Patent Publication Number: US-9902017-B2

Title: Laser machining device and laser machining method

Description:
TECHNICAL FIELD 
     One aspect of the present invention relates to a laser processing device and a laser processing method. 
     BACKGROUND ART 
     Known as a conventional laser processing device is one which converges laser light at an object to be processed, so as to form a modified region within the object along a line to cut (see, for example, Patent Literature 1). In such a laser processing device, a spatial light modulator modulates laser light emitted from a laser light source to the object, so as to converge the laser light at a plurality of positions in the object. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-Open No. 2011-51011 
     SUMMARY OF INVENTION 
     Technical Problem 
     The above-mentioned conventional technique has been demanded to improve processing quality along with its wide-spreading use in recent years. For example, concerning the cut object, there have been increasing demands for ameliorating straightness in cut sections, improving bending strength, and suppressing damages in the laser light entrance surface and its opposite surface. 
     In view of the foregoing circumstances, it is an object of one aspect of the present invention to provide a laser processing device and laser processing method which can improve the processing quality. 
     Solution to Problem 
     The inventors conducted diligent studies for solving the problem mentioned above and, as a result, have found a possibility that, when a laser light converging part has a form elongated in the laser light incident direction of the laser light (hereinafter referred to as “vertically long”) in the object so as to form a vertically long modified region in the object, the amount of fractures (cracks) occurring within the object can decrease, so as to ameliorate the straightness in the cut sections and the bending strength, thereby improving the processing quality. 
     In this case, however, there is a fear of the energy density decreasing in the laser light converging part so that the modified region itself may fail to be formed or a large amount of energy being consumed for processing. In this regard, the inventors further conducted diligent studies and have found that such a fear can be coped with by modulating the laser light as appropriate by using a spatial light modulator, thereby completing the present invention. 
     That is, the laser processing device in accordance with one aspect of the present invention is a laser processing device for converging laser light at an object to be processed so as to form a plurality of rows of modified regions along a laser light irradiation direction within the object along a line to cut, the laser processing device comprising a laser light source emitting the laser light, a spatial light modulator modulating the laser light emitted from the laser light source, and a converging optical system converging the laser light modulated by the spatial light modulator at the object; the plurality of rows of modified regions include at least an entrance-surface-side modified region located on the laser light entrance surface side, an opposite-surface-side modified region located on the opposite surface side of the laser light entrance surface, and a middle modified region located between the entrance-surface-side modified region and opposite-surface-side modified region; when forming the middle modified region, the spatial light modulator displays an axicon lens pattern as a modulation pattern so as to form converging points at a plurality of positions juxtaposed close to each other along a laser light irradiation direction; and when forming the entrance-surface-side modified region and opposite-surface-side modified region, the spatial light modulator is restrained from displaying the axicon lens pattern as the modulation pattern. By the “axicon lens pattern” is meant a modulation pattern generated such as to achieve an action of an axicon lens (the same shall apply hereinafter). 
     When forming the middle modified region, this laser processing device can converge laser light so as to form converging points at a plurality of positions juxtaposed close to each other along the laser light irradiation direction by causing the spatial light modulator to display the axicon lens pattern as a modulation pattern. This forms a pseudo vertically long converging part while fully keeping its energy density, whereby a vertically long middle modified region is formed in the object. As with the above-mentioned finding, such a middle modified region can reduce the amount of fractures occurring within the object, thereby improving the straightness in the cut sections and the bending strength. When forming the entrance-surface-side modified region and the opposite-surface-side modified region, on the other hand, the spatial light modulator is not caused to display the axicon lens pattern as the modulation pattern, whereby damages can be inhibited from becoming easier to occur in the laser light entrance surface and its opposite surface because of the fact that the entrance-surface-side modified region and opposite-surface-side are vertically long. Therefore, one aspect of the present invention can improve the processing quality. 
     Specifically, for exhibiting the operation and effect mentioned above, the laser processing device may form a plurality of modified spots within the object along the line and let a plurality of the modified regions form the modified region; when forming the middle modified region, the spatial light modulator may form modified dots respectively at the plurality of positions juxtaposed close to each other along the laser light irradiation direction; and a plurality of the modified dots may constitute the modified spot elongated in the laser light irradiation direction. 
     In a display part of the spatial light modulator, the axicon lens pattern may have a circular region located at a center of the laser light incident thereon and a plurality of circular ring regions demarcated about the center region concentrically with the circular region, each of the circular region and the plurality of circular ring regions being set such as to increase brightness gradually from radially outer side to inner side. 
     The laser processing method in accordance with one aspect of the present invention is a laser light processing method comprising modulating laser light emitted from a laser light source with a spatial light modulator and converging the laser light modulated by the spatial light modulator at an object to be processed so as to form a plurality of rows of modified regions along a laser light irradiation direction within the object along a line to cut; the method comprising at least the steps of forming an entrance-surface-side modified region located on the laser light entrance surface side, forming an opposite-surface-side modified region located on the opposite surface side of the laser light entrance surface, and forming a middle modified region located between the entrance-surface-side modified region and the opposite-surface-side modified region; the step of forming the middle modified region causes the spatial light modulator to display an axicon lens pattern as a modulation pattern so as to converge the laser light such that converging points are formed at a plurality of positions juxtaposed adjacent to each other along the laser light irradiation direction; and the steps of forming the entrance-surface-side modified region and the opposite-surface-side modified region restrain the spatial light modulator from displaying the axicon lens pattern. 
     When forming the middle modified region, this laser processing method also forms a pseudo vertically long converging part while fully keeping its energy density, thereby producing a vertically long middle modified region. When forming the entrance-surface-side modified region and the opposite-surface-side modified region, on the other hand, damages can be inhibited from becoming easier to occur in the laser light entrance surface and its opposite surface because of the fact that the entrance-surface-side modified region and opposite-surface-side are vertically long. Therefore, the above-mentioned operation and effect of being able to improve the processing quality is exhibited. 
     Advantageous Effects of Invention 
     One aspect of the present invention can provide a laser processing device and laser processing method which can improve the processing quality. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic structural diagram of a laser processing device used for forming a modified region; 
         FIG. 2  is a plan view of an object to be processed for which the modified region is formed; 
         FIG. 3  is a sectional view of the object taken along the line III-III of  FIG. 2 ; 
         FIG. 4  is a plan view of the object after laser processing; 
         FIG. 5  is a sectional view of the object taken along the line V-V of  FIG. 4 ; 
         FIG. 6  is a sectional view of the object taken along the line VI-VI of  FIG. 4 ; 
         FIG. 7  is a schematic structural diagram illustrating the performing laser processing device in accordance with an embodiment; 
         FIG. 8  is a partial sectional view of a reflective spatial light modulator; 
         FIG. 9  is a diagram illustrating an axicon lens pattern displayed in a liquid crystal layer of the reflective spatial light modulator; 
         FIG. 10  is a plan view illustrating the object to be subjected to laser processing; 
         FIG. 11  is a schematic sectional view for explaining the laser processing method in accordance with a first embodiment; 
         FIG. 12  is a photograph illustrating an example of modified spots formed by the laser processing method in accordance with the first embodiment; 
         FIG. 13  is a photograph for explaining effects of the laser processing method in accordance with the first embodiment; 
         FIG. 14  is another photograph for explaining effects of the laser processing method in accordance with the first embodiment; 
         FIG. 15  is a schematic sectional view for explaining the laser processing method in accordance with a second embodiment; 
         FIG. 16  is a schematic sectional view illustrating a sequel to  FIG. 15 ; and 
         FIG. 17  is a diagram for explaining aberrations occurring at converging positions of laser light. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, preferred embodiments in accordance with one aspect of the present invention will be explained in detail with reference to the drawings. In the following explanations, the same or equivalent constituents will be referred to with the same signs while omitting their overlapping descriptions. 
     The laser processing device and laser processing method in accordance with an embodiment converges laser light at an object to be processed with laser light, so as to form a modified region along a line to cut. Therefore, the forming of the modified region will be explained at first with reference to  FIGS. 1 to 6 . 
     As illustrated in  FIG. 1 , a laser processing device  100  comprises a laser light source  101  for causing laser light L to oscillate in a pulsating manner, a dichroic mirror  103  arranged such as to change the direction of the optical axis (optical path) of the laser light L by 90°, and a converging lens  105  for converging the laser light L. The laser processing device  100  further comprises a support table  107  for supporting an object to be processed  1  which is irradiated with the laser light L converged by the converging lens  105 , a stage  111  for moving the support table  107 , a laser light source controller  102  for regulating the laser light source  101  in order to adjust the output, pulse width, pulse waveform, and the like of the laser light L, and a stage controller  115  for regulating the movement of the stage  111 . 
     In the laser processing device  100 , the laser light L emitted from the laser light source  101  changes the direction of its optical axis by 90° with the dichroic mirror  103  and then is converged by the converging lens  105  into the object  1  mounted on the support table  107 . At the same time, the stage  111  is shifted, so that the object  1  moves relative to the laser light L along a line to cut  5 . This forms a modified region in the object  1  along the line  5 . While the stage  111  is shifted here for relatively moving the laser light L, the converging lens  105  may be shifted instead or together therewith. 
     Employed as the object  1  is a planar member (e.g., a substrate or a wafer), examples of which include semiconductor substrates formed of semiconductor materials and piezoelectric substrates formed of piezoelectric materials. As illustrated in  FIG. 2 , the line  5  for cutting the object  1  is set in the object  1 . The line  5  is a virtual line extending straight. When forming a modified region within the object  1 , the laser light L is relatively moved along the line  5  (i.e., in the direction of arrow A in  FIG. 2 ) while locating a converging point P within the object  1  as illustrated in  FIG. 3 . This forms a modified region  7  within the object  1  along the line  5  as illustrated in  FIGS. 4 to 6 , whereby the modified region  7  formed along the line  5  becomes a cutting start region  8 . 
     The converging point P is a position at which the laser light L is converged. The line  5  may be curved instead of being straight, a three-dimensional one combining them, or one specified by coordinates. The line  5  may be one actually drawn on a front face  3  of the object  1  without being restricted to the virtual line. The modified region  7  may be formed either continuously or intermittently. The modified region  7  may be formed either in rows or dots and is only required to be formed at least within the object  1 . There are cases where fractures are formed from the modified region  7  acting as a start point, and the fractures and modified region  7  may be exposed at outer surfaces (the front face  3 , rear face  21 , and outer peripheral surface) of the object  1 . The laser light entrance surface for forming the modified region  7  is not limited to the front face  3  of the object  1  but may be the rear face  21  of the object  1 . 
     Here, the laser light L is absorbed in particular in the vicinity of the converging point within the object  1  while being transmitted therethrough, whereby the modified region  7  is formed in the object  1  (i.e., internal absorption type laser processing). Therefore, the front face  3  of the object  1  hardly absorbs the laser light L and thus does not melt. In the case of forming a removing part such as a hole or groove by melting it away from the front face  3  (surface absorption type laser processing), the processing region gradually progresses from the front face  3  side to the rear face side in general. 
     By the modified region  7  formed in this embodiment are meant regions whose physical characteristics such as density, refractive index, and mechanical strength have attained states different from those of their surroundings. Examples of the modified region  7  include molten processed regions (meaning at least one of regions resolidified after having being once molten, those in the molten state, and those in the process of resolidifying from the molten state), crack regions, dielectric breakdown regions, refractive index changed regions, and their mixed regions. Other examples of the modified region include areas where the density of the modified region has changed from that of an unmodified region and areas formed with a lattice defect in a material of the object (which may also collectively be referred to as high-density transitional regions). 
     The molten processed regions, refractive index changed regions, areas where the modified region has a density different from that of the unmodified region, or areas formed with a lattice defect may further incorporate a fracture (cut or microcrack) therewithin or at an interface between the modified and unmodified regions. The incorporated fracture may be formed over the whole surface of the modified region or in only a part or a plurality of parts thereof. Examples of the object  1  include those containing or made of silicon (Si), glass, silicon carbide (SiC), LiTaO 3 , and sapphire (Al 2 O 3 ). 
     This embodiment forms a plurality of modified spots (processing scars) along the line  5 , thereby producing the modified region  7 . The modified spots, each of which is a modified part formed by a shot of one pulse of pulsed laser light (i.e., one pulse of laser irradiation; laser shot), gather to yield the modified region  7 . Examples of the modified spots include crack spots, molten processed spots, refractive index changed spots, and those in which at least one of them is mixed. As for the modified spots, their size and lengths of fractures occurring therefrom can be controlled as necessary in view of the required cutting accuracy, the demanded flatness of cut surfaces, the thickness, kind, and crystal orientation of the object, and the like. 
     The first embodiment will now be explained. 
       FIG. 7  is a schematic structural diagram illustrating a laser processing device for performing the laser processing method in accordance with this embodiment. As illustrated in  FIG. 7 , the laser processing device  300  of this embodiment comprises a laser light source  202 , a reflective spatial light modulator  203 , a 4f optical system  241 , and a converging optical system  204  within a housing  231 . The laser light source  202 , which emits laser light L having a wavelength of 1080 nm to 1200 nm, for example, uses a fiber laser, for instance. Here, the laser light source  202  is secured to a top plate  236  of the housing  231  with a screw and the like so as to emit the laser light L horizontally. 
     The laser light source  202  may emit short pulse laser light L or ultrashort pulse laser light L. The short pulse laser light L may have a pulse width of sub-ns to sub-ms, for example. When the ultrashort pulse laser light L has a pulse width less than sub-ns, there are cases where the pulse width is about 100 ps and about 10 ps. The laser light source  202  of this embodiment emits the ultrashort pulse laser light L. The beam profile of the laser light L may be a Gaussian form (so-called Gaussian beam) or a top-hat form (so-called top-hat beam). Here, the object  1  is irradiated with the laser light L having the top-hat form. 
       FIG. 8  is a partial sectional view of the reflective spatial light modulator in the laser processing device of  FIG. 7 . As illustrated in  FIG. 8 , the reflective spatial light modulator  203  comprises a silicon substrate  213 , a drive circuit layer  914 , a plurality of pixel electrodes  214 , a reflecting film  215  such as a dielectric multilayer mirror, an alignment film  999   a , a liquid crystal layer (display part)  216 , an alignment film  999   b , a transparent conductive film  217 , and a transparent substrate  218  such as a glass substrate, which are stacked in this order. 
     The transparent substrate  218  has a front face  218   a  extending along an XY plane, while the front face  218   a  constitutes a front face of the reflective spatial light modulator  203 . The transparent substrate  218  is mainly composed of a light-transmitting material such as glass, for example, and transmits therethrough the laser light L having a predetermined wavelength incident thereon from the front face  218   a  of the reflective spatial light modulator  203  to the inside of the latter. The transparent conductive film  217  is formed on a rear face of the transparent substrate  218  and mainly composed of a conductive material (e.g., ITO) which transmits therethrough the laser light L. 
     The plurality of pixel electrodes  214  are arranged two-dimensionally according to the arrangement of a plurality of pixels on the silicon substrate  213  along the transparent conductive film  217 . Each pixel electrode  214  is made of a metal material such as aluminum, for example, while its front face  214   a  is processed flat and smooth. The plurality of pixel electrodes  214  are driven by an active matrix circuit provided in the drive circuit layer  914 . 
     The active matrix circuit is disposed between the plurality of pixel electrodes  214  and the silicon substrate  213  and controls the voltage applied to the pixel electrodes  214  according to a light image to be outputted from the reflective spatial light modulator  203 . An example of such an active matrix circuit has a first driver circuit for controlling pixel rows each aligning in the X direction and a second driver circuit for controlling pixel columns each aligning in the Y direction, which are not depicted, and is constructed such that a controller  250  (which will be explained later) applies a predetermined voltage to the pixel electrode  214  of a pixel specified by both of the driver circuits. 
     The alignment films  999   a ,  999   b  are arranged on both end faces of the liquid crystal layer  216 , respectively, so as to align a group of liquid crystal molecules in a fixed direction. As the alignment films  999   a ,  999   b , those made of a polymer material such as polyimide, whose surfaces coming into contact with the liquid crystal layer  216  have been subjected to rubbing, for example, are employed. 
     The liquid crystal layer  216  is arranged between the plurality of pixel electrodes  214  and the transparent conductive film  217  and modulates the laser light L according to an electric field formed between each pixel electrode  214  and the transparent conductive film  217 . That is, when the active matrix circuit applies a voltage to a given pixel electrode  214 , an electric field is formed between the transparent conductive film  217  and this pixel electrode  214 . 
     The electric field is applied to the reflecting film  215  and the liquid crystal layer  216  at a ratio of their respective thicknesses. The alignment direction of liquid crystal molecules  216   a  changes according to the magnitude of the electric field applied to the liquid crystal layer  216 . The laser light L entering the liquid crystal layer  216  through the transparent substrate  218  and transparent conductive film  217 , if any, is modulated by the liquid crystal molecules  216   a  while passing through the liquid crystal layer  216 , then reflected by the reflecting film  215 , and thereafter modulated again by the liquid crystal layer  216  before being taken out. 
     At this time, the controller  250 , which will be explained later, applies a voltage to each of the pixel electrodes  214  opposing the transparent conductive film  217 , whereby the refractive index in a part held between the transparent conductor film  217  and each of its opposing pixel electrodes  214  changes (the refractive index of the liquid crystal layer  216  at a position corresponding to each pixel changes). Such a change in refractive index can alter the phase of the laser light L for each pixel of the liquid crystal layer  216  according to the voltage applied. That is, a phase modulation corresponding to a hologram pattern can be imparted to the liquid crystal layer  216  for each pixel (i.e., the liquid crystal layer  216  of the reflective spatial light modulator  203  can be caused to display a modulation pattern as a hologram pattern providing a modulation). 
     As a result, the wavefront of the laser light L incident on and transmitted through the modulation pattern is adjusted, so that individual rays constituting the laser light L vary in phases of components in a predetermined direction orthogonal to their advancing direction. Therefore, appropriately setting the modulation pattern displayed in the reflective spatial light modulator  203  can modulate the laser light L (in terms of its intensity, amplitude, phase, and polarization, for example). 
     As will be explained later, the reflective spatial light modulator  203  of this embodiment displays an axicon lens pattern as a modulation pattern in the liquid crystal layer  216 , thereby modulating the laser light L, with which the object  1  is irradiated, such that the laser light L is converged at the object  1  so as to form converging points at a plurality of positions juxtaposed close to each other along the laser light irradiation direction. This forms, as illustrated in  FIG. 11 , modified dots d at the plurality of positions, respectively. 
     The plurality of modified dots d constitute one modified spot Sx to become a multipoint long modified spot. The modified spot Sx is elongated in the laser light irradiation direction (vertically long) as compared with a modified spot subjected to laser processing without displaying modulation patterns in the liquid crystal layer  216 . That is, the plurality of modified dots d are densely juxtaposed close to each other in a row along the laser light irradiation direction, while the modified spot Sx formed by a series of the plurality of modified dots d has an elongated form which is longer in the laser light irradiation direction than in directions intersecting the former direction. 
     Returning to  FIG. 7 , the 4f optical system  241  adjusts the wavefront form of the laser light L modulated by the reflective spatial light modulator  203 . The 4f optical system  241  has first and second lenses  241   a ,  241   b . The lenses  241   a ,  241   b  are arranged between the reflective spatial light modulator  203  and the converging optical system  204  such that the distance between the reflective spatial light modulator  203  and the first lens  241   a  equals the focal length f 1  of the first lens  241   a , the distance between the converging optical system  204  and the second lens  241   b  equals the focal length f 2  of the lens  241   b , the distance between the first and second lenses  241   a ,  241   b  equals f 1 +f 2 , and the first and second lenses  241   a ,  241   b  constitute a double-telecentric optical system. This 4f optical system  241  can inhibit the laser light L modulated by the reflective spatial light modulator  203  from changing its wavefront form through spatial propagation and thereby increasing aberrations. 
     The converging optical system  204  converges the laser light L modulated by the 4f optical, system  241  into the object  1 . The converging optical system  204 , which includes a plurality of lenses, is placed on a bottom plate  233  of the housing  231  while interposing therebetween a drive unit  232  including a piezoelectric device and the like. 
     In the laser processing device  300  constructed as in the foregoing, the laser light L emitted from the laser light source  202  advances horizontally within the housing  231  and then is reflected downward by a mirror  205   a , whereupon its light intensity is adjusted by an attenuator  207 . Then, the laser light L is reflected horizontally by a mirror  205   b  and, with its intensity distribution homogenized by a beam homogenizer  260 , enters the reflective spatial light modulator  203 . 
     The laser light L having entered the reflective spatial light modulator  203  passes through the modulation pattern displayed in the liquid crystal layer  216 , so as to be modulated according to the modulation pattern, then is reflected upward by a mirror  206   a , changes its polarization direction with a half-wave plate  228 , and is horizontally reflected by a mirror  206   b , so as to enter the 4f optical system  241 . 
     The laser light L having entered the 4f optical system  241  has its wavefront form adjusted so as to be incident as parallel light on the converging optical system  204 . Specifically, the laser light L converges by passing through the first lens  241   a , is reflected downward by a mirror  219 , diverges at a confocal point O, and passes through the second lens  241   b , so as to be converged again to become parallel light. Then, the laser light L passes through dichroic mirrors  210 ,  238  in sequence, so as to enter the converging optical system  204 , thereby being converged into the object  1  mounted on the stage  111 . 
     The laser processing device  300  of this embodiment also comprises, within the housing  231 , a surface observation unit  211  for observing the laser light entrance surface of the object  1  and an AF (AutoFocus) unit  212  for finely adjusting the distance between the converging optical system  204  and the object  1 . 
     The surface observation unit  211  has an observation light source  211   a  for emitting visible light VL 1  and a detector  211   b  for receiving and detecting reflected light VL 2  of the visible light VL 1  reflected by the laser light entrance surface of the object  1 . In the surface observation unit  211 , the visible light VL 1  emitted from the observation light source  211   a  is reflected by or transmitted through a mirror  208  and dichroic mirrors  209 ,  210 ,  238 , so as to be converged by the converging optical system  204  to the object  1 . The reflected light VL 2  reflected by the laser light entrance surface of the object  1  is converged by the converging optical system  204 , so as to be transmitted through or reflected by the dichroic mirrors  238 ,  210 , and then transmitted through the dichroic mirror  209 , so as to be received by the detector  211   b.    
     The AF unit  212  emits AF laser light LB 1  and receives and detects reflected light LB 2  of the AF laser light LB 1  reflected by the laser light entrance surface of the object  1 , thereby acquiring displacement data of the laser light entrance surface along the line  5 . Then, when forming the modified region  7 , the AF unit  212  drives the drive unit  232  according to thus obtained displacement data, so as to move the converging optical system  204  to and fro in its optical axis direction along undulations of the laser light entrance surface of the object  1 . 
     The laser processing device  300  further comprises the controller  250 , constituted by CPU, ROM, RAM, and the like, for controlling the laser processing device  300 . The controller  250  controls the laser light source  202 , so as to adjust the output, pulse width, and the like of the laser light L emitted from the laser light source  202 . When forming the modified region  7 , the controller  250  controls at least one of the positions of the housing  231  and stage  111  and the driving of the drive unit  232  so that the converging point P of the laser light L relatively moves along the line  5  while being located at a predetermined distance from the front face  3  of the object  1 . 
     When forming the modified region  7 , the controller  250  applies a predetermined voltage to each electrode  214  in the reflective spatial light modulator  203 , so as to cause the liquid crystal layer  216  to display a predetermined modulation pattern, thereby enabling the reflective spatial light modulator  203  to modulate the laser light L as desired. 
     Here, the modulation pattern displayed in the liquid crystal layer  216  is derived beforehand from the position to be formed with the modified region  7 , the wavelength of the laser light L emitted, the material of the object  1 , refractive indexes of the converging optical system  204  and object  1 , and the like and stored in the controller  250 . This modulation pattern includes at least one of an individual difference correction pattern for correcting individual differences occurring in the laser processing device  300  (e.g., distortions occurring in the liquid crystal layer  216  of the reflective spatial light modulator  203 ), a spherical aberration correction pattern for correcting a spherical aberration, and an axicon lens pattern. 
       FIG. 9  is a diagram illustrating an axicon lens pattern displayed in the liquid crystal layer. The depicted axicon lens pattern Ax shows a state of the liquid crystal layer  216  in its front view. As illustrated in  FIG. 9 , the axicon lens pattern Ax is a modulation pattern generated so as to achieve an action of an axicon lens. The axicon lens pattern Ax converges the laser light L at the object  1  such that converging points are formed at a plurality of positions juxtaposed close to each other along the laser light irradiation direction. In other words, the laser light L is modulated so as to have a plurality of intensity distributions in the laser light irradiation direction. Here, the axicon lens pattern Ax is an inverted conical optical pattern which is downward convex. 
     Specifically, the axicon lens pattern Ax has a circular region a 1  located at the center with respect to the incident laser light L and a plurality of circular ring regions a 2  demarcated about the circular region a 1 . The circular ring regions  2   a  are formed concentrically with the circular region a 1  and have circular ring forms with or without partial cutouts. The circular region a 1  and plurality of circular ring regions a 2  are configured such as to increase brightness gradually from their radially outer sides to inner sides. 
     When performing laser processing while causing the liquid crystal layer  216  to display the axicon lens pattern Ax, the number of converging points (number of modified dots d) juxtaposed close to each other along the laser light irradiation direction increases with a parameter number which is the number of circular ring regions a 2  (the number of turns of brightness from the center), thereby forming the vertically long modified spot Sx corresponding to the parameter number. Here, as the parameter number increases and decreases, the number of modified dots d increases and decreases on the front side of (on the upstream side of) the laser light L, whereby the resulting modified spot Sx tends to expand and contract on the front side of the laser light L. 
     A laser processing method using the above-mentioned laser processing device  300  will now be explained in detail. 
       FIG. 10  is a plan view illustrating the object to be subjected to laser processing in accordance with this embodiment, while  FIG. 11  is a sectional view for explaining the laser processing method in this embodiment. The laser processing method of this embodiment is used as a chip manufacturing method for manufacturing a plurality of chips by laser-processing the object  1 . As illustrated in  FIG. 10 , the object  1 , examples of which include silicon substrates, sapphire substrates, SiC substrates, glass substrates (tempered glass substrates), semiconductor substrates, and transparent insulated substrates, has a planar form. The thickness of the object  1 , which may be 150 μm to 350 μm, is 200 μm or 250 μm here. 
     The front face  3  of the object  1  is provided with a plurality of functional device formation regions  15  arranged in a matrix. A plurality of lines to cut  5  extending so as to pass between the functional device formation regions  15  adjacent to each other are set on the front face  3  of the object  1 . The plurality of lines  5  extend like grids and include lines to cut  5   a ,  5   b  which extend along directions substantially parallel and perpendicular to an orientation flat  6  of the object  1 , respectively. When the object  1  is a sapphire substrate, its c-plane is taken as principal surfaces (the front face  3  and rear face  21 ), and the lines  5  are configured so as to extend in directions along the r-plane of the sapphire substrate. 
     First, when laser-processing the object  1 , an expandable tape is attached to the rear face  21  of the object  1 , and the object  1  is mounted on the stage  111 . Subsequently, as illustrated in  FIGS. 7 and 11 , the controller  250  controls the reflective spatial light modulator  203 , so as to cause the liquid crystal layer  216  to display the axicon lens pattern Ax as a modulation pattern, whereupon the object  1  is irradiated with the laser light L in a pulsating manner while using the front face  3  of the object  1  as a laser light entrance surface (laser light irradiation surface) and converging the laser light L at the object  1  so as to form converging points at a plurality of positions juxtaposed close to each other along the laser light irradiation direction. At the same time, the object  1  and the laser light L are relatively moved (scanned) along the lines  5 . 
     This forms a plurality of vertically long modified spots Sx, each having modified dots d formed at a plurality of positions juxtaposed close to each other along the laser light irradiation direction, along the lines  5  at a predetermined depth in the thickness direction of the object  1 . The plurality of modified spots Sx form the modified region  7 . Thereafter, the expandable tape is expanded, so as to cut the object  1  along the lines  5  from the modified region  7  acting as a cutting start point, whereby a plurality of cut chips are obtained as semiconductor devices (examples of which include memories, IC, light-emitting devices, and light-receiving devices). 
     When forming the modified spot Sx, the following axicon lens pattern production step (axicon lens pattern production control) may be performed. In the axicon lens pattern production step, the controller  250  produces the axicon lens pattern Ax, for example, according to a state of a normal modified spot formed within the object  1  without causing the liquid crystal layer  216  to display the modulation pattern (which may simply be referred to as “normal modified spot” in the following). 
     There are cases where the modified spot Sx varies its length in the laser light irradiation direction depending on the material of the object  1  and the energy of the laser light L, thereby changing its position in the thickness direction. Therefore, the following converging point position correction step (converging point position correction control) may be performed when forming the modified spot Sx. 
     With reference to the deepest position (position closest to the rear face  21 ) of the laser light L concerning the modified spot Sx to be formed, the converging point position correction step corrects the converging point position (Z height) of the laser light in the thickness direction according to the state of the normal modified spot, for example. This is because the modified spot Sx tends to expand and contract on the front side of the laser light L according to the parameter number. 
     When forming the modified spot Sx, an axicon lens pattern adjustment step (axicon lens pattern adjustment control) for adjusting the parameter number of the axicon lens pattern Ax (the number of circular ring regions a 2 ) may be performed. In the axicon lens pattern adjustment step, the parameter number of the axicon lens pattern Ax is set according to the state of the normal modified spot, for example, so as to correspond to the length in the laser light irradiation direction of the modified spot Sx (modified region  7 ) to be formed. Specifically, the parameter number is made greater and smaller when the modified spot Sx is to be formed longer and shorter in the laser light irradiation direction, respectively. 
     As in the foregoing, when forming the modified region  7 , this embodiment can cause the reflective spatial light modulator  203  to display the axicon lens pattern Ax as a modulation pattern, thereby converging laser light so as to form converging points at a plurality of positions juxtaposed close to each other in the laser light irradiation direction. That is, the longitudinally fine multipoint-diverged laser light L can perform laser processing (so-called simultaneous multipoint elongation processing), so as to connect the diverged multipoint converging points to each other, thereby forming a pseudo vertically long converging spot. This can form the modified spot Sx having a plurality of modified dots d juxtaposed close to each other in the laser light irradiation direction. 
     This modified spot Sx forms a simulatively (and substantially) vertically long converging part while fully keeping its energy density, thereby producing the vertically long modified region  7 . This can reduce the number of fractures occurring within the object  1  and make the fractures harder to extend, thereby ameliorating the straightness in cut sections and the bending strength and making it possible to improve the processing quality. 
     The modified region  7  becomes vertically long and thus can improve throughput. This embodiment can reduce the number of fractures and thus is effective in particular when the fractures within the object  1  are to be controlled (e.g., when the lines  5  do not align with the crystal orientation or when a glass material is to be processed). 
     The above-mentioned operation and effect that it can reduce the amount of fractures is remarkably effective in decreasing fractures in the c-plane direction when the object  1  is an SiC substrate or sapphire substrate. While an optical system using an axicon lens to make converging points vertically long typically lowers the energy density so that normal processing is difficult or a large amount of energy is necessary for processing, this embodiment can converge the laser light L while frilly keeping the energy density. Since the spatial phase modulator  203  is used for forming the vertically long modified spot Sx, the modified region  7  can instantaneously be formed with a given pitch at a given position. 
     When the object  1  is a sapphire substrate taking the c-plane as a principal surface, fractures are typically hard to extend along the r-plane in the sapphire substrate. Therefore, when the modified region  7  is formed along the line  5  extending along the r-plane, resulting fractures are easy to extend in directions intersecting the line  5 , thereby yielding a fear of breaking along directions intersecting the line  5 . By contrast, this embodiment can reduce the amount of fractures and thus can suppress the fear, thereby being effective in particular when the line  5  is set so as to extend in a direction along the r-plane of the sapphire substrate. The above-mentioned operation and effect of improving the processing quality is remarkable when the object  1  is amorphous glass. 
       FIG. 12  is a photograph illustrating an example of modified spots formed by the laser processing method of this embodiment.  FIG. 12  illustrates the object  1  as seen from a side, while the depicted vertical direction corresponds to the thickness direction. It can be seen from  FIG. 12  that converging points are formed at a plurality of positions juxtaposed close to each other in the laser light irradiation direction, whereby the vertically long modified spot Sx having a plurality of modified dots d juxtaposed close to each other in the laser light irradiation direction is made. 
       FIG. 13  is a photograph for explaining effects of the laser processing method of this embodiment.  FIG. 13( a )  is a picture in which the object  1  is irradiated with the laser light L while no modulation pattern is displayed in the liquid crystal layer  216 , so as to form modified spots Sy within the object  1 .  FIG. 13( b )  is a picture in which the object  1  is irradiated with the laser light L while the axicon lens pattern Ax is displayed in the liquid crystal layer  216 , so as to form the modified spot Sx within the object  1 . The pictures in  FIG. 13  illustrate the inside of the object  1  formed with the modified spots as seen from the laser light entrance surface. 
     It can be seen from  FIG. 13  that the vertically long modified spot Sx formed while the axicon lens pattern Ax is displayed in the liquid crystal layer  216  can reduce the amount of fractures occurring within the object  1  as compared with the modified spots Sy formed while no modulation pattern is displayed in the liquid crystal layer  216 . 
       FIG. 14  is another photograph for explaining effects of the laser processing method of this embodiment.  FIG. 14( a )  illustrates a cut section  25   y  at the time when the object  1  is irradiated with the laser light L while no modulation pattern is displayed in the liquid crystal layer  216 .  FIG. 14( b )  illustrates a cut section  25   x  at the time when the object  1  is irradiated with the laser light L while the axicon lens pattern Ax is displayed in the liquid crystal layer  216 .  FIG. 14  illustrates the object  1  as seen from a side, while the depicted vertical direction corresponds to the thickness direction. 
     It can be seen from  FIG. 14  that laser processing with the axicon lens pattern Ax tends to reduce fractures extending in directions intersecting the thickness direction, for example, in particular, whereby the straightness at chip end faces improves in the cut section  25   x  as compared with that in the cut section  25   y  formed by laser processing without displaying modulation patterns in the liquid crystal layer  216 . 
     The following will illustrate results of measurement of bending strength concerning a chip produced by laser-processing the object  1  without displaying modulation patterns in the liquid crystal layer  216  and a chip produced by laser-processing the object  1  while displaying the axicon lens pattern Ax in the liquid crystal layer  216 . Here, a load is exerted on the laser light entrance surface side.
         Displaying no modulation pattern: bending strength 75.3 MPa   Displaying the axicon lens pattern: bending strength 109.6 MPa       

     It can be seen from the above-mentioned bending strength measurement results that laser processing with the axicon lens pattern Ax improves the bending strength of the chip as compared with the laser processing without displaying modulation patterns in the liquid crystal layer  216 . 
     Since converging points are formed at a plurality of positions juxtaposed close to each other in the optical axis direction (a vertically long multipoint converging point is formed), the following operations and effects are exhibited. The modified region  7  is formed like a perforation, which makes it easier to cut the object  1  along the modified region  7 . This easily enables laser processing independent of the cleavage characteristic and crystal orientation. It also allows laser processing to be performed with an amount of energy smaller than that in the case of converging at a single vertically long converging spot, thereby making it easier to secure a sufficient energy density even when forming the modified region  7  within the object  1  at a position which is deep from the laser light entrance surface, so that the modified region  7  having a sufficient size (large width) can be formed. 
     With respect to one converging point, another converging point is located nearby in the optical axis direction, so as to enhance the destructive force in the object  1  at the time of cutting, whereby the object  1  can be cut easily. The latter converging point also brings a heat-inducing effect to the former converging point, whereby the object  1  can be cut easily. This can further enhance a destressing effect at the time when the laser light L modifies the object  1 , whereby the object  1  can be cut easily. 
     The second embodiment will now be explained in detail. This embodiment will be explained mainly in terms of differences from the above-mentioned first embodiment. 
       FIGS. 15 and 16  are sectional views for explaining the laser processing method in this embodiment. By converging the laser light L at the object  1 , this embodiment forms a plurality of (3 here) rows of modified regions  7  within the object  1  along the line  5 . This embodiment is applicable to the thick object  1 , whose thickness is not limited in particular but may be as thick as 5000 μm, for example, and ranges from 30 μm to 2000 μm here. 
     In this embodiment, as illustrated in  FIG. 15( a ) , the controller  250  controls the reflective spatial light modulator  203  at first so that no modulation pattern is displayed in the liquid crystal layer  216 . In this state, the object  1  is irradiated with the laser light L in a pulsating manner from the front face  3  such that a converging point is located within the object  1  on the rear face  21  side, while the object  1  and the laser light L are moved relative to each other along the line  5 . As a result, a plurality of normal modified spots Sy are formed within the object  1  on the rear face  21  side along the line  5 , and the plurality of modified spots Sy form one row of a rear-face-side modified region (opposite-surface-side modified region)  7   1 . 
     Subsequently, as illustrated in  FIG. 15( b ) , the controller  250  controls the reflective spatial light modulator  203 , so as to cause the liquid crystal layer  216  to display the axicon lens pattern Ax. In this state, the object  1  is irradiated with the laser light L in a pulsating manner from the front face  3  such that a converging point is located within the object  1  between the front face  3  side and the rear face  21  side (between the rear-face-side modified region  7   1  and a front-face-side modified region  7   3  which will be explained later; at the center in the thickness direction here). This converges the laser light L at the object  1  such that converging points are formed at a plurality of positions juxtaposed close to each other along the laser light irradiation direction within the object  1  between the front face  3  side and the rear face  21  side. At the same time, the object  1  and the laser light L are moved relative to each other along the line  5 . 
     As a result, a plurality of vertically long modified spots Sx each having modified dots d formed at a plurality of positions juxtaposed close to each other along the laser light irradiation direction are produced along the line  5  within the object  1  between the front face  3  side and the rear face  21  side, and the plurality of modified spots Sx form one row of a middle modified region  7   2 . 
     Finally, as illustrated in  FIG. 16 , the controller  250  controls the reflective spatial light modulator  203  such that no modulation pattern is displayed in the liquid crystal layer  216 . In this state, the object  1  is irradiated with the laser light L in a pulsating manner from the front face  3  such that a converging point is located within the object  1  on the front face  3  side, while the object  1  and the laser light L are moved relative to each other along the line  5 . This forms a plurality of normal modified spots Sy within the object  1  on the rear face  21  side along the line  5 , and the plurality of modified spots Sy form one row of the front-face-side modified region (entrance-surface-side modified region)  7   3 . 
     As in the foregoing, when forming the middle modified region  7   2 , this embodiment can converge the laser light so as to form converging points at a plurality of positions juxtaposed close to each other along the laser light irradiation direction by causing the reflective spatial light modulator  203  to display the axicon lens pattern Ax as a modulation pattern. This forms the pseudo vertically long modified spot Sx while fully keeping its energy density, whereby the vertically long middle modified region  7   2  is formed. As in the above-mentioned embodiment, this middle modified region  7   2  can reduce the amount of fractures occurring within the object  1 , so as to ameliorate the straightness in the cut sections and the bending strength, thereby improving the processing quality. 
     When forming the rear-face-side modified region  7   1  and front-face-side modified region  7   3 , on the other hand, this embodiment does not cause the reflective spatial light modulator  203  to display the axicon lens pattern Ax as a modulation pattern, but uses no modulation pattern here as mentioned above. This can inhibit the rear-face-side modified region  7   1  and front-face-side modified region  7   3  from becoming vertically long and making damages (dents and the like) easier to occur in the rear face  21  and front face  3 . 
     That is, when laser-processing the thick object  1  by forming a plurality of rows of modified regions  7 , this embodiment does not perform so-called simultaneous multipoint elongation processing (formation of the modified spot Sx) in regions close to the front face  2  and rear face  21  and inhibits damages from occurring in the front face  3  and rear face  21  while enhancing the capability of the object  1  to divide, but can perform the so-called simultaneous multipoint elongation processing in the center region, so as to improve the straightness and bending strength of chips. Therefore, the processing quality can be improved while keeping the capability of the object  1  to divide. 
     Though embodiments in accordance with one aspect of the present invention are explained in the foregoing, the present invention is not limited to the above-mentioned embodiments but may be modified or applied to others within the scope not changing the gist set forth in each claim. 
     For example, while the above-mentioned second embodiment forms the modified regions  7   1  to  7   3  in this order, it is not restrictive; a plurality of rows of modified regions  7  may be formed in any order in the object  1 . In the above-mentioned second embodiment, four or more rows of modified regions  7  may be formed within the object  1  at positions different from one another in the thickness direction. For example, any of the rear-face-side modified region  7   1 , middle modified region  7   2 , and front-face-side modified region  7   3  may be formed by a plurality of rows. 
     Though the above-mentioned embodiments employ the front face  3  and rear face  21  as the “laser light entrance surface” and “the surface opposite to the laser light entrance surface,” respectively, the front face  3  becomes the “surface opposite to the laser light entrance surface” when the rear face  21  is the “laser light entrance surface.” In the above-mentioned embodiments, a fracture reaching at least one of the front face  3  and rear face  21  may be generated from the modified region  7 . One aspect of the present invention can also be regarded as a chip manufactured by the above-mentioned laser processing device or method. 
       FIG. 17  is a diagram for explaining aberrations occurring at converging positions of laser light. The laser light L converges geometrically at a single point when it is a plane wave (has a planar wavefront (phase)). On the other hand, while the plane-wave laser light L typically changes its wavefront under various influences, the laser light L converged at the object  1  may fail to converge at the single point, i.e., aberrations may occur naturally. Examples of the aberrations include Seidel aberrations (astigmatism, coma, spherical aberration, curvature of field, and distortion), longitudinal aberration which is an aberration in the longitudinal direction (along the optical axis direction), and lateral aberration which is an aberration in a direction intersecting the longitudinal direction. 
     When the laser light L is made incident on the object  1  by the converging optical system  204  (see  FIG. 11 ) and the like in the process of being converged at the object  1 , a spherical aberration in which light beams having different angles of incidence converge at different positions by refraction (Snell&#39;s law) naturally occurs as illustrated in  FIG. 17 , for example. That is, as depicted, since the laser light L is converged at the object  1 , an aberration occurs naturally at the converging position, whereby a range of the aberration along the optical axis direction (a range where the intensity of the laser light L is a processing threshold or greater) exists as a reference aberration range H. 
     It is found that adding a new aberration to the aberration including a spherical aberration and the like caused by such converging of the laser light L (hereinafter referred to as “converging-induced aberration”) can control the processing quality. An example of techniques for adding the new aberration is a phase modulation of the laser light L with the reflective spatial light modulator  203  as an aberration providing part for providing an aberration as in the above-mentioned embodiments. By the phase modulation is meant modulating a wavefront (phase) of the laser light L into a given form. 
     Examples of the phase modulation include those achieving an action of an axicon lens, attaining an action of a diffraction grating, and generating a predetermined spherical aberration. These examples can be performed by causing the reflective spatial light modulator  203  to display an axicon lens pattern, a diffraction grating pattern, and a predetermined spherical aberration pattern, respectively, for instance. As a technique for adding a new aberration, a lens providing an aberration may be used, or a medium may be inserted in the process of converging, while each of the lens and medium constitutes the aberration providing part in such a case. 
     Therefore, in the above-mentioned embodiments, the laser light L may include at its converging position not only the aberration provided by the phase modulation with the axicon lens pattern Ax but the converging-induced aberration. When no modulation patterns are displayed in the liquid crystal layer  216  of the reflective spatial light modulator  203  at the time of forming the rear-face-side modified region  7   1  and front-face-side modified region  7   3 , the laser light L may include the converging-induced aberration at its converging position. Here, other phase modulations may be added to the phase modulation caused by the axicon lens pattern Ax (other patterns may be additionally displayed in the liquid crystal layer  216 ). 
     INDUSTRIAL APPLICABILITY 
     One aspect of the present invention can provide a laser processing device and laser processing method which can improve the processing quality. 
     REFERENCE SIGNS LIST 
       1 : object to be processed;  5 ,  5   a ,  5   b : line to cut;  7 : modified region;  7   1 : rear-face-side modified region (opposite-surface-side modified region);  7   2 : middle modified region;  7   3 : front-face-side modified region (entrance-surface-side modified region);  100 ,  300 : laser processing device;  101 ,  202 : laser light source;  203 : reflective spatial light modulator (spatial light modulator);  204 : converging optical system;  216 : liquid crystal layer (display part); a 1 : circular region; a 2 : circular ring region; Ax: axicon lens pattern; d: modified dot; Sx: modified spot; L: laser light.