Method for preparing a multi-level stepped microstructure

Abstract

The present invention provides a method for preparing a multi-level stepped microstructure, which comprises the following steps: (1) preparing a photoresist coating on a substrate; (2) designing an initial exposure layout, the initial exposure layout being a grayscale image, and the grayscale value of each pixel in the grayscale image being 0-255 gray levels; the grayscale value of each pixel on the initial exposure layout is correlated with the exposure dose of the ultraviolet beam to each point on the photoresist coating; (3) optimizing the initial exposure layout to obtain an optimized exposure layout; (4) importing the optimized exposure layout into an ultraviolet lithography system, and using ultraviolet lithography equipment to perform exposure processing on the photoresist coating; (5) performing a developing process on the exposed photoresist coating to generate a photoresist stepped microstructure corresponding to the optimized exposure layout on the photoresist coating. The method of the present invention can prepare stepped microstructures with small sizes, multiple step orders, and arbitrary heights with high precision.

Description

Technical Field

[0001] The present invention belongs to the field of microfabrication. Specifically, the present invention relates to a method for preparing a multi-level stepped microstructure. Background Art

[0002] Currently, most methods for preparing stepped microstructures adopt the method of multiple lithographic alignments. During the preparation process of this method, multiple alignment processes are required. It is precisely this multiple alignment that leads to a large error in this method, thereby affecting the precision of the prepared stepped microstructure.

[0003] Laser direct writing technology has attracted much attention in the field of processing various microstructures and devices due to its characteristics of not requiring a mask, high processing precision (sub-micron level), and the ability to process any structure. The current laser direct writing technology is also limited to the preparation of two-dimensional patterns, mainly because it is restricted by the modulation range of the acousto-optic modulator inside the laser direct writing equipment system.

[0004] In order to meet the high-precision processing requirements for high-performance three-dimensional structures in application fields such as diffractive optical elements, micro-optical lenses, and optical communications, especially for periodic or aperiodic stepped microstructures with arbitrary heights, new processes need to be developed to achieve the controllable processing of stepped microstructures with small sizes, multiple step orders, and arbitrary heights. Summary of the Invention

[0005] The object of the present invention is to provide a new method for preparing a multi-level stepped microstructure, which has high precision and can prepare stepped microstructures with small sizes, multiple step orders, and arbitrary heights.

[0006] The above object of the present invention is achieved by the following technical solutions.

[0007] In the context of the present invention, the term "length of the stepped microstructure" refers to the length of the stepped microstructure in a plane parallel to the photoresist coating.

[0008] In the context of the present invention, the term "width of the stepped microstructure" refers to the width of the stepped microstructure in a plane parallel to the photoresist coating.

[0009] In the context of the present invention, the term "thickness of the stepped microstructure" refers to the longitudinal depth of the stepped microstructure in a dimension perpendicular to the photoresist coating.

[0010] The present invention provides a method for preparing a multi-level stepped microstructure, which comprises the following steps:

[0011] (1) Prepare a photoresist coating on a substrate;

[0012] (2) Design the initial exposure layout, where the initial exposure layout is a grayscale image, and the grayscale value of each pixel in the grayscale image is in the range of 0 - 255 gray levels; the grayscale value of each pixel on the initial exposure layout is correlated with the exposure dose of the ultraviolet beam at each point on the photoresist coating;

[0013] (3) Optimize the initial exposure layout to obtain an optimized exposure layout, where optimizing the initial exposure layout is carried out by a method including the following steps:

[0014] (i) Measure the optical parameters of the used photoresist using an optical characterization device;

[0015] (ii) Draw a grayscale layout with gray levels from 0 to 255;

[0016] (iii) Expose the grayscale layout, and after exposure, obtain a relationship curve between the photoresist thickness and the gray value, i.e., the contrast curve;

[0017] (iv) Input the initial exposure layout, the optical parameters of the photoresist, the contrast curve, and the relationship curve between the gray value and the corresponding desired thickness value in the initial exposure layout into the optimization software to obtain the optimized exposure layout;

[0018] (4) Import the optimized exposure layout into an ultraviolet lithography system, and use the ultraviolet lithography equipment to perform exposure processing on the photoresist coating;

[0019] (5) Perform a development process on the exposed photoresist coating to generate a photoresist step microstructure corresponding to the optimized exposure layout on the photoresist coating.

[0020] The inventors of the present application unexpectedly found that the laser direct writing exposure technology can not only be used for the preparation of two-dimensional patterns, but also for the preparation of step microstructures, and can prepare step microstructures with small sizes, multiple step orders, and arbitrary heights with high precision. Without wishing to be bound by theory, this may be because the laser direct writing technology has certain three-dimensional processing potential, and through the regulation of relevant parameters, such as exposure dose and optimization of the exposure layout, the preparation of three-dimensional structures, such as multi-step and continuous phase relief microstructures, can be achieved. Therefore, the application field of the laser direct writing technology can also be further expanded to optical fields such as diffractive optical elements (DOEs) and microlenses, and even applied to the optical communication field.

[0021] The present invention provides a method for preparing a multi-level stepped microstructure by using a laser direct writing exposure technique. In this method, the gray value of the structural unit is defined, and then the gray value distribution (the relationship between the gray value and the exposure dose) is calculated and iteratively optimized by software, and the exposure and development are simulated. The dose assigned to each unit (pixel) is reasonably allocated so that the height of the stepped microstructure obtained by exposure has a certain correlation with the set gray value of the structural unit, which can be a negative correlation or a positive correlation. This method greatly exerts the potential of the laser direct writing technique in preparing three-dimensional structures and greatly improves the maximum energy control order of the laser direct writing equipment system.

[0022] Limited by the modulation range of the acousto-optic modulator inside the laser direct writing equipment system, the three-dimensional processing ability of laser direct writing is limited. Moreover, the topography generated by photolithography is mainly related to the geometric characteristics of the exposure pattern, the optical proximity effect, and the non-linear relationship between the photoresist and the exposure dose, and it is also difficult to control the stepped height of gray-scale processing. Through the method of the present invention, the above technical problems can be solved, and the preparation of stepped microstructures with small size, multiple stepped orders, and arbitrary height can be realized.

[0023] Preferably, in the method of the present invention, the optical characterization device is an ellipsometer.

[0024] Preferably, in the method of the present invention, the gray-scale layout of drawing 0-255 gray levels is carried out by taking 10 as the step and drawing 25 squares with different gray values.

[0025] Preferably, in the method of the present invention, the optical parameters are the refractive index n and the extinction coefficient k.

[0026] Preferably, in the method of the present invention, the substrate is selected from glass, quartz or silicon.

[0027] Preferably, in the method of the present invention, the photoresist is an ultraviolet photoresist.

[0028] Preferably, in the method of the present invention, the ultraviolet photoresist is sensitive to the exposure beam in the wavelength band of 350nm-450nm.

[0029] Preferably, in the method of the present invention, the ultraviolet photoresist is selected from one or more of AZ 4562, AZ4620 and AZ 6130.

[0030] Preferably, in the method of the present invention, the correlation in step (2) includes negative correlation and positive correlation.

[0031] Preferably, in the method of the present invention, the ultraviolet photolithography equipment is a laser direct writing exposure equipment, and its laser beam wavelength is 350nm-450nm.

[0032] Preferably, in the method of the present invention, the defocus amount of the laser direct writing exposure apparatus is 10 μm - 60 μm.

[0033] Preferably, in the method of the present invention, the spot size of the laser beam used for exposure by the laser direct writing exposure apparatus is 100 nm - 1 μm.

[0034] Preferably, in the method of the present invention, the length of the stepped micro-structure is 1 μm - 10 μm, the width is 1 μm - 10 μm, and the longitudinal thickness is 0 - 15 μm.

[0035] Preferably, in the method of the present invention, the method further includes the following steps after step (5):

[0036] Directionally etching the photoresist layer and the substrate to convert the photoresist stepped micro-structure into a substrate stepped micro-structure; or

[0037] Conformally depositing a material layer on the photoresist layer so that the photoresist stepped micro-structure is transferred to the deposited material layer.

[0038] In a specific embodiment of the present invention, preparing a photoresist coating includes: spin-coating a liquid photoresist onto the surface of a substrate to form a semi-solidified coating; baking the spin-coated photoresist to obtain a solid photoresist coating.

[0039] In a specific embodiment of the present invention, designing an exposure layout file includes drawing an initial exposure layout and optimizing the initial exposure layout. The exposure layout includes pixel points assigned with different gray values to form a gray-scale image. Optimizing the exposure layout includes optimizing the gray value distribution of the designed layout.

[0040] In a specific embodiment of the present invention, performing laser direct writing exposure on the photoresist coating includes using the exposure layout before and after optimization to expose the photoresist coating, and then performing the step of developing the exposed photoresist coating.

[0041] In a specific embodiment of the present invention, the flowchart of the method for preparing a photoresist stepped micro-structure can refer to Figure 1。In step 1, a photoresist coating is prepared. A cleaned substrate, such as a silicon wafer, a quartz wafer, or other substrates with a flat and smooth surface, is placed on the coating stage of a spin coater. Then, a disposable dropper is used to evenly drop the photoresist (such as AZ 6130 photoresist) on the substrate, and the photoresist is evenly spun at a speed of 1000 rpm / min. The preliminarily formed photoresist coating is placed on a baking stage at 100 °C for drying. It should be noted that the parameters described in this application are provided as examples to enable those skilled in the art to implement the embodiments provided herein. However, the parameter values described here can be changed according to actual applications, such as the type of photoresist used, or the desired thickness of the photoresist, etc., without departing from the teachings of the present invention.

[0042] In a specific embodiment of the present invention, it may be desirable to obtain a photoresist coating with a certain thickness of the photoresist. Therefore, the above steps of spin coating and baking can also be repeated to obtain a photoresist coating with a greater thickness, so as to achieve a microstructure with a greater step height span.

[0043] In a specific embodiment of the present invention, refer to Figure 1 Step 2, design the layout file for exposure as a grayscale image. In some embodiments, software such as matlab or other drawing software can be used to draw the desired exposure grayscale layout. Taking the layout required for the step microstructure as an example, certain grayscale values are assigned to the pixel points corresponding to the step squares of the desired size. Among them, the grayscale value corresponds to the exposure dose. Taking 256-level (0 - 255 levels) grayscale as an example, the maximum grayscale value of 255 corresponds to the relatively maximum exposure dose, and the minimum grayscale value of 0 corresponds to the minimum exposure dose. When the photoresist used is a positive photoresist, the parts with a larger exposure dose are more easily removed during development, while the parts with a smaller exposure dose are more easily retained during development.

[0044] In a specific embodiment of the present invention, refer to Figure 1Step 3: The laser direct writing exposure equipment can use the exposure layout designed in Step 2 to perform laser direct writing exposure on the photoresist coating prepared in Step 1. The so-called laser direct writing exposure refers to the method of directly irradiating the photoresist with a laser beam for exposure. The laser direct writing exposure system can read the gray value of each pixel in the exposure layout and determine the exposure position on the photoresist coating according to the pixel coordinate value. At the same time, the laser direct writing exposure equipment can also determine the laser energy intensity or exposure time corresponding to the exposure position according to the gray value of the pixel point, so that the exposure dose at the exposure position is proportional to the gray value of the corresponding pixel point. For simplicity, the exposure positions on the photoresist can also be called pixel points, which correspond one by one to the pixel points on the exposure layout. The laser direct writing exposure equipment can move the laser spot between each pixel point under the control of the driving device, such as moving in a row-by-row scanning manner, to complete the direct writing exposure of all pixel points. At the same time, during the scanning process of the laser beam, the energy intensity of the laser beam can be continuously adjusted according to the designed exposure layout, or the irradiation time of the laser beam can be adjusted to adjust the exposure dose corresponding to each pixel point, so that the thickness of the photoresist obtained after development is correlated with the gray value of the corresponding pixel. Since laser direct writing exposure is a method of exposing pixels one by one with a laser beam, it is necessary to pay attention to ensuring the change of the exposure dose between adjacent pixels and reducing the influence caused by beam stitching and optical proximity effects. Therefore, when designing the exposure layout, the Beamer software is used to iteratively optimize the gray value distribution (the relationship between the gray value and the exposure dose), simulate exposure and development, and reasonably allocate the dose to each unit (pixel).

[0045] In a specific embodiment of the present invention, in Step 4, the exposed photoresist is developed. The type of developer can be selected or formulated according to the type of photoresist. For example, AZ 6130 photoresist can be developed with AZ 300MIF developer. The development time can be determined according to the thickness of the photoresist to be developed and the composition of the developer, generally within the range of 1 min - 30 min. After development is completed, it is rinsed clean with deionized water to remove the residual developer on the surface, and the moisture is blown dry with a nitrogen gun.

[0046] In some embodiments, the photoresist layer and the substrate can also be etched directionally, such as vertically etched, so that the stepped three-dimensional microstructures at any height of the photoresist layer can be transferred to the substrate. In some embodiments, a material layer, such as a metal film layer, can also be conformally deposited on the photoresist layer, so that the stepped structure of the photoresist layer is transferred to the deposited material layer.

[0047] The present invention has the following beneficial effects:

[0048] The preparation method of the present invention has high precision and can prepare stepped microstructures with small sizes, multiple step orders, and arbitrary heights. This method greatly exerts the potential of laser direct writing technology in preparing three-dimensional structures and significantly improves the maximum energy control order of the laser direct writing equipment system. Through the method of the present invention, the application field of laser direct writing technology can be further expanded to optical fields such as diffractive optical elements (DOE) and microlenses, and can even be applied to the field of optical communication. Brief Description of the Drawings

[0049] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, wherein:

[0050] Figure 1 is a flowchart of a method for preparing a photoresist stepped microstructure according to a specific embodiment of the present invention;

[0051] Figure 2 is an initial exposure layout drawn using drawing software;

[0052] Figure 3 is to apply Beamer software to Figure 2 the optimized exposure layout after optimization;

[0053] Figure 4 is prepared using Figure 2 the initial exposure layout of the photoresist stepped microstructure;

[0054] Figure 5 is prepared using Figure 3 the optimized exposure layout of the photoresist stepped microstructure;

[0055] Figure 6 is Figure 5 a partial enlarged view of the black frame of;

[0056] Figure 7 is a grayscale layout of 0-255 gray levels;

[0057] Figure 8 is Figure 7 the relationship curve (i.e., contrast curve) between the photoresist thickness and the gray value obtained after exposing the grayscale layout of;

[0058] Figure 9 is the photoresist stepped microstructure prepared in Comparative Example 1;

[0059] Figure 10 is the photoresist stepped microstructure prepared in Comparative Example 2. Detailed Description of the Embodiments

[0060] The present invention will be further described in detail below in conjunction with the specific embodiments. The embodiments given are only for clarifying the present invention and not for limiting the scope of the present invention.

[0061] Example 1

[0062] (1) Prepare a photoresist coating on the substrate:

[0063] Place the cleaned quartz substrate on the coating stage of the spin coater. Then, use a disposable dropper to evenly drip the photoresist AZ 6130 on the substrate and spin coat it evenly at a speed of 1000 rpm / min. Place the preliminarily formed photoresist coating on a baking stage at 100 °C for drying.

[0064] (2) Design the initial exposure layout:

[0065] Use matlab software to draw the initial exposure gray-scale layout. The drawn initial exposure layout is as shown in Figure 2 Figure.

[0066] (3) Optimize the initial exposure layout:

[0067] The initial exposure layout is optimized by a method including the following steps:

[0068] (i) Use an ellipsometer to measure the optical parameters of the used photoresist: refractive index n and extinction coefficient k; among them, the measured refractive index is 1.6 and the extinction coefficient k is 0.

[0069] (ii) Draw a gray-scale layout with gray levels from 0 to 255, draw 25 squares with different gray values at a step of 10, as shown in Figure 7 Figure;

[0070] (iii) Expose the gray-scale layout. After exposure, obtain the relationship curve between the photoresist thickness and the gray value, that is, the contrast curve, as shown in Figure 8 Figure; After exposure and development, use a step profiler to measure the remaining photoresist thickness of each small square.

[0071] (iv) Input the initial exposure layout, the optical parameters of the photoresist, the contrast curve, and the relationship curve between the gray value and the corresponding expected thickness in the initial exposure layout into the Beamer optimization software to obtain the optimized exposure layout, as shown in Figure 3 Figure.

[0072] (4) Import the optimized exposure layout into the ultraviolet lithography system and use the ultraviolet lithography equipment to perform exposure processing on the photoresist coating.

[0073] (5) Perform a development process on the exposed photoresist coating to produce a photoresist step microstructure corresponding to the optimized exposure layout on the photoresist coating, where the development is about 1 min.

[0074] The photoresist step microstructure prepared in this example is as shown in Figure 5-6 Figure. Figure 5-6It is shown that the stepped microstructure of the present invention has high precision, small size (about 4 - 6 μm), many stepped orders (up to 8 stepped orders), and adjustable thickness.

[0075] Comparative Example 1

[0076] The operating steps of this comparative example are the same as those of Example 1, except that the defocus amount of the laser direct writing exposure equipment is adjusted to 8 μm. The photoresist stepped microstructure prepared in this comparative example is as Figure 9 shown. Figure 9 It is shown that when the defocus amount of the laser direct writing exposure equipment is less than 10 μm, the stepped edge is blurred and the stepped steepness is very poor.

[0077] Comparative Example 2

[0078] The operating steps of this comparative example are the same as those of Example 1, except that the defocus amount of the laser direct writing exposure equipment is adjusted to 62 μm. The photoresist stepped microstructure prepared in this comparative example is as Figure 10 shown. Figure 10 It is shown that when the defocus amount of the laser direct writing exposure equipment is greater than 60 μm, the stepped edge is blurred and the stepped steepness is very poor.

[0079] Comparative Example 3

[0080] The operating steps of this comparative example are the same as those of Example 1, except that the spot size of the laser beam is adjusted to 95 nm. The photoresist stepped microstructure prepared in this comparative example is similar to Figure 9 , the stepped edge is blurred and the stepped steepness is very poor.

[0081] Comparative Example 4

[0082] The operating steps of this comparative example are the same as those of Example 1, except that the spot size of the laser beam is adjusted to 1.1 μm. The photoresist stepped microstructure prepared in this comparative example is similar to Figure 10 , the stepped edge is blurred and the stepped steepness is very poor.

Claims

1. A method for preparing a multi-level stepped microstructure, which comprises the following steps: (1) Prepare a photoresist coating on a substrate; (2)Design the initial exposure layout, where the initial exposure layout is a grayscale image, and the grayscale value of each pixel in the grayscale image is in the range of 0 - 255 gray levels; The gray value of each pixel point on the initial exposure layout is correlated with the exposure dose of each point on the photoresist coating by an ultraviolet light beam; (3) Optimize the initial exposure layout to obtain an optimized exposure layout, wherein the initial exposure layout is optimized by a method comprising the following steps: (i) Measure the optical parameters of the used photoresist by an optical characterization device; (ii) Draw a gray-scale layout with a gray scale of 0-255; (iii) Expose the gray-scale layout, and after exposure, obtain a relationship curve between the photoresist thickness and the gray value, that is, a contrast curve; (iv) Input the initial exposure layout, the optical parameters of the photoresist, the contrast curve, and the relationship curve between the gray value and the corresponding desired thickness value in the initial exposure layout into optimization software to obtain an optimized exposure layout; (4) Import the optimized exposure layout into an ultraviolet lithography system, and use an ultraviolet lithography device to perform exposure processing on the photoresist coating; (5) Perform a developing process on the exposed photoresist coating to generate a photoresist stepped microstructure corresponding to the optimized exposure layout on the photoresist coating.

2. The method according to claim 1, wherein The optical characterization device is an ellipsometer.

3. The method according to claim 1, wherein The drawing of the gray-scale layout with a gray scale of 0-255 in step (ii) is carried out by taking 10 as the step and drawing 25 squares with different gray values.

4. The method according to claim 1, wherein, The optical parameters are the refractive index n and the extinction coefficient k.

5. The method according to claim 1, wherein, The substrate is selected from glass, quartz or silicon.

6. The method according to claim 1, wherein The photoresist is an ultraviolet photoresist.

7. The method according to claim 6, wherein The ultraviolet photoresist is sensitive to an exposure light beam in the wavelength band of 350 nm - 450 nm.

8. The method according to claim 6, wherein The ultraviolet photoresist is selected from one or more of AZ 4562, AZ 4620 and AZ 6130.

9. The method according to claim 1, wherein The correlation in step (2) includes negative correlation and positive correlation.

10. The method according to claim 1, wherein, The ultraviolet lithography device is a laser direct writing exposure device, and the wavelength of its laser beam is 350 nm - 450 nm.

11. The method according to claim 10, wherein, The defocus amount of the laser direct writing exposure device is 10 μm - 60 μm.

12. The method according to claim 10, wherein The spot size of the laser beam used for exposure by the laser direct writing exposure device is 100 nm - 1 μm.

13. The method according to claim 1, wherein, The length of the stepped microstructure is 1 μm - 10 μm, the width is 1 μm - 10 μm, and the thickness is 0 - 15 μm.

14. The method according to claim 1, wherein The method further includes the following steps after step (5): Perform directional etching on the photoresist layer and the substrate to convert the photoresist stepped microstructure into a substrate stepped microstructure; Or Conformally deposit a material layer on the photoresist layer so that the photoresist stepped microstructure is transferred to the deposited material layer.

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