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  Apparatus for grinding a waferThe Patent Description & Claims data below is from USPTO Patent Application 20070287363. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001]The present invention generally relates to an apparatus for grinding a wafer. More particularly, the present invention relates to an apparatus for back grinding a wafer with half precut. BACKGROUND [0002]Wafers need to be back ground (ground on the side opposite the active chip side) in order to achieve the small thicknesses currently required for semiconductor device packages. A schematic diagram of a known back grinding technique is shown in FIG. 1. A wafer 10 having a chip active side 11 is ground down from its back side 12 to make it thinner using a grinding wheel 1. The grinding wheel 1 has a grinding surface 2 which is entirely covered with grinding material. The wheel 1 partially overlaps the wafer 10 and rotates eccentrically about the wafer 10 so as to grind down the whole wafer surface. [0003]However, special copper metallization layers on the wafer, which are used in today's chip designs, cause the wafer to warp after back grinding has taken place. For this reason, the wafer is partially cut on the chip active side prior to the back grinding process to release some of the stress generated by the metallization, This cutting process causes the formation of some partial singulated chips. [0004]A problem with current back grinding techniques is that, as the wafer becomes thinner during the back grind operation, such partial singulated chips can become loose and stand up above the wafer surface. The loose chips can be pulled away from the wafer surface by the edge of the grinding wheel, which can cause the remaining wafer to be damaged by being shattered or scratched. SUMMARY [0005]The present invention has been devised with the foregoing in mind. [0006]Thus, the present invention provides an apparatus for grinding a wafer on a wafer surface generally opposing a chip active surface, the apparatus comprising: a grinding member, the grinding member having a grinding element for grinding a first section of a said wafer surface and a wafer contact element planar with and adjacent the grinding element so as to contact a second section of the said wafer surface when the grinding element is grinding the first section. Thus, the wafer contact element supports the wafer surface while grinding of the wafer is taking place. [0007]Preferably, the wafer contact element should be peripheral to and surrounding the grinding element and the grinding member should be generally disk-shaped and operable to rotate about a central axis of rotation as a grinding wheel. The grinding member should also be capable of moving in the same plane as the wafer surface so that the entire wafer surface can be background. For example, the grinding member can be configured to rotate eccentrically about its central axis of rotation. This can be achieved by having an actuation means, such as a shaft, to establish a driveable connection between the grinding member and a drive means, for example an electric motor. Control means, such as a microprocessor, can also be provided to control the movement of the apparatus with respect to a wafer surface, and control the amount of grinding to achieve the desired wafer thickness. [0008]The minimum diameter of the grinding member should preferably be equal to twice the diameter of the wafer so that the grinding member completely covers the surface of the wafer. The grinding element and the wafer contact element can be integrally formed or the wafer contact element can be provided as a substrate on the grinding member such that the grinding element is formed on the substrate, In this way the outer area of the grinding member will be covered with the same substrate as used for keeping the grinding material in the grinding element. This means that the surface of the grinding member is flatter overall; therefore the whole surface of the grinding member is able to get close to the wafer surface being ground. [0009]The present invention also provides a method of separating a semiconductor chip from a wafer, the method comprising cutting the wafer on a chip active surface to form a plurality of blind slit apertures defining chip boundaries, grinding the wafer on a wafer surface generally opposing the chip active surface in a first section and providing a contact means to contact a second section of the wafer surface when grinding of the first section is taking place, and applying stress to the first section of the wafer surface so as to break the wafer between the wafer surface and a blind end of each of said apertures. Stress can be applied to the wafer surface by tape laminating the newly ground wafer surface and stretching the tape in a direction parallel to the wafer surface. The stress on the wafer breaks up the small pieces of wafer between the wafer surface and the blind end of the apertures or cuts. This causes chip singulation and individual chips can then be removed from the wafer. After stretching, stress can also be applied to the ground wafer surface perpendicular to the surface so as to further assist in bending the surface and breaking the chip boundaries. BRIEF DESCRIPTION OF THE DRAWINGS [0010]Further characteristics and advantages of the invention ensue from the description below of a preferred embodiment and from the accompanying drawings, in which: [0011]FIG. 1 is a schematic diagram of a known back grinding technique; [0012]FIG. 2 is a side view of a section of an apparatus for grinding a wafer according to the invention; [0013]FIG. 3 is a schematic view of a grinding surface of an apparatus for grinding a wafer according to the invention. DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS [0014]Referring now to FIG. 2, a wafer 10 having a chip active side 11 and a back side 12 has undergone metallization with a metal layer 13, for example copper, and has been partially cut through from the active chip side 11 to form cuts 14 defining blind slit apertures at chip boundaries in the chip active side 11 of the wafer 10. A protective film 15 has been attached to the active chip side 11 after the formation of cuts 14 so as to prepare the wafer 10 for back grinding. [0015]A grinding wheel 20 (only the half of the grinding wheel covering the chip surface is shown in FIG. 2) has a grinding element 21 and a wafer contact element 22 provided on a grinding surface 23. The wafer contact element 22 can form a substrate upon which the grinding element 21 can be formed. Alternatively, the wafer contact element 22 can itself be formed on a substrate separately from the grinding element 21. The grinding element 21 is formed by depositing grinding material in the substrate so that the grinding element 21 is provided in the centre of the grinding surface 23 and is surrounded by the wafer contact element 22. This is shown more clearly in FIG. 3. The grinding wheel 20 is generally disk-shaped, such that its thickness is small in comparison with its diameter, and its central axis of rotation is perpendicular to the grinding surface 23. A shaft 24 connects the grinding wheel 20 with a drive means (not shown). The central vertical axis of the shaft 24 is coincident with the central axis of rotation of the grinding wheel 20. The shaft 24 is driven so as to allow the grinding wheel 20 to simultaneously rotate about its central axis of rotation and move in the same plane as the grinding surface 23. [0016]To grind down the wafer 10 from its back side 12, the grinding wheel 20 is held against the surface of the back side 12 of the wafer 10 and rotated by the shaft 24 about its central axis of rotation, perpendicular to the surface of the wafer 10, such that it rotates over the surface of the wafer 10. The grinding wheel 20 can be made to rotate by any suitable drive means (not shown), for example an electric motor. [0017]As the grinding wheel 20 rotates over the surface of the wafer 10, the grinding element 21 grinds down the section of the surface of the wafer 10 that is in contact with the grinding element 21, while the wafer contact element 22 provided in the outer section of the wheel 20 contacts the part of the surface of the wafer 10 that is not undergoing grinding and supports it. Therefore, as the wafer 10 is ground down and becomes thinner, loose chips are prevented from sticking up above the surface of the back side 12 of the wafer 10. Ideally, the grinding surface 23 should be in contact with the wafer 10 at all times as it is moved across the surface of the backside 12 of the wafer 10. [0018]At the same time as the grinding wheel 20 is rotated about its central axis of rotation against the wafer 10, it is also moved over surface of the wafer 10 (by moving the shaft so that the wheel 20 rotates eccentrically about its central axis of rotation, or otherwise) so that the outer edge of the grinding element 21 is coincident with the centre of the wafer 10 and the grinding element 21 is eventually brought into contact with the entire surface of the back side 12 of the wafer 10, while the section of the wafer 10 that is not being background is supported by the wafer contact element 22. In this way the whole of the wafer 10 can be ground down to the same desired thickness. [0019]Provision of the wafer contact element 22 on the grinding surface 23 for supporting the wafer 10 during grinding provides the advantage that the wafer 10 can be ground down to a smaller thickness without damage. This enables chips to be separated from the wafer 10 after back grinding without a further cutting operation. To separate chips from the wafer 10 after back grinding, the surface of the wafer 10 is simply stretched on the back side 12 in a direction parallel with the wafer surface, and/or bent by applying stress to the back side 12 in a direction perpendicular to the wafer surface, so that the wafer 10 breaks between the blind ends of the cuts 14 and the back side 12 to form individual chips. [0020]Although the present invention has been described hereinabove with reference to specific embodiments, it is not limited to these embodiments and no doubt further alternatives will occur to the skilled person which lie within the scope of the invention as claimed. 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