Abstract:
A method and apparatus for transporting and dispersing microstructures on a substrate by fluidic self-assembly. The apparatus has an assembly vessel that is tilted and rotated to apply uncaptured microstructures back onto the substrate as the assembly vessel rotates. The assembly vessel has ramp structures that collect the microstructures that have not been captured by the substrate at the lower edge of the assembly vessel, carry the microstructures as the assembly vessel rotates, and release the microstructures back on to the substrate at the upper edge of the assembly vessel. Vibrational energy may also be applied to the assembly vessel to assist in the dispersal and location of the microstructures on the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is related to and claims benefit under 35 U.S.C. 119(e) of copending U.S. Provisional Application No. 60/490,194, titled “Self-Location method and Apparatus,” filed Jul. 25, 2003. The contents of U.S. Provisional Application No. 60/490,194 are incorporated herein by reference in their entirety. 

   BACKGROUND 
   1. Field 
   The present disclosure relates to a method and apparatus for assembly of device, integrated circuit, and/or passive components on a substrate to provide hybrid electronic, optoelectronic, or other types of integrated systems. For example, the present disclosure describes a method and apparatus for transporting and dispersal of microstructures by fluidic self-assembly onto a substrate wafer. 
   2. Description of Related Art 
   Increasingly complex integrated electronic and optoelectronic systems require larger numbers of integrated circuits and devices to implement increasingly complex system functions. However, to achieve cost and weight goals, it is preferred that these integrated systems be implemented with as few separate device structures as possible. One approach is to fabricate all of the integrated circuits and devices on a single wafer or portion of a wafer, which provides the structural base for the system and minimizes the interconnect distances between circuits and devices. Such fabrication may be referred to as “wafer-scale” integration. 
   Many complex integrated electronic and optoelectronic systems require the use of integrated circuits and devices that utilize different semiconductor technologies. One approach known in the art for wafer-scale integration of different semiconductor technologies is heteroepitaxy. The heteroepitaxy approach may limit the number of different devices and material systems that can be successfully integrated. Moreover, growth and fabrication procedures optimized for a single device technology often must be compromised to accommodate dissimilar material systems. Finally, testing of individual portions of the integrated system may be made difficult by the fabrication techniques used to accommodate dissimilar material systems on a single wafer. 
   Since it may be difficult to fabricate high performance systems with multiple types of devices using heteroepitaxy approaches, it may be preferable to fabricate separate arrays of devices or circuit modules and couple these separately fabricated components to a host wafer. This approach allows each individual component to have state-of-the-art performance and high yield (due to pre-testing). Each component may use proven device and circuit architectures, while optimum epitaxial growth and/or device processing sequences are employed to fabricate each component. 
   The separate components may be individually integrated with the host wafer using any one of several established methods for chip-level integration. These methods generally rely upon surface-mounting techniques for attaching complete die assemblies using solder bumps or wire bonding. The most advanced of these methods is the “flip-chip” technique that can support integration of a wide variety of device technologies and fully utilizes the costly, high-performance device wafer real estate. However, flip-chip is generally limited to relatively large size components, typically greater than 1 square millimeter, and is inefficient for the placement of large numbers of components due to its serial nature. 
   Pick-and-place assembly techniques for positioning components with sizes less than a millimeter on a substrate are known in the art. See, for example, Saitou et al., “Externally Resonated Linear Microvibromotor for Microassembly,”  J. Microelectromech. Syst ., vol. 9, pp. 336–346, September 2000. However, these techniques are known to suffer limitations due to the surface adhesion forces based on the extremely small size of the components. Further, these techniques are also inefficient for the placement of large numbers of components, again due to the serial nature of the techniques. 
   At the wafer-scale level, self-assembly methods generally provide the best capability to allow integration of arbitrary configurations and densities of components. The most advanced of the self-assembly methods use a fluid medium to transport components to a host substrate or wafer for assembly. Two different fluidic self-assembly methods are known in the art, which differ in the underlying mechanism used to locate, position, and connect the components on the host substrate or wafer. 
   The first method of fluidic self-assembly uses gravitational forces and geometrical constraints to integrate components with a host substrate. The components are fabricated with specific shapes and complementary shaped receptacles are formed on the substrate for receiving the shaped components. The components are typically formed using semiconductor fabrication techniques and the receptacles are formed by using wet or dry etching techniques. A solvent such as water or ethanol is used to transport the individual components to the host substrate with the receptacles. The receptacles trap the components, which come to rest in predictable orientations due to their specific shapes. The driving potential is primarily gravitational in origin, but the fluid and surface forces may also play a role in the assembly process. 
   The second method of fluidic self-assembly utilizes chemically-based driving forces to govern the assembly process, where the attraction, positioning, orientation, and ordering of components is controlled by molecular interactions at the surfaces of the components and the host substrate. Molecular-based self-assembly techniques generally use surface coatings that consist of chemically-bonded films which are either hydrophobic or hydrophilic by nature. Thermodynamic driving forces control the assembly of complex arrays of components by minimizing the surface energies of the components and host substrate. 
   Both methods may be used together to provide for integration of electronic and opto-electronic devices into hybrid electronic systems. See, for example, A. Terfort, et al., “Self-Assembly of an Operating Electrical Circuit Based on Shape Complementarity and the Hydrophobic Effect,”  Adv. Material,  10, No. 6, 1998, pp. 470–473. See also A. Terfort, et al., “Three-dimensional Self-Assembly of Millimeter-scale Components,”  Nature , Vol. 386, Mar. 13, 1997, pp. 162–164. 
   Various apparatus and methods are known in the art for assembling microstructures onto a substrate through fluid transport. For example, U.S. Pat. No. 5,904,545, issued on May 18, 1999, to Smith et al. describes an apparatus used for fabricating electronic systems using fluidic self-assembly methods. A schematic of the apparatus is depicted in  FIG. 1 . 
   The Smith apparatus, as shown in  FIG. 1 , consists of a vessel that contains the substrate that is to receive the microstructures, a fluid medium with the microstructures therein, and a pumping system. The pumping system uses gas bubbles to circulate the fluid and microstructures throughout the system. A funnel shaped drain collects and concentrates the microstructures that have not been assembled onto the substrate, and directs them for re-circulation to a column where bubbles are injected. The bubbles push the fluid and microstructures up the return line and the fluid containing the microstructures is then re-dispersed over the substrate through the spout. Changing the gas flow into the return line controls the pump rates. 
   However, problems associated with assembling microstructures using the Smith apparatus are reported in the Ph.D. thesis of Mark Hadley (University of California—Berkeley, 1994). In that thesis, it is disclosed that tests were performed using large (1.2 mm×1.0 mm×0.235 mm) and small (150 micron×150 micron×35 micron) microstructures with substrates having complementary shaped receptacle holes. Assembly tests were performed with the Smith apparatus using 500 large Si blocks with a substrate having 191 receptacles. Tests were also performed with 30,000 small Si blocks with a substrate having ˜4096 receptacles. When using water as the transport fluid, bubbles were found to attach to the Si blocks causing them to float. For the larger blocks, the addition of a surfactant, which altered the surface properties of the microstructures, was found to stop the attachment of the bubbles. However, smaller microstructures could not escape the forces at the water/air interface. Assembly of small microstructures required non-aqueous media such as ethanol or methanol. The results disclosed in the Hadley thesis were based on the use of a gravity-based process that employed shape matching between the microstructure blocks and the substrate receptacles. 
   As briefly mentioned above, other procedures for assembling microstructures do not rely on gravity to aid the placement of the microstructures on the substrate. Some procedures employ selective surface coatings (i.e., rendering surfaces hydrophobic or hydrophilic) on the microstructures and/or the substrate to guide the placement. See, for example, Gracias, et al., “Forming Electrical Networks in Three Dimensions by Self-Assembly,”  Science , Vol. 289, Aug. 18, 2000, pp. 1170–1172. Other procedures employ long-range forces, such as electro-static attraction, for placement. Generally, these procedures require water as the transport medium and cannot tolerate air/water interfaces (i.e., no bubbles). These interfaces are extremely high-energy surfaces that strongly attract microstructures, which leads to clumping or trapping of the coated microstructures. This behavior decreases the efficiency of the location process. 
   Another procedure for assembling microstructures on a substrate involves applying the microstructures in a fluid medium through the use of a pipette. See, for example, Srinivasan, et al., “Microstructure to Substrate Self-Assembly Using Capillary Forces,”  J. Microelectromech. Syst ., vol. 10, pp. 17–24, March 2001. Those skilled in the art will understand that this procedure does not lend itself to a manufacturing environment in which large numbers of microstructures are to be assembled on multiple substrate wafers. 
   Therefore, there exists a need in the art for a method and apparatus that facilitates the transport and dispersal of microstructures onto a substrate for fluidic self-assembly. There exists a further need for a method and apparatus that allows for the microstructures to be repeatedly and at least somewhat uniformly dispersed over a substrate wafer. There exists a further need for a method and apparatus that eliminates or, at least, minimizes air/fluid interfaces to avoid the attractive forces between such interfaces and small-sized microstructures. Finally, there exists a need for a method and apparatus for fluidic assembly that may support the assembly of large numbers of microstructures on multiple substrates in a manufacturing environment. 
   SUMMARY 
   Embodiments of the present invention provide a method and apparatus for self-assembly of electronic micro-systems on a wafer-scale level. These embodiments provide for the transportation, dispersal, and positioning of individual device and integrated circuit microstructure components into host circuits for fabricating mixed-technology systems or to populate assembly templates for subsequent wafer-scale component printing. The embodiments provide an environment to efficiently transport device microstructures within a liquid medium and to position the microstructures in or on host substrates. 
   Embodiments of the present invention do not require the flow of the liquid medium or internally moving parts or external pumps to circulate the microstructures over the surface of a host substrate. Instead, gravity is used to mechanically manipulate the microstructures through a set of ramps that are generally disposed circumferentially inside an assembly vessel. The host substrate is disposed within the center of the assembly vessel. By disposing the assembly vessel at an incline and rotating the inclined vessel, the ramps act to gradually move the microstructures upward, collecting the unincorporated microstructures from the bottom portion of the vessel and releasing them over the top of substrate. 
   Preferably, low friction, low energy surface coatings are used on the interior surfaces of the assembly vessel. Further, the use of low frequency vibration of the assembly vessel is also preferred to facilitate the sliding of the microstructures across the substrate and to help prevent adhesion of the microstructures to the walls of the vessel. The vessel may be filled entirely with a fluid to exclude air from within the assembly vessel. 
   According to embodiments of the present invention, the unincorporated microstructures may be continuously passed over the surface of the substrate until all of the positions on the substrate to be filled are filled or until a desired yield is reached. Changing the rotation speed of the assembly template assists in controlling the flow rates and patterns of the microstructures over the substrate surface. Hence, a uniform or near uniform distribution of the microstructures across the substrate surface may be achieved. 
   Embodiments of the present invention may be used with a variety of liquids, such as water, methanol, etc., to assemble microstructures using both gravity- and chemical coating-based self assembly processes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  (prior art) is an illustration of an apparatus for assembling microstructures onto a substrate using processes known in the art. 
       FIG. 2  is a photograph of a substrate with a molded receptacle layer containing several microstructures. 
       FIG. 3  shows an assembly vessel according to an embodiment of the present invention with microstructures disposed within the vessel for assembly onto a substrate. 
       FIG. 4  is an exploded view of the assembly vessel depicted in  FIG. 3  showing some components of the vessel in additional detail. 
       FIG. 5  shows a closer view of the assembly vessel depicted in  FIG. 3 . 
       FIG. 6  shows the assembly vessel depicted in  FIG. 5  with the cover and sight glass removed. 
       FIG. 7  show the assembly vessel depicted in  FIG. 6  with the substrate ring removed. 
       FIG. 8  shows the assembly vessel depicted in  FIG. 7  with the assembly substrate removed. 
       FIG. 9A  shows a close-up view of a portion of the assembly vessel depicted in  FIG. 7 . 
       FIG. 9B  shows a further close-up view of a portion of the assembly vessel depicted in  FIG. 9A  illustrating the path that a microstructure takes during the assembly process. 
       FIG. 10  shows a cross-section view of the assembly vessel depicted in  FIG. 5 . 
       FIG. 11  shows a perspective view of an assembly structure holding an embodiment of the assembly vessel according to the present invention. 
       FIG. 12  shows another perspective view of the structure shown in  FIG. 11 . 
       FIG. 13  shows a close-up view of the rotation and vibration components (with the assembly vessel removed) of the assembly structure depicted in  FIGS. 11 and 12 . 
       FIG. 14A  shows a close-up view of an alternative embodiment of the present invention in which no substrate ring is used to transition between the ramp structures and the substrate wafer. 
       FIG. 14B  shows a further close-up view of the portion of the assembly vessel depicted in  FIG. 14A  illustrating the path that a microstructure takes during the assembly process in this embodiment. 
       FIG. 15  shows a cross-section of the assembly vessel depicted in  FIGS. 14A and 14B . 
       FIG. 16  shows an embodiment of an assembly vessel according to another embodiment of the present invention in which pixel guides are used. 
       FIG. 17  shows an embodiment of an assembly vessel according to another embodiment of the present invention in which different length pixel guides are used. 
   

   DETAILED DESCRIPTION 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Further, the dimensions of layers and other elements shown in the accompanying drawings may be exaggerated to more clearly show details. The present invention should not be construed as being limited to the dimensional relations shown in the drawings, nor should the individual elements shown in the drawings be construed to be limited to the dimensions shown. 
   As used herein, “microstructures” are used interchangeably with “components,” “pixels,” “integrated circuit components,” “electronic devices,” or “semiconductor structures” and generally refer to devices fabricated by semiconductor techniques that are to be assembled on a separate host structure. Further, as used herein, “receptacle” is used interchangeably with “receptacle site” and generally refers to a region on a host substrate at which a microstructure is to be positioned. 
     FIG. 3  shows an embodiment of an assembly vessel  100  according to the present invention.  FIG. 3  shows the use of ramp structures  200  within the assembly vessel  100  to deliver microstructures  50  to the surface of a substrate wafer  500 .  FIG. 3  shows that the assembly vessel  100  provides for assembly of the microstructures  50  on the substrate wafer in desired locations.  FIG. 3  also shows that the use of the ramp structures  200  to capture the microstructures  50  at the bottom of the substrate wafer  500 , to carry the microstructures  50  to the top of the wafer  500 , and to release the microstructures back onto the surface of the wafer  500  at its top. Components of the assembly vessel  100  will be described in additional detail below. 
     FIG. 4  presents an exploded view of the assembly vessel  100  depicted in  FIG. 3 .  FIG. 4  shows that the assembly vessel  100  comprises a vessel body  150 , a vessel cover  110 , a sight glass  120 , a cover seal  115 , and a wafer ring  130 .  FIG. 4  also shows an axle  155 , attached to the vessel body  150 , that provides the axis around which the assembly vessel  100  may be rotated.  FIG. 4  also shows a substrate wafer  500  that is positioned on the wafer supports  250  within the vessel body  150 . The ramp structures  200  disposed within the vessel body  150  are described in additional detail below. 
   Preferred embodiments of the assembly vessel  100  comprise precision-machined metal parts. However, those skilled in the art will understand that other embodiments of the assembly vessel may be manufactured from molded metal, plastic, or other materials. Further, the embodiments depicted and described within the present disclosure generally present embodiments configured to handle 3 inch wafers. Those skilled in the art will understand that other embodiments may be scaled up or down to handle larger or smaller-sized wafers. 
   The assembly vessel  100  is made ready for the self-assembly process by first placing the substrate wafer  500  to be populated with the microstructures  50  on the wafer supports  250 . The substrate ring  130  is then disposed around and above the edge of the substrate wafer  500 . A liquid to facilitate the self-assembly process is then poured into the assembly vessel  100 . The microstructures  50  to be applied to the substrate wafer  500  may be already contained within the liquid or separately placed in the assembly vessel  100 . The cover  110 , sight glass  120 , and cover seal  115  are then placed on the vessel body  150  and fastened so as to seal the assembly vessel  100 . The cover seal  115  assists in seating the sight glass  120  and the vessel cover  110  on the vessel body  150 . The cover seal  115  also assists in making sure that all air is forced out of the assembly vessel  100  when the vessel  100  is sealed. 
     FIG. 4  shows screw openings  112  in the cover  110  and screw holes  152  in the vessel body  150  to allow the cover  110  to be fastened to the vessel body  150 . However, other means for fastening the cover  110  to the body  150  may be used. As briefly indicated above, it is preferred that a tight seal between the cover  110  and the body  150  be achieved to allow air to be forced from liquid contained within the assembly vessel  100  and to prevent leaks of the liquid. Of course, the vessel body  150  and other components of the assembly vessel  100  are also preferably manufactured to achieve the desired tight seal. 
   The sight glass  120  shown in  FIG. 4  allows the assembly process to be viewed while the assembly vessel is being rotated (described in additional detail below). However, to simplify the fabrication of the assembly vessel  100 , the sight glass  120  may be eliminated and the cover  110  fabricated as a single piece to cover and seal the assembly vessel  100 . 
     FIG. 5  provides a close-up view of the assembly vessel  100  when sealed for the assembly process.  FIG. 6  shows the assembly vessel with the cover  110 , sight glass  120 , and cover seal  112  removed.  FIG. 5  shows the use of screws  113  to fasten the cover  110  to the vessel body  130 .  FIGS. 5 and 6  illustrate the ramp structures  200  used to transport microstructures around the substrate wafer  500  and deliver the microstructures  50  to the wafer  500  during the assembly process. 
   Preferably, the ramp structures  200  are disposed in a continuous fashion around the inner wall  142  of the vessel body  150 , as shown in  FIGS. 5 and 6 . Each ramp structure  200  comprises a receiving ramp portion  210 , a ramp transition portion  220 , a delivery ramp portion  230 , and a ramp wall  240 . The outer wall of each ramp portion  200  is defined by the inner wall  142  of the vessel body  150 .  FIGS. 5 and 6  also show a portion of the inner wall  205  of each ramp portion  200 . These portions will be described in additional detail below. 
     FIG. 5  also shows the vessel axle  155  and vessel rotator  910 . The vessel rotator  910  provides that the assembly vessel  100  may be continuously rotated at a selected speed. As will be described in additional detail below, the rotation of the vessel  100  provides that the microstructures  50  may be applied across the surface of the substrate wafer  500 . 
     FIG. 7  shows the assembly vessel  100  with the substrate ring  130  removed.  FIG. 7  also shows the complete inner wall  205  of each ramp portion  200 , which is defined by a wall that projects from the top of the delivery ramp portion  230  to the floor  154  of the vessel body  150 . Preferably, when installed, the outer edge of the substrate ring  130  rests against or nearly against the inner wall  205  of each ramp portion  200 . The substrate ring  130  preferably has a thickness that is greater than the horizontal gap between the ramp portion inner wall  205  and the outer edge of the substrate wafer  500 . The substrate ring  130  rests on the wafer supports  250 . The wafer supports  250  have a ring support portion  252  that holds the substrate ring  130  above the surface of the substrate wafer  500  such that there is a vertical gap between the bottom of the substrate ring  130  and the top of the substrate wafer  500 . 
     FIG. 8  shows the assembly vessel  100  without the substrate wafer  500  and substrate ring  130 . Particularly,  FIG. 8  shows the wafer supports  250  disposed on the floor  154  of the vessel body  150  that are used to hold the substrate wafer  500 . The wafer supports  250  comprise the ring support portion  252  that holds the substrate ring  130 , as described above, and a lower support portion  254  that holds the substrate wafer  500 . Preferably, the lower support portion  254  holds the substrate wafer  500  above the floor  154  of the vessel body  150  so that there is a vertical gap between the floor  154  and the bottom of a substrate wafer  500 . Preferably, this vertical gap is sized so as to allow the microstructures  50  to be positioned on the substrate wafer  500  to move beneath the wafer  500 .  FIG. 8  shows four wafer supports  250 , but those skilled in the art will understand that the wafer supports  250  may number more than or less than four. Those skilled in the art will also understand that other embodiments of the present invention may use different wafer supports than those depicted in  FIG. 8  or that no wafer supports may be used. 
     FIG. 9A  shows a close up view of several of the ramp structures  200  according to an embodiment of the present invention.  FIG. 9A  shows a portion of a substrate wafer  500  positioned on a wafer support  250  with no substrate ring  130 . As previously described, each ramp structure  200  comprises a receiving ramp portion  210 , a ramp transition portion  220 , a delivery ramp portion  230 , and a ramp wall  240 . The ramp structure  200  provides a continuous path from at or below the floor  154  of the vessel body  150  to a position above the wafer ring  130 . 
   As can be seen in  FIG. 9A , the receiving ramp portion  210  projects upwards from the floor  154  of the vessel body  150  towards the inner wall  142  of the vessel body. The receiving ramp portion  210  may additionally comprise a recess  212  at the base of the receiving ramp portion  210 . The recess  212  projects beneath the floor  154  of the vessel body  150  to facilitate the capture of the microstructures  50  falling from the surface of the substrate wafer  500  or moving along the surface of the floor  154  of the vessel body  150 . 
   The delivery ramp portion  230  projects from the inner wall  142  of the vessel body  150  to a position at and above the edge of the wafer ring  130 . The ramp transition portion  220  provides a smooth transition from the receiving ramp portion  210  to the delivery ramp portion  230  at the inner wall  142  of the vessel body  150 . The ramp wall  240  provides that the microstructures will properly traverse the ramp structure  200  as the assembly vessel  100  rotates. 
     FIG. 9B  shows the paths that a microstructure  50  will follow as the assembly vessel  100  rotates in a counterclockwise manner (shown by line  199 ). Those skilled in the art will understand that the ramp structure  200  shown in  FIG. 9B  should be constructed with the receiving ramp and delivery ramp portions  210 ,  230  swapped if the assembly vessel  100  is rotated in a clockwise manner. As noted above, the assembly vessel  100  is generally oriented at an angle so that as the assembly vessel  100  rotates, each ramp structure  200  will, at one point in time, be located below the substrate wafer  500  and, at another point in time, will be located above the substrate wafer  500 . 
   As shown in  FIG. 9B , curve  200   a  shows the path that the microstructure  50  will take from the surface of the substrate wafer  500  to the receiving ramp portion  210  when the ramp structure  200  is rotated so that the ramp structure  200  is located below the substrate wafer  500 . At this point, the angle of orientation of the assembly vessel  100  is preferably such that the microstructure will continue to move in the direction of line  200   b  towards the inner wall  142  of the vessel body  150 . As the assembly vessel  100  rotates, the microstructure  50  will move towards the ramp wall  240  as shown by line  200   c . As the ramp structure  200  is rotated to be above the substrate wafer  500 , the microstructure  50  will move in the direction indicated by line  200   d . When the microstructure  50  leaves the top of the delivery ramp portion  230 , it will first fall onto the wafer ring  130  as shown by curve  200   e  in  FIG. 9B . The microstructure  50  will then fall from the wafer ring  130  onto the substrate wafer  500  as shown by curve  200   f . Gravity will then cause the microstructure  50  to move across the surface of the substrate wafer  500 , where the microstructure  50  may be captured and located at a specific position on the wafer  500 , or fall to the bottom of the assembly vessel  100 , where the microstructure  50  will be captured by one of the ramp structures  200  for transport back to above the substrate wafer  500 . 
   To further illustrate the embodiment of the invention depicted in  FIG. 5 ,  FIG. 10  presents a cross-sectional view of that embodiment.  FIG. 10  shows the preferable disposition of the cover seal  115  being beneath the cover  110  and sight glass  120 .  FIG. 10  also shows that the receiving ramp portion  210  of the ramp structure  200  preferably projects slightly below the floor  154  of the vessel body  150 . The delivery ramp portion  230  ends at a height above the wafer ring  130  and the substrate wafer  500 .  FIG. 10  also shows the preferred gap between the wafer ring  130  and the substrate wafer  500  and the gap between the substrate wafer  500  and the floor  154  of the vessel body  150 . 
   Preferably, all surfaces within the assembly vessel  100  which may be contacted by the microstructures  50  during the assembly process are coated with or comprise low friction, low energy surface coatings to facilitate the sliding of the microstructures  50  within the assembly vessel  100  and to prevent adhesion of the microstructures  50  to the surfaces within the assembly vessel  100 . For example, surface coatings of SU-8 epoxy may be used on all internal surfaces of the assembly vessel  100 . SU-8 epoxy is a low friction coating that should prevent unwanted adhesion, should facilitate sliding of the microstructures, and provide a soft material that will reduce the possibility of damage to the microstructures  50  while they are being moved within the assembly vessel  100 . 
   The assembly vessel  100  is also preferably filled with a fluid and sealed so as to force all air out of the assembly vessel  100 . Preferably, the fluid includes a surfactant, such as polyoxyethylene (2) sorbitan monolaurate, which is commercially available under the tradename Tween® 20. 
   The apparatus  100  described above and shown in  FIGS. 3–10  uses the substrate ring  130  to provide a transition between each delivery ramp portion  230  and the surface of the substrate wafer  500 . However, alternative embodiments of an apparatus according to the present invention may eliminate the use of this substrate ring.  FIG. 14A  shows a portion of the assembly vessel  100  according to the present invention in which no substrate ring  130  is used. 
   In  FIG. 14A , the outer edge of the substrate wafer  500  is disposed adjacent to the inner wall  205  of each ramp portion  200 . Preferably, the spacing between the outer edge of the substrate wafer  500  and each inner wall  205  is less than the sizes of the microstructures to be positioned on the substrate wafer  500 . Those skilled in the art will understand that such a close spacing may require precise fabrication or machining of the apparatus  100  and precise fabrication of the substrate wafer  500 , but such precise fabrication and/or machining techniques are well known in the art. 
   Not shown in  FIG. 14A  are the wafer supports  250  and the recesses  212  that may be disposed at the bottom of each receiving ramp portion  210 . The wafer support elements  250  may be similar to those as earlier described, but sized so that the substrate wafer  500  is elevated more in relation to the floor  154  of the vessel body. The recesses  212  may be used to further facilitate the capture of the microstructures  50  as they slide off the substrate wafer  500 . 
     FIG. 14B  depicts the path that a microstructure may take when the assembly vessel  100  shown in  FIG. 14A  is rotated. Curve  299   a  shows the path that a microstructure  50  will take from the surface of the substrate wafer  500  to the receiving ramp portion  210  when the ramp structure  200  is rotated so that the ramp structure  200  is located below the substrate wafer  500 . At this point, the angle of orientation of the assembly vessel  100  is preferably such that the microstructure  50  will continue to move in the direction of line  299   b  towards the inner wall  142  of the vessel body  150 . As the assembly vessel  100  rotates, the microstructure  50  will move towards the ramp wall  240  as shown by line  299   c . As the ramp structure  200  is rotated to be above the substrate wafer  500 , the microstructure  50  will move in the direction indicted by line  299   d . When the microstructure  50  leaves the top of the delivery ramp portion  230 , it will fall onto the substrate wafer  500  as shown by curve  299   e . Gravity will then cause the microstructure  50  to move across the surface of the substrate wafer  500 , where the microstructure  50  may be captured and located at a specific position on the wafer  500 , or fall to the bottom of the assembly vessel  100 , where the microstructure  50  will be captured by one of the ramp structures  200  for transport back to above the substrate wafer  500 . 
   To further illustrate the embodiment depicted in  FIGS. 14A and 14B  and its difference from the earlier described embodiment,  FIG. 15  presents a cross-sectional view of the embodiment shown in  FIGS. 14A and 14B . As can be seen from  FIG. 15 , there is no substrate ring  130  and the outer edge of the substrate wafer  500  is adjacent to the inner wall  205  of each ramp portion  200 .  FIG. 15  also shows the wafer supports  250  that are disposed on the floor  154  of the vessel body  150  to elevate the substrate wafer  500  from the floor  154 . 
   The assembly vessel  100  is preferably disposed at an angle to the horizontal plane, so that gravity facilitates the movement of the microstructures  50  across the substrate wafer  500  as the assembly vessel  100  rotates.  FIG. 11  shows an assembly structure  900  adapted to dispose the assembly vessel  100  at a desired tilt angle. As discussed in more detail below, the assembly structure  900  may also have components that provide for both the rotation of the assembly vessel  100  and vibration of the vessel  100 . 
     FIG. 11  shows the assembly structure  900  comprising a base  901 , two vertical supports  903  projecting from the base  901 , a pivot axle  905 , and a vessel bracket  907 . The pivot axle  905  supports the vessel bracket  907  from the vertical supports  903 . The bracket  907  pivots around the pivot axle  905 , which allows the assembly vessel  100  disposed at the end of the bracket  907  to be disposed at a wide range of tilt angles. A bolt  917  positioned through a slot  919  in one of the vertical supports  903  allows the vessel bracket  907  to be fixed at a desired pivot angle. Those skilled in the art will understand that other means may be used to position and hold the bracket  907  at a desired pivot angle.  FIG. 11  also shows the vessel vibrators  920 , which are discussed in additional detail below. 
     FIG. 12  shows the assembly structure  900  from the opposite side to highlight the components used to rotate the assembly vessel  100 . A motor  913  is coupled to a rotation mechanism  911 , which is coupled to the vessel rotator  910 . The motor  913  provides the power to rotate the vessel rotator  910  at a desired rotational rate. As shown in  FIG. 12 , the vessel rotator  910  is coupled to the vessel axle  155 , which rotates the assembly vessel  100 . Those skilled in the art will understand that other components or mechanisms may be used to rotate the assembly vessel  100 . Preferably, the assembly structure  900  is operated to complete a rotation about once per minute, but other rotational speeds may be used. 
     FIG. 13  shows a close-up view of the vessel rotator  910  and the vessel vibrators  920  (with the assembly vessel removed) of the assembly structure  900  depicted in  FIGS. 11 and 12 . Preferably, two vessel vibrators  920  are coupled to the vessel axle  155  to provide vibrational movement to the assembly vessel  100  in orthogonal directions. The use of small amplitude mechanical vibration facilitates the capture and assembly of the microstructures  50  into receptacle sites on the substrate wafer  500 . Preferable frequencies for the vibration range from 200 Hz to 600 Hz, but other frequencies may be used. The vibration also reduces the adherence of the microstructures  50  to the internal surfaces of the assembly vessel  100 . 
   The assembly vessel depicted in  FIG. 5  and described above was tested in the use of the assembly of silicon microstructures (55 microns×55 microns×20 microns) into a 30×30 array of complementary-shaped receptacle sites on a substrate wafer. The receptacle sites were micro-molded into the surface of an SU-8 coated silicon wafer. The receptacle wafer was placed into the assembly vessel along with approximately 100 microstructures in an ethanol solution containing Tween® 20 surfactant. The assembly vessel was positioned at a 45 degree angle, which allowed the microstructures not captured within the receptacle sites to easily slide off the wafer. The entire assembly vessel was also mechanically vibrated at 400 Hz. 
   In typical test runs, yields of approximately 95% for populating the arrays with the microstructures were achieved after about 5 minutes of circulating the microstructures over the receptacle array by rotating the assembly vessel.  FIG. 2  shows an example of an array filled by using the apparatus and method described above. Specifically,  FIG. 2  shows microstructures  50  positioned on the substrate wafer  500  and also shows receptacles  501  in which no microstructure  50  was positioned. The tests showed that the apparatus provides for efficient location and orientation of microstructures into receptacle sites on a substrate. 
   As the substrate wafer  500  becomes populated with microstructures  50 , the assembly process may be impeded by the presence of microstructures  50  that have been positioned on the substrate. The assembly process described above results in the microstructures  50  sliding across the surface of the substrate wafer  500 . If the microstructures  50 , when positioned in the substrate wafer, project above the substrate wafer  500 , and/or the microstructures  50  are tightly spaced on the substrate wafer  500 , the sliding of the unpositioned microstructures  50  across the substrate wafer  500  may be impeded. The result may be that, even after several rotations of the assembly vessel  100 , not all areas of the substrate wafer  500  may be populated with the microstructures  50 . This problem may be particularly seen when several different types of microstructures  50  are to be positioned on the substrate wafer  500  in several passes. While a large percentage of the microstructures  50  may be positioned during the earlier passes, the positioning of different microstructures  50  during the later passes may again be inhibited by the presence of the earlier positioned microstructures  50 . 
   This particular problem may be addressed by placing pixel guides on top of the substrate wafer  500  during the assembly process.  FIG. 16  shows a set of pixel guides  950  placed on top of the substrate wafer  500  to direct the microstructures  50  to a particular area of the substrate wafer  500 . As can be seen from  FIG. 16 , the pixel guides  950  may cover all of the substrate wafer  500 , except the portion of the substrate wafer  500  to which the microstructures  50  are to be directed. The pixel guides  950  are shaped to as to have outer edges  951  that are preferably directly adjacent to the inner walls  205  of the ramp structures. The pixel guides  950  also preferably have raised walls  953  that serve to direct the microstructures towards the desired area of the substrate wafer  500 . 
   Although  FIG. 16  depicts the pixel guides  950  as having the same lengths, the lengths of the pixel guides  950  may be varied to provide for application of the microstructures over various portions of the substrate wafer  500 .  FIG. 17  shows pixel guides  950  with varying lengths. These varying lengths allow the microstructures  50  to be applied to different areas of the substrate wafer  500 . This may then allow the direction of the microstructures  50  to particular inner areas of the substrate wafer  500  when the outer areas have been completely populated or nearly completely populated during earlier phases of the assembly process. 
   From the foregoing description, it will be apparent that the present invention has a number of advantages, some of which have been described herein, and others of which are inherent in the embodiments of the invention described herein. Also, it will be understood that modifications can be made to the method and apparatus described herein without departing from the teachings of subject matter described herein. As such, the invention is not to be limited to the described embodiments except as required by the appended claims.