrode  31  is vertically moved, and thereby, a digital variable capacitor is formed. In the digital variable capacitor, the capacitance value between the bottom electrodes  32  and  33  has a binary change. 
     The following is an explanation about a hybrid type actuator controlling an inter-electrode distance of the variable capacitance member  21  and the spring structure member  15 . The hybrid type actuator has the electrostatic type actuators  22 - 1 ,  22 - 2 , and the piezoelectric type actuators  23 - 1  and  23 - 2 . The electrostatic type actuator  22 - 1  is arranged on one end side of the variable capacitance member  21 . The electrostatic type actuator  22 - 1  is composed of the top electrode  34 - 1  and the bottom electrode  35 - 1 . The electrostatic type actuator  22 - 2  is arranged on the other end side of the variable capacitance member  21  via the spring structure member  15 . The electrostatic type actuator  22 - 2  is composed of the top electrode  34 - 2  and the bottom electrode  35 - 2 . 
     Piezoelectric type actuators  23 - 1  and  23 - 2  are provided between electrostatic type actuator  22 - 1  and anchor  24 - 1  and between electrostatic type actuator  22 - 2  and anchor  24 - 2 , respectively. These piezoelectric type actuators  23 - 1  and  23 - 2  have piezoelectric films  36 - 1 ,  36 - 2 , piezoelectric drive top electrodes  37 - 1 ,  37 - 2 , and bottom electrodes  38 - 1 ,  38 - 2 , respectively. In this case, these top electrodes  37 - 1  and  37 - 2  are provided on the bottom electrodes  38 - 1  and  38 - 2  via piezoelectric films  36 - 1  and  36 - 2 . For example, AlN and PZT are used as the material for these piezoelectric films  36 - 1  and  36 - 2 . 
     An insulating film  41  is formed on the top electrode  31  of the variable capacitance member  21 , the top electrodes  34 - 1 ,  34 - 2  of electrostatic type actuators  22 - 1 ,  22 - 2  and the top electrodes  37 - 1 ,  37 - 2  of piezoelectric type actuators  23 - 1 ,  23 - 2 . An insulating film  42  is formed under bottom electrodes  38 - 1  and  38 - 2  of piezoelectric type actuators  23 - 1  and  23 - 2 . Moreover, an insulating film  43  is formed on the semiconductor substrate  10 . The bottom electrodes  32  and  33  of the variable capacitance member  21  and the bottom electrodes  35 - 1  and  35 - 2  of the electrostatic type actuators  22 - 1  and  22 - 2  are formed on the insulating film  43 . An insulating film  44  is formed on these bottom electrodes  32 ,  33 ,  35 - 1 ,  35 - 2  and the insulating film  43 . As shown in  FIG. 8 , the top electrodes  31 ,  34 - 1  and  34 - 2  are formed with holes  17  like a matrix. The holes  17  are used for performing uniform etching in an etching process for forming the cavity  25 . These holes  17  contribute for reducing air resistance, and serve to perform high-speed switching. Of course, the holes  17  are not always formed, and even if no hole  17  is formed, the essential effect of this embodiment has no change. 
     The foregoing structure is given, and thereby, potential difference is applied between the top electrodes  37 - 1 ,  37 - 2  and the bottom electrodes  38 - 1 ,  38 - 2  in the piezoelectric type actuators  23 - 1  and  23 - 2 . By doing so, the piezoelectric films  36 - 1  and  36 - 2  displace so that the center portion of the elastic member  16  downwardly displaces. Both unimolf type and bimolf type actuator are used as the piezoelectric type actuators  23 - 1  and  23 - 2 . The first potential difference is applied between the top electrodes  37 - 1  and  37 - 2  and the bottom electrodes  38 - 1  and  38 - 2  in the piezoelectric type actuators  23 - 1  and  23 - 2  to downwardly displace top electrodes  31 ,  34 - 1  and  34 - 2 . By doing so, the top electrodes  34 - 1 ,  34 - 2  close to the bottom electrodes  35 - 1 ,  35 - 2 . In this state, the second potential difference is applied between the top electrodes  34 - 1 ,  34 - 2  and the bottom electrodes  35 - 1 ,  35 - 2 , respectively. The second potential difference may be the same as the first potential difference, or may be smaller or larger than the first potential difference. Thus, the top electrode  31  of the variable capacitance member  21  further downwardly displaces, and thereby, the distance between the top electrode  31  and bottom electrodes  32 ,  33  becomes narrow. Therefore, the capacitance value of the variable capacitance member  21  has a binary change. 
     In order to restore the top electrode  31  of the variable capacitance member  21  after to upwardly displace it, the following operation is made. Specifically, the potential difference of electrostatic type actuators  22 - 1  and  22 - 2  is offset, and thereafter or simultaneously, the potential difference of piezoelectric type actuators  23 - 1  and  23 - 2  is offset. 
     According to the foregoing structure, the spring structure member  15  is interposed between the variable capacitance member  21  and the electrostatic type actuator  22 - 2 . In other words, the spring structure member  15  is interposed between the top electrodes  31  and  34 - 2 . Thus, tensile stress of the elastic member  16  between the anchors  24 - 1  and  24 - 2  is relaxed. By doing so, the displacement of the top electrode  31  of the variable capacitance member  21  increases. Therefore, it is possible to reduce a force required for downwardly displacing the top electrode  31 . 
     Moreover, the spring structure member  15  is interposed between one side of the variable capacitance member  21  and the electrostatic type actuator  22 - 2 . In this case, the top electrode  31  is restored from a state that it contacts with the insulating film  44 . Thus, a force of restoring the top electrode  31  is larger than the case where the spring structure member is arranged on both side of the top electrode  31 . 
     As described above, according to the second embodiment, the spring structure member  15  is interposed between one side of the variable capacitance member  21  and the electrostatic type actuator  22 - 2 . By doing so, the tensile stress between anchors is relaxed. Moreover, the displacement of the top electrode  31  increases while it is possible to reduce a force required for downwardly displacing the top electrode  31 . Simultaneously, a restoring force acting on the top electrode  31  is improved in a state that the top electrode  31  contacts with the insulating film  44 . 
     According to the second embodiment, the spring structure member  15  is arranged on one side only of the top electrode  31  of the variable capacitance member  21 . For example, the spring structure members may be arranged on both sides of the top electrode of the variable capacitance member. In this case, the spring constant of one spring structure member may be set to a value different from that of the other spring structure member. By doing so, the same effect as above is obtained. 
     Third Embodiment 
     A MEMS device according to a third embodiment of the invention will be hereinafter described. The same reference numerals are used to designate portions identical to the second embodiment, and the details are omitted. According to the third embodiment, a variable capacitor using a MEMS device having a piezoelectric type actuator will be explained. 
       FIG. 10  is a top plan view showing a variable capacitor using a MEMS device according to the third embodiment.  FIG. 11  is a cross-sectional view taken along the line  11 - 11  of  FIG. 10 . 
     The variable capacitor using the MEMS device includes the variable capacitance member  21 , piezoelectric type actuators  23 - 1 ,  23 - 2  and the spring structure member  15 . According to the third embodiment, these piezoelectric type actuators  23 - 1  and  23 - 2  are actuated, and thereby, the distance between the top electrode  31  and the bottom electrodes  32 ,  33  is changed. By doing so, it is possible to form a digital variable capacitor in which the capacitance value between bottom electrodes  32  and  33  has a binary change. 
     The piezoelectric type actuator controlling the inter-electrode distance of the variable capacitance member  21  and the spring structure member  15  will be described below. The piezoelectric type actuator  23 - 1  is arranged on one end side of the variable capacitance member  21 . The piezoelectric type actuator  23 - 2  is arranged on the other end side of the variable capacitance member  21  via the spring structure member  15 . These piezoelectric type actuators  23 - 1  and  23 - 2  are further connected to anchors  24 - 1  and  24 - 2 , respectively. 
     An insulating film  41  is formed on the top electrode  31  of the variable capacitance member  21  and the top electrodes  37 - 1 ,  37 - 2  of the piezoelectric type actuators  23 - 1 ,  23 - 2 . An insulating film  42  is formed under bottom electrodes  38 - 1 ,  38 - 2  of the piezoelectric type actuators  23 - 1 ,  23 - 2 . Moreover, an insulating film  43  is formed on the semiconductor substrate  10 . Bottom electrodes  32  and  33  of the variable capacitance member  21  are formed on the insulating film  43 . An insulating film  44  is formed on these bottom electrodes  32 ,  33  and insulating film  43 . 
     The foregoing structure is given, and thereby, a potential difference is applied between the top electrodes  37 - 1 ,  37 - 2  and the bottom electrodes  38 - 1 ,  38 - 2  in piezoelectric type actuators  23 - 1 ,  23 - 2 . By doing so, piezoelectric films  36 - 1  and  36 - 2  displace; as a result, the beam downwardly displaces. Thus, the top electrode  31  of the variable capacitance member  21  displaces onto the side of bottom electrodes  32 ,  33 . Therefore, the distance between the top electrode  31  and the bottom electrodes  32 ,  33  becomes narrow, and the capacitance value of the variable capacitance member  21  has a binary change. In order to restore the top electrode  31  of the variable capacitance member  21  after to upwardly displace it, the potential difference of piezoelectric type actuators  23 - 1 ,  23 - 2  is offset. 
     According to the foregoing structure, the spring structure member  15  is interposed between the variable capacitance member  21  and the piezoelectric type actuator  23 - 2 . Thus, the tensile stress acting on the beam between anchors  24 - 1  and  24 - 2  is relaxed. By doing so, the displacement of the top electrode  31  increases in the variable capacitance member  21 . This serves to reduce a force required for downwardly displacing the top electrode  31 . 
     Moreover, the spring structure member  15  is interposed between one side of the variable capacitance member  21  and the piezoelectric type actuator  23 - 2 . When the top electrode  31  is restored from the insulating film  44  on bottom electrodes  32  and  33 , the restoring force is larger than the case where spring structure members are arranged on both side of the top electrode  31 . Other structure and effect are the same as the second embodiment. 
     Fourth Embodiment 
     A MEMS device according to a fourth embodiment of the invention will be hereinafter described. The same reference numerals are used to designate portions identical to the second embodiment, and the details are omitted. According to the fourth embodiment, a variable capacitor using a MEMS device having an electrostatic type actuator will be explained. 
       FIG. 12  is a top plan view showing a variable capacitor using a MEMS device according to the fourth embodiment.  FIG. 13  is a cross-sectional view taken along the line  13 - 13  of  FIG. 12 . 
     The variable capacitor using the MEMS device includes the variable capacitance member  21 , the electrostatic type actuators  22 - 1 ,  22 - 2  and the spring structure member  15 . According to the fourth embodiment, electrostatic type actuators  22 - 1  and  22 - 2  are actuated, and thereby, the distance between the top electrode  31  and bottom electrodes  32 ,  33  is changed. By doing so, it is possible to form a digital variable capacitor in which the capacitance value between bottom electrodes  32  and  33  has a binary change. 
     The electrostatic type actuator controlling the inter-electrode distance of the variable capacitance member  21  and the spring structure member  15  will be described below. The electrostatic type actuator  22 - 1  is arranged on one end side of the variable capacitance member  21 . The electrostatic type actuator  22 - 2  is arranged on the other end side of the variable capacitance member  21  via the spring structure member  15 . These electrostatic type actuators  22 - 1  and  22 - 2  are further connected to anchors  24 - 1  and  24 - 2 , respectively. 
     An insulating film  41  is formed on the top electrode  31  of the variable capacitance member  21  and the top electrodes  34 - 1 ,  34 - 2  of the electrostatic type actuators  22 - 1 ,  22 - 2 . Moreover, an insulating film  43  is formed on the semiconductor substrate  10 . The bottom electrodes  32  and  33  of the variable capacitance member  21  and the bottom electrodes  35 - 1 ,  35 - 2  of electrostatic type actuators  22 - 1 ,  22 - 2  are formed on the insulating film  43 . An insulating film  44  is formed on these bottom electrodes  32 ,  33 ,  35 - 1 ,  35 - 2  and insulating film  43 . 
     The foregoing structure is given, and thereby, potential difference is applied between the top electrodes  34 - 1 ,  34 - 2  and the bottom electrodes  35 - 1 ,  35 - 2  in electrostatic type actuators  22 - 1  and  22 - 2 , respectively. By doing so, the center portion of the elastic member  16  downwardly moves. Thus, the top electrode  31  of the variable capacitance member  21  displaces onto the side of bottom electrodes  32  and  33 . Therefore, the distance between the top electrode  31  and the bottom electrodes  32 ,  33  becomes narrow, and thereby, the capacitance value of the variable capacitance member  21  has a binary change. In order to restore the top electrode  31  of the variable capacitance member  21  after to upwardly displace it, the potential difference applied to electrostatic type actuators  22 - 1  and  22 - 2  is offset. 
     According to the foregoing structure, the spring structure member  15  is interposed between the variable capacitance member  21  and the electrostatic type actuator  22 - 2 . Thus, the tensile stress acting on the beam between anchors  24 - 1  and  24 - 2  is relaxed. By doing so, the displacement of the top electrode  31  increases in the variable capacitance member  21 . This serves to reduce a force required for downwardly displacing the top electrode  31 . 
     Moreover, the spring structure member  15  is interposed between one side of the variable capacitance member  21  and the electrostatic type actuator  22 - 2 . When the top electrode  31  is restored from the insulating film  44  on bottom electrodes  32  and  33 , the restoring force is larger than the case where spring structure members are arranged on both side of the top electrode  31 . Other structure and effect are the same as the second embodiment. 
     Fifth Embodiment 
     A MEMS device according to a fifth embodiment of the invention will be hereinafter described. The same reference numerals are used to designate portions identical to the second embodiment, and the details are omitted. According to the fifth embodiment, a capacitive switch using a MEMS device having an electrostatic type actuator will be explained. 
       FIG. 14  is a top plan view showing a capacitive switch using a MEMS device according to the fifth embodiment.  FIG. 15  is a cross-sectional view taken along the line  15 S- 15 S of  FIG. 14 . 
     The capacitive switch using the MEMS device includes a switch member  51  and the spring structure member  15 . An insulating film  52  is formed on a semiconductor substrate  10 . A bottom electrode  53  of the switch member  51  is formed on the insulating film  52 . Moreover, an insulating film  54  is formed on the bottom electrode  53  and the insulating film  52  to cover the bottom electrode  53 . 
     Insulating films  55 - 1  and  55 - 2  are formed on the semiconductor substrate  10  separate from each other. Both ends of a top electrode  56  are fixed onto these insulating films  55 - 1  and  55 - 2 . A cavity  25  is formed between the semiconductor substrate  10  and the top electrode  56 . 
     The switch member  51  is composed of top electrode  56 , bottom electrode  53 , and insulating film  54  on the bottom electrode  53 . The switch  51  functions as an electrostatic actuator, and the top electrode  56  and the bottom electrode  53  are used as an electrode of the electrostatic actuator. The spring structure member  15  is interposed between the top electrode  56  forming the switch member  51  and the top electrode  56  on the insulating film  55 - 2 . The top electrode  56  is supplied with a ground voltage while the bottom electrode  53  is supplied with a signal voltage. In the capacitive switch having the foregoing structure, the top electrode  56  vertically moves, and thereby, the distance between the bottom electrode  53  and the top electrode  56  changes. By doing so, the capacitance value between the bottom electrode  53  and the top electrode  56  changes. 
     According to the foregoing structure, the spring structure member  15  is interposed between the top electrode  56  forming the switch member  51  and the top electrode  56  on the insulating film  55 - 2 . The tensile stress of the top electrode  56  between insulating films  55 - 1  and  55 - 2  is relaxed. By doing so, the displacement of the top electrode  56  is increased; therefore, this serves to reduce a force required for downwardly displacing the top electrode  56 . 
     Moreover, the spring structure member  15  is interposed between one side of the switch member  51  and the insulating film  55 - 2  using the switch member as a reference. When the top electrode  56  is restored from a state that it contacts with the insulating film  54 , the restoring force is larger as compared with the case where the spring structure members are arranged on both sides of the switch member  51 . 
     According to the fifth embodiment, the spring structure member  15  is interposed between one side of the switch member  51  and the insulating film  55 - 2 . By doing so, the tensile stress of the top electrode  56  between insulating films  55 - 1  and  55 - 2  is relaxed. Thus, the displacement of the top electrode  56  is increased, and therefore, this serves to reduce a force required for downwardly displacing the top electrode  56 . Simultaneously, a restoring force acting on the top electrode  56  is improved in a state that the top electrode  56  contacts with the insulating film  54 . 
     Sixth Embodiment 
     A MEMS device according to a sixth embodiment of the present invention will be hereinafter described. The same reference numerals are used to designate portions having the same configuration and effect as the second embodiment, and the explanation is omitted. According to the foregoing embodiments, the MEMS device is applied to the variable capacitor and the capacitive switch. In this case, the MEMS device is applicable to a contact type switch. The sixth embodiment relates to a contact type switch using a MEMS device having an electrostatic actuator. 
       FIG. 16  is a top plan view showing a switch using a MEMS device according to a sixth embodiment.  FIG. 17  is a cross sectional view taken along the line  17 - 17  of  FIG. 16 . 
     The switch using the MEMS device includes a switch  61 , insulators  62 - 1 ,  62 - 2 , electrostatic actuators  22 - 1 ,  22 - 2  and a spring structure member  15 . According to the sixth embodiment, the foregoing electrostatic actuators  22 - 1  and  22 - 2  are actuated, and thereby, the top electrode  31  contacts with the bottom electrodes  32  and  33 . By doing so, a conductive state is given between the bottom electrodes  32  and  33 . In other words, when the electrostatic actuators  22 - 1  and  22 - 2  are not actuated, a shut-off state is given between the bottom electrodes  32  and  33 . On the other hand, when the electrostatic actuators  22 - 1  and  22 - 2  are actuated so that the top electrode  31  contacts with the bottom electrodes  32  and  33 , a conductive state is given between the bottom electrodes  32  and  33 . As is evident from the foregoing description, the switch can take a shut-off state (off state) and a conductive state (on state). 
     The electrostatic actuator controlling the inter-electrode distance of the switch  61  and the spring structure member  15  will be hereinafter described. The electrostatic actuator  22 - 1  is arranged on one side of the switch  61 . The electrostatic actuator  22 - 2  is arranged on the other side of the switch  61  via the spring structure member  15 . An insulator  62 - 1  is interposed between the top electrode  31  of the switch  61  and a top electrode  63 - 1  of the electrostatic actuator  22 - 1  to insulate these components. An insulator  62 - 2  is interposed between the top electrode  31  of the switch  61  and the spring structure member  15  to insulate these components. The top electrode  63 - 1  of the electrostatic actuator  22 - 1  is connected to the anchor  24 - 1 . Moreover, a top electrode  63 - 2  of the electrostatic actuator  22 - 2  is connected to the anchor  24 - 2 . An elastic member  16  is configured so that the top electrode  63 - 1  of the electrostatic actuator  22 - 1 , the insulator  62 - 1 , the top electrode  31  of the switch  61 , the insulator  62 - 2 , the spring structure member  15  and the top electrode  63 - 2  of the electrostatic actuator  22 - 2  are linearly arranged along one direction. 
     An insulating film  43  is formed on a semiconductor substrate  10 . Bottom electrodes  32 ,  33  of the switch  61  and the bottom electrodes  35 - 1 ,  35 - 2  of the electrostatic actuators  22 - 1 ,  22 - 2  are formed on the insulating film  43 . An insulating film  44  is formed on these bottom electrodes  35 - 1 ,  35 - 2  and the insulating film  43 . 
     The foregoing structure is given, and thereby, a potential difference is applied between the top electrodes  63 - 1 ,  63 - 2  and the bottom electrodes  35 - 1 ,  35 - 2  in the electrostatic actuators  22 - 1  and  22 - 2 . By doing so, the center portion of the elastic member  16  downwardly moves. Thus, the top electrode  31  of the switch  61  displaces onto the side of the bottom electrodes  32  and  33  so that it contacts with these bottom electrodes  32  and  33 . Therefore, a conductive state is given between the bottom electrodes  32  and  33 , and thereby, the switch  61  becomes an on state. In order to restore the top electrode  31  of the switch  61  by upwardly displacing it, a potential difference is not given between the electrostatic actuators  22 - 1  and  22 - 2 . 
     According to the structure, the spring structure member  15  is interposed between one side of the switch  61  and the electrostatic actuator  22 - 2 . The tensile stress of the elastic member  16  between the anchors  24 - 1  and  24 - 2  is relaxed. By doing so, the displacement of the top electrode  31  of the switch  61  is increased. This serves to reduce a force required for downwardly displacing the top electrode  31 . Moreover, when the top electrode  31  is restored from the bottom electrodes  32  and  33 , the restoring force is larger in the case where the spring structure member  15  is arranged on one side only of the top electrode  31  as compared with the case where the spring structure member is arranged on both sides of the top electrode  31 . 
     According to the sixth embodiment, the spring structure member  15  is interposed between one side of the switch  61  and the electrostatic actuator  22 - 2 . By doing so, the tensile stress between anchors is relaxed, and the displacement of the top electrode  31  increases. Therefore, this serves to reduce a force required for downwardly displacing the top electrode  31 . In addition, in a state that the top electrode  31  contacts with the bottom electrodes  32  and  33 , a restoring force acting on the top electrode  31  is improved. 
     According to the foregoing embodiments, the electrostatic actuator and the piezoelectric actuator are used. In this case, an electromagnetic or a thermal type actuator may be used in place of the preceding actuators. 
     According the foregoing embodiments of the invention, there is provided a MEMS device, which can relax the tensile stress of the beam provided with an electrode, and improve a restoring force of the electrode. 
     In the foregoing, each embodiment is solely carried out, and in addition, may be combined. Various inventive steps are included in the foregoing embodiments. Constituent components disclosed in the foregoing embodiments are properly combined, and thereby, various inventive steps are extracted. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.