Patent Publication Number: US-2007111480-A1

Title: Wafer product and processing method therefor

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is based on and incorporates herein by reference Japanese Patent Applications No. 2005-331209 filed on Nov. 16, 2005, No. 2005-331218 filed on Nov. 16, 2005 and No. 2006-196890 filed on Jul. 19, 2006.  
     FIELD OF THE INVENTION  
      The present invention relates to a wafer product and a processing method for the wafer product. In particular, the present invention relates to a wafer product that is cut and separated by cutting with a reformed region due to multiphoton absorption, formed by irradiation with laser light, taken as a starting point for cutting and a processing method for the wafer product.  
     BACKGROUND OF THE INVENTION  
      Laser dicing technologies have been developed for cutting and separating (dividing) a wafer-like object to be processed into a plurality of chips using laser light.  
      For example, a wafer-like object such as a semiconductor substrate to be processed is irradiated with laser light with a light-converging point positioned inside the object. Thus, a reformed (modified) region due to multiphoton absorption is formed inside the object. The reformed region may be a reformed region including crack region, a reformed region including molten processed region, and a reformed region including region with changed refractive index. A region to be a starting point for cutting is formed inside the object by this reformed region. This region is formed within the object inside by a predetermined distance from the laser light incident face of the object along a line along which the object to be processed should be cut. The object to be processed is cut by cutting with this region taken as the starting point. This is disclosed in U.S. Pat. No. 6,992,026 (JP 3408805) for instance.  
      As described above, an object to be processed is irradiated with laser light with the light-converging point of the laser light positioned inside the object. A reformed region is thereby formed within the object along a line along which the object should be cut. At the same time, the position of the light-converging point of laser light in the direction of incidence of the laser light, applied to the object, on the object is changed. The reformed region is thereby plurally formed so that they are lined in the direction of incidence. This is disclosed in U.S. Pat. No. 6,992,026.  
      According to this technology, multiple reformed regions are formed so that they are lined in the direction of incidence. This increases a number of points that make a starting point when an object to be processed is cut. As a result, even a thick object can be cut.  
      An expansible sheet is attached to either face of a flat object to be processed including a substrate, and the other face of the object to be processed is taken as laser light incident face. Then, the object is irradiated with laser light with the light-converging point positioned inside the substrate, and a reformed region (molten processed region) due to multiphoton absorption is thereby formed. A starting point region for cutting is formed by this reformed region inside by a predetermined distance from the laser light incident face along a line along which the object should be cut. Then, the sheet is stretched to cut the object into multiple portions, starting with the starting point region for cutting, so that spacing is provided between the individual portions. This is disclosed in US 2005/0202596 (JP 2005-1001A).  
      According to this technology, the starting point region for cutting is formed inside the substrate, and then the sheet is stretched. Therefore, tensile stress can be favorably applied to the starting point region for cutting, and the substrate can be accurately split and cut by relatively small force, starting with the starting point region for cutting.  
      An attempt has been recently made to implement the following using the above laser dicing technology: a reformed region (reformed layer) is formed inside a wafer; and the wafer is cut and separated into individual chips (semiconductor chips) by cutting using the reformed region as starting point for cutting.  
      However, in this technology, the light-converging point cannot be positioned inside the wafer in the cases, where there is variation from wafer to wafer and the wafer to be processed is too thin, or where the setting of the light-converging point of laser light is inappropriate. In these cases, the light-converging point can be positioned beyond the face (rear face) of the wafer opposite its laser light incident face (front face).  
      That is, the focal point of laser light cannot be positioned inside the wafer when the wafer is too thin or when the setting of the focal point is inappropriate. As a result, the focal point can be positioned beyond the face of the wafer opposite its laser light incident face.  
      According to US 2005/0202596, for example, an expansible sheet attached to the rear face of a wafer can be melted and damaged due to laser light when the light-converging point of the laser light is positioned inside the sheet. Thus, when the sheet is stretched to cut and separate the wafer, the tensile stress from the sheet cannot be evenly applied to the wafer. Therefore, it becomes difficult to appropriately cut and separate the wafer.  
      In cases where the light-converging point of laser light is positioned within the stage (specimen support) of a laser machine with a wafer placed on it, the stage can be melted and damaged by the laser light and lose planarity. Thus, when the next wafer is placed on the stage and irradiated with laser light, the light-converging point cannot be positioned in a desired position inside the wafer, and a reformed region cannot be formed in a due position. As a result, it becomes difficult to accurately cut and separate the wafer with the reformed region taken as the starting point.  
      Further, according to US 2005/0202596, as illustrated in  FIG. 16A , a wafer W that is formed of semiconductor, such as silicon, and has semiconductor devices D formed over its light incident face is prepared. Its rear face opposite the light incident face is bonded to an extensible resin sheet S. A bonding layer B to which ultraviolet curing adhesive or the like is applied is formed over the entire face of the sheet S to which the wafer W is to be bonded. The entire rear face of the wafer W is bonded to the bonding layer B.  
      A laser head H that projects laser light L is provided with a condenser lens CV that condenses the laser light L, and converges the laser light L at a predetermined focal position. In a reformed region formation process, the laser head H is moved along a planned dividing line DL along which the wafer W is to be divided (to the near side in the figure). At this time, the laser head is moved under laser light irradiation conditions so set that the light-converging point P of the laser light L is positioned in a place at a depth of Dp from the front face of the wafer W. Then, the wafer W is irradiated with the laser light L from its front face side. Thus, a reformed region R due to multiphoton absorption is formed in the pass at a depth of Dp through which the light-converging point P of the laser light L is caused to make scanning movement.  
      The reformed region R can be plurally formed in multiple places at arbitrary depths within the range of the thickness of the wafer W by taking the following procedure: the depth Dp of the light-converging point P is adjusted along a planned dividing line DL, and the light-converging point P is moved in the direction of the thickness of the wafer W.  
      Multiphoton absorption is defined as a substance absorbing multiple homogeneous or heterogeneous photons. Because of this multiphoton absorption, a phenomenon designated as optical damage occurs at the light-converging point P and in the vicinity of the point within the wafer W. This induces thermal distortion and cracks to occur in that area. As a result, a layer in which cracks aggregate, that is, a reformed region R is formed.  
      Subsequently, stress is applied to the wafer W in the in-plane direction indicated by arrows F 1  and F 2  in  FIG. 16B . The cracks are thereby developed in the direction of the substrate thickness with the reformed regions R taken as starting point, and the wafer W is thereby divided along the planned dividing lines DL to obtain semiconductor chips C.  
      However, the reformed region R is introduced in an area in the vicinity of the rear face, a face to be joined with the sheet S. In cases where, at this time, the laser light L passes through the wafer W and its light-converging point P is positioned inside the bonding layer B or the sheet S, their quality can be changed by heat affection. Portions whose quality has been changed in the bonding layer B and the sheet S lose extensibility and become frangible. For this reason, when the wafer W is divided, they can fly as powder in all directions and stick to semiconductor devices D.  
      To avoid this phenomenon, the area in the vicinity of the rear face of the wafer W can be protected from being irradiated with laser light L. In cases where this is done, a sufficient amount of reformed regions R cannot be formed aiming at the area in the vicinity of the rear face that becomes a starting point for division. As a result, great force is required to divide the substrate. This will become a cause of a portion left undivided in the wafer W.  
     SUMMARY OF THE INVENTION  
      The present invention has an object to provide a wafer product and a processing method for the wafer product, wherein a light-converging point of laser light can be positioned without being positioned beyond the face opposite a laser light incident face.  
      According to the present invention, a wafer product separable by cutting at a reformed region formed by laser light comprises a wafer, a dicing sheet and a protection layer. The wafer has two faces, one of which is a laser light incident face and the other of which is opposite the light incident face in a direction of wafer thickness. The dicing sheet is attached to the other face of the wafer for cutting the wafer into a plurality of chips. The protection layer is provided between the wafer and the dicing sheet for scattering or reflecting the laser light passing though the wafer thereat to protect the dicing sheet from the laser light.  
      The protection layer may be projections and depressions uniformly formed on the other face of the wafer, or a large number of particles provided on the other face, so that the laser light may be scattered not to enter the dicing sheet. Alternatively, the protection layer may be a reflector that reflects the laser light not to enter the dicing sheet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:  
       FIG. 1  is an enlarged sectional view illustrating how the wafer is irradiated with laser light to form a reformed region in a first embodiment of the invention;  
       FIG. 2  is an enlarged sectional view illustrating how the wafer is irradiated with laser light to form a reformed region in the first embodiment of the invention;  
       FIG. 3  is an enlarged sectional view illustrating how the wafer whose rear face is a smooth surface is irradiated with laser light from the front face side to form a reformed region;  
       FIG. 4  is an enlarged sectional view illustrating how the wafer whose rear face is a smooth surface is irradiated with laser light from the front face side to form a reformed region;  
       FIG. 5  is an enlarged sectional view illustrating how the wafer whose rear face is a roughened surface is irradiated with laser light from the rear face side with the rear face taken as incident face to form a reformed region;  
       FIG. 6  is an enlarged sectional view illustrating how the wafer whose rear face is a roughened surface is irradiated with laser light from the rear face side with the rear face taken as incident face to form a reformed region;  
       FIG. 7  is a graph showing the result of experiments conducted to examine whether or not a reformed region is formed, taking the following procedure: YAG laser with a wavelength of 1.064 μm is used as the laser light, and the maximum height in surface roughness of the rear faces of wafers is varied;  
       FIG. 8  is an enlarged sectional view illustrating how the wafer is irradiated with laser light to form a reformed region in a second embodiment of the invention;  
       FIG. 9  is an enlarged sectional view illustrating how the wafer is irradiated with laser light to form a reformed region in a third embodiment of the invention;  
       FIG. 10A  is an enlarged sectional view illustrating how the wafer is irradiated with laser light to form a reformed region in a fourth embodiment of the invention;  
       FIG. 10B  is an enlarged sectional view illustrating how the wafer is irradiated with laser light to form a reformed region in a fifth embodiment of the invention;  
       FIG. 11A  is a plan view of a wafer in a sixth embodiment of the present invention;  
       FIG. 11B  is a sectional view taken along line  11 B- 11 B in  FIG. 11A .  
       FIG. 12  is a schematic sectional view illustrating a method for irradiating a wafer with laser light;  
       FIG. 13  is an enlarged sectional view illustrating how laser light is reflected by an aluminum sheet formed over a wafer;  
       FIG. 14  is an enlarged sectional view illustrating a method for converging laser light, reflected by an aluminum sheet, to form a reformed region in a seventh embodiment;  
       FIG. 15  is an enlarged sectional view of a construction with which an aluminum sheet is formed on a rear face at least on planned dividing lines;  
       FIG. 16A  is an enlarged sectional view illustrating a process in which a reformed region is formed by laser light irradiation; and  
       FIG. 16B  is an enlarged sectional view illustrating a process in which a wafer is divided. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
      Referring first to  FIGS. 1 and 2 , a bulk silicon wafer  10  is formed of a bulk material of single crystal silicon, and its rear face  10   a  is a roughened surface in which substantially uniform projections and depressions  10   c  are formed as a protection layer. To make the rear face  10   a  of the wafer  10  a roughened surface, that is, protection layer, any processing method may be used. Examples of the processing method include: a method in which the rear face  10   a  is immersed in acid solution or alkaline solution that affects the material for forming the wafer  10  and chemically treated; and a method in which the rear face is processed by mechanical polishing, such as sandblast.  
      To cut and separate the wafer  10  using a laser dicing technology, the following procedure is taken: a dicing sheet (dicing film, dicing tape, expand tape)  11  is stuck to the rear face  10   a  of the wafer  10 . The dicing sheet  11  is composed of expansible plastic sheet material that is stretched by applying heat or force in the direction of expansion. It is bonded to the entire rear face  10   a  of the wafer  10  by a binding material (not shown). Thus, the wafer  10  and the dicing sheet  11  form a wafer product. The wafer product is placed on a stage (specimen support)  12  of a laser machine (not shown) with the rear face  10   a  of the wafer  10  facing downward. This brings the dicing sheet  11  into contact with the upper face of the stage  12 .  
      The laser machine includes a laser light source (not shown) that projects laser light L and a condenser lens CV. The laser light L is applied to the front face (laser light incident face)  10   b  of the wafer  10  through the condenser lens CV with the optical axis OA of the laser light L perpendicular to the front face  10   b  of the wafer  10 . Thus, the light-converging point (focal point) P where the laser light L is converged is positioned in a predetermined position inside the wafer  10 . As a result, a reformed region (reformed layer) R is formed at the location. of the light-converging point P inside the wafer  10 .  
      For example, laser light of YAG (Yttrium Aluminum Garnet) laser with a wavelength of 1.064 μm in the infrared light region can be used as the laser light L. The reformed region R includes a molten processed region mainly due to multiphoton absorption, formed by irradiation with the laser light L.  
      The location of the light-converging point P within the wafer  10  is locally heated by the multiphoton absorption of the laser light L. It is melted by this heat, and then set again. Thus, a region melted and then set again within the wafer  10  becomes the reformed region R.  
      That is, the molten processed region is a region whose phase or crystal structure has been changed. In other words, the molten processed region is any of a region where the single crystal silicon has been transformed into amorphous silicon inside the wafer  10 , a region where single crystal silicon has been transformed into polycrystalline silicon, and a region where single crystal silicon has been transformed into a structure containing amorphous silicon and polycrystalline silicon. Since the wafer  10  is a bulk silicon wafer, the molten processed region is mainly composed of polycrystalline silicon.  
      The molten processed region is not formed by the laser light L being absorbed within the wafer  10 . That is, it is not formed by ordinary heating by laser light. The molten processed region is formed mainly by multiphoton absorption. For this reason, the laser light L is hardly absorbed in the locations inside the wafer  10  other than that of the light-converging point P, and the front face  10   b  of the wafer  10  is not melted.  
      With the depth position of the light-converging point P inside the wafer  10  kept constant, the laser machine applies the laser light L in a pulse pattern and further causes it to make scanning movement. The laser machine thereby moves the light-converging point P along a straight line DL along which the wafer  10  should be cut. Instead, with the position of laser light L application fixed, the stage  12  may be moved in the direction orthogonal to the direction of laser light L application. The direction of laser light L application is the direction of incidence of the laser light L on the front face  10   b  of the wafer  10 .  
      That is, the light-converging point P is moved relative to the wafer  10  along the line DL along which the wafer  10  should be cut by causing the laser light L to make scanning movement or moving the wafer  10 .  
      As stated above, with the depth position of the light-converging point P within the wafer  10  kept constant, the wafer  10  is irradiated with the laser light L in a pulse pattern and further the light-converging point P is moved relative to the wafer. Thus, a reformed region group composed of multiple reformed regions R at certain intervals in the direction parallel to the front and rear faces  10   b  and  10   a  of the wafer  10  is formed in a certain depth position from the front face  10   b  of the wafer  10 . That is, the reformed region group is formed in a position inside an area at a certain distance from the laser light L incident face.  
      The depth of the light-converging point P within the wafer  10  is equivalent to the distance from the front face (laser light L incident face)  10   b  of the wafer  10  to the light-converging point P.  
      Thus, the reformed region group composed of the multiple reformed regions R is formed inside the wafer  10 . Thereafter, the dicing sheet  11  is stretched in the horizontal direction (the directions indicated by arrows β and β′ in  FIG. 1 ) with respect to the line DL along which the wafer should be cut, and tensile stress is thereby applied to each reformed region R.  
      Thus, shearing stress is produced within the wafer  10 , and a crack is made in the direction of the depth of the wafer  10  with each reformed region R taken as starting point. When the grown cracks reach the front and rear faces  10   b  and  10   a  of the wafer  10 , the wafer  10  is thereby cut and separated.  
      As mentioned above, each reformed region R is formed along the line DL along which the wafer should be cut. Therefore, the following can be implemented by stretching the dicing sheet  11  to favorably apply tensile stress to each reformed region R, and thereby cutting the wafer with each reformed region R taken as starting point for cutting: the wafer  10  can be accurately cut and separated by relatively small force without causing an undesired crack in the wafer  10 .  
      Over the front face  10   b  of the substantially disk-shaped wafer  10  of thin plate, there are aligned and arranged a large number of chips (not shown) in a grid pattern. The lines DL along which the wafer should be cut are disposed between the chips. That is, the multiple lines DL along which the wafer  10  should be cut are disposed in a lattice pattern in the front face  10   b  of the wafer  10 .  
      For this reason, the wafer  10  can be cut and separated into individual chips by forming each reformed regions R with respect to each of the lines DL along which the wafer should be cut, and then stretching the dicing sheet  11 .  
      The light-converging point P cannot be positioned inside the wafer  10  in the following cases: cases where there is variation from wafer  10  to wafer  10  and the wafer  10  to be processed is too thin; and cases where the setting of the light-converging point P of laser light L is inappropriate. In these cases, the light-converging point P can be positioned beyond the rear face  10   a  of the wafer  10  opposite its front face (laser light L incident face)  10   b.    
      The focal point P of laser light L cannot be positioned inside the wafer  10  when the wafer  10  is too thin or when the setting of the focal point P is inappropriate. As a result, the focal point P can be positioned beyond the face (rear face)  10   a  of the wafer  10  opposite its laser light L incident face (front face)  10   b.    
      Referring to  FIG. 3  and  FIG. 4  explaining how the wafer  10  is irradiated with the laser light L from its front face  10   b  side to form a reformed region when the rear face  10   a  of the wafer is a smooth surface. They schematically illustrate a longitudinal section of a wafer  10 .  
      When the light-converging point P of laser light L is positioned inside the dicing sheet  11  as illustrated in  FIG. 3  as an example, the dicing sheet  11  can be melted and damaged by the laser light L. Consequently, when the dicing sheet  11  is stretched to cut and separate the wafer  10 , tensile stress from the dicing sheet  11  cannot be evenly applied to the wafer  10 . This makes it difficult to normally cut and separate the wafer  10 .  
      When the light-converging point P of laser light L is positioned inside the stage  12 , as illustrated in  FIG. 4 , the stage  12  can be melted and damaged by the laser light L and can lose planarity. Consequently, when the next wafer  10  is placed on the stage  12  and irradiated with laser light L, the light-converging point P cannot be positioned in a desired position inside the wafer  10  and the reformed region R cannot be formed in a due position. This makes it difficult to accurately cut and separate the wafer  10  with the reformed region R taken as starting point.  
      In the first embodiment, meanwhile, the rear face  10   a  of the wafer  10  is roughened as illustrated in  FIG. 2 . Therefore, even when the light-converging point P is erroneously set to a point beyond the rear face  10   a , the above problem does not arise. Since the laser light L is scattered by the rear face  10   a , as indicated by arrows γ, the light-converging point P is not formed. As a result, the energy of laser light L at the erroneously set light-converging point P is significantly attenuated.  
      According to the first embodiment, therefore, the light-converging point P of laser light L is not positioned inside the dicing sheet  11 , and the dicing sheet  11  can be prevented from being melted and damaged. Further, the light-converging point P of laser light L is not positioned within the stage  12 , as illustrated in  FIG. 2 , and the stage  12  can be protected from being over-heating and damaged.  
       FIG. 5  and  FIG. 6  show how the reformed region R is formed by irradiating the wafer  10  with the laser light L from its rear face  10   a  side with the rear face  10   a  taken as incident face when the rear face  10   a  is roughened. It schematically illustrates a longitudinal section of the wafer  10 .  
      When the laser light L passes through the rear face  10   a  of the wafer  10 , as illustrated in  FIG. 5 , the energy of the laser light L at the light-converging point P is considerable. As a result, the reformed region R is formed inside the wafer  10 .  
      When the laser light L is scattered by the rear face  10   a  of the wafer  10 , as illustrated in  FIG. 6 , the energy of the laser light L at the light-converging point P is attenuated. For this reason, the reformed region R is not formed inside the wafer  10 .  
       FIG. 7  shows the result of experiments conducted to examine whether the reformed region R is formed inside the wafer  10 , taking the following procedure: YAG laser with a wavelength of 1.064 μm is used as the laser light L, and the maximum height Rmax in the surface roughness of the laser light incident face (the rear face  10   a  of the wafer  10 ) is varied.  
      The maximum height Rmax (Ry) in surface roughness is a value according to a measuring method laid down in the JIS Standard “JIS B0601-1982.” It expresses the maximum height value, obtained by taking the following procedure, in micrometers (μm): the maximum height of a portion extracted from a profile curve by a reference length is measured in the direction of the axial magnification of the profile curve. This is equivalent to the distance between two straight lines parallel to an average line when the extracted portion is sandwiched between the two straight lines.  
      The following is understood from  FIG. 7 : when the maximum height Rmax in the surface roughness of the rear face  10   a  of the wafer  10  is equal to or larger than the wavelength λ (=1.064 μm) of laser light L, the reformed region R is not formed. This is indicated with the mark X in  FIG. 7 , while the mark  0  indicates occurrence of the reformed region.  
      There is no difference in the state of scattering of laser light L between the following cases: cases where the wafer  10  is irradiated with laser light L from its front face  10   b  side with the front face  10   b  taken as the light incident face (cases in  FIG. 1  and  FIG. 2 ); and cases where the wafer  10  is irradiated with the laser light L from its rear face  10   a  side with the rear face  10   a  taken as the incident face (cases illustrated in  FIG. 5  and  FIG. 6 ).  
      It is noted that even in the following cases, the reformed region R is not formed as long as the maximum height Rmax in the surface roughness of the rear face  10   a  of the wafer  10  is equal to or larger than the wavelength λ of the laser light L: cases where laser light with any other wavelength than 1.064 μm is used; and cases where any other kind of laser than YAG laser is used.  
      Examples of other kinds of laser than YAG laser include solid state laser, such as ruby laser and glass laser, semiconductor laser, such as gallium arsenide laser and indium gallium arsenide laser, and gas laser, such as excimer laser and carbon dioxide laser.  
      Therefore, a light-converging point P can be positioned without fail not to be positioned beyond the rear face  10   a  of the wafer  10  opposite its front face (laser light L incident face)  10   b  by taking the following measure: the maximum height Rmax in the surface roughness of the rear face  10   a  of the wafer  10  is so set that it is equal to or larger than the wavelength λ of laser light L used (Rmax≧λ). This can be implemented even when the wafer  10  is irradiated with laser light L from its front face  10   b  side with the front face  10   b  taken as the incident face, as in the first embodiment ( FIG. 1  and  FIG. 2 ). This protects the dicing sheet  11  or the stage  12  from being melted and damaged by the laser light L.  
      Thus, the laser light L can be restricted without fail from forming the focal point P beyond the face (rear face)  10   a  of the wafer  10  opposite its laser light L incident face (front face)  10   b  by taking the following measure: the maximum height Rmax in the surface roughness of the rear face  10   a  of the wafer  10  is so set that it is equal to or larger than the wavelength λ of laser light L used.  
     Second Embodiment  
      The second embodiment shown in  FIG. 8  is different from the first embodiment in the following points.  
      (2.1) The rear face  10   a  of the wafer  10  is a smooth surface.  
      (2.2) The dicing sheet  11  is composed of a sheet base material  11   a  and a binding material  11   b , and the binding material  11   b  is applied to the entire front face of the sheet base material  11   a.    
      (2.3) The sheet base material  11   a  is formed of an expansible plastic sheet material, and its front face is a smooth surface. The binding material  11   b  is formed of a thin sheet of adhesive having the property of bonding together the wafer  10  and the sheet base material  11   a . An example of this adhesive is acrylic adhesive. The front face of the binding material  11   b  is a roughened surface in which substantially uniform projections and depressions are formed, so that the binding material functions as a protection layer for protecting the base material  11   a  from the laser light L.  
      (2.4) The rear face  10   a  of the wafer  10  is in contact only with the projections of the projections and depressions formed in the front face of the binding material  11   b . That is, an air gap is formed between the rear face  10   a  of the wafer  10  and the depressions of the projections and depressions formed in the front face of the binding material  11   b.    
      Thus, the front face of the binding material  11   b  (the face joined with the rear face  10   a  of the wafer  10 ) is a roughened surface. Therefore, even when the light-converging point P is erroneously set to a point beyond the rear face  10   a  of the wafer  10 , the above problem does not arise. Since the laser light L is scattered by the front face of the binding material  11   b , as indicated by arrows γ, the light-converging point P is not formed. As a result, the energy of laser light L at the erroneously set light-converging point P is significantly attenuated.  
      Therefore, the same effect as in the first embodiment is provided in the second embodiment. That is, the light-converging point P of laser light L is not formed in the dicing sheet  11  or the stage  12 , and the dicing sheet  11  and the stage  12  can be protected from being melted and damaged.  
      Also, in the second embodiment, laser light L can be restricted without fail from forming a focal point P beyond the face (rear face)  10   a  of the wafer  10  opposite its laser light L incident face (front face)  10   b  due to the same operation as in the first embodiment by taking the following measure: the maximum height Rmax in the surface roughness of the front face of the binding material  11   b  is so set that it is equal to or larger than the wavelength λ of laser light L used (Rmax≧λ).  
      To make the front face of the binding material  11   b  a roughened surface, any processing method may be used. Examples of the processing method include: a method in which the dicing sheet  11  is immersed in acid solution or alkaline solution that affects the adhesive that is a material for forming the binding material  11   b  and chemically treated; a method in which the binding material is processed by mechanical polishing, such as sandblast; and a method in which the binding material is processed by pressing, that is, a jig with projections and depressions formed in its front face is pressed against the partially set binding material  11   b.    
     Third Embodiment  
      In the third embodiment shown in  FIG. 9 , the rear face  10   a  of the wafer  10  is a smooth surface and the dicing sheet  11  is formed of the sheet base material  11   a  and the binding material  11   b  as described in Sections 2.1 and 2.2 with regard to the second embodiment.  
      The third embodiment is different from the second embodiment only in the following:  
      (3.1) The front face of the sheet base material  11   a  is a roughened surface in which substantially uniform projections and depressions are formed. The front face of the binding material  11   b  is a smooth surface.  
      (3.2) The entire rear face  10   a  of the wafer  10  is stuck to the front face of the binding material  11   b . The front face (in contact with the binding material  11   b ) of the sheet base material  11   a  is a roughened surface.  
      Therefore, even when a light-converging point P is erroneously set to a point beyond the rear face  10   a  of the wafer  10 , the above problem does not arise. Since the laser light L is scattered by the front face of the sheet base material  11   a , as indicated by arrows γ, the light-converging point P is not formed. As a result, the energy of laser light L at the erroneously set light-converging point P is significantly attenuated. Therefore, the same effect as in the first embodiment is provided in the third embodiment.  
      Also, in the third embodiment, the above effect can be provided without fail due to the same operation as in the first embodiment by taking the following measure: the maximum height Rmax in the surface roughness of the front face of the sheet base material  11   a  is so set that it is equal to or larger than the wavelength λ of laser light L used.  
      To make the front face of the sheet base material  11   a  a roughened surface, any processing method may be used. Examples of the processing method include: a method in which the sheet base material  11   a  is immersed in acid solution or alkaline solution that affects the sheet base material  11   a  and chemically treated; a method in which the sheet base material is processed by mechanical polishing, such as sandblast; and a method in which the sheet base material is processed by pressing, that is, a jig with projections and depressions formed in its front face is pressed against the sheet base material  11   a.    
     Fourth Embodiment  
      In the fourth embodiment shown in  FIG. 10A , the rear face  10   a  of the wafer  10  is a smooth surface and the dicing sheet  11  is formed of the sheet base material  11   a  and the binding material  11   b  as described in Sections 2.1 and 2.2 with respect to the second embodiment.  
      The fourth embodiment is different from the second embodiment in the following points:  
      (4.1) The front faces of the sheet base material  11   a  and the binding material  11   b  are smooth surfaces.  
      (4.2) A large number of substantially spherical particles  13  are scattered and bonded to the flat front face of the binding material  11   b  to provide the protective layer.  
      (4.3) The rear face  10   a  of the wafer  10  is in contact with the front face of the binding material  11   b  only at its portions without particles  13 .  FIG. 10A  does not show the state in which the rear face  10   a  of the wafer  10  is in contact with the front face of the binding material  11   b . However, since the particles  13  are small in particle diameter and the binding material  11   b  is highly flexible, the following can be implemented by pressing the dicing sheet  11  against the rear face  10   a  of the wafer  10 : the dicing sheet  11  can be stuck to the wafer  10  by the portions of the front face of the binding material  11   b  where particles  13  are not placed.  
      In the fourth embodiment, a large number of particles  13  are substantially evenly scattered on the front face (the face joined with the wafer  10 ) of the binding material  11   b . This makes the front face of the binding material  11   b  an apparent roughened surface. Therefore, even when the light-converging point P is erroneously set to a point beyond the rear face  10   a  of the wafer  10 , the above problem does not arise. Since laser light L is scattered by the particles  13  as indicated by arrows γ, the light-converging point P is not formed. As a result, the energy of laser light L at the erroneously set light-converging point P is significantly attenuated.  
      Therefore, the same effect as in the first embodiment is provided in the fourth embodiment. Also, in the fourth embodiment, the above effect can be provided without fail due to the same operation as in the first embodiment by taking the following measure: the maximum height Rmax in the apparent surface roughness of the front face of the binding material  11   b  where the particles  13  are scattered is so set that it is equal to or larger than the wavelength λ of laser light L used.  
      The maximum height Rmax in the apparent surface roughness of the front face of the binding material  11   b  where the particles  13  are scattered is substantially equal to the particle diameter of the particles  13 .  
      Any material (e.g. glass, ceramics, plastic) can be used as the material for forming the particles  13 . The same material as the material for forming the sheet base material  11   a  or the binding material  11   b  may be used or a material different from them may be used. With respect to the material for forming the particles  13  and the refractive index and reflectance of this material, optimum ones can be experimentally selected by cut-and-try methods so that the above-mentioned action and effect can be provided.  
     Fifth Embodiment  
      The fifth embodiment shown in  FIG. 10B  is different from the fourth embodiment in the following points:  
      (5.1) A large number of substantially spherical particles  13  are substantially evenly buried in the binding material  11   b . The refractive index and reflectance of the particles  13  are different from the refractive index and reflectance of the binding material  11   b . This means that a large number of the particles  13  are substantially evenly scattered and fixed on the front face of the sheet base material  11   a.    
      (5.2) The entire rear face  10   a  of the wafer  10  is bonded to the front face of the binding material  11   b.    
      In the fifth embodiment, as described above, a large number of particles  13  are substantially evenly scattered on the front face (the face in contact with the binding material  11   b ) of the sheet base material  11   a . This makes the front face of the sheet base material  11   a  an apparent roughened surface. Therefore, even when the light-converging point P is erroneously set to a point beyond the rear face  10   a  of the wafer  10 , the above problem does not arise. Since laser light L is scattered by the particles  13 , as indicated by arrows γ, the light-converging point P is not formed. As a result, the energy of laser light L at the erroneously set light-converging point P is significantly attenuated. Therefore, the same effect as in the first embodiment is provided in the fifth embodiment.  
      Also, in the fifth embodiment, the above effect can be provided without fail due to the same operation as in the first embodiment by the taking the following measure: the maximum height Rmax in the apparent surface roughness of the front face of the sheet base material  11   a  where the particles  13  are scattered is so set that it is equal to or larger than the wavelength λ of laser light L used.  
      The maximum height Rmax in the apparent surface roughness of the front face of the sheet base material  11   a  where the particles  13  are scattered is substantially equal to the particle diameter of the particles  13 .  
      (Modifications)  
      The first to fifth embodiments may be modified in the following manner.  
      (1) The bulk silicon wafer  10  may be replaced with a wafer formed of semiconductor material for forming a wafer of multilayer structure. Example of wafers used in this case include: wafer of bonding SOI (Semiconductor On Insulator) structure; wafer of SIMOX (Separation by IMplanted OXygen) structure; wafer of SOI structure in which polycrystalline silicon or amorphous silicon is formed over an insulating substrate of glass or the like by solid phase epitaxy or melt recrystallization; wafer for use in light-emitting devices, obtained by crystal growing a III-V compound semiconductor layer over a substrate of sapphire or the like; and wafer formed by sticking together a silicon substrate and a glass substrate using anodic bonding.  
      (2) The bulk silicon wafer  10  may be replaced with any kind of wafer as long as the wafer is formed of semiconductor material (e.g. gallium arsenide) for forming a wafer (e.g. gallium arsenide substrate).  
      Further it may be replaced with a wafer formed of any of various materials (e.g. material containing glass). In this case, the reformed region R due to multiphoton absorption is not limited to those including a molten processed region as in the above embodiments. It may be appropriately formed in accordance with the material for forming the wafer. For example, in cases where the material for forming a wafer contains glass, the reformed region R due to multiphoton absorption can be so formed that it includes a crack region or a region where the refractive index is varied.  
      (3) The wafer  10  may be cut and separated by taking the following measure: the curved surface (convex face) of an object having a curvature (e.g. semi-spherical object) is pressed against a line along which the wafer  10  should be cut, and pressing force is applied. Shearing stress is thereby produced in the reformed region R to cut and separate the wafer  10 .  
      (4) Aside from the dicing sheet  11 , a dedicated light scattering member may be provided; the light scattering member may be bonded to the rear face  10   a  of the wafer  10 , which is in turn irradiated with laser light L to form the reformed region R; thereafter, the light scattering member maybe removed from the wafer  10 , and subsequently, the dicing sheet  11  may be bonded to the rear face  10   a  of the wafer  10 .  
     Sixth Embodiment  
      In the sixth embodiment, as shown in  FIGS. 11A and 11B , a thin-plate, disk-shaped wafer  21  is formed of silicon. The wafer  21  is bonded to a resin sheet  41  at its rear face  21   b  opposite its laser light incident face  21   a , which is a substrate surface face. The sheet has a bonding layer  52  ( FIG. 11B ) formed of an adhesive or the like over its entire surface, and has extensibility. The peripheral portion of the sheet  41  is held by an annular frame  42  so that the sheet  41  is tightened to provide a wafer product.  
      At part of the peripheral portion of the wafer  21 , there is formed an orientation flat OF that indicates crystal orientation. Over the light incident face  21   a  of the wafer  21 , semiconductor devices  24  formed through a diffusion process and the like are aligned and disposed in a grid pattern.  
      In the light incident faces  21   a  between the individual semiconductor devices  24 , planned dividing lines DL 1  to DL 14  are established so that they extend toward the rear face  21   b  in the direction of the thickness of the wafer  21 . The planned dividing lines are lines along which the wafer  21  is to be diced and divided in the direction of its thickness. The planned dividing lines DL 1  to DL 7  are provided in the direction substantially perpendicular to the orientation flat OF so that they are parallel to one another. The planned dividing lines DL 8  to DL 14  are provided in the direction substantially parallel to the orientation flat OF so that they are parallel to one another. That is, the planned dividing lines DL 1  to DL 7  and the planned dividing lines DL 8  to DL 14  perpendicularly intersect each other.  
      Each semiconductor device  24  is surrounded with planned dividing lines DL on its four sides. The wafer  21  is divided in the direction of its thickness along the planned dividing lines DL, and multiple semiconductor chips  22  having a semiconductor device  24  are thereby obtained.  
      An aluminum sheet  25 , several micrometer in thickness, is formed over the entire rear face  21   b  of the wafer  21  by sputtering ( FIG. 11B ). The aluminum sheet  25  is easily formed and is high in adhesion to the wafer  21  and in efficiency of reflecting laser light. The aluminum sheet  25  is formed by a sputtering technique, one of dry processes; therefore, it is unlikely that semiconductor devices  24  are affected in a sheet formation process. The sheet  25  is provided as a protection layer that protects the sheet  41  from the laser light.  
      In the following description, portions that have not been divided from a wafer  21  and are supposed to become semiconductor chips after the substrate is divided will also be referred to as semiconductor chips. These semiconductor chips  22  are formed by dividing a wafer along the planned dividing lines DL in the direction of its thickness in a dicing process. Thereafter, they are subjected to various processes, such as mount process, bonding process, and encapsulating process, and thereby brought to completion as packaged ICs and LSIs.  
      As illustrated in  FIG. 11B , six semiconductor chips  22   a  to  22   f  having respective semiconductor devices  24  thereon are formed over the wafer  21  along line  11 B- 11 B. The wafer  21  has its rear face  21   b  bonded to the bonding layer  52  with the aluminum sheet  25  in-between. The aluminum sheet  25 , bonding layer  52  and sheet  41  are disposed in this order from the rear side  21   b  of the wafer  21 .  
      To part these semiconductor chips  22   a  to  22   f  from one another, seven planned dividing lines DL 1  to DL 7  and planned dividing lines DL 11  and DL 12  ( FIG. 11A ) that are not shown in  FIG. 11B  are established. Reformed regions R that become starting points for division are formed on the planned dividing lines DL 1  to L 7 , DL 11 , and DL 12  in the direction of the thickness of the wafer  21  by the method described later.  
      As illustrated in  FIG. 12 , a manufacturing machine  1  such as a laser machine for semiconductor chips is provided with a laser head  31  that projects laser light L. The laser head  31  has a condenser lens  32  that converges laser light L and is capable of converging laser light L at a predetermined focal position. In this example, the laser head is so set that the light-converging point P of laser light L is formed at a place at a depth of Dpp from the light incident face  21   a  within the wafer  21 .  
      To form the reformed region R within the wafer  21 , one of the planned dividing lines DL illustrated in  FIG. 11A  is scanned with laser light L for wafer detection, and a range to be irradiated with laser light L is set. It is assumed here that the reformed regions R are formed on the planned dividing line DL 4  as an example.  
      The laser head  31  is caused to make scanning movement along the planned dividing line DL 4  (in the direction indicated by arrow F 4 ), as illustrated in  FIG. 12 . Then, laser light L is applied from the light incident face  21   a  side. As a result, the reformed region R due to multiphoton absorption is appropriately formed in the path at a depth of Dp through which the light-converging point P of the laser light L is caused to make scanning movement.  
      At this time, an arbitrary number of layers of reformed region R can be formed at an arbitrary depth within the range of the thickness of the wafer  21  by adjusting the depth Dp of the light-converging point P of the laser light L. In cases where the wafer  21  is relatively thick, for example, the light-converging point P is moved in the direction of its thickness, and the reformed region R is formed continuously or at multiple points on the planned dividing line DL in the direction of the thickness of the substrate. Thus, the wafer  21  can be divided without fail.  
      It is assumed that a reformed region Rs is formed on the planned dividing line DL 4  within the wafer  21  in the vicinity of its rear face  2   b , as illustrated in  FIG. 13 . Even when the light-converging point of laser light L is shifted toward the sheet  41  from an intended light-converging point Pa at which the reformed region Rs is supposed to be formed and is moved to a light-converging point Pb inside the sheet  41  and outside the wafer  21 , no problem arises. Since the aluminum sheet  25  is formed over the rear face  21   b  of the wafer  21 , laser light L is reflected by the aluminum sheet  25  before it is converged at the position Pb. Therefore, since the laser light L is not really converged at the light-converging point Pb, it is unlikely that the quality of the bonding layer  52  or the sheet  41  is changed by heat affection.  
      That is, even when the wafer  21  is irradiated with laser light L in the vicinity of its rear face  21   b , it is unlikely that the laser light L passes through the wafer  21  to be converged within the sheet  41 . Therefore, the reformed region R can be formed aiming at the vicinity of the rear face  21   b  that becomes a starting point for division. Also, with respect to the other planned dividing lines DL, the reformed regions R are formed in the vicinity of the rear face  21   b  as with the planned dividing line DL 4 .  
      Subsequently, the sheet  41  is expanded in the direction of plane to apply stress to the wafer  21 . Cracks or cuts are thereby developed with the reformed regions R taken as starting points to divide the wafer  21  along the planned dividing lines DL in the direction of thickness.  
      An example of methods for expanding the sheet  41  is a publicly known method in which the following procedure is taken: with the frame  42  fixed, a pressing apparatus, not shown, having a flat face in substantially the same size as the rear face  21   b  of the wafer  21  is used; using this apparatus, the wafer  21  is pressed from the rear side of the sheet  41  so that the wafer  21  is pushed up; and the sheet  41  is thereby expanded in the direction of plane to apply stress to the wafer  21  in the in-plane direction.  
      When the reformed regions R are formed in the vicinity of the rear face  21   b  of the wafer  21 , they effectively act as starting points for cracking when the sheet  41  is expanded to divide the wafer  21 . Therefore, the cracks can be developed by small force, and the wafer  21  can be divided without fail.  
      In the sixth embodiment, any material other than the aluminum sheet  25  can be used as the sheet formed over the rear face  21   b  of the wafer  21  as long as the material reflects laser light L. For example, any other metal sheet, such as a titanium sheet, may be formed. The sheet formation method is not limited to sputtering, and plating or the application of coating material that reflects laser light may be adopted. In cases where a metal sheet is formed by plating, the metal sheet high in adhesion to the wafer can be formed in a short time at low cost.  
      The sixth embodiment provides the following advantages.  
      (1) The aluminum sheet  25  that reflects the laser light L is formed over the rear face  21   b  of the wafer  21 . Therefore, the light-converging point P can be restricted from being positioned within the sheet  41  by the laser light L passing through the wafer  21 .  
      Even when the wafer  21  is irradiated with laser light L in the vicinity of its rear face  21   b , it is unlikely that the laser light L passes through the wafer  21  and the sheet  41  is irradiated with it. Therefore, a sufficient amount of reformed regions R that become starting points for division can be formed aiming at the area in the vicinity of the rear face  21   b.    
      Thus, the sheet  41  is protected from being changed by the laser light L, applied to the planned dividing lines DL, passing through the wafer  21  and being converged within the sheet  41 . At the same time, the reformed regions R sufficient for wafer cutting can be formed in the vicinity of the rear face  21   b  of the wafer  21 .  
     Seventh Embodiment  
      In the seventh embodiment shown in  FIG. 14 , the reformed region R is formed by converging laser light L reflected by the aluminum sheet  25 .  
      As illustrated in  FIG. 12 , the position of the light-converging point P is determined by a distance M between the laser light L projection face of the laser head  31  and the light incident face  21   a . In cases where the aluminum sheet  25  is not formed, the light-converging point P is shifted more toward the sheet  41  with decrease in this distance M. When beams of laser light L 1  to L 5  are applied, as illustrated in  FIG. 14 , the laser head is so set that the distance M is shortened in this order.  
      The laser light L 1  is converged at the light-converging point P 1  that is set to a position closest to the light incident face  21   a  among the light-converging points P 1  to P 5 , and the reformed region R 1  is formed just below the light incident face  21   a . Similarly, the reformed region P 2  is formed around the light-converging point P 2  by the laser light L 2 , and the reformed region R 3  is formed around the light-converging point P 3  by the laser light L 3 . That is, the reformed regions R 1  to R 3  are formed by the beams of laser light L 1  to L 3 , incident on the wafer  21 , being directly converged at the light-converging points P 1  to P 3 .  
      Without the aluminum sheet  25 , the laser light L 4  would be converged at a light-converging point Pm within the bonding layer  52 . With the aluminum sheet  25  provided, however, it is reflected by the sheet  25  before it is converged there. It is converged at the light-converging point P 4  set between the light-converging point P 3  and the rear face  21   b , and the reformed region R 4  is formed around it. Similarly, without the aluminum sheet  25 , the laser light L 5  would be converged at a light-converging point Pn within the sheet  41 . With the aluminum sheet  25  provided, it is reflected by the sheet before it is converged there. It is converged at the light-converging point P 5  set between the light-converging point P 2  and the light-converging point P 3 , and the reformed region R 5  is formed around it.  
      When reformed regions R are formed in the increasing order of distance from the light incident face  21   a  when multiple layers of reformed region R are introduced in the direction of the thickness of the wafer  21 , the laser light L passes through the reformed region R already having been formed before and it is scattered. Thus, the light-converging point P becomes less prone to be formed. As a result, the reformed region R having sufficient dimensions may not be formed. To cope with this, it is desirable to form the reformed regions R in the decreasing order of distance from the light incident face  21   a.    
      Therefore, it is desirable that the reformed regions R should be formed in the order of R 4 , R 3 , R 5 , R 2  to R 1 . The distance M ( FIG. 12 ) between the laser head  31  and the light incident face  21   a  is so controlled that the beams of laser light L are projected in the order of L 4 , L 3 , L 5 , L 2  to L 1 .  
      The seventh embodiment provides the following advantages.  
      (1) In the reformed region formation process, laser light L applied to the inside of wafer  21  is reflected by the aluminum sheet  25  formed over its rear face  21   b . The light-converging point P of the reflected laser light L is positioned within the wafer  21 , and the reformed region R is thereby formed. Therefore, the reformed region R can be efficiently formed by making good use of the energy of the reflected laser light L.  
      (2) The laser light L reflected by the aluminum sheet  25  is converged to form reformed regions R 4  and K 5  in the vicinity of the rear face  21   b . Thus, a sufficient number or amount of reformed regions R to become starting points for division can be formed in the vicinity of the rear face  21   b . The reformed region R formed in the vicinity of the rear face  21   b  becomes a starting point for the development of cracks on a planned dividing line DL when the wafer  21  is divided by expanding the sheet  41 . Therefore, the wafer  21  can be divided by smaller force by forming a sufficient number or amount of reformed regions R. Consequently, the wafer  21  can be divided easily, and portions left undivided can be eliminated.  
     Eighth Embodiment  
      In the eighth embodiment shown in  FIG. 15 , the aluminum sheet  25  is formed over the rear face  21   b  only along the planned dividing lines DL. The aluminum sheet  25  is formed on the planned dividing lines DL 3  to DL 5  in shape of strip having a width of, for example, approximately 1/10 of the length of a side of each semiconductor chip  22 . When the entire rear face  21   b  of the wafer  21  is viewed, the aluminum sheet  25  is formed on the planned dividing lines DL 1  to DL 14  in a grid pattern. Even in cases where this construction is adopted, the aluminum sheet  25  is capable of reflecting laser light L applied to the planned dividing lines DL; therefore, the same advantage as in the first and second embodiments can be provided. Further, most of regions in the rear face  21   b  other than the regions where the aluminum sheet  25  is formed are directly bonded to the bonding layer  52 , and the wafer  21  can be firmly bonded to the sheet  41 . In the dividing process, therefore, stress can be applied to the wafer  21  with reliability, and the substrate can be divided without fail.  
      Instead, metal foil, such as aluminum foil, may be placed in a lattice pattern on the bonding layer  52  in the positions corresponding to the planned dividing lines DL.  
      In the sixth to eighth embodiments, the wafer  21  may be an oxide sheet composed of oxide silicon formed over its light incident face  21   a  and a wafer of SOI (Silicon On Insulator) and the like.