Patent Publication Number: US-2023135465-A1

Title: Micro led display device and method forming the same

Description:
BACKGROUND 
     Technical Field 
     The present disclosure is related to a micro light-emitting diode (μLED) display device, and in particular it is related to a configuration of a reflective structure and a black structure of the micro light-emitting diode display device and a method forming the same. 
     Description of the Related Art 
     As optoelectronic technology advances, the feature size of an optoelectronic device continues to reduce. In comparison with organic light emitting diode (OLED), micro light-emitting diode (μLED) has several advantages, such as higher efficiency, longer lifetime, relatively stable materials that are insensitive to environmental influences, and capability of providing images with higher resolution. Therefore, a display device manufactured with micro light-emitting diodes arranged in an array has gradually gained importance in the market. 
     Color-conversion materials are quantum dots (QD) that consist of semiconductor particles of II-VI or III-V group elements. The emitting light color of the color-conversion materials may be adjusted through the dimension, structure, or composition of the color-conversion material, in order to achieve higher color-conversion efficiency. While the characteristics of the color-conversion materials play a crucial role to the overall performance, a plurality of bank structures separating the color-conversion materials of different colors may also affect the display device in operation. Under turn-off state, the ambient light may reflect at the reflective layer, which will decrease the display quality, for example the contrast ratio of the displayed image. These related issues need to be addressed. 
     SUMMARY 
     In an embodiment, a micro LED display device includes: a substrate; a plurality of micro light-emitting diodes disposed on the substrate; a reflective layer and a black layer sequentially stacked on the substrate, the reflective layer and the black layer cover a surface of the substrate, wherein a top surface of each micro light-emitting diode is exposed through the reflective layer and the black layer, and a sidewall of each micro light-emitting diode is covered by the reflective layer and the black layer; a plurality of reflective banks and a plurality of black banks sequentially disposed on the black layer and exposing the top surface of the plurality of micro light-emitting diodes, wherein the reflective layer, the black layer, the plurality of reflective banks, and the plurality of black banks overlap each other in a display direction; and a color-conversion material covering the top surface of at least one of the plurality of micro light-emitting diodes, wherein the color-conversion material is laterally disposed between the plurality of reflective banks. 
     In another embodiment, a method forming a micro LED display device, includes: providing a substrate; bonding a plurality of micro light-emitting diodes on the substrate by a mass transfer process; forming a first reflective layer on the substrate, wherein the first reflective layer covers the substrate and a sidewall of each micro light-emitting diode; forming a first black layer to cover the first reflective layer and the plurality of micro light-emitting diodes; etching back the first black layer until a top surface of each micro light-emitting diode is exposed through the first black layer; forming a second reflective layer and a second black layer on the plurality of micro light-emitting diodes and the first black layer; patterning the second reflective layer and the second black layer into a plurality of reflective banks and a plurality of black banks, respectively; and filling a color-conversion material laterally between the plurality of reflective banks, wherein the color-conversion material covers the top surface of at least one of the plurality of micro light-emitting diodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be more fully understood from the following detailed description when read with the accompanying figures. It is worth noting that, in accordance with standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG.  1    is a cross-sectional view of a display device, according to some embodiments of the present disclosure. 
         FIGS.  2 A- 2 H  are various cross-sectional views of intermediate stages in manufacturing the display device, according to some embodiments of the present disclosure. 
         FIGS.  3 A and  3 B  are cross-sectional views of display devices, according to other embodiments of the present disclosure. 
         FIG.  4    is a cross-sectional view of a display device, according to yet other embodiments of the present disclosure. 
         FIGS.  5 A and  5 B  are cross-sectional views of display devices, according to other embodiments of the present disclosure. 
         FIGS.  6 A and  6 B  are cross-sectional views of display devices, according to yet other embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the subject matter provided. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, a first feature is formed on a second feature in the description that follows may include embodiments in which the first feature and second feature are formed in direct contact, and may also include embodiments in which additional features may be formed between the first feature and second feature, so that the first feature and second feature may not be in direct contact. 
     It should be understood that additional steps may be implemented before, during, or after the illustrated methods, and some steps might be replaced or omitted in other embodiments of the illustrated methods. 
     Furthermore, spatially relative terms, such as “beneath,” “below,” “lower,” “on,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to other elements or features as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     In the present disclosure, the terms “about,” “approximately” and “substantially” typically mean±20% of the stated value, more typically ±10% of the stated value, more typically ±5% of the stated value, more typically ±3% of the stated value, more typically ±2% of the stated value, more typically ±1% of the stated value and even more typically ±0.5% of the stated value. The stated value of the present disclosure is an approximate value. That is, when there is no specific description of the terms “about,” “approximately” and “substantially”, the stated value includes the meaning of “about,” “approximately” or “substantially”. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It should be understood that terms such as those defined in commonly used dictionaries should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined in the embodiments of the present disclosure. 
     The present disclosure may repeat reference numerals and/or letters in following embodiments. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     The quality of an image resulted from a display device may be compromised when the contrast ratio is reduced or when the blackness is insufficient. In order to increase contrast ratio and/or blackness, ambient light reflection must be suppressed. A conventional structure of the display device implements a single reflective layer and a plurality of black banks. The reflective layer surrounds each of a plurality of micro light-emitting diodes, while the plurality of black banks are disposed on the reflective layer and laterally between and exposing the plurality of micro light-emitting diodes. Even though the conventional configuration is simpler with lower cost, such configuration is inadequate to suppress ambient light reflection. For example, the plurality of black banks may absorb photons emitted from the plurality of micro light-emitting diodes, resulting in lower luminance. In other words, the configuration of the conventional display device may concurrently eliminate desired light rays and allow undesired light rays, hence degrading the overall quality of the resulting image displayed. 
     The present disclosure provides an innovative way to solve both the luminance issue and the ambient light reflection issue. According to some embodiments of the present disclosure, a black layer is added above the reflective layer to reduce ambient light reflection, and a plurality of reflective banks is added below the plurality of black banks to preserve the emitted photons. When the bank structure includes materials of reflective nature, the emitted photons may be more easily funneled toward a designated direction in order to improve luminance. Furthermore, capping the plurality of reflective banks with the plurality of black banks may more effectively absorb unwanted ambient light 
       FIG.  1    is a cross-sectional view of a display device  10 , according to some embodiments of the present disclosure. In some embodiments, display devices may contain millions of pixels in reality. For the sake of brevity,  FIG.  1    only illustrates two exemplary pixels P of an actual display device. Each exemplary pixel P may include sub-pixel regions that are red, green, and blue (for example, from left to right, as shown in  FIG.  1   ). According to some embodiments of the present disclosure, the display device  10  includes a substrate  100 , a plurality of micro light-emitting diodes  102  (for example, blue micro light-emitting diodes  102 A), a reflective layer  104 , a black layer  106 , a plurality of reflective banks  108 , a plurality of black banks  110 , color-conversion materials  112 , a plurality of light-shielding structures  114 , a plurality of color filter units  116 , a cover plate  120 , and an optical layer  122 . In some embodiment, the plurality of micro light-emitting diodes  102  may also be near violet (in a wavelength between 365 nm and 405 nm), ultra violet, green, or the like. Moreover, the color-conversion materials  112  include a red color-conversion material  112 - 1  and a green color-conversion material  112 - 2 . In some embodiments, the plurality of color filter units  116  include a red color filter unit  116 - 1 , a green color filter unit  116 - 2 , and a blue color filter unit  116 - 3 . 
     According to some embodiments of the present disclosure, the red color-conversion material  112 - 1  and the green color-conversion material  112 - 2  may convert blue light emitted from their corresponding blue micro light-emitting diodes  102 A underneath into red light and green light, respectively. The converted red light and the converted green light may then be directed through the red color filter unit  116 - 1  and the green color filter unit  116 - 2 , respectively. Since the red light, the green light, and the blue light are all available in the structure shown, they may constitute the exemplary pixel P of the display device  10  and an array of pixels would display an image in a display direction A. 
     Referring to  FIG.  1   , the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , and the plurality of black banks  110  are sequentially stacked, so reflective elements and black elements are alternately arranged. For example, the black layer  106  is sandwiched between the reflective layer  104  and the plurality of reflective banks  108  in the display direction A, while the plurality of reflective banks  108  are sandwiched between the black layer  106  and the plurality of black banks  110  in the display direction A. In some embodiments, the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , and the plurality of black banks  110  overlap each other in the display direction A. When viewed from top, except for the top surface of the plurality of micro light-emitting diodes  102 , the display device  10  may appear to be substantially covered with black material, which makes it more efficient for suppressing ambient light reflection. 
     In some embodiments, the substrate  100  may be, for example, a wafer or a chip, but the present disclosure is not limited thereto. In some embodiments, the substrate  100  may be a semiconductor substrate, for example, silicon substrate. Furthermore, in some embodiments, the semiconductor substrate may also be an elemental semiconductor including germanium, a compound semiconductor including gallium nitride (GaN), silicon carbide (SiC), gallium arsenide (GaAs), gallium phosphide (GaP), indium phosphide (InP), indium arsenide (InAs), and/or indium antimonide (InSb), an alloy semiconductor including silicon germanium (SiGe) alloy, or a combination thereof. In some embodiments, the substrate  100  may be a photoelectric conversion substrate, such as a silicon substrate or an organic photoelectric conversion layer. 
     In other embodiments, the substrate  100  may also be a semiconductor on insulator (SOI) substrate. The semiconductor on insulator substrate may include a base plate, a buried oxide layer disposed on the base plate, and a semiconductor layer disposed on the buried oxide layer. In some embodiments, the substrate  100  may be a glass substrate with a thin film transistor (TFT) array. Furthermore, the substrate  100  may be an N-type or a P-type conductive type. 
     In some embodiments, the substrate  100  may be a backplane for the plurality of micro light-emitting diodes  102 . The backplane may further include additional elements (not shown for simplicity), such as the thin film transistors (TFT), complementary metal-oxide semiconductor (CMOS), printed circuit board (PCB), driving components, suitable conductive features, the like, or combinations thereof. Conductive features may include, but not limited to, cobalt (Co), ruthenium (Ru), aluminum (Al), tungsten (W), copper (Cu), titanium (Ti), tantalum (Ta), silver (Ag), gold (Au), platinum (Pt), nickel (Ni), zinc (Zn), chromium (Cr), molybdenum (Mo), niobium (Nb), the like, combinations thereof, or the multiple layers thereof. These elements provide circuitry that connects to the plurality of micro light-emitting diodes  102 . 
     Referring to  FIG.  1   , the plurality of micro light-emitting diodes  102  are disposed on the substrate  100 . In some embodiments, the plurality of micro light-emitting diodes  102  are arranged in an array on the surface of the substrate  100 . The plurality of micro light-emitting diodes  102  may each include an N-type semiconductor layer, a light-emitting layer, and a P-type semiconductor layer. The light-emitting layer may be disposed between the N-type semiconductor layer and the P-type semiconductor layer. The light emitted from each of the plurality of micro light-emitting diodes  102  is determined by the light-emitting layer. For example, the blue micro light-emitting diodes  102 A shown in  FIG.  1    may emit blue light. As mentioned previously, the light-emitting layers of other micro light-emitting diodes  102  may emit near violet light, ultra violet light, green light, the like, or combinations thereof. 
     The N-type/P-type semiconductor layers may include materials of II-VI group (for example, zinc selenide (ZnSe)) or III-V group (for example, gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN), or aluminum indium gallium nitride (AlInGaN)). Moreover, the semiconductor layers may include dopants (such as silicon or germanium), but the present disclosure is not limited thereto. The light-emitting layer may include at least one undoped semiconductor layer or at least one lightly doped layer. For example, the light-emitting layer may be a multiple quantum well (MQW) layer. 
     Referring to  FIG.  1   , the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , and the plurality of black banks  110  are sequentially formed on the substrate  100 . In some embodiments, the reflective layer  104  and the black layer  106  are coated onto the surface of the substrate  100 , followed by an etching back process to expose the plurality of micro light-emitting diodes  102 . From top view, the plurality of reflective banks  108  and the plurality of black banks  110  are patterned-grid structures, in order to compartmentalize each of the plurality of micro light-emitting diodes  102 . The color-conversion materials  112  may be disposed above at least one of the plurality of micro light-emitting diodes  102 , and fills the space laterally between the plurality of reflective banks  108 . The manufacturing process of the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , the plurality of black banks  110 , and the color-conversion materials  112  will be described in detail in reference with  FIGS.  2 A- 2 H . 
     According to some embodiments of the present disclosure, the reflective layer  104  is formed on the substrate  100 . The reflective layer  104  may reflect lights emitted from the plurality of micro light-emitting diodes  102 . Since the lights emitted from the light-emitting layer are irradiated in all directions, the reflective layer  104  surrounds the plurality of micro light-emitting diodes  102  in order to reflect as much emitted light as possible toward the display direction A, so the light efficiency and the brightness may be increased. When the black layer  106  is not present thereon, the reflective layer  104  may also reflect the ambient light inadvertently. The presence of the black layer  106  may suppress unwanted ambient light from reflecting. Materials of the reflective layer  104  may include organic layer (photo-resistant, epoxy, the like, or combinations thereof) with titanium dioxide (TiO 2 ), zirconium dioxide (ZrO 2 ), or other reflective nanoparticles. The thickness of the reflective layer  104  may be approximately between 4.0 μm and 6.0 μm. The reflective layer  104  may be formed by spin-on coating or other suitable process. 
     The black layer  106  may be formed on the reflective layer  104 . According to some embodiments of the present disclosure, the black layer  106  may absorb the ambient light, in order to suppress ambient light reflection. Materials of the black layer  106  may include acrylic resin (polymeric) photo-resist material with black pigment or dye, the like, or combinations thereof. The thickness of the black layer  106  may be approximately between 1.0 μm and 2.0 μm. The formation of the black layer  106  may be similar to that of the reflective layer  104 , and the details are not described again herein to avoid repetition. In the present embodiment, the surface of the black layer  106  may be coplanar with the top surface of the plurality of micro light-emitting diodes  102 . 
     The plurality of reflective banks  108  are disposed on the surface of the black layer  106 , and not on the top surface of the plurality of micro light-emitting diodes  102 . When the light rays are emitted from the top surface of the plurality of micro light-emitting diodes  102 , the plurality of reflective banks  108  may function as a light pipe structure to reflect the light rays toward the plurality of color filter units  116  located above. Additionally, the plurality of reflective banks  108  may allow the color-conversion materials  112  to be filled in between. In other words, when the light rays are transmitted toward the plurality of color filter units  116 , the plurality of reflective banks  108  may isolate the light rays within the sub-pixel region therebetween to prevent interference between adjacent sub-pixel regions (causing color cross-talk, resulting in inaccurate pixel color, and affecting the displayed quality). Materials of the plurality of reflective banks  108  are similar to those of the reflective layer  104 , and the details are not described again herein to avoid repetition. The thickness of the plurality of reflective banks  108  may be approximately between 5.0 μm and 15.0 μm. It should be appreciated that in the display direction A, the sum of the thickness of the reflective banks  108  and the thickness of the black banks  110  disposed above is larger than the sum of the thickness of the underlying black layer  106  and the thickness of the reflective layer  104  disposed below. When the color-conversion materials  112  are laterally disposed between the plurality of reflective banks  108 , the plurality of reflective banks  108  may fully encompass the color-conversion materials  112 . The formation of the plurality of reflective banks  108  may include any suitable deposition and patterning processes. 
     The plurality of black banks  110  may be disposed on the plurality of reflective banks  108 . When the ambient light enters the display device  10 , the plurality of black banks  110  may prevent the ambient light to contact and reflect on the plurality of reflective banks  108 . As mentioned previously, the placement of the plurality of black banks  110  suppresses the ambient light reflection from the plurality of reflective banks  108 . Materials of the plurality of black banks  110  are similar to those of the black layer  106 , and the details are not described again herein to avoid repetition. The thickness of the plurality of black banks  110  may be approximately between 1.0 μm and 2.0 μm. The formation of the plurality of black banks  110  may be similar to that of the plurality of reflective banks  108 , and the details are not described again herein to avoid repetition. 
     Referring to  FIG.  1   , the top width of the plurality of reflective banks  108  is substantially equal to the bottom width of the plurality of black banks  110 . The plurality of reflective banks  108  and the plurality of black banks  110  laterally surround and expose each of the plurality of micro light-emitting diodes  102 . As stated earlier, the plurality of reflective banks  108  may function as a light pipe structure to reflect the light toward the plurality of color filter units  116  located above, and the plurality of black banks  110  may suppress ambient light reflection. In other words, the plurality of reflective banks  108  and the plurality of black banks  110  constitute a partition wall that prevent emitted light of adjacent micro light-emitting diodes  102  from interfering with each other. The configuration of the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , and the plurality of black banks  110  may suppress the ambient light reflection and improve the contrast ratio and/or the blackness simultaneously, so a resulting image of higher quality may be displayed. 
     In some embodiments, the color-conversion materials  112  are disposed on at least one of the plurality of micro light-emitting diodes  102  and a portion of the black layer  106 , and corresponding to the respective color filter units  116 . More specifically, the color-conversion materials  112  are located between at least one of the plurality of micro light-emitting diodes  102  and at least one of the plurality of color filter units  116  in the display direction A. In some embodiments, the color-conversion materials  112  may be colored red, green, blue, or colors of other wavelength. In the present embodiment, the color-conversion materials  112  include the red color-conversion material  112 - 1  and the green color-conversion material  112 - 2 . According to some embodiments of the present disclosure, the color-conversion materials  112  may change the wavelength of incident light rays (from the plurality of micro light-emitting diodes  102 ). In some embodiments, the red light has a wavelength between 605 nm to 650 nm, the green light has a wavelength between 500 nm to 550 nm, and the blue light has a wavelength between 400 nm to 460 nm. 
     For example, the red color-conversion material  112 - 1  and the green color-conversion material  112 - 2  may absorb the emitted blue light and change its wavelength into that of the red light and the green light, respectively. Materials of the color-conversion materials  112  may include CdSe, CdS, CdTe, ZnO, InP, CsPbX 3  (perovskite quantum dots), KSF/β-SiAlON (phosphor materials) the like, or combinations thereof. The thickness of the color-conversion materials  112  may be equal to or less than the thickness of the plurality of reflective banks  108 . In other words, the color-conversion materials  112  are formed within at least one of the areas defined by the plurality of reflective banks  108 . The color-conversion materials  112  may be formed in sequence by a coating, exposure, and development process at different steps. Alternatively, the color-conversion materials  112  may be formed by ink-jet printing. 
     Referring to  FIG.  1   , the optical layer  122  is formed on the intermediate structure of the display device  10 . The optical layer  122  may cover the substrate  100 , the plurality of micro light-emitting diodes  102 , the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , the plurality of black banks  110 , and the color-conversion materials  112 . According to some embodiments of the present disclosure, the optical layer  122  may provide structural support and maintain the space between the substrate  100  and the subsequently formed cover plate  120 . As shown in  FIG.  1   , the optical layer  122  is vertically interposed between the plurality of micro light-emitting diodes  102  and the plurality of color filter units  116 . The optical layer  122  may have a thickness T between the substrate  100  and the cover plate  120  in the display direction A. The thickness T of the optical layer  122  may be approximately between 15 μm and 100 μm, and the thickness T between 20 μm and 50 μm is preferred for display quality. The optical layer  122  may be formed by spin-on coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), high-density plasma chemical vapor deposition (HDP-CVD), plasma-enhanced chemical vapor deposition (PECVD), flowable chemical vapor deposition (FCVD), sub-atmospheric chemical vapor deposition (SACVD), sputtering, the like, or combinations thereof. 
     The cover plate  120  may function as a base structure for the plurality of light-shielding structures  114  and the plurality of color filter units  116  to be formed thereon, and may also provide mechanical protection toward the underlying structure. In other words, the plurality of light-shielding structures  114  and the plurality of color filter units  116  are arranged on the cover plate  120 , and the cover plate  120  is flipped upside down and adhered to the substrate  100  through the optical layer  122 . It means that, the plurality of color filter units  116  are disposed between the cover plate  120  and the substrate  100 . In the present embodiment, the red color filter unit  116 - 1 , the green color filter unit  116 - 2 , and the blue color filter unit  116 - 3  are placed in correspondence with the red color-conversion material  112 - 1 , the green color-conversion material  112 - 2 , and the blue micro light-emitting diodes  102 A. The cover plate  120  may be formed of, for example, a light-transmissive insulation material, such as glass or transparent resins. Exemplary transparent resins include polyethylene terephthalate (PET) resins, polycarbonate (PC) resins, polyimide (PI) resins, polymethylmethacrylates (PMMA), the like, or combinations thereof. The thickness of the cover plate  120  may be approximately between 200 μm and 600 μm. In some embodiments, the cover plate  120  may be formed by any of the deposition methods described above, and the details are not described again herein to avoid repetition. 
     The plurality of light-shielding structures  114  are disposed on the cover plate  120 . In some embodiments, the plurality of light-shielding structures  114  are arranged to prevent the light rays transmitting through adjacent color filter units  116  from interfering with each other, which may affect the quality of the displayed image. Materials of the plurality of light-shielding structures  114  may include acrylic resin (polymeric) photo-resist material with black pigment or dye, the like, or combinations thereof. The thickness of the plurality of light-shielding structures  114  may be approximately between 1 μm and 2 μm. The plurality of light-shielding structures  114  may be formed by depositing a black material layer on the cover plate  120  and then patterning the black material layer using photolithography and etching processes, but the present disclosure is not limited thereto. 
     The plurality of color filter units  116  are disposed on the cover plate  120 , and are laterally separated by the plurality of light-shielding structures  114 . In the present embodiment, as stated earlier, the plurality of color filter units  116  may include the red color filter unit  116 - 1 , the green color filter unit  116 - 2 , and the blue color filter unit  116 - 3 . The purpose of the plurality of color filter units  116  may further filter the generated lights to ensure the exemplary pixel P may display pure red light, pure green light, and pure blue light. The plurality of color filter units  116  may be formed in sequence by a coating, exposure, and development process at different steps. Alternatively, the plurality of color filter units  116  may be formed by ink-jet printing. 
       FIGS.  2 A- 2 H  are various cross-sectional views of intermediate stages in manufacturing the display device  10 , according to some embodiments of the present disclosure. It should be appreciated that the plurality of light-shielding structures  114 , the plurality of color filter units  116 , the cover plate  120 , and the optical layer  122  are omitted for simplicity. The illustration of  FIGS.  2 A- 2 H  will emphasize the formation of the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , and the plurality of black banks  110 . 
     Initially, the substrate  100  is provided with the plurality of micro light-emitting diodes  102  formed thereon, and the plurality of micro light-emitting diodes  102  may be bonded onto the substrate  100  using mass transfer process (to transfer and bond a few thousands to hundreds of thousands micro light-emitting diodes at a time), as shown in  FIG.  2 A . The reflective layer  104  is over-coated on the substrate  100  to cover the plurality of micro light-emitting diodes  102 , as shown in  FIG.  2 B . Next, the over-coated reflective layer  104  is etched back until a top surface  1021  and a portion of a sidewall  1022  of the plurality of micro light-emitting diodes  102  protrude above the reflective layer  104 . In other words, the reflective layer  104  may be lower than the plurality of micro light-emitting diodes  102 , as shown in  FIG.  2 C . The black layer  106  is then formed conformally on the plurality of micro light-emitting diodes  102  and the reflective layer  104 , as shown in  FIG.  2 D . After that, the black layer  106  is etched back to expose the top surface  1021  of the plurality of micro light-emitting diodes  102 . It is preferred that the surface of the black layer  106  is coplanar with the top surface  1021  of the plurality of micro light-emitting diodes  102 , as shown in  FIG.  2 E . It should be noted that, at this stage, the top surface  1021  of the plurality of micro light-emitting diodes  102  are uncovered by the reflective layer  104  and the black layer  106 . 
     After that, a second reflective layer  108 ′ and a second black layer  110 ′ are sequentially formed on the plurality of micro light-emitting diodes  102  and the black layer  106 , as shown in  FIG.  2 F . Next, the second reflective layer  108 ′ and the second black layer  110 ′ are respectively patterned into the plurality of reflective banks  108  and the plurality of black banks  110  through lithography and etching processes, forming multiple accommodating spaces and exposing the top surface  1021  of the micro light-emitting diodes  102  and a portion of the black layer  106 , as shown in  FIG.  2 G . Then, the red color-conversion material  112 - 1  and the green color-conversion material  112 - 2  are filled into the accommodating spaces to be disposed above two of the plurality of micro light-emitting diodes  102  (or the blue micro light-emitting diodes  102 A) and laterally between the plurality of reflective banks  108 , as shown in  FIG.  2 H . 
       FIGS.  3 A and  3 B  are cross-sectional views of display devices  20  and  20 ′, according to other embodiments of the present disclosure. The display device  20  of  FIG.  3 A  illustrates an alternative design. In comparison with the display device  10  of  FIG.  1   , the reflective layer  104  of the display device  20  includes concave portions  105  located between the plurality of micro light-emitting diodes  102 . The features of the substrate  100 , the plurality of micro light-emitting diodes  102  (for example, the blue micro light-emitting diodes  102 A), the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , the plurality of black banks  110 , the color-conversion materials  112  (including the red color-conversion material  112 - 1  and the green color-conversion material  112 - 2 ), the plurality of light-shielding structures  114 , the plurality of color filter units  116  (including the red color filter unit  116 - 1 , the green color filter unit  116 - 2 , and the blue color filter unit  116 - 3 ), the cover plate  120 , and the optical layer  122  are similar to those illustrated in  FIG.  1   , and the details are not described again herein to avoid repetition. 
     Referring to  FIG.  3 A , the reflective layer  104  is etched back for a longer period of time to form the concave portions  105 . The subsequently deposited black layer  106  may adopt the same topology of the reflective layer  104 , so the black layer  106  may have a concave surface  105 S. The concave surface  105 S of the concave portions  105  is lower than the top surface  1021  of the plurality of micro light-emitting diodes  102 , and the plurality of reflective banks  108  sit on the concave portions  105 . In the presence of the concave portions  105 , the concave profile of the black layer  106  may absorb lights emitted from the plurality of micro light-emitting diodes  102  in lateral directions and absorb the ambient light reflection to further increase the contrast ratio, in which the displayed image quality may be further enhanced. 
     Referring to  FIG.  3 B , the display device  20 ′ further includes a sealant layer  124 , where an optical layer  122 ′ is confined by the sealant layer  124  and fills in the space between the substrate  100  and the cover plate  120 . In the present embodiments, the sealant layer  124  surrounds the array of exemplary pixels P when viewed from top to adhere the cover plate  120  and the substrate  100 , and the optical layer  122 ′ is interposed between the plurality of micro light-emitting diodes  102  and the plurality of color filter units  116 . Materials of the optical layer  122 ′ are similar to those of the optical layer  122 , and the details are not described again herein to avoid repetition. The lateral width of the sealant layer  124  may be approximately between 200 μm and 8 mm. In comparison with the display device  20  of  FIG.  3 A , the display device  20 ′ with the sealant layer  124  surrounding the perimeter of the space (when viewed from top) may exhibit stronger adhesion and structural support than the display device  20  with only the optical layer  122 . 
       FIG.  4    is a cross-sectional view of a display device  30 , according to yet other embodiments of the present disclosure. In comparison with the display device  20 ′ of  FIG.  3 B , the display device  30  includes an air gap  126 , one more type of the plurality of micro light-emitting diodes  102  and one less type of the color-conversion materials  112 . The features of the substrate  100 , the plurality of micro light-emitting diodes  102 , the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , the plurality of black banks  110 , the color-conversion materials  112 , the plurality of light-shielding structures  114 , the plurality of color filter units  116  (including the red color filter unit  116 - 1 , the green color filter unit  116 - 2 , and the blue color filter unit  116 - 3 ), the cover plate  120 , and the sealant layer  124  are similar to those illustrated in  FIG.  3 B , and the details are not described again herein to avoid repetition. 
     Referring to  FIG.  4   , the air gap  126 , instead of the optical layer  122 ′, is interposed between the plurality of micro light-emitting diodes  102  and the plurality of color filter units  116 . It should be noted that, the refractive index of air is lower than the refractive index of the material of the optical layer  122  or  122 ′. Since there will be less refraction within the space, the resulting image may appear to be brighter. However, the display device  20 ′ with the optical layer  122 ′ filling up the space may exhibit stronger integrity and higher reliability than the display device  30 . 
     The refractive index is a characteristic of a substance that changes the speed of light, and is a value obtained by dividing the speed of light in vacuum by the speed of light in the substance. When light travels between two different materials at an angle, its refractive index determines the angle of light transmission (refraction). Therefore, the light rays transmitted through the materials of the optical layer  122  or  122 ′ will be refracted from the light rays transmitted through the air gap  126 , thus the displayed image may be affected. 
     Referring to  FIG.  4   , the display device  30  includes a green micro light-emitting diode  102 B of the plurality of micro light-emitting diodes  102 , which replaces the green color-conversion material  112 - 2  of the color-conversion materials  112 . The green micro light-emitting diode  102 B may emit green light, so the color-conversion materials  112  is not required to convert the emitted light&#39;s wavelength. The light of each sub-pixel region may be generated from the red color-conversion material  112 - 1 , the green micro light-emitting diode  102 B, and the blue micro light-emitting diode  102 A, respectively. It should be understood that a red micro light-emitting diode is rarely used, thus embodiments of which are not introduced herein. Depending on the design or application requirement, the number of types of micro light-emitting diodes  102  and the number of types of color-conversion materials  112  may be selected accordingly, as long as the display function may be properly realized. 
       FIGS.  5 A and  5 B  are cross-sectional views of display devices  40  and  40 ′, according to other embodiments of the present disclosure. In comparison with the display device  30  of  FIG.  4   , the display device  40  includes a photon recycling layer  118  formed onto one of the plurality of color filter units  116 . The features of the substrate  100 , the plurality of micro light-emitting diodes  102  (including the blue micro light-emitting diode  102 A and the green micro light-emitting diode  102 B), the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , the plurality of black banks  110 , the color-conversion materials  112  (for example, the red color-conversion material  112 - 1 ), the plurality of light-shielding structures  114 , the plurality of color filter units  116  (including the red color filter unit  116 - 1 , the green color filter unit  116 - 2 , and the blue color filter unit  116 - 3 ), the cover plate  120 , the sealant layer  124 , and the air gap  126  are similar to those illustrated in  FIG.  4   , and the details are not described again herein to avoid repetition. 
     Referring to  FIG.  5 A , when the blue light emitting from the blue micro light-emitting diode  102 A is being transmitted through the red color-conversion material  112 - 1 , the red color-conversion material  112 - 1  may not completely convert emitted light&#39;s original wavelength into the desired wavelength. For the blue light that is not converted into the red light, the photon recycling layer  118  of the display device  40  may reflect the unconverted blue light back to the red color-conversion material  112 - 1 , instead of allowing the unconverted blue light being absorbed then filtered off by the red color filter unit  116 - 1 . For this embodiment, the photon recycling layer  118  may also be referred to as a blue photon recycling layer. It should also be appreciated that the photon recycling layer  118  is placed to correspond to the color-conversion material  112 . 
     The photon recycling layer  118  (or the blue photon recycling layer) is characterized with high reflection for the blue light and high transmission for the red/green light. Instead of being simply filtered off by the red color filter unit  116 - 1 , the unconverted blue light may be reflected back to the red color-conversion material  112 - 1  for another attempt of conversion. If successfully converted, the converted red light can then be transmitted through the red color filter unit  116 - 1 . Therefore, implementing the photon recycling layer  118  may improve the color-conversion efficiency of the display device  40 . The photon recycling layer  118  may be a distributed Bragg reflector (DBR), a patterned Cholesteric liquid crystal (P-CLC), and a blue anti-transmission film (BATF). The distributed Bragg reflector is a structure formed from multiple layers of alternating materials with varying refractive index, resulting in periodic variation in the effective refractive index in a dielectric waveguide. The patterned cholesteric liquid crystal may create arbitrarily patterned circular polarized optical devices. The blue anti-transmission film may improve the color-conversion efficiency and the stability of color-conversion materials. 
     Referring to  FIG.  5 B , the display device  40 ′ includes the optical layer  122  between the substrate  100  and the cover plate  120 . While the sealant layer  124  of the display device  40  only partially fills the space between the substrate  100  and the cover plate  120  with the air gap  126  generated in the confined space, the optical layer  122  of the display device  40 ′ completely fills the space between the substrate  100  and the cover plate  120 . Similar to the display device  20  of  FIG.  3 A , the optical layer  122  may exhibit stronger integrity and higher reliability than the display device  40  with the sealant layer  124  disposed only along the perimeter of the space from top view. However, the display device  40  with the air gap  126  confined in the space may result in brighter image than the display device  40 ′ with the optical layer  122  filling the space. 
       FIGS.  6 A and  6 B  are cross-sectional views of display devices  50  and  50 ′, according to yet other embodiments of the present disclosure. In comparison with  FIG.  5 A , the display device  50  includes the photon recycling layer  118  formed on more than one of the plurality of color filter units  116 . The features of the substrate  100 , the plurality of micro light-emitting diodes  102  (for example, the blue micro light-emitting diode  102 A), the reflective layer  104 , the black layer  106 , the plurality of reflective banks  108 , the plurality of black banks  110 , the color-conversion materials  112  (including the red color-conversion material  112 - 1  and the green color-conversion material  112 - 2 ), the plurality of light-shielding structures  114 , the plurality of color filter units  116  (including the red color filter unit  116 - 1 , the green color filter unit  116 - 2 , and the blue color filter unit  116 - 3 ), the photon recycling layer  118 , the cover plate  120 , the sealant layer  124 , and the air gap  126  are similar to those illustrated in  FIG.  5 A , and the details are not described again herein to avoid repetition. 
     Referring to  FIG.  6 A , the photon recycling layer  118  is formed on the red color filter unit  116 - 1  and the green color filter unit  116 - 2 . Since the display device  50  includes only the blue micro light-emitting diode  102 A with the red color-conversion material  112 - 1  and the green color-conversion material  112 - 2 , the photon recycling layer  118  should be placed to correspond to both the red color-conversion material  112 - 1  and the green color-conversion material  112 - 2 . In some embodiments, when the blue light emitted from the blue micro light-emitting diode  102 A is not completely converted to the red light through the red color-conversion material  112 - 1  and to the green light through the green color-conversion material  112 - 2 , the photon recycling layer  118  may allow the converted red light and the converted green light to transmit through, and reflect the unconverted blue light back to the red color-conversion material  112 - 1  and the green color-conversion material  112 - 2 . As explained previously, implementing the photon recycling layer  118  may improve the color-conversion efficiency of the display device  50 . 
     Referring to  FIG.  6 B , the optical layer  122  of the display device  50 ′ completely fills the space between the substrate  100  and the cover plate  120 . Similar to the display device  40 ′ of  FIG.  5 B , the optical layer  122  may exhibit stronger integrity and higher reliability than the display device  50  with the sealant layer  124  disposed only along the perimeter of the space from top view. However, the display device  50  with the air gap  126  confined in the space may result in brighter image than the display device  50 ′ with the optical layer  122  filling the space. 
     The present disclosure replaces the conventional reflective layer with the reflective layer  104  and the black layer  106 , and replaces the conventional plurality of black banks with the plurality of reflective banks  108  and the plurality of black banks  110 . The refined structure of the display devices may help suppress the ambient light reflection and improves the contrast ratio and/or the blackness, thus an image of higher quality may be displayed. In addition, the present disclosure also illustrates the concave portions  105  within the reflective layer  104 , different types of micro light-emitting diodes  102  or different types of color-conversion materials  112 , and the photon recycling layer  118  on at least one of the plurality of color filter units  116  to correspond to the color-conversion materials  112 . Depending on the design or application requirements, any of the aforementioned features may be selected to produce the display device of superior performance. 
     The foregoing outlines features of several embodiments so that those skilled in the art will better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. Therefore, the scope of protection should be determined through the claims. In addition, although some embodiments of the present disclosure are disclosed above, they are not intended to limit the scope of the present disclosure. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present disclosure should be or are in any single embodiment of the disclosure. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present disclosure. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the disclosure may be combined in any suitable manner in one or more embodiments. One skilled in the prior art will recognize, in light of the description herein, that the disclosure can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the disclosure.