Gases containing electrically
ch a r g e d p a r t i c l e s o ffe r a n
exc e l l e n t way o f m a n i p u l a t i n g
a n d t r a n s fo r m i n g m a t e r i a l s i n
a c o n t r o l l e d way
Queen's University Belfast
Technological plasmas
An inductively
coupled plasma
VISIONS 14
B R I E F I N G P A P E R S F O R
P O L I C Y M A K E R S
A hot plasma
from a DC arc
jet reactor
P
lasmas are the most common
form of matter in the visible
Universe. They are gases
containing charged particles –
usually electrons and atoms that
have lost electrons (ions). All the
stars we see are essentially
plasma, as is much of tenuous
matter found between the stars.
Interactions between charged
Plasma processing for industry
particles, and with any neutral
atoms or molecules, produces
light. We can see the effect in
the beautiful aurora – as plasma
from the Sun (the solar wind)
interacts with molecules in the
Earth’s atmosphere – and, closer
to home, in fire, lightning and in
brightly coloured neon signs.
Plasmas behave rather
differently from electrically
neutral gases, solids and liquids
(for this reason they have been
called the fourth state of matter).
Because they are charged,
plasmas created on Earth can be
manipulated with electric and
magnetic fields, and can be used
to effect technologically useful
physical and chemical
transformations in controllable
ways. Over recent decades,
there has been an explosion of
activity in exploiting plasmas
industrially. Today, the world
market for plasma technologies
is worth well over £100 billion.
Industrial plasmas are employed
to manufacture a huge variety of
products from computer chips to
medical implants. They are also
used in environmental clean-ups
and to purify drinking water.
Plasmas come in many forms,
existing over a wide range of
temperatures and pressures. In
the dense core of the Sun, the
temperature is 100 million
degrees – hot enough for nuclear
fusion reactions to occur. Fusion
reactors on Earth try to emulate
these plasma conditions to
create energy. ‘Thermal’
plasmas at about 10,000
degrees and at atmospheric
pressure, generated by arc
discharges, are used in welding
and in furnaces to anneal
materials, and to vitrify or
destroy toxic or biological waste.
Low pressure plasmas
However, industry is mostly
interested in much cooler, very
low pressure plasmas, which
contain neutral atoms and
molecules as well as ions and
electrons. They have the unusual
but useful characteristic that the
THANKS
GO TO
BILL GRAHAM
OF
QUEEN’S UNIVERSITY BELFAST
electrons are extremely
energetic, or ‘hot’, while the
remaining gas constituents
remain cold.
Such low pressure plasmas
are generated by applying a
radiofrequency voltage in an
evacuated chamber through
which the rarefied gas flows.
Any electrons present are
accelerated and they release
more electrons from atoms until
there are sufficient to produce
the familiar steady glow
associated with a plasma. In
these low pressure plasmas, the
lightweight electrons move
much faster than the atoms or
molecules. In effect, they can
provide enough energy to
transform the other, colder gas
constituents into chemically
active species, which then react
at surfaces. Plasmas thus offer a
uniquely sensitive environment
which allows otherwise fragile
molecules to undergo ‘hot’
chemistry.
Such plasma processing has
been crucial to the development
and growth of the electronics
industry. The circuits on
semiconductor chips are created
by depositing or removing
FOR HIS HELP WITH THIS PAPER
University of Bristol
various materials, and
plasmas are an ideal
conduit. The plasma set-up
can be subtly tailored to
regulate the chemistry. A
good example is the use of
carbon tetrafluoride gas to
etch silicon chips. It first
breaks up into carbon and
fluorine fragments in the
plasma reactor, and the
highly reactive fluorine
atoms attack and remove
the insulating silicon dioxide
layer on the chip. When all
the exposed silicon dioxide
has been removed, the carbon
then deposits as a protective
layer which prevents further
attack by the fluorine on the
silicon underneath. Clever
plasma chemistry based on
hydrocarbons and hydrogen is
also used to deposit coatings
of diamond.
Another hugely important area
for plasma technology is lighting.
Every fluorescent tube contains
a plasma. This emits UV light
which interacts with the
phosphor coating on the tube
wall causing it to fluoresce and
emit white light. One of the
challenges today is to make strip
lighting more efficient and replace
the mercury which is the major
light-emitter in the phosphor.
Scientists are also working on
modulating the plasma physics to
alter the colour of the light. Highintensity arc lamps used in
outside public areas rely directly
on the plasma to produce the
light and their colours depend
directly on the chemical
elements in the plasma, not on
a phosphor coating.
A recent development to hit
the high-street, that exploits
plasma light-emission, are the
plasma displays used in large
screen TVs. They operate at half
an atmosphere pressure but rely
on the same principle as
fluorescent tubes. The pixels in
the screen contain phosphors
which can glow red, green or
blue as the result of a UVemitting plasma discharge.
Surface engineering
Plasma technology still has
enormous potential in many
areas of manufacturing: surface
engineering using plasmas is
now big business. Aero-engine
parts are surface-hardened by
laying down just a few atomic
layers of a ceramic using plasma
processing. Anti-reflective and
anti-scratch coatings, similarly
prepared, are an option when
you go to buy new glasses at
the opticians.
The non-thermal nature of
low pressure plasmas is being
increasingly exploited to deposit
coatings that render a soft
polymer surface water-attracting
or repelling, or improve the
uptake of dyes. Jas Pal Badyal
at Durham University has a
company called Surface
Innovations which markets
plasma technologies for making
surfaces highly water and oilrepellent, and also for
manufacturing protein chips for
gene-sequencing. Robert Short
and colleagues at the University
of Sheffield has developed
pioneering techniques to deposit
polymer coatings using low
power plasmas, which are used
for culturing skin cells and
delivering them to wounds that
won’t heal. The research is now
being marketed through a spinout company CellTran.
Increasingly, plasma
researchers are trying to devise
such systems that operate at
atmospheric pressure, thus
avoiding the need for vacuum
pumps, so that they can be used
to deposit, for example, polymer
coatings over large areas of
fabric or packaging materials.
Another application at
atmospheric pressure is in
environmental clean-up.
Christopher Whitehead at
Manchester University is
working on an ‘end-of-pipe’
plasma system to treat diesel
exhaust and remove volatile
organics from waste solvents.
The process, which relies on
creating highly reactive chemical
species called radicals, can also
be applied to kill airborne
microbes, say, in hospitals
where infection with antibioticresistant bacteria is becoming
an increasing risk.
There are a huge range of
plasma techniques available
and the process used has to
be carefully tailored to the
application. Much of the
development in industry has
been based on trial and error,
but physicists are trying to
understand and model the
complex behaviour in
technological plasmas so as
to predict optimum operating
conditions. The UK has a
network of academic research
groups working in this field to
take forward this commercially
important and growing
technology.
Jas Pal Badyal and his
team at Durham
University use plasma
technology to create
super-repellent surfaces
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