JP7159861B2 - Double-headed grinding method - Google Patents

Description

The present invention relates to a double-headed grinding method.

Conventionally, the object to be ground is ground by rotating the object to be ground, supplying a grinding fluid to both main surfaces of the object to be ground, and bringing the grindstones of the grinding wheel into contact with both main surfaces of the object to be ground. A double-headed grinding method is known (see Patent Document 1, for example).
The method described in Patent Document 1 reduces the amount of grinding fluid supplied as the height of the grindstone decreases, thereby reducing the hydroplaning effect between the object to be ground and the grindstone. It keeps the grinding condition of the object constant.

JP 2009-16842 A

However, in the method disclosed in Patent Document 1, the temperature of the working atmosphere changes due to changes in the flow rate of the grinding fluid, which may lead to fluctuations in quality such as thickness.

SUMMARY OF THE INVENTION An object of the present invention is to provide a double-headed grinding method capable of obtaining a workpiece having a desired thickness and excellent nanotopography.

In the double-headed grinding method of the present invention, an object to be ground is rotated, a grinding liquid is supplied to both main surfaces of the object to be ground, and grindstones of a grinding wheel are brought into contact with both main surfaces of the object to be ground. using a double-sided grinding machine comprising grinding means for grinding the object to be ground and thickness measuring means for measuring the thickness of the object to be ground, and measuring the thickness of the object to be ground based on the measurement result of the thickness measuring means; A double-headed grinding method for grinding an object until the thickness of the object reaches a predetermined thickness, wherein a predetermined amount of grinding fluid is supplied to both main surfaces of the first object to be ground, and the first object to be ground is A first grinding step of grinding until the thickness reaches the predetermined thickness, a nanotopography measuring step of measuring the nanotopography of the first ground object, and based on the measurement results of the nanotopography measuring step a second grinding step of adjusting the grinding conditions so that the nanotopography of the second object to be ground approaches 0, and grinding until the thickness of the second object to be ground reaches the predetermined thickness; In the second grinding step, while maintaining the total amount of grinding fluid supplied in the first grinding step, the amount of grinding fluid supplied to one main surface of the second object to be ground and the other The second object to be ground is ground by adjusting the ratio of the amount of grinding liquid supplied to the main surface.

In the double-headed grinding method of the present invention, the thickness measuring means has a pair of contactors that respectively contact both main surfaces of the object to be ground, and outputs a signal corresponding to the position of the pair of contactors. It is preferable to use a differential transformer type displacement gauge for measuring the thickness of the object to be ground.

In the double-headed grinding method of the present invention, the second grinding step includes measuring the first object to be ground on the second object to be ground based on the measurement result of the nanotopography measurement step of the first object to be ground. It is preferable to adjust the ratio so that the supply amount of the grinding fluid to the main surface of the concave side of the object is larger than the supply amount of the grinding fluid to the other main surface.

According to the present invention, it is possible to obtain an object to be ground having good nanotopography and a desired thickness.

BRIEF DESCRIPTION OF THE DRAWINGS The related technology of this invention and the schematic diagram of the double-disc grinding apparatus which concerns on one Embodiment. Partially enlarged view of the double-headed grinding machine FIG. 2 is a block diagram of a control system of the double-headed grinding machine; 10 is a graph showing the results of Experiment 1 for leading the present invention and showing the relationship between the amount of grinding fluid supplied to each of the first and second main surfaces and the nanotopography at the center of the wafer. 4 is a graph showing the relationship between the amount of grinding fluid supplied to each of the first and second main surfaces of the wafer and the thickness of the center of the wafer obtained in Experiment 1; 10 is a result of Experiment 2 for leading the present invention, and is a graph showing the relationship between the measurement environment temperature of the differential transformer type displacement gauge and the measured value. FIG. 10 is a result of Experiment 3 for leading the present invention, and is a graph showing the relationship between the supply ratio of the grinding fluid to each of the first and second main surfaces and the nanotopography at the center of the wafer; 10 is a graph showing the results of Experiment 3 and showing the relationship between the supply ratio of the grinding liquid to each of the first and second main surfaces and the thickness of the center of the wafer; 4 is a flowchart of a double-headed grinding method according to the embodiment;

[Related technology of the present invention]
First, the technology related to the present invention will be described.
[Structure of Double Disc Grinding Device]
As shown in FIGS. 1 to 3, the double-sided grinding apparatus 1 includes grinding means 2, a differential transformer type displacement gauge 3 as thickness measuring means, a processing chamber 4, and control means 5. As shown in FIGS.

The grinding means 2 includes a carrier ring 21, wafer rotating means 22, first and second grinding wheels 23 and 24, first and second wheel rotating means 25 and 26, and first and second wheels. Equipped with advancing/retreating means 27 and 28 and grinding liquid supply means 29 .

The carrier ring 21 has an annular shape and holds the wafer W therein.
The wafer rotating means 22 is controlled by the control means 5 to rotate the carrier ring 21 around the center of the wafer W. As shown in FIG.

The first and second grinding wheels 23 and 24 are composed of substantially disk-shaped wheel bases 23A and 24A and a plurality of grindstones 23B and 24B provided at predetermined intervals along the outer edge of one surface of the wheel bases 23A and 24A. and Grinding liquid supply holes 23C and 24C are provided in the centers of the wheel bases 23A and 24A so as to penetrate both sides of the wheel bases 23A and 24A.
The first and second wheel rotating means 25, 26 are controlled by spindles 25A, 26A which hold the first and second grinding wheels 23, 24 at their distal ends, respectively, and the control means 5 to rotate the spindles 25A, 26A, respectively. Rotation motors 25B and 26B for rotating are provided. The first wheel rotating means 25 is provided on the left side of the wafer W in FIG. 1, and the second wheel rotating means 26 is provided on the right side.
The first and second wheel advancing/retreating means 27 and 28 are controlled by the control means 5 to advance/retreat the first and second wheel rotating means 25 and 26 with respect to the wafer W. As shown in FIG.

Grinding fluid supply means 29 is controlled by control means 5 and supplies grinding fluid into first and second grinding wheels 23 and 24 via grinding fluid supply holes 23C and 24C of first and second grinding wheels 23 and 24. , supply the grinding fluid.

The differential transformer type displacement meter 3 includes a pair of signal output means 31 , arms 32 extending downward from each signal output means 31 , and contacts 33 provided at the ends of each arm 32 . A pair of contactors 33 are provided so as to contact the first and second main surfaces W1 and W2 of the wafer W, respectively, and move according to the thickness of the wafer W. As shown in FIG. The signal output means 31 outputs signals corresponding to the positions of the contactors 33 to the control means 5 .

The processing chamber 4 is formed in the shape of a box in which at least the wafer W, the first and second grinding wheels 23 and 24, and the differential transformer type displacement gauge 3 can be arranged. is prevented from scattering to the outside of the processing chamber 4.

The control means 5 is connected to a memory (not shown), and grinds the wafer W based on various conditions stored in the memory.

[Double-headed grinding method of related technology]
Next, a related double-sided grinding method using the above-described double-sided grinding apparatus 1 will be described.
First, the first and second grinding wheels 23 and 24 are positioned at the positions indicated by solid lines in FIG. , W2, the control means 5 controls the wafer rotation means 22, the first and second wheel rotation means 25 and 26, the first and second wheel advance/retreat means 27 and 28, and the grinding fluid supply means. 29 to press the first and second grinding wheels 23 and 24 against the first and second main surfaces W1 and W2 of the wafer W, respectively, as indicated by the two-dot chain line in FIG. The wafer W is ground by supplying a grinding liquid into the first and second grinding wheels 23 and 24 and rotating the carrier ring 21 and the first and second grinding wheels 23 and 24 .

At this time, as shown in FIG. 2, the control means 5 rotates the wafer W and the second grinding wheel 24 in the clockwise direction (right-hand direction) as viewed from the left side of the figure, and rotates the first grinding wheel 23 is rotated counterclockwise (counterclockwise direction). Further, the control means 5 supplies the same amount of grinding liquid to the first main surface W1 and the second main surface W2. The rotating directions of the first and second grinding wheels 23 and 24 are not limited to the directions described above.
Then, the control means 5 manages the thickness of the wafer W based on the signal output from the differential transformer displacement gauge 3, and when it determines that the wafer W has been ground to a predetermined thickness, the first, The second grinding wheels 23, 24 are separated from the wafer W to finish grinding.

[Circumstances leading to the present invention]
The present inventor has obtained the following knowledge as a result of earnest research.
[Experiment 1]
When the nanotopography of the wafer W obtained by the double-sided grinding method of the related art was measured, it was confirmed that the center of the wafer W had undulations in the concave direction when viewed from the first main surface W1 side. Note that nanotopography is undulation in the nanometer range that exists in a period of millimeters when the wafer W is placed with non-adsorption or weak adhesion, and is included in flatness in a broad sense.
The inventor of the present invention considered the cause of such a phenomenon, and found that the first and second grinding wheels 23 24, a difference occurs in the state of wear and the cutting edge, and the front and back removal amount difference appears particularly remarkably in the central portion of the wafer W, which is always in contact with the first and second grinding wheels 23 and 24 during grinding, and the central portion It was speculated that a concave or convex habit occurs in the
Therefore, the present inventors considered that it is possible to improve the nanotopography of the wafer W by adjusting the supply amount of the grinding liquid, and conducted the following experiment.

First, a double-headed grinding machine 1 (manufactured by Koyo Machine Industries Co., Ltd., model: DXSG320) was prepared. Then, while supplying a grinding fluid of 1.2 L/min each to the first main surface W1 and the second main surface W2, the double-sided grinding method of the related art is performed to obtain a wafer W having a diameter of 300 mm. It was ground to a thickness (Experimental Example 1-1).
Further, the supply amount of the grinding fluid to the first main surface W1 and the second main surface W2 was set to 1.5 L/min each (Experimental Example 1-2) and 1.8 L/min each (Experimental Example 1-3). 10 wafers W were ground under the same conditions as in Experimental Example 1 except that the wafers W were

Ten wafers W were each ground by the grinding methods of Experimental Examples 1-1 to 1-3, and the first main surface W1 was measured with a nanotopography measuring machine (manufactured by Mizojiri Kogaku Kogyo Co., Ltd., model: FT-300U). We measured the nanotopography of The nanotopography at this time measures the unevenness profile of the surface shape of the first main surface W1 when the position of the outermost periphery of the first main surface W1 is 0 nm. The nanotopography of the center of the surface W1 was measured to obtain cross-sectional profile data passing through the center of the wafer W, and the numerical value of the central portion of the wafer W in the profile was used as an evaluation index. Note that the value of the outermost periphery is taken as a reference (0 nm). The results are shown in FIG.
In FIG. 4, when the value of nanotopography is less than 0, it means that the center of the first main surface W1 is recessed, and when it exceeds 0, it means that the center protrudes. Also, the larger the absolute value of the nanotopography, the larger the amount of depression or protrusion.

As shown in FIG. 4, it was confirmed that the nanotopography changed by adjusting the supply amount of the grinding fluid.
From this, it was found that there is a possibility that the nanotopography of the wafer W can be improved by adjusting the supply amount of the grinding liquid to the first main surface W1 and the second main surface W2.

[Experiment 2]
The present inventor found that the nanotopography of the wafer W could be improved by adjusting the supply amount of the grinding fluid from the results of Experiment 1 above. When the thickness of the center of the wafer W was measured, as shown in FIG. 5, it was confirmed that Experimental Example 1-1 was thicker than Experimental Example 1-3 by about 1 μm.
Even if the nanotopography of the wafer W can be improved, it is undesirable for the thickness to differ from the target value.
Therefore, the inventor of the present invention conducted a study and found that the temperature of the processing chamber 4 changed due to the adjustment of the supply amount of the grinding fluid, and the measurement error of the differential transformer type displacement gauge 3 was caused due to this temperature change. , the thickness of the wafer W may have been different from the target value, and the following experiment was conducted.

First, the relationship between the measurement environment temperature of the differential transformer type displacement meter 3 and the measured value was investigated.
A differential transformer type displacement gauge 3 (manufactured by Tokyo Seimitsu Co., Ltd., model: PULCOM series) is prepared, and a temperature sensor (manufactured by T&D, model: TR-52i) is attached to the housing of the signal output means 31 of the differential transformer type displacement gauge 3. ) was attached. The contactor 33 was brought into contact with a wafer W having a predetermined thickness, and the thickness was measured while changing the measurement environment temperature. The measurement results are shown in FIG.
As shown in FIG. 6, it was confirmed that the measured value of the differential transformer type displacement meter 3 decreased as the environmental temperature increased.
From this fact, when the same thickness is targeted for grinding using the differential transformer type displacement gauge 3, the higher the temperature in the processing chamber 4, the higher the wafer W reaches the target value before the grinding progresses. Since a measurement result indicating that the thickness has been reached is obtained, it can be estimated that the thickness of the wafer W is increased.

Next, the relationship between the supply amount of the grinding fluid and the temperature inside the processing chamber 4 was examined.
The double-sided grinding apparatus 1 having the temperature sensor attached to the signal output means 31 was prepared, and the wafer W was ground under the same conditions as in Experimental Example 1-1 with the same amount of grinding fluid supply as in Experimental Example 1-1. A change in temperature was measured every second (Experimental Example 2-1).
In addition, the wafer W was ground under the same conditions as in Experimental Example 2-1 except that the amount of the grinding fluid supplied was the same as in Experimental Example 1-3, and the temperature change during grinding was measured (Experimental Example 2- 2).
Table 1 shows the average values of the measurement results.
As shown in Table 1, the temperature of Experimental Example 2-2 was about 0.7° C. lower than that of Experimental Example 2-1. It is considered that the larger the amount of grinding liquid, the higher the cooling effect of the wafer W during grinding, and as a result, the temperature of the processing chamber 4 became lower in Experimental Example 2-2 in which the amount of supply was large.

From the results of FIG. 5 and Table 1, it is considered that the higher the temperature of the processing chamber 4, the thicker the wafer W after grinding.
From this, it was confirmed that the thickness of the wafer W after grinding differs from the target value due to the measurement error of the differential transformer type displacement gauge 3 when the supply amount of the grinding liquid is adjusted.

[Experiment 3]
From the results of Experiment 1, it was found that there is a possibility that the nanotopography of the wafer W can be improved by adjusting the supply amount of the grinding liquid to the first main surface W1 and the second main surface W2. Further, from the result of Experiment 2, it was confirmed that the thickness of the wafer W after grinding was different from the target value when the supply amount of the grinding liquid was adjusted.
Based on the results of Experiments 1 and 2, the inventor of the present invention, as a result of extensive research, found that while maintaining the total supply amount of the grinding fluid to the first and second main surfaces W1 and W2, the first main surface W1 By adjusting the ratio of the supply amount of the grinding liquid to the second main surface W2 and the supply amount of the grinding liquid to the second main surface W2, it is possible to obtain a wafer W with a desired thickness while improving the nanotopography of the wafer W. Therefore, the following experiment was conducted.

A double-headed grinding machine 1 identical to Experiment 2 and a wafer W having a thickness of about 870 μm and a diameter of 300 mm were prepared. Then, based on the conditions shown in Table 2 below, a double-sided grinding method having the same processing content as the related art is performed, and 10 wafers W are each ground, and the temperature change in the processing chamber 4 during grinding is measured. It was measured every second (Experimental Examples 3-1 to 3-3).
That is, in Experimental Examples 3-1 to 3-3, the total supply amount of the grinding fluid to the first and second main surfaces W1 and W2 was fixed at 2.8 L/min, and Adjusted the feed rate.

Table 3 shows the average values of the measurement results of the temperature inside the processing chamber 4 .
As shown in Table 3, the maximum temperature difference in Experimental Examples 3-1 to 3-3 is 0.1°C. It was confirmed that the temperature in the processing chamber 4 hardly changed even when the ratio of the supply amount to W2 was changed.

FIG. 7 shows the calculation result of the nanotopography at the center of the first principal surface W1 when the position of the outermost periphery of the first principal surface W1 is set to 0 nm.
As shown in FIG. 7, while maintaining the total supply amount of the grinding fluid to the first and second main surfaces W1 and W2, the ratio of the supply amount to the first and second main surfaces W1 and W2 is changed. Therefore, it was confirmed that nanotopography can be adjusted. In particular, by adjusting the ratio so that the supply amount of the grinding fluid to the recessed first main surface W1 is larger than the supply amount of the grinding fluid to the other second main surface W2, It was confirmed that the topography could be brought close to 0 nm.

FIG. 8 shows the center thickness of the wafer W and its average value.
As shown in FIG. 8, even if the ratio of the supply amount to the first and second main surfaces W1 and W2 is changed, the total supply amount of the grinding fluid to the first and second main surfaces W1 and W2 remains the same. It has been confirmed that the thickness of the wafer W is almost unchanged if there is.

From the results shown in FIGS. 7 and 8, while maintaining the total amount of grinding fluid supplied to the first and second main surfaces W1 and W2, the amount of grinding fluid supplied to the first main surface W1 and the second main surface W1 It was confirmed that a wafer W having a desired thickness can be obtained while improving the nanotopography of the wafer W by adjusting the ratio of the amount of grinding liquid supplied to the surface W2.

[Embodiment]
Next, a double-headed grinding method according to an embodiment of the present invention will be described.
First, a related art double-sided grinding machine 1, a first wafer _Wt as a first object to be ground, and a second wafer _Wp as a second object to be ground are prepared. The first wafer W _t and the second wafer W _p are substantially the same in material and shape, for example, from one silicon single crystal ingot, or different silicon single crystal ingots manufactured under the same manufacturing conditions. are extracted from each.

After the first wafer _Wt is set on the carrier ring 21, the control means 5 grinds the first wafer _Wt as shown in FIG. 9 (step S1: first grinding step ). The first wafer _Wt used in the first grinding process may be a dummy wafer for pre-grinding or a product wafer of the previous lot.
In this first grinding process, the differential transformer type displacement gauge 3 measures the thickness of the first wafer _Wt and outputs a signal corresponding to the measurement result to the control means 5 . The control means 5 supplies a predetermined amount of grinding fluid to the first and second main surfaces W1 and W2 of the first wafer _Wt , and moves the first wafer Wt based on the signal from the differential transformer type displacement gauge 3. When it is determined that the thickness of _Wt has been ground to a predetermined thickness, the grinding is terminated. The amount of grinding fluid supplied to the first and second main surfaces W1 and W2 in the first grinding process may be the same or different, but the total supply amount in the first grinding process is set to be the same as the total supply amount in the second grinding process described later.

Next, the operator measures the nanotopography of the first wafer Wt using a _{nanotopography} measuring instrument (not shown) (step S2: nanotopography measurement step).
Thereafter, the control means 5 grinds the second wafer _Wp set on the carrier ring 21 (step S3: second grinding step).
In this second grinding process, first, the operator sets grinding conditions such that the _{nanotopography} of the second wafer Wp approaches zero based on the measurement results in the nanotopography measurement process. Specifically, based on the _{nanotopography} of the center of the wafer W, the operator performs the While maintaining the total supply amount of the grinding liquid, the ratio between the supply amount of the grinding liquid to the first main surface W1 and the supply amount of the grinding liquid to the second main surface W2 is set.
For example, it is known that the higher the ratio of the supply amount to the first main surface W1, the smaller the recess amount of the first main surface _W1 . When the center of is recessed, the operator increases the ratio of the supply amount to the first main surface W1, and when the center of the first main surface W1 protrudes, the supply amount to the first main surface W1 reduce the ratio of Conversely, when the center of the second main surface W2 of the first wafer _Wt is recessed, the operator increases the ratio of the second main surface W2 to the center of the second main surface W2. protrudes, the ratio to the second main surface W2 is reduced. In other words, it is sufficient to increase the ratio of the amount of supply to the main surface having a recessed center. At this time, as for the supply ratio of the grinding fluid, the value obtained by dividing the supply amount of the higher ratio by the supply amount of the lower one is preferably 200% or less. For example, the supply amount of the higher ratio is 2 L/min. , the lower feed rate is preferably 1 L/min.

Then, the control means 5 controls the second wafer W _p under the same grinding conditions as in the pre-grinding step, except for the supply ratio of the grinding liquid to the first and second main surfaces W1 and W2, based on the setting of the operator. grinding.

[Action and effect of the embodiment]
According to the above embodiment, in the second grinding step, while maintaining the total amount of grinding liquid supplied to the first and second main surfaces W1 and W2 based on the _{nanotopography} of the first wafer Wt, The supply ratio of the grinding fluid to the first and second main surfaces W1, W2 is adjusted. In this way, by adjusting the supply ratio of the grinding liquid, the _{nanotopography} is improved and the total supply amount of the _grinding liquid is maintained. can be made almost the same as Therefore, it is possible to obtain a second wafer _Wp having a desired thickness with good nanotopography.
In particular, since the supply ratio is changed while maintaining the total supply amount of the grinding liquid, the temperature in the processing chamber 4 can be made substantially the same between the first grinding process and the second grinding process. Therefore, even if the differential transformer type displacement meter 3, which causes measurement errors due to the environmental temperature, is used, the thickness of the wafer W can be made substantially equal to the target value in both the first grinding process and the second grinding process. can do. Since the differential transformer type displacement meter 3 has a high measurement accuracy, the second wafer _Wp whose thickness is adjusted with higher accuracy can be obtained.

[Modification]
The present invention is not limited to the above-described embodiments, and various improvements and design changes are possible without departing from the scope of the present invention.

For example, the object to be ground may be a wafer other than silicon, or a disk-shaped object other than the wafer W, such as ceramics or stone.

Although the second grinding process was performed based on the operator's settings, the following may be performed.
First, in a state in which the total supply amount of the grinding fluid to the first and second main surfaces W1 and W2 is maintained at a predetermined amount, the amount of the grinding fluid supplied to the first main surface W1 and the second main surface are stored in the memory. Supply ratio adjustment information is stored that indicates how the nanotopography changes when the ratio of the supply amount of the grinding fluid to W2 is adjusted. For example, as in the result obtained in Experiment 3, the supply ratio adjustment information is stored such that the higher the ratio of the supply amount to the first main surface W1, the smaller the recess amount of the first main surface W1. back. At this time, the relationship between the material and size of the wafer W, the target thickness after grinding, the total supply amount of the grinding liquid, and the rotation direction of the wafer W and the first and second grinding wheels 23 and 24 It is preferable to store the supply ratio adjustment information with different contents in the second. The supply ratio adjustment information may be created based on experimental results using the double-headed grinding machine 1, or may be created by simulation.
Then, based on the _{nanotopography} of the first wafer Wt and the supply ratio adjustment information, the control means 5 may adjust the supply ratio so that the _{nanotopography} of the second wafer Wp approaches zero. .

REFERENCE SIGNS LIST 1 double-headed grinding device, 2 grinding means, 3 differential transformer type displacement gauge (thickness measuring means), 23, 24 first and second grinding wheels, 23B, 24B grindstone, 33 contactor, W... Wafer (object to be ground), _Wt ... First wafer (first object to be ground), _Wp ... First wafer (second object to be ground), W1... First main surface (one main surface), W2 . . . second main surface (the other main surface).

Claims (3)

The object to be ground is ground by rotating the object to be ground, supplying a grinding liquid to both main surfaces of the object to be ground, and bringing the grindstones of the grinding wheel into contact with both main surfaces of the object to be ground. A double-headed grinding machine comprising grinding means and thickness measuring means for measuring the thickness of the object to be ground is used, and the thickness of the object to be ground is determined to be a predetermined thickness based on the measurement result of the thickness measuring means. A double-headed grinding method for grinding until
a first grinding step of performing grinding until the thickness of the first object to be ground reaches the predetermined thickness while supplying a predetermined amount of grinding fluid to both main surfaces of the first object to be ground;
a nanotopography measuring step of measuring the nanotopography of the first object to be ground;
a second grinding step of performing grinding until the thickness of the second object to be ground reaches the predetermined thickness while supplying the predetermined amount of grinding liquid to both main surfaces of the second object to be ground; prepared,
In the second grinding step, the nanotopography of the second object to be ground is adjusted by an operator or a control means so that the nanotopography of the second object to be ground approaches 0 based on the measurement result of the nanotopography measurement step. A double-sided grinding method, characterized in that the grinding liquid is supplied according to the ratio of the amount of the grinding liquid supplied to one main surface of the workpiece to the amount of the grinding liquid supplied to the other main surface. In the double-headed grinding method according to claim 1,
The thickness measuring means has a pair of contactors that contact both main surfaces of the object to be ground, respectively, and outputs a signal corresponding to the position of the pair of contactors to measure the thickness of the object to be ground. A double-headed grinding method characterized by using a differential transformer type displacement gauge for measuring. In the double-headed grinding method according to claim 1 or claim 2,
In the second grinding step, based on the measurement result of the nanotopography measurement step of the first object to be ground, the main part of the recessed side of the first object to be ground in the second object to be ground is ground. A double-sided grinding method, wherein the ratio is adjusted so that the amount of grinding fluid supplied to one surface is larger than the amount of grinding fluid supplied to the other main surface.

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