Abstract:
A micro-electromechanical (MEMS) actuator and relay are implemented using a copper coil and a magnetic core. The magnetic core includes a base section that lies within the copper coil, and a cantilever section that lies outside of the copper coil. The presence of a magnetic field in the coil causes the cantilever section to move horizontally away from a rest position, while the absence of the magnetic field allows the cantilever section to return to the rest position.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to actuators and relays and, more particularly, to a MEMS actuator and relay with horizontal actuation. 
   2. Description of the Related Art 
   A switch is a well-known device that connects, disconnects, or changes connections between devices. An electrical switch is a switch that provides a low-impedance electrical pathway when the switch is “closed,” and a high-impedance electrical pathway when the switch is “opened.” A mechanical-electrical switch is a type of switch where the low-impedance electrical pathway is formed by physically bringing two electrical contacts together, and the high-impedance electrical pathway is formed by physically separating the two electrical contacts from each other. 
   An actuator is a well-known mechanical device that moves or controls a mechanical member to move or control another device. Actuators are commonly used with mechanical-electrical switches to move or control a mechanical member that closes and opens the switch, thereby providing the low-impedance and high-impedance electrical pathways, respectively, in response to the actuator. 
   A relay is a combination of a switch and an actuator where the mechanical member in the actuator moves in response to electromagnetic changes in the conditions of an electrical circuit. For example, electromagnetic changes due to the presence or absence of a current in a coil can cause the mechanical member in the actuator to close and open the switch. 
   One approach to implementing actuators and relays is to use micro-electromechanical (MEMS) technology. MEMS devices are formed using the same fabrication processes that are used to form conventional semiconductor devices, such as bipolar and CMOS transistors. Although a number of approaches exist for forming MEMS actuators and relays, there is a need for an additional approach to forming MEMS actuators and relays. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-11A  are plan views illustrating a method of forming a MEMS-based actuator  100  in accordance with the present invention. 
       FIGS. 1B-11B  are cross-sectional views taken along lines  1 B- 1 B of FIG.  1 A through  11 B- 11 B of  FIG. 11A , respectively. 
       FIGS. 1C-11C  are cross-sectional views taken along lines  1 C- 1 C of FIGS.  1 A through  11 C- 11 C of  FIG. 11A , respectively. 
       FIGS. 1D-11D  are cross-sectional views taken along lines  1 D- 1 D of FIGS.  1 A through  11 D- 11 D of  FIG. 11A , respectively. 
       FIGS. 1E-11E  are cross-sectional views taken along lines  1 E- 1 E of FIGS.  1 A through  11 E- 11 E of  FIG. 11A , respectively. 
       FIGS. 12A-20A  are plan views illustrating a method of forming a MEMS-based relay  1200  in accordance with the present invention. 
       FIGS. 12B-20B  are cross-sectional views taken along lines  12 B- 12 B of FIGS.  12 A through  20 B- 20 B of  FIG. 20A , respectively. 
       FIGS. 12C-20C  are cross-sectional views taken along lines  12 C- 12 C of FIGS.  12 A through  20 C- 20 C of  FIG. 20A , respectively. 
       FIGS. 12D-20D  are cross-sectional views taken along lines  12 D- 12 D of FIGS.  12 A through  20 D- 20 D of  FIG. 20A , respectively. 
       FIGS. 12E-20E  are cross-sectional views taken along lines  12 E- 12 E of FIGS.  12 A through  20 E- 20 E of  FIG. 20A , respectively. 
       FIGS. 12F-20F  are cross-sectional views taken along lines  12 F- 12 F of FIGS.  12 A through  20 F- 20 F of  FIG. 20A , respectively. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A-11A ,  1 B- 11 B,  1 C- 11 C,  1 D- 11 D, and  1 E- 11 E show a series of views that illustrate a method of forming a MEMS actuator  100  in accordance with the present invention. As shown in  FIGS. 1A-1E , the method utilizes a conventionally formed single-crystal silicon semiconductor wafer  110  that has an overlying dielectric layer  112 . 
   Dielectric layer  112  can represent a dielectric layer that includes no metal structures, or a dielectric layer that includes metal structures, such as the dielectric layer of a metal interconnect structure. When formed as the dielectric layer of a metal interconnect structure, dielectric layer  112  includes levels of metal traces, which are typically aluminum, a large number of contacts that connect the bottom metal trace to electrically conductive regions on wafer  110 , and a large number of inter-metal vias that connect the metal traces in adjacent layers together. Further, selected regions on the top surfaces of the metal traces in the top metal layer function as pads which provide external connection points. 
   In the present example, dielectric layer  112  represents the dielectric layer of a metal interconnect structure that also includes pads P 1  and P 2 . Pads P 1  and P 2  are selected regions on the top surfaces of two of the metal traces in the top layer of metal traces that provide electrical connections for a to-be-formed square coil. (Only pad P 2 , and not the entire metal interconnect structure, is shown in cross-section in  FIGS. 1C-11C  for clarity.) 
   Referring again to  FIGS. 1A-1E , the method begins by forming a seed layer  114  on the top surface of dielectric layer  112 . In the present example, since dielectric layer  112  represents the dielectric layer of a metal interconnect structure, seed layer  114  is also formed on the pads P 1  and P 2 . 
   Seed layer  114  typically includes a layer of titanium (e.g., 300 Å thick) and an overlying layer of copper (e.g., 3000 Å thick). The titanium layer enhances the adhesion between the aluminum in the underlying metal traces and the overlying layer of copper. Once seed layer  114  has been formed, a mask  116 , such as a layer of photoresist, is formed and patterned on the top surface of seed layer  114 . 
   As shown in  FIGS. 2A-2E , following the formation and patterning of mask  116 , copper is deposited by electroplating to form a number of spaced-apart copper lower sections  120 . The copper lower sections  120  form the lower sides of the to-be-formed square coil. Since dielectric layer  112  represents the dielectric layer of a metal interconnect structure in the present example, the ends of the copper lower sections  120  that correspond with the opposite ends of the square coil are electrically connected to pads P 1  and P 2 . After the copper lower sections  120  have been formed, mask  116  is removed, followed by the removal of the underlying regions of seed layer  114 . 
   Next, as shown in  FIGS. 3A-3E , a dielectric layer  122 , such as an oxide layer, is conformally deposited on dielectric layer  112  and the copper lower sections  120 . Once dielectric layer  122  has been formed, a mask  126 , such as a layer of photoresist, is then formed and patterned on the top surface of dielectric layer  122 . 
   As shown in  FIGS. 4A-4E , after the formation and patterning of mask  126 , a seed layer  130  is formed on the top surface of dielectric layer  122  and mask  126 . After seed layer  130  has been formed, a mask  132 , such as a layer of photoresist, is formed and patterned on the top surface of seed layer  130 . 
   Following the formation and patterning of mask  132 , as shown in  FIGS. 5A-5E , a magnetic material, such as an alloy of nickel and iron like permalloy, is deposited by electroplating to form an actuation member  134 . Once actuation member  134  has been formed, as shown in  FIGS. 6A-6E , mask  132 , the underlying regions of seed layer  130 , and mask  126  are removed. 
   The removal of these materials leaves actuation member  134  with a core section  136  and a floating cantilever section  138 . Core section  136 , which is defined by the opening in mask  126  and the overlying portion of mask  132 , touches dielectric layer  122 . Further, core section  136  has a first end  136 -E 1  and a spaced apart second end  136 -E 2 . 
   Floating cantilever section  138 , in turn, is defined by the opening in mask  132  that lies over mask  126 . Thus, floating cantilever section  138  is vertically spaced apart from dielectric layer  122  by underlying mask  126 , and thereby floats after underlying mask  126  has been removed. As a result, the thickness of mask  126  determines an offset gap  128 , which is the vertical spacing that lies between dielectric layer  122  and floating cantilever section  138 . Further, floating cantilever section  138  has a first end  138 -E 1  and a spaced apart second end  138 -E 2 . 
   In addition, as further shown in  FIGS. 6A-6E , the second end  136 -E 2  of core section  136  and the second end  138 -E 2  of floating cantilever section  138  are horizontally spaced apart by an actuation gap  139 . The size of actuation gap  139  is defined by the patterns in masks  126  and  132 . Thus, as a result of offset gap  128  and the actuation gap  139 , floating cantilever section  138  is horizontally movable so that the second end  138 -E 2  can move towards the second end  136 -E 2  of core section  136  to touch the second end  136 -E 2  of core section  136 . 
   Next, as shown in  FIGS. 7A-7E , a dielectric layer  140 , such as an oxide layer, is conformally deposited on dielectric layer  122  and actuation member  134 . After dielectric layer  140  has been formed, a mask  142 , such as a layer of photoresist, is then formed and patterned on the top surface of dielectric layer  140 . 
   Following the formation and patterning of mask  142 , as shown in  FIGS. 8A-8E , the exposed regions of the dielectric layer  140  and underlying dielectric layer  122  are etched to form vertical openings  144  that expose the top surfaces of the ends of the copper lower sections  120  that form the lower sides of the to-be-formed square coil. Mask  142  is then removed. 
   Once mask  142  has been removed, as shown in  FIGS. 9A-9E , a seed layer  146  is formed on the exposed ends of the copper lower sections  120  and the top surface of dielectric layer  140 . After seed layer  146  has been formed, a mask  150 , such as a layer of photoresist, is formed and patterned on the top surface of seed layer  146 . The pattern in mask  150  is shown hatched in  FIG. 9A . 
   Next, as shown in  FIGS. 10A-10E , following the formation and patterning of mask  150 , copper is deposited by electroplating to form a number of copper side sections  156  of the square coil, and a number of copper upper sections  158  of the square coil. The copper upper sections  158  of the square coil are shown hatched in  FIG. 10A . Following this, as shown in  FIGS. 11A-11E , mask  150  and the underlying regions of seed layer  146  are removed to complete the process. 
   Thus, a method of forming actuator  100  has been described. As shown in  FIGS. 11A-11E , actuator  100  has a square coil  160  that lies on dielectric layer  112 . In the present example, coil  160  is formed by connecting together the copper lower sections  120 , the copper side sections  156 , and the copper upper sections  158 . 
   Actuator  100  also has actuation member  134  which, in turn, has core section  136  and floating cantilever section  138 . Core section  136  lies within and is isolated from coil  160  by dielectric layer  122  and dielectric layer  140 . In addition, core section  136  has first and second ends  136 -E 1  and  136 -E 2  that lie outside of the outer lower sections  120  of coil  160 . 
   Floating cantilever section  138 , which has first end  138 -E 1  and second end  138 -E 2 , floats vertically above dielectric layer  122  by offset gap  128 , while the second end  138 -E 2  of floating cantilever section  138  is horizontally spaced apart from the second end  136 -E 2  of core section  136  by actuation gap  139 . 
   As a result, the second end  138 -E 2  of floating cantilever section  138  is horizontally movable towards the second end  136 -E 2  of core section  136 . In addition, the first end  138 -E 1  of floating cantilever section  138  touches the first end  136 -E 1  of core section  136 . Further, actuation member  134  is implemented with a magnetic material, such as an alloy of nickel and iron like permalloy. 
   In operation, when no current is present in coil  160 , floating cantilever section  138  has the shape shown in  FIG. 11A . As shown, the second end  136 -E 2  of core section  136  and the second end  138 -E 2  of floating cantilever section  138  are spaced apart by actuation gap  139 , thereby providing a first actuation position. 
   On the other hand, when a current flows through coil  160  and generates an electromagnetic field that is stronger than the spring force of floating cantilever section  138 , the electromagnetic field causes the second end  138 -E 2  of floating cantilever section  138  to move towards the second end  136 -E 2  of core section  136 , thereby providing a second actuation position. 
   The force required to achieve good movement is in the range of 100 μN. Modeling of actuator  100  gives forces in the range of 100 μN for a coil with five windings, and a core member that is 10 μm wide, 10 μm high, and 500 μm long with a Young&#39;s modulus of steel (210 GPa). The modeling of actuator  100  also assumed a gap of 3 μm, and 2.75V of bias passed across the coil (approximately 20 mA of current) whose resistance (the coils) is 3×10 −8  Ωm −1 . 
     FIGS. 12A-20A ,  12 B- 20 B,  12 C- 20 C,  12 D- 20 D,  12 E- 20 E, and  12 F- 20 F show a series of views that illustrate a method of forming a MEMS relay  1200  in accordance with the present invention. The method of forming MEMS relay  1200  is similar to the method of forming actuator  100  and, as a result, utilizes the same reference numerals to designate the structures which are common to both methods. 
   As shown in  FIGS. 12A-12F , the method of forming relay  1200  utilizes a conventionally formed single-crystal silicon semiconductor wafer  1210  and an overlying dielectric layer  1212 . Like dielectric layer  112 , dielectric layer  1212  can represent a dielectric layer that includes no metal structures, or a dielectric layer that includes metal structures, such as the dielectric layer of a metal interconnect structure. 
   When formed as the dielectric layer of a metal interconnect structure, dielectric layer  1212  includes levels of metal traces, a large number of contacts that connect the bottom metal trace to electrically conductive regions in and on wafer  1210 , and a large number of inter-metal vias that connect metal traces in adjacent layers together. Further, selected regions on the top surfaces of the metal traces in the top metal layer function as pads which provide external connection points. 
   In the present example, dielectric layer  1212  represents the dielectric layer of a metal interconnect structure that also includes pads P 1 -P 4 . Pads P 1  and P 2  are selected regions on the top surfaces of two of the metal traces in the top layer of metal traces that provide electrical connections for a to-be-formed square coil, while pads P 3  and P 4  are selected regions on the top surfaces of the metal traces that provide electrical connections for a to-be-formed switch. (Only pads P 2 -P 4 , and not the entire metal interconnect structure, are shown in cross-section for clarity.) 
   Referring again to  FIGS. 12A-12F , the method of forming relay  1200  begins the same as the method for forming actuator  100 , except that seed layer  114  is also formed on pads P 3  and P 4  in addition to pads P 1  and P 2 . Once seed layer  114  has been formed, mask  116  is formed and patterned as before except that the pattern also exposes the regions of seed layer  114  that lie over pads P 3  and P 4  in addition the regions of seed layer  114  that lie over pads P 1  and P 2 . 
   As shown in  FIGS. 13A-13F , following the formation and patterning of mask  116 , copper is deposited by electroplating as before to form the copper lower sections  120  (the lower sides of the to-be-formed square coil). In addition, copper structures  1214  and  1216  are formed and electrically connected to pads P 3  and P 4  at the same time that the copper lower sections  120  are formed. After the copper lower sections  120  have been formed, mask  116  is removed, followed by the removal of the underlying regions of seed layer  114 . 
   The method of forming MEMS relay  1200  then follows the same process as described above with respect to  FIGS. 3A-3E  through  7 A- 7 E up to the formation of mask  142 . As shown in  FIGS. 14A-14F , mask  142  is formed as above except that the pattern also exposes the regions of dielectric layer  140  that lie over copper structures  1214  and  1216 . 
   Following the formation and patterning of mask  142 , as shown in  FIGS. 15A-15F , the exposed regions of the dielectric layer  140  and underlying dielectric layer  122  are etched as before to form vertical openings  144 . In addition, the etch also forms a vertical opening  1220  that exposes the top surface of copper structure  1214 , and a vertical opening  1222  that exposes the top surface of copper structure  1216 . Mask  142  is then removed. 
   Once mask  142  has been removed, as shown in  FIGS. 16A-16F , seed layer  146  is formed as before except that seed layer  146  is also formed on the exposed top surfaces of copper structures  1214  and  1216 . After seed layer  146  has been formed, mask  150  is formed and patterned as before, except that mask  150  also exposes the regions of seed layer  146  that lie on the top surface of dielectric layer  140  adjacent to core section  136 , the top surface of dielectric layer  140  over floating cantilever section  138 , and the top surfaces of copper structures  1214  and  1216 . The pattern (openings) in mask  150  is shown hatched in  FIG. 16A . 
   Next, as shown in  FIGS. 17A-17F , following the formation and patterning of mask  150 , copper is deposited by electroplating as before to form the copper side sections  156  and the copper upper sections  158  of the square coil. In addition, a copper first strip  1224  is formed adjacent to and along core section  136 , and a copper second strip  1226  is formed over floating cantilever section  138  at the same time that side and upper sections  156  and  158  are formed. Copper first strip  1224  is connected to copper structure  1214 , and copper second strip  1226  is connected to copper structure  1216  to provide electrical connectivity for the to-be-formed switch. Copper first strip  1224 , copper second strip  1226 , and the copper upper sections  158  of the square coil are shown hatched in  FIG. 17A . 
   Following this, as shown in  FIGS. 18A-18F , mask  150  and the underlying regions of seed layer  146  are removed. The removal of mask  150  and the underlying regions of seed layer  146  leaves the seed layer  146  (that lies under a portion of second copper strip  1226 ) connected to dielectric layer  122  as shown by the arrow X in  FIG. 18D . 
   Next, as shown in  FIGS. 19A-19F , the seed layer  146  connected to dielectric layer  122  as shown by the arrow X is wet etched for a predetermined period of time to thereby free floating cantilever structure  138  from any connection with underlying dielectric layer  122 . Once free, the end wall face of second copper second strip  1226  can contact the end wall face of first copper strip  1224  when end  138 -E 2  of floating cantilever section  138  moves a distance horizontally towards end  138 -E 1  of core section  136 . 
   Following this, a conductive layer  1230 , such as a layer of titanium, nickel, or chrome, and an overlying layer of gold, is deposited on dielectric layer  140 , the copper upper sections  158 , and the first and second strips  1224  and  1226 . Conductive layer  1230  is electrically isolated from core section  136  and floating cantilever section  138  by regions of dielectric layer  140 . 
   When sputtered, titanium, nickel, chrome, and gold provide good coverage on the high-aspect ratio (vertical) end walls of the core and floating cantilever sections  136  and  138  that face each other. Titanium, nickel, and chrome, in turn, improve the adhesion of gold. After conductive layer  1230  has been formed, a mask  1232  is formed and patterned on conductive layer  1230 . The regions of conductive layer  1230  that are protected by mask  1232  are shown hatched in  FIG. 19A . 
   As shown in  FIGS. 20A-20F , following the formation and patterning of mask  1232 , the exposed regions of conductive layer  1230  are etched away to form a first end plate  1234  that lies adjacent to the second end  136 -E 2  of core section  136 , and a trace  1236  that electrically connects first end plate  1234  to conductive structure  1214 . The etch also forms a second end plate  1240  that lies adjacent to the second end  138 -E 2  of floating cantilever section  138 , and a trace  1242  that electrically connects second end plate  1240  to conductive structure  1216 . 
   In addition, as further shown in  FIG. 20D , first end plate  1234  and second end plate  1240  are horizontally spaced apart by a switch gap  1243 . The size of switch gap  1243  is defined by the patterns in mask  150  and the thickness of conductive layer  1230 . Mask  1232  is then removed to complete the process. 
   Thus, a method of forming relay  1200  has been described. As shown in  FIGS. 20A-20F , relay  1200  is the same as actuator  100  except that relay  1200  includes a switch  1244  that has a first electrode  1246  and a second electrode  1248 . First electrode  1246  is implemented with first end plate  1234 , trace  1236 , and first copper strip  1224 . Second electrode  1248 , which rides on floating cantilever section  138 , is implemented with second end plate  1240 , trace  1242 , and second copper strip  1226 . 
   In operation, when no current is present, floating cantilever section  138  has the shape shown in  FIG. 20A . As shown, first electrode  1246  and second electrode  1248  are spaced apart by switch gap  1243 , thereby providing a high-impedance electrical pathway. On the other hand, when a current flows through coil  160  and generates an electromagnetic field that is stronger than the spring force of floating cantilever section  138 , the floating end  138 -E 2  of floating cantilever section  138  bends towards the second end  136 -E 2  of core section  136  so that the first end plate  1234  of first electrode  1246  touches the second end plate  1240  of second electrode  1248 , thereby providing a low-impedance electrical pathway. 
   As noted above, dielectric layers  112  and  1212  can represent a dielectric layer that is free of metal structures. When free of metal structures, the electrical connections to coil  160  can be made, for example, by wire bonding to points on the copper upper sections  158  that represent opposite ends of coil  160 . In addition, connections to the first and second electrodes  1246  and  1248  can be made, for example, by wire bonding to traces  1236  and  1242 . 
   One of the advantages of the present invention is that the present invention requires relatively low processing temperatures. As a result, the present invention is compatible with conventional backend CMOS processes. 
   It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. For example, the various seed layers can be implemented as copper seed layers, or as tungsten, chrome, or combination seed layers as need to provide the correct ohmic and mechanical (peel) characteristics. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.