Scanning tunneling microscopy investigations of the Si(111) topography produced by etching in 40% NH4F: Observation of an optimum etch duration

The time evolution of the topography of an oxidized Si(111) surface immersed in an aqueous 40% NH4F etching solution is investigated ex situ by STM. The topography of a rough oxide layer of thickness ≊24 Å becomes smoother as this layer is thinned. An exposure of 75 s completely removes the surface oxide and a minimum obtainable roughness of ≊1.4 Å rms is observed ≊15 s, hence. With continued etching, two processes—etching of the silicon surface and gas evolution—operate in conjunction to produce pillars of silicon at locations on the Si(111) surface which are shielded from etching by adsorbed gas bubbles. Gas evolution and roughening are suppressed by the addition of the reductant NaCNBH3 (0.50%) to the etching solution.