Plasma Chemistry Applications of Barrel Reactors Photo Resist Removal
The removal of photo resist from silicon or GaAs semiconductor
slices is a very safe and effective process. The thickness
of resist is normally 1-3 microns and has usually been baked on for
about
1 hour at 100°C. Using O2 at full power, 1 micron should disappear
in about 20-30 minutes. Ion implanted resist is very hard and
in some cases will take longer to etch, up to 2 hours. One
way of
reducing time is by fitting a larger rotary pump therefore
allowing more gas flow for the same pressure. Normal operating
pressure
is between 0.4-0.2 mbar. Silicon Etching
To etch silicon and silicon nitride from slices a fluorinated
gas is required, usually carbon tetrafluoride. Films are normally
in the order of 100-500 microns and are quickly removed, 2-5 minutes
normally being sufficient. Visual color changes make complete removal
obvious. Carbon tetrafluoride will also attack the chamber and
window. Quartz, which does not contain boron, is often specified
instead of glass when using carbon tetrafluoride to avoid boron
contamination. Asbestos Detection
Plasma is now accepted as the standard process for SEM examination
of asbestos fibers caught in acetate filter papers. Filters are
surface etched in O2 at approximately 100 watts for between 2-8
minutes, this being sufficient to etch away the filter surface
leaving the asbestos fibers for SEM examination. Atomic Absorption Spectroscopy Preparation
Plasma is widely used in A.A.S. as a convenient way of removing
the organic binder frpm a host of samples making trace metal determination
easier. Samples are presented dry and crushed and etched in O2.
Ashing times vary but are usually 1-3 hours. General Cleaning
Delicate parts such as SEM chamber parts or cleaning of small
items can be achieved using plasma. For super-clean surfaces 99.999%
ultra pure argon or high purity oxygen is used. Plastic Treatment
There are numerous applications being tried on plastic surfaces,
the most common being the requirement to convert hydrophobic surfaces
to be hydrophilic (surface wettable). The advantages being the
ease of printing, painting, marking, etc. and the great improvement
in the bond strength of adhesives. These reactions take place in
high purity O2 at relatively low temperatures and power input (10-40W).
Process times are relatively short, 15 seconds-2 minutes usually
being sufficient.
Interset has been shown in putting down "barrier
layers" on
plastic containers to increase shelf life by stopping the ingress
of CO2 and stopping
the contents of the container reacting with the plastic. The films are
normally inert fluorine films and are produced using carbon
tetrafluoride. Oil
Generally in plasma environments the rotary pumps are required
to pump large amounts of oxygen. As most rotary pump oils are organic
based there is a very remote chance of explosion. There are two
ways to minimize this risk:
- Use a synthetic based oil, such as "Fomblin." The
only drawback to this is cost.
- It is possible to break into the pipe between the plasma unit
and pump and dilute the oxygen with argon. However this
reduces the pumping speed of the pump, requires an argon supply
and
would be ineffective if the argon supply failed
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