Optimization of Near-Field Image Capture With Millimeter-Wave Bow-Tie Probes

A 60 GHz high-spatial resolution millimeter nearfield scanning microwave microscope system is developed. Like in scanning near-field optical microscopy, it involves a modulation of the probe-sample distance. An analytical model of the near-field detection versus the minimum probe-sample distance is derived as a function of the harmonic rank used for lock-in detection. The model is validated owing to various probe sizes and materials. Both the model and experiments exhibit the filtering out of low spatial frequencies, the best efficiency being obtained at highest harmonics. It yields a resolution of ≈2 μm (i.e., λ/2500) with an 18 μm gap bow-tie probe and an optimized detection setup operating on the third harmonic.