In order to meet the needs of the semi-conductor industry as it requires finer lithography nodes, a method of feedback control for scanning probe microscopy generates a microwave frequency comb of harmonics in a tunneling junction by irradiating the junction with mode-locked pulses of electromagnetic radiation. Utilizing power measurements within one or more harmonics, the tip-sample distance in the tunneling junction may be regulated for maximum efficiency and avoid tip crash when used with resistive samples. Optionally, no DC bias is required to use the method. Utilization of this method contributes to true sub-nanometer resolution of images of carrier distribution in resistive samples such as semi-conductors.

The superimposition of a periodic potential wave to the tip movement control or the bias supply of an STM, in which a microwave frequency comb is generated in its tunneling junction, may be used to reduce or eliminate artifacts or other noise generated from outside the tunneling junction.