Abstract:
The invention describes a method and apparatus for deploying micromachined actuators in a plane which is orthogonal to the original fabrication plane of the devices. Using batch-processing, photolithographic procedures known in the micromachined electro-mechanical system (MEMS) art, a plurality of devices is constructed on a suitable substrate. The devices are then separated one from another by sawing and dicing the original fabrication wafer. The devices are rotated into an orthogonal orientation and affixed to a second wafer. The second wafer also contains circuitry for addressing and manipulating each of the devices independently of the others. With this method and apparatus, arrays of actuators are constructed whose plane of actuation is perpendicular to the plane of the array. This invention is useful for constructing N×M fiber optic switches, which direct light from N input fibers into M output fibers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This U.S. patent application is a divisional application of U.S. application Ser. No. 09/764,913, filed Jan. 17, 2001 now U.S. Pat. No. 6,812,061. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
   Not applicable. 
   REFERENCE TO MICROFICHE APPENDIX 
   Not applicable. 
   FIELD OF THE INVENTION 
   This invention relates to micromachined electromechanical systems (MEMS), and their use in switching technology for optical telecommunications. 
   BACKGROUND OF THE INVENTION 
   Arrays of high speed, high precision actuation devices are becoming required for a proliferating number of applications, in diverse fields. Deep space astronomical observatories may use multifaceted mirrors, each facet independently controlled by a precision actuator. Digital projection cameras manipulate a plurality of reflectors, in order to cast an image onto a projection screen. 
   But optical telecommunications in particular, have an urgent need for high bandwidth, low inertia, microactuated reflectors. This industry uses optical wavelength radiation in communication channels, spanning long distances via optical fiber. The fiber generally carries single mode light, but with a bandwidth such that multiple frequencies can be transmitted by a single fiber strand. Called dense wavelength divisional multiplexing (DWDM), each of the multiple frequencies carries a single channel on a specific frequency within the fiber bandwidth. 
   A major technical challenge for optical telecommunications is switching the optical beams from a set of input fibers to a set of output fibers. Presently, this switching is achieved by converting the optical signal into an electrical signal, switching the signal electronically to the output circuit, and then converting the electrical signal back into modulated light that is inserted into an output fiber. This procedure has many disadvantages, including high cost, complexity, large footprint, and is relatively difficult to upgrade as bandwidths and frequencies change with time. 
   It is desirable therefore, to accomplish this switching without converting the light into electrical signals. An array of optical switches would address this application, wherein each optical switch (a micromirror) would redirect light from one of N input optical fibers into one of M output fibers. N×M array of such switches would constitute an all-optical N×M fabric for a telecommunications fiber network. 
   For such high speed, high precision applications, stringent design criteria are set on the physical and mechanical properties of the actuator. It should have low inertia and low power requirements. For low cost applications, it should also be mechanically simple. These considerations have led to the miniaturization of familiar electromechanical devices, using photolithographic processing rather than machining bulk components. Formation of sub-millimeter scale electromechanical systems is now well known in the art, as micromachined electromechanical systems, or MEMS. 
   Micromachined solenoidal magnetic actuators are known in the MEMS art as micro-solenoid switches. Typically, a slug of magnetic material is affixed to a piston or plunger, and a coil is provided whose diameter is sufficient to admit the slug into its interior. The coil is then energized to repel or attract the slug, depending on the direction of current in the coil. The resulting linear mechanical motion is used to actuate various linear devices, such as opening and closing a switch or valve, or driving a piston. An embodiment of a linear, solenoidal micro-actuator is found for example, in Guckel, et al., U.S. Pat. No. 5,644,177 (1997), “Micromechanical magnetically actuated devices.” 
   Another design option is a rotary actuator. This device resembles a miniaturized electromagnetic motor, with a ferromagnetic core material deposited on the substrate and wound with an electrical coil. The core is patterned with some arrangement of gaps, into each of which protrudes a driven member which interacts magnetostatically with the flux across the gap. A plurality of such elements, when driven in the proper sequence and timing, can produce a positive torque on a freely rotating member. Magnetostatic micromotors can be used as rotary actuators by mounting the device of interest onto the moving member, i.e. the rotor. This concept is clearly described in Mehregany, et al. in U.S. Pat. No. 6,029,337 (2000), “Methods of fabricating micromotors with utilitarian features.” 
   However each of these MEMS devices suffer from a common drawback, which is that the actuation motion is constrained by design and fabrication considerations, to be in the fabrication plane of the device. For example with solenoidal electromagnetic actuator, such as described in the aforementioned prior art, motion of the magnetic slug is required to be in the fabrication plane of the device. For the rotary micromotor, the device is mounted on the rotor, and so rotates in the plane of fabrication of the motor. 
   Many applications require motion in the orthogonal plane, i.e. vertical to the original plane of fabrication of the MEMS device, and an array of such devices is needed. Examples of such applications include ailerons composed of a multitude of fluid flow diverters, actuated or pivoting in the vertical direction relative to the array. N×M optical switches require an array of independently addressable shutters or reflectors, in which the plane of actuation of the individual devices is orthogonal to the plane of the array, in order to intercept the beams of light passing over the horizon of the array. Projection cameras and multi-faceted reflectors require a tilting motion of the individual mirrors. 
   Therefore the MEMS fabrication substrate, cannot serve as the array plane for these applications, because the plane of actuation would be parallel to, not perpendicular to, the plane of the array. 
   Additional beams, gears and bearings can translate actuator motion out-of-plane, as in Ho et al., in U.S. Pat. No. 5,629,918 (1997), “Electromagnetically actuated micromachined flap.” In this invention a flap, which is the moving member of the actuator, is coupled by one or more beams to a substrate and thereby cantilevered out of the plan of the substrate. While conceptually this invention allows larger motions in out-of-plane directions, the need for multiple beams and pivots seriously complicates the design and fabrication of the device, and deleteriously affects tolerances and rigidity. 
   Therefore, an assembly of MEMS actuation devices, operating as an array, and with the plane of motion of each device being substantially perpendicular to the plane of the array, is not heretofore known in the art. Accordingly, there is a distinctly felt need for such an assembly of MEMS devices in a wide variety of applications, and in particular, within the optical telecommunications industry. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention described herein is an assembly method and apparatus used to mount a plurality of MEMS devices into an array. Each MEMS device is a low inertia, high bandwidth microactuator carrying the device of interest (shutter, piston, optical element, etc.). In the embodiment described here, each MEMS actuator carries an optical micromirror on the actuator arm. A plurality of like devices is fabricated on a silicon substrate, using processes known in the MEMS art. 
   Each of the devices is separated into individual dies after fabrication, by sawing the wafer into rows, and sawing the rows into individual dies. The dies are then individually mounted and adjusted on another substrate using an apparatus equipped with vertical and azimuthal actuation means. This second carrier wafer serves as a miniature optical table for the dies. 
   To achieve the desired plane of motion, the dies are rotated out of their original plane of fabrication, before being affixed to the second carrier wafer. In the preferred embodiment, a 90 degree rotation transforms the original plane of motion into one orthogonal to the plane of the array, as desired. Fine adjustment of the positioning is then accomplished with a feedback mechanism which optimizes the placement before the die is affixed to the second carrier wafer. 
   The carrier wafer has previously been photolithographically processed using techniques well known in semiconductor device fabrication, to form the supporting electronic circuitry to address and control each MEMS device. This second wafer, carrying the electronic circuitry, is henceforth termed the “circuit wafer”, as to distinguish from the original MEMS fabrication wafer. 
   As a result, the finished circuit wafer comprises an array of micromechanical reflectors, which each reflector independently addressable by associated circuitry. The array of reflectors in designed for use in the optical telecommunications industry, with each mirror capable of extension and retraction out of the plane of the array. This invention gives rise to the following significant advantages:
         1. Ability to assemble switching devices in which the plane of the mirror is in the MEMS fabrication plane (which provides the highest quality, lowest loss mirror), but the mirror is deployed in the transverse plane, to intercept beams of light.   2. Ability to pre-test individual dies or rows and assemble only properly functioning devices (“yielded dies”). This is in contrast to optical switch arrays built on a wafer in which a non-yielding die means an imperfect assembled device. Such arrays consequently have either very demanding yield requirements or must be designed with sufficient duplicity to overcome the yield problems.   3. Ability to assemble optical switches with precise alignment independent of warping of substrate materials, and various other manufacturing defects, using a feedback mechanism during assembly.   4. Ability to use one type of optical switch for a large number of different assemblies for different applications. For example, this includes the ability to assemble N×M switch arrays where N and M are any positive integer, using the same building block die.   5. Ability to assemble more than one type of die into an assembly of dies.   6. Ability to assemble a packaged device that is hermetically sealed, the hermetic seal can be confirmed, and the gas enclosed is such that the cooling of the device inside the package is optimized or the mechanical damping of the device is optimized.       

   The method and apparatus for constructing this array is the subject of this invention disclosure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a, b  are simplified diagrams of the MEMS actuator in the retracted (a) and extended (b) positions; 
       FIG. 2   a  depicts a simplified view of an individual MEMS device occupying a single die;  FIG. 2   b  shows the device as residing on its fabrication wafer;  FIG. 2   c  shows the MEMS fabrication wafer in perspective view;  FIG. 2   d  illustrates a single row sawed from the wafer.  FIG. 2   e  shows the wafer rotated 90 degrees from the original plane of fabrication. 
       FIG. 3  illustrates an individual die mounted to the circuit substrate; 
       FIG. 4  depicts an array of dies on the circuit substrate; 
       FIG. 5  illustrates the reflection of fight from a number of input fibers to a number of output fibers; 
       FIG. 6  illustrates the attachment method and apparatus according to the present invention; 
       FIG. 7  is an end-view of an individual die, showing the electrical bonding pads and the mechanical bonding pads. 
       FIG. 8  shows an alternative method, in which holes are formed in the circuit substrate and plated with a conducting film; 
       FIG. 9   a  is a simplified diagram of the tooling fixture for array assembly;  FIG. 9   b  shows the elevation mechanism of the tooling assembly; 
       FIG. 10  is a simplified top-down view of the alignment process underway. 
       FIG. 11  is a simplified side view of the array assembly with the output fiber block in place; 
       FIG. 12  is a simplified schematic diagram of the feedback apparatus used for aligning the individual die; 
       FIG. 13  shows the packaging of the finished array device. 
   

   REFERENCE NUMERALS IN THE DRAWINGS 
     9  Substrate 
     10  Driven member 
     11  Pivot point 
     12  Magnetic core 
     13  Magnetic tab 
     14  Gap 
     16  Coil 
     21  MEMS fabrication wafer 
     22  circuit wafer 
     28  mirror 
     30  die 
     32  bond pads 
     34  bond pads 
     36  conductive trace 
     40  conductive trace 
     44  external bonding pads 
     48  external bond pads 
     60  solder bump 
     70  eutectic solder bump 
     72  mechanical bonding pad 
     80  plated conducting film 
     90  die holder 
     94  vacuum hole 
     96  die holder cutout 
     100  elevation actuator 
     102  azimuth actuator 
     104  actuator body 
     106  piezoelectric 
     110  light source 
     112  focus lens 
     120  optical input fiber 
     122  lens 
     124  optical fiber 
     126  lens 
     128  fiber mounting block 
     130  fiber mounting block 
     230  XYZ bench 
     220  light source 
     222  light detector 
     224  amplifier 
     226  feedback logic 
     228  computer controller 
     520  lid 
     522  eutectic seal 
     524  gas 
   DETAILED DESCRIPTION OF THE INVENTION 
   A unique MEMS actuator is used for the embodiment, because it possesses uniquely large throw and multiple stable positions. Although the design of this low inertia microactuator is not the subject of this invention, it is used for this embodiment because of its advantageous features. As shown diagrammatically in  FIGS. 1   a  and  1   b , the actuator comprises a magnetic core  12  with a gap  14  affixed to the substrate  9 , and wound with an electrical coil  16 , as in a micromotor. Energizing of the coil  16  induces a magnetic flux through the core material, and across the gap  14  in the core. The driven member  10  is a hinge-mounted structure, which pivots about a stationary point  11 . The driven member  10  includes a tab of magnetic material  13 , which interacts with the core gap field, to impel motion of the member  10  about the pivot point  11 . A micromirror  28  is fabricated on the driven member  10  of the low inertia micromechanical actuator die  30 . The operational side retains its flat surface and supports the optical coatings. 
   These two constituents, the low inertia microactuator die  30 , carrying the micromirror  28 , are fabricated on a composite silicon-on-insulator (SOI) substrate. Using batch lithographic processes well known in the MEMS are, a plurality of like devices are constructed on a single SOI substrate. It is desired to separate each individual die, in order to mount them on the circuit wafer which contains the electronic circuitry to drive the individual devices. Further details of the manufacture of the MEMS actuator die  30  shown in  FIGS. 1   a  and  1   b  are given in U.S. Pat. No. 6,831,380 B2 to Rybnicek, et al, which was incorporated by reference in the parent application, U.S. patent application Ser. No. 09/764,913 filed Jan. 17, 2001, now U.S. Pat. No. 6,812,061 B1. However, since this MEMS actuator is exemplary only, and any other MEMS device may be used to practice this invention, these details are not essential to the understanding of the present invention, they are not set forth herein. 
   Accordingly, it should be understood that the MEMS actuator die  30  shown in  FIGS. 1   a  and  1   b  is only one exemplary embodiment, and that many other types of MEMS devices may be used in other exemplary embodiments. For example, the MEMS actuator shown in  FIGS. 1   a  and  1   b  is shown carrying a micromirror  28 . However, it should be understood that the driven member  10  may serve any of a number of alternative purposes. For example, instead of micromirror  28 , the driven member  10  may carry an electrical contact, such that the MEMS actuator becomes a MEMS switch, rather than an optical element. In this case, a conducting feature may replace the micromirror  28  shown in  FIG. 1   a . In yet another embodiment, the MEMS actuator die  30  may be used to restrict flow in a channel, such that MEMS actuator die  30  becomes a fluid flow diverter. As such applications are well known to those of ordinary skill in the art, these alternative features are not shown in  FIGS. 1   a  and  1   b . However, it should be understood that the invention described herein is intended to encompass such alternative embodiments. 
     FIG. 2   a  shows an individual MEMS device, which forms a single cell in the MEM fabrication wafer of  FIG. 2   b . The plane of the mirror is in the plane of the MEMS wafer. Rows of these dies are cut from the wafer using a technique well known in the silicon and disk drive recording head industries. These rows can be further cut into individual dies or shorter rows of fewer dies.  FIG. 2   a  shows die  30  that is sliced from MEMS fabrication wafer  21 . The rows and/or dies are then rotated 90 degrees and presented to the circuit wafer. The rotation changes the plane of the mirror to be perpendicular to the circuit wafer. These processes are depicted in  FIG. 2   c – 2   e.    
     FIG. 3  shows die  30  mounted to the circuit wafer. Electrically conductive traces  36  and  40  connect bond pads  32  and  34  of die  30  to bond pads  44  and  48 . External electrical connection can be made to bond pads  44  and  48  by means well known in the art of packaging electronic devices. 
     FIG. 4  depicts an array of dies and rows on circuit wafer  22 , with an array of electrical connections.  FIG. 5  illustrates the way in which light from a number of input fibers can be directed to different output fibers. For each input fiber, a mirror can be raised (put in the extended position) to direct the light to a given output fiber. To redirect the light to a different fiber, that mirror is lowered (put in the retracted position) and the appropriate mirror is put in the up position. Circuit wafer  22  allows the electrical connection to all the actuator/mirror dies. 
     FIG. 6  shows a schematic for the preferred attachment method between die  30  and circuit wafer  22 . An array of eutectic solder bumps  70  is attached to the circuit wafer. These eutectics are well known in the bonding industries, and the skilled reader will understand that a variety of glues, adhesives, and other materials could be substituted and the bond can be made anodically or by another means. Solder bumps  60  are also attached to the circuit wafer, and they are in turn electrically connected to traces  36  and  40  by direct contact.  FIG. 7  shows and end-view of die  30  which depicts the exposed contact bond pads  32  and  34  of die  30 , as well as exposed mechanical bond pads  72  of die  30 . Note that mechanical bond pads  72  are located on both sides of die  30 , and there are corresponding eutectic solder bumps  70  for each mechanical bond pad  72 . 
   By placing the eutectic solder bumps  70  on both sides of the die, the assembled part of die  30  and circuit wafer  22  is made relatively insensitive to changes in stress at the contact area. The changes in stress could be from a variety of material or environmental factors, such as temperature, stress-relief or aging with the passing of time, etc. With a bond on one side of die  30  only, such changes could result in a change in the angle of die  30  with respect to circuit wafer  22 , or a translation between the two pieces. In the current invention, with solder bumps on both sides, there can be changes in stress with very little change in the angles, and virtually no translation. 
   In the assembly process, die  30  is pressed against the circuit wafer and heat is applied and then removed. At that time, an electrical connection is made between bond pads  32  and  34  and solder bumps  60 , respectively. Also, a mechanical connection is made between solder bumps  70  and mechanical bond pads  72  which are located on die  30 . In this way, the die is held fast and the electrical connection is made to the die. 
   In the preferred embodiment, solder bumps  70  are applied to the circuit wafer in discrete shapes instead of a continuous film. This allows the eutectic to flow onto the die or row directly instead of flowing to an adjacent die, aided by the surface tension of the eutectic. 
     FIG. 8  shows another embodiment in which holes are formed in circuit wafer (many methods are well known in the art) and electrically conductive material is attached to the inside of the holes, shown in the diagram as conductive plated films  80 . In this case, the electrical connection is made between bond pads  32  and  34  and bond pads  44  and  48 , but  44  and  48  exist on the other side of the circuit wafer. This allows a number of benefits, including separation of the circuitry of circuit wafer from the packing of dies. 
   An additional embodiment is one in which the electrical connections made between bond pads  32  and  34  and the circuit elements on circuit substrate  22  are made by using ball bonding, a technique well known and long practiced in the electronics industry. 
   Precision placement of the die onto the circuit wafer is accomplished by an articulated tooling fixture equipped with a feedback device.  FIG. 9   a  shows a simplified diagram of the tooling fixture. Die  30  is held in die holder  90  by vacuum holes  94  and pressure between tensioner  92  and die holder  90 . Two-dimensional actuation of die  30  is performed by elevation actuator  100  and azimuth actuator  102 . These actuators provide the necessary range for alignment of die  30  with respect to circuit substrate  22 . A schematic of elevation actuator  100  is shown in  FIG. 9   b . Elevation actuator  100  is comprised of actuator body  104  and piezoelectric  106 . Piezoelectric  106  would typically be made of a ceramic piezoelectric material, capable of moving several microns with application of hundreds of volts. Actuator body  104  flexes with the expansion or contraction of piezoelectric  106 , giving rise to rotation. This method of rotation is well known in the art. It will be understood by the reader that azimuth actuator  102  is a similar mechanism to elevation actuator  100 , but rotated 90 degrees. 
   Light source  110  emits light which passes through line focus lens  112 , whose function is to generate a line focus of light along the intersection of die  30  and the circuit wafer, heating each along their line of contact. This light source will provide heat to activate the adhesive, when the die is properly oriented. 
   Orientation of the die holder by the articulated fixture, is under feedback control. The feedback mechanism is shown in  FIGS. 10 and 11 . A light source  220  is delivered by an optical fiber  120  through a lens  122 . Cutout  96  in die holder  90  is a through-hole that allows light from light source  220  to pass through the die holder to the mirror  28  on die  30 . The mirror reflects that light toward lens  126  and fiber  124 . Measuring the intensity of the light in fiber  124  produces the feedback signal. The die is affixed to the circuit wafer when the light intensity is optimized. 
     FIG. 10  shows the view from above during alignment. (Die holder  90  and various components from  FIG. 9  have been omitted from the drawing for simplicity). Fiber mounting blocks  128  and  130  are mounted to circuit substrate  22 . Fibers  120  and  124  and lenses  122  and  126  are mounted to fiber mounting blocks  128  and  120  as shown.  FIG. 11  shows the side view, with the output block, fiber and lens not shown for simplicity. 
   The feedback assembly is shown schematically in  FIG. 12 . The assembly procedure is as follows: (For simplicity, the presence of die holder  90  and various components from  FIG. 9  have been omitted from the drawing). XYZ bench  230  is used to properly position circuit wafer  22  with respect to the die by translating in three dimensions. After the die is loaded into the die holder, XYZ bench  230  is activated to bring the circuit wafer into close proximity to the die. Light from light source  220  is applied to input fiber  120  and input lens  122  collimates the light. The resulting beam of light reflects off mirror  28  to output lens  126  that focuses the light onto output fiber  124 . (The angular relationship between input fiber  120 , mirror  28  and output fiber  124  is as shown in  FIG. 10  and their relationship is shown in  FIG. 12  is purely schematic.) The light intensity is then detected by light detector  222 , amplified by amplifier  224  and the resulting signal is provided to feedback electronics  226 . 
   Under computer  228  control, feedback electronics  226  drives actuators  100  and  102  to optimize the signal intensity of light detector  222 . By optimizing the mirror angles in azimuth and elevation, the signal strength is increased. Computer  228  determines the rate of progress of the optimization and at such time that the progress is determined to be sufficient, the feedback electronics halts and locks the signal to actuators  100  and  102  and computer  228  sends a trigger signal to light source  110 . 
   Light emitted from light source  110  is focused by line focus lens  112  onto the area of proximity between die  30  and the circuit wafer, sufficiently heating the region to cause the eutectic  70  to melt, bridging between die  30  and the circuit wafer, aided by surface tension of the eutectic. (See  FIG. 7  for the spatial relationship between eutectic  50  and the bond pads of die  30 .) After a brief time, computer  228  signals light source  110  to turn off, the eutectic  70  cools and solidifies, and the bonding operation is complete with the die  30  accurately placed with respect to circuit wafer and input and output optical fibers and lenses. 
   It should be understood that all references to mounting, aligning and bonding dies can also be applied to rows of various lengths which include some number of dies, and that the invention herein covers these cases. In addition, the assembly technique with feedback can be applied to a single die at a time in order to generate a multi-mirror switch array, or can be applied to many dies or rows at a time. 
   A further embodiment of this invention is the method used in sealing the packaging around circuit wafer and the input and output fibers. The problem to be solved is to hermetically seal the package, allow testing of the hermetic seal, and to optimize the cooling or mechanical damping of interior parts in the finished packaged device. Referring to  FIG. 13 , lid  520  is applied to circuit wafer with a eutectic seal  522  around all edges. This technique is well known in packaging art. The environmental atmosphere at the time of sealing is controlled to be a specific gas at a specific pressure. To accomplish this, the unsealed package must be placed in a vacuum chamber and the pressure reduced to allow sufficient out gassing of the materials. Then the environment is filled with the proper gas at the proper pressure, and the two pieces of the package, the circuit wafer and lid are pressed together, with a combination of pressure and temperature, as required by the eutectic. Gas  524  is thereby hermetically sealed inside the package between circuit substrate  22  and lid  520 . 
   To check the eutectic seal, the chamber is again pumped to a high vacuum and leak checking is performed with a residual gas analyzer, well known in the art of vacuum engineering. 
   The specific gas and specific pressure chosen for the package depends on the requirements for the device. For optimum cooling, helium gas is chosen with the pressure such that the mean free path of the helium molecules in the device is approximately the dimension of a characteristic length of the device, i.e. the height of the inside of the package. For mechanical damping, the gas chosen could be xenon and the pressure is adjusted for the proper damping, potentially greater than 1 atmosphere. 
   While the invention has been particularly described and illustrated with reference to a preferred embodiment, it will be understood by those skilled in the art that changes in the description and illustrations may be made with respect to form and detail without departing from the spirit and scope of the invention. Accordingly, the present invention is to be considered as encompassing all modifications and variations coming within the scope defined by the following claims.