JP5899513B2 - Substrate manufacturing method and modified layer forming apparatus - Google Patents

Description

  The present invention provides, for example, a substrate manufacturing method and a modified layer in which a modified layer having a low adhesion force is formed using a nonlinear absorption effect accompanying multiphoton absorption of laser light energy and a part of a workpiece is peeled off. The present invention relates to a forming apparatus.

  For example, a substrate used for manufacturing a semiconductor device can be manufactured from a workpiece which is a columnar or prismatic columnar ingot made of a crystal such as silicon (Si) or gallium arsenide (GaAs).

  As a method of separating a substrate from a workpiece, a method of physically cutting the workpiece using a wire saw or a diamond blade saw is known.

  At present, since it takes time to cut wafers one by one using a diamond blade saw, a method of cutting a plurality of wafers simultaneously using a plurality of wire saws has become the mainstream.

  However, in the method of physically cutting such a workpiece, it is difficult to obtain a thin wafer having a thickness of 100 μm or less.

  Moreover, in the said method, it is easy to increase the waste cutting allowance which will be pulverized as a chip.

  On the other hand, as another method for separating the substrate from the work piece, a modified layer having a low adhesion force is formed by using a nonlinear absorption effect accompanying multiphoton absorption of laser light energy, and a part of the work piece is formed. A method of peeling is known.

  Therefore, such a conventional substrate manufacturing method will be specifically described with reference mainly to FIGS. 7 and 8 (see, for example, Patent Document 1).

  FIG. 7 is a schematic perspective view for explaining the formation of the modified layer 120 in the conventional substrate manufacturing method.

  FIG. 8 is a schematic partial cross-sectional view for explaining the formation of the modified layer 120 in the conventional substrate manufacturing method.

  First, a laser beam 141 from a direction perpendicular to the side surface of the workpiece 110 is irradiated to the side surface of the workpiece 110 so that a body-wound notch 110a is formed.

  Next, the laser beam 140 from the direction perpendicular to the upper surface of the workpiece 110 is aligned with the planned separation surface 130 where the focal point of the laser beam 140 that has passed through the condenser lens 160 is determined by the notch 110a. The upper surface of the workpiece 110 is irradiated such that the modified layer 120 is formed as a planar processing region extending from the notch 110a toward the inside of the workpiece 110.

  Thus, a thin wafer can be obtained by peeling a part of the workpiece 110 on the upper surface side from the modified layer 120.

JP 2006-245498 A

  However, it has been found that the above-described conventional substrate manufacturing method has a disadvantage in order to obtain higher manufacturing efficiency.

  More specifically, the reforming layer 120 is composed of a number of reforming portions that are substantially spheroidal, such as rugby balls.

  Such a modified portion 120a is generated by polycrystallization accompanying multiphoton absorption of the energy of the laser beam 140, and thus extends in the direction indicated by the arrow X1. That is, the direction of the major axis of the spheroid is the direction indicated by the arrow X1, and the direction of the minor axis of the spheroid is the direction perpendicular to the direction indicated by the arrow X1.

  Here, the direction indicated by the arrow X1 is a direction in which the laser beam 140 is incident on the condensing lens, but is a direction perpendicular to the upper surface of the workpiece 110, and the direction of the optical axis 150 of the condensing lens 160. Substantially coincides with the direction.

  The laser beam 140 is irradiated by scanning a pulsed laser so that the modified layer 120 extends in a plane parallel to the upper surface of the workpiece 110.

  The processing pitch p1 in the scanning direction for irradiating the laser beam 140 has a size obtained by adding some space to the diameter of the modified portion 120a in the scanning direction.

  For this reason, since the scanning direction is the direction of the short axis of the spheroid, it is difficult to sufficiently widen the processing pitch p1.

  Furthermore, since the direction perpendicular to the upper surface of the workpiece 110 is the direction of the long axis of the spheroid, the cutting allowance for separating the substrate may increase.

  Note that when the modified layer 120 is formed by using only the laser beam 141 without using the laser beam 140, the transmission distance in the crystal becomes long, and the laser transmission near the center of the workpiece 110 occurs. Since the rate decreases, it is difficult to manufacture a substrate having a large area from a workpiece having a large diameter.

  In view of the above-described conventional problems, an object of the present invention is to provide a substrate manufacturing method and a modified layer forming apparatus capable of obtaining higher manufacturing efficiency.

The first aspect of the present invention is a substrate manufacturing method in which a modified layer is formed on a separation plane parallel to the surface of a workpiece, and the substrate is separated from the workpiece with the modified layer as a boundary,
The condensing lens is arranged so that the optical axis of the condensing lens is included in the planned separation surface, laser light is incident on the condensing lens, and the laser light transmitted through the condensing lens is used as the workpiece. A reforming part forming step for forming a reforming part on the planned separation surface by being incident on the surface of
Reforming to form the modified layer composed of a plurality of the modified parts by repeating the modified part forming step while changing the relative positional relationship between the workpiece and the condenser lens. A layer forming step;
A substrate separating step for separating the substrate from the workpiece with the modified layer as a boundary;
Is a substrate manufacturing method.

  The second aspect of the present invention is the substrate manufacturing method according to the first aspect of the present invention, wherein in the modified portion forming step and the modified layer forming step, multi-branch simultaneous laser processing using a plurality of laser beams is performed.

  The third aspect of the present invention is the substrate according to the first aspect of the present invention, wherein the condensing lens is a lens obtained by removing a portion that should physically interfere with the workpiece from the convex lens when it is disposed. It is a manufacturing method.

  According to a fourth aspect of the present invention, the condensing lens is a lens obtained by cutting the convex lens with a planar lens cutting surface parallel to the optical axis and removing a portion including the optical axis from the convex lens. It is a substrate manufacturing method according to a third aspect of the present invention.

  According to a fifth aspect of the present invention, in the modified portion forming step and the modified layer forming step, the condensing lens is disposed so that the lens cutting surface is close to the surface of the workpiece, and the condensing In the substrate manufacturing method according to the fourth aspect of the present invention, the laser light shaped into the shape of the incident surface of the lens is incident on the condenser lens in parallel to the optical axis.

  According to a sixth aspect of the present invention, in the modified layer forming step, the workpiece and the workpiece are arranged so that the condenser lens and the modified portion are separated from each other with respect to a direction in which the laser beam is incident on the condenser lens. It is the board | substrate manufacturing method of 1st this invention which changes the relative positional relationship with the said condensing lens.

  The seventh aspect of the present invention is the substrate manufacturing method according to the first aspect of the present invention, wherein the workpiece is a silicon ingot.

The eighth aspect of the present invention is a modified layer forming apparatus for forming a modified layer on a separation scheduled plane parallel to the surface of a workpiece,
A laser light source that emits laser light;
A condensing lens which is arranged so that an optical axis is included in the planned separation surface, and forms a modified portion on the planned separation surface by causing the laser light to enter the surface of the workpiece;
A positional relationship changing unit that changes a relative positional relationship between the workpiece and the condenser lens;
With
The modified layer including a plurality of the modified portions is a modified layer forming apparatus formed by changing a relative positional relationship between the workpiece and the condenser lens.

  According to the present invention, it is possible to provide a substrate manufacturing method and a modified layer forming apparatus capable of obtaining higher manufacturing efficiency.

(A) The typical partial top view for demonstrating formation of a modified layer of the board | substrate manufacturing method of embodiment in this invention, (B) The modified layer of the board | substrate manufacturing method of embodiment in this invention (C) Typical partial right view for demonstrating formation of a modified layer of the board | substrate manufacturing method of embodiment in this invention. The typical fragmentary enlarged plan view for demonstrating formation of a modified layer of the board | substrate manufacturing method of embodiment in this invention (A) Typical partial cross-sectional view for explaining the formation of the modified layer in the substrate manufacturing method of the comparative example in the present invention, (B) Formation of the modified layer in the substrate manufacturing method of the comparative example in the present invention. Schematic partial right side view for explaining Typical partial top view for demonstrating multi-branch simultaneous laser processing of the board | substrate manufacturing method of embodiment in this invention Typical fragmentary sectional drawing for demonstrating the change of the relative positional relationship of a to-be-processed object and a condensing lens of the board | substrate manufacturing method of embodiment in this invention Schematic partial sectional view of a modified layer forming apparatus according to an embodiment of the present invention Schematic perspective view for explaining formation of a modified layer in a conventional substrate manufacturing method Schematic partial cross-sectional view for explaining the formation of a modified layer in a conventional substrate manufacturing method

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  First, the substrate manufacturing method of the present embodiment will be described with reference mainly to FIGS.

  1A is a schematic partial plan view for explaining the formation of the modified layer 20 in the substrate manufacturing method according to the embodiment of the present invention, and FIG. 1B is an implementation in the present invention. FIG. 1C is a schematic partial cross-sectional view taken along the line AA for explaining the formation of the modified layer 20 in the substrate manufacturing method according to the embodiment. FIG. 1C is a modification of the substrate manufacturing method according to the embodiment of the present invention. 3 is a schematic partial right side view for explaining formation of a mass layer 20. FIG.

  In the substrate manufacturing method according to the present embodiment, a modified layer 20 is formed on a planned separation surface 30 parallel to the surface of the workpiece 10, and the substrate is separated from the workpiece 10 with the modified layer 20 as a boundary. The method includes a modified portion forming step, a modified layer forming step, and a substrate separating step.

  In the modified portion forming step, the condenser lens 60a is arranged so that the optical axis 50a of the condenser lens 60a is included in the planned separation surface 30, the laser light 40a is incident on the condenser lens 60a, and the condenser lens 60a is By allowing the transmitted laser beam 40 a to be incident on the surface of the workpiece 10, the modified portion 20 a is formed on the planned separation surface 30.

  In the modified layer forming step, the modified layer formed of a plurality of modified portions 20a is formed by repeating the modified portion forming step while changing the relative positional relationship between the workpiece 10 and the condenser lens 60a. 20 is formed.

  In the substrate separation step, the substrate is separated from the workpiece 10 with the modified layer 20 as a boundary.

  The condensing lens 60a is a lens obtained by removing, from the convex lens, a portion that should physically interfere with the workpiece 10 when placed.

  More specifically, the condensing lens 60a is a lens obtained by cutting a convex lens with a planar lens cutting surface parallel to the optical axis 50a and removing a portion including the optical axis 50a from the convex lens.

  Further, in the modified portion forming step and the modified layer forming step, the condenser lens 60a is arranged so that the lens cut surface is close to the surface of the workpiece 10, and is shaped into the shape of the incident surface of the condenser lens 60a. The laser beam 40a is incident on the condenser lens 60a in parallel with the optical axis 50a.

  The workpiece 10 is a silicon ingot.

  Next, the substrate manufacturing method of the present embodiment will be described more specifically.

  Separation of the substrate from the workpiece 10 such as a columnar ingot is performed using the modified layer 20 formed in the vicinity of the planned separation surface 30 inside the workpiece 10 as a boundary.

  The reforming layer 20 is composed of a number of reforming portions that are substantially spheroidal, such as rugby balls.

  Such a modified portion 20a is generated by polycrystallization accompanying multiphoton absorption of the energy of the laser beam 40a, and thus extends in the direction indicated by the arrow Xa. That is, the major axis direction of the spheroid is the direction indicated by the arrow Xa, and the minor axis direction of the spheroid is the direction perpendicular to the direction indicated by the arrow Xa.

  Here, the direction indicated by the arrow Xa is a direction in which the laser beam 40a is incident on the condenser lens 60a, but is a direction parallel to the upper surface of the workpiece 110, and the optical axis 50a of the condenser lens 60a. Substantially coincides with the direction.

  The laser beam 40a is incident obliquely on the upper surface of the workpiece 10. However, since the optical axis 50a is disposed in the planned separation surface 30 parallel to the upper surface of the workpiece 10, the laser beam 40a is It has many components with a small incident angle with respect to the upper surface of the workpiece 10.

  Therefore, as compared with the case where the laser beam is simply incident on the upper surface of the workpiece 10 obliquely as described below (see FIG. 3), the modified portion 20a is formed on the workpiece 10. It extends considerably long in the direction of the long axis of the spheroid parallel to the upper surface.

  The laser beam 40a is irradiated by scanning a pulse laser so that the modified layer 20 extends in a plane parallel to the upper surface of the workpiece 10.

  The processing pitch pa in the scanning direction for irradiating the laser beam 40a has a size obtained by adding some space to the diameter of the modified portion 20a in the scanning direction.

  For this reason, since the scanning direction is the direction of the long axis of the spheroid, the processing pitch pa can be sufficiently widened.

  Furthermore, since the direction perpendicular to the upper surface of the workpiece 10 is the direction of the short axis of the spheroid, there is very little possibility that the cutting allowance for separating the substrate will be large.

  Next, the substrate manufacturing method of the present embodiment will be described more specifically.

  The condensing lens 60a is a round convex lens having a disk-shaped bottom surface with a radius r, and a planar shape parallel to the optical axis 50a having a height h from the outermost point on the outer peripheral side toward the center of the bottom surface. It is obtained by cutting at the lens cutting surface.

  Here, since the height h is larger than the radius r, the lens cut surface is located beyond the optical axis 50a, and the portion including the optical axis 50a (shown by a dotted line) is removed.

  That is, the condensing lens 60a is a cut lens from which a portion physically interfering with the workpiece 10 is removed so that the optical axis 50a is disposed in the planned separation surface 30 inside the workpiece 10. It is a lens piece.

  The condensing lens 60 a is disposed close to the workpiece 10 so that the lens cutting surface is parallel to the upper surface of the workpiece 10.

  The laser light 40a is incident on the condensing lens 60a as laser parallel light shaped according to the incident aperture of the condensing lens 60a.

  Of course, since the laser beam 40a before being shaped is equivalent to the laser beam 140, the energy of both is the same if neglecting a minute loss that occurs when shaping the laser beam 40a.

  If the workpiece 10 is a silicon block made of a single crystal of silicon, that is, a silicon ingot, the wavelength of the laser beam 40a is selected so that the laser beam 40a is transparent to silicon.

  Since the refractive index of silicon is higher than that of air, the laser light 40 a incident obliquely with respect to the upper surface of the workpiece 10 is refracted and enters the workpiece 10.

  Then, the laser beam 40a is condensed by the condenser lens 60a so as to be focused on the planned separation surface 30 inside the workpiece 10.

  The energy of the laser beam 40 a is compressed temporally and spatially in the vicinity of the condensing point in the separation planned surface 30.

  Therefore, in the vicinity of the condensing point, a region extending along the optical axis 50a having a very high energy density state is formed locally, and a multiphoton of the energy of the laser beam 40a having a very high absorption characteristic locally. It is obtained by a non-linear absorption effect accompanying absorption.

  In the vicinity of the condensing point, the crystal lattice of silicon is disturbed, and polycrystallization occurs due to high-density dislocations, so that a modified layer 20 composed of a large number of modified parts having a spheroidal shape is formed. .

  Regarding such a modified portion 20a, since the volume of silicon locally expands, horizontal cracks are generated at both ends in the direction of the major axis of the spheroid.

  Therefore, in the modified layer 20 formed as a planar processing region extending from the side surface of the workpiece 10 toward the inside of the workpiece 10, horizontal cracks are sequentially generated, and the starting point of cleaving. Is formed.

  Thus, a wafer can be obtained by peeling off a part of the workpiece 10 on the upper surface side of the modified layer 20 having a weak adhesion force by cleaving with external stress or thermal stress.

  According to the present embodiment, a thin wafer having a thickness of approximately 100 μm can be obtained.

  For example, the condensing lens 60a is a round convex lens having a disk-shaped bottom surface having a radius r of 5 mm, and a height h from the outermost point toward the center of the bottom surface is 5.5 mm. What is necessary is just to obtain by cut | disconnecting by the planar lens cut surface parallel to the optical axis 50a which is.

  At this time, the height H of the condenser lens 60a is 4.5 mm, the gap g between the lens cut surface and the upper surface of the workpiece 10 is 0.4 mm, and the depth d of the modified layer 20 Is 100 μm, which is approximately equal to the thickness of the wafer where the cutting allowance is ignored (in the drawing, for the sake of easy understanding, the ratio of the dimensions shown is different from the actual one).

  Furthermore, for example, as shown in FIG. 2 which is a schematic partial enlarged plan view for explaining the formation of the modified layer 20 in the substrate manufacturing method of the embodiment of the present invention, the modified layer 20 Is composed of a number of substantially spheroidal modified parts having a major axis direction diameter δa of 30 μm and a minor axis direction diameter δb of 5 μm, and the machining pitch pa in the major axis direction is 50 μm. The processing pitch pb in the direction of the minor axis may be 10 μm.

  At this time, the space σa (= pa−δa) between adjacent reforming portions in the long axis direction is 20 μm, and the space σb (= pb−δb) between adjacent reforming portions in the short axis direction is 5 μm. It is.

  Of course, the smaller the space between adjacent modified portions, the easier it is to peel off by cleaving.

  However, such a space does not necessarily have to be zero.

  This is because even if there is a space of the above size, peeling can be easily performed and the processing pitch can be sufficiently widened without any problems.

  That is, in the present embodiment, as described above, the reforming portion 20a extends considerably long in the direction of the long axis.

  Accordingly, the diameter δa is sufficiently large, and even if the processing pitch pa is widened, the space σa does not become so large, so that the peeling can be easily performed.

  Needless to say, the separation plane 30 on which the modified layer 20 is formed spreads two-dimensionally, so that processing in one direction is repeated alternately while moving in a direction perpendicular to the one direction. .

  For example, the scheduled separation surface 30 extends in the direction indicated by the arrow Xa and the direction indicated by the arrow Xb perpendicular to the direction indicated by the arrow Xa.

  Then, as indicated by a plurality of arrows Y, on the planned separation surface 30, one row of processing is performed from one end of the workpiece 10 to the other end in the direction indicated by the arrow Xb, and is indicated by the arrow Xa. The next line of processing shifted in the direction opposite to the direction to be processed is performed from the other end of the workpiece 10 to one end in the direction opposite to the direction indicated by the arrow Xb, and such processing is repeated. It is.

  By the way, FIG. 3A which is a schematic partial cross-sectional view taken along the line AA for explaining the formation of the modified layer 220 in the substrate manufacturing method of the comparative example in the present invention, and the substrate manufacturing of the comparative example in the present invention. As shown in FIG. 3B, which is a schematic partial right side view for explaining the formation of the modified layer 220 in the method, the laser beam 240 is incident on the condenser lens 260. If the direction indicated by the arrow X2 that substantially coincides with the direction of the optical axis 250 of the condensing lens 260 is simply an angle inclined with respect to the upper surface of the workpiece 210, the modification The portion 220a only extends long in the direction indicated by the arrow X2.

  This is because a region having a very high energy density state locally extends along the optical axis 250.

  Therefore, if the substrate is separated from the workpiece 210 using the modified layer 220 formed in the vicinity of the planned separation surface 230 as a boundary, the processing pitch p2 cannot be sufficiently widened, and the substrate is separated. Therefore, there is a risk that the cutting allowance will increase.

  In addition, as shown in FIG. 4 which is a schematic partial plan view for explaining the multi-branch simultaneous laser processing of the substrate manufacturing method according to the embodiment of the present invention, the reforming portion forming step and the reforming are performed. In the layer formation step, it is desirable to perform multi-branch simultaneous laser processing using a plurality of laser beams.

  Laser light 40 incident from the direction indicated by arrow X is branched by half mirrors 80a, 80b,..., 80n, reflected in the direction indicated by arrow Xa, and condensed lenses 60a, 60b,. Is incident on.

  The optical axes 50, 50a, 50b,..., 50n are optical axes of the laser beams 40, 40a, 40b,.

  The direction of the optical axis 50 is the direction indicated by the arrow X, and the directions of the optical axes 50a, 50b, ..., 50n are the directions indicated by the arrow Xa.

  The laser beams 40a, 40b, ..., 40n may be further branched by diffractive optical elements 70a, 70b, ..., 70n inserted in front of the condenser lenses 60a, 60b, ..., 60n, respectively.

  Of course, continuous laser processing using one laser beam may be performed, but if multi-branch simultaneous laser processing using multiple laser beams is performed, processing in one direction can be performed at a time. Therefore, the variation in processing becomes smaller, the processing time is shortened, the processing cost is reduced, and the quality uniformity is improved.

  If the degree of multi-branching is increased, the processing in one direction is more performed simultaneously, so the above effect is more remarkable.

  For example, if the pulse frequency of the laser beam 40 is 1 kHz and the processing pitch pa in one direction indicated by the arrow Xa is 0.1 mm, the scanning speed in that direction is 100 mm / second.

  With respect to the workpiece 10 having a rectangular shape with an upper surface having a side length of 160 mm, which is generally used for applications such as solar cells, the scanning in this direction is completed in 1.6 seconds.

  In such a case, if a laser beam having a short picosecond class pulse width is used, processing energy of about 10 μJ is required. Therefore, if the processing pitch pa is 8 μm, the number of branches is 20000. Yes, the required average power is 200W.

  If thinning is possible and the processing pitch pa is 80 μm, the number of branches is 2000, and the required average output is 20 W.

  5 is a schematic partial sectional view for explaining a change in the relative positional relationship between the workpiece 10 and the condenser lens 60a in the substrate manufacturing method of the embodiment of the present invention. As shown, in the modified layer forming step, the workpiece 10 and the condensing lens are arranged such that the condensing lens 60a and the modifying portion 20a are away from each other in the direction in which the laser beam 40a is incident on the condensing lens 60a. It is desirable to change the relative positional relationship with 60a.

  This is because in the case where the processing is performed in the backward direction, which is the direction opposite to the direction indicated by the arrow Xa, the laser beam 40a has a temperature at which the laser beam 40a has passed when the modified portion was previously formed. This is because the portion 10a that may have risen is not passed.

  If the laser beam 40a is incident on the condensing lens 60a, the relative positional relationship between the workpiece 10 and the condensing lens 60a is changed so that the condensing lens 60a and the modified portion 20a approach each other. If so, the characteristics of the focused laser beam 40a may change.

  This is because in such a case where the processing is performed in the forward direction, which is the direction indicated by the arrow Xa, the laser beam 40a passes through the portion through which the laser beam 40a has passed when the modified portion was previously formed, This is because the temperature may increase considerably in such a part.

  Of course, there is almost no possibility of such a temperature rise in the direction indicated by the arrow Xb perpendicular to the direction indicated by the arrow Xa, and the above-described multi-branch simultaneous laser processing can be performed without any problem.

  Further, as shown in FIG. 6 which is a schematic partial sectional view of the modified layer forming apparatus according to the embodiment of the present invention, the modified layer is formed on the planned separation surface 30 parallel to the surface of the workpiece 10. A modified layer forming apparatus for forming 20 is included in the present invention.

  Such a modified layer forming apparatus includes a laser light source 91, a condenser lens 60a, and a positional relationship changing unit 92.

  The laser light source 91 is means for emitting laser light 40a.

  The condensing lens 60 a is arranged so that the optical axis 50 a is included in the planned separation surface 30, and the modified portion 20 a is formed on the planned separation surface 30 by causing the laser light 40 a to enter the surface of the workpiece 10. Means.

  The positional relationship changing unit 92 is a means for changing the relative positional relationship between the workpiece 10 and the condenser lens 60a.

  The modified layer 20 including a plurality of modified portions 20a is formed by changing the relative positional relationship between the workpiece 10 and the condenser lens 60a.

  Therefore, according to the present invention, the processing time can be shortened and the processing cost can be reduced, and at the same time, the device can be manufactured stably and inexpensively.

  The substrate manufacturing method and the modified layer forming apparatus of the present invention can obtain higher manufacturing efficiency. For example, the substrate manufacturing method and the modified layer forming apparatus can improve the weakness of fixing by using the non-linear absorption effect accompanying multiphoton absorption of laser light energy. It is useful for forming a quality layer and exfoliating part of the workpiece.

DESCRIPTION OF SYMBOLS 10 Workpiece 20 Modified layer 20a Modified portion 30 Planned separation surface 40a Laser beam 50a Optical axis 60a Condensing lens

Claims (8)

A substrate manufacturing method for forming a modified layer on a planned separation plane parallel to a surface of a workpiece, and separating the substrate from the workpiece with the modified layer as a boundary,
The condensing lens is arranged so that the optical axis of the condensing lens is included in the planned separation surface, laser light is incident on the condensing lens, and the laser light transmitted through the condensing lens is used as the workpiece. A reforming part forming step for forming a reforming part on the planned separation surface by being incident on the surface of
Reforming to form the modified layer composed of a plurality of the modified parts by repeating the modified part forming step while changing the relative positional relationship between the workpiece and the condenser lens. A layer forming step;
A substrate separating step for separating the substrate from the workpiece with the modified layer as a boundary;
A substrate manufacturing method comprising:   The substrate manufacturing method according to claim 1, wherein in the modified portion forming step and the modified layer forming step, multi-branch simultaneous laser processing using a plurality of laser beams is performed.   The substrate manufacturing method according to claim 1, wherein the condensing lens is a lens obtained by removing, from the convex lens, a portion that should physically interfere with the workpiece when placed.   The said condensing lens is a lens obtained by cut | disconnecting the said convex lens by the planar lens cutting surface parallel to the said optical axis, and removing the part containing the said optical axis from the said convex lens. Substrate manufacturing method.   In the modified portion forming step and the modified layer forming step, the condensing lens is disposed so that the lens cutting surface is close to the surface of the workpiece, and the shape of the incident surface of the condensing lens is set. The substrate manufacturing method according to claim 4, wherein the shaped laser beam is incident on the condenser lens in parallel to the optical axis.   In the modified layer forming step, relative to the workpiece and the condensing lens so that the condensing lens and the modifying portion are separated from each other with respect to the direction in which the laser beam is incident on the condensing lens. The substrate manufacturing method according to claim 1, wherein the positional relationship is changed.   The substrate manufacturing method according to claim 1, wherein the workpiece is a silicon ingot. A modified layer forming apparatus for forming a modified layer on a planned separation surface parallel to the surface of a workpiece,
A laser light source that emits laser light;
A condensing lens which is arranged so that an optical axis is included in the planned separation surface, and forms a modified portion on the planned separation surface by causing the laser light to enter the surface of the workpiece;
A positional relationship changing unit that changes a relative positional relationship between the workpiece and the condenser lens;
With
The modified layer forming apparatus is formed by changing a relative positional relationship between the workpiece and the condenser lens, the modified layer including a plurality of the modified portions.

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