Abstract:
The present disclosure provides a light emitting device including a serially-connected LED array comprising a plurality of LED cells on a substrate. The serially-connected LED array includes a first LED cell, a second LED cell, and a serially-connected LED sub-array comprising at least three LED cells intervening between the first and second LED cell; and a plurality of conducting metals formed on the LED cells to electrically connect the plurality of LED cell in series; wherein among the LED sub-array which are continuously and sequentially connected by the conducting metals, each LED cell connects to a previous LED cell by a first connecting direction and connects to a next LED cell by a second connecting direction, the first connecting direction is not parallel to the second connecting direction.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This application a continuation application of U.S. patent application Ser. No. 13/198,396, entitled “LIGHT EMITTING DEVICE”, filed on Aug. 4, 2011, which claims the right of priority based on U.S. provisional application Ser. No. 61/378,191, filed on Aug. 30, 2010, and the content of both applications are hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosure relates to a light emitting device, and more particularly to a light emitting device with the ability of electrostatic discharge (ESD) protection. 
       DESCRIPTION OF BACKGROUND ART 
       [0003]      FIG. 1  shows an illustration of a light emitting device  10 . The light emitting device  10  comprises a plurality of LED cells  11 (A, B, C, C 1 , C 2 , C 3 ) connecting in series by conducting metals  13  on a single substrate  15 , wherein each LED cell  11  comprises a first semiconductor layer  17  on the substrate  15 , a second semiconductor layer  19  on the first semiconductor layer  17 , an active layer  47  (not shown in  FIG. 1 ) arranged between the first semiconductor layer  17  and the second semiconductor layer  19 , and a conducting metal  13  arranged on the second semiconductor layer  19 . When one polarity of the AC input passes from conducting region α to conducting region β, the current flows through the LED cells  11  in the following order: A→C 1 →C 2 →C 3 →C→B. The largest potential difference of the LED cells  11  occurs between LED cells A and B. As shown in  FIG. 1 , the serially-connected LED array further comprises a serially-connected sub-array with four LED cells  11  (C 1 , C 2 , C 3 , C) intervening the terminal LED cells A and B in the series connection. 
         [0004]    As shown in  FIG. 1 , LED A and LED B further comprise a first side (A 1 , B 1 ) and a second side (A 2 , B 2 ), respectively. The first sides (A 1 , B 1 ) of LED A and LED B neighbor to the sub-array, and the second sides (A 2 , B 2 ) of LED A and LED B neighbor to each other. Besides, a trench T is formed between LED A and LED B. Namely, the trench T is formed between the second sides of LED A and LED B. 
         [0005]    Normally, the forward voltage for one LED cell  11  is about 3.5 volt, so the voltage difference between LED cells A and B should be about 3.5*6=21 volts under normal working situation. Because the distance between LED cells A and B is very short (about 10˜100 μm), the electric field strength (E=V/D, V=potential difference, D=distance) between LED cells A and B is high. 
         [0006]    Besides, if there is suddenly a strong electrostatic field from the outside environment (such as from the human body or the working machine) injecting into the conducting region α, an ultra-high electrical voltage is further inputting to LED cell A, and causes the largest potential difference between LED cells A and B. When the value of the electric field strength reaches a certain value by the strong electrostatic field from the outside environment, the mediums (air, glue, or other dielectric materials) therebetween may be ionized, and parts of LED cells A and B within the electrical field strength are damaged (the damage region  12 ) by discharging, which is called the ESD (electrostatic discharge) damage. The SEM picture of the ESD damage situation is shown in  FIG. 2 , wherein the ordinary current flow  14  flows in the direction as the arrows indicated in the figure. 
       SUMMARY OF THE DISCLOSURE 
       [0007]    The present disclosure provides a light emitting device, including a serially-connected LED array comprising a plurality of LED cells on a substrate. The serially-connected LED array includes a first LED cell, a second LED cell, and a serially-connected LED sub-array comprising at least three LED cells intervening between the first and second LED cell; and a plurality of conducting metals formed on the LED cells to electrically connect the plurality of LED cell in series; wherein among the LED sub-array which are continuously and sequentially connected by the conducting metals, each LED cell connects to a previous LED cell by a first connecting direction and connects to a next LED cell by a second connecting direction, the first connecting direction is not parallel to the second connecting direction. 
         [0008]    The present disclosure provides another light emitting device, including a substrate; a plurality of LED cells formed on the substrate and arranged in an array; and a plurality of conducting metals formed on the LED cells to electrically connect the plurality of LED cell in series, and each conducting metal which directly connects two adjacent LED cells defining a connecting direction; wherein the plurality of LED cells comprises N LED cells, wherein the N LED cells are continuously and sequentially connected by the conducting metals, wherein N≧6 and the connecting directions change N−2 times. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates the structure of a conventional light emitting device; 
           [0010]      FIG. 2  illustrates an SEM picture of a light emitting device in accordance with an embodiment of the disclosure; 
           [0011]      FIG. 3  illustrates a light emitting device in accordance with an embodiment of the disclosure; 
           [0012]      FIG. 4  illustrates a light emitting device in accordance with an embodiment of the disclosure; 
           [0013]      FIG. 5  illustrates a light emitting device in accordance with an embodiment of the disclosure; 
           [0014]      FIG. 6  illustrates a cross-section of a light emitting device along line  6 - 6 ′ in  FIG. 5  in accordance with an embodiment of the disclosure; 
           [0015]      FIG. 7  illustrates an electric circuit of a light emitting device in accordance with an embodiment of the disclosure; 
           [0016]      FIG. 8  illustrates an electric circuit of a light emitting device in accordance with an embodiment of the disclosure; 
           [0017]      FIG. 9  illustrates a cross-section of a light emitting device in accordance with an embodiment of the disclosure; 
           [0018]      FIG. 10A  illustrates a light emitting device in accordance with an embodiment of the disclosure; 
           [0019]      FIG. 10B  illustrates a cross-section of a light emitting device in accordance with an embodiment of the disclosure; 
           [0020]      FIG. 11  illustrates a light emitting device in accordance with an embodiment of the disclosure; 
           [0021]      FIG. 12  illustrates a light emitting device in accordance with an embodiment of the disclosure; 
           [0022]      FIG. 13  illustrates light emitting devices in accordance with an embodiment of the disclosure; 
           [0023]      FIG. 14  illustrates a light emitting device in accordance with an embodiment of the disclosure; 
           [0024]      FIG. 15  illustrates a light emitting device in accordance with an embodiment of the disclosure; 
           [0025]      FIG. 16  illustrates a cross-section of a light emitting device in accordance with an embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    The embodiments are described hereinafter in accompany with drawings. To better and concisely explain the disclosure, the same name or the same reference number given or appeared in different paragraphs or figures along the specification should has the same or equivalent meanings while it is once defined anywhere of the disclosure. 
         [0027]    In order to solve the ESD damage problem,  FIG. 3  shows a light emitting device  20  in accordance with one embodiment of the present disclosure. The light emitting device  20  comprises a plurality of LED cells  11 (A, B, C, C 1 , C 2 , C 3 ) connecting in series by conducting metals  13  on a single substrate  15 , wherein each LED cell  11  comprises a first semiconductor layer  17  on the substrate  15 , a second semiconductor layer  19  on the first semiconductor layer  17 , an active layer  47  (not shown in  FIG. 3 ) arranged between the first semiconductor layer  17  and the second semiconductor layer  19 , and a conducting metal  13  arranged on the second semiconductor layer  19 . As can be seen in  FIG. 3 , each conducting metal  13  further comprises the extending part with at least two divided metal lines. The number of the divided metal lines extended from each extending part is not limited to what is disclosed herein. In order to have the LED array connected in series, the first semiconductor layer  17  of LED A is electrically connected to the second semiconductor layer  19  of the adjacent LED cell, for example, LED C 1 . When one polarity of the AC input passes from conducting region α to conducting region β, the current flows through the LED cells  11  in the following order: A→C 1 →C 2 →C 3 →C→B. The largest potential difference of the LED cells  11  occurs between LED cells A and B. As shown in  FIG. 3 , the serially-connected LED array further comprises a serially-connected sub-array with four LED cells  11  (C 1 , C 2 , C 3 , C) intervening the terminal LED cells A and B in the series connection. 
         [0028]    As shown in  FIG. 3 , LED A and LED B further comprise a first side (A 1 , B 1 ) and a second side (A 2 , B 2 ), respectively. The first sides (A 1 , B 1 ) of LED A and LED B neighbor to the sub-array and the second sides (A 2 , B 2 ) of LED A and LED B neighbor to each other. Besides, a trench T is formed between LED A and LED B. Namely, the trench T is formed between the second sides of LED A and LED B. 
         [0029]    To prevent the ESD damage, a protecting structure is formed near the trench T to prevent the light emitting device from being damaged at a region near the trench by a surge voltage higher than a normal operating voltage. In this embodiment, a first insulating layer  23  is formed between the LED cells, and a second insulating layer  21  is further formed over the first insulating layer  23  in the region between two LED cells  11  with high electrical field strength, for example, the trench T. The second insulating layer  21  can be optionally thicker than the first insulating layer  23 . Taking the light emitting device  20  in  FIG. 3  for instance, LED A and LED B are electrically connected in series with four (more than three) LED cells  11  connected in-between and therefore suffer a high electric field strength over a certain value, and therefore the second insulating layer  21  is formed to cover the part of the top surfaces of the first insulating layer  23  and part of the top surfaces of LED A and LED B to isolate the LED cells  11  from the ESD damage. Besides, without the first insulating layer  23 , the second insulating layer  21  only can also be the protecting structure to cover the region between two LED cells  11  with high electrical field strength, for example, the exposed surfaces of the substrate  15 , the side surface of the first semiconductor layer  17 , and the side surface of the second semiconductor layer  19  between LED cells A and B. Besides, the materials of the insulating layer  21  and/or  23  can be insulating materials such as AlO x1 , SiO x2 , SiN x3 , and so on, and the insulating layer  21  and/or  23  may be a composite structure with multi layers formed by different materials. For example, the second insulating layer  21  may be formed by the combination of one layer of SiO x4  with the thickness of 2100 Å and one layer of AlO x5  with the thickness of 2000 Å, and the first insulating layer  23  may be formed by only one layer of SiO x4  with the thickness of 2100 Å. (The index words X1-X5 here are numbers, which could be integers or decimals, and can be the same or different.) 
         [0030]      FIG. 4  shows a light emitting device  30  in accordance with another embodiment of the present disclosure. As can be seen, the second insulating layer  21  covers most part of the top surfaces of LED A and LED B. For the same reason mentioned above, the insulating layers  21  and  23  can also be a composite structure with multi layers formed by different materials or a thick single layer, and the number or the thickness of the second insulating layer  21  on the covered top surfaces of LED A and LED B can be more than those in other regions. 
         [0031]      FIG. 5  shows a light emitting device  40  in accordance with another embodiment of the present disclosure. In the embodiment, reducing the electrical field strength is another method to prevent the ESD damage. As shown in  FIG. 5 , an insulating wall  41  is formed between LED A and LED B (the region with a high electric field strength or between two adjoining LED cells  11  which are electrically connected in series and with more than three LED cells  11  connected in-between). Because the insulating wall  41  is formed by the insulating material, the electric lines originating from LED A cannot be directed to LED B by penetrating the insulating wall  41  directly and should be extended along the contours of the insulating wall  41  instead. The length of the electric line is extended, and therefore the electrical field strength (E=V/D, V=potential difference, D=distance) between LED A and LED B is reduced. In order to extend the length of the electric lines between LED A and LED B, the insulating wall  41  located between LED A and LED B should be formed at the shortest route from LED A to LED B to shield the electric lines coming from LED A or LED B. In other words, as shown in  FIG. 5 , the thickness of the insulating wall  41  in the trench should be substantially larger than the sum of the thickness of the first semiconductor layer  17 , the active layer  47 , the second semiconductor layer  19 , and the conducting metal  13 . Preferably, the thickness of the insulating wall  41  in the trench should be larger than 1.5 times the sum of the thicknesses of the first semiconductor layer  17 , the active layer  47 , the second semiconductor layer  19 , and the conducting metal  13 . 
         [0032]      FIG. 6  is the cross-section of the  6 - 6 ′ line shown in  FIG. 5 . In  FIG. 6 , the first insulating layer  23  covers conformably along the side walls of LED A and LED B (including the first semiconductor layers  17 , the second semiconductor layers  19 , and the active layers  47 ), part of the top surfaces of the LED A and LED B, and part of the top surface of the substrate in the trench T directly. Besides, the insulating wall  41  can be formed on the first insulating layer  23  and is higher than the LED A and LED B, therefore, the electric lines from LED A to LED B can be shielded by the insulating wall  41 . However, the exact position of the insulating wall  41  can be modified and should not be limited. For example, the insulating wall  41  can also be formed on the top surface of the substrate  15  directly, or the insulating wall  41  can have a specific pattern formed by the traditional CVD method and the photolithography method. 
         [0033]      FIG. 7  shows an electric circuit of a light emitting device  50  in accordance with another embodiment of the present disclosure. As the experimental result indicates, a floating conductive line  55  electrically connecting to the LED cell  11  with the electric potential level between the highest potential and the lowest potential and located between the LED cell  11  with the highest potential and the LED cell  11  with the lowest potential can reduce the ESD damage. As shown in  FIG. 7 , a floating conductive line  55  can connect to the conducting metal  13  between LED C 2  and LED C 3 . 
         [0034]    Similar to  FIG. 7 ,  FIG. 8  is an electric circuit of a light emitting device  60  in accordance with another embodiment of the present disclosure. Rather than forming a floating conductive line  55  connecting to the conducting metal  13 , one grounding conductive line  65  is formed between the LED cell  11  with the highest potential and the LED cell  11  with the lowest potential, and the grounding conductive line  65  is grounded by connecting to outside. 
         [0035]      FIG. 9  is the cross-section of the embodiment shown in  FIG. 7  and  FIG. 8 . The floating (grounding) conductive line  55 ( 65 ) can be formed on the insulating layer  23  or on the substrate  15  directly between two LED cells  11 . 
         [0036]      FIG. 10A  discloses a light emitting device  70  in accordance with another embodiment of the present disclosure. Since the ESD damage normally causes the failure of the LED and is not easy to be avoided, additional ESD damage regions  75  are formed to confine the ESD damage happened in the specific regions. As shown in  FIG. 10A , there are two additional ESD damage regions  75  extending from the conductive metals  13 . Because the two additional ESD damage regions  75  are facing to each other closely, the higher electric field strength is caused therebetween, and the ESD damage may happen between the additional ESD damage regions  75  more easily. The additional ESD damage regions  75  are two additional metal plates that do not function with the LED cells  11 , therefore, they can help to maintain the working function of the LED cells  11 . Besides, the edge of the additional ESD damage region  75  facing the other one can be roughened to form tips, which raises the probability of the ESD phenomenon happening in the predetermined regions. 
         [0037]      FIG. 10B  is the cross-section of the embodiment shown in  FIG. 10A . The two additional ESD damage regions  75  are formed on the insulating layer  23  or on the substrate  15  directly between two LED cells  11 . 
         [0038]      FIG. 11  shows a light emitting device  80  in accordance with another embodiment of the present disclosure. Because the ESD damage comes from the high electric field strength, and the electric field strength between two objects depends on the potential difference and the distance of the two objects. As shown in  FIG. 11 , indicated by the experimental result, the distance (D) between two adjoining LED cells  11  with more than three LED cells  11  connected in-between should be larger than 15 μm. Preferably, the distance (D) between two adjoining LED cells  11  with more than three LED cells  11  connected therein should be more than 30 μm. The distance (D) here is identified as the shortest distance between two first semiconductor layers  17  of two adjoining LED cells  11 . In addition, the “adjoining LED cells” here means any two LED cells  11  with the shortest distance from the first semiconductor layers  17  of the LED cell to the first semiconductor layer of the other LED cell, wherein the distance (D) is preferred to be smaller than 50 μm. 
         [0039]      FIG. 12  shows a light emitting device  90  in accordance with another embodiment of the present disclosure. Similar to the embodiment shown in  FIG. 11 , to prevent the ESD damage happened between the conducting metals  13  of the two adjoining LED cells  11 , the distance (d) of the conducting metals  13  of the two adjoining LED cells  11  should be more than 100 μm. This design is suitable for the two adjoining LED cells  11  with large potential difference therebetween and/or with more than three LED cells  11  connected in-between. Preferably, the distance (d) of the conducting metals  13  of the two adjoining LED cells  11  should be more than 80 μm. The “distance of the conducting metals” here is defined as the shortest distance from the conducting metal  13  of the LED cell  11  to the conducting metal  13  of the adjoining LED cell. In addition, the “adjoining light-emitting diode cells” here means any two LED cells  11  with the shortest distance from the first semiconductor layers  17  of the LED cell  11  to the first semiconductor layer  17  of the other LED cell  11 , wherein the distance (d) is preferred to be smaller than 50 μm. 
         [0040]    As shown in  FIG. 13 , the high electric field strength often occurs between two adjoining LED cells  11  with more than three LED cells  11  connected in-between (with large potential difference), such as LED cells A and B shown in  FIG. 12 . In order to prevent the LED cells  11  with large potential difference from being too close, when a series of LED cells  11  formed on a substrate  15 , the series of LED cells  11  should change its arranging direction when certain amount of LED cells  11  are aligned with one direction, for example, which more than three LED cells  11  aligned with. In other words, the arranging direction of the series of the LED cells  11  should be changed often to avoid any two LED cells  11  with large potential difference or with more than three LED cells  11  connected in-between located adjoining to each other.  FIG. 13  shows three diagrams of different arrange configurations of a series LED cells  11  on a single substrate  15  in accordance with embodiments of the present disclosure. In each diagram, a series of the LED cells  11  disposed (formed by epitaxy or attached to the substrate by metal bonding or glue bonding) on a single substrate  15  with the bonding pads  105  formed at two ends of the series of the LED cells. The arrow here indicates the extending direction  103  (the connection order) of the LED cells  11 . In each arrangement, any two of the adjoining LED cells do not have the large potential difference therebetween. In detail, as shown in  FIG. 13 , each serially-connected LED array comprises at least eight LED cells and at least more than two branches. In order not to let any two of the adjoining LED cells have too large potential difference therebetween, each LED array shown in the figure changes its arranging direction with every two successive LED cells. 
         [0041]      FIG. 14  shows a light emitting device  110  in accordance with another embodiment of the present disclosure. According to the surface electric charge distribution of the objects, the surface electric discharge density per unit area of the object is large when the object has a small curvature radius. Another cause of the damage of the LED cells  11  called “point discharge” often happens at the position with high surface electric discharge density per unit area. Therefore, to prevent the “point discharge” phenomenon, the contours of the LED cells  11  are modified in this embodiment. As shown in  FIG. 14 , the upper corners of the first semiconductor layer  17  between LED cells A and B are patterned, for example, rounded. The above modification is not limited to the identified positions, and all of the corners of the LED cells  11  can be patterned to be rounded. Furthermore, not only the first semiconductor layer  17  but the second semiconductor layer  19  can also be rounded, especially for the edges of the LED cell close to the second side, which has the smaller distance from the adjoining LED cell. Preferably, the radius of the curvature of the patterned corner is not less than 15 μm. 
         [0042]    With the similar concept of the embodiment disclosed in  FIG. 14 , to prevent the “point discharge” phenomenon,  FIG. 15  shows that the terminal  123  of each divided metal line of the LED cells  11  can be patterned by forming the round metal plates  123 . The shape of the terminal metal  123  is not limited to the round shape. As indicated by the experimental result, any shape of the terminal metal  123  formed at the terminal with the enlarged portion or the radius larger than the line width of the conducting metal  13  can be formed to reduce the “point discharge” damage. 
         [0043]      FIG. 16  shows a cross-section of a light emitting device  130  in accordance with another embodiment of the present disclosure. In order to reduce the undesirable discharge, a smoother path facilitating the current flow can help. As shown in  FIG. 16 , in order to let the current spread from the conducting metal widely, a current blocking layer  133  is provided beneath the conducting metal  13 . The current blocking layer  133  is made by a dielectric material which is an insulator, such as SiO y1  or SiN y2 . However, the current blocking layer also blocks most paths that the current flows. If the current cannot disperse along the normal path, it leaks out in other forms such as ESD or point discharge. Therefore, in this embodiment, the current blocking layer  133  is formed beneath the conducting metal  13 , not under the terminal of the conducting metal  13 . This design let the current at the terminal of the conducting metal  133 , where the discharge is caused most easily, to flow more smoothly and reduce the probability the current leaking out along the discharge path. Besides, a transparent conducting layer  135  such as ITO, IZO, ZnO, AZO, thin metal layer, or the combination thereof can also be optionally formed on the second semiconductor layer to help the current spreading. 
         [0044]    The embodiments mentioned above are used to describe the technical thinking and the characteristic of the invention and to make the person with ordinary skill in the art to realize the content of the invention and to practice, which could not be used to limit the claim scope of the present invention. That is, any modification or variation according to the spirit of the present invention should also be covered in the claim scope of the present disclosure. For example, the electric connecting method is not limited to the serial connection. The ESD protection methods shown as the embodiments above can be applied to any two adjoining light emitting diode cells with a high electric field strength over a certain value or with more than three LED cells connected in-between electrically connect in parallel or in the combination of serial and parallel.