Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2018 / Paper 1 / Section A(ii) / 4 / i / b

An approximately isotropic wet etch produces lateral undercut as well as downward removal. In the hydrofluoric acid and nitric acid mixture, oxidation of silicon and dissolution of the oxidized material cooperate. This is not the strongly crystal-anisotropic KOH-type picture.

If the exposed horizontal surface recedes about $200\mathrm{nm}$ and the local rate is isotropic, the etch front advances a comparable distance beneath each resist edge. A section through the front has rounded corners, and the silicon support under the mask narrows. The mouth can consequently widen laterally by roughly $400\mathrm{nm}$ in total at the original surface level. The precise contour depends on transport, chemistry and mask integrity; the diagram represents the isotropic geometrical limit.

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.