Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1999-11-05
2002-08-20
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192120, C204S192150, C204S192200
Reexamination Certificate
active
06436248
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to thin film magnetic disks of high storage density for use in hard magnetic disks and more particularly to magnetron sputtering of thin films on magnetic disks.
BACKGROUND OF THE INVENTION
Methods of producing thin film magnetic disks systems where a large number of substrates are coated at one time, or in single disk sputtering system, are already known. The substrates for the production of thin film magnetic disks presently consist of glass or of highly polished aluminum disks with a nickel-phosphorous coating. In the future, various alternative materials such as aluminum borocarbide might be used.
Whatever the substrate material, a number of thin films are applied by physical and/or chemical sputtering of the films. There are one or more nonmagnetic underlayer films, which are often chromium based, followed by one or more magnetic layers, which typically consist of composite alloys of cobalt, chromium and other metals, and an overcoat, which is typically carbon based. DC magnetron sputtering is the most commonly used coating method in the production of thin film magnetic disks.
For high-quality thin film magnetic disks, a storage density in excess of 3 Gbit/in2 is presently required. The resulting fly height of the magnetic heads is below 40 nm. A reduction in noise and in gaps on the storage surface and effective adaptation to the heads are prerequisites for the realization of this storage density.
Methods of achieving the low flying heads by improving the topography of the surface of the finished thin film magnetic disk and reducing the thickness of the films, especially the magnetic coat and the top coat, is already known. By using new alloys for the magnetic storage layer and specially adapted underlying structures, thin film magnetic disks with coercive field strengths of ≧2500 Oe and remanent magnetization layer thickness (Mrt) of ≧0.5 can be produced with good reproducibility.
New problems, not previously encountered, have arisen chiefly due to demands for increasing storage density and reducing the thickness of the films.
A decisive shortcoming is the fact that increased corrosion occurs with the use of aluminum substrates. This corrosion gives rise to blooming and warping on the surface of the magnetic disk, which results in areas which can no longer be written on or read. There is a danger that these may come into contact with the heads, as the flying head is around the same height as the bloom. Two factors have an effect on corrosion. On one hand, errors in polishing the substrates and residues from wet processes are triggers for corrosion. On the other hand, the atomized films are only tight and non-porous to a limited extent. Among other things, the reduction in film thickness from 30 nm to 10 nm has the effect of worsening corrosion behavior. The top film can only perform the function of the mechanical protection of the magnetic film against contact with the heads and, in conjunction with the lubricant, safeguards the sliding capability of the heads. (The problem of corrosion is described in detail by Chia, Wang, Tang and Lee in “Overview of Accelerated Corrosion Tests on Thin Film Magnetic Media” in The Minerals, Metals & Materials Society, 1998, p. 311-319.) Although the substrates are polished to a mirror finish, the topography of the finished magnetic disks is less than ideal. Microscopic cracks and pinholes can be detected. These defects on the surface of the substrates stand in the way of optimum cleaning during the washing process. This decreases the evenness of the sputtering of the films and results in less effective covering of the edges. It is known that corrosion can be reduced by increasing the thickness of the underlayer film and applying a negative DC voltage to the substrate during the sputtering of the underlayer film.
Neither of these measures solves the problem. They only reduce the number of corrosion points revealed in acid tests with HCl or HNO
3
. The usefulness of the measures is not unlimited. The increase in thickness of the underlayer film is in conflict with the requirements as regards magnetism. The application of a negative DC voltage to the substrate can only be used in the case of conductive substrates.
SUMMARY OF THE INVENTION
An object of the invention is improving the production of thin film magnetic disks to such an extent that, in the case of metallic substrates, the corrosion resistance of magnetic disks is significantly increased and, in the case of nonmetallic substrates, the diffusion of water to the substrate and of freely moveable ions from the substrate is drastically reduced. The method is intended for use with all substrate materials—both conductive and insulating. It is intended that the method can be implemented for industrial use in the usual plants. A further object of the invention is to create a thin film magnetic disk with increased corrosion resistance.
The invention places a barrier layer between the substrate and the series of typical nonmagnetic and magnetic films in such a way that the magnetic values such as coercive field strength and remanent magnetization layer thickness are not influenced. This reduces the corrosion to such an extent that practically no gaps caused by corrosion can be detected on the magnetic disks using known testing methods. This represents an improvement by one to two orders of magnitude. Surprisingly, the effect is achieved at a barrier layer thickness as low as just a few nanometers (nm), e.g., less than 10 nm. which is thinner than the typical underlayer. In accordance with the method, aluminum or chromium are the preferred materials for application as barrier layers.
Very good properties are achieved where the sputtering process gas contains a proportion of oxygen and/or nitrogen. Depositing a chromium film for the barrier layer is very advantageous, as the crystal formation in the subsequent steps is not disturbed.
It is of particular advantage that the effect of the barrier layer can be adapted to the relevant substrate material by means of the choice of material to be sputtered, the film thickness and the selection of the frequency and pulse ratio for the sputtering process. A frequency of 50 kHz has been shown to be particularly suitable. This frequency evidently brings about a favorable sputtering rate to the plasma excitation ratio, to form very tight, defect-free films with a high yield of sputtered excited particles. The freedom from defects here relates to films without pinholes or a columnar structure.
The method in accordance with the invention can be implemented for all magnetic disk coating systems. This suitability for integration into production plants is a great advantage of the method.
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R. Chia, et al., “Overview of Accelerated Corrosion Tests on Thin Film Magnetic Media”, The Minerals, Metals, & Materials Society, 1998, pp. 311-319.
Baur Heinz
Bluethner Ralf
Buchberger Hans
Goedicke Klaus
Junghaehnel Michael
Knight G. Marlin
McDonald Rodney G.
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