Improvement of sensitivity of eddy current sensors for nano-scale thickness measurement of Cu films

• Sensitivity analysis of metal film thickness measurement of eddy current method.
• Sensitivity of an eddy current sensor is not proportional to its Q factor.
• Sensitivity can be improved by reducing the resistance of the sensor coil.
• Multi-thread wire is better than the single wire for getting higher sensitivity.

Measurement of nano-scale copper film thickness is of great importance in the semiconductor industry. The eddy current method is used for the purpose due to its non-destructive and fast dynamic response features. In this paper, an equivalent circuit model is used to get the relationship between the measurement sensitivity and sensor parameters. It is found that the internal resistance of an eddy current sensor plays a primary role in the improvement of the measurement sensitivity beside of the Q factor of the sensor. A simple experimental setup is established and a series of Cu films with the thicknesses ranging from 20 nm to 350 nm are prepared as test samples. Test results indicate that the sensitivity of an optimized sensor made of a lower resistant multi-wire Cu line has better sensitivity than that wound with a higher resistant single Cu wire under large lift-off.