Application Notes

Atmospheric Pressure Plasma Technology
By Dr. Robert F. Hicks

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Atmospheric pressure plasmas are valuable tools for surface treatment of materials prior to bonding, gluing, coating and other applications.  They are used in manufacturing many products ranging from stents, catheters and prosthetics for the medical device industry to large, three-dimensional composites for the aerospace industry.  Atmospheric plasma treatment is an active area of research and the number of publications on this topic is growing exponentially.  The purpose of this application note is to introduce the reader to how atmospheric pressure plasmas work and to highlight the types of plasma sources available on the market.  It is important to recognize that atmospheric plasmas do not all perform the same way.  The engineer must run tests to find out which device works best for his or her application.

Out there in the marketplace there is a bewildering array of plasma technologies for materials processing.  Not only are there multiple types of atmospheric plasma sources, but there are vacuum-based plasmas that may be used when batch treatment is convenient.  In order to choose among different types, one needs to know the “Figures of Merit” that govern their operation.  The key Figures of Merit for plasmas are:

  • Average electron temperature
  • Average electron concentration
  • Average neutral temperature
  • Breakdown voltage
  • Types of reactive gas species
  • Average concentration of reactive gas species
  • Remote versus direct contact with the substrate

Surfx® Technologies’ products utilize specially designed electrodes and helium gas to produce a capacitive discharge at atmospheric pressure and temperatures between 50 and 200 oC. A radio frequency (RF) power supply (at 27.12 MHz) and a matching network are used to generate the plasma.

The Figures of Merit of the RF capacitive discharge plasma have been determined in many scientific studies.  The electron temperature is about 1.5 eV and the electron concentration approaches 1012 cm-3.  This may be compared to the density of the gas, which at room pressure and 200 oC is 1019 cm-3.  In other words, the plasma contains one free electron for every 50 million neutral species.  The highly energetic free electrons dissociated a lot of molecules inside the plasma, so that about 1016 to 1017 cm-3 of reactive atoms are produced.  These atoms amount to about 1 in every 500 neutral species in the gas stream.  They can convert a polymer surface from a hydrophobic to a hydrophilic state in a fraction of a second.

The two other important Figures of Merit for the RF capacitive discharge are: (1) a breakdown voltage of 170 V; and (2) remote contact with the substrate.  The low breakdown voltage means there is no danger of electrocution.  In addition, the power supply can have a small compact design.  Remote contact with the substrate means that the plasma is confined to the applicator upstream of the substrate.  No electrons or ions can stream out onto the substrate and cause any damage.  Furthermore, the substrate is not subject to any electric fields, nor is it part of an electric circuit.  It does not matter whether the substrate is a metal, or an insulator, this has no effect on the performance of the plasma process.

One last point about RF capacitive discharges: they are clean plasma sources that can be used in the most stringent clean room conditions, such as encountered in making integrated circuits or photomasks.  The weakly ionized state of the plasma combined with the collisional sheath around the electrodes, guarantees that there is no ion bombardment of the electrode surface, and subsequent ejection of sputtered material into the gas flow.  Surfx Technologies’ Atomflo™ sources are well suited for plasma cleaning of the most sensitive electronic devices…

To request a copy of the complete Application Note on Atmospheric Plasma Technology please click on the following link Surfx Application Notes.  To ask Dr. Hicks any questions you may have about plasma physics, chemistry or engineering, please use our contact us form for further information.