Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2018 / Paper 1 / Section A(i) / 2 / i

A tip-change artefact occurs when the effective apex changes during scanning, for example through material pickup, loss of an attached particle or damage. In tapping-mode atomic force microscopy, a sudden change can introduce a false height offset between parts of the image, even on a continuous terrace.

The change alters the probe–surface interaction or the effective point at which the chosen amplitude setpoint is reached. The controller moves the vertical scanner to recover that setpoint, recording a spurious step or a change in line baseline. The sign is not fixed: different apex changes can move the apparent surface up or down.

The amplitude channel usually shows a transient deviation as the interaction changes; with stable feedback it returns toward its setpoint. It therefore need not retain the same persistent offset as the recorded height. A change in probe resonance or free amplitude can also change the operating condition. If the apex geometry has changed, widths and shapes can change too, or duplicated structures can appear as a double-tip artefact. A baseline correction cannot recover these lost geometric data.