Abstract:
An apparatus  101  for singulating an object is disclosed. The apparatus  101  comprises a laser  103  configured to emit a laser beam  105  with a Gaussian irradiance profile, as well as a beam-shaping device  115  configured to reshape the Gaussian irradiance profile of the laser beam  105  emitted from the laser  103 . In particular, the beam-shaping device  115  has a plurality of aspherical lenses  117, 119  to redistribute irradiance of the laser beam  105 , so as to reduce variation of the irradiance in an effective irradiation spectrum of the laser beam  105  for singulating the object. By redistributing the irradiance of the laser beam  105 , irradiation energy may be more efficiently delivered to the semiconductor wafer  102  for laser singulation, compared with conventional laser beams with Gaussian irradiance profiles which are non-uniform. A method of singulating an object is also disclosed.

Description:
FIELD OF THE PRESENT INVENTION 
     This invention relates to a laser apparatus for singulating an object using optics, and a method of singulating the same. The laser apparatus is particularly, but not exclusively, configured to singulate semiconductor wafers for fabricating semiconductor devices. 
     BACKGROUND OF THE INVENTION 
     Multiple semiconductor devices are fabricated in a matrix on a semiconductor wafer, which is typically made of material such as sapphire, copper, silicon, and/or their compounds. The semiconductor wafer is then cut by a laser to divide or assist in dividing the semiconductor devices into separate pieces. Laser singulation may include any of the following processes: i) laser scribing, in which linear grooves (or scribe lines) are formed on the semiconductor wafer surface to facilitate breakage along the grooves; or ii) laser cutting, in which the semiconductor wafer is cut through from its top surface to its bottom surface. 
     Laser singulation is contingent on delivering irradiance (i.e. fluence or energy) to the semiconductor wafer that exceeds its material ablation threshold. By focusing a Gaussian laser beam using an objective lens, a laser output width of the Gaussian laser beam can be made small in the order of 1 to 20 μm. Such dimensions of the laser beam ensure that its irradiance exceeds the material ablation threshold of the semiconductor wafer for laser singulation. 
     However, when the laser beam width is made small, it is important to ensure that a distance between two consecutive laser pulses is within a maximum possible distance D pulse  in order to effect singulation. The relation between the maximum possible distance D pulse  of two consecutive laser pulses, the feeding speed V feeding  of the laser beam, and the pulse repetition frequency f pulse  of the laser beam is governed by the following equation:
 
 D   pulse   =V   feeding   /f   pulse  (measured in units of mm/pulse or μm/pulse)
 
     It is therefore seen that the feeding speed V feeding  of the laser beam is constrained by the maximum possible distance D pulse . One way to increase the feeding speed V feeding  of the laser beam is by increasing its pulse repetition frequency f pulse . However, although the laser beam gives higher average power at higher pulse reception frequencies f pulse , its pulse energy drops rapidly as its pulse repetition frequency f pulse  exceeds a certain threshold. Accordingly the feeding speed V feeding  of the laser beam is ultimately limited by the constraints of both its optimum pulse repetition frequency f pulse  and the maximum possible distance D pulse . 
     In addition, scribe lines on the semiconductor wafer as formed by the Gaussian laser beam typically have a trough-like scribe depth along the scribing direction. This is because the irradiance distribution of the laser beam is of a Gaussian nature. Accordingly, portions of the scribe line that receives a weaker irradiance from the laser beam will have smaller depths compared with other portions that receive a stronger irradiance. To ensure a consistent scribe depth along the entire linear groove, further constraints may have to be imposed on the feeding speed of the laser beam. 
     Thus, it is an object of this invention to relax the aforesaid constraints on the feeding speed of the laser output to improve overall throughput. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of this invention will now be described, by way of example only, with reference to the drawings in which: 
         FIG. 1  shows a laser system according to an embodiment of the present invention; 
         FIG. 2   a  shows an irradiance profile of a conventional laser output, and  FIG. 2   b  shows an irradiance profile of a laser output from the laser system of  FIG. 1 ; 
         FIGS. 3   a  and  3   b  show corresponding 3D irradiance profiles of  FIGS. 2   a  and  2   b  respectively; 
         FIG. 4  shows a boundary of optimum aspect ratios of the laser output from the laser system of  FIG. 1 ; 
         FIG. 5  shows the laser system of  FIG. 1  in operation during laser scribing of a semiconductor wafer; 
         FIG. 6  shows a cross-sectional view of the semiconductor wafer when viewed along line A-A′ as shown in  FIG. 5 ; 
         FIG. 7  shows a plan view of the semiconductor wafer in  FIG. 6  from direction B as shown in  FIG. 6 ; 
         FIGS. 8   a  and  8   b  show the cumulative irradiance profiles of the conventional laser spot and the laser output of  FIGS. 2   a  and  2   b  respectively during laser scribing; 
         FIGS. 9   a  to  9   e  show various cross-sectional views of a semiconductor wafer during laser scribing using the conventional laser output; 
         FIGS. 10   a  to  10   e  show various cross-sectional views of a semiconductor wafer during laser scribing using the laser output from the laser system of  FIG. 1 ; 
         FIGS. 11   a  to  11   c  show different variations of the irradiance profile of the laser output from the laser system of  FIG. 1 ; 
         FIG. 12  shows an alternative 3D irradiance profile of the laser output from the laser system of  FIG. 1 . 
     
    
    
     SUMMARY OF THE INVENTION 
     A first aspect of the invention is an apparatus for singulating an object. The apparatus comprises: i) a laser configured to emit a laser beam with a Gaussian irradiance profile; and ii) a beam-shaping device configured to reshape the Gaussian irradiance profile of the laser beam emitted from the laser. In particular, the beam-shaping device has a plurality of aspherical lenses to redistribute irradiance of the laser beam, so as to reduce variation of the irradiance in an effective irradiation spectrum of the laser beam for singulating the object. 
     It should be noted that embodiments of the claimed apparatus not only include lasers that are configured to emit laser beams with strictly-defined Gaussian irradiance profiles, but also lasers that are configured to emit laser beams with approximately-defined Gaussian irradiance profiles. 
     By redistributing the irradiance of the laser beam, more irradiation energy of the laser beam may be suitably utilised to singulate the semiconductor wafer, compared with conventional laser beams with irradiance of Gaussian profiles which are non-uniform. Advantageously therefore, embodiments of the claimed apparatus may provide a more efficient use of the irradiation energy from the laser beams. 
     Moreover, a higher depth uniformity of a scribe line may be achieved through the use of embodiments of the claimed apparatus, compared with conventional lasers. Accordingly, embodiments of the claimed apparatus would not be constrained by a smaller feeding speed if higher depth uniformity of a scribe line is desired. By contrast, the feeding speed of the conventional lasers may be compromised in order to increase their respective pulse overlapping ratios to ensure a higher scribe depth uniformity. As embodiments of the claimed apparatus are less likely to be constrained by their feeding speeds, their throughput capacities would advantageously be higher than that of conventional lasers. 
     Some optional features of the claimed apparatus have been defined herein. 
     For instance, embodiments of the claimed apparatus may include a first beam-expanding device configured to magnify the laser beam emitted from the laser. Embodiments of the claimed apparatus may also include a second beam-expanding device configured to modify the laser beam with the reshaped irradiance profile to define a major longitudinal axis and a minor axis orthogonal to the major longitudinal axis, wherein the modified laser beam has a larger width along the major longitudinal axis than along the minor axis. By including the first and second beam-expanding devices, a desired aspect ratio of the laser output—i.e. a ratio of its width along the major longitudinal axis to its width along the orthogonal minor axis—may be advantageously manipulated for optimal performance of the laser singulation process. 
     A second aspect of the invention is a method of singulating an object. The method comprises the steps of: emitting a laser beam with a Gaussian irradiance profile; redistributing the irradiance of the emitted laser beam using a plurality of aspherical lenses to reshape the Gaussian irradiance profile, so as to reduce variation of an effective irradiance spectrum of the laser beam for singulating the object. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows a laser system  101  for scribing a semiconductor wafer  102 . The laser system  101  includes: i) a laser  103  for emitting a circular Gaussian collimated beam  105 ; ii) a first beam expander  107  having two optical components (shown in  FIG. 1  as aberration-corrected spherical lenses  109 ,  111 ) for magnifying the collimated beam  105  to form an expanded collimated beam  113 ; iii) a beam shaper  115  having two optical components (shown in  FIG. 1  as a pair of aspherical lenses  117 ,  119 ) for shaping irradiance of the expanded collimated beam  113  to form a ‘flattened’ collimated beam  121  (details below); iv) a second beam expander  123  having two optical components (shown in  FIG. 1  as cylindrical lenses  125 ,  127 ) for modifying an aspect ratio of the flattened collimated beam  121  (i.e. a ratio of respective widths of the beam along orthogonal axes); v) a mirror  129  for reflecting the flattened collimated beam  121  with the modified aspect ratio; and vi) a beam-focusing device (shown in  FIG. 1  as a focusing lens assembly  131 ) for focusing the reflected flattened collimated beam  121  to form a ‘flattened’ laser output  133  on the surface of a semiconductor wafer  102 . 
     Specifically the first beam expander  107  magnifies the Gaussian collimated beam  105  to a suitable spot diameter configured for the beam shaper  115 . The beam shaper  115  then distributes the irradiance of the expanded collimated beam  113 , so that the flattened collimated beam  121  has a uniform irradiance profile. The aspherical lenses  117 ,  119  of the beam shaper  115  may be fabricated from micro-lens or using holographic techniques. By shaping surface profiles of the aspherical lenses  117 ,  119  accordingly, a desired refractive index that ensures controlled phase distribution of electromagnetic waves within an operating range of wavelength can be configured. In particular the beam shaper  115  distributes the irradiance of the expanded collimated beam  113  by first causing it to converge with predetermined aberration characteristics through the aspherical lens  117 , and subsequently causing the collimated beam  113  to diverge with the aspherical lens  119  to collect the redistributed electromagnetic waves. 
     The second beam expander  123  modifies the aspect ratio of the flattened collimated beam  121  for optimum processing and thus higher throughput for cost-effective manufacturing. The combination of the lenses  109 ,  111 ,  117 ,  119 ,  125 ,  127  also allows precise aberration control of the flattened collimated beam  121  (with the modified aspect ratio) within a desired spatial range. The flattened collimated beam  121  (with the modified aspect ratio) is then reflected by the mirror  129  to the focusing lens assembly  131 , which focuses the flattened laser output  133  on the semiconductor wafer  102 .  FIG. 1  shows the flattened laser output  133  having a major longitudinal axis as well as a minor axis orthogonal to the major longitudinal axis. In particular, the flattened laser output  133  has a larger width along the major longitudinal axis than along the minor axis. Also, the flattened laser output  133  is focused on the semiconductor wafer  102  such that its major longitudinal axis is aligned with a direction of singulation (i.e. a feeding direction) of the semiconductor wafer  102 . 
     Typically, an irradiance profile of a laser output can be quantified by defining its encircled power. The irradiance profile is useful to control the precise amount of irradiance delivered to a workpiece surface such as a semiconductor wafer to maximise efficiency of laser singulation. From the irradiance profile, an ‘energy circle’ of the laser output can be derived by determining the 60%-limit based on its maximum encircled power. A characteristic beam width of the laser output is then accordingly defined based on the width of the energy circle. 
       FIGS. 2   a  and  2   b  compare the irradiance profiles of: i) a conventional laser spot; and ii) the flattened laser output  133  along their feeding axes respectively. 
       FIG. 2   a  shows a Gaussian irradiance profile  201  of the conventional laser output with a characteristic beam width  203  and a peak irradiance  205 . By contrast, the irradiance profile  207  of the flattened laser output  133  shown in  FIG. 2   b  has a more uniform (and thus flatter and less varied) peak irradiance  211  than the shaper peak irradiance  205  of the conventional laser output. 
     Moreover the irradiance profile  207  of the flattened laser output  133  has a larger characteristic beam width  209  than that of the conventional laser output. This means that a larger proportion of the irradiance of the flattened laser output  133  falls within its energy circle than in the case of the conventional laser output. Accordingly a larger proportion of the irradiance profile of the flattened laser output  133  may have irradiance above the material ablation threshold of the semiconductor wafer  102 —i.e. an effective irradiation spectrum for singulating the semiconductor wafer  102 —thereby leading to better utilisation of the laser irradiance for laser singulation. 
       FIGS. 3   a  and  3   b  show the corresponding 3D irradiance profiles of the conventional laser output and the flattened laser output  133  respectively. It can be seen from these 3D irradiance profiles that the conventional laser output and the flattened laser output  133  both have diffraction-limited widths  301 ,  303  in their respective minor axes on the semiconductor wafer plane and orthogonal to their major longitudinal feeding axes. However, the diffraction-limited width of the conventional laser output having the Gaussian irradiation profile is typically larger than that of the flattened laser output  133 . It can be seen from  FIG. 3   b  that the 3D irradiance profile of the flattened laser output  133  is substantially trapezoidal. 
     A diffraction-limited width of a laser output allows reduction of a scribe line width, and thereby increases the device density on the semiconductor wafer that is allowable for laser singulation. Since the irradiance profile  207  of the flattened laser output  133  has a diffraction-limited width  303  in its minor axis, it thus provides a sufficiently narrow scribe line width that increases the device density on the semiconductor wafer  102  whilst reducing wastage of the irradiance energy as is the case of the conventional laser output. 
     Typically, irradiance wastage is proportional to an aspect ratio of a laser output—i.e. the ratio of the laser output width along its major axis or feeding direction to the laser output width along its orthogonal minor axis. An aspect ratio of the flattened laser output  133  can be varied by adjusting the distance and/or the focal length of the cylindrical lenses  125 ,  127  of the second beam expander  123 . The present inventors have found an optimum range—in terms of speed and energy optimisation—of the aspect ratio of the flattened laser output  133  to be between 1.5:1 and 5:1 for laser singulation that involves laser scribing or laser cutting. 
       FIG. 4  shows a boundary of the optimum aspect ratios of the flattened laser output  133  with a width along its major feeding axis of between 4.5 and 100 μm and a width along its orthogonal minor axis of between 3 and 20 μm. 
     It should of course be appreciated that the major axis width may be set to a value of between 20 and 80 μm, or to a value of between 40 and 60 μm. Depending on the targeted (kerf) width of the scribe line, the minor axis width may also be set to a value of between 5 and 15 μm, or between 8 and 12 μm. Accordingly, the optimum aspect ratio range of the flattened laser output  133  may be between 3:1 and 5:1, or between 4:1 and 5:1. 
       FIG. 5  shows an operation of the laser system  101  during laser scribing. 
     The semiconductor wafer  102  is carried on a chuck table  501 . As the chuck table  501  moves along the Y-direction indicated in  FIG. 5 , the semiconductor wafer  102  is scribed by the flattened laser output  133 . An adhesive tape  503  is further arranged between the semiconductor wafer  102  and the chuck table  501  to secure the semiconductor wafer  102  during scribing. The focusing assembly  131  illustrated by solid lines in  FIG. 5  shows its position relative to the semiconductor wafer  102  before the chuck table  501  moves, whereas the focusing assembly  131  illustrated by dotted lines shows its position relative to the semiconductor wafer  102  after the chuck table  501  has moved in the Y-direction for a certain time period. Thus, although the focusing assembly  131  is stationary, its position relative to the semiconductor wafer  102  actually shifts in a direction opposite to the Y-direction moved by the chuck table  501 . 
     The laser system  101  deposits distinct laser pulses both in time and in space on the surface of the semiconductor wafer  102 —this means that no two separate laser pulses are deposited on the same location on the semiconductor wafer surface, but are instead deposited with a constant separation (i.e. the pulse distance). 
     The laser pulse repetition rate of the laser  103  is selected to optimise use of its pulse energy and its irradiance on the semiconductor wafer  102 . Depending on the pulse repetition frequency of the laser  103  and the feeding speed constraints, the chuck table  501  moves at an appropriate feeding speed to ensure precise and effective laser irradiation on the semiconductor wafer  102  to form the scribe line. 
     Thus, the semiconductor wafer  102  receives a substantially constant amount of energy along the feeding direction of the flattened laser output  133  to form a scribe line on the semiconductor wafer  102 . Each pulse of the flattened laser output  133  typically has operational pulse energies of between 1 and 30 μJ. Alternatively, the operational pulse energies may be between 5 and 100 μJ, between 20 and 80 μJ, or between 40 and 60 μJ. 
       FIG. 6  shows a cross-sectional view of the semiconductor wafer  102  when viewed along line A-A′ indicated in  FIG. 5 . 
     It is seen that electronic devices  601  are fabricated on the semiconductor wafer  102  and adjacent electronic devices  601  are separated from each other by a distance W. The space between distance W is known as a street, the width of which is sufficient to accommodate a scribe line  603  on the semiconductor wafer  102  by laser scribing so that the electronic devices  601  can be separated by breakage along the scribe line  603 . 
       FIG. 7  shows a plan view of the semiconductor wafer  102  when viewed from the direction B indicated in  FIG. 6 . As the flattened laser output  133  has a diffraction-limited width along its minor axis, the width of the scribe line  603  may therefore be made the narrowest possible to increase the device density that is allowable on the semiconductor wafer  102  for laser scribing. 
       FIGS. 8   a  and  8   b  compare the cumulative irradiance profiles  801 ,  803  delivered on the semiconductor wafer  102  by the conventional laser output and by the flattened laser output  133  during laser scribing. From these figures, it is seen that portions of adjacent irradiance profiles overlap each other as the semiconductor wafer  102  moves in the Y-direction to form a scribe line. It should be appreciated that the corresponding pulse overlapping ratio decreases accordingly with increasing feeding speed of the respective laser output. 
       FIGS. 9   a - 9   e  and  10   a - 10   e  compare qualities of the scribe lines as formed on the semiconductor wafer  102  by the conventional laser output and by the flattened laser output  133  respectively. 
       FIGS. 9   a  to  9   c  show the various states of the semiconductor wafer surface during laser scribing using the conventional laser output. Specifically  FIG. 9   a  shows the semiconductor wafer  102  just before receiving the irradiance from the conventional laser output;  FIG. 9   b  shows the semiconductor wafer  102  when it has partially received the irradiance from the conventional laser output; and  FIG. 9   c  shows the semiconductor wafer  102  when it has completely received the irradiance from the conventional laser output. In particular portions  901  of the semiconductor wafer  102  are not removed by the conventional laser output because the irradiance level at the corresponding portions of the conventional laser output is lower than the ablation threshold of the semiconductor wafer  102 . 
       FIGS. 9   d  and  9   e  show the resultant scribe line  903  formed by the laser scribing using the conventional laser output as the semiconductor wafer  102  moves in the Y-direction. It can be seen that the resultant scribe line  903  has a varying scribe depth. This is due to the irradiance profile of the conventional laser output along its major axis (or feeding direction) having a Gaussian nature, and thus, the variance of the irradiance level of the conventional laser output is high. Accordingly parts of the resultant scribe line  903  irradiated by a lower irradiance level of the conventional laser output have smaller scribe depths, whereas other parts of the scribe line irradiated by a higher irradiance level of the conventional laser output have larger scribe depths. 
       FIGS. 10   a  to  10   c  show the various states of a semiconductor wafer surface during laser scribing using the flattened laser output  133 . Specifically  FIG. 10   a  shows the semiconductor wafer  102  just before receiving the irradiance from the flattened laser output  133 ;  FIG. 10   b  shows the semiconductor wafer  102  when it has partially received the irradiance from the flattened laser output  133 ; and  FIG. 10   c  shows the semiconductor wafer  102  when it has completely received the irradiance from the flattened laser output  133 . In particular portions  1001  of the semiconductor wafer  102  are not removed by the flattened laser output  133  because the irradiance level at the corresponding portions of the flattened laser output  133  is lower than the ablation threshold of the semiconductor wafer  102 . 
       FIGS. 10   d  and  10   e  show the resultant scribe line  1003  formed by the laser scribing using the flattened laser output  133  as the semiconductor wafer  102  moves in the Y-direction. In contrast with  FIGS. 9   d  and  9   e , the resultant scribe line  1003  formed by the flattened laser output  133  has a more uniform scribe depth than the resultant scribe line  903  formed by the conventional laser output. This is due to a lower variance of the irradiance level of the flattened laser output  133  along its major axis (or feeding direction), and thus, the resultant scribe line  1003  has a more uniform scribe depth, compared with the resultant scribe line  903  formed by the conventional laser output. 
     In order to avoid the varying scribe depth of the resultant scribe line  903  formed by the conventional laser output, the pulse overlapping ratio of the conventional laser output can be increased to reduce fluctuation of the average irradiance received in each unit of the scribing length along the semiconductor wafer  102 . This, however, affects the feeding speed of the conventional laser output as more pulses per unit of the scribing length are required. Accordingly, the use of the conventional laser output may impose a further constraint to its feeding speed if a uniform scribe depth is required. 
     Therefore, it is seen that using the flattened laser output  133  in laser scribing achieves uniformity of the scribe depth whilst optimising its feeding speed during laser scribing. 
     It is further seen that by flattening the irradiance profile of the expanded collimated beam  113  through irradiance redistribution and beam shaping, more energy may be delivered to a workpiece for material removal compared with the conventional laser output that has a Gaussian irradiance profile. Thus, the laser system  101  advantageously improves overall efficiency and processing speed for laser singulation by, for example, increasing the feeding speed of the flattened laser output  133 . 
     It should of course be appreciated that many variations of the described embodiment are possible without departing from the scope and spirit of this invention. 
     For example, while  FIG. 2   b  shows the flattened laser output  133  having a constant peak irradiance  211  within its characteristic beam width, such a characteristic is merely preferred for laser singulation but not essential.  FIGS. 11   a  to  11   c  show different irradiance profiles of the flattened laser output  133  without a constant peak irradiance within their characteristic beam width but is nonetheless of substantial uniformity. Specifically,  FIGS. 11   a  to  11   c  shows alternative configurations of the flattened laser output  133  having respective oscillatory, concave, and convex irradiation spectrums at the central portion of the laser output  133 . It is seen that irradiation variation at the respective central portions is limited to ±15% from their respective average (nominal) value 1101. The non-constant irradiance of these different irradiance profiles of the flattened laser output  133  has minimal effect on the feeding speed and efficiency of laser singulation. In fact, it may actually improve the ease of producing the flattened laser output  133  for laser singulation. 
     It should of course be appreciated that variation of the irradiance level of the flattened laser output  133  may be limited to ±12%, or ±10%, or ±5% from its average (nominal) value at its central portion within its characteristic beam width. Any desired variation of the irradiance of the flattened laser output  133  from its average (nominal) value may be achieved by altering the surface profiles of the pair of aspherical lenses  117 ,  119 . 
     Furthermore, although it has been described that the irradiance of the flattened laser output  133  has been redistributed along its major longitudinal axis that is aligned to the direction of singulation of the semiconductor wafer  102 , it should be appreciated that the irradiance of the flattened laser output  133  is also redistributed along its orthogonal minor axis on the plane of the semiconductor wafer  102 . 
     Moreover, although the flattened laser output  133  has a diffraction-limited width along its minor axis to provide a sufficiently narrow scribe width, a nominally flat but wider scribe width along its minor axis may be provided by the laser system  101  if desired.  FIG. 12  shows an alternative 3D irradiance profile of the flattened laser output  133  that has generally flat irradiance profiles in both its major and minor axes  1201 ,  1203  to give a more cuboidal appearance. In contrast with the 3D irradiance profile shown in  FIG. 3   b , the diffraction-limited width of this alternative 3D irradiance profile is larger. 
     In addition, it should be appreciated that the first and second beam expanders  107 ,  123  are optional features of the laser system  101 . Without either or both of these beam expanders  107 ,  123 , the beam shaper  115  would still be operable to redistribute the irradiance of the laser beam  105  to flatten its irradiance profile. The circular laser spot can then be focused on the semiconductor wafer  102 , where its effective irradiation spectrum is used for singulating the semiconductor wafer  102 . Also, the laser system  101  may be used to singulate other objects besides semiconductor wafers. 
     Optionally, the beam expander  107  may also be incorporated in the beam shaper  115  so that the laser  101  directly emits the collimated beam  105  to the beam shaper  115  for both magnification and shaping. In addition, while laser scribing using the laser system  101  has been described, other laser singulation processes such as laser cutting can also be undertaken using the laser system  101 .