Abstract:
A light-emitting diode (LED) structure includes a substrate; a first semiconductor layer on the substrate; a light emitting layer on the first semiconductor layer; a second semiconductor layer on the light emitting layer; and an electrode on the semiconductor layer composed of a body and an extension body, wherein, the electrode extension portion is in a certain angle with the contacting semiconductor layer and separates the electrode body from the light emitted to its top surface and sides with a semi-wrapping structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of, and claims priority to, PCT/CN2015/078637, filed May 11, 2015, which claims priorities to Chinese Patent Application No. CN 201410600560.0, filed Oct. 31, 2014. The disclosures of the above applications are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    Solid state lighting, in particular, light emitting diodes (LEDs), due to long service life, environmental friendliness and high efficiency, has became a new generation of lighting sources in replace of fluorescent lamps/incandescent bulbs. However, directly-fabricated LEDs only emit monochromatic light and require mixture of several colors to obtain white light. At present, to produce white light LEDs, blue/ultra violet LEDs are used to activate light conversion materials, like fluorescent powder. 
         [0003]    In most LEDs, electrodes are made of highly conductive materials like Au, which will absorb some light. Therefore, electrode light absorption greatly reduces light emitting efficiency. In white light chips of the prior art as shown in  FIG. 1 , an electrode  105  mainly absorbs: 1) the light r 1  from the light emitting layer  103  to the electrode bottom; 2) the light r 2  from the light emitting layer  103  to the electrode sides; and 3) the light r 3  scattered/excited from the light conversion material  107 , like fluorescent powder, to the electrode top surface and sides. 
         [0004]    To solve electrode light absorption, LED reflective electrodes in the prior art are divided into two types: 1) the electrode bottom is a reflector and its sides and top surface are still made of light-absorption metal. For LED chips with such electrodes, in particular, for white light chips, the light emitting efficiency is still reduced due to electrode light absorption; 2) the entire electrode is wrapped by reflective metal. However, the reflective metal, in general, Ag/Al, may cause electrode degradation due to metal electromigration during usage of LED chips. 
       SUMMARY 
       [0005]    The present disclosure relates to an LED structure and the fabrication method thereof, which reduces electrode light absorption and electromigration of metal reflecting electrodes. 
         [0006]    According to the first aspect of the present disclosure, an LED structure, comprising: a substrate; a first semiconductor layer on the substrate; a light emitting layer on the first semiconductor layer; a second semiconductor layer on the light emitting layer; an electrode on the semiconductor layer, composed of a body and an extension portion, wherein, the electrode extension portion is in a certain angle with the contacting semiconductor layer and separates the electrode body from the light emitted to its top surface and sides with a semi-wrapping structure. 
         [0007]    In some embodiments, the electrode body is a multi-layer structure, wherein, the bottom layer is made of reflective metal for reflecting the light emitted from the light emitting layer to the electrode bottom surface; and the top layer is made of a metal not prone to electromigration for preventing the reflective metal on the bottom surface from electromigration during conductive process. 
         [0008]    In some embodiments, in the electrode extension portion, the surface close to the electrode body is made of electromigration inert metal, and the outer side surface far from the electrode body is made of reflective metal for reflecting the light emitted to the upper surface and sides of the electrode body. 
         [0009]    In some embodiments, the electrode extension portion is not higher than the electrode body. 
         [0010]    In some embodiments, the electrode extension portion is in a(an) straight, obtuse and acute angle or any of their combinations with the contacting semiconductor layer. 
         [0011]    In some embodiments, the electrode extension portion surface is a regular plane, an irregular plane or their combination. 
         [0012]    In some embodiments, besides electrode regions, the LED structure also comprises light conversion materials. 
         [0013]    According to a second aspect of the present disclosure, a fabrication method for LED structures, comprising: a) providing an LED wafer, which comprises a substrate for supporting and protecting the LED, a first semiconductor layer on the substrate, a light emitting layer on the first semiconductor layer and a second semiconductor layer on the first semiconductor layer; b) dividing the upper surface of the second semiconductor layer into an electrode region and a non-electrode region, and forming a shield layer in the non-electrode region; c) depositing an electrode layer on the LED wafer in a certain angle with the LED wafer surface to plate the electrode layer at the side wall of the shield layer next to the electrode region; d) removing the shield layer and its upper electrode layer and leaving the electrode layers in the electrode region and at the side wall of the shield layer, wherein, the electrode layer in the electrode region is the electrode body, and that at the side wall of the shield layer is the electrode extension portion. 
         [0014]    In some embodiments, the angle θ between the depositing direction and the LED wafer surface meets that: 0°&lt;θ&lt;90°. 
         [0015]    In some embodiments, the angle θ between the depositing direction and the LED wafer surface meets that: 30°≦θ≦80°. 
         [0016]    In some embodiments, the deposited electrode layer is a multi-layer structure, where the initially-deposited metal layer is made of reflective metal. 
         [0017]    In some embodiments, the deposited electrode layer is a multi-layer structure, where the lastly-deposited metal layer is made of an anti-electromigration metal. 
         [0018]    In some embodiments, after fabrication of the electrode, light conversion material is deposited in other areas except the electrode. 
         [0019]    According to a third aspect of the present disclosure, a fabrication method for LED structures, comprising: a) providing an LED wafer, which comprises a substrate for supporting and protecting the LED, a first semiconductor layer on the substrate, a light emitting layer on the first semiconductor layer and a second semiconductor layer on the first semiconductor layer; b) dividing the upper surface of the second semiconductor layer into an electrode region and a non-electrode region, and forming a light conversion layer in the non-electrode region; c) depositing a shield layer on the light conversion layer and exposing the electrode region; d) depositing an electrode layer on the LED wafer in a certain angle with the LED wafer surface to plate the electrode layer at the side wall of the light conversion layer next to the electrode region; e) removing the shield layer and its upper electrode layer and leaving the electrode layers in the electrode region and at the side wall of the light conversion layer, wherein, the electrode layer in the electrode region is the electrode body, and that at the side wall of the shield layer is the electrode extension portion. 
         [0020]    In some embodiments, the angle θ between the depositing direction and the LED wafer surface meets that: 0°&lt;θ&lt;90°. 
         [0021]    In some embodiments, the angle θ between the depositing direction and the LED wafer surface meets that: 30°≦θ≦80°. 
         [0022]    In some embodiments, the deposited electrode layer is a multi-layer structure, where the initially-deposited metal layer is made of reflective metal. 
         [0023]    In some embodiments, the deposited electrode layer is a multi-layer structure, where the lastly-deposited metal layer is made of an anti-electromigration metal. While the invention will be described in conjunction with exemplary embodiments and methods of use, it will be understood by those skilled in the art that such description is not intended to limit the scope of the present disclosure, and various alternations, modifications and equivalents may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
         [0024]    In another aspect, a light-emitting system is provided including a plurality of the LEDs described above. The light-emitting system can be used, for example, for lighting, display, signage, etc. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a schematic diagram of electrode light absorption of a conventional white light LED. 
           [0026]      FIG. 2  is a sectional view of an LED structure according to Embodiment 1, wherein, the electrode extension portion is in a straight angle with the semiconductor layer surface. 
           [0027]      FIG. 3  is a flow diagram for one mode of implementation of Embodiment 1. 
           [0028]      FIG. 4  illustrates a first step in manufacturing the LED structure of Embodiment 1. 
           [0029]      FIG. 5  illustrates a second step in manufacturing the LED structure of Embodiment 1. 
           [0030]      FIG. 6  illustrates a third step in manufacturing the LED structure of Embodiment 1. 
           [0031]      FIG. 7  is an SEM image for the electrode before removal of the shield layer of Embodiment 1. 
           [0032]      FIG. 8  is a top-view SEM image for the electrode after removal of the shield layer of Embodiment 1. 
           [0033]      FIG. 9  is a sectional view SEM image for the electrode after removal of the shield layer of Embodiment 1. 
           [0034]      FIG. 10  is a structural sectional view of Embodiment 2 comprising a light conversion layer based on Embodiment 1. 
           [0035]      FIG. 11  is a flow diagram for one mode of implementation of Embodiment 2. 
           [0036]      FIG. 12  is a sectional view of an LED in a first step of fabrication shown in  FIG. 11 . 
           [0037]      FIG. 13  is a sectional view of an LED in a second step of fabrication shown in  FIG. 11 . 
           [0038]      FIG. 14  is a flow diagram for another mode of implementation of Embodiment 2. 
           [0039]      FIG. 15  is a sectional view of an LED in a first step of fabrication shown in  FIG. 14 . 
           [0040]      FIG. 16  is a sectional view of an LED in a second step of fabrication shown in  FIG. 14 . 
           [0041]      FIG. 17  is a sectional view of an LED in a third step of fabrication shown in  FIG. 14 . 
           [0042]      FIG. 18  is a sectional view of an LED structure of Embodiment 3, wherein, the electrode extension portion is in an obtuse angle with the semiconductor layer surface. 
           [0043]      FIG. 19  is a sectional view of an LED structure of Embodiment 4, wherein, the electrode extension portion is in an acute angle with the semiconductor layer surface. 
           [0044]      FIG. 20  is a sectional view of an LED structure of Embodiment 5, wherein, the electrode extension portion is irregular. 
       
    
    
       [0045]    In the drawings: 
         [0046]      101 ,  201 ,  301 ,  401 ,  501 ,  601 : substrate; 
         [0047]      102 ,  202 ,  302 ,  402 ,  502 ,  602 : first semiconductor layer; 
         [0048]      103 ,  203 ,  303 ,  403 ,  503 ,  603 : light emitting layer; 
         [0049]      104 ,  204 ,  304 ,  404 ,  504 ,  604 : second semiconductor layer; 
         [0050]      204   a,    304   a : electrode region; 
         [0051]      105 ,  205 ,  305 ,  405 ,  505 ,  605 : electrode; 
         [0052]      106 ,  306 : light conversion layer; 
         [0053]      107 ,  307 : light conversion material; 
         [0054]      208 ,  308 ,  408 ,  508 ,  608 : electrode body; 
         [0055]      209 ,  309 ,  409 ,  509 ,  609 : electrode extension portion; 
         [0056]      210 ,  310 ,  410 ,  510 ,  610 : reflective metal layer; 
         [0057]      211 ,  311 ,  411 ,  511 ,  611 : anti-electromigration metal layer; 
         [0058]      212 ,  312 : shield layer; 
         [0059]      212   a : side wall of the shield layer next to the electrode region; 
         [0060]      214 : metal evaporation source. 
       DETAILED DESCRIPTION 
       [0061]    The embodiments of the present disclosure will be described in detail with reference to the accompanying drawings and examples, to help understand and practice the disclosed embodiments, regarding how to solve technical problems using technical approaches for achieving the technical effects. It should be understood that the embodiments and their characteristics described in this disclosure may be combined with each other and such technical proposals are deemed to be within the scope of this disclosure without departing from the spirit of this invention. 
       Embodiment 1 
       [0062]      FIG. 2  is a sectional view of the LED structure according to a first preferred embodiment of the present disclosure. This embodiment will be described in detail with reference to the fabrication method thereof. Refer to  FIG. 3  for the fabrication process. 
         [0063]    As shown in  FIG. 4 , provide an LED wafer, comprising a substrate  201  for supporting and protecting the LED, a first semiconductor layer  202  on the substrate, a light emitting layer  203  on the first semiconductor layer  202  and a second semiconductor layer  204  on the light emitting layer  203 . In this embodiment, the substrate  201  is made of high conductive material (e.g., Si, Cu, CuW alloy). Of course, besides conductive material, the substrate  201  can be made of insulating material (e.g., substrate) based on specific devices. When the first semiconductor layer  202  is a p-type semiconductor, the second semiconductor layer  204  can be an n-type semiconductor in different electrical property and vice versa. The light emitting layer  203  is between the first semiconductor layer  202  and the second semiconductor layer  204 , which can be a neutral-type, a p-type or an n-type semiconductor. When the applied current passes through the light-emitting epitaxial laminated layer, the light emitting layer  203  is excited to emit light. When the light emitting layer  203  is made of nitride-based material, blue or green light will be emitted; when made of AlInGaP-based material, red, orange or yellow light in amber color will be emitted. 
         [0064]    S 01 : divide the upper surface of the second semiconductor layer  204  into an electrode region  204   a  and a non-electrode region; form a shield layer  212  in the non-electrode region and expose the electrode region, wherein, the angle of the side wall  212   a  of the shield layer next to the electrode region is designed based on electrode structures. As shown in  FIG. 5 , the designed angle is a straight angle; preferably, the shield layer  212  is made of photosensitive material like photoresist. 
         [0065]    S 02 : as shown in  FIG. 5 , deposit an electrode layer on the LED wafer in a certain angle θ with the LED wafer surface to plate the electrode layer at the side wall  212   a  of the shield layer next to the electrode region. Referring to  FIG. 6 , in some embodiments, the electrode layer is a multi-layer structure, wherein, the reflective metal layer  210  is initially deposited and the anti-electromigration metal layer  211  is lastly deposited; and the angle θ between the depositing direction and the LED wafer surface is from 0° to 90°. Preferable, the angle between the depositing direction of the electrode layer and the LED wafer is 60°; the reflective metal  210  is Ag, Al or their alloy; and the anti-electromigration metal  211  is Au, Pt or their alloy. Referring to  FIG. 7 , as shown in the SEM image, the electrode layer is formed in the electrode region  204   a  of the second semiconductor layer and at the side wall  212   a  of the shield layer next to the electrode region. 
         [0066]    S 03 : remove the shield layer and its upper electrode layer and leave the electrode layers in the electrode region and at the side wall of the shield layer. Refer to  FIGS. 8-9  for the structure, wherein, the electrode layer  208  in the electrode region is the electrode body and the electrode layer  209  at the side wall of the shield layer is the electrode extension portion, which constitute an electrode  205 . 
         [0067]    In this embodiment, metal reflecting layers  210  are formed at the bottom and side walls of the LED electrode, which can reflect the light reflected by the light emitting layer, thus reducing light emitting efficiency decrease caused by electrode light absorption. The reflective layer  211  of the electrode extension portion is exposed but the metal in the extension portion is not prone to electromigration because the electrode extension portion is not for conduction. Though the electrode body is mainly for conduction, the anti-electromigration metal on the upper of the metal reflecting layer reduces the electromigration risk to the minimum. Therefore, the present disclosure reduces electrode light absorption and electromigration of metal reflecting electrodes. 
       Embodiment 2 
       [0068]      FIG. 10  is a sectional view of the LED structure according to a second preferred embodiment of the present disclosure. This embodiment will be described in detail with reference to the fabrication method thereof. 
         [0069]    As shown in  FIG. 10 , different from Embodiment 1, in this embodiment, a light conversion layer  306  is covered in the non-electrode region of the second semiconductor layer  304 . Refer to  FIG. 11  for the fabrication process of this embodiment. 
         [0070]    First, fabricate an LED structure basically same as that in Embodiment 1 following S 11 -S 13 , which are basically similar to S 01 -S 03  in Embodiment 1. In this embodiment, as the electrode extension portion  309  is to reflect the light emitted from the light conversion layer  306  to the electrode  305 , the height of the electrode extension portion  309  is not lower than the thickness of the light conversion layer  306 ; preferably, the electrode extension portion  309  is thicker than 10 μm; accordingly, the shield layer formed in S 11  is preferably thicker than 10 μm; Futurrex, AZ electronic material or JSR Corporation photoresist can be used. 
         [0071]    S 14 : referring to  FIG. 12 , cover a shield layer  312  on the electrode  305 . 
         [0072]    S 15 , referring to  FIG. 13 , on the second semiconductor layer  304 , fill in a light conversion layer  307  on areas except the electrode  305 . Specifically, coat the light conversion layer on the entire plane and grind the light conversion layer above the electrode  305  to expose the shield layer. Preferably, the light conversion layer is made of mixture of silica gel and fluorescent powder. 
         [0073]    S 16 : remove the shield layer  312 . 
         [0074]    In this embodiment, the LED electrode completely reflects the light emitted by the light conversion layer to the electrode, which greatly reduces electrode light absorption and improves light emitting efficiency, which can be directly applied in white light LED structures. 
         [0075]    The LED structure of this embodiment can be fabricated through another method as shown below. Refer to  FIG. 14  for the fabrication process. 
         [0076]    First, provide an LED wafer, comprising a substrate  301  for supporting and protecting the LED, a first semiconductor layer  302  on the substrate  301 , a light emitting layer  303  on the first semiconductor layer  302  and a second semiconductor layer  304  on the light emitting layer  303 . 
         [0077]    S 21 : divide the upper surface of the second semiconductor layer  304  into an electrode region  304   a  and a non-electrode region, wherein, form a patterned light conversion layer  306  in the non-electrode region and expose the electrode region, as shown in  FIG. 15 . Preferably, the light conversion layer is made of mixture of silica gel and fluorescent powder. 
         [0078]    S 22 : form a shield layer  312  on the light conversion layer  306  and expose the electrode region, as shown in  FIG. 16 . 
         [0079]    S 23 : deposit an electrode layer on the LED wafer in a certain angle θ with the LED wafer surface to plate the electrode layer at the side wall of the light conversion layer  306  next to the electrode region. Specifically, deposit metal layers in sequence on the LED wafer by initially depositing the reflective metal layer  310  and lastly depositing the anti-electromigration metal layer  311 , as shown in  FIG. 17 . Preferably, the angle between the metal depositing direction and the LED wafer surface is from 30° to 80°. 
         [0080]    S 24 : remove the shield layer  312  and its upper electrode layer and leave the electrodes in the electrode region and at the side wall of the light conversion layer. 
         [0081]    In this embodiment, the electrode is made through the electrode region defined by the light conversion layer and the light conversion layer can protect the electrode extension portion, which reduces difficulty in mass production. 
       Embodiment 3 
       [0082]      FIG. 18  is a sectional view of the LED structure according to a third preferred embodiment. Different from Embodiment 1, in this embodiment, the electrode extension portion  409  is in an acute angle with the LED device surface and forms a “W” shape with the electrode body  408 . The fabrication process is similar to that stated in Embodiment 1. Key step S 01 : form a shield layer on the semiconductor layer and expose the electrode region, and the side wall of the shield layer next to the electrode region is designed as an obtuse angle. During actual fabrication, the angle can be controlled through the type of shield layers, exposure energy and development time. Preferably, in this embodiment, the shield layer is made of positive photoresist. 
         [0083]    The LED structure according to this embodiment, on the one hand, reduces electrode light absorption and electromigration of metal reflecting electrodes; and on the other hand, reflects more light back to the back surface due to angle control of the electrode extension portion, which is more suitable for flip chips. 
       Embodiment 4 
       [0084]      FIG. 19  is a sectional view of the LED structure according to a fourth preferred embodiment. Different from Embodiment 1, in this embodiment, the electrode extension portion  509  is in an acute angle with the LED device surface. The fabrication process is similar to that stated in Embodiment 1. Key step S 01 : form a shield layer on the semiconductor layer and expose the electrode region, and the side wall of the shield layer next to the electrode region is designed as an obtuse angle. During actual fabrication, the angle can be controlled through the type of shield layers, exposure energy and development time. Preferably, in this embodiment, the shield layer is made of negative photoresist. 
         [0085]    The LED structure according to this embodiment, on the one hand, reduces electrode light absorption and electromigration of metal reflecting electrodes; and on the other hand, emits more light to the front surface due to angle control of the electrode extension portion, which is more suitable for applications requiring large light emitting angle. 
         [0086]    Due to limited contents, in Embodiments 1, 3 and 4, the angle between the electrode extension portion and the semiconductor layer surface in each embodiment has only one type from obtuse angle, straight angle or acute angle. In more complicated designs, however, different angles are possible in one embodiment, i.e., the angle between the electrode extension portion and the semiconductor layer surface can be any one of obtuse angle, straight angle or acute angle or their combinations. 
       Embodiment 5 
       [0087]    In the aforesaid embodiments, the electrode extension portion surfaces are regular planes, but in actual applications, they can be irregular structures.  FIG. 20  is a sectional view of the LED structure according to a fifth preferred embodiment. Different from Embodiment 1, in this embodiment, the electrode extension portion  609  is an irregular structure. The fabrication process is similar to that stated in Embodiment 1. Key step S 01 : form a patterned shield layer on the semiconductor layer and expose the electrode region, and the side wall of the shield layer next to the electrode region is designed as an obtuse angle. To obtain an irregular surface structure: 
         [0088]    1) Subtly design the thickness of the shield layer to make the light of the exposure source diffract on the semiconductor layer surface, thus forming a wave-shape electrode extension portion; 
         [0089]    2) After fabrication of the shield layers in Embodiments 1, 3 and 4 following step S 01 , take chemical etching or ICP etching for the shield layer surface to make it a special form, thereby obtaining an irregular electrode extension portion. 
         [0090]    The LED structure according to this embodiment, on the one hand, reduces electrode light absorption and electromigration of metal reflecting electrodes; and on the other hand, is suitable for special applications due to angle control of the electrode extension portion. 
         [0091]    All references referred to in the present disclosure are incorporated by reference in their entirety. Although specific embodiments have been described above in detail, the description is merely for purposes of illustration. It should be appreciated, therefore, that many aspects described above are not intended as required or essential elements unless explicitly stated otherwise. Various modifications of, and equivalent acts corresponding to, the disclosed aspects of the exemplary embodiments, in addition to those described above, can be made by a person of ordinary skill in the art, having the benefit of the present disclosure, without departing from the spirit and scope of the disclosure defined in the following claims, the scope of which is to be accorded the broadest interpretation so as to encompass such modifications and equivalent structures.