Abstract:
A light emitting diode (LED) is fabricated using an underfill layer that is deposited on either the LED or the submount prior to mounting the LED to a submount. The deposition of the underfill layer prior to mounting the LED to the submount provides for a more uniform and void free support, and increases underfill material options to permit improved thermal characteristics. The underfill layer may be used as support for the thin and brittle LED layers during the removal of the growth substrate prior to mounting the LED to the submount. Additionally, the underfill layer may be patterned to and/or polished back so that only the contact areas of the LED and/or submount are exposed. The patterns in the underfill may also be used as a guide to assist in the singulating of the devices.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related to the fabrication of a light emitting diode. 
       BACKGROUND 
       [0002]    Semiconductor light-emitting diodes (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors; for example, binary, ternary, and quaternary alloys of gallium, aluminum, indium, nitrogen, phosphorus, and arsenic. III-V devices emit light across the visible spectrum. GaAs- and GaP-based devices are often used to emit light at longer wavelengths such as yellow through red, while III-nitride devices are often used to emit light at shorter wavelengths such as near-UV through green. 
         [0003]    Gallium nitride LEDs typically use a transparent sapphire growth substrate due to the crystal structure of sapphire being similar to the crystal structure of gallium nitride. 
         [0004]    Some GaN LEDs are formed as flip chips, with both electrodes on the same surface, where the LED electrodes are bonded to electrodes on a submount without using wire bonds. A submount provides an interface between the LED and an external power supply. Electrodes on the submount bonded to the LED electrodes may extend beyond the LED or extend to the opposite side of the submount for wire bonding or surface mounting to a circuit board. 
         [0005]      FIGS. 1A-1D  are simplified cross-sectional views of the process of mounting GaN LEDs  10  to a submount  12  and removing the sapphire growth substrate  24 . The submount  12  may be formed of silicon or may be a ceramic insulator. If the submount  12  is silicon, an oxide layer may insulate the metal pattern on the submount surface from the silicon, or different schemes of ion implantation can be realized for added functionality such as electrostatic discharge protection. 
         [0006]    As can be seen in  FIG. 1A , a number of LED dies  10  are formed with a thin GaN LED layer  18  formed on a sapphire growth substrate  24 . Electrodes  16  are formed in electrical contact with the n-type and p-type layers in the GaN layer  18 . Gold stud bumps  20  are placed on the electrodes  16  on the LEDs  10  or alternatively on the metal pads  14  on the submount  12 . The gold stud bumps  20  are generally spherical gold balls placed at various points between the LED electrodes  16  and the submount metal pads  14 . The LED layers  18  and electrodes  16  are all formed on the same sapphire substrate  24 , which is then diced to form the individual LED dies  10 . 
         [0007]    As illustrated in  FIG. 1B , the LEDs  10  are bonded to the substrate  12  with the metal pads  14  on the submount  12  electrically bonded to the metal electrodes  16  on the GaN layers  18 . Pressure is applied to the LED structure while an ultrasonic transducer rapidly vibrates the LED structure with respect to the submount to create heat at the interface. This causes the surface of the gold stud bumps to interdiffuse at the atomic level into the LED electrodes and submount electrodes to create a permanent electrical connection. Other types of bonding methods include soldering, applying a conductive paste, and other means. 
         [0008]    Between the LED layers  18  and the surface of the submount  12  there is a large void that is filled with an epoxy to provide mechanical support and to seal the area, as illustrated in  FIG. 1C . The resulting epoxy is referred to as an underfill  22 . Underfilling is very time-consuming since each LED dies  10  must be underfilled separately, and a precise amount of underfill material needs to be injected. The underfill material must be a low enough viscosity that it can flow under the LED dies  10 , which may include a complicated geometry of electrodes, without trapping any bubbles that could result in poorly supported regions, as illustrated as region  22   a . The underfill material, however, must not spread in an uncontrolled fashion onto undesirable surfaces, such as the top of the LED device, as illustrated at  22   b , or pads on the submount where wire bonds must be subsequently applied. 
         [0009]    The sapphire substrates  24  are removed after the LED dies  10  are bonded to the submount  12  and the submount  12  is separated into individual elements to form the LED structures illustrated in  FIG. 1D . Since the LED layers  18  are very thin and brittle, the underfill serves the additional purpose to provide the necessary mechanical support to prevent fracturing of the fragile LED layers when the supporting substrate  24  is removed. The gold stud bumps  20  do not provide sufficient support by themselves to prevent fracturing of the LED layers since, given their limited shape and are spaced far apart. Conventionally used underfill materials are typically composed of organic substances and possess very different thermal expansion properties from metal and semiconductor materials. Such spurious expansion behavior is particularly aggravated at high operating temperatures—typical of high power LED applications—where underfill materials approach their glass transition point and begin to behave as elastic substances. The net effect of such mismatch in thermal expansion behavior is to induce stresses on the LED devices that limit or reduce their operability at high power conditions. Lastly, underfill materials have low thermal conductivity properties that result in unnecessarily high temperature operation for the semiconductor devices. 
         [0010]    What are needed are techniques for mechanically supporting the thin LED layers during a substrate removal process which provides a more uniform and void free support; provides support with more closely matched thermal expansion behavior, provide a support with high temperature operability, not limited by the glass transition point of organic materials; and provides a support with improved thermal conductivity for superior heat sinking. 
       SUMMARY 
       [0011]    A light emitting diode (LED) is fabricated using an underfill layer that is deposited on either the LED or the submount prior to mounting the LED to a submount. The deposition of the underfill layer prior to mounting the LED to the submount provides for a more uniform and void free support, and increases underfill material options to permit improved thermal characteristics. In one embodiment, the underfill layer may be deposited on the LEDs and used support for the thin and brittle LED layers during the removal of the growth substrate. The growth substrate can then be removed at the wafer level prior to mounting the LED to the submount. In other embodiments, the underfill layer may be patterned and/or polished back so that only the contact areas of the LED and/or submount are exposed. The LEDs and submount can then be bonded with the underfill layer between them. The patterned underfill layer may also be used as a guide to assist in the singulating of the devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIGS. 1A-1D  are simplified cross-sectional views of the process of mounting LEDs to a submount, followed by injecting an underfill and removing the sapphire growth substrate. 
           [0013]      FIGS. 2A-2E  are simplified cross-sectional views of a process of removing the growth substrate from LEDs on the wafer level and mounting the LEDs to a submount in accordance with one embodiment of the present invention. 
           [0014]      FIG. 3  illustrates a portion of the LED structure including a sapphire growth substrate and GaN layers. 
           [0015]      FIGS. 4A and 4B  illustrate the deposition of the underfill material over a portion of the LEDs at the wafer level. 
           [0016]      FIGS. 5A-5C  are simplified cross-sectional views of mounting LEDs to a submount and removing the growth substrate after the LEDs are mounted to the submount. 
           [0017]      FIGS. 6A-6E  are simplified cross-sectional views of a process of mounting LEDs to a submount that has an underfill coating. 
           [0018]      FIGS. 7A-7D  illustrate another embodiment in which wafer level LEDs with a patterned underfill layer are mounted to a submount. 
           [0019]      FIGS. 8A-8G  illustrate another embodiment in which individual LED dies are mounted to a submount on which an underfill layer has been deposited. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIGS. 2A-2E  are simplified cross-sectional views of a process of mounting GaN LEDs to a submount and removing the growth substrate in accordance with one embodiment of the present invention. 
         [0021]      FIG. 2A  illustrates a portion of a wafer level LED structure  100  including a growth substrate  102 , which may be, e.g., sapphire, upon which has been formed the thin GaN LED layers  104 . The GaN LED layers  104  may be conventionally grown on the sapphire substrate, as described, e.g., in U.S. Publication No. 2007/0096130, which is incorporated herein by reference.  FIG. 3  illustrates a portion of the wafer structure  100  including a sapphire substrate  102  over which an n-type GaN layer  104   n  is grown using conventional techniques. The GaN layer  104   n  may be multiple layers including a clad layer. The GaN layer  104   n  may include Al, In, and an n-type dopant. An active layer  104   a  is then grown over the GaN layer  104   n.  The active layer  104   n  will typically be multiple GaN-based layers and its composition (e.g., In x AlyGa 1-x-y N) depends on the desired wavelength of the light emission and other factors. The active layer  104   a  may be conventional. A p-type GaN layer  104   p  is then grown over the active layer  104   a.  The GaN layer  104   p  may be multiple layers including a clad layer and may also be conventional. The GaN layer  104   p  may include Al, In, and a p-type dopant. The LED structure of  FIG. 3  is referred to as a double heterostructure. 
         [0022]    In one embodiment, the growth substrate is about 90 microns thick, and the GaN layers  104  have a combined thickness of approximately 4 microns. 
         [0023]    Although a GaN based LED with a sapphire growth substrate is used in the example, other types of LEDs using other substrates such as SiC (used to form an InAlGaN LED) and GaAs (used to form an AlInGaP LED) may benefit from the present invention. 
         [0024]    Metal bonding layers are formed over the wafer to form n-contacts  108   n  and p-contacts  108   p , referred to herein as contacts  108 . The contacts  108  may be patterned by forming a masking layer at positions where the metal contacts are not desired, then depositing the metal contact layer over the entire wafer, and then stripping the masking layer to lift off the metal deposited over it. The metal layers could also be negatively patterned by depositing similarly stacked blanket metal layers and then selectively etching them back using a masking scheme. The contacts may be formed from one or more metals, such as TiAu, Au, Cu, Al, Ni or other ductile material, or a combination of such layers. Stud bumps  110 , which may be, e.g., gold, are then formed over the contacts  108 . The stud bumps  110  are generally spherical gold balls placed at various points on the contact  108 . The stud bumps  110  serve as part of the contacts for the LED and are used to bond the LED to a submount. If desired, other types of bonding material or structures such as plates may be used in place of stud bumps  110 . 
         [0025]    As illustrated in  FIG. 2 , the underfill  120  is then deposited over the GaN layers  104 , contacts  108  and bumps  110 . Because the underfill  120  is applied prior to bonding the LEDs to a submount, any suitable material may be used for the underfill  120  without depending on the flow characteristics that are needed in the case of traditional underfill. For example, a polymer polyimide based materials, which have high glass transition temperature, may be used as the underfill layer  120 . With the use of polyimide material, a solvent may be used to adjust the viscosity to 400-1000 Pa*s to assist in the deposition of the material. A filler powder, such as small particle SiO 2 , may be added to the polyimide material, e.g., in the amount of 50% to 90%) in order to match the CTE (coefficient of thermal expansion). During deposition, the LEDs and polyimide material may be heated below the glass transition temperature, and then allowed to cool down to cure. Such materials will help the device withstand high temperature/high current condition without deformation of thin LEDs. The underfill material may be a bi-stage cured material, using a low temperature cure for cross-linking of additives, e.g., epoxy additives, in the underfill material that are responsible for the first stage cure. The underfill material should have B-stage cure properties in order to adhere to both LED and support wafer surfaces. 
         [0026]      FIGS. 4A and 4B  illustrate the deposition of the underfill  120  material over a portion of the wafer  100 . As shown in  FIG. 4A , the underfill  120  may be blanket deposited over the surface of the GaN layers  104 . In one embodiment, the underfill  120  is patterned, e.g., using stencil printing or mesh screen-printing, so that areas  122 , such as area where the LED dies are to be separated, have no underfill  120 . For example, the material may be deposited in the form of a viscous paste using, e.g., stencil printing techniques. Areas where underfill is not desired, e.g., the areas around the LEDs where there is wire bonding, are protected by a mask. Openings in the mask allow the underfill material to be deposited in desired areas. After deposition, e.g., by screen printing, the underfill layer is cured at low temperature, e.g., 120°-130° C., until it is sufficiently hard to be polished. As shown in  FIG. 4B , the underfill  120  is then polished back until the metal connection, i.e., bumps  110 , are exposed. In one embodiment, the underfill  120  has a final thickness of 30 μm. 
         [0027]    The underfill  120  advantageously serves as a support layer for the GaN layers  104  and, accordingly, the growth substrate  102  can be removed, as illustrated in  FIG. 2C . The substrate  102  can be removed, e.g., by laser lift-off using an excimer laser beam that is transmitted through the transparent sapphire substrate  102  and evaporates a top layer of the n-GaN layer  104   n.  The removal of the substrate  102  produces tremendous pressure at the substrate/n-GaN layer  104   n  interface. The pressure forces the substrate  102  off the n-GaN layer  104   n , and the substrate  102  is removed. The support provided by the underfill  120  prevents the high pressure during the substrate lift-off from fracturing the thin brittle LED layers  104 . Additionally, if desired, the exposed n layer  104   n  (shown in  FIG. 3 ) may be roughened for increasing light extraction, e.g., using photo-electro-chemical etching, or by small scale imprinting or grinding. Alternatively, roughening can include forming prisms or other optical elements on the surface for increased light extraction and improved control of the radiation pattern. 
         [0028]    After the substrate is removed, the GaN layers  104  with the underfill  120  are scribed and separated into individual LED elements. The scribing and separation may be accomplished using, e.g., a saw that uses the areas  122  in the underfill  120  as a guide. Alternatively, a laser scribe process may be used. Prior to separating, the wafer of LEDs is adhered to a stretchable plastic sheet, and after the wafer is broken along the scribe lines, the sheet is stretched to separate the dies while the dies remain adhered to the stretchable sheet. An automatic pick and place device then removes each die  125  from the sheet, and mounts the die  125  on a submount  130  as illustrated in  FIG. 2D . The bonding metal, i.e., bumps  110  on the LED dies  125  are ultrasonically or thermosonically welded directly to corresponding bonding metal patterns  132  on the submount  130 , which may be, e.g., gold or other appropriate material. The submount  130  may be formed of silicon or a ceramic insulator. If the submount  130  is silicon, an oxide layer may insulate the metal pattern on the submount surface from the silicon, or different schemes of ion implantation can be realized for added functionality such as Zener diodes for electro-discharge protection. If the submount is a ceramic instead of silicon, the metal patterns can be directly formed on the ceramic surfaces. 
         [0029]    An ultrasonic transducer (thermosonic metal-to-metal interdiffusion process) may be used to apply a downward pressure to the dies  125  and rapidly vibrates the dies  125  with respect to the submount  130  so that the atoms from the opposing bonding metals merge to create an electrical and mechanical connection between the die  125  and the submount  130 . Other methods for LED die-to-submount interconnection can also be used, such as using a soldering layer. During die attach process, e.g., with an Au—Au interconnect, the substrate temperature is maintained above (e.g., 40-50° C.) the glass transition temperature Tg, which causes the underfill material to be in the elastic stage to soften and comply with the LEDs and to prevent the formation of voids. Subsequently, the underfill layer is permitted to cure at approximately the glass transition temperature, e.g., 200° C., for 1-2 hours in order to harden. The submount  130  can then be scribed and singulated to form LEDs  140  as illustrated in  FIG. 2E . 
         [0030]    In another embodiment, the growth substrate  102  is not removed until after the wafer is separated into separate dies and mounted to the submount  130 , as illustrated in  FIGS. 5A ,  5 B, and  5 C. As shown in  FIG. 5A , the sapphire substrate  102  with LED GaN layers  104  are scribed and separated into individual dies  150 . The scribing and separation may be accomplished using, e.g., a saw that cuts through the sapphire substrate  102  using areas  122  in the underfill  120  as a guide. The separate dies  150  are then mounted to the submount  130 , as described above. Once the dies  150  are mounted to the submount  130 , the sapphire substrate  102  can be lifted-off described above and as illustrated in  FIG. 5B . The submount  130  is then singulated to form LEDs  140 , as illustrated in  FIG. 5C . 
         [0031]    In another embodiment, the underfill may be deposited on the submount instead of the GaN layers.  FIGS. 6A-6E  are simplified cross-sectional views of a process of mounting GaN LEDs to a submount with an underfill coating. 
         [0032]      FIG. 6A  illustrates a portion of a submount  202  with bonding metal patterns  204  with bumps  206 . By way of example, a silicon submount  202  with Au bonding metal patterns  204  and Au bumps  206  may be used. Alternatively, other materials may be used if desired. An underfill layer  210  is deposited over submount  202 , bonding metal patterns  204  and bumps  206 . As discussed above, the underfill layer  210  may be patterned, e.g., using stencil printing or mesh screen-printing, so that areas, such as area where the LED dies are to be separated, have no underfill material. Because the underfill  210  is applied prior to bonding LEDs to the submount, any suitable material may be used for the underfill  210  without depending on the flow characteristics that are needed in the case of traditional underfill. For example, a polymer polyimide based materials, which have high glass transition temperature, may be used as the underfill layer  210 . Such materials will help the device withstand high temperature/high current condition without deformation of thin LEDs. The underfill material should have B-stage cure properties in order to adhere to both LED and support wafer surfaces. Moreover, an inorganic dielectric material may be used if desired. As illustrated in  FIG. 6B , the underfill layer  210  is polished back until the metal connection, i.e., bumps  206 , are exposed. 
         [0033]    A growth substrate  230  with an LED GaN layer  232  and contacts  234  is separated into separate LED dies  235 , which are then mounted on the submount  202  with underfill layer  210  as illustrated in  FIG. 6C . The contacts on the LED dies  234  are ultrasonically or thermosonically welded directly to corresponding the bonding bumps  206  on the submount  130 , as described above. Once mounted, the growth substrate  230  is removed, e.g., using a laser lift-off process, resulting in the structure illustrated shown in  FIG. 6D . The submount  202  is then singulated to form LEDs  240 , as illustrated in  FIG. 6E . 
         [0034]      FIGS. 7A-7D  illustrate wafer level LEDs with a patterned underfill layer that are mounted to a submount in accordance with another embodiment. 
         [0035]      FIG. 7A  is a simplified cross-sectional view of portions of wafer level LEDs  300  with a patterned underfill layer  310  and  FIG. 7B  is a plan view of the bottom surface of the wafer level LEDs  300  along line AA shown in  FIG. 7A . The wafer level LEDs  300  include a growth substrate  302  along with LED layers  304  and contacts  306 . The underfill layer  310  is deposited, e.g., using stencil printing or mesh screen-printing to form a pattern that exposes the contacts  306  on the bottom surface of the LED layers  304  and cured. The underfill pattern matches the pattern of contacts  322  that are present on the submount  320 , as can be seen in  FIG. 7C  so that only the areas where the LEDs will be attached are exposed. 
         [0036]    As illustrated in  FIGS. 7C and 7D , the wafer level LEDs  300  are mounted at the wafer level with a submount  320  with contacts  322 . The waver level LEDs  300  are mounted to the submount  320 , e.g. ultrasonically or thermosonically, to bond the contacts  306  on the LED layers  304  with the contacts  322  on the submount  320 , as described above. The growth substrate  302  can then be lift-off, e.g., using a laser lift-off process, and the LEDs singulated into individual dies. 
         [0037]      FIGS. 8A-8G  illustrate another embodiment in which individual LED dies are mounted to a submount on which an underfill layer has been deposited.  FIG. 8A  is a simplified cross-sectional view of portions of a submount  402  with contacts  404 .  FIG. 8B  is a plan view of the top surface of the submount  402  along line BB shown in  FIG. 8A . As illustrated in  FIG. 8C , an underfill layer  410  is deposited over the submount  402  and in particular is patterned to cover the contacts  404 . Once the underfill layer  410  cures, the underfill layer  410  is polished back to expose the contacts  404 , as illustrated in  FIG. 8D  and  FIG. 8E , which illustrates another plan view of the submount  402 . 
         [0038]    LED dies  420 , each of which includes a growth substrate  422 , LED layers  424  and contacts  426  on the bottom surface of the LED layers  424  are then individually mounted to the submount  402 , as illustrated in  FIG. 8F . Once mounted, the growth substrate  422  can be lifted off, as illustrated in  FIG. 8G , and the submount  402  can be singulated to form individual LED elements as described above. 
         [0039]    Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.