Technologies and Designs MTR 96W0000044 for Electronic Nanocomputers July 1996 Michael S. Montemerlo J. Christopher Love Gregory J. Opiteck David Goldhaber-Gordon James C. Ellenbogen MITRE McLean, Virginia Technologies and Designs MTR 96W0000044 for Electronic Nanocomputers July 1996 Michael S. Montemerlo J. Christopher Love Gregory J. Opiteck David Goldhaber-Gordon James C. Ellenbogen Sponsor MITRE MSR Program Project No. 51CCG89G Dept. W062 Approved for public release; distribution unlimited. Copyright © 1996 by The MITRE Corporation. All rights reserved. MITRE McLean, Virginia ABSTRACT This paper reviews research developments aimed at the design of electronic computers that contain components with dimensions of only a few nanometers. A nanometer, one billionth of meter, is only about 10 atomic diameters. Such nanometer- scale electronic computers–i.e., electronic “nanocomputers”–that contain molecular-scale components are likely to be up to 10,000 times more densely integrated than today’s smallest microcomputers. Electronic technology is one of several alternative technologies (e.g., mechanical, chemical, quantum) that have been proposed for implementing a nanocomputer. Electronic technology for nanometer-scale computers has the advantage, though, that it builds upon nearly a half century of experience and infrastructure developed for electronic computing. Electronic nanocomputers could be orders of magnitude faster than current electronic computers, as well as many times smaller or more densely integrated. Although some of the operating principles for electronic nanocomputers could be similar to present-day electronic microcomputers, there is a limit to how far the designs and fabrication technologies for present-day microcomputers can be scaled down. This has led some investigators to propose radically dierent “wireless” designs, quantum cellular automata, and nanometer-scale neural networks, all to be constructed from nanoscopic quantum-eect devices. These devices and designs take advantage of some of the very eects that have been obstacles to making smaller conventional transistors and circuits. Still other investigators have proposed the “self-assembly” of electronic devices with nanometer-scale components in order to circumvent some of the diculties that have inhibited the fabrication of sub-micron structures using conventional lithographic techniques. This review examines critically a range of such technologies and designs for electronic nanocomputers. It describes and compares the operating principles, advantages, disadvantages, and status of the new technologies and designs that promise to continue the miniaturization of the electronic computer down to the scale of a few tens of nanometers and, ultimately, to the molecular scale. This information is presented in non-mathematical terms intended for a general, technically interested readership. Copyright c 1996 The MITRE Corporation, McLean, VA iii Contents List of Figures vii List of Tables viii I Introduction 1 II Genesis of Nanotechnology 1 A Mechanical Nanocomputers............................................................................. 4 B Chemical Nanocomputers................................................................................. 4 C Quantum Nanocomputers................................................................................. 5 D Electronic Nanocomputers............................................................................... 5 III Current Computer Technology: Microelectronic Two-State Devices 6 A How a MOSFET Works .................................................................................. 8 B Obstacles to the Functioning of Nanometer-Scale Transistors: Limitations to Scaling ................................................................... 8 1 Shrinkage of Depletion Regions .......................................................... 8 2 High Electric Fields ............................................................................. 8 3 Tunneling ............................................................................................. 9 4 Heat Dissipation ................................................................................... 9 5 Vanishing Bulk Properties ................................................................... 9 IV FutureTechnologies:LikelyApproachestoNanoelectronicTwo-StateDevices 9 A Resonant-Tunneling Transistor ......................................................................... 10 B Single-Electron Transistor ................................................................................. 13 C Electrostatic Quantum Dot Cells ....................................................................... 13 D Molecular Shuttle Switch .................................................................................. 16 E Atom Relay ........................................................................................................ 16 F Refinedned Molecular Relay ............................................................................. 18 V Logic Designs and Architectures for Electronic Nanocomputers 20 A Wireless Ground State Computing ..................................................................... 20 B Modifications of Wireless Computing Designs .................................................. 25 1 Lent-Porod Adiabatic Wireless Computing Approach ......................... 25 2 Korotkov's Wireless Dissipative Computing Approach ....................... 25 C Adaptations o fWireless Computing Designs ..................................................... 27 1 Quantum Cellular Neural Networks ..................................................... 27 2 Nanometer-Scale Nonlinear Networks ................................................. 27 D Considerations and Constraints for Nanocomputer Logic Designs..................... 27 1 Approaches to Parallelism..................................................................... 27 2 Interconnects.......................................................................................... 28 3 Error Correction..................................................................................... 28 VI Fabrication 29 A Traditional Methods of Fabrication..................................................................... 30 1 UV Lithography..................................................................................... 30 2 X-ray Lithography................................................................................. 30 3 Electron-beam Lithography................................................................... 30 4 Molecular Beam Epitaxy....................................................................... 30 B Emerging Fabrication Technologies.................................................................... 31 1 Mechanosynthesis.................................................................................. 31 2 Chemosynthesis..................................................................................... 33 Copyrightc © 1996 The MITRE Corporation, McLean, VA v VII Emerging Technologies for the Implementation of Nanoelectronics 38 A Molecular Electronics....................................................................................... 38 B Silicon Nanoelectronics.................................................................................... 40 VIII Modeling 40 IX Remaining Challenges for Nanoelectronics 41 X Conclusion 41 XI The Authors 42 XII Acknowledgements 43 XIII Bibliography 45 XIV Glossary 55 vi Copyrightc © 1996 The MITRE Corporation, McLean, VA List of Figures No. Title Page 1 Genesis of Nanotechnology ........................................... 2 2 Cross-Section of a NMOS Transistor ..................................... 7 3 Schematic of a Resonant-Tunneling Diode (RTD) .............................. 12 4 Conductance Peak of an RTD ......................................... 14 5 Resonant-Tunneling Transistor ........................................ 14 6 Projected Timeline for the Arrival of Hybrid Nanoelectronics ....................... 15 7 Single Electron Transistor ........................................... 15 8 Concept of a Quantum Dot .......................................... 17 9 Communicating between Quantum Dots ................................... 17 10 Lent-Porod Quantum Dot Cells ........................................ 17 11 Reversible Molecular Switch .......................................... 19 12 Atom Relay ................................................... 19 13 Rened Molecular Switch, Type 1 ...................................... 19 14 Rened Molecular Switch, Type 2 ...................................... 21 15 Communicating between Quantum Dot Cells ................................ 21 16 A Majority Gate Constructed with Quantum Dot Cells .......................... 22 17 A Full Adder Constructed with Quantum Dot Cells ............................ 22 18 Example of a Diculty Encountered in the Design of Ground State Computers ............. 24 19 Elements of Korotkov’s Wireless Dissipative Computer ........................... 26 20 Operation of Korotkov’s Wireless Computer ................................. 26 21 Switching with an STM Tip .......................................... 32 22 SEM Micrograph of a Micro-STM ...................................... 34 23 SEM Micrograph of a Micro-STM Tip .................................... 34 24 Side View of a Self-Assembled Array of Gold Clusters with Molecular Electronic Properties ...... 36 25 Top View of a Self-Assembled Array of Gold Clusters