Preparation method of small numerical aperture microlens array

Abstract

The invention discloses a method for preparing a small numerical aperture microlens array, which comprises the following steps of: preparing a silicon-based surface adhesive layer, exposing and overlay, developing, hot melting, etching and the like, setting photoresist spin-coating to be multi-layer spin-coating, enabling photoresist materials and spin-coating parameters of different adhesive layers to be different, enabling the viscosities of three layers of photoresist to be obviously distinguished, and improving the resolution of the three layers of photoresist. The factor is a key factor for influencing subsequent small numerical aperture microlens forming; circular masks with different areas are designed, precise control on the aperture of the micro lens after hot melting of the lens can be realized after overlay exposure, and the surface uniformity of the micro lens array in the silicon-based wafer is improved; the temperature and time of hot melting and baking are the key of fusing the double-layer photoresist structure to form the micro lens; the etching parameters with the etching speed higher than that of silicon are selected, the curvature radius of the microlens can be further increased, the small numerical aperture and the size of the microlens are reduced, and therefore preparation of the small numerical aperture microlens array is effectively achieved.

Description

Technical Field

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

[0002] Microlenses and microlens arrays have been increasingly widely used in displays, stereoscopic imaging, fiber coupling, detector arrays, and optical processor interconnection. Silicon-based microlenses have been successfully used in infrared charge-coupled devices and infrared sensors. The hot-melt method for making microlenses is simple and low-cost, and can be completed using conventional photolithography processes. However, this method has a high failure rate when the numerical aperture of the microlens is small, because the lens is relatively large in terms of lateral and longitudinal dimensions, and the shape of the microlens is prone to collapse and cannot form a spherical surface. Small numerical aperture microlens arrays have the advantages of small size, small field of view, and good collimation. The preparation of silicon-based small numerical aperture microlens arrays is still challenging. Summary of the invention

[0003] In view of the shortcomings of the prior art, the present invention provides a method for preparing a small numerical aperture microlens array, which prepares the small numerical aperture microlens array through the process steps of preparing a silicon-based surface glue layer, exposure overlay, development, hot melting and etching.

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

[0005] A method for preparing a small numerical aperture microlens array comprises the following steps:

[0006] 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;

[0007] 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;

[0008] 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;

[0009] 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.

[0010] 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.

[0011] 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.

[0012] Further, step S1 comprises:

[0013] 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;

[0014] 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;

[0015] S1.3, third layer coating: prepare positive high viscosity photoresist, and evenly spin coat the positive high viscosity photoresist on the second 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;

[0016] Furthermore, the baking temperatures of the three layers of photoresist in step S1 are compared as follows: the first layer of photoresist> the second layer of photoresist> the third layer of photoresist.

[0017] Furthermore, 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] Furthermore, in step S4, the temperature of the hot plate is 160-170° C., and the baking time is 30-60 min.

[0019] Furthermore, in step S5, the silica gel etching ratio is adjusted by adjusting the O2, CF4, and SF6 gas parameters.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention adopts the process steps of preparing a silicon-based surface glue layer, exposure overlay, development, hot melting and etching, and sets the photoresist coating as a multi-layer coating. The photoresist materials and coating parameters of different glue layers are different, and the viscosity of the three-layer photoresist is obviously different, which is a key factor affecting the subsequent formation of a small numerical aperture microlens. Circular mask plates with different areas are designed, and the aperture of the microlens after hot melting can be accurately controlled after overlay exposure, thereby improving the surface uniformity of the microlens array in the silicon-based wafer. The temperature and time of hot melting baking are the key to the fusion of the double-layer photoresist structure to form the microlens. The etching parameter selection of the etching parameter with a faster etching rate than silicon can further improve the curvature radius of the microlens, reduce the small numerical aperture and volume of the microlens, and effectively realize the preparation of the small numerical aperture microlens array. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work, among which:

[0023] Figure 1 is a flow chart of a method for preparing a small numerical aperture microlens array;

[0024] Figure 2 Flow chart for the preparation of silicone-based surface adhesive layer;

[0025] Figure 3 It is a schematic diagram of the structure of mask No. 1, mask No. 2 and mask No. 3;

[0026] Figure 4 A schematic diagram of exposure overlay;

[0027] Figure 5 Schematic diagram of development, heat fusing and etching.

[0028] Marked in the figure: 1, silicon-based wafer; 2, first layer of photoresist; 3, second layer of photoresist; 4, third layer of photoresist; 5, mask No. 1; 6, mask No. 2; 7, mask No. 3. DETAILED DESCRIPTION

[0029] In the description of the present invention, it should be noted that directional words, such as the terms "center", "lateral (X)", "longitudinal (Y)", "vertical (Z)", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc., indicating directions and positional relationships are based on the directions or positional relationships shown in the accompanying drawings, which are only for the convenience of narrating the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and cannot be understood as limiting the specific scope of protection of the present invention.

[0030] In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features. Therefore, the definition of "first" and "second" features can explicitly or implicitly include one or more of the features. In the description of the present invention, "several" and "a number" mean two or more, unless otherwise clearly and specifically defined.

[0031] A method for preparing a small numerical aperture microlens array comprises the following steps:

[0032] S1, preparation of silicon-based surface adhesive layer: take silicon-based wafer 1, and spin-coat three layers of photoresist on the surface of silicon-based wafer 1, namely, first layer of photoresist 2, second layer of photoresist 3 and third layer of photoresist 4, and bake and cool to room temperature after each layer of photoresist is coated before the next spin coating;

[0033] S2.1, exposure overlay: prepare three masks, mask No. 1 5 is a small-area circular array, mask No. 2 6 is a medium-area circular array, and mask No. 3 7 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;

[0034] S2.2, respectively put the first mask 5, the second mask 6, and the third mask 7 into the exposure machine to expose the silicon-based wafer 1 with adhesive, and adjust the exposure parameters according to the thickness of the adhesive layer;

[0035] S3, development: after three exposures, the silicon-based wafer 1 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.

[0036] S4, hot melting: setting the temperature of the hot plate. When the temperature of the hot plate reaches the set temperature, the developed silicon-based wafer 1 is placed on the hot plate and begins to be baked. After baking, the silicon-based wafer 1 is taken out and cooled to room temperature.

[0037] S5, etching: placing the baked silicon-based wafer 1 in an etching device, setting etching parameters, adjusting the silica gel etching ratio, using process parameters with a faster photoresist etching rate, further reducing the numerical aperture of the silicon-based microlens, and increasing the lens curvature radius. After etching is completed, taking out the silicon-based wafer 1 to complete the preparation.

[0038] Preferably, the step S1 comprises:

[0039] S1.1, first layer coating: prepare positive low-viscosity photoresist, and evenly spin-coat the positive low-viscosity photoresist onto the surface of the silicon-based wafer 1 through a coating machine. The coating speed of the coating machine can be modified according to actual needs. After the 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;

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

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

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

[0043] Preferably, 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, and the baking temperature in step S1.3 is 80° C. and the baking time is 30 min.

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

[0045] Preferably, in step S5, the silica gel etching ratio is adjusted by adjusting the gas parameters of O2, CF4 and SF6. Specifically, by increasing the O2 flow rate and reducing the CF4 and SF6 flow rates, the highest silica gel etching ratio that can be achieved is 1:3.

[0046] Working principle and advantages:

[0047] 1. In the silicon-based surface preparation stage: by setting the baking temperature of the three layers of photoresist to 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 the exposure of mask No. 1 5, the development rate of the first layer of photoresist 2 is the slowest, the development speed of the second layer of photoresist 3 is medium, and the development speed of the third layer of photoresist 4 is the fastest.

[0048] 2. Exposure and overlay stage: Since the existing technology usually only uses one glue for uniform coating and one mask for exposure, the final result after development is that there is photoresist covering the entire wafer at the bottom. The thickness of the bottom glue is difficult to accurately control, and it is also difficult to control the aperture size of the microlens after reflow. Therefore, in order to accurately control the aperture of the microlens after reflow, it is necessary to introduce other masks to limit the aperture of the microlens after hot melting. Therefore, in the present invention, by designing multiple circular masks of different areas, the aperture of the microlens after lens hot melting can be accurately controlled after overlay exposure, thereby improving the surface uniformity of the microlens array in the silicon-based wafer 1.

[0049] 3. During the silicon-based surface preparation and exposure stage: By modifying the coating parameters of different adhesive layers, a combination of cylinders of different thicknesses can be achieved, and a pyramid-shaped cylindrical structure can be achieved by matching masks of different circular areas. Using this method, the aperture of the lens can be controlled after hot melting, and free matching can achieve microlens arrays of different sizes.

[0050] 4. In the development and hot-melt stage: after development, because the three layers of photoresist are mainly made of the same material, the contact angle between the layers is smaller than that of the photoresist on the silicon base, and the area of ​​the photoresist is the first layer> the second layer> the third layer, so the photoresist has good fluidity from top to bottom; the multi-layer cylindrical step structure can first allow the third layer of photoresist 4 to be hot-melted into a microlens through baking. Since the microlens formed by the third layer has a small width-to-length ratio, the spherical surface is easier to form and no top depression will occur; the third layer of microlens continues to flow and merge with the second layer of photoresist 3 during the hot-melt process. After the fusion, the viscosity of the photoresist is lower than that of the third layer of photoresist 4, which increases the fluidity of the photoresist and forms a new microlens with a medium width-to-length ratio; the new microlens after the third layer and the second layer are fused continues to flow and merge with the third layer of photoresist 2 during the hot-melt process. After the fusion, the viscosity of the photoresist continues to decrease, which further increases the fluidity of the photoresist and forms a photoresist microlens structure with a large width-to-length ratio and a small numerical aperture.

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

[0052] The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should also be regarded as 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; 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 evenly spin coat the positive high viscosity photoresist on the second 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; The baking temperatures of the three layers of photoresist in step S1 are compared: the first layer of photoresist> the second layer of photoresist> the third layer of photoresist.

3. The method for preparing a microlens array with a small numerical aperture 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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