Abstract:
In some embodiments, provided is a microstructure assembly having a capture receptacle, a key, and an actuator associated therewith. The capture receptacle is associated with a substrate and includes alignment projections projecting upward from the base of the capture receptacle. The key is associated with a microstructure device and configured to mate in the capture receptacle, the key has alignment receptacles in a bottom surface of the key constructed to mate with the alignment projections. The actuator is adjacent to the key so as to be capable of contacting the key to trap the key against the capture receptacle.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is related to copending U.S. application Ser. No. 12/258,407 entitled IMPROVED KEY STRUCTURE AND EXPANSION ENHANCED ALIGNMENT OF SELF-ASSEMBLED MICROSTRUCTURES, by Brewer et al., filed on a date even herewith, herein incorporated by reference in its entirety. 
     BACKGROUND 
     In wafer scale integrated circuits, separate component chips may be individually integrated with a host wafer using any of several established methods for chip level integration. 
     The alignment accuracy is a critical parameter in determining the utility of this technology. The accuracy of alignment directly impacts the integration densities, interconnect line widths and pitches, and the ability to fabricate 3D stacks of chips. Furthermore, in cooperative radiating or detecting systems, alignment is critical to the functionality of the system. 
     To insure proper placement and registration of the microstructures, the microstructures are formed as geometric blocks and recesses are etched from the wafer to provide receptacle sites with geometric profiles that are complementary to the profiles of the blocks. One example is shown in U.S. Pat. No. 5,545,291, by Smith et al., entitled METHOD FOR FABRICATING SELF-ASSEMBLING MICROSTRUCTURES, herein incorporated by reference in its entirety. Fluidic self-assembly may be used to integrate the individual device microstructures into receptacle sites on host electronic circuits using a liquid medium for transport. Placement and registration of the device microstructures into receptacles on a substrate carrying electronic microcircuits is controlled by shape recognition or by selective chemical adhesion or both. 
     Other examples of microstructure placement techniques and structures include U.S. Pat. No. 7,223,635 by Brewer, entitled ORIENTED SELF-LOCATION OF MICROSTRUCTURES WITH ALIGNMENT STRUCTURES; and U.S. Pat. No. 7,018,575, by Brewer et al., entitled METHOD FOR ASSEMBLY OF COMPLEMENTARY-SHAPED RECEPTACLE SITE AND DEVICE MICROSTRUCTURES; both herein incorporated by reference in their entireties. Further examples may be found in U.S. Pat. Nos. 6,946,322 and 5,783,856, herein incorporated by reference. 
     SUMMARY 
     In various embodiments, provided is a microstructure assembly having a capture receptacle, a key, and an actuator associated therewith. The capture receptacle is associated with a substrate and includes alignment projections projecting upward from the base of the capture receptacle. The key is associated with a microstructure device and configured to mate in the capture receptacle, the key has alignment receptacles in a bottom surface of the key constructed to mate with the alignment projections. The actuator is adjacent to the key so as to be capable of contacting the key to trap the key against the capture receptacle. 
     In various embodiments, a microstructure assembly is provided having a generally circular capture receptacle associated with a substrate, the capture receptacle includes alignment projections projecting upward from a central region of the base of the capture receptacle. A cylindrical key is associated with a microstructure device and configured to mate in the capture receptacle. The key includes alignment receptacles in a bottom surface of the cylindrical key constructed to mate with the alignment projections and an alignment notch located on a periphery of the cylindrical key. An actuator is positioned to be capable of engaging the alignment notch so as to urge the projection receptacles against the alignment projections. 
     In various embodiments, a microstructure system is provided which includes a capture receptacle associated with a substrate, the capture receptacle having alignment projections projecting upward from the base of the capture receptacle. A key is associated with a microstructure device and is configured to mate in the capture receptacle. The key has alignment receptacles in a bottom surface of the key constructed to mate with the alignment projections. An actuator is located adjacent to the key and is constructed to be capable of engaging the key to trap the key in the receptacle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention will be better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1A  is a simplified illustration showing cross sectional side view of a capture receptacle in a substrate taken along the  1 A- 1 A line of  FIG. 1B . 
         FIG. 1B  is a simplified illustration showing a top view of the capture receptacle. 
         FIG. 2A  is a simplified illustration showing a cross sectional side view of the capture receptacle with a key and associated device, viewed from the  2 A- 2 A line of  FIG. 2B . 
         FIG. 2B  is a simplified illustration showing a view along line  2 A- 2 A of  FIG. 2A . 
         FIG. 3A  is a simplified illustration showing a cross sectional side view of the capture receptacle with a key and associated device, viewed from the  3 A- 3 A line of  FIG. 3B . 
         FIG. 3B  is a simplified illustration showing a view along line  3 B- 3 B of  FIG. 3A . 
         FIG. 4A  is a simplified illustration showing a cross sectional side view of the capture receptacle with a key and associated device, viewed from the  4 A- 4 A line of  FIG. 4B . 
         FIG. 4B  is a simplified illustration showing a view along line  4 B- 4 B of  FIG. 4A . 
         FIG. 5  is a simplified illustration showing a bottom view of a key of a possible embodiment. 
     
    
    
     DESCRIPTION 
     Self-assembly methods can potentially dramatically lower the costs of electronically steered antennas, mm-wave imaging systems, and MMIC technology. In areas such as phased array antennas, low cost MMIC technology, and millimeter wave imaging, the alignment accuracy of components that are assembled using self-assembly or other techniques such as automated placement, can be critically important. The alignment accuracy is a critical parameter in determining the utility of this technology since it sets the design rules for the integration process. The accuracy of alignment directly impacts the integration densities, interconnect line widths and pitches, and the ability to fabricate 3D stacks of chips. 
     Precise registration and alignment of a self-assembly structure is difficult to obtain with current systems. Prior U.S. Pat. Nos. 6,946,322, 5,545,291, 5,783,856, and 5,904,545, herein incorporated by reference, describe methods for fabricating self-assembling microstructures. The self-assembling microstructures are material blocks, or device and IC components that are either geometrically shaped or have external alignment keys for assembly and positioning. 
     In the case of the shaped blocks, these fit into recessed regions of a substrate and become integral with the substrate. The fabrication of the geometrically shaped microstructure utilizes crystallogaphically selective wet-chemical etching to tailor the sidewall profile of the microstructures. It has been realized by the present inventor that this approach has a number of disadvantages including: limited selection of sidewall profiles (difficult to implement for extremely thin device microstructures), poor orientation capability—limited selection of geometric shapes (IC or device design are typically to squares or rectangles), poor use of device and circuit area due to shaping of microstructure, limited applicability to materials that are difficult to etch (Al 2 O 3 , SiC, GaN, etc.), and poor compatibility of etch chemistry to shape the microstructure with fabricated devices and circuits on the microstructure. 
     In the case of the components with external alignment keys, the alignment accuracy is limited by the mechanical tolerance between the key and the complimentary shaped receptacle. There is a tradeoff between the optimum assembly throughput and the precision of the alignment. Typically, to achieve the highest assembly throughput the mechanical fit must be loose. 
     Some implementations provide a means for improving the alignment accuracy of keyed device or integrated circuit components that are assembled and positioned in receptacles on a substrate using self-assembly methods (i.e. chip printing or fluidic self-assembly) or other high throughput assembly means (i.e. pick and place). In various implementations and embodiments, improved alignment is desired without compromising the assembly throughput or initial orienting process. Various implementations and embodiments may be utilized to improve assembly and transfer of integrated devices or integrated circuit components on a host circuit, as well as the processes for keying the components and making the complementary receptacle arrays. 
     To facilitate manufacturing, it is desirable to achieve precise alignment without compromising the assembly throughput, and preferably without significantly changing the orientation process. One way to achieve this is to improve the alignment mechanism of the self-assembled device or IC microstructures with their complementary shaped host receptacles. The improved microstructures ultimately will be transferred and integrated into host circuits. 
     As described in further detail below, various implementations and embodiments employ components that have alignment key structures and receptacles that are complementary shaped. In one embodiment of the invention, the receptacle is fashioned with an actuating structure that is used to guide the key into a higher degree of alignment. This solves a long-standing problem of how to quickly assemble loosely fitting parts and achieve very high levels of alignment accuracy. 
     FIGS.  1 A- 5   
       FIG. 1A  is a simplified illustration showing cross sectional side view of a capture receptacle  205  in a substrate  201  viewed along the  1 A- 1 A line of  FIG. 1B .  FIG. 1B  is a simplified illustration showing a top view of the capture receptacle  205 . In the embodiment of  FIGS. 1A &amp; 1B , the capture receptacle  205  is a cylindrical cavity with alignment projections  210  projecting from a base  205   b  of the capture receptacle  205 . As used herein, the term substrate refers to any material in which one or more capture receptacles may be formed. Thus, a substrate as used herein, may be a base material, i.e. a wafer or the like, or, it may be one or more layers on a base material or carrier. 
     In this embodiment, there are two “L” shaped projections  210  arranged with the long segments  210   a  of the projections  210  generally parallel so that the projections  210  are symmetric about and axis (not shown) extending between and parallel to the projections  210 . In this embodiment, the short segments  210   b  of the projections extend in opposite directions away from each other and the axis. Thus, the “L” shaped projections  210  are “back-to-back” and spaced apart. 
     An actuator  225  is associated with the substrate  201  to enhance alignment as discussed below. The actuator  225  may be actuated by thermal, magnetic, electrical, gravitational, or capillary forces. The actuator may be on a top surface of the substrate  201 , or contained within a recess in the substrate  201 , or in the receptacle  205 , in various embodiments. 
       FIG. 2A  is a simplified illustration showing an cross sectional side view of the capture receptacle  205  with a key  255  and associated device  250  from the  2 A- 2 A line of  FIG. 2B .  FIG. 2B  is a simplified illustration of the key  255  in the capture receptacle  205  viewed along the  2 B- 2 B line of  FIG. 2A . In  FIGS. 2A and 2B , although captured by the capture receptacle  205 , the key  255  is not properly oriented within the capture receptacle  205  to allow the projection receptacles  290  ( FIG. 2B ) to mate with the projections  210 . 
     The circular cross section of the receptacle  205  allows easy capture of the key  255 . Orientation of the key  255  is accomplished by engaging the projections  210 . The key  255  is captured by the receptacle  205  and oriented until it engages the projections  210 , as shown in  FIGS. 3A and 3B . 
       FIG. 3A  is a simplified illustration showing an cross sectional side view of the capture receptacle  205  with a key  255  and associated device  250 , viewed from the  3 A- 3 A line of  FIG. 3B .  FIG. 3B  is a simplified illustration showing an oriented key  255  in the capture receptacle  205  viewed along the  3 B- 3 B line of  FIG. 3A . The mating of the projections  210  with the corresponding projection receptacles  290  fix the orientation of the key, along with the device  250 . As the key  255  becomes aligned, it drops into the receptacle  205 . This may be done by causing the key to spin, such as by vibrating of the system. Alternatives to vibration include without limitation, asymmetric fluid flow (jet of fluid in one direction on one end of a diameter, opposite direction of fluid flow on the opposite end of a diameter), applying a magnetic field to a key made responsive to magnetic fields, or any other means for causing the key to rotate. The device  250  may seat on the top surface of the substrate  201 . In other embodiments, the key  255  may drop to seat in the base  205   b  of the receptacle  205 , or the key  255  may seat on the tops  210   t  of the projections  210 . In this way, in some embodiments, the height of the device  250  can be controlled. 
     Turning to  FIGS. 4A &amp; 4B ,  FIG. 4A  is a simplified illustration showing an cross sectional side view of the capture receptacle  205  with a key  255  and associated device  250  viewed from the  4 A- 4 A line of  FIG. 4B .  FIG. 4B  is a simplified illustration showing alignment of the key  255  in the capture receptacle  205 , viewed along the  4 B- 4 B line of  FIG. 4A . After the key  255  is oriented, an actuator  225  associated with the capture receptacle  205  engages the key  255  to align the key  255  (and device  250 ), as shown in  FIG. 4B . Prior to actuation, shown in  FIGS. 2B and 3B , the actuator  225  may be withdrawn so that it does not interfere with the capture and orientation of the key  255 . The capture receptacle  205  may be fitted with an actuator  225  that changes shape or length to guide the key  255  into a high accuracy alignment. The actuator  225  urges the key projection receptacles  290  against the projections  210  and traps the key  255  in the receptacle  205 . The actuator  225  may engage the key  255  in an actuator receptacle  275 , shown in  FIGS. 2B ,  3 B, and  4 B as a “V” shaped notch in the periphery of the key  255 . 
     The actuator  225  may be, or include, a shape memory alloy, shape memory polymer, or a material that changes dimension or moves through other means, such as but not limited to swelling (solvent uptake, thermal, state change), or application of an additional magnetic or electrical force. The actuator  225  may include an alignment pin that is urged by the expansion of a material from behind, for example. Or, it may be pulled or pushed toward the actuator receptacle  275 , with one or more electrodes, a solenoidal means, piezoelectric, piezomagnetic, etc. Shown in  FIG. 1B , a controllable expansion material  225   m  is shown behind the actuator pin  225   p  to urge the actuator pin  225   p  forward to engage the key (not shown in  FIG. 1B ). 
     In other embodiments (not shown) it may be possible to utilize the actuator  225  and the actuator receptacle  275  as part of the orientation of the key  255 , to also function as a coarse alignment projection, or even a fine alignment projection, depending on its location with relation to the receptacle  205 . 
       FIG. 5  is a simplified illustration showing a bottom view of a key  355  of a possible embodiment. In  FIG. 5 , the long segments  390   a  of the projection receptacles  390  are non-parallel, i.e. the long segments  390   a  are slanted with respect to the other. In this configuration, when the actuator (not shown) engages the actuator receptacle  375 , the force of the actuator normal to the key  355  will push both legs  390   a  and  390   b  of the key  355  more directly against both legs (not shown in  FIG. 5 ) of the corresponding projections (not shown in  FIG. 5 ). 
     In contrast, in some embodiments in accordance with  FIG. 4B , as the actuator  255  slides against the tapered sidewall of the actuator receptacle  275  to a fully seated position within the actuator receptacle  275 , the key  255  moves transverse to the actuation direction of the actuator  225 , to abut the long legs  290   a  of the projections  290 . The key  255  is guided transversely by the actuator  225 . In some embodiments in accordance with  FIG. 5 , however, the key  355  is guided transversely by the legs  390   a  and  390   b  causing less stress to the actuator (not shown in  FIG. 5 ). 
     Photolithographic methods may be used to fabricate alignment key structures on circuit and device-sized components for shape matching self-assembly. Referring to  FIGS. 1A-4B , these “keyed” devices or components  250  may be designed to match corresponding receptacle  205  sites on a patterned assembly template (not shown). The alignment key enables close positioning and orientation of arrays of microstructures without shaping the microstructures themselves. The key  255  structures may be designed for 100 micron sized devices  250 . 
     In one method for fabricating the alignment key  255  structure on small device structures  250 , a photoresist process is used. The key  255  fabrication uses a single-step process in which thick SU-8 (a negative photo-epoxy resist) is applied and exposed to pattern the shape of the key  255 . After exposure, the SU-8 film is developed, the resulting structure undergoes a final hard-bake process to form the key  255  on the device  250 . 
     Expansion of the material of the key  255  also may be used to pre-align, to assist actuator alignment, or to further refine the actuator alignment of the key  255  within the receptacle  205 , as discussed in related application Ser. No. 12/258,407 by the present inventor entitled IMPROVED KEY STRUCTURE AND EXPANSION ENHANCED ALIGNMENT OF SELF-ASSEMBLED MICROSTRUCTURES, filed on a date even herewith, incorporated by reference. As discussed therein, when the key  255  swells within the receptacle  205 , it creates a tighter fit, which improves precision. In some implementations, final alignment is achieved by heating the keyed component (not shown) that is oriented in the receptacle  255 . The large difference between the thermal expansion coefficients of the SU-8 (112×10 −6 ° C. −1 ) key  155  and the silicon (2.6×10 −6 ° C. −1 ) receptacle material forms the basis of the enhanced alignment of the positioned components in the receptacles. This difference in the thermal expansion of a factor of approximately 40 allows large dimensional changes to be induced during moderate heating (150° C.). For example, a 1 millimeter key will undergo approximately a 16.8 micron expansion in diameter during a 150 degree Celsius heating, whereas the silicon receptacle will decrease in cross section diameter by approximately 0.4 microns. These dimensional changes are enhancing the alignment of a keyed component (not shown), which is captured and oriented in a receptacle  205 , to the receptacle  205  itself. 
     There are a number of possible mechanisms that can induce changes between the shape of a key  205  and a receptacle  255 . These include but are not limited to the following: thermal expansion differences between the key and the receptacle materials, swelling of the key structure by solvent uptake, solvent state change, piezoelectric, piezomagnetic, etc. 
     In some embodiments, a heating process for either/both types of alignment may be carried out solely for alignment, or as part of a solder process so that the component (not shown) is simultaneously aligned, secured, and electrically connected to a substrate (not shown). 
     Using an actuator, after capture by the receptacle facilitates greater precision alignment than is otherwise achievable with conventional alignment means. Further, when key spins within the capture receptacle, the design inhibits skewing and/or wedging of the key within the receptacle so that it does not seat cockeyed within the receptacle. 
     In some embodiments (not shown) it is possible to have the projections extend from the key, with the projection receptacles being recessed within the base  205   b  of the generally circular capture receptacle  205  expand around protruding “receptacle” projection. In some embodiments (not shown) it is possible to have key material expand around protruding “receptacle” projection. 
     The example embodiments herein are not intended to be limiting, various configurations and combinations of features are possible. Having described this invention in connection with a number of embodiments, modification will now certainly suggest itself to those skilled in the art. As such, the invention is not limited to the disclosed embodiments, except as required by the appended claims.