Patent Publication Number: US-2021193521-A1

Title: Device chip manufacturing method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a device chip manufacturing method for subjecting a device wafer to laser processing and thereafter dividing the device wafer into a plurality of device chips. 
     Description of the Related Art 
     There has been known a method in which a plate-shaped workpiece with a plurality of streets set in a grid pattern on a front surface side and with a device formed in each of regions partitioned by the plurality of streets is processed by a laser beam and is thereafter divided (see, for example, Japanese Patent Laid-open No. 2002-192370). At the time of processing the workpiece by the laser beam, for example, first, a dicing tape is adhered to a back surface located on a side opposite to the front surface of the workpiece. Next, the back surface side of the workpiece is held by a chuck table. In this instance, the workpiece is disposed such that the front surface of the workpiece is on the upper side and the back surface is on the lower side. 
     Thereafter, a laser beam is applied to the workpiece from above the workpiece. In this instance, in a state in which the focal point of the laser beam is positioned inside the workpiece, the workpiece and the focal point are relatively moved along the street. Multiphoton absorption is generated at the focal point and in the vicinity thereof, and a modified region (modified layer) as a brittle region where mechanical strength is lowered is formed along the path of movement of the focal point. After the modified layers are formed along all the streets, the dicing tape is radially expanded. As a result, an external force is applied to the workpiece, cracks extend from the upper surface to the lower surface with the modified layers as start points, and the workpiece is divided along the streets. In short, the workpiece is divided into a plurality of device chips. 
     Incidentally, in-plane variability may be present in the thickness (height) of the workpiece. However, there has been developed a technology in which, for forming the modified layers at a uniform depth from the upper surface even in such a case, the height of the upper surface of the workpiece is preliminarily measured and a laser beam is applied while the height of the focal point is adjusted according to the results of measurement (see, for example, Japanese Patent Laid-open No. 2005-193286). 
     SUMMARY OF THE INVENTION 
     However, even if the modified layers are formed at a substantially uniform depth relative to the upper surface, the modified layers are not necessarily formed at a substantially uniform depth relative to the lower surface. Therefore, even it is intended to divide the workpiece by expanding the dicing tape after the formation of the modified layers, the cracks may not reach the lower surface, and it may be impossible to divide the workpiece. For example, in the case where variability in thickness (height) in excess of ±10 μm relative to a predetermined reference height is present in the upper surface of the workpiece, the cracks may not reach the lower surface in a region where the height exceeds +10 μm (namely, a thick region). In this case, the workpiece cannot be divided. The present invention has been made in consideration of such a problem. It is an object of the present invention to restrain generation of defective division even when in-plane variability is present in the thickness of the workpiece. 
     In accordance with an aspect of the present invention, there is provided a device chip manufacturing method including an adhering step of adhering a protective member to a side of one surface of a front surface of a workpiece including a device wafer having on the front surface side a device region having a device formed in each of regions partitioned by streets and a back surface located on a side opposite to the front surface, a holding step of positioning the one surface on a lower side and holding the workpiece under suction by a holding surface of a chuck table through the protective member, a height measuring step of measuring a height of a lower surface of the workpiece along the streets, based on results of measurement of reflected light from the lower surface obtained by applying measurement light from above an upper surface located on a side opposite to the lower surface of the workpiece held by the holding surface or results of measurement of reflected light from the holding surface obtained by applying measurement light to the holding surface, and applying measurement light from above the workpiece to measure the height of the upper surface along the streets, based on results of measurement of reflected light from the upper surface, a laser processing step of applying a laser beam having such a wavelength as to be transmitted through the workpiece along the streets while adjusting the height of a focal point of the laser beam inside the workpiece according to the heights of the lower surface and the upper surface, to thereby form two or more modified layers at different heights inside the workpiece, after the height measuring step, and a dividing step of breaking the workpiece along the streets with the modified layers as start points, to thereby divide the workpiece into a plurality of device chips, after the laser processing step. The laser processing step includes a first processing step of applying the laser beam along the streets while adjusting the height of the focal point according to the height of the lower surface measured in the height measuring step, to thereby form a first modified layer on the lower surface side, and a second processing step of applying the laser beam along the streets while adjusting the height of the focal point according to the height of the upper surface measured in the height measuring step, to thereby form a second modified layer on the upper surface side. 
     Preferably, in the adhering step, the protective member is adhered to the front surface side, and, in the laser processing step, the laser beam is applied from the back surface side of the workpiece. 
     In addition, preferably, the device chip manufacturing method further includes a protective film adhering step of adhering a protective film to the side of other surface located on a side opposite to the one surface to which the protective member has been adhered, after the adhering step and before the holding step, in which in the holding step, an upper surface of the protective film is the upper surface of the workpiece, and, in the laser processing step, the laser beam is applied to the workpiece through the protective film. 
     The laser processing step of the device chip manufacturing method according to one aspect of the present invention includes the first processing step and the second processing step. In the first processing step, the first modified layer is formed on the lower surface side by applying the laser beam along the streets while the height of the focal point is adjusted according to the height of the lower surface of the workpiece measured in the height measuring step. Besides, in the second processing step, the second modified layer is formed on the upper surface side by applying the laser beam along the streets while the height of the focal point is adjusted according to the height of the upper surface of the workpiece measured in the height measuring step. In this way, the positions of the modified layers are adjusted according to the heights of both the lower surface and the upper surface of the workpiece, and therefore, generation of defective division can be restrained even when in-plane variability is present in the thickness of the workpiece. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a wafer and the like; 
         FIG. 1B  is a partial sectional view of the wafer and the like; 
         FIG. 2  is a perspective view of a laser processing apparatus; 
         FIG. 3A  is a diagram for explaining a height measuring step; 
         FIG. 3B  is a diagram for explaining reflected light; 
         FIG. 4A  is a diagram for explaining a first processing step; 
         FIG. 4B  is a diagram for explaining a second processing step; 
         FIG. 5  is a partial sectional view of the wafer after laser processing and a dicing tape; 
         FIG. 6A  is a partly sectional side view depicting a tape expanding device; 
         FIG. 6B  is a diagram depicting a dividing step; 
         FIG. 7  is a flow chart of a device chip manufacturing method according to a first embodiment; 
         FIG. 8A  is a perspective view of the wafer and the like; 
         FIG. 8B  is a partial sectional view of the wafer and the like; 
         FIG. 9  is a diagram depicting reflected light from a holding surface in a lower surface height measuring step; 
         FIG. 10  is a partial sectional view of the wafer and a protective film after laser processing and the dicing tape; and 
         FIG. 11  is a flow chart of a device chip manufacturing method according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments according to modes of the present invention will be described referring to the attached drawings. First, a wafer (workpiece)  11  as an object of processing and the like will be described.  FIG. 1A  is a perspective view of the wafer  11  and the like, and  FIG. 1B  is a partial sectional view of the wafer  11  and the like. The wafer  11  in the present embodiment is a disk-shaped substrate formed from silicon (Si). It is to be noted that the wafer  11  is not limited to silicon wafer, but may be formed from a semiconductor material such as gallium arsenide (GaAs), silicon carbide (SiC), and gallium nitride (GaN), or sapphire, or any of various glasses. 
     A plurality of streets  13  are set in a grid pattern on a front surface  11   a  side of the wafer  11 , and a device  15  such as an integrated circuit (IC) or a large scale integration (LSI) is formed in each of a plurality of regions partitioned by the streets  13 . In other words, the wafer  11  in the present embodiment is a device wafer having a plurality of devices  15 . It is to be noted that test element group (TEG) may be formed on the streets  13  in a device region  15   a  where the plurality of devices  15  are formed. In addition, a peripheral surplus region  15   b  where the devices  15  are not formed is present in the periphery of the device region  15   a.    
     In the case where a back surface  11   b  located on a side opposite to the front surface  11   a  of the wafer  11  is taken as a reference of height, the wafer  11  has variability in the distance (thickness) from the back surface  11   b  to the front surface  11   a  (see  FIG. 1B ). For example, the thickness in region A depicted in  FIG. 1B  is thinner than the thickness in region B. The wafer  11  is processed in a state in which a dicing tape (protective member)  17  formed of a resin is adhered to the front surface  11   a  side, as illustrated in  FIG. 1A . The dicing tape  17  has a diameter larger than that of the wafer  11 , and the wafer  11  is adhered to a substantially central portion of the dicing tape  17 . 
     The dicing tape  17  has a stacked structure of a base material layer formed of a resin such as a polyolefin, and an adhesive layer formed of a ultraviolet ray-curing resin, or the like, for example. The adhesive layer exhibits a strong sticking force to the wafer  11  and the like, and on the other hand, is cured and is lowered in the sticking force when irradiated with ultraviolet rays. It is to be noted that the dicing tape  17  may not necessarily have the stacked structure of the base material layer and the adhesive layer. The dicing tape  17  may have only the base material layer. In this case, the dicing tape  17  is adhered to the wafer  11  by thermocompression bonding, for example. One surface of an annular frame  19  formed of a metal is adhered to a peripheral portion of the dicing tape  17 . As a result, the wafer  11  is supported by the frame  19  through the dicing tape  17 . In the present embodiment, the unit of the wafer  11 , the dicing tape  17 , and the frame  19  is referred to as a wafer unit  21 . 
     Next, a laser processing apparatus  2  for applying laser processing to the wafer  11  will be described.  FIG. 2  is a perspective view of the laser processing apparatus  2 . It is to be noted that in  FIG. 2 , a component is represented by a functional block. The laser processing apparatus  2  includes a base  4  for supporting each structure. The base  4  includes a rectangular parallelepiped base section  6 , and a wall section  8  extending upward at a rear end of the base section  6 . The upper side of the base section  6  is covered with a metallic cover member (not illustrated), and a touch panel  8   a  functioning as an input device and a display device is disposed at a side surface of the cover member located at a front end of the base section  6 . 
     A cassette elevator  8   b  is provided on a right side of the laser processing apparatus  2 , facing the touch panel  8   a . A cassette  8   c  in which a plurality of wafer units  21  are accommodated is mounted on a lift base of the cassette elevator  8   b . A pair of guide rails  10  are provided on a rear side of the cassette  8   c . A clamp carrying mechanism (not illustrated) for drawing the wafer unit  21  from the cassette  8   c  onto the guide rails  10  is provided on an upper side of the pair of guide rails  10 . 
     A carrying unit  12  is provided above the pair of guide rails  10 . The carrying unit  12  carries the wafer unit  21  between the guide rails  10  and a holding table (chuck table)  14  in a state in which the frame  19  is sucked by a suction pad. The holding table  14  has a metallic frame body having a disk-shaped recess on an upper surface side. A disk-shaped porous plate formed of a porous ceramic or the like is fixed in the recess of the frame body. A flow path (not illustrated) is formed inside the frame body, and a suction source (not illustrated) such as an ejector is connected to one end of the flow path. 
     When a negative pressure generated by the suction source is made to act on the porous plate through the flow path, a negative pressure is generated at an upper surface of the porous plate. Therefore, the upper surface of the porous plate functions as a holding surface  14   a  for holding the wafer unit  21  under suction. The holding table  14  is supported in a rotatable manner by a support base  16  having a rotational drive source (not illustrated) such as a motor. In addition, the support base  16  is supported by an X-axis moving table  20  of an X-axis moving mechanism  18 . 
     The X-axis moving table  20  is supported in the manner of being slidable in an X-axis direction by a pair of X-axis guide rails  22  substantially parallel to the X-axis direction. A nut section (not illustrated) is provided on a back surface side (lower surface side) of the X-axis moving table  20 , and an X-axis ball screw  24  disposed in substantially parallel to the X-axis guide rails  22  is coupled to the nut section in a rotatable manner. An X-axis pulse motor  26  is connected to one end of the X-axis ball screw  24 . When the X-axis ball screw  24  is rotated by the X-axis pulse motor  26 , the X-axis moving table  20  is moved in the X-axis direction (processing feeding direction) along the X-axis guide rails  22 . 
     A Y-axis moving mechanism  28  is provided on the back surface side (lower surface side) of the X-axis moving table  20 . The Y-axis moving mechanism  28  has a Y-axis moving table  30  that supports the X-axis moving mechanism  18 . The Y-axis moving table  30  is supported in the manner of being slidable in a Y-axis direction by a pair of Y-axis guide rails  32  substantially parallel to the Y-axis direction. A nut section (not illustrated) is provided on the back surface side (lower surface side) of the Y-axis moving table  30 , and a Y-axis ball screw  34  disposed in substantially parallel to the Y-axis guide rails  32  is coupled to the nut section in a rotatable manner. 
     A Y-axis pulse motor  36  is connected to one end of the Y-axis ball screw  34 . When the Y-axis ball screw  34  is rotated by the Y-axis pulse motor  36 , the Y-axis moving table  30  is moved in the Y-axis direction (indexing feeding direction) along the Y-axis guide rails  32 . One end of a support arm  38  extending forward is fixed to a front surface of an upper portion of the wall section  8 . Part of a laser applying unit  40  is fixed to the support arm  38 . The laser applying unit  40  includes a laser generating section (not illustrated). 
     The laser generating section has a laser oscillator (not illustrated) having a laser medium such as Nd:YAG or Nd:YVO 4  suitable for laser oscillation. The laser generating section generates a pulsed laser beam L (see  FIG. 4A  and the like) having a predetermined wavelength (for example, 1,064 nm) such as to be transmitted through the wafer  11 . A head section  40   a  is disposed at the other end of the support arm  38 . A condenser lens (not illustrated) for concentrating the laser beam L is provided at the head section  40   a . The condenser lens is disposed in such a manner that the optical axis is parallel to a Z-axis direction. The laser beam L is applied from the head section  40   a  toward the holding surface  14   a.    
     An actuator (not illustrated) including a piezo element is connected to the condenser lens. By adjusting a voltage supplied to the actuator to thereby adjust the position of the condenser lens in the Z-axis direction, the position of a focal point P (see  FIG. 4A  and the like) of the laser beam L is adjusted. A microscope unit  42  is provided at a position adjacent to the laser applying unit  40 . The microscope unit  42  images the wafer  11  held by the holding surface  14   a . The microscope unit  42  has a head portion  42   a  in which an imaging lens (not illustrated) is disposed such as to face the holding surface  14   a.    
     Light taken in through the imaging lens is guided to an imaging element (not illustrated) provided inside the microscope unit  42 . The imaging element includes a complementary metal oxide semiconductor (CMOS) image sensor, a charge coupled device (CCD) image sensor, or the like. A height measuring instrument  44  is provided at a position adjacent to the microscope unit  42 . The height measuring instrument  44  is, for example, a spectral interference type laser displacement meter, and measures the heights of the front surface  11   a , the back surface  11   b  of the wafer  11  held by the holding surface  14   a , and the like. 
     The laser displacement meter has a light source (not illustrated) such as a superluminescent diode (SLD). Infrared light of a wide band is emitted from the light source as inspection light. Part of the light emitted from the light source is reflected by a half-mirror (not illustrated) provided inside the head section  44   a  of the height measuring instrument  44 , and goes toward the object of measurement such as the wafer  11 . Part of the light reflected by the object of measurement is transmitted through the half-mirror, to be incident on a spectrometer (not illustrated). The spectrometer has a diffraction grating (not illustrated) for spectrally separating the incident light to obtain a spectrum. A light receiving element (not illustrated) such as a CCD is disposed in the vicinity of the diffraction grating. The light spectrally separated by the diffraction grating is converted into an electrical signal according to light intensity on a wavelength basis in the light receiving element. The light receiving element is connected to a control unit  46  which will be described later. The control unit  46  analyzes the waveform of the electrical signal outputted from the light receiving element by Fourier transformation or the like, for example. 
     Incidentally, a reference plate (not illustrated) serving as a reference for height measurement is provided at a lower end portion of the head section  44   a . The reference plate is formed, for example, of glass, quartz, or the like. A part of the light incident on one surface (reference surface) of the reference plate is reflected by the reference surface, and other part of the light incident on the reference surface is transmitted through the reference plate and is reflected by the object of measurement. In other words, first reflected light reflected by the reference surface and second reflected light reflected by the object of measurement are incident on the spectrometer. The first reflected light and the second reflected light are intensified by each other at a predetermined wavelength according to the distance (optical distance) from the reference surface to the object of measurement. By utilizing this principle, the distance from the reference surface to a predetermined surface (for example, an upper surface, a lower surface, or the like of the workpiece) of the object of measurement is calculated by the control unit  46 . In addition to the calculation of the distance, the control unit  46  controls the components of the laser processing apparatus  2 . 
     The control unit  46  controls the cassette elevator  8   b , the pair of guide rails  10 , the carrying unit  12 , the X-axis moving mechanism  18 , the Y-axis moving mechanism  28 , the laser applying unit  40 , the microscope unit  42 , the height measuring instrument  44 , and the like. The control unit  46  includes a computer including a processing device such as, a central processing unit (CPU), a main storage device such as a dynamic random access memory (DRAM), and an auxiliary storage device such as a flash memory or a hard disk drive, for example. By operating the processing device and the like according to a software stored in the auxiliary storage device, functions of the control unit  46  are realized. 
     Next, a manufacturing method for the device chips  23  according to a first embodiment will be described referring to  FIGS. 1A to 6B . It is to be noted that  FIG. 7  is a flow chart of the manufacturing method for the device chips  23  according to the first embodiment. In the present embodiment, first, as depicted in  FIG. 1A , the dicing tape  17  is adhered to the front surface  11   a  side of the wafer  11  and one surface of the frame  19 , to form the wafer unit  21  (adhering step S 10 ). 
     The adhering step S 10  may be performed by a tape adhering device (not illustrated) or may be manually performed by an operator. After the adhering step S 10 , the cassette  8   c  accommodating each wafer unit  21  is mounted on the lift base of the cassette elevator  8   b . Next, the wafer unit  21  is drawn out from the cassette  8   c  onto the guide rails  10  by a clamp carrying mechanism (not illustrated). The wafer unit  21  adjusted in the position in the X-axis direction by the pair of guide rails  10  is carried to the holding table  14  by the carrying unit  12 . In this instance, the wafer unit  21  is mounted on the holding surface  14   a  in such a manner that the surface (in the present embodiment, the front surface  11   a ) with the dicing tape  17  adhered thereto is on the lower side. In other words, the front surface  11   a  is the lower surface of the wafer  11 , and the back surface  11   b  is the upper surface of the wafer  11 . 
     Subsequently, the suction source is operated to generate a negative pressure at the holding surface  14   a . As a result, the lower surface (front surface  11   a ) side is held by the holding surface  14   a  through the dicing tape  17  (holding step S 20 ). After the holding step S 20 , the height of the lower surface (front surface  11   a ) and the height of the upper surface (back surface  11   b ) are measured along the streets  13  by use of the height measuring instrument  44  or the like (height measuring step S 30 ).  FIG. 3A  is a diagram for explaining the height measuring step S 30 . 
     In the height measuring step S 30 , first, alignment of the wafer  11  is conducted by use of the microscope unit  42  or the like. Next, a lower end of the head section  44   a  of the height measuring instrument  44  is positioned on an extension line of one street  13 . Then, while measurement light is applied to the wafer  11  from above the wafer  11 , the wafer  11  is moved along the X-axis direction relatively to the head section  44   a  by the X-axis moving mechanism  18 . In this instance, reflected light from the wafer  11  is measured by the height measuring instrument  44 . 
       FIG. 3B  is a diagram for explaining the reflected light. In the present embodiment, results of measurement of first reflected light C 1  (not illustrated) reflected by the reference surface and second reflected light C 2  reflected by the upper surface (back surface lib) are analyzed by the control unit  46 . As a result, the height of the upper surface (back surface lib) relative to the reference surface is measured (upper surface measuring step S 32 ). Similarly, results of measurement of the first reflected light C 1  (not illustrated) reflected by the reference surface and third reflected light C 3  reflected by the lower surface (front surface  11   a ) are analyzed by the control unit  46 . As a result, the height of the lower surface (front surface  11   a ) relative to the reference surface is measured (lower surface measuring step S 34 ). It is to be noted that the height of the lower surface is not necessarily flat since ruggedness or the like of the holding surface  14   a  is reflected. 
     In this way, the height of the upper surface (back surface  11   b ) and the height of the lower surface (front surface  11   a ) are measured at once based on the results of measurement of the reflected light. After the heights of the upper surface and the lower surface are measured along one street  13 , the wafer  11  is put into indexing feeding along the Y-axis direction. As a result, the head section  44   a  is positioned on an extension line of another street  13  adjacent to the one street  13  to which the measurement light has been applied. Then, the heights of the upper surface and the lower surface are similarly measured along another street  13 . 
     After the heights of the upper surface and the lower surface are measured along all the streets  13  set along one direction, the rotational drive source is operated to rotate the holding table  14  by 90 degrees. Then, the heights of the upper surface and the lower surface are measured along all the streets  13  in another direction orthogonal to the one direction. XY coordinates (positions) on the upper surface and the lower surface and information concerning the heights (the heights of the upper surface and the lower surface) at each position measured in the height measuring step S 30  are stored in a storage section (for example, the auxiliary storage device) of the control unit  46 . 
     After the height measuring step S 30 , the laser beam L is applied to the wafer  11  from above the wafer  11  to process the wafer  11  along the streets  13  (laser processing step S 40 ). Specifically, in a state in which the height of the focal point P of the laser beam L is positioned inside the wafer  11 , the focal point P of the laser beam L and the wafer  11  are relatively moved in the X-axis direction along the street  13 . As a result, a modified region (modified layer) as a brittle region where mechanical strength is lowered is formed inside the wafer  11  along the street  13 . In the present embodiment, firstly, a first modified layer  11   c  is formed on the lower surface side of the wafer  11  (first processing step S 42 ). 
       FIG. 4A  is a diagram for explaining the first processing step S 42 . In the first processing step S 42 , information obtained in the height measuring step S 30  is read out from the storage section, and while the height of the focal point P is adjusted according to the height of the lower surface of the wafer  11  by controlling the abovementioned actuator according to the XY coordinates, the laser beam L is applied along the street  13 . It is to be noted that since the processing feeding speed of the holding table  14  at the time of applying the laser beam L is predetermined, the position of the focal point P in the Z-axis direction can be adjusted along the street  13  by operating the abovementioned actuator at a predetermined timing according to the processing feeding speed. 
     In the first processing step S 42 , after the laser beam L is applied along all the streets  13  along one direction, the holding table  14  is rotated by 90 degrees, and the laser beam L is applied along all the streets  13  along another direction orthogonal to the one direction. As a result, the first modified layers  11   c  are formed along all the streets  13 . It is to be noted that in the first processing step S 42  of the present embodiment, two kinds of first modified layers  11   c   1  and  11   c   2  are formed at different heights on the lower surface side of the wafer  11  (see  FIG. 5 ). The first modified layers  11   c   1  are formed at positions of 30 μm to 50 μm from the lower surface, whereas the first modified layers  11   c   2  are formed at positions of 130 μm to 150 μm from the lower surface. 
     In order to avoid scattering, splashing, or the like of the laser beam L to the lower surface (front surface  11   a ), it is preferable to form the first modified layers  11   c   2  after the first modified layers  11   c   1  are formed. It is to be noted that the number of the kinds of the first modified layers  11   c  is not limited to two, and the number may be one or may be three or more. After the first processing step S 42 , second modified layers  11   d  are formed on the upper surface side of the wafer  11  (second processing step S 44 ).  FIG. 4B  is a diagram for explaining the second processing step S 44 . 
     In the second processing step S 44 , also, the information is read out from the storage section, and while the height of the focal point is adjusted according to the height of the upper surface (back surface lib) by operating the actuator according to the processing feeding speed, the laser beam L is applied along the streets  13 . 
     The second modified layers  11   d  are formed, for example, at positions spaced downward by a predetermined distance of 100 μm to 120 μm from the height of the upper surface.  FIG. 5  is a partial sectional view of the wafer  11  after formation of the second modified layers  11   d  and the dicing tape  17 . In the second processing step S 44  of the present embodiment, one kind of second modified layers  11   d  are formed on the upper surface side of the wafer  11 . It is to be noted that the number of kinds of the second modified layers  11   d  is not limited to one, and the number may be two or more. In the present embodiment, in a state in which the front surface  11   a  is positioned on the lower side, the laser beam L is applied from above the wafer  11 . Therefore, generation of defective processing due to reflection of the laser beam L by the TEG formed on the streets  13  on the front surface  11   a  side can be prevented. 
     After the laser processing step S 40 , the wafer  11  is divided into a plurality of device chips  23  (dividing step S 50 ). In the dividing step S 50 , a tape expanding device  50  is used.  FIG. 6A  is a partially sectional side view depicting the tape expanding device  50 . The tape expanding device  50  has a cylindrical drum  52  having a diameter larger than the diameter of the wafer  11 . A plurality of rollers (not illustrated) are provided at an upper end portion of the drum  52  along the circumferential direction. 
     An annular frame holding table  54  having an inside diameter larger than the diameter of the drum  52  is provided at a peripheral portion of the drum  52 . An upper surface of the frame holding table  54  is a substantially flat mount surface  54   a  on which the frame  19  is mounted. A plurality of clamp unit  56  are provided at a peripheral portion of the frame holding table  54 . In addition, an upper end portion of a rod  58  movable along the height direction of the drum  52  is fixed to a lower portion of the frame holding table  54 . 
     A part on the lower side of the rod  58  is disposed inside an air cylinder  60 . When the rod  58  is drawn into the air cylinder  60 , the mount surface  54   a  is lowered relative to an upper end of the drum  52 . Next, the dividing step S 50  conducted using the tape expanding device  50  will be described.  FIG. 6B  is a diagram depicting the dividing step S 50 . In the dividing step S 50 , in a state in which the upper end of the drum  52  and the mount surface  54   a  are set at substantially the same height, the wafer unit  21  is mounted on the drum  52  and the mount surface  54   a.    
     Next, the position of the frame  19  is fixed by the clamp units  56 . Then, the rod  58  is drawn into the air cylinder  60 , whereby the mount surface  54   a  is lowered relative to the upper end of the drum  52 . As a result, the dicing tape  17  is radially expanded, and an external force is applied to the wafer  11 . The wafer  11  is broken along the streets  13 , with the first modified layers  11   c  and the second modified layers  11   d  as start points and are divided into a plurality of device chips  23 . In the present embodiment, two or more kinds of modified layers including the first modified layers  11   c  according to the height of the lower surface and the second modified layers  11   d  according to the height of the upper surface are formed. In this way, the positions of the modified layers are adjusted according to the heights of both the lower surface and the upper surface, and therefore, generation of defective division can be restrained even when in-plane variability is present in the thickness of the wafer  11 . 
     Next, a second embodiment will be described. In the second embodiment, a resin-made protective film  25  is adhered to the back surface  11   b  located on the side opposite to the front surface  11   a  to which the dicing tape  17  has been adhered.  FIG. 8A  is a perspective view of the wafer  11  and the like according to the second embodiment. The protective film  25  has, for example, a stacked structure of a base material layer and an adhesive layer, and the adhesive layer side is adhered to the upper surface of the wafer  11 . With the protective film  25  thus provided, for example, generation of chipping (lacking) in the dividing step S 50  can be reduced.  FIG. 8B  is a partial sectional view of the wafer  11  with the protective film  25  adhered thereto and the like. 
     In the second embodiment, a stacked body of the wafer  11  and the protective film  25  is a workpiece to be processed by the laser beam L. In addition, in the second embodiment, the front surface  11   a  of the wafer  11  is a lower surface of the workpiece, whereas an upper surface  25   a  of the protective film  25  is an upper surface of the workpiece. Besides, one unit of the wafer  11 , the dicing tape  17 , the frame  19  and the protective film  25  is a wafer unit  21 . Next, referring to  FIGS. 9 and 10 , a manufacturing method for the device chips  23  according to the second embodiment will be described.  FIG. 11  is a flow chart of the manufacturing method for the device chips  23  according to the second embodiment. In the following, differences from the first embodiment will be described primarily. 
     In the second embodiment, after the adhering step S 10 , the protective film  25  is adhered to the back surface  11   b  side (protective film adhering step S 12 ). Next, the height of the holding surface  14   a  of the holding table  14  is measured (lower surface height measuring step S 14 ).  FIG. 9  is a diagram depicting fourth reflected light C 4  from the holding surface  14   a  in the lower surface height measuring step S 14 . In the lower surface height measuring step S 14 , measurement light is applied from the head section  44   a , and results of measurement of first reflected light C 1  (not illustrated) reflected by the reference surface of the head section  44   a  and fourth reflected light C 4  reflected by the holding surface  14   a  are analyzed by the control unit  46 . As a result, the height of the holding surface  14   a  as a whole relative to the reference surface is measured. 
     Then, the thickness (for example, 100 μm) of the dicing tape  17  is added to the height of the holding surface  14   a  measured. As a result, the height of the lower surface (front surface  11   a ) relative to the reference surface is calculated. In this way, in the second embodiment, the height position of the lower surface (front surface  11   a ) in the case where the lower surface side is held by the holding surface  14   a  is measured indirectly. Note that it is sufficient for the lower surface height measuring step S 14  to be conducted before the first processing step S 42  and may be performed before the adhering step S 10  or before the protective film adhering step S 12 . 
     The XY coordinates (positions) on the lower surface (front surface  11   a ) and the information concerning the height of the lower surface at each position are stored in the storage section (for example, the auxiliary storage device) of the control unit  46 , as in the first embodiment. After the lower surface height measuring step S 14 , the holding step S 20  and the upper surface height measuring step S 32  are sequentially carried out. In the upper surface height measuring step S 32 , measurement light is applied from the head section  44   a , and results of measurement of first reflected light C 1  (not illustrated) reflected by the reference surface and fifth reflected light C 5  (not illustrated) reflected by the upper surface  25   a  of the protective film  25  are analyzed by the control unit  46 . As a result, the height of the upper surface  25   a  relative to the reference surface is measured. 
     The XY coordinates (positions) on the upper surface  25   a  and the information concerning the height of the upper surface  25   a  at each position are stored in the storage section (for example, the auxiliary storage device) of the control unit  46 . Subsequently, the laser processing step S 40  is sequentially conducted, as in the first embodiment. It is to be noted that in the laser processing step S 40  of the second embodiment, the laser beam L is applied to the inside of the wafer  11  through the protective film  25 . As a result, in the first processing step S 42 , the first modified layers  11   c   1  and  11   c   2  are formed at different positions on the lower surface (front surface  11   a ) side of the wafer  11 . 
     Besides, in the second processing step S 44 , second modified layers  11   d   1  are formed on the upper surface (back surface  11   b ) side of the wafer  11 , and in addition, second modified layers  11   d   2  are formed in the protective film  25 .  FIG. 10  is a partial sectional view of the wafer  11  and the protective film  25  after laser processing and the dicing tape  17 . It is to be noted that at the time of forming the second modified layers  11   d   1  and  11   d   2 , the laser beam L is applied along the streets  13  while the height of the focal point P is adjusted according to the height of the upper surface  25   a  of the protective film  25  (namely, the upper surface of the workpiece). 
     In the second embodiment, also, since the positions of the first modified layers  11   c  and the second modified layers  11   d  are adjusted according to both the lower surface and the upper surface of the workpiece (namely, the wafer  11  and the protective film  25 ), generation of defective division can be restrained even when in-plane variability is present in the thickness of the wafer  11  and the like. Other than the above, the structures, methods, and the like according to the above embodiment can be modified as required insofar as the modifications do not depart from the scope of the object of the present invention. 
     In the first and second embodiments described above, the wafer  11  and the like have been processed in a state in which the front surface  11   a  of the wafer  11  is positioned on the lower side and the back surface  11   b  is positioned on the upper side. However, the wafer  11  and the like may be processed in a state in which the back surface  11   b  of the wafer  11  is positioned on the lower side and the front surface  11   a  is positioned on the upper side. In the case where the back surface  11   b  is on the lower side, the dicing tape (protective member)  17  is adhered to the back surface  11   b  side. In addition, the protective film  25  may be adhered to the side of the front surface  11   a  located on the upper side, as in the second embodiment. 
     Incidentally, the laser processing step S 40  may be conducted while the height of the workpiece is measured by the height measuring instrument  44 , instead of performing the laser processing step S 40  after the height of the workpiece is measured by the height measuring instrument  44 . For example, in the case where the laser processing apparatus  2  is provided with one height measuring instrument  44 , the laser processing step S 40  is conducted using the laser applying unit  40  while the height of the workpiece is measured by the height measuring instrument  44  at the time of processing feeding toward one side in the X-axis direction. It is to be noted that, however, in the case where the height measuring instruments  44  are provided at both sides of the laser applying unit  40 , the laser processing step S 40  can be performed while the height of the workpiece is measured not only at the time of processing feeding toward one side in the X-axis direction but also at the time of processing feeding toward the other side in the X-axis direction. 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.