Head–disk spacing measurement based on Michelson interferometry using white light illumination

Abstract

Head–disk spacing of hard disk drives was measured based on Michelson interferometry using white light illumination. This method utilizes the phase comparison of two interference fringe patterns formed respectively on the inner surface of a glass disk and the air bearing surface of a slider through the glass disk. To suppress interference noise and to further enhance measurement accuracy, low coherent white light was adopted as an illuminating source to replace high coherent laser. A high sensitive CCD digital camera was used to compensate for the attenuation of the brightness and contrast of the interference fringe pattern in the white light-based Michelson interferometer. The captured images and the measured results indicate that the application of white light illumination effectively improves the interference fringe pattern, and the measurement error of extracting the ridgeline can be consequently reduced five times in comparison with a laser. Spacing resolution (=λ/2w) was enhanced to 0.68 nm/pixel in our experiment, closely approaching the extreme spacing resolution (at the largest fringe interval wmax) of 0.64 nm/pixel. Indirect and direct methods for determining fringe interval were proposed, and the latter provides higher accuracy. Head–disk spacing was measured at different spacing resolutions of 2.61 and 0.68 nm/pixel to clarify the effect of fringe interval on measurement accuracy. The results show that tested spacing at larger fringe interval achieves not only better agreement with the calculated example, but also better deviation tolerance characteristics. Based on the advantages, measurement of 10 nm spacing was realized with high accuracy and good repeatability.