Study On Symmetrical Common-Path Heterodyne Interferometer For Magnetic Head Flying Height Measurement

The flying height (FH) of the slider in the hard disk device (HDD) is lower than 10nm now. However, higher density of HDD could only be achieved with the slider of even lower FH and smaller FHM (Flying height modulation). Therefore, it is critical and difficult to explore the real-time methods to measure the FH accurately and dynamically. In this dissertation, the Common-Path Double-focus FH Measuring system is studied for the head/disk spacing measurement. In addition, an improved scheme, the Symmetrical Common-Path Heterodyne Interferometer(SCHI), is proposed and studied experimentally and theoretically.To begin with, some simulative experimental results prove that the former system reaches a high measuring resolution of 0.09nm, and therefore it is capable of measuring minor displacement as well as minor velocity. Besides, the result of the stability test shows that, the system drifts less than 4nm in the laboratory without any air-conditioning (a temperature variation about 1℃), which also proved that the system has some ability of overcoming environmental disturbance due to its common path design with a birefringent lens. However, the system failed in dynamically measuring the FH because of its poor adaptation to the runout of the rotating disk surface.Consequently, the SCHI is proposed for the FH measurement. The system utilizes a tranverse Zeeman laser and a symmetrical layout of two Wollaston differential interferometers with the common gage beams, and therefore it meets the needs of higher stability and adaptation for the in-situ FH measurement, and the Abbe error introduced by the shift of the disk-rotor equilibrium position, which may not be compensated by the former methods, could now get a real-time compensation. Moreover, with the high-speed phase meter, the system could measure the FH at a resolution of 0.1nm and at a sampling frequency of 100kHz. Lastly, the interferometer is not only suitable for the FH measurement with real magnetic disk, but also with apparent glass disk. Furthermore, the dissertation studies the motion of the disk of a portable HDD (Hitachi HTS541040g9at00) and the flyability of the head-gimbal assembly. It can be observed that the assembly flying-height modulation is no larger than 10nm when the runout of rotating disk surface is around 1.2μm, and that the radial placement of measuring points is crucial to obtain the measurement with the least nonlinear error.Last but not least, the dissertation also discusses the problem of uncertainty of FH measurement. Moreover, the resources and effect of nonlinear errors are discussed for DCHI by means of theoretical simulation and experimental study, based on which the method of compensating the nonlinear error and system design are suggested.