2005 IRISH SCIENTIST YEAR BOOK

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Dublin City University
Angus Mc Carter
Plasmoids in an inductively coupled plasma

Radiofrequency (rf) plasma discharges are used extensively in the semiconductor and other materials processing industries. There are numerous types and configurations of rf discharge, however they are all based on the underlying principle of an rf electric field accelerating free electrons, and causing ionising collisions with a background gas. The reason for using radiofrequency (1-40 MHz) is because the more mobile electrons respond to the fast oscillating field whereas the much heavier ions are too slow to respond, and only feel a time averaged electric field either generated or applied. This allows ions to be generated in the plasma and then extracted for use in processes such as etching and sputtering, relatively unaffected by the plasma producing mechanism.

Here at the National Centre for Plasma Science and Technology (NCPST) we carry out research into a particular type of rf discharge known as an inductively coupled plasma (ICP), which is used extensively in the semiconductor industry. An ICP uses a radiofrequency antenna to radiate electromagnetic waves into a vacuum discharge chamber. Free electrons are accelerated in the gas causing the ionising collisions. An essential feature of this type of discharge in relation to semiconductor processing is plasma uniformity. If the plasma exhibits non uniformities or instabilities then the processed batch may have defects affecting the entire production line.

A particular type of instability exhibited in ICP discharges is the phenomenon of plasmoids. These are gross spatial instabilities observed as bright lobes of dense plasma within the discharge. They can be made to 'dance' around the discharge changing in size, number and even colour, depending on the plasma discharge conditions. Although aesthetically pleasing they are detrimental to any discharge requiring uniformity. This type of instability has been observed in various types of discharges since the 1930s, however as yet, no satisfactory explanation has been given to their formation.

At the NCPST we have been carrying out experimental and theoretical work to explain this aesthetically pleasing, yet ultimately undesirable phenomenon.

Contact: Angus McCarter, NCPST,
Dublin City University, Glasnevin, Dublin 9;
E-mail: [email protected]