Abstract:
Disclosed herein is a processing apparatus using a laser beam, which includes a holder for holding a workpiece, and laser beam applicator for irradiating the workpiece, held by the holder, with a pulsed laser beam capable of passing through the workpiece, thereby deteriorating the workpiece. The laser beam applicator includes a pulsed laser beam oscillator and a transmitter/focuser for transmitting and focusing the pulsed laser beam oscillated by the pulsed laser beam oscillator. The transmitter/focuser focuses the pulsed laser beam, with a time difference provided, to at least two focus points that are displaced in the optical axis direction.

Description:
FIELD OF THE INVENTION 
   This invention relates to a processing apparatus using a laser beam and, more particularly, a processing apparatus comprising holding means for holding a workpiece, and laser beam application means for irradiating the workpiece, which is held by the holding means, with a pulsed laser beam capable of passing through the workpiece, thereby deteriorating the workpiece. 
   DESCRIPTION OF THE PRIOR ART 
   In the production of a semiconductor device, for example, it is well known that many semiconductor circuits are formed on the face of a wafer, including a suitable 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 produce individual semiconductor circuits, namely, semiconductor devices. Various modes using a laser beam are proposed as methods for dividing the wafer. 
   U.S. Pat. No. 6,211,488 and Japanese Patent Application Laid-Open No. 2001-277163 each disclose a wafer dividing method which focuses a pulsed laser beam onto an intermediate portion in the thickness direction of a wafer, moves the pulsed laser beam and the wafer relative to each other along a division line to generate a deterioration region along the division line in the intermediate portion in the thickness direction of the wafer, and then exerts an external force on the wafer to break the wafer along the deterioration region. 
   It is conceivable not only to generate the deterioration region in the intermediate portion in the thickness direction of the wafer, but also to form a deterioration region along the division line in a portion ranging from the back of the wafer to a predetermined depth, or in a portion ranging from the face of the wafer to a predetermined depth, instead of or in addition to the intermediate portion in the thickness direction of the wafer. In each case, in order to exert an external force on the wafer to break the wafer along the division line sufficiently precisely, it is necessary to provide a relatively large thickness of the deterioration region, namely, a relatively large dimension of the deterioration region in the thickness direction of the wafer. To increase the thickness of the deterioration region, there is need to displace the position of the focus point of the pulsed laser beam in the thickness direction of the wafer, and repeatedly move the pulsed laser beam and the wafer relative to each other along the division line, because the deterioration region is generated in the vicinity of the focus point of the pulsed laser beam. If the thickness of the wafer is relatively large, therefore, it takes a relatively long time to generate the deterioration region of a necessary thickness to break the wafer sufficiently precisely. 
   In an attempt to solve the above-described problems, the specification and drawings of Japanese Patent Application No. 2003-273341, filed by the applicant (the assignee) of the present application, disclose a processing apparatus adapted to focus a pulsed laser beam to at least two focus points displaced in the direction of the optical axis. According to such a processing apparatus, deterioration regions can be simultaneously generated on at least two sites displaced in the thickness direction of a workpiece, namely, a wafer. However, the processing apparatus is still not sufficiently satisfactory, but poses the following problems to be solved: Deterioration at one of the two focus points, more specifically, the focus point at a shorter distance from pulsed laser beam oscillation means, inhibits application of the pulsed laser beam to the other focus point, more specifically, the focus point at a longer distance from the pulsed laser beam oscillation means, thereby inhibiting the generation of desired deterioration. 
   SUMMARY OF THE INVENTION 
   It is a principal object of the present invention, therefore, to provide a novel and improved processing apparatus using a laser beam, the processing apparatus being capable of generating deterioration, as desired, on at least two focus points displaced in the thickness direction of a workpiece, while ingeniously avoiding a situation where deterioration of one of the two focus points inhibits application of a pulsed laser beam to the other focus point, thereby generating a deterioration region of a required thickness in the workpiece. 
   We, the inventors, diligently conducted studies and experiments, and have found the following facts: A pulsed laser beam is focused, with some time difference provided, to at least two focus points displaced in the optical axis direction of the pulsed laser beam, accordingly, displaced in the thickness direction of a workpiece. As a result, a situation where deterioration at one of the focus points inhibits application of the pulsed laser beam to the other focus point can be avoided. Thus, the aforementioned principal object can be attained. 
   That is, according to the present invention, as a processing apparatus using a laser beam for attaining the above-mentioned principal object, there is provided a processing apparatus using a laser beam, which comprises holding means for holding a workpiece, and laser beam application means for irradiating the workpiece, held by the holding means, with a pulsed laser beam capable of passing through the workpiece, thereby deteriorating the workpiece, the laser beam application means including pulsed laser beam oscillation means and transmitting/focusing means for transmitting and focusing the pulsed laser beam oscillated by the pulsed laser beam oscillation means, wherein the transmitting/focusing means focuses the pulsed laser beam, with a time difference provided, to at least two focus points displaced in the optical axis direction of the pulsed laser beam. 
   Preferably, the transmitting/focusing means focuses the laser beam to the focus point at a longer distance from the pulsed laser beam oscillation means earlier than the focus point at a shorter distance from the pulsed laser beam oscillation means. The time difference during focusing between the focus points adjacent in the optical axis direction is preferably not smaller than one pulse duration of the pulsed laser beam, but not larger than a time interval between successive pulses of the pulsed laser beam. In a preferred embodiment, the transmitting/focusing means includes a splitter for dividing the pulsed laser beam from the pulsed laser beam oscillation means into a first pulsed laser beam and a second pulsed laser beam; a plurality of mirrors for aligning the optical axis of the second pulsed laser beam with the optical axis of the first pulsed laser beam; diameter changing means for changing the diameter of one of the first pulsed laser beam and the second pulsed laser beam; and a common focusing lens. Preferably, the transmitting/focusing means includes optical path length increasing means for increasing the optical path length of one of the first pulsed laser beam and the second pulsed laser beam. The optical path length increasing means preferably includes an optical fiber or a plurality of mirrors. 
   In the processing apparatus using a laser beam according to the present invention, a time difference is caused to exist in the focusing of the pulsed laser beam at each of at least two focus points. Hence, a situation where deterioration generated at one of the focus points inhibits application of the pulsed laser beam to the other focus point is avoided. Consequently, deterioration, as desired, can be generated on at least two focus points. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing a preferred embodiment of a processing apparatus constructed in accordance with the present invention. 
       FIG. 2  is a graph showing a time difference during focusing between a first pulsed laser beam and a second pulsed laser beam in the processing apparatus of  FIG. 1 . 
       FIG. 3  is a schematic view showing another embodiment of a processing apparatus constructed in accordance with the present invention. 
       FIG. 4  is a graph showing a time difference during focusing among a first pulsed laser beam, a second pulsed laser beam, and a third pulsed laser beam in the processing apparatus of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of a processing apparatus using a laser beam, which is constructed in accordance with the present invention, will now be described in greater detail by reference to the accompanying drawings. 
     FIG. 1  schematically shows a preferred embodiment of a processing apparatus constructed in accordance with the present invention. The illustrated processing apparatus comprises holding means  4  for holding a workpiece  2 , and laser beam application means indicated entirely at the numeral  6 . 
   The holding means  4  is composed of a holding member  8  formed, for example, from a porous member or having a plurality of suction holes or grooves formed therein, and suction means (not shown) annexed to the holding member  8 . The holding means  4  may be of a form in which the workpiece  2 , for example, a wafer, is attracted to the surface of the holding member  8  by suction. 
   The laser beam application means  6  includes pulsed laser beam oscillation means  10 , and transmitting/focusing means  12  for transmitting and focusing a pulsed laser beam oscillated by the pulsed laser beam oscillation means  10 . Importantly, the pulsed laser beam oscillation means  10  oscillates a pulsed laser beam  14  which can pass through the workpiece  2 . If the workpiece  2  is a wafer including a silicon substrate, a sapphire substrate, a silicon carbide substrate, a lithium tantalate substrate, a glass substrate, or a quartz substrate, the pulsed laser beam oscillation means  10  can be advantageously formed from a YVO4 pulsed laser oscillator or a YAG pulsed laser oscillator which oscillates the pulsed laser beam  14  having a wavelength of, for example, 1064 nm. 
   With reference to  FIG. 1 , the transmitting/focusing means  12  in the laser beam application means  6  is interposed between the pulsed laser beam oscillation means  10  and the workpiece  2  held on the holding means  4 . The transmitting/focusing means  12  in the illustrated embodiment includes a half mirror  16  functioning as a splitter, a mirror  18 , a mirror  20 , a half mirror  22 , diameter changing means  24  disposed between the half mirror  16  and the half mirror  22 , and optical path length increasing means  26  disposed between the mirror  18  and the mirror  20 . The diameter changing means  24  is composed of an expander having two convex lenses  28  and  30 . The optical path length increasing means  26  for increasing the optical path between the mirror  18  and the mirror  20  by, for example, several meters can be composed of an optical fiber extending, for example, over several meters. Alternatively, the optical path length increasing means  26  can be composed of a plurality of mirrors instead of, or in addition to, the optical fiber. The transmitting/focusing means  12  further includes a focusing lens  32  for focusing the pulsed laser beam  14 . 
   In the above-described processing apparatus, the pulsed laser beam  14  oscillated from the pulsed laser beam oscillation means  10  is separated by the half mirror  16  into two pulsed laser beams  14   a  and  14   b , namely, the first pulsed laser beam  14   a  which passes through the half mirror  16  and advances straightly, and the second pulsed laser beam  14   b  which is reflected by the half mirror  16  and changed in direction to a substantially perpendicular direction. The first pulsed laser beam  14   a  passes through the diameter changing means  24 , and thereby has its diameter changed, more specifically, is converted into a form in which its diameter gradually increases as the first pulsed laser beam  14   a  goes farther from the diameter changing means  24 . Then, the first pulsed laser beam  14   a  passes through the half mirror  22 , and is focused by the focusing lens  32  to a focus point  34   a  in the workpiece  2 . On the other hand, the second pulsed laser beam  14   b  is reflected by the mirror  18 , the mirror  20 , and the half mirror  22  to be changed in direction to a substantially perpendicular direction at each time, and is finally brought into a state where its optical axis aligns with the optical axis of the first pulsed laser beam  14   a . Then, the second pulsed laser beam  14   b  is focused by the focusing lens  32  to a focus point  34   b  in the workpiece  2 . As clearly shown in  FIG. 1 , the focus point  34   a  and the focus point  34   b  are displaced in the optical axis direction of the first and second pulsed laser beams  14   a  and  14   b , and the focus point  34   a  is located more remotely from the pulsed laser beam oscillation means  10  than the focus point  34   b . The position of the focus point  34   a  can be adjusted, as appropriate, by moving the expander, which constitutes the diameter changing means  24 , in the optical axis direction, or by moving the convex lens  28  or  30  of the expander in the optical axis direction. 
   In the illustrated embodiment, the optical path of the second pulsed laser beam  14   b  extends to the half mirror  22  via the half mirror  16 , the mirror  18 , the optical path length increasing means  26 , and the mirror  20 , so that the optical path length of the second pulsed laser beam  14   b  is increased by, for example, several meters as compared with the optical path length of the first pulsed laser beam  14   a . Thus, the second pulsed laser beam  14   b  arrives at the focus point  34   b  later than a point in time, when the first pulsed laser beam  14   a  reaches the focus point  34   a , by a time required for the second pulsed laser beam  14   b  to pass over the increased optical path length. As shown in  FIG. 2 , it is preferred that a time difference dt between the point in time, when the first pulsed laser beam  14   a  arrives at the focus point  34   a , and a point in time, when the second pulsed laser beam  14   b  arrives at the focus point  34   b , is not smaller than one pulse duration (pulse width) Δt of the pulsed laser beam  14   a  (or  14   b ), but not larger than a time interval gt between successive pulses of the pulsed laser beam  14   a  (or  14   b ), and that each pulse of the second pulsed laser beam  14   b  is located between respective pulses of the first pulsed laser beam  14   a . For example, let each pulse duration of the pulsed laser beams  14   a  and  14   b  be Δt (seconds), and the repetition frequency of the pulsed laser beams  14   a  and  14   b  be W (Hz), and the difference between the optical path length (L 1 ) of the first pulsed laser beam  14   a  and the optical path length (L 2 ) of the second pulsed laser beam  14   b  be DL. Since the velocity of light is c (=3×10 8  m/second), the above time difference dt is (L 2 −L 1 )÷c. Thus, the second optical path length L 2  may be increased so as to satisfy Δt×c≦(L 2 −L 1 )≦( 1 /W−Δt)×c. For example, if the pulse duration Δt is 10 ns, and the pulse repetition frequency W is 100 k (Hz), then it is recommendable to increase the optical path length L 2  of the second pulsed laser beam  14   b  by about 3 m as compared with the optical path length L 1  of the first pulsed laser beam  14   a.    
   When the first pulsed laser beam  14   a  is focused to the focus point  34   a , deterioration is generated in the workpiece  2 , because of this focusing, in the vicinity of the focus point  34   a , usually, in a region having some width W 1  from the focus point  34   a  upward. When the second pulsed laser beam  14   b  is focused to the focus point  34   b , deterioration is generated in the workpiece  2  in the vicinity of the focus point  34   b , usually, in a region having some width W 2  from the focus point  34   b  upward. If the arrival of the first pulsed laser beam  14   a  at the focus point  34   a , and the arrival of the second pulsed laser beam  14   b  at the focus point  34   b  are substantially simultaneous, there is a tendency that the first pulsed laser beam  14   a  focused to the focus point  34   a  is adversely affected by the deterioration generated at the focus point  34   b , and the generation of desired deterioration in the vicinity of the focus point  34   a  is inhibited. Assume, by contrast, that the first pulsed laser beam  14   a  is focused to the focus point  34   a  to start the generation of deterioration in the vicinity of the focus point  34   a , and then, with a time difference provided, the second pulsed laser beam  14   b  is focused to the focus point  34   b  to start the generation of deterioration in the vicinity of the focus point  34   b . In this case, the situation where the focusing of the first pulsed laser beam  14   a  to the focus point  34   a  is inhibited by the generation of deterioration by the second pulsed laser beam  14   b  in the vicinity of the focus point  34   b  is fully avoided, and desired deterioration can be generated in the vicinity of the focus point  34   a  and in the vicinity of the focus point  34   b . The above-mentioned deterioration in the workpiece  2  occurs as melting and resolidification (namely, melting when the laser beams  14   a  and  14   b  are focused, and solidification after the focusing of the pulsed laser beams  14   a  and  14   b  is completed), and appears as voids or cracks, although this is dependent on the material for the workpiece  2 , or the intensity of the pulsed laser beams  14   a  and  14   b  focused. When the laser beam application means  6  and the holding means  4  are relatively moved along the division line extending, for example, in the right-and-left direction in  FIG. 1 , two deterioration portions continuously extending along the division line with a width W 1  and a width W 2  (if the spots of the laser beams  14   a  and  14   b  at the focus points  34   a  and  34   b , the spots being adjacent in the relative movement direction, overlap partially), or many deterioration portions located with spacing along the division line with the width W 1  and the width W 2  (if the spots of the laser beams  14   a  and  14   b  at the focus points  34   a  and  34   b , the spots being adjacent in the relative movement direction, are located with spacing) are formed in the workpiece  2 . As noted here, according to the first embodiment of the processing apparatus constructed in accordance with the present invention, the deterioration portions of the width W 1  and the width W 2  can be simultaneously formed, as desired, by the single laser beam application means  6  in the workpiece  2  in two regions displaced in the thickness direction of the workpiece  2 . 
   If the deterioration portions of the width W 1  and the width W 2  are insufficient to divide the workpiece  2  sufficiently precisely along the division line, the following procedure is recommendable: The laser beam application means  6  and the holding means  4  are moved relative to each other over a predetermined distance in the optical axis direction, namely, in the up-and-down direction in  FIG. 1 , whereby the focus points  14   a  and  14   b  are displaced in the optical axis direction, accordingly, in the thickness direction of the workpiece  2 . Further, the laser beam application means  6  and the holding means  4  are moved relative to each other along the division line. In this manner, in addition to the aforementioned formation of the previous deterioration portions, two deterioration portions continuously extending along the division line with the width W 1  and the width W 2 , or many deterioration portions located with spacing along the division line with the width W 1  and the width W 2  are formed in the workpiece  2  at sites displaced in the thickness direction of the workpiece  2 . 
     FIG. 3  shows another embodiment of a processing apparatus constructed in accordance with the present invention. The processing apparatus illustrated in  FIG. 3  comprises holding means  104  for holding a workpiece  102 , and laser beam application means  106 . The holding means  104  may be of the same configuration as that of the holding means  4  in the embodiment illustrated in  FIG. 1 . 
   The laser beam application means  106  in the embodiment shown in  FIG. 3  includes pulsed laser beam oscillation means  110 , and transmitting/focusing means  112  for transmitting and focusing a pulsed laser beam  114  oscillated by the pulsed laser beam oscillation means  110 . The pulsed laser beam oscillation means  110  may be substantially the same as the pulsed laser beam oscillation means  10  shown in  FIG. 1 . The transmitting/focusing means  112  in the embodiment illustrated in  FIG. 3  includes a half mirror  116  functioning as a first splitter, a half mirror  117  functioning as a second splitter, a mirror  118 , a mirror  119 , a mirror  120 , a mirror  121 , a half mirror  122 , a half mirror  123 , first diameter changing means  124 , second diameter changing means  125 , first optical path length increasing means  126 , second optical path length increasing means  127 , and a common focusing lens  132 . Each of the first diameter changing means  124  and the second diameter changing means  125  may be substantially the same as the diameter changing means  24  shown in  FIG. 1 , and each of the first optical path length increasing means  126  and the second optical path length increasing means  127  may be of substantially the same configuration as that of the optical path length increasing means  26  shown in  FIG. 1 . (However, as will be further mentioned later, the optical path length increased by the first optical path length increasing means  126 , and the optical path length increased by the second optical path length increasing means  127  need to be different by a required length.) 
   In the embodiment shown in  FIG. 3 , the pulsed laser beam  114  from the pulsed laser beam oscillation means  110  is separated into a first pulsed laser beam  114   a  which passes through the half mirror  116  and advances straightly, and a second pulsed laser beam  114   b  which is reflected by the half mirror  116  and changed in direction to a substantially perpendicular direction. The first pulsed laser beam  114   a  passes through the half mirror  117 , and advances. During this motion, a third pulsed laser beam  114   c , which is reflected by the half mirror  117  substantially perpendicularly, is separated from the first laser beam  114   a . The first pulsed laser beam  114   a  passes through the diameter changing means  124 , and thereby has its diameter changed, more specifically, is converted into a form in which its diameter gradually increases as the first pulsed laser beam  114   a  goes farther from the diameter changing means  124 . Then, the first pulsed laser beam  114   a  passes through the half mirrors  122  and  123 , and is focused by the focusing lens  132  to a focus point  134   a  in the workpiece  102 . The second pulsed laser beam  114   b  is reflected by the mirror  118  and the mirror  119  to be changed in direction to a substantially perpendicular direction at each time, and then passes through the diameter changing means  125 , thereby having its diameter changed, more specifically, being converted into a form in which its diameter gradually increases as the second pulsed laser beam  114   b  goes farther from the diameter changing means  125 . Then, the second pulsed laser beam  114   b  is reflected by the half mirror  123  to be changed in direction to a substantially perpendicular direction, and is thereby brought into a state where its optical axis aligns with the optical axis of the first pulsed laser beam  114   a . Then, the second pulsed laser beam  114   b  is focused by the focusing lens  132  to a focus point  134   b  in the workpiece  102 . The third pulsed laser beam  114   c  is reflected by the mirror  120 , the mirror  121  and the half mirror  122  to be changed in direction to a substantially perpendicular direction at each time, and is thereby brought into a state where its optical axis aligns with the optical axis of the first laser beam  114   a . Then, the third pulsed laser beam  114   c  passes through the half mirror  123 , and is focused by the focusing lens  132  to a focus point  134   c  in the workpiece  102 . The focus point  134   a , the focus point  134   b , and the focus point  134   c  are displaced in the optical axis direction of the first pulsed laser beam  114   a , the second pulsed laser beam  114   b , and the third pulsed laser beam  114   c . In addition, the second pulsed laser beam  114   b  passes through the first optical path length increasing means  126 , and is thereby focused to the focus point  134   b  later by a required time difference dt 1  than the first pulsed laser beam  114   a  being focused to the focus point  134   a . The third pulsed laser beam  114   c  passes through the second optical path length increasing means  127 , and is thereby focused to the focus point  134   c  later by a required time difference dt 2  than the second pulsed laser beam  114   b  being focused to the focus point  134   b . As shown in  FIG. 4 , these time differences dt 1  and dt 2  are preferably arranged such that the first pulsed laser beam  114   a  focused to the focus point  134   a , the second pulsed laser beam  114   b  focused to the focus point  134   b , and the third pulsed laser beam  114   c  focused to the focus point  134   c  are sequentially focused without overlapping each other in terms of time. 
   In the processing apparatus shown in  FIG. 3 , deterioration portions are formed in the workpiece  102 , as required, in the vicinity of the focus points  134   a ,  134   b  and  134   c , usually, in regions having some width W 1 , some width W 2  and some width W 3  from the focus points  134   a ,  134   b  and  134   c  upwards. After the first pulsed laser beam  114   a  is focused to the focus point  134   a , the second pulsed laser beam  114   b  is focused to the focus point  134   b . Thus, the deterioration in the region having the width W 2  does not inhibit the focusing of the first pulsed laser beam  114   a . After the second pulsed laser beam  114   b  is focused to the focus point  134   b , the third pulsed laser beam  114   c  is focused to the focus point  134   c . Thus, the deterioration in the region having the width W 3  does not inhibit the focusing of the second pulsed laser beam  114   b . Thus, the required deterioration can be generated in the respective regions having the widths W 1 , W 2  and W 3 . When the laser beam application means  106  and the holding means  104  are relatively moved along the division line extending, for example, in the right-and-left direction in  FIG. 3 , three deterioration portions continuously extending along the division line with the width W 1 , the width W 2  and the width W 3 , or many deterioration portions located with spacing along the division line with the width W 1 , the width W 2  and the width W 3  are formed in the workpiece  102 . If the deterioration portions of the width W 1 , the width W 2  and the width W 3  are insufficient to divide the workpiece  102  sufficiently precisely along the division line, the following procedure is recommendable: The laser beam application means  106  and the holding means  104  are moved relative to each other over a predetermined distance in the optical axis direction, namely, in the up-and-down direction in  FIG. 3 , whereby the focus points  134   a ,  134   b  and  134   c  are displaced in the optical axis direction, accordingly, in the thickness direction of the workpiece  102 . Further, the laser beam application means  106  and the holding means  104  are moved relative to each other along the division line. In this manner, in addition to the aforementioned formation of the previous deterioration portions, three deterioration portions continuously extending along the division line with the width W 1 , the width W 2  and the width W 3 , or many deterioration portions located with spacing along the division line with the width W 1 , the width W 2  and the width W 3  are formed in the workpiece  102  at sites displaced in the thickness direction of the workpiece  102 .