New insights into fundamental ion-surface interactions

Collisions of ions with surfaces at low energy ($<$1 keV) are important in reactive ion etching of semiconductors, dielectrics, and metals. For example, ion bombardment strongly influences etch rates, anisotropy, and selectivity through physical sputtering, momentum-assisted product removal, and modification of reaction rates. Fundamental understanding of these issues requires detailed information about scattering dynamics. We report results from beam scattering experiments involving mass-filtered ions (F$^{+}$ and CF$_{x}^{+})$ with tunable energy (50-1000 eV) and high flux off several surfaces (Si, Al, Ag). Topics to be discussed include: (1) electronic excitations in hard collision events (inelastic losses and F$^{++}$ formation); (2) pre-collision fragmentation of CF$_{x}^{+}$ ions which result in fast exit products such as C$^{+}$, F$^{-}$, and CF$^{-}$; (3) high yields of fast F$^{-}$; and (4) F$_{2}^{-}$ formation via an Eley-Rideal mechanism. For instance, energy losses for single-scatter events of F$^{+}$ off Si and Al show that F$^{++}$ can be formed through double electron promotion which ``turns-on'' above a critical collision energy. Velocity analysis of daughter fragments from CF$_{3}^{+}$ impact on Si and Ag point to projectile fragmentation before the hard collision step. Finally, energy spectra of F$^{+}$, F$^{-}$, and F$_{2}^{-}$ leaving Si and Ag show three distinct scattering channels: single-scatter binary-like elastic events, another at low energy that cannot be explained as simple sputtering, and still another where fast F$_{2}^{-}$ is formed via abstraction. These results illustrate that product species can suffer significant inelastic losses as well as show faster-than-SIMS behavior which may have dramatic effects on profile evolution in plasma etching.