Detecting electrical forces in noncontact atomic force microscopy

A scanning electrical force microscope (SEFM) is presented, for which the mechanical cantilever modulation frequency is close to the mechanical resonance, whereas the fundamental of an electrical modulation far below the mechanical resonance is used for Kelvin feedback and the second harmonic of the electrical modulation is used for detecting the derivative of the capacitance with respect to the tip-sample spacing. The operation of the microscope is demonstrated for a test sample with structured chromium electrodes on a glass substrate. The imaging of surface potential distributions as well as of local capacitances is discussed. For gold films on silicon, variations in capacitance caused by variations of the tip-sample geometry show the sensitivity of the method.