Atmospheric Plasma Processing

     Plasma reactors are used in approximately a third of all the process steps used to manufacture integrated circuits.  The ionized gas in the plasma generates a high concentration of reactive species at 50-100°C, and provides a means of cleaning, etching and depositing materials at much lower temperatures than is possible by thermally driven reactions alone.  Sales of plasma reactors for semiconductor processing exceed several billion dollars annually.

     Atmospheric pressure plasmas are emerging as important tools for the surface treatment of materials. The advantage of these systems is that they can be used for in-line manufacturing of 3-dimensional objects of any size or shape. For further information on atmospheric pressure plasma systems, you can visit Surfx® Technologies, LLC, one of the leading suppliers of this equipment.

     A picture of the Atomflo™ plasma source operating at 1.0 atmosphere is shown below courtesy of Surfx® Technologies, LLC.  The Atomflo™ plasma source produces a large flux of atoms and/or radicals, depending on the gas fed to the device.  This flux of reactive species is well suited for high-tech materials applications. 

The Atomflo™ operating with oxygen and helium feed gases at 1 atm and 75°C.

     At UCLA, we have investigated the chemistry of oxygen, nitrogen, hydrogen and fluorine containing atmospheric-pressure plasmas.  The oxygen plasma is capable of etching photoresist films (baked at 120°C) at several microns per second.  This plasma can be combined with hexamethyldisilazane (HMDSN) or other silicon precursors to deposit high quality silicate glass films.  It should be noted that the device depicted in the picture above is a downstream plasma source.  The ions and electrons are rapidly consumed by collisions prior to impinging on the substrate.

     Through a combination of numerical modeling, ultraviolet absorption and optical emission spectroscopy, we have characterized the concentrations of reactive neutral species appearing in the downstream afterglow from the oxygen plasma.  The most abundant radicals are ground-state oxygen atoms.   Shown in the figure below is the distribution of reactive species as a function of time after shutting off the power to the plasma.  Note that about 5x10¹⁵ cm^-3 of O atoms are generated by the plasma.  This species lasts for up to 1.5 cm downstream of the source.  To learn more about these (and other) research results, go to our Publications link.

[Left:] The dependence of the O, O₂ (¹D_g), O₂ (¹S_g⁺) and O₃ concentrations on time and distance in a helium-oxygen discharge.. [Right:] The distribution of relevant reactive species in an argon-oxygen discharge generated by the X-flo plasma source (see Publication 109).

     Patents are pending on the plasma sources developed at UCLA, and these have been licensed to Surfx® Technologies LLC. If you are interested in acquiring one of these, you may contact them.

    We are actively pursuing research on new surface treatments using atmospheric pressure plasmas.  Click here to see a video of some of the preliminary hydrophobic work on silicon we completed in 2006, with the help of some of our partners from Asia.  Traditional HF dip treatment of silicon wafers yields water contact angles of 60 - 80°, while our process yields stable contact angles greater than 100°.

     We have demonstrated the low-temperature PECVD of silicon dioxide on plastic and on aluminum, and of aluminum-doped zinc oxide on silicon and glass.  The latter may be used in the fabrication of copper-indium-gallium diselenide (CIGS) solar cells.  These studies have confirmed the special applicability of our atmospheric pressure plasma sources for processing thermally-sensitive materials.

[Left:] Dielectric strength and breakdown voltages for PECVD glass on aluminum. [Right:] Resistivity of aluminum-doped zinc oxide (on silicon) grown by PECVD with CO₂ (▼,●) and by conventional thermal CVD with O₂ (█) at low-T (≤ 220°C).