WAFER GRINDING METHOD

Abstract

A wafer grinding method includes loading a wafer onto a first chuck, first lowering a spindle having a wheel from a first vertical position to a second vertical position, second lowering the spindle from the second vertical position to a third vertical position, grinding the wafer by the wheel, identifying the third vertical position of the spindle by determining whether amounts of change in an operation current of the spindle exceed a reference amount of change in current, calculating spindle feedback displacement, and applying the spindle feedback displacement to the second vertical position of the first chuck.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2023-0145095, filed on Oct. 26, 2023, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety.

BACKGROUND

Aspects of the inventive concept relate to a wafer grinding method, and more particularly, to a wafer grinding method using spindle position feedback.

A process of manufacturing semiconductor chips includes a wafer grinding process. During the wafer grinding process, a wafer may be loaded onto a chuck, and the wafer may be ground and polished by a wheel. Since wafer grinding and polishing are performed within a fine numerical range, the moving speed of a spindle that moves the wheel in contact with the wafer may be limited.

The position (e.g., with respect to a vertical direction) of the wafer may differ from a reference position of the wafer for various reasons. When the wafer is located lower than the reference position, it takes more time for the wheel to approach the wafer than when the wafer is located at the reference position, which may adversely affect production volume per unit time.

SUMMARY

Aspects of the inventive concept provide a wafer grinding process having improved productivity and reliability.

The aspects of the inventive concept are not limited to the object mentioned above, but other aspects not described herein will be clearly understood by those skilled in the art from the following description.

According to an aspect of the inventive concept, there is provided a wafer grinding method including loading a wafer onto a first chuck, first lowering a spindle having a wheel from a first vertical position to a second vertical position, second lowering the spindle from the second vertical position to a third vertical position, grinding the wafer by the wheel, identifying the third vertical position of the spindle by determining whether amounts of change in an operation current of the spindle exceed a reference amount of change in current, calculating spindle feedback displacement, and applying the spindle feedback displacement to the second vertical position of the first chuck, wherein the third vertical position of the spindle is defined as a vertical position at which the wheel comes into contact with the wafer, and the spindle feedback displacement is defined as a difference between a reference vertical position of a reference chuck and the third vertical position of the spindle, and the reference vertical position of the reference chuck is defined as the third vertical position of the reference chuck.

According to another aspect of the inventive concept, there is provided a wafer grinding method including loading a wafer onto a first chuck, first lowering a spindle having a wheel from a first vertical position to a second vertical position, second lowering the spindle from the second vertical position to a third vertical position, grinding the wafer by the wheel, identifying the third vertical position of the spindle by determining whether amounts of change in an operation current of the spindle exceed a reference amount of change in current, calculating spindle feedback displacement, and applying the spindle feedback displacement to the second vertical position of the first chuck, wherein the third vertical position of the spindle is defined as a vertical position at which the wheel comes into contact with the wafer, the spindle feedback displacement is defined as a difference between a reference vertical position of the first chuck and the third vertical position of the spindle, and the reference vertical position of the first chuck is defined as the third vertical position of the first chuck during a previous grinding process.

According to another aspect of the inventive concept, there is provided a wafer grinding method including loading a wafer onto a first chuck, first lowering a spindle having a wheel from a first vertical position to a second vertical position, second lowering the spindle from the second vertical position to a third vertical position, grinding the wafer by the wheel, identifying the third vertical position of the spindle by determining whether amounts of change in an operation current of the spindle exceed a reference amount of change in current, calculating spindle feedback displacement, and applying the spindle feedback displacement to the second vertical position of the first chuck, wherein the third vertical position of the spindle is defined as a vertical position at which the wheel comes into contact with the wafer, the spindle feedback displacement is calculated by adding an amount of wear of the wheel to a difference between a reference vertical position of a reference chuck and the third vertical position of the spindle, and the reference vertical position of the reference chuck is defined as the third vertical position of the reference chuck, identifying as the third vertical position of the spindle, a position of the spindle at the moment when an amount of change in the operation current of the spindle, which first exceeds the reference amount of change in current among the amounts of change in the operation current of the spindle, occurs, the reference amount of change in current is a positive number and three times a current change standard deviation, which is a standard deviation of the amounts of change in the operation current of the spindle, the current change standard deviation is calculated based on the amounts of change in the operation current of the spindle from a time the spindle reaches the second vertical position to a time the grinding of the wafer is completed, and a number of rotations per unit time of the wheel is maintained constant after reaching the second vertical position of the spindle.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a plan view of a wafer grinding device according to an embodiment;

FIG. 2 illustrates a cross-sectional view of the wafer grinding device according to an embodiment;

FIG. 3 illustrates a cross-sectional view of the wafer grinding device with respect to operation of a wafter grinding method according to an embodiment;

FIG. 4 is a flowchart for describing the wafer grinding method according to an embodiment;

FIG. 5 is a graph for describing the wafer grinding method according to an embodiment;

FIG. 6 is a graph for describing the wafer grinding method according to an embodiment; and

FIG. 7 is a graph for describing the wafer grinding method according to an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the inventive concept are described in detail with reference to the accompanying drawings.

The embodiments of the inventive concept are provided to more fully describe the inventive concept to those skilled in the art. The following embodiments may be modified in many different forms, and the scope of the inventive concept is not limited to the following embodiments. Rather, these embodiments are provided so that the inventive concept will be thorough and complete, and will fully convey the inventive concept to those skilled in the art. Also, in the drawings, the thickness and size of each layer are exaggerated for convenience and clarity of illustration.

FIG. 1 a plan view of a wafer grinding device according to an embodiment. FIG. 2 illustrates a cross-section view of the wafer grinding device according to an embodiment. FIG. 3 illustrates a cross-sectional view of the wafer grinding device with respect to operation of a wafter grinding method laccording to an embodiment.

Referring to FIGS. 1 to 3, a wafer grinding device may include a plurality of chucks, a spindle SP, a wheel WH, and a controller 200.

As used herein, a first direction (e.g., a first horizontal) may refer to an X-direction and a second direction (e.g., a second horizontal) may refer to a Y-direction. The first direction and the second direction may be perpendicular to each other. A third direction (e.g., a vertical direction) may refer to a Z-direction, and the third direction may be perpendicular to both the first direction and the second direction. A horizontal plane or a plane refers to an X-Y plane. An upper surface of a specific object refers to a surface located in the positive third direction with respect to the specific object and a lower surface of a specific object refers to a surface located in the negative third direction with respect to the specific object.

As illustrated in FIG. 2, a plurality of wafers (i.e., W_A, W_B, W_C, and W_D) may be subject to wafer grinding. The plurality of wafer may be respectively disposed on the plurality of chucks. The plurality of chucks may include a first chuck CH_A, a second chuck CH_B, a third chuck CH_C, and a fourth chuck CH_D. In this specification, a case in which a plurality of chucks include four chucks is described as an example, but the embodiment is not limited thereto.

The spindle SP may include a wheel WH at one end of the spindle SP. The wheel WH may polish and grind each of the wafers loaded on (e.g., disposed on) the plurality of chucks. A plurality of blades may be provided on the lower surface of the wheel WH. Each of the blades may have certain roughness. When the roughness of the blade is high, the wafer may be polished more during the same amount of time compared to the blades having lower roughness. The roughness of the blade may be generally affected by the average size of abrasive particles in the blade or may vary depending on the shape of the abrasive particles or a state in which the abrasive particles are arranged.

The blade may include abrasive particles and binder. The abrasive particles may include, for example, diamond, cubic boron nitride (CBN), calcium carbonate, emery, novaculite, ferric oxide, ceramic, alumina, glass, silica, silicon carbide, or zirconia. In an embodiment, the abrasive particles may include diamond. The abrasive particles may be arranged while mixed with the binder. For example, the binder may surround the abrasive particles.

The abrasive particles may include crushed single crystalline diamond. In some embodiments, the abrasive particles may include particles obtained by crushing polycrystalline natural diamond, sintered polycrystalline diamond, or polycrystalline artificial diamond. The plurality of abrasive particles in the blade may have an average particle size of about 300 to about 1000 mesh.

The roughness of the plurality of blades in the wheel WH, the shape of the plurality of blades in the wheel WH, the number of the plurality of blades, and the arrangement of the plurality of blades may vary depending on the purpose of grinding and workpieces to be ground.

The spindle SP is configured to perform translational motion in the first direction, the second direction, and the third direction while the spindle SP rotates the wheel WH. That is, as shown in FIG. 2, the spindle SP may move in the first direction and the second direction so that the wheel WH is positioned above a first wafer W_A. In addition, the spindle SP may move in the third direction such that the wheel WH approaches the first wafer W_A in the vertical direction, which is the third direction. Accordingly, the wheel WH may come into contact with the first wafer W_A and grind the first wafer W_A. As shown in FIG. 2, the distance in the third direction between the lower surface of the wheel WH and the upper surface of the first wafer W_A may be referred to as a vertical distance HZ.

When the wheel WH rotates to grind the wafer, the wafer may rotate in the same direction as the wheel WH or rotate in a different direction from the wheel WH, or the wafer may be in a stationary state. For example, as shown in FIG. 1, the wheel WH may rotate counterclockwise, and the first wafer W_A may rotate clockwise, which is opposite to the rotation direction of the wheel WH, due to the rotation of the first chuck CH_A. However, aspects of the inventive concept are not limited by the rotation directions of the wheel WH and the wafer.

The controller 200 may be connected to each of the plurality of chucks and control the rotation movement and operation of the plurality of chucks. The controller 200 may be connected to the spindle SP and control rotational and translational motion of the spindle SP.

As shown in FIG. 3, the process of moving the spindle SP to grind the wafer may include starting high-speed lowering, lowering the spindle SP in a high-speed lowering section, lowering the spindle SP in an air cut section, and reaching a grinding section. In FIG. 3, a case in which the first wafer W_A is ground by the spindle SP while the first wafer W_A is placed on the first chuck CH_A is described as an example. As used herein, the air cut section refers to a section (e.g., a vertical distance) between the surface of a wafer and a reference height (i.e., in the vertical direction) above the surface of the wafer. The operation (e.g., rotational speed, a movement speed in the vertical direction, etc.) of the spindle SP is different than in other sections (e,g., a vertical distance above the air cut section). As illustrated in FIG. 3, second section R2 represents the air cut section and first section R1 represents a section different from the second section R2. For example, first section R1 represents a vertical distance above the air cut section.

The spindle SP may be lowered in the vertical direction through the first section R1 and, subsequently, the second section R2 (i.e., the air cut section). The speed at which the spindle SP is lowered through the first section R1 (i.e., first lowering speed) is greater than the speed at which the spindle SP is lowered through the second section R2 (i.e., second lowering speed). The first section R1 may extend from a first vertical position to a second vertical position. For example, the first vertical position may refer to a point at which the spindle SP is placed above the first wafer W_A and begins to descend for grinding. Alternatively, the first vertical position may refer to a point at which the wheel WH begins to rotate while the spindle SP is placed above the first wafer W_A. Aspects of the inventive concept are not limited by the definitions of the first vertical position.

The second vertical position may refer to a position of a start point PS of the air cut section (i.e., the second section R2). The second section R2 may have the second vertical position as the start point PS. The second section R2 may refer to a section in which the air cut is performed, and the air cut may refer to a section in which the wheel WH is lowered toward the wafer in a state, in which the wheel WH is not in contact with the wafer, and then the spindle SP moves assuming that the wheel WH is about to come into contact with the wafer.

The descending speed of the spindle SP may be different between the first section R1 and the second section R2. For example, in the second section R2, the spindle SP may move downward at a speed of about 0.5 μm per second to about 1 μm per second. However, in the first section R1 in which the spindle SP begins to descend at high speed and reaches the air cut section, the spindle SP may move downward at a speed greater than that in the second section R2. For example, in the first section R1, the spindle SP may move downward at a speed of about 1 μm per second to about 4 μm per second.

As discussed above, the descending (i.e., lowering) speed of the spindle SP in the first section R1 is different from the descending (i.e., lowering) speed of the spindle SP in the second section R2. This is because the spindle SP in the second section R2 is likely to come into contact with the wafer, unlike the first section R1. When the spindle SP in the first section R1 is lowered at high speed and comes into contact with the wafer, the equipment stability, wafer quality, and yield may be significantly adversely affected.

The plurality of chucks in wafer grinding device are intended to be located at substantially the same vertical level in the third direction. That is, the first chuck CH_A, the second chuck CH_B the third chuck CH_C, and the fourth chuck CH_D are intended to have substantially the same vertical level in the third direction. However, due to various reasons including mechanical errors and device setting errors, there may be slight differences in the vertical levels between the plurality of chucks. For example, a difference in the vertical levels between the upper surface of the first wafer W_A disposed on the first chuck CH_A and the upper surface of a second wafer W_B disposed on the second chuck CH_B may be about 5 μm to about 10 μm. Aspects of the inventive concept are not limited by the numerical examples of the differences in vertical levels.

As described above, the spindle SP in the second section R2 moves downward at a speed of about 0.5 μm per second to about 1 μm per second as the descending speed. For example, the vertical level of the upper surface of the second wafer W_B may be lower than that of the first wafer W_A, the difference in the vertical levels between the upper surface of the first wafer W_A and the upper surface of the second wafer W_B may be 10 μm, and the descending speed of the spindle SP in the second section R2 is 0.5 μm per second. Under the above conditions, the time for the wheel WH to come into contact with the second wafer W_B and grind the second wafer W_B may be about 20 seconds later than the time for the wheel WH to come into contact with the first wafer W_A and grind the first wafer W_A. That is, due to a slight difference in the vertical levels between the plurality of chucks, the time required to grind the wafer may increase. Accordingly, the productivity of the wafer grinding device may deteriorate.

On the contrary, for example, the vertical level of the upper surface of the second wafer W_B may be higher than that of the upper surface of the first wafer W_A, and the difference in the vertical levels between the upper surface of the first wafer W_A and the upper surface of the second wafer W_B may be 8 μm. At the same time, when the distance of the second section R2 is m, the spindle SP is lowered by 2 μm. Accordingly, the wheel WH may reach the upper surface of the second wafer W_B relatively quickly, but the second wafer W_B may be located close to the start point PS of the second section R2. As a result, there is a risk that the wheel WH and the second wafer W_B may collide with each other when the spindle SP in the first section R1 is lowered at high speed. Accordingly, the reliability of the wafer grinding device may deteriorate.

FIG. 4 is a flowchart for describing the wafer grinding method 1 according to an embodiment. FIG. 5 is a graph for describing the wafer grinding method 1 according to an embodiment. FIG. 6 is a graph for describing the wafer grinding method 1 according to an embodiment. Repeated descriptions as those given above may be omitted.

Referring to FIGS. 4 and 5, the wafer grinding method 1 according to an embodiment may include loading a wafer onto a first chuck (S110); first lowering a spindle having a wheel from a first vertical position to a second vertical position (S120); second lowering the spindle from the second vertical position to a third vertical position (S130); grinding the wafer by the wheel (S140); determining whether an amount (ΔIsp) of change in the operation current of the spindle (hereinafter, referred to as a spindle operation current change ΔIsp) exceeds a reference value (S200); calculating spindle feedback displacement ΔZsp (S210); and applying the spindle feedback displacement to the second vertical position of the first chuck (S220).

When the spindle operation current change ΔIsp does not exceed the reference value during the determining of whether the spindle operation current change ΔIsp exceeds the reference value (S200), the wafer may be loaded again onto the first chuck (S110) without calculating the spindle feedback displacement ΔZsp (S210) and applying the spindle feedback displacement to the second vertical position of the first chuck (S220). Alternatively, when the spindle operation current change ΔIsp exceeds the reference value during the determining of whether the spindle operation current change ΔIsp exceeds the reference value (S200), the wafer may be loaded again onto the first chuck (S110) after calculating the spindle feedback displacement ΔZsp (S210) and applying the spindle feedback displacement to the second vertical position of the first chuck (S220).

Hereinafter, in describing the wafer grinding method 1 according to an embodiment, the first chuck CH_A is described as an example of a reference chuck, and the second chuck CH_B is described as an example of a first chuck during the loading of the wafer on the first chuck (S110). Also, the process of calculating the spindle feedback displacement ΔZsp for each of the chucks is described with reference to FIG. 6.

As described above, the first vertical position may refer to a point at which the spindle SP is placed above the first wafer W_A and begins to descend for grinding. The second vertical position may refer to the position of the start point PS of the air cut section. The third vertical position refers, to the end point PE, which is the vertical position of the lower surface of the wheel WH at the moment the wheel WH comes into contact with the wafer.

Spindle feedback displacement ΔZsp may represent the difference between a reference vertical position and the third vertical position. The reference vertical position may refer to the third vertical position of a reference chuck that serves as a setting reference among the plurality of chucks.

As shown in FIG. 5, the section between the second vertical position and the upper surface of the first wafer W_A disposed on the first chuck CH_A may be defined as a reference air cut section AC1R. That is, a user of the wafer grinding device may set the reference for vertical positions on the basis of the first chuck CH_A. For example, the reference air cut section AC1R may be 10 μm. Wafer thicknesses TW, which represent the vertical thicknesses of the first wafer W_A, the second wafer W_B, a third wafer W_C, and a fourth wafer W_D, may be substantially equal to each other.

The reference air cut section AC1R may refer to a section between a reference start point RS and a reference end point RE. That is, the second section R2, with respect to the first chuck CH_A as the reference chuck, may refer to the reference air cut section AC1R.

Beyond the reference start point RS and the reference end point RE of the reference air cut section AC1R, the top surface of the second chuck CH_B may be located below top surface of the first chuck CH_A. Accordingly, the upper surface of the second wafer W_B disposed on the second chuck CH_B may be located below the reference end point RE by second spindle feedback displacement ΔZspB. Since the upper surface of the second wafer W_B is lower than the reference end point RE, the second spindle feedback displacement ΔZspB may have a negative number.

Similarly, beyond the reference start point RS and the reference end point RE of the reference air cut section AC1R, the top surface of the third chuck CH_C may be located below the top surface of the first chuck CH_A. Accordingly, the upper surface of the third wafer W_C disposed on the third chuck CH_C may be located below the reference end point RE by third spindle feedback displacement ΔZspC. Since the upper surface of the third wafer W_C is lower than the reference end point RE, the third spindle feedback displacement ΔZspC may have a negative number. In addition, the absolute value of the third spindle feedback displacement ΔZspC may be greater than the absolute value of the second spindle feedback displacement ΔZspB. For example, the second spindle feedback displacement ΔZspB may be −3 μm and the third spindle feedback displacement ΔZspC may be −5 μm.

Between the reference start point RS and the reference end point RE of the reference air cut section AC1R, the top surface of the fourth chuck CH_D may be located above the top surface of the first chuck CH_A. Accordingly, the upper surface of the fourth wafer W_D disposed on the fourth chuck CH_D may be located above the reference end point RE by fourth spindle feedback displacement ΔZspD. Since the upper surface of the fourth wafer W_D is higher than the reference end point RE, the fourth spindle feedback displacement ΔZspD may have a positive number. For example, the fourth spindle feedback displacement ΔZspD may be +2 μm.

The spindle feedback displacements may be respectively applied to the second vertical positions of the spindle SP for the chucks. For example, the second spindle feedback displacement ΔZspB may be added to the second vertical position with respect to the second chuck CH_B. Accordingly, as shown in the right diagram of FIG. 5, a second air cut section AC2 of the second chuck CH_B may start at a second start point RS2 lower than the reference start point RS of the reference air cut section AC1R. The spindle SP may be lowered at a high speed by the reduced distance of the second air cut section AC2. Accordingly, the time for which the spindle SP moves to grind the second wafer W_B may be shortened. For example, when the high-speed lowering of the spindle SP in the first section R1 is at a speed of 2 μm per second and the lowering of the spindle SP in the second section R2 of the first chuck CH_A is at a speed of 0.5 μm per second, the time for which the spindle SP moves may be reduced by 4.5 seconds assuming that the second spindle feedback displacement ΔZspB is −3 μm. Specifically, given the example of the second spindle feedback displacement ΔZspB is −3 μm, a speed of 2 μm per second would be utilized to traverse the second spindle feedback displacement ΔZspB (i.e., −3 μm) as oppose to a speed of 0.5 μm per second.

The third spindle feedback displacement ΔZspC may be added to the second vertical position with respect to the third chuck CH_C. Accordingly, as shown in the right diagram of FIG. 5, a third air cut section AC3 of the third chuck CH_C may start at a third start point RS3 lower than the reference start point RS of the reference air cut section AC1R. The spindle SP may be lowered at a high speed by the reduced distance of the third air cut section AC3. Accordingly, the time for which the spindle SP moves to grind the third wafer W_C may be shortened. For example, when the high-speed lowering of the spindle SP in the first section R1 is at a speed of 2 μm per second and the lowing of the spindle SP in the second section R2 of the third chuck CH_C is at a speed of 0.5 μm per second, the time for which the spindle SP moves may be reduced by 7.5 seconds assuming that the third spindle feedback displacement ΔZspC is −5 μm. Specifically, given the example of the second spindle feedback displacement ΔZspC is −5 μm, a speed of 2 μm per second would be utilized to traverse the second spindle feedback displacement ΔZspC (i.e., −5 μm) as oppose to a speed of 0.5 μm per second. Therefore, since the time for which the spindle SP moves to the wafer is reduced, the productivity of wafer grinding may be improved by the wafer grinding method 1 according to an embodiment.

The fourth spindle feedback displacement ΔZspD may be added to the second vertical position with respect to the fourth chuck CH_D. Accordingly, as shown in the right diagram of FIG. 5, a fourth air cut section AC4 of the fourth chuck CH_D may start at a fourth start point RS4 lower than the reference start point RS of the reference air cut section AC1R. In this case, the time for which the spindle SP moves may increase, but the air cut section may be secured within a range that does not exceed the required air cut section. If the required air cut section is secured, the possibility of the wheel WH colliding with the wafer when the spindle SP is lowered at high speed before reaching the air cut section may be reduced or eliminated. Therefore, the reliability of wafer grinding may be improved by the wafer grinding method 1 according to an embodiment.

If an excessive spindle feedback displacement is applied to the second vertical position of each chuck, the spindle may come into high-speed contact with the wafer before entering the air-cut section. To prevent this, the excessive spindle feedback displacement may not be reflected in the second vertical position. For example, if the magnitude of the second spindle feedback displacement ΔZspB derived from the second chuck CH_B is greater than the height difference between the second vertical position and the third vertical position, the feedback displacement may not be reflected in each of the chucks.

Referring to FIG. 6, the wheel WH may be rotated by the spindle SP that has been supplied with current and may grind the wafer. For example, the controller 200 supplies a spindle operation current Isp to the spindle SP, and at the same time, the controller 200 records and stores the spindle operation current Isp and processes a numerical value of the spindle operation current Isp. Accordingly, the controller 200 may obtain a numerical value derived from the spindle operation current Isp. For example, the controller 200 may calculate and store the spindle operation current Isp, the spindle operation current change ΔIsp, and a standard deviation (asp) of the amounts of change in current (hereinafter, referred to as a current change standard deviation asp) based on the spindle operation current change ΔIsp.

The controller 200 may be provided inside the spindle SP or may be configured as a separate device. The controller 200 may be configured by hardware, software, or both the hardware and software. The controller 200 may be provided in the form of hardware or software or in the form of a plurality of pieces of hardware or software. A function (processing) may be performed by a computer having a central processing unit (CPU) or memory. For example, a program for performing a method (a control method) according to embodiments may be stored in a storage device, and each function may be performed by executing the program stored in the storage device on the CPU.

The program may be stored and supplied to a computer using various types of non-transitory computer readable media. The non-transitory computer-readable media include various types of tangible storage media. The non-transitory computer readable media include, for example, magnetic recording media (e.g. flexible discs, magnetic tapes, and hard disk drives), magneto-optical recording media (e.g., optical magnetic discs), compact disc read-only memory (CDROM), compact disc readable (CD-R), compact disc rewritable (CD-R/W), and semiconductor memory (e.g., mask ROM, programmable ROM (PROM), erasable PROM (EPROM), flash ROM, and random access memory (RAM)). Also, the program may be supplied to the computer via various types of transitory computer readable media.

The upper graph of FIG. 6 shows the spindle operation current Isp and the lower graph of FIG. 6 shows the spindle operation current change ΔIsp. The X-axis of the graph in FIG. 6 represents time in units of seconds. The Y-axis of the upper graph of FIG. 6 represents current in units of mA. The Y-axis of the lower graph of FIG. 6 represents the amount of change in current in units of mA/sec. In each of the upper graph and lower graph of FIG. 6, the time interval between a point indicated by data and a point adjacent thereto is 0.5 seconds.

For the wafer grinding method 1 according to an embodiment, FIG. 6 illustrates a method of calculating the second spindle feedback displacement ΔZspB for the second chuck CH_B when the first chuck CH_A is set as the reference chuck. The graph of the spindle operation current Isp and the graph of the spindle operation current change ΔIsp in FIG. 6 may represent a graph of a grinding process for the second chuck CH_B.

A second end point of the second air cut section AC2, which is the actual air cut section of the second chuck CH_B, may be lowered in the vertical direction than the reference end point RE of the reference air cut section AC1R that is based on the first chuck CH_A. As described in the above example, the second end point of the second air cut section AC2, which is the actual air cut section of the second chuck CH_B, may be lowered by −3 μm in the vertical direction than the reference end point RE of the reference air cut section AC1R. That is, the time it takes for the spindle SP to descend in the second air cut section AC2 may be greater than the time it takes for the spindle SP to descend in the reference air cut section AC1R.

For a stable grinding process of wafers, the spindle SP may maintain the constant number of rotations per unit time so that the wheel WH rotates uniformly. When the wheel WH is in contact with the upper surface of the wafer, resistance due to contact with the wafer may occur in the wheel WH. Despite the resistance occurring in the wheel WH due to contact with the wafer, the spindle SP may maintain the constant number of rotations per unit time so that the wheel WH rotates uniformly. Accordingly, the current supplied to the spindle SP may increase. For example, the average value of air cut current lac in a section in which the spindle SP rotates without contacting the wafer may be about 7200 mA and the average value of grinding operation current Igr in a grinding section in which the spindle SP rotates in contact with the wafer may be about 13800 mA.

When the grinding process is performed in the air cut section, the spindle operation current Isp may increase as shown in the upper graph of FIG. 6. The spindle operation current change ΔIsp calculated from the spindle operation current Isp increases significantly at the moment the spindle SP comes into contact with the upper surface of the second wafer W_B. That is, the contact of the spindle SP with the upper surface of the second wafer W_B may be confirmed using the spindle operation current change ΔIsp.

For example, in the lower graph of FIG. 6 showing the spindle operation current change ΔIsp, the spindle operation current change ΔIsp is calculated up to a value of about 1920 mA/s, which is close to 2000 mA/s. In the controller 200, the position of the spindle SP which is adjacent to a reference end point RE of the reference air cut section AC1R and at which the spindle operation current change ΔIsp significantly increases may be determined as the actual position at which the wheel WH comes into contact with the upper surface of the second wafer W_B and begins to grind the second wafer W_B. That is, the third vertical position may represent the position of the spindle SP, which is adjacent to the reference end point RE of the reference air cut section AC1R and at which the spindle operation current change ΔIsp significantly increases.

That is, the controller 200 determines whether the spindle operation current change ΔIsp exceeds a reference amount (ΔIr) of change in current (hereinafter, referred to as a reference current change ΔIr). When the spindle operation current change ΔIsp exceeds the reference current change ΔIr, the controller 200 may determine the third vertical position, at which the grinding process starts, on the basis of the spindle operation current change ΔIsp.

The reference current change ΔIr may be determined by a standard deviation (a) of the amounts of change in current (hereinafter, referred to as a current change standard deviation a) that is calculated based on the spindle operation current change ΔIsp. For example, the reference current change ΔIr may be set to a value that is three times the current change standard deviation a. In FIG. 6, the current change standard deviation a is 291 mA/s. Therefore, the reference current change ΔIr may be 873 mA/s. For example, the current change standard deviation a may be calculated from values of the spindle operation current change ΔIsp, from the time the spindle SP reaches the second vertical position of the spindle SP to the time the grinding process of the second wafer W_B is completed, among the values of the spindle operation current change ΔIsp. That is, the current change standard deviation a may be calculated from some values of the spindle operation current change ΔIsp other than values of the spindle operation current change ΔIsp which are relatively less related to the grinding of the second wafer W_B.

In the lower graph of FIG. 6 showing the spindle operation current change ΔIsp, a value exceeding three times the current change standard deviation a, which is defined as the reference current change ΔIr, is observed at three points. In the controller 200, a point appearing first among the three points having the values exceeding three times the current change standard deviation a defined as the reference current change ΔIr may be identified as a point at which the wheel WH and the second wafer W_B come into contact with each other.

In the spindle operation current change ΔIsp, a value which is a negative number and exceeds three times the current change standard deviation a may be calculated. This value of the spindle operation current change ΔIsp may cause an error in determining the third vertical position at which the grinding starts. Accordingly, only positive values of the spindle operation current change ΔIsp may be used to determine the third vertical position of the spindle SP. That is, there is a point having a value of the spindle operation current change ΔIsp, which is a positive number and exceeds the reference current change ΔIr for the first time among the values of the spindle operation current change ΔIsp. The controller 200 may identify this point as the point at which the wheel WH and the second wafer W_B come into contact with each other.

In addition, there is a point having a value of the spindle operation current change ΔIsp, which is a positive number and exceeds the reference current change ΔIr for the first time among the values of the spindle operation current change ΔIsp within 60 seconds from the time the spindle SP reaches the second vertical position. This point may be identified as the point at which the wheel WH and the second wafer W_B come into contact with each other. The above condition of within 60 seconds from the time the spindle SP reaches the second vertical position may prevent determining whether the spindle operation current change ΔIsp exceeds the reference current change ΔIr in a section in which the wheel WH and the second wafer W_B are unlikely to come into contact with each other. Therefore, the possibility that an incorrect position is calculated as the point at which the wheel WH and the second wafer W_B come into contact with each other may be reduced.

The speed at which the spindle SP descends and moves in the air cut section is generally constant, or the speed at which the spindle SP descends and moves is recorded by the controller 200. Therefore, the second spindle feedback displacement ΔZspB may be calculated from a second spindle movement time difference ΔTspB which is defined as a time difference between the reference end point RE of the reference air cut section AC1R and the time point at which the wheel WH and the second wafer W_B come into contact with each other. For example, when the average descending speed of the spindle SP moving through the second air cut section AC2 is 0.5 μm per second and the second spindle movement time difference ΔTspB is 3 seconds as shown in FIG. 6, −1.5 μm, which is obtained by multiplying these two values, may represent the value of the second spindle feedback displacement ΔZspB.

After the second spindle feedback displacement ΔZspB is calculated, the second spindle feedback displacement ΔZspB may be added to a second start point RS2, which is a start point of the second air cut section AC2 of the second chuck CH_B. For example, as shown in FIG. 5, the second spindle feedback displacement ΔZspB having a negative value is reflected in the second start point RS2, and the second air cut section AC2 of the second chuck CH_B may start from the second start point RS2 which is defined by changing the position of a first start point RS1 of the reference chuck, which is the same as the reference start point RS, by the second spindle feedback displacement ΔZspB. Therefore, since the time for which the spindle SP moves in the second air cut section AC2 is reduced, the productivity of wafer grinding may be improved by the wafer grinding method 1 according to an embodiment.

Similarly, the third spindle feedback displacement ΔZspC having a negative value is reflected in a third start point RS3, and the third air cut section AC3 of the third chuck CH_C may start from the third start point RS3 which is defined by changing the position of the first start point RS1 of the first chuck CH_A, which is the reference chuck, by the third spindle feedback displacement ΔZspC.

The fourth spindle feedback displacement ΔZspD having a positive value is reflected in a fourth start point RS4, and the fourth air cut section AC4 of the fourth chuck CH_D may start from the fourth start point RS4 which is defined by changing the position of the first start point RS1 of the first chuck CH_A, which is the reference chuck, by the fourth spindle feedback displacement ΔZspD. That is, with respect to the fourth chuck CH_D, the fourth air cut section AC4 of the fourth chuck CH_D may be secured, which is an air cut section that is not excessive but has a sufficient distance. Accordingly, the risk of collision between the wheel WH and the fourth wafer W_D when the spindle SP is lowered at high speed is reduced. The reliability of the wafer grinding device may be improved by the wafer grinding method 1 according to an embodiment.

As the wafer is ground, the wheel WH may be worn. In order to more accurately calculate and apply the spindle feedback displacement ΔZsp, the expected amount of wear of the wheel WH may be added to each of the above-described second to fourth spindle feedback displacements ΔZspB, ΔZspC, and ΔZspD.

For example, as shown in FIGS. 1 and 5, four chucks are provided, and grinding of the wafer disposed on the second chuck CH_B is performed. Subsequently, grinding of the three wafers respectively arranged on the other three chucks (i.e., CH_C, CH_D, CH_A) may be performed, and then grinding of the wafer disposed on the second chuck CH_B may be performed again. That is, grinding may be performed four times, and then grinding on the second chuck CH_B may be performed again. In this case, the expected amount of wear of the wheel WH for four grinding processes is calculated, and the amount of wear of the wheel WH during four times may be added to the second spindle feedback displacement ΔZspB of the second chuck CH_B. Accordingly, the position of the second start point RS2 with respect to the second chuck CH_B may be determined by considering the amount of wear of the wheel WH. For example, the amount of wear of the wheel WH may be a value based on a predicted value or may be a value measured using a separate device.

FIG. 7 is a graph for describing a wafer grinding method 1A according to an embodiment. Repeated descriptions as those given above may be omitted.

Referring to FIG. 7, the reference chuck may not be set separately, unlike the wafer grinding method 1 according to an embodiment described with reference to FIG. 5. For convenience of description, calculation of spindle feedback displacement ΔZsp for each of a plurality of chucks is described in a state in which all of a first start point RS1 of a spindle SP of a first chuck CH_A, a second start point RS2 of the spindle SP of a second chuck CH_B, a third start point RS3 of the spindle SP of a third chuck CH_C, and a fourth start point RS4 of the spindle SP of a fourth chuck CH_D are equal to each other.

Similarly, the case, in which all of a first end point RE1 of the spindle SP of the first chuck CH_A, a second end point RE2 of the spindle SP of the second chuck CH_B, a third end point RE3 of the spindle SP of the third chuck CH_C, and a fourth end point RE4 of spindle SP of fourth chuck CH_D are equal to each other, is given as an example.

That is, without the reference chuck, the spindle feedback displacement ΔZsp is calculated based on the first start point RS1 to the fourth start point RS4 corresponding to the first chuck CH_A to the fourth chuck CH_D, respectively, in a previous process (i.e., a previous grinding process), and the spindle feedback displacement ΔZsp may be applied to a first start point RS1, a second start point RS2, a third start point RS3, and a fourth start point RS4, which are for a subsequent grinding process.

The upper surface of the first wafer W_A disposed on the first chuck CH_A may be located below the position of the first end point RE1. As described above, in order for the spindle SP to efficiently grind the first wafer W_A, a first air cut section AC1 of the first chuck CH_A may be increased by an amount of first spindle feedback displacement ΔZspA.

Through the same process as described with reference to FIG. 6, the point at which the wheel WH and the first wafer W_A come into contact with each other may be identified. Therefore, the first spindle feedback displacement ΔZspA may be calculated, and the first spindle feedback displacement ΔZspA may be reflected in the first start point RS1. Accordingly, as shown on the right side of FIG. 7, the first air cut section AC1 of the first chuck CH_A may start from the first start point RS1 that is changed by the first spindle feedback displacement ΔZspA.

Therefore, since the time for which the spindle SP moves in the second air cut section AC2 is reduced, the productivity of wafer grinding may be improved by the wafer grinding method 1A according to an embodiment.

Regarding the second chuck CH_B to the fourth chuck CH_D, the spindle feedback displacement ΔZsp may be calculated based on the second start point RS2 to the fourth start point RS4 in the previous process, which is similar to the first chuck CH_A. That is, spindle feedback displacement ΔZsp is calculated separately from the first chuck CH_A to the fourth chuck CH_D, and the spindle feedback displacement ΔZsp may be applied to the start point of the air cut section, which is the second section of the spindle SP, for each of the first chuck CH_A to the fourth chuck CH_D. A detailed description of calculating the spindle feedback displacement ΔZsp for the second chuck CH_B to the fourth chuck CH_D overlaps with that described above and may be thus omitted. Also, a detailed description of calculating the spindle feedback displacement ΔZsp for the first chuck CH_A overlaps with the detailed description of calculating the spindle feedback displacement ΔZsp for the second chuck CH_B to the fourth chuck CH_D and may be thus omitted.

While aspects of the inventive concept have been particularly shown and described with reference to embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the following claims.

Claims

1. A wafer grinding method comprising: loading a wafer onto a first chuck;first lowering a spindle having a wheel from a first vertical position to a second vertical position;second lowering the spindle from the second vertical position to a third vertical position;grinding the wafer by the wheel;identifying the third vertical position of the spindle by determining whether amounts of change in an operation current of the spindle exceed a reference amount of change in current;calculating spindle feedback displacement; andapplying the spindle feedback displacement to the second vertical position of the first chuck,wherein the third vertical position of the spindle is defined as a vertical position at which the wheel comes into contact with the wafer, andthe spindle feedback displacement is defined as a difference between a reference vertical position of a reference chuck and the third vertical position of the spindle, and the reference vertical position of the reference chuck is defined as the third vertical position of the reference chuck.

2. The wafer grinding method of claim 1, wherein the identifying of the third vertical position of the spindle comprises identifying, as the third vertical position of the spindle, a position of the spindle at the moment when an amount of change in the operation current of the spindle, which first exceeds the reference amount of change in current among the amounts of change in the operation current of the spindle, occurs.

3. The wafer grinding method of claim 2, wherein the reference amount of change in current is three times a current change standard deviation, which is a standard deviation of the amounts of change in the operation current of the spindle.

4. The wafer grinding method of claim 3, wherein the reference amount of change in current is a positive number.

5. The wafer grinding method of claim 4, wherein the current change standard deviation is calculated based on the amounts of change in the operation current of the spindle from a time the spindle reaches the second vertical position to a time the grinding of the wafer is completed.

6. The wafer grinding method of claim 4, wherein the current change standard deviation is calculated based on the amounts of change in the operation current of the spindle within 60 seconds from a time when the spindle reaches the second vertical position.

7. The wafer grinding method of claim 4, wherein a spindle feedback time is defined as a difference between a reference time when the spindle reaches the reference vertical position and a reference time when the spindle reaches the third vertical position, and the spindle feedback displacement is calculated by multiplying the spindle feedback time and an average descending speed during the second lowering of the spindle.

8. The wafer grinding method of claim 4, wherein the spindle feedback displacement is calculated by adding an amount of wear of the wheel to the difference between the reference vertical position of the reference chuck and the third vertical position of the spindle.

9. The wafer grinding method of claim 4, wherein the reference vertical position of the reference chuck is at a distance of 8 μm to 20 μm downward from the second vertical position of the reference chuck.

10. The wafer grinding method of claim 4, wherein a number of rotations per unit time of the wheel is maintained constant after reaching the second vertical position of the spindle.

11. The wafer grinding method of claim 4, wherein the applying of the spindle feedback displacement to the second vertical position of the first chuck comprises not applying the spindle feedback displacement to the second vertical position of the first chuck when the spindle feedback displacement is greater than a vertical distance from the second vertical position to the third vertical position of the spindle.

12. A wafer grinding method comprising: loading a wafer onto a first chuck;first lowering a spindle having a wheel from a first vertical position to a second vertical position;second lowering the spindle from the second vertical position to a third vertical position;grinding the wafer by the wheel;identifying the third vertical position of the spindle by determining whether amounts of change in an operation current of the spindle exceed a reference amount of change in current;calculating spindle feedback displacement; andapplying the spindle feedback displacement to the second vertical position of the first chuck,wherein the third vertical position of the spindle is defined as a vertical position at which the wheel comes into contact with the wafer,the spindle feedback displacement is defined as a difference between a reference vertical position of the first chuck and the third vertical position of the spindle, andthe reference vertical position of the first chuck is defined as the third vertical position of the first chuck during a previous grinding process.

13. The wafer grinding method of claim 12, wherein the identifying of the third vertical position of the spindle comprises identifying, as the third vertical position of the spindle, a position of the spindle at the moment when an amount of change in the operation current of the spindle, which first exceeds the reference amount of change in current among the amounts of change in the operation current of the spindle, occurs.

14. The wafer grinding method of claim 13, wherein the reference amount of change in current is three times a current change standard deviation, which is a standard deviation of the amounts of change in the operation current of the spindle, and the reference amount of change in current is a positive number.

15. The wafer grinding method of claim 14, wherein the current change standard deviation is calculated based on the amounts of change in the operation current of the spindle from a time the spindle reaches the second vertical position to a time the grinding of the wafer is completed.

16. The wafer grinding method of claim 14, wherein the current change standard deviation is calculated based on the amounts of change in the operation current of the spindle within 60 seconds from a time when the spindle reaches the second vertical position.

17. The wafer grinding method of claim 14, wherein a spindle feedback time is defined as a difference between a reference time when the spindle reaches the reference vertical position and a reference time when the spindle reaches the third vertical position, and the spindle feedback displacement is calculated by multiplying the spindle feedback time and an average descending speed during the second lowering of the spindle.

18. The wafer grinding method of claim 13, wherein the spindle feedback displacement is calculated by adding an amount of wear of the wheel to the difference between the reference vertical position of the first chuck and the third vertical position of the spindle.

19. The wafer grinding method of claim 14, wherein a number of rotations per unit time of the wheel is maintained constant after reaching the second vertical position of the spindle.

20. A wafer grinding method comprising: loading a wafer onto a first chuck;first lowering a spindle having a wheel from a first vertical position to a second vertical position;second lowering the spindle from the second vertical position to a third vertical position;grinding the wafer by the wheel;identifying the third vertical position of the spindle by determining whether amounts of change in an operation current of the spindle exceed a reference amount of change in current;calculating spindle feedback displacement; andapplying the spindle feedback displacement to the second vertical position of the first chuck,wherein the third vertical position of the spindle is defined as a vertical position at which the wheel comes into contact with the wafer,the spindle feedback displacement is calculated by adding an amount of wear of the wheel to a difference between a reference vertical position of a reference chuck and the third vertical position of the spindle, and the reference vertical position of the reference chuck is defined as the third vertical position of the reference chuck,identifying, as the third vertical position of the spindle, a position of the spindle at the moment when an amount of change in the operation current of the spindle, which first exceeds the reference amount of change in current among the amounts of change in the operation current of the spindle, occurs,the reference amount of change in current is a positive number and three times a current change standard deviation, which is a standard deviation of the amounts of change in the operation current of the spindle,the current change standard deviation is calculated based on the amounts of change in the operation current of the spindle from a time the spindle reaches the second vertical position to a time the grinding of the wafer is completed, anda number of rotations per unit time of the wheel is maintained constant after reaching the second vertical position of the spindle.