Multi-layer texture layer

Details Multi-layer texture layer Multi-layer texture layer

2000-05-11

2002-11-19

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S433000, C428S612000, C428S667000, C428S680000, C428S690000, C428S690000

Reexamination Certificate

active

06482505

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to techniques for manufacturing magnetic disks employed in data storage applications, to disks formed as described herein, and disk drives incorporating such disks. More particularly, the present invention relates to magnetic disks and manufacturing techniques therefor which improve corrosion resistance and magnetic performance.
The use of a substrate such as glass, or aluminum covered with plated nickel phosphorus (NiP), as a base to manufacture magnetic disks for data storage is well known. For purposes of discussion,
FIG. 1
shows an exemplary aluminum alloy substrate
102
. Above substrate
102
, there is disposed a layer of nickel phosphorus
104
, typically formed by a deposition process such as electroless plating. Typically, the NiP layer is plated to a thickness of several microns. This thickness is required because a substantial amount is subsequently polished off to create a smooth surface. The polishing is relatively extensive because the as-deposited surface of the NiP layer is rough, which in turn is due to the relatively rough surface of the aluminum substrate. A high thickness of NiP is also required to provide a hard surface compared to that of the soft aluminum substrate, to reduce the damage caused by sudden head impact (“dinging”) during operation of the disk drive.
There are typically additional layers disposed above NiP layer
104
, such as an underlayer typically comprising chromium (Cr) (as used herein, Cr or a layer of Cr shall be understood to include Cr alloys), an overlayer of magnetic material (such as a cobalt alloy or iron alloy) disposed above the Cr underlayer, and a protective overcoat.
By way of background, the NiP layer is typically textured to provide a preferential degree of orientation of magnetic moments in the overlying magnetic layer. Generally, the NiP layer is textured by forming texture grooves in the downtrack direction. As the term is employed herein, the downtrack direction shall be understood to be generally orthogonal or near orthogonal to the radial direction of the disk and may include concentric, crosshatch, or at times non-parallel patterns. The texture grooves cause a preferential alignment of magnetic moments along the downtrack direction in the cobalt alloy layer.
As is well known to those skilled in the art, this preferential alignment of magnetic moments allows for increased coercivity and hysteresis squareness in the downtrack direction which makes it possible to reliably store bits of data at high density in the magnetic layer as compared with an isotropic layer. The high squareness is important because it results in a higher magnetic remanence (Mr) in the downtrack direction. As is known, the signal strength is proportional to Mr times the thickness (T) of the magnetic layer, or MrT. While it is desirable to have a high MrT for the signal, it is also imperative to reduce the effective space loss between the read/write element and the magnetic layer to 1 microinch (&mgr;″) or lower. The effective space loss is the distance between approximately the center of the magnetic layer and the read and write element. Thus, there has been a continuing trend towards reduced magnetic layer thickness, T. With the higher Mr provided by preferential orientation therefore, a lower thickness magnetic layer can be employed while still maintaining sufficient MrT. In addition to reducing the thickness of the magnetic layer, other methods to reduce the space loss include reducing the thickness of the protective overcoat layer, and reducing the head-media spacing during read and write operations.
Returning to the texture process, in the current art, the NiP layer is typically textured using a mechanical abrasion process. In one case, the mechanical abrasion process essentially abrades the NiP layer along the downtrack direction using a tape having thereon abrasive particles. Unfortunately, it has been found that the mechanical abrasion process tends to gouge the NiP layer forming some grooves that are excessively deep. Additionally, high ridges are formed along the gouged groove.
FIG. 2
is an atomic force microscope (AFM) scan of a textured NiP layer. It will be appreciated that the horizontal and vertical scales of
FIG. 2
are not the same as one another. As can be seen, some grooves, such as groove
202
, are excessively deep and narrow, while others are of approximately the desired depth for inducing the preferred magnetic orientation. In addition, along deep groove
202
is ridge
203
, which is higher than desired. Although the non-uniformity among groves can be minimized by using abrasive slurries having a more uniform distribution of particles and by controlling the abrasion process more precisely, the very mechanical nature of the mechanical abrasion process renders it impossible to eliminate the nonuniformity completely.
FIG. 3A
illustrates a problem that occurs with deep gouges. It will be appreciated that the drawings of the grooves such as that shown in
FIG. 3A
are not necessarily to scale. In
FIG. 3A
, layer
306
represents the overcoat layer. It will be appreciated that there are other layers, not shown in
FIG. 3A
, underlying layer
306
. Such layers may include, for example, one or more underlayers, one or more magnetic layers, one or more overcoat layers, and one or more additional layers that may be deposited by e.g. sputter deposition. The overcoat layer
306
typically comprises a carbon-containing layer. As layer
306
is deposited, the depth and profile of deep groove
302
makes it difficult for layer
306
to adequately cover the NiP surface. As a result, voids or gaps in layer
306
may be present near the vicinity of deep groove
302
. The layers underlying layer
306
may or may not have voids in deep grooves, depending on their thickness and other factors. As a result of the gaps, one or more of the various layers under layer
306
, and/or the NiP layer and/or the substrate are now exposed to moisture, which causes corrosion. The Co alloy layer is particularly susceptible to corrosion, and is the primary cause of concern. Additionally, the other layers, and the substrate material are also susceptible to corrosion to varying degrees. In any event, corrosion will lead to the generation of particles that are picked up by the head resulting in degraded drive performance
In contrast to groove
302
of
FIG. 3A
, groove
304
is of about the desired profile, and layer
306
can cover the entire surface in the region of groove
304
. Because layer
306
is a good moisture barrier, corrosion is prevented because moisture can not penetrate to the layers underneath layer
306
. The formation of gaps in layer
306
becomes more likely as the thickness of layer
306
is reduced, so that the problem depicted in
FIG. 3A
can be expected to get worse in future products.
As mentioned above, a further problem that may occur during texturing is the formation of ridges, such as ridge
305
in
FIG. 3A
, along the gouged grooves. While the deposition coverage of the various layers over high points is generally good, there may be a failure to cover extremely sharp points, particularly by the thin protective overcoat layer, so that underlying layers are exposed and therefore susceptible to corrosion. An additional concern arises with respect to ridges sufficiently high to collide with magnetoresistive heads, giant magnetoresistive heads and the like, because such collisions cause a temperature rise of the magnetoresistive element, which generates a false signal. This failure mechanism is referred to as thermal asperity. Because of this, as one of the later stages of manufacture, after all layers have been deposited, a burnish step is performed which effectively knocks off any high points. When the asperity is knocked off during burnish, a portion of layer
306
is knocked off as well. This problem is also particularly severe with overcoat layers having a low thickness. In any event, because of this one or more layers, such as the Co alloy layer, will be expos

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