Performance Limitations and Measurement Analysis of a Near-Field Microwave Microscope for Nondestructive and Subsurface Detection

Near-field microwave microscopy provides a means for nondestructive localized characterization of both surface and subsurface materials and devices. This paper details the design and implementation of a near-field microwave measurement system capable of achieving large scan areas (>; 1 mm²) with micrometer spatial resolution, good signal-to-noise and long-term measurement stability. We discuss the measurement topology, system design, and qualification, as well as the design and optimization of the probe circuit and equivalent microwave circuit. The system noise floor is measured and noise-limiting elements are identified. A calibration method is discussed that allows for the quantitative extraction of the tip-sample impedance and enables tip-sample modeling. Several demonstration measurements are provided that show differentiation of not just metals and dielectrics, but compound targets. Additionally, resolution of 10-μm features at the surface are demonstrated with a factor of 4 degradation in spatial resolution at a depth approximately equal to the tip diameter.