Preparation Method of Micro-Lens Array with Small Numerical Aperture

Abstract

The present invention discloses a preparation method for a micro-lens array with a small numerical aperture. Through process steps such as the preparation of a silicon-based surface glue layer, exposure and overlay, development, hot melting, and etching, by setting the spin coating of photoresist as multi-layer spin coating, the photoresist materials and spin coating parameters of different glue layers are different, and the viscosities of the three layers of photoresist are significantly distinguishable, which is the key factor affecting the subsequent formation of micro-lenses with a small numerical aperture; circular masks with different areas are designed, and precise control of the micro-lens aperture after hot melting can be achieved through overlay exposure, improving the surface flatness uniformity of the micro-lens array within the silicon-based wafer; the temperature and time of hot melting baking are the key to the formation of micro-lenses by the fusion of the double-layer photoresist structure; selecting etching parameters with an etching rate of the glue faster than that of silicon can further increase the curvature radius of the micro-lens, reduce the small numerical aperture and volume of the micro-lens, thereby effectively realizing the preparation of the micro-lens array with a small numerical aperture.

Description

Technical Field

[0001] The present invention relates to the technical field of microlenses, and more specifically to a method for fabricating a microlens array with a small numerical aperture. Background Art

[0002] Microlenses and microlens arrays have been increasingly widely used in displays, three-dimensional imaging, fiber optic coupling, detector arrays, and optical processor interconnections. Silicon-based microlenses have been successfully applied to infrared charge-coupled devices and infrared sensors. The hot-melt method for fabricating microlenses is simple and low-cost, and can be completed using conventional lithography processes. However, this method has a high failure rate when the numerical aperture of the microlens is small. Due to the large aspect ratio of the lens, the shape of the microlens is prone to collapse and cannot form a spherical surface. The silicon-based microlens array with a small numerical aperture has the advantages of small volume, small field of view angle, and good collimation. The preparation of a silicon-based microlens array with a small numerical aperture is still challenging. Summary of the Invention

[0003] In view of the deficiencies of the prior art, the present invention provides a method for fabricating a microlens array with a small numerical aperture. The fabrication method prepares the microlens array with a small numerical aperture through process steps such as silicon-based surface adhesive layer preparation, exposure and overlay, development, hot melting, and etching.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] A method for fabricating a microlens array with a small numerical aperture, comprising the following steps:

[0006] S1. Silicon-based surface adhesive layer preparation: Take a silicon-based wafer, and spin-coat three layers of photoresist on the surface of the silicon-based wafer. After each layer of photoresist is coated, it is baked and cooled to room temperature before the next spin-coating;

[0007] S2.1. Exposure and overlay: Prepare three masks. The first mask is a small-area circular array, the second mask is a medium-area circular array, and the third mask is a large-area circular array. The circular area is opaque and the non-circular area is transparent. There are alignment marks on the upper, lower, left, and right edges of the mask. The circular centers of the same position of the three masks coincide;

[0008] S2.2. Respectively place the first, second, and third masks into the exposure machine to expose the silicon-based wafer with the adhesive layer, and adjust the exposure parameters according to the thickness of the adhesive layer;

[0009] S3. Development: Place the silicon-based wafer with the adhesive layer after three exposures into the developer, and develop it using a developer containing potassium hydroxide. Adjust the concentration of the developer and the development time according to the thickness of the adhesive layer. Develop until there is no bottom adhesive residue in the non-circular area of the first layer, then rinse it with DI water by rotation and dry it with nitrogen;

[0010] S4, Heat Melting: Set the temperature of the hot plate. After the hot plate temperature reaches the set temperature, place the developed silicon-based wafer on the hot plate for baking. After baking is completed, take out the silicon-based wafer and cool it to room temperature;

[0011] S5, Etching: Place the baked silicon-based wafer in the etching device, set the etching parameters, adjust the silicone etching ratio, and use process parameters with a faster etching rate for the photoresist to further reduce the numerical aperture of the silicon-based microlens and increase the lens curvature radius. After etching is completed, take out the silicon-based wafer to complete the preparation.

[0012] Further, step S1 includes:

[0013] S1.1, First Layer Spin Coating: Prepare a positive low-viscosity photoresist, and spin coat the positive low-viscosity photoresist evenly on the surface of the silicon-based wafer through a spin coater. After spin coating is completed, put it into a baking device for baking. After baking is completed, take it out and cool it to room temperature;

[0014] S1.2, Second Layer Spin Coating: Prepare a positive medium-viscosity photoresist, and spin coat the positive medium-viscosity photoresist evenly on the first layer of photoresist on the surface of the silicon-based wafer through a spin coater. After spin coating is completed, put it into a baking device for baking. After baking is completed, take it out and cool it to room temperature;

[0015] S1.3, Third Layer Spin Coating: Prepare a positive high-viscosity photoresist, and spin coat the positive high-viscosity photoresist evenly on the second layer of photoresist on the surface of the silicon-based wafer through a spin coater. After spin coating is completed, put it into a baking device for baking. After baking is completed, take it out and cool it to room temperature;

[0016] Further, for the baking temperatures of the three layers of photoresist in step S1, the first layer of photoresist > the second layer of photoresist > the third layer of photoresist.

[0017] Further, the baking temperature in step S1.1 is 110°C, and the baking time is 5 min. The baking temperature in step S1.2 is 100°C, and the baking time is 10 min. The baking temperature in step S1.3 is 80°C, and the baking time is 30 min.

[0018] Further, the temperature of the hot plate in step S4 is 160 - 170°C, and the baking time is 30 - 60 min.

[0019] Further, in step S5, adjust the silicone etching ratio by adjusting the gas parameters of O2, CF4, and SF6.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] The present invention prepares a silicon-based surface glue layer, performs exposure and overlay, develops, hot-melts, and etches. By setting the photoresist spin coating to multi-layer spin coating, the photoresist materials and spin coating parameters of different glue layers are different, and the viscosities of the three-layer photoresist are significantly different, which is the key factor affecting the formation of micro-lenses with small numerical apertures in the subsequent process. Circular masks with different areas are designed. After exposure and overlay, precise control of the micro-lens aperture after hot-melting of the lens can be achieved, improving the surface flatness uniformity of the micro-lens array in the silicon-based wafer. The temperature and time of hot-melting baking are the keys to the formation of micro-lenses by the fusion of the double-layer photoresist structure. Selecting etching parameters with an etching rate of the glue faster than that of silicon can further increase the curvature radius of the micro-lens, reduce the small numerical aperture and volume of the micro-lens, thereby effectively realizing the preparation of a micro-lens array with a small numerical aperture. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for description in the embodiments. Obviously, the 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 drawings can also be obtained based on these drawings, where:

[0023] Figure 1 is a flowchart of a preparation method for a micro-lens array with a small numerical aperture;

[0024] Figure 2 is a flowchart of the preparation of a silicon-based surface glue layer;

[0025] Figure 3 are schematic structural diagrams of a first mask, a second mask, and a third mask;

[0026] Figure 4 is a schematic diagram of exposure and overlay;

[0027] Figure 5 are schematic diagrams of development, hot-melting, and etching.

[0028] The labels in the figure are: 1, silicon-based wafer; 2, first-layer photoresist; 3, second-layer photoresist; 4, third-layer photoresist; 5, first mask; 6, second mask; 7, third mask. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] In the description of the present invention, it should be noted that for the orientation terms, such as the terms "center", "transverse (X)", "longitudinal (Y)", "vertical (Z)", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., the indicated orientation and positional relationships are based on the orientation or positional relationships shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and should not be construed as limiting the specific protection scope of the present invention.

[0030] In addition, such terms as "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, the meanings of "several" and "a number of" are two or more, unless otherwise specifically defined.

[0031] A method for preparing a micro-lens array with a small numerical aperture includes the following steps:

[0032] S1. Preparation of a silicon-based surface glue layer: Take a silicon-based wafer 1, and spin-coat three layers of photoresist on the surface of the silicon-based wafer 1, namely the first layer of photoresist 2, the second layer of photoresist 3, and the third layer of photoresist 4. After each layer of photoresist is coated, it is baked and cooled to room temperature before the next spin-coating.

[0033] S2.1. Exposure and alignment: Prepare three masks. The first mask 5 is a small-area circular array, the second mask 6 is a medium-area circular array, and the third mask 7 is a large-area circular array. The circular areas are opaque and the non-circular areas are transparent. There are alignment marks on the upper, lower, left, and right edges of the masks, and the circular centers at the same positions of the three masks coincide.

[0034] S2.2. Respectively place the first mask 5, the second mask 6, and the third mask 7 into an exposure machine to expose the silicon-based wafer 1 with glue, and the exposure parameters are adjusted according to the thickness of the glue layer.

[0035] S3. Development: Put the silicon-based wafer 1 with glue after being exposed three times into a developing machine, and use a developer containing potassium hydroxide for development. The concentration of the developer and the development time are adjusted according to the thickness of the glue layer. Develop until there is no bottom glue residue in the non-circular area of the first layer, then rinse it with DI water by rotation and dry it with nitrogen.

[0036] S4. Heat melting: Set the temperature of a hot plate. After the temperature of the hot plate reaches the set temperature, place the developed silicon-based wafer 1 on the hot plate to start baking. After baking is completed, take out the silicon-based wafer 1 and cool it to room temperature.

[0037] S5, Etching: Place the baked silicon-based wafer 1 in an etching device, set the etching parameters, adjust the silicone etching ratio, adopt process parameters with a faster etching rate for the photoresist, further reduce the numerical aperture of the silicon-based microlens, increase the lens curvature radius, take out the silicon-based wafer 1 after etching is completed, and complete the preparation.

[0038] Preferably, the step S1 includes:

[0039] S1.1, First-layer spin coating: Prepare a positive low-viscosity photoresist, and spin coat the positive low-viscosity photoresist evenly on the surface of the silicon-based wafer 1 through a spin coater. The spin coating speed of the spin coater can be modified according to actual needs. After spin coating is completed, put it into a baking device for baking. After baking is completed, take it out and cool it to room temperature;

[0040] S1.2, Second-layer spin coating: Prepare a positive medium-viscosity photoresist, and spin coat the positive medium-viscosity photoresist evenly on the first-layer photoresist 2 on the surface of the silicon-based wafer 1 through a spin coater. The spin coating speed of the spin coater can be modified according to actual needs. After spin coating is completed, put it into a baking device for baking. After baking is completed, take it out and cool it to room temperature;

[0041] S1.3, Third-layer spin coating: Prepare a positive high-viscosity photoresist, and spin coat the positive high-viscosity photoresist evenly on the second-layer photoresist 3 on the surface of the silicon-based wafer 1 through a spin coater. The spin coating speed of the spin coater can be modified according to actual needs. After spin coating is completed, put it into a baking device for baking. After baking is completed, take it out and cool it to room temperature;

[0042] Preferably, for the baking temperatures of the three layers of photoresist in the step S1, the first-layer photoresist 2 > the second-layer photoresist 3 > the third-layer photoresist 4.

[0043] Preferably, the baking temperature in the step S1.1 is 110 °C and the baking time is 5 min. The baking temperature in the step S1.2 is 100 °C and the baking time is 10 min. The baking temperature in the step S1.3 is 80 °C and the baking time is 30 min.

[0044] Preferably, the temperature of the hot plate in the step S4 is 160 - 170 °C and the baking time is 30 - 60 min.

[0045] Preferably, in the step S5, adjust the silicone etching ratio by adjusting the gas parameters of O2, CF4, and SF6. Specifically, by increasing the O2 flow rate and decreasing the CF4 and SF6 flow rates, the highest achievable silicone etching ratio is 1:3.

[0046] Working principle and advantages:

[0047] 1. In the silicon-based surface preparation stage: By setting the baking temperatures of the three layers of photoresist as the first layer of photoresist 2 > the second layer of photoresist 3 > the third layer of photoresist 4, the activity of the photosensitizer in the first layer of photoresist 2 is the lowest, the second layer of photoresist 3 is the second, and the third layer of photoresist 4 is the highest. The exposure energy required for different photoresist layers is the first layer of photoresist 2 > the second layer of photoresist 3 > the third layer of photoresist 4. Therefore, after exposure with the first mask 5, the development rate of the first layer of photoresist 2 is the slowest, the development rate of the second layer of photoresist 3 is medium, and the development rate of the third layer of photoresist 4 is the fastest.

[0048] 2. Exposure and overlay stage: Since only one glue is usually used for spin coating and one mask is used for exposure in the existing technology, the result after final development is that there is photoresist covering the entire wafer at the bottom, and it is difficult to precisely control the bottom glue thickness. At the same time, it is also difficult to control the diameter size after the micro-lens reflux; therefore, in order to precisely control the diameter of the micro-lens after reflux, other masks need to be introduced to limit the diameter of the micro-lens after heat melting. Thus, in the present invention, by designing multiple circular masks with different areas, precise control of the diameter of the micro-lens after heat melting of the lens can be achieved through overlay exposure, improving the surface uniformity of the micro-lens array in the silicon-based wafer 1.

[0049] 3. In the silicon-based surface preparation and exposure stage: Different combinations of cylinders with different thicknesses can be achieved by modifying the spin coating parameters of different glue layers, and a pyramid-shaped cylinder structure can be achieved by matching masks with different circular areas. Using this method, the diameter of the lens after heat melting can be controlled, and different-sized micro-lens arrays can be achieved through free combination.

[0050] 4. In the development and heat melting stage: After development, since the main materials of the three layers of photoresist are the same, the contact angle between the glue layers is smaller than that of the photoresist on the silicon-based surface, and the area of the photoresist is the first layer > the second layer > the third layer. Therefore, the photoresist has good fluidity from top to bottom; the structure of the multi-layer cylindrical steps can first make the third layer of photoresist 4 heat melt into a micro-lens through baking. Since the aspect ratio of the micro-lens formed by the third layer is small, the spherical surface is relatively easy to form and there will be no top depression; during the heat melting process of the third layer of micro-lens, it continues to flow and fuse with the second layer of photoresist 3. After fusion, the viscosity of the photoresist is lower than that of the third layer of photoresist 4, increasing the fluidity of the photoresist and forming a new micro-lens with a medium aspect ratio; the new micro-lens formed by the fusion of the third layer and the second layer continues to flow and fuse with the first layer of photoresist 2 during the heat melting process. After fusion, the viscosity of the photoresist continues to decrease, further increasing the fluidity of the photoresist and forming a photoresist micro-lens structure with a large aspect ratio and a small numerical aperture.

[0051] 5. In the etching stage: The etching rate of the photoresist and silicon can be adjusted according to actual needs to further reduce the numerical aperture, and the photoresist micro-lens array pattern can be transferred into a silicon-based micro-lens array with a larger aspect ratio and a smaller numerical aperture.

[0052] The above are only the preferred embodiments of the present invention. The protection scope of the present invention is not limited to the above embodiments. Any technical solutions falling within the concept of the present invention belong to the protection scope of the present invention. It should be noted that for those of ordinary skill in the art, several improvements and refinements made without departing from the principle of the present invention should also be regarded as within the protection scope of the present invention.

Claims

1. A method for preparing a microlens array with a small numerical aperture, characterized in that: The steps include: S1. Preparation of silicon-based surface adhesive layer: take a silicon-based wafer, and spin-coat three layers of photoresist on the surface of the silicon-based wafer. After each layer of photoresist is coated, it is baked and cooled to room temperature before the next spin coating. The first layer of photoresist is a positive low-viscosity photoresist, the second layer of photoresist is a positive medium-viscosity photoresist, and the third layer of photoresist is a positive high-viscosity photoresist. The baking temperatures of the three layers of photoresist are compared: the first layer of photoresist> the second layer of photoresist> the third layer of photoresist; S2.1, Exposure overlay: Prepare three masks, mask No. 1 is a small-area circular array, mask No. 2 is a medium-area circular array, and mask No. 3 is a large-area circular array. The circular area is opaque, and the non-circular area is transparent. There are alignment marks on the upper, lower, left and right edges of the masks, and the center points of the circles at the same position of the three masks coincide; S2.2, respectively put the No. 1, No. 2 and No. 3 masks into the exposure machine to expose the silicon-based wafer with glue, and adjust the exposure parameters according to the thickness of the glue layer; S3, development: after three exposures, the silicon-based wafer with adhesive is placed in a developer, and developed with a developer containing potassium hydroxide. The developer concentration and the development time are adjusted according to the thickness of the adhesive layer. The first layer of non-circular area is developed until there is no primer residue, and then it is rinsed with DI water and blown dry with nitrogen. S4, hot melting: set the temperature of the hot plate. When the temperature of the hot plate reaches the set temperature, place the developed silicon-based wafer on the hot plate and start baking. After baking, take out the silicon-based wafer and cool it to room temperature. S5, etching: Place the baked silicon-based wafer in the etching device, set the etching parameters, adjust the silica gel etching ratio, use process parameters with a faster photoresist etching rate, further reduce the numerical aperture of the silicon-based microlens, and increase the lens curvature radius. After etching, take out the silicon-based wafer to complete the preparation.

2. The method for preparing a small numerical aperture microlens array according to claim 1, characterized in that: Step S1 includes: S1.1, first layer of photoresist: prepare positive low-viscosity photoresist, and evenly spin-coat the positive low-viscosity photoresist onto the surface of the silicon-based wafer through a photoresist spinner. After spin coating, put it into a baking device for baking. After baking, take it out and cool it to room temperature; S1.2, second layer coating: prepare positive medium viscosity photoresist, and evenly spin coat the positive medium viscosity photoresist on the first layer of photoresist on the surface of the silicon-based wafer through a coating machine. After the spin coating is completed, put it into a baking device for baking. After the baking is completed, take it out and cool it to room temperature; S1.3, third layer coating: prepare positive high-viscosity photoresist, and use a coating machine to evenly spin-coat the positive high-viscosity photoresist onto the second layer of photoresist on the surface of the silicon-based wafer. After spin coating, put it into a baking device for baking. After baking, take it out and cool it to room temperature.

3. The method for preparing a small numerical aperture microlens array according to claim 2, characterized in that: The baking temperature in step S1.1 is 110° C. and the baking time is 5 min. The baking temperature in step S1.2 is 100° C. and the baking time is 10 min. The baking temperature in step S1.3 is 80° C. and the baking time is 30 min.

4. The method for preparing a microlens array with a small numerical aperture according to claim 1, characterized in that: In step S4, the temperature of the hot plate is 160-170° C., and the baking time is 30-60 minutes.

5. The method for preparing a microlens array with a small numerical aperture according to claim 1, characterized in that: In step S5, the silica gel etching ratio is adjusted by adjusting the gas parameters of O2, CF4, and SF6.

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