Electromagnetic Characterization of Thin Films by Using Non-Contacting Waveguides

Surface impedance represents a crucial parameter for the characterization of thin films. Indeed, materials with a sheet impedance varying as a function of their elongation could be used in radio frequency piezoresistive sensors. To estimate this quantity while having the possibility of stretching the sample under test, a waveguide-based non-contact approach is proposed. The absence of contact between the sample under test and the waveguides determines an electromagnetic field leakage, which is prevented by adopting an electromagnetic band gap (EBG) structure. The surface impedance of the sample is retrieved through an inversion procedure exploiting the scattering parameters measured using the proposed setup. The inversion procedure is based on a circuit representation of the waveguide-air-waveguide Section as a \pi -junction. The reliability of the proposed measurement method has been experimentally assessed using a WR137 waveguide. The proposed method allows to accurately determine the real part of surface impedance (the sheet resistance), while higher uncertainty is achieved in the estimation of the imaginary part.