JP7670580B2 - Wafer grinding method - Google Patents

Description

The present invention relates to a method for grinding wafers.

In a grinding device that uses a grinding wheel to grind a wafer held on the holding surface of a chuck table, the height of the holding surface and the height of the wafer's top surface are measured, and the difference between the height of the holding surface and the height of the wafer's top surface is calculated as the wafer thickness. The wafer is then ground until the calculated wafer thickness reaches a preset finishing thickness (see, for example, Patent Document 1).

The height of the holding surface is measured at the height position of the probe where its tip touches the holding surface. Similarly, the height of the top surface of the wafer is measured at the height position of the probe where its tip touches the wafer. As a result, grinding debris may adhere to the holding surface and cause the probe to ride up on the grinding debris, or grinding debris may adhere to the top surface of the wafer and cause the probe to ride up on the grinding debris, making it impossible to measure the height accurately.

In particular, the grinding waste fluid containing grinding debris flows radially outward over the top surface of the wafer due to the centrifugal force of the rotating grinding wheel, and the probe that measures the height of the holding surface measures the top surface flush with the holding surface, outside the wafer held by the holding surface. The grinding debris from the grinding waste fluid that flows out from the top surface of the wafer tends to adhere to the top surface. Therefore, when the probe that measures the height of the holding surface measures the height of the top surface with grinding debris attached, it recognizes the thickness as thinner than it actually is. In this case, there is a problem that grinding ends even though the actual thickness of the wafer has not reached the finishing thickness. On the other hand, if grinding debris is present between the top surface of the wafer and the tip of the probe, the thickness of the wafer is recognized as thicker than it actually is.

To solve this problem, it has been proposed to spray water onto the tip of the probe to prevent grinding debris from getting in (see, for example, Patent Document 2).

JP 2008-073785 A JP 2005-246491 A

However, under grinding conditions that generate a lot of grinding debris, it is necessary to spray a large amount of water onto the tip of the probe to remove the debris. In that case, the water pressure can cause the probe to float up, making it impossible to measure the height accurately.

Therefore, a grinding device that grinds a wafer while measuring its thickness by contacting it with a probe has the problem of grinding the wafer to the specified finishing thickness even under grinding processing conditions that produce a large amount of grinding debris.

The present invention relates to a method for grinding a wafer using a grinding device that grinds a wafer with a grinding wheel, the grinding device comprising: a chuck table that holds a wafer on a holding surface; a grinding unit that grinds the wafer held on the holding surface with a grinding wheel; a grinding feed unit that feeds the grinding wheel in a direction perpendicular to the holding surface; a holding surface height measuring device that measures the height of the holding surface outside the wafer held on the holding surface ; an upper surface height measuring device that measures the height of the upper surface of the wafer held on the holding surface; and a thickness calculation unit that calculates the difference between the value measured by the holding surface height measuring device and the value measured by the upper surface height measuring device, and further comprising a finishing thickness setting step of setting a finishing thickness; and a confirmation step of confirming the thickness of the wafer before it reaches the finishing thickness. the thickness calculation unit calculates the difference for a predetermined time period by the thickness calculation unit while keeping the lower surface of the grinding wheel in contact with the upper surface of the wafer; and a further grinding step of further grinding the remaining amount of grinding calculated by subtracting the difference measured for the predetermined time period from the finished thickness after the thickness confirmation step.

In the present invention, even if grinding debris adheres between the tip of the probe of the holding surface height measuring device and the upper surface of the chuck table, and between the tip of the probe of the upper surface height measuring device and the upper surface of the wafer, the grinding feed of the grinding wheel is stopped before separating the grinding wheel from the wafer, and the thickness of the wafer is then confirmed, and the remaining amount of grinding to reach the finish thickness is confirmed. Therefore, even if there is an error in the wafer thickness measured during grinding, grinding is not ended as is, but additional grinding is performed, so that it is possible to grind the wafer to the specified thickness even under grinding processing conditions that produce a large amount of grinding debris.
In addition, since there is no need to spray water onto the measurement surface for measuring the holding surface height or onto the upper surface of the wafer in order to prevent grinding debris from adhering thereto, water can be saved.

FIG. 1 is a perspective view showing an example of a grinding device. 4 is a graph showing a first example of a grinding method. 11 is a graph showing a second example of a grinding method. 13 is a graph showing a third example of a grinding method. 11 is a graph showing intervals at which the thickness of a wafer is calculated in a thickness confirmation process. 11 is a graph showing an example of calculated values of wafer thickness in a thickness grinding process.

1 Grinding Apparatus The grinding apparatus 1 shown in FIG. 1 is an apparatus that grinds a wafer 10 held on a chuck table 2 using a grinding unit 3. The chuck table 2 is driven by a chuck table feed mechanism (not shown) to be movable in the Y-axis direction, and the grinding unit 3 is driven by a grinding feed unit 4 to be movable in the Z-axis direction.

The chuck table 2 includes a suction section 20 formed of a porous material and a frame body 21 that supports the suction section 20, and the surface of the suction section 20 forms a holding surface 200 that holds the wafer. The holding surface 200 and the upper surface 210 of the frame body 21 are flush with each other. The chuck table 2 is driven by a rotation drive mechanism (not shown) and is rotatable about the chuck rotation shaft 22, and is movable in the Y-axis direction with the expansion and contraction of the bellows 23.

The grinding unit 3 includes a spindle 30 having a rotation axis 300 in the Z-axis direction, a spindle rotation mechanism 31 for rotating the spindle 30, a spindle housing 32 for rotatably supporting the spindle 30, a mount 33 connected to the lower end of the spindle 30, and a grinding wheel 34 attached to the mount 33. When the spindle rotation mechanism 31 rotates the spindle 30, the grinding wheel 34 also rotates. The grinding wheel 34 is composed of a base 340 fixed to the mount 33 and a plurality of grinding stones 341 fixed in an annular shape to the lower surface of the base 340. Inside the spindle 30, a flow path is formed for allowing the grinding water flowing in from the inlet 35 to flow out from the base 340 of the grinding wheel 34, and the grinding water is supplied to the wafer 10 during grinding of the wafer 10. A nozzle for spraying the grinding water may be provided next to the chuck table 2.

The grinding feed unit 4 includes a ball screw 40 having a rotating shaft 400 in the Z-axis direction, a motor 41 for rotating the ball screw 40, a pair of guide rails 42 arranged parallel to the ball screw 40, a lift plate 43 whose side is in sliding contact with the guide rails 42 and has a nut (not shown) inside that screws into the ball screw 40, and a holder 44 connected to the lift plate 43 and supports the spindle housing 32. When the ball screw 40 rotates, the lift plate 43 is guided by the guide rails 42 and moves in a direction perpendicular to the holding surface 200, and the grinding unit 3 also moves in the same direction, and the grinding wheel 341 approaches or moves away from the holding surface 200. The position of the grinding unit 3 in the Z-axis direction can be recognized by the reading of the encoder of the motor 41.

A thickness calculation unit 5 is disposed on the side of the movement path of the chuck table 2 in the Y-axis direction to measure the thickness of the wafer 10 held on the holding surface 200. The thickness calculation unit 5 includes a holding surface height measuring device 51 having a probe whose tip contacts the upper surface 210 of the outer frame 21 of the wafer 10 held on the holding surface 200 and measures the height of the holding surface 200, and an upper surface height measuring device 52 having a probe whose tip contacts the upper surface 100 of the wafer 10 held on the holding surface 200 and measures the height of the wafer 10, and measures the thickness of the wafer 10 based on the difference between the measurement value of the holding surface height measuring device 51 and the measurement value of the upper surface height measuring device 52.

The grinding device 1 is equipped with a control unit 6 that controls the chuck table 2, the grinding unit 3, the grinding feed unit 4, and the thickness calculation unit 5. The control unit 6 is equipped with a CPU and a memory element.

2 Grinding Method (1) Finishing Thickness Setting Step When grinding the top surface 100 of the wafer 10 using the grinding unit 3, the value of the finishing thickness of the wafer 10 is stored in the memory of the control unit 6 before grinding starts. The value of the finishing thickness is input by an operator, for example, using a touch panel (not shown) provided in the grinding device 1. Here, the value of the finishing thickness is set to, for example, 792 μm.

(2) Confirmation Thickness Setting Process In addition to the value of the finishing thickness, the confirmed thickness value is stored in the memory of the control unit 6. Here, the confirmed thickness is, for example, the thickness of the wafer 10 just before it reaches the finishing thickness, i.e., a thickness value slightly thicker than the finishing thickness, and this value is also input by the operator using, for example, a touch panel (not shown) provided on the grinding device 1. The confirmed thickness is, for example, a value obtained by adding a few μm to the finishing thickness, and here, the confirmed thickness value is, for example, 794 μm. Note that when the amount of grinding is small, the confirmed thickness may be set to the same value as the finishing thickness.

(3) Holding Step A protective tape 11 is attached to the lower surface 101 of the wafer 10 to be ground. The protective tape 11 side is then suction-held on the holding surface 200 of the chuck table 2, and the upper surface 100 of the wafer 10 is exposed.

(4) Grinding Process The chuck table 2 holding the wafer 10 on the holding surface 200 is rotated around the chuck rotation axis 22, while the spindle rotation mechanism 31 rotates the spindle 30 and the grinding wheel 34 around the rotation axis 300, and the grinding feed unit 4 lowers the grinding unit 3. Here, the rotation speed of the chuck table 2 is, for example, 191 rpm, and the rotation speed of the spindle 30 is, for example, 2000 rpm. For example, grinding water is supplied to the wafer 10 from the base 340 of the grinding wheel 34. At time T1, the height 61 of the lower surface (grinding surface) of the grinding wheel 341 shown by the solid line in FIG. 2 is the stage where the grinding wheel 341 is brought close to the upper surface 100 of the wafer 10 before grinding, and the grinding wheel 341 is lowered at high speed. The wafer thickness 62 indicated by the dashed line in FIG. 2 is the measured value of the thickness of the wafer 10 measured by the thickness calculation unit 5 as the difference between the measurement value of the holding surface height measuring device 51 and the measurement value of the upper surface height measuring device 52, and before grinding begins, the value is, for example, 815 μm.

Next, in the grinding process 81 from time T2, the grinding feed rate is significantly slower than at time T1, for example, at a grinding feed rate of 0.3 μm/sec, and the grinding unit 3 is fed for grinding, performing air cutting by slowly moving the grinding wheel 341 closer to the top surface 100 of the wafer 10. As a result, the bottom surface (grinding surface) of the grinding wheel 341 comes into contact with the top surface 100 of the wafer 10, and the value of the wafer thickness 62 indicated by the dashed line in FIG. 2 gradually decreases.

After the grinding surface of the grinding wheel 341 comes into contact with the upper surface of the wafer 10 at time T2, the grinding wheel 341 is brought closer to the holding surface 200 at time T3 under the same grinding feed rate (0.3 μm/sec) as T2 to continue grinding. Thus, the wafer thickness 62 is reduced.
At time T3, when the wafer 10 is being ground with the grinding wheel 341, the thickness calculation unit 5 calculates the thickness of the wafer 10 as the difference between the measurement value of the holding surface height measuring device 51 and the measurement value of the upper surface height measuring device 52.
Moreover, the control unit 6 performs grinding while checking every 100 milliseconds whether the thickness of the wafer 10 calculated by the thickness calculation unit 5 has reached a preset confirmation thickness.
In addition, to check the thickness of the wafer 10 during grinding, the holding surface height measuring instrument 51 and the upper surface height measuring instrument 52 each transfer their measurement results every 0.5 milliseconds to the thickness calculation unit 5. The thickness calculation unit 5 calculates the average value of the 40 measurements of the upper surface height measuring instrument 52 and the holding surface height measuring instrument 51 sent every 0.5 milliseconds, and calculates the difference between the average value of the measurement values of the upper surface height measuring instrument 52 and the average value of the measurement values of the holding surface height measuring instrument 51 every 20 milliseconds, as shown in FIG.

(5) Thickness Confirmation Step As described above, even during grinding at time T3, the thickness of the wafer 10 is measured using the thickness calculation unit 5. Then, when the measured value of the thickness calculation unit 5 reaches the confirmation thickness 71 (794 μm) set in the confirmation thickness setting step, the operation of approaching the grinding wheel 341 to the holding surface 200 is stopped, and the process proceeds to a thickness confirmation step 82. In the thickness confirmation step 82, the grinding feed is stopped, and the calculation of the thickness of the wafer 10 is continued while the lower surface of the grinding wheel 341 is kept in contact with the upper surface 100 of the wafer 10. In the thickness confirmation process, even if grinding debris is caught between the probe of the holding surface height measuring device 51 and the upper surface 210 of the frame 21, or between the tip of the probe of the upper surface height measuring device 52 and the upper surface 100 of the wafer 10, the grinding feed is stopped and the grinding wheel 341 is rotating together with the rotation of the chuck table 2, so that the grinding debris is removed while the grinding feed of the grinding wheel 341 is stopped.

For example, in the example shown in Figure 2, grinding debris was caught between the probe of the holding surface height measuring device 51 and the upper surface 210 of the frame 21, causing the measured height of the holding surface 200 to be higher than the actual value, resulting in the calculated thickness of the wafer 10 being thinner than its actual thickness. When the grinding debris was removed in the thickness confirmation process 82, the measured height of the holding surface 200 became lower, and the calculated thickness of the wafer 10 in the thickness confirmation process increased.

The example shown in Figure 3 is one in which grinding debris was caught between the probe of the top surface height measuring device 52 and the top surface 100 of the wafer 10, causing the measured value of the top surface 100 to be higher than the actual value, resulting in the calculated thickness of the wafer 10 being thicker than it actually was. When the grinding debris was removed in the thickness confirmation process 85, the measured value of the height of the holding surface 200 became lower, and the calculated value of the thickness of the wafer 10 in the thickness confirmation process decreased.

4 is an example in which the confirmed thickness 71 and the finished thickness 72 are the same (the same value is set). In this case, the grinding amount is small and the grinding feed speed is relatively slow, and by stopping the grinding feed after the confirmed thickness 71 is reached at time T3, even if grinding debris is sandwiched between the probe of the holding surface height measuring device 51 and the upper surface 210 of the frame 21, or between the probe of the upper surface height measuring device 52 and the upper surface 100 of the wafer 10, the grinding debris is removed, and it can be confirmed that the finished thickness has been reached.
If grinding debris is caught between the top surface 100 of the wafer 10 and the tip of the probe of the top surface height measuring device 52, the value calculated by the thickness calculation unit 5 will be thicker than the actual thickness of the wafer 10. Therefore, the wafer may be over-ground, but if the grinding feed rate is relatively slow, the thickness will not fall significantly below the set finishing thickness 72, so the confirmed thickness 71 and the finishing thickness 72 may be the same.

In the thickness confirmation process, the holding surface height measuring instrument 51 and the top surface height measuring instrument 52 each transfer their measurement results to the thickness calculation unit 5 every 0.5 milliseconds. The thickness calculation unit 5 calculates the average value of the 40 measurements of the top surface height measuring instrument 52 and the holding surface height measuring instrument 51 sent every 0.5 milliseconds, and calculates the difference between the average value of the measurements of the top surface height measuring instrument 52 and the average value of the measurements of the holding surface height measuring instrument 51 every 20 milliseconds, as shown in FIG. 5, and sets the calculated value as the thickness 66 of the wafer 10.

The control unit 6 requests the thickness calculation unit 5 for the thickness value of the wafer 10 every 100 milliseconds, and the thickness calculation unit 5, in response to the request, transfers five calculation values of the thickness of the wafer 10 together to the control unit 6. For example, as shown in Fig. 5, when the thickness confirmation process takes 200 milliseconds, the values are transferred twice every 100 milliseconds, and the control unit 6 obtains ten calculation values.
The thickness calculation unit 5 includes a storage unit 55 for temporarily storing five calculation values of the thickness of the wafer 10 .
In the thickness confirmation step, the control unit 6 may request the thickness value of the wafer 10 from the thickness calculation unit 5 every 20 milliseconds.

The control unit 6 excludes the maximum and minimum values from the obtained 10 thickness calculation values, and calculates the average value of the remaining 8 thickness calculation values. For example, in the example shown in FIG. 6, if the maximum value of 794 μm and the minimum value of 792 μm are excluded to obtain the median value, the median value is 793.8 μm. Note that during the first 100 milliseconds, the thickness values of the wafer 10 are 792 μm and 792.3 μm, but the values suddenly decrease. At this time, grinding debris was sandwiched between the tip of the probe of the holding surface height measuring device 51 and the holding surface 200, but it is considered that the grinding debris was removed during the subsequent measurement of the wafer thickness.
In other words, it is considered that grinding chips were attached to the measurement surface with which the tip of the probe of the holding surface height measuring device 51 came into contact, and the upper surface of the grinding chips was measured.
The median may be calculated without excluding the maximum and minimum values.

In the thickness confirmation process, when starting the thickness measurement, the probe of the holding surface height measuring device 51 and the probe of the top surface height measuring device 52 may be raised and withdrawn once, and then the probe may be lowered again to start the measurement. By doing so, grinding debris caught between the holding surface 200 or the top surface 100 of the wafer 10 and the tip of the probe can be reliably removed. In this case, after the probe is raised, the thickness measuring unit 5 stops measuring the thickness of the wafer 10 until the subsequent additional grinding process begins.

(6) Additional Grinding Step The remaining amount to be ground in the additional grinding step is 1.8 μm obtained by subtracting the finishing thickness 72 of 792 μm from the median thickness of the wafer 10 of 793.8 μm found in the thickness confirmation step. As shown in Figures 2 and 3, if there is an amount to be ground in the additional grinding steps 83 and 86, the grinding feed is resumed to grind the upper surface 100 of the wafer 10. The grinding feed speed here is the same as the grinding feed speed when grinding for time T3.

In the additional grinding steps 83 and 86, the control unit 6 requests the thickness calculation unit 5 to calculate the thickness of the wafer 10 every 20 milliseconds. When the calculated value matches the value of the finishing thickness 72, the grinding feed is stopped, and at time T5, spark-out is performed, which is grinding with the grinding wheel 341 stopped descending. At time T5, the grinding wheel 341 and the top surface 100 of the wafer 10 eventually no longer come into contact, and the spark-out ends.

When grinding at time T5 is completed, an escape cut is performed at time T6 in which grinding is performed while raising the grinding wheel 341 at a slow speed, for example, 0.3 um/sec.

When the escape cut at time T6 is completed, the process moves to time T7, at which the grinding unit 3 is raised at high speed to separate the grinding wheel 341 from the wafer 10, and grinding is completed.

As described above, in the thickness confirmation process, the grinding feed of the grinding wheel is stopped to confirm whether the thickness of the wafer is the finishing thickness or not. Therefore, even if the measured thickness of the wafer measured in the grinding process reaches the finishing thickness but actually does not reach the finishing thickness, the wafer can be finished to the specified thickness. Conversely, even if the measured thickness of the wafer measured in the grinding process does not reach the finishing thickness but actually does reach the finishing thickness, the wafer can be finished to the specified thickness.

In this embodiment, the height of the holding surface 200 and the height of the upper surface 210 of the frame body 21 are the same, and a case has been described where the height of the holding surface 200 can be measured by measuring the height of the upper surface 210, but there are also cases where the upper surface 210 of the frame body 21 is located at a lower position than the holding surface 200. In such cases, the control unit 6 recognizes in advance the value of the height difference between the height of the upper surface 210 of the frame body 21 and the height of the holding surface 200, and calculates the thickness of the wafer 10 taking this height difference into account.

1: Grinding device 2: Chuck table 20: Suction unit 200: Holding surface 21: Frame 210: Top surface 22: Chuck rotation axis 3: Bellows 3: Grinding unit 30: Spindle 300: Rotation axis 31: Spindle rotation mechanism 32: Spindle housing 33: Mount 34: Grinding wheel 340: Base 341: Grinding wheel 35: Inlet 4: Grinding feed unit 40: Ball screw 400: Rotation axis 41: Motor 42: Guide rail 43: Lift plate 44: Holder 5: Thickness calculation unit 51: Holding surface height measuring device 52: Top surface height measuring device 55: Memory unit 6: Control unit 61: Height 62: Wafer thickness 71: Confirmed thickness 72: Finished thickness 81, 84, 87: Grinding process 82, 85, 88: Thickness confirmation process 83, 86: Additional grinding process 10: Wafer 100: Upper surface 101: Lower surface 11: Protective tape

Claims (1)

A method for grinding a wafer using a grinding device that grinds a wafer with a grinding wheel, comprising the steps of:
The grinding device comprises:
a chuck table for holding a wafer on a holding surface, a grinding unit for grinding the wafer held on the holding surface with a grinding wheel, a grinding feed unit for feeding the grinding wheel in a direction perpendicular to the holding surface, a holding surface height measuring device for measuring the height of the holding surface outside the wafer held on the holding surface, an upper surface height measuring device for measuring the height of the upper surface of the wafer held on the holding surface, and a thickness calculation unit for calculating a difference between a value measured by the holding surface height measuring device and a value measured by the upper surface height measuring device,
A finishing thickness setting process for setting a finishing thickness;
a confirmation thickness setting step of setting a confirmation thickness, which is a thickness of the wafer before the finishing thickness is reached;
a holding step of holding a wafer on the holding surface;
a grinding step of measuring the height of the holding surface by the holding surface height measuring device, measuring the height of the upper surface of the wafer by the upper surface height measuring device, and moving the grinding wheel closer to the holding surface by the grinding feed unit until the difference calculated by the thickness calculation unit becomes the confirmed thickness, thereby grinding the wafer;
a thickness confirmation step of stopping the operation of bringing the grinding wheel closer to the holding surface, and measuring the thickness of the wafer by calculating the difference for a predetermined time using the thickness calculation unit while keeping the lower surface of the grinding wheel in contact with the upper surface of the wafer;
After the thickness confirmation step,
and a further grinding step of further grinding a remaining amount of grinding obtained by subtracting the difference measured for the predetermined time from the finished thickness.

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