An Inverted Micro Light-Emitting Diode and Its Manufacturing Method

Abstract

The present invention discloses an inverted micro light-emitting diode and a preparation method thereof. The inverted micro light-emitting diode includes a plurality of light-emitting units arranged in a matrix. Each light-emitting unit includes a main structure and a light-emitting surface structure stacked along the light-emitting direction of the inverted micro light-emitting diode. The main structure includes an n-type semiconductor structure, and the n-type semiconductor structure is integrally formed with the light-emitting surface structure. The light-emitting surface structure includes an inclined surface structure, and the inclined surface structure includes n-level inclined surfaces distributed from the edge of the n-type semiconductor structure to the center of the n-type semiconductor structure, where n is greater than or equal to 2. The angle between the inclined surface and the horizontal plane gradually decreases along the light-emitting direction. In the inverted micro light-emitting diode of the present invention, the multi-segment inclined surfaces adopted can reduce total internal reflection, increase the possibility of light emission, and allow more light to escape, thereby improving the top light extraction efficiency.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and particularly to an inverted micro light-emitting diode capable of improving the top light extraction efficiency and a preparation method thereof. Background Art

[0002] Micro light-emitting diodes (Micro LED) have the advantages of high peak brightness, long service life, low power consumption, high resolution, etc., and have gradually shown an application prototype in wearable devices, AR / VR devices, vehicle-mounted display devices, and micro projectors. However, due to the influence of total internal reflection (TIR) and Fresnel reflection, the light extraction efficiency of micro-LEDs is relatively low, especially the light extraction efficiency in the vertical direction decreases as the size of the micro-LED gradually decreases.

[0003] In view of this, many solutions have been proposed to solve the problem of low light extraction efficiency of micro-LEDs, including setting anti-reflection films, surface micro-nano structures, and microlens arrays. However, although depositing an anti-reflection film on the surface of a micro-LED to improve the light effect is a relatively convenient method, it requires high-precision coating equipment; the method of setting surface micro-nano structures requires relatively complex structure design, and may also require high-precision equipment and complex process technology to fabricate the micro-structures; while the microlens array is added on the micro-LED chip, and the microlens array is integrated into the micro-LED to improve the top light extraction efficiency. However, at present, the chip size of the micro-LED is continuously decreasing, the integration difficulty is increasing, the required alignment accuracy is also increasing, and a suitable bonding scheme also needs to be considered. Summary of the Invention

[0004] In view of this, the object of the present invention is to provide an improved inverted micro light-emitting diode, which can effectively improve the top light extraction efficiency of the micro light-emitting diode.

[0005] The present invention provides an inverted micro light-emitting diode, including a plurality of light-emitting units arranged in a matrix. Each light-emitting unit includes a main structure and a light-emitting surface structure stacked along the light-emitting direction of the inverted micro light-emitting diode. The main structure includes an n-type semiconductor structure, and the n-type semiconductor structure is integrally formed with the light-emitting surface structure; the light-emitting surface structure includes an inclined surface structure, and the inclined surface structure includes n-level inclined surfaces distributed from the edge of the n-type semiconductor structure to the center of the n-type semiconductor structure, where n is greater than or equal to 2; the angle between the inclined surface and the horizontal plane gradually decreases along the light-emitting direction.

[0006] According to some preferred implementation aspects of the present invention, the n-type semiconductor structure has the same material as the light-emitting surface structure, and the vertical height of the inclined surface gradually decreases along the light-emitting direction.

[0007] According to some preferred implementation aspects of the present invention, the light-emitting surface structure further includes a sidewall structure, and the sidewall structure is disposed between the n-type semiconductor structure and the inclined surface structure along the light-emitting direction; the height of the sidewall structure is greater than or equal to 100 nm.

[0008] According to some preferred implementation aspects of the present invention, the light-emitting surface structure includes a first plane located between adjacent inclined surfaces, and the length of the first plane is 0.01 - 2 μm.

[0009] According to some preferred implementation aspects of the present invention, the light-emitting surface structure includes a second plane located at the center of the light-emitting surface structure, and the length of the second plane is 37% - 64% of the length of the micro light-emitting diode.

[0010] According to some preferred implementation aspects of the present invention, the maximum thickness of the light-emitting surface structure is 1.2 - 1.5 μm.

[0011] According to some preferred implementation aspects of the present invention, the size of the micro light-emitting diode is 5 - 20 μm, n is less than or equal to 3, that is, preferably, the light-emitting surface structure of the flip-chip micro light-emitting diode with a main body structure size of 5 μm - 20 μm has three-level inclined surfaces.

[0012] According to some preferred implementation aspects of the present invention, the n-level inclined surface includes a first-level inclined surface, a second-level inclined surface, and a third-level inclined surface. The angle between the first-level inclined surface and the horizontal plane is 45° - 80°, the angle between the second-level inclined surface and the horizontal plane is 30° - 60°, and the angle between the third-level inclined surface and the horizontal plane is 20° - 45°.

[0013] According to some preferred implementation aspects of the present invention, the height of the first-level inclined surface is greater than the height of the second-level inclined surface, and the height of the second-level inclined surface is greater than the height of the third-level inclined surface.

[0014] Preferably, the height of the first-level inclined surface is 300 nm - 500 nm, the height of the second-level inclined surface is 200 nm - 400 nm; the height of the third-level inclined surface is 100 nm - 300 nm.

[0015] The present invention also provides a method for manufacturing a flip-chip micro light-emitting diode as described above, including the following steps:

[0016] Provide a substrate, and prepare an epitaxial layer on the substrate; wherein, the epitaxial layer includes a first n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer stacked.

[0017] Etch the epitaxial layer to form a second n-type semiconductor layer and a plurality of spaced-apart body structures; wherein, each body structure includes an n-type semiconductor structure, a light-emitting structure, and a p-type semiconductor structure stacked on the second n-type semiconductor layer in sequence; the second n-type semiconductor layer is the remaining part of the first n-type semiconductor layer after etching away the n-type semiconductor structure;

[0018] Remove the substrate;

[0019] Perform multiple etching processes on the second n-type semiconductor layer to obtain the light-emitting surface structure, so as to fabricate a micro light-emitting diode.

[0020] In some embodiments of the present invention, the method for fabricating a flip-chip micro light-emitting diode specifically includes the following steps:

[0021] Step S1: Use the MOCVD process to grow an epitaxial layer on a substrate, and the epitaxial layer sequentially includes a first n-type semiconductor layer (n-GaN layer), a light-emitting layer (MQW layer), a p-type semiconductor layer (p-GaN layer), and an ITO layer;

[0022] Step S2: Etch the epitaxial layer to form a second n-type semiconductor layer and a plurality of spaced-apart body structures; wherein, each body structure includes an n-type semiconductor structure, a light-emitting structure, and a p-type semiconductor structure stacked on the second n-type semiconductor layer in sequence; the second n-type semiconductor layer is the remaining part of the first n-type semiconductor layer after etching away the n-type semiconductor structure; the overall etching depth is 1 - 2 μm, and the size of the body structure is 5 - 20 μm;

[0023] Step S3: Use PECVD to prepare a silicon oxide insulating layer outside the body structure;

[0024] Step S4: Prepare an N electrode and a P electrode by EBE electron beam thermal evaporation, the N electrode is located between adjacent body structures, and the P electrode is located on the ITO layer;

[0025] Step S5: Use the laser lift-off technology to remove the substrate;

[0026] Step S6: Repeat the photolithography process on the second n-type semiconductor layer to form an n-level inclined surface, obtain the light-emitting surface structure, and finally obtain a micro light-emitting diode.

[0027] According to some preferred implementation aspects of the present invention, the performing multiple etching processes on the second n-type semiconductor layer to obtain the light-emitting surface structure includes the following steps:

[0028] Coat a photoresist layer on the surface of the second n-type semiconductor layer and perform photolithography to form a current pattern;

[0029] Using the current pattern as a mask, etch the second n-type semiconductor layer to form an i-th level inclined surface on the surface of the second n-type semiconductor layer; where 1 ≤ i ≤ n;

[0030] Remove the photoresist layer;

[0031] Repeat the above steps to form n-level inclined surfaces on the surface of the second n-type semiconductor layer, obtaining the light-emitting surface structure.

[0032] In the present invention, preferably n of the n-level inclined surfaces is 3, that is, the n-level inclined surfaces include a first-level inclined surface, a second-level inclined surface, and a third-level inclined surface. Obtaining the light-emitting surface structure by performing multiple etching processes on the second n-type semiconductor layer includes the following steps:

[0033] Coat a first photoresist layer on the surface of the second n-type semiconductor layer and perform photolithography to form a first pattern;

[0034] Use a plasma etching process to transfer the first pattern to the second n-type semiconductor layer, so as to form a first-level inclined surface on the surface of the second n-type semiconductor layer;

[0035] Remove the first photoresist layer;

[0036] Coat a second photoresist layer on the surface of the second n-type semiconductor layer having a first-level inclined surface and perform photolithography to form a second pattern;

[0037] Use a plasma etching process to transfer the second pattern to the second n-type semiconductor layer having a first-level inclined surface, so as to form a second-level inclined surface on the surface of the second n-type semiconductor layer having a first-level inclined surface;

[0038] Remove the second photoresist layer;

[0039] Coat a third photoresist layer on the surface of the second n-type semiconductor layer having a first-level inclined surface and a second-level inclined surface and perform photolithography to form a third pattern;

[0040] Use a plasma etching process to transfer the third pattern to the second n-type semiconductor layer having a first-level inclined surface and a second-level inclined surface, so as to form a third-level inclined surface on the surface of the second n-type semiconductor layer having a first-level inclined surface and a second-level inclined surface, obtaining the light-emitting surface structure.

[0041] In some embodiments, obtaining the light-emitting surface structure by performing multiple etching processes on the second n-type semiconductor layer specifically includes the following steps:

[0042] Step S61: Coat a first photoresist layer on the surface of the second n-type semiconductor layer and perform photolithography to form a first pattern;

[0043] Step S62: Transfer the first pattern to the second n-type semiconductor layer using a plasma etching process, forming a first-level inclined surface on the surface of the second n-type semiconductor layer, with an etching depth of 300 nm - 500 nm, preferably 400 nm, and the angle between the first-level inclined surface and the horizontal plane being 45° - 80°;

[0044] Step S63: Remove the first photoresist layer; coat a second photoresist layer on the surface of the second n-type semiconductor layer with the first-level inclined surface and perform lithography to form a second pattern;

[0045] Step S64: Transfer the second pattern to the second n-type semiconductor layer with the first-level inclined surface using a plasma etching process, forming a second-level inclined surface on the surface of the second n-type semiconductor layer with the first-level inclined surface, with an etching depth of 200 nm - 400 nm, preferably 300 nm, which is less than the etching depth of the first-level inclined surface; the angle between the second-level inclined surface and the horizontal plane is 30° - 60°, which is less than the angle between the first-level inclined surface and the horizontal plane;

[0046] Step S65: Remove the second photoresist layer; coat a third photoresist layer on the surface of the second n-type semiconductor layer with the first-level inclined surface and the second-level inclined surface and perform lithography to form a third pattern;

[0047] Step S66: Transfer the third pattern to the second n-type semiconductor layer with the first-level inclined surface and the second-level inclined surface using a plasma etching process, forming a third-level inclined surface on the surface of the second n-type semiconductor layer with the first-level inclined surface and the second-level inclined surface. The etching depth is 100 nm - 300 nm, preferably 200 nm, and the etching depth is less than the etching depth of the second-level inclined surface; the angle between the second-level inclined surface and the horizontal plane is 20° - 45°, which is less than the angle between the second-level inclined surface and the horizontal plane.

[0048] Thus, a light-emitting surface structure with three levels of inclined surfaces is obtained on the second n-type semiconductor layer.

[0049] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art. The flip-chip micro light-emitting diode of the present invention adopts multiple inclined surfaces, which can reduce total internal reflection, increase the possibility of light emission, and avoid most of the light emitted from the light-emitting layer being absorbed by the semiconductor layer or the electrode layer after total internal reflection and returning to the medium. More light can escape, thereby improving the top light extraction efficiency; moreover, there is no need to design and fabricate a microlens array additionally, reducing the subsequent alignment and gluing processes with the micro-LED. The process preparation is relatively convenient, simple and feasible, and the yield is high. Description of the Drawings

[0050] To more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the accompanying drawings required for the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0051] Figure 1 is a schematic diagram of light extraction from the top of a micro light-emitting diode;

[0052] Figure 2 is a schematic structural diagram of a flip-chip micro light-emitting diode provided by an embodiment of the present invention;

[0053] Figure 3 is a three-dimensional schematic diagram of the top surface light extraction structure of a flip-chip micro light-emitting diode provided by an embodiment of the present invention;

[0054] Figure 4 is a cross-sectional schematic diagram of the top surface light extraction structure of a flip-chip micro light-emitting diode provided by an embodiment of the present invention;

[0055] Figure 5 is a schematic diagram of the manufacturing process of a flip-chip micro light-emitting diode provided by an embodiment of the present invention;

[0056] Figure 6 is a schematic diagram of the manufacturing process of the top surface light extraction structure of a flip-chip micro light-emitting diode provided by an embodiment of the present invention;

[0057] Figure 7 is a schematic structural diagram of a mask plate when manufacturing the top surface light extraction structure provided by Embodiment 6 of the present invention;

[0058] Figure 8 is a comparison diagram of the results of the top LEE of the micro-LEDs of the embodiments and comparative examples of the present invention;

[0059] The reference numerals include: flip-chip micro light-emitting diode - 1, light-emitting unit - 2, main body structure - 3, light extraction surface structure - 4, substrate - 5, first n-type semiconductor layer - 6, light-emitting layer - 7, p-type semiconductor layer - 8, ITO layer - 9, second n-type semiconductor layer - 10, n-type semiconductor structure - 11, silicon oxide insulating layer - 12, N electrode - 13, P electrode - 14, sidewall structure - 15, first-stage inclined surface - 16, second-stage inclined surface - 17, third-stage inclined surface - 18, first plane - 19, second plane - 20, photoresist layer - 21, mask plate - 22, light-shielding region - 23. Detailed Embodiments

[0060] To enable those skilled in the art to better understand the solution of the present invention, the following will clearly and completely describe the technical solution in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0061] For traditional GaN-based micro light-emitting diodes, the light emitted from the active layer passes through a semiconductor material (a medium with a large refractive index) and exits into the air (a medium with a small refractive index). Only light at a certain angle can escape into the air, and most of the light returns to the medium after total internal reflection and is absorbed by the light-emitting layer or the electrode inside. As shown in Figure (a) below, that is, only the light within the critical angle can escape into the air, and the critical angle between the GaN material and the air is 23.4°. The flip-chip micro light-emitting diode provided by the present invention can increase the probability of some light exiting from the inclined surface and allow more light to escape by etching multiple inclined surfaces on the surface, thereby increasing the probability of light exiting from the surface and further improving the light extraction efficiency at the top, as shown in Figure (b) below. The following will provide a detailed description of the flip-chip micro light-emitting diode provided by the present invention. Figure 1 As shown in Figure (a) below, that is, only the light within the critical angle can escape into the air, and the critical angle between the GaN material and the air is 23.4°. The flip-chip micro light-emitting diode provided by the present invention can increase the probability of some light exiting from the inclined surface and allow more light to escape by etching multiple inclined surfaces on the surface, thereby increasing the probability of light exiting from the surface and further improving the light extraction efficiency at the top, as shown in Figure (b) below. The following will provide a detailed description of the flip-chip micro light-emitting diode provided by the present invention. Figure 1 As shown in Figure (b) below. The following will provide a detailed description of the flip-chip micro light-emitting diode provided by the present invention.

[0062] Please refer to Figure 2 , Figure 2 which is a schematic structural diagram of a flip-chip micro light-emitting diode provided by an embodiment of the present invention. The flip-chip micro light-emitting diode 1 includes a plurality of light-emitting units 2 arranged in a matrix.

[0063] Each light-emitting unit 2 includes a main body structure 3 and a light-emitting surface structure 4 stacked along the light-emitting direction of the flip-chip micro light-emitting diode 1. The main body structure 3 includes an n-type semiconductor structure 11, and the n-type semiconductor structure 11 is integrally formed with the light-emitting surface structure 4.

[0064] Furthermore, as shown in Figure 3 and Figure 4 , the light-emitting surface structure 4 includes an inclined surface structure. The inclined surface structure includes n levels of inclined surfaces distributed from the edge of the n-type semiconductor structure 11 to the center of the n-type semiconductor structure 11, where n is greater than or equal to 2; the angle between the inclined surface and the horizontal plane gradually decreases along the light-emitting direction. Figure 3 Figure (a) below shows the light-emitting surface structure 4 in the shape of a frustum pyramid, Figure 3 and Figure (b) below shows the light-emitting surface structure 4 in the shape of a truncated cone pyramid.

[0065] In one embodiment, n is less than or equal to 3.

[0066] In one embodiment, the n-type semiconductor structure 11 and the light-emitting surface structure 4 are made of the same material, and the vertical height of the inclined surface gradually decreases along the light-emitting direction.

[0067] In one embodiment, as Figure 4 shown, the light-emitting surface structure 4 further includes a sidewall structure 15, and the sidewall structure 15 is disposed between the n-type semiconductor structure 11 and the inclined surface structure along the light-emitting direction; the height of the sidewall structure 15 is greater than or equal to 100 nm.

[0068] In one embodiment, the light-emitting surface structure 4 includes a first plane 19 located between adjacent inclined surfaces, and the length of the first plane 19 is 0.01 - 2 μm. The provision of the first plane 19 can increase the light-emitting area, further improve the light-emitting probability, and thus improve the light extraction efficiency at the top.

[0069] In one embodiment, the light-emitting surface structure 4 includes a second plane 20 located at the center of the light-emitting surface structure 4, and the length of the second plane 20 is 37% - 64% of the length of the micro light-emitting diode. For example, the length of the second plane 20 is 37%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 56%, 58%, 60%, 62% or 64% of the length of the micro light-emitting diode. The length of the micro light-emitting diode is the size of the micro light-emitting diode, and more specifically, the size of the main structure of the micro light-emitting diode.

[0070] In one embodiment, the maximum thickness of the light-emitting surface structure 4 is 1.2 - 1.5 μm, and the maximum thickness of the light-emitting surface structure 4 is the height of the light-emitting surface structure 4 in the light-emitting direction. For example, the maximum thickness of the light-emitting surface structure 4 can be 1.2 μm, 1.22 μm, 1.24 μm, 1.26 μm, 1.28 μm, 1.3 μm, 1.32 μm, 1.34 μm, 1.36 μm, 1.38 μm, 1.4 μm, 1.42 μm, 1.44 μm, 1.46 μm, 1.48 μm or 1.5 μm.

[0071] In one embodiment, the n-level inclined surface includes a first-level inclined surface 16, a second-level inclined surface 17, and a third-level inclined surface 18. The angle between the first-level inclined surface 16 and the horizontal plane is 45° - 80°, for example: 45°, 55°, 60°, 65°, 70°, 75° or 80°; the angle between the second-level inclined surface 17 and the horizontal plane is 30° - 60°, for example: 30°, 35°, 40°, 45°, 50°, 55° or 60°; the angle between the third-level inclined surface 18 and the horizontal plane is 20° - 45°, for example: 20°, 25°, 30°, 35°, 40° or 45°.

[0072] Further, the height of the first - stage inclined surface 16 is greater than the height of the second - stage inclined surface 17, and the height of the second - stage inclined surface 17 is greater than the height of the third - stage inclined surface 18.

[0073] Further, the height of the first - stage inclined surface 16 is 300 nm - 500 nm, the height of the second - stage inclined surface 17 is 200 nm - 400 nm; the height of the third - stage inclined surface 18 is 100 nm - 300 nm. Preferably, the height of the first - stage inclined surface 16 is 400 nm, the height of the second - stage inclined surface 17 is 300 nm, and the height of the third - stage inclined surface 18 is 200 nm.

[0074] In one embodiment, the size of the micro - light - emitting diode is 5 - 20 μm. For example, the size of the micro - light - emitting diode is 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm.

[0075] As Figures 2 - 6 shown, the present invention also provides a method for manufacturing the above - mentioned flip - chip micro - light - emitting diode 1, which specifically includes the following steps:

[0076] Step S1: Provide a substrate 5, and use the Metal - organic Chemical Vapor Deposition (MOCVD) process to grow an epitaxial layer on the substrate 5. The epitaxial layer sequentially includes a first n - type semiconductor layer 6 (which can be an n - GaN layer), a light - emitting layer 7 (i.e., a multi - quantum well layer), a p - type semiconductor layer 8 (which can be a p - GaN layer), and a conductive layer (which can be an ITO layer 9).

[0077] The material of the substrate 5 is preferably sapphire or gallium nitride.

[0078] Step S2: Etch the epitaxial layer so that the epitaxial layer forms a second n - type semiconductor layer 10 and a plurality of spaced - apart main structures 3.

[0079] The overall etching depth is 1 - 2 μm. The size of the main structure 3 is 5 - 20 μm. The main structure 3 includes an n - type semiconductor structure 11, a light - emitting structure, and a p - type semiconductor structure that are sequentially stacked on the second n - type semiconductor layer 10. The second n - type semiconductor layer 10 is the remaining part of the first n - type semiconductor layer 6 after removing the n - type semiconductor structure 11 by etching.

[0080] Step S3: Use plasma enhanced chemical vapor deposition (PECVD) to prepare a silicon oxide insulating layer 12 outside the main structure 3.

[0081] A window for subsequent preparation of the P electrode 14 is left on the silicon oxide insulating layer 12.

[0082] Step S4: Prepare the N electrode 13 and the P electrode 14 by electron beam evaporation (EBE). The N electrode 13 is located between adjacent main structures 3 and is connected to the second n-type semiconductor layer 10. The P electrode 14 is located on the ITO layer 9 and penetrates through the silicon oxide insulating layer 12 and is connected to the conductive layer.

[0083] Step S5: Completely remove the substrate 5 by using the laser lift-off technology.

[0084] Step S6: Perform photolithography on the side of the second n-type semiconductor layer 10 away from the main structure 3 multiple times to form an n-level inclined surface, obtain the light-emitting surface structure 4, and finally obtain the micro light-emitting diode.

[0085] Preferably, as Figures 3 to 6 shown, n of the n-level inclined surface is 3, that is, the n-level inclined surface includes a first-level inclined surface 16, a second-level inclined surface 17, and a third-level inclined surface 18. Perform multiple etching processes on the second n-type semiconductor layer 10 to obtain the light-emitting surface structure 4, which specifically includes the following steps:

[0086] Step S61: Coat a first photoresist layer 21 on the surface of the second n-type semiconductor layer 10 and perform photolithography to form a first pattern.

[0087] Step S62: Use the plasma etching process to transfer the first pattern to the second n-type semiconductor layer 10, so that a first-level inclined surface 16 is formed on the surface of the second n-type semiconductor layer 10.

[0088] The etching depth is 300 nm - 500 nm, preferably 400 nm. The angle between the first-level inclined surface 16 and the horizontal plane is 45° - 80°.

[0089] Step S63: Remove the first photoresist layer 21; coat a second photoresist layer 21 on the surface of the second n-type semiconductor layer 10 with the first-level inclined surface 16 and perform photolithography to form a second pattern.

[0090] Step S64: Use the plasma etching process to transfer the second pattern to the second n-type semiconductor layer 10 with the first-level inclined surface 16, so that a second-level inclined surface 17 is formed on the surface of the second n-type semiconductor layer 10 with the first-level inclined surface 16.

[0091] The etching depth is 200 nm - 400 nm, preferably 300 nm, and the etching depth is less than that of the first-level inclined surface 16. The angle between the second-level inclined surface 17 and the horizontal plane is 30° - 60°, which is less than the angle between the first-level inclined surface 16 and the horizontal plane.

[0092] Step S65: Remove the second photoresist layer 21; coat a third photoresist layer 21 on the surface of the second n-type semiconductor layer 10 having the first-level inclined surface 16 and the second-level inclined surface 17 and perform photolithography to form a third pattern.

[0093] Step S66: Use a plasma etching process to transfer the third pattern to the second n-type semiconductor layer 10 having the first-level inclined surface 16 and the second-level inclined surface 17, so that a third-level inclined surface 18 is formed on the surface of the second n-type semiconductor layer 10 having the first-level inclined surface 16 and the second-level inclined surface 17.

[0094] The etching depth is 100 nm - 300 nm, preferably 200 nm, and the etching depth is less than the etching depth of the second-level inclined surface 17; the angle between the second-level inclined surface 17 and the horizontal plane is 20° - 45°, which is less than the angle between the second-level inclined surface 17 and the horizontal plane. Thus, a light-emitting surface structure 4 with three-level inclined surfaces is obtained on the second n-type semiconductor layer 10.

[0095] In some other embodiments, the above steps can be repeated to form a light-emitting surface structure 4 with more inclined surfaces where n > 3; or the number of repetitions of the above steps can be reduced to form a light-emitting surface structure 4 with two-level inclined surfaces where n = 2. Preferably, the light-emitting surface structure 4 of the flip-chip micro light-emitting diode 1 with a main body structure 3 sized 5 μm - 20 μm has three-level inclined surfaces (n = 3), and the light-emitting surface structure 4 of the flip-chip micro light-emitting diode 1 with a main body structure 3 sized 20 μm - 30 μm has four-level inclined surfaces (n = 4), and so on. The larger the size of the main body structure 3, the larger n is. Embodiment 1

[0096] Please refer to Figures 1 to 4 , the flip-chip micro light-emitting diode 1 provided in this embodiment includes a plurality of light-emitting units 2 arranged in a matrix. Each light-emitting unit 2 includes a main body structure 3 and a light-emitting surface structure 4 stacked along the light-emitting direction of the flip-chip micro light-emitting diode 1. The main body structure 3 includes an n-type semiconductor structure 11, and the n-type semiconductor structure 11 is integrally formed with the light-emitting surface structure 4. The n-type semiconductor structure 11 and the light-emitting surface structure 4 are made of the same material.

[0097] The maximum thickness of the light-emitting surface structure 4 is 1.38 μm, and the maximum thickness of the light-emitting surface structure 4 is the height of the light-emitting surface structure 4 in the light-emitting direction. The light-emitting surface structure 4 includes an inclined surface structure, a side wall structure 15, a first plane 19 located between adjacent inclined surfaces, and a second plane 20 located at the center of the light-emitting surface structure 4. The side wall structure 15 is disposed between the n-type semiconductor structure 11 and the inclined surface structure along the light-emitting direction, and the height of the side wall structure 15 is equal to 150 nm. The length of the first plane 19 is 0.5 μm. The setting of the first plane 19 can increase the light-emitting area, further improve the light-emitting probability, and then improve the light extraction efficiency at the top. Preferably, the length of the second plane 20 is 37% of the length of the micro light-emitting diode.

[0098] The inclined surface structure includes n-level inclined surfaces distributed from the edge of the n-type semiconductor structure 11 to the center of the n-type semiconductor structure 11. In this embodiment, n is equal to 2. That is, the n-level inclined surfaces include a first-level inclined surface 16 and a second-level inclined surface 17. The angle between the first-level inclined surface 16 and the horizontal plane is 45°; the angle between the second-level inclined surface 17 and the horizontal plane is 30°, and the angle between the inclined surface and the horizontal plane gradually decreases along the light-emitting direction.

[0099] The height of the first-level inclined surface 16 is 400 nm, and the height of the second-level inclined surface 17 is 300 nm. The vertical height of the inclined surface gradually decreases along the light-emitting direction. Embodiment 2

[0100] In this embodiment, the structure of the flip-chip micro light-emitting diode 1 is basically the same as that in Embodiment 1, except that in the light-emitting surface structure 4 of this embodiment, the angle between the first-level inclined surface 16 and the horizontal plane is 80°; the angle between the second-level inclined surface 17 and the horizontal plane is 60°. Embodiment 3

[0101] As Figures 5 - 6 shown, in this embodiment, the second n-type semiconductor layer 10 is etched multiple times to obtain the light-emitting surface structure 4, which specifically includes the following steps:

[0102] Step S61: Coat a first photoresist layer 21 on the surface of the second n-type semiconductor layer 10 and perform the first photolithography to form a first pattern.

[0103] Step S62: Use a plasma etching process to transfer the first pattern to the second n-type semiconductor layer 10, so that a first-level inclined surface 16 is formed on the surface of the second n-type semiconductor layer 10.

[0104] The etching depth corresponding to the plasma etching process is 400 nm, and the angle between the first-level inclined surface 16 and the horizontal plane is 45°.

[0105] Step S63: Remove the first photoresist layer 21; coat a second photoresist layer 21 on the surface of the second n-type semiconductor layer 10 with the first-level inclined surface 16 and perform second photolithography to form a second pattern.

[0106] During the second photolithography, the exposure time is increased, so that the area covered by the photoresist is reduced.

[0107] Step S64: Adjust the gas flow rate and its proportion, and use the plasma etching process to transfer the second pattern to the second n-type semiconductor layer 10 with the first-level inclined surface 16, so that a second-level inclined surface 17 is formed on the surface of the second n-type semiconductor layer 10 with the first-level inclined surface 16.

[0108] The etching depth is 300 nm, and the etching depth is less than that of the first-level inclined surface 16; the angle between the second-level inclined surface 17 and the horizontal plane is 30°.

[0109] Step S65: Remove the second photoresist layer 21; coat a third photoresist layer 21 on the surface of the second n-type semiconductor layer 10 with the first-level inclined surface 16 and the second-level inclined surface 17 and perform third photolithography to form a third pattern.

[0110] During the third photolithography, the exposure time is continuously increased, so that the area covered by the photoresist is continuously reduced.

[0111] Step S66: Adjust the gas flow rate and its proportion, and use the plasma etching process to transfer the third pattern to the second n-type semiconductor layer 10 with the first-level inclined surface 16 and the second-level inclined surface 17, so that a third-level inclined surface 18 is formed on the surface of the second n-type semiconductor layer 10 with the first-level inclined surface 16 and the second-level inclined surface 17.

[0112] The etching depth corresponding to the plasma etching process is 200 nm, which is less than the etching depth of the second-level inclined surface 17; the angle between the third-level inclined surface 18 and the horizontal plane is 20°, which is less than the angle between the second-level inclined surface 17 and the horizontal plane. Thus, a light-emitting surface structure 4 with three-level inclined surfaces is obtained on the second n-type semiconductor layer 10. Example 4

[0113] The difference between this example and Example 3 is that the angles between the inclined surfaces on the light-emitting surface structure 4 and the horizontal plane are different. In this example, the angle between the first-level inclined surface 16 and the horizontal plane is 80°, the angle between the second-level inclined surface 17 and the horizontal plane is 60°, and the angle between the third-level inclined surface 18 and the horizontal plane is 45°. The remaining structures and preparation steps are basically the same as those in Example 3 and will not be elaborated here. Example 5

[0114] This example is a preparation method of a flip-chip micro light-emitting diode 1 with a multi-level inclined surface light-emitting structure, including the following steps:

[0115] Step S1: Provide a sapphire substrate 5, and grow an epitaxial layer on the substrate 5 using the MOCVD process. The epitaxial layer sequentially includes a first n-type semiconductor layer 6 with a thickness of 5 μm and a material of n-GaN, a multi-quantum well layer with a thickness of 100 nm, a p-type semiconductor layer 8 with a thickness of 100 nm and a material of p-GaN, and an ITO layer 9 with a thickness of 100 nm.

[0116] Step S2: Perform ICP dry etching on the epitaxial layer so that the epitaxial layer forms a second n-type semiconductor layer 10 and a plurality of main structures 3 arranged at intervals.

[0117] The etching depth is 2 μm. The size of the main structure 3 is 10 μm; the main structure 3 includes an n-type semiconductor structure 11, a light-emitting structure, and a p-type semiconductor structure sequentially stacked on the second n-type semiconductor layer 10. The second n-type semiconductor layer 10 is the remaining part of the first n-type semiconductor layer 6 after removing the n-type semiconductor structure 11 by etching.

[0118] Step S3: Prepare a silicon oxide insulating layer 12 outside the main structure 3 using PECVD, and a window for subsequently preparing the P electrode 14 is left on the silicon oxide insulating layer 12.

[0119] Step S4: Prepare the N electrode 13 and the P electrode 14 by EBE electron beam thermal evaporation. The N electrode 13 is located between adjacent main structures 3, and the P electrode 14 is located on the ITO layer 9 and penetrates through the silicon oxide insulating layer 12.

[0120] Step S5: Completely remove the substrate 5 using the laser lift-off technique.

[0121] Step S6: Perform multiple photolithographies on the side of the second n-type semiconductor layer 10 away from the main structure 3 to form a three-stage inclined surface, obtain a light-emitting surface structure 4, and finally obtain a micro light-emitting diode.

[0122] Step S6 adopts the method in Embodiment 3, which will not be elaborated here. Embodiment 6

[0123] The etching method for forming a multi-stage inclined surface light-emitting structure on the light-emitting surface of the flip-chip micro light-emitting diode 1 in this embodiment is different from that in Embodiment 3 in that: as Figure 7 shown, in the second photolithography in Step S63 and the third photolithography in Step S65 of this embodiment, the light-shielding area 23 of the mask 22 corresponding to the preparation of the second-stage inclined surface 17 and the third-stage inclined surface 18 is gradually reduced without changing the exposure time of the photolithography, achieving the same purpose as in Embodiment 3. The remaining steps are basically the same as those in Embodiment 3 and will not be elaborated here. Figure 7In (a) is a schematic structural diagram of the mask plate 22 during the first photolithography, (b) is a schematic structural diagram of the mask plate 22 during the second photolithography, and (c) is a schematic structural diagram of the mask plate 22 during the third photolithography. Comparative Example 1

[0124] In this comparative example, the structure of the flip-chip micro light-emitting diode 1 is basically the same as that of Example 1. The difference is that in the light-emitting surface structure 4 of this comparative example, there is no inclined surface and the first plane 19, the entire light-emitting surface structure 4 is rectangular, and the top of the light-emitting surface structure 4 is a plane. Comparative Example 2

[0125] In this comparative example, the structure of the flip-chip micro light-emitting diode 1 is basically the same as that of Example 1. The difference is that in the light-emitting surface structure 4 of this comparative example, n of the n-level inclined surface is 1, that is, there is only the first-level inclined surface 16, and the included angle between the first-level inclined surface 16 and the horizontal plane is 45°. Comparative Example 3

[0126] In this comparative example, the structure of the flip-chip micro light-emitting diode 1 is basically the same as that of Example 1. The difference is that in the light-emitting surface structure 4 of this comparative example, n of the n-level inclined surface is 1, that is, there is only the first-level inclined surface 16, and the included angle between the first-level inclined surface 16 and the horizontal plane is 80°. Comparative Example 4

[0127] In this comparative example, the structure of the flip-chip micro light-emitting diode 1 is basically the same as that of Example 1. The difference is that the entire light-emitting surface structure 4 of this comparative example is hemispherical, and there is no inclined surface and the first plane 19.

[0128] Simulation and Results

[0129] The software based on the finite-difference time-domain (FDTD) method was used to perform simulation calculations on the top light extraction efficiency (Light Extraction Efficiency, LEE) of the surfaces of the flip-chip micro-LEDs in the above examples and comparative examples. Examples 1 and 3 are relatively gentle multi-level inclined surface schemes without strong steepness; Examples 2 and 4 are micro-LED structural models with larger multi-level inclined surface angles and better steepness. The calculation results of the top light extraction efficiency of the two are as Figure 8 shown, where Figure 8 In (a) is the top light extraction efficiency result of the flip-chip micro-LEDs in Example 1, Example 3, Comparative Example 1, Comparative Example 2, and Comparative Example 4, Figure 8 In (b) is the top light extraction efficiency result of the flip-chip micro-LEDs in Example 2, Example 4, Comparative Example 1, Comparative Example 3, and Comparative Example 4.

[0130] Figure 8 The results show that the light-emitting surface structures 4 in Examples 1-4 can effectively improve the light extraction efficiency at the top of the flip-chip micro light-emitting diode 1. The inclined surface angles of Example 1 and Example 3 are gentler than those of Example 2 and Example 4. The top LEE of Example 1 and Example 2 with two inclined surfaces is basically the same, and it is also significantly better than the hemispherical light-emitting surface structure of Comparative Example 4. That is, when only etching twice to form two inclined surfaces, the improvement effect of the top LEE of the flip-chip micro-LED is basically the same, but the included angle between the inclined surface of Example 2 and the horizontal plane is larger, and it is easier to control during preparation.

[0131] The top LEE of Example 3 and Example 4 with three inclined surfaces is further improved, showing a significant improvement compared to the hemispherical light-emitting surface structure of Comparative Example 4; and the effect of Example 3 is significantly better than that of Example 4, reaching 1.43 times that of Comparative Example 4. That is, when performing three etching operations, the small-angle three-inclined surface structure of Example 3 can obtain a larger top LEE, but the process of the small-angle multi-inclined surface is more difficult to implement than that of Example 4, and the inclination angle is not easy to control. Example 4 is easier to prepare.

[0132] The present invention does not involve the design and manufacture of any microlenses, and there is no need to add optical lenses or optical elements on the light-emitting surface to focus light and direct it. It does not add other optical elements on the plane additionally to achieve the function. It is realized through traditional photolithography and etching processes on the original basis, and can also achieve the improvement of light extraction efficiency. The light-emitting surface design of multiple inclined surfaces in the present invention is not a circular arc-shaped light-emitting surface, but a light-emitting surface with multiple inclined surfaces, which is different from spherical or arc-shaped surfaces, and the effect is better than spherical or arc-shaped surfaces. The multiple inclined surfaces improve the light extraction efficiency at the top according to the principle of light refraction and increasing the light-emitting area, thereby improving the light output efficiency. And the light-emitting surface structure of the present invention is in the semiconductor gallium nitride layer, so multiple inclined surfaces are etched on the surface of gallium nitride, so it is not a substrate material. Compared with the prior art, the present invention has the following beneficial effects:

[0133] 1. The present invention directly prepares a multi-inclined light-emitting surface on the GaN surface of the micro-LED, without the need to additionally design complex surface micro-nano structures to improve the light output efficiency. Such multi-inclined surfaces correspond one-to-one with the micro-LED pixels. The light output from such multi-inclined surfaces is more convenient and feasible in the process preparation, and the effect is better than that achieved by arc-shaped or hemispherical shapes. At the same time, the chip structure is designed as a flip-chip structure, which is more conducive to surface light output; arc-shaped and hemispherical light output are usually difficult to be formed in one step, and the yield is low. Especially, the hemispherical surface increases the light crosstalk between adjacent pixels.

[0134] 2. The multi-segment inclined surface of the present invention is directly on the micro-LED device. Compared with the traditional microlens array, the difference is that there is no need to design and fabricate the microlens array additionally, and the subsequent alignment and gluing processes with the micro-LED are also reduced.

[0135] 3. Due to total internal reflection, most of the light emitted within the light-emitting layer returns to the interior of the medium and is absorbed by the semiconductor layer or the electrode layer after total reflection. The multi-segment inclined surface adopted in the design of the present invention can reduce total internal reflection, increase the possibility of light emission, and allow more light to escape, thereby improving the top light extraction efficiency.

[0136] The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. It is not intended to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

[0137] In the ranges disclosed herein, the endpoints and any values are not limited to the precise ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and a single point value, and between single point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

Claims

1. A flip-chip micro light-emitting diode, characterized in that The flip-chip micro light-emitting diode includes a plurality of light-emitting units arranged in a matrix. Each light-emitting unit includes a main structure and a light-emitting surface structure stacked along the light-emitting direction of the flip-chip micro light-emitting diode. The main structure includes an n-type semiconductor structure, and the n-type semiconductor structure is integrally formed with the light-emitting surface structure. The light-emitting surface structure includes an inclined surface structure, and the inclined surface structure includes n-level inclined surfaces distributed from the edge of the n-type semiconductor structure to the center of the n-type semiconductor structure, where n is greater than or equal to 2. The angle between the inclined surface and the horizontal plane gradually decreases along the light-emitting direction. The n-level inclined surfaces include a first-level inclined surface, a second-level inclined surface, and a third-level inclined surface. The angle between the first-level inclined surface and the horizontal plane is 45°-80°, the angle between the second-level inclined surface and the horizontal plane is 30°-60°, and the angle between the third-level inclined surface and the horizontal plane is 20°-45°.

2. The flip-chip micro light-emitting diode according to claim 1, wherein The n-type semiconductor structure and the light-emitting surface structure are made of the same material, and the vertical height of the inclined surface gradually decreases along the light-emitting direction.

3. The flip-chip micro light-emitting diode according to claim 1, wherein The light-emitting surface structure further includes a sidewall structure, and the sidewall structure is disposed between the n-type semiconductor structure and the inclined surface structure along the light-emitting direction. The height of the sidewall structure is greater than or equal to 100 nm.

4. The flip-chip micro light-emitting diode according to claim 1, wherein The light-emitting surface structure includes a first plane located between adjacent inclined surfaces, and the length of the first plane is 0.01-2 μm. And / or, the light-emitting surface structure includes a second plane located at the center of the light-emitting surface structure, and the length of the second plane is 37%-64% of the length of the micro light-emitting diode.

5. The flip-chip micro light-emitting diode according to claim 1, wherein, The maximum thickness of the light-emitting surface structure is 1.2-1.5 μm. And / or, the size of the micro light-emitting diode is 5-20 μm. And / or, n is less than or equal to 3.

6. The flip-chip micro light-emitting diode according to claim 5, wherein The height of the first-level inclined surface is greater than the height of the second-level inclined surface, and the height of the second-level inclined surface is greater than the height of the third-level inclined surface.

7. The flip-chip micro light-emitting diode according to claim 6, wherein The height of the first-level inclined surface is 300 nm-500 nm, and the height of the second-level inclined surface is 200 nm-400 nm. The height of the third-level inclined surface is 100 nm-300 nm.

8. A method for preparing an inverted micro light-emitting diode according to any one of claims 1-7, characterized in that, Including the following steps: Provide a substrate and prepare an epitaxial layer on the substrate. Wherein, the epitaxial layer includes a first n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer stacked. Etch the epitaxial layer to form a second n-type semiconductor layer and a plurality of spaced main structures. Wherein, the main structure includes an n-type semiconductor structure, a light-emitting structure, and a p-type semiconductor structure stacked in sequence on the second n-type semiconductor layer. The second n-type semiconductor layer is the remaining part of the first n-type semiconductor layer after etching away the n-type semiconductor structure. Remove the substrate. Perform multiple etching processes on the second n-type semiconductor layer to obtain the light-emitting surface structure, so as to fabricate a micro light-emitting diode.

9. The preparation method according to claim 8, characterized in that, The performing multiple etching processes on the second n-type semiconductor layer to obtain the light-emitting surface structure includes the following steps: Coat a photoresist layer on the surface of the second n-type semiconductor layer and perform photolithography to form a current pattern. Using the current pattern as a mask, etching the second n-type semiconductor layer to form an i-th level inclined surface on the surface of the second n-type semiconductor layer; where 1 ≤ i ≤ n; Removing the photoresist layer; Repeating the above steps to form n-level inclined surfaces on the surface of the second n-type semiconductor layer to obtain the light-emitting surface structure.

10. The preparation method according to claim 8 or 9, characterized in that, The process of performing multiple etching processes on the second n-type semiconductor layer to obtain the light-emitting surface structure includes the following steps: Coating a first photoresist layer on the surface of the second n-type semiconductor layer and performing photolithography to form a first pattern; Using a plasma etching process to transfer the first pattern to the second n-type semiconductor layer to form a first-level inclined surface on the surface of the second n-type semiconductor layer; Removing the first photoresist layer; Coating a second photoresist layer on the surface of the second n-type semiconductor layer with a first-level inclined surface and performing photolithography to form a second pattern; Using a plasma etching process to transfer the second pattern to the second n-type semiconductor layer with a first-level inclined surface to form a second-level inclined surface on the surface of the second n-type semiconductor layer with a first-level inclined surface; Removing the second photoresist layer; Coating a third photoresist layer on the surface of the second n-type semiconductor layer with a first-level inclined surface and a second-level inclined surface and performing photolithography to form a third pattern; Using a plasma etching process to transfer the third pattern to the second n-type semiconductor layer with a first-level inclined surface and a second-level inclined surface to form a third-level inclined surface on the surface of the second n-type semiconductor layer with a first-level inclined surface and a second-level inclined surface to obtain the light-emitting surface structure.

Images