Development of a near-field scanning microwave microscope using a tunable resonance cavity for high resolution

We report on the operational principles and the practical implementation of a near-field scanning microwave microscope system consisting of lambda/4 coaxial resonator with a tunable resonance cavity coupled to a sharp probe tip. The changes of sensitivity and spatial resolution as a function of cavity length are clearly modulated by tuning the resonance cavity. By tuning the resonance cavity, we demonstrate improved sensitivity and spatial resolution better than 4 (mmu) of the near-field images of a YBa2Cu3Oy thin film on a MgO substrate at an operating frequency of f = 1-1.5 GHz. We modelled the lambda/4 coaxial resonator by a LCR circuit and the principles of operation can be explained by using perturbation theory, considering the radius of the probe tip, the quality factor, impedance matching, and the sample-tip distance.