Abstract:
A method of making micro-actuator devices including a silicon wafer, a magnet positioned inside an insulated actuating chamber having electrical coil wound around its circumference thereby forming an electromagnet assemblage. A plurality of etched holes in silicon wafer receives the electromagnet assemblage and is adapted to produce a magnetic field in response to an applied current that acts on the magnet to cause the axial reciprocating motion of the magnet.

Description:
FIELD OF THE INVENTION 
     This invention relates to fabrication of integrated hybrid silicon-based micro electromechanical devices. This invention specifically utilizes the conventional silicon planar technology to build integrated three-dimensional arrays of hybrid silicon-based micro-actuators. 
     BACKGROUND OF THE INVENTION 
     Micro-Electromechanical Systems (MEMS) is a rapidly growing field that is impacting many applications today. Three dimensional micro-engineered devices and systems involving silicon planar technology can be mass produced with features from one to a few hundred microns having tolerances in micron or sub-micron level. Most of the current micro-engineering technologies are evolved from the adaptation of thin films, photolithographic and etching technologies generally applied to silicon wafers on which silicon monoxide, silicon dioxide, silicon nitride and the like thin films are deposited and etched thereafter yielding planar configuration. 
     Although the planar silicon technology is capable of building a three dimensional array, the process steps involved in building those structures are many and very often exceed 20 to 30 steps thus making the process less attractive for many applications. Furthermore, there are many complicated structures that are not possible to be incorporated in the silicon planar technology because of certain limitations of the thin film technology. 
     Moreover, experience indicates that the current planar technologies using silicon substrates are inadequate for the fabrication of an integrated and self-contained three-dimensional arrays of micro-devices which can be used as solenoids, actuators, transformers and the like. 
     The limitation of the planar silicon technologies stems from the fact that the multi-step thin film technology along with etching processes which are usually used to build three dimensional structures on a silicon wafer can not produce complex structures. As for example, one of the greatest drawbacks of the silicon technology is that it is not possible to build a buried helical coil or a uniform vertical cylindrical column having higher length to radius aspect ratio, and similar complex configurations. Furthermore, building three-dimensional multi-layered structures using thin film technology involves multiple process steps, generally twenty or more, and therefore makes this process not economically feasible. 
     Therefore, there persists a need for a method to fabricate integrated hybrid silicon-based micro electromechanical devices, such as solenoids, actuators, and transformers, that requires substantially fewer steps thereby reducing production cycle time and increasing cost efficiency. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a method of making integrated hybrid silicon-based micro-actuator devices which is cost effective and requires minimal production steps. 
     It is another object of the invention to provide a method of making integrated hybrid micro-actuator devices that uses silicon wafer technology in conjunction with micro-molding technology consisting of ceramic, magnetic and polymeric materials. 
     It is a feature of the invention that a method of fabricating integrated hybrid silicon-based micro-actuator devices comprises the step of arranging an actuator assemblage having a displaceable ferromagnetic member into a cavity of a silicon wafer, the ferromagnetic member being reciprocatingly axially displaced in an actuating chamber in response to an applied current to conductive pads on the silicon wafer. 
     Accordingly, for accomplishing these and other objects, features and advantages of the invention, there is provided, in one aspect of the invention, a method of fabricating hybrid micro-actuator comprising the step of providing a generally planar silicon wafer having at least one generally cylindrical, etched cavity therein and at least two electrical contacts arranged about the at least one cavity. A micro-molded ferromagnetic member is provided having a first and second ends and a non-magnetic actuator arm extending from any one of the first and second ends. Also provided is an electrically insulated actuating chamber for receiving the ferromagnetic member and for enabling reciprocating displacements of the ferromagnetic member therein. The actuating chamber has a first electrical coil wound partially lengthwise about a first half portion of the chamber and a second electrical coil wound lengthwise about a second half portion of the chamber. Each of the first and second coils having a pair of free end portions for electrically connecting to the electrical contacts of the silicon wafers. The ferromagnetic member is arranged for axial reciprocating displacement inside the actuating chamber with the actuator arm protruding outwardly from the actuating chamber. This arrangement forms an electromagnetic actuator assemblage. A closure member having an opening is securely applied to the open end of the actuating chamber containing the ferromagnetic member such that the actuator arm protrudes through the opening in the closure member. The electromagnetic actuator assemblage is then arranged securely into one cavity in the silicon wafer such that the pair of free end portions of the first and second coils surrounding the actuating chamber provides an electrically conductive path to corresponding electrical contacts arranged about the at least one cavity in the silicon wafer. Thus, current flow through the electrically conductive path causes reciprocating axial displacements of the ferromagnetic member inside the actuating chamber in response to the current flow thereby defining a micro-actuator. 
     It is, therefore, an advantage of the present invention that the disclosed method of fabrication surmounts the several problems associated with the planar silicon technology in that the buried coil structure, monolithic ceramic columnar structure and other complex three dimensional features can be integrated as a hybrid device. Moreover, the present invention offers a unique solution to the conventional silicon technology by integrating the micro-molded three-dimensional monolithic components within a silicon wafer. Further advantages of this invention include its cost-effectiveness associated with the manufacturing of three-dimensional arrays of micro-devices. Also, the method of the invention overcomes many of the disadvantages of planar silicon-based thin film technology. The method of the invention further enables the manufacture of micro-devices utilizing automated silicon technology for integrated electronics. Finally, the method of the invention enables the utilization of the cost-effective criteria of silicon technology in conjunction with micro-molding technologies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
     FIG. 1 is an enlarged fragmentary diagrammatic view of an integrated three-dimensional silicon-based hybrid actuator of the invention; 
     FIG. 2 is a micro-molded ferromagnet having actuator arm attached to it; 
     FIG. 3 is a micro-molded ceramic end cap; 
     FIG. 4 is an enlarged cut off view of the electromagnet assembly having a single wound electrical coil; 
     FIG. 5 is a perspective of a micro-molded electro-magnet assembly comprising electrically insulating ceramic cylinder with wound and counter-wound electrical conductor and inserted therein the magnet having an actuator arm; and 
     FIG. 6 is an exploded view showing the assembly of integrated silicon-based hybrid micro-actuator of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings, and particularly to FIGS. 1 and 6, the integrated hybrid silicon-based micro-actuator  100  of the invention illustrates the incorporation of micro-molded monolithic ferromagnetic member or magnet  40  in an electrically insulating ceramic actuating chamber or cylinder  50 . Shown in FIG. 2, actuator arm  70 , preferably made of plastic although skilled artisans will appreciate that other similar materials may be used, is fixedly attached to ferromagnetic member  40  forming an actuating member  72 . As perhaps best seen in FIGS. 1,  4 ,  5  &amp;  6 , actuating member  72  is then disposed for axial displacement inside actuating chamber  50 . 
     According to FIGS. 1 and 5, in one embodiment of the invention, actuating chamber  50  has a first and second current carrying coil  54   a,    54   b  wound around its circumference. In this embodiment, first current carrying coil  54   a  is wound clockwise around a top half portion of actuating chamber  50  and second current carrying coil  54   b  is wound counter-clockwise around a bottom half portion of actuating chamber  50 . It should be appreciated that the winding direction of coils  54   a  and  54   b  are not important; but, it is within the contemplation of the invention that coils  54   a  and  54   b  are wound in opposite directions about the circumference of actuating chamber  50 . Further, skilled artisans will appreciate that conventional winding technology equipped with tools to handle micro-components may be used to accomplish the windings. 
     According to FIGS. 1 &amp; 6, a micro-molded insulating ceramic end cap  30  (see FIG.  3  and description below) has a central opening  32  through which actuator arm  70  passes to confine the ferromagnet  40  within the ceramic actuating chamber  50 . Actuator arm  70 , preferably a cylindrical rod, made from engineering plastic such as nylon, polyimides, pvc and the like, having the dimension not exceeding 100 μm in diameter in cross section and a suitable length, is attached to one end of the ferromagnet  40  using any known conventional bonding technique. Thus, the preferably plastic actuating arm  70  is the primary actuating element for the micro-actuator  100 . Those skilled in the art will appreciate that actuator arm  70  can be made from other engineering materials such as aluminum, magnesium, graphite reinforced epoxy composites, and other materials having attributes of good mechanical strength and light weight. It is important to the invention that the length of the ferromagnet  40  is shorter than the inside length of the ceramic actuating chamber  50  to insure ample space for axial displacement inside actuating chamber  50  when the actuating member  72  is energized, as described below. 
     Referring again to FIGS. 1 &amp; 6, integrated hybrid silicon-based micro-actuator  100  of the invention further comprises a generally planar silicon wafer  10  having at least one substantially cylindrical hole or cavity  15 . Preferably cavity  15  is etched into silicon wafer  10  using any conventional etching process such that the cavity  15  has a continuous vertical sidewall  19  surrounding the cavity  15 . It is also important to the invention that cavity  15  has a diameter slightly larger than the outside diameter of the ceramic actuating chamber  50 . 
     Referring to FIG. 1, micro-molded monolithic ferromagnet  40  inside ceramic actuating chamber  50  is disposed inside in a cavity  15  of silicon wafer  10 . Preferably, any free space between the actuating chamber  50  and sidewall  19  of cavity  15  is filled with electrically insulating epoxy  20  to bond those components to the silicon wafer  10 . If desired, the epoxy  20  can be conveniently replaced with ceramic or glass sol-gel or ceramic slurry and cured at a relatively higher temperature but not exceeding 300° C. Ceramic slurry or sol-gel will offer greater rigidity and higher strength for the devices. As seen in FIG. 5, active electrical leads  60  coming off from the wound coils  54   a,    54   b  are soldered or bonded to conducting pads  64  which are generated on the silicon wafer  10  by conventional thin film technology. The micro-device thus fabricated can be used as an actuator or a solenoid which may be used as a component for an ink jet engine or related applications. 
     Skill artisans will appreciate that micro-molding technology is made possible by designing and fabricating molding tools using MEMS technology. Micro-molding ceramics with features as small as 100 μm has been fully described by the inventors in U.S. patent application Ser. No. 08/749,256, filed Nov. 15, 1996, entitled “Method For Micro-Molding Ceramic Structures”, by Furlani et al. Similarly details of micro-molding magnets and electromechanical parts have been fully described by the inventors as set forth below. 
     Referring to FIG. 2, micro-molded ferromagnet  40  is preferably generally cylindrical and made from hard magnetic materials, such as NdFeB, barium ferrite, and SmCo which can be produced using injection molding process. The details of the preferred method of forming micro-magnets are described in the commonly assigned U.S. patent application Ser. No. 08/866,991, filed Jun. 2, 1997, entitled “Method For Making Ceramic Micro-Electromechanical Parts and Tools”, by Furlani et al, and commonly assigned U.S. patent application Ser. No. 08/795,960, filed Jan. 31, 1997, entitled “Method For Making Ceramic Tools For The Production Of Micro-Magnets”, by Furlani et al, both hereby incorporated herein by reference 
     Referring now to FIG. 3, micro-molded end cap  30  is preferably made from insulating ceramic materials. The details a preferred method of forming micro-molded ceramic components are described in the commonly assigned U.S. patent application Ser. No. 08/749,256, filed Nov. 15, 1996, entitled “Method For Micro-Molding Ceramic Structures”, by Furlani et al, hereby incorporated herein by reference. The end cap  30  can be formed by dry pressing or injection molding insulating ceramic such as alumina, silica, magnesia and zirconia. 
     Referring to FIG. 4, an alternative embodiment, ferromagnet  40  is shown disposed in micro-molded ceramic actuating chamber  80 . In this embodiment, actuating chamber  80  has a single electrical conductor  54  with free ends  60  wound about its outer circumference and only partially along its axial length. As shown, micro-molded magnet  40  disposed has actuator arm  70  protruding outward through opening  32  in closure member or end cap  30 . Thus, magnet  40  coupled with the electrical coil  54  wound around its outer circumference having the insulating ceramic actuating chamber  50  therebetween forms an electromagnet when the coil is energized by a power supply (not shown). 
     Referring again to FIGS. 4 &amp; 5, in operations, a source of power (not shown) causes current to flow through the coil  54  (single wind as illustrated in FIG.  4 )) in a first direction that propels the actuator arm  70  upwardly inside actuating chamber  80 . The motion of the actuator arm  70  in response to the current can be understood by considering the interaction of the magnetic field of the electromagnet with the magnetic poles of the magnet  40 . Specifically, the coil  54  produces a magnetic field substantially along its axis that imparts an upward force in the north pole of the magnet  40  that moves upward inside the actuating chamber  80  as a consequence of these forces. According to FIG. 5, similar forces are operative in the alternative embodiment wherein the first and second current carrying coils  54   a  and  54   b  respectively, are used. Coils  54   a,    54   b  wrapped about the circumference of actuating chamber  50  interacts with the north pole of the magnet  40  and axially displaces the magnet  40  along with the actuator arm  70  upward in the actuating chamber  50 . In a similar fashion, the coils  54   a,    54   b  produces a magnetic field substantially along its axis that again imparts an upward force to the south pole (not shown) of the magnet  40 . The advantage of this alternative embodiment is that the combined interaction forces of the north and south poles deliver more energy to the actuator arm  70 . 
     Referring to FIG. 6, depicted are the hybrid components integral to the invention integrated into a silicon wafer  10 . An array of cylindrical cavities  15 , described above, are etched on the generally planar surface of silicon wafer  10 . Deposited onto silicon wafer  10  in close proximity to each cavity is a plurality of conducting electrical pads  64 . Electrical pads  64  are formed on silicon wafer  10  using conventional thin film technology. The free ends  60  from coils  54   a,    54   b,  are soldered onto the conducting pads  64  to build the integrated hybrid silicon-based micro-devices. 
     Thus, the invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood and appreciated that variations and modifications can be effected within the spirit and scope of the invention. 
     Parts List 
       10  silicon wafer 
       15  cylindrical hole or cavity 
       19  continuous vertical sidewall 
       20  epoxy or other insulating glass or ceramic 
       30  micro-molded insulating ceramic end cap 
       32  central opening of end cap  30   
       40  micro-molded monolithic ferromagnetic member or magnet 
       50  ceramic actuating chamber or cylinder 
       54  single electrical conductor or coil 
       54   a  first current carrying coil 
       54   b  second current carrying coil 
       60  electrical conductor leads or free ends 
       64  conducting pads 
       70  actuator arm 
       72  actuating member 
       80  micro-molded ceramic actuating chamber 
       100  integrated hybrid silicon-based micro-actuator