A method for preparing a high aspect ratio nano-metallic hole

Abstract

The application provides a preparation method of a high-aspect-ratio nano metal hole, comprising the following steps: forming a negative photoresist layer on a substrate, performing exposure and development on the negative photoresist layer to form a photoresist column; depositing a metal layer on the substrate with the photoresist column to cover the photoresist column and the substrate; thinning the metal layer to a target metal hole height by using a chemical mechanical polishing process to expose the top of the photoresist column; removing the photoresist column to form a nano metal hole at the position originally occupied by the photoresist column; wherein the deposition thickness of the metal layer is greater than or equal to 1.5 times the target metal hole height. The application converts the direct etching of the high-aspect-ratio small-size metal hole in the traditional process into a combined process of preparing a high-aspect-ratio photoresist column, metal full-deposition, chemical mechanical polishing process thinning and selective photoresist removal, realizes wafer-level global planarization and ensures that the metal hole depth of the whole wafer surface reaches height uniformity.

Description

Technical Field

[0001] This invention belongs to the field of nano-metal hole processing technology, and in particular relates to a method for preparing high aspect ratio nano-metal holes. Background Technology

[0002] Metal pore structures have extremely important applications in gas sensors and biochemical detection. With the trend of device miniaturization, the market demand for metal pore structures with high aspect ratios and ultra-small sizes (such as nanoscale pore sizes) is becoming increasingly urgent.

[0003] Currently, existing metal hole fabrication technologies generally have limitations. Although traditional photolithography and lift-off methods are relatively mature, these methods are more suitable for fabricating micron-sized metal hole structures. To achieve the fabrication of small-sized (such as nanometer-sized) metal holes, expensive electron beam lithography equipment must be relied upon, and the lift-off process faces the problem of uneven metal layer removal at the nanoscale.

[0004] While focused ion beam etching can fabricate nanopores, it requires precise positioning and multiple steps to form the nanopores. This not only demands extremely high operational precision but also results in very low processing efficiency, making it difficult to meet the needs of large-area fabrication. Nanosphere photolithography and template methods involve complex steps such as nanosphere transfer and template fabrication, further complicating the metal hole fabrication process.

[0005] Furthermore, in the direct etching process of high aspect ratio structures, the fabrication of small-sized metal holes with high aspect ratios is particularly difficult due to technical bottlenecks such as limited mass transfer, low etching rate, and difficulty in controlling sidewall morphology. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention aims to propose a method for preparing high aspect ratio nano-metal holes, which solves the problems of complex processes, low efficiency, and low yield in the existing technology of directly etching small-sized metal hole structures with high aspect ratio.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A method for preparing high aspect ratio nanoporous metal pores includes the following steps: A negative photoresist layer is formed on a substrate, and the negative photoresist layer is exposed and developed to form photoresist pillars; A metal layer is deposited on the substrate on which the photoresist pillars are formed to cover the photoresist pillars and the substrate; The metal layer is thinned to the height of the target metal hole using a chemical mechanical polishing process to expose the top of the photoresist pillar; Remove the photoresist pillars to form nano-metal holes at the locations originally occupied by the photoresist pillars; The deposition thickness of the metal layer is greater than or equal to 1.5 times the height of the target metal hole.

[0008] Furthermore, the negative photoresist used to form the negative photoresist layer is a hydrogen silsesquioxane photoresist.

[0009] Furthermore, the thickness of the negative photoresist layer is greater than or equal to 1.1 times the height of the target metal hole.

[0010] Furthermore, the exposure is performed using electron beam exposure technology.

[0011] Furthermore, the exposure pattern of the electron beam exposure technology is a circle, a periodic circular dot matrix, or a ring.

[0012] Furthermore, the exposure parameters of the electron beam exposure technology are: accelerating voltage of 100kV, beam current of 100pA, and exposure dose of 10000μC / cm² to 20000μC / cm².

[0013] Furthermore, the specific steps of the developing process are as follows: A 2.35%–2.40% tetramethylammonium hydroxide developer was used, and the mixture was immersed in a 50°C water bath for 3 minutes for development; then deionized water was used for fixing for 30 seconds.

[0014] Furthermore, after fixing, supercritical carbon dioxide drying is performed to form photoresist pillars.

[0015] Furthermore, the material of the metal layer is gold, platinum, or chromium.

[0016] Furthermore, the specific steps for removing the photoresist pillars are as follows: The thinned substrate was immersed in a 6:1 BOE solution for 30 seconds to remove the photoresist pillars.

[0017] Compared with existing technologies, the method for preparing high aspect ratio nanoporous metal pores described in this invention has the following advantages: This invention transforms the traditional direct etching of high aspect ratio, small-sized metal holes into a combined process of fabricating high aspect ratio photoresist pillars, full-coverage metal deposition, chemical mechanical polishing thinning, and selective photoresist removal. The synergistic effect of these steps avoids the technical bottlenecks faced in etching high aspect ratio, small-sized metal hole structures, such as limited mass transfer, low etching rate, and difficulty in controlling sidewall morphology. This significantly improves the yield and reliability of high aspect ratio nano-metal hole fabrication and enables efficient fabrication of nano-metal holes over large areas.

[0018] This invention employs chemical mechanical polishing (CMP) for thinning. Compared to existing techniques that require spin-coating a photoresist planarization layer before ion beam polishing, this invention controls the metal deposition thickness to be at least 1.5 times the target metal hole height, ensuring sufficient thinning margin in the metal layer itself. Excess metal can be removed directly without the need for a photoresist planarization layer. Simultaneously, CMP achieves wafer-level global planarization, overcoming the limitations of ion beam polishing, such as limited processing area and poor uniformity. The polished metal surface roughness can reach the nanometer level, ensuring high uniformity of the metal hole depth across the entire wafer surface, significantly reducing surface scattering loss for plasmonic optics devices. Attached Figure Description

[0019] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a schematic diagram of the photoresist pillars in Embodiment 1 provided by the present invention; Figure 2 This is a schematic diagram of the deposited metal layer in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of thinning in Embodiment 1 provided by the present invention; Figure 4 This is a schematic diagram of the nano-metal pores in Example 1 provided by the present invention; Figure 5 This is a top view of the photoresist pillar array in Embodiment 2 of the present invention; Figure 6 The top view of the photoresist ring in Embodiment 3 provided by the present invention.

[0020] Explanation of reference numerals in the attached figures: 1. Substrate; 2. Photoresist pillars; 3. Metal layer; 4. Nanometal holes; 5. Photoresist rings. Detailed Implementation

[0021] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0022] The high depth-to-width ratio referred to in this invention means that the ratio of the depth to the width of the structure is relatively large, usually between 5:1 and 20:1. The calculation formula is: depth-to-width ratio = through hole depth / through hole diameter.

[0023] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] Example 1: Preparation of single-pore circular nanoporous metal pores like Figures 1 to 4 As shown, a method for preparing high aspect ratio nanoporous metal pores includes the following steps: A negative photoresist layer is formed on a substrate, and the negative photoresist layer is exposed and developed to form photoresist pillars; A metal layer is deposited on the substrate on which the photoresist pillars are formed to cover the photoresist pillars and the substrate; The metal layer is thinned to the height of the target metal hole using a chemical mechanical polishing process to expose the top of the photoresist pillar; Remove the photoresist pillars to form nano-metal holes at the locations originally occupied by the photoresist pillars; The deposition thickness of the metal layer is greater than or equal to 1.5 times the height of the target metal hole.

[0025] This embodiment first uses a negative photoresist layer to fabricate a solid photoresist pillar with a high aspect ratio. Then, a metal layer is deposited around the photoresist pillar, completely encapsulating it. Next, a global thinning process is used to cut the metal layer from top to bottom until the top of the photoresist pillar is exposed. Finally, the original photoresist pillar is selectively peeled off. Specifically: S1. First, a substrate is provided. The substrate can be various substrate materials commonly used in the art, such as silicon wafers or sapphire substrates, but it does not react with buffer oxide etching solution. Then, a negative photoresist layer is formed on the substrate. This can be achieved using a spin coating process, where liquid negative photoresist is dropped onto the substrate surface, and the desired photoresist thickness is obtained by controlling the spin coating speed and time. Next, the negative photoresist layer is exposed and developed. After development, fixing and drying processes can be performed as needed to obtain a photoresist pillar with a complete structure.

[0026] S2. A metal layer is deposited on the substrate with photoresist pillars using a magnetron sputtering deposition process. The thickness of the deposited metal layer is greater than or equal to 1.5 times the height of the target metal hole. During the deposition process, the metal material is simultaneously deposited on the top surface and sidewalls of the photoresist pillars, as well as on the substrate surface not covered by the photoresist pillars, completely covering the photoresist pillars and ensuring that the top surface and sidewalls of the photoresist pillars are covered by the metal material.

[0027] Specifically, taking a target metal hole height of 220nm as an example, the deposition thickness should be greater than or equal to 330nm. This ensures that during sidewall coverage, the metal layer can completely cover the root and sidewalls of the high aspect ratio photoresist pillars, filling any gaps that may be caused by the shadowing effect. At the same time, it provides ample process buffer for subsequent global thinning processes, helping to eliminate macroscopic undulations on the initial metal surface caused by the undulations of the photoresist pillars, allowing planar polishing to proceed more smoothly to the bottom.

[0028] S3. The metal layer is thinned as a whole using chemical mechanical polishing. The goal of thinning is to reduce the thickness of the metal layer from the initial deposition thickness to the target metal hole height, and to expose the top of the photoresist pillar at the end of the thinning process.

[0029] Specifically, chemical mechanical polishing (CMP) is a mature process in the semiconductor industry capable of achieving wafer-level global planarization. It can process entire substrates (4 inches to 12 inches) simultaneously, achieving surface flatness at the nanometer level, providing a feasible technical path for the large-scale fabrication of large-area nanopore arrays. CMP equipment is standard equipment in the integrated circuit manufacturing field, with a large number of existing machines, high process maturity, and good prospects for industrialization.

[0030] The surface roughness Ra of the metal after CMP processing is less than 1 nm, and the surface is smooth and dense without obvious scratches or damage layers. For plasmonic optics devices, a smooth metal surface can significantly reduce surface scattering loss and improve device performance.

[0031] Compared with the angled ion beam polishing used in existing technologies, CMP is a uniform thinning process across the entire surface. It does not have the problem of ion beam polishing being unable to achieve uniform thinning on large-area wafers, nor does it introduce a damage layer on the metal surface due to high-energy particle bombardment.

[0032] S4. Selectively etch away the photoresist pillars while leaving the metal layer unaffected. Once the photoresist pillars are completely removed, a hollow pore structure, i.e., a nano-metal pore, is formed in the space originally occupied by the photoresist pillars.

[0033] In a preferred embodiment of the present invention, a hydrogen silsesquioxane negative photoresist is used to form the negative photoresist layer.

[0034] Specifically, by using hydrogen silsesquioxane as a negative photoresist, it is possible to stably prepare key size structures at the nanoscale (such as the 10nm level). During the subsequent metal deposition process, the morphology can be maintained without brittle collapse or softening collapse, thereby ensuring the morphological fidelity of the final nano-metal pores.

[0035] In a preferred embodiment of the present invention, the thickness of the negative photoresist layer is greater than or equal to 1.1 times the height of the target metal hole.

[0036] Specifically, the thickness of the negative photoresist layer is more than 10% greater than the height of the target metal via. For example, when the target metal via height is 200nm, the thickness of the negative photoresist layer is set to be greater than or equal to 220nm. This provides sufficient thickness margin for the entire process flow. Even if some photoresist loss occurs during exposure, development, deposition, and thinning, the height of the photoresist pillars can still be guaranteed to be sufficient to form a complete via structure in the metal layer, effectively improving the process's fault tolerance and product yield.

[0037] In a preferred embodiment of the present invention, the exposure is performed using electron beam exposure technology, and the exposure pattern of the electron beam exposure technology is circular.

[0038] Specifically, the exposure parameters for the electron beam lithography technique are set as follows: accelerating voltage of 100kV, beam current of 100pA, and exposure dose of 10000μC / cm² to 20000μC / cm². The exposure pattern of the electron beam lithography technique is set as an isolated circular pattern with a target design diameter of 10nm. After exposure, development, and drying, isolated photoresist pillars with completely upright morphology, no bending or collapse, circular cross-section, and a height greater than 220nm are obtained on the substrate. Finally, after metal deposition, thinning, and photoresist removal, individual nano-metal holes are formed.

[0039] Electron beam lithography can achieve extremely high resolution pattern definition. By simply changing the design of the exposure pattern, different morphologies of nanostructures, including single nanopores, nanopore arrays, and nanoring arrays, can be fabricated in the same process flow. There is no need to develop different process routes for different structures, which greatly expands the application scope of this embodiment and enables it to meet the diverse needs of different devices for nanostructure morphologies.

[0040] In a preferred embodiment of the present invention, the specific steps of the developing process are as follows: A tetramethylammonium hydroxide developer with a concentration of 2.35% to 2.40%, preferably a tetramethylammonium hydroxide developer with a concentration of 2.38%, is used. The mixture is immersed in a water bath at 50°C for 3 minutes for development. Fixing was performed using deionized water for 30 seconds.

[0041] Specifically, heating the development process at 50°C increases the development reaction rate, resulting in more thorough and uniform development. A development time of 3 minutes, optimized through experiments under the aforementioned concentration and temperature conditions, ensures complete removal of unexposed areas while preventing over-development that could damage exposed areas. Fixing with deionized water for 30 seconds effectively terminates the development reaction and cleans the sample surface, preventing developer residue from adversely affecting subsequent processes.

[0042] In a preferred embodiment of the present invention, after fixing, supercritical carbon dioxide drying is performed to obtain photoresist pillars.

[0043] Specifically, supercritical carbon dioxide drying is performed immediately after fixing to eliminate the collapse or bending of high aspect ratio photoresist pillars caused by liquid surface tension during conventional drying, ensuring that the photoresist pillars remain upright, and ultimately obtaining high aspect ratio photoresist pillars with small linewidths (such as photoresist pillars with a diameter of about 10nm and a height of more than 220nm). In a preferred embodiment of the present invention, the material of the metal layer is gold, platinum or chromium.

[0044] Specifically, the aforementioned metal materials all exhibit good resistance to buffered oxide etching solutions. They remain chemically stable and do not undergo dissolution or corrosion reactions during the subsequent removal of hydrogen silsesquioxane (HSQ) photoresist pillars using buffered oxide etching solution (BOE solution), thereby ensuring the integrity and dimensional accuracy of the nano-metal pore structure.

[0045] In a preferred embodiment of the present invention, the specific steps for removing the photoresist pillars are as follows: The thinned substrate was immersed in a 6:1 BOE solution for 30 seconds to remove the photoresist pillars.

[0046] Specifically, the 6:1 BOE solution is a mixture of 40% ammonium fluoride aqueous solution and 49% hydrofluoric acid aqueous solution in a volume ratio of 6:1.

[0047] After selectively etching away the hydrogen silsesquioxane (HSQ) photoresist pillars using a buffered oxide etchant (BOE solution), the spaces previously occupied by the photoresist pillars are transformed into hollow nano-metal pore structures. Because the metal layer remains stable in the BOE solution, the size, shape, and positional accuracy of the nano-metal pores are effectively preserved.

[0048] Example 2: Fabrication of Nanoscale Metal Pore Arrays like Figure 5 As shown, this embodiment provides a method for fabricating high aspect ratio nano-metal holes, used to process large-area, regularly arranged micro / nano-hole array structures on a substrate. The basic process flow and parameters of this embodiment are largely the same as those of Embodiment 1; the core difference lies in the definition of the electron beam exposure pattern in S1 and the subsequent arrangement structure: In this embodiment, the exposure pattern of the electron beam lithography technology is set as a periodic circular dot pattern (for example, the array period of the dot pattern is set to 50nm, and the target design diameter of each circular unit is 10nm). The diameter and array period can be designed according to actual needs.

[0049] The same scanning and writing process was performed using an accelerating voltage of 100 kV, a beam current of 100 pA, and an exposure dose of 15000 μC / cm². Subsequently, the substrate was immersed in a 2.38% tetramethylammonium hydroxide developer at 50°C for 3 minutes, fixed with deionized water for 30 seconds, and immediately subjected to supercritical carbon dioxide drying. After drying, multiple circular negative photoresist pillars arranged in a periodic array were obtained on the substrate. These circular negative photoresist pillars were independent of each other and remained highly upright, without any clumping or adhesion caused by capillary forces.

[0050] The subsequent steps are exactly the same as in Example 1. After metal deposition, thinning and resist removal, multiple nano-metal holes (i.e., nano-metal hole arrays) are formed in a periodic lattice arrangement in the metal layer on the substrate surface, corresponding to the positions originally occupied by the photoresist pillars.

[0051] Example 3: Preparation of Nanoscale Metal Ring Structures like Figure 6 As shown, this embodiment provides a method for fabricating high aspect ratio nano-metal holes for processing large-area nano-metal ring arrays on a substrate. The basic process flow of this embodiment is largely the same as that of Embodiment 1, with the core difference lying in the definition of the exposure pattern in step one and the subsequent arrangement structure: In this embodiment, the exposure pattern of the electron beam exposure technology is set as a ring pattern (for example, the outer diameter of the ring pattern is set to 50nm, the inner diameter to 30nm, and the ring width to 10nm). In actual production, different specifications of nano-metal ring arrays can be realized by adjusting the outer and inner diameters of the ring pattern.

[0052] The same scanning and writing process was performed using an accelerating voltage of 100 kV, a beam current of 100 pA, and an exposure dose of 15000 μC / cm². Subsequently, the substrate was immersed in a 2.38% tetramethylammonium hydroxide developer at 50°C for 3 minutes, fixed with deionized water for 30 seconds, and immediately subjected to supercritical carbon dioxide drying. After drying, a negative photoresist pillar (i.e., a photoresist ring) with a circular cross-sectional shape was finally formed on the substrate.

[0053] The subsequent steps are exactly the same as in Example 1. After metal deposition, thinning, and resist removal, nano-metal pores (i.e., nano-metal ring structures) with a circular cross-section are formed on the substrate.

[0054] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing high aspect ratio nanoporous metal pores, characterized in that, Includes the following steps: A negative photoresist layer is formed on a substrate, and the negative photoresist layer is exposed and developed to form photoresist pillars; A metal layer is deposited on the substrate on which the photoresist pillars are formed to cover the photoresist pillars and the substrate; The metal layer is thinned to the height of the target metal hole using a chemical mechanical polishing process to expose the top of the photoresist pillar; Remove the photoresist pillars to form nano-metal holes at the locations originally occupied by the photoresist pillars; The deposition thickness of the metal layer is greater than or equal to 1.5 times the height of the target metal hole.

2. The method for preparing high aspect ratio nanoporous metal pores according to claim 1, characterized in that: The negative photoresist used to form the negative photoresist layer is hydrogen silsesquioxane photoresist.

3. The method for preparing high aspect ratio nanoporous metal pores according to claim 1, characterized in that: The thickness of the negative photoresist layer is greater than or equal to 1.1 times the height of the target metal hole.

4. The method for preparing high aspect ratio nanoporous metal pores according to claim 1, characterized in that: The exposure is performed using electron beam exposure technology.

5. The method for preparing high aspect ratio nanoporous metal pores according to claim 4, characterized in that: The exposure pattern of the electron beam exposure technology is a circle, a periodic circular dot matrix, or a ring.

6. The method for preparing high aspect ratio nanoporous metal pores according to claim 4, characterized in that: The exposure parameters of the electron beam exposure technology are: accelerating voltage of 100kV, beam current of 100pA, and exposure dose of 10000μC / cm²~20000μC / cm².

7. The method for preparing high aspect ratio nanoporous metal pores according to claim 1, characterized in that: The specific steps of the development process are as follows: A 2.35%–2.40% tetramethylammonium hydroxide developer was used, and the mixture was immersed in a 50°C water bath for 3 minutes for development; then deionized water was used for fixing for 30 seconds.

8. The method for preparing high aspect ratio nanoporous metal pores according to claim 5, characterized in that: After fixing, supercritical carbon dioxide drying is performed to form photoresist pillars.

9. The method for preparing high aspect ratio nanoporous metal pores according to claim 1, characterized in that: The metal layer is made of gold, platinum, or chromium.

10. The method for preparing high aspect ratio nanoporous metal pores according to claim 1, characterized in that: The specific steps for removing the photoresist pillars are as follows: The thinned substrate was immersed in a 6:1 BOE solution for 30 seconds to remove the photoresist pillars.

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