SG189628A1

SG189628A1 - Glass substrate for magnetic recording medium and magnetic recording medium using the glass substrate for magnetic recording medium

Info

Publication number: SG189628A1
Application number: SG2012073581A
Authority: SG
Inventors: Tamada Minoru
Original assignee: Asahi Glass Co Ltd
Priority date: 2011-10-17
Filing date: 2012-10-03
Publication date: 2013-05-31
Also published as: CN103050133B; CN103050133A; JP2013089268A; MY152421A; JP5212530B2

Abstract

-35- GLASS SUBSTRATE FOR MAGNETIC RECORDING MEDIUM AND MAGNETICRECORDING MEDIUM USING THE GLASS SUBSTRATE FOR MAGNETIC RECORDING MEDIUMPROBLEM5 It is an object to provide a glass substrate formagnetic recording medium and a magnetic recording medium using the glass substrate for magnetic recording medium in which, when a surface roughness Ra is measured for each of evaluation areas in a lattice form that are set in at10 least one of entire principal surfaces (recordable and reproducible surfaces) of the glass substrate for magnetic recording medium, a maximum value thereof falls within a predetermined range.MEANS TO SOLVE 15 There is provided a glass substrate for magneticrecording medium, comprising: a pair of principal surfaces, wherein, in at least one of the principal surfaces, a maximum value of a surface roughness Ra measured in eachof evaluation areas in a lattice form that are set on an90 entire surface of. the principal surface is less than or equal to 1.7 times a mean value of the surface roughness Ra.SELECTED DRAWINGFIG. 125

Description

~1

DESCRIPTION

GLASS SURSTRATE FOR MAGNETIC BECORDING MEDIUM

AND MAGNETIC RECORDING MEDIUM USING THE GLASS SUBSTRATE

FOF MAGRETIC RECORDING MEDIUM

The present invention relates to a glass substrate for magnetic recording medium and a magnetic recording medium using the glass substrate for magnetic recording medium.

BACKGROUND ART i5 Conventionally, an aluminum alloy substrate has peen used for a substrate for magnetic recording medium used for a magnetic disk recording apparatus or the like.

However, with a demand for high-density recording in recent years, a glass substrate, which is harder than the aluminum alloy substrate and excellent in flatness and amoothness, has become a mainstream.

Then, with high-density recording of a magnetic disk (hereinafter, may be referred to as “magnetic racording medium”) in recent years, magnetic signals have been recorded minutely on the magnetic disk, thereby making the signals very weak. In order to read and record such weak signals, a reguest for shortening as much as possible a distance between s magnetic disk and a magnetic head has besn made.

In order to reduce a distance between a magnetic head and & magnetic disk rotating at & high speed, that is, to reduce a floating amcunt of the magnetic head, the

Ee surface of a glass substrate for magnetic recording media, which is a substrate of the magnetic disk, must be an extremely uniform surface so that the magnetic disk and the magnetic head are not brought inte contact with each other.

Various studies have been made on surface characteristics of the glass substrate for magnetic recording medium in order to reduce a distance between a magnetic head and a magnetic disk. For example, Patent

Document 1 teaches reducing a surface roughness Ra to be less than or equal to a gredetermined value on the assumption that a fom of a defect existing on a glass substrate for magnetic disks gives an influence thereto.

FHIOR ART DOCUMENT

PATENT DOCRGENT

Patent Document 1: WO2010/001843

SUMMARY OF THE INVENTION

PROBLEM 10 BE SOLVED BY THE INVENTION

However, in the above-mentioned Patent Document 1, a surface roughness Ka of only an area of a limited portion of a recordable and reproducible area of a glass substrate for magnetic recording medium is merely £5 evaluated. Accordingly, a surface roughness Ra is not measured in a large part of the recordable and reproducible area. Thersfore, there may be a problem in that. an area where a surface roughness Ra is large in the recordable and reproducible area. For example, there ars problems that it is difficult to reduce a floating amount of a magnetic head and a distance beltween a magnetic disk and a magnetic head is not stable, which causes generation of a magnetic noise.

When manufacturing a magnetic recording medium, a magnetic layer is deposited on a surface of a glass substrate for magnetic recording medium, however, theze may be a case where nonuniformity occurs in a crystal grain size of the deposited magnetic layer if there is an area having a high surface roughness Ba {an area of which surface roughness Ra is nonuniform) in the principal surface of the glass substrate for magnetic recording medium. In such a magnetic recording medium having a nonuniform magnetic layer {a magnetic layer of which crystal grain size is nonuniform}, an amount of magnetic noise may be increased in the area where the crystal grain size iz nonuniform. Accordingly, there way be a problem 15% in that an accuracy of reading and writing on a magnetic recording medium by a magnetic head decreases, a recording density decreases, and so forth.

In consideration of the above-mentioned problems in the conventional techniques, it is an object of the present invention to provide a glass substrate for magnetic recording medium in which a surface roughness Ra falls within a predetermined range in an entire surface of at least one of the principal surfaces of the glass substrate for magnetic recording medium.

In order to solve the above-mentioned problems, the present invention provides a glass substrate lor magnetic recording medium, comprising: a palr of principal surfaces, wherein, in at least cone of the principal surfaces, a maximum value of a surface roughness Ra measured in each of evaluation areas in a lattice form yp. that are set in an entire surface of the principal surface is less than or egual to 1.7 times a mean value of the surface roughness Ra.

According to the glass substrate for magnetic recording medium of the present invention (hereinafter, may be simply referred to as “glass substrate”), when thse surface roughness Ra {arithmetic mean roughness) is measured in each evaluation area set in at least one of the principal surfaces of the glass substrate, the maximum value thereof has a predetermined relationship with the mean value of the measured values of the surface roughness

Ra. i5 For this reason, it is possible to form a glass substrate in which the values of the surface roughness Ra fall within a predetermined ranges over the entire surface of the at least cone of the principal surfaces of the glass substrate, and does not have an area of which surface roughness Ra 1s locally high, and the entire surface of the principal surface is uniform and smooth.

According to the glass substrate for magnetic recording medium of the present invention, crystal grains are prevented from being ccarse and large when forming a 2h magnetic layer on a surface of the glass substrate, and, thershy, a magnetle recording medium having a smooth surface with a magnetic layer of which crystal grain size is uniform can be provided.

Thereby, a magnetic noise of the magnetic recording medium can be prevented from being generated, a distance between the magnetic recording medium and a magnetic head can be sel smaller than a conventional one,

and a distance between the magnetic recording medium and the magnetic head can be stabilized, and, therefore, a reading and writing accuracy of records on the magnetic recording medium can be made higher than a conventional one.

FIG. 1 is a view for explaining a glass substrate for magnetic recording medium according a first mode for carrying out the present invention and evaluation areas thereof.

Although a mode for carrying out the present dnvention is hereafter explained with reference to the drawing, the present invention is not limited to the mode for carrying out the invention explained below, and various modifications and replacements can be made to the made For carrying out the invention mentioned below without departing from the scope of the present invention. (First Mode for carrying out the Invention}

In the present mode for carrying out the invention, a description is given of a glass substrate for magnetic recording medium of the present invention.

The glass substrate for magnetic recording medium of the present invention features that a maximum value of a surface roughness Ba measured in each of evaluation areas in a lattice form set in an entire area of the principal surface is less than or egual to 1.7 times a mean value of measured values of the surface roughnass Ra.

A description will now be given, with reference

To FIG, 1, of a specific evaluation method.

The glass substrate for magnetic recording medium has a disk shape having a circular aperture at a center part thereof, az illustrated in FIG, 1-{3&}. In the present invention, a surface roughness Ra 1s measured fox each of evaluation areas in a lattice form set on at least cng of principal surfaces of the glass substrate, that is, an entire surface of a recordable and reproducible area when the glass substrate is formed as a magnetic rveccerding medium. As illustrated in FIGS. 1-{A) and {BB}, the evziuation areas are defined by dividing the principal surface inte a plurality of portions without a gap therebetween. It is preferable to make the evaluation areas identical in shape.

Measuring means for measuring the surface roughness {arithmetic mean roughness) Ra is not limited to a specific means, and can be any means which can measure a surface roughness of each of the evaluation areas set in a lattice form on an entire surface. For exampls, the surface roughness Ra can be measured by a scanning interference microscope or the like,

A shape and size of sach of the evaluation areas set in a lattice form are not limited to a particular shape and size, and, as a shape of the evaluation areas, there iz a lattice form such as, for example, a sguare lattice, a triangular lattice, a hexagonal lattice, a rhombic lattice, a rectangular lattice, a parallelogram lattice, ste.

As a size of each evaluation area, for example, if the shape of the evaluation area is a square lattice, a length of one side may be zet in a ranges of 5 um to 50 um, and can be set to a desired size such az, for example, a

58 um square, 40 pm square, 30 um square, 20 pm square, etc.

Moreover, if the form of each evaluation area is a form other than a sguare lattice, a size thereof may be get to a size pursuant to the square lattice, that is, the size of each svaluation area may be set so that an area of sach evaluation area is the same as the case where the area of the sguare lattice {for example, 25 we’ to 2500 ra’

It should be noted that the size of the evaluation area will be hereinafter described bassd on a length of one side of a sguare lattice, however, it is not limited to the sguare lattice, and, if the evaluation area is set in z form other than the snouare lattice, the size of each evaluation area means a size {area) pursuant to the form of each of the evaluation areas. furthermore, the shape and size of each of the evaluation areas set in a lattice form may he different over an entire surface of the principal surface (for example, the entire surface of the principal surface is divided into predetermined areas, and a shape and size is set to each of the areas, or the like}, but preferably be the sams shape and size.

Because it is particularly preferable fo perform an evaluation for each area of a size the same as a 2% magnetic head or each area having a size smaller than the magnetic head, it is preferable to perform an evaluation with # sire less than or sgual to 50 um square, and, more preferably, a size less than or equal to 30 um square.

However, if the size of the evaluation area is excessively small, it becomss difficult to perform a process of evaluation because the number of the evaluation areas is large, and it is preferable to set each

- 8] —~ evaluation area to be 5 pm sguare or larger, and more praferably, 10 um square or larger.

Here, a description is given of the evaluation area. FIG, I-{A} and (B)} schematically illustrate each evaluation area. It should be noted that although lines are indicated in the drawings, they ars for explaining evaluation areas, and the lines are not actually drawn on a glass substrate. Here, a description is given of an example in which the evaluation area is a sguare lattice, 1G FIG. 1-{B} is an enlarged illustration of a part of FIG. 1-{&} for the purpose of explaining the evaluation area. Fach block of the square lattice {square shape) illustrated in FIG, 1-(B) indicates the evaluation area, and, for example, a length A to ¥ of each side is set to is 30 um.

Then, in the glass substrate for magnetic recording medium of the present invention, a mean value of a surface roughness Ra calculated from results of measurement of a surface roughness with respect to each evaluation area in an entire avea of one of the principal surfaces is set to Bde. Further, when a maximum valus of the surface roughness Ra, that is, the surface roughness

Ra of an evaluation area having a maximum surface roughness value from among the svaluation areas in the aforementioned entire principal surface, is sab 0 Rag, a ralationship Roper = 1.7 Rau. is satisfisd, that is, the maximum value of the surface roughness Ra is less than or egual bo 1.7 times the mean value of the surface roughness

Ra, 36 Hare, the relationship between Rise and Rape, 18 preferadly Ripe = 1.4°Rige, and particoularviy preferably

Bags: = 1.3 Réuue.

—Qe-

This is because the variation in the surface roughness Ra becomes smaller in the entire principal surface as a ratio of the maximum value and the mean value of the surface roughness Ra iz closer to 1.0, and because it indicates that the principal surface of the glass substrate becomes smoother {that is, Bags = 1.0Raawe}-

It should be noted that although there are {wo principal surfaces on upper and lower sides of the glass substrate for magnetic recording medium, there may be a case where only one of the two principal surfaces sets a recordable and reproducible ares when the glass substrate is formed as a magnetic disk. In such a case, it is sufficient that the one of the principal surfaces satisfies the above-mentioned conditions. If both the 1% upper and lower surfaces are set as a recordable and reproducible area when formed in a magnetic disk, it is desirable that the mean value and the maximum value of the surface roughness Ra measured and calculated for each of the two surfaces satisfy the above-mentioned conditions. 2 The same applies to a mean value and a standard deviation mentioned later.

Further, the standard deviation of the surface roughness Ra measured in each svaluation area set in the entire surface of the above-mentioned principal surface is preferably less than or equal te 0.012 rm, and, more praferably, less than or equal to §.010 nm, and, particularly preferable to be less than or equal to 0.008

Ni.

This iz because a reduction In the standard deviation means a reduction in a variation of the surface roughness Ra, and indicates that the principal surface of the glass substrate becomes smoother.

wd

Thus, because the entire surface of the principal surface is smooth and there is ne area having a high surface roughness Ra locally, crystal grains are prevented from becoming coarse locally, when forming a magnetic layer to make a magnetic recording medium, and a magnetic film having a uniform crystal grain size and a smooth surface can be obiained,

For this reason, a distance between a magnetic disk (magnetic recording medium) and a magretic head can be reduced. Moreover, because the distance betwesn the magnetic disk and the magnetic head becomes stable, necurvence of a magnetic noise can be suppressed, and it becomes possible to seb a writing-and-reading accuracy of records and a recording density higher than a conventional one. Furthermore, a bit size can be reduced by using the magnetic recording medium having the magnetic layer of which crystal grain size is small and uniform, which permits attempting an improvement in the surface recording density of the magnetic recording medium.

Moreover, it is desirable that the mean value of the surface roughness Ra measured in each evaluation area in the lattice form set in the entire surface of the above-mentioned principal surface is less than or equal to 0.48 nm, and, move preferably, less than or equal to 0.07 2% nm.

This is because a reduction in the mean value of the surface roughness Ra indicates that the entire principal surface of the glass substrate is caused to be smoother,

Tf the entire principal surface of the glass substrate is smooth, crystal grains are prevented from being coarse to reduce and uniformize the size of the

“ll~ crystal grains, when forming a msgnetic layer, thereby permitting acquisition of a magnetic film having a uniform and smooth surface.

A bit size can be reduced by reducing and unifermizing a size of crystal grains, which permits attempting an improvement in the surface recording density of a magnetic recording medium. Moreover, it becomes possible to reduce a distance petween a magnetic disk and a magnetic head and reduce a variation in the distance between the magnetic disk and the magnetic head, which permits suppressing generation of a magnetic neise and it becomes possible toe set a writing-and-reading accuracy of records and a recording density higher than a conventional one.

Here, a description will be given of a manufacturing method of the glass substrate for magnetic recording madien of the present invention.

The glass substrate for magnetic recording medium can be manufactured by a manufacturing method including the following processes 1 through 4. {Process 1} A shaping process of chamfering an inner peripheral surface and an outer peripheral surface after forming a glass substrate material into a glass substrate of a disc shape having a circular hole at a central part thereof. {Process 2) An edge surface polishing process of polishing the edge surfaces of the glass substrate {an inner peripheral surface and an outer peripheral surface]. {Process 3) A principal surface polishing process of polishing principal surfaces of the above mentioned glass substrate. (Process 4) A cleaning process of precision

- cleaning and drying the above-mentioned glass substrate.

Then, the glass substrate for magnetic recording medium obtained by the manufacturing method including the above mentioned processes can be made into a magnetic recording medium by further performing a process of forming a thin film such as a magnetic layer on the glass substrate.

Here, the shaping process of {Process 1} is to process a glags substrate material, which ig formed by a float process, a fusion method, a press-—molding process, a down~draw method, or a re-draw process, into a glass substrate ¢f a disc shape having a circular hele at the center part thereof. It should be noted that the glass substrate material used may be amorphous glass or crystal glass, or may be strengthened glass having a compression stress layer (strengthening laver) on a surface of a glass subgtrate,

Then, the edge surface polishing process of {Process 2} 1s toe polish edges of the glass substrate {side surface parts and chamfered surfacss!.

The principal surface polishing process of (Frocess 3) is to simultaneously polish upper and lowsy principal surfaces of the glass substrate while supplying a polishing liguid onto the principal surfaces of the glass substrate. The polishing of the glass substrate of the present invention may be only a primary polishing, or may perform a primary polishing and a secondary polishing may be performed, or a tertiary polishing may be performed after a secondary polishing.

Then, it is desirable to use a pad, which has been subiected to a cleaning process oy pure water for 10 minutes or longer, as a polishing pad used in the

=i 3 polishing process. This ls to suppress aggregation of abrasive grains contained in the polishing liquid.

It should be noted that, as for the polishing

Tiguid used for polishing performed at the end of the principal surface polishing process, it is desirable to wee a polishing liguid containing a colloidal silica of which primary particle diameter is greater than or egual to 5 nm as an abrasive grain and having a pH value of 3 or higher. iG This is because, if the primary particle diameter of the colloidal silica contained in the polishing liquid is smaller than 5 nm, the colleidal zilica tends to aggregate, which may cause a problem such that a polishing cannot be performed stably, a surface 15% roughness Ra of the principal surface becomes large, eto.

The primary particle diameter of the colloidal silica is preferably greater than or equal to 5 nm, and, more preferably greater than or egual to 8 nm, and particularly preferably greater than or equal to 10 nm.

Additionally, the primary particle diameter of the colloidal silica is preferably less than or egual to 30 mm, and, more preferably less than or equal to 28 nm, and particularly preferably less than or equal to 18 nm.

Moreover, if the pH value of the polishing liguid is lower than 3, the suriace roughness Ra of the principal surface of the polished glass substrate may be high due to an influence of acid. Thus, the pH value of the polishing liquid is preferably 3 or higher, and more particularly 3.5 or higher, and particularly preferably 4 or higher.

Prior to the above-mentioned principal suriace palishing process of (Process 3), a lapping (for example,

free abrasive lapping, fixed abrasive lapping, eto.) may be performed. Moreover cleaning of the glass substrate {in process cleaning) or etehing of a surface of the glass substrate {in process etching) may be performed between 3 the processes. It should be noted that the lapping of a principal surface corvesponds Lo polishing of a principal surface in a bread ssnss.

Further, if a large mechanical strength is required for the glass substrate for magnetic recording medium, a strengthening process {for example, a chemical strengthening process} of forming a compression stress layer {strengthening laver} on a surface layer of a glass substrate may be performed before the polishing process or after the polishing process or may be performed during the polishing process. {Second Mode for Carrying Out the Invention)

In the present mode for carrying cut the invention, a description is given of a magnetic recording medium {magnetic disk) using the glass substiate for magnetic recording medium explained in the first mode for carrying out the invention.

The magnetic recording medium of the present invention is oot limited to a specific structure, if the giass substrate for magnetic recording medium explained in the first mode for carrying out the invention is used, and, there ig, for example, a magnetic recording medium having a magnetic layer, a protective layer and a lubrication layer thereon.

Although there are a horizontal magnetic recording method and a vertical magnetic recording method for a magnetic recording medium, a specific manufacturing method 1s explained below with the vertical magnetic

—1 he recording method as an example.

The magnetic recording medium is provided with a magnetic layer, a protective layer and a lubrication layer on at least a surface thereof. Then, in a case of the vertical magnetic recording method, it is usual to provide a soft magnetic substrate layer, which plays a role of circulating a recording magnetic field fram a magnetic head. Thus, for example, a soft magnetic substrate layer, a non-magnetic intermediate layer, a vertical recording 1 magnetic laver, a protective layer and a lubrication layst are laminated on the principal surface of the glass substrate in that order.

A description will be given below of each layer.

As a soft magnetic substrate layer, for example,

CoNiFe, FeCoB, CoCufe, NiFe, FeAlSi, FelalN, Fel, FeTal,

CoFeB, CoZrM, eto., may ke usad.

A nonmagnetic intermediate layer ls formed of Ru, an Eu alloy, etc. The nonmagnetic intermediate layer has a function of facilitating epitaxial growth of a vertical recording magnetic layer and a function of cutting a magnetic exchange coupling between the soft magnetic substrate laver and the recording magnetic layer.

A vertical recording magnetic laver ig a magnetic film of which axis of easy magnetization is directed in a perpendicular direction with respect to a substrate surface, and contains Co and Pt at least. In order to reduce an intergranular exchange coupling, which may be a cause of a high intrinzic medium noise, it is desirable to form a fine particle structure {granular structure} in which grains are well-isoclated.

Specifically, it is desirable to use a material of a Cobt alloy or the like to which an oxide (810z, SiO, Cr0;, CoO;

—1 fH

Tasly, Til, eto.) or Cr, B, Cu, Ta, Zr, =2tc., iz added.

The soft magnetic substrate layer, the nonmagnetic intermediate layer and the vertical recording magnetic laver mentioned thus far can be manufactured according to an inline sputter method, a DC magnetron sputter method, or the like in a continuous fashion.

A protection layer is provided for preventing the vertical recording magnetic layer from being corroded znd preventing damage on a medium surface aven when a magnetic head contacts the medium, and is provided on the vertical recording magnetic layer. As a protective layer, a material containing C, Zr0;, 5i0;, stc., may be used.

As a formation method thereof, for example, an inline sputter method, a CVD method, a spin coal method, 1% eto., may be used.

A lubrication laver iz formed on the surface of the protective layer in order to reduce friction between a magnetics head and the recording medium {magnetic disk).

For example, perfluorce polyather, Iluorinated alcohol, fluorinated carboxylic acid, setc., may be used to form the lubrication layer. The lubrication layer can be formed according to a dip method, a spray method, ato.

Because the magnetic recording medium {magnetic disk) produced using the glass substrate for magnetic recording medium of the present invention according to the procedure mentioned above has a surface roughness Ha of the entire principal surface of the glass substrate falling within a pradeterminad range, crystal grains ars suppressed from being coarse, when forming the magnetic layers, and the size of the crystal grains can be made small and uniform. Thereby, the bit size of the magnetic recording medium can be made small, and an improvement in

-1 7 — a surface recording density of the magnetic recording medium can be asitempted. Morsover, because a distance petwesn the magnetic recording medium and the magnetic head can be smaller than a conventional one, and a fluctuation in the distance between the magnetic recording medium and the magnetic head can be made small, it is possible to suppress generation of a magnetic noise and raise a reading-and-writing accuracy of records and a recording density can be made higher than a conventional one. (Embodiment)

Although a description will be given below of specific examples, the present invention iz not limited to those examples.

First, a description is given of an evaluation method of a glass substrate for magnetic recording medium in the examples and comparative examples mentioned below, and an evaluation method of a magnetic recording medium in which a thin film such as a magnetic layer, etc., is deposited on a glass substrate surface. {1) Surface Roughness (arithmetic mean roughness) Ra

A surface roughness Ra was measured using a scanning interference microscope (ZeMapper manufactured by

Zygo corporation). The measurement area of the surface roughness Ra was sel to a range including an entire surface of a recordable and reproducible avea of the glass substrate. in the present embodiment, evaluation areas each 3¢ of which has a square lattice shape having a side of 30 um ware set on one of the entire principal surfaces of the glass substrate for magnetic recording medium, and a

~18- surface roughness Ra was acquired from each of the evaluation areas. {27 Certify Test

A certify test evaluates z defect (signal guality) of a magnetic laver or the like of a magnetic recording medium. Evaluation was made by performing writing, reproducing, erasing, re-reproducing, etc., of a write signal for each track of a disk using a test head in which a magnetic head for a certify test is provided to a head slider, and reproducing a relationship betwesn a magnetic head of a magnetic disk apparatus and a magnetic disk. In the present embodiment, an BP (Extra Pulse) error was evaluated.

The EP srror is a reproduction signal, which is largely deviated from amplitude of a reproduction signal of the test head when there exists on a magnebic recording medium an area where damage, 3 foreign substance or a roughness defect is present or a size of crystal grains is nonuniform.

In the present embodiment, 10800 pieces of magnetic recording medium were svaluated, and a ratio of magnetic recording medium in which an BE error ccourved was sat az an EP ocourrence rate.

A description 1s given below of Examples 1 through 13, which are glass substrates for magnetic recording medium and magnetic recording media made by forming a magnetic recording layer or the like on a glass substrate for magnetic recording mediwg, Here, Hxamples 1 through 7 are examples which satisfy requirements specified by the present invention, and Examples 8 through 130 are comparative examples.

The glass substrates for magnetic recording

-} Ge medium of Examples 1 through 10 explained below were produced according te the following procedures.

A glass substrate formed by a {leat method and containing SiC; as a main component was processed into a disk shaped glass substrate having a circular hole at a center part therscef so that a glass substrate for magnetic recording medium having an outer diameter of 65% mm, an inner diameter of 20 mm and a thickness of 0.830 mm is obtained, 1G An inner peripheral surface and an outer peripheral surface of the disk shaped glass substrate were chamfered so that a glass substrate for magnetic recording medium having a chamfer width of $0.15 mm and a chamfer angle of 45 degrees {an inner periphery chamfering process and an outer periphery chamfering process).

After the chamfering, upper and lower principal surfaces of the glass substrate were lapped using an alumina abrasive, and the alumina abrasive was cleaned and removed. 23 Then, the cuter circumferentisl side suriace part and the outer circumferential chamfered part of the glass substrate for magnetic recording medium were polished using a polishing brush and a polishing liguid containing a cerium oxide abrasive, and a process modified layer (such as a flaw) of the outer ciyvcumferential side surface part and the outer circumferential chamfered part was removed, and the cuter peripheral surface was polished go as to be a mirror surface {an outer peripheral surface polishing process),

After the peripheral surface polishing, the inner peripheral side surface part and the inner periphery chamfered part of the glass substrate for magnetic

—2 Hh — the same as the case of Example 1.

Moreover, evaluation was performed with respect to the cbhtained glass substrate for magnetic recording medium and magnetic recording medium similar to Example 1.

Results are indicated in Table 1. {Example 3}

In the finish polishing (tertiary polishing process) of the principal surface polishing, the glass substrate for magnetic recording medium and the magnetic in recording medium {magnetic disk) were manufactursd according to the same method as Example 1 except for setting the polishing liquid to pHS and setting a flow amount of the polishing liguid supplied to the surfaces to be polished in the double side polisher to 2 ml/min per 15% cone sheet of the glass substrate for magnetic recording medium,

It should be noted that the cohesive property of the abrasive grains {colloidal silica) contained in the polishing ligaid used in the finish polishing was good (A) the same as the case of Example 1.

Moreover, evaluation was performed with respect to the cbtained glass substrate for magnetic recording medium and magnetic recording medium similar to Example 1.

Results are indicated in Table 1. {Bzawmple 4}

In the finish pelishing (tertiary polishing process; of the principal surface polishing, the glass substrate for magnetic recording medium and the magnetic recording medium {magnetic disk) were manufactured according to the same method as Bxample 1 except for using a polishing liquid of which colloidal silica contained in the polishing ligudd has a primary particle diameter of 20

~2 0 recording medium were polished using a polishing brush and a polishing liguld containing a cerium oxide sbrasive, and a process modified layer {such as a fiow) of the immer peripheral side surface part and thes cuter periphery chamfered part was removed, and the inner peripheral surface was polished so as to be a mirror surface {an inner peripheral surface polishing process). The abrasive on the immer peripheral surface polished glass substrate was cleaned and removed.

After processing the edge surfaces cof the glass substrate, the upper and lower principal surfaces of the glase substrate was lapped using a fixed particle tool containing diamond abrasives and a grinding liguid, and was cleaned.

Then, glass substrate was primarvy-polished by a 228~type double side polisher (product name: DSMIZRE-GEV-

AME, manufactured by Spsedfam Co., Lid.) using a polishing brush made of rigid urethane and a polishing liguid containing a cerium oxide abrasive {a polishing liguid 28 composite material containing a cerium oxide of an average diameter of particles (hereinafter, abbreviated as an average particle diameter) of about 1.3 wn, so that a polished amount of the uppser and lower principal surfaces ig 20 um, and the cerium oxide was olsaned and removed.

Tt should be noted that 216 shests of glass substrate of ong lot were simultaneously polished.

The glass substrate after the primary-polishing was sacondarv-polished by the ZUB-type double side polisher using a polishing liguid containing a cerium oxide sbrasive having an average particle diameter smaller than the above-mentioned cerium oxide {a polishing liguid composite material containing a cerium oxide having an average particle diameter of about 0.5% wm} so that a polished amount of upper and lower principal surfaces is 5 wn, and the cerium oxide was cleanad and removed.

Tha glass substrate after the sscondary- polishing was subijectad to a tertiary-pelishing. The upper and lower principal surfaces were polished by a double side polisher using a rigid urethane made polishing pad as an abrasive tool of the tertiary-poelishing and a polishing liguid containing colleidal silica. The above- mentioned rigid urethane made polishing pad was attached to a polishing table, and, thereafter, a dressing process is applied, and, cleaning by pure water ls applied for a pradetermined Lime in some cases.

Moreover, the double side polisher was configured to be capable of supplying and ejecting a polishing liguid, and the polishing waz performed while adiusting an amount of supply of the polishing liguid so that a supply flow amcunt of the polishing liquid becomes a predetermined value.

A clsaning time of the polishing pad, a primary particle diameter of the colloidal silica, a pH value of the polishing liquid, a cohesive property of the colloidal silica contained in the polishing liquid, and a flow amount of the polishing liguid are described in the

Fzamples 1 through 10 mentioned later.

The cohesive property of the colloidal silica centained in the polishing liguid was evaluated by measuring a particle size distribution of the polishing liquid before polishing a glass subsirate and a particle size distribution of the polishing liquid after polishing a glass substrate using a particle size distribution measuring apparatus (FPAR~1G00 manufactured by QTSURA

ELECTRONICS CO., LTD), and a db0 value iz acquired from each of the particle size distributions, and calculating a changed amount {difference} in the d30 value before and after polishing the glass substrate.

It should ke noted that the above-mentioned dbl value meana a particle diameter causing an integrated value to be 50% when a number conversion is periommed from a scattered intensity distribution.

Specifically, evaluation was performed according fo an amount of change in the 450 value before and after polishing, which was caloulated hy the following relationship.

Frprassed by [450 value of polishing liquid after polishing {rm)] - [d50 value of polishing iiguid 1% beicre polishing {rom} |.

It was assumed that if an amcunt of change in the db0 values, which is calculated according to the above mentioned expression, is less than or sgual to 15 nm, aggregation of abrasive grains {colloidal silica) contained in the polishing licuid seldom ocours, theraby {A} dispersibility is good, and it was assumed that (B} aggregation occurs a little if the amount of change is 18 nm to 30 mm. Further, it was assumed that, 1f the amount . of change is egal to or 31 nm, {C) aggregation ocours to the extent of giving an influence to polishing of a principal surface of the glass substrate. in order to obtain the glass substrate for magnetic recording medium, which zatisfiss reguirements specified by the present invention, it is desirable to use the polishing liguid of (A) with which aggregation of abrasive grains contained in the polishing liguid seldom occurs and dispersibility is good. This is because, if a polishing liguid in which abrasive grains tend to aggregate is used, it is possible to cause creation of a part where a surface roughness Ra is locally high on a surface of a glass substrate for magnetic recording medium.

The glass substrate which had been finish- polished (tertiary polishing} was subjected to scrub cleaning, ultrasonic cleaning in a state where the glass substrate is immersad into a detergent solution, and ultrascnic cleaning in a state where the glass substrate is immersed into pure water, sequentially in that order (precizion cleaning).

The surface roughness Ha, which had been acguired according to the above-mentioned procedure with respect to an entire principal surface of the glass 13 substrate for magnetic recording medium, was evaluated according to the above-mentioned method.

Moreover, a magnetic recording medium was produced by depositing a multi-layer film having a magnetic laver according te the following procedures on a surface of the glass substrate for magnetic recording medium obtained according to the above-mentioned procedure, and evaluation was made with respect to the BP occurrence rate.

An NiFe layer as a soft magnetic substrate layer, an Ru laver as a nonmagnetic intermediate layer, and a gramilar structure layer of ColrPrSiC; were sequentially laminated on a surface of the glass substrate for magnetic recording medium, which has been cleaned before film deposition using an inline sputtering apparatus.

Subsecguently, a diamond-like carbon film was formed according to a CVD method. Thersafter, a lubrication film containing perflucre polyether was formed according to a

—F dG - dip method.

The EP error waz evaluated according to the above-mentioned method with respect to the obtained magnetic recording medium so as fo acquire an EP oocurrengs rate.

The conditions of the polishing ldiguid used in the finish polishing (tertiary polishing) of a principal surface are described in the following Example 1 through

Example 10. Examples 1 through 7 are practical examples, 1 and Examples § through 10 ave comparative examples.

The surface roughness Ra maximum value / surface roughness Ra mean value, the surface roughness Fa mean value {nm}, and the surface roughness Ba standard deviation (nm) in the entire principal surface of the glass substrate for magnetic recording medium processed according to the processing conditions of Examples 1 through 10 are indicated in Table 1. Morsover, the BF gocurrence rate (F) of the magnetic recording medium is aise indicated in Tables 1. 24 {Examples 1)

The finish polishing {tertiary polishing process) of the principal surface polishing was performed on the glass substrate for magnetic recording medium, which has been subjected to the processes until the secondary polishing of the principal surface polishing according to the above-mentioned procedure.

The finish polishing was performed using a rigid urethane made polishing pad, which has been cleaned by pure water for 10 minutes, a polishing liquid having pH4 and containing colloidal silica of which primary particle diameter as an abrasive grain is 12 mm. When the cohesive property of the abrasive grains {colicidal silica

— 7 5 - contained in the polishing liguid was evaluated, an amount of change in the d5%0 value of the polishing liguid after polishing was less than or equal to 15 nm, which indicates good {A}.

Moreover, the polishing was performed so that a flow amount of the polishing liquid supplisd to the surface to be polished in the double side polisher is set to & ml/min per one sheet of the glass substrate for magnetic recording medium.

The glass substrate for magnetic recording medium was obtained by applying precision cleaning on the glass substrate after the principal surface processing, and a surface roughness Ra of the principal surface of the glass substrate for magnetic recording medium was measured and evaluated.

Moreover, a magnetic recording medium was Iormed by depositing a multi-layer £ilm having a magnetic layer ont the surface of the glass substrate for magnetic recording medium, and the EF occurrence rate was evaluated. {(Ezampie 2}

In the finish polishing {tertiary polishing procasa) of the principal surface polishing, the glass substrate for magnetic recording medium and the magnetic recording medium (magnetic disk) were manufactured 25% according to the same method as Example 1 except for setting a flow amount of the polishing liguid supplied to tha surfaces to be polished in the double side polisher to 2 ml/min per one sheet of the glass substrate for magnetic vanording medium. 36 Tt should be noted that the cohesive property of the abrasive grains {colloidal silica) contained in the polishing liguid used in the finish polishing was good {A}

mm and setting a flow amount of the polishing liguld supplied to the surfaces to be polished in the double side polisher to 2 ml/min per one sheet of the glass substrate for magnetlce recording medium.

It should be noted that the cohesive property of the abrasive grains (colloidal silica) contained in the polishing liquid used in the finish polishing was good (A) the same as the case of Example 1.

Moreover, evaluation was performed with respect to the obtained glass substrate for magnetic recording madium and magnetic recording medium similar to Example 1.

Results are indicated in Table 1. {Example 5}

In the finish polishing {tertiary polishing 1% process) of the principal surface polishing, the glass substrate for magnetic recording medium and the magnetic recording medium {magnetic disk) were manufactured according to the same method as Example 1 except for using a polishing liquid of which colloidal silica contained in the polishing liguid has a primary particle diameter of 2% nm and setting a flow amount of the polishing liguid supplied to the surfaces to be polished in the double side polisher to © ml/min per one sheet of the glass substrate for magnetic rseording medium.

It should be noted that the cchesive property of the abrasive grains {ccolleidal silica) contained in the polishing liquid used in the finish polishing was good (A) the same as the case of Example 1.

Moreover, evaluation was performed with respect to the obtained glass substrate for magnetic recording medium and magnetic recording medium similar to Example 1.

Results are indicated in Table 1.

{Example 6)

In the finish polishing {tertiary polishing process) of the principal surface polishing, the glass substrate for magnetic recording medium and the magnetic recording medium {magnetic disk) were manufacturad according to the same method as Example 1 except for using a polishing Liguid of which colloidal silica contained in the polishing liguid has a primary particle diameter of 35 nim and setting a flow amount of the polishing liguid supplied to the surfaces to be polished in the double side colisher to 2 ml/min per one sheet of the glass substrate for magnetic recording medium.

It should be noted that the cohesive property of the abrasive grains (colloidal silica) contained in the 1% polishing iliguid used in the finish polishing was good {A}

The sams as the case of Examples 1. : Moreover, evaluation was performed with respect to the obtained glass substrate for magnetic recording medium and magnetic recording medium similar to Example 1.

Results are indicated in Table 1. {Example 7}

In the finish polishing (tertiary polishing procaesa) of the principal surface polishing, the glass substrate for magnetic recording medium and the magnetic <5 recording medium (magnetic disk) were manufacturad according to the same method as Exsaple 1 sxzcept for setting the polishing liguid to pH and setting a flow amount of the polishing ligaid supplied to the surfaces to be polished in the double side polisher to 2 ml/min per one sheet of the glass substrate for magnetic recording medium.

It should be noted that the ccheslive property of

Pe the abrasive grains {colloidal silica} contained in the polishing liguid used in the finish polishing was good {A} the same as the case of Example 1.

Results ave indicated in Table 1. {Example 8)

In the finish polishing {tertiary polishing process) of the principal surface polishing, the glass substrate for magnetic recording medion and the magnetic recording medium {magnetic disk) were manufactured according to the same method as Example 1 except for using za polishing pad which had not been subjected to cleaning by pure water and setting a flow amount of the polishing liguid supplied te the surfaces to be polished in the double side polisher to 2 ml/min per one sheet of the glass substrate for magnetic recording medium.

It should be noted that because the particle size distribution d50 of the polishing liquid was changed by 31 nm or more before and after the polishing, the cohesive property of the abrasive grains {colloidal silica) contained in the polishing liguid used in the finish polishing was (C).

Results are indicated in Table 1. (Example 9} 34 In the finish polishing (tertiary polishing process) of the principal surface polishing, the glass substrate for magnetic recording medium and the magnetic

~3 {0 - recording medium (magnetic disk) were manufactured according to the same method as Example 1 except for setting the cleaning time of the polishing pad by pure water to 5 minutes and seriting a flow amount of the polishing liquid supplied te the surfaces to be pelished in the double side polisher to 2 ad/min per one sheet of

Lhe glass substrate for magnetic rseording medium,

Tt should be noted that because an amount of change in the d50 value of the pelishing liguid was greater than or egual to 186 nm and less than or egual to 30 nm, the cohesive property of the abrasive grains {colloidal silica) contained in the polishing liquid used in the finish polishing was (B).

Moreover, evaluation was performed with respect 1% to the obtained glass substrate for magnetic recording medium and magnetic recording medium similar fo Example 1.

Results are indicated in Table 1. {Example 10}

In the finish polishing (tertiary polishing process) of the principal surface polishing, the glass substrate for magnetic recording medium and the magnetic recording medium {magnetic disk) were manufactured according to the same method as Example 1 except for setting the polishing liguid to pHEZ and setting a flow amount of the polishing liquid te 2 ml/min per one sheet of the glass substrate for magnetic recording medium.

Tt should be noted that because an amount of change in the d50 value of the polishing liguid was less than or egual to 15 nm, the cohesive property of the abrasive grains {colloidal silica’ contained in the polishing liguid used in the finish polishing was good (A).

Results are indicated in Table 1.

According to the results of Table 1, 1t was confirmed that in any one of the magnetic recording medium produced using the glass substrates for magnetic recording medium of Examples 1 through 7, which satisiy reguirements specified by the present invention, the EP cccurrence rate was a small value less than or equal to 1.46. On the other 1% hand, it is appreciated that the EP occurrence rate of

Examples 8 through 10, which are comparative examples, 1s as high as 2.5% even for one having a low BEF cocourrence rate,

It is considered that this is because the crystal grains are suppressed from being large locally when forming the magnetic recording medium by forming a magnetic layer or the like because the glass substrate for magnetic recording medium of the present invention does not: have an area where a surface roughness Ra iz locally high on an entire principal surface thereof and the entire surface of the principal surface is flat and smooth.

For this reason, it is considered that a magnetic nolse is suppressed from being generated, and an aceuracy of reading-and-writing on a magnetic recording medium by a magnetic head can be improved and a recording density can be prevented from being decreased.

=37- iTable 13

Free g Fe) = 3 i } 3 = w=

ES — ofS 3 * o : <

Li © o 3 —

Ee a | <2 = on ir3 5 on <3 < od = oS <i id

To 4 \ ar > id Er = 5 eS = x LS = ad

Maacatteeesast: A A 4 A SE RR] rr a

IIR a o 3 2 « gs < — < < o x < < ul 3 ; . . hosssonfesmsssssssedeinmm ms

ES 2 5 o —

SEE IE 3 = oz

ITD D0 => os 5 = 2 SC EZF = & 4

FO ol = 7 = oe == & i -2 Lid i we = =i TS DE wd Bug buen in wd [SR

ESExxiE HiEgx od ee 3 =

Bh @ 2 1%] ees SE

WB Le bm 22 TD bm memo Ta a0 SEP EEL Zn

A wd £3 LL o> <> sRL-584 E82 = Eg = = =

Claims

1. A glass substrate for magnetic recording medium, comprising: a palr of principal surfaces; an outer circumference edge surface; and inner clrcumferential edge surface, wherein, in at least one of the principal surfaces, a mean value of surface roughness Ra measured in each of evaluation areas in a lattice foxm that are set on an entire surface of the principal surface, which contains a recording and reproduction area when used as a magnetic recording medium, is less than or equal to

0.07 nm, each of the evaluation areas having an area of um? to 2500 pm®, and a maximum value of said surface roughness Ra is less than or equal to 1.4 times the mean value of said surface roughness Ra.

2. The glass substrate for magnetic recording medium as claimed in claim 1, wherein the maximum value of the surface roughness Ra measured in each of the evaluation areas in a lattice form that are set -on the antire surface of the principal surface, which contains the recording and reproduction area when used as said magnetic recording medium, is less than or egual to 1.3 times the mean value of said surface roughness Ra.

3. The glass substrate for magnetic recording medium as claimed in claim 1 or 2, wherein a standard deviation of the surface roughness Ra measured in each of the evaluation areas in a lattice form that are set on the entire surface of the principal surface, which contains the recording and reproduction area when used as sald magnetic recoxding medium, is less than or equal toe 0.012 nm.

4. The glass substrate for magnetic recording medium as claimed in any one of claims 1 to 3, wherein the mean value of the surface roughness Ra measured in each of the evaluation areas in a lattice form that are set on the entire surface of the principal surface, which contains the recording and reproduction area when used ag sald magnetic recording medium, is less than or equal to 0.06 nm.

5. The glass substrate for magnetic recording medium as claimed in any one of claims 1 to 4, wherein a shape of each of the evaluation areas in a lattice form that are set on the entire surface of the principal surface, which contains the recording and reproduction area when used as said magnetic recording medium, is one or more kinds of shapes selected from a group consisting of a square lattice, a triangular lattice, a hexagonal Lattice, a diamond lattice, a rectangulax lattice and parallelogram lattice.

6. The glass substrate for magnetic recording medium as claimed in any one of claims 1 to 5, wherein said surface roughness Ra is measured using a scanning interference microscope.

7. The glass substrate for magnetic recording medium as claimed in any one of claims 1 to 6, wherein a shape of each of the evaluation areas in a lattice form that are set on the entire surface of the principal surface, which contains the recording and reproduction area when used as said magnetic recording medium, is a square lattice having a side of which length is 5 um to 50 um.

8. A magnetic recording medium using the glass substrate for magnetic recording medium as claimed in any one of claims 1 to 7.