Optical modulation device

Abstract

To provide an optical modulation device that eliminates the time and effort for position adjustment of a Faraday rotator and prevents loss due to attenuation of light by the Faraday rotator.SOLUTION: An optical modulation device comprises: a separation element (polarization beam splitter 44) having a light-branching surface (44b) that separates incident light entering along a first direction into first incident light and second incident light, transmits the first incident light, and reflects the second incident light in a first reflection direction parallel or inclined with respect to a second direction; a first spatial light modulator (46) that is disposed directly opposite to the separation element in the first direction, phase-modulates the first incident light transmitted through the light-branching surface to generate first modulated light (L1), and returns the first modulated light to the light-branching surface; and a second spatial light modulator (48) that is disposed directly opposite to the separation element in the first reflection direction, phase-modulates the second incident light reflected by the light-branching surface to generate second modulated light (L2), and returns the second modulated light to the light-branching surface. The light-branching surface combines the first modulated light incident from the first spatial light modulator and the second modulated light incident from the second spatial light modulator.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an optical modulation device that separates and modulates incident light, and to a laser processing device that includes this optical modulation device. [Background technology]

[0002] There is known a laser processing device that uses a focusing lens to focus the laser light inside a workpiece such as a silicon wafer (hereinafter abbreviated as wafer), thereby forming a laser processing area inside the wafer along a planned cutting line of the wafer (see, for example, Patent Documents 1 and 2). The laser processing area here refers to an area where the physical properties of the inside of the wafer, such as density, refractive index, and mechanical strength, become different from those of the surrounding area due to the irradiation of laser light, resulting in a lower strength than the surrounding area.

[0003] The laser processing devices described in Patent Documents 1 and 2 simultaneously form a pair of laser processing areas by simultaneously focusing laser light at two different positions in the thickness direction of the wafer using the focusing lens while moving the focusing lens relative to the wafer along the line to cut. This makes it possible to form two rows of laser processing areas within the wafer for one line to cut in one scan.

[0004] Such laser processing devices are provided with an optical modulation device to simultaneously focus laser light at two different positions in the thickness direction within the wafer using a focusing lens (see, for example, Patent Documents 2 to 4 above). This optical modulation device includes a polarizing beam splitter, a first reflective optical spatial modulator, a second reflective optical spatial modulator, a first Faraday rotator, and a second Faraday rotator (see, in particular, Figure 1 of Patent Document 2 above).

[0005] The polarizing beam splitter polarizes and splits the laser light emitted from the laser light source into P-polarized light and S-polarized light. The first reflective optical spatial modulator phase-modulates the P-polarized light that has passed through the polarizing beam splitter to generate first modulated light and reflects this first modulated light toward the polarizing beam splitter. The second reflective optical spatial modulator phase-modulates the S-polarized light reflected by the polarizing beam splitter to generate second modulated light and reflects this second modulated light toward the polarizing beam splitter.

[0006] The first Faraday rotator is disposed between the polarizing beam splitter and the first reflective optical spatial modulator and rotates, by 45 degrees, the polarization plane of the P-polarized light emitted from the polarizing beam splitter toward the first reflective optical spatial modulator and the polarization plane of the first modulated light reflected from the first reflective optical spatial modulator toward the polarizing beam splitter. The second Faraday rotator is disposed between the polarizing beam splitter and the second reflective optical spatial modulator and rotates, by 45 degrees, the polarization plane of the S-polarized light emitted from the polarizing beam splitter toward the second reflective optical spatial modulator and the polarization plane of the second modulated light reflected from the second reflective optical spatial modulator toward the polarizing beam splitter. This causes the first modulated light (S-polarized light) and the second modulated light (P-polarized light) to be incident on the polarizing beam splitter. The polarizing beam splitter then combines the first modulated light and the second modulated light, and reflects the combined light in a direction perpendicular to the incident direction of the laser light (the direction opposite to the second reflective spatial optical modulator). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-051011 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-202956 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-202957 [Patent Document 4] Japanese Patent Application Laid-Open No. 2014-202958 Summary of the Invention [Problem to be solved by the invention]

[0008] Incidentally, the optical modulation devices described in Patent Documents 2 to 4 require a first Faraday rotator to be disposed between the polarizing beam splitter and the first reflective optical spatial modulator, and a second Faraday rotator to be disposed between the polarizing beam splitter and the second reflective optical spatial modulator. In this case, it is necessary to adjust the position of each Faraday rotator, and furthermore, loss occurs due to attenuation of each light and each modulated light in each Faraday rotator.

[0009] The present invention has been made in consideration of the above circumstances, and aims to provide an optical modulation device and a laser processing device that can eliminate the need for adjusting the position of a Faraday rotator and prevent loss of light due to attenuation by the Faraday rotator. [Means for solving the problem]

[0010] The optical modulation device for achieving the object of the present invention includes a separation element having an optical branching surface that separates incident light incident along a first direction among a first direction, a second direction, and a third direction that are orthogonal to each other, into a first incident light and a second incident light, and transmits the first incident light and reflects the second incident light in a first reflection direction that is parallel to or tilted from the second direction when viewed from the third direction; a first spatial light modulator that is disposed directly opposite the separation element in the first direction, modulates the first incident light that has transmitted through the optical branching surface to generate first modulated light, and returns the first modulated light to the optical branching surface; and a second spatial light modulator that is disposed directly opposite the separation element in the second direction. and a second spatial light modulator that modulates the second incident light reflected by the light branching surface to generate second modulated light and returns the second modulated light to the light branching surface, wherein the first spatial light modulator reflects the first modulated light in a second reflection direction that is parallel to a plane perpendicular to the second direction and inclined with respect to the incident light when viewed from the second direction, causing the first modulated light to be incident on the light branching surface, the second spatial light modulator reflects the second modulated light toward the point of incidence of the first modulated light on the light branching surface, and the light branching surface combines the first modulated light incident from the first spatial light modulator and the second modulated light incident from the second spatial light modulator, and outputs the combined light in the second reflection direction.

[0011] According to this light modulation device, it is not necessary to provide a Faraday rotator between the separation element and the first spatial light modulator, and between the separation element and the first spatial light modulator.

[0012] In an optical modulation device according to another aspect of the present invention, the separation element is a cube-shaped polarizing beam splitter, and in the polarizing beam splitter, the surface onto which the incident light enters is the same as the surface from which the first modulated light and the second modulated light exit from the optical branching surface, thereby eliminating the need to provide a Faraday rotator in the optical modulation device.

[0013] In an optical modulation device according to another aspect of the present invention, a polarizing beam splitter splits incident light into P-polarized light, which is a first incident light, and S-polarized light, which is a second incident light.

[0014] In another aspect of the optical modulation device of the present invention, the polarization direction of the P-polarized light and the first modulated light is maintained between the optical branching surface and the first spatial light modulator, and the polarization direction of the S-polarized light and the second modulated light is maintained between the optical branching surface and the second spatial light modulator.

[0015] In another aspect of the optical modulation device of the present invention, the distance traveled by the first incident light that has passed through the optical branching surface to reach the first spatial light modulator is the same as the distance traveled by the second incident light that has been reflected by the optical branching surface to reach the second spatial light modulator.

[0016] A laser processing apparatus for achieving the object of the present invention is a laser processing apparatus that forms a laser processing area inside a workpiece along a planned cutting line of the workpiece by irradiating the workpiece with a laser beam with a focal point aligned with the inside of the workpiece, the laser processing apparatus comprising: a laser light source that outputs laser beam; a laser beam splitter that splits the laser beam output from the laser light source into a first incident beam and a second incident beam; a first modulated beam that is generated by modulating the first incident beam and a second modulated beam that is generated by modulating the second incident beam; and a laser beam splitter that combines the first modulated beam and the second modulated beam. the light modulation device according to any one of claims 1 to 5, which outputs a modulated light beam; a focusing lens which focuses the first modulated light and the second modulated light inside the workpiece; a relative movement unit which moves the focusing lens relative to the workpiece along the planned cutting line; and a control unit which controls the first spatial light modulator and the second spatial light modulator to form the focusing points of the first modulated light and the second modulated light which are focused inside the workpiece by the focusing lens at positions which are different from each other in the thickness direction of the workpiece and which are equal to each other in the direction of relative movement of the focusing lens. [Effects of the Invention]

[0017] The present invention eliminates the need for adjusting the position of a Faraday rotator and prevents loss of light due to attenuation by the Faraday rotator. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram of a laser processing device. [Figure 2] FIG. 1 is a perspective view of an optical modulation device. [Figure 3] 3 is a top view of the optical modulation device in FIG. 2 as seen from above. [Figure 4] 3 is a side view of the optical modulation device in FIG. 2 as seen from the lateral side. [Figure 5] 3 is an explanatory diagram showing an example of a hologram pattern presented by a first spatial light modulator and a second spatial light modulator. FIG. [Figure 6] FIG. 10 is a diagram showing a state in which the first modulated light and the second modulated light are individually focused inside the wafer. [Figure 7]7 is a diagram showing a state in which laser processed regions are formed at the focusing positions of the first modulated light and the second modulated light shown in FIG. 6. FIG. [Figure 8] FIG. 10 is a diagram showing a state in which two rows of laser processing regions are formed inside the wafer along the line to cut. [Figure 9] FIG. 10 is a perspective view showing a modified example of the optical modulation device. [Figure 10] FIG. 10 is a top view of a modified example of the optical modulation device as viewed from above. [Figure 11] FIG. 10 is a side view of a modified example of the optical modulation device as viewed from the side. DETAILED DESCRIPTION OF THE INVENTION

[0019] 1 is a schematic diagram of a laser processing apparatus 10 equipped with an optical modulator 28 of the present invention. In the figure, the X, Y, and Z directions are perpendicular to one another, the X and Y directions being horizontal and the Z direction being up and down. θ is the direction around an axis parallel to the Z direction as the rotation axis. This laser processing apparatus 10 moves a condenser lens 38 relative to the wafer W, which is the workpiece, in the X direction along the lines to be cut, and forms two rows of laser processing areas within the wafer W for each line to be cut in one scan.

[0020] 1, the laser processing apparatus 10 includes a stage 11, a laser processing head (also referred to as a laser engine) 20, and a control unit 50. In this embodiment, the laser processing head 20 and the control unit 50 are configured separately, but the laser processing head 20 may include part or all of the control unit 50.

[0021] The stage 11 corresponds to the relative moving unit of the present invention and holds the wafer W by suction. The stage 11 includes a stage movement mechanism (not shown) and is configured to be movable in the X, Y, Z, and θ directions by this stage movement mechanism. This allows the wafer W to be moved in the X, Y, Z, and θ directions relative to the laser processing head 20 (condenser lens 38) described below. This stage movement mechanism is configured using various mechanisms (actuators), such as a ball screw mechanism or a linear motor mechanism. Note that in this embodiment, the stage 11 is configured to be movable in the X, Y, Z, and θ directions, but is not particularly limited as long as the wafer W can be moved in the X, Y, Z, and θ directions relative to the laser processing head 20. For example, the stage 11 may be configured to be movable in the X, Y, Z, and θ directions, and the laser processing head 20 may be configured to be movable in the Z direction.

[0022] The wafer W is divided into a plurality of regions by cutting lines arranged in a grid pattern, and various devices constituting semiconductor chips are formed in each of these divided regions. An adhesive backgrinding tape is attached to the surface (device surface) of the wafer W on which the devices are formed, and the wafer W is placed on the stage 11 with its back surface facing upward. Alternatively, an adhesive dicing tape may be attached to one surface of the wafer W, and the wafer W may be placed on the stage 11 in a state where it is integrated with a frame via this dicing tape.

[0023] Under the control of the control unit 50, the laser processing head 20 simultaneously forms a pair of laser processed regions by simultaneously focusing the first modulated light L1 and the second modulated light L2 of the laser light L at two different positions in the thickness direction within the wafer W using a focusing lens 38. The laser processing head 20 includes a laser light source 22, and a beam expander 24, a mirror 25, a λ / 2 wave plate 26, an optical modulator 28, mirrors 30 and 31, a first lens 32a (4f optical system 32), mirrors 33 and 34, a second lens 32b (4f optical system 32), and a focusing lens 38, which are arranged along the optical path of the laser light L (first modulated light L1 and second modulated light L2) emitted from the laser light source 22.

[0024] The laser light source 22 emits laser light L, which is used to form a laser processed region inside the wafer W, toward the beam expander 24. For example, a semiconductor laser pumped Nd:YAG (Yttrium Aluminum Garnet) laser is used as the laser light source 22. The conditions of the laser light L are, for example, a wavelength of 1.1 μm and a laser light spot cross-sectional area of ​​3.14×10 -8 cm 2 The oscillation form is Q-switched pulse, the repetition frequency is 80 to 120 kHz, the pulse width is 180 to 280 ns, and the output is 8 W.

[0025] The beam expander 24 expands the laser light L output from the laser light source 22 to a beam diameter appropriate for phase modulation by the optical modulator 28, which will be described later, and then emits the expanded laser light L toward the mirror 25. The mirror 25 reflects the laser light L incident from the beam expander 24 toward the λ / 2 wave plate 26. The λ / 2 wave plate 26 adjusts the incident polarization plane of the laser light L incident on the optical modulator 28. Note that the λ / 2 wave plate 26 may be omitted.

[0026] The optical modulator 28, which will be described in detail later, polarizes and separates the laser light L that has passed through the λ / 2 wavelength plate 26, performs phase modulation on each of the polarized and separated laser light L to generate first modulated light L1 and second modulated light L2, and further combines the first modulated light L1 and the second modulated light L2 to output to the mirror 30.

[0027] The mirror 30 and the mirror 31 sequentially reflect the first modulated light L1 and the second modulated light L2 incident from the light modulation device 28 and guide them to the first lens 32a (4f optical system 32).

[0028] The 4f optical system 32 is an afocal optical system (a double-telecentric optical system) including a first lens 32a and a second lens 32b, and projects the first modulated light L1 and the second modulated light L2 modulated by the optical modulator 28 onto a focusing lens 38 in a reduced size (or enlarged size).

[0029] The mirrors 33 and 34 sequentially reflect the first modulated light L1 and the second modulated light L2 that have passed through the first lens 32a and guide them to the second lens 32b, whereby the first modulated light L1 and the second modulated light L2 that have passed through the second lens 32b are projected onto the condenser lens 38 at a reduced size.

[0030] The condenser lens 38 is an objective lens (infrared objective lens) that condenses the first modulated light L1 and the second modulated light L2 inside the wafer W, and condenses the first modulated light L1 and the second modulated light L2 at two different positions in the Z direction inside the wafer W. The numerical aperture (NA) of this condenser lens 38 is, for example, 0.65.

[0031] The condenser lens 38 also includes a correction collar 40 for correcting aberrations of the first modulated light L1 and the second modulated light L2 that occur inside the wafer W. The correction collar 40 is manually rotatable, and rotating the correction collar 40 in a predetermined direction changes the spacing between the lens groups that make up the condenser lens 38. This makes it possible to adjust the amount of aberration correction so that the aberrations of the first modulated light L1 and the second modulated light L2 are equal to or less than a predetermined aberration at a predetermined depth from the irradiation surface (back surface) of the wafer W. Note that the "aberration correction amount" is a value converted into a depth from the irradiation surface of the wafer W. Since aberration correction using the correction collar 40 is a well-known technique, a detailed description thereof will be omitted.

[0032] The correction collar 40 may be configured to be rotatably driven electrically by a correction collar drive unit (not shown). In this case, the control unit 50 controls the correction collar drive unit to rotate the correction collar 40, thereby correcting the aberrations of the first modulated light L1 and the second modulated light L2 to a desired state.

[0033] The control unit 50 is a control device that controls the operation of each part of the laser processing apparatus 10, and includes, for example, a CPU (Central Processing Unit) that functions as a controller that executes various processes, a RAM (Random Access Memory) and a ROM (Read Only Memory) that function as memories that store various information, etc. The control unit 50 controls the formation of a laser processed region inside the wafer W by comprehensively controlling the operation of each part of the laser processing apparatus 10, including the movement of the stage 11, control of the laser light source 22, and control of the first spatial light modulator 46 and second spatial light modulator 48 of the light modulation device 28, which will be described later, based on processing information (processing conditions, etc.) specified by an operator.

[0034] [Light modulation device] FIG. 2 is a perspective view of the optical modulation device 28. FIG. 3 is a top view of the optical modulation device 28 in FIG. 2, as seen from above. FIG. 4 is a side view of the optical modulation device 28 in FIG. 2, as seen from the side. In each figure, the X1 direction is parallel to the incident direction of the laser light L to the optical modulation device 28 (corresponding to the first direction), and the Z1 direction (corresponding to the second direction) is parallel to the reflection direction of the reflected light LS by the polarized beam splitter 44, which will be described later. The Y1 direction (corresponding to the third direction) is perpendicular to both the X1 and Z1 directions.

[0035] As shown in FIGS. 2 to 4, the light modulation device 28 includes a cube-type polarizing beam splitter 44, a first spatial light modulator 46, and a second spatial light modulator 48.

[0036] The polarizing beam splitter 44 corresponds to the separation element of the present invention, and has a light incident / exit surface 44a, a light splitting surface 44b, a first opposing surface 44c, and a second opposing surface 44d.

[0037] The light incident / exit surface 44a is located on one end side in the X1 direction and is a surface perpendicular to the X1 direction. This light incident / exit surface 44a functions as an incident surface onto which the laser light L from the λ / 2 wave plate 26 is incident along the X1 direction, and also functions as an exit surface from which first modulated light L1 and second modulated light L2, which will be described later, are emitted toward the mirror 30. Note that in Fig. 2, symbol SA1 indicates the incident point of the laser light L within the light incident / exit surface 44a, and symbol SA2 indicates the exit point of the first modulated light L1 and second modulated light L2 within the light incident / exit surface 44a.

[0038] The light splitting surface 44b is located between the light incident / exit surface 44a and the first opposing surface 44c in the X1 direction and is inclined at 45 degrees when viewed from the Y1 direction. The light splitting surface 44b splits the laser light L incident on the light incident / exit surface 44a along the X1 direction at an incident point SB1 into P-polarized transmitted light LP (corresponding to the first incident light) and S-polarized reflected light LS (corresponding to the second incident light). The light splitting surface 44b emits the transmitted light LP toward the first opposing surface 44c and reflects the reflected light LS toward the second opposing surface 44d. The reflected light LS is reflected by the light splitting surface 44b in a reflection direction perpendicular to the X1 direction, that is, in other words, in a direction parallel to the Z1 direction when viewed from the Y1 direction (the first reflection direction of the present invention). In this case, if the laser light L is linearly polarized, the ratio of the transmitted light LP to the reflected light LS can be adjusted by adjusting the rotation angle of the λ / 2 wavelength plate 26 to adjust the angle of the polarization plane (polarization direction) of the laser light L.

[0039] Furthermore, the light splitting surface 44b, which will be described in detail later, combines the first modulated light L1 reflected by the first spatial light modulator 46 and returned, and the second modulated light L2 reflected by the second spatial light modulator 48 and returned, and emits the first modulated light L1 and the second modulated light L2 toward an emission point SA2 on the light incident / exit surface 44a. Note that the symbol SB2 in Fig. 2 indicates the incident point of the first modulated light L1 and the second modulated light L2 within the light splitting surface 44b.

[0040] The first opposing surface 44c is located on the other end side in the X1 direction, is a surface perpendicular to the X1 direction, and faces a first spatial light modulator 46, which will be described later. This first opposing surface 44c functions as an exit surface that emits the transmitted light LP that has passed through the light branching surface 44b toward the first spatial light modulator 46, and also functions as an entrance surface onto which the first modulated light L1 reflected by the first spatial light modulator 46 is incident. Note that in FIG. 2, symbol SC1 indicates the exit point of the transmitted light LP within the first opposing surface 44c, and symbol SC2 indicates the entrance point of the first modulated light L1 within the first opposing surface 44c.

[0041] The second opposing surface 44d is a surface perpendicular to the Z1 direction and faces the second spatial light modulator 48, which will be described later. This second opposing surface 44d functions as an exit surface that emits the reflected light LS reflected by the light branching surface 44b toward the second spatial light modulator 48, and also functions as an entrance surface onto which the second modulated light L2 reflected by the second spatial light modulator 48 is incident. Note that in FIG. 2, symbol SD1 indicates the exit point of the reflected light LS within the second opposing surface 44d, and symbol SD2 indicates the entrance point of the second modulated light L2 within the second opposing surface 44d.

[0042] 5 is an explanatory diagram showing an example of hologram patterns 52, 54 presented by the first spatial light modulator 46 and the second spatial light modulator 48. As shown in FIG. 5 and the previously described FIGS. 2 to 4, the first spatial light modulator 46 and the second spatial light modulator 48 are of a phase modulation type, and for example, a reflective liquid crystal (LCOS: Liquid Crystal on Silicon) spatial light modulator (SLM: Spatial Light Modulator) is used. The first spatial light modulator 46 performs phase modulation on the transmitted light LP, and the second spatial light modulator 48 performs phase modulation on the reflected light LS. Note that when the first spatial light modulator 46 and the second spatial light modulator 48 are LCOS-type SLMs, only linearly polarized light components whose vibration direction is parallel to the liquid crystal alignment direction are modulated. Therefore, the first spatial light modulator 46 is positioned to match the angle of the polarization plane (polarization direction) of the transmitted light LP, and the second spatial light modulator 48 is positioned to match the angle of the polarization plane of the reflected light LS.

[0043] The first spatial light modulator 46 is disposed directly opposite the first opposing surface 44c of the polarizing beam splitter 44. The second spatial light modulator 48 is disposed directly opposite the second opposing surface 44d of the polarizing beam splitter 44. Here, "directly facing" means that no optical element that rotates the polarization plane of light, such as a Faraday rotator, is disposed between the first spatial light modulator 46 and the first opposing surface 44c, and between the second spatial light modulator 48 and the second opposing surface 44d.

[0044] The first spatial light modulator 46 has a reflective surface (modulation surface) on which a plurality of pixels are two-dimensionally arranged and which is positioned conjugate with the pupil plane of the condenser lens 38. Under the control of the control unit 50, a hologram pattern 52 that modulates the phase of transmitted light LP incident from the first opposing surface 44c is presented on the reflective surface of the first spatial light modulator 46. As a result, the reflective surface of the first spatial light modulator 46 phase-modulates the transmitted light LP using the hologram pattern 52 to generate first modulated light L1, and reflects this first modulated light L1 toward the light splitting surface 44b through the first opposing surface 44c.

[0045] The first spatial light modulator 46 is held by a movement mechanism (not shown) so that its position can be adjusted in the X1 direction, and is held by a rotation mechanism (not shown) so that its rotation can be adjusted around a rotation axis parallel to the Z1 direction. The first spatial light modulator 46 is adjusted in position and rotation by the movement mechanism and the rotation mechanism so that the distance from the transmitted light LP that has passed through the light branching surface 44b to the first spatial light modulator 46 is a predetermined distance d (see FIG. 4).

[0046] Furthermore, the first spatial light modulator 46 is positionally and rotationally adjusted by a moving mechanism and a rotating mechanism so as to reflect the first modulated light L1 in a reflection direction RD (see FIGS. 2 and 3). This reflection direction RD (corresponding to a second reflection direction of the present invention) is a direction parallel to an X1Y1 plane (corresponding to a plane perpendicular to the second direction of the present invention) that is perpendicular to the Z1 direction, and is tilted with respect to the X1 direction (incident direction of laser light L) when viewed from the Z1 direction side, and further is a direction passing through the light branching surface 44b and the light incident / exit surface 44a.

[0047] As described above, no Faraday rotator or the like is disposed between the first spatial light modulator 46 and the first opposing surface 44c, and therefore the transmitted light LP (P-polarized light) emitted from the first opposing surface 44c is incident on the reflecting surface of the first spatial light modulator 46 while maintaining its plane of polarization (polarization direction). Furthermore, the first modulated light L1 (P-polarized light) reflected by the reflecting surface of the first spatial light modulator 46 is also incident on the light splitting surface 44b from the first opposing surface 44c at the incident point SB2 while maintaining its plane of polarization (polarization direction). Therefore, the first modulated light L1 passes through the light splitting surface 44b as is and exits from the light incident / exit surface 44a.

[0048] The second spatial light modulator 48 has a reflective surface similar to that of the first spatial light modulator 46. A hologram pattern 54 that modulates the phase of the reflected light LS incident from the second opposing surface 44d is presented on the reflective surface of this second spatial light modulator 48 under the control of the control unit 50. As a result, the reflective surface of the second spatial light modulator 48 phase-modulates the reflected light LS using the hologram pattern 54 to generate second modulated light L2, and reflects this second modulated light L2 toward the light splitting surface 44b through the second opposing surface 44d.

[0049] The second spatial light modulator 48 is held by a movement mechanism (not shown) so that its position can be adjusted in the Z1 direction, and is held by a rotation mechanism (not shown) so that its rotation can be adjusted around a rotation axis parallel to the X1 direction. The second spatial light modulator 48 is position-adjusted and rotated by the movement mechanism and rotation mechanism so that the distance over which the reflected light LS reflected by the light branching surface 44b reaches the second spatial light modulator 48 is a predetermined distance d (see FIG. 4). As a result, the distance over which the transmitted light LP reaches the first spatial light modulator 46 from the light branching surface 44b and the distance over which the reflected light LS reaches the second spatial light modulator 48 from the light branching surface 44b are both the same, at the predetermined distance d.

[0050] Furthermore, the second spatial light modulator 48 is position-adjusted and rotated by the moving mechanism and rotating mechanism so that the second modulated light L2 is incident on the incident point SB2 of the optical branching surface 44b, i.e., the incident point SB2 of the first modulated light L1 on the optical branching surface 44b, and further so that the second modulated light L2 is reflected in the reflection direction RD at this incident point SB2.

[0051] As described above, no Faraday rotator or the like is disposed between the second spatial light modulator 48 and the second opposing surface 44d, so the reflected light LS (S-polarized) emitted from the second opposing surface 44d is incident on the reflecting surface of the second spatial light modulator 48 while maintaining its plane of polarization (polarization direction). The second modulated light L2 (S-polarized) reflected by the reflecting surface of the second spatial light modulator 48 is also incident on the light splitting surface 44b from the second opposing surface 44d at an incident point SB2 while maintaining its plane of polarization (polarization direction). Therefore, the second modulated light L2 is reflected by the light splitting surface 44b in the reflection direction RD. The first modulated light L1 and the second modulated light L2 are combined at the incident point SB2 on the light splitting surface 44b, and then emitted from the exit point SA2 of the light incident / exit surface 44a along the reflection direction RD.

[0052] The hologram patterns 52 and 54 are derived in advance based on the position where the laser processing area is to be formed, the wavelength of the laser light L, and the refractive indexes of the condenser lens 38 and wafer W, and are stored in the control unit 50.

[0053] The hologram pattern 52 is a modulation pattern for modulating the phase of the transmitted light LP. Specifically, the hologram pattern 52 is obtained by superimposing a focusing hologram pattern for making the focusing position of the first modulated light L1 focused by the focusing lens 38 different from the focusing position of the second modulated light L2 in the thickness direction of the wafer W, and a correction hologram pattern for correcting aberration of the first modulated light L1 that occurs inside the wafer W. The correction hologram pattern may include a pattern for correcting aberration generated by the optical system of the laser processing head 20.

[0054] The hologram pattern 54 is a modulation pattern for modulating the phase of the reflected light LS, and is a correction hologram pattern for correcting aberration of the second modulated light L2 that occurs inside the wafer W.

[0055] The hologram patterns 52 and 54 may be reversed. Furthermore, the hologram pattern 54 may be formed by superimposing a condensing hologram pattern and a correction hologram pattern, similar to the hologram pattern 52. In other words, the hologram patterns 52 and 54 are not particularly limited as long as the condensing positions of the first modulated light L1 and the second modulated light L2 condensed inside the wafer W by the condenser lens 38 can be adjusted to different positions in the thickness direction of the wafer W.

[0056] As described above, in this embodiment, the transmitted light LP and the reflected light LS are individually phase-modulated using the two spatial light modulators, the first spatial light modulator 46 and the second spatial light modulator 48, thereby reducing the amount of light incident on each of the first spatial light modulator 46 and the second spatial light modulator 48. As a result, the temperature rise of the first spatial light modulator 46 and the second spatial light modulator 48 can be suppressed, and distortion due to heat of the first spatial light modulator 46 and the second spatial light modulator 48 can be suppressed, and the risk of damage can be reduced.

[0057] It is also possible to divide the reflecting surface of one spatial light modulator into two parts and individually phase-modulate the transmitted light LP and the reflected light LS for each divided area of ​​the reflecting surface, instead of using two first spatial light modulators 46 and second spatial light modulators 48 as in this embodiment. However, in this case, the first modulated light L1 and the second modulated light L2 that are individually focused for each divided area cannot equally occupy the aperture of the focusing lens 18, which reduces the focusing ability of the first modulated light L1 and the second modulated light L2 and also reduces the processing quality.

[0058] In contrast to this, in this embodiment, the first spatial light modulator 46 and the second spatial light modulator 48 are used to individually phase-modulate the transmitted light LP and the reflected light LS, so that the first modulated light L1 and the second modulated light L2 are combined and overlapped, and each of them occupies the same amount of space in the aperture of the condenser lens 18. As a result, deterioration in the condensing ability of the first modulated light L1 and the second modulated light L2 and deterioration in processing quality are suppressed.

[0059] [Laser processing using laser processing equipment] Next, a flow of laser processing of the wafer W using the laser processing apparatus 10 configured as described above will be described. First, aberration correction is performed as necessary using the correction collar 40 of the condenser lens 38. Next, the wafer W is held by suction on the stage 11, and then the control unit 50 controls an alignment optical system (not shown) and the stage 11 (stage movement mechanism) to perform alignment by aligning the optical axis of the condenser lens 38 with the processing start position of the intended cutting line.

[0060] Once the alignment is complete, the control unit 50 starts emitting laser light L from the laser light source 22, presenting a hologram pattern 52 by the first spatial light modulator 46, and presenting a hologram pattern 54 by the second spatial light modulator 48, and also drives the stage 11 to move the focusing lens 38 relative to the wafer W in the X direction (processing feed direction).

[0061] The laser light L emitted from the laser light source 22 passes through a beam expander 24, a mirror 25, and a λ / 2 wave plate 26 and enters an optical modulator .

[0062] The laser light L incident on the light modulation device 28 is incident on the light incident / exit surface 44a of the polarizing beam splitter 44, and then polarized and separated by the light splitting surface 44b into transmitted light LP and reflected light LS. The transmitted light LP passes through the light splitting surface 44b and is emitted from the first opposing surface 44c toward the first spatial light modulator 46, and enters the first spatial light modulator 46 while maintaining its plane of polarization (polarization direction). As a result, the transmitted light LP is phase-modulated into first modulated light L1 by the hologram pattern 52 presented on the reflecting surface of the first spatial light modulator 46. The first modulated light L1 is then reflected by the reflecting surface of the first spatial light modulator 46, passes through the first opposing surface 44c along the reflection direction RD while maintaining its plane of polarization, and enters the light splitting surface 44b at an incident point SB2.

[0063] On the other hand, the reflected light LS is reflected by the light splitting surface 44b and then emitted from the second opposing surface 44d toward the second spatial light modulator 48, and enters the second spatial light modulator 48 while maintaining its plane of polarization (polarization direction). As a result, the reflected light LS is phase-modulated into second modulated light L2 by the hologram pattern 54 presented on the reflecting surface of the second spatial light modulator 48. The second modulated light L2 is then reflected by the reflecting surface of the second spatial light modulator 48, and while maintaining its plane of polarization, passes through the second opposing surface 44d and enters the light splitting surface 44b at an incident point SB2.

[0064] The first modulated light L1 and the second modulated light L2 are combined at the incident point SB2, then travel along the reflection direction RD, and are emitted from the emission point SA2 of the light incident / exit surface 44a toward the mirror 30. In this way, in this embodiment, by adjusting the arrangement of the first spatial light modulator 46 and the second spatial light modulator 48 with respect to the polarizing beam splitter 44 and causing the first modulated light L1 and the second modulated light L2 to emit from the light incident / exit surface 44a where the laser light L is incident, it is possible to omit the arrangement of a Faraday rotator or the like between the polarizing beam splitter 44 and each spatial light modulator 46, 48.

[0065] The first modulated light L1 and the second modulated light L2 emitted from the light incident / exit surface 44a pass through mirrors 30 and 31, the first lens 32a, mirrors 33 and 34, and the second lens 32b and are incident on the condenser lens 38. The first modulated light L1 and the second modulated light L2 are then condensed by the condenser lens 38 at two different positions inside the wafer W.

[0066] Fig. 6 is a diagram showing a state in which the first modulated light L1 and the second modulated light L2 are individually focused inside the wafer W. Fig. 7 is a diagram showing a state in which laser processing areas P1, P2 are formed at the focusing point Q1 of the first modulated light L1 and the focusing point Q2 of the second modulated light L2 shown in Fig. 6. Fig. 8 is a diagram showing a state in which two rows of laser processing areas P1, P2 are formed inside the wafer W along the line to cut.

[0067] As shown in Fig. 6, the first modulated light L1 and the second modulated light L2 are simultaneously focused by the focusing lens 38 at two positions (focus points Q1 and Q2) inside the wafer W that are different from each other in the thickness direction and are equal to each other in the X direction (corresponding to the relative movement direction). As a result, a pair of laser processing areas P1 and P2 are formed near the two focus points Q1 and Q2, as shown in Fig. 7. Furthermore, fissures K1 and K2 (also referred to as cracks) are formed starting from the laser processing areas P1 and P2 and extending in the thickness direction of the wafer W. When one scan is performed along the line to cut, two rows of laser processing areas P1 and P2 are formed inside the wafer W along the line to cut, as shown in Fig. 8.

[0068] Next, under the control of the control unit 50, the stage 11 is indexed and fed by one pitch in the Y direction, and the next cutting line is similarly formed into laser processing areas P1 and P2.

[0069] Then, when the laser processing areas P1, P2 are formed along all the lines to cut that are parallel to the X direction, the stage 11 is rotated 90° under the control of the control unit 50, and the laser processing areas P1, P2 are similarly formed along all the lines to cut that are perpendicular to the previous lines to cut. As a result, the laser processing areas P1, P2 are formed along all the lines to cut.

[0070] After the laser processing areas P1 and P2 have been formed along the intended cutting line in the manner described above, a backside grinding process is performed in which the backside of the wafer W is ground using a grinding device (not shown) to process the thickness (initial thickness) T1 of the wafer W to a predetermined thickness (final thickness) T2 (e.g., 30 to 50 μm).

[0071] After the back grinding process, an expandable tape (dicing tape) is applied to the back surface of the wafer W, and after the BG tape applied to the front surface of the wafer W is peeled off, an expanding process is performed in which tension is applied to the expandable tape applied to the back surface of the wafer W to stretch it. As a result, the wafer W is cut from cracks that have extended to the device surface (front surface) of the wafer W. In other words, the wafer W is cut along the planned cutting lines and divided into multiple chips.

[0072] As described above, in this embodiment, the Faraday rotator can be omitted from the optical modulation device 28, which eliminates the need for the conventional effort of adjusting the position of the Faraday rotator and prevents loss of light due to attenuation by the Faraday rotator.

[0073] [others] 9 is a perspective view showing a modified example of the light modulation device 28. In the above embodiment, the light modulation device 28 is provided with a cube-type polarizing beam splitter 44. However, as shown in FIG. 9, the light modulation device 28 may be provided with a plate-type polarizing beam splitter 60 instead of the cube-type. This polarizing beam splitter 60 has a light splitting surface 44b similar to that of the polarizing beam splitter 44 in the above embodiment, and, similar to the above embodiment, performs polarization separation of the laser light L, emits transmitted light LP to the first spatial light modulator 46, emits reflected light LS to the second spatial light modulator 48, combines the first modulated light L1 and the second modulated light L2, and emits the first modulated light L1 and the second modulated light L2 to the mirror 30.

[0074] In the above-described embodiments, the laser light L is polarized and separated by the polarizing beam splitters 44, 60. However, for example, a half mirror may be used as the separation element of the present invention to separate the laser light L into a first incident light and a second incident light, and the first incident light may be emitted to the first spatial light modulator 46 and the second incident light may be emitted to the second spatial light modulator 48. However, if the first spatial light modulator 46 and the second spatial light modulator 48 are LCOS-type SLMs, only the linearly polarized light component whose vibration direction is parallel to the liquid crystal alignment direction is modulated, and therefore, using a half mirror reduces the light utilization efficiency. For this reason, it is preferable to use the polarizing beam splitters 44, 60.

[0075] In the above embodiment, a two-stage process is performed in which laser processing areas P1, P2 are simultaneously formed at two different focusing points Q1, Q2 in the thickness direction inside the wafer W, and then a back-grinding process and an expanding process are performed. However, the present invention is not limited to this. For example, laser processing may be performed multiple times while changing the positions at which the first modulated light L1 and the second modulated light L2 are focused inside the wafer W (the processing depth of the laser processing areas P1, P2) as needed. In this case, by superimposing a correction pattern for correcting aberrations inside the wafer W (in this case, a pattern in a direction that cancels the correction by the correction collar 40) on the hologram patterns 52, 54 according to the processing depth of the laser processing areas P1, P2, appropriate aberration correction can be performed even in a relatively shallow portion from the back surface of the wafer W.

[0076] Fig. 10 is a top view of a modified example of the light modulation device 28 seen from above. Fig. 11 is a top view of a modified example of the light modulation device 28 seen from above. Note that in order to avoid complication of the drawing, the second spatial light modulator 48 is not shown in Fig. 10.

[0077] In the above embodiment, the reflected light LS is reflected by the light splitting surface 44b in a direction perpendicular to the X1 direction (Z1 direction), but the present invention is not limited to this. For example, as shown in Figures 10 and 11, by adjusting the positions and attitudes of the polarizing beam splitter 44 and each spatial light modulator 46, 48, the reflected light LS may be reflected by the light splitting surface 44b in a reflection direction tilted with respect to the Z1 direction (corresponding to the first reflection direction of the present invention) when viewed from the Y1 direction. In this case, too, the second modulated light L2 reflected by the reflecting surface of the second spatial light modulator 48 is incident on the light splitting surface 44b while maintaining its polarization plane, and is combined with the first modulated light L1 as in the above embodiment, and then travels along the reflection direction RD together with the first modulated light L1.

[0078] In the above embodiment, an LCOS type SLM is used as each of the spatial light modulators 46, 48, but a MEMS (Micro Electro Mechanical Systems) type SLM or a deformable mirror device may also be used. Furthermore, each of the spatial light modulators 46, 48 is not limited to a reflective type and may also be a transmissive type. Furthermore, each of the spatial light modulators 46, 48 may be a liquid crystal cell type or an LCD (Liquid Crystal Display) type. [Explanation of symbols]

[0079] 10...laser processing device, 11...stage, 18...condensing lens, 20...laser processing head, 22...laser light source, 24...beam expander, 25...mirror, 26...two-wavelength plate, 28...light modulator, 30...mirror, 31...mirror, 32...4f optical system, 32a...first lens, 32b...second lens, 33...mirror, 34...mirror, 38...condensing lens, 40...correction collar, 44...polarizing beam splitter, 44a...light incident / exit surface, 44b...light branching surface, 44c...first opposing surface , 44d...second opposing surface, 46...first spatial light modulator, 48...second spatial light modulator, 50...controller, 52...hologram pattern, 54...hologram pattern, 60...polarizing beam splitter, K1...crack, K2...crack, L...laser light, L1...first modulated light, L2...second modulated light, LP...transmitted light, LS...reflected light, Nd...semiconductor laser excitation, P1...laser processing area, P2...laser processing area, Q1...focus point, Q2...focus point, RD...reflection direction, W...wafer, d...predetermined distance

Claims

[Claim 1] An optical modulation device that is provided in a laser processing device that forms a pair of laser processing regions at different processing depth positions inside a workpiece by focusing phase-modulated first modulated light and second modulated light inside the workpiece using a focusing lens, and generates the first modulated light and the second modulated light, a separation element having a light branching surface that separates incident light incident along a first direction among a first direction, a second direction, and a third direction that are orthogonal to each other, into a first incident light and a second incident light, and transmits the first incident light and reflects the second incident light in a first reflection direction that is parallel to or inclined with respect to the second direction when viewed from the third direction; a first spatial light modulator that is disposed directly opposite the splitting element in the first direction, that phase-modulates the first incident light that has passed through the light splitting surface to generate the first modulated light, and that returns the first modulated light to the light splitting surface; a second spatial light modulator that is disposed directly opposite the splitting element in the first reflection direction, that phase-modulates the second incident light reflected by the light splitting surface to generate the second modulated light, and that returns the second modulated light to the light splitting surface; Equipped with the first spatial light modulator reflects the first modulated light in a second reflection direction that is parallel to a plane perpendicular to the second direction and is inclined with respect to the incident light when viewed from the second direction, and causes the first modulated light to be incident on the light branching surface; the second spatial light modulator reflects the second modulated light toward an incident point of the first modulated light on the light splitting surface; the light splitting surface combines the first modulated light incident from the first spatial light modulator and the second modulated light incident from the second spatial light modulator and outputs the combined light in the second reflection direction; Light modulation device.

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