U.S. patent application number 11/114256 was filed with the patent office on 2005-10-27 for glass substrate for perpendicular magnetic recording disk and method of producing same.
This patent application is currently assigned to Nihon Microcoating Co., Ltd.. Invention is credited to Horie, Yuji, Kumasaka, Noriyuki, Tanifuji, Tatsuya.
Application Number | 20050238927 11/114256 |
Document ID | / |
Family ID | 35136840 |
Filed Date | 2005-10-27 |
United States Patent
Application |
20050238927 |
Kind Code |
A1 |
Horie, Yuji ; et
al. |
October 27, 2005 |
Glass substrate for perpendicular magnetic recording disk and
method of producing same
Abstract
A glass substrate for perpendicular magnetic recording, having a
surface with an average surface roughness of 2.0 .ANG. or less and
surface height variations of 1 .ANG. or less with wavelengths in
the range of 0.05 mm-0.5 mm in both radial and circumferential
directions, is produced by rotating a glass substrate, supplying
polishing slurry containing a specified amount of abrading
particles of artificial diamond on its surface, pressing a
polishing tape on the surface and causing this polishing tape to
travel in a direction opposite to the direction of rotation of the
glass substrate.
Inventors: |
Horie, Yuji; (Tokyo, JP)
; Tanifuji, Tatsuya; (Tokyo, JP) ; Kumasaka,
Noriyuki; (Tokyo, JP) |
Correspondence
Address: |
BEYER WEAVER & THOMAS LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
Nihon Microcoating Co.,
Ltd.
|
Family ID: |
35136840 |
Appl. No.: |
11/114256 |
Filed: |
April 25, 2005 |
Current U.S.
Class: |
428/846.9 ;
427/11; 427/299; G9B/5.288; G9B/5.299 |
Current CPC
Class: |
B24B 37/04 20130101;
C03C 2204/08 20130101; C03C 19/00 20130101; C03C 21/002 20130101;
G11B 5/8404 20130101; B24B 21/04 20130101; G11B 5/73921
20190501 |
Class at
Publication: |
428/846.9 ;
427/299; 427/011 |
International
Class: |
B05D 001/00; B05D
003/00; B32B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 2004 |
JP |
2004-129140 |
Claims
What is claimed is:
1. A glass substrate for a perpendicular magnetic recording disk,
said glass substrate having a surface with an average surface
roughness of 2.0 .ANG. or less and surface height variations of 1
.ANG. or less with wavelengths in the range of 0.05 mm-0.5 mm in
both radial and circumferential directions.
2. A method of producing a glass substrate for a perpendicular
magnetic recording disk, said method comprising the steps of:
rotating a glass substrate; supplying polishing slurry on a surface
of said glass substrate; and pressing a polishing tape on said
surface and causing said polishing tape to travel in a direction
opposite to the direction of rotation of said glass substrate
wherein said surface comes to have an average surface roughness of
2.0 .ANG. or less and surface height variations of 1 .ANG. or less
with wavelengths in the range of 0.05 mm-0.5 mm in both radial and
circumferential directions; wherein said polishing slurry comprises
abrading particles and a dispersant, said abrading particles
comprising artificial diamond particles with diameters less than 50
nm, said polishing slurry containing said abrading particles in an
amount of 0.005 weight %-0.5 weight % of said polishing slurry;
wherein said polishing tape has a contact part comprising a
material selected from the group consisting of woven cloths,
unwoven cloths, flocked cloths and raised cloths comprising fibers
with thickness of 0.1 .mu.m -5.0 .mu.m.
3. The method of claim 2 wherein said polishing slurry contains
said abrading particles in an amount of 0.005 weight %-0.1 weight %
of said polishing slurry.
4. The method of claim 2 wherein said dispersant consists of water
and an additive, said additive comprising one or more selected from
the group consisting of glycol compounds, higher aliphatic amides,
organic phosphoric acid esters and surfactants, said polishing
slurry containing said additive in an amount of 1 weight %-10
weight %.
5. The method of claim 2 further comprising the step of preparing
said artificial diamond particles by a shock wave process.
Description
[0001] Priority is claimed on Japanese Patent Application
2004-129140 filed Apr. 26, 2004.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a glass substrate for a
perpendicular magnetic recording disk and a method of producing
such a substrate.
[0003] Data processors for recording and reproducing data such as
characters, images and sounds are coming to be installed not only
in computers but also in apparatus such as televisions, cameras and
telephones. Such data processors are now required to have improved
processing capabilities (with increased recording capacities) and
accuracy in reproduction and to be smaller in size. Data are
magnetically recorded on a magnetic recording medium and reproduced
therefrom by means of a magnetic head of the data processor.
[0004] As disclosed in http://www.trl.ibm.com/projects/perpen/
("Perpendicular Magnetic Recording", IBM Tokyo Research Laboratory)
and http://spin.pe.titech.ac.jp/hp/research/nfts2/ ("Production of
Co--Cr High-Density Perpendicular Magnetic Recording Medium",
Nakagawa Group, Department of Electronic Physical Engineering,
Tokyo Engineering University), perpendicular magnetic recording
disks are now under consideration as a magnetic recording medium.
Such disks are produced by sequentially forming a magnetic layer
and a protective layer on the surface of a disk-shaped glass
substrate by using a thin film technology such as sputtering. The
magnetic layer comprises an assembly of columnar crystalline
elements having a segregated structure by composition separation of
a magnetic layer material deposited on the surface of a
high-temperature glass substrate, and each crystalline element is
comprised of a ferromagnetic columnar center part extending in a
direction perpendicular to the surface of the glass substrate and a
non-magnetic surrounding part formed around this center part. These
columnar crystalline elements form the recording bits that are
magnetizable in the direction perpendicular to the surface of the
glass substrate.
[0005] Because a magnetic layer is thus formed with columnar
crystalline elements extending perpendicularly to the surface of
the glass substrate, the surface of a perpendicular magnetic
recording disk is particularly required to be flat and smooth such
that the average surface roughness should be 2 .ANG. or less.
[0006] The increase in the capacity for data recording and the
accuracy in reproduction both depend largely on the distance of
separation between the surface of the magnetic disk and the
magnetic head. Since data are recorded by outputting a magnetic
signal from the magnetic head to form small magnets on the magnetic
layer and reproduced by reading the magnetic signals from these
small magnets by means of the magnetic head, an increased distance
of separation between the surface of the magnetic disk and the
magnetic head means that the magnetic signals outputted from the
magnetic head is dispersed more such that the quantity of recording
per unit area (the recording density or recording capacity) is
reduced. Thus, in order to increase the capacity of data recording
and to improve the accuracy of reproduction, the distance of
separation between the surface of the magnetic disk and the
magnetic head must be made smaller. Moreover, the magnetic disk can
be made smaller if the recording quantity per unit area is
increased. For this reason, the distance of separation between the
surface of the magnetic disk and the magnetic head is now required
to be 15 nm or less.
[0007] Magnetic heads are either of the floating type or the
contacting type, as explained, say, in
http://www.jst.go.jp/pr/report/report22/ ("Success in Development
of Contacting Type Thin Film Magnetic Heads for Hard Disk", Report
No. 22, Kagaku Gijutsu Shingo Jigyodan). Magnetic heads of the
floating type are provided with a slider on the side opposite the
magnetic disk so as to stabilize the head at a floating distance
(the distance to the magnetic disk) of 15 nm or less. If the
unevenness in the height of the magnetic disk surface is large, the
slider of the magnetic head may contact or collide with the uneven
surface to damage the magnetic disk and it will not be possible to
stably maintain a floating distance of 15 nm or less. Magnetic
heads of the contacting type are adapted to contact the surface of
the magnetic disk through an elastic pad but if the magnetic disk
has an uneven or rough surface, the magnetic head may be caused to
oscillate and damaged.
[0008] The glass substrate for a perpendicular magnetic recording
disk is therefore required to have a high level of smoothness (with
average surface roughness of 2.0 .ANG. or less) and a high level of
flatness (with the surface height variations of 1 .ANG. or less
with wavelengths in the range of 0.05 mm-0.5 mm in both radial and
circumferential directions).
[0009] In general, glass substrates are polished with free abrading
particles by using a lapping plate or a tape. According to Japanese
Patent Publication Tokkai 9-314458, slurry with abrading particles
with average particle diameter of 10 nm-1 .mu.m of a material such
as artificial or natural diamond, cerium oxide and zirconium oxide
dispersed at a rate of 0.5 weight %-20 weight % with respect to the
whole of the slurry is used for the polishing. With prior art
slurry of this kind, however, the surface roughness of the polished
surface of a glass substrate exceeds 5 .ANG., and it is not
possible to obtain a surface with average roughness equal to or
less than 2 .ANG., as desired.
SUMMARY OF THE INVENTION
[0010] It is therefore an object of this invention to provide a
glass substrate for a perpendicular magnetic recording disk, having
an average surface roughness of 2.0 .ANG. or less and surface
height variations of 1 .ANG. or less with wavelengths in the range
of 0.05 mm-0.5 mm in both radial and circumferential directions, as
well as a method of producing such a glass substrate.
[0011] The invention therefore relates on one hand to a glass
substrate for a perpendicular magnetic recording disk, having a
surface with an average surface roughness of 2.0 .ANG. or less and
surface height variations of 1 .ANG. or less with wavelengths in
the range of 0.05 mm-0.5 mm in both radial and circumferential
directions. The invention also relates to a method of producing
such a glass substrate for perpendicular magnetic recording,
characterized as comprising the steps of rotating a glass
substrate, supplying polishing slurry on a surface of the glass
substrate, pressing a polishing tape on the substrate surface and
causing it to travel in a direction opposite to the direction of
rotation of the glass substrate.
[0012] In the above, the polishing slurry comprises abrading
particles and a dispersant, the abrading particles comprising
artificial diamond particles with diameters less than 50 nm, say,
obtained by a shock wave method. The polishing slurry contains such
abrading particles in an amount of 0.005 weight %-0.5 weight % of
the polishing slurry. The dispersant consists of water and an
additive. Examples of the additive include one or more selected
from glycol compounds, higher aliphatic amides, organic esters of
phosphoric acid and surfactants. The amount of dispersant to be
contained is 1 weight %-10 weight % of the whole of the polishing
slurry.
[0013] The polishing tape has a contact part comprising a material
selected from the group consisting of woven cloths, unwoven cloths,
flocked cloths and raised cloths comprising fibers with thickness
of 0.1 .mu.m-5.0 .mu.m.
[0014] By such a method according to this invention, the surface of
a glass substrate can be polished so as to have an average surface
roughness of 2.0 .ANG. or less and surface height variations of 1
.ANG. or less with wavelengths in the range of 0.05 mm-0.5 mm in
both radial and circumferential directions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic drawing of a polishing machine which
may be used for the production of a glass substrate according to
this invention.
[0016] FIG. 2 is a computer-generated image that shows the surface
condition of a glass substrate after the rough polishing
process.
[0017] FIG. 3 is a computer-generated image that shows the surface
condition of a glass substrate after the surface reinforcing
process.
[0018] FIG. 4 is a computer-generated image that shows the surface
condition of a glass substrate of a test example after the
polishing process.
[0019] FIG. 5 is a computer-generated image that shows the surface
condition of a glass substrate of a comparison example after the
polishing process.
DETAILED DESCRIPTION OF THE INVENTION
[0020] This invention relates to a glass substrate for a
perpendicular magnetic recording disk, having an average surface
roughness of 2.0 .ANG. or less and surface height variations of 1
.ANG. or less with wavelengths in the range of 0.05 mm-0.5 mm in
both radial and circumferential directions, as well as to a method
of producing such a glass substrate.
[0021] Examples of the glass material that may be used according to
this invention include soda-lime glass with silicon dioxide
(SiO.sub.2), sodium oxide (Na.sub.2O) or calcium oxide (CaO) as
main component, alumisilicate glass with silicon dioxide
(SiO.sub.2), aluminum oxide (Al.sub.2O.sub.3) and R.sub.2O (where R
is potassium (K), sodium (Na) or lithium (Li)) as main components,
borosilicate glass, lithium oxide (Li.sub.2O)-SiO.sub.2 glass,
Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2 glass,
R'O--Al.sub.2O.sub.3--SiO.sub.2 glass (where R' is magnesium (Mg),
calcium (Ca), strontium (Sr) or barium (Ba)), and chemically
reinforced glass obtained by adding zirconium oxide (ZrO.sub.2),
titanium oxide (TiO.sub.2), etc. to the above. Glass substrates
with a surface subjected to a chemical surface-reinforcing process
can be used. Crystalline glass with main crystals of
.alpha.-crysto-balite (.alpha.-SiO.sub.2) and lithium monosilicate
(Li.sub.2O.cndot.SiO.sub.2) may also be used as glass
substrate.
[0022] Such a glass substrate for a perpendicular magnetic
recording disk may be produced by using a polishing machine 10
shown in FIG. 1 to polish the surface of a glass substrate.
Although the polishing machine 10 shown in FIG. 1 is of a type for
polishing both surfaces of a glass substrate, a polishing machine
of a type for polishing only one side of a glass substrate may be
employed.
[0023] After a glass substrate 15 is set on a shaft (not shown)
connected to a driving motor, the driving motor is activated to
polish the glass substrate 15 by rotating it in the direction of
arrow R, as shown in FIG. 1. Polishing slurry is supplied through
nozzles 12 to both surfaces of the glass substrate 15, and
polishing tapes 14 are pressed upon both surfaces of the substrate
15 by means of contact rollers 11. The polishing tapes 14 are
caused to advance in the direction of arrows T opposite to the
rotation of the substrate 15. After the polishing, a washing liquid
such as water is blown through nozzles 13 onto the surfaces of the
substrate 15 while the latter is kept rotating in the direction of
R.
[0024] It is preferable to preliminarily carry out a rough
polishing process on the surfaces of the substrate 15. Unwanted
unevenness is formed on the surfaces of a glass substrate if the
time spent on the polishing process is too long. The time required
for the polishing process can be reduced if a rough polishing
process is carried out preliminarily.
[0025] The preliminary rough polishing is carried out such that the
average roughness of the surfaces of the substrate will become 2
.ANG.-5 .ANG. and the surface height variations with wavelengths in
the range of 0.05 mm-0.5 mm in both radial and circumferential
directions will be in the range of 1 .ANG.-10 .ANG.. This process
of rough polishing may be carried out by the conventional
technology of using a lapping plate or a polishing tape, described,
say, in Japanese Patent Publications Tokkai 11-114792 and
11-221741.
[0026] A surface reinforcing process may preferably be carried out
after the aforementioned preliminary rough polishing process and
before the final polishing process. This surface reinforcing
process may be carried out chemically, for example, by immersing
the glass substrate in a heated solution of mixed molten salt with
potassium nitrate and sodium nitrate so as to exchange a part of
the ions on the substrate surfaces with ions having larger
diameters.
[0027] The polishing slurry is comprised of abrading particles and
a dispersant. Use as the abrading particles is made of artificial
diamond particles of diameters less than 50 nm produced by a
conventional shock wave method (or explosion method) of a known
type such as described in Japanese Patent Publication Tokkai
2000-136376. According to this method, a diamond material
comprising graphite powder is compressed at a high temperature by a
shock wave and thereafter impurities are removed to obtain
artificial diamond powder of density in the range of 3.2
g/cm.sup.3-3.4 g/cm.sup.3 (the density of natural diamond being
3.51 g/cm.sup.3). Artificial diamond particles thus obtained are
chemically processed by using hydrochloric acid or nitric acid in
order to dissolve the impurities and thereafter washed with water.
Since marks as shown in FIG. 5 may be formed on the surface of the
glass substrate or the substrate surface may be rendered rough if
the diameter of the abrading particles is 50 nm or greater, only
artificial diamond particles with diameters less than 50 nm are
kept by a classification process and used as abrading
particles.
[0028] Artificial diamond particles to be used as abrading
particles according to this invention are required only to have a
diameter less than 50 nm and may be either primary particles or
secondary particles. Secondary particles are adapted to break up
into smaller primary and secondary particles as they are pressed
onto the surface of the glass substrate by a polishing tape during
a polishing process such that these resultant smaller particles act
on the substrate surface. It goes without saying, however, that the
secondary particles themselves act on the substrate surface before
they break up. Thus, even if secondary particles break up into
smaller particles with diameters less than 50 nm, they may leave
marks on the substrate surface or make the substrate surface rough
before they break up if their diameters before breaking up is
larger than 50 nm. This is why the invention requires that even
secondary particles according to this invention should have
diameters smaller than 50 nm.
[0029] The amount of abrading particles that are contained is 0.005
weight %-0.5 weight %, and preferably in the range of 0.005 weight
%-0.1 weight % with respect to the whole of the polishing slurry.
If the content of the abrading particles is less than 0.005 weight
%, the polishing power is insufficient and the time required for
the polishing process becomes excessively long, causing unwanted
unevenness to result on the surface of the glass substrate. If the
content of the abrading particles exceeds 0.1 weight %, on the
other hand, marks begin to be formed on the surface. If the content
reaches 0.5 weight %, not only are marks formed on the surface but
the surface also becomes rough. If a magnetic head of the
contacting type is used, the surface unevenness due to such marks
and roughness causes the magnetic head to vibrate and becomes the
cause of a breakdown.
[0030] The dispersant for the polishing slurry is comprised of
water and an additive. Examples of the additive include one or more
selected from glycol compounds, higher aliphatic amides, organic
esters of phosphoric acid and surfactants. The amount of dispersant
to be contained is 1 weight %-10 weight % of the whole of the
polishing slurry.
[0031] Glycol compounds have affinity with abrading particles and
function well as a dispersant. Glycol compounds also serve to
prepare a uniform dispersant because they have the effect of
reducing the viscosity of the dispersant when the dispersant is
prepared. Since they also have affinity with water, the glass
substrate can be washed efficiently after the polishing process.
Examples of glycol compound that can be used include alkylene
glycol, polyethylene glycol, polypropylene glycol and diethylene
butylether.
[0032] Higher aliphatic amides function as a polishing accelerator
that improves the speed of polishing. Examples of higher aliphatic
amide that may be used include oleic acid diethanol amide, stearic
acid diethanol amide, lauric acid diethanol amide, retinoic acid
diethanol amide, retinoic acid isopropanol amide, ersinic acid
diethanol amide and tall oil aliphatic acid diethanol amide. Those
with 12-22 carbon atoms are preferred.
[0033] Organic esters of phosphoric acid have the finction of
controlling the generation of abnormal protrusions (burs that are
formed by polishing debris and become attached to the surface of
the glass substrate) on the substrate surfaces. They are esters
obtained by replacing a hydrogen atom of phosphoric acid
(H.sub.3PO.sub.4) with alkyl or allyl group. Examples of organic
esters of phosphoric acid that may be used include aliphatic salts
and aromatic salts such as phosphates of polyoxyethylene
nonylphenolether.
[0034] Surfactants have the effect of improving the dispersion
capability of abrading particles. Examples of surfactant that may
be used include nonionic and anionic surfactants.
[0035] Polishing slurry may be produced by adding abrading
particles to water, dispersing the abrading particles by means of
ultrasonic waves, thereafter further adding additives and still
further thereafter using ultrasonic waves to disperse the abrading
particles.
[0036] A tape of woven cloth, unwoven cloth, flocked cloth (having
hair known as piles attached to the surface) or raised cloth with
at least the surface portion (or the portion that contacts and
actually acts on the surface of the glass substrate) comprised of
fibers with thickness in the range of 0.1 .mu.m-5.0 .mu.m may be
used as the polishing tape. If the thickness of these fibers is
less than 0.1 .mu.m, the contact between the fibers on the surface
portion of the polishing tape and the abrading particles in the
polishing slurry diminishes and the abrading particles cannot act
on the surface of the glass substrate sufficiently effectively. If
the thickness of the fibers exceeds 5.0 .mu.m, on the other hand,
the step differences among the fibers forming the surface portion
of the polishing tape increase and the surface of the glass
substrate cannot be polished uniformly.
[0037] The invention is described next by way of test and
comparison examples.
TEST EXAMPLE
[0038] A glass substrate for a perpendicular magnetic recording
disk was produced by a method according to this invention.
[0039] The surfaces of a crystalline glass substrate with diameter
2.5 inches was subjected to a rough polishing process and after a
surface reinforcing process was carried out, a polishing machine as
shown in FIG. 1 was used to polish its surfaces.
[0040] The rough polishing process was carried out by using a
double-surface polisher of a known kind (trade name: Hamai 9B,
produced by Hamai Seisakusho Corporation). It was carried out by
sandwiching the glass substrate between the upper and lower lapping
plates each having a suede pad attached to its surface, supplying
polishing slurry having cerium oxide particles with average
diameter of 2.mu. dispersed therein onto both surfaces of the glass
substrate and causing the glass substrate to undergo a planetary
motion while the upper and lower lapping plates were rotated. FIG.
2 shows the surface condition of the glass substrate after the
rough polishing.
[0041] The surface reinforcing process was carried out chemically
by immersing the glass substrate in a heated molten liquid of mixed
molten salts of potassium nitrate and sodium nitrate to thereby
exchange a part of the ions on the substrate surfaces with ions
having larger diameters. FIG. 3 shows the surface condition of the
glass substrate after the surface reinforcing process.
[0042] After the rough polishing and surface reinforcing processes,
the surfaces of the glass substrate were polished under the
conditions shown in Table 1 given below by using tapes of non-woven
cloth of thickness 660 .mu.m comprised of nylon fibers of thickness
2.0 .mu.m. FIG. 4 shows the surface condition of the glass
substrate after the polishing process.
1TABLE 1 Rotational speed of glass substrate 1600 rpm Travel speed
of polishing tape 5 inches/minute Supply rate of polishing slurry
15 ml/minute Hardness of rubber contact rollers 45 duro Pressure by
contact rollers 5 pounds Frequency (total amplitude) of oscillation
5 Hz (1 mm) Polishing time 20 seconds
[0043] The polishing slurry used in Test Example was obtained by
adding artificial diamond particles produced by the shock wave
method to pure water and dispersing them by means of ultrasonic
waves. The average diameter (D50) of the artificial diamond
particles after the dispersion was 20 nm. An additive with
composition shown in Table 2 given below was added and stirred, and
the artificial diamond particles were dispersed again by means of
ultrasonic waves. There were no artificial diamond particles either
in the form of primary or secondary particles contained in the
polishing slurry thus obtained. Table 3 shows the composition of
this polishing slurry.
2 TABLE 2 Glycol compositions 50 weight % Organic phosphoric acid
esters 15 weight % Higher aliphatic amides 15 weight % Nonionic
surfactant 20 weight %
[0044]
3 TABLE 3 Artificial diamond particles 0.01 weight % (diameters
less than 50 nm) Additive 5.0 weight % Pure water 94.99 weight
%
COMPARISON EXAMPLE
[0045] Another glass substrate of comparison example for a
perpendicular magnetic recording disk was prepared by polishing
both surfaces of a glass substrate on which rough polishing and
surface reinforcing processes as explained above have been carried
out (of which the surface condition after the rough polishing
process is shown in FIG. 2 and that after the surface reinforcing
process is shown in FIG. 3) by using polishing slurry of a
different kind, containing abrading particles including artificial
diamond particles with average secondary particle diameter of 100
nm. FIG. 5 shows the surface condition of this glass substrate
after the polishing process. Table 4 shows the composition of the
additive used for preparing the polishing slurry and Table 5 shows
the composition of the polishing slurry thus prepared for
Comparison Example.
4 TABLE 4 Glycol compositions 50 weight % Organic phosphoric acid
esters 15 weight % Higher aliphatic amides 15 weight % Nonionic
surfactant 20 weight %
[0046]
5 TABLE 5 Artificial diamond particles 0.01 weight % (average
secondary particle diameter = 100 nm) Additive 5.0 weight % Pure
water 94.99 weight %
[0047] Comparison Experiment
[0048] The glass substrates of Test and Comparison Examples were
compared in terms of average surface roughness and unevenness in
the radial and circumferential directions after the rough polishing
process, after the surface reinforcing process and after the
polishing process.
[0049] The average surface roughness was measured by using an
atomic-force microscope AFM (trade name: Dimension 3100 Series,
produced by Digital Instrument Corporation). The computer-generated
drawings shown in FIGS. 2-5 were produced by using this AFM to scan
an arbitrarily selected surface area of 0.87 mm.times.0.65 mm on
the glass substrates (at 512 points) and converting the result into
a three-dimensional image.
[0050] The unevenness in the radial and circumferential directions
was measured by using a white-light microscope (trade name: New
View 5020 produced by Zygo Corporation) to measure the unevenness
in an arbitrarily selected surface area of 0.87 mm.times.0.65 mm on
the glass substrates surface with wavelengths in the range of 0.05
mm-0.5 mm in both radial and circumferential directions.
[0051] Results of Comparison
[0052] The average surface roughness and unevenness in the radial
and circumferential directions of glass substrates after the rough
polishing process, after the surface reinforcing process and the
polishing process according to the test example and the comparison
examples are shown in Table 6.
6 TABLE 6 Average Unevenness in Unevenness surface circumferential
in radial roughness (.ANG.) direction direction After rough 5.0 1.8
1.5 polishing After surface 3.5 0.7 0.8 reinforcing Text Example
0.8 0.5 0.7 Comparison 2.8 1.3 1.5 Example
[0053] Table 6 clearly shows that a glass substrate for a
perpendicular magnetic recording disk having an average surface
roughness of 2.0 .ANG. or less and surface height variations of 1
.ANG. or less with wavelengths in the range of 0.05 mm-0.5 mm in
both radial and circumferential directions could be produced by a
method according to this invention (Test Example). A comparison
between FIGS. 4 and 5 also indicates that no unwanted marks of the
kind appearing in the case of a comparison example (FIG. 5) are
visible in the case of a test example (FIG. 4). In other words, a
smoother and flatter surface can be produced by a method according
to this invention.
* * * * *
References