Method of laser texturing glass or glass-ceramic substrates for magnetic recording media

The height of protrusions formed during laser texturing a glass or glass-ceramic substrate is controlled by controlling the quench rate of the protrusions during resolidification. In an embodiment, the quench rate is controlled by heating the substrate during laser texturing. Heating can be initiated prior or subsequent to, or simultaneously with, initial exposure of the substrate surface to a pulsed, focused CO.sub.2 laser beam for texturing.

TECHNICAL FIELD 
The present invention relates to the recording, storage and reading of 
magnetic data, particularly rotatable magnetic recording media, such as 
thin film magnetic disks having textured surfaces for contact with 
cooperating magnetic transducer heads. The invention has particular 
applicability to high density magnetic recording media for mobile computer 
data storage applications. 
BACKGROUND ART 
Thin film magnetic recording disks and disk drives are conventionally 
employed for storing large amounts of data in magnetizable form. 
Typically, one or more disks are rotated on a central axis in combination 
with data transducer heads. In operation, a typical contact start/stop 
(CSS) method commences when the head begins to slide against the surface 
of the disk as the disk begins to rotate. Upon reaching a predetermined 
high rotational speed, the head floats in air at a predetermined distance 
from the surface of the disk due to dynamic pressure effects caused by air 
flow generated between the sliding surface of the head and the disk. 
During reading and recording operations, the transducer head is maintained 
at a controlled distance from the recording surface, supported on a 
bearing of air as the disk rotates, such that the head can be freely moved 
in both the circumferential and radial directions allowing data to be 
recorded on and retrieved from the surface of the disk at a desired 
position. Upon terminating operation of the disk drive, the rotational 
speed of the disk decreases and the head again begins to slide against the 
surface of the disk and eventually stops in contact with and pressing 
against the disk. Thus, the transducer head contacts the recording surface 
whenever the disk is stationary, accelerated from the stop and during 
deceleration just prior to completely stopping. Each time the head and 
disk assembly is driven, the sliding surface of the head repeats the 
cyclic operation consisting of stopping, sliding against the surface of 
the disk, floating in the air, sliding against the surface of the disk and 
stopping. 
It is considered desirable during reading and recording operations to 
maintain each transducer head as close to its associated recording surface 
as possible, i.e., to minimize the flying height of the head. Thus, a 
smooth recording surface is preferred, as well as a smooth opposing 
surface of the associated transducer head, thereby permitting the head and 
the disk to be positioned in close proximity with an attendant increase in 
predictability and consistent behavior of the air bearing supporting the 
head. However, if the head surface and the recording surface are too flat, 
the precision match of these surfaces gives rise to excessive stiction and 
friction during the start up and stopping phases, thereby causing wear to 
the head and recording surfaces eventually leading to what is referred to 
as a "head crash." Thus, there are competing goals of reduced head/disk 
friction and minimum transducer flying height. 
Conventional practices for addressing these apparent competing objectives 
involve providing a magnetic disk with a toughened recording surface to 
reduce the head/disk friction by techniques generally referred to as 
"texturing." Conventional texturing techniques involve polishing the 
surface of a disk substrate to provide a texture thereon prior to 
subsequent deposition of layers, such as an underlayer, a magnetic layer, 
a protective overcoat, and a lubricant topcoat, wherein the textured 
surface on the substrate is intended to be substantially replicated in the 
subsequently deposited layers. 
A typical magnetic recording medium is depicted in FIG. 1 and comprises a 
substrate 10, typically an aluminum (Al)-base alloy, such as an 
aluminum-magnesium (Al-Mg) alloy, plated with a layer of amorphous 
nickel-phosphorous (NIP). Substrate 10 typically contains sequentially 
deposited thereon a chromium (Cr) or Cr-alloy underlayer 11, a magnetic 
layer 12 which is usually a cobalt (Co)-base alloy, a protective overcoat 
13 which usually comprises carbon, and a lubricant topcoat 14. Cr or 
Cr-alloy underlayer 11, Co-base alloy magnetic layer 12 and protective 
carbon overcoat 13 are typically deposited by sputtering techniques. A 
conventional Al-alloy substrate is provided with a NiP plating primarily 
to increase the hardness of the Al substrate, serving as a suitable 
surface for polishing to provide the requisite surface roughness or 
texture, which is intended to be substantially replicated on the disk 
surface. 
The escalating requirements for high areal recording density impose 
increasingly greater requirements on thin film magnetic media in terms of 
coercivity, stiction, squareness, low medium noise and narrow track 
recording performance. In addition, increasingly high density and 
large-capacity magnetic disks require increasingly smaller flying heights, 
i.e., the distance by which the head floats above the surface of the disk 
in the CSS drive. The requirement to further reduce the flying height of 
the head renders it particularly difficult to satisfy the requirements for 
controlled texturing to avoid head crash. 
Conventional techniques for providing a disk substrate with a textured 
surface comprise a mechanical operation, such as polishing. See, for 
example, Nakamura et al., U.S. Pat. No. 5,202,810. Conventional mechanical 
texturing techniques are attendant with numerous disadvantages. For 
example, it is extremely difficult to provide a clean textured surface due 
to debris formed by mechanical abrasions. Moreover, the surface inevitably 
becomes scratched during mechanical operations, which contributes to poor 
glide characteristics and higher defects. In addition, various desirable 
substrates are difficult to process by mechanical texturing. This 
undesirably limiting facet of mechanical texturing, virtually excludes the 
use of many materials for use as substrates. 
An alternative texturing technique to mechanical texturing comprises the 
use of a laser light beam focused on an upper surface of a non-magnetic 
substrate. See, for example, Ranjan et al., U.S. Pat. No. 5,062,021, 
wherein the disclosed method comprises polishing an NiP plated Al 
substrate to a specular finish, and then rotating the disk while directing 
pulsed laser energy over a limited portion of the radius, to provide a 
textured landing zone leaving the data zone specular. The landing zone 
comprises a plurality of individual laser spots characterized by a central 
depression surrounded by a substantially circular raised rim. 
Another laser texturing technique is reported by Baumgart et al. "A New 
Laser Texturing Technique for High Performance Magnetic Disk Drives," IEEE 
Transactions on Magnetics, Vol. 31, No. 6, pp. 2946-2951, November 1995. 
The laser texturing technique disclosed by Baumgart et al. employs a 
single focusing lens, and the shape of the resulting protrusions are shown 
to be altered by adjusting the pulse energy. At low pulse energies, the 
bump or protrusion shape comprises a central depression and a surrounding 
rim, similar to that reported by Ranjan et al. As the pulse energy is 
increased, the bottom of the depression flattens into a rounded, smooth, 
central dome resembling a "sombrero." At higher powers, the central dome 
broadens and decreases in height to eventually become equal to or lower 
than the rim. 
In copending application Ser. No. 08/666,374 filed on Jun. 27, 1996 a laser 
texturing technique is disclosed employing a multiple lens focusing system 
for improved control of the resulting topographical texture. In copending 
application Ser. No. 08/666,374 filed on Jun. 27, 1996, a laser texturing 
technique is disclosed wherein a pulsed, focused laser light beam is 
passed through a crystal material to control the spacing between resulting 
protrusions. 
Conventional laser texturing techniques have previously been applied to 
metal-containing substrates or substrates having a metal-containing 
surface, such as Ni-P plated Al or Al-base alloys. Such substrates, 
however, exhibit a tendency toward corrosion and are relatively 
deformable, thereby limiting their utility so that they are not 
particularly desirable for use in mobile computer data storage 
applications, such as laptop computers. Glass and glass-ceramic substrates 
exhibit superior resistance to shock than Ni-P coated Al or Al-alloy 
substrates. Accordingly, glass and glass-ceramic substrates are desirable 
candidates for use in mobile computer data storage applications. However, 
it is extremely difficult to provide an adequate texture on a glass or a 
glass-ceramic substrate, particularly in view of the escalating 
requirements for high areal recording density. 
Conventional practices for texturing a glass or glass-ceramic substrate 
comprise heat treatment. Goto et al., U.S. Pat. No. 5,391,522, discloses a 
glass ceramic substrate suitable for use in a magnetic recording medium. A 
textured surface is provided by heat treatment, during which the 
recrystallization temperature is maintained for about 1 to about 5 hours 
to generate secondary crystal grains forming the surface texture 
characterized by irregular protrusions with surrounding valleys extending 
into substrate. 
Hoover et al., U.S. Pat. No. 5,273,834 discloses the use of alternate 
substrates, such as glass-ceramic substrates. The substrate material is 
provided with ions for absorbing radiation in the near infrared portion of 
the spectrum, thereby rendering the material capable of attaining elevated 
temperatures during film deposition. 
The use of heat treatment to form a textured surface on alternate 
substrates, such as glass or glass-ceramic substrates, is undesirably slow 
and inefficient in terms of energy consumption. Significantly, it is 
extremely difficult to exercise control over the size and shape of the 
secondary crystal grains due to inherent limitations in controlling 
temperature uniformity. Accordingly, it is virtually impossible to provide 
a glass or glass-ceramic substrate with a controlled textured landing zone 
for optimizing flying height and maximizing data zone recording density. 
Moreover, the resulting texture comprises irregularly shaped protrusions 
with surrounding valleys extending into the substrate, thereby creating 
undesirable stress profiles during subsequent deposition of layers by 
sputtering at elevated temperatures. Such undesirable stress profiles 
render it extremely difficult to accurately replicate the texture in 
subsequently deposited layers. 
In copending PCT application Serial No. PCT/US96/06830, a method is 
disclosed for laser texturing a glass or glass-ceramic substrate employing 
a laser light beam derived from a CO.sub.2 laser source. The textured 
glass or glass-ceramic substrate surface comprises a plurality of 
protrusions which extend above the substrate surface, without surrounding 
valleys extending substantially into the substrate as is characteristic of 
a laser textured metallic substrate. The effect of laser parameters, such 
as pulse width, spot size and pulse energy, and substrate composition on 
the protrusion or bump height of a laser textured glass or glass-ceramic 
substrate is reported by Kuo et al., in an article entitle "Laser Zone 
Texturing on Glass and Glass-Ceramic Substrates," presented at The 
Magnetic Recording Conference (TMRC), Santa Clara, Calif., Aug. 19-21, 
1996. 
There remains a need for a magnetic recording medium comprising a glass or 
glass-ceramic substrate having an accurately controlled texture, and for a 
method of laser texturing a glass or glass-ceramic substrate wherein the 
height of the protrusions extending above the substrate surface is 
controlled. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is a method of accurately texturing a 
glass or glass-ceramic substrate to provide a controllable topography. 
Another object of the present invention is a method of laser texturing a 
glass or glass-ceramic substrate and controlling the height of the 
resulting protrusions formed on the substrate surface. 
Additional objects, advantages and other features of the invention will be 
set forth in the description which follows and in part will become 
apparent to those having ordinary skill in the art upon examination of the 
following or may be learned from the practice of the invention. The 
objects and advantages of the invention may be realized and obtained as 
particularly pointed out in the appended claims. 
According to the present invention the foregoing and other objects are 
achieved in part by a method of manufacturing a magnetic recording medium, 
which method comprises: texturing a surface of a glass or glass-ceramic 
substrate with a pulsed, focused laser light beam to form a plurality of 
protrusions on and extending above the substrate surface; and controlling 
the height of the protrusions by controlling the quench rate during 
resolidification of the laser formed protrusions. 
Another aspect of the present invention is a method of manufacturing a 
magnetic recording medium, which method comprises: texturing a surface of 
a glass or glass-ceramic substrate with a pulsed, focused laser light beam 
to form a plurality of protrusions on and extending above the substrate 
surface; and controlling the height of the protrusions by heating the 
substrate to reduce the quench rate during resolidification of the laser 
formed protrusions. 
Additional objects and advantages of the present invention will become 
readily apparent to those skilled in the art from the following detailed 
description, wherein embodiments of the invention are described, simply by 
way of illustration of the best mode contemplated for carrying out the 
invention. As will be realized, the invention is capable of other and 
different embodiments, and its several details are capable of 
modifications in various obvious respects, all without departing from the 
invention. Accordingly, the drawings and description are to be regarded as 
illustrative in nature, and not as restrictive.

DESCRIPTION OF THE INVENTION 
In laser texturing a glass or glass-ceramic substrate, as with a CO.sub.2 
laser, the resulting textured topography comprises a plurality of rounded 
protrusions extending above the substrate surface, without surrounding 
valleys extending substantially into the substrate as in texturing a 
metal-containing surface, such as an NiP plated Al or Al-alloy substrate. 
Such relatively uniform protrusions improve the tribological performance 
of the resulting magnetic recording medium. However, the height of the 
protrusions is one of the most critical parameters in that it directly 
impacts glide and tribological performance. Accordingly, the present 
invention comprises a method of laser texturing a glass or glass-ceramic 
substrate wherein the height of the resulting protrusions extending above 
the substrate surface is controlled to optimize glide and tribological 
performance. 
After considerable experimentation and investigation, it was found that a 
net volume gain is experienced in forming laser protrusions (bumps) on 
glass or glass-ceramic substrates; whereas, laser protrusions formed on 
NiP/Al substrates typically exhibit a negligible volume change. It was 
also found that the height of laser protrusions formed on a glass or 
glass-ceramic substrate is extremely sensitive to pulse energy. In 
accordance with the present invention, the height of laser generated 
protrusions is controlled by controlling the quench rate during laser 
texturing, i.e., during resolidification of the laser formed protrusions. 
In an embodiment of the present invention, the quench rate is controlled 
by heating the region of the substrate surface undergoing laser texturing. 
Heating of the substrate surface can be initiated prior to, simultaneously 
with, or subsequent to exposing the substrate to a laser light beam for 
texturing. Heating can be effected by any conventional means, as by an 
external heating source, e.g., a radiant heater, or by employing a laser 
beam. Heating of the substrate surface can be discontinued subsequent to 
resolidification of the laser formed protrusions. 
Thus, in accordance with the present invention, the bump height or 
protrusion height of laser formed protrusions during laser texturing of a 
glass or glass-ceramic substrate is controlled to less than about 150 nm, 
preferably less than about 30 nm, e.g., to within a range of about 3 nm to 
about 30 nm, by heating the substrate during laser texturing. The 
temperature to which the substrate is heated depends upon the particular 
substrate material. It has been found suitable to heat the substrate to a 
temperature less than the reflow temperature of the particular substrate 
material. Most glass materials suitable for use as a non-magnetic 
substrate in a magnetic recording medium have a reflow temperature of 
about 600.degree. C. to about 615.degree. C. Accordingly, in laser 
texturing a glass substrate for a magnetic recording medium, it has been 
found suitable to typically heat the substrate in proximity to the area of 
the substrate undergoing laser texturing at a temperature up to about 
615.degree. C., for example, at a temperature between about 100.degree. C. 
to about 615.degree. C. 
Most glass-ceramic substrates suitable for use as a substrate in a magnetic 
recording medium have a reflow temperature of about 700.degree. C. to 
about 750.degree. C. Accordingly, in laser texturing a glass-ceramic 
substrate in accordance with the present invention by controlling the 
height of the protrusions, it has been found suitable to typically heat 
the substrate to a temperature up to about 750.degree. C. as, for example, 
at a temperature between about 100.degree. C. and about 750.degree. C., 
during laser texturing. 
During laser texturing of a glass or glass-ceramic substrate, a minute 
portion of the substrate is melted and a dome-shaped bump grows in height 
and size. As pulse energy increases, the dome-shaped bumps grow and the 
dome gradually flattens out and eventually collapses to form crater-shaped 
bumps at elevated pulse energy. Once the protrusion has resolidified, it 
is not necessary to continue application of heat in that quenching is 
essentially completed. The present invention focuses upon controlling the 
bump/protrusion height, i.e., minimizing the bump/protrusion height 
sensitivity to laser power variation by lowering the quench rate during 
resolidification of the laser formed protrusions. In addition, the 
inventive method also reduces the pulse energy required to form 
protrusions with a specific height. Consequently, higher throughput is 
achieved with same laser power. This can be efficiently achieved by 
applying heat to the substrate during protrusion formation. 
The exact mechanism operative which enables the protrusion height to be 
minimized by lowering the quench rate during resolidification of the laser 
formed protrusions is not known with certainty. However, it is believed 
that a reduced quenching rate, i.e., a lower cooling rate, increases the 
density of the individual protrusion and, hence, results in a decrease in 
the protrusion height in that the protrusion is provided with sufficient 
time to settle. In practicing the claimed invention, one having ordinary 
skill in the art can easily optimize the temperature to which the 
substrate is heated as well as the duration of heating, dependent upon the 
particular substrate material employed, given the objectives of the 
present invention. 
The inventive method can be practiced employing the apparatus schematically 
depicted in FIG. 2 which comprises a CO.sub.2 laser 20 pulsed by RF driver 
21. An alternative way is to utilize a CW CO.sub.2 laser with an external 
AO modulator to split the laser beam. Emitted laser light beam 22 passes 
through variable beam attenuator 23 and beam expander 44. Expanded laser 
light beam 22 is then focused by lens 25 onto the surface of substrate 26 
which is driven by spindle 27 powered by motor 28. Substrate 26 and 
spindle 27 are mounted on a linear slide 29. A thermopile detector 30 
measures the average laser power, which can be easily translated into 
pulse energy. 
In accordance with the present invention, the surface of the substrate 
undergoing laser texturing is heated during laser texturing in order to 
control the height of the laser formed protrusions. Such heating can be 
effected by radiant heater 31 shown in FIG. 2. In another embodiment of 
the present invention, the laser beam is split into first and second laser 
sub-beams. The first laser sub-beam is expanded and directed to heat the 
substrate surface undergoing laser texturing. The second laser sub-beam is 
focused onto the substrate to effect laser texturing while the substrate 
surface is being heated by the expanded first laser sub-beam. The 
inventive method can be employed to accurately form a landing zone with 
improved tribological performance by virtue of the precisely controlled 
uniform protrusions having a controlled height extending above the 
substrate surface. 
Consistent with conventional practices, opposite surfaces of a glass or 
glass-ceramic substrate can be laser textured in accordance with the 
present invention. The present invention enables accurate control of the 
height of laser formed protrusions, thereby optimizing tribologic and 
magnetic requirements compatible with the escalating requirements for high 
areal density and mobile computer data storage applications, such as 
laptop computers. In practicing the present invention, conventional and 
commercially available glass or glass-ceramic substrates can be employed, 
such as O'Hara glass. The substrate is initially polished to provide a 
specular surface and a landing zone accurately formed thereon by the 
inventive laser texturing technique, leaving a specular data zone with 
maximized areal recording density. 
The magnetic layers deposited in accordance with the present invention can 
be any of those conventionally employed in the production of magnetic 
recording media. Such conventional magnetic alloys, include, but are not 
limited to, cobalt (Co)-base alloys, such as cobalt-chromium (CoCr), 
cobalt-samarium (CoSm), cobalt-chromium-tantalum (CoCrTa), 
cobalt-nickel-chromium (CoNiCr), cobalt-chromium-samarium (CoCrSm), 
cobalt-chromium-platinum-tantalum (CoCrPtTa), cobalt-chromium-platinum 
(CoCrPt), cobalt-nickel-platinum (CoNiPt), cobalt-nickel-chromium-platinum 
(CoNiCrPt) and cobalt-chromium-platinum-boron (CoCrPtB). The thickness of 
the magnetic layer is consistent with conventional practices and 
manufacturing a magnetic recording medium. Cobalt-base alloys having a 
thickness of about 100.ANG. to about 1000.ANG., such as 200.ANG. to about 
500.ANG., has been found suitable. 
As in conventional practices, an underlayer can be deposited on the 
textured substrate prior to depositing the magnetic layer. The underlayer 
can comprise chromium or a chromium-alloy, such as chromium-vanadium or 
chromium-titanium, oxygen-doped chromium, tungsten or a tungsten alloy. 
In addition, a protective overcoat, such as a carbon overcoat, can be 
deposited on the magnetic layer, and a lubricant topcoat deposited on the 
protective overcoat. The underlayer, magnetic layers and protective 
overcoat can be applied in a conventional manner, by any of various 
sputtering techniques, deposited in conventional thicknesses employed in 
production of magnetic recording media. 
The present invention can be employed to produce any of various types of 
magnetic recording media including thin film disks, with an attendant 
improvement in flying stability, glide performance and head-medium 
interface reliability. Moreover, the precise manner in which a landing 
zone is laser textured enables increased areal recording density, e.g., an 
increase of 40% or more, and a reduction in the size of head sliders. 
Only the preferred embodiment of the invention and but a few examples of 
its versatility are shown and described in the present disclosure. It is 
to be understood that the invention is capable of use in various other 
combinations and environments and is capable of changes or modifications 
within the scope of the inventive concept as expressed herein.