A focused Ga⁺ beam is rastered over a top-down height-field grid; every ion impact knocks a few surface atoms loose (sputtering). The number of atoms ejected per ion — the sputter yield — depends strongly on the local angle θ between the beam and the surface normal, following the empirical Yamamura form:
Y(θ) = Y₀ · cos(θ)⁻ᶠ · exp[ −Σ·(cos(θ)⁻¹ − 1) ]
f ≈ 1.7, Σ ≈ 1.0 (typical for Ga⁺ on Si/metals)
At normal incidence (θ=0) the yield is just Y₀. As θ rises the yield first increases — ions deposit their energy closer to the surface — then collapses toward grazing incidence as ions increasingly reflect off the surface instead of sputtering it. Numerically evaluating this exact formula (f=1.7, Σ=1.0) puts the peak at θ≈54°, not the 60–75° range sometimes quoted for this parameterisation — this sibling's theory text and readouts use the checked value.
This simulator computes θ at every grid cell from the local slope of the milled surface itself, each frame, so the feedback is real: as a trench floor deepens, its walls tilt and present a higher θ to the vertical beam than the floor does — walls mill faster than floors, which is exactly why real FIB-cut features end up with curved, tapered sidewalls (V-grooves) rather than perfect vertical walls, and why deep structures need staircase-compensated scanning. The top-down map shades each cell by remaining height; the strip beneath it plots the live cross-section along the beam's current row so you can watch the taper angle form directly.
- Beam energy — scales the base yield Y₀ (higher energy, more atoms per ion, up to the point of ion channeling losses).
- Beam current / dose — sets how much material is removed per unit dwell time (ions delivered per second).
- Redeposition — a fraction of sputtered atoms don't escape; they land back on nearby low-yield (flatter) surfaces, building the debris ridges seen around real milled features. Toggle it off to see the idealised, redeposition-free profile.
- Drag / scroll — drag the top-down map to pan, scroll or pinch to zoom into the milled feature.