Manufacturing method of terahertz slow-wave structure

Abstract

The invention belongs to the technical field of terahertz vacuum electronic device manufacturing, and particularly relates to a terahertz slow wave structure manufacturing method which comprises the steps that a device to be machined is divided into a substrate and a structural thin plate, and the substrate is used for mechanical supporting, heat dissipation and vacuum sealing; a slow wave structure for realizing electron and electromagnetic wave energy exchange is arranged on the structural thin plate, and the substrate is thicker than the structural thin plate; a slow wave structure is machined on the structural thin plate through femtosecond laser rotary cutting or slow wire cutting, and surface polishing is conducted through electrochemical polishing; the substrate is machined through a conventional precision machining method, and surface polishing is carried out; and carrying out diffusion bonding on the structure thin plate subjected to electrochemical polishing and the substrate subjected to surface polishing. A slow wave structure is machined through femtosecond laser rotary cutting or low-speed wire cutting, the surface quality of a structural thin plate is improved through electrochemical polishing, loss in the signal transmission process is reduced, and high vacuum sealing performance of a terahertz vacuum electronic device in a complex working environment is ensured through diffusion bonding.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of terahertz vacuum electron device manufacturing, and particularly relates to a terahertz slow wave structure manufacturing method. BACKGROUND

[0002] With the working frequency of vacuum electron devices entering the terahertz frequency band (such as 220 GHz to 1 THz or above), the size of the core component slow wave structure (such as a folded waveguide) is rapidly reduced to the order of hundreds of microns or even tens of microns, and the structure depth can reach tens to hundreds of microns. Such small size and complex structure put extremely high requirements on the manufacturing technology. The traditional mechanical processing and micro-EDM (electrical discharge machining) technology has reached its processing limit and is difficult to meet the manufacturing requirements.

[0003] The current mainstream manufacturing technologies include deep reactive ion etching (DRIE), ultraviolet-lithography, electroforming (UV-LIGA), and nanometer numerical control milling. The DRIE technology needs to be metallized after etching the structure on silicon, but there is a problem of poor adhesion of the metal film layer to the substrate, and the silicon material has poor heat dissipation; the UV-LIGA technology can manufacture all-metal structures, but when processing ultra-high frequency (such as >670 GHz) structures, it faces the process problems of photoresist uniformity, electroforming stress control, high aspect ratio de-gluing, etc., resulting in the need to improve the yield and high-frequency performance stability; as a pure mechanical micro-milling technology, the nanometer numerical control milling has a processing precision of ±0.5 μm and a roughness of ≥50 nm, but for high aspect ratio (such as depth 200 μm, slit width 50 μm) folded waveguide structures, the micro-milling cutter is prone to wear and breakage, the processing efficiency is low, and the control of the side wall perpendicularity and the bottom roughness is difficult.

[0004] Therefore, a terahertz slow wave structure manufacturing method is provided. SUMMARY

[0005] In order to overcome the problems in the prior art, the application provides a terahertz slow wave structure manufacturing method.

[0006] The technical scheme for solving the above technical problems is as follows: The application provides a terahertz slow wave structure manufacturing method, which comprises the following steps: The device to be processed is divided into a substrate and a structure sheet, the substrate is used for mechanical support, heat dissipation and vacuum sealing, and the structure sheet is provided with a slow wave structure for realizing energy exchange between electrons and electromagnetic waves; The substrate and the structure sheet are processed respectively, and then the substrate and the structure sheet are bonded together to obtain the required slow wave structure, which is specifically as follows: The substrate and the structure sheet are split in the thickness direction of the slow wave structure to be processed, and the thickness of the substrate is greater than that of the structure sheet. The required slow wave structure is machined on the structure sheet by femtosecond laser rotary cutting or slow wire cutting, and then surface polishing treatment is performed by electrochemical polishing; the substrate is machined by conventional precision mechanical machining method, and surface polishing treatment is performed; The structure sheet after electrochemical polishing and the substrate after surface polishing are aligned and diffusion bonded.

[0007] Further, the substrate is provided with a fiducial mark, a flow channel or a welding surface; and the slow wave structure comprises a waveguide groove and an electron beam passage profile.

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

[0009] Further, the substrate and the structure sheet are made of the same material.

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

[0011] Further, the electrochemical polishing is that the sheet machined by femtosecond laser is immersed in a phosphoric acid-ethylene glycol electrolyte, and a voltage is applied to polish the structure sheet in the electrolyte.

[0012] Further, the volume ratio of phosphoric acid-ethylene glycol in the electrolyte is 3:7; the applied voltage is a direct current voltage in the range of 5V-8V; and the polishing time is 30s-60s.

[0013] Further, the thickness T2 of the structure sheet satisfies the following condition: 0.1mm≤T2≤1mm.

[0014] Further, the structure sheet after electrochemical polishing and the substrate after surface polishing are aligned and diffusion bonded, comprising: The structure sheet after electrochemical polishing and the substrate after surface polishing are aligned, so that the slow wave structure on the structure sheet matches the corresponding area on the substrate; The structure sheet and the substrate that are bonded together are heated at a preset temperature in a vacuum or a protective atmosphere furnace, and a pressure is applied, and then heat preservation is performed under the preset temperature and pressure conditions.

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

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

[0017] Compared with the prior art, the present application has the following technical effects: (1) The present application splits the slow-wave structure into a substrate and a structure sheet for processing respectively, the substrate is used for mechanical support, heat dissipation and vacuum sealing, and the structure sheet is used for processing the slow-wave structure for realizing the energy exchange between electrons and electromagnetic waves. This split processing mode avoids the problems of difficult degumming in traditional electroforming process, fast tool wear in nanometer numerical control milling process, and difficult control of bottom roughness, etc. Selecting the substrate and the structure sheet made of the same material such as oxygen-free copper not only ensures good combination between the two, but also fully plays the excellent electrical conductivity and thermal conductivity of oxygen-free copper, avoids the process of metallization after traditional deep reactive ion etching, and the adhesion and heat dissipation problems of the metal film layer and the substrate. The slow-wave structure processed by femtosecond laser rotary cutting or slow wire cutting has high precision and high perpendicularity of the cutting surface. The 'cold processing' characteristics of femtosecond laser can minimize the heat affected zone, slag and recast layer. Electrochemical polishing uniformly removes the micron / submicron surface unevenness, burrs and recast layer generated by laser processing, further improves the surface quality of the structure sheet, and reduces the loss in the signal transmission process. The metallurgical bonding formed by diffusion bonding enables the bonding body to withstand high-temperature soldering in the subsequent device assembly process without problems such as bubbling, deformation or air leakage, ensuring the high vacuum sealing of the terahertz slow-wave structure in complex working environment.

[0018] (2) The manufacturing method proposed in the present application greatly simplifies the process flow, avoids the complex and tedious photoetching, electroforming and degumming process in UV-LIGA technology, and also eliminates the etching and metallization steps in DRIE technology. The traditional process chain is long, and each link may introduce uncertain factors, which makes it difficult to ensure the product yield. The shortened process chain of the present method not only reduces the 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, which can more accurately control the processing parameters, effectively reduce the waste rate, greatly improve the production efficiency and product yield, and lay a solid foundation for large-scale and high-quality production of terahertz vacuum electron devices. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, and the advantages thereof, the drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0020] Figure 1 It is an exploded view of the substrate and the sheet of the present application. Figure 2 It is a laser rotary cutting processing sheet of the present application. Figure 3The present application is slow wire cutting processing thin plate; Figure 4 The present application is a schematic diagram of the whole after the substrate and the thin plate diffusion bonding. DETAILED DESCRIPTION

[0021] In order to further illustrate the technical means and effects adopted by the present application to achieve the predetermined object, the specific embodiments, structures, features and effects of the technical solutions according to the present application are described in detail below in combination with the drawings and preferred embodiments. The specific features, structures or characteristics in one or more embodiments can be combined in any suitable form. Unless otherwise defined, all technical and scientific terms used in the present application have the same meaning as understood by those skilled in the art of the technology to which the present application belongs.

[0022] The present application aims to provide a terahertz slow wave structure manufacturing method. The method aims to simplify the manufacturing process, reduce the overall processing difficulty, improve the yield, and ensure that the final device has excellent heat conductivity and structural strength brought by pure metal structure by concentrating the complex microstructure with high precision and high surface quality requirements on a relatively thin metal plate, and then high-precision bonding with a thick substrate.

[0023] In the present embodiment, referring to Figures 1-4 , a terahertz slow wave structure manufacturing method is provided, comprising the following steps: The device to be processed is divided into a substrate and a structure thin plate, the substrate is used for mechanical support, heat dissipation and vacuum sealing; the structure thin plate is provided with a slow wave structure for realizing the energy exchange between its electrons and electromagnetic waves, and the thickness of the substrate is greater than that of the structure thin plate; The required slow wave structure is machined on the structure thin plate by femtosecond laser rotary cutting or slow wire cutting, and then surface polishing treatment is performed by electrochemical polishing; the substrate is machined by conventional precision machining method, and surface polishing treatment is performed; The structure thin plate after electrochemical polishing is diffusion bonded with the substrate after surface polishing.

[0024] The above steps are described in detail as follows: Step 100: The device to be processed is divided into a substrate and a structure thin plate in the thickness direction, the thickness of the substrate is greater than that of the structure thin plate; the substrate is used for mechanical support, heat dissipation and vacuum sealing; the structure thin plate is provided with a slow wave structure for realizing the energy exchange between its electrons and electromagnetic waves.

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

[0026] For example, when H = 5mm, T1 = 4.8mm and T2 = 0.2mm can be set. The thickness distribution is not fixed, but is flexibly adjusted according to actual functional requirements and manufacturing conditions. The thickness of the structure sheet ranges from 0.1mm to 1mm, which can meet the design requirements of slow wave structures with different complexity. At the same time, considering the key role of the waveguide micro slot in the slow wave structure, its depth is mostly greater than 0.1mm, which requires the structure sheet to have sufficient thinness and be able to process waveguide micro slots that meet the design requirements.

[0027] The substrate and the structure sheet have different functions in the split slow wave structure. The substrate is used for mechanical support, heat dissipation and vacuum sealing. Since its main function does not involve complex slow wave structure processing, only some simple alignment marks, flow channels or welding surfaces need to be processed on the substrate. These simple structure processes are relatively easy to implement and have relatively low requirements on the manufacturing process. For example, the alignment marks can provide positioning reference for the assembly of the substrate and the structure sheet, ensuring the relative position accuracy between the two; the design of the flow channel helps to improve the heat dissipation performance of the slow wave structure, dissipating heat generated during operation in time to ensure stable operation of the device; the welding surface is used to realize reliable connection between the substrate and other components, ensuring the vacuum sealing effect.

[0028] The part containing all the complex slow wave structures is designed on the T2-thickness structure sheet, which includes waveguide slots, electron beam channel profiles, etc. They are the key to the energy exchange between electrons and electromagnetic waves in the slow wave structure. The shape, size and position accuracy of the folded waveguide slot directly affect the transmission characteristics of electromagnetic waves, and the accuracy of the electron beam channel profile relates to the transmission quality of the electron beam and the interaction effect with electromagnetic waves. Therefore, during the design and manufacturing process of the structure sheet, high-precision processing technology needs to be used to ensure that these complex folded waveguides can be accurately formed to meet the high-performance requirements of the slow wave structure in the terahertz frequency band.

[0029] Step 200: Process the required slow wave structure on the structure sheet by femtosecond laser rotary cutting or slow wire cutting, and then perform surface polishing treatment by electrochemical polishing; process the substrate by conventional precision mechanical processing method, and perform surface polishing treatment.

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

[0031] Step 210: Process the structure sheet by femtosecond laser rotary cutting or slow wire cutting to carve out the waveguide slots and electron beam channel profiles of the slow wave structure.

[0032] The first method is suitable for vacuum electron devices in the frequency band of 340 GHz and above. The material of the structure sheet is oxygen-free copper. In the first step of processing the structure sheet, a femtosecond pulse laser system is used to spin cut the oxygen-free copper structure sheet. Conventional laser processing methods mostly use galvanometer scanning. However, due to the focal depth limitation of the focused laser beam, and the reflection phenomenon existing in the inner wall during processing, the material ablation rate decreases sharply with the increase of the cutting depth, and the cutting section inevitably appears taper, which is difficult to meet the requirements of high-precision microstructure processing.

[0033] The femtosecond spin cutting technology solves this problem. Femtosecond laser spin cutting is a material processing technology that realizes high precision, low thermal effect and controllable taper. The core principle is that the laser beam rotates around the optical axis at a certain deflection angle after passing through a special optical module. This deflection angle can be adjusted to control the taper of the cutting section, realizing taper-controllable drilling and cutting. In actual processing, the running track of the platform is set to directly carve out the entire profile of the slow wave structure, etc. The application of the femtosecond laser spin cutting process is not related to the improvement of the femtosecond laser spin cutting process, so the deeper cutting control is not described here.

[0034] Femtosecond laser has unique cold processing characteristics, which is an important advantage of its application in high-precision microstructure processing. In the processing process, the pulse duration of femtosecond laser is extremely short, and the energy is released in a very short time, which makes the material vaporize instantly and minimizes the heat-affected zone. Compared with traditional laser processing, it can effectively avoid the problems of material melting, slag and recast layer caused by heat accumulation, so as to obtain the preliminary high-precision structure morphology.

[0035] The second method is suitable for vacuum electron devices in the frequency band of 220 GHz and above. Slow wire cutting (WEDM-LS) is an important branch of wire electrical discharge machining technology. Its core principle is to use a continuously moving thin metal wire as an electrode to remove metal materials by high temperature generated by pulse spark discharge to realize precision machining.

[0036] The material of the structure sheet is oxygen-free copper. In the processing of terahertz slow wave structure structure sheet, a very thin electrode wire is used with the slow wire electrical discharge machining machine tool. The diameter of the electrode wire is between 0.02 mm and 0.1 mm. The oxygen-free copper structure sheet has good electrical conductivity and thermal conductivity, and also has high processing performance. Combined with the slow wire cutting processing method, the advantages of both can be fully utilized.

[0037] In the processing, the precision numerical control system controls the electrode wire to move along the pre-designed slow wave structure plane profile trajectory, and cuts on the oxygen-free copper structure sheet according to the established route. The one-time through cutting thickness of the sheet is T2, and all the vertical side wall profiles of the structure can be directly obtained, which greatly simplifies the processing flow and improves the processing efficiency. The slow wire cutting has many obvious advantages, the structure processed by the slow wire cutting has excellent straightness, can ensure that the side wall of the slow wave structure is straight and regular, the size consistency is very high, ensures that each processed structure meets the strict design requirements, and guarantees the quality stability of the product. Meanwhile, the smaller corner error makes the transition of the slow wave structure at the corner more smooth, reduces the loss and interference in the signal transmission process, and provides a powerful guarantee for the high-performance operation of the terahertz slow wave structure.

[0038] Step 220: electrochemical polishing is performed on the structure sheet after femtosecond laser rotary cutting or slow wire cutting processing, so as to reduce the roughness of the surface of the sheet.

[0039] After the femtosecond laser rotary cutting processing or the slow wire cutting, although the preliminary structure topography has been formed, there are still some micron / submicron level unevenness, burrs and recast layers on the surface, which will affect the performance and service life of the slow wave structure. Therefore, the second processing-electrochemical polishing is needed.

[0040] The electrochemical polishing is that the sheet processed by the femtosecond laser is immersed in a phosphoric acid-ethylene glycol electrolyte, the volume ratio of the electrolyte is 3:7; then a direct current voltage of 5V-8V is applied, and the structure sheet is polished in the electrolyte for 30s-60s. In the electrochemical polishing process, the surface of the sheet acts as an anode and undergoes an electrochemical dissolution reaction under the action of the current. Due to the uniformity of the current distribution, the material can be uniformly removed, so as to effectively eliminate the surface unevenness, burrs and recast layers generated by the laser processing.

[0041] Through the electrochemical polishing, the roughness of the surface of the sheet can be significantly reduced to the nanometer level (Ra<50 nm), and the side wall perpendicularity can be controlled to be ≤0.5°. At the same time, the process can further trim the structure edge, improve the size consistency, and make the microstructure of the processed sheet more accurately meet the design requirements, meet the demand of the terahertz band slow wave structure for high precision and high performance.

[0042] In summary, the process combining the femtosecond laser rotary cutting processing or the slow wire cutting with the electrochemical polishing can give full play to the advantages of the two processing technologies, realize the high-precision and high-quality processing of the microstructure of the oxygen-free copper sheet, and provide an effective solution for the manufacturing of the terahertz band slow wave structure.

[0043] Step 300: Diffusion bonding the electrochemically polished structure sheet with the surface polished substrate.

[0044] The substrate, as a key basic component in the manufacture of slow wave structure, is consistent with the structure sheet in material and structure, taking oxygen-free copper as an example. In the forming link, conventional precision machining and electric spark machining are two main ways. Conventional precision machining uses mature processes such as turning, milling, planing, and grinding, uses high-precision equipment and tools, strictly controls parameters such as cutting speed, feed rate, and cutting depth, and can process oxygen-free copper raw materials into substrates of specific shape and size, which is suitable for the manufacture of substrates with relatively regular shapes. Electric spark machining uses the electric corrosion phenomenon generated by electric spark discharge, which is not limited by material hardness, and can process complex structures such as deep and narrow cavities that cannot be achieved by conventional machining, and can process shallow cavities on the substrate that are suitable for the structure sheet.

[0045] After the substrate is formed, the surface roughness often cannot meet the requirements, and needs to be precisely polished to control the surface roughness within the range of Ra≤50nm, effectively reduce the contact resistance between the substrate and the sheet, avoid problems such as partial discharge, and ensure the signal transmission efficiency and the performance of the slow wave structure.

[0046] Step 400: Aligning and diffusion bonding the electrochemically polished structure sheet with the surface polished substrate.

[0047] The electrochemically polished structure sheet is aligned with the surface polished substrate. The aligned and bonded sheet and substrate are placed in a vacuum or protective atmosphere furnace, and the key process of diffusion bonding is started. The vacuum environment can effectively isolate air to prevent the metal from reacting with oxygen, nitrogen, and other gases at high temperatures, prevent oxidation and nitridation, and ensure the purity of the bonding interface; the protective atmosphere furnace creates an oxygen-free or low-oxygen environment similar to a vacuum by introducing inert gases such as argon and nitrogen, which also protects the metal surface.

[0048] In the furnace, appropriate temperature and pressure are applied to the structure sheet and the substrate. The temperature is set between 400°C and 600°C, which is the balance point of the activity of metal atoms and the stability of material performance. At this temperature, the metal atoms have enough energy to become active, but the material performance will not change irreversibly due to excessive temperature. The pressure is controlled at 5MPa-20MPa, and appropriate pressure makes the sheet and substrate tightly bonded, providing the necessary contact conditions for the diffusion of metal atoms. After 30-60 minutes of heat preservation, the metal atoms at the interface begin to diffuse. This metallurgical bonding is different from simple physical bonding, which tightly connects the sheet and substrate at the atomic level, achieving complete sealing in structure and vacuum, and providing reliable physical support and sealing protection for the slow wave structure.

[0049] The bonding body has strong high-temperature stability due to the unique structure of the metallurgical bond. The metal bond formed by the metallurgical bond has high strength and stability. At high temperatures, the bonding force between metal atoms can still be maintained and will not be easily damaged. At the same time, the uniform bonding interface does not have obvious interface defects and stress concentration phenomena, and can effectively resist the thermal stress and thermal shock generated during high-temperature brazing. In addition, the good vacuum sealing performance is also continued at high temperatures, preventing the penetration and leakage of gas molecules and ensuring that the vacuum environment inside the slow wave structure is always stable. Therefore, the bonding body can remain 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 examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application.

Claims

1. A method of manufacturing a terahertz slow wave structure, characterized by, The method comprises the following steps: The slow wave structure to be processed is divided into a substrate and a structure sheet, the substrate is used for mechanical support, heat dissipation and vacuum sealing, the structure sheet is provided with a slow wave structure for realizing energy exchange between electrons and electromagnetic waves, and the thickness of the substrate is greater than that of the structure sheet; The slow wave structure required is processed on the structure sheet by femtosecond laser rotary cutting or slow wire cutting, and then surface polishing treatment is performed by electrochemical polishing; The substrate is processed by a conventional precision mechanical processing method, and surface polishing treatment is performed; The structure sheet after electrochemical polishing is diffusion bonded with the substrate after surface polishing.

2. The method of claim 1, wherein The substrate is provided with a quasi-mark, a flow channel or a welding surface, and the slow wave structure comprises a waveguide groove and an electron beam passage contour.

3. The method of claim 1 or 2, wherein The substrate and the structure sheet are made of the same material.

4. The method of claim 3, wherein The substrate and the structure sheet are made of oxygen-free copper.

5. The method of claim 2, wherein The electrochemical polishing is that the structure sheet after femtosecond laser processing is immersed in a phosphoric acid-ethylene glycol electrolyte, and voltage is applied to perform polishing treatment of the structure sheet in the electrolyte.

6. The method of claim 5, wherein The volume ratio of phosphoric acid to ethylene glycol in the electrolyte is 3:7, the applied voltage is a direct current voltage in the range of 5V-8V, and the polishing treatment time is 30s-60s.

7. The method of claim 2, wherein the method further comprises: The surface roughness Ra of the substrate is less than or equal to 50nm.

8. The method of claim 3, wherein the method further comprises: The structure sheet after electrochemical polishing is aligned and diffusion bonded with the substrate after surface polishing, comprising: The structure sheet after electrochemical polishing is aligned with the substrate after surface polishing, so that the slow wave structure on the structure sheet matches the corresponding area on the substrate; In a vacuum or protective atmosphere furnace, the structure sheet and the substrate that are attached together are heated at a preset temperature and pressure, and then are kept at a preset temperature and pressure.

9. The method of claim 8, wherein The preset temperature is set at 400°C-600°C, and the pressure is controlled at 5MPa-20MPa.

10. The method of claim 8 or 9, wherein the method further comprises, The keeping time is 30min-60min.

Images