Patent Publication Number: US-10790192-B2

Title: Wafer processing method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method for processing a wafer in which patterns including a metal layer are formed on streets. 
     Description of the Related Art 
     A wafer formed with devices such as integrated circuits (ICs) on the front surface thereof is divided along streets (division lines), whereby a plurality of device chips each including the device are obtained. The division of the wafer is conducted, for example, by using a cutting apparatus in which an annular cutting blade for cutting the wafer is mounted. The cutting blade is rotated and is made to cut into the wafer along the streets, whereby the wafer is cut and divided. 
     The device chips obtained by the division of the wafer are incorporated into various electronic apparatuses; in recent years, attendant on reductions in the size and thickness of electronic apparatuses, reductions in the size and thickness of the device chips have also been demanded. In view of this, a technique of grinding the back surface of the wafer by a grindstone to thin the device chips has been used. When the back surface of the wafer is ground by the grindstone, minute ruggedness (minute projections and recesses) and cracks are formed in the ground region. Since the presence of the region formed with the ruggedness or cracks (crushed layer) would lower the die strength of the wafer, the crushed layer is removed by polishing with a polishing pad or by dry etching or the like after the grinding. 
     On the other hand, it is known that when the crushed layer is left on the wafer, a gettering effect is obtained by which metallic elements such as copper contained in the inside of the wafer are captured into the crushed layer. Therefore, when the crushed layer is formed on the back surface side of the wafer, the metallic elements are captured to the back surface side of the wafer, whereby the metallic elements can be prevented from moving into the devices formed on the front surface side of the wafer. As a result, generation of defects (current leakage or the like) in the devices due to the metallic elements inside the wafer is restrained. When the crushed layer is removed for enhancing the die strength of the wafer, however, the gettering effect also is lost. In view of this, there has been proposed a method in which minute ruggedness or cracks (strain) finer than those of the crushed layer are formed on the back surface of the wafer, after removal of the crushed layer, and the metallic elements are captured in the region formed with this strain (strain layer). By this, a gettering effect on the metallic elements can be obtained without largely lowering the die strength of the wafer. Japanese Patent Laid-open No. 2010-177430 discloses a technique in which after a crushed layer formed by grinding the back surface of a wafer is removed by a plasma etching treatment, a plasmatized inert gas is applied to the back surface of the wafer to form a strain layer (gettering layer). 
     SUMMARY OF THE INVENTION 
     Patterns including a metal layer that are not constituent elements of devices, such as a test element group (TEG) for device evaluation and columnar metallic patterns (pillars) for supporting on the streets an insulating film or the like formed at the time of manufacturing the devices, are often formed on the streets partitioning the devices formed on a wafer. Since the patterns (dummy patterns) do not relate to operations of the device chips obtained by dividing the wafer, they are cut and removed together with the wafer at the time of cutting the wafer by a cutting blade. However, when the dummy patterns including the metal layer are cut by the cutting blade, the cutting blade and the metal layer contact each other, and protrusions (burs) of the metal are generated. When the plasma processing for the formation of the aforementioned strain layer or the like is applied to the wafer formed with the burs, electric discharge may be generated in the region formed with the burs, possibly leading to breakage of the devices. 
     The present invention has been made in consideration of the above-mentioned problems. It is therefore an object of the present invention to provide a wafer processing method by which generation of electric discharge at the time of plasma processing of a wafer can be restrained and breakage of devices can be prevented. 
     In accordance with an aspect of the present invention, there is provided a method for processing a wafer in which devices are formed respectively in regions on a front surface side partitioned by a plurality of streets and in which patterns including a metal layer are formed on the streets, the method including: a laser processing step of applying a laser beam of such a wavelength as to be absorbed in the wafer along the streets formed with the patterns, to form laser processed grooves while removing the patterns; a cut groove forming step of forming cut grooves having a depth in excess of a finished thickness of the wafer, inside the laser processed grooves, by a cutting blade thinner than a width of the laser processed grooves; a protective member adhering step of adhering a protective member to the front surface side of the wafer formed with the cut grooves; a grinding step of holding the wafer by a chuck table through the protective member, grinding a back surface side of the wafer to thin the wafer to the finished thickness, and to expose the cut grooves to the back surface of the wafer, thereby dividing the wafer into a plurality of device chips; a crushed layer removing step of removing a crushed layer formed on the back surface side of the wafer by the grinding of the wafer; and a strain layer forming step of forming a strain layer on the back surface side of the wafer deprived of the crushed layer, by plasma processing using an inert gas. 
     Note that, preferably, in the crushed layer removing step, the crushed layer is removed by polishing with a polishing pad. Besides, preferably, in the crushed layer removing step, the crushed layer is removed by plasma etching using a halogen-containing gas. In addition, preferably, the method for processing the wafer further includes a protective film forming step of forming a water-soluble protective film on the front surface side of the wafer before the laser processing step, and a protective film removing step of removing the protective film from the front surface side of the wafer after the laser processing step. 
     In the method for processing the wafer according to the described aspect of the present invention, after the dummy patterns including a metal layer that are formed on the streets of the wafer are removed by application of the laser beam, the wafer is cut along the streets. By this, cutting of the dummy patterns by the cutting blade is avoided, and generation of metal burs at the time of cutting the wafer is restrained. Therefore, at the time of applying plasma processing to the wafer to form the strain layer on the back surface of the wafer, generation of electric discharge is restrained, and damaging of the devices is prevented. 
     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 a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view depicting a configuration example of a wafer; 
         FIG. 2  is a perspective view of the wafer in the state of being supported by an annular frame; 
         FIG. 3  is a partly sectional side view depicting the manner in which a protective film is formed on the wafer; 
         FIG. 4A  is a partly sectional side view depicting the manner in which a laser beam is applied to the wafer; 
         FIG. 4B  is an enlarged plan view of a street formed with patterns including a metal layer; 
         FIG. 4C  is an enlarged plan view of the street from which the patterns including the metal layer have been removed and in which a laser processed groove has been formed; 
         FIG. 5  is a partly sectional side view depicting the manner in which the protective film formed on the wafer is removed; 
         FIG. 6A  is a partly sectional side view depicting the manner in which a cut groove is formed in the wafer; 
         FIG. 6B  is an enlarged plan view of the street formed with the cut groove; 
         FIG. 7A  is a perspective view depicting the manner in which a protective member is adhered to the wafer; 
         FIG. 7B  is a perspective view of the wafer to which the protective member has been adhered; 
         FIG. 8  is a side view depicting the manner in which the back surface side of the wafer is ground; 
         FIG. 9  is a side view depicting the manner in which the back surface side of the wafer is polished; 
         FIG. 10  is a sectional schematic view depicting a configuration example of a plasma treatment apparatus; and 
         FIG. 11  is an enlarged sectional view of the wafer in the state of having been formed with a strain layer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will be described below, referring to the attached drawings. In the first place, an example of a wafer to be processed by a method for processing a wafer according to the present embodiment will be described.  FIG. 1  is a plan view depicting a configuration example of a wafer  11 . 
     The wafer  11  is formed in a disk shape from such a material as silicon, and is partitioned into a plurality of regions by a plurality of streets (division lines)  13  arranged in a grid pattern. In addition, a device  15  including an IC or the like is formed in each of the regions partitioned by the streets  13  on the front surface side of the wafer  11 . Note that while the disk-shaped wafer  11  formed from such a material as silicon is used in the present embodiment, the material, shape, structure, size, and the like of the wafer  11  are not limited. For example, a wafer  11  formed from such a material as other semiconductor than silicon, a ceramic, a resin, or a metal may also be used. Similarly, the kind, number, shape, structure, size, layout, and the like of the devices  15  are not limited. 
     In addition, at least part of the streets  13  are formed with patterns  17  including a metal layer. The patterns  17  are patterns (dummy patterns) including a metal layer not being a constituent element of the devices  15 , and are composed, for example, of TEG for device evaluation, or columnar metal patterns (pillars) for supporting an insulating film formed at the time of device production or the like on the streets  13 . Note that an example in which the patterns  17  are disposed on part of the streets  13  is depicted in  FIG. 1 , the patterns  17  may be disposed on all the streets  13 . 
     By dividing the wafer  11  along the streets  13 , a plurality of device chips respectively including the devices  15  are obtained. In the present embodiment, description will be made of an example in which grooves having a depth of less than the thickness of the wafer  11  are first formed (half cut) on the front surface side of the wafer  11 , and thereafter the back surface side of the wafer  11  is ground by a grindstone, to thereby divide the wafer  11  into a plurality of device chips. 
     At the time of forming the grooves on the front surface side of the wafer  11 , for example, a cutting apparatus equipped with an annular cutting blade can be used. In the case of using the cutting apparatus, the cutting blade is rotated and made to cut into the wafer  11 , thereby to form the grooves in the wafer  11 . It is to be noted, however, that since at least part of the streets  13  are formed with the patterns  17  including the metal layer, formation of the grooves along the streets  13  by this method results in that the patterns  17  are cut by the cutting blade. The patterns  17  include the metal layer, so that when the metal layer is cut by the cutting blade, the metal layer is extended by the cutting blade, to generate metal protrusions (burs). Where the burs are left on the wafer  11 , when plasma processing is conducted in a later step (for example, a strain layer forming step which will be described later), electric discharge may be generated in the regions where the burs are formed, possibly leading to breakage of the device  15 . 
     In view of this, in the wafer processing method according to the present embodiment, prior to the cutting of the wafer  11  by the cutting blade, a laser beam is applied to the wafer  11  to remove the patterns  17 . By this, cutting of the patterns  17  by the cutting blade is avoided, whereby the generation of burs can be prevented, and the generation of electric discharge at the time of plasma processing can be restrained. The wafer processing method according to the present embodiment will be specifically described below. 
     First, in order to hold the wafer  11  on each of various processing apparatuses such as a cutting apparatus and a laser processing apparatus, the wafer  11  is supported by an annular frame.  FIG. 2  is a perspective view of the wafer  11  in the state of being supported by the annular frame  21 . Note that the patterns  17  are omitted from illustration in  FIG. 2 . The annular frame  21  is adhered along an outer periphery of a disk-shaped adhesive tape  19  formed from a resin or the like, and a back surface  11   b  side of the wafer  11  is adhered to the adhesive tape  19 . As a result, the wafer  11  is supported on the annular frame  21  in the state in which its front surface  11   a  is exposed to the upper side. 
     Next, a water-soluble protective film is formed on the front surface  11   a  side of the wafer  11  (protective film forming step).  FIG. 3  is a partly sectional side view depicting the manner in which the water-soluble protective film  23  is formed on the front surface  11   a  side of the wafer  11 . The protective film  23  can be formed by use of a spin coater  2 , for example. The spin coater  2  includes a spinner table  4  that supports the wafer  11 , a plurality of clamps  6  that fix the annular frame  21  supporting the wafer  11 , and a nozzle  8   a  for jetting a material for the protective film  23 , which includes a water-soluble resin or the like, toward the wafer  11 . 
     First, the wafer  11  is disposed on the spinner table  4 , and the annular frame  21  is fixed by the clamps  6 . Part of an upper surface of the spinner table  4  constitutes a holding surface  4   a  for suction holding the wafer  11  through the adhesive tape  19 . The holding surface  4   a  is connected to a suction source (not illustrated) through a suction passage (not illustrated) or the like formed inside the spinner table  4 . A negative pressure of the suction source is made to act on the holding surface  4   a , whereby the wafer  11  is suction held by the spinner table  4 . Note that a chuck table for holding the wafer  11  by a mechanical method, an electrical method, or the like may be used in place of the spinner table  4 . 
     Then, while the spinner table  4  suction holding the wafer  11  by the holding surface  4   a  is being rotated around a rotational axis substantially parallel to the vertical direction, a water-soluble resin such as polyvinyl alcohol (PVA) or polyethylene glycol (PEG) is jetted from the nozzle  8   a  disposed on the upper side of the spinner table  4 . As a result, the water-soluble resin adhering to the wafer  11  flows to an outer peripheral portion of the wafer  11  under a centrifugal force, whereby the water-soluble protective film  23  is formed on the front surface  11   a  side of the wafer  11 . The protective film  23  is formed for preventing fine particles (debris) scattered from a processing region, at the time of application of a laser beam to the front surface  11   a  of the wafer  11  in a later step, from adhering to the front surface  11   a  of the wafer  11 . It is to be noted, however, that the protective film forming step may not necessarily be carried out, and the protective film forming step may be omitted, for example, in the case where the application of the laser beam is conducted under such processing conditions that debris is hardly generated, or in the case where adhesion of debris is not liable to become a problem. 
     Subsequently, a laser beam of such a wavelength as to be absorbed in the wafer  11  is applied along the streets  13  formed with the patterns  17 , to form laser processed grooves while removing the patterns  17  (laser processing step). The application of the laser beam to the wafer  11  is performed using a laser processing apparatus.  FIG. 4A  is a partly sectional side view depicting the manner in which the laser beam is applied to the wafer  11 . 
     The laser processing apparatus  10  includes a chuck table  12  that suction holds the wafer  11 , a laser applying unit  14  that applies the laser beam of such a wavelength as to be absorbed in the wafer  11 , and a plurality of clamps  16  that fix the annular frame  21  supporting the wafer  11 . 
     First, the wafer  11  is disposed on the chuck table  12 , and the annular frame  21  is fixed by the clamps  16 . Part of an upper surface of the chuck table  12  constitutes a holding surface  12   a  that suction holds the wafer  11  through the adhesive tape  19 . The holding surface  12   a  is connected to a suction source (not illustrated) through a suction passage (not illustrated) or the like formed inside the chuck table  12 . Note that a chuck table for holding the wafer  11  by a mechanical method, an electrical method, or the like may be used in place of the chuck table  12 . In a state in which the wafer  11  is disposed on the holding surface  12   a  of the chuck table  12  through the adhesive tape  19 , a negative pressure of the suction source is made to act on the holding surface  12   a , whereby the wafer  11  is suction held by the chuck table  12 . Then, the chuck table  12  with the wafer  11  held thereon is moved to a position beneath the laser applying unit  14 . 
     Next, the laser beam is applied from the laser applying unit  14  to the wafer  11 . The laser applying unit  14  has a function of applying a pulsed laser beam of such a wavelength as to be absorbed in the wafer  11 . Note that in the case where the protective film  23  is formed on the front surface  11   a  side of the wafer  11 , the laser beam is applied to the wafer  11  through the protective film  23 . While applying the laser beam from the laser applying unit  14  to the wafer  11 , the chuck table  12  is moved along the longitudinal direction of the street  13  such that the laser beam is applied along the street  13 . By this, the wafer  11  is subjected to ablation processing, and a rectilinear laser processed groove  11   c  is formed along the street  13  on the front surface  11   a  side of the wafer  11 . 
     In addition, when the laser beam is applied to the street  13  formed with the pattern  17 , the patterns  17  are removed by the application of the laser beam.  FIG. 4B  is an enlarged plan view of the street  13  formed with the patterns  17 .  FIG. 4B  depicts an example in which TEG  25  for device evaluation and pillars  27  as columnar metal patterns for supporting on the street an insulating film or the like formed at the time of production of devices  15  are formed as the patterns  17 . Note that in  FIG. 4B , the regions where the TEG  25  and the pillars  27  are formed are hatched. 
     When the laser beam is applied along the street  13 , the laser beam is applied also to the TEG  25  and the pillars  27 , whereby the TEG  25  and the pillars  27  are removed.  FIG. 4C  is an enlarged plan view of the street  13  from which the TEG  25  and the pillar  27  have been removed and in which the laser processed groove  11   c  has been formed. In  FIG. 4C , the region where the laser processed groove  11   c  is formed is hatched. Note that the patterns  17  are used mainly in the production process of the devices  15 , and do not relate to operations of the device chips obtained by dividing the wafer  11 , so that removal of the patterns  17  does not affect the function of the device chips. Laser beam application conditions (the power, spot diameter, and repetition frequency of the laser beam, etc.) are set in such a manner that the laser processed grooves  11   c  can be formed in the wafer  11  and the patterns  17  can be removed. 
     Note that in the laser processing step, the laser beam may be applied along the same street  13  multiple times. In this case, where the application position of the laser beam is changed stepwise in the width direction of the street  13 , the width of the laser processed groove  11   c  formed along the street  13  can be thereby controlled. In addition, it is unnecessary to completely remove the patterns  17  by the application of the laser beam, and it is sufficient to apply the laser beam in such a manner that at least the width of the laser processed groove  11   c  will be greater than the width of the cutting blade to be used for cutting the wafer  11  in a later step.  FIG. 4C  denotes an example in which part of the pillars  27  are left on the street  13 . Besides, the application of the laser beam in the laser processing step may be performed for all the streets  13 , or may be conducted for only the streets  13  which are formed with the patterns  17 . In the case where the application of the laser beam is carried out for only the streets  13  formed with the patterns  17 , the streets  13  formed with the patterns  17  are preliminarily grasped. 
     Subsequently, the protective film  23  is removed from the front surface  11   a  side of the wafer  11  (protective film removing step).  FIG. 5  is a partly sectional side view depicting the manner in which the water-soluble protective film  23  formed on the front surface  11   a  side of the wafer  11  is removed. 
     First, like in the protective film forming step, the wafer  11  is suction held by the spinner table  4 . Then, while the spinner table  4  with the wafer  11  suction held by the holding surface  4   a  thereof is being rotated around a rotational axis substantially parallel to the vertical direction, pure water is jetted from a nozzle  8   b  located on the upper side of the spinner table  4 . By this, the water-soluble protective film  23  formed on the front surface  11   a  side of the wafer  11  is removed together with debris deposited on the wafer  11 . Thus, since the protective film  23  is formed using a water-soluble resin (protective film forming step), it can be washed away with pure water. Therefore, the protective film  23  can be removed extremely easily. Note that the removal of the protective film  23  may be conducted by jetting to the wafer  11   a  mixed fluid of pure water with a gas (air, etc.). Note that in the case where the protective film forming step is not performed and no protective film is formed on the front surface  11   a  of the wafer  11 , it is unnecessary to perform the protective film removing step. 
     Next, using a cutting blade thinner than the width of the laser processed grooves  11   c  formed in the laser processing step, cut grooves having a depth in excess of a finished thickness of the wafer  11  are formed inside the laser processed grooves  11   c  (cut groove forming step).  FIG. 6A  is a partly sectional side view depicting the manner in which the cut groove  11   d  is formed in the wafer  11 . 
     In the cut groove forming step, the cut grooves are formed by use of a cutting apparatus  18 . As illustrated in  FIG. 6A , the cutting apparatus  18  includes a chuck table  20  that suction holds the wafer  11 , a cutting unit  22  that cut the workpiece held by the chuck table  20 , and a plurality of clamps  24  that fix the annular frame  21  supporting the wafer  11 . 
     First, the wafer  11  is disposed on the chuck table  20 , and the annular frame  21  is fixed by the clamps  24 . Part of an upper surface of the chuck table  20  constitutes a holding surface  20   a  for suction holding the wafer  11  through the adhesive tape  19 . The holding surface  20   a  is connected to a suction source (not illustrated) through a suction passage (not illustrated) or the like formed inside the chuck table  20 . A negative pressure of the suction source is made to act on the holding surface  20   a , whereby the wafer  11  is suction held by the chuck table  20 . Note that a chuck table for holding the wafer  11  by a mechanical method, an electrical method, or the like may be used in place of the chuck table  20 . 
     The cutting unit  22  for cutting the wafer  11  is disposed on the upper side of the chuck table  20 . The cutting unit  22  includes a spindle  26  having an axis in a direction substantially parallel to the holding surface  20   a , and an annular cutting blade  28  is mounted to a tip portion of the spindle  26 . The cutting blade  28  is composed, for example, of an electroformed grindstone in which diamond abrasive grains are bound by nickel plating. In addition, the spindle  26  is connected to a rotational drive source (not illustrated) such as a motor, and the cutting blade  28  mounted to the spindle  26  is rotated by a force transmitted from the rotational drive source. 
     In the cut groove forming step, first, the wafer  11  is disposed on the holding surface  20   a  of the chuck table  20  through the adhesive tape  19 , and a negative pressure of the suction source is made to act. As a result, the wafer  11  is held by the chuck table  20  in a state in which the front surface  11   a  side thereof is exposed to the upper side. 
     Next, the spindle  26  is rotated, the cutting blade  28  is made to cut into a bottom portion of the laser processed groove  11   c  from the front surface  11   a  side of the wafer  11  along the street  13 , and the chuck table  20  is moved in a processing feeding direction, namely, a direction which is substantially parallel to the holding surface  20   a  and is substantially perpendicular to the axis of the spindle  26 . As a result, a rectilinear cut groove  11   d  is formed in the bottom portion of the laser processed groove  11   c  along the street  13 . At the time of making the cutting blade  28  cut into the wafer  11 , the distance from the front surface  11   a  of the wafer  11  to a lower end of the cutting blade  28  is set to a value which is less than the thickness of the wafer  11  and is in excess of the finished thickness of the wafer  11 . The finished thickness of the wafer  11  corresponds to the thickness of device chips when the wafer  11  is processed finally into the device chips. 
     Note that the width of the cutting blade  28  is smaller than the width of the laser processed grooves  11   c  formed in the laser processed groove forming step. In the cut groove forming step, the cutting blade  28  is made to cut into part of the bottom portion of the laser processed groove  11   c . Therefore, the cut groove  11   d  is formed inside the laser processed groove  11   c .  FIG. 6B  is an enlarged plan view of the street  13  formed with the cut groove  11   d . The cut groove  11   d  has a width narrower than the width of the laser processed groove  11   c , and is formed in a region inside the laser processed groove  11   c  in plan view. Note that in  FIG. 68 , the region formed with the cut groove  11   d  is hatched. 
     The bottom portion of the laser processed groove  13   c  formed in the laser processing step is a region where the patterns  17  have been removed and no metal layer is left. In the cut groove forming step, the bottom portion of the laser processed groove  13   c  is cut by the cutting blade  28 . Therefore, cutting of the patterns  17  by the cutting blade  28  does not occur, so that generation of burs due to cutting of the metal layer is prevented. 
     Subsequently, a protective member is adhered to the front surface  11   a  side of the wafer  11  (protective member adhering step).  FIG. 7A  is a perspective view depicting the manner in which the protective member  29  is adhered to the wafer  11 . As illustrated in  FIG. 7A , the protective member  29 , disk-like in shape, is adhered to the front surface  11   a  side of the wafer  11  such as to cover the devices  15 . As the protective member  29 , there can be used, for example, a tape formed from a flexible resin or the like.  FIG. 7B  is a perspective view depicting the wafer  11  to which the protective member  29  has been adhered. By the protective member  29 , the devices  15  formed on the front surface  11   a  side of the wafer  11  are protected in later steps (a grinding step, a crushed layer removing step, and a strain layer forming step). Note that  FIG. 7B  denotes a state in which the wafer  11  has been peeled from the adhesive tape  19  and been detached from the annular frame  21 . 
     Next, the back surface  11   b  side of the wafer  11  is ground to thin the wafer  11  to the finished thickness and to expose the cut grooves  11   d  to the back surface  11   b , thereby dividing the wafer  11  into a plurality of device chips (grinding step).  FIG. 8  is a side view depicting the manner in which the back surface  11   b  side of the wafer  11  is ground. 
     The grinding of the wafer  11  is conducted, for example, by using a grinding apparatus  30  depicted in  FIG. 8 . The grinding apparatus  30  includes a chuck table  32  for suction holding the wafer  11 . The chuck table  32  is connected to a rotational drive source (not illustrated) such as a motor, and is rotated around a rotational axis substantially parallel to the vertical direction. In addition, a moving mechanism (not illustrated) is provided under the chuck table  32 , and the moving mechanism has a function of moving the chuck table  32  in a horizontal direction. 
     An upper surface of the chuck table  32  constitutes a holding surface  32   a  for suction holding the wafer  11 . The holding surface  32   a  is connected to a suction source (not illustrated) through a suction passage (not illustrated) or the like formed inside the chuck table  32 . In the state in which the wafer  11  is disposed on the holding surface  32   a  through the protective member  29 , a negative pressure of the suction source is made to act on the holding surface  32   a , whereby the wafer  11  is suction held by the chuck table  32 . Note that a chuck table for holding the wafer  11  by a mechanical method, an electrical method, or the like may be used in place of the chuck table  32 . 
     A grinding unit  34  is disposed on the upper side of the chuck table  32 . The grinding unit  34  includes a spindle housing (not illustrated) supported by a lift mechanism (not illustrated). A spindle  36  is accommodated in the spindle housing, and a disk-shaped mount  38  is fixed to a lower end portion of the spindle  36 . A grinding wheel  40  substantially equal in diameter to the mount  38  is mounted to a lower surface of the mount  38 . The grinding wheel  40  includes a wheel base  42  formed from a metallic material such as stainless steel or aluminum. A plurality of grindstones  44  are arranged on a lower surface of the wheel base  42 . A rotational drive source (not illustrated) such as a motor is connected to the upper end side (base end side) of the spindle  36 , and the grinding wheel  40  is rotated around a rotational axis substantially parallel to the vertical direction by a force generated from the rotational drive source. In the inside or the vicinity of the grinding unit  34 , there is provided a nozzle (not illustrated) for supplying a grinding liquid such as pure water to the wafer  11  and the like. 
     At the time of grinding the wafer  11 , first, in a state in which the wafer  11  is disposed on the holding surface  32   a  through the protective member  29 , a negative pressure of the suction source is made to act on the holding surface  32   a . As a result, the wafer  11  is suction held by the chuck table  32  in the state in which the back surface  11   b  side thereof is exposed to the upper side. 
     Subsequently, the chuck table  32  is moved to a position beneath the grinding unit  34 . Then, while rotating the chuck table  32  and the grinding wheel  40  respectively and while supplying the grinding liquid to the back surface  11   b  side of the wafer  11 , the spindle  36  is lowered. Note that the position and the lowering speed of the spindle  36  are adjusted in such a manner that lower surfaces of the grindstones  44  are pressed against the back surface  11   b  side of the wafer  11  with an appropriate force. By this, the back surface  11   b  side is ground, and the wafer  11  is thinned. When the wafer  11  is thinned and the cut grooves  11   d  are exposed to the back surface  11   b  of the wafer  11 , the wafer  11  is divided into the plurality of device chips each of which includes the device  15 . Then, when the wafer  11  is thinned to the finished thickness, the grinding is completed. Note that the wafer  11  is ground by use of one set of grinding unit in the present embodiment, the wafer  11  may be ground by use of two or more sets of grinding units. In that case, for example, rough grinding is conducted using grindstones containing abrasive grains larger in diameter, and finish grinding is performed using grindstones containing abrasive grains smaller in diameter, whereby flatness of the back surface  11   b  can be enhanced without largely prolonging the time required for grinding. 
     When the back surface  11   b  side of the wafer  11  is ground by the grindstones, minute ruggedness (minute projections and recesses) or cracks are formed on the back surface  11   b  side of the wafer  11 . The presence of a region (crushed layer) where the ruggedness or cracks are formed leads to a lowering in the die strength of the device chips obtained by dividing the wafer  11 . Therefore, the crushed layer formed on the back surface  11   b  side of the wafer  11  is removed (crushed layer removing step). The crushed layer can be removed, for example, by polishing conducted using a polishing pad.  FIG. 9  is a side view depicting the manner in which the back surface  11   b  side of the wafer  11  is polished by a polishing pad. 
     The polishing of the wafer  11  is performed, for example, by use of a polishing apparatus  46  depicted in  FIG. 9 . The polishing apparatus  46  includes a chuck table  48  for suction holding the wafer  11 . The chuck table  48  is connected to a rotational drive source (not illustrated) such as a motor, and is rotated around a rotational axis which is substantially parallel to the vertical direction. In addition, a moving mechanism (not illustrated) is provided under the chuck table  48 , and the moving mechanism has a function of moving the chuck table  48  in a horizontal direction. An upper surface of the chuck table  48  constitutes a holding surface  48   a  for suction holding the wafer  11 . The holding surface  48   a  is connected to a suction source (not illustrated) through a suction passage (not illustrated) or the like formed inside the chuck table  48 . In the state in which the wafer  11  is disposed on the holding surface  48   a  through the protective member  29 , a negative pressure of the suction source is made to act on the holding surface  48   a , whereby the wafer  11  is suction held by the chuck table  48 . Note that a chuck table for holding the wafer  11  by a mechanical method, an electrical method, or the like may be used in place of the chuck table  48 . 
     A polishing unit  50  is disposed on the upper side of the chuck table  48 . The polishing unit  50  includes a spindle housing (not illustrated) supported by a lift mechanism (not illustrated). A spindle  52  is accommodated in the spindle housing, and a disk-shaped mount  54  is fixed to a lower end portion of the spindle  52 . A polishing pad  56  is mounted to a lower surface of the mount  54 . The polishing pad  56  includes a polishing fabric formed from nonwoven fabric, polyurethane foam, or the like. A rotational drive source (not illustrated) including a motor or the like is connected to the upper end side (base end side) of the spindle  52 , and the polishing pad  56  is rotated around a rotational axis substantially parallel to the vertical direction by a force generated by the rotational drive source. Note that the polishing unit  50  may be formed therein with a supply passage (not illustrated) for supplying a polishing liquid to the wafer  11  held by the chuck table  48 . For example, a slurry containing abrasive grains dispersed in a liquid reactive to the wafer  11  can be supplied as the polishing liquid to the wafer  11  through the supply passage. 
     In the crushed layer removing step, first, the protective member  29  is put in contact with the holding surface  48   a  of the chuck table  48 , and the negative pressure of the suction source is made to act there. As a result, the wafer  11  is suction held by the chuck table  48  in the state in which the back surface  11   b  side thereof is exposed to the upper side. Note that the wafer  11  has been divided into the plurality of device chips  31  by the grinding step. Next, the chuck table  48  is moved to a position beneath the polishing unit  50 . Then, while supplying the polishing liquid to the back surface  11   b  side of the wafer  11 , the chuck table  48  and the polishing pad  56  are rotated respectively, and the spindle  52  is lowered. Note that the lowering amount of the spindle  52  is adjusted to such an extent that a lower surface (polishing surface) of the polishing pad  56  is pressed against the back surface  11   b  side of the wafer  11 . In this way, the back surface  11   b  of the wafer  11  is polished, whereby the crushed layer formed on the back surface  11   b  side of the wafer  11  by the grinding in the grinding step is removed. By the removal of the crushed layer, the die strength of the device chips  31  can be enhanced. 
     Note that while wet polishing in which the polishing liquid is supplied to the wafer  11  has been descried above, dry polishing in which the polishing liquid is not used may also be used for polishing the wafer  11 . In addition, the method for removing the crushed layer is not limited to the polishing by the polishing pad  56 . For example, plasma etching using a halogen gas may be used to remove the crushed layer. The details of the plasma etching will be described later. 
     Next, using plasma etching using an inert gas, strain including minute ruggedness (minute projections and recesses) or cracks is formed on the back surface  11   b  side of the wafer  11  (strain layer forming step). For the strain formation, a plasma treatment apparatus can be used.  FIG. 10  is a sectional schematic view denoting a configuration example of a plasma treatment apparatus  60  which can be used for forming the strain. 
     The plasma treatment apparatus  60  includes a vacuum chamber  64  forming a treatment space  62 . The vacuum chamber  64  is formed in a rectangular parallelepiped shape including a bottom wall  64   a , an upper wall  64   b , a first side wall  64   c , a second side wall  64   d , a third side wall  64   e , and a fourth side wall (not illustrated), with the second side wall  64   d  being provided with an opening  66  for carrying the wafer  11  in and out. On the outside of the opening  66  is provided a gate  68  for closing and opening the opening  66 . The gate  68  is moved upward and downward by an opening/closing mechanism  70 . The opening/closing mechanism  70  includes an air cylinder  72  and a piston rod  74 . The air cylinder  72  is fixed to the bottom wall  64   a  of the vacuum chamber  64  through a bracket  76 , and a tip of the piston rod  74  is connected to a lower portion of the gate  68 . With the gate  68  opened by the opening/closing mechanism  70 , the wafer  11  can be carried into the treatment space  62  of the vacuum chamber  64  through the opening  66 , or the wafer  11  can be carried out of the treatment space  62  of the vacuum chamber  64 . An exhaust port  78  is formed in the bottom wall  64   a  of the vacuum chamber  64 . The exhaust port  78  is connected to an exhaust mechanism  80  such as a vacuum pump. 
     A lower electrode  82  and an upper electrode  84  are disposed opposite to each other, in the treatment space  62  of the vacuum chamber  64 . The lower electrode  82  is formed from a conductive material, and includes a disk-shaped holding section  86  and a cylindrical support section  88  projecting downward from the center of a lower surface of the holding section  86 . The support section  88  is inserted in an opening  90  formed in the bottom wall  64   a  of the vacuum chamber  64 . In the opening  90 , an annular insulating member  92  is disposed between the bottom wall  64   a  and the support section  88 , whereby the vacuum chamber  64  and the lower electrode  82  are insulated from each other. The lower electrode  82  is connected to a high-frequency power supply  94  in the exterior of the vacuum chamber  64 . 
     An upper surface of the holding section  86  is formed with a recess, and a table  96  on which to place the wafer  11  is provided in the recess. The table  96  is formed therein with a suction passage (not illustrated), which is connected to a suction source  100  through a flow path  98  formed inside the lower electrode  82 . In addition, the holding section  86  is formed therein with a cooling flow path  102 . One end of the cooling flow path  102  is connected to a coolant circulation mechanism  106  through a coolant introduction passage  104  formed in the support section  88 , whereas the other end of the cooling flow path  102  is connected to a coolant circulation mechanism  106  through a coolant discharge passage  108  formed in the support section  88 . When the coolant circulation mechanism  106  is operated, a coolant flows through the coolant introduction passage  104 , the coolant flow path  102 , and the coolant discharge passage  108  in this order, to cool the lower electrode  82 . 
     The upper electrode  84  is formed from a conductive material, and includes a disk-shaped gas jetting section  110  and a cylindrical support section  112  projecting upward from the center of an upper surface of the gas jetting section  110 . The support section  112  is inserted in an opening  114  formed in the upper wall  64   b  of the vacuum chamber  64 . In the opening  114 , an annular insulating member  116  is disposed between the upper wall  64   b  and the support section  112 , whereby the vacuum chamber  64  and the upper electrode  84  are insulated from each other. The upper electrode  84  is connected to a high-frequency power supply  118  in the exterior of the vacuum chamber  64 . In addition, a support arm  122  connected to a lift mechanism  120  is attached to an upper end portion of the support section  112 , and the upper electrode  84  is moved upward and downward by the lift mechanism  120  and the support arm  122 . A lower surface of the gas jetting section  110  is provided with a plurality of jet ports  124 . The jet ports  124  are connected to a first gas supply source  130  and a second gas supply source  132  through a flow path  126  formed in the gas jetting section  110  and a flow path  128  formed in the support section  112 . The first gas supply source  130 , the second gas supply source  132 , the flow paths  126  and  128 , and the jet ports  124  constitute a gas introduction section for introducing gases into the vacuum chamber  64 . 
     The opening/closing mechanism  70 , the exhaust mechanism  80 , the high-frequency power supply  94 , the suction source  100 , the coolant circulation mechanism  106 , the high-frequency power supply  118 , the lift mechanism  120 , the first gas supply source  130 , the second gas supply source  132 , and the like are connected to a control device  134 . Information on the pressure inside the treatment space  62  is inputted from the exhaust mechanism  80  to the control device  134 . In addition, information on the temperature of the coolant (or information on the temperature of the lower electrode  82 ) is inputted from the coolant circulation mechanism  106  to the control device  134 . Further, information on the flow rates of gasses is inputted from the first gas supply source  130  and the second gas supply source  132  to the control device  134 . Based on these pieces of information and other information inputted from the user and the like, the control device  134  outputs control signals for controlling the aforementioned components of the plasma treatment apparatus  60 . 
     In the strain layer forming step, first, the gate  68  of the plasma treatment apparatus  60  is lowered by the opening/closing mechanism  70 . Next, the wafer  11  is carried via the opening  66  into the treatment space  62  of the vacuum chamber  64 , and is placed on the table  96  of the lower electrode  82 , with its back surface  11   b  side exposed to the upper side. Note that at the time of carrying the wafer  11  in, it is preferable to preliminarily raise the upper electrode  84  by the lift mechanism  120  to enlarge the spacing between the lower electrode  82  and the upper electrode  84 . Thereafter, a negative pressure of the suction source  100  is made to act, thereby fixing the wafer  11  on the table  96 . Besides, the gate  68  is raised by the opening/closing mechanism  70 , to hermetically close the treatment space  62 . Further, the height position of the upper electrode  84  is adjusted by the lift mechanism  120  in such a manner that the upper electrode  84  and the lower electrode  82  are put into a predetermined positional relation suitable for plasma processing. In addition, the exhaust mechanism  80  is operated to establish a vacuum (low pressure) in the treatment space  62 . Note that in the case where it is difficult to hold the wafer  11  by the negative pressure of the suction source  100  after reducing the pressure inside the treatment space  62 , the wafer  11  is held on the table  96  by an electrical force (typically, an electrostatic force) or the like. For example, by embedding electrodes inside the table  96  and supplying the electrodes with electric power, an electrical force can be made to act between the table  96  and the wafer  11 . 
     In this state, while a gas for plasma processing is supplied at a predetermined flow rate, predetermined high-frequency electric power is supplied to the lower electrode  82  and the upper electrode  84 . In the strain layer forming step according to the present embodiment, while a predetermined pressure (for example, a pressure of 5 to 50 Pa) is maintained in the treatment space  62 , predetermined high-frequency electric power (for example, 1,000 to 3,000 W) is supplied to the lower electrode  82  and the upper electrode  84  while supplying an inert gas such as a rare gas at a predetermined flow rate from the first gas supply source  130 . As a result, a plasma is generated between the lower electrode  82  and the upper electrode  84 , ions generated from the plasmatized inert gas are attracted to the lower electrode  82  side, and are applied to the back surface  11   b  of the wafer  11 . Then, the back surface  11   b  of the wafer  11  is sputtered, whereby minute ruggedness (minute projections and recesses) or cracks (strain) are formed in the back surface  11   b . The region formed with the strain (strain layer) functions as a gettering layer for capturing metallic elements contained in the inside of the wafer  11 . 
       FIG. 11  is an enlarged sectional view of the wafer  11  in the state of having been formed with a strain layer  33 . The wafer  11  having been divided into the plurality of device chips  31  through the grinding step is disposed on the table  96  through the protective member  29 , and the back surface  11   b  side of the wafer  11  is exposed toward the upper electrode  84  (see  FIG. 10 ). When the plasma processing using the inert gas is applied to the wafer  11 , the strain layer  33  is formed on the back surface  11   b  side of the wafer  11 . In the strain layer forming step, the plasma processing is applied to the wafer  11  which has been subjected to removal of the patterns  17  (see  FIG. 1 , etc.) by application of the laser beam and then cut by the cutting blade (the laser processing step, the cut groove forming step) and in which generation of burs has been restrained. Therefore, generation of electric discharge at the time of the plasma processing is restrained, and breakage of the devices  15  is prevented. By the formation of the strain layer  33 , a gettering effect is obtained by which metallic elements contained in the inside of the wafer  11  are captured to the back surface  11   b  side of the wafer  11 . Note that the strain layer  33  formed by the plasma processing has an extremely small thickness (for example, 1/10 times or below) as compared to the crushed layer formed by the grinding step. Therefore, the formation of the strain layer  33  would not lead to a large lowering in the die strength of the wafer  11 . 
     As has been described above, in the wafer processing method according to the present embodiment, the wafer  11  is cut along the streets  13  after the patterns  17  formed on the streets  13  of the wafer  11  are removed by application of the laser beam. As a result, cutting of the patterns  17  by the cutting blade  28  is avoided, and generation of burs at the time of cutting the wafer  11  is restrained. Therefore, at the time of applying plasma processing to the wafer  11  to form the strain layer  33  on the back surface  11   b  of the wafer  11 , damaging of the devices due to electric discharge to the burs is prevented. 
     Note that while the crushed layer removing step of removing the crushed layer by polishing with the polishing pad  56  has been described in the present embodiment (see  FIG. 9 ), the crushed layer can also be removed by plasma etching using the plasma treatment apparatus  60  denoted in  FIG. 10  in the crushed layer removing step. In the case of removing the crushed layer by the plasma etching, the wafer  11  having been divided into the plurality of device chips  31  is disposed on the table  96  of the plasma treatment apparatus  60  through the protective member  29 , like in  FIG. 11 . In this state, high-frequency electric power is supplied to the lower electrode  82  and the upper electrode  84  while supplying a gas for etching. Specifically, the inside of the treatment space  62  is maintained at a predetermined pressure (for example, 50 to 300 Pa), and predetermined high-frequency electric power (for example, 1,000 to 3,000 W) is supplied to the lower electrode  82  and the upper electrode  84  while supplying a halogen-containing gas such as SF6 from the second gas supply source  132  at a predetermined flow rate. Note that the distance between the lower electrode  82  and the upper electrode  84  is set larger than that at the time of performing the strain layer forming step. By this, the voltage to be impressed on the wafer  11  is lowered as compared to that at the time of performing the strain layer forming step. 
     By the above-mentioned step, a plasma is generated between the lower electrode  82  and the upper electrode  84 , and an active substance generated by the plasma acts on the back surface  11   b  of the wafer  11 , whereby the back surface  11   b  of the wafer  11  is etched. In this way, the crushed layer formed on the back surface  11   b  of the wafer  11  is removed. This plasma etching also is applied to the wafer  11  which has been subjected to the laser processing step and the cut groove forming step and in which generation of burs has been restrained. Therefore, in the crushed layer removing step, also, electric discharge can be restrained, and damaging of the devices  15  is prevented. 
     Other than the foregoing, the structures, methods, and the like according to the present embodiment can be modified, as required, in carrying out the present invention, within the scope of the object of the invention. 
     The present invention is not limited to the details of the above described preferred embodiment. 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.