Contactless Measurement of Sheet Resistance of Nanomaterial Using Waveguide Reflection Method

Conductive nanomaterials are widely studied and used. The four-point probe method has been widely used to measure nanomaterials' sheet resistance, denoted as R-s. However, for materials sensitive to contamination or physical damage, contactless measurement is highly recommended if not required. Feasibility of R-s evaluation using a one-port rectangular waveguide working on the microwave band in a contact-free mode is studied. Compared with existed waveguide methods, the proposed method has three advantages: first, by introducing an air gap between the waveguide flange and the sample surface, it is truly contactless; second, within the specified range of R-s, the substrate's effect may be neglected; third, it does not require a matched load and/or metallization at the sample backside. Both theoretical derivation and simulation showed that the magnitude of the reflection coefficient S-11 decreased monotonously with increasing R-s. Through calibration, a quantitative correlation of S-11 and R-s was established. Experimental results with various conductive glasses showed that, for R-s in the range of similar to 10 to 400 Ohm/sq, the estimation error of sheet resistance was below similar to 20%. The potential effects of air gap size, sample size/location and measurement uncertainty of S-11 are discussed. The proposed method is particularly suitable for characterization of conductive glass or related nanomaterials with R-s in the range of tens or hundreds of Ohm/sq.