Noncontact Surface Resistivity Measurement Using a Cylindrical Surface Potential Detector With a Corona Charger

A noncontact surface resistivity tester was developed by improving the charge decay measurement. The tester has a corona charger and surface potential detectors in a cylindrical grounded body. The corona charger provides a surface charge on the test material, and the surface potential detectors measure the traveling motion of the surface charge. The traveling speed of the surface charge is dependent on the surface resistivity of the test material; therefore, the surface resistivity can be determined without contact to the material. An equivalent circuit with a 1-D ladder circuit was predicted, which includes the surface resistance of the material, the volume resistance of air, and the capacitance between the cylinder and the test material. From analysis of the equivalent circuit, the surface resistivity for an insulative surface was found to be a function of ρ_s/(ρ_vδ), where ρ_s is the surface resistivity of the test material, ρ_v is the volume resistivity of air, and δ is the gap between the test surface and the cylinder. For conductive and dissipative surfaces, the surface resistivity is dependent on ε₀ρ_s/δ, where ε₀ is the permittivity of air. The measurement concepts were experimentally verified using a needle-type corona charger, two induction probes, and a surface potential meter. The predicted surface resistivity measured using this method was consistent with the surface resistivity measured with a contact-type surface resistivity tester.