Abstract:
An intermediate transfer surface includes a substrate, a two-dimensional array of electrodes, a dielectric spacer layer on the two-dimensional array of electrodes, and a voltage controller electrically connected to the array of electrodes. A method of manufacturing an intermediate transfer surface, depositing an array of etch stops on a conductive surface, etching the conductive surface to form mesas of the conductive surface separated by gaps, and coating the mesas with a dielectric coating. A microassembly system includes an assembly surface having a first two dimensional array of potential wells on a first surface, a first voltage source electrically connected to the first array of potential wells, an intermediate transfer surface having a second two dimensional array of potential wells on a second surface arranged to face the first surface, and a second voltage source electrically connected to the second array of potential wells.

Description:
TECHNICAL FIELD 
       [0001]    These disclosures relate to microassemblers, more particularly to microassemblers that use assembly templates. 
       BACKGROUND 
       [0002]    Microassembler systems use a dynamic, electrostatic template, such as a two-dimensional array of controllable voltage electrodes to create electric field patterns. The field patterns manipulate and align particles or chiplets, typically suspended in a solution, into an assembly. The pitch of the electrodes defines the design grid of the assembly and determines at what resolution a designer may be able to place and align particles in an assembly. The design grids may have a scale of 100s of microns down to a micron. 
         [0003]    Assemblers can assemble a single particle type, or could be used to assemble different particle types. Assembling multiple particle types can be done simultaneously using feedback control to appropriately place different types of particles or in a sequence of assembly steps. In all these cases, a challenge exists when transferring an assembly from the assembly template to a final substrate, if a dielectric layer covering the assembly template does not become the final substrate. Maintaining fidelity of the assembly presents a first level of challenge, which increases when the process involves multiple assemblies that require proper alignment with high fidelity. 
       SUMMARY 
       [0004]    A first embodiment is an intermediate transfer surface includes a substrate, a two-dimensional array of electrodes, a dielectric spacer layer on the two-dimensional array of electrodes, and a voltage controller electrically connected to the array of electrodes. A second embodiment is a method of manufacturing an intermediate transfer surface, depositing an array of etch stops on a conductive surface, etching the conductive surface to form mesas of the conductive surface separated by gaps, and coating the mesas with a dielectric coating. A third embodiment is a microassembly system that includes an assembly surface having a first two dimensional array of potential wells on a first surface, a first voltage source electrically connected to the first array of potential wells, an intermediate transfer surface having a second two dimensional array of potential wells on a second surface arranged to face the first surface, and a second voltage source electrically connected to the second array of potential wells. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIGS. 1-3  show cross sectional views of an assembly template and an intermediate transfer surface. 
           [0006]      FIGS. 4-6  show cross sectional view of an assembly template and an intermediate transfer surface for multi-element transfers. 
           [0007]      FIG. 7  shows an embodiment of a local optical cure on an intermediate transfer surface. 
           [0008]      FIG. 8  shows an embodiment of a transfer from an intermediate transfer surface to a final substrate. 
           [0009]      FIGS. 9-12  show an embodiment of a process of manufacturing an intermediate transfer surface. 
           [0010]      FIGS. 13-15  show an embodiment of a transfer between an assembly surface and an intermediate transfer surface with contact. 
           [0011]      FIGS. 16-18  show embodiments of control electrode architectures for an intermediate transfer surface. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0012]    A microassembler system uses a dynamic electrostatic template, which typically consists of two-dimensional array of voltage electrodes controlled by software. The software controls the electrodes to create electric field patterns. The field patterns manipulate and align particles or chiplets, suspended in a solution, into an assembly. For purposes of this discussion, the particles or chiplets suspended in solution will be referred to as elements or assembly elements. The pitch of the electrodes defines the design grid of assembly and determines with what resolution a designer might be able to place and align particles in an assembly. The challenge lies in transferring an assembly from the assembly template to the substrate. 
         [0013]      FIG. 1  shows a cross-sectional view of a microassembly system  10  employing an intermediate transfer surface. The system includes an assembly plane  12 , controlled by some sort of voltage controller  24 . Elements such as  18  and  26  are attracted to the electrostatic potential wells between the electrodes such as  16 . Once all of the elements are placed on the assembly plane they form an assembly. The assembly would typically be transferred to the final substrate. Registration when transferring to the final substrate can be difficult to maintain. A thin dielectric separator  11  will typically be on the assembly plane  12  and another  13  on the multipass transfer plane  14 . For simplicity in the drawings, these layers are only shown in  FIG. 1 , but must be understood to exist in all embodiments of both surfaces. 
         [0014]    The embodiments here make it possible to maintain the registration of the elements to one and other by using an intermediate transfer surface. The intermediate transfer surface  14  shown in the embodiment of  FIG. 1  also has a two dimensional array of potential wells, in this embodiment formed by an array of electrodes such as  20 . One should note that because of the cross-sectional view, the electrodes appear to be only one line, but are actually repeated going away from the front of the drawing. In  FIG. 1 , a computer or other voltage controller actuates the assembly template to assemble the elements into their desired locations and orientations. In addition, the elements could extend over multiple electrodes and have charge patterns commensurate with the grid. 
         [0015]    In  FIG. 2 , the assembly transfers to the intermediate transfer surface. In some embodiment, the transfer surface may be referred to as a multipass transfer surface, as it may be employed multiple times. In  FIG. 2 , the intermediate transfer surface  14  moves towards the assembly surface  12 , which assists in maintaining fidelity of the original assembly. Similar to xerography, the template voltage control  24  can be selectively reversed to repulsively assist in the transfer of the particles to the transfer assembly. Because mechanical transfer can disturb the assembly, maintaining a small gap and then electrostatically levitating the elements from the assembly template to the intermediate will help maintain the fidelity. The controller  22  can increase the amplitude of the electric field on the intermediate transfer surface while the controller  24  decreases, or reverses the polarity of, the signal on the assembly surface. The gap between the two surfaces during the transfer should be on the order of the lateral electrode spacing in order to maintain fidelity. The electrostatic field patterns of the two assemblies can be commensurate and aligned or incommensurate and uncorrelated. For example, the electrodes on the transfer assembly could be on a finer pitch and simply grab transferred particles dielectrophoretically. Dielectrophoresis (DEP) exerts a force on a dielectric particle when it is subjected to a non-uniform electric field. This force does not require the particle to be charged. All particles exhibit dielectrophoretic activity in the presence of electric fields. 
         [0016]      FIG. 3  shows the completion of the transfer. The elements are now electrostatically held on the transfer surface  14  with the same alignment as on the assembly template. 
         [0017]    As mentioned above, the transfer surface may be referred to as a multipass transfer surface.  FIGS. 4-6  show an example of this type of transfer. In  FIG. 4 , the elements  18  and  26  have been assembled into their desired positions on the assembly template and transferred to the transfer surface. Block  30  is assembled to a position between former positions of elements  18  and  26 . If the elements  18  and  26  were left on the assembly surface, it would be extremely difficult or impossible to guide the block  30  around the elements and then place it in close proximity to them. 
         [0018]    In the first pass, shown in  FIG. 5 , the elements  18  and  26  pass to the transfer surface. The block  30  then passes to the transfer surface. The resulting assembly on the transfer surface  14  is shown on  FIG. 6 . One should note that the elements shown in  FIG. 6  are symmetric elements. The assembler has the ability to place and align asymmetric objects such as micro-wires. The potential wells on the transfer surface will maintain the position and orientation of these objects as well. In addition, the dielectric separator shown in  FIG. 1  can be removable from the transfer plane to become the final substrate for the elements. 
         [0019]    One variation of this process allows the assembly to be fused together with a cure on the transfer surface. This forms a single, monolithic assembly made up of the particles assembled on the assembly surface and then transferred to the transfer surface.  FIG. 7  shows an example of this. The transfer surface  14  holds the assembly of elements  18 ,  20  and  26  with an applied voltage from the voltage source and/or controller  22 . In this embodiment, a light source  40 , such as an array of light emitting diodes (LEDs) that emit UV light  42  cure the assembly locally. 
         [0020]    Another approach could be to use patterned light from a DLP that is focused on the assembly through appropriate optics to direct optical energy only at the interfaces one would like to fuse. A local cure means that the cure energy is directed only at the interfaces that we wish to cure, rather than the entire assembly. This may be advantageous as some materials in the assembly may be sensitive to excessive energy. The local cure serves to create a single assembly of assembled particles. The local cure may only be a temporary or partial cure which still allows the cured assembly to be moved to the final substrate. 
         [0021]      FIG. 8  shows an embodiment of an application of a global cure. The locally cured assembly transfers from the transfer surface  14  to the final substrate  50  and is then globally cured into place by global cure  52 . One should note that global cure  52  and the local cure  42  may be one of many different types. The UV cure activation is just an example, the cure may be other types of radiant, thermal, chemical, etc. 
         [0022]    Regardless of how the transfer is accomplished, the control of the electrostatic fields remains an important aspect of the embodiments. Generation of the potential wells may occur in many ways. In addition, the transfer surface may constitute a planar substrate or it may mount on a roll to achieve transfer a line at a time. Manufacture of the transfer surface may occur in many different ways.  FIGS. 9-12  show one embodiment of a method of manufacturing a transfer surface. 
         [0023]      FIG. 9  shows a conductive surface  60  that can undergo texturing to form potential wells. The conductive surface  60  has formed upon it disks such as  62  that act as etch stops in  FIG. 10 . The surface then undergoes etching to produce mesas of the conductive surface such as  66  shown in  FIG. 11 . Gaps such as  64  separate the mesas filled with dielectric  68 . The disks such as  62  are removed. In  FIG. 12 , the gaps  64  in  FIG. 11  have been filled with the dielectric  68 . This may planarize the surface, or at least nearly planarize to leave small gaps between the mesas to allow for better registration of the elements when they are transferred to the transfer surface. The same or a different dielectric will also form a layer such as  11  on the mesas. Again, for simplicity, this layer is not shown in subsequent layers. 
         [0024]    With a transfer surface, however it is manufactured, one can apply a bias between the assembly template and the transfer surface, as shown in  FIGS. 13-15 . The important aspect of this embodiment is that an array of electrodes  70  will concentrate the electric field on the transfer surface at the edges of the electrodes. In this embodiment, the voltage controller  22  is in contact with both the assembly surface and the transfer surface, rather than two separate controllers. For example, the controller could be a computer connected to a voltage source or sources. In  FIG. 13 , the assembly surface  14  and the transfer surface  12  are connected to a common controller  22 . The assembly surface  12  consists of the conductive surface  60  with mesas such as  66 . The application of a bias will create areas  70  of high field gradient around each tip which will create a dielectrophoretic force. This force will attract the edges of the particles to help maintain fidelity during transfer. This will only assist with motion or perturbations in the assembly plane and will not levitate the particle  72  to the transfer plane or hold it against the transfer plane as was discussed in the previous embodiments. In this embodiment, the two planes  14  and  12  will come close enough for the assembled element to come into contact with the assembly plane as shown in  FIG. 14 , relying on van der Walls forces to hold the particle on the transfer surface. The DEP force will ensure the particles end up with the correct placement during the physical transfer. As shown in  FIG. 15 , the physical features will serve to create an electrostatic design grid on the transfer plane that will hold the fidelity of the assembly as it is transferred. This approach has the benefit of simplicity. Only a single electrical connection is needed on the transfer surface. One consideration lies with surface energies chosen in order to ensure particles will transfer from the assembly template to the transfer template. 
         [0025]    Alternatively, the surface could be created by micropatterning electrodes that have at least as fine a pitch as the assembly template electrodes. The electrode array could be patterned on a planar surface that is rigid or flexible and then mounted on a roll. The circuitry that drives the electrodes could take one of many forms depending upon the requirements of the system design, each with varying degrees of complexity.  FIG. 16  shows one example of one such circuit and its signal inputs. The polarity of the electrodes are fixed and simple circuitry will allow control over the amplitude of the electric field. The electrodes  80  and  82  have opposite polarity. The polarity of the electrodes are fixed and simple circuitry will allow control over the amplitude of the electric field. The transfer process may be controlled by the same computer system which drives the assembly template and has the ability to control the amplitude of the voltage on all electrodes on the transfer template, and as a result, the reach of the potential wells. 
         [0026]    In an alternative embodiment, the circuitry driving the electrode array would have the ability to control the amplitude of the voltage on each line in the cross-process direction of the transfer surface, shown in  FIG. 17 . The electrodes  80  and  82  still have opposite polarity, but the driving signals are on the line level, instead of on the array level. The controller will then sequentially transfer the assembly in a line by line fashion giving the system another control with which to help ensure fidelity, at the expense of complexity of the drive circuit and back plane that makes up the multipass transfer surface. 
         [0027]    Another alternative design is shown in  FIG. 18 . The circuitry driving the micropatterned electrode array used to generate potential wells would have the ability to control the amplitude of the voltage on each individual electrode, addressed by driving lines such as  90  and  92 . In this embodiment, the control computer can use the transfer surface to do sophisticated, particle-by-particle transfer that could help maintain fidelity as well. While providing the most degrees of freedom, this approach also requires the most complex circuit design and deterministic control for the transfer surface. 
         [0028]    In this manner, one can vary the design of the surface electrodes to match the desired complexity and control. The resulting microassembly system allows for successful transfer of assembled elements from an assembly surface to a final substrate, maintaining alignment and orientation without disruption. This allows for the possibility of building up multiple layers of the same type of particle in very close proximity that may not be possible in a single assembly process due to path planning and neighboring particle field limitations. It also allows for the possibility of creating heterogeneous assemblies by sequentially aligning multiple assemblies on a single surface. Once all assemblies are in place and aligned to each other on the transfer surface it is possible to use optical or thermal curing to fuse the assembly together to provide some mechanical support to maintain fidelity when the multiple assembly is transferred to the final substrate via mechanical means. 
         [0029]    It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.