Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 1 / Section A(i) / 2 / ii

In tapping-mode atomic force microscopy, the cantilever is driven near a resonance and its oscillation amplitude is monitored. Tip-surface interaction changes that amplitude. A controller compares it with a chosen setpoint and moves the vertical piezoelectric actuator to reduce the error. The recorded actuator displacement supplies the topography channel. A smaller amplitude during approach commonly triggers withdrawal and an increased amplitude during separation triggers approach, with the actual feedback sign set by the instrument's operating convention. Correct tracking requires enough response bandwidth without destabilizing the loop.

A conventional tapping tip tapers toward its apex, so its shank contacts an overhanging feature before its apex can probe underneath it. Critical-dimension atomic force microscopy instead uses a calibrated flared or laterally extended tip, with a relatively narrow neck and side-sensitive control. The lateral extension can contact a reentrant sidewall while the neck clears the upper edge. The measured profile still includes the tip's finite shape and must be interpreted with a tip calibration; changing the tip does not automatically eliminate all convolution errors.^[ref]

Lateral oscillation or dithering is added to the vertical oscillation. It provides a tip-sidewall interaction signal that can drive lateral feedback while the vertical channel maintains height control. This is how a suitable flared tip can track steep or reentrant walls. Treating the additional signal as merely a larger vertical tapping amplitude would not supply the required sidewall sensing.^[ref]