Fabrication of Micrometer-Scale, PatternedPolyhedra by Self-Assembly**By David H. Gracias, Vikram Kavthekar,J. Christopher Love, Kateri E. Paul, andGeorge M. Whitesides*We recently proposed and demonstrated a strategy for fab-ricating self-assembling, three-dimensional (3D) electricalnetworks.[1]In this demonstration, we used millimeter scalebuilding blocks (polyhedra) whose faces were patterned withcopper connectors and devices (light-emitting diodes). Onesignificant hurdle to implementing self-assembly in practicalsystems is that of miniaturizing the assemblies. To do so wouldrequire us to construct building blocks similar to those of~1 mm scale,[1]but on the micrometer scale. The buildingblocks must have the following characteristics: a) polyhedralstructures, b) faces patterned with arbitrary patterns thatwould serve as connectors, and c) microelectronic devicesattached to the faces of the polyhedron.It is difficult to fabricate micrometer scale polyhedral struc-tures. Structures with these dimensions are usually fabricatedby projection lithography,[2]and this technique is inherentlyplanar. Most methods of fabrication in 3D utilize processessuch as surface micromachining[3]that are precise and versa-tile, but also expensive and limited in the range of materialsthat can be used and the types of structures that can be gener-ated. It is also difficult to generate arbitrarily patterned struc-tures in 3D or on curved surfaces. Techniques for patterninghave been limited to microcontact printing,[4±6]projection li-thography on spherical substrates using elaborate optics,[7]and shell plating onto die-cast mandrills.[8]Fabricating deviceson 3D objects is extremely difficult; this is because processes(e.g., ion implantation) used to build silicon-based microde-vices[9]are inherently planar techniques.This paper describes the fabrication of patterned polyhedra,having 100±300 lm sides, by the spontaneous folding of two-di-mensional (2D) structures under the influence of the surfacetension of liquid solder. Our examination of this approach wasstimulated by the early work of Pister[10]and Shimoyama[11]onmicromachined hinges and by the extensive research of Symsand others on the use of capillary forces in liquid solder[12±17]and similar methods for directly shrinking polymer joints[18±20]for the assembly of non-planar microstructures. The structureswe describe can be patterned and processed in 2D using con-ventional techniquesÐphotolithography, evaporation, electro-deposition, etchingÐthat have been extensively developed bythe semiconductor industry.[9]In the past, auto-folding[12±16]has been used primarily to actuate micrometer scale compo-nents in microelectromechanical systems (MEMS) devices. Inour work, we demonstrate that the self-assembling process ofauto-folding can be used as a strategy for fabricating patterned3D components from 2D precursors. We have also demonstrat-ed that it is possible to build 3D polyhedra whose faces containsingle crystal silicon chipsÐthe most primitive electronicdevice, i.e., a resistor.The approach we demonstrate has four steps: 1) The de-sired structures are designed in planar form as a series of un-connected but adjacent faces. 2) The faces are fabricated in2D on a sacrificial layer using a combination of photolithogra-phy, evaporation, etching, and electrodeposition. 3) The en-semble of faces is covered with a thin film of liquid solder bydip coating. 4) The structure is released from the substrate bydissolving the sacrificial layer, and allowed to fold under theinfluence of the surface tension of the molten solder. Thisstrategy is sketched in Figure 1.We experimented with many different materials, structures,and processes. Figure 2 shows scanning electron microscopy(SEM) images of folded metallic polyhedra and the 2D pre-cursors of these structures. The metallic faces of the polyhedracontained either holes (the trigonal pyramid in Fig. 2) or solidfaces (as seen in the tetragonal pyramid, cube, and hexagonalprism). The faces ranged in size between 100±300 lm (on aside). The 2D precursors contained faces that were nothinged; the faces were aligned as close to each other as possi-ble (given the mask and photolithographic capabilities). For200±300 lm faces, spacings between 8 and 15 lm worked well;for 100 lm faces, a spacing of 8 to 10 lm was required. Whenthe 2D structures were dipped in solder, the solder bridgedthe faces and formed a continuous layer. The 2D precursorswere released from the wafer by dissolving a sacrificial layeron which they were built. The precursors were heated abovethe melting point of the solder. The liquid solder tried to mini-mize its surface area (capillarity); this process drew the facestogether to form a compact 3D polyhedron. The equilibrated3D polyhedron was, at this point, filled with solder; the fold-ing thus worked best when the volume of the solder presentwas equal to the volume of the polyhedron.Since the volume of solder present was equal to that depos-ited on the 2D precursor, the critical step controlling the yieldof the process was the deposition of solder. We controlled theamount of solder deposited by changing the surface tension ofthe liquid solder, as well as by changing the solder±copper in-terfacial energy.[21]The surface tension of liquids decreasesapproximately linearly with increasing temperatures;[22]as aresult, when the solder dip-coating was carried out at elevatedtemperatures (100 C for a solder with melting point, m.p.,47 C), a smaller volume of solder was deposited. The solder±copper interfacial energy was also controlled using fluxes andacids that aid in cleaning organic contaminants and dissolvingoxide layers at the solder and copper surfaces. When the con-Adv. Mater. 2002, 14, No. 3, February 5 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/0302-0235 $ 17.50+.50/0 235COMMUNICATIONS±[*] Prof. G. M. Whitesides, Dr. D. H. Gracias, V. Kavthekar, J. C. Love,K. E. PaulDepartment of Chemistry and Chemical Biology, Harvard UniversityCambridge, MA 02138 (USA)E-mail: [email protected][**] This work was funded by the National Science Foundation CHE-9901358and the Defense Advanced Research Projects Agency/Space and NavalWarfare System Center, San Diego. J. C. Love thanks the U.S. Depart-ment of Defense for a graduate fellowship. The authors thank HongkaiWu, Pascal Deschatelets, and Osahon Omoregie for helpful suggestions.ditions were optimized, the solder