Abstract:
A fabrication method of an RF MEMS switch includes forming a signal transmission line having a first signal transmission line and a second signal transmission line electrically separated from each other for transmitting a signal and forming an on/off component for turning on/off the signal transmission line. The forming the on/off component further includes forming a suspension layer, forming a piezoelectric capacitor disposed at the suspension layers, and actuated with a piezoelectric characteristic by receiving an external power, forming a contact electrode disposed at the suspension layers, and electrically separated from the piezoelectric capacitors, and forming a ground line adjacent to the signal transmission line, wherein the ground line is electrically connected to the signal transmission line by a connection line.

Description:
This application is a Divisional of application Ser. No. 11/036,039 filed on Jan. 18, 2005 (now U.S. Pat. No. 7,151,425), and for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 10-2004-0003972 filed in Republic of Korea on Jan. 19, 2004 under 35 U.S.C. §119; the entire contents of all are hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The present invention relates to an RF MEMS switch and a fabrication method thereof, and more particularly, to an RF MEMS switch capable of controlling an RF signal by using a piezoelectric capacitor and a fabrication method thereof. Description of the Conventional Art 
   As information communication develops recently, an information communication electronic system is required to be small, light, and actuated in a high function. According to this, miniature components which constitute the information communication electronic system are required to be developed. Among the miniature components, a radio frequency micro-electromechanical system (RF MEMS) switch for controlling a system signal is being widely used. 
   Currently, an FET switch or a semiconductor switch such as a PIN diode, etc. is being widely used as a switch of information communication system. diode, etc. is being widely used as a switch of information communication system. However, the switches have many disadvantages such as a high power loss, a distortion, and a non-linear characteristic at the time of being actuated. 
   Also, the RF MEMS switch using an electrostatic force requires a high voltage and has a low reliability in operation. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to provide an RF MEMS switch capable of being actuated with a low driving voltage and a low consumption power by using a piezoelectric capacitor and capable of obtaining a high yield and a high reliability, and a fabrication method thereof. 
   To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided an RF MEMS switch comprising: a cap substrate having via holes at both sides thereof, the via hole provided with a connection line; a transmission line having a first transmission line and a second transmission line spaced from each other with a certain gap at a lower surface of the cap substrate; a ground line formed at both sides of the transmission line; a connection pad formed at an upper surface of the cap substrate, and electrically connected to the transmission line and the ground line by the connection line; a bottom substrate having a certain gap from the cap substrate; a piezoelectric capacitor formed at one side of a suspension layer formed at an upper surface of the bottom substrate, and actuated with a piezoelectric characteristic by receiving an external power; and a contact electrode formed at another side of the suspension layer and moved up and down by a displacement of the piezoelectric capacitor, for selectively turning on/off RF signals of the first signal transmission line and the second signal transmission line. 
   To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is also provided a fabrication method of an RF MEMS switch comprising: sequentially forming a first suspension layer of a silicon nitride and a second suspension layer of a silicon oxide on an upper surface of a bottom substrate, and forming a piezoelectric capacitor composed of a first electrode layer, a piezoelectric layer, and a second electrode at one side of an upper surface of the second suspension layer; forming a contact electrode at another side of the upper surface of the second suspension layer; etching the first suspension layer and the second suspension layer of the rest region except the piezoelectric capacitor and the contact electrode; etching a part of the bottom substrate to release a gap between the first suspension layer and the bottom substrate, thereby completing the bottom substrate; etching a center part of a lower surface of a cap substrate to form a groove; forming via holes at both sides of the cap substrate and filling a metal in the via hole, thereby forming a connection line; forming a signal transmission line and a ground line at an upper surface of the cap substrate; forming a connection pad at the lower surface of the cap substrate; electrically connecting the signal transmission line and the ground line to the connection pad by the connection line, thereby completing the cap substrate; and bonding the cap substrate to the upper surface of the bottom substrate with a certain gap by using a bump. 
   According to a second embodiment of the present invention, there is provided an RF MEMS switch comprising: a cap substrate having via holes at both sides thereof, the via hole provided with a connection line; a signal transmission line formed at a lower surface of the cap substrate; a ground line formed at both sides of the signal transmission line; a connection pad corresponding to the signal transmission line and formed at an upper surface of the cap substrate so as to be electrically connected to the signal transmission line and the ground line by the connection line; a bottom substrate having a certain gap from the cap substrate; a piezoelectric capacitor formed at one side of a suspension layer formed at an upper surface of the bottom substrate with a first metal layer thereof being exposed, and actuated with a piezoelectric characteristic by receiving an external power; and a capacitor contact electrode formed at another side of the suspension layer and connected to the exposed first metal layer, for selectively turning on/off an RF signal of the signal transmission line by being moved up and down by a displacement of the piezoelectric capacitor. 
   According to a second embodiment of the present invention, there is also provided a fabrication method of an RF MEMS switch comprising: sequentially forming a first suspension layer of a silicon nitride and a second suspension layer of a silicon oxide on an upper surface of a bottom substrate, and forming a piezoelectric capacitor composed of a first electrode layer, a piezoelectric layer, and a second electrode at one side of an upper surface of the second suspension layer; patterning the second electrode layer and the piezoelectric layer and then etching so that a part of the first electrode layer can be exposed; forming a capacitor contact electrode at another side of the upper surface of the second suspension layer so that the exposed part of the first electrode layer can be covered; etching the first suspension layer and the second suspension layer of the rest region except the piezoelectric capacitor and the contact electrode; etching a part of the bottom substrate to release a gap between the first suspension layer and the bottom substrate, thereby completing the bottom substrate; etching a center part of a lower surface of a cap substrate to form a groove; forming via holes at both sides of the cap substrate and filling a metal in the via hole, thereby forming a connection line; forming a signal transmission line and a ground line at an upper surface of the cap substrate; forming a connection pad at the lower surface of the cap substrate; electrically connecting the signal transmission line and the ground line to the connection pad by the connection line, thereby completing the cap substrate; and bonding the cap substrate to the upper surface of the bottom substrate with a certain gap by using a bump. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     In the drawings: 
       FIG. 1  is a perspective view showing an operation of a piezoelectric capacitor using a piezoelectric power; 
       FIG. 2  is a schematic diagram showing a structure of a resistive type RF MEMS switch; 
       FIG. 3  is an equivalent circuit showing the structure of the resistive type RF MEMS switch; 
       FIG. 4  is a construction view showing a capacity type RF MEMS switch; 
       FIG. 5  is a circuit showing a structure of a resistive type RF MEMS switch; 
       FIG. 6  is a perspective view showing an RF MEMS switch according to a first embodiment of the present invention; 
       FIG. 7  is a longitudinal section view showing the RF MEMS switch according to the first embodiment of the present invention; 
       FIG. 8  is a longitudinal section view taken along line I-I of  FIG. 7 ; 
       FIG. 9  is a plane view showing a bottom substrate of the RF MEMS switch according to the first embodiment of the present invention; 
       FIG. 10  is a plane view showing another example of the bottom substrate of the RF MEMS switch according to the first embodiment of the present invention; 
       FIG. 11  is a plane view showing still another example of the bottom substrate of the RF MEMS switch according to the first embodiment of the present invention; 
       FIGS. 12A to 12K  are section views showing a fabrication process of an RF MEMS switch according to a first embodiment of the present invention; 
       FIGS. 13A to 13F  are section views showing a process for fabricating a cap substrate in the fabrication process of the MEMS switch according to the first embodiment of the present invention; 
       FIG. 14  is a perspective view showing an RF MEMS switch according to a second embodiment of the present invention; 
       FIG. 15  is a longitudinal section view showing the RF MEMS switch according to the second embodiment of the present invention; 
       FIG. 16  is a longitudinal section view taken along line II-II of  FIG. 15 ; 
       FIG. 17  is a plane view showing a bottom substrate in the RF MEMS switch according to the second embodiment of the present invention; 
       FIG. 18  is a plane view showing another example of the bottom substrate in the RF MEMS switch according to the second embodiment of the present invention; 
       FIG. 19  is a plane view showing still another example of the bottom substrate in the RF MEMS switch according to the second embodiment of the present invention; and 
       FIGS. 20A to 20L  are section views showing a fabrication process of the RF MEMS switch according to the second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
   Hereinafter, an RF MEMS switch and a fabrication method thereof according to the present invention will be explained. To this end, an operation of a piezoelectric capacitor that serves as a piezoelectric actuating unit using a piezoelectric power, and structures of a resistive type RF MEMS switch and a capacity type RF MEMS switch will be explained in brief. 
     FIG. 1  is a perspective view showing an operation of a piezoelectric capacitor using a piezoelectric power. 
   As shown, a piezoelectric capacitor  160  is formed on a suspension layer  151 . The piezoelectric capacitor  160  has a structure that a piezoelectric body  162  is formed between an upper electrode  163  and a lower electrode  161 . 
   When a power is applied to the upper electrode  163  and the lower electrode  161 , the piezoelectric body  162  is contracted or expanded thereby to be displaced as much as “h” upwardly. The “h” is determined by a thickness ratio of the suspension layer  151  and a piezoelectric constant of the piezoelectric body  162 . When PZT such as Pb(Zr(Zr,Ti)O 3  is used, power applied to the piezoelectric body  162  can be lowered, the piezoelectric body is smoothly actuated, and a fast switching speed can be obtained. 
     FIG. 2  is a schematic diagram showing a structure of a resistive type RF MEMS switch, and  FIG. 3  is an equivalent circuit showing the structure of the resistive type RF MEMS switch. 
   As shown, a contact electrode  170  is moved up and down by the displacement of the piezoelectric body  160 , thereby turning on/off a first signal line  121  and a second signal line  122  spaced from each other with a certain gap. According to this, an RF signal flowing to the first signal line  121  and the second signal line  122  is controlled. 
   The first signal line  121  denotes an RF inline, and the second signal line  122  denotes an RF outline. 
   When the piezoelectric capacitor is operated, an RF signal is turned on and vice versa. 
     FIG. 4  is a construction view showing a capacity type RF MEMS switch, and  FIG. 5  is a circuit showing a structure of a resistive type RF MEMS switch. 
   As shown, a capacitor contact electrode  270  connected to the piezoelectric capacitor  160  and composed of a first metal layer, a dielectric layer, and a second metal layer is spaced from a signal transmission line  220  with a certain gap. 
   When a power is applied to the piezoelectric capacitor  160 , the piezoelectric body  162  is contracted or expanded thereby to be displaced as much as “h” upwardly. At this time, the capacitor contact electrode  270  is moved and served as a variable capacitor. According to this, a capacitor is increased towards a ground line (not shown), thereby turning off an RF signal flowing to the signal transmission line  220 . When the piezoelectric capacitor  160  is operated, an RF signal is turned off and vice versa. 
   In the RF MEMS switch and the fabrication method thereof according to the present invention, an RF signal can be fast and easily controlled by using a piezoelectric capacitor actuated as electric energy thereof is converted into mechanical energy when an electric field is applied thereto. 
   According to this, the RF MEMS switch of the present invention can be actuated with a voltage lower than 5V by using a piezoelectric power, and the RF MEMS switch can obtain a high reliability, a low voltage loss, and a fast switching speed. 
   Hereinafter, an RF MEMS switch and a fabrication method thereof according to the present invention will be explained with reference to the attached drawings. 
     FIG. 6  is a perspective view showing an RF MEMS switch according to a first embodiment of the present invention,  FIG. 7  is a longitudinal section view showing the RF MEMS switch according to the first embodiment of the present invention,  FIG. 8  is a longitudinal section view taken along line I-I of  FIG. 7 , and  FIG. 9  is a plane view showing a bottom substrate of the RF MEMS switch according to the first embodiment of the present invention. 
   As shown, an RF MEMS switch  100  according to the first embodiment of the present invention comprises: a cap substrate  110  having via holes  111  at both sides thereof, the via hole  111  provided with a connection line  112 ; a signal transmission line  120  formed at a lower surface of the cap substrate  110 , and having a first signal transmission line  121  and a second signal transmission line  122  spaced from each other with a certain gap; a ground line  130  formed at both sides of the signal transmission line  120 ; a connection pad  140  formed at an upper surface of the cap substrate  110 , and electrically connected to the signal transmission line  120  and the ground line  130  by the connection line  112 ; a bottom substrate  150  having a certain gap from the cap substrate  110 ; a piezoelectric capacitor  160  formed at one side of a suspension layer  151  formed at an upper surface of the bottom substrate  150 , and actuated with a piezoelectric characteristic when an external power is applied thereto; and a contact electrode  170  formed at another side of the suspension layer  151 , for selectively turning on/off RF signals of the first signal transmission line  121  and the second signal transmission line  122  by being moved up and down by a displacement of the piezoelectric capacitor  160 . 
   The cap substrate  110  is formed as one signal transmission line  120  that connects an input terminal to an output terminal and two ground lines  130  are deposited on a silicon or a glass by a plating method. 
   A bump  180  is formed between the cap substrate  110  and the bottom substrate  150  in order to maintain a certain gap between the cap substrate  110  and the bottom substrate  150 . As the bump  180 , a solder bump or an organic bump can be used. 
   The contact electrode  170  has a basic structure of Ti/Au or Cr/Au for a low resistance and an excellent bonding with the suspension layer. In order to prevent a contact part between the contact electrode and the signal transmission line from being adhered to each other during an operation or in order to prevent a surface of the contact electrode from being damaged, Mo, W, and Ir for a high mechanical intensity and melting point can be thinly deposited. 
   That is, the contact electrode  170  preferably has one structure of Ti/Au/Mo, Cr/Au/Mo, Ti/Au/W, Cr/Au/W, Ti/Au/Ir, and Cr/Au/Ir. 
   The piezoelectric capacitor  160  is composed of a first metal layer  161 , a piezoelectric layer  162 , and a second metal layer  163 . Preferably, the piezoelectric layer  162  is Pb(Zr, Ti) and a ratio between the Zr and Ti is 6:4. 
     FIG. 10  is a plane view showing another example of the bottom substrate of the RF MEMS switch according to the first embodiment of the present invention, and  FIG. 11  is a plane view showing still another example of the bottom substrate of the RF MEMS switch according to the first embodiment of the present invention. 
   As shown in  FIG. 10 , in a bottom substrate  150   a , four piezoelectric cantilevers  151   a  support a square contact electrode  170   a.    
   As shown in  FIG. 11 , in a bottom substrate  150   b , four piezoelectric cantilevers  151   b  support a square contact electrode  170   b.    
   As the contact electrodes  170   a  and  170   b  move in parallel with the bottom substrates  150   a  and  150   b , a contact area between the aforementioned signal transmission line (not shown) and each contact electrode  170   a  and  170   b  becomes wider and thereby an RF signal can be more efficiently controlled. 
   In the RF MEMS switch  100  according to the first embodiment of the present invention, when power is supplied to the RF MEMS switch through the second metal layer  163 , the suspension layer  151  that serves as a cantilever is deformed. At this time, the contact electrode  170  connects the first signal transmission line  121  and the second signal transmission line  122 , thereby turning on the RF MEMS switch. On the contrary, when a power supply to the RF MEMS switch  100  is cut off, the deformed suspension layer  151  is restored to the original state, thereby turning off the RF MEMS switch. 
     FIGS. 12A to 12K  are section views showing a fabrication process of an RF MEMS switch according to a first embodiment of the present invention. 
   A fabrication method of an RF MEMS switch according to a first embodiment of the present invention comprises: sequentially forming a first suspension layer of a silicon nitride and a second suspension layer of a silicon oxide on an upper surface of a bottom substrate, and forming a piezoelectric capacitor composed of a first electrode layer, a piezoelectric layer, and a second electrode at one side of an upper surface of the second suspension layer; forming a contact electrode at another side of the upper surface of the second suspension layer; etching the first suspension layer and the second suspension layer of the rest region except the piezoelectric capacitor and the contact electrode; etching a part of the bottom substrate to release a gap between the first suspension layer and the bottom substrate, thereby completing the bottom substrate; etching a center part of a lower surface of a cap substrate to form a groove; forming via holes at both sides of the cap substrate and filling a metal in the via hole, thereby forming a connection line; forming a signal transmission line and a ground line at an upper surface of the cap substrate; forming a connection pad at the lower surface of the cap substrate; electrically connecting the signal transmission line and the ground line to the connection pad by the connection line, thereby completing the cap substrate; and bonding the cap substrate to the upper surface of the bottom substrate with a certain gap by using a bump. 
   As shown in  FIG. 12A , a first suspension layer  152  of a silicon nitride and a second suspension layer  153  of a silicon oxide are sequentially formed on an upper surface of a bottom substrate  150 . Also, suspension layers  154  and  155  of a silicon nitride are formed on a lower surface of the bottom substrate  150 . Then, a piezoelectric capacitor  160  composed of a first electrode layer  161 , a piezoelectric layer  162 , and a second electrode layer  163  is formed at one side of an upper surface of the second suspension layer  153 . Preferably, the first suspension layer  152  and the second suspension layer  153  are deposited on the upper surface of the bottom substrate  150  by using a low pressure chemical vapor deposition method. 
   As shown in  FIG. 12B , a part of the piezoelectric capacitor  160  is patterned and then etched, thereby removing unnecessary parts thereof. 
   As shown in  FIG. 12C , a contact electrode  170  is formed at another side of the upper surface of the second suspension layer  153 . 
   As shown in  FIG. 12D , the first suspension layer  152  and the second suspension layer  153  of the rest region except the piezoelectric capacitor and the contact electrode are etched. 
   As shown in  FIG. 12E , a part  157  of the bottom substrate  150  is etched to release a gap between the first suspension layer  152  and the bottom substrate  150 , thereby completing the bottom substrate  150 . 
   In the step for completing the bottom substrate, a wet etching is preferably performed by using one of KOH, hydrofluoric nitric acetic (HNA), tetra methyl ammonium hydroxide (TMAH), ethylene diamine pyrocatechol (EDP), and NaOH. 
   As shown in  FIG. 12F , a center part of a lower surface of a mother substrate to be served as a cap substrate  110  is etched, thereby forming a groove  113 . 
   Preferably, the groove of the cap substrate is formed by using one of a plasma dry etching method, a sand blaster, or a laser cutting method. 
   As shown in  FIGS. 12G to 12I , a via hole  111  is formed at both sides of the cap substrate  110 , and a rear surface of the cap substrate  110  is polished until the via hole  111  is exposed by using a chemical mechanical polishing method. Then, metal such as gold, copper, or silver is filled in the via hole  111  by using an electro-analysis or a metal paste, thereby forming a connection line  112 . 
   As shown in  FIG. 12J , a signal transmission line  120 , a ground line  130  (referring to  FIG. 12K ), and a connection pad  140  are sequentially patterned to be deposited on the lower surface of the cap substrate  110 . 
   The signal transmission line  120  and the ground line  130  are electrically connected to the connection pad  140  by the connection line  112 , thereby completing the cap substrate  110 . 
   As shown in  FIG. 12K , the cap substrate  110  is bonded to the upper surface of the bottom substrate  150  by using a bump  180 , thereby completing the RF MEMS switch  100 . Herein, the gap between the contact electrode and the signal transmission line approximately corresponds to a sum between the height of the bump and the height of the etched part of the cap substrate. 
   Although not shown, it is preferable that the ground line of the piezoelectric capacitor and the ground line of the cap substrate are independently constructed in order to control the gap between the signal transmission line and the contact electrode. 
     FIGS. 13A to 13F  are section views showing a process for fabricating a cap substrate in the fabrication process of the MEMS switch according to the first embodiment of the present invention. 
   As shown in  FIG. 13A , both parts of a lower surface of a mother substrate to be served as a cap substrate  110 ′ are etched, thereby forming a groove  113 ′. 
   The groove  113 ′ of the cap substrate  110 ′ is formed by using one of a plasma dry etching method, a sand blaster, or a laser cutting method. 
   As shown in  FIGS. 13B to 13D , a via hole  111 ′ is formed at both sides of the cap substrate  110 ′, and metal is filled in the via hole  111 ′ thereby to form a connection line  112 ′. The lower surface of the cap substrate  110 ′ is polished by using a chemical mechanical polishing method, thereby forming the via hole  111 ′. 
   As shown in  FIGS. 13E to 13F , a signal transmission line  120 ′, a ground line  130 ′, and a connection pad  140 ′ are sequentially patterned to be deposited on a lower surface of the cap substrate  110 ′. Then, the signal transmission line  120 ′ and the ground line  130 ′ are electrically connected to the connection pad  140 ′ by the connection line  112 ′, thereby completing the cap substrate  110 ′. 
   In the above process, both sides of the lower surface of the cap substrate  110 ′ are etched to form the groove  113 ′ and thereby the center part of the cap substrate  110 ′ is downwardly protruded. According to this, when the RF MEMS switch is completed, the gap between the contact electrode and the signal transmission line is minimized. 
     FIG. 14  is a perspective view showing an RF MEMS switch according to a second embodiment of the present invention,  FIG. 15  is a longitudinal section view showing the RF MEMS switch according to the second embodiment of the present invention,  FIG. 16  is a longitudinal section view taken along line II-II of  FIG. 15 , and  FIG. 17  is a plane view showing a bottom substrate in the RF MEMS switch according to the second embodiment of the present invention. 
   As shown, an RF MEMS switch  200  according to the second embodiment of the present invention comprises: a cap substrate  210  having via holes  211  at both sides thereof, the via hole  211  provided with a connection line  212 ; a signal transmission line  220  formed at a lower surface of the cap substrate  210 ; a ground line  230  formed at both sides of the signal transmission line  220 ; a connection pad  240  corresponding to the signal transmission line  220  and formed at an upper surface of the cap substrate  210  so as to be electrically connected to the signal transmission line  220  and the ground line  230  by the connection line  212 ; a bottom substrate  250  having a certain gap from the cap substrate  210 ; a piezoelectric capacitor  260  formed at one side of a suspension layer  251  formed at an upper surface of the bottom substrate  250  with a part of a first metal layer  261  being exposed, and actuated with a piezoelectric characteristic when an external power is applied thereto; and a capacitor contact electrode  270  formed at another side of the suspension layer  251  and connected to the exposed first metal layer  261 , for selectively turning on/off an RF signal of the signal transmission line  220  by being moved up and down by a displacement of the piezoelectric capacitor  260 . 
   The piezoelectric capacitor  260  is formed as a first metal layer  261 , a dielectric layer  262 , and a second metal layer  263  are sequentially deposited. A part of the first metal layer  261  is exposed to outside. 
   The capacitor contact electrode  270  preferably has a deposited structure composed of a first metal layer  271 , a dielectric layer  272 , and a second metal layer  273 . 
   The capacitor contact electrode  270  has a basic structure of Ti/Au or Cr/Au for a low resistance and an excellent bonding with the suspension layer. In order to prevent a contact part between the contact electrode and the signal transmission line from being adhered to each other during an operation or in order to prevent a surface of the contact electrode from being damaged, Mo, W, and Ir for a high mechanical intensity and melting point can be thinly deposited. 
   That is, the capacitor contact electrode  270  preferably has one structure of Ti/Au/Mo, Cr/Au/Mo, Ti/Au/W, Cr/Au/W, Ti/Au/Ir, and Cr/Au/Ir. 
   The suspension layer  251  can be formed as a second suspension layer  253  of a silicon oxide is deposited on a first suspension layer  252  of a silicon nitride, as a second suspension layer  253  of a silicon oxide is deposited on a first suspension layer  252  of a silicon oxide, or as a second suspension layer  253  of a silicon nitride is deposited on a first suspension layer  252  of a silicon nitride. 
   Referring to  FIG. 17 , the second metal layer  272  of the capacitor contact electrode  270  is connected to the piezoelectric capacitor  260  by the first metal layer  261 , thereby turning off the RF MEMS switch by flowing an RF signal to the ground line  230  when the capacitor contact electrode  270  is connected to the signal transmission line  220 . 
     FIG. 18  is a plane view showing another example of the bottom substrate in the RF MEMS switch according to the second embodiment of the present invention, and  FIG. 19  is a plane view showing still another example of the bottom substrate in the RF MEMS switch according to the second embodiment of the present invention. 
   As shown in  FIG. 18 , a second metal layer (not shown) of a square capacitor contact electrode  270   a  is connected to a piezoelectric capacitor  260   a  by a first metal layer  261   a.    
   As shown in  FIG. 19 , a second metal layer (not shown) of a capacitor contact electrode  270   b  having a diamond shape is connected to a piezoelectric capacitor  260   b  by a first metal layer  261   b.    
   In the RF MEMS switch  200  according to the second embodiment of the present invention, when power is supplied to the RF MEMS switch through the second metal layer  263 , the suspension layer  251  that serves as a cantilever is deformed. At this time, the capacitor contact electrode  270  serves as a variable capacitor thereby to ground an RF signal flowing to the signal transmission line  220 , thereby turning off the RF MEMS switch. On the contrary, when a power supply to the RF MEMS switch is cut off, the deformed suspension layer  251  is restored to the original state and thereby the RF MEMS switch is turned on. 
     FIGS. 20A to 20L  are section views showing a fabrication process of the RF MEMS switch according to the second embodiment of the present invention. 
   According to a second embodiment of the present invention, there is also provided a fabrication method of an RF MEMS switch comprising: sequentially forming a first suspension layer of a silicon nitride and a second suspension layer of a silicon oxide on an upper surface of a bottom substrate, and forming a piezoelectric capacitor composed of a first electrode layer, a piezoelectric layer, and a second electrode at one side of an upper surface of the second suspension layer; patterning the second electrode layer and the piezoelectric layer and then etching so that a part of the first electrode layer can be exposed; forming a capacitor contact electrode at another side of the upper surface of the second suspension layer so that the exposed part of the first electrode layer can be covered; etching the first suspension layer and the second suspension layer of the rest region except the piezoelectric capacitor and the contact electrode; etching a part of the bottom substrate to release a gap between the first suspension layer and the bottom substrate, thereby completing the bottom substrate; etching a center part of a lower surface of a cap substrate to form a groove; forming via holes at both sides of the cap substrate and filling a metal in the via hole, thereby forming a connection line; forming a signal transmission line and a ground line at an upper surface of the cap substrate; forming a connection pad at the lower surface of the cap substrate; electrically connecting the signal transmission line and the ground line to the connection pad by the connection line, thereby completing the cap substrate; and bonding the cap substrate to the upper surface of the bottom substrate with a certain gap by using a bump. 
   Hereinafter, a fabrication method of an RF MEMS switch according to a second embodiment of the present invention will be explained with reference to  FIGS. 20A to 20L . 
   As shown in  FIG. 20A , a first suspension layer  252  of a silicon nitride and a second suspension layer  253  of a silicon oxide are sequentially formed on an upper surface of a bottom substrate  250 . Then, a piezoelectric capacitor  260  composed of a first electrode layer  261 , a piezoelectric layer  262 , and a second electrode layer  263  is formed at one side of an upper surface of the second suspension layer  253 . 
   As shown in  FIGS. 20B and 20C , the piezoelectric layer  262  and the second electrode layer  263  are patterned and then etched so that a part of the first electrode layer  261  can be exposed. 
   As shown in  FIG. 20D , a capacitor contact electrode  270  is formed at another side of the upper surface of the second suspension layer  253  so that the exposed part of the first electrode layer  261  can be covered. 
   As shown in  FIG. 20E , the first suspension layer  252  and the second suspension layer  253  of the rest region except the piezoelectric capacitor  260  and the capacitor contact electrode  270  are etched. 
   As shown in  FIG. 20F , a part  257  of the bottom substrate  250  is etched to release a gap between the first suspension layer  252  and the bottom substrate  250 , thereby completing the bottom substrate  250 . 
   As shown in  FIG. 20G , a center part of a lower surface of a mother substrate to be served as a cap substrate  210  is etched, thereby forming a groove  213 . 
   Preferably, the groove of the cap substrate is formed by using one of a plasma dry etching method, a sand blaster, or a laser cutting method. 
   As shown in  FIGS. 20H to 20J , a via hole  211  is formed at both sides of the cap substrate  210 , and then a metal is filled in the via hole  211 , thereby forming a connection line  212 . The via hole  211  is formed by polishing the lower surface of the cap substrate  210  with a chemical mechanical polishing method. 
   As shown in  FIG. 20K , a signal transmission line  220 , a ground line  230  (referring to  FIG. 20L ), and a connection pad  240  are formed on the lower surface of the cap substrate  210 . 
   The signal transmission line  220  and the ground line  230  are electrically connected to the connection pad  240  by the connection line  212 , thereby completing the cap substrate  210 . 
   As shown in  FIG. 20L , the cap substrate  210  is bonded to the upper surface of the bottom substrate  250  by using a bump  280 , thereby completing the RF MEMS switch  200 . 
   In the step for completing the bottom substrate  250  by etching the part  257  of the bottom substrate  250  and thereby releasing the gap between the first suspension layer  252  and the bottom substrate  250 , a wet etching is preferably performed by using one of KOH, HNA, TMAH, EDP, NaOH, and XeF 2  gas phase. 
   Preferably, in the step for forming the groove of the cap substrate, one of a plasma dry etching method, a sand blaster, or a laser cutting method is preferably used. 
   In the step for forming via holes at both sides of the cap substrate and then filling a metal in the via hole thereby to form a connection line, the lower surface of the cap substrate is polished by using a chemical mechanical polishing method. 
   As aforementioned, the RF MEMS switch according to the present invention can be actuated with a low voltage and a low consumption power by using a piezoelectric capacitor, thereby having a high reliability and a high yield. 
   As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to be embraced by the appended claims.