A method for manufacturing a terahertz slow wave structure

Abstract

The present application belongs to the technical field of terahertz vacuum electron device manufacturing, and particularly relates to a terahertz slow wave structure manufacturing method, comprising: dividing a device to be processed into a substrate and a structure sheet, the substrate being used for mechanical support, heat dissipation and vacuum sealing; the structure sheet being provided with a slow wave structure for realizing energy exchange between electrons and electromagnetic waves, the substrate thickness being greater than the structure sheet thickness; machining the slow wave structure on the structure sheet through femtosecond laser rotary cutting or slow wire cutting, and performing surface polishing through electrochemical polishing; machining the substrate through a conventional precision machining method, and performing surface polishing; and diffusion bonding the structure sheet after electrochemical polishing and the substrate after surface polishing. The slow wave structure is machined through femtosecond laser rotary cutting or slow wire cutting, the electrochemical polishing improves the surface quality of the structure sheet, reduces loss in the signal transmission process, and the diffusion bonding ensures high vacuum sealing of the terahertz vacuum electron device under complex working environment.

Description

Technical Field

[0001] This invention belongs to the field of terahertz vacuum electronic device manufacturing technology, specifically relating to a method for manufacturing a terahertz slow-wave structure. Background Technology

[0002] As vacuum electronic devices operate at frequencies exceeding the terahertz band (e.g., 220 GHz to 1 THz), the size of their core components—slow-wave structures (such as folded waveguides)—has drastically shrunk to the order of hundreds or even tens of micrometers, with structural depths reaching tens to hundreds of micrometers. Such minute dimensions and complex structures place extremely high demands on manufacturing technology. Traditional machining and micro-electrical discharge machining techniques have reached their processing limits and are insufficient to meet manufacturing requirements.

[0003] Currently, mainstream manufacturing technologies include deep reactive ion etching (DRIE), ultraviolet lithography, electroforming (UV-LIGA), and nano-CNC milling. DRIE technology requires metallization after etching the structure on silicon; however, it suffers from poor adhesion between the metal film and the substrate, and silicon has poor heat dissipation. While UV-LIGA technology can fabricate all-metal structures, it faces challenges in processing ultra-high frequency (e.g., >670 GHz) structures, such as photoresist uniformity, electroforming stress control, and high aspect ratio photoresist removal, leading to lower yield and lower high-frequency performance stability. Nano-CNC milling, as a purely mechanical micro-milling technology, can achieve machining accuracy of ±0.5 μm and roughness ≥50 nm. However, for folded waveguide structures with high aspect ratios (e.g., depth 200 μm, slot width 50 μm), the micro-milling cutter is prone to wear and breakage, resulting in low machining efficiency and difficulty in controlling sidewall perpendicularity and bottom roughness.

[0004] Based on this, a method for manufacturing terahertz slow wave structures is proposed. Summary of the Invention

[0005] To overcome the problems in the prior art, this invention proposes a method for manufacturing terahertz slow wave structures.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: This invention provides a method for manufacturing a terahertz slow-wave structure, comprising the following steps: The device to be processed is divided into a substrate and a structural thin plate. The substrate is used for mechanical support, heat dissipation and vacuum sealing. The structural thin plate is provided with a slow-wave structure to realize the exchange of electron and electromagnetic wave energy. The substrate and structural thin plate are processed separately, and then the substrate and structural thin plate are bonded together to obtain the desired slow-wave structure, as detailed below: The substrate and the structural sheet are split in the thickness direction of the slow wave structure to be processed, and the thickness of the substrate is greater than the thickness of the structural sheet. The required slow-wave structure is processed on the structural thin plate by femtosecond laser rotary cutting or slow wire cutting, and then the surface is polished by electrochemical polishing; the substrate is processed by conventional precision machining methods and the surface is polished. The electrochemically polished structural sheet is aligned and diffusion bonded to the surface-polished substrate.

[0007] Furthermore, the substrate is provided with a quasi-marker, flow channel or welding surface; the slow wave structure includes waveguide groove and electron beam channel outline.

[0008] Furthermore, the surface roughness Ra of the substrate is ≤50nm.

[0009] Furthermore, the substrate and the structural sheet are made of the same material.

[0010] Furthermore, the substrate and the structural sheet are made of oxygen-free copper.

[0011] Furthermore, the electrochemical polishing involves immersing the thin plate processed by femtosecond laser into a phosphoric acid-ethylene glycol electrolyte and applying voltage to polish the structural thin plate in the electrolyte.

[0012] Furthermore, the volume ratio of phosphate to ethylene glycol in the electrolyte is 3:7; the applied voltage is a DC voltage, ranging from 5V to 8V; and the polishing time is 30s–60s.

[0013] Furthermore, the thickness T2 of the structural thin plate satisfies the following condition: 0.1mm≤T2≤1mm.

[0014] Further, the electrochemically polished structural sheet is aligned and diffusion-bonded to the surface-polished substrate, including: Align the electrochemically polished structural sheet with the surface-polished substrate to match the slow-wave structure on the structural sheet with the corresponding area on the substrate. In a vacuum or protective atmosphere furnace, the structural sheet and substrate bonded together are heated to a preset temperature and pressure is applied, and then kept warm under preset temperature and pressure conditions.

[0015] Furthermore, the preset temperature is set at 400°C - 600°C, and the pressure is controlled at 5MPa - 20MPa.

[0016] Furthermore, the heat preservation time is 30 min - 60 min.

[0017] Compared with the prior art, the present invention has the following technical effects: (1) This invention separates the slow-wave structure into a substrate and a structural thin plate for separate processing. The substrate is used for mechanical support, heat dissipation, and vacuum sealing, while the structural thin plate is used to process the slow-wave structure that realizes the exchange of electron and electromagnetic wave energy. This split processing mode avoids the problems of difficult glue removal, fast tool wear, and difficulty in bottom roughness control in traditional electroforming processes. Using the same material as the substrate and structural thin plate, such as oxygen-free copper, not only ensures a good bond between the two, but also fully utilizes the excellent electrical and thermal conductivity of oxygen-free copper, avoiding the metallization process required after traditional deep reactive ion etching, as well as the adhesion problem between the metal film and the substrate, and the heat dissipation problem. The slow-wave structure processed by femtosecond laser spin cutting or slow wire cutting has high precision and high cut perpendicularity. The "cold processing" characteristic of femtosecond laser can minimize the heat-affected zone, slag, and recast layer. Electrochemical polishing uniformly removes the micron / submicron level surface unevenness, burrs, and recast layer generated by laser processing through electrochemical dissolution, further improving the surface quality of the structural thin plate and reducing signal transmission loss. The metallurgical bond formed by diffusion bonding enables the bond to withstand the high-temperature brazing during subsequent device assembly without problems such as bubbling, deformation or leakage, thus ensuring the high vacuum sealing of the terahertz slow wave structure in complex working environments.

[0018] (2) The manufacturing method proposed in this invention significantly simplifies the process flow, avoiding the complex and cumbersome photolithography, electroforming, and resist removal processes in UV-LIGA technology, and also eliminating the etching and metallization steps in DRIE technology. Traditional process chains are lengthy, and each step may introduce uncertainties, making it difficult to guarantee product yield. The shortened process chain of this method not only reduces processing steps but also enhances the controllability of the entire manufacturing process. Especially in the manufacturing of ultra-high frequency structures, the advantages of this simplified process are more prominent, enabling more precise control of processing parameters, effectively reducing scrap rate, and greatly improving production efficiency and product yield, laying a solid foundation for the large-scale, high-quality production of terahertz vacuum electronic devices. Attached Figure Description

[0019] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is an exploded view of the substrate and thin plate of the present invention; Figure 2 This invention relates to laser rotary cutting of thin plates; Figure 3This invention relates to slow wire EDM for processing thin plates; Figure 4 This is a schematic diagram of the substrate and the thin plate after diffusion bonding according to the present invention. Detailed Implementation

[0021] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the specific implementation methods, structures, features, and effects of the technical solutions proposed according to the present invention are described in detail below with reference to the accompanying drawings and preferred embodiments. Specific features, structures, or characteristics in one or more embodiments may be combined in any suitable form. Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0022] This invention aims to provide a method for manufacturing terahertz slow-wave structures. This method simplifies the manufacturing process, reduces overall processing difficulty, improves yield, and ensures the final device possesses the excellent thermal conductivity and structural strength inherent in pure metal structures by centrally processing complex microstructures requiring high precision and high surface quality onto a relatively thin metal plate, followed by high-precision bonding to a thick substrate.

[0023] In this embodiment, refer to Figures 1-4 A method for manufacturing a terahertz slow-wave structure is provided, comprising the following steps: The device to be processed is divided into a substrate and a structural thin plate. The substrate is used for mechanical support, heat dissipation and vacuum sealing. The structural thin plate is provided with a slow-wave structure to realize the exchange of electron and electromagnetic wave energy. The thickness of the substrate is greater than the thickness of the structural thin plate. The required slow-wave structure is processed on the structural thin plate by femtosecond laser rotary cutting or slow wire cutting, and then the surface is polished by electrochemical polishing; the substrate is processed by conventional precision machining methods and then the surface is polished. The structural thin plate, after electrochemical polishing, is diffusion bonded to the surface-polished substrate.

[0024] The following is a detailed explanation of each of the above steps: Step 100: The device to be processed is split into a substrate and a structural thin plate in the thickness direction. The thickness of the substrate is greater than that of the structural thin plate. The substrate is used for mechanical support, heat dissipation and vacuum sealing. The structural thin plate is provided with a slow wave structure to realize the exchange of electron and electromagnetic wave energy.

[0025] The complete slow-wave structure (taking a folded waveguide as an example) is split in the thickness direction. Specifically, assuming a slow-wave structure with a total height of H, it is decomposed into a substrate with a thickness of T1 and a structural thin plate with a thickness of T2, wherein the thickness of the substrate T1 is greater than the thickness of the structural thin plate T2.

[0026] For example, when H = 5mm, T1 = 4.8 mm and T2 = 0.2 mm can be set. The thickness distribution is not fixed but can be flexibly adjusted according to actual functional requirements and manufacturing conditions. The thickness range of the structural sheet is between 0.1mm and 1mm, which can meet the design requirements of slow-wave structures of varying complexity. Meanwhile, considering the crucial role of waveguide microgrooves in slow-wave structures, their depth is mostly greater than 0.1mm. This requires that the structural sheet, while possessing sufficient thinness, must also be able to be fabricated to produce waveguide microgrooves that meet design requirements.

[0027] The substrate and structural sheet serve different functions in the disassembled slow-wave structure. The substrate provides mechanical support, heat dissipation, and vacuum sealing. Since its primary function does not involve complex slow-wave structure fabrication, only simple alignment marks, flow channels, or solder surfaces need to be fabricated on the substrate. These simple structural fabrications are relatively easy to implement and have relatively low requirements for manufacturing processes. For example, alignment marks provide a positioning reference for subsequent assembly of the substrate and structural sheet, ensuring the relative positional accuracy between them; the design of flow channels helps improve the heat dissipation performance of the slow-wave structure, dissipating the heat generated during operation in a timely manner and ensuring stable device operation; the solder surfaces are used to achieve reliable connections between the substrate and other components, ensuring the vacuum sealing effect.

[0028] The components containing all the complex slow-wave structures are concentrated on a structural thin plate with a thickness of T2. These slow-wave structures include waveguide grooves and electron beam channel profiles, which are crucial for the energy exchange between electrons and electromagnetic waves. The shape, size, and positional accuracy of the folded waveguide grooves directly affect the transmission characteristics of the electromagnetic waves, while the accuracy of the electron beam channel profiles relates to the transmission quality of the electron beam and its interaction with the electromagnetic waves. Therefore, high-precision processing techniques are required in the design and manufacturing of the structural thin plate to ensure that these complex folded waveguides can be accurately formed, meeting the high-performance requirements of the slow-wave structures in the terahertz frequency band.

[0029] Step 200: The required slow-wave structure is processed on the structural thin plate by femtosecond laser rotary cutting or slow wire cutting, and then the surface is polished by electrochemical polishing; the substrate is processed by conventional precision machining method and then the surface is polished.

[0030] In the manufacturing of slow-wave structures in the terahertz band, the processing of thin-plate microstructures is a key step, and its processing quality directly affects the performance of the slow-wave structure.

[0031] Step 210: Perform femtosecond laser rotary cutting or slow wire cutting on the structural thin plate to carve out the contours of the waveguide grooves, electron injection channels, etc. of the slow wave structure.

[0032] The first method is applicable to vacuum electronic devices in the 340 GHz and above frequency bands. The material of the structural thin plate is oxygen-free copper. In the first step of the structural thin plate processing, a femtosecond pulsed laser system is used to spin-cut the oxygen-free copper structural thin plate. Conventional laser processing methods mostly use galvanometer scanning. However, due to the focal depth limitation of the focused laser beam and the reflection phenomenon on the inner wall during processing, the material ablation rate decreases sharply with the increase of cutting depth, which inevitably leads to the taper of the cut section, making it difficult to meet the requirements of high-precision microstructure processing.

[0033] Femtosecond laser spin cutting technology solves this problem. Femtosecond laser spin cutting is a material processing technology that utilizes femtosecond lasers to achieve high precision, low heat effect, and controllable taper. Its core principle is that the laser beam, after passing through a special optical module, rotates at high speed around the optical axis at a certain yaw angle. This yaw angle is adjustable, thereby controlling the taper of the cutting section, achieving taper-controlled drilling and cutting. In actual processing, by setting the platform's running trajectory, the entire outline of slow-wave structures, such as slow-wave structures, can be directly engraved. This solution is an application of femtosecond laser spin cutting technology and does not involve improvements to the femtosecond laser spin cutting process; therefore, deeper levels of cutting control will not be elaborated upon.

[0034] Femtosecond lasers possess unique cold-working characteristics, a significant advantage for their application in high-precision microstructure fabrication. During processing, the extremely short pulse duration of a femtosecond laser releases energy in a very short time, causing the material to vaporize instantaneously and minimizing the heat-affected zone. Compared to traditional laser processing, it effectively avoids problems such as material melting, slag formation, and recast layers caused by heat accumulation, thus achieving a preliminary high-precision structural morphology.

[0035] The second method is applicable to vacuum electronic devices in the 220GHz and above frequency bands. Wire EDM-LS is an important branch of wire electrical discharge machining technology. Its core principle is to use a continuously moving fine metal wire as an electrode, and to remove metal materials through high-temperature erosion generated by pulsed spark discharge to achieve precision machining.

[0036] The structural sheet material is oxygen-free copper. In the processing of terahertz slow-wave structural sheets, extremely fine electrode wires are used in conjunction with slow-wire EDM machines, with wire diameters ranging from 0.02 mm to 0.1 mm. Oxygen-free copper structural sheets possess excellent electrical and thermal conductivity, along with high machinability. Combined with slow-wire EDM, the advantages of both materials are fully utilized.

[0037] During the machining process, a precision CNC system controls the electrode wire to move along a pre-designed slow-wave structure plane contour trajectory, cutting on the oxygen-free copper structural thin plate according to a predetermined route. A single cut through a thin plate of thickness T2 directly obtains the contours of all vertical sidewalls of the structure, greatly simplifying the machining process and improving efficiency. Slow wire EDM offers numerous significant advantages: the resulting structures exhibit excellent straightness, ensuring straight and regular sidewalls; extremely high dimensional consistency, ensuring that each machined structure meets stringent design requirements and guaranteeing product quality stability; and smaller corner errors result in smoother transitions at corners, reducing signal transmission loss and interference, providing a strong guarantee for the high-performance operation of terahertz slow-wave structures.

[0038] Step 220: Perform electrochemical polishing on the structural thin plate after femtosecond laser rotary cutting or slow wire EDM to reduce the surface roughness of the thin plate.

[0039] Although the structural thin plates processed by femtosecond laser rotary cutting or slow wire EDM have formed a preliminary structural morphology, some micron / submicron-level unevenness, burrs, and recast layers still exist on the surface. These problems will affect the performance and service life of the slow-wave structure. Therefore, a second processing step—electrochemical polishing—is required.

[0040] Electrochemical polishing involves immersing a femtosecond laser-processed thin plate in a phosphoric acid-ethylene glycol electrolyte with a volume ratio of 3:7. A DC voltage of 5V-8V is then applied, allowing the thin plate to polish for 30-60 seconds. During electrochemical polishing, the plate surface acts as the anode, undergoing an electrochemical dissolution reaction under the influence of current. Due to the uniformity of the current distribution, material can be removed evenly, effectively eliminating surface irregularities, burrs, and recast layers generated during laser processing.

[0041] Electrochemical polishing significantly reduces the surface roughness of the thin plate to the nanometer level (Ra < 50 nm), and the sidewall perpendicularity can be controlled to ≤ 0.5°. Simultaneously, this process further refines the structural edges, improves dimensional consistency, and ensures that the processed thin plate microstructure more precisely meets design requirements, satisfying the high precision and high performance demands of terahertz slow-wave structures.

[0042] In summary, the combination of femtosecond laser rotary cutting or slow wire cutting with electrochemical polishing can fully leverage the advantages of both technologies to achieve high-precision, high-quality machining of oxygen-free copper thin-plate microstructures, providing an effective solution for the manufacture of terahertz slow-wave structures.

[0043] Step 300: Diffusion bonding is performed between the electrochemically polished structural sheet and the surface-polished substrate.

[0044] As a key component in the manufacturing of slow-wave structures, the substrate is made of the same material as the structural sheet, using oxygen-free copper as an example. In the forming process, conventional precision machining and electrical discharge machining (EDM) are the two main methods. Conventional precision machining utilizes mature processes such as turning, milling, planing, and grinding, employing high-precision equipment and tools, and strictly controlling parameters such as cutting speed, feed rate, and depth of cut. This allows oxygen-free copper raw materials to be processed into substrates of specific shapes and sizes, suitable for manufacturing relatively regular-shaped substrates. EDM, on the other hand, utilizes the electro-corrosion phenomenon generated by electrical discharge. It is not limited by material hardness and can process complex structures that are difficult to achieve with conventional machining, such as deep, narrow, and shallow cavities. Shallow cavities adapted to the structural sheet can be machined on the substrate.

[0045] After the substrate is formed, the surface roughness often fails to meet the requirements and needs to be precision polished to control the surface roughness within the range of Ra≤50nm. This effectively reduces the contact resistance between the substrate and the thin plate, avoids problems such as partial discharge, and ensures signal transmission efficiency and slow wave structure performance.

[0046] Step 400: Align and diffuse bond the electrochemically polished structural sheet with the surface-polished substrate.

[0047] The electrochemically polished structural sheet is aligned with the surface-polished substrate. The aligned and bonded sheet and substrate are then placed in a vacuum or protective atmosphere furnace to initiate the crucial diffusion bonding process. The vacuum environment effectively isolates the metal from air, preventing reactions between the metal and oxygen, nitrogen, etc., at high temperatures, thus preventing adverse phenomena such as oxidation and nitriding and ensuring the purity of the bonding interface. The protective atmosphere furnace creates a vacuum-like oxygen-free or low-oxygen environment by introducing inert gases such as argon or nitrogen, which also serves to protect the metal surface.

[0048] Inside the furnace, appropriate temperature and pressure are applied to the structural sheet and substrate. The temperature is set between 400°C and 600°C, a range considered a balance between the activity of metal atoms and the stability of material properties. At this temperature, metal atoms gain sufficient energy to become active without causing irreversible changes in material properties due to excessive heat. The pressure is controlled between 5 MPa and 20 MPa, ensuring a tight bond between the sheet and substrate and providing the necessary contact conditions for metal atom diffusion. After a holding period of 30-60 minutes, metal atoms at the interface begin to diffuse into each other. This metallurgical bonding differs from simple physical bonding; it tightly connects the sheet and substrate at the atomic level, achieving complete structural and vacuum sealing, providing reliable physical support and sealing for the slow-wave structure.

[0049] This bonded structure, with its unique metallurgical bonding, exhibits strong high-temperature stability. The metallic bonds formed by the metallurgical bonding possess high strength and stability; even at high temperatures, the binding force between metal atoms remains intact and is not easily broken. Simultaneously, the uniform bonding interface lacks obvious interface defects and stress concentration, effectively resisting the thermal stress and thermal shock generated during high-temperature brazing. Furthermore, excellent vacuum sealing performance is maintained at high temperatures, preventing the penetration and leakage of gas molecules and ensuring a consistently stable vacuum environment within the slow-wave structure. Therefore, this bonded structure remains intact during high-temperature brazing, providing a solid guarantee for the subsequent assembly and long-term stable operation of the slow-wave structure.

[0050] The above 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 the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A method for manufacturing a terahertz slow-wave structure, characterized in that, Includes the following steps: The slow-wave structure to be processed is divided into a substrate and a structural thin plate. The substrate and the structural thin plate are made of the same material. The substrate is used for mechanical support, heat dissipation and vacuum sealing. The structural thin plate is provided with a slow-wave structure to realize the exchange of electron and electromagnetic wave energy. The thickness of the substrate is greater than the thickness of the structural thin plate. The required slow-wave structure is processed on the structural thin plate by femtosecond laser rotary cutting or slow wire cutting, and then the surface is polished by electrochemical polishing. The substrate is processed using conventional precision machining methods and then its surface is polished. The electrochemically polished structural sheet is diffusion bonded to a surface-polished substrate, including: Align the electrochemically polished structural sheet with the surface-polished substrate to match the slow-wave structure on the structural sheet with the corresponding area on the substrate. In a vacuum or protective atmosphere furnace, the structural sheet and substrate bonded together are heated to a preset temperature and pressure is applied, and then kept warm under preset temperature and pressure conditions.

2. The method for manufacturing a terahertz slow-wave structure according to claim 1, characterized in that, The substrate is provided with alignment marks, flow channels or welding surfaces; the slow wave structure includes waveguide grooves and electron beam channel contours.

3. The method for manufacturing a terahertz slow-wave structure according to claim 2, characterized in that, The substrate and the structural sheet are made of oxygen-free copper.

4. The method for manufacturing a terahertz slow-wave structure according to claim 2, characterized in that, The electrochemical polishing process involves immersing a thin plate processed by a femtosecond laser into a phosphoric acid-ethylene glycol electrolyte and applying a voltage to polish the structural thin plate in the electrolyte.

5. The method for manufacturing a terahertz slow-wave structure according to claim 4, characterized in that, The volume ratio of phosphate to ethylene glycol in the electrolyte is 3:7; the applied voltage is a DC voltage, ranging from 5V to 8V; and the polishing time is 30s–60s.

6. The method for manufacturing a terahertz slow-wave structure according to claim 2, characterized in that, The surface roughness Ra of the substrate is ≤50nm.

7. The method for manufacturing a terahertz slow-wave structure according to claim 1, characterized in that, The preset temperature is set between 400°C and 600°C, and the pressure is controlled between 5MPa and 20MPa.

8. A method for manufacturing a terahertz slow-wave structure according to claim 1 or 7, characterized in that, The heat preservation time is 30 min - 60 min.

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