Paper PS2-TuA10
Molecular Dynamics Analysis of Si Etching with Cl and Br Beams: Ion Incident Angle and Neutral Radical Flux Dependence

Tuesday, October 30, 2012, 5:00 pm, Room 25

Profile anomalies and surface roughness are critical issues to be resolved in plasma etching of nanometer-scale microelectronic devices, which in turn requires a better understanding of the effects of ion incident energy and angle on surface reaction kinetics. In addition, incident neutral radicals also affect the surface reaction kinetics during plasma etching, and thus the etching characteristics are varied by the neutral-to-ion flux ratio. This paper presents a classical molecular dynamics (MD) simulation of Si etching by energetic Cl⁺ and Br⁺ ion beams and low-energy neutral Cl and Br radicals, using an improved Stillinger-Weber interatomic potential model for Si/Cl and Si/Br systems. Emphasis is placed on a systematic understanding of plasma-surface interactions of Si/Cl and Si/Br systems, which are widely used in manufacturing microelectronic devices.

In the MD simulation, the substrate has a Si(100) surface, which is a square 32.58 Å on a side and contains 72 Si atoms in a monolayer (ML). The simulation cell initially contains 1440 Si atoms (20 ML) in a depth of 26.0 Å, where Si atoms in the bottom layer are fixed during simulation, while periodical boundaries are imposed in the horizontal direction. Energetic Cl⁺ and Br⁺ ions are injected toward the substrate surface from randomly selected horizontal locations above the target, and neutral Cl and Br atoms are introduced onto the surface prior to every ion incidence. The neutral-to-ion flux ratio was varied in the range Γ_n/Γ_i = 0–100. The ion incident energy was in the range E_i = 20–300 eV, and the ion incident angle was in the range θ_i = 0–90˚. The kinetic energy of neutral atoms was taken to be 1 eV.

Numerical results indicated that in etching by beam only (Γ_n/Γ_i = 0), the surface reaction kinetics exhibit a characteristic of the ion-enhanced etching at lower ion energies, where the etching yield is maximum at normal incidence (θ_i = 0˚), while a characteristics of the physical sputtering at higher energies, where the yield is maximum at off-normal incidence (θ_i = 60–70˚). Similar inclinations for the ion incident angle were obtained with increasing Γ_n/Γ_i, although the thickness of the surface reaction layer increases a little and the etch yield increases significantly with increasing Γ_n/Γ_i. These imply that impinging ions disarrange the surface Si lattice and weaken the binding force of Si atoms on the top surface, and incident neutral radicals etch surface Si atoms whose binding force with the neighbor Si atoms becomes lower than that of Si atoms in the original diamond lattice. These effects are clearly observed for Si/Br system, as compared to for Si/Cl system.