Non-contact characterization of printed resistors

In this work we demonstrate the use of a high resolution non-contact scanning microwave microscope for characterization of printed resistors. The resonant microwave probe operates at a frequency of 5.73 GHz and it is based on a dielectric resonator coupled to a gold-coated tungsten tip with radius of 200 μm protruding from a cavity wall. Direct print additive manufacturing was used to produce the resistive films. Non-contact measurements of the resonant frequency fr and quality factor Q of the resonant microwave probe at a standoff distance of 20 μm were performed. Quality factor images were obtained over a scan area of 160 μm × 1670 μm in steps of 10 μm. Measurements reveal that Q varies from 214 to 233 over the studied region. In this work, variations in Q are associated with non-uniformities on the resistor surface. The quality factor of the probe was also acquired as a function of the tip-sample distance and measured data was fitted to a polynomial equation. We converted Q images to sheet resistance images using the polynomial equation and the material resistivity (400 Ω/sq/mil). Using the proposed approach, we found that the average sheet resistance over the scan area is Rs = 1027 W/sq and that Rs variations up to 662 Ω/sq, due to non-uniformities in the resistor's thickness, were detected by the microwave microscope. The localized microwave characterization capability demonstrated by the non-contact microscope could be of interest for defect detection in printed microwave circuits.