AVS 50th International Symposium
    Plasma Science and Technology Thursday Sessions
       Session PS-ThA

Paper PS-ThA10
Measurement of Absolute Radical and Metastable Species Densities in O@sub 2@ and N@sub 2@ Plasmas using Modulated Beam Appearance Ionization Mass Spectrometry

Thursday, November 6, 2003, 5:00 pm, Room 315

Session: Plasma Diagnostics: Mechanisms
Presenter: E.S. Aydil, University of California, Santa Barbara
Authors: S. Agarwal, University of California, Santa Barbara
G.W.W. Quax, Eindhoven University of Technology, The Netherlands
B. Hoex, Eindhoven University of Technology, The Netherlands
M.C.M. van de Sanden, Eindhoven University of Technology, The Netherlands
D. Madoudas, University of Massachusetts, Amherst
E.S. Aydil, University of California, Santa Barbara
Correspondent: Click to Email
Measurement of radical densities in an electrical gas discharge is important for understanding and improving plasma etching and plasma-assisted deposition processes. We have designed, developed and demonstrated an experimental apparatus for measuring the density of the radicals and electronically excited molecular species in a plasma using modulated beam line-of-sight appearance ionization mass spectrometry (LOS-AIMS). In LOS-AIMS, the species in the plasma are sampled through an aperture on the substrate platen and detected using a quadrupole mass spectrometer (QMS) placed in line-of-sight with this aperture in a three-stage differentially pumped vacuum chamber. Although LOS-AIMS is a versatile tool for measuring absolute radical densities, we show that it requires careful vacuum design and calibration which should take into account various sources of error such as the contribution to the QMS signal from the background gases, the ion mass-to-charge ratio dependence in the sensitivity of the QMS, and space-charge limitations in the QMS ionizer. In addition, collisions within the extracted molecular beam must be taken into account for higher operating pressures in the plasma chamber. Careful consideration of these effects and modulation of the sampled radical beam with a chopper allows the determination of the absolute radical densities, parent molecule concentrations, and the neutral gas temperature near the substrate plane. Specifically, we have measured densities of O and Ar atoms and O@sub 2@ molecules in O@sub 2@/Ar plasma mixtures and N atoms and metastable N@sub 2@ molecules in N@sub 2@ plasmas. In addition, we find that at low pressures, the O@sub 2@ translational temperature is higher than that for Ar. We attribute this difference in Ar and O@sub 2@ translational temperatures to hot O@sub 2@ molecules that are created by O-O recombination reactions on the walls of the plasma chamber which do not equilibrate effectively with Ar atoms at lower pressures.