US Patent 7173417 Eddy current sensor with concentric confocal distance sensor

A metrology instrument includes an eddy current sensor that is mounted to and concentric with a confocal distance sensor. By measuring the precise vertical placement of the eddy current probe with respect to the surface of the sample using the confocal distance sensor, the accuracy and precision of the eddy current measurement is improved. Because the confocal distance sensor and the eddy current sensor are confocal, there is no need to move the relative lateral positions between the sample and instrument, between the distance measurement and the eddy current measurement, thereby reducing error in the measurement as well are maximizing the throughput by minimizing the required stage travel for a single measurement.