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. (C) 1999 American Institute of Physics. [S0003-6951(99)03518-4].