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Implications of Emerging Micro- and Nanotechnologies 1 Introduction BACKGROUND The prediction of the progress of technology over long periods is an uncertain exercise. The temptations are to be either too conservative, acknowledging the current limitations of technology and not foreseeing the breakthroughs in conception and capability that will inevitably occur, or too exuberant, brushing aside real physical limitations in an excess of futuristic zeal. Such a challenge particularly applies to nanotechnology, which is an exciting and relatively unexplored scientific and technological frontier offering many new insights and applications but at the same time evoking much speculation and hyperbole. To cite Shermer in one of the many recent journal overviews of nanotechnology, “The rub in exploring the borderlands is finding that balance between being open-minded enough to accept radical new ideas but not so open-minded that your brains fall out.”1 From an applications perspective, the combination of microtechnologies and nanotechnologies offers a particularly powerful combination for future Air Force missions and deserves careful consideration. Two particularly pervasive themes of microtechnology, now extending into nanotechnology, have been miniaturization of electronic systems and the resulting increase in information density. The miniaturization trends of the last 50 years will undoubtedly continue and even accelerate over the next 50 years. In 1950, we had five-transistor radios, and computers were vacuum-tube-filled rooms with a very limited mean time between failure, available only to governments and large corporations. Today we have inexpensive, 100-million-transistor computer chips in our homes, which we replace not because they have failed, but
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Implications of Emerging Micro- and Nanotechnologies because the technology has advanced. We complain about the demands of ubiquitous connectivity as we attach cell phones to our belts and of information overload as we put more and more material on our Internet servers. Over this time span, transistors have gone from macroscopic, ~1-millimeter junction-length devices, to ~90-nanometer gates in the latest commercial chips and to ~10-nanometer gates in laboratory devices. This linear scaling clearly must end as devices approach the size of atoms (~0.2 nanometers). This does not, however, mean that progress in electronics and information technology will come to a halt. The integrated circuit paradigm that has enabled this dramatic scaling improvement is a planar, two-dimensional concept based on an interconnection of three-terminal switching elements (transistors).2 Moving to a volumetric approach, new materials, and different computing strategies will probably allow continuation and even acceleration of the capabilities and function per weight/volume/power of electronics. The practical success of miniaturization has been the result of the accompanying dramatic reduction in cost per function achieved by the integration of so many electronic devices onto single chips and using parallel, or batch, fabrication technologies to allow this cost scaling. Less well understood is the acceleration in other micro- and nanotechnologies, which is being driven by miniaturization and is contributing to the increasing density of information transmitted, stored, and processed. The growth in magnetic information storage in recent years has been even more rapid than growth in electronic information processing.3 Advances in magnetic memory storage range from new giant magnetoresistive nanoscale layered materials to read heads flying 10 nanometers over the surface of magnetic discs moving at speeds of 20 meters/second. To appreciate the challenge in control of tolerances for this technology, scaling to the macro world by the relative lengths of a magnetic read head and an F-18 jet fighter would correspond to flying the F-18 only 100 micrometers above the ground, which has been polished to a smoothness of 10 micrometers and staying on course within an accuracy of 100 micrometers. Optical information transmission has also been increasing at growth rates comparable to that for magnetic memories, aided by control of materials—for example, in optical fibers with ultrahigh-purity microscale cores and semiconducting lasers with nanoscale quantum wells. Mechanical devices at the microscale and below promise to further extend the reach of miniaturized technologies. Microelectromechanical systems (MEMS) build on the manufacturing paradigm of microelectronics and offer the promise of large-scale batch fabrication at low cost. Currently this emerging technology is primarily focused on simple devices such as inertial sensors for air bag release in automobiles and microscale mirrors for optical projection and switching. However, future applications of MEMS for airfoil control, inertial sensing, or satellite maneuverability could significantly broaden the scope of this technology. The integration of MEMS technologies with electronics and optics is also being explored for chemical sensing, so-called lab-on-a-chip systems. Indeed, the current
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Implications of Emerging Micro- and Nanotechnologies status of this technology appears to be similar to that of microelectronics some 25 years ago. The emerging breadth of microscale technologies (mechanical, optical, magnetic, chemical, and biological, as well as electronic) and the promise of future nanoscale technologies suggest that revolutionary advances in systems are likely. Miniaturization and high information density will be particularly important where performance requirements place weight and size at a premium. The potential of low cost, if achieved, implies the ubiquitous use of devices, as is now happening in microelectronics with the embedding of computer chips throughout systems. The widespread ability to embed high information density in combination with local detection, processing, and response in small packages will allow large networks of distributed systems and increasingly autonomous systems. The overarching theme that emerges is increased functionality and autonomy of systems. Low cost, ubiquitous, distributed systems will raise new questions such as the role of autonomous control and decision making and the integration of such system capabilities into military conduct of operations. STATEMENT OF TASK This study was requested by the Deputy Assistant Secretary of the Air Force for Science, Technology and Engineering. The Committee on Implications of Emerging Micro- and Nanotechnologies, established by the National Research Council, was asked to perform the following tasks: Conduct a study to examine the role that emerging micro- and nanotechnologies can play in improving current Air Force capabilities and enabling new weapons, systems, and capabilities. Assess the current state of the art in micro- and nanotechnologies. Discuss how current and future Air Force mission capabilities may be impacted or enabled by these technologies. Review the current Air Force and Department of Defense (DoD) investment strategies and the Air Force plan of execution in micro- and nanotechnologies for adequacy; recommend directions for accelerating the operational success of these technologies in Air Force missions. Recommend research initiatives that are needed to explore promising micro- and nanotechnologies. WHAT WE MEAN BY “MICRO” AND “NANO” In undertaking this study, the committee decided not to put hard size limitations on micro- and nano- objects and technologies. It understands these concepts as relating roughly to scale but also as having significant differences in underlying physical and chemical mechanisms. There is no hard line between
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Implications of Emerging Micro- and Nanotechnologies micro and nano, but there are some clear differences in the way the scientific and technology communities approach these regimes. It is very difficult to come up with sufficiently inclusive definitions for these concepts that hold across the multiple disciplines that are being explored, but it is not as difficult to decide in an individual case which category it fits into—in some sense, “you know it when you see it” holds here as well as in less noble realms of human activity. So, the committee defines micro and nano by example (see Figures 1-1, 1-2, and Box 1-1). Science and technology are always heavily intertwined and impossible to discuss, or indeed to advance, independently. Understanding the science enables the technology, and harnessing the technology allows further advances in the science. For conciseness in this report the committee speaks of micro- and nanotechnology, but this should always be understood to mean both micro- and nanoscience and micro- and nanotechnology. Microtechnology is characterized by a top-down fabrication paradigm, where the starting point is macroscopic and material is added or taken away in processes such as lithography to define patterns on surfaces, etching to remove material, and deposition to add material and thus allow complex structures to be made. The integrated circuit is an example of this paradigm. The starting point is an almost perfect wafer of silicon. Areas are defined on this wafer for introducing electrically active dopants, for adding various electrodes (source, gate, and drain contacts of transistors), and for making interconnections. When it was first conceived FIGURE 1-1 Model of a MEMS safety switch. SOURCE: Sandia National Laboratories. 2002. Available online at <http://mems.sandia.gov/scripts/images.asp> [June 21, 2002].
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Implications of Emerging Micro- and Nanotechnologies FIGURE 1-2 Atomic force microscopic image of InAs quantum dots. SOURCE: Courtesy of Kevin Malloy, University of New Mexico. in the late 1950s by Jack Kilby at Texas Instruments and Bob Noyce at Intel, the integrated circuit was a response to the difficulty of reliably packaging together numbers of transistors, resistors, capacitors, and other circuit elements to make large-scale circuits. The technology was known initially as “the monolithic solution.”4 MEMS devices, which now cover a very broad range of application, from accelerometers and angular rate sensors to switches to infrared bolometer focal plane arrays, are further examples of what we are calling micro. Typical scales range from a few hundreds of micrometers down to one micrometer and less. At the microscale, objects have greatly reduced inertia, and turbulence, convection, and momentum become negligible. At this scale, the surface and interface properties of materials begin to play an increasingly dominant role in the behavior of structures. A defining feature of the nanoscale is that there is a qualitative difference in material behavior, which does not scale from the macro and micro scales. New physics and chemistry come into play. Another way to say this is that dimensions, in addition to composition and structure, impact material properties in nanoscale materials. At least two factors dominate this transition. The first is that dimensions in the nanometer regime approach characteristic (quantum) wavefunction scales of excitations in the material—electrons and holes, photons, spin waves, and magnons, among others. The second factor is the very large surface to volume ratio of these structures, which means that no atom is very far from an interface; atomic forces and chemical bonds dominate. The first factor is the domain of quantum physics. Electronic wave functions (the de Broglie length) in semiconductors are typically on the order of 10 to 100 nanometers. The solid-state physics community has long been exploring the properties of quantum wells, in which one dimension (the growth direction) is on this scale. Modern telecommunications is based on semiconductor quantum-well lasers that exploit the unique properties of these structures. More recently, attention
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Implications of Emerging Micro- and Nanotechnologies BOX 1-1 A Matter of Scale We are used to thinking in a linear world. Changes in scale by many factors of 10 challenge both our intuition and our imagination. In going to the world of micro-and nanoscale phenomena we move to smaller dimensions by factors of 1,000, the micrometer being one one-thousandth of a millimeter (the diameter of the tip of your pen) and the nanometer being a million times smaller than the pen tip, or one one-thousandth of a micrometer. Figure 1-1-1 shows examples of the impact of the nanoscale on macroscopic objects for both biological and artificial systems. To visualize this scale change for everyday objects, consider your morning cup of coffee. The width of the mug is about 80 millimeters (mm). Now consider the diameter of a human hair, which is typically 50 micrometers (mm), a reduction in scale by about 1,000. To reach the nanoworld yet another reduction of 1,000 is needed. An object 50 nanometers (nm) high corresponds to a stack of about 200 atoms. If packed without space between them, a billion of these nano-objects would fit within a 50- × 50- × 50-micrometer cube! The speed of moving objects is another way to visualize large changes in scale. Consider a baby crawling. A person walking goes about 10 times as fast, a car traveling at 60 mph is 100 times faster, and a jet fighter at the speed of sound is 1,000 times faster than the crawling baby. Factors of 1,000 in moving between the macro-, micro-, and nanoworlds are truly large changes that challenge our intuitive capabilities. FIGURE 1-1-1 Dimensional scale. SOURCE: Wilson, B. 2001. AFRL Nano Science and Technology Initiative. Briefing by Barbara Wilson, Chief Technologist, Air Force Research Laboratory, to the Committee on Implications of Emerging Micro and Nanotechnologies, National Academy of Sciences, Irvine, Calif., December 18.
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Implications of Emerging Micro- and Nanotechnologies has turned to quantum dot structures that have all three dimensions in this regime. In some sense, these are “designer” atoms and molecules that can be engineered to provide the needed functionality. Another example is the wavelength of visible light, which is 400 to 800 nanometers. When periodic structures are created in optical materials at these dimensional scales by varying the dielectric constant, the propagation of light can be strongly influenced in analogy to electrons in semiconductors. While these properties are only now being explored, the possibilities include confining and steering light down to unprecedented small scales and creating low loss-optical devices such as near-thresholdless lasers. The second factor is a consequence of the large surface areas and unique chemical reactivity of nanostructures. This is the basis for much of the excitement at the juncture of nano- and biotechnologies. The information stored in the genome and the exquisite selectivity of biochemical interactions based on chemical recognition and matching are examples of nanoscale properties where the interfaces play a determining role. Nanoparticles have size-dependent chemical and electronic structure, reactivity, etc. that can be exploited to produce improved catalysts as well as electronic, magnetic, optical, and biomaterials. Materials constituted of nano particles are different from bulk materials and different from molecules. An easy characterization is to say that nanoscale objects contain a large (more than a simple molecule) but countable (for example a box of 100 atoms on a side containing 1 million atoms) number of atoms. With our increasing ability to fabricate structures with well-defined nanoscale features, new materials are emerging that promise both evolutionary and unexpected new properties. Another major thrust of nanoscale research is integration, where the aim is to preserve the unique properties of nanoscale structures as they are incorporated into macroscopic objects. Nanotechnology is generally anticipated to require a fundamentally different approach to fabrication than microtechnology. Whereas microscale structures are typically formed by top-down techniques such as patterning, deposition, and etching, the practical formation of structures at nanoscale dimensions will require an additional component—bottom-up self-assembly. This is the process whereby structures are built up from atomic- or molecular-scale units into larger and increasingly complex structures—as is widely used by biological systems. In practice some combination of top-down (lithographic) and bottom-up (self-assembly) techniques likely will be necessary for the efficient manufacturing and integration of nanoscale systems. Many tools now exist for investigating structure and properties at the nanoscale, including scanning tunneling probes, electron microscopies, and various diffraction techniques. An important development in nanoscale tools occurred in 1981 with the introduction of the scanning tunneling microscope for imaging individual atoms on surfaces. This development, which earned Bennig and Rohrer the Nobel Prize in Physics, allowed the imaging and manipulation of single atoms and set the stage for an entire family of scanning microscopy, with atomic force microscopy being the most widely used.
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Implications of Emerging Micro- and Nanotechnologies As will be seen in the following pages, four overarching themes emerged from the committee’s study of micro- and nanotechnologies: Increased information capabilities, Miniaturization of systems, New materials resulting from new science at these scales, and Increased functionality and autonomy. These themes emerge as a natural consequence of the advances in micro- and nanotechnologies resulting from scaling to small size. They will have far-reaching consequences for Air Force missions. Finally, the committee notes that not all things “nano” adhere to the usual nanometer dimensional scale, nanosatellites being a notable example. In this case nanosatellites have overall dimensions of many centimeters—the name evolved as a way to designate systems that are significantly smaller in a revolutionary way from today’s large, expensive satellite technology (see Box 1-2). However, even here the basis for developing nano satellites is provided by advances in micro- and nanotechnologies. REPORT ORGANIZATION AND METHODOLOGY This report documents the committee’s analysis, findings, and recommendations. The Air Force asked the committee to address short-term impacts as well as longer-term impacts, 20 to 50 years out. Both micro- and nanotechnologies were included because in combination they cover the near- and long-term trends in modern technology that will impact Air Force missions. These trends are most apparent in microelectronics and include the miniaturization of components, increased capability (information density), reduced cost per function, and increased reliability and ruggedness. Advances in microtechnology are evolving smoothly into other areas, such as MEMS for micromechanical components, and control at the nanoscale is helping to improve the performance of microscale systems. At the same time new, more revolutionary advances in materials, properties, and, ultimately, systems are emerging at the nanoscale. In Chapter 2, “Expectations for Future Micro- and Nanotechnologies,” a brief overview of current perspectives in micro- and nanotechnologies is presented. Chapter 3, “Major Areas of Opportunity,” addresses advances in micro-and nanotechnology areas most relevant to the Air Force. The committee included sections on information technology, sensors, biologically inspired materials and systems, structural materials, aerodynamics, and propulsion and power. These are all areas that could be of great interest to the Air Force; however, they do not necessarily merit the same level of emphasis. In Chapter 4, “Enabling Manufacturing Technologies,” the challenges and trends faced by the practical realization of micro- and nanoscale materials, components, and systems are dis-
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Implications of Emerging Micro- and Nanotechnologies BOX 1-2 Small Satellites: How Small Can We Go? The term “nano” has taken on a different meaning in the context of satellites; yet these future miniaturized systems are firmly based on advances in micro- and nanotechnologies. Popular early in the space age because of payload limitations on launch vehicles, “microsatellites” are satellites with a mass between 10 and 100 kilograms. More recently, satellites with mass between 1 and 10 kilograms have been called “nanosatellites,” while those with mass between 0.1 and 1 kilogram are now called “picosatellites.” Even smaller are “femtosatellites,” between 10 and 100 grams. The first artificial Earth satellite, Sputnik-1, launched October 4, 1957, had a mass of only 83.6 kilograms. The continually improving payload capabilities of launch vehicles have led to ever-larger active satellites with greater spacecraft power and communications data rate. For the most part, communications satellites have migrated from low Earth orbit (LEO—below 1,500-kilometer altitude) to fixed geosynchronous orbit (GEO— 35,786-kilometer altitude), requiring relatively large and costly spacecraft. During the 1980s, integrated circuit and radio frequency communications technology advanced to the point where microsatellites in LEO could provide competitive communications and data relay support, including the support of military forces in the field. In 1990 the Defense Advanced Research Projects Agency launched two experimental 66-kilogram-mass multiple access communications satellites (MACSATs), providing store-and-forward communications at up to 2.4 kilobits per second.1 The MACSATs were used for logistics communications in support of a Marine air wing in Desert Shield and Desert Storm. Today, technology has evolved to the point that nanosatellites and even picosatellites can perform complex scientific, communications, Earth observation, and satellite assistance missions. Figure 1-2-1 shows a modern nanosatellite fabricated by Surrey Satellite Technologies in the United Kingdom and designed to perform a satellite inspection mission. 1 Martin, D.H. 1996. Communication Satellites, 1958-1995. El Segundo, Calif.: The Aerospace Press. FIGURE 1-2-1 The SNAP-1 nanosatellite. Courtesy of Surrey Satellite Technology Limited, Centre for Satellite Engineering Research, University of Surrey, Guildford, Surrey, United Kingdom.
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Implications of Emerging Micro- and Nanotechnologies cussed. Chapter 5, “Air Force Micro- and Nanotechnology Programs and Opportunities,” briefly summarizes the current investments by the Air Force in micro-and nanotechnologies and considers the role of Air Force science and technology in this area relative to the commercial sector. Chapter 6, “Opportunities in Micro-and Nanotechnologies,” focuses on the systems implications of micro- and nanotechnologies and suggests areas for further consideration. Such mission considerations provide a methodology to focus on and prioritize investments for those technologies discussed in Chapters 3 and 4 that are most critical to the Air Force. Finally, Chapter 7, “Findings and Recommendations,” provides a summary of the findings and recommends ways in which the Air Force might focus its attention and resources in the areas of micro- and nanotechnology. REFERENCES 1. Shermer, M. 2001. Nano nonsense and cryonics. Scientific American 285(3): 29. 2. Reid, T.R. 2001. The Chip: How Two Americans Invented the Microchip and Launched a Revolution. New York, N.Y.: Random House, Inc. 3. National Research Council. 2001. Physics in a New Era: An Overview. Washington, D.C.: National Academy Press. 4. Reid, T.R. 2001. The Chip: How Two Americans Invented the Microchip and Launched a Revolution. New York, N.Y.: Random House, Inc.
Representative terms from entire chapter: