Patent Publication Number: US-2010126834-A1

Title: Switch and esd protection element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-302759, filed Nov. 27, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     As devices using micro-electro-mechanical systems (MEMS), a variable capacitor, a switch, an acceleration sensor, a pressure sensor, a radio frequency (RF) filter, a gyroscope, a mirror device and the like are mainly investigated and developed. 
     Among these devices, the MEMS switch does not cause any leakage when the switch turns off, and hence the switch is effective for reducing the power consumption (electric current consumption) of a chip. Moreover, the MEMS switch can be formed on interconnection layers of a semiconductor integrated circuit (IC), and hence a chip area can be reduced as compared with a case where an element or a circuit formed on the surface of a wafer (the chip) is used as the switch as in, for example, a CMOS inverter. 
     Therefore, a technology is suggested in which the MEMS switch is used for a technique referred to as “power gating” (e.g., see A. Raychowdhury, Jeong I l Kim, D. Peroulis, K. Roy, “Integrated MEMS switches for Leakage Control of Battery Operated Systems”, Porc. IEEE 2006 Custom Integrated Circuits Conference (CICC), pp. 457 to 460). Power gating is a technology for reducing the power consumption of an IC. In this technology, the supply of a power voltage to an unused circuit block is blocked by a switch (hereinafter referred to as a power gating switch) to suppress the power consumption due to leakage. 
     The MEMS switch is provided as such a power gating switch between the circuit block and a power supply to decrease the power consumption. 
     In the literature of Raychowdhury et al., a thermal driving type MEMS switch is used as a power gating switch. The thermal driving type MEMS switch has an advantage that the switch operates at a low voltage. On the other hand, the thermal driving type MEMS switch uses a large driving current, and eventually the power consumption increases. Furthermore, the thermal driving type MEMS switch has a large size and a low switching speed as compared with a MEMS switch of another driving type. 
     Moreover, based on the fact that the MEMS switch should be interposed between the circuit block and the outside (or the power supply, a pad), it is expected that the MEMS switch is used in an electrostatic discharge (ESD) protection element. In this case however, the same problems as in the power gating technology may arise. 
     SUMMARY 
     A switch according to an aspect of the present invention comprising: first and second electrodes provided on a substrate; an anchor provided on the first electrode; a movable structure of which a first end is supported by the anchor, extending from the anchor to a position above the second electrode, using a conductor, and configured to move in a vertical direction with respect to the second electrode; a contact portion provided at a second end of the movable structure and disposed above the second electrode; a film having a different stress value with respect to the stress value of the movable structure, and warping the contact portion toward the second electrode; and a cap provided on the substrate to cover the movable structure, configured to be in contact with the film, and functioning as a driving electrode. 
     A switch according to an aspect of the present invention comprising: first and second electrodes provided on a substrate; a third electrode provided on the substrate and between the first electrode and the second electrode; an anchor provided on the first electrode; a movable structure of which a first end is supported by the anchor, extending from the anchor to a position above the second electrode, using a conductor, and configured to move in a vertical direction with respect to the second electrode; a contact portion provided at a second end of the movable structure and disposed above the second electrode; a first film provided on the upper surface of the movable structure, the first film having a different stress value with respect to the stress value of the movable structure, and warping the contact portion toward the second electrode; a second film provided on the bottom surface of the movable structure, the second film having a different stress value with respect to the stress value of the movable structure, warping the movable structure toward the third electrode and configured to be in contact with the third electrode; and a cap provided on the substrate to cover the movable structure and being into contact with the first film. 
     An ESD protection element according to an aspect of the present invention comprising: a first terminal provided on a substrate of an IC chip and connected to a first interconnection which is connected to a device to be protected; and a switch provided on the substrate between the first terminal and a second interconnection and including a movable structure, wherein when the IC chip is non-active, the switch has a low impedance state, and when the IC chip is active, the switch has a high impedance state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1A  is a plan view showing a structure of an MEMS switch; 
         FIG. 1B  is a sectional view cut along the line A-A′ of  FIG. 1A ; 
         FIG. 10  is a sectional view cut along the line B-B′ of  FIG. 1A ; 
         FIGS. 2A to 2D  are sectional views showing the manufacturing steps of the MEMS switch; 
         FIGS. 3A and 3B  are diagrams for explaining the operation of the MEMS switch of  FIG. 1 ; 
         FIGS. 4A ,  4 B,  5 A and  5 B are diagrams for explaining the structure and operation of the MEMS switch; 
         FIGS. 6 ,  7 A,  7 B,  8 A,  8 B and  8 C are diagrams for explaining an application example of the MEMS switch; 
         FIGS. 9 and 10  are diagrams for explaining a constitution example of the ESD protection element; 
         FIG. 11  is a diagram for explaining the structure of the ESD protection element; 
         FIGS. 12A ,  12 B,  13 A and  13 B are diagrams for explaining the structure and operation of the ESD protection element; and 
         FIG. 14  is a diagram for explaining a constitution example of the ESD protection element. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. It is to be noted that in the following description, components having the same functions and constitutions are denoted with the same reference numerals, and the detailed description thereof will be given where required. 
     First Embodiment 
     In a first embodiment, a fine switch to which an MEMS is applied (hereinafter referred to as the MEMS switch) will be described. 
     (1) Example 1 
     (a) Structure 
     The structure of an MEMS switch according to one example of the first embodiment will be described with reference to  FIGS. 1A ,  1 B and  10 . 
       FIG. 1A  is a plan view showing the structure of an MEMS switch  1  according to the first example of the present embodiment.  FIG. 1B  is a sectional view of the MEMS switch  1  along the line A-A′ of  FIG. 1A . Moreover,  FIG. 10  is a sectional view of the MEMS switch  1  along the line B-B′ of  FIG. 1A . 
     The MEMS switch  1  shown in  FIGS. 1A to 10  is a normally-on type MEMS switch. The driving system of this switch  1  is an electrostatic driving system. 
     An insulating substrate  10  is made of an insulating layer formed on a silicon substrate or a glass substrate. 
     The substrate  10  is provided with three electrodes  11 ,  12  and  13 . Two electrodes  11 ,  12  are arranged in an x-direction (a first direction) side by side, and the one remaining electrode  13  is disposed so as to surround the peripheries of the two electrodes  11 ,  12  in the x-direction and y-direction (a second direction). The three electrodes  11 ,  12  and  13  are electrically separated from one another. 
     The electrodes  11 ,  12  and  13  are connected to interconnections  21 ,  22  and  23  in the substrate  10  via, for example, plugs P 1 , P 2  and P 3 . 
     The first electrode (the first port)  11  is one electrode of the switch  1 , and a potential Vs is supplied to the electrode  11 . The electrode  11  is connected to the first interconnection  21  via, for example, the plug P 1 . 
     The second electrode (the second port)  12  is the other electrode of the switch  1 , and a potential Vd is supplied to the electrode  12 . The electrode  12  is connected to the second interconnection  22  via, for example, the plug P 2 . 
     Moreover, a potential Vg for driving the switch is supplied to the third electrode  13 . The electrode  13  is connected to the third interconnection  23  via, for example, the plug P 3 . 
     A movable structure  15  is provided above the electrodes  11 ,  12 . The movable structure  15  extends in a direction (the x-direction) in which the two electrodes  11 ,  12  are arranged side by side. The planar shape of the movable structure is, for example, rectangular. 
     One end of the movable structure  15  in the x-direction thereof is supported by an anchor  14  provided on the electrode  11 , and the movable structure  15  has a cantilever structure (cantilever beam structure). Moreover, the sectional shape of the movable structure  15  along the x-direction is an upwardly convex shape. 
     The movable structure  15  moves in a vertical direction, that is, from an electrode  12  side to a cap  20  side, or from the cap  20  side to the electrode  12  side. 
     The movable structure  15  and the anchor  14  are made of, for example, conductors. The anchor  14  is electrically connected to the electrode  11 . Therefore, the movable structure  15  is electrically connected to the electrode  11 . The potential Vs is supplied from the electrode  11  to the movable  15 . 
     The other end of the movable structure  15  in the x-direction is provided with a contact portion  16 . The contact portion  16  is provided above the electrode  12 . The contact portion  16  warps downwards, that is, toward the electrode  12 . The contact portion  16  is brought into a contact state or a non-contact state with respect to the electrode  12  in accordance with the operation of the movable structure  15 . The contact portion  16  is made of, for example, the same conductor as that of the movable structure. 
     On the movable structure  15 , a regulation film  18  is provided, and the movable structure  15  and the regulation film  18  form a laminated structure. The regulation film  18  covers, for example, the upper side of the contact portion  16 . 
     The internal stress of the regulation film  18  has relatively strong compressive properties as compared with that of the movable structure  15 . Due to the compressive internal stress (hereinafter referred to as the compressive force) of the regulation film  18 , the movable structure  15  has an upwardly convex shape on a contact portion  16  side, and the contact portion  16  is directed downwards (on a substrate side). Hereinafter, the upwardly convex portion of the movable structure  15  that is formed due to the internal stress of the regulation film  18  will be referred to as a convex portion  17 . 
     As a material for the regulation film  18 , for example, an insulator is used. However, any material may be used as the regulation film  18  as long as it has a compressive force larger than that of the movable structure  15 . 
     The cap  20  is provided on the electrode  13  so as to cover the movable structure  15 . A region between the movable structure  15  and the cap  20  is hollow, for example, a vacuum. Note that in  FIGS. 1B and 10 , the cap  20  having a single layer structure is shown for simplicity of description, but the cap may have a multilayered structure. 
     The cap  20  is in contact with a part of the regulation film  18 . The cap  20  presses the convex portion  17  of the movable structure  15  from the upper side via the regulation film  18  to bring the contact portion  16  into contact with the electrode  12 . In consequence, the force at the time of contact between the contact portion  16  and the electrode  12  (the contact force) is increased. The cap  20  includes a conductor, and is electrically connected to the electrode  13 . The potential Vg is supplied from the electrode  13  to the cap  20 . 
     In a case where the material of the regulation film  18  is an insulating film, a conductor is used as the material of the cap  20  which directly is in contact with the regulation film  18 . 
     This cap  20  functions as a substantial electrode (the driving electrode) for driving the movable structure  15 . That is, the on/off state of the MEMS switch  1  of the present example is controlled by a relation between the potential Vg supplied to the cap  20  and the potential Vs supplied to the movable structure  15 . 
     For example, the MEMS switch  1  sets a potential difference between the potentials Vg and Vs to 0 V, whereby the top portion (the convex portion  17 ) of the movable structure  15  is pressed from the upper side to the substrate  10  side by the cap  20  to maintain the contact state between the contact portion  16  and the electrode  12 . 
     Moreover, the MEMS switch  1  sets the potential difference between the potentials Vg and Vs to a certain potential difference or more to generate an electrostatic attraction between the movable structure  15  and the cap  20 , thereby moving the movable structure  15 . In consequence, the non-contact state between the contact portion  16  and the electrode  12  is maintained. 
     A space H between the bottom surface of the upper portion of the cap  20  and the surface of the substrate  10  enables that the contact state between the contact portion  16  and the electrode  12  to be maintained by the cap  20  when the potential difference between the movable structure  15  and the cap  20  is substantially 0 V, and also enables the contact portion  16  and the electrode  12  to be brought into a non-contact state when the potential difference between the movable structure  15  and the cap  20  becomes larger than a predetermined pull-in voltage Vpi at which the movable structure  15  starts to move. The upper limit value of the space H is more preferably 0.5 μm or less. 
     Note that since the regulation film  18  using the insulator is interposed between the movable structure  15  using the conductor and the cap  20  using the conductor, the movable structure  15  is not in electrical contact (short-circuit) with the cap  20 . 
     The MEMS switch  1  shown in  FIGS. 1A to 10  can realize the operation of the movable structure  15 , that is, the control of the on/off state of the MEMS switch by controlling the potential difference between the movable structure  15  and the cap  20 . When the switch  1  is turned on, the cap  20  presses the movable structure  15  from the upper side to electrode  12 , and hence the potential difference between the movable structure  15  and the cap  20  is not necessary. Moreover, when the switch  1  is turned off, the movable structure  15  is driven by the electrostatic attraction caused by the potential difference between the movable structure  15  and the cap  20 . Therefore, the power consumption of the MEMS switch  1  of the present example is small. 
     Furthermore, according to the above constitution, an actuator for operating the MEMS switch  1  does not have to be separately provided. Therefore, the MEMS switch described in the present example can decrease the area occupied by the MEMS switch. 
     Note that as to the MEMS switch  1  of the present embodiment, the constitution of the switch  1  shown in  FIGS. 1A to 10  shows a basic constitution for realizing the present embodiment, and the present invention is not limited to this constitution. For example, in  FIG. 1A , the planar shape of the movable structure  15  is rectangular, but the planar shape may be a convex on the contact portion  16  side. Moreover, the same material may be used in the movable structure  15  and the contact portion  16 , and different materials may be used to decrease a contact resistance between the contact portion  16  and the electrode  12  or optimize the operation of the movable structure  15  or the like. 
     (b) Manufacturing Method 
     The manufacturing method of the MEMS switch  1  of the present example will be described with reference to  FIGS. 2A to 2D .  FIGS. 2A to 2D  are sectional views showing the manufacturing steps of the MEMS switch  1 , respectively. 
     As shown in  FIG. 2A , the interconnections  21 ,  22  and  23  and the plugs P 1 , P 2  and P 3  are formed in the predetermined positions of the substrate  10 . Then, a conductive layer is deposited on the substrate  10 , and this conductive layer is patterned into a predetermined shape. In consequence, the electrodes  11 ,  12  and  13  are formed on the substrate  10 . The electrodes  11 ,  12  and  13  are formed so as to come in contact with the plugs P 1 , P 2  and P 3 , respectively. 
     Next, a first sacrificial layer  40  is deposited on the substrate  10  and the electrodes  11 ,  12  and  13 , and the upper surface of this sacrificial layer  40  is flattened. Then, a conductive layer, which becomes the movable structure  15  is deposited on the sacrificial layer  40 , and the conductive layer is patterned into a predetermined shape. 
     Afterward, the sacrificial layer on the electrode  11  is removed to expose the surface of the electrode  11 . Then, the anchor  14  for supporting the movable structure  15  is formed on the exposed electrode  11 . The material of the anchor  14  is a conductor. Note that the anchor  14  may simultaneously be formed by using the same material as that of the movable structure  15 . 
     The regulation film  18  having a predetermined shape is formed on the movable structure  15 . The regulation film  18  covers the contact portion  16  from the upper side thereof. The material of the regulation film  18  is, for example, an insulator. 
     In these steps, the movable structure  15  is closely attached to the sacrificial layer  40 . Therefore, even when the regulation film  18  is formed on the upper surface of the movable structure  15  and the compressive internal stress caused by the regulation film  18  is applied to the movable structure  15 , the movable structure  15  maintains a substantially horizontal state with respect to the surface of the substrate  10 . 
     As shown in  FIG. 2B , a second sacrificial layer  41  is formed on the movable structure  15  and the regulation film  18 . The upper surface of the sacrificial layer  41  is flattened so that the height of the upper surface of the sacrificial layer  41  from the surface of the substrate  10  is a predetermined height. Furthermore, the sacrificial layer  40  and  41  is patterned into a predetermined shape to expose the upper surface of the electrode  13 . 
     Moreover, on the electrode  13  and the sacrificial layer  41 , a conductive layer which becomes the cap  20  is deposited. 
     As shown in  FIG. 2C , an opening Q is formed in the cap  20 . Then, the sacrificial layer is removed through this opening Q by, for example, wet etching. 
     Since the sacrificial layer closely attached to the movable structure  15  is removed, the movable structure  15  curves owing to the difference in stress between the movable structure and the regulation film  18 . Thus, the movable structure  15  has the convex portion  17  having an upwardly convex shape, and the contact portion  16  warps toward the electrode  12 . 
     Moreover, the cap  20  presses the convex portion  17  from the upper side to the substrate  10  side via the regulation film  18 . In consequence, the contact portion  16  is in contact with the electrode  12 . This is realized by flattening the upper surface of the sacrificial layer  41  so that the height of the upper surface of the sacrificial layer (the film thickness of the sacrificial layer  41 ) is a predetermined height (the film thickness) to regulate the height of the cap  20  as in the step shown in  FIG. 2C . 
     Then, to vacuum-seal the cavity between the cap  20  and the movable structure  15 , a seal layer  25  is formed on the cap  20  as shown in, for example,  FIG. 2D . 
     As described above, the MEMS switch according to the present example can be formed by a simple manufacturing method. 
     (c) Operation 
     The operation of the MEMS switch  1  of the present example will be described with reference to  FIGS. 3A and 3B .  FIGS. 3A and 3B  are diagrams for explaining the operation of the MEMS switch. 
       FIG. 3A  shows the state of the MEMS switch  1  when the potential Vs supplied to the movable structure  15  and the potential Vg supplied to the cap  20  are substantially equal. 
     The potential Vs is supplied to the movable structure  15  via the electrode  11 . The potential Vg is supplied to the cap  20  via the electrode  13 . A potential difference (|Vg−Vs|) between the potentials Vs and Vg is substantially set to 0 V. 
     In the MEMS switch  1  of the present example, the convex portion  17  of the movable structure  15  is pressed from the upper side to the substrate  10  side by the cap  20  via the regulation film  18 . 
     When the potential difference (|Vg−Vs|) is 0 V, no electrostatic attraction is generated between the movable structure  15  (the regulation film  18 ) and the cap  20 , and the movable structure  15  is not attracted to the cap  20  as a driving electrode. Therefore, the movable structure  15  has an upwardly convex shape, and the contact portion  16  comes into contact with the electrode  12 . Therefore, the electrodes  11  and  12  corresponding to ports of the MEMS switch  1  conduct. Thus, the state of the MEMS switch  1  before driving the movable structure  15  is an on-state. This on-state is held until the potential difference (|Vg−Vs|) between the movable structure  15  and the cap  20  becomes larger than the pull-in voltage Vpi. 
     Moreover,  FIG. 3B  shows the state of the MEMS switch  1  when the potential difference between the potentials Vg and Vs is larger than a predetermined potential difference. 
     The potential Vs is supplied to the movable structure  15 , and the potential Vg having a size different from that of the potential Vs is supplied to the cap  20 . In this case, an electrostatic attraction is generated between the movable structure  15  and the cap  20  as the driving electrode. 
     In a case where the potential difference (|Vg−Vs|) between the potential Vs of the movable structure  15  and the potential Vg of the cap  20  is set to a value larger than the pull-in voltage Vpi at which the movable structure  15  starts to move, the contact portion  16  is attracted to the cap  20  side (the upper side). In consequence, a portion of the movable structure  15  from the convex portion  17  to the contact portion  16  moves upwards (to the cap  20  side), and is in contact with the cap  20  via the regulation film  18 . In consequence, the contact portion  16  and the electrode  12  are brought into a non-contact state, and are electrically separated from each other. That is, the electrodes  11  and  12  corresponding to the ports of the MEMS switch  1  do not conduct. Thus, the state of the MEMS switch  1  after driving the movable structure  15  is an off-state. 
     In the MEMS switch  1  of the present example, the convex portion  17  of the movable structure  15  constantly is in contact with the cap  20  via the regulation film  18 . Therefore, when the movable structure  15  is driven and the MEMS switch  1  changes from the on-state to the off-state, a portion of the movable structure  15  from the convex portion  17  to the tip thereof successively is in contact with the cap  20  via the regulation film  18  in a zipper-like state. Such a zipper-like operation of the movable structure  15  means that the MEMS switch  1  can be driven at a low voltage. 
     In particular, when a space between the bottom surface of the upper portion of the cap  20  and the surface of the substrate  10  is set to 0.5 vim or less, the pull-in voltage Vpi which is the driving potential of the MEMS switch  1  can be decreased to 1 V or less. Therefore, the MEMS switch  1  of the present example can be driven at a low voltage. 
     As described above, the MEMS switch  1  described in Example 1 of the present embodiment presses the fine movable structure  15  (the convex portion  17 ) from the upper side to the substrate  10  side by use of the cap  20 , and mechanically (structurally) brings the contact portion  16  into contact with the electrode  12 , to hold the on-state. Moreover, the MEMS switch  1  of the present example electrically separates the contact portion  16  from the electrode  12  by electrostatic attraction, to hold the off-state. 
     In consequence, to hold the on/off state, the MEMS switch  1  of the present embodiment does not require a large power (voltage). Moreover, when the MEMS switch  1  is changed from the on-state to the off-state, the movable structure  15  is driven in a zipper-like operation by electrostatic attraction, and hence the driving voltage of the switch is small. 
     Therefore, according to the MEMS switch  1  of the present example, the power consumption of the MEMS switch  1  can be decreased. 
     In the MEMS switch  1  of the present example, the movable structure  15  is driven by the electrostatic attraction generated between the movable structure  15  and the cap  20 . Therefore, it is not necessary to separately provide an actuator for driving the movable structure  15 . Therefore, the occupying area of the MEMS switch can be decreased. 
     Moreover, the MEMS switch  1  of the present example is a normally-on type MEMS switch, and hence when the contact portion  16  and the electrode  12  are brought into a non-contact state even slightly, the switch immediately enters the off-state. That is, the MEMS switch  1  of the present example has a high switching speed. Furthermore, to realize the high-speed switching characteristics of the MEMS switch  1 , the inside of the cap  20  is preferably brought into a vacuum state. When a common MEMS switch is sealed in the vacuum state, ringing occurs. On the other hand, in the MEMS switch  1  of the present example, since the top portion (the convex portion  17 ) of the movable structure  15  is constantly fixed to the cap  20 , there is only a slight ringing effect. Therefore, the MEMS switch  1  of the present example can realize high-speed switching. 
     In addition, the level of electrostatic attraction between two conductors indicates dependence on the permittivity of the insulator interposed between the two conductors. Therefore, when a material having a large permittivity is used in the regulation film  18  as the insulator, the electrostatic attraction generated between the cap  20  and the movable structure  15  can be increased. When the material of the regulation film  18  is optimized in this manner, it is possible to contribute to an increase in the switching speed and a decrease in the power consumption. 
     As described above, according to Example 1 of the present embodiment, the power consumption of the MEMS switch can be decreased. Moreover, a small-sized MEMS switch can be provided. Furthermore, the switching characteristics of the MEMS switch can be improved. 
     (2) Example 2 
     An MEMS switch  2  according to one example of the first embodiment will be described with reference to  FIGS. 4A and 4B . 
       FIGS. 4A and 4B  are diagrams for explaining the constitution and operation of the MEMS switch according to Example 2 of the present embodiment. Note that the planar shape of the MEMS switch  2  of the present example is substantially the same as that of the structure shown in  FIG. 1A . 
     The MEMS switch  2  shown in  FIGS. 4A and 4B  is a normally-on type MEMS switch, and the driving system of the switch is an electrostatic driving system similar to that as in the MEMS switch  1  described in Example 1. 
     As described above, there is no particular restriction on the material of the regulation film for obtaining the upwardly convex shape of the movable structure as long as the internal stress of the regulation film has relatively strong compressibility with respect to the internal stress of the movable structure. Therefore, as shown in  FIGS. 4A and 4B , a conductor may be used as the material of a regulation film  18 X. 
     In this case, a cap (the lower-layer cap)  20 A using an insulator is provided between the regulation film  18 X using the conductor and a cap (the upper-layer cap)  25 A as a driving electrode. 
     The cap  20 A using the insulator is in direct contact with the regulation film  18 X using the conductor. The cap  25 A using the conductor is electrically connected to the electrode  13 , and a potential Vg is supplied to the cap  25 A via an electrode  13 . 
     The operation of the MEMS switch  2  of the present example is substantially the same as that of the MEMS switch  1  shown in  FIGS. 1A to 10 . 
       FIG. 4A  shows the state of the MEMS switch  2  when the potential Vg is substantially equal to a potential Vs. 
     When a potential difference between the potential Vs of a movable structure  15  and the potential Vg of the cap  25 A is substantially 0 V, no electrostatic attraction is generated between the cap  20 A or  25 A and the movable structure  15 . Therefore, when a convex portion  17  of the movable structure  15  is pressed from the upper side to the electrode  12  side by the caps  20 A,  25 A, a contact portion  16  is in contact with an electrode  12 , and the two electrodes  11 ,  12  conduct. Therefore, the MEMS switch  2  before driving the movable structure  15  is an on-state. 
     Moreover,  FIG. 4B  shows the state of the MEMS switch  2  when a potential difference between the potentials Vg and Vs is larger than a certain potential difference. 
     When the potential difference between the potentials Vs and Vg is larger than a pull-in voltage Vpi, a portion of the movable structure  15  between the convex portion  17  and a tip is attracted to a cap  25 A side in a zipper-like manner due to the electrostatic attraction generated between the cap  25 A and the movable structure  15 . In consequence, the contact portion  16  and the electrode  12  are brought into a non-contact state, and the two electrodes  11  and  12  corresponding to ports of the MEMS switch  2  do not conduct. Therefore, the MEMS switch  2  after driving the movable structure  15  is an off-state. 
     As described above, as the material of constituent members of the MEMS switch  2  shown in  FIGS. 4A and 4B , a material different from that of the constituent members of the MEMS switch  1  shown in  FIGS. 1A to 1C  is used. Also in this case, the MEMS switch  2  shown in  FIGS. 4A and 4B  is driven by a small driving voltage and holds the on/off state with a low power consumption in the same manner as in the MEMS switch  1  shown in  FIGS. 1A to 1C . 
     Therefore, needless to say, an effect similar to that of the MEMS switch shown in  FIGS. 1A to 1C  is also obtained in the MEMS switch  2  shown in  FIGS. 4A and 4B . 
     Note that the MEMS switch  2  of the present embodiment is different from the MEMS switch  1  shown in  FIGS. 1A to 1C  only in a material used in each constituent element, and the manufacturing method of the MEMS switch is substantially the same as the manufacturing steps described with reference to  FIGS. 2A to 2D . 
     (3) Example 3 
     (a) Structure 
     An MEMS switch  3  according to one example of the first embodiment will be described with reference to  FIGS. 5A and 5B . 
       FIGS. 5A and 5B  are diagrams for explaining the constitution and operation of the MEMS switch according to Example 3 of the present embodiment. Note that the planar shape of the MEMS switch  3  of the present example has the same structure as that shown in  FIG. 1A . 
     In Examples 1 and 2, the normally-on type MEMS switches  1 ,  2  have been described, but a normally-off type MEMS switch can be realized by using a regulation film laminated on a movable structure and a cap which is in contact with a portion (a convex portion) of the movable structure via the regulation film. 
     The MEMS switch  3  shown in  FIGS. 5A and 5B  is a normally-off type MEMS switch. The driving system of the MEMS switch  3  is an electrostatic driving system in the same manner as in the MEMS switches of Examples 1 and 2. 
     A movable structure  15  is supported by an anchor  14  provided on an electrode  11 . A contact portion  16  is provided on the tip side of the movable structure  15 . The contact portion  16  is disposed above an electrode  12 . When the MEMS switch  3  has an on-state, the contact portion  16  is in contact with the electrode  12 . When the MEMS switch  3  has an off-state, the contact portion  16  is not in contact with the electrode  12 . 
     A cap  29  is provided on, for example, a dummy layer  19 . The same material as that of the electrodes  11  and  12  and an electrode  13  is used in the dummy layer  19 . 
     In the normally-off type MEMS switch  3  of the present example, the cap  29  does not function as a driving electrode. The driving electrode  13  of the MEMS switch  3  of the present example is provided on a substrate  10 . 
     The driving electrode  13  is provided between the electrode (the first port)  11  connected to the anchor  14  and the electrode (the second port)  12  which is in contact with the contact portion  16 , and is disposed together with electrodes  11 ,  12  in an x-direction. 
     Two regulation films  18 A,  18 B are provided on the movable structure  15 . The first regulation film (first film)  18 A is provided on the upper surface of the movable structure  15  to cover the contact portion  16  from the upper side. The second regulation film (second film)  18 B is provided on the bottom surface of the movable structure  15 , and is disposed between the regulation film  18 A and the anchor  14  so as to be in contact with the driving electrode  13 . 
     The regulation films  18 A,  18 B have an internal stress with relatively strong compressive properties with respect to the internal stress of the movable structure  15 . Due to this stress, the movable structure  15  curves. 
     A portion  17 A in the movable structure  15  provided with the regulation film  18 A is deformed into an upwardly (the cap side) convex shape. In consequence, the contact portion  16  warps on a substrate side (the electrode  12  side). Thus, the contact portion  16  has a curved structure, whereby the contact portion  16  obtains a large contact force with respect to the electrode  12  during the contact with the electrode  12 . 
     A portion  17 B in the movable structure  15  provided with the regulation film  18 B is deformed into a downwardly (the substrate side) convex shape. In consequence, the movable structure  15  warps upwardly (a cap  29  side), and a portion of the movable structure  15  provided with the regulation film  18 B has a downwardly convex shape. 
     Hereinafter, the portion  17 A of the movable structure  15  which is upwardly convex is referred to as the convex portion  17 A, and the portion  17 B of the movable structure  15  which is downwardly convex is referred to as the downwardly convex portion  17 B. 
     The driving electrode  13  is disposed under the regulation film  18 B. Moreover, the convex portion  17 A is pressed from the upper side to the substrate  10  side by the cap  29  so that the driving electrode  13  is in direct contact with a part (the downwardly convex portion  17 B) of the regulation film  18 B. However, the regulation film  18 B may be disposed slightly away from the driving electrode  13 . Noted that a space H′ between the cap  29  and the surface of the substrate  10  prevents the contact portion  16  from coming into contact with the electrode  12  when a potential difference (|Vg−Vs|) between the electrode  11  and the driving electrode  13  is substantially 0 V. 
     Thus, in the MEMS switch  3  of the present example, the regulation film  18 B which imparts a compressive internal stress to the movable structure  15  is provided on the bottom surface of the movable structure  15 , and the portion  17 B of the movable structure provided with the regulation film  18 B warps upward. In consequence, according to the MEMS switch  3  of the present example, there is provided a normally-off type MEMS switch in which the contact portion  16  does not come into contact with the electrode  12  when a potential difference between the movable structure  15  and the driving electrode  13  is substantially 0 V, that is, in a state before driving the movable structure  15 . 
     Note that the manufacturing method of the MEMS switch  3  shown in  FIGS. 5A and 5B  is different from that shown in  FIGS. 2A to 2D  in the following steps. On a sacrificial layer, the regulation film  18 B for warping the movable structure  15  upwards is formed, and then a conductive film which becomes the movable structure  15  and the anchor  14  is formed on the regulation film  18 B. Then, the regulation film  18 A for warping the contact portion  16  of the movable structure  15  downwards is formed above the contact portion  16  in the movable structure. Afterward, the cap  29  is formed and the sacrificial layer is removed in the same manner as in the steps shown in  FIGS. 2A to 2D . In consequence, the MEMS switch  3  shown in  FIGS. 5A and 5B  is completed. 
     (b) Operation 
     The operation of the normally-off type MEMS switch  3  of the present example will be described with reference to  FIGS. 5A and 5B . 
       FIG. 5A  shows the state of the MEMS switch  3  when potentials Vg and Vs are substantially equal. 
     The potential Vs is supplied to the anchor  14  and the movable structure  15  via the electrode  11 . The potential Vg is supplied to the driving electrode  13 . The potential difference (|Vg−Vs|) between the potentials Vs and Vg is substantially set to 0 V. 
     In this case, since there is no potential difference between the driving electrode  13  and the movable structure  15 , no electrostatic attraction is generated between the driving electrode  13  and the movable structure  15 . Therefore, the movable structure  15  does not move, and the contact portion  16  is not in contact with the electrode  12 . That is, the two electrodes  11 ,  12  corresponding to ports of the MEMS switch  3  do not conduct. Therefore, the state of the MEMS switch  3  before driving the movable structure  15  is an off-state. 
       FIG. 5B  shows the state of the MEMS switch  3  when the potential difference between the potentials Vg and Vs is larger than a certain potential difference. 
     When the potential difference (|Vg−Vs|) between the potential Vg of the driving electrode  13  and the potential Vs of the movable structure  15  increases, electrostatic attraction is generated between the driving electrode  13  and the movable structure  15 . Then, when the potential difference (|Vg−Vs|) is larger than a pull-in voltage Vpi, the movable structure  15  is attracted to the electrode  13  by electrostatic attraction, and moves downwards (a substrate  10  side). By the movement of the movable structure  15 , the regulation film  18 A comes away from the cap  29 , and the contact portion  16  on the tip side of the movable structure  15  comes into direct contact with the electrode  12 . In consequence, the two electrodes  11 ,  12  corresponding to the ports of the MEMS switch  3  conduct. Therefore, the state of the MEMS switch  3  after driving the movable structure  15  is an on-state. 
     When the MEMS switch  3  changes from the off-state to the on-state, a portion of the movable structure  15  on a contact portion  16  side from a contact portion between the regulation film  18 B and the electrode  13  successively is in contact with the electrode  13  in a zipper-like manner. Thus, the movable structure  15  moves in a zipper-like manner, and hence the MEMS switch can be driven at a low voltage. 
     Moreover, as described above, the contact portion  16  has such a structure as to warp on the electrode  12  side and the whole bottom surface of the regulation film  18 B directly is in contact with the driving electrode  13 , whereby a large contact force is obtained between the contact portion  16  and the electrode  12 . 
     As described above, also in the normally-off type MEMS switch of the present example, a power consumption can be decreased in the same manner as in the normally-on type MEMS switch described in Example 1. 
     (4) Application Example 
     Hereinafter, application examples of the MEMS switches  1 ,  2  and  3  described in the present embodiment will be described with reference to  FIGS. 6 to 8C . 
     (a) Power Gating Switch 
     The MEMS switches  1 ,  2  and  3  of the present embodiment can be applied to a power gating switch. 
     For example, as shown in  FIG. 6 , a semiconductor integrated circuit (hereinafter referred to as the IC) is formed on a semiconductor substrate (the chip)  50 . Note that in  FIG. 6 , for simplicity of description, only one field effect transistor Tr is shown. 
     An element forming region of the semiconductor substrate  50  is defined by an isolation insulating film  59 . The field effect transistor Tr is provided in the element forming region. 
     The field effect transistor Tr has two diffusion layers  51 S,  51 D as a source/drain in the semiconductor substrate  50 . On the surface (the channel region) of the semiconductor substrate  50  between the two diffusion layers  51 S and  51 D, a gate insulating film  52  is provided. On the gate insulating film  52 , a gate electrode  53  is provided. On the diffusion layers  51 S,  51 D, contact plugs CP are provided, and the diffusion layers  51 S,  51 D are connected to interconnections  55 ,  56  via the contact plugs CP. 
     To cover the field effect transistor Tr, interlayer insulating films  10 ,  57  and  58  are provided on the semiconductor substrate  50 . A plurality of interconnections are laminated by a multi-level interconnection technology, and the plurality of interlayer insulating films  10 ,  57  and  58  are provided with a plurality of interconnections  21 ,  22 ,  23 ,  55  and  56 , respectively. 
     An MEMS switch  1  is provided as the power gating switch above the semiconductor substrate  50 . That is, the MEMS switch  1  as the power gating switch uses the top layer  10  of the interlayer insulating films as the substrate  10  provided with the MEMS switch. Then, the interconnections  21 ,  55  are electrically connected to the transistor Tr on the semiconductor substrate  50  via plugs P 1 , V 0  and the plugs CP. 
     Thus, in the MEMS switch  1 , the interlayer insulating films are used as the substrate and can be laminated on the semiconductor integrated circuit (IC), and hence the chip size of the IC chip does not increase owing to the use of the MEMS switch  1 . Therefore, when the MEMS switch  1  of the present embodiment is used as the power gating switch, the chip size can be reduced as compared with the power gating switch (hereinafter referred to as the transistor switch) using the field effect transistor formed on the surface of the substrate as in, for example, a CMOS inverter. 
     Even when the transistor switch has an off-state, leakage is generated. Moreover, when the transistor switch is miniaturized to suppress an increase in the chip size due to the use of a power gating technology, leakage is generated owing to the short channel effect of the transistor or the like. Therefore, the transistor switch suffers from leakage and a power consumption due to such leakage, and hence cannot sufficiently perform the function as a power gating switch. 
     On the other hand, when the MEMS switch  1  has an off-state, there is substantially no leakage, whereby the switch  1  is very effective for decreasing the power consumption of the whole chip. 
     Furthermore, in a transistor switch, when there is no driving voltage (gate voltage), it is difficult to hold the on-state. In consequence, power has to be supplied to the power gating switch to reduce the power consumption in order to operate the transistor of the switch. 
     On the other hand, as in the MEMS switch  1  shown in  FIGS. 1A to 10 , in a case where the normally-on type MEMS switch  1  has a constitution such that the cap  20  presses the movable structure  15  from the upper side to mechanically bring the contact portion  16  into contact with the electrode  12  as the port, the power for holding the on-state is not necessary. Moreover, in the MEMS switch  1 , since the movable structure  15  moves in a zipper-like manner due to electrostatic attraction, the power consumption (the driving voltage) required for obtaining the off-state is decreased. Furthermore, the normally-on type MEMS switch  1  immediately turns off when the contact portion  16  comes away from the electrode  12 , thus the switching operation of the switch is fast. 
     Moreover, since the transistor switch has a diffusion layer in the semiconductor substrate  50 , a parasitic capacity/parasitic resistance due to the diffusion layer is present, and the operation of the IC is delayed or adversely affected. On the other hand, the MEMS switch  1  is advantageous in that the on-resistance is small, parasitic capacity is small and no devices are distorted, and hence has only slight adverse influence on the IC. 
     Therefore, the MEMS switch of the present embodiment is very effectively used as a power gating switch. 
     A more specific application example of the power gating technology using the MEMS switch of the present embodiment will be described with reference to  FIGS. 7A and 7B .  FIGS. 7A and 7B  are block diagrams for explaining an application example of power gating technology using a MEMS switch. 
     As shown in  FIG. 7A , in the power gating technology, a switch matrix  60  constituted of a plurality of power gating switches is provided between a power supply circuit  61  in an IC chip and a circuit block  62  of the IC chip. Note that  FIG. 7A  shows only one circuit block, but needless to say, one IC chip may include a plurality of circuit blocks. 
     The power supply circuit  61  generates a power voltage VDD. The circuit block  62  is constituted of a plurality of sub-circuit blocks  63   1  to  63   n . 
     The MEMS switches  1  of the present embodiment are used in the power gating switches  1  constituting the switch matrix  60 . 
     The power voltages VDD are supplied to the plurality of circuit blocks  63   1  to  63   n  via the switch matrix  60 , respectively. 
     In this case, the switch matrix  60  controls the on/off state of the MEMS switch  1  in the switch matrix  60  so that the power voltage VDD is not supplied to the non-operating circuit blocks  63   1  to  63   n . 
     As shown in  FIG. 7A , the power supply circuit  61  is preferably connected to one of the sub-circuit blocks  63   1  to  63   n , respectively, by use of a constitution in which two or more MEMS switches  1  are connected in series. This is because even in a case where a defect such that the one MEMS switch  1  does not switch from the on-state to the off-state is generated owing to stiction, when the other switch connected in series to the defective switch is turned off, the supply of the power voltages to the sub-circuit blocks can be blocked. 
     Moreover, as shown in  FIG. 7A , the power supply circuit  61  may be connected to one of the sub-circuit blocks  63   1  to  63   n  by two current paths. Thus, the plurality of MEMS switches  1  connected to the respective sub-circuit blocks  63   1  to  63   n  are arranged in parallel, whereby even in a case where the MEMS switch  1  which does not turn on is present in the one path connecting the power supply circuit  61  to the sub-circuit blocks  63   1  to  63   n , the normal on/off state of the MEMS switch  1  can be controlled to supply the power voltage VDD from the power supply circuit  61  to the predetermined sub-circuit blocks  63   1  to  63   n  via the other path connected in parallel. 
     As shown in  FIG. 7B , there may further be provided a voltage monitor circuit  65  which monitors the operation situations of the sub-circuit blocks  63   1  to  63   n  in the circuit block  62  and a switch matrix control circuit  66  which controls the operation of the switch matrix  60 . 
     In a constitution shown in  FIG. 7B , the voltage monitor circuit  65  monitors a potential in the sub-circuit blocks  63   1  to  63   n  in the circuit block  62 , and feed the monitored voltage back to the switch matrix control circuit  66 . According to such a constitution, the operation state of the respective circuit blocks  63   1  to  63   n  and the presence/absence of the supply power are mutually reflected, whereby a defective switch in the switch matrix  60  can be identified. Then, the identified defective switch is considered to be non-selective, and may not be driven. Moreover, the defective switch may be replaced with a spare switch. 
     As described above, the MEMS switches  1 ,  2  and  3  of the present embodiment are used as power gating switches, which can contribute to a decrease in the chip size and can decrease the power consumption of the circuit block  62 . 
     (b) Circuit 
     The MEMS switches  1 ,  2  and  3  of the present embodiment can be applied to a logical circuit or a storage circuit.  FIGS. 8A ,  8 B and  8 C show equivalent circuit diagrams of a circuit using the MEMS switch. 
     The MEMS switch can be shown by a circuit element shown in  FIG. 8A . In  FIG. 8A , a control terminal (the gate) to which a potential Vg is applied corresponds to the driving electrode of the MEMS switch. One end Vs and the other end Vd of a current path correspond to two ports of the MEMS switch. 
     The normally-on type MEMS switch turns on when the potential difference is a pull-in potential Vpi or less, and turns off when the potential difference is larger than the pull-in potential Vpi. When the potential not more than the pull-in potential Vpi is an “L” level and the potential larger than the pull-in potential Vpi is an “H” level, the normally-on type MEMS switch is equivalent to a P-channel MOS transistor using silicon. 
     On the other hand, the normally-off type MEMS switch turns off when the potential difference is the pull-in potential Vpi or less, and turns on when the potential difference is larger than the pull-in potential Vpi. That is, the normally-off type MEMS switch is equivalent to an N-channel transistor using silicon. 
     Therefore, by using the normally-on type MEMS switch  1  and the normally-off type MEMS switch  3  described in the present embodiment, a logical gate equivalent to a circuit using the MOS transistor can be constituted. 
     As shown in, for example,  FIG. 8B , by using each of the normally-on/off type MEMS switches  1 A,  3 A, a CMOS inverter (the NOT gate) can be constituted. 
     In an inverter MI using the MEMS switch, control gates (the driving electrodes) are connected in parallel to form input node. Moreover, the ends (the second ports) of two MEMS switches constituting the inverter MI are connected in series to form output node n 1 . 
     Moreover, to the other end (the first port) of the normally-on type MEMS switch, a driving potential Vdd is supplied, and to the other end (the first port) of the normally-off type MEMS switch, a ground potential Vss is supplied. 
     The operation of the inverter MI using this MEMS switch is similar to that of the CMOS inverter. That is, one of signals of the “H” level (e.g., the potential Vdd) and the “L” level (e.g., a potential of 0 V) is input into the input node of the inverter MI. In accordance with the level of the signal, the one MEMS switch turns on, and the other MEMS switch turns off. The MEMS switch which has turned off becomes a load with respect to the MEMS switch which has been turned on. In consequence, the signal output from an output node n 1  is the inverted signal of the input signal. 
     As described above, MEMS switches  1 A,  3 A have substantially no leakage. Therefore, the MEMS switches can be used to realize a leak-less logic having very small leakage. 
     Note that here an example has been described in which the MEMS switch is applied to an inverter (the NOT gate), but needless to say, the normally-on/off type MEMS switch can be used to constitute another circuit (a logical gate) such as an NAND gate or an NOR gate. 
     Moreover, as shown in  FIG. 8C , MEMS switches can be used to constitute a storage circuit. In  FIG. 8C , a static random access memory (SRAM) is constituted by using the MEMS switches. 
     The SRAM shown in  FIG. 8C  is constituted of two inverters MI shown in  FIG. 8B , that is, four MEMS switches  1 A,  1 B,  3 A and  3 B. 
     The connection relationship between inverters MI 1 , MI 2  and the respective MEMS switches  1 A,  1 B,  3 A and  3 B is similar to that in an SRAM constituted by flip-flop connecting CMOS inverters. That is, an input node of the one inverter MI 1  is connected to an output node n 2  of the other inverter MI 2 , and an input node of the other inverter MI 2  is connected to an output node n 1  of the one inverter MI 1 . Note that the nodes n 1 , n 2  are connected to a word line and two bit lines via selective switches, respectively, but this is not shown in  FIG. 8C . 
     The potentials of the word line and the bit lines are controlled to execute the writing and reading of data in and from the SRAM using the MEMS switches. 
     The SRAM using the MEMS switches can be used as a resistor for temporarily storing the calculated data of the sub-circuit blocks  63   1  to  63   n  in the circuit block  62  shown in, for example,  FIGS. 7A and 7B . As described above, since the MEMS switch has a small power consumption, the power consumption can be decreased in the register (SRAM) using the MEMS switches as compared with a register using the CMOS inverter. Moreover, since the MEMS switch can be laminated above the semiconductor substrate (the chip), the chip size can be decreased. Furthermore, since the MEMS switch hardly has any leakage, the retention characteristics of the data can be improved. 
     As described above, the normally-on/off type MEMS switch described in the present embodiment is used in the constituent element of the logical gate or the storage circuit, which can contribute to a decrease in the power consumption, decrease in the chip size, stabilization of data retention and the like. 
     Second Embodiment 
     As described above, an MEMS structure and a switch (an MEMS switch) having the MEMS structure can be applied to various devices. 
     Besides the use shown in  FIGS. 6 to 8C , it is expected that the MEMS switch (the MEMS structure) will be used as an element to protect various circuits and elements from ESD, that is, a so-called ESD protection element. 
     Hereinafter, an ESD protection element using the MEMS switch will be described. 
     (1) Example 1 
     One example of an ESD protection element using an MEMS switch will be described with reference to  FIG. 9 .  FIG. 9  is a block diagram showing a chip (IC) provided with an ESD protection element  68  using the MEMS switch. 
     The ESD protection element  68  shown in  FIG. 9  uses, for example, a normally-on type MEMS switch  1  shown in  FIG. 1 . The ESD protection element  68  protects a circuit block (device)  62  to be protected from ESD. Hereinafter, circuit block  62  to be protected from ESD will be referred to as an ESD protection target circuit. Note that in  FIG. 9 , the one circuit block  62  is connected to the one ESD protection element  68 , but needless to say, the one circuit block  62  may be connected to a plurality of ESD protection elements  68 . 
     As shown in  FIG. 9 , one end of a current path of the circuit block  62 , for example, an input/output portion  78  (e.g., the inverter) is connected to a pad (an input/output terminal)  70 . Moreover, the other end of the current path of the circuit block  62  is connected to a ground line (a low potential power interconnection)  73 . To a signal line  72 , a signal potential corresponding to an input or output signal is supplied, and to the ground line  73 , a ground potential is supplied. When the ESD is generated, an ESD pulse is applied from the pad  70  to the circuit block  62 . 
     The ESD protection element  68  is connected in parallel with an ESD protection target circuit (the circuit block  62 ). That is, one terminal (a second port (Vd)) of an MEMS switch  1  as the ESD protection element  68  is electrically connected to the signal line  72  which connects the ESD protection target circuit (the circuit block  62 ) to the pad  70 . The other terminal (a first port (Vs)) of the MEMS switch  1  is electrically connected to the same ground line  73  as in the other end of the ESD protection target circuit (the circuit block  62 ). Note that the ESD protection element  68  using the MEMS switch  1  and the ESD protection target circuit (the circuit block  62 ) may be connected to different ground lines. 
     When the normally-on type MEMS switch  1  shown in  FIGS. 1A to 10  is used in the ESD protection element  68 , for example, a terminal (the pad) drawn from the signal line  72  is connected to an electrode, the electrode is brought into contact/non-contact with the contact portion  16 , and the movable structure  15  and the anchor  14  are connected to the ground line  73  common to the circuit block  62 . Therefore, the ground line  73  corresponds to the one electrode (the first port) of the MEMS switch, and the signal line  72  corresponds to the other electrode (the second port) of the MEMS switch. Moreover, the cap  20  becomes a control terminal (the gate) which controls the operation of the MEMS switch  1  as the ESD protection element  68 , and a driving potential Vg is supplied to the terminal. 
     The operation of the ESD protection element  68  using the MEMS switch  1  shown in  FIGS. 1A to 10  is as follows. Note that the operation of the MEMS switch  1  has been described with reference to  FIGS. 3A and 3B , and hence a detailed description thereof is omitted. 
     When the circuit block  62  is non-active, that is, no power voltage is supplied to the circuit block  62 , the MEMS switch  1  as the ESD protection element  68  is turned on. Therefore, the pad  70  is connected to the ground line  73  via two ports of the MEMS switch  1 . Note that an active state of a chip may be defined as a state in which a power voltage is supplied to the chip or a state in which a power voltage is supplied to the chip and a control signal for enabling the chip is input in the chip. 
     When the ESD pulse is input into the pad  70 , the ESD pulse is applied to the MEMS switch  1  as the ESD protection element  68 . That is, the ESD pulse is discharged from the contact portion  16  electrically connected to the signal line  72  to the ground line  73  via the movable structure  15  and the anchor  14 . 
     Therefore, an excessively large breakdown voltage/current due to ESD is not applied into the circuit block  62 , and the circuit block  62  is not damaged by the ESD pulse. 
     On the other hand, when the circuit block  62  is active, that is, the power voltage is supplied to the circuit block  62 , the MEMS switch  1  as the ESD protection element  68  is turned off by supplying the driving potential Vg to the control terminal of the switch (the cap  20 ). In consequence, the ESD protection element  68  is electrically separated from the circuit block  62 . 
     As described above, the ESD protection element using the MEMS switch of the present example holds the on-state when the IC chip (the circuit block) is non-active, and holds the off-state when the IC chip (the circuit block) is active. 
     In a case where the normally-on type MEMS switch  1  shown in, for example,  FIGS. 1A to 1C  is used in the ESD protection element  68  as in the present example, when no driving voltage is supplied to the gate of the MEMS switch, the switch is turned on. That is, even when the whole chip is electrically separated from the power supply, the normally-on type MEMS switch  1  can hold the on-state. 
     There is a high possibility that the ESD pulse is generated by the electrostatic charging of the constituent members of the chip in a step of mounting the chip on the circuit substrate or a step of testing a semiconductor device. On the other hand, while a packaged chip is incorporated in an electronic device, the breakdown of the IC chip (the circuit block  62 ) by the ESD pulse noticeably decreases. When the power voltage is supplied to the chip and electronic device, the breakdown of the chip by the ESD pulse further decreases. That is, there is a high possibility that the ESD pulse is generated when any power voltage is not supplied to the IC chip (the circuit block  62 ). 
     Thus, the ESD pulse is easily generated when the IC chip and the circuit block  62  of the chip are electrically separated from the power supply, and hence the normally-on type MEMS switch  1  which does not require any power voltage for holding the on-state as shown in  FIGS. 1A to 1C  is very effectively used in the ESD protection element  68 . In consequence, during the generation of the ESD pulse, the MEMS switch  1  does not require any switching operation, and hence the switching of the MEMS switch and the speed of the switching do not adversely affect the protection from the ESD. 
     Moreover, the ESD protection element  68  using the MEMS switch  1  does not include any PN junction (diffusion layer) formed in the semiconductor substrate, and hence has a small parasitic capacity as compared with an ESD protection element using a resistor, a diode or a transistor. Furthermore, when the circuit block  62  is active, the MEMS switch  1  is turned off, and the MEMS switch  1  is electrically separated from the signal line  72 . 
     Therefore, the ESD protection element  68  using the MEMS switch  1  does not cause any interconnection delay of the circuit block  62  due to the parasitic capacity or the leakage, and the input/output of data or command in/from the circuit block  62  can be speeded up. 
     Moreover, the MEMS switch  1  having the off-state has substantially no leakage. Therefore, the ESD protection element  68  using the MEMS switch  1  does not cause a decrease in the power voltage with respect to the circuit block  62 . Furthermore, the MEMS switch  1  of  FIGS. 1A to 1C  has an electrostatic driving system, and hence the driving potential for holding the off-state is also low. Therefore, a large increase in the power consumption of the whole chip is not caused. 
     As described above, when the MEMS switch  1  is used as the ESD protection element  68 , the power consumption can be decreased, and the breakdown of the circuit block  62  by the ESD can be prevented without deteriorating the performance of the circuit block  62 . Therefore, there can be provided an ESD protection element of an MEMS structure having a high performance and a low power consumption. 
     Note that the ESD protection element  68  of the present example is effective against all the ESD of a human body model (HBM), a machine model (MM) and a charged device model (CDM). 
     (2) Example 2 
     One example of an ESD protection element using an MEMS switch will be described with reference to  FIG. 10 .  FIG. 10  is a block diagram showing the more specific constitution of an ESD protection element  68  using the MEMS switch and a device to be protected. 
     As shown in  FIG. 10 , a power potential VDD and a ground potential VSS are supplied to a circuit block  62  via pads  71 ,  74 . 
     An ESD protection element  68 X using a PN junction is provided between the power potential VDD and the ground potential VSS. This is because in a case where the ESD protection element using a normally-on type MEMS switch is provided between the power potential VDD and the ground potential Vss, short-circuit occurs between the power potential and the ground potential owing to the MEMS switch having the on-state, and no potential for turning off the MEMS switch con be generated. 
     The ESD protection element  68 X using the PN junction is connected in series between the pad  71  to which the power voltage is supplied and the pad  74  to which the ground potential is supplied. The ESD protection element  68 X using the PN junction is, for example, a diode. This diode  68 X has a cathode connected to the pad (the power supply terminal)  71  and an anode connected to the pad (the ground terminal)  74 . However, this ESD protection element  68 X is not limited to a diode, and needless to say, an ESD protection element using a transistor may be used. 
     In the present example, an ESD protection element (in the present example, the diode) using a PN junction is provided between the power potential VDD and the ground potential VSS. The parasitic capacity of this ESD protection element  68 X is larger than that of the MEMS switch, but the parasitic capacities of an interconnection to which the power potential VDD is supplied and an interconnection to which the ground potential VSS themselves are large, whereby the ESD protection element  68 X has only little influence on the operation of the IC chip (the circuit block  62 ). 
     A power-on detecting circuit  67 A is connected to the pad  71  to which the power potential VDD is supplied. The power-on detecting circuit  67 A detects the introducing situation of the power potential VDD with respect to the chip (the circuit block  62 ), and outputs a power-on detecting signal SPO to a potential generation circuit (control circuit)  67 B. 
     The potential generation circuit  67 B is connected to the power-on detecting circuit  67 A. The potential generation circuit  67 B generates a driving potential Vg of the MEMS switch  1  as the ESD protection element  68  based on the power-on detecting signal SPO. 
     The normally-on type MEMS switch  1  as the ESD protection element  68  is an MEMS switch having a structure shown in, for example,  FIGS. 1A to 10 . The control terminal (the cap  20 ) of the normally-on type MEMS switch  1  is connected to the potential generation circuit  67 B. 
     One end (the second port) of the MEMS switch  1  is connected to a current path (the signal line) of the pad  70  for the input/output of the signal and an input/output portion of the circuit block  62 , and the other end (the first port) of the MEMS switch  1  is connected to the pad  74  via an interconnection  73 . 
     As shown in  FIG. 10 , the diode (the ESD protection element)  68 X is interposed between the power potential VDD and the ground potential Vss, whereby there is no short-circuit between two power supplies due to the ESD protection element  68  using the normally-on type MEMS switch. Therefore, the operation (on/off) of the normally-on type MEMS switch can be controlled by using the power potential VDD common to the IC chip (the circuit block  62 ). 
     The operation of the chip including the ESD protection element using the MEMS switch shown in  FIG. 10  is as follows. Note that before the power potential is introduced, that is, when the chip is non-active, the MEMS switch of the ESD protection element has an on-state. This is similar to the operation described with reference to  FIG. 9 , and hence a detailed description thereof is omitted. 
     On detecting that the power is introduced to the IC chip (the circuit block  62 ) via the interconnection (a high potential power interconnection), the power-on detecting circuit  67 A outputs the power-on detecting signal SPO to the potential generation circuit  67 B. When the power potential VDD is introduced, the IC chip (the circuit block  62 ) becomes active. 
     The potential generation circuit  67 B, in which the power-on detecting signal is input, generates the driving potential Vg of the MEMS switch  1 . Then, the generated potential Vg is supplied to a control terminal of the MEMS switch  1 . Note that the size of the driving potential Vg may be equal to or different from that of the power potential VDD. 
     Due to the generated potential Vg, the normally-on type MEMS switch  1  as the ESD protection element  68  is turned off. That is, the MEMS switch  1  is electrically separated from the signal line  72 . In consequence, the input/output of the signal is executed between the circuit block  62  and the pad  70 . 
     As described above, the on/off operation of the ESD protection element  68  using the normally-on type MEMS switch is controlled. 
     As described above, the normally-on type MEMS switch  1  shown in  FIGS. 1A to 1C  does not require any potential for holding the on-state of the switch, and the potential for holding the off-state of the switch is also small. Therefore, the ESD protection element with a small power consumption can be provided. 
     Moreover, according to the constitution shown in  FIG. 10 , MEMS switches  1  as the ESD protection elements  68  can further decrease the parasitic capacity in the off-state, and do not cause any distortion with respect to the elements and the IC chip (the circuit block  62 ). Therefore, according to a constitution in which the ESD protection element  68  using the MEMS switch is mounted in the chip as in the present example, the switch is especially effective for a high speed IC, a high speed memory and an RF-CMOS chip. 
     Therefore, an effect similar to that of the ESD protection element  68  using the MEMS switch described with reference to  FIG. 9  is obtained, and the ESD protection element of an MEMS structure having a high performance and a low power consumption can be provided. 
     (3) Example 3 
     (a) Structure 
     One example of the ESD protection element using an MEMS switch will be described with reference to  FIGS. 11 ,  12 A and  12 B.  FIG. 11  shows a plan view of an ESD protection element  68  using the MEMS switch, and  FIGS. 12A and 12B  show sectional views of the ESD protection element using the MEMS switch. 
     The MEMS switch used in the ESD protection element  68  shown in  FIGS. 9 and 10  may be of a normally-on type, and is not limited to the MEMS switch shown in  FIGS. 1A to 1C . As shown in, for example,  FIGS. 11 ,  12 A and  12 B, a normally-on type MEMS switch  4  driven by an actuator  9  may be used in the ESD protection element  68 . 
     The ESD protection element  68  shown in  FIGS. 11 ,  12 A and  12 B uses the normally-on type MEMS switch  4  of an electrostatic driving system in the same manner as in the ESD protection element shown in  FIG. 9 . This MEMS switch is driven in an on-state or an off-state by the actuator  9 . 
     The structure of the MEMS switch will be described with reference to  FIGS. 11 ,  12 A and  12 B. 
     MEMS switch  4  as the ESD protection element, a circuit block (ESD protection target circuit)  62  to be protected from ESD, as various interconnections  72 ,  73  and  35 , pads  70 ,  74  and  76  are provided on a substrate  100 . To the interconnection (the signal line)  72 , a signal potential Vsig is supplied via the pad (the input/output terminal)  70 . Moreover, to the interconnection (the ground line)  73 , a ground potential Vgnd is supplied via the pad (the ground terminal)  74 . 
     A beam portion constituting a movable structure  90  is connected to anchors  94 A,  94 B via torsion bars  93 A,  93 B. A conductor are used in the movable structure  90 , the torsion bars  93 A,  93 B and the anchors  94 A,  94 B. 
     The one anchor  94 A of the two anchors  94 A,  94 B is connected to the pad  74  via the interconnection  73 . The ground line  73  and the ground terminal  74  correspond to one electrode (the first port) of the MEMS switch. The other anchor  94 B is disposed on a dummy interconnection. Note that the ground terminal  74  may be shared by the MEMS switch  4  (the ESD protection element  68 ) and the circuit block  62 , or ground terminals may separately be provided. 
     The tip of the movable structure  90  on a signal line  72  side is provided with a contact portion  91 . 
     The contact portion  91  is disposed above the signal line  72 , and warps toward the signal line  72 . 
     The planar shape of the contact portion  91  includes, for example, a tip having a pointed shape, and is a hooked shape. According to such a planar shape, the contact resistance of the contact portion  91  can be decreased, and the loss of the MEMS switch can be decreased. Note that the number of the contact portions  91  may be one, two or more. 
     As the material of the contact portion  91 , the same conductor as that of the movable structure is used. However, when the MEMS switch is applied to the ESD protection element, as the material of the contact portion  91 , for example, gold (Au), aluminum (Al) and alloy including Al may be used. Al and an alloy of Al are formed into a natural oxidation film on its surface, but a stress voltage due to an ESD pulse causes a fritting phenomenon in which the natural oxidation film breaks down (dielectric breakdown). Due to this phenomenon, an ohmic contact between the contact portion  91  and the electrode (the interconnection) is realized. As suggested herein, the ESD protection element  68  using the normally-on type MEMS switch does not have to have a fully on-state when the IC chip is non-active. That is, the MEMS switch only need to be in a low impedance state in which the MEMS switch (the ESD protection element  68 ) immediately turns on when the chip is in a non-active state and the ESD pulse is applied. For a similar reason, in the non-active state of the chip, even in a case where the contact portion  91  is disposed a little away from an electrode (the signal line  72 , when the ESD pulse is applied, the contact portion  91  immediately comes into contact with the electrode (the signal line  72 ), and the MEMS switch  4  may have an on-state. 
     However, to decrease a parasitic capacity in the active state of the IC chip, a space between the contact portion  91  and the electrode (the signal line  72 ) in the active state of the chip needs to be larger than a space between the contact portion  91  and the electrode in the non-active state thereof. This means that in the active state of the IC chip, the MEMS switch has a high impedance state. 
     As described above, the required conditions of the MEMS switch which functions as the ESD protection element  68  are that the switch has a high impedance state when the IC chip (or the circuit block) is active and that the switch has a low impedance state when the IC chip is non-active. 
     In the example shown in  FIG. 11 , the signal line  72  as the electrode (the second port) of the MEMS switch  4 . Note that the electrode (the terminal) which functions as the second port may be an electrode separately drawn from the signal line  72 , as long as the drawn electrode is in contact with the contact portion  91 . 
     The warping of the contact portion  91  is realized by a regulation film  95  on the movable structure (the beam)  90 . The regulation film  95  uses, for example, an insulator, and is provided so as to cover the contact portion  91 . The regulation film  95  has a compressive internal stress larger than that of the movable structure  90  and the contact portion  91 . Due to this different stress value between the regulation film  95  and the contact portion  91 , the contact portion  91  (the tip of the movable structure  90 ) warps downwards. Note that when the regulation film  95  can impart a compressive internal stress to the contact portion  91 , the material of the regulation film  95  is not limited to an insulator. 
     The electrostatic actuator  9  is provided at the end of the movable structure  90  on a side opposite to the contact portion  91  side. The electrostatic actuator  9  has two spring structures  97 A,  97 B, an upper electrode  98 A and a lower electrode  98 B. 
     The upper electrode  98 A is connected to the movable structure  90  via the two spring structures  97 A,  97 B. The material of the upper electrode  98 A is, for example, a conductor. 
     When the spring structures  97 A,  97 B are made of a conductor, a potential Vsa supplied to the upper electrode  98 A is supplied from the ground line  73  via the movable structure  90 . Therefore, the potential Vsa of the upper electrode  98 A has a size substantially equal to that of a potential Vgnd of the ground line  73 . 
     The lower electrode  98 B is provided on the substrate  100 . The surface of the lower electrode  98 B is provided with an insulating film  79 , and the upper electrode  98 A is in contact with the lower electrode  98 B via this insulating film  79 . 
     The lower electrode  98 B is connected to the pad  76  via the interconnection  75 . To the pad  76 , a potential Vga for driving the actuator  9  is supplied. As the material of the lower electrode  98 B, for example, the same material (the conductor) as that of the interconnection  75  is used. 
     On the upper electrode  98 A of the electrostatic actuator  9 , an insulating film  99  is provided. This is provided to tilt the upper electrode  98 A on a lower electrode  98 B side and to narrow a space between the upper electrode  98 A and the lower electrode  98 B and to decrease the driving voltage of the actuator  9 . In this case, a space between the upper electrode  98 A and the lower electrode  98 B broadens toward a movable structure  90  side. 
     In a portion (hereinafter referred to as the spring portion)  96  of the movable structure  90  from the torsion bars  93 A,  93 B to the spring structures  97 A,  97 B, the spring constant (a spring constant k 1 ) of the spring portion  96  is larger than the spring constant (a spring constant k 2 ) of the spring structures  97 A,  97 B. To increase the spring constant k 1  of the spring portion  96 , for example, the dummy metal  79  is provided on the substrate  100  under the movable structure  90 . 
     The actuator  9  moves the movable structure  90  by the electrostatic attraction generated by a potential difference (|Vga−Vsa|) between the upper electrode  98 A and the lower electrode  98 B. 
     In the MEMS switch  4  of the present example, when the actuator  9  is not driven, that is, when the potential difference between the upper electrode  98 A and the lower electrode  98 B is substantially 0 V, the contact portion  91  is in contact with the signal line  72  as the electrode (the port). At this time, the MEMS switch  4  (the ESD protection element  68 ) has an on-state. Moreover, when the potential difference between the upper electrode  98 A and the lower electrode  98 B is larger than a pull-in voltage Vpia of the actuator  9 , the contact portion  91  is electrically separated from the signal line  72 . At this time, the MEMS switch  4  (the ESD protection element  68 ) has an off-state. 
     Thus, the MEMS switch  4  of the present example is a normally-on type MEMS switch. 
     (b) Operation 
     The operation of the ESD protection element using the MEMS switch of the present example will be described with reference to  FIGS. 12A and 12B .  FIG. 12A  shows a sectional structure when the MEMS switch  4  (the ESD protection element  68 ) has the on-state, and  FIG. 12B  shows a sectional structure when the MEMS switch  4  (the ESD protection element  68 ) has the off-state. Note that the operation of the whole chip is similar to that in the example described with reference to  FIGS. 9 and 10 , and hence a detailed description thereof is omitted here. 
     As shown in  FIG. 12A , when the IC chip (the circuit block  62 ) is non-active, the potential difference (|Vga−Vsa|) between the upper electrode  98 A and the lower electrode  98 B constituting the actuator  9  is substantially set to 0 V. Therefore, the contact portion  91  warps downwards, and hence the contact portion  91  comes into contact with the signal line  72 . That is, the MEMS switch  4  is held in the on-state even when there is no driving potential. Note that the contact portion  91  may be disposed a little away from the signal line  72 . This is because when the ESD pulse flows through the signal line  72 , due to the potential difference between the signal line  72  and the contact portion  91 , electrostatic attraction is generated between the signal line  72  and the contact portion  91 , and the MEMS switch  4  turns on by the electrostatic attraction. 
     When the potential Vsig of the signal line  72  is much larger than the ground potential Vgnd of the ground line owing to ESD, an ESD pulse is generated. In the present embodiment, the MEMS switch  4  has an on-state (a low impedance state), and the signal line  72  and the ground line  73  conduct. Therefore, the generated ESD pulse and a current caused by the pulse are discharged from the signal line  72  to the ground line  73  via the contact portion  91 , the movable structure  90  and the torsion bars  93 A,  93 B. 
     Therefore, the ESD pulse does not flow into the circuit block  62 , but is discharged to the ground terminal  74 . 
     As shown in  FIG. 12B , when the circuit block  62  is active, the contact portion  91  and the signal line  72  are brought into a non-contact state, that is, the MEMS switch  4  is turned off. This specific operation is as follows. 
     To the lower electrode  98 B, the driving potential Vga of the actuator  9  is supplied via the pad  76  and the interconnection  75 . On the other hand, to the upper electrode  98 A, the potential Vsa (e.g., the ground potential Vgnd) is supplied. 
     As described above, the spring constant k 1  of the spring portion  96  is larger than the spring constant k 2  of the spring structures  97 A,  97 B. Therefore, when the potential difference (|Vga−Vsa|) between the upper electrode  98 A and the lower electrode  98 B in the electrostatic actuator  9  is larger than such a pull-in potential Vpia of the actuator  9  so as to attract the upper electrode  98 A to a lower electrode  98 B side, the spring structures  97 A,  97 B are noticeably deformed as compared with the spring portion  96 , and the contact portion  91  floats upwards in a seesaw manner around the torsion bars  93 A,  93 B as a rotation axis. In consequence, the ESD protection element  68  using the MEMS switch  4  is brought into the off-state (the high impedance state) when the circuit block  62  is active. 
     As described above, the MEMS switch  4  as the ESD protection element is turned on when the IC chip (the circuit block  62 ) is non-active, and is turned off when the IC chip (the circuit block  62 ) is active. 
     As described with reference to  FIGS. 11 ,  12 A and  12 B, the structure and specific operation of the normally-on type MEMS switch  4  used in the ESD protection element  68  are different from those of the normally-on type MEMS switch used in the ESD protection element  68  described with reference to  FIGS. 1A to 10  and  FIG. 9 . 
     However, also in the present example, the MEMS switch  4  used in the ESD protection element is of the normally-on type and operates by an electrostatic driving system. Therefore, even in the ESD protection element using the MEMS switch  4  of the present example, the driving potential for holding the on-state of the element is not required, and the power consumption for holding the off-state of the element is small. 
     Therefore, the same effect as that of the ESD protection element  68  described with reference to  FIGS. 1A to 10  and  FIG. 9  is obtained, and an ESD protection element of the MEMS structure having a high performance and a low power consumption can be provided. 
     (4) Example 4 
     One example of an ESD protection element using an MEMS switch will be described with reference to  FIGS. 13A and 13B . In an ESD protection element  68  shown in  FIGS. 13A and 13B , a normally-on type MEMS switch  5  is used. However, this MEMS switch  5  is different from the MEMS switch described in Examples 1, 2 and 3 in that the switch is a thermal driving system. 
     (a) Structure 
       FIG. 13A  shows a plan view of the ESD protection element  68  using the MEMS switch  5  of the present example, and  FIG. 13B  schematically shows the sectional structure of the ESD protection element using the MEMS switch  5  of the present example. 
     In the MEMS switch  5  shown in  FIGS. 13A and 13B , a movable structure  80  has a V-shaped planar shape. The tip of this V-shape is a contact portion  81 . 
     Portions branched from the V-shaped tip (the contact portion  81 ) in two directions are connected to anchors  82 A,  82 B, respectively. The movable structure  80  is supported by the two anchors  82 A,  82 B. The movable structure  80  and the anchors  82 A,  82 B are made of a conductor. 
     The anchor  82 A is disposed on a ground line  73 . 
     To this anchor  82 A, a ground potential Vgnd is supplied via a pad  75 . 
     The anchor  82 B is disposed on an interconnection  74 . To this anchor  82 B, a driving potential Vg of the MEMS switch  5  is supplied via a pad  76 . 
     The contact portion  81  is provided at the tip of the V-shaped movable structure  80 , and is disposed above a signal line  72 . The planar shape of the contact portion  81  has a pointed tip, and is a hooked shape. Moreover, the contact portion warps to a signal line  72  side (downwards). This warping of the contact portion  81  is obtained by a regulation film  85  provided on the upper surface of the movable structure  80 . The regulation film  85  has a compressive internal stress larger than that of the movable structure  80 . 
     Moreover, as described above, the MEMS switch  5  of the present example is of a thermal driving type. Therefore, in the movable structure  80  and the contact portion  81 , a material having a thermal expansion ratio larger than that of the regulation film  85  is used. Note that in general, the thermal expansion coefficient of the conductor is larger than that of the insulating film. In the present example, the regulation film  85  uses an insulator, but there is no special restriction on the material as long as the internal stress and thermal expansion ratio of the regulation film  85  satisfy the above conditions of the internal stresses and thermal expansion ratios of the movable structure  80  and contact portion  81 . 
     In the normally-on type MEMS switch  5  of the thermal driving system of the present example, a potential difference (|Vg−Vgnd|) between the two anchors  82 A and  82 B is set to a value larger than a predetermined potential to pass a current through the movable structure  80 , and due to a thermal function caused by the current, the MEMS switch  5  is turned off. Note that this MEMS switch  5  is turned on by setting the potential difference between the two anchors  82 A and  82 B to substantially 0 V. 
     (b) Operation 
     The operation of the ESD protection element  68  using the MEMS switch  5  of the present example will be described with reference to  FIG. 13B . Note that  FIG. 13B  schematically shows the sectional structure of the MEMS switch  5  in an on/off state. Note also that the operation of the whole chip is similar to that of the example described with reference to  FIGS. 9 and 10 , and hence a detailed description thereof is omitted here. 
     As shown in  FIG. 13B , when an IC chip (a circuit block  62 ) is non-active, in the MEMS switch  5  of the present example, the signal line  72  is in contact with the contact portion  81  by the warping of the contact portion  81  due to the internal stress of the regulation film  85 . Therefore, the MEMS switch  5  (the ESD protection element  68 ) has an on-state even when no driving potential is present. 
     Moreover, when an ESD pulse is supplied to the signal line  72 , the ESD pulse is discharged to the ground line  73  via the MEMS switch  5  as the ESD protection element  68 . Therefore, the IC chip (the circuit block  62 ) is not damaged by the ESD pulse. 
     When the IC chip (the circuit block  62 ) is active, the potential difference (|Vg−Vgnd|) of a predetermined value or more is supplied between the two pads  75  and  76  shown in  FIG. 13A . Then, a current Ia flows into the movable structure  80 , and due to this current Ia, the movable structure  80  generates heat. 
     When the thermal expansion ratio of the movable structure  80  is larger than that of the regulation film  85 , the movable structure  80  causes such a shape change as to cancel the compressive internal stress due to the regulation film  85  owing to the difference of the thermal expansion ratio. Due to the shape change due to this thermal expansion, the movable structure  80  moves upwards with respect to the signal line  72 , and the contact portion  81  comes away from the signal line  72 . 
     Moreover, as described above, there is a high probability of the ESD pulse being generated in the manufacturing steps of the chip, and a very low probability of generation during the actual use of the IC chip (the circuit block  62 ). Therefore, after the manufacturing steps, an excessively large current is passed through the MEMS switch  5 , and the movable structure  80  is heated and irreversibly plastically changed, whereby the off-state of the MEMS switch  5  may be held. In consequence, to hold the MEMS switch  5  in the off-state, no driving potential is necessary, and in addition to a decrease in the power consumption in the on-state (during ESD protection), the power consumption can be decreased also in the off-state of the MEMS switch  5 . Moreover, the ESD protection element  68  using the MEMS switch  5  is completely electrically separated from the IC chip (the circuit block  62 ), and hence does not have any adverse influence on the IC chip during the actual use thereof. 
     Thus, when the IC chip (the circuit block  62 ) is active, a current for changing the shape of the movable structure  80  is supplied from the one end (the anchor  82 B) to the other end (the anchor  82 A) of the movable structure, thereby turning off the MEMS switch. In consequence, the ESD protection element  68  is electrically separated from the circuit block  62 . 
     Further in the ESD protection element  68  using the normally-on type MEMS switch  5  of the thermal driving system as in the present example, the MEMS switch  5  as the ESD protection element is turned on when the IC chip (the circuit block  62 ) is non-active, and is turned off when the IC chip (the circuit block  62 ) is active. 
     Therefore, an effect similar to that in Examples 1, 2 and 3 described above, and an ESD protection element using the MEMS structure having a high performance and a low power consumption can be provided. 
     (5) Example 5 
     One example of an ESD protection element using the MEMS switch will be described with reference to  FIG. 14 .  FIG. 14  is a block diagram showing the constitution of an ESD protection element  68  of the present example. 
     In the example described with reference to  FIGS. 9 to 13B , a constitution has been described in which an IC made of a transistor is protected from an ESD pulse by the ESD protection element using a normally-on type MEMS switch. However, needless to say, the ESD protection element using the MEMS switch of each example may be used to protect an MEMS device from ESD. 
     In, for example,  FIG. 14 , in the ESD protection element  68  using a normally-on type MEMS switch  1 , an RF-MEMS device  69  as the MEMS device is a target of ESD protection. The RF-MEMS device  69  outputs a high frequency (radio frequency: RF) in accordance with the operation of the device. 
     One end of the RF-MEMS device  69  is connected to a signal line  72 , and the other end of the RF-MEMS device  69  is connected to a ground line  73 . Moreover, to connect the ESD protection element  68  in parallel with the RF-MEMS device  69 , the one end and other end of the ESD protection element  68  are connected to the signal line  72  and the ground line  73 , respectively. 
     The ESD protection element  68  using the normally-on type MEMS switch  1  has an on-state when the RF-MEMS device  69  is non-active. Moreover, the ESD protection element  68  is turned off when the RF-MEMS device  69  is active. 
     Further, the ESD protection element  68  using the normally-on type MEMS switch  1  can protect the RF-MEMS device  69  to be protected from the ESD pulse. Therefore, an ESD protection element which uses an MEMS structure having a high performance and a low power consumption can also be provided by the present example in the same manner as in Examples 1 to 4. 
     Note that, in the present example, an RF-MEMS device is the target of ESD protection, but needless to say, a similar effect can be obtained even in another device using a MEMS structure. Moreover, in the present example, as the MEMS switch as the ESD protection element  68 , needless to say, the MEMS switches  4 ,  5  shown in  FIGS. 11 and 13A  may be used. 
     (6) Conclusion 
     As described above, in the second embodiment of the present invention, the normally-on type MEMS switch is applied to the ESD protection element as described in Examples 1 to 5. 
     Moreover, when no power voltage is supplied to the circuit (the IC or the MEMS device) as the target of ESD protection (non-active state), the MEMS switch as the ESD protection element is turned on (low impedance state). On the other hand, when the power voltage is supplied to the circuit as the target of ESD protection (active state), the MEMS switch as the ESD protection element is turned off (high impedance state). 
     In consequence, when the circuit is non-active and the probability of generation of an ESD pulse is high, the ESD protection element  68  using the MEMS switch does not require any driving potential, but can hold the on-state. In this case, since the MEMS switch holds the on-state, there is no adverse influence on the switching. 
     Moreover, when the circuit as the target of the ESD protection is active, the ESD protection element  68  using the MEMS switch is turned off. Since the MEMS switch causes substantially no leakage, the corresponding ESD protection element can substantially completely electrically be separated from the IC chip (the circuit block), and the generation of a voltage decrease and delay with respect to the circuit can be decreased. 
     As described above, according to the second embodiment of the present invention, the power consumption of the ESD protection element can be decreased, and any adverse influence on the circuit can be decreased. 
     Therefore, an ESD protection element using a MEMS structure having a high performance and low power consumption can be provided. 
     [Others] 
     The examples of the present invention are not limited to the above embodiment, and the respective constituent elements can be changed and embodied without departing from the scope of the present invention. Moreover, by the appropriate combination of a plurality of constituent elements described in the above embodiments, various inventions can be achieved. For example, several constituent elements may be deleted from all the constituent elements described in the above embodiment, and the constituent elements of different embodiments may appropriately be combined.