Metal film thickness measurement using phase linearity feature for immunity to lift-off effect

Abstract

In eddy current (EC) thickness measurements, the presence of the lift-off effect will significantly impact the measurement signal, leading to inaccuracies. It has been reported that the phase signal demonstrates a certain level of immunity to the lift-off effect compared to the amplitude signal, as its variation in response to lift-off is relatively less prominent. However, it should be noted that the phase is not entirely unaffected by the lift-off effect. In this paper, a linear relationship between the tangent of the phase angle and the lift-off has been found, termed the phase linear feature. Firstly, the phase linear feature was validated through theoretical derivations and experimental investigations. Subsequently, a new method of thickness measurement based on the fixed lift-off deviation was proposed by using this feature, and a corresponding double-coil sensor was also designed. Finally, to validate the proposed method, measurements of aluminum and copper films were carried out using the developed measurement system under different lift-off distances. The experimental results demonstrate that the measurement relative error of the proposed method remains within 5% in the 0–3 mm lift-off range. This indicates that the proposed method can provide accurate thickness measurements and is independent of the lift effect. Furthermore, the proposed method also has the advantages of single-frequency excitation, simplicity of the sensor, and straightforward relationship between features and thickness. Therefore, this study presents an effective solution for practical online thickness measurement.