Abstract:
A processing method and apparatus using a laser beam, which can expel as much debris, produced upon application of a laser beam, as possible out of a workpiece to minimize the debris remaining on side surfaces of grooves. The processing method and apparatus superpose a first laser beam ( 30 A) having a width D 1  of a focal spot, and a second laser beam ( 30 B) having a focal spot ( 32 B) upstream, in a beam advancing direction, of the focal spot of the first laser beam, and having a width D 2  of a beam spot at the focal spot of the first laser beam, D 2  being larger than D 1  (D 2 &gt;D 1 ); and apply the superposed laser beams to the workpiece.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a processing method and apparatus using a laser beam and, more particularly, a processing method and apparatus which move a workpiece, such as a semiconductor wafer, and a laser beam relative to each other while applying the laser beam to the workpiece.  
       DESCRIPTION OF THE PRIOR ART  
       [0002]     In the production of a semiconductor device, for example, it is well known that many rectangular regions are defined by streets arranged in a lattice pattern on the face of a semiconductor wafer including a substrate, such as a silicon substrate, a sapphire substrate, a silicon carbide substrate, a lithium tantalate substrate, a glass substrate, or a quartz substrate, and a semiconductor circuit is formed in each of such rectangular regions. Then, the semiconductor wafer is divided along the streets to obtain the individual semiconductor circuits.  
         [0003]     As methods and apparatuses for dividing the semiconductor wafer along the streets, processing methods and apparatuses using a laser beam have been proposed in recent times. JP-B 62-39539 and JP-A 6-120334 disclose processing methods and apparatuses which move a semiconductor wafer and a laser beam relative to each other along the streets on the face of the workpiece while applying the laser beam to the streets to form grooves along the streets on the face of the semiconductor wafer, and then exert an external force on the semiconductor wafer to rupture the semiconductor wafer along the grooves.  
         [0004]     According to experiments conducted by the inventors, however, if the semiconductor wafer is divided to produce the individual semiconductor circuits by the above-described processing methods and apparatuses using a laser beam, the deflective strength of the products is relatively low. Such decreases in the deflective strength have been found to result from the following facts: Upon application of the laser beam to the workpiece, the workpiece is melted at the site of laser beam application. According to the conventional processing methods and apparatuses, so-called debris generated by melting is not fully removed from the workpiece, but adheres to and remains on the side surfaces of the resulting grooves, with the result that heat distortion due to heat transmitted from the debris is caused to the neighborhood of the grooves.  
       SUMMARY OF THE INVENTION  
       [0005]     It is a principal object of the present invention, therefore, to provide a processing method and apparatus using a laser beam, which can expel as much debris, produced upon application of the laser beam, as possible out of the semiconductor wafer to minimize the debris remaining on the side surfaces of the grooves, thereby avoiding or suppressing the generation of heat distortion due to the debris, and thus sufficiently avoiding or suppressing the decrease in the deflective strength of the workpiece.  
         [0006]     According to the present invention, the above principal object is attained by superposing a first laser beam having a width D 1  of a focal spot, and a second laser beam having a focal spot upstream, in a beam advancing direction, of the focal spot of the first laser beam, and having a width D 2  of a beam spot at the focal spot of the first laser beam, D 2  being larger than D 1  (D 2 &gt;D 1 ), and applying the superposed laser beams to a workpiece.  
         [0007]     According to a first aspect of the present invention, there is provided, as a processing method for attaining the above principal object, a processing method which moves a workpiece and a laser beam relative to each other while applying the laser beam to the workpiece, and comprising superposing a first laser beam having a width D 1  of a focal spot, and a second laser beam having a focal spot upstream, in a beam advancing direction, of the focal spot of the first laser beam, and having a width D 2  of a beam spot at the focal spot of the first laser beam, D 2  being larger than D 1  (D 2 &gt;D 1 ), and applying the superposed laser beams to the workpiece.  
         [0008]     According to a second aspect of the present invention, there is provided, as a processing apparatus for attaining the above principal object, a processing apparatus comprising holding means for holding a workpiece, laser beam application means for applying a laser beam to the workpiece held on the holding means, and moving means for moving the holding means and the laser beam application means relative to each other, and  
         [0009]     wherein the laser beam application means superposes a first laser beam having a width D 1  of a focal spot, and a second laser beam having a focal spot upstream, in a beam advancing direction, of the focal spot of the first laser beam, and having a width D 2  of a beam spot at the focal spot of the first laser beam, D 2  being larger than D 1  (D 2 &gt;D 1 ), and applying the superposed laser beams to the workpiece.  
         [0010]     It is preferred that at the focal spot of the first laser beam, the beam spot shape of the first laser beam is an elliptic shape, the beam spot shape of the second laser beam is a circular shape, and the diameter of the circular shape is smaller than the major diameter of the elliptic shape and is larger than the minor diameter of the elliptic shape. It is advantageous to align the focal spot of the first laser beam with the face of the workpiece. Preferably, the laser beam application means includes a common laser beam source for generating a parallel laser beam, splitting means for splitting the laser beam from the laser beam source into the first laser beam and the second laser beam, nonparallel lens means for converting the second laser beam into a nonparallel laser beam, and focusing lens means for focusing the first laser beam and the second laser beam, and the focusing lens means is composed of a first cylindrical lens and a second cylindrical lens, the focusing directions of the first cylindrical lens and the second cylindrical lens being orthogonal to each other.  
         [0011]     According to the processing method and apparatus of the present invention, the workpiece is melted by the application of the first laser beam and the second laser beam in the superposed state. Debris, which has been formed by the melting and is about to adhere to and remain on the side surfaces of the grooves, is expelled out of the grooves by the action of a widthwise outward portion of the second laser beam present beyond the width of the beam spot of the first laser beam. Thus, heat distortion due to remaining debris is avoided or suppressed. Consequently, the decrease in the deflective strength of the workpiece is fully avoided or suppressed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic view showing a preferred embodiment of a processing apparatus constructed according to the present invention.  
         [0013]      FIG. 2  is a side view of a focusing means in the processing apparatus shown in  FIG. 1 .  
         [0014]      FIG. 3  is an enlarged view showing the neighborhood of the focal spots of a first laser beam and a second laser beam in the processing apparatus shown in  FIG. 1 .  
         [0015]      FIG. 4  is a schematic view showing the beam spot shape of the first laser beam and the beam spot shape of the second laser beam on the surface of a workpiece.  
         [0016]      FIG. 5  is a sectional view showing a groove formed in the workpiece. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     Preferred embodiments of the processing method and apparatus constituted in accordance with the present invention will be described in further detail by reference to the accompanying drawings.  
         [0018]      FIG. 1  schematically shows the preferred embodiment of the processing apparatus constructed according to the present invention. The illustrated processing apparatus is composed of a holding means  4  for holding a workpiece  2  such as a semiconductor wafer, and a laser beam application means indicated entirely at the numeral  6 . The holding means  4  may be a vacuum attraction chuck which is composed of, for example, a porous member or a member having a plurality of suction holes and/or grooves, and which is brought into selective communication with a vacuum source (not shown). The holding means  4  is moved by a suitable drive means (not shown) in a right-and-left direction in  FIG. 1  and a direction perpendicular to the sheet face of  FIG. 1 , and is also rotated about the axis of rotation extending in an up-and-down direction in  FIG. 1 . On the other hand, the laser beam application means  6  is moved in the up-and-down direction in  FIG. 1 , whereby the state of application of a laser beam to the workpiece  2  is adjusted.  
         [0019]     The laser beam application means  6  in the illustrated embodiment is composed of a common laser beam source  8 , and an optical means  10  for applying a laser beam delivered from the laser beam source  8  to the workpiece  2 . The laser beam source  8  may be a YVO 4  pulsed laser or a YAG pulsed laser which generates a parallel laser beam, for example, with a wavelength of 532 nm, 355 nm or 266 nm. The repetition frequency of the laser beam may be 10 kHz, and its average output may be of the order of 3W to 5W.  
         [0020]     The optical means  10  for applying a parallel laser beam generated by the laser beam source  8  to the workpiece  2  includes a splitting means  12 , which can be composed of a half mirror, a first reflecting mirror  14 , a dielectric mirror  16 , a focusing means  18 , a second reflecting mirror  20 , an expander  22 , and a nonparallel lens means  24  which can be composed of a finely diameter-reducing lens. The focusing means  18  is composed of a first cylindrical lens  26  and a second cylindrical lens  28 . As will be clearly understood by reference to  FIG. 1  and  FIG. 2  as a side view of the focusing means  18 , the focusing direction of the first cylindrical lens  26  and the focusing direction of the second cylindrical lens  28  are orthogonal to each other. That is, the focusing direction of the first cylindrical lens  26  is the right-and-left direction in  FIG. 1  and a direction perpendicular to the sheet face of  FIG. 2 , while the focusing direction of the second cylindrical lens  28  is the direction perpendicular to the sheet face of  FIG. 1  and a right-and-left direction in  FIG. 2 .  
         [0021]     With further reference to  FIG. 1 , a parallel laser beam  30  projected from the laser beam source  8  is split by the splitting means  12  into a first laser beam  30 A and a second laser beam  30 B. Then, the first laser beam  30 A is reflected by the first reflecting mirror  14 , passed through the dielectric mirror  16 , and entered into the focusing means  18 . Then, as is clearly illustrated in  FIG. 3 , the first laser beam  30 A is focused to a focal spot  32 A by the focusing action of the first cylindrical lens  26  and the second cylindrical lens  28  of the focusing means  18 . A beam spot shape at the focal spot  32 A is an elliptic shape having a minor diameter (width) D 1  and a major diameter (length in a direction of relative movement) D 3 , as shown in  FIG. 4 . Advantageously, the minor diameter D 1  is of the order of 15 μm, and the major diameter D 3  is of the order of 200 μm. The focal spot  32 A of the first laser beam  30 A preferably lies on the face of the workpiece  2  or the neighborhood of the face.  
         [0022]     On the other hand, the second laser beam  30 B is reflected by the reflecting mirror  20 , and entered into the expander  22  to be increased in the beam diameter by the expander  22 . Then, the second laser beam  30 B is incident on the nonparallel lens means  24  to be converted into a nonparallel laser beam progressively decreasing in diameter toward the front in the advancing direction. Then, the second laser beam  30 B is reflected by the dielectric mirror  16  and entered into the focusing means  18 . As is clearly illustrated in  FIG. 3 , the second laser beam  30 B is focused to a focal spot  32 B by the focusing action of the first cylindrical lens  26  and the second cylindrical lens  28  of the focusing means  18 . It is important that the focal spot  32 B of the second laser beam  30 B be located upstream, in the beam advancing direction, of the focal spot  32 A of the first laser beam  30 A by a predetermined distance x. The distance x may be of the order of 20 μm. The beam spot shape of the second laser beam  30 B at the focal spot  32 B is a circular shape. The second laser beam  30 B further advances from the focal spot  32 B, is superposed on the first laser beam  30 A, and is projected to the face of the workpiece  2 . During this process, as the second laser beam  30 B goes farther from the focal spot  32 B, its beam spot diameter is progressively increased. At the focal spot  32 A of the first laser beam  30 A, the beam spot of the second laser beam  30 B has a circular shape of a diameter (width and length) D 2 . At the focal spot  32 A of the first laser beam  30 A, the beam spot diameter D 2  (i.e., width) of the second laser beam  30 B is importantly larger than the aforementioned minor diameter D 1  (i.e., width) of the beam spot of the first laser beam  30 A, and is preferably smaller than the aforementioned major diameter D 3  of the beam spot of the first laser beam  30 A. The diameter D 2  of the beam spot of the second laser beam  30 B may be of the order of 20 μm.  
         [0023]     When the holding means  4  holding the workpiece  2  is moved in the right-and-left direction in  FIG. 1 , with the first laser beam  30 A and the second laser beam  30 B being applied to the face of the workpiece  2  in the above-described manner, a groove  34  having a sectional shape as illustrated in  FIG. 5  and extending in the right-and-left direction in  FIG. 1  is formed in the face of the workpiece  2 . The formation of the groove  34  will be described in further detail. According to the processing method and apparatus constituted in accordance with the present invention, the face of the workpiece  2  is melted in the region of superposition of the first laser beam  30 A and the second laser beam  30 B to form the groove  34 . Debris generated by the melting of the workpiece  2  is about to adhere to the side surface of the groove  34  and remain there. However, a widthwise outward portion of the second laser beam  30 B present beyond the width of the beam spot of the first laser beam  30 A acts on the debris, which is about to adhere to the side surface of the groove  34  and remain there, thereby effectively expelling the debris outside. Thus, the groove  34 , where the adhesion and remaining of debris have been fully avoided or suppressed, is formed, so that the generation of heat distortion due to debris can be fully avoided or suppressed. The workpiece  2  having the grooves  34  formed therein can be broken along the grooves  34  by exerting an external force, as appropriate, on the workpiece  2 .  
         [0024]     If, on the other hand, the laser beam  30  from the laser beam source  8  is not split into the first laser beam  30 A and the second laser beam  30 B, but is caused to be incident on the focusing means  18  via the first reflecting mirror  14  and the dielectric mirror  16 , for example, and is applied to the workpiece  2 , there is a tendency that debris  36  adheres to the side surface of the groove  34  and remains there, as indicated by a dashed double-dotted line in  FIG. 5 .  
         [0025]     While the preferred embodiments of the present invention have been described in detail by reference to the accompanying drawings, it is to be understood that the invention is not limited to such embodiments, but various changes and modifications may be made without departing from the scope of the invention.