Wafer grinding method

Abstract

To provide a method for grinding a wafer that can grind the wafer in an even thickness efficiently in a short time.SOLUTION: A method for grinding a wafer W, which grinds the wafer W while contacting a rotating ring-like grindstone 26b with a radius part of the rotating wafer W held on a conical holding surface 11a of a chuck table 10, grinds the wafer W through a holding step of holding the wafer W on the holding surface 11a, a first grinding step of grinding an outer periphery portion Wb of the wafer W slightly thicker while forming a small recess W1 at a central portion Wa of the wafer W, by contacting a lower surface of the ring-like grindstone 26b with the radius portion of the wafer W held on the holding surface 11a, and a second grinding step of grinding a portion other than the central portion Wa of the wafer W, by changing inclinations of the chuck table 10 and of the ring-like grindstone 26b relatively so that the lower surface of the ring-like grindstone 26b does not contact the center of the wafer W, after the first grinding step.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a wafer grinding method for grinding a wafer held on a holding surface of a chuck table with an annular grinding wheel. [Background Art]

[0002] In the manufacturing process of semiconductor devices such as ICs and LSIs used in various electronic devices, the back surface of a wafer is ground to thin the wafer to a predetermined thickness for reducing the size and weight of semiconductor devices. For example, when the radial portion of a wafer held on a chuck table is ground by the lower surface of an annular grinding wheel using the grinding apparatus disclosed in Patent Document 1, as described in Patent Document 2, a slight depression is formed in the central portion of the ground wafer. It is considered that this depression is formed because the holding surface of the chuck table holding the wafer is formed conically, so that the central portion of the wafer becomes convex at the central portion of the holding surface, and this convex central portion is ground away more.

[0003] Therefore, Patent Document 3 proposes a holding surface forming method for removing the center of the holding surface of the chuck table, that is, the apex of the cone, which prevents the formation of depressions caused by excessive grinding of the central portion of the wafer. [Prior Art Documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2018-114573 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2021-146416 [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2020-175472 [Summary of the Invention] [Problems to be Solved by the Invention]

[0005] However, the method for forming the holding surface proposed in Patent Document 3 has the problem that it takes a long time to form the holding surface of the chuck table, resulting in poor work efficiency.

[0006] The present invention has been made in view of the above problems, and its objective is to provide a wafer grinding method that can grind a wafer to a uniform thickness efficiently in a short amount of time. [Means for solving the problem]

[0007] To achieve the above objective, the present invention provides a wafer grinding method comprising: a holding step of holding the wafer on the holding surface; and a step of bringing the lower surface of the annular grinding wheel into contact with the radial portion of the wafer, which is held and rotated on the conical holding surface of a chuck table, thereby grinding the outer circumference of the wafer while forming a slight depression in the central portion of the wafer. From the central part A first grinding step in which the wafer is ground slightly thicker, and after the first grinding step, the relative inclination between the chuck table and the annular grinding wheel is changed so that the lower surface of the annular grinding wheel does not come into contact with the center of the wafer. In the first grinding step, the annular grinding wheel is brought into contact with the outer periphery, which is formed thicker than the central portion, and the wafer is ground to a uniform thickness so that the thickness of the outer periphery matches the thickness of the central portion. It is characterized by comprising a second grinding step. [Effects of the Invention]

[0008] According to the present invention, in the first grinding step, a slight depression is formed in the central part of the wafer while the outer periphery of the wafer is ground to a slightly thicker thickness. In the second grinding step, the relative inclination between the chuck table and the annular grinding wheel is changed so that the lower surface of the annular grinding wheel does not come into contact with the center of the wafer, thereby grinding the slightly thicker outer periphery of the wafer other than the central part. As a result, the thickness of the wafer can be uniformly finished to the thickness of the depression formed in the central part of the wafer in the first grinding step. Therefore, it is not necessary to remove the apex of the holding surface of the chuck table to prevent a depression from forming in the central part of the wafer, and the formation of the holding surface does not take a long time. Thus, the effect of efficiently grinding the wafer to a uniform thickness in a short time is obtained. [Brief explanation of the drawing]

[0009] [Figure 1] This is a perspective view showing a section of a wafer grinding apparatus for carrying out the method of the present invention. [Figure 2] This is a plan view showing the positional relationship between the wafer and the annular grinding wheel. [Figure 3] This is a partial cross-sectional view in the direction of arrow A in Figure 2, showing the first grinding step in the method of the present invention. [Figure 4] This is a partial cross-sectional view in the direction of arrow A in Figure 2, showing the second grinding step in the method of the present invention. [Figure 5] (a) is a longitudinal cross-sectional view of a wafer ground in the first grinding step of the present invention, and (b) is a longitudinal cross-sectional view of a wafer ground in the second grinding step of the present invention. [Figure 6] (a) is a figure showing the radial thickness distribution of a wafer ground by the method of the present invention, and (b) is a figure showing the radial thickness distribution of a wafer ground by a conventional grinding method. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below with reference to the accompanying drawings.

[0011] First, the configuration of the wafer grinding apparatus for carrying out the method of the present invention will be described below with reference to Figures 1 and 2. In the following description, the arrow directions shown in Figure 1 will be the X-axis direction (left-right direction), the Y-axis direction (front-back direction), and the Z-axis direction (up-down direction), respectively.

[0012] [Grinding equipment] The grinding apparatus 1 shown in Figure 1 is used to grind a disc-shaped wafer W, which is the workpiece, and comprises the following components.

[0013] That is, the grinding apparatus 1 mainly includes: a chuck table 10 that holds a wafer W and rotates around the axis CL1 (see FIGS. 3 and 4) of the wafer W; a grinding unit 20 that grinds the wafer W suction-held on the chuck table 10; a thickness measuring instrument 30 that measures the thickness of the wafer W during grinding; a grinding feed means 40 that lifts and lowers the grinding unit 20 in the direction perpendicular to the holding surface 11a of the chuck table 10 (Z-axis direction); a nozzle 50 that supplies grinding water to a contact area (grinding area) R (see FIG. 2) between the annular grinding wheel 26b of the grinding unit 20 and the wafer W; a tilt adjustment mechanism 70 that adjusts the tilt of the chuck table 10; and a horizontal movement mechanism 80 that moves the chuck table 10 in the horizontal direction (Y-axis direction) relative to the holding surface 11a.

[0014] Here, the wafer W is formed of a single-crystal silicon base material, and a plurality of devices (not shown) are formed on the downward-facing surface in the state shown in FIG. 1, and these devices are protected by a protective tape T attached to the surface of the wafer W. The surface (the lower surface in FIG. 1) of the wafer W is suction-held on the holding surface 11a of the chuck table 10 via the protective tape T, and the back surface (the upper surface in FIG. 1) is ground by the grinding wheel 26b of the grinding unit 20 while receiving grinding water supplied from the nozzle 50.

[0015] Next, the configurations of the main components of the grinding apparatus 1, namely the chuck table 10, the grinding unit 20, the thickness measuring instrument 30, the grinding feed means 40, the nozzle 50, the tilt adjustment mechanism 70, and the horizontal movement mechanism 80, will each be described.

[0016] (Chuck Table) The chuck table 10 is a disk-shaped member, and is constituted by a disk-shaped frame 10A and a porous member 11 incorporated in a circular recess 10a formed in the center of the frame 10A. Here, the porous member 11 is formed of porous ceramic or the like, and the upper surface thereof constitutes a holding surface 11a that suction-holds the disk-shaped wafer W.

[0017] The chuck table 10 is rotationally driven around the axis CL1 in the arrow direction (counterclockwise direction) shown in FIG. 2 by a rotation driving mechanism (not shown). That is, the chuck table 10 includes a rotating shaft 12 that extends vertically and integrally downward from the center thereof, and the rotating shaft 12 is rotationally driven at a predetermined speed by the rotation driving mechanism (not shown). Although not illustrated, the porous member 11 of the chuck table 10 is selectively connected to a suction source (not shown) such as a vacuum pump.

[0018] Further, the grinding apparatus 1 according to the present embodiment includes a rectangular box-shaped base 100 elongated in the Y-axis direction (front-rear direction), and the chuck table 10 faces a rectangular opening 100a elongated in the Y-axis direction opened in the base 100. The periphery of the chuck table 10 at the opening 100a is covered with a rectangular plate-shaped cover 101, and the front and rear portions (in the -Y direction and +Y direction) of the cover 101 at the opening 100a are respectively covered with bellows-shaped expandable covers 102 and 103 that expand and contract while moving together with the cover 101. Therefore, no matter what position on the Y-axis the chuck table 10 is located at, the opening 100a is always closed by the cover 101 and the expandable covers 102 and 103, preventing foreign matter from entering the base 100 through the opening 100a.

[0019] (Grinding Unit) The grinding unit 20 includes a spindle housing 22 fixed to a holder 21, a spindle motor 23 accommodated in the spindle housing 22, a vertical spindle 24 rotationally driven by the spindle motor 23, a disk-shaped mount 25 attached to the lower end of the spindle 24, and a grinding wheel 26 detachably mounted on the lower surface of the mount 25. Here, the grinding wheel 26 is constituted by a disk-shaped base 26a and a plurality of annular grinding stones 26b that are processing tools attached in an annular shape to the lower surface of the base 26a. The annular grinding stones 26b are rectangular block-shaped processing tools for grinding a wafer W, and the lower surface thereof constitutes a grinding surface that contacts the upper surface (surface to be ground) of the wafer W.

[0020] (Thickness measuring instrument) The thickness measuring instrument 30 is a height gauge for measuring the thickness of a wafer W being ground while it is held in the chuck table 10. It comprises a first contact 31 that contacts the upper surface of the wafer W and a second contact 32 that contacts the upper surface of the frame 10A of the chuck table 10. The height of the upper surface of the wafer W being ground is measured by the first contact 31, and the thickness of the wafer W is determined by the difference between the height of the upper surface of the wafer W measured by the first contact 31 and the height of the upper surface of the frame 10A measured by the second contact 32.

[0021] (Grinding feed mechanism) The grinding feed mechanism 40 raises and lowers the grinding unit 20 in a direction perpendicular to the holding surface 11a of the chuck table 10 (Z-axis direction), and is positioned on the -Y-axis end face (front face) of a rectangular box-shaped column 41 that is erected vertically on the +Y-axis end (rear end) of the upper surface of the base 100. This grinding feed mechanism 40 raises and lowers a rectangular plate-shaped lifting plate 42 attached to the back of the holder 21 in the Z-axis direction along a pair of left and right guide rails 43, together with the holder 21 and the spindle 24 and grinding wheel 26 held in the holder 21. Here, the pair of left and right guide rails 43 are arranged perpendicularly and parallel to each other on the front face of the column 41.

[0022] Furthermore, a rotatable ball screw 44 is erected vertically along the Z-axis direction (up and down direction) between a pair of left and right guide rails 43, and the upper end of the ball screw 44 is connected to a reversible electric motor 45, which is the drive source. Here, the electric motor 45 is mounted vertically on the column 41 via a rectangular plate-shaped bracket 46 attached to the upper surface of the column 41. The lower end of the ball screw 44 is rotatably supported by the column 41, and a nut member (not shown) that protrudes horizontally toward the rear (+Y-axis direction) from the back of the lifting plate 42 is screwed onto this ball screw 44.

[0023] Therefore, by starting the electric motor 45 and rotating the ball screw 44 in both forward and reverse directions, the lifting plate 42, to which a nut member (not shown) that screws onto the ball screw 44 is attached, moves up and down along a pair of guide rails 43 together with the grinding unit 20, causing the grinding unit 20 to move up and down and setting the amount of grinding (grinding allowance) of the grinding wheel 26b relative to the wafer W.

[0024] (nozzle) The nozzle 50 supplies grinding water, such as pure water, to the grinding region R (see Figure 2), which is the contact area between the grinding wheel 26b and the wafer W during grinding. The nozzle 50 ejects the grinding water from the inside of the grinding wheel 26b, which covers the wafer W and rotates during grinding. More specifically, the nozzle 50 is a member bent in an inverted L shape that ejects the grinding water linearly in the radial direction from the outer circumference of the wafer W toward the center.

[0025] (Tilt adjustment mechanism) The tilt adjustment mechanism 70 is a mechanism for adjusting the tilt of the chuck table 10, and as shown in Figure 1, it is provided between the flange 13 of the chuck table 10 and the slider 82 described later. Specifically, as shown in Figure 2, this tilt adjustment mechanism 70 is composed of two actuators 71 and one pivot 72, and these actuators 71 and pivot 72 are arranged at equal angular pitches (120° pitch) in the circumferential direction.

[0026] Here, each actuator 71 adjusts the inclination of the chuck table 10 with respect to the horizontal plane by tilting the chuck table 10 around the pivot 72 by moving a rod (not shown) up and down. Each actuator 71 may also be equipped with a vertical load measuring device such as a load cell for measuring the vertical load acting perpendicularly from the annular grinding wheel 26b to the wafer W.

[0027] (Horizontal movement mechanism) The horizontal movement mechanism 80 is a mechanism for moving the chuck table 10 horizontally (in the Y-axis direction) relative to the holding surface 11a, and as shown in Figure 1, it is disposed on a rectangular block-shaped internal base 81 housed inside the base 100. This horizontal movement mechanism 80 is equipped with a block-shaped slider 82, which is slidable in the Y-axis direction along a pair of left and right guide rails 83 that are arranged parallel to each other along the Y-axis direction (front-rear direction). Therefore, the chuck table 10 and a rotational drive mechanism (not shown), supported by the slider 82, are slidable along the Y-axis direction together with the slider 82.

[0028] A rotatable ball screw 84 extending in the Y-axis direction (front-to-back direction) is positioned between a pair of left and right guide rails 83 on the internal base 81. One end of the ball screw 84 in the Y-axis direction (left end in Figure 1) is connected to a reversible electric motor 85, which is the drive source. The other end of the ball screw 84 in the Y-axis direction (right end in Figure 1) is rotatably supported on the internal base 81 by a bearing 86 erected on the internal base 81. A nut member (not shown) protruding downward from the slider 82 is screwed onto the ball screw 84.

[0029] Therefore, when the electric motor 85 is started to rotate the ball screw 84 in both forward and reverse directions, a nut member (not shown) that is screwed onto the ball screw 84 slides along the ball screw 84 in the Y-axis direction (forward and backward direction) together with the slider 82. As a result, the chuck table 10 also moves integrally along the Y-axis direction together with the slider 82. Consequently, the wafer W held by suction on the holding surface 11a of the chuck table 10 also moves along the Y-axis direction.

[0030] [Wafer grinding method] Next, the method for grinding a wafer W according to the present invention using the grinding apparatus 1 configured as described above will be explained below with reference to Figures 3 to 5.

[0031] Prior to grinding the wafer W, the holding surface 11a of the chuck table 10 that holds the wafer W is formed by self-grinding into a conical surface with its apex at the center, that is, a slope that inclines downward radially outward from the center, as shown in Figures 3 and 4. Note that the conical shape of the holding surface 11a is exaggerated in Figures 3 and 4, but in reality, the inclination of this conical surface is so minute that it is not visible to the naked eye.

[0032] The present invention is a method for grinding a wafer W through 1) a holding step, 2) a first grinding step, and 3) a second grinding step. The holding step, the first grinding step, and the second grinding step will be described below.

[0033] 1) Holding process: The holding process involves holding the wafer W to be ground on the holding surface 11a of the chuck table 10. In this holding process, the wafer W is placed on the holding surface 11a of the chuck table 10 with the protective tape T (see Figure 1) facing downwards. The porous member 11 of the chuck table 10 is then connected to a suction source (not shown), such as a vacuum pump, and the porous member 11 is evacuated. This generates negative pressure in the porous member 11, and the wafer W is attracted and held on the holding surface 11a of the chuck table 10 by this negative pressure.

[0034] 2) First grinding process: In the first grinding step, the horizontal movement mechanism 80 shown in Figure 1 is driven to move the chuck table 10 in the +Y axis direction (rearward), positioning the wafer W, which is held by suction on the chuck table 10, below the grinding wheel 26 of the grinding unit 20. That is, when the electric motor 85 is started and the ball screw 84 rotates, a slider 82, to which a nut member (not shown) that screws onto the ball screw 84 is attached, slides along a pair of left and right guide rails 83 in the +Y axis direction together with the chuck table 10, etc., so that the wafer W held on the holding surface 11a of the chuck table 10 is positioned below the grinding wheel 26 of the grinding unit 20. At this time, the horizontal positional relationship between the annular grinding wheel 26b and the wafer W is adjusted so that the lower surface (machining surface) of the annular grinding wheel 26b passes through the center of the wafer W (see Figure 2).

[0035] Furthermore, a rotational drive mechanism (not shown) is driven to rotate the chuck table 10, causing the wafer W held on the holding surface 11a of the chuck table 10 to rotate at a predetermined rotational speed (for example, 1,000 rpm), while the spindle motor 23 is driven to rotate the grinding wheel 26.

[0036] In this first grinding step, as shown in Figure 3, with the axis CL1 of the chuck table 10 tilted by a predetermined first angle α with respect to the vertical axis CL2 of the spindle 24, the lower surface of the annular grinding wheel 26b is brought into contact with the radial portion of the wafer W held on the holding surface 11a of the chuck table 10, and the annular grinding wheel 26b is lowered in the -Z axis direction (direction approaching the holding surface) by the grinding feed means 40 until the thickness of the wafer W (thickness measured by the thickness measuring instrument 30) reaches a predetermined thickness.

[0037] In other words, when the electric motor 45 of the grinding feed means 40 is driven and the ball screw 44 rotates, the lifting plate 42, which is provided with a nut member (not shown) that screws onto the ball screw 44, descends in the -Z axis direction together with the grinding wheel 26 and the like. Then, the lower surface (grinding surface) of the annular grinding wheel 26b of the grinding wheel 26 comes into contact with the upper surface (back surface) of the wafer W. In this state, when the grinding wheel 26 is further lowered by a predetermined amount in the -Z axis direction from the lower surface of the annular grinding wheel 26b to the upper surface of the wafer W, the upper surface of the wafer W is ground by a predetermined amount by the annular grinding wheel 26b. While the wafer W is being ground as described above, grinding water is sprayed linearly from the nozzle 50 in the radial direction from the outer circumference of the wafer W toward the center.

[0038] As described above, when the wafer W is ground by the annular grinding wheel 26b, a slight depression W1 with a thickness t1 is formed in the central portion Wa of the upper surface of the wafer W, as shown in Figure 5(a), and the outer peripheral portion Wb other than the central portion Wa where the depression W1 is formed is ground to a thickness slightly thicker than the thickness t1 of the central portion Wa. Here, the maximum thickness at the outer edge of the wafer W is denoted as t2. The processing area R of the wafer W by the annular grinding wheel 26b in this first grinding step is the arc-shaped portion shown in Figure 2.

[0039] 3) Second grinding process: In the second grinding step, which is performed after the first grinding step described above, as shown in Figure 4, the axis CL1 of the chuck table 10 is tilted by a second angle β with respect to the vertical axis CL2 of the spindle 24 (annular grinding wheel 26b) so that the lower surface of the annular grinding wheel 26b does not come into contact with the center of the wafer W, and the downward movement of the annular grinding wheel 26b by the grinding feed means 40 (movement in the direction approaching the holding surface 11a) is stopped, and the outer peripheral portion Wb of the wafer W, excluding the central portion Wa where the recess W1 is formed, is ground by the annular grinding wheel 26b.

[0040] As a result of the above, the outer peripheral portion Wb of the wafer W shown in Figure 5(a), which was ground in the first grinding step, is ground in the second grinding step, and as shown in Figure 5(b), the overall thickness of the wafer W becomes equal to the thickness t1 of the central portion Wa where the recess W1 is formed.

[0041] As described above, in the wafer grinding method according to the present invention, the grinding process is divided into two stages: a first grinding step and a second grinding step. In the first grinding step, a slight depression W1 is formed in the central portion Wa of the wafer W, while the outer peripheral portion Wb of the wafer W is slightly thickened. In the second grinding step, the relative inclination between the chuck table 10 and the annular grinding wheel 26b is changed so that the lower surface of the annular grinding wheel 26b does not come into contact with the center of the wafer W, thereby grinding the slightly thicker outer peripheral portion Wb other than the central portion Wa of the wafer W. As a result, the thickness of the wafer W can be uniformly finished to the thickness t1 of the depression W1 formed in the central portion Wa of the wafer W in the first grinding step. Therefore, it is not necessary to remove the apex of the holding surface 11a of the chuck table 10 so that a depression W1 does not occur in the central portion Wa of the wafer W, and the formation of the holding surface 11a does not require a long time. Therefore, the effect is obtained that the wafer W can be efficiently ground to a uniform thickness t1 in a short amount of time.

[0042] Here, Figure 6(a) shows the radial thickness distribution of a wafer ground by the method of the present invention, and Figure 6(b) shows the radial thickness distribution of a wafer ground by a conventional grinding method. As shown in Figure 6(b), the wafer ground by the conventional grinding method has a depression in the central part, whereas the wafer ground by the method of the present invention does not have a depression in the central part, as shown in Figure 6(a), and the radial thickness shows a nearly uniform distribution. In Figure 6, the horizontal axis is the radial distance from the center of the wafer (center is set to 0), and the vertical axis is the thickness of the wafer.

[0043] In the above embodiment, in the first and second grinding steps, the wafer W was ground with the annular grinding wheel 26b while the axis CL1 of the chuck table 10 was tilted at a first angle α and a second angle β with respect to the axis CL2 of the spindle 24, respectively. However, the opposite may be performed, where the axis CL2 of the spindle 24 is tilted at a first angle α and a second angle β with respect to the axis CL1 of the chuck table 10, respectively, while the wafer W was ground with the annular grinding wheel 26b.

[0044] Furthermore, the present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the technical idea described in the claims, specification, and drawings. [Explanation of Symbols]

[0045] 1: Grinding device, 10: Chuck table, 10A: Frame, 10a: Recess of frame, 11: Porous member, 11a: Holding surface, 12: Rotating shaft, 13: Flange, 20: Cutting unit, 21: Holder, 22: Spindle housing, 23: Spindle motor, 24: Spindle, 25: Mount, 26: Grinding wheel 26a: Base, 26b: Ring-shaped grinding wheel, 30: Thickness measuring instrument, 31: First contact element, 32: Second contact element, 40: Grinding feed mechanism, 41: Column, 42: Lifting plate, 43: Guide rail, 44: Ball screw, 45: Electric motor, 46: Bracket, 50: Nozzle, 70: Tilt adjustment mechanism, 71: Actuator, 72: Pivot, 80: Horizontal movement mechanism, 81: Internal base, 82: Slider, 83: Guide rail, 84: Ball screw, 85: Electric motor, 86: Bearing, 100: Base, 100a: Base opening, 101: Cover, 102, 103: Expandable cover, CL1: Chuck table axis, CL2: Spindle axis, R: Grinding area T: protective tape, t1: thickness of the central part of the wafer, t2: Maximum thickness of the outer edge of the wafer, W: Wafer, W1: Wafer recess, Wa: Central part of the wafer, Wb: Outer edge of the wafer

Claims

[Claim 1] A wafer grinding method comprising grinding a wafer by bringing a rotating annular grinding wheel into contact with the radial portion of a wafer that is held and rotated on the conical holding surface of a chuck table, A holding step of holding the wafer on the holding surface, A first grinding step involves bringing the lower surface of the annular grinding wheel into contact with the radial portion of the wafer held on the holding surface, thereby grinding the outer periphery of the wafer to be slightly thicker than the central portion while forming a slight depression in the central portion of the wafer. A wafer grinding method comprising: a first grinding step, followed by a second grinding step in which the inclination between the chuck table and the annular grinding wheel is changed relative to the first grinding step so that the lower surface of the annular grinding wheel does not come into contact with the center of the wafer, and the annular grinding wheel is brought into contact with the outer peripheral portion which is thicker than the central portion in the first grinding step, thereby grinding the wafer to a uniform thickness so that the thickness of the outer peripheral portion matches the thickness of the central portion.

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