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Dr. Bob on Plasma Science

  2. Dr. Bob on Plasma Science

Science of Atmospheric Pressure Plasmas

A Quick Introduction

  • Plasma is an ionized gas with free electrons (e¯), positive ions (n+) and negative ions (n¯).
  • Plasmas are neutral¹¯³: [e¯] + [n+] + [n¯] = 0

[x] = number density of x in cm¯³

Two Types of Atmospheric Pressure Plasmas

Strongly ionized arc1,4:

  • Electron density, [e-] > 0.1% gas density, [n]
    • Electron temperature, Te- ~ 17,500 K
  • Neutral temperature, Tn, inside arc > 5,000 K

    Weakly ionized, uniform glow4,5:

    • Electron density, [e-] < 400 parts per billion gas density, [n]
      • Electron temperature, Te- ~ 17,500 K
    • Neutral temperature, Tn, in plasma = 425 – 465 K

    [n] equals neutral species density, which at 1.0 atm equals 2.4×1019 cm¯³

    Surfx Plasma Image
    Argon Plasma:

    weakly ionized, uniform glow

    Surfx Air Plasma
    Air Plasma:

    strongly ionized, blown arc

    Why two types of atmospheric pressure plasma?

    Plasma type is determined by:

    Purple Number 1

    Breakdown voltage (VB) – electric potential required to cause gas ionization.

    Purple Number 2

    Ionization rate (Ri) – rate free electrons collide into neutral species and convert them into ions.

    Paschen curve describes gas breakdown voltage1,4

    • VB depends on pressure (P) times electrode gap (d)
    • Vacuum plasmas operate near minimum in the curve
    • At atmospheric pressure, the gap must be 1-2 mm
    • Air (N2) has a very high VB, greater than 2 kV
    Paschen curve chart

    Paschen Curve

    Current and Voltage Chart

    Ionization at 1.0 atm

    • VB depends on the gas:
      • Helium* = 170 V
      • Argon* = 600 V
      • Air > 2,000 V

    *RF powered

    Current and Voltage Chart

    *RF powered

    • The blue line shows how the voltage and current increase linearly as power is applied to the electrodes surrounding the gas.
    • With argon gas, breakdown occurs with formation of a weakly ionized, uniform glow (pink line).
    • With air, breakdown occurs with formation of an arc (red line).
    • Arcs are characterized by a low voltage and high current.

    Free electrons are generated by ionization in the gas

    Ionization rate: Ri = ki[n][e¯]

    Electron collision graphic

    Every collision doubles the number of electrons

    Free electrons are lost at the wall

    Termination rate: Rt = kt[e¯]

    Termination rate graphic

    Every collision removes one electron

    Arc or uniform glow?

    Depends on rates of ionization relative to termination:
    d[e¯]/dt = Ri – Rt = (ki[n] – kt)*[e¯]
    [e¯] = [e¯0]exp((ki[n] – kt)*t)

    If (ki[n] – kt > 0), you get an exponential increase to an arc.

    If (ki[n] – kt < 0), you get an exponential decrease to a stable uniform glow.

    Argon versus Nitrogen

    Ratio of ionization rate constants6:
    ki(N2)/kt(Ar) = 2,300

    Argon: Ri is slow, so (ki [n] – Kt) < 0 produces a uniform glow

    Nitrogen: Ri is fast, so (ki [n] – kt) > 0 yields an arc

    Electrons Drive Chemistry

    Hydrogen plasma reaction:
    H2  + e¯ Simple arrow H + H + e¯
    Rate = A • exp (-E/RTe)[H2][e¯]

    Ratio of H atoms in atmospheric (AP) and vacuum (V) plasma:
    RAP/RV = [H2]AP[e¯]AP/[H2]V[e¯]V
    RAP/RV = [7.6][2×10¹²]/[0.008][1×10¹¹] = 19,000

    Processing rates in atmospheric pressure plasmas are fast

    • An atmospheric pressure argon plasma produces 19,000 times more H radicals than a vacuum plasma.
    • Hydrogen radical removal of oxide layers from copper, tin and indium is hundreds of times faster with AP plasma compared to vacuum plasma.
    • An atmospheric pressure argon plasma produces 1,500 times more O biradicals than a vacuum plasma.
    • Oxygen radical cleaning and activation of glass and polymer surfaces is at least ten times faster with AP plasma compared to vacuum plasma.

    Uniform glow plasmas have a sheath1-3

    • Strong electric fields form at the walls.
    • The walls repel electrons, thereby maintaining a stable gas plasma.
    • Positive ions are accelerated to the walls by the sheath potential.
    Sheath and plasma chart
    Sheath and plasma graph

    Sheath Collisions

    No sheath collisions in vacuum plasmas

    • Vacuum plasmas (<100 mTorr) have collisionless sheaths.
    • Mean free path = 600 microns; sheath thickness = 250 microns.
    • Ion collisions = 0.
    • Positive ions accelerated by plasma potential.
    • Ion bombardment of walls and wafer creates many particles in vacuum plasma.

    Many sheath collisions in AP argon plasma

    • AP argon plasma has a highly collisional sheath.
    • Mean free path = 0.08 microns; sheath thickness = 20 microns.
    • Ion collisions = 250.
    • As long as sheath is intact, positive ions will not strike wall with high energy.
    • Few, if any, particles created in AP argon plasma.

    Air plasmas produce particles

    • Arcs do not have protective sheaths.
    • In an arc, free electrons are generated at the wall by thermionic emission.
    • To maintain charge neutrality at the wall, positively charged ions collide with the wall at high velocity.
    • Positively charge ions smashing the wall sputter away material, generating particles.

    Atmospheric argon plasmas are safe on integrated circuits

    The RF capacitive discharge produces no sparks, no ions, no particles, and no ESD.
    Enlarged Plasma Beam Image

    Atmospheric argon plasmas are low temperature

    The table below shows the chip surface temperature for several offset distances between the plasma head and the chip and for several plasma head scan rates.

    Treatment achieved while maintaining temperature below 40°C

    Atmospheric Table
    Atmospheric Treatment image

    Atmospheric argon plasmas do not generate particles

    • Native oxide on 200 mm silicon wafer scanned with argon and oxygen plasma in a class 100 cleanroom.
    • Water contact angle after plasma scan was less than 5 degrees.
    • Particle counts mapped out on wafer with a light scattering instrument.
    • No particles added by AP argon plasma exposure.
    Particle count chart before treatment
    Particle counts before treatment
    Particle count chart after treatment
    Particle counts after plasma treatment

    Conclusions

    • The atmospheric pressure argon plasma is a weakly ionized discharge with an electron concentration of 80 parts per billion and it has a protective sheath.
    • Radical concentrations generated by the AP argon plasma, are thousands of times higher than in a vacuum plasma.
    • Atmospheric argon plasmas have unique properties with wide applications in semiconductor manufacturing.

    Reference Publications

    1. Y. P. Raizer, Gas Discharge Physics, 1st edition, Springer-Verlag, Berlin (1991).
    2. F.F. Chen and J.P. Chang, Lecture Notes on Principles of Plasma Processing, 1st edition, pp. 1-208, Kluwer Academic/Plenum Publishers, New York (2003).
    3. M.A. Lieberman and A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, 2nd edition, John Wiley & Sons, New York (2005).
    4. A. Schutze, J. Y. Jeong, S. E. Babayan, J. Park, G. S. Selwyn and R. F. Hicks, The atmospheric-pressure plasma jet: A review and comparison to other plasma sources, IEEE Trans. Plasma Sci., Vol. 26, 1685-1694 (1998).
    5. M. Moravej, X. Yang, G. R. Nowling, J. P. Chang, R. F. Hicks and S. E. Babayan, Physics of high-pressure helium and argon radio-frequency plasmas, J. Appl. Phys., Vol. 96, 7011-7017 (2004).
    6. J. Annaloro, V. Morel, A. Bultel and P. Omaly, Physics of Plasmas, Vol. 19, 073515 (2012); and M. H. Bortner, NBS Technical Note 484 (May 1969).