Patent Publication Number: US-11652089-B2

Title: Panel for display by micro LED and method for making same

Description:
FIELD 
     The subject matter herein generally relates to LED displays. 
     BACKGROUND 
     A method for making a micro LED display panel can include transferring a plurality of micro LEDs from a carrier substrate onto a thin film transistor (TFT) substrate. However, since each of the micro LEDs is extremely small in size, no more than one hundred micrometers, methods for making the micro LED display panel require high precision. 
     Therefore, there is room for improvement in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present disclosure will now be described, by way of embodiment, with reference to the attached figures. 
         FIG.  1    is a cross-sectional view shown during step S 1  of a method for making a micro LED display panel according to one embodiment. 
         FIG.  2    is a cross-sectional view shown during step S 2  of a method for making a micro LED display panel according to one embodiment. 
         FIG.  3    is a cross-sectional view shown during step S 3  of a method for making a micro LED display panel according to one embodiment. 
         FIG.  4    is a cross-sectional view shown during step S 4  of a method for making a micro LED display panel according to one embodiment. 
         FIG.  5    is a cross-sectional view shown during step S 5  of a method for making a micro LED display panel according to one embodiment. 
         FIG.  6    is a cross-sectional view shown during step S 6  of a method for making a micro LED display panel according to one embodiment. 
         FIG.  7    is a cross-sectional view shown during steps S 7  and S 8  of a method for making a micro LED display panel according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that the exemplary embodiments described herein may be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the exemplary embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features of the present disclosure. 
     The term “comprising” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like. The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references can mean “at least one”. The term “circuit” is defined as an integrated circuit (IC) with a plurality of electric elements, such as capacitors, resistors, amplifiers, and the like. 
     The embodiments of the present disclosure are shown in the drawings, and the disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments may not provide a complete disclosure of the disclosure and the scope of the disclosure. For clarity, the dimensions of the layers and regions are enlarged in the figures. 
     Certain terms used in this specification have meanings predetermined by the inventors. In particular, as used in the disclosure, “micro LED” refers to a light emitting diode (LED) having dimension of from one micrometer to about one hundred micrometers, and more specifically to an LED having at least one dimension less than one hundred micrometers. 
     A method for making a micro LED display panel according to one embodiment of the present disclosure is as follows. 
     Block S 1 : As shown in  FIG.  1   , a carrier substrate  10  with a plurality of micro LEDs  20  is provided. A first electrode  25  is on an end of each micro LED  20  away from the carrier substrate  10 . 
     As shown in  FIG.  1   , the carrier substrate  10  includes a substrate  11  and an adhesive layer  12  on a surface of the substrate  11 . The micro LEDs  20  are spaced apart from each other and embedded in the adhesive layer  12 . The adhesive layer  12  is made of a material decomposable by ultraviolet irradiating or heating, causing non-adhesiveness. 
     As shown in  FIG.  1   , each micro LED  20  includes a P-type doped light emitting layer  21 , an active layer  22 , and an N-type doped light emitting layer  23 . The active layer  22  is between the P-doped light emitting layer  21  and the N-doped light emitting layer  23 . A second electrode  26  is on one end of each micro LED  20  adjacent to the carrier substrate  10 . The N-doped light emitting layer  23  is electrically coupled to the second electrode  26 , and the P-doped light emitting layer  21  is electrically coupled to the first electrode  25 . 
     In one embodiment, each micro LED  20  is further provided with a protective layer  24  that wraps around side surfaces of the micro LED  20  and exposes the first electrode  25  and the second electrode  26 . The protective layer  24  may be made of an oxide of silicon. 
     Block S 2 : As shown in  FIG.  2   , a thin film transistor (TFT) substrate  50  is provided, the TFT substrate  50  includes a driving circuit  51 , and a plurality of conductive connecting elements  52  electrically coupled to the driving circuit  51  is formed on the TFT substrate  50 .  FIG.  2    shows only one conductive connecting element  52 . 
     In one embodiment, the TFT substrate  50  is a multilayer structure and the driving circuit  51  includes a plurality of data lines  60 , a plurality of scan lines  70 , a plurality of storage capacitors C, a plurality of first TFTs M 1 , a plurality of second TFTs M 2 , and a plurality of third TFTs M 3 .  FIG.  2    shows only one data line  60 , one scan line  70 , one storage capacitor C, one first TFT M 1 , one second TFT M 2 , and one third TFT M 3 . The data lines  60  extend in a first direction (not shown), the scan lines  70  extend in a second direction (not shown) intersecting the first direction. One sub-pixel unit (not shown) is defined by all adjacent scan lines  70  and all adjacent data lines  60 . Each micro LED  20  corresponds to one sub-pixel unit. Through the TFT substrate  50 , the driving circuit  51  drives the micro LEDs  20  to emit light. 
       FIG.  2    shows an equivalent circuit diagram of one sub-pixel unit. As shown in  FIG.  2   , each sub-pixel unit includes one storage capacitor C, one first TFT M 1 , one second TFT M 2 , and one third TFT M 3 . The storage capacitor C includes a first end a and a second end b electrically coupled to a direct current voltage Vdd. The first TFT M 1  includes a source electrode electrically coupled to one data line  60 , a gate electrode electrically coupled to one scan line  70 , and a drain electrode electrically coupled to the first end a of the storage capacitor C. The second TFT M 2  includes a source electrode electrically coupled to the direct current voltage Vdd, a gate electrode electrically coupled to the first end a of the storage capacitor C, and a drain electrode electrically coupled to one conductive connecting element  52 . The third TFT M 3  includes a source electrode electrically coupled to a reference voltage Vref, a gate electrode electrically coupled to a control voltage V-off, and a drain electrode electrically coupled to one conductive connecting element  52 . The direct current voltage Vdd is different from the reference voltage Vref. 
     In one embodiment, the conductive connecting elements  52  formed on the TFT substrate  50  are spaced, each conductive connecting element  52  corresponds to one micro LED  20  and is configured to electrically connect to the first electrode  25  of the micro LED  20 . The conductive connecting element  52  includes a conductive solder layer  523  on the TFT substrate  50 . In the present embodiment, the conductive connecting element  52  further includes a base conductive layer  521  on the TFT substrate  50  and a conductive barrier layer  522  on a side of the base conductive layer  521  away from the TFT substrate  50 . The conductive barrier layer  522  is between the base conductive layer  521  and the conductive solder layer  523  and prevents the material of the conductive solder layer  523  from diffusing into the base conductive layer  521 . 
     In one embodiment, the base conductive layer  521  includes a first portion  5211  and a second portion  5212  spaced apart and insulated from the first portion  5211 . The conductive barrier layer  522  and the conductive solder layer  523  are sequentially stacked on the first portion  5211 , and a side of the second portion  5212  away from the TFT substrate  50  is not covered by the conductive barrier layer  522  and the conductive solder layer  523 . The drain electrode of the second TFT M 2  is electrically coupled to the first portion  5211 . The drain electrode of the third TFT M 3  is electrically coupled to the second portion  5212 . 
     In one embodiment, the base conductive layer  521  may be made of indium tin oxide, the conductive solder layer  523  may be made of tin, and the conductive barrier layer  522  may be made of nickel or gold. The conductive barrier layer  522  prevents the material of the conductive solder layer  523  from diffusing to the base conductive layer  521 . The base conductive layer  521  is configured for reflecting the light emitted by the micro LEDs  20  toward the side away from the TFT substrate  50 , thereby improving light emitting efficiency of the micro LED display panel. 
     Block S 3 : As shown in  FIG.  3   , an insulating layer  53  covering the conductive connecting element  52  is formed on the TFT substrate  50 . 
     As shown in  FIG.  3   , the insulating layer  53  defines a contact hole  531  exposing the second portion  5212  of the base conductive layer  521 . In one embodiment, the insulating layer  53  may be made of an oxide of silicon. 
     Block S 4 : As shown in  FIG.  4   , a contact electrode layer  54  is formed on a side of the insulating layer  53  away from the TFT substrate  50 , and the contact electrode layer  54  extends through the insulating layer  53  to be electrically coupled to the driving circuit  51 . 
     As shown in  FIG.  4   , the contact electrode layer  54  fills the contact hole  531  and is electrically coupled to the driving circuit  51  by the second portion  5212  of the base conductive layer  521 . 
     Block S 5 : As shown in  FIG.  5   , the insulating layer  53  and the contact electrode layer  54  are patterned to define a through hole  55  in the contact electrode layer  54  and the insulating layer  53 . The through hole  55  extends through the contact electrode layer  54  and the insulating layer  53  to expose the conductive connecting element  52 . 
     As shown in  FIG.  5   , the through hole  55  exposes a portion of the conductive solder layer  523 . 
     In one embodiment, the contact electrode layer  54  is patterned to form a plurality of spaced contact electrodes  541  ( FIG.  5    only shows one contact electrode  541 ). Each contact electrode  541  corresponds to one conductive connecting element  52 . 
     Block S 6 : As shown in  FIG.  6   , during the process of transferring the micro LEDs  20 , the first electrode  25  of the micro LED  20  resists against the contact electrode layer  54 . The reference voltage Vref is applied to the contact electrode layer  54  by the driving circuit  51 , and the direct current voltage Vdd (different from the reference voltage Vref) is applied to the conductive connecting element  52  by the driving circuit  51 , causing electrostatic attraction between the first electrode  25  of the micro LED  20  and the conductive connecting element  52 . 
     In one embodiment, each micro LED  20  is aligned with one conductive connecting element  52 . The first electrode  25  of each micro LED  20  resists against one contact electrode  541  so as to be in direct contact with the contact electrode  541 . During the process of transferring the micro LEDs  20 , the first TFT M 1 , the second TFT M 2 , and the third TFT M 3  are powered on while the TFT substrate  50 , particularly the contact electrode  541 , resists against the micro LEDs  20 . The reference voltage Vref is thus applied to both the contact electrode layer  54  and the first electrode  25  of the micro LED  20 , and the direct current voltage Vdd is applied to the conductive connecting element  52 . The voltage difference between Vdd and Vref creates electrostatic attraction between the first electrode  25  of the micro LED  20  and the conductive connecting element  52  of the TFT substrate  50 , the micro LED  20  and the TFT substrate  50  are thus attracted to each other. 
     As shown in  FIG.  6   , a projection of the first electrode  25  of the micro LED  20  on the TFT substrate  50  completely covers a projection of the through hole  55  on the TFT substrate  50 . 
     Since the through hole  55  completely penetrates the contact electrode layer  54  and the insulating layer  53 , a surface of the insulating layer  53  away from the TFT substrate  50  is completely covered by the contact electrode layer  54 . Therefore, when the first electrode  25  of the micro LED  20  resists against the contact electrode layer  54 , the first electrode  25  of the micro LED  20  cannot be in contact with the insulating layer  53  due to the through hole  55 . High-precision accuracy in transferring of the micro-LEDs  20  to the TFT substrate  50  is reduced as a condition. 
     Block S 7 : As shown in  FIG.  7   , the micro LED  20  and the first electrode  25  are peeled off from the carrier substrate  10  and transferred to the TFT substrate  50 . 
     In one embodiment, during the process of transferring the micro LED  20 , the adhesive layer  12  is irradiated with ultraviolet light or heated in order to remove the adhesiveness of the adhesive layer  12 , when the first TFT M 1 , the second TFT M 2 , and the third TFT M 3  are to be powered on. The micro LED  20  and the first electrode  25  are thus peeled off from the adhesive layer  12  of the carrier substrate  10 . After the micro LED  20  and the first electrode  25  are separated from the carrier plate  10 , the attraction between the first electrode  25  of the micro LED  20  and the conductive connecting element  52  of the TFT substrate  50  renders the micro LED  20  stable on the TFT substrate  50  and effectively immovable on the TFT substrate  50 . 
     Block S 8 : As shown in  FIG.  7   , the conductive connecting element  52  is treated so as to bond the first electrode  25  of the micro LED  20 . Therefore, the micro LED  20  is permanently fixed to the TFT substrate  50 . 
     In one embodiment, the conductive connecting element  52  is heated to melt the conductive solder layer  523  while in direct contact with the first electrode  25  of the micro LED  20 , then the conductive solder layer  523  is solidified to firmly bond the first electrode  25 . As such, the micro LED  20  becomes fixed to the TFT substrate  50 . 
     As shown in  FIG.  7   , the drain electrode of the second TFT M 2  and the drain electrode of the third TFT M 3  are electrically coupled to the first electrode  25  of the micro LED  20 . The drain electrode of the second TFT M 2  is electrically coupled to the first electrode  25  of the micro LED  20  by the conductive connecting element  52 . The drain electrode of the third TFT M 3  is electrically coupled to the micro LED  20  by the contact electrode layer  54 . 
     In one embodiment, the third TFT M 3  has a function only during the process of transferring the micro LEDs  20  onto the TFT substrate  50 . When the micro LED display panel is in use and displaying images, the third TFT M 3  is not powered and only the first TFT M 1  and the second TFT M 2  operate. Therefore, the reference voltage Vref is not applied to the first electrode  25  of the micro LED  20  and thus does not affect emission of light from the micro LED  20 . For example, when the micro LED display panel performs display function, the driving circuit  51  applies a direct current voltage Vdd to the first electrode  25  of the micro LED  20  by the first TFT M 1  and the second TFT M 2 , and the driving circuit  51  applies another lesser voltage to the second electrode  26  of the micro LED  20 . The micro LED  20  emits light under the forward bias. The first electrode  25  is generally referred to as an anode of the micro LED  20 , and the second electrode  26  is generally referred to as a cathode of the micro LED  20 . 
     The preparation method of the micro LED display panel of the present disclosure simultaneously patterns the insulating layer  53  and the contact electrode layer  54 , the preparation process is simple. In addition, the high-accuracy standard in transferring the first electrode  25  of the micro LED  20  against the contact electrode  541  is reduced due to the through hole  55 . 
     The present disclosure further provides a micro LED display panel made by the above method. The micro LED display panel may be a mobile phone, a tablet computer, a smart watch, or the like. 
     It is to be understood, even though information and advantages of the present exemplary embodiments have been set forth in the foregoing description, together with details of the structures and functions of the present exemplary embodiments, the disclosure is illustrative only. Changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the present exemplary embodiments to the full extent indicated by the plain meaning of the terms in which the appended claims are expressed.