Thickness Measurement of Metallic Film Based on a High-Frequency Feature of Triple-Coil Electromagnetic Eddy Current Sensor

Previously, various techniques have been proposed for reducing the lift-off effect on the thickness measurement of the nonmagnetic films, including the peak-frequency feature and phase feature in the Dodd–Deed analytical formulation. To realize a real-time feedback response on the thickness monitoring, the phase term in the Dodd–Deeds formulation must be taken off the integration. Previous methods were based on the slow change rate of the phase term when compared to the rest of the term—the magnitude term. However, the change rate of the phase term is still considerable for a range of working frequencies. In this article, a high-frequency feature has been found. That is, the ratio between the imaginary and real parts of the phase term is proportional to the integral variable $\alpha $ under high frequencies. Based on this proportion relationship, the phase term has been taken out; and a thickness algorithm has been proposed. By combing the measured impedance from the custom-built sensor (three coils), the thickness of the metallic film can be reconstructed. Experiments have been carried out for the verification of the proposed scenario. Results show that the thickness of the metal film can be reconstructed with a small error of less than 2%, and immune to a reasonable range of lift-offs.