Patent Publication Number: US-10784241-B2

Title: Method of manufacturing micro-LED array display devices with CMOS cells

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application Ser. No. 15/613,233, filed Jun. 4, 2017, which claims the benefit of Korean Patent Application No. 10-2016-0090600, filed Jul. 18, 2016. The contents of these applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to micro-LED array display devices, and more specifically to micro-LED array display devices in which a plurality of micro-LED pixels are arrayed on one micro-LED panel by etching for the production of LED chips and the pixel-arrayed micro-LED panel is flip-chip bonded to a CMOS backplane through bumps so that the micro-LED pixels can be individually driven, thus being suitable for microdisplay applications. 
     BACKGROUND 
     Demand for light emitting diodes (LEDs) has expanded exponentially in terms of low power consumption and environmental friendliness. LEDs are used as backlights for lighting apparatuses and LCD devices and are widely applied to display devices. 
     LEDs are kinds of solid-state elements that convert electrical energy into light. LEDs are based on the principle that when a voltage is applied between two doped layers, i.e. an n-type semiconductor layer and a p-type semiconductor layer, between which an active layer is interposed, electrons and holes are injected into and recombine in the active layer to emit light. LEDs can be driven at relatively low voltage and have high energy efficiency. Due to these advantages, LEDs release a small amount of heat. LEDs can be produced in various types. Particularly, micro-LED array display devices are fabricated based on types of LEDs in which a plurality of micro-LED pixels are formed on one wafer. According to a conventional method for the fabrication of a micro-LED array display device in which a plurality of micro-LED pixels are formed on one wafer, a p-type terminal and an n-type terminal are formed in each pixel through a chip production process and are arrayed along the longitudinal and transverse axes of signal lines to drive the pixel. In this case, elements responsible for signal control in the micro-LED pixels should be disposed in the vicinity of the micro-LED pixels, resulting in an increase in the size of the micro-LED array display device. Further, data lines arrayed along the longitudinal and transverse axes should be connected to the micro-LED pixels by wire bonding, making the process complicated and inconvenient. 
     The formation of a plurality of micro-LED pixels on one substrate technically limits the production of structures emitting red, green, and blue light. Because of this technical difficulty, the use of LEDs as light sources in micro-LED array display devices inevitably leads to the emission of monochromatic light. Thus, there is a need in the art for an approach that can provide a solution to the problems of the prior art. 
     SUMMARY 
     One object of the present invention is to provide a micro-LED array display device in which micro-LED pixels are flip-chip bonded to corresponding CMOS cells formed on a CMOS backplane through bumps, thus avoiding the complexity and inconvenience of wire bonding for connecting micro-LED pixels to various data lines while enabling individual control of the micro-LED pixels. 
     A further object of the present invention is to provide a micro-LED array display device in which micro-LED panels, each including a plurality of micro-LED pixels, are flip-chip bonded to a single CMOS backplane, thus overcoming the difficulties of the prior art in forming red, green, and blue light emitting structures including micro-LED pixels formed on one substrate. 
     According to one aspect of the present invention, there is provided a micro-LED array display device including: a micro-LED panel including a plurality of micro-LED pixels; a CMOS backplane including a plurality of CMOS cells corresponding to the micro-LED pixels to individually drive the micro-LED pixels; and bumps electrically connecting the micro-LED pixels to the corresponding CMOS cells in a state in which the micro-LED pixels are arranged to face the CMOS cells, wherein the micro-LED pixels are flip-chip bonded to the corresponding CMOS cells formed on the CMOS backplane through the bumps so that the micro-LED pixels are individually controlled. 
     According to one embodiment, the micro-LED pixels are formed by growing a first conductivity-type semiconductor layer, an active layer, and a second conductivity-type semiconductor layer in this order on a substrate and etching the layers, the micro-LED pixels have a vertical structure including the first conductivity-type semiconductor layer, the active layer, and the second conductivity-type semiconductor layer formed in this order, and the active layer and the second conductivity-type semiconductor layer are removed from the exposed portions of the first conductivity-type semiconductor layer where none of the micro-LED pixels are formed. 
     According to one embodiment, a first conductivity-type metal layer is formed over the portions of the first conductivity-type semiconductor layer where none of the micro-LED pixels are formed and is spaced apart from the micro-LED pixels. 
     According to one embodiment, the first conductivity-type metal layer is formed along the periphery of the micro-LED panel on the first conductivity-type semiconductor layer. 
     According to one embodiment, the first conductivity-type metal layer has the same height as the micro-LED pixels. 
     According to one embodiment, the first conductivity-type metal layer functions as a common electrode of the micro-LED pixels. 
     According to one embodiment, the CMOS backplane includes a common cell formed at a position corresponding to the first conductivity-type metal layer and the first conductivity-type metal layer is electrically connected to the common cell through a common bump. 
     According to one embodiment, the first conductivity-type is n-type and the second conductivity-type is p-type. 
     According to one embodiment, the substrate is made of a material selected from sapphire, SiC, Si, glass, and ZnO. 
     According to one embodiment, the bumps are formed on the CMOS cells and are melted by heating such that the CMOS cells are electrically connected to the corresponding micro-LED pixels. 
     According to a further aspect of the present invention, there is provided a micro-LED array display device including: first, second, and third micro-LED panels emitting light of different wavelength bands, each of the micro-LED panels including a plurality of micro-LED pixels; a single CMOS backplane including a plurality of CMOS cells corresponding to the micro-LED pixels of the first, second, and third micro-LED panels to individually drive the micro-LED pixels; and bumps electrically connecting the micro-LED pixels of the first, second, and third micro-LED panels to the corresponding CMOS cells in a state in which the micro-LED pixels of the first, second, and third micro-LED panels are arranged to face the CMOS cells, wherein the micro-LED pixels of the first, second, and third micro-LED panels are flip-chip bonded to the corresponding CMOS cells formed on the CMOS backplane through the bumps so that the micro-LED pixels are individually controlled. 
     In the new concept of micro-LED array display device according to the present invention, micro-LED pixels are flip-chip bonded to micro-LED pixels formed on a CMOS backplane through bumps, avoiding the complexity and inconvenience of wire bonding for connecting micro-LED pixels to various data lines while enabling individual control of the micro-LED pixels. The micro-LED array display device of the present invention in which a plurality of micro-LED panels emitting red, green, and blue light are flip-chip bonded to a single CMOS backplane through bumps can focus three colors on the same area using an optical system to achieve full color. Therefore, the micro-LED array display device of the present invention is effective in overcoming the technical difficulties of the prior art in forming red, green, and blue light emitting structures including a plurality of micro-LED pixels on one substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  illustrates an exemplary micro-LED panel  100  of a micro-LED array display device according to one embodiment of the present invention; 
         FIG. 2  illustrates the micro-LED panel  100  of  FIG. 1  including micro-LED pixels and a CMOS backplane  200  including a plurality of CMOS cells adapted to individually drive the micro-LED pixels of the micro-LED panel  100 ; 
         FIG. 3  illustrates a state in which the micro-LED panel  100  and the CMOS backplane  200  illustrated in  FIG. 2  are electrically connected to each other through bumps  300  arranged on the CMOS backplane  200 ; 
         FIG. 4  illustrates a state in which the micro-LED panel  100  and the CMOS backplane  200  illustrated in  FIG. 3  are arranged to face each other through the bumps  300  to electrically connect the micro-LED pixels of the micro-LED panel  100  to the CMOS cells of the CMOS backplane  200 ; 
         FIG. 5  illustrates a state in which red, green, and blue micro-LED panels  1100 ,  1200 , and  1300 , a single CMOS backplane  2000  having CMOS cell areas  2100 ,  2200 , and  2300  where the micro-LED panels  1100 ,  1200 , and  1300  are to be electrically connected to CMOS cells, and bumps  3000  arranged on the CMOS cells to achieve full color in a micro-LED array display device according to one embodiment of the present invention; 
         FIG. 6  illustrates a state in which the red, green, and blue micro-LED panels  1100 ,  1200 , and  1300  are electrically connected to the single CMOS backplane  2000  through the bumps  3000  in the micro-LED array display device of  FIG. 5 ; and 
         FIG. 7  is a view for briefly explaining the driving of the micro-LED array display device illustrated in  FIG. 5  to achieve full color. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to a micro-LED array display device in which micro-LED pixels are arrayed by MESA etching and are flip-chip bonded to a CMOS backplane, thus being applicable to a micro display, such as a head mounted display (HMD) or head up display (HUD). In the micro-LED array display device of the present invention, micro-LED pixels arrayed by MESA etching for the production of LED chips are flip-chip bonded to a CMOS backplane so that they can be individually driven. The present invention is also directed to a micro-LED array display device in which three red, green, and blue light emitting elements, i.e. micro-LED panels, are arrayed on a CMOS backplane to achieve full color. 
     Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. The drawings and embodiments described with reference to the drawings are simplified and illustrated such that those skilled in the art can readily understand the present invention. Accordingly, the drawings and the embodiments should not be construed as limiting the scope of the present invention. 
       FIG. 1  illustrates an exemplary micro-LED panel  100  of a micro-LED array display device according to one embodiment of the present invention,  FIG. 2  illustrates the micro-LED panel  100  including micro-LED pixels and a CMOS backplane  200  including a plurality of CMOS cells adapted to individually drive the micro-LED pixels of the micro-LED panel  100 ,  FIG. 3  illustrates a state in which the micro-LED panel  100  and the CMOS backplane  200  are electrically connected to each other through bumps  300  arranged on the CMOS backplane  200 , and  FIG. 4  illustrates a state in which the micro-LED panel  100  and the CMOS backplane  200  are arranged to face each other through the bumps  300  to electrically connect the micro-LED pixels of the micro-LED panel  100  to the CMOS cells of the CMOS backplane  200 . 
     Referring first to  FIGS. 1 to 4 , a description will be given of a micro-LED array display device according to one embodiment of the present invention. The micro-LED array display device includes a micro-LED panel  100 , a CMOS backplane  200 , and bumps  300 . The micro-LED panel  100  includes a plurality of micro-LED pixels  130 . The CMOS backplane  200  includes a plurality of CMOS cells  230  corresponding to the micro-LED pixels  130  to individually drive the micro-LED pixels  130 . The micro-LED pixels  130  are electrically connected to the corresponding CMOS cells  230  through the bumps  300  in a state in which the micro-LED pixels  130  are arranged to face the CMOS cells  230 . In  FIGS. 1 to 4 , only one of the micro-LED pixels and only one of the CMOS cells are denoted by reference numerals  130  and  230 , respectively, for the purpose of convenience. The micro-LED pixels  130  are flip-chip bonded to the corresponding CMOS cells  230  formed on the CMOS backplane  200  through the bumps  300 . Due to this construction, the micro-LED pixels  130  can be individually controlled. 
     The micro-LED pixels  130  of the micro-LED panel  100  are formed by growing a first conductivity-type semiconductor layer  132 , an active layer  134 , and a second conductivity-type semiconductor layer  136  in this order on a substrate  110  and etching the layers. The micro-LED pixels have a vertical structure including the first conductivity-type semiconductor layer  132 , the active layer  134 , and the second conductivity-type semiconductor layer  136  formed in this order on the substrate  110 . The substrate  110  may be made of a material selected from sapphire, SiC, Si, glass, and ZnO. The first conductivity-type semiconductor layer  132  may be an n-type semiconductor layer and the second conductivity-type semiconductor layer  136  may be a p-type semiconductor layer. The active layer  134  is a region where electrons from the first conductivity-type semiconductor layer  132  recombine with holes from the second conductivity-type semiconductor layer  136  when power is applied. 
     The second conductivity-type semiconductor layer  136  and the active layer  134  are removed from the etched portions  120  of the micro-LED panel  100  where none of the micro-LED pixels  130  are formed, and as a result, the first conductivity-type semiconductor layer is exposed in the etched portions. The micro-LED panel  100  includes a first conductivity-type metal layer  140  formed over the portions  120  of the first conductivity-type semiconductor layer  132  where none of the micro-LED pixels  130  are formed. The first conductivity-type metal layer  140  is spaced apart from the micro-LED pixels  130 . The first conductivity-type metal layer  140  is formed with a predetermined width along the periphery of the micro-LED panel  100  on the first conductivity-type semiconductor layer  132 . The first conductivity-type metal layer  140  has substantially the same height as the micro-LED pixels  130 . The first conductivity-type metal layer  140  is electrically connected to the CMOS backplane  200  through the bumps  300 . As a result, the first conductivity-type metal layer  140  functions as a common electrode of the micro-LED pixels  130 . For example, the first conductivity-type metal layer  140  may be a common ground. 
     The plurality of CMOS cells  230  of the CMOS backplane  200  serve to individually drive the micro-LED pixels  130 . The CMOS cells  230  are electrically connected to the corresponding micro-LED pixels through bumps  330 . The CMOS cells  230  are integrated circuits for individually driving the corresponding micro-LED pixels. The CMOS backplane  200  may be, for example, an active matrix (AM) panel. Specifically, each of the CMOS cells  230  may be a pixel driving circuit including two transistors and one capacitor. When the micro-LED panel  100  is flip-chip bonded to the CMOS backplane  200  through the bumps  300 , each of the micro-LED pixels may be arranged between a drain terminal and a common ground terminal (e.g., reference numeral  240 ) of a transistor of the pixel driving circuit to form an equivalent circuit. 
     The CMOS backplane  200  includes a common cell  240  formed at a position corresponding to the first conductivity-type metal layer  140 . The first conductivity-type metal layer  140  is electrically connected to the common cell  240  through a common bump  340 . Herein, the bumps  300  is often intended to include the bumps  330  electrically connecting the plurality of CMOS cells to the micro-LED pixels and the common bump  340  electrically connecting the first conductivity-type metal layer  140  to the common cell  240 . 
     As illustrated in  FIG. 3 , the CMOS backplane  200  on which the CMOS cells  230  are arranged faces the micro-LED panel  100 . After the CMOS cells  230  are brought into contact with the micro-LED pixels  130  in a one-to-one relationship, the bumps  330  and the common bump  340  are melted by heating. As a result, the CMOS cells  230  are electrically connected to the corresponding micro-LED pixels  130 , as illustrated in  FIG. 4 . 
     Referring next to  FIGS. 5 and 6 , a description will be given of a micro-LED array display device capable of achieving full color according to a further embodiment of the present invention.  FIG. 5  illustrates a state in which red, green, and blue micro-LED panels  1100 ,  1200 , and  1300 , a single CMOS backplane  2000  having CMOS cell areas  2100 ,  2200 , and  2300  where the micro-LED panels  1100 ,  1200 , and  1300  are to be electrically connected to CMOS cells, and bumps  3000  arranged on the CMOS cells to achieve full color in a micro-LED array display device according to one embodiment of the present invention and  FIG. 6  illustrates a state in which the red, green, and blue micro-LED panels  1100 ,  1200 , and  1300  are electrically connected to the single CMOS backplane  2000  through the bumps  3000 . 
     Referring to these figures, the micro-LED array display device capable of achieving full color includes a first micro-LED panel  1100 , a second micro-LED panel  1200 , and a third micro-LED panel  1300 , each of which includes a plurality of arrayed micro-LED pixels. The first  1100 , second  1200 , and third micro-LED panels  1300  emit light of different wavelength bands. For example, the first, second, and third micro-LED panels  1100 ,  1200 , and  1300  may be constructed to emit red light, green light, and blue light, respectively. The micro-LED array display device capable of achieving full color includes a single CMOS backplane  2000  adapted to individually drive the micro-LED pixels of the first, second, and third micro-LED panels  1100 ,  1200 , and  1300 . The single CMOS backplane  2000  includes a plurality of CMOS cells corresponding to the micro-LED pixels of the first, second, and third micro-LED panels  1100 ,  1200 , and  1300 . CMOS cell areas  2100 ,  2200 , and  2300  are formed in the CMOS backplane  2000  such that the micro-LED panels  1100 ,  1200 , and  1300  are arranged on the CMOS backplane  2000 . The CMOS cell areas  2100 ,  2200 , and  2300  are formed corresponding to the micro-LED panels  1100 ,  1200 , and  1300 , respectively. The micro-LED panels  1100 ,  1200 , and  1300  are flip-chip bonded to the CMOS cell areas  2100 ,  2200 , and  2300 , respectively. A plurality of CMOS cells corresponding to the micro-LED pixels of the micro-LED panels  1100 ,  1200 , and  1300  are formed in the CMOS cell areas  2100 ,  2200 , and  2300 , respectively. With this arrangement, the micro-LED panels  1100 ,  1200 , and  1300  are flip-chip bonded to the single CMOS backplane  2000  to electrically connect the micro-LED pixels to the CMOS cells. The CMOS cells are electrically connected to the micro-LED pixels through bumps  3000 . The flip-chip bonding of the micro-LED panels  1100 ,  1200 , and  1300  to the single CMOS backplane  2000  is performed in the same manner as that of the micro-LED panel  100  to the CMOS backplane  200  explained with reference to  FIGS. 1 to 4 . 
     Common cells are formed in the CMOS cell areas  2100 ,  2200 , and  2300  on the single CMOS backplane  2000  and are electrically connected to first conductivity-type metal layers of the micro-LED panels  1100 ,  1200 , and  1300  through common bumps. 
     As described before, the micro-LED array display device of the present invention in which the plurality of independently fabricated micro-LED panels emitting light of different wavelength bands, i.e. red, light, and blue light, are flip-chip bonded to the single CMOS backplane  2000  can focus three colors on the same area using an optical system to achieve full color, thus overcoming the technical difficulties of the prior art in forming red, green, and blue light emitting structures on one substrate in the fabrication of micro-LEDs. In addition, the micro-LED array display device of the present invention can avoid the inconvenience or difficulty of wire bonding for connecting LED chips to various data lines, which run along the longitudinal and transverse axes and are responsible for the control of the LED chips. Furthermore, the micro-LED array display device of the present invention can eliminate the need to dispose elements responsible for signal control in LED chips at positions away from the LED chips, contributing to a reduction in the overall size of the display device. 
     Finally,  FIG. 7  is a view for briefly explaining the driving of the micro-LED array display device illustrated in  FIG. 5  to achieve full color. As illustrated in  FIG. 7 , the micro-LED array display device is driven in response to control signals from a drive IC  700 . The control signals from the drive IC  700  are transmitted to the micro-LED pixels by the CMOS cells (i.e. CMOS integrated circuits) formed in the CMOS backplane  2000 . The control signals from the drive IC  700  may be analog or digital signals. The digital signals may also be pulse width modulation (PWM) signals. 
     EXPLANATION OF REFERENCE NUMERALS 
       100 ,  1100 ,  1200 ,  1300 : Micro-LED panels 
       110 : Substrate 
       120 ,  132 : First conductivity-type semiconductor layers 
       130 : Micro-LED pixel 
       134 : Active layer 
       136 : Second conductivity-type semiconductor layer 
       140 : First conductivity-type metal layer 
       200 ,  2000 : CMOS backplanes 
       230 : CMOS cell 
       240 : Common cell 
       340 : Common bump 
       300 ,  330 ,  3000 ,  3100 ,  3200 ,  3300 : Bumps 
       2100 ,  2200 ,  2300 : CMOS cell areas 
       700 : Drive IC