Abstract:
A MEMS switch includes a lower substrate having a signal line on an upper surface of the lower substrate; an upper substrate, having a cavity therein, disposed apart from the upper surface of the lower substrate by a distance, and having a membrane layer on a lower surface of the upper substrate; a bimetal layer formed in the cavity of the upper substrate on the membrane layer; a heating layer formed on a lower surface of the membrane layer; and a contact member formed on a lower surface of the heating layer. The contact member can come into contact with or separate from the signal line. A method for manufacturing the MEMS switch includes preparing the upper and lower substrates and combining them so that a surface having the signal line faces a surface having the contact member and the upper and lower substrates are disposed apart by a distance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority from Korean Patent Application No. 10-2005-0064798 filed on Jul. 18, 2005, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Apparatuses and methods consistent with the present invention relate to a Micro-Electro-Mechanical Systems (MEMS) switch and a method for manufacturing the same. 
   2. Description of the Related Art 
   Electronic systems for use in a high frequency bandwidth are getting slimmer, smaller, lighter, and better in performance. Ultra-small microswitches using a new technology such as micromachining are being developed to substitute for semiconductor devices such as Field Effect Transistors (FET) and pin diodes, which have been used for controlling such electronic systems. 
   Among radio frequency (RF) devices using MEMS technologies, most devices manufactured are switches. RF switches are frequently applied in signal transmission circuits and impedance matching circuits for use in wireless terminals and systems using micro- or millimeter-wavelength bandwidth. 
   In conventional MEMS switches, electrification is caused when a DC voltage is applied to a fixed switch and thus a movable electrode is attracted to a substrate due to an electrostatic attraction. As the movable electrode is attracted to the substrate, a contact member provided on the movable electrode comes into contact with a signal line provided on the substrate. The switch operates so that the switch is turned on and off as the contact member comes into contact with and is separated from the signal line in response to the voltage application. 
   However, MEMS switches performing their switching operations by electrostatic attraction have disadvantages as discussed below. 
   First, such conventional MEMS switches operate at a high driving voltage. 
   Second, in manufacturing the MEMS switches on a wafer, structures constituting the MEMS switches are not the same over the entire area of the wafer, that is, the uniformity of the structures manufactured in the wafer is not good. 
   Third, since the manufacturing method of the MEMS switches includes lots of process steps, the MEMS switches are manufactured in low yield. 
   Here, “uniformity” means that distances between fixed electrodes and movable electrodes in lots of cells are constant all over the wafer. 
   Fourth, since a contact force of the contact member to the signal line is not stable, an insertion loss also increases as the number of switching operations increases. 
   SUMMARY OF THE INVENTION 
   An exemplary embodiment of the present invention provides a MEMS switch driven at a low voltage, having a stable contact force, and being capable of manufacture in a high yield, and a method for manufacturing the MEMS switch where the method is capable of enhancing a production yield by including a smaller number of process steps than conventional methods. 
   According to one exemplary embodiment of the present invention, there is provided an MEMS switch including a lower substrate having a signal line on an upper surface thereof; an upper substrate, having a cavity therein, being disposed apart from the upper surface of the lower substrate by a distance and having a membrane layer on a lower surface thereof; a bimetal layer formed in the cavity on the membrane layer; a heating layer formed on a lower surface of the membrane layer; and a contact member formed on a lower surface of the heating layer and coming into contact with or separating from a signal line. 
   The MEMS switch further includes a sealing layer disposed between the upper and lower substrates for maintaining the distance between the upper and lower substrates and for sealing an inner space between the upper and lower substrates. 
   The MEMS switch may further include a cover disposed over the upper substrate for covering the cavity. 
   The membrane layer may be made, for example, of an oxide material and the heating layer may be made, for example, of a polysilicon material. 
   The heating layer may have an electrical resistance heating body and the electrical resistance heating body may have, for example, a helical shape. 
   The electrical resistance heating body may further have a power supply unit for supplying a voltage. The power supply unit may include an upper voltage application pad connected to the resistance heating body, a lower voltage application pad formed on the upper surface of the lower substrate and connected to the upper voltage application pad, a voltage connection part buried in the lower substrate through a hole and connected to the lower voltage application pad, and an external voltage application pad formed on a lower surface of the lower substrate and connected to an external voltage application pad connected to the voltage connection part. 
   The MEMS switch may further include a signal line connection unit on the lower substrate for connecting the signal line to an external circuit. The signal line connection unit may include a signal line connection part buried in the lower substrate through a hole and connected to the signal line, and a signal line pad formed on the lower surface of the lower substrate and connected to the signal line connection part. 
   The upper and lower substrates may be made, for example, of a silicon material and the cover may be made, for example, of a glass material. The upper substrate and the cover may be joined, for example, by an anodic bonding method. 
   The signal line, contact member, and sealing layer may be made, for example, of a bondable conductive material and the conductive material may be one of Au, AuSn, and PbSn. 
   According to another embodiment of the present invention, there is provided a method for manufacturing an MEMS switch, including preparing a lower substrate by depositing a conductive layer and forming a signal line on a substrate by patterning the conductive layer; preparing an upper substrate by depositing a membrane layer on a lower surface of an upper substrate; depositing a heating layer on a lower surface of the membrane layer; forming a cavity by selectively etching the upper substrate; forming a bimetal on the membrane layer in the cavity; depositing a conductive layer on a lower surface of the heating layer and patterning the conductive layer to form a contact member; and combining the upper substrate and the lower substrate such that a surface having the signal line of the lower substrate faces a surface having the contact member of the upper substrate and the upper and the lower substrates are disposed apart by a distance. 
   The method further includes patterning the heating layer in a helical shape after the patterning the contact member. 
   A lower sealing layer for sealing the upper and lower substrates may be patterned while patterning the signal line, and an upper sealing layer for sealing the upper and lower substrates may be patterned while patterning the conductive layer to form a contact member. 
   The method further includes forming a signal line connection unit for connecting the signal line and the heating layer to an external circuit. 
   Forming the signal line connection unit may include: forming a plurality of holes to be extended to the signal line and the heating layer in the lower substrate before the forming the signal line; polishing the lower substrate after the upper and lower substrates are bonded to expose a surface of a conductive layer buried in the hole, where the conductive layer is formed for the signal line; and patterning an external voltage application pad and a signal line pad after depositing a conductive layer on the lower surface of the lower substrate. 
   The membrane layer may be made, for example, of an oxide material and the heating layer may be made, for example, of a polysilicon material. 
   The method further includes bonding a cover for covering the cavity to the upper surface of the upper substrate after the forming the bimetal layer. 
   The upper and lower substrates may be made, for example, of a silicon material and the cover may be made; for example, of a glass material. 
   The signal line, contact member, and sealing layer may be made, for example, of a bondable conductive material and the conductive material may be one of Au, AuSn, and PbSn. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features of the present invention will be described in reference to certain exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a layout view illustrating an MEMS switch according to an embodiment of the present invention; 
       FIG. 2  is a sectional view taken along line II-II′ of the MEMS switch shown in  FIG. 1 ; 
       FIG. 3  is a sectional view taken along line III-III′ of the MEMS switch shown in  FIG. 1 ; 
       FIG. 4  is a top plan view illustrating a lower substrate of the MEMS switch shown in  FIG. 1 ; 
       FIG. 5  is a bottom plan view illustrating an upper substrate of the MEMS switch shown in  FIG. 1 ; 
       FIGS. 6A and 6B  are sectional views illustrating process steps of forming the lower substrate shown in  FIG. 2 , where the views are taken along the line II-II′ shown in  FIG. 1 ; 
       FIGS. 7A and 7B  are sectional views illustrating process steps of forming the lower substrate shown in  FIG. 2 , where the views are taken along the line III-III′ shown in  FIG. 1 ; 
       FIGS. 8A to 8E  are sectional views illustrating process steps of forming the upper substrate shown in  FIG. 2 , where the views are taken along the line II-II′ shown in  FIG. 1 ; 
       FIGS. 9A to 9C  are sectional views illustrating the process steps of completing the MEMS switch by combining the upper substrate and the lower substrate, where the views are taken along the line II-II′ shown in  FIG. 1 ; and 
       FIGS. 10A to 10C  are sectional views illustrating the process steps of completing the MEMS switch by combining the upper substrate and the lower substrate, where the views are taken along the line III-III′ shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. 
     FIG. 1  illustrates a layout view of a MEMS switch according to one exemplary embodiment of the present invention,  FIG. 2  illustrates a sectional view of the MEMS switch, where the view is taken along a line II-II′ shown in  FIG. 1 , and  FIG. 3  illustrates a sectional view of the MEMS switch where the view is taken along a line III-III′ shown in  FIG. 1 . 
   Referring to  FIGS. 1 to 3 , the MEMS switch  100  includes a signal part  110  and a driving part  150 . 
   The signal part  110  includes a lower substrate  111 , a signal line  113  formed on an upper surface of the lower substrate  111 , a signal line connection unit  130  for connecting external circuits, and a power supply unit  120  for supplying a voltage to a heating layer  155  in the driving part  150  to be described later. The lower substrate  111  may be made, for example, of a silicon material. 
   The driving part  150  includes an upper substrate  151  having a cavity  151   a  therein, a membrane layer  153  formed on a lower surface of the upper substrate  151 , the heating layer  155  formed on a lower surface of the membrane layer  153 , a bimetal layer  157  formed on an upper surface of the membrane layer  153 , and a contact member  159  formed on a lower surface of the heating layer  155 . 
   The upper substrate  151  may be made, for example, of a silicon material and the membrane layer  153  may be formed, for example, of an oxide material. 
   The heating layer  155  is an electrical resistance heating body  155   a  and may be formed, for example, of a polysilicon material. The heating layer  155  may be formed to have a coil shape and is movable by expansibility of the bimetal layer  157 . 
   The contact member  159  is disposed on the lower surface of the heating layer  155 , which is movable due to the expansibility of the bimetal layer  157  and serves to transfer RF signals when in contact with a signal line  113 . The contact member  159  is made of a conductive material such as, for example, Au, AuSn, or PbSn. 
   The bimetal layer  157  is a switch formed of two different metal layers  157   a  and  157   b  joined together to form one unit having a differential expansion rating. The bimetal layer  157  will bend if there is a temperature change, that is, the metal layer  157   a  having a relatively high expansion rate bends toward the metal layer  157   b  having a relatively low expansion rate. The contact member  159  comes into contact with the signal line  113  due to this characteristic of the bimetal layer  157 . 
     FIG. 4  illustrates a top plan view of the lower substrate of the MEMS switch shown in  FIG. 1 , and  FIG. 5  illustrates a bottom plan view of the upper substrate of the MEMS switch shown in  FIG. 1 . 
   Referring to  FIG. 3  and  FIG. 5 , there is provided the power supply unit  120  for supplying a voltage to the heating layer  155 . The power supply unit  120  can include upper voltage application pads  121   a  and  121   b  connected to the electrical resistance heating body  155   a , lower voltage application pads  127   a  and  127   b  formed on the upper surface of the lower substrate  111  and connected to the upper voltage application pads  121   a  and  121   b , voltage connection parts  123   a  and  123   b  buried in the lower substrate  111 , passing through holes  111   a  formed in the lower substrate  111  and connected to the lower voltage application pads  127   a  and  127   b  via the holes  111   a , and external voltage application pads  125   a  and  125   b  formed on the lower surface of the lower substrate  111  and connected to the voltage connection parts  123   a  and  123   b.    
   Referring to  FIG. 2  and  FIG. 4 , there is provided the signal line connection unit  130  for connecting the MEMS switch to an external circuit. The signal line connection unit  130  is buried in the lower substrate  111  through the holes  111   a  and can include signal line connection parts  131   a  and  131   b  connected to the signal line  113 , and signal line pads  133   a  and  133   b  formed on the lower surface of the lower substrate  111  and connected to the signal line connection parts  131   a  and  131   b.    
   Referring to  FIG. 2 , a sealing layer  141  is provided between the upper substrate  151  and the lower substrate  111  to keep a distance between the upper substrate  151  and the lower substrate  111  and seal the inside space between the substrates  151  and  111 . 
   The sealing layer  141  can be simultaneously patterned with the contact member  159  and the signal line  113 . In this instance, the contact member  159  and the signal line  113  are made of the same material. Further, an upper sealing layer  141   a  formed on the upper substrate  151  and a lower sealing layer  141   b  formed on the lower substrate  111  are joined by a bonding method. Bondable conductive materials include, for example, Au, AuSn, and PbSn. 
   On the other hand, a cover  161  is provided on the upper surface of the upper substrate  151  to cover the cavity  151   a . The cover  161  is formed of, for example, a glass material, and the upper substrate  151  and the cover  161  can be joined by an anodic bonding method. 
   In the MEMS switch having the structure described above, when a certain voltage is supplied to the MEMS switch through the external voltage application pads  125   a  and  125   b , the voltage is supplied to the electrical resistance heating body  155   a  of the heating layer  155  through the voltage connection parts  123   a  and  123   b  and the upper and lower voltage application pads  121   a ,  121   b ,  127   a , and  127   b . Accordingly, the electrical resistance heating body  155   a  generates heat which is transferred to the bimetal layer  157 . At this time, the bimetal layer  157  bends down due to the differential expansion rating of the metal layers  157   a  and  157   b . In association with the bending of the bimetal layer  157 , the membrane layer  153  and the heating layer  155  also bend down together so that the contact member  159  comes into contact with the signal line  113 . 
   Hereinafter, a method for manufacturing an MEMS switch will be described. 
     FIGS. 6A and 6B  and  FIGS. 7A  and  FIG. 7B  illustrate the process steps of forming the structure of the lower substrate, and  FIGS. 6A and 6B  are views taken along the line II-II′ and  FIGS. 7A to 7B  are views taken along the line III-III′. 
   Referring to  FIG. 4 ,  FIG. 6A , and  FIG. 7A , a plurality of holes  111   a  is formed on the upper surface of the lower substrate  111 . 
   Referring to  FIG. 4 ,  FIG. 6B , and  FIG. 7B , for example, a conductive layer is formed on the upper surface of the lower substrate  111  and is made of An, AuSn, or PbSn. In this instance, the conductive layer is buried in the lower substrate  111  through the holes  111   a , so that the voltage connection parts  123   a  and  123   b  and the signal line connection parts  131   a  and  131   b  are formed. Further, the conductive layer deposited is patterned by an etching process to form the signal line  113  and the lower voltage application pads  127   a  and  127   b . Here, the lower sealing layer  141   b  can be formed on the edges of the lower substrate  111 . 
   As such, after finishing processing of the lower substrate  111 , the upper substrate  151  providing the switch driving part  150  is processed. The method for processing the upper substrate  151  will be described below. 
     FIGS. 8A to 8E  are views illustrating sequential process steps of manufacturing the upper substrate shown in  FIG. 2  and the views are taken along the line II-II′ shown in  FIG. 1 . 
   Referring to  FIG. 8A , for example, the membrane layer  153  and the heating layer  155  are sequentially deposited on a lower surface of the upper substrate  151 , which may be, for example, a silicon substrate. Here, the membrane layer  153  may be formed, for example, of an oxide layer and the heating layer  155  may be formed, for example, of a polysilicon layer. 
   Referring to  FIG. 8B , the cavity  151   a  is formed in the upper substrate  151 . 
   Referring to  FIG. 8C , the bimetal layer  157  is formed in the cavity  151   a  on the membrane layer  153 . The bimetal layer  157  is formed by sequentially depositing two different metal layers  157   a  and  157   b  having a different expansion rate, where the metal layer  157   a  preferably has a higher expandability than that of the metal layer  157   b.    
   Referring to  FIG. 8D , the cover  161 , that may be made, for example, of a glass material, is bonded on the upper surface of the upper substrate  151 . In this instance, the upper substrate  151  and the cover  161  can be joined by an anodic bonding method. 
   Referring to  FIG. 8E , a conductive layer is deposited on the lower surface of the heating layer  155  and patterned to form the contact member  159 . Further, the heating layer  155  is patterned in a helical shape to complete the electrical resistance heating body  155   a . In this instance, the upper voltage application pads  121   a  and  121   b  for supplying a voltage to the electrical resistance heating body  155   a  are formed and the upper sealing layer  141   a  can be patterned along edges of the upper substrate  151 . 
   Referring to  FIGS. 9A to 9C , the upper substrate and the lower substrate are combined together to complete the MEMS switch.  FIGS. 9A to 9C  are sectional views taken along the line II-II′ shown in  FIG. 1  and  FIGS. 10A to 10C  are sectional views taken along the line III-III′ shown in  FIG. 1 . 
   Referring to  FIG. 9A  and  FIG. 10A , the upper substrate  151  and the lower substrate  111  are bonded using the upper and lower sealing layers  141   a  and  141   b . Here, the bondable conductive material may include, for example, Au, AuSn, or PbSn. 
   Referring to  FIG. 9B  and  FIG. 10B , the lower surface of the lower substrate  111  is subject to a polishing process to expose the voltage connection parts  123   a  and  123   b  and the signal line connection parts  131   a  and  131   b  buried in the holes  111   a . 
   Referring to  FIG. 9C  and  FIG. 10C , a conductive layer is deposited on the lower surface of the lower substrate  111  and patterned to form the external voltage application pads  125   a  and  125   b  and the signal line pads  133   a  and  133   b  to be connected to the voltage connection parts  123   a  and  123   b  and the signal line connection parts  131   a  and  131   b . 
   As described above, the MEMS switch according to the present invention has at least the following advantages. 
   First, the MEMS switch according to the present invention operates at a lower driving voltage compared to conventional MEMS switches. 
   Second, since an additional packaging process is not needed, a yield of producing the MEMS switches is enhanced. 
   Third, since the contact member comes into contact with the signal line by the bimetal switching operation, a contact force is enhanced compared to the conventional switches. 
   While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.