Clamping jig for glass substrate, buffer sheet, method for processing glass substrate, and glass substrate

Abstract

The present invention provides a method which can simultaneously grind inner peripheral edges and outer peripheral edges of a multiplicity of glass substrates used for magnetic storage media or the like.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a clamping jig for holding a multiplicity of annular glass substrates with the annular glass substrates being stacked, a buffer sheet to be interposed between the glass substrates, a method for grinding (chamfering or polishing) inner peripheral edges and outer peripheral edges of the multiplicity of annular glass substrates, and a glass substrate obtained by this method.

[0003] 2. Description of the Related Art

[0004] Glass is superior to aluminum in shock resistance, rigidity, hardness, and strength, so that the glass has been increasingly used as a magnetic storage medium which is built into a hard disc drive or the like.

[0005] In particular, a configuration of a personal computer has been gradually changed from a desktop type to a notebook type or a mobile type recently, thus the demand for glass substrates which are excellent in flatness and densification has been increased instead of aluminum substrates.

[0006] In order to obtain a glass substrate to be used for the magnetic storage medium, a disc-like (a doughnut-like) glass substrate is cut out from an untreated glass plate, and then, as also disclosed in Japanese Patent Laid-Open No. 10-154321 for example, peripheral edges (an inner peripheral edge and an outer peripheral edge) of this disc-like glass substrate are subjected to lapping which is conducted by means of a diamond grindstone and to polishing which is conducted by means of a cerium oxide suspension, and subsequently, two recording sides (main surfaces) of this glass substrate are subjected to lapping by means of alumina abrasive grains and to polishing by means of a cerium oxide suspension.

[0007] The above described method is ineffective because the glass substrates should be processed every single substrate. Consequently, a method in which a multiplicity of glass substrates are simultaneously polished has been disclosed in Japanese Patent Laid-Open No. 11-221742. According to this disclosure, a multiplicity of glass substrates are stacked as they are and clamped to make them into a block, then a polishing brush or a polishing pad is allowed to become contact with an inner hole of this block, and subsequently, the inner hole, in other words, inner peripheral edges of the multiplicity of glass substrates are simultaneously polished while supplying a polishing agent thereto.

[0008] However, the prior art as described above has problems as follows.

[0009] Firstly, although the inner peripheral edges of the multiplicity of glass substrates can be simultaneously polished, the outer peripheral edges have to be polished by other steps completely different from the polishing steps which have been conducted for the inner peripheral edges, because the outer peripheral edges of the glass substrates are still remained within a substrate case. That is, the multiplicity of glass substrates are removed from the substrate case, and then the outer peripheral edges are polished every single substrate, or the multiplicity of glass substrates are newly held by another damper to simultaneously polish the outer peripheral edges. However, the processing becomes inefficient when the polishing is conducted every single substrate, on the other hand, the use of another damper not only complicate the polishing steps but also causes misalignment of centers of the glass substrates at a time of newly holding the glass substrates by the another clamper, so that the processing precision deteriorates.

[0010] Secondly, cracks are easily produced on the recording surfaces since the glass substrates are directly contacted with each other. That is, a great amount of cullets (glass powder) are produced in a working field of the polishing, so that the cullets may be interposed between the glass substrates, and consequently, a pressure applied by the clamp to the glass substrates causes deep cracks (25 μm or more) on the surfaces. In this way, the presence of the cracks on the recording surface of the glass substrate has a disadvantage for forming a uniform magnetic film, so that the polishing has to be continued until the cracks disappear. The polishing time becomes longer when a polishing margin becomes thicker, and the glass substrate itself also requires to be manufactured in consideration of the polishing margin, so that the glass substrate inevitably becomes thicker.

SUMMARY OF THE INVENTION

[0011] In order to solve the above described problems, a clamping jig according to the present invention comprises: a first pair of plates, each plate of which is pressed against each side of a glass substrate block fabricated by stacking the multiplicity of glass substrates and each plate of which is provided with a fastening tool insertion hole at a center of the each plate; a first fastening tool which is inserted through an inner hole of the above described glass substrate block, end portions of the first fastening tool being attached to the fastening tool insertion holes of the above described first plates; a second pair of plates, each plate of which is disposed outside the above described first plate and each plate of which has an inner diameter smaller than an outer diameter of the first plate; and a second fastening tool for fastening the second pair of plates at a portion outside the above described glass substrate block.

[0012] According to this clamping jig, inner peripheral edges and outer peripheral edges can be continuously subjected to lapping and polishing, without setting free the clamping situation of the glass substrate block which is fabricated by stacking the multiplicity of glass substrates.

[0013] And in a method for processing a glass substrate according to the present invention, a stack of a multiplicity of annular glass substrates is clamped at an outer side or an inner side thereof while buffer sheets are respectively interposed between the annular glass substrates such that the buffer sheets do not protrude from inner peripheral edges and outer peripheral edges of the glass substrates, the outer peripheral edges of the multiplicity of glass substrates are simultaneously ground with the stack being clamped at the inner side thereof, the inner peripheral edges of the multiplicity of glass substrates are simultaneously ground with the stack being clamped at the outer side thereof, and further, the clamping situation of the glass substrates is maintained even at a time of switching between an operation for grinding the above described inner peripheral edges and an operation for grinding the above described outer peripheral edges.

[0014] In the above described method, the grinding may be started from either of the outer peripheral edges and the inner peripheral edges. The grinding includes chamfering, rough grinding (lapping), and polishing.

[0015] According to the present invention, the inner peripheral edges and the outer peripheral edges of the multiplicity of glass substrates can be continuously ground, and further, the polishing can be subsequently performed after the completion of the lapping.

[0016] In order to perform chamfering as one of the grinding operations, a grindstone in which diamond abrasive grains are bonded to a back metal for example is allowed to come into rotationally contact with the glass substrate. In addition, in order to perform lapping or polishing, a polishing brush or a polishing pad is allowed to come into rotationally contact with the glass substrate while supplying a polishing agent thereto, for example.

[0017] As above described buffer sheet, a resin sheet having a thickness of 0.2 mm or less whose inner diameter is larger than that of the glass substrate and whose outer diameter is smaller than that of the glass substrate is preferably used.

[0018] Setting the inner and outer diameters within a range described above is required to prevent poor lapping or poor polishing, and setting the thickness at 0.2 mm or less provides an RC (Radial Curvature) of 40 nm or less.

[0019] Further, as above described buffer sheet, it is appropriate to use a sheet which is made of a flexible material having a Rockwell hardness of 40 or less, a sheet comprising a two-layered structure including a flexible layer and a hard layer (for example, its compressive elasticity modulus is 100 MPa or more and preferably 1000 MPa or more), a sheet comprising three-layered structure in which a hard layer is sandwiched between flexible layers, or the like.

[0020] A relation between the Rockwell hardness and the compressive elasticity modulus is shown in FIG. 14. It is apparent from this figure that the materials for the flexible layers include PO (polyolefin), PU (polyurethane), PE (polystyrene), and S (suede) and that the materials for the hard layers include PP (polypropylene), PVC (polyvinyl chloride), PET (polyethylene terephthalate), and PS (polyester).

[0021] In addition, the above described buffer sheet to be used has a convex portion at an inner periphery or an outer periphery of the buffer sheet; the buffer sheet and the glass substrate are stacked such that the convex portion reaches to the inner peripheral edge or the outer peripheral edge of the glass substrate, and are subjected to the grinding; and a portion of the inner periphery or the outer periphery of the glass substrate, corresponding to the above described convex portion, is left as an insufficiently ground portion. In this manner, markings for the quality control can be made on a surface of the glass substrate with simultaneously performing the grinding.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a block diagram showing a fabrication sequence of a glass substrate for a magnetic storage medium;

[0023] FIG. 2 is a perspective view of a clamping jig;

[0024] FIG. 3 is a side view of a glass substrate block in which a multiplicity of glass substrates are stacked;

[0025] FIG. 4 is a top view showing a situation in which a buffer sheet is overlaid on a glass substrate;

[0026] FIG. 5(a) and (b) are cross sectional views showing other examples of the buffer sheet;

[0027] FIG. 6 is a graph showing a relation between an RC (Radial Curvature) and a buffer sheet thickness;

[0028] FIG. 7 is a conceptual view of the RC (Radial Curvature);

[0029] FIG. 8 is a side view showing a situation in which outer peripheral edges of the glass substrates are chamfered;

[0030] FIG. 9 is a side view showing a situation in which inner peripheral edges of the glass substrates are chamfered;

[0031] FIG. 10 is a side view showing a situation in which outer edges of the glass substrate are polished;

[0032] FIG. 11 is a side view showing a situation in which inner edges of the glass substrates are polished;

[0033] FIG. 12(a) is a top view of a buffer sheet having convex portions at its outer periphery;

[0034] FIG. 12(b) is a top view of a buffer sheet having convex portions at its inner periphery;

[0035] FIG. 13 shows a glass substrate manufactured by the use of the buffer sheet having the convex portions; and

[0036] FIG. 14 shows a relation between a compressive elasticity modulus and a Rockwell hardness.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Embodiments of the present invention will now be described with reference to accompanying drawings. FIG. 1 is a block diagram showing a fabrication sequence of a glass substrate used for a magnetic storage medium. Firstly, an untreated glass plate is made into an annular (disc-like) glass substrate, then an outer edge and an inner edge of this glass substrate are subjected to chamfering and polishing, before performing lapping and polishing of a recording surface of this glass substrate. Subsequently, after performing scrub washing, the surface is allowed to be chemically strengthened before washing the substrate, then inspection is carried out to make a final product. Among the steps described above, the present invention is applied to the chamfering of the glass substrate and to the grinding of the edges.

[0038] FIG. 2 is a perspective view of a clamping jig used for practicing the present invention. The clamping jig is comprised of a first pair of plates 1, 1, a second plates 2, 2, a first fastening tool 3 inserted between the first pair of plates 1, 1, and a second fastening tool 4 inserted between the second pair of plates 2, 2.

[0039] The first plate 1 is an annular plate having an outer diameter smaller than that of a glass substrate g and an inner diameter generally identical with that of the glass substrate g, while the second plate 2 is an annular plate 2 having an outer diameter larger than that of the glass substrate g and an inner diameter smaller than the outer diameter of the first plate 1. Each of the first and second fastening tools 3 and 4 comprises a combination of a bolt and nuts, and three second fastening tools 4 are equally spaced apart in a circumferential direction of the second plate 2.

[0040] When the outer edge of the glass substrate g is ground by means of the above described clamping jig, the first plates 1,1 and the first fastening tool 3 are used as will be described below in detail with reference to FIG. 8. On the other hand, when the inner edge of the glass substrate g is ground, the first plates 1,1, the second plates 2, 2, and the second fastening tool 4 are used as will be described below in detail with reference to FIG. 9.

[0041] FIG. 3 shows a glass substrate block G in which a multiplicity of glass substrates g are stacked, and in the present invention, this glass substrate block G is subjected to chamfering, lapping, and polishing for example. The glass substrate block G has buffer sheets 5 each of which is interposed between the glass substrates g.

[0042] If foreign substances such as cullets are present between the glass substrates g, cracks may be produced on the recording surface when the glass substrates g are directly stacked to each other. Therefore, the buffer sheet 5 is interposed between the glass substrates 9 to allow the foreign substances such as cullets to be embedded within the buffer sheet in order to prevent the occurrence of the cracking. For this purpose, the buffer sheet 5 is required to be made of a more flexible material than the glass substrate g. However, since the glass substrates g are liable to become misaligned during the polishing operation if the buffer sheets are too flexible, a preferable Rockwell hardness of the buffer sheet 5 is set at 5 or more.

[0043] In addition, if the buffer sheet 5 is protruded from the inner edge or from the outer edge of the glass substrate at the time of grinding thereof, the portion of the glass substrate covered with the buffer sheet can not be ground. Therefore, an inner diameter of the buffer sheet 5 is made larger than that of the glass substrate g by 0.01 mm to 5.0 mm for example and is also made smaller than an outer diameter of the glass substrate g by 0.03 mm to 10.0 mm for example, as shown in FIG. 4.

[0044] In order to prevent the buffer sheet 5 from protruding from an outer periphery of the glass substrate g even when the buffer sheets 5 are interposed between the glass substrates g with the buffer sheets 5 misaligned with each other, a difference between an outer diameter of the glass substrate g and an outer diameter of the buffer sheet 5 is preferably set at a certain value which is at least two times larger than a difference between an inner diameter of the glass substrate g and an inner diameter of the buffer sheet 5.

[0045] FIG. 5(a) and FIG. 5(b) are cross sectional views showing another embodiments of a buffer sheet. As a buffer sheet 5, a two-layered structure including a flexible layer 51 and a hard layer 52 can be used as shown in FIG. 5(a), or a three-layered structure having a flexible layer 51 sandwiched between hard layers 52 as shown in FIG. 5(b) can also be used. In addition, a material which has a compressive elasticity modulus of 100 MPa, and preferably of 1000 MPa, is used as the hard layer 52.

[0046] A relation between the compressive elasticity modulus and the Rockwell hardness is as shown in FIG. 14, and among the materials having their compressive elasticity modulus of 100 MPa or more are PP (polypropylene), PVC (polyvinyl chloride), PET (polyethylene terephthalate), and PS (polyester).

[0047] The buffer sheet of the present invention including the hard layer and the flexible layer comprises one of the sheets described below or a laminate thereof. That is, a relation between compressive elasticity modulus Y (MPa) and a Rockwell hardness R of each layer to be used for the buffer sheet is defined so as to be included within a region between two dashed lines in FIG. 14, the two dashed lines respectively representing two formulae, log Y=0.022R+0.78 and log Y=0.022R−0.42.

[0048] FIG. 6 is a graph showing a relation between the RC (Radial Curvature) and the buffer sheet thickness. In this case, the RC is indicated by a maximum distance which is measured between points A and B of a sagging portion of the glass substrate g, the sagging portion being created radially between a point at a predetermined distance R1 from a center of the disc-like glass substrate g and a point at a distance R2 from the center of the disc-like glass substrate g substantially corresponding to an end of the recording surface, as shown in FIG. 7. Generally, the RC requires to be 42 μm or less. To meet this requirement, it can be seen from FIG. 6 that the thickness of the buffer sheet 5 should be 0.2 mm or less.

[0049] Next, a procedure for carrying out the chamfering of the outer edge and the inner edge of the glass substrate g will be described with reference to FIG. 8 and FIG. 9.

[0050] Firstly, a multiplicity of annular glass substrates g are stacked alternately with buffer sheets 5 to form a glass substrate block G. Then, the first plates 1, 1 of the above described clamping jig are applied to both sides of the glass substrate block G, and the first fastening tool 3 is inserted through center holes of the first plates 1, 1 and the glass substrate block G to clamp the glass substrate block G from an inside of the block, then the outer edges are subjected to the chamfering by means of a grindstone 6.

[0051] The grindstone 6 has diamond abrasive grains bonded to a back metal, and an outer periphery of the grindstone 6 has a rugged shape adapted to an intended chamfering shape. Then, in order to chamfer the inner edges of the glass substrates g, the glass substrate block G is allowed to be rotated while simultaneously rotating the grindstone 6.

[0052] After simultaneously subjecting the outer edges of the multiplicity of glass substrates g to the chamfering process as described above, the second plates 2, 2 of the clamping jig are applied to the outside of the first plates 1, 1, while keeping this clamping situation without setting free the glass substrate block G. Further, three second fastening tools 4 are inserted between the second pates 2, 2, and the glass substrate block G is clamped from an outside of the block. Then, the first fastening tool 3 is removed and another grindstone 7 is inserted through the center hole of the glass substrate block G as shown in FIG. 9 to simultaneously chamfer the inner edges of the multiplicity of glass substrates g.

[0053] After this chamfering, as shown in FIG. 10 and FIG. 11, a polishing brush or a polishing pad 8, 9 such as nylon is used to polish the chamfered outer edges and inner edges of the glass substrate block g without setting free the block G, while supplying thereto a polishing agent in which cerium oxide particulates are suspended.

[0054] The outer edges of the glass substrates g can be firstly processed before processing the inner edges thereof as described in this example, and vice versa.

[0055] FIG. 12 shows another example of a buffer sheet, a buffer sheet 5 shown in FIG. 12(a) having convex portions 5a at an outer periphery thereof and a buffer sheet 5 shown in FIG. 12(b) having convex portions 5b at an inner periphery thereof. The buffer sheet 5 having convex portions 5a, 5b at the outer periphery or the inner periphery thereof as described above is interposed between the glass substrates g such that the convex portions 5a, 5b reach to the outer edges or inner edges of the glass substrates g, then lapping or polishing is performed with the buffer sheet kept at this position. Consequently, portions of the glass substrate g covered with the convex portions 5a, 5b will not be sufficiently lapped or polished, so that marks m can be positively produced at the outer edge or the inner edge of the glass substrate g as shown in FIG. 13.

[0056] The above described marks m can be used for the quality control, that is, the marks can be used for checking a product number, a production plant, and a date and time of the production, for example, for every lot. However, the quality of the glass substrates may be affected, if such portions which have not been sufficiently polished exist excessively. Therefore, the sizes and the numbers of the convex portions 5a, 5b are limited within a range which dose not affect the quality.

[0057] Next, results of the experiments conducted by the use of various buffer sheets which are different in their materials and their laminar structures are shown in the following table. The experiments were conducted under the conditions as follows.

[0058] Chamfering of the outer peripheral edges and chamfering the inner peripheral edges were respectively performed by using ten grooves type of grindstones having an outer diameter of 80 mm φ and an inner diameter of 22 mm φ respectively. In this case, ten sheets of glasses which were stacked by the clamping jig were simultaneously processed by the ten grooves type of grindstone, each groove having a predetermined cross sectional shape. As for each of the ten grooves of the grindstone, a #500 diamond grindstone was fixed to a back metal. In this case, various buffer sheets having different materials were interposed between the glass plates. Then, polishing was performed with a nylon brush roll, while supplying thereto a polishing agent in which cerium oxide particles were suspended.

[0059] After the polishing, the glasses were removed from the clamping jig to be released from the stacked arrangement. Then, main surfaces (recording surfaces) of the glass plate were polished by 25 μm for each surface using a polishing pad and a polishing agent including cerium oxide grinding particles whose mean particle sizes are 0.5 to 1.7 μm, and were subsequently washed and dried.

[0060] Next, the glass plate was subjected to a chemically strengthening treatment which was performed by ion exchange which uses mixed molten salt of potassium nitrate and sodium nitrate, and the treated glass plate was washed and dried again. After this treatment, the inspection was conducted for cracks on the main surfaces of the glass plate. Alternatively, processing (grinding) of the outer peripheral edges and the inner peripheral edges of the glass substrates using the diamond grindstone may be performed every sheet of glasses, and then the multiplicity of the ground glass substrates may be stacked by the clamping jig to be collectively subjected to the polishing such that the inner peripheral edges and the outer peripheral edges of the glass substrates may be simultaneously polished by a polishing pad or a polishing brush while supplying a cerium oxide polishing agent thereto.

	TABLE 1
							Effects
					Com-				Workability at a time of
				press-		Yield after	Radial	buffer sheet
								treatment
					ive	Laminar	the	curvature	At a	At a
				elas-	struc-	inspection	after	time of	time of
					ture
				ti-	val-	for cracks	edge	setting a	removing a
					ues			buffer
		Thick-	Rockwell	city	are	thickness	on main	polish-	sheet on	buffer sheet
	Material	ness (mm)	hardness	modulus	in	surfaces	ing (nm)	a glass	from a
					μms.)			surface	glass
Example 1	Flexible	Polyurethane	0.275	10	40	Three	100(PU) +	94%	178	4	1
	surface	(PU)			MPa	layers	75 (PET) +
							100(PU)
Example 2		Polyurethane	0.2	10	10	Single	200	100%	45	1	1
		(PU)		MPa		layer	(PU)
Example 3		Polyolefin	0.15	5	3	Single	150	92%	8	3	3
		(PO)			MPa	layer	(PO)
Example 4		Polyolefin	0.1	5	3	Single	100	96%	4	2	3
		(PO)			MPa	layer
Example 5		Polyurethane	0.19	10	40	Three	90(PU) + 35	98%	11	5	5
		(Satin-			MPa	layers	(PET) +
		like) (PU)				50(PU)
Example 6		Polyethylene	0.1	40	20	Single	100	92%	5	5	5
		(PE)			MPa	layer	(PE)
Example 7	Hard-	Polyurethane	0.175	10	40	Two	100(PU) +	86%	20	4	2
	flexible	(PU)			MPa	layers	75(PET)
	surface
Comparative	Hard	Polyester	0.1	140	1400	Single	100	58%	7	4	4
Example 1	surface	(PS)			MPa	layer	(Polyester)
Comparative		Polypropylene	0.3	110	1300	Single	300	78%	187	4	4
Example 2		(PP)			MPa	layer	(PP)
Comparative		Hard polyvinyl	0.15	120	1500	Single	150	50%	10	4	4
Example 3		chloride (PVC)			MPa	layer	(PVC)
Workability evaluation
1: Wrinkles easily occur on a sheet surface, and the setting of the sheet requires a certain technique. It is extremely difficult to remove the sheet from the glass.
2: Workability between 1 and 3
3: The setting can be performed at a predetermined position. The sheet can be removed from the glass in a short time
4: Workability between 3 and 5
5: The setting can be easily performed within a predetermined position. The sheet can be easily removed.

[0061] In the above described table, each of Examples 1 to 6 used a buffer sheet comprising a single layer whose Rockwell hardness was 40 MPa or less, and the occurrence of cracks on a main surface of the glass was prevented. That is, in a visual inspection which was carried out by illuminating the glass surface with a lamp of 500 W, an yield after passing the inspection provided that no cracks were found on the glass surface was as high as 92% or more. Among these Examples, Example 1 showed that a total thickness of the buffer sheet was 0.275 mm which was the largest value among these examples and also showed that the radial curvature was as large as 178 nm. In contrast to Example 1, thickness of each buffer sheets used for Examples 2 to 7 was 0.2 mm or less, so that each of their radial curvatures was as small as 45 nm or less.

[0062] In each of Comparative Examples 1 to 3 where single layered buffer sheet was used, it was found that cracks occurred on a main surface of the glass substrate and its yield after the inspection for cracks was as low as 78% or less. In Comparative Example 2 where a buffer sheet whose thickness was the largest among three Comparative Examples was used, the radial curvature exhibited a large value, so that it was found that this buffer sheet was not suitable for manufacturing a substrate used for a magnetic storage medium.

[0063] In addition, evaluation points as for the workability of the buffer sheets at the time of setting the buffer sheets on the glass surfaces and at the time of removing the buffer sheets from the wetting glass surfaces may vary depending on the thickness of the buffer sheets, adhesion properties to a surface of the glass substrate under the wet condition caused by the material to be used, and the nerve, for example.

[0064] According to the present invention as described above, a multiplicity of glass substrates can be simultaneously treated at a time of lapping or polishing inner peripheral edges and outer peripheral edges of the glass substrates to be used as magnetic storage media, and therefore, the cost-reduction can be substantially carried out.

[0065] In addition, a clamping situation of the multiplicity of glass substrates can be kept as it is without setting free the clamping situation when the outer peripheral edges are subjected to lapping or polishing after continuously performing lapping or polishing of the inner peripheral edges thereof, so that the misalignment of the glass substrates will not occur at the time of newly holding the glass substrates by the another clamping jig.

[0066] In addition, a buffer sheet is interposed between the glass substrates at a time of stacking the multiplicity of glass substrates. Therefore, even if foreign substances such as cullets are sandwiched between the glass substrates, such foreign substances will be embedded into the buffer sheets and cracks will not be produced on a surface of the recording surface.

[0067] According to the buffer sheet of the present invention which is used at a time of stacking a multiplicity of glass substrates, its Rockwell hardness exhibits a predetermined value, so that the occurrence of cracks on a main surface (a recording surface) of the glass substrate is prevented. In addition, a buffer sheet having a multi-layered structure including a flexible layer prevents the glass substrate from becoming misaligned in its lateral direction, so that the chamfering and the polishing of the edges can be performed completely roundly.

[0068] In addition, convex portions are provided for a certain portion of the buffer sheet, and lapping or polishing is performed such that the convex portions reach to the inner peripheral edge or the outer peripheral edge of the glass substrate, so that marks used for the quality control can be easily produced on the glass substrate.

Claims

1. A clamping jig for holding a multiplicity of annular glass substrates with the annular glass substrates being stacked, comprising: a first pair of plates, each plate of which is pressed against each side of a glass substrate block fabricated by stacking the multiplicity of glass substrates and each plate of which is provided with a fastening tool insertion hole at a center of the each plate; a first fastening tool which is inserted through an inner hole of said glass substrate block, end portions of the first fastening tool being attached to the fastening tool insertion holes of said first plates; a second pair of plates, each plate of which is disposed outside said first plate and each plate of which has an inner diameter smaller than an outer diameter of the first plate; and a second fastening tool for fastening the second pair of plates at a portion outside said glass substrate block.

2. A buffer sheet which is interposed between annular glass substrates, wherein the buffer sheet is made of a flexible material whose Rockwell hardness is 40 or less.

3. A buffer sheet which is interposed between annular glass substrates, wherein the buffer sheet comprises a two-layered structure including a flexible layer and a hard layer.

4. A buffer sheet which is interposed between annular glass substrates, wherein the buffer sheet comprises a three-layered structure in which a hard layer is sandwiched between flexible layers.

5. The buffer sheet according to claim 3 or 4, wherein said hard layer has a compressive elasticity modulus of 100 MPa or more.

6. The buffer sheet according to claims 2 to 5, wherein a thickness of the buffer sheet is 0.2 mm or less.

7. The buffer sheet according to claims 2 to 6, wherein the buffer sheet is shaped such that an inner diameter of the buffer sheet is larger than an inner diameter of the glass substrate and an outer diameter of the buffer sheet is smaller than an outer diameter of the glass substrate.

8. A method for processing a glass substrate, wherein a multiplicity of annular glass substrates are stacked alternately with the buffer sheets according to claims 2 to 7, said glass substrates are clamped at an outer or inner peripheral side of the glass substrates, and said outer or inner peripheral side which is not clamped is ground.

9. A method for processing a glass substrate, wherein a stack of a multiplicity of annular glass substrates is clamped at an outer side or an inner side thereof while buffer sheets are respectively interposed between the annular glass substrates such that the buffer sheets do not protrude from inner peripheral edges and outer peripheral edges of the glass substrates, the outer peripheral edges of the multiplicity of glass substrates are simultaneously ground with the stack being clamped at the inner side thereof, the inner peripheral edges of the multiplicity of glass substrates are simultaneously ground with the stack being clamped at the outer side thereof, and further, the clamping situation of the glass substrates is maintained even at a time of switching between an operation for grinding said inner peripheral edges and an operation for grinding said outer peripheral edges.

10. The method for processing a glass substrate according to claim 9, wherein the grinding of said outer peripheral edges and said inner peripheral edges is chamfering which is conducted by allowing a grindstone to come into rotationally contact with the outer peripheral edges and the inner peripheral edges, the grindstone being made by bonding diamond abrasive grains to an alloy.

11. The method for processing a glass substrate according to claim 9, wherein the grinding of said outer peripheral edges and said inner peripheral edges is polishing which is conducted by allowing a polishing brush or a polishing pad to come into rotationally contact with the outer peripheral edges and the inner peripheral edges while supplying a polishing agent thereto.

12. The method for processing a glass substrate, wherein the outer peripheral edges and the inner peripheral edges are polished by allowing the polishing brush or the polishing pad to come into rotationally contact with the outer peripheral edges and the inner peripheral edges while supplying the polishing agent thereto, with the clamping situation of the glass substrates being continuously maintained without releasing respective glass substrates from the clamping situation, after the grinding according to claim 9.

13. The method for processing a glass substrate according to claims 9 to 12, wherein said buffer sheet to be used is a buffer sheet defined in claims 2 to 7.

14. The method for processing a glass substrate according to claims 9 to 12, wherein said buffer sheet to be used has a convex portion at an inner periphery or an outer periphery of the buffer sheet, the buffer sheet and the glass substrate are stacked such that the convex portion reaches to the inner peripheral edge or the outer peripheral edge of the glass substrate, and are subjected to the grinding, and a portion of the inner periphery or the outer periphery of the glass substrate, corresponding to said convex portion, is left as an insufficiently ground portion.

15. A glass substrate, wherein the glass substrate is obtained by the processing methods according to claims 8 to 12.

16. A glass substrate, wherein the glass substrate is obtained by the method for processing the glass substrate according to claims 13.