GRINDING APPARATUS

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a grinding apparatus including a chuck table having a holding surface that is polygonal in plan view and a workpiece grinding method for grinding a workpiece held on the holding surface.

Description of the Related Art

Integrated circuit (IC) packages are indispensable components for electronic appliances including mobile phones and personal computers. Downsizing and thinning of electronic appliances in recent years have caused demands for thinner IC packages. In order to thin the IC packages, a rectangular substrate is formed by sealing a plurality of semiconductor device chips disposed in a matrix by mold resin, and thereafter, one side (for example, a rear side) of the rectangular substrate is thinned by being ground by a grinding apparatus.

The grinding apparatus includes a chuck table that is rotatable about a rotational axis. A rotation center of this rotational axis is disposed at a position corresponding to a center of a surface (that is, a holding surface) on which a rectangular substrate is held. According to the shape of the rectangular substrate that is square or rectangular in plan view, the shape of the holding surface in plan view is also square or rectangular.

Above the chuck table, a grinding unit including a spindle is provided. On a lower end portion of the spindle, a circular ring-shaped grinding wheel is mounted. The grinding wheel has a circular ring-shaped base, on a lower surface of which a plurality of grindstones are disposed at substantially equal intervals along a circumferential direction of the base. When the grinding wheel rotates by rotation of the spindle, a circular ring-shaped grinding surface formed by trajectories of the plurality of grindstones passes over a center of the holding surface of the chuck table.

Grinding one side of the rectangular substrate by the grinding apparatus is, for example, performed in the following order. First, the other side (for example, the face side) of the rectangular substrate is held under suction on the holding surface of the chuck table. Next, the position of the chuck table is adjusted such that a center of the one side of the rectangular substrate and the trajectories of the plurality of grindstones overlap in a vertical direction. Subsequently, the grinding unit is moved downward at a predetermined grinding feed speed while both the chuck table and the spindle are being rotated.

While a portion that is part of the one side of the rectangular substrate and that comes into contact with the plurality of grindstones is ground, since the chuck table is rotating at the time of grinding, the one side of the rectangular substrate is ground in whole. However, in association with the rotation of the chuck table, an area of contact (grinding area) between the rectangular substrate and the plurality of grindstones alternately increases and decreases in a direction moving around the rotational axis.

For example, the area ground in the rectangular substrate when portions between the center of the one side of the rectangular substrate and each corner of the rectangular substrate are ground is larger than the area ground when portions between the center of the one side of the rectangular substrate and each side of the rectangular substrate are ground. Hence, when the one side of the rectangular substrate is ground, portions in the vicinity of the corners of the rectangular substrate tend to become relatively thick, while other portions tend to become relatively thin.

In light of such points, for the purpose of reducing thickness variations that occur at the time of grinding a rectangular substrate, there is proposed a method for grinding the holding surface of the chuck table before grinding the workpiece (see, for example, Japanese Patent Laid-open No. 2020-55080). The method described in Japanese Patent Laid-open No. 2020-55080 seeks to reduce thickness variations in the ground rectangular substrate by grinding in advance the holding surface and thereby making the shape formed on one surface of the holding surface due to the increase and decrease in the grinding area at the time of grinding the holding surface and the shape formed on one side of the rectangular substrate due to the increase and decrease in the grinding area at the time of grinding the rectangular substrate substantially the same.

However, the holding surface and the rectangular substrate are made of different materials. For example, the holding surface is formed by ceramics such as alumina, while the area to be ground in the rectangular substrate is formed by epoxy resin in which a filler made of silica is mixed. Moreover, the holding surface includes a frame area formed by non-porous dense ceramics and a porous area formed by porous ceramics. Such differences in material and structure make it typically difficult to make the shape of the ground holding surface and the shape of the ground one side of the rectangular substrate completely the same.

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems described above, and one object thereof is to reduce a difference between a shape of a ground holding surface and a shape of a ground one side of a workpiece when the holding surface is polygonal in plan view.

In accordance with an aspect of the present invention, there is provided a grinding apparatus including a chuck table that has a holding surface polygonal in plan view and is rotatable about a predetermined rotational axis by a rotary drive source including a first motor, a grinding unit that includes a spindle mounted at a higher position than the holding surface and a spindle motor for rotating the spindle and has a grinding wheel mounted on the spindle, a moving unit that includes a second motor and moves the chuck table and the grinding unit relative to each other along a predetermined direction, a load detection unit that detects a load applied to one of or both the grinding unit and the chuck table, and a controller that includes a processor and a memory and receives information regarding the load from the load detection unit, in which the controller includes a load storage section that stores one of or both a predetermined load value and the information regarding the load, a holding surface grinding command section that controls the load applied to one of or both the grinding unit and the chuck table, by controlling a parameter related to grinding processing when the holding surface is ground by the grinding wheel, and a workpiece grinding command section that controls the load that is applied to one of or both the grinding unit and the chuck table and that is the same type of load as that controlled by the holding surface grinding command section, by controlling the parameter related to grinding processing when a workpiece held on the holding surface is ground, and the controller controls the parameter related to grinding processing such that the load controlled by the holding surface grinding command section and the load controlled by the workpiece grinding command section have the same value.

Preferably, the load detection unit includes at least any one of a first load sensor configured to detect a first load that is a load applied to the grinding unit, a second load sensor configured to detect a second load that is a load applied to the chuck table, or a load ammeter configured to measure a load current flowing in the spindle motor, the load current being a load of the spindle motor, the load applied to the grinding unit includes the first load and the load current, the load applied to the chuck table includes the second load, and the parameter related to grinding processing includes a relative moving speed of the chuck table and the grinding unit along the predetermined direction, a rotational speed of the chuck table, a rotational speed of the spindle, and a flow rate of grinding water to be supplied per unit hour.

Preferably, the parameter related to grinding processing is a relative moving speed of the chuck table and the grinding unit along the predetermined direction, the holding surface grinding command section controls the moving speed such that the load applied to one of or both the grinding unit and the chuck table has the predetermined load value, when the holding surface is ground by the grinding wheel, and the workpiece grinding command section controls the moving speed such that the load that is applied to one of or both the grinding unit and the chuck table and that is the same type of load as that controlled by the holding surface grinding command section has the predetermined load value, when the workpiece held on the holding surface is ground.

In accordance with another aspect of the present invention, there is provided a workpiece grinding method for grinding a workpiece held on a holding surface, polygonal in plan view, of a chuck table, after the holding surface is ground, the method including grinding the holding surface of the chuck table by a first grinding wheel mounted on a spindle, while detecting, by a load detection unit, a load applied to one of or both a grinding unit including the spindle and the chuck table that is rotatable about a predetermined rotational axis and bringing the grinding unit and the chuck table relatively close to each other along a predetermined direction, after the grinding the holding surface, holding the workpiece having a polygonal shape corresponding to the holding surface on the holding surface, and, after the holding, grinding the workpiece held on the holding surface by a second grinding wheel mounted on the spindle, while detecting, by the load detection unit, the load applied to one of or both the grinding unit and the chuck table and bringing the grinding unit and the chuck table relatively close to each other along the predetermined direction, in which a parameter related to grinding processing is controlled such that the load applied to one of or both the grinding unit and the chuck table and detected by the load detection unit in the grinding the holding surface and the load that is applied to one of or both the grinding unit and the chuck table and detected by the load detection unit in the grinding the workpiece and that is the same type of load as that controlled in the grinding the holding surface have the same value.

Preferably, the load applied to the grinding unit includes a first load applied to the grinding unit and a load current that is a load of a spindle motor that rotates the spindle, the load applied to the chuck table includes a second load applied to the chuck table, and the parameter related to grinding processing includes a relative moving speed of the chuck table and the grinding unit along the predetermined direction, a rotational speed of the chuck table, a rotational speed of the spindle, and a flow rate of grinding water to be supplied per unit hour.

Preferably, in the grinding the holding surface, the parameter related to grinding processing is controlled such that the load applied to one of or both the grinding unit and the chuck table has the same value as the load that is to be detected by the load detection unit when the workpiece is ground in the grinding the workpiece and that is known beforehand.

Preferably, in the grinding the workpiece, the parameter related to grinding processing is controlled such that the load applied to one of or both the grinding unit and the chuck table is of the same type and has the same value as the load detected by the load detection unit when the holding surface is ground in the grinding the holding surface.

Preferably, the workpiece grinding method includes, before the grinding the holding surface, setting the predetermined load value as a target value in the grinding the holding surface and the grinding the workpiece, in which the parameter related to grinding processing is controlled such that the load controlled has the predetermined load value in both the grinding the holding surface and the grinding the workpiece.

Preferably, the first grinding wheel that has last ground the holding surface in the grinding the holding surface and the second grinding wheel used throughout the grinding the workpiece are the same grinding wheel.

The controller of the grinding apparatus according to the aspect of the present invention includes the holding surface grinding command section and the workpiece grinding command section. The holding surface grinding command section controls the load applied to one of or both the grinding unit and the chuck table, by controlling the parameter related to grinding processing when the holding surface is ground by the grinding wheel.

In contrast, the workpiece grinding command section controls the load that is applied to one of or both the grinding unit and the chuck table and that is the same type of load as that controlled by the holding surface grinding command section, by controlling the parameter related to grinding processing when the workpiece held on the holding surface is ground.

That is, the controller controls the parameter related to grinding processing such that the load controlled by the holding surface grinding command section and the load controlled by the workpiece grinding command section have the same value, so that, compared to the case in which such control is not performed, the difference between the shape of the ground holding surface and the shape of the ground one side of the workpiece can be reduced.

Also in the grinding method according to the other aspect of the present invention, the difference between the shape of the ground holding surface and the shape of the ground one side of the workpiece can be reduced compared to the case in which controlling the parameter related to grinding processing to have the same load value is not performed. Hence, the thickness variation in the ground workpiece can be reduced.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view, partly in cross section, illustrating a grinding apparatus according to a first embodiment of the present invention;

FIG. 2A is a perspective view of a chuck table;

FIG. 2B is an A-A cross sectional view taken along line A-A of FIG. 2A;

FIG. 2C is a B-B cross sectional view taken along line B-B of FIG. 2A;

FIG. 3 is a side elevational view, partly in cross section, illustrating a rotation mechanism and a support mechanism of the chuck table;

FIG. 4 is a plan view of a table base;

FIG. 5 is a plan view of a grinding unit;

FIG. 6 is a view illustrating some trajectories of a plurality of grindstones;

FIG. 7 is a side elevational view, partly in cross section, of a grinding unit in the vicinity of a grinding wheel;

FIG. 8 is a flowchart of a rectangular substrate grinding method according to the first embodiment;

FIG. 9 is a side elevational view, partly in cross section, illustrating a holding surface grinding step;

FIG. 10 is a side elevational view, partly in cross section, illustrating a holding step;

FIG. 11 is a side elevational view, partly in cross section, illustrating a workpiece grinding step;

FIG. 12A is a perspective view of the rectangular substrate that has undergone the workpiece grinding step;

FIG. 12B is a side elevational view of the rectangular substrate that has undergone the workpiece grinding step;

FIG. 12C is a plan view of one side of the rectangular substrate that has undergone the workpiece grinding step;

FIG. 13 is a flowchart of a rectangular substrate grinding method according to a second modification of the first embodiment;

FIG. 14 is a flowchart of a rectangular substrate grinding method according to a second embodiment of the present invention;

FIG. 15A is a plan view of a chuck table having a plurality of holding surfaces each having a reed shape;

FIG. 15B is a perspective view of the chuck table illustrated in FIG. 15A;

FIG. 15C is a plan view of a chuck table having a plurality of holding surfaces each having a square shape; and

FIG. 15D is a perspective view of the chuck table illustrated in FIG. 15C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

An embodiment according to one aspect of the present invention is described with reference to the attached drawings. FIG. 1 is a side elevational view, partly in cross section, of a grinding apparatus 2. Note that, in FIG. 1, some of the components are illustrated in functional blocks.

An X-axis, a Y-axis, and a Z-axis each illustrated in FIG. 1 are perpendicular to one another. In the present specification, a direction parallel to the X-axis is referred to as an X-axis direction, a direction parallel to the Y-axis is referred to as a Y-axis direction, and a direction parallel to the Z-axis is referred to as a Z-axis direction. The Z-axis is parallel to a vertical direction and an up-down direction, and an XY plane corresponds to a horizontal plane perpendicular to the Z-axis.

The grinding apparatus 2 includes a base 4 for supporting or housing components. In an upper surface of the base 4, a rectangular parallelepiped recess 4a which has a longitudinal portion disposed along the X-axis is formed. The recess 4a is provided with a moving mechanism 6 of a ball screw type.

The moving mechanism 6 includes a pair of guide rails (not illustrated). The pair of guide rails are disposed substantially parallel to the X-axis and fixed to the base 4. On an upper side of the pair of guide rails, a moving plate 8 is attached in a slidable manner.

On a lower surface of the moving plate 8, a nut 10 is provided. To the nut 10, a screw shaft 12 is rotatably coupled via a plurality of balls (not illustrated). The screw shaft 12 is disposed along the X-axis between the pair of guide rails.

To one end portion of the screw shaft 12, a drive source 14 such as a servomotor or a step motor is coupled. When the screw shaft 12 is rotated by the drive source 14, the moving plate 8 moves along the X-axis. Above the moving plate 8, a chuck table 16 whose outer shape is a disk shape is provided.

Here, with reference to FIGS. 2A through 2C, explanation is given on the chuck table 16. FIG. 2A is a perspective view of the chuck table 16, FIG. 2B is an A-A cross sectional view taken along line A-A of FIG. 2A, and FIG. 2C is a B-B cross sectional view taken along line B-B of FIG. 2A.

In FIGS. 2A through 2C, the shape of a holding surface 16a is illustrated in an exaggerated manner for the sake of description. Moreover, the holding surface 16a illustrated in FIGS. 2A through 2C has a shape that is available after a holding surface grinding step S10 to be described later is performed but before a holding step S20 to be described later is carried out, that is, a shape that is available immediately before a rectangular substrate (workpiece) 11 is held thereon.

The chuck table 16 has a non-porous dense disk-shaped first frame 18 formed of ceramics such as alumina. The first frame 18 includes a large diameter portion which has an outer peripheral portion in which a plurality of screw holes are provided and a small diameter portion disposed concentrically with the large diameter portion.

On an upper surface of the small diameter portion of the first frame 18, a second frame 20 that is square (that is, polygonal) in plan view is provided. Similarly to the first frame 18, the second frame 20 is also formed of ceramics such as alumina and is a non-porous dense frame.

The second frame 20 includes a recess 20 that is square in plan view. To the recess 20a, a square porous plate 22 formed of ceramics such as alumina is fixed via an adhesive agent or the like. An outer shape of the porous plate 22 is the same as an outer shape of the recess 20a.

On a bottom surface of the recess 20a, a plurality of flow channels are formed. As illustrated in FIG. 2B, the plurality of flow channels include a first flow channel 20b disposed on a bottom surface of the recess 20a and a second flow channel 20c that is positioned at a center of the bottom surface of the recess 20a and that penetrates the second frame 20 from the bottom surface of the recess 20a up to a bottom surface 20d of the second frame 20.

To the second frame 20, a vacuum apparatus (not illustrated) such as a vacuum pump and an ejector is connected via a rotary joint (not illustrated). The porous plate 22 is a continuous porous material in which pores are connected continuously, and vacuum generated by the vacuum apparatus can be transmitted to an upper surface of the porous plate 22 through the first flow channel 20b, the second flow channel 20c, and the like.

An upper surface of the second frame 20 and the upper surface of the porous plate 22 are substantially mesh with each other and form the holding surface 16a that holds under suction the rectangular substrate 11 (see FIG. 1). The holding surface 16a according to the present embodiment is square (that is, polygonal) in plan view, and each side of the holding surface 16a is a straight line in plan view.

Yet, as illustrated in the perspective view of FIG. 2A, a corner 16b of each side of the holding surface 16a is protruding compared to an intermediate point 16c of each side. That is, when the second frame 20 and the porous plate 22 (that is, the holding member) are viewed in side elevation, one side of the holding surface 16a is not a straight line but a curved line curved in such a manner that a central portion is slightly recessed (see FIG. 12B).

FIG. 2B illustrates the cross section of the chuck table 16 that passes through a center 16a₀ of the holding surface 16a and the corner 16b of the holding surface 16a and that is taken at a plane parallel to the upper surface of the first frame 18. As illustrated in FIG. 2B, the center 16a₀ is protruding upward compared to the corners 16b by a predetermined length Δ (for example, 25 μm).

Nevertheless, the amount of protrusion of the center 16a₀ relative to the corners 16b is sufficiently small compared to the length of each side of the holding surface 16a. The length of one side of the holding surface 16a in plan view is a predetermined value within the range of 320 to 700 mm. In FIG. 2B, the amount of protrusion of the center 16a₀ is illustrated in an exaggerated manner.

The A-A cross section illustrated in FIG. 2B includes a conical area 16a₁ which has the center 16a₀ as the vertex and a recessed curved surface area 16a₂ in which a height of the holding surface 16a gradually becomes lower from the corner 16b toward the center 16a₀.

As described above, the holding surface 16a gradually becomes lower from the corner 16b toward the intermediate point 16c of one side along the one side of the holding surface 16a (see an arrow C in FIG. 2A). As illustrated in FIG. 2C, in a cross section that passes through the intermediate point 16c of one side and the center 16a₀, the recessed curved surface area 16a₂ disappears.

The holding surface 16a at an end face of the B-B cross section illustrated in FIG. 2C corresponds to the two sides of an isosceles triangle. Note that, in FIG. 2C, the corner 16b that is positioned in a father direction on the sheet than the end face of the B-B cross section is positioned higher than the intermediate point 16c.

The holding surface 16a that is square in plan view has the conical area 16a₁ at the central portion and the recessed curved surface area 16a₂ in the vicinity of each of the corners 16b in such a manner as to fill the gap between the conical area 16a₁ at the central portion and each of the four corners 16b.

The rectangular substrate 11 (see FIG. 1) that is held under suction on the holding surface 16a has, in plan view, a square shape that is substantially the same as the holding surface 16a (a polygonal shape corresponding to the holding surface 16a), and has substantially the same size as the holding surface 16a.

The rectangular substrate 11 is formed by sealing a semiconductor package substrate including a die pad, a lead frame, and the like and a plurality of semiconductor device chips by molded resin (for example, epoxy resin in which a filler made of silica is mixed).

The rectangular substrate 11 includes one side (rear side) 11b which is to be ground and a to-be-held side (face side) 11a that faces the holding surface 16a at the time of grinding. Here, other configurations of the grinding apparatus 2 are described with reference to FIG. 3.

FIG. 3 is a side elevational view, partly in cross section, illustrating a rotation mechanism and a support mechanism of the chuck table 16. In FIG. 3, some of the components are illustrated in functional blocks. The chuck table 16 is rotatably supported by an air bearing 24.

The air bearing 24 includes a rotor 24a (a predetermined rotational shaft) that is coupled to a lower surface of the chuck table 16. The rotor 24a has a large diameter portion 24a₁ provided on an apex in the longitudinal direction and a small diameter portion 24a₂ located on a lower side of the large diameter portion 24a1.

Below the large diameter portion 24a₁ and around the small diameter portion 24a₂, a ring-shaped stator 24b is disposed. The rotor 24a and the stator 24b are not in contact with each other and have a slight gap (not illustrated) formed between them. To this gap, compressed air is supplied from an unillustrated air supply source.

To a lower end portion of the rotor 24a, a driven pulley (not illustrated) is fixed. Further, in the vicinity of the rotor 24a, a rotary drive source 26 that has a motor (first motor) 26a as exemplified by a servomotor is provided. The rotary drive source 26 is supported by the moving plate 8 described above.

To an output shaft of the motor 26a, a driving pulley (not illustrated) is fixed, and around the driving pulley and the driven pulley, an endless belt (not illustrated) is wound. When the motor 26a is operated, the power of rotation is transmitted to the rotor 24a.

The rotor 24a rotates by the power transmitted from the rotary drive source 26 while maintaining the state of not being in contact with the stator 24b. By the rotation of the rotor 24a, the chuck table 16 rotates about the rotor 24a (is rotatable).

The stator 24b of the air bearing 24 is supported by an annular table base 28. At a central portion of the table base 28 in a radial direction, a through hole 28a into which the rotor 24a is to be inserted is formed.

On an upper surface of the table base 28 and around this through hole 28a, a plurality of lower side load sensors (that is, second load sensors) 30 each having a disk shape are provided. The lower side load sensors 30 detect a downward load (second load) that is a load applied downward to the chuck table 16 (see FIG. 1).

Each of the lower side load sensors 30 is supported by the table base 28. FIG. 4 is a plan view of the table base 28. In the present embodiment, three lower side load sensors 30 are disposed at substantially equal intervals along a circumferential direction of the through hole 28a.

Yet, the number of lower side load sensors 30 is not limited to three. The table base 28 is only required to be provided with at least one lower side load sensor 30. The lower side load sensor 30 is, for example, a diaphragm load cell.

Alternatively, the lower side load sensor 30 may be a column load cell. The load cell includes a sensor for converting the load into an electric signal. For example, the load cell has a piezoelectric sensor including piezoelectric elements, but instead, may have a strain-gage sensor or a capacitive sensor, for example.

An upper surface of the lower side load sensor 30 is in contact with a lower surface of the stator 24b (see FIG. 3). Force pressing the chuck table 16 downward is transmitted to the lower side load sensor 30 via the stator 24b.

A downward load value 30a (that is, information 84a regarding a load) (see FIG. 1) that is applied to each of the lower side load sensos 30 is transmitted to a controller 86 to be described later. The controller 86 can acquire the downward load in real time and also calculate an average value or total value of the downward load applied to the plurality of lower side load sensors 30.

Note that, in FIG. 1, for easy viewing of the figure, one lower side load sensor 30 and the controller 86 are connected to each other by a line, but in reality, all of the lower side load sensors 30 and the controller 86 are connected to each other.

Described with reference to FIG. 3 again, the table base 28 is supported by a tilt adjustment mechanism 32. The tilt adjustment mechanism 32 adjusts the tilt of the table base 28 relative to the XY plane. The tilt adjustment mechanism 32 includes a fixed support mechanism 32a, a first movable support mechanism 32b, and a second movable support mechanism 32c.

The fixed support mechanism 32a, the first movable support mechanism 32b, and the second movable support mechanism 32c are separated from one another at substantially equal intervals along the circumferential direction of the through hole 28a, and each have an apex fixed to the lower surface of the table base 28.

The fixed support mechanism 32a, the first movable support mechanism 32b, and the second movable support mechanism 32c are provided at such positions that they do not overlap with the lower side load sensors 30 in the Z-axis direction. For example, the fixed support mechanism 32a, the first movable support mechanism 32b, the second movable support mechanism 32c, and the three lower side load sensors 30 are located at the positions of the vertices of a regular hexagon (see FIG. 4).

The fixed support mechanism 32a includes a fixed shaft 34a of a predetermined length. In contrast, the first movable support mechanism 32b includes a movable shaft 34b which has a distal end portion in which a male screw is formed, and the second movable support mechanism 32c includes a movable shaft 34c which has a distal end portion in which a male screw is formed.

An upper portion of each of the movable shafts 34b and 34c is rotatably coupled to a screw hole of an upper support fixed to the lower surface of the table base 28. To lower end portions of the movable shafts 34b and 34c, drive sources 36b and 36c as exemplified by servomotors and pulse motors are coupled, respectively.

The movable shafts 34b and 34c are rotatable by the drive sources 36b and 36c. The movable shafts 34b and 34c are supported by the moving plate 8. Adjusting the amount of screwing the movable shafts 34b and 34c into the upper support allows the tilt of the table base 28 relative to the XY plane to be adjusted.

The rotor 24a tilts in line with the tilt of the table base 28 in such a manner as to correspond to the tilt of the table base 28. A rotation center 24a3 of the rotor 24a is tilted by only a minute angle α relative to the Z-axis in a YZ plane, and is also titled by only a minute angle relative to the Z-axis in an XZ plane.

Here, with reference to FIG. 1 again, other configurations of the grinding apparatus 2 are described. On both sides of the chuck table 16 in the X-axis direction, a bellows-like cover member 40 that is extendable and contractable along the X-axis direction is provided. The cover member 40 prevents the moving mechanism 6 from being contaminated by grinding water 70 (see FIG. 7), grinding swarf generated during grinding, and the like.

On a rear side (one side in the X-axis direction) of the grinding apparatus 2, a rectangular parallelepiped support structure 4b is provided in a manner protruding upward. On a side surface on the front side (the other side in the X-axis direction) of the support structure 4b, a grinding feed mechanism (moving unit) 42 of a ball screw type is provided.

The grinding feed mechanism 42 moves the chuck table 16 and a grinding unit 54 to be described later relative to each other along the Z-axis direction by moving the grinding unit 54 along the Z-axis direction (predetermined direction).

The grinding feed mechanism 42 includes a pair of guide rails 44 disposed along the Z-axis. To the pair of guide rails 44, a rectangular moving plate 46 is fixed in a slidable manner along the Z-axis direction. On a rear side surface of the moving plate 46, a nut 48 is provided.

To the nut 48, a screw shaft 50 is rotatably coupled via a plurality of balls (not illustrated). The screw shaft 50 is disposed along the Z-axis between the pair of guide rails 44. To an upper end portion of the screw shaft 50, a motor (second motor) 52 such as a servomotor or a step motor is coupled.

When the screw shaft 50 is rotated by the motor 52, the moving plate 46 moves along the Z-axis. To a front side surface of the moving plate 46, a cylindrical holding member 56 for holding the grinding unit 54 is fixed. The grinding unit 54 includes a cylindrical spindle housing 58.

The spindle housing 58 is disposed in a hollow portion of the holding member 56 and supported by the holding member 56. More specifically, the spindle housing 58 is supported by the holding member 56 via a plurality of upper side load sensors (first load sensors) 60.

A lower surface of each of the upper side load sensors 60 is fixed to a bottom plate of the holding member 56, while an upper surface of each of the upper side load sensors 60 is fixed to a bottom surface of the spindle housing 58. The upper side load sensors 60 are each, for example, a diaphragm or column load cell.

On each of the upper side load sensors 60, downward force (that is, compression force) along the Z-axis direction is acting at all times due to force caused by the own weight of the grinding unit 54, for example. However, when the grinding unit 54 moved by the grinding feed mechanism 42 presses the holding surface 16a or the rectangular substrate 11 downward at the time of grinding, the grinding wheel 68 is subjected to upward force as reaction force.

At this time, with the spindle housing 58 also being subjected to upward force, upward force (that is, tensile force) acts on each of the upper side load sensors 60. This tensile force reduces the compression force acting on the upper side load sensors 60. In other words, an upward load (first load) applied upward to the grinding unit 54 is detected according to an increase or decrease in compression force acting on the upper side load sensors 60.

The controller 86 is notified of an upward load value 60a (that is, information 84a regarding a load) that is applied to each of the upper side load sensors 60. The controller 86 can acquire the upward load value in real time and can also calculate an average value or total value of the upward load applied to the plurality of upper side load sensors 60.

Note that, in FIG. 1, for easy viewing of the figure, one upper side load sensor 60 and the controller 86 are connected to each other by a line, but in reality, all of the upper side load sensors 60 and the controller 86 are connected to each other.

FIG. 5 is a plan view of the grinding unit 54, illustrating the plurality of upper side load sensors 60. In the present embodiment, three upper side load sensors 60 are disposed at substantially equal intervals along the circumferential direction of the spindle housing 58. Yet, the number of upper side load sensors 60 is not limited to three. The table base 28 may be provided with at least one upper side load sensor 60.

Reference is made to FIG. 1 again. In the spindle housing 58, part of a cylindrical spindle 62 is housed in a rotatable manner. The spindle 62 and the spindle housing 58 are disposed at higher positions than the holding surface 16a of the chuck table 16. The longitudinal direction of the spindle 62 is arranged along the Z-axis.

Inside the spindle housing 58, a spindle motor 64 such as a direct current (DC) servomotor is disposed around the spindle 62. The spindle motor 64 rotates the spindle 62 at a rotational frequency (rotational speed) corresponding to the supplied electric power in a state in which there is no load for stopping the rotation (that is, at the time of no load).

A lower end portion of the spindle 62 is protruding downward than the bottom surface of the holding member 56, and has a disk-shaped wheel mount 66 fixed thereto. To a bottom surface of the wheel mount 66, a circular ring-shaped grinding wheel 68 is fixed by bolts (not illustrated). That is, the grinding wheel 68 is mounted on the lower end portion of the spindle 62.

The grinding wheel 68 includes a circular ring-shaped wheel base 68a which has an outer diameter substantially the same as that of the wheel mount 66. The wheel base 68a is formed of metal such as an aluminum alloy. To a lower surface of the wheel base 68a, a plurality of grindstones 68b are fixed at substantially equal intervals along the circumferential direction of the wheel base 68a.

Each of the grindstones 68b includes abrasive grains formed of diamond, cubic boron nitride (cBN), or the like and a bonding material, such as a resin bond, a metal bond, or a vitrified (or ceramic) bond, that holds the abrasive grains together.

When the spindle 62 is rotated, the grinding wheel 68 rotates about the spindle 62. When the grinding wheel 68 rotates, a circular ring-shaped grinding surface 68c (see FIG. 6) is formed by trajectories of bottom surfaces of the plurality of the grindstones 68b.

FIG. 6 is a view illustrating some of the moving trajectories of the plurality of grindstones 68b when the holding surface 16a is viewed in plan. The grinding surface 68c lies in such a manner as to pass through the center 16a₀ of the holding surface 16a in plan view.

In the spindle 62, the wheel mount 66, and the wheel base 68a, flow channels 62a, 66a, and 68a₁ for supplying the grinding water 70 such as pure water to the grindstones 68b are formed (see FIG. 7).

FIG. 7 is a side elevational view, partly in cross section, of the grinding unit 54 in the vicinity of the grinding wheel 68. The grinding water 70 is used for removing heat and grinding swarf generated at the processing area. To the flow channel 62a of the spindle 62, a grinding water supply source 72 is connected.

The grinding water supply source 72 includes a tank in which the grinding water 70 is stored, a pump for supplying the grinding water 70 from this tank, and the like (none of which are illustrated). The grinding water supply source 72 is usually an apparatus provided separately from the grinding apparatus 2.

When the holding surface 16a and the rectangular substrate 11 are to be ground, the chuck table 16 and the spindle 62 are each being rotated, and the grinding feed mechanism 42 moves the grinding unit 54 downward at a predetermined speed while the grinding water 70 is being supplied to the grindstones 68b at a predetermined flow rate from the grinding water supply source 72.

At the time of grinding, the spindle 62 is rotated by electric power being supplied to the spindle motor 64 by a power source 80 (see FIG. 1). The power source 80 includes a DC power source (not illustrated) and a predetermined circuit (not illustrated) for changing the driving voltage to be supplied to the spindle motor 64 from the DC power source. The predetermined circuit is, for example, a circuit for changing the driving voltage by a pulse width modulation (PWM) system or a linear system.

At the time of grinding, a load current that flows to the spindle motor 64 (that is, the load applied to the grinding unit 54) changes according to the load torque (that is, the grinding load) of the spindle 62. As illustrated in FIG. 1, to the spindle motor 64 and the power source 80, a load ammeter 82 is connected in series, and the load current flowing to the spindle motor 64 is measured by the load ammeter 82.

The controller 86 of the grinding apparatus 2 is notified of a load current value 82a (the information 84a regarding the load) measured by the load ammeter 82. The controller 86 can acquire the load current value 82a in real time and calculate the average value of the load current.

For example, when the load current value 82a becomes greater than a predetermined reference value, this means that the grinding load is greater than the predetermined reference value, that is, the rotational speed of the spindle 62 is lower than the predetermined reference value.

The plurality of lower side load sensors 30, the plurality of upper side load sensors 60, and the load ammeter 82 configure a load detection unit 84. That is, the load detection unit 84 according to the present embodiment can detect the loads applied to both the grinding unit 54 and the chuck table 16.

The load detection unit 84 according to the present embodiment includes three lower side load sensors 30, three upper side load sensors 60, and the load ammeter 82, but the load detection unit 84 may include at least any one of the lower side load sensor 30, the upper side load sensor 60, or the load ammeter 82.

Note that the load detection unit 84 may detect either only the load applied to one of the grinding unit 54 or the chuck table 16 or only the load current (load) applied to the grinding unit 54. The load detection unit 84 may alternatively detect a combination of a load and a load current.

The load applied to the grinding unit 54 and detected by the load detection unit 84 includes an upward load and a load current, and the load applied to the chuck table 16 and detected by the load detection unit 84 includes a downward load.

The controller 86 is, for example, configured by a computer including a processor 86a typified by a central processing unit (CPU) and a memory 86b. The memory 86b includes a main storage unit such as a dynamic random access memory (DRAM) and an auxiliary storage unit such as a flash memory.

In the auxiliary storage unit, software including a predetermined program is stored. When the processor 86a and the like are operated in accordance with this software, the functions of the controller 86 are implemented.

The controller 86 receives, from the load detection unit 84, the information 84a regarding the load (that is, at least any one of the downward load value 30a, the upward load value 60a, or the load current value 82a), and controls the grinding apparatus 2 according to the received information 84a regarding the load.

Here, with reference to FIG. 9, the functions of the controller 86 at the time of grinding are described. Part of the storage area of the memory 86b functions as a load storage section 88. The load storage section 88 stores one of or both the predetermined load value and the information 84a regarding the load.

Note that the predetermined load value according to the present specification refers to a value set beforehand for at least any one of the following: (i) a load (that is, an upward load) applied to the grinding unit 54 and measured by the upper side load sensors 60; (ii) a load (that is, a downward load) applied to the chuck table 16 and measured by the lower side load sensors 30; or (iii) a load (that is, a load current) measured by the load ammeter 82.

A holding surface grinding command section 90 is embodied by the processor 86a executing the programs stored in the memory 86b, for example. The holding surface grinding command section 90 controls the load applied to one of or both the grinding unit 54 and the chuck table 16, by controlling parameters related to grinding processing, when the holding surface 16a is ground by the grinding wheel 68.

The parameters related to grinding processing include (a) a relative moving speed (μm/s) of the chuck table 16 and the grinding unit 54 along the Z-axis direction (predetermined direction), (b) a rotational speed (rpm) of the chuck table 16, (c) a rotational speed (rpm) of the spindle 62, and (d) a flow rate (L/min) of the grinding water 70 to be supplied per unit hour.

The downward load, the upward load, and the load current (a) increase when the relative moving speed is increased, (b) decrease when the rotational speed of the chuck table 16 is increased, (c) decrease when the rotational speed of the spindle 62 is increased, and (d) decrease when the flow rate of the grinding water 70 is increased.

Conversely, the downward load, the upward load, and the load current (a) decrease when the relative moving speed is decreased, (b) increase when the rotational speed of the chuck table 16 is decreased, (c) increase when the rotational speed of the spindle 62 is decreased, and (d) increase when the flow rate of the grinding water 70 is decreased.

As described above, when the holding surface 16a is ground, the holding surface grinding command section 90 controls the load applied to one of or both the grinding unit 54 and the chuck table 16, by controlling at least any one of the parameter (a), the parameter (b), the parameter (c), or the parameter (d) related to grinding processing.

A workpiece grinding command section 92 is also embodied by the processor 86a executing the programs stored in the memory 86b, for example. The workpiece grinding command section 92 controls the load applied to one of or both the grinding unit 54 and the chuck table 16, by controlling the parameters related to grinding processing, when the rectangular substrate 11 held under suction on the holding surface 16a is ground.

In the present embodiment, the load controlled by the workpiece grinding command section 92 is the same type of load as the load controlled by the holding surface grinding command section 90. Further, as in the case of grinding the holding surface 16a, the parameters related to grinding processing at the time of grinding the rectangular substrate 11 as the workpiece include the abovementioned parameters (a) through (d).

The workpiece grinding command section 92 controls the load applied to one of or both the grinding unit 54 and the chuck table 16, by controlling at least any one of the parameter (a), the parameter (b), the parameter (c), or the parameter (d) related to grinding processing, when the rectangular substrate 11 is ground.

In particular, the controller 86 controls the parameters related to grinding processing such that the load controlled by the holding surface grinding command section 90 and the load controlled by the workpiece grinding command section 92 have the same value.

For example, when the load controlled by the holding surface grinding command section 90 is a downward load, the controller 86 sets the load to be controlled by the workpiece grinding command section 92 to also be a downward load. The controller 86 controls at least any one of the abovementioned parameters (a) through (d) such that the downward load applied at the time of grinding the rectangular substrate 11 has the same value as the downward load applied at the time of grinding the holding surface 16a.

Similarly, when the load controlled by the holding surface grinding command section 90 is an upward load, the controller 86 sets the load to be controlled by the workpiece grinding command section 92 to also be an upward load. The controller 86 controls at least any one of the abovementioned parameters (a) through (d) such that the upward load applied at the time of grinding the rectangular substrate 11 has the same value as the upward load applied at the time of grinding the holding surface 16a.

Similarly, when the load controlled by the holding surface grinding command section 90 is a load current, the controller 86 sets the load to be controlled by the workpiece grinding command section 92 to also be a load current. The controller 86 controls at least any one of the abovementioned parameters (a) through (d) such that the load current supplied at the time of grinding the rectangular substrate 11 has the same value as the load current supplied at the time of grinding the holding surface 16a.

More specifically, the controller 86 performs proportional-integral derivative (PID) control, for example, such that the load applied at the time of grinding the holding surface 16a and the load applied at the time of grinding the rectangular substrate 11 have the same value.

In the case of performing PID control, for example, the controller 86 controls at least any one of the abovementioned parameters (a) through (d) such that the value of the load that is currently being applied while the rectangular substrate 11 is ground (that is, an output value of the PID control) has the value of the load applied when the holding surface 16a was ground (that is, a target value of the PID control).

Alternatively, for example, the controller 86 controls at least any one of the abovementioned parameters (a) through (d) such that the value of the load that is currently being applied while the holding surface 16a or the rectangular substrate 11 is ground (that is, an output value of the PID control) has a predetermined load value (that is, the target value of the PID control).

As one specific example, when the holding surface 16a is ground by the grinding wheel 68, the holding surface grinding command section 90 controls (a) the moving speed such that the load applied to one of or both the grinding unit 54 and the chuck table 16 has the predetermined load value, and when the rectangular substrate 11 held on the holding surface 16a is ground after the holding surface 16a is ground, the workpiece grinding command section 92 controls (a) the moving speed such that the load that is applied to one of or both the grinding unit 54 and the chuck table 16 and that is the same type of load as that controlled at the time of grinding the holding surface 16a by the grinding wheel 68 has the predetermined load value.

Since the controller 86 can instantaneously give an instruction to the motor 52 by an electric signal and (a) the moving speed immediately responds to the instruction, more precise control can be realized compared to the case of controlling (b) the rotational speed of the chuck table 16, (c) the rotational speed of the spindle 62, and (d) the flow rate of the grinding water 70.

According to the present embodiment, the controller 86 performs such control when the rectangular substrate 11 is ground, so that the difference between the shape of the ground holding surface 16a and the shape of ground one side 11b of the rectangular substrate 11 can be reduced compared to the case in which such control is not performed.

Needless to say, the shape of the ground holding surface 16a and the shape of the ground one side 11b of the rectangular substrate 11 cannot be made completely the same at all times, but the difference in shape between the two components can reliably be reduced compared to the case in which such control is not performed. Accordingly, the thickness variation (for example, a total thickness variation (TTV)) in the ground rectangular substrate 11 can be reduced.

Next, with reference to FIGS. 8 through 12C, a method for grinding the rectangular substrate 11 held on the holding surface 16a, after the holding surface 16a is ground, is described. FIG. 8 is a flowchart of the method for grinding the rectangular substrate 11 according to the first embodiment. In the present embodiment, the grinding apparatus 2 described above is used to perform the steps including the holding surface grinding step S10, the holding step S20, and a workpiece grinding step S30, in this order.

FIG. 9 is a side elevational view, partly in cross section, illustrating the holding surface grinding step S10. In the holding surface grinding step S10, the holding surface 16a is formed to have the shape illustrated in FIGS. 2A through 2C.

In the holding surface grinding step S10, the grinding wheel 68 (first grinding wheel) grinds the holding surface 16a, while the load detection unit 84 detects the load applied to one of or both the grinding unit 54 and the chuck table 16, and the grinding unit 54 and the chuck table 16 are brought relatively close to each other along the Z-axis direction.

In the holding surface grinding step S10 according to the present embodiment, based on the presumption that the average value of the downward load to be applied to the chuck table 16 in the subsequent workpiece grinding step S30 is experimentally known to be 50 N beforehand, (a) the moving speed as one of the parameters related to grinding processing is controlled such that the load applied to the chuck table 16 becomes 50 N (that is, the predetermined load value).

Note that the load controlled in the holding surface grinding step S10 may be either a preliminarily known load to be applied to only the grinding unit 54 or preliminarily known loads to be applied to both the grinding unit 54 and the chuck table 16 in the workpiece grinding step S30. In this case as well, the parameters related to grinding processing are controlled such that the load controlled in the holding surface grinding step S10 has the same value as the preliminarily known load to be applied in the workpiece grinding step S30.

Note that the parameter to be controlled is not limited to (a) the moving speed, and at least any one of (a) the moving speed, (b) the rotational speed of the chuck table 16, (c) the rotational speed of the spindle 62, or (d) the flow rate of the grinding water 70 may be controlled.

After the holding surface grinding step S10, the rectangular substrate 11 is held under suction on the holding surface 16a such that the one side 11b is exposed upward and the other side 11a faces the holding surface 16a (holding step S20). FIG. 10 is a side elevational view, partly in cross section, illustrating the holding step S20.

After the holding step S20, the grinding wheel 68 (second grinding wheel) grinds the one side 11b of the rectangular substrate 11 held on the holding surface 16a, while the load detection unit 84 detects the load applied to one of or both the grinding unit 54 and the chuck table 16, and the grinding unit 54 and the chuck table 16 are brought relatively close to each other in the Z-axis direction (workpiece grinding step S30).

The controller 86 controls the parameters related to grinding processing such that the load applied to one of or both the grinding unit 54 and the chuck table 16 and detected by the load detection unit 84 in the holding surface grinding step S10 and the load applied to one of or both the grinding unit 54 and the chuck table 16 and detected by the load detection unit 84 in the workpiece grinding step S30, the load being the same type of load as that controlled in the holding surface grinding step S10, have the same value.

According to the present embodiment, the controller 86 controls (a) the moving speed such that the average value of the load applied to the chuck table 16 in the holding surface grinding step S10 and the average value of the load applied to the chuck table 16 in the workpiece grinding step S30 both have the value 50 N (predetermined load value). FIG. 11 is a side elevational view, partly in cross section, illustrating the workpiece grinding step S30.

In the workpiece grinding step S30 according to the present embodiment, the same grinding wheel (that is, the grinding wheel 68) as the grinding wheel 68 that has last ground the holding surface 16a in the holding surface grinding step S10 is used throughout the workpiece grinding step S30. Processing conditions in the holding surface grinding step S10 and the workpiece grinding step S30 are, for example, adjusted as needed within the following range.

• • (a) Moving speed: 1.0 to 10 μm/s
  • (b) Rotational speed of chuck table: 100 to 500 rpm
  • (c) Rotational speed of spindle: 1,000 to 7,000 rpm
  • (d) Flow rate of grinding water: 1.0 to 10 L/min

FIG. 12A is a perspective view of the rectangular substrate 11 that has undergone the workpiece grinding step S30. In the workpiece grinding step 30, as illustrated in FIG. 12A, the one side 11b of the rectangular substrate 11 is ground to follow the shape of the holding surface 16a formed in the holding surface grinding step S10.

This can reduce the difference between the shape of the ground holding surface 16a and the shape of the ground one side 11b of the rectangular substrate 11 compared to the case in which controlling the parameters related to grinding processing to have the same load value in the holding surface grinding step 10 and the workpiece grinding step S30 is not performed. Hence, the thickness variation in the ground rectangular substrate 11 can be reduced.

FIG. 12B is a side elevational view of the rectangular substrate 11 that has undergone the workpiece grinding step S30. FIG. 12C is a plan view of the one side 11b of the rectangular substrate 11 that has undergone the workpiece grinding step S30. In FIG. 12C, radial saw marks (that is, grinding marks) 11c are also illustrated.

In FIG. 12C, a brighter color means the closer the one side 11b and the holding surface 16a are to the bottom surface 20d (that is, the thinner the relevant portion is) on a straight line orthogonal to the flat bottom surface 20d of the second frame 20, while a darker color means the farther the one side 11b and the holding surface 16a are from the bottom surface 20d (that is, the thicker the relevant portion is).

First Modification

Next, a first modification of the first embodiment is described. In the first modification, first, the holding surface grinding step S10 is performed, and the load applied to the chuck table 16 in the holding surface grinding step S10 is detected. For example, suppose that the average value of the downward load applied to the chuck table 16 is 30 N.

In this case, in the subsequent workpiece grinding step S30, (a) the moving speed as one of the parameters related to grinding processing is controlled such that the average value of the downward load applied to the chuck table 16 becomes 30 N.

However, in the holding surface grinding step S10, the load applied to only the grinding unit 54 or the load applied to both the grinding unit 54 and the chuck table 16 may be detected, and the parameters related to grinding processing may be controlled such that the load controlled in the workpiece grinding step S30 is the same (that is, is of the same type and has the same value) as the load controlled in the holding surface grinding step S10.

Needless to say, the parameter to be controlled is not limited to (a) the moving speed, and at least any one of (a) the moving speed, (b) the rotational speed of the chuck table 16, (c) the rotational speed of the spindle 62, or (d) the flow rate of the grinding water 70 may be controlled.

According to the first modification, compared to the first embodiment, unit per hour (UPH) and a grinding quality of the rectangular substrate 11 may become low, but compared to the case in which controlling the parameters related to grinding processing to have the same load value in the holding surface grinding step S10 and the workpiece grinding step S30 is not performed, the thickness variation in the ground rectangular substrate 11 can be reduced.

Second Modification

Next, a second modification of the first embodiment is described. FIG. 13 is a flowchart of a method for grinding the rectangular substrate 11 according to the second modification. In the second modification, before the holding surface grinding step S10, a predetermined load value as the target value in the holding surface grinding step S10 and the workpiece grinding step S30 is set (load value setting step S5). In the load value setting step S5, an operator inputs a predetermined load value via a display input unit such as a touch panel (not illustrated) provided in the grinding apparatus 2.

In both the holding surface grinding step S10 and the workpiece grinding step S30, the controller 86 controls the parameters related to grinding processing such that the load controlled (for example, the average value of the downward load applied to the chuck table 16) has the predetermined load value (for example, 40 N).

Needless to say, in the holding surface grinding step 10 and the workpiece grinding step S30, the load applied to only the grinding unit 54 or the load applied to both the grinding unit 54 and the chuck table 16 may be detected, and the controller 86 may control the parameters related to grinding processing such that the loads controlled in the two steps have the same value.

Moreover, the parameter to be controlled is not limited to (a) the moving speed, and at least any one of (a) the moving speed, (b) the rotational speed of the chuck table 16, (c) the rotational speed of the spindle 62, or (d) the flow rate of the grinding water 70 may be controlled.

Incidentally, making the load applied to the load detection unit 84 in the holding surface grinding step 10 (that is, the load controlled by the holding surface grinding command section 90) and the load applied to the load detection unit 84 in the workpiece grinding step 30 (that is, the load controlled by the workpiece grinding command section 92) have the same value is not limited to having the same average value throughout the two steps.

The shape of the ground surface in the holding surface grinding step S10 and the workpiece grinding step 30 is largely affected by the grinding feed performed for the last several tens of nanometers in each step. Moreover, in the workpiece grinding step S30, in order to improve the throughput, grinding feed for the last several tens of nanometers is sometimes performed at a relatively low grinding feed speed after grinding feed is performed at a relatively high grinding feed speed. Grinding feed speed is (a) the relative moving speed described above, and hence, the load applied to the load detection unit 84 may vary according to the grinding feed speed.

As such, the controller 86 may control the parameters related to grinding processing such that the average value of the load applied to the load detection unit 84 when grinding feed for the last several tens of nanometers is performed in the holding surface grinding step S10 and the average value of the load applied to the load detection unit 84 when grinding feed for the last several tens of nanometers is performed at a relatively low grinding feed speed in the workpiece grinding step S30 have the same value. Needless to say, in both the holding surface grinding step S10 and the workpiece grinding step S30, the same type of load is monitored and controlled.

Note that, in the holding surface grinding step S10, the grinding feed speed is in some cases made constant over the entire period. In this case, the controller 86 may control the parameters related to grinding processing such that the average value of the load applied to the load detection unit 84 throughout the holding surface grinding step S10 and the average value of the load applied to the load detection unit 84 when the grinding feed for the last several tens of nanometers is performed at a relatively low grinding feed speed in the workpiece grinding step S30 have the same value.

Second Embodiment

Next, a second embodiment of the present invention is described with reference to FIG. 14. FIG. 14 is a flowchart of a method for grinding the rectangular substrate 11 according to the second embodiment. In the second embodiment, first, a first holding surface grinding step S12 is performed with use of a grinding wheel exclusively used for the holding surface (not illustrated).

In the first holding surface grinding step S12, the holding surface 16a is ground by the grinding wheel exclusively used for the holding surface, so that the upper surface of the second frame 20 and the upper surface of the porous plate 22 are formed to have a substantially conical shape (at this time, the recessed curved surface area 16a₂ is not formed).

Thereafter, the grinding wheel exclusively used for the holding surface is replaced with a workpiece grinding wheel (which corresponds to the grinding wheel 68 described above) (wheel replacing step S14). After the wheel replacing step S14, the holding surface 16a is ground by the workpiece grinding wheel, so that a holding surface 16a including a conical area 16a₁ and a recessed curved surface area 16a₂ is formed (second holding surface grinding step S16).

After completing the holding surface 16a by the steps described above, the holding step S20 is performed, and the one side 11b of the rectangular substrate 11 is ground with the workpiece grinding wheel continuously being used (workpiece grinding step S30).

The grinding wheel exclusively used for the holding surface includes a plurality of grindstones each including abrasive grains having an average grain size that is relatively large. The grain size of the abrasive grains of the grinding wheel exclusively used for the holding surface is, for example, #320, and a resin bond is used as the bonding material, for example.

The grain size represents the size of the abrasive grains. The grain size in this description follows the standard described in Japanese Industrial Standards (JIS) R 6001-2:2107 (Bonded abrasives—Determination and designation of grain size distribution—Part 2: Microgrits) defined in JIS or conforms to the standard.

The workpiece grinding wheel includes a plurality of grindstones each including abrasive grains having an average grain size that is relatively small. The average grain size of the abrasive grains of the workpiece grinding wheel is small compared to the average grain size of the abrasive grains of the grinding wheel exclusively used for the holding surface. The grain size of the abrasive grains of the workpiece grinding wheel is, for example, #3000, and a vitrified bond is used as the bonding material, for example.

According to the second embodiment, the holding surface 16a can be formed efficiently compared to the case in which only the workpiece grinding wheel is used to form the holding surface 16a including the conical area 16a₁ and the recessed curved surface area 16a₂ from a substantially flat holding surface 16a including no conical area 16a₁ and recessed curved surface area 16a₂.

Note that grinding the holding surface 16a by use of only the grinding wheel exclusively used for the holding surface cannot form a holding surface 16a including the recessed curved surface area 16a₂. When the grinding wheel exclusively used for the holding surface that has relatively high grinding power is to be used, the holding surface 16a needs to be ground by the workpiece grinding wheel that has relatively low grinding power.

(Modification of Chuck Table 16)

Next, with reference to FIGS. 15A through 15D, a modification of the chuck table 16 is described. FIG. 15A is a plan view of a chuck table 96 including a plurality of holding surfaces 96a each having a reed shape.

FIG. 15B is a perspective view of the chuck table 96 illustrated in FIG. 15A. In the chuck table 96 illustrated in FIGS. 15A and 15B, three sets of the second frame 20 and the porous plate 22, each set having a reed shape, are disposed in parallel on an upper surface of the first frame 18.

However, the shape of each of the plurality of holding surfaces 96a formed through the holding surface grinding step S10 is different from one another. As can be recognized by analogy based on the shape of the holding surface 16a illustrated in FIGS. 2A through 2C, the conical area 16a₁ is formed in the vicinity of the rotation center 24a3 of the holding surface 96a located at the central portion in plan view.

Further, in each of the two holding surfaces 96a that are located at both end portions in plan view, the recessed curved surface area 16a₂ is formed in the vicinity of each corner 96b that is relatively far from the rotation center 24a3, and this corner 96b is protruding compared to an intermediate point 96c on a long side that is on a side far from the rotation center 24a3.

This is because, in the holding surface grinding step S10, in association with the rotation of the chuck table 96 about the rotation center 24a3, the area where the contact area (grinding area) between the holding surface 96a and the grinding surface 68c (see FIG. 6) is large becomes relatively thick, while the area where the contact area (grinding area) between the holding surface 96a and the grinding surface 68c is small becomes relatively thin.

FIG. 15C is a plan view of a chuck table 106 including a plurality of holding surfaces 106a each having a square shape, and FIG. 15D is a perspective view of the chuck table 106 illustrated in FIG. 15C.

In the chuck table 106 illustrated in FIGS. 15C and 15D, three sets of the second frame 20 and the porous plate 22, each set having a square shape, are disposed in such a manner as to surround the rotation center 24a3 on the first frame 18. However, the shape of each of the holding surfaces 106a formed through the holding surface grinding step S10 is different from one another.

While a description regarding the specific shapes of the holding surfaces 106a is omitted, in association with the rotation of the chuck table 106 about the rotation center 24a3, the area where the contact area (grinding area) between the holding surface 106a and the grinding surface 68c (see FIG. 6) is large becomes relatively thick, and the area where the contact area (grinding area) between the holding surface 106a and the grinding surface 68c is small becomes relatively thin.

Also in cases where the chuck table 96 illustrated in FIGS. 15A and 15B or the chuck table 106 illustrated in FIGS. 15C and 15D is used, the parameters related to grinding processing are controlled such that the load has the same value in both the holding surface grinding step S10 and the workpiece grinding step S30.

As a result, compared to the case in which such control is not performed, a difference between the shape of the ground holding surface 96a or 106a and the shape of the ground one side 11b of the rectangular substrate 11 can be reduced. This leads to a reduction in the thickness variation in the ground rectangular substrate 11.

Besides, the structure, methods, and the like according to the embodiments can be modified as appropriate within the range not departing from the scope of object of the present invention. The shape of each of the holding surfaces 16a, 96a, and 106a is not limited to a square, a rectangular, or the like, and may be a tetragon including a parallelogram, a rhombus, and a trapezoid, a triangle, or a polygon with five or more sides.

Further, in the controller 86, the function of the holding surface grinding command section 90 and the function of the workpiece grinding command section 92 are essentially the same, and hence, the controller 86 may include only either the holding surface grinding command section 90 or the workpiece grinding command section 92. In this case, one of the two sections is used in both the holding surface grinding step S10 and the workpiece grinding step S30.

The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.