Laser beam machining device and laser beam machining method

【0001】

[Technical field to which the invention belongs]

The present invention relates to a laser processing apparatus and a laser processing method for forming a modified region by multiphoton absorption inside a wafer-shaped object to be processed by irradiating with a laser beam.

【0002】

[Conventional technology]

The following Patent Document 1 can be exemplified as a document that discloses this kind of technology in the past. In the specification of Patent Document 1, the following technology is described as a ninth example. That is, after aligning the condensing point near the back surface of the inside of the object to be processed and irradiating the laser beam to form a modified region by multiphoton absorption along the planned cutting line, the area near the inside surface of the object to be processed The laser beam is irradiated at the same focusing point, and a modified region by multiphoton absorption is further formed along the above-mentioned scheduled cutting line.

【0003】

In this way, by forming a plurality of modified regions inside the work target for one planned cutting line, even when the thickness of the work target is larger than the modified region, Compared to the one in which only one modified region is formed, it is possible to cut the workpiece with high accuracy along the planned cutting line with a small force.

【0004】

In addition, Patent Document 2 below describes a technique of irradiating a laser beam by locating a condensing point near the front surface and the back surface of the object to be processed to melt the object to be processed. The technique described in Patent Document 2 is for reducing the variation in energy density in the thickness direction of the object to be processed and reliably heating and melting the object to be processed.

【0005】

[Patent Document 1]

International Publication No. 02/22301 Pamphlet [Patent Document 2]

Japanese Patent No. 2664625 [0006]

[Problems to be Solved by the Invention]

As described above, since the technique described in Patent Document 1 is an extremely effective technique when the thickness of the object to be processed is large with respect to the modified region, a plurality of lines are used for one planned cutting line. A technique for forming a modified region more efficiently has been desired.

【0007】

Therefore, the present invention has been made in view of such circumstances, and laser machining is capable of efficiently forming a plurality of modified regions inside an object to be machined for one scheduled cutting line. It is an object of the present invention to provide an apparatus and a laser processing method.

【0008】

[Means for solving problems]

In order to achieve the above object, the laser processing apparatus according to the present invention is a laser processing apparatus that forms a modified region by absorption of multiple photons inside a wafer-shaped object to be processed, and is the first laser processing apparatus having different spread angles. The laser beam and the second laser beam are focused inside the object to be processed, and multiphoton absorption is performed at the position of the focusing point of the first laser beam and the position of the focusing point of the second laser beam. It is provided with a condensing lens to be generated and a moving means for relatively moving the condensing point of the first laser light and the condensing point of the second laser light along the planned cutting line of the object to be processed. It is a feature.

【0009】

In this laser processing apparatus, the first laser beam and the second laser beam are condensed inside the object to be processed by the condenser lens. At this time, since the first laser beam and the second laser beam have different spread angles, the first laser beam and the second laser beam are deep from the surface of the object to be processed on the condenser lens side. Will be focused on different positions. Then, since the first laser beam condensing point and the second laser beam condensing point are relatively moved along the scheduled cutting line by the moving means, cutting is performed according to this laser processing apparatus. Multiple modified regions can be formed in one scan along the scheduled line, and thus multiple modified regions can be efficiently formed for one scheduled cutting line. .. Note that "for one scheduled cutting line" does not mean that only one scheduled cutting line is set for the machining target, and a plurality of scheduled cutting lines are set for the machining target. Of course, this is included.

【0010】

Further, the condenser lens condenses the measurement laser beam for measuring the displacement of the laser beam irradiation surface of the object to be processed toward the object to be processed on the same axis as the first and second laser beams. By detecting the reflected light of the measurement laser light reflected on the laser light irradiation surface, the depth of the first laser light focusing point and the second laser light focusing point from the laser light irradiation surface is detected. It is preferable to provide a depth control means for controlling the light.

【0011】

According to this configuration, the measurement laser beam for measuring the displacement of the laser beam irradiation surface of the workpiece is directed toward the workpiece by the condenser lens on the same axis as the first and second laser beams. It is focused. At this time, the depth control means detects the reflected light of the measurement laser light reflected on the laser light irradiation surface, and the condensing point of the first laser light and the second laser light from the laser light irradiation surface. The depth of the focusing point is controlled. In this way, the formation of the modified region by the first and second laser beams and the measurement of the displacement of the laser beam irradiation surface by the measurement laser beam are performed on the same axis. Even when the mounted stage vibrates, it is possible to prevent the depth of each condensing point from deviating from a predetermined depth. Therefore, it is possible to accurately form a plurality of modified regions at a predetermined depth for one planned cutting line.

【0012】

Further, the laser beam irradiation surface is the surface of the object to be processed on the condensing lens side, and the depth control means has the depth of the condensing point of the first laser light and the condensing point of the second laser light. It is preferable to control the depths of the first laser beam focusing point and the second laser beam focusing point so as to be constant from the surface. As a result, when the work target is cut along the planned cutting line, the cutting accuracy on the surface side of the work target can be stabilized. Such stabilization of cutting accuracy on the surface side can prevent damage to the functional elements when a plurality of functional elements are formed on the surface of the object to be processed and the object to be processed is cut for each functional element. Therefore, it is particularly effective. The functional element means 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.

【0013】

Further, the depth control means controls the depth of the first laser beam condensing point and the second laser beam condensing point by changing the distance between the condensing lens and the object to be processed. Is preferable. That is, by moving at least one of the condensing lens and the object to be processed, it is possible to adjust the depth of each condensing point from the laser beam irradiation surface to a predetermined depth.

【0014】

Further, in order to achieve the above object, the present invention also relates to a laser processing method, which is a laser processing method for forming a modified region by absorption of multiple photons inside a wafer-shaped object to be processed, and has a widening angle with each other. The different first laser beam and second laser beam are focused inside the object to be processed, and the position of the focusing point of the first laser beam and the position of the focusing point of the second laser beam are It is characterized by causing multiphoton absorption and relatively moving the focusing point of the first laser beam and the focusing point of the second laser beam along the planned cutting line of the object to be processed.

【0015】

Further, the measurement laser beam for measuring the displacement of the laser beam irradiation surface of the workpiece is focused toward the workpiece on the same axis as the first and second laser beams, and the laser beam is irradiated. By detecting the reflected light of the measurement laser light reflected on the surface, the depth of the focusing point of the first laser light and the focusing point of the second laser light from the laser light irradiation surface is controlled. Is preferable.

【0016】

The laser beam irradiation surface is a surface on which the first and second laser beams are incident on the object to be processed, and is the depth of the first laser beam focusing point and the second laser beam focusing point. It is preferable to control the depths of the first laser beam focusing point and the second laser beam focusing point so that the light is constant from the surface.

【0017】

Further, by changing the distance between the condensing lens that condenses the first and second laser beams and the object to be processed, the condensing point of the first laser light and the condensing point of the second laser light can be changed. It is preferable to control the depth.

【0018】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the laser processing apparatus and the laser processing method according to the present invention will be described in detail with reference to the drawings.

【0019】

The laser processing apparatus of the present embodiment forms a plurality of modified regions for one planned cutting line inside a wafer-shaped object to be processed by multiphoton absorption. Therefore, prior to the description of the laser processing apparatus of the present embodiment, the formation of the modified region by multiphoton absorption will be described.

【0020】

Material absorption bandgap E_GWhen the photon energy hν is smaller than that, it becomes optically transparent. Therefore, the condition for absorption in the material is hν> E_GIs. However, even if it is optically transparent, if the intensity of the laser beam is made very large, nhν> E_GAbsorption occurs in the material under the condition of (n = 2,3,4, ...). This phenomenon is called multiphoton absorption. In the case of a pulse wave, the intensity of the laser beam is the peak power density (W / cm) of the focusing point of the laser beam.²), For example, the peak power density is 1 × 10.⁸(W / cm²) Multiphoton absorption occurs under the above conditions. The peak power density is obtained by (energy per pulse of laser light at the focusing point) ÷ (beam spot cross-sectional area of laser light × pulse width). In the case of continuous waves, the intensity of the laser beam is the electric field intensity (W / cm) at the focusing point of the laser beam.²).

【0021】

The principle of laser processing according to the present embodiment utilizing such multiphoton absorption will be described with reference to FIGS. 1 to 6. FIG. 1 is a plan view of the processing target 1 during laser processing, FIG. 2 is a cross-sectional view of the processing object 1 shown in FIG. 1 along the line II-II, and FIG. 3 is a processing target after laser processing. It is a plan view of the object 1, FIG. 4 is a cross-sectional view of the workpiece 1 shown in FIG. 3 along the IV-IV line, and FIG. 5 is a cross section of the workpiece 1 shown in FIG. 3 along the VV line. FIG. 6 is a plan view of the cut object 1 to be processed.

【0022】

As shown in FIGS. 1 and 2, the surface 3 of the workpiece 1 has a desired scheduled cutting line 5 on which the workpiece 1 should be cut. The planned cutting line 5 is a virtual line extending in a straight line (a line may be actually drawn on the object 1 to be processed to be the planned cutting line 5). In the laser processing according to the present embodiment, the processing target 1 is formed by irradiating the processing target 1 with the laser beam L by aligning the condensing point P inside the processing target 1 under the condition that multiphoton absorption occurs. The focusing point is a point where the laser beam L is focused.

【0023】

By moving the laser beam L relatively along the planned cutting line 5 (that is, along the direction of arrow A), the focusing point P is moved along the planned cutting line 5. As a result, as shown in FIGS. 3 to 5, the modified region 7 is formed only inside the workpiece 1 along the scheduled cutting line 5, and the modified region 7 forms the planned cutting portion 8. .. In the laser machining method according to the present embodiment, the machining target 1 does not generate heat by absorbing the laser beam L to form the modified region 7. The modified region 7 is formed by transmitting the laser beam L through the object to be processed 1 and generating multiphoton absorption inside the object 1 to be processed. Therefore, since the laser beam L is hardly absorbed by the surface 3 of the object to be processed 1, the surface 3 of the object to be processed 1 is not melted.

【0024】

In cutting the object to be processed 1, if there is a starting point at the cutting point, the object 1 to be processed is cracked from the starting point, so that the object 1 to be processed can be cut with a relatively small force as shown in FIG. Therefore, the machining object 1 can be cut without causing unnecessary cracks on the surface 3 of the machining object 1.

【0025】

The following two methods can be considered for cutting the object to be processed starting from the planned cutting portion. One is a case where an artificial force is applied to an object to be machined after the planned portion to be cut is formed, so that the object to be machined is cracked starting from the planned portion to be cut and the object to be machined is cut. This is, for example, cutting when the thickness of the object to be processed is large. When an artificial force is applied, for example, thermal stress is generated by applying bending stress or shear stress to the workpiece along the planned cutting portion of the workpiece, or by giving a temperature difference to the workpiece. To let them do it. The other is when the planned cutting portion is formed so that the planned cutting portion is naturally cracked in the cross-sectional direction (thickness direction) of the machining target, and as a result, the machining target is cut. Is. This is possible, for example, when the thickness of the object to be processed is small, the planned cutting portion is formed by one row of modified regions, and when the thickness of the object to be processed is large, the thickness direction This is possible by forming a planned cutting portion by the modified regions formed in a plurality of rows. In addition, even in the case of this spontaneous cracking, in the cutting portion, the crack does not advance to the surface of the portion corresponding to the portion where the planned cutting portion is not formed, and the portion corresponding to the portion where the planned cutting portion is formed. Since only can be divided, the division can be controlled well. In recent years, the thickness of an object to be processed such as a silicon wafer tends to be thin, so such a cutting method with good controllability is very effective.

【0026】

The modified regions formed by multiphoton absorption in this embodiment include the following (1) to (3).

【0027】

(1) When the modified region is a crack region containing one or more cracks The object to be processed (for example, glass or LiTaO)₃The electric field strength at the condensing point is 1 × 10 by aligning the condensing point inside the piezoelectric material consisting of⁸(W / cm²) Or more and the pulse width is 1 μs or less. The magnitude of this pulse width is a condition in which a crack region can be formed only inside the object to be processed without causing extra damage to the surface of the object to be processed while causing multiphoton absorption. As a result, a phenomenon called optical damage due to multiphoton absorption occurs inside the object to be processed. This optical damage induces thermal strain inside the work piece, which forms a crack region inside the work piece. The upper limit of the electric field strength is, for example, 1 × 10.¹²(W / cm²). The pulse width is preferably 1 ns to 200 ns, for example. The formation of crack regions by multiphoton absorption is described in, for example, "Inside the glass substrate by solid-state laser harmonics" on pages 23 to 28 of the 45th Laser Thermal Processing Workshop Proceedings (December 1998). It is described in "Marking".

【0028】

The present inventor experimentally determined the relationship between the electric field strength and the size of cracks. The experimental conditions are as follows.

【0029】

(A) Object to be processed: Pilex (registered trademark) glass (thickness 700 μm) (B) Laser light source: Semiconductor laser excitation Nd: YAG Laser wavelength: 1064 nm Laser light spot Cross-sectional area: 3.14 × 10^-8cm²Oscillation form: Q-switch Pulse repetition frequency: 100kHz Pulse width: 30ns Output: Output <1mJ / Pulsed laser light Quality: TEM₀₀Polarization characteristics: Linearly polarized light (C) Condensing lens Transmittance to laser light wavelength: 60% (D) Movement speed of the mounting table on which the object to be processed is placed: 100 mm / sec [0030]

The laser light quality is TEM.₀₀Means that the light collecting property is high and the light can be collected up to the wavelength of the laser beam.

【0031】

FIG. 7 is a graph showing the results of the above experiment. The horizontal axis is the peak power density, and since the laser light is a pulsed laser light, the electric field strength is represented by the peak power density. The vertical axis shows the size of the crack portion (crack spot) formed inside the workpiece by the 1-pulse laser beam. Crack spots gather to form a crack area. The size of the crack spot is the size of the portion having the maximum length in the shape of the crack spot. The data indicated by the black circles in the graph is when the magnification of the condensing lens (C) is 100 times and the numerical aperture (NA) is 0.80. On the other hand, the data indicated by white circles in the graph is when the magnification of the condensing lens (C) is 50 times and the numerical aperture (NA) is 0.55. Peak power density is 10¹¹(W / cm²It can be seen that crack spots are generated inside the object to be processed from the degree of), and the crack spots also increase as the peak power density increases.

【0032】

Next, in the laser machining according to the present embodiment, the mechanism of cutting the workpiece by forming the crack region will be described with reference to FIGS. 8 to 11. As shown in FIG. 8, under the condition that multiphoton absorption occurs, the condensing point P is aligned with the inside of the work target 1, the laser light L is irradiated to the work target 1, and the crack region inside along the planned cutting line. Form 9. The crack region 9 is a region containing one or a plurality of cracks. A planned cutting portion is formed at the crack region 9. As shown in FIG. 9, the crack grows further starting from the crack region 9 (that is, starting from the planned cutting portion), and as shown in FIG. 10, the crack reaches the front surface 3 and the back surface 17 of the workpiece 1. As shown in FIG. 11, when the object to be processed 1 is cracked, the object to be processed 1 is cut. The cracks that reach the front and back surfaces of the object to be processed may grow naturally, or may grow when a force is applied to the object to be processed.

【0033】

(2) When the modified region is a melt processing region The condensing point is set inside the object to be processed (for example, a semiconductor material such as silicon), and the electric field strength at the condensing point is 1 × 10.⁸(W / cm²) Or more and the pulse width is 1 μs or less. As a result, the inside of the object to be processed is locally heated by multiphoton absorption. By this heating, a melt processing region is formed inside the object to be processed. The melting treatment region is a region that has been once melted and then resolidified, a region that is exactly in a molten state, or a region that is in a state of being resolidified from a molten state, and can also be said to be a region in which the phase has changed or a region in which the crystal structure has changed. Further, the melt-treated region can be said to be a region in which one structure is changed to another in a single crystal structure, an amorphous structure, or a polycrystalline structure. That is, for example, it means a region changed from a single crystal structure to an amorphous structure, a region changed from a single crystal structure to a polycrystalline structure, and a region changed from a single crystal structure to a structure including an amorphous structure and a polycrystalline structure. To do. When the object to be processed has a silicon single crystal structure, the melt processing region is, for example, an amorphous silicon structure. The upper limit of the electric field strength is, for example, 1 × 10.¹²(W / cm²). The pulse width is preferably 1 ns to 200 ns, for example.

【0034】

The present inventor has experimentally confirmed that a melt processing region is formed inside the silicon wafer. The experimental conditions are as follows.

【0035】

(A) Object to be processed: Silicon wafer (thickness 350 μm, outer diameter 4 inches) (B) Laser light source: Semiconductor laser excitation Nd: YAG Laser wavelength: 1064 nm Laser light spot Cross-sectional area: 3.14 × 10^-8cm²Oscillation form: Q-switch pulse repetition frequency: 100kHz Pulse width: 30ns Output: 20μJ / pulse laser light quality: TEM₀₀Polarization characteristics: Linearly polarized light (C) Condensing lens Magnification: 50 times NA: 0.55 Transmittance to laser light wavelength: 60% (D) Movement speed of the mounting table on which the object to be processed is placed: 100 mm / sec [0036] ]

FIG. 12 is a diagram showing a photograph of a cross section of a part of a silicon wafer cut by laser processing under the above conditions. A melt processing region 13 is formed inside the silicon wafer 11. The size of the melt processing region 13 formed under the above conditions in the thickness direction is about 100 μm.

【0037】

It will be described that the melt processing region 13 is formed by multiphoton absorption. FIG. 13 is a graph showing the relationship between the wavelength of the laser beam and the transmittance inside the silicon substrate. However, the reflective components on the front surface side and the back surface side of the silicon substrate are removed, and the transmittance is shown only inside. The above relationship was shown for each of the thickness t of the silicon substrate of 50 μm, 100 μm, 200 μm, 500 μm, and 1000 μm.

【0038】

For example, at 1064 nm, which is the wavelength of the Nd: YAG laser, when the thickness of the silicon substrate is 500 μm or less, it can be seen that 80% or more of the laser light is transmitted inside the silicon substrate. Since the thickness of the silicon wafer 11 shown in FIG. 12 is 350 μm, the melt processing region 13 by multiphoton absorption is formed near the center of the silicon wafer, that is, at a portion of 175 μm from the surface. In this case, the transmittance is 90% or more with reference to a silicon wafer having a thickness of 200 μm, so that the laser beam is hardly absorbed inside the silicon wafer 11 and most of it is transmitted. This does not mean that the laser beam is absorbed inside the silicon wafer 11 and the melt processing region 13 is formed inside the silicon wafer 11 (that is, the melt processing region is formed by normal heating by the laser light). It means that the melt processing region 13 was formed by multiphoton absorption. The formation of the melt processing region by multiphoton absorption can be found in, for example, "Evaluation of Silicon Machining Characteristics by Picosecond Pulsed Laser" on pages 72-73 of the 66th Annual Meeting of the Welding Society (April 2000). Are listed.

【0039】

In the silicon wafer, cracks are generated in the cross-sectional direction starting from the planned cutting portion formed in the melt processing region, and the cracks reach the front surface and the back surface of the silicon wafer, resulting in cutting. Will be done. These cracks that reach the front and back surfaces of the silicon wafer may grow naturally, or may grow when a force is applied to the silicon wafer. When cracks naturally grow from the planned cutting portion to the front surface and the back surface of the silicon wafer, the cracks grow from the state where the melt processing region forming the planned cutting portion is melted, and the planned cutting portion. In both cases, cracks grow when the melt-treated region forming the wafer is resolidified from the molten state. However, in both cases, the melt processing region is formed only inside the silicon wafer, and the melt treatment region is formed only inside as shown in FIG. 12 on the cut surface after cutting. When the planned cutting portion is formed inside the object to be processed by the melt processing region, unnecessary cracking off the line of the scheduled cutting portion is unlikely to occur at the time of cutting, so that the cutting control becomes easy.

【0040】

(3) When the modified region is the refractive index change region The light field strength at the light collection point is 1 × 10 by aligning the light collection point inside the work object (for example, glass).⁸(W / cm²) Or more and the pulse width is 1 ns or less to irradiate the laser beam. When the pulse width is extremely shortened and multiphoton absorption is caused inside the work object, the energy due to the multi photon absorption is not converted into thermal energy, and the ionic valence changes and crystallizes inside the work object. Alternatively, a permanent structural change such as polarization orientation is induced to form a refractive index change region. The upper limit of the electric field strength is, for example, 1 × 10.¹²(W / cm²). The pulse width is preferably 1 ns or less, more preferably 1 ps or less, for example. The formation of the refractive index change region by multiphoton absorption is described, for example, in "Proceedings of the 42nd Laser Thermal Processing Study Group (November 1997)" on pages 105 to 111, "Inside the glass by femtosecond laser irradiation. Photoinduced structure formation .

【0041】

Next, the laser processing apparatus of this embodiment will be described with reference to FIGS. 14 to 16.

【0042】

As shown in FIG. 14, the laser machining apparatus 20 is an apparatus in which two modification regions 7a and 7b are formed inside a wafer-shaped object 1 to be processed by multiphoton absorption with respect to one scheduled cutting line 5. Is. Here, the object to be processed 1 is a semiconductor wafer such as a silicon wafer, and the modified regions 7a and 7b are melt processing regions. Further, a plurality of scheduled cutting lines 5 are set on the surface 3 of the object 1 to be processed, for example, in a grid pattern.

【0043】

The laser processing apparatus 20 has a stage (moving means) 21 on which the object to be processed 1 is placed, and the stage 21 has an X-axis direction, a Y-axis direction, and a Z-axis with the vertical direction as the Z-axis direction. It can be moved in each direction. Above the stage 21, a housing 23 containing a laser light source 22 or the like that generates a laser beam for processing is arranged. The laser light source 22 is, for example, an Nd: YAG laser, and emits a processing laser beam which is a pulse laser beam having a pulse width of 1 μs or less downward.

【0044】

An electric revolver 24 is attached to the lower end surface of the housing 23, and the electric revolver 24 has an observation objective lens 26 for observing the object 1 to be processed and a laser beam for processing to be focused. A processing objective lens 27 is attached. The optical axes of the objective lenses 26 and 27 are aligned with the Z axis (axis) by the rotation of the electric revolver 24. An actuator 28 using a piezo element is interposed between the processing objective lens 27 and the electric revolver 24, and the position of the processing objective lens 27 is in the Z-axis direction (vertical direction) by this actuator 28. Fine-tuned.

【0045】

As shown in FIG. 15, the processing objective lens 27 has a cylindrical lens holder 29, and the lens holder 29 has a condensing lens 31 having a numerical aperture of 0.80 formed by combining a plurality of lenses inside the lens holder 29. keeping. An entrance opening 32 is formed at the upper end of the lens holder 29 as an entrance pupil of the processing laser light for the condenser lens 31, and an exit opening 33 for the processing laser light is formed at the lower end of the lens holder 29. ing.

【0046】

Further, as shown in FIG. 14, below the laser light source 22 in the housing 23, a beam expander 34 for expanding the beam size of the processing laser light emitted from the laser light source 22 and a processing laser light are provided. The output adjustment optical system 35 that adjusts the output, the output observation optical system 36 that observes the output of the processing laser light adjusted by the output adjustment optical system 35, and the polarization adjustment optical system 37 that adjusts the polarization of the processing laser light. And the electromagnetic shutter 38 that passes or blocks the processing laser beam are arranged in this order from top to bottom.

【0047】

Further, below the electromagnetic shutter 38, a half mirror 39 is arranged which reflects about half of the processing laser light that has passed through the electromagnetic shutter 38 in the lateral direction and allows the rest to travel straight through. As shown in FIG. 16, the processing laser light transmitted through the half mirror 39 is reflected laterally by the mirror 41 and then reflected downward by the polarization beam splitter 42 located on the Z axis, and is reflected by the first laser. It travels downward on the Z axis as light L1.

【0048】

On the other hand, the processing laser beam reflected laterally by the half mirror 39 passes through the 1/2 wave plate 43 and is then reflected downward by the mirror 44 located on the Z axis. The processing laser light reflected by the mirror 44 passes through the concave lens 45 arranged on the Z axis, so that the spread angle is wider than that of the first laser light L1, and the Z axis is used as the second laser light L2. Proceed upward and downward. The second laser beam L2 passes straight through the polarization beam splitter 42 located below the concave lens 45, is combined with the first laser beam L1, and travels downward on the Z-axis. The first laser beam L1 and the second laser beam L2 are linearly polarized light, but by arranging the 1/2 wavelength plate 43 on the optical path of the processing laser beam that becomes the second laser beam, When the first laser beam L1 and the second laser beam L2 are combined, the directions of linear polarization of the first laser beam L1 and the second laser beam L2 are almost the same.

【0049】

Therefore, in the laser processing apparatus 20, the first laser beam L1 and the second laser beam L2 having a larger spread angle than the first laser beam L1 are directed downward on the Z axis as the processing laser beam. Then, it enters the condensing lens 31 of the processing objective lens 27. As a result, the condensing point P1 of the first laser beam L1 and the condensing point P2 of the second laser beam L2 condensed by the condensing lens 31 have different depths from the surface 3 of the object 1 to be processed. Will be located at. Specifically, since the second laser beam L2 has a larger spread angle than the first laser beam L1, the focusing point P2 of the second laser beam L2 is the focusing point of the first laser beam L1. It will be located deeper than surface 3 compared to P1. The peak power density of the processing laser beams L1 and L2 at each of the focusing points P1 and P2 by the condenser lens 31 is 1 × 10.⁸(W / cm²) That's all.

【0050】

Further, between the polarizing beam splitter 42 and the processing objective lens 27, a diaphragm member 46 for narrowing the beam size of the processing laser beams L1 and L2 is arranged. As shown in FIG. 15, the diaphragm member 46 is attached to the housing 23 at a position above the incident aperture 32 of the processing objective lens 27, and is an aperture that narrows and passes the processing laser beams L1 and L2. Has 47. The aperture diameter of the aperture 47 is formed to be equal to or less than the diameter of the incident aperture 32 of the processing objective lens 27, and the central axis of the aperture 47 is the central axis of the incident aperture 32 by the adjusting screw 48 provided on the aperture member 46. It can be exactly aligned (ie, the Z axis).

【0051】

By arranging the diaphragm member 46 configured in this way on the side where the processing laser beams L1 and L2 are incident on the processing objective lens 27, the following actions and effects are exhibited. That is, in the processing laser light L1 and L2, the outer peripheral portion of the laser light larger than the aperture 47 is cut by the diaphragm member 46, whereby the diameter of the processing laser light L1 and L2 that has passed through the aperture 47 is changed to the processing objective. It is less than or equal to the diameter of the incident aperture 32 of the lens 27. Therefore, it is possible to prevent the lens holder 29 from being heated by the irradiation of the processing laser light L1 and L2 by almost eliminating the amount of the processing laser light L1 and L2 cut by the peripheral portion of the incident aperture 32. Therefore, it is possible to suppress the positional fluctuation of the focusing points P1 and P2 of the processing laser beams L1 and L2 mainly caused by the heating of the lens holder 29 during laser processing.

【0052】

Further, the laser processing apparatus 20 condenses the light so that the depths of the condensing point P1 of the first laser beam L1 and the condensing point P2 of the second laser beam L2 are constant from the surface 3 of the object 1 to be processed. It has a depth control means 50 that controls the depth of the point P1 and the focusing point P2. The depth control means 50 will be described with reference to FIGS. 15 and 17 to 19.

【0053】

As shown in FIG. 15, the measurement laser beam L3 emitted from the measurement laser light source 51 such as a laser diode sequentially passes through the pinhole 52 and the beam expander 53, and then is sequentially reflected by the mirror 54 and the half mirror 56. Then, it is guided to a dichroic mirror 57 arranged between the polarizing beam splitter 42 and the narrowing member 46. The measurement laser light L3 reflected by the dichroic mirror 57 is combined with the processing laser light L1 and L2, travels downward on the Z axis, passes through the aperture 47 of the aperture member 46, and then is used for processing. The light is collected by the condenser lens 31 of the objective lens 27 and irradiated to the object 1 to be processed. The processing laser beams L1 and L2 pass through the dichroic mirror 57.

【0054】

Then, the reflected light L4 of the measurement laser light reflected by the surface (laser light irradiation surface) 3 of the processing object 1 re-enters the condenser lens 31 of the processing objective lens 27 and moves upward on the Z axis. It travels toward it, passes through the aperture 47 of the aperture member 46, and is then reflected by the dichroic mirror 57. The reflected light L4 of the measurement laser beam reflected by the dichroic mirror 57 passes through the half mirror 56 and the filter 58 in that order. This filter 58 passes or blocks light according to the wavelength, and while passing the reflected light L4 of the measurement laser light, the processing laser light reflected by the front surface 3 and the back surface 17 of the processing object 1 Blocks the reflected light of L1 and L2. The reflected light L4 of the measurement laser light that has passed through the filter 58 is focused by the shaping optical system 59 composed of the cylindrical lens and the plano-convex lens, and is condensed on the quadrant position detection element 61 formed by dividing the photodiode into four equal parts. Be irradiated.

【0055】

The condensed image pattern of the reflected light L4 of the measurement laser light focused on the 4-division position detection element 61, which is the light receiving element, is a collection of the measurement laser light L3 by the condenser lens 31 of the processing objective lens 27. It changes depending on the position of the light spot (that is, the focal point) with respect to the surface 3 of the workpiece 1. Here, the relationship between the position of the condensing point of the measurement laser beam L3 and the condensing image pattern of the reflected light L4 of the measurement laser beam will be described.

【0056】

As shown in FIG. 17, when the condensing point P3 of the measurement laser beam L3 is located on the surface 3 of the object 1 to be processed, the reflected light L4 of the measurement laser beam is the same as the measurement laser beam L3. The condensing lens 31 of the processing objective lens 27 is reversed, passes through the shaping optical system 59, and a perfect circular condensing image pattern F is formed on the 4-division position detection element 61.

【0057】

Further, as shown in FIG. 18, when the condensing point P3 of the measurement laser beam L3 is located at the tip of the surface 3 of the machining object 1 (that is, inside the machining object 1), the measurement laser beam Unlike the measurement laser light L3, the reflected light L4 reverses the condenser lens 31 of the processing objective lens 27 while diffusing, passes through the shaping optical system 59, and is a vertically long ellipse on the 4-division position detection element 61. The condensed image pattern F of is formed.

【0058】

Further, as shown in FIG. 19, when the condensing point P3 of the measurement laser light L3 is located in front of the surface 3 of the object 1 to be processed, the reflected light L4 of the measurement laser light is the measurement laser light. Unlike L3, the condensing lens 31 of the processing objective lens 27 is reversed while being focused, and passes through the shaping optical system 59 to form a horizontally long elliptical condensing image pattern F on the 4-division position detection element 61. ..

【0059】

As described above, the condensed image pattern F of the reflected light L4 of the measurement laser light on the 4-division position detection element 61 changes according to the position of the focusing point P3 of the measurement laser light L3. Therefore, based on the output signal from the 4-division position detection element 61 (the difference between the output from the light receiving surfaces facing each other in the vertical direction and the output from the light receiving surfaces facing each other in the horizontal direction), the surface 3 of the workpiece 1 is relative to the surface 3. The position of the focusing point P3 of the measurement laser beam L3 can be obtained.

【0060】

Therefore, as shown in FIG. 15, the depth control means 50 includes a position detection calculation circuit 62 and an actuator control unit 63. The position detection calculation circuit 62 calculates the position of the condensing point P3 of the measurement laser beam L3 with respect to the surface 3 of the workpiece 1 based on the output signal from the 4-division position detection element 61. Then, the actuator control unit 63 has the focusing point P1 of the first laser beam L1 and the focusing point P2 of the second laser beam L2 based on the position of the focusing point P3 obtained by the position detection calculation circuit 62. The actuator 28 is constantly feedback-controlled during laser machining so that the depth of the light is constant from the surface 3 of the object 1 to be machined, and the position of the objective lens 27 for machining is finely adjusted in the Z-axis direction.

【0061】

Further, as shown in FIG. 14, the laser processing apparatus 20 has an observation light source 71 outside the housing 23 that generates visible light for observation in order to observe the processing object 1 placed on the stage 21. However, the CCD camera 72 is contained in the housing 23.

【0062】

That is, the observation visible light emitted by the observation light source 71 is guided into the housing 23 by the light guide 73 made of an optical fiber, and after passing through the field diaphragm 74, the aperture diaphragm 76, the dichroic mirror 77, etc. in sequence, It is reflected by the dichroic mirror 78 arranged between the aperture member 46 and the incident aperture 32 of the processing objective lens 27. The reflected visible light for observation travels downward on the Z-axis, passes through the observation objective lens 26 arranged on the Z-axis by the rotation of the electric revolver 24, and is irradiated to the object 1 to be processed. .. The processing laser light L1 and L2, the measuring laser light L3, and the reflected light L4 are transmitted through the dichroic mirror 78.

【0063】

Then, the reflected visible light for observation reflected on the surface 3 of the object 1 to be processed re-enters the objective lens 26 for observation, travels upward on the Z-axis, and is reflected by the dichroic mirror 78. To. The reflected light reflected by the dichroic mirror 78 is further reflected by the dichroic mirror 77, passes through the filter 79, the imaging lens 81, and the relay lens 82 in that order, and is incident on the CCD camera 72.

【0064】

The imaged data captured by the CCD camera 72 is taken into the overall control unit 83, and the overall control unit 83 displays an image of the surface 3 or the like of the object 1 to be processed on the TV monitor 84. The overall control unit 83 executes various processes, moves the stage 21, rotates the electric revolver 24, opens and closes the electromagnetic shutter 38, captures images with the CCD camera 72, and performs the entire operation of the laser processing apparatus 20. It controls.

【0065】

Next, the laser processing method using the laser processing apparatus 20 described above will be described with reference to FIG. Here, when the focusing point P1 of the first laser beam L1 is aligned with the position of the depth D1 from the surface 3 of the object 1 to be processed, the focusing point P2 of the second laser beam L2 is the depth from the surface 3. It shall match the position of D2 (> D1), and the modified regions 7a and 7b shall be formed at the position of depth D1 and the position of depth D2 from the surface 3 along the planned cutting line 5, respectively.

【0066】

First, the processing target 1 is placed on the stage 21, and the stage 21 is moved so that the focusing points P1 and P2 coincide with the formation start positions of the modified regions 7a and 7b in the processing target 1. The initial position of the stage 21 is determined based on the thickness and refractive index of the workpiece 1 and the numerical aperture of the condenser lens 31.

【0067】

Subsequently, the processing laser light is emitted from the laser light source 22, the measurement laser light L3 is emitted from the measurement laser light source 51, and the first laser light L1 and the second laser light L1 are condensed by the condenser lens 31. The stage 21 is driven in the X-axis direction and the Y-axis direction so that the laser light L2 and the measurement laser light L3 scan on the line 5 scheduled to be cut. At this time, the depth control means 50 detects the reflected light L4 of the laser beam for measurement, and the depth D1 of the condensing point P1 of the first laser beam L1 is always constant from the surface 3 of the object 1 to be processed. The actuator 28 is feedback-controlled so that the position of the processing objective lens 27 is finely adjusted in the vertical direction. As a result, the depth D2 of the condensing point P2 of the second laser beam L2 is always constant from the surface 3 of the object 1 to be processed.

【0068】

Therefore, as shown in FIG. 20, even if the surface 3 of the object to be processed 1 has a surface runout, the modified regions 7a and 7b are once formed from the surface 3 to the position of the depth D1 and the position of the depth D2. It can be formed by scanning. When the work target 1 having the modified regions 7a and 7b following the surface runout of the surface 3 along the planned cut line 5 is cut along the cut line 5, the surface 3 side of the work target 1 is cut. The cutting accuracy is stabilized. Therefore, a plurality of functional elements such as a light receiving element or a light emitting element are formed on the surface 3 of the processing object 1 which is a semiconductor wafer, and when the processing object 1 is cut for each of these functional elements, the functional element by cutting is formed. It becomes possible to prevent damage.

【0069】

As described above, in the laser processing apparatus 20, the first laser beam L1 and the second laser beam L2 having different spread angles are condensed inside the processing object 1 by the condenser lens 31. .. At this time, since the second laser beam L2 has a larger spread angle than the first laser beam L1, the focusing point P1 of the first laser beam L1 is located at the depth D1 from the surface 3 of the object 1 to be processed. , The focusing point P2 of the second laser beam L2 is aligned with the position of the depth D2 (> D1) from the surface 3. Then, the condensing point P1 and the condensing point P2 are moved along the planned cutting line 5 of the workpiece 1 by driving the stage 21 in the X-ray axis direction and the Y-axis direction with respect to the condensing lens 31. Be done. Therefore, according to the laser machining apparatus 20, two modified regions 7a and 7b can be formed in one scan along the planned cutting line 5, and two for one planned cutting line 5. It becomes possible to efficiently form the modified region of. In this way, by forming two modified regions 7a and 7b inside the work target 1 with respect to one planned cutting line 5, the thickness of the work target 1 becomes each modified region 7a, Even when the height is larger than the height of 7b, it is possible to cut the workpiece 1 with high accuracy along the planned cutting line 5 with a small force compared to the one in which only one modified region is formed. become.

【0070】

Further, in the laser machining apparatus 20, the measuring laser beam L3 for measuring the displacement of the surface 3 of the machining object 1 is on the same axis (that is, the Z axis) as the machining laser beams L1 and L2. The light is focused toward the object 1 to be processed by 31. At this time, the depth control means 30 detects the reflected light L4 of the measurement laser light reflected on the surface 3, and controls the depths of the condensing point P1 and the condensing point P2 from the surface 3. In this way, the formation of the modified regions 7a and 7b by the processing laser light L1 and L2 and the measurement of the displacement of the surface 3 by the measurement laser light L3 are performed on the same axis. Even if the stage 21 on which 1 is placed vibrates, it is possible to prevent the depths of the focusing points P1 and P2 from deviating from the predetermined depths D1 and D2. Therefore, it is possible to accurately form two modified regions 7a and 7b at predetermined depths D1 and D2 for one scheduled cutting line 5, respectively.

【0071】

Further, when the modified regions 7a and 7b by multiphoton absorption are formed inside the thin plate-shaped object 1 to be processed, the numerical aperture of the condensing lens 31 of the processing objective lens 27 is as large as "0.80". Therefore, the condenser lens 31 and the object to be processed 1 are close to each other up to about 10 mm. In such a state, it is extremely difficult to arrange a sensor or the like for measuring the displacement of the surface 3 side by side on the side of the condenser lens 31, but according to the laser processing apparatus 20, it is such. There is no need to adopt a difficult configuration.

【0072】

The present invention is not limited to the above embodiment. For example, the above embodiment is a case where the first laser beam L1 and the second laser beam L2 are irradiated to the processing object 1 as the processing laser light, but if the spreading angles are different from each other, the processing is performed. As the laser beam for use, three or more laser beams may be focused inside the object to be processed 1 by the condenser lens 31. According to this, three or more modified regions can be formed inside the workpiece 1 by one scan along the scheduled cutting line 5.

【0073】

Further, in the above embodiment, in order to relatively move the condensing point P1 and the condensing point P2 of the processing laser beams L1 and L2 along the planned cutting line 5 of the processing object 1, the processing object 1 is moved. The mounted stage 21 is driven in the X-axis direction or the Y-axis direction. For example, the condenser lens 31 side may be driven in the X-axis direction or the Y-axis direction with respect to the stage 21.

【0074】

Further, the above embodiment is a case where the depth control means 50 detects the reflected light L4 of the laser beam for measurement on the surface 3 of the object 1 to be processed, but the present invention is not limited to this. For example, the reflected light of the measurement laser light L3 on the back surface (laser light irradiation surface) 17 of the processing object 1 may be detected together with the reflected light L4 of the measurement laser light on the front surface 3 of the processing object 1. .. As a result, the displacement of the front surface 3 and the displacement of the back surface 17 of the processing object 1 can be measured, and therefore the thickness of the processing object 1 directly under the condenser lens 31 of the processing objective lens 27 can be accurately obtained. be able to. Therefore, for example, the condensing point P1 of the first laser beam L1 is aligned with the position of half the thickness of the object to be processed 1, or the thickness of the object of processing 1 is aligned with the position of 1/3 from the surface 3 side. As such, it becomes possible to control the depth of the focusing point P1 and the focusing point P2 in various modes.

【0075】

Further, the above embodiment is a case where the focusing points P1 and P2 of the processing laser beams L1 and L2 are aligned with the positions of the depths D1 and D2 from the surface 3 of the processing object 1, but each focusing point. The depth control of the condensing point P1 and the condensing point P2 may be performed so that the positions where P1 and P2 are aligned are changed along the scheduled cutting line 5. For example, the position where the focusing points P1 and P2 are aligned may be changed in a wavy line, or the depth of the position where the focusing points P1 and P2 are aligned may be changed in the middle.

【0076】

[Effect of the invention]

As described above, according to the laser processing apparatus and the laser processing method according to the present invention, it is possible to efficiently form a plurality of modified regions inside an object to be processed for one planned cutting line. Become.

[Simple explanation of drawings]

FIG. 1 is a plan view of an object to be machined during laser machining by the laser machining method according to the present embodiment.

FIG. 2 is a cross-sectional view of the work object shown in FIG. 1 along the line II-II.

FIG. 3 is a plan view of an object to be machined after laser machining by the laser machining method according to the present embodiment.

FIG. 4 is a cross-sectional view of the work object shown in FIG. 3 along the IV-IV line.

FIG. 5 is a cross-sectional view of the object to be machined shown in FIG. 3 along the VV line.

FIG. 6 is a plan view of a processing object cut by the laser processing method according to the present embodiment.

FIG. 7 is a graph showing the relationship between the electric field strength and the size of crack spots in the laser processing method according to the present embodiment.

FIG. 8 is a cross-sectional view of a processing object in the first step of the laser processing method according to the present embodiment.

FIG. 9 is a cross-sectional view of a processing object in the second step of the laser processing method according to the present embodiment.

FIG. 10 is a cross-sectional view of a processing object in the third step of the laser processing method according to the present embodiment.

FIG. 11 is a cross-sectional view of a processing object in the fourth step of the laser processing method according to the present embodiment.

FIG. 12 is a diagram showing a photograph of a cross section of a part of a silicon wafer cut by the laser processing method according to the present embodiment.

FIG. 13 is a graph showing the relationship between the wavelength of laser light and the transmittance inside a silicon substrate in the laser processing method according to the present embodiment.

FIG. 14 is a schematic configuration diagram of a laser processing apparatus according to the present embodiment.

15 is a schematic configuration diagram showing a depth control means of the laser machining apparatus shown in FIG. 14. FIG.

16 is a schematic configuration diagram showing an optical system of a laser beam for processing of the laser processing apparatus shown in FIG. 14. FIG.

FIG. 17 is a diagram for explaining a condensed image pattern of reflected light of the laser beam for measurement when the focusing point of the laser beam for measurement is located on the surface of the object to be processed.

FIG. 18 is a diagram for explaining a condensed image pattern of reflected light of the laser beam for measurement when the focusing point of the laser beam for measurement is located ahead of the surface of the object to be processed.

FIG. 19 is a diagram for explaining a condensed image pattern of reflected light of the laser beam for measurement when the focusing point of the laser beam for measurement is located in front of the surface of the object to be processed.

20 is a diagram showing a state of laser processing by the laser processing apparatus shown in FIG. 14. FIG.

[Explanation of symbols]

1 ... object to be machined, 3 ... front surface (laser light irradiation surface), 5 ... planned cutting line, 7a, 7b ... modified region, 17 ... back surface (laser light irradiation surface), 20 ... Laser Machining Equipment, 21 ... Stage (Movement Means), 31 ... Condensing Lens, 50 ... Depth Control Means, L1 ... 1st Laser Light, L2 ... 1st 2 laser light, L3 ... measurement laser light, L4 ... measurement laser light reflected light, P1, P2 ... focusing point, Z ... Z axis (axis).