Patent Publication Number: US-2018047781-A1

Title: Dot matrix light-emitting diode light source for a wafer-level microdisplay and method for fabricating the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microdisplay and, more particularly, to a dot matrix light-emitting diode (LED) light source for a wafer-level microdisplay and a method for fabricating the same. 
     2. Description of the Related Art 
     Display devices at the earliest stage adopting raster scan theory of cathode ray tube (CRT) were born in 1922, enabling the ways of information dissemination to migrate from static texts to dynamic pictures of images and videos. Those early-stage displays had turned a new page back then in terms of information dissemination and recording. 
     However, CRT displays have the issues of being bulky and taking up too much space. Such size issue failed to be successfully tackled with numerous attempts been made until the emergence of liquid crystal displays. As liquid crystal materials are not self-illuminating, a backlight source is required for liquid crystal displays (LCD) to display information. To further diminish the size of displays, the blue light-emitting diode (LED) was developed in 1993. Subsequently, the white LED with higher luminance and lighting efficiency was introduced. In view of the advantages in small size, high luminance and high lighting efficiency, LEDs have been used as the backlight sources for LCDs, small displays and projectors. 
     Current technology involved with an LED light source can be implemented on a substrate, such as sapphire, gallium arsenide (GaAs) and gallium phosphide (GaP) substrates. An epitaxial layer is deposited on the substrate by epitaxial growth methods, such as metal organic chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE), liquid phase epitaxy (LPE) or molecular beam epitaxy (MBE). The epitaxial layer can be divided into multiple LEDs spaced apart from each other by a photolithography process, an etching process, a lift-off process, a thin film deposition process, a metal deposition process, a spin process, and an alloy process for each LED to be equipped with electrodes for conducting power and packaging. A grinding process is further applied to thin the thickness of the substrate, a dicing process is applied to cut the multiple LEDs into multiple LED dices, and a packaging process is applied to the LED light source. 
     The foregoing photolithography process includes coating, exposure and development processes and serves to generate a photoresist layer on a surface of the epitaxial layer, a surface of a thin film or a surface of a thin metal film. The photoresist layer is formed by a photosensitive material. The exposure process serves to print a pattern of a mask having spaces arranged at spaced intervals on the photoresist layer. The etching process first etches away portions of the epitaxial layer not covered by the photoresist layer and then removes the photoresist layer for the epitaxial layer to form multiple LEDs spaced apart from each other by gaps, the thin film to form a pattern with spaces arranged at spaced intervals, or the thin metal film to form a pattern with spaces arranged at spaced intervals. The lift-off process removes the photoresist layer with an organic chemical solution for the thin metal film grown on the photoresist layer to he removed and portions of the thin metal film not covered by the photoresist layer to remain. The etching process may be a dry etching process or a wet etching process. Specifically, the dry etching process is an inductively coupled plasma reactive ion etching (ICP-RIE) and the wet etching process utilizes a chemical solution to perform etching via chemical reaction. The thin film deposition process deposits thin metal films on the multiple LEDs, and the photolithography process and the etching process are further applied to form electrodes. The thin film deposition process targets at growing non-metal thin film on surfaces of the multiple LEDs or portions among the multiple LEDs, and the photolithography process and the etching process are further applied to remove unnecessary portions of the thin film to serve the purpose of insulation, support or electrical conduction depending on the nature of the thin film. The alloy process forms good ohmic contact between the electrodes and the LEDs for electrical conduction through high-temperature baking. 
     There is another conventional LED light source, which has an epitaxial layer formed on a first substrate. The LED wafer fabrication process develops multiple LEDs on the epitaxial layer. A wafer bonding process bonds the multiple LEDs to a second substrate, which is highly thermally and electrically conductive or even transparent. A laser lift-off process is further applied to remove the first substrate to enhance efficacy of the multiple LEDs in operation, a grinding process is applied to thin the second substrate, the dicing process separates multiple LED dies from the wafer, and the packaging process packages the multiple LED dies to form the LED light source 
     Most LED light sources arranged in current LED displays take the form of arrays, such as seven-segment displays, dot matrix displays or regular LCD displays. As usually tending to be relatively large in size and limited by requirements of working accuracy for positioning and spatial arrangement, the packaged LED light sources are not applicable to small displays or are applicable to displays with limitations in size and the number of LED light sources equipped, which compromise display performance and operational convenience. Additionally, production cost of the LED array displays inevitably increases due to the array assembly processes. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a dot matrix light-emitting diode (LED) light source for a wafer-level microdisplay and a method for fabricating the same, which allow multiple LEDs connected in series to constitute a dot matrix LED light source for assurance of compact size and low production cost without requiring additional processes in dicing, packaging and assembly. 
     To achieve the foregoing objective, the dot matrix LED light source for a wafer-level microdisplay includes a substrate, an LED epitaxial layer, a first electrode assembly, and a second electrode assembly. 
     The LED epitaxial layer is formed on a top surface of the substrate and has multiple LED sets formed on the LED epitaxial layer and arranged at spaced intervals. Each LED set has multiple LEDs aligned in a first direction. Each LED has a first epitaxial layer, a light-emitting layer and a second epitaxial layer. The first epitaxial layers of the LEDs of each LED set are connected to form a first epitaxial platform. The LEDs of the multiple LED sets that are aligned along any two adjacent rows in a second direction are aligned with each other. 
     The first electrode assembly has multiple first electrodes. Each first electrode is formed on a top surface of the first epitaxial platform of a corresponding LED set to connect in series to the LEDs of the corresponding LED set. 
     The second electrode assembly has multiple second electrodes. The second electrodes are each respectively formed on top surfaces of the LEDs of the multiple LED sets aligned in a corresponding row along the second direction to connect the LEDs of the multiple LED sets aligned in the corresponding row along the second direction. 
     From the foregoing description, manufacturers of microdisplays can connect the first electrode assembly and the second electrode assembly to the multiple LEDs that are arranged at spaced intervals to constitute a dot matrix LED light source, which can be directly packaged and assembled in a wafer-level microdisplay. As there is no additional dicing, packaging and array assembly involved for producing the microdisplay, the microdisplay can be implemented at a reduced size and lower production cost. 
     To achieve the foregoing objective, the method for fabricating a dot matrix light-emitting diode (LED) light source for a wafer-level microdisplay includes: 
     preparing a substrate; 
     forming an LED epitaxial layer on a top surface of the substrate; 
     forming multiple LED sets arranged at spaced intervals by applying an LED wafer fabrication process to the LED epitaxial layer, wherein each LED set has multiple LEDs aligned in a first direction, each LED set has a first epitaxial platform, and the LEDs of the multiple LED sets that are aligned along any two adjacent rows in a second direction are aligned with each other; 
     forming a first electrode assembly on top surfaces of the first epitaxial platforms of the multiple LED sets; and 
     forming a second electrode assembly on top surfaces of the LEDs of the multiple LED sets aligned in the second direction. 
     The foregoing fabrication method is involved with fabrication processes of forming the LED epitaxial layer on the substrate, forming the multiple LED sets arranged at spaced intervals on the LED epitaxial layer through the LED wafer fabrication process. Each LED set includes the multiple LEDs and the first epitaxial platform, such that the first electrode assembly can be formed on the first epitaxial platform and the second electrode assembly can be formed on the LEDs of the multiple LED sets aligned in the second direction to constitute a dot matrix LED light source, thereby fulfilling the implementation of a wafer-level microdisplay using the dot matrix LED light source. 
     To achieve the foregoing objective, the dot matrix light-emitting diode (LED) light source for a wafer-level microdisplay includes a substrate, an LED epitaxial layer, a bonding layer, a first electrode assembly, and a second electrode assembly. 
     The LED epitaxial layer is formed on a top surface of the substrate and has multiple LED sets formed on the LED epitaxial layer and arranged at spaced intervals. Each LED set has multiple LEDs aligned in a first direction. Each LED has a first epitaxial layer, a light-emitting layer and a second epitaxial layer. The first epitaxial layers of the LEDs of each LED set are connected to form a first epitaxial platform. The LEDs of the multiple LED sets that are aligned along any two adjacent rows in a second direction are aligned with each other. 
     The bonding layer is formed between the substrate and the multiple LED sets. 
     The first electrode assembly is formed between the bonding layer and the multiple LED sets and has multiple first electrodes each of which is formed between a corresponding LED set and the bonding layer to connect in series to the LEDs of the corresponding LED set. 
     The second electrode assembly has multiple second electrodes. The second electrodes are each respectively formed on top surfaces of the LEDs of the multiple LED sets aligned in a corresponding row along the second direction to connect the LEDs of the multiple LED sets aligned in the corresponding row along the second direction. 
     From the foregoing description, manufacturers of microdisplays can connect the first electrode assembly and the second electrode assembly to the multiple LEDs that are arranged at spaced intervals to constitute a dot matrix LED light source. As there is no additional dicing, packaging and array assembly involved for producing the microdisplay, the microdisplay can be implemented at a reduced size and lower production cost. 
     To achieve the foregoing objective, the method for fabricating a dot matrix light-emitting diode (LED) light source for a wafer-level microdisplay includes: 
     preparing a first substrate; 
     forming an LED epitaxial layer on a top surface of the substrate; 
     forming a first electrode assembly on a top surface of the LED epitaxial layer; and 
     preparing a second substrate; 
     forming a bonding layer on a top surface of the second substrate; 
     bonding the first electrode assembly to a top surface of the bounding layer; 
     forming multiple LED sets arranged at spaced intervals by applying an LED wafer fabrication process to the LED epitaxial layer, wherein each LED set has multiple LEDs aligned in a first direction, each LED set has a first epitaxial platform, and the LEDs of the multiple LED sets that are aligned along any two adjacent rows in a second direction are aligned with each other; and 
     forming a second electrode assembly on top surfaces of the LEDs of the multiple LED sets aligned in the second direction. 
     The foregoing fabrication method is involved with fabrication processes of forming the LED epitaxial layer on the first substrate, forming the first electrode assembly on the LED epitaxial layer, then providing the second substrate, forming the bonding layer on the second substrate, bonding the first electrode assembly to the bonding layer, and removing the second substrate with a laser lift-off or etching technique, such that an LED wafer fabrication process is applied to the LED epitaxial layer to form the multiple LED sets arranged at spaced intervals on the LED epitaxial layer, and the second electrode assembly can be formed on the LEDs of the multiple LED sets aligned in the second direction to constitute a dot matrix LED light source, thereby fulfilling the implementation of a wafer-level microdisplay using the dot matrix LED light source. 
     Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a first embodiment of a dot matrix LED light source for a wafer-level microdisplay in accordance with the present invention; 
         FIG. 2  is a cross-sectional view of the dot matrix LED light source in  FIG. 1  taken along line  2 - 2 ; 
         FIG. 3  is a cross-sectional view of the dot matrix LED light source in  FIG. 1  taken along line  3 - 3 ; 
         FIG. 4  is a cross-sectional view of the dot matrix LED light source in  FIG. 3  fabricated under a first fabrication process; 
         FIG. 5  is a cross-sectional view of the dot matrix LED light source in  FIG. 3  fabricated under a second fabrication process; 
         FIG. 6  is a cross-sectional view of the dot matrix LED light source in  FIG. 3  fabricated under a third fabrication process; 
         FIG. 7  is a cross-sectional view of the dot matrix LED light source in  FIG. 3  fabricated under a fourth fabrication process; 
         FIG. 8  is another top view of the dot matrix LED light source in  FIG. 1 ; 
         FIG. 9  is a top view of a second embodiment of a dot matrix LED light source for a wafer-level microdisplay in accordance with the present invention; 
         FIG. 10  is a cross-sectional view of the dot matrix LED light source in  FIG. 9  taken along line  10 - 10 ; 
         FIG. 11  is a cross-sectional view of the dot matrix LED light source in FIG.  10  fabricated under a first fabrication process; 
         FIG. 12  is a cross-sectional view of the dot matrix LED light source in  FIG. 10  fabricated under a second fabrication process; 
         FIG. 13  is a cross-sectional view of the dot matrix LED light source in  FIG. 10  fabricated under a third fabrication process; 
         FIG. 14  is a cross-sectional view of the dot matrix LED light source in  FIG. 10  fabricated under a fourth fabrication process; 
         FIG. 15  is a cross-sectional view of the dot matrix LED light source in  FIG. 10  fabricated under a fifth fabrication process; 
         FIG. 16  is a cross-sectional view of the dot matrix LED light source in  FIG. 10  fabricated under a sixth fabrication process; and 
         FIG. 17  is another top view of the dot matrix LED light source in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIGS. 1-3 , a first embodiment of a dot matrix light-emitting diode (LED) light source  100  for a wafer-level microdisplay in accordance with the present invention includes a first substrate  10 , multiple LED sets  20 , a first electrode assembly  30 , and a second electrode assembly. 
     The multiple LED sets  20  are arranged at spaced intervals. Each LED set  20  has multiple LEDs  21  spaced apart from each other and aligned in a first direction (Y-axis direction). Each LED  21  has a first epitaxial layer  211 , a light-emitting layer  212  and a second epitaxial layer  213 , which are sequentially formed on a top surface of the first substrate  10 . The first epitaxial layers  211  of the LEDs  21  of each LED set  20  are mutually connected to form a first epitaxial platform  22 . The LEDs  21  of the multiple LED sets  20  that are aligned along any two adjacent rows in a second direction (X-axis direction) are aligned with each other, such that the LEDs  21  of the multiple LED sets  20  are aligned in the form of a matrix. The multiple LED sets  20  further have multiple first slots  23  and multiple second slots  24 . Each first slot  23  is formed between adjacent two of the multiple LED sets  20 . Each second slot  24  is formed between the LEDs  21  of the LED sets  20  aligned in two adjacent rows along the second direction (X-axis direction). The multiple first slots  23  and the multiple second slots  24  are aligned in the first direction (Y-axis direction) and the second direction (X-axis direction) respectively. The first direction (Y-axis direction) is perpendicular to the second direction (X-axis direction). In the present embodiment, the first substrate  10  is a transparent substrate. 
     The first electrode assembly  30  includes multiple first electrodes  31  each of which is formed on a top surface of the first epitaxial platform  22  of a corresponding LED set  20  to connect the LEDs  21  of the corresponding LED set  20  in series, and the second electrode assembly includes multiple second electrodes  41  each respectively formed on top surfaces of the LEDs  21  of the multiple LED sets  20  aligned in a corresponding row along the second direction (X-axis direction) to connect the LEDs  21  of the multiple LED sets  20  aligned in the corresponding row along the second direction (X-axis direction), such that at least one dot matrix LED light source  100  can be constituted. 
     In the present embodiment, a packaging area  50  is formed around a perimeter of the multiple LED sets  20 . The packaging area  50  includes a first area and a second area. The first area is aligned in the first direction (Y-axis direction) with multiple first electrode terminals  51  formed on the first area and connecting with the respective first electrodes  31  of the first electrode assembly  30 , and the second area is aligned in the second direction (X-axis direction) with multiple second electrode terminals  52  formed on the second area and connecting with the respective second electrodes  41  of the second electrode assembly, facilitating subsequent packaging of each LED light source  100 . In the present embodiment, a scribe channel  200  is formed between each adjacent two columns of the multiple dot matrix LED light sources  100  for each dot matrix LED light source  100  to be easily separated by a dicing process and packaged. 
     To depict a method for fabricating the dot matrix LED light source  100 , with reference to  FIGS. 1 and 4 , an LED epitaxial layer is formed on a surface of the first substrate  10  by way of epitaxial growth. The multiple LED sets  20  that are spaced apart from each other are formed on the LED epitaxial layer through an LED wafer fabrication process. Each LED set  20  includes the multiple LEDs  21  aligned along a straight line in the first direction (Y-axis direction). Each LED  21  has the first epitaxial layer  211 , the light-emitting layer  212  and the second epitaxial layer  213  sequentially stacked on the surface of the first substrate  10 . The first epitaxial layers  211  of the multiple LEDs  21  of each LED set  20  are mutually connected to form the first epitaxial platform  22 . Each first slot  23  is formed between corresponding adjacent two of the multiple LED sets  20 . Each second slot  24  is formed between the LEDs  21  of the LED sets  20  aligned in two adjacent rows along the second direction (X-axis direction). In the present embodiment, the LED wafer fabrication process adopts and combines a photolithography process, an etching process, a lift-off process, a thin film deposition process, a coating process, a wafer bonding process, a wafer laser de-bonding process, a laser lift-off process, a metal deposition process, and an alloy process. The LED epitaxial layer may be made from gallium nitride (GaN), indium gallium nitride (InGaN) or aluminum gallium indium nitride (AlGaInN). The size of each LED  21  is in a range of 1 μm˜500 μm. 
     With reference to  FIGS. 1 and 5 , the first electrode assembly  30  includes the multiple first electrodes  31  and a first insulation layer  32 . Each first electrode  31  is formed on a top surface of the first epitaxial platform  22  of a corresponding LED set  20  to connect the LEDs  21  of the corresponding LED set  20  in series. 
     The first insulation layer  32  is formed on top surfaces of the second epitaxial layers  213  of the multiple LED sets  20  and the first electrodes  31  and is filled in the first slots  23  to protect the multiple LED sets  20  and the first electrodes  31  and support the second electrode assembly with a portion of a top surface of the second epitaxial layer  213  of each LED  21  exposed. In the present embodiment, the first insulation layer  32  is formed by silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ). 
     With reference to  FIGS. 1, 6 and 7 , the second electrode assembly includes the multiple second electrodes  41 , a second insulation layer  42  and a grating layer. Each second electrode  41  is formed on a top surface of the first insulation layer  32  and the top surfaces of the second epitaxial layers  213  of the LEDs  21  of the multiple LED sets  20  aligned in a corresponding row along the second direction (X-axis direction) to connect the LEDs  21  aligned in the corresponding row along the second direction (X-axis direction). The multiple second electrodes  41  cover the respective second slots  24  to prevent light emitted from the LEDs  21  of the multiple LED sets  20  from coming out from the multiple second slots  24  in an upward direction and effectively concentrate light emitted from the LEDs  21 . The multiple first electrodes  31 , the multiple second electrodes  41  and the LEDs  21  of the multiple LED sets  20  are formed with good electric conductivity by using the alloy process. In the present embodiment, each first electrode  31  is formed by overlapping titanium, aluminum and gold in layers of Ti/Al/Ti/Au or overlapping platinum, titanium and gold in layers of Pt/Ti/Pt/Au. 
     With reference to  FIGS. 7 and 8 , the second insulation layer  42  is formed on top surfaces of the multiple second electrodes  41  and the first insulation layer  32  to protect the multiple second electrodes  41  and support the grating layer. The grating layer is further formed on a top surface of the second insulation layer  42  and has multiple gratings  43 . The multiple gratings  43  respectively block the first epitaxial platforms  22  and the multiple first slots  23  to prevent light emitted from the LEDs  21  of the multiple LED sets  20  from coming out from the multiple first slots  23  and the first epitaxial platforms  22  in an upward direction and effectively concentrate light emitted from the LEDs  21 . In the present embodiment, the second insulation layer  32  is formed by silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ), and the gratings are formed by opaque materials. 
     The LEDs  21  of the multiple LED sets  20  are connected in series through the first electrodes  31  and the second electrodes  41  to constitute the dot matrix LED light source  100 . During manufacture of a wafer-level microdisplay, the dot matrix LED light source  100  just needs to be packaged in a microdisplay. Thus, the LEDs  21  are unnecessarily cut into separate LEDs  21  first before packaging and assembly. Therefore, cutting expense can be reduced. As no additional packaging and array assembly are required, compact size of the LEDs can be secured to facilitate subsequent assembly. 
     With reference to  FIGS. 9 and 10 , a second embodiment of a dot matrix LED light source  100 A for a wafer-level microdisplay in accordance with the present invention includes a first substrate  60 , a bonding layer  61 , multiple LED sets  70 , a first electrode assembly  80  and a second electrode assembly  90 . 
     The multiple LED sets  70  are arranged at spaced intervals. Each LED set  70  has multiple LEDs  71  spaced apart from each other and aligned in a first direction (Y-axis direction). Each LED  71  has a first epitaxial layer  711 , a light-emitting layer  712  and a second epitaxial layer  713 , which are sequentially formed on a top surface of the first substrate  60 . The first epitaxial layers  711  of the multiple LEDs  71  are mutually connected. The LEDs  71  of the multiple LED sets  70  that are aligned along any two adjacent rows in the second direction (X-axis direction) are aligned with each other, such that the LEDs  71  of the multiple LED sets  70  are aligned in the form of a matrix. There are multiple first slots  72  and multiple second slots  73 . Each first slot  72  is formed between adjacent two of the multiple LED sets  20 . Each second slot  73  is formed between the LEDs  71  of the LED sets  70  aligned in two adjacent rows along the second direction (X-axis direction). The multiple first slots  72  and the multiple second slots  73  are aligned in the first direction (Y-axis direction) and the second direction (X-axis direction) respectively. The first direction (Y-axis direction) is perpendicular to the second direction (X-axis direction). In the present embodiment, the first substrate  60  is a transparent substrate with high thermal dissipation and high conductivity, and the size of each LED is in a range of 1 μm˜500 μm. 
     The first electrode assembly  80  is formed between the bonding layer  61  and the multiple LED sets  70  and includes multiple first electrodes  81  each of which is formed between a corresponding LED set  70  and the bonding layer  61  to connect the LEDs  71  of the corresponding LED set  70  in series. The second electrode assembly  90  includes multiple second electrodes  91  each respectively formed on top surfaces of the LEDs  71  of the multiple LED sets  70  aligned in a corresponding row along the second direction (X-axis direction) to connect the LEDs  71  of the multiple LED sets  70  aligned in the corresponding row along the second direction (X-axis direction), such that at least one dot matrix LED light source  100 A can be constituted. 
     In the present embodiment, a packaging area  50 A is formed around a perimeter of the multiple LED sets  70 . The packaging area  50 A includes a first area and a second area. The first area is aligned in the first direction (Y-axis direction) with multiple first electrode terminals  51 A formed on the first area and connecting with the respective first electrodes  81  of the first electrode assembly  80 , and the second area is aligned in the second direction (X-axis direction) with multiple second electrode terminals  52 A formed on the second area and connecting with the respective second electrodes  91  of the second electrode assembly  90 , facilitating subsequent packaging of each LED light source  100 A. In the present embodiment, a scribe channel  200 A is formed between each adjacent two columns of multiple dot matrix LED light sources  100 A for each dot matrix LED light source  100 A to be easily separated by a dicing process and packaged. 
     With reference to  FIG. 11 , a second substrate  62  is prepared first, and an LED epitaxial layer is formed on a surface of the second substrate  62  by way of epitaxial growth. The LED epitaxial layer includes a first epitaxial layer  711 , a light-emitting layer  712  and a second epitaxial layer  713  sequentially stacked on the surface of the first substrate  62 . A third substrate  63  is further prepared. A bonding layer  64  is then formed on a top surface of the third substrate  63 . A wafer bonding fabrication process is used to bond a surface of the second epitaxial layer  713  to a surface of the bonding layer  64 . A laser lift-off technique is applied to remove the second substrate  62  for a surface of the first epitaxial layer  711  to be exposed. In the present embodiment, the bonding layer  64 A is formed by indium (In)/tin (Sn)/gold (Au)/Silicon (Si)/Germanium (Ge) or Bzocyclobutene (BCB) adhesive or spin-on glass (SOG). 
     With reference to  FIG. 12 , the first electrode assembly  80  includes the multiple first electrodes  81 , a first insulation layer  82 , a reflective layer and a second insulation layer  84 . The multiple first electrodes  81  are formed on a top surface of the first epitaxial layer  711 . The first insulation layer  82  is formed on the top surface of the first epitaxial layer  711  and portions between each adjacent two of the multiple first electrodes  81  to support the reflective layer. Top surfaces of the multiple LED sets  70  are partially exposed for the multiple first electrodes  81  to be respectively formed on the exposed portions of the top surfaces of the multiple LED sets  70 . The reflective layer is formed on the top surfaces of the multiple first electrodes  81  and a top surface of the first insulation layer  82 . The reflective layer has multiple reflective strips  83  correspondingly covering the respective first electrodes  81  to reflect light emitted from the multiple LED sets  70 . The second insulation layer  84  is formed on a top surface of the reflective layer and the top surface of the first insulation layer  82  to protect the reflective layer. A top surface of the second insulation layer  84  is flat. 
     In the present embodiment, the first insulation layer  82  and the second insulation layer are formed by silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ), the reflective strips  83  are formed by silver (Ag), aluminum (Al) or distributed Bragg reflector (DBR), and the first electrodes  81  are formed by overlapping titanium (Ti), aluminum (Al) and gold (Au) in layers of Ti/Al/Ai/Au or overlapping platinum (Pt), titanium (Ti), gold (Au) in layers of Pt/Ti/Pt/Au. 
     With reference to  FIG. 13 , the first substrate  60  is prepared first, the bonding layer  61  is formed on the top surface of the first substrate  60 , a bottom surface of the second insulation layer  84  is bonded to the top surface of the bonding layer  61 , and the bonding layer  64  and the third substrate  63  are removed to expose the top surface of the second epitaxial layer  713 . 
     With reference to  FIGS. 9 and 14 , the LED epitaxial layer is fabricated into the multiple LED sets  70  spaced apart from each other. The multiple LED sets  70  include the LEDs aligned in the first direction (Y-axis direction) with the first epitaxial layers  711  of the LEDs  71  mutually connected. Each first slot  72  is formed between adjacent two of the multiple LED sets  70 . Each second slot  73  is formed between the LEDs of the multiple LED sets  70  aligned in two adjacent rows along the second direction (X-axis direction). 
     The multiple LED sets  70  respectively correspond to the multiple first electrodes  81  and the multiple reflective strips  83 . Each first electrode  81  is connected with the LEDs  71  of a corresponding LED set  70 . The reflective strips  83  match and cover the respective LED sets  70  to reflect light emitted from the multiple LED sets  70  to come out in an upward direction. 
     With reference to  FIG. 15 , the second electrode assembly  90  includes the multiple second electrodes and a first insulation layer  92 . The first insulation layer  92  is formed on top surfaces of the LEDs  71  of the multiple LED sets  70  aligned in the second direction (X-axis direction), is filled in the multiple first slots  72  and the multiple second slots  73  to protect the LEDs  71  of the multiple LED sets  70  aligned in the second direction (X-axis direction) and support the multiple second electrodes  91 . The second epitaxial layers  713  of the LEDs  71  aligned in the second direction (X-axis direction) are partially exposed. The multiple second electrodes  91  are connected in series to the respective LEDs aligned in the second direction (X-axis direction) and cover the respective second slots  73 . In the present embodiment, each second electrode is formed by overlapping titanium, aluminum and gold in layers of Ti/Al/Ti/Au or overlapping platinum, titanium and gold in layers of Pt/Ti/Pt/Au. 
     With reference to  FIGS. 16 and 17 , the second electrode assembly  90  further includes a second insulation layer  93  and a grating layer. The second insulation layer  93  is formed on top surfaces of the second electrodes  91  and the first insulation layer  92  to protect the second electrodes  91  and support the grating layer. The top surfaces of the LEDs  71  of the LED sets  70  aligned in the second direction (X-axis direction) are partially exposed for the second electrodes  91  to be respectively formed on the partially exposed portions of the top surfaces of the LEDs  71 . The grating layer is formed on a top surface of the second insulation layer  93  and includes multiple gratings  94  covering the respective first slots to prevent light emitted from the multiple LEDs  71  aligned in the second direction (X-axis direction) from coming out from the first slots  72  and effectively concentrate light emitted from the LEDs  71 . In the present embodiment, the first insulation layer  92  and the second insulation layer  93  are formed by silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ), and the multiple gratings  94  are formed by an opaque material. 
     The dot matrix LED light source  100 A can be constructed by virtue of the multiple first electrodes  81  and the multiple second electrodes  91  respectively connected in series to the LEDs  71  of the multiple LED sets  70 . During manufacture of a wafer level microdisplay, the dot matrix LED light source  100 A just needs to be packaged in a microdisplay and the wafer level microdisplay is formed. Thus, the LEDs  71  are unnecessarily cut into separate LEDs  71  before packaging and assembly. Therefore, cutting expense can be reduced. As no additional packaging and array assembly are required, compact size of the LEDs can be secured to facilitate subsequent assembly. 
     Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only. Changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.