Wafer back surface grinding
The device with a shim addresses the issue of uneven thickness and flatness in the back grinding process by creating a uniform surface, resulting in a consistently thick and structurally sound wafer with minimal surface deviation.
Patent Information
- Application Number
- JP2024150458
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-08-31
- Filing Date
- 2024-09-02
- Publication Date
- 2025-03-18
AI Technical Summary
The back grinding process of wafers for IC chip production faces challenges due to uneven thickness and flatness caused by the presence of solder balls, which can lead to instability and non-uniform thickness during grinding.
A device with a shim is used on a grinding table to create a flat surface, where the shim is shaped to match the cross-sectional profile of the step gap formed by the grinding tape covering the solder balls, ensuring uniform thickness and stability during back grinding.
The use of this device ensures a wafer with consistent thickness and structural integrity, preventing bending and cracking, and achieving a surface deviation of less than 5 μm, thereby improving the back grinding operation.
Smart Images
Figure 2025037834000001_ABST
Abstract
Description
[Technical field]
[0001] This description relates to backgrinding wafers for the fabrication of integrated circuit (IC) chips. [Background technology]
[0002] Wafer-level chip-scale packaging (WCSP) is a semiconductor packaging method that reduces package dimensions by closely matching the size of the chip, achieving small size and high integration. This results in shorter electrical connections and improved performance in terms of both electrical and thermal properties. WCSP offers cost and space efficiencies that make it a preferred choice in space-constrained applications such as mobile and wearable devices, where efficient integration is preferred to achieve superior performance.
[0003] In contrast to conventional processing techniques that involve molding operations, WCSP avoids the use of molding for chip encapsulation. Instead, WCSP uses a redistribution layer (RDL) process to establish connections between the pads of the microchip and external solder balls directly on the wafer c. This eliminates the need for a separate molded plastic or ceramic package to encapsulate the microchip.
[0004] Backgrinding, 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 accomplished by precision grinding the backside of the wafer with an abrasive material. By thinning the wafer, benefits such as improved thermal performance, reduced signal delays, and improved electrical performance due to shorter interconnect lengths are realized. After grinding, a polishing operation is often used to ensure uniform thickness. Backgrinding is used to create thin and lightweight devices such as mobile computing devices, and contributes to the 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 a device for a grinding table that includes a shim having a cross-section shaped to mirror the cross-section of a step gap formed in an edge of a grinding tape adhered to a first side of a wafer covering a solder ball, such that the shim and grinding tape form a flat surface on the grinding table for the wafer.
[0006] A second example relates to a method for backgrinding a wafer including placing a device on a grinding table, the device including a shim for the wafer. The method includes placing a grinding tape adhered to a first side of the wafer over the device. The shim has a cross-section shaped to mirror a cross-section of a step formed at an edge of the grinding tape covering the solder balls on the first side of the wafer, such that the shim and the grinding tape form a flat surface on the grinding table. The method also includes backgrinding a second side of the wafer to form a flat surface on the second side of the wafer.
[0007] A third example relates to a method for backgrinding a wafer including placing a device on a grinding table, the device including a shim for the wafer. The method includes placing a grinding tape adhered to a first side of the wafer over the device, the grinding tape covering the solder balls on the first surface of the wafer. The method also includes backgrinding a second side of the wafer to form a flat surface on the second side of the wafer. The backgrinding includes applying pressure to the second side of the wafer. The shim of the device is shaped to inhibit bending of the wafer around those of the solder balls that are closest to an edge of the grinding tape. The method further includes removing a portion of the second side of the wafer to provide a background wafer, the background wafer having a continuous lattice. [Brief description of the drawings]
[0008] [Figure 1]1 illustrates an exemplary device for facilitating backgrinding of a wafer.
[0009] [Diagram 2] 2 illustrates an enlarged view of a portion of the wafer illustrated in FIG. 1;
[0010] [Figure 3A] 2 illustrates a cross-sectional view of the device illustrated in FIG. 1, where the wafer has been removed from the view.
[0011] [Figure 3B] 2 illustrates a top view of the device shown in FIG. 1, where the wafer has been removed from the view.
[0012] [Figure 4] 1 illustrates a first stage of an exemplary method for forming IC chips from a wafer.
[0013] [Diagram 5] 1 illustrates a second stage of an exemplary method for forming IC chips from a wafer.
[0014] [Figure 6] 1 illustrates a third stage of an exemplary method for forming IC chips from a wafer.
[0015] [Figure 7] 1 illustrates a fourth stage of an exemplary method for forming IC chips from a wafer.
[0016] [Figure 8] 1 illustrates a fifth stage of an exemplary method for forming IC chips from a wafer.
[0017] [Figure 9] 1 shows a photograph of a background wafer.
[0018] [Figure 10]1 illustrates a series of graphs plotting the undulations of a background wafer as a function of distance from the edge of the background wafer.
[0019] [Figure 11] 1 illustrates a flow chart of an exemplary method for forming IC chips from a wafer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] The present description relates to a device designed to facilitate the backgrinding process of wafers used for integrated circuit (IC) chips. The device is mountable on or integrated with a grinding table. The device has a circular or oval shape, and the device may be integrated with or removable from the grinding table. The device is intended for use with wafers such as wafer-level chip-scale package (WCSP) wafers, but may also be used with other types of wafers that have edge planarity issues. The wafer has a silicon layer with an embedded circuit layer on a first side. The circuit layer includes circuit elements for the IC chips. A set of solder balls are positioned to provide electrical continuity to pads on the circuit layer, and a grinding tape is adhered to the first side of the wafer to cover the set of solder balls. The solder balls are positioned near the center of the wafer and do not extend to the edge.
[0021] The grinding tape has a varying thickness across the wafer due to the absence of solder balls around it, resulting in non-uniform flatness. To compensate for this varying thickness (referred to as a step gap), the device incorporates a shim that surrounds the periphery of the grinding tape. The shim has a shape that reflects the non-uniformity caused by the varying thickness of the grinding tape, and is intended to create a more uniform and flat surface. The grinding tape and shim together act to form a flat (and level) surface on the vacuum table of the grinding table. This ensures stability during the backgrinding process, where a downward force (pressure) is applied to the second side (opposite the first side) of the wafer to reduce the thickness of the silicon layer of the wafer. The shim prevents the wafer from moving (e.g., rocking), maintains a uniform thickness reduction, and prevents cracking, resulting in consistent thickness and structural integrity of the background wafer. Therefore, the device including the shim is referred to as a planarization fixture in some instances.
[0022] Thus, the device improves wafer backgrinding operations in IC chip production. By using shims in conjunction with abrasive tape, the device ensures uniform flatness and stability during the grinding process, resulting in wafers with consistent thickness and structural integrity. Use of the device does not significantly increase the production steps or costs associated with IC chip manufacturing.
[0023] FIG. 1 illustrates a cross-sectional view of a device 100 for facilitating backgrinding of a wafer 104. The device 100 can be mounted on a grinding table 108. The device 100, the grinding table 108, and the wafer 104 have a circular or elliptical shape in some examples. In some examples, the device 100 is integrated with the grinding table 108. In other examples, the device 100 is removable from the grinding table 108. In some examples, the wafer 104 is a wafer-level chip-scale package (WCSP) wafer. In other examples, the wafer 104 is a different type of wafer that generally has edge planarity issues. The wafer 104 includes a silicon layer 112 with a first side 116 and a second side 120 opposite the first side 116. The first side 116 includes a circuit layer 124 underlying the first side 116 of the silicon layer 112. Circuitry layer 124 contains circuitry embedded in silicon to perform the functions of the integrated circuit (IC) chips formed from wafer 104 .
[0024] A set of solder balls 128 are formed in contact with the pads of the circuit layer 124. A grinding tape 132 is attached to the first side 116 of the wafer 104 over the set of solder balls 128. The set of solder balls 128 is located proximate to the center of the wafer 104, and the set of solder balls 128 does not extend to the periphery of the wafer 104. Thus, a solder ball 128 of 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 near the periphery of the wafer 104. FIG. 2 illustrates this concept. More specifically, FIG. 2 illustrates an enlarged view of an area 136 of the wafer 104 of FIG. 1. Therefore, FIGS. 1 and 2 use the same reference numbers to indicate the same structures. Also, other components have been removed for clarity, and wafer 104 is shown inverted in FIG.
[0025] As shown in detail in FIG. 2, the area of the grinding tape 132 overlying the set of solder balls 128 has a first thickness, and the area of the grinding tape 132 extending to the periphery of the wafer 104 has a second thickness (forming a taper) that is less than the first thickness. Thus, the grinding tape 132 has a non-uniform flatness across the wafer 104. The difference in height between a line 200 extending from a portion of the grinding tape 132 at the first thickness (maximum thickness) and a line 204 extending from the top of the solder ball 128 closest to the tapered edge of the grinding tape 132 is referred to as a step 208. In other words, the step 208 has a height measured from a point of maximum thickness of the grinding tape 132 to a point on the solder ball 128 proximate to the edge of the grinding tape 132. The step can be, for example, from about 40 micrometers (μm) to about 60 μm.
[0026] Referring again to FIG. 1, to offset the uneven flatness of the grinding tape 132 (formed by the step 208), the device 100 includes a shim 140 that extends around the grinding tape 132. As mentioned above, the device 100 has a circular or elliptical shape, as FIG. 1 illustrates two portions of the shim 140. The shim 140 is shaped so that the cross section of the shim 140 reflects the step 208 formed in the tapered edge of the grinding tape 132. That is, the shim 140 is shaped to offset the step 208 of the grinding tape 132. In some instances, the shim 140 is referred to as having a gum height to compensate for the step 208 of FIG. 2. The grinding tape 132 and the shim 140 work in concert to form a flat surface 148 on the vacuum table 152 of the grinding table 108. The vacuum table 152 draws in air and exerts a downward force in the direction indicated by arrow 156 to keep the wafer 104 stationary during the grinding operation.
[0027] 3A and 3B illustrate the device 100 with the wafer 104 removed. More specifically, FIG. 3A illustrates a cross-sectional view of the device 100 (similar to FIG. 1) and FIG. 3B illustrates a top view of the device 100. Therefore, FIG. 1, FIG. 3A, and FIG. 3B use the same reference numbers to indicate the same structures. As illustrated in FIG. 3B, the shim 140 has a circular shape that is placed on the vacuum table 152 of the grinding table 108. The shape of the shim 140 allows the wafer 104 (not shown in FIG. 3A and FIG. 3B) to be placed on the device 100 in the manner illustrated in FIG. 1.
[0028] 1, as described above, the shim 140 is shaped to ensure that the grinding tape 132 and the shim 140 form a flat surface 148 on the vacuum table 152 of the grinding table 108. During the backgrinding process, a downward force (e.g., pressure) in a direction indicated by arrow 156 is applied to the second side 120 of the silicon layer 112. The backgrinding operation removes a layer of the silicon layer 112 to reduce the overall thickness of the silicon layer 112. The shim 140 also cooperates with the grinding tape 132 to provide the flat surface 148 and prevent rocking, bending, and / or damping of the wafer 104 during the backgrinding process. In this manner, the device 100 operates as a planarization fixture. Thus, the shim 140 holds the wafer 104 in place (stationary) during the backgrinding operation to ensure that the thickness variation (e.g., waviness) of the silicon layer 112 is 5 μm or less across the entire surface of the silicon layer 112 after the backgrinding is performed. The shim 140 also prevents cracking during the backgrinding operation and ensures that the resulting wafer 104 has a continuous lattice. Additionally, the inclusion of the device 100 does not increase the process steps and / or costs for forming IC chips from the wafer 104.
[0029] Figures 4-8 illustrate steps in a method for forming an IC chip from a WCSP wafer. For simplicity, Figures 4-8 use the same reference numbers to indicate the same structures.
[0030] At 300, in a first stage of the method, as illustrated in FIG. 4, a device 400 (e.g., device 100 of FIG. 1) is placed on a grinding table 404 (e.g., grinding table 108 of FIG. 1). The device 400 includes a shim 402 for receiving a wafer 408 (e.g., a WCSP wafer). The grinding table 404 includes a vacuum table 406. In some examples, the shim 402 is held in place by the suction created by the vacuum table 406.
[0031] Also at 300, a grinding tape 412 that is adhered to a first side 416 of a wafer 408 is placed on the shim 402. More specifically, the grinding tape is adhered to an underside of a silicon layer 420 of the wafer 408. The grinding 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 an edge of the grinding tape 412. The shim 402 is shaped such that a cross section of a step (e.g., step 208 in FIG. 2) formed at the edge of the polishing tape 412 is mirrored by the shim 402. To place the grinding tape 412 on the shim 402, the wafer 408 is moved toward the device 400 in a direction indicated by an arrow 418.
[0032] 5, in a second stage of the method, the wafer 408 is placed in the device 100 such that a flat surface 424 is formed on the vacuum table 406. Also, at 305, a backgrinding operation of the wafer 408 is initiated. In the backgrinding operation, a portion of the silicon layer 420 of the wafer 408 is removed. In the backgrinding operation, a portion of the second side 428 of the wafer 408 is removed, the second side 428 being distal from the set of solder balls 414. More specifically, in the illustrated example, a portion of the silicon layer 420 above the marked line 432 is removed during the backgrinding operation.
[0033] In various examples, different materials may be used to perform the backgrinding operation in 305. The choice of material for the backgrinding operation depends on several factors, including the type of wafer, the desired thickness reduction, and the manufacturing tool. Materials used for the backgrinding operation include diamond grinding wheels to reduce damage to the wafer 408. Also, in some examples, complementary grinding fluids and / or coolants are used to lubricate and / or cool the process to prevent overheating and potential cracking. In some examples, a polishing pad with fine abrasives is applied to achieve a smoother surface of the wafer 408 after backgrinding. Also, a protective coating may be applied to shield the backside of the wafer during backgrinding. In some examples, a chemical mechanical planarization (CMP) slurry is used for further thinning and polishing, a blend of chemical reaction with mechanical abrasion. During the backgrinding operation, a downward force (pressure) is applied in the direction shown by arrow 436. The shim 402 of the device 400 is shaped to inhibit bending of the wafer 408 around the solder ball 414 of the set of solder balls 414 that is closest to the edge of the grinding tape 412. A flat surface 424 formed on the vacuum table 406 inhibits rocking of the wafer 408 during the grinding operation. The backgrinding operation may include, for example, multiple passes to reduce the thickness of the silicon layer 420 to a desired level. Backgrinding the wafer 408 removes a portion of the second side of the wafer 408 to provide a background wafer.
[0034] At 310, in the third stage of the method, the wafer 408 is removed from the polishing table 404, as illustrated in FIG. 6. The resulting silicon layer 420 has a surface deviation of about 5 μm or less. Also, the wafer 408 has a continuous lattice (e.g., no cracks) as a result of the backgrinding operation. As illustrated in FIG. 6, the wafer 408 is background (at 305) so that the marked line 432 is at the top of the silicon layer 420 of the wafer 408. To remove the wafer 408 from the grinding table 404, the wafer 408 is lifted in the direction indicated by the arrow 440. The wafer 408 can be moved to a dicing table.
[0035] At 315, in a fourth stage of the method, as shown in FIG. 7, the wafer 408 is placed on a dicing table 444. Also at 315, the dies of the wafer 408 are singulated with a saw 448. The saw 448 may be, for example, a laser saw or a plasma saw. The resulting singulated dies are removed from the dicing table 444.
[0036] At 320, the grinding tape 412 is removed from the singulated die 450 prior to mounting the singulated die 450 on a printed circuit board (PCB). Removal of the grinding tape 412 exposes a set of solder balls 414 on the singulated die 450 to allow electrical connection to circuitry embedded in the singulated die 450.
[0037] FIG. 9 shows a photograph of a background wafer, which is a wafer (e.g., wafer 104 of FIG. 1) after backgrinding. FIG. 9 includes a first photograph 500 illustrating a conventional approach in which a planarization fixture is not used, so that the first background 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 rocking of the first background wafer 504 during the backgrinding operation, as the downward pressure is distributed non-uniformly across the first background wafer 504. In addition, due to this non-uniform downward pressure on the first wafer 504, the first background wafer 504 contains several cracks 516. These cracks 516 can cause failures in the singulated dies, thereby reducing the yield of the wafer 504.
[0038] FIG. 9 also includes a second photograph 528 illustrating how a planarization fixture (e.g., device 100 FIG. 1 including shim 140) is used during backgrinding of the second background wafer 530. The planarization fixture prevents rocking of the second background wafer 530 during the backgrinding operation. The second background wafer 530 has a uniform thickness, as evidenced by the relatively straight first cut line 534, second cut line 538, and third cut line 542. The second background wafer 530 has a top surface 546 with a deviation (e.g., waviness) of less than 5 μm. The second background wafer 530 also has a continuous lattice and is free of cracks (e.g., some of the cracks 516 in the first photograph 500). Thus, as shown, the inclusion of the planarization fixture improves the results of the backgrinding operation.
[0039] FIG. 10 illustrates a set of graphs 600 (six graphs) that plot the waviness of a background wafer (e.g., wafer 104 of FIG. 1 after backgrinding) in μm as a function of distance (μm) from the edge of the background wafer. Each graph in the set of graphs 600 includes a plot of a background wafer that was background without a fixture using a conventional backgrinding operation and a plot of a background wafer that was background using a planarization fixture (e.g., device 100 with shim 140 of FIG. 1). As illustrated, the conventionally background wafer has waviness (e.g., top side variation) of up to about 10 μm. In contrast, in the illustrated example, the wafer that was background while resting on the planarization fixture has waviness (e.g., top side variation) of about 0.5 μm to about 1 μm.
[0040] FIG. 11 illustrates a flow chart of an example method 700 for forming IC chips from a wafer, such as wafer 104 of FIG. 1. Thus, in some examples, the wafer is a WCSP wafer. At 710, a planarization fixture (e.g., device 100) is placed on a grinding table (e.g., grinding table 108 of FIG. 1). The planarization fixture includes a shim (e.g., shim 140 of FIG. 1). At 715, a grinding tape adhered to a first side of the wafer is placed on the planarization fixture. The shim has a cross-section shaped to reflect the cross-section of the step formed at the edge of the grinding tape covering the solder balls on the first side of the wafer, such that the shim and grinding tape form a flat surface on the grinding table. That is, the shim has a rubber height that matches the step of the grinding tape. The step has a height of about 40 μm to about 60 μm. In some examples, the grinding tape is placed on a vacuum table of the grinding table 108 (eg, vacuum table 152 in FIG. 1).
[0041] At 720, the second side of the wafer is back-ground to form a flat surface on the second side of the wafer. The back-grinding operation includes applying pressure (e.g., a downward force) to the second side of the wafer. The shims of the device are shaped to inhibit bending of the wafer around certain of the solder balls closest to the edge of the grinding tape. The back-grinding removes a portion of the second side of the wafer to provide a back-ground wafer, the back-ground wafer having a continuous lattice (e.g., free of cracks). In some examples, the deviation of the flat surface on the second side of the wafer formed by the back-grinding is 5 micrometers or less.
[0042] At 725, the wafer is removed from the grinding table. At 730, the dies on the wafer are singulated using a saw, such as a laser saw or a plasma saw (e.g., saw 448 of FIG. 7 ). At 735, the grinding tape is removed from the singulated dies to expose the solder balls. With the grinding tape removed, the resulting singulated dies can be placed (mounted) onto a PCB.
[0043] As used herein, unless otherwise indicated, "about" preceding a parameter means within + / - 10% of that parameter. Modifications may be made in the exemplary embodiments described, and other embodiments are possible, within the scope of the present 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 formed at an edge of an abrasive tape adhered to the first side of the wafer covering the solder balls; Thus, the shim and the grinding tape form a flat surface for the wafer on the grinding table. device.
2. The device of claim 1 , wherein the device is a planarization fixture.
3. 2. The device of claim 1, wherein the step has a height measured from a point at a maximum thickness of the abrasive tape to a point on a solder ball proximate the edge of the abrasive tape.
4. The device of claim 3 , wherein the step has a height of about 40 micrometers to about 60 micrometers.
5. 2. The device of claim 1, wherein the flat surface is formed by the shim and the abrasive tape is configured to rest on a vacuum table of the abrasive tape.
6. The device of claim 1 , wherein the shim has a circular shape.
7. The device of claim 1 , wherein the wafer is a wafer level chip scale package (WCSP) wafer.
8. 1. A method for backgrinding a wafer, comprising: placing a device including a shim for the wafer on a grinding table; placing a grinding tape adhered to a first side of a wafer on the device, the shim having a cross-section shaped to mirror the cross-section of a step at an edge of the grinding tape covering a solder ball on the first side of the wafer, such that the shim and the grinding tape form a flat surface on the grinding table; backgrinding the second side of the wafer to form a flat surface on the second side of the wafer; A method comprising:
9. 9. The method of claim 8, further comprising: removing the wafer from the grinding table; singulating die in the wafer; A method comprising:
10. 9. The method of claim 8, further comprising removing the grinding tape from the singulated die to expose the solder balls.
11. 9. The method of claim 8, wherein the planar surface of the second side of the wafer has a deviation of 5 micrometers or less.
12. The method of claim 8 , wherein the device is a planarization fixture.
13. 9. The method of claim 8, wherein the step has a height measured from a point at a maximum thickness of the abrasive tape to a point on a solder ball proximate the edge of the abrasive tape.
14. The method of claim 11 , wherein the step has a height of about 40 micrometers to about 60 micrometers.
15. 9. The method of claim 8, wherein the flat surface is formed by the shim and the abrasive tape is configured to rest on a vacuum table of the abrasive tape.
16. The method of claim 8 , wherein the shim has a circular shape.
17. 9. The method of claim 8, wherein the wafer is a wafer level chip scale package (WCSP) wafer.
18. 1. A method for backgrinding a wafer, comprising: placing a device including a shim for the wafer on a grinding table; placing a grinding tape adhered to a first side of a wafer over the device, the grinding tape covering solder balls on the first side of the wafer; backgrinding the second side of the wafer to form a flat surface on the second side of the wafer; Including, The back grinding step comprises: applying pressure to the second side of the wafer, the shim of the device being shaped to inhibit bending of the wafer around those of the solder balls that are closest to an edge of the grinding tape; removing a portion of the second side of the wafer to provide a background wafer, the background wafer having a continuous grating; A method comprising:
19. The method of claim 8 , wherein the device is a planarization fixture.
20. 20. The method of claim 19, wherein the shim of the device has a cross-section shaped to mirror the cross-section of a step formed in the edge of the grinding tape, such that the shim and the grinding tape form a flat surface on the grinding table.