Topographical scanning in non-contact/tapping mode relies on cantilever operation at resonant frequency to maintain regular oscillation of tip in the z-direction. Tip engagement with surface may alter this behavior and require re-tuning of tip.
Often when engaged signal quality may be too low for effective computer generation of topographical image. Lowering set-point allows for harder tip-surface contact and measurement of surface features. Note: set-point is relative to cantilever (sum) signal when not in contact with surface (“free air amplitude"). Low set-point relative to free air amplitude can result in harder contact with surface and blunting/damage to tip.
When engaged on surface the z-piezo controls tip approach or withdrawal form surface in order to maintain good contact. The amount/length of adjustment in the direction for tracking up and down vertical features is controlled by the Gain value. A high (>50) versus low (<5) gain value allows for greater step sizes when controller adjust tip in z-direction. This, however, is relative to how fast a scan is being conducted as slower scan speeds (≤5 nm/s) allow for greater time to climb up and down surface features. Hysteresis of drift between trace and retrace curves can be improved by increasing gain. Note: Higher gain results harder contact with surface and blunting/damage of tip.
Gain can also result in the creation of additional noise in a scan through over/underestimating of necessary step length and repeated corrections to maintain good contact with surface. This typically results in the appearance of a sinusoidal waveform in image that is not actually there. Reducing gain will reduce or remove waveform all together.
Tip-surface interaction can also play a critical role in image quality. Specifically phase shifting of tip when in close proximity to surface can effect attractive and repulsive interactions between tip-surface. “Attractive" forces can be observed when Phase is > 90˚, “repulsive" when < 90˚. Tuning typically attempts to center phase @ 90˚, but further adjustments can be made by altering "phase" offset during tuning.