JP3641171B2 - Method for manufacturing glass substrate for magnetic recording medium - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a glass substrate for a magnetic recording medium and a method for producing a magnetic recording medium using the glass substrate.
[0002]
[Prior art]
As the capacity of a magnetic disk storage device increases, the flying height of the magnetic head is 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 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 performance is remarkably affected, there is an increasing demand for a substrate having excellent smoothness. In response to such demands, a glass substrate has a feature that the surface can be smoothed relatively easily by polishing. Therefore, the glass substrate has begun to be adopted as a substrate for a magnetic recording medium.
[0003]
Processing of a glass substrate for a magnetic recording medium usually includes the following steps in the order of processing, and the glass substrate manufactured through this step can be applied to a magnetic disk device having a magnetic head flying height of about 75 nm. .
1. Disk processing step: a process lapping step for processing plate glass into a disk-shaped glass substrate. 2. Lapping step: A step of processing a glass substrate to a predetermined plate thickness. Polishing step: A step of polishing and smoothing the surface of the glass substrate. Chemical strengthening step: a step of chemically strengthening a glass substrate. The polishing step is usually a first step for removing a work-affected layer such as a crack generated in the glass substrate in the 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 stage polishing.
[0004]
On the other hand, 3. 3. The second stage polishing in the polishing step. A method performed after the chemical strengthening step is known (Japanese Patent Laid-Open No. 63-175219). Although this reason is not clarified in the said literature, it is guessed that it intended to make the surface of a glass substrate smooth further.
[0005]
[Problems to be solved by the invention]
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 can be obtained, but warpage of the glass substrate occurs. It turned out to be easy. This warpage of the glass substrate causes an increase in the axial acceleration of the magnetic recording medium, which degrades the flying characteristics of the magnetic head and hinders the improvement in recording density.
[0006]
In view of the above circumstances, the present invention provides an efficient production of a glass substrate for a magnetic recording medium that can be applied to a magnetic disk device having excellent smoothness, little warpage, and a magnetic head flying height of about 50 nm. It aims to provide a method.
[0007]
[Means for Solving the Problems]
The present invention provides a method for producing a glass disk substrate for a magnetic recording medium in which the main surface of a glass disk substrate subjected to chemical strengthening treatment is polished and smoothed, and both surfaces of the main surface of the glass disk substrate are 0.02 μm to 0 mm. Polishing at the same time using a 2 μm grain size abrasive and a polishing pad, the glass thickness to be reduced is 0.1 μm or more and 0.7 μm or less for each polished surface, and the difference in the reduced thickness on both polished surfaces to 0.15μm or less, the glass disk for a magnetic recording medium characterized to Rukoto the average of the difference between the maximum value and the minimum value of irregularities per 12 [mu] m □ as measured by atomic force microscopy in the polished surface 15nm or less A method for manufacturing a substrate.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Chemical strengthening treatment refers to generating compressive stress on the glass surface by substituting ions in the vicinity of the glass surface with ions having a larger ionic radius in the temperature region below the glass transition point of the glass used, for example, It is performed by immersing the glass in molten potassium nitrate and replacing sodium ions in the glass with potassium ions in the molten salt.
[0010]
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.
[0011]
Incidentally, examples of the abrasive that can be used in the present invention include cerium oxide, alumina abrasive grains, diamond abrasive grains, colloidal silica abrasive grains, zirconium oxide abrasive grains, etc., from the viewpoint of improving the smoothness of the polished surface, Free abrasive grains such as colloidal silica using ultrafine succinic anhydride as a colloidal solution and ultrafine particles of zirconium oxide are desirable. In general, the smaller the grain size of the abrasive grains, the better the surface smoothness. On the other hand, the price of the abrasive grains also rises. Therefore, abrasive grains having a grain size of 0.02 μm to 0.2 μm are particularly suitable for carrying out the present invention. preferable. Furthermore, the shape of the abrasive grains is preferably a spherical shape from the viewpoint of improving smoothness.
[0012]
According to the present invention, the amount of polishing the glass substrate surface and reducing the glass thickness is within a range that does not make the glass substrate warp to be a certain value or more, but more than a range necessary to ensure a certain surface smoothness. Therefore, a chemically strengthened glass substrate having excellent surface smoothness and little warpage can be produced.
[0013]
As shown in FIG. 5, the stress distribution of the glass substrate subjected to the chemical strengthening treatment has a very large compressive stress in the vicinity of the surface, while the stress value sharply decreases when proceeding from the surface to the inside. For this reason, when polishing a glass substrate after chemical strengthening treatment, if there is a difference in the glass thickness to be reduced between the polished surfaces of the glass substrate, even if this difference is small, the stress between the polished surfaces is reduced. 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 thin plate thickness, it easily causes a warp.
[0014]
Since the difference in thickness to be reduced is caused by the difference in the polishing rate on both sides of the glass substrate, it is necessary to take care that the polishing conditions are the same on both sides of the substrate, but it is extremely difficult to make these conditions exactly the same. Therefore, in order to avoid the warp, the glass thickness to be reduced must be a certain value or less.
[0015]
However, if the thickness to be reduced is too small, the surface smoothness may be lost. In particular, on the glass substrate after chemical strengthening, as confirmed in Examples described later by the inventor, protrusions of several tens of nanometers are formed, and it is necessary to polish the surface at least to the extent that these protrusions are removed. It is believed that there is.
[0016]
According to the present invention, the relationship between the glass thickness to be reduced by polishing 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 is controlled within a certain range. Accordingly, since the substrate warpage is within a range that does not impede practically 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]
【Example】
(Example 1)
1. Disc processing step First, plate glass made of soda lime silicate glass of 40 mm square and 0.7 mm thick is disked into a donut shape with an outer diameter of 34 mm and an inner diameter of 8 mm using a diamond tool. A predetermined chamfering process was performed on the end face.
[0018]
2. Lapping process The lapping process was performed using the lapping apparatus shown in FIG. As the lapping abrasive grains, alumina abrasive grains 25a having a grain size of # 1000 are used, the polishing pressure is set to about 200 g / cm ² , and the inner gear 21 and the outer gear 22 are rotated to be installed in the carrier 23 made of FRP. The both surfaces of the glass substrate 1 were wrapped. By this processing, the plate thickness of the glass substrate was set to 0.45 mm, and the surface roughness was set to about Rmax 2 μm.
[0019]
3. Polishing First Process Using the polishing apparatus shown in FIG. 7, a work-affected layer such as a crack generated in the lapping process was removed. Here, the polishing apparatus shown in FIG. 7 uses a surface plate 32 in which a polishing pad 31 is bonded to the inner surface instead of the cast iron surface plate 24 in the lapping apparatus shown in FIG. Instead, it differs from the lapping apparatus only in that a polishing slurry 25b in which cerium oxide abrasive grains are mixed with water is used, but the rest is the same. This first polishing step was carried out under the following polishing conditions using a hard pad (polyurethane pad manufactured by Speedfam Co., Ltd .; trade name: MHC15A) as the polishing pad 31.
[0020]
Polishing slurry: cerium oxide (average particle size: about 1.5 μm) + water polishing pressure: 200 g / cm ²
Polishing time: 30 minutes Removal amount: 60 μm (both sides)
[0021]
By this first polishing step, the surface roughness of the glass substrate 1 is determined by an atomic force microscope (manufactured by Digital Instruments Co., Ltd .; trade name NanoScope: hereinafter referred to as “AFM”) with irregularities per surface of 12 μm □. The difference between the maximum value and the minimum value (hereinafter simply referred to as “maximum / minimum value”) was about 18 nm on average and about 35 nm on maximum. Moreover, the curvature of the glass substrate 1 was about 1 micrometer on the average from the measurement by a surface shape measuring apparatus (ZYGOMark 4 made by ZYGO Co., Ltd.). In this step, it is desirable that the surface roughness be less than Rmax 50 nm. This is because the production efficiency is increased in relation to the second polishing step.
[0022]
4). Chemical strengthening process A glass substrate was immersed in molten potassium nitrate heated to 450 ° C. for 20 hours for chemical strengthening treatment to form a compressive stress layer on the surface. According to the optical measuring machine, the thickness of the stress layer was about 60 μm, and the compressive stress on the surface was about 60 kgf / mm ² .
[0023]
As a result of this chemical strengthening step, the surface roughness of the glass substrate was an average maximum of 12 μm square of 27 μm and a maximum of 45 nm by AFM, and the surface smoothness was worse than before the step.
[0024]
When the main surface of the glass substrate after the chemical strengthening treatment was observed by AFM, a large number of many protrusions 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, the protrusions are generated on the glass substrate along with the chemical strengthening treatment.
[0025]
FIG. 8 shows a cross section of the polished surface including the largest of the protrusions.
[0026]
Further, the warpage of the glass substrate was about 1 μm on average from the measurement by the surface shape measuring device described above, and was the same as before the chemical strengthening treatment.
[0027]
The thickness of the stress layer formed in this step is suitably 10 μm to 200 μm by controlling the temperature and time during ion exchange.
[0028]
5. Polishing Second Step Both surfaces of the glass substrate were simultaneously polished for each sheet using the polishing apparatus shown in FIGS. That is, the polishing pad 4a, which holds the glass substrate 1 in a vertical state by the guide rollers 2a, 2b, 2c and is bonded to the surface plates 3a, 3b provided opposite to both sides of the glass substrate 1 with double-sided tape, respectively. Polishing both surfaces of the glass substrate 1 at the same time by rotating the polishing pads 4a and 4b via the drive belts 8a and 8b by the motors 7a and 7b while supplying the polishing slurry 6 from the polishing liquid supply pipe 5 while applying pressure by the polishing liquid supply pipe 5b. did. At this time, the glass substrate 1 was rocked by driving the rocking jig 9 attached with the guide rollers 2a, 2b, and 2c up and down. The oscillation width 10 was determined so that the glass thickness to be polished and reduced was almost uniform in the radial direction. The pressing of the polishing pads 4 a and 4 b was performed by pressing the polishing pads 4 a and 4 b against the glass substrate 1 through the holding table 12 and the shaft 13 using the spring-type pressing jig 11. The polishing slurry 6 was supplied from the polishing liquid tank 14 by a pump 15.
[0029]
As a polishing pad, a soft pad (Suede pad manufactured by Speed Fam Co., Ltd .; trade name Polytex) was used under the following polishing conditions.
[0030]
Polishing slurry: zirconium oxide (average particle size: about 0.2 μm) + water polishing pressure: 100 g / cm ²
Polishing time: 4 minutes [0031]
Here, the polishing rate under this polishing condition was about 0.036 μm / min per side, and therefore the reduced thickness of the glass substrate in this second polishing step was about 0.15 μm per side.
[0032]
This process removes the protrusions generated in the above-described chemical strengthening process, and at the same time removes fine scratches, irregularities, etc. remaining on the main surface of the glass substrate after the first polishing process. The surface roughness of the glass substrate produced through the process was sufficiently small, with an average of 10 nm and a maximum of 14 nm, being the maximum and minimum values of 12 μm □ measured by the AFM described above.
[0033]
When the main surface of the polished glass substrate was observed by AFM, it was confirmed that the protrusions generated in the chemical strengthening step could be almost removed.
[0034]
FIG. 9 shows a cross section of the polished surface after polishing.
[0035]
Further, the warpage of the glass substrate was about 1.2 μm on average from the measurement by the surface shape measuring device, which was slightly increased from that before chemical strengthening, but was smaller than the allowable value of 2 μm.
[0036]
Here, the second polishing step is not performed by so-called batch polishing for polishing a plurality of sheets, but is performed by so-called single wafer polishing for polishing one by one, reflecting the variation in glass thickness before polishing. This is to prevent variation in the thickness reduction amount of each glass substrate. That is, the polishing force does not sufficiently act on the glass substrate having a thin plate thickness in the batch, and the planned polishing cannot be prevented sufficiently.
[0037]
Variation in the polishing thickness can also be prevented by measuring the plate thickness of all the glass substrates and batch polishing only the glass substrates selected by 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 units and to have a certain amount of stock. Therefore, in the present invention, as the second polishing step, a step of polishing while reliably controlling the polishing thickness within a certain range by a single wafer polishing apparatus is adopted.
[0038]
(Example 2)
FIG. 3 shows the maximum and minimum values of 12 μm square measured by AFM and the surface shape measuring device for each glass substrate obtained by variously changing the reduced thickness of the glass substrate in the second polishing step in Example 1. It shows the warpage and the reduced thickness of the glass substrate.
[0039]
From this, regarding the smoothness of the surface, 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 fairly smooth surface is obtained. It can be seen that when the thickness is .1 μm or more, the maximum and minimum values of AFM are 15 nm or less on average and a very smooth surface is obtained. As for warpage, if the thickness to be reduced is 0.7 μm or less, the allowable value of 2 μm is not exceeded. Especially when the reduction thickness is 0.3 μm or less, it is 1.4 μm or less and good glass with little warpage. It can be seen that a substrate is obtained.
[0040]
This is because if the reduced thickness is 0.05 μm or less, the protrusions generated by the above-described chemical strengthening and the fine scratches and irregularities remaining in the first polishing step cannot be sufficiently removed. This is because if the thickness exceeds 0.7 μm, the difference in the reduced thickness between both polished surfaces becomes large, and the bending stress resulting therefrom increases.
[0041]
As is clear from the above, if the reduced thickness on one side is in the range of 0.05 μm or more and 0.7 μm or less, a chemically strengthened glass substrate for a magnetic recording medium having an extremely smooth surface and sufficiently small warpage is obtained. Can be obtained.
[0042]
Moreover, the glass thickness to be reduced is particularly preferably not less than 0.15 μm and not more than 0.3 μm from the viewpoint of surface unevenness in mass production, variation in warpage and shortening of polishing time.
[0043]
(Example 3)
A glass substrate was produced under the same conditions as in Example 1 except that the polishing slurry in the second polishing stage was changed to colloidal silica (average particle diameter: about 0.05 μm). Here, the polishing rate under this polishing condition was about 0.014 μm / min for each polishing surface, and thus the reduction thickness in this polishing second stage was about 0.06 μm.
[0044]
The surface roughness of the glass substrate according to this example was as small as an average of 8 nm and a maximum of 10 nm, with a maximum and minimum value of 12 μm □ measured by AFM. Further, the warpage of the glass substrate 1 was about 1.1 μm on average from the measurement by the surface shape measuring device and increased slightly before chemical strengthening, but was smaller than the allowable value of 2 μm.
[0045]
(Example 4)
Only one side of the chemically strengthened glass substrate having an outer diameter of 34 mm, an inner diameter of 8 mm, and a plate thickness of 0.381 mm is reduced and polished with various thicknesses, and the thickness to be reduced and the size of the generated warp are measured by a surface shape measuring device. The measurement results are shown in FIG. It can be seen that the warpage exceeding the allowable value of 2 μm occurs due to the difference in the reduced thickness of 0.15 μm.
[0046]
(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.
[0047]
First, a 100 nm thick Ti film, a 150 nm thick Cr film, a 50 nm thick Co—Cr—Ta alloy film, and a 20 nm thick C film are sequentially sputtered onto the main surface of the glass substrate obtained in Example 1. Was formed. 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 underlayer are base films that improve the magnetic properties of the magnetic film, and the C film is a protective film.
[0048]
The touchdown height (hereinafter referred to as “TDH”) was measured for several magnetic disks using a glide height tester (manufactured by Eaton Corporation; product No. 005G). An 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 floated, and the rotational speed of the magnetic disk 42 is gradually lowered in this state, and 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.
[0049]
The TDH of the magnetic disk according to this example was 20 nm on average and 25 nm at maximum, which was extremely good. Such a magnetic disk can be easily applied to a magnetic disk apparatus in which the flying height of the magnetic head is 50 nm or less, even in consideration of various margins during production.
[0050]
【The invention's effect】
According to the present invention, it is possible to efficiently produce a glass substrate suitable for a magnetic recording medium that can also be used in a magnetic disk device having a magnetic head flying height of about 50 nm.
[0051]
In particular, the reduction range of the thickness of the glass substrate after chemical strengthening that can realize surface smoothness while suppressing warpage of the glass substrate that hinders the reduction of the flying height of the magnetic head has been clarified, and this is reflected in the polishing conditions. As a result, a method for producing the glass substrate more efficiently than the conventional method 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 polished portion of the apparatus suitable for carrying out the present invention. Fig. 4 shows the relationship between the amount of polishing, the smoothness of the substrate surface, and the warpage of the substrate. Fig. 4 shows the relationship between the reduced thickness and the warpage of the substrate when only one side of the glass substrate after chemical strengthening is polished. 5 is a diagram showing the stress distribution in the cross-sectional direction of the glass subjected to chemical strengthening treatment. FIG. 6 is a schematic diagram of the main part of the apparatus used in the lapping process of the example. FIG. 7 is used in the first polishing process of the example. Schematic diagram of the main part of the device [Fig. 8] Fig. 9 shows the result of measuring the cross section of the glass substrate after chemical strengthening with an atomic force microscope. [Fig. 9] Measuring the cross section of the glass substrate after the second polishing step with an atomic force microscope. Fig. 10 shows the result of the measurement. Fig. 10 shows the outline of the touchdown height measurement.
1 glass substrate,
2a, 2b, 2c guide rollers,
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 Oscillation width,
11 Spring-type pressure jig,
12 holding stand,
13 axes,
14 Polishing liquid tank,
15 pump,
21 inner jig,
22 Outer jig,
23 Career,
24 Cast iron surface plate,
25a Abrasive slurry containing alumina abrasive,
25b A polishing slurry containing cerium oxide,
31 Polishing pad,
32 A surface plate with a polishing pad bonded,
41 magnetic head,
42 Magnetic disk

Claims (1)

In the method for manufacturing a glass disk substrate for a magnetic recording medium, wherein the main surface of the glass disk substrate subjected to chemical strengthening treatment is polished and smoothed, both surfaces of the main surface of the glass disk substrate are formed with grains of 0.02 μm to 0.2 μm. Polishing simultaneously using diameter abrasive grains and polishing pad, the glass thickness to be reduced is 0.1 μm or more and 0.7 μm or less for each polishing surface, and the difference in reduction thickness between both polishing surfaces is 0.15 μm or less to method of manufacturing a magnetic recording medium for a glass disk substrate, wherein to Rukoto the average of the difference between the maximum value and the minimum value of irregularities per 12 [mu] m □ as measured by atomic force microscopy in the polished surface 15nm or less .

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