Patent Publication Number: US-2005127299-A1

Title: Apparatus for checking a laser processed deteriorated layer

Description:
FIELD OF THE INVENTION  
      The present invention relates to an apparatus for checking a deteriorated layer formed in the inside of a workpiece along a dividing line by applying a laser beam capable of passing through the workpiece along the dividing line formed on the workpiece.  
     DESCRIPTION OF THE PRIOR ART  
      In the production process of a semiconductor device, a plurality of areas are sectioned by dividing lines called “streets” arranged in a lattice pattern on the front surface of a substantially disk-like semiconductor wafer, and a circuit such as IC, LSI or the like is formed in each of the sectioned areas. Individual semiconductor chips are manufactured by cutting this semiconductor wafer along the dividing lines to divide it into the areas in which the circuits are formed. An optical device wafer comprising gallium nitride-based compound semiconductors formed on the front surface of a sapphire substrate is also cut along dividing lines to be divided into individual optical devices such as light emitting diodes or laser diodes, and these devices are widely used in electric equipment.  
      Cutting along the dividing lines of the above semiconductor wafer or optical device wafer is generally carried out by a cutting machine called “dicer”. This cutting machine comprises a chuck table for holding a workpiece such as a semiconductor wafer or optical device wafer, a cutting means for cutting the workpiece held on the chuck table, and a cutting-feed means for moving the chuck table and the cutting means relative to each other. The cutting means has a spindle unit which comprises a rotary spindle, a cutting blade mounted to the spindle and a drive mechanism for rotary-driving the rotary spindle. The cutting blade comprises a disk-like base and an annular cutting edge which is mounted to the side wall periphery portion of the base and formed as thick as about 20 μm by fixing diamond abrasive grains having a diameter of about 3 μm to the base by electroforming.  
      Since a sapphire substrate, silicon carbide substrate, etc. have high Mohs hardness, cutting with the above cutting blade is not always easy. Further, since the cutting blade has a thickness of about 20 μm, the dividing lines for sectioning devices must be as thick as about 50 μm. Therefore, in the case of a device measuring about 300 μm×300 μm, the area occupied by the dividing lines is large, thereby reducing productivity.  
      Meanwhile, as a means of dividing a plate-like workpiece such as a semiconductor wafer, a laser beam processing method for applying a pulse laser beam capable of passing through the workpiece with its focusing point set to the inside of the area to be divided is attempted and disclosed by JP-A 2003-88975, for example. In the dividing method using this laser beam processing technique, the workpiece is divided by applying a pulse laser beam of an infrared range capable of passing through the workpiece from one side of the workpiece with its focusing point set to the inside to continuously form deteriorated layers in the inside of the workpiece along the dividing lines and exerting external force along the dividing lines whose strength has been reduced by the formation of the deteriorated layers.  
      To divide the workpiece having deteriorated layers formed in the inside along the dividing lines without fail by applying a pulse laser beam, the deteriorated layers must be reliably formed at a predetermined position in the inside of the workpiece. However, when a pulse laser beam is applied without positioning the focusing point of the pulse laser beam to the predetermined position in the inside of the workpiece, the deteriorated layers cannot be formed at the predetermined position in the inside of the workpiece. Since the deteriorated layers formed in the inside of the workpiece cannot be checked from the outside, there is a problem that when external force is exerted to the workpiece having no deteriorated layers in the inside along the dividing lines, the workpiece may be broken.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide an apparatus for reliably checking a laser processed deteriorated layer formed in the inside of a workpiece by applying a laser beam to the workpiece.  
      To attain the above object, according to the present invention, there is provided an apparatus for checking a deteriorated layer formed in the inside of a workpiece by applying a laser beam capable of passing through the workpiece to the workpiece, comprising: 
          a workpiece holding means for holding the workpiece;     a light application means for applying light capable of passing through the workpiece held on the workpiece holding means to the exposed surface of the workpiece at a predetermined angle;     a light receiving means for receiving light that is applied from the light application, passes through the inside of the workpiece and is reflected from the workpiece; and     a display means for displaying the state of light received by the light receiving means.        

      Preferably, the apparatus comprises which comprises a scanning-feed means for moving the light application means, the light receiving means and the workpiece holding means in a predetermined scanning-feed direction relative to one another. Preferably, the light application means irradiates an infrared laser beam.  
      Since the apparatus for checking a laser processed deteriorated layer according to the present invention is constituted as described above, the deteriorated layer which is formed in the inside of the workpiece and cannot be checked from the outside can be checked without fail. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of an apparatus for checking a laser processed deteriorated layer constituted according to the present invention;  
       FIG. 2  is a graph showing the transmittances of materials of a semiconductor wafer;  
       FIG. 3  is a diagram showing the positional relationship between a light application means and a light receiving means provided in the apparatus for checking a laser processed deteriorated layer shown in  FIG. 1  and a workpiece;  
       FIG. 4  is a diagram showing a scanning state by the apparatus for checking a laser processed deteriorated layer shown in  FIG. 1 ;  
       FIG. 5  is a diagram showing light applied from the light application means and its reflected light;  
       FIG. 6  is a diagram showing an image of the inside of the workpiece;  
       FIG. 7  is a perspective view of a semiconductor wafer as the workpiece; and  
      FIGS.  8 ( a ) and  8 ( b ) are diagrams for explaining laser processing for forming a deteriorated layer in the inside of the semiconductor wafer shown in  FIG. 7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 7  is a perspective view of a semiconductor wafer  10  comprising a silicon substrate as the workpiece. In the semiconductor wafer  10  shown in  FIG. 7 , a plurality of dividing lines  11  are formed in a lattice pattern on the front surface  10   a , and a circuit  12  such as IC, LSI or the like is formed in each of a plurality of areas sectioned by the plurality of dividing lines  11 . The laser processing method for forming a deteriorated layer in the inside of the semiconductor wafer  10  along the dividing line  11  will be described with reference to FIGS.  8 ( a ) and  8 ( b ).  
      To form the deteriorated layer in the inside of the semiconductor wafer  10  along the dividing line  11 , the semiconductor wafer  10  is placed on the chuck table  20  of a laser beam processing machine in such a manner that the back surface  10   b  faces up and suction-held on the chuck table  20  as shown in FIGS.  8 ( a ) and  8 ( b ). After the semiconductor wafer  10  is suction-held on the chuck table  20 , the dividing line  11  is detected from the back surface  10   b  by an infrared aligning means (not shown), and the chuck table  20  is moved to a laser beam application range where the condenser  21  of laser beam application means for applying a laser beam is located, to bring one end (left end in  FIG. 8 ( a )) of the predetermined dividing line  11  to a position right below the condenser  21  of the laser beam application means as shown in  FIG. 8 ( a ). The chuck table  20 , that is, the semiconductor wafer  10  is moved in the direction indicated by the arrow X 1  in  FIG. 8 ( a ) at a predetermined processing-feed rate while a pulse laser beam capable of passing through the semiconductor wafer  10  is applied to the workpiece  10  from the condenser  21 . Then, when the application position of the condenser  21  of the laser beam application means reaches the other end (right end in  FIG. 8 ( b )) of the dividing line  11  as shown in  FIG. 8 ( b ), the application of the pulse laser beam is suspended, and the movement of the chuck table  20 , that is, the semiconductor wafer  10  is stopped. In this laser processing, by setting the focusing point P of the pulse laser beam to a predetermined position in the inside of the semiconductor wafer  10 , a deteriorated layer  110  is formed in the inside of the semiconductor wafer  10  along the dividing line  11 . This deteriorated layer  110  is formed as a molten-resolidified layer in which the wafer has been once molted and then re-solidified.  
      The laser processing conditions in the above laser processing are set as follows, for example. 
          Laser: pulse laser having a wavelength of 1,064 nm     Repetition frequency: 100 kHz     Pulse width: 25 ns     Peak power density: 3.2×10 10  W/cm 2       Focusing spot diameter: 1 μm     Processing-feed rate: 100 mm/sec        

      After the laser processing is carried out along the dividing line  11  in the predetermined direction formed on the wafer  10  as described above, the chuck table  20  or the laser beam application means is indexing-fed a distance corresponding to the interval between the dividing lines  11  in the indexing-feed direction perpendicular to the sheet surface in FIGS.  8 ( a ) and  8 ( b ) to further carry out the above laser processing. After the above laser processing is carried out on all the dividing lines  11  formed in the predetermined direction, the chuck table  20  is turned at 90° to carry out the above laser processing along dividing lines formed in the direction perpendicular to the above predetermined direction subsequently, thereby making it possible to form deteriorated layers  110  in the inside of the semiconductor wafer  10  along all the dividing lines  11 . When a low-dielectric insulating film (Low-k film) or test element group (Teg) is not formed on the top surface of the dividing lines  11  formed on the front surface  10   a  of the semiconductor wafer  10 , a pulse laser beam may be applied to the workpiece from the front surface  10   a  side of the semiconductor wafer  10  to form the deteriorated layers  110 .  
      The deteriorated layer  110  formed in the inside of the semiconductor wafer  10  along the dividing line  11  cannot be checked from the outside as described above. Therefore, it is necessary to check whether the deteriorated layer  110  is formed at the predetermined position in the inside of the semiconductor wafer  10  without fail. The apparatus for checking a laser processed deteriorated layer in the inside of the workpiece will be described hereinbelow with reference to  FIG. 1 .  
       FIG. 1  is a perspective view of the apparatus for checking a laser processed deteriorated layer constituted according to the present invention. The apparatus for checking a laser processed deteriorated layer shown in  FIG. 1  comprises a stationary base  2 , a chuck table mechanism  3  for holding a workpiece, which is mounted on the stationary base  2  in such a manner that it can move in a direction indicated by an arrow X, a laser beam application means  4  for applying light capable of passing through a workpiece to the workpiece held on the chuck table mechanism  4 , a light receiving means  5  for receiving light, which is applied from the light application means  4 , passes through the inside of the workpiece and is reflected from the workpiece, a control means  6  and a display means  7 .  
      The above chuck table mechanism  3  comprises a pair of guide rails  31  and  31  that are mounted on the stationary base  2  and arranged parallel to each other in the direction indicated by the arrow X, a first sliding block  32  mounted on the guide rails  31  and  31  in such a manner that it can move in the direction indicated by the arrow X, a second sliding block  33  mounted on the first sliding block  32  in such a manner that it can move in the direction indicated by the arrow Y, a support table  35  supported on the second sliding block  33  by a cylindrical member  34 , and a chuck table  36  as a workpiece holding means. This chuck table  36  is made of a porous material, and a semiconductor wafer as the workpiece is held on the chuck table  36  by a suction means that is not shown. The chuck table  36  is turned by a pulse motor (not shown) installed in the cylindrical member  34 . An infrared aligning means (not shown) is arranged above the chuck table  36 .  
      The above first sliding block  32  has, on its undersurface, a pair of to-be-guided grooves  321  and  321  to be fitted to the above pair of guide rails  31  and  31  and has, on its top surface, a pair of guide rails  322  and  322  formed parallel to each other in the direction indicated by the arrow Y. The first sliding block  32  constituted as described above can move in the direction indicated by the arrow X along the pair of guide rails  31  and  31  by fitting the to-be-guided grooves  321  and  321  to the pair of guide rails  31  and  31 , respectively. The chuck table mechanism  3  in the illustrated embodiment comprises an indexing-feed means  37  for moving the first sliding block  32  along the pair of guide rails  31  and  31  in the indexing-feed direction indicated by the arrow X. The indexing feed means  37  has a male screw rod  371  arranged between the above pair of guide rails  31  and  31  and in parallel to them, and a drive source such as a pulse motor  372  for rotary-driving the male screw rod  371 . The male screw rod  371  is, at its one end, rotatably supported onto a bearing block  373  fixed on the above stationary base  2  and is, at the other end, transmission-coupled to the output shaft of the above pulse motor  372  by a speed reducer that is not shown. The male screw rod  371  is screwed into a threaded through-hole formed in a female screw block (not shown) projecting from the undersurface of the center portion of the first sliding block  32 . Therefore, by driving the male screw rod  371  in a normal direction or reverse direction with the pulse motor  372 , the first sliding block  32  is moved along the guide rails  31  and  31  in the indexing-feed direction indicated by the arrow X.  
      The above second sliding block  33  has, on its undersurface, a pair of to-be-guided grooves  331  and  331  to be fitted to the pair of guide rails  322  and  322  on the top surface of the above first sliding block  32  and can move in the scanning-feed direction indicated by the arrow Y by fitting the to-be-guided grooves  331  and  331  to the pair of guide rails  322  and  322 , respectively. The chuck table mechanism  3  in the illustrated embodiment comprises a scanning-feed means  38  for moving the second sliding block  33  in the scanning-feed direction indicated by the arrow Y along the pair of guide rails  322  and  322  on the first sliding block  32 . The scanning-feed means  38  has a male screw rod  381  which is arranged between the above pair of guide rails  322  and  322  and in parallel to them, and a drive source such as a pulse motor  382  for rotary-driving the male screw rod  381 . The male screw rod  381  is, as its one end, rotatably supported onto a bearing block  383  fixed on the top surface of the above first sliding block  32  and is, at the other end, transmission-coupled to the output shaft of the above pulse motor  382  by a speed reducer that is not shown. The male screw rod  381  is screwed into a threaded through-hole formed in a female screw block (not shown) projecting from the undersurface of the center portion of the second sliding block  33 . Therefore, by driving the male screw rod  381  in a normal direction or reverse direction with the pulse motor  382 , the second sliding block  33  is moved along the guide rails  322  and  322  in the scanning-feed direction indicated by the arrow Y.  
      The above light application means  4  and the above light receiving means  5  are opposed to each other along the pair of guide rails  31  and  31  of the above chuck table mechanism  3  with the above chuck table  36  interposed therebetween. That is, the light application means  4  and the light receiving means  5  are opposed to each other in the direction perpendicular to the scanning-feed direction indicated by the arrow Y.  
      The above light application means  4  is so constituted as to apply light capable of passing through the workpiece. Here, the light capable of passing through the workpiece will be described hereinbelow.  FIG. 2  is a graph showing the transmittances of silicon (Si), gallium arsenic (GaAs) and indium (InP) crystals used as the materials of a semiconductor wafer. In the graph, the horizontal axis shows the wavelength of light and the vertical axis shows transmittance. As understood from  FIG. 2 , all of the above materials have a high transmittance at an infrared range of 1 to 10 μm of the wavelength. Therefore, when the workpiece is made of any one of the above materials, the light application means  4  may be designed to apply infrared radiation or infrared laser beam having a wavelength of 1 to 10 μm. In the illustrated embodiment, the light application means  4  comprises a light source such as a 1.3 μm laser diode or a 1.5 μm laser diode for applying a laser beam having a single wavelength at a spot diameter of 400 μm. The light application means  4  thus constituted applies an infrared laser beam at a predetermined angle θ (10 to 45°) to the exposed surface (top surface) of a wafer  10  as the workpiece held on the chuck table  36  as shown in  FIG. 3 .  
      The above light receiving means  5  comprises an infrared image pick-up device (infrared CCD), and its light receiving surface is inclined to form the same angle θ as the angle θ at which an infrared laser beam is applied to the exposed surface (top surface) of the wafer  10  held on the chuck table  36  as shown in  FIG. 3 . Therefore, an infrared laser beam applied from the above light application means  4  goes into the inside of the wafer  10  as the workpiece, is reflected on the interface (undersurface) and received by the light receiving means  5 . The light receiving means  5  that has thus received reflected light of the infrared laser beam applied from the light application means  4  outputs an electric signal corresponding to the intensity of the received light. The electric signal from the light receiving means  5  is sent to the control means  6  shown in  FIG. 1 . The control means  6  carries out predetermined processing such as image processing, etc. based on the input electric signal, and displays the result of processing on the display means  7 .  
      The apparatus for checking a laser processed deteriorated layer in the illustrated embodiment is constituted above, and its function will be described hereinbelow with reference to  FIGS. 1 and 4  to  6 .  
      The semiconductor wafer  10  having deteriorated layers formed in the inside along the dividing lines  11  by laser processing as described above is placed on the chuck table  36  of the checking apparatus shown in  FIG. 1  in such a manner that the back surface  10   b  faces up and suction-held on the chuck table  36 . The chuck table  36  suction-holding the semiconductor wafer  10  is detected and aligned by an infrared aligning means (not shown) such that dividing lines  11  formed in a lattice pattern on the semiconductor wafer  10  become parallel to and perpendicular to the indexing-feed direction indicated by the arrow X and the scanning-feed direction indicated by the arrow Y in  FIG. 1 , respectively. After the semiconductor wafer  10  is suction-held on the chuck table  36 , the chuck table  36  is moved to a scanning area shown in  FIG. 1  where one end (left end in  FIG. 4 ) of a predetermined dividing line  11  is brought to a position opposed to the light application means  4  as shown in  FIG. 4 .  
      Thereafter, as shown in  FIG. 5 , an infrared laser beam  41  is applied from the light application means  4  to the semiconductor wafer  10  held on the chuck table  36  at a predetermined incident angle θ, and the scanning-feed means  38  is activated to move the chuck table  36 , that is, the semiconductor wafer  10  in the direction indicated by the arrow Y 1  in  FIG. 4  at a predetermined scanning speed. The infrared laser beam  41  applied from the light application means  4  goes into the inside from the exposed surface (top surface) of the semiconductor wafer  10  and is reflected on the interface (undersurface) as shown in  FIG. 5 , and the reflected light  42  passes through the inside of the semiconductor wafer  10  and goes out from the exposed surface (top surface) toward the light receiving surface of the light receiving means  5  at a predetermined reflection angle θ. This reflected light  42  is received by the light receiving means  5 . However, the reflected light  42   a  passing through the deteriorated layer  110  formed in the inside of the semiconductor wafer  10  is diffracted. That is, since the deteriorated layer  110  is a molten re-solidified layer as described above, it differs from other portions in crystal structure and diffracts light. Consequently, there is an area where the reflected light  42   a  passing through the deteriorated layer  110  is not received by the light receiving means  5 , and the light receiving means  5  does not receive light in the area shown by a broken line in  FIG. 5 . Light received by the light receiving means  5  is converted into an electric signal, which is then sent to the control means  6 . The control means  6  carries out image processing based on the electric signal from the light receiving means  5  and displays the obtained image on the display means  7 .  
       FIG. 6  shows an example of the image displayed on the display means  7 .  
      In  FIG. 6 , the area  110   a  displayed dark shows the above deteriorated layer  110  and the length in the vertical direction of the area  110   a  displayed dark shows the thickness of the deteriorated layer  110 . Thus, it is possible to check whether the deteriorated layer  110  is formed in the inside of the semiconductor wafer  10  or not without fail and also to check the thickness of the deteriorated layer  110 . The reason that the area  110   a  displayed dark in  FIG. 6  is curved and not straight is that the deteriorated layer  110  is not formed uniformly at a predetermined position in the thickness direction. As described above, by checking the existence of the deteriorated layer, the thickness of the deteriorated layer and a defect site such as the undulation of the semiconductor wafer or the like, re-processing may be carried out as the case may be and the analysis of the defect can be carried out effectively.  
      While the present invention has been described as related to the embodiment shown in the accompanying drawings, it is to be understood that various changes and modifications may be made in the invention without departing from the spirit and scope thereof. For example, in the illustrated embodiment, the light receiving means  5  is inclined at the same angle θ as the angle θ at which an infrared layer beam is applied by the light application means  4 . The light receiving means  5  may be arranged in the diffraction direction of the infrared laser beam so as to receive the diffracted infrared laser beam.