Apparatus and method for wafer backside grinding
By using a device with a gasket and a grinding belt during the back of the wafer, the uneven flatness problem caused by the uneven thickness of the grinding belt is solved, and a more uniform and stable grinding effect is achieved, ensuring the structural integrity and consistent thickness of the wafer.
Patent Information
- Application Number
- CN202411497706.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2023-08-31
- Filing Date
- 2024-10-25
- Publication Date
- 2025-02-28
AI Technical Summary
During the grinding process on the back of the wafer, the uneven thickness of the grinding belt leads to uneven flatness, affecting the grinding effect and the structural integrity of the wafer.
A device is designed including a gasket and a grinding belt whose cross-section is shaped to mirror the step gap formed at the edge of the grinding belt to form a flatter surface to ensure stability and uniformity during the grinding process.
By using this device, a more uniform and flatter surface can be formed on the back of the wafer, reducing shaking and bending during the grinding process, ensuring structural integrity and consistent thickness of the wafer.
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Figure CN119526262A_ABST
Abstract
Description
Technical Field
[0001] This description relates to back grinding of wafers for use in manufacturing integrated circuit (IC) chips. Background Art
[0002] Wafer chip scale packaging (WCSP) is a semiconductor packaging method that achieves compact and highly integrated chip packaging by closely aligning the size of the chip to reduce the package size. This produces shorter electrical connections and enhances performance in both electrical and thermal properties. WCSP provides cost and space efficiencies, making WCSP the first choice in space-constrained applications such as mobile devices and wearable devices, where achieving excellent performance through efficient integration is preferred.
[0003] Compared to conventional processing techniques involving molding operations, WCSP avoids the use of molding for chip encapsulation. In fact, WCSP uses a redistribution layer (RDL) process to establish connections between the pads of the microchip and the external solder balls directly on the wafer. This avoids the need for a separate molded plastic or ceramic package to encapsulate the microchip.
[0004] Back grinding, also known as backside grinding or wafer thinning, is a semiconductor manufacturing operation that involves reducing the thickness of a silicon wafer after front-side processing. This is achieved via precision grinding of the back side of the wafer using an abrasive material. By making the wafer thinner, benefits such as improved thermal performance, reduced signal delays, and enhanced electrical performance are achieved due to shorter interconnect lengths. After grinding, a polishing operation is often employed to ensure uniform thickness. Back grinding is used to produce thin and lightweight devices such as mobile computing devices, facilitating miniaturization and improved functionality of such devices by allowing better heat dissipation and smaller form factors. Summary of the invention
[0005] A first example relates to an apparatus for a grinding table. The apparatus includes a pad having a cross section shaped to mirror the cross section of a step gap formed at an edge of a grinding tape adhered to a first side of a wafer covering solder balls, so that the pad and the grinding tape form a flatter surface for the wafer on the grinding table.
[0006] A second example relates to a method for back grinding a wafer, the method comprising placing an apparatus on a grinding table, wherein the apparatus comprises a pad for the wafer. The method comprises placing a grinding tape adhered to a first side of the wafer on the apparatus. The pad has a cross section shaped to mirror a cross section of a step gap formed at an edge of the grinding tape covering solder balls on the first side of the wafer, so that the pad and the grinding tape form a flatter surface on the grinding table. The method also comprises back grinding a second side of the wafer to form a flatter surface on the second side of the wafer.
[0007] A third example relates to a method for back grinding a wafer, the method comprising placing an apparatus on a grinding table, the apparatus comprising a pad for the wafer. The method comprises placing a grinding tape adhered to a first side of the wafer on the apparatus, wherein the grinding tape covers solder balls on the first side of the wafer. The method also comprises back grinding a second side of the wafer to form a flatter surface on the second side of the wafer. The back grinding comprises applying pressure on the second side of the wafer. The pad of the apparatus is shaped to reduce bending of the wafer around solder balls of the solder balls that are closest to the grinding tape at an edge thereof. The method further comprises removing a portion of the second side of the wafer to provide a back ground wafer, and the back ground wafer has a continuous lattice. BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Figure 1 An example apparatus for facilitating backside grinding of a wafer is shown.
[0009] Figure 2 Show Figure 1 An enlarged view of a portion of a wafer is shown.
[0010] Figure 3A Show Figure 1 A cross-sectional view of the device is shown with the wafer removed from view.
[0011] Figure 3B Show Figure 1 A top view of the device is shown with the wafer removed from view.
[0012] Figure 4 The first stage of an example method of forming an IC chip from a wafer is shown.
[0013] Figure 5 A second stage of an example method for forming IC chips from a wafer is shown.
[0014] Figure 6 The third stage of an example method for forming IC chips from a wafer is shown.
[0015] Figure 7 A fourth stage of an example method for forming IC chips from a wafer is shown.
[0016] Figure 8 A fifth stage of an example method for forming IC chips from a wafer is shown.
[0017] Fig. 9 A photograph of a back-ground wafer is shown.
[0018] Fig.10A set of graphs are shown that plot the relief of a back ground wafer as a function of distance from the edge of the back ground wafer.
[0019] Fig.11 A flow chart illustrating an example method for forming IC chips from a wafer. DETAILED DESCRIPTION
[0020] The present specification relates to a device designed to facilitate the back grinding process of wafers used in integrated circuit (IC) chips. The device can be mounted on a grinding table or integrated with a grinding table. The device has a circular or oval shape, and the device can be integrated with the grinding table or can be removed from the grinding table. The device is intended for use with wafers such as wafer-level chip scale packaging (WCSP) wafers, but can also be used with other types of wafers with edge flatness issues. The wafer has a silicon layer having an embedded circuit layer on a first side. The circuit layer contains the circuit system of the IC chip. A group of solder balls are arranged to provide conductivity for pads on the circuit layer, and a grinding tape is adhered to the first side of the wafer to cover the group of solder balls. The solder balls are positioned near the center of the wafer and do not extend to the edge.
[0021] Since there are no solder balls at the periphery, the grinding tape has different thicknesses across the wafer, resulting in uneven flatness. In order to compensate for this varying thickness (called step gap), the device incorporates a pad around the periphery of the grinding tape. The shape of the pad mirrors the unevenness caused by the varying thickness of the grinding tape, thereby aiming to produce a more uniform and flatter surface. The grinding tape and the pad work together to form a flatter (and horizontal) surface on the vacuum table of the grinding table. This ensures stability during the back grinding process, in which a downward force (pressure) is applied to the second side of the wafer (opposite to the first side) to reduce the thickness of the silicon layer of the wafer. The pad prevents the wafer from moving (e.g., shaking), maintains a uniform thickness reduction, and prevents cracking, thereby producing a back-ground wafer with consistent thickness and structural integrity. Therefore, in some instances, a device containing a pad is referred to as a leveling fixture.
[0022] Accordingly, the device improves the back grinding operation of wafers in IC chip production. By using the pad in combination with the grinding tape, the device ensures uniform flatness and stability during the grinding process, thereby producing wafers with consistent thickness and structural integrity. The use of the device does not significantly increase the production steps or costs associated with manufacturing IC chips from wafers.
[0023] Figure 1A cross-sectional view of an apparatus 100 for facilitating backside grinding of a wafer 104 is shown. The apparatus 100 may be mounted on a grinding table 108. In some instances, the apparatus 100, the grinding table 108, and the wafer 104 have a circular or oval shape. In some instances, the apparatus 100 is integrated with the grinding table 108. In other instances, the apparatus 100 may be removed from the grinding table 108. In some instances, the wafer 104 is a wafer-level chip scale package (WCSP) wafer. In other instances, the wafer 104 is a different type of wafer that typically experiences edge flatness issues. The wafer 104 includes a silicon layer 112 having a first side 116 and a second side 120 opposite to the first side 116. The first side 116 includes a circuit layer 124 underlying the first side 116 of the silicon layer 112. The circuit layer 124 includes a circuit system embedded in the silicon to perform the functions of an integrated circuit (IC) chip formed by the wafer 104.
[0024] A set of solder balls 128 are formed to contact the pads of the circuit layer 124. A grinding tape 132 covers the set of solder balls 128 and is adhered to the first side 116 of the wafer 104. The set of solder balls 128 are arranged close to the center of the wafer 104, and the set of solder balls 128 do not extend to the periphery of the wafer 104. Accordingly, the solder balls 128 in the set of solder balls 128 closest to the periphery of the wafer 104 may be about 1 millimeter (mm) to about 3 mm from the periphery of the wafer 104. The grinding tape 132 has a tapered edge near the periphery of the wafer 104 due to the absence of solder balls 128 in this area. Figure 2 More specifically, Figure 2 Show Figure 1 FIG. 1 is a magnified view of region 136 of wafer 104. Thus, Figure 1 and 2 The same reference numerals are used to represent the same structures. In addition, for clarity, other components have been removed and the wafer 104 is shown in FIG. Figure 2 is displayed upside down.
[0025] like Figure 2 1, the region of the lapping tape 132 that overlies the set of solder balls 128 has a first thickness, and the region of the lapping tape 132 that extends to the periphery of the wafer 104 has a second thickness (forming a taper) that is less than the first thickness. Thus, the lapping tape 132 has a non-uniform flatness across the wafer 104. The height difference at a line 200 extending from the portion of the lapping tape 132 that is at the first thickness (maximum thickness) and a line 204 extending from the top of the solder balls 128 closest to the tapered edge of the lapping tape 132 is referred to as a step gap 208. In other words, the step gap 208 has a height measured from a point at the maximum thickness of the lapping tape 132 to a point on the solder balls 128 that is close to the edge of the lapping tape 132. The step gap may be, for example, about 40 micrometers (μm) to about 60 μm.
[0026] Return to reference Figure 1 To counteract the uneven flatness of the polishing belt 132 (formed by the step gap 208), the device 100 includes a spacer 140 extending around the periphery of the polishing belt 132. As mentioned, the device 100 has a circular or oval shape such that Figure 1 Two (2) portions of the shim 140 are shown. The shim 140 is shaped so that a cross section of the shim 140 mirrors the step gap 208 formed at the tapered edge of the lapping tape 132. That is, the shim 140 is shaped to offset the step gap 208 of the lapping tape 132. In some examples, the shim 140 is referred to as having a shape for compensating for the step gap 208. Figure 2 The lapping tape 132 and pad 140 cooperate to form a flatter surface 148 on the vacuum table 152 of the lapping table 108. The vacuum table 152 draws air to apply a downward force in the direction indicated by arrow 156 to keep the wafer 104 stationary during the lapping operation.
[0027] Figure 3A and 3B The device 100 is shown with the wafer 104 removed. More specifically, Figure 3A A cross-sectional view of the device 100 is shown (similar to Figure 1 ),and Figure 3B 1 shows a top view of the device 100. Thus, Figure 1 , 3A 3B use the same reference numerals to represent the same structure. Figure 3B As shown, the pad 140 has a circular shape located on the vacuum table 152 of the grinding table 108. The shape of the pad 140 allows the wafer 104 ( Figure 3A and 3B (not shown) can be Figure 1 The method shown is mounted on the device 100.
[0028] Return to reference Figure 1As mentioned, the pad 140 is shaped to ensure that the grinding tape 132 and the pad 140 form a flatter surface 148 on the vacuum table 152 of the grinding table 108. During the back grinding process, a downward force (e.g., pressure) in the direction indicated by the arrow 156 is applied to the second side 120 of the silicon layer 112. The back grinding operation removes layers of the silicon layer 112 to reduce the overall thickness of the silicon layer 112. In addition, the pad 140 cooperates with the grinding tape 132 to provide a flatter surface 148, thereby preventing the wafer 104 from rocking, bending and / or braking during the back grinding process. In this way, the device 100 operates as a leveling fixture. Therefore, during the back grinding operation, the pad 140 holds the wafer 104 in place (stationary) to ensure that after the back grinding is performed, the thickness of the silicon layer 112 varies (e.g., undulates) by 5 μm or less across the surface of the silicon layer 112. Furthermore, the spacer 140 prevents cracking during the back grinding operation, so that the resulting wafer 104 has a continuous crystal lattice. In addition, the inclusion of the device 100 does not increase the process steps and / or cost of forming an IC chip from the wafer 104.
[0029] Figure 4-8 The various stages of the method of forming an IC chip from a WCSP wafer are shown. For simplicity, Figure 4-8 Like reference numerals are used to refer to like structures.
[0030] At 300, in a first stage of the method, as Figure 4 As shown, the device 400 (eg, Figure 1 The device 100 is placed on a grinding table 404 (e.g., Figure 1 The apparatus 400 includes a pad 402 for receiving a wafer 408 (e.g., a WCSP wafer). The grinding table 404 includes a vacuum table 406. In some examples, the pad 402 is held in place by suction created by the vacuum table 406.
[0031] Also at 300, a lapping tape 412 adhered to a first side 416 of a wafer 408 is placed on the pad 402. More specifically, the lapping tape is adhered to the bottom side of a silicon layer 420 of the wafer 408. The lapping tape 412 covers a set of solder balls 414 formed on the first side 416 of the wafer 408. The solder balls 414 have the solder balls 414 closest to the edge of the lapping tape 412. The pad 402 is shaped so that the step gap (e.g., Figure 1 The cross section of the step gap 208 is mirrored by the pad 402. In order to place the grinding tape 412 on the pad 402, the wafer 408 is moved toward the apparatus 400 in the direction indicated by the arrow 418.
[0032] At 305, in the second stage of the method, as Figure 5 As shown, the wafer 408 is placed in the apparatus 100 so that a flatter surface 424 is formed on the vacuum table 406. And, at 305, a back grinding operation of the wafer 408 is initiated. In the back grinding operation, a portion of the silicon layer 420 of the wafer 408 is removed. In the back grinding operation, a portion of the second side 428 of the wafer 408 is removed, and the second side 428 is away from the set of solder balls 414. More specifically, in the example shown, during the back grinding operation, a portion of the silicon layer 420 located above the mark line 432 is removed.
[0033] In various examples, different materials may be used to perform the back grinding operation at 305. The selection of materials for the back grinding operation depends on several factors, including the wafer type, the desired thickness reduction, and the manufacturing tool. The materials used for the back grinding operation include diamond grinding wheels to reduce damage to the wafer 408. In addition, in some examples, supplementary grinding fluids and / or coolants are used to lubricate and / or cool the process to prevent overheating and potential cracks. In some examples, polishing pads with fine abrasives are applied to achieve a smoother surface of the wafer 408 after back grinding. In addition, during back grinding, a protective coating may be applied to shield the back side of the wafer. In some examples, chemical mechanical planarization (CMP) slurry is used for further thinning and polishing, thereby mixing chemical reactions with mechanical abrasion. During the back grinding operation, a downward force (pressure) is applied in the direction indicated by arrow 436. The pad 402 of the device 400 is shaped to reduce the bending of the wafer 408 around the solder balls 414 of the group of solder balls 414 closest to the edge of the grinding belt 412. The flatter surface 424 formed on the vacuum table 406 reduces the shaking of the wafer 408 during the grinding operation. The back grinding operation may include, for example, multiple passes to reduce the thickness of the silicon layer 420 to a desired level. The back grinding of the wafer 408 removes a portion of the second side of the wafer 408 to provide a back ground wafer.
[0034] At 310, in the third stage of the method, as Figure 6 As shown, wafer 408 is removed from grinding station 404. The resulting silicon layer 420 has a surface variation of about 5 μm or less. In addition, wafer 408 has a continuous crystal lattice (e.g., no cracks) due to the back grinding operation. Figure 6 As shown, wafer 408 is back ground (at 305) so that mark line 432 is the top of silicon layer 420 of wafer 408. To remove wafer 408 from grinding station 404, wafer 408 is lifted in the direction indicated by arrow 440. Wafer 408 may be moved to a singulation station.
[0035] At 315, in the fourth stage of the method, as Figure 7As shown, the wafer 408 is placed on a singulation station 444. Also at 315, the die of the wafer 408 are singulated using a saw 448. The saw 448 can be, for example, a laser saw or a plasma saw. The resulting singulated die are removed from the singulation station 444.
[0036] At 320, the grinding tape 412 is removed from the singulated die 450 before mounting it on a printed circuit board (PCB). The removal of the grinding tape 412 exposes the set of solder balls 414 of the singulated die 450 to enable electrical connection to circuit components embedded in the singulated die 450.
[0037] Fig. 9 A photograph showing a back-ground wafer is shown, which is a wafer after back-ground (e.g., Figure 1 of chip 104). Fig. 9 A first photograph 500 is included showing a conventional method in which a leveling fixture is not used, so that the first back ground wafer 504 has a non-uniform thickness, as shown by the non-uniform cut lines 508 and 512. This non-uniform thickness is caused by the shaking of the first back ground wafer 504 during the back grinding operation because the downward pressure is unevenly distributed on the first back ground wafer 504. In addition, due to this non-uniform downward pressure on the first wafer 504, the first back ground wafer 504 includes several cracks 516. These cracks 516 may cause the singulated dies to fail, thereby reducing the yield of the wafer 504.
[0038] Fig. 9 Also included is a second photograph 528 showing the use of a leveling fixture (e.g., including pad 140) during back grinding of a second back ground wafer 530. Figure 1 The second back-ground wafer 530 is provided with a method of forming a back-ground wafer 530 by a flattening fixture. The flattening fixture prevents the second back-ground wafer 530 from shaking during the back-ground operation. The second back-ground wafer has a uniform thickness, as evidenced by the relatively straight first cut line 534, the second cut line 538, and the third cut line 542. The second back-ground wafer 530 has a top surface 546 that varies (e.g., undulations) by less than 5 μm. In addition, the second back-ground wafer 530 has a continuous lattice and does not include cracks (e.g., the several cracks 516 in the first photo 500). Therefore, as shown, the inclusion of the flattening fixture improves the results of the back-ground operation.
[0039] Fig.10 A set of graphs 600 (six (6) graphs) are shown that plot the thickness of a back-ground wafer (e.g., after back-grinding) as a function of the distance (in μm) from the edge of the back-ground wafer. Figure 1Each of the set of graphs 600 includes a graph for a back-ground wafer that has been back-ground without a fixture and a graph for a back-ground wafer that has been back-ground with a flattening fixture (e.g., Figure 1 1 and 1 . As shown, the wafer back ground with conventional methods has a undulation (e.g., top side variation) of up to about 10 μm. In contrast, in the example shown, the wafer back ground while mounted on a leveling fixture has a undulation (e.g., top side variation) of about 0.5 μm to about 1 μm.
[0040] Fig.11 Shown for example Figure 1 104 or the like. Thus, in some examples, the wafer is a WCSP wafer. At 710, a flattening fixture (e.g., apparatus 100) is placed on a grinding table (e.g., Figure 1 The leveling fixture includes a shim (e.g., Figure 1 At 715, the lapping tape adhered to the first side of the wafer is placed on a leveling fixture. The cross-section of the lapping tape is shaped to mirror the cross-section of the step gap formed at the edge of the lapping tape covering the solder balls on the first side of the wafer, so that the lapping tape and the lapping tape form a flatter surface on the lapping table. That is, the lapping tape has a gum height that matches the step gap of the lapping tape. The height of the step gap is about 40 μm to about 60 μm. In some examples, the lapping tape is placed on a vacuum table (e.g., Figure 1 on a vacuum table 152).
[0041] At 720, the second side of the wafer is back ground to form a flatter surface on the second side of the wafer. The back grinding operation includes applying pressure (e.g., a downward force) on the second side of the wafer. The pads of the device are shaped to reduce bending of the wafer around the solder balls of the solder balls that are closest to the edge of the grinding zone. Back grinding removes a portion of the second side of the wafer to provide a back ground wafer, and the back ground wafer has a continuous lattice (e.g., without cracks). In some examples, the flatter surface on the second side of the wafer formed by back grinding has a variation of 5 microns or less.
[0042] At 725, the wafer is removed from the grinding station. At 730, the wafer is sawed (e.g., Figure 7The die on the wafer are singulated by saw 448. At 735, the grinding tape is removed from the singulated die to expose the solder balls. The removal of the grinding tape enables the resulting singulated die to be mounted (installed) on a PCB.
[0043] In this specification, "about" preceding a parameter means within + / - 10% of the parameter, unless otherwise stated. Modifications are possible in the described embodiments, and other embodiments are possible within the scope of the claims.
Claims
1. A device for a grinding table, comprising: A shim having a cross section shaped to mirror the cross section of a step gap formed at an edge of a grinding tape adhered to a first side of a wafer covering solder balls so that the shim and the grinding tape form a flatter surface for the wafer on the grinding table.
2. The device of claim 1, wherein the device is a leveling fixture. 3 . The apparatus of claim 1 , wherein the step gap has a height measured from a point at a maximum thickness of the grinding tape to a point on a solder ball close to the edge of the grinding tape.
4. The device of claim 3, wherein the step gap has a height of about 40 microns to about 60 microns.
5. The apparatus of claim 1, wherein the flatter surface formed by the pad and the polishing tape is configured to be placed on a vacuum table of the polishing table. The device according to claim 1 , wherein the spacer has a circular shape.
7. The apparatus of claim 1, wherein the wafer is a wafer scale chip package (WCSP) wafer.
8. A method for back grinding a wafer, comprising: placing an apparatus on a lapping table, wherein the apparatus includes a spacer for a wafer; placing a grinding tape adhered to a first side of a wafer on the apparatus, wherein a cross section of the pad is shaped to mirror a cross section of a step gap formed at an edge of the grinding tape covering solder balls on the first side of the wafer so that the pad and the grinding tape form a flatter surface on the grinding table; as well as The second side of the wafer is back ground to form a more planar surface on the second side of the wafer.
9. The method according to claim 8, further comprising: removing the wafer from the grinding station; as well as The die in the wafer are singulated.
10. The method of claim 9, further comprising removing the grinding tape from the singulated die to expose the solder balls.
11. The method of claim 8, wherein the flatter surface on the second side of the wafer has a variation of 5 microns or less.
12. The method of claim 8, wherein the device is a leveling fixture.
13. The method of claim 8, wherein the step gap has a height measured from a maximum thickness of the grinding tape to a point on a solder ball close to the edge of the grinding tape.
14. The method of claim 11, wherein the step gap has a height of about 40 microns to about 60 microns.
15. The method of claim 8, wherein the flatter surface formed by the pad and the polishing tape is configured to be placed on a vacuum table of the polishing table. The method of claim 8 , wherein the spacer has a circular shape.
17. The method of claim 8, wherein the wafer is a wafer scale chip package (WCSP) wafer.
18. A method for back grinding a wafer, comprising: placing an apparatus on a lapping table, wherein the apparatus includes a spacer for a wafer; placing an abrasive tape adhered to a first side of a wafer on the apparatus, wherein the abrasive tape covers solder balls on the first side of the wafer; as well as back grinding the second side of the wafer to form a more planar surface on the second side of the wafer, wherein the back grinding comprises: applying pressure on the second side of the wafer, wherein the pads of the device are shaped to reduce bending of the wafer about edges of the solder balls that are closest to the grinding zone; as well as A portion of the second side of the wafer is removed to provide a back ground wafer, and the back ground wafer has a continuous crystal lattice.
19. The method of claim 18, wherein the device is a leveling fixture.
20. The method of claim 19, wherein a cross-section of the pad of the device is shaped to mirror a cross-section of a step gap formed at the edge of the polishing tape so that the pad and the polishing tape form a flatter surface on the polishing table.