Appearing in International Conference on Computer-Aided Design, November 2002 (ICCAD’02) Molecular Electronics: Devices, Systems and Tools for Gigagate, Gigabit Chips Michael Butts Andre´ DeHon Seth Copen Goldstein Cadence Design Systems, Inc. Dept. of CS, 256-80 School of Computer Science 13221 SW 68th Pkwy California Institute of Technology Carnegie Mellon University Portland, OR 97223 Pasadena, CA 91125 Pittsburgh, PA 15213 [email protected] [email protected] [email protected] h i h i h i Abstract In the next section we survey some of the promising, emerging device technologies. Section 3 surveys some fabrication techniques New electronics technologies are emerging which may carry us and their implications on the kinds of systems we can build. Next, beyond the limits of lithographic processing down to molecular- in Section 4, we review proposals for designing complete electronic scale feature sizes. Devices and interconnects can be made from a systems. Defect and fault tolerance, discussed in Section 5, will be variety of molecules and materials including bistable and switch- major issues for any technology at this feature size where single able organic molecules, carbon nanotubes, and, single-crystal molecules and countable electrons are used to form devices and semiconductor nanowires. They can be self-assembled into orga- hold state. In Section 6 we highlight some of the new challenges nized structures and attached onto lithographic substrates. This tu- for CAD created by bottom-up, nanoscale technologies. torial reviews emerging molecular-scale electronics technology for 2. Materials and Devices CAD and system designers and highlights where ICCAD research can help support this technology. 2.1. Robust Digital Abstraction Over the past several decades, we have developed a series of abstractions and properties that, when met, make it easy to assem- 1. Introduction ble and reason about robust digital systems, including: switching, isolation, noise margins, restoration, and state storage. Isolation al- Researchers have reported a number of interesting develop- lows active devices to be combined in large circuits and define a ments in the field of molecular-scale electronics during the past single, intended direction of signal flow. Signal restoration makes two years, prompting the AAAS journal Science to name Molec- output logic levels match or exceed input logic levels, assuring that ular Electronics as its “Breakthrough of the Year” for 2001. With it is possible to cascade an arbitrary number of devices in series and these developments and an accelerated rate of progress, interest- for circuits to operate correctly with feedback cycles. Good noise ing molecular-scale circuits are moving from fantasy into reality. margins reject noise in the system from crosstalk and voltage drops This decade it may become feasible to self assemble digital sys- across dissapitive media. Gain in circuits is needed both for signal tems with billions of components. The impact on electronics, and restoration and to support fanout. ICCAD tools in particular, will be profound. We have successfully transitioned from vacuum tubes, to diodes, Molecular-scale devices actually operating today include: to bipolar transistors, and to MOS transitors as basic logic devices FETs, junction transistors, diodes, and, molecular and mechanical by maintaining these features and transporting our experience from switches. Logic gates with voltage gain have been built, and many one form of circuit technology to the next. Today’s digital logic, techniques have been demonstrated to assemble nanometer wide memory and hardware system architectures can be built on any im- wires into large arrays. Programmable and non-volatile devices plementation technology that satisfies these requirements, includ- which hold their state in a few molecules or in square nanometers of ing molecular-scale circuits. Molecular electronics, however, may material have been demonstrated. Researchers project densities of require new ways of thinking about these properties. As we will de- 11 12 2 10 to 10 devices/cm . (In comparison, the 2001 ITRS roadmap scribe below, the molecular devices, contacts, and wires often have [1] projects 3 109 transistors/cm2 for 2016 for ASICs and MPUs.) × far different characteristics than traditional devices. In many cases, Building systems of this size requires a completely new set of the characteristics are currently inferior, but these weakness do not approaches to manufacturing and fabrication which will have a sig- appear fundamental. For example, none of the current molecular nificant impact on circuits and architectures. These approaches devices have all of the above properties, but rather a conglomera- are characterized by bottom-up molecular-scale fabrication which tion of molecular devices can be assembled to satisfy them. avoid the needs for lithographic patterning in order to create the nanometer scale dimensions for these devices and assemblies. We 2.2. Carbon Nanotubes can expect batch chemical and physical processes which will be In 1991 Sumio Iijima discovered a tubular variant of the C60 able to form semi-regular arrays with 1% to 5% defect rates. In- ”buckyball” carbon molecule [24]. A carbon nanotube (NT) is a teresting circuit functionality can be realized by a combination of molecular sheet a single atom thick, which has wrapped around into statistical design and post-fabrication programming to avoid defects a tube, as little as one nanometer wide and, so far, up to millimeters and define desired computations. long. Since it’s a single molecule, nanotubes are extremely strong Cheap gigagate, gigabit, gigaHertz non-volatile FPGAs and and flexible. RAMs, built with molecular-scale electronic arrays on microelec- Depending on their lattice geometry, NTs behave as metals or tronic silicon substrates, may be ready for commercial production semiconductors, acting as good electrical conductors. So far there by 2010. is no way to synthesize a pure batch of just one type [35]. Some 1 Vc1 Vc1 Vc1 Vbias n-type NW Vi1 Vi1 Vi1 Vc1 Vi2 Vo i2 Vbias FET Vo V Vo Vi2 n Vo Vbias p-type NW Vi1Vi2 Vbias FET Figure 1: CNW-diode AND gate Figure 2: CNW-FET NOR gate success in selecting NTs by voltage breakdown of undesired types has been reported [9]. Carbon nanotube technology is very new. NTs may become more predictable and manageable in time. 2.3. Single-crystal Nanowires Silicon may be the best understood and most manageable of all solids. Single-crystal silicon nanowires (NWs) have been fabri- cated in bulk by laser-assisted catalytic growth, with diameters of 6 to 20 nm and lengths ranging from 1 to 30 microns [25]. Germa- nium, gold, gallium phosphide, gallium nitride and indium phos- phide NWs have also been made [22, 31]. Evaluating nanoscale wires is difficult because the wire/contact/device system is highly intertwined. In the best case, the wire behaves like a 1-D quantum wire, i.e., it transports electrons ballistically, and has a length-independent resistance Figure 3: I-V curve for a “typical NDR device in multiples of h=e2 [26]. A perfectly conducting nanotube has a minimal resistance of h=4e2 or a resistance of approximately IV-curve (See Figure 3.) Devices with useful Peak-to-valley ratio 6 KΩ; which agrees with experiment [36]. Capacitance continues have been measured at room temperature [7]. These molecules can to scale with length. be used to as the basis of logic families [17, 14] or as the core of 2.4. Transistors, Diodes, and NDRs a molecular latch which also provides signal restoration and I/O- isolation [20]. Nanowires and nanotubes can be used as more than just inter- connect wires, they can also be used as active devices. Metallic 2.5. Programmable Switches wires can be grown with molecules which act as two-terminal de- Molecular electronics has the potential to not only reduce the vices embedded in them [33]. Controlled doping of silicon and scale of systems, but also to introduce novel devices with fea- gallium nitride NWs with phosphorus and boron has been success- tures not found in any silicon based systems. An example are pro- ful, producing p-type and n-type semiconducting nanowires [10]. grammable switches which hold their own state and can be pro- Carbon nanotubes can also act as diodes or transistors [21]. grammed using the signal wires, yet take up no more area than a Two semiconductor NWs, one p-type and one n-type, form wiring crossing. In conventional silicon systems, a programmable a junction diode at their crossing. Three NWs with two cross- crosspoint costs 40–100 the area of a wire crossing or via. Con- ings form a bipolar junction transistor [11]. Small working NW sequently, these devices× reduce the overhead (in both device area diode arrays have been made, with 85% to 95% yield, showing in- and interconnect area) for programmable circuits. dependent operation. Turn-on voltages of 1V are observed, and logic gates may be made with crossed nanowire diodes (CNW- diodes) [21] (See Figure 1). Molecular Switches Organic molecules exist which have two CNW-diodes can be made with 5V turn-on by increasing the mechanically distinct parts, such as a ring and a rod or interlocking oxide thickness at the junctions. This oxide can be grown by pass- rings. Applying a programming voltage across the molecule adds ing high current through a low turn-on diode in air, oxidizing the or subtracts an electron (oxidation-reduction), shifting the ring and junction by joule heating [21]. This device can be used as a one- changing the molecule’s conductivity. It functions as a non-volatile time-programmable crosspoint for ROM or logic arrays, or it can programmable molecular switch. Catenane is one such molecule be used as an FET. which opens at 2 volts, closes at 2 volts, is read at 0.1 volt, and CNW-diodes act as FETs in their non-conducting region.