Laser beam machining method and laser beam machining device

Abstract

Provided is a laser beam machining method for forming a modified region by irradiating the interior of a wafer with a laser beam, the laser beam machining method including a first irradiation step for irradiating a first street of the wafer with the laser beam, and a second irradiation step for irradiating a second street that crosses the first street with the laser beam after the first irradiation step, where, in the second irradiation step, the total output of the laser beam at the intersection of the first street and the second street is changed.

Description

Laser processing method and laser processing device

[0001] The present invention relates to a laser processing method and a laser processing apparatus. This application claims priority to Japanese Patent Application No. 2024-055602, filed on March 29, 2024, the contents of which are incorporated herein by reference.

[0002] Conventionally, a laser processing apparatus is known that performs internal focusing processing to form a laser processing area that serves as a starting point for cutting inside a wafer along a plurality of streets (streets CH1, CH2) formed in a grid pattern (Patent Document 1).

[0003] When performing internal focusing processing of a wafer using the laser processing apparatus described in Patent Document 1, first, a laser beam is irradiated along street CH1 to form a laser processing region inside the wafer, and then a laser processing region is formed inside the wafer along street CH2 that intersects with street CH1.

[0004] When the laser beam along the street CH2 reaches the intersection with the street CH1, the laser beam is scattered by the laser processing area formed earlier at the intersection, which may cause thermal damage around the street and adversely affect the device layer formed around the street.

[0005] Here, by switching the laser beams that perform internal focusing processing at the intersection, scattering of the laser beam in the laser processing region previously formed at the intersection can be suppressed. Therefore, the occurrence of the thermal damage can be suppressed. Conventionally, optical modulators capable of switching a single laser beam to multiple laser beams are known (Patent Document 2). Patent Document 2 discloses a reflective crystal (LCOS: Liquid Crystal on Silicon) spatial light modulator (SLM) (LCOS-SLM) as an example of an optical modulator. This reflective crystal spatial light modulator can switch a single laser beam to multiple laser beams. Furthermore, it is also possible to change the intensity ratio of each laser beam among the multiple laser beams. By switching the laser beams or changing the intensity ratio of the laser beams at the intersection, the occurrence of the thermal damage can be suppressed.

[0006] Japanese Patent Publication No. 2022-006548 Japanese Patent Publication No. 2016-107334

[0007] Here, the switching of the laser beam by the LCOS-SLM is usually performed when the scanning direction of the laser beam is changed (when the laser beam returns from back-and-forth processing). In addition, in the case of a wafer having a diameter of 300 mm, the time required to process one line of the wafer (for example, to form a crack inside) is approximately 270 ms to 500 ms. On the other hand, the switching of the laser beam by the LCOS-SLM takes approximately 50 ms to 150 ms. Therefore, because the time required for switching is long, it is not possible to switch the laser beam during processing.

[0008] Other known means for switching a single laser beam into multiple laser beams include the use of a diffractive optical element (DOE), a half mirror, or a polarizing beam splitter (PBS). When a DOE, a half mirror, or a polarizing beam splitter (PBS) is used, the response speed is insufficient to switch the laser beams only around the intersection, as in the case of an LCOS-SLM, and it is difficult to suppress thermal damage around the intersection while maintaining processing quality.

[0009] The present invention has been made in view of the above circumstances, and has an object to provide a laser processing method and a laser processing apparatus that can suppress thermal damage that can occur around intersections in a wafer.

[0010] The present invention has been made to solve the above-mentioned problems and proposes the following means. <1> A laser processing method according to one aspect of the present invention is a laser processing method for irradiating a wafer with laser light to form a modified region, the method including a first irradiation step of irradiating a first street of the wafer with the laser light, and a second irradiation step of irradiating a second street intersecting the first street with the laser after the first irradiation step, wherein the total output of the laser light at an intersection between the first street and the second street is changed in the second irradiation step. <2> In the laser processing method described in <1> above, the total output of the laser light irradiated at the intersection may be changed by an acousto-optical element in the second irradiation step. <3> In the laser processing method described in <1> or <2> above, at least one of the intensity and the number of the laser light irradiated at the intersection may be changed in the second irradiation step. <4> In the laser processing method described in <3> above, in the second irradiation step, a plurality of focal points are generated in the depth direction of the wafer by a plurality of the laser beams, and the total output of the laser beams may be changed so that the laser irradiation intensity of the focal point with the smallest intensity among the plurality of focal points is less than the modification limit.

[0011] <5> Furthermore, a laser processing apparatus according to one aspect of the present invention is a laser processing apparatus that irradiates laser light into the interior of a wafer to form a modified region, and changes the total output of the laser light at the intersection of a first street of the wafer and a second street that intersects with the first street.

[0012] According to the present invention, it is possible to suppress thermal damage that may occur around the intersections in the wafer.

[0013] 7 is a schematic diagram showing an outline of thermal damage evaluation using a wafer having a low-melting-point metal film. FIG. 7 is an optical microscope image of a Sn film after scanning with laser light L. FIG. 7 is an optical microscope image of a Sn film after scanning with laser light L on a first street CH1 and a second street CH2 perpendicular to the first street CH1. FIG. 7 is a schematic configuration diagram of a laser processing apparatus according to a first embodiment of the present invention. FIG. 7 is a conceptual diagram for explaining laser light L. FIG. 7 is a conceptual diagram showing examples of the intensities of three laser beams. FIG. 7 is a graph showing an example of the modification limit energy. FIG. 7 is an enlarged view of region VII in FIG. 7. FIG. 7 is an optical microscope image of a cross section of a wafer W after irradiating with laser light L of different energies. FIG. 7 is a schematic diagram of a cross section near an intersection on a second street where a modified region is formed by a single laser beam. FIG. 7 is a schematic diagram of a cross section near an intersection CR on a second street processed by laser light. FIG. 7 is a schematic diagram of a cross section near an intersection CR on a second street processed by multiple laser beams. FIG. 7 is a flowchart explaining the flow of a laser processing method according to an embodiment of the present invention. FIG. 7 is a flowchart explaining the flow of a second irradiation step in the same embodiment.

[0014] Before describing the embodiments of the present invention, thermal damage occurring in a wafer will be described. FIG. 1 is a schematic diagram illustrating an outline of thermal damage evaluation using a wafer having a low-melting-point metal film. FIG. 1 is a cross-sectional view of a wafer W along the scanning direction of a laser beam L, and schematically illustrates how modified regions R are formed on the wafer W along the scanning direction of the laser beam L. The wafer W having a low-melting-point metal film is a wafer for evaluating thermal damage to product wafers in advance, and has a Sn film formed on the underside of the substrate. The low-melting-point metal film is, for example, a Sn film made of Sn (tin). The melting point of Sn is 232°C. As shown in FIG. 1, laser beam L is irradiated into the interior of the wafer W from the side opposite the side on which the Sn film is formed, forming modified regions R, and thermal damage occurs in the Sn film located directly below each modified region R in the Z direction. Thermal damage to the wafer W after the modified regions R (modified layers) are formed is evaluated using the Sn film. If the irradiation of the laser light L is inappropriate, thermal damage may occur not only directly below the modified region R but also in the surrounding area. Evaluation of thermal damage using a wafer W having a Sn film is performed to determine in advance the locations where thermal damage may occur.

[0015] Figure 2 is an optical microscope image of the Sn film after scanning with laser light L. As shown in Figure 2, thermal damage has occurred in the Sn film due to the laser light L. By observing the Sn film in this way, thermal damage can be evaluated. Note that in Figure 2, a crack extending in the direction of the thermal damage can be seen. This crack was generated when a crack that had occurred in the Si (substrate) above the Sn film reached the Sn film.

[0016] FIG. 3 shows an optical microscope image of the Sn film after laser light L is scanned on a first street CH1 and a second street CH2 perpendicular to the first street CH1. When a modified region is formed on the first street CH1 and the second street CH2 by irradiating laser light using a conventional method, the laser light L may be reflected and scattered by the modified region R formed on the first street CH1 at the intersection between the first street CH1 and the second street CH2 and reach the periphery of the intersection. As a result, as shown in FIG. 3, thermal damage may occur around the street, particularly around the intersection. Such thermal damage to a wafer carrying devices may result in product defects. Therefore, it is important to prevent thermal damage from occurring around the street.

[0017] First Embodiment Hereinafter, a laser processing apparatus 1 according to a first embodiment of the present invention will be described with reference to the drawings. Fig. 4 is a schematic diagram of the processing apparatus according to the first embodiment of the present invention. In the XYZθ directions shown in each figure, the + direction is the positive direction, and the - direction is the negative direction. As shown in Fig. 4, the θ direction is the direction of rotation parallel to the XY plane with the Z axis as the central axis.

[0018] As shown in FIG. 1, the laser processing apparatus 1 according to this embodiment includes a wafer moving unit 11, a laser head 20, and a control unit 50.

[0019] The wafer moving unit 11 includes a suction stage 13 that holds the wafer W by suction, and an XYZθ table 12 that is provided on the main body base 16 of the laser processing apparatus 1 and that precisely moves the suction stage 13 in the XYZθ directions. The wafer W is precisely moved in the XYZθ directions by this wafer moving unit 11.

[0020] The wafer W has a backgrinding tape (hereinafter referred to as BG tape) having an adhesive attached to the surface on which devices are formed, and is placed with the back surface facing upward on the suction stage 13. Alternatively, the wafer W may have a dicing sheet having an adhesive attached to one surface, and be placed on the suction stage 13 in a state where it is integrated with a frame via this dicing sheet.

[0021] The laser head 20 mainly includes a laser light source 22, a spatial light modulator 28, a condenser lens 38, and the like.

[0022] The laser light source 22 outputs a processing laser light L for forming a modified region inside the wafer W under the control of the control unit 50. The laser light L is, for example, a semiconductor laser-pumped Nd:YAG laser light source, a wavelength of 1.1 μm, a repetition frequency of 80 to 200 kHz, a pulse width of 180 to 400 ns, and an output of 10 to 40 W. The wavelength of the laser light L is preferably 1.0 μm or more and 1.5 μm or less. When forming a modified region R inside the wafer using the laser light L, using a longer wavelength for the laser light L can form wider cracks. Furthermore, under the wavelength conditions of the laser light L described above, the pulse pitch of the laser light L can be increased. Therefore, the pulse pitch of the laser light L is preferably 4 μm or more and 12 μm or less. Based on the above wavelength and pulse pitch conditions, the processing speed of the modified region R using the laser light L can be improved. The relative speed at which the laser light L travels with respect to the wafer W is, for example, 800 to 1600 mm / s.

[0023] The laser light source 22 includes an intensity modulation element (not shown). The intensity modulation element modulates the intensity of the laser light (laser irradiation intensity). As the intensity modulation element, an acousto-optic element, an electro-optic modulator, a Mach-Zehnder interferometer, or the like is used. As the acousto-optic element, for example, an AOM (acousto-optic modulator) is mentioned. Since high-speed intensity modulation is possible, the acousto-optic element is preferable as the intensity modulation element.

[0024] The spatial light modulator 28 is a phase modulation type that inputs the laser light L output from the laser light source 22, presents a predetermined hologram pattern that modulates the phase of the laser light L in each of a plurality of two-dimensionally arranged pixels, and outputs the phase-modulated laser light L. This hologram pattern is a superposition of a plurality of Fresnel lens patterns with different focusing positions. As a result, a plurality of laser beams are focused at an arbitrary intensity ratio at positions within the wafer W that are at different depths from the laser light irradiation surface (the back surface of the wafer W). In other words, the spatial light modulator 28 generates a plurality of focusing points in the depth direction of the wafer W.

[0025] The spatial light modulator 28 can modulate the laser light L, for example, so that the laser light L comprises any number of laser beams. FIG. 5 is a conceptual diagram for explaining the laser light L. FIG. 5( a) shows a single laser beam L (L10). The spatial light modulator 28 may, for example, modulate the laser light L ( FIG. 5( a)) comprising only the single laser beam L10 so that the laser light L comprises a first laser beam L11 and a second laser beam L12, as shown in FIG. 5( b), or may modulate the laser light L so that the laser light L comprises a third laser beam L13, a fourth laser beam L14, and a fifth laser beam L15, as shown in FIG. 5( c).

[0026] 6A and 6B are conceptual diagrams showing examples of the intensities of three laser beams. In Fig. 6A, the relationship between the intensity P3 of the three branched laser beams L13, the intensity P4 of the laser beam L14, and the intensity P5 of the laser beam L15 is P3<P4<P5. In Fig. 6B, the relationship between the intensities of the three branched laser beams is P3=P4=P5. In Fig. 6C, the relationship between the intensities of the three branched laser beams is P3>P4>P5. As described above, the spatial light modulator 28 can change the intensity ratio of the plurality of laser beams.

[0027] An LCOS-SLM, for example, is used as the spatial light modulator 28. The operation of the spatial light modulator 28 and the hologram pattern presented by the spatial light modulator 28 are controlled by the control unit 50. Note that the specific configuration of the spatial light modulator 28 and the hologram pattern presented by the spatial light modulator 28 are already known, and therefore a detailed description thereof will be omitted here. Note that the hologram pattern is derived in advance based on the formation position of the modified region, the wavelength of the irradiated laser light L, and the refractive indexes of the condenser lens 38 and the wafer W, and is stored in the control unit 50.

[0028] The intensity modulation element modulates the intensity (total output) of the laser light L. By reducing the intensity of the laser light L before branching, the intensity modulation element can make the intensity of at least one of the multiple laser beams after branching less than the modification limit at which the modified region R is formed. For example, the total output of the laser light L can be changed so that the laser irradiation intensity of the focal point with the smallest intensity among the multiple focal points is less than the modification limit.

[0029] FIG. 7 is a graph showing an example of the modification energy limit. The modification energy limit refers to the minimum energy of laser light capable of forming a modified region on a wafer. The modification limit varies depending on the depth at which the modified region is formed, the pulse width, pulse shape, frequency, and beam condition of the laser light, etc. FIG. 7 is a graph plotting the width of the modified region R formed by irradiation with a single laser beam in the thickness direction of the wafer W as a function of the energy (μJ) of the single laser beam per pulse. FIG. 8 is an enlarged view of region VII in FIG. 7. As shown in FIGS. 7 and 8, when the laser beam energy was 3.5 μJ or less, the width of the modified region R in the thickness direction of the wafer W was 0.0 μm. On the other hand, when the laser beam energy was 3.6 μJ or more, the width of the modified region R in the thickness direction of the wafer W was approximately 10.0 μm or more. FIG. 9 is an optical microscope image of a cross section of a wafer W along the scanning direction of the laser beam after irradiation with laser beams L of different energies. As shown in Figure 9, when the energy of the laser beam was 3.6 μJ or more, a modified region was formed along the scanning direction. On the other hand, when the energy of the laser beam was 3.4 μJ or less, little or no modified region was formed. In this way, the formation of a modified region can be controlled by modulating the energy, i.e., intensity, of the laser beam. Therefore, in the case of multiple branched laser beams, by reducing the total output of the laser beam before branching, the intensity of one or more of the multiple branched laser beams can be made below the modification limit at which no modified region is formed.

[0030] 10A and 10B are schematic diagrams of a cross section of a wafer W near an intersection on a second street where a modified region is formed by a single laser beam. FIG. 10A shows a cross section of a wafer W in which a modified region R is formed along a second street CH2 by a laser beam of excessive intensity. FIG. 10B shows a cross section of a wafer W when a modified region R is formed along a second street CH2 by reducing the intensity of the laser beam near the intersection CR. FIGS. 10A and 10B are schematic diagrams of a cross section of a wafer W along a second street CH2. The ratio of the intensity of the laser beam before and after the reduction in intensity in FIG. 10B is, for example, 8:7, and the intensity of the laser beam near the intersection CR in FIG. 10B is less than the modification limit.

[0031] 10A, the modified region R near the intersection CR is smaller than the modified region R formed at a position other than the intersection CR due to reflection and scattering of the laser light in the modified region R formed on the first street CH1. Furthermore, heat damage may occur around the street.

[0032] On the other hand, in FIG. 10( b), the intensity of the laser light is reduced near the intersection CR, thereby preventing thermal damage caused by reflection and scattering by the modified region R formed on the first street CH1. Also, in FIG. 10( b), the degree of propagation of the crack formed near the intersection CR is slightly reduced compared to the case of FIG. 10( a). However, the crack continues uninterrupted in the Y direction. Therefore, after a crack is formed in the wafer W, when the wafer W is divided along the formed crack, linearity in the division can be maintained.

[0033] 11A and 11B are schematic diagrams of a cross section of a second street near an intersection processed by a laser beam. Fig. 11A is a schematic diagram showing a modified region R and a crack formed along the second street CH2 by a single laser beam, and Fig. 11B is a schematic diagram showing a modified region R and a crack formed along the second street CH2 near the intersection CR by a laser beam split into two by a spatial light modulator after increasing the total output of the laser beam by an intensity modulation element. For example, the total output of the laser beam before splitting is set to 80%, and the total output of the split laser beam is set to 100%.

[0034] 11( a), when a single laser beam is used to form a modified region R and a crack, the laser beam is reflected and scattered by the modified region R formed on the first street CH1, resulting in a modified region R formed near the intersection CR being smaller than the modified region R formed outside the intersection CR. Furthermore, the quality of the modified region R may also deteriorate. As a result, the degree of extension of the crack formed near the intersection CR may be reduced compared to the crack formed outside the intersection CR.

[0035] 11(b), modified regions are formed near the intersection CR by the two laser beams at positions close to the surface and at deeper positions. As a result, the crack formed near the intersection CR has the same degree of extension as cracks formed outside the intersection CR. As a result, after a crack is formed in the wafer W, when the wafer W is divided along the formed crack, the straightness of the division can be maintained.

[0036] FIG. 12 is a schematic diagram of a cross section near an intersection on the second street processed by multiple laser beams. In FIG. 12, the laser beam L is branched into a first laser beam, a second laser beam, and a third laser beam. The first laser beam is irradiated at a depth D1, the second laser beam at a depth D2, and the third laser beam at a depth D3. If the intensity of the first laser beam is P1, the intensity of the second laser beam is P2, and the intensity of the third laser beam is P3, the intensities of the three laser beams are modulated so that P1 > P2 > P3. FIG. 12(a) shows a cross section of a wafer W in which a modified region R is formed along the second street CH2 by laser beams with an excessive total output. FIG. 12(b) shows a cross section of a wafer W in which the total output of the laser beam L at the intersection CR is reduced by an intensity modulation element (20% reduction compared to when the intensity is not reduced). 12A and 12B are schematic cross-sectional views of the wafer W taken along the second street CH2.

[0037] When a modified region R is formed at a deeper position in the Z direction (thickness direction) of the second street CH2, the laser light irradiated at the deeper position, for example, the second laser light irradiated at position D2 and the third laser light irradiated at position D3, is easily affected by the modified region R previously formed on the first street CH1. Specifically, cracks in the first street CH1 may cause a portion of the second laser light and the third laser light to be reflected and scattered, resulting in the formation of incompletely modified regions at the respective depth positions. Furthermore, the reflection and scattering may cause thermal damage around the street. For example, in FIG. 12( a), homogeneous modified regions R are formed at regular intervals in the scanning direction of the laser light at depth D1. Meanwhile, incompletely modified regions R are formed near the intersections CR at depths D2 and D3. When incompletely modified regions R are formed, cracks caused by the incompletely modified regions R may prevent the wafer W from being divided in a straight line. Furthermore, thermal damage may occur around the street.

[0038] In FIG. 12( b), the intensity modulation element reduces the total output of the laser beam L near the intersection CR, thereby reducing the intensities of the first, second, and third laser beams. As a result, the intensities of each laser beam are below the modification limit at depths D2 and D3. Compared to FIG. 12( a), no modified region is formed even in areas farther from the first street CH1. However, because the modified region R is continuously formed at depth D1, cracks occur along the second street CH2 near the surface of the wafer W. This maintains the dividing force, allowing the wafer W to be divided. Furthermore, because the formation of incompletely modified regions is suppressed, linearity can be maintained during division. Furthermore, because reflection and scattering of the second and third laser beams are suppressed, thermal damage can also be prevented.

[0039] In the present embodiment, the laser light L is described as including a single laser light L10 in the portion of the second street CH2 other than the intersection CR, but this is not limiting. For example, the laser light L may include a plurality of laser lights in the portion of the second street CH2 other than the intersection CR.

[0040] 4, the configuration of the laser processing apparatus 1 will be described. The condenser lens 38 is an objective lens (infrared objective lens) that condenses the laser light L inside the wafer W. The condenser lens 38 has a numerical aperture (NA) of 0.65 to 0.85 (for example, 0.85).

[0041] In addition to the above configuration, the laser head 20 also includes a beam expander 24, a λ / 2 wave plate 26, a reduction optical system 36, and the like.

[0042] The beam expander 24 expands the laser light L output from the laser light source 22 to a beam diameter appropriate for the spatial light modulator 28. The λ / 2 wave plate 26 adjusts the polarization plane of the laser light incident on the spatial light modulator 28. The reduction optical system 36 is an afocal optical system (a double-telecentric optical system) consisting of a first lens 36 a and a second lens 36 b, and reduces and projects the laser light L modulated by the spatial light modulator 28 onto a condenser lens 38.

[0043] Although not shown in the figure, the laser head 20 is also equipped with an alignment optical system for aligning with the wafer W, an autofocus unit for maintaining a constant distance (working distance) between the wafer W and the focusing lens 38, and the like.

[0044] The control unit 50 is configured by an arithmetic device such as a personal computer, and includes an arithmetic circuit configured with various processors, memories, etc. The various processors include a central processing unit (CPU), a graphics processing unit (GPU), an application specific integrated circuit (ASIC), and a programmable logic device (e.g., a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), and a field programmable gate array (FPGA)). The various functions of the control unit 50 may be realized by a single processor or by multiple processors of the same or different types.

[0045] The control unit 50 also controls the operation of the spatial light modulator 28 to cause the spatial light modulator 28 to present a predetermined hologram pattern.

[0046] The control unit 50 also receives operation input from an operator via an operation unit (not shown) and transmits control signals corresponding to the operation input to each part of the laser processing apparatus 1 to control the operation of each part. The operation unit (not shown) is a means for receiving operation input from the operator. The operation unit (not shown) includes, for example, a keyboard, a mouse, or a touch panel.

[0047] The laser processing device 1 may also be composed of a wafer transport means, an operation panel, a monitor, an indicator light, and the like, all of which are not shown.

[0048] The operation panel is fitted with switches and a display device for controlling the operation of each part of the laser processing device 1. The monitor displays wafer images captured by a CCD camera (not shown), or program contents and various messages, etc. The indicator lights display the operating status of the laser processing device 1, such as when processing is in progress, processing is complete, and when an emergency stop is made.

[0049] Next, a laser processing method according to an embodiment of the present invention will be described with reference to Figures 13 and 14. Figure 13 is a flowchart illustrating the flow of the laser processing method according to an embodiment of the present invention. Figure 14 is a flowchart illustrating the flow of the second irradiation step in the same embodiment.

[0050] The laser processing method according to this embodiment is a laser processing method for forming a modified region by irradiating a laser into the interior of a wafer. As shown in Fig. 13, the laser processing method according to this embodiment includes a first irradiation step (step S1) of irradiating a first street with a laser, and a second irradiation step (step S2) of irradiating a second street intersecting the first street with the laser after the first irradiation step. In the second irradiation step, the total output of the laser at the intersection of the first street and the second street is changed.

[0051] In the second irradiation step, when the laser light L reaches the vicinity of the intersection CR, at least one of the total output power of the laser light L and the number of laser beams (number of focal points) is changed (step S21). At the intersection CR, the wafer W is processed with the changed laser light L (step S22). After passing through the intersection CR, at least one of the total output power of the laser light L and the number of laser beams is changed. In other words, the conditions changed in step S21 are restored (step S23), and the portions following the second street are processed. The second irradiation step (steps S21 to S23) is performed for each second street as many times as the number of intersections. The second irradiation step is performed, for example, several hundred times for each second street. Furthermore, the wafer W has multiple second streets. For example, a wafer W with a diameter of 300 mm has several hundred second streets. The second irradiation step is performed for the multiple second streets arranged on the wafer W, and processing of one wafer W is completed.

[0052] Although the embodiments of the present invention have been described above, the technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

[0053] The condenser lens 38 may simultaneously condense the laser light L at two positions spaced apart from each other in the wafer movement direction.

[0054] In the above embodiment, the control unit 50 comprehensively controls the laser processing apparatus 1, but multiple control units may control the laser processing apparatus 1. The control units may be provided integrally with the processing apparatus or separately.

[0055] In the above embodiment, in step S23, the conditions changed in step S21 are returned to the original conditions, but they may be changed to other conditions, or the conditions at the intersection CR may be maintained.

[0056] Furthermore, within the scope of the present invention, the components in the above-described embodiments may be replaced with well-known components as appropriate, and the above-described modifications may be combined as appropriate.

[0057] REFERENCE SIGNS LIST 1 laser processing device 11 wafer moving unit 12 table 13 suction stage 20 laser head 22 laser light source 24 beam expander 26 λ / 2 wave plate 28 spatial light modulator 36 reduction optical system 38 condenser lens 50 control unit

Claims

1. A laser processing method for forming a modified region by irradiating the interior of a wafer with laser light, comprising: a first irradiation step of irradiating a first street of the wafer with the laser light; and a second irradiation step of irradiating a second street that intersects with the first street after the first irradiation step, with a laser, wherein in the second irradiation step, the total output of the laser light at the intersection of the first street and the second street is changed.

2. The laser processing method according to claim 1, wherein in the second irradiation step, the total output of the laser light irradiated onto the intersection is changed by an acousto-optical element.

3. A laser processing method according to claim 1 or 2, wherein in the second irradiation step, at least one of the intensity and the number of laser beams irradiated onto the intersection is changed.

4. A laser processing method as described in claim 3, wherein in the second irradiation step, a plurality of focal points are generated in the depth direction of the wafer by a plurality of the laser beams, and the total output of the laser beams is changed so that the laser irradiation intensity of the focal point with the smallest intensity among the plurality of focal points is less than the modification limit.

5. A laser processing device that irradiates a laser beam into the interior of a wafer to form a modified region, and that changes the total output of the laser beam at the intersection of a first street of the wafer and a second street that intersects with the first street.

Images