Patent Publication Number: US-2022238377-A1

Title: Chip manufacturing method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a chip manufacturing method for manufacturing chips by dividing a wafer in which a substrate and a laminate are laminated. 
     Description of the Related Art 
     Semiconductor chips are generally manufactured by dividing a wafer along streets (planned dividing lines) set on a front surface of a substrate, the wafer including the substrate formed of a semiconductor such as silicon (Si) and a laminate of a Low-k film and a conductive layer or the like formed on the front surface of the substrate. Incidentally, a test element group (TEG) may be formed in partial regions of the laminate which regions are on the streets. In order to divide the wafer along the streets, laser-processed grooves are first formed along the streets by, for example, applying, along the streets, a pulsed laser beam having a wavelength absorbed by the substrate and the laminate. In the laser-processed grooves, the laminate is partly removed, so that the front surface of the substrate is exposed along the streets. 
     After the laser-processed grooves are formed, modified layers in which mechanical strength is decreased are formed within the substrate by moving a condensing point of a pulsed laser beam having a wavelength transmitted through the substrate and the wafer relative to each other along the streets in a state in which the condensing point is positioned within the substrate. Incidentally, at this time, cracks extending to the front surface side of the substrate with the modified layers as a starting point are also formed. After the modified layers are formed, the cracks are further extended by applying an external force to the wafer by grinding or the like. The wafer is thereby divided into a plurality of semiconductor chips (see Japanese Patent Laid-Open No. 2007-173475, for example). 
     However, when the laser-processed grooves are formed in the laminate, the front surface side of the substrate may be affected by a thermal effect of the laser beam, and consequently the crystal orientation of a partial region of the substrate which region is in proximity to a bottom portion of a processed groove may change. The extending direction of the cracks extending with the modified layers as a starting point depends on the crystal orientation of the substrate. Thus, when a change in the crystal orientation occurs, the cracks appear on the front surface of the substrate in such a manner as to avoid the laser-processed grooves, so that the wafer is not divided along the streets (that is, a processing defect occurs). 
     For this problem, a processing method in which the laser-processed grooves are formed along the streets after the modified layers are formed, and the wafer is thereafter divided by grinding the back surface side of the substrate is conceivable. It is conceivable that, if this processing method is adopted, abnormal extension of the cracks due to a change in the crystal orientation can be suppressed. However, after the modified layers are formed with a back surface of the wafer exposed upward, the laser-processed grooves need to be formed with a front surface of the wafer exposed upward, and the substrate needs to be thereafter ground with the back surface of the wafer exposed upward again. Thus, work of inverting the wafer upside down needs to be performed at least twice during a period from the formation of the modified layers to the grinding. Productivity is consequently decreased. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of such problems. It is an object of the present invention to divide a wafer reliably by suppressing abnormal extension of cracks when dividing the wafer having a laminate on the front surface of a substrate, and reduce the number of times of inverting the wafer upside down. 
     In accordance with an aspect of the present invention, there is provided a chip manufacturing method for manufacturing chips by dividing a wafer including a device formed in each region demarcated by a plurality of intersecting planned dividing lines set on a front surface of a substrate and a laminate formed at least on the plurality of planned dividing lines, the chip manufacturing method including a modified layer forming step of forming a modified layer along the planned dividing lines and forming a crack extending from the modified layer to a front surface side of the substrate by applying, along the planned dividing lines, a first laser beam having a wavelength transmitted through the substrate, in a state in which a back surface side of the substrate is exposed and a condensing point of the first laser beam is positioned within the substrate from the back surface side of the substrate, a grinding step of thinning the wafer to a predetermined thickness by grinding the back surface side of the substrate exposed in the modified layer forming step, after the modified layer forming step, and a laser-processed groove forming step of forming a laser-processed groove in the laminate by applying, along the planned dividing lines, a second laser beam having a wavelength absorbed by the substrate, from a front surface side of the wafer, after the grinding step. 
     In the laser-processed groove forming step, the laser-processed groove may be formed in such a manner as to cover the crack extending from the modified layer to the front surface side of the wafer. 
     In the laser-processed groove forming step, the laser-processed groove may be formed in such a manner as not to divide the laminate completely. In the laser-processed groove forming step, the laser-processed groove may be formed in such a manner as to divide the laminate completely. 
     In the modified layer forming step, the crack extending to the front surface side of the wafer may be formed in such a manner as not to sever the laminate. In the modified layer forming step, the crack extending to the front surface side of the wafer may be formed in such a manner as to sever the laminate. 
     The chip manufacturing method may further include a tape affixing step of affixing a tape having elasticity to the back surface side of the substrate, after the grinding step but before the laser-processed groove forming step, and an expanding step of expanding the tape affixed to the back surface side of the substrate, after the laser-processed groove forming step. 
     The chip manufacturing method according to one aspect of the present invention performs the modified layer forming step and the grinding step in order in a state in which the back surface side is exposed, and further performs the laser-processed groove forming step in a state in which the front surface side is exposed, after the grinding step. Thus, the number of times of inversion of the wafer in a period from the modified layer forming step to the laser-processed groove forming step can be reduced to one. In addition, because the laser-processed groove forming step is performed after the modified layer forming step, abnormal extension of the crack due to a change in crystal orientation can be suppressed. It is therefore possible to reduce the number of times that the wafer is inverted upside down, and reliably divide the wafer having the laminate on the front surface of the substrate, along the planned dividing lines. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a wafer; 
         FIG. 2  is a flowchart of a chip manufacturing method according to a first embodiment; 
         FIG. 3  is a partially sectional side view depicting a modified layer forming step; 
         FIG. 4A  is an enlarged sectional view of the wafer, the enlarged sectional view depicting an example of cracks; 
         FIG. 4B  is an enlarged sectional view of the wafer, the enlarged sectional view depicting another example of the cracks; 
         FIG. 5  is a perspective view depicting a grinding step; 
         FIG. 6A  is an enlarged sectional view depicting an example of the wafer obtained after the grinding step; 
         FIG. 6B  is an enlarged sectional view depicting another example of the wafer obtained after the grinding step; 
         FIG. 7  is a diagram depicting an outline of a peeling step; 
         FIG. 8  is a partially sectional side view depicting a laser-processed groove forming step; 
         FIG. 9A  is an enlarged sectional view of the wafer, the enlarged sectional view depicting laser-processed grooves; 
         FIG. 9B  is an enlarged sectional view of the wafer, the enlarged sectional view depicting laser-processed grooves having a sufficiently large width; 
         FIG. 10A  is an enlarged sectional view of the wafer, the enlarged sectional view depicting two laser-processed grooves; 
         FIG. 10B  is an enlarged sectional view of the wafer, the enlarged sectional view depicting wide laser-processed grooves; 
         FIG. 11A  is an enlarged sectional view of the wafer, the enlarged sectional view depicting laser-processed grooves in a first stage; 
         FIG. 11B  is an enlarged sectional view of the wafer, the enlarged sectional view depicting laser-processed grooves in a second stage; 
         FIG. 12  is a flowchart of a chip manufacturing method according to a second embodiment; 
         FIG. 13  is an enlarged sectional view of the wafer, the enlarged sectional view depicting laser-processed grooves not reaching a front surface of a substrate; 
         FIG. 14A  is a partially sectional side view of an expanding apparatus; and 
         FIG. 14B  is a partially sectional side view of the expanding apparatus that has undergone an expanding step. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments according to one aspect of the present invention will be described with reference to the accompanying drawings.  FIG. 1  is a perspective view of a wafer  11  to be processed. The wafer  11  includes a substantially disk-shaped substrate  13 . The substrate  13  is formed by a semiconductor material such as Si. It is to be noted that there are no limitations on the material, shape, structure, size, and the like of the substrate  13 . For example, the substrate  13  may be formed by another semiconductor material (GaAs, InP, GaN, or the like) or a material such as sapphire, glass, ceramic, resin, or a double oxide (LiNbO 3  or LiTaO 3 ). 
     The diameter of the substrate  13  in the present example is approximately 300 mm (12 inches). The thickness of the substrate  13  from a front surface  13   a  to a back surface  13   b  is approximately 700 μm. However, the diameter and thickness of the substrate  13  are not limited to those in the present example. A plurality of planned dividing lines (streets)  17  are set in a lattice manner on the front surface  13   a  of the substrate  13 . A device  19  such as an integrated circuit (IC) is formed in each of regions demarcated by the planned dividing lines  17  intersecting one another. The external shape of the device  19  in the present example is a rectangular shape of 4 mm×5 mm. However, there are no limitations on the kind, quantity, shape, structure, size, and the like of the device  19 . 
     A laminate  15  including a low dielectric constant insulator layer (insulating layer formed of what is generally called a Low-k material) and a conductive layer formed of metal is formed on the front surface  13   a  of the substrate  13 . The thickness of the laminate  15  is, for example, approximately a few micrometers, and is sufficiently thin as compared with the thickness of the substrate  13  that has not yet been processed. The laminate  15 , including the planned dividing lines  17 , is formed on the whole of the front surface  13   a . Regions of the laminate  15  in which regions the devices  19  are formed protrude more than regions in which the planned dividing lines  17  are set. 
     Description will next be made of a method of manufacturing a plurality of device chips (chips)  35  (see  FIG. 9A  and the like) by dividing the wafer  11  into units of the devices  19 .  FIG. 2  is a flowchart of a method of manufacturing the device chips  35  according to a first embodiment. First, a first protective tape  21  is affixed to the laminate  15  side of the wafer  11  (that is, a front surface  11   a  of the wafer  11 ) and one surface of an annular frame  23  (see  FIG. 3 ) made of metal which annular frame has an opening portion of a diameter larger than that of the wafer  11  (tape affixing step S 10 ). 
     In the present example, a wafer unit  25  is formed in which the wafer  11  is thus supported by the annular frame  23  via the first protective tape  21 . However, the annular frame  23  is not essential. The first protective tape  21  having substantially the same diameter as that of the wafer  11  may be affixed to the front surface  11   a  side. The first protective tape  21  is, for example, a resin-made film in which a glue layer (adhesive layer) is provided on a base material layer formed of resin. In the present example, the base material layer is formed of a polyolefin resin, and the adhesive layer is formed of an ultraviolet (UV) curable resin. 
     After the tape affixing step S 10 , modified layers  13   c  are formed in the substrate  13  by using a first laser processing apparatus  2  (modified layer forming step S 20 ). The first laser processing apparatus  2  depicted in  FIG. 3  is used in the modified layer forming step S 20 . The first laser processing apparatus  2  includes a disk-shaped chuck table  4  having a circular holding surface  4   a  that holds the front surface  11   a  side of the wafer  11  under suction. The chuck table  4  includes a disk-shaped frame body and a disk-shaped porous plate (not depicted) having a diameter smaller than that of the frame body. 
     A negative pressure from a suction source such as an ejector acts on the porous plate via a flow passage (not depicted) formed in the frame body. The chuck table  4  is configured to be rotatable about a predetermined rotational axis. The rotation of the chuck table  4  is performed by a rotational driving source (not depicted) that rotates a base member on which the chuck table  4  is mounted. In addition, the chuck table  4  is configured to be movable along a processing feed direction  6  and an indexing feed direction orthogonal to the processing feed direction  6  in a horizontal plane. Movement in the processing feed direction  6  is performed by an X-axis direction moving mechanism of a ball screw type. Movement in the indexing feed direction is performed by a Y-axis direction moving mechanism of a ball screw type. 
     On the side of the chuck table  4 , a plurality of clamp mechanisms  8  are provided along the circumferential direction of the chuck table  4 . A head section  12  of a laser beam irradiating unit  10  is provided above the holding surface  4   a . The laser beam irradiating unit  10  includes a laser oscillator (not depicted) and the head section  12  including a condensing lens (not depicted). A pulsed laser beam (first laser beam)  14  having a wavelength transmitted through the substrate  13  is applied from the head section  12  toward the holding surface  4   a.    
     In the modified layer forming step S 20 , first, the front surface  11   a  side of the wafer  11  is held under suction by the holding surface  4   a , and the annular frame  23  is held by each clamp mechanism  8 . At this time, a back surface  11   b  of the wafer  11  (that is, the back surface  13   b  of the substrate  13 ) is exposed upward. Then, an offset of the planned dividing lines  17  with respect to the processing feed direction  6  (what is generally called a θ offset) is corrected by rotating the chuck table  4  as appropriate. Next, a condensing point  14   a  of the laser beam  14  and the chuck table  4  are moved relative to each other along the processing feed direction  6  in a state in which the condensing point  14   a  is positioned at a predetermined depth within the substrate  13  from the back surface  13   b  side of the substrate  13 . In the modified layer forming step S 20 , processing conditions are set as follows, for example, and the wafer  11  is processed. 
     Wavelength of the laser beam: 1064 nm 
     Average power: 1 W 
     Pulse repetition frequency: 100 kHz 
     Processing feed speed: 800 mm/s 
     Distance from the condensing point to the front surface of the substrate: 70 μm 
     Number of pass(es): 1 
     Incidentally, the number of pass(es) means the number of time(s) that the laser beam  14  is applied along one planned dividing line  17 . However, the number of passes may be set to two or more, and two or more modified layers  13   c  may be formed at different depth positions closer to the front surface  13   a  side than the back surface  13   b . For example, it is preferable to set the number of passes to three, and form three modified layers  13   c  at different depth positions closer to the front surface  13   a  side than the back surface  13   b . In addition, a distance from the condensing point  14   a  to the front surface  13   a  of the substrate  13  (that is, a distance from the modified layer  13   c  to the front surface  13   a ) is adjusted as appropriate in such a manner as to be larger than a finished thickness of the substrate  13 . For example, in a case where the finished thickness of the substrate  13  is 50 μm, the distance from the modified layer  13   c  to the front surface  13   a  of the substrate  13  is adjusted to 70 μm or the like. 
     After the modified layer  13   c  is formed along one planned dividing line  17 , the chuck table  4  is indexing-fed by a predetermined length in the indexing feed direction, and a modified layer  13   c  is similarly formed along another planned dividing line  17  adjacent to the one planned dividing line  17 . After modified layers  13   c  are formed along all of the planned dividing lines  17  along one direction, the chuck table  4  is rotated by 90 degrees by the rotational driving source, and modified layers  13   c  are similarly formed along all of the planned dividing lines  17  along another direction orthogonal to the one direction. The modified layers  13   c  refer to regions of mechanical strength lower than that of regions in which no modified layer  13   c  is formed in the substrate  13 . In the modified layer forming step S 20 , when the modified layers  13   c  are formed, cracks  13   d  extend from the modified layers  13   c  to the front surface  13   a  side and the back surface  13   b  side. 
       FIG. 4A  is an enlarged sectional view of the wafer  11 , the enlarged sectional view depicting an example of the cracks  13   d  formed by performing the modified layer forming step S 20  according to the above-described processing conditions. In the example depicted in  FIG. 4A , the cracks  13   d  reach the front surface  13   a  of the substrate  13 , but do not sever the laminate  15 . In a case where the cracks  13   d  do not sever the laminate  15  as mentioned above, the wafer  11  is not divided into the individual device chips  35 , and therefore movement of the device chips  35  does not occur at a time of a tape affixing step S 50  to be described later. Incidentally, the same is true for a case where the cracks  13   d  do not sever the laminate  15  and the extension of the cracks  13   d  is stopped within the laminate  15 . When the movement of the device chips  35  does not occur, intervals between the device chips  35  are uniform, and there is thus an advantage in that a laser-processed groove forming step S 70  to be described later is performed easily as compared with a case where the intervals between the device chips  35  vary due to the movement of the device chips  35 . 
     However, the cracks  13   d  may be formed in the laminate  15 , and the laminate  15  may be divided by the cracks  13   d , by application of one or a plurality of the following: increasing at least one of pulse energy and repetition frequency; bringing the condensing point  14   a  close to the front surface  13   a ; and decreasing processing feed speed, for example. For example, as depicted in  FIG. 4B , the laminate  15  can be divided by the cracks  13   d  by forming the modified layers  13   c  at a position further closer to the front surface  11   a  side than in the above-described example. This provides an advantage of being able to divide the wafer  11  more reliably.  FIG. 4B  is an enlarged sectional view of the wafer  11 , the enlarged sectional view depicting another example of the cracks  13   d . Incidentally, in the examples depicted in  FIG. 4A  and  FIG. 4B , the cracks  13   d  do not reach the back surface  11   b  (that is, the back surface  13   b ). 
     After the modified layer forming step S 20 , the back surface  11   b  side exposed in the modified layer forming step S 20  is ground, an external force is applied to the back surface  11   b  side, and the wafer  11  is thinned to a predetermined finished thickness (grinding step S 30 ). A grinding apparatus  20  depicted in  FIG. 5  is used in the grinding step S 30 . The grinding apparatus  20  includes a disk-shaped chuck table  22 . The chuck table  22  has a structure similar to that of the above-described chuck table  4 . Repeated description of the chuck table  22  will therefore be omitted. 
     A rotational driving source (not depicted) is coupled to a lower portion of the chuck table  22 , and the chuck table  22  is rotated at high speed about a predetermined rotational axis  24 . A grinding unit  26  is provided above a holding surface of the chuck table  22 . The grinding unit  26  includes a cylindrical spindle  28 . An upper end portion of the spindle  28  is provided with a motor (not depicted). A lower end portion of the spindle  28  is provided with a disk-shaped mounter  30 . 
     An annular grinding wheel  32  is coupled to a lower surface of the mounter  30 . The grinding wheel  32  includes an annular wheel base  34  made of metal. A plurality of grinding stones  36  are fixed to the bottom surface side of the wheel base  34  along the circumferential direction of the wheel base  34 . The grinding stones  36  are, for example, formed by mixing abrasive grains of diamond, cubic boron nitride (cBN), or the like in a binding material of metal, ceramic, resin, or the like.  FIG. 5  is a perspective view depicting the grinding step S 30 . Before the grinding step S 30  is performed, however, the first protective tape  21  is first cut out along the contour of the wafer  11  without the wafer unit  25  being inverted upside down, and a wafer unit  27  of the wafer  11  and the first protective tape  21  is thereby formed. Incidentally, the first protective tape  21  does not need to be cut out in a case where the annular frame  23  is not used in the tape affixing step S 10 . 
     Next, in the grinding step S 30 , the grinding unit  26  is grinding-fed downward at a predetermined grinding feed speed (for example, 0.5 μm/s) in a state in which the front surface  11   a  side is held under suction by the chuck table  22 , the chuck table  22  is rotated at a high speed (for example, 100 rpm), and further, the grinding wheel  32  is rotated at a predetermined speed (for example, 3000 rpm) while grinding water such as pure water is supplied to a processing point. Consequently, the back surface  13   b  side is ground, and the substrate  13  is thinned to a predetermined finished thickness (for example, approximately 50 μm).  FIG. 6A  is an enlarged sectional view depicting an example of the wafer  11  obtained after the grinding step S 30 . 
     The external force applied to the wafer  11  in the grinding step S 30  causes the cracks  13   d  to extend to the front surface  13   a  and the back surface  13   b  of the substrate  13 , and, for example, as depicted in  FIG. 6A , each crack  13   d  continues from the front surface  13   a  to the back surface  13   b . Incidentally, in a case where the cracks  13   d  are already formed also in the laminate  15  (see  FIG. 4B ) or in a case where the external force applied to the wafer  11  in the grinding step S 30  is relatively large, the cracks  13   d  may continue from the front surface  11   a  to the back surface  11   b  of the wafer  11  (see  FIG. 6B ).  FIG. 6B  is an enlarged sectional view depicting another example of the wafer  11  obtained after the grinding step S 30 . 
     After the grinding step S 30 , the wafer  11  is transported to a tape reaffixing apparatus (not depicted). The tape reaffixing apparatus performs a UV irradiating step S 40  and a tape affixing step S 50  to be described later. The tape reaffixing apparatus includes a UV irradiating unit. The UV irradiating unit includes a table formed of a transparent material that transmits UV. A UV lamp is disposed below the table. 
     In the UV irradiating step S 40  after the grinding step S 30 , first, the wafer  11  is transported onto the table while the front surface  11   a  side of the wafer  11  remains oriented downward. Then, the adhesive force of the first protective tape  21  is decreased by irradiating the first protective tape  21  with UV. The tape reaffixing apparatus includes an affixing unit that affixes a second protective tape  29  to the wafer  11  or the like; and a cutting-out unit for cutting out the second protective tape  29  affixed to the wafer  11  or the like into a circular shape. The cutting-out unit includes an arm and a cutting blade provided to a distal end portion of the arm. 
     After the UV irradiating step S 40 , in a state in which an annular frame  31  made of metal which annular frame has a diameter larger than that of the wafer  11  is disposed on the periphery of the wafer  11 , the affixing unit affixes the second protective tape  29  to one surface of the annular frame  31  and the back surface  11   b  side of the wafer  11  (tape affixing step S 50 ). Next, the cutting-out unit cuts out the second protective tape  29  into a predetermined diameter. The tape reaffixing apparatus further includes an inversion moving unit that vertically inverts the wafer  11  and a peeling unit for peeling the first protective tape  21 . The wafer  11  having the second protective tape  29  affixed to the back surface  11   b  side is vertically inverted by the inversion moving unit, and is transported to a holding table of the peeling unit. 
     Consequently, the first protective tape  21  is exposed upward, and the second protective tape  29  is held under suction by the holding table. Then, the peeling unit peels off the first protective tape  21  whose adhesive force is decreased in the UV irradiating step S 40  (peeling step S 60 ). Thus, the wafer  11  is transferred from the first protective tape  21  to the second protective tape  29 , and a wafer unit  33  (see  FIG. 7 ) in which the wafer  11  is supported by the annular frame  31  via the second protective tape  29  is formed.  FIG. 7  is a diagram depicting an outline of the peeling step S 60 . Incidentally, as with the first protective tape  21 , the second protective tape  29  is a resin-made film in which a glue layer (adhesive layer) is provided on a base material layer formed of resin. However, the second protective tape  29  is a film having elasticity, in particular. 
     After the peeling step S 60 , a laser-processed groove forming step S 70  which forms a laser-processed groove  15   a  (see  FIG. 8 ) along each planned dividing line  17  is performed. A second laser processing apparatus  40  depicted in  FIG. 8  is used in the laser-processed groove forming step S 70 . The second laser processing apparatus  40  includes a chuck table  42  similar to the chuck table  4  and clamp mechanisms  44  similar to the clamp mechanisms  8 . Incidentally, a rotational driving source, an X-axis direction moving mechanism, a Y-axis direction moving mechanism, and the like are provided as in the first laser processing apparatus  2 . A head section  48  of a laser beam irradiating unit  46  is provided above a holding surface  42   a.    
     The laser beam irradiating unit  46  includes a laser oscillator (not depicted) and the head section  48  including a condensing lens (not depicted). A pulsed laser beam (second laser beam)  50  having a wavelength absorbed by the substrate  13  and the laminate  15  is applied from the head section  48  toward the holding surface  42   a . The laser beam  50  has an intensity distribution close to a Gaussian distribution in a plane perpendicular to the optical axis of the condensing lens in the laser beam irradiating unit  46 . 
     When the laser-processed groove forming step S 70  is to be performed, first, a coating and cleaning apparatus (not depicted) is used to apply a water-soluble resin formed of polyvinyl alcohol or the like to the whole of the front surface  11   a  side, and thereby form a protective film (not depicted). Next, the back surface  11   b  side of the wafer  11  is held under suction by the holding surface  42   a  of the second laser processing apparatus  40 , and the annular frame  31  is held by each clamp mechanism  44 . Incidentally, at this time, the front surface  11   a  of the wafer  11  is exposed upward. 
     Then, after an offset of the planned dividing lines  17  with respect to the processing feed direction  6  is corrected by rotating the chuck table  4 , a condensing point  50   a  of the laser beam  50  and the chuck table  4  are moved relative to each other along the processing feed direction  6  in a state in which the condensing point  50   a  is positioned at the front surface  11   a .  FIG. 8  is a partially sectional side view depicting the laser-processed groove forming step S 70 . In the laser-processed groove forming step S 70 , processing conditions are set as follows, for example, and the wafer  11  is processed. 
     Wavelength of the laser beam: 355 nm 
     Average power: 2 W 
     Pulse Repetition frequency: 200 kHz 
     Processing feed speed: 400 mm/s 
     Number of pass(es): 1 
     Next, the condensing point  50   a  is positioned at the front surface  11   a  in such a manner as to be superimposed on a region irradiated with the laser beam  14  in the modified layer forming step S 20  in a thickness direction A 1  (see  FIG. 9A ) of the wafer  11 , and the laser beam  50  is applied along each planned dividing line  17 . Laser-processed grooves  15   a  are thereby formed.  FIG. 9A  is an enlarged sectional view of the wafer  11 , the enlarged sectional view depicting an example of the laser-processed grooves  15   a.    
     The laminate  15  is ablation-processed, so that a laser-processed groove  15   a  in which the laminate  15  is removed (that is, the laminate  15  is completely severed) from the front surface  11   a  of the wafer  11  to the front surface  13   a  of the substrate  13  is formed along each planned dividing line  17  in the laminate  15 . After laser-processed grooves  15   a  are formed along all of the planned dividing lines  17 , the coating and cleaning apparatus is used again to clean the protective film by pure water or the like together with debris produced during the laser processing, and thereafter dry the protective film. A plurality of device chips  35  obtained by dividing the wafer  11  along the plurality of planned dividing lines  17  are thus formed. 
     In the present embodiment, the modified layer forming step S 20  and the grinding step S 30  are performed in a state in which the back surface  11   b  side is exposed, and after the grinding step S 30 , the laser-processed groove forming step S 70  is performed in a state in which the front surface  11   a  side is exposed. Thus, the number of times of inversion of the wafer  11  in a period from the modified layer forming step S 20  to the laser-processed groove forming step S 70  can be reduced to one. In addition, because the laser-processed groove forming step S 70  is performed after the modified layer forming step S 20 , abnormal extension of the cracks  13   d  due to a change in crystal orientation of the substrate  13  can be suppressed. It is therefore possible to reduce the number of times that the wafer  11  is inverted upside down, and reliably divide the wafer  11  having the laminate  15  on the front surface  13   a  of the substrate  13 , along the planned dividing lines  17 . 
     Incidentally, the cracks  13   d  may meander in the thickness direction A 1  of the substrate  13  and an extending direction A 2  of the planned dividing lines  17 . If a crack  13   d  appearing on the front surface  13   a  is present in a region different from a bottom portion of the laser-processed groove  15   a , the laser-processed groove  15   a  and the crack  13   d  may not be connected to each other. Accordingly, the width of the laser-processed groove  15   a  (width in a direction orthogonal to the extending direction A 2  of the planned dividing line  17  in a plane substantially parallel with the back surface  11   b ) may be made wider than a meandering width of the crack  13   d  appearing on the front surface  13   a.    
     (First Modification)  FIG. 9B  is an enlarged sectional view of the wafer  11 , the enlarged sectional view depicting a laser-processed groove  15   a   1  that is superimposed on the crack  13   d  in the thickness direction A 1  and has a width  15   b   1  sufficiently wider than the meandering width of the crack  13   d . Incidentally, the laser-processed groove  15   a   1  completely severs the laminate  15 . While the laser-processed groove  15   a  depicted in  FIG. 9A  can also cover the crack  13   d  appearing on the front surface  13   a , the laser-processed groove  15   a   1  depicted in  FIG. 9B  can sufficiently cover the crack  13   d  appearing on the front surface  13   a , so that the laser-processed groove  15   a   1  and the crack  13   d  can be connected to each other more reliably. 
     (Second Modification) In the foregoing example, a laser-processed groove  15   a  is formed by applying the laser beam  50  once along one planned dividing line  17 . However, the laser beam  50  may be applied a plurality of times. For example, the laser beam  50  having an intensity distribution similar to a Gaussian distribution as in the cases of forming the laser-processed grooves  15   a  and  15   a   1  is branched into two by using a diffractive optical element or the like, and two condensing points  50   a  are applied along one planned dividing line  17 . 
     Two laser-processed grooves  15   a  that completely sever the laminate  15  along each planned dividing line  17  are thus formed.  FIG. 10A  is an enlarged sectional view of the wafer  11 , the enlarged sectional view depicting the two laser-processed grooves  15   a  formed by applying a first laser beam  50 . Next, the laminate  15  remaining between the two laser-processed grooves  15   a  is irradiated with a laser beam  50  (what is generally called a top-hat beam) that has a substantially uniform intensity distribution in the plane perpendicular to the optical axis of the condensing lens and that is wider than the first laser beam  50 .  FIG. 10B  is an enlarged sectional view of the wafer  11 , the enlarged sectional view depicting a wide laser-processed groove  15   a   2  formed by applying the second laser beam  50 . 
     The second modification has an advantage of being able to solve, by forming the two relatively narrow laser-processed grooves  15   a , a problem that the laminate  15  in the vicinity of a planned dividing line  17  is split and peeled, what is generally called delamination. Incidentally, because a width  15   b   2  of the laser-processed groove  15   a   2  is wider than that of the crack  13   d , there is also an advantage of being able to connect the laser-processed groove  15   a   2  and the crack  13   d  to each other reliably. 
     (Third Modification) Incidentally, the laminate  15  does not necessarily have to be severed completely in the application of the first laser beam  50 . For example, in the application of the first laser beam  50 , two shallow laser-processed grooves  15   a   3  not severing the laminate  15  are formed.  FIG. 11A  is an enlarged sectional view of the wafer  11 , the enlarged sectional view depicting two shallow laser-processed grooves  15   a   3 . In the application of the second laser beam  50  following the first laser beam  50 , the laser-processed groove  15   a   2  is formed by applying the above-described top-hat beam. 
       FIG. 11B  is an enlarged sectional view of the wafer  11 , the enlarged sectional view depicting a wide laser-processed groove  15   a   2  formed by applying the second laser beam  50 . The third modification also has an advantage of being able to connect the laser-processed groove  15   a   2  and the crack  13   d  to each other reliably. In addition, the third modification can reduce thermal damage to the substrate  13  at a time of forming the two shallow laser-processed grooves  15   a   3 , and can therefore increase the strength of the device chips  35  as compared with the second modification. Incidentally, in addition to the foregoing modifications, the wide laser-processed groove  15   a   2  may be formed in a state in which a plurality of condensing points  50   a  of the relatively thin laser beam  50  used to form the laser-processed grooves  15   a  are arranged along a direction intersecting the planned dividing line  17 . 
     The above-described method of manufacturing the device chips  35  brings about effects that the number of times of inverting the wafer  11  upside down in a period from the modified layer forming step S 20  to the laser-processed groove forming step S 70  can be reduced and that the wafer  11  having the laminate  15  on the front surface  13   a  of the substrate  13  can be divided reliably along the planned dividing lines  17 . 
     (Second Embodiment) A second embodiment will next be described.  FIG. 12  is a flowchart of a method of manufacturing device chips  35  according to the second embodiment. As depicted in  FIG. 13 , the laser-processed groove forming step S 70  according to the second embodiment forms a laser-processed groove  15   a   4  having a predetermined depth such that the laminate  15  is not severed completely.  FIG. 13  is an enlarged sectional view of the wafer  11 , the enlarged sectional view depicting one laser-processed groove  15   a   4  not reaching the front surface  13   a  of the substrate  13 . The laser-processed groove  15   a   4  can be formed by, for example, decreasing average power and repetition frequency, increasing processing feed speed, or changing the height position of the condensing point  50   a . Damage to the front surface  13   a  side of the substrate  13  in the laser-processed groove forming step S 70  can be reduced by forming the laser-processed groove  15   a   4  not reaching the front surface  13   a.    
     In the second embodiment, after the laser-processed groove forming step S 70 , an expanding step S 80  is performed by using an expanding apparatus  52  that expands the second protective tape  29 .  FIG. 14A  is a partially sectional side view of the expanding apparatus  52 . The expanding apparatus  52  has a cylindrical drum  54 . An upper end portion of the drum  54  is provided with a plurality of rollers  56  along the circumferential direction of an opening of the drum  54 . A plurality of leg portions  58  (three or more leg portions, for example, four leg portions) are arranged on the outside of the drum  54 . Incidentally,  FIG. 14A  depicts two leg portions  58 . A lower end portion of each of the plurality of leg portions  58  is provided with an air cylinder (not depicted) for moving the leg portion  58  in an upward-downward direction. 
     One annular table  60  is disposed on upper end portions of the plurality of leg portions  58 . The annular table  60  has a rectangular external shape, and has a circular opening through which the drum  54  can pass. A plurality of clamp mechanisms  62  that each sandwich the annular frame  31  of the wafer unit  33  are arranged on a peripheral portion of the annular table  60 . 
     In the expanding step S 80 , the upper end portion of the drum  54  and an upper surface of the annular table  60  are arranged at a substantially same height, and then the wafer unit  33  is disposed on the drum  54  and the annular table  60 . Then, the annular frame  31  is sandwiched by the plurality of clamp mechanisms  62 . Next, the annular table  60  is lowered with respect to the drum  54  by operating the air cylinders. The second protective tape  29  is thereby expanded in a radial direction. 
       FIG. 14B  is a partially sectional side view of the expanding apparatus  52  that has undergone the expanding step S 80 . As the second protective tape  29  is expanded, an external force is applied also to the wafer  11  affixed to the second protective tape  29 , and the wafer  11  is divided into a plurality of device chips  35  with the cracks  13   d  and the laser-processed grooves  15   a   4  as a boundary. The second embodiment can divide the wafer  11  into the plurality of device chips  35  by using the expanding apparatus  52  even when the laminate  15  is not completely severed in the laser-processed groove forming step S 70 . Needless to say, the second embodiment can also reduce the number of times of inverting the wafer  11  upside down, and reliably divide the wafer  11  having the laminate  15  along the planned dividing lines  17 . 
     Incidentally, while the laser-processed groove  15   a   4  does not completely sever the laminate  15  in the second embodiment, the laminate  15  may be severed completely. In addition, while  FIG. 13  depicts a case where the crack  13   d  does not sever the laminate  15 , the crack  13   d  may sever the laminate  15 . 
     In a case where the wafer  11  is already divided into the device chips  35  before the expanding step S 80 , the expanding step S 80  may be performed to facilitate pickup of the device chips  35  by widening intervals between the device chips  35 . Besides, structures, methods, and the like according to the foregoing embodiments can be modified and implemented as appropriate without departing from the objective scope of the present invention. For example, also in the first embodiment, the expanding step S 80  can be performed after the laser-processed groove forming step S 70 . 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.