Near-field microwave microscopy on nanometer length scales

The Near-field scanning microwave microscope (NSMM) can be used to measure ohmic losses of metallic thin films. We report on the presence of an interesting length scale in the probe-to-sample interaction for the NSMM. We observe that this length scale plays an important role when the tip-to-sample separation is less than about 10 nm. Its origin can be modeled as a tiny protrusion at the end of the tip. The protrusion causes deviation from a logarithmic increase of capacitance versus a decrease in the height of the probe above the sample. We model this protrusion as a cone at the end of a sphere above an infinite plane. By fitting the frequency shift of the resonator versus height data (which is directly related to capacitance versus height) for our experimental setup, we find the protrusion size to be 3-5 nm. For one particular tip, the frequency shift of the NSMM relative to 2 μm away saturates at a value of about −1150 kHz at a height of 1 nm above the sample, where the nominal range of sheet resistance values of the sample is 15-150 Ω. Without the protrusion, the frequency shift would have followed the logarithmic dependence and reached a value of about −1500 kHz.