11/8/ Damage Profiles of Low-Energy Ion Bombardment: Application to Ion Milling SFR Workshop November 8, 2000 Dave Humbird, David Graves Berkeley, CA 2001 GOAL: Write and test a Molecular Dynamics code to study energy deposition profiles. Conduct preliminary sputtering studies. Compare results to phenomenological theory, by 9/30/2001

11/8/ Motivation Crystalline damage caused by low-energy (~200 eV) ion impacts is localized to a very narrow region near the surface This may permit the etching of very small features using a highly focused ion beam (ion milling) Tractable simulation size and feature size of interest are rapidly converging

11/8/ Molecular Dynamics (MD) Simulation Interatomic PotentialInteratomic Forces typical MD time step: initial configuration update positionsupdate velocities evaluate forces Simulation is non- equilibrium! Kinetic + potential energy is always conserved

11/8/ Ion Damage Simulations Simulation cell size is 20 x 20 x 20 ų (512 atoms) Introduce energetic Ar with velocity normal to surface Follow motion of all Si atoms until collision cascade subsides Repeat x 2000 for statistics Formulate a distribution (in r and z, relative to the point of impact) of atoms’ displacements 1 Expense: 100 impacts = 40 min (PIII-700) 1 in the spirit of Winterbon, Sigmund, and Sanders, Mat. Fys. Medd. Dan. Vid. Selsk. 37 (14) (1970)

11/8/ Ion Damage Profile Atomic Displacement Distribution 200 eV Ar Impacts, Normal Incidence Si atoms are displaced from their origins by a distance proportional to the color temperature. (red → ~6 Å, blue → 0 Å) ion trajectory Relative coordinate system:

11/8/ Ion Milling Simulation Simulation cell is substantially larger (60 x 60 x 40 ų, 9200 atoms) Introduce energetic Ar ion above surface, confined to a “beam” of radius 12 Å about the center Follow collision cascade Delete sputtered Si atoms Cool cell to room temperature Accumulate impacts on the same cell Expense: 100 impacts = 20 hours (PIII-700)

11/8/ Feature Evolution Representations of the surface after 330 impacts (200 eV Ar, normal incidence) Stereograph top view Tissue-paper plots left: side view, +y direction right: top view, 30° toward viewer

11/8/ Observations and Questions Sputtering yield is much lower than anticipated for 200 eV Ar –0.06 for first 100 impacts, 0.04 for the second 100 –Feature has been created chiefly by displacement –Could it withstand annealing? –Could stress cause a crater rim to form? Feature is conical in shape –Self-perpetuating: ions are repulsively funneled towards the center –Is there a depth limitation? –What ion energy will overcome this limit?

11/8/ and 2003 Plans Write and test code to study ion-induced surface diffusion. Compare results to phenomenological theory and experiment by 9/30/2002. Conduct Molecular Dynamics studies of ion bombardment. Simulate energy deposition and surface diffusion and compare to LER experiments by 9/30/2003.