US20070287363A1 - Apparatus for Grinding a Wafer - Google Patents

Abstract

An apparatus for grinding a wafer comprises a grinding member. The grinding member has a grinding element for grinding a first section of a wafer surface and a wafer contact element planar with and adjacent the grinding element. The wafer contact element is configured to contact a second section of the said wafer surface when the grinding element is grinding the first section.

Description

• 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
• 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 inFIG. 1. Awafer10 having a chipactive side11 is ground down from itsback side12 to make it thinner using a grinding wheel1. The grinding wheel1 has agrinding surface2 which is entirely covered with grinding material. The wheel1 partially overlaps thewafer10 and rotates eccentrically about thewafer10 so as to grind down the whole wafer surface.
• 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.
• 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
• The present invention has been devised with the foregoing in mind.
• 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.
• 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.
• 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.
• 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
• Further characteristics and advantages of the invention ensue from the description below of a preferred embodiment and from the accompanying drawings, in which:
• FIG. 1 is a schematic diagram of a known back grinding technique;
• FIG. 2 is a side view of a section of an apparatus for grinding a wafer according to the invention;
• 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
• Referring now toFIG. 2, awafer10 having a chipactive side11 and aback side12 has undergone metallization with ametal layer13, for example copper, and has been partially cut through from theactive chip side11 to formcuts14 defining blind slit apertures at chip boundaries in the chipactive side11 of thewafer10. Aprotective film15 has been attached to theactive chip side11 after the formation ofcuts14 so as to prepare thewafer10 for back grinding.
• A grinding wheel20 (only the half of the grinding wheel covering the chip surface is shown inFIG. 2) has agrinding element21 and awafer contact element22 provided on agrinding surface23. Thewafer contact element22 can form a substrate upon which thegrinding element21 can be formed. Alternatively, thewafer contact element22 can itself be formed on a substrate separately from thegrinding element21. Thegrinding element21 is formed by depositing grinding material in the substrate so that thegrinding element21 is provided in the centre of thegrinding surface23 and is surrounded by thewafer contact element22. This is shown more clearly inFIG. 3. The grindingwheel20 is generally disk-shaped, such that its thickness is small in comparison with its diameter, and its central axis of rotation is perpendicular to thegrinding surface23. Ashaft24 connects the grindingwheel20 with a drive means (not shown). The central vertical axis of theshaft24 is coincident with the central axis of rotation of thegrinding wheel20. Theshaft24 is driven so as to allow thegrinding wheel20 to simultaneously rotate about its central axis of rotation and move in the same plane as thegrinding surface23.
• To grind down thewafer10 from itsback side12, thegrinding wheel20 is held against the surface of theback side12 of thewafer10 and rotated by theshaft24 about its central axis of rotation, perpendicular to the surface of thewafer10, such that it rotates over the surface of thewafer10. The grindingwheel20 can be made to rotate by any suitable drive means (not shown), for example an electric motor.
• As thegrinding wheel20 rotates over the surface of thewafer10, thegrinding element21 grinds down the section of the surface of thewafer10 that is in contact with thegrinding element21, while thewafer contact element22 provided in the outer section of thewheel20 contacts the part of the surface of thewafer10 that is not undergoing grinding and supports it. Therefore, as thewafer10 is ground down and becomes thinner, loose chips are prevented from sticking up above the surface of theback side12 of thewafer10. Ideally, thegrinding surface23 should be in contact with thewafer10 at all times as it is moved across the surface of thebackside12 of thewafer10.
• At the same time as thegrinding wheel20 is rotated about its central axis of rotation against thewafer10, it is also moved over surface of the wafer10 (by moving the shaft so that thewheel20 rotates eccentrically about its central axis of rotation, or otherwise) so that the outer edge of thegrinding element21 is coincident with the centre of thewafer10 and thegrinding element21 is eventually brought into contact with the entire surface of theback side12 of thewafer10, while the section of thewafer10 that is not being background is supported by thewafer contact element22. In this way the whole of thewafer10 can be ground down to the same desired thickness.
• Provision of thewafer contact element22 on the grindingsurface23 for supporting thewafer10 during grinding provides the advantage that thewafer10 can be ground down to a smaller thickness without damage. This enables chips to be separated from thewafer10 after back grinding without a further cutting operation. To separate chips from thewafer10 after back grinding, the surface of thewafer10 is simply stretched on theback side12 in a direction parallel with the wafer surface, and/or bent by applying stress to theback side12 in a direction perpendicular to the wafer surface, so that thewafer10 breaks between the blind ends of thecuts14 and theback side12 to form individual chips.
• 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.
• For example, the grinding member is not limited to being disk-shaped and rotatably moveable relative to the wafer surface. The grinding member could be rectangular, for example, and be configured to move across the surface of the wafer in a straight line backwards and forwards relative to the surface of the wafer.

Claims (10)

1. 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, the wafer contact element being configured to contact a second section of the said wafer surface when the grinding element is grinding the first section.

2. The apparatus according toclaim 1, wherein the wafer contact element is peripheral to and surrounding the grinding element.

3. The apparatus according toclaim 1, wherein the grinding member is generally disk-shaped.

4. The apparatus according toclaim 1, wherein the grinding member is operable to rotate about a central axis of rotation.

5. The apparatus according toclaim 1, wherein the grinding member is operable to move relative to the wafer surface in the same plane as the wafer surface.

6. The apparatus according toclaim 1, wherein a minimum diameter of the grinding member is equal to twice the wafer diameter.

7. The apparatus according toclaim 1, wherein the grinding element and the wafer contact element are integrally formed.

8. The apparatus according toclaim 1, wherein the wafer contact element is provided as a substrate on the grinding member and the grinding element is formed on the substrate.

9. The apparatus according toclaim 1, further comprising a control means for controlling movement of the grinding member against the wafer surface.

10. A method of separating a semiconductor chip from a wafer, the method comprising the steps of:

cutting the wafer on a chip-active surface to form a plurality of blind apertures defining chip boundaries;

grinding the wafer on a wafer surface generally opposing the chip-active surface in a first section by means of a grinding member, which includes a grinding element and a contact element, whereby the wafer contact element is contacting a second section of the wafer surface, which is not contacted by the grinding element when the grinding of the first section is taking place; and

applying pressure to the first section of the wafer surface thereby breaking the wafer between the wafer surface and a blind end of each of said apertures.

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