Grinding method for workpiece

Abstract

To provide a workpiece grinding method by which the air-cut period in which the rotation of a grinding wheel drops at a low speed to be idled, is shortened compared to heretofore.SOLUTION: A workpiece grinding method includes: a first movement step of relatively moving a chuck table and a grinding wheel at a first speed to make approach until a first distance while forming the film by flowing of a grinding liquid on the top face of the workpiece held on a holding face of the chuck table in such a state as no rotation of the chuck table but rotating the grinding wheel; and a second movement step of grinding the workpiece by moving relatively the above table and wheel at a second speed to make approach until a second distance smaller than thickness of the workpiece in the state where the chuck table and the grinding wheel are rotated. The first movement step is terminated by considering as the first distance, the distance between the table and wheel at the point of time at which the rotation load of the wheel fluctuates more largely than a threshold level.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for grinding a workpiece. [Background technology]

[0002] In order to realize small and lightweight device chips, there are increasing opportunities to thin wafers with devices such as integrated circuits on their front side. For example, the front side of the wafer is held by a chuck table, and the chuck table and a grinding wheel with a grinding stone containing abrasive grains fixed to it are rotated relative to each other, and the grinding stone is pressed against the back side of the wafer while a liquid (grinding fluid) such as pure water is supplied, thereby grinding the wafer to make it thinner.

[0003] Specifically, first, a grinding wheel is placed above a chuck table that holds a wafer. Next, the grinding wheel descends toward the chuck table at high speed. When the grinding wheel approaches a predetermined distance from the chuck table, the speed of the grinding wheel's descent is switched to low speed, and grinding of the wafer begins. Immediately after the speed of the descent is switched to low speed, there is a period called air cut, during which the grinding wheel rotates idly without contacting the wafer (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-24145 Summary of the Invention [Problem to be solved by the invention]

[0005] In the above-mentioned method, there is an air-cut period during which the grinding wheel descends slowly and rotates idly, so the grinding wheel descending at high speed does not collide with the wafer. On the other hand, during the air-cut period when the grinding wheel descends slowly, no grinding of the wafer progresses at all, so if this air-cut period can be shortened, the time required to grind the wafer will also be shortened, and it is thought that productivity will improve.

[0006] The timing at which the grinding wheel's descent speed is switched to low (i.e., the timing at which the air cut in which the grinding wheel descends at low speed begins) is determined based on the wafer thickness (height of the wafer's upper surface) measured by, for example, a contact-type height gauge. However, with this method, the grinding wheel's descent speed must be switched at a height position with a certain degree of leeway, taking into account the measurement accuracy of the height gauge, and the period of the air cut in which the grinding wheel descends at low speed cannot necessarily be sufficiently shortened.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method for grinding a workpiece in which the period of air cutting, during which the grinding wheel descends at a low speed and rotates idly, is shorter than in the past. [Means for solving the problem]

[0008] According to one aspect of the present invention, a method for grinding a workpiece includes a holding step of holding the workpiece on a holding surface of a chuck table, a placement step of placing a grinding wheel attached to the tip of a spindle and having grinding stones arranged in a ring shape above the chuck table, a first movement step of moving the chuck table and the grinding wheel relatively at a first speed so as to approach each other to a first distance while forming a film of flowing grinding fluid on the upper surface of the workpiece while rotating the grinding wheel without rotating the chuck table, and a second movement step of moving the chuck table and the grinding wheel relatively at a first speed while forming a film of flowing grinding fluid on the upper surface of the workpiece after the first movement step. and a second movement step of rotating the chuck table and the grinding wheel and moving them relatively at a second speed to bring them closer to a second distance that is smaller than the thickness of the workpiece, thereby grinding the workpiece. The first movement step ends when the rotating grinding wheel comes into contact with the film of grinding fluid flowing on the top surface of the workpiece and the rotation load of the grinding wheel fluctuates to a value greater than a threshold value, and the distance between the chuck table and the grinding wheel is set to the first distance.

[0009] In one aspect of the present invention, preferably, the second speed is lower than the first speed. Also, in one aspect of the present invention, after the first moving step and before the second moving step, a separating step of relatively moving the chuck table and the grinding wheel so as to separate them may be further included. Also, in one aspect of the present invention, after the placing step and before the first moving step, a preparatory moving step of relatively moving the chuck table and the grinding wheel at a preparatory speed higher than the first speed so as to approach each other to a first moving step start distance which is larger than the first distance may be further included. [Effects of the Invention]

[0010] A method for grinding a workpiece according to one aspect of the present invention utilizes the phenomenon that the rotational load of the grinding wheel fluctuates significantly when the rotating grinding wheel approaches the workpiece while in contact with a film of grinding fluid flowing on the workpiece's upper surface, and terminates the first movement step when this rotational load fluctuates more than a threshold value. Therefore, compared to conventional methods that use measurements from a height gauge, the second movement step can be started from a state where the grinding wheel and workpiece are close to each other. In other words, the period of air cutting during which grinding of the workpiece does not progress during the second movement step in which the grinding wheel descends at a low speed is shorter than in conventional methods. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view schematically showing a grinding device. [Figure 2] FIG. 2 is a cross-sectional view showing a grinding wheel disposed above a chuck table that holds a workpiece. [Figure 3] FIG. 3 is a cross-sectional view showing the state in which the chuck table and the grinding wheel move relatively to each other and approach each other to the start distance. [Figure 4] FIG. 4 is a cross-sectional view showing the state in which the chuck table and the grinding wheel move relatively to each other and approach each other to a first distance. [Figure 5] FIG. 5 is a graph showing the relationship between the time elapsed while the chuck table and the grinding wheel are moving relative to each other and the current flowing through the rotary drive source that rotates the grinding wheel. [Figure 6] FIG. 6 is a cross-sectional view showing the state in which the chuck table and the grinding wheel move relatively to each other and approach each other to a second distance. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a cross-sectional view schematically showing a grinding apparatus 2 used in a method for grinding a workpiece according to this embodiment. As shown in Fig. 1, the grinding apparatus 2 includes a chuck table 4 configured to hold a disk-shaped workpiece.

[0013] The chuck table 4 includes a disk-shaped (cylindrical) frame 6 made of, for example, ceramics or the like. A recess 6a having a circular opening at its upper end is formed on the upper surface of the frame 6. A holding plate 8 made of, for example, ceramics or the like and formed in a porous disk shape is fixed to this recess 6a.

[0014] The upper surface 8a of the holding plate 8 is configured in a shape corresponding to the side surface of a cone, for example, and functions as a holding surface for holding a workpiece. Note that although the shape of the upper surface 8a of the holding plate 8 is exaggerated in Fig. 1, in reality, the difference in height (height difference) between the center of the upper surface 8a, which corresponds to the apex of the cone, and the outer periphery of the upper surface 8a is approximately 10 µm to 30 µm.

[0015] The lower surface side of the holding plate 8 is connected to a suction source (not shown) such as an ejector via a flow path 6b provided inside the frame 6 and a valve (not shown) arranged outside the frame 6. Therefore, when a workpiece is brought into contact with the upper surface 8a of the holding plate 8, the valve is opened, and negative pressure from the suction source is applied, the workpiece is sucked by the chuck table 4. In other words, the workpiece is held by the chuck table 4.

[0016] A rotary drive source (not shown), such as a motor, is connected to the lower part of the frame 6. The chuck table 4 rotates, for example, around a vertical rotation axis by the power generated by this rotary drive source, so that the center of the upper surface 8a of the holding plate 8 becomes the center of rotation. The frame 6 is also supported by, for example, a ball screw type chuck table moving mechanism (not shown), and the chuck table 4 moves, for example, horizontally by the power generated by this chuck table moving mechanism.

[0017] A grinding unit 10 is disposed above the chuck table 4. The grinding unit 10 includes a cylindrical spindle housing (not shown). This spindle housing is supported by, for example, a ball screw type grinding unit moving mechanism (not shown), and the grinding unit 10 moves in the vertical direction by the power generated by this grinding unit moving mechanism.

[0018] A columnar spindle 12 is accommodated in the space inside the spindle housing. A disk-shaped mount 14 is fixed to the lower end of the spindle 12. A plurality of holes (not shown) are formed in the outer edge of the mount 14, penetrating the mount 14 in the thickness direction, and a bolt (not shown) or the like is inserted into each hole.

[0019] A disk-shaped grinding wheel 16 having roughly the same diameter as the mount 14 is attached to the underside of the mount 14 with bolts or the like. In other words, the grinding wheel 16 is attached to the lower end of the spindle 12 via the mount 14 or the like. The grinding wheel 16 includes a disk-shaped wheel base 18 made of a metal such as stainless steel or aluminum. The outer edge of the upper surface of the wheel base 18 is provided with threaded holes into which the above-mentioned bolts or the like are inserted.

[0020] A plurality of grinding wheels 20 are fixed in an annular region on the underside of the wheel base 18 along the circumferential direction of the wheel base 18. That is, a plurality of grinding wheels 20 are arranged in an annular pattern on the underside of the wheel base 18. Each grinding wheel 20 has a structure in which abrasive grains made of, for example, diamond or the like are dispersed in a binder such as resin, and is suitable for grinding workpieces.

[0021] A rotational drive source (not shown), such as a motor, is connected to the upper end of the spindle 12. The grinding wheel 16 rotates, for example, around a vertical rotation axis by the power generated by this rotational drive source, so that the intersection with the axis of the spindle 12 (the center of the grinding wheel 16) becomes the center of rotation. Note that the rotation axis of the grinding wheel 16 is generally slightly tilted with respect to the rotation axis of the chuck table 4.

[0022] A nozzle 22 configured to supply a grinding fluid to the workpiece or the like held on the chuck table 4 is provided directly below the grinding wheel 16. Pure water, for example, is used as the grinding fluid. Note that instead of or together with the nozzle 22, a supply port configured to supply the grinding fluid to the workpiece or the like may be provided inside the grinding wheel 16.

[0023] A control unit (not shown) is connected to each of the above-mentioned elements of the grinding device 2. This control unit is configured by, for example, a computer including a processing device, a storage device, and an input device, and controls the operation of each of the above-mentioned elements of the grinding device 2 so that the workpiece 11 is appropriately ground.

[0024] The processing device is typically a CPU (Central Processing Unit) that performs various processes necessary to control the above-mentioned elements. The storage device includes, for example, a main storage device such as a DRAM (Dynamic Random Access Memory) and an auxiliary storage device such as a hard disk drive or flash memory.

[0025] The input device is, for example, a touch panel, which also serves as an output device (display device). Note that a keyboard, mouse, etc. may also be used as an input device. The functions of the control unit configured in this manner are realized, for example, by the processing device operating in accordance with software stored in the storage device.

[0026] In this embodiment, some or all of the steps required for grinding the workpiece are performed under the control of the control unit of the grinding device 2. However, some or all of the steps do not necessarily have to be performed under the control of the control unit. For example, some or all of the steps may be performed under the control of an operator.

[0027] In the method for grinding a workpiece according to this embodiment, first, the workpiece is held on the holding surface of the chuck table 4 (holding step), and then the grinding wheel 16 is placed above the chuck table 4 (placement step). Fig. 2 is a cross-sectional view that schematically shows how the grinding wheel 16 is placed above the chuck table 4 that holds the disk-shaped workpiece 11.

[0028] 2, the workpiece 11 is placed on the chuck table 4 so that the lower surface 11a of the workpiece 11 contacts the upper surface 8a of the holding plate 8. Next, the valve is opened, and negative pressure generated by the suction source acts on the upper surface 8a of the holding plate 8. As a result, the lower surface 11a of the workpiece 11 is sucked onto the upper surface 8a of the holding plate 8, and the workpiece 11 is held on the chuck table 4 with the upper surface 11b exposed.

[0029] After the workpiece 11 is held on the holding surface of the chuck table 4, a grinding wheel 16 is placed above the chuck table 4. Specifically, as shown in Fig. 2, the chuck table moving mechanism moves the chuck table 4 to directly below the grinding wheel 16 under the control of the control unit. In other words, the grinding wheel 16, on which a plurality of grinding stones 20 are arranged in a ring shape, is placed above the chuck table 4 on which the workpiece 11 is held.

[0030] For example, when the grinding wheel 16 is placed above the chuck table 4, the grinding fluid 21 is supplied to the upper surface 11b of the workpiece 11 from the nozzle 22. In this embodiment, the grinding wheel 16 is placed above the chuck table 4 after the workpiece 11 is held on the holding surface of the chuck table 4, but the workpiece 11 may also be held on the holding surface of the chuck table 4 after the grinding wheel 16 is placed above the chuck table 4.

[0031] After the workpiece 11 is held on the holding surface of the chuck table 4 and the grinding wheel 16 is placed above the chuck table 4, the chuck table 4 and the grinding wheel 16 move relatively at a predetermined preparatory speed and approach each other to a predetermined start distance (first movement step start distance) (preparatory movement step). Figure 3 is a cross-sectional view showing the state in which the chuck table 4 and the grinding wheel 16 move relatively and approach each other to the start distance (first movement step start distance) D0.

[0032] 3, the grinding unit moving mechanism lowers the grinding unit 10 to a position at a height of the starting distance D0 from the chuck table 4 under the control of the control unit. The preparation speed required for the lowering of the grinding unit 10 is typically set arbitrarily within a range of 1 mm / s to 100 mm / s. However, the preparation speed is not limited to this range.

[0033] For example, when the chuck table 4 and the grinding wheel 16 approach each other by a predetermined start distance D0, the grinding unit 10 stops descending at the preparation speed, i.e., the relative movement between the chuck table 4 and the grinding wheel 16 stops. Here, the start distance D0 is typically set to a value obtained by adding 10 μm to 50 μm to the thickness of the workpiece 11. However, the start distance D0 is not limited to this range.

[0034] The relative movement of the chuck table 4 and the grinding wheel 16 to approach each other to the start distance D0 is typically performed without rotating the chuck table 4 and the grinding wheel 16. In this case, the grinding wheel 16 starts to rotate after the chuck table 4 and the grinding wheel 16 have approached each other to the start distance D0. The rotation speed (number of rotations) of the grinding wheel 16 is typically set arbitrarily in the range of 1000 rpm to 6000 rpm. Note that the relative movement of the chuck table 4 and the grinding wheel 16 may also be performed while the grinding wheel 16 is rotating.

[0035] As described above, the relative movement of the chuck table 4 and the grinding wheel 16 to bring them closer to the start distance D0 is performed while the nozzle 22 supplies the grinding fluid 21 to the upper surface 11b of the workpiece 11. By supplying the grinding fluid 21 from the nozzle 22, a film of the flowing grinding fluid 21 is formed on the upper surface 11b of the workpiece 11.

[0036] The amount of grinding fluid 21 supplied is set, for example, so that the thickness of the film of grinding fluid 21 is 300 μm to 1000 μm in the area where the grinding wheel 20 and the workpiece 11 come into contact during grinding. Therefore, when the chuck table 4 and the grinding wheel 16 are brought close to the start distance D0, the grinding wheel 20 of the grinding wheel 16 comes into contact with the film of grinding fluid 21.

[0037] However, the amount of grinding fluid 21 supplied is not limited to a range in which the thickness of the film of grinding fluid 21 is 300 μm to 1000 μm. The amount of grinding fluid 21 supplied may be set, for example, within a range in which the grinding wheel 20 does not come into contact with the film of grinding fluid 21 when the chuck table 4 and the grinding wheel 16 are brought close to each other to the start distance D0.

[0038] Furthermore, the relative movement between the chuck table 4 and the grinding wheel 16 may be performed without the nozzle 22 supplying the grinding fluid 21 to the upper surface 11b of the workpiece 11. In this case, the nozzle 22 starts supplying the grinding fluid 21 after the chuck table 4 and the grinding wheel 16 have approached each other to the start distance D0.

[0039] After the chuck table 4 and the grinding wheel 16 have approached each other by the start distance D0, the chuck table 4 and the grinding wheel 16 move relatively at a predetermined first speed while the chuck table 4 is not rotating and the grinding wheel 16 is rotating, and approach each other by the first distance (first movement step). Figure 4 is a cross-sectional view showing the state in which the chuck table 4 and the grinding wheel 16 move relatively and approach each other by the first distance D1.

[0040] 4, the grinding unit moving mechanism lowers the grinding unit 10 to a position at a height of a first distance D1 from the chuck table 4 under the control of the control unit. The first speed at which the grinding unit 10 is lowered is set arbitrarily in the range of 5 μm / s to 80 μm / s when rough grinding the workpiece 11, and in the range of 1 μm / s to 10 μm / s when finish grinding the workpiece 11. However, the first speed is not limited to these ranges.

[0041] As described above, the relative movement of the chuck table 4 and the grinding wheel 16 to approach each other to the first distance D1 is performed while the nozzle 22 supplies the grinding fluid 21 to the upper surface 11b of the workpiece 11. As the grinding fluid 21 is supplied from the nozzle 22, a film of the flowing grinding fluid 21 is formed on the upper surface 11b of the workpiece 11.

[0042] When the distance between the upper surface 11b of the workpiece 11 and the grinding wheel 20 becomes sufficiently small while the grinding wheel 20 is in contact with this film of grinding fluid 21, the load on the rotation of the grinding wheel 16 increases significantly. This phenomenon is presumably caused by the increased influence of the viscosity of the grinding fluid 21 as the workpiece 11 and the grinding wheel 20 approach each other. In this embodiment, this increase in the load on the rotation is used to control the relative movement between the chuck table 4 and the grinding wheel 16.

[0043] Specifically, the control unit stops the lowering of the grinding wheel 16 at the first speed by the grinding unit moving mechanism when an increase in the load on the rotation of the grinding wheel 16 is detected. That is, the distance between the chuck table 4 and the grinding wheel 16 at the time when the increase in the load is detected is set to a first distance D1, and the relative movement between the chuck table 4 and the grinding wheel 16 at the first speed ends.

[0044] The magnitude of the load applied to the rotation of the grinding wheel 16 corresponds to, for example, the magnitude of the current flowing through the rotary drive source that rotates the spindle 12. Therefore, in this embodiment, the current flowing through the rotary drive source is measured in real time by an ammeter (not shown) and used to detect the load. Figure 5 is a graph showing the relationship between the time t that has elapsed while the chuck table 4 and the grinding wheel 16 are moving relative to each other and the current I that flows through the rotary drive source that rotates the grinding wheel 16. In Figure 5, the horizontal axis represents time t, and the vertical axis represents current I.

[0045] 5, before time t1, when the workpiece 11 and the grinding wheel 20 are sufficiently separated, the current I flowing through the rotary drive source that rotates the grinding wheel 16 is kept at a value smaller than the first current I1. On the other hand, after time t1, when the workpiece 11 and the grinding wheel 20 are sufficiently close to each other, the current I flowing through the rotary drive source that rotates the grinding wheel 16 becomes larger than the first current I1.

[0046] Therefore, when a current I greater than a preset first current I1 is measured, the control unit causes the grinding unit moving mechanism to terminate lowering of the grinding wheel 16 at the first speed at that point in time. In other words, the distance between the chuck table 4 and the grinding wheel 16 at the point in time when the rotational load of the grinding wheel 16 fluctuates more than a first threshold value corresponding to the first current I1 is set to a first distance D1, and the relative movement between the chuck table 4 and the grinding wheel 16 at the first speed is terminated.

[0047] The first current I1 is set arbitrarily within a range of, for example, 1.05 to 1.20 times the current I (typically, the average value of the current I for a given period before time t1) flowing through the rotary drive source that rotates the grinding wheel 16 when the workpiece 11 and the grinding wheel 20 are sufficiently separated. However, the first current I1 is not limited to this range. Furthermore, the rotation load of the grinding wheel 16 may be detected by any other method that does not utilize the current I flowing through the rotary drive source.

[0048] As described above, this embodiment utilizes the phenomenon that the rotational load of the grinding wheel 16 fluctuates significantly when the rotating grinding wheel 16 approaches the workpiece 11 while in contact with the film of grinding fluid 21 flowing on the upper surface 11b of the workpiece 11, and the relative movement at the first speed between the chuck table 4 and the grinding wheel 16 ends when this rotational load fluctuates beyond a first threshold value. Therefore, compared to conventional methods that use measurements from a height measuring device, the grinding wheel 16 and the workpiece 11 can be brought closer together by relative movement at the first speed. Specifically, this method can achieve a first distance D1 of 1 μm to 5 μm.

[0049] In this embodiment, the chuck table 4 does not rotate when the chuck table 4 and the grinding wheel 16 move relatively at the first speed, so even if, for example, control by the control unit is slightly delayed in relation to the first speed and the grinding wheel 20 comes into contact with the workpiece 11, almost no problems occur with the grinding wheel 20 or the workpiece 11. On the other hand, if the grinding wheel 20 comes into contact with the workpiece 11, the load on the rotary drive source that rotates the chuck table 4 becomes significantly large, and the rotation of the chuck table 4 does not start properly.

[0050] Therefore, when the grinding wheel 20 comes into contact with the workpiece 11, it is desirable that the chuck table 4 and the grinding wheel 16 then move relatively at a predetermined separation speed until they are separated by the separation distance (separation step). Specifically, it is desirable that the grinding unit moving mechanism, under the control of the control unit, raises the grinding unit 10 to a position at the height of the separation distance from the chuck table 4.

[0051] Whether or not the workpiece 11 and the grinding wheel 20 are in contact can be determined based on an increase in the load applied to the rotation of the grinding wheel 16. As shown in Fig. 5, after time t2, when the workpiece 11 and the grinding wheel 20 are in contact, the current I flowing through the rotation drive source that rotates the grinding wheel 16 becomes larger than the second current I2.

[0052] Therefore, when the current I of the rotary drive source that rotates the grinding wheel 16 becomes larger than a preset second current I2, the control unit determines that the workpiece 11 and the grinding wheel 20 have come into contact. In other words, when the rotational load of the grinding wheel 16 becomes larger than a second threshold value corresponding to the second current I2, the control unit determines that the workpiece 11 and the grinding wheel 20 have come into contact.

[0053] Here, the second current I2 is greater than the first current I1, and the second threshold is greater than the first threshold. The second current I2 is set arbitrarily within a range of, for example, 1.20 to 2.00 times the current I (typically, the average value of the current I for a given period before time t1) flowing through the rotary drive source that rotates the grinding wheel 16 when the workpiece 11 and the grinding wheel 20 are sufficiently separated. However, the second current I2 is not limited to this range.

[0054] Furthermore, the determination of whether the workpiece 11 and the grinding wheel 20 are in contact with each other may be performed by any other method that does not utilize the current I flowing through the rotation drive source. Also, the chuck table 4 and the grinding wheel 16 may always be separated from each other regardless of whether the workpiece 11 and the grinding wheel 20 are in contact with each other. In this case, it is not necessary to determine whether the workpiece 11 and the grinding wheel 20 are in contact with each other.

[0055] The separation speed at which the grinding unit 10 is raised is typically set to any value within the range of 1 μm / s to 80 μm / s. However, the separation speed is not limited to this range. The separation distance is desirably set within a range in which the effect on productivity is sufficiently small, and is typically set to any value within the range of 1 μm to 10 μm. However, the separation distance is also not limited to this range.

[0056] After the relative movement between the chuck table 4 and the grinding wheel 16 at the first speed, the chuck table 4 starts to rotate. However, if the chuck table 4 and the grinding wheel 16 are separated from each other after the relative movement between the chuck table 4 and the grinding wheel 16 at the first speed, the chuck table 4 starts to rotate after this separation. The rotation speed (number of rotations) of the chuck table 4 is typically set arbitrarily within the range of 100 rpm to 300 rpm.

[0057] Then, while the chuck table 4 and the grinding wheel 16 are both rotating, the chuck table 4 and the grinding wheel 16 move relatively to each other at a predetermined second speed until they approach each other to a second distance D2, which is smaller than the thickness of the workpiece 11 (second movement step). In other words, the workpiece 11 is ground by the grinding wheel 16. Figure 6 is a cross-sectional view showing the chuck table 4 and the grinding wheel 16 moving relatively to each other until they approach each other to a second distance D2.

[0058] 6, the grinding unit moving mechanism lowers the grinding unit 10 to a position at a height of the second distance D2 from the chuck table 4 under the control of the control unit. The second speed at which the grinding unit 10 is lowered is smaller than the first speed, and is set arbitrarily in the range of 1.0 μm / s to 10.0 μm / s when rough grinding the workpiece 11, and in the range of 0.1 μm / s to 1.5 μm / s when finish grinding the workpiece 11. However, the second speed is not limited to this range.

[0059] The relative movement between the chuck table 4 and the grinding wheel 16 at the second speed is also performed while the nozzle 22 supplies grinding fluid 21 to the upper surface 11b of the workpiece 11. For example, when the chuck table 4 and the grinding wheel 16 approach each other by a predetermined second distance D2, the lowering of the grinding unit 10 at the second speed, i.e., the relative movement between the chuck table 4 and the grinding wheel 16, is completed. The second distance D2 is set to, for example, an arbitrary thickness required for the workpiece 11 after grinding.

[0060] As described above, the method for grinding a workpiece according to this embodiment utilizes the phenomenon that the rotational load of the grinding wheel 16 fluctuates significantly when the rotating grinding wheel 16 approaches the workpiece 11 while in contact with the film of grinding fluid 21 flowing on the upper surface of the workpiece 11. The relative movement between the chuck table 4 and the grinding wheel 16 at a first speed (first movement step) ends when this rotational load fluctuates more than a first threshold. Therefore, compared to conventional methods that use measurements from a height gauge, the relative movement between the chuck table 4 and the grinding wheel 16 at a second speed (second movement step) can be started from a state in which the grinding wheel 16 and the workpiece 11 are approaching each other. In other words, the period of air cutting, during which grinding of the workpiece 11 does not progress when the chuck table 4 and the grinding wheel 16 move relatively at the second speed, which is slower than the first speed, is shortened compared to conventional methods.

[0061] For example, if a conventional air cut performed for 40 seconds (20 μm) at a speed of 0.5 μm / s (second speed) is replaced with an air cut for 1.8 seconds (18 μm) at a speed of 10 μm / s (first speed) and an air cut for 4 seconds (2 μm) at a speed of 0.5 μm / s (second speed), the air cut period will be shortened by more than 31 seconds, even if it takes 3 seconds to rotate the chuck table 4 before the air cut at 0.5 μm / s.

[0062] The present invention is not limited to the above-described embodiments and may be implemented with any modifications. Furthermore, the structures, methods, etc. according to the above-described embodiments may be implemented with any modifications without departing from the scope of the present invention. [Explanation of symbols]

[0063] 2: Grinding equipment 4: Chuck table 6: Frame 6a: Recess 6b: Flow path 8: Holding plate 8a:Top surface 10: Grinding unit 12: Spindle 14: Mount 16: Grinding wheel 18: Wheel base 20: Grinding wheel 22: Nozzle 11: Workpiece 11a: Bottom surface 11b:Top surface 21: Grinding fluid

Claims

1. A method for grinding a workpiece, comprising: a holding step of holding the workpiece on a holding surface of the chuck table; a placement step of placing a grinding wheel, which is attached to the tip of a spindle and has grinding stones arranged in an annular shape, above the chuck table; a first moving step of relatively moving the chuck table and the grinding wheel at a first speed so as to approach each other to a first distance while forming a film of flowing grinding fluid on the upper surface of the workpiece while rotating the grinding wheel without rotating the chuck table; a second moving step of, after the first moving step, moving the chuck table and the grinding wheel relatively at a second speed so as to approach each other to a second distance that is smaller than the thickness of the workpiece while rotating the chuck table and the grinding wheel, thereby grinding the workpiece; The method for grinding a workpiece ends the first movement step by setting the distance between the chuck table and the grinding wheel at the time when the rotating grinding wheel comes into contact with the film of grinding fluid flowing on the top surface of the workpiece and the rotation load of the grinding wheel fluctuates to a value greater than a threshold value as the first distance.

2. 2. The method of claim 1, wherein the second speed is less than the first speed.

3. 3. The method for grinding a workpiece according to claim 1, further comprising a separating step of relatively moving the chuck table and the grinding wheel so as to separate them from each other after the first moving step and before the second moving step.

4. 4. The method for grinding a workpiece according to claim 1, further comprising, after the positioning step and before the first moving step, a preparatory moving step of relatively moving the chuck table and the grinding wheel at a preparatory speed higher than the first speed so as to bring them closer to each other to a first moving step start distance that is larger than the first distance.

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