JPH07134823A - Manufacture of glass base for magnetic recording medium and magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain an efficient manufacture of a glass base for a magnetic recording medium which is also applicable to a magnetic disk device of which a flying amount of a magnetic head is about 50nm. CONSTITUTION:On the occasion of grinding the main surface of a chemically reinforced glass base, a glass thickness to be reduced by the grinding is made to be within a range that it is 0.05-0.7mum for each surface to be ground or that it is 0.05mum or more and a difference in the thickness to be reduced between two surfaces to be ground is 0.15mum or less. Thereby the glass base for a magnetic recording medium having the smooth surface can be manufactured efficiently while a wrap of the base is held down to be within an allowable range.

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 With the increase in capacity of magnetic disk storage devices, the flying height of magnetic heads has been reduced in order to improve recording density. For this purpose, a magnetic recording medium excellent in smoothness is required. However, in a normal thin film magnetic recording medium, the magnetic film thickness is as thin as about 0.5 μm or less, and the surface condition of the substrate is smooth. Therefore, there is an increasing demand for a substrate having excellent smoothness because it significantly affects the property. In response to such a demand, the glass substrate has a feature that its surface can be relatively easily smoothed by polishing, and is therefore being adopted as a substrate for a magnetic recording medium.

The processing of a glass substrate for a magnetic recording medium usually includes the following steps in the order of processing. The glass substrate manufactured through these steps is applicable to a magnetic disk device having a magnetic head flying height of about 75 nm. It is possible. Disc processing process: Process of processing plate glass into a disk-shaped glass substrate Lapping process: Process of processing glass substrate to a predetermined thickness Polishing process: Process of polishing and smoothing the surface of glass substrate Chemical strengthening process: For glass substrate The step of chemically strengthening the polishing step is usually a first-step polishing for removing work-affected layers such as cracks generated in the glass substrate in the previous step, and the surface smoothness of the glass substrate. It comprises a two-step polishing process of a second-stage polishing for attaining a predetermined level.

On the other hand, there is known a method of performing the second polishing step of the polishing step after the chemical strengthening step (JP-A-63-175219). The reason for this is not clarified in the above literature, but it is presumed that it was intended to make the surface of the glass substrate smoother.

[0005]

However, in order to further reduce the flying height of the magnetic head, when the glass substrate after chemical strengthening is further polished by the above method, a glass substrate having a smoother surface cannot be obtained. Although it was possible, it was found that the glass substrate was likely to warp. The warp of the glass substrate causes an increase in the axial acceleration of the magnetic recording medium, deteriorates the flying characteristics of the magnetic head, and becomes a factor that hinders the improvement of the recording density.

In view of the above circumstances, the present invention has an excellent smoothness, little warpage, and is applicable to a magnetic disk device having a magnetic head flying height of about 50 nm. The object is to provide a conventional manufacturing method.

[0007]

The first object of the present invention is to reduce the glass thickness by polishing in a method for manufacturing a glass substrate for a magnetic recording medium in which the main surface of a glass substrate subjected to a chemical strengthening treatment is polished and smoothed. This is a method for manufacturing a glass substrate for a magnetic recording medium, characterized in that each polished surface is 0.05 μm or more and 0.7 μm or less.

A second aspect of the present invention is a method for manufacturing a glass substrate for a magnetic recording medium in which the main surface of a glass substrate subjected to a chemical strengthening treatment is polished to be smooth, and the glass thickness to be polished and reduced is equal to each polished surface. The method for producing a glass substrate for a magnetic recording medium is characterized in that it is 0.05 μm or more, and the difference in reduced thickness between both polished surfaces is 0.15 μm or less.

Here, the chemical strengthening treatment means that in the temperature range below the glass transition point of the glass used, ions near the glass surface are replaced with ions having a larger ionic radius to generate compressive stress on the glass surface. This is performed by, for example, 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:
Cerium oxide, alumina abrasive grains, diamond abrasive grains, colloidal silica abrasive grains, zirconium oxide abrasive grains, etc. can be mentioned, but from the viewpoint of improving the smoothness of the polished surface, colloidal using ultrafine particles of silicic acid anhydride as a colloidal solution. silica,
Free abrasive grains such as zirconium oxide ultrafine particles are desirable.
Further, generally, the smaller the grain size of the abrasive grains is, the more the surface smoothness is improved, but on the other hand, the price of the grain grains is also increased. Therefore, the grain size of 0.02 μm to 0.2 μm is particularly preferable in the practice of the present invention. preferable. Further, the shape of the abrasive grains is preferably close to a spherical shape from the viewpoint of improving smoothness.

[0012]

According to the present invention, the amount of polishing the surface of the glass substrate to reduce the glass thickness is necessary to secure a certain surface smoothness within the range in which the warp of the glass substrate is not more than a certain value. Since it is at least the above range, a chemically strengthened glass substrate having excellent surface smoothness and less warp can be manufactured.

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

Since the difference in the thickness to be reduced is caused by the difference in the polishing rate on both sides of the glass substrate, it is necessary to keep the same polishing conditions on both sides of the substrate, but these conditions should be exactly the same. Since it is extremely difficult to reduce the warp, the glass thickness to be reduced must be set to a certain value or less.

However, if the thickness to be reduced is too small, the surface smoothness may be lost. Particularly, on the glass substrate after the chemical strengthening, as confirmed by the present inventor in Examples described later, projections of several tens nm are formed, and it is necessary to polish the surface at least to remove the projections. It is believed that there is.

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

[0017]

【Example】

(Example 1) Disc processing step First, a plate glass made of soda lime silicate glass having a size of 40 mm square and a thickness of 0.7 mm was processed into a donut shape having an outer diameter of 34 mm and an inner diameter of 8 mm by using a diamond tool, and further, A predetermined chamfering process was applied to the outer peripheral end face and the inner peripheral end face. Lapping Process A lapping process was performed using the lapping device shown in FIG.
Alumina abrasive grains with a grain size of # 1000 as lapping grains 25
a, the polishing pressure is set to about 200 g / cm ² ,
The glass substrate 1 installed in the FRP carrier 23 by rotating the inner gear 21 and the outer gear 22.
Wrapping both sides. By this processing, the plate thickness of the glass substrate was 0.45 mm and the surface roughness was R _{max of} about 2 μm. First Step of Polishing Using the polishing apparatus shown in FIG. 7, work-affected layers such as cracks generated in the lapping step were removed. Here, in the polishing apparatus shown in FIG. 7, instead of the cast iron surface plate 24 in the lapping device shown in FIG. 6, a surface plate 32 having a polishing pad 31 adhered to its inner surface is used, and the polishing machine shown in FIG. Instead of the lapping device, only a polishing slurry 25b in which cerium oxide abrasive grains are mixed with water is used, but the other points 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: about 1.5μ
m) + water Polishing pressure: 200 g / cm ² Polishing time: 30 minutes Removal amount: 60 μm (both sides) By this polishing first step, the surface roughness of the glass substrate 1 is
Difference between the maximum value and the minimum value of the irregularities per 12 μm square on the surface by an atomic force microscope (manufactured by Digital Instruments Co., Ltd .; trade name NanoScope: hereinafter referred to as “AFM”) (hereinafter simply referred to as “maximum value”) .), The average was 18 nm and the maximum was about 35 nm. Further, the warp of the glass substrate 1 is about 1 on average from the measurement by a surface shape measuring device (ZYGO Mark 4; ZYGOMark 4).
was μm.

In this step, the surface roughness is set to R _max.
It is desirable that the thickness is less than 50 nm. This is because the production efficiency is increased in relation to the second polishing step. Chemical Strengthening Step A glass substrate was immersed in a molten salt of potassium nitrate heated to 450 ° C. for 20 hours for chemical strengthening treatment to form a compressive stress layer on the surface. According to an optical measuring machine, the thickness of this stress layer is about 60 μm, and the compressive stress on the surface is about 60 kgf / m.
It was m ² .

By this chemical strengthening process, the surface roughness of the glass substrate was 27 nm on average and 45 nm on average at 12 μm □ by AFM, and the surface smoothness was worse than before the process.

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

FIG. 8 shows a cross section of the polishing surface including the largest one of the protrusions.

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

The thickness of the stress layer formed in this step is controlled by controlling the temperature and time during ion exchange.
It is suitable that the thickness is from 200 to 200 μm. Second Step of Polishing Using the polishing apparatus shown in FIGS. 1 and 2, both surfaces of glass substrates were simultaneously polished one by one. That is, the glass substrate 1
Are held vertically by the guide rollers 2a, 2b, 2c, and are provided on both sides of the glass substrate 1 so as to face each other,
Polishing pad 4 adhered on each side 3b with double-sided tape
Both sides of the glass substrate 1 are rotated by rotating the polishing pads 4a, 4b via the drive belts 8a, 8b by the motors 7a, 7b while supplying the polishing slurry 6 from the polishing liquid supply pipe 5 while applying pressure with a and 4b. Polished at the same time. At this time, by vertically moving the swing jig 9 to which the guide rollers 2a, 2b, 2c are attached,
The glass substrate 1 was rocked. The swing width 10 was determined so that the glass thickness to be polished / reduced was substantially uniform in the radial direction. Pressurization of the polishing pads 4a and 4b is performed by the spring type pressing jig 11
Using the polishing pad 4 via the holder 12 and the shaft 13.
It was performed by pressing a and 4b against the glass substrate 1. The polishing slurry 6 was supplied from the polishing liquid tank 14 by the pump 15.

As the polishing pad, a soft pad (Suede Pad manufactured by Speed Fam Co., Ltd .; trade name Polytex) was used, and the polishing was performed 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 this polishing condition is about 0.036 μm / min per one side, and therefore, the reduction of the glass substrate in the second polishing step. The thickness is about 0.
It was 15 μm.

In this step, the protrusions generated in the above-mentioned chemical strengthening step are removed, and at the same time, the minute scratches, irregularities, etc. 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 step was 10 μm on average and 14 nm on average, which were sufficiently small with the maximum and minimum values of 12 μm □ measured by the above-mentioned AFM.

When the main surface of the glass substrate after polishing was observed by AFM, it was confirmed that the protrusions generated in the chemical strengthening step could be almost removed.

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

Further, the warp of the glass substrate was about 1.2 μm on average as measured by the surface profile measuring apparatus, which was slightly larger than that 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 each sheet is polished. This is to prevent variations in the reduction amount of the thickness of each glass substrate by reflecting the above. That is, the polishing force is not sufficiently exerted on the glass substrate having a thin plate thickness during the batch, and it is intended to prevent the planned polishing from being not sufficiently performed.

The plate thickness was measured for all glass substrates,
Variation in the polishing thickness can also be prevented by batch-polishing only the glass substrate selected according to the plate thickness. However, this method is not preferable in terms of production efficiency because it is necessary to inspect the plate thickness for all the pieces and it is necessary to have a fixed amount of stock. Therefore, in the present invention, as the second polishing step, the step of polishing is performed while the polishing thickness is surely controlled within a certain range by the single-wafer polishing apparatus. (Embodiment 2) FIG. 3 shows 12 μm □ measured by AFM for each glass substrate obtained by changing the reduced thickness of the glass substrate in the second polishing step in Example 1 in various ways.
2 shows the maximum and minimum values of the warp, the warpage measured by the surface shape measuring device, and the reduced thickness of the glass substrate.

With respect to the smoothness of the surface, it can be seen that when the thickness to be reduced is 0.05 μm or more, the maximum and minimum values of AFM are 20 nm or less on average, and a considerably smooth surface is obtained. Is 0.1 μm or more, the maximum and minimum values of AFM are 15 nm or less on average, and it can be seen that a very smooth surface is obtained. Regarding the warp, if the thickness to be reduced is 0.7 μm or less, the allowable value is 2 μm.
It can be seen that the glass thickness does not exceed m, and particularly when the reduced thickness is 0.3 μm or less, it becomes 1.4 μm or less, and a good glass substrate with less warpage can be obtained.

This is because if the reduced thickness is 0.05 μm or less, it is not possible to sufficiently remove the protrusions and fine scratches and irregularities remaining in the first polishing step, which are caused by the aforementioned chemical strengthening. Further, if the reduced thickness exceeds 0.7 μm, the difference in the reduced thickness between both polished surfaces becomes large, and the bending stress resulting from this increases.

As is clear from the above, if the reduced thickness on one side is within the range of 0.05 μm or more and 0.7 μm or less,
It is possible to obtain a chemically strengthened glass substrate for a magnetic recording medium which has an extremely smooth surface and has a sufficiently small warpage.

Further, from the viewpoints of surface irregularities during production, unevenness of warpage, and shortening of polishing time, the glass thickness to be reduced is particularly preferably 0.15 μm or more and 0.3 μm or less. Example 3 A glass substrate was produced under the same conditions as in Example 1 except that the polishing slurry used in the second polishing step was changed to colloidal silica (average particle size: about 0.05 μm).

Here, the polishing rate under this polishing condition is about 0.014 μm / min for each polishing surface, so the reduced thickness in the second stage of polishing is about 0.06.
was μm.

The surface roughness of the glass substrate according to this embodiment is
An average of 8 with a maximum and minimum value of 12 μm □ measured by AFM
The maximum was 10 nm, which was even smaller. Also,
The warp of the glass substrate 1 was about 1.1 μm on average as measured by the surface profile measuring device, and was slightly increased from that before the chemical strengthening, but it was smaller than the allowable value of 2 μm. (Example 4) Outer diameter 34 mm, inner diameter 8 mm, plate thickness 0.38
FIG. 4 shows the results obtained by measuring and reducing the reduced thickness and the amount of warpage generated by reducing / polishing only one surface of the glass substrate having various thicknesses after the chemical strengthening of 1 mm. It can be seen that due to the difference in the reduced thickness of 0.15 μm, a warp exceeding the allowable value of 2 μm occurs. (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 above.

First, a 100 nm-thickness Ti film and a 150 nm-thickness C film were formed on the main surface of the glass substrate obtained in Example 1.
r film, Co-Cr-Ta alloy film with a film thickness of 50 nm, film thickness 2
A 0 nm C film was sequentially formed by sputtering. Next, a perfluoropolyether lubricant was applied to the surface to obtain a magnetic disk.

Here, the Co-Ni-Cr alloy film is a magnetic film, the Cr film and the Ti film as the underlayers thereof are the underlayer films for improving the magnetic characteristics of the magnetic film, and the C film is the protective film.

For several magnetic disks, glide height tester (manufactured by Eaton Co., Ltd .; 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, the magnetic head 41 is levitated, and in this state, the rotational speed of the magnetic disk 42 is gradually lowered, and contact between the magnetic disk 42 and the magnetic head 41 begins to occur. The flying height of the magnetic head 42 was taken 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 this 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 50n even if various margins at the time of production are taken into consideration.
It can be easily applied to a magnetic disk device having a size of m or less.

[0041]

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

In particular, since the range of reduction in the thickness of the glass substrate after chemical strengthening which can realize the surface smoothness while suppressing the warp of the glass substrate which hinders the reduction of the flying height of the magnetic head is clarified, this is defined as the polishing condition. By reflecting this in the above, a method for manufacturing the above glass substrate more efficiently than ever has been realized.

[Brief description of drawings]

FIG. 1 is a schematic diagram 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 amount of polishing of the glass plate thickness of the glass substrate after the chemical strengthening treatment, the smoothness of the substrate surface, and the warp of the substrate.

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

FIG. 5 is a diagram showing the stress distribution in the cross-sectional direction of glass that has been chemically strengthened.

FIG. 6 is a schematic diagram of a main part of an apparatus used in a lapping process of an example.

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

FIG. 8 is a diagram showing the results of measuring the cross section of a glass substrate after chemical strengthening with an atomic force microscope.

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

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

[Explanation of symbols]

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; rocking jig, 10; rocking Width, 11; spring type pressure jig, 12; holding table, 13; shaft 14, polishing liquid tank, 15; pump, 21; inner jig,
22; outer jig, 23; carrier, 24; cast iron surface plate, 2
5a; polishing slurry containing alumina abrasive sulphate; 25b; polishing slurry containing cerium oxide; 31; polishing pad; 32; surface plate to which polishing pad is adhered; 41;
Magnetic head, 42; magnetic disk

Claims (5)

[Claims] 1. In a method for producing a glass substrate for a magnetic recording medium, which comprises polishing and smoothing a main surface of a chemically strengthened glass substrate, the glass thickness to be polished and reduced is 0.05 μm or more for each polished surface. A method of manufacturing a glass substrate for a magnetic recording medium, wherein the glass substrate has a thickness of 7 μm or less. 2. In a method for producing a glass substrate for a magnetic recording medium, which polishes and smoothes the main surface of a glass substrate which has been chemically strengthened, the glass thickness to be polished and reduced is 0.05 μm or more for each polished surface. The difference in the reduced thickness between the two polished surfaces is 0.15 μm or less, and the method for manufacturing a glass substrate for a magnetic recording medium. 3. The method for manufacturing a glass substrate for a magnetic recording medium according to claim 1, wherein both main surfaces of the glass substrate are simultaneously polished. 4. The method for manufacturing a glass substrate for a magnetic recording medium according to claim 3, wherein the glass substrates are polished one by one with respect to one set of polishing pads. 5. A method of manufacturing a magnetic recording medium, which comprises forming a magnetic film on the main surface of a glass substrate manufactured by the method of claim 1, 2, 3, or 4.