Abstract:
In a Sapphire Collector (SC), one or more features, both structural and parametric, are included for capturing the die-size sapphire chips that are removed from a semiconductor structure during die-level laser lift-off (LLO). These features are designed to increase the likelihood that each sapphire chip is securely captured by the Sapphire Collector immediately after it is released from the semiconductor structure. The Sapphire Collector includes a vacuum-enhance collector with a pickup element that lifts each released chip into the collector, and air pushers that direct the chips further into the collection tunnel leading to a discard bin.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the field of light emitting devices, and in particular to a system that reduces damage to the light emitting devices during laser lift-off of the sapphire substrate upon which the light emitting element is grown. 
       BACKGROUND OF THE INVENTION 
       [0002]    Semiconductor devices, including semiconductor light emitting elements, are formed/grown on a substrate, sapphire wafer substrates being common. In the example of a light emitting element, a GaN nucleation layer may be formed on a sapphire substrate, followed by one or more n-type layers, one or more active layers, and one or more p-type layers. Metallic conductors may be formed through and upon the layers to provide coupling of the n-type and p-type layers to an external source of power to activate the active layer(s) of the light emitting element, via contact pads above the uppermost (p-type) layer. 
         [0003]    Because the metallic contact pads are generally opaque or reflective, the light emitting element is designed to emit lights from the surface opposite the contact pads and through the substrate. To improve light extraction efficiency, the substrate may be removed, exposing the semiconductor surface. The semiconductor surface may be processed to further enhance the light extraction efficiency. In some cases one or more contact pads may be placed on the light emitting side of the device. 
         [0004]    Laser lift-off is a process that is commonly used to remove the sapphire substrate from the light emitting element. A laser pulse is projected through the sapphire substrate and is absorbed by the semiconductor layer at the Sapphire-semiconductor interface, producing a localized explosive shockwave, due to the instant thermal decomposition of the semiconductor layer at the interface. 
         [0005]    If the laser lift-off (LLO) is performed at the wafer level, the wafer-size sapphire substrate is removed after the whole wafer has been processed. If, on the other hand, the laser lift-off is performed for each individual die, the dies are flip-chip mounted on a sub-mount tile, with the sapphire facing upward. The laser is applied to each die, and the die-size sapphire chips pop up into a “Sapphire Collector”, or “Confetti Catcher” immediately after the laser is incident on each die, leaving the semiconductor structure on the sub-mount tile. The sub-mount tile is subsequently processed to create, for example, lens elements over each die, then sliced/diced to provide the individual light emitting devices. 
         [0006]    Between the time that the sapphire is removed and the dies are covered, the relatively fragile semiconductor surface is exposed, and susceptible to mechanical damage. During an example set of production runs, the yield lost to such mechanical damage has been measured to be about 0.236%. 
       SUMMARY OF THE INVENTION 
       [0007]    It would be advantageous to reduce the likelihood of mechanical damage to a light emitting element after laser lift-off. 
         [0008]    To better address this concern, in an embodiment of this invention, one or more features, both structural and parametric, are included in a Sapphire Collector (SC) for capturing the die-size sapphire chips that are removed from a semiconductor structure during die-level laser lift-off (LLO). These features are designed to increase the likelihood that each sapphire chip is securely captured by the Sapphire Collector immediately after it is released from the semiconductor structure. The Sapphire Collector includes a vacuum-enhance collector with a pickup element that lifts each released chip into the collector, and air pushers that direct the chips further into the collection tunnel leading to a discard bin. 
         [0009]    In embodiments of this invention, the features that enhance the likelihood of a released sapphire chip being securely captured by the Sapphire Collector include one or more of the following. 
         [0010]    To reduce the likelihood of the chip striking a top surface of the collector and bouncing back exiting the pickup element, an angled air pusher may be situated near the top of the collector to direct the chips away from the top surface and farther into the collection tunnel At the same time, a complementary angled air pusher may be situated near the bottom of the collector to also direct the picked-up chips farther into the collection tunnel, and further direct any ricocheted chips away from the pickup element. To further enhance the efficiency of these air pushers, the air pushers may be shaped as air knives with high velocity, low volume output. 
         [0011]    The entry to the collection tunnel may be flared to maximize the collection cross-section area, and to reduce the likelihood of a chip ricocheting back toward the pickup area. A trench may be created around the interior of the pickup element, to prevent any chips that manage to slide out of the flared tunnel opening, or otherwise coming near to the pickup element from exiting the pickup element. 
         [0012]    The exterior surface of the collector facing the dies on the sub-mount may be chamfered, to reduce the likelihood of a ‘wild chip’ ricocheting repeatedly between the sub-mount and the lower surface of the collector. The pickup element, and the provided vacuum may also be designed to optimize the likelihood that a released chip will be forced to enter the collector. 
         [0013]    In an example embodiment of this invention, the yield loss due to mechanical damage after laser lift-off was reduced by an order of magnitude, from 0.236% to 0.023%. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
           [0015]      FIG. 1A  illustrates an example prior art Sapphire Collector. 
           [0016]      FIGS. 1B and 1C  illustrate examples of adverse travel of a sapphire chip in the prior art Sapphire Collector of  FIG. 1A . 
           [0017]      FIGS. 2A-2C  illustrate an example embodiment of a Sapphire Collector that substantially reduces the likelihood of mechanical damage to the light emitting element after laser lift-off. 
           [0018]      FIGS. 3A-3C  illustrates example dimensions of the Sapphire Collector of  FIG. 2 . 
       
    
    
       [0019]    Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention. 
       DETAILED DESCRIPTION 
       [0020]    In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
         [0021]      FIG. 1A  illustrates an example prior art Sapphire Collector (SC)  120 . The SC  120  includes a collection cavity  125  that is open to two tunnels  130 A and  130 B. These tunnels  130  are under negative pressure, causing vacuum flows  135 A and  135 B in tunnels  130 A and  130 B respectively. The SC  120  also includes input pipes  140 A and  140 B to the cavity  125  that provides a pressure flows  145 A and  145 B in pipes  140 A and  140 B respectively. Additional tunnels and pipes may also be provided. 
         [0022]    A laser element  110  provides a pulsed laser beam  115  that enters SC  120  via a gate  128 . The gate  128  is designed so as not to block the laser beam  115 , but to prevent the escape of any sapphire chips  170  after they enter the cavity  125 . The gate  128  may be, for example, a lens element, or simply a grate. 
         [0023]    Below the SC  120 , a plurality of light emitting elements  160  with attached sapphire substrate chips  170  are mounted on a submount  150 . During laser lift-off, the SC  120  is situated over a light emitting element  160  with an attached sapphire chip  170 , either by moving SC  120  relative to the submount  150 , or moving the submount  150  relative to the opening  122  to the cavity  125  of SC  120 . 
         [0024]    With the light emitting element  160  and chip  170  situated beneath the opening  122 , the pulsed laser beam  115  is applied, causing the chip  170  to be explosively released from the light emitting element  160 . The upward force causes the released chip  170  to enter the opening  122  and the vacuum flows  135 A and  13 B causes it to travel toward the tunnels  130 A and  130 B. The pressurized air flows  145 A and  145 B exiting the pipes  140 A and  140 B also serve to push the traveling chip  170  toward the tunnels  130 . 
         [0025]    Depending upon the initial liftoff trajectory direction and velocity of the chip  170  relative to the vacuum  130  and pressurized air  145 , the chip  170  may enter one of the vacuum tunnels  130  directly, or after a few ricochets. Ideally, even if the chip  170  ricochets around within the cavity  125 , the chip  170  will eventually enter one of the tunnels  130 A,  130 B because its velocity will be continually decreasing, and thus increasingly more influenced by the vacuums  135 A,  135 B and pressurized air flows  145 A,  145 B. 
         [0026]    The inventors have recorded the laser lift-off operation with respect to the submount  150  and the opening  122  using a high speed camera, and have observed that some chips  170  exit the opening  122  and cause damage. 
         [0027]    In some cases, the chips  170  are hovering below the opening  122  and are eventually sucked back into the cavity  125 , causing no adverse effects. In other cases, however, the chips  170  are traveling at a sufficient downward speed that the vacuum flows  135 A,  135 B and pressurized air flows  145 A,  145 B is insufficient to reverse or alter its direction before it exits the opening  122  and strikes the submount  150 , as illustrated in  FIG. 1B . The likely cause of this downward travel is a ricochet of the chip  170  off the walls or top surface of the cavity  125 . Most ricocheting chips  170  are likely to eventually be sucked into the tunnels  130 A,  130 B, due to the vacuum flows  135 A,  135 B and the pressurized air flows  145 A,  145 B, but some chips  170  escape through the opening  122  and strike the sub-mount  150  with mounted light emitting elements  160 . 
         [0028]    If the exiting chip  170  strikes the submount  150  at a location where the light emitting element  160  is situated without an attached sapphire chip  170  (i.e. elements  160  with chips  170  laser removed), even at a low speed, the fragile nature of the semiconductor surface will likely result in the ruin of the element  160 . 
         [0029]      FIG. 1C  illustrates an observed failure mechanism wherein the exiting chip  170  ricochets repeatedly between the lower exterior surface  126  of SC  120  and the submount  150 , causing substantial damage, often to multiple elements  160  on the submount  150 . 
         [0030]    As noted above, the yield loss due to mechanical damage after laser lift-off has been observed in one set of production runs to amount to 0.236%; and, as subsequently been determined, most (90%) of this lost yield is due to the damage that exiting chips  170  inflict. It was also observed that a substantial majority of damage was produced due to the repeated ricochets illustrated in  FIG. 1C . 
         [0031]      FIG. 2A  illustrates an example embodiment of a Sapphire Collector (SC)  220  that substantially reduces the likelihood of mechanical damage to the light emitting element after laser lift-off. 
         [0032]    Of particular note, SC  220  includes a single tunnel  230 , with a tubular portion  231  and a flared portion  232 . A narrow end of the flared portion  232  is connected to tubular portion  231  and a wide end is connected to a cavity  225 . Flared portion  232  may be a conic section, such as a cone shape with a pointed end removed. Both the flared portion  232  and the tubular portion  231  may have a circular cross section, or the shapes may be more complex. For example, flared portion  232  may have a rectangular cross section at its wide end, where it opens into the cavity  225 , and a circular cross section at its narrow end, where it couples to the tubular portion  231 . In the alternative, the cross sections for the either the flared portion  232  or the tubular portion  231  may have any suitable cross section e.g. square, triangular, elliptical. Likewise cavity  225  may have a rectangular cross section, a round cross section or any suitable cross section. The cross section of cavity  225  may be the same along its entire height or it may differ. 
         [0033]    The tunnel  230  is held at negative pressure, resulting in a vacuum force or vacuum flow  235 . Although this wider tunnel  230  may require a greater vacuum force  235  than the narrower tunnels  130 A,  130 B of  FIG. 1 , features that reduce the loss of the created vacuum may be provided, as detailed further below. 
         [0034]    A laser element  110  provides a pulsed laser beam  115  that enters SC  220  via a gate  128 . The gate  128  is designed so as not to block the laser beam  115 , but to prevent the escape of any sapphire chips  170  after they enter the cavity  225 . The gate  128  may be, for example, a lens element, or simply a grate. 
         [0035]    Below the SC  220 , a plurality of light emitting elements  160  with attached sapphire substrate chips  170  are mounted on a submount  150 . During laser lift-off, the SC  220  is situated over a light emitting element  160  with an attached sapphire chip  170 , either by moving SC  220  relative to the submount  150 , or moving the submount  150  relative to the opening  222  to the cavity  225  of SC  220 . 
         [0036]    With the light emitting element  160  and chip  170  situated beneath the opening  222 , the pulsed laser beam  115  is applied from laser source  110 , causing the chip  170  to be explosively released from the light emitting element  160 . 
         [0037]    Although cavity  225  is shown as having a single side connected to tunnel  230  and a flat side opposite the connection to tunnel  230 , other configurations are contemplated and are included within the scope of the invention. Cavity  225  may have a circular cross section or any other suitable cross section e.g. square, triangular, elliptical. Cavity  225  may be formed of any suitable combination of cross sections e.g. a cylindrical portion next to the laser  110  and a square cross section near wafer  150 . 
         [0038]    SC  220  includes two angled nozzles, or air pushers  250 A and  250 B, collectively air pushers  250 , that couple the pipes  240 A and  240 B to the cavity  225 , and may protrude into the cavity  225 . These pipes  240 A,  240 B direct pressure flows  245 A and  245 B into the cavity  225  via the air pushers  250 A,  250 B. One of the air pushers  250 A is situated near the top of the cavity  225 , and is angled downward i.e. toward the opening  222 , to reduce the likelihood that the released chip  170  will strike and ricochet from a top surface of the cavity  225  and/or reduce the likelihood that ricocheting chips will be directed toward the pickup opening  222 . Another air pusher  250 B is situated near the bottom of the cavity  225 , and is angled upward i.e. away from opening  222 , to direct the chip  170 , or any ricocheting chips  170  into the flared portion  232 , further reducing the likelihood that ricocheting chips  170  will be directed toward the pickup opening  222 . 
         [0039]    The air pushers  250 A,  250 B may be shaped as air knives that have volume shaped as a parallelepiped with a thin but long opening/slits  255 A,  255 B (not shown in  FIG. 2A ) into the cavity  225 , as illustrated in  FIG. 2B  (side view, cross-section of  250 A), and  FIG. 2C  (front view of  250 A). The air knives may extend across the inner surface of the cavity  225 , and direct the flows  240 A,  240 B through the narrow slits  255 A,  255 B toward the flared portion  232 , thereby creating high velocity laminar air flows in the cavity  225 . These laminar flows of air create shearing layers that minimizes the likelihood of a chip  170  passing through the shearing layers, and in particular, the likelihood of ricocheting chips  170  passing through both shearing layers without being directed toward the flared portion  232 . The thin openings  255 A,  255 B of the air knives  250 A,  250 B also limits the volume of air that enters the cavity  225 , thereby reducing loss of vacuum pressure  235  within the cavity  225 , and increasing the vacuum force at the pickup opening  222  of SC  220 . Although this example air knife has a rectangular cross section with a slit shaped as a milled rectangle, other suitable shapes for the air knife and the slit are contemplated, such as a plurality of orifices arranged in a line, and are included within the scope of the invention. 
         [0040]    Because of the air knife shape of the air pushers  250 A,  250 B, most of the chips  170  will generally enter the flared portion  232 , particularly ricocheting chips  170  whose speed continually decrease with each ricochet. However, some chips  170  may land on the sloped wall  233 . Because these chips  170  are out of the primary airflow into the tunnel  230 , there may not be enough of a vacuum force to pull the chips  170  up the sloped wall  233 , and some of the chips  170  may slide down toward the pickup opening  222 . To prevent these chips from falling out of the pickup opening  222 , a barrier  280  may be placed around the pickup opening  222 , forming a trench wherein the chips  170  are contained. Means may be provided for periodically removing the captured chips  170  from the trench formed by the barrier  280 . 
         [0041]    The exterior of SC  220  may be shaped to reduce the likelihood of extensive damage due to a ricocheting chip  170 , such as illustrated in  FIG. 1C . Of particular note, the pickup opening  222  may be formed by sloped walls  270  that serve to substantially deflect a ricocheting chip  170 , minimizing the chance of the repetitious ricochet pattern of  FIG. 1C . In like manner, the lower structure  275  may be sloped to further prevent a repetitious ricochet pattern. 
         [0042]    In an embodiment of this invention, the amount of vacuum force  235  and pressure force  245  may be adjustable to provide an optimal air flow for the particular size and shape of the sapphire chips  170  that are being lifted off. In like manner the height  272  of SC  220  above the substrate  150  may be adjustable to optimize the airflow into the opening  222 , while at the same time being as high as practical over the submount  150 , to avoid damage by chips  170  that exit the opening  222  at low velocity and are sucked back into the opening  222  before they strike an exposed semiconductor  160 . This elevated height also serves to reduce the force with which a falling chip  170  may strike the submount  150 , by increasing the time that the falling chip  170  is exposed to the vacuum forces in the opposite direction. 
         [0043]      FIG. 3  illustrates example dimensions of the Sapphire Collector of  FIG. 2 , and Table 1 illustrates example values for each parameter or dimension. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Feature 
                 Parameter 
                 Example Value 
               
               
                   
               
             
             
               
                 Air Pushers 250 
                 Angles a1, a2 
                 ≦90°, adjustable 
               
               
                   
                 Number of Pushers 
                 ≧1 
               
               
                   
                 Slit thickness, S 
                 75 μm ± 25 μm 
               
               
                   
                 Slit width, W 
                 ≧10x chip size 
               
               
                 Flared Vacuum 
                 Taper angle, b 
                 ≦45° 
               
               
                 Tunnel 230 
                 Tunnel diameter, D 
                 ≧ O; ≧ K 
               
               
                   
                 Position 
                 aligned with pushers 250 
               
               
                 Pickup Opening 
                 Opening width, O 
                 as small as practical without 
               
               
                 222 
                   
                 blocking laser beam 
               
               
                   
                 Opening shape 
                 Square or rectangular 
               
               
                   
                 Trench height, H 
                 1x ≦ H ≦ 3x chip size 
               
               
                 Air Flow 
                 Vacuum, V 
                 &lt;−6 kPa 
               
               
                   
                 Pressure, P 
                 &gt;0.6 MPa 
               
               
                 Exterior 
                 Chamfer angle, g 
                 &gt;45° 
               
               
                   
                 Wall thickness at tip 
                 &lt;1x chip size 
               
               
                 Velocity 
                 vacuum velocity, Vv 
                 Vector sum must point 
               
               
                   
                 staging velocity, Vs 
                 inside opening 222 
               
               
                   
               
             
          
         
       
     
         [0044]    As illustrated, the number of air pushers  250 A,  250 B may be equal to or greater than one. If only one air pusher  250 A is provided, it may be situated near the top of the cavity  225 , to prevent chips from ricocheting off the top of the cavity  225 . If more than two air pushers  250 A,  250 B are provided, their orientation angles may be a continual change from angle a 1  to angle a 2 . 
         [0045]    Although the position of the tunnel is aligned with the pushers  250 A,  250 B, the alignment need only be approximate, and may be dependent upon the relative strengths of the pressure from the air pushers  250 A,  250 B and the vacuum in the tunnel  230 . For example, if the vacuum force is great, so that most chips enter the tunnel  230  without an assist from the air pushers  250 A,  250 B, the air pushers  250 A,  250 B may be situated higher in the cavity  225 , their function being primarily to redirect those chips that have a high vertical velocity toward the horizontal and into the tunnel  230 . 
         [0046]    To optimize production time, the submount  150  or the SC  220  is moved quickly to place each next semiconductor  160  with chip  170  beneath the opening  222 . In some embodiments, the submount  150  travels at a varying velocity Vs due to acceleration and deceleration of stage movement, and the laser is activated when the semiconductor  160  with chip  170  is staged beneath the opening  222 . The resulting velocity of the chip upon liftoff will be equal to the vector sum of the staging velocity Vs and the velocity induced by the vacuum Vv, including the initial velocity due to the laser separation of the chip  170  from the semiconductor  160 . When the laser is activated, this vector sum Vvs must point into the opening  222 . Accordingly, the opening  222  may be elongated (rectangular) to accommodate the offset produced by the lateral staging velocity Vs. In like manner, if SC  220  travels at velocity Vs and the submount  150  is held stationary, the opening  222  may also be elongated to accommodate the movement of SC  220 . In some embodiments, the submount  150  is moved to place the next semiconductor  160  with chip  170  beneath the opening and stopped completely before the laser is activated. 
         [0047]    The other parameters and dimensions in TABLE 1 are self explanatory to one of skill in the art and need no further details. 
         [0048]    As noted above, high speed camera recordings have provided evidence that the yield loss using the aspects of this invention has been reduced by an order of magnitude, in one example, from 0.236% to 0.023%. Additionally, the rate of fallout (the number of chips that exit the pickup opening per the number of chips processed) has been reduced by well over an order of magnitude, from an average of 4.31% (25/580) to 0.24% (1.4/580). 
         [0049]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.