JP2009160705A - Wafer grinding method and grinding apparatus - Google Patents

Abstract

In a method for grinding a wafer, the thickness of the wafer is reliably detected and a high-speed feed operation is performed until just before contacting the wafer, thereby preventing collision between the wafer and the grindstone and reducing the actual processing time as much as possible. Provided is a method for grinding a wafer.
A maximum thickness of a wafer 1 held on a chuck table 20 is detected in a thickness detection step, and the maximum thickness is supplied to a storage means. When the chuck table 20 holding the wafer 1 is positioned at the grinding position, the control unit controls the operation of the feed mechanism 43 based on the maximum thickness of the wafer 1 supplied to the storage unit, and waits for the grinding unit 30. Approach from the position P to the machining start position H is performed at the speed of the high-speed feed operation. Next, the control means switches the feed speed of the grinding unit 30 to the work feed speed, and the wafer 1 is ground by the grinding unit 30.
[Selection] Figure 3

Description

  The present invention relates to a method of grinding a wafer using a grinding apparatus.

  Semiconductor wafers are divided into a large number of rectangular areas on the surface by grid-like division lines, and electronic circuits such as IC and LSI are formed on the surface of these rectangular areas, and then the back surface is ground to the desired thickness. Is done. Thereafter, the wafer is cut along all the planned division lines to obtain a device used for an electronic device such as a mobile phone.

  Generally, the grinding of the wafer is performed by exposing the back side to the holding surface of a vacuum suction type chuck table, which is a wafer holding means, and holding and holding the wafer. It is performed by a grinding apparatus configured to press and grind against the back surface while rotating. In such a grinding apparatus, the grinding means moves at a relatively high speed until it comes into contact with the wafer (high-speed feed operation). In general, the grinding means moves at a relatively slow grinding speed immediately before contacting the wafer. However, since the wafer to be ground by the grinding apparatus has a thickness distribution of several μm, it has not been known how close the grinding means should be to the holding means by the high-speed feeding operation. For this reason, the grinding means and the holding means are brought closer to each other at a grinding speed before the position where the grinding means actually contacts the wafer. Therefore, there is a problem that the time for processing the wafer becomes longer than necessary.

  As a method for solving the above problem, for example, there is a method described in Patent Document 1. According to Patent Document 1, the thickness of a plurality of locations on a wafer is measured to detect the maximum thickness of the wafer, and the high-speed feed operation is controlled based on this maximum thickness.

JP 2003-39318 A

  In the above-mentioned patent document, it is necessary to transfer the wafer to a position where it is actually ground after measuring the thickness of the wafer. Therefore, the wafer state may be different at each position due to differences in wafer holding state during wafer thickness measurement, wafer transfer and wafer processing, and external factors that the wafer receives during transfer. There is a problem that the thickness of the wafer during processing cannot be obtained with high accuracy.

  Therefore, according to the present invention, when the wafer is ground, the thickness of the wafer is reliably detected, and the high-speed feed operation is performed until just before contacting the wafer, thereby preventing the wafer and the grindstone from colliding with each other and reducing the actual processing time as much as possible. An object of the present invention is to provide a method for grinding a wafer that can be shortened.

  The present invention includes a chuck table having a holding surface for holding a wafer, and a grinding wheel provided with a grinding wheel arranged to face the holding surface of the chuck table and grinding the wafer held on the holding surface of the chuck table. Grinding means, feed means for moving the grinding means toward or away from the chuck table, and at least the feed speed of the feed means from the standby position where the grinding means is waiting to the wafer held on the chuck table by the grinding wheel The first feed speed at which the grinding means is sent to the machining start position, which is the position immediately before the grinding wheel touches, and the grinding means of the grinding wheel is brought into contact with the wafer held on the chuck table from the machining start position and the grinding means is fed by grinding. Grinding process comprising at least control means for controlling to a second feed speed lower than the first feed speed This is a wafer grinding method that grinds the wafer by placing it in a thickness detection process that detects the maximum value of the wafer thickness while the wafer is held on the holding surface of the chuck table, and a thickness detection process. An approaching step of approaching the grinding means from the standby position to the processing start position at the first feed rate based on the maximum value of the wafer thickness, and a grinding step of grinding the wafer at the second feed rate. It is a feature.

  In the grinding method of the present invention, by detecting the maximum value of the wafer thickness in advance, the grinding means can be approached at the first feed speed until just before the wafer in the approaching step. In addition, an accurate measurement value can be obtained by detecting the maximum value of the thickness of the wafer while being held on the chuck table. Thereby, the grinding means can be brought closest to the wafer without causing the grinding means to collide with the wafer. Therefore, production efficiency can be improved.

  The thickness detection step of the present invention is preferably performed at a place other than the processing position where the wafer is ground. The processing start position of the present invention is preferably a position obtained by adding 7 to 15 μm to the maximum value of the wafer thickness detected in the thickness detection step. The first feed speed is preferably 10 mm / second to 50 mm / second, and the second feed speed is preferably 0.1 μm / second to 3 μm / second. Thus, the first feed speed can be reliably switched to the second feed speed immediately before the wafer. As a result, the grinding means does not come into contact with the wafer at the first feed speed, and the wafer can be ground in an optimum grinding time.

  In the thickness detection step of the present invention, it is preferable to detect the wafer thickness distribution and use the maximum value as the maximum value of the wafer thickness.

  Next, a grinding apparatus of the present invention is an apparatus that can suitably carry out the above grinding method, and is disposed opposite to a chuck table having a holding surface for holding a wafer and the holding surface of the chuck table. Grinding means including a grinding wheel provided with a grinding wheel for grinding a wafer held on the holding surface, a feed means for moving the grinding means toward or away from the chuck table, and a feed speed of the feed means, At least a first feed speed for feeding the grinding wheel from a standby position where the grinding wheel of the grinding wheel is waiting to a processing start position which is a position immediately before the grinding wheel contacts the wafer held by the chuck table, and a processing start position The first feed speed is such that the grinding wheel of the grinding wheel is brought into contact with the wafer held on the chuck table and the grinding means is ground and fed. A control means for controlling the speed of the feed means by a second feed speed that is lower, a thickness detection means for detecting the maximum value of the thickness of the wafer while the wafer is held on the holding surface of the chuck table, Storage means for storing the maximum value of the thickness of the wafer detected by the thickness detection means. The control means of this grinding apparatus controls the distance from the standby position to the processing start position, which is the range in which the grinding means moves at the first feed rate, based on the maximum value of the wafer thickness stored in the storage means. Then, the grinding means is approached at the first feed speed from the standby position to the processing start position, and the wafer held on the chuck table is ground at the second feed speed.

  Preferably, the thickness detection means detects the maximum value of the thickness of the wafer at a place other than a processing position where the wafer is ground. The processing start position is a position obtained by adding 7 to 15 μm to the maximum value of the wafer thickness detected by the thickness detection means. Further, the thickness detecting means of the grinding apparatus of the present invention is characterized in that the thickness distribution of the wafer is detected and the maximum value is set as the maximum value of the wafer thickness. The first feed speed of the feed means of the grinding apparatus of the present invention is 10 mm / second to 50 mm / second. The second feed rate of the grinding apparatus of the present invention is 0.1 μm / second to 3 μm / second.

  According to the present invention, the processing start position can be corrected to an appropriate position by detecting the maximum value of the thickness of the wafer held on the chuck table in the thickness detection step. Collision of grinding means and useless processing feed can be suppressed. As a result, there is an effect that the apparatus can be safely operated and the production efficiency can be improved.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
[1] Grinding apparatus FIG. 2 shows a grinding apparatus to which the present invention is applied. The grinding apparatus 10 grinds the surface of a semiconductor wafer such as a silicon wafer (hereinafter abbreviated as a wafer). FIG. 1 shows an example of a wafer to be ground. This wafer 1 is obtained by slicing a raw material ingot, and then the thickness is adjusted by lapping, and then mechanical damage on both sides formed by lapping. The material stage is the layer removed by etching. The thickness of the wafer 1 is, for example, about 800 μm, but the thickness is not uniform, and there is an in-plane thickness variation of about 2 to 3 μm due to etching. Accordingly, the wafer 1 is removed by the grinding apparatus 10 to a thickness of about 10 to 20 μm, for example.

FIG. 2 shows the entire grinding apparatus 10. The apparatus 10 has a rectangular parallelepiped processing side base 11A whose upper surface is horizontal, and a detachable side base 11B whose upper surface is higher than the upper surface of the processing side base 11A. And. In FIG. 2, the longitudinal direction, the width direction, and the vertical direction of the device 10 are shown as a Y direction, an X direction, and a Z direction, respectively. A column 13 arranged in the X direction is erected in a pair at one end of the processing side base 11A in the Y direction. On the processing side base 11A is a processing area 12A for grinding the wafer 1, and on the detachable side base 11B, the unprocessed wafer 1 is supplied to the processing area 12A and the processed wafer 1 is recovered. It is set as the detachable area 12B.
Hereinafter, the grinding area 12A and the attachment / detachment area 12B will be described.

(I) Grinding area In the grinding area 12A, a disk-shaped turntable 14 whose rotation axis is parallel to the Z direction and whose upper surface is horizontal is rotatably provided. The turntable 14 is rotated in the direction of arrow R by a rotation drive mechanism (not shown). A plurality of (in this case, three) disc-shaped chuck tables 20 are rotatably arranged at equal intervals in the circumferential direction on the outer peripheral portion of the turntable 14.

  These chuck tables 20 are of a generally known vacuum chuck type, and suck and hold the wafer 1 placed on the upper surface. As shown in FIG. 3, the chuck table 20 has a circular suction area 21 made of porous ceramics on the upper surface, and the wafer 1 is sucked and held on the upper surface 21 a of the suction area 21. ing. An annular frame 22 is formed around the suction area 21, and an upper surface 22 a of the frame 22 is continuous with the upper surface 21 a of the suction area 21 and forms the same plane. Each chuck table 20 is independently rotated or rotated in one direction or both directions by a rotation drive mechanism (not shown) provided in the turntable 14. When the turntable 14 rotates, the chuck table 20 revolves. It becomes a state.

  As shown in FIG. 2, in a state where two chuck tables 20 are arranged in the X direction on the column 13 side, grinding units 30 are respectively disposed immediately above the chuck tables 20. Each chuck table 20 is positioned at three positions, that is, each grinding position below each grinding unit 30 and the attachment / detachment position closest to the attachment / detachment area 12B by the rotation of the turntable 14. There are two grinding positions, and a grinding unit 30 is provided for each of these grinding positions. In this case, the grinding position on the upstream side (front side in FIG. 2) in the transfer direction indicated by the arrow R of the chuck table 20 by the rotation of the turntable 14 is the primary grinding position, and the downstream grinding position is the secondary grinding position. Yes. In secondary grinding, rough grinding is performed, and in secondary grinding, finish grinding is performed.

  A slider 40 is attached to the column 13 so as to be movable up and down. The slider 40 is slidably mounted on a guide rail 41 extending in the Z direction, and can be moved in the Z direction by a ball screw type feed mechanism 43 driven by a servo motor 42. The front surface on the front side in the Y direction of each slider 40 is a vertical surface with respect to the upper surface of the processing-side base 11A. The grinding unit 30 is attached to the front surface of the slider 40. The mounting structure of each grinding unit 30 to each column 13 is the same. Each grinding unit 30 is moved up and down in the Z direction by the feed mechanism 43 and lowered to grind the exposed surface of the wafer 1 held on the chuck table 20 by a feed operation approaching the chuck table 20. The operation of the feed mechanism 43 is controlled by a feed operation control means (not shown). This feed operation control means controls the high-speed feed operation and the machining feed based on the maximum thickness of the wafer 1 measured by the thickness measuring device 54 described later. In the grinding apparatus 10 of one embodiment, the processing start position, which is the end position of the high-speed feed operation, is set to a position obtained by adding 7 to 15 μm to the maximum thickness of the wafer 1 detected by the thickness measuring device 54. Moreover, the speed of the high-speed feed operation of one embodiment is 10 mm / second to 50 mm / second. In addition, the processing feed speed of one embodiment is 0.1 μm / second to 3 μm / second.

  The grinding unit 30 is fixed to a cylindrical spindle housing 31 whose axial direction extends in the Z direction, a spindle shaft 32 coaxially and rotatably supported in the spindle housing 31, and an upper end portion of the spindle housing 31. A motor 33 that rotationally drives the spindle shaft 32 and a disk-like flange 34 that is coaxially fixed to the lower end of the spindle shaft 32 are provided. A grinding wheel 35 is detachably attached to the flange 34 by attachment means such as screwing.

  The grinding wheel 35 is configured such that a plurality of grinding wheels 37 are annularly arranged and fixed to the lower end surface of an annular frame 36 over the entire outer periphery of the lower end surface. A grinding wheel 35 having a grinding wheel 37 containing abrasive grains # 320 to # 400, for example, is attached to the flange 34 of the grinding unit 30 for primary grinding disposed above the primary grinding position. Further, a grinding wheel 35 including a grinding wheel 37 containing abrasive grains of # 2000 to # 8000 or more is attached to the flange 34 of the grinding unit 30 for secondary grinding disposed above the secondary grinding position. The flange 34 and the grinding wheel 35 are provided with a grinding water supply mechanism (not shown) for supplying grinding water for cooling and lubrication of the grinding surface or discharging grinding scraps, and a water supply line is connected to the mechanism. Yes. The grinding outer diameter of the grinding wheel 35, that is, the diameter of the outer peripheral edge of the plurality of grinding wheels 37 is set to be at least equal to or larger than the radius of the wafer 1 and generally equal to the diameter of the wafer 1.

  When grinding the wafer 1, the thickness is measured using a thickness measuring device (not shown), and the grinding amount of the wafer is controlled. As the thickness measuring device, for example, a contact type using a probe, which includes a reference-side height gauge and a wafer-side height gauge is used. In that case, the height gauge on the reference side is such that the tip of the probe contacts the upper surface 22a of the frame 22 of the chuck table 20 that is not covered with the wafer 1, and the height position of the upper surface 22a is detected. The height gauge on the wafer side detects the height position of the upper surface of the wafer 1 when the tip of the probe contacts the upper surface of the wafer 1 held by the chuck table 20, that is, the surface to be ground. According to this thickness measuring device, the thickness of the wafer is measured based on a value obtained by subtracting the measurement value of the height gauge on the reference side from the measurement value of the height gauge on the wafer side.

  The wafer 1 is first ground (roughly ground) by the grinding unit 30 at the primary grinding position, and then transferred to the secondary grinding position by the turntable 14 rotating in the R direction shown in FIG. Then, secondary grinding (finish grinding) is performed by the grinding unit 30.

(II) Attachment / Removal Area As shown in FIG. 2, a two-bar link pickup robot 50 that moves up and down is installed in the center of the attachment / detachment area 12B. Around the pickup robot 50, the supply cassette 51a, the positioning table 52, the supply means 53, the thickness measuring device 54, the recovery means 55, the spinner type cleaning device 56, the recovery cassette are counterclockwise as viewed from above. 51b are respectively arranged. The thickness measuring device 54 measures the thickness distribution of the wafer 1 held on the chuck table 20 and detects the maximum thickness of the wafer 1. The thickness measurement device 54 includes a measurement unit 54a that measures the thickness distribution of the wafer 1 held on the chuck table 20, and an arm unit 54b that has the measurement unit 54a fixed to the tip and moves the measurement unit 54a in the Z direction. It is comprised by. For example, a laser displacement meter type “LD-1300L-200 manufactured by Ono Sokki Co., Ltd.” or a capacitance type “Microsense manufactured by Japan AED Co., Ltd.” is used for the measuring unit 54a. As long as the measurement part 54a is an apparatus which can measure the thickness of the wafer 1 and can detect the maximum thickness, you may use things other than the above-mentioned apparatus. The maximum thickness of the wafer 1 detected by the thickness measuring device 54 is supplied to a storage means (not shown). Based on the maximum thickness supplied to the storage means, the high-speed feed operation and the machining feed are controlled by the control means.

  The supply means 53 is made of a porous material, and includes a suction pad 53a that sucks the wafer 1 on a horizontal lower surface by a vacuum action, and a horizontal swiveling recovery arm 53b having the suction pad 53a fixed to the tip. ing. The collection unit 55 has the same configuration as the supply unit 53, and includes a suction pad 55a and a collection arm 55b. The cassettes 51a and 51b are for storing a plurality of wafers in a horizontal posture and in a stacked state at regular intervals in the vertical direction, and are set at one end of the detachable side base 11B.

[2] Operation of grinding method and grinding apparatus Next, the operation of the grinding method and the grinding apparatus of one embodiment will be described.
The wafer 1 to be ground is first taken out from the supply cassette 51a by the pick-up robot 50, placed on the positioning table 52, and determined at a fixed position. Next, the wafer 1 is picked up from the positioning table 52 by the supply means 53 and placed on the chuck table 20 waiting at the attachment / detachment position with the surface to be ground facing up. When the wafer 1 is held on the chuck table 20, the arm 54b of the thickness measuring device 54 brings the measuring unit 54a closer to the wafer 1 on the chuck table 20, and the measuring unit 54a detects the maximum thickness of the wafer 1. (Thickness detection process). At this time, the supply means 53 and the recovery means 55 have moved to positions that do not hinder the thickness measurement of the wafer 1. The maximum thickness of the wafer 1 detected by the thickness measuring device 54 is supplied to the storage means.

  The wafer 1 is transferred to the primary grinding position by the rotation of the turntable 14 in the R direction. When the wafer 1 is placed at the primary grinding position, the servo motor 42 controlled by the feed operation control means is moved to move the grinding unit 30 from the standby position P shown in FIG. Send (approach process). At this time, the feed operation control unit corrects the processing start position H based on the maximum thickness L of the wafer 1 stored in the storage unit. Thereby, a safe and lean high-speed feeding operation is performed. After completion of the high-speed feed operation, the surface of the wafer 1 is ground from the machining start position H by the grinding unit 30 at a machining feed rate (grinding process). When grinding the wafer 1, the grinding amount is controlled while measuring the thickness of the wafer 1 one by one with a thickness measurement gauge. When the wafer 1 reaches a desired thickness, the grinding unit 30 is retracted to the standby position P. After the primary grinding, the wafer 1 is moved to the secondary grinding position by the turntable 14 rotating in the R direction. Next, the wafer 1 is finish-ground in the same manner as the primary grinding position. The wafer 1 that has been subjected to the secondary grinding is returned to the attachment / detachment position when the turntable 14 further rotates in the R direction.

  The wafer 1 on the chuck table 20 returned to the attachment / detachment position is taken up by the recovery means 55, transferred to the spinner type cleaning device 56, washed with water and dried. Then, the wafer 1 cleaned by the spinner cleaning device 56 is transferred and accommodated in the recovery cassette 51b by the pickup robot 50. The above is the overall operation of the grinding apparatus 10 of one embodiment, and this operation is repeatedly performed to grind a large number of wafers 1 continuously.

  In one embodiment, the maximum thickness of the wafer 1 is detected by the thickness measuring device 54, and the grinding unit 30 is controlled by the control means, thereby approaching the processing start position H at the speed of the high-speed feeding operation in the approaching process. Can do. In one embodiment, the maximum thickness of the wafer 1 is detected while being held on the chuck table 20. Thereby, the maximum thickness of the wafer 1 can be detected in the same state as when the wafer 1 is ground, and the thickness of the wafer during processing can be obtained with high accuracy. Further, by setting the processing start position H, the speed of the high-speed feed operation, and the speed of the work feed as described above, the speed of the high-speed feed operation can be reliably switched from the speed of the high-speed feed operation immediately before the wafer 1. This prevents the grinding unit 30 from coming into contact with the wafer 1 at a relatively high speed of the high-speed feed operation, and the wafer 1 can be ground in an optimum processing time. As a result, the apparatus can be operated safely and the production efficiency can be improved.

  In one embodiment, the maximum thickness of the wafer 1 is detected by the thickness measuring device 54 at the attachment / detachment position. This makes it possible to detect the maximum thickness of the wafer waiting at the attachment / detachment position while grinding other wafers, reducing the loss of time to detect the maximum thickness of the wafer and producing it. Efficiency can be improved. Further, the wafer holding method is the same during conveyance from the attachment / detachment position to the processing position, and the wafer during that time is sucked and held by the chuck table 20 and thus is less susceptible to external factors.

  In one embodiment, the maximum thickness of the wafer 1 is detected by the thickness measurement device 54 at the attachment / detachment position. However, the attachment / detachment is performed if the wafer 1 is held on the chuck table 20 before being ground. The maximum thickness of the wafer 1 may be detected at a place other than the position. Furthermore, when there are a plurality of processing positions as in the embodiment, the thickness may be detected before each processing is performed.

It is sectional drawing which shows the wafer ground with the grinding method of one Embodiment of this invention. It is a perspective view which shows the grinding device which can implement suitably the grinding method of one Embodiment. It is sectional drawing which shows the positional relationship of the grinding unit with which the grinding processing apparatus shown in FIG. 2 is equipped, and the wafer hold | maintained at the chuck table.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Wafer 10 ... Grinding apparatus 20 ... Chuck table 21a ... Upper surface (holding surface) of adsorption area
30 ... Grinding unit (grinding means)
35 ... grinding wheel 37 ... grinding wheel 43 ... feed mechanism (feed means)
P: Standby position H ... Processing start position L ... Maximum wafer thickness

Claims (8)

A chuck table having a holding surface for holding a wafer;
A grinding means comprising a grinding wheel disposed opposite to the holding surface of the chuck table and provided with a grinding wheel for grinding a wafer held on the holding surface of the chuck table;
Feed means for moving the grinding means toward or away from the chuck table;
The feed speed of the feed means is ground at least from a standby position where the grinding means is waiting to a processing start position which is a position immediately before the grinding wheel of the grinding wheel contacts the wafer held by the chuck table. The first feed speed for feeding the means and the second feed speed lower than the first feed speed for feeding the grinding means by bringing the grinding wheel of the grinding wheel into contact with the wafer held on the chuck table from the processing start position are controlled. A wafer grinding method for grinding a wafer by a grinding apparatus comprising at least a control means for
A thickness detecting step of detecting a maximum value of the thickness of the wafer while the wafer is held on the holding surface of the chuck table;
An approaching step of approaching the grinding means from the standby position to the processing start position at the first feed speed based on the maximum value of the wafer thickness detected in the thickness detecting step;
And a grinding step of grinding the wafer at the second feed rate. The thickness detection step is performed at a place other than the processing position where the wafer is ground.
2. The wafer grinding method according to claim 1, wherein the processing start position is a position obtained by adding 7 to 15 [mu] m to the maximum value of the thickness of the wafer detected in the thickness detection step.   3. The wafer grinding method according to claim 1, wherein in the thickness detection step, the thickness distribution of the wafer is detected, and the maximum value is set as the maximum value of the thickness of the wafer.   The first feed rate is 10 mm / sec to 50 mm / sec, and the second feed rate is 0.1 μm / sec to 3 μm / sec. Wafer grinding method. A chuck table having a holding surface for holding a wafer;
A grinding means comprising a grinding wheel disposed opposite to the holding surface of the chuck table and provided with a grinding wheel for grinding a wafer held on the holding surface of the chuck table;
Feed means for moving the grinding means toward or away from the chuck table;
The feed speed of the feed means is at least from a standby position where the grinding wheel of the grinding wheel is waiting to a processing start position which is a position immediately before the grinding wheel contacts the wafer held by the chuck table. The first feeding speed for feeding the wafer and the second feeding speed lower than the first feeding speed for feeding the grinding means by bringing the grinding wheel of the grinding wheel into contact with the wafer held on the chuck table from the processing start position. Control means for controlling the speed of
A thickness detecting means for detecting a maximum value of the thickness of the wafer in a state where the wafer is held on the holding surface of the chuck table;
A wafer grinding apparatus comprising at least storage means for storing the maximum value of the thickness of the wafer detected by the thickness detection means,
The control means controls the distance from the standby position to the processing start position, which is a range in which the grinding means moves at the first feed speed, based on the maximum value of the wafer thickness stored in the storage means, A wafer grinding apparatus, wherein a grinding means is approached from a standby position to a processing start position at a first feed speed, and the wafer held on the chuck table is ground at the second feed speed. The thickness detection means detects the maximum value of the thickness of the wafer at a place other than the processing position where the wafer is ground.
6. The wafer grinding apparatus according to claim 5, wherein the processing start position is a position obtained by adding 7 to 15 [mu] m to the maximum value of the wafer thickness detected by the thickness detection means.   7. The wafer grinding apparatus according to claim 5, wherein the thickness detecting unit detects a wafer thickness distribution and uses the maximum value as a maximum value of the wafer thickness.   The first feed rate is 10 mm / sec to 50 mm / sec, and the second feed rate is 0.1 μm / sec to 3 μm / sec. Wafer grinding machine.

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