Abstract:
A novel MEMS assembly and testing system that utilizes a scanning electron microscope (SEM) having 5 axes of freedom as the imaging instrument. Microgrippers or other tools mounted at the end of a linear motion feed through device having a motion resolution of about 10 nanometers are used as the manipulator. All of the assembly features are located inside of a vacuum chamber to permit operation of the SEM imaging system. A variety of other auxiliary devices that support the MEMS assembly and testing system are also included to enhance the capabilities thereof.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for the assembly of microelectromechanical systems (MEMS). 
     BACKGROUND OF THE INVENTION 
     Assembly of MEMS is recognized as difficult for three principal reasons. First, the optical imaging methods commonly used to image small MEMS parts during assembly lack sufficient resolution and depth of field to make accurate imaging of the very small parts practical. Secondly, the parts undergoing assembly are usually manipulated with a micromanipulator equipped with a small set of grippers. The conventional micromanipulator has barely the control necessary for assembly of small mechanical and electromechanical parts. Thirdly, most light microscopes do not have stages of sufficient axes of freedom to manipulate the device so that it is placed properly for precise placement of parts by the grippers located on the micromanipulator. 
     U.S. Pat. No. 5,559,329 issued Sep. 24, 1996 describes an apparatus for measuring the interfacial properties of fiber-matrix composites. The apparatus includes a linear motion feedthrough for pushing an indentor on a fiber end, a load cell for sensing indentor load and a scanning electron microscope (SEM) for magnifying the material in order to align the indentor with a fiber end undergoing testing. The SEM includes a vacuum chamber for housing the indentor, the load cell and a hot stage module. Data acquisition and recording devices as well as an imaging computer for recording images of the materials during testing are also described. 
     OBJECT OF THE INVENTION 
     It is an object of the present invention to relate the capabilities of the devices described in the foregoing U.S. Pat. No. 5,559,329 to the field of MEMS assembly through the presentation of a novel MEMS assembly device that utilizes the general approach of this patent. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a novel MEMS assembly and testing system that utilizes a scanning electron microscope (SEM) having 5 axes of freedom as the imaging instrument. This imaging instrument has a resolution and depth of focus about 100 times greater than currently used optically based imaging systems. Microgrippers or other tools mounted at the end of a linear motion feed through device having a motion resolution of about 10 nanometers are used as the manipulator. All of the assembly features are located inside of a vacuum chamber to permit operation of the SEM imaging system. A variety of other auxiliary devices that support the MEMS assembly and testing system are also included to enhance the capabilities thereof. 
    
    
     DESCRIPTIONS OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the MEMS assembly device of the present invention. 
     FIG. 2 is perspective representation of the assembly sage of the MEMS assembly device of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, the MEMS assembly device  10  of the present invention comprises: a vacuum chamber  12 , an SEM  14 , a linear motion feedthrough device  16 , connected via a shaft  18  to a microgripper or other suitable tool as described hereinafter  20  and including an alignment guide  17 , an assembly stage  22 , a hot stage heater  24 , an SEM stage  26  and a stage platform  28 . The various other elements of MEMS assembly device  10  that support the forgoing essential elements include: stage controls  30  that provide control of stage platform  28  movement in the x, y, and z directions as well as permit tilting and rotation of stage platform  28  and associated assembly stage  22  and SEM stage  26 , linear motion feedthrough controller  32 , acoustic emission sensor  34 , load cell  36 , piezoelectric positioner  35 , vacuum gauge  38 , X-ray microanalysis system  40  and SEM column  42 . The roles of these various elements and devices will be explained more fully below. 
     A suitable linear motion feedthrough  16  is manufactured by Huntington Mechanical Laboratories as model MFL-275-4. Linear motion feedthrough  16  is connected to feedthrough controller  32  by cable  44 . A suitable controller  32  is manufactured by Huntington Laboratories and includes a model MLC-1 indexer and a model ssp-500 hand held programmer. Such a feedthrough device provides a 20 pound load force capacity and a minimum drive velocity of about 0.127 μm/s or about 7.62 μm/min. This load force capacity and relatively slow drive speed are suitable for MEMS assembly operations. Linear motion feedthrough device  16  can also be equipped to activate a buzzer or other audible alarm whenever microgripper  20  contacts a conductive surface. Such a modification can be helpful when approaching a surface or part very slowly. It typically provides an earlier warning of contact than the load cell described hereinafter and is somewhat more reliable than the acoustic emission sensor described below for determining when a surface has been contacted. The use of a variety of redundant systems of this type to provide feedback regarding various situations encountered during an assembly operation is highly desirable and preferred. 
     SEM  14  includes vacuum chamber  12 . This provides an excellent inert environment that assures that no contamination due to dust or other contaminants occurs. Vacuum chamber  12  is preferably mounted on a vibration isolation table  46  that dampens out external vibrations for even greater accuracy in the assembly operation. 
     Vacuum chamber  12  also houses a linear motion feedthrough shaft  18  microgrippers  20 , load cell  36 , acoustic emission sensor  34  and a piezoelectric positioner  35 , as well as the various stages, assembly  22 , hot stage  24 , and SEM  26  and platform  28 . Alignment guide  17  aligns feedthrough shaft  18  through a passageway in vacuum chamber  12 . 
     A suitable SEM  14  is one previously manufactured by Cambridge as Stereoscan  90 , Model B includes a backscatter electron detector  50 , a secondary electron detector  52  as well as X-ray microanalysis system  40 . X-ray microanalysis system  40  includes a digital imaging computer  72  that is interconnected with secondary electron detector  52  and X-ray microanalysis system  40  via cables  74  and running suitable image analysis and display software. A suitable digital imaging computer  72  can be any of the well known such devices such as a 486 or better PC. Computer  72  is used to acquire, record, transmit and view the assembly operation. Imaging in an SEM is also advantageous in that exposed conductors when under power image significantly brighter under the SEM thereby allowing an operator to readily check circuitry during operation. 
     Acoustic emission sensor  34  mounted on feedthrough shaft  18  is one form, in addition to optical imaging, for sensing contact between two parts being assembled within vacuum chamber  12  with microgrippers  20 . The sound of contact between two parts is readily detected by such a device and provides an additional sensory perception of activity occurring within vacuum chamber  12  during assembly. A suitable acoustic emission sensor is manufactured by Physical Acoustics Corporation as model μ30. 
     Locating and positioning of parts in the assembly is preferably performed in two primary ways. The “shop floor” or “assembly area” can be precisely moved using the X, Y, and Z tilt and rotate movements of SEM  14 . Alternatively, using piezoelectric positioner  35  connected to linear motion feedthrough device  16  the X, Y, Z movements of microgrippers  20  (or another suitable tool) can be performed. 
     Suitable piezoelectric positioning devices  35 , sometimes referred to as “nanopositioners”, are commercially available from Polytec PI, Inc., 23 Midstate Drive, Suite 212, Auburn, Mass. 01501 as models P-280, P-281 and P-282 XY and XYZ PZT Flexure NanoPositioners. 
     Similarly, load cell  36  connected to the linear axis of feedthrough shaft  18  within vacuum chamber  12  permits careful control of various “pushing” or “pulling” operations occurring during an assembly operation by providing feed back to the operator as individual parts as inserted into one another or other pressure sensitive operations are performed. A suitable load cell is manufactured by Sensotec as Model 31 that can be fitted with a variety of sensing heads ranging in capacity from about one or two pounds up to about 25 pounds. 
     Both load cell  36  and acoustic emission sensor  34  are monitored by a suitable detection device  68  such as a computer connected to these devices via cables  70 . 
     Vacuum gauge  38  provides the means for monitoring the pressure within vacuum chamber  12 . Such devices are well known in the art and commonly associated with vacuum chambers of the type utilized in conjunction with SEMs. 
     Referring now to FIG. 2 that depicts schematically the MEMS assembly stage  22  of the apparatus of the present invention, MEMS assembly stage  22  includes a hot stage heater  24  such as that manufactured Oxford Instruments Limited as model H1005 controlled by a suitable digital temperature controller (not shown). The hot stage heater  24  permits careful and accurate control of temperature on the “shop floor” or assembly area  58 , i.e. the area atop SEM sample holder or stage  28 . Additionally, the presence of hot stage  24  permits heating of parts being assembled to cure, for example, thermoset polymers or to braze or solder parts. Elevated temperature testing of parts, materials or assemblies or “burn in” temperature operating conditions of electronic or mechanical assemblies can also be accomplished with this configuration. The temperature of hot stage  24  is monitored and controlled by hot stage controller  54  connected to hot stage  24  via cable  56 . 
     Mounted about the periphery of SEM stage  26  are one or more holders, two in number in the accompanying drawings, identified by the numerals  60  and  62 . Each of holders  60  and  62  includes apertures or other containment devices such as apertures  64  for placement of parts necessary in the microassembly operation. Another use of apertures  64 , as shown in the case of holder  62  in accompanying FIG. 2 is for the placement of, for example, a container  66  of a vacuum stable adhesive and an applicator  68  therefor. The placement of parts, adhesives etc. in these locations allows ready accessibility thereto during assembly operations. Unless it is desired that “tools”, parts or other elements placed in apertures  64  be heated prior to use, it is highly desirable that an air gap  70  or some suitable insulation layer be provided between assembly stage  58  and holders  60 - 62 . 
     The MEMS assembly and testing device described herein can be equipped with a wide array of additional imaging methods including: secondary electron imaging, that provides imaging of the structure being assembled; backscatter imaging that can be set to vary contrast with respect to atomic number; specimen current imaging that identifies electrically conductive pathways; electron fluorescent imaging that images based upon the sample&#39;s ability to give off light while being excited by an electron beam as well as X-ray microanalysis that can provide elemental analysis of virtually any location being imaged. A light microscope can also be incorporated. 
     As will be apparent to the skilled artisan, a wide variety of “tools” can be substituted for the microgrippers referred to hereinabove and commonly used in MEMS assembly operations. For example, voltage probes, punches, hooks, cutting instruments etc. can all be used in place of the microgripper  20  and such additional “tools” can be stored in, for example, apertures  64  in holders  62  of the accompanying drawings or other similar retaining means. 
     In use, the appropriate “tools”, parts and other materials are located in holders  60  and  62  or upon assembly area  58  and a microgripper or other suitable assembly or testing device attached to feedthrough shaft  18 . The vacuum chamber is then sealed and evacuated in accordance with normal procedures. Once scanning is commenced, the initial MEMS part (generally attached to the assembly stage surface  58 ) is brought to the appropriate location and orientation through manipulation of stage controls  30 . Linear motion feedthrough device  16  is slowly brought into view. Careful movement of both the stage controls  30  and the linear motion feedthrough device  16  allows for the lifting of various components from holders  60  and  62  and permits them to be placed precisely in their appropriate assembly locations. 
     Attachment of the various parts being assembled can be achieved in a variety of ways, for example, by friction fit, through the use of vacuum stable thermoset or thermoplastic polymers, heat, electron beam or UV cured, or by soldering or brazing. 
     During assembly, the force used to press or hold the parts in place can be measured by load cell  36  and acoustic emission sensor  34  can be used to provide an audible confirmation of contact between parts, as can the previously described conductive path technique. 
     As will be apparent to the skilled artisan, more than one linear motion feedthrough device  16  could be installed in a single MEMS assembly device according to the present invention. 
     Among the myriad of sensors and machines that MEMS assembly device  10  could be used to assemble are included, but not limited to: accelerometers such as those used in airbags, automobile control, pacemakers, games, automotive brakes, image stabilizers and other inertial measurement systems, so-called laboratory on a chip type sensors; flow sensors, optical switches, projection and handheld displays; pressure sensors such as those developed for tire pressure and other automotive and industrial applications; miniature read/write heads; cell phone parts; MEMS devices for radar applications and steerable antennas; microrelays; and a variety of sensors to measure such physical properties as humidity, temperature, vibration etc. 
     While a significant advantage of MEMS assembly device  10  of the present invention is that assembly of prototype and production micro-devices can be made manually guided by the high resolution and depth of focus of the SEM, the device is also capable of being partly or entirely automated for high volume production and testing. 
     Through the use of appropriate tools, MEMS device  10  is capable of being used to test virtually any material or device that can be attached to assembly stage  26  in tensile, creep, or compression loading. Among the tests possible is the measurement of properties of virtually any component manufactured using the MEMS process where the smallest dimension needed to be resolved is greater than about 0.05μ. This would include the testing of gears, posts, and other structural members. MEMS device  10  can also be used to measure the mechanical properties of coatings or films with such tests as scratch tests, impact tests and hardness tests. Such testing would not only disclose the information about the strength of such materials, but it would also provide micromechanical information leading to a micromechanical understanding of the mode of failure which is sometimes the most valuable piece of information. For example, placing a microhardness indentor in place of the microgrippers described above allows for the measurement of microhardness while observing the sample at the SEM level. Additionally, the fine control possible with MEMS device  10  will allow the measurement of nanohardness. Furthermore, all such tests can be done at elevated temperature and all while imaging the test while it is in progress to detect minute changes such as detecting if failure occurs through the formation of a crack at a load point rather than through deformation. Other tests that can be performed include a variety of fiber tests such as 3-point and 4-point bend tests, crush tests, and tensile tests. Abrasion testing can also be performed by continually increasing the load placed by an indentor while simultaneously moving the sample beneath the indentor all while the micromechanics of the test are being observed. 
     As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.