Mechanical texturing of glass and glass-ceramic substrates

Details Mechanical texturing of glass and glass-ceramic substrates Mechanical texturing of glass and glass-ceramic substrates

1999-05-24

2001-06-19

Pianalto, Bernard (Department: 1762)

Coating processes

Magnetic base or coating

Magnetic coating

C427S128000, C427S307000

Reexamination Certificate

active

06248395

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to methods for texturing glass and glass-ceramic substrates and other silica-containing substrates. This invention also relates to slurries used for polishing glass and glass ceramic substrates and other silica-containing substrates. This invention also relates to apparatus for polishing glass and glass ceramic substrates and other silica-containing substrates.
Magnetic disks are typically manufactured by the following process.
1. First, an aluminum alloy substrate is electroless plated with a hard material such as NiP.
2. The NiP is polished and then textured. In one type of process, the NiP is mechanically textured to form a set of grooves in the NiP.
3. An underlayer such as Cr, a magnetic alloy such as a Co alloy, and a protective carbon overcoat are then sputtered onto the textured NiP, in that order.
4. A lubricant layer is applied to the protective overcoat.
Of importance, the texture in the NiP affects the magnetic properties of the Co magnetic alloy. In particular, the texture in the NiP causes anisotropic magnetic characteristics in the Co alloy which are beneficial. For example, the coercivity of the magnetic film is increased in the direction of the texture grooves (which is substantially parallel to the data tracks in the magnetic film). This helps to increase the data storage density of the magnetic disk.
Another type of magnetic disk uses a glass or glass ceramic substrate. Of importance, glass and glass ceramic are harder and more rigid than aluminum. Thus, a magnetic disk comprising a glass substrate is less susceptible to wobbling in response to rotation at high rotational velocities. The glass substrate is also less susceptible to “head slap” or “non-operating shock” which is a severe failure mode due to the continual motion of the disk drive during travel, assembly and delivery. A magnetic disk comprising a glass substrate is typically manufactured by the following process:
1. A glass substrate is subjected to a grinding process to ensure that the substrate is substantially flat. This is accomplished by using a grinding stone embedded with diamond particles, or by using a slurry comprising large abrasive Al
2
O
3
particles. This process typically leaves the glass substrate with cracks, fractures or other mechanical defects that are subsequently removed by polishing.
2. The glass substrate is then subjected to a coarse polishing step, during which between 20 and 50 &mgr;m of the glass is removed. This polishing step is accomplished using a slurry comprising large (2 to 3 &mgr;m) CeO
2
particles. One can remove about 1 &mgr;m/minute of glass using CeO
2
particles. During this step, the glass substrate is made smoother, and many of the mechanical defects such as cracks and fractures are removed.
3. Thereafter, the glass substrate is subjected to a fine polishing step, during which between 2 and 5 &mgr;m of the glass is removed. This polishing step is typically accomplished using smaller (0.5 to 1 &mgr;m) CeO
2
particles. At the conclusion of this step, the surface roughness of the glass substrate has a Ra of about 3Å. (“Ra” is a well-known measure of surface roughness.)
4. Thereafter, an underlayer such as NiAl (e.g. 60 to 80 nm thick), a magnetic Co alloy, and a protective overcoat (e.g. hydrogenated carbon) are sputtered, in that order, on the polished glass substrate.
5. A lubricant layer is applied to the magnetic disk.
Although diamond is harder than CeO
2
particles, typically CeO
2
particles polish glass more rapidly than diamond. This is because the mechanism by which CeO
2
polishes glass is a chemical-mechanical process. The CeO
2
reacts with the glass, and forms a hydrated type of material on the glass surface that can easily be abraded away. In contrast, the process by which diamond polishes glass is purely mechanical, and is slower. Further, because the process by which diamond abrades glass is mechanical, and fractures the glass, the glass surface at the conclusion of diamond polishing is typically very irregular. Thus, CeO
2
is used very of ten as the polishing agent when polishing glass.
Unlike the manufacturing process using a NiP-coated aluminum substrate and mechanical texturing, the above-mentioned manufacturing process using a glass substrate and CeO
2
particles does not result in a texture comprising circumferential grooves. Thus, this process does not enhance the coercivity of the magnetic film along the direction of texture grooves.
Another manufacturing process known in the art comprises the following steps:
1. A glass substrate is subjected to grinding and coarse polishing as described above.
2. A seed layer (e.g. a palladium alloy) is sputtered on the glass substrate.
3. A NiP layer is electroless plated on the seed layer.
4. The NiP layer is then polished and mechanically textured to form texture grooves in the circumferential direction of the substrate.
5. An underlayer comprising Cr, a Co magnetic alloy, and a hydrogenated carbon overcoat are then sputtered, in that order, on the textured NiP layer,
6. A lubricant layer is applied to the hydrogenated carbon overcoat.
This process results in the formation of texture grooves in the circumferential direction. These grooves enhance the coercivity of the Co magnetic alloy in the circumferential direction. However, this manufacturing process requires additional expensive steps, e.g. depositing the seed layer, electroless plating NiP on the seed layer, and mechanically texturing the NiP.
SUMMARY
A method for texturing a glass substrate comprises the step of using a slurry comprising diamond particles (either single crystal or polycrystalline diamond or both) and cerium oxide particles (CeO
2
). The slurry particles are typically in a liquid such as water. Of importance, we have discovered that a slurry comprising both diamond and CeO
2
particles exhibits an increased material removal rate compared to a slurry of only CeO
2
particles. We have also discovered that a slurry comprising both diamond and CeO
2
particles can be used to form texture grooves in the circumferential direction. In one embodiment, these grooves are used to enhance the magnetic characteristics (e.g. coercivity) of a magnetic film formed on the glass substrate in the circumferential direction. Thus, a magnetic disk in accordance with our invention can exhibit a higher recording density in the direction of the texture grooves.
We have also discovered that a slurry comprising both diamond and CeO
2
particles does not leave a damaged surface layer in the glass substrate the way that diamond particles alone would. Thus, our slurry results in an improved surface morphology for the glass substrate.
After polishing and texturing, a seed layer such as Al, Cr, Ti, Ni
3
P, MgO, C, W, Zr, AlN or NiAl is deposited on the substrate, and a magnetic Co alloy is deposited on the seed layer (e.g. by sputtering). In one embodiment, the circumferential texture of the present invention results in an improved orientation of the magnetic domains of the Co alloy, with a resulting improvement in the recording performance of the magnetic disk. Thereafter, a protective overcoat and lubricant layer are typically deposited on the disk.
A texture in accordance with our invention reduces the number and heights of surface asperities on the disk surface, thus permitting a read-write head to fly closer to the magnetic Co alloy, and permitting correspondingly high recording densities.
The slurry of our invention can be used to texture glass substrates that have been subjected to chemical strengthening. (Chemical strengthening is a process by which ions such as potassium ions are diffused into the glass to create a compressive stress region at the surface of the substrate. This stress region prevents cracks from propagating. As the name of this process implies, chemical strengthening results in a stronger glass substrate.)
The slurry of our invention can be used to texture glass substrates having any of a number of additives, e.g. soda lime glass, borosilicate glass, and glass having

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