Contactless Measurement of Sheet Resistance of Nanomaterial Using Waveguide Reflection Method

Abstract

Conductive nanomaterials are widely studied and used. The four-point probe method has been widely used to measure nanomaterials’ sheet resistance, denoted as Rs. However, for materials sensitive to contamination or physical damage, contactless measurement is highly recommended if not required. Feasibility of Rs 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 Rs, 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 S11 decreased monotonously with increasing Rs. Through calibration, a quantitative correlation of S11 and Rs was established. Experimental results with various conductive glasses showed that, for Rs in the range of ~10 to 400 Ohm/sq, the estimation error of sheet resistance was below ~20%. The potential effects of air gap size, sample size/location and measurement uncertainty of S11 are discussed. The proposed method is particularly suitable for characterization of conductive glass or related nanomaterials with Rs in the range of tens or hundreds of Ohm/sq.

Contactless Waveguide Characterization of Piezoresistive Materials for Wireless Strain Sensors

“…Resonant methods [12][13][14] show good accuracy and sensitivity; however, they require an ad hoc cavity and sample preparation can be complicated. Non-resonant methods [15][16][17][18] are based on the measurement of the signal reflected and transmitted by the sample. These methods allow measurements to be made using different experimental setups and require fewer precautions [10].…”