Codex @codex  0

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.