WO2014104368A1 - 磁気ディスク用ガラス基板および磁気ディスク - Google Patents
磁気ディスク用ガラス基板および磁気ディスク Download PDFInfo
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- WO2014104368A1 WO2014104368A1 PCT/JP2013/085270 JP2013085270W WO2014104368A1 WO 2014104368 A1 WO2014104368 A1 WO 2014104368A1 JP 2013085270 W JP2013085270 W JP 2013085270W WO 2014104368 A1 WO2014104368 A1 WO 2014104368A1
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- glass substrate
- magnetic disk
- outer peripheral
- magnetic
- center
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/62—Record carriers characterised by the selection of the material
- G11B5/73—Base layers, i.e. all non-magnetic layers lying under a lowermost magnetic recording layer, e.g. including any non-magnetic layer in between a first magnetic recording layer and either an underlying substrate or a soft magnetic underlayer
- G11B5/739—Magnetic recording media substrates
- G11B5/73911—Inorganic substrates
- G11B5/73921—Glass or ceramic substrates
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/74—Record carriers characterised by the form, e.g. sheet shaped to wrap around a drum
- G11B5/82—Disk carriers
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/84—Processes or apparatus specially adapted for manufacturing record carriers
- G11B5/8404—Processes or apparatus specially adapted for manufacturing record carriers manufacturing base layers
Definitions
- the present invention relates to a glass substrate for a magnetic disk and a magnetic disk.
- a personal computer or a DVD (Digital Versatile Disc) recording device has a built-in hard disk device (HDD: Hard Disk Drive) for data recording.
- HDD Hard Disk Drive
- a hard disk device used in a portable computer such as a notebook personal computer
- a magnetic disk in which a magnetic layer is provided on a glass substrate is used, and the magnetic head slightly floats above the surface of the magnetic disk.
- magnetic recording information is recorded on or read from the magnetic layer.
- a glass substrate is preferably used because it has a property that it is less likely to be plastically deformed than a metal substrate (aluminum substrate) or the like.
- the density of magnetic recording has been increased.
- the magnetic recording information area is miniaturized by using a perpendicular magnetic recording method in which the magnetization direction in the magnetic layer is perpendicular to the surface of the substrate.
- the storage capacity of one disk substrate can be increased.
- a magnetic head equipped with a DFH (Dynamic Flying Height) mechanism is used to significantly shorten the flying distance from the magnetic recording surface, so that the recording / reproducing element of the magnetic head and the magnetic It is also practiced to increase the accuracy of recording / reproducing information (to improve the S / N ratio) by reducing the magnetic spacing between the magnetic recording layers of the disk. Even in this case, the surface irregularities of the substrate of the magnetic disk are required to be as small as possible in order to read and write magnetic recording information by the magnetic head stably over a long period of time.
- DFH Dynamic Flying Height
- Servo information used for positioning the magnetic head on the data track is recorded on the magnetic disk.
- the roundness of the end face on the outer peripheral side of the magnetic disk hereinafter also referred to as the outer peripheral end face
- the flying of the magnetic head is stabilized, the servo information can be read well, and the magnetic head can read and write.
- the technique described in Patent Document 1 discloses a glass substrate for a magnetic disk in which the roundness of the outer peripheral end face is 4 ⁇ m or less. According to this glass substrate, LUL (load-unload) test durability is improved by reducing the roundness of the outer peripheral end face.
- HDDs that employ a single write method in which recording is performed so that adjacent tracks partially overlap are known.
- signal deterioration due to recording on adjacent tracks is extremely small, so that the track recording density (hereinafter also referred to as TPI) can be dramatically improved.
- TPI track recording density
- an extremely high track of 500 kTPI (track per inch) or more. Recording density can be realized.
- EAMR energy assisted magnetic recording
- the followability of the magnetic head to the servo signal is required more severely than before.
- the servo signal is read at the outer peripheral end of the magnetic disk.
- the servo signal is read at the outer peripheral end of the magnetic disk.
- An object of the present invention is to provide a glass substrate for a magnetic disk and a magnetic disk capable of suppressing servo signal reading errors at the outer peripheral end of the magnetic disk.
- the present inventor In order to eliminate servo signal reading errors in the vicinity of the outer peripheral end of the magnetic disk, the present inventor first considered eliminating the influence caused by the rattling of the inner hole of the magnetic disk.
- the HDD was assembled by precisely aligning the center of the magnetic disk with the center of the spindle.
- the influence of the inner hole is eliminated, so that the blur in the in-plane direction of the outer peripheral end surface of the magnetic disk is less than the roundness of the outer peripheral end surface.
- the influence of the roundness of the end face on the inner peripheral side of the magnetic disk and the concentricity of the inner peripheral end face and the outer peripheral end face was not affected, but the servo signal reading error was not improved.
- the probe contacts at a position that protrudes most outward from the substrate in the thickness direction.
- the contour line of the outer peripheral end portion that is the basis of roundness measurement reflects the shape that protrudes most outward from the substrate regardless of the shape of the outer peripheral end portion in the thickness direction.
- the conventional roundness measurement method does not reflect the three-dimensional shape in the thickness direction of the side wall surface of the outer peripheral end portion. If the roundness of the outer peripheral edge of the magnetic disk is sufficiently improved by the conventional roundness measurement method, the influence of other factors other than roundness on the flutter becomes relatively large. Therefore, it was thought that there was no correlation between roundness and flutter.
- the present inventor has looked at the shape of the magnetic disk in the plate thickness direction in addition to the parameters in the in-plane direction of the magnetic disk such as roundness.
- the thickness variation at the outer peripheral end of the magnetic disk was examined, but the variation was extremely small and no problem was found. Therefore, as a result of diligent research on various other shape parameters, the inclination and unevenness in the thickness direction of the side wall surface (the surface extending in the direction perpendicular to the main surface) of the outer peripheral end surface of the magnetic disk are It has been revealed for the first time that it affects the flutter at the outermost periphery, and in turn affects the reading of servo signals. That is, it has been clarified that the shape of the outer peripheral end face in the thickness direction affects flutter only when the roundness of the outer peripheral end face of the magnetic disk is made extremely small.
- a glass substrate for a magnetic disk according to the present invention comprises a pair of main surfaces, a side wall surface formed at an outer peripheral end, and a chamfered surface interposed between the side wall surface and the main surface.
- the roundness of the side wall surface is 1.5 ⁇ m or less, and the radius of the inscribed circle and the circumscribed circle of the plurality of contour lines of the side wall surface at a plurality of positions including the center position and different in the plate thickness direction. The difference is 5 ⁇ m or less.
- the glass substrate for a magnetic disk according to the present invention acquires circumferential contour lines at two positions separated by 200 ⁇ m in the plate thickness direction on the side wall surface on the outer circumferential side, and obtains two minimum values respectively obtained from these contour lines.
- the center point between the centers of the square circles is set as the center point A, and the contour lines in the circumferential direction are respectively obtained at the positions of the center in the thickness direction on the two chamfered surfaces on the outer peripheral side, and obtained from these contour lines.
- the distance between the midpoint A and the center B is preferably 1 ⁇ m or less.
- the glass substrate for magnetic disk of the present invention is suitably used when the plate thickness is 0.5 mm or less.
- the magnetic disk of the present invention is suitably used when a magnetic layer is formed on the main surface of the glass substrate for magnetic disk described above.
- the top view of the glass substrate for magnetic discs of this embodiment Sectional drawing of the plate
- Aluminosilicate glass, soda lime glass, borosilicate glass, or the like can be used as the material for the magnetic disk glass substrate in the present embodiment.
- aluminosilicate glass can be suitably used in that it can be chemically strengthened and a glass substrate for a magnetic disk excellent in the flatness of the main surface and the strength of the substrate can be produced. More preferably, it is an amorphous aluminosilicate glass.
- the composition of the glass substrate for a magnetic disk of this embodiment is not limited, the glass substrate of this embodiment is preferably converted to an oxide standard and expressed in mol%, SiO 2 is 50 to 75%, Al 2 to O 3 to 1 to 15%, at least one component selected from Li 2 O, Na 2 O and K 2 O in total 5 to 35%, selected from MgO, CaO, SrO, BaO and ZnO 0-20% in total of at least one component, and at least one selected from ZrO 2 , TiO 2 , La 2 O 3 , Y 2 O 3 , Ta 2 O 5 , Nb 2 O 5 and HfO 2 An amorphous aluminosilicate glass having a composition having a total of 0 to 10% of components.
- the glass substrate of the present embodiment is preferably, for example, in terms of mass%, SiO 2 is 57 to 75%, Al 2 O 3 is 5 to 20% (however, the total amount of SiO 2 and Al 2 O 3 is 74% or more), ZrO 2 , HfO 2 , Nb 2 O 5 , Ta 2 O 5 , La 2 O 3 , Y 2 O 3 and TiO 2 in total exceed 0%, 6% or less, Li 2 O 1 %, 9% or less, Na 2 O 5 to 28% (where the mass ratio Li 2 O / Na 2 O is 0.5 or less), K 2 O 0 to 6%, MgO 0 to 4% , CaO exceeds 0% and 5% or less (however, the total amount of MgO and CaO is 5% or less and the content of CaO is larger than the content of MgO), and SrO + BaO is 0 to 3% An amorphous aluminosilicate glass having a composition may be used.
- the glass substrate of this embodiment is, for example, SiO 2 : 45.60 to 60%, Al 2 O 3 : 7 to 20%, and B 2 O 3 : 1.00 to 8 in mass% based on oxide. %, And P 2 O 5 : 0.50 to 7%, and TiO 2 : 1 to 15%, and the total amount of RO: 5 to 35% (where R is Zn and Mg), content of CaO 3.00% or less, the content of BaO is equal to or less than 4%, PbO component, As 2 O 3 component and Sb 2 O 3 component and Cl -, NO -, SO 2- , F - component
- the main crystal phase contains RAl 2 O 4 , R 2 TiO 4 , where R is one or more selected from Zn and Mg, and the crystal grain size of the main crystal phase Is in the range of 0.5 nm to 20 nm, and the crystallinity is 15% or less It may be crystallized glass characterized by having a specific gravity of 2.95 or less.
- the composition of the glass substrate for a magnetic disk according to the present embodiment includes, as an essential component, at least one alkaline earth metal selected from the group consisting of SiO 2 , Li 2 O, Na 2 O, and MgO, CaO, SrO, and BaO.
- the molar ratio of the CaO content to the total content of MgO, CaO, SrO and BaO (CaO / (MgO + CaO + SrO + BaO)) is 0.20 or less and the glass transition temperature is 650 ° C. or more. Also good.
- a glass substrate for a magnetic disk having such a composition is suitable for a glass substrate for a magnetic disk used for a magnetic disk for energy-assisted magnetic recording.
- the glass substrate for magnetic disk in this embodiment is an annular thin glass substrate.
- the size of the glass substrate for magnetic disks is not ask
- FIG. 1A and 1B show a glass substrate G for magnetic disk of this embodiment.
- FIG. 1A is a plan view of the glass substrate G for magnetic disks
- FIG. 1B is a cross-sectional view in the thickness direction of the glass substrate G for magnetic disks.
- the glass substrate G for magnetic disk includes a pair of main surfaces 11p and 12p, a side wall surface 11w formed at the outer peripheral end, and chamfered surfaces 11c and 12c interposed between the side wall surface 11w and the main surfaces 11p and 12p.
- the glass substrate G has a circular hole in the center.
- the side wall surface 11w includes a center position 32 (see FIG. 2) in the thickness direction of the glass substrate G.
- the inclination angle of the chamfered surfaces 11c and 12c with respect to the main surfaces 11p and 12p is not particularly limited, and is 45 °, for example.
- the boundary between the side wall surface 11w and the chamfered surfaces 11c and 12c is not limited to the shape having an edge as shown in the figure, and may be a curved surface that is smoothly continuous.
- FIG. 3 is a diagram for explaining a method of measuring the cylindricity of the outer peripheral end surface of the glass substrate G, and a plurality of contours of the side wall surface 11w at a plurality of positions 31, 32, 33 including the center position 32 and different in the plate thickness direction. Lines 31a, 32a, 33a (see FIG. 3) are shown.
- the cylindricity refers to the difference R between the radii of the inscribed circle C1 and the circumscribed circle C2 (see FIG.
- the roundness may be measured by a known method. For example, by arranging a plate-like probe longer than the thickness of the glass substrate so as to face the outer peripheral end surface in a direction perpendicular to the main surface of the glass substrate, the contour line can be obtained by rotating the glass substrate in the circumferential direction. And the difference in radius between the inscribed circle and the circumscribed circle of the contour line can be calculated as the roundness of the glass substrate.
- a roundness / cylindrical shape measuring device can be used for measuring the roundness.
- the roundness of the side wall surface 11w is adjusted, for example, by end face grinding described later and end face polishing using a magnetorheological fluid (hereinafter abbreviated as “MRF”) performed as necessary. Done.
- MRF magnetorheological fluid
- the cylindricity of the side wall surface 11 w is obtained using each contour line acquired at a plurality of measurement positions 31, 32, and 33 different in the thickness direction on the side wall surface 11 w.
- the measurement position 32 is the center position of the glass substrate G in the thickness direction.
- the measurement positions 31 and 33 are positions away from the measurement position 32 by 200 ⁇ m in the thickness direction, for example. In this embodiment, there are three measurement positions on the side wall surface 11w.
- the plate thickness from the measurement position 32 may be positions separated by 100 ⁇ m in the direction.
- the number of measurement positions may be greater than three.
- the contour lines 31a, 32a, and 33a can be distinguished and acquired at the measurement positions 31 to 33 of the side wall surface 11w. Is used. The same measuring device as that described above can be used.
- the stylus it is preferable to use a small hole measuring element having a relatively small diameter such as a curvature radius of the tip of ⁇ 0.4 mm or less.
- the stylus 3 is arranged so as to face each measurement position 31 to 33 of the measurement wall 11w of the glass substrate G, and the measurement is performed one by one in order.
- the outlines 31a to 33a of the measurement positions 31 to 33 are acquired by rotating the glass substrate G once in a state where the stylus 3 is arranged to face the measurement positions 31 to 33.
- the circumscribed circle C2 that is in contact with the outermost side as in the case of the roundness described above, and An inscribed circle C1 in contact with the innermost side is determined. Then, a radius difference R between the circumscribed circle C2 and the inscribed circle C1 is obtained as the cylindricity of the side wall surface 11w.
- the degree of cylindricity of the side wall surface 11w is adjusted by, for example, end face grinding described later and end face polishing using MRF performed as necessary.
- the shape evaluation value is an index value for evaluating the degree of coaxiality between the outer peripheral side wall surface and the chamfered surface of the glass substrate G.
- 4 and 5 are diagrams illustrating a method for measuring the shape evaluation value of the outer peripheral end face of the magnetic disk glass substrate G of the present embodiment.
- FIG. 4 shows a cross section in the thickness direction of the outer peripheral end face of the glass substrate G.
- the inclination angle of the side wall surface 11w is not particularly limited, and is, for example, 40 ° to 70 °.
- the boundary between the side wall surface 11w and the chamfered surfaces 11c and 12c is not limited to the shape having an edge as shown in the figure, and may be a curved surface that is smoothly continuous.
- the shape evaluation values are obtained by obtaining circumferential contour lines at two positions 37 and 38 separated by 200 ⁇ m in the plate thickness direction on the side wall surface 11w, respectively, and two least square circles 37c obtained from the contour lines, respectively.
- a midpoint between the centers 37o and 38o of 38c is defined as a midpoint A, and a contour line in the circumferential direction is formed at the center positions 34 and 35 of the plate thickness direction length on the two chamfered surfaces 11c and 12c.
- the center 34o obtained from one chamfered surface 11c is the center B and the center 35o obtained from the other chamfered surface 12c.
- the shape evaluation value of the glass substrate G is preferably 1.0 ⁇ m or less. More preferably, it is 0.5 ⁇ m or less.
- the two positions 37 and 38 on the side wall surface 11w are, for example, positions separated from the center position in the thickness direction of the glass substrate G by 100 ⁇ m toward the main surfaces 11p and 12p.
- the measurement positions 34 and 35 for acquiring the contour lines of the chamfered surfaces 11c and 12c are, for example, positions (for example, the surface of the glass substrate G) that approach the center position side in the plate thickness direction from the main surfaces 11p and 12p, respectively. When the length in the thickness direction of the chamfering surface is 0.15 mm, it is a position that approaches the central position by 0.075 mm from the main surfaces 11p and 12p of the glass substrate G).
- a roundness / cylindrical shape measuring device can be used as a measuring device for measuring the shape of the outer peripheral end face at each of the measurement positions 37, 38, 34, 35.
- the stylus 3 of the roundness / cylindrical shape measuring apparatus can be moved in the vertical direction (plate thickness direction) in units of microns. Prior to the measurement, the thickness of the glass substrate G is measured in advance with a micrometer.
- the contour shape measuring machine measures in advance the shape of the chamfered surface in the radial section, the length in the plate thickness direction and the radial direction, the angle with respect to the main surface, and the length of the side wall surface.
- the position of the boundary between the chamfered surface and the side wall surface can be determined by the intersection of the extended line of the side wall surface and the extended line of the chamfered surface when any of the outlines is linear.
- the contour line of the chamfered surface or the side wall surface has an arc shape, for example, it can be approximated by one circle that best overlaps the contour line, and can be determined by the intersection with the obtained circle.
- the main surface of the glass substrate G is horizontal with the reference surface of the roundness / cylindrical measuring device, and further, the center of the glass substrate G is aligned with the rotation center of the measuring device, A glass substrate G is set in the measuring device.
- the position of the tip of the stylus 3 that contacts the glass substrate G at the time of measurement is matched with the height of the upper main surface of the glass substrate G set in the measuring device.
- the stylus 3 is arranged at the center height of the thickness of the glass substrate G.
- the outline of the outer peripheral edge of the glass substrate G is measured at a point 37 where the stylus 3 is raised by 100 ⁇ m from the center of the plate thickness and a point 38 where the stylus 3 is lowered by 100 ⁇ m from the center of the plate thickness.
- the centers 37o and 38o of the two least square circles 37c and 38c of the side wall surface 11w are determined, and further, the midpoint A between these two centers 37o and 38o is determined. Further, the position of the stylus 3 is set to be an intermediate height between the two chamfered surfaces in the respective plate thickness directions, and the contour line of the outer peripheral edge of the glass substrate G is measured at each of the positions 34 and 35. The Based on these contour lines, the centers B and C of the least squares 34c and 35c of the chamfered surfaces 11c and 12c are determined. Next, the shape evaluation value is obtained by summing the distance a between the midpoint A and the center B and the distance b between the midpoint A and the center C.
- the intermediate positions 34 and 35 of the height in the plate thickness direction of the chamfered surface are the eccentricity of the cylinder corresponding to the chamfered surface portion when the structure having three cylinders having different diameters is considered. It is considered to be the point that best represents the degree. Further, the position is considered to be the point that has the most influence on the air flow in the vicinity of the chamfered surface. For these reasons, it is preferable to measure the contour line at this position.
- the shape evaluation value determined by the side wall surface 11w and the chamfered surfaces 11c and 12c is adjusted by, for example, end surface grinding described later and end surface polishing using MRF performed as necessary.
- the glass substrate G for magnetic disk described above has extremely small roundness and cylindricity. For this reason, the airflow is hardly disturbed at the outer peripheral end, and flutter is suppressed. Thereby, the followability to the servo information at the outer peripheral end is maintained.
- a high track recording density such as a magnetic disk adopting a single write method requires severe followability to servo information, but this glass substrate G can be suitably used for a magnetic disk.
- the reason why the servo information reading is stabilized due to the small cylindricity is considered as follows. When the roundness of the outer peripheral end of the glass substrate G is large, the amount of air pushed out by the outer peripheral end surface of the magnetic disk in the horizontal direction (plane direction) fluctuates.
- the present inventor has found that the outer peripheral end surface of the glass substrate G causes a disturbance in the air flow that cannot be solved by a change in the design of the HDD, resulting in a glass substrate G having an extremely small cylindricity of the outer peripheral end surface. .
- the plate thickness of the glass substrate G of the present embodiment is, for example, 0.8 mm and 0.635 mm, and is, for example, 0.5 mm or less.
- the glass substrate G When the glass substrate G is used for a magnetic disk, it tends to flutter as the plate thickness decreases, and flutter tends to increase.
- the glass substrate G has a cylindricity of 5 ⁇ m or less as described above, the turbulence of the air current at the outer peripheral side end portion is suppressed and flutter is suppressed when used for a magnetic disk.
- the glass substrate G of the present embodiment has a very small shape evaluation value and the shape of the outer peripheral end surface is less likely to cause turbulence of the airflow.
- shape evaluation value is small, flutter can be further suppressed when used in a magnetic disk. Thereby, the followability to the servo information of the magnetic head in the HDD is further improved.
- the glass substrate for a magnetic disk of this embodiment is suitable for a magnetic disk having the above-described high recording density.
- the evaluation index of the outer peripheral end portion on the main surface is 30 nm or less.
- the dove-off value is preferably greater than zero.
- the dub-off value is the profile of the main surface in the radial direction of the glass substrate G. When the profile between two points having a radius of 31.2 to 32.2 mm is measured and the two points are connected by a virtual line, the virtual line The maximum distance from the profile of the main surface of the glass substrate G.
- the dub-off value is expressed as a plus value when the virtual straight line is closer to the center in the thickness direction when the virtual straight line and the profile of the main surface are compared.
- the main surface contour line is closer to the center side of the plate thickness, it is represented by a negative value.
- the shape of the main surface in the vicinity of the outermost periphery is flatter and better, and the magnetic head floats more stably. Therefore, in combination with the extremely small roundness and cylindricity, the effect of further improving the followability to the servo signal of the magnetic head in the HDD can be obtained.
- the dub-off value can be measured using, for example, an optical surface shape measuring device.
- the dub-off value of this case measures the area
- the nanowaveness (NW-Rq) of the main surface on the outermost peripheral side of the outer peripheral side end portion is 0.5 ( ⁇ ) or less.
- the nano waveness can be expressed by an RMS (Rq) value calculated as a roughness of a wavelength band of 50 to 200 ⁇ m in an area of a radius of 30.5 to 31.5 mm. Can be measured. By doing so, the flying of the magnetic head is further stabilized. Accordingly, in this case as well, in addition to the extremely small roundness and cylindricity, the effect of improving the followability to the servo signal of the magnetic head in the HDD can be obtained.
- a glass substrate having a predetermined shape that is a base of the glass substrate for magnetic disk is cut out from the plate glass.
- the glass substrate may be molded by press molding using an upper mold and a lower mold, for example.
- a glass substrate can also be manufactured using well-known manufacturing methods, such as not only these methods but a downdraw method, a redraw method, and a fusion method. Note that rough grinding using loose abrasive grains may be performed on both main surfaces of the glass substrate as necessary.
- circular shaped glass substrate is performed.
- the grinding process for the end surface of the glass substrate is performed in order to form a chamfered surface on the outer peripheral side end portion and the inner peripheral side end portion of the glass substrate.
- the grinding process for the outer peripheral side end surface of the glass substrate may be, for example, a known chamfering process using a general-purpose grindstone using diamond abrasive grains.
- the grinding process on the outer peripheral side end surface of the glass substrate is performed by grinding the end surface of the glass substrate and the grindstone so that the locus of the grindstone contacting the end surface of the glass substrate is not constant.
- the grinding process with respect to the outer peripheral side end surface of a glass substrate is demonstrated below with reference to FIG. 6A and FIG. 6B.
- 6A and 6B are diagrams showing a processing method for the outer peripheral side end face of the glass substrate.
- 6B is a diagram showing a YY cross section of FIG. 6A.
- the grinding wheel 41 used for grinding the outer peripheral side end surface of the glass substrate G is formed in an annular shape and has a groove 43.
- the groove 43 is formed so that both the side wall surface 11w and the chamfered surface 11c on the outer peripheral side of the glass substrate G can be ground simultaneously.
- the groove 43 includes the side wall portion 43a and its side. It has a groove shape composed of chamfered portions 43b and 43b existing on both sides.
- the side wall portion 43a and the chamfered portion 43b of the groove 43 are formed in a predetermined dimensional shape in consideration of the final dimensional shape of the ground surface of the glass substrate G.
- the glass substrate G is inclined with respect to the groove direction of the groove 43 formed in the grinding wheel 41, that is, the rotation axis of the glass substrate G with respect to the rotation axis 46 of the grinding wheel 41.
- grinding is performed by rotating both the glass substrate G and the grinding wheel 41 while bringing the grinding wheel 41 into contact with the outer peripheral side end face 11w of the glass substrate G.
- the locus of the grinding wheel 41 that contacts the outer peripheral end surface of the glass substrate G does not become constant, and the abrasive grains of the grinding wheel 41 contact and act on the substrate end surface at random positions.
- the surface roughness and in-plane variation of the ground surface can be reduced, and the ground surface can be finished with a higher level of smoothness, that is, a quality that can meet higher quality requirements. Furthermore, it also has the effect of improving the wheel life.
- the contact state between the grinding wheel 41 and the glass substrate G is a surface contact state between the groove 43 of the grinding wheel 41 and the outer diameter arc of the glass substrate G.
- the contact area with the glass substrate G increases. Therefore, the contact length (cutting blade length) of the grinding wheel 41 with respect to the glass substrate G can be extended and the sharpness of the abrasive grains can be maintained. Therefore, stable grinding performance can be secured even when grinding is performed using a fine abrasive wheel that is advantageous for machining surface quality, and good grinding surface quality (mirror surface quality) by plastic mode-based grinding is stable. Can get to.
- by maintaining the sharpness of the grinding wheel and stably ensuring the grindability for realizing the plastic mode it is possible to ensure good dimensional shape accuracy by chamfering the outer peripheral side end face of the glass substrate.
- the inclination angle ⁇ of the glass substrate G with respect to the groove direction of the grinding wheel 41 can be arbitrarily set. However, in order to achieve the above-described effects better, it is set within a range of 2 to 8 degrees, for example. Is preferred.
- the grinding wheel 41 used for grinding is preferably a grinding wheel (resin bond grinding stone) in which diamond abrasive grains are bonded with a resin (resin).
- the count of the diamond grindstone is preferably # 2000 to # 3000.
- the peripheral speed of the grinding wheel 41 is about 800 to 1000 m / min, and the peripheral speed of the glass substrate G is about 10 m / min.
- the ratio of the peripheral speed of the grinding wheel 41 to the peripheral speed of the glass substrate G is preferably in the range of 80 to 200.
- the elastic modulus of the resin bond grindstone is preferably in the range of 2000 to 3000 [N / mm].
- the grindstone elastic modulus is an index having a correlation with the bond strength between diamond abrasive grains and resin.
- the outer peripheral side end surface after grinding can be made a quasi-mirror surface, so that the machining allowance can be reduced in the subsequent end surface polishing step, while maintaining the high surface quality,
- the shape accuracy of the end including the cylindricity can be increased.
- the grindstone elastic modulus can be calculated, for example, by measuring the displacement when the HRF indenter is pressed against the surface of the grindstone with a predetermined load (for example, 15 kgf) using a bending strength measurement tester. .
- circular shaped glass substrate is performed.
- the polishing process for the end surface of the glass substrate is performed in order to improve the surface properties on the outer peripheral side and inner peripheral side end surfaces (side wall surface and chamfered surface) of the glass substrate.
- end face polishing using MRF is performed on the outer peripheral side end face of the glass substrate.
- a magnetic slurry lump is formed by holding the magnetic slurry at the lines of magnetic force, and the lump and the outer peripheral end surface of the glass substrate are brought into contact with each other to move relative to each other. Polish the end face on the side.
- end face polishing using MRF the side wall surface and the chamfered surface can be polished simultaneously.
- the machining allowance in end face polishing using MRF is, for example, about 1 ⁇ m to 5 ⁇ m.
- polishing is performed using the slurry (free abrasive grain) containing a cerium oxide abrasive grain as an abrasive grain.
- the end surface polishing using MRF may also be performed on the inner peripheral side end surface of the glass substrate.
- FIG. 7A to FIG. 7C and FIG. 8 are diagrams for explaining an example of a polishing method for the outer peripheral side end face of the glass substrate by end face polishing using MRF.
- the apparatus 20 for polishing the end face using MRF polishes the end face of the glass substrate by using magnetism generating means and magnetic slurry.
- fine particles such as cerium oxide and zirconium oxide are used as the magnetorheological fluid and abrasive grains.
- Nonpolar oil or polar oil has a viscosity of 1 to 20 (Pa ⁇ sec) in a non-magnetized state at room temperature (20 ° C.), for example.
- the apparatus 20 includes a pair of magnets 22 and 24 that are permanent magnets, a spacer 26, and a nonmagnetic material such as stainless steel. And a cylindrical pipe 28. Magnets 22 and 24 and a spacer 26 are built in the pipe 28.
- a glass substrate to be subjected to end surface polishing using MRF is held by a holder (not shown).
- the pipe 28 is disposed opposite to the outer peripheral portion of the glass substrate held by the holder, and a later-described magnetic slurry lump 30 (see FIGS. 7C and 8) and the outer peripheral end surface of the glass substrate are brought into contact with each other.
- the outer peripheral end face of the glass substrate is polished by relatively moving the lump 30 formed by the magnets 22 and 24 in the pipe 28 and the outer peripheral end face of the glass substrate in contact with each other.
- a holder (not shown) that holds the pipe 28 and the glass substrate of the apparatus 20 is mechanically connected to a drive motor (not shown).
- the outer peripheral end surface of the glass substrate can be polished by relatively rotating at 500 to 2000 rpm. .
- the magnet 22 and the magnet 24 are close to each other and function as magnetism generating means to form a magnetic force line 29 as shown in FIG. 7B.
- the magnetic field lines 29 proceed so as to protrude outward from the centers of the magnets 22 and 24, and proceed in the thickness direction of the glass substrate.
- a spacer 26 made of a non-magnetic material is provided in order to produce a lump 30 of magnetic slurry on the outer periphery of the pipe 28 as shown in FIG. 7C.
- the magnetic flux density in the magnetism generating means may be set to such an extent that the magnetic slurry lump 30 is formed, but is preferably 0.3 to 5 Tesla from the viewpoint of efficient end face polishing.
- a permanent magnet is used as the magnetism generating means, but an electromagnet can also be used.
- the magnets 22 and 24 are fixed to the pipe 28 without using the spacer 26, and the separation distance between the N pole end face of the magnet 22 and the S pole end face of the magnet 24 can be secured constant.
- abrasive grains contained in the magnetic slurry known glass substrate abrasive grains such as cerium oxide, colloidal silica, zirconia oxide, alumina abrasive grains and diamond abrasive grains can be used.
- the particle size of the abrasive grains is, for example, 0.5 to 3 ⁇ m. By using abrasive grains in this range, the inner end face of the glass substrate can be satisfactorily polished.
- the abrasive grains are contained, for example, in an amount of 1 to 20 vol% in the magnetic slurry.
- Precision grinding process In a precision grinding process, it grinds with respect to the main surface of a glass substrate using a double-sided grinding apparatus.
- the double-sided grinding apparatus has a pair of upper and lower surface plates (upper surface plate and lower surface plate), and a glass substrate is sandwiched between the upper surface plate and the lower surface plate. And, by moving either the upper surface plate or the lower surface plate, or both, the glass substrate and each surface plate are moved relatively to grind both main surfaces of the glass substrate. Can do.
- loose abrasive grains can be used in addition to diamond fixed abrasive grains.
- polishing is given to the main surface of the ground glass substrate.
- the purpose of the first polishing is to remove scratches and distortions remaining on the main surface by precise grinding, and to adjust surface irregularities (micro-waveness, roughness).
- the main surface of the glass substrate is polished using a double-side polishing apparatus equipped with a planetary gear mechanism.
- the double-side polishing apparatus has an upper surface plate and a lower surface plate.
- a flat polishing pad (resin polisher) is attached to the upper surface of the lower surface plate and the bottom surface of the upper surface plate.
- One or more glass substrates accommodated in the carrier are sandwiched between the upper surface plate and the lower surface plate, and either or both of the upper surface plate and the lower surface plate are moved by a planetary gear mechanism.
- the main surfaces of the glass substrate can be polished by relatively moving the glass substrate and each surface plate.
- the resin polisher is preferably made of polyurethane.
- the hardness is preferably 70-100 in terms of Asker C hardness.
- the main surface of the glass substrate is polished by the abrasive contained in the polishing liquid.
- the abrasive is, for example, cerium oxide or zirconium oxide.
- the average particle size of the abrasive is preferably 0.3 to 3 ⁇ m.
- the glass substrate after the first polishing step is chemically strengthened.
- the chemical strengthening liquid for example, a mixed liquid of potassium nitrate and sodium sulfate or the like can be used, and the glass substrate can be immersed in the chemical strengthening liquid.
- the glass substrate by immersing the glass substrate in the chemical strengthening solution, lithium ions and sodium ions on the surface layer of the glass substrate are respectively replaced with sodium ions and potassium ions having a relatively large ion radius in the chemical strengthening solution, The glass substrate is strengthened.
- Second Polishing (Final Polishing) Step Next, second polishing is performed.
- the second polishing is intended for mirror polishing of the main surface.
- a polishing apparatus similar to that used in the first polishing is used.
- the difference from the first polishing is that the type and particle size of the free abrasive grains are different and the hardness of the resin polisher is different.
- the resin polisher preferably has an Asker C hardness of 60 to 90.
- it is made of polyurethane foam and is preferably a suede type.
- the free abrasive grains used for the second polishing for example, fine particles (particle size: diameter of about 10 to 100 nm) such as colloidal silica made turbid in the slurry are used. Thereby, the surface roughness of the main surface of a glass substrate can further be reduced, and an edge part shape can be adjusted to a preferable range. By cleaning the polished glass substrate, a glass substrate for a magnetic disk can be obtained.
- a magnetic disk is obtained as follows using a magnetic disk glass substrate.
- the magnetic disk is, for example, on the main surface of a glass substrate for magnetic disk (hereinafter simply referred to as “substrate”), in order from the closest to the main surface, at least an adhesion layer, an underlayer, a magnetic layer (magnetic recording layer), and a protection A layer and a lubricating layer are laminated.
- the substrate is introduced into a film forming apparatus that has been evacuated, and a film is sequentially formed from an adhesion layer to a magnetic layer on the main surface of the substrate in an Ar atmosphere by a DC magnetron sputtering method.
- a CoPt alloy can be used as the adhesion layer
- CrRu can be used as the underlayer.
- a CoPt alloy can be used. It is also possible to form a CoPt-based alloy and FePt based alloy L 10 regular structure and magnetic layer for heat-assisted magnetic recording.
- a magnetic recording medium can be formed by forming a protective layer using, for example, C 2 H 4 by a CVD method and subsequently performing nitriding treatment for introducing nitrogen into the surface. Thereafter, for example, PFPE (perfluoropolyether) is applied on the protective layer by a dip coating method, whereby a lubricating layer can be formed.
- PFPE perfluoropolyether
- the manufactured magnetic disk is preferably a magnetic disk drive device (HDD) as a magnetic recording / reproducing device, which includes a magnetic head equipped with a DFH (Dynamic Flying Height) control mechanism and a spindle for fixing the magnetic disk. (Hard Disk Drive)).
- HDD magnetic disk drive device
- DFH Dynamic Flying Height
- Glass composition SiO 2 65 mol%, Al 2 O 3 6 mol%, Li 2 O 1 mol%, Na 2 O 9 mol%, MgO 17 mol%, CaO 0 mol%, SrO 0 mol%, BaO 0 mol%, ZrO 2 2 mol%
- the molar ratio of the CaO content to the total content of MgO, CaO, SrO and BaO is 0 and is an amorphous aluminosilicate glass having a glass transition temperature of 671 ° C.
- the glass substrate for magnetic disks of an Example it produced by performing each process of the manufacturing method of the glass substrate for magnetic disks of this embodiment in order.
- the glass substrate of (1) was formed using a press molding method. In rough grinding, loose abrasive grains were used.
- the end surface grinding step of (3) first, chamfering processing was performed on the outer peripheral side end surface of the glass substrate with a general-purpose grindstone using diamond abrasive grains.
- the angle of inclination of the glass substrate with respect to the groove direction of the grinding wheel ( ⁇ in FIG. 6A) is set to 5 degrees using a resin bond grindstone of # 2500 diamond abrasive grains. The other conditions were adjusted as appropriate.
- end surface polishing step (4) end surface polishing using MRF is performed on the outer peripheral side end surface of the glass substrate, and brush polishing is performed on the inner peripheral side end surface of the glass substrate using slurry containing cerium oxide abrasive grains as polishing abrasive grains. Went.
- the polishing slurry was obtained by further dispersing cerium oxide abrasive grains in a magnetic fluid in which fine particles of Fe are dispersed in nonmagnetic oil.
- magnets used in the examples permanent magnets having a magnetic flux density of 3 Tesla were used.
- polishing was performed using a double-side polishing apparatus equipped with a planetary gear.
- a slurry containing cerium oxide abrasive grains having an average particle diameter of 1.5 ⁇ m and a hard urethane pad (Asker C hardness: 85) were used as a polishing pad.
- a mixed melt of potassium nitrate and sodium nitrate was used as the chemical strengthening liquid.
- glass substrates for magnetic disks of comparative examples and examples were produced.
- the roundness and cylindricity of the side wall surface of the outer peripheral end surface are different between the glass substrates for magnetic disks of the comparative example and the example.
- Glass substrates having different roundness and cylindricity of the side wall surfaces were produced by appropriately changing the grindstone elastic modulus of the resin bond grindstone used mainly for grinding on the outer peripheral side end surface.
- the roundness of the side wall surface of a magnetic disk glass substrate is measured by obtaining a contour line by placing a plate-shaped probe longer than the glass substrate thickness in a direction perpendicular to the main surface of the glass substrate. did.
- the cylindricity of the side wall surface was calculated as shown in FIG. That is, the center position of the side wall surface in the thickness direction and the contour line at a position 200 ⁇ m apart from the center position are obtained, the radius of the inscribed circle of the three contour lines is obtained, and then the three contour lines The difference between the maximum value and the minimum value of the inscribed circle radii was determined as the cylindricity of the side wall surface. All measurements were performed using a roundness / cylindrical measuring machine.
- Magnetic disk glass substrates of comparative examples and examples were prepared, and a magnetic layer was formed to produce a magnetic disk.
- the magnetic disk is incorporated into a 2.5-inch HDD with a disk rotational speed of 7200 rpm together with a DFH head, and after recording a magnetic signal at a track density of 500 kTPI, servo signals are read in an area of a radius of 30.4 to 31.4 mm. A test was conducted.
- glass substrates for magnetic disks of Examples 7 to 12 were produced.
- Examples 7 to 9 were produced based on the production conditions of Example 1
- Examples 10 to 12 were produced based on the production conditions of Example 3.
- Glass substrates having different shape evaluation values on the outer peripheral end face were made separately. As the tilt angle is increased, the surface quality after grinding is improved, and the allowance for subsequent polishing can be reduced. Therefore, the shape evaluation value can be improved.
- a magnetic disk having a magnetic layer formed on the obtained glass substrate for magnetic disk was produced. Thereafter, fluttering was evaluated for each magnetic disk using a laser Doppler vibrometer.
- a laser Doppler vibrometer For evaluation of fluttering, first, a magnetic disk is mounted on a spindle of a hard disk drive (HDD) having a rotational speed of 7200 rpm, and laser light is irradiated from the laser Doppler vibrometer to the main surface of the rotating magnetic disk. Next, the laser beam reflected by the magnetic disk is received by the laser Doppler vibrometer, thereby obtaining a vibration value in the thickness direction of the magnetic disk. This vibration value is called fluttering characteristic value. More details are as follows.
- a magnetic disk is mounted on a 2.5-inch HDD spindle, the magnetic disk is rotated, and the main surface of the rotating magnetic disk is irradiated with laser light from a laser Doppler vibrometer. .
- a cover is properly attached and a hole for laser irradiation is formed in the HDD cover.
- the laser Doppler vibrometer receives the laser light reflected by the magnetic disk, and the amount of shake in the thickness direction of the magnetic disk is measured as a fluttering characteristic value.
- fluttering characteristic values were measured under the following conditions. ⁇ Environment of HDD and measurement system: Maintain the temperature at 25 ° C.
Abstract
Description
さらに、記憶容量の一層の増大化のために、DFH(Dynamic Flying Height)機構を搭載した磁気ヘッドを用いて磁気記録面からの浮上距離を極めて短くすることにより、磁気ヘッドの記録再生素子と磁気ディスクの磁気記録層との間の磁気的スペーシングを低減して情報の記録再生の精度をより高める(S/N比を向上させる)ことも行われている。この場合においても、磁気ヘッドによる磁気記録情報の読み書きを長期に亘って安定して行うために、磁気ディスクの基板の表面凹凸は可能な限り小さくすることが求められる。
例えばシングルライト方式を採用するなどして500kTPI以上としたHDDにおいて、磁気ディスクの外周端面の真円度を1.5μm以下に低減しても、磁気ディスクの外周側の端部ではサーボ信号の読み取りが不安定となる現象が生じていた。特に、磁気ディスクの外周側端部の最外周側は、それより内周側の領域と比べ、安定した読み取りが困難になっていた。
従来、磁気ディスクの真円度を下げるとフラッタが小さくなることから、真円度とフラッタの間には相関性があると考えられていた。しかし、本発明者の研究によれば、真円度を1.5μm以下にしても、フラッタは少なくならず、真円度が極めて小さい場合には、真円度とフラッタの間には相関性が見られないことが明らかとなった。その理由は以下のように考えられた。すなわち、従来は、ガラス基板の板厚よりも長い板状のプローブをガラス基板の主表面に対して垂直方向に立てて外周端部に当てることで外周端部の真円度を測定していた。このとき、プローブは、板厚方向において最も基板の外側へ突出した位置で接する。したがって、真円度測定の基礎となる外周端部の輪郭線には、外周端部の板厚方向の形状とは無関係に、基板の外側へ最も突出した形状が反映されることになる。そのため、従来の真円度の測定方法では、外周端部の側壁面の板厚方向での3次元形状を反映したものとはなっていなかった。そして、従来の真円度の測定方法で磁気ディスクの外周端部の真円度を十分に良好にした場合には、真円度以外の別の要因がフラッタに及ぼす影響が相対的に大きくなり、それによって真円度とフラッタの間には相関性が見られなくなったと考えられた。
そこで、本発明者は、真円度のような磁気ディスクの面内方向のパラメータに加え、磁気ディスクの板厚方向の形状に目を向けた。まず、磁気ディスクの外周側端部における板厚のバラつきを調べたが、バラつきは極めて小さく、問題は見出だせなかった。そこで、他の様々な形状パラメータについて鋭意研究を行った結果、磁気ディスクの外周端面のうち、側壁面(主表面と直交する方向に延びる面)の板厚方向の傾きや凹凸が、磁気ディスクの最外周部のフラッタに影響を与え、ひいてはサーボ信号の読み取りに影響を与えていることを初めて明らかにした。つまり、磁気ディスクの外周端面の真円度を極めて小さくしたことによって初めて、外周端面の板厚方向の形状がフラッタに影響を与えることが、明らかにされた。
本実施形態における磁気ディスク用ガラス基板の材料として、アルミノシリケートガラス、ソーダライムガラス、ボロシリケートガラスなどを用いることができる。特に、化学強化を施すことができ、また主表面の平坦度及び基板の強度において優れた磁気ディスク用ガラス基板を作製することができるという点で、アルミノシリケートガラスを好適に用いることができる。アモルファスのアルミノシリケートガラスとするとさらに好ましい。
磁気ディスク用ガラス基板Gは、一対の主表面11p,12p、外周側端部に形成された側壁面11w、及び、側壁面11wと主表面11p,12pの間に介在する面取面11c,12cとを備える。
ガラス基板Gは、中心部に円孔を有する。側壁面11wは、ガラス基板Gの板厚方向の中心位置32(図2参照)を含む。図2は、ガラス基板Gの外周端面の円筒度の測定方法を説明する図であり、ガラス基板Gの外周端面の板厚方向の断面を示す。面取面11c,12cの主表面11p,12pに対する傾斜角度は、特に制限されず、例えば45°である。また、側壁面11w及び面取面11c,12cの境界は、図示されるようなエッジを有する形状に限定されるものではなく、滑らかに連続する曲面状であってもよい。
真円度の測定方法は、公知の方法でよい。例えば、ガラス基板の板厚よりも長い板状のプローブをガラス基板の主表面に対して垂直方向に、外周端面と対向するように配置し、ガラス基板を円周方向に回転させることで輪郭線を取得し、この輪郭線の内接円と外接円との半径の差をガラス基板の真円度として算出することができる。なお、真円度の測定には、例えば、真円度・円筒形状測定装置を用いることができる。
側壁面11wの真円度の調節は、例えば、後で説明する端面研削加工および必要に応じて行われる磁性粘性流体(MagnetoRheological Fluid;以下、「MRF」と略記する。)を用いた端面研磨によって行われる。
図2に示すように、側壁面11wの円筒度は、側壁面11wにおいて板厚方向に異なる複数の測定位置31,32,33で取得される各輪郭線を用いて求められる。測定位置32は、ガラス基板Gの板厚方向の中心位置である。測定位置31,33は、例えば、測定位置32から板厚方向に200μm離れた位置である。なお、側壁面11w上の複数の測定位置は、本実施形態では3箇所ある。板厚が0.635mm以下の場合や、面取り量が大きいことなどによって、上述した測定位置の決定方法による測定位置31及び33が側壁面上から外れてしまう場合には、測定位置32から板厚方向にそれぞれ100μmずつ離れた位置を測定位置31、33としてもよい。測定位置の数は3よりも多くても構わない。
各測定位置31~33でのガラス基板Gの外周端面の形状を測定するための測定装置としては、側壁面11wの測定位置31~33において各輪郭線31a,32a,33aを区別して取得できるものが用いられる。測定装置は、前記した真円度と同じものを用いることができる。スタイラスは、例えば先端の曲率半径がφ0.4mm以下等の、比較的小径の小穴用測定子を用いることが好ましい。測定の際には、スタイラス3は、ガラス基板Gの測定壁11wの各測定位置31~33に対向するよう配置され、一箇所ずつ順に測定を行う。
各測定位置31~33の輪郭線31a~33aは、スタイラス3を各測定位置31~33に対向して配置した状態で、ガラス基板Gを一周回転させることで取得される。そして、取得された3つの輪郭線31a~33aを重ねて得られる輪郭線から最小二乗法で求めた中心Oに基づいて、上記の真円度のときと同様に最も外側に接する外接円C2及び最も内側に接する内接円C1が決められる。そして、これら外接円C2及び内接円C1の半径の差Rが、側壁面11wの円筒度として求められる。
側壁面11wの円筒度は、例えば、後述する端面研削加工および必要に応じて行われるMRFを用いた端面研磨によって調節される。
図4及び図5を参照して、ガラス基板Gの形状評価値について説明する。形状評価値とは、ガラス基板Gの外周の側壁面と面取面の同軸の程度を評価するための指標値である。
図4及び図5は、本実施形態の磁気ディスク用ガラス基板Gの外周端面の形状評価値の測定方法を説明する図である。図4は、ガラス基板Gの外周端面の板厚方向の断面を示す。側壁面11wの傾斜角度は、特に制限されず、例えば40°~70°である。また、側壁面11w及び面取面11c,12cの境界は、図示されるようなエッジを有する形状に限定されるものではなく、滑らかに連続する曲面状であってもよい。
側壁面11w上の2つの位置37,38は、例えば、ガラス基板Gの板厚方向の中心位置から100μmずつ主表面11p,12p側に離れた位置である。面取面11c,12cの輪郭線を取得するための測定位置34,35は、例えば、主表面11p,12pからそれぞれ板厚方向の中心位置側に等距離近づく位置(例えば、ガラス基板Gの面取面の板厚方向長さが0.15mmの場合、ガラス基板Gの主表面11p,12pから中心位置に0.075mmずつ近づく位置)である。
なお、測定に先立って、マイクロメータで予めガラス基板Gの板厚が測定される。また、輪郭形状測定機により、半径方向の断面における面取面の、形状、板厚方向および半径方向の各長さ、主表面に対する角度、さらに、側壁面の長さ、が予め測定される。面取面と側壁面との境界の位置は、いずれの外形線も直線状である場合は、側壁面の延長線と面取面の延長線との交点によって定めることができる。面取面や側壁面の外形線が円弧状である場合は、例えば、当該外形線と最もよく重なる1つの円で近似し、求めた円との交点によって定めることができる。
測定の際には、ガラス基板Gの主表面が真円度・円筒形状測定装置の基準面と水平になるように、さらには、ガラス基板Gの中心が測定装置の回転中心と合うように、ガラス基板Gが測定装置にセットされる。そして、スタイラス3の先端の、測定時にガラス基板Gと接触する位置が、測定装置にセットされたガラス基板Gの上側の主表面の高さと合わせられる。この状態で、スタイラス3を、板厚の半分の距離を板厚方向に下げると、スタイラス3は、ガラス基板Gの板厚の中央の高さに配される。そして、スタイラス3を板厚の中央から100μm上げた点37、および、板厚の中央から100μm下げた点38において、ガラス基板Gの外周端部の輪郭線が測定される。これらの輪郭線から、側壁面11wの2つの最小二乗円37c,38cの中心37o,38oが決められ、さらに、これら2つの中心37o,38o間の中点Aが決められる。
また、スタイラス3の位置が、2つの面取面の、それぞれの板厚方向における中間の高さとなるよう設定され、それぞれの位置34,35でガラス基板Gの外周端部の輪郭線が測定される。これらの輪郭線に基づいて、面取面11c,12cの最小二乗円34c,35cの中心B,Cが決められる。次いで、中点Aおよび中心B間の距離aと、中点Aおよび中心C間の距離bとを合計することで、形状評価値が求められる。
なお、面取面の板厚方向の高さの中間の位置34,35は、前述の径の異なる3つの円筒を有する構造体を考えた場合に、面取面部分に相当する円筒の偏心の程度を最もよく表す点と考えられる。また、当該位置は、面取面近傍の空気の流れに最も多くの影響を与える点であると考えられる。これらの理由から、当該位置で輪郭線を測定することが好ましい。
側壁面11wと面取面11c,12cによって決定される形状評価値は、例えば、後述する端面研削加工および必要に応じて行われるMRFを用いた端面研磨によって調節される。
円筒度が小さいことによりサーボ情報の読み取りが安定する理由は、次のように考えられる。ガラス基板Gの外周端部の真円度が大きい場合は、磁気ディスクの外周端面が水平方向(面方向)に押し出す空気の量が変動するため、大きな気流の乱れが起きやすい。しかし、外周端面の真円度が極めて小さいと、そのような大きな気流の乱れは生じにくい。外周端面の真円度が極めて小さい状況では、水平方向の気流の代わりに、ガラス基板Gの外周端部とHDD内壁との隙間を、いかに空気が磁気ディスクを跨ぐように板厚方向にスムーズに流れるかが重要である。
本発明者の研究によれば、HDDの内部において、HDD内壁と磁気ディスクの外周端面との間の隙間には、定常的に板厚方向の空気の流れが存在しており、この流れを乱し不規則になる現象が生じると、フラッタが発生して磁気ヘッドの浮上が不安定となることが分かった。逆に、ガラス基板Gの外周端面の円筒度が小さいと、HDD内壁と磁気ディスクの外周端面との間の隙間において、板厚方向の空気は定常的にスムーズに流れ、フラッタが発生しにくい。
上述の通り、極めて高いトラック記録密度のHDDでは、HDDの内部の空気の流れの乱れが、磁気ヘッドのサーボ情報への追従性を改善する上で重要である。このような空気の乱れによって、フラッタは大きくなる。この空気の乱れには、周期的(定常的)に発生する乱れと、突発的に発生する乱れとの2種類がある。このうち、周期的に発生する乱れについては、HDDの設計を変えることで解消できる場合が多いが、突発的に発生する乱れについては、HDDの設計を変えることでは改善できないため、他の手段によって低減を図ることが求められる。本発明者は、ガラス基板Gの外周端面が、HDDの設計の変更によっては解決できない空気の流れの乱れを引き起こすことを見出して、外周端面の円筒度が極めて小さいガラス基板Gをなすに至った。
こうすることにより、磁気ヘッドの浮上がさらに安定する。したがって、ここでも、真円度および円筒度が極めて小さいことと合わせて、HDDにおける磁気ヘッドのサーボ信号への追従性が良好になる効果が得られる。
以下、本実施形態の磁気ディスク用ガラス基板の製造方法について、工程毎に説明する。ただし、各工程の順番は適宜入れ替えてもよい。
例えばフロート法によって板状ガラスを形成した後、この板状ガラスから、磁気ディスク用ガラス基板の元となる所定形状のガラス基板が切り出される。フロート法の代わりに、例えば上型と下型を用いたプレス成形によってガラス基板を成形してもよい。なお、ガラス基板は、これらの方法に限らず、ダウンドロー法、リドロー法、フュージョン法などの公知の製造方法を用いて製造することもできる。
なお、ガラス基板の両主表面に対して、必要に応じて、遊離砥粒を用いた粗研削加工を行ってもよい。
円筒状のドリルを用いて、円盤状ガラス基板の中心部に内孔を形成し、円環状のガラス基板とする。なお、ダイヤモンドカッター等によるスクライビングを用いることもできる。
次に、円環状のガラス基板の端面に対する研削加工が行われる。ガラス基板の端面に対する研削加工は、ガラス基板の外周側端部と内周側端部に対して面取面を形成するために行われる。ガラス基板の外周側端面に対する研削加工は、例えば、ダイヤモンド砥粒を用いた総形砥石による公知の面取り加工でよい。
ガラス基板の外周側端面に対する研削加工は、ガラス基板の端面に当接する砥石の軌跡が一定とならないように、ガラス基板の端面と砥石とを接触させる研削加工で行う。ガラス基板の外周側端面に対する研削加工について、図6Aおよび図6Bを参照して以下で説明する。
図6Aおよび図6Bに示すように、ガラス基板Gの外周側端面の研削加工に用いる研削砥石41は、全体が円環状に形成されているとともに溝43を有する。溝43は、ガラス基板Gの外周側の側壁面11wと面取面11cとの両方の面を同時に研削加工できるように形成されており、具体的には、溝43は、側壁部43a及びその両側に存在する面取部43b,43bからなる溝形状を備えている。上記溝43の側壁部43a及び面取部43bは、ガラス基板Gの研削加工面の仕上がり目標の寸法形状を考慮して、所定の寸法形状に形成されている。
ガラス基板の外周側端面の加工では、研削砥石41に形成された溝43の溝方向に対してガラス基板Gを傾けた状態、つまり研削砥石41の回転軸46に対してガラス基板Gの回転軸45を角度αだけ傾けた状態で、ガラス基板Gの外周側端面11wに研削砥石41を接触させながら、ガラス基板Gと研削砥石41の両方を回転させて研削加工を行う。これによって、ガラス基板Gの外周側端面に当接する研削砥石41の軌跡が一定とはならないで、研削砥石41の砥粒が基板端面に対してランダムな位置に当接、作用するため、基板へのダメージが少なく、研削加工面の表面粗さやその面内ばらつきも小さくなり、研削加工面をより高平滑に、すなわちより高い品質要求に応えられるレベルの品位に仕上げることができる。さらには砥石寿命の向上効果も有する。
研削砥石41の周速度は例えば、800~1000m/分、ガラス基板Gの周速度は、10m/分程度である。また、ガラス基板Gの周速度に対する研削砥石41の周速度の比(周速度比)は、80~200の範囲内であることが好ましい。
発明者は、様々な特性のレジンボンド砥石を用いて外周側端面の研削加工を行いガラス基板の端面の加工品質を観察した結果、レジンボンド砥石におけるダイヤモンド砥粒と樹脂との結合強度が、上記研削加工後のガラス基板の外周端面の側壁面の円筒度に大きく影響を与えることを見出した。すなわち、砥石弾性率が高過ぎるレジンボンド砥石を用いて外周端面の研削加工を行うと、加工レートは良好となるが表面にキズが入り易くなって側壁面の円筒度は悪化し、砥石弾性率が低過ぎるレジンボンド砥石を用いて外周側端面の研削加工を行うと、側壁面の円筒度は良好となるが加工レートが著しく低下することがわかった。換言すると、砥石弾性率を変化させることでガラス基板の側壁面の円筒度を調節することができる。その結果、砥石弾性率の範囲は、上記範囲が好ましいことがわかった。上記範囲内とすることで、研削加工後の外周側端面を準鏡面とすることができるので、その後の端面研磨工程では取代を少なくすることができ、高い表面品質を維持しつつ、側壁面の円筒度を含む端部の形状精度を高めることができる。
砥石弾性率は、例えば、抗折強度測定試験機を用いて、HRF圧子を砥石の表面に対して所定の荷重(例えば15kgf)で押圧させたときの変位を測定することにより算出することができる。
次に、円環状のガラス基板の端面に対する研磨加工が行われる。ガラス基板の端面に対する研磨加工は、ガラス基板の外周側及び内周側端面(側壁面及び面取面)に対する表面性状を良好にするために行われる。
本実施形態では、ガラス基板の外周側端面についてMRFを用いた端面研磨が施される。MRFを用いた端面研磨では、磁性スラリーを磁力線に保持させることにより磁性スラリーの塊を形成させ、この塊と、ガラス基板の外周端面とを接触させて相対移動させることにより、ガラス基板の内周側の端面の研磨を行う。MRFを用いた端面研磨では、側壁面と面取面とを同時に研磨することができる。MRFを用いた端面研磨おける取り代は、例えば1μm~5μm程度である。
ガラス基板の内周側端面については、研磨砥粒として酸化セリウム砥粒を含むスラリー(遊離砥粒)を用いてブラシ研磨を行う。なお、ブラシ研磨に代えて、ガラス基板の内周側端面についてもMRFを用いた端面研磨を施すようにしてもよい。
MRFを用いた端面研磨を行う装置20は、磁気発生させる手段と磁性スラリーを用いてガラス基板の端面の研磨を行う。磁性スラリーには、磁気粘性流体と、研磨砥粒として、例えば、酸化セリウムや酸化ジルコニウム等の微粒子が用いられる。磁気粘性流体は、例えば、0.1~10μmのFeからなる磁性体微粒子を3~5g/cm3含む非極性オイル、及び界面活性剤を含んだ流体が用いられる。非極性オイルあるいは極性オイルは、例えば、室温(20℃)において非磁化状態で1~20(Pa・秒)の粘度を有する。
磁気発生手段における磁束密度は、磁性スラリーの塊30を形成させる程度に設定すればよいが、端面研磨を効率よく行う点で、0.3~5テスラであることが好ましい。
なお、図7A~図7C及び図8に示す例では、磁気発生手段として永久磁石を用いたが、電磁石を用いることもできる。また、スペーサ26を用いず、パイプ28に磁石22,24が固定されて、磁石22のN極の端面と磁石24のS極の端面との間の離間距離を一定に確保することもできる。
精研削工程では、両面研削装置を用いてガラス基板の主表面に対して研削加工を行う。両面研削装置は、上下一対の定盤(上定盤および下定盤)を有しており、上定盤および下定盤の間にガラス基板が狭持される。そして、上定盤または下定盤のいずれか一方、または、双方を移動操作することにより、ガラス基板と各定盤とを相対的に移動させることで、このガラス基板の両主表面を研削することができる。精研削工程では、例えばダイヤモンドの固定砥粒のほか、遊離砥粒を用いることができる。
次に、研削されたガラス基板の主表面に第1研磨が施される。第1研磨は、精研削により主表面に残留したキズ、歪みの除去、表面凹凸(マイクロウェービネス、粗さ)の調整を目的とする。
第1研磨工程では、遊星歯車機構を備えた両面研磨装置を用いてガラス基板の主表面に対する研磨を行う。両面研磨装置は、上定盤および下定盤を有している。下定盤の上面および上定盤の底面には、平板の研磨パッド(樹脂ポリッシャ)が取り付けられている。上定盤および下定盤の間に、キャリアに収容した1又は複数のガラス基板が狭持され、遊星歯車機構により、上定盤または下定盤のいずれか一方、または、双方を移動操作することにより、ガラス基板と各定盤とを相対的に移動させることで、このガラス基板の両主表面を研磨することができる。樹脂ポリッシャは、ポリウレタン製のものが好ましい。また、硬度は、アスカーC硬度で70~100とすることが好ましい。
上記相対運動の動作中には、上定盤がガラス基板に対して(つまり、鉛直方向に)所定の荷重で押圧され、ガラス基板に対して研磨パッドが押圧されるとともに、ガラス基板と研磨パッドの間に研磨液が供給される。この研磨液に含まれる研磨剤によってガラス基板の主表面が研磨される。研磨剤は、例えば酸化セリウムや酸化ジルコニウムである。研磨剤の平均粒径は、0.3~3μmとすることが好ましい。
次に、第1研磨工程後のガラス基板は化学強化される。
化学強化液として、例えば硝酸カリウムと硫酸ナトリウムの混合液等を用い、ガラス基板を化学強化液中に浸漬することによって行うことができる。
このように、ガラス基板を化学強化液に浸漬することによって、ガラス基板の表層のリチウムイオン及びナトリウムイオンが、化学強化液中のイオン半径が相対的に大きいナトリウムイオン及びカリウムイオンにそれぞれ置換され、ガラス基板が強化される。
次に、第2研磨が施される。第2研磨は、主表面の鏡面研磨を目的とする。第2研磨では例えば、第1研磨で用いたものと同様の研磨装置を用いる。このとき、第1研磨と異なる点は、遊離砥粒の種類及び粒子サイズが異なることと、樹脂ポリッシャの硬度が異なることである。樹脂ポリッシャの硬度は、アスカーC硬度で60~90とすることが好ましい。また、発泡ポリウレタン製で、スウェードタイプのものが好ましい。
第2研磨に用いる遊離砥粒として、例えば、スラリーに混濁させたコロイダルシリカ等の微粒子(粒子サイズ:直径10~100nm程度)が用いられる。これにより、ガラス基板の主表面の表面粗さをさらに低減でき、端部形状を好ましい範囲に調整できる。
研磨されたガラス基板を洗浄することで、磁気ディスク用ガラス基板が得られる。
磁気ディスクは、磁気ディスク用ガラス基板を用いて以下のようにして得られる。
磁気ディスクは、例えば磁気ディスク用ガラス基板(以下、単に「基板」という。)の主表面上に、主表面に近いほうから順に、少なくとも付着層、下地層、磁性層(磁気記録層)、保護層、潤滑層が積層された構成になっている。
例えば基板を、真空引きを行った成膜装置内に導入し、DCマグネトロンスパッタリング法にてAr雰囲気中で、基板の主表面上に付着層から磁性層まで順次成膜する。付着層としては例えばCrTi、下地層としては例えばCrRuを用いることができる。磁性層としては、例えばCoPt系合金を用いることができる。また、L10規則構造のCoPt系合金やFePt系合金を形成して熱アシスト磁気記録用の磁性層とすることもできる。上記成膜後、例えばCVD法によりC2H4を用いて保護層を成膜し、続いて表面に窒素を導入する窒化処理を行うことにより、磁気記録媒体を形成することができる。その後、例えばPFPE(パーフルオロポリエーテル)をディップコート法により保護層上に塗布することにより、潤滑層を形成することができる。
作製された磁気ディスクは、好ましくは、DFH(Dynamic Flying Height)コントロール機構を搭載した磁気ヘッドと、磁気ディスクを固定するためのスピンドルとを備えた、磁気記録再生装置としての磁気ディスクドライブ装置(HDD(Hard Disk Drive))に組み込まれる。
本実施形態の磁気ディスク用ガラス基板の効果を確認するために、製造した磁気ディスク用ガラス基板から2.5インチの磁気ディスク(外径65mm、内径20mm、板厚0.635mm)を作製した。作製した磁気ディスク用ガラス基板のガラスの組成は、下記の通りである。
(ガラスの組成)
SiO2を65モル%、Al2O3を6モル%、Li2Oを1モル%、Na2Oを9モル%、MgOを17モル%、CaOを0モル%、SrOを0モル%、BaOを0モル%、ZrO2を2モル%
なお、MgO、CaO、SrOおよびBaOの合計含有量に対するCaOの含有量のモル比(CaO/(MgO+CaO+SrO+BaO))は0であり、ガラス転移温度が671℃のアモルファスのアルミノシリケートガラスである。
実施例の磁気ディスク用ガラス基板については、本実施形態の磁気ディスク用ガラス基板の製造方法の各工程を順序通りに行うことで作製した。
ここで、
(1)のガラス基板の成形は、プレス成形方法を用いた。粗研削では、遊離砥粒を用いた。
(3)の端面研削工程では、まず、ガラス基板の外周側端面に対して、ダイヤモンド砥粒を用いた総形砥石による面取り加工を行った。そしてさらに、別の総型砥石を用いて、ガラス基板の端面に当接する砥石の軌跡が一定とならないように、ガラス基板の端面と砥石とを接触させながら端部の表面を準鏡面に仕上げる研削加工を行った。
ガラス基板の外周側端面に対する2段目の研削加工では、#2500のダイヤモンド砥粒のレジンボンド砥石を用いて、研削砥石の溝方向に対するガラス基板の傾斜角度(図6Aのα)を5度とし、その他の条件については適宜調整して行った。
(6)の第1研磨では、遊星歯車を備えた両面研磨装置を用いて研磨した。平均粒径1.5μmの酸化セリウム砥粒を含むスラリーと、研磨パッドとして硬質ウレタンパッド(アスカーC硬度:85)を使用した。
(7)の化学強化では、化学強化液として硝酸カリウムと硝酸ナトリウムの混合溶融液を用いた。
(8)の第2研磨では、第1研磨と同様の研磨装置を用いて平均粒径50nmのコロイダルシリカの微粒子を混濁させたスラリーとスウェードタイプの発泡ポリウレタンの研磨パッド(アスカーC硬度:65)を用いて研磨した。第2研磨後のガラス素板を洗浄し、磁気ディスク用ガラス基板を得た。
磁気ディスク用ガラス基板の側壁面の真円度は、ガラス基板の板厚よりも長い板状のプローブをガラス基板の主表面に対して垂直方向に配置することで輪郭線を取得することにより測定した。側壁面の円筒度は、図2に示したようにして算出した。つまり、側壁面の板厚方向の中心位置、及び中心位置から上下に200μm離れた位置の輪郭線を取得し、その3個の輪郭線の内接円の半径をもとめ、次いで3個の輪郭線の内接円の半径のうちの最大値と最小値の差をもとめ、その半径の差を側壁面の円筒度とした。いずれの測定も真円度・円筒形状測定機を用いて行った。
比較例及び実施例の磁気ディスク用ガラス基板を用意し、磁性層他を形成して磁気ディスクを作製した。その磁気ディスクをディスク回転数が7200rpmの2.5インチ型HDDにDFHヘッドと共に組み込み、500kTPIのトラック密度で磁気信号を記録した後、半径位置30.4~31.4mmの領域においてサーボ信号の読み取り試験を行った。
[評価基準]
HDDのサーボ信号の読み取りエラー回数を評価した。結果を、表1に示す。30カウント以下であれば実用上合格である。
また、表2の評価結果により、円筒度が5μm以下である場合は、板厚が0.5mmであっても(実施例6)、サーボ信号の読み取りは合格レベルを維持し、板厚が薄くても磁気ヘッドのサーボ信号に対する追従性はほとんど悪化しないことがわかった。一方、円筒度が5μmを超える場合は、板厚が0.5mmであると(比較例5)、サーボ信号の読み取りは著しく乱れ、磁気ヘッドのサーボ信号に対する追従性が極めて悪くなることがわかった。
より詳細には、下記のとおりである。
フラッタリング特性値の測定では、磁気ディスクを2.5インチ型HDDのスピンドルに装着して磁気ディスクを回転させ、回転中の磁気ディスクの主表面に対してレーザドップラー振動計からレーザ光を照射する。なお、外気の影響を受けないように、きちんとカバーを取り付け、HDDのカバーにはレーザ照射用の穴を開けてある。次に、磁気ディスクで反射したレーザ光をレーザドップラー振動計が受光することにより、磁気ディスクの板厚方向の振れ量をフラッタリング特性値として測定する。このとき、以下の条件でフラッタリング特性値を測定した。
・HDD及び測定システムの環境:恒温恒湿チャンバー内で温度を25℃に維持
・磁気ディスクの回転数:7200rpm
・レーザ光の照射位置:磁気ディスクの中心から半径方向に31mm(外周端から1.5mm)の位置
・HDDの筐体のディスク装着部の内壁直径の最小値:65.880mm
[評価基準]
測定されたフラッタリング特性値に対する評価結果を、良好な順に(つまり、フラッタリング特性値が小さい順に)4つのレベル1~4に分けて示した。レベル1、2であれば実用上合格である。結果を表3に示す。
レベル1:20nm以下
レベル2:20nmより大きく、30nm以下
レベル3:30nmより大きく、40nm以下
レベル4:40nmより大きい
Claims (4)
- 一対の主表面、外周側端部に形成された側壁面、及び、前記側壁面と主表面の間に介在する面取面とを備える磁気ディスク用ガラス基板であって、
前記側壁面の真円度が1.5μm以下であり、
前記中心位置を含み板厚方向で異なる複数の位置における側壁面の複数の輪郭線の内接円及び外接円の半径の差が5μm以下であることを特徴とする、
磁気ディスク用ガラス基板。 - 外周側の側壁面上の板厚方向に200μm離れた2点の位置における円周方向の輪郭線をそれぞれ取得し、これら輪郭線からそれぞれ求められる2つの最小二乗円の中心間の中点を中点Aとし、
外周側の2つの面取面上の板厚方向長さの中心の位置において円周方向の輪郭線をそれぞれ取得し、これら輪郭線から求められる最小二乗円の中心のうち、一方の面取面から求められる中心を中心B、他方の面取面から求められる中心を中心Cとしたとき、
中点Aおよび中心B間の距離と、中点Aおよび中心C間の距離との合計が1μm以下であることを特徴とする、
請求項1に記載された磁気ディスク用ガラス基板。 - 板厚が0.5mm以下であることを特徴とする、
請求項1又は2に記載された磁気ディスク用ガラス基板。 - 請求項1から3のいずれかに記載された磁気ディスク用ガラス基板の主表面上に、磁性層を形成したことを特徴とする、磁気ディスク。
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US14/652,365 US9595283B2 (en) | 2012-12-29 | 2013-12-27 | Glass substrate for magnetic disk and magnetic disk |
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