Abstract:
A laser processing method is disclosed, comprising the steps of: directing a laser beam to a workpiece; and effecting a relative motion between the laser beam and the workpiece. In particular, the step of directing the laser beam to the workpiece comprises focusing the laser beam within the workpiece until an internal damage forms within the workpiece and a crack propagates from the internal damage to at least one surface of the workpiece to form a surface crack on the workpiece. Further, the step of effecting the relative motion between the laser beam and the workpiece is such that the surface crack on the workpiece propagates along a line of separation on the workpiece. A laser processing apparatus is also disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a laser processing method and apparatus, which are particularly but not exclusively for singulating a semiconductor wafer using laser. 
       BACKGROUND OF THE INVENTION 
       [0002]    Multiple semiconductor devices are fabricated in a matrix on a semiconductor wafer, which is typically made of material such as sapphire, silicon, gallium 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. 
         [0003]    Conventional 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. 
         [0004]    Specifically, 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 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. 
         [0005]      FIG. 1  shows a conventional laser scribing process  100 , in which a laser beam  102  is focused at a point on a surface of a semiconductor wafer  104  having semiconductor devices (not shown), before a relative motion between the laser beam  102  and the semiconductor wafer  104  is effected along a scribing direction  110  to form a scribe line  106  on the surface of the semiconductor wafer  104 . However, when the laser beam  102  is focused on the surface of the semiconductor wafer  104  with the delivered irradiance at or above its material ablation threshold, debris  108  will be removed from the semiconductor wafer  104  and may redeposit back onto the surface of the semiconductor wafer  104 . This may contaminate the semiconductor devices on the semiconductor wafer  104 . Thus, the conventional laser singulation process has the problem of surface contamination of the semiconductor wafer  104 . 
         [0006]    One way to avoid the debris  108  from contaminating the semiconductor devices on the semiconductor wafer  104  is by performing surface coating and washing before and after laser processing. Unfortunately, the surface coating process has its own limitations. For instance, the surface and side-wall recast molten layer may affect the appearance and/or the performance of the semiconductor device after singulation. Other post-processing approaches, such as side-wall etching, have been proposed to mitigate this problem. However, extra pre- and post-processing of the surface-scribed wafer would ultimately limit the overall production yield and increase the running cost. 
         [0007]    Thus, it is an object of this invention to at least seek to ameliorate the problems among conventional laser singulation processes. 
       SUMMARY OF THE INVENTION 
       [0008]    A first aspect of the invention is a laser processing method comprising the steps of: directing a laser beam to a workpiece; and effecting a relative motion between the laser beam and the workpiece. In particular, the step of directing the laser beam to the workpiece comprises focusing the laser beam within the workpiece until an internal damage forms within the workpiece and a crack propagates from the internal damage to at least one surface of the workpiece to form a surface crack on the workpiece. Further, the step of effecting the relative motion between the laser beam and the workpiece is such that the surface crack on the workpiece propagates along a line of separation on the workpiece. 
         [0009]    A second aspect of the invention is a laser processing apparatus comprising: a supporting device for holding a workpiece; a laser-emitting device for directing a laser beam to the workpiece; and a positioning device operative to effect a relative motion between the laser-emitting device and the supporting device. In particular, the laser-emitting device is configured to focus the emitted laser beam within the workpiece during operation until an internal damage forms within the workpiece and a crack propagates from the internal damage to at least one surface of the workpiece to form a surface crack on the workpiece. The positioning device is also configured to effect the relative motion between the laser-emitting device and the supporting device such that the surface crack on the workpiece propagates along a line of separation on the workpiece. 
         [0010]    Some preferred but optional steps/features of the invention have been defined in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: 
           [0012]      FIG. 1  shows a conventional surface scribing process; 
           [0013]      FIG. 2  shows a laser processing apparatus according to a preferred embodiment; 
           [0014]      FIG. 3  shows an operation of the laser processing apparatus of  FIG. 2 ; 
           [0015]      FIGS. 4   a - 4   h  show a singulation process of a semiconductor wafer using the laser processing apparatus of  FIG. 2 ; 
           [0016]      FIG. 5   a  and  FIG. 5   b  are respective plan and isometric views of the semiconductor wafer after the singulation process using the laser processing apparatus of  FIG. 2 ; and 
           [0017]      FIG. 6   a  and  FIG. 6   b  are respective plan and cross-sectional views of the semiconductor wafer after multiple singulation processes using the laser processing apparatus of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]      FIG. 2  is an isometric view of the laser processing apparatus  200  according to the preferred embodiment. The laser processing apparatus  200  comprises: i) a laser-emitting device  202 ; and ii) an XY-chuck table  204  for supporting and moving a semiconductor wafer  206 . In particular, the laser-emitting device  202  is operative to direct a laser beam  208  to the semiconductor wafer  200 . The XY-chuck table  204  is also operative to move the semiconductor wafer  206  relative to the laser-emitting device  202  with respect to a scribing axis  210  and also with respect to an indexing axis  212  that is perpendicular to the scribing axis  210 . 
         [0019]    Specifically, the laser-emitting device  202  comprises: i) a laser  202   a  for generating the laser beam  208 ; ii) an optical attenuator  202   b  for optically attenuating the laser beam  208 ; iii) a beam expander  202   c  for magnifying the laser beam  208 ; iv) a mirror  202   d  for reflecting the laser beam  208  towards the semiconductor wafer  206 ; and v) an objective lens  202   e  for focusing the laser beam  208  within the semiconductor wafer  206  along a focusing axis  214 . 
         [0020]      FIG. 3  is a side view of the laser processing apparatus  200  showing a singulation operation of the laser processing apparatus  200 . At the start of the operation, the laser-emitting device  202  is originally positioned with respect to the left side of the semiconductor wafer  206 . As the XY-chuck table  204  move leftwards relative to the laser-emitting device  202  and in a direction parallel to the scribing axis  210 —shown by arrow  300 —the laser-emitting device  202  is consequently repositioned (even though the laser-emitting device  202  does not move) with respect to the right side of the semiconductor wafer  206 , as shown by the dotted lines of the laser-emitting device  202  in  FIG. 3 . This also means that the movement of the XY-chuck table  204  effects a relative motion of the laser-emitting device  202  in a direction parallel to the scribing axis  210 . 
         [0021]    The singulation operation of the laser processing apparatus  200  will now be explained with reference to  FIGS. 4   a - 4   h  showing cross-sectional views of the semiconductor wafer  206  as viewed along line B-B′. 
         [0022]    First, the laser beam  208  is focused at a focal point  400  inside the semiconductor wafer  206 , as shown in  FIG. 4   a . The step of focusing the laser beam  208  at the focal point  400  inside the semiconductor wafer  206  continues until an internal damage  402  forms and grows around the focal point  400  inside the semiconductor wafer  206 , as shown in  FIGS. 4   b - 4   d.  By further continuing focus of the laser beam  208  within the semiconductor wafer  206 , a crack  404  propagates from the internal damage  402  to a surface of the semiconductor wafer  206  to form a surface crack on the semiconductor wafer  206 , as shown in  FIGS. 4   e - 4   f.  As the XY-chuck table moves in the direction parallel to the scribing axis  210  relative to the laser-emitting device  202 , the semiconductor wafer  206  would consequently be semi-scribed along a desired line of separation. 
         [0023]    Optionally, continuous focus of the laser beam  208  within the semiconductor  206  may result in another crack  406  propagating from the internal damage  402  to the opposite surface of the semiconductor wafer  206  to form another surface crack on the semiconductor wafer  206 , as shown in  FIG. 4   g . As the XY-chuck table  204  moves in the direction parallel to the scribing axis  210  relative to the laser-emitting device  202 , the semiconductor wafer  206  would consequently be split into separate pieces along a desired line of separation, as shown in  FIG. 4   h . Such a process is particularly suitable if complete dicing of the semiconductor wafer  206  is desired, instead of semi-scribing of the semiconductor wafer  206 . 
         [0024]    The present inventors have found that formation of the crack(s)  404 ,  406  depends on factors such as the energy density of the laser beam  208  and the focus level of the laser beam  208  with respect to the surface of semiconductor wafer  206 . In particular, the energy density of the laser beam  208  depends on parameters such as the laser wavelength and the motion of the XY-chuck table  204 , whereas the focus level of the laser beam  208  depends on the parameters of the optics (e.g. the numerical aperture of the objective lens  202   e .) 
         [0025]    Specifically, the energy density of the laser beam  208  is defined as follows: 
         [0000]      Energy density (uJ/um)=[Pulse energy (uJ/pulse)×Pulse repetition rate (KHz, Kpulse/s)]/Scribing speed (mm/s)
 
         [0026]    Accordingly, the volumetric energy (uJ/um 3 ) of the laser beam  208  that is delivered at the focal point  400  inside the semiconductor wafer  206  is defined as follows: 
         [0000]    
       
         
           
             
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         [0027]    (where r and z are respectively the radius (um) and longitudinal height (um) along the laser beam path; P(t) is the time-dependent laser power profile of the laser pulse in a period of T, or a laser pulse repetition rate of 1/T; and 
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         [0000]    which determines the length over which the laser beam can propagate without diverging significantly. The position z=0 in the equation above corresponds to the beam waist w 0  or focus where the laser beam radius is at its minimum.) 
         [0028]    Based on the relevant factors for forming the crack(s)  404 ,  406 , the laser processing apparatus  200  should preferably be configured such that the laser-emitting device  202  emits the laser beam  208  with a pulse energy density of between 0.3 and 0.8 uJ/um, with the emitted laser beam  208  being focused within the semiconductor wafer  206  at a distance of between 10 and 25 um from the surface of the semiconductor wafer  206 . Preferably also, the volumetric energy of the laser beam  208  is between 35-140 KJ/m 3 . Alternatively, the laser processing apparatus  200  may be configured such that the laser-emitting device  202  emits the laser beam with a pulse energy density of at least 0.5 uJ/um, with the emitted laser beam  208  being focused within the semiconductor wafer  206  at a distance of between 25 and 40 um from the surface of the semiconductor wafer  206 . 
         [0029]      FIG. 5   a  and  FIG. 5   b  show respective plan and isometric views of the semiconductor wafer  206  after the laser processing apparatus  200  has completed a singulation operation. It can be seen that a surface crack  500  is formed on the semiconductor wafer  206 , which corresponds to a desired line of separation of the semiconductor wafer  206 . In particular, the surface crack  500  is formed by the propagation of the crack  404  along the line of separation as the XY-chuck table  204  moves in the direction parallel to the scribing axis  210 . It should be appreciated that the surface crack  500  is not necessarily a continuous line and may comprise a plurality of broken surface cracks instead. It should also be appreciated that the surface crack  500  is not necessarily a straight line and may have a deviation of a few microns (e.g. 1-2 microns). 
         [0030]      FIG. 6   a  is a plan view of the semiconductor wafer  206  after the laser processing apparatus  200  has completed a plurality of singulation operations, while  FIG. 6   b  is a cross-section view of the semiconductor wafer  206  as viewed along line C-C′ in  FIG. 6   a . In particular, the plurality of singulation operations include actuating the XY-chuck table  204  in a direction parallel to an indexing axis  212 , in order to index the semiconductor wafer  206  relative to the laser-emitting device  202  each time a surface crack  500  is formed on the semiconductor wafer  206 . 
         [0031]    It can be seen from the plan view of the semiconductor wafer  206  in  FIG. 6   a  that multiple surface cracks  500  are formed on the semiconductor wafer  206 , each of which corresponds to a desired line of separation of the semiconductor wafer  206 . It can also be seen from the cross-sectional view of the semiconductor wafer  206  in  FIG. 6   b  that each of the surface cracks  500  is formed by the propagation of the corresponding crack  404  along the line of separation as the XY-chuck table  204  moves in the direction parallel to the scribing axis  210 . 
         [0032]    By focusing the laser beam  208  inside the semiconductor wafer  206  until the crack  404  propagates from the internal damage  402  to the surface of the semiconductor wafer  206  to form the surface crack  500  on the semiconductor wafer  206 , little or no debris is formed at the surface of the semiconductor wafer  206  during the singulation operation. Thus, the laser apparatus  200  advantageously addresses the problem of surface contamination of the semiconductor wafer  206  due to the creation of debris by using conventional laser scribing apparatus and processes. In addition, the surface crack  500  formed on the surface of the semiconductor wafer  206  can advantageously serve as a reference marker for dicing the semiconductor wafer  206  into separate pieces. If the surface crack  500  were not formed and the surface of the semiconductor wafer  206  were intact, it would be technically difficult to determine the exact location of the internal damage  402  inside the semiconductor wafer  206  to perform a subsequent dicing process, to divide the semiconductor wafer  206  into separate pieces along the internal damage  402 . 
         [0033]    Various embodiments of this invention can also be envisaged without departing from the scope of the invention. For example, instead of the XY-chuck table  204  moving relative to the laser-emitting device  202 , the laser-emitting device  202  may itself be operable to move relative to the XY-chuck table  204  (without the XY-chuck table  204  moving) in orthogonal directions parallel to the scribing and indexing axes  210 ,  212  when performing the singulation process. Furthermore, the XY-chuck table  204  may be configured to support other types of workpieces beside the semiconductor wafer  206 . The laser processing apparatus  200  may also perform singulation of semiconductor wafers made from sapphire, silicon, gallium and/or their compounds.