Laser processing methods

Abstract

Laser processing method comprising: a first step of converging laser light (L) onto a workpiece (1) having a semiconductor substrate formed with several functional devices on a front surface (3), using a rear surface of the semiconductor substrate as a laser light entry surface, and while maintaining a distance between the front surface (3) of the semiconductor substrate and a first convergence point (P1) of the laser light (L) at a first distance, moving the first convergence point (P1) of the laser light (L) along a cutting line (5) passing between the adjacent functional devices to form a first modified area (7a) along the cutting line (5);after the first step, a second step of converging the laser light (L) on the object to be processed (1), using the back surface of the semiconductor substrate as the laser light entry surface, and while maintaining a distance between the front surface (3) of the semiconductor substrate and a second convergence point (P2) at a second distance greater than the first distance, and while the second convergence point (P2) of the laser light (L) is offset in a direction perpendicular to both a thickness direction of the semiconductor substrate and an extent direction of the cutting line (5) with respect to a position where the first convergence point (P1) of the laser light (L) has converged, moving the second convergence point (P2) of the laser light (L) along the cutting line (5) to form a second modified area (7b) along the cutting line (5);and after the second step, a third step of removing a predetermined section (K1) which includes the back surface and at least the second modified region (7b) in the semiconductor substrate.

Description

Technical FieldOne aspect of the present invention relates to a laser processing method.Prior ArtThere is known a laser processing method in which laser light is emitted onto an object to be processed, which includes a silicon substrate formed with a plurality of functional devices on a front surface in a matrix, using a rear surface of a silicon substrate as a laser light input surface to form a modified region in the vicinity of the front surface of the silicon substrate along a cut line formed in a lattice pattern to pass between adjacent functional devices, and then the rear surface of the silicon substrate is polished such that the silicon substrate has a predetermined thickness to cut the object to be processed for each functional device (see, for example, Patent Literature 1).Citeration listPatent LiteraturePatent Literature 1: International Publication No. WO 03 / 077 295 A1US 2008 / 0 135 975 A1 and JP 2015-65 209 A disclose methods for cutting a semiconductor wafer with a laser beam.SUMMARY OF THE INVENTIONTechnical ProblemIn the above-described laser processing method, it is important to reduce the number of scanning operations of the laser light for one cutting line (that is, the number of formed rows of the modified region for one cutting line) from the viewpoint of improving processing efficiency. Accordingly, in the formation of the modified region, a crack may greatly expand from the modified region in the thickness direction of the semiconductor substrate. However, in this case, when the laser light converges on the semiconductor substrate, damage may occur on the front side of the semiconductor substrate on the side opposite to the laser light input surface, whereby the characteristics of the functional device may deteriorate.Moreover, in the above-described laser processing method, it is desirable to increase the yield of a plurality of chips obtained by cutting the object to be processed. For this reason, for example, it is necessary to suppress occurrence of a height difference on a cut surface and improve straightness of the cut surface.An aspect of the present invention is to provide a laser processing method capable of suppressing the occurrence of damage on a front surface of an object to be processed on the side opposite to the laser light input surface and smoothing the cut surface.Solution of the ProblemA laser processing method according to an aspect of the present invention includes: a step of converging laser light on an object to be processed including a semiconductor substrate formed with a plurality of function devices on a front surface using a rear surface of the semiconductor substrate as a laser light input surface, and while maintaining a distance between the front surface of the semiconductor substrate and a first converging point of the laser light at a first distance, moving the first converging point of the laser light along a cutting line set to pass between the adjacent function devices to form a first modified region along the cutting line; after the first step, a second step of converging the laser light on the object to be processed using the back surface of the semiconductor substrate as the laser light input surface, and while maintaining a distance between the front surface of the semiconductor substrate and a second convergence point at a second distance larger than the first distance, and while shifting the second convergence point of the laser light in a direction vertical to both a thickness direction of the semiconductor substrate and an extension direction of the cutting line with respect to a position where the first convergence point of the laser light converges, moving the second convergence point of the laser light along the cutting line to form a second modified region along the cutting line; and after the second step, a third step of removing a predetermined portion including the back surface and at least the second modified region in the semiconductor substrate.In this laser processing method, when the second modified region is formed in the second step, the second converging point of the laser light is offset in the direction vertical to both the thickness direction of the semiconductor substrate and the extending direction of the cutting line (hereinafter, simply referred to as "the vertical direction of the cutting line") with respect to the position at which the first converging point of the laser light converges. Consequently, the occurrence of damage on the front surface of the object to be processed on the side opposite to the laser light input surface can be prevented. By the displacement, a position of the second modified region has a height difference (deviation) from a position of the first modified region in the vertical direction of the cut line; however, at least the second modified region is removed together with the removal of the predetermined portion in the third step. Accordingly, in the object to be machined cut along the cut line, after the removal of the predetermined portion, it is possible to suppress occurrence of a height difference on the cut surface due to the second modified region and to smooth the cut surface to improve straightness.In the laser processing method according to an aspect of the present invention, in the third step, a predetermined portion further including the first modified region in the semiconductor substrate may be removed. With this configuration, not only the second modified region but also the first modified region is removed by the third step. Accordingly, in the object to be machined cut along the cut line, after the removal of the predetermined portion, it is possible to suppress deterioration of the straightness of the cut surface due to the first modified region.In the laser processing method according to an aspect of the present invention, the semiconductor substrate may be a silicon substrate, and the laser light may have a wavelength greater than 1064 nm. Consequently, in the formation of the first modified region and the second modified region, compared to a case where laser light having a wavelength of 1064 nm or less is used, the crack can be greatly expanded in the thickness direction of the silicon substrate from the first modified region and the second modified region.In the laser processing method according to an aspect of the present invention, the laser light may have a wavelength of 1099 nm or more and 1342 nm or less. In this case, in the formation of the first modified region and the second modified region, the crack may be more greatly expanded in the thickness direction of the silicon substrate from the first modified region and the second modified region.In the laser processing method according to an aspect of the present invention, a distance by which the second convergence point of the laser light is displaced in the direction vertical to both the thickness direction of the silicon substrate and the extension direction of the cut line with respect to the position at which the first convergence point of the laser light converges may be 24 μm or less. In this case, the crack can reliably establish a connection between the first modified region and the second modified region, and the crack can be reliably expanded in the thickness direction of the silicon substrate from the first modified region and the second modified region in forming the first modified region and the second modified region.In the laser processing method according to an aspect of the present invention, the distance by which the second convergence point of the laser light is displaced in the direction vertical to both the thickness direction of the silicon substrate and the extension direction of the cut line with respect to the position at which the first convergence point of the laser light converges may be 4 μm or more and 18 μm or less. In this case, the crack can more reliably establish a connection between the first modified region and the second modified region, and the crack can more reliably be expanded from the first modified region and the second modified region in the thickness direction of the silicon substrate in forming the first modified region and the second modified region.In the laser processing method according to an aspect of the present invention, in the second step, by forming the second modified region, a crack extending in the thickness direction of the semiconductor substrate from the first modified region and the second modified region may be made to reach the front surface of the semiconductor substrate, and in the third step, by removing the predetermined portion, the crack extending in the thickness direction of the semiconductor substrate from the first modified region and the second modified region in the second step may be made to reach the back surface of the semiconductor substrate from which the predetermined portion has been removed. In this case, the object to be processed can be accurately cut along the line to be cut.Advantageous Effects of the InventionAccording to an aspect of the present invention, it is possible to provide a cutting method for an object to be processed and a cutting apparatus for an object to be processed, which are capable of smoothing the cut surface.Brief Description of the DrawingsFIG. 1 is a schematic configuration diagram of a laser processing apparatus used for forming a modified region. FIG. 2 is a plan view of an object to be processed in which the modified region is formed. FIG. 3 is a sectional view of the object to be processed taken along the line III-III of FIG. 2. FIG. 4 is a plan view of the object to be processed after laser processing. FIG. 5 is a sectional view of the object to be processed taken along the line V-V of FIG. 4. FIG. 6 is a sectional view of the object to be processed taken along the line VI-VI of FIG. 4. FIG. 7( a) is a sectional view along a section line of the object to be processed during laser processing. FIG. 7( b) is a plan view of the object to be processed after cutting. FIG. 8( a) is a sectional view along the cutting line of the object to be processed during laser processing. FIG. 8( b) is a plan view of the object to be processed after cutting. FIG. 9( a) is a sectional view along the cutting line of the object to be processed during laser processing. FIG. 9( b) is a plan view of the object to be processed after cutting. FIG. 10( a) is a sectional view along the cutting line of the object to be processed during laser processing. FIG. 10( b) is a plan view of the object to be processed after cutting. FIG. 11( a) is a view illustrating a photograph of a surface parallel to the cut line of the silicon substrate after cutting. FIG. 11( b) is a view illustrating a photograph of the front surface side of the silicon substrate after cutting. FIG. 12( a) is a view illustrating a photograph of a surface parallel to the cut line of the silicon substrate after forming a first modified region and a second modified region. FIG. 12( b) is a view illustrating a photograph of a surface vertical to the cut line of the silicon substrate after the formation of the first modified region and the second modified region. FIG. 13( a) is a view illustrating a photograph of a surface parallel to the cut line of the silicon substrate after the formation of the first modified region and the second modified region. FIG. 13( b) is a view illustrating a photograph of a surface vertical to the cut line of the silicon substrate after the formation of the first modified region and the second modified region. FIG. 14 is a graph showing a relationship between a displacement value and a length of a crack. FIG. 15 is a graph showing a relationship between the displacement value and the number of spatters. FIG. 16( a) is a view illustrating a photograph of a surface parallel to the cut line of the silicon substrate after cutting. FIG. 16( b) is a view illustrating a photograph of the front surface side of the silicon substrate after cutting. FIG. 17( a) is a view illustrating a photograph of the front surface side of the silicon substrate after cutting in the case where the displacement value is 2 μm. FIG. 17( b) is a view illustrating a photograph of the front surface side of the silicon substrate after cutting in a case where the displacement value is 4 μm. FIG. 17( c) is a view illustrating a photograph of the front surface side of the silicon substrate after cutting in the case where the deviation value is 6 μm. FIG. 18( a) is a view illustrating a surface vertical to the cut line of the silicon substrate in the case where the displacement value is small. FIG. 18( b) is a view illustrating a surface vertical to the cut line of the silicon substrate in the case where the displacement value is large. FIG. 19 is a sectional view for explaining a semiconductor chip manufacturing method using a laser processing method according to an embodiment. FIG. 20 is a sectional view for explaining the semiconductor chip manufacturing method using the laser processing method according to the embodiment. FIG. 21 is a sectional view for explaining the semiconductor chip manufacturing method using the laser processing method according to the embodiment. FIG. 22 is a sectional view for explaining the semiconductor chip manufacturing method using the laser processing method according to the embodiment. FIG. 23 is a sectional view for explaining the semiconductor chip manufacturing method using the laser processing method according to the embodiment. FIG. 24 is a sectional view for explaining the semiconductor chip manufacturing method using the laser processing method according to the embodiment. FIG. 25( a) is a sectional view taken along the line of intersection of the object to be processed before polishing. FIG. 25( b) is a sectional view along the line of intersection of the object to be processed after polishing.DESCRIPTION OF THE EMBODIMENTSHereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant description is omitted.A laser processing apparatus and a laser processing method according to the embodiments of the present invention converge laser light on an object to be processed to form a modified region within the object to be processed along a cut line. Thus, first, the formation of the modified region will be described with reference to FIGS. 1, 2, 3, 4, 5 to 6.As shown in FIG. 1, a laser processing apparatus 100 includes a laser light source 101 that causes laser light L to oscillate in a pulsating manner, a dichroic mirror 103 arranged to change an optical axis (optical path) direction of the laser light L by 90°, and a converging lens 105 for converging the laser light L. The laser processing apparatus 100 further includes a stage 107 for holding an object to be processed 1 irradiated with the laser light L converged by the converging lens 105, a stage 111 for moving the stage 107, a laser light source controller 102 for controlling the laser light source 101 to adjust the output, the pulse width, the pulse waveform, and the like of the laser light L, and a stage controller 115 for controlling the movement of the stage 111.In the laser processing apparatus 100, the laser light L emitted from the laser light source 101 changes the optical axis direction thereof by 90° using the dichroic mirror 103, and then converges by the converging lens 105 within the object to be processed 1 mounted on the travel table 107. At the same time, the stage 111 is shifted so that the object to be processed 1 is moved along a cut line 5 with respect to the laser light L. Consequently, a modified region is formed along the line of intersection 5 in the object 1 to be processed. while the step 111 is being moved for relative movement of the laser light L, the converging lens 105 may be moved instead of or together with it.As the object to be processed 1, a planar member (for example, a substrate or a wafer) including, for example, semiconductor substrates formed of semiconductor materials and piezoelectric substrates formed of piezoelectric materials is used. As shown in FIG. 2, in the object to be processed 1, the cutting line 5 for cutting the object to be processed 1 is set. The cut line 5 is a virtual line that is straight. In forming a modified region inside the object to be processed 1, the laser light L is relatively moved along the cutting line 5 (that is, in the direction of the arrow A in FIG. 2 ) while setting a convergence point (convergence position) P inside the object to be processed 1 as shown in FIG. 3. Consequently, a modified region 7 is formed within the object 1 to be machined along the cutting line 5 as shown in FIGS. 4, 5, and 6, and the modified region 7 formed along the cutting line 5 becomes an initial cutting region 8.A convergence point P is a position where the laser light L converges. The cut line 5 may be formed three-dimensionally instead of being straight curved or by combining them, or may be a line defined by coordinates. The cutting line 5 is not limited to the virtual line, and may be a real line drawn on a front surface 3 of the object 1 to be processed. The modified region 7 may be formed either continuously or intermittently. The modified region 7 can be formed either in rows or dots and only has to lie at least within the object 1 to be processed. A crack may be formed as a starting point of the modified region 7, and the crack and the modified region 7 may be exposed on the outer surface (the front surface 3, a rear surface, or an outer circumferential surface) of the object 1 to be processed. A laser light entrance surface is not limited to the front surface 3 of the object to be processed 1 in forming the modified region 7, but may be the back surface of the object to be processed 1.Incidentally, in the case where the modified region 7 is formed inside the object to be processed 1, the laser light L passes through the object to be processed 1 and is absorbed, in particular, in the vicinity of the convergence point P disposed inside the object to be processed. Consequently, the modified region 7 is formed in the object 1 to be processed (that is, internal absorption type laser processing). In this case, the front surface 3 of the object to be processed 1 hardly absorbs the laser light L and thus does not melt. On the other hand, in a case where the modified region 7 is formed on the front surface 3 of the object to be processed 1, the laser light L is partially absorbed in the vicinity of the convergence point P disposed on the front surface 3, and ablation portions such as holes and grooves (surface absorption type laser processing) are formed by being melted and removed from the front surface 3.The modified region 7 is a region in which the density, the refractive index, the mechanical strength, and other physical properties are different from the environments. Examples of the modified region 7 include a melted worked region (that is, a region that has resolidified after melting and / or a region in the melted state and / or a region during resolidified from the melted state), a crack region, a dielectric breakdown region, a refractive index change region, and a mixed region thereof. Other examples of the modified region 7 include a region in which the density of the modified region 7 has changed compared to the density of an unmodified region in a material of the object 1 to be processed, and a region having a lattice defect. In a case where the material of the object to be processed 1 is single crystal silicon, the modified region 7 may also be referred to as a high dislocation density region.The melted processed region, the refractive index changing region, the region where the density of the modified region 7 has changed compared to the density of the unmodified region, and the region having the lattice defect may further have the crack (a crack or a micro crack) therein or at an interface between the modified region 7 and the unmodified region. The introduced crack may be formed over the entire surface of the modified region 7 or only over a part or more portions thereof. The object to be processed 1 includes a substrate made of a crystalline material having a crystal structure. For example, the object to be processed 1 includes a substrate formed of at least gallium nitride (GaN), silicon (Si), silicon carbide (SiC), LiTaO 3 and sapphire (Al 2 O 3). In other words, the object to be processed 1 includes, for example, a gallium nitride substrate, a silicon substrate, a SiC substrate, a LiTaO 3- substrate, or a sapphire substrate. The crystalline material may be either an anisotropic crystal or an isotropic crystal. Further, the object to be processed 1 may include a substrate made of a non-crystalline material having a non-crystalline structure (an amorphous structure), and include, for example, a glass substrate.In the embodiment, the modified region 7 may be formed by forming a plurality of modified sites (machining marks) along the cut line 5. In this case, the plurality of modified sites are gathered to form the modified region 7. Each of the modified sites is a modified portion formed by one shot of a pulse of the pulsed laser light (that is, laser irradiation of a pulse: laser shot). Examples of the modified sites include cracking sites, melted processed sites, refractive index changing sites, and those in which at least one of them occurs. As for the modified sites, the sizes and lengths of the cracks resulting therefrom can be controlled as needed in view of the required cutting accuracy, the required flatness of the cut surfaces, the thickness, the kind and the crystal orientation of the object 1 to be processed, and the like. In addition, according to the present embodiment, the modified site along the cut line 5 may be formed as the modified region 7.In the following, inspection results of a fault or spatter will be described. It should be noted that "damage occurring on the front surface of the object to be processed on the opposite side to the laser light input surface in a case where the laser processing as described above is performed on the object to be processed including the semiconductor substrate 1" is referred to as the "spatter". Hereinafter, the silicon substrate will be described as an example of the semiconductor substrate.As shown in FIGS. 7, 8, 9 to 10, as the object to be processed, a silicon substrate 10 having a metal film 11 on a front surface 10 ais manufactured. The metal film 11 is formed by forming a Cr film having a thickness of 20 μm as a base on the front surface 10 aof the silicon substrate 10 and forming an Au film having a thickness of 50 μm on the Cr film.As shown in FIG. 7( a), the laser light L 0 having a wavelength of 1064 nm converges inside the silicon substrate 10 using a back surface 10 bof the silicon substrate 10 as the laser light input surface, and a convergence point P of the laser light L 0 is moved along the cut line 5, thereby forming the modified region 7 inside the silicon substrate 10 along the cut line 5. At this time, the irradiation conditions of the laser light L 0 are set such that a crack F extending in the thickness direction of the silicon substrate 10 from the modified region 7 in the formation of the modified region 7 (that is, the crack F occurs in the formation of the modified region 7 without applying an external force to the silicon substrate 10) reaches the front surface 10 aof the silicon substrate 10. In this case, the damage does not occur on the metal film 11 as shown in FIG. 7( b).As shown in FIG. 8( a), using the back surface 10 bof the silicon substrate 10 as the laser light entrance surface, the laser light L 1 having a wavelength of 1342 nm converges inside the silicon substrate 10, and the convergence point P of the laser light L 1 is moved along the cut line 5, thereby forming the modified region 7 inside the silicon substrate 10 along the cut line 5. At this time, the irradiation conditions of the laser light L 1 are adjusted such that the crack F extending from the modified region 7 reaches the front surface 10 aof the silicon substrate 10. In particular, except that the wavelength is different, the irradiation conditions of the laser light L 1 are the same as the irradiation conditions of the laser light L 0 described above. In this case, a spatter S is formed on the metal film 11 as shown in FIG. 8( b).As shown in FIG. 9( a), using the back surface 10 bof the silicon substrate 10 as the laser light input surface, the laser light L 1 having a wavelength of 1342 nm converges in the silicon substrate 10, and the convergence point P of the laser light L 1 is moved along the cut line 5, thereby forming the modified region 7 in the silicon substrate 10 along the cut line 5. At this time, the irradiation conditions of the laser light L 1 are adjusted such that the crack F extending from the modified region 7 is formed in the silicon substrate 10 without reaching the front surface 10 aof the silicon substrate 10. Specifically, the pulse energy of the laser light L 1 is set to be smaller than in the case of FIG. 8. In this case, no spatter is formed on the metal film 11 as shown in Fig. 9(b).As shown in FIG. 10( a), the laser light L 1 having a wavelength of 1342 nm converges in the silicon substrate 10 using the back surface 10 bof the silicon substrate 10 as the laser light input surface, and the convergence point P of the laser light L 1 is moved along the cut line 5, thereby forming a first modified region 7 aand a second modified region 7 bin the silicon substrate 10 along the cut line 5. Here, the irradiation conditions of the laser light L 1 are set such that the crack F does not reach the front surface 10 aof the silicon substrate 10 when only the first modified region 7 ais formed, and the crack F reaches the front surface 10 aof the silicon substrate 10 when the second modified region 7 bis formed on the back surface 10 bside of the silicon substrate 10 with respect to the first modified region 7 a. In this case, a spatter S is formed on the metal film 11 as shown in FIG. 10( b).FIG. 11 is a view illustrating a photograph of the silicon substrate 10 when the first modified region 7 aand the second modified region 7 bare formed in the silicon substrate 10 under the condition of the case of FIG. 10. Specifically, FIG. 11( a) is a view illustrating a photograph of a surface parallel to the cut line of the silicon substrate 10 after cutting. FIG. 11( b) is a view illustrating a photograph of the front surface 10 aside (metal film 11) of the silicon substrate 10 after cutting. Referring to FIG. 11( b), it can be confirmed that a dark portion is present in a region surrounded by the one-dot chain line in the metal film 11. This is the problem sprayer S.When the laser light L 1 having a wavelength of more than 1064 nm, such as 1342 nm, is used, compared to a case where the laser light L 0 having a wavelength of 1064 nm or less is used, the crack F can be greatly expanded in the thickness direction of the silicon substrate 10 from the modified region 7. When the laser light L 1 having a wavelength of more than 1064 nm, such as 1342 nm, is used, compared to the case where the laser light L 0 having a wavelength of 1064 nm or less is used, the modified region 7 can be formed at a deeper position from the laser light input surface of the silicon substrate 10. This results from the fact that the laser light L 1 having a wavelength of more than 1064 nm has a higher transmittance for silicon than the laser light L 0 having a wavelength of 1064 nm or less. Thus, from the viewpoint of reducing the number of scanning operations of the laser light L for a cut line 5 (that is, the number of formed rows of the modified region 7 for a cut line 5), laser light L 1 having a wavelength of more than 1064 nm is preferably used for improving the processing efficiency.However, when attempting to make the crack F reach the front surface 10 aof the silicon substrate 10 using the laser light L 1 having a wavelength of more than 1064 nm, the spatter S occurs on the metal film 11 as in the case of FIGS. 8 and 10 described above. When the spatter S occurs in a case where a functional device (for example, a semiconductor operation layer formed by crystal growth, a light receiving device such as a photodiode, a light emitting device such as a laser diode, or a switching device formed as a circuit) is provided on the front surface 10 aof the silicon substrate 10 on the opposite side to the laser light input surface, the characteristics of the functional device may deteriorate.Thus, the case where the crack F is caused to reach the front surface 10 aof the silicon substrate 10 using the laser light L 1 having a wavelength of more than 1064 nm may be of technical importance when the occurrence of the spatter S can be suppressed.The present inventors have considered that the occurrence of the spatter S occurs on the front surface 10 aof the silicon substrate 10 due to the fact that when the laser light L 1 having a wavelength of more than 1064 nm is used, the laser light L 1 converges on the crack F that extends far from the modified region 7 already formed, thereby increasing the influence of leaking light (light in the laser light L 1 that does not contribute to the formation of the modified region 7 and leaks to the front surface 10 aside of the silicon substrate 10). Based on this finding, the present inventors have considered that in forming the second modified region 7 bin the case of FIG. 10, when the convergence point P of the laser light L 1 is displaced, the influence of the leaking light that causes the occurrence of the spatter S can be reduced, and the subsequent examination is performed. It should be noted that, in the formation of the second modified region 7 b, the fact that "the convergence point P of the laser light L 1 is displaced in a direction vertical to both the thickness direction of the silicon substrate 10 and an extension direction of the cut line 5 (a direction vertical to the cross section of the silicon substrate 10 in FIG. 10( a) ) with respect to a position at which the convergence point P of the laser light L 1 converges when the first modified region 7 ais formed" may be simply referred to as "the convergence point P of the laser light L 1 is displaced". A "distance by which the convergence point P of the laser light L 1 is offset" is referred to as an "offset value".First, a check is performed for the direction of the crack F extending from the first modified region 7 ato the front surface 10 aside of the silicon substrate 10. FIG. 12 is a view illustrating a photograph of the silicon substrate 10 in a case where the convergence point P of the laser light L 1 is not displaced in forming the second modified region 7 b. Specifically, FIG. 12( a) is a view illustrating a photograph of a surface parallel to the cut line of the silicon substrate 10 after the formation of the first modified region 7 aand the second modified region 7 b. FIG. 12( b) is a view illustrating a photograph of a surface vertical to the cut line of the silicon substrate 10 after the formation of the first modified region 7 aand the second modified region 7 b. Referring to FIG. 12( b), it can be confirmed that in a case where the convergence point P of the laser light L 1 is not displaced in forming the second modified region 7 b, the crack F extends straight (along the thickness direction of the silicon substrate 10) to the front side 10 aside of the silicon substrate 10 from the first modified region 7 a.FIG. 13 is a view illustrating a photograph of the silicon substrate 10 in a case where the convergence point P of the laser light L 1 is offset in the formation of the second modified region 7 b(in a case where the offset value is 8 μm). Specifically, FIG. 13( a) is a view illustrating a photograph of a surface parallel to the cut line of the silicon substrate 10 after the formation of the first modified region 7 aand the second modified region 7 b. FIG. 13( b) is a view illustrating a photograph of a surface vertical to the cut line of the silicon substrate 10 after the formation of the first modified region 7 aand the second modified region 7 b. Referring to FIG. 13( b), it can be confirmed that even in a case where the convergence point P of the laser light L 1 is offset in forming the second modified region 7 b, the crack F extends straight (along the thickness direction of the silicon substrate 10) to the front side 10 aside of the silicon substrate 10 from the first modified region 7 a.Subsequently, a check is performed for the length of the crack F extending from the first modified region 7 ato the front surface 10 aside of the silicon substrate 10. FIG. 14 is a graph illustrating a relationship between the displacement value and the length of the crack F. The length of the crack F is a length of the crack F extending from the first modified region 7 ato the front surface 10 aside of the silicon substrate 10. Referring to FIG. 14, it can be confirmed that even if the convergence point P of the laser light L 1 is offset or not offset (even in a case where the offset value is 0 μm), when the second modified region 7 bis formed, the length of the crack F extending from the first modified region 7 ato the front surface 10 aside of the silicon substrate 10 is not changed.Subsequently, a check is performed for a generation amount of the spatter S. FIG. 15 is a graph showing a relationship between the displacement value and the number of spatters S. The number of spatters S is the number of spatters S generated in regions of 20 μm or more away from both sides of the cut line 5 (the number per 15 mm of the length of the cut line 5). Referring to FIG. 15, it can be confirmed that when the convergence point P of the laser light L 1 is displaced in forming the second modified region 7 b, the number of spatters S decreases compared to a case where the convergence point P is not displaced (in the case where the displacement value is 0 μm). It should be noted that the reason why the number of the spatters S generated in the regions of 20 μm or more away from the cut line 5 is counted is that, in particular, such spatters S cause the problem that the characteristics of the functional device formed on the front surface 10 aof the silicon substrate 10 deteriorate. Since a dicing line (the area between adjacent functional devices) is often provided in the areas within 20 μm on both sides of the dicing line 5, the possibility that the spatter S occurring in the areas causes the problem in which the characteristics of the functional device deteriorate is low.From the examination results of FIGS. 12, 13, 14 to 15, it was found that even when the convergence point P of the laser light L 1 is displaced in forming the second modified region 7 b, the crack F extends straight (along the thickness direction of the silicon substrate 10) to the front surface 10 aside of the silicon substrate 10 from the first modified region 7 aand the length of the crack F extending from the first modified region 7 ato the front surface 10 aside of the silicon substrate 10 does not change. On the other hand, it is found that when the convergence point P of the laser light L 1 is displaced in forming the second modified region 7 b, the number of spatters S decreases. In the inspection of FIGS. 12, 13, 14 to 15, except for the offset value, the irradiation conditions of the laser light are the same.The considerations of the present inventors regarding the decrease in the number of spatters S are as follows. FIG. 16 is a view illustrating a photograph of the silicon substrate 10 in the case where the convergence point P of the laser light L 1 is offset in forming the second modified region 7 b. Specifically, FIG. 16( a) is a view illustrating a photograph of a surface parallel to the cut line 5 of the silicon substrate 10 after cutting. FIG. 16( b) is a view illustrating a photograph of the front surface 10 aside (metal film 11) of the silicon substrate 10 after cutting. Referring to FIG. 16( a), it can be confirmed that the convergence point P of the laser light L 1 is offset in forming the second modified region 7 b, thereby suppressing the laser light L 1 from converging on the crack F extending from the second modified region 7 band the already formed first modified region 7 aand forming the second modified region 7 binto a large size. That is, it is considered that the proportion of the laser light L 1 contributing to the formation of the second modified region 7 bincreases and the proportion of the leaking light decreases. Referring to FIG. 16( b), it can be confirmed that no spatter S occurs.On the other hand, referring to FIG. 11( a) illustrating a photograph of the silicon substrate 10 in the case where the convergence point P of the laser light L 1 is not displaced in forming the second modified region 7 b, it can be confirmed that the second modified region 7 bis formed small. It is assumed that this is due to the fact that the laser light L 1 converges on the crack F extending from the second modified region 7 band the already formed first modified region 7 a, and the leaking light has an increased value. In the inspection of FIGS. 11 and 16, except for the offset value, the irradiation conditions for the laser light are the same.FIG. 17 is a view illustrating a photograph of the front surface 10 aside (metal film 11) of the silicon substrate 10 after cutting. Specifically, FIG. 17( a) shows a case where the displacement value is 2 μm. FIG. 17( b) shows a case where the displacement value is 4 μm. FIG. 17( c) shows a case where the displacement value is 6 μm. In either case, except for the offset value, the irradiation conditions of the laser light are the same. Referring to FIGS. 17( a) and 17( b), it can be confirmed that the spatter S occurs on an opposite side to a side where the convergence point P of the laser light L 1 is displaced in the formation of the second modified region 7 band the spatter S occurs with increasing displacement value away from the cutting line 5. Referring to FIGS. 17( a), 17( b), and 17( c), it can be confirmed that an occurrence range of the spatter S is decreased as the displacement value is increased. Note that even in the cases of FIGS. 17( a) and 17( b), as compared with the case where the convergence point P of the laser light L 1 is not displaced in forming the second modified region 7 b, the appearance range of the spatter S is reduced.The reasons why the results of FIGS. 17( a), 17( b) and 17( c) are obtained are as follows. FIG. 18( a) is a view illustrating a surface vertical to the cut line 5 of the silicon substrate 10 in a case where the displacement value is small. FIG. 18( b) is a view illustrating a surface vertical to the cut line 5 of the silicon substrate 10 in a case where the displacement value is large. It should be noted that the term "the convergence point P of the laser light L 1 in forming the first modified region 7 a" is referred to as "a first convergence point P 1.". The term "the convergence point P of the laser light L 1 in forming the second modified region 7 b" is referred to as "a second convergence point P 2".As shown in FIG. 18( a), in the case where the displacement value is small, in the crack F extending from the second modified region 7 band the already formed first modified region 7 a, a portion F 1 at which the second convergence point P 2 of the laser light L 1 converges is inclined by a small angle with respect to a thickness direction D of the silicon substrate 10. For this reason, an inclination angle θ of the laser light L 1 with respect to the portion F 1 becomes large. Thus, the leaking light L 2 that does not contribute to the formation of the second modified region 7 bin the laser light L 1 moves at a small angle with respect to the thickness direction D of the silicon substrate 10 toward the opposite side from the side where the convergence point P of the laser light L 1 is displaced. Consequently, the optical path length of the leaking light L 2 reaching the front surface 10 aof the silicon substrate 10 becomes short, and the absorption height and the scattering degree of the leaking light L 2 in the silicon substrate 10 become small. It should be noted that the terms "small", "large", "short" and the like are used in comparison with the case of FIG. 18( b).On the other hand, as shown in FIG. 18( b), in the case where the displacement value is large, in the crack F extending from the second modified region 7 band the already formed first modified region 7 a, the portion F 1 at which the second convergence point P 2 of the laser light L 1 converges inclines at a large angle with respect to the thickness direction D of the silicon substrate 10. Thus, the leaking light L 2 that does not contribute to the formation of the second modified region 7 bin the laser light L 1 moves at a large angle with respect to the thickness direction D of the silicon substrate 10 toward the opposite side from the side where the convergence point P of the laser light L 1 is offset. Consequently, the optical path length of the leaking light L 2 reaching the front surface 10 aof the silicon substrate 10 becomes long, and the absorption amount and the scattering degree of the leaking light L 2 in the silicon substrate 10 become large. It should be noted that the terms "large", "small", "long" and the like are used in comparison with the case of FIG. 18( a).From the above consideration of FIG. 18, it is assumed that the spatter S occurs on the opposite side to the side where the convergence point P of the laser light L 1 is offset in forming the second modified region 7 b, and as the offset value of the spatter S increases, the occurrence range of the spatter S decreases from the cutting line, and as the offset value increases.Next, a semiconductor chip manufacturing method using a laser processing method of the embodiment will be described. First, as shown in FIG. 19, the object to be processed 1 is manufactured with the silicon substrate 10 having a functional device layer 15 on the front surface 10 a. The functional device layer 15 side of the object to be processed 1 is bonded to a protective film 22 held by an annular holding member 20. The functional device layer 15 includes a plurality of functional devices arranged in a matrix.Subsequently, the first modified region 7 ais formed along each cut line 5 arranged in a lattice pattern so as to extend between adjacent functional devices. Specifically, using the back surface 10 bof the silicon substrate 10 as the laser light input surface, the laser light L 1 having a wavelength of more than 1064 nm converges on the silicon substrate 10, and while a distance between the front surface 10 aof the silicon substrate 10 and the first convergence point P 1 of the laser light L is maintained at a first distance, the first convergence point P 1 of the laser light L 1 is moved along the cut line 5, thereby forming the first modified region 7 aalong the cut line 5 (first step). At this time, while the distance is maintained at 0, whereby the first convergence point P 1 of the laser light L 1 is displaced in the direction vertical to both the thickness direction of the silicon substrate 10 and the extension direction of the cut line 5, with respect to the cut line 5, the first convergence point P 1 of the laser light L 1 is moved along the cut line 5. That is, while maintaining a state in which the first convergence point P 1 of the laser light L is disposed on the cutting line 5 as viewed from the thickness direction of the silicon substrate 10, the first convergence point P 1 of the laser light L is moved along the cutting line 5. Consequently, the first modified region 7 ais formed inside the silicon substrate 10 along the cut line 5 in a state of being positioned on the cut line 5 as viewed from the thickness direction of the silicon substrate 10.Subsequently, the second modified region 7 bis formed along each cut line 5 arranged in a lattice pattern so as to extend between adjacent functional devices. Specifically, using the back surface 10 bof the silicon substrate 10 as the laser light input surface, the laser light L 1 having a wavelength of more than 1064 nm converges on the silicon substrate 10, and while a distance between the front surface 10 aof the silicon substrate 10 and the second convergence point P 2 of the laser light L 1 is maintained at a second distance larger than the first distance, and while the second convergence point P 2 of the laser light L 1 is displaced, the second convergence point P 2 of the laser light L 1 is moved along the cut line 5, thereby forming the second modified region 7 balong the cut line 5 (second step). That is, while maintaining a state in which the second convergence point P 2 of the laser light L is located away from the cutting line 5 by a predetermined distance as viewed in the thickness direction of the silicon substrate 10, the second convergence point P 2 of the laser light L is moved along the cutting line 5 (parallel to the cutting line 5). Consequently, the second modified region 7 bis formed inside the silicon substrate 10 along the cut line 5 (parallel to the cut line 5) in a state of being away from the cut line 5 by the predetermined distance as viewed in the thickness direction of the silicon substrate 10.Consequently, the crack F extending in the thickness direction of the silicon substrate 10 from the first modified region 7 aand the second modified region 7 bachtains the front surface 10 aof the silicon substrate 10, and the functional device layer 15 is cut for each functional device. For example, the thickness of the silicon substrate 10 is 775 μm, and the first modified region 7 aand the second modified region 7 bare formed in a region from the front surface 10 aof the silicon substrate 10 to a depth of 160 μm.The aforementioned first step and second step are performed by the laser processing apparatus 100 described above. That is, the stage 107 holds the object to be processed 1. the laser light source 101 emits the laser light L 1 having a wavelength of more than 1064 nm. The converging lens (converging optical system) 105 converges the laser light L 1 emitted from the laser light source 101 on the object to be processed 1 held by the stage 107, so that the back surface 10 bof the silicon substrate 10 is the laser light input surface. Subsequently, the stage controller (controller) 115 and the laser light source controller (controller) 102 control the operations of the stage 107 and the laser light source 101 such that the above-described first step and second step are performed. It should be noted that the movement of the first convergence point P 1 and the second convergence point P 2 of the laser light L with respect to the cut line 5 is performed by operating the converging lens 105 side, or by operating both the stage 107 side and the converging lens 105 side.Subsequently, as shown in FIGS. 19 and 20, a predetermined portion K including the back surface 10 bof the second modified region 7 bin the silicon substrate 10 is removed (third step). Specifically, the back surface 10 bof the silicon substrate 10 on which the first modified region 7 adisposed on the front surface 10 aside and the second modified region 7 bdisposed on the back surface 10 bside of the first modified region 7 aare mechanically ground and polished using a grinding machine or the like. Consequently, a predetermined portion K 1 including the second modified region 7 band not including the first modified region 7 aare removed from the back surface 10 band the object to be processed 1 is thinned to a predetermined thickness. That is, the back surface 10 bof the silicon substrate 10 is polished such that the predetermined portion K 1 having the second modified region 7 bis removed.In the object to be processed 1, after polishing of the first modified region 7 aand the second modified region 7 b, the first modified region 7 aremains. In the object to be processed 1, only the first modified region 7 aand the crack F are included after polishing. By removing the predetermined portion K 1, the crack F reaches the back surface 10 bof the silicon substrate 10 from which the predetermined portion K 1 has been removed, and the object to be processed 1 is cut for each functional device. For example, the silicon substrate 10 is thinned to a thickness of 200 μm.Subsequently, as shown in FIG. 21, an expansion film 23 is bonded to the back surface 10 bof the silicon substrate 10 and the holding member 20. As shown in FIG. 22, the protective film 22 is removed. As shown in Fig. 23, a pressing member 24 is pressed against the expanding film 23 to expand the expanding film 23. Consequently, the object to be processed 1 cut for each functional device 15 a, that is, a plurality of semiconductor chips 1A, is separated from each other. Subsequently, as shown in FIG. 24, the expansion film 23 is irradiated with ultraviolet light to reduce the adhesive force of the expansion film 23. Each semiconductor chip 1A is accommodated.As described above, in the laser processing method of the embodiment, the second convergence point P 2 of the laser light L 1 is offset when the second modified region 7 bis formed. Consequently, it is possible to suppress the occurrence of the spatter S on the front surface 3 of the object to be processed 1 on the opposite side to the laser light input surface.By offsetting the second convergence point P 2, a position of the second modified region 7 bin an offset direction (vertical direction to the intersecting line 5) has a height difference with respect to a position of the first modified region 7 a. However, the second modified region 7 bis removed together with the predetermined portion K 1. Accordingly, in the object to be processed 1 cut along the cutting line 5 after the removal of the predetermined portion K 1, it is possible to suppress the occurrence of the difference in height on the cutting surface due to the second modified region 7 b. It is possible to improve straightness by smoothing the cutting surface. It is possible to obtain a satisfactory end face of each semiconductor chip 1A.In the laser processing method of the embodiment, the silicon substrate 10 is used as a semiconductor substrate. The laser light L1 has a wavelength of more than 1064 nm. Consequently, compared with the case where the laser light L 0 having a wavelength of 1064 nm or less is used, the crack F can be greatly expanded in the thickness direction of the silicon substrate 10 from the first modified region 7 aand the second modified region 7 bin the formation of the first modified region 7 aand the second modified region 7 b.When laser light L 1 having a wavelength of 1099 nm or more and 1342 nm or less is used, the crack F in the thickness direction of the silicon substrate 10 can be greatly expanded by the first modified region 7 aand the second modified region 7 bin the formation of the first modified region 7 aand the second modified region 7 b. In particular, the laser light L 1 having a wavelength of 1342 μm can greatly expand the crack F.When the displacement value for displacing the second convergence point P 2 of the laser light L 1 in forming the second modified region 7 bis 24 μm or less, the crack F can reliably establish a connection between the first modified region 7 aand the second modified region 7 b. The crack F can be reliably expanded in the thickness direction of the silicon substrate 10 from the first modified region 7 aand the second modified region 7 bin the formation of the first modified region 7 aand the second modified region 7 b. Further, when the displacement value is set to 4 μm or more and 18 μm or less, the crack F can more reliably establish a connection between the first modified region 7 aand the second modified region 7 b. The crack F can be more reliably expanded in the thickness direction of the silicon substrate 10 from the first modified region 7 aand the second modified region 7 b. In particular, when the displacement value is set to 6 μm or more and 16 μm or less, a balanced relationship can be obtained with respect to suppressing the occurrence of the spatter S and bonding and expansion of the crack F.In the laser processing method of this embodiment, by forming the second modified region 7 b, the crack F extending from the first modified region 7 aand the second modified region 7 bin the thickness direction of the silicon substrate 10 is made to reach the front surface 10 a. Subsequently, by removing the predetermined portion K 1, the crack F is caused to reach the back surface 10 bof the silicon substrate 10 from which the predetermined portion K 1 has been removed. Consequently, the object to be processed 1 can be cut accurately along the cutting line 5.FIG. 25 is a view illustrating a laser processing method according to a modification. FIG. 25( a) is a sectional view taken along the line of intersection 5 of the object 1 to be processed before polishing. FIG. 25( b) is a sectional view along the section line 5 of the object to be processed 1 after polishing. As shown in FIGS. 25( a) and 25( b), in the case where the back surface 10 bof the silicon substrate 10 is polished, a predetermined portion K 2 including the back surface 10 bof the silicon substrate 10 and both the first and second modified regions 7 aand 7 bmay be removed. That is, the back surface 10 bof the silicon substrate 10 may be polished such that the predetermined portion K 2 further including the first modified region 7 ais removed with respect to the predetermined portion K 1 (see FIG. 20 ).Consequently, the predetermined portion K2 including the first and second modified regions 7a and 7b is removed from the silicon substrate 10, and the object to be processed 1 is thinned to a predetermined thickness. Neither the first nor the second modified regions 7 aand 7 bremain in the object to be processed 1 after polishing, so that only the crack F is present.In the laser processing method illustrated in FIG. 25, after the formation of the first and second modified regions 7 aand 7 b, not only the second modified region 7 bbut also the first modified region 7 aare removed from the silicon substrate 10. Accordingly, in the object to be machined 1 cut along the cutting line 5, after removing the predetermined portion K 2, it is possible to suppress deterioration of the straightness of the cut surface due to the first modified region 7 a. It is possible to obtain an even better end face of each semiconductor chip 1A.The preferred embodiments have been described above; however, the present invention is not limited to the above-described embodiments and can be modified or applied to other aspects in the light of the gist of the invention described in each claim.For example, when the first modified region 7 ais formed, the first convergence point P 1 of the laser light L 1 may be displaced to a side in the direction vertical to both the thickness direction of the silicon substrate 10 and the extension direction of the cut line 5 with respect to the cut line 5. In forming the second modified region 7 b, the second convergence point P 2 of the laser light L 1 may be displaced to the other side in the direction vertical to both the thickness direction of the silicon substrate 10 and the extension direction of the cut line 5 with respect to the cut line 5. That is, in forming the first modified region 7 a, while maintaining a state in which the first convergence point P 1 of the laser light L is apart from the cutting line 5 to the one side by a predetermined distance from the cutting line 5 in the thickness direction of the silicon substrate 10, the first convergence point P 1 of the laser light L can be moved along the cutting line 5 (parallel to the cutting line 5). When the second modified region 7 bis formed while maintaining a state in which the second convergence point P 2 of the laser light L is away from the cut line 5 by a predetermined distance from the other side as viewed in the thickness direction of the silicon substrate 10, the second convergence point P 2 of the laser light L can be moved along the cut line 5 (parallel to the cut line 5). Consequently, the first modified region 7 ais formed inside the silicon substrate 10 along the cut line 5 (parallel to the cut line 5) in a state of being apart from the cut line 5 by a predetermined distance toward one side as viewed in the thickness direction of the silicon substrate 10. The second modified region 7 bis formed inside the silicon substrate 10 along the cut line 5 (parallel to the cut line 5) in a state of being away from the cut line 5 by a predetermined distance toward the other side as viewed in the thickness direction of the silicon substrate 10. In this case, the first modified region 7 aand the second modified region 7 bmay be formed on the one side and the other side with respect to the cut line 5 in a balanced relationship.The above-described embodiments are not limited to an example in which a step (first step) of forming the first modified region 7 afor all the cut lines 5 arranged in a grid pattern and then a step (second step) of forming the second modified region 7 bfor all the cut lines 5 arranged in a grid pattern are performed. As another example, the step (first step) of forming the first modified region 7 aand the step (second step) of forming the second modified region 7 bmay be performed as follows. First, the step (first step) of forming the first modified region 7 ais performed for the cut line 5 extending in a first direction from all the cut lines 5 arranged in a grid pattern. Subsequently, the step (second step) of forming the second modified region 7 bfor the cut line 5 extending in the first direction is performed. Subsequently, for the cut line 5 extending in a second direction (direction vertical to the first direction) from all the cut lines 5 arranged in a grid pattern, the step (first step) of forming the first modified region 7 ais performed. Subsequently, the step (second step) of forming the second modified region 7 bfor the cut line 5 extending in the second direction is performed. For a plurality of cut lines 5, for each individual cut line 5, the step (first step) of forming the first modified region 7 ais performed. Subsequently, the step (second step) of forming the second modified region 7 bmay be performed. That is, the step (first step) of forming the first modified region 7 aand the step (second step) of forming the second modified region 7 bmay be performed for one cutting line 5, and the step (first step) of forming the first modified region 7 aand the step (second step) of forming the second modified region 7 bmay be performed for another cutting line 5.The removal of the predetermined portion K may be performed by chemical polishing such as etching instead of or in addition to polishing by mechanical grinding. The predetermined portion K may be removed by various known techniques. In the above-described embodiment, the semiconductor substrate is not limited to the silicon substrate 10 as long as the modified region 7 can be formed by converging the laser light L 1. In the above-described embodiments, the wavelength of the laser light L 1 is not limited to a wavelength greater than 1064 nm as long as the modified region 7 can be formed by converging the laser light L 1.In the above-described embodiments, by causing the crack F to reach the back surface 10 bof the silicon substrate 10 when the predetermined portion K 1 is removed, the object to be processed 1 is cut along the cut line 5. However, instead of or in addition to this, the object to be processed 1 may be cut along the cut line 5 by causing the crack F to reach the back surface 10 bof the silicon substrate 10 at the extension of the extension film 23 to which the silicon substrate 10 from which the predetermined portion K 1 has been removed is bonded.Industrial applicabilityAccording to an aspect of the present invention, it is possible to provide a laser processing method capable of suppressing the occurrence of spatter on the front surface of the object to be processed on the side opposite to the laser light input surface and smoothing the cut surface.List of reference characters1 Object to be processed, 5 cut line, 7 afirst modified region, 7 bsecond modified region, 10 silicon substrate (semiconductor substrate), 10 afront surface, 10 bback surface, 15 afunction device, F crack, K 1, K 2 predetermined portion, L 1 laser light, P 1 first convergence point, P 2 second convergence point.

Claims

A laser processing method comprising: a first step of converging laser light (L) onto an object to be processed (1) having a semiconductor substrate formed with a plurality of functional devices on a front surface (3) using a rear surface of the semiconductor substrate as a laser light entrance surface, and while maintaining a distance between the front surface (3) of the semiconductor substrate and a first convergence point (P1) of the laser light (L) at a first distance, moving the first convergence point (P1) of the laser light (L) along a cutting line (5) passing between the adjacent functional devices to form a first modified region (7a) along the cutting line (5); after the first step, a second step of converging the laser light (L) on the object to be processed (1), using the back surface of the semiconductor substrate as the laser light entrance surface, and while maintaining a distance between the front surface (3) of the semiconductor substrate and a second convergence point (P 2) at a second distance larger than the first distance, and while shifting the second convergence point (P 2) of the laser light (L) in a direction vertical to both a thickness direction of the semiconductor substrate and an extending direction of the cut line (5) with respect to a position where the first convergence point (P 1) of the laser light (L) has converged, moving the second convergence point (P 2) of the laser light (L) along the cut line (5) to form a second modified region (7 b) along the cut line (5); and after the second step, a third step of removing a predetermined portion (K1) including the back surface and at least the second modified region (7b) in the semiconductor substrate.The laser processing method according to claim 1, wherein in the third step, the predetermined portion (K1) further including the first modified region (7a) in the semiconductor substrate is removed.The laser processing method according to claim 1 or 2, wherein the semiconductor substrate is a silicon substrate, and the laser light (L) has a wavelength of more than 1064 nm.The laser processing method according to claim 3, wherein the laser light (L) has a wavelength of 1099 nm or more and 1342 nm or less.The laser processing method according to claim 3 or 4, wherein a distance by which the second convergence point (P2) of the laser light (L) is offset in the direction perpendicular to both the thickness direction of the silicon substrate and the extension direction of the cut line (5) with respect to the position at which the first convergence point (P1) of the laser light (L) converges is 24 μm or less.The laser processing method according to claim 5, wherein the distance by which the second convergence point (P2) of the laser light (L) is 4 μm or more and 18 μm or less in the direction perpendicular to both the thickness direction of the silicon substrate and the extension direction of the cut line (5) with respect to the position at which the first convergence point (P1) of the laser light (L) converges.The laser processing method according to any one of claims 1 to 6, wherein in the second step, by forming the second modified region (7b), a crack (F) extending in the thickness direction of the semiconductor substrate from the first modified region (7a) and the second modified region (7b) is made to reach the front surface (3) of the semiconductor substrate, and in the third step, by removing the predetermined portion (K1), the crack (F) extending in the thickness direction of the semiconductor substrate from the first modified region (7a) and the second modified region (7b) in the second step is made to reach the rear surface of the semiconductor substrate from which the predetermined portion has been removed.

Images