U.S. patent application number 11/543578 was filed with the patent office on 2007-04-19 for method of texturing magnetic hard disk substrate.
This patent application is currently assigned to NIHON Micro Coating Co., Ltd.. Invention is credited to Takashi Arahata, Hiromitsu Okuyama, Yasuyuki Yokota.
Application Number | 20070087668 11/543578 |
Document ID | / |
Family ID | 38035074 |
Filed Date | 2007-04-19 |
United States Patent
Application |
20070087668 |
Kind Code |
A1 |
Yokota; Yasuyuki ; et
al. |
April 19, 2007 |
Method of texturing magnetic hard disk substrate
Abstract
Texturing marks are formed on the surface of a substrate of a
magnetic hard disk first by forming approximately concentric
circular preliminary marks in a first step and then forming in a
second step approximately concentric circular texturing marks on
the surface of the substrate based on the preliminary marks formed
in the first step. The surface after the texturing marks are formed
in the second step has average surface roughness in the range of 1
.ANG. or more and 6 .ANG. or less and a ratio of maximum surface
roughness to the average surface roughness in the range of less
than 10. A foamed tape and a lubricant not containing any abrading
particles are used in the second step. The foamed tape has a foamed
layer having average diameter of air bubbles in the range of 1
.mu.m or more and 50 .mu.m or less, compressibility in the range of
3% or more and 7% or less, compression recovery ratio in the range
of 40% or more and 60% or less, Shore D hardness in the range of 20
degrees or more and 30 degrees or less, and thickness in the range
of 50 .mu.m or more and 80 .mu.m or less.
Inventors: |
Yokota; Yasuyuki; (Tokyo,
JP) ; Arahata; Takashi; (Tokyo, JP) ; Okuyama;
Hiromitsu; (Tokyo, JP) |
Correspondence
Address: |
BEYER WEAVER LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
NIHON Micro Coating Co.,
Ltd.
|
Family ID: |
38035074 |
Appl. No.: |
11/543578 |
Filed: |
October 4, 2006 |
Current U.S.
Class: |
451/41 ; 451/57;
G9B/5.293; G9B/5.299 |
Current CPC
Class: |
B24B 21/04 20130101;
G11B 5/8404 20130101; G11B 5/82 20130101; B24D 11/00 20130101; B24B
19/028 20130101 |
Class at
Publication: |
451/041 ;
451/057 |
International
Class: |
B24B 7/30 20060101
B24B007/30; B24B 1/00 20060101 B24B001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 14, 2005 |
JP |
2005-299521 |
Claims
1. A texturing method for forming texturing marks on the surface of
a substrate of a magnetic hard disk, said method comprising: a
first step of forming approximately concentric circular preliminary
marks on the surface of said substrate; and a second step of
forming approximately concentric circular texturing marks on said
surface of said substrate based on said preliminary marks, wherein
said surface after said texturing marks are formed in said second
step has average surface roughness in the range of 1 .ANG. or more
and 6 .ANG. or less and a ratio of maximum surface roughness to
said average surface roughness in the range of less than 10;
wherein said first step comprises the steps of rotating said
substrate, supplying slurry having abrading particles dispersed to
the surface of said substrate and pressing a woven or non-woven
cloth tape to the surface of said substrate; wherein said second
step comprises the steps of rotating said substrate, supplying a
lubricant to the surface of said substrate with said preliminary
marks formed thereon, and pressing a foamed tape on the surface of
said substrate; wherein said foamed tape comprises a base material
formed as a tape, and a foamed layer formed on the surface of said
base material; and wherein said foamed layer has average diameter
of air bubbles in the range of 1 .mu.m or more and 50 .mu.m or
less, compressibility in the range of 3% or more and 7% or less,
compression recovery ratio in the range of 40% or more and 60% or
less and Shore D hardness in the range of 20 degrees or more and 30
degrees or less.
2. The texturing method of claim 1 wherein said foamed layer has
average diameter of air bubbles in the range of 1 .mu.m or more and
30 .mu.m or less.
3. The texturing method of claim 1 wherein said foamed layer
comprises polyurethane resin.
4. The texturing method of claim 1 wherein said first step further
includes the step of washing the surface of said substrate after
the step of forming said approximately concentric circular
marks.
5. The texturing method of claim 1 wherein said second step further
includes the step of washing the surface of said substrate after
the step of forming said approximately concentric circular
texturing marks.
Description
[0001] This application claims priority on Japanese Patent
Application 2005-299521 filed Oct. 14, 2005.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a texturing method for forming
approximately concentric circular texturing marks on the surface of
a magnetic hard disk substrate.
[0003] Magnetic hard disks are being used as a medium for recording
data such as sound and image for data recording and reproducing
apparatus such as computers. A magnetic hard disk is generally
produced by mirror-polishing the surface of a non-magnetic
substrate such as a glass substrate or an aluminum substrate with
Ni--P plating, carrying out a texturing process on its surface to
form approximately concentric circular texturing marks thereon and
sequentially forming a magnetic layer, a protective layer, etc. on
this textured surface by using a known thin-film technology such as
sputtering.
[0004] As known to persons skilled in the art, the texturing
process is for forming approximately concentric circular line marks
on the surface of a magnetic hard disk similar to the texturing
marks formed on the surface of the substrate in order to prevent
the adsorption of the magnetic head to the magnetic hard disk.
[0005] For the purpose of carrying out magnetization of a magnetic
hard disk accurately for accurate recording and reproduction, the
surface of the magnetic hard disk must satisfy the following four
topological conditions.
[0006] (1) Firstly, the pitch of the line marks formed on the
surface of the magnetic hard disk must be made smaller. In other
words, if the number of line marks per unit length in the radial
direction of the disk is increased, the number of protruding parts
of the linear marks per unit area facing the magnetic head (or the
surface portions of the magnetic hard disk near the magnetic head)
increases such that it becomes possible to carry out the
magnetization of the magnetic hard disk more accurately. In recent
years, the number of line marks per unit length in the radial
direction of the disk, or the line density, is coming to be
required to be 40 lines/.mu.m or more.
[0007] (2) Secondly, deep indentations (deep indentations of line
marks and scratches) must not be formed on the surface of the
magnetic hard disk. This is because, if these indented parts are
too deep, magnetic flux from the magnetic head does not reach the
magnetic layer near the bottom of the indented parts and cannot
magnetize these parts. This makes accurate recording and
reproduction impossible. It is also because a magnetic layer may
fail to be formed near the bottom of the indented parts at the time
of forming a thin film by sputtering.
[0008] (3) Thirdly, abnormal protrusions reaching the floating
distance of the magnetic head must not be formed such that the
magnetic head can fly at a low height. This is because, if the
magnetic head collides with such protrusions, the magnetic head may
be damaged and the pieces of the protrusions will become attached
to the surface of the magnetic hard disk such that accurate
recording to and reproduction from the magnetic hard disk become
impossible. In recent years, floating distances of 10 nm or less
are being required.
[0009] (4) Fourthly, the surface roughness of the magnetic hard
disk must be made low such that the magnetic head can slide
smoothly on the surface of the magnetic hard disk after landing
thereon and before floating up therefrom.
[0010] In summary, it is required to form line marks having
indentations with an appropriate depth and protrusions with an
appropriate height, and such topological surface conditions of a
magnetic hard disk depends largely on the texturing process carried
out on the surface of its substrate.
[0011] As described in Japanese Patent Publication Tokkai
2005-131711, texturing is conventionally carried out by supplying
slurry having abrading particles dispersed therein to the surface
of the substrate and pressing a tape onto the surface of the
substrate and it is becoming possible to form texturing marks with
a small pitch without forming abnormally high protrusions by
correctly selecting the kind and size of the abrading particles and
the kind of the tape.
[0012] With such prior art technologies, however, abrading
particles and polishing debris that remain on the substrate surface
after the texturing process are removed only by blowing a washing
liquid onto the substrate surface. Thus, the magnetic layer and the
protective layer are now being formed on a textured surface under
such conditions that abnormally deep indentations and scratches may
be left, the surface roughness may not be reduced to the level for
allowing the magnetic head to slide smoothly on the surface or be
adjusted to the level for preventing the adsorption of the magnetic
head to the surface. FIG. 4 is a computer-generated image of a
substrate surface after a prior art texturing process.
[0013] In view of the above, there is a demand for forming
texturing line marks also on the surface of a substrate having
indentations with an appropriate depth and protrusions with an
appropriate height in order to satisfy the aforementioned
topological conditions required of the surface of a magnetic hard
disk. As one of judgment standards in the technological field of
texturing, the magnitude of the ratio of the maximum surface
roughness (Rmax) with respect to the average surface roughness in
the peripheral direction of the substrate (Ra) (or the average
value of the height difference of unevenness formed on the surface)
is coming to be considered on the textured surface in recent years.
In recent years, the value of this ratio Rmax/Ra is required to be
less than 10. It is also being required that the average surface
roughness Ra of the substrate be 1 .ANG. or more and 6 .ANG. or
less.
SUMMARY OF THE INVENTION
[0014] It is therefore an object of this invention to provide a
texturing method capable of forming texturing marks having
indentations with an appropriate depth and protrusions with an
appropriate height on the surface of a magnetic hard disk
substrate.
[0015] The present invention therefore relates to a texturing
method for forming texturing marks on the surface of a substrate of
a magnetic hard disk, and the method of this invention is
characterized as comprising a first step of forming approximately
concentric circular preliminary marks on the surface of the
substrate and a second step of forming approximately concentric
circular texturing marks on the surface of the substrate based on
the preliminary marks formed in the first step. The surface after
the texturing marks are formed in the second step is characterized
as having average surface roughness in the range of 1 .ANG. or more
and 6 .ANG. or less and a ratio of maximum surface roughness to the
average surface roughness in the range of less than 10.
[0016] The first step comprises the steps of rotating the
substrate, supplying slurry having abrading particles dispersed to
the surface of the substrate and pressing a tape to the surface of
the substrate. The tape to be used may be of a woven or non-woven
cloth material. The first step may further include the step of
washing the surface of the substrate after the aforementioned
approximately concentric circular marks have been formed.
[0017] The second step comprises the steps of rotating the
substrate, supplying a lubricant (not containing any abrading
particles) to the surface of the substrate with the preliminary
marks already formed thereon, and pressing a foamed tape on the
surface of the substrate. The second step may further include the
step of washing the surface of the substrate after the
aforementioned approximately concentric circular texturing marks
have been formed.
[0018] The foamed tape used in the second step comprises a base
material formed in the shape of a tape and a foamed layer formed on
the surface of the base material. A plastic sheet with thickness in
the range of 25 .mu.m or more and 125 .mu.m or less may be used as
the base material.
[0019] The foamed layer is characterized as having average diameter
of air bubbles in the range of 1 .mu.m or more and 50 .mu.m or
less, compressibility in the range of 3% or more and 7% or less,
compression recovery ratio in the range of 40% or more and 60% or
less, Shore D hardness in the range of 20 degrees or more and 30
degrees or less, and thickness in the range of 50 .mu.m or more and
80 .mu.m or less.
[0020] With a cleaning sheet of this invention, the surface area
portion of the foamed layer (exclusive of the air bubble portions)
is large because the average diameter of the air bubbles inside is
as small as in the range of 1 .mu.m or more and 50 .mu.m or less
and preferably 30 .mu.m or less. Since the compressibility of the
foamed layer is in the range of 3% or more and 7% or less, the
foamed layer is compressed such that its surface will follow the
shape of the surface of the workpiece when the surface of the
foamed layer is pressed against the surface of the workpiece. Since
the compression recovery ratio of the foamed layer is in the range
of 40% or more and 60% or less, the surface of the foamed layer
moves on the surface of the workpiece such that the surface of the
foamed layer follows the surface of the workpiece as the foamed
layer is moved relative to the workpiece. In other words, the
surface of the foamed layer has a good characteristic of following
the surface of the workpiece. Since the Shore D hardness of the
foamed layer is in the range of 20 degrees or more and 30 degrees
or less, that is, since its hardness is sufficiently low, unwanted
protrusions formed on the surface of the workpiece and foreign
objects and dirt attached to the surface of the workpiece can be
easily removed.
[0021] Because the foamed layer has such mechanical
characteristics, the foreign objects and dirt attached to the
surface of the substrate can be removed in the second step without
forming scratches on the surface of the substrate and without
excessively scraping the surface of the substrate and indentations
with an appropriate depth and protrusions with an appropriate
height can be formed such that the ratio Rmax/Ra is less than 10
and Ra is in the range 1.ANG. or more and 6 .ANG. or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic drawing of a polishing machine which
may use a method of this invention.
[0023] FIG. 2 is a sectional view of a foamed tape used in the
second step of the method of this invention.
[0024] FIG. 3 is a computer-generated surface image of the foamed
layer of the foamed tape of FIG. 2, obtained by a scanning electron
microscope (SEM).
[0025] FIG. 4 is a computer-generated surface image of a substrate
after the first step of the method of this invention (which is
substantially the same as a conventional texturing step), obtained
by a scanning electron microscope.
[0026] FIG. 5 is a computer-generated surface image of a substrate
after the second step of the method of this invention, obtained by
a scanning electron microscope.
DETAILED DESCRIPTION OF THE INVENTION
[0027] This invention relates to a texturing method for forming
texturing marks on the surface of a magnetic hard disk substrate
such as a glass substrate and an aluminum substrate.
[0028] A method of this invention may be carried out by using a
double-side polishing machine as shown at 10 in FIG. 1 to form
texturing marks on both surfaces at the same time or by using a
single-surface polishing machine (not shown) of any kind known to
persons skilled in the art to form texturing marks on only one
surface at a time. Throughout herein "surface of a substrate" and
"surfaces of a substrate" are both intended to mean not only one of
the surfaces of the substrate but also both surfaces of the
substrate.
[0029] A texturing method of this invention comprises a first step
of forming approximately concentric circular preliminary marks on
the surface of a substrate and a second step of forming
approximately concentric circular texturing marks on the surface of
the substrate based on the aforementioned preliminary marks such
that the average surface roughness of the surface of the substrate
after the texturing marks are formed is in the range of 1 .ANG. or
more and 6 .ANG. or less and that the ratio of the maximum surface
roughness Rmax to the average surface roughness Ra on the surface
of the substrate with the texturing line marks formed is in the
range of 10 or less. The first step and the second step of the
method of this invention may be carried out by using the same
double-side or single-side polishing machine or separately by using
different polishing machines but it is preferable to carry them out
by using the same polishing machine.
[0030] With reference to FIG. 1, the first step comprises the steps
of rotating the substrate 21 in the direction of arrow R, supplying
slurry having abrading particles dispersed to the surfaces of the
substrate 21 through nozzles 12 and pressing tapes 20 onto the
surfaces of the substrate 21 by means of contact rollers 11. The
tapes 20 are delivered in the direction of arrow T opposite to the
direction R of rotation of the substrate 21. Approximately
concentric circular line marks are thus formed on the surfaces of
the substrate 21, say, as shown in FIG. 4.
[0031] After approximately concentric circular line marks are thus
formed on the surface of the substrate 21, a washing liquid such as
water is blown onto the surfaces of the substrate 12 through
nozzles 13 such that foreign objects such as abrading particles
remaining on the surfaces of the substrate 21 and polishing debris
are removed. The removal of these foreign objects is carried out by
blowing the washing liquid onto the surfaces of the substrate 12
while the substrate 12 is kept in the rotating condition after the
tapes 20 are separated from the surfaces of the substrate 21 and
discharging these objects from the surfaces of the substrate 12
together with the washing liquid by utilizing the centrifugal force
of the rotating substrate 21.
[0032] The slurry is obtained by dispersing abrading particles in a
dispersant. Particles of materials selected from alumina, ceria,
silica and diamond are used as the abrading particles. The size of
the abrading particles is such that their average diameter is in
the range of 0.02 .mu.m or more and 0.5 .mu.m or less. A preferred
example of the abrading particles is cohesive polycrystalline
diamond particles which are secondary particles having diameters
within the range of 20 nm or more and 150 nm or less with a
plurality of polycrystalline diamond particles combined together,
wherein the primary particles of these polycrystalline diamond
particles have diameters in the range of 30 nm or less and an
average diameter in the range of 4 nm or more and 10 nm or less.
The content of abrading particles in the slurry is 0.001 weight %
or more and 0.5 weight % or less.
[0033] The dispersant comprises water and an additive, the additive
including at least two kinds selected from higher fatty acid
amides, glycol compounds, organic esters of phosphoric acid and
surfactants. The content of the additive with respect to the whole
of the slurry is in the range of 0.5 weight % or more and 5.0
weight % or less. Since no significant change is obtained on the
surface of the substrate after the first step even if the slurry
contains more than 5.0 weight % of additive, it is preferable to
set the upper limit of the content at 5.0 weight % in order to
reduce the cost of the slurry.
[0034] Higher fatty acid amides function as a process accelerator
for increasing the processing speed of the first step. Examples of
higher fatty acid amide that may be used include oleic acid
diethanolamide, stearic acid diethanolamide, lauric acid
diethanolamide, ricinolic acid diethanolamide, ricinolic acid
isopropanolamide, ersinic acid diethanolamide, and tol fatty acid
diethanolamide. Among these example, those with 12-22 carbon atoms
are preferred. The content of higher fatty acid amide with respect
to the whole of the additive is in the range of 20 weight %-60
weight %. If the content is less than 20 weight %, the process
speed becomes low. If it exceeds 60 weight %, abnormal protrusions
are generated.
[0035] Glycol compounds have affinity with abrading particles and
function as a dispersant. If a glycol compound is used when a
dispersing medium is prepared, it serves to reduce the viscosity of
the medium and hence a medium can be prepared more uniformly. Since
they have affinity with water, the substrate can be washed more
effectively after the first step. Examples of glycol compound that
may be used include alkylene glycol, polyethylene glycol,
polypropylene glycol and diethylene glycol butylether. The content
of glycol compound with respect to the whole of the additive is in
the range of 20 weight %-60 weight %. If it is less than 20 weight
%, the dispersion characteristic of abrading particle is adversely
affected such that it becomes easier for abrading particles to sink
and large cohesive particles are formed. If it exceeds 60 weight %,
it becomes hard to form clear texturing marks.
[0036] Organic esters of phosphoric acid have the function of
inhibiting the generation of abnormal protrusions (burrs formed by
polishing debris attaching to the substrate surface) on the surface
of the substrate. They are esters obtained by replacing hydrogen of
phosphoric acid H.sub.3PO.sub.4 with alkyl group or allyl group.
Fatty acid salt type and aromatic salt type may be used. For
example, phosphoric acid salt of polyoxyethylene nonylphenolether
may be used. The content of organic ester of phosphoric acid with
respect to the whole of the additive is in the range of 5 weight
%-40 weight %. If it is less than 5 weight %, abnormal protrusions
are generated. If it exceeds 40 weight %, it becomes hard to form
clear texturing marks.
[0037] Surfactants have the effect of improving dispersing
characteristic of abrading particles. Surfactants of nonion or
anion type can be used. The content of surfactant with respect to
the whole of the additive is in the range of 20 weight % or
less.
[0038] The slurry is obtained by adding abrading particles into
water, further adding thereinto an additive including at least two
agents selected from higher fatty acid amides, glycol compounds,
organic esters of phosphoric acid and surfactants and mixing them
by using a homo-mixer.
[0039] As the tape 20, a porous tape capable of acting elastically
on the substrate surface and taking in foreign objects such as
polishing debris inside is used. A woven or non-woven cloth tape
having at least its surface portion made of fibers with diameter in
the range of 0.1 .mu.m or more and 2.0 .mu.m or less may be used as
such a tape.
[0040] FIG. 4 shows approximately concentric circular line marks
formed on a substrate after the first step. Although no abnormal
protrusions higher than 10 nm are formed on the substrate surface,
there are abnormal protrusions with sectional shape of a pointed
tower formed locally. Such abnormal protrusions cast shadows at the
time of forming a thin film, say, by sputtering, and cause spots on
the formed film such as the magnetic layer. These abnormal
protrusions can also cause collisions with the magnetic head
gliding above the substrate surface, adversely affecting the
quality of recording and reproduction by the magnetic hard
disk.
[0041] According to prior art technologies, a magnetic layer and a
protective layer are formed on the substrate surface left with
these line marks after the first step by using a thin film
technology such as sputtering. According to the present invention,
on the other hand, the second step is carried out to remove these
abnormal protrusions such as spots having ill effects on the
recording and reproduction by the magnetic hard disk, making the
average surface roughness Ra in the range of 1 .ANG. or more and 6
.ANG. or less and the ratio Rmax/Ra less than 10.
[0042] In other words, the second step of the texturing method
according to this invention is for removing unwanted protrusions,
burrs, scratches, foreign objects and dirt remaining on the
substrate surface after the first step such that texturing marks
are formed on the substrate surface more accurately and this is
done by trimming and cleaning the substrate surface. FIG. 5 shows
the approximately circular texturing marks formed on the substrate
surface after the second step. As shown, the abnormal protrusions
left on the substrate surface after the first step are removed by
the second step.
[0043] The same polishing machine used in the first step may be
used for the second step or a different machine may be used for the
purpose. Preferably, the polishing machine 10 used in the first
step is also used for the second step. For the purpose, therefore,
the tapes 20 (of woven or non-woven cloth) used in the first step
are removed after the first step and replaced with tapes 30 of a
foamed material.
[0044] With reference to FIG. 1, the second step comprises the
steps of rotating the substrate 21 in the direction shown by arrow
R, supplying a lubricant not containing any abrading particles to
the surfaces of the substrate 21 having the aforementioned line
marks formed thereon through nozzles 14 and pressing the foamed
tapes 30 onto the surfaces of the substrate 21 through the contact
rollers 11. The foamed tapes 30 are advanced in the direction shown
by arrow T opposite the direction of rotation R of the substrate
21. In this manner, the surfaces of the substrate 21 are trimmed
and cleaned and texturing line marks as described above are formed
on them.
[0045] After the texturing marks are formed on the substrate
surfaces based on the aforementioned line marks, a washing liquid
such as water is blown onto the surfaces of the substrate 12 though
the nozzles 13 such that foreign objects remaining on the surfaces
are removed. The removal of these foreign objects is carried out by
blowing the washing liquid onto the surfaces of the substrate 12
while the substrate 12 is kept in the rotating condition after the
foamed tapes 30 are separated from the surfaces of the substrate 21
and discharging the foreign objects from the surfaces of the
substrate 12 together with the washing liquid by utilizing the
centrifugal force of the rotating substrate 21.
[0046] Water or an aqueous solution may be used as the lubricant.
The aqueous solution is prepared by adding to water an additive
that can react chemically with the surfaces of the substrate. At
least two kinds of agents selected from higher fatty acid amides,
higher fatty acids, metallic salts of higher fatty acids, glycol
compounds and organic esters of phosphoric acid are added to water
to produce such an additive.
[0047] The amount of the additive to be added with respect to the
whole of the lubricant is in the range of 0.5 weight % or more and
10 weight % or less. Since no significant change is found on the
substrate surfaces after the second step even if the additive is
added to the lubricant in an amount in excess of 10 weight % and it
only takes more time for the washing in the second step, it is
preferable to set the upper limit of the content to be 10 weight
%.
[0048] The lubricant is alkaline, it being preferable that its pH
value be pH7 or over and pH12 or less. This is such that the
surfaces of the substrate will not be overly scraped and that
foreign objects such as particles and oils attached to the
substrate surfaces can be removed. If the lubricant is acidic, the
unevenness of the textured marks formed on the surfaces of the
substrate in the first step are excessively polished and the
texturing marks become unclear.
[0049] Higher fatty acid amides are used for removing the burrs and
abnormal protrusions remaining on the substrate surfaces after the
first step. Examples of higher fatty acid amide that may be used
include oleic acid diethanolamide, stearic acid diethanolamide,
lauric acid diethanolamide, ricinolic acid diethanolamide,
ricinolic acid isopropanolamide, ersinic acid diethanolamide, and
tol fatty acid diethanolamide. Among these example, those with
12-22 carbon atoms are preferred. The content of higher fatty acid
amide with respect to the whole of the additive is in the range of
10 weight % or more and 50 weight % or less. If the content is less
than 10 weight %, the removal of attached objects from the
substrate surfaces become less effective. If it exceeds 50 weight
%, the texturing marks formed on the substrate surfaces are
excessively scraped and the marks become unclear.
[0050] Higher fatty acids and their metallic salts are effective
agents for removing foreign objects remaining attached to the
substrate surfaces after the first step. The metallic salts of
higher fatty acids includes metallic salts such as Na, K, Al and Ba
of saturated or unsaturated fatty acid. Examples of fatty acid
include stearic acid, palmitic acid, myristic acid, oleic acid,
lauric acid and behenic acid and those with 12-22 carbon atoms are
preferred. The content of salts of higher fatty acid with respect
to the whole of the additive is in the range of 10 weight % or more
and 50 weight % or less. If it is less than 10 weight %, the
removal of attached objects from the substrate surfaces become less
effective. The removal characteristic is not much affected, on the
other hand, if the content exceeds 50 weight %.
[0051] When higher fatty acid is added, alkanolamine is further
added in order to improve the affinity with water. The amount of
alkanolamine to be added with respect to the whole of the additive
is in the range of 10 weight % or more and 60 weight % or less. If
it is less than 10 weight %, the lubricant may become white and
opaque. Although it is increased to more than 60 weight %, the
affinity to water does not change significantly.
[0052] Glycol compounds serve to reduce the viscosity of the
lubricant when it is prepared and makes it easier to wash the
substrate after the first step. Examples of glycol compound that
may be used include alkylene glycol, polyethylene glycol,
polypropylene glycol and diethylene glycol butylether. The content
of glycol compound with respect to the whole of the additive is in
the range of 5 weight %-50 weight %.
[0053] Organic esters of phosphoric acid have the function of
inhibiting the generation of abnormal protrusions (burrs formed by
polishing debris attaching to the substrate surface) on the surface
of the substrate. They are esters obtained by replacing hydrogen of
phosphoric acid H.sub.3PO.sub.4 with alkyl group or allyl group.
Fatty acid salt type and aromatic salt type may be used. For
example, phosphoric acid salt of polyoxyethylene nonylphenolether
may be used. The content of organic ester of phosphoric acid with
respect to the whole of the additive is in the range of 5 weight
%-40 weight %. If it is less than 5 weight %, abnormal protrusions
are generated. If it exceeds 40 weight %, the texturing marks
formed on the substrate surfaces in the first step are excessively
scraped and their indentations and protrusions become unclear.
[0054] As shown in FIG. 2, the foamed tape 30 is comprised of a
tape-shaped base material 31 and a foamed layer 32 formed on the
surface of this base material 31. FIG. 3 shows a computer-generated
surface image of this foamed layer by a scanning electron
microscope (SEM).
[0055] The thickness of the base material 31 is in the range of 25
.mu.m or more and 125 .mu.m or less. The base material 31 is a
plastic sheet with a flat and smooth surface and has a uniform
thickness. A sheet made of a synthetic resin material such as
polyester and. polyethylene terephthalate (PET) is used as the
plastic sheet.
[0056] The average air bubble diameter of the foamed layer is in
the range of 1 .mu.m or more and 50 .mu.m or less, and preferably
in the range of 1 .mu.m or more and 30 .mu.m or less.
[0057] Since the aforementioned second step is carried out as a wet
process by supplying a liquid lubricant between the surface of the
foamed layer 32 and the surface of the substrate 21, if the average
air bubble diameter is less than 1 .mu.m, the lubricity of the
liquid between the surfaces of the foamed layer 32 and the
substrate 21 becomes low and it becomes difficult to take in the
foreign objects removed from the surface of the substrate 21 into
the interior of the foamed layer 32. If the average air bubble
diameter exceeds 50 .mu.m, on the other hand, the surface portion
of the foamed layer 32 (exclusive of the air bubble portions)
acting on unit surface area of the substrate 21 such that not only
does it take longer for the trimming and cleaning of the surface of
the substrate 21 but the lubricity of the liquid also becomes too
large inside the foamed layer 32 and between the surface of the
foamed layer 32 and the substrate 21 and the foreign objects once
taken in are ejected out and cause to scrape the surface of the
substrate 21 excessively or to form scratches on the surface of the
substrate 21.
[0058] The compressibility of the foamed layer 32 is in the range
of 3% or more and 7% or less, the compressibility being defined as
the change in the thickness of the foamed layer when the load
thereon is 16 psi from the thickness at the time when the load
therein is 1.4psi as measured under the environmental condition of
23.+-.3.degree. C.
[0059] The surface of the foamed layer 32 becomes compressed as it
is pressed against the surface of the substrate 21. If the
compressibility of the foamed layer 32 is less than 3%, it becomes
difficult for the surface of the foamed layer 32 to be pressed by
following the shape of the surface of the substrate 21 such that
the compressive force of the surface part (exclusive of the air
bubble portions) of the foamed layer 32 on the surface of the
substrate 21 becomes non-uniform and spots come to be formed on the
finished or cleaned surface of the substrate 21. If the
compressibility of the foamed layer 32 exceeds 7%, on the other
hand, the thickness of the foamed layer 32 becomes too small when
the surface of the foamed layer 32 is pressed against the surface
of the substrate 21 and the volume for taking is liquid such as the
cleaning liquid becomes significantly reduced. As a result, the
lubricity of the liquid such as the cleaning liquid inside the
foamed layer 32 and between the surface of the foamed layer 32 and
the surface of the substrate 21 becomes low and it becomes
difficult to take in the foreign objects scraped off the surface of
the substrate into the interior of the foamed layer 32.
[0060] The compression recovery ratio of the foamed layer 32 is in
the range of 40% or more and 60% or less, the compression recovery
ratio being obtained by measuring the displacement of the foamed
layer 32 under a load of 12 psi under the environmental condition
of 23.+-.3.degree. C. After the load is reduced to 1.6 psi, the
recovered displacement in 30 seconds is measured and this measured
displacement is divided by the aforementioned displacement at the
time of the load of 16 pse, that is, the percentage ratio of
recovered displacement with respect to the compressed
displacement.
[0061] If the compression recovery ratio of the foamed layer 32 is
less than 40%, the force of recovery by the compressed foamed layer
32 is too low, and the pressure of the surface portion (exclusive
of the air bubble portions) of the foamed layer 32 on the surface
of the substrate becomes low such that the force for removing the
foreign objects attached to the surface of the substrate 21 becomes
low. If the compressive recovery ratio of the foamed layer 32
exceeds 60%, on the other hand, the recovery force of the
compressed foamed layer 32 becomes too strong and the pressure of
the surface portion (exclusive of the air bubble portions) of the
foamed layer 32 on the surface of the substrate becomes high such
that, as the objects scraped off the surface of the substrate 21
are pressed onto the surface of the substrate 21, scratches are
formed by them on the surface of the substrate 21.
[0062] The Shore D hardness of the foamed layer 32 is in the range
of 20 degrees or more and 30 degrees or less, the Shore D hardness
being the measured value under the environmental condition of
23.+-.3.degree. C. by using a Shore D hardness meter according to
JIS-L-1096.
[0063] If the Shore D hardness of the foamed layer 32 is less than
20 degrees, the force of removing foreign objects attached to the
surface of the substrate 21 and the unwanted protrusions (abnormal
protrusions) formed on the surface of the substrate 21 becomes low.
If the Shore D hardness of the foamed layer 32 exceeds 30 degrees,
on the other hand, not only the unwanted protrusions (abnormal
protrusions) formed on the surface of the substrate 21 but also
necessary protrusions formed on the surface of the substrate 21
such as the protrusion parts of the texturing lines are scraped
off, and it also becomes easier to form scratches on the surface of
the substrate 21.
[0064] The thickness of the foamed layer 32 is in the range of 50
.mu.m or more and 800 .mu.m or less. If the foamed layer 32 is too
thin, lubricity of the liquid such as the cleaning liquid inside
the foamed layer 23 and between the surface of the foamed layer 32
and the surface of the substrate 21 cannot be maintained at a high
level and foreign objects cannot be effectively taken into the
interior of the foamed layer 21 for a long time. If the foamed
layer 32 is too thick, spots are generated as foreign objects
attached to the surface of the substrate 21 and dirt are removed.
This is considered to be because the foamed layer 32 deforms
significantly in the direction of its surface during the trimming
and cleaning processes and the geometrical structure of the foamed
layer 32 itself is significantly deformed.
[0065] The foamed tapes 30 are obtained by cutting a foamed sheet
produced as will be explained below into the form of a tape.
[0066] For producing the foamed sheet, a resin solution is
mechanically stirred to obtain a paint having air bubbles with
average diameter in the range of 1 .mu.m or more and 50 .mu.m or
less (preferably in the range of 1 .mu.m or more and 30 .mu.m or
less) and foam magnification in the range of 2.times. or more and
5.times. or less.
[0067] The resin solution is one containing urethane resin or acryl
resin, and preferably self-emulsifying aqueous urethane resin. In
the above, aqueous urethane means waterborne polyurethane
dispersion (WBPUD) obtained either by introducing into the main
chain of polyurethane a hydrophilic component for dispersing stably
in water or by dispersing with an external emulsifier. Those
obtained by the former method, or by directly introducing a
hydrophilic component into the main chain of polyurethane, are
referred to as self-emulsifying aqueous urethane resin. (See, for
example, "Recent Development in Technology of Waterbome
Polyurethane Dispersion" by Toshifumi Tamaki, Dainippon Ink and
Chemicals, Inc.; http://www.dic.co.jp/rd/tech/rev0301/index.html).
If self-emulsifying aqueous urethane is used, particles like
aluminum hydroxide powder functioning as abrading particles need
not be used. In other words, hard particles which may become one of
the causes for scraping the substrate surface excessively need not
be used as external emulsifier.
[0068] The resin solution may further contain an agent for
accelerating the foaming of this resin solution and for dispersing
air bubbles stably inside the paint. Such an agent is selected from
higher fatty acids, denaturations of higher fatty acids and alkali
salts of higher fatty acids. This agent is contained preferably at
a rate of 30 weight parts or less as solid component and more
preferably at a rate of 20 weight parts or less as solid component
for 100 weight parts of resin solution as solid component. If more
than 30 weight parts as solid component are contained, there is no
significant change in the function of accelerating the foaming of
the resin solution or dispersing air bubbles stably inside the
paint. Higher fatty acid ammonium may be used conveniently as an
example of this agent.
[0069] The resin solution can be mechanically stirred by placing
the resin solution inside a container and rotating stirring vanes.
For example, a continuous high-pressure foaming machine (such as
TW-70 (trade name) produced by Aikosha Seisakusho) may be used. The
size of the air bubbles dispersed inside the paint and their
foaming magnification can be adjusted by appropriately setting the
rotational speed of the stirring vanes, the quantities of the resin
solution and air and the time of stirring.
[0070] Next, this paint is applied to the surface of the sheet-like
base material to form a film comprising this paint on the surface
of the base material. The application of the paint can be carried
out by any of the known coating methods such as the blade method,
the gravier roll method, the knife method, the extrusion method,
the reverse roll method and the cast method.
[0071] The coated film is dried next to form on the surface of the
base material a foamed layer with average bubble diameter in the
range of 1 .mu.m or more and 50 .mu.m or less, compressibility in
the range of 3% or more and 7% or less, the compression recovery
ratio in the range of 40% or more and 60% or less and the Shore D
hardness in the range of 20 degrees or more and 30 degrees or
less.
[0072] The coated film is dried in an environment of 90.degree.
C.-160.degree. C. In order to completely harden the coated film,
far-infrared light may be used. A foamed layer described above is
thus formed.
[0073] The invention is described next by way of sample substrates
of Test Examples 1-3 which were produced according to this
invention by carrying out texturing process on substrates of
magnetic hard disks which were 2.5-inch aluminum substrate with the
surface Ni--P plated and mirror-polished. These sample substrates
were produced under the same conditions except that the average
diameter D50 of the abrading particles in the slurry used in the
first step were different.
[0074] The double-side polishing machine shown and described above
was used for the first step under conditions shown in Table 1.
TABLE-US-00001 TABLE 1 First Step Rotational speed of substrate 400
rpm Supply speed of tapes 60 mm/minute Supply rate of slurry 15
ml/minute Hardness of contact rollers 40 duro Oscillation frequency
5 Hz (amplitude = 1 mm) Compressive pressure on tapes 1.5 kg Time
of processing 30 seconds
After the first step, pure water was blown on the surface of each
substrate to wash it while the substrate was rotated. FIG. 4 shows
the surface condition of the substrate after the first step.
[0075] The composition of the slurry is shown in Table 2. The
average diameter (D50) of the abrading particles was 0.05 .mu.m for
Test Example 1, 0.10 .mu.m for Test Example 2 and 0.15 .mu.m for
Test Example 3. TABLE-US-00002 TABLE 2 Composition Cohesive
polycrystalline diamond 0.03 weight % of slurry particles (abrading
particles) Pure water 94.97 weight % Additive 5 weight %
Composition of Glycol compound 20 weight % additive (total = Ester
of phosphoric acid 40 weight % 100 weight %) Metal salt of higher
fatty acid 40 weight %
[0076] The tapes were made of non-woven cloth of thickness 700
.mu.m comprising nylon fibers with thickness 1 .mu.m.
[0077] The second step was carried out by replacing the tapes with
foamed tapes on the same double-side polishing machine under the
conditions shown in Table 3 after the first step was completed.
After the time of processing mentioned in Table 3 has elapsed, pure
water was blown onto the surface of the substrate for washing. FIG.
5 shows the surface condition of the substrate after the second
step. TABLE-US-00003 TABLE 3 Second step Rotational speed of
substrate 800 rpm Supply speed of tapes 30 mm/minute Supply rate of
lubricant 5 ml/minute Hardness of contact rollers 40 duro
Oscillation frequency 5 Hz (amplitude = 1 mm) Compressive pressure
on tapes 0.5 kg Time of processing 5 seconds
[0078] The foamed tapes were obtained by cutting a foamed sheet
which was produced as follows. First, a resin solution containing
self-emulsifying waterborne polyurethane dispersion was prepared.
When this resin solution was prepared, an adjuster of foam
formation and an adjuster of bubble size and shape were added in
order to accelerate the foaming of this resin solution and to
disperse air bubbles stably inside the paint. The composition of
this resin solution is shown in Table 4. The solid component of
this self-emulsifying waterborne polyurethane dispersion was 40%.
TABLE-US-00004 TABLE 4 Waterborne polyurethane dispersion 90 weight
parts (self-emulsifying type): Product name: Superflex 410 Produced
by: Daiichi Kogyo Seiyaku Kabushiki Kaisha Adjuster of foam
formation: 4 weight parts N-beef fat
alkylsulpho-succinanamate/sodium sulfite Product name: FCU-305
Produced by: Sanko Kagaku Kogyo Kabushiki Kaisha Adjuster of bubble
size and shape: 7 weight parts Higher aliphatic ammonium Product
name: DC-100A Produced by: Sannopco Kabushiki Kaisha
[0079] Next, this resin solution was stirred by using a known type
of continuous foaming device (with the rotational speed of the
rotary vanes=2000 rpm) to produce a paint with foaming
magnification 3x and having dispersed air bubbles with average
diameter 30 .mu.m.
[0080] Next, this paint was applied to the surface of a PET sheet
of thickness 50 .mu.m by using a cylindrical blade coater of a
known kind to form a membrane comprising this paint on the surface
of this sheet. This membrane was completely dried in an environment
of 100.degree. C. to form a foaming layer of thickness 400 .mu.m on
the surface of the PET sheet to produce a foamed sheet. Mechanical
characteristics of the foamed layer of this foamed sheet are
summarized in Table 5. TABLE-US-00005 TABLE 5 Average diameter of
air bubbles 26.mu. Compressibility 5.3% Compression recovery ratio
50.4% Shore D hardness 26 degrees
[0081] The composition of the lubricant that was used was as shown
in FIG. 6. TABLE-US-00006 TABLE 6 Lubricant Pure water 95 weight %
Additive 5 weight % Additive Higher fatty acid 35 weight % (total =
100 weight %) Glycol compound 30 weight % Metallic salt of higher
fatty acid 5 weight % Alkanol amine 30 weight %
[0082] A comparison test was carried out by comparing the surface
condition (the average roughness (Ra) and the maximum roughness
(Rmax) of these substrates after the texturing process with
Comparison Examples for which texturing process was carried out
according to prior art technologies.
[0083] In what follows, Comparison Examples will mean what were
obtained only after the first step in Test Examples. In other
words, comparisons were made between the surface conditions of the
substrates after the first step (Comparison Examples 1-3) and after
the second step was done thereafter (Test Examples 1-3).
[0084] The average surface roughness Ra and the maximum roughness
Rmax were measured by using a scanning electron microscope
(Nanoscope Dimension 3100 Series (trade name) produced by Digital
Instruments, Inc.) The results of the comparison are shown in
Tables 7 and 8. TABLE-US-00007 TABLE 7 Average surface Maximum
surface Rmax/ roughness (Ra) roughness (Rmax) Ra Comparison Example
1 2.2 .ANG. 30 .ANG. 13.64 Comparison Example 2 4.5 .ANG. 60 .ANG.
13.33 Comparison Example 3 5.3 .ANG. 85 .ANG. 16.04
[0085] TABLE-US-00008 TABLE 8 Average surface Maximum surface Rmax/
roughness (Ra) roughness (Rmax) Ra Test Example 1 2.0 .ANG. 18
.ANG. 9.00 Test Example 2 4.3 .ANG. 41 .ANG. 9.53 Test Example 3
5.1 .ANG. 50 .ANG. 9.80
[0086] As shown in Tables 7 and 8, the ratio Rmax/Ra becomes less
than 10 if the texturing process is carried out on the surface of
the substrate by a method according to this invention, and it can
be understood that texturing marks having indentations with an
appropriate depth and protrusions with an appropriate height can be
formed by a method according to this invention.
* * * * *
References