Micro-LED structure capable of emitting light with high collimation

Abstract

The invention provides a Micro-LED structure with high collimation light emission, and belongs to the field of semiconductor luminescent devices. The device structurally comprises a metal reflecting layer, a filling layer, an N-type semiconductor, a multi-quantum well, a P-type semiconductor, an ITO electrode, a filling layer and a metal reflecting layer from top to bottom in sequence. According to the structural design of the device, the light intensity of emergent light can be improved, the emergent light collimation of the device is improved, meanwhile, the emergent light wave band is screened, the Micro-LED emergent light color purity is higher, the half-peak width is narrower, and the utilization rate of emergent light beams is improved. An upper layer confocal resonant cavity and a lower layer confocal resonant cavity are formed through metal coating, the light emitting performance of the Micro-LED is improved, and the application range of the semiconductor visible light emitting device is further expanded.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor light-emitting devices, and in particular to a Micro-LED structure with high collimated light output. Background Art

[0002] As a new generation of interactive display technology that promotes the interaction between user digital content and the real world, augmented reality (AR) plays an important role in future display methods. With the development of AR technology, computer-generated visual information continues to enhance the interactivity between real-world objects and virtual-world objects. AR displays usually consist of two parts: a light engine and an optical synthesizer. The optical synthesizer ensures that the displayed image is imaged in high quality in the wearer's eyes while allowing ambient light to pass through. The light engine, as the display image source, directly determines several basic properties of the AR system, such as image brightness, power consumption, frame rate, latency, and color purity.

[0003] Micro-LED can be used as a light engine for AR display systems due to its small chip size (<100μm), high brightness (>10000000cd / cm2), high pixel density per inch (>5000) and high contrast. Among them, the application of high-brightness display panels in various augmented reality scenarios is crucial, especially in outdoor scenes. In outdoor scenes, due to the high-brightness crosstalk of ambient light, a high-brightness light engine must be used as the image source to improve the applicability and contrast of AR displays. Currently, several types of micro displays have been used, including micro organic light-emitting diodes (micro-OLED), liquid crystal on silicon (LCoS), digital micromirror devices (DMD), and laser beam scanning (LBS) based on microelectromechanical systems (MEMS). As a light engine for AR displays, Micro-LED can cope with the problem of high ambient light brightness crosstalk due to its higher brightness characteristics.

[0004] In addition, in order to achieve a higher field of view and ensure an angular resolution of more than 60PPD, the pixel resolution of the optical engine must be improved. Micro-LED can provide a higher pixel density due to its smaller chip size to improve pixel resolution. Since AR needs to interact with the real environment through perspective, the AR display must be placed next to the eye, and an optical element is used to couple the screen image with the eye. The mainstream method currently used is the differential diffraction waveguide solution.

[0005] At present, Micro-LED and diffraction waveguide solutions still have several disadvantages when applied to the AR display field: for example, the divergence angle of Micro-LED is about 180°, and the LED light source in the plane has a Lambertian radiation mode, which can be approximately regarded as a Lambertian radiation light source. Although the inherent Lambertian radiation mode of LED chips has advantages in the application of wide-view television (TV), in order to make the image quality clearer and to achieve higher image contrast and brightness in the AR system, the light source image light needs to be collimated. Since only the light source emitted in the near-normal direction can be effectively utilized, the optical efficiency will be relatively low for the uncollimated Micro-LED. However, the addition of the collimation system will usually increase the size of the AR system at the same time, which makes the miniaturization process of the AR system difficult. The present invention directly utilizes the design of a confocal resonant cavity in a single pixel of Micro-LED, and the directional emission display concentrates the display power to the normal view. This not only improves the light utilization efficiency of the entire system, but also contributes to the miniaturization and compactness of the AR system when applied to the AR system. In addition, the wider emission spectrum of the conventional LED will cause the problem of diffraction dispersion, which will cause rainbow phenomenon and halo in the image, thereby reducing the imaging quality. Therefore, it is crucial to realize a Micro-LED light engine with high collimation, high brightness, high color purity and dense pixel density. This is also one of the important research topics of Micro-LED light-emitting devices in the future. Summary of the invention

[0006] In view of this, the purpose of the present invention is to provide a Micro-LED structure with highly collimated light output. Through the optimized design of the resonant cavity structure, the light-emitting angle of the Micro-LED pixel can be significantly reduced, and the light can be collimated within a divergence angle of nearly 30° in the vertical direction of the pixel light-emitting surface, thereby greatly improving the brightness of the entire product such as projection or near-eye display, reducing power consumption and improving optical efficiency.

[0007] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a Micro-LED structure with high collimation light output, the Micro-LED structure comprising a substrate, a Micro-LED table, a resonant cavity, an electrode and a filling layer; the Micro-LED table is composed of an n-doped region, a multi-quantum well layer and a p-doped region; the resonant cavity is prepared on the surface of the substrate or the surface of the Micro-LED table, and a reflector structure with upper and lower curved surfaces coated with a metal reflective film is formed to form a resonant cavity to realize spontaneous collimation of the light source; the curvature radius of the upper and lower curved surfaces of the resonant cavity are both R, and are equal to the cavity length L of the resonant cavity, and the resonant cavity has a common focus, which is limited to a confocal resonant cavity structure; the Micro-LED epitaxial layer is placed in the cavity of the confocal resonant cavity to realize the optical mode of the active resonant cavity; the confocal resonant cavity emits plane wave or spherical wave output light, and the output light direction is perpendicular to the direction of the Micro-LED table.

[0008] In a preferred embodiment, the substrate is made of silicon carbide SiC or silicon Si, or other low-refractive-index transparent media including sapphire, K9 glass, fused quartz glass, and flexible organic transparent materials with low refractive index.

[0009] In a preferred embodiment, the filling layer is prepared on the surface of the substrate or the surface of the Micro-LED table; the filling layer is an oxide insulating thin film material, which provides a passivation protection layer for the Micro-LED device.

[0010] In a preferred embodiment, the Micro-LED table includes GaN-based, InAlP-based binary, ternary or quaternary materials, and the chip size has a side length of 0.5μm to 100μm; the Micro-LED structure is made of N-type electrodes and P-type electrodes, and both electrodes are made of high-transmittance transparent thin film materials to maintain high transmittance and high light extraction efficiency.

[0011] In a preferred embodiment, the oxide insulating thin film material has a spherical or quasi-spherical morphology on the upper and lower sides of the Micro-Led table. The spherical radius is approximately equal to the cavity length of the optical resonant cavity, ranging from 3μm to 20μm, and the thickness ranges from 10 to 200nm. The methods for preparing spherical oxide films include magnetron sputtering, ion beam sputtering, chemical vapor deposition, thermal oxidation, and gel-sol method.

[0012] In a preferred embodiment, the reflector structure with upper and lower curved surfaces coated with metal reflective films is respectively coated on the upper and lower hemisphere filling layer films, and the thickness of the upper and lower hemispheres ranges from 10nm to 200nm; the material used to make the reflector structure has the characteristics of low photon absorption rate and high photon reflectivity, including Au, Ag, Al, and Ge; wherein the confocal resonant cavity is grown to form the upper and lower spherical structures, and the spherical structure is formed on the polymethyl methacrylate PMMA spin-coated on the ITO current diffusion layer, and the pattern on the pre-designed mold is transferred to the polymethyl methacrylate PMMA by ultraviolet curing. The process involves depositing metallic silver on the PMMA spherical structure using electron beam evaporation technology to form a reflector. After the reflector is formed, a second thicker layer of PMMA is spin-coated on the reflector to cover and fill the reflector to ensure a smooth transition and good bonding with the next layer of material. The processed Micro-LED structure is bonded to the silicon substrate through epitaxial bonding using Sn / Au alloy. This step achieves structural fixation by forming a metal bonding layer between the Micro-LED structure and the silicon substrate.

[0013] In a preferred embodiment, after substrate bonding and laser stripping are completed to remove the original sapphire substrate, the GaN sacrificial layer is polished away using chemical mechanical polishing (CMP) technology to expose the n-GaN layer; a new PMMA film is selectively coated on the n-GaN layer, and a pattern of the top reflector is formed again using nanoimprinting and other technologies; a spherical structure of the top reflector is formed on the n-GaN layer using nanoimprinting technology; after a contact hole for making an electrode is reserved on the top, a third mask is used to make the electrode in the ITO and the reserved hole on the top; an electrode contact hole is formed on the ITO layer and the top layer through a precise photolithography process so that the electrode can be led out to achieve electrical connection;

[0014] The preparation methods of the high refractive index Micro-LED diode include electron beam evaporation, chemical vapor deposition, thermal evaporation, atomic layer deposition, and solution method.

[0015] In a preferred embodiment, the process steps for preparing a Micro-LED chip with a microcavity structure include: first, using magnetron sputtering to prepare an ITO current diffusion layer on a GaN epitaxial layer on a sapphire substrate, then spin-coating PMMA on the ITO, and using UV-curing nanoimprinting to transfer the pattern to PMMA to form a spherical structure; then, using electron beam evaporation of metallic silver on the PMMA pattern to form a reflector, using spin coating to cover a second thicker layer of PMMA on the reflector, and after filling, bonding the epitaxial layer to a silicon substrate on a plane, and removing the original sapphire substrate of the epitaxial layer by laser stripping and polishing the GaN sacrificial layer by chemical mechanical polishing (CMP), then selectively coating a new PMMA film on the top of the n-GaN and nanoimprinting the pattern of the top reflector using the same method, and using electron beam evaporation to form a thinner layer of metallic silver.

[0016] In a preferred embodiment, the cavity length L of the microcavity structure is determined according to the following steps:

[0017] Step S1, construct a flip-chip GaN-based Micro-LED and set the wavelength of the incident light, then determine the reflectivity r1 and r2 of the upper and lower metal reflective layers. The light extraction efficiency of the resonant cavity structure Micro-LED is defined as:

[0018]

[0019] m is the cavity order, n is the refractive index of the emitting medium GaN; the top reflection layer thickness h1 and the bottom reflection layer thickness h2 are selected to obtain the Ag film reflectivity r1 and r2 of the corresponding band; substituting into the formula, the light extraction efficiency is obtained as an expression related to the cavity order;

[0020] Step S2: Determine the overall cavity length at three wavelengths of R, G, and B according to the light collimation effect to be achieved. The cavity length is derived as:

[0021]

[0022] Where L is the thickness of the GaN layer table plus the two filling layers. When the emission center wavelength remains unchanged, a smaller m-order cavity corresponds to a shorter cavity length. When the thickness decreases, the supported mode and FWHM of the resonant wavelength both decrease, and the light extraction efficiency is improved.

[0023] Step S3: When the cavity resonates, the electric field inside the cavity is distributed as a standing wave.

[0024] In a preferred embodiment, the steps of preparing the microcavity Micro-LED structure include:

[0025] S1. Select a sapphire substrate and use magnetron sputtering technology to deposit an ITO current diffusion layer on the surface of the GaN epitaxial layer of the sapphire substrate; the ITO layer has a thickness of 130nm and serves as the current diffusion layer;

[0026] S2. Spin-coat a 500nm thick PMMA film on the ITO layer. The PMMA film coating process precisely controls the spin-coating parameters to ensure that the film layer is uniform and free of bubbles. Subsequently, the coated PMMA film is transferred into a spherical microcavity structure pattern by UV-curing nanoimprinting technology.

[0027] S3. Using electron beam evaporation technology, a 50nm thick layer of metallic silver is deposited on the transferred PMMA pattern to form a reflector layer; this layer of silver reflector will effectively improve the reflectivity of light, enhance the light efficiency of the chip, and ensure the guidance and enhancement of light to maximize light output;

[0028] S4, covering with a second layer of PMMA and filling the structure, and applying a 3 μm thick PMMA film again by spin coating;

[0029] S5. On the filled PMMA layer, the epitaxial structure is precisely bonded to the silicon substrate through Sn / Au alloy solder. This bonding process heats the alloy solder at a relatively low temperature to form a strong bond between the epitaxial structure and the silicon substrate, thereby providing stable mechanical support for the chip.

[0030] S6. After bonding is completed, the sapphire substrate is peeled off from the GaN epitaxial layer using laser lift-off technology. The laser precise lift-off process is used to ensure that the separation of the sapphire substrate and the GaN layer does not damage the GaN layer.

[0031] S7. Use chemical mechanical polishing (CMP) technology to polish the GaN surface. The CMP process polishes the GaN layer to a high degree of flatness, preparing for the subsequent microcavity structure preparation.

[0032] S8. After the GaN surface treatment is completed, a new PMMA film with a thickness of 1 μm is applied; the designed top reflector pattern is transferred to the PMMA film through UV-curing nanoimprinting technology to form a microcavity top structure; then, a thin layer of metallic silver 20 nm is deposited on the PMMA layer using electron beam evaporation technology to construct the top reflector structure.

[0033] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a highly collimated light-emitting Micro-LED structure built into a confocal resonant cavity. The device structure design can improve the light intensity of the emitted light, improve the device light collimation, and filter the light band, so that the Micro-LED light color purity is higher, the half-peak width is narrower, and the utilization rate of the emitted light beam is improved. The metal coating is used to form the upper and lower confocal resonant cavities to improve the light-emitting performance of the Micro-LED. The present invention further expands the application range of semiconductor visible light emitting devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 The front view of the nanostructure unit of the metasurface in Example 1 of the present invention

[0035] Figure 2 1 is a top view of the nanostructure unit of the metasurface in Example 1 of the present invention.

[0036] Figure 3 Schematic diagram of the working principle of the metasurface in Example 1 of the present invention.

[0037] Figure 4 This is the angle distribution diagram of the Micro-LED in Example 1 of the present invention when the wavelength is 460nm.

[0038] Figure 5 This is the angle distribution diagram of Micro-LED in Example 1 of the present invention when the wavelength is 550nm.

[0039] Figure 6 This is the angle distribution diagram of the Micro-LED in Example 1 of the present invention when the wavelength is 680nm.

[0040] Figure 7 This is a diagram of the light intensity achieved by the Micro-LED in Example 1 of the present invention at a wavelength of 460nm.

[0041] Figure 8 This is a diagram of the light intensity achieved by the Micro-LED in Example 1 of the present invention at a wavelength of 550nm.

[0042] Fig. 9 This is a diagram of the light intensity achieved by the Micro-LED in Example 1 of the present invention when the wavelength is 680nm.

[0043] Fig.10 4 is a process flow chart of the Micro-LED microcavity structure in Example 1 of the present invention. DETAILED DESCRIPTION

[0044] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0045] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0046] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application; as used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or their combinations.

[0047] refer to Figure 1-10 , a Micro-LED structure with high collimation light output, suitable for projection, near-eye display, backlight source, and spotlight with high collimation light output requirements, the Micro-LED structure includes a substrate, a Micro-LED table, a resonant cavity, an electrode, and a filling layer. The Micro-LED table is composed of an n-doped region, a multi-quantum well layer, and a p-doped region. It is characterized in that the resonant cavity structure can be prepared on the surface of the substrate or the surface of the Micro-LED table, and a reflector structure with a metal reflective film coated on the upper and lower curved surfaces is formed to form a resonant cavity to realize the spontaneous collimation of the light source. The curvature radius of the upper and lower reflection surfaces of the resonant cavity is R, and is equal to the cavity length L of the resonant cavity. This resonant cavity has a common focus and is limited to a confocal resonant cavity structure. This confocal resonant cavity can realize a stable cavity structure and function, and the Micro-LED epitaxial layer is placed in this cavity to realize the optical mode of the active resonant cavity. The confocal resonant cavity can emit light including but not limited to plane waves and spherical waves, and the direction of the emitted light is perpendicular to the direction of the Micro-LED table. The confocal resonant cavity structure allows the paraxial light to oscillate back and forth multiple times, thereby reducing lateral overflow, reducing light emitted from the side walls of the Micro-LED, and constraining the collimation of the light beam emitted by the Micro-LED.

[0048] The substrate material may be silicon carbide (SiC) and silicon (Si), or other low-refractive-index transparent media including but not limited to sapphire, K9 glass, fused quartz glass, flexible organic transparent materials and other low-refractive-index materials.

[0049] The filling layer can be prepared on the surface of the substrate or the surface of the Micro-LED table. The filling layer is a common oxide insulating thin film material, which can provide a passivation protection layer for the Micro-LED device. At the same time, it should have a low refractive index and good light transmittance in the visible light band, and good stability and mechanical properties. High hardness can allow the optical device to maintain a high light extraction efficiency. Such materials include but are not limited to silicon dioxide, titanium dioxide, etc.

[0050] The Micro-LED table includes but is not limited to binary, ternary, and quaternary materials such as GaN-based and InAlP-based materials, and the chip size has a side length of 0.5μm to 100μm.

[0051] The thin film material in the structure has an overall spherical or quasi-spherical morphology on the upper and lower sides of the Micro-Led table. The spherical radius is approximately equal to the cavity length of the optical resonant cavity, ranging from 3μm to 20μm, and the thickness ranges from 10 to 200nm. Methods for preparing oxide films with spherical morphology include but are not limited to magnetron sputtering, ion beam sputtering, chemical vapor deposition, thermal oxidation, gel-sol method, etc.

[0052] The two curved metal reflectors are respectively coated on the upper and lower hemisphere filling layer films, and the thickness of the upper and lower hemispheres ranges from 10nm to 200nm. The selected metal reflector material should have the characteristics of low photon absorption rate and high photon reflectivity, including but not limited to Au, Ag, Al, Ge, etc. The confocal resonant cavity grows to form a spherical upper and lower layer structure, and forms a spherical structure on the PMMA (polymethyl methacrylate) spin-coated on the ITO current diffusion layer using conformal technologies including but not limited to embossing and coating. The pattern on the pre-designed mold is transferred to the PMMA layer by ultraviolet light curing, thereby forming the required spherical structure. Electron beam evaporation technology is used to deposit metal silver on the PMMA spherical structure to form a reflector. This step is carried out on the spherical PMMA structure that has been formed. By accurately controlling the evaporation process, a uniform metal silver layer can be formed on the PMMA spherical surface, thereby realizing the spherical shape of the confocal resonant cavity. Its preparation method includes but is not limited to preparing a metal film by DC sputtering, vacuum thermal evaporation, and magnetron sputtering or evaporation. After forming the reflector, a second thicker layer of PMMA needs to be spin-coated on the reflector to cover and fill the reflector to ensure a smooth transition and good bonding with the next layer of material. The processed Micro-LED structure is bonded to the silicon substrate through epitaxial bonding with Sn / Au alloy. This step fixes the structure by forming a metal bonding layer between the Micro-LED structure and the silicon substrate.

[0053] The Micro-LED structure needs to go through processes including but not limited to coating, photolithography, etching, etc. to make N-type electrodes and P-type electrodes. Both electrodes are made of high-transmittance transparent thin film materials to maintain high transmittance and high light extraction efficiency. According to claim 6, after completing substrate bonding and laser stripping to remove the original sapphire substrate, the GaN sacrificial layer is polished away using chemical mechanical polishing (CMP) technology to expose the n-GaN layer. A new PMMA film is selectively applied on the n-GaN layer, and nanoimprinting and other technologies are used again to form the pattern of the top reflector. This step is similar to the step of forming the bottom reflector, and the spherical structure of the top reflector is formed on the n-GaN layer by nanoimprinting technology. After reserving the contact hole for making the electrode at the top, the third mask is used to make the electrode in the reserved hole position of ITO and the top. This step forms electrode contact holes on the ITO layer and the top layer through precise photolithography technology so that the electrode can be led out to achieve electrical connection.

[0054] The preparation method of the high refractive index Micro-LED diode includes but is not limited to electron beam evaporation, chemical vapor deposition, thermal evaporation, atomic layer deposition, and solution method. The complete process flow steps for preparing the Micro-LED chip with this microcavity structure include: first, using magnetron sputtering, prepare an ITO current diffusion layer on the GaN epitaxial layer of the sapphire substrate, then spin-coating PMMA on the ITO, and using ultraviolet curing nanoimprinting to transfer the pattern to PMMA to form a spherical structure. Then, electron beam evaporation of metallic silver is used on the PMMA pattern to make a reflector. The second thicker layer of PMMA is covered on the reflector by spin coating, and after filling, the epitaxial bonding is performed on the plane to the silicon substrate, and the original sapphire substrate of the epitaxial is removed by laser stripping and the GaN sacrificial layer is polished by chemical mechanical polishing (CMP). Then, a new PMMA film is selectively coated on the n-GaN top and the pattern of the top reflector is nano-imprinted in the same way, and a thinner layer of metallic silver is evaporated by electron beam.

[0055] A highly collimated light emitting Micro-LED structure, the structure of which is as follows Figure 1 and Figure 2As shown, they are the main view and top view of the coaxial spherical cavity Micro-LED in Example 1. Wherein R is the spherical radius of the hemispherical filling layer film, h2 and h3 are the filling layers, and the Ag reflective layer constitutes the resonant cavity. The n-GaN, MQWs, and p-GaN are the table tops of the Micro-LED structure. In this Example 1, the cavity length L, the upper reflective layer curvature radius R1, and the lower reflective layer curvature radius R2 must satisfy the condition R1=R2=L. The center of this cavity is the common focus of the two curved reflectors. The application of the resonant cavity is essentially the process of reciprocating reflection of light in the cavity. Because the efficiency of extracting photons from the GaN epitaxial layer is low, the use of curved reflectors can reduce the geometric deflection loss of the cavity. The filling layers h2 and h3 select TiO2 and SiO2 reflective layers that have low light wave absorption and low refractive index, and select metal film Ag with high reflection efficiency.

[0056] The microcavity structure proposed in the present invention can realize the multiplexing of different wavelengths, such as Figure 3 The figure is a schematic diagram of the working principle of the cofocus cavity in Example 1.

[0057] The microcavity structure proposed in the present invention realizes the collimation of three colors of light with central wavelengths of 460nm, 550nm and 680nm. The cavity length L that should be possessed is determined according to the following steps to achieve Figure 3 The functional specific steps shown include:

[0058] S1. Construct a common inverted GaN-based Micro-LED and set the wavelength of the incident light. Then determine the reflectivity r1 and r2 of the upper and lower metal reflective layers. The light extraction efficiency of the resonant cavity structure Micro-LED can be defined as:

[0059]

[0060] m is the cavity order, and n is the refractive index of the emitting medium GaN. When the thickness of the top increases, the transmittance decreases, and more light is confined in the cavity, resulting in a narrower spectral width but relatively low extraction efficiency. In order to achieve the resonant cavity effect and ensure light extraction at the same time, h1 and h2 are the thickness of the metal reflective layer. Here, the top reflective layer thickness h1 = 20nm and the bottom reflective layer thickness h2 = 200nm are selected to obtain the Ag film reflectivities r1 and r2 in the corresponding bands. Substituting into the formula, we get an expression for the light extraction efficiency as a function of the cavity order.

[0061] S2. According to the desired light collimation effect, the overall cavity length at the three wavelengths of R, G, and B can be derived as:

[0062]

[0063] Where L is the thickness of the GaN layer table plus two filling layers. When the emission center wavelength remains unchanged, a smaller m-order cavity corresponds to a shorter cavity length. When the thickness decreases, the supported mode and FWHM of the resonant wavelength decrease, and the light extraction efficiency increases. However, as the GaN layer becomes thinner, the active area is more easily damaged. Considering these factors, the order of the cavity is m = 16λ = 460nm. The corresponding thickness required for the central wavelength blue light is L = 1223nm.

[0064] S3, such as Figure 3 When the cavity is resonant, the electric field distribution inside is a standing wave. The cavity order m is set to 16, and the simulated emission spectrum of the designed resonant cavity Micro-LED is shown in Figure 4 shown.

[0065] Figure 4 , Figure 5 and Figure 6 This is the light field distribution diagram achieved by the confocal cavity Micro-LED proposed in this embodiment at three wavelengths: blue light (460nm), green light (550nm) and red light (680nm). Figure 7 , Figure 8 and Fig. 9 This is the far-field distribution diagram of the confocal cavity Micro-LED proposed in this embodiment at three wavelengths of blue light (460nm), green light (550nm) and red light (680nm). The light collimation of the three colors of RGB are 4.98°, 5.1° and 3.87° respectively.

[0066] Specifically, when the target wavelength λ=460nm, the light extraction efficiency of the structure is 13.7%. When the target wavelength λ=550nm, the light extraction efficiency of the structure is 18.9%. When the target wavelength λ=680nm, the light extraction efficiency of the structure is 15.4%.

[0067] The process flow of preparing a Micro-LED resonant cavity structure with high collimation light is as follows: Fig.10 As shown, the specific preparation process includes using nanoimprinting to form the upper spherical resonant cavity and the lower spherical resonant cavity, and using electron beam evaporation to deposit a metal silver reflector on the PMMA spherical resonant cavity. The following steps determine the specific process flow of the micro-LED structure with a microcavity, which can be realized Fig.10 The specific steps of the preparation process shown include:

[0068] S1. Select a sapphire substrate and use magnetron sputtering technology to deposit an ITO current diffusion layer on the surface of the GaN epitaxial layer of the sapphire substrate. The ITO layer is 130nm thick and, as a current diffusion layer, effectively improves the uniform distribution of current and provides good current input conditions for the formation of subsequent light emission structures.

[0069] S2. Spin-coat a 500nm thick PMMA film on the ITO layer. The PMMA film coating process precisely controls the spin-coating parameters to ensure that the film layer is uniform and free of bubbles. Subsequently, the coated PMMA film is transferred into a spherical microcavity structure pattern by UV-curing nanoimprinting technology. In this step, the designed microcavity pattern is successfully transferred to the surface of the PMMA layer using the precise structure of the nanoimprinting mold, forming a microspherical structure with optical enhancement function.

[0070] S3, using electron beam evaporation technology, deposit a 50nm thick layer of metallic silver on the transferred PMMA pattern to form a reflector layer. This layer of silver reflector will effectively improve the reflectivity of light, enhance the light efficiency of the chip, and ensure the guidance and enhancement of light to maximize light output.

[0071] S4, cover with a second layer of PMMA and fill the structure. In order to maintain the flatness of the surface and optimize the light transmission, a thicker PMMA film of 3 μm is applied again by spin coating. The PMMA layer fills the gap between the reflector surface and the microcavity structure to ensure the overall surface flatness.

[0072] S5. On the filled PMMA layer, the epitaxial structure is precisely bonded to the silicon substrate through Sn / Au alloy solder. This bonding process heats the alloy solder at a relatively low temperature to form a strong bond between the epitaxial structure and the silicon substrate, thereby providing stable mechanical support for the chip.

[0073] S6. After bonding is completed, the sapphire substrate is peeled off from the GaN epitaxial layer using laser lift-off technology. The laser precision lift-off process ensures that the separation of the sapphire substrate and the GaN layer does not damage the GaN layer. In this process, the sapphire substrate is completely removed and the GaN layer is further processed by laser lift-off.

[0074] S7. In order to remove the residual sacrificial layer on the GaN layer, the GaN surface is polished by chemical mechanical polishing (CMP). The CMP process polishes the GaN layer to a high degree of flatness, preparing for the subsequent microcavity structure preparation.

[0075] S8. After the GaN surface is treated, a new PMMA film with a thickness of 1 μm is applied. The designed top reflector pattern is transferred to the PMMA film by UV-curing nanoimprinting technology to form the top structure of the microcavity. Subsequently, a thin layer of 20 nm of metallic silver is deposited on the PMMA layer using electron beam evaporation technology to construct the top reflector structure. This reflector enhances the reflection effect of light and further improves the light output efficiency.

Claims

1. A Micro-LED structure with highly collimated light, characterized in that: The Micro-LED structure includes a substrate, a Micro-LED table, a resonant cavity, an electrode and a filling layer; the Micro-LED table is composed of an n-doped region, a multi-quantum well layer and a p-doped region; the resonant cavity is prepared on the surface of the substrate or the surface of the Micro-LED table, and a reflector structure with upper and lower curved surfaces coated with a metal reflective film is formed to form a resonant cavity to realize spontaneous collimation of the light source; the curvature radius of the upper and lower curved surfaces of the resonant cavity are both R, and are equal to the cavity length L of the resonant cavity, and the resonant cavity has a common focus, which is limited to a confocal resonant cavity structure; the Micro-LED epitaxial layer is placed in the cavity of the confocal resonant cavity to realize the optical mode of the active resonant cavity; the confocal resonant cavity emits plane wave or spherical wave light, and the direction of the emitted light is perpendicular to the direction of the Micro-LED table.

2. The highly collimated light emitting Micro-LED structure according to claim 1, characterized in that: The substrate is made of silicon carbide SiC or silicon Si, or other low-refractive-index transparent media including sapphire, K9 glass, fused quartz glass, and flexible organic transparent materials with low refractive index.

3. The highly collimated light emitting Micro-LED structure according to claim 1, characterized in that: The filling layer is prepared on the surface of the substrate or the surface of the Micro-LED table; the filling layer is an oxide insulating thin film material, which provides a passivation protection layer for the Micro-LED device.

4. The highly collimated light emitting Micro-LED structure according to claim 1, characterized in that: The Micro-LED table includes GaN-based, InAlP-based binary, ternary or quaternary materials, and the chip size has a side length of 0.5μm to 100μm; the Micro-LED structure is made of N-type electrodes and P-type electrodes, both of which are high-transmittance transparent thin film materials to maintain high transmittance and high light extraction efficiency.

5. The highly collimated light emitting Micro-LED structure according to claim 3, characterized in that: The oxide insulating thin film material has a spherical or quasi-spherical morphology on the upper and lower sides of the Micro-Led table. The spherical radius is approximately equal to the cavity length of the optical resonant cavity, ranging from 3μm to 20μm, and the thickness ranges from 10 to 200nm. The methods for preparing the spherical oxide film include magnetron sputtering, ion beam sputtering, chemical vapor deposition, thermal oxidation, and gel-sol method.

6. The highly collimated light emitting Micro-LED structure according to claim 1, characterized in that: The reflector structure with upper and lower curved surfaces coated with metal reflective films is respectively coated on the upper and lower hemisphere filling layer films, and the thickness of the upper and lower hemispheres ranges from 10nm to 200nm; the material used to make the reflector structure has the characteristics of low photon absorption rate and high photon reflectivity, including Au, Ag, Al, and Ge; the confocal resonant cavity grows to form the upper and lower spherical structures, and the spherical structure is formed on the polymethyl methacrylate PMMA spin-coated on the ITO current diffusion layer, and the pattern on the pre-designed mold is transferred to the polymethyl methacrylate PMMA layer by ultraviolet curing, so as to form the required spherical structure; the metal silver is deposited on the PMMA spherical structure by electron beam evaporation technology to form a reflector; After forming the reflector, a second thicker layer of PMMA is spin-coated on the reflector to cover and fill the reflector to ensure a smooth transition and good bonding with the next layer of material; epitaxial bonding is performed using Sn / Au alloy to bond the processed Micro-LED structure to the silicon substrate; this step achieves structural fixation by forming a metal bonding layer between the Micro-LED structure and the silicon substrate.

7. The highly collimated light emitting Micro-LED structure according to claim 6, characterized in that: After substrate bonding and laser stripping to remove the original sapphire substrate, the GaN sacrificial layer is polished away using chemical mechanical polishing (CMP) technology to expose the n-GaN layer; a new PMMA film is selectively coated on the n-GaN layer, and nanoimprinting and other technologies are used again to form the pattern of the top reflector; the spherical structure of the top reflector is formed on the n-GaN layer using nanoimprinting technology; after reserving contact holes for making electrodes on the top, the third mask is used to make electrodes in the ITO and the reserved holes on the top; Through precise photolithography, electrode contact holes are formed on the ITO layer and the top layer to lead out the electrodes and achieve electrical connection; The preparation methods of the high refractive index Micro-LED diode include electron beam evaporation, chemical vapor deposition, thermal evaporation, atomic layer deposition, and solution method.

8. The highly collimated light emitting Micro-LED structure according to claim 7, characterized in that: The process steps for preparing a Micro-LED chip with a microcavity structure include: first, using magnetron sputtering to prepare an ITO current diffusion layer on the GaN epitaxial layer of a sapphire substrate, then spin-coating PMMA on the ITO, and using UV-curing nanoimprinting to transfer the pattern to PMMA to form a spherical structure; then, using electron beam evaporation of metallic silver on the PMMA pattern to form a reflector, using spin coating to cover the reflector with a second thicker layer of PMMA, and after filling, the epitaxial layer is bonded to a silicon substrate on a plane, and the original sapphire substrate of the epitaxial layer is removed by laser lift-off and the GaN sacrificial layer is polished off by chemical mechanical polishing (CMP), and then a new PMMA film is selectively coated on the top of the n-GaN and the pattern of the top reflector is nano-embossed in the same way, and a thinner layer of metallic silver is evaporated by electron beam.

9. The highly collimated light emitting Micro-LED structure according to claim 1, characterized in that: The cavity length L of the microcavity structure is determined according to the following steps: Step S1, construct a flip-chip GaN-based Micro-LED and set the wavelength of the incident light, then determine the reflectivity r1 and r2 of the upper and lower metal reflective layers. The light extraction efficiency of the resonant cavity structure Micro-LED is defined as: m is the cavity order, n is the refractive index of the emitting medium GaN; the top reflection layer thickness h1 and the bottom reflection layer thickness h2 are selected to obtain the Ag film reflectivity r1 and r2 of the corresponding band; substituting into the formula, the light extraction efficiency is obtained as an expression related to the cavity order; Step S2: Determine the overall cavity length at three wavelengths of R, G, and B according to the light collimation effect to be achieved. The cavity length is derived as: Where L is the thickness of the GaN layer table plus the two filling layers. When the emission center wavelength remains unchanged, a smaller m-order cavity corresponds to a shorter cavity length. When the thickness decreases, the supported mode and FWHM of the resonant wavelength both decrease, and the light extraction efficiency is improved. Step S3: When the cavity resonates, the electric field inside the cavity is distributed as a standing wave.

10. The highly collimated light emitting Micro-LED structure according to claim 1, characterized in that: The steps of preparing the micro-cavity Micro-LED structure include: S1. Select a sapphire substrate and use magnetron sputtering technology to deposit an ITO current diffusion layer on the surface of the GaN epitaxial layer of the sapphire substrate; the ITO layer has a thickness of 130nm and serves as the current diffusion layer; S2. Spin-coat a 500nm thick PMMA film on the ITO layer. The PMMA film coating process precisely controls the spin-coating parameters to ensure that the film layer is uniform and free of bubbles. Subsequently, the coated PMMA film is transferred into a spherical microcavity structure pattern by UV-curing nanoimprinting technology. S3. Using electron beam evaporation technology, a 50nm thick layer of metallic silver is deposited on the transferred PMMA pattern to form a reflector layer; this layer of silver reflector will effectively improve the reflectivity of light, enhance the light efficiency of the chip, and ensure the guidance and enhancement of light to maximize light output; S4, covering with a second layer of PMMA and filling the structure, and applying a 3 μm thick PMMA film again by spin coating; S5. On the filled PMMA layer, the epitaxial structure is precisely bonded to the silicon substrate through Sn / Au alloy solder. This bonding process heats the alloy solder at a relatively low temperature to form a strong bond between the epitaxial structure and the silicon substrate, thereby providing stable mechanical support for the chip. S6. After bonding is completed, the sapphire substrate is peeled off from the GaN epitaxial layer using laser lift-off technology. The laser precise lift-off process is used to ensure that the separation of the sapphire substrate and the GaN layer does not damage the GaN layer. S7. Use chemical mechanical polishing (CMP) technology to polish the GaN surface. The CMP process polishes the GaN layer to a high degree of flatness, preparing for the subsequent microcavity structure preparation. S8. After the GaN surface treatment is completed, a new PMMA film with a thickness of 1 μm is applied; the designed top reflector pattern is transferred to the PMMA film through UV-curing nanoimprinting technology to form a microcavity top structure; then, a thin layer of metallic silver 20 nm is deposited on the PMMA layer using electron beam evaporation technology to construct the top reflector structure.

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