= Solution
\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.
Back to article page
