Diamond-like carbon for magnetic storage disks

doi:10.1016/j.surfcoat.2003.10.146

Surface and Coatings Technology

Volumes 180–181, 1 March 2004, Pages 190-206

https://doi.org/10.1016/j.surfcoat.2003.10.146 Get rights and content

Abstract

Diamond-like carbon films form a critical protective layer on magnetic hard disks and their reading heads. The ultimate limit to storage density is the super-paramagnetic limit, where the thermal energy is able to overcome the coercive energy of the magnetic bit. Perpendicular recording should allow storage densities up to ∼1 Tbit/inch². This requires the read head to approach closer to the magnetic layer and ever-thinner layers of carbon 1–2 nm thick. A critical review of the properties of the main classes of carbon films used for magnetic storage disks is presented. Tetrahedral amorphous carbon can provide the atomic smoothness, continuity and density required for magnetic storage applications down to a few atomic layers thickness. The main approaches to assess the structural and morphological properties of ultra-thin carbon layers are reviewed. Raman spectroscopy, X-ray reflectivity, atomic force microscopy and surface acoustic waves based methods allow a full non-destructive characterization of ultra-thin carbon layers.

Section snippets

Magnetic storage technology

Magnetic storage is the most economic form of non-volatile storage for many applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Its great advantage is that the storage density is increasing at a very rapid rate [1], [2], [3], [4], [5] (Fig. 1). Recently, with the introduction of giant magneto-resistive heads, storage densities are increasing at 100% per year. This is much faster than the Moore's law rate for silicon devices (∼50% per year).

Data are stored in

Classification of carbon films

The main properties of the various carbon films are well known, at least for films thicker than 10 nm [20]. Here we present a general classification of carbon films in terms of their bonding and some useful correlations linking mechanical and structural properties of ta-C.

The great versatility of carbon materials arises from the strong dependence of their physical properties on the ratio of sp² (graphite-like) to sp³ (diamond-like) bonds [20]. There are many forms of sp² bonded carbons with

Requirements for the carbon overcoat

When first introduced, the role of carbon films was to provide protection against corrosion. Simple a-C was used, deposited by magnetron sputtering. Later, a-C:H was used, produced by the reactive sputtering of graphite in an Ar/hydrogen atmosphere [27], in order to provide also some protection against mechanical wear and damage during head crashes [28], [29], [30], [31]. Many groups have emphasized this role of DLC for mechanical protection. However, hardness is not the critical parameter. For

Optical storage technology

Optical storage is the preferred technology when high-density storage on removable and exchangeable media is required. The advantages are its low cost, media exchangeability (standardization) and robustness. Over the past 20 years, the storage capacity in optical recording has been raised by increasing the numerical aperture of the focussing optics and decreasing the laser wavelength (Fig. 8a.) The new generation of optical storage devices will have approximately a 20-fold higher capacity than

Ultra-thin carbon films characterization

Finding reliable characterization tools for ultra-thin carbon layers down to a few atomic layers thickness is one of the most decisive factors for technology development and production [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65]. The carbon performance is judged in terms of its coverage, lubricant compatibility and mechanical hardness. The mechanical hardness is directly related to the fraction of CC sp³ bonds and the carbon film density. The

Evolution of ta-C properties with thickness

Fig. 14, Fig. 20a allow some interesting conclusions on the thickness evolution of ta-C properties. The density, sp³ fraction and Young's modulus all decrease for films below 8-nm thickness [48]. However, there are distinct trends. The XRR density of a 2.2-nm ta-C film, 2.8 g/cm³, corresponds, by using Eq. (1), to ∼60% sp³ content, similar to that found by direct electron energy loss spectroscopy measurements (∼45%) [48]. In contrast, its Young's modulus (∼100 GPa) would correspond to a much

Conclusions

The status of DLC films to be used as overcoat for magnetic and optical storage disks has been reviewed. The main requirements, such as smoothness, density, corrosion protection have been highlighted. In order to achieve ∼1 Tbit/inch² storage density the challenges is to provide ∼1-nm films with suitable properties and to be able to assess these properties in a lab and then on the production line. The main non-destructive measurement techniques for structural evaluation of the carbon overcoats

Acknowledgements

The author wishes to thank all the people who allowed the work reviewed in this paper to be performed, in particular the research team of the Mainz IBM STD plant. The author thanks J. Robertson, C. Casiraghi, F. Piazza, R. Ohr, M. Von Gradowsky, C. Shug, H. Hilgers, D. Schneider, M.G. Beghi, C.E. Bottani, A. Libassi, B.K. Tanner. The author acknowledges financial support from the Royal Society, from the project ‘Innovative Reaktoren und In-Situ Analytik für Nano-Schutzschichten’ funded by the

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Diamond-like carbon for magnetic storage disks

Abstract

Section snippets

Magnetic storage technology

Classification of carbon films

Requirements for the carbon overcoat

Optical storage technology

Ultra-thin carbon films characterization

Evolution of ta-C properties with thickness

Conclusions

Acknowledgements

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Tribol. Int.

Diamond Relat. Mater.

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