Abstract:
In a method of manufacturing a flip-chip light emitting diode, semiconductor layers ( 14, 16, 16′, 18, 18′, 20, 20 ′) that define a light emitting electrical junction ( 18, 18 ′) are epitaxially deposited on a principle surface of an epitaxy substrate ( 12, 12 ′). A light-emitting device mesa ( 24, 24 ′) is formed from the epitaxially deposited semiconductor layers. A first electrode ( 30, 30′, 54 ) is formed on a portion of the device mesa distal from the epitaxy substrate. The first electrode electrically contacts the device mesa. A second electrode ( 32, 32′, 56 ) is disposed on the principle surface of the substrate. First and second electrodes are flip-chip bonded to bonding pads ( 40, 40′, 42, 42 ′). The epitaxy substrate is removed. An electrically conductive, light-transmissive window layer ( 14, 14 ′) is arranged over the device mesa and the second electrode. The window layer forms an electrical connection between the device mesa and the second electrode.

Description:
[0001]    This application claims the benefit of provisional application Ser. No. 60/464,512 filed on Apr. 22, 2003. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates to the lighting arts. It especially relates to phosphide-based high power flip chip light emitting diodes for signaling and lighting applications, and will be described with particular reference thereto. However, the invention will also find application in conjunction with other types of flip-chip light emitting diodes and with other types of optoelectronic devices.  
           [0003]    Light emitting diodes are increasingly being employed in outdoor displays and signal lights, indoor illumination, and other applications. High brightness AlInGaP light emitting diodes provide light in the red to amber spectral range. A mature epitaxial growth technology base exists for generating high-quality AlInGaP layers on GaAs substrates. Light emitting devices are typically fabricated by forming a mesa and depositing front and back electrodes on the topmost AlInGaP layer and on the conductive GaAs substrate, respectively. Alternatively, two front-side contacts can be employed.  
           [0004]    Heretofore, AlInGaP light emitting diodes have not typically employed a flip-chip arrangement, such as is well-known for GaN-based ultraviolet and blue light emitting diodes. A reason for this is that unlike the transparent sapphire epitaxy substrates generally used for GaN epitaxy, GaAs substrates used in AlInGaP epitaxy are absorbing for red and amber light.  
           [0005]    The present invention contemplates an improved apparatus and method that overcomes the above-mentioned limitations and others.  
         BRIEF SUMMARY  
         [0006]    According to one aspect, a method of manufacturing a light emitting diode is provided. A plurality of semiconductor layers are deposited on a deposition substrate. At least some of the deposited semiconductor layers are removed from a selected trench region of the deposition substrate to define a light-emissive mesa. An electrode is formed on the mesa. The mesa is flip-chip bonded to a first electrical bonding pad of a thermally conductive support. The deposition substrate is removed.  
           [0007]    According to another aspect, a flip-chip light emitting diode is disclosed. A thermally conductive support structure includes first and second electrical pads arranged on a surface of the support structure for delivering electrical power. A plurality of light-generating semiconductor layers define a light-emissive mesa electrically contacting the first electrical pad. A window layer is disposed over the light-emissive mesa and the second electrical pad. The window layer electrically contacts the second electrical pad. The window layer is light-transmissive with respect to light generated by the light-generating semiconductor layers. The window layer further is electrically conductive to define a current-spreading electrical path between the light-emissive mesa and the second electrical pad.  
           [0008]    According to yet another aspect, a method of manufacturing a flip-chip light emitting diode is provided. Semiconductor layers that define a light emitting electrical junction are epitaxially deposited on a principle surface of an epitaxy substrate. A light-emitting device mesa is formed from the epitaxially deposited semiconductor layers. A first electrode is formed on a portion of the device mesa distal from the epitaxy substrate. The first electrode electrically contacts the device mesa. A second electrode is disposed on the principle surface of the substrate. First and second electrodes are flip-chip bonded to bonding pads. The epitaxy substrate is removed. An electrically conductive, light-transmissive window layer is arranged over the device mesa and the second electrode. The window layer forms an electrical connection between the device mesa and the second electrode.  
           [0009]    Numerous advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the present specification. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.  
         [0011]    [0011]FIG. 1 shows a side view of an AlInGaP light emitting diode prior to flip-chip bonding.  
         [0012]    [0012]FIG. 2 shows a side view of the AlInGaP light emitting diode epitaxial structure of FIG. 1 flip-chip bonded to a support, with the epitaxy substrate removed.  
         [0013]    [0013]FIG. 3A shows a top view of the p-type and n-type electrodes of the AlInGaP light emitting diode epitaxial structure of FIG. 1.  
         [0014]    [0014]FIG. 3B shows a top view of alternative p-type and n-type electrodes with a larger p-type electrode-to-n-type electrode area ratio.  
         [0015]    [0015]FIG. 4 shows a side view of another AlInGaP light emitting diode epitaxial structure, flip-chip bonded to a support.  
         [0016]    [0016]FIG. 5 shows a side view of the flip-chip bonded AlInGaP light emitting diode epitaxial structure of FIG. 4, with the epitaxy substrate removed and a window layer deposited.  
     
    
       [0017]    It is to be appreciated that the FIGURES are for exemplary purposes only and are not drawn to scale.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    With reference to FIG. 1, an AlInGaP light emitting diode epitaxial structure  10  includes a GaAs epitaxy substrate  12 , a window layer  14  epitaxially deposited on the GaAs substrate  12 , and a plurality of light-emitting semiconductor layers  16 ,  18 ,  20  epitaxially deposited on the window layer  14 . In the exemplary illustrated device structure  10 , the light-emitting semiconductor layers  16 ,  18 ,  20  include an n-type AlInGaP layer  16  and a p-type AlInGaP layer  20 , with an active region  18  of low-doped (typically not intentionally doped) AlInGaP disposed therebetween. In a preferred embodiment, the semiconductor layers  16 ,  18 ,  20  are epitaxially deposited using metalorganic chemical vapor deposition (also known as organometallic vapor phase epitaxy or the like). In another suitable embodiment, molecular beam epitaxy is used to deposit semiconductor layers  16 ,  18 ,  20 .  
         [0019]    In operation, holes and electrons are supplied to the active region  18  that is arranged at the electrical junction of the p-type and n-type layers  16 ,  20 . Electron-hole pairs radiatively recombine in the active region  18  to emit light. The spectral distribution of emitted light is generally determined by characteristics of the active region  18 , such as the composition and bandgap of the AlInGaP layer  18 .  
         [0020]    The illustrated light-emitting semiconductor layers  16 ,  18 ,  20  are exemplary only. Those skilled in the art can readily include other or additional semiconductor layers for achieving specific light emission characteristics. For example, the active region can include quantum wells. Additional heavily doped p- and/or n-type layers can be incorporated to improving electrical current injection into the active region  18 . The alloy compositions of the AlInGaP layers are selected to obtain desired bandgap, optical, and other material characteristics while remaining substantially lattice-matched to GaAs. However, as is known in the art some lattice mismatch can be included, especially in thinner layers such as active region quantum wells, to produce coherently strained layers. Incorporation of one or more strain-relaxed lattice-mismatched layers is also contemplated. However, in view of material degradation that typically occurs due to defects introduced by strain relaxation, lattice-matched or coherently strained layers are generally preferred.  
         [0021]    The window layer  14  is selected to be substantially electrically conductive and light-transmissive for light generated by the light-emitting semiconductor layers  16 ,  18 ,  20 . Unless the semiconductor layers  16 ,  18 ,  20  are intended to be strain-relaxed, the window layer  14  should also be lattice-matched to the GaAs substrate. For the exemplary AlInGaP light emitting diode structure, suitable materials for the window layer  14  include ternary AlGaAs, which is lattice-matched to GaAs for most compositions, and wide-bandgap AlInGaP materials such as AlInP. To provide adequate light extraction and electrical current spreading, the window layer  14  preferably has a thickness of at least two microns.  
         [0022]    A device mesa  24  is defined by selectively removing the semiconductor layers  16 ,  18 ,  20  in selected trench regions  26  of the substrate  12 . In a preferred embodiment, the trench regions  26  are formed photolithographically. A p-type electrode  30  is formed on a surface of the device mesa  24  distal from the substrate  12 . An n-type electrode  32  is disposed on the substrate  12  by forming the electrode  32  on the window layer  14  in the trench region  26 . Optionally, an insulating layer  34  is deposited between the device mesa  24  and the n-type electrode  32  to ensure substantial electrical isolation therebetween. The electrodes  30 ,  32  and optional insulator  34  are suitably formed by evaporation, sputtering, or otherwise depositing the metal or insulator material, in conjunction with photolithographic processing. The electrodes  30 ,  32  can be formed of the same or different material. Typically, each electrode includes a metal layer stack designed to provide an approximately ohmic contact with the semiconductor  20  or the window layer  14 .  
         [0023]    Those skilled in the art recognize that the AlInGaP light emitting diode epitaxial structure  10  is similar to a conventional AlInGaP light emitting diode epitaxial structure, except that the window layer  14  is arranged adjacent to the epitaxy substrate  12 . In contrast, in a conventional AlInGaP light emitting diode epitaxial structure the window layer, if present, is arranged as a topmost layer or a nearly topmost layer of the epitaxial semiconductor layer stack. That is, in a conventional AlInGaP light emitting diode epitaxial structure, the window is arranged in the semiconductor stack distal from the substrate, rather than adjacent thereto.  
         [0024]    With reference to FIG. 2, the AlInGaP light emitting diode epitaxial structure  10  is flip-chip bonded to p-type and n-type electrical bonding pads  40 ,  42  of a thermally conductive sub-mount  44  via solder bumps  46 ,  48  respectively. After flip-chip bonding, the GaAs substrate  12  is removed by chemical etching, plasma etching, or another suitable technique to expose the window layer  14 . Preferably, the window layer  14  acts as an etch stop. For example, certain phosphoric acid/hydrogen peroxide aqueous solutions are known to be highly selective for etching GaAs over phosphide-based semiconductors. Rather than or in conjunction with chemical or plasma etching, mechanical polishing can be used to remove or thin the epitaxy substrate  12 .  
         [0025]    After substrate removal, the exposed surface  50  of the window layer  14  is optionally roughened, patterned, or otherwise modified to further improve light extraction. Light extraction can be improved by forming a Fresnel lensing pattern on the surface  50 , or by applying an epoxy coating, plastic coating, refractive index matching coating after removal of the epitaxy substrate  12 . A micro-lens can also be bonded to the exposed surface  50 . Optionally, the device mesa  24  can additionally include a distributed Bragg reflector or other reflective layer or layers (not shown) disposed between the p-type AlInGaP layer  20  and the p-type electrode  30  during epitaxy to further increase reflectivity.  
         [0026]    In operation, electrical bias applied at the electrical bonding pads  40 ,  42  energize the device through a current path (traced in the following from positive to negative) including: the p-type bonding pad  40 ; the solder bump  46 ; the p-type electrode  30 ; the device mesa  24 ; the current-spreading window layer  14 ; the n-type electrode  32 ; the solder bump  48 ; and the n-type bonding pad  42 . Light emitted by the device mesa  24  passes out the window layer  14  via the surface  50  as the device light output. Light generally directed toward the p-type electrode  30  is reflected toward the window layer  14  and also contributes to the device light output through the surface  50 .  
         [0027]    To enable chip-scale package design, bonding pads  40 ,  42  for a plurality of light emitting diodes can be interconnected by printed circuitry on the sub-mount  44  or other support structure. Moreover, the support structure can include driving electronics connected to the light emitting diodes by the printed circuitry.  
         [0028]    With reference to FIG. 3A, the electrodes  30 ,  32  are arranged with a gap  52  therebetween. However, other electrode configurations can also be employed, such as the configuration shown in FIG. 3B, in which a p-type electrode  54  is made larger compared with the electrode  30 , while an n-type electrode  56  is made smaller. A large p-type electrode-to-n-type electrode area ratio is advantageous since the light emitting mesa underlies the p-type electrode. (However, it will be recognized that if the polarity of the mesa is reversed, so that the n-type electrode is on the mesa, then a large n-type electrode-to-p-type electrode area ratio is advantageous). The optimal electrode area ratio depends upon various factors including chip size, current spreading capability of the window layer, and the ohmic contact quality of the electrodes.  
         [0029]    With reference to FIG. 4, another AlInGaP light emitting diode epitaxial structure  10 ′ includes a GaAs epitaxy substrate  12 ′ and a plurality of light-emitting semiconductor layers  16 ′,  18 ′,  20 ′ epitaxially deposited on the GaAs epitaxy substrate  12 ′. A device mesa  24 ′ is defined by selectively removing the semiconductor layers  16 ′,  18 ′,  20 ′ in selected trench regions  26 ′ of the substrate  12 ′. A p-type electrode  30 ′ is formed on a surface of the device mesa  24 ′ distal from the substrate  12 ′. An n-type electrode  32 ′ is disposed on the substrate  12 ′ by forming the electrode  32 ′ on the epitaxy substrate  12 ′ in the trench region  26 ′. Optionally, an insulating layer  34 ′ is deposited between the device mesa  24 ′ and the n-type electrode  32 ′ to ensure substantial electrical isolation therebetween.  
         [0030]    It will be recognized that the epitaxial structure  10 ′ is generally similar to the epitaxial structure  10  of FIG. 1, except that the epitaxially grown window layer  14  of the epitaxial structure  10  is omitted. The epitaxial structure  10 ′ is flip-chip bonded to p-type and n-type electrical bonding pads  40 ′,  42 ′ of a thermally conductive sub-mount  44 ′ via solder bumps  46 ′,  48 ′ respectively.  
         [0031]    With reference to FIG. 5, after flip-chip bonding, the GaAs substrate  12 ′ is removed by chemical etching, plasma etching, or another suitable technique. Removal of the epitaxy substrate  12 ′ exposes the n-type AlInGaP layer  16 ′ and a surface of the n-type electrode  32 ′ where it had contacted the substrate  12 ′. Removal of the GaAs substrate  12 ′ effects a physical separation of the mesas  24 ′ at the trenches  26 ′. After removal of the epitaxy substrate  12 ′, a substantially electrically conductive and light-transmissive window layer  14 ′ is deposited over the n-type AlInGaP layer  16 ′ and the exposed surface of the n-type electrode  32 ′ to form an electrically conductive connection therebetween.  
         [0032]    Because the window layer  14 ′ is deposited after epitaxial growth of the semiconductor layers  16 ′,  18 ′,  20 ′, the window layer  14 ′ is generally not epitaxial with respect to the semiconductor layers  16 ′,  18 ′,  20 ′. Rather, the window layer  14 ′ can be selected for preferred electrical and optical characteristics. Suitable materials for the window layer  14 ′ include sputtered indium tin oxide, GaP or AlGaAs grown by liquid phase epitaxy, or the like. The window layer  14 ′ is preferably at least two microns thick; hence, a fast deposition technique such as sputtering or liquid phase epitaxy is preferably employed. In contrast, the epitaxial window  14  of the embodiment of FIGS. 1 and 2 is grown along with semiconductor layers  16 ,  18 ,  20  by metalorganic chemical vapor deposition or molecular beam epitaxy, which are slower layer deposition techniques.  
         [0033]    The electrodes  30 ′,  32 ′ are typically metal layer stacks optimized to provide approximately ohmic contact with the p-AlInGaP layer  20 ′ and the window layer  14 ′, respectively. Moreover, if the electrical characteristics of a direct junction between material of the window layer  14 ′ and material of the solder bump  48 ′ are sufficiently ohmic, the n-type electrode  32 ′ is optionally omitted.  
         [0034]    A surface  50 ′ of the window layer  14 ′ is optionally roughened or patterned (for example with a Fresnel lens pattern) to further improve light extraction. Additionally, an encapsulant  60  can be included adjacent to the surface  50 ′ and surrounding the light emitting diode device to hermetically seal the light emitting diode and to improve light extraction. Similarly, a microlens or other light-coupling element can be bonded to the surface  50 ′ of the window layer  14 ′.  
         [0035]    Although AlInGaP light emitting diodes have been illustrated, those skilled in the art can readily adapt these exemplary devices to the manufacture flip-chip bonded light emitting diodes in other materials systems that are grown on epitaxy substrates that substantially absorb the light output.  
         [0036]    The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.