Patent Publication Number: US-2022231197-A1

Title: Flip-chip light emitting device and production method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 17/002,423 filed on Aug. 25, 2020, which claims priority of Chinese Patent Application No. 201910809304.5, filed on Aug. 29, 2019. The content of each of the aforesaid prior applications is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The disclosure relates to a light emitting device and a production method thereof, and more particularly to a flip-chip light emitting device and a production method thereof. 
     BACKGROUND 
     Referring to  FIG. 1 , a conventional flip-chip light emitting device includes a transparent substrate  11 , a transparent bonding layer  12 , an epitaxial light-emitting structure, a first electrode  18 , a second electrode  17 , and a protective insulating layer  16 . The light-emitting structure is connected with the transparent substrate  11  through the transparent bonding layer  12 , and includes a first-type electrically conductive layer  13 , an active layer  14 , and an second-type electrically conductive layer  15  that are sequentially disposed on the transparent bonding layer  12  in such order. The active layer  14  is composed of quantum wells. The first and second electrodes  18 ,  17  are respectively disposed on the first-type and second-type electrically conductive layers  13 ,  15 . The protective insulating layer  16  is disposed over the light-emitting structure and the bonding layer  12 . 
     In order to enhance the light emission efficiency, a roughened interface  19  is formed between the first-type electrically conductive layer  13  and the transparent bonding layer  12 , and between the protective insulating layer  16  and the transparent bonding layer  12 . However, in one aspect, interstices exist at the part of the roughened interface  19  between the transparent bonding layer  12  and the protective insulating layer  16 , such that liquids for processing the flip-chip light emitting device, water vapor, metal ions from solder, and so forth might undesirably pass through these interstices and hence lead to damages of the light-emitting structure. In another aspect, cracks might be generated in the protective insulating layer  6  during the cutting process of the flip-chip light emitting device. Thus, the protective insulating layer  6  might be undesirably detached due to the aforesaid drawbacks, leading to failure of the protective function of the protective insulating layer  6 . 
     SUMMARY 
     Therefore, an object of the disclosure is to provide a flip-chip light emitting device that can alleviate at least one of the drawbacks of the prior art. 
     The flip-chip light emitting device includes a substrate, an epitaxial light-emitting layer, a bonding layer, and a protective insulating layer. The light-emitting layer has a top surface and a bottom surface that is opposite to the top surface and faces toward the substrate. The bonding layer is disposed between the substrate and the light-emitting layer. The protective insulating layer is disposed over the light-emitting layer and the bonding layer. The bonding layer has a first upper surface that faces away from the substrate and that the protective insulating layer is disposed thereon, and a second upper surface that faces away from the substrate, and that the light-emitting layer is disposed thereon. The first and second upper surfaces respectively have first and second roughnesses. The first roughness of the first upper surface is different from the second roughness of the second upper surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiment(s) with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic sectional view illustrating a conventional flip-chip light emitting device; 
         FIGS. 2 and 9  are schematic sectional views respectively illustrating steps  1  to  8  in a first embodiment of a method for producing a flip-chip light emitting device according to the present disclosure; 
         FIG. 10  is a fragmentary, enlarged sectional view illustrating first and second contact surfaces of a first embodiment of a flip-chip light emitting device produced by the first embodiment of the method; 
         FIG. 11  is a schematic sectional view illustrating a second embodiment of the flip-chip light emitting device according to the present disclosure, a part of the second embodiment being enlarged in  FIG. 11  to show first and second upper surfaces of a bonding layer of the flip-chip light emitting device; and 
         FIG. 12  is a schematic sectional view illustrating a third embodiment of the flip-chip light emitting device according to the present disclosure, a part of the third embodiment being enlarged in  FIG. 12  to show the first and second upper surfaces of the bonding layer of the flip-chip light emitting device. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 2 to 9  illustrate a first embodiment of a method for producing a flip-chip light emitting device according to the present disclosure. Thus, the flip-chip light emitting device produced by the first embodiment of the method as shown in  FIG. 9  is a first embodiment of a flip-chip light emitting device according to the present disclosure. 
     The first embodiment of the method includes steps  1  to  8 . 
     In step  1 , referring to  FIG. 2 , a growth substrate  20  and an epitaxial light-emitting structure formed thereon are provided. The light-emitting structure includes a first-type electrically conductive layer  23 , an active layer  24  that is disposed on the first-type electrically conductive layer  23  and that is composed of quantum wells, and an second-type electrically conductive layer  25  that is disposed on the active layer  24  opposite to the first-type electrically conductive layer  23 . The light-emitting structure is formed on the growth substrate  20  in a manner that the second-type electrically conductive layer  25  is disposed between the growth substrate  20  and the active layer  24 . 
     The term “first-type” refers to being doped with a first conductivity type dopant, and the term “second-type” refers to being doped with a second conductivity type dopant that is opposite in conductivity type to the first conductivity type dopant. For instance, the first conductivity type dopant may be a p-type dopant, and the second conductivity type dopant may be an n-type dopant, and vice versa. 
     The active layer  24  of the light-emitting structure may be made from a material selected from the group consisting of Al x In y Ga (1-x-y) P (0≤x≤1, 0≤y≤1, 0≤x+y≤1), and Al x Ga (1-x) As (0≤x≤1). The active layer  24  of the light-emitting structure may be configured to emit light having a wavelength ranging from 570 nm and 950 nm (such as yellow light, orange light, red light, and infrared light). For instance, the active layer  24  may emit red light having a wavelength ranging from 610 nm to 650 nm. 
     In other embodiments, an intermediate layer (not shown in the drawings) may be disposed between the growth substrate  20  and the epitaxial light-emitting structure. The intermediate layer may be selected from the group consisting of a buffer layer (which may be made from GaAs), an etch stop layer (which may be made from InGaP), and a combination thereof. 
     In step  2 , referring to  FIG. 3 , a bottom surface of the first-type electrically conductive layer  23 , which faces away from the growth substrate  20 , and which is also a bottom surface of the light-emitting structure, is roughened (for example, through random roughening) to have a roughness (Ra) of 300 nm. 
     The light-emitting structure further has a top surface which is opposite to the roughened bottom surface of the light-emitting structure, and which is also a top surface of the second-type electrically conductive layer  25  facing toward the growth substrate  20 , and side walls which interconnect the top and bottom surfaces of the light-emitting structure, and which are composed of side walls of the second-type electrically conductive layer  25 , the active layer  24 , and the first-type electrically conductive layer  23 . 
     In step  3 , referring to  FIG. 4 , a transparent bonding layer  22  is formed on the roughened bottom surface of the first-type electrically conductive layer  23  through deposition, and is subsequently subjected to polishing for facilitating connection of the transparent bonding layer  22  to a transparent substrate  21  as described below. 
     I-n step  4 , referring to  FIG. 5 , the growth substrate  20  is removed (through, for example, grinding and etching) so as to expose the second-type electrically conductive layer  25  of the light-emitting structure, and the transparent substrate  21  is connected to the transparent bonding layer  22  (through, for example, high-temperature and high-pressure bonding), so that the transparent substrate  21  is disposed on the transparent bonding layer  22  opposite to the light-emitting structure (i.e. the light-emitting structure is connected with the transparent substrate  21  through the transparent bonding layer  22 ). In other embodiments, when the aforesaid intermediate layer is disposed between the growth substrate  20  and light-emitting structure, the aforesaid intermediate layer is removed with the growth substrate  20  in step  4 . 
     In this embodiment, the transparent substrate  21  is made from sapphire and has a thickness of 90 μm, and the transparent bonding layer  22  is made from an insulation material. (e.g. silicon dioxide). 
     Light emitted from the light-emitting structure passes through the transparent bonding layer  22  and the transparent substrate  21 , thereby being emitted out from the flip-chip light emitting device. 
     In step  5 , referring to  FIG. 6 , through a photolithography process employing a photoresist, the second-type electrically conductive layer  25  and the active layer  24  are partially removed so as to partially expose the first-type electrically conductive layer  23 . 
     In step  6 , referring to  FIG. 7 , through a photolithography process employing a photoresist, a periphery of the first-type electrically conductive layer  23  is removed so as to expose a portion of the transparent bonding layer  22 . Subsequently, via etching, the exposed portion of the transparent bonding layer  22  is subjected to thickness reduction, so that a first contact surface  30  of the transparent bonding layer  22  facing away from the transparent substrate  21  is formed and exposed. 
     The transparent bonding layer  22  further has a second contact surface  29  that faces away from the transparent substrate  21 , and that meshes with and is bonded to the roughened bottom surface of the first-type electrically conductive layer  23 . The first and second contact surfaces  30 ,  29  of the transparent bonding layer  22  are different in roughness and maximum height. 
     In this embodiment, the transparent bonding layer  22  further has smooth lateral walls that interconnect the first and second contact surfaces  30 ,  29 . 
     The transparent bonding layer  22  may have a thickness ranging from 1 μm to 5 μm. 
     In this embodiment, the first and second contact surfaces  30 ,  29  of the transparent bonding layer  22  respectively have first and second maximum heights (H 1 , H 2 ) measured from the transparent substrate  21  (see  FIG. 10 ). 
     Namely, a larger-thickness section of the transparent bonding layer  22 , which is interposed between the first-type electrically conductive layer  23  and the transparent substrate  21 , has the second contact surface  29  and hence a maximum thickness equal to the second maximum height (H 2 ). Moreover, a smaller-thickness section of the transparent bonding layer  22 , which extends from the larger-thickness section, has the first contact surface  30  and a maximum thickness equal to the first maximum height (H 1 ). 
     In this embodiment, the first maximum height (H 1 ) of the first contact surface  30  is lower than a second minimum height of the second contact surface  29  measured from the transparent substrate  21 . 
     In this embodiment, the second maximum height (H 2 ) of the second contact surface  29 , i.e. the maximum thickness of the larger-thickness section of the transparent bonding layer  22 , is 3 μm. The first maximum height (H 1 ) of the first contact surface  30 , i.e. the maximum thickness of the smaller-thickness section of the transparent bonding layer  22 , is 2 μm. However, in another embodiment, the first maximum height (H 1 ) of the first contact surface  30  may be lower than the second maximum height (H 2 ) of the second contact surface  29  by at least 200 nm. 
     The first and second contact surfaces  30 ,  29  of the transparent bonding layer  22  respectively have first and second roughnesses (Ra). The first roughness of the first contact surface  30  is less than the second roughness of the second contact surface  29 . The first roughness of the first contact surface  30  may be not greater than 50 nm, and the second roughness of the second contact surface  29  may be equal to or greater than 100 nm and not greater than 500 nm. 
     In this embodiment, the first roughness of the first contact surface  30  is 20 nm, and the second roughness of the second contact surface  29  is 300 nm. 
     The first contact surface  30  of the transparent bonding layer  22  may have a width ranging from 10 μm to 20 μm. 
     In step  7 , referring to  FIG. 8 , via deposition, a protective insulating layer  26  is formed over the second-type electrically conductive layer  25  (over the top surface and the side walls thereof), over the active layer  24  (over the side walls thereof), over the first-type electrically conductive layer  23  (over a top surface and the side walls thereof), and over the transparent bonding layer  22  (over the first contact surface  30  and the smooth lateral walls thereof). Therefore, the protective insulating layer  26  hence has a roughened lower surface which faces toward the transparent substrate  21 , and the first contact surface  30  of the transparent bonding layer  22  meshes with and is bonded to the roughened lower surface of the protective insulating layer  26 . Accordingly, the protective insulating layer  26  sufficiently covers the light-emitting structure and can hence effectively protect the same. It should be noted that the protective insulating layer  26  may be partially or completely disposed over the first contact surface  30  of the bonding layer  22 . 
     In step  8 , referring to  FIGS. 9 and 10 , the protective insulating layer  26  is subjected to a hole-forming process, so that a first through hole is formed to partially expose the first-type electrically conductive layer  23 , and so that a second through hole is formed to partially expose the second-type electrically conductive layer  25 . Further, a first electrode  28  and a second electrode  27  are respectively disposed in the first and second through holes of the protective insulating layer  26  to be correspondingly electrically connected with the first-type electrically conductive layer  23  and the second-type electrically conductive layer  25 . Accordingly, the flip-chip light emitting device is produced. 
     The flip-chip light emitting device thus produced may be configured to emit, through the transparent substrate  21 , light such as red light or infrared light. 
     Since the first roughness of the first contact surface  30  of the transparent bonding layer  22  is lower than the second roughness of the second contact surface  29  of the transparent bonding layer  22 , the number of interstices at the interface between the first contact surface  30  of the transparent bonding layer  22  and the protective insulating layer  26  can be greatly reduced. In addition, the vertical, smooth lateral walls of the transparent bonding layer  22  interconnecting the first and second contact surfaces  30 ,  29  of the transparent bonding layer  22  not only separate the first and second contact surfaces  30 ,  29 , but also allow the protective insulating layer  26  to be well disposed thereover (i.e. no interstice is formed between the smooth lateral walls of the transparent bonding layer  22  and the protective insulating layer  26 ). In view of the foregoing, the protective insulating layer  26  can be prevented from cracking and undesired detachment attributed to interstices, thus effectively protecting the light-emitting structure. 
     In addition, the larger-thickness section of the transparent bonding layer  22  not only can provide sufficient bonding strength between the light-emitting structure and the transparent substrate  21 , but also is sufficiently large in area to be polished for facilitating the connection of the transparent bonding layer  22  to the transparent substrate  21 . 
     Referring to  FIG. 11 , the present disclosure provides a second embodiment of the flip-chip light emitting device, which is similar to the first embodiment of the flip-chip light emitting device, except for following differences. 
     The substrate  21  in the second embodiment is not necessarily transparent. 
     The epitaxial light-emitting structure is an epitaxial light-emitting layer. Nevertheless, the epitaxial light-emitting layer in the second embodiment is structurally the same as the epitaxial light-emitting structure in the first embodiment. 
     The bonding layer  22  in the second embodiment is not necessarily transparent. Nevertheless, the bonding layer  22  may be made from a transparent insulation material. 
     The bonding layer  22  has an inner section that is bonded to the light-emitting layer, and an outer section that extends from the inner section and that is bonded to the protective insulating layer  26 . The bonding layer  22  has a first bonding sub-layer  221  that is disposed on the substrate  21 , and a second bonding sub-layer  222  that is disposed on the first bonding sub-layer  221  opposite to the substrate  21 . The inner section has the first and second bonding sub-layers  221 , 222 . The outer section has both of the first bonding sub-layer  221  and the second bonding sub-layer  222 . 
     The outer section of the bonding layer  22  has a first upper surface  30  that faces away from the substrate  21  and that the protective insulating layer  26  is disposed thereon. The inner section of the bonding layer  22  has a second upper surface  29  that faces away from the substrate  21 , and that the light-emitting layer is disposed thereon. The first and second upper surfaces  30 ,  29  respectively having the first and second roughnesses described above. The inner section of the bonding layer  22  has a thickness larger than that of the outer section of the bonding layer  22 . 
     The first and second upper surfaces  30 ,  29  of the bonding layer  22  in the second embodiment are similar to the first and second contact surfaces  30 ,  29  of the transparent bonding layer  22  in the first embodiment, and respectively have the first and second maximum heights (H 1 , H 2 ) described above. 
     The bonding layer  22  further has a lateral wall that interconnects the first and second upper surfaces  30 ,  29  of the bonding layer  22 , and the light-emitting layer further has a side wall that interconnects the top and bottom surfaces of the light-emitting layer. 
     The protective insulating layer  26  is disposed over the top surface and the side wall of the light-emitting layer, and over the first upper surface  30  and the lateral wall of the bonding layer  22 . 
     The lateral wall of the bonding layer  22  has a roughness smaller than that of the first upper surface  30  of the bonding layer  22 . 
     The second bonding sub-layer  222  of the bonding layer  22  may be thinner than the first bonding sub-layer  221  of the bonding layer  22 . A refractive index of the second bonding sub-layer  222  of the bonding layer  22  may be higher than a refractive index of the first bonding sub-layer  221  of the bonding layer  22 . The refractive index of the second bonding sub-layer  222  of the bonding layer  22  is lower than a refractive index of the light-emitting layer, such that light emission, not light reflection, can be enhanced. 
     The second bonding sub-layer  222  of the bonding layer  22  has an inner segment disposed between the light-emitting layer and the first bonding sub-layer  221  of the bonding layer  22 , and an outer segment disposed between the protective insulating layer  26  and the first bonding sub-layer  221  of the bonding layer  22 . The inner segment of the second bonding sub-layer  222  having a thickness larger than that of the outer segment of the second bonding sub-layer  222 . 
     The first bonding sub-layer  221  has an inner segment disposed between the substrate  21  and the inner segment of the second bonding sub-layer  222 . The inner segment of the second bonding sub-layer  222  is smaller in thickness than the inner segment of the first bonding sub-layer  221 . 
     The second bonding sub-layer  222  of the bonding layer  22  may be made from aluminum oxide, and the first bonding sub-layer  221  of the bonding layer  22  may be made from silicon oxide. 
     A thickness of the inner segment of the second bonding sub-layer  222  of the bonding layer  22  may be smaller than one fifth of a thickness of the inner segment of the first bonding sub-layer  221  of the bonding layer  22 . 
     The inner segment of the second bonding sub-layer  222  of the bonding layer  22  may have a thickness ranging from 1 nm to 500 nm. The second bonding sub-layer  222  of the bonding layer  22  serves to enhance the adhesion of the first bonding sub-layer  221  of the bonding layer  22  to the light-emitting layer, such that the substrate  21  underneath the first bonding sub-layer  221  of the bonding layer  22  can be sufficiently connected with the light-emitting layer. 
     The inner segment of the first bonding sub-layer  221  of the bonding layer  22  may have a thickness greater than 1 μm, for instance, a thickness greater than 2 μm, but may not be greater than 5 um. 
     Referring to  FIG. 12 , the present disclosure provides a third embodiment of the flip-chip light emitting device, which is similar to the second embodiment of the flip-chip light emitting device, except for the following difference. 
     The outer section of the bonding layer  22  has only the first bonding sub-layer  221 . The first bonding sub-layer  221  of the bonding layer  22  has the first upper surface  30  of the bonding layer  22 , and contacts the protective insulating layer  26 . 
     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.