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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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.
