SINCE the invention of the scanning tunnelling microscope1, the value of establishing a physical connection between the macroscopic world and individual nanometre-scale objects has become increasingly evident, both for probing these objects2–4 and for direct manipulation5–7 and fabrication8–10 at the nanometre scale. While good progress has been made in controlling the position of the macroscopic probe of such devices to sub-angstrom accuracy, and in designing sensitive detection schemes, less has been done to improve the probe tip itself4. Ideally the tip should be as precisely defined as the object under investigation, and should maintain its integrity after repeated use not only in high vacuum but also in air and water. The best tips currently used for scanning probe microscopy do sometimes achieve sub-nanometre resolution, but they seldom survive a 'tip crash' with the surface, and it is rarely clear what the atomic configuration of the tip is during imaging. Here we show that carbon nanotubes11,12 might constitute well defined tips for scanning probe microscopy. We have attached individual nanotubes several micrometres in length to the silicon cantilevers of conventional atomic force microscopes. Because of their flexibility, the tips are resistant to damage from tip crashes, while their slenderness permits imaging of sharp recesses in surface topography. We have also been able to exploit the electrical conductivity of nanotubes by using them for scanning tunnelling microscopy.

Nanotubes as nanoprobes in scanning probe microscopy

Part I: Background and Fundamentals Introduction, Mohamed Gad-el-Hak, University of Notre Dame Scaling of Micromechanical Devices, William Trimmer, Standard MEMS, Inc., and Robert H. Stroud, Aerospace Corporation Mechanical Properties of MEMS Materials, William N. Sharpe, Jr., Johns Hopkins University Flow Physics, Mohamed Gad-el-Hak, University of Notre Dame Integrated Simulation for MEMS: Coupling Flow-Structure-Thermal-Electrical Domains, Robert M. Kirby and George Em Karniadakis, Brown University, and Oleg Mikulchenko and Kartikeya Mayaram, Oregon State University Liquid Flows in Microchannels, Kendra V. Sharp and Ronald J. Adrian, University of Illinois at Urbana-Champaign, Juan G. Santiago and Joshua I. Molho, Stanford University Burnett Simulations of Flows in Microdevices, Ramesh K. Agarwal and Keon-Young Yun, Wichita State University Molecular-Based Microfluidic Simulation Models, Ali Beskok, Texas A&M University Lubrication in MEMS, Kenneth S. Breuer, Brown University Physics of Thin Liquid Films, Alexander Oron, Technion, Israel Bubble/Drop Transport in Microchannels, Hsueh-Chia Chang, University of Notre Dame Fundamentals of Control Theory, Bill Goodwine, University of Notre Dame Model-Based Flow Control for Distributed Architectures, Thomas R. Bewley, University of California, San Diego Soft Computing in Control, Mihir Sen and Bill Goodwine, University of Notre Dame Part II: Design and Fabrication Materials for Microelectromechanical Systems Christian A. Zorman and Mehran Mehregany, Case Western Reserve University MEMS Fabrication, Marc J. Madou, Nanogen, Inc. LIGA and Other Replication Techniques, Marc J. Madou, Nanogen, Inc. X-Ray-Based Fabrication, Todd Christenson, Sandia National Laboratories Electrochemical Fabrication (EFAB), Adam L. Cohen, MEMGen Corporation Fabrication and Characterization of Single-Crystal Silicon Carbide MEMS, Robert S. Okojie, NASA Glenn Research Center Deep Reactive Ion Etching for Bulk Micromachining of Silicon Carbide, Glenn M. Beheim, NASA Glenn Research Center Microfabricated Chemical Sensors for Aerospace Applications, Gary W. Hunter, NASA Glenn Research Center, Chung-Chiun Liu, Case Western Reserve University, and Darby B. Makel, Makel Engineering, Inc. Packaging of Harsh-Environment MEMS Devices, Liang-Yu Chen and Jih-Fen Lei, NASA Glenn Research Center Part III: Applications of MEMS Inertial Sensors, Paul L. Bergstrom, Michigan Technological University, and Gary G. Li, OMM, Inc. Micromachined Pressure Sensors, Jae-Sung Park, Chester Wilson, and Yogesh B. Gianchandani, University of Wisconsin-Madison Sensors and Actuators for Turbulent Flows. Lennart Loefdahl, Chalmers University of Technology, and Mohamed Gad-el-Hak, University of Notre Dame Surface-Micromachined Mechanisms, Andrew D. Oliver and David W. Plummer, Sandia National Laboratories Microrobotics Thorbjoern Ebefors and Goeran Stemme, Royal Institute of Technology, Sweden Microscale Vacuum Pumps, E. Phillip Muntz, University of Southern California, and Stephen E. Vargo, SiWave, Inc. Microdroplet Generators. Fan-Gang Tseng, National Tsing Hua University, Taiwan Micro Heat Pipes and Micro Heat Spreaders, G. P. "Bud" Peterson, Rensselaer Polytechnic Institute Microchannel Heat Sinks, Yitshak Zohar, Hong Kong University of Science and Technology Flow Control, Mohamed Gad-el-Hak, University of Notre Dame) Part IV: The Future Reactive Control for Skin-Friction Reduction, Haecheon Choi, Seoul National University Towards MEMS Autonomous Control of Free-Shear Flows, Ahmed Naguib, Michigan State University Fabrication Technologies for Nanoelectromechanical Systems, Gary H. Bernstein, Holly V. Goodson, and Gregory L. Snider, University of Notre Dame Index

The MEMS Handbook

Shape-Persistent Molecular Architectures of Nanoscale Dimension

The chemistry of metal oxide surfaces has long been thought to be dominated by reactions involving defects1,2. These are minority sites such as oxygen vacancies. Thus far, it has proved difficult to obtain direct experimental evidence to support this idea, although some progress has been made3,4,5. Here, we use the scanning tunnelling microscope (STM) to image the reaction of water molecules with bridging-oxygen vacancies on a model oxide surface, rutile TiO2(110). In a form of single-molecule chemistry, individual oxygen vacancies are observed being transformed into OH species as a water molecule dissociates in the vacancy. We use the STM tip to selectively desorb individual H atoms, whilst leaving the vacancies intact. This allows us to distinguish between vacancies and OH, which have a similar appearance in STM. In a very clear way, these results validate the view that defects can play a key role in oxide surface reactions.

Direct visualization of defect-mediated dissociation of water on TiO2(110)

Advances in techniques for the nanoscale manipulation of matter are important for the realization of molecule-based miniature devices with new or advanced functions. A particularly promising approach involves the construction of hybrid organic-molecule/silicon devices. But challenges remain--both in the formation of nanostructures that will constitute the active parts of future devices, and in the construction of commensurately small connecting wires. Atom-by-atom crafting of structures with scanning tunnelling microscopes, although essential to fundamental advances, is too slow for any practical fabrication process; self-assembly approaches may permit rapid fabrication, but lack the ability to control growth location and shape. Furthermore, molecular diffusion on silicon is greatly inhibited, thereby presenting a problem for self-assembly techniques. Here we report an approach for fabricating nanoscale organic structures on silicon surfaces, employing minimal intervention by the tip of a scanning tunnelling microscope and a spontaneous self-directed chemical growth process. We demonstrate growth of straight molecular styrene lines--each composed of many organic molecules--and the crystalline silicon substrate determines both the orientation of the lines and the molecular spacing within these lines. This process should, in principle, allow parallel fabrication of identical complex functional structures.

Self-directed growth of molecular nanostructures on silicon

The scanning tunneling microscope has been used to desorb hydrogen from hydrogen-terminated silicon (100) surfaces. As a result of control of the dose of incident electrons, a countable number of desorption sites can be created and the yield and cross section are thereby obtained. Two distinct desorption mechanisms are observed: (i) direct electronic excitation of the Si-H bond by field-emitted electrons and (ii) an atomic resolution mechanism that involves multiple-vibrational excitation by tunneling electrons at low applied voltages. This vibrational heating effect offers significant potential for controlling surface reactions involving adsorbed individual atoms and molecules.

https://science.sciencemag.org/content/sci/268/5217/1590.full.pdf

Atomic scale desorption through electronic and vibrational excitation mechanisms

PtSi source/drain p-type metal–oxide–semiconductor field-effect transistors (MOSFETs) have been fabricated at sub-40 nm channel lengths with 19 A gate oxide. These devices employ gate-induced field emission through the PtSi ∼0.2 eV hole barrier to achieve current drives of ∼350 μA/μm at 1.2 V supply. Delay times estimated by the CV/I metric extend scaling trends of conventional p-MOSFETs to ∼2 ps. Thermal emission limits on/off current ratios to ∼20–50 in undoped devices at 300 K, while ratios of ∼107 are measured at 77 K. Off-state leakage can be reduced by implanting a thin layer of fully depleted donors beneath the active region to augment the Schottky barrier height or by use of ultrathin silicon-on-insulator substrates.

Sub-40 nm ptsi schottky source/drain metal-oxide-semiconductor field-effect transistors

This letter explores regulation of current flow within a silicon field‐effect transistor by gate‐induced tunneling through a Schottky barrier located at the interface between a metallic source electrode and the Si channel. The goal here is to forestall short‐channel effects which are expected to prevent further size reductions in conventional devices when linewidths reach ∼1000 A. Control of tunneling appears to be possible at minimum channel lengths L∼250 A or less while simultaneously eliminating the need for large‐area source and drain contacts, so that scaling of Si transistors could be significantly extended if this principle proves technically feasible.

/pdf/silicon-field-effect-transistor-based-on-quantum-tunneling-521mjt85h1.pdf

Silicon field-effect transistor based on quantum tunneling

In this article we investigate the subthreshold behavior of PtSi source/drain Schottky barrier metal–oxide–semiconductor field-effect transistors. We demonstrate very large on/off ratios on bulk silicon devices and show that slight process variations can result in anomalous leakage paths that degrade the subthreshold swing and complicate investigations of device scaling.

Suppression of leakage current in Schottky barrier metal–oxide–semiconductor field-effect transistors

Ultrathin oxide patterns of a linewidth of 50 A have been created on Si(100)‐2×1 surfaces by a scanning tunneling microscope operating in ultrahigh vacuum. The oxide thickness is estimated to be 4–10 A. The morphology and spectroscopy of the oxide region are obtained. Hydrogen passivation is used as an oxidation mask. The defects caused by oxidation in the passivated region before and after the hydrogen desorption are compared and discussed. The multistep silicon processings by an ultrahigh vacuum scanning tunneling micropscope is thus demonstrated.

/pdf/nanoscale-oxide-patterns-on-si-100-surfaces-5f6nbxbe2h.pdf

C. Wang

Papers

Atomic scale desorption through electronic and vibrational excitation mechanisms

Sub-40 nm ptsi schottky source/drain metal-oxide-semiconductor field-effect transistors

Silicon field-effect transistor based on quantum tunneling

Suppression of leakage current in Schottky barrier metal–oxide–semiconductor field-effect transistors

Nanoscale oxide patterns on Si(100) surfaces