In-situ n & k phase compensation in an interferometric flying height tester

Conventional multiwavelength flying height testers rely upon an external ellipsometer for measurement of the slider´s n and k. These values are used to correct the phase offset of the interferometric signal. Typically n and k measurements obtained from a sampling of 20 to 30 sliders are averaged and the results are used indefinitely in production testing. This approach to n and k phase correction may be adequate for high flying heights but it becomes less attractive as flying heights decrease and the correction becomes a large fraction of the nominal. In the new method described here phase offset is measured when a standard retract or rpm calibration is performed. Using the calibration data, an estimate is obtained of the absolute reflectivity of the slider. Next the value of the real part n of the complex index of the slider is inferred from an independent correlation of n to the slider reflectivity. Given these two numbers-the slider reflectivity R and the real part n of the index-the phase correction for flying height is calculated using a well-known relationship giving phase shift on reflection as a function of n and R