Invited Paper PS2-WeM9
New Insight into Fundamental Ion-Surface Interactions

Wednesday, October 17, 2007, 10:40 am, Room 607

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 can have a strong effect on etch rates, profile anisotropy, and selectivity through physical sputtering, momentum-assisted product removal, and modification of reaction rates. Fundamental understanding of these issues requires detailed information about the scattering processes which occur under different bombardment conditions. To this end, we have conducted scattering experiments involving mass-filtered ions (F⁺, CF_x⁺, NF_x⁺) with tunable energy (50-1000 eV) and high flux (monolayers/s) on several surfaces (Si, Al, Ag) to look critically at collision kinematics, charge exchange processes, and surface reaction products. 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) bimodal energy distributions of F⁺ and F^- species leaving Si and Ag surfaces. For instance, energy losses measured for single-scatter events of F⁺ off Si and Al show that F⁺⁺ can be formed through a double electron promotion mechanism which “turns-on” above a critical collision energy. Velocity analysis of daughter fragments from CF₃⁺ impact on Si and Ag point to several situations where fast exit species (C⁺+F^- and F⁺+CF⁺ with energies > binary collision predictions) are formed as a result of the projectile ion breaking apart before the hard collision step. Finally, energy analysis of F⁺ and F^- leaving Si and Ag surfaces shows two distinct scattering channels: one associated with a binary-like, single-scatter elastic event and another narrow, low-energy channel that cannot be explained as simple sputtering. These results illustrate that in many instances, product species can show significant inelastic losses as well as faster-than-SIMS behavior which may have a dramatic impact on profile evolution in plasma etching. In addition, energy analysis of both the positive and negative ion products associated with fluorinated ion scattering provides indispensable clues about the physics of reactive ion etching.