Thickness Measurement Based on Eddy Current Sensor With Coaxial Double Coil for Chemical Mechanical Polishing

Real-time and accurate thickness measurements of copper films on 300-mm silicon wafers are utilized to improve global planarization, control polishing pressure, and avoid dishing and erosion caused by underpolishing and overpolishing during chemical mechanical polishing (CMP) of metals. Nevertheless, the requirements of nanometer-scale measurement sensitivity, wide measurement range from tens of nanometers to
$2~\mu \mathrm {m}$
, and high edge measurement performance make eddy current thickness measurement extremely challenging. Therefore, we proposed a concise and effective theoretical model to analyze the measurement range and sensitivity in both the amplitude- and phase-based methods. The advantages and disadvantages of the two methods are compared and discussed systematically through simulations and experiments. The effects of the liftoff distance and frequency of excitation signal on measurement sensitivity, range, and edge measurement performance are also examined. The results show that the proposed theory can be used to improve measurement range and sensitivity. Also, the double-sensor system demonstrated better edge measurement performance than a single coil. Finally, a double-coil eddy current thickness measurement system designed with an average sensitivity of 1.9 mV/nm, a measurement range of 60 nm–
$2 \mu \mathrm {m}$
, and an edge attenuation position of 141.5 mm at a liftoff distance of 3.5 mm can be utilized in the amplitude-based method for advanced Cu-CMP.