Abstract:
Disclosed herein is a latchable MEMS switch device capable of retaining its ON or OFF state even after the external power source is turned off. It is unnecessary not only to introduce novel materials such as magnetic material but also to form complicated structures. At least one of the cantilever and pull-down electrode of a cold switch is connected to a second MEMS switch. A capacitor between the cantilever and pull-down electrode of the cold switch is charged by the second MEMS switch. Thereafter since the cold switch is isolated in the device, the charge remains stored. Therefore, the cold switch can remain in the ON state since the charge continues to create electrostatic attraction between the cantilever and the pull-down electrode.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to semiconductor devices using MEMS (Micro Electro Mechanical System) switches which operate mechanically by converting electrostatic force to actuating force, and more particularly to a semiconductor device using MEMS switches capable of remaining turned on or off even if power from a power source to the MEMS switches is stopped.  
       RELATED ARTS  
       [0002]     Due to the progress in lithography technology of semiconductor manufacture, semiconductor devices with design rules of 130 nm to 90 nm are being produced. In addition, the wafer size is advancing from 200 mm to 300 mm in diameter with advancing semiconductor manufacture equipment. Where 300 mm diameter wafers are combined with a design rule of 130 nm or finer, chips are produced in large quantities at a time. In this situation, cell-based system LSI development is not allowed unless the system LSI is expected to be consumed in large quantities. For a user demanding various kinds of products in small quantities, cell-based IC development does not pay in many cases due to the rising cost for masks, test production and development.  
         [0003]     Directed to these demands, reconfigurable logic devices (or reconfigurable processors) are now under development. A reconfigurable logic device has a programmable logic device (such as an FPGA) combined with a microcomputer therein and allows the user to immediately realize a custom LSI by configuring user-defined functions into the programmable logic device. An FPGA is needed for where the configuration is implemented according to a program. In this FPGA block, each cell is composed of, for example, a 4-input look-up table and a flip-flop. Upon power on, configuration data is sent from a ROM (such as a flash memory) where the user program is stored. Logical operation begins after the control register is set so as to indicate the operation of the flip-flop in each cell has been programmed with the configuration data. In this architecture, since configuration data, namely, a user program is recorded as the flip-flop operations of the cells, the logical states cannot be retained if the power source is stopped.  
         [0004]     Application of these reconfigurable logic LSIs to communication equipment and mobile devices is being considered. In particular in the case of mobile devices, chip size reduction and power saving are required. Accordingly, we have considered using MEMS switches with latch mechanism instead of flip-flops. The MEMS switch is an ideal switch showing an on-resistance of substantially 0 and a substantially infinite off-resistance since it mechanically connects/disconnects a contact to/from another contact. If bistable MEMS switches, that is, MEMS switches with latch mechanism are used, not only the voltage-keeping circuit can be omitted but also power consumption can be reduced since no power is required to keep the state of each switch.  
         [0005]     In addition, MEMS switches can also be used to dynamically power on/off circuit blocks on each block basis. Although attempts to use MOS transistors for source power control have so far been made, they must enlarge the chip size if all circuit blocks are controlled since the channel width of each transistor must be enlarged according to the magnitude of current flowing through the corresponding circuit block. Contrastingly in the case of MEMS switches, it is not necessary to enlarge the chip size since metal contacts allow a large magnitude of current to flow therethrough and they can be formed in a wiring layer not like those of transistors that must be formed on the surface of the Si substrate.  
         [0006]     To add latch mechanism to a MEMS switch, various attempts have so far been made. For example, in Patent Document 1 (Japanese Patent Laid-open No. 2001-176369), a magnetic material is used to make a MEMS switch latchable as shown in  FIG. 15 . This switch is on when a contact  14  on a cantilever  13  is brought into contact with a contact  16  on another substrate  18  opposite to the cantilever  13 . In this switch, a magnetic element  15  is placed on the cantilever  13  formed on a substrate  11  and a magnetic element  17  is placed on the pull-down electrode  18 . The magnetic element  15  is magnetized by a coil  12  placed below the cantilever  13  to create a magnetic force which is used to keep the switch in the on state.  
         [0007]     In another method disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 1997-63293), a diaphragm  23  is used as a latch to form a memory cell (MEMS switch). This switch turns off if the diaphragm  23  becomes curved upward away from the support. If the diaphragm  23  becomes curved downward into the open region to come in contact with a pull-down electrode  22  formed on a substrate  21 , the switch turns on.  
         [0008]     Further, such methods as to mechanically implement a latch by thermal actuation and implement a latch by a devised mechanical structure have been proposed.  
       SUMMARY OF THE INVENTION  
       [0009]     In most of these known examples, latch mechanism is implemented by introducing a novel material such as a magnetic material or forming a complicated structure on the device surface. If a novel material, particularly a magnetic material, is used, contamination control and special cleaning must be added since such a material has been treated as contaminant material for semiconductor devices. In addition, if a complicated structure is formed, the process may probably become complicated since it must be formed on the semiconductor wafer concurrently with other conventional elements.  
         [0010]     Therefore, it is an object of the present invention to implement a simply structured MEMS switch with latch mechanism without introducing a novel material such as a magnetic material.  
         [0011]     According to the present invention, two or more MEMS switches are combined to make it possible for an MEMS switch to remain in the on state or in the off state even if the external power supply is stopped. There are two types of MEMS switches: hot switches and cold switches. In a hot switch, a cantilever and a contact on cantilever are at the same voltage, that is, the cantilever also serves as a contact on cantilever to propagate an electrical signal. In a cold switch, a cantilever is insulated from a contact on cantilever so that the electrical signal to be propagated can be controlled independently of the actuation of the cantilever.  
         [0012]     According to the present invention, two MEMS switch are connected in series. The rear switch is a cold switch whereas the front switch is a hot switch. In the cold switch, a capacitor is formed by the cold switch&#39;s main portion (cantilever) carrying the switch terminal of the cold switch and a pull-down electrode placed opposite to the cantilever. This capacitor is charged via the front MEMS switch to create attraction between the respective electrodes (cantilever and pull-down electrode). This attraction is used to actuate the cold switch. Charging the capacitor via the front MEMS switch turns on the cold switch whereas discharging the capacitor via the front MEMS switch turns off the cold switch.  
         [0013]     As described above, according to the present invention, two or more MEMS switches, including the rear cold switch, are combined so as to make the rear cold switch latchable by accumulating charge between the cantilever and pull-down electrode of the rear cold switch. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIGS. 1A through 1C  are diagrams for explaining a MEMS switch with latch mechanism according to a first embodiment of the present invention, in which  FIG. 1A  illustrates a cross section of the switch device,  FIG. 1B  is a top view of the switch, and  FIG. 1C  is a timing chart indicating how the switch device is operated;  
         [0015]      FIG. 2  shows a modified embodiment of the MEMS switch with latch mechanism according to the first embodiment of the present invention;  
         [0016]      FIGS. 3A through 3D  are cross-sectional views partly indicating how the MEMS switches in the first embodiment of the present invention are fabricated;  
         [0017]      FIGS. 4A through 4D  are cross-sectional views partly indicating how the MEMS switches in the first embodiment of the present invention are fabricated;  
         [0018]      FIGS. 5A through 5D  are cross-sectional views partly indicating how the MEMS switches in the first embodiment of the present invention are fabricated;  
         [0019]      FIGS. 6A through 6C  are top views of the MEMS switches in the first embodiment of the present invention;  
         [0020]      FIGS. 7A through 7C  are top views of MEMS switches according to a second embodiment of the present invention;  
         [0021]      FIG. 8  is a timing chart for explaining how the MEMS switches in the second embodiment of the present invention are operated;  
         [0022]      FIG. 9A through 9C  are diagrams for explaining a MEMS switch with latch mechanism according to a third embodiment of the present invention, in which  FIG. 9A  illustrates a cross section of the switch device,  FIG. 9B  is a top view of the switch device and  FIG. 9C  is a timing chart indicating how the switch device is operated;  
         [0023]      FIGS. 10A through 10D  are cross-sectional views partly indicating how the MEMS switches in the third embodiment of the present invention are fabricated;  
         [0024]      FIGS. 11A through 11D  are cross-sectional views partly indicating how MEMS switches in the third embodiment of the present invention are fabricated;  
         [0025]      FIGS. 12A through 12D  are cross-sectional views partly indicating how the MEMS switches in a fourth embodiment of the present invention are fabricated;  
         [0026]      FIGS. 13A through 13C  are cross-sectional views partly indicating how the MEMS switches in the fourth embodiment of the present invention are fabricated;  
         [0027]      FIGS. 14A through 14D  are cross-sectional views partly indicating how the MEMS switches in the fourth embodiment of the present invention are fabricated;  
         [0028]      FIG. 15  is a cross-sectional view of a first prior art MEMS switch with latch function; and  
         [0029]      FIG. 16  is a cross-sectional view of a second prior art MEMS switch with latch mechanism. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [heading-0030]     &lt;Embodiment 1&gt; 
         [0031]     Referring to  FIG. 1 , the following describes a MEMS switch device according to a first embodiment of the present invention.  FIG. 1A  is a section view of the structure of the MEMS switch device according to the present invention while  FIG. 1B  is a top view of the MEMS switch device. The sectional structure in  FIG. 1A  is depicted along line D-D′ in  FIG. 1B . This MEMS switch is composed of two switches, i.e., a front switch S 1  and a rear switch S 2 . In this embodiment, the front switch S 1  is fabricated as a hot switch while the rear switch S 2  as a cold switch.  
         [0032]     The hot switch S 1  is turned on when a voltage is applied to between two electrodes of a capacitor, a cantilever  116  and a pull-down electrode  118 , since the cantilever  116  is attracted toward the pull-down electrode  118  and therefore short-circuited with a contact of signal line  120  (or a stationary contact). In the rear cold switch S 2 , an insulator  110  is sandwiched between a cantilever  117  and a contact  109  on the cantilever (or a mobile contact). When a voltage is applied to between two electrodes, the cantilever  117  and a pull-down electrode  119 , the cantilever  117  is attracted toward the pull-down electrode  119  likewise in the hot switch and thus the contact  109  on the cantilever short-circuits two stationary contacts (wiring lines) Y 1  and Y 2  with each other so as to allow a signal to be propagated between them. This operation is described below with reference to the top view in  FIG. 1B  and a timing chart in  FIG. 1C .  
         [0033]     To turn on the switch S 2 , a cantilever electrode terminal A 2  of the switch S 1  is set to +Vcc and a pull-down electrode terminal A 1  of the switch S 1  and a pull-down electrode terminal B 1  of the switch S 2  are set to GND. Since this forms a potential difference of |Vcc| between the cantilever  116  and pull-down electrode  118  of the switch S 1 , the switch S 1  is turned on to short-circuit the cantilever  116  with the contact  120  of signal line. While the switch S 1  is in the ON state, the cantilever electrode terminal A 2  is set to +Vcc, so the cantilever electrode terminal B 2  of the switch S 2  is set to +Vcc. This forms a potential difference of |Vcc| between the cantilever  117  and the pull-down electrode  119  of the switch S 2  and therefore turns on the switch S 2 . In this state, the contact  109  on the cantilever short-circuits the two wiring terminals (stationary contacts) Y 1  and Y 2  with each other, resulting in Y 1 =Y 2 .  
         [0034]     Thereafter, if the cantilever terminal A 2  of the switch S 1  is set to GND, the switch S 1  goes into the OFF state whereas the switch S 2  remains in the ON state since the potential difference between the cantilever  117  and pull-down electrode  119  can be retained due to the charge accumulated to the cantilever  117 .  
         [0035]     Actually, however, when the switch S 1  is turned off, the charge accumulated between the cantilever  117  and pull-down electrode  119  of the switch S 2  is partly released. If this discharge is large in quantity, the potential difference between the cantilever  117  and pull-down electrode  119  of the switch decreases, perhaps making it impossible to retain the switch S 2  in the ON state. Therefore, the electrode size of the capacitor in the switch S 2  is designed larger than that in the switch S 1  in order to raise the quantity of charge. In addition, the gap between the upper and lower electrodes (cantilever to pull-down electrode gap) of the switch S 2  is designed so narrow that the switch S 2  can remain in the ON state even if the potential difference somewhat decreases when the switch S 1  is turned off.  
         [0036]     Then, if the stationary electrode terminal A 1  of the switch S 1  is set to +Vcc, a potential difference of |Vcc| is formed between the cantilever  116  and pull-down electrode  118  of the switch S 1 , which turns on the switch S 1  to short-circuit the cantilever  116  with the contact  120  of signal line. Thus, the voltage B 2  of the cantilever electrode  117  in the switch S 2  is set to GND since the cantilever terminal A 2  is set to GND. This turns off the switch S 2  since the charge accumulated in the capacitor of the switch S 2  is released due to no potential difference between the cantilever  117  and pull-down electrode  119 .  
         [0037]     As shown in  FIG. 1A , each of the terminals A 1  and A 2  of the hot switch S 1  is connected to a MOS transistor T 1  in a voltage supply circuit C 1 . Likewise, each of the stationary contact terminals Y 1  and Y 2  of the cold switch S 2  is connected to a MOS transistor T 2  in a signal circuit C 2 .  
         [0038]     Theoretically, the ON-OFF control of the cold switch S 2  can also be implemented by using a MOS transistor instead of the hot switch S 1  and switching on/off the MOS transistor. Practically, however, it is impossible to keep the cold switch S 2  in the ON state since the charge in the cold switch S 2  is gradually released due to the leak current flowing through the MOS transistor in the OFF state. Accordingly, by using the MEMS switch S 1  capable of physically disconnecting the voltage supply circuit, the present invention makes it possible to surely retain the ON state.  
         [0039]     Although in the above description of the cold switch S 2 , a potential difference of |Vcc| is formed between the electrodes of the capacitor by setting the pull-down electrode B 1  to GND and giving +Vcc to the cantilever B 2 , it is also possible to turn on the switch S 2  and keep the switch S 2  in the ON state by giving +Vcc to the pull-down electrode B 1  and GND to the cantilever B 2  so as to form the potential difference of |Vcc| between the electrodes. In this case, the contact  120  of the switch S 1  is connected not to the cantilever  117  of the switch S 2  as shown in  FIGS. 1A and 1B  but to the pull-down electrode  119  of the switch S 2  since the switch S 2  cannot retain the ON state due to the leak current flowing through the MOS transistor in the voltage supply circuit C 1  if the pull-down electrode B 1  is directly connected to the voltage supply circuit C 1 .  
         [0040]     In addition although in the above description of the cold switch S 2 , the contact  109  on the cantilever short-circuits the two stationary contacts Y 1  and Y 2  which are connected to the signal circuit C 2 , the switch S 2  may also be configured in such a manner that as shown in  FIG. 2 , it has a mobile contact Y 1  and a stationary contact Y 2  and short-circuits them which are connected to the signal circuit C 2 . In this cold switch S 2  shown in  FIG. 2 , however, the mobile portion is unbalanced due to the center of electrostatic force deviated from the center of actuation since wiring is required to electrically draw the mobile contact. From the viewpoint of design, it is therefore preferable to configure the cold switch S 2  as shown in  FIG. 1B .  
         [0041]     The following describes how to manufacture a MEMS switch device of the present embodiment.  
         [0042]     In  FIG. 3A , MEMS switches are being formed on the top of a wafer where a voltage supply circuit C 1  and a signal circuit C 2  are formed. Note that the signal circuit is omitted in the figure. There are underlayer metal lines  102  buried in an interlayer dielectric film  101 . The underlayer metal lines  102  are connected to transistors T 1  via plugs  103 . SiN is deposited as a cap film  104  for the interlayer dielectric film  101  and holes are formed in the SiN cap film  104  and the interlayer dielectric film  101 . After the plugs  103  are buried in the holes, planarization is made. Then, an underlayer metal film  105  is deposited which is to be used to form the pull-down electrodes and stationary contacts of the MEMS switches. Here, poly-Si is used. A pattern for the pull-down electrode and stationary contacts is transferred to a resist  100  on the poly-Si film  105  by photo-lithography process. This resist is removed after used as a mask to etch the poly-Si film  105  ( FIG. 3B ).  
         [0043]     After the surface is cleaned, plasma TEOS is deposited as a sacrifice film  106 , which is to be removed to form a gap in the switches. A pattern that has holes corresponding to the mobile contacts of the switches is transferred to a resist  107  as shown in  FIG. 3C .  
         [0044]     After dents  108  are formed by etching, using this resist as a mask, the resist is removed as shown in  FIG. 3D . Although it is also possible to form the cantilever and mobile contact without forming these dents  108 , these dents  108  make the switches more reliable since the cantilever  116  and mobile contact  109  can have projections respectively for contact with the stationary contacts  120  and  105 .  
         [0045]     Then, after the surface is cleaned, poly-Si is deposited as a metal film  99  to be used to form a mobile contact. Then, by photo-lithography process, a resist pattern  98  is formed only for the mobile contact on the side of the cold switch S 2  ( FIG. 4A ).  
         [0046]     As shown in  FIG. 4B , the resist  98  is removed after used as a mask to etch the electrode terminal  109 .  
         [0047]     Then, after an insulator  110  is deposited on the electrode terminal  109 , a mobile contact of the cold switch S 2 , and on the sacrifice layer  106 , a resist pattern  111  is formed as to cover the electrode terminal  109  of the cold switch S 2  as shown in  FIG. 4C . In the present embodiment, aluminum oxide is used to form the insulator  110 .  
         [0048]     Then, the insulator  110  is removed by dry etching and the resist  111  is removed as shown in  FIG. 4D .  
         [0049]     Further, after cleaning process is done, a resist pattern  112  is formed by photo-lithography process to make contact holes for the cantilevers as shown in  FIG. 5A .  
         [0050]     After the sacrifice layer  106  is etched by using this resist  112  as a mask until the contact holes  113  reach the surface of the underlayer metal film  105 , the resist  112  is removed as shown in  FIG. 5B .  
         [0051]     Into these contact holes  113  and onto the sacrifice layer  106 , a metal film  114  is deposited. Thereafter, a pattern for the cantilevers of the switches is transferred to a resist  115  as shown in  FIG. 5C . In the present embodiment, the cantilevers are made of poly-Si.  
         [0052]     The cantilever  116  of the hot switch and that  117  of the cold switch are formed by etching the metal film  114  by using the resist pattern  115  as a mask. Thereafter, the resist  115  is removed ( FIG. 5D ).  
         [0053]     Then, after the sacrifice layer  106  is removed by wet etching, the wafer is dried to complete the switch structure shown in  FIG. 1A . In the present embodiment, buffered hydrogen fluoride is used to remove the sacrifice layer  106 . The wafer is cleaned with water after the wet etching. If the wafer is dried just after cleaned with water, the cantilevers  116  and  117  may stick respectively to the pull-down electrodes  118  and  119  due to the surface tension of water. Therefore the wafer is dipped with methanol before super critical carbon dioxide drying is done finally.  
         [0054]     After the MEMS switch structure is formed, its top surface is sealed with glass or ceramics for isolation from the outer environment. In this sealing, it is preferable to fill the inside with an inert gas or depressurize the inside.  
         [0055]      FIGS. 6A through 6C  are top views of the MEMS switch device.  FIG. 6A  is depicted after the underlayer metal film  105  is patterned as shown in  FIG. 3B .  FIG. 6B  corresponds to  FIG. 5B  and shows the positional relations among the cantilever terminal (mobile contact)  109  of the cold switch S 2 , the contact holes  113  to respectively connect the cantilevers to the underlayer metal lines  105 , and the underlayer metal lines  105 . Further,  FIG. 6C  shows the positional relations among the mobile contact  109 , the underlayer metal lines  105 , the contact holes  113  and the cantilevers  116  and  117 . The contact holes  113  are filled with mobile electrode material poly-Si. This figure is a top view of a latchable MEMS switch device composed of one hot switch S 1  and one cold switch S 2 . Its operation has been described earlier with reference to  FIG. 1 .  
         [heading-0056]     &lt;Embodiment 2&gt; 
         [0057]     The following describes a second embodiment of the present invention where a latchable cold switch is realized by using two hot switches.  
         [0058]     Its manufacture process is similar to that shown in  FIG. 3  through  FIG. 5 .  FIGS. 7A through 7C  show top views of the MEMS switch device. Also in this embodiment, the electrode size of the capacitor to keep the cold switch (S 3  in  FIG. 7 ) in the ON or OFF state should be larger than that of the front switches (S 1  and S 2  in  FIG. 7 ) as mentioned earlier. In the case of the MEMS switch device shown in  FIG. 7 , all electrodes have the same size. Even such a MEMS switch device can operate reliably if the switch ON voltage is designed comparatively smaller than |Vcc|.  FIG. 7A  corresponds to  FIG. 3B  in the progress of process wherein the underlayer metal film is patterned.  FIG. 7B  corresponds to  FIG. 5B  in the progress of process and shows the positional relations among an electrode terminal (mobile contact)  109  of the cold switch S 3 , underlayer metal lines  105 , and contact holes  113  to connect cantilevers respectively to the underlayer metal lines  105 . Further,  FIG. 7C  shows the positional relations among the mobile contact  109 , the underlayer metal lines  105 , the contact holes  113  and cantilevers  116  and  117 . The contact holes  113  are filled with cantilever material poly-Si.  
         [0059]     The latchable MEMS switch device composed of the hot switches S 1  and S 2  and the cold switch S 3  is operated according to a timing chart in  FIG. 8 . The contact of cantilever A 2  and the contact of pull-down electrode A 1  of the hot switch S 1  are respectively set to GND and +VCC to turn on the switch S 1  and therefore set the pull-down electrode C 1  of the cold switch S 3  to GND. Meanwhile, the contact of pull-down electrode B 1  and contact of cantilever B 2  of the hot switch S 2  are respectively set to GND and +VCC to turn on the switch S 2  and therefore set the cantilever C 2  of the cold switch S 3  to +Vcc. At this time, since a potential difference of |Vcc| is formed between the pull-down electrode C 1  and cantilever C 2  of the cold switch S 3 , the switch S 3  turns on and therefore short-circuits signal terminals (stationary contacts) Y 1  and Y 2  with each other via the contact  109  of cantilever.  
         [0060]     Then, the hot switches S 1  and S 2  are turned off by switching the contact of pull-down electrode A 1  and the contact of cantilever B 2  to GND. However, the cold switch can remain in the ON state since the electrostatic attraction continues to work between the cantilever C 2  and pull-down electrode C 1  due to the charge accumulated between them although the potential difference between the pull-down electrode C 2  and cantilever electrode C 1  decreases from |Vcc| as mentioned earlier since the charge is partially released from the cantilever C 2  when the hot switches S 1  and S 2  are turned off.  
         [0061]     To turn off the cold switch S 3 , the contact of pull-down electrode A 1  and contact of cantilever A 2  of the switch are respectively set to +Vcc and GND and the contact of pull-down electrode B 1  and the contact of cantilever B 2  are respectively set to +Vcc and GND. Since this turns on the hot switches S 1  and S 2  but sets both pull-down electrode C 1  and cantilever C 2  of the cold switch S 3  to GND, the accumulated charge is released to turn off the cold switch S 3 . Note that as indicated by a broken line in  FIG. 8 , the hot switch S 1  must not necessarily be turned on to turn off the cold switch S 3  since the pull-down electrode C 1  is set to GND while the cold switch is in the ON state. The cold switch S 3  can be turned off by turning on only the hot switch S 2 .  
         [0062]     The aforementioned first embodiment can be implemented by a smaller area than the present embodiment since the first embodiment is composed of two switches. Meanwhile, the present embodiment can retain the cold switch S 3  more reliably than the first embodiment since the pull-down electrode of the cold switch S 3  is completely floating while the cold switch S 3  is kept in the ON state. Note that the present invention can also be configured in such a manner that like the cold switch S 2  in  FIG. 2 , the cold switch S 3  has one contact of pull-down electrode and one contact of cantilever that are connected to a signal circuit.  
         [heading-0063]     &lt;Embodiment 3&gt; 
         [0064]     In the aforementioned first and second embodiments, a latchable MEMS switch device is made by combining one or more hot switches with a cold switch. The same function can also be implemented by combining cold switches. The following describes such a third embodiment of the present invention.  
         [0065]      FIGS. 9A through 9C  show an example of a latchable MEMS switch device configured by using three cold switches S 1 , S 2  and S 3 .  FIG. 9B  is its top view.  FIG. 9A  is a cross-sectional view of the structure depicted along line D-D′ in  FIG. 9B .  FIG. 9C  is a timing chart showing its latching mechanism. In this configuration, the switch S 3  is a switch with latch function. While the switch S 3  is in the ON state, two signal terminals (stationary contacts) Y 1  and Y 2  are short-circuited with each other (Y 1 =Y 2 ). While the switch S 3  is in the OFF state, the signal terminals Y 1  and Y 2  are brought into an open-circuit state. How to fabricate this MEMS switch device having cold switches connected in series is described later. In each of the cold switches S 1 , S 2  and S 3 , the cantilever  220  is electrically isolated from the contact of cantilever (mobile contact)  212  by the insulator  215  as shown in  FIG. 9A . Each cold switch is designed so that it is turned on by electrostatic force between the pull-down electrode  221  and the cantilever  220  when the potential difference between these electrodes is |Vcc| or slightly small voltage than |Vcc|. This MEMS switch device operates as described below.  
         [0066]     The contact of pull-down electrode A 1  and contact of cantilever A 2  of the switch S 1  are respectively set to GND and +Vcc to turn on the switch S 1 . Thus the terminal X 1  is short-circuited to the contact of pull-down electrode C 1  of the switch S 3  through the top mobile contact  212  of the switch S 1 . Since X 1  is set to GND, the pull-down electrode  221  of the switch S 3  is also set to GND. Meanwhile, the contact of pull-down electrode B 1  and contact of cantilever B 2  of the switch S 2  are also set to GND and +Vcc respectively to turn on the switch S 2 . Thus, the terminal X 2  of the switch S 3  is short-circuited to the cantilever of the switch S 3  through the top mobile contact  212  of the switch S 2 . At this time, setting the terminal X 2  to +Vcc turns on the switch S 3  since a potential difference of |Vcc| is formed between the cantilever  220  and pull-down electrode  221  of the switch S 3 , resulting in the signal terminals (stationary contacts) Y 1  and Y 2  short-circuited with each other by the top mobile contact  212  of the switch S 3 .  
         [0067]     If the switches S 1  and S 2  are turned off at this time by setting the contact of cantilever A 2  of the switch S 1  and the contact of cantilever B 2  of the switch S 2  to GND, the switch S 3  can remain in the ON state since the electrostatic force continues to work between the cantilever  220  and pull-down electrode  221  of the switch S 3  due to the charge accumulated between them. Note that the terminal X 1  is set to GND after the switch S 3  remains in the ON state.  
         [0068]     Since the switches S 1  and S 2  are cold switches, the accumulated charge is not released from the switch S 3  when the switches S 1  and S 2  are turned off. Therefore, as compared with the aforementioned first and second embodiments, the present embodiment can keep the MEMS switch device in the ON state more reliably.  
         [0069]     To turn off the switch S 3 , the switches S 1  and S 2  are turned on by setting the contact of cantilever A 2  of the switch S 1  and the contact of cantilever B 2  of the switch S 2  to +Vcc and the contact of pull-down electrode A 1  of the switch S 1  and the contact of pull-down electrode B 1  of the switch S 2  to GND. Further, the terminal X 2  is set to GND to release the charge accumulated between the cantilever  220  and pull-down electrode  221  of the switch S 3 , which turns off the switch S 3  since the electrostatic force eliminates between the cantilever  220  and pull-down electrode  221  of the switch S 3 .  
         [0070]     Although in the above description, the terminal X 1  (contact of pull-down electrode C 1 ) and terminal X 2  (contact of cantilever C 2 ) are respectively set to GND and +Vcc in order to turn on the switch S 3 , the switch S 3  may also be turned on by inversely setting the terminals X 1  and X 2  (contact of pull-down electrode C 1  and contact of cantilever C 2 ) to +Vcc and GND as indicated by dotted lines in the timing chart of  FIG. 9C . Further, any voltages other than GND and +Vcc can be set to the terminals X 1  and X 2  (contact of pull-down electrode C 1  and contact of cantilever C 2 ) if a potential difference of |Vcc| or larger is formed between the terminal X 1  (contact of pull-down electrode C 1 ) and the terminal X 2  (contact of cantilever C 2 ). The same holds for the switches S 1  and S 2  when they are turned on.  
         [0071]     In addition, although in the above description, the terminal X 1  (contact of pull-down electrode C 1 ) and terminal X 2  (contact of cantilever C 2 ) are set to GND to turn off the switch S 3 , they must not necessarily be set to GND. They may be any voltages other than GND if the potential difference between the terminal X 1  (contact of pull-down electrode C 1 ) and the terminal X 2  (contact of cantilever C 2 ) is made smaller than |Vcc|. However, setting them to the same voltage can turn off the switch S 3  more reliably. The same holds for the switches S 1  and S 2  when they are turned off.  
         [0072]     The following describes how to fabricate the latchable MEMS switch device that is composed of cold switches as shown in  FIG. 9A . Until the structure shown in  FIG. 3D  is obtained, the manufacturing procedure is the same as for a MEMS switch device composed of hot and cold switches.  
         [0073]     Then, after the surface is cleaned, a metal film  210  is deposited which is to be used to form mobile contacts. In the present embodiment, poly-Si is used as the metal film. A resist pattern  211  for the cantilever of each cold switch is formed by photolithography process ( FIG. 10A ). After the metal film  210  is, the resist pattern  211  is removed ( FIG. 10B ).  
         [0074]     Then, after aluminum oxide is deposited as an insulation film  213  on the surface, a resist pattern  214  is formed so as to cover the mobile contact or contact of cantilever  212  of each cold switch as shown in  FIG. 10C .  
         [0075]     Then, after the aluminum oxide insulation film  213  is removed by dry etching, the resist  214  is removed as shown in  FIG. 10D . At this time, the contacts of cantilevers (mobile contacts)  215  are covered by aluminum oxide insulators  215 .  
         [0076]     Further, after cleaning process is done, a resist pattern  216  to form the contact hole of each cantilever is formed by photolithography process ( FIG. 11A ). Then after the sacrifice layer  207  is etched to the surface of the underlayer metal lines  205 , the resist  216  is removed as shown in  FIG. 11B .  
         [0077]     On this surface, poly-Si is deposited as a metal film  218  to form the cantilevers  218 . Then, a pattern for the cantilevers is transferred to a resist  219  as shown in  FIG. 11C .  
         [0078]     Using this resist pattern as a mask, the metal film  218  is etched to form the cantilever  220  of each cold switch. After that, the resist  219  is removed ( FIG. 1D ).  
         [0079]     Then, after the sacrifice layer  207  is removed by wet etching, drying is done to complete the switch structure shown in  FIG. 9A .  
         [0080]     The present embodiment is advantageous in that the switching voltage can be designed easily since all switches in the MEMS switch device are cold switches and they can have the same configuration. The aforementioned first and second embodiments are preferable to the present embodiment in that they can be implemented by smaller areas since the terminals X 1  and X 2  to supply voltages to the cantilever  220  and pull-down electrode  221 , shown in  FIG. 10B , must not be formed.  
         [heading-0081]     &lt;Embodiment 4&gt; 
         [0082]     The MEMS switch device according to the present invention is characterized in that a charge is accumulated between mobile and a pull-down electrode and a cantilever and the charge is kept so that an electrostatic force between the pull-down electrode and cantilever continues to work in order to retain the MEMS switch device in the ON state. In the aforementioned embodiments, each MEMS switch device is operated in a depression atmosphere or inert gas-filled environment. In such an environment, small leak current may flow along the surfaces of electrodes while the MEMS switch device is kept in the ON state, decreasing the quantity of the accumulated charge.  
         [0083]     To prevent this, covering the surfaces of the pull-down electrode and cantilever with insulator film. The following describes such a fourth embodiment of the present invention.  
         [0084]     In the progress of process,  FIG. 12A  corresponds to  FIG. 3B . In  FIG. 12A , poly-Si underlayer electrodes  305  are formed after a SiN film is deposited on the surface of an interlayer dielectric film  304 .  
         [0085]     Then, as shown in  FIG. 12B , after an aluminum oxide insulator  306  is deposited thereon, a resist pattern  307  for each switch is formed by photolithography process with a portion to come into contact area corresponding to a mobile contact of cantilever. The deposited aluminum oxide insulator  306  covers the surface of each pull-down electrode for each switch in order to minimize the surface leak current.  
         [0086]     After the insulator  306  is etched by using the resist pattern  307  as a mask to form a contact hole  308 , the resist  307  is removed as shown in  FIG. 12C .  
         [0087]     Then after the surface is cleaned, plasma TEOS is deposited as a sacrifice layer  309  which will be removed to form a gap in each switch. Thereon, a pattern for each switch is transferred to a resist  310  by photolithography process with a portion corresponding to the mobile contact of cantilever as shown in  FIG. 12D .  
         [0088]     Then after dents  311  are formed on the sacrifice layer by using this resist as a mask, the resist is removed as shown in  FIG. 13A .  
         [0089]     Then after the surface is cleaned, a metal film  312  is deposited to form the contact of cantilever. Poly-Si is used as the metal film in the present embodiment, too. A resist pattern  313  that masks the mobile contact area of cantilever for each cold switch is formed by photolithography process ( FIG. 13B ).  
         [0090]     Then, after the metal film  312  is patterned by using this mask to form the mobile contact of cantilever  314  of each switch, the resist  313  is removed ( FIG. 13C ).  
         [0091]     Then, after aluminum oxide is deposited on the surface as an insulator  315 , a resist pattern  316  corresponding to a contact hole for the base of cantilever is formed by photolithography process ( FIG. 13D ).  
         [0092]     Using this resist  316  as a mask, the aluminum oxide insulator  315 , the sacrifice layer  309  and the aluminum oxide insulator  306  are continuously etched to form contact holes  317  down to the surface of the underlayer metal film  305 . Thereafter, the resist  316  is removed as shown in  FIG. 14A .  
         [0093]     On the surface, poly-Si is deposited as a metal film  318  to form the cantilever of each switch. Then, a pattern for the cantilevers is transferred to a resist  319  as shown in  FIG. 14B .  
         [0094]     Then after the metal film  318  and the aluminum insulator  315  thereunder are etched using this resist pattern  319  as a mask to form the cantilever  320  of each cold switch, the resist  319  is removed.  
         [0095]     Then, after the sacrifice layer  309  is removed by wet etching, drying is performed to complete the switch structure shown in  FIG. 14D .  
         [0096]     In the present embodiment, since not only the top surfaces of the pull-down electrodes and other underlayer electrodes  305  are covered but also the bottom surfaces of the cantilevers  320  are covered respectively by aluminum oxide films  306  and  315 , it is possible to improve the reliability of the MEMS switch device by reducing the surface leak current between the pull-down electrode and cantilever while the MEMS switch device is kept in the ON state. However, since each poly-Si cantilever  320  is stacked on an aluminum oxide film  315 , deliberate stress control is required to minimize the warping of the cantilever  320 . Therefore, it is most preferable to cover only the pull-down electrodes with the aluminum oxide  306 .  
         [0097]     The present embodiment, combined with any of the aforementioned embodiments, allows the MEMS switch device to be kept in the ON state more reliably.  
         [heading-0098]     Explanation of Reference Numerals:  
         [0099]      11 . Substrate,  12 . Coil,  13 . Cantilever,  14 . Contact on Cantilever,  15 . Magnetic Material on Cantilever,  16 . Contact of Pull-down Electrode,  17 . Magnetic Material on Pull-down Electrode,  18 . another substrate,  21 . Substrate,  22 . Lower Electrode,  23 . Diaphragm,  101 . Interlayer Insulator Film,  102 . Underlayer Metal Line,  103 . Plug,  104 . Cap Film of Interlayer Insulator Film,  105 . Underlayer Metal Film,  106 . Sacrifice Layer,  107 . Resist,  108 . Partial Etched Pattern on Sacrifice Layer,  109 . Contact-electrode Pattern of Cantilever,  110 . Insulator Film,  111 . Resist,  112 . Resist,  113 . Etched Pattern to Connect Cantilever,  114 . Metal Film,  115 . Resist,  116 . Cantilever of Hot Switch,  117 . Cantilever of Cold Switch,  118 . Pull-down electrode of Hot Switch,  119 . Pull-down electrode of Cold Switch,  205 . Underlayer Electrode,  207 . Sacrifice Layer,  210 . Metal Film,  211 . Resist,  212 . Mobile Contact-electrode Pattern of Cantilever,  213 . Insulator Film,  214 . Resist,  215 . Insulator Film Covering Mobile Contact-electrode Pattern of Electrode,  216 . Resist,  217 . Etched Pattern to Connect cantilever,  218 . Metal Film,  219 . Resist,  220 . Cantilever,  221 . Pull-down Electrode,  304 . Cap Film of Interlayer insulator Film,  305 . Underlayer Metal Line,  306 . Insulator Film,  307 . Resist,  308 . Contact Hole corresponding to Mobile Contact,  309 . Sacrifice Layer,  310 . Resist,  311 . Etched Pattern on Sacrifice Layer,  312 . Metal Film,  313 . Resist,  314 . Contact-electrode pattern of Cantilever,  315 . Insulator Film,  316 . Resist,  317 . Etched Pattern to Connect Cantilever,  318 . Metal Film,  319 . Resist,  320 . Cantilever