SPM mode

    Noncontact EFM

Generally Electric Force Microscopy (EFM) can be used in several modes, depending on the type of the sample under investigation and kind of the required information.
The most useful of them is Non-Contact EFM mode based on the two-pass technique. During the second pass the cantilever is piezodrived at resonant frequency and cantilever is grounded or biased by dc voltage V. Capacitive tip-sample electric force (or rather its derivative) leads to resonance frequency shift. Accordingly amplitude of cantilever oscillation decreases and phase of oscillation changes [1]. Both amplitude and (or) phase of oscillation deviations can be measured and electric potential distribution over the sample surface can be imaged.
This mode of operation has some advantages comparatively to Scanning Kelvin Microscopy (SKM). Registered amplitude or phase deviations images are determined by capacitive tip-sample electric force derivative, e.g. second derivative of tip-sample capacity. As a result Non-Contact EFM leads to higher resolution because the ratio of the parasitic capacitance of the tip side and of the flat part of the cantilever to the useful capacitance of the tip apex should is minimized [2, 3].

    References 

1.  J. Appl. Phys. 61, 4723 (1987).
2. Appl. Phys. Lett. 52, 1103 (1988).
3. Nanotechnology 12, 485 (2001).
   
   
    

    Contact EFM

As in Noncontact EFM in Contact EFM the cantilever is biased directly by V_tip=V_dc + V_ac sin(wt), where V_ac is referred to as the driving voltage. Scanning is executed as in usual Constant Force mode and simultaneously the electric forces are measured. The capacitive force F_cap(z) between the tip and a surface at potential V_s is

F_cap(z) =(1/2) (V_tip - V_s)²(dC/dz)

where C(z) is the tip-surface capacitance dependent on tip geometry, surface topography and tip-surface separation z. 
In contact EFM the influence of the first harmonic of F_cap(z) on the cantilever oscillation is registered. 
Contact EFM can be applied to semiconductor structures. The current flow through the contact area may decrease those forces, but it is extremely small, since there are usually thick (a few nanometers) oxide layers on the tip and sample surface.
As an example data of GaAlAs geterostructures measuring are given.

    References

1. Rev. Sci. Instr. 70, 1735 (1999).