Plasma Science Advancing Knowledge in the National Interest (2007) / Chapter Skim
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2 Low-Temperature Plasma Science and Engineering
2 Low-Temperature Plasma Science and Engineering
Pages 38-74
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From page 38... ...
. 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)
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From page 39... ...
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.
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From page 40... ...
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.
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From page 41... ...
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)
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From page 42... ...
Many low temperature plasmas exhibit turbulent, chaotic, and stochastic behavior. Arc generated plasmas used to spray coat turbine blades are usually turbulent.
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From page 43... ...
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.
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From page 44... ...
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.
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From page 45... ...
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.
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From page 46... ...
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.
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From page 47... ...
During a single plasma pulse, complex phenomena occur as shown in this three-dimensional simulation of optical emission. Courtesy of Plasma Dynamics Corporation.
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From page 48... ...
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.
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From page 49... ...
Low-temperature plasmas are often used in environments requiring extreme re producibility over large areas or volumes. One example is plasma deposition over many square meters of substrate area for photovoltaics or flat panel displays with uniformities of a fraction of a percent; another is the etching of a single atomic layer of material for a microelectronic component (Figure 2.4)
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From page 50... ...
Plasmas with continuous power deposition at levels approaching MW/cm3 at pressures exceeding 1 atm are approaching the realm where quantum phenomena in plasmas may become important. Collective ef fects, transition to a liquid plasma state, and blurring of the boundary between gas- and condensed-phase plasmas hold unusual promise for discovering new phenomena (Figure 2.5)
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From page 51... ...
The current state-of-the-art light source is a mercury arc lamp whose pressure is more than 100 atm, with power dissipation exceeding an average of 100 W/cm3 and ap proaching 1 MW/cm3 based on arc volume. Fundamental science issues must be addressed for this class of photon sources to be advanced.
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From page 52... ...
In both ozone and ultraviolet plasma sources there is a trade-off between power density and efficiency, so a scientific breakthrough to more selectively generate ozone or ultraviolet light in a more compact space could spread the use of these proven, nonchlorine treatment methods. The supporting atomic physics must also advance beyond the current state of the art, requiring, for example, a detailed understanding of far-wing line broadening that occurs at extremely high pressure.
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From page 53... ...
Low-pressure plasmas are commonly used to modify the properties of high-value materials such as those used in microelectronics devices. High-pressure, filamentary plasmas are typically FIGURE 2.6 The profound coupling between plasmas and surfaces in low-temperature plasma science is illustrated by this molecular dynamics simulation of a semiconductor surface during plasma etching.
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From page 54... ...
Turbulent, Stochastic, and Chaotic Behavior of Complex Plasmas and Plasmas in Liquids Diagnosing, predicting, and understanding the unique properties of plasmas sustained in liquids, supercritical fluids, and multiphase media such as aerosols (e.g., dusty plasmas) will reveal new and unexpected physical phenomena and will provide a knowledge base for new technologies.
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From page 55... ...
. The band bend ing that occurs at the surface of microplasma sources with electric fields of many hundreds of kilovolts per centimeter is sufficient to merge the continua of the solid and gas phases.
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From page 56... ...
represents a fundamental challenge for technological applications such as plasma spraying. Spatial gradients can be so steep that a con tinuum description of heat and mass transfer may break down even at pressures of many atmospheres.
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From page 57... ...
. At the other extreme, plasma-assisted combustion may facilitate the development of advanced propulsion concepts such as SCRAMjets or the use of plasma coatings on turbine blades in jet engines to shape the airflow and allow conventional propulsion systems to operate more efficiently.
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From page 58... ...
These are chemically complex plasmas far from Boltzmann or Saha equilibrium. Because only a tiny fraction of the data needed to understand their operation is available for metal halides from traditional measurement techniques, computational models have been built that make extensive use of ab initio and semiempirical methods to generate the required input data (electron-impact cross sections, and gas and surface reaction rate coefficients)
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From page 59... ...
Although these abilities exist, in principle, by in tersecting electron and molecular beams, technologically important methods may require such selectivity over several square meters and so require less expensive and more easily scaled techniques. Scientific advances in chemically selective plasmas will make it practical to apply these unique conditions to large surfaces.
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From page 60... ...
Methods to Describe the Behavior of Plasmas That Contain Chaotic and Stochastic Processes Low-temperature plasmas have always been considered as being "hierarchical," "multiscale," or "hybrid." That is, the important plasma phenomena were catego rized according to the spatial scale or the timescale and linkages made between those hierarchies. It has not to date been practical to integrate electron trajectories in a plasma torch or to consider the molecular dynamics of a surface exposed to incident radicals in a manner that is fully integrated with reactor-scale phenomena.
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From page 61... ...
Scientific advances on the interaction of plasma species with living tissue may lead to much more selective and beneficial use of plasmas in medicine, analogous to the fine control that is now exercised in semiconductor processing plasmas. Courtesy of K.R.
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From page 62... ...
For example, how might an atmospheric-pressure-glow discharge be sustained in a highly attaching gas mixture over many square meters of nonplanar surface with a uniformity of processing to within a few percent? It is important to develop a fundamental understanding of the instabilities that occur in these plasmas and to identify methods to manage them.
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From page 63... ...
The field is in need of new diagnostics that are general and can be used by nonspecialists as well as highly specialized diagnostics for specific purposes. For example, at one extreme are tomographic methods that provide nanosecond, three dimensional resolution of the onset of instabilities at atmospheric pressure.
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From page 64... ...
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.
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From page 65... ...
• 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.
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From page 66... ...
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)
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From page 67... ...
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.
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From page 68... ...
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. Low-temperature plasmas already have global importance, and their impact is likely to grow.
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From page 69... ...
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 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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From page 70... ...
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.
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From page 71... ...
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.
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From page 72... ...
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.
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From page 73... ...
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.
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From page 74... ...
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.
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