EFM of aluminum dots on gold substrate

EFM uses the shift in resonance frequency of the cantilever oscillation to monitor variations in the electrostatic field between sample and tip. The electrostatic force is always attractive and increases at larger contact potential differences between tip and sample, causing a lower phase at the (fixed) oscillation frequency of the cantilever.

The phase shift as function of potential difference can be elegantly measured while running a tip voltage spectrum. The tip voltage at maximum phase (i.e. miminal attractive force) differs for different metals with different work functions. This can be seen from the horizontally shifted phase spectra measured on gold or aluminum.

Similar to MFM, EFM is measured with the tip lifted above the sample. The electrostatic interaction depends on distance between tip and sample. For flat samples, the electrostatic interaction can be measured in constant height mode. For samples with larger variations, like the Al-Au microstructure, the distance variation also causes phase shifts. To reduce this mingling, the C3000 controller and the CoreAFM offer the contour following mode.

In both constant height and contour following mode, the topography is measured in a first run, though it can be omitted for constant height mode. For constant height the tip is moved with constant slope over the sample in a second run. With contour following, the separation is kept constant, by moving the tip up and down according to the previously measured topography.

As can be derived from the tip voltage spectra, the phase difference between two metals with different work functions increases with increasing tip voltage. When measuring at +3V the phase on aluminum is significantly higher than that of gold. When applying a negative tip potential of –3V the contrast inverts, as would be expected from the tip voltage spectra on Aluminum and gold.

More quantitative imaging of potential variations can be done in Kelvin Probe Force Microscopy (KPFM).