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Low-Temperature Plasmas

INTRODUCTION

During the last half century, low-temperature plasmas have made a dramatic impact on society, significantly improved the quality of life, and provided challenging scientific problems. Examples are the fluorescent lights that can be found in almost every home in America; high-power switches that control the electrical grid of the United States and divert electrical power on command; gas discharge lasers, including the red He-Ne laser, which was the first gas laser invented, and the high-power, infrared, CO2 lasers that are used daily in surgery and metal working; and plasma sources that provide positive and negative ions for ion-beam accelerators. These ion sources are used to implant ions into materials, including semiconductor chips for the computer industry, and to harden bearings to increase the life and reliability of high-performance engines. Provided the opportunity, the field of low-temperature plasmas will continue to make significant contributions.

Based on the preceding paragraph, it is not surprising that low-temperature plasmas are important in many disciplines. Typically, they are high-pressure collision-dominated plasmas that have average electron energies of 1–10 eV. The purity of the gas is often important, and the physics and chemistry of the excited atomic states dominate the discharge characteristics. In industrial applications, the stability of the discharge frequently impacts the design and utility of the process, and the heterogeneous wall chemistry often impacts its reproducibility and reliability.

Unfortunately, because basic research in this area has been neglected for



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Plasma Science: From Fundamental Research to Technological Applications 1 Low-Temperature Plasmas INTRODUCTION During the last half century, low-temperature plasmas have made a dramatic impact on society, significantly improved the quality of life, and provided challenging scientific problems. Examples are the fluorescent lights that can be found in almost every home in America; high-power switches that control the electrical grid of the United States and divert electrical power on command; gas discharge lasers, including the red He-Ne laser, which was the first gas laser invented, and the high-power, infrared, CO2 lasers that are used daily in surgery and metal working; and plasma sources that provide positive and negative ions for ion-beam accelerators. These ion sources are used to implant ions into materials, including semiconductor chips for the computer industry, and to harden bearings to increase the life and reliability of high-performance engines. Provided the opportunity, the field of low-temperature plasmas will continue to make significant contributions. Based on the preceding paragraph, it is not surprising that low-temperature plasmas are important in many disciplines. Typically, they are high-pressure collision-dominated plasmas that have average electron energies of 1–10 eV. The purity of the gas is often important, and the physics and chemistry of the excited atomic states dominate the discharge characteristics. In industrial applications, the stability of the discharge frequently impacts the design and utility of the process, and the heterogeneous wall chemistry often impacts its reproducibility and reliability. Unfortunately, because basic research in this area has been neglected for

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Plasma Science: From Fundamental Research to Technological Applications many years, there is a severe lack of quantitative and experimental understanding of a wide range of phenomena that occur in low-temperature collision-dominated plasmas. Most low-temperature plasma applications involve complex reactions between electrons and a host of atomic, molecular, and ionic species. These species are found in highly excited states not encountered in nonplasma environments. Operation of plasmas in applications ranging from lasers to materials processing and lighting requires optimization of the densities of these species. Scientists modeling these systems require a broader range of diagnostics to characterize species densities in benchmark plasmas, and more powerful methods for measuring, calculating, or approximating the cross sections that dominate the rate equations. In some cases, such as microwave breakdown, the positive column of dc metal-vapor rare-gas discharges, and wall-stabilized arcs, researchers have obtained experimental data, theoretical understanding, and predictive models. However, much of this basic research was performed before 1960. In some cases with immediate industrial and government applications the information was updated in the 1970s, using modern experimental and modeling techniques. Examples include fluorescent lamps, high-intensity lamps, electron-beam-controlled discharge lasers, some specific plasma processes, and arcs (e.g., in discharge-limiting situations, such as transport in weakly ionized swarms and near thermal equilibrium). This research produced a significant improvement in the performance of devices using these plasmas. Recent research was driven by interest in high-power lasers for ballistic missile defense. The decline of interest in that use has severely reduced related funding. In other areas, there has been limited progress during the last 30 years, including understanding phenomena such as collisional discharges in magnetic fields in the presence of boundaries, transient discharges and sheaths, discharge stability, and plasma interactions with practical surfaces. For example, recently there has been much interest in the dc cathode fall, since modeling and experiments are much further ahead for bulk-phase plasmas than for cases, such as the cathode fall, in which plasma contact with surfaces is important. Lack of research support in the physics of low-temperature plasmas has resulted in a low level of training in collision-dominated low-temperature plasmas and in the training of engineers and physicists for plasma processing. No federal agency claims responsibility for this area. The existing support has emphasized short-term goals and work only on current government- and industry-related topics. In FY 1991, there were only two long-term projects, and neither is currently funded. It is our understanding that since the beginning of FY 1992, there has been essentially no low-temperature plasma research project with more than a one-year time scale, since research in this area is dominated by the current needs of the radio-frequency plasma processing and lighting industries. This short time scale severely discourages new, innovative, or thorough research. Novel experimental and modeling tech-

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Plasma Science: From Fundamental Research to Technological Applications niques should be developed to explore new areas and provide more quantitative work in existing areas. A serious problem in low-temperature collision-dominated plasmas has been the lack of reproducible experimental verification of theoretical predictions. This is partly due to the critical dependence of the relevant phenomena on surface conditions and gas purity. It is also due to the fact that the models are too limited and qualitative to be tested or to be of general use. While gas purification techniques have been known for many years, the role of impurity effects in practical systems is often poorly understood. The problem of the reproducibility of practical surfaces is very difficult, and few useful and successful recipes exist. Even fewer techniques exist for characterizing practical surfaces with regard to their interactions with plasmas. As a result, most of the successful quantitative gas-discharge investigations are of phenomena that are relatively free of surface effects (i.e., microwave breakdown, the positive column, swarm transport, and near-equilibrium radiation). However, given adequate funds, more realistic models could be developed to investigate these complex phenomena with modern computer facilities. The Japanese government has long supported an active program in basic gas discharge research, particularly in its engineering schools. Japanese research is recognized internationally for its quality and impact. In spite of a major focus on plasma processing, many Japanese faculty still devote a significant fraction of their time and resources to basic, undirected research. In recent years the French government also has supported a large effort in low-temperature collision-dominated plasma research, which has produced a large fraction of the invited papers at recent international meetings. To change the situation in the United States will require strong support for research in applied physics and engineering in the area of the basic physics of low-temperature plasmas. Experimental programs emphasizing quantitative and reproducible results will be necessary to properly test the predictions of theoretical models. Improved understanding of these plasmas is necessary for applications such as plasma processing and environmental cleanup. This basic research can also be expected to yield innovative experimental techniques and novel modeling methods, and it will provide highly trained scientists and engineers in low-temperature plasma science. That low-temperature plasmas are crucial in so many technologies is both a strength and a weakness. These plasmas are indispensable in today's highly technical world, but since they are useful in many apparently disconnected disciplines, no agency has taken responsibility for research in low-temperature plasmas. This chapter focuses on the following important areas of low-temperature plasma physics: lighting, gas discharge lasers, plasma isotope separation, space propulsion, magnetohydrodynamics, and the use of plasmas for pollution control and reduction. Another major area is plasma processing, which was addressed in

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Plasma Science: From Fundamental Research to Technological Applications detail in the recent report of the National Research Council (NRC) Panel on Plasma Processing of Materials.1 The findings and recommendations of that study are summarized below in the section ''Plasma Processing of Materials.1 The findings and recommendations of that study are summarized below in the section "Plasma Processing of Materials." LIGHTING Lighting has been one of the principal areas contributing to the understanding of low-temperature plasmas, and historically it has been responsible for much of the low-temperature plasma research in industry. Unfortunately, due to the severe recession of the late 1980s and early 1990s, this research effort has declined rapidly. Westinghouse and GTE-Sylvania have sold their lighting divisions to foreign investors, leaving General Electric the only large U.S. lighting company. By contrast, low-temperature plasma research in the Far East has been increasing rapidly; it is now three to four times larger than that in the United States. Important contributions from lighting in the past 10 years include the control and modification of the electron-energy distribution function and novel laser diagnostics that provide valuable microscopic information about discharge parameters. Other important contributions include sophisticated and predictive models of lighting discharges and an understanding of the effects of isotopic gas mixtures in low-pressure mercury rare-gas discharges. Major technological innovations have been made in the last decade in many areas. They include lower-power compact fluorescent and high-intensity discharges (HID), a variety of electrodeless discharges such as microwave, rf, and surface wave discharges for practical lighting applications, and the electronic ballasting of light sources. Other important innovations include an improved understanding of heterogeneous chemistry, resulting in superior performance and better compatibility with existing and novel materials, and the development of a systems approach to light sources that integrates principles of plasma discharges, materials, electronics, and homogeneous and heterogeneous chemistry. Although most of the R&D for lighting plasmas is performed by the lighting industry, the field has also benefited from advances in other disciplines. For example, solid-state, plasma processing, and materials advances made in other industries and in academic and research institutions have contributed to the progress in the lighting industry. A fundamental understanding of many processes is necessary for the lighting industry to design and fabricate higher-efficiency lamps. Therefore, university and government research has and will continue to impact the industry. Areas of research include, for example, local thermodynamic equilibrium (LTE) 1   National Research Council, Plasma Processing of Materials: Scientific Opportunities and Technological Challenges, National Academy Press, Washington, D.C., 1991.

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Plasma Science: From Fundamental Research to Technological Applications and non-LTE low-temperature plasmas, sophisticated diagnostics, and measurement of atomic and molecular cross-section data for electron-impact processes of importance to the lighting industry. Because industrial R&D goals have become increasingly short-term over the last 10 years, it is important that the academic and government communities initiate longer-term R&D in plasmas related to lighting. Examples of research and development that would be of great benefit to the lighting industry include the application of massive computation techniques to lighting problems and improved diagnostics that will provide detailed information about the behavior of lighting plasmas. New approaches to modeling and probing complex sheaths associated with thermionic electrodes, for example, would help the lighting industry reduce mercury and thorium usage and would also increase lamp efficiency and life. Other important topics include methods to obtain higher conversion efficiency of electrical energy into radiation and the evaluation and exploitation of solid-state sources for lighting applications. Scientific opportunities in lighting plasmas include the exploration of novel ways of producing monoenergetic or narrow electron-energy distributions in discharges to selectively excite electronic states, resulting in the more efficient production of radiation and the reduction of long-wavelength emission and thereby enhancing visible emission, using the principles of quantum electrodynamics and quantum interference. Radiation from lamps can have important applications in environmental cleanup and other areas, including water purification with light, promoting algae growth with special metal-halide sources to reduce heavy metal concentrations in water, accelerating food growth, and a variety of display applications. Lighting plasmas are synergistic with the fields of plasma deposition and etching, materials science, electronics, and lasers. However, increased scientific productivity in this area will require new basic experimental facilities. GAS DISCHARGE LASERS The field of gas discharge lasers has had considerable government support over the last 40 years. Strong support in the 1970s and 1980s led to an improved understanding of the basic phenomena in high-pressure plasmas, including electron-impact excitation cross sections of vibrational and electronic excited states, the physics of the stability of high-pressure discharges, and the homogeneous chemistry and products of excited-state reactions. Advances in the understanding of discharge physics include improved understanding and predictive capability of the discharge parameters and the stability of the plasma, discovery of the dominant impact that excited states have on discharge physics and laser chemistry, and an increased knowledge of electronic kinetics and the interaction between secondary electrons and excited states. This research made significant contributions to advancing the state of the

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Plasma Science: From Fundamental Research to Technological Applications technology by increasing the efficiencies of lasers from a fraction of a percent to 10%. This improvement was particularly dramatic in excimer lasers. Examples include the rare-gas lasers that radiate in the vacuum ultraviolet (VUV), the rare-gas halide lasers that lase in the ultraviolet (UV), the rare-gas triatomic excimers that have broadband emission in the visible, and the metal excimers that emit in the visible and UV. Gas lasers have produced many important technological capabilities. Examples include optical lithography, where rare-gas fluoride lasers have extended the resolution to less than 0.5 µm; laser working of metals, where high power CO2 lasers are now used routinely for welding, cutting, and marking in industry; and medicine, where lower-power gas lasers have made a significant impact, including the use of surgical CO2 lasers and excimer lasers for treating eyes and occluded arteries. The combined gas laser market is presently of the order of $300 million and is predicted to grow at an annual rate of approximately 5%. There are also other emerging uses for these lasers, such as LIDAR (laser radar) for airports that can measure the location of wind shear and thereby increase the safety of air travel, and laser-produced x-ray sources for microlithography. PLASMA ISOTOPE SEPARATION Funding for plasma isotope separation has decreased dramatically in the post-Cold War era. Isotope separation has been actively investigated for the last 20 years, principally by plasma centrifuge, laser (AVLIS), and ion cyclotron resonance techniques. Of these methods, the AVLIS program at Lawrence Livermore National Laboratory has been the most strongly supported. The results are classified. Briefly, the separation process involves the selective ionization of one isotope and the subsequent collection of this ion. Lasers are used to ionize the desired isotopes, which form a low-temperature plasma. Plasma physics issues that have to be addressed include excited and ionic species reactions, homogeneous chemistry, and the physics and chemistry of the sheath near the collection electrodes. More conventional plasma isotope separation has been investigated on a much smaller scale by several groups including the FOM Institute for Plasma Physics in the Netherlands, Yale University, the Max Planck Institute in Germany, the Sydney University School of Plasma Physics in Australia, the National Space Research Institute in Brazil, and TRW in the United States. There are important uses of isotope separation besides nuclear fuels enrichment, including medical diagnostics, chemistry, and basic research. Thus, the development of plasma centrifuge technology offers a number of potential opportunities. The demand for stable, enriched isotopes for medical applications grows each year. The plasma centrifuge offers an improved means of meeting this need. However, at present, many important basic plasma phenomena remain

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Plasma Science: From Fundamental Research to Technological Applications to be understood in the rotating plasmas utilized in plasma centrifuges, and further research in this area will be necessary to fully exploit their potential. In addition to isotope separation, the plasmas developed for plasma centrifuges can also be expected to be useful for other applications, including use in imploding, "z-pinch," plasma x-ray sources and in plasma switches. PLASMAS FOR ELECTRIC PROPULSION OF SPACE VEHICLES Space electric propulsion has been studied for the last three decades. Concepts that have been investigated include expanding electrically heated plasmas, accelerating plasmas with thrusters and plasma guns, accelerating ions, and laser propulsion. Research groups have included TRW in Redondo Beach, California; Avco Everett Research Laboratory in Everett, Massachusetts; the National Aeronautics and Space Administration (NASA) Lewis Research Center; and several university research groups. Electric plasma propulsion requires less fuel mass than chemical systems, potentially making launching of satellites and space exploration less expensive. Decreasing the weight of fuel and hence the overall payload could have a significant impact on the $9.5 billion currently spent annually for launches: $5 billion by the Department of Defense (DOD), $3 billion by NASA, and $1.5 billion by industry. A plasma propulsion device (an ion accelerator) has been tested and has worked successfully in space for 13 years. A key issue for any such device that is launched into space is its reliability and longevity in both its on-the-shelf and operating lives. This makes the use of electrodes problematic. TRW has been pursuing electrodeless thrusters, and these plasma accelerators potentially could satisfy the demanding reliability requirements of space qualifiable systems. Other areas that require research include a better understanding of the plasma, identification of the appropriate gas or fuel, and matching the electrical driver to the nonlinear plasma load. There is presently a pressing need for low power (e.g., of order 100 W) thrusters for long term maintenance of orbits. The requirements for interplanetary missions will require significantly higher-power thrusters. The development of space plasma propulsion systems also is synergistic with other applications. For example, plasma accelerators can be used in plasma processing and in the simulation of space plasmas to determine the chemistry of these reactive media on satellites. MAGNETOHYDRODYNAMICS The branch of magnetohydrodynamics (MHD) of interest here is that concerned with plasmas at low temperatures (2000–10,000 K) and high pressures (1–10 atm). Most of the current work in this area is engineering development rather than scientific research. These efforts have focused primarily on electrical power generation, with some effort directed toward space vehicle thrusters. The power

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Plasma Science: From Fundamental Research to Technological Applications generation community is mostly industrial, and much of this work has been performed by Textron Defense Systems (formerly the Avco Everett Research Laboratory). There is some university involvement, such as the work at the University of Tennessee Space Institute. Workers in the field are generally engineers with plasma and fluid, thermoscience, mechanical engineering, or electrical engineering backgrounds. There are several technological opportunities for applications, including electric power plants; multimegawatt portable power supplies for land, air, or space uses; high-enthalpy test facilities for testing high-speed propulsion systems; magnetoplasmadynamic (MPD) thrusters for space vehicles; and MHD boost of oxygen-fuel jets in coating applications. The ultimate users of MHD power generation technology may be the utilities and independent power producers who generate electric power and who seek economic and environmentally benign methods of generating it. Major achievements during the past 10 years include the development of equipment capable of operating for long durations and a greater understanding of the physical phenomena associated with the corrosion and erosion of plasma-facing surfaces. Industry has played a key role in the engineering development of components and subsystems of power-generating systems for proof-of-concept demonstrations. The ultimate objective of this work is commercialization of the technology. The science community involved in this area is small. There is a need for a better understanding of basic MHD phenomena. Specific needs include a detailed understanding of conditions that lead to plasma instabilities at high power densities; better understanding and analysis of electric discharges in flowing, reacting gases; analysis and experiments to better understand electrode and boundary layer phenomena in the presence of strong magnetic fields, slag-layer shorting effects, and associated electrical nonuniformities; and a better understanding of scaling laws. Additional research could also foster the development of high-temperature, nonslagging channel walls and high-temperature direct-fired air preheaters. PLASMAS FOR POLLUTION CONTROL AND REDUCTION With the increased concern for the environment, the use of plasmas for pollution control and reduction is predicted to be an area of considerable growth in the next decade. In particular, American industry is beginning to realize the importance of low-temperature plasmas for pollution control (e.g., flue gas treatment, air toxics treatment), while international efforts at pilot-plant scale are much more advanced. At present, researchers in this community are focusing mainly on studies of the plasma chemistry and discharge physics of nonequilibrium plasmas. The plasmas are usually created by electrical or electron-beam-driven discharges. This field is very old in terms of the phenomenological un-

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Plasma Science: From Fundamental Research to Technological Applications derstanding of the phenomena involved and the identification of potential practical applications. Modern research in this field is highly applications oriented, with basic research focused primarily on the measurement and computer-based modeling of transient events. Applied research in this area is divided between a fundamental approach, involving basic discharge physics and plasma chemistry, and an Edisonian approach, centered on plasma-based pollution control devices. Nonequilibrium plasma technology has been applied to the chemical processing of gaseous media for more than a century. Two major applications are chemical synthesis, exemplified by ozone generation, and the removal of undesirable compounds from flue gases, exemplified by the electrostatic precipitator. During the past two decades, interest in applying nonequilibrium plasmas to the removal of hazardous chemicals from gaseous media has been growing, particularly because of heightened concerns over the pollution of our environment and a growing body of environmental regulations. These more recent applications have involved efforts to destroy toxic chemical agents, to remove harmful acid rain gases such as sulfurous and nitrous oxides, and to treat other environmentally hazardous hydrocarbon and halocarbon compounds. Major contributions in the last 10 years include the decontamination of wastewater, flue/stack gas processing for SO2 and NOx reduction, military applications (nerve agent destruction), and toxic chemical/vapor processing. Industry is a potential user and market for plasma technology for pollution control. The utility industry is faced with more stringent environmental regulations, which demand improved technology for effluent cleanup (both for power plants and for utility customers). Industry can play a strong role as an advocate for technology development and as a technical contributor by working cooperatively with researchers in the field on applications. Many institutions are actively involved in this area, including Lawrence Livermore National Laboratory, Los Alamos National Laboratory, Sandia National Laboratories, and several universities and industrial laboratories. To date, funding in this area has been small, but given the increasing concern about environmental issues, it could increase dramatically in the next decade. There is much that has to be learned before low-temperature plasmas can be used in the cleanup and preservation of the environment. This includes developing a better database for plasma chemical processes, reaction-rate constants, and the resulting products, and developing diagnostics to determine the physics and chemistry of the cleanup process. There are many scientific and technical opportunities, including developing basic plasma data on the reaction of excited states and radicals with various contaminants and sophisticated modeling of the physics and chemistry of plasmas they apply to the cleanup problem. Fortunately, there are many facilities in the various national laboratories, universities, and industry that can be used to perform the initial proof-of-principle experiments. There is a strong synergism between environmental applications and the many other disciplines that also depend on low-temperature plasmas.

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Plasma Science: From Fundamental Research to Technological Applications PLASMA PROCESSING OF MATERIALS Plasmas used for the processing of materials affect several of the largest manufacturing industries, including national defense, automobiles, biomedicine, computers, waste management, paper, textiles, aerospace, and telecommunications. The importance of plasma processing to the electronics industry is illustrated in Figure 1.1. An NRC study reviewed plasma processing of materials in detail in 1991.2 This section briefly summarizes its findings and recommendations. Applications of plasma-based systems used to process materials are diverse because of the broad range of plasma conditions, geometries, and excitation methods that may be used. This technology is multidisciplinary and, ideally, the researchers should have a basic knowledge of several scientific disciplines, including elements of electrodynamics, atomic science, surface science, computer science, and industrial process control. The impact of—and urgent need for—plasma-based materials processing is overwhelming for the electronics industry. In its report, the NRC study panel made the following statements:3 "In recent years, the number of applications requiring plasmas in the processing of materials has increased dramatically. Plasma processing [such as that illustrated in Plate 1] is now indispensable to the fabrication of electronic components and is widely used in the aerospace and other industries. However, the United States is seeing a serious decline in plasma reactor development that is critical to plasma processing steps in the manufacture of VLSI [very large scale integrated] microelectronic circuits. In the interest of the U.S. economy and national defense, renewed support for low-energy plasma science is imperative." (p. 2) "The demand for technology development is outstripping scientific understanding of many low-energy plasma processes. The central scientific problem underlying plasma processing concerns the interaction of low-energy collisional plasmas with solid surfaces. Understanding this problem requires knowledge and expertise drawn from plasma physics, atomic physics, condensed matter physics, chemistry, chemical engineering, electrical engineering, materials science, computer science, and computer engineering. In the absence of a coordinated approach, the diversity of the applications and of the science tends to diffuse the focus of both." (p. 2) "Currently, computer-based modeling and plasma simulation are inadequate for developing plasma reactors. As a result, the detailed descriptions required to guide the transfer of processes from one reactor to another or to scale 2   See footnote 1, p. 36. 3   See footnote 1, p. 36.

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Plasma Science: From Fundamental Research to Technological Applications FIGURE 1.1 The world electronics "food chain." Although revenues for plasma technology are a small portion of the world electronics market, plasma technology is a critical component upon which the industry rests. Note that the plasma reactor business is expected to quadruple in this decade. (Courtesy of R.A. Gottscho. Adapted from the National Advisory Committee on Semiconductors report, Preserving the Vital Base, Arlington, Va., July 1990, and from data in "Semiconductor Equipment Manufacturing and Materials Worldwide," Dataquest, Inc., 1994.) processes from a small to a large reactor are not available. Until we understand how geometry, electromagnetic design, and plasma-surface interactions affect material properties, the choice of plasma reactor for a given process will not be obvious, and costly trial-and-error methods will continue to be used. Yet there is no fundamental obstacle to improved modeling and simulation nor to the eventual creation of computer-aided design (CAD) tools for designing plasma reactors. The key missing ingredients are the following: (1) A reliable and extensive plasma data base against which the accuracy of simulations of plasmas can be compared.… (2) A reliable and extensive input data base for calculating plasma generation, transport, and surface interactions. … (3) Efficient numerical algorithms and supercomputers for simulating magnetized plasmas in three dimensions." (p. 3) "In the coming decade, custom-designed and custom-manufactured chips, i.e., application-specific integrated circuits (ASICs), will gain an increasing fraction of the world market in microelectronic components. This market, in turn, will belong to the flexible manufacturer who uses a common set of processes and

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Plasma Science: From Fundamental Research to Technological Applications equipment to fabricate many different circuit designs. Such flexibility in processing will result only from real understanding of processes and reactors. On the other hand, plasma processes in use today have been developed using a combination of intuition, empiricism, and statistical optimization. Although it is unlikely that detailed, quantitative, first-principles-based simulation tools will be available for process design in the near future, design aids such as expert systems, which can be used to guide engineers in selecting initial conditions from which the final process is derived, could be developed if gaps in our fundamental understanding of plasma chemistry were filled." (p. 4) "Three areas were recognized by the PLSC [Plasma Science Committee] panel as needing concerted, coordinated experimental and theoretical research: surface processes, plasma generation and transport, and plasma-surface interactions. For surface processes, studies using well-controlled reactive beams impinging on well-characterized surfaces are essential for enhancing our understanding and developing mechanistic models. For plasma generation and transport, chemical kinetic data and diagnostic data are needed to augment the basic plasma reactor CAD tool. For studying plasma-surface interactions, there is an urgent need for in situ analytical tools that provide information on surface composition, electronic structure, and material properties." (p. 4) "Breakthroughs in understanding the science will be paced by development of tools for the characterization of the systems. To meet the coming demands for flexible device manufacturing, plasma processes will have to be actively and precisely controlled. But today no diagnostic techniques exist that can be used unambiguously to determine material properties related to device yield. Moreover, the parametric models needed to relate diagnostic data to process variable are also lacking." (p. 4) "The most serious need in undergraduate education is adequate, modern teaching laboratories. Due to the largely empirical nature of many aspects of plasma processing, proper training in the traditional scientific method, as provided in laboratory classes, is a necessary component of undergraduate education. The Instrumentation and Laboratory Improvement Program sponsored by the National Science Foundation has been partly successful in fulfilling these needs, but it is not sufficient.'' (p. 5) "Research experiences for undergraduates made available through industrial cooperative programs or internships are essential for high-quality technical education. But teachers and professors themselves must first be educated in low-energy plasma science and plasma processing before they can be expected to educate students. Industrial-university links can also help to impart a much needed, longer-term view to industrial research efforts." (p. 5) Plasma processing of materials is a technology that is of vital importance to several of the largest manufacturing industries in the world. Foremost among these industries is the electronics industry, in which plasma-based processes are

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Plasma Science: From Fundamental Research to Technological Applications indispensable for the manufacture of VLSI microelectronic circuits (or chips). Plasma processing of materials is also a critical technology in the aerospace, automotive, steel, biomedical, and toxic waste management industries. Because plasma processing is an integral part of the infrastructure of so many American industries, it is important for both the economy and the national security that the United States maintain a strong leadership role in this technology. As in the case of other disciplines that use low-temperature plasmas, there is no centralized agency that takes responsibility for R&D for this area. The NRC plasma processing study determined that there are approximately 14 agencies within the federal government that invest approximately $17 million in plasma process science and technology. It concluded that this funding was inadequate and uncoordinated, given the impact of this vital area on the country. CONCLUSIONS AND RECOMMENDATIONS Conclusions Low-temperature plasma science has significantly improved the quality of our lives. These contributions will continue by providing solutions to several present and future problems and by preserving our industrial base and providing challenging opportunities in the post-Cold War era. Examples of technical areas that will benefit from low-temperature plasmas include the plasma processing of materials, environmental cleanup, "cold" sterilization of medical products, "cold" pasteurization of food, advanced imaging devices that can be used in medicine and in the detection of explosives and drugs, and isotope separation. Research in low-temperature plasmas has decreased substantially, primarily because the largest source of funding, the federal government, has had a shrinking budget for such activities in the last several years. Research has also been adversely affected by the recent recession and a general move of large U.S. companies to divest themselves of manufacturing. The shrinking budgets of the last few years have resulted in a sharp decrease in the population of scientists working in low-temperature plasma science. The supply of PhD-level scientists would be sufficient to reverse this trend, if funding were available. If this trend is not reversed, the United States will be creating a future problem. Recommendations To fully exploit the potential of low-temperature plasma science and maximize its impact on the many relevant technological applications, the panel recommends that one agency within the government be given the responsibility for coordinating research in low-temperature plasma science. Given the multidisciplinary nature of the field, the Advanced Research Projects Agency (ARPA) and

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Plasma Science: From Fundamental Research to Technological Applications the National Institute of Standards and Technology (NIST) are possible candidate agencies for this responsibility. The panel recommends that the responsible agency focus on funding low-temperature plasma science. Such projects would include the physics and chemistry of the plasma sheath; plasma stability; electrodeless plasma production; magnetic field effects on plasmas; improved diagnostics to help understand surface and sheath effects and plasma stability; energy and charge transfer from plasmas to particulates; and improved utilization of computers.