Materials for future electronics by Kwan S. Kwok The Defense Advanced Research Projects Agency A previous article in Materials Today1described (DARPA) is continuing its far-reaching vision, from DARPA’s Moletronics Program and its intense interest the invention of stealth fighters to creating a new in developing molecular-based electronic materials for use in both memory architectures and circuit class of computers for the future. Future computers elements2-9. This article is a continuation of that must be small, lightweight, low-power, and have previous one and provides an update on the program properties that will eventually yield computational and its latest accomplishments. capabilities reaching or exceeding those of the human brain. The Moletronics Program at DARPA was Memories of the future created first to optimize size, weight, power, and The primary objective of DARPA’s Moletronics Program is to deliver a prototype 16 kbit nanocomputer memory by early parallelism, and then to design the required elements 2005. This prototype will be integrated on the molecular for computation around basic building units. These scale and have dimensions of ~10 µm x 10 µm, which is will one day revolutionize the current semiconductor approximately the footprint of a human cell. As a result, the 2 industry by providing Si devices with additional target device density for the nanomemory is 100 Gbit/cm . At this level of integration, the prototype will be ten times functions that will create new applications. more dense than the dynamic random access memory (DRAM) that the Si industry projects it will deliver at the end of its roadmap in 2016. Additional objectives for the Moletronics Program’s nanomemory are that it should be nonvolatile and low-power, as well as defect- and fault- tolerant. It should also have an input/output interface with microelectronic systems and be fabricated using new techniques for hierarchical self-assembly, which will eventually provide a path to cost-effective mass manufacturing of the system. Several complementary technical approaches are being Moletronics Program, applied successfully by the Moletronics Program to Microsystems Technology Office, Defense Advanced Research Projects Agency, demonstrate molecular-scale electronic nanomemory arrays. 3701 North Fairfax Drive, These include: Arlington, VA 22203-1714, USA E-mail: [email protected] • Switches made from electrically active molecules 20 December 2003 ISSN:1369 7021 © Elsevier Ltd 2003 PROGRESS REPORT sandwiched between imprinted metal nanowires at the intersections of a nanowire crossbar array; • Arrays of nanoscale diodes and transistors formed at the intersections of doped, self-assembled semiconductor nanowires; and • Stacks of electrically active porphyrin molecules, which are used to enhance the density of conventional Si CMOS (complementary metal-oxide semiconductor) DRAM. Key inventions and innovations of the program have included: novel molecular switches and wires, molecular-scale diodes and transistors built from nanowires, plus techniques for the Fig. 1 An atomic force micrograph of an 8 x 8 crossbar circuit. precise, mass fabrication and assembly/imprinting of very large numbers of molecule-sized nanowires, new molecular The basic element in the circuit is the Pt/molecule/Ti electronic devices, and system simulations that have been junction formed at each cross-point, which acts as a written especially for the Moletronics Program. These are reversible and nonvolatile switch; 64 such switches are being used extensively to guide the research and to ensure connected to form an 8 x 8 crossbar circuit within a 1 µm2 the functionality of proposed designs and architectures in area. To demonstrate demultiplexer/multiplexer functionality advance, thereby shortening the entire development cycle. integrated with memory, a defect-free 8 x 8 crossbar was configured into a 4 x 4 memory and two 4 x 4 decoders by Molecular electronic circuits setting the resistances at specific cross-points. In the Hewlett-Packard Laboratories has developed a process to demultiplexer/multiplexer logic circuits, various address codes fabricate crossbar molecular circuits with the highest-density were used to select and transmit signals from multiple input electronically addressable memory reported to date. The lab wires to a single output wire (and/or from a single input wire demonstration circuit – a 64 bit memory using molecules as to one of several output wires) to read the resistances in the switches – occupies an area of 1 µm2,as shown in Fig. 1. The memory. bit density of the circuit is more than ten times greater than In addition to the development of high-density molecular today’s Si memory chips. It also combines both memory and electronic circuits, which form critical building blocks for a logic for the first time, using rewritable, nonvolatile molecular computing system, molecular memory devices molecular-switch devices. The circuits were fabricated using were incorporated onto CMOS to form a hybrid system. This nano-imprint lithography. approach provides an early opportunity for the inclusion of In the circuits, the bottom nanowire electrodes are defined molecular devices in conventional Si-based memory products. by imprint lithography. A molecular monolayer of rotaxane is deposited using the Langmuir Blodgett method9-11(Fig. 2). Hybrid memory devices Fabrication of the top electrodes begins with the blanket Researchers at the University of California-Riverside, North evaporation of a 7.5 nm Ti protective layer, which minimizes Carolina State University (NCSU), and ZettaCore, Inc. have subsequent disruption of the molecular monolayer and also functions as a direct electrical contact to the molecules. Patterned top electrodes of 5 nm Ti and 10 nm Pt are then fabricated by the same process, using the same imprint mold oriented perpendicularly to the bottom electrodes. Finally, reactive ion etching (RIE) is used to remove the Ti protective layer, leaving the Pt electrodes intact. The molecules and Ti layer under the Pt top electrode are protected. After RIE, the crossbar circuits with the molecular monolayer sandwiched between bottom and top nanowires remain. Fig. 2 A schematic diagram showing the circuit structure. December 2003 21 PROGRESS REPORT While the hybrid device represents an early opportunity for the inclusion of molecular devices into a computing product, a random assembly approach based on a cell containing nano-sized components, or NanoCell, represents a bold new way to achieve manufacturing and assembly. The NanoCell concept combines present Si-based technology and that based purely on molecular switches and wires. If successful, the NanoCell random assembly approach will revolutionize the way in which computers are built and Fig. 3 A porphyrin memory molecule. circuits are reconfigured and perform computation. recently demonstrated the first fully-functional hybrid NanoCells for molecular computing CMOS/molecular memory device12. The development of Nanoelectronic architectures could prove themselves capable hybrid devices is an important first step in establishing a of complementing traditional solid-state devices13-15. Most transitional technology that will lead to a class of fully proposed architectures are dependent on precise order and molecular computers. building devices with exact arrays of nanostructures, which In the hybrid device, porphyrin molecules, as shown in are painstakingly interfaced with microstructure16-20. Fig. 3, are attached to a lithographically fabricated Si Conversely, a team from Rice, NCSU, Yale, South Carolina platform in an array of memory cells, as shown in Fig. 4. Bits (USC) and Penn State Universities, as well as Motorola Corp., of information are stored in the discrete redox states of the has been developing the NanoCell concept for nanoelectronic porphyrin molecules. The porphyrins are particularly assemblies. The NanoCell approach13-21is not dependent attractive candidates for memory applications because they upon placing molecules or nanowires in precise orientations have the ability to store multiple bits of information (certain or locations. The internal portions are, for the most part, porphyrin-based molecules are capable of storing three bits disordered and there is no need to precisely locate any of the of information). This will greatly reduce the complexity and switching elements. The nano-sized switches are added in increase the storage capacity of the memory chip. In the abundance, yet only a small percentage are needed to prototypical CMOS/molecular device (Fig. 4), the memory array is fully integrated with on-chip sense amplifiers, which are fabricated by conventional methods. These sense amplifiers have been designed to read multiple bits. Fig. 5 A scanning electron micrograph of the NanoCell after assembly of the Au nanowires and molecule 134. The upper image shows the five juxtaposed pairs of fabricated leads across the NanoCell; some Au nanowires are barely visible on the internal rectangle of the discontinuous Au film. The lower image is the NanoCell's central portion, at a higher magnification, showing the disordered discontinuous Au film with an attached Au nanowire that is affixed via the OPE-dithiol (not observable) derived from molecule 1. Fig. 4 A prototype hybrid CMOS/molecular memory chip. (Reprinted with permission from34. © 2003 American Chemical Society.) 22 December 2003 PROGRESS REPORT Molecular programmability Molecular electronics can be developed if we are able to program