Industrial Applications of
Low Temperature Plasma Physics
Plasmas are gases
that are so hot that electrons have come off the atoms, forming a negatively
charged fluid which is mixed with the positively charged ions. The entire
plasma is electrically neutral, but its behavior is complex because the
particles' motions are controlled by electric and magnetic fields. Though
the main applications of plasma physics are to space exploration and to
controlled fusion (reproducing on earth the energy source of stars), the aspect
that affects our everyday lives is the use of plasmas in manufacturing.
For example, metals, such as propeller blades, are hardened with a plasma
surface treatment; plastics, such as automobile bumpers and potato chip bags,
are plasma treated for better paint adhesion; camera lenses have optical
coatings applied with a plasma; and synthetic diamonds can be made with a plasma
arc. The most important practical use of plasmas is, however, in the
manufacture of semiconductor circuits, particularly computer chips.
In making a
computer chip, which may contain as many as 100M transistors, the feature sizes
have to be smaller than 0.13mm. Purely chemical
etching is much too crude; plasma etching is required. In processing a
silicon wafer, perhaps half of the steps require plasma processing, and the
machines required for this account for over half the cost of the factory.
These plasma "reactors" were developed originally by trial and error
without the help of plasma physicists. In recent years there has been a
continuous improvement in the accuracy and efficiency of these reactors, and it
is becoming more clear how they work.
In UCLA's Low
Temperature Plasma Technology Laboratory (LTPTL), experimental and theoretical
work is done to understand the physical mechanisms in plasma generators so that
one can design
better ones. For instance, the helicon source, which we have been
studying for a decade, can produce denser plasmas with less radiofrequency (RF)
power. The reasons for its superiority, however, were mysterious, and the
understanding of this source led to many fascinating research results. One
ongoing project is to make an array of helicon sources which can produce a wide
plasma capable of etching a large substrate uniformly, as shown by the diagram
on the left. Click on the thumbnail to expand the image, and
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has to do with Inductively Coupled Plasmas (ICPs), which are commonly
used today. These machines do not need a magnetic field, as do helicon
sources, and it is a puzzling how the RF energy gets into the center of the
plasma to produce a uniform density, when the RF field should be shielded by the
plasma. By studying these sources in a laboratory device such as shown on
the right, we have discovered that nonlinear effects can cause a number of
energetic electrons to reach the interior and produce plasma there. An
interesting problem is how an ICP turns into a helicon source as a small
magnetic field is applied. Nothing much happens until the field is large
enough to support a helicon wave. A computer code, HELIC, has been
developed to predict this behavior and enable the design of future helicon
transistors smaller and smaller, one also has to scale the thickness of the
insulating oxide layer on the "gate" down to less than 50
Angstroms. Etching these oxide layers is problematic, because plasma
processing can cause large voltages to build up across these gates and ruin
their electrical properties. A mechanism called Electron Shading Damage
was postulated some time ago to explain why this happens. To test this
hypothesis, we have constructed an experiment and a computational code to model
the effect on a macroscopic scale. As shown by this diagram, we plan to
measure the charging currents in a structure of mm size rather than the
sub-micron size of real semiconductor features. To do this required making
a very tenuous plasma source so that the plasma sheath thickness scales along
with the feature size.
The physics of
partially ionized plasmas has been considered a "dirty science"
because adding neutral atoms to ions and electrons can complicate the problem so
that the solutions are messy. Our aim has been to show that
low-temperature plasma physics need not be low-brow. There are challenging
problems with elegant solutions. As an added bonus, this line of research
has an impact on devices and materials which we use everyday.
information, please visit the website for the LTPTL.