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UNC-Chapel Hill PHYS 53 - Forming Electrical Networks in Three Dimensions by Self-Assembly

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Forming Electrical Networks inThree Dimensions bySelf-AssemblyDavid H. Gracias, Joe Tien, Tricia L. Breen, Carey Hsu,George M. Whitesides*Self-assembly of millimeter-scale polyhedra, with surfaces patterned with sol-der dots, wires, and light-emitting diodes, generated electrically functional,three-dimensional networks. The patterns of dots and wires controlled thestructure of the networks formed; both parallel and serial connections weregenerated.Most fabrication of microelectronic devicesis carried out by photolithography and isintrinsically two-dimensional (2D) (1). The3D interconnections required in current de-vices are fabricated by the superposition ofstacked, parallel planes and by their connec-tion using perpendicular vias (2–4). We dem-onstrate self-assembly as a strategy to forminterconnections between electronic devicesand prefabricated circuits, and to form 3Delectrical circuits.Previous uses of self-assembly to fabri-cate electronic devices include shape-directedfluidic self-assembly of light-emitting diodes(LEDs) on silicon substrates (5) and coplanarintegration of segmented integrated circuit(IC) devices (6 ) into 2D “superchips” usingcapillary forces at the surface of a flotationliquid (7 ). We demonstrate the formation oftwo classes of 3D electrical networks—par-allel and serial— by self-assembly, as an ear-ly step toward a strategy for fabricating 3Dmicroelectronic devices. The basic unit inthese assemblies is a polyhedron [a truncatedoctahedron ( TO)], on whose faces electricalcircuits are printed. In the present demonstra-tions, these circuits include LEDs to demon-strate electrical connectivity and trace thenetworks; in the future, they will includedevices with more complex functionality(e.g., processors). The LEDs are wired topatterns of solder dots on adjacent faces ofthe polyhedron. The TOs are suspended in anapproximately isodense liquid at a tempera-ture above the melting point of the solder(m.p. ⬃ 47°C), and allowed to tumble gentlyinto contact with one another. The drops ofmolten solder fuse, and the minimization oftheir interfacial free energy generates theforces that assemble the TOs into regularstructures (8). Processes based on capillaryinteractions between solder drops have beenused previously to assemble electronic andmechanical structures: examples include“flip-chip” technology (9) and the rotation ofparts of microstructures into nonplanar orien-tations (10, 11). During assembly, recogni-tion of the pattern of dots on one face by thaton another orients and registers the patterns,and generates dot-on-dot electrical connec-tions among polyhedra. Self-assembly ofpolyhedra can generate networks in which theLEDs are connected either in parallel or inseries. Figure 1 outlines both the fabricationof the patterned polyhedra and their self-assembly into 3D structures that include elec-trical networks (12).We used a scheme in which LEDs weremounted on the hexagonal faces of the TO,and the solder dots were placed on the squarefaces. To maximize the rate of self-assembly,all of the square faces of the TO had the samefourfold symmetric pattern of solder dots.With this pattern, correct registration of pat-terns on juxtaposed faces occurred in one offour indistinguishable ways; dots on the pat-terns that transformed into each other underfourfold rotational symmetry were equivalentand served the same function. On the 3 mmby 3 mm square face, the width of all of thesolder dots was ⬃1 mm (Fig. 2). A commonsize was required: the solder wetted the cop-per with a well-defined contact angle, andeach drop of the same size therefore had thesame height. Empirical testing suggested thatthe optimum distance between adjacent sol-der dots was approximately one-half theirwidth. Smaller separations resulted in electri-cal shorting between dots due to bridgingwith solder; larger separations resulted inmisalignment. We designed the shapes ofsolder dots to give an energy diagram forself-assembly having one large (global) min-imum and relatively small local minima.The wires that connect different solderdots electrically on each TO were fabricatedin the same way as the dots. When the pat-terned TOs were dipped in solder, these wireswere also covered with a solder layer. Bymaking the wires substantially narrower(⬃150 ␮m) than the diameter of the dots (⬃1mm), we limited the height of the solder filmon the wires to ⬃15% that of the dots. Whenthe faces self-assembled, the larger dots fusedinto connections, but the smaller wires didnot touch and fuse (Fig. 2C). It was, as aresult, unnecessary to insulate the wires toprevent shorting, even when they crossed onjuxtaposed faces of two TOs.We wished to demonstrate, by self-as-sembly in 3D, networks that are widelyused in current 2D IC technology. In thesesystems, pins on processors belong to oneof three groups: bus lines (driving voltage,clock), inputs, and outputs. Bus lines con-nect processors in parallel; outputs of oneprocessor connect serially to inputs of ad-jacent processors.In the pattern of solder dots (Fig. 2D), thefive dots that lie on reflection axes compriseDepartment of Chemistry and Chemical Biology, Har-vard University, Cambridge, MA 02138, USA.*To whom correspondence should be addressed. E-mail: [email protected]. 1. The procedure used to form electricalnetworks in 3D by self-assembly (12). (A)Anarray of the basic pattern of copper dots, con-tact pads, and wires was defined on a flexiblecopper-polyimide sheet using photolithographyand etching. (B) These pattern elements werecut out along the dotted line, (C) glued on thefaces of the polyhedron, and (D) LEDs weresoldered manually onto the contact pads. (E)The copper dots and wires on the TOs werecoated with solder, and self-assembly occurredin hot, isodense, aqueous KBr solution.R EPORTS18 AUGUST 2000 VOL 289 SCIENCE www.sciencemag.org1170two sets of dots differentiated by symmetry:{1} and {2, 5, 8, and 11}. During self-assem-bly, dots from one set on a TO connect to dotsfrom the same set on another TO. These dotsare used for parallel or bus-line connections.The other dots, {3, 6, 9, and 12} and {4, 7,10, and 13}, form two distinct sets related byreflection symmetry. Upon assembly, dotsfrom one set on a TO (e.g., outputs from oneprocessor) connect to those from the other seton another TO (e.g., inputs to a second pro-cessor). These dots are used for serial, input/output connections.Figure 3 shows the realization of a 3Dnetwork


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UNC-Chapel Hill PHYS 53 - Forming Electrical Networks in Three Dimensions by Self-Assembly

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