High-Precision Thickness Measurement of Cu Film on Si-Based Wafer Using Erasable Printed Eddy Current Coil and High-Sensitivity Associated Circuit Techniques

Abstract

The eddy current coil is widely adopted to measure the thickness of copper (Cu) film on the silicon (Si) based wafer. However, the lift-off distance (LOD) variation between the coil and Cu film has a strong negative effect on the accuracy and repetitiveness of the thickness measurement. Therefore, it is necessary to eliminate the impact of LOD variation. In this article, a high-precision thickness measurement method is proposed to determine the thickness of Cu film on the Si-based wafer by using the techniques of the erasable printed eddy current coil (EPECC) and the high-sensitivity associated circuit. The theoretical feasibility and sensitivity improvement of the conceptual design are analyzed in detail. As a proof of concept, a prototype of the proposed method is fabricated. The EPECC is prepared on the back surface of the wafer, and the LOD is unvaried during the measurement. Experimental results show that comparing with the traditional eddy current coil, the measurement stability, precision, and accuracy of the Cu film thickness by the proposed method are significantly improved even under severe vibration conditions. The relative error is 2% in repeated measurements.