A method for fabricating an all-metal tip terahertz near-field scanning probe

Abstract

This invention relates to the field of terahertz application technology, specifically to a method for fabricating an all-metal tip terahertz near-field scanning probe. The method uses an SOI wafer as a substrate, prepares an adhesion layer and a seed layer by magnetron sputtering, and then uses ultraviolet thick-film photolithography and electroplating to form the tip body. Combined with electron beam exposure and evaporation coating, a tip tip with a curvature radius of less than 10 nm is formed. Finally, a cantilever beam and needle holder are integrally formed by photolithography and deep silicon etching. This approach uses standard semiconductor wafer-level processes to replace traditional electrochemical etching and adhesive bonding methods, significantly improving probe dimensional accuracy and structural consistency. It can stably obtain all-metal probes with a tip height greater than 80 μm and a tip curvature radius of less than 10 nm, while also enabling batch production, greatly improving production efficiency, reducing costs, and eliminating adhesive bonding risks. It is more suitable for high-resolution, high signal-to-noise ratio terahertz near-field scanning imaging applications.

Description

Technical Field

[0001] This invention relates to the field of terahertz application technology, specifically to a method for preparing an all-metal tip terahertz near-field scanning probe. Background Technology

[0002] Scanning probe microscopy is a core tool for nanoscale measurement of material surfaces. Its resolution limit is determined by the sharpness of the probe tip. Combining scanning probes with spectroscopy is an important means of obtaining the physicochemical properties of materials at the nanoscale. Terahertz waves, due to their unique penetrability to many materials and fingerprint spectrum recognition capabilities, have shown great potential in advanced materials, biomedicine and other fields when combined with near-field scanning technology (THz-SNOM).

[0003] In THz-SNOM, ultra-high spatial resolution imaging is mainly achieved by scattering probes. The principle is to use the antenna effect of the metal tip to localize and strongly enhance the incident terahertz wave in a very small area at the tip, forming an optical hotspot with a size comparable to the tip's radius of curvature. The theoretical resolution of this technique is determined only by the radius of curvature at the tip and is independent of the terahertz wavelength. Therefore, it can break the diffraction limit and achieve nanometer or even sub-10 nanometer resolution. To achieve efficient field enhancement and scattering, the tip of the probe must be an all-metal structure. Conventional silicon cone tips are not suitable for THz-SNOM systems.

[0004] However, the existing method for preparing metal needle tips is a traditional process combining electrochemical etching and manual bonding. This method first prepares the metal needle tip and cantilever beam individually through electrochemical etching, and then manually bonds them to the base. This process has inherent and serious drawbacks: Extremely poor accuracy and consistency: The electrochemical corrosion process is difficult to control precisely, resulting in drastic fluctuations in the needle tip geometry parameters (such as height and radius of curvature), and low product consistency. According to publicly available data, the cantilever beam length error is as high as ±15%, and the needle tip height error is ±25%. The repeatability of the tip nanostructure cannot be guaranteed, which seriously affects the stability and reliability of imaging resolution.

[0005] Low preparation efficiency and high cost: This process is a single-probe tandem preparation, which can only process one probe at a time, making it impossible to achieve mass production. This results in extremely low production efficiency and high cost per probe.

[0006] All-metal cantilever beams are prone to fatigue, which makes it impossible to guarantee detection consistency, and the reduced service life leads to higher operating costs.

[0007] There are potential structural reliability issues: The probe tip and the base rely on adhesive bonding. Under long-term use or in environments such as temperature changes or laser irradiation, there is a potential risk that the adhesive layer will age, crack, or fall off, affecting the probe's lifespan and signal stability.

[0008] In summary, existing technologies cannot meet the urgent needs of THz-SNOM for high-performance, highly consistent, and mass-producible all-metal probes, which severely restricts the widespread application and development of this technology. Summary of the Invention

[0009] To address the problems existing in the prior art, this invention provides a method for preparing an all-metal tip terahertz near-field scanning probe, which solves the problem that the existing preparation process is difficult to control precisely, resulting in drastic fluctuations in tip geometric parameters, low product consistency, and large errors in cantilever beam length and tip height.

[0010] To solve the above problems, the technical solution of the present invention is as follows: A method for preparing an all-metal tip terahertz near-field scanning probe, comprising the following steps: S1. Prepare an SOI wafer and prepare an adhesion layer and a seed layer on top of the SOI wafer. The SOI wafer includes a top silicon layer on the upper surface of the SOI wafer for fabricating cantilever beams and a substrate layer on the lower surface of the SOI wafer for fabricating pin holders. S2. Spread ultraviolet thick adhesive over the seed layer to prepare the first photoresist circular hole, and electroplate a metal pillar in the first photoresist circular hole as the needle tip body. S3. Spread electron beam resist over the ultraviolet thick resist, engrave a second photoresist circular hole over the needle tip body, evaporate and deposit a film over the electron beam resist to form the tip of the needle, remove the ultraviolet thick resist and electron beam resist, and peel off the excess metal. S4. Spray photoresist, expose and develop the cantilever beam mask pattern, and etch the seed layer and top silicon to form a cantilever beam. S5. Apply ultraviolet photoresist under the substrate to form a needle holder mask pattern, etch to form the needle holder, remove the photoresist and clean the sample to obtain an all-metal tip terahertz near-field scanning probe.

[0011] Furthermore, in S1, the material of the adhesive layer is Ti or Cr, and the thickness is 10nm-50nm.

[0012] Furthermore, in S1, the seed layer is made of one or more of Au, Pt, and Ir, and has a thickness of 100nm-500nm.

[0013] Furthermore, in S2, the thickness of the first photoresist is ≥80μm.

[0014] Furthermore, in S2, the material of the electroplated metal column is one or more of Au, Pt, and Ir.

[0015] Furthermore, in S3, the thickness of the electron beam gel is 0.5 μm–2 μm.

[0016] Furthermore, in S3, the material of the evaporated coating is one or more of Au, Pt, and Ir.

[0017] Furthermore, in S3, the radius of curvature R of the tip of the needle is less than 10 nm, taking advantage of the phenomenon that the opening of the second photoresist circular hole gradually shrinks and closes during the evaporation coating process.

[0018] Furthermore, in S4, the photoresist thickness is >85μm.

[0019] Furthermore, in S5, the thickness of the ultraviolet photoresist is 6μm–15μm.

[0020] Compared with existing technologies, this invention has the following advantages: It uses a standard semiconductor process combining photolithography, film deposition, electroplating and deep silicon etching to prepare an all-metal tip terahertz near-field scanning probe, replacing the traditional electrochemical etching and adhesive bonding process. This significantly improves the precision and consistency of the probe structure, effectively reduces the error in cantilever beam length and tip height, improves the repeatability of the tip detail structure, and ensures stable and reliable terahertz near-field imaging resolution. Moreover, the metal layer deposited on the silicon cantilever beam replaces the metal cantilever beam, improving the problem of fatigue of metal cantilever beams.

[0021] By electroplating to form a high aspect ratio metal cylinder and then evaporating to form a nanoscale needle tip, and by utilizing the phenomenon that the opening of the second photoresist circular hole gradually shrinks and closes during the evaporation process, a needle tip with a curvature radius of less than 10 nm can be stably prepared, so that the probe meets the requirements of nanoscale high-resolution terahertz near-field imaging, and the imaging resolution is not limited by the terahertz wavelength.

[0022] Multiple probes can be fabricated simultaneously on the entire SOI wafer, enabling wafer-level mass production, significantly improving fabrication efficiency, reducing production costs, and solving the problems of low efficiency and high cost of traditional single probe fabrication.

[0023] The probe tip, cantilever beam, and probe holder are integrated into a single unit without adhesive bonding, thus avoiding reliability issues caused by adhesive aging, cracking, or detachment and improving probe lifespan and terahertz signal acquisition stability.

[0024] The tip height can be flexibly adjusted to be greater than 80μm by adjusting the thickness of the adhesive and the electroplating parameters, so as to meet the requirements of different terahertz near-field scanning test scenarios for tip height. It has strong process compatibility and high adjustability. Attached Figure Description

[0025] Figure 1 This is a flowchart of the preparation method of the present invention; Figure 2 This is a schematic diagram of the all-metal tip terahertz near-field scanning probe structure of the present invention.

[0026] In the figure: 1. SOI wafer; 2. Adhesive layer and seed layer; 3. UV thick resist; 4. First photoresist circular hole; 5. Tip body; 6. Electron beam resist; 7. Second photoresist circular hole; 8. Coating; 9. Tip top; 10. Photoresist; 11. UV photoresist; 12. Needle holder; 13. Cantilever beam. Detailed Implementation

[0027] This invention discloses a method for fabricating an all-metal tip terahertz near-field scanning probe. The entire process utilizes standard semiconductor processes such as photolithography, magnetron sputtering, electroplating, electron beam exposure, evaporation deposition, and deep silicon etching. This method achieves integrated fabrication of the tip body 5, tip tip 9, cantilever beam 13, and tip holder 12. The fabricated probe meets the technical requirements of an all-metal structure, tip height > 80 μm, and tip tip tip 9 curvature radius < 10 nm. The fabrication steps are as follows: Figure 1 As shown, the specific steps are as follows: Step S1: Prepare SOI sheet 1, and prepare an adhesion layer and a seed layer 2 on top of SOI sheet 1.

[0028] SOI wafer 1 is selected as the substrate material. SOI wafer 1 includes a three-layer structure: a top silicon layer on the upper surface, which is used to fabricate the cantilever beam 13 structure; and a substrate layer on the lower surface, which is used to fabricate the pin holder 12 structure.

[0029] An adhesion layer and a seed layer 2 are sequentially prepared on the upper surface of the top silicon layer using a magnetron sputtering method. The adhesion layer is made of Ti or Cr and has a thickness of 10 nm to 50 nm. It is used to enhance the adhesion between the metal layer and the silicon substrate and prevent peeling in subsequent processes. The seed layer is made of one or more of Au, Pt, and Ir and has a thickness of 100 nm to 500 nm. It serves as a conductive substrate for subsequent electroplating processes.

[0030] The adhesive layer and seed layer 2 prepared by magnetron sputtering are dense and uniform, with a thickness control accuracy of ±5%, which is far superior to traditional manual processes. Ti or Cr forms a stable compound interface with silicon, and the Au, Pt, and Ir seed layers have excellent chemical stability and conductivity, providing a reliable electrochemical interface for subsequent electroplating processes and ensuring the uniformity and density of the electroplated metal pillars.

[0031] Step S2: Spread ultraviolet thick adhesive 3 evenly over the seed layer to prepare the first photoresist circular hole 4, and electroplate a metal pillar inside the first photoresist circular hole 4 as the tip body 5.

[0032] A UV thick adhesive 3 is uniformly coated on the seed layer using a spin coating or spray coating process. The thickness of the UV thick adhesive 3 is ≥80μm. A first photoresist circular hole 4 structure is prepared in the UV thick adhesive 3 by exposure and development using a UV photolithography process. The diameter of the circular hole is 10μm-30μm, and the bottom of the hole exposes the surface of the seed layer.

[0033] Using the seed layer as the electroplating substrate, metal pillars are electroplated within the first photoresist circular hole 4 to form the tip body 5. The electroplated metal pillar material is one or more of Au, Pt, and Ir, matching the seed layer material. The electroplating employs a constant current electroplating or pulse electroplating process, with a current density of 5 mA / cm²-20 mA / cm². The height of the metal pillar is maintained >80 μm by controlling the electroplating time.

[0034] By employing a thick-film photolithography combined with electroplating, arrayed metal pillars can be simultaneously fabricated on the entire wafer surface, enabling mass production. The first photoresist circular hole 4 serves as the electroplating mold, precisely defining the diameter and position of the metal pillars. The uniformity error of the pillar diameter is less than 5%. The electroplating process forms a dense metal layer through ion reduction deposition. The height of the metal pillars can be precisely controlled by electroplating time and current density, with the height error controlled within ±5%, far superior to the ±25% error of traditional electrochemical etching processes. The height-to-diameter ratio of the metal pillars can reach more than 2:1, meeting the requirements of terahertz near-field scanning for tip height.

[0035] Step S3: Spread electron beam resist 6 evenly over the UV thick resist 3, etch a second photoresist circular hole 7 over the needle tip body 5, evaporate and deposit a film 8 over the electron beam resist 6 to form the needle tip 9, remove the UV thick resist 3 and electron beam resist 6, and peel off the excess metal.

[0036] The overlay accuracy is in the hundreds of nanometers. For the probe, the tip 9 is much smaller than the body 5 below it. Even if there is a slight error in the position of the tip 9 relative to the center of the body 5 below it, it will not affect the operation.

[0037] Electron beam resist 6 is uniformly coated on top of the ultraviolet thick resist 3 and the metal pillar. The thickness of the electron beam resist 6 is 0.5μm-2μm. Using electron beam lithography, a second photoresist circular hole 7 is formed on top of the metal pillar. The diameter of the second photoresist circular hole 7 is 1μm-5μm, which is smaller than the diameter of the first photoresist circular hole 4, and it is aligned with the center of the metal pillar.

[0038] Metal material is deposited in the second photoresist circular hole 7 using an electron beam evaporation process. The evaporation material is one or more of Au, Pt, and Ir, or a Pt / Ir alloy. During the evaporation process, the opening formed by the electron beam photoresist 6 gradually shrinks until it closes, naturally forming an extremely fine needle tip 9 structure on the top of the metal pillar. After the evaporation is completed, the sample is soaked in a resist remover solution to remove the UV thick resist 3 and the electron beam resist 6, while the excess metal layer is peeled off and the sample is cleaned.

[0039] During electron beam evaporation, metal atoms are deposited perpendicularly at the bottom of the circular hole, while a small number of atoms are deposited on the sidewalls, causing the effective diameter of the circular hole to gradually shrink. When the sidewall deposits converge at the center, a nanoscale tip with a curvature radius of <10nm is naturally formed. This process does not require additional mechanical grinding or chemical etching sharpening, avoiding the uncontrollable damage to the tip structure caused by traditional processes. The tip top formed by the gradual closure effect of the opening is atomically smooth, with good symmetry and high repeatability. The tip curvature radius can be stably controlled below 10nm, meeting the requirements of nanoscale high-resolution terahertz near-field imaging.

[0040] Step S4: Spray photoresist 10, expose and develop the mask pattern of cantilever beam 13, etch the seed layer and top silicon to form cantilever beam 13.

[0041] Photoresist 10 is sprayed onto the upper surface using a spray gun. The thickness of the photoresist 10 is >85μm, ensuring that it covers the formed metal tip structure with a margin. Ultraviolet lithography is then used to expose and develop a mask pattern of the cantilever beam 13 structure. The cantilever beam 13 pattern includes a support area connecting the tip, the cantilever beam 13 body, and an area connecting to the subsequent needle holder 12.

[0042] The seed layer metal in the exposed area is removed by wet etching, and then the top silicon is etched by deep reactive ion etching (DRIE) to form the cantilever beam 13 structure. The deep reactive ion etching adopts the Bosch process, alternating between SF6 etching and C4F8 passivation, and is cycled 100-500 times to achieve high aspect ratio silicon structure etching. After etching, the photoresist 10 is removed by photoresist remover and the sample is cleaned.

[0043] The thick photoresist spraying process can form 10 layers of photoresist of uniform thickness on uneven surfaces (including metal tips), ensuring the integrity of the cantilever beam 13 mask. Deep reactive ion etching has advantages such as good anisotropy, high selectivity, and high sidewall perpendicularity. The fabricated cantilever beam 13 has precise structural dimensions, smooth sidewalls, and thickness uniformity better than ±3%. The cantilever beam 13 is integrally formed with the tip without adhesive bonding, avoiding the risk of photoresist aging, cracking, or detachment, and significantly improving the mechanical reliability and service life of the probe. The length error of the cantilever beam 13 can be controlled within ±2%, which is far better than the ±15% error of traditional processes.

[0044] Step S5: Spread ultraviolet photoresist 11 under the substrate to form a mask pattern for the needle holder 12, etch to form the needle holder 12, remove the photoresist and clean the sample to obtain an all-metal tip terahertz near-field scanning probe.

[0045] UV photoresist 11 is uniformly applied to the lower surface of the substrate. The thickness of UV photoresist 11 is 6μm-15μm. A double-sided lithography machine is used for overlay alignment with an alignment accuracy of ±1μm. Exposure is then used to form a mask pattern of pin holder 12 structure.

[0046] Deep reactive ion etching or wet etching is used to penetrate the substrate layer to form a probe holder 12 structure. The probe holder 12 provides mechanical support and a mounting interface, forming an integral probe frame with the cantilever beam 13. After etching, the resist is removed using a resist remover, and the sample is cleaned to obtain the final all-metal tip terahertz near-field scanning probe, such as... Figure 2 As shown.

[0047] The double-sided photolithography process ensures precise alignment between the probe holder 12 structure and the cantilever beam 13 structure on the upper surface, resulting in high overall probe geometric accuracy and good assembly consistency. The probe holder 12, cantilever beam 13, and probe tip are integrally formed to create a complete all-metal probe structure with no adhesive bonding points, significantly improving structural reliability. Through whole-wafer processing, 50-100 probes (4-inch wafer) or 200-400 probes (6-inch wafer) can be produced in a single batch, greatly improving mass production efficiency and significantly reducing the cost per probe.

[0048] To further illustrate the technical solution of the present invention, specific embodiments are described below. These embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0049] Example 1 S1: Prepare a 4-inch SOI wafer 1 with a top silicon layer of 20μm and a substrate of 400μm. Use magnetron sputtering to deposit a 20nm Ti adhesion layer and a 200nm Pt seed layer on the top surface. The deposition power is 150W and 200W respectively, and the carrier disk rotation speed is 5-10rpm.

[0050] S2: Spin-coating with 80μm thick UV-coated photoresist 3, UV photolithography to prepare the first photoresist circular hole with a diameter of 20μm 4, exposure dose 1500mj / cm 2 Electroplated Pt metal column, electroplating temperature 30-50℃, current density 10mA / cm², time 60 minutes, column height 100μm.

[0051] S3: Spin-coating with 1μm PMMA electron beam resist 6, electron beam exposure to etch a second photoresist circular hole with a diameter of 2μm 7, exposure dose 300μC / cm 2 8. Beam current 1-2 nA, evaporate and deposit Pt / Ir alloy or Pt metal at a rate of 0.5-1 Å / s and a thickness of 300 nm. 9. Utilize the opening and closing phenomenon to form the tip of the needle. 10. Immerse in NMP stripper solution at 80°C for 2 hours to remove photoresist and strip off excess metal.

[0052] S4: Spray 85μm photoresist 10 with a spray gun, spray atomization pressure 0.2MPa, nozzle height 20mm, scanning speed 10mm / s, UV lithography cantilever beam 13 mask, remove Pt seed layer in exposed area with aqua regia, DRIE etch top silicon 20μm to form cantilever beam 13.

[0053] S5: Apply 8-10μm UV photoresist 11 to the lower surface, perform double-sided photolithography alignment, and DRIE through-etch 400μm into the substrate to form a needle holder 12. Select Bosch etching process for etching, remove the photoresist and clean to obtain an all-metal terahertz probe with a tip height of 100μm and a top curvature radius of <10nm.

[0054] Example 2 The difference from Example 1 is that: in S2, a 100μm thick UV resist 3 is coated and the electroplating time is extended to 80 minutes, resulting in a column height of 120μm; in S4, a 110μm photoresist 10 is sprayed, ultimately obtaining an all-metal terahertz probe with a tip height >100μm.

[0055] Example 3 The difference from Example 1 is that: in S1, a 20nm Cr adhesive layer and a 200nm Au seed layer are sputtered; in S2, Au metal pillars are electroplated; in S3, an 8Au film is evaporated to form the tip 9, and finally an all-metal tip terahertz probe with a tip height > 80μm and a tip curvature radius < 10nm is obtained.

[0056] This application uses an SOI wafer 1 as a substrate, prepares an adhesion layer and a seed layer by magnetron sputtering, and uses ultraviolet thick adhesive 3 for photolithography and electroplating to form the tip body 5. Combined with electron beam exposure and evaporation coating 8, a tip tip 9 with a curvature radius of less than 10nm is formed. Then, a cantilever beam 13 and a needle holder 12 are formed in an integrated manner by photolithography and deep silicon etching. This solution adopts standard semiconductor wafer-level processes to replace traditional electrochemical etching and adhesive bonding methods, which significantly improves the probe size accuracy and structural consistency. It can stably obtain all-metal probes with a tip height greater than 80μm and a tip curvature radius of less than 10nm, while realizing batch production, greatly improving production efficiency, reducing costs, and eliminating adhesive bonding risks. It is more suitable for high-resolution, high signal-to-noise ratio terahertz near-field scanning imaging applications.

[0057] The above specific embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for fabricating an all-metal tip terahertz near-field scanning probe, characterized in that, Includes the following steps: S1. Prepare an SOI wafer (1), and prepare an adhesive layer and a seed layer (2) on top of the SOI wafer (1). The SOI wafer (1) includes a top silicon layer located on the upper surface of the SOI wafer for fabricating a cantilever beam (13) and a substrate layer located on the lower surface of the SOI wafer (1) for fabricating a pin holder (12). S2. Apply UV thick adhesive evenly above the seed layer (3) to prepare the first photoresist circular hole (4), and electroplate a metal pillar inside the first photoresist circular hole (4) as the tip body (5). S3. Spread electron beam resist (6) over the ultraviolet thick resist (3), engrave a second photoresist circular hole (7) over the needle tip body (5), evaporate and deposit a film (8) over the electron beam resist (6) to form the tip tip (9), remove the ultraviolet thick resist (3) and electron beam resist (6), and peel off the excess metal. S4. Spray photoresist (10), expose and develop the cantilever beam mask pattern, etch the seed layer and top silicon to form the cantilever beam (13). S5. Apply ultraviolet photoresist (11) under the substrate to form a needle holder mask pattern, etch to form a needle holder (12), remove the photoresist and clean the sample to obtain an all-metal terahertz near-field scanning probe.

2. The preparation method according to claim 1, characterized in that: In S1, the material of the adhesive layer is Ti or Cr, and the thickness is 10nm-50nm.

3. The preparation method according to claim 2, characterized in that: In S1, the seed layer is made of one or more of Au, Pt, and Ir, and has a thickness of 100nm-500nm.

4. The preparation method according to claim 3, characterized in that: In S2, the thickness of the first photoresist (4) is ≥80μm.

5. The preparation method according to claim 4, characterized in that: In S2, the material of the electroplated metal column is one or more of Au, Pt, and Ir.

6. The preparation method according to claim 5, characterized in that: In S3, the thickness of the electron beam adhesive (6) is 0.5 μm–2 μm.

7. The preparation method according to claim 6, characterized in that: In S3, the material of the evaporated coating is one or more of Au, Pt, and Ir.

8. The preparation method according to claim 7, characterized in that: In S3, the radius of curvature R of the tip (9) is less than 10 nm, taking advantage of the phenomenon that the opening of the second photoresist circular hole (7) gradually shrinks and closes during the evaporation coating process.

9. The preparation method according to claim 8, characterized in that: In S4, the thickness of the photoresist (10) is >85μm.

10. The preparation method according to claim 9, characterized in that: In S5, the thickness of the ultraviolet photoresist (11) is 6μm–15μm.

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