AVS 51st International Symposium
    Plasma Science and Technology Thursday Sessions
       Session PS-ThA

Paper PS-ThA9
Validity of Binary Collision Theory in Ion-Surface Interactions at 50-500 eV

Thursday, November 18, 2004, 4:40 pm, Room 213A

Session: Plasma-Surface Interaction
Presenter: K.P. Giapis, California Institute of Technology
Authors: M. Gordon, California Institute of Technology
K.P. Giapis, California Institute of Technology
Correspondent: Click to Email
Ion-surface interactions in the 50-500 eV regime have become increasingly important in plasma processing. Profile evolution simulations rely on the binary collision approximation (BCA) to estimate the energy of scattered ions. Concerns exist in literature about the validity of the BCA theory at low impact energies because peculiarities are frequently seen in the scattered ion energy distribution. Sub-surface processes, multiple bouncing, and super-elastic phenomena have all been hypothesized. This talk will explore the usefulness of BCA theory in predicting energy transfer during ion-surface collisions in the 50-500 eV energy range. Well-defined beams of rare gas ions (Ne, Ar) were scattered off semiconductor (Si, Ge) and metal surfaces (Al, Mg, Ti, Ag, Au, Ni, Ga) to measure energy loss upon impact. The ion beams were produced from a floating ICP reactor coupled to a small accelerator beamline for transport and mass filtering. Exit energies were measured using a 90 deg electrostatic sector coupled to a quadrupole mass filter with single ion detection capability. Although the BCA presents an over-simplified picture of the collision process, our results demonstrate that it is remarkably accurate in the 50-500 eV range for a variety of projectile-target combinations. However, large deviations from BCA exist for combinations where electron promotions may occur during the hard collision. We find that to be the case for Si, Al, and Mg at energies greater than 500 eV. The promotion occurs at a target-dependent threshold energy and is surface mediated. Further, doubly-charged projectiles maybe generated during the hard collision at the same energy threshold. The implications of these findings for profile evolution during plasma etching will be discussed.