METHOD OF MANUFACTURING WAFER

Abstract

A method of manufacturing a wafer from a workpiece that has a first surface, a second surface positioned opposite the first surface, and a side surface joined to outer peripheral edges of the first and second surfaces, the first and second surfaces containing warpage, the method includes grinding the first surface of the workpiece while holding the workpiece on a first holding table such that the second surface faces the first holding table and the first surface is exposed, by gripping and securing the side surface of the workpiece with at least two fixing members of the first holding table without holding the second surface under suction on the first holding table, grinding the second surface of the workpiece, forming a peel-off layer in the workpiece, and peeling off the wafer from the workpiece along the peel-off layer that acts as peel-off initiating points.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method of manufacturing a wafer from a workpiece that has a first surface, a second surface positioned opposite the first surface, and a side surface joined to an outer peripheral edge of the first surface and an outer peripheral edge of the second surface, the first and second surfaces containing warpage.

Description of the Related Art

Device chips for use in electronic appliances are fabricated by a plurality of devices being formed on the face side of a wafer, e.g., a silicon wafer, as a monocrystalline substrate made of a semiconductor material such as silicon and then the wafer being divided into pieces as device chips each including one of the devices. The silicon wafer from which the device chips are fabricated is usually produced from a cylindrical ingot being sliced with a wire saw.

The wire saw has a thickness that is relatively large compared to the thickness of the wafer to be fabricated. Moreover, since the wafer cut off from the ingot by the wire saw has surface irregularities left on its face and reverse sides by the wire saw, it is necessary to polish the face and reverse sides of the wafer to remove the surface irregularities therefrom. Consequently, the volume of the ingot material that is discarded per wafer, also known as saw dust, is relatively large.

It has been proposed to fabricate a wafer from an ingot by use of a laser beam rather than the wire saw (see, for example, Japanese Patent Laid-open No. 2019-12765). Specifically, a laser beam is applied to an ingot to form a peel-off layer in the ingot at a predetermined depth from a face side of the ingot, and then a flat ingot fragment is peeled off as a wafer from the ingot along the peel-off layer acting as peel initiating points. Thereafter, the surface of the ingot from which the wafer has been peeled off is planarized by grinding. The above sequence of processing steps is repeated on the ingot to fabricate a plurality of wafers successively from the ingot.

In preparation for forming a peel-off layer in an ingot with a laser beam, it has been customary to grind a surface of the ingot to planarize the surface that is to be irradiated with the laser beam.

If an ingot to be processed by a laser beam is an ingot as sliced from a straight cylindrical part, except a tail and a cone, of a cylindrical ingot manufactured by a monocrystalline crystal growth method such as the Czochralski method after the straight cylindrical part has been cut into a plurality of longitudinally separate sections, then the ingot has warpage in its face and reverse sides.

When the ingot is held under suction on the flat holding surface of a chuck table of a grinding apparatus, the ingot is elastically deformed into a shape in line with the holding surface to correct itself out of the warpage. After the ingot has been ground by the grinding apparatus, the ingot is removed from the holding surface of the chuck table. Upon removal from the holding surface, the ingot is deformed back into the shape containing the warpage.

Therefore, if a peel-off layer is formed in the ingot that has the warpage after the grinding and a wafer is peeled off from the ingot along the peel-off layer that acts as peel-off initiating points, then the wafer has a shape that contains the warpage from the ingot. The warpage remaining in the wafer is a problem.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method of manufacturing a wafer from an ingot having warpage by use of a laser beam in a manner of restraining the wafer from containing the warpage from the ingot.

In accordance with an aspect of the present invention, there is provided a method of manufacturing a wafer from a workpiece that has a first surface, a second surface positioned opposite the first surface, and a side surface joined to an outer peripheral edge of the first surface and an outer peripheral edge of the second surface, the first and second surfaces containing warpage, the method including a first grinding step of grinding the first surface of the workpiece while holding the workpiece on a first holding table such that the second surface faces the first holding table and the first surface is exposed, by gripping and securing the side surface of the workpiece with at least two fixing members of the first holding table without holding the second surface under suction on the first holding table, after the first grinding step, a second grinding step of grinding the second surface of the workpiece, after the second grinding step, a peel-off layer forming step of forming in the workpiece a peel-off layer that includes modified layers and cracks developed from the modified layers, by positioning a focused spot of a laser beam having a wavelength transmittable through the workpiece in the workpiece at a predetermined depth corresponding to a thickness of the wafer to be manufactured from the workpiece and moving the focused spot and the workpiece relatively to each other in a predetermined direction perpendicular to a thicknesswise direction of the workpiece, and after the peel-off layer forming step, a peeling step of peeling off the wafer from the workpiece along the peel-off layer that acts as peel-off initiating points.

Preferably, the second grinding step includes holding the first surface that has been ground in the first grinding step under suction on a holding surface of a second holding table such that the second surface is exposed.

Preferably, the first surface that is exposed in the first grinding step has protruding warpage, and the first grinding step includes reducing the warpage of the first surface by grinding the first surface.

Preferably, the method of manufacturing a wafer further includes, before the peel-off layer forming step, a planarizing step of reducing surface roughness of one, to be irradiated with the laser beam in the peel-off layer forming step, of the first surface that has been ground in the first grinding step or the second surface that has been ground in the second grinding step.

With the method of manufacturing a wafer according to the aspect of the present invention, in the first grinding step, the first surface of the workpiece is ground while the workpiece is being held on the first holding table such that the first surface is exposed by gripping and securing the side surface of the workpiece with at least the two fixing members of the first holding table without holding the second surface that has warpage under suction on the first holding table.

Consequently, warpage that remains on the first surface when the ground workpiece is removed from the first holding table is reduced compared to warpage that remains when the first surface is ground while the workpiece is being elastically deformed by holding the second surface under suction on the first holding table. Therefore, warpage of the wafer that is peeled off from the workpiece after the peeled-layer forming step and the peeling step is 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 a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method of manufacturing a wafer according to an embodiment of the present invention;

FIG. 2A is a perspective view of an ingot;

FIG. 2B is a side elevational view of the ingot;

FIG. 3A is a plan view of a first holding table;

FIG. 3B is a side elevational view of the ingot that is fixedly positioned on the first holding table and the first holding table;

FIG. 4 is a side elevational view illustrating the manner in which a first grinding step is started;

FIG. 5 is a side elevational view illustrating the manner in which the first grinding step is ended;

FIG. 6 is a graph illustrating an example of cross-sectional curves of a first surface of the ingot available before and after the first grinding step;

FIG. 7 is a side elevational view, partly in cross section, illustrating the manner in which a second grinding step is started;

FIG. 8 is a side elevational view, partly in cross section, illustrating the manner in which the second grinding step is ended;

FIG. 9 is a perspective view illustrating the manner in which a peel-off layer forming step is carried out;

FIG. 10A is a plan view illustrating the manner in which the peel-off layer forming step is carried out;

FIG. 10B is a cross-sectional view of the ingot with a peel-off layer formed therein;

FIG. 11A is a side elevational view illustrating the manner in which a peeling step is started;

FIG. 11B is a side elevational view illustrating the manner in which the peeling step is ended;

FIG. 12A is a flowchart illustrating a first modification of the method of manufacturing a wafer;

FIG. 12B is a flowchart illustrating a second modification of the method of manufacturing a wafer; and

FIG. 13 is a side elevational view illustrating the manner in which a second grinding step of a third modification of the method of manufacturing a wafer is carried out.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiment

An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a flowchart illustrating a method of manufacturing a wafer 13 (see FIG. 11B) from an ingot 11 (see FIG. 2A), i.e., a workpiece, according to the present embodiment.

According to the present embodiment, as illustrated in FIG. 1, the method includes a first grinding step S10, a second grinding step S20, a peel-off layer forming step S30, a peeling step S40, a decision step S50, and a peeled-surface grinding step S60 that are successively carried out in the order named. First, the ingot 11 on which the steps of the method are performed will be described below with reference to FIGS. 2A and 2B.

FIG. 2A illustrates the ingot 11 in perspective. The ingot 11 is made of monocrystalline silicon (Si). The ingot 11 is not limited to any particular conductivity type, and may be of the p-type or the n-type. According to the present embodiment, the ingot 11 is of a cylindrical shape and has a diameter of 12 inches, i.e., approximately 300 mm, and a thickness of 14 mm. While the diameter and thickness of the ingot 11 are not limited to these values, the thickness of the ingot 11 should preferably be 3 mm or more in view of the structure of a first holding table 12 to be described later, and should preferably be 1000 mm or less to meet safety requirements for processing the ingot 11.

The ingot 11 has a first surface 11a, a second surface 11b positioned opposite the first surface 11a in a thicknesswise direction 11c of the ingot 11, and a round side surface 11d joined to an outer peripheral edge 11a1 of the first surface 11a and an outer peripheral edge 11b1 of the second surface 11b. According to the present embodiment, the side surface 11d has a notch 11d1 defined therein that extends along the thicknesswise direction 11c of the ingot 11. However, the side surface 11d may have an unillustrated orientation flat, instead of the notch 11d1.

As illustrated in FIG. 2B, the ingot 11 has warpage on the first surface 11a and the second surface 11b. FIG. 2B illustrates the ingot 11 in side elevation. According to the present embodiment, the first surface 11a has protruding warpage 11e₁ that is approximately 50 μm high, for example. According to the present embodiment, the second surface 11b has recessed warpage 11e₂ that is approximately 50 μm deep, for example.

The warpage of the first surface 11a and the second surface 11b can be measured using a measuring instrument, e.g., a contact-type microfigure measuring instrument or a non-contact-type height measuring instrument that uses a measuring, not processing, laser beam, that moves an unillustrated stage on which the ingot 11 is to be placed and an unillustrated sensor head relatively to each other.

A grinding apparatus 10 (see FIG. 4) to be used in the first grinding step S10 will be described below with reference to FIGS. 3A through 5. The grinding apparatus 10 has a first holding table 12 (see FIG. 3A). FIG. 3A illustrates the first holding table 12 in plan, and FIG. 3B illustrates the ingot 11 that is fixedly positioned on the first holding table 12 and the first holding table 12 in side elevation.

The first holding table 12 has a disk-shaped inner frame 14 made of non-porous ceramic. The inner frame 14 has an unillustrated circular recess defined in an upper portion thereof in concentric relation to the inner frame 14. A circular porous plate 16 made of porous ceramic is fixedly fitted in the recess in the inner frame 14. The porous plate 16 has a substantially flat upper surface. The inner frame 14 has a substantially flat upper surface extending around the recess. The upper surface of the inner frame 14 and the upper surface of the porous plate 16 lie substantially flush with each other, jointly providing a substantially flat holding surface of the first holding table 12. Yet, the holding surface of the first holding table 12 may be of a conical shape whose central portion slightly protrudes upwardly beyond an outer circumferential portion thereof.

The inner frame 14 is fluidly connected through an unillustrated fluid channel to an unillustrated suction source such as a vacuum pump. When actuated, the suction source generates a negative pressure that is transmitted via the fluid channel to the porous plate 16. The fluid channel includes an unillustrated valve such as a solenoid-operated valve. When the valve is opened and closed, it selectively transmits and interrupts the negative pressure from the suction source to the porous plate 16. For example, when the valve is opened, it transmits the negative pressure from the suction source to the porous plate 16, enabling the first holding table 12 to function as a chuck table for holding the ingot 11 under suction on the holding surface of the first holding table 12. Conversely, when the valve is closed, it prevents the negative pressure from the suction source from being transmitted to the porous plate 16.

The first holding table 12 is fixedly mounted on a rotatable shaft 12a (schematically depicted in FIG. 4) that extends vertically along a vertical Z-axis indicated by the arrow Z. An unillustrated driven pulley is coaxially mounted on the shaft 12a. An unillustrated drive pulley is coaxially mounted on the output shaft of an unillustrated electric motor. An unillustrated endless belt is trained around the driven pulley and the drive pulley. When the electric motor is energized, its rotary power is transmitted from the drive pulley through the endless belt to the driven pulley, rotating the shaft 12a and hence the first holding table 12.

An unillustrated tilt adjusting mechanism is disposed beneath the first holding table 12. When actuated, the tilt adjusting mechanism adjusts the tilt of the first holding table 12 to make the upper surfaces of the inner frame 14 and the porous plate 16, i.e., the holding surface of the first holding table 12, substantially parallel to a horizontal plane that lies perpendicular to the Z-axis. In a case where the holding surface is of a conical shape as described above, the tilt adjusting mechanism adjusts the tilt of the first holding table 12 to make a portion of the holding surface substantially parallel to the horizontal plane. At this time, the shaft 12a is oriented slightly obliquely to the Z-axis.

As illustrated in FIGS. 3A and 3B, a clamp jig 18 is fixed to an outer circumferential side of the inner frame 14. The clamp jig 18 has an annular outer frame 20 made of metal or ceramic. The outer frame 20 has four unillustrated internally threaded holes defined therein that extend radially therethrough from an outer circumferential side to an inner circumferential side thereof. The four internally threaded holes are angularly spaced at substantially equal intervals circumferentially around the outer frame 20. Four bolts 22 have respective heads 22a and four externally threaded shanks 22b extending radially inwardly from the respective heads 22a and rotatably threaded radially in the respective internally threaded holes in the outer frame 20.

When the heads 22a of the bolts 22 are turned in predetermined directions, the shanks 22b have their tip ends pressed against the outer circumferential side of the inner frame 14. The bolts 22 on the outer frame 20 are thus tightened to grip the inner frame 14, so that the outer frame 20 is securely mounted on the inner frame 14. At this time, the outer frame 20 and the inner frame 14 have respective upper surfaces lying flush with each other on a plane.

Clamps or fixing members 24 are disposed on the upper surface of the outer frame 20 in alignment with the respective internally threaded holes in the outer frame 20. According to the present embodiment, the clamps 24 include four clamps 24. Each of the clamps 24 has a base 26 shaped as a rectangular parallelepiped fixed to the outer frame 20.

The base 26 includes an unillustrated hollow cylindrical internally threaded hole defined therein that is open at an upper surface thereof. A bolt 28 has a head 28a and an externally threaded shank 28b extending downwardly from the head 28a and rotatably threaded in the threaded hole in the base 26. When the bolt 28 is turned about its central axis, the bolt 28 is moved along the Z-axis with respect to the base 26.

A movable member 30 shaped as a rectangular parallelepiped is disposed between the head 28a of the bolt 28 and an upper surface of the base 26 of each of the clamps 24. The movable member 30 is movable in directions perpendicular to the longitudinal directions of the shank 28b of the bolt 28. The movable member 30 has a slot defined therein that extends longitudinally therealong. The slot also extends vertically through the movable member 30 between its upper and lower surfaces. The shank 28b of the bolt 28 is inserted vertically through the slot.

When the heads 28a of the bolts 28 are turned to grip the movable members 30 between the heads 28a and the upper surfaces of the bases 26, the movable members 30 are fixed in position with respect to the outer frame 20. Flat presser plates 32 are fixedly attached to respective inner ends of the movable members 30 that are closer to the center of the first holding table 12.

According to the present embodiment, four presser plates 32 are spaced at substantially equal intervals circumferentially around the first holding table 12. Two of the four presser plates 32 face each other in diametrically opposite relation with respect to the first holding table 12, and the other two also face each other in diametrically opposite relation with respect to the first holding table 12. The presser plates 32 have respective tip end faces closer to the center of the first holding table 12 that act as contact faces for contacting the side surface 11d of the ingot 11.

The contact faces of the respective presser plates 32 may be flat faces or curved faces having a radius of curvature corresponding to the radius of curvature of the round side surface 11d of the ingot 11. Moreover, the contact faces of the respective presser plates 32 may be made of a relatively pliable elastic material such as resin.

According to the present embodiment, the first holding table 12 has four clamps 24 as described above. However, two or three clamps 24 or five or more clamps 24 may be provided as long as they can grip the side surface 11d of the ingot 11, for example, in position diametrically across the ingot 11. Since the first holding table 12 rotates at a speed of 100 rpm or higher, for example, while the ingot 11 is being processed on the first holding table 12, the first holding table 12 should preferably have three or more clamps 24.

As illustrated in FIG. 4, the grinding apparatus 10 also includes a grinding unit 34 disposed above the first holding table 12. The grinding unit 34 is coupled to an unillustrated Z-axis moving mechanism including a ball screw for moving the grinding unit 34 along the Z-axis. The grinding unit 34 has a cylindrical spindle 36 whose longitudinal axis extends substantially parallel to the Z-axis. The spindle 36 has an upper portion rotatably housed in an unillustrated spindle housing and coupled to an unillustrated electric motor. The spindle 36 functions as a rotor when it is rotated by the electric motor.

A disk-shaped mount 38 is attached to a lower end of the spindle 36. An annular grinding wheel 40 is mounted on a lower surface of the mount 38 by unillustrated bolts. The grinding wheel 40 includes a wheel base 40a that is essentially identical in diameter to the mount 38. The wheel base 40a has a lower surface on which there are disposed an annular array of grindstones 40b, each shaped as a block, spaced at substantially equal intervals circumferentially around the grinding wheel base 40a. Each of the grindstones 40b is made up of abrasive grains and a bonding material that binds the abrasive grains together.

The abrasive grains of the grindstones 40b have a grain size in the range of #240 to #1200, for example. In other words, the grinding wheel 40 is classified as a coarse grinding wheel. The levels of granularity represented by #and numerals are defined by R6001-2:2017 (Bonded abrasives-Determination and designation of grain size distribution-Part 2: Microgrits) of the Japanese Industrial Standards (JIS). The grain sizes of #240 to #1200 may be assessed by a sedimentation test or an electrical resistance test.

The first grinding step S10 for grinding the first surface 11a of the ingot 11 will be described below. In the first grinding step S10, an operator specifies which of the opposite faces of the ingot 11 is the first surface 11a of the protruding shape and which of the opposite faces of the ingot 11 is the second surface 11b of the recessed shape, by using the measuring instrument referred to above.

Then, as illustrated in FIG. 3B, the operator places the ingot 11 on the first holding table 12 such that the recessed second surface 11b faces the first holding table 12 and the protruding first surface 11a is exposed upwardly. At this time, the operator keeps the center of the first surface 11a and the center of the porous plate 16 in substantial alignment with each other. Then, the operator moves the movable members 30 radially inwardly toward the center of the first holding table 12 until the presser plates 32 are brought into contact with the side surface 11d of the ingot 11, and then tightens the bolts 28 to cause their shanks 28b to be threaded into the threaded hole in the base 26, securing the movable members 30 positionally with respect to the outer frame 20.

Particularly, in the first grinding step S10, the valve in the fluid channel between the suction source and the porous plate 16 remains closed, interrupting the transmission of the negative pressure from the suction source to the porous plate 16. Hence, a disk-shaped gap 17 is created between the recessed second surface 11b and the upper surface of the porous plate 16. With the first surface 11a exposed upwardly while the second surface 11b is not being held under suction on the porous plate 16 of the first holding table 12, the ingot 11 is gripped and secured in position by the clamps 24 of the first holding table 12.

With the ingot 11 thus held on the first holding table 12, grinding water such as pure water is supplied to an area where the grindstones 40b and the ingot 11 are to be held in abrasive contact with each other, the first holding table 12 and the spindle 36 are rotated about their central axes in respective directions by their respective electric motors, and the grinding unit 34 is grinding-fed, i.e., moved downwardly, at a predetermined grinding feed speed by the Z-axis moving mechanism. When the grinding unit 34 is thus grinding-fed, the grindstones 40b are brought into abrasive contact with the first surface 11a of the ingot 11, performing in-feed grinding on the first surface 11a. FIG. 4 illustrates in side elevation the manner in which the first grinding step S10 is started. For example, processing conditions in the first grinding step S10 are established as follows:

• • Rotational speed of the spindle: 3200 rpm
  • Rotational speed of the first holding table: 300 rpm
  • Grinding-feed speed: 3.0 μm/s

According to the present embodiment, inasmuch as the protruding first surface 11a is exposed upwardly, the ingot 11 is prevented from being tilted and positionally shifted and hence is ground more stably than if the recessed second surface 11b were exposed upwardly and the protruding first surface 11a faced the first holding table 12.

After the grinding unit 34 has been grinding-fed to grind the first surface 11a over a predetermined period of time, reducing the warpage 11e₁ of the first surface 11a, e.g., substantially planarizing the first surface 11a, the grinding unit 34 stops being grinding-fed, bringing the first grinding step S10 to an end. FIG. 5 illustrates in side elevation the manner in which the first grinding step S10 is ended. After being stopped from being grinding-fed, the grinding unit 34 is moved upwardly to keep the grinding wheel 40 away from the ingot 11 by a sufficient distance. Then, the first holding table 12 stops being rotated, after which the operator loosens the bolts 28 and moves the movable members 30 away from the ingot 11. Then, the operator removes the ingot 11 from the first holding table 12.

FIG. 6 is a graph illustrating an example of cross-sectional curves of the first surface 11a, i.e., the actual surface, of the ingot 11 before and after first grinding step S10. The horizontal axis of the graph represents positions (mm) on the first surface 11a along a diametrical direction thereof, and the vertical axis represents heights (μm) at those positions of the first surface 11a from a reference height, i.e., 0 μm on the vertical axis. The graph lacks data on its right end, though the first surface 11a has a diameter of 300 mm.

The pieces of data of the graph of FIG. 6 were measured using a microfigure measuring instrument, type ET4100S, sold by Kosaka Laboratory Ltd. As illustrated in FIG. 6, whereas the protruding warpage 11e₁ of the first surface 11a was approximately 50 μm high before the first grinding step S10, the warpage 11e₁ of the first surface 11a was reduced to approximately 5 μm after the first grinding step S10.

After the first grinding step S10, the second grinding step S20 is carried out to grind the second surface 11b of the ingot 11. A grinding apparatus 50 to be used in the second grinding step S20 will be described below with reference to FIG. 7. As illustrated in FIG. 7, the grinding apparatus 50 has a second holding table 52 that is different from the first holding table 12. The second holding table 52 is different from the first holding table 12 in that it is free of the clamp jig 18. Other structural details of the second holding table 52 are the same as those of the first holding table 12. Those structural details that are shared by the first holding table 12 are denoted by reference characters identical to those of the first holding table 12 and will be omitted from detailed description.

The second holding table 52 functions as a chuck table. As with the first holding table 12, the second holding table 52 has an inner frame 14 and a porous plate 16 that is fitted in the inner frame 14 and that has an upper surface that is substantially flat. The upper surface of the inner frame 14 and the upper surface of the porous plate 16 lie substantially flush with each other, jointly providing a substantially flat holding surface 52a. As in the first holding table 12, the holding surface 52a may be of a conical shape whose central portion slightly protrudes upwardly beyond an outer circumferential portion thereof. As with the grinding apparatus 10, the grinding apparatus 50 also includes a grinding unit 34 disposed above the second holding table 52. The grinding unit 34 of the grinding apparatus 50 is structurally identical to the grinding unit 34 of the grinding apparatus 10.

For grinding the second surface 11b of the ingot 11 in the second grinding step S20, the operator places the ingot 11 on the second holding table 52 such that the second surface 11b is exposed upwardly and the first surface 11a that has been ground faces the second holding table 52 (see FIG. 7). Then, the operator opens the valve to transmit the negative pressure from the suction source to the porous plate 16, so that the first surface 11a that has been ground substantially flatwise in the first grinding step S10 is held under suction on the holding surface 52a.

Then, with the ingot 11 thus held under suction on the second holding table 52, grinding water is supplied to an area where the grindstones 40b and the ingot 11 are to be held in abrasive contact with each other, the second holding table 52 and the spindle 36 are rotated about their central axes in respective directions by their respective electric motors, and the grinding unit 34 is grinding-fed, i.e., moved downwardly, at a predetermined grinding feed speed by the Z-axis moving mechanism.

FIG. 7 illustrates in side elevation, partly in cross section, the manner in which the second grinding step S20 is started. Processing conditions that are established in the second grinding step S20 are essentially the same as those in the first grinding step S10, for example. The grinding unit 34 is grinding-fed to grind the second surface 11b over a predetermined period of time, reducing the warpage 11e₂ of the second surface 11b, e.g., substantially planarizing the second surface 11b, after which the grinding unit 34 stops being grinding-fed, bringing the second grinding step S20 to an end.

FIG. 8 illustrates in side elevation, partly in cross section, the manner in which the second grinding step S20 is ended. After being stopped from being grinding-fed, the grinding unit 34 is moved upwardly to keep the grinding wheel 40 away from the ingot 11 by a sufficient distance. Then, the second holding table 52 stops being rotated, after which the operator closes the valve to interrupt the negative pressure applied to the porous plate 16, releasing the ingot 11 from the holding surface 52a. Then, the operator removes the ingot 11 from the second holding table 52.

The second grinding step S20 is followed by the peel-off layer forming step S30. In the peel-off layer forming step S30, a peel-off layer 15 (see FIG. 10B) is formed in the ingot 11 along a plane therein at a predetermined depth 11f from the first surface 11a, with use of a laser processing apparatus 60 (see FIG. 9).

First, the laser processing apparatus 60 will be described below with reference to FIGS. 9, 10, and 10B. The laser processing apparatus 60 has a third holding table 62 that also functions as a chuck table. The third holding table 62 includes a frame and a porous plate that are equivalent respectively to the inner frame 14 and the porous plate 16 of the first holding table 12 described above.

The frame and porous plate of the third holding table 62 have respective upper surfaces that are substantially flat and that lie substantially flush with each other, jointly providing a substantially flat holding surface 62a (see FIG. 10B) for holding the ingot 11 under suction thereon. The third holding table 62 is movable along an X-axis indicated by an arrow X and a Y-axis indicated by an arrow Y. The X and Y axes extend horizontally perpendicularly to the Z-axis extending vertically.

For example, the third holding table 62 is movable along the X-axis by an unillustrated ball-screw-type X-axis moving mechanism and is also movable along the Y-axis by an unillustrated ball-screw-type Y-axis moving mechanism. Moreover, the third holding table 62 is rotatable in a predetermined angular range about a vertical axis along the Z-axis.

The laser processing apparatus 60 further includes a laser beam applying unit 64 that is disposed above the holding surface 62a and that includes a beam condenser 66. The laser beam applying unit 64 includes an unillustrated laser oscillator having an unillustrated laser medium made of Nd:YAG crystal or Nd:YVO₄, for example, an unillustrated exciting light source, such as a lamp for applying exciting light to the laser medium, and a Q switch.

The laser oscillator emits a pulsed laser beam L having a wavelength of 1064 nm, for example, that is transmittable through the ingot 11. The emitted laser beam L is reflected by an unillustrated mirror and applied through an unillustrated condensing lens disposed in the beam condenser 66 toward the holding surface 62a.

The beam condenser 66 is coupled to an unillustrated ball-screw-type Z-axis moving mechanism and is movable vertically along the Z-axis by the Z-axis moving mechanism. When actuated, the Z-axis moving mechanism moves the beam condenser 66 vertically to adjust the position of the beam condenser 66 along the Z-axis, thereby adjusting the position of a focused spot P of the laser beam L along the Z-axis.

FIG. 9 illustrates in perspective the manner in which the peel-off layer forming step S30 is carried out. FIG. 10A illustrates in plan the manner in which the peel-off layer forming step S30 is carried out, and FIG. 10B illustrates in cross section the ingot 11 with the peel-off layer 15 formed therein. In the peel-off layer forming step S30, the position of the focused spot P of the laser beam L along the Z-axis is placed at the predetermined depth 11f in the ingot 11 that corresponds to the thickness of a wafer 13 (see FIG. 11B) to be peeled off from the ingot 11. The distance from the first surface 11a of the ingot 11 to the plane at the depth 11f represents the thickness of the wafer 13.

After the position along the Z-axis of the focused spot P of the laser beam L has been adjusted, the focused spot P and the ingot 11 are moved relatively to each other along the X-axis. According to the present embodiment, the thicknesswise direction 11c of the ingot 11 extends substantially parallel to the Z-axis, and the X-axis extends perpendicularly to the thicknesswise direction 11c of the ingot 11.

The focused spot P and the ingot 11 are moved relatively to each other as follows: As illustrated in FIG. 9, for example, the focused spot P is fixed in position, and the X-axis moving mechanism moves the third holding table 62 in a processing feed direction along the X-axis. After the X-axis moving mechanism has moved the third holding table 62 until the focused spot P traverses the outer peripheral edge 11a1 of the first surface 11a along the processing feed direction, the X-axis moving mechanism stops moving the third holding table 62. While the focused spot P is moving in the ingot 11 in the processing feed direction along the X-axis, it forms a modified layer 15a (see FIG. 10A) and cracks 15b developed from the modified layer 15a in the ingot 11. Then, the Y-axis moving mechanism moves the third holding table 62 a predetermined distance in an indexing feed direction along the Y-axis.

Then, the X-axis moving mechanism moves the third holding table 62 until the focused spot P traverses the outer peripheral edge 11a1 of the first surface 11a along the processing feed direction in the same manner as described above. In this fashion, the third holding table 62 is repeatedly moved alternately in the processing feed direction and the indexing feed direction. The focused spot P thus moved in the ingot 11 forms a peel-off layer 15 (see FIG. 10A) including a succession of modified layers 15a and cracks 15b developed from each of the modified layers 15a, substantially all along the plane at the depth 11f in the ingot 11. For example, processing conditions for the peel-off layer forming step S30 are established as follows:

• • Laser beam wavelength: 900 nm to 1400 nm
  • Laser beam average power output: 2.5 W
  • Laser beam pulse frequency: 40 kHz
  • Processing feed speed: 240 mm/s
  • Indexing feed distance: 500 μm

The modified layers 15a refer to a region of the ingot 11 that has a physical property, e.g., an amorphous structure, different from the crystallinity, e.g., a monocrystalline structure, of a region of the ingot 11 that has not been irradiated with the laser beam L, and has a lower level of mechanical strength than the non-irradiated region. As illustrated in FIG. 10A, the cracks 15b extend along the indexing feed direction, i.e., the Y-axis, perpendicular to the thicknesswise direction 11c and the processing feed direction, i.e., the X-axis, from each of the modified layers 15a that are formed straight along the X-axis as the ingot 11 is viewed in plan.

After the peel-off layer forming step S30, the peeling step S40 is carried out to peel off the wafer 13 from the ingot 11 along the peel-off layer 15 that acts as peel-off initiating points. The peeling step S40 is carried out using a peeling apparatus 70 (see FIGS. 11A and 11B). First, the peeling apparatus 70 will be described below with reference to FIG. 11A. The peeling apparatus 70 has a fourth holding table 72.

The fourth holding table 72 is essentially equal in diameter and structure to the third holding table 62 described above. The fourth holding table 72 has a substantially flat upper surface that functions as a holding surface 72a for holding the ingot 11 under suction thereon. Thus, the fourth holding table 72 also functions as a chuck table. The peeling apparatus 70 further includes a peeling unit 74 disposed above the fourth holding table 72. The peeling unit 74 has a cylindrical movable member 76 whose longitudinal axis extends substantially parallel to the Z-axis. The movable member 76 has an upper portion coupled to an unillustrated Z-axis moving mechanism.

The Z-axis moving mechanism is a ball-screw-type moving mechanism, for example, though it may be another actuator. The movable member 76 is movable along the Z-axis by the Z-axis moving mechanism. The movable member 76 has a lower end to which a disk-shaped suction head 78 is attached. The suction head 78 includes a frame and a porous plate equivalent to those of the fourth holding table 72 described above. The frame and the porous plate have respective lower surfaces lying flush with each other parallel to a horizontal plane defined as an XY plane extending along the X-axis and the Y-axis, jointly providing a holding surface 78a.

FIG. 11A illustrates in side elevation the manner in which the peeling step S40 is started. In the peeling step S40, the holding surface 72a of the fourth holding table 72 holds the ground second surface 11b under suction thereon, and the holding surface 78a holds the ground first surface 11a under suction thereon. Then, the Z-axis moving mechanism lifts the suction head 78, peeling off the wafer 13 from the remainder of the ingot 11 along the peel-off layer 15.

FIG. 11B illustrates in side elevation the manner in which the peeling step S40 is ended. Before the suction head 78 is lifted, an external force may be exerted on the ingot 11. The external force may be applied by the operator or a dedicated device that drives an unillustrated wedge, into the side surface 11d of the ingot 11 at a vertical position aligned with the peel-off layer 15. At this time, it is preferable to drive the wedge into the side surface 11d at respective vertical positions spaced at intervals circumferentially around the ingot 11, rather than driving the wedge into the side surface 11d of the ingot 11 at a single vertical position.

The external force thus applied to the side surface 11d of the ingot 11 assists in further developing the cracks 15b in the ingot 11 at the depth where the peel-off layer 15 is present, making it easy to peel off the wafer 13. Rather than driving the wedge into the side surface 11d, ultrasonic waves, i.e., elastic vibration waves in a frequency range in excess of 20 kHz, may be applied to the ingot 11 to exert an external force thereon.

Providing ultrasonic waves are applied to the ingot 11, they are applied to an exposed surface, i.e., the ground first surface 11a in FIG. 11A, of the ingot 11 through liquid such as pure water, for example, before the ingot 11 is held under suction on the suction head 78. Specifically, liquid to which ultrasonic waves are applied is ejected from a nozzle to the ingot 11. Alternatively, ultrasonic waves are applied from an ultrasonic horn via liquid to the ingot 11.

In a case where a nozzle or an ultrasonic horn is used, ultrasonic waves are applied as an external force to a circular local area of the ground first surface 11a that has a diameter ranging from approximately 5 mm to 50 mm. Then, the nozzle or the ultrasonic horn and the fourth holding table 72 are moved relatively to each other, and ultrasonic waves are applied as an external force to another circular local area of the ground first surface 11a. In this manner, the nozzle or the ultrasonic horn and the fourth holding table 72 are incrementally moved relatively to each other until finally the first surface 11a is covered in its entirety with local areas where external forces are applied.

The external forces thus applied to the first surface 11a are effective to interconnect the cracks 15b between adjacent ones of the modified layers 15a, making the mechanical strength of the peel-off layer 15 lower than in the case where the cracks 15b between adjacent ones of the modified layers 15a are not interconnected. Both a nozzle and an ultrasonic horn may be used to apply external forces to the first surface 11a. Providing both a nozzle and an ultrasonic horn are used to apply external forces to the first surface 11a, external forces may be applied from the nozzle and the ultrasonic horn to the first surface 11a in different local areas and/or the ultrasonic waves from the nozzle and the ultrasonic horn may have different frequencies.

After the peeling step S40, if no more wafer 13 is to be peeled off (NO in the decision step S50), then the method represented by the flowchart illustrated in FIG. 1 comes to an end. Conversely, if another wafer 13 is to be peeled off (YES in the decision step S50), then the peeled-surface grinding step S60 is carried out to grind a peeled surface 11g (see FIG. 11B) of the ingot 11 that has been exposed after the peeling step S40 with the grinding apparatus 10 or 50, reducing the surface roughness of the peeled surface 11g to a level lower than the level immediately after the peeling step S40.

Then, back to the peel-off layer forming step S30, another peel-off layer 15 is formed in the ingot 11 by use of the laser processing apparatus 60, after which another wafer 13 is peeled off with use of the peeling apparatus 70. In this manner, a plurality of wafers 13 are successively manufactured from the single ingot 11.

According to the present embodiment, as described above, in the first grinding step S10, the first surface 11a of the ingot 11 is ground while the ingot 11 is being held on the first holding table 12 with the first surface 11a being exposed by gripping and securing the side surface 11d of the ingot 11 with two or more clamps 24, i.e., at least two clamps 24, without holding the second surface 11b with the warpage of the ingot 11 under suction on the first holding table 12.

Consequently, warpage that remains on the first surface 11a when the ground ingot 11 is removed from the first holding table 12 is reduced compared to warpage that remains when the first surface 11a is ground while the ingot 11 is being elastically deformed by holding the second surface 11b under suction on the first holding table 12 such that the second surface 11b is held in its entirety in contact with the upper surface of the porous plate 16. Therefore, warpage of the wafer 13 that is peeled off from the ingot 11 after the peeled-layer forming step S30 and the peeling step S40 is reduced.

First Modification

A first modification of the above embodiment will be described below with reference to FIG. 12A. FIG. 12A is a flowchart illustrating a first modification of the method of manufacturing a wafer 13 according to the above embodiment. According to the first modification, the method includes a planarizing step S15 for reducing the surface roughness of the first surface 11a that has been ground in the first grinding step S10.

The planarizing step S15 is an optionally selective step. If the surface roughness of the first surface 11a that has been ground in the first grinding step S10 is relatively large, then the planarizing step S15 is carried out to reduce the surface roughness before the peel-off layer forming step S30. The planarizing step S15 may be omitted as with the method according to the above embodiment. In the planarizing step S15, the surface roughness of the first surface 11a that is to be irradiated with the laser beam L in the peel-off layer forming step S30 is reduced to make the arithmetic mean roughness Ra (see, for example, B0601: 2013 of JIS) of the contour curve of the first surface 11a equal to or smaller than 20 nm.

After the surface roughness of the first surface 11a has thus been reduced, it is easier for the laser beam L to enter the ingot 11 through the first surface 11a in the peel-off layer forming step S30 than in the case where the arithmetic mean roughness Ra exceeds 20 nm. In other words, the amount of energy of the laser beam L that would otherwise be lost due to the scattering caused by the rough first surface 11a is reduced, and hence the amount of energy of the laser beam L that enters the ingot 11 via the planarized first surface 11a is increased.

In the planarizing step S15, a grinding wheel having a plurality of grindstones whose abrasive grains have an average grain size smaller than the average grain size of the abrasive grains of the grindstones 40b used in the first grinding step S10 is used. For example, the grinding wheel used in the planarizing step S15 is classified as a finishing grinding wheel, for example. The finishing grinding wheel has a plurality of finishing grindstones whose abrasive grains have grain sizes ranging from #2000 to #8000. Processing conditions for the planarizing step S15 may be the same as or may be appropriately changed from those for the first grinding step S10.

Second Modification

A second modification of the above embodiment will be described below with reference to FIG. 12B. FIG. 12B is a flowchart illustrating a second modification of the method of manufacturing a wafer 13 according to the above embodiment. According to the second modification, the method includes a planarizing step S25 for reducing the surface roughness of the second surface 11b that has been ground in the second grinding step S20.

The planarizing step S25 is carried out after the second grinding step S20 but before the peel-off layer forming step S30. In the planarizing step S25, a grinding wheel having a plurality of grindstones whose abrasive grains have an average grain size smaller than the average grain size of the abrasive grains of the grindstones 40b used in the second grinding step S20, i.e., a finishing grinding wheel, is used.

In the planarizing step S25, the arithmetic mean roughness Ra of the contour curve of the second surface 11b is equal to or smaller than 20 nm. In the peel-off layer forming step S30, the second surface 11b whose surface roughness has been reduced in the planarizing step S25 is irradiated with the laser beam L. According to the first or second modification, therefore, the surface roughness of one of the first surface 11a ground in the first grinding step S10 or the second surface 11b ground in the second grinding step S20 that is to be irradiated with the laser beam L in the peel-off layer forming step S30 is reduced in the planarizing step S15 or S25.

Third Modification

A third modification of the above embodiment will be described below with reference to FIG. 13. FIG. 13 illustrates in side elevation the manner in which the second grinding step 20 of the third modification of the method of manufacturing a wafer 13 according to the above embodiment is carried out.

In the second grinding step 20 according to the above embodiment, the second holding table 52 that is free of the clamp jig 18 is used. In the second grinding step 20 according to the third modification, the first holding table 12 having the clamp jig 18 is used as in the first grinding step S10. However, in the second grinding step 20 according to the third modification, the ground first surface 11a is held under suction on the upper surface of the inner frame 14 and the upper surface of the porous plate 16. At this time, the side surface 11d of the ingot 11 is not secured by the clamps 24. However, the side surface 11d of the ingot 11 may be secured by the clamps 24.

The structural and methodical details according to the embodiment and the modifications may be changed or modified without departing from the scope of the present invention. For example, unlike in the above embodiment and modifications, the first surface 11a may be of a recessed shape whereas the second surface 11b may be of a protruding shape.

In the first grinding step S10, the grinding process itself can be performed, even if one of the first surface 11a or the second surface 11b that is recessed is exposed upwardly and the other surface that is protruding is oriented downwardly. However, when the protruding other surface faces the first holding table 12, since the central portion of the protruding other surface is held in contact with the porous plate 16, the ingot 11 is more likely to move when ground, making the ground surface of the ingot 11 less flat, than in the case where the recessed surface faces the first holding table 12. Therefore, it is preferable to orient the recessed surface downwardly in facing relation to the first holding table 12.

The grinding apparatuses 10 and 50 are different apparatuses. However, the grinding apparatus 10 may be used as the grinding apparatus 50 by removing the clamp jig 18 from the first holding table 12 of the grinding apparatus 10.

In the peel-off layer forming step S30, the first surface 11a that has been ground is exposed. However, the second surface 11b that has been ground may be exposed, and the peel-off layer 15 may be formed in the ingot 11 at the depth 11f from the ground second surface 11b.

Each of the first holding table 12, the second holding table 52, the third holding table 62, and the fourth holding table 72 may be an electrostatic chuck table for holding the ingot 11 under electrostatic forces thereon, rather than the negative pressure. Further alternatively, each of those holding tables may hold the ingot 11 thereon by a mechanism other than the chucks using the negative pressure and the electrostatic forces.

The present invention is not limited to the details of the above described preferred embodiment. 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.

Claims

1. A method of manufacturing a wafer from a workpiece that has a first surface, a second surface positioned opposite the first surface, and a side surface joined to an outer peripheral edge of the first surface and an outer peripheral edge of the second surface, the first and second surfaces containing warpage, the method comprising: a first grinding step of grinding the first surface of the workpiece while holding the workpiece on a first holding table such that the second surface faces the first holding table and the first surface is exposed, by gripping and securing the side surface of the workpiece with at least two fixing members of the first holding table without holding the second surface under suction on the first holding table;after the first grinding step, a second grinding step of grinding the second surface of the workpiece;after the second grinding step, a peel-off layer forming step of forming in the workpiece a peel-off layer that includes modified layers and cracks developed from the modified layers, by positioning a focused spot of a laser beam having a wavelength transmittable through the workpiece in the workpiece at a predetermined depth corresponding to a thickness of the wafer to be manufactured from the workpiece and moving the focused spot and the workpiece relatively to each other in a predetermined direction perpendicular to a thicknesswise direction of the workpiece; andafter the peel-off layer forming step, a peeling step of peeling off the wafer from the workpiece along the peel-off layer that acts as peel-off initiating points.

2. The method of manufacturing a wafer according to claim 1, wherein the second grinding step includes holding the first surface that has been ground in the first grinding step under suction on a holding surface of a second holding table such that the second surface is exposed.

3. The method of manufacturing a wafer according to claim 1, wherein the first surface that is exposed in the first grinding step has protruding warpage, and the first grinding step includes reducing the warpage of the first surface by grinding the first surface.

4. The method of manufacturing a wafer according to claim 1, further comprising: before the peel-off layer forming step, a planarizing step of reducing surface roughness of one, to be irradiated with the laser beam in the peel-off layer forming step, of the first surface that has been ground in the first grinding step or the second surface that has been ground in the second grinding step.