Head-Disk Spacing Measurement Using Michelson Laser Interferometry as Observed Through Glass Disk

Abstract

A spacing measurement using Michelson laser interferometry was developed for measuring the head-disk spacing in a hard disk drive. This method utilizes the phase comparison of two kinds of interference fringe patterns formed respectively over the inner surface of a glass disk and the air bearing surface of a slider through the glass disk. To increase resolution and reproducibility, four principal techniques were newly introduced: an optical system on which a high-definition CCD (charge-coupled device) camera was mounted, external identification of the fringe interval by driving the reference mirror, a wobbling technique to freeze ridgelines at a prescribed position when capturing the fringe image, and a powerful method for extracting ridgelines from the fringe pattern. In this method, the phase shift generated by light reflection on the slider surface incurring multiple beam interference effects must be compensated. For this purpose, first the phase shift was quantified by simulating the multiple beam interferences over a stratified medium inclusive of the lubricating air film, the DLC (diamond-like-carbon) protective layer, the silicon underlayer and the substrate. Then, the shift was subtracted from the optical spacing to obtain the mechanical spacing. Finally, the repeatedly measured mechanical spacings were compared with the theoretically calculated spacings, showing a good accordance between them.