Abstract
A systematic and quantitative comparison of electrical detection systems in scanning microwave microscopy is reported. Scanning microwave microscopy (SMM) is capable of investigating nanoscale electrical properties with high accuracy over a broad frequency range of 1–20 GHz. However, due to the passive matching network only discrete frequencies can be used every 1 GHz with varying signal-to-noise ratio (SNR). Here we study in detail the impedance matching mechanism using an interferometric network where a two-port measurement is implemented with a reduction of the trace noise due to signal subtraction. The interferometer setup shows superior performance resulting in a 2–8 fold increased SNR with respect to the standard shunt solution, in addition to stable broadband performance over the full frequency range. We perform a comparison of the electrical sensitivity obtained using a direct connection from the network analyser to probe, the typically implemented shunt-resonator impedance matching network, and the proposed interferometer setup. The interferometer SMM allows us also for calibrated impedance measurements, which we demonstrate on Tobacco mosaic viruses with 18-nm diameter, with a capacitance resolution of 0.67 attoFarads at 10 ms acquisition time per pixel.
Highlights
S CANNING Microwave Microscopy (SMM) is a valuable tool that allows for simultaneous investigation of mechanical and high frequency electrical properties of materials at the nanoscale
Since no impedance matching network is used and the ensemble made of SMM probe and nosecone has impedance much higher than 50 Ω, the poor impedance matching leads to a lack of standing waves, and of notches in the frequency domain, along with a ripple pattern and a tendency of increasing attenuation with increasing frequency, due to the long transmission line used
In order to determine empirically the power threshold that allows for noise minimisation, we repeated the spectroscopy experiment that lead to the results presented in Fig. 4, using the best-observed detection setup, i.e., interferometer with low noise amplifier (LNA) and voltage variable attenuator (VVA)
Summary
S CANNING Microwave Microscopy (SMM) is a valuable tool that allows for simultaneous investigation of mechanical and high frequency electrical properties of materials at the nanoscale. To this end, it interfaces an atomic force microscope (AFM) with a vector network analyser (VNA). This makes it possible to break the resolution limit of the wavelength of the probing microwaves and to resolve sample features with a lateral resolution down to nm range [2], theoretically limited only by the tip size. The most versatile SMM studies in terms of frequency range can be performed using a VNA as excitation and detection tool. The best hardware configuration, employed at the optimal power level, was used for an imaging application that had previously pushed the limits of standard SMM sensitivity
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