RF Plasma sources

The most widely used family of plasma sources for industrial processes might be the RF (radio frequency) one. Within this family, the plasma discharges are sustained by means of electrodes or antennas excited in the radiofrequency domain, typically between 1 MHz and 100 MHz. Roughly, three different types of RF driven discharges can be distinguished: capacitive, inductive and wave heated, more known as helicon discharges in the radio frequency domain. Capacitive discharges lead to low density plasmas (typ. 10⁹-10¹⁰ electrons/cm³), which is not very good in terms of processes rates, but on another hand large area industrial capacitive reactors were successfully developed, although limits in these reactors upscaling now appear. Inductive discharges lead to higher densities (typ. 10¹¹-10¹² electrons/cm³), at least in a cylindrical geometry, but up to now large area inductive reactors gave deceiving results. Helicon discharges can lead to very high densities (typ. 10¹²-10¹⁴ electrons/cm³) which should be excellent in terms of processes rates improvement, but they are almost never used for industrial applications. This is due to the fact that conventional helicon sources suffer from severe drawbacks with regards to industrial applications.

The main limitations of these conventional helicon sources in terms of industrial use are:

The operating pressure range: conventional sources can operate under pressures around 0.1-1 Pa (10^-3-10^-2 mbar), which is too low for many industrial applications which generally require a 1-100 Pa operating pressure range.

The size: conventional helicon sources are typically of about 10 cm in diameter, which is small if one considers for example the 30-40 cm Si wafers that are processed in semiconductors industries.

The electronegative discharges: many industrial processes rely on the use of precursors that leads to electronegative discharges, such as oxygen or SF6. Conventional helicon source can not sustain such discharges, or only with very low precursor flows (typ. 10 sccm), which are useless from the industrial applications point of view.