Abstract:
A light-emitting-diode (LED) array is disclosed which comprises a first LED device having a first electrode, a second LED device having a second electrode, wherein the first and the second LED device are formed on the same substrate and separated by a gap, at least one polymer material substantially filling the gap, and an interconnect, formed on top of the at least one polymer material, electrically connecting the first and the second electrode.

Description:
CROSS REFERENCE 
       [0001]    This application claims the benefits of U.S. Provisional Patent Application Ser. No. 61/351,779, which was filed on Jun. 4, 2010 and entitled “Interconnect structure &amp; process for multi-mesa based LED chip by gap-filling with polymer and low melting temperature metal connect.” 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to semiconductor-based light emitting devices, and, more particularly, to a structure of such devices and a method for manufacturing the same. 
         [0003]    A Light-emitting diode (LED) is a semiconductor diode based light source. When a diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. When used as a light source, the LED presents many advantages over incandescent light sources. These advantages include lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. 
         [0004]      FIG. 1  is a perspective view of a LED die  100  which comprises a substrate  102 , an N-type layer  110 , a light-emitting layer  125  and a P-type layer  130 . N-contact and p-contact  115  and  135  are formed on the N-type layer  110  and the P-type layer  130 , respectively, for making electrical connections thereto. When a proper voltage is applied to the N- and P-contacts  115  and  135 , electrons depart the N-type layer  110  and combine with holes in the light-emitting layer  125 . The electron-hole combination in the light-emitting layer  125  generates light. Sapphire is a common material for making the substrate  102 . The N-type layer  110  may be made of, for example, AlGaN doped with Si or GaN doped with Si. The P-type layer  240  may be made of, for example, AlGaN doped with Mg or GaN doped with Mg. The light-emitting layer  125  is typically formed by a single quantum well or multiple quantum wells, e.g. InGaN/GaN. 
         [0005]    In some cases, a series or parallel LED array is formed on an insulating or highly resistive substrate (e.g. sapphire, SiC, or other III-nitride substrates). The individual LEDs are separated from each other by trenches, and interconnects deposited on the array electrically connect the contacts of the individual LEDs in the arrays. Typically, to make sure complete electrical isolation of individual LEDs, a dielectric material is deposited over the LED array before the interconnects deposition, then patterned and removed in places to open contact holes on N-type layer and P-type layer, such that dielectric material is left in trench between the individual LEDs on the substrate and on the mesa walls between the exposed P-type layer and N-type layer of each LED. Dielectric material may be, for example, oxides of silicon, nitrides of silicon, oxynitrides of silicon, aluminum oxide, or any other suitable dielectric material. 
         [0006]    However, deposition of dielectric material is a slow and costly process. Moreover, subsequently formed interconnects which poses reliability concern due to complex profiles and sharp corners of the interconnects. As such, what is desired is a system and method for manufacturing a LED array device cost-effectively and with improved long term reliability. 
       SUMMARY 
       [0007]    A light-emitting-diode (LED) array is disclosed which comprise a first LED device having a first electrode, a second LED device having a second electrode, wherein the first and the second LED device are formed on the same substrate and separated by a gap, at least one polymer material substantially filling the gap, and an interconnect, formed on top of the at least one polymer material, electrically connecting the first and the second electrode. 
         [0008]    In another aspect of the present invention, a method for forming a light-emitting-diode (LED) array is disclosed which comprises forming a LED structure on a substrate, dividing the LED structure into at least a first and a second LED device with a gap, depositing at least one polymer material over the LED structure substantially filling the gap, removing portions of the at least one polymer material to expose a first electrode of the first LED device and a second electrode of the second LED device, and forming an interconnect on top of the at least one polymer material electrically connecting the first and second electrode. 
         [0009]    The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer conception of the invention, and of the components and operation of systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore non-limiting, embodiments illustrated in the drawings, wherein like reference numbers (if they occur in more than one view) designate the same elements. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. 
           [0011]      FIG. 1  is a perspective view of a LED die. 
           [0012]      FIGS. 2A and 2B  are top views of an LED array in a single substrate. 
           [0013]      FIG. 3  is a cross-sectional view of the conventional LED array shown in  FIG. 2B . 
           [0014]      FIGS. 4A-4C  illustrates processing steps for forming a LED device according to an embodiment of the present invention. 
           [0015]      FIG. 5  illustrates a trench formed in the substrate to separate two LED devices according to another embodiment of the present invention. 
           [0016]      FIGS. 6A and 6B  illustrate some alternative patterns of the interconnects. 
           [0017]      FIG. 7  illustrates a LED chip being flip mounted on a board. 
       
    
    
     DESCRIPTION 
       [0018]    The present invention discloses a LED array structure and a process method for manufacturing the same. The LED array is formed by multiple LED devices for producing significant amount of light at relatively low current density. Low current density generates less heat and allows polymer materials to be used in forming the LED array. Details of the LED array structure and the process for manufacturing the same are described hereinafter. 
         [0019]      FIGS. 2A and 2B  is a top view of an of LED array  200  in a single substrate  205 . Referring to  FIG. 2A  and for illustration purpose, the LED array  200  has four rows (Y) and four columns (X) of separated yet identical LED devices  210 [0:3, 0:3], each shaped like a mesa. The LED devices  210  may be separated by laser etching or Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE). As an example, neighboring LED devices,  210 [2, 3] and  210 [3, 3] are separated by a gap  220 [2]. The LED device  210 [2, 3] has two electrodes, i.e., pads  213 [2, 3] and  215 [2, 3] serving as anode and a cathode of the LED device  210 [2, 3], respectively. The electrodes can be formed on P-GaN and N-GaN (either P-side up or N-side up). One LED device&#39;s anode pad is placed close to a neighboring LED device&#39;s cathode pad, so that the LED devices  210  can be easily connected in series. 
         [0020]    Referring now to  FIG. 2B , the pad  213 [2, 3] and the pad  215 [3, 3] are connected by an interconnect  230 [2, 3]. The pads  213  and  215  are typically formed by a metal, and so is the interconnect  230 . The pads  213  and  215  and the interconnect  230  may not necessarily be formed by the same metal. 
         [0021]      FIG. 3  is a cross-sectional view of the conventional LED array  202  at a location A-A′ shown in  FIG. 2B . On a single substrate  205 , multiple LED devices  210  are built with cross-sections of two adjacent ones,  210 [1, 3] and  210 [2, 3] shown in  FIG. 3 . The pad  213 [1, 3], for instance, is an anode of the LED device  210 [1, 3]. The pad  215 [2, 3] is a cathode of the LED device  210 [2, 3]. Conventionally, an oxide layer  310  is deposited in the gap  220 [1] between the LED devices  210  to electrically isolate the pads  213  and  215  from adjacent structures. Then the metal interconnect  230 [1, 3] is formed on top of the oxide layer  310  to connect the pads  213 [1, 3] and  215 [2, 3]. Due to the depth of gap  220 , the oxide layer  310  cannot fill up the gap  220 , and causing the metal interconnect  230  to form a complex profile with sharp corners. The sharp corners are relatively easy to break hence become a reliability concern. 
         [0022]      FIGS. 4A-4C  illustrates processing steps that uses a polymer to fill up the gap  220  between the LED devices  210  according to an embodiment of the present invention. Because the LED devices in accordance with the present invention are intended to be used at high efficiency with little heat generated, it is feasible to leave polymer material in a finished LED device. 
         [0023]    Referring to  FIG. 4A , after each individual LED devices  210  and respective pads  213  and  215  are formed, a polymer layer  410  is deposited over the LED devices  210 . The polymer layer  410  fills up the gap  220 . Photoresist, such as polymethylglutarimide (PMGI) or SU-8, is a preferred polymer material. Refractive index of the polymer layer  410  ranges from 1 to 2.6 (between air and semiconductor) to enhance light extraction. Optical transparency of the polymer layer  410  is equal to or more than 90%, and preferably equal to or more than 99%. Typically, a thickness of the polymer layer  410  measured on top of the pad  213  is approximately 2 micron meter. The polymer layer  410  can be pre-mixed with phosphor (about 30 weight percentage loading) to adjust the output light color. However, the relative dimension between polymer coating thickness and phosphor particle size should be coordinated. For example, when a thickness of the polymer layer  410  at the pad  213  is about 3 micron meter, proper phosphor particle size is approximately 3 micron meter or less. 
         [0024]    Referring to  FIG. 4B , a patterned mask  420  is applied over the polymer layer  410 . The mask  420  has openings  423  at the locations of pads  213  and  215  to allow the removal of the polymer layer  410  thereon. The polymer removal process also smooth out surface profile of the polymer layer  410 . 
         [0025]    After the polymer removal process and pads  213  and  215  being exposed, a surface hydrophilic modification is performed on the polymer surface (e.g., oxygen plasma) to transform the originally hydrophobic surface into hydrophilic surface. Therefore, a subsequently formed metal-based interconnect can have improved adhesion to the polymer layer  410 . 
         [0026]    Referring to  FIG. 4C , a interconnect  430  is then formed on top of the polymer layer  410  to connect the pad  213  and pad  215 . Because of the smooth surface profile of the polymer layer  410 , the subsequently formed metal-based interconnect  430  can have thin and smooth profile with improved endurance. In comparison, conventional interconnect ( 230  in  FIG. 3 ) is easy to brake due to complex profiles and sharp corners. Even though the fragileness of the conventional interconnect  230  can be slightly improved by increasing the thickness of the interconnect  230 , this is done at increased cost due to both additional material used and additional processing time. 
         [0027]    In the present invention, as mentioned above, the LED devices  210  are intended to be used at high efficiency with little heat generated, metals with lower melting points, such as Al, In, Sn or related alloy metals, can be used to form the major component of interconnect  430  (equal to or more than 90 vol %), which further lowers the cost of producing the LED device. Fabrication processes, such as chemical vapor deposition, sputtering or evaporation of the metal can be used for forming the interconnect  430 . In an exemplary process, three layers of metal, Ti/Al/Pt, are sputtered to form the interconnect  430 . 
         [0028]    Furthermore, mixture of metal powder and polymer (e.g. silver paste) can also be used to form the interconnect  430 . Corresponding fabrication process may be screen printing or stencil printing process with even lower manufacture cost. 
         [0029]    In addition, the smoothness of the polymer layer  410  allows sizes of the pads  213  and  215  and interconnect  430  to be smaller than the conventional ones shown in  FIG. 3 , so that less LED area will be shielded by the opaque pads  213  and  215  and interconnects  430 . 
         [0030]    In addition to the aforementioned providing a smooth surface, the polymer layer  410  can also absorb and dissipate heat from neighboring LED devices  210 , especially when the polymer layer  410  is mixed with some special materials such as ceramics and carbon-based nanostructures. 
         [0031]    Ceramics and carbon-based nanostructures absorb heat energy and emit it as far-infrared wavelength energy. Infrared radiation is a form of electromagnetic radiation with wavelengths longer than those at the red-end of the visible portion of the electromagnetic spectrum but shorter than microwave radiation. This wavelength range spans roughly 1 to several hundred microns, and is loosely subdivided—no standard definition exists—into near-infrared (0.7-1.5 microns), mid-infrared (1.5-5 microns) and the far-infrared (5 to 1000 microns). 
         [0032]    Ceramics which are inorganic oxides, nitrides, or carbides are considered as the most effective far infrared ray emitting bodies. A number of studies on ceramic far infrared ray emitting bodies have been reported, including zirconia, titania, alumina, zinc oxides, silicon oxides, boron nitride and silicon carbides. Oxides of transition elements such as MnO2, Fe2O3, CuO, CoO, and the like are considered more effective far infrared ray emitting bodies. Other far infrared ray emitting body includes carbon-based nanostructures, such as carbon nanocapsule and carbon nanotubes. They also show a high degree of radiation activity. These materials are very close to a black body exhibiting a high degree of radiation activity throughout the entire infrared range. In accordance with an embodiment of the present invention, the polymer layer  410  is pre-mixed with ceramics or carbon-based nanostructures which absorb the heat from nearby LED devices  210  and/or phosphors, and then dissipate the heat as far infrared radiation. This characteristic can be used to allow heat to escape from the LED devices  210  even when the LED devices  210  are in a sealed enclosure without heat sinks or cooling fans. Of course, with the addition of heat sinks or cooling fans heat can be better dissipated. 
         [0033]      FIG. 5  illustrates a trench  502  formed in the substrate  205  to separate two LED devices according to another embodiment of the present invention. The trench  502  is typically laser etched into the substrate during the formation of the gap between two LED devices  210  in order to allow more light to come out the lateral sides of the LED devices  210 . As a result, light extraction efficiency of a whole LED chip that incorporates an array of the LED devices  210  will be increased. The deeper the trench  502  is, the higher the light extraction efficiency the LED chip attains. Typically, a depth of the trench  502  measured from an original surface of the substrate  205  to the bottom of the trench  502  is controlled at a range between 20 microns and 100 microns. 
         [0034]    However, the trench  502  is more difficult to fill. As shown in  FIG. 5 , a PMGI layer  510  is first deposited in the trench  502 , and then followed by a SU-8 layer  520  in accordance with the embodiment of the present invention. The PMGI layer  510  has better filling characteristic. The SU-8 layer  520  deposited on top of the PMGI layer  510  also serves as a barrier layer protecting the underneath PMGI layer  510  from reacting with developers in subsequent photoresist processes. One of such photoresist processes is for forming the interconnect  430  by metal sputtering in which a NR-7 patterning photoresist is used. The developer used with the NR-7 photoresist can react with the PMGI layer  510  if not for the protection of the SU-8 layer  520 . However, if the interconnect  430  is formed by a silver paste in a printing process, a single PMGI layer can be used for filling the entire gap, including the trench  502 , between the two LED device  210  for further saving processing cost. 
         [0035]      FIGS. 6A and 6B  illustrate some alternative patterns of the interconnect  430 . Referring to  FIG. 6A , interconnects  630   a  and  630   b  are moved to edges of the LED devices  210  corresponding to relocations of electrode pads (not shown). Referring to  FIG. 6B , interconnects  635   a  and  635   b  are T-shaped to connect neighboring LED devices  210 . Varying the interconnect patterns is to reduce areas of the interconnects, so that less light generated by the LED devices is shielded by the interconnects. 
         [0036]      FIG. 7  illustrates a LED chip  702  being flip mounted on a board  720 . The LED chip  702  is produced through the processes shown in  FIGS. 4A˜4C , i.e., a plurality of the LED devices  210  are formed on the same substrate  205  (not shown in  FIG. 7 ). When the substrate  205  is a sapphire which is highly transparent to light, the LED chip  702  can be flip mounted on a board  720 . In such case, the substrate  205  of the LED chip  702  is on the top, the plurality of the LED devices  210  are below the substrate  205 . Before the LED chip  702  being flip mounted on the board  720 , solder balls  710  are first formed on the terminals of the LED chip  702 . Then the LED chip  702  is flipped over and placed on the board  720  with the solder balls  710  aligned to corresponding terminal interconnects  722 . After a melting process, the solder balls  710  bonds the LED chip  702  to the board  720  through the terminal interconnects  722 . Apparently, the flip-chip technology yields the shortest board-level interconnects and better electrical characteristics. When multiple LED chips  702  are mounted on the same board  720 , mounting density for the flip-chip mounting can be higher than conventional wire bonding. In addition, after the LED chip  702  being flip mounted on the board  720 , the substrate (not shown in  FIG. 7 ) on which the LED chip  702  is grown can be removed for even better light emission. 
         [0037]    The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
         [0038]    Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.