Method of texturing magnetic hard disk substrate

Info

Publication number: 20070087668
Type: Application
Filed: Oct 4, 2006
Publication Date: Apr 19, 2007
Applicant:
Inventors: Yasuyuki Yokota (Tokyo), Takashi Arahata (Tokyo), Hiromitsu Okuyama (Tokyo)
Application Number: 11/543,578

Abstract

Texturing marks are formed on the surface of a substrate of a magnetic hard disk first by forming approximately concentric circular preliminary marks in a first step and then forming in a second step approximately concentric circular texturing marks on the surface of the substrate based on the preliminary marks formed in the first step. The surface after the texturing marks are formed in the second step has average surface roughness in the range of 1 Å or more and 6 Å or less and a ratio of maximum surface roughness to the average surface roughness in the range of less than 10. A foamed tape and a lubricant not containing any abrading particles are used in the second step. The foamed tape has a foamed layer having average diameter of air bubbles in the range of 1 μm or more and 50 μm or less, compressibility in the range of 3% or more and 7% or less, compression recovery ratio in the range of 40% or more and 60% or less, Shore D hardness in the range of 20 degrees or more and 30 degrees or less, and thickness in the range of 50 μm or more and 80 μm or less.

Description

Description

This application claims priority on Japanese Patent Application 2005-299521 filed Oct. 14, 2005.

BACKGROUND OF THE INVENTION

This invention relates to a texturing method for forming approximately concentric circular texturing marks on the surface of a magnetic hard disk substrate.

Magnetic hard disks are being used as a medium for recording data such as sound and image for data recording and reproducing apparatus such as computers. A magnetic hard disk is generally produced by mirror-polishing the surface of a non-magnetic substrate such as a glass substrate or an aluminum substrate with Ni—P plating, carrying out a texturing process on its surface to form approximately concentric circular texturing marks thereon and sequentially forming a magnetic layer, a protective layer, etc. on this textured surface by using a known thin-film technology such as sputtering.

As known to persons skilled in the art, the texturing process is for forming approximately concentric circular line marks on the surface of a magnetic hard disk similar to the texturing marks formed on the surface of the substrate in order to prevent the adsorption of the magnetic head to the magnetic hard disk.

For the purpose of carrying out magnetization of a magnetic hard disk accurately for accurate recording and reproduction, the surface of the magnetic hard disk must satisfy the following four topological conditions.

(1) Firstly, the pitch of the line marks formed on the surface of the magnetic hard disk must be made smaller. In other words, if the number of line marks per unit length in the radial direction of the disk is increased, the number of protruding parts of the linear marks per unit area facing the magnetic head (or the surface portions of the magnetic hard disk near the magnetic head) increases such that it becomes possible to carry out the magnetization of the magnetic hard disk more accurately. In recent years, the number of line marks per unit length in the radial direction of the disk, or the line density, is coming to be required to be 40 lines/μm or more.

(2) Secondly, deep indentations (deep indentations of line marks and scratches) must not be formed on the surface of the magnetic hard disk. This is because, if these indented parts are too deep, magnetic flux from the magnetic head does not reach the magnetic layer near the bottom of the indented parts and cannot magnetize these parts. This makes accurate recording and reproduction impossible. It is also because a magnetic layer may fail to be formed near the bottom of the indented parts at the time of forming a thin film by sputtering.

(3) Thirdly, abnormal protrusions reaching the floating distance of the magnetic head must not be formed such that the magnetic head can fly at a low height. This is because, if the magnetic head collides with such protrusions, the magnetic head may be damaged and the pieces of the protrusions will become attached to the surface of the magnetic hard disk such that accurate recording to and reproduction from the magnetic hard disk become impossible. In recent years, floating distances of 10 nm or less are being required.

(4) Fourthly, the surface roughness of the magnetic hard disk must be made low such that the magnetic head can slide smoothly on the surface of the magnetic hard disk after landing thereon and before floating up therefrom.

In summary, it is required to form line marks having indentations with an appropriate depth and protrusions with an appropriate height, and such topological surface conditions of a magnetic hard disk depends largely on the texturing process carried out on the surface of its substrate.

As described in Japanese Patent Publication Tokkai 2005-131711, texturing is conventionally carried out by supplying slurry having abrading particles dispersed therein to the surface of the substrate and pressing a tape onto the surface of the substrate and it is becoming possible to form texturing marks with a small pitch without forming abnormally high protrusions by correctly selecting the kind and size of the abrading particles and the kind of the tape.

With such prior art technologies, however, abrading particles and polishing debris that remain on the substrate surface after the texturing process are removed only by blowing a washing liquid onto the substrate surface. Thus, the magnetic layer and the protective layer are now being formed on a textured surface under such conditions that abnormally deep indentations and scratches may be left, the surface roughness may not be reduced to the level for allowing the magnetic head to slide smoothly on the surface or be adjusted to the level for preventing the adsorption of the magnetic head to the surface. FIG. 4 is a computer-generated image of a substrate surface after a prior art texturing process.

In view of the above, there is a demand for forming texturing line marks also on the surface of a substrate having indentations with an appropriate depth and protrusions with an appropriate height in order to satisfy the aforementioned topological conditions required of the surface of a magnetic hard disk. As one of judgment standards in the technological field of texturing, the magnitude of the ratio of the maximum surface roughness (Rmax) with respect to the average surface roughness in the peripheral direction of the substrate (Ra) (or the average value of the height difference of unevenness formed on the surface) is coming to be considered on the textured surface in recent years. In recent years, the value of this ratio Rmax/Ra is required to be less than 10. It is also being required that the average surface roughness Ra of the substrate be 1 Å or more and 6 Å or less.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a texturing method capable of forming texturing marks having indentations with an appropriate depth and protrusions with an appropriate height on the surface of a magnetic hard disk substrate.

The present invention therefore relates to a texturing method for forming texturing marks on the surface of a substrate of a magnetic hard disk, and the method of this invention is characterized as comprising a first step of forming approximately concentric circular preliminary marks on the surface of the substrate and a second step of forming approximately concentric circular texturing marks on the surface of the substrate based on the preliminary marks formed in the first step. The surface after the texturing marks are formed in the second step is characterized as having average surface roughness in the range of 1 Å or more and 6 Å or less and a ratio of maximum surface roughness to the average surface roughness in the range of less than 10.

The first step comprises the steps of rotating the substrate, supplying slurry having abrading particles dispersed to the surface of the substrate and pressing a tape to the surface of the substrate. The tape to be used may be of a woven or non-woven cloth material. The first step may further include the step of washing the surface of the substrate after the aforementioned approximately concentric circular marks have been formed.

The second step comprises the steps of rotating the substrate, supplying a lubricant (not containing any abrading particles) to the surface of the substrate with the preliminary marks already formed thereon, and pressing a foamed tape on the surface of the substrate. The second step may further include the step of washing the surface of the substrate after the aforementioned approximately concentric circular texturing marks have been formed.

The foamed tape used in the second step comprises a base material formed in the shape of a tape and a foamed layer formed on the surface of the base material. A plastic sheet with thickness in the range of 25 μm or more and 125 μm or less may be used as the base material.

The foamed layer is characterized as having average diameter of air bubbles in the range of 1 μm or more and 50 μm or less, compressibility in the range of 3% or more and 7% or less, compression recovery ratio in the range of 40% or more and 60% or less, Shore D hardness in the range of 20 degrees or more and 30 degrees or less, and thickness in the range of 50 μm or more and 80 μm or less.

With a cleaning sheet of this invention, the surface area portion of the foamed layer (exclusive of the air bubble portions) is large because the average diameter of the air bubbles inside is as small as in the range of 1 μm or more and 50 μm or less and preferably 30 μm or less. Since the compressibility of the foamed layer is in the range of 3% or more and 7% or less, the foamed layer is compressed such that its surface will follow the shape of the surface of the workpiece when the surface of the foamed layer is pressed against the surface of the workpiece. Since the compression recovery ratio of the foamed layer is in the range of 40% or more and 60% or less, the surface of the foamed layer moves on the surface of the workpiece such that the surface of the foamed layer follows the surface of the workpiece as the foamed layer is moved relative to the workpiece. In other words, the surface of the foamed layer has a good characteristic of following the surface of the workpiece. Since the Shore D hardness of the foamed layer is in the range of 20 degrees or more and 30 degrees or less, that is, since its hardness is sufficiently low, unwanted protrusions formed on the surface of the workpiece and foreign objects and dirt attached to the surface of the workpiece can be easily removed.

Because the foamed layer has such mechanical characteristics, the foreign objects and dirt attached to the surface of the substrate can be removed in the second step without forming scratches on the surface of the substrate and without excessively scraping the surface of the substrate and indentations with an appropriate depth and protrusions with an appropriate height can be formed such that the ratio Rmax/Ra is less than 10 and Ra is in the range 1Å or more and 6 Å or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a polishing machine which may use a method of this invention.

FIG. 2 is a sectional view of a foamed tape used in the second step of the method of this invention.

FIG. 3 is a computer-generated surface image of the foamed layer of the foamed tape of FIG. 2, obtained by a scanning electron microscope (SEM).

FIG. 4 is a computer-generated surface image of a substrate after the first step of the method of this invention (which is substantially the same as a conventional texturing step), obtained by a scanning electron microscope.

FIG. 5 is a computer-generated surface image of a substrate after the second step of the method of this invention, obtained by a scanning electron microscope.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a texturing method for forming texturing marks on the surface of a magnetic hard disk substrate such as a glass substrate and an aluminum substrate.

A method of this invention may be carried out by using a double-side polishing machine as shown at 10 in FIG. 1 to form texturing marks on both surfaces at the same time or by using a single-surface polishing machine (not shown) of any kind known to persons skilled in the art to form texturing marks on only one surface at a time. Throughout herein “surface of a substrate” and “surfaces of a substrate” are both intended to mean not only one of the surfaces of the substrate but also both surfaces of the substrate.

A texturing method of this invention comprises a first step of forming approximately concentric circular preliminary marks on the surface of a substrate and a second step of forming approximately concentric circular texturing marks on the surface of the substrate based on the aforementioned preliminary marks such that the average surface roughness of the surface of the substrate after the texturing marks are formed is in the range of 1 Å or more and 6 Å or less and that the ratio of the maximum surface roughness Rmax to the average surface roughness Ra on the surface of the substrate with the texturing line marks formed is in the range of 10 or less. The first step and the second step of the method of this invention may be carried out by using the same double-side or single-side polishing machine or separately by using different polishing machines but it is preferable to carry them out by using the same polishing machine.

With reference to FIG. 1, the first step comprises the steps of rotating the substrate 21 in the direction of arrow R, supplying slurry having abrading particles dispersed to the surfaces of the substrate 21 through nozzles 12 and pressing tapes 20 onto the surfaces of the substrate 21 by means of contact rollers 11. The tapes 20 are delivered in the direction of arrow T opposite to the direction R of rotation of the substrate 21. Approximately concentric circular line marks are thus formed on the surfaces of the substrate 21, say, as shown in FIG. 4.

After approximately concentric circular line marks are thus formed on the surface of the substrate 21, a washing liquid such as water is blown onto the surfaces of the substrate 12 through nozzles 13 such that foreign objects such as abrading particles remaining on the surfaces of the substrate 21 and polishing debris are removed. The removal of these foreign objects is carried out by blowing the washing liquid onto the surfaces of the substrate 12 while the substrate 12 is kept in the rotating condition after the tapes 20 are separated from the surfaces of the substrate 21 and discharging these objects from the surfaces of the substrate 12 together with the washing liquid by utilizing the centrifugal force of the rotating substrate 21.

The slurry is obtained by dispersing abrading particles in a dispersant. Particles of materials selected from alumina, ceria, silica and diamond are used as the abrading particles. The size of the abrading particles is such that their average diameter is in the range of 0.02 μm or more and 0.5 μm or less. A preferred example of the abrading particles is cohesive polycrystalline diamond particles which are secondary particles having diameters within the range of 20 nm or more and 150 nm or less with a plurality of polycrystalline diamond particles combined together, wherein the primary particles of these polycrystalline diamond particles have diameters in the range of 30 nm or less and an average diameter in the range of 4 nm or more and 10 nm or less. The content of abrading particles in the slurry is 0.001 weight % or more and 0.5 weight % or less.

The dispersant comprises water and an additive, the additive including at least two kinds selected from higher fatty acid amides, glycol compounds, organic esters of phosphoric acid and surfactants. The content of the additive with respect to the whole of the slurry is in the range of 0.5 weight % or more and 5.0 weight % or less. Since no significant change is obtained on the surface of the substrate after the first step even if the slurry contains more than 5.0 weight % of additive, it is preferable to set the upper limit of the content at 5.0 weight % in order to reduce the cost of the slurry.

Higher fatty acid amides function as a process accelerator for increasing the processing speed of the first step. Examples of higher fatty acid amide that may be used include oleic acid diethanolamide, stearic acid diethanolamide, lauric acid diethanolamide, ricinolic acid diethanolamide, ricinolic acid isopropanolamide, ersinic acid diethanolamide, and tol fatty acid diethanolamide. Among these example, those with 12-22 carbon atoms are preferred. The content of higher fatty acid amide with respect to the whole of the additive is in the range of 20 weight %-60 weight %. If the content is less than 20 weight %, the process speed becomes low. If it exceeds 60 weight %, abnormal protrusions are generated.

Glycol compounds have affinity with abrading particles and function as a dispersant. If a glycol compound is used when a dispersing medium is prepared, it serves to reduce the viscosity of the medium and hence a medium can be prepared more uniformly. Since they have affinity with water, the substrate can be washed more effectively after the first step. Examples of glycol compound that may be used include alkylene glycol, polyethylene glycol, polypropylene glycol and diethylene glycol butylether. The content of glycol compound with respect to the whole of the additive is in the range of 20 weight %-60 weight %. If it is less than 20 weight %, the dispersion characteristic of abrading particle is adversely affected such that it becomes easier for abrading particles to sink and large cohesive particles are formed. If it exceeds 60 weight %, it becomes hard to form clear texturing marks.

Organic esters of phosphoric acid have the function of inhibiting the generation of abnormal protrusions (burrs formed by polishing debris attaching to the substrate surface) on the surface of the substrate. They are esters obtained by replacing hydrogen of phosphoric acid H₃PO₄with alkyl group or allyl group. Fatty acid salt type and aromatic salt type may be used. For example, phosphoric acid salt of polyoxyethylene nonylphenolether may be used. The content of organic ester of phosphoric acid with respect to the whole of the additive is in the range of 5 weight %-40 weight %. If it is less than 5 weight %, abnormal protrusions are generated. If it exceeds 40 weight %, it becomes hard to form clear texturing marks.

Surfactants have the effect of improving dispersing characteristic of abrading particles. Surfactants of nonion or anion type can be used. The content of surfactant with respect to the whole of the additive is in the range of 20 weight % or less.

The slurry is obtained by adding abrading particles into water, further adding thereinto an additive including at least two agents selected from higher fatty acid amides, glycol compounds, organic esters of phosphoric acid and surfactants and mixing them by using a homo-mixer.

As the tape 20, a porous tape capable of acting elastically on the substrate surface and taking in foreign objects such as polishing debris inside is used. A woven or non-woven cloth tape having at least its surface portion made of fibers with diameter in the range of 0.1 μm or more and 2.0 μm or less may be used as such a tape.

FIG. 4 shows approximately concentric circular line marks formed on a substrate after the first step. Although no abnormal protrusions higher than 10 nm are formed on the substrate surface, there are abnormal protrusions with sectional shape of a pointed tower formed locally. Such abnormal protrusions cast shadows at the time of forming a thin film, say, by sputtering, and cause spots on the formed film such as the magnetic layer. These abnormal protrusions can also cause collisions with the magnetic head gliding above the substrate surface, adversely affecting the quality of recording and reproduction by the magnetic hard disk.

According to prior art technologies, a magnetic layer and a protective layer are formed on the substrate surface left with these line marks after the first step by using a thin film technology such as sputtering. According to the present invention, on the other hand, the second step is carried out to remove these abnormal protrusions such as spots having ill effects on the recording and reproduction by the magnetic hard disk, making the average surface roughness Ra in the range of 1 Å or more and 6 Å or less and the ratio Rmax/Ra less than 10.

In other words, the second step of the texturing method according to this invention is for removing unwanted protrusions, burrs, scratches, foreign objects and dirt remaining on the substrate surface after the first step such that texturing marks are formed on the substrate surface more accurately and this is done by trimming and cleaning the substrate surface. FIG. 5 shows the approximately circular texturing marks formed on the substrate surface after the second step. As shown, the abnormal protrusions left on the substrate surface after the first step are removed by the second step.

The same polishing machine used in the first step may be used for the second step or a different machine may be used for the purpose. Preferably, the polishing machine 10 used in the first step is also used for the second step. For the purpose, therefore, the tapes 20 (of woven or non-woven cloth) used in the first step are removed after the first step and replaced with tapes 30 of a foamed material.

With reference to FIG. 1, the second step comprises the steps of rotating the substrate 21 in the direction shown by arrow R, supplying a lubricant not containing any abrading particles to the surfaces of the substrate 21 having the aforementioned line marks formed thereon through nozzles 14 and pressing the foamed tapes 30 onto the surfaces of the substrate 21 through the contact rollers 11. The foamed tapes 30 are advanced in the direction shown by arrow T opposite the direction of rotation R of the substrate 21. In this manner, the surfaces of the substrate 21 are trimmed and cleaned and texturing line marks as described above are formed on them.

After the texturing marks are formed on the substrate surfaces based on the aforementioned line marks, a washing liquid such as water is blown onto the surfaces of the substrate 12 though the nozzles 13 such that foreign objects remaining on the surfaces are removed. The removal of these foreign objects is carried out by blowing the washing liquid onto the surfaces of the substrate 12 while the substrate 12 is kept in the rotating condition after the foamed tapes 30 are separated from the surfaces of the substrate 21 and discharging the foreign objects from the surfaces of the substrate 12 together with the washing liquid by utilizing the centrifugal force of the rotating substrate 21.

Water or an aqueous solution may be used as the lubricant. The aqueous solution is prepared by adding to water an additive that can react chemically with the surfaces of the substrate. At least two kinds of agents selected from higher fatty acid amides, higher fatty acids, metallic salts of higher fatty acids, glycol compounds and organic esters of phosphoric acid are added to water to produce such an additive.

The amount of the additive to be added with respect to the whole of the lubricant is in the range of 0.5 weight % or more and 10 weight % or less. Since no significant change is found on the substrate surfaces after the second step even if the additive is added to the lubricant in an amount in excess of 10 weight % and it only takes more time for the washing in the second step, it is preferable to set the upper limit of the content to be 10 weight %.

The lubricant is alkaline, it being preferable that its pH value be pH7 or over and pH12 or less. This is such that the surfaces of the substrate will not be overly scraped and that foreign objects such as particles and oils attached to the substrate surfaces can be removed. If the lubricant is acidic, the unevenness of the textured marks formed on the surfaces of the substrate in the first step are excessively polished and the texturing marks become unclear.

Higher fatty acid amides are used for removing the burrs and abnormal protrusions remaining on the substrate surfaces after the first step. Examples of higher fatty acid amide that may be used include oleic acid diethanolamide, stearic acid diethanolamide, lauric acid diethanolamide, ricinolic acid diethanolamide, ricinolic acid isopropanolamide, ersinic acid diethanolamide, and tol fatty acid diethanolamide. Among these example, those with 12-22 carbon atoms are preferred. The content of higher fatty acid amide with respect to the whole of the additive is in the range of 10 weight % or more and 50 weight % or less. If the content is less than 10 weight %, the removal of attached objects from the substrate surfaces become less effective. If it exceeds 50 weight %, the texturing marks formed on the substrate surfaces are excessively scraped and the marks become unclear.

Higher fatty acids and their metallic salts are effective agents for removing foreign objects remaining attached to the substrate surfaces after the first step. The metallic salts of higher fatty acids includes metallic salts such as Na, K, Al and Ba of saturated or unsaturated fatty acid. Examples of fatty acid include stearic acid, palmitic acid, myristic acid, oleic acid, lauric acid and behenic acid and those with 12-22 carbon atoms are preferred. The content of salts of higher fatty acid with respect to the whole of the additive is in the range of 10 weight % or more and 50 weight % or less. If it is less than 10 weight %, the removal of attached objects from the substrate surfaces become less effective. The removal characteristic is not much affected, on the other hand, if the content exceeds 50 weight %.

When higher fatty acid is added, alkanolamine is further added in order to improve the affinity with water. The amount of alkanolamine to be added with respect to the whole of the additive is in the range of 10 weight % or more and 60 weight % or less. If it is less than 10 weight %, the lubricant may become white and opaque. Although it is increased to more than 60 weight %, the affinity to water does not change significantly.

Glycol compounds serve to reduce the viscosity of the lubricant when it is prepared and makes it easier to wash the substrate after the first step. Examples of glycol compound that may be used include alkylene glycol, polyethylene glycol, polypropylene glycol and diethylene glycol butylether. The content of glycol compound with respect to the whole of the additive is in the range of 5 weight %-50 weight %.

Organic esters of phosphoric acid have the function of inhibiting the generation of abnormal protrusions (burrs formed by polishing debris attaching to the substrate surface) on the surface of the substrate. They are esters obtained by replacing hydrogen of phosphoric acid H₃PO₄with alkyl group or allyl group. Fatty acid salt type and aromatic salt type may be used. For example, phosphoric acid salt of polyoxyethylene nonylphenolether may be used. The content of organic ester of phosphoric acid with respect to the whole of the additive is in the range of 5 weight %-40 weight %. If it is less than 5 weight %, abnormal protrusions are generated. If it exceeds 40 weight %, the texturing marks formed on the substrate surfaces in the first step are excessively scraped and their indentations and protrusions become unclear.

As shown in FIG. 2, the foamed tape 30 is comprised of a tape-shaped base material 31 and a foamed layer 32 formed on the surface of this base material 31. FIG. 3 shows a computer-generated surface image of this foamed layer by a scanning electron microscope (SEM).

The thickness of the base material 31 is in the range of 25 μm or more and 125 μm or less. The base material 31 is a plastic sheet with a flat and smooth surface and has a uniform thickness. A sheet made of a synthetic resin material such as polyester and. polyethylene terephthalate (PET) is used as the plastic sheet.

The average air bubble diameter of the foamed layer is in the range of 1 μm or more and 50 μm or less, and preferably in the range of 1 μm or more and 30 μm or less.

Since the aforementioned second step is carried out as a wet process by supplying a liquid lubricant between the surface of the foamed layer 32 and the surface of the substrate 21, if the average air bubble diameter is less than 1 μm, the lubricity of the liquid between the surfaces of the foamed layer 32 and the substrate 21 becomes low and it becomes difficult to take in the foreign objects removed from the surface of the substrate 21 into the interior of the foamed layer 32. If the average air bubble diameter exceeds 50 μm, on the other hand, the surface portion of the foamed layer 32 (exclusive of the air bubble portions) acting on unit surface area of the substrate 21 such that not only does it take longer for the trimming and cleaning of the surface of the substrate 21 but the lubricity of the liquid also becomes too large inside the foamed layer 32 and between the surface of the foamed layer 32 and the substrate 21 and the foreign objects once taken in are ejected out and cause to scrape the surface of the substrate 21 excessively or to form scratches on the surface of the substrate 21.

The compressibility of the foamed layer 32 is in the range of 3% or more and 7% or less, the compressibility being defined as the change in the thickness of the foamed layer when the load thereon is 16 psi from the thickness at the time when the load therein is 1.4psi as measured under the environmental condition of 23±3° C.

The surface of the foamed layer 32 becomes compressed as it is pressed against the surface of the substrate 21. If the compressibility of the foamed layer 32 is less than 3%, it becomes difficult for the surface of the foamed layer 32 to be pressed by following the shape of the surface of the substrate 21 such that the compressive force of the surface part (exclusive of the air bubble portions) of the foamed layer 32 on the surface of the substrate 21 becomes non-uniform and spots come to be formed on the finished or cleaned surface of the substrate 21. If the compressibility of the foamed layer 32 exceeds 7%, on the other hand, the thickness of the foamed layer 32 becomes too small when the surface of the foamed layer 32 is pressed against the surface of the substrate 21 and the volume for taking is liquid such as the cleaning liquid becomes significantly reduced. As a result, the lubricity of the liquid such as the cleaning liquid inside the foamed layer 32 and between the surface of the foamed layer 32 and the surface of the substrate 21 becomes low and it becomes difficult to take in the foreign objects scraped off the surface of the substrate into the interior of the foamed layer 32.

The compression recovery ratio of the foamed layer 32 is in the range of 40% or more and 60% or less, the compression recovery ratio being obtained by measuring the displacement of the foamed layer 32 under a load of 12 psi under the environmental condition of 23±3° C. After the load is reduced to 1.6 psi, the recovered displacement in 30 seconds is measured and this measured displacement is divided by the aforementioned displacement at the time of the load of 16 pse, that is, the percentage ratio of recovered displacement with respect to the compressed displacement.

If the compression recovery ratio of the foamed layer 32 is less than 40%, the force of recovery by the compressed foamed layer 32 is too low, and the pressure of the surface portion (exclusive of the air bubble portions) of the foamed layer 32 on the surface of the substrate becomes low such that the force for removing the foreign objects attached to the surface of the substrate 21 becomes low. If the compressive recovery ratio of the foamed layer 32 exceeds 60%, on the other hand, the recovery force of the compressed foamed layer 32 becomes too strong and the pressure of the surface portion (exclusive of the air bubble portions) of the foamed layer 32 on the surface of the substrate becomes high such that, as the objects scraped off the surface of the substrate 21 are pressed onto the surface of the substrate 21, scratches are formed by them on the surface of the substrate 21.

The Shore D hardness of the foamed layer 32 is in the range of 20 degrees or more and 30 degrees or less, the Shore D hardness being the measured value under the environmental condition of 23±3° C. by using a Shore D hardness meter according to JIS-L-1096.

If the Shore D hardness of the foamed layer 32 is less than 20 degrees, the force of removing foreign objects attached to the surface of the substrate 21 and the unwanted protrusions (abnormal protrusions) formed on the surface of the substrate 21 becomes low. If the Shore D hardness of the foamed layer 32 exceeds 30 degrees, on the other hand, not only the unwanted protrusions (abnormal protrusions) formed on the surface of the substrate 21 but also necessary protrusions formed on the surface of the substrate 21 such as the protrusion parts of the texturing lines are scraped off, and it also becomes easier to form scratches on the surface of the substrate 21.

The thickness of the foamed layer 32 is in the range of 50 μm or more and 800 μm or less. If the foamed layer 32 is too thin, lubricity of the liquid such as the cleaning liquid inside the foamed layer 23 and between the surface of the foamed layer 32 and the surface of the substrate 21 cannot be maintained at a high level and foreign objects cannot be effectively taken into the interior of the foamed layer 21 for a long time. If the foamed layer 32 is too thick, spots are generated as foreign objects attached to the surface of the substrate 21 and dirt are removed. This is considered to be because the foamed layer 32 deforms significantly in the direction of its surface during the trimming and cleaning processes and the geometrical structure of the foamed layer 32 itself is significantly deformed.

The foamed tapes 30 are obtained by cutting a foamed sheet produced as will be explained below into the form of a tape.

For producing the foamed sheet, a resin solution is mechanically stirred to obtain a paint having air bubbles with average diameter in the range of 1 μm or more and 50 μm or less (preferably in the range of 1 μm or more and 30 μm or less) and foam magnification in the range of 2× or more and 5× or less.

The resin solution is one containing urethane resin or acryl resin, and preferably self-emulsifying aqueous urethane resin. In the above, aqueous urethane means waterborne polyurethane dispersion (WBPUD) obtained either by introducing into the main chain of polyurethane a hydrophilic component for dispersing stably in water or by dispersing with an external emulsifier. Those obtained by the former method, or by directly introducing a hydrophilic component into the main chain of polyurethane, are referred to as self-emulsifying aqueous urethane resin. (See, for example, “Recent Development in Technology of Waterbome Polyurethane Dispersion” by Toshifumi Tamaki, Dainippon Ink and Chemicals, Inc.; http://www.dic.co.jp/rd/tech/rev0301/index.html). If self-emulsifying aqueous urethane is used, particles like aluminum hydroxide powder functioning as abrading particles need not be used. In other words, hard particles which may become one of the causes for scraping the substrate surface excessively need not be used as external emulsifier.

The resin solution may further contain an agent for accelerating the foaming of this resin solution and for dispersing air bubbles stably inside the paint. Such an agent is selected from higher fatty acids, denaturations of higher fatty acids and alkali salts of higher fatty acids. This agent is contained preferably at a rate of 30 weight parts or less as solid component and more preferably at a rate of 20 weight parts or less as solid component for 100 weight parts of resin solution as solid component. If more than 30 weight parts as solid component are contained, there is no significant change in the function of accelerating the foaming of the resin solution or dispersing air bubbles stably inside the paint. Higher fatty acid ammonium may be used conveniently as an example of this agent.

The resin solution can be mechanically stirred by placing the resin solution inside a container and rotating stirring vanes. For example, a continuous high-pressure foaming machine (such as TW-70 (trade name) produced by Aikosha Seisakusho) may be used. The size of the air bubbles dispersed inside the paint and their foaming magnification can be adjusted by appropriately setting the rotational speed of the stirring vanes, the quantities of the resin solution and air and the time of stirring.

Next, this paint is applied to the surface of the sheet-like base material to form a film comprising this paint on the surface of the base material. The application of the paint can be carried out by any of the known coating methods such as the blade method, the gravier roll method, the knife method, the extrusion method, the reverse roll method and the cast method.

The coated film is dried next to form on the surface of the base material a foamed layer with average bubble diameter in the range of 1 μm or more and 50 μm or less, compressibility in the range of 3% or more and 7% or less, the compression recovery ratio in the range of 40% or more and 60% or less and the Shore D hardness in the range of 20 degrees or more and 30 degrees or less.

The coated film is dried in an environment of 90° C.-160° C. In order to completely harden the coated film, far-infrared light may be used. A foamed layer described above is thus formed.

The invention is described next by way of sample substrates of Test Examples 1-3 which were produced according to this invention by carrying out texturing process on substrates of magnetic hard disks which were 2.5-inch aluminum substrate with the surface Ni—P plated and mirror-polished. These sample substrates were produced under the same conditions except that the average diameter D50 of the abrading particles in the slurry used in the first step were different.

The double-side polishing machine shown and described above was used for the first step under conditions shown in Table 1.

TABLE 1 First Step Rotational speed of substrate 400 rpm Supply speed of tapes 60 mm/minute Supply rate of slurry 15 ml/minute Hardness of contact rollers 40 duro Oscillation frequency 5 Hz (amplitude = 1 mm) Compressive pressure on tapes 1.5 kg Time of processing 30 seconds

After the first step, pure water was blown on the surface of each substrate to wash it while the substrate was rotated. FIG. 4 shows the surface condition of the substrate after the first step.

The composition of the slurry is shown in Table 2. The average diameter (D50) of the abrading particles was 0.05 μm for Test Example 1, 0.10 μm for Test Example 2 and 0.15 μm for Test Example 3.

TABLE 2 Composition Cohesive polycrystalline diamond 0.03 weight % of slurry particles (abrading particles) Pure water 94.97 weight % Additive 5 weight % Composition of Glycol compound 20 weight % additive (total = Ester of phosphoric acid 40 weight % 100 weight %) Metal salt of higher fatty acid 40 weight %

The tapes were made of non-woven cloth of thickness 700 μm comprising nylon fibers with thickness 1 μm.

The second step was carried out by replacing the tapes with foamed tapes on the same double-side polishing machine under the conditions shown in Table 3 after the first step was completed. After the time of processing mentioned in Table 3 has elapsed, pure water was blown onto the surface of the substrate for washing. FIG. 5 shows the surface condition of the substrate after the second step.

TABLE 3 Second step Rotational speed of substrate 800 rpm Supply speed of tapes 30 mm/minute Supply rate of lubricant 5 ml/minute Hardness of contact rollers 40 duro Oscillation frequency 5 Hz (amplitude = 1 mm) Compressive pressure on tapes 0.5 kg Time of processing 5 seconds

The foamed tapes were obtained by cutting a foamed sheet which was produced as follows. First, a resin solution containing self-emulsifying waterborne polyurethane dispersion was prepared. When this resin solution was prepared, an adjuster of foam formation and an adjuster of bubble size and shape were added in order to accelerate the foaming of this resin solution and to disperse air bubbles stably inside the paint. The composition of this resin solution is shown in Table 4. The solid component of this self-emulsifying waterborne polyurethane dispersion was 40%.

TABLE 4 Waterborne polyurethane dispersion 90 weight parts (self-emulsifying type): Product name: Superflex 410 Produced by: Daiichi Kogyo Seiyaku Kabushiki Kaisha Adjuster of foam formation: 4 weight parts N-beef fat alkylsulpho-succinanamate/sodium sulfite Product name: FCU-305 Produced by: Sanko Kagaku Kogyo Kabushiki Kaisha Adjuster of bubble size and shape: 7 weight parts Higher aliphatic ammonium Product name: DC-100A Produced by: Sannopco Kabushiki Kaisha

Next, this resin solution was stirred by using a known type of continuous foaming device (with the rotational speed of the rotary vanes=2000 rpm) to produce a paint with foaming magnification 3x and having dispersed air bubbles with average diameter 30 μm.

Next, this paint was applied to the surface of a PET sheet of thickness 50 μm by using a cylindrical blade coater of a known kind to form a membrane comprising this paint on the surface of this sheet. This membrane was completely dried in an environment of 100° C. to form a foaming layer of thickness 400 μm on the surface of the PET sheet to produce a foamed sheet. Mechanical characteristics of the foamed layer of this foamed sheet are summarized in Table 5.

TABLE 5 Average diameter of air bubbles 26μ Compressibility 5.3% Compression recovery ratio 50.4% Shore D hardness 26 degrees

The composition of the lubricant that was used was as shown in FIG. 6.

TABLE 6 Lubricant Pure water 95 weight % Additive 5 weight % Additive Higher fatty acid 35 weight % (total = 100 weight %) Glycol compound 30 weight % Metallic salt of higher fatty acid 5 weight % Alkanol amine 30 weight %

A comparison test was carried out by comparing the surface condition (the average roughness (Ra) and the maximum roughness (Rmax) of these substrates after the texturing process with Comparison Examples for which texturing process was carried out according to prior art technologies.

In what follows, Comparison Examples will mean what were obtained only after the first step in Test Examples. In other words, comparisons were made between the surface conditions of the substrates after the first step (Comparison Examples 1-3) and after the second step was done thereafter (Test Examples 1-3).

The average surface roughness Ra and the maximum roughness Rmax were measured by using a scanning electron microscope (Nanoscope Dimension 3100 Series (trade name) produced by Digital Instruments, Inc.) The results of the comparison are shown in Tables 7 and 8.

TABLE 7 Average surface Maximum surface Rmax/ roughness (Ra) roughness (Rmax) Ra Comparison Example 1 2.2 Å 30 Å 13.64 Comparison Example 2 4.5 Å 60 Å 13.33 Comparison Example 3 5.3 Å 85 Å 16.04

TABLE 8 Average surface Maximum surface Rmax/ roughness (Ra) roughness (Rmax) Ra Test Example 1 2.0 Å 18 Å 9.00 Test Example 2 4.3 Å 41 Å 9.53 Test Example 3 5.1 Å 50 Å 9.80

As shown in Tables 7 and 8, the ratio Rmax/Ra becomes less than 10 if the texturing process is carried out on the surface of the substrate by a method according to this invention, and it can be understood that texturing marks having indentations with an appropriate depth and protrusions with an appropriate height can be formed by a method according to this invention.

Claims

1. A texturing method for forming texturing marks on the surface of a substrate of a magnetic hard disk, said method comprising:

a first step of forming approximately concentric circular preliminary marks on the surface of said substrate; and

a second step of forming approximately concentric circular texturing marks on said surface of said substrate based on said preliminary marks, wherein said surface after said texturing marks are formed in said second step has average surface roughness in the range of 1 Å or more and 6 Å or less and a ratio of maximum surface roughness to said average surface roughness in the range of less than 10;

wherein said first step comprises the steps of rotating said substrate, supplying slurry having abrading particles dispersed to the surface of said substrate and pressing a woven or non-woven cloth tape to the surface of said substrate;

wherein said second step comprises the steps of rotating said substrate, supplying a lubricant to the surface of said substrate with said preliminary marks formed thereon, and pressing a foamed tape on the surface of said substrate;

wherein said foamed tape comprises a base material formed as a tape, and a foamed layer formed on the surface of said base material; and

wherein said foamed layer has average diameter of air bubbles in the range of 1 μm or more and 50 μm or less, compressibility in the range of 3% or more and 7% or less, compression recovery ratio in the range of 40% or more and 60% or less and Shore D hardness in the range of 20 degrees or more and 30 degrees or less.

2. The texturing method of claim 1 wherein said foamed layer has average diameter of air bubbles in the range of 1 μm or more and 30 μm or less.

3. The texturing method of claim 1 wherein said foamed layer comprises polyurethane resin.

4. The texturing method of claim 1 wherein said first step further includes the step of washing the surface of said substrate after the step of forming said approximately concentric circular marks.

5. The texturing method of claim 1 wherein said second step further includes the step of washing the surface of said substrate after the step of forming said approximately concentric circular texturing marks.