Grinding apparatus and wafer grinding method

Abstract

To prevent slight arc-like unevenness from being formed on a ground wafer.SOLUTION: A rotation control part 8 calculates a maximum thickness difference in a wafer 100 by using a non-contact thickness measuring device 60. When the maximum thickness difference exceeds an allowable range, a rotational speed of a chuck table 31 is relatively varied relative to a rotational speed of a spindle 72. A rotation frequency ratio RN which is a ratio between the rotational speed of the spindle 72 and the rotational speed of the chuck table 31 can be shifted from an integer. Consequently, in spark-out machining, the maximum thickness difference can be reduced. As a result, a surface 102 to be ground of the wafer 100 can be made flat by reducing a variation in a periodic thickness value formed on the wafer 100.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a grinding device. and wafer grinding method Regarding. [Background technology]

[0002] A grinding machine that uses a grinding wheel to grind a wafer held by a chuck table is equipped with, for example, a non-contact thickness gauge that measures the thickness of the wafer in a non-contact manner. In this configuration, grinding is continued until the thickness of the wafer measured by the non-contact gauge during grinding reaches a predetermined thickness (see Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-184604 A [Patent Document 2] JP 2018-034283 A [Patent Document 3] JP 2020-026010 A Summary of the Invention [Problem to be solved by the invention]

[0004] A wafer that has been ground to a specified thickness may have slight arc-shaped irregularities formed along the path of the grinding wheel. These slight irregularities may adversely affect devices.

[0005] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to prevent slight arc-shaped irregularities from being formed on a ground wafer. [Means for solving the problem]

[0006] The grinding device of the present invention (the present grinding device) comprises: a chuck table that holds a wafer by a holding surface; a table rotation mechanism that rotates the chuck table about an axis at the center of the holding surface; a grinding mechanism that grinds the wafer by a grinding wheel that is attached to the tip of a spindle and rotates so as to pass through the center of the holding surface; and a spindle rotation mechanism that rotates the spindle. a non-contact thickness gauge for measuring the thickness of the wafer held by the holding surface in a non-contact manner; A grinding apparatus comprising: a control unit which executes a grinding step of grinding the wafer held on the chuck table by rotating the chuck table and the spindle and moving the rotating grinding wheel in a direction approaching the chuck table until the measured thickness of the wafer reaches a preset target value for the thickness of the wafer after grinding, and a spark-out step of continuing grinding the wafer after the grinding step while stopping the movement of the grinding wheel; , In the spark-out process, the thickness of the wafer is measured by the non-contact thickness gauge, The difference between the minimum and maximum values ​​of the periodic thickness changes of a wafer along the direction of wafer rotation. The maximum thickness difference is determined based on the target thickness value. If the tolerance is exceeded, The ratio of the rotational speed of the spindle to the rotational speed of the chuck table is not an integer. and a rotation control unit that changes the rotation speed of the chuck table relatively to the rotation speed of the spindle. The method for grinding a wafer of the present invention is a method for grinding a wafer using a grinding device including a chuck table for holding a wafer by a holding surface, a table rotation mechanism for rotating the chuck table about an axis of the center of the holding surface, a grinding mechanism for grinding the wafer by a grinding wheel attached to the tip of a spindle and rotating so as to pass through the center of the holding surface, a spindle rotation mechanism for rotating the spindle, and a non-contact thickness gauge for measuring the thickness of the wafer held by the holding surface in a non-contact manner, the method including rotating the chuck table and the spindle and rotating the rotating grinding wheel in a direction approaching the chuck table until the thickness of the wafer measured by the non-contact thickness gauge reaches a preset target value for the thickness of the wafer after grinding, and a spark-out step of continuing grinding of the wafer after the grinding step with the movement of the grinding wheel stopped, the spark-out step including: determining a maximum thickness difference, which is the difference between a minimum value and a maximum value of periodic changes in thickness value of the wafer appearing along the rotation direction of the wafer, by measuring the thickness of the wafer with the non-contact thickness gage; and, when the maximum thickness difference exceeds an allowable range defined with the target thickness value as a reference, changing the rotation speed of the chuck table relatively to the rotation speed of the spindle so that the rotational speed ratio between the rotation speed of the spindle and the rotation speed of the chuck table is no longer an integer. Effect of the Invention

[0007] In this grinding device, a non-contact thickness measuring device is used to calculate the difference between the minimum and maximum values ​​of the periodic thickness value change (maximum thickness difference), and when this maximum thickness difference exceeds the allowable range, the rotation control unit changes the rotation speed of the chuck table relative to the rotation speed of the spindle. As a result, even if the rotation speed ratio, which is the ratio between the rotation speed of the spindle and the rotation speed of the chuck table, is an integer, the rotation speed ratio can be shifted from the integer by changing the rotation speed of the chuck table. Therefore, the maximum thickness difference formed on the ground surface of the wafer in the grinding process can be reduced. As a result, it is possible to reduce the periodic thickness value change formed on the wafer and flatten the ground surface of the wafer. Therefore, it is possible to suppress the formation of slight arc-shaped irregularities on the ground wafer. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is a perspective view showing a configuration of a grinding device. [Diagram 2] 1 is a graph showing the change in height of a grinding wheel over time. [Diagram 3]3(a) to (c) are explanatory diagrams showing grinding marks formed on the ground surface of a wafer. [Figure 4] FIG. 4(a) is an explanatory diagram showing segment grinding marks formed on the ground surface of a wafer when the rotation speed ratio is an integer, and FIG. 4(b) is an explanatory diagram showing segment grinding marks formed on the ground surface of a wafer when the rotation speed ratio is not an integer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] As shown in FIG. 1, the grinding apparatus 1 according to this embodiment is an apparatus for grinding a wafer 100 as a workpiece, and includes a rectangular parallelepiped base 10 and a column 11 extending upward.

[0010] The wafer 100 is, for example, a circular semiconductor wafer. One surface of the wafer 100 facing downward in Fig. 1 is a device surface 101 that holds multiple devices and is protected by a protective tape 103 attached thereto. The other surface of the wafer 100 is a grinding surface 102 that is subjected to grinding processing.

[0011] An opening 13 is provided on the upper surface side of the base 10. A wafer holding mechanism 30 is disposed within the opening 13. The wafer holding mechanism 30 includes a chuck table 31 having a holding surface 32 for holding the wafer 100, and a table rotation mechanism 35 that supports and rotates the chuck table 31.

[0012] The chuck table 31 includes a holding surface 32 for holding the wafer 100, and a frame 33 for supporting the holding surface 32. The holding surface 32 is made of a porous material, and is connected to a suction source (not shown) to hold the wafer 100 by suction. That is, the chuck table 31 holds the wafer 100 by means of the holding surface 32. The holding surface 32 is formed so as to be flush with the upper surface of the frame 33.

[0013] The table rotation mechanism 35 supports the chuck table 31 from below. The table rotation mechanism 35 is configured to rotate the chuck table 31 around the center of the holding surface 32 as an axis.

[0014] A cover plate 39 that moves along the Y-axis direction together with the chuck table 31 is provided around the chuck table 31. A bellows cover 12 that expands and contracts in the Y-axis direction is connected to the cover plate 39. A Y-axis direction moving mechanism 40 is provided below the wafer holding mechanism 30.

[0015] The Y-axis movement mechanism 40 relatively moves the chuck table 31 and the grinding mechanism 70 in the Y-axis direction parallel to the holding surface 32. In this embodiment, the Y-axis movement mechanism 40 is configured to move the chuck table 31 in the Y-axis direction relative to the grinding mechanism 70.

[0016] The Y-axis direction moving mechanism 40 includes a pair of Y-axis guide rails 42 parallel to the Y-axis direction, a Y-axis moving table 45 that slides on these Y-axis guide rails 42, a Y-axis ball screw 43 parallel to the Y-axis guide rails 42, a Y-axis motor 44 connected to the Y-axis ball screw 43, and a holding base 41 that holds these.

[0017] The Y-axis moving table 45 is slidably installed on the Y-axis guide rail 42. A nut portion (not shown) is fixed to the underside of the Y-axis moving table 45. The Y-axis ball screw 43 is screwed into this nut portion. The Y-axis motor 44 is connected to one end of the Y-axis ball screw 43 as shown in FIG.

[0018] In the Y-axis direction moving mechanism 40, the Y-axis motor 44 rotates the Y-axis ball screw 43, thereby moving the Y-axis moving table 45 in the Y-axis direction along the Y-axis guide rail 42. The table rotation mechanism 35 of the wafer holding mechanism 30 is placed on the Y-axis moving table 45. Therefore, as the Y-axis moving table 45 moves in the Y-axis direction, the wafer holding mechanism 30 including the table rotation mechanism 35 and the chuck table 31 moves in the Y-axis direction.

[0019] In this embodiment, the wafer holding mechanism 30 is moved along the Y-axis direction by the Y-axis moving mechanism 40 between a workpiece loading position on the -Y direction side for holding the wafer 100 on the holding surface 32 and a grinding position on the +Y direction side where the wafer 100 is ground.

[0020] 1, a column 11 is provided in an upright position at the rear (+Y direction side) of the base 10. A grinding mechanism 70 for grinding the wafer 100 and a grinding feed mechanism 50 are provided in front of the column 11.

[0021] The grinding feed mechanism 50 relatively moves the chuck table 31 and the grinding wheel 77 of the grinding mechanism 70 in the Z-axis direction (grinding feed direction) perpendicular to the holding surface 32. In this embodiment, the grinding feed mechanism 50 is configured to move the grinding wheel 77 in the Z-axis direction relative to the chuck table 31.

[0022] The grinding feed mechanism 50 includes a pair of Z-axis guide rails 51 parallel to the Z-axis direction, a Z-axis moving table 53 that slides on the Z-axis guide rails 51, a Z-axis ball screw 52 parallel to the Z-axis guide rails 51, a Z-axis motor 54, an encoder 55 for detecting the rotation angle of the Z-axis ball screw 52, ​​and a support case 56 attached to the Z-axis moving table 53. The support case 56 supports the grinding mechanism 70.

[0023] The Z-axis moving table 53 is slidably installed on the Z-axis guide rail 51. A nut portion (not shown) is fixed to the Z-axis moving table 53. A Z-axis ball screw 52 is screwed into this nut portion. The Z-axis motor 54 is connected to one end of the Z-axis ball screw 52.

[0024] In the grinding feed mechanism 50, the Z-axis motor 54 rotates the Z-axis ball screw 52, ​​thereby moving the Z-axis moving table 53 in the Z-axis direction along the Z-axis guide rail 51. As a result, the support case 56 attached to the Z-axis moving table 53 and the grinding mechanism 70 supported by the support case 56 also move in the Z-axis direction together with the Z-axis moving table 53.

[0025] The encoder 55 is rotated by the Z-axis motor 54 rotating the Z-axis ball screw 52, ​​and can recognize the rotation angle of the Z-axis ball screw 52. Then, based on the recognition result of the encoder 55, the height position of the grinding wheel 77 of the grinding mechanism 70 moved in the Z-axis direction can be detected.

[0026] The grinding mechanism 70 includes a spindle housing 71 fixed to the support case 56, a spindle 72 rotatably held in the spindle housing 71, a spindle motor 73 that rotates the spindle 72, a wheel mount 74 attached to the lower end of the spindle 72, and a grinding wheel 75 supported by the wheel mount 74.

[0027] The spindle housing 71 is held by the support case 56 so as to extend in the Z-axis direction. The spindle 72 extends in the Z-axis direction so as to be perpendicular to the holding surface 32 of the chuck table 31, and is supported by the spindle housing 71 so as to be rotatable.

[0028] The spindle motor 73 is connected to the upper end side of the spindle 72. The spindle 72 is rotated about a rotation axis extending in the Z-axis direction by the spindle motor 73. The spindle motor 73 is an example of a spindle rotation mechanism that rotates the spindle 72.

[0029] The wheel mount 74 is formed in a disk shape, and is fixed to the lower end (tip) of the spindle 72. The wheel mount 74 supports the grinding wheel 75.

[0030] The grinding wheel 75 is formed so that its outer diameter is approximately the same as the outer diameter of the wheel mount 74. The grinding wheel 75 includes an annular wheel base (annular base) 76 made of a metal material. An annular grinding wheel 77 is fixed to the underside of the wheel base 76, the grinding wheel 77 being made of a plurality of segmented grinding wheels arranged in an annular shape around the entire circumference.

[0031] The annular grinding wheel 77 is disposed so that its underside passes through the center of the holding surface 32 of the chuck table 31 , and has an inner diameter that protrudes horizontally beyond the holding surface 32 . The annular grinding wheel 77 has segmented grindstones arranged in an annular shape with gaps between adjacent segmented grindstones.

[0032] This annular grinding wheel 77 is rotated by the spindle motor 73 via the spindle 72, wheel mount 74, and wheel base 76 about a rotation axis that passes through its center and extends in the Z-axis direction so as to pass through the center of the holding surface 32 of the chuck table 31. As a result, the grinding wheel 77 comes into contact with the radial portion of the wafer 100 held on the holding surface 32 of the chuck table 31 arranged at the grinding position, and grinds the wafer 100. A grinding area parallel to the lower surface of the grinding wheel 77 is formed in the radius portion of the holding surface 32. The grinding area is parallel to the grinding wheel 77 when the wafer 100 is rotated around the center of the holding surface 32. The annular grinding wheel 77 may be a continuous grinding wheel in which a plurality of segmented grinding wheels are arranged annularly with no gaps between them.

[0033] In this manner, the grinding mechanism 70 grinds the wafer 100 held on the rotating holding surface 32 using a grinding wheel 77 that is attached to the tip of the spindle 72 via a wheel mount 74 and a wheel base 76 and rotates so as to pass through the center of the holding surface 32.

[0034] 1, a non-contact thickness gauge 60 is disposed on the side of the opening 13 in the base 10. The non-contact thickness gauge 60 can measure the thickness of the wafer 100 held on the holding surface 32 in a non-contact manner.

[0035] The non-contact thickness gauge 60 may include one non-contact type thickness measurement unit, for example, a laser type thickness measurement unit. For example, this thickness measurement unit irradiates the wafer 100 with a laser beam having a wavelength that transmits through the wafer 100, receives reflected light from the lower surface of the wafer 100 and reflected light from the upper surface of the wafer 100, and measures the thickness of the wafer 100 based on the optical path difference between the reflected lights. The non-contact type thickness measurement unit may be a spectroscopic interference type wafer thickness gauge that measures the thickness of the wafer 100 by analyzing the interference light between the reflected light from the lower surface of the wafer 100 and the reflected light from the upper surface of the wafer 100. An SLD (Super Luminescent Diode) may be provided as a light source that emits measurement light.

[0036] 1, the non-contact thickness measuring device 60 may have a holding surface height measuring unit 61 and a wafer height measuring unit 62. The holding surface height measuring unit 61 and the wafer height measuring unit 62 are non-contact height gauges, and may be laser distance measuring devices.

[0037] For example, the holding surface height measuring unit 61 irradiates a laser beam onto the upper surface of the frame 33 of the chuck table 31, and measures the height of the upper surface of the frame 33 (i.e., the height of the holding surface 32) based on the reflected light. On the other hand, the wafer height measuring unit 62 irradiates a laser beam onto the upper surface (surface 102 to be ground) of the wafer 100, and measures the height of the upper surface of the wafer 100 based on the reflected light. Then, the non-contact thickness measuring device 60 calculates the thickness of the wafer 100 based on the difference between the measured height of the holding surface 32 and the height of the wafer 100 .

[0038] In addition, the holding surface height measuring unit 61 and the wafer height measuring unit 62 may be configured to measure the height of the holding surface 32 and the height of the upper surface of the wafer 100 by irradiating sound waves to the frame 33 and the upper surface of the wafer 100.

[0039] The grinding device 1 also includes a control unit 7 and a rotation control unit 8. The control unit 7 includes a CPU that performs calculation processing according to a control program, and a storage medium such as a memory, etc. The control unit 7 controls the above-mentioned members of the grinding device 1 to perform grinding processing on the wafer 100.

[0040] The rotation control unit 8 has a function of controlling the rotation of the chuck table 31 by the table rotation mechanism 35 and the rotation of the spindle 72 by the spindle motor 73 during grinding.

[0041] The grinding process in the grinding device 1 will be described below. In the grinding process in the grinding apparatus 1, first, an operator holds the wafer 100 on the holding surface 32 (see FIG. 1) of the chuck table 31 in the mounting position so that the surface to be ground 102 faces up.

[0042] Next, the control unit 7 controls the Y-axis direction moving mechanism 40 to move the chuck table 31 in the Y-axis direction so that the chuck table 31 is disposed at the grinding position below the grinding mechanism 70. At the grinding position, the grinding wheel 77 is positioned at the rotation center of the wafer 100 held on the holding surface 32 of the chuck table 31.

[0043] The control unit 7 also controls the spindle motor 73 of the grinding mechanism 70 to rotate the grinding wheel 77 together with the spindle 72. The rotation speed of the spindle 72 is 1650 rpm in this embodiment. Furthermore, the rotation control unit 8 controls the table rotation mechanism 35 to rotate the holding surface 32 of the chuck table 31 that holds the wafer 100. The rotation speed of the chuck table 31 is 150 rpm in this embodiment.

[0044] Next, the control unit 7 uses the grinding feed mechanism 50 to move the grinding mechanism 70, which is located above the holding surface 32 at an origin height position where the lower surface (grinding surface) of the grinding wheel 77 does not come into contact with the wafer 100 held by the holding surface 32, closer to the chuck table 31.

[0045] Fig. 2 shows the change over time in the height of the grinding wheel 77 (the height of the grinding surface (lower surface) of the grinding wheel 77). As shown in Fig. 2, the control unit 7 first lowers the grinding mechanism 70 at a relatively high initial speed V1 so as to approach the chuck table 31 until the height of the grinding wheel 77 reaches a predetermined air-cut start height h1 (time range T1).

[0046] The control unit 7 detects the height position of the grinding wheel 77 in the Z-axis direction, for example, appropriately using the encoder 55. Then, after the grinding surface of the grinding wheel 77 reaches a predetermined air-cut start height h1, the control unit 7 sets the descent speed of the grinding mechanism 70 to an air-cut speed V2 slower than the initial speed V1 using the grinding feed mechanism 50. Then, the control unit 7 causes the grinding mechanism 70 to approach the chuck table 31 at the air-cut speed V2 by the grinding feed mechanism 50 (time range T2).

[0047] In the time range T2, the grinding wheel 77 has not yet come into contact with the wafer 100 and air cutting is being performed. Thereafter, the control unit 7 controls the grinding feed mechanism 50 to bring the grinding wheel 77 closer to the holding surface 32 of the chuck table 31, and brings the grinding surface of the grinding wheel 77 into contact with the surface to be ground 102 of the wafer 100 held on the holding surface 32.

[0048] When the grinding surface of the grinding wheel 77 reaches a height h2 at which it comes into contact with the grinding surface 102 of the wafer 100, the control unit 7 grinds the grinding surface 102 of the wafer 100 with the grinding wheel 77 at a first grinding speed V3 (time range T3; first grinding process). The first grinding speed V3 is slower than the initial speed V1, and is, for example, the same speed as the air cut speed V2. In this first grinding process, the grinding surface 102 of the wafer 100 is ground by, for example, about 30 μm.

[0049] The control unit 7 appropriately uses the non-contact thickness measuring device 60 to measure the thickness of the wafer 100 being ground. Then, when the minimum thickness of the wafer 100 measured by the non-contact thickness measuring device 60 approaches a predetermined value (target value), the control unit 7 uses the grinding feed mechanism 50 to move the grinding mechanism 70 closer to the chuck table 31 at a second grinding speed V4 slower than the first grinding speed V3 (time range T4). That is, the control unit 7 further slows the descending speed of the grinding mechanism 70 from the first grinding speed V3 to the second grinding speed V4 to continue the grinding process (second grinding process). In this second grinding process, the grinding marks generated on the wafer 100 in the first grinding process are reduced. In this second grinding process, the grinding surface 102 of the wafer 100 is ground by, for example, about 10 μm.

[0050] Then, when the minimum thickness of the wafer 100 measured by the non-contact thickness gauge 60 reaches a predetermined value, the control unit 7 stops the lowering operation of the grinding mechanism 70 and performs a process known as spark-out processing (time range T5).

[0051] That is, the control unit 7 stops the lowering operation of the grinding mechanism 70 to approach the holding surface 32 of the chuck table 31. By such spark-out processing, grinding spots on the ground surface 102 of the wafer 100 are removed, and the rotation control unit 8 performs rotation control, which will be described later, to rotate the chuck table 31 and reduce periodic changes in the thickness value of the wafer 100 measured by the non-contact thickness gauge 60, thereby flattening the ground surface 102 of the wafer 100.

[0052] After the spark-out process is completed, the control unit 7 performs the escape cut process (time range T6). At this time, the control unit 7 uses the grinding feed mechanism 50 to slowly raise the grinding mechanism 70 at a preset escape cut process speed V6, thereby performing the escape cut process on the wafer 100 with the grinding wheel 77.

[0053] The escape cut processing is performed, for example, until the grinding wheel 77 leaves the grinding surface 102 of the wafer 100. After the escape cut processing is completed, the control unit 7 uses the grinding feed mechanism 50 to retract the grinding mechanism 70 to the origin height position at a relatively high retraction speed V7 (time range T7).

[0054] Here, the rotation control by the above-mentioned rotation control unit 8 for reducing the periodic change in thickness value formed on the wafer 100 and flattening the ground surface 102 of the wafer 100 will be described. First, the mechanism by which the periodic thickness change is formed on the wafer 100 will be described.

[0055] When the wafer 100 is ground by the grinding wheel 77, a periodic change in thickness value may appear in the rotation direction of the wafer 100. That is, as shown in Fig. 3(a), grinding marks (thickness unevenness, waviness) 130 as a periodic change in thickness value may be formed periodically on the ground surface 102 of the wafer 100 along the rotation direction of the wafer 100. The grinding marks 130 correspond to a trajectory 200 of the grinding surface of the grinding wheel 77 during grinding, and include a convex portion 131 and a concave portion 132 that appear at a predetermined period. That is, grinding marks 130 are formed by the trajectories of the grinding surfaces of the respective segment grindstones that form the annular grinding wheel 77 . The grinding marks 130 are marks made by the outer circumference of the segment grindstone. Therefore, the grinding marks 130 are easily formed in the case of an annular grinding wheel 77 in which gaps are arranged between the segment grindstones. In addition, even in the case of a continuous type annular grinding wheel 77 in which there are no gaps between the segment grindstones, the grinding marks 130 are formed due to the different loads applied to the segment grindstones and the different amounts of compression of the segment grindstones.

[0056] The height H1 of the grinding marks 130 (i.e., the height difference between the convex portion 131 and the concave portion 132) becomes prominent when the rotation speed ratio RN is an integer. Here, the rotation speed ratio RN is expressed as RN=N1 / N2, where N1 is the rotation speed of the spindle 72 (the rotation speed of the grinding wheel 77) and N2 is the rotation speed of the chuck table 31 (the rotation speed of the wafer 100).

[0057] When the rotation speed ratio RN is an integer (for example, when RN=9), as shown in FIG. 4(a), during one rotation of the chuck table 31, RN (9) segment grinding marks 121 to 129, each of which has approximately the same size as the other, are lined up without gaps on the grinding surface 102 of the wafer 100, formed by each segment grinding wheel of the annular grinding wheel 77. The segment grinding marks 121 to 129 may be formed by only one segment grindstone, or may be formed by overlapping tracks of a plurality of segment grindstones. The segment grinding marks 121 to 129 are grinding marks 130 shown in FIG.

[0058] Here, the segment grinding marks 121-129 are regions of the ground surface 102 of the wafer 100 that have undergone geometric interference with the grinding surface of the grinding wheel 77 during one rotation of the grinding wheel 77. The segment grinding marks 121-129 have a shape surrounded by two approximately arc-shaped curves extending from the center of the wafer 100 toward the outer periphery and a part of the outer periphery of the wafer 100.

[0059] When the rotation speed ratio RN is an integer, as shown by the thick line in FIG. 4(a), the start position 1211 of the first segment grinding mark 121 during the first rotation of the chuck table 31 coincides with the end position 1292 of the last segment grinding mark 129.

[0060] Here, the end position 1292 of the last segment grinding mark 129 is the start position of the first segment grinding mark 121 in the second rotation. Therefore, even in the second rotation and the third rotation and thereafter of the chuck table 31, the start position 1211 of the first segment grinding mark 121 is substantially the same position as in the first rotation.

[0061] Therefore, when the rotation speed ratio RN is an integer, even if the chuck table 31 rotates multiple times during spark out processing, the arrangement of the segment grinding marks 121-129 on the grinding surface 102 of the wafer 100 will always be approximately the same, and the grinding surface of the grinding wheel 77 will follow approximately the same trajectory on the wafer 100.

[0062] Therefore, the height difference of the grinding surface of the grinding wheel 77 always coincides with the unevenness on the existing ground surface 102, making it difficult for the grinding surface of the grinding wheel 77 to grind off the convex portion 131 shown in Fig. 3(a). As a result, even if the chuck table 31 is rotated many times during spark out, it is difficult to sufficiently reduce the grinding marks 130, and grinding marks 130 having relatively large unevenness remain.

[0063] On the other hand, when the rotation speed ratio RN is not an integer (for example, RN=8.2), RN (eight) segment grinding marks 121-128 each having approximately the same size are lined up on the ground surface 102 of the wafer 100 during one rotation of the chuck table 31, as shown in Fig. 4(b). However, in this case, a deviation amount θ is formed between the start position 1211 of the first segment grinding mark 121 and the end position 1282 of the last segment grinding mark 128, and they do not match.

[0064] Therefore, there is a deviation in the start position of the first segment grinding mark 121 between the first and second rotations of the chuck table 31. Similarly, there is a deviation in the start position of the first segment grinding mark 121 between the second and third rotations of the chuck table 31. There is also a deviation in the start position of the first segment grinding mark 121 between the first and third rotations.

[0065] Therefore, in the second rotation of the chuck table 31, the arrangement of the segment grinding marks 121-128 on the grinding surface 102 of the wafer 100 is different from the arrangement of the segment grinding marks 121-128 in the first rotation. Therefore, the grinding surface of the grinding wheel 77 forms a track on the wafer 100 different from that in the first rotation. Therefore, a deviation occurs between the height difference of the grinding surface of the grinding wheel 77 and the unevenness on the grinding surface 102 already formed, and these interfere with each other to grind off the convex portion 131. Therefore, when the rotation speed ratio RN is not an integer, the grinding marks 130 after spark out are smaller than when the rotation speed ratio RN is an integer.

[0066] Therefore, the rotation control unit 8 performs rotation control, for example, immediately after the start of spark-out processing. In this rotation control, the rotation control unit 8 first measures the thickness value of the wafer 100 during grinding using the non-contact thickness measuring device 60. Then, the rotation control unit 8 detects periodic changes in the thickness value (corresponding to the grinding marks 130 described above) that appear along the rotation direction of the wafer 100.

[0067] Furthermore, the rotation control unit 8 calculates the difference between the minimum and maximum values ​​(hereinafter referred to as the maximum thickness difference) in the periodic change in thickness value of the wafer 100. The maximum value is the height of the convex portion 131 shown in FIG. 3(a), the minimum value is the height of the concave portion 132, and the maximum thickness difference is the height difference (height H1) between the convex portion 131 and the concave portion 132.

[0068] Then, the rotation control unit 8 judges whether or not this maximum thickness difference exceeds an allowable range. This allowable range is determined, for example, based on a predetermined thickness of the wafer 100 that is set in advance, that is, a target value for the thickness of the wafer 100 after grinding.

[0069] When the maximum thickness difference (height H1) exceeds the allowable range, it is considered that the above-mentioned rotation speed ratio RN is substantially an integer. Therefore, in this case, the rotation control unit 8 controls the table rotation mechanism 35 (see FIG. 1) to change the rotation speed (150 rpm) of the chuck table 31 relative to the rotation speed (1650 rpm) of the spindle 72, for example, within a range of 145 to 155 rpm. As a result, the rotation speed ratio RN is no longer an integer.

[0070] After such rotation control, the control unit 7 continues the spark-out processing while maintaining the rotation speed of the chuck table 31 changed as described above. In this spark-out processing, as shown in FIG. 3(b), in the first rotation of the chuck table 31 after the change in the rotation speed, a first trajectory 201 (dashed line) which is the trajectory of the grinding surface of the grinding wheel 77 changes from an initial trajectory 200 (solid line) which is the trajectory before the change in the rotation speed. Therefore, the convex portion 131 of the grinding mark 130 formed up to that point is ground off by the grinding wheel 77. Therefore, the maximum thickness difference (height H2) becomes smaller than the maximum thickness difference (height H1) up to that point.

[0071] 3(c), in the second rotation of the chuck table 31 after the change in rotation speed, a second locus 202 (dash line) which is the locus of the grinding surface of the grinding wheel 77 differs from both the initial locus 200 (solid line) and the first locus 201 (dashed line). Therefore, the convex portion 131 of the grinding mark 130 is further ground away by the grinding wheel 77. Therefore, the maximum thickness difference (height H3) becomes even smaller than the previous maximum thickness difference (height H2). In this manner, in this embodiment, the maximum thickness difference can be reduced by the spark-out processing.

[0072] Then, after the maximum thickness difference falls within the allowable range, the control unit 7 determines that the ground surface 102 of the wafer 100 has been flattened, terminates the spark-out process, and performs the escape cut process described above (time range T6 in Figure 2).

[0073] On the other hand, the control unit 7 stops the grinding operation if the maximum thickness difference does not fall within the allowable range even after a predetermined time has elapsed after the rotation speed of the chuck table 31 has been changed. Then, the control unit 7 uses a display member (not shown) to display to the operator that the maximum thickness difference does not fall within the allowable range as a processing error.

[0074] As described above, in this embodiment, the rotation control unit 8 calculates the maximum thickness difference in the wafer 100 using the non-contact thickness measuring device 60 during the spark-out process, and when this maximum thickness difference exceeds the allowable range, the rotation speed of the chuck table 31 is changed relatively to the rotation speed of the spindle 72. As a result, even if the rotation speed ratio RN, which is the ratio between the rotation speed of the spindle 72 and the rotation speed of the chuck table 31, is an integer, the rotation speed ratio RN can be shifted from an integer by changing the rotation speed of the chuck table 31. Therefore, in the spark-out process, the maximum thickness difference formed on the grinding surface 102 of the wafer 100 can be reduced. As a result, it is possible to reduce the periodic change in thickness value formed on the wafer 100 and flatten the grinding surface 102 of the wafer 100. Therefore, it is possible to suppress the formation of slight arc-shaped irregularities on the ground wafer 100.

[0075] It is preferable to initially set the rotation speed of the spindle 72 and the rotation speed of the chuck table 31 before the grinding process so that the ratio between them, that is, the rotation speed ratio RN, is not an integer. However, even if such initial settings are performed, the rotation speeds of the spindle 72 and the chuck table 31 may not actually be as initially set due to the loads applied to the grinding wheel 77 and the chuck table 31, and the rotation speed ratio RN may become an integer. For this reason, even if the above-mentioned initial settings are performed, it is preferable to perform rotation operation control by the rotation control unit 8 as in this embodiment, and to appropriately set the rotation speed of the chuck table 31 according to the measurement result of the thickness measurement of the wafer 100 by the non-contact thickness measuring device 60.

[0076] Furthermore, in this embodiment, when the maximum thickness difference exceeds the allowable range, the rotation control unit 8 changes the rotation speed (150 rpm) of the chuck table 31 relatively, for example, within a range of 145 to 155 rpm, with respect to the rotation speed (1650 rpm) of the spindle 72. In this regard, it is preferable that the rotation control unit 8 makes the rotation speed of the chuck table 31 slower than the initial value (150 rpm) when the wafer 100 is hard (for example, when it is a SiC wafer), whereas it is preferable that the rotation control unit 8 makes the rotation speed of the chuck table 31 faster than the initial value (150 rpm) when the wafer 100 is a wafer that is easy to grind, such as a silicon wafer.

[0077] In this embodiment, the rotation control unit 8 performs the rotation control to control the rotation speed of the chuck table 31 just after the start of the spark-out process, that is, when the descent of the grinding wheel 77 is stopped. The rotation control may be performed not only just after the start of the spark-out process, but also just before the end of the second grinding process before the start of the spark-out process. The rotation control unit 8 may also perform the control of the rotation speed across the second grinding process and the spark-out process. That is, just before the end of the second grinding process, a part of the rotation control operation (for example, up to the determination of whether or not the maximum thickness difference exceeds the allowable range) may be performed, and the remaining operation may be performed just after the start of the spark-out process. Alternatively, the rotation control unit 8 may perform the rotation control after a predetermined time has elapsed from the start of the spark-out process. The rotation control unit 8 may control the rotation speed of the spindle 72 instead of the rotation speed of the chuck table 31. [Explanation of symbols]

[0078] 1: grinding device, 7: control unit, 8: rotation control unit, 10: Base, 11: Column, 12: Bellows cover, 13: Opening, 30: wafer holding mechanism, 31: chuck table, 32: holding surface, 33: frame, 35: Table rotation mechanism, 39: Cover plate, 40: Y-axis direction movement mechanism, 41: support stand, 42: Y-axis guide rail, 43: Y-axis ball screw, 44: Y-axis motor, 45: Y-axis moving table, 50: Grinding feed mechanism, 51: Z-axis guide rail, 52: Z-axis ball screw, 53: Z-axis moving table, 54: Z-axis motor, 55: Encoder, 56: Support case, 60: Non-contact thickness measuring device, 61: Holding surface height measuring unit, 62: Wafer height measuring unit, 70: grinding mechanism, 71: spindle housing, 72: spindle, 73: Spindle motor, 74: Wheel mount, 75: Grinding wheel, 76: Wheel base, 77: Grinding wheel, 100: wafer, 101: device surface, 102: grinding surface, 103: Protective tape, 121-129: segment grinding marks, 130: grinding marks, 131: convex portion, 132: concave portion

Claims

1. A grinding apparatus comprising: a chuck table that holds a wafer by a holding surface; a table rotation mechanism that rotates the chuck table about an axis at the center of the holding surface; a grinding mechanism that grinds the wafer by a grinding wheel that is attached to a tip of a spindle and rotates so as to pass through the center of the holding surface; a spindle rotation mechanism that rotates the spindle; and a non-contact thickness gauge that measures the thickness of the wafer held by the holding surface in a non-contact manner, a control unit which executes a grinding step of grinding the wafer held on the chuck table by rotating the chuck table and the spindle and moving the rotating grinding wheel in a direction approaching the chuck table until the thickness of the wafer measured by the non-contact thickness gauge reaches a preset target value for the thickness of the wafer after grinding, and a spark-out step of continuing grinding the wafer after the grinding step while stopping the movement of the grinding wheel; a rotation control unit which, in the spark-out process, measures the thickness of the wafer with the non-contact thickness measuring device to determine a maximum thickness difference which is the difference between the minimum and maximum values ​​of periodic thickness changes of the wafer occurring along the rotation direction of the wafer, and when this maximum thickness difference exceeds an allowable range determined based on a target thickness value, changes the rotation speed of the chuck table relatively to the rotation speed of the spindle so that the rotational speed ratio between the rotation speed of the spindle and the rotation speed of the chuck table is no longer an integer; A grinding device comprising:

2. A method for grinding a wafer using a grinding device comprising: a chuck table for holding a wafer by a holding surface; a table rotation mechanism for rotating the chuck table about an axis at the center of the holding surface; a grinding mechanism for grinding the wafer by a grinding wheel attached to the tip of a spindle and rotating so as to pass through the center of the holding surface; a spindle rotation mechanism for rotating the spindle; and a non-contact thickness gauge for non-contact measuring the thickness of the wafer held by the holding surface, a grinding step of rotating the chuck table and the spindle and moving the rotating grinding wheel in a direction approaching the chuck table, thereby grinding the wafer held on the chuck table, until the thickness of the wafer measured by the non-contact thickness gauge reaches a preset target value for the thickness of the wafer after grinding; A spark-out process for continuing grinding of the wafer after the grinding process while stopping the movement of the grinding wheel, The spark out process includes: Measuring the thickness of the wafer with the non-contact thickness gauge to determine a maximum thickness difference between the minimum and maximum values ​​of periodic thickness changes of the wafer occurring along the rotation direction of the wafer; and and when the maximum thickness difference exceeds an allowable range that is determined based on the target thickness value, changing the rotational speed of the chuck table relatively to the rotational speed of the spindle so that the ratio of the rotational speeds of the spindle and the chuck table is not an integer. A method for grinding wafers.

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