JP2000105922A - Manufacture of glass substrate for magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an efficient manufacturing method for glass substrates for a magnetic recording medium which is applicable even for a magnetic disk device with a magnetic head floating hight of about 50 nm. SOLUTION: When the principal surfaces of the chemically reinforced glass substrates are ground, the glass thicknesses to be reduced by grinding are controlled to be >=0.05 μm and <=0.7 μm for each ground surface or >=0.05 μm and the difference in thicknesses to be reduced on both ground surfaces is controlled to be in the range of <=0.15 μm. Both principal surfaces of the glass substrates are ground at the same time or the glass substrates are ground one by one.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a glass substrate for a magnetic recording medium and a method for manufacturing a magnetic recording medium using the glass substrate.

[0002]

2. Description of the Related Art As the capacity of magnetic disk storage devices has increased, the flying height of a magnetic head has been reduced in order to improve the recording density. For this purpose, a magnetic recording medium having excellent smoothness is required. However, in a normal thin film type magnetic recording medium, the magnetic film thickness is as thin as about 0.5 μm or less, and the surface state of the substrate is smooth. Since the substrate has a remarkable effect, the demand for a substrate having excellent smoothness is increasing. In response to such demands, glass substrates have the characteristic that the surface can be relatively easily smoothed by polishing, and thus glass substrates have begun to be used as substrates for magnetic recording media.

The processing of a glass substrate for a magnetic recording medium usually includes the following steps in the processing order, and the glass substrate manufactured through this step is applicable to a magnetic disk drive having a magnetic head flying height of about 75 nm. It is possible. 1. 1. Disk processing step: Step of processing sheet glass into a disk-shaped glass substrate Lapping step 2. Lapping step: a step of processing a glass substrate to a predetermined thickness. 3. Polishing step: a step of polishing and smoothing the surface of the glass substrate. Chemical strengthening step: a step of chemically strengthening the glass substrate. The polishing step is usually a first-step polishing for removing a process-altered layer such as a crack generated in a glass substrate in a previous step, and a second step for bringing the surface smoothness of the glass substrate to a predetermined level. It consists of a two-stage polishing process of a stage polishing.

On the other hand, 3. 3. The polishing in the second stage of the polishing step A method of performing the method after the chemical strengthening step is known (JP-A-63-175219). Although the reason has not been clarified in the above-mentioned literature, it is presumed that the reason was intended to further smooth the surface of the glass substrate.

[0005]

However, when the glass substrate after chemical strengthening is further polished by the above method in order to further reduce the flying height of the magnetic head, it is impossible to obtain a glass substrate having a smoother surface. Although it was possible, it was found that the glass substrate was likely to be warped. The warpage of the glass substrate causes an increase in the axial acceleration of the magnetic recording medium, deteriorating the flying characteristics of the magnetic head and hindering the improvement of the recording density.

In view of the above circumstances, the present invention provides a glass substrate for a magnetic recording medium having excellent smoothness, little warpage, and applicable to a magnetic disk drive having a magnetic head flying height of about 50 nm. It is intended to provide a simple manufacturing method.

[0007]

SUMMARY OF THE INVENTION A first aspect of the present invention is a method of manufacturing a glass substrate for a magnetic recording medium for polishing and smoothing a main surface of a glass substrate subjected to a chemical strengthening treatment. The method is a method for manufacturing a glass substrate for a magnetic recording medium, wherein each polished surface has a thickness of 0.05 μm or more and 0.7 μm or less.

A second aspect of the present invention is a method of manufacturing a glass substrate for a magnetic recording medium for polishing and smoothing the main surface of a glass substrate subjected to a chemical strengthening treatment. The method of manufacturing a glass substrate for a magnetic recording medium, wherein the difference between the reduced thicknesses on both polished surfaces is 0.15 μm or less.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Chemical strengthening treatment means that in a temperature range below the glass transition point of a glass used, ions near the glass surface are replaced with ions having a larger ionic radius to generate a compressive stress on the glass surface. This is performed, for example, by immersing glass in a molten salt of potassium nitrate and replacing sodium ions in the glass with potassium ions in the molten salt.

The glass that can be used in the present invention is not particularly limited as long as it can be chemically strengthened, and soda lime glass, borosilicate glass, aluminoborosilicate glass and the like can be used.

The abrasives usable in the present invention include:
Examples thereof include cerium oxide, alumina abrasive grains, diamond abrasive grains, colloidal silica abrasive grains, and zirconium oxide abrasive grains. silica,
Free abrasive grains such as ultrafine particles of zirconium oxide are desirable.
Generally, as the particle size of the abrasive grains is smaller, the surface smoothness is improved, but on the other hand, the price of the abrasive grains is also increased. preferable. Further, the shape of the abrasive grains is preferably close to a spherical shape from the viewpoint of improving smoothness.

According to the present invention, the amount of reduction of the glass thickness by polishing the surface of the glass substrate is required to ensure a constant surface smoothness while keeping the warpage of the glass substrate within a range not exceeding a certain value. Since it is more than the above range, it is possible to manufacture a chemically strengthened glass substrate having excellent surface smoothness and less warpage.

As shown in FIG. 5, the stress distribution of the glass substrate subjected to the chemical strengthening treatment is such that the compressive stress in the vicinity of the surface is very large, while the stress value decreases sharply from the surface to the inside. For this reason, when polishing the glass substrate after the chemical strengthening treatment, if there is a difference in the glass thickness to be reduced between the polished surfaces of the glass substrates, even if the difference is minute, the stress between the polished surfaces is small. The balance is lost and a large bending stress is generated. As a result, particularly in a glass substrate for a magnetic recording medium having a small thickness, the glass substrate easily causes warpage.

Since the difference in thickness to be reduced is caused by the difference in polishing rate between both surfaces of the glass substrate, it is necessary to pay attention to the same polishing conditions on both surfaces of the substrate. Is extremely difficult, the glass thickness to be reduced must be reduced to a certain value or less in order to avoid the warpage.

However, if the thickness to be reduced is too small, the surface smoothness may be lost. In particular, on the glass substrate after the chemical strengthening, protrusions of several tens of nm have been formed, as confirmed by the present inventors in Examples described later, and it is necessary to polish the surface at least to the extent that the protrusions are removed. It is believed that there is.

According to the present invention, the relationship between the glass thickness reduced by polishing the glass substrate after the chemical strengthening treatment and the surface smoothness or warpage of the glass substrate is confirmed by an embodiment described later, and the reduced thickness is kept within a certain range. By controlling so that the warpage of the substrate is within a range that does not hinder practical use while having excellent surface smoothness, it is possible to efficiently manufacture a glass substrate that can reduce the flying height of the magnetic head.

[0017]

Embodiment (Embodiment 1) Disk Processing Step First, a 40 mm square, 0.7 mm thick soda lime silicate glass plate glass is processed into a donut shape having an outer diameter of 34 mm and an inner diameter of 8 mm using a diamond tool. Predetermined chamfering was performed on the end face.

2. Lapping step The lapping step was performed using the lapping apparatus shown in FIG.
Alumina abrasive grains 25 of grain size # 1000 as lap abrasive grains
a, the polishing pressure was set to about 200 g / cm ² ,
By rotating the inner gear 21 and the outer gear 22, the glass substrate 1 set in the carrier 23 made of FRP
Wrapped on both sides. By this processing, the thickness of the glass substrate was set to 0.45 mm, and the surface roughness was set to Rmax of about 2 μm.

3. First Polishing Step By using the polishing apparatus shown in FIG. 7, an affected layer such as a crack generated in the above lapping step was removed. Here, the polishing apparatus shown in FIG. 7 is different from the lapping apparatus shown in FIG. 6 in that instead of the cast iron platen 24, a platen 32 having a polishing pad 31 adhered to the inner surface thereof is used. The only difference is that a polishing slurry 25b in which cerium oxide abrasive grains are mixed with water is used instead of the lapping device, but the others are the same. The first polishing step was performed under the following polishing conditions using a hard pad (polyurethane pad manufactured by Speed Fam Co., Ltd .; trade name: MHC15A) as the polishing pad 31.

Polishing slurry: cerium oxide (average particle size:
Polishing pressure: 200 g / cm ² Polishing time: 30 minutes Removal amount: 60 μm (both sides)

In the first polishing step, the surface roughness of the glass substrate 1 can be adjusted by using an atomic force microscope (manufactured by Digital Instruments Co., Ltd .; trade name: NanoScope: hereinafter referred to as “AF”).
M ”. ), The difference between the maximum value and the minimum value of the unevenness per 12 μm square of the surface (hereinafter simply referred to as “maximum / minimum value”) was 18 nm on average and 35 nm at maximum.
The warpage of the glass substrate 1 is measured by a surface shape measuring device (ZY
The average was about 1 μm as measured by ZYGOMark 4) manufactured by GO Co., Ltd. In this step, it is desirable that the surface roughness is less than Rmax 50 nm. This is because the production efficiency is increased in connection with the second polishing step.

4. Chemical strengthening step A glass substrate was immersed in a molten salt of potassium nitrate heated to 450 ° C. for 20 hours to perform a chemical strengthening treatment, thereby forming a compressive stress layer on the surface. According to the optical measuring device, the thickness of the stress layer is about 60 μm, and the compressive stress on the surface is about 60 kgf / m
m ² .

As a result of this chemical strengthening step, the maximum and minimum values of the 12 μm square by AFM were 27 nm and 45 nm, respectively, and the surface smoothness of the glass substrate was worse than before the same step.

The main surface of the glass substrate after the chemical strengthening treatment is A
Observation by FM revealed that a large number of projections having a diameter of about 0.2 μm and a height of several tens of nm were present on the surface of the glass substrate. Since such a large number of protrusions are not observed on the glass substrate before the chemical strengthening treatment, these protrusions are generated on the glass substrate with the chemical strengthening treatment.

FIG. 8 shows a cross section of the polished surface including the largest one of the projections.

The average warpage of the glass substrate was about 1 μm as measured by the above-mentioned surface profile measuring apparatus, and was the same as before the chemical strengthening treatment.

The thickness of the stress layer formed in this step can be controlled by controlling the temperature and time during ion exchange.
It is appropriate that the thickness be from μm to 200 μm.

5. Second Polishing Step Both sides of the glass substrate were simultaneously polished one by one using the polishing apparatus shown in FIGS. That is, the glass substrate 1
Is held vertically by guide rollers 2a, 2b and 2c, and a surface plate 3a provided on both sides of the glass substrate 1 so as to face each other.
Polishing pad 4 adhered on both sides with double-sided tape
While the polishing pads 4a and 4b are rotated by the motors 7a and 7b via the driving belts 8a and 8b while the polishing slurry 6 is supplied from the polishing liquid supply pipe 5 while pressing by the pressures a and 4b, both surfaces of the glass substrate 1 are pressed. Polished at the same time. At this time, the swing jig 9 to which the guide rollers 2a, 2b, 2c are attached is driven up and down,
The glass substrate 1 was swung. The swing width 10 was determined so that the glass thickness to be polished and reduced was substantially uniform in the radial direction. The pressing of the polishing pads 4a and 4b is performed by a spring type pressing jig 11.
And the polishing pad 4 via the holding table 12 and the shaft 13.
a, 4b were pressed against the glass substrate 1. The polishing slurry 6 was supplied from a polishing liquid tank 14 by a pump 15.

As a polishing pad, a soft pad (a suede pad manufactured by Speed Fam Co., Ltd .; trade name: Polytex) was used under the following polishing conditions.

Polishing slurry: zirconium oxide (average particle size: about 0.2 μm) + water Polishing pressure: 100 g / cm ² Polishing time: 4 minutes

Here, the polishing rate under these polishing conditions is about 0.036 μm / min per one side.
The reduced thickness of the glass substrate in the second polishing step was about 0.15 μm per side.

In this step, the projections generated in the above-mentioned chemical strengthening step are removed, and at the same time, minute scratches, irregularities, and the like remaining on the main surface of the glass substrate after the first polishing step are also removed. The surface roughness of the glass substrate manufactured through this process was a sufficiently small value of 10 nm on average and 14 nm on average at the maximum and minimum values of 12 μm □ measured by the aforementioned AFM.

When the main surface of the polished glass substrate was observed by AFM, it was confirmed that the projections generated in the chemical strengthening step were almost completely removed.

FIG. 9 shows a cross section of the polished surface after polishing.

Further, the warpage of the glass substrate was about 1.2 μm on average from the measurement by the surface shape measuring device, and slightly increased from before the chemical strengthening, but was smaller than the allowable value of 2 μm.

Here, the second polishing step is not performed by so-called batch polishing in which a plurality of sheets are polished, but is performed by so-called single-wafer polishing in which polishing is performed for each sheet. This is to prevent the variation in the amount of reduction in the thickness of each glass substrate from being reflected. In other words, this is to prevent the polishing force from sufficiently acting on the glass substrate having a small plate thickness during the batch and preventing the intended polishing from being performed sufficiently.

The thickness of all glass substrates was measured,
Variations in the polished thickness can also be prevented by batch polishing only the glass substrates selected according to the plate thickness. However, this method is not preferable in terms of production efficiency because it is necessary to inspect the thickness of all the sheets and it is necessary to have a certain amount of stock. Therefore, in the present invention, as the second step of polishing, a step of performing polishing while reliably controlling the abrasion thickness within a certain range by a single-wafer polishing apparatus is employed.

Example 2 FIG. 3 shows the maximum and minimum values of 12 μm square measured by AFM for each glass substrate obtained in Example 1 by changing the reduced thickness of the glass substrate in the second polishing step. 2 shows warpage measured by a surface shape measuring device and a reduced thickness of a glass substrate.

From the above results, regarding the surface smoothness, when the thickness to be reduced is 0.05 μm or more, the maximum and minimum values of the AFM are 20 nm or less on average, and a considerably smooth surface is obtained. When the thickness is 0.1 μm or more, the maximum and minimum values of the AFM are 15 nm or less on average, and it can be seen that a very smooth surface is obtained. The warpage is 2 μm, which is an allowable value if the thickness to be reduced is 0.7 μm or less.
m, and especially when the reduced thickness is 0.3 μm or less, it is 1.4 μm or less, and it can be seen that a good glass substrate with little warpage can be obtained.

This is because if the reduced thickness is 0.05 μm or less, it is not possible to sufficiently remove protrusions generated due to the above-mentioned chemical strengthening and fine scratches and irregularities remaining in the first polishing step. On the other hand, if the reduced thickness exceeds 0.7 μm, the difference in the reduced thickness between the two polished surfaces becomes large, and the bending stress resulting therefrom increases.

As is clear from the above, if the reduced thickness on one side is in the range of 0.05 μm to 0.7 μm,
A chemically strengthened glass substrate for a magnetic recording medium having an extremely smooth surface and having sufficiently small warpage can be obtained.

Further, from the viewpoints of surface irregularities during mass production, variations in warpage, and reduction in polishing time, the glass thickness to be reduced is particularly preferably 0.15 μm or more and 0.3 μm or less.

Example 3 The polishing slurry in the second polishing step was made of colloidal silica (average particle size: about 0.05 μm).
A glass substrate was produced under the same conditions as in Example 1 except for changing to m). Here, the polishing rate under these polishing conditions is about 0.014 μm / min for each polished surface,
Therefore, the reduced thickness in the second polishing step is about 0.0
It was 6 μm.

The surface roughness of the glass substrate according to the present embodiment is:
Averaged at the maximum and minimum value of 12 μm □ measured by AFM, 8
The nm was as small as 10 nm at maximum. Also,
The warpage of the glass substrate 1 was about 1.1 μm on average from the measurement by the surface shape measuring device, slightly increased from before chemical strengthening, but smaller than the allowable value of 2 μm.

(Example 4) Outer diameter 34 mm, inner diameter 8 mm,
FIG. 4 shows the results obtained by reducing and polishing only one side of a glass substrate having a thickness of 0.381 mm after chemical strengthening with various thicknesses, and measuring the reduced thickness and the amount of warpage by a surface shape measuring device. Show. It can be seen that warpage exceeding the allowable value of 2 μm occurs due to the reduced thickness difference of 0.15 μm.

Example 5 Next, a magnetic disk as a magnetic recording medium was manufactured by the following method using the glass substrate obtained in Example 1 described above.

First, a Ti film having a thickness of 100 nm and a C film having a thickness of 150 nm were formed on the main surface of the glass substrate obtained in Example 1.
r film, Co-Cr-Ta alloy film with a thickness of 50 nm, film thickness 2
0 nm C films were sequentially formed by sputtering. Next, a perfluoropolyether-based lubricant was applied to the surface to obtain a magnetic disk. Here, Co-Ni-Cr
The alloy film is a magnetic film, and its underlying Cr film and Ti
The film is a base film for improving the magnetic characteristics of the magnetic film, and the C film is a protective film.

About several magnetic disks, a glide height tester (manufactured by Eaton Corporation; product No. 00)
5G) was used to measure the touchdown height (hereinafter referred to as “TDH”). The outline of this measurement is shown in FIG.
That is, the magnetic disk 42 is rotated at a sufficiently high speed to float the magnetic head 41, and in this state, the rotation speed of the magnetic disk 42 is gradually reduced, and the contact between the magnetic disk 42 and the magnetic head 41 begins to occur. The flying height of the magnetic head 42 was defined as TDH. The presence or absence of contact was detected by an acoustic emission sensor attached to the magnetic head.

The TDH of the magnetic disk according to the present embodiment is
The average was 20 nm and the maximum was 25 nm, which was extremely good. In such a magnetic disk, the flying height of the magnetic head is 50 n even when various margins during production are taken into consideration.
It is easily applicable to a magnetic disk device having a diameter of m or less.

[0050]

According to the present invention, it is possible to efficiently produce a glass substrate suitable for a magnetic recording medium which can be used in a magnetic disk drive having a magnetic head flying height of about 50 nm.

In particular, the range of reduction in the thickness of the glass substrate after chemical strengthening, which can achieve surface smoothness while suppressing warpage of the glass substrate, which is an obstacle to reducing the flying height of the magnetic head, has been clarified. Thus, a method of manufacturing the above glass substrate more efficiently than before was realized.

[Brief description of the drawings]

FIG. 1 is a schematic view of an apparatus suitable for carrying out the present invention.

FIG. 2 is a schematic view of a polishing portion of an apparatus suitable for carrying out the present invention.

FIG. 3 is a diagram showing the relationship between the abrasion amount of the glass thickness of the glass substrate after the chemical strengthening treatment, the smoothness of the substrate surface, and the warpage of the substrate.

FIG. 4 is a diagram showing the relationship between the reduced thickness and the warpage of the substrate when only one side of the glass substrate after the chemical strengthening treatment is polished.

FIG. 5 is a view showing a stress distribution in a cross-sectional direction of glass subjected to a chemical strengthening treatment.

FIG. 6 is a schematic view of a main part of the apparatus used in the lapping process of the embodiment.

FIG. 7 is a schematic view of a main part of the apparatus used in the first polishing step of the embodiment.

FIG. 8 is a view showing a result of measuring a cross section of a glass substrate after a chemical strengthening treatment with an atomic force microscope.

FIG. 9 is a view showing the result of measuring the cross section of the glass substrate after the second polishing step with an atomic force microscope.

FIG. 10 is a diagram showing an outline of a touchdown height measurement.

[Explanation of symbols]

 Reference Signs List 1 glass substrate, 2a, 2b, 2c guide roller, 3 surface plate, 4a, 4b polishing pad, 5 polishing liquid supply pipe, 6a, 6b polishing slurry, 7a, 7b motor, 8a, 8b drive belt, 9 swing jig , 10 swing width, 11 spring type pressing jig, 12 holding table, 13 shaft, 14 polishing liquid tank, 15 pump, 21 inner jig, 22 outer jig, 23 carrier, 24 cast iron platen, 25a alumina abrasive Polishing slurry containing sulfuric acid, 25b polishing slurry containing cerium oxide, 31 polishing pad, 32 surface plate with polishing pad bonded thereto, 41 magnetic head, 42 magnetic disk

Claims (4)

[Claims] 1. A method for manufacturing a glass substrate for a magnetic recording medium, in which a main surface of a glass substrate subjected to a chemical strengthening treatment is polished and smoothed, wherein a glass thickness to be polished and reduced is 0.05 μm or more per polished surface. A method for producing a glass substrate for a magnetic recording medium, which is not more than 0.7 μm. 2. A method for manufacturing a glass substrate for a magnetic recording medium, in which a main surface of a glass substrate subjected to a chemical strengthening treatment is polished and smoothed, wherein a glass thickness to be polished and reduced is 0.05 μm or more for each polished surface. A method for manufacturing a glass substrate for a magnetic recording medium, wherein the difference in reduced thickness between the two polished surfaces is 0.15 μm or less. 3. The method of manufacturing a glass substrate for a magnetic recording medium according to claim 1, wherein both main surfaces of the glass substrate are polished simultaneously. 4. The method of manufacturing a glass substrate for a magnetic recording medium according to claim 3, wherein the glass substrates are polished one by one for a set of polishing pads.