Patent Publication Number: US-7719066-B2

Title: Electrostatic micro switch, production method thereof, and apparatus provided with electrostatic micro switch

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   A present invention relates to an electrostatic micro switch which performs switching by drive of electrostatic attraction, an electrostatic micro switch production method, and an apparatus provided with the electrostatic micro switch. 
   2. Description of the Related Art 
   An RF-MEMS (Radio Frequency Micro Electro Mechanical Systems) element which is of a conventional electrostatic micro switch will be described below with reference to  FIG. 20  to  FIG. 26 . 
     FIGS. 20A and 20B  show an outline of the RF-MEMS element. A RF-MEMS element  81  of  FIG. 20  functions as a switching element of a coplanar line while incorporated into a high-frequency circuit. The RF-MEMS element  81  has a substrate  82 . A coplanar line (CPW line)  83  which is of a line for transmitting a high-frequency signal is formed on the substrate  82 . In the coplanar line  83 , a signal line  83   s  is located between two ground lines  83   g   1  and  83   g   2  at certain intervals. 
   A movable body  84  is provided in the substrate  82 . The movable body  84  is arranged above the coplanar line  83  at certain intervals while commonly facing the signal line  83   s  and parts of the ground lines  83   g   1  and  83   g   2  of the coplanar line  83 . The movable body  84  is supported by the substrate  82  through beams  85  and support portions  89  such that displacement is vertically allowed with respect to the substrate  82 . A movable electrode  86  is formed on a surface on the side of the substrate  82  in the movable body  84 . 
     FIG. 21A  simplistically shows an example of an arrangement relationship between the movable electrode  86  and the coplanar line  83  when viewed from above the RF-MEMS element  81 , and  FIG. 21B  shows an example of the arrangement relationship between the movable electrode  86  and the coplanar line  83  when laterally viewed. As shown in  FIG. 21 , the movable electrode  86  is formed so as to stride across the ground line  83   g   1 , the signal line  83   s , and the ground line  83   g   2  of the coplanar line  83 , and the movable electrode  86  faces the lines  83   s ,  83   g   1 , and  83   g   2  while separated from the lines  83   s ,  83   g   1 , and  83   g   2  at certain intervals. 
   Returning to  FIGS. 20A and 20B , a protection insulating film  87  is formed on a surface of the movable electrode  86 . In the substrate  82 , a fixed electrode for moving  88  ( 88   a  and  88   b ) is formed in a region which faces the movable body  84 . 
   In the MEMS element  81  having the above configuration, movable body displacing means for displacing the movable body  84  is formed by the movable body  84  which is of the electrode and the fixed electrodes for moving  88   a  and  88   b . When a direct-current voltage is applied between the movable body  84  and the fixed electrode for moving  88  from the outside, electrostatic attraction is generated between the movable body  84  and the fixed electrode for moving  88 . As shown in  FIG. 20B , the movable body  84  is attracted toward the side of the fixed electrodes for moving  88  by the electrostatic attraction. Thus, the movable body  84  can be displaced by utilizing the electrostatic attraction with the movable body  84  and the fixed electrode for moving  88 . The displacement changes an electrostatic capacitance between the movable electrode  86  and the coplanar line  83 , which allows to signal conduction to be turned on and off in the coplanar line  83 . 
   Because the MEMS element  81  having the above configuration is formed by a MEMS technology, the small, low-loss electrostatic micro switch having good high-frequency (transmission) characteristics can be realized. 
   The movable body  84  is made of a high-resistance semiconductor whose resistivity ranges from 1 kΩcm to 10 kΩcm. The high-resistance semiconductor shall mean a semiconductor which behaves as an insulating material for the high-frequency signal (for example, signals having frequencies not lower than about 5 GHz) while behaving as the electrode for a low-frequency signal (for example, signals having frequencies not more than about 100 kHz) and a direct-current signal. That is, the movable body  84  made of the high-resistance semiconductor has good dielectric-loss characteristics for the high-frequency signal, whereas the movable body  84  functions as the electrode for the direct-current signal (direct-current voltage). 
   There are the following problems in the conventional electrostatic micro switch. When the direct-current voltage is applied between the movable body  84  and the fixed electrode for moving  88  to displace the movable body  84 , a depletion layer  90  ( 90   a  and  90   b ) is formed in a region of the movable body  84 , where the movable body  84  faces the fixed electrode for moving  88 . 
   The above phenomenon will be described in detail with reference to models shown in  FIGS. 22 and 23 .  FIGS. 22A and 23A  show models in which counterparts of the movable body  84  and the fixed electrode for moving  88  are modeled as a capacitor, and  FIGS. 22B and 23B  show equivalent circuits of the models respectively. In the models, a gap  91  located between the movable body  84  and the fixed electrode for moving  88  is an insulator and the movable body  84  is the semiconductor. Therefore, the models have a MIS structure (Metal Insulator Semiconductor) structure which is one of modes of the transistor. 
     FIGS. 22A and 22B  show the state in which the direct-current voltage is not applied between the movable body  84  and the fixed electrode for moving  88 . In this case, as shown in  FIG. 22B , a total capacitance C of the capacitor is equal to a capacitance Co of a capacitor which is formed through the gap  91  by the movable body  84  and the fixed electrode for moving  88 . 
   On the other hand,  FIGS. 23A and 23B  show the state in which the direct-current voltage is applied between the movable body  84  and the fixed electrode for moving  88 . In this case, as shown in  FIG. 23A , the depletion layer  90  is formed in the region of the movable body  84 , where the fixed electrode for moving  88  faces the movable body  84  made of the semiconductor. This leads to the state in which the new capacitor is formed in the movable body  84 , and the new capacitor and the capacitor formed through the gap  91  are connected in series as shown in  FIG. 23B . Accordingly, the total capacitance of the capacitor becomes 1/C=(1/Co)+(1/Cs) and the total capacitance is decreased, so that the voltage at the gap  91  is decreased. 
   An expression in which the capacitance C of the MIS structure shown in  FIGS. 22 and 23  is normalized by the capacitance Co is obtained as follows: 
   
     
       
         
           
             
               
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   Where ∈0 is a dielectric constant of vacuum, ∈o is a dielectric constant of an insulator, q is a charge amount of electron, Na is a carrier concentration, Xo is a thickness of an insulator, ∈Si is a dielectric constant of a semiconductor, and V is an applied voltage. 
     FIG. 24  shows a relationship between the ratio of C/Co and the applied voltage when the resistivity of a silicon semiconductor is variously changed based on the above expression (1). Referring to  FIG. 24 , it is found that the ratio of C/Co is decreased as the semiconductor resistivity is increased. That is, when the resistivity is high, the depletion layer is increased and the capacitance Cs is also increased. Therefore, the voltage drop at the gap  91  by the capacitance Cs is increased as the resistivity is increased. Accordingly, in order to perform the desired operation of the movable body  84  which is of the high-resistance semiconductor, it is necessary that the high direct-current voltage be applied between the movable body  84  and the fixed electrode for moving  88  when compared with the case where the movable body  84  is made of the low-resistance semiconductor. 
     FIG. 25  shows the equivalent circuit of the state in which a direct-current power supply  92  applies the voltage between the movable body  84  and the fixed electrode for moving  88 . In  FIG. 25 , R is a resistance of the movable body  84 , vc is a terminal voltage of the capacitor, vR is a terminal voltage of the resistance, and ic is a current passed through the movable body  84 . 
   Because the circuit shown in  FIG. 25  becomes an RC circuit, the following expression holds. 
   
     
       
         
           
             
               
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   Where ∈ is a base of a natural logarithm and t is time. As can be seen from the expression (2), the time t during which the voltage vc is brought close to the applied voltage V is lengthened, when a product of the resistance R and the capacitance C is increased. 
     FIG. 26  is a graph showing the relationship between resistance R and time t, in which a terminal voltage vc of the capacitor becomes V, when the capacitance C of the capacitor is set at 1 μF in the equivalent circuit shown in  FIG. 25 . As can be seen from  FIG. 26 , a charging time to the capacitance is lengthened as the resistance R is increased. That is, the charging time to the capacitor is lengthened, when the resistivity of the semiconductor which is of the movable body  84  is increased. 
   When the direct-current voltage is applied between the movable body  84  and the fixed electrode for moving  88 , the movable body  84  is brought close to the fixed electrode for moving  88 , which increases the capacitance C of the capacitor. Therefore, the charging time to the capacitor is further lengthened, which decreases an operation speed of the electrostatic micro switch. 
   In order to avoid the above problems, it is thought that the resistivity of the movable body  84  is decreased. However, in this case, transmission characteristics of the high-frequency signal are lowered. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention provide an electrostatic micro switch in which drive voltage rise and operation speed lowering are never generated while the high-frequency characteristics are maintained. 
   In accordance with one aspect of the present invention, an electrostatic micro switch comprises a fixed electrode which is provided in a fixed substrate; a movable substrate which includes a movable electrode, the movable electrode being arranged while facing the fixed electrode, the movable substrate being elastically supported by the fixed substrate; a fixed-side signal conducting unit which is provided in the fixed substrate; and a movable-side signal conducting unit which provided in the movable substrate, the movable-side signal conducting unit displacing the movable substrate by electrostatic attraction between the movable electrode and the fixed electrode to perform switching between the movable-side signal conducting unit and the fixed-side signal conducting unit, wherein the movable substrate is made of a semiconductor including a plurality of regions having different values of resistivity; at least a portion where the movable-side signal conducting unit is provided and a portion which faces the fixed-side signal conducting unit have high resistivity in the movable substrate; and at least a part of the movable electrode has low resistivity. 
   An embodiment of the present invention, at least the portion where the movable-side signal conducting unit is provided, the portion which faces the fixed-side signal conducting unit, and peripheral portions of the portions have the high resistivity in the movable substrate. 
   An embodiment of the present invention, the peripheral portions cover outsides which are at least 100 μm away from the portion where the movable-side signal conducting unit is provided and the portion which faces the fixed-side signal conducting unit in the movable substrate respectively. 
   An embodiment of the present invention, the movable substrate is formed by bonding a low-resistivity semiconductor substrate provided with the movable electrode and a high-resistivity semiconductor substrate provided with the movable-side signal conducting unit. 
   An embodiment of the present invention, the low-resistivity region of the movable electrode is formed by doping. 
   An embodiment of the present invention, the high resistivity is not lower than 800 Ωcm. 
   An embodiment of the present invention, the low resistivity is not more than 300 Ωcm. 
   In accordance with one aspect of the present invention, a radio communication device comprises an antenna; an internal processing circuit; and an electrostatic micro switch which is connected between the antenna and the internal processing circuit, the electrostatic micro switch comprising a fixed electrode which is provided in a fixed substrate; a movable substrate which includes a movable electrode, the movable electrode being arranged while facing the fixed electrode, the movable substrate being elastically supported by the fixed substrate; a fixed-side signal conducting unit which is provided in the fixed substrate; and a movable-side signal conducting unit which provided in the movable substrate, the movable-side signal conducting unit displacing the movable substrate by electrostatic attraction between the movable electrode and the fixed electrode to perform switching between the movable-side signal conducting unit and the fixed-side signal conducting unit, wherein the movable substrate is made of a semiconductor including a plurality of regions having different values of resistivity; at least a portion where the movable-side signal conducting unit is provided and a portion which faces the fixed-side signal conducting unit have high resistivity in the movable substrate; and at least a part of the movable electrode has low resistivity. 
   In accordance with one aspect of the present invention, an electrostatic micro switch production method comprises the steps of: providing a fixed electrode and a fixed-side signal conducting unit in a fixed substrate; forming a movable substrate which is formed with a low-resistivity region in a part of a high-resistivity semiconductor substrate and is made of a semiconductor including a plurality of regions having different values of resistivity; providing a movable-side signal conducting unit in the movable substrate; and bonding integrally the movable substrate to the fixed substrate. 
   An embodiment of the present invention, the low-resistivity region is formed to form the movable substrate by performing doping into a region which faces the fixed electrode of the high-resistivity semiconductor substrate in the step of forming the movable substrate. 
   An embodiment of the present invention, the region which faces the fixed electrode of the high-resistivity semiconductor substrate is removed and a low-resistivity semiconductor film is formed to form the movable substrate in the removed region in the step of forming the movable substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an exploded view of a structure of an electrostatic micro switch according to an embodiment of the invention. 
       FIG. 2  shows a plan view of the electrostatic micro switch. 
       FIG. 3  shows a sectional view taken on line A-A′ of  FIG. 2 . 
       FIG. 4  shows a lower surface view of a movable substrate in the electrostatic micro switch. 
       FIG. 5  shows a sectional view taken on line B-B′ of  FIG. 2 . 
       FIG. 6A  shows an equivalent circuit when a voltage is applied between a fixed electrode and one connection pad, and  FIG. 6B  shows an equivalent circuit when the voltage is applied between the fixed electrode and two connection pads. 
       FIGS. 7A to 7F  show a sectional view of an example of a movable substrate production process. 
       FIGS. 8A to 8G  show a sectional view of another example of the movable substrate production process. 
       FIG. 9  shows a simulation result of studying a relationship between resistivity and insertion loss with respect to a semiconductor used as the movable substrate. 
       FIG. 10  shows a simulation result of studying a relationship between a frequency of a signal to be switched and the insertion loss in the electrostatic micro switch. 
       FIG. 11  shows a model utilized for a simulation for studying a frequency of signal to be turned on and off and the insertion loss when a width of a high-resistivity region is changed in the electrostatic micro switch,  FIG. 11A  shows a sectional view, and  FIG. 11B  shows a plan view. 
       FIG. 12  shows a result of the simulation. 
       FIG. 13  shows a distribution of response time when the electrostatic micro switch is driven. 
       FIG. 14  shows a structure of an electrostatic micro switch according to another embodiment of the invention,  FIG. 14A  shows a sectional view, and  FIG. 14B  shows a lower surface view of the movable substrate in the electrostatic micro switch. 
       FIG. 15  shows a structure of an electrostatic micro switch according to still another embodiment of the invention,  FIG. 15A  shows a sectional view, and  FIG. 15B  shows a lower surface view of the movable substrate in the electrostatic micro switch. 
       FIG. 16  shows a structure of an electrostatic micro switch according to still another embodiment of the invention,  FIG. 16A  shows a sectional view,  FIG. 16B  shows a lower surface view of the movable substrate in the electrostatic micro switch, and  FIG. 16C  shows a sectional view taken on line C-C′ of  FIG. 16B . 
       FIG. 17  shows a block diagram of a schematic configuration of a radio communication device according to still another embodiment of the invention. 
       FIG. 18  shows a block diagram of a schematic configuration of a measuring device according to still another embodiment of the invention. 
       FIG. 19  shows a circuit diagram of a main-part configuration of a handheld terminal according to still another embodiment of the invention. 
       FIG. 20  schematically shows a sectional view of a conventional RF-MEMS element,  FIG. 20A  shows a state in which the voltage is not applied between a movable body and a fixed electrode for moving in the RF-MEMS element, and  FIG. 20B  shows a state in which the voltage is applied. 
       FIG. 21  simplistically shows of an example of arrangement relationship between a movable electrode and a coplanar line in the conventional RF-MEMS element,  FIG. 21A  shows a plan view, and  FIG. 21B  shows a sectional view. 
       FIG. 22A  shows modeling of a state in which the voltage is not applied between the movable body and the fixed electrode for moving, and  FIG. 22B  shows an equivalent circuit of the modeling. 
       FIG. 23A  shows modeling of a state in which the voltage is applied between the movable body and the fixed electrode for moving, and  FIG. 23B  shows an equivalent circuit of the modeling. 
       FIG. 24  shows a relationship between a ratio of C/Co and the applied voltage when resistivity of a silicon semiconductor is variously changed in the equivalent circuit shown in  FIG. 23B . 
       FIG. 25  shows an equivalent circuit of a state in which a power supply applies the voltage between the movable body and the fixed electrode for moving. 
       FIG. 26  shows a relationship between resistance R and time t in the equivalent circuit shown in  FIG. 25 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
   A first embodiment of the invention will be described below with reference to  FIGS. 1 to 13 .  FIGS. 1 to 3  show a structure of an electrostatic micro switch according to the first embodiment.  FIG. 1  is an exploded view showing the structure of an electrostatic micro switch of the first embodiment,  FIG. 2  shows a plan view, and  FIG. 3  shows a sectional view taken on line A-A′ of  FIG. 2 .  FIG. 4  shows a bottom surface view of a movable substrate in the electrostatic micro switch. In the drawings, the same component is designated by the same numeral. 
   An electrostatic micro switch  1  is one in which a movable substrate  20  is integrated with an upper surface of a fixed substrate  10 . In the fixed substrate  10 , a fixed electrode  12  and two signal lines (fixed-side signal conducting unit)  13  and  14  are provided on the upper surface of a glass substrate  10   a . The surface of the fixed electrode  12  is coated with an insulating film  17 . The fixed electrode  12  is connected to connection pads  12   b   1  and  12   b   2  through interconnect  12   a   1 , the fixed electrode  12  is connected to a connection pad  12   b   3  through an interconnect  12   a   2 , the fixed electrode  12  is connected to connection pads  12   b   4  and  12   b   5  through an interconnect  12   a   3 , and the fixed electrode  12  is connected to an connection pad  12   b   6  through an interconnect  12   a   4 . The signal lines  13  and  14  are arranged in the same straight line. End portions of the signal lines  13  and  14 , which are opposite each other, form fixed contacts  13   a  and  14   a  which are provided at predetermined intervals, and the other ends are connected to connection pads  13   b  and  14   b  respectively. 
   The fixed electrodes  12  are formed on both sides of the signal lines  13  and  14  with predetermined intervals, and the fixed electrodes  12  are also used as a high-frequency GND electrode, which forms a coplanar structure. The fixed electrodes  12  and  12  located on both the sides of the signal lines  13  and  14  are connected to each other between fixed contacts  13   a  and  14   a  of the signal lines  13  and  14 . Because electric flux lines generated by a switching signal are terminated at the high-frequency GND electrode located between the fixed contacts  13   a  and  14   a , isolation characteristics is improved. The upper surfaces of the fixed electrodes  12  and  12  are formed so as to be lower than the upper surfaces of the signal lines  13  and  14 . 
   The movable substrate  20  is formed by a substantially rectangular plate-shaped semiconductor substrate. In the movable substrate  20 , movable electrodes  23  and  23  are elastically supported through first elastic support portions  22  and  22  by anchors  21   a  and  21   b . In a central portion of the movable substrate  20 , a contact setting portion  25  is elastically supported through second support portions  24  and  24  by the anchors  21   a  and  21   b . A silicon substrate can be cited as an example of the semiconductor substrate. 
   The anchors  21   a  and  21   b  are vertically provided at two points on the upper surface of the fixed substrate  10 . The anchors  21   a  and  21   b  are electrically connected to connection pads  16   b  and  15   b  through interconnects  16   a  and  15   a  provided on the upper surface of the fixed substrate  10  respectively. The first elastic support portions  22  and  22  are formed by slits  22   a  and  22   a  provided along both side-end portions of the movable substrate  20 , and the first elastic support portions  22  and  22  are integrated with the anchors  21   a  and  21   b  at the lower surfaces of the end portions. 
   The movable electrode  23  facing the fixed electrode  12  is attracted to the fixed electrode  12  by the electrostatic attraction which is generated by applying the voltage between the electrodes  12  and  23 . The second support portions  24  and  24  and the contact setting portion  25  are formed by notch portions  26   a  and  26   b  which are provided toward the central portion from the centers of the both side-end portions of the movable substrate  20 . In the movable electrode  23 , portions which face at least the signal lines  13  and  14  are removed because of the notch portions  26   a  and  26   b.    
   The second support portions  24  and  24  are narrow beams which couple the contact setting portion  25  and the movable electrodes  23  and  23 . The second support portions  24  and  24  are configured to obtain elastic force larger than the first elastic support portions  22  and  22  in closing the contact. The contact setting portion  25  is supported by the second support portions  24  and  24 , and a movable contact (movable-side signal conducting unit)  28  is provided in the lower surface of the contact setting portion  25  through an insulating film  27 . A movable contact unit  29  includes the contact setting portion  25 , the insulating film  27 , and the movable contact  28 . The movable contact  28  faces the fixed contacts  13   a  and  14   a , and the movable contact  28  performs the closing to the fixed contacts  13   a  and  14   a  to electrically connect the signal lines  13  and  14 . 
   In the first embodiment, as shown in  FIGS. 3 and 4 , the region which faces the fixed electrode  12  of the fixed substrate  10  is a low-resistivity region in the lower surface of the movable substrate  20  made of the semiconductor, i.e., in the surface side on which the fixed substrate  10  is arranged. Therefore, the generation of the depletion layer can be suppressed in the region facing the fixed electrode  12  and the drive voltage rise can be avoided. Since the region of the movable substrate  20  has the low resistivity, the operation speed lowering can be suppressed. 
   The regions except for the region facing the fixed electrode  12 , i.e., the regions near the signal lines  13  and  14  through which the high-frequency signal is passed are a high-resistivity region HR. Therefore, the insertion loss can be decreased to maintain the good high-frequency characteristics. 
   The control of the semiconductor resistivity can be realized by selectively doping a need amount of impurity by ion implantation or diffusion only into a portion where the resistivity is changed in the semiconductor substrate having certain resistivity. 
   In the case of the electrostatic micro switch  1  having the structure shown in  FIGS. 1 to 4 , it is desirable that the electrostatic attraction be generated more evenly in planes facing each other in the movable electrode  23  and fixed electrode  12  when the voltage is applied between the movable electrode  23  and the fixed electrode  12 . Therefore, it is desirable that the voltage be applied to both the connection pads  15   b  and  16   b  of the fixed substrate  10  electrically connected to the movable electrode  23 . The reason will be described below with reference to  FIGS. 5 and 6 . 
     FIG. 5  shows a sectional view taken on line B-B′ of  FIG. 2 . In the first embodiment, the fixed electrodes  12  and  12  located on the both sides of the signal lines  13  and  14  are connected to each other between the fixed contacts  13   a  and  14   a . For the capacitor formed by the movable electrodes  23  and  23  and the fixed electrodes  12  and  12 , as shown in  FIG. 5 , a capacitor C 1  exists on the side of the anchor  21   a  and a capacitor C 2  exists on the side of the anchor  21   b.    
     FIG. 6A  shows the equivalent circuit when a voltage is applied only between the fixed electrode  12  and the connection pad  16   b . In the case of  FIG. 6A , only a low-resistance component LR is connected in series between a power supply PS and the capacitor C 1 , and a high-resistance component HR is connected in series between the power supply PS and the capacitor C 2 . Therefore, as described above with reference to  FIGS. 25 and 26 , although there is no problem in the charging characteristics of the capacitor C 1 , there is the problem that the charging time is lengthened in the capacitor C 2 . 
   On the other hand,  FIG. 6B  shows the equivalent circuit when the voltage is applied between the fixed electrode  12  and both the connection pad  16   b  and the connection pad  15   b . In the case of  FIG. 6B , similarly to the capacitor C 1 , the low-resistance component LR is connected in series between the power supply PS and the capacitor C 2 . Therefore, there is also no problem in the charging characteristics of the capacitor C 2 . 
   A method of producing the electrostatic micro switch  1  having the above configuration will be described below. Particularly, a method of forming the movable substrate  20  will be described in detail with reference to  FIGS. 7 and 8 . A general-purpose MEMS process or a general-purpose semiconductor production process can be utilized as the individual process technique, and it is not necessary to use the unique process. 
     FIGS. 7A to 7F  show an example of the method of producing the movable substrate  20 . As shown in  FIG. 7A , a high-resistivity semiconductor substrate  30  which becomes the movable substrate  20  is prepared, and a mask  31  is formed by an insulating film or the like in the region where the low resistivity is not necessary in the lower surface of the semiconductor substrate  30 . As shown in  FIG. 7B , the doping is performed by the ion implantation or the diffusion to the lower surface of the semiconductor substrate  30  to form the desired depth and region having the low resistivity. Then, as shown in  FIG. 7C , the mask  31  is removed. 
   As shown in  FIG. 7D , in order to adjust the thickness or to form a recess at the desired position by etching, a mask  32  is formed by the insulating film or the like in the region where the etching is not necessary. As shown in  FIG. 7E , the etching is performed. As shown in  FIG. 7F , the mask  32  is removed to complete the movable substrate  20 . In the case where plural recesses are formed while the recesses have the different recesses, it is necessary that the proper mask be formed in each case to repeat the processes shown in  FIGS. 7D to 7F . 
     FIGS. 8A to 8G  show another example of the method of producing the movable substrate  20 . As shown in  FIG. 8A , the high-resistivity semiconductor substrate  30  which becomes the movable substrate  20  is prepared, and the mask  31  is formed by the insulating film or the like in the region where the low resistivity is not necessary in the lower surface of the semiconductor substrate  30 . As shown in  FIG. 8B , the etching is performed to region where the low resistivity is necessary in the lower surface of the semiconductor substrate  30 . After the mask  31  is removed, a sacrifice layer  33  is formed in the region where the low resistivity is not necessary. As shown in  FIG. 8C , a low-resistivity semiconductor film  34  having the desired thickness is deposited by CVD (Chemical Vapor Deposition) or the like. As shown in  FIG. 8D , the semiconductor substrate  30  in which the low-resistivity region is embedded is obtained by etching the sacrifice layer  33 . 
   As shown in  FIG. 8E , in order to adjust the thickness or to form the recess at the desired position by etching, the mask  32  is formed by the insulating film or the like in the region where the etching is not necessary. As shown in  FIG. 8F , the etching is performed. As shown in  FIG. 8G , the mask  32  is removed to complete the movable substrate  20 . In the case where plural recesses are formed while the recesses have the different recesses, it is necessary that the proper mask be formed in each case to repeat the processes shown in  FIGS. 8E to 8G . 
   After the contact portions and the like are formed in the movable substrate  20  produced in the above manner by the general purpose MEMS process, the movable substrate  20  is bonded to the fixed substrate  10  in which the interconnects and the like are formed. The movable electrode  23 , the first elastic support portions  22 , and  22  and the second support portions  24  and  24  are formed by photolithography and the etching, and the electrostatic micro switch  1  is completed. 
   The ranges of the high-resistivity and the low-resistivity will be described below with reference to  FIGS. 9 and 10 .  FIG. 9  is a graph showing a simulation result of studying a relationship between resistivity and insertion loss which one of high-frequency characteristics with respect to a semiconductor used as the movable substrate  20 . The Model used in the simulation corresponds to the electrostatic micro switch  1  of the first embodiment, and numerical values indicating various characteristics are as follows. 
   That is, the material of the semiconductor substrate  30  is silicon, the thickness of the semiconductor substrate  30  is 20 μm, a relative dielectric constant of the semiconductor substrate  30  is 11.36, tan δ which is of the dielectric loss characteristic of the semiconductor substrate  30  is 0.013, the thickness of the movable contact  28  of the movable substrate  20  is 1 μm, the width of the movable contact  28  of the movable substrate  20  is 100 μm, the material of the fixed substrate  10  is Pyrex (registered trademark), the thickness of the fixed substrate  10  is 500 μm, the thicknesses of the fixed contacts  13   a  and  14   a  of the fixed substrate  10  are 2 μm, the widths of the fixed contacts  13   a  and  14   a  of the fixed substrate  10  are 300 μm, and the interval between the two fixed contacts  13   a  and  14   a  is 40 μm. Only one kind of the resistivity is used for the semiconductor substrate  30 . 
   As can be seen from  FIG. 9 , the insertion loss is rapidly decreased up to the semiconductor resistivity of 300 Ωcm, saturation of the insertion loss is started at 800 Ωcm, and then the insertion loss is gently decrease. That is, for the high resistivity, it is desirable that the resistivity be not lower than 800 Ωcm. 
     FIG. 10  is a graph showing a simulation result of studying a relationship between a frequency of a signal to be switched and the insertion loss in the electrostatic micro switch  1  of the first embodiment. In  FIG. 10 , a curve connecting x-marks indicates the first embodiment. In the first embodiment, as shown in  FIGS. 3 and 4 , the 800-Ωcm high-resistivity region is formed in the predetermined portion of the semiconductor which is of the movable substrate  20 , and the 300-Ωcm low-resistivity region is formed in other portions. On the other hand, a curve connecting rhombic marks indicates a comparative example in which the 300-Ωcm low-resistivity region is formed in all the portions of the semiconductor which is of the movable substrate. A curve connecting square marks also indicates a comparative example in which the 800-Ωcm high-resistivity region is formed in all the portions of the semiconductor which is of the movable substrate. As can be seen from  FIG. 10 , the electrostatic micro switch  1  of the first embodiment has the excellent high-frequency characteristics similar to the case where the high-resistivity region is formed in all the portions of the semiconductor which is of the movable substrate. 
   As described above, in the movable substrate  20  of the first embodiment, the high-resistivity region HR is formed near the signal lines  13  and  14  through which the high-frequency signal is passed in the surface on the arrangement side of the fixed substrate  10  as shown in  FIGS. 3 and 4 . For the movable substrate  20  of the first embodiment, the region where the high-resistivity region HR is formed should cover how far the range from the region facing the signal lines  13  and  14  will be described with reference to  FIGS. 11 and 12 . 
     FIGS. 11 and 12  shows the simulation result of the study of the relationship between a frequency f of the signal to be turned on and off and the insertion loss when an area (width) of the high-resistivity region HR is changed in the electrostatic micro switch  1  of the first embodiment.  FIG. 11A  simply shows the movable substrate  20 , the movable contact  28 , the glass substrate  10   a , and the fixed contacts  13   a  and  14   a  for the model utilized for the simulation.  FIG. 11B  shows the signal lines  13  and  14  such that the width, the interval, and the arrangement can be seen. 
   In the model the high resistivity is set at 800 Ωcm and the low resistivity is set at 300 Ωcm. As shown in  FIG. 11A , in the movable substrate  20 , the high-resistivity region HR is formed in the region which is enlarged from the region facing the signal lines  13  and  14  by a predetermined width W, and the simulation is performed in the case of the widths W of 0, 70, 100, 130, and 160 μm. 
     FIG. 12  is a graph showing the result of the simulation. As can be seen from  FIG. 12 , it is necessary that the high-resistivity region HR be formed in the region where the width W is enlarged not lower than 100 μm from the region facing the signal lines  13  and  14 . This is attributed to the fact that an electric field generated by the high-frequency signal passed through the signal line propagates through a space near the signal line. Accordingly, even if the movable substrate  20  has any structure, it is found that the high-resistivity region is formed in the region enlarged not lower than 100 μm from the region facing the signal line through which the high-frequency signal is passed. 
   In the first embodiment, because the widths (290 μm) of the signal lines  13  and  14  located in the fixed substrate  10  is wider than the width (100 μm) of the movable contact  28  of the movable substrate  20 , the high-resistivity region HR is determined while the region facing the signal lines  13  and  14  is set at the reference region. However, in the case where the width of the movable contact  28  is wider than the widths of the signal lines  13  and  14 , the high-resistivity region HR may be determined while the regions of signal lines  13  and  14  are set at the reference region. 
   A response time of the electrostatic micro switch  1  of the first embodiment will be described with reference to  FIG. 13 .  FIG. 13  shows a distribution of the response time when the electrostatic micro switch is driven. In  FIG. 13 , a gray bar graph indicates the first embodiment. In the first embodiment, as shown in  FIGS. 3 and 4 , the 800-Ωcm high-resistivity region is formed in the predetermined portion of the semiconductor which is of the movable substrate  20 , and the 300-Ωcm low-resistivity region is formed in other portions. On the other hand, a hatched bar graph indicates a comparative example in which the 800-Ωcm high-resistivity region is formed in all the portions of the semiconductor which is of the movable substrate. 
   As can be seen from  FIG. 13 , when the high-resistivity region is formed in all the portions of the semiconductor which is of the movable substrate, the response time is lengthened due to influences such as the formation of the depletion layer and the charging characteristics of the CR circuit. On the contrary, in the electrostatic micro switch  1  of the first embodiment, since the low-resistivity region is formed in the portions where the drive voltage is applied, the formation of the depletion layer and the charging characteristics of the CR circuit have the small influence on the electrostatic micro switch  1 , which results in the response time as short as 100 μsec or less. 
   Thus, it can be understood that the electrostatic micro switch  1  of the first embodiment has the little insertion loss and the excellent high-frequency characteristics while the drive voltage rise and the response speed lowering never occur. 
   It is desirable that the required thickness of the low-resistivity region be determined by the thickness of the depletion layer  90  and the charging characteristics of the CR circuit. The thickness of the depletion layer  90  is generated in the movable substrate  20  when the voltage is applied to the movable substrate  20  and the fixed electrode  10 . The CR circuit is formed by the total resistance value R of the movable substrate  20  and the capacitance C between the movable substrate  20  and the fixed electrode  12 . 
   The thickness of the depletion layer  90  is determined by a threshold voltage of the MIS structure modeled by the movable substrate  20  and the fixed electrode  12 , the resistivity of the movable substrate  20 , the dielectric constant of vacuum, and the like. The threshold voltage of the MIS structure is determined by sizes such as an area of a structure and a gap. The total resistance value R of the movable substrate  20  is determined by the resistivity and distribution of the movable substrate  20 , a volume of the movable substrate  20 , and the like. Accordingly, it is necessary to design the required thickness of the low-resistivity region in consideration of various features such as the material and structure of the movable substrate  20  and the positional relationship between the movable substrate  20  and the fixed electrode  12 . 
   A boundary between the low-resistivity region and the high-resistivity region is clear in the first embodiment. As long as the thickness of the region and the resistivity are properly set, it is obvious that the same effect is obtained even in the case where the resistivity is gradually changed at the boundary. 
   Second Embodiment 
   A second embodiment of the invention will be described below with reference to  FIG. 14 . The electrostatic micro switch  1  according to the second embodiment differs from the electrostatic micro switch  1  of the first embodiment shown in  FIGS. 1 to 5  only in the high-resistivity and the low-resistivity regions in the movable substrate  20 . In other configurations, the electrostatic micro switch  1  of the second embodiment is similar to the electrostatic micro switch  1  of the first embodiment. In the electrostatic micro switch  1  of the second embodiment, the component having the same function as the first embodiment is designated by the same numeral as the first embodiment, and the description will not be given. 
     FIG. 14  shows a structure of the electrostatic micro switch  1  of the second embodiment, and  FIGS. 14A and 14B  correspond to  FIGS. 3 and 4  respectively. Referring to  FIG. 14 , in the movable substrate  20  of the second embodiment, the high-resistivity region HR is formed only near the signal lines  13  and  14  through which the high-frequency signal are passed, and the low-resistivity region is formed in other regions. The movable substrate  20  of the second embodiment can be produced by preparing the low-resistivity semiconductor substrate to form the high-resistivity semiconductor film in a predetermined region on the semiconductor substrate. 
   The same effect as the first embodiment can be obtained even in the electrostatic micro switch  1  of the second embodiment. The width and height of the high-resistivity region HR can be determined by performing the simulation shown in  FIGS. 11 and 12 . 
   Third Embodiment 
   A third embodiment of the invention will be described below with reference to  FIG. 15 . The electrostatic micro switch  1  according to the third embodiment differs from the electrostatic micro switch  1  of the first embodiment shown in  FIGS. 1 to 5  only in the high-resistivity and the low-resistivity region in the movable substrate  20 . In other configurations, the electrostatic micro switch  1  of the third embodiment is similar to the electrostatic micro switch  1  of the first embodiment. In the electrostatic micro switch  1  of the third embodiment, the component having the same function as the first embodiment is designated by the same numeral as the first embodiment, and the description will not be given. 
     FIG. 15  shows a structure of the electrostatic micro switch  1  of the third embodiment,  FIGS. 15A and 15B  correspond to  FIGS. 3 and 4 , respectively. Referring to  FIG. 15 , in the movable substrate  20  of the third embodiment, the high-resistivity region HR is formed from the region near the signal lines  13  and  14  through which the high-frequency signal are passed in the lower surface to the corresponding region in the upper surface, and the low-resistivity region is formed in other regions. The movable substrate  20  of the third embodiment can be produced by utilizing a bonded semiconductor substrate in which a high-resistivity semiconductor substrate is sandwiched by two low-resistivity semiconductor substrates. 
   The same effect as the above embodiments can be obtained in the third embodiment. Further, production period shortening and production cost reduction can be realized because the resistivity control by the doping shown in  FIG. 7  or the semiconductor film formation shown in  FIG. 8  is not required. In the third embodiment, similarly to the above embodiments, in order to generate more evenly the electrostatic attraction in the planes facing each other in the movable electrode  23  and the fixed electrode  12 , it is desirable that the voltage be applied to both the connection pads  15   b  and  16   b  of the fixed substrate  10  electrically connected to the movable electrode  23 . 
   Fourth Embodiment 
   A fourth embodiment of the invention will be described below with reference to  FIG. 16 . The electrostatic micro switch  1  according to the fourth embodiment differs from the electrostatic micro switch  1  of the third embodiment shown in  FIG. 15  only in that the notch portions  26   a  and  26   b  are not formed toward the central portions from the both side-edge portions of the movable substrate  20 . In other configurations, the electrostatic micro switch  1  of the fourth embodiment is similar to the electrostatic micro switch  1  of the third embodiment. In the electrostatic micro switch  1  of the fourth embodiment, the component having the same function as the third embodiment is designated by the same numeral as the third embodiment, and the description will not be given. 
     FIG. 16  shows a structure of the electrostatic micro switch of the fourth embodiment,  FIGS. 16A and 16B  correspond to  FIGS. 15A and 15B .  FIG. 16C  shows a sectional view taken on line C-C′ of  FIG. 16B . Referring to  FIG. 16 , in the movable substrate  20  of the fourth embodiment, when compared with the movable substrate  20  shown in  FIG. 15 , the notch portions  26   a  and  26   b  are not formed toward the central portions from the both side-edge portions of the movable substrate  20 , but a recess  26   c  is formed. 
   The recess  26   c  faces the signal lines  13  and  14  and the recess  26   c  has the high resistivity, so that the excellent high-frequency characteristics with little insertion loss can be maintained. Since the notch portions  26   a  and  26   b  are not provided, not only rigidity is improved to enhance strength of the movable substrate  20 , but also the influence of residual stresses of the insulating film  27  formed in the movable substrate  20 , the film of the movable contact  28 , and the like is decreased. Therefore, the influence of warping is decreased to improve dimensional accuracy. 
   In the above embodiments, in the electrostatic micro switch  1 , the switching is performed by bringing the contacts into contact with each other. However, it is obvious that the same effect is obtained, even if the invention is applied to the electrostatic micro switch disclosed in Japanese Patent Laid-Open No. 2003-258502 (Published Sep. 12, 2003) in which the switching is performed by the change in electrostatic capacitance. 
   Fifth Embodiment 
   A fifth embodiment of the invention will be described below with reference to  FIG. 17 .  FIG. 17  shows a schematic configuration of a radio communication device  41  according to the fifth embodiment. In the radio communication device  41 , an electrostatic micro switch  42  is connected between an internal processing circuit  43  and an antenna  44 . Turning on or off the electrostatic micro switch  42  enables the internal processing circuit  43  to switch the state in which the signal is transmitted or received through the antenna  44  and the state in which the signal is not transmitted or received. In the fifth embodiment, the electrostatic micro switch  1  shown in  FIGS. 1 to 16  is utilized as the electrostatic micro switch  42 . Therefore, the electrostatic micro switch  42  can be suppress the insertion loss of the high-frequency signal transmitted or received by the internal processing circuit  43  while the drive voltage rise and the response speed lowering are not generated. 
   Sixth Embodiment 
   A sixth embodiment of the invention will be described below with reference to  FIG. 18 .  FIG. 18  shows a schematic configuration of a measuring device  51  according to the sixth embodiment. In the measuring device  51 , plural electrostatic micro switches  52  are connected in midpoints of plural signal lines  57  from one internal processing circuit  56  to plural measuring objects  58 . Turning on or off each of the electrostatic micro switches  52  enables the internal processing circuit  56  to switch the measuring objects  58  to be transmitted or received. 
   In the sixth embodiment, the electrostatic micro switch  1  shown in  FIGS. 1 to 16  is utilized as the electrostatic micro switch  52 . Therefore, the electrostatic micro switch  52  can be suppress the insertion loss of the high-frequency signal transmitted or received by the internal processing circuit  56  while the drive voltage rise and the response speed lowering are not generated. 
   Seventh Embodiment 
   A seventh embodiment of the invention will be described below with reference to  FIG. 19 .  FIG. 19  shows a main-part configuration of a handheld terminal  61  according to the seventh embodiment. In the handheld terminal  61 , two electrostatic micro switches  62   a  and  62   b  are utilized. The electrostatic micro switch  62   a  performs a function of switching an internal antenna  63  and an outer antenna  64 , and the electrostatic micro switch  62   b  perform a function of switching signal flow between an electric power amplifier  65  on the transmission circuit side and a low-noise amplifier  66  on the reception circuit side. 
   In the sixth embodiment, the electrostatic micro switch  1  shown in  FIGS. 1 to 16  is utilized as the electrostatic micro switches  62   a  and  62   b . Therefore, the electrostatic micro switches  62   a  and  62   b  can be suppress the insertion loss of the high-frequency signal, which is transmitted by the electric power amplifier  65  and received by the low-noise amplifier  66 , while the drive voltage rise and the response speed lowering are not generated. 
   As described above, the electrostatic micro switch according to the invention can pass through the signal ranging from the direct-current signal to the high-frequency signal with low loss while maintaining the stable characteristics for a long time. Accordingly, the adoption of the electrostatic micro switch of the invention to the radio communication device  41 , the measuring device  51 , and the handheld terminal  61  enables the signal to be accurately transmitted for a long time while the load onto the amplifier used in the internal processing circuit or the like is suppressed. Further, the electrostatic micro switch of the invention is small and power consumption is also small, so that the effectiveness is exerted particularly in the battery-powered devices such as the radio communication device and handheld terminal and in the case where the plural measuring devices are used. 
   In the above embodiments, the resistivity is set at 300 Ωcm in the low-resistivity portion of the semiconductor which is of the movable substrate  20 . From the viewpoint of response speed, it is preferable that the resistivity of the low-resistivity portion be lowered as much as possible. For example, because the resistivity ranges from 3 to 4 Ωcm in the semiconductor usually used in the MEMS element, the semiconductor usually used in the MEMS element may be used as the low-resistivity portion. 
   The invention is not limited to the above embodiments, but various changes could be made without departing from the scope shown in claims. Another embodiment obtained by appropriately combining technical means disclosed in the different embodiments is also included in the technical range of the invention. 
   Thus, in the electrostatic micro switch according to the invention, the drive voltage rise can be avoided, the operation speed lowering can be prevented, and the good high-frequency characteristics can be maintained. Therefore, the electrostatic micro switch of the invention can be applied to other MEMS elements in which the high-frequency signal is utilized.