Quantitative electrostatic force measurement in AFM

Abstract We describe a method for measuring forces in the atomic force microscope (AFM), in which a small amplitude oscillation (∼1 Ap–p) is applied to a stiff (∼40 N/m) cantilever below its first resonant frequency, and the force gradient is measured directly as a function of separation. We have used this instrument to measure electrostatic forces by applying an ac voltage between the tip and the sample, and observed a variation in contact potential difference with separation. We also show how the benefits of this instrument may be exploited to make meaningful capacitance measurements, especially at small tip–surface separations, and demonstrate the potential of this technique for quantitative dopant profiling in semiconductors.