Abstract:
Elements are added to a light emitting device to reduce the stress within the light emitting device caused by thermal cycling. Alternatively, or additionally, materials are selected for forming contacts within a light emitting device based on their coefficient of thermal expansion and their relative cost, copper alloys being less expensive than gold, and providing a lower coefficient of thermal expansion than copper. Elements of the light emitting device may also be structured to distribute the stress during thermal cycling.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of light emitting device, and in particular to the manufacture of light emitting devices (LEDs) with reduced epi stress. 
     BACKGROUND OF THE INVENTION 
     As the light emitting capabilities of Light Emitting Diodes (LEDs) continues to improve, their use in conventional lighting applications continues to increase, as do the competitive pressures to provide reliable, long-lasting products in a cost-effective manner. Even though the cost of LED products is relatively low, the savings of even a few cents per device can have a significant impact on profit margin, due to the increasingly growing market for these devices. 
     To reduce the cost of LED devices, copper can replace gold as the bulk metal for electrical contacts for LED dies. However gold still remains the preferred metal to provide efficient and reliable electrical and mechanical interconnections between the LED and its submount in a flip-chip configuration, wherein the upper layer of a LED die is attached to a submount, and light from the LED is emitted from a surface opposite the submount. 
       FIG. 1A  illustrates a conventional flip-chip submount configuration of a light emitting device  100 . The submount may include a base  110  upon which contacts  120  are formed; the contacts may be plated  125  to facilitate connections  145  to the flip chip contacts  150 . The flip chip may comprise a growth substrate  170 , a light emitting element  160 , interconnect layers  165 , and contacts  150 . The growth substrate  170 , commonly sapphire or other rigid material, may be removed after the flip chip is attached to the submount. 
     Two contacts  120  are illustrated in  FIG. 1A , separated by a channel  130  that provides electrical isolation between the two contacts  120 . In like manner, the contacts  150  are illustrated as being separated by a channel  135 . The channel  135  may be smaller than the channel  130 , in order to increase an amount of support provided to the interconnect layers  165  and light emitting element  170  by the contacts  150 . This increased support may be particularly beneficial during the removal of the growth substrate  170 . Also, the channel  130  may be larger than the channel  135  is order to accommodate potential alignment inaccuracy when the flip-chip is placed on the submount. 
       FIG. 1B  illustrates an example thermal deformation  190  that may be caused when the light emitting device  100  is subject to high temperatures after the growth substrate  170  is removed. This deformation  190  may occur during manufacturing, and each time the light emitting device  100  is cycled from off to on. The deformation  190  may induce repeated stress to the interconnect layers  165  and the light emitting element  160 , and may cause the device  100  to fail prematurely. Additionally, the upper layer  175  of the light emitting device may be etched to increase the light extraction efficiency of the light emitting element  170 , which may cause the upper layer  175  to be more susceptible to stress induced failures. 
     SUMMARY OF THE INVENTION 
     It would be advantageous to mitigate the amount of stress in a light emitting device that is caused by thermal cycling. It would be advantageous to mitigate this stress without significantly increasing the cost of the light emitting device. 
     In an embodiment of this invention, elements are added to the light emitting device to reduce the stress caused by thermal cycling. Alternatively, or additionally, the materials are selected for forming contacts within a light emitting device based on their coefficient of thermal expansion and their relative cost, copper alloys providing a lower coefficient of thermal expansion than copper. Elements of the light emitting device may also be structured to distribute the stress during thermal cycling. 
     The light emitting device may include a submount, a light emitting structure having a metal layer with contacts separated by a channel, and one or more elements that are added to reduce a thermally induced stress in the light emitting structure in a vicinity of the channel. The added elements may include, for example, a buffer layer between the metal layer and a light emitting element in the light emitting structure, one or more gaps in the metal layer, a filler material within the channel, a filler material between contacts on the submount, and additional micro bumps in an area adjacent the channel. 
     The light emitting device may also, or alternatively, use an alloy with a relatively low CTE for the metal layer. A copper alloy may be used, including, for example, CuNi, CuNiTi, CuW, CuFe, and CuMo. The CTE of the alloy is preferably lower than the CTE of copper (about 16 ppm/K), more preferably less than 10 ppm/K, and more preferably less than 8 ppm/K. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
         FIGS. 1A-1B  illustrates an example flip-chip on submount light emitting device. 
         FIG. 2  illustrates an example light emitting device with a buffer layer above the metal layer. 
         FIG. 3  illustrates an example light emitting device with an increased density of connection material adjacent the channel. 
         FIG. 4  illustrates an example light emitting device with gaps added to the metal layer. 
     
    
    
     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 
     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. 
     For ease of reference, because the stress may be shown to be most significant at the uppermost/surface layer  175  (hereinafter the epi-layer) of the light emitting element  160 , this disclosure will address the stress at the epi-layer  175 , although one of skill in the art will recognize that a stress induced failure may occur anywhere within the light emitting element  160  or the interconnects  165 . Accordingly, terms such as ‘cracking the epi-layer’ are to be interpreted as ‘cracking the epi-layer or any layer below the epi-layer’. In like manner, the layer comprising the contacts  150  may include other elements than the contacts; for ease of reference, the term ‘metal layer  150 ’ is used hereinafter to identify the layer of metal that provides support to the light emitting element  160 . 
     Gold has been shown to be a suitable material for forming the metal layer  150  of the light emitting device of  FIGS. 1A and 1B . To reduce costs, copper has been proposed for use instead of gold for this metal layer  150 . However, a copper-to-copper interconnect may not provide the desired reliability for the light emitting device  100 ; accordingly, gold may be used as the connection material  145 , which may be in the form of a micro bump layer. In this manner, if the plating  125  of the metal layer  120  is also gold, a gold-to-gold interconnect may be formed, providing a more reliable electrical and/or thermal interconnection between the flip-chip and the submount. 
     Copper has a Young&#39;s modulus of 110 GPa, which is stronger than that of gold which is 77 GPa (or 26 GPa for annealed gold wires). In addition, copper has much less plastic effect than gold. Accordingly, the use of copper for the metal layer  150  reduces the probability of cracking the epi-layer  175  if/when the growth substrate  170  is removed. However, during thermal cycling, a copper metal layer will introduce significantly more deformation  190  than a gold metal layer, which may increase the likelihood of cracking the epi-layer  175  during thermal cycling. 
     Further, if gold micro bumps  145  are used between the copper metal layer  150  and the submount, the amount of deformation  190  caused by copper metal layer  150  is likely to be more significant, because gold is a relatively compliant material, allowing the edges of the copper metal layer  150  at the channel  135  to lift even further. 
     In an embodiment, the material selected for the metal layer  150  is selected based on its coefficient of thermal expansion (CTE). In particular, an alloy having a lower coefficient of thermal expansion than copper may be used to form the metal layer  150 . For example, this alloy may include CuNi, CuNiTi, CuW, CuFe, CuMo, etc. The NiTi alloy may be quite effective because it has a negative CTE. 
     Copper has a CTE of 16-18.5 ppm/K within a temperature range of 20-250 C. This CTE is much higher than a majority of the other materials used to form the light emitting device, and much higher than that of Alumina, which may be used as the submount, with a CTE of less than 10 ppm/K. Alloying copper with a low or even negative CTE material would provide an alloy with a CTE less than copper. 
     Finite Element Analysis (FEA) has demonstrated that a maximum stress caused by thermal cycling may be reduced from 1481 MPa down to 384.5 MPa when the CTE of the metal layer is reduced from 18 ppm/K to 8 ppm/K. To achieve a CTE of 8 ppm/K, a plating process may be used to form a copper alloy of Ni, TiNi, W, Fe, Mo, and so on. Particularly, Ti 0.507 Ni 0.493  alloy has a negative CTE of −21 ppm/K, and may be the most effective. 
     As illustrated in  FIG. 2 , alternatively, or additionally, a compliant metallization layer  210 , such as gold or aluminum, may be introduced between the metal layer  150  and the interconnects  165 , to act as a buffer between the metal layer  150  and the interconnects  165 , to absorb some of the stress caused by thermal cycling. 
     A layer  210  of softer material, such as gold or aluminum may be applied, corresponding to the pattern used to create the metal layer  150 . This layer  210  acts as a buffer to alleviate the CTE mismatch between the metal layer  150  and the upper layers  160  and  165 . It has been estimated that a 1 um thick layer of gold may reduce the maximum principle stress within the epi-layer  150  by as much as 42%, and a 3 um thick layer of gold can reduce the maximum principle stress within the epi-layer  150  by 49%. In lieu of a continuous layer of this compliant material, a layer of micro bumps may also be used to further enhance the compliancy of this buffer layer. 
     Also alternatively or additionally, the compliancy of the micro bump layer  145  can be reduced. Just as introducing a buffer to absorb a portion of the deformation caused by thermal cycling, reducing the compliancy of the micro bump layer will serve to restrict this distortion. The compliancy may be reduced, for example, by reducing the height of the micro bump layer  145 , or by increasing the density or size of the micro bumps, particularly in the vicinity of the channel  135 , as illustrated at  310  of  FIG. 3 . 
     Alternatively or additionally, the channel areas  130  or  135  may be filled with a material that has a closer CTE to the material of the metal layer  150 , thereby providing a more thermally consistent layer, reducing the distortion  190 . 
     The LED  100  may be overmolded with a silicone resin that molded or shaped to form a lens. Because the lens overmold material will likely flow into the channels  130  and  135 , and may have a CTE around 200 ppm/K, its thermal expansion will further increase the distortion of the metal layer  150  and the corresponding stress within the epi-layer  175 . By filling the channel  130  on the submount side with a material with a lower CTE, the thermal expansion within the channel and the effects from this expansion will be reduced. Also, by filling the channel  135  with a material having a CTE closer to the CTE of the metal layer  150 , the expansion or warping of the metal layer  150  will be reduced. 
     As illustrated in  FIG. 4 , alternatively, or additionally, the metal layer  150  may be structured or patterned to reduce the stress caused by thermal cycling. 
     For example, the mask used to create the metal layer  150  may include small gaps or trenches  410 , i.e. un-metallized areas, that serve to redistribute the effects of the CTE mismatch between the metal layer  150  and the upper layers  160  and  165 . These gaps  410  split the lateral stresses and strains that are incurred in the upper layers  160  and  165  due to the thermal expansion of the layer  150 , thereby mitigating the stress at the region above the channel  135  as well. 
     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. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.