Highly Sensitive Phase-Variation Microwave Sensor for Measuring the Thickness of Dielectric Films on Metals with Micrometer-Scale Resolution

A reflective-mode phase-variation sensor consisting of a microstrip transmission line terminated with a complementary split ring resonator (CSRR), the sensing element (etched in the ground plane), is presented. The one-port sensor operates at a single frequency and the phase of the reflection coefficient, the output variable, is very sensitive to the dielectric properties of the material under test (MUT), which should be placed in contact (or in close proximity) with the CSRR. Such high sensitivity is achieved by virtue of the strong phase variation with frequency in the vicinity of the operating frequency, in turn obtained by weakly coupling the CSRR to the line. Thanks to the high sensitivity, and due to the fact that the sensing resonator (CSRR) is etched in the ground plane, the sensor can be useful to detect changes experienced in the surface of metals (e.g., caused by corrosion), of potential interest for structural health monitoring (SHM) of urban or civil structures. For validation purposes, the sensor is preliminarily applied in this paper to the measurement of the thickness of different micrometer-scale layers of polyether-ether-ketone (PEEK), a dielectric material with dielectric constant of 3.22, on top of copper, as a preliminary means to emulate an oxidation-related corrosion process.