In situ characterization of microstrip line using near-field scanning microwave microscopy

With the continuous scaling and increasing operating frequencies of microwave integrated circuits, accurate characterization of on-chip microwave signal propagation and field distributions has become increasingly important for ensuring circuit reliability and performance stability. To address this challenge, we have developed a beat-frequency-enhanced near-field scanning microwave microscopy, which enables in situ, non-destructive characterization of the microwave field distributions on microstrip line surfaces under operating conditions. When the probe–sample distance and the surrounding dielectric environment are kept constant, the oscillation amplitude of the transmission coefficient S21, ΔS21can serve as a relative indicator of the surface microwave field strength of the microstrip line. The experimental results demonstrate that ΔS21 exhibits a positive correlation with the input signal power applied to the sample. Furthermore, as the input signal frequency of the sample approaches the resonant frequency of the probe–sample coupling, the detection sensitivity of ΔS21 to the input signal power is significantly enhanced, with a minimum detectable power as low as −40 dBm. Surface scanning results reveal clear visualization of microwave field strength distributions for both standard and defective microstrip lines, in good agreement with simulation data. The proposed technique offers a promising pathway for real-time diagnostics of high-frequency integrated circuits, combining high sensitivity, non-destructive evaluation, and compatibility with practical operating environments.