A method for preparing nano frustum

Abstract

The present invention provides a method for preparing a nano-cone, comprising the following steps: step 1: constructing an FEM model of a wafer bare chip coated with photoresist, establishing an exposure energy value and an exposure focal length matrix of the wafer bare chip according to a required frustum size, performing exposure and development using the exposure energy value and exposure focal length matrix parameters of the wafer bare chip to obtain a frustum structure, measuring a key dimension of the frustum structure and comparing it with the required key dimension of the frustum structure to obtain an optimal exposure energy value and an optimal exposure focal length matrix; step 2: performing a single exposure on the wafer coated with photoresist according to 40% of the optimal exposure energy value and an exposure focal length of 0 μm; and constructing a secondary exposure FEM model, wherein the secondary exposure energy base value is 70% of the optimal exposure energy value and the secondary exposure basic focal length base value is the optimal exposure focal length, establishing a secondary exposure energy value and a secondary exposure focal length matrix, and using the method in step 1 to obtain an optimal secondary exposure energy value and an optimal secondary exposure focal length.

Description

Technical Field

[0001] The present invention belongs to the technical field of micro-electromechanical systems, and in particular relates to a method for preparing a nano-truncated cone. Background Art

[0002] In the MEMS (micro-electromechanical systems) manufacturing process, the key to achieving high performance lies in precise control of microstructures. Although traditional single-shot exposure technology performs well in many application scenarios, it still has the following limitations when preparing high-precision microstructures:

[0003] 1. Complex shape limitations: For microstructures that require fine three-dimensional configurations, a single exposure is often difficult to achieve the required geometric accuracy. This usually requires the use of multi-layer exposure and more complex lithography steps to construct, which leads to increased production costs and longer process cycles. 2. Uniformity issues: When a large area of ​​wafer is exposed at one time, the uneven distribution of light intensity may cause deviations in the size and morphology of microstructures between different regions on the same wafer, thereby affecting the consistency and stability of the product. 3. Difficulty in achieving high-aspect ratio structures: For certain specific MEMS devices, it is necessary to create fine structures with high aspect ratio features. However, as the depth of these structures increases, the difficulty of maintaining the sidewall inclination and overall consistency using a single exposure method also increases, which brings additional technical obstacles to actual operation.

[0004] Therefore, it is necessary to improve the existing photolithography technology to further improve the performance of MEMS devices. Summary of the Invention

[0005] The present invention provides a method for preparing nano-frustums, which utilizes a double exposure technique to precisely control the size and morphology of microstructures, thereby achieving precise preparation of nano-frustums with high aspect ratios.

[0006] Other purposes and advantages of the present invention can be further understood from the technical features disclosed in the present invention.

[0007] To achieve one or part or all of the above-mentioned purposes or other purposes, a technical solution of the present invention provides a method for preparing a nano-cone, characterized in that it includes the following steps: Step 1: constructing a wafer bare die FEM model, setting an exposure energy base value and an exposure focal length base value according to the required frustum size, establishing a wafer bare die exposure energy value and exposure focal length matrix, using the parameters in the wafer bare die exposure energy value and exposure focal length matrix to perform exposure and development to obtain a frustum structure, measuring the critical dimensions of the frustum structure and comparing them with the required critical dimensions of the frustum structure to obtain the optimal exposure energy value and optimal exposure focal length; Step 2: performing a single exposure according to 40% of the optimal exposure energy value and an exposure focal length of 0 μm; And construct a double exposure FEM model, the double exposure energy base value is 70% of the optimal exposure energy value obtained in step 1, the double exposure basic focal length base value is the optimal exposure focal length obtained in step 1, establish a double exposure energy value and a double exposure focal length matrix, use the double exposure energy value and the parameters in the double exposure focal length matrix to perform double exposure and development on the wafer exposed once to obtain a truncated cone structure, measure the key dimensions of the truncated cone structure and compare them with the required key dimensions of the truncated cone to obtain the optimal double exposure energy value and the optimal double exposure focal length; step three: use the single exposure parameters and double exposure parameters obtained in step 2 to perform exposure and development on a wafer coated with photoresist to obtain the required nano-truncated cone structure. The beneficial effect of this technical solution is that the present invention combines the double exposure technology with the construction of an FEM model, uses the FEM model to simulate the parameters of the double exposure, and optimizes the parameters of the double exposure to ensure high precision and consistency of the lithography. At the same time, the double exposure technology can significantly improve the preparation accuracy, consistency and reliability of the nano-truncated cone, and improve the manufacturing accuracy and high aspect ratio requirements of the MEMS structure.

[0008] In the step 1, the exposure focal length base value is set to 0um.

[0009] In step one, the exposure energy value of the wafer bare chip and the parameters in the exposure focal length matrix are exposed one by one on the wafer bare chip using the same mask; in step two, the secondary exposure energy value and the parameters in the secondary exposure focal length matrix are exposed one by one on the wafer that has been exposed once.

[0010] If the exposure energy value and exposure focal length matrix of the wafer bare chip and the measurement results of the parameters in the secondary exposure energy value and the secondary exposure focal length matrix that are closest to the required frustum critical size among the critical sizes of the frustum structure after exposure and development are within a range of ±10% from the required frustum critical size, then the parameters corresponding to the measurement results that are closest to the required frustum critical size are the optimal exposure parameters; otherwise, the exposure energy value and exposure focal length matrix are reconstructed until the measurement results of the parameters in the matrix that are closest to the required frustum critical size among the critical sizes of the frustum after exposure are within a range of ±10% from the required frustum critical size.

[0011] The key dimension of the truncated cone structure is the bottom diameter of the truncated cone structure.

[0012] In the step 2, the overlap accuracy between the second exposure pattern and the first exposure pattern is less than 0.2 μm.

[0013] In the step 2, the exposure equipment, temperature and humidity used in the second exposure are the same as those used in the first exposure.

[0014] The step three includes the following steps: S3.1 cleaning the back side of the wafer; S3.2 uniformly coating a BARC layer with a thickness of 30nm and a photoresist located above the BARC layer on the wafer; S3.3 performing a first exposure according to the first exposure parameters obtained in step two; S3.4 performing a second exposure according to the second exposure parameters obtained in step two; S3.5 developing the exposed wafer surface to obtain a nano-cone structure; S3.6 removing impurities from the surface of the nano-cone structure to remove surface residues.

[0015] The developer is a 2.38% concentration tetramethylammonium hydroxide solution.

[0016] In step S3.6, a plasma cleaning machine is used to perform surface cleaning for 60 seconds.

[0017] Compared with existing technologies, the present invention's significant advantages include combining double exposure technology with FEM modeling, using the FEM model to simulate and optimize double exposure parameters, ensuring high lithography precision and consistency. Furthermore, double exposure significantly improves the accuracy, consistency, and reliability of nano-frustum fabrication, enhancing the manufacturing precision of MEMS structures and meeting high aspect ratio requirements.

[0018] In order to make the above and other objects, features and advantages of the present invention more clearly understood, preferred embodiments are given below with reference to the accompanying drawings for detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 Schematic diagram of the preparation process of the nano frustum of the present invention.

[0021] Figure 2 This is a scanning electron microscope morphology image of the critical size of the nano-cone prepared in Example 1.

[0022] Figure 3 This is a critical size measurement image of the nano-cone prepared in Example 1 under a scanning electron microscope. DETAILED DESCRIPTION

[0023] The foregoing and other technical aspects, features, and functions of the present invention are clearly presented in the following detailed description of a preferred embodiment with reference to the accompanying drawings. Directional terms such as up, down, left, right, front, and back, used in the following embodiments, are intended solely to refer to the directions in the accompanying drawings. Therefore, the directional terms used are for illustrative purposes only and are not intended to limit the present invention.

[0024] Example 1

[0025] Embodiment 1 provides a method for preparing a nano-truncated cone, comprising the following steps:

[0026] Step 1: Construct a FEM (energy focus matrix) model of the wafer with photoresist coating.

[0027] The construction of the FEM model is a systematic process, including the following steps: demand analysis, determining the size and shape of the required microstructure. In the first embodiment of the frustum preparation method, the microstructure size and shape that need to be determined are mainly the bottom diameter and height of the frustum;

[0028] Parameter selection: In Example 1, exposure energy and focal length are selected as the main parameters affecting the geometric characteristics of the frustum; experimental design: the selected parameters are designed, and the exposure value and the basic value of the focal length are changed so that the parameters can cover the expected space; data collection: using statistical methods, experiments are conducted according to the experimental parameters preset in the experimental design stage, and the frustum size and shape under different experimental parameters are collected; verification and optimization: based on the collected frustum size and shape under different experimental parameters and the required frustum size and shape, the experimental parameters in the FEM model are adjusted to optimize the FEM model.

[0029] According to the required frustum size, an exposure energy base value is set, which can be preset based on photolithography experience, and an exposure focus base value is set, which is preset to 0um. The set exposure energy base value and exposure focus base value are adjusted, and multiple values ​​near the set base value are selected based on the set base value to establish a wafer bare die exposure energy value and exposure focus length matrix. The parameters in the wafer bare die exposure energy value and exposure focus length matrix are used to perform exposure and development to obtain a frustum structure. The critical dimensions of the frustum structure are measured and compared with the required critical dimensions of the frustum structure, and the exposure energy value and exposure focus length closest to the required values ​​are selected as the optimal exposure energy value and optimal exposure focus length.

[0030] Step 2: First, perform a single exposure on the photoresist-coated wafer according to 40% of the optimal exposure energy value obtained in step 1 and an exposure focal length of 0 μm to obtain a wafer after single exposure;

[0031] Reconstruct a secondary exposure FEM model, and after the first exposure is completed, perform secondary exposure on the wafer after the first exposure. The secondary exposure FEM model established during the secondary exposure is selected as the secondary exposure energy base value which is 70% of the optimal exposure energy value obtained in step one, and the secondary exposure base focal length base value which is the optimal exposure focal length in step one. Establish a secondary exposure energy value and a secondary exposure focal length matrix. The selection of the values ​​in the secondary exposure energy value and the secondary exposure focal length matrix is ​​the same as that in step one. Use the parameters in the secondary exposure energy value and the secondary exposure focal length matrix to perform secondary exposure and development on the wafer after the first exposure to obtain a frustum structure. Measure the critical dimensions of the frustum structure and compare them with the required critical dimensions of the frustum to obtain the optimal secondary exposure energy value and the optimal secondary exposure focal length.

[0032] Step 3: Use the first exposure parameter and the second exposure parameter obtained in step 2 to perform exposure and development on the wafer coated with photoresist to obtain the desired nano-cone structure.

[0033] In order to ensure the implementation of the FEM model and to find the optimal lithography parameters, in step one, the same mask is used with the same pattern on the mask, and the exposure energy value of the wafer bare chip and the parameters in the exposure focal length matrix are exposed one by one on the wafer bare chip. Except for the exposure energy value and the exposure focal length parameters, other parameters during exposure are the same; in step two, the same mask is used, and the secondary exposure energy value and the parameters in the secondary exposure focal length matrix are secondary exposed one by one on the wafer exposed once to ensure the identity of the exposure. At the same time, the mask during the secondary exposure should be in the same position as the mask during the primary exposure.

[0034] In step one and step two, the method for obtaining the optimal exposure energy value and exposure focal length of the wafer bare chip and the optimal secondary exposure energy value and secondary exposure focal length is that the measurement results of the parameters in the wafer bare chip exposure energy value and exposure focal length matrix and the secondary exposure energy value and secondary exposure focal length matrix after exposure and development are closest to the required frustum structure key dimensions, and the difference range with the required frustum structure key dimensions is within ±10%, then the exposure parameters corresponding to the measurement results closest to the required frustum structure key dimensions are the optimal exposure parameters; otherwise, the exposure energy value and exposure focal length matrix are reconstructed until the measurement results of the parameters in the matrix after exposure are closest to the required frustum structure key dimensions, and the difference range with the required frustum structure key dimensions is within ±10%.

[0035] In step 2, the overlap accuracy of the second exposure pattern and the first exposure pattern is less than 0.2 μm. The second exposure uses the same exposure equipment, temperature, and humidity as the first exposure to reduce the impact of variables on the results.

[0036] See also Figure 1 Schematic diagram of the nano-truncated cone preparation process in FIG. 3 , step three includes the following steps:

[0037] S3.1 cleans the back side of the wafer; S3.2 uniformly coats a BARC layer with a thickness of 30 nm and a photoresist located above the BARC layer on the wafer; S3.3 performs a first exposure according to the first exposure parameters obtained in step 2; S3.4 performs a second exposure according to the second exposure parameters obtained in step 2; S3.5 develops the exposed wafer surface using a 2.38% concentration of tetramethylammonium hydroxide solution as the developer to obtain a nano-cone structure; S3.6 removes impurities from the surface of the nano-cone structure using a plasma cleaning machine for 60 seconds to remove surface residues.

[0038] The following is a detailed description of the process using the example of fabricating a truncated cone with a bottom diameter of 500 nm. The truncated cone structure is fabricated on an 8-inch wafer coated with a photoresist with a thickness of 1.2 μm, and the microstructure designed on the photomask is a 500 nm circular structure.

[0039] The preparation process is as follows:

[0040] a) An FEM model was established on the bare wafer. The base exposure energy was set to 225 mj based on experience. Five energy groups were selected around 225 mj: 175 mj, 200 mj, 225 mj, 250 mj, and 275 mj. The base exposure focal length was set to 0 μm, and seven focal length groups were selected: 0 μm, -0.2 μm, -0.3 μm, -0.4 μm, -0.5 μm, -0.6 μm, and -0.7 μm. An FEM model was established on the bare wafer. The parameters in the above-established bare wafer FEM model were exposed and developed one by one on a photolithography machine to obtain the corresponding frustum structure. The bottom diameter of the frustum was measured using a CD-SEM (critical dimension scanning electron microscope). The measurement results are shown in Table 1.

[0041] Table 1 Wafer bare die FEM model parameters CD-SEM data after exposure

[0042]

[0043]

[0044] The NA in the table above indicates that no data could be measured due to image distortion. After comparison, we selected an energy of 250mJ and a focal length of -0.2µm as the optimal exposure energy and focal length for the wafer die. Alternatively, we could select other exposure energy and focal length values, as shown in Table 1, where the measurement result is 0.5µm.

[0045] b) The exposure energy value X obtained from step a is 250 mj, and the exposure energy of one exposure is selected as 40% of the optimal exposure energy value in step a, that is, the exposure energy value of one exposure is 100 mj, and the exposure focal length is selected as 0 μm. According to the selected exposure energy value and exposure focal length of one exposure, one exposure is performed on the wafer bare chip.

[0046] c) The energy base value for the second exposure is 70% of the optimal exposure energy value in step a, that is, the secondary exposure energy base is 175mj, and the focal length base is the optimal exposure focal length of -0.2um in step a. Referring to the base value, five energy value groups of 160mj, 170mj, 180mj, 90mj, and 200mj are selected, and six focal length groups of -0.6um, -0.4um, -0.3um, 0um, 0.2um, and 0.4um are selected. A secondary exposure FEM model is established, and each parameter in the secondary exposure FEM model is used to perform a secondary exposure on the wafer bare die after the primary exposure. The overlap accuracy of the primary exposure and secondary exposure patterns should be less than 0.2um. After the secondary exposure, the corresponding truncated cone structure is obtained by development, and the bottom diameter of the truncated cone is measured by CD-SEM (critical dimension scanning electron microscope). The measurement results are shown in Table 2 below.

[0047] Table 2. CD-SEM data after secondary exposure FEM model parameters

[0048]

[0049] After comparison, after the second exposure, when the energy value is 190mj and the focal length is -0.6um, the obtained frustum size is 0.52um. The bottom diameter of the frustum is also the data closest to the target frustum bottom diameter of 0.5um in the second exposure FEM model in Table 2. The difference between the measured result and the required frustum bottom diameter is less than 10% of the ratio of the target frustum bottom diameter, which meets the design requirements. Therefore, the exposure energy of the second exposure is 190mj and the focal length is -0.6um.

[0050] d) Based on the first exposure parameters and the second exposure parameters obtained in step c and step d, preparing the desired nano-truncated cones on an 8-inch wafer coated with a photoresist thickness of 1.2 μm, specifically comprising the following steps:

[0051] d1 cleaning, cleaning the back side of the wafer;

[0052] d2 coating: evenly coating a 30nm BARC layer and a 1.2um photoresist on the wafer. The BARC layer is an anti-reflective layer used to reduce the reflection of the light source during photolithography and improve the photolithography effect;

[0053] D3 single exposure, set the exposure energy value to 100msec, focal length to 0um, and perform a single exposure;

[0054] D4 secondary exposure, set the exposure energy value to 190msec, focal length -0.6um, and perform secondary exposure;

[0055] d5 development, using 2.38% TMAH (tetramethylammonium hydroxide) solution for development;

[0056] D6 post-processing uses a plasma cleaner to perform a de-impurity modification process on the wafer surface after development to remove surface residues.

[0057] The process in step d is as follows Figure 1 Schematic diagram of the nanotruncated cone preparation process.

[0058] See also Figure 2 It can be found that when the exposure energy is 190mj and the exposure focal length is -0.6um, the sidewall angle and top flatness of the frustum can be optimized, so that the frustum structure meets the requirements of the MEMS structure for a high aspect ratio. Figure 3After the second exposure, the frustum structure was measured under CD-SEM (critical dimension scanning electron microscope) to confirm that the bottom size of the frustum was 537nm, the height was 1.2um, and the aspect ratio of the nano frustum was about 2.4. The size of the prepared frustum structure met the required frustum structure size, and the preparation process met the requirements.

[0059] The above describes in detail the method for preparing a nano-truncated cone provided by the present invention. Specific examples are used herein to illustrate the structure and working principle of the present invention. The description of the above embodiments is intended only to facilitate understanding of the method and core concept of the present invention. It should be noted that, without departing from the principles of the present invention, a number of improvements and modifications may be made to the present invention by those skilled in the art, and such improvements and modifications fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a nano-truncated cone, characterized in that: The steps include: Step 1: Construct a wafer bare die FEM model, set the exposure energy base value and the exposure focal length base value according to the required frustum size, establish a wafer bare die exposure energy value and exposure focal length matrix, use the parameters in the wafer bare die exposure energy value and exposure focal length matrix to perform exposure and development to obtain a frustum structure, measure the critical dimensions of the frustum structure and compare them with the required critical dimensions of the frustum structure to obtain the optimal exposure energy value and optimal exposure focal length; Step 2: Perform one exposure on the photoresist-coated wafer at 40% of the optimal exposure energy value obtained in step 1 and an exposure focal length of 0 μm; A secondary exposure FEM model is constructed, wherein the secondary exposure energy base value is 70% of the optimal exposure energy value obtained in step 1, and the secondary exposure base focal length base value is the optimal exposure focal length obtained in step 1. A secondary exposure energy value and a secondary exposure focal length matrix are established, and secondary exposure and development are performed on a single-exposure wafer using parameters in the secondary exposure energy value and the secondary exposure focal length matrix to obtain a frustum structure. The critical dimensions of the frustum structure are measured and compared with the required critical dimensions of the frustum to obtain the optimal secondary exposure energy value and the optimal secondary exposure focal length. Step 3: Using the primary exposure parameters and secondary exposure parameters obtained in step 2, performing exposure and development on the wafer coated with photoresist to obtain the desired nano-truncated cone structure.

2. The method for preparing a nano-truncated cone according to claim 1, wherein: In the step 1, the exposure focal length base value is set to 0um.

3. The method for preparing a nano-truncated cone according to claim 1, wherein: In the step 1, the exposure energy values ​​of the wafer bare chips and the parameters in the exposure focus matrix are exposed one by one on the wafer bare chips using the same photomask; In step 2, the same photomask is used to perform secondary exposure on the wafer that has been exposed once, using the secondary exposure energy values ​​and the parameters in the secondary exposure focal length matrix one by one.

4. The method for preparing a nano-truncated cone according to claim 1, wherein: The measurement result of the critical dimensions of the frustum structure of the wafer bare die exposure energy value and the exposure focal length matrix and the parameters in the secondary exposure energy value and the secondary exposure focal length matrix after exposure and development is closest to the required frustum critical dimension, and the difference range with the required frustum critical dimension is within ±10%, then the parameter corresponding to the measurement result closest to the required frustum critical dimension is the optimal exposure parameter; Otherwise, the exposure energy value and exposure focus matrix are reconstructed until the measurement result of the frustum critical size closest to the required frustum critical size among the parameters in the matrix after exposure is within ±10% of the required frustum critical size.

5. The method for preparing a nano frustum according to claim 1, wherein: The key dimension of the truncated cone structure is the bottom diameter of the truncated cone structure.

6. The method for preparing a nano-truncated cone according to claim 1, characterized in that: In the step 2, the overlap accuracy between the second exposure pattern and the first exposure pattern is less than 0.2 μm.

7. The method for preparing a nano-truncated cone according to claim 1, characterized in that: In the step 2, the exposure equipment, temperature and humidity used in the second exposure are the same as those used in the first exposure.

8. The method for preparing a nano-truncated cone according to claim 1, wherein: The step three includes the following steps: S3.1 Clean the back side of the wafer; S3.2 uniformly coating a BARC layer with a thickness of 30 nm on the wafer and a photoresist on the BARC layer; S3.3 Perform a single exposure based on the single exposure parameters obtained in step 2; S3.4 Perform secondary exposure according to the secondary exposure parameters obtained in step 2; S3.5 developing the exposed wafer surface to produce a nano-truncated cone structure; S3.6 De-impurify and modify the surface of the nano-truncated cone structure to remove surface residues.

9. The method for preparing a nano-truncated cone according to claim 8, characterized in that: The developer is a 2.38% tetramethylammonium hydroxide solution.

10. The method for preparing a nano-truncated cone according to claim 8, characterized in that: In step S3.6, a plasma cleaning machine is used to perform surface decontamination and modification for 60 seconds.

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