Accurate Electrostatic Force Measurements by Atomic Force Microscopy Using Proper Distance Control

Electrostatic force microscopy (EFM) enables us to examine the local electrostatic force between an atomic force microscopy (AFM) tip and sample surface, from which an electrical capacitance between them and surface potential can be evaluated, by applying direct current (dc) and alternating current (ac) voltages. From the dependences of the electrostatic force on the dc voltage and the frequency of ac voltage in EFM, carrier density, carrier type, and deep-level states in a semiconductor can also be investigated. Since EFM is basically operated as an AFM, special care should be taken in an effect of the electrostatic force on a distance control between the tip and the sample, and robust distance control is necessary even under the strong electrostatic force to realize proper measurements of both topography and electrostatic force. In this article, we have examined the effectiveness of the usage of the oscillation amplitude of a cantilever as a feedback target for the distance control, which is referred to as
$\Delta A$
-mode operation, while the cantilever oscillation frequency is always kept at its primary resonance, like the conventional frequency modulation AFM (FM-AFM) which uses a resonant frequency shift as the feedback target. First, we have verified that a distance change due to the strong electrostatic force was significantly reduced compared with FM-AFM. Second, we have confirmed that the
$\Delta A$
-mode operation in EFM realized proper measurements of the dependence of a tip–sample capacitance including a surface depletion capacitance on the dc voltage and the frequency of ac voltage, indicating good robustness of the method against the strong electrostatic force.