Grinding wheel and wafer grinding method

1. A grinding wheel comprising:

an annular base having a free end portion; and

a plurality of segment grindstones fixed on the free end portion of the annular base in a state of being spaced from one another in a circumferential direction extending along a circumference of the annular base,

wherein the plurality of segment grindstones are divided into a plurality of grindstone groups each including a predetermined number of segment grindstones, and wherein adjacent grindstone groups at least partially overlap each other in a diametric direction extending along the circumference of the annular base, the diametric direction extending from an outer circumferential side toward an inner circumferential side of the annular base and substantially transverse to a longitudinal axis of each of the grindstone groups,

each of the segment grindstones included in each of the grindstone groups has a grinding surface formed into a rectangular shape having a long side and a short side, and

wherein the segment grindstones of each of the grindstone groups are sequentially fixed on the annular base from the outer circumferential side toward the inner circumferential side such that a direction in which the long side of each of the segment grindstones extends changes from the circumferential direction to the diametric direction,

wherein a first segment grindstone at a first end of each of the grindstone groups extends substantially in the circumferential direction and a second segment grindstone at an opposing second end of each of the grindstone groups extends substantially in the diametric direction such that a width of a grinding area of each of the grindstone groups is greater at the second end than a width of the grinding area at the first end, wherein the width of the grinding area of each of the grindstone groups gradually increases from the first end to the second end, and

wherein an adjacent segment grindstone that is adjacent to said first segment grindstone at said first end of each of the grindstone groups has a longitudinal axis that extends through all of the subsequent segment grindstones between the adjacent segment grindstone and said second end of each of the grindstone groups.

2. The grinding wheel according to claim 1 , wherein grindstone segments in each of the grindstone groups are identical.

3. A wafer grinding method using a grinding wheel that includes an annular base having a free end portion and a plurality of segment grindstones fixed on the free end portion of the annular base in a state of being spaced from one another in a circumferential direction extending along a circumference of the annular base, the plurality of segment grindstones being divided into a plurality of grindstone groups each including a predetermined number of segment grindstones, each of the segment grindstones included in each of the grindstone groups having a grinding surface formed into a rectangular shape having a long side and a short side, wherein adjacent grindstone groups at least partially overlap each other in a diametric direction along a circumference of the annular base, the diametric direction extending from an outer circumferential side toward an inner circumferential side of the annular base and substantially transverse to a longitudinal axis of each of the grindstone groups, wherein the segment grindstones of the grindstone groups being sequentially fixed on the annular base from the outer circumferential side toward the inner circumferential side such that a direction in which the long side of the segment grindstones extends change from the circumferential direction to the diametric direction, wherein a first segment grindstone at a first end of each of the grindstone groups extends substantially in the circumferential direction and a second segment grindstone at an opposing second end of each of the grindstone groups extends substantially in the diametric direction such that a width of a grinding area of each of the grindstone groups is greater at the second end than a width of the grinding area at the first end, and wherein the width of the grinding area of each of the grindstone groups gradually increases from the first end to the second end and wherein an adjacent segment grindstone that is adjacent to said first segment grindstone at said first end of each of the grindstone groups has a longitudinal axis that extends through all of the subsequent segment grindstones between the adjacent segment grindstone and said second end of each of the grindstone groups, the wafer grinding method comprising:

a holding step of holding a wafer by a rotatable chuck table while a center of the wafer is positioned at a rotational center of the chuck table;

a positioning step of rotating the grinding wheel in a direction extending from the segment grindstones on the outer circumferential side whose long side is disposed along the circumferential direction, to the segment grindstones on the inner circumferential side whose long side is disposed along the diametric direction, the segment grindstones on the outer circumferential side and the segment grindstones on the inner circumferential side being included in a same grindstone group, and positioning the grinding wheel such that the segment grindstones on an outermost circumferential side passes through the center of the wafer; and

a grinding step of bringing the segment grindstones of the rotating grinding wheel into contact with the wafer held by the rotating chuck table, to grind the wafer while grinding water is supplied from a central portion of the grinding wheel, after the positioning step is carried out.

4. The wafer grinding method according to claim 3 , wherein grindstone segments in each of the grindstone groups are identical.