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Plasma Chemistry

Photo resist removal

The removal of photo resist by plasma etching 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 CF4. Films are normally in the order of 100-500 microns and are quickly removed; 2-5 minutes normally being sufficient. Visual colour changes make complete removal obvious. CF4 will also attack the chamber and window. Quartz, which does not contain Boron, is offered as standard instead of glass to avoid Boron contamination.

Asbestos detection

Plasma is now accepted as the standard process for SEM examination of asbestos fibres caught in Acetate filter papers. Filters are surface etched in O2 at approximately 100 Watts for 2-8 minutes, this being sufficient to etch away the filter surface leaving the asbestos fibres for SEM examination.

Atomic absorption spectroscopy preparation

Plasma is widely used in A.A.S. as a convenient way of removing the organic binder from 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

Cleaning of delicate parts, such as SEM gun assemblies or chamber parts, 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 using plasma, 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 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.
Interest 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 of inert Fluorine and are produced using CF4.

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 minimise this risk:-
i) Use a synthetic based oil, for example "Fomblin" is highly recommended.
ii) 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 which can be costly and would be ineffective if the Argon supply failed.