RF Plasma Sources

When the frequency of applied field lies in the range of radio waves only the electrons can follow the electric field, whereas the ions remain at rest. These electrons collide with the gas atoms/molecules and ionize them to generate plasma. Such plasma sources are called RF plasma sources.

Figure 1 Schematic of the steel plasma reactor in capacitively coupled configurations as RF power is connected to the powered electrode through the matching network containing capacitor. The bottom electrode is grounded. (DOI: https://pubs.acs.org/doi/10.1021/acs.jpcb.5b07309)

The experimental setup consists of two parallel electrodes where RF power is coupled either in capacitive (through a capacitor) or inductive (antenna coils) to one of the electrodes. The polarity of powered electrode is changing with the applied frequency while the other electrode is kept grounded. In such case, the current is dominated by displacement current rather than by conduction current. These plasmas are classified as low-pressure plasmas as they operate in few mTorr range and produce 2-5 eV electron temperature and 10⁹-10¹¹ cm^-3 electron density. These sources find vital importance in almost all kind of industrial applications.

Figure 4 (a) Schematic of the experimental setup used for plasma characterization The employed vacuum vessel was equipped with mini ICP electronics as well as with an ICP plasma source located at the top. (b) Photographs taken through the vessel viewport with the plasma ON/OFF. (c) Schematic of mini ICP (DOI: https://doi.org/10.1116/6.0001636)

Above is a miniature plasma source heated with 13.56 MHz RF in inductively coupled configuration. This source of Hydrogen plasma was developed to be used for in situ cleaning of functional surfaces (such as sensors, fiducials and interferometer mirrors) in extreme ultraviolet lithographic scanner. This system has promised improved cleaning speed while simultaneously reducing the overall thermal load. As an additional benefit, this plasma source has a minimized and flexible building volume to allow easy integration into various locations in the EUV lithographic scanner.