WO2018110238A1 - Laser machining device and laser machining method - Google Patents

Abstract

A laser machining device, provided with: a measurement light source; a condensing lens; a displacement detector for detecting the displacement of a laser beam incidence surface on the basis of reflected light of measurement light reflected by the laser beam incidence surface; and an imaging state adjustment unit for moving the imaging state of the measurement light and/or the reflected light of the measurement light. The displacement detector has: a branching unit for branching the reflected light of the measurement light into a plurality of branched reflected lights; a plurality of astigmatism imparting units for imparting astigmatism amounts of different sizes to each of the plurality of branched reflected lights; a plurality of beam shape detectors for detecting the respective beam shapes of each of the branched reflected lights to which astigmatism has been imparted; and a signal acquisition unit for selecting, from the light paths of the plurality of branched reflected lights, a light path corresponding to the imaging state to be adjusted by the imaging state adjustment unit, and acquiring a signal relating to displacement on the basis of the result of detection by the beam shape detector on the light path of the selected branched reflected light.

Description

Laser processing apparatus and laser processing method

One aspect of the present invention relates to a laser processing apparatus and a laser processing method.

Conventionally, there has been known a laser processing apparatus that forms a modified region on a processing object by condensing a processing laser beam on the processing object (see, for example, Patent Document 1). Such a laser processing apparatus includes a measurement light source that emits measurement light, a condensing lens that condenses the processing laser light and measurement light on the processing target, and a laser light incident surface of the processing target. A displacement detector that detects a displacement of the laser light incident surface (hereinafter also simply referred to as “displacement”) based on the reflected reflected light of the measurement light. The displacement detection unit adds astigmatism to the reflected light of the measurement light, detects the beam shape of the reflected light to which the astigmatism is added, and acquires a signal related to the displacement based on the detection result.

Japanese Patent Laid-Open No. 2015-186825

In the laser processing apparatus as described above, the signal fluctuation (signal inclination) with respect to the acquired displacement may become too gentle. In this case, for example, if the laser light incident surface is a ground surface, the measurement light is scattered by the grinding marks (also referred to as saw marks), and a different signal occurs even with the same displacement. Become prominent. As a result, it may be difficult to accurately detect the displacement of the laser light incident surface.

An object of one aspect of the present invention is to provide a laser processing apparatus and a laser processing method capable of accurately detecting a displacement of a laser light incident surface.

A laser processing apparatus according to one aspect of the present invention is a laser processing apparatus that forms a modified region on a processing object by condensing the processing laser light on the processing object, and emits measurement light. Based on a measurement light source, a condensing lens for condensing the processing laser light and the measurement light on the object to be processed, and a reflected light of the measurement light reflected by the laser light incident surface of the object to be processed A displacement detector that detects the displacement of the light incident surface; and an imaging state adjustment unit that moves at least one of the imaging states of the measurement light and the reflected light of the measurement light. Provided in each of the branch part for branching the reflected light of the light into a plurality of branched reflected light and the optical path of the plurality of branched reflected light, and adding an astigmatism amount of a different size to each of the plurality of branched reflected light Multiple astigmatism addition sections and multiple branched reflected light A plurality of beam shape detectors that detect the beam shape of each of the plurality of branched reflected lights provided with astigmatism, respectively, and the imaging state adjustment unit move from the optical paths of the plurality of branched reflected lights. A signal acquisition unit that selects one corresponding to the imaging state and acquires a signal related to displacement based on a detection result of the beam shape detection unit in the optical path of the selected branched reflected light.

The inventors of the present invention have made extensive studies and found that the inclination of the acquired signal has a correlation with the imaging state of at least one of the measurement light and the reflected light of the measurement light. Furthermore, the present inventors have found that the reason why the signal inclination becomes too gentle is due to a mismatch between the imaging state and the amount of astigmatism added to the reflected light. Therefore, in the laser processing apparatus according to one aspect of the present invention, the reflected light of the measurement light is branched into a plurality of branched reflected lights, and the astigmatism amount having a different size is added to the optical path of each branched reflected light. The beam shape of the reflected light is detected. Then, one of the optical paths of the branched reflected light is selected according to the imaging state moving by the imaging state adjusting unit, and a signal based on the beam shape of the branched reflected light of the selected optical path is acquired. . Thereby, the amount of astigmatism added to the branched reflected light related to the signal to be acquired can be made in accordance with the imaging state. It is possible to suppress the inclination of the acquired signal from becoming too gentle. Therefore, it is possible to detect the displacement of the laser light incident surface with high accuracy.

In the laser processing apparatus according to one aspect of the present invention, the imaging state adjustment unit adjusts the offset amount by moving the imaging state, and the signal acquisition unit connects the optical paths of the plurality of branched reflected lights. One corresponding to the offset amount to be adjusted by the image state adjustment unit may be selected. Here, the offset amount is a scale indicating the relative displacement of the laser incident surface of the workpiece when the signal for the displacement of the laser incident surface of the workpiece is a reference value (typically zero). For example, when the offset amount is 0 μm, when parallel light having the same wavelength as the measurement light is incident on the condensing lens, the arrangement is such that the laser incident surface of the object to be processed is minimized, and a signal corresponding to the displacement signal at that time Is a geometric arrangement state in which is a reference value. Further, for example, an offset amount of −180 μm means that a geometry whose signal becomes a reference value when the laser incident surface of the object to be processed approaches 180 μm from the above arrangement when the offset amount is 0 μm. This is a general arrangement state. When the value of the offset amount is small (minus width is large), the offset or the offset amount is said to be deep. When the value of the offset amount is large (minus width is small), the offset or the offset amount is shallow.

It is found that the slope of the signal to be acquired specifically correlates with the offset amount and becomes too gentle due to a mismatch between the offset amount and the astigmatism amount. Therefore, in the laser processing apparatus according to one aspect of the present invention, one corresponding to the offset is selected from the optical paths of the plurality of branched reflected lights. Thereby, the amount of astigmatism added to the branched reflected light related to the signal to be acquired can be set according to the offset amount. It is possible to suppress the inclination of the acquired signal from becoming too gentle.

In the laser processing apparatus according to one aspect of the present invention, the branching unit branches the reflected light of the measurement light into at least the first branched reflected light and the second branched reflected light, and the astigmatism adding unit is the first branched reflection. A first astigmatism adding unit that is provided in the optical path of the light and adds the first astigmatism amount to the first branched reflected light; and an optical path of the second branched reflected light that is provided in the optical path of the second branched reflected light. A second astigmatism addition unit for adding a large second astigmatism amount to the second branched reflected light, and the signal acquisition unit has an offset amount adjusted by the imaging state adjustment unit in the first range. In this case, the optical path of the first branched reflected light is selected, and when the offset amount adjusted by the imaging state adjustment unit is in the second range deeper than the first range, the optical path of the second branched reflected light is selected. May be.

More specifically, it is found that the slope of the acquired signal becomes more gradual as the offset becomes deeper when the same astigmatism amount is added. Therefore, in the laser processing apparatus according to one aspect of the present invention, when the offset amount is in the first range, the optical path of the first branched reflected light is selected, and the offset amount adjusted by the imaging state adjustment unit is the first amount. If the second range is deeper than the range, the optical path of the second branched reflected light is selected. Thereby, the amount of astigmatism added to the branched reflected light related to the signal to be acquired can be increased when the offset is deep (small when it is shallow). It is possible to suppress the inclination of the acquired signal from becoming too gentle.

A laser processing apparatus according to an aspect of the present invention includes an offset amount setting unit that sets an offset amount that is adjusted by an imaging state adjustment unit, and an imaging state adjustment unit that has an offset amount that is set by an offset amount setting unit. An imaging state control unit that controls According to this configuration, the imaging state can be automatically adjusted so that the set offset amount is obtained.

A laser processing apparatus according to one aspect of the present invention includes a drive mechanism that operates at least one of a processing target and a condensing lens along the optical axis direction of the condensing lens, and a signal acquired by a signal acquisition unit. And a drive mechanism control unit that operates the drive mechanism so as to maintain the target value. According to this configuration, the focusing lens can be relatively moved along the optical axis direction so as to follow the laser light incident surface.

The laser processing apparatus according to an aspect of the present invention may include an optical axis adjustment mechanism that aligns the optical axis of the measurement light with the optical axis of the processing laser light. According to this configuration, the optical axis of the measurement light can be accurately aligned with the optical axis of the processing laser light.

In the laser processing apparatus according to one aspect of the present invention, the measurement light source can emit any of a plurality of lights having different wavelengths, and has the highest reflectivity with respect to the processing object among the light of the plurality of wavelengths. Light having a high wavelength may be emitted as measurement light. In this case, it becomes possible to easily detect the reflected light obtained by reflecting the measurement light on the laser light incident surface.

A laser processing method according to one aspect of the present invention is a laser processing method for forming a modified region on a processing object by condensing the processing laser light on the processing object, While condensing the laser light with the condensing lens, the measuring light is condensed on the object to be processed by the condensing lens, and the reflected light of the measuring light reflected by the laser light incident surface of the object to be processed is reflected. At least the first branched reflected light and the second branched reflected light are branched, and the beam shape of the first branched reflected light to which the first astigmatism amount is added in the optical path of the first branched reflected light is detected and the second branched reflected light is detected. The beam shape of the second branched reflected light to which the second astigmatism amount larger than the first astigmatism amount is added is detected in the optical path of, and a signal relating to the displacement of the laser light incident surface based on the detection result of the beam shape And the acquired signal maintains the target value. And a laser processing step for operating at least one of the object to be processed and the condensing lens along the optical axis direction of the condensing lens, and the laser processing step includes a first step for setting an offset amount, When the offset amount set in one step is the first range, the optical path of the first branched reflected light is selected, and when the offset amount set in the first step is the second range deeper than the first range, the second range is selected. A second step of selecting the optical path of the branched reflected light, and a third step of moving the imaging state of at least one of the measurement light and the reflected light of the measurement light so that the offset amount set in the first step is obtained. And after the third step and the fourth step, the fourth step of operating at least one of the processing object and the condensing lens so as to be the offset amount set in the first step, After the fifth step of obtaining the target value and the fifth step, the beam shape is changed in the optical path of the branched reflected light selected in the second step while condensing the processing laser light on the processing object with the condensing lens. Detect and acquire a signal based on the detection result of the beam shape, and at least one of the processing object and the condensing lens along the optical axis direction of the condensing lens so that the acquired signal maintains the target value And a sixth step for operating the device.

Also in this laser processing method, the amount of astigmatism added to the branched reflected light related to the acquired signal can be increased when the offset is deep (small when it is shallow). Based on the above knowledge, it is possible to suppress the inclination of the acquired signal from becoming too gentle. Therefore, it is possible to detect the displacement of the laser light incident surface with high accuracy.

According to one aspect of the present invention, it is possible to provide a laser processing apparatus and a laser processing method capable of accurately detecting the displacement of the laser light incident surface.

FIG. 1 is a schematic configuration diagram of a laser processing apparatus used for forming a modified region. FIG. 2 is a plan view of a workpiece to be modified. FIG. 3 is a cross-sectional view taken along the line III-III of the workpiece of FIG. FIG. 4 is a plan view of an object to be processed after laser processing. FIG. 5 is a cross-sectional view taken along the line VV of the workpiece in FIG. 6 is a cross-sectional view of the workpiece of FIG. 4 along the line VI-VI. FIG. 7 is a schematic configuration diagram of a laser processing apparatus according to an embodiment. FIG. 8 is a partial cross-sectional view of the reflective spatial light modulator of the laser processing apparatus of FIG. FIG. 9 is a schematic configuration diagram of an autofocus control system including the AF unit of the laser processing apparatus of FIG. FIG. 10A is a diagram illustrating a case where the beam shape of the reflected light is a vertically long ellipse. FIG. 10B is a diagram illustrating a case where the beam shape of the reflected light is a perfect circle. (C) is a figure explaining the case where the beam shape of reflected light is a horizontally long ellipse. FIG. 11 is a graph illustrating an example of the error signal. FIG. 12 is an example of a flowchart showing a laser processing method performed by the laser processing apparatus of FIG. FIG. 13 is a graph showing an error signal generated based only on the beam shape detected by the first beam shape detection unit. FIG. 14 is a graph showing an error signal generated in the laser processing apparatus of FIG. FIG. 15 is a graph showing an error signal generated based only on the beam shape detected by the second beam shape detection unit. FIG. 16 is a schematic configuration diagram of an AF unit according to a modification. FIG. 17A is a graph for explaining the effect of the AF unit of FIG. 16, and is an error signal when the first beam shape detection unit is fixed. FIG. 17B is a graph for explaining the effect of the AF unit of FIG. 16, and is an error signal when the first beam shape detection unit is movable.

Hereinafter, embodiments will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the same or an equivalent part, and the overlapping description is abbreviate | omitted.

In the laser processing apparatus and the laser processing method according to the embodiment, the modified region is formed in the processing object along the planned cutting line by condensing the laser beam on the processing object. First, the formation of the modified region will be described with reference to FIGS.

As shown in FIG. 1, a laser processing apparatus 100 includes a laser light source 101 that is a processing laser light source that pulsates laser light L that is a processing laser light, an optical system 103 that guides the laser light L, A condensing lens 105 for condensing the laser light L. The laser processing apparatus 100 includes a support base 107 for supporting the workpiece 1 irradiated with the laser light L condensed by the condensing lens 105, a stage 111 for moving the support base 107, a laser, A laser light source controller 102 for controlling the laser light source 101 to adjust the output (pulse energy, light intensity), pulse width, pulse waveform, etc. of the light L, and a stage controller 115 for controlling the movement of the stage 111. I have.

In the laser processing apparatus 100, the laser light L emitted from the laser light source 101 is guided by the optical system 103 and condensed by the condensing lens 105 inside the processing target 1 placed on the support base 107. Is done. At the same time, the stage 111 is moved, and the workpiece 1 is moved relative to the laser beam L along the planned cutting line 5. Thereby, a modified region along the planned cutting line 5 is formed on the workpiece 1. Here, the stage 111 is moved in order to move the laser light L relatively, but the condensing lens 105 may be moved, or both of them may be moved.

As the processing object 1, a plate-like member (for example, a substrate, a wafer, or the like) including a semiconductor substrate formed of a semiconductor material, a piezoelectric substrate formed of a piezoelectric material, or the like is used. As shown in FIG. 2, a scheduled cutting line 5 for cutting the workpiece 1 is set in the workpiece 1. The planned cutting line 5 is a virtual line extending linearly. When the modified region is formed inside the workpiece 1, the laser beam L is cut in a state where the condensing point (condensing position) P is aligned with the inside of the workpiece 1 as shown in FIG. 3. It moves relatively along the planned line 5 (that is, in the direction of arrow A in FIG. 2). Thereby, as shown in FIGS. 4, 5, and 6, the modified region 7 is formed on the workpiece 1 along the planned cutting line 5, and the modified region formed along the planned cutting line 5. 7 becomes the cutting start region 8.

The condensing point P is a portion where the laser light L is condensed. The planned cutting line 5 is not limited to a straight line, but may be a curved line, a three-dimensional shape in which these lines are combined, or a coordinate designated. The planned cutting line 5 is not limited to a virtual line, but may be a line actually drawn on the surface 3 of the workpiece 1. The modified region 7 may be formed continuously or intermittently. The modified region 7 may be in the form of a line or a dot. In short, the modified region 7 only needs to be formed at least inside the workpiece 1. In addition, a crack may be formed starting from the modified region 7, and the crack and the modified region 7 may be exposed on the outer surface (front surface 3, back surface, or outer peripheral surface) of the workpiece 1. . The laser light incident surface when forming the modified region 7 is not limited to the front surface 3 of the workpiece 1 and may be the back surface of the workpiece 1.

Incidentally, when the modified region 7 is formed inside the workpiece 1, the laser light L passes through the workpiece 1 and is near the condensing point P located inside the workpiece 1. Especially absorbed. Thereby, the modified region 7 is formed in the workpiece 1. In this case, since the energy density of the laser beam L is low on the surface 3 of the workpiece 1, the surface 3 of the workpiece 1 is not melted. On the other hand, when the modified region 7 is formed on the front surface 3 or the back surface of the workpiece 1, the laser light L is particularly absorbed in the vicinity of the condensing point P located on the front surface 3 or the back surface, and from the front surface 3 or the back surface. It is melted and removed to form removed portions such as holes and grooves.

The modified region 7 is a region where the density, refractive index, mechanical strength and other physical characteristics are different from the surroundings. Examples of the modified region 7 include a melt treatment region (meaning at least one of a region once solidified after melting, a region in a molten state, and a region in a state of being resolidified from melting), a crack region, and the like. In addition, there are a dielectric breakdown region, a refractive index change region, etc., and there is a region where these are mixed. Further, the modified region 7 includes a region where the density of the modified region 7 in the material of the workpiece 1 is changed compared to the density of the non-modified region, and a region where lattice defects are formed. When the material of the workpiece 1 is single crystal silicon, the modified region 7 can be said to be a high dislocation density region.

The area where the density of the melt processing area, the refractive index changing area, the density of the modified area 7 is changed as compared with the density of the non-modified area, and the area where lattice defects are formed are further included in the interior of these areas or the modified areas In some cases, cracks (cracks, microcracks) are included in the interface between the region 7 and the non-modified region. The included crack may be formed over the entire surface of the modified region 7, or may be formed in only a part or a plurality of parts. The workpiece 1 includes a substrate made of a crystal material having a crystal structure. For example, the workpiece 1 includes a substrate formed of at least one of gallium nitride (GaN), silicon (Si), silicon carbide (SiC), LiTaO ₃ , and sapphire (Al ₂ O ₃ ). In other words, the workpiece 1 includes, for example, a gallium nitride substrate, a silicon substrate, a SiC substrate, a LiTaO ₃ substrate, or a sapphire substrate. The crystal material may be either an anisotropic crystal or an isotropic crystal. Moreover, the workpiece 1 may include a substrate made of an amorphous material having an amorphous structure (amorphous structure), for example, a glass substrate.

In the embodiment, the modified region 7 can be formed by forming a plurality of modified spots (processing marks) along the planned cutting line 5. In this case, the modified region 7 is formed by collecting a plurality of modified spots. For the modified spot, the size and length of cracks to be generated are appropriately determined in consideration of the required cutting accuracy, required flatness of the cut surface, thickness, type, crystal orientation, etc. of the workpiece 1. Can be controlled. In the embodiment, the modified spot can be formed as the modified region 7 along the planned cutting line 5.

Next, a laser processing apparatus and a laser processing method according to the embodiment will be described. Below, the case where the back surface 21 of the workpiece 1 is a laser light incident surface is illustrated. The description will be made assuming that the thickness direction of the workpiece 1 is the Z direction.

As shown in FIG. 7, the laser processing apparatus 300 includes a laser light source 202, a reflective spatial light modulator 203, a 4f optical system 241 and a condensing optical system 204 in a housing 231. The laser processing apparatus 300 focuses the laser beam L on the workpiece 1 to form the modified region 7 on the workpiece 1 along the planned cutting line 5.

The laser light source 202 emits laser light L. The laser light source 202 emits pulsed laser light, which is laser light having a pulse width of 1 μs or less, as laser light L. The laser light source 202 includes an ultrashort pulse laser light source as a laser oscillator. As the laser oscillator, for example, a solid laser, a fiber laser, an external modulation element, or the like can be used. The laser light source 202 includes an output adjusting unit that adjusts the output of the laser light L. The output adjustment unit can be composed of a λ / 2 wavelength plate unit, a polarizing plate unit, and the like. Further, the laser light source 202 includes a beam expander that parallelizes the laser light L while adjusting the diameter thereof.

The wavelength of the laser light L emitted from the laser light source 202 is included in any wavelength band of 500 to 550 nm, 1000 to 1150 nm, or 1300 to 1400 nm. The wavelength of the laser beam L here is 1064 nm. Such a laser light source 202 is fixed to the top plate 236 of the housing 231 with screws or the like so as to emit the laser light L in the horizontal direction.

The reflective spatial light modulator 203 modulates the laser light L emitted from the laser light source 202. The reflective spatial light modulator 203 is, for example, a reflective liquid crystal (LCOS: Liquid Crystal on Silicon) spatial light modulator (SLM: Spatial Light Modulator). The reflective spatial light modulator 203 modulates the laser beam L incident from the horizontal direction and reflects the laser beam L obliquely upward with respect to the horizontal direction.

As shown in FIG. 8, the reflective spatial light modulator 203 includes a silicon substrate 213, a drive circuit layer 914, a plurality of pixel electrodes 214, a reflective film 215 such as a dielectric multilayer mirror, an alignment film 999a, and a liquid crystal layer 216. , An alignment film 999b, a transparent conductive film 217, and a transparent substrate 218 such as a glass substrate are stacked in this order. The transparent substrate 218 has a surface 218a along a predetermined plane. The surface 218 a of the transparent substrate 218 constitutes the surface of the reflective spatial light modulator 203. The transparent substrate 218 is made of a light transmissive material such as glass, for example. The transparent substrate 218 transmits the laser light L having a predetermined wavelength incident from the surface 218 a of the reflective spatial light modulator 203 into the reflective spatial light modulator 203. The transparent conductive film 217 is formed on the back surface of the transparent substrate 218. The transparent conductive film 217 is made of a conductive material (for example, ITO) that transmits the laser light L.

The plurality of pixel electrodes 214 are arranged in a matrix on the silicon substrate 213 along the transparent conductive film 217. The plurality of pixel electrodes 214 are formed of a metal material such as aluminum, for example. The surfaces 214a of the plurality of pixel electrodes 214 are processed flat and smoothly. The plurality of pixel electrodes 214 are driven by an active matrix circuit provided in the drive circuit layer 914.

The active matrix circuit is provided between the plurality of pixel electrodes 214 and the silicon substrate 213. The active matrix circuit controls the voltage applied to each pixel electrode 214 in accordance with the light image to be output from the reflective spatial light modulator 203. For example, the active matrix circuit includes a first driver circuit that controls an applied voltage of each pixel column aligned in one direction along the surface 218a, and each pixel column orthogonal to the one direction and aligned in the other direction along the surface 218a. And a second driver circuit for controlling the applied voltage. Such an active matrix circuit is configured such that a predetermined voltage is applied to the pixel electrode 214 of the pixel specified by both driver circuits by the control unit 250 (see FIG. 7).

The alignment films 999a and 999b are disposed on both end faces of the liquid crystal layer 216, and align liquid crystal molecule groups in a certain direction. The alignment films 999a and 999b are formed of a polymer material such as polyimide, for example. The contact surfaces of the alignment films 999a and 999b with the liquid crystal layer 216 are rubbed.

The liquid crystal layer 216 is disposed between the plurality of pixel electrodes 214 and the transparent conductive film 217. The liquid crystal layer 216 modulates the laser light L in accordance with the electric field formed by each pixel electrode 214 and the transparent conductive film 217. That is, when a voltage is applied to each pixel electrode 214 by the active matrix circuit of the drive circuit layer 914, an electric field is formed between the transparent conductive film 217 and each pixel electrode 214, and the electric field formed in the liquid crystal layer 216. The alignment direction of the liquid crystal molecules 216a changes depending on the size of the liquid crystal molecules. When the laser light L passes through the transparent substrate 218 and the transparent conductive film 217 and enters the liquid crystal layer 216, the laser light L is modulated by the liquid crystal molecules 216 a while passing through the liquid crystal layer 216, and is reflected on the reflective film 215. After reflection, the light is again modulated by the liquid crystal layer 216 and emitted.

At this time, the voltage applied to each pixel electrode 214 is controlled by the control unit 250 (see FIG. 7), and a portion sandwiched between the transparent conductive film 217 and each pixel electrode 214 in the liquid crystal layer 216 according to the voltage. The refractive index of the liquid crystal layer 216 at the position corresponding to each pixel changes. With this change in refractive index, the phase of the laser light L can be changed for each pixel of the liquid crystal layer 216 in accordance with the applied voltage. That is, phase modulation corresponding to the hologram pattern can be applied to each pixel by the liquid crystal layer 216. The wavefront of the laser light L that enters and passes through the modulation pattern is adjusted, and the phase of the component in the direction orthogonal to the traveling direction is shifted in each light beam constituting the laser light L. Therefore, by appropriately setting the modulation pattern to be displayed on the reflective spatial light modulator 203, the laser light L can be modulated (for example, the intensity, amplitude, phase, polarization, etc. of the laser light L can be modulated).

Returning to FIG. 7, the 4f optical system 241 is an adjustment optical system that adjusts the wavefront shape of the laser light L modulated by the reflective spatial light modulator 203. The 4f optical system 241 includes a first lens 241a and a second lens 241b. In the first lens 241a and the second lens 241b, the distance of the optical path between the reflective spatial light modulator 203 and the first lens 241a is the first focal length f1 of the first lens 241a. The distance of the optical path between the second lens 241b is the second focal distance f2 of the second lens 241b, and the distance of the optical path between the first lens 241a and the second lens 241b is the first focal distance f1 and the second focal distance. on the optical path between the reflective spatial light modulator 203 and the condensing optical system 204 so that the first lens 241a and the second lens 241b are both-side telecentric optical systems. Has been placed. According to the 4f optical system 241, the laser beam L modulated by the reflective spatial light modulator 203 can be suppressed from changing its wavefront shape due to spatial propagation and increasing aberration.

The condensing optical system 204 includes a laser beam L emitted from the laser light source 202 and modulated by the reflective spatial light modulator 203, and a measurement beam LB1 emitted from an AF unit 212 described later. Condensed to The condensing optical system 204 is installed on the bottom plate 233 of the housing 231 via a drive unit 232 configured to include a piezoelectric element and the like. The condensing optical system 204 is a condensing lens, and includes a plurality of lenses.

In the laser processing apparatus 300 configured as described above, the laser light L emitted from the laser light source 202 travels in the horizontal direction in the housing 231, is then reflected downward by the mirror 205 a, and is reflected by the attenuator 207. Strength is adjusted. Thereafter, the laser light L is reflected in the horizontal direction by the mirror 205 b, the intensity distribution of the laser light L is made uniform by the beam homogenizer 260, and is incident on the reflective spatial light modulator 203.

The laser light L incident on the reflective spatial light modulator 203 is modulated in accordance with the modulation pattern by transmitting the modulation pattern displayed on the liquid crystal layer 216. Thereafter, the laser light L is reflected upward by the mirror 206a, the polarization direction is changed by the λ / 2 wave plate 228, reflected by the mirror 206b in the horizontal direction, and enters the 4f optical system 241.

The wavefront shape of the laser light L incident on the 4f optical system 241 is adjusted so as to be incident on the condensing optical system 204 as parallel light. Specifically, the laser light L is transmitted and converged through the first lens 241a, reflected downward by the mirror 219, diverges through the condensing point O, and transmits through the second lens 241b to become parallel light. So that it converges again. Then, the laser light L sequentially passes through the dichroic mirrors 210 and 238 and enters the condensing optical system 204, and is condensed by the condensing optical system 204 in the workpiece 1 placed on the stage 111. .

The laser processing apparatus 300 also includes a surface observation unit 211 for observing the laser light incident surface of the workpiece 1 and an AF (AutoFocus) for finely adjusting the distance between the condensing optical system 204 and the workpiece 1. ) Unit 212 and housing 231.

The surface observation unit 211 includes an observation light source 211a that emits visible light VL1, and a detector 211b that receives and detects the reflected light VL2 of the visible light VL1 reflected by the laser light incident surface of the workpiece 1. Have. In the surface observation unit 211, the visible light VL1 emitted from the observation light source 211a is reflected and transmitted by the mirror 208, the half mirror 209, and the dichroic mirrors 210 and 238, and directed toward the workpiece 1 by the condensing optical system 204. Focused. The reflected light VL2 reflected by the laser light incident surface of the workpiece 1 is condensed by the condensing optical system 204, transmitted and reflected by the dichroic mirrors 238 and 210, and then transmitted through the half mirror 209 to be detected. Light is received at 211b.

The AF unit 212 emits the measurement light LB1 and receives and detects the reflected light LB2 of the measurement light LB1 reflected by the laser light incident surface, whereby the displacement of the laser light incident surface along the scheduled cutting line 5 is detected. An error signal (signal related to displacement) that is data is acquired. The AF unit 212 outputs the acquired error signal to the control unit 250 when forming the modified region 7. The control unit 250 drives the drive unit 232 based on the error signal, and reciprocates the condensing optical system 204 in the optical axis direction so as to follow the undulation of the laser light incident surface. The configuration and operation of the AF unit 212 will be described later in detail.

The laser processing apparatus 300 includes a control unit 250 that controls the operation of each unit of the laser processing apparatus 300. The control unit 250 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like.

The controller 250 controls the operation of the laser light source 202 and emits the laser light L from the laser light source 202. The controller 250 controls the operation of the laser light source 202 and adjusts the output, pulse width, and the like of the laser light L emitted from the laser light source 202. When the control unit 250 forms the modified region 7, the condensing point P of the laser light L is located at a predetermined distance from the front surface 3 or the back surface 21 of the workpiece 1 and the condensing point P of the laser light L is cut. At least one of the housing 231, the position of the stage 111, and the drive of the drive unit 232 is controlled so as to move relatively along the scheduled line 5. The control unit 250 has functions of the laser light source control unit 102 and the stage control unit 115.

When forming the modified region 7, the control unit 250 applies a predetermined voltage to each pixel electrode 214 in the reflective spatial light modulator 203 and causes the liquid crystal layer 216 to display a predetermined modulation pattern. Thereby, the control unit 250 modulates the laser light L as desired by the reflective spatial light modulator 203. The modulation pattern displayed on the liquid crystal layer 216 includes, for example, the position where the modified region 7 is to be formed, the wavelength of the laser beam L to be irradiated, the material of the workpiece 1, and the condensing optical system 204 or the workpiece 1. Is derived in advance based on the refractive index and the like and stored in the control unit 250. The modulation pattern includes an individual difference correction pattern for correcting individual differences generated in the laser processing apparatus 300 (for example, distortion generated in the liquid crystal layer 216), a spherical aberration correction pattern for correcting spherical aberration, and the like.

Next, the configuration of the AF unit 212 will be specifically described.

As shown in FIG. 9, the AF unit 212 includes a measurement light source 30, a displacement detection unit 50, and an imaging state adjustment unit 70. The measurement light source 30 emits measurement light LB1. The measurement light source 30 can emit any of a plurality of lights having different wavelengths. The measurement light source 30 includes a plurality of SLD (Super Luminescent Diode) light sources 31 and 32. In the measurement light source 30, the control unit 250 selects one of the plurality of SLD light sources 31 and 32 that emits light having a wavelength with high reflectivity with respect to the workpiece 1. The measurement light source 30 emits light having a wavelength with high reflectivity with respect to the workpiece 1 as measurement light LB1 from one of the selected SLD light sources 31 and 32. The SLD light source 31 emits light having a wavelength of 650 nm, for example. The SLD light source 32 emits light having a wavelength of, for example, 830 nm.

The measurement light source 30 is not limited to the SLD light sources 31 and 32, and may include, for example, an LED (Light Emitting Diode) light source or an LD (Laser Diode) light source. The wavelength of the measurement light LB1 only needs to have a reflectance greater than zero on the back surface 21 that is the laser light incident surface. The measurement light source 30 does not have to be capable of emitting light of a plurality of wavelengths. For example, the measurement light source 30 may include only one of the SLD light sources 31 and 32 and may emit only light of one wavelength.

The measurement light source 30 transmits the emitted measurement light LB1 to the adjustment optical system 60 via a WDM (Wavelength Division Multiplexing) 33 and a single-mode optical fiber 34 used for the synthesis of laser beams having a plurality of wavelengths. Note that the WDM 33 is not required when the measurement light source 30 emits only one wavelength of the measurement light LB1. Instead of the optical fiber 34, a spatial light transmission device may be used. The adjustment optical system 60 includes a plurality of types of lenses and adjusts the measurement light LB1 so as to have an appropriate beam diameter.

The displacement detector 50 detects the displacement of the back surface 21 based on the reflected light LB2 of the measurement light LB1 reflected by the back surface 21 that is the laser light incident surface of the workpiece 1. The displacement detection unit 50 includes a first branching unit 51, a second branching unit (branching unit) 52, first and second astigmatism adding units (astigmatism adding units) 53 and 54, and first and second beam shapes. Detection units (beam shape detection units) 55 and 56 and an error signal generation unit 57 are included.

The first branching unit 51 is a beam splitter that branches the measurement light LB1 and the reflected light LB2. The first branching unit 51 divides the optical paths of the measurement light LB1 and the reflected light LB2 into an optical path of the measurement light LB1 and an optical path of the reflected light LB2. The first branch unit 51 transmits the measurement light LB1 and reflects the reflected light LB2. The first branching unit 51 is provided between the imaging state adjusting unit 70 and the adjusting optical system 60 in the optical paths of the measurement light LB1 and the reflected light LB2.

The second branching unit 52 is a beam splitter that branches the reflected light LB2 branched by the first branching unit 51 into the first branched reflected light LS1 and the second branched reflected light LS2. The second branching unit 52 divides the optical path OP of the reflected light LB2 into a first branched optical path OP1 that is an optical path of the first branched reflected light LS1 and a second branched optical path OP2 that is an optical path of the second branched reflected light LS2. The second branch portion 52 transmits the second branch reflected light LS2, while reflecting the first branch reflected light LS1. The second branch portion 52 is provided downstream of the first branch portion 51 in the optical path OP of the reflected light LB2.

The first astigmatism adding unit 53 is provided downstream of the second branching unit 52 in the first branching optical path OP1. The first astigmatism adding unit 53 adds a first astigmatism amount smaller than the astigmatism amount added by the second astigmatism adding unit 54 to the first branched reflected light LS1. The amount of astigmatism is a measure representing the magnitude of astigmatism, and is defined here as follows. When a parallel beam having the same wavelength as the measurement light LB1 is incident on the astigmatism adding portion, the short axis of the beam width projected onto the plane perpendicular to the optical axis of the beam emitted from the astigmatism adding portion is minimized. The short axis direction on the plane perpendicular to the optical axis is the characteristic axis of the astigmatism adding portion. When the focal length in the characteristic axis direction of the astigmatism adding unit is fL1, and the focal length in the direction perpendicular to the characteristic axis is fL2, fL2 / fL1 is the amount of astigmatism. The first astigmatism adding unit 53 is configured by a combination of a convex lens 53a and a cylindrical lens 53b. For example, the focal length of the convex lens 53a is 40 mm, and the focal length of the cylindrical lens 53b is 100 mm.

The second astigmatism adding unit 54 is provided via the mirror 58 downstream of the second branching unit 52 in the second branching optical path OP2. The second astigmatism adding unit 54 adds an astigmatism amount different from the first astigmatism amount to the second branched reflected light LS2. The second astigmatism adding unit 54 adds a second astigmatism amount larger than the first astigmatism amount to the second branched reflected light LS2. The second astigmatism adding unit 54 is configured by a combination of a convex lens 54a and a cylindrical lens 54b. For example, the focal length of the convex lens 53a is 75 mm, and the focal length of the cylindrical lens 53b is 75 mm.

The first beam shape detection unit 55 is provided in the first branch optical path OP1. The first beam shape detection unit 55 receives the first branched reflected light LS1 to which the first astigmatism amount is added via the filter 59a, and detects the beam shape of the first branched reflected light LS1. The second beam shape detection unit 56 is provided in the second branch optical path OP2. The second beam shape detection unit 56 receives the second branched reflected light LS2 to which the second astigmatism amount is added via the filter 59b, and detects the beam shape of the second branched reflected light LS2.

The filter 59a attenuates the light having the wavelength of the laser light L in the first branch reflected light LS1. The filter 59 a prevents light having the wavelength of the laser light L from entering the first beam shape detection unit 55. The filter 59b attenuates the light having the wavelength of the laser light L in the second branched reflected light LS2. The filter 59 b prevents light having the wavelength of the laser light L from entering the second beam shape detection unit 56.

As the first beam shape detector 55 and the second beam shape detector 56, a four-quadrant detector can be used. The first beam shape detection unit 55 and the second beam shape detection unit 56 output the detection results to the error signal generation unit 57. Specifically, each of the first beam shape detection unit 55 and the second beam shape detection unit 56 divides and receives the beam shape formed on the light receiving surface, and outputs an output value (voltage) corresponding to each light amount. Value) to the error signal generator 57. The first beam shape detection unit 55 is not particularly limited as long as the beam shape can be detected. For example, a two-dimensional PD (Photo Diode) array may be used.

The error signal generator 57 receives outputs from the first beam shape detector 55 and the second beam shape detector 56 and generates an error signal. Specifically, the error signal generation unit 57 is one of the first and second beam shape detection units 55 and 56 provided on one of the first and second branch optical paths OP1 and OP2 selected by the control unit 250. An error signal is generated based on the detection result. When the first branch optical path OP1 is selected by the control unit 250, the error signal generation unit 57 generates an error signal using the detection result of the first beam shape detection unit 55. The error signal generation unit 57 generates an error signal using the detection result of the second beam shape detection unit 56 when the control unit 250 selects the second branch optical path OP2. The error signal generator 57 outputs the generated error signal to the controller 250.

Here, the error signal and its acquisition principle will be specifically described below.

The AF unit 212 passes the displacement (relative displacement) of the back surface 21 that is the laser light incident surface of the workpiece 1 using the Through the Lens method, that is, the condensing optical system 204 that condenses the laser light L. Measurement is performed using the measurement light LB1. The AF unit 212 measures the displacement of the back surface 21 using astigmatism. In the AF unit 212, the distance of the optical system changes due to a change in the relative displacement between the condensing optical system 204 and the back surface 21, and the position of the image point of the reflected light LB2 of the measurement light LB1 moves through the optical system. Take advantage of what you do.

In the AF unit 212, the beam shape of the reflected light LB2 changes on the beam shape detectors 55 and 56 such as a four-quadrant detector due to the displacement of the back surface 21 from a reference position described later on the workpiece 1. Specifically, the reflected light LB2 reflected by the back surface 21 has different beam divergence angles depending on the displacement of the back surface 21, and different beams on the beam shape detection units 55 and 56 according to the beam divergence angle. It becomes a shape. For example, as shown in FIG. 10, the beam shape H of the reflected light LB2 is a vertically long ellipse (see FIG. 10A), a perfect circle (see FIG. 10B), and a horizontally long ellipse (see FIG. 10C). And change between. In the AF unit 212, the beam shape changing in this manner is detected by being divided into light receiving surfaces S _A , S _B , S _C , and S _D in the beam shape detectors 55 and 56. Then, the AF unit 212 generates an error signal by the calculation of the following equation (1) based on the beam shape detection result.
Error signal = [(I _A + I _C ) − (I _B + I _D )] / [(I _A + I _B + I _C + I _D )]
... (1)
However,
I _A : a signal value output based on the amount of light on the light receiving surface S _A ,
I _B: signal value outputted based on the amount of light at the light receiving surface S _B,
I _C : a signal value output based on the amount of light on the light receiving surface S _C ,
_ID : A signal value output based on the amount of light on the light receiving surface _SD .

FIG. 11 is a graph showing an example of the error signal. In the graph shown in FIG. 11, the horizontal axis indicates the displacement of the laser light incident surface from the position where the error signal becomes zero, and the vertical axis indicates the magnitude of the error signal. The smaller the displacement is (the closer to the left in the figure), the closer the laser light incident surface is to the condensing optical system 204. The larger the displacement is (the more it goes to the right side in the figure), the more the laser light incident surface is positioned away from the condensing optical system 204.

As shown in FIG. 11, the error signal changes in an S-curve shape on the graph. The displacement when the error signal becomes zero is the displacement when the beam shape becomes a perfect circle on the beam shape detectors 55 and 56. The range that can be used in the error signal is a range that monotonously decreases around zero (hereinafter, this range is referred to as a “measurement range”). The length measurement range of the present embodiment is at least ± 10 μm in consideration of practicality from the variation in displacement of the back surface 21 at the laser processing start position due to warpage of the workpiece 1. The length measurement range of this embodiment is ± 20 μm or more.

Referring back to FIG. 9, the imaging state adjustment unit 70 moves the imaging state of the measurement light LB1 and the reflected light LB2. The imaging state adjusting unit 70 is provided between the first branching unit 51 and the dichroic mirror 238 in the optical paths of the measurement light LB1 and the reflected light LB2. The imaging state adjustment unit 70 includes a concave lens 71 and a convex lens 72. The imaging state adjustment unit 70 changes the distance between the concave lens 71 and the convex lens 72 based on a command from the control unit 250 and moves the imaging state. Thereby, the imaging state adjustment unit 70 adjusts the offset amount. The movement of the imaging state includes copying any set of imaging position relationships on the optical path to another imaging position relationship set (that is, moving the imaging position).

The reference position is the depth position of the laser light incident surface set at the time of reference position described later (step S5). Specifically, the reference position is the position of the back surface 21 when the back surface 21 is imaged by the front surface observation unit 211 and the contrast of the projected reticle is maximized.

When the workpiece 1 is a silicon substrate having a maximum thickness of 775 μm, the imaging state adjusting unit 70 moves the imaging state of the measurement light LB1 and the reflected light LB2 to move from 0 μm to −180 μm. Vary the offset amount in the range.

When the processing object 1 is a silicon substrate having a thickness of 775 μm and the modified region 7 is formed at a shallow position from the back surface 21 in the processing object 1, the convergence power of the imaging state adjustment unit 70 is weakened, and the offset amount Is a value close to 0 μm to 0 μm, and the distance between the condensing optical system 204 and the back surface 21 is a long distance. On the other hand, when the processing object 1 is a silicon substrate having a thickness of 775 μm and the modified region 7 is formed at a deep position from the back surface 21 in the processing object 1, the convergence power of the imaging state adjusting unit 70 is The offset is set to a value close to −180 μm to −180 μm, and the distance between the condensing optical system 204 and the surface 3 is set to a short distance.

The control unit 250 sets an offset amount based on a command from a host system such as a host controller. The control unit 250 controls the imaging state adjustment unit 70 so that the set offset amount is obtained. Specifically, the control unit 250 stores in advance a data table relating to the position of the concave lens 71 determined for each offset amount. The control unit 250 obtains the position of the concave lens 71 having the set offset amount by referring to the data table, and outputs a command to move the concave lens 71 to the obtained position of the concave lens 71 to the imaging state adjustment unit 70. .

The control unit 250 selects one of the first branching optical path OP1 and the second branching optical path OP2 according to the imaging state that is moved by the imaging state adjusting unit 70. Specifically, the control unit 250 selects one of the first branch optical path OP1 and the second branch optical path OP2 according to the offset amount adjusted by the imaging state adjustment unit 70. More specifically, when the set offset amount is in the first range, the control unit 250 selects the first branched optical path OP1 that is the optical path of the first branched reflected light LS1. When the set offset amount is in the second range, the second branch optical path OP2 that is the optical path of the second branch reflected light LS2 is selected. The second range is a range deeper than the first range. The first range is a range of 0 μm or less and greater than −40 μm. The second range is a range of −40 μm or less and −180 μm or more. The control unit 250 outputs an instruction related to the selection result of the first branch optical path OP1 and the second branch optical path OP2 to the error signal generation unit 57. The controller 250 operates the drive unit 232 so that the error signal generated by the error signal generator 57 maintains the target value (here, zero).

The AF unit 212 further includes a first steering mirror 81 and a second steering mirror 82. The first and second steering mirrors 81 and 82 are disposed between the imaging state adjusting unit 70 and the dichroic mirror 238 in the optical paths of the measurement light LB1 and the reflected light LB2. The first and second steering mirrors 81 and 82 align (coalign) the optical axis of the measurement light LB1 with the optical axis of the laser light L. The first and second steering mirrors 81 and 82 constitute an optical axis adjustment mechanism.

Next, a laser processing method performed in the laser processing apparatus 300 will be described.

The laser processing method of the present embodiment is used as a chip manufacturing method for manufacturing a plurality of chips by laser processing the workpiece 1. The workpiece 1 has a plate shape. The workpiece 1 is, for example, a sapphire substrate, a SiC substrate, a glass substrate (tempered glass substrate), a silicon substrate, a semiconductor substrate, a transparent insulating substrate, or the like. The processing object 1 here is a silicon substrate. A functional element layer is formed on the processing object 1 on the surface 3 side opposite to the back surface 21 side which is the laser light incident surface side. The functional element layer includes a plurality of functional elements (for example, a light receiving element such as a photodiode, a light emitting element such as a laser diode, or a circuit element formed as a circuit) arranged in a matrix. The back surface 21 side of the workpiece 1 is ground so that the workpiece 1 is thinned to a desired thickness. A plurality of cutting lines 5 that extend so as to pass between adjacent functional elements are set in the workpiece 1. The plurality of scheduled cutting lines 5 extend in a lattice shape.

In the laser processing method of the present embodiment, first, the processing object 1 is placed on the support base 107 of the stage 111 so that the back surface 21 becomes the laser light incident surface. Laser light L is emitted from the laser light source 202, and the laser light L is condensed inside the workpiece 1 by the condensing optical system 204. At the same time, the movement of the stage 111 is controlled by the control unit 250, and the laser light L is moved (scanned) relatively in the processing progress direction along the planned cutting line 5 to be modified inside the processing target 1. The region 7 is formed along the planned cutting line 5. Thereafter, the expanding tape attached to the front surface 3 or the back surface 21 of the processing object 1 is expanded to cut the processing object 1 and cut the processing object 1 into a plurality of chips.

Here, the back surface 21 of the workpiece 1 has warping or waviness due to the influence of stress or the like resulting from the formation of the functional element layer on the front surface 3. Therefore, in order to focus the laser beam L and stably form the modified region 7 at the intended depth, control is performed to keep the relative displacement between the condensing optical system 204 and the back surface 21 at the intended displacement. There is a need.

Therefore, in the laser processing method of the present embodiment, the measurement light LB1 is focused on the processing target 1 while condensing the laser light L on the processing target 1. The first branched reflected light LS1 obtained by branching the reflected light LB2 of the measurement light LB1 reflected by the back surface 21 into the first and second branched reflected lights LS1 and LS2 and adding the first astigmatism amount in the first branched optical path OP1. And a beam shape of the second branched reflected light LS2 to which the second astigmatism amount is added in the second branch optical path OP2. An error signal is acquired based on the detection result of the beam shape, and the condensing optical system 204 is operated in the Z direction by the drive unit 232 so that the error signal maintains a target value. Specifically, the following steps are executed.

That is, as shown in FIG. 12, the control unit 250 sets an offset amount based on a command from the host system (step S1). Based on the command from the host system, the control unit 250 selects one of the SLD light sources 31 and 32 of the measurement light source 30 that emits light having a wavelength with high reflectivity with respect to the workpiece 1 (step S2). ).

Control unit 250 selects branch optical paths OP1 and OP2 that generate error signals based on the set offset amount (step S3). In step S3, when the set offset amount is in the first range (−40 μm <offset amount ≦ 0 μm), the first branch optical path which is an optical path for adding the first astigmatism amount to the first branch reflected light LS1. Select OP1. When the set offset amount is in the second range (−180 μm ≦ offset amount ≦ −40 μm), an optical path for adding a second astigmatism amount larger than the first astigmatism amount to the second branched reflected light LS2. The second branch optical path OP2 is selected.

The image forming state adjusting unit 70 is controlled by the control unit 250 so that the set offset amount is obtained, and the image forming states of the measurement light LB1 and the reflected light LB2 are moved (step S4). In step S4, the position of the concave lens 71 corresponding to the set offset amount is derived with reference to the data table, and the concave lens 71 is moved to this position.

The reference positioning is performed to position the workpiece 1 at the reference position (step S5). In step S5, the back surface 21 which is the laser light incident surface is imaged by the front surface observation unit 211, and the stage is positioned by the control unit 250 so that the back surface 21 is positioned at a depth where the contrast of the projected reticle is maximized. 111 is moved in the Z direction. When the wavelength of the light that projects the reticle and the wavelength of the measurement light LB1 are equal, the magnitude of the error signal at this stage is zero when the offset amount is 0 μm. On the other hand, when the wavelength of the light that projects the reticle is different from the wavelength of the measurement light LB1, the magnitude of the error signal at this stage is the same as that of the reticle projection light of the condensing optical system 204 when the offset amount is 0 μm. It becomes a value corresponding to the magnitude of chromatic aberration with respect to the light beam LB1. The reference position and the offset amount are associated in this way. Thereafter, the control unit 250 moves the stage 111 so that the set offset is obtained, and causes the processing target object 1 to approach the condensing optical system 204 (step S6).

The target value of the error signal is acquired and stored in the control unit 250 (step S7). In step S7, the measurement light LB1 is emitted from one of the SLD light sources 31 and 32 of the measurement light source 30 selected in step S2. The measurement light LB1 has its beam diameter adjusted by the adjusting optical system 60, passes through the first branching unit 51, and after the image forming state is adjusted by the image forming state adjusting unit 70, the first and second steering mirrors 81 , 82 and the dichroic mirror 238 are sequentially reflected, condensed on the workpiece 1 by the condensing optical system 204, and reflected by the back surface 21.

The reflected light LB2 reflected by the back surface 21 passes through the condensing optical system 204, is sequentially reflected by the dichroic mirror 238, the second and first steering mirrors 82, 81, and the imaging state is adjusted by the imaging state adjustment unit 70. After being reflected by the first branch part 51, the second branch part 52 is branched into the first and second branch reflected lights LS 1 and LS 2. After the first astigmatism amount is added to the first branched reflected light LS1 by the first astigmatism adding unit 53 in the first branch optical path OP1, the first beam shape detecting unit 55 passes through the filter 59a. Received light. After the second astigmatism amount is added to the second branched reflected light LS2 by the second astigmatism adding unit 54 in the second branch optical path OP2, the second beam shape detecting unit 56 passes through the filter 59b. Received light. When the first branch optical path OP1 is selected by the control unit 250 in step S3, the error signal generation unit 57 generates an error signal corresponding to the beam shape detected by the first beam shape detection unit 55 using the above equation (1). Generate according to On the other hand, when the second branch optical path OP2 is selected by the control unit 250 in step S3, an error signal corresponding to the beam shape detected by the second beam shape detection unit 56 is generated according to the above equation (1). The generated error signal is stored in the control unit 205 as a target value.

Subsequently, laser processing is started (step S8). In step S8, while scanning the laser beam L along the planned cutting line 5, an error signal is acquired in the same manner as in step S7, and the drive unit 232 collects the error signal so that the acquired error signal maintains the target value. The optical system 204 is operated in the Z direction. Thereby, along with the scanning of the laser beam L, feedback control is performed in which the relative displacement between the condensing optical system 204 and the back surface 21 is kept constant, and the condensing optical system 204 follows the displacement of the back surface 21. . Thereafter, it is determined whether or not laser processing along all the scheduled cutting lines 5 has been completed (step S9). In the case of No in step S9, the above steps S1 to S9 are repeatedly performed along the scheduled cutting line 5 where the laser processing is not completed, whereas in the case of Yes in step S9, the laser processing ends.

FIG. 13 is a graph showing an error signal generated based only on the beam shape detected by the first beam shape detection unit 55. In the figure, each error signal when the offset amount is changed from 0 μm to −180 μm in increments of 10 μm to 20 μm is shown. As shown in FIG. 13, the slope of the error signal has a correlation with the offset amount (that is, the imaging state moved by the imaging state adjustment unit 70). In addition, it can be seen that as the offset becomes deeper in the workpiece 1, the slope of the error signal becomes too gentle. Note that the slope of the error signal is the fluctuation of the error signal with respect to the acquired displacement. The slope of the error signal is the rate of change of the error signal related to displacement. The slope of the error signal corresponds to the proportionality constant when the error signal monotonously decreases proportionally. The slope of the error signal corresponds to the amount of fluctuation when the error signal fluctuates with a change in displacement.

Here, a mismatch between the amount of astigmatism added to the reflected light LB2 of the measurement light LB1 and the amount of offset is found as a factor in which the inclination of the error signal becomes too gentle. Therefore, in the laser processing apparatus 300, one of the first and second branch optical paths OP1 and OP2 is selected according to the offset amount, and the beam shape detected in one of the selected first and second branch optical paths OP1 and OP2 is obtained. Based on this, an error signal is generated. As a result, the amount of astigmatism added to one of the first and second branched reflected lights LS1 and LS2 used for generating the error signal can be made in accordance with the offset amount. As a result, it is possible to suppress the inclination of the error signal from becoming too gentle. Therefore, it is possible to accurately detect the displacement of the back surface 21 that is the laser light incident surface.

In particular, since the back surface 21 is ground until the workpiece 1 is thinned to a desired thickness, the back surface 21 is in a state in which grinding marks are formed (a state in which many grooves having extremely shallow depths are formed). ). In this case, even if the measurement light LB1 is scattered on the back surface 21 and the displacement is the same, the error signal may vary. Therefore, if the inclination of the error signal becomes too gentle, there may be a problem in practicality. Therefore, when the grinding mark is formed on the back surface 21 as described above, the above-described effect of suppressing the inclination of the error signal from becoming too gentle is remarkable.

In the laser processing apparatus 300, when the offset amount is in the first range, the first branch optical path OP1 is selected, and when the offset amount is in the second range deeper than the first range, the second branch optical path OP2 is selected. Select. In this case, when the offset is shallow, the first branched reflected light LS1 to which a small amount of astigmatism is added is used to generate an error signal. When the offset is deep, the first astigmatism having a large amount of astigmatism is added. The bifurcated reflected light LS2 can be used for generating an error signal. It becomes possible to suppress the inclination of the error signal from becoming too gentle and to detect the displacement of the back surface 21 with high accuracy.

FIG. 14 is a graph showing an error signal generated in the laser processing apparatus 300. In the figure, each error signal when the offset amount is changed from 0 μm to −180 μm in increments of 10 μm to 20 μm is shown. In each item (series) name in the figure, when the error signal is based on the beam shape of the first branch optical path OP1, “OP1” is added, and when the error signal is based on the beam shape of the second branch optical path OP2. “OP2” is attached. As shown in FIG. 14, according to the laser processing apparatus 300, it can be seen that the inclination of the error signal can be suppressed from becoming too gentle. For example, in the laser processing apparatus 300, the error signal has a certain inclination or more so that the measurement error due to the grinding mark is within the practical range. For example, the error signal may have an absolute value of a slope of 0.025 / μm or more at a displacement at which the error signal becomes zero. For example, the error signal may have an absolute value of a slope of 0.0275 / μm or more at a displacement at which the error signal becomes zero.

When the offset amount is −40 μm, the slope is constant regardless of whether the error signal is based on the beam shape of the first branch optical path OP1 or the error signal based on the beam shape of the second branch optical path OP2. It turns out that it becomes the above. Therefore, in this embodiment, the first range is 0 μm or less and larger than −40 μm, and the second range is −40 μm or less and −180 μm or more, but the first range is 0 μm or less and −40 μm or more. The second range may be less than −40 μm and greater than −180 μm.

FIG. 15 is a graph showing an error signal generated based only on the beam shape detected by the second beam shape detection unit 56. In the figure, each error signal when the offset amount is changed from 0 μm to −180 μm in increments of 10 μm to 20 μm is shown. According to the results shown in FIG. 15, it can be seen that the shallower the offset is in the workpiece 1, the steep error signal becomes steep and the measurement range becomes insufficient. On the other hand, in the laser processing apparatus 300, it is possible to suppress the error signal from becoming too steep, and to ensure a sufficient length measurement range (see FIG. 14).

In the laser processing apparatus 300, the offset amount is set by the control unit 250, and the imaging state adjusting unit 70 is controlled so as to be the set offset amount. According to this configuration, the imaging states of the measurement light LB1 and the reflected light LB2 can be automatically adjusted so that the set offset amount is obtained.

The laser processing apparatus 300 includes a drive unit 232 that operates the condensing optical system 204 in the Z direction, and the drive unit 232 is operated by the control unit 205 so that the error signal maintains a target value. According to this configuration, the condensing optical system 204 can be moved in the Z direction so as to follow the back surface 21.

The laser processing apparatus 300 includes first and second steering mirrors 81 and 82 for aligning the optical axis of the measurement light LB1 with the optical axis of the laser light L. According to this configuration, the optical axis of the measurement light LB1 can be accurately aligned with the optical axis of the laser light L.

In the laser processing apparatus 300, the measurement light source 30 emits, as the measurement light LB1, light having a wavelength with a high reflectivity with respect to the workpiece 1 among the light having a plurality of wavelengths. As a result, the measurement light LB1 can be easily reflected by the back surface 21.

Incidentally, the inclination of the error signal can be reduced by shortening the physical distance between the imaging state adjusting unit 70 and the condensing optical system 204, or by inserting a 4f lens system therebetween to shorten the optical distance. It is also possible to suppress the change. However, it is difficult to shorten the physical distance due to restrictions on the device configuration, and inserting the 4f lens system also leads to an increase in the size of the device, which may be difficult to realize. In particular, when the first and second steering mirrors 81 and 82 are arranged, it is difficult to shorten the physical distance. In this respect, the laser processing apparatus 300 is less likely to be restricted in terms of the apparatus configuration, and can suppress an increase in the size of the apparatus. In the laser processing apparatus 300, the first and second steering mirrors 81 and 82 can be disposed.

FIG. 16 is a configuration diagram showing a part of an AF unit 212B according to a modification. As shown in FIG. 16, each of the first and second beam shape detection units 55 and 56 of the AF unit 212B moves in accordance with the movement of the concave lens 71 of the imaging state adjustment unit 70. , OP2 may be movable along each. Specifically, the first and second beam shape detection units 55 and 56 are moved away from the first and second astigmatism addition units 53 and 54 as the concave lens 71 is moved so that the offset is deepened by the control unit 250. The first and second beam shape detectors 55 and 56 may be moved closer to the first and second astigmatism adding units 53 and 54 as the offset becomes deeper.

FIG. 17 is a graph for explaining the effect of the AF unit 212B of FIG. FIG. 17 shows an error signal generated based on the beam shape detected by the first beam shape detection unit 55. FIG. 17A shows an error signal when the first beam shape detection unit 55 is fixed. FIG. 17B shows an error signal generated by the AF unit 212B, that is, an error signal when the first beam shape detection unit 55 is movable. As shown in FIGS. 17A and 17B, in the AF unit 212B according to the modified example, the S-curve of the error signal is leveled around zero on the horizontal axis for any offset amount. The shape can be improved. This contributes to responsiveness improvement such as PID control.

In the AF unit 212B, instead of or in addition to the movement of the first and second beam shape detectors 55 and 56, the convex lenses 53a and 54a and the cylindrical lenses 53b and 54b of the first and second astigmatism adding units 53 and 54 are used. At least one of these may be moved similarly. Even in this case, the same effect is produced.

As mentioned above, although embodiment was described, this invention is not restricted to the said embodiment, You may change in the range which does not change the summary described in each claim, or may apply it to another thing.

In the above embodiment, the optical path OP of the reflected light LB2 is branched into two optical paths (first and second branched optical paths OP1 and OP2) by the second branching unit 52, but may be branched into three or more optical paths. In each of the three or more optical paths, a plurality of astigmatism adding sections for adding different amounts of astigmatism and a plurality of beam shapes for detecting the beam shapes of the plurality of branched reflected lights to which astigmatism is added. And a beam shape detecting unit of the above-mentioned need only be provided. In this case, an optical path is selected from a plurality of optical paths so that a larger astigmatism amount is added to the branched reflected light as the offset is deeper, and based on the detection result of the beam shape of the branched reflected light in the selected optical path. An error signal may be generated.

In the embodiment described above, the imaging state adjustment unit 70 is disposed between the first branching unit 51 and the dichroic mirror 238 in the optical paths of the measurement light LB1 and the reflected light LB2. It is not limited. Instead of or in addition to the arrangement of the above embodiment, the first branch 51 and the second branch 52 are arranged upstream of the first branch 51 in the optical path of the measurement light LB1 and in the optical path OP of the reflected light LB2. The imaging state adjustment unit 70 may be disposed at least at any point between the two.

In the above embodiment, the imaging state adjustment unit 70 is configured by the concave lens 71 and the convex lens 72, but the imaging state adjustment unit 70 is not particularly limited, and may be a variable focal length lens, for example. The optical system of the above embodiment includes the dichroic mirror 238 that transmits the laser light L and reflects the measurement light LB1 and the reflected light LB2. Instead, the optical system reflects the laser light L and reflects the measurement light. A configuration including a dichroic mirror that transmits LB1 and reflected light LB2 may also be used. Similarly, the optical system of the above embodiment may be configured to reflect the measurement light LB1 and transmit the reflected light LB2 at the first branching unit 51. Similarly, the optical system of the above embodiment may be configured to transmit the first branched reflected light LS1 and reflect the second branched reflected light LS2 in the second branch portion 52.

In the above embodiment, the bias offset values of the first and second beam shape detectors 55 and 56 (the first and second beam shapes in a state in which the beam shape is not detected) between the step S6 and the step S7. The output values of the detection units 55 and 56) may be acquired and adjusted. In the above embodiment, the offset amount is set as an optical arrangement in which the error signal becomes zero. However, the present invention is not limited to the case where the error signal becomes zero, and the offset amount may be set as an optical arrangement in which the error signal becomes a reference value. Good.

In the above embodiment, the offset amount is set based on a command from the host system. However, the offset amount may be set by an operator's operation, or the offset amount is set in advance according to the position of the reforming region 7 to be formed. It may be. In the above embodiment, the imaging state adjustment unit 70 is controlled by the control unit 250 so that the set offset amount is obtained, but the imaging state adjustment unit 70 may be controlled by an operator's operation.

The above embodiment includes the reflective spatial light modulator 203 as the spatial light modulator. However, the spatial light modulator is not limited to the reflective type, and may include a transmissive spatial light modulator. . In the above embodiment, the back surface 21 of the workpiece 1 is a laser light incident surface, but the front surface 3 of the workpiece 1 may be a laser light incident surface. In the above, the control unit 250 and the error signal generation unit 57 constitute a signal acquisition unit. The control unit 250 configures an offset amount setting unit, an imaging state control unit, and a drive mechanism control unit.

DESCRIPTION OF SYMBOLS 1 ... Work object, 3 ... Front surface, 7 ... Modified area | region, 21 ... Back surface, 30 ... Light source for measurement, 50 ... Displacement detection part, 52 ... 2nd branch part (branch part), 53 ... 1st astigmatism Addition unit (astigmatism addition unit), 54 ... second astigmatism addition unit (astigmatism addition unit), 55 ... first beam shape detection unit (beam shape detection unit), 56 ... second beam shape detection unit (Beam shape detection unit), 57 ... error signal generation unit (signal acquisition unit), 70 ... imaging state adjustment unit, 81 ... first steering mirror (optical axis adjustment mechanism), 82 ... second steering mirror (optical axis adjustment) Mechanism), 100, 300 ... laser processing apparatus, 204 ... condensing optical system (condensing lens), 232 ... drive unit (drive mechanism), 250 ... control unit (signal acquisition unit, offset amount setting unit, imaging state) Control unit, drive mechanism control unit), L ... laser beam (laser beam for processing) LB1 ... measurement light, LB2 ... reflected light, LS1 ... first branched reflected light (branched reflected light), LS2 ... second branched reflected light (branched reflected light), OP1 ... first branched optical path (first branched reflected light Optical path), OP2... Second branched optical path (optical path of the second branched reflected light).

Claims (8)

1. A laser processing apparatus that forms a modified region on the processing object by condensing the processing laser beam on the processing object,
   A measurement light source that emits measurement light; and
   A condensing lens that condenses the processing laser light and the measurement light on the object to be processed;
   A displacement detector that detects the displacement of the laser light incident surface based on the reflected light of the measurement light reflected by the laser light incident surface of the workpiece;
   An imaging state adjustment unit that moves an imaging state of at least one of the measurement light and reflected light of the measurement light, and
   The displacement detector is
   A branching section for branching the reflected light of the measurement light into a plurality of branched reflected lights;
   A plurality of astigmatism adding units that are provided in each of the plurality of branched reflected light paths and add astigmatism amounts of different sizes to each of the plurality of branched reflected lights;
   A plurality of beam shape detection units that are provided in each of the optical paths of the plurality of branch reflected lights and detect the beam shape of each of the plurality of branch reflected lights to which astigmatism is added;
   One of the plurality of branched reflected light paths is selected according to the imaging state to be adjusted by the imaging state adjusting unit, and the detection result of the beam shape detection unit in the selected optical path of the branched reflected light And a signal acquisition unit that acquires a signal related to the displacement based on the laser processing apparatus.
2. The imaging state adjustment unit adjusts the offset amount by moving the imaging state,
   The laser processing apparatus according to claim 1, wherein the signal acquisition unit selects one of a plurality of optical paths of the branched reflected light according to the offset amount adjusted by the imaging state adjustment unit.
3. The branching unit branches the reflected light of the measurement light into at least a first branched reflected light and a second branched reflected light,
   The astigmatism adding unit is
   A first astigmatism adding unit that is provided in the optical path of the first branched reflected light and adds a first astigmatism amount to the first branched reflected light;
   A second astigmatism adding unit that is provided in the optical path of the second branched reflected light and adds a second astigmatism amount larger than the first astigmatism amount to the second branched reflected light; ,
   The signal acquisition unit
   When the offset amount to be adjusted by the imaging state adjustment unit is in the first range, the optical path of the first branched reflected light is selected,
   3. The laser processing apparatus according to claim 2, wherein an optical path of the second branched reflected light is selected when the offset amount adjusted by the imaging state adjusting unit is in a second range deeper than the first range. .
4. An offset amount setting unit for setting the offset amount to be adjusted by the imaging state adjusting unit;
   The laser processing apparatus according to claim 2, further comprising: an imaging state control unit that controls the imaging state adjustment unit so as to be the offset amount set by the offset amount setting unit.
5. A drive mechanism for operating at least one of the object to be processed and the condensing lens along the optical axis direction of the condensing lens;
   The laser processing apparatus according to any one of claims 1 to 4, further comprising: a drive mechanism control unit that operates the drive mechanism so that the signal acquired by the signal acquisition unit maintains a target value.
6. The laser processing apparatus according to any one of claims 1 to 5, further comprising an optical axis adjustment mechanism that aligns the optical axis of the measurement light with the optical axis of the processing laser light.
7. The measurement light source can emit any of a plurality of lights having different wavelengths, and the light having the highest reflectivity with respect to the object to be processed is selected from the plurality of wavelengths. 6. The laser processing apparatus according to claim 1, wherein the laser processing apparatus emits as
8. A laser processing method for forming a modified region in a processing object by condensing a processing laser beam on the processing object,
   While condensing the processing laser light on the processing object with a condensing lens, the measurement light is condensed on the processing object with the condensing lens, and the laser light incident surface of the processing object is The reflected light of the reflected measurement light is branched into at least a first branched reflected light and a second branched reflected light, and a first astigmatism amount is added in the optical path of the first branched reflected light. Detecting a beam shape of the second branched reflected light to which a second astigmatism amount larger than the first astigmatism amount is added in the optical path of the second branched reflected light, and detecting the beam shape of the second branched reflected light; A signal related to the displacement of the laser light incident surface is acquired based on the detection result of the beam shape, and the workpiece and the processing object along the optical axis direction of the condensing lens so that the acquired signal maintains a target value. At least one of the condensing lenses Comprising a laser processing step of operation,
   The laser processing step includes
   A first step of setting an offset amount;
   When the offset amount set in the first step is in the first range, the optical path of the first branched reflected light is selected, and the offset amount set in the first step is a second deeper than the first range. A second step of selecting an optical path of the second branched reflected light in the case of a range;
   A third step of moving an imaging state of at least one of the measurement light and the reflected light of the measurement light so as to be the offset amount set in the first step;
   A fourth step of operating at least one of the object to be processed and the condensing lens so as to be the offset amount set in the first step;
   A fifth step for obtaining the target value after the third step and the fourth step;
   After the fifth step, the beam shape is detected in the optical path of the branched reflected light selected in the second step while condensing the processing laser light on the processing object with the condensing lens, The signal is acquired based on the detection result of the beam shape, and the object to be processed and the condensing lens along the optical axis direction of the condensing lens so that the acquired signal maintains the target value. And a sixth step of operating at least one of the laser processing method.

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