A Thickness Measurement System for Metal Films Based on Eddy-Current Method With Phase Detection

This paper proposes a high-speed measurement system for the thickness of metal films based on the eddy-current method. Theoretical derivation shows the effects of film thickness on the impedance of the sensor coil. In a voltage division circuit, the thickness information is converted to the phase difference of two measured signals. The tangent value of the phase difference is proportional to thickness h, which is a result that has been verified theoretically and experimentally. The prototype used in this paper can cover a measurement range of 10 μm–0.5 mm for copper films, and can achieve a high measurement speed (up to excitation frequency) without a complex demodulation circuit. The measurement error caused by the temperature variation is roughly –0.2%/°C. The influence of external magnetic field on the prototype is also tested. The entire sensor size can be easily controlled in 50 × 50 × 30 mm3 as a handheld device. The instrument can meet many industrial requirements for both online measurement and postinspection.