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Abstract

A system is provided for measuring the thickness of a ferromagnetic layer formed over a conductive base layer. An eddy current probe is provided for measuring the thickness of the ferromagnetic layer. The eddy current probe can be placed in direct contact with the ferromagnetic layer, or it may be spaced above its surface by an unknown standoff distance. This spacing may be caused by the presence of an overlying nonferrous, nonconductive layer applied over the ferromagnetic layer, or it may be an air gap over the ferromagnetic layer. An analog detector connected to the probe provides output values in two dimensions corresponding to the modulation of the probe's magnetic field by the ferromagnetic and conductive layers. These values are utilized to determine a mapping between the detector output values and ferromagnetic layer thickness and standoff values. The output of the mapping function provides two values, a ferromagnetic layer thickness and a standoff distance, which corresponds to the detected values. The mapping function is calibrated by measuring several test objects having known ferromagnetic layer thicknesses and standoff values. Interpolation techniques are used to generate data points for the mapping function between those measured for the test objects. The mapping function can be efficiently performed using a lookup table.

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