U.S. patent application number 10/646506 was filed with the patent office on 2004-02-26 for substrate for perpendicular magnetic recording hard disk medium and method for producing the same.
Invention is credited to Tsumori, Toshihiro.
Application Number | 20040038082 10/646506 |
Document ID | / |
Family ID | 31890554 |
Filed Date | 2004-02-26 |
United States Patent
Application |
20040038082 |
Kind Code |
A1 |
Tsumori, Toshihiro |
February 26, 2004 |
Substrate for perpendicular magnetic recording hard disk medium and
method for producing the same
Abstract
Proposed are a substrate that comprises an easily producible
soft magnetic backing film and can reduce spike noise from the soft
magnetic backing film, and a method for producing the substrate.
More specifically, provided are a substrate for a perpendicular
magnetic recording hard disk medium, comprising an Si single
crystal substrate (1) having a diameter of 65 mm or less, a
thickness of 1 mm or less and an average surface roughness (Rms) of
1 nm or more and 1000 nm or less; an under-plated layer (2) formed
on the substrate, the layer (2) comprising one or more metals
selected from a group consisting of Ni, Cu and Ag and having a
thickness of 1 nm to 300 nm; and a plated soft magnetic layer (3)
formed on the under-plated layer, the layer (3) having a thickness
of 50 nm or more and less than 1000 nm, a coercivity of 20
Oe(oersteds) or less and a saturation magnetization of 1T or more,
wherein the average surface roughness (Rms) of the plated soft
magnetic layer is 0.1 nm or more and 5 nm or less.
Inventors: |
Tsumori, Toshihiro;
(Fukui-ken, JP) |
Correspondence
Address: |
Myers Bigel Sibley & Sajovec
Post Office Box 37428
Raleigh
NC
27627
US
|
Family ID: |
31890554 |
Appl. No.: |
10/646506 |
Filed: |
August 25, 2003 |
Current U.S.
Class: |
428/832 ;
428/846; G9B/5.288; G9B/5.299 |
Current CPC
Class: |
G11B 5/667 20130101;
G11B 5/8404 20130101 |
Class at
Publication: |
428/694.0SG ;
428/694.00R |
International
Class: |
G11B 005/66 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 26, 2002 |
JP |
2002-244837 |
Aug 26, 2002 |
JP |
2002-244838 |
Claims
1. A substrate for a perpendicular magnetic recording hard disk
medium, comprising an Si single crystal substrate having a diameter
of 65 mm or less, a thickness of 1 mm or less and an average
surface roughness (Rms) of 1 nm or more and 1000 nm or less; an
under-plated layer formed on said substrate, the under-plated layer
comprising one or more metals selected from a group consisting of
Ni, Cu and Ag and having a thickness of 1 nm to 300 nm; and a
plated soft magnetic layer formed on said under-plated layer, the
plated soft magnetic layer having a thickness of 50 nm or more and
less than 1000 nm, coercivity of 20 Oe(oersteds) or less and a
saturation magnetization of 1T or more, wherein the average surface
roughness (Rms) of said plated soft magnetic layer is 0.1 nm or
more and 5 nm or less.
2. The substrate for a perpendicular magnetic recording hard disk
medium according to claim 1, having induced anisotropy on the
surface thereof.
3. A method for producing a substrate for a perpendicular magnetic
recording hard disk medium, comprising steps of carrying out
under-plating to form an under-plated layer comprising one or more
metals selected from a group consisting of Ni, Cu and Ag on an Si
single crystal substrate having a diameter of 65 mm or less, a
thickness of 1 mm or less and an average surface roughness (Rms) of
1 nm or more and 1000 nm or less; forming a plated soft magnetic
layer having coercivity of 20 Oe(oersteds) or less and a saturation
magnetization of 1T or more on said under-plated layer; and
polishing said plated soft magnetic layer so as to have an average
surface roughness (Rms) of 0.1 nm or more and 5 nm or less.
4. The method for producing a substrate for a perpendicular
magnetic recording hard disk medium according to claim 3,
comprising a pretreatment step of etching said Si single crystal
substrate preceding the step of carrying out the under-plating.
5. The method for producing a substrate for a perpendicular
magnetic recording hard disk medium according to claim 3, wherein
the step of carrying out the under-plating comprises electroless
plating.
6. The method for producing a substrate for a perpendicular
magnetic recording hard disk medium according to claim 3, wherein
the pretreatment step comprises chemically etching the Si single
crystal substrate.
7. The method for producing a substrate for a perpendicular
magnetic recording hard disk medium according to claim 4, wherein
the pretreatment step comprises etching in an alkaline aqueous
solution comprising one or more selected from the group consisting
of NaOH, KOH and ammonia.
8. The method for producing a substrate for a perpendicular
magnetic recording hard disk medium according to claim 4, wherein
the pretreatment step comprises etching in an acidic aqueous
solution comprising one or more selected from the group consisting
of hydrofluoric acid, hydrochloric acid and nitric acid.
9. The method for producing a substrate for a perpendicular
magnetic recording hard disk medium according to claim 3, wherein
said substrate has induced anisotropy on the surface thereof.
10. The method for producing a substrate for a perpendicular
magnetic recording hard disk medium according to claim 3, wherein
said step of forming said plated soft magnetic layer is carried out
in a magnetic field having an intensity of 10 G or more and 1000 G
or less.
11. The method for producing a substrate for a perpendicular
magnetic recording hard disk medium according to claim 3, preceding
the step of carrying out the under-plating, comprising a step of
mirror-polishing said Si single crystal substrate and a subsequent
pretreatment step of etching in a mixed aqueous solution of ammonia
and hydrogen peroxide.
12. The method for producing a substrate for a perpendicular
magnetic recording hard disk medium according to claim 3, wherein
the sep of forming said plated soft magnetic layer comprises
electroless plating.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to Japanese Patent
Application Nos. 2002-244837 and 2002-244838, both filed Aug. 26,
2002, the disclosures of which are incorporated herein by reference
in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a substrate for use in a
perpendicular magnetic recording hard disk and a method for
producing the same.
[0004] 2. Description of the Related Art
[0005] In the field of magnetic recording, information recording by
hard disk apparatus is indispensable for primary external storage
apparatuses in computers including personal computers. In recent
years, the improvement in magnetic recording density in hard disk
apparatus has been significant, rising at a rate of 100% or more
per year. The recording density has reached close to 60
Gbits/inch.sup.2 at research levels and 30 Gbits/inch.sup.2 even at
product levels.
[0006] This high recording density has been attained by remarkable
improvement in the performance of various mechanical and electronic
elements such as electronic components constituting hard disk
apparatus and related software. In particular, this high recording
density is largely attributed to the progress of magnetic heads
(thin-film heads, MR heads, GMR heads, etc.) for reading/writing
recorded information and the progress of error correction methods
(software) for improving the reliability of signals having been
read. Nevertheless, there are no particular changes in the basic
recording system and the configuration of the apparatus, that is,
the apparatus is configured on the basis of the horizontal magnetic
recording system.
[0007] However, because of the improvement in magnetic recording
density, the volume of a recording layer per bit for magnetic
recording has decreased abruptly. In order to improve the recording
density, it is necessary to improve both the linear recording
density in the circumferential direction and the track density in
the radial direction. However, problems are caused particularly in
the improvement in the linear recording density because of reasons
in the principle of the magnetic recording system. This will be
described below in detail.
[0008] The magnetic recording system is broadly divided into the
horizontal magnetic recording system and the perpendicular magnetic
recording system as schematically shown in FIG. 2 and FIG. 3
depending on the magnetic unit (bit) arrangement system for holding
information on a recording medium.
[0009] The horizontal magnetic recording system is a system for
carrying out recording so that the magnetic information units
formed of S-N magnetic poles become parallel with the plane of a
recording medium, and this system is used for conventional hard
disk media. On the other hand, the perpendicular magnetic recording
system is a system for carrying out recording so that the magnetic
information units become perpendicular to the plane of a recording
medium, and this system is widely used for videotapes and the like
requiring high-density recording.
[0010] In the case when the recording density per unit area is
improved in magnetic recording, it is necessary to reduce the
volume of the magnetic recording unit (bit), as a matter of
course.
[0011] However, because of problems in the principle of magnetic
theory, when the volume of the magnetic substance generating the
effects of a ferromagnetic material for carrying out recording is
decreased, it is known that stability is not always maintained as
the volumes decrease. Owing to the competitiveness between thermal
energy kT (k: Boltzmann constant, T: absolute temperature) at room
temperature and anisotropic energy K.sub.uV (K.sub.u: anisotropic
energy, in particular, crystalline magnetic anisotropic energy in
the case of magnetic recording, V: the volume of a unit recording
bit) for holding the ferromagnetic substance in one direction, it
is known that the volume of the magnetic recording unit is
extremely small and that the magnetized state of the ferromagnetic
substance becomes unstable even at room temperature when
kT.about.K.sub.uV is nearly established. In the case when the
magnetized volume per bit is extremely small as described above,
the state wherein a ferromagnetic substance becomes a paramagnetic
substance is referred to as super paramagnetism. It is known that
there is a limit dimension (critical volume) at the time when the
ferromagnetic substance becomes super paramagnetic, although the
dimension differs depending on the magnetic recording material.
[0012] In actual magnetic recording, when the recording unit volume
is decreased close to the critical dimension by raising the
recording density, a problem becomes manifest before the super
paramagnetism is reached. In other words, a problem of
deteriorating magnetically recorded information (reducing the S/N
ratio of the signal read by the magnetic head) occurs, since the
ferromagnetic state by magnetic recording decays with time in a
relatively short time and the magnetization direction becomes
random. If this phenomenon occurs in a magnetic recording, recorded
information that was written becomes unable to be read after a
lapse of some time or writing itself cannot be carried out. In
recent years, this decaying of recording bits owing to this super
paramagnetism, referred to as a "Brownian motion" problem, has
become an extremely serious problem resulting in determining the
limits of magnetic recording.
[0013] Although the specific numerical value of the recording limit
owing to Brownian motion in the conventional horizontal magnetic
recording system is not known, it is assumed to be approximately
100 Gbits/inch.sup.2 in terms of the recording density of a hard
disk medium.
[0014] As systems for solving the problem of the recording limit
owing to Brownian motion in the conventional horizontal magnetic
recording disk medium, various new recording systems have been
proposed. A system regarded and examined as the most promissing
system is the perpendicular magnetic recording system. In the
perpendicular magnetic recording system, the magnetic field from
adjacent bits becomes the same direction as the magnetization
direction, whereby the stability of the recorded and magnetized
bits is supported. In other words, a closed magnetic circuit is
formed between adjacent bits, whereby a self-demagnetizing field
(hereafter referred to as a demagnetizing field) by
self-magnetization in the perpendicular magnetic recording system
is small in comparison with the horizontal magnetic recording
system, and the magnetized state becomes stable. On the other hand,
in the horizontal magnetic recording system, as the linear
recording density is raised, adjacent recording bits become closer
to each other, and the demagnetizing field becomes larger. In order
to raise the linear recording density further, it is necessary to
extremely reduce the thickness of the magnetic recording layer so
that a rotation magnetization mode does not occur inside the
magnetic recording layer. In the horizontal magnetic recording
system, as the recording density rises, the volume of the recording
bit decreases three-dimensionally. In the perpendicular magnetic
recording system, it is not necessary to reduce the thickness of
the magnetic film in accordance with the improvement in recording
density. In consideration of these, in the perpendicular magnetic
recording system, the demagnetizing field can be reduced and the
value of K.sub.uV can be obtained securely, whereby the stability
of magnetization against Brownian motion is high. Therefore, it is
possible to say that the perpendicular magnetic recording system is
a recording system that can extend the recording limit much
further. The recording medium for the perpendicular magnetic
recording system is highly compatible with the horizontal recording
medium, and technologies basically similar to those used
conventionally can also be used for writing and reading of
magnetically recorded information.
[0015] However, in detail, there are some points causing problems
in commercialization of the perpendicular magnetic recording
system. One of them is the construction of a magnetic medium. FIG.
2 is a schematic sectional view showing the film construction of a
horizontal magnetic recording medium, and FIG. 3 is a schematic
sectional view showing the film construction of a perpendicular
magnetic recording medium. In the horizontal magnetic recording
medium shown in FIG. 2, a nonmagnetic under layer 103 having a
thickness of 20 to 30 nm and a recording layer 104 having a
thickness of 20 to 30 nm are formed on a substrate 101. In the
perpendicular magnetic recording medium shown in FIG. 3, a soft
magnetic layer 105 having a thickness of 100 to 500 nm and a
recording layer 104 having a thickness of 20 to 30 nm are formed on
a substrate 101.
[0016] As substrates for the horizontal magnetic recording system,
Al-Mg alloy substrates plated with NiP are mainly used for 3.5-inch
substrates, and glass substrates are mainly used for 2.5-inch
substrates. On each substrate, a nonmagnetic under film (mainly
made of Cr or Cr alloy), a recording film (mainly made of
Co-Cr-based alloy), a protection film (mainly made of DLC:
diamond-like carbon), a lubrication film, etc., are formed.
[0017] In reality, one or more buffer layers are frequently formed
between the substrate and the under film or between the under film
and the recording film. In a typical construction of films with
respect to the thickness values thereof, the thickness of the under
film is approximately up to 30 nm and the thickness of the
recording film is approximately up to 20 nm at a density of
approximately 20 Gbits/inch.sup.2.
[0018] On the other hand, a perpendicular magnetic recording medium
comprises a soft magnetic backing layer (typically made of
permalloy or the like), a recording film (candidate materials
include a CoCr-based alloy, a multi-layer film obtained by
alternately laminating a PtCo layer and ultra-thin films of Pd and
Co to form several layers, and a SmCo amorphous film.), a
protection film, a lubrication film, etc., on a substrate. There
are two significant differences between the horizontal magnetic
recording medium and the perpendicular magnetic recording medium,
that is, the Cr-based nonmagnetic under layer for the horizontal
magnetic recording medium and the soft magnetic backing layer for
the perpendicular magnetic recording medium, and the compositions
of the recording layer. In particular, the backing layer of the
perpendicular recording medium is required to have soft magnetism
and a thickness of approximately 100 nm to 500 nm. The soft
magnetic backing film is a path for magnetic flux from the upper
recording film and also a path for writing magnetic flux from the
recording head. Hence, the film plays the same role as the iron
yoke of a permanent magnetic circuit. The film is required to be
relatively very thick in comparison with the film of the horizontal
recording medium as described above.
[0019] Forming the soft magnetic backing film of the perpendicular
recording medium is not easy in comparison with forming the
nonmagnetic Cr-based under film of the horizontal recording
medium.
[0020] Usually, all the films of the horizontal recording medium
are formed by a dry process (mainly magnetron sputtering). Even in
the perpendicular recording medium, film formation by a dry process
is a natural trend.
[0021] However, forming the soft magnetic backing layer of the
perpendicular recording medium by sputtering has problems.
Magnetron sputtering is a physical deposition process widely used
to form not only magnetic recording media but also metallic thin
films. In this process, a target is placed in an atmosphere of thin
inert gas, an electrode placed near the target or the target itself
is used as an electrode, and target atoms are physically driven
away by gas plasma obtained by applying a high frequency wave
across the electrodes thereby to form a film. In order to increase
the speed of film formation, a method wherein a permanent-magnet
magnetic circuit is disposed on the rear side of the target and the
magnetic force leakage to the front side is used to raise the
density of plasma is generally used. However, in the case when an
attempt is made to form a soft magnetic layer for perpendicular
magnetic recording by this magnetron sputtering system, many
problems occur. Since the target has soft magnetism, most of the
magnetic flux generated from the magnetic circuit passes through
the inside of the target and hardly leaks to the outside of the
surface of the target. When the leakage amount of the magnetic flux
is low, generated plasma becomes weak and unstable, whereby the
film formation speed by sputtering cannot be sufficiently attained.
In addition, the magnetic flux leakage portion of the target is
preferentially subjected to sputtering. However, the leakage of the
magnetic flux at the portion subjected to sputtering is larger than
that at the fringe portion because of the magnetic flux having
intrinsically passed through the inside of the target, and the
leakage portion is increasingly subjected to sputtering and is
pitted, whereby partial wear of the target occurs. In other words,
when a soft magnetic target is subjected to magnetron sputtering,
the sputtered portion is worn in the shape of a V-groove, and the
backing plate is exposed in a relatively short time, whereby the
life of the target is shortened. On the other hand, if a thin
target is used in order to increase the leakage of magnetic flux on
the target, the life of the target is shortened, and the target is
required to be exchanged frequently. If an attempt is made to
increase the thickness of the target in order to extend the life of
the target, most of-the magnetic flux from the magnetic circuit at
the bottom passes through the inside of the target, and the
external leakage of the magnetic flux becomes nearly lost.
Therefore, the thickness cannot be increased significantly. Since
the leakage of the magnetic field cannot be made large and since
local sputtering is apt to occur, the number of sputtering vacuum
baths serving as components of the sputtering apparatus is required
to be increased. Otherwise, thick films cannot be formed.
Furthermore, partial wear of the target affects the uniformity of
the thickness of a formed film and the uniformity of the
composition of an alloy. On the other hand, since the recording
layer formed on the soft magnetic backing film is relatively thin,
film formation is possible without problems in a dry process and
any other process. Although the forming of the soft magnetic
backing film for a perpendicular recording medium can be carried
out in principle by the conventional sputtering method, the film
formation has big problems in mass production efficiency and
productivity as described above.
[0022] Moreover, as a problem peculiar to the perpendicular
magnetic recording medium, noise occurs from the magnetic film of
the perpendicular recording medium. The noise is broadly divided
into medium noise from the recording magnetic film and spike noise
from the soft magnetic backing film. The former also occurs in the
case of horizontal recording. However, the latter, that is, the
spike noise from the soft magnetic backing film, is peculiar to the
perpendicular recording film. It has been recently thought that the
spike noise occurs since the magnetic head picks up the magnetic
field leaked from magnetic domain walls present in the soft
magnetic backing layer. Reducing the spike noise from the soft
magnetic backing film is one of the critical points required to be
attained in order to commercialize the perpendicular recording
film.
SUMMARY OF THE INVENTION
[0023] In consideration of the above-mentioned conventional film
forming method and substrate construction, the present invention
proposes a substrate that comprises an easily producible soft
magnetic backing film and can reduce spike noise from the soft
magnetic backing film, and also proposes a method for producing the
substrate.
[0024] The present invention is intended to provide a hard disk
substrate for perpendicular magnetic recording excellent in
magnetic characteristics and productivity by forming a under layer
and a soft magnetic metal backing layer having satisfactory
adhesiveness and magnetic characteristics on a single crystal Si
substrate and by smoothing the surface by polishing to obtain a
satisfactory metallic surface. In other words, the present
invention provides a hard disk substrate wherein a soft magnetic
film is formed on a substrate made of an Si single crystal by a wet
process. The soft magnetic film preferably has induced anisotropy
on the surface thereof.
[0025] A schematic sectional view showing the film construction of
a perpendicular magnetic recording medium in accordance with the
present invention is shown in FIG. 1.
[0026] The present invention provides a substrate for a
perpendicular magnetic recording hard disk medium, comprising an Si
single crystal substrate 1 having a diameter of 65 mm or less, a
thickness of 1 mm or less and an average surface roughness (Rms) of
1 nm or more and 1000 nm or less; an under-plated layer 2 formed on
the substrate, the layer 2 having a thickness of 1 nm to 300 nm and
comprising Ni and/or Cu and/or Ag; and a plated soft magnetic layer
3 formed on the under-plated layer 2, the layer 3 having a
thickness of 50 nm or more and less than 1000 nm, coercivity
(coercive force) of 20 Oe(oersteds) or less and a saturation
magnetization of 1T or more, wherein the average surface roughness
(Rms) of the plated soft magnetic layer is 0.1 nm or more and 5 nm
or less. The substrate has preferably induced anisotropy on the
surface thereof. In addition, the present invention provides a
method for producing a substrate for a perpendicular magnetic
recording hard disk medium, comprising a step of carrying out
under-plating to form an under-plated layer comprising Ni and/or Cu
and/or Ag on an Si single crystal substrate having a diameter of 65
mm or less, a thickness of 1 mm or less and an average surface
roughness (Rms) of 1 nm or more and 1000 nm or less; a step of
forming a plated soft magnetic layer having coercivity of 20
Oe(oersteds) or less and a saturation magnetization of 1T or more
on the under-plated layer in a magnetic field having an intensity
of 10 G or more and 1000 G or less; and a step of polishing the
plated soft magnetic layer so as to have an average surface
roughness (Rms) of 0.1 nm or more and 5 nm or less. The substrate
has induced anisotropy on the surface thereof.
[0027] According to the present invention, a thick soft magnetic
backing film can be easily formed on an Si single crystal
substrate, and a satisfactory Si single crystal substrate for
perpendicular magnetic recording, having secured surface roughness,
can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a schematic sectional view showing the film
construction of a perpendicular magnetic recording medium in
accordance with the present invention;
[0029] FIG. 2 is a schematic sectional view showing the film
construction of the conventional horizontal magnetic recording
medium; and
[0030] FIG. 3 is a schematic sectional view showing the film
construction of the conventional perpendicular magnetic recording
medium.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The present invention will be described below in detail.
[0032] According to the present invention, an Si single crystal
substrate may be selected as a substrate. The Si single crystal
substrate is excellent in rigidity, satisfactory in surface
smoothness and very stable in the state of the surface, thereby
being excellent as a high-density magnetic recording substrate. A
Si substrate is well-known to be used as a magnetic recording
substrate, and variety thereof has been proposed. Such substrates
have been disclosed in Japanese Patent Publication (JP-B) Nos.
142048/'89, 2-41089/'90, 2-59523/'90, 1-45140/'89, Japanese Patent
Provisional Publication (JP-A) Nos. 57-105826/'82, 668463/'94,
6-28655/'94, 4-259908/'92, etc., for example. Among these, a
recording medium made by forming a under layer on an Si single
crystal substrate and then by forming a recording layer thereon has
also been disclosed (Japanese Patent Publication (JP-B) No.
2-41089). It is thus known that a Si single crystal is used as a
horizontal magnetic recording substrate. However, since the
substrate is a horizontal magnetic recording medium as described
above, the problem of Brownian motion is not resolved.
[0033] The present invention is particularly intended to make
improvements to resolve the problem of Brownian motion and relates
to a substrate for perpendicular magnetic recording.
[0034] The Si single crystal substrate in accordance with the
present invention is a substrate having a diameter of 65 mm or less
and a thickness of 1 mm or less, and is used for small-diameter
HDDs. If the diameter is more than 65 mm, the ratio of the material
cost of the Si single crystal may become too high, whereby the
substrate may be undesirable as a recording medium substrate. When
a Si substrate having a diameter of 65 mm or less is used, since it
is high in rigidity, less vibration occurs even if it is made thin,
whereby it is suited for mobile applications. In addition, if the
thickness is more than 1 mm, it may be technically difficult to
make scattering values in the thickness at the various parts of the
substrate constant by polishing, thereby being undesirable.
Regarding the minimum dimension, a diameter of 20 mm or more may be
desirable in view of the relationship with the other components
constituting the hard disk drive and in view of the degree of
difficulty and cost in component production. The lower limit of the
thickness may be preferably 0.1 mm. The thickness may be more
preferably in the range of 0.3 to 0.7 mm.
[0035] The square mean roughness (Rms) of the surface of the Si
single crystal substrate in accordance with the present invention
may be preferably 1 nm or more and 1000 nm or less. If the
roughness is less than 1 nm, the adhesiveness of an under-plated
layer formed on the substrate may become insufficient. If the
roughness is more than 1000 nm, surface smoothness required for the
hard disk may not be obtained. The square mean roughness (Rms) of
the surface is the square root of the mean of the squares of
deviations from the measurement average line to the measurement
lines and can be measured by AFM (atomic force microscopy).
[0036] As described in Description of the Related Art, the
inventors of the present invention thought that a dry process alone
to form all films for perpendicular recording caused problems.
Generally, the molding of a hard disk substrate material and the
polishing of the substrate as surface finish are done by wet
processes. Hence, considering that the portion up to the soft
magnetic backing film was a part of the substrate, the inventors
earnestly examined a process wherein the soft magnetic backing film
was formed by a wet process (electroplating, electroless plating,
etc.) and the smoothness was ensured by mechanochemical polishing
(CMP).
[0037] When the formation of the under layer, the wet-type film
formation of the soft magnetic layer and the subsequent smoothening
are regarded as a part of substrate processing, the present
invention is very compatible with the conventional substrate
production process.
[0038] Another reason for selecting the Si single crystal substrate
is that film formation can be carried out stably regardless of
whether the pH value of the bath is acidic or alkaline in the
wet-type film formation (film formation by plating) Further, since
the substrate is made of a single material, extremely excellent
coating uniformity is obtained in film formation by plating,
thereby solving the problem of mutual action at the interface
between-the film and the substrate. In addition, by using the Si
single crystal substrate, the crystallinity and structure
refinement of the layers formed on the surface are satisfactory,
whereby a high-quality soft magnetic backing layer can be
formed.
[0039] Since the surface of a hard disk medium is frequently
subjected to impact owing to the hitting of the head body referred
to as "head slap" during usage, if the soft magnetic backing layer
is only formed by a wet process, the film is low in adhesiveness
and peeled off easily during usage. Moreover, according to the
present invention, CMP polishing after plating is required to
ensure surface smoothness. If the film formed by plating is low in
adhesiveness, a problem of the plated film peeling off occurs in
the middle of the CMP polishing.
[0040] The inventors investigated the adhesiveness between the Si
substrate and the soft magnetic film and found that a plated film
having satisfactory adhesiveness was able to be formed by carrying
out appropriate plating treatment and plating pretreatment and that
a smooth medium was able to be obtained by polishing this film.
[0041] In other words, the present invention is characterized in
that an under-plated layer is formed between the Si substrate and
the soft magnetic metal layer in order to enhance the adhesiveness
between the Si substrate and the soft magnetic metal layer and to
enhance the above-mentioned breaking strength.
[0042] According to the present invention, a layer having
satisfactory adhesiveness can be formed between the Si substrate
and the plated magnetic film by carrying out the pretreatment
and/or the under-plating.
[0043] While investigating a method for removing an oxidized film
formed naturally on the surface of an Si single crystal wafer in
order to improve the adhesiveness, the inventors found that by
etching the surface of the substrate preferably under particular
conditions before under-plating the surface of the Si substrate,
the oxidized film was removed and appropriate unevenness capable of
appropriately improving the adhesiveness of the plated film to the
surface of the substrate was produced, whereby the adhesiveness
between the Si substrate and the magnetic film was improved
significantly.
[0044] In other words, in order to form a satisfactory film by
plating on the Si substrate, it is necessary to remove grease and
naturally oxidized film in the same manner as conventional
plating.
[0045] In addition, the process for removing the naturally oxidized
film is an strongly recommended process for embodying the present
invention.
[0046] According to the present invention, etching the surface of
the Si substrate in the pretreatment prior to plating may be
carried out by a wet process etching in an acid or an alkali. It is
possible to have the etching both in acid and in alkali wherein
alkali treatment is preferably carried out before acid treatment.
By this plating pretreatment, the square mean roughness (Rms) of
the Si single crystal substrate may be preferably set at 1 nm or
more and 1000 nm or less.
[0047] More specifically, in the case of etching in an acid, the
substrate may be immersed in an aqueous solution of one or more
selected from the group consisting of hydrofluoric acid,
hydrochloric acid and nitric acid.
[0048] As the condition of the etching solution, it is preferable
that the concentration of the aqueous solution of hydrofluoric acid
is 2 to 10% by weight, the concentration of the aqueous solution of
nitric acid is 5 to 30% by weight, and the concentration of the
aqueous solution of hydrochloric acid is 2 to 15% by weight.
[0049] However, according to the present invention, it may not be
preferable to remove the oxidized film by etching in the aqueous
solution of HF and to carry out plating.
[0050] When the Si substrate is etched in the aqueous solution of
HF, it is known that the Si atoms on the surface are selectively
bonded to H ions in the solution so that the surface is coated with
H atoms. Consequently, hydrophobicity and ability for inhibiting
reoxidation are kept for several hours. The metal film formed by
plating on the substrate has very low adhesiveness to the surface
of the substrate and is easily peeled off by a weak stress such as
the stress of film washing.
[0051] It is possible to obtain the adhesiveness of the metal film
by a mechanical anchor effect generated by roughening the surface
of the substrate. However, in consideration of the application of
the substrate for HDDs, excessive roughening on the surface to the
degree of microns is not suited for forming a thin magnetic
recording film thereon.
[0052] While investigating the method for removing the naturally
oxidized film from the Si single crystal wafer in order to improve
adhesiveness, the inventors found that the adhesiveness of the
plated film was realized by etching the surface of the substrate
having a slightly high surface roughness in an alkaline aqueous
solution, thereby completing the present invention. According to
etching in an alkali, the substrate may be immersed in an aqueous
solution of one or more selected from the group consisting of NaOH,
KOH and ammonia, for example.
[0053] A particularly preferable alkaline aqueous solution includes
an aqueous solution obtained by mixing 0.3 to 10% by weight ammonia
and 0.5 to 25% by weight hydrogen peroxide, preferably at a pure
substance weight ratio of 2:1 to 1:2; and an aqueous solution of 2
to 50% by weight sodium hydroxide. By using such a solution,
extremely satisfactory adhesiveness can be attained.
[0054] The treatment conditions are as follows: the immersion
treatment can be carried out for 30 seconds to 1 hour at a
temperature between 10.degree. C. and the boiling point of the
solution. In the case of acid treatment, electrolytic etching may
be carried out by flowing electric current at 1 to 20 mA/cm.sup.2
for 1 second to 1 minute.
[0055] In particular, the aqueous solution in which ammonia and
hydrogen peroxide are mixed has a weak effect in removing the
oxidized film from the surface in comparison with the aqueous
solution of HF, and is used as a cleaning liquid for a chemical
cleaning process, generally referred to as the Raytheon process,
that is used to clean semiconductor wafers, glass for the
electronic industry or the like.
[0056] As a result of investigation, when the Si single crystal
substrate having been polished so as to leave predetermined minute
roughness thereon was treated in this mixed etching solution, the
inventors found that the oxidized film was able to be removed
appropriately by etching without deteriorating the surface of the
substrate and that the treatment had extremely preferable
characteristics required for the plating pretreatment.
[0057] The Si single crystal substrate may be mirror-polished,
preceding the pretreatment in this mixed etching solution prior to
plating. The mirror polishing may be carried out to smoothen the
surface of the substrate to obtain a surface roughness (Rms) of 1
to 1000 nm by using colloidal silica having an average grain
diameter of 10 to 200 nm, for example.
[0058] The plating after the etching in the pretreatment can result
in a film having satisfactory adhesiveness, formed between the Si
substrate and the plated magnetic film. It may be conceivable that
the adhesiveness between the plated film and the Si substrate is
considerably attributable to a chemical microscopic mechanism by
judging from the fact that sufficient adhesiveness is not obtained
when a substrate having similar roughness and not being pretreated
is subjected to plating, although the adhesiveness is also
attributed to the surface roughness obtained by somewhat physical
initial polishing, that is, an anchor effect.
[0059] When the substrate has been subjected to the prescribed
pretreatment, it does not matter whether or not the subsequent
under-plating is electrolytic plating or electroless plating.
Although the under-plating is not particularly limited, Ni and/or
Cu is preferably be used for film formation. The electroless
plating may be rather preferable since its plating conditions are
not dependent on the electrical characteristics of the Si
substrate.
[0060] An under-plated layer can be formed, for example, by adding
0.2 to 1N ammonium chloride to 0.01 to 0.5N nickel sulfate. At that
time, pH is adjusted in a range of 8 to 10, and plating can be
carried out at a temperature of 75 to 95.degree. C. The
under-plated layer can also be formed by other methods. The plated
layer formed preferably comprises one or more metals (in which
alloys thereof may be included) selected from the group consisting
of Ni, Cu and Ag. More preferably, Ni or Cu is used.
[0061] The thickness of the under-plated layer is preferably 1 nm
or more and 300 nm or less. If the thickness is less than 1 nm, the
surface of the substrate may not be coated uniformly. If the
thickness is more than 300 nm, the crystal of the under film may be
enlarged.
[0062] The soft magnetic backing layer formed on the Si single
crystal substrate may comprise various kinds of alloy composition.
In order to have satisfactory soft magnetism (low coercivity Hc),
it is desirable to use an alloy which satisfies the requirement in
which values of magnetocrystalline anisotropy K.sub.u and
magnetostriction .lambda..sub.u are both zero simultaneously. In
addition, regarding its saturation magnetization (Bs), a high
permeability material having Bs of 1T or more is preferable. A high
permeability material having Bs of approximately 1.5T such as a
permalloy comprising 45 mol % Ni and 55 mol % Fe is particularly
preferable. In addition, the soft magnetic layer material that can
be used in the present invention includes materials capable of
being used for film formation in a wet process and having soft
magnetism, such as permalloy (NiFe-based alloys), CoNi-based
alloys, CoFe-based alloys and CoFeNi-based alloys. Furthermore, in
view of magnetic force, an alloy composition which can satisfy both
the high saturation magnetization and low coercivity is desirable.
The CoFeNi-based alloys have a relatively high saturation
magnetization (Bs). Then, if the condition in which the values of
both K.sub.u and .lambda..sub.u are equal to or close to "0" is
satisfied, low Hc can be attained. In particular, the thin film of
the CoFeNi-based alloy formed by electrolytic plating, reported by
Osaka et al., has a fine-grained structure, and can satisfy both
saturation magnetization of approximately 2T and soft magnetism
with Hc being no more than 20 Oe (oersteds). Hence, the
CoFeNi-based alloy may be a very desirable material for the present
invention. The material has been reported, for example, by T. Osaka
et al., Nature 387, (1998), 796; by K. Ohashi et al., IEEE Trans.
Mag. 35, (1999), 2538; by K. Ohashi et al., IEEE Trans. Mag. 34,
(1998), 1462.
[0063] Although an FeTaC film and a Co-based amorphous film are
also candidate materials for the backing film, since these films
are formed by a dry process such as sputtering, they may not be
included in some cases, according to the present invention. It is
desirable that alloys being usable for film formation in a wet
process are used as materials applicable to the present
invention.
[0064] The permeability of the soft magnetic layer is set at 1T or
more in consideration of the limitation in film thickness because
of the above-mentioned reasons. When a material having Bs of less
than 1T is used, its film thickness is required to be made larger
in order to have performance necessary for a soft magnetic backing
layer. Generally, when a thick metal layer is formed by a wet
process or a dry process, the grains in the metal layer structure
grow as the film thickness increases. In order to improve the S/N
ratio in magnetic recording, it is very important to restrict the
enlargement of the grain boundaries of the recording layer and the
soft magnetic layer serving as a magnetic path during replay of
record. In the wet process in accordance with the present
invention, grain growth advances remarkably when the thickness of
the soft magnetic layer is more than 1000 nm. In addition, the
grain diameter distribution of the finely grained structure in the
soft magnetic film expands and the nonuniformity of the grain size
increases. Therefore, the film thickness is set at 50 to 1000
nm.
[0065] It is known that if the coercivity (iHc) is more than 20
Oe(oersteds), significant obstruction occurs when the magnetic flux
generated from the head during writing passes through the soft
magnetic layer, whereby the S/N ratio is lowered greatly in the
case when such the film having such coercivity is used as a medium.
According to the present invention, in view of this finding, as a
requirement for specifying the soft magnetic layer, the coercivity
is 20 Oe (oersteds) or less, preferably 5 Oe or less.
[0066] In order to obtain firm chemical binding in forming a soft
magnetic film by electroplating on an Si single crystal substrate,
it is necessary that metal ions in a plating solution directly
receive electrons from the surface of the Si single crystal
substrate when the metal under film is formed. When a film is
formed by electroplating, it is desirable to use a material doped
with impurities so as to include excessively paired electrons and
have the negative polarity, instead of an intrinsic semiconductor
Si single crystal. When the soft magnetic film is formed by
electroless plating, the Si substrate may be an N-type, P-type or
non-doped intrinsic single crystal.
[0067] Noise occurring from the magnetic film of a perpendicular
recording medium is broadly divided into medium noise from the
recording magnetic film and spike noise from the soft magnetic
backing film. It is recently believed that the latter, that is, the
spike noise from the soft magnetic backing film, occurs because the
magnetic head picks up a magnetic field leaked from magnetic domain
walls present in the soft magnetic backing film, as already
described.
[0068] In order to reduce the spike noise, it is effective to
eliminate the magnetic domain walls inside the soft magnetic
backing film. In order to attain this, several proposals have been
made. For example, it is proposed that the magnetic moment of the
soft magnetic backing film is pinned to the lower portion of the
film by exchange binding in a dry process. It is said that when a
ferromagnetic hard film or antiferromagnetic film is added as the
pinning layer, the soft magnetic backing film is loosely pinned
effectively by the exchange magnetic field from the lower film,
whereby the magnetic domain walls are reduced significantly,
thereby being effective. However, the ferromagnetic hard film
serving as the pinned layer is a source for supplying magnetic flux
to the soft magnetic backing film and lowers permeability. Thus,
the antiferromagnetic film is more desirable. In addition, it is
said that the soft magnetic backing film having a layer structure
comprising a soft magnetic layer and a nonmagnetic layer is
effective in reducing magnetic domain walls.
[0069] Although it is confirmed that these methods are effective,
these methods are proposed because it is difficult to reduce
magnetic domain walls by using the soft magnetic backing film
itself. The problem is that the film structure becomes complicated
and the film production is not necessarily easy in view of
efficient mass production.
[0070] According to the present invention, magnetic domain walls
are reduced by using the soft magnetic backing film itself. The
soft magnetic backing film is formed by a wet method (typically a
plating method). It is generally known that when a soft magnetic
material is heat-treated, cooled or plated for film formation in a
magnetic field, induced anisotropy is generated in the application
direction of the magnetic field. In particular, regarding the
induced anisotropy of FeNi alloys (permalloys), the explanations
given by Neel and Taniguchi et al. are famous as the mechanism of
directionally ordered array. The inventors thus carried out plating
to form a soft magnetic backing film in a magnetic field by
applying the magnetic field in the substantially radial or
circumferential direction of the substrate, and found that a soft
magnetic backing film having in-plane anisotropy, not causing any
magnetic domain structure (in other words, not having magnetic
domain walls) was able to be formed.
[0071] According to the present invention, since the soft magnetic
backing film having in-plane induced anisotropy can be obtained as
a single layer, it is not necessary to use a multi-layer structure
inside the backing film or to use a layer for pinning the lower
portion of the backing film.
[0072] The intensity of the magnetic field required for plating in
the magnetic field is 10 G or more and 1000 G or less. If the
intensity is lower than 10G, sufficient induced anisotropy is not
obtained. Even if the intensity is more than 100G, satisfactory
induced anisotropy is obtained. However, devices such as a
permanent-magnet magnetic circuit and an electromagnet for applying
the magnetic field become large and too expensive, thereby being
undesirable.
[0073] The soft magnetic layer is formed, for example, as follows.
The 0.001 to 0.1N DMAB (dimethylamine borane), 0.002 to 0.2N nickel
sulfate, 0.002 to 0.2N iron sulfate, 0.01 to 1N cobalt sulfate and
others are used. A brightening agent such as saccharin, a chelating
agent such as tartaric acid, citric acid or EDTA, and/or a stress
moderating agent such as methylcarbobenzodiazothiazol may be added
appropriately. The mixture is adjusted to pH 6 to pH 13, and the
film can be formed at a temperature of 55 to 75.degree. C.
[0074] The surface roughness of the soft magnetic film formed by
the wet process on the Si single crystal substrate is not
satisfactory, although the roughness differs depending on the
thickness of the film. The surface roughness of the film formed by
the wet process is ensured by polishing the surface of the backing
film. The polishing can be carried out by mechanical polishing or
CMP. Unlike the ordinary polishing method using only polishing
slurry, CMP is conducted together with chemical polishing using an
acid or alkaline polishing solution. A polishing medium includes
colloidal alumina, colloidal silica or the like. The CMP using a
colloidal polishing medium is high in polishing speed and offers
significantly improved surface roughness, thereby being highly
suited as a method for polishing perpendicular magnetic recording
media. This is because the grain diameter of the colloidal
polishing medium is a very small value of 10 to 100 nm and because
the shape of the grain is nearly spherical, whereby excellent
smoothness can be attained. In addition, according to CMP, the
surface is not simply shaved away mechanically but polished in
chemically dissolving manner. Hence, even when a finely grained
spherical polishing medium is used, industrially sufficient
polishing speed can be attained.
[0075] The type and pH value of the polishing slurry to be used are
different depending on the alloy composition of a material to be
polished (the soft magnetic backing film in the present invention).
For example, in the case of a CoFeNi film, the polishing slurry is
preferably alkaline, pH 10 or more. However, in the case of a
permalloy film, the polishing slurry is preferably acidic in view
of the chemical etching action.
[0076] The polishing conditions are optimized so that the surface
roughness of each alloy composition film becomes satisfactory.
Parameters affecting the polishing include the type and size of a
machine, polishing slurry (polishing material, pH value and
solution temperature), buffing, rotation speed. The conditions are
required to be optimized in consideration of these parameters.
[0077] The present invention relates to the under film of a
high-density recording hard disk. It is preferable that polishing
is carried out so that the smoothness of the film after the
polishing is an average surface roughness (Ra) or a square mean
roughness (Rms) of 0.1 nm or more and 5 nm or less, particularly
preferably 0.1 to 0.5 nm. If it is less than 0.1 nm, multi-stage
CMP is required or the ranges of the conditions become extremely
rigid, thereby being undesirable. Furthermore, if it is more than 5
nm, the surface roughness of a recording film to be placed on the
backing film is affected adversely, whereby the surface roughness
is desired to be not more than this value. The average surface
roughness (Ra) is the average of the absolute deviations from the
measurement average line to the measurement lines. The square mean
roughness (Rms) is the square root of the mean of the squares of
deviations from the measurement average line to the measurement
lines. These can be measured by AFM.
[0078] The substrate, the smoothness of which is improved by the
CMP, may be cleaned by brush cleaning or the like to remove
particles attached to the surface.
[0079] Although ordinary mechanical polishing or the like using
oxide slurry for glass polishing can also be used, this polishing
may be low in polishing speed and require multi-stage polishing to
obtain a satisfactory polished surface as described above. Hence,
CMP is preferable. When the Si single crystal substrate having the
smooth surface of backing film by CMP is used, an alloy-based
recording film or a multi-layer film, each having a various
composition, can be formed on the substrate, whereby an excellent
perpendicular magnetic recording medium can be obtained.
[0080] Examples of the present invention will be described below;
however, the present invention is not construed to be limited to
these examples.
EXAMPLE 1
[0081] Both surfaces of a (100) Si single crystal (a P-doped N-type
substrate) having a diameter of 65 mm, which had been produced by
cutout, edge-removal and lapping of a single crystal substrate with
diameter of 200 mm fabricated by the CZ (Czochralski) method, were
polished by colloidal silica having a mean particle size of 95 nm
so as to be smoothened to an average surface roughness (Rms) of 5
nm (measured by AFM (atomic force microscopy)). This substrate was
immersed and etched for 5 minutes at 80.degree. C. in an aqueous
solution in which saturated aqueous hydrogen peroxide solution was
mixed with 30 wt % aqueous ammonia, the concentration of each being
2 wt %, thereby removing a thin oxidized film on the surface of the
substrate. This substrate was subjected to electroless plating at
80.degree. C. in a plating solution prepared by appropriately
adding ammonium chloride to an aqueous solution of 0.1N nickel
sulfate and 0.1N sodium tartrate so as to have pH 8. Consequently,
the surface was coated with an Ni plated film having a thickness of
100 nm. Then, a CoNiFe soft magnetic film having a thickness of
approximately 1000 nm was formed as a soft magnetic layer. The
plating was carried out to form the film at 80.degree. C. in an
ammonium chloride bath mainly comprising Co, Ni and Fe and having a
value of pH 9, to which hypophosphrous acid serving as a reducing
agent and saccharin and the like serving as brightening and stress
moderating agents were added appropriately.
[0082] After the film formation, elemental analysis was carried out
according to the wave length dispersion method by EPMA. The
composition of the film was approximately 15 mol % Ni, 75 mol % Fe
and 10 mol % Co. When the magnetic characteristics were measured by
using VSM (vibrating sample magnetometer), Bs was 1.9T and iHc was
15 Oe(oersteds). The film exhibited a low coercivity and soft
magnetism.
[0083] The substrate with the soft magnetic film formed thereon was
polished for 6 minutes by a double-sided polishing machine with a
surface plate having a diameter of 700 mm and covered with nonwoven
fabric while a pressure of 180 gf/cm.sup.2 was applied by using a
polishing solution including colloidal silica having a mean
particle size of 80 nm at pH 11 and at a solution temperature of
30.degree. C., whereby a soft magnetic film having a thickness of
approximately 400 nm was obtained. When the surface roughness after
the polishing was measured by AFM, Rms was 0.6 nm, and nearly the
entire surface of the substrate was smoothened.
[0084] In order to check the adhesiveness of the plated film, the
surface of the substrate was coated with a diamond-like carbon
(DLC) protection film having a thickness of 20 nm by magnetron
sputtering and built into a 2.5-inch hard disk unit MK-4313MAT
manufactured by Toshiba Corp. In a state wherein the head was in
close contact with the substrate, a durability test (a head slap
test) to apply an impact of 750 G was carried out by using SM-105
MP manufactured by AVEX Inc. When the surface of the substrate was
observed with an optical microscope having a magnification of 30
after the test, it was confirmed that the plated film was not
peeled off at all, although impact traces were confirmed.
EXAMPLE 2
[0085] Both surfaces of a (100) Si single crystal (a P-doped N-type
substrate) having a diameter of 65 mm, which had been produced by
cutout, edge-removal and lapping of a single crystal substrate with
diameter of 200 mm fabricated by the CZ method, were polished by
using colloidal silica (having a mean particle size of 95 nm) so as
to be smoothened to an average surface roughness (Rms) of 0.4 nm
(measured by AFM). This substrate was immersed and etched for 10
minutes at 50.degree. C. in an aqueous solution of 40 wt % caustic
soda, thereby removing a thin oxidized film on the surface of the
substrate and roughening the surface to a surface roughness Rms of
10 nm. This substrate was subjected to electroless plating at
80.degree. C. in a plating solution prepared by appropriately
adding sodium hydroxide to an aqueous solution of 0.1N nickel
sulfate and 0.1N sodium tartrate so as to have pH 9. Consequently,
the surface was coated with a Ni plated film having a thickness of
150 nm. Then, a CoNiFe soft magnetic film having a thickness of
approximately 1000 nm was formed as a soft magnetic layer. The
plating was carried out to form the film at 80.degree. C. in an
ammonium chloride bath mainly comprising Co, Ni and Fe and having a
value of pH 9, to which hypophosphrous acid serving as a reducing
agent and saccharin and the like serving as brightening and stress
moderating agents were added appropriately.
[0086] After the film formation, elemental analysis was carried out
according to the wave length dispersion method by EPMA. The
composition of the film was approximately 60 mol % Co, 10 mol % Ni
and 30 mol % Fe. When the magnetic characteristics were measured by
using VSM, Bs was 1.9T and iHc was 2 Oe(oersteds). The film
exhibited a low coercivity and soft magnetism.
[0087] The substrate with the soft magnetic film formed thereon was
polished for 6 minutes by a double-sided polishing machine with a
surface plate having a diameter of 700 mm and covered with nonwoven
fabric while a pressure of 180 gf/cm.sup.2 was applied by using a
polishing solution including colloidal silica having a mean
particle size of 80 nm at pH 11 and at a solution temperature of
30.degree. C., whereby a soft magnetic film having a thickness of
approximately 400 nm was obtained. When the surface roughness after
the polishing was measured by AFM, Rms was 0.6 nm, and nearly the
entire surface of the substrate was smoothened.
[0088] In order to check the adhesiveness of the plated film, the
surface of the substrate was coated with a diamond-like carbon
(DLC) protection film having a thickness of 20 nm by magnetron
sputtering and built into a 2.5-inch hard disk unit MK-4313MAT
manufactured by Toshiba Corp. In a state wherein the head was in
close contact with the substrate, a durability test (a head slap
test) to apply an impact of 750 G was carried out by using SM-105
MP manufactured by AVEX Inc. When the surface of the substrate was
observed with an optical microscope having a magnification of 30
after the test, it was confirmed that the plated film was not
peeled off at all, although impact traces were confirmed.
EXAMPLE 3
[0089] Both surfaces of a (100) Si single crystal (a P-doped N-type
substrate) having a diameter of 65 mm, which had been produced by
cutout, edge-removal and lapping of a single crystal substrate with
diameter of 200 mm fabricated by the CZ method, were polished by
using colloidal silica (having a mean particle size of 95 nm) so as
to be smoothened to an average surface roughness (Rms) of 0.4 nm
(measured by AFM). This substrate was immersed in 4 wt % aqueous
solution of HF for 1 minute at 18.degree. C. and electric current
(5 mA/cm.sup.2) was flown for 10 seconds, thereby removing a thin
oxidized film on the surface of the substrate by anodic oxidation
and roughening the surface to a surface roughness Rms of 12 nm.
This substrate was subjected to electroless plating at 80.degree.
C. in a plating solution prepared by appropriately adding sodium
hydroxide to an aqueous solution of 0.1N nickel sulfate and 0.1N
sodium citrate so as to have pH 8. Consequently, the surface was
coated with a Ni plated film having a thickness of 180 nm. Then, a
CoNiFe soft magnetic film having a thickness of approximately 1000
nm was formed as a soft magnetic layer. The plating was carried out
to form the film at 80.degree. C. in an ammonium chloride bath
mainly comprising Co, Ni and Fe and having a value of pH 9, to
which dimethylamine borane serving as a reducing agent and
saccharin and the like serving as brightening and stress moderating
agents were added appropriately.
[0090] After the film formation, elemental analysis was carried out
according to the wavelength dispersion method by EPMA. The
composition of the film was approximately 60 mol % Co, 10 mol % Ni
and 30 mol % Fe. When the magnetic characteristics were measured by
using VSM, Bs was 1.9T and iHc was 2 Oe(oersteds). The film
exhibited a low coercivity, and soft magnetism.
[0091] The substrate with the soft magnetic film formed thereon was
polished for 6 minutes by a double-sided polishing machine with a
surface plate having a diameter of 700 mm and covered with nonwoven
fabric while a pressure of 180 gf/cm.sup.2 was applied by using a
polishing solution including colloidal silica having a mean
particle size of 80 nm at pH 11 and at a solution temperature of
30.degree. C., whereby a soft magnetic film having a thickness of
approximately 400 nm was obtained. When the surface roughness after
the polishing was measured by AFM, Rms was 0.6 nm, and nearly the
entire surface of the substrate was smoothened.
[0092] In order to check the adhesiveness of the plated film, the
surface of the substrate was coated with a diamond-like carbon
(DLC) protection film having a thickness of 20 nm by magnetron
sputtering and built into a 2.5-inch hard disk unit MK-4313MAT
manufactured by Toshiba Corp. In a state wherein the head was in
close contact with the substrate, a durability test (a head slap
test) to apply an impact of 750 G was carried out by using SM-105
MP manufactured by AVEX Inc. When the surface of the substrate was
observed with an optical microscope having a magnification of 30
after the test, it was confirmed that the plated film was not
peeled off at all, although impact traces were confirmed.
[0093] Hence, a satisfactory soft magnetic film for a perpendicular
recording medium substrate was formed by a wet process, and
satisfactory soft magnetism and the smoothening of the soft
magnetic film were attained. As a result, a substrate highly suited
for a perpendicular magnetic recording medium was provided.
EXAMPLE 4
[0094] Both surfaces of a (100) Si single crystal (a P-doped N-type
substrate) having a diameter of 65 mm, which had been produced by
cutout, edge-removal and lapping of a single crystal substrate with
diameter of 200 mm fabricated by the CZ method, were polished by
using colloidal silica (having a mean particle size of 95 nm) so as
to be smoothened to an average surface roughness (Rms) of 5 nm
(measured by AFM). This substrate was immersed and etched at
80.degree. C. in an aqueous solution in which saturated aqueous
hydrogen peroxide solution was mixed with 30 wt % aqueous ammonia,
the concentration of each being 2 wt %, thereby removing a thin
oxidized film on the surface of the substrate. This substrate was
subjected to electroless plating at 80.degree. C. in a plating
solution prepared by appropriately adding ammonium chloride to an
aqueous solution of 0.1N nickel sulfate and further adding a small
amount of ammonia water so as to have pH 8. Consequently, the
surface was coated with an Ni plated film having a thickness of 50
nm. Then, a CoNiFe soft magnetic film having a thickness of
approximately 1000 nm was formed as a soft magnetic layer. The
plating was carried out to form a film at 80.degree. C. in an
ammonium chloride bath mainly comprising Co, Ni and Fe and having a
value of pH 9, to which dimethylamine borane serving as a chelating
agent and saccharin and the like serving as brightening and stress
moderating agents were added appropriately. The Ni plated film and
the soft magnetic CO.sub.2Ni.sub.14Fe plated film were formed in a
magnetic field having an intensity of 200 G. The magnetic field was
generated from a magnetic circuit comprising NdFeB rare earth
permanent magnets opposed to each other and disposed so as to hold
the plating bath therebetween.
[0095] After the film formation, elemental analysis was carried out
according to the wavelength dispersion method by EPMA. The
composition of the film was approximately 60 mol % Co, 10 mol % Ni
and 30 mol % Fe. When the magnetic characteristics were measured by
using VSM (vibrating sample magnetometer), Bs was 1.9T and iHc was
2 Oe(oersteds). The film exhibited a low coercivity and soft
magnetism.
[0096] The substrate with the soft magnetic film formed thereon was
polished for 6 minutes by a double-sided polishing machine with a
surface plate having a diameter of 700 mm and covered with nonwoven
fabric while a pressure of 180 Gf/cm.sup.2 was applied by using a
polishing solution including colloidal silica having a mean
particle size of 80 nm at pH 11 and at a solution temperature of
30.degree. C., whereby a soft magnetic film having a thickness of
approximately 400 nm was obtained. When the surface roughness after
the polishing was measured by AFM, Rms was 0.6 nm, and nearly the
entire surface of the substrate was smoothened. In addition, when
the presence or absence of magnetic domain walls was checked by MFM
at 20 different sampling points in the substrate, no magnetic
domain walls were found.
[0097] In order to check the adhesiveness of the plated film, the
surface of the substrate was coated with a diamond-like carbon
(DLC) protection film having a thickness of 20 nm by magnetron
sputtering and built into a 2.5-inch hard disk unit MK-4313MAT
manufactured by Toshiba Corp. In a state wherein the head was in
close contact with the substrate, a durability test (a head slap
test) to apply an impact of 750 G was carried out by using SM-105
MP manufactured by AVEX Inc. When the surface of the substrate was
observed with an optical microscope having a magnification of 30
after the test, it was confirmed that the plated film was not
peeled off at all, although impact traces were confirmed.
[0098] Hence, a satisfactory soft magnetic film for a perpendicular
recording medium substrate was formed by a wet process, and
satisfactory soft magnetism and the smoothening of the soft
magnetic film were attained. As a result, a substrate highly suited
for a perpendicular magnetic recording medium was provided.
* * * * *