JP5322418B2 - Laser processing method and laser processing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser processing method and a laser processing apparatus by which a reformed region where is nearest to a prescribed surface can be formed extremely close to the prescribed surface and the reformed region where is nearest to the incoming surface of laser light can be formed extremely close to the incoming surface of the laser light. <P>SOLUTION: By emitting the reflected light of the laser light L which is reflected by the surface 17a of a metal film 17 opposing to the surface 3 which is the incoming surface of the laser light of a processing object 1 onto a silicon wafer 11, a melting treated region 13<SB>1</SB>nearest to the surface 17a of the metal film 17 among the six rows of the melting treated regions 13<SB>1</SB>, 13<SB>2</SB>. is formed. In this way, the melting treated region 13<SB>1</SB>can be formed extremely close to the surface 17a of the metal film 17. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to a laser processing method and a laser processing apparatus for cutting a plate-like processing object along a scheduled cutting line.

As a conventional laser processing method, by irradiating a plate-shaped processing target with laser light, it becomes the starting point of cutting so that it is aligned in the thickness direction of the processing target along the planned cutting line of the processing target. A method for forming a plurality of rows of modified regions inside a workpiece is known (see, for example, Patent Document 1).
JP 2004-343008 A

  By the way, in the laser processing method as described above, a predetermined surface (for example, the back surface of the processing object) facing the laser light incident surface (for example, the surface of the processing object) on which the laser light is incident on the processing object is the most. The near modified region is preferably formed in the very vicinity of the predetermined surface. The modified region closest to the laser light incident surface is preferably formed in the immediate vicinity of the laser light incident surface. Because, when these modified regions are formed at a position away from a predetermined surface or laser light incident surface, when the workpiece is cut, each end of the cut surface in the thickness direction of the workpiece is This is because there is a risk that the line will be greatly separated from the planned cutting line.

  However, in the laser processing method as described above, even if an attempt is made to form the modified region closest to the predetermined surface in the very vicinity of the predetermined surface, for example, the thickness of the processing object follows the planned cutting line. If there is a change, the modified region closest to the predetermined surface may be partially formed at a position away from the predetermined surface. Further, even if an attempt is made to form the modified region closest to the laser light incident surface in the very vicinity of the laser light incident surface, for example, due to the temperature dependence of the absorption coefficient (details will be described later) Risk of damage. As described above, it is often difficult to form the modified region closest to the predetermined surface in the very vicinity of the predetermined surface.

  Therefore, the present invention has been made in view of such circumstances, and the modified region closest to the predetermined surface is formed in the very vicinity of the predetermined surface, or the modified region closest to the laser light incident surface. An object of the present invention is to provide a laser processing method and a laser processing apparatus capable of forming a laser beam near the laser light incident surface.

In order to achieve the above object, a laser processing method according to the present invention irradiates a plate-like processing object with laser light, thereby along the planned cutting line of the processing object, in the thickness direction of the processing object. Is a laser processing method for forming a plurality of rows of modified regions as starting points of cutting inside a workpiece so as to be aligned with each other, and facing the laser light incident surface on which laser light is incident on the workpiece and processing by irradiating the reflected light of the laser light incident surface side of the surface der Ru predetermined surface in reflected laser light in the metal layer the object is provided in the object, among the modified regions in a plurality of rows, a predetermined When forming one or a plurality of modified regions including at least one of the modified region closest to the surface and the modified region closest to the laser light incident surface, and forming the modified region closest to the predetermined surface At the same time as forming the modified region The weakened area along the line to cut and forming the metal film.

  In this laser processing method, a plurality of rows of modified regions are irradiated by irradiating the processing object with reflected light of the laser light reflected by a predetermined surface facing the laser light incident surface on which the laser light is incident on the processing object. Among them, one or more modified regions including at least one row of the modified region closest to the predetermined surface and the modified region closest to the laser light incident surface are formed. This makes it possible to form the modified region closest to the predetermined surface in the vicinity of the predetermined surface, or to form the modified region closest to the laser light incident surface in the vicinity of the laser light incident surface. It becomes.

  Each modified region is formed by causing multiphoton absorption or other light absorption inside the processing object by irradiating the processing object with laser light.

  In the laser processing method according to the present invention, it is preferable to cut the object to be processed along the scheduled cutting line using a plurality of modified regions as starting points for cutting. As a result, the workpiece can be accurately cut along the scheduled cutting line.

  In the laser processing method according to the present invention, the workpiece may include a semiconductor substrate, and the modified region may include a melt processing region.

  According to the present invention, the modified region closest to the predetermined surface is formed in the vicinity of the predetermined surface with good controllability, or the modified region closest to the laser light incident surface is formed in the vicinity of the laser light incident surface. Or can be formed.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the laser processing method of the present embodiment, a phenomenon called multiphoton absorption is used in order to form a modified region inside the workpiece. Therefore, first, a laser processing method for forming a modified region by multiphoton absorption will be described.

Photon energy hν is smaller than the band gap E _G of absorption of the material becomes transparent. Therefore, a condition under which absorption occurs in the material is hv> E _G. However, even when optically transparent, increasing the intensity of the laser beam very Nhnyu> of _{E G} condition (n = 2,3,4, ···) the intensity of laser light becomes very high. This phenomenon is called multiphoton absorption. In the case of a pulse wave, the intensity of the laser beam is determined by the peak power density (W / cm ² ) at the condensing point of the laser beam. For example, the multiphoton is obtained under conditions where the peak power density is 1 × 10 ⁸ (W / cm ² ) or more. Absorption occurs. The peak power density is determined by (energy per pulse of laser light at the condensing point) / (laser beam cross-sectional area of laser light × pulse width). In the case of a continuous wave, the intensity of the laser beam is determined by the electric field intensity (W / cm ² ) at the condensing point of the laser beam.

  The principle of the laser processing method according to this embodiment using such multiphoton absorption will be described with reference to FIGS. As shown in FIG. 1, there is a planned cutting line 5 for cutting the workpiece 1 on the surface 3 of the wafer-like (plate-like) workpiece 1. The planned cutting line 5 is a virtual line extending linearly. In the laser processing method according to the present embodiment, as shown in FIG. 2, the modified region 7 is formed by irradiating the laser beam L with the focusing point P inside the processing object 1 under the condition that multiphoton absorption occurs. Form. In addition, the condensing point P is a location where the laser light L is condensed. Further, the planned cutting line 5 is not limited to a straight line but may be a curved line, or may be a line actually drawn on the workpiece 1 without being limited to a virtual line.

  Then, the condensing point P is moved along the planned cutting line 5 by relatively moving the laser light L along the planned cutting line 5 (that is, in the direction of arrow A in FIG. 1). Thereby, as shown in FIGS. 3 to 5, the modified region 7 is formed inside the workpiece 1 along the planned cutting line 5, and this modified region 7 becomes the cutting start region 8. Here, the cutting starting point region 8 means a region that becomes a starting point of cutting (cracking) when the workpiece 1 is cut. The cutting starting point region 8 may be formed by continuously forming the modified region 7 or may be formed by intermittently forming the modified region 7.

  The laser processing method according to the present embodiment does not form the modified region 7 by causing the workpiece 1 to generate heat when the workpiece 1 absorbs the laser beam L. The modified region 7 is formed by transmitting the laser beam L through the workpiece 1 and generating multiphoton absorption inside the workpiece 1. Therefore, since the laser beam L is hardly absorbed by the surface 3 of the workpiece 1, the surface 3 of the workpiece 1 is not melted.

  If the cutting start region 8 is formed inside the processing target 1, cracks are likely to occur from the cutting start region 8, so that the processing target 1 is cut with a relatively small force as shown in FIG. 6. be able to. Therefore, the processing object 1 can be cut with high accuracy without causing unnecessary cracks on the surface 3 of the processing object 1.

  The following two types of cutting of the workpiece 1 starting from the cutting start region 8 are conceivable. First, after the cutting start region 8 is formed, when the artificial force is applied to the processing target 1, the processing target 1 is broken starting from the cutting start region 8 and the processing target 1 is cut. It is. This is, for example, cutting when the workpiece 1 is thick. The artificial force is applied by, for example, applying bending stress or shear stress to the workpiece 1 along the cutting start region 8 of the workpiece 1 or giving a temperature difference to the workpiece 1. It is to generate thermal stress. The other one forms the cutting start region 8 so that it is naturally cracked from the cutting start region 8 in the cross-sectional direction (thickness direction) of the processing target 1, and as a result, the processing target 1 is formed. This is the case when it is disconnected. This is possible, for example, when the thickness of the workpiece 1 is small, and the cutting start region 8 is formed by one row of the modified region 7, and when the thickness of the workpiece 1 is large. This is made possible by forming the cutting start region 8 by the modified region 7 formed in a plurality of rows in the thickness direction. Even in the case of natural cracking, cracks do not run on the surface 3 of the portion corresponding to the portion where the cutting start region 8 is not formed in the portion to be cut, and the portion where the cutting start region 8 is formed. Since only the corresponding part can be cleaved, the cleaving can be controlled well. In recent years, since the thickness of the workpiece 1 such as a silicon wafer tends to be thin, such a cleaving method with good controllability is very effective.

  In the laser processing method according to the present embodiment, there are the following cases (1) to (3) as modified regions formed by multiphoton absorption.

(1) In the case where the modified region is a crack region including one or a plurality of cracks, the focusing point is set inside the object to be processed (for example, a piezoelectric material made of glass or LiTaO ₃ ), and the electric field strength at the focusing point is Irradiation with laser light is performed under conditions of 1 × 10 ⁸ (W / cm ² ) or more and a pulse width of 1 μs or less. The magnitude of this pulse width is a condition that allows a crack region to be formed only inside the workpiece without causing extra damage to the surface of the workpiece while causing multiphoton absorption. As a result, a phenomenon of optical damage due to multiphoton absorption occurs inside the workpiece. This optical damage induces thermal strain inside the workpiece, thereby forming a crack region inside the workpiece. The upper limit value of the electric field strength is, for example, 1 × 10 ¹² (W / cm ² ). The pulse width is preferably 1 ns to 200 ns, for example. The formation of the crack region by multiphoton absorption is described in, for example, “Inside of glass substrate by solid-state laser harmonics” on pages 23-28 of the 45th Laser Thermal Processing Research Papers (December 1998). It is described in “Marking”.

  The inventor obtained the relationship between the electric field strength and the size of the cracks by experiment. The experimental conditions are as follows.

(A) Workpiece: Pyrex (registered trademark) glass (thickness 700 μm)
(B) Laser
Light source: Semiconductor laser pumped Nd: YAG laser
Wavelength: 1064nm
Laser light spot cross-sectional area: 3.14 × 10 ⁻⁸ cm ²
Oscillation form: Q switch pulse
Repeat frequency: 100 kHz
Pulse width: 30ns
Output: Output <1mJ / pulse
Laser light quality: TEM ₀₀
Polarization characteristics: Linearly polarized light (C) Condensing lens
Transmittance with respect to laser beam wavelength: 60% (D) Moving speed of mounting table on which workpiece is mounted: 100 mm / second

Note that the laser light quality TEM ₀₀ means that the light condensing performance is high and the light can be condensed up to the wavelength of the laser light.

FIG. 7 is a graph showing the results of the experiment. The horizontal axis represents the peak power density. Since the laser beam is a pulsed laser beam, the electric field strength is represented by the peak power density. The vertical axis represents the size of a crack portion (crack spot) formed inside the workpiece by one pulse of laser light. Crack spots gather to form a crack region. The size of the crack spot is the size of the portion having the maximum length in the shape of the crack spot. Data indicated by black circles in the graph is for the case where the magnification of the condenser lens (C) is 100 times and the numerical aperture (NA) is 0.80. On the other hand, the data indicated by the white circles in the graph is when the magnification of the condenser lens (C) is 50 times and the numerical aperture (NA) is 0.55. From the peak power density of about 10 ¹¹ (W / cm ² ), it can be seen that a crack spot is generated inside the workpiece, and the crack spot increases as the peak power density increases.

  Next, the mechanism of cutting the workpiece by forming the crack region will be described with reference to FIGS. As shown in FIG. 8, under the condition that multiphoton absorption occurs, the converging point P is aligned with the inside of the workpiece 1 and the laser beam L is irradiated to form a crack region 9 along the planned cutting line. The crack region 9 is a region including one or more cracks. The crack region 9 thus formed becomes a cutting start region. As shown in FIG. 9, the crack further grows from the crack region 9 (that is, from the cutting start region), and the crack is formed on the front surface 3 and the back surface 21 of the workpiece 1 as shown in FIG. 10. As shown in FIG. 11, the workpiece 1 is broken and the workpiece 1 is cut. A crack that reaches the front surface 3 and the back surface 21 of the workpiece 1 may grow naturally, or may grow when a force is applied to the workpiece 1.

(2) When the reforming region is a melt processing region The focusing point is set inside the object to be processed (for example, a semiconductor material such as silicon), and the electric field strength at the focusing point is 1 × 10 ⁸ (W / cm ^2). ) Irradiation with laser light is performed under the above conditions with a pulse width of 1 μs or less. As a result, the inside of the workpiece is locally heated by multiphoton absorption. By this heating, a melt processing region is formed inside the workpiece. The melt treatment region is a region once solidified after melting, a region in a molten state, or a region re-solidified from a molten state, and can also be referred to as a phase-changed region or a region in which the crystal structure has changed. The melt treatment region can also be said to be a region in which one structure is changed to another structure in a single crystal structure, an amorphous structure, or a polycrystalline structure. In other words, for example, a region changed from a single crystal structure to an amorphous structure, a region changed from a single crystal structure to a polycrystalline structure, or 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 has, for example, an amorphous silicon structure. The upper limit value of the electric field strength is, for example, 1 × 10 ¹² (W / cm ² ). The pulse width is preferably 1 ns to 200 ns, for example.

  The inventor has confirmed through experiments that a melt-processed region is formed inside a silicon wafer. The experimental conditions are as follows.

(A) Workpiece: silicon wafer (thickness 350 μm, outer diameter 4 inches)
(B) Laser
Light source: Semiconductor laser pumped Nd: YAG laser
Wavelength: 1064nm
Laser light spot cross-sectional area: 3.14 × 10 ⁻⁸ cm ²
Oscillation form: Q switch pulse
Repeat frequency: 100 kHz
Pulse width: 30ns
Output: 20μJ / pulse
Laser light quality: TEM ₀₀
Polarization characteristics: Linearly polarized light (C) Condensing lens
Magnification: 50 times
N. A. : 0.55
Transmittance with respect to laser beam wavelength: 60% (D) Moving speed of mounting table on which workpiece is mounted: 100 mm / second

  FIG. 12 is a view 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 in the thickness direction of the melt processing region 13 formed under the above conditions is about 100 μm.

  The fact that the melt processing region 13 is formed by multiphoton absorption will be described. FIG. 13 is a graph showing the relationship between the wavelength of the laser beam and the transmittance inside the silicon substrate. However, the reflection components on the front side and the back side of the silicon substrate are removed to show the transmittance 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.

  For example, when the thickness of the silicon substrate is 500 μm or less at the wavelength of the Nd: YAG laser of 1064 nm, it can be seen that the laser light is transmitted by 80% or more 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 11, 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. Therefore, the laser beam is hardly absorbed inside the silicon wafer 11 and almost all is transmitted. This is not because 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 with laser light) It means that the melt processing region 13 is formed by multiphoton absorption. The formation of the melt processing region by multiphoton absorption is described in, for example, “Evaluation of processing characteristics of silicon by picosecond pulse laser” on pages 72 to 73 of the 66th Annual Meeting of the Japan Welding Society (April 2000). Have been described.

  Note that a silicon wafer is cracked as a result of generating cracks in the cross-sectional direction starting from the cutting start region formed by the melt processing region and reaching the front and back surfaces of the silicon wafer. The The cracks that reach the front and back surfaces of the silicon wafer may grow naturally or may grow by applying force to the silicon wafer. And when a crack naturally grows from the cutting start region to the front and back surfaces of the silicon wafer, the case where the crack grows from a state where the melt treatment region forming the cutting starting region is melted, and the cutting starting region There are both cases where cracks grow when the solidified region is melted from the molten state. However, in either case, the melt processing region is formed only inside the silicon wafer, and the melt processing region is formed only inside the cut surface after cutting as shown in FIG. In this way, when the cutting start region is formed by the melt processing region inside the workpiece, unnecessary cracking off the cutting start region line is unlikely to occur during cleaving, so that cleaving control is facilitated. Incidentally, the formation of the melt processing region is not only caused by multiphoton absorption, but may also be caused by other absorption effects.

(3) When the modified region is a refractive index changing region The focusing point is set inside the object to be processed (for example, glass), and the electric field intensity at the focusing point is 1 × 10 ⁸ (W / cm ² ) or more. Laser light is irradiated under the condition that the pulse width is 1 ns or less. When the pulse width is made extremely short and multiphoton absorption occurs inside the workpiece, the energy due to the multiphoton absorption is not converted into thermal energy, and the ion valence change and crystallization occur inside the workpiece. Alternatively, a permanent structural change such as polarization orientation is induced to form a refractive index change region. The upper limit value of the electric field strength is, for example, 1 × 10 ¹² (W / cm ² ). For example, the pulse width is preferably 1 ns or less, and more preferably 1 ps or less. The formation of the refractive index changing region by multiphoton absorption is described in, for example, “The Femtosecond Laser Irradiation to the Inside of Glass” on pages 105 to 111 of the 42nd Laser Thermal Processing Research Institute Proceedings (November 1997). Photo-induced structure formation ”.

  As described above, the cases of (1) to (3) have been described as the modified regions formed by multiphoton absorption. However, considering the crystal structure of the wafer-like workpiece and its cleavage property, the cutting origin region is described below. If it forms in this way, it will become possible to cut | disconnect a process target object with much smaller force from the cutting | disconnection starting point area | region as a starting point with still smaller force.

That is, in the case of a substrate made of a single crystal semiconductor having a diamond structure such as silicon, the cutting start region is formed in a direction along the (111) plane (first cleavage plane) or the (110) plane (second cleavage plane). Is preferred. In the case of a substrate made of a zinc-blende-type III-V group compound semiconductor such as GaAs, it is preferable to form the cutting start region in the direction along the (110) plane. Further, in the case of a substrate having a hexagonal crystal structure such as sapphire (Al ₂ O ₃ ), the (1120) plane (A plane) or (1100) plane ( It is preferable to form the cutting start region in a direction along the (M plane).

  Note that the orientation flat on the substrate along the direction in which the above-described cutting start region is to be formed (for example, the direction along the (111) plane in the single crystal silicon substrate) or the direction perpendicular to the direction in which the cutting start region is to be formed. By using the orientation flat as a reference, it is possible to easily and accurately form the cutting start area along the direction in which the cutting start area is to be formed on the substrate.

  Next, a preferred embodiment of the present invention will be described.

  As shown in FIGS. 14 and 15, the workpiece 1 is a so-called MEMS wafer, which is formed on the surface of the silicon wafer 11 including a silicon wafer (semiconductor substrate) 11 having a thickness of 300 μm and a plurality of functional elements 15. The functional element layer 16 and the metal film 17 formed on the back surface of the silicon wafer 11 are provided. The functional elements 15 are, for example, machine element parts, sensors, actuators, electronic circuits, and the like, and are formed in a matrix in a direction parallel to and perpendicular to the orientation flat 6 of the silicon wafer 11. The metal film 17 is made of gold and has a thickness of 3 μm.

  The workpiece 1 configured as described above is cut for each functional element 15 as follows. First, as shown in FIG. 16, the expanded tape 23 is attached to the back surface 21 of the workpiece 1, that is, the back surface of the metal film 17. Then, the workpiece 1 is fixed on a mounting table (not shown) of the laser processing apparatus with the functional element layer 16 facing upward.

  Subsequently, as shown in FIG. 17, the surface 3 of the workpiece 1, that is, the surface of the functional element layer 16 is set as the laser light incident surface, and a position 320 μm from the surface of the silicon wafer 11 (outside of the silicon wafer 11) is collected. When the laser light is transmitted through a laser light reflecting surface (here, the surface 17a of the metal film 17) facing the position of the condensing lens serving as the light spot (the laser light incident surface (here, the surface 3) of the workpiece). Assuming that the same applies to the following, the laser beam L is irradiated, and the planned cutting line 5 (broken line in FIG. 14) set in a lattice shape so as to pass between the adjacent functional elements 15 and 15 by moving the mounting table. The laser beam L is scanned along the reference).

At this time, the laser light L is reflected by the surface (predetermined surface) 17a of the metal film 17 facing the surface 3 of the workpiece 1, that is, the laser light incident surface side of the metal film 17, and the reflected light RL is reflected. The silicon wafer 11 is irradiated and condensed near the back surface 21 inside the silicon wafer 11. Thus, in the immediate vicinity of the back surface 21 in the interior of the silicon wafer 11, the molten processed region 13 ₁ and the microcavity 14 is formed along the line to cut 5. In this case, the laser light irradiation conditions are a pulse width of 150 ns and an energy of 15 μJ. In addition, the above-mentioned “position of 320 μm from the surface of the silicon wafer 11” means a theoretical “position where the condensing point P is aligned” without considering spherical aberration or the like.

Here, the microcavity 14 will be described. Generally, a condensing point is set inside the silicon wafer 11, and laser light is irradiated under conditions where the peak power density at the condensing point is 1 × 10 ⁸ (W / cm ² ) or more and the pulse width is 1 μs or less. Then, the melt processing region 13 and the microcavity 14 may be formed in pairs inside the silicon wafer 11. The microcavity 14 may be formed away from the melt processing region 13 or may be formed continuously with the melt processing region 13, but on the downstream side of the melt processing region 13 in the laser beam traveling direction. It is formed. In the case described above, since the microcavity 14 is formed on the surface 3 side of the object 1 relative to the molten processed region 13 _1, the reflected light RL contributes to the formation of the molten processed region 13 ₁ and the microcavity 14 It can be said that. Note that the principle of forming the melt processing region 13 and the microcavity 14 in pairs is described in detail in JP-A-2005-57257.

  Further, as shown in FIG. 18, the surface 3 of the workpiece 1 is used as the laser beam incident surface, and the laser beam L is irradiated with the focusing point P inside the silicon wafer 11. The laser beam L is scanned along the scheduled cutting lines 5 set in a lattice shape so as to pass between the functional elements 15 and 15.

The scanning of the laser beam L along the planned cutting line 5 is performed five times on one planned cutting line 5, but the distance between the surface of the silicon wafer 11 and the position where the condensing point P is aligned is changed each time. it is, between the molten processed region 13 ₁ and the surface of the silicon wafer 11, to form the molten processed region 13 ₂ of the five columns along the line to cut 5. Incidentally, the number of columns of the molten processed region 13 ₂ formed within the silicon wafer 11 with respect to one line to cut 5, which changes according to the thickness and the like of the silicon wafer 11, in five rows It is not limited. Further, the rear face 21 side of the object 1 with respect to each molten processed region 13 _2, there is a case where the minute cavities 14 are formed to become molten processed region 13 ₂ a pair. Further, cracks may be mixed in the melt processing regions 13 ₁ and 13 ₂ .

Subsequently, as shown in FIG. 19, the expandable tape 23 is expanded, as a starting point for cutting the molten processed region 13 _1, 13 _2, to cut the object 1 along the line to cut 5. At this time, since the expanded tape 23 is expanded, the plurality of semiconductor chips 25 obtained by cutting are separated from each other.

  The above-described laser processing method is performed by the laser processing apparatus shown in FIG. As shown in FIG. 26, the laser processing apparatus 100 includes a laser light source 101 that emits laser light L, a dichroic mirror 103 that is arranged so as to change the direction of the optical axis of the laser light L, and a laser light L. A condensing lens 105 for condensing light. Further, the laser processing apparatus 100 includes a mounting table 107 for supporting the workpiece 1 irradiated with the laser light L collected by the condensing lens 105, and the mounting table 107 in the X, Y, and Z axis directions. And a control unit 115 that controls the entire laser processing apparatus 100, such as adjusting the output and pulse width of the laser beam L, moving the stage 111, and the like.

  In this laser processing apparatus 100, the laser light L emitted from the laser light source 101 has its optical axis changed by 90 ° by the dichroic mirror 103, and is directed toward the processing object 1 placed on the placing table 107. Then, the light is condensed by the condensing lens 105. 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. As a result, a modified region serving as a starting point for cutting is formed on the workpiece 1 along the planned cutting line 5.

As described above, in the laser processing method described above, the reflected light RL of the laser light L reflected from the surface 17a of the metal film 17 facing the surface 3 that is the laser light incident surface of the workpiece 1 is converted into the silicon wafer 11. by irradiating the molten processed region ₁₃ 1 of 6 rows, 13 of the _two, to form a closest molten processed region 13 ₁ on the surface 17a of the metal film 17. Thus, as shown in FIG. 20, the molten processed region 13 ₁ can be formed very close to the surface 17a of the metal film 17. When the thickness of the silicon wafer 11 changes along the planned cutting line 5, or when the silicon wafer 11 is a highly doped wafer or the like and the transmittance of the laser light L is low, the vicinity of the surface 17a and the same high also it is difficult to form along the line to cut 5 the molten processed region 13 ₁ while maintaining the position, reflected Thus, the nearest formation of a molten processed region 13 ₁ on the surface 17a of the metal film 17 by utilizing light RL, can be stably form a molten processed region 13 ₁ along the line to cut 5 with a high density very close to the surface 17a of the metal film 17. Therefore, when the workpiece 1 is cut, the end on the back surface 21 side of the cut surface is prevented from coming off the scheduled cutting line 5, and the workpiece 1 is accurately cut along the scheduled cutting line 5. Is possible.

Here, in the case of using reflected light RL of the laser light L reflected by the surface 17a of the metal film 17, a description will be given of the principle that the molten processed region 13 ₁ is formed very close to the surface 17a of the metal film 17.

The first principle estimated by the present inventor is as follows. As shown in FIG. 21, when the laser beam L is irradiated with the condensing point on the back surface of the silicon wafer 11, that is, in the vicinity of the surface 17a of the metal film 17, the degree of condensing between the central ray and the surrounding rays is affected by the spherical aberration. As a result, the respective light beams are not condensed at one point, and the condensing points of the respective light beams, particularly the surrounding light beams, are shifted in the optical axis direction of the laser light L. As a result, the collection point of the light beam that converges on the surface 17a of the metal film 17 is reflected by the reflection point on the surface 17a of the metal film 17 at the condensing portion of the light beam that travels so as to be condensed below the surface 17a of the metal film 17. It is complemented by the light spot. Therefore, the molten processed region 13 ₁ along the line to cut 5 are formed in high density very close to the surface 17a of the metal film 17. Theoretically to think the spherical aberration, although the molten processed region 13 ₁ should be formed on the back surface of the silicon wafer 11 is the position of the focal point, formation of the molten processed region 13 ₁ the influence by the reflected It can be said that the position is shifted upward.

The second principle estimated by the present inventor is as follows. As shown in FIG. 22A, when the laser beam L is irradiated with the focusing point positioned below the surface 17a of the metal film 17, that is, the position of the focusing lens where the outside of the silicon wafer 11 becomes the focusing point, The laser light L is reflected by the surface 17 a of the metal film 17, and the reflected light RL is condensed inside the silicon wafer 11. The laser beam L is hardly absorbed by the silicon wafer 11 before being reflected by the surface 17a of the metal film 17, so that the laser light L is hardly absorbed by the silicon wafer 11 and locally becomes high at the position of the condensing point P of the reflected light RL. For this reason, the absorption coefficient increases at the position of the condensing point P due to the temperature dependence of the absorption coefficient, and from the position of the condensing point P of the reflected light RL upstream of the condensing point P in the traveling direction of the reflected light RL (the reflecting surface 17a). The reflected light RL is easily absorbed on the side). As a result, the molten processed region 13 ₁ along the line to cut 5 with a high density very close to the surface 17a of the metal film 17 (i.e., as a highly divided reforming region) will be formed.

  As shown in FIG. 22B, when the laser beam L is irradiated with the condensing point P on the inside of the silicon wafer 11, that is, on the surface 17 a of the metal film 17, even at a position above the condensing point P. The temperature rises. Therefore, the absorption coefficient increases at a position above the condensing point P due to the temperature dependence of the absorption coefficient, and the absorption of the laser light L starts. As a result, the laser light L traveling in the vicinity of the condensing point P in the traveling direction of the laser light L is reduced, and the upper part of the condensing point P is locally heated along the optical axis of the laser light L. . Therefore, the absorption coefficient increases in the upper part of the condensing point P due to the temperature dependence of the absorption coefficient, and the laser light L is absorbed. As a result, the energy of the laser beam L that can be used for forming the melt processing region 13 is reduced, so that the melt processing region 13 is slightly separated from the surface 17a (in the vicinity) of the metal film 17 along the planned cutting line 5. It will be formed at a low density at the position. This is because, in a thick wafer, the deeper the position from the laser light incident surface, the greater the influence of absorption, the energy of the laser light decreases, and in order to exceed the processing threshold, the temperature-dependent influence due to absorption cannot be ignored. It is guessed.

  The present invention is not limited to the above embodiment.

For example, in the above embodiment, by irradiating the reflected light RL of the laser light L reflected by the surface 17a of the metal film 17 on the silicon wafer 11, the molten processed region 13 ₁ very close to the surface 17a of the metal film 17 It was formed, as shown in FIGS. 23 and 24, may be formed molten processed region 13 ₁ very close to the surface 3 of the object 1. In this case, the surface 3 of the workpiece 1 is used as the laser light incident surface, the focusing point is set at a position 600 μm from the surface of the silicon wafer 11 (outside the silicon wafer 11), and the pulse width is 150 ns and the energy is 15 μJ. The laser beam L is irradiated under conditions. Accordingly, the surface 3 of the workpiece 1 is prevented from being damaged due to melting due to the temperature dependence of the absorption coefficient described above, and the melt processing region 13 is located in the immediate vicinity of the surface 3 of the workpiece 1. ₁ can be formed. In addition, when the workpiece 1 is cut, the end on the surface 3 side of the cut surface is prevented from coming off from the scheduled cutting line 5, and the workpiece 1 is accurately cut along the scheduled cutting line 5. Is possible.

  It should be noted that the surface of the metal film 17 in the plurality of rows of the melt processing regions 13 as well as the melt processing region 13 closest to the surface 17 a of the metal film 17 and the melt processing region 13 closest to the surface 3 of the workpiece 1. Lasers reflected by the surface 17a of the metal film 17 in a plurality of rows of the melt processing region 13 including at least one row of the melt processing region 13 closest to 17a and the melt processing region 13 closest to the surface 3 of the workpiece 1 You may form using the reflected light RL of the light L. FIG.

Further, as shown in FIG. 25, at the same time forms the closest molten processed region 13 ₁ on the surface 17a of the metal film 17, the rear face 21 of the object 1, cutting line of the weakened regions 18 having a predetermined depth 5 may be formed. In this case, the surface 3 of the workpiece 1 is used as the laser light incident surface, the focusing point is aligned with the position of 305 μm from the surface of the silicon wafer 11 (outside of the silicon wafer 11), and the pulse width is 150 ns and the energy is 15 μJ. The laser beam L is irradiated under conditions. At this time, the back surface 21 of the workpiece 1 is the back surface of the metal film 17. Even in this case, the weakened region 18 having a predetermined depth is formed on the metal film 17 along the planned cutting line 5. Therefore, the workpiece 1 can be accurately cut along the scheduled cutting line 5 with a relatively small external force. Moreover, since the molten processed region 13 ₁ is formed within the silicon wafer 11, it can be prevented from the molten processed region 13 ₁ of particles are generated.

  Moreover, in the said embodiment, although the surface which reflects the laser beam L was the surface 17a of the metal film 17, the workpiece 1 does not have the metal film 17, for example, the surface which reflects the laser beam L is a silicon wafer. 11 may be the back surface. In this case, the laser light L is partially reflected on the back surface of the silicon wafer 11 and the reflected light RL is irradiated onto the silicon wafer 11. The functional element 15 may be, for example, a semiconductor operation layer formed by crystal growth, 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.

Moreover, in the said embodiment, although the surface 3 of the workpiece 1 was made into the laser beam incident surface, when the workpiece 1 is not provided with the metal film 17, the back surface 21 of the workpiece 1 is made into the laser beam incident surface. It is good. When the back surface 21 of the workpiece 1 is a laser light incident surface, the workpiece 1 is cut into a plurality of semiconductor chips 25 as follows as an example. That is, a protective tape is affixed to the surface of the functional element layer 16, and the protective tape holding the workpiece 1 is fixed to the mounting table of the laser processing apparatus in a state where the functional element layer 16 is protected by the protective tape. Then, by irradiating the laser beam L into the silicon wafer 11 a rear face 21 of the object 1 as a laser light entrance surface along the line to cut 5 forms a molten processed region 13 _1, 13 _2. Subsequently, the protective tape fixed to the mounting table is separated from the workpiece 1. Then, paste the expandable tape 23 on the back surface 21 of the object 1, after peeling off the protective tape from the surface of the functional device layer 16, by expanding the expandable tape 23, the molten processed region 13 _1, 13 ₂ cutting As a starting point, the workpiece 1 is cut along the planned cutting line 5 and a plurality of semiconductor chips 25 obtained by cutting are separated from each other.

In the above embodiment, inside molten processed region 13 1 of the silicon wafer _11, 13 is ₂ was formed, glass or a piezoelectric material such as, inside a wafer made of other materials, crack region or a refractive index changed regions, Other modified regions may be formed.

  Further, the modified region 7 may be formed on the workpiece 1 as follows. First, as shown in FIG. 27A, the position near the laser light reflecting surface (here, the back surface 21) facing the laser light incident surface (here, the front surface 3) of the workpiece 1 is a condensing point. The modified region 7a is formed by irradiating the laser beam L to P. Thereafter, as shown in FIG. 27B, when it is assumed that the laser light is transmitted through the laser light reflecting surface, the laser light source is downstream in the traveling direction of the laser light L (the laser light source with respect to the laser light reflecting surface). By irradiating the laser beam L so that the position on the side opposite to the laser beam emission side becomes the condensing point P, the modified region 7b is formed by the reflected light RL. In this way, by forming the modified region 7a and the modified region 7b so as to overlap each other, the modified region 7 having a high density (that is, having a high splitting property) can be formed on the workpiece 1.

  By irradiating the workpiece 1 with the laser light L, a plurality of rows of the modified regions 7 are arranged at least on the workpiece 1 so as to be aligned in the thickness direction of the workpiece 1 along one cutting scheduled line 5. The present invention is not limited to the case of forming inside, and one row of modified regions 7 may be formed at least inside the workpiece 1 along one cutting scheduled line 5.

It is a top view of the processing target object during laser processing by the laser processing method concerning this embodiment. It is sectional drawing along the II-II line of the workpiece shown in FIG. It is a top view of the processing target after laser processing by the laser processing method concerning this embodiment. It is sectional drawing along the IV-IV line of the workpiece shown in FIG. It is sectional drawing along the VV line of the workpiece shown in FIG. It is a top view of the processed object cut | disconnected by the laser processing method which concerns on this embodiment. It is a graph which shows the relationship between the peak power density and the magnitude | size of a crack spot in the laser processing method which concerns on this embodiment. It is sectional drawing of the process target object in the 1st process of the laser processing method which concerns on this embodiment. It is sectional drawing of the process target object in the 2nd process of the laser processing method which concerns on this embodiment. It is sectional drawing of the process target object in the 3rd process of the laser processing method which concerns on this embodiment. It is sectional drawing of the process target object in the 4th process of the laser processing method which concerns on this embodiment. It is a figure showing the photograph of the section in the part of silicon wafer cut by the laser processing method concerning this embodiment. It is a graph which shows the relationship between the wavelength of the laser beam and the internal transmittance | permeability of a silicon substrate in the laser processing method which concerns on this embodiment. It is a top view of the processing target used as the object of the laser processing method of this embodiment. It is a fragmentary sectional view along the XV-XV line shown in FIG. It is a fragmentary sectional view of the processing object for explaining the laser processing method of this embodiment. It is a fragmentary sectional view of the processing object for explaining the laser processing method of this embodiment. It is a fragmentary sectional view of the processing object for explaining the laser processing method of this embodiment. It is a fragmentary sectional view of the processing object for explaining the laser processing method of this embodiment. It is a fragmentary sectional view along the XX-XX line shown in FIG. It is a figure for demonstrating the 1st principle of the laser processing method of this embodiment. It is a figure for demonstrating the 2nd principle of the laser processing method of this embodiment. It is a fragmentary sectional view of the processing object for explaining other laser processing methods of this embodiment. It is a fragmentary sectional view along the XX-XX line shown in FIG. It is a fragmentary sectional view along the XX-XX line shown in FIG. It is a schematic block diagram of the laser processing apparatus of this embodiment. It is a fragmentary sectional view of the processing object for explaining other laser processing methods of this embodiment.

Explanation of symbols

1 ... workpiece, 3 ... surface (laser light entrance surface), 5 ... line to cut, 11 ... silicon wafer (semiconductor substrate), ₁₃ 1, 13 2 _... molten processed region (modified region), 17 ... metal film 17a ... surface (predetermined surface), L ... laser light, RL ... reflected light.

Claims (9)

By irradiating a plate-like workpiece with laser light, modification of a plurality of rows serving as starting points of cutting so as to line up in the thickness direction of the workpiece along the planned cutting line of the workpiece A laser processing method for forming a region inside the workpiece,
The workpiece the laser light to face the incident surface and the workpiece reflected light of the reflected laser beams in the plane der Ru predetermined surface of the laser light incident surface side of the metal film included in the laser light is incident at 1 to include at least one row of a modified region closest to the predetermined surface and a modified region closest to the laser light incident surface among a plurality of the modified regions. Forming a modified region of a row or multiple rows ,
When forming the modified region closest to the predetermined surface, the weakened region is formed in the metal film along the planned cutting line at the same time as forming the modified region. Method. As a starting point for cutting said modified regions of a plurality of rows, the laser processing method according to claim 1, wherein the cutting the workpiece along the cutting line. The workpiece comprises a semiconductor substrate, a laser processing method according to claim 1 or 2, wherein the modified region is characterized by containing a molten processed region. By irradiating a plate-like workpiece with laser light , modification of a plurality of rows serving as starting points of cutting so as to line up in the thickness direction of the workpiece along the planned cutting line of the workpiece A laser processing method for forming a region inside the workpiece,
When said laser beam opposite to the incident surface and the surface der of the the metal film workpiece comprises laser light entrance surface side Ru laser light reflecting surface of the laser beam of the object is assumed to transmit a laser beam The downstream position of the laser light reflecting surface in the traveling direction is a condensing point of the laser light collected by a condensing lens for condensing the laser light on the workpiece. By arranging a condensing lens and irradiating the workpiece with laser light , among the modified regions in a plurality of rows, the modified region closest to the laser light reflecting surface and the laser light incident surface Forming one or more modified regions including at least one row of near modified regions ;
When forming the modified region closest to the laser light reflecting surface, a weakened region is formed in the metal film along the planned cutting line simultaneously with forming the modified region. Processing method. By irradiating a plate-like workpiece with laser light , modification of a plurality of rows serving as starting points of cutting so as to line up in the thickness direction of the workpiece along the planned cutting line of the workpiece A laser processing apparatus for forming a region inside the workpiece,
When said laser beam opposite to the incident surface and the surface der of the the metal film workpiece comprises laser light entrance surface side Ru laser light reflecting surface of the laser beam of the object is assumed to transmit a laser beam The downstream position of the laser light reflecting surface in the traveling direction is a condensing point of the laser light collected by a condensing lens for condensing the laser light on the workpiece. By arranging a condensing lens and irradiating the workpiece with laser light , among the modified regions in a plurality of rows, the modified region closest to the laser light reflecting surface and the laser light incident surface Forming one or more modified regions including at least one row of near modified regions ;
When forming the modified region closest to the laser light reflecting surface, a weakened region is formed in the metal film along the planned cutting line simultaneously with forming the modified region. Processing equipment.   By irradiating the object to be processed with the reflected light of the laser beam reflected by the predetermined surface, the modified region is formed as a pair of a melt treatment region and a microcavity, and the microcavity is formed by the melting. The laser processing method according to claim 1, wherein the laser processing method is formed on the laser light incident surface side with respect to a processing region. When irradiating the object to be processed with the reflected light of the laser beam reflected by the predetermined surface, theoretically, the condensing point of the laser light is adjusted to the object to be processed unless spherical aberration is considered. The laser processing method according to claim 6 . By irradiating the object to be processed with the reflected light of the laser light reflected by the laser light reflecting surface, the modified region is formed as a pair of a melt treatment region and a microcavity, and the microcavity is The laser processing method according to claim 4 , wherein the laser processing method is formed on the laser light incident surface side with respect to the melt processing region. When irradiating the workpiece with the reflected light of the laser beam reflected by the laser beam reflecting surface, theoretically, if the spherical aberration is not considered, the condensing point of the laser beam is aligned with the workpiece. The laser processing method according to claim 8, wherein:

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