Polarisation interferometry flying height testing

Abstract

The in-depth analyses of polarisation interferometry flying height testing are presented. The drawbacks of the oblique incidence polarisation interferometry method are discussed. The application of the dual-beam normal incidence polarisation interferometry method is illustrated. It is shown that with this normal incidence polarisation interferometer, not only the flying height can be measured down to contact without losing accuracy, but the optical parameters of the head-slider can also be determined.

Introduction

Flying height or spacing between the read-write head-slider and the magnetic medium is a critical parameter in hard disk design. The flying height is below $\text{25}\text{nm}$ for most current disk drives, and drives with $\text{5–10}\text{nm}$ head-disk spacing will be common in the next few years. Although there exists a range of methods for the characterisation of this parameter [1], [2], [3], [4], [5], the data storage industry still relies mainly on optical testers for directly quantifying the head-slider's flying height in both new product design and manufacturing of the head-slider.

Up to now, the methods that are widely accepted by the industry are intensity interferometry and polarisation interferometry. A detailed analysis of the characteristics of these two methods can be found in [6]. Using the intensity interferometry technique, the flying height can be measured down to $\text{20}\text{nm}$ or even $\text{10}\text{nm}$ with acceptable accuracy. However, because of the nature and shape of the intensity signal curve with respect to the flying height, when the spacing is reduced to under $\text{10}\text{nm}$, the sensitivity of this intensity-based technique worsens considerably. In real measurement instruments, this means that a small amount of noise will result in a significant measurement error. In this paper, detailed analyses to the characteristics of the two polarisation interferometry methods are provided. The drawbacks of the oblique incidence polarisation interferometry method are revealed in Section 2. The advantage and capability of the normal incidence polarisation interferometry method are instantiated in Section 3.