A novel STM-assisted microwave microscope with capacitance and loss imaging capability

We report a new technique of scanning capacitance microscopy at microwave frequencies. A near field scanning microwave microscope probe is kept at a constant height of about 1 nm above the sample with the help of scanning tunneling microscope (STM) feedback. The microwaves are incident onto the sample through a coaxial resonator that is terminated at one end with a sharp tip (the same tip is used to conduct STM), and capacitively coupled to a feedback circuit and microwave source at the other end. The feedback circuit keeps the source locked onto the resonance frequency of the resonator and outputs the frequency shift and quality factor change due to property variations of the sample. The spatial resolution due to capacitance variations is congruent with 2.5 nm. The microwave microscope is sensitive to sample sheet resistance, as demonstrated through measurements on a doped silicon sample. We develop a quantitative transmission line model treating the tip to sample interaction as a series combination of capacitance and sheet resistance in the sample.