Tip–sample distance feedback control in a scanning evanescent microwave microscope

We have developed a means of tip–sample distance regulation for a scanning evanescent microwave microscope over conductive samples. Changes in resonant frequency and quality factor are measured, where changes in resonant frequency are related to the tip–sample capacitance and changes in quality factor are related to microwave absorption. With the analytical expression of the tip–sample capacitance as a function of tip–sample distance, we can quantitatively regulate the tip–sample separation. We demonstrated simultaneous noncontact imaging of topography and surface resistance with high spatial resolution.