Abstract:
A machining apparatus utilizing a laser beam, the machining apparatus being capable of efficiently forming a deterioration zone of a required thickness along a division line. A laser beam from laser beam generation means is focused not to a single focused spot, but to at least two focused spots displaced in the direction of an optical axis.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a machining apparatus utilizing a laser beam and, more particularly, to a machining apparatus comprising holding means for holding a workpiece, laser beam generation means, and optical means for applying a laser beam from the laser beam generation means to the workpiece.  
       DESCRIPTION OF THE PRIOR ART  
       [0002]     It is well known, for example, in the production of a semiconductor device that many semiconductor circuits are formed on the face of a 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 then the wafer is divided to form the individual semiconductor circuits. Various methods utilizing a laser beam have been proposed for dividing the wafer.  
         [0003]     U.S. Pat. No. 6,211,488 and Japanese Patent Application Laid-Open No. 2001-277163 each disclose a wafer dividing method which comprises focusing a laser beam onto an intermediate portion in the thickness direction of a wafer, relatively moving the laser beam and the wafer along a division line, thereby forming a deterioration zone along the division line in the intermediate portion in the thickness direction of the wafer, and then exerting an external force on the wafer to break the wafer along the deterioration zone.  
         [0004]     The methods of dividing the wafer are not limited to the formation of the deterioration zone in the intermediate portion in the thickness direction of the wafer. It is also conceivable to form the deterioration zone along the division line in a region ranging from the back of the wafer to a depth of a predetermined thickness, or in a region ranging from the face of the wafer to a predetermined depth. In any of these cases, sufficiently precise breakage along the division line by exerting an external force on the wafer requires that the thickness of the deterioration zone, namely, the dimension of the deterioration zone in the thickness direction of the wafer, be rendered relatively large. Under certain circumstances, the deterioration zone needs to cover the entire thickness of the wafer. To increase the thickness of the deterioration zone, it is necessary to displace the position of the focused spot of the laser beam in the thickness direction of the wafer and repeatedly move the laser beam and the wafer relative to each other along the division line, because the deterioration zone is formed in the vicinity of the focused spot of the laser beam. Particularly when the thickness of the wafer is relatively large, therefore, a relatively long time is taken for forming the deterioration zone of a necessary thickness to break the wafer sufficiently precisely.  
       SUMMARY OF THE INVENTION  
       [0005]     A principal object of the present invention is to provide a novel and improved machining apparatus utilizing a laser beam, the apparatus being capable of efficiently forming a deterioration zone of a required thickness along a division line.  
         [0006]     According to the present invention, the principal object is attained by focusing a laser beam from laser beam generation means not to a single focused spot, but to at least two focused spots displaced in the direction of an optical axis.  
         [0007]     That is, according to the present invention, as a machining apparatus utilizing a laser beam, aimed at attaining the above-described principal object, there is provided a machining apparatus utilizing a laser beam, which comprises holding means for holding a workpiece, laser beam generation means, and optical means for applying a laser beam from the laser beam generation means to the workpiece held by the holding means, and which is characterized in that the optical means focuses the laser beam from the laser beam generation means to at least two focused spots displaced in the direction of an optical axis.  
         [0008]     In a preferred embodiment, the optical means includes at least two focusing lenses arranged in column in the direction of the optical axis and having different apertures. In another preferred embodiment, the optical means includes a splitter for separating the laser beam from the laser beam generation means into a first laser beam and a second laser beam; a plurality of mirrors for bringing the optical axis of the second laser beam into conformity with the optical axis of the first laser beam; diameter varying means for varying one of the diameters of the first laser beam and the second laser beam; and a common focusing lens. The diameter varying means preferably can adjust the degree to which the diameter is varied. The diameter varying means may be an expander for increasing the diameter.  
         [0009]     In the machining apparatus of the present invention, the laser beam from the laser beam generation means is focused to at least two focused spots displaced in the direction of the optical axis. Thus, deterioration zones can be simultaneously formed in at least two regions displaced in the thickness direction of the workpiece. Consequently, deterioration zones of a required thickness can be formed sufficiently efficiently. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic view showing a first embodiment of a machining apparatus constructed in accordance with the present invention.  
         [0011]      FIG. 2  is a schematic view showing a second embodiment of a machining apparatus constructed in accordance with the present invention.  
         [0012]      FIG. 3  is a schematic view showing a third embodiment of a machining apparatus constructed in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Preferred embodiments of the machining apparatus constructed in accordance with the present invention will now be described in greater detail by reference to the accompanying drawings.  
         [0014]      FIG. 1  schematically shows a first embodiment of a machining apparatus constructed in accordance with the present invention. The illustrated machining apparatus comprises holding means  4  for holding a workpiece  2 , laser beam generation means  6 , and optical means  8 .  
         [0015]     The holding means  4  is composed of, for example, a holding member  10 , which is a porous member or a member having a plurality of suction holes and/or suction grooves, and suction means (not shown) annexed to the holding member  10 . The holding means  4  may be of a type attracting the workpiece  2 , for example, a wafer, to the surface of the holding member  10  by suction.  
         [0016]     It is important for the laser beam generation means  6  to be one generating a laser beam capable of passing through the workpiece  2 . If the workpiece  2  is a 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, the laser beam generation means  6  can advantageously be composed of a YVO4 pulse laser or YAG pulse laser which generates a laser beam having a wavelength of, for example, 1064 nm. In the illustrated embodiment, the laser beam generation means  6  emits a pulse laser beam  12  toward the workpiece  2  held on the holding means  4 .  
         [0017]     The optical means  8 , interposed between the laser beam generation means  6  and the workpiece  2 , is composed of two focusing lenses  16  and  18  placed in column in the direction of an optical axis. The aperture of the focusing lens  16  is relatively large, while the aperture of the focusing lens  18  is relatively small. The lower surface of the focusing lens  16  is downwardly convex, and its upper surface is a flat surface. The lower surface of the focusing lens  18  is a flat surface, and its upper surface is upwardly convex. The lower surface of the focusing lens  18  is superposed on the upper surface of the focusing lens  16 . If desired, the focusing lens  16  and the focusing lens  18  can be formed integrally.  
         [0018]     In the above-described machining apparatus, the laser beam  12  from the laser beam generation means  6  is focused to two focused spots  20  and  22 , which are displaced in the direction of the optical axis in the workpiece  2 , by the optical focusing action of the optical means  8  composed of the two focusing lenses  16  and  18 . In further detail, part of the laser beam  12 , namely, its diametrically peripheral edge portion, passes through the focusing lens  16  alone, and is then focused to the focused spot  20  in the workpiece  2 . The remainder of the laser beam  12 , namely, its diametrically central portion, passes through the focusing lens  16  along with the focusing lens  18 , and is then focused to the focused spot  22  in the workpiece  2 . The focused spot  20  and the focused spot  22  are displaced from each other in the direction of the optical axis of the laser beam  12 . When the laser beam  12  is focused to the focused spots  20  and  22 , deterioration zones are formed in the workpiece  2  in the vicinity of the focused spots  20  and  22 , normally, in regions having certain widths, width W 1  and width W 2 , measured upwardly from the focused spots  20  and  22 . The width W 1  and the width W 2  may be substantially the same, or may be different from each other. The deterioration zone of the width W 1  and the deterioration zone of the width W 2  may be formed with spacing in the thickness direction of the workpiece  2 , as clearly shown in  FIG. 2 , or may be formed substantially continuously in the thickness direction of the workpiece  2 . Deterioration in the deterioration zone depends on the material for the workpiece  2  and the intensity of the laser beam  12  focused. Normally, the deterioration is melting/resolidification (namely, melting taking place when the laser beam  12  is focused, followed by solidification occurring after the focusing of the laser beam  12  is completed), voids, or cracks. Hence, when the combination of the laser beam generation means  6  and the optical means  8 , and the holding means  4  are relatively moved along a division line extending, for example, in the right-and-left direction in  FIG. 1 , there are formed, in the workpiece  2 , two deterioration zones extending continuously with the width W 1  and the width W 2  along the division line (if the spots constituting the focused spots  20  and  22  of the laser beam  12 , the spots adjacent in the direction of the relative movement, partially overlap), or many deterioration zones of the width W 1  and the width W 2  located at intervals along the division line (if the spots constituting the focused spots of the laser beam  12 , the spots adjacent in the direction of the relative movement, are located at intervals). That is, according to the first embodiment constituted in accordance with the present invention, the deterioration zones of the width W 1  and the width W 2  can be formed simultaneously, by the single laser beam generation means  6 , in two regions displaced in the thickness direction of the workpiece  2 .  
         [0019]     If the deterioration zones of the width W 1  and the width W 2  are not enough to divide the workpiece  2  sufficiently precisely along the division line, it is permissible to take the following measure: The combination of the laser beam generation means  6  and the optical means  8 , and the holding means  4  are relatively moved by a predetermined distance in the direction of the optical axis, namely, in the up-and-down direction in  FIG. 1 , whereby the focused spots  20  and  22  are displaced in the direction of the optical axis, accordingly in the thickness direction of the workpiece  2 . Furthermore, the combination of the laser beam generation means  6  and the optical means  8 , and the holding means  4  are relatively moved along the division line. By so doing, in addition to the previously formed deterioration zones, two deterioration zones extending continuously with the width W 1  and the width W 2  along the division line, or many deterioration zones of the width W 1  and the width W 2  located at intervals along the division line, are formed in regions displaced in the thickness direction of the workpiece  2 .  
         [0020]     In the embodiment shown in  FIG. 1 , the laser beam  12  is focused to the two focused spots  20  and  22 , which are displaced in the direction of the optical axis, by use of the optical means  8  including the two focusing lenses  16  and  18  having different apertures. If desired, the laser beam can be focused to three or more focused spots, which are displaced in the direction of the optical axis, by use of the optical means including three or more focusing lenses having different apertures.  
         [0021]      FIG. 2  shows a second embodiment of a machining apparatus constructed in accordance with the present invention. The machining apparatus illustrated in  FIG. 2  comprises holding means  104  for holding a workpiece  102 , laser beam generation means  106 , and optical means  108 . The holding means  104  and the laser beam generation means  106  may be of the same configuration as the holding means  4  and the laser beam generation means  6  in the embodiment shown in  FIG. 1 .  
         [0022]     The optical means  108  in the embodiment shown in  FIG. 2  is composed of a half mirror  124  which functions as a splitter; a mirror  126 ; a mirror  128 ; a half mirror  130 ; an expander  132  which functions as diameter varying means; and a common focusing lens  134 . The expander  132  includes two convex lenses  136  and  138 . A laser beam  112  from the laser beam generation means  106  is separated into two laser beams, i.e., a first laser beam  112   a  which passes through the half mirror  124  and travels straight, and a second laser beam  112   b  which is reflected by the half mirror  124  and travels in a changed direction, a substantially perpendicular direction. The first laser beam  112   a  passes through the expander  132 , whereby the first laser beam  112   a  is turned into a form in which its diameter is varied, in more detail, its diameter is gradually increased as the first laser beam  112   a  goes farther away from the expander  132 . Then, the first laser beam  112   a  passes through the half mirror  130 , and is focused to a focused spot  120  within the workpiece  102  by the focusing lens  134 . On the other hand, the second laser beam  112   b  is reflected by the mirror  126 , the mirror  128  and the half mirror  130 , to be thereby changed in direction to a substantially perpendicular direction upon each reflection, and to be brought into a state in which its optical axis conforms with the optical axis of the first laser beam  112   a . Then, the second laser beam  112   b  is focused to a focused spot  122  within the workpiece  102  by the focusing lens  134 . The focused spot  120  and the focused spot  122  are displaced with respect to each other in the direction of the optical axis of the first laser beam  112   a  and the second laser beam  112   b . The position of the focused spot  120  of the first laser beam  112   a  can be adjusted appropriately, for example, by moving the expander  132  in the direction of the optical axis, or by moving the lens  136  or  138  of the expander  132  in the direction of the optical axis. If desired, a single convex lens may be used instead of the expander  132 , and may be disposed such that the focal point of such a convex lens will come upstream of the focusing lens  134 . By this measure, the laser beam can pass through the focal point of the convex lens, have its diameter gradually increased, and enter the focusing lens  134 .  
         [0023]     In the machining apparatus shown in  FIG. 2  as well, deterioration zones are formed in the workpiece  102  in the vicinity of the focused spots  120  and  122 , normally, in regions having certain widths, width W 1  and width W 2 , measured upwardly from the focused spots  120  and  122 . Hence, when the combination of the laser beam generation means  106  and the optical means  108 , and the holding means  104  are relatively moved along a division line extending, for example, in the right-and-left direction in  FIG. 2 , there are formed, in the workpiece  102 , two deterioration zones extending continuously with the width W 1  and the width W 2  along the division line, or many deterioration zones of the width W 1  and the width W 2  located at intervals along the division line. If the deterioration zones of the width W 1  and the width W 2  are not enough to divide the workpiece  102  sufficiently precisely along the division line, it is permissible to take the following measure: The combination of the laser beam generation means  106  and the optical means  108 , and the holding means  104  are relatively moved by a predetermined distance in the direction of the optical axis, namely, in the up-and-down direction in  FIG. 2 , whereby the focused spots  120  and  122  are displaced in the direction of the optical axis, accordingly in the thickness direction of the workpiece  102 . Furthermore, the combination of the laser beam generation means  106  and the optical means  108 , and the holding means  104  are relatively moved along the division line. By so doing, in addition to the previously formed deterioration zones, two deterioration zones extending continuously with the width W 1  and the width W 2  along the division line, or many deterioration zones of the width W 1  and the width W 2  located at intervals along the division line, are formed in regions displaced in the thickness direction of the workpiece  102 .  
         [0024]      FIG. 3  shows a third embodiment of a machining apparatus constructed in accordance with the present invention. The machining apparatus illustrated in  FIG. 3  comprises holding means  204  for holding a workpiece  202 , laser beam generation means  206 , and optical means  208 . The holding means  204  and the laser beam generation means  206  may be of the same configuration as the holding means  4  and the laser beam generation means  6  in the embodiment shown in  FIG. 1 .  
         [0025]     The optical means  208  in the embodiment shown in  FIG. 3  is composed of a half mirror  224  which functions as a first splitter; a half mirror  225  which functions as a second splitter; a mirror  226 ; a mirror  227 ; a mirror  228 ; a mirror  229 ; a half mirror  230 ; a half mirror  231 ; an expander  232  which functions as a first diameter varying means; an expander  233  which functions as a second diameter varying means; and a common focusing lens  234 . The expander  232  includes two convex lenses  236  and  237 . The expander  233  also includes two convex lenses  238  and  239 . A laser beam  212  from the laser beam generation means  206  is separated into two laser beams, i.e., a first laser beam  212   a  which passage through the half mirror  224  and travels straight, and a second laser beam  212   b  which is reflected by the half mirror  224  and travels in a changed direction, a substantially perpendicular direction. The first laser beam  212   a  passes through the half mirror  225  and proceeds afterwards. On this occasion, a third laser beam  212   c , which is reflected by the half mirror  225  substantially perpendicularly, is separated from the first laser beam  212   a . By passing through the expander  232 , the first laser beam  212   a  is turned into a form in which its diameter is varied, in more detail, its diameter is gradually increased as the first laser beam  212   a  goes farther away from the expander  232 . Then, the first laser beam  212   a  passes through the half mirrors  230  and  231 , and is focused by the focusing lens  234  to a focused spot  220  within the workpiece  202 . The second laser beam  212   b  is reflected by the mirror  226  and the mirror  227 , to be thereby changed in direction to a substantially perpendicular direction upon each reflection, and is then passed through the expander  233 . As a result, the second laser beam  212   b  is turned into a form in which its diameter is varied, in more detail, its diameter is gradually increased as the second laser beam  212   b  goes farther away from the expander  233 . Then, the second laser beam  212   b  is reflected by the half mirror  231  to undergo a change of direction to a substantially perpendicular direction, and also to have its optical axis brought into conformity with the optical axis of the first laser beam  212   a . Then, the second laser beam  212   b  is focused by the focusing lens  234  to a focused spot  222  within the workpiece  202 . The third laser beam  212   c  is reflected by the mirror  228 , the mirror  229  and the half mirror  230 , to be thereby changed in direction to a substantially perpendicular direction upon each reflection, and to be brought into a state in which its optical axis conforms with the optical axis of the first laser beam  212   a . Then, the third laser beam  212   c  passes through the half mirror  231 , and is focused by the focusing lens  234  to a focused spot  223  within the workpiece  202 . The focused spot  220 , the focused spot  222 , and the focused spot  223  are displaced with respect to each other in the direction of the optical axes of the first laser beam  212   a , the second laser beam  212   b  and the third laser beam  212   c . The position of the focused spot  220  of the first laser beam  212   a  can be adjusted appropriately, for example, by moving the expander  232  in the direction of the optical axis, or by moving the lens  236  or  237  of the expander  232  in the direction of the optical axis. Similarly, the position of the focused spot  222  of the second laser beam  212   b  can be adjusted appropriately, for example, by moving the expander  233  in the direction of the optical axis, or by moving the lens  238  or  239  of the expander  233  in the direction of the optical axis.  
         [0026]     In the machining apparatus shown in  FIG. 3 , deterioration zones are formed in the workpiece  202  in the vicinity of the focused spots  220 ,  222  and  223 , normally, in regions having certain widths, a width W 1 , a width W 2  and a width W 3 , measured upwardly from the focused spots  220 ,  222  and  223 . Hence, when the combination of the laser beam generation means  206  and the optical means  208 , and the holding means  204  are relatively moved along a division line extending, for example, in the right-and-left direction in  FIG. 3 , there are formed, in the workpiece  202 , three deterioration zones extending continuously with the width W 1 , the width W 2  and the width W 3  along the division line, or many deterioration zones of the width W 1 , the width W 2  and the width W 3  located at intervals along the division line. If the deterioration zones of the width W 1 , the width W 2  and the width W 3  are not enough to divide the workpiece  202  sufficiently precisely along the division line, it is permissible to take the following measure: The combination of the laser beam generation means  206  and the optical means  208 , and the holding means  204  are relatively moved by a predetermined distance in the direction of the optical axis, namely, in the up-and-down direction in  FIG. 3 , whereby the focused spots  220 ,  222  and  223  are displaced in the direction of the optical axis, accordingly in the thickness direction of the workpiece  202 . Furthermore, the combination of the laser beam generation means  206  and the optical means  208 , and the holding means  204  are relatively moved along the division line. By so doing, in addition to the previously formed deterioration zones, three deterioration zones extending continuously with the width W 1 , the width W 2  and the width W 3  along the division line, or many deterioration zones of the width W 1 , the width W 2  and the width W 3  located at intervals along the division line are formed in regions displaced in the thickness direction of the workpiece  202 .