Patent Publication Number: US-2022238596-A1

Title: Micro light emitting diode display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Taiwanese Invention Patent Application No. 110103079, filed on Jan. 27, 2021. 
     FIELD 
     The disclosure relates to a micro light emitting diode (micro-LED) display device, and more particularly to a common-cathode micro-LED display device. 
     BACKGROUND 
     Light emitting diodes (LEDs) have become the mainstream of illumination and light source for displays due to various advantages such as small volume, high brightness, long lifetime, low heat emission, improved energy efficiency, etc. Micro-LEDs, which shares advantages similar to those of traditional LEDs, such as high brightness, high efficiency and high reliability, include dies having dimensions which are shrank to be less than one-tenth the size of a conventional LED die, such that the micro-LEDs might have more extracted lights and an increased number of dies per unit area compared with those of traditional LEDs. Therefore, the micro-LEDs could be applied to thin, highly efficient and flexible displays, and are considered to be the next-generation display technology. 
     Although the micro-LEDs have superior properties due to their reduced dimensions, mass transfer of the micro-LEDs in commercialization thereof still faces problems. Further, the micro-LEDs applied to the displays might have a wide-angle light distribution which might result in light loss. 
     SUMMARY 
     Therefore, an object of the disclosure is to provide a micro light emitting diode (micro-LED) display device that can alleviate at least one of the drawbacks of the prior art. 
     According to the disclosure, the micro-LED display device includes a light-transmissive unit, a plurality of light emitting units and a plurality of converting units. The light-transmissive unit includes a protective layer which has a first surface and a second surface opposite to the first surface. The light emitting units are arranged in an array on the second surface of the protective layer, and each of the light emitting units includes a first light emitting portion, a second light emitting portion, and a third light emitting portion which emit lights with the same original wavelength. 
     Each of the first, second and third light emitting portions includes a first type semiconductor layer, a light emitting layer and a second type semiconductor layer which are sequentially stacked on the second surface of the protective layer such that the lights respectively from the first, second and third light emitting portions are permitted to pass through the light-transmissive unit to emit outward from the first surface of the protective layer, and such that the first type semiconductor layers of the first, second and third light emitting portions are integrally formed while the light emitting layers of the first, second and third light emitting portions are spaced apart from one another. 
     The converting units are disposed on the first surface of the protective layer in positions corresponding to the light emitting units, respectively. Each of the converting units includes a reflecting feature, a first wavelength converting element and a second wavelength converting element. The reflecting feature is formed on the first surface of the protective layer, and includes three inner peripheral surfaces which respectively define three through holes in positions corresponding to the first, second and third light emitting portions of the respective light emitting unit, respectively. An included angle between the first surface of the protective layer and each of the inner peripheral surfaces is greater than 90 degrees. 
     The first and second wavelength converting elements are respectively formed in two of the through holes in positions corresponding to the first and second light emitting portions of the respective light emitting unit such that when the lights from the first and second light emitting portions respectively pass through the first and second wavelength converting elements, the lights from the first and second light emitting portions are respectively converted to have a first predetermined wavelength and a second predetermined wavelength which are different from the original wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiments with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic bottom view showing a first embodiment of a micro-LED display device according to the disclosure, except that a reflecting layer and a circuit board are omitted therefrom; 
         FIG. 2  is a schematic cross-sectional view taken along line II-II of  FIG. 1 , but includes the reflective layer and the circuit board; 
         FIGS. 3 and 4  are similar to  FIG. 2 , and illustrate consecutive steps for making the micro-LED display device shown in  FIG. 2 ; 
         FIG. 5  is a schematic cross-sectional view of the first embodiment taken along line V-V of  FIG. 1 , but includes the reflective layer and the circuit board; 
         FIG. 6  is a schematic cross-sectional view of the first embodiment taken along line VI-VI of  FIG. 1 , but includes the reflective layer and the circuit board; 
         FIG. 7  is a schematic cross-sectional view illustrating a variation of the first embodiment shown in  FIG. 2 ; 
         FIG. 8  is a circuit diagram illustrating an operation of the first embodiment shown in  FIG. 1 ; and 
         FIG. 9  is a partial bottom view showing a second embodiment of a micro-LED display device according to the disclosure, except that a reflecting layer and a circuit board are omitted therefrom. 
     
    
    
     DETAILED DESCRIPTION 
     Before the disclosure is described in greater detail, it should be noted that where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated among the figures to indicate corresponding or analogous elements, which may optionally have similar characteristics. It should be noted that the drawings, which are for illustrative purposes only, are not drawn to scale, and are not intended to represent the actual sizes or actual relative sizes of the components of the micro-LED display device. 
     Referring to  FIGS. 1 to 6 , a first embodiment of a micro-LED display device  2  according to the disclosure, such as a common-cathode micro-LED display device, includes a light-transmissive unit  20 , a plurality of light emitting units  22  and a plurality of converting units  23 . The light-transmissive unit  20  includes a protective layer  210  which has a first surface  201  and a second surface  202  opposite to the first surface  201 , and a light transmissible substrate  211  disposed to separate the protective layer  210  from the light emitting units  22 . 
     The protective layer  210  is made of an organic material or an inorganic material, and has a thickness that is equal to or smaller than 2 μm. The light transmissible substrate  211  is made of a native substrate for epitaxial growth of the light emitting units  22 . The native substrate may be made of a material selected from a light transmissible material (such as sapphire or gallium nitride), silicon (for large-area fabrication) or a semiconductor material suitable for epitaxial growth. In this embodiment, in order to avoid influencing the light extraction of the light emitting units  22 , the light transmissible substrate  211  is made of, but is not limited to, sapphire. 
     The light emitting units  22  are arranged in an array on the second surface  202  of the protective layer  210 . Each of the light emitting units  22  includes a first light emitting portion  22 A, a second light emitting portion  22 B, and a third light emitting portion  22 C which emit lights (L) with the same original wavelength. In this embodiment, the original wavelength of lights (L) emitted from the first, second and third light emitting portions  22 A,  22 B,  22 C ranges from 440 nm to 490 nm, i.e., the first, second and third light emitting portions  22 A,  22 B,  22 C respectively emit blue lights. 
     Each of the first, second and third light emitting portions  22 A,  22 B,  22 C includes a first type semiconductor layer  221 , a light emitting layer  222  and a second type semiconductor layer  223  which are sequentially stacked on the second surface  202  of the protective layer  210 . The first type semiconductor layer  221  may be one of a p-type semiconductor layer and an n-type semiconductor layer, and the second type semiconductor layer  223  is the other one of the p-type semiconductor layer and the n-type semiconductor layer. In this embodiment, the first type semiconductor layer  221  is the n-type semiconductor layer, and the second type semiconductor layer  223  is the p-type semiconductor layer. 
     The first type semiconductor layer  221 , the light emitting layer  222  and the second type semiconductor layer  223  of each of the first, second and third light emitting portions  22 A,  22 B,  22 C may be respectively formed of semiconductor materials to permit the first, second and third light emitting portions  22 A,  22 B,  22 C to emit the desired original color of lights, and may be stacked together in various arrangements. In some embodiments, the first type semiconductor layer  221 , the light emitting layer  222  and the second type semiconductor layer  223  of each of the first, second and third light emitting portions  22 A,  22 B,  22 C may be made of the same semiconductor material but with different conductive types of dopants and doping concentrations. In some embodiments, the first type semiconductor layer  221 , the light emitting layer  222  and the second type semiconductor layer  223  of each of the first, second and third light emitting portions  22 A,  22 B,  22 C may be made of different semiconductor materials. 
     The lights (L) respectively from the first, second and third light emitting portions  22 A,  22 B,  22 C are permitted to pass through the light-transmissive unit  20  to emit outward from the first surface  201  of the protective layer  210 . The first type semiconductor layers  221  of the first, second and third light emitting portions  22 A,  22 B,  22 C are integrally formed, while the light emitting layers  222  of the first, second and third light emitting portions  22 A,  22 B,  22 C are spaced apart from one another. The second type semiconductor layers  223  of the first, second and third light emitting portions  22 A,  22 B,  22 C are also spaced apart from one another. 
     The light emitting layer  222  has a length and a width, each of which is not greater than 100 μm. In some embodiments, each of the length and the width of the light emitting layer  222  ranges from 10 μm to 20 μm. In this embodiment, each of the light emitting units  22  includes three light emitting portions  22 A,  22 B,  22 C. In some embodiments, the number of light emitting portions in each of the light emitting units  22  may be varied based on demands or designs. 
     Each of the light emitting units  22  further includes a reflecting layer  24  which is formed to cover the first, second and third light emitting portions  22 A,  22 B,  22 C opposite to the light-transmissive unit  20  so as to direct the lights (L) from the first, second and third light emitting portions  22 A,  22 B,  22 C toward the light-transmissive unit  20 . 
     The converting units  23  are disposed on the first surface  201  of the protective layer  210  in positions corresponding to the light emitting units  22 , respectively. Each of the converting units  23  includes a reflecting feature  231  formed on the first surface  201  of the protective layer  210 . The reflecting feature  231  includes three inner peripheral surfaces  2311  which respectively define three through holes  233  in positions corresponding to the first, second and third light emitting portions  22 A,  22 B,  22 C of the respective light emitting unit  22 , respectively. The reflecting feature  231  is made of a material which reflects a broad wavelength range of light. An included angle (θ) between the first surface  201  of the protective layer  210  and each of the inner peripheral surfaces  2311  is greater than 90 degrees and less than 180 degrees, thereby forming tapered through holes  233 . In some embodiments, the reflecting features  231  of the converting units  23  may be integrally formed. 
     Each of the converting units  23  further includes a first wavelength converting element  232 A and a second wavelength converting element  232 B which are respectively formed in two of the through holes  233  in positions corresponding to the first and second light emitting portions  22 A,  22 B of the respective light emitting unit  22 . The protective layer  210  is disposed to protect the first and second wavelength converting elements  232 A,  232 B. In the rest of the through holes  233  in positions corresponding to the third light emitting portion  22 C of the respective light emitting unit  22 , no wavelength converting element is positioned therein. In some embodiments, scattering particles may be disposed in the through holes  233  in positions corresponding to the third light emitting portions  22 C of the light emitting units  22  by inkjet printing or other suitable semiconductor processes. 
     Each of the first and second wavelength converting elements  232 A,  232 B includes quantum dots which are excited by the light from a respective one of the first and second light emitting portions  22 A,  22 B so as to vary the wavelength of the light outputted therefrom. The quantum dots may have different sizes according to demands, and may be formed of a material selected from cadmium selenide (CdSe), cadmium sulfide (CdS), zinc selenide (ZnSe), perovskite or combinations thereof. In this embodiment, when the lights (L) from the first and second light emitting portions  22 A,  22 B respectively pass through the first and second wavelength converting elements  232 A,  232 B, the lights (L) are respectively converted to have a first predetermined wavelength and a second predetermined wavelength which are different from the original wavelength. In some embodiments, the first predetermined wavelength ranges from 610 nm to 720 nm (red light), and the second predetermined wavelength ranges from 500 nm to 600 nm (green light). With such arrangement, the inner peripheral surfaces  2311  in positions corresponding to the first and second light emitting portions  22 A,  22 B of the respective light emitting unit  23  may respectively reflect red and green lights, while the inner peripheral surface  2311  in position corresponding to the third light emitting portion  22 C of the respective light emitting unit  22  may reflect blue light emitted therefrom so that the first, second and third light emitting portions  22 A,  22 B,  22 C function as high-directional light sources. 
     Each of the reflecting feature  231  and the reflecting layer  24  has a micro-feature with curved and uneven surfaces. In some embodiments, each of the reflecting feature  231  and the reflecting layer has a Bragg reflection structure, such as a distributed Bragg reflector having different refraction indices. Each of the reflecting feature  231  and the reflecting layer  24  is independently made of a material selected from a metal, a metal oxide or a combination thereof. If the reflecting layer  24  is made of a metal, an insulating layer should be formed to separate the reflecting layer  24  from the first type semiconductor layer  221 , the light emitting layer  222 , and the second type semiconductor layer  223 . In some embodiments, the material of each of the reflecting feature  231  and the reflecting layer  24  may be nitride, composite oxide or a combination thereof, such as SiN X /SiO x  or SiO 2 /TiO 2 . In this embodiment, each of the reflecting feature  231  and the reflecting layer  24  is formed of, but not limited to, a composite material SiO 2 /Al/SiO 2 . 
     By forming the tapered through holes  233 , the first and second wavelength converting elements  232 A,  232 B may be retained in the reflecting feature  231 , and the blue light emitted from the third light emitting portion  22 C of each of the light emitting units  22  and the red and green lights outputted from the first and second wavelength converting elements  232 A,  232 B of the respective converting unit  23  may be reflected by the inner peripheral surfaces  2311  of the reflecting feature  231  to travel away from the respective converting unit  23 . Therefore, the red, green and blue lights reflected by the inner peripheral surfaces  2311  are high-directional lights with small light exit angle. In addition, with the provision of the reflecting layer  24 , more lights emitted from the first, second and third light emitting portions  22 A,  22 B,  22 C can be ensured to be outputted from the first surface  201  of the protective layer  210 . 
     Each of the converting units  23  further includes a selective reflection layer  251  which is disposed to cover the first and second wavelength converting elements  232 A,  232 B opposite to the light-transmissive unit  20  so as to prevent the lights (L) with the original wavelength (which are emitted from the first and second light emitting portions  22 A,  22 B of the respective light emitting unit  22  and are not converted to have the first or second predetermined wavelength by the first or second wavelength converting elements  232 A,  232 B) from passing through the selective reflection layer  251 . In this embodiment, the selective reflection layer  251  is a long-pass filter which transmit longer wavelengths of lights (i.e., red and green lights) and reflects shorter wavelengths of lights (i.e., blue lights). By forming the selective reflection layer  251  on the first and second wavelength converting elements  232 A,  232 B, the lights (L) with the original wavelength would be reflected and the quantum dots in the first and second wavelength converting elements  232 A,  232 B may convert the reflected lights into red and green lights. In this case, the number of quantum dots may be reduced and thus, the thicknesses of the first and second wavelength converting elements  232 A,  232 B and the reflecting feature  231  may be decreased. In some embodiments, the selective reflection layers  251  of the converting units  23  may be integrally formed to have openings  251   a  (see  FIG. 6 ) in positions corresponding to the third light emitting portions  22 C of the light emitting units  22  so as to permit the lights from the third light emitting portions  22 C to pass through the openings  251   a  of the selective reflection layers  251 . 
     Each of the converting units  23  further includes a first filter  252 A, a second filter  252 B and an absorbing layer  253  disposed between the respective first and second filters  252 A,  252 B. Each of the first and second filters  252 A,  252 B is disposed downstream of a respective one of the first and second wavelength converting elements  232 A,  232 B and the selective reflection layer  251  so as to permit the light (L) with a respective one of the first and second predetermined wavelength to pass therethrough. In this embodiment, the first filter  252 A may be a red color filter for transmitting red light only, and the second filter  252 B may be a green color filter for transmitting green light only. The absorbing layer  253  is formed to prevent adjacent lights from interfering each other. In some embodiments, the absorbing layers  253  of the converting units  23  may be integrally formed to have openings  253   a  (see  FIG. 6 ) in positions corresponding to the third light emitting portions  22 C of the light emitting units  22  so as to permit the lights from the third light emitting portions  22 C to pass through the openings  253   a  of the absorbing layers  253 . In some embodiments, a third filter (not shown) may be disposed in each of the openings  253   a  of the absorbing layers  253  to filter the light (L) from the third light emitting portion  22 C of a respective one of the light emitting units  22 . 
     The micro-LED display device  2  further includes a light transmissible cover plate  26  disposed to cover the converting units  23  opposite to the light-transmissive unit  20  for protecting the converting units  23 , the light emitting units  22  and the light-transmissive unit  20 . 
     The micro-LED display device  2  further includes a circuit board  3  disposed on the light emitting units  22  opposite to the light-transmissive unit  20 . Each of the light emitting units  22  further includes a first electrode  2241  and a plurality of second electrodes  2242 . After the first electrode  2241  and the second electrodes  2242  are formed on each of the light emitting units  22  (see  FIG. 3 ), the reflecting layer  24  is formed to cover the first, second and third light emitting portions  22 A,  22 B,  22 C while exposing the first and second electrodes  2241 ,  2242  for electrical connection with the circuit board  3  (see  FIGS. 2 and 4 ). The first electrode  2241  electrically connects the first type semiconductor layers  221  of the first, second and third light emitting portions  22 A,  22 B,  22 C with the circuit board  3 . Each of the second electrodes  2242  electrically connects the second type semiconductor layer  223  of a respective one of the first, second and third light emitting portions  22 A,  22 B,  22 C with the circuit board  3 , and includes a transparent conductive layer  2243  and an electrode layer  2244  disposed between the transparent conductive layer  2243  and the circuit board  3 . In this embodiment, the first electrode  2241  is an n-type electrode, and each of the second electrodes  2242  is a p-type electrode. The transparent conductive layer  2243  may be made of a material selected from ITO (In 2 O 3 :Sn), IZO (ZnO:In) or AZO (ZnO:Al 2 O 3 ). Each of the first electrode  2241  and the electrode layer  2244  of each of the second electrodes  2242  may be independently made of a material selected from metal or metal alloy, such as Au, In, Cu or Cu/Sn. In some embodiments, each of the first electrode  2241  and the electrode layer  2244  of each of the second electrodes  2242  may be independently formed in a single layer or multi-layers. 
     The light-transmissive unit  20 , the light emitting units  22  and the converting units  23  cooperatively form a micro-LED display structure. The micro-LED display structure is electrically connected to the circuit board  3  by flip chip bonding instead of mass transfer. 
     Referring to  FIG. 7 , a variation of the first embodiment of the micro-LED display device  2  according to the disclosure is shown. In this variation, the light-transmissive unit  20  only includes a protective layer  210 . During fabrication, the light emitting units  22  are firstly formed on the light transmissible substrate  211  (see  FIGS. 3 and 4 ) and the resulted structure is electrically connected to the circuit board  3  by flip chip bonding. Then, the light transmissible substrate  211  is removed, and the protective layer  210  is directly formed to permit the second surface  202  of the protective layer  210  to be in contact with the first type semiconductor layers  221  of the light emitting units  22 , followed by forming the converting units  23  on the first surface  201  of the protective layer  210  in positions corresponding to the light emitting units  22 . 
     Referring to  FIG. 8 , a circuit diagram illustrating an operation of the first embodiment shown in  FIG. 1  is shown. The first, second and third light emitting portions  22 A,  22 B,  22 C shown in  FIGS. 1 to 7  are arranged in rows and columns, and are all represented by the same numeral  220  in  FIG. 8  to illustrate micro-LEDs. The circuit board  3  includes a first driving circuit  31 , a second driving circuit  32 , and a plurality of transistors  33 . The first driving circuit  31  includes a plurality of scan lines  311  which extend in a row direction, and which are displaced from one another in a column direction transverse to the row direction. The second driving circuit  32  includes a plurality of data lines  321  which extend in the column direction, and which are displaced from one another in the row direction. The transistors  33  are arranged in rows and columns. Each of the transistors  33  includes a first electrode  331 , a second electrode  332 , and a third electrode  333 . The first electrodes  331  of the transistors  33  are electrically connected to the first, second and third light emitting portions  220  (i.e., elements  22 A,  22 B,  22 C shown in  FIGS. 1-7 ), respectively. Each of the scan lines  311  is electrically connected to the second electrodes  332  of a corresponding row of the transistors  33 , and each of the data lines  321  is electrically connected to the third electrodes  333  of a corresponding column of the transistors  33 . 
     In this embodiment, the transistors  33  are p-channel transistors. To be specific, the first electrode  331  is a drain electrode, the second electrode  332  is a gate electrode, and the third electrode  333  is a source electrode. Therefore, each of the data lines  321  is electrically connected to the source electrodes  333  of the corresponding column of the transistors  33  for providing driving current to the corresponding column of the transistors  33 , and each of the scan lines  311  is electrically connected to the gate electrodes  332  of the corresponding row of the transistor  33  so as to permit the corresponding row of the transistor  33  to receive timing signals, and so as to control the on and off states of the corresponding row of the transistors  33 . The anode of each of the micro-LEDs  220  is electrically connected to the drain electrode  331  of each of the transistors  33 , and the cathode of each of the micro-LEDs  220  is electrically connected to a ground circuit. In this manner, the transistors  33  may drive each of the micro-LEDs  220  according to the timing signal with the driving current sequentially provided to each of the micro-LEDs  220 . In some embodiments, the transistors  33  may be n-channel transistors. In this case, the first electrode  331  is a source electrode, the second electrode  332  is a gate electrode, and the third electrode  333  is a drain electrode. That is, each of the data lines  321  is electrically connected to the drain electrodes  333  of the corresponding column of the transistors  33 , and the anode of each of the micro-LEDs is electrically connected to the source electrode  331  of each of the transistors  33 . 
     It should be noted that the choice of using re-channel or p-channel transistors depends on the substrate material of the circuit board  3 . If the substrate of the circuit board  3  is made of glass, an n-channel amorphous silicon thin-film transistor may be fabricated or p-channel or n-channel low-temperature polycrystalline silicon (LIPS) thin-film transistor may be fabricated. If the substrate of the circuit board  3  is made of silicon, a p-channel transistor or an n-channel transistor may be fabricated. 
     Referring to  FIG. 9 , a second embodiment of a micro-LED display device  2  according to the disclosure is similar to the first embodiment except for the second electrodes  2242 . In this embodiment, the transparent conductive layer  2243  is disposed on a central region of the second type semiconductor layer  223  to expose a peripheral region of the second type semiconductor layer  223 . The electrode layer  2244  of the second electrode  2242  of each of the first, second and third light emitting portions  22 A,  22 B,  22 C includes a ring-shaped electrode part  2246  formed on the transparent conductive layer  2243  and a detecting electrode part  2247  disposed to be in contact with a portion of the ring-shaped electrode part  2246  and a portion of the peripheral region of the second type semiconductor layer  223  (see  FIGS. 1  and  2 ). Uniform current spreading may be achieved with the ring-shaped electrode part  2246  and the transparent conductive layer  2243 , and detecting convenience may be achieved with the detecting electrode part  2247 . 
     Each of the first, second and third light emitting portions  22 A,  22 B,  22 C further includes an insulating layer  225  which is disposed to separate the first type semiconductor layer  221  and the light emitting layer  222  (see  FIGS. 1 and 2 ) from a corresponding one of the second electrodes  2242 . 
     In overall, the micro-LED display device  2  includes a plurality of common-cathode light emitting units  22  which is electrically connected to the circuit board  3  by flip chip bonding through the first and second electrodes  2241 ,  2242 . The micro-LED display device  2  according to the disclosure may be fabricated to avoid the problem of using mass transfer technique, i.e., each light emitting portions should be individually transferred to the native substrate first and then applied to different displays. Further, by providing the included angle (e) greater than 90 degrees, the lights (L) emitted from the micro-LED display device  2  are high-directional and have small angle light distribution which reduces the light loss therefrom. Moreover, with the formation of the reflecting layer  24  on the first, second and third light emitting portions  22 A,  22 B,  22 C, more reflected lights may be generated to increase the amount of lights (L) exiting from the first surface  201  of the protective layer  210 . Additionally, the deposition of the selective reflection layer  251  may reduce the amount of the quantum dots in the wavelength converting elements  232 A,  232 B, thus shortening the height of the first and second wavelength converting elements  232 A,  232 B and the reflecting feature  231 , while achieving the same color conversion efficiency. 
     In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments. It will be apparent, however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. It should also be appreciated that reference throughout this specification to “one embodiment,” “an embodiment,” an embodiment with an indication of an ordinal number and so forth means that a particular feature, structure, or characteristic may be included in the practice of the disclosure. It should be further appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects, and that one or more features or specific details from one embodiment may be practiced together with one or more features or specific details from another embodiment, where appropriate, in the practice of the disclosure. 
     While the disclosure has been described in connection with what are considered the exemplary embodiments, it is understood that this disclosure is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.