= Solution
\b[Near-normal <ion milling> preferentially removes exposed horizontal <silicon> and gives a more directional trench with much less lateral undercut.] Energetic argon ions eject surface atoms by momentum transfer. The resist blocks direct trajectories to the protected regions.
After about $200\,\mathrm{nm}$ removal of exposed <silicon>, expect an approximately flat recessed floor and steep transitions near the mask edges. The mask is also sputtered: finite <etch selectivity>, angular yield variations, mask-edge faceting and redeposition can produce sloped walls, fences or residue. No resist erosion depth can be deduced without its sputter rate. Thus a perfectly unchanged rectangular mask is only the ideal sketch.
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