We present the design and experimental results of a near-field scanning microwave microscope working at a frequency of 1GHz. Our microscope is unique in that the sensing probe is separated from the excitation electrode to significantly suppress the common-mode signal. Coplanar waveguides were patterned onto a silicon nitride cantilever interchangeable with atomic force microscope tips, which are robust for high speed scanning. In the contact mode that we are currently using, the numerical analysis shows that contrast comes from both the variation in local dielectric properties and the sample topography. Our microscope demonstrates the ability to achieve high resolution microwave images on buried structures, as well as nanoparticles, nanowires, and biological samples.

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We emphasize that the simulation only considers the sensing unit as an enclosed system. The field distribution is different if the contribution from the rest of the cantilever and the matching network is taken into account, which is beyond the scope of this article and will not be described here.

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Since the NSMM is built on an AFM platform, one may attempt to use the z-height control from laser feedback to separate the topographical signal from electrical signal. This, however, does not apply because much coupling to the sample actually comes from the excitation ring. While the AFM function can keep the tip in constant height on every detail of the topography, the entire sensing unit still moves up and down with respect to the overall sample surface.

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For a summary of the biological samples in this study, refer to Professor Luo’s group website http://www.stanford. edu/group/luolab/
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