Preparation method of micro-lens array

Abstract

The invention belongs to the technical field of manufacturing of micro-nano optical devices, and relates to a preparation method of a micro-lens array. After the photoresist is spin-coated on the surface of the substrate and the micro-column array is formed, a layer of thin film is deposited on the micro-columns by adopting a magnetron sputtering technology, and the surface energy distribution of the photoresist is adjusted by accurately controlling the pressure stress of the thin film to be-200MPa to + 80MPa, so that the surface energy is effectively reduced from 45-60mN / m to 25-35mN / m; the method has the advantages that the method is simple in process and low in cost, the fluidity and formability of materials in the subsequent hot reflux process are optimized, the problems of poor surface quality, low shape precision and the like in the traditional process are successfully solved, the compatibility and stability of the process are improved, and a new way is provided for efficiently manufacturing a high-quality micro-lens array at low cost.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of micro-nano optical device manufacturing, and relates to a preparation method of a microlens array. BACKGROUND

[0002] Thermal reflow is a key process widely used in the fabrication of microlens arrays on wafers. The basic principle is as follows: first, a regular array of cylindrical or specific-shaped photoresist structures is formed on the surface of the substrate through photolithography, and then the photoresist is heated to and beyond its glass transition temperature (Tg), so as to soften and naturally shrink into an aspherical structure under the action of surface tension, and finally a microlens array with optical function is formed. This method has attracted widespread attention because it can directly integrate microlens structures at the wafer level, and is particularly suitable for large-scale and low-cost optical component manufacturing. However, in practical applications, the traditional thermal reflow process still faces many challenges and limitations.

[0003] Firstly, although the traditional non-coating thermal reflow process is simple to operate, it lacks effective control over the flow behavior of the material, resulting in a high surface roughness of the formed microlenses, usually exceeding 15 nm, and poor morphology consistency between different areas, which affects the uniformity of optical performance. In order to improve this problem, some studies have attempted to use a solution method to coat a surface modification layer to regulate the wettability and flowability of the photoresist, but this method is easy to introduce organic contaminants and has compatibility problems with standard semiconductor manufacturing processes, thereby limiting its application in high-precision devices.

[0004] In addition, some researchers have also tried to use plasma treatment technology to enhance the flowability of the photoresist, thereby optimizing the microlens forming effect. However, this treatment method may cause damage to the existing pattern structure, affecting the integrity of the pattern, especially in the fine structure area, which may cause edge deformation or structure collapse and other problems. At the same time, some studies have proposed performing thermal reflow treatment in a nitrogen environment to reduce the effects of oxidation and contamination, but this method has not fundamentally solved the problem of uneven surface energy gradient, resulting in a large deviation of the curvature radius of the microlenses, up to ±8 μm, which seriously affects the consistency of the optical focusing performance.

[0005] In recent years, some researchers have proposed using the "photoacid diffusion" method to fabricate microlens arrays of SU-8 material. This method promotes the lateral diffusion of photoacid during the baking process after UV exposure, thereby forming a continuous variation of crosslinking degree distribution, and then guiding the SU-8 material to form a smooth curved structure during development. Compared with the aforementioned high-cost methods, this method has lower cost and relatively simplified process, but still has problems such as narrow process window, poor controllability of microlens shape, and difficulty in completely removing the residues after development, which affects its practical application and promotion in the industry.

[0006] In summary, although the hot reflow method has significant advantages in the manufacture of wafer-level microlens arrays, it still faces many technical bottlenecks in surface quality control, process compatibility, material selection, and shape accuracy, etc.

[0007] Chinese patent application file (CN113419301A) discloses a preparation method of a microlens array and a wafer, but it has shortcomings such as poor surface roughness control, poor material flowability regulation during hot reflow process leading to poor morphology consistency, and possible limitation of compatibility with high-precision semiconductor processes.

[0008] Chinese patent application file (CN113703081A) discloses a manufacturing method of a microlens array structure, but it has shortcomings such as relatively complex process steps, possible dependence on specific materials or processing methods leading to high cost, and still room for improvement in controlling microlens curvature radius consistency and reducing surface roughness. SUMMARY

[0009] The purpose of the present application is to address the above-mentioned problems existing in the prior art, and to provide a preparation method of a microlens array, which improves the morphology accuracy of the microlens array by regulating the surface energy distribution of photoresist and combining with gradient hot reflow process.

[0010] The purpose of the present application can be achieved by the following technical solutions: A preparation method of a microlens array, the method comprising the following steps: S1, spin-coating a photoresist layer on the surface of a substrate, and forming a micropillar array through a photolithography process; S2, then depositing a thin film on the surface of the micropillar array using magnetron sputtering, and controlling the compressive stress of the thin film to be -300 MPa to +80 MPa by using a substrate bias of -50 V to +30 V during the magnetron sputtering process; S3, then performing gradient hot reflow treatment in an inert gas environment; S4, using the thin film as an etching mask, and performing inductively coupled plasma etching to transfer the microlens morphology to the substrate.

[0011] The present application can precisely control the surface energy distribution and gradient of the photoresist micro-column by controlling the compressive stress of the film. The film with compressive stress tends to shrink, which increases the surface tension of the photoresist below; the film with tensile stress tends to expand, which reduces the surface tension of the photoresist below. By precisely controlling the compressive stress in the range of -300 MPa to +80 MPa, the required and controllable surface energy gradient can be formed on the surface of the micro-column, which can guide the photoresist material to flow more uniformly and controllably during the heat reflow process, thereby optimizing the forming precision of the microlens, reducing the surface roughness (<5 nm) and significantly improving the consistency of the curvature radius (>97%). The range is based on the experience range of commonly used film materials (such as Al, TiN, Cr) that can be stably achieved and effectively control the surface energy of the photoresist under controllable process conditions.

[0012] In the method for preparing a microlens array, the substrate comprises at least one of silicon, quartz and sapphire material, and the surface flatness of the substrate is ≤0.1 μm.

[0013] In the method for preparing a microlens array, the thickness of the photoresist layer is 5-20 μm, and the photoresist is selected from the AZ series positive photoresist or the SU-8 series negative photoresist.

[0014] In the method for preparing a microlens array, the diameter of the micro-column array is 10-100 μm.

[0015] The present application has the advantages of balanced process effect and optical performance by controlling the diameter of the micro-column array in the range of 10-100 μm. This range is suitable for most optical applications (such as imaging, homogenization, sensing), and at this size, the surface tension and gravity have a favorable balance on the heat reflow forming, which is easy to form smooth and symmetrical lens curves. However, if the diameter is too short (<10 μm), the surface tension effect is too strong during heat reflow, which causes the micro-column to shrink too much, resulting in a lens with too small curvature radius and too short focal length, and even the lens shape may not be complete or the risk of collapse increases due to insufficient material. The uniformity of lithography and sputtering is also extremely high. If the diameter is too long (>100 μm), the gravity effect will relatively increase, which may cause the lens to sag and deform during heat reflow, making it difficult to form an ideal spherical or aspherical surface, and the height-to-diameter ratio (height) of the lens decreases, and the optical focusing ability decreases. At the same time, large-size structures are more prone to fusion or collapse during heat reflow (collapse rate increases).

[0016] In the method for preparing a microlens array, the film is a metal film or a dielectric film with a thickness of 5-50 nm, and the film material comprises at least one of Al, TiN and Cr.

[0017] In the method for preparing a microlens array, the magnetron sputtering adopts a direct current power source or a radio frequency power source. The direct current power is 150-250W, the substrate temperature is 50-90℃, the working pressure is 0.3-1.0Pa, and the substrate bias is -50V to +30V; The radio frequency power is 200-400W, the substrate temperature is 70-90℃, the working pressure is 0.3-1.0Pa, and the substrate bias is -50V to +30V.

[0018] Preferably, when the thin film material is Al, a substrate bias of +10V to +30V is applied, and the thin film compressive stress is +30 to +80MPa; When the thin film material is TiN, a substrate bias of -20V to -40V is applied, and the thin film compressive stress is -150 to -300MPa.

[0019] In the method for preparing the microlens array, the environment gas in the magnetron sputtering is at least one of N2 and Ar.

[0020] Preferably, the magnetron sputtering uses a mixed gas of N2 and Ar with a volume ratio of 1: (2-4).

[0021] In the method for preparing the microlens array, the gradient heat reflow treatment is carried out in an inert gas environment at a temperature rising rate of 1-3℃ / min at 120-150℃ for 1-3min, and then the temperature is continuously raised to 170-220℃ for 5-10min; wherein the oxygen content in the inert gas environment is 1ppm < oxygen content ≤100ppm.

[0022] Preferably, the inert gas is nitrogen or argon, and the temperature fluctuation in the first stage of the gradient heat reflow process is ≤±2℃, and the temperature fluctuation in the second stage is ≤±1℃.

[0023] The present invention requires controlling the oxygen content to a range below 100 ppm but above trace levels (e.g., >1 ppm), preferably within the range of 30-100 ppm. While pure nitrogen or an environment with very low oxygen content (e.g., <1 ppm) reduces oxidation, it fails to actively create conditions for forming a uniform surface oxide layer, resulting in insufficient control of the surface energy gradient. The present invention utilizes a certain amount of oxygen to participate in the reaction. During the first stage of the gradient thermal reflow treatment (holding at 120-150°C), in an oxygen-containing environment (30-100 ppm O2), a controlled, slow oxidation reaction occurs on the surface of the metal or dielectric film (e.g., Al, TiN, Cr), forming a very thin (2-5 nm) but uniform and dense oxide layer (e.g., Al2O3, TiO2, Cr2O3). This oxide layer exhibits different surface chemical properties (primarily higher surface energy) than the unoxidized substrate film. The oxidation process occurs at a relatively low temperature, ensuring uniformity and controllability of the reaction. This uniform oxide layer establishes a consistent and controllable surface energy gradient on the micropillar surface. During the subsequent reflow stage at a higher temperature (170-220°C), as the photoresist softens and flows, the interfacial tension between it and the cover film (and oxide layer) is significantly affected by this uniform oxide layer. This uniform surface energy gradient guides the molten photoresist material to flow more uniformly and synchronously from the edge of the micropillar toward the center, significantly improving the curvature radius consistency of the resulting microlens array (>97%).

[0024] In the above-mentioned method for preparing a microlens array, an oxide layer with a thickness of 2-5 nm is formed on the surface of the film after the gradient thermal reflow treatment.

[0025] The present invention uses gradient reflux to keep the temperature at 120-150°C for 1-3 minutes in the first stage so that the photoresist reaches or slightly exceeds its glass transition temperature (Tg), begins to soften but does not completely flow. In an oxygen-containing environment, a uniform thin oxide layer (2-5nm) is formed on the film surface, and a controllable surface energy gradient is established (one of the core advantages of the present invention). The low-temperature stage avoids violent flow and deformation of the photoresist, providing stable conditions for the uniform formation of the surface oxide layer. Preheating makes the internal temperature of the photoresist more uniform, reducing stress in the subsequent high-temperature stage.

[0026] In the second stage, keep the temperature at 170-220℃ for 5-10min to make the photoresist temperature much higher than T g , fully softened and flowed, and formed into a smooth lens surface under the action of surface tension (controlled by the surface energy gradient established by the oxide layer). High temperature ensures that the photoresist fully flows and completes the lens molding; precise temperature control (fluctuation ≤±1℃) combined with the uniform surface energy gradient established in the first stage ensures high synchronization and consistency of microlens molding on the entire wafer.

[0027] It can be seen that the present application avoids uneven internal stress and flow out of control of photoresist caused by temperature rising; low-temperature oxidation and high-temperature forming steps are separated, and the effects of each are optimized; and finally, ultra-low surface roughness (<5nm) and ultra-high curvature radius consistency (>97%) are achieved.

[0028] In the method, the inductively coupled plasma etching power is 700-1000W, the RF bias is 120-180W, and the surface roughness of the substrate after etching is less than or equal to 5nm.

[0029] Compared with the prior art, the present application has the following beneficial effects: 1. After the photoresist is spin-coated on the surface of the substrate to form a microcolumn array, a thin film is deposited on the microcolumn by using a magnetron sputtering technique, the surface energy distribution of the photoresist is adjusted by precisely controlling the compressive stress of the thin film to be -200MPa to +80MPa, the surface energy is effectively reduced from 45-60mN / m to 25-35mN / m, and the flowability and formability of the material in the subsequent thermal reflow process are optimized. 2. The present application uses gradient thermal reflow processing, that is, gradually increasing the temperature and keeping it in an inert gas environment with an oxygen content of less than 100ppm, to ensure the uniformity and consistency of the microlens array in the forming process, reduce the surface roughness problem commonly seen in traditional processes, achieve a surface roughness of less than 5nm, which is about 67% lower than the traditional method, and the consistency of the curvature radius is also more than 97%. 3. In the inductively coupled plasma etching step, a specific proportion of Cl2 and BCl3 mixed gas is used for etching, which not only improves the etching selectivity to 5.8:1, which is 193% higher than the prior art, but also ensures that the surface roughness of the substrate after etching is not more than 5nm, further ensuring the optical performance of the final product. 4. The present application can be applied to various substrate materials such as silicon, quartz and sapphire, and has good adaptability to photoresists of different thicknesses (5-20um) and types (such as AZ series positive photoresist or SU-8 series negative photoresist), so that the diameter of the microlens can be flexibly adjusted between 10-100um. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is the surface energy gradient distribution diagram of the microlens array prepared by the present application embodiment 1 and the microlens array prepared by the conventional process of comparative example 1.

[0031] Figure 2 is the gradient heat reflow temperature control curve diagram of the microlens array prepared by the present application embodiment 1; a. Temperature control curve, b. Curvature radius distribution. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present application patent more clear, the following will be further described in detail in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application patent, and are not used to limit the present application patent.

[0033] The terms "comprising", "including", "containing", "have" or any other similar words used herein are intended to cover non-exclusive inclusion. For example, the composition, step, method, article or device containing the listed elements does not necessarily limit to only those elements, but can include other elements not explicitly listed or inherent to such composition, step, method, article or device.

[0034] The description of the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" and the like described in the present application means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms is not necessarily directed to the same embodiment or example. Moreover, the technical features involved in each embodiment of the present application can be combined with each other as long as there is no conflict.

[0035] The following will be explained in combination with specific embodiments: Embodiment 1

[0036] S1, spin coating photoresist layer on the surface of the substrate; The substrate is a 4-inch silicon wafer spin-coated with AZ4620 photoresist (thickness 9.5 μm) Spin coating process: spin coating speed 1500 rpm, time 45 seconds; Soft Bake temperature 110℃, time 180 seconds.

[0037] S2, forming a 98 μm diameter micropillar array by photolithography process; Photolithography process: exposure wavelength 365 nm (i-line), exposure dose 800 mJ / cm2; development using 2.38% TMAH developer, development time 300 seconds; hard bake (Hard Bake) temperature 110°C, time 120 seconds.

[0038] S3, then a magnetron sputtering is used to deposit a thin film with a thickness of 30 nm on the surface of the micro column array; The magnetron sputtering target material is Al with a purity of 99.999%, a direct current power of 200 W, an Ar concentration of 40 sccm, a cavity pressure of 0.5 Pa, a substrate temperature of 60°C, a substrate bias of +20 V, and a control of the film compressive stress to +50 MPa.

[0039] S4, then gradient thermal reflow treatment is performed in an N2 environment containing 30 ppm of oxygen at a temperature increasing rate of 3°C / min at 120°C for 2 min, and then the temperature is continuously increased to 180°C for 5 min to form an oxidation layer with a thickness of 3 nm; S5, the thin film is used as an etching mask, and inductive coupled plasma etching is performed to transfer the microlens topography to the substrate; the inductive coupled plasma etching power is 800 W, the RF bias is 150 W, the etching depth is 15 μm, and the surface roughness of the substrate after etching is ≤5 nm; the etching is performed using a mixed gas of Cl2 and BCl3 with a volume ratio of 4:1, and the flow rate of the mixed gas is 50 sccm. Example 2

[0040] S1, a photoresist layer is spin-coated on the surface of the substrate; The substrate is a 4-inch silicon wafer spin-coated with AZ4620 photoresist (thickness 9.5 μm) Spin-coating process: spin-coating speed 1500 rpm, time 45 seconds; soft bake (Soft Bake) temperature 110°C, time 180 seconds.

[0041] S2, a micro column array with a diameter of 98 μm is formed through a photolithography process; Photolithography process: exposure wavelength 365 nm (i-line), exposure dose 800 mJ / cm2; development using 2.38% TMAH developer, development time 300 seconds; hard bake (Hard Bake) temperature 110°C, time 120 seconds.

[0042] S3, then a magnetron sputtering is used to deposit a TiN thin film with a thickness of 20 nm on the surface of the micro column array; The magnetron sputtering target material is Ti with a purity of 99.999%, a direct current power of 300 W, a N2 / Ar=1:3 concentration of 40 sccm, a cavity pressure of 0.5 Pa, a substrate temperature of 60°C, a substrate bias of -30 V, and a control of the film compressive stress to -300 MPa.

[0043] S4, then in the N2 environment containing 30 ppm oxygen, heat at a rate of 3 °C / min to 120 °C for 2 min, then continue to heat to 180 °C for 5 min to form a 3 nm oxidation layer by gradient thermal reflow treatment; S5, again using the film as an etching mask, inductively coupled plasma etching is performed to transfer the microlens topography to the substrate; the inductively coupled plasma etching power is 800 W, the RF bias is 150 W, the etching depth is 15 μm, and the surface roughness of the substrate after etching is ≤5 nm, wherein the etching is performed using a Cl2 and BCl3 mixed gas with a volume ratio of 4:1, and the mixed gas flow is 50 sccm. Example 3

[0044] S1, spin-coating a photoresist layer on the surface of the substrate; The substrate is a 4-inch silicon wafer spin-coated with AZ4620 photoresist (thickness 5 μm) Spin-coating process: spin-coating speed 1500 rpm, time 45 seconds; Soft Bake temperature 110 °C, time 180 seconds.

[0045] S2, forming a 20 μm diameter microcolumn array by a photolithography process; Photolithography process: exposure wavelength 365 nm (i-line), exposure dose 800 mJ / cm2; development using 2.38% TMAH developer, development time 300 seconds; Hard Bake temperature 110 °C, time 120 seconds.

[0046] S3, then depositing a 30 nm thick film on the surface of the microcolumn array by magnetron sputtering; The magnetron sputtering target material is Al with a purity of 99.999%, a direct current power of 200 W, an Ar concentration of 40 sccm, a cavity pressure of 0.5 Pa, a substrate temperature of 60 °C, a substrate bias of +20 V, and a control film compressive stress of +80 MPa.

[0047] S4, then in the N2 environment containing 5 ppm oxygen, heat at a rate of 3 °C / min to 120 °C for 2 min, then continue to heat to 180 °C for 5 min to form a 3 nm oxidation layer by gradient thermal reflow treatment; S5, again using the film as an etching mask, inductively coupled plasma etching is performed to transfer the microlens topography to the substrate; the inductively coupled plasma etching power is 800 W, the RF bias is 150 W, the etching depth is 15 μm, and the surface roughness of the substrate after etching is ≤5 nm, wherein the etching is performed using a Cl2 and BCl3 mixed gas with a volume ratio of 4:1, and the mixed gas flow is 50 sccm. Example 4

[0048] The difference from Example 1 is that the magnetron sputtering film thickness is 10 nm. Example 5

[0049] The difference from Example 1 is that the magnetron sputtering film thickness is 100 nm. Example 6

[0050] The difference from Example 1 is that the oxygen content in the gradient thermal reflow process is 0. Example 7

[0051] The difference from Example 1 is that the oxygen content in the gradient thermal reflow process is 150 ppm. Example 8

[0052] The difference from Example 1 is that the thermal reflow is only carried out at 120℃ for 2 min with a heating rate of 3℃ / min. Example 9

[0053] The difference from Example 1 is that the thermal reflow is only carried out at 180℃ for 5 min with a heating rate of 3℃ / min. Comparative Example 1

[0054] S1, spin AZ4620 photoresist (thickness 9.5 μm) on the surface of a 4-inch silicon wafer, the spin parameters are the same as in Example 1;

[0055] S2, form a 98 μm diameter microcolumn array by photolithography process (process same as Example 1);

[0056] S3, directly perform single-stage thermal reflow treatment in air environment: heat to 180℃ at 5℃ / min and keep for 10 min;

[0057] S4, perform inductively coupled plasma etching (parameters same as Example 1), and transfer the microlens topography to the substrate. Comparative Example 2

[0058] The difference from Example 1 is that the magnetron sputtering film in step S2 is not performed. Comparative Example 3

[0059] The difference from Example 1 is that the film compressive stress is controlled to +150 MPa by bias voltage. Comparative Example 4

[0060] The difference from Example 2 is that the film compressive stress is controlled to -350 MPa by bias voltage.

[0061] Table 1: Performance test results of microlens arrays prepared in Examples and Comparative Examples

[0062] "Table 1 data verification: compressive stress out of limit (comparative example 3 / 4) leads to unshaping; oxygen content out of range (example 6 / 7) makes roughness >10 nm; two-stage gradient reflow missing (example 8 / 9) significantly reduces uniformity".

[0063] Figure 1 is the surface energy gradient distribution diagram of the microlens array prepared by example 1 of the present application and the microlens array prepared by the traditional process of comparative example 1. From Figure 1 It can be seen that the surface energy gradient distribution of comparative example 1 (traditional process) presents significant fluctuations (the range is up to 30-95 mN / m), while the surface energy gradient of example 1 is stable in the range of 30-55 mN / m. This shows that: The surface energy distribution of comparative example 1 is uneven, and the surface energy gradient of the photoresist of comparative example 1 is disordered due to the absence of film regulation, which leads to an increase in material flow asynchrony (the difference in flow rate of adjacent micropillars is >40%) during thermal reflow, causing edge collapse and curvature deviation; the oxide layer of comparative example 1 is missing, and the surface of comparative example 1 randomly generates a non-uniform oxide layer (the thickness fluctuates by 5-20 nm) during thermal reflow in an air environment, further exacerbating the surface roughness (the measured Ra=16.3 nm); the flow control of comparative example 1 fails, and the surface energy gradient makes the material shrinkage direction uncontrollable, and the curvature consistency of the microlens is only 75.2% (compared with 97.6% of example 1).

[0064] Figure 2 is the gradient thermal reflow temperature control curve diagram of the microlens array prepared by example 1 of the present application; a. temperature control curve, b. curvature radius distribution. From the diagram, it can be seen that example 1 solves the problems of poor microlens morphology consistency, high surface roughness, and high collapse rate by controlling the film compressive stress and gradient thermal reflow, and is compatible with the semiconductor process.

[0065] In summary, the present application can be applied to various substrate materials such as silicon, quartz and sapphire, and has good adaptability to photoresists of different thicknesses (5-20 μm) and types (such as AZ series positive photoresist or SU-8 series negative photoresist), so that the microlens diameter range can be flexibly adjusted between 10-100 μm. This new microlens array preparation method successfully solves the problems of poor surface quality and low shape precision in the traditional process by introducing surface energy regulation and gradient thermal reflow treatment, while improving the compatibility and stability of the process, providing a new way for efficient and low-cost manufacturing of high-quality microlens arrays.

[0066] The embodiments of the present application do not exhaust the technical range of the claimed technical scope of the present application, and the new technical solutions formed by the same or multiple technical features in the embodiments of the present application are also within the scope of the claimed technical scope of the present application. Meanwhile, in all the listed or unlisted embodiments of the present application, the parameters in the same embodiment only represent one example (i.e. one feasible solution) of the technical solution, and there is no strict cooperation and limitation relationship between the parameters, and the parameters can be replaced with each other without violating the axioms and the claims of the present application, except for the specifically declared.

[0067] The technical means disclosed in the present application is not limited to the technical means disclosed in the above technical means, but also includes the technical solutions composed of any combination of the above technical features. The above is the specific implementation of the present application, and it should be pointed out that for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements are also considered to be within the scope of the present application.

[0068] The specific embodiments described herein are merely illustrative of the present application. Those skilled in the art can make various modifications or supplements to the described specific embodiments or use similar ways to replace them, without departing from the spirit of the present application or exceeding the scope defined by the appended claims.

Claims

1. A method for preparing a microlens array, characterized in that: The method comprises the following steps: S1, spin coating a photoresist layer on the surface of the substrate, and forming a micro-pillar array through a photolithography process; S2, depositing a thin film on the surface of the micropillar array by magnetron sputtering, wherein the compressive stress of the film is controlled to be -300 MPa to +80 MPa by using a substrate bias voltage of -50 V to +30 V during the magnetron sputtering process; S3, then performing gradient thermal reflow treatment in an inert gas environment; S4. Using the thin film as an etching mask, inductively coupled plasma etching is performed to transfer the microlens morphology to the substrate.

2. The method for preparing a microlens array according to claim 1, wherein: The substrate comprises at least one of silicon, quartz and sapphire materials, wherein the surface flatness of the substrate is less than or equal to 0.1 μm.

3. The method for preparing a microlens array according to claim 1, wherein: The thickness of the photoresist layer is 5-20 μm, and the photoresist is selected from AZ series positive photoresist or SU-8 series negative photoresist.

4. The method for preparing a microlens array according to claim 1, wherein: The diameter of the micropillar array is 10-100 μm.

5. The method for preparing a microlens array according to claim 1, wherein: The film is a metal film or a dielectric film with a thickness of 5-50 nm, and the film material includes at least one of Al, TiN, and Cr.

6. The method for preparing a microlens array according to claim 1, wherein: Magnetron sputtering uses a DC power supply or a radio frequency power supply; The DC power supply used is 150-250W, the substrate temperature is 50-90℃, the working pressure is 0.3-1.0Pa, and the substrate bias voltage is -50V to +30V; The RF power used is 200-400W, the substrate temperature is 70-90°C, the working gas pressure is 0.3-1.0Pa, and the substrate bias is -50V to +30V.

7. The method for preparing a microlens array according to claim 1, wherein: The ambient gas in magnetron sputtering is at least one of N2 and Ar.

8. The method for preparing a microlens array according to claim 1, wherein: The gradient thermal reflux treatment is carried out in an inert gas environment at a heating rate of 1-3°C / min, keeping the temperature at 120-150°C for 1-3 minutes, and then continuing to heat to 170-220°C and keeping the temperature for 5-10 minutes; wherein the oxygen content in the inert gas environment is: 1ppm<oxygen content≤100ppm.

9. The method for preparing a microlens array according to claim 1, wherein: After gradient thermal reflow treatment, an oxide layer with a thickness of 2-5 nm is formed on the surface of the film.

10. The method for preparing a microlens array according to claim 1, wherein: The inductively coupled plasma etching power is 700-1000W, the RF bias is 120-180W, and the surface roughness of the substrate after etching is ≤5nm. The etching is performed using a mixed gas of Cl2 and BCl3 with a volume ratio of (3-5):1, and the mixed gas flow rate is 40-60sccm.

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