3.7.3 Oxidation sharpening
The all-important radius of curvature of the tip can be made smaller – both in the silicon nitride and pure silicon tips – by the trick of oxidation sharpening. Figure 15 shows the principle. The original tip is first heated to around 1000 °C in an oxidising atmosphere, such that the outermost micrometre or so of the material is converted to an oxide of silicon. This oxide is more easily etched in buffered hydrofluoric acid than the underlying silicon or silicon nitride, so when the etch is carried out it stops when all the oxide has been consumed. This makes the process highly repeatable. The diagram shows, in crude geometrical terms, how the effect of removing this newly formed oxide layer results in a reduction in the radius of curvature of the tip.
The image in Figure 16 is of a silicon tip before and after the oxidation-sharpening process. It is possible to achieve radii of curvature less than 10 nm by this means.