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2 Low-Temperature Plasma Science and Engineering Low-temperature plasma science and engineering is that area of plasma re- search addressing partially ionized gases with electron temperatures typically below about 100,000 K (10 eV). Such plasmas are often known as “collisional plasmas” or “weakly ionized plasmas” because input power first couples with the charged electrons and ions and then is collisionally transferred to neutral atoms and mol- ecules, creating chemically active species. The richness of the field comes from the intimate contact between energetic plasmas and ordinary matter in all its phases: gas, liquid, and solid. When these interactions can be accomplished in a stable, reproducible, controlled way, the result can be practical products or processes that benefit society (Figure 2.1). A particular challenge for low-temperature plasma research is the diversity of parameter space and conditions that are encountered: Size. From ever larger, stable plasmas (5 m2 plasmas are used to make liquid • crystal display television panels) to tiny (100 µm2) plasmas so intense that the plasma electrons merge with the electrons inside the solid electrodes. • Pressure. From ever lower pressures used in semiconductor processing equipment (<1 millitorr) to increasing pressures, now more than 100 atm (76,000 torr), for the lamps that power projection displays. • Chemistry. From simple rare gas plasmas used to propel spacecraft to ever more complex and reactive hydrocarbon and halogen chemistries for plasma-augmented combustion and material processing. 

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering  and FIGURE 2.1 The many beneficial applications of low-temperature plasmas are realized most effectively when plasma behavior can be accurately, reliably, and rapidly predicted. A robust predictive capability rests, in turn, on a healthy foundation of low-temperature plasma science and a robust effort to improve and extend the scientific understanding in key areas. Low-temperature plasma science and engineering is a highly interdisciplinary field because of its widespread applications. The field is driven by both fundamental science issues and the societal benefits that result from application of these plasmas. As such, there are often parallel approaches to furthering the state of the art. Like research in other fields of science and engineering, research in low-temperature plasmas strives to gain a deeper understanding of the underlying fundamental principles governing plasmas. At the same time, the research can be motivated by the need to develop detailed understanding of application-specific phenomena that may have important consequences for practical applications. Because the to- tal worldwide effort in applications dwarfs the efforts devoted to basic science, it

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Plasma science 40 is typically the case that an application attracts the science in an effort to replace empirical development with scientific rigor. However, the greatest success stories are often found when the science and application advance together. Advances in the science of low-temperature plasmas have produced great so- cietal benefits. Some of the products and processes include these: • Computer chips, fabricated using multiple plasma processing steps to de- posit, pattern, and remove material at the nanometer scale of modern integrated circuits. • Plasma television, which has leveraged scientific advances in high-pressure dielectric barrier discharges to become one of the best-selling video dis- plays. They are the forerunners of microplasmas having unique properties approaching quantum effects. • Textiles and polymers, functionalized by plasmas to produce stain-resistant carpets and waterproof jackets and to prepare plastic surfaces for printing and painting. • Artificial joints and arterial stents, treated in plasmas to make them bio- compatible, reducing the risk of rejection by the patient. • Fluorescent and high-intensity-discharge lamps, which supply four-fifths of the artificial light for offices, stores, roadways, stadiums, and parking lots. Their higher efficiencies allow them to consume only one-fifth as much power as incandescent lamps. • Jet engines, which rely on protective plasma spray coatings to protect com- ponents subject to the highest temperatures. • Plasma thrusters and rockets that maintain the orbit of many satellites and propel deep space probes. • Environmental improvements realized from low-temperature plasma tech- nologies and enabled by improved energy usage and renewable energy sources including plasma-aided combustion, fabrication of large-area pho- tovoltaics, plasma remediation of greenhouse and toxic gases, and plasma destruction of hazardous wastes. • Low-temperature plasma production of nanoscale materials, from super- hard nanocomposites to photonic nanocrystals to nanowires and nano- tubes, is one of the key enablers of the nanotechnology revolution. • Unique materials and coatings for transportation applications, produced using arc-generated direct current and radio frequency thermal plasmas. These range from superhard coatings to nanophase materials that have enabled advances in current and next-generation automotive and aerospace technologies. The breadth of the science and the importance of the applications place a high

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering 41 and premium on the ability to quantitatively predict the behavior of low-temperature plasmas. Obtaining experimental, theoretical, and model-based predictive capa- bility is crucial to integrating the intellectual diversity of the field and speeding advances in low-temperature plasma science that benefits society. Each box in this chapter tells the story of an application of low-temperature plasmas (Boxes 2.1 through 2.7). Each application has its own flavor, giving some idea of the diversity of the approaches that are needed to make effective use of scientific breakthroughs. The chapter is organized around the scientific topics, issues, and opportunities that underlie the diverse applications of low-temperature plasmas. INTRODuCTION AND uNIFYINg SCIENTIFIC PRINCIPLES There are recurring and unifying scientific principles behind the extraordinary range of practical uses for low-temperature plasmas. The list of scientific themes is similar to that found in other branches of plasma science, but the details are unique to low-temperature plasmas and their broad range of operating conditions. A no- table feature throughout low-temperature plasmas is the close coupling of plasmas with surfaces, leading to unique complexities and feedback mechanisms. Plasma Heating, Stability, and Control Depending on the plasma requirements, low-temperature plasmas can be heated by electromagnetic energy ranging from zero frequency (direct current) up to microwave frequency (several gigahertz). The ability to deposit a high density of power is important for many applications, from waste processing to lighting to rockets. The need to control plasmas is illustrated by the extreme cases where plasmas are used to remove a single atomic layer of material or maintain unifor- mity over square meters of area. The scientific challenge is to connect charged and neutral particle collisional and collective processes at the atomic level to the behavior of a plasma that can span an area of several square meters. Efficiency and Selectivity The desirable end-product of many low-temperature plasmas is an excited plasma species. In certain environmental applications the goal is to produce ozone, O3; hydroxyl, OH; or atomic oxygen, O(1D). For many plasma lamps the goal is to produce mercury atoms in a particular electronic state, Hg(63P1). In fact, 10 percent of all electric power produced in the United States is used to create this one ex- cited atomic state in lamps. The scientific challenge is to understand the whole of the plasma, quantitatively follow the flow of energy and material, maximize the desirable end product, and minimize deleterious processes.

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Plasma science 42 BOX 2.1 Reaching the Planets Plasma-based propulsion systems are already keeping satellites in their proper orbit, and they propelled the Deep Space 1 probe to Comet Borelly. They may also take the first humans to Mars. Plasmas will never launch a rocket into orbit because the instantaneous power requirement is too high, but once in space, the plasma is highly efficient and can reduce fuel requirements by a factor of 100 (Figure 2.1.1). Plasma based electric rockets could have significant commercial advantage over conventional chemical rockets to propel space cargo, said President Bush in his speech, “The United States Vision for Space Exploration.”1 The advantage of plasma propulsion is that its exhaust speed can be very high. This high speed produces a very high efficiency in terms of the momentum that the rocket can give to the spacecraft relative to the mass of fuel consumed (the specific impulse). Instead of being limited by the temperature of a chemical reaction, as in conventional rockets, these devices utilize electric and magnetic fields to provide the driving forces that ultimately accelerate the exhaust particles to much higher speeds. Since the ejected particles move faster, fewer of them are required to achieve the same propulsive effect. This results in lower fuel consumption and higher payload. To be competitive, plasma rockets must be lightweight and able to handle increasing levels of power in a relatively small package. In addition, given that they must be on for long periods of time, they must be reliable and have long-lived components. One way to meet these goals is to use electrodeless systems where the plasma is created and accelerated by the action of electromagnetic waves rather than by the presence of physical electrodes immersed in the flow. (The latter are severely limited by erosion and wear due to plasma bombardment.) A favored plasma generator for such applications is the helicon discharge developed in the 1970s for the plasma materials processing industry. Significant advances in our understanding of the physics and engineering of these devices has been driven by their application to space propulsion. Major efforts in the packaging of high-power electrical supplies are also under way in support of these technologies. 1 George W. Bush, speaking at NASA, “The United States Vision for Space Exploration,” January 14, 2004. Stochastic, Chaotic, and Collective Behavior Quiescent, uniform plasmas are rarely found outside textbooks. Many low- temperature plasmas exhibit turbulent, chaotic, and stochastic behavior. Arc- generated plasmas used to spray coat turbine blades are usually turbulent. Stream- ers (filamentary plasmas similar to lightning) branch and wander in high-pressure gases and liquids in unpredictable ways. Even apparently quiescent glows may have striations and surprising collective motion (Figure 2.2.) The scientific challenge is to understand the conditions that govern the transitions among the different regimes of behavior and to uncover mechanisms for controlling them. Plasma Interactions with Surfaces Low-temperature plasmas are in contact with surfaces that profoundly affect the plasma properties. Even a simple chamber wall intended to be nothing more

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering 4 and FIGURE 2.1.1 This Hall thruster is just one example of several plasma-based space propulsion technologies. Plasmas are uniquely able to convert electric input power into gas momentum with high efficiency. The tech- nical challenges include the need for very high reliability and long life, which is addressed by managing the plasma–surface interactions within the thruster. Courtesy of NASA. than an inactive part of the vacuum system can alter a plasma process by collecting or releasing material or by becoming electrically charged. In material processing plasmas, the basic purpose of the plasma is to alter the properties of a surface, depositing or removing material or chemically functionalizing the surface, and returning species to the plasma. Thus the surface is an integral part of the process and can be very complex, up to and including living tissue. The scientific challenge is to quantify, characterize, and predict the interactions between reactive plasmas with complex surfaces. Plasmas in Dusty and Other Nonideal Media Small clusters (tens of atoms), nanoparticles (a few to tens of nanometers), and larger particles (up to tens of microns) are present in many plasmas. Particles are sometimes a desirable product of a plasma process, as in the case of nanomaterial

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Plasma science 44 Pressure (torr) FIGURE 2.2 Plasma interactions with surfaces drive collective effects in near atmospheric-pressure microdischarges. These top-down views show the visible emission from a 750-micrometer-diameter low-temperature plasma in xenon. The patterns result from interactions of the plasma with its metallic and insulating boundaries. SOURCE: K.H. Schoenbach, M. Moselhy, and W. Shi, “Self-organization in cathode boundary layer microdischarges,” Plasma Sources Science and Technology 13: 177 (2004); © IOP Publishing Limited. synthesis or spray coating of jet engine components. Conversely, unwanted plasma- generated particles can cause killer defects during microelectronics fabrication. Dusty plasmas exhibit nucleation dynamics, crystal formation, and phase transi- tions that in many cases are found only in plasmas. The scientific challenges include leveraging the unique plasma–particle interactions to create new structures and ma- terials and to diagnose nonlinear phenomena. Diagnostics and Predictive Modeling The ability to quantitatively predict the behavior of low-temperature plasmas is not only a test of our fundamental understanding but also has important eco- nomic implications because it can reduce the time, cost, and risk of developing new plasma applications. There has been tremendous progress in the development of

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering 4 and BOX 2.2 Making Nanoparticles with Plasmas A new and exciting application of low-temperature plasmas is their use as controllable sources of nanometer-sized structures (e.g., nanowires, quantum dots, nanoparticles) that have novel physical and chemical properties. For example, low-temperature plasmas can be used to fabricate self-aligned carbon nanotubes, at both low and high pressure, and self-limiting nanowires on electronic materials. Plasma- engineered nanoparticles, often smaller than 10 nm, are being studied for their potential to enhance the properties of bulk materials for strength or ductility or to be used as building blocks for new photonic devices (Figure 2.2.1). Compared to other gas-phase methods for synthesizing such nanoparticles, plasmas have unique advantages. Among these are their ability to reduce particle agglomeration by charging all particles negatively and so have them be self-repulsive, their ability to anneal particles in situ in the plasma by unique plasma–particle interactions, and their ability to keep particles suspended in the synthesis reac- tor virtually indefinitely until they are used, thereby reducing possible contamination. Plasma-synthesized nanoparticles have already enabled development of new materials and devices, including mixed-phase nanocrystal/amorphous silicon films with improved optoelectronic properties, luminescent quantum dots, particles with improved magnetic properties, nanocrystal-based memory devices, single electron transis- tors, and cold electron emitters. Given the fast-paced growth of nanotechnology, it is expected that more such applications of “nanodusty plasmas”—plasmas containing nanoparticles—will rapidly emerge. FIGURE 2.2.1 Laboratory plasmas can create an environment having conditions able to uniquely pro- duce nanoparticles. In this example the pristine cleanliness of the plasma environment is needed to synthesize silicon nanocrystals with unique optoelectronic properties. SOURCE: A. Bapat et al., “Plasma synthesis of single-crystal silicon nanoparticles for novel electronic device applications,” Plasma Physics and Controlled Fusion 46 (2004) B97; © IOP Publishling Limited.

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Plasma science 4 BOX 2.3 Plasma Televisions and Displays Ask the average person what is meant by a “plasma” and the answer will probably be “plasma television,” a big change from 10 years ago, when the answer would probably have been “blood.” Each pixel in a plasma television set is a self-contained fluorescent lamp capable of switching on and off rapidly enough to display moving images. A dielectric-barrier discharge in a mixture of rare gases produces ultraviolet radiation to excite phosphors and produce a red, green, or blue pixel. As cathode-ray tubes fall into disuse, many displays will soon be powered by plasmas in one form or another. Plasma televisions and computer displays form an im- age by filtering the light from fluorescent plasma lamps behind the screen, and computer data projectors are powered by very intense, high-pressure plasma lamps operating at internal pressures well above 100 atm and power densities above 100 W/mm3. The success of plasmas in displays is a significant technological achieve- ment and offers lessons for the future of low-temperature plasma science. Applications Motivate Science That Impacts Daily Life The challenges of tiny dimensions (100 µ) and transient operation (50 kHz) of plasma display panel pixels motivated a large effort to develop transient, three-dimensional models of pixel operation and corresponding diagnostics to measure their properties (Figure 2.3.1). The extreme conditions in a projector lamp have driven the need to quantitatively understand the lack of collisional equilibrium even at high pressures, where power transport is dominated by radiation. While it is true that commercial success depends on many factors, plasmas have emerged as a dominant display technology in large part because they are efficient, compact, and inex- pensive. Understanding plasma transport is of scientific interest; but it is also required to design the product and meet the performance requirements. The Large Potential for Economic Impact The global market for displays is about $110 billion. Once the initial materials and electronics advances had been developed in laboratories in the United States, federal programs quickly ramped down support for continued research in the area. The Japanese and Korean governments, on the other hand, poured millions of dollars into the fundamental science of plasma displays in partnerships with industry. It was those government- led and -funded partnerships that produced the advances that enabled Japanese (and now Korean) manufac- turing to take the lead. Because these firms achieved a dominant global market share, they are now able to dictate future trends in the industry. As a result, the United States has a small part of this global market. The absence of a distinctly supported low-temperature plasma science community (in contrast to engineering and application-development work supported by industry) may have contributed to this chain of events. It is beyond the committee’s scope to draw conclusions but within its scope to point out that there are lessons to be learned from this bit of recent history. science-based, predictive models (Figure 2.3). Detailed diagnostic measurements and modeling can not only reveal the complex dynamics of a plasma but are also part of the work to develop and improve applications of plasmas such as plasma televisions. Nevertheless, modeling and simulation, diagnostics, and the allied

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering 4 and FIGURE 2.3.1 Each pixel of a plasma-display television has three electric discharges (red, blue, and green) having dimensions of a few hundred microns. During a single plasma pulse, complex phenomena occur as shown in this three-dimensional simulation of optical emission. Courtesy of Plasma Dynamics Corporation. sciences face extreme challenges to develop comprehensive and validated theo- ries, computer models, and material property databases (collision cross sections, reaction and transport coefficients, etc.) that place predictive capabilities in the hands of technologists. Developing a predictive capability to quantify and advance

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Plasma science 4 Torr FIGURE 2.3 Advanced particle-in-cell simulation techniques provide a first-principles representation of advanced materials processing reactors such as this magnetron reactor. The electron density is shown as a function of position. This reactor may be used to etch nanometer-sized features or deposit only a few monolayers of metal on 300-mm wafers for fabrication of microelectronics. Courtesy of fig 2.3 K. Nanbu, Institute for Fluid Science, Tohoku University. revised text our understanding of low-temperature plasmas and to leverage that understanding by speeding the development of technologies that benefit society represents the highest level of challenge and the highest potential return. RECENT PROgRESS AND TRENDS Low-temperature plasma science and engineering have long been driven by technological applications in disparate fields. For example, the jet turbine coat- ing industry and the microelectronics industry both depend on plasmas, yet their researchers typically have few technical interactions. Advances in nonequilibrium electron transport that resulted from higher dimensional solutions of Boltzmann’s equation benefited nearly the entire low-temperature discipline. However, when these advances were applied to investigating phenomena in different technology areas, the discipline fragmented.

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Plasma science 4 Fundamental Data Predictive models and optical diagnostics in low-temperature plasmas rely on fundamental data such as material properties, cross sections, and reaction rate coef- ficients for both gas-phase and surface processes. Although plasma chemistry mod- els for complex systems interacting with surfaces may have hundreds of reactions, the corresponding fundamental data are not available in the archival literature. The experience of the field throughout the development of semiconductor processing plasmas over the past two decades is that traditional laboratory measurements of these properties cannot keep pace with what is needed for the rapid development of the applications and changes and investigation of process chemistries. In the past decade, the appetite for input data has motivated significant ef- forts to develop databases using a variety of techniques, ranging from ab initio to semiempirical methods and scaling laws. The multipronged approach has been suc- cessful in several applications, notably (1) metal deposition chemistries for semi- conductor manufacturing and (2) lighting plasmas. The success of this approach rests on the recognition that it is more important to develop a data set or reaction mechanism that describes the plasma as a whole rather than a deep understand- ing of any given microscopic process. As such, a data set is a self-consistent list of reactions and corresponding data that can be used to predict plasma behavior with sufficient fidelity over a specified range of conditions. The best data sets are a careful trade-off between accuracy and generality on the one hand and the effort to develop them and the computational effort to make use of them on the other. Good data sets can even be used to identify critical processes where additional accuracy is justified. The refinement of these data estimation methods so that they can be used with confidence by plasma scientists is an important activity. Even with robust data estimation methods, low-temperature plasma science will continue to support the atomic and molecular physics community, particularly the collision physics community, as a vital source of fundamental data without which progress in low- temperature plasmas would be much slower. Because the stewardship of this re- search has been almost entirely ad hoc, there are few guarantees for the future. Thus, in spite of its importance, the ability to make fundamental measurements—of, for example, electron impact cross sections—or the ability to compute such values is in danger of being lost in the United States unless the priorities change. Moreover, the lack of a clear federal commitment to this research makes such research unattractive to universities, and they are unlikely to hire new faculty with this expertise. THE INTERNATIONAL PERSPECTIvE The German Ministry for Education and Research (BMBF) has published sev- eral reports on low-temperature plasma research. Evaluierung Plasmatechnik stands

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering  and out for its extensive use of surveys, data analysis, and economic assessment. Plasma Technology: Process Diversity and Sustainability is an English-language document that generally parallels and amplifies the applications and opportunities cited in the German-language report. From Evaluierung Plasmatechnik one learns that • The United States is world-class in the development of low-temperature plasma devices and systems, along with Germany and Japan; France, the United Kingdom, Italy, and Russia are in the middle. China and Korea are investing heavily. • In Japan some $30 million is devoted to research in low-temperature plas- mas by various Japanese agencies. The focus areas are plasmas for transi- tioning microelectronics to nanoelectronics, solar cell production, carbon nanotube production, and catalysis. • Cross-disciplinary programs and industrial group projects are important, and the German model uniquely brings academic research together with medium and large companies. Over the period 1996-2003, the BMBF in- vested €63.7 million (approximately $80 million) into 34 such cooperative projects. • Some 350,000 German manufacturing jobs depend on plasma processes that are indispensable for the technology involved, representing $64 billion per year of economic activity. Sales of plasma sources and systems amount to $35 billion per year. • In the United States there is no centralized organization to promote plasma technology development, and correspondingly no multiyear vision for the field. • U.S. priorities are shaped by a long and complex process involving many people. U.S. organizations have no specific plasma emphasis. Indeed, a national initiative to support cross-disciplinary plasma research is lacking altogether in the United States. • The emerging use of plasmas in life science is a U.S. strength not only because it necessitates interdisciplinary research but also because of U.S. strength in biotechnology. • The United States is weak in the training of new plasma scientists, but it compensates by attracting scientists from all over the world. Evaluierung Plasmatechnik notes a confusing divergence of opinion about the progress of the United States in low-temperature plasmas. The United States is rated as strong by most of the rest of the world but as weak by those working here. The committee proposes that this disparity occurs because external assessors base their observations on end products like computer chips. The United States is indeed a formidable competitor in this and other areas that involve plasma science, but for reasons that go far beyond the state of the science. Although this committee is not

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Plasma science  expert in global economic trend analysis, it believes that the entrepreneurial spirit, system of laws, and access to capital are also important for commercial success. From another perspective, one can examine the level of U.S. participation in the professional and international low-temperature plasma community. Recent inter- national benchmarking exercises have proposed looking at the proportion of papers presented by U.S. university researchers at scientific conferences. For instance, at the recent 2006 Gaseous Electronics Conference, the premier such conference in the United States, fewer than half of the papers came from U.S. authors. Fifteen years ago, this conference would have been dominated by papers from U.S. authors. Journals such as Transactions on Plasma Science, once dominated by U.S. authors in the subdisciplines of low-temperature plasmas, now are highly international. In turn, U.S. authors have low participation rates in foreign journals such as Journal of Physics D in the subdisciplines of low-temperature plasmas. THE ACADEMIC PERSPECTIvE There is currently no regular federal program dedicated to support the science of low-temperature plasmas at universities in the United States (see Appendix D for a brief survey of identifiable sources of public funding). Rather, the science is advanced within larger programs, both private and public, to develop specific technical applications that use plasmas. For example, the National Nanotechnol- ogy Initiative is a notable source of funding for developing nanotechnologies that use low-temperature plasmas. Much good plasma science is done within such programs. In fact most of the scientific highlights described earlier in this chapter came out of such applications-directed work. However, the amount of research on fundamental low-temperature plasmas attributable to areas such as materials processing and nanotechnology is tiny at best and the arrangement is ultimately unstable. Faculty appointments are based, in part, on the prospect for substantial, continued funding, leading to commensurate scientific breakthroughs and recogni- tion in a science area. It is the committee’s judgment that without a reliable source of funding for fundamental investigations in low-temperature plasmas, there will be soon be no faculty. Without faculty there can be no course development, text- books, workshops, graduate theses, or scientists educated in the field entering the workforce. It is for this reason that the committee concludes that in the absence of clear action, low-temperature plasma science as an academic discipline will probably soon cease to exist in the United States. The loss of an academic basis for low-temperature plasma science would not only undermine the U.S. ability to train experts in this field but would also significantly reduce the capacity for U.S. innovation in the field. In K-12 education, exposure to plasma science is essentially nonexistent. Plas- mas are not a standard topic in introductory or required physics courses at the

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering  and undergraduate level. At the graduate level, the highly interdisciplinary nature of low-temperature plasma science and engineering has caused plasma-related educa- tion to be fragmented across several academic disciplines. While physics depart- ments are obvious homes for courses in plasma physics, the majority of scientists and engineers involved in low-temperature plasmas are trained not in physics departments but in any of several engineering disciplines (e.g., chemical, electrical, mechanical, aeronautical), chemistry, or materials science. Only a few universities in the United States offer graduate courses in low-temperature plasma physics, and in only a few academic universities does one find a critical mass (more than a single faculty member) of research activity in low-temperature plasmas. This situation stands in stark contrast to several relatively large research laboratories dedicated to low-temperature plasmas at academic institutions in Europe (Ireland, Italy, France, Germany, and the Netherlands) and in the Far East (Japan and Korea). The U.S. funding situation deteriorated in stages since the last decadal survey. At that time, some low-temperature plasma science was supported by the Office of Naval Research, the Air Force Office of Scientific Research, the Office of Basic Energy Sciences at DOE, and the National Science Foundation (NSF). The NSF ERC for Plasma-Aided Manufacturing was still active at the University of Wis- consin and the University of Minnesota, and some research has been supported through Presidential Young Investigator grants. The NSF-DOE Partnership on Basic Plasma Science provided some funding during this time as well. Since the last decadal study, more than half of the funding sources for low-temperature plasma science have either disappeared or been dramatically reduced. As the committee prepares this decadal survey it can say that U.S. public funding is insufficient for young researchers to build and sustain a research program in the field. A result is that few if any openings for junior faculty exist in low-temperature plasma science, because academic departments are unlikely to seek faculty in areas that have such poor prospects for funding. The interdisciplinary nature of low-temperature plasma science has impeded the kind of discipline-based evolution that enabled other fields to maintain large centers of research, education, and training at U.S. universities. At the same time, however, it provides exceptionally fertile ground for interdisciplinary education and training activities, provided that appropriate linkages can be built across aca- demic departments, institutions, and private industry. This will require proactive and sustained support at the national level. For example, a new application of plasma science usually brings with it the need for a new, completely different skill set, such as a clinical researcher who is developing surgical plasma instruments. A highly effective approach, in view of the cross-disciplinary nature of the oppor- tunities, is to have a balanced mix of investigators from very diverse disciplines. The fundamental plasma science is investigated in the context of an application, to optimize the relevancy of the science while speeding the development of the

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Plasma science  technology. It is difficult to imagine a more fertile environment for the education of young scientists and engineers. THE INDuSTRIAL PERSPECTIvE The true industrial viewpoint is the global perspective, in that companies op- erate in a globally competitive environment, and low-temperature plasma science transcends national boundaries. The U.S. perspective reflects a concern for the health of U.S. science, education, and industry within the global environment. Industries that rely on low-temperature plasma technologies are no different than other industries that must globally compete. There is a constant need to inno- vate, to protect intellectual property, to focus on the highest value-added activities, to move quickly, and to manage risk. In short, it is an environment where time is money and where great value is placed on predictive capabilities that are accurate and reliable. The ability to understand and predict plasma behavior from a solid foundation of plasma science is the central theme of this report. A robust U.S. effort in low-temperature plasma science, reinforced by the competitive strength and entrepreneurial spirit of the United States, can convert the benefits of the ap- plications not only into benefits for our nation but also into global benefits. From the perspective of industry, education, training, and texts in low- temperature plasmas are scarce at all levels, from B.S. to Ph.D., pointing to a dearth of plasma science faculty to develop and teach such curricula. There is no core set of diagnostics, codes, and data to be nurtured, so that improvements and break- throughs are not leveraged across the field. This points to a lack of coordination and stewardship of the field. There have been, and continue to be, cooperative arrangements between industry and academia—for example, the Semiconductor Research Corporation—but such arrangements are far more common outside the United States—for example, Germany’s BMBF and Japan’s MITI.1 Low-temperature plasmas already have global importance, and their impact is likely to grow. Companies of all sizes, from one-person start-ups to the world’s largest industrial companies, contribute to and benefit from these growth areas. There is no lack of opportunity. The question for low-temperature plasma science and engineering as a discipline is whether the scientific progress will be led by open, public research or will be confined within companies that sometimes view the dissemination of knowledge as the loss of competitive advantage. Immigration has been an important source of scientists for U.S. industry and for low-temperature plasma science in particular since the beginnings of industrial 1 The committee notes this pattern in passing; it certainly might be worthy of further study by a more qualified group to understand if it is more widespread and whether it arises from a structural difference in the U.S. university system.

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering  and research in the 19th century. Over the past 15 years the former Soviet Union has been a key source of scientific talent, and a current trend is the establishment of research facilities by U.S. industry in low-cost countries with abundant scientific talent, examples being India, China, and portions of Eastern Europe. The constant is that whatever the condition of U.S. academic plasma science, U.S. industry will draw on a global talent pool and, if expedient, will go where the talent is. Will the United States prosper in this global environment? Here, as expressed in a recent NRC report, one has cause for concern.2 Can the United States continue to rely on immigration as the primary source of scientific talent? Will subsidized industrial consortia in Europe and the Far East attract U.S. companies to operate there? Will U.S. companies continue to support U.S. graduate student research when it is less costly to hire an experienced Ph.D. in an overseas lab? The answers to these questions have impacts far beyond the health of low-temperature plasma science industries. STEWARDSHIP OF THE FIELD The fields of thermodynamics and aeronautics have historically benefited from the leadership and coordinating role of NASA through works such as the Joint Army Navy Air Force (JANAF) database. Genetic research moves forward faster and more effectively with the guidance and assistance of the National Institutes of Health (NIH); in fact, although DOE’s Office of Biological and Environmental Research contributed significantly to the successful Human Genome Project, were it not for the home base of this research at NIH, it would have never moved forward so effectively. Low-temperature plasma science and engineering could be similarly propelled forward if there were a good steward for the field. However, it is not practical, and perhaps not even desirable, for a single agency or entity to become the steward for all of the science and applications given the diffuse nature of low- temperature plasma science, the diversity of the applications, and the advantage, in many cases, of involving private companies, from start-ups to conglomerates. Rather, some imaginative new paradigm may be required that captures the inter- disciplinary nature of the field: one that supports the fundamental science while integrating the applications-oriented research across constituency groups. The commercial importance of low-temperature plasmas might lead one to assume that industry should pay for the research and that public funding should have no role. In addition to improving the fundamental knowledge base, public funding can have a large, positive impact because it can be targeted at common scientific issues that have a broad impact across the discipline and across the 2 See Rising Above the Gathering Storm: Energizing and Employing America for a Brighter Economic Future, Washington, D.C.: The National Academies Press, 2007.

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Plasma science 0 industrial effort to apply plasmas for practical benefit. Public funding also has a role because companies tend to see basic research as a risky way to gain commercial advantage and its open publication as a loss of that advantage. Private funding of academic research and training is under extreme pressure because globalization has made it more costly for a company to fund a graduate student in the United States than hire an experienced staff member in other countries. U.S. policy makers and funding agencies represent the public’s interest, which goes well beyond the competitive advantage of any one company. Public funding for low-temperature plasma science can ensure that research is conducted and disseminated in a way that promotes scientific progress, trains the next generation of scientists, and serves the national interest. Unless concerted effort is applied, fundamental research and development in low-temperature plasmas for U.S. companies will continue to be progressively and perhaps irreversibly performed offshore, a trend that will probably also result in high technology manufacturing being performed offshore. As notably observed in the NRC report Globalization of Materials R&D: Time for a National Strategy,3 the movement of high-technology manufacturing offshore is an inevitable response to free market forces and is not intrinsically problematic. However, the longer-term strategic concern is whether the United States will be able to maintain access to and competency in the latest scientific and technical developments if the bulk of the basic and applied research moves offshore. Active stewardship of low-temperature plasma science and engineering in the United States is required. CONCLuSIONS AND RECOMMENDATIONS FOR THIS TOPIC Low-temperature plasma science is an indispensable part of entire sectors of our high-technology economy. The unique, chemically active plasma environment can produce materials, fabricate structures, modify surfaces, propel vehicles, pro- cess gas streams, and make light in ways that are not otherwise possible. The prac- tical contributions can be measured in real economic terms. The worldwide $250 billion semiconductor microelectronics industry is built on plasma technologies. The $2 trillion telecommunications industry, and all of the commerce, research, and technology enabled by microelectronics, would not exist in its present form in the absence of plasma etching and deposition. The entire state of worldwide technology would be dramatically different in the absence of plasma-assisted mi- croelectronics manufacturing, perhaps stalling at a 1990 level. Let’s consider some examples. Gene sequencing, which is enabling huge advances in health care, would not be possible if the researchers were forced to use 1990 computing technologies. 3 NRC, Globalization of Materials R&D: Time for a National Strategy, Washington, D.C.: The Na- tional Academies Press, 2005, pp. 3-5.

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering 1 and Lighting consumes 22 percent of all electric power produced in the United States; the power consumption would be would be three to five times higher in the absence of plasmas. The majority of turbine blades in state-of-the-art jet engines are coated using plasma spray techniques. Worldwide air-based commerce would not exist in its present form without plasmas. There would not be any two-engine, transoceanic commercial aircraft nor would there be high-performance fighters. Conclusion: Low-temperature plasma science and engineering is an area that makes indispensable contributions to the nation’s economic strength, is vital to national security, and is very much a part of everyday life. It is a highly interdisciplinary, intellectually diverse area with a rich set of scientific challenges. Low-temperature plasma science and engineering is a vital and continually evolving field. Within the last decade, startling new science developments have led to new applications such as hypersensitive optical detectors using microplasmas, plasma augmented combustion, plasma surgery, and plasma propulsion. The so- lutions to society changing problems (e.g., energy sufficiency, high-performance materials, sustainable manufacturing) can be partly found in the science and ap- plication of low-temperature plasmas. Decadal surveys like this one often ask what opportunities will be lost if the United States does not support low-temperature plasma science and engineering. In this report, the more important question is about the consequences of failing to exploit the scientific challenges and opportunities outlined in this chapter. Moore’s law for microelectronics and for developing the generations of microelectron- ics devices beyond current technologies can only be sustained with advances in low-temperature plasma science. Advanced materials for the entire realm of tech- nologies that improve energy usage, from solar cells to fuel cells to high-efficiency combustion, will rely on advances in low-temperature plasma science. The next generation of biotechnology devices, from labs-on-a-chip to human implants, will require advances in low-temperature plasma science. There is a one-to-one map- ping of these societal benefits with addressing and solving the science challenges described here. Certainly, low-temperature plasma science and its many applications will con- tinue to advance but at an ad hoc and unplanned rate. The question addressed in this decadal survey is whether or not the United States will propel the science and claim the benefits. Low-temperature plasma science and engineering are not recognized or funded as a scientific discipline in the United States. Progress in low- temperature plasma science occurs, for the most part, as a hidden part of programs whose emphasis is to develop applications that use low-temperature plasmas. Plasma science is now more often than not accomplished under the umbrella of a project funded to develop, for example, superhard refractory plasma deposited

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Plasma science 2 coatings, but not as a main thrust of the activity. As a result, the science lags the application and the plasma is viewed as a mysterious black box that is as likely to misbehave and ruin a promising application as it is to be the scientific cornerstone of an application with major societal impact. Conclusion: The science and technology benefits from low-temperature plasma science and engineering, and the health of the field itself, depend on strong connections both with the applications—biology, environment, microelectronics, medicine, etc.—and with several closely allied sciences, notably atomic and molecular physics, chemistry, and materials science. The close coupling between science and application imparts a special vitality to the scientific work. When science and applications are in close contact, the science impacts the applications in positive ways that are readily understood by a wider audience. Low-temperature plasma science seeks to maintain this positive relation- ship. What is undesirable is the current imbalance, where effectively all scientific work occurs within mission- and objective-oriented programs whose fundamental purpose is something other than advancing plasma science. It is duplicative and wasteful because each application resolves the same science issue. It does not take the science to a point mature enough for general use that can translate the science across the entire field. It damages the credibility of plasma science and technology as a whole. That is, progress in low-temperature science is hindered by research programs that are perhaps too tightly coupled to applications. Conquering the intellectual challenges now facing the field requires a more coordinated, funda- mental approach that advances the science in a manner that will also ultimately benefit applications. Interagency collaborations such as the NNI have been effective in promoting and advocating intrinsically interdisciplinary fields of science. National consortia of companies, such as the Semiconductor Research Corporation, have successfully contributed to the vibrancy and health of a research sector that is critical to the economic well-being of the country. Conclusion: Low-temperature plasma science and engineering share much intellectual space with other subfields of plasma science such as basic plasma science, magnetic fusion science, and space plasma science and will benefit from stewardship that is integrated with plasma science as a whole. Low-temperature plasmas share scientific challenges with other branches of plasma research. For instance, the principles underlying plasma heating, stabil- ity, and control in the low-temperature regime are the same as those that govern processes in magnetic fusion, just as the emergence of collective behavior is shared with many other areas of plasma science. Another crosscutting topic is plasma in- teractions with surfaces: These interactions are often the desired outcome of certain low-temperature engineering procedures, but in fusion, they must be controlled

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l ow - t e m P e r at u r e P l a s m a s c i e n c e engineering  and and minimized. Finally, basic plasma science studies of dusty plasmas have shed enormous light on the mechanism for controlling the rates and purities of plasma etching reactions. There is substantial overlap between the scientific objectives of low-temperature plasma research and the other branches of plasma science. The time is now to tap into this synergy. Conclusion: There is no dedicated support within the federal government for research in low-temperature plasma science and engineering. The field has no steward because of its interdisciplinary nature and its connection to applications. As a result, the basic research, conducted primarily at u.S. universities, and the host of potential future applications underpinned by it are eroding and are at substantial risk of collapse. The field is in danger of becoming subcritical and disappearing as a research discipline in the united States. Low-temperature plasma science and engineering are recognized as a scientific discipline internationally and are nurtured and funded as such. It would be desir- able to have a more data-centered discussion of this topic, but the fact is that no U.S. entity has taken up the role of steward for this field, even to the extent of col- lecting data. In the absence of data, the committee reverted to foreign assessments and anecdotal information. Recommendation: To fully address the scientific opportunities and the intellectual challenges within low-temperature plasma science and engi- neering and to optimally meet economic and national security goals, one federal agency should assume lead responsibility for the health and vitality of this subfield by coordinating an explicitly funded, interagency effort. This coordinating office could appropriately reside within the Department of En- ergy’s Office of Science. Low-temperature plasmas are pervasive and critical to the nation’s economy and security; they pose intellectual challenges of the highest caliber that stand inde- pendent of their practical use. There is, however, no coordinated national steward- ship of the field. That is, even if an initiative in federal support for low-temperature plasma research were to be undertaken, there is no entity within the government to oversee and lead it. (By contrast, NSF has clear stewardship over the NNI.) Establishing a dedicated program within the Department of Energy’s Of- fice of Science would provide a science-based infrastructure for research in low-temperature plasmas. Support for the fundamental science would also ap- propriately reside in this lead agency. Because of the strong interdependence of low-temperature plasma science and its application, reflected in the ties between academia and industry, a low-temperature plasma science program would have to be well coordinated with related activities across the federal government. Coordination of agency efforts is facilitated by the White House Office of

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Plasma science 4 Science and Technology Policy; in some cases, such as the NNI, interagency coor- dination is also guided by a full-time director and coordination office. Just as the NNI effort is not a monolithic federal investment, neither would low-temperature plasma science and engineering be one. Instead, it would comprise a lead science effort with connections and collaborations in NSF, DOD, NIST, and even other parts of DOE. This new paradigm for low-temperature plasma research would also include U.S. industry. It should focus on scientific research topics, but in view of the many technical applications and the cross-disciplinary nature of the field, it should also • Integrate across institutions (universities, national laboratories, and industry); • Integrate across disciplines (from physics to engineering to medicine); • Ensure that the research portfolio aligns with applications addressing na- tional needs; and • Develop the fundamental research component and clarify its connections to the diverse applications. Seamlessly bringing together disciplines is difficult enough.4 Seamlessly inte- grating institutions with very different purposes and legal structures (e.g., national laboratories and industry) is even more difficult. The committee emphasizes, how- ever, that these difficulties are very real and must be overcome. One such model might build on the success of the NNI by employing a full- time director for low-temperature plasmas. The director, assisted by a board of advisors from industry similar to the boards convened for the directorates of NSF and the DOD Offices of Scientific Research, would be responsible for maintaining and growing the initiative and setting priorities for funding. The director would also act as an advocate for the field with federal agencies in setting agency priorities, with the public, and with the popular media. This consortium might be unique among the federal agencies sponsoring research in having strong participation from industry as both advisory and funding partners. A model for coordinating the funding of basic research with applied research is the Semiconductor Research Cor- poration. The coordinating office and director could appropriately reside within the Office of Science at DOE. 4 The NRC report Facilitating Interdisciplinary Research, Washington, D.C.: The National Academies Press, 2004, explores some techniques for responding to these issues on campus.