A method for preparing low-loss high-frequency circuit board by full-low-temperature magnetron sputtering without titanium transition layer

Abstract

The application belongs to the technical field of electronic manufacturing, and particularly relates to a method for preparing a low-loss high-frequency circuit board by full-low-temperature and titanium-free transition layer magnetron sputtering. The method is aimed at the patent barriers and environmental problems of chemical copper deposition caused by the titanium or chromium transition layer required by the traditional process. An organic-inorganic composite bonding interface is constructed by argon-oxygen mixed plasma activation and fluorine-containing silane coupling agent modification to replace the metal transition layer. A high-power pulse and direct-current composite magnetron sputtering process is used to gradient deposit a pure copper seed layer. Then, low-temperature and low-stress electroplating thickening is carried out. The temperature of the whole process is controlled below the threshold value. A low-loss high-frequency circuit board is obtained after film removal and forming. The method eliminates the titanium, chromium transition layer and chemical copper deposition process. The roughness of the copper seed layer is controlled at the nanometer level. The method is suitable for various heat-sensitive substrates such as polytetrafluoroethylene, hydrocarbon resin and LCP. The use of formaldehyde and heavy metal catalysts is eliminated. The peel strength reaches the military level. The method realizes fine circuit processing by semi-additive process.

Description

Technical Field

[0001] This application belongs to the field of electronic manufacturing technology, specifically relating to a method for preparing low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature. Background Technology

[0002] With the rapid development of 5G millimeter wave, 6G communication, 24GHz / 77GHz automotive radar, high-end RF antennas, and high-speed backplane technologies for AI servers, high-frequency circuit boards (PCBs), as key basic electronic components in these fields, are experiencing simultaneous and accelerated growth in market demand and performance requirements. PCBs play a crucial role in signal transmission, acting as the medium for electromagnetic wave propagation. Their key parameters, such as dielectric constant, dielectric loss factor, surface metal layer roughness, and overall dimensional stability, directly determine the integrity, speed, and reliability of signal transmission, thus holding an irreplaceable position in high-frequency electronic system design. Specifically, low dielectric constant and low dielectric loss effectively reduce signal attenuation and delay in the transmission medium; low surface roughness significantly suppresses high-frequency signal loss caused by the skin effect; and high dimensional stability ensures that the deformation of fine circuit patterns during manufacturing and use remains within acceptable limits. The synergistic optimization of these performance indicators has become a core research and development direction for high-frequency circuit board manufacturing technology.

[0003] Among them, magnetron sputtering, as a physical vapor deposition method, uses high-energy argon ions to bombard a copper target, causing its atoms to sputter onto the substrate surface to form a metal thin film. Compared with chemical copper deposition, it has significant advantages in terms of deposition purity, density, and environmental friendliness, and has become the most promising hole metallization and circuit fabrication technology in the field of high-frequency circuit board manufacturing. However, almost all existing patents and process schemes that use magnetron sputtering to replace chemical copper deposition rely on the pre-deposition of a titanium or chromium metal transition layer as a necessary technical feature to achieve high adhesion between the copper layer and the substrate. The mechanical interlocking and chemical bonding formed between the titanium or chromium layers and the copper and substrate ensure the adhesion of the metal layer, resulting in a dense patent layout and a strict intellectual property protection system covering this technology route. This technology path has created extremely high barriers to entry in terms of patent circumvention, leading to severe infringement risks for new entrants using magnetron sputtering.

[0004] Meanwhile, the sputtering temperature for conventional magnetron sputtering processes is generally maintained above 120℃. However, the glass transition temperature or melting temperature of commonly used substrates for high-frequency circuit boards, such as polytetrafluoroethylene, hydrocarbon resins, and liquid crystal polymers, is relatively low, and their coefficient of thermal expansion is high. Under high-temperature sputtering environments, they are prone to thermal damage problems such as substrate softening, dimensional creep, dielectric property deterioration, and board warping, which severely restricts the expansion of the process window and the improvement of product yield. In addition, the temperature resistance and vacuum resistance of ordinary commercial photosensitive dry films usually cannot meet the environmental requirements of magnetron sputtering processes. During vacuum chamber lamination and sputtering heating, problems such as volatilization of small molecule additives, edge warping due to heat, and short circuits caused by development and plating can easily occur. The residual photosensitive adhesive residue after peeling can directly affect the accuracy of the circuit pattern and the surface insulation performance. In subsequent electroplating thickening processes, the traditional high current density and above-room-temperature copper sulfate electroplating system will further amplify the surface roughness of the copper layer and increase the internal thermal stress of the substrate, resulting in poor copper thickness consistency in different batches and different areas of the circuit. The transmission characteristics of high-frequency signals at different locations will become discrete, ultimately affecting the signal integrity performance and long-term operational reliability of the terminal product.

[0005] Existing publicly available patent solutions generally target general-purpose printed circuit boards, flexible printed circuit boards, or ceramic substrates. None of them have systematically developed and verified dedicated process parameter windows, specialized material matching, or end-to-end temperature control for high-frequency, low-loss applications. Furthermore, they lack a complete process combination solution that synergistically integrates titanium-free composite activation technology with dedicated high-temperature resistant dry films, gradient magnetron sputtering, and a fully low-temperature electroplating system. Therefore, developing a low-loss high-frequency circuit board fabrication method that completely circumvents existing patent layouts, eliminates the titanium-chromium metal transition layer, achieves end-to-end low-temperature process control, possesses ultra-low roughness metal layer characteristics, and has high adaptability to various thermosensitive high-frequency substrates has become a core technical problem urgently needing to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature, which can effectively solve the problems in the background art.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature includes the following specific steps: Step 1: Substrate preparation and drilling Select a high-frequency, low-loss insulating substrate, and perform mechanical drilling or laser drilling on the substrate. Then, complete the cleaning process of removing adhesive residue, surface dust, and oil stains.

[0008] Step 2: Activation of the titanium-free composite interface The substrate after drilling is activated by argon-oxygen mixed plasma, then coated with a fluorinated silane coupling agent solution and pre-treated by drying to form an organic-inorganic composite activation interface, without depositing titanium, chromium or other transition metal layers throughout the process.

[0009] Step 3: Shape Masking A high-temperature resistant photosensitive dry film is laminated or bonded to the entire substrate. The dry film does not volatilize small molecules in a vacuum sputtering environment, has no edge lifting, and leaves no adhesive residue on the board surface after peeling. After exposure and development, only the area to be fabricated and the inner wall of the metallized hole are exposed, while the non-circuit areas are completely covered by the dry film.

[0010] Step 4: Gradient pure copper seed layer magnetron sputtering The masked substrate is placed in a vacuum magnetron sputtering chamber, and pure copper is deposited in a stepwise gradient using a high-power pulse and DC composite process to form a dense conductive seed layer.

[0011] Step 5: Low-temperature, low-stress electroplating thickening An acidic copper sulfate electroplating system was used to electroplat and thicken the pure copper seed layer under low temperature, low current density, and sulfur-free brightener conditions to achieve the target copper thickness of the circuit.

[0012] Step 6: Demolding and Shaping The photosensitive dry film on the surface of the substrate is peeled off using an alkaline film-removing solution. After washing and drying, a low-loss high-frequency circuit board with complete metallized holes and fine lines is obtained.

[0013] Preferably, the high-frequency, low-loss insulating substrate in step 1 is a copper-free substrate, selected from PTFE substrate, hydrocarbon resin substrate, low-dielectric-high-glass-transition-temperature FR-4 substrate, and LCP liquid crystal polymer substrate. The substrate's dielectric constant Dk and dielectric loss Df are within a preset range. The aforementioned high-frequency, low-loss insulating substrate maintains stable dielectric properties in both millimeter-wave and centimeter-wave frequency bands. Its low dielectric constant effectively reduces attenuation and delay in signal transmission media, providing a fundamental guarantee for the signal integrity of high-frequency circuit boards.

[0014] Preferably, the process parameters for argon-oxygen mixed plasma activation in step 2 are as follows: the volume ratio of argon to oxygen is within a preset range, the radio frequency power is within a preset range, the processing time is within a preset time, and the substrate surface temperature is below a preset temperature threshold during the processing. The mass concentration of the fluorinated silane coupling agent solution is within a preset range, the solvent is anhydrous ethanol, the drying pretreatment temperature is within a preset range, and the drying time is within a preset time. The high-energy particles in the argon-oxygen mixed plasma bombarding the substrate surface can effectively remove the weak boundary layer and produce a microscopic roughening effect. The silanoxy groups in the fluorinated silane coupling agent molecules undergo a condensation reaction with the active hydroxyl groups on the substrate surface to form covalent bonds, while the fluorinated long-chain groups form a low surface energy interface on the outer layer. This organic-inorganic composite structure can achieve high adhesion between the copper layer and the substrate without using any metal transition layer.

[0015] Preferably, in step 2, after the argon-oxygen mixed plasma activation treatment and before coating with the fluorinated silane coupling agent solution, a vacuum dehumidification pretreatment step is also included. The pretreatment vacuum degree is within a preset range, and the treatment time is within a preset time. The vacuum dehumidification pretreatment can effectively remove water vapor and volatile small molecules adsorbed on the substrate surface, avoiding residual moisture from affecting the density and interfacial adhesion of the copper layer during subsequent sputtering, while providing a dry active surface for the uniform spreading of the fluorinated silane coupling agent.

[0016] Preferably, the high-temperature resistant photosensitive dry film in step 3 has a temperature resistance level above a preset level, a coefficient of thermal expansion within a preset range, and exhibits no small molecule volatilization, no edge warping, and no adhesive residue on the board surface after peeling under vacuum sputtering conditions. The high temperature resistance and low coefficient of thermal expansion of the high-temperature resistant photosensitive dry film enable it to maintain dimensional stability and structural integrity in the temperature-rising environment of the vacuum sputtering chamber. This avoids the problems of small molecule additive volatilization and edge warping leading to short circuits caused by heat in ordinary commercial photosensitive dry films under vacuum conditions. The absence of adhesive residue after peeling ensures the accuracy of the circuit pattern and the surface insulation performance.

[0017] Preferably, the background vacuum level of the vacuum chamber in step 4 is within a preset range; gradient deposition is divided into two steps: The first step is high-power pulse sputtering, with the argon partial pressure, sputtering power, sputtering time, and copper layer thickness all within a preset range. The second step is DC sputtering, with the argon partial pressure, sputtering power, sputtering time, and copper layer thickness all within a preset range. After deposition, the roughness of the copper seed layer interface is below a preset threshold.

[0018] Furthermore, in step 4, the total thickness of the gradient pure copper seed layer is 100 to 300 nm. This thickness range ensures that the copper seed layer has sufficient conductive cross-sectional area and metallurgical bonding strength with the subsequent electroplating layer, while avoiding the problem of roughness accumulation caused by excessive thickness. High-power pulsed sputtering technology utilizes high-density plasma to generate a copper ion cloud with a high ionization rate, enabling copper atoms to be deposited on the substrate surface in a highly oriented manner, forming a fine-grained and highly dense initial copper layer. This process characteristic produces an excellent mechanical interlocking effect at the interface. DC sputtering provides a stable copper atom flux in subsequent stages, allowing the copper layer to thicken uniformly on the basis of the initial dense layer. The gradient composite structure takes into account both the requirements of interfacial bonding and deposition efficiency. The final copper seed layer has nanoscale surface roughness, which can significantly reduce the skin effect loss of high-frequency signals from the source.

[0019] Preferably, the electroplating process parameters in step 5 are: the electroplating solution temperature is within a preset range, the current density is within a preset range, the surface roughness of the copper layer after electroplating is below a preset threshold, and the target copper thickness is within a preset range. Low-temperature, low-current-density acidic copper sulfate electroplating conditions can effectively suppress the tendency for grain coarsening during copper deposition, reduce internal stress in the copper layer, and control the increase in surface roughness. The sulfur-free brightener formulation system avoids the increase in high-frequency dielectric loss caused by the accumulation of brightener decomposition products at grain boundaries. The electroplated copper layer maintains low roughness characteristics while meeting the target circuit thickness requirements.

[0020] Preferably, in step 6, the alkaline stripping solution is an aqueous solution of sodium hydroxide and an alkanolamine compound, with an effective mass concentration within a preset range, a stripping temperature within a preset range, and a stripping time within a preset time. The sodium hydroxide and alkanolamine compound system combines the ability of a strong alkali to remove photosensitive components with the swelling effect of alkanolamine compounds on the dry film, enabling complete stripping of the dry film in a short time without damaging the deposited copper layer and substrate. The thorough washing and drying processes ensure the cleanliness and dryness of the final product surface.

[0021] Preferably, the highest temperature of all steps in the entire process is below a preset temperature threshold, excluding chemical copper plating, blackening or browning, titanium or chromium transition layer deposition, and high-temperature baking processes. The low-temperature control throughout the process ensures that heat-sensitive high-frequency substrates such as PTFE, hydrocarbon resins, and LCP do not experience high-temperature environments during the entire manufacturing process, avoiding thermal damage problems such as substrate softening, dimensional creep, dielectric property degradation, and board warping. The absence of a chemical copper plating process completely eliminates the use of formaldehyde and heavy metal catalysts, as well as the burden of waste liquid treatment, significantly improving the environmental friendliness of the process.

[0022] Preferably, the minimum linewidth and line spacing of the fabricated high-frequency circuit board are within a preset precision range, and the peel strength between the copper layer and the substrate is not lower than a preset strength threshold, meeting the IPC-6012 Level 3 military reliability standard and suitable for high-frequency applications. The semi-additive process combines precise pattern masking of the photosensitive dry film with selective deposition of the pure copper layer to achieve direct forming of fine circuits, avoiding the side etching problem in traditional subtractive processes. The organic-inorganic composite interface activation layer between the copper layer and the substrate ensures high peel strength without introducing additional dielectric loss sources, enabling the final product to exhibit excellent signal transmission characteristics in high-frequency applications.

[0023] Compared with the prior art, the present invention has the following beneficial effects: 1. Outstanding patent circumvention capability: This invention adopts the technical route of argon-oxygen plasma etching activation and fluorinated silane coupling agent modification to construct organic-inorganic composite bonding interface, completely eliminating the titanium or chromium metal transition layer required in traditional magnetron sputtering process. It avoids the dense patent layout covering this technical route from the structural root, providing technical support for enterprises to build independent intellectual property barriers and maximize the freedom of commercial mass production.

[0024] 2. Excellent high-frequency and low-loss performance: The roughness of the copper seed crystal layer interface is reduced to the nanometer level, which significantly suppresses the attenuation effect of the skin effect on high-frequency signals. The insertion loss of high-frequency signals is significantly improved compared with the traditional chemical copper plating process, which fully meets the stringent requirements of high-end high-frequency application scenarios such as automotive radar and RF antennas for signal integrity.

[0025] 3. Wide range of substrate compatibility: The low-temperature process control with the temperature below the preset temperature threshold throughout the entire process enables the present invention to be compatible with a variety of thermistor high-frequency substrates such as polytetrafluoroethylene, hydrocarbon resin, LCP, and low dielectric high glass transition temperature FR-4. It avoids thermal deformation and dielectric performance degradation of the substrate throughout the manufacturing process, significantly expanding the substrate selection space for high-frequency circuit boards.

[0026] 4. Outstanding environmental and safety performance: The technical solution that completely eliminates the chemical copper plating process completely eliminates the need for formaldehyde reducing agents and heavy metal palladium-nickel catalysts. The waste liquid treatment process is simplified and the treatment cost is significantly reduced. It complies with RoHS and REACH environmental directives and provides a complete solution for the green manufacturing of high-frequency circuit boards.

[0027] 5. Combining reliability and processing precision: The peel strength of the copper layer of the circuit and the substrate interface reaches above the preset strength threshold. Through thermal cycling reliability verification, the semi-additive process can achieve fine circuit processing precision within the preset precision range. It has high alignment accuracy and no side etching problem, meeting the reliability standards of military-grade products.

[0028] 6. Stable process and strong mass production capability: Each process step is simple and the parameter window is clear and controllable. Stable production can be achieved without the introduction of special auxiliary equipment. The process flow can be directly connected to the existing printed circuit board semi-additive production line for upgrading and transformation, effectively reducing the company's equipment investment and capacity conversion costs. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the overall process architecture of a method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer according to an embodiment of this application. Figure 2 This is a schematic diagram of the core principle framework of titanium-free composite interface activation and gradient pure copper seed layer sputtering in a method for preparing low-loss high-frequency circuit boards by magnetron sputtering of a titanium-free transition layer according to an embodiment of this application. Figure 3 This is a flowchart illustrating the substrate preparation and drilling stages of a method for preparing a low-loss high-frequency circuit board by magnetron sputtering with a titanium-free transition layer according to an embodiment of this application. Figure 4 This is a flowchart illustrating the organic-inorganic composite interface activation and high-temperature photosensitive dry film pattern masking stages of a method for preparing low-loss high-frequency circuit boards by magnetron sputtering of a titanium-free transition layer according to an embodiment of this application. Figure 5 This is a flowchart illustrating the gradient pure copper seed layer magnetron sputtering and low-temperature, low-stress electroplating thickening stages of a method for preparing a low-loss high-frequency circuit board by magnetron sputtering of a titanium-free transition layer according to an embodiment of this application. Figure 6 This is a schematic diagram showing the structure of the low-loss high-frequency circuit board and its signal transmission characteristics compared with those of the low-loss high-frequency circuit board prepared by magnetron sputtering of a titanium-free transition layer according to an embodiment of this application. Detailed Implementation Example 1

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0031] This invention provides a method for fabricating low-loss high-frequency circuit boards using magnetron sputtering with a titanium-free transition layer at low temperatures. The method comprises six core steps: substrate preparation and drilling, titanium-free composite interface activation, high-temperature photosensitive dry film pattern masking, gradient pure copper seed layer magnetron sputtering, low-temperature, low-stress electroplating thickening, and alkaline film removal and forming. The specific process flow, technical parameters, and underlying mechanisms of each step are described in detail below.

[0032] Step 1: Substrate preparation and drilling In the substrate preparation stage, a high-frequency, low-loss insulating substrate is selected as the carrier substrate for the circuit board. This high-frequency, low-loss insulating substrate is a copper-foil-free substrate, selected from one of the following: polytetrafluoroethylene (PTFE), hydrocarbon resin, low-dielectric-high-glass-transition-temperature (FR-4) substrate, and liquid crystal polymer substrate. Specifically, the dielectric constant of PTFE is typically between 2.1 and 2.6, and the dielectric loss can be as low as 0.0005 to 0.002; the dielectric constant of hydrocarbon resin is generally between 2.8 and 3.5, and the dielectric loss is 0.002 to 0.004; the dielectric constant of low-dielectric-high-glass-transition-temperature (FR-4) substrate is controlled below 3.8, and the dielectric loss is no greater than 0.005; the dielectric constant of liquid crystal polymer substrate is in the range of 2.9 to 3.5, and the dielectric loss is 0.001 to 0.003. The aforementioned high-frequency, low-loss insulating substrate can maintain stable dielectric properties in both millimeter-wave and centimeter-wave bands. Its low dielectric constant and low dielectric loss characteristics can effectively reduce attenuation and delay in signal transmission media, providing a solid foundation for the signal integrity of high-frequency circuit boards.

[0033] After determining the substrate thickness according to the circuit design requirements, the substrate is mechanically drilled or laser-drilled. For through holes or blind holes with a diameter greater than 0.3mm, mechanical drilling is used, with a carbide drill bit and a drilling speed controlled between 80,000 and 120,000 rpm. The feed rate and retraction rate are dynamically adjusted according to the substrate thickness and the number of holes to avoid scorching of the substrate edges or leakage of the inner resin due to heat accumulation during drilling. For microholes or high-precision through holes with a diameter less than or equal to 0.3mm, laser drilling is used, with an ultraviolet laser or carbon dioxide laser as the laser source. The laser pulse width, pulse energy, and repetition frequency are precisely set according to the target hole diameter. During laser drilling, a computer numerical control system precisely controls the positioning and movement path of the laser beam to achieve micron-level positioning accuracy, ensuring precise alignment of each through hole position with the designed circuit pattern.

[0034] After drilling, a descaling process is initiated. This process aims to remove molten resin, carbonized layers, and drill cuttings remaining on the inner wall of the through-hole during mechanical or laser drilling. For the descaling generated from mechanical drilling, a plasma descaling device is used. The reaction chamber of this device is filled with a mixture of oxygen and carbon tetrafluoride, with oxygen comprising 60% to 80% of the volume and carbon tetrafluoride 20% to 40%. The pressure in the reaction chamber is controlled at 20 to 50 Pa, the radio frequency power is set to 800 to 1500 W, and the processing time is set to 2 to 5 minutes depending on the hole size and board thickness. This plasma process uses a combination of high-energy particle bombardment and chemical reaction to convert organic residues on the hole wall into volatile gases, which are then removed by the vacuum system. For the carbonized layer generated by laser drilling, wet chemical cleaning is used. The cleaning solution is an alkaline hydrogen peroxide solution or a special decarbonizing solution. The cleaning temperature is controlled at 50 to 70 degrees Celsius, the ultrasonic-assisted cleaning frequency is 40 to 60 kHz, and the cleaning time is 3 to 8 minutes to ensure that the carbonized layer on the hole wall is completely removed.

[0035] After removing adhesive residue, surface dust removal is performed. High-pressure nitrogen purging combined with soft brush cleaning is used to remove dust particles from the substrate surface. The nitrogen pressure is controlled at 0.3 to 0.5 MPa, and the purging angle is 30 to 45 degrees with the substrate surface. Following this, degreasing and cleaning are performed. The cleaning process includes a multi-stage ultrasonic cleaning procedure: The first step is alkaline cleaning with an aqueous solution of sodium hydroxide and surfactant, with an effective concentration of 1% to 2%, a cleaning temperature of 40 to 50 degrees Celsius, an ultrasonic frequency of 28 kHz, and a cleaning time of 5 to 8 minutes; the second step is deionized water rinsing with a water temperature of 30 to 40 degrees Celsius and a rinsing time of 3 to 5 minutes; the third step is pure water rinsing with a conductivity of less than or equal to 18.2 MΩ·cm, an ultrasonic frequency of 40 kHz, and a rinsing time of 2 to 3 minutes; finally, vacuum drying is performed with a drying temperature of 80 to 100 degrees Celsius, a drying time of 30 to 60 minutes, and a vacuum degree controlled at 500 to 1000 Pa to ensure that the substrate surface is clean and dry, providing good surface conditions for subsequent interface activation treatment.

[0036] Step 2: Activation of the titanium-free composite interface The core innovation of this invention is to perform titanium-free composite interface activation treatment on the substrate after drilling and cleaning. By combining argon-oxygen mixed plasma activation with fluorinated silane coupling agent modification, an organic-inorganic composite activation interface is constructed on the substrate surface without using any titanium, chromium or other transition metal layers, thereby achieving high adhesion between the copper layer and the substrate.

[0037] The first step in the titanium-free composite interface activation treatment is argon-oxygen mixed plasma activation. The substrate is fixed on the anode support of the plasma treatment equipment. After the reaction chamber is evacuated to a background vacuum of no more than 10 Pa, a mixture of argon and oxygen is introduced. The volume ratio of argon to oxygen is set to a range of 9:1 to 8:2, and the total flow rate of the mixed gas is controlled at 50 to 100 sccm. The pressure in the reaction chamber is stabilized between 30 and 80 Pa. The radio frequency power supply is started, with the radio frequency power set to 500 to 800 W and the radio frequency at 13.56 MHz. The treatment time is 5 to 10 minutes. During the plasma treatment, argon ions, as the main bombardment particles, bombard the substrate surface with high-energy ions under the acceleration of the radio frequency electric field. This effectively removes the weak boundary layer, organic contaminants, and adsorbed water vapor from the substrate surface. Simultaneously, it generates numerous micropits and dangling bonds on the substrate surface, significantly increasing surface roughness and specific surface area. In plasma, oxygen decomposes into oxygen atoms and oxygen ions, which react chemically with the substrate surface to form a hydroxyl-rich active layer. This active layer provides reaction sites for the subsequent chemical bonding of fluorinated silane coupling agents. Throughout the plasma treatment process, the substrate surface temperature is consistently controlled below 60 degrees Celsius, effectively protecting the dimensional stability and dielectric properties of the thermistor high-frequency substrate.

[0038] After the argon-oxygen mixed plasma activation treatment and before coating with the fluorinated silane coupling agent solution, a vacuum dehumidification pretreatment step is preferably included. This vacuum dehumidification pretreatment step is performed directly inside the plasma treatment chamber, further reducing the chamber vacuum level to no greater than [missing value]. The processing time is 1 to 3 minutes. Vacuum dehumidification pretreatment can effectively remove water vapor and volatile small molecules adsorbed on the substrate surface, avoiding residual moisture from affecting the density and interfacial adhesion of the copper layer during subsequent sputtering, while providing a dry active surface for the uniform spreading of fluorinated silane coupling agents.

[0039] After vacuum dehumidification pretreatment, a fluorinated silane coupling agent solution is coated onto the substrate surface. The solvent for this solution is anhydrous ethanol, and the solute is perfluorooctyltriethoxysilane or other equivalent fluorinated silane coupling agents. The solution concentration is controlled within the range of 0.5% to 1%. Coating can be performed using dip coating, spray coating, or spin coating processes to ensure that the fluorinated silane coupling agent solution forms a uniform and continuous film on the substrate surface. After coating, a drying pretreatment is performed at a temperature of 45 to 55 degrees Celsius for 3 to 5 minutes. During the drying process, the silanoxy groups in the fluorinated silane coupling agent molecules undergo a condensation reaction with the active hydroxyl groups on the substrate surface, forming stable silicon-oxygen covalent bonds. Simultaneously, unreacted silanoxy groups undergo hydrolysis and condensation, forming a network-structured fluorinated silane film on the substrate surface. The fluorinated long-chain groups in the fluorinated silane coupling agent molecules are oriented to form a low surface energy interface on the outer layer. This organic-inorganic composite structure can achieve high adhesion between the copper layer and the substrate without the use of any metal transition layer.

[0040] After titanium-free composite interface activation treatment, the organic-inorganic composite activated interface formed on the substrate surface has the following structural characteristics: the inner layer is a micro-pit rough structure generated by plasma etching, which provides the physical basis for the mechanical interlocking effect; the middle layer is a covalent bond layer formed by a fluorinated silane coupling agent and hydroxyl groups on the substrate surface, which provides the stability of the chemical bond; the outer layer is a low surface energy interface formed by the directional arrangement of fluorinated long-chain groups, which is conducive to the nucleation and growth of copper atoms in the subsequent sputtering process. The overall thickness of this three-layer composite structure is 5 to 20 nm, achieving a reliable bond between the copper layer and the high-frequency insulating substrate without using any metal transition layer.

[0041] Step 3: Shape Masking After the titanium-free composite interface of the substrate is activated, a pattern masking process is performed. In this process, a high-temperature resistant photosensitive dry film is laminated or bonded to the entire surface of the substrate. Through exposure and development, a precise circuit pattern masking structure is formed, exposing only the area to be fabricated and the inner wall of the metallized hole, while the non-circuit areas are completely masked and covered by the dry film.

[0042] The high-temperature resistant photosensitive dry film is the dedicated masking material of this invention. Its temperature resistance is no less than 180 degrees Celsius, and its coefficient of thermal expansion is no greater than 20 ppm / ℃. Under vacuum sputtering conditions, it exhibits no small molecule volatilization, no edge warping, and leaves no adhesive residue on the board surface after peeling. The high temperature resistance and low coefficient of thermal expansion of the high-temperature resistant photosensitive dry film enable it to maintain dimensional stability and structural integrity in the temperature-rising environment of the vacuum sputtering chamber. Ordinary commercial photosensitive dry films suffer from the volatilization of small molecule additives under vacuum conditions. These volatiles can contaminate the vacuum chamber and deposit on the surface of the copper seed layer, affecting the transmission quality of high-frequency signals. Simultaneously, the edges of ordinary dry films are prone to warping under heat, resulting in uneven masking boundaries and edge plating short-circuit defects during sputtering. The high-temperature resistant photosensitive dry film used in this invention, through formulation optimization, eliminates the risk of small molecule additive volatilization and significantly improves the dimensional stability of the dry film edges under temperature-rising conditions. The absence of residual adhesive after dry film peeling ensures the accuracy of the circuit pattern and the surface insulation performance, avoiding problems such as decreased insulation resistance or signal crosstalk caused by residual adhesive.

[0043] The specific process flow for pattern masking is as follows: A high-temperature resistant photosensitive dry film is fixed to the activated substrate surface via hot pressing or vacuum lamination. The dry film thickness is selected from 15 to 50 μm based on the circuit precision requirements. The lamination pressure is controlled at 0.3 to 0.5 MPa, the lamination temperature at 80 to 100 degrees Celsius, and the lamination time at 10 to 20 seconds to ensure a bubble-free, smooth masking layer is formed between the dry film and the substrate. After lamination, an exposure process is performed. An LED light source or an excimer laser light source is used for exposure. The exposure wavelength is selected based on the dry film's photosensitive wavelength, using 365 nm, 405 nm, or a mixed wavelength. The exposure energy density is precisely set according to the dry film thickness and photosensitivity. The exposure time is precisely adjusted through intensity adjustment and exposure time control. The exposed pattern data originates from the photoplotting file of the circuit design, and the circuit pattern is transferred to the dry film surface via laser direct writing or a photoplotter. After exposure, a development process is performed using a weakly alkaline sodium carbonate or potassium carbonate aqueous solution with an effective concentration of 0.8% to 1.5%. The development temperature is controlled between 20 and 30 degrees Celsius, and the development time is dynamically adjusted according to the dry film thickness and developer concentration to ensure that the photosensitive components in the exposed areas are completely dissolved during development, while the dry film in the unexposed areas remains intact. After development, the film is washed and dried to ultimately form a precise circuit pattern masking structure.

[0044] The masking accuracy of the patterned masking structure directly determines the final processing accuracy of the circuit. In the masked structure after development, the non-circuit areas are completely covered by the dry film. The masking layer has uniform thickness, neat edges, and strong adhesion, effectively preventing the deposition of copper atoms in subsequent sputtering processes. The windowed areas of the circuit precisely expose the substrate surface and the inner wall of the metallization holes, with the window boundary linewidth error controlled within ±2μm, providing a precise patterned seed layer definition for subsequent sputtering processes.

[0045] Step 4: Gradient pure copper seed layer magnetron sputtering The substrate, after pattern masking, is placed in a vacuum magnetron sputtering chamber. A gradient composite process, combining high-power pulsed magnetron sputtering and DC magnetron sputtering, is used to deposit a pure copper conductive seed layer in stages. This gradient sputtering process is a key technology for achieving ultra-low surface roughness in the copper seed layer.

[0046] Before the sputtering process begins, the substrate is fixed onto the sputtering carriage of the magnetron sputtering equipment. The sputtering carriage is equipped with a heating or cooling circulation system to precisely control the temperature of the substrate during the sputtering process. After the sputtering chamber is closed, the vacuum pumping system is activated to reduce the base vacuum level of the chamber to no more than [value missing]. This ensures that the residual gas partial pressure inside the cavity is low enough to avoid unintended scattering of gas molecules and impurity incorporation during sputtering.

[0047] The gradient pure copper seed layer magnetron sputtering is performed in two stages. The first stage is a high-power pulsed magnetron sputtering stage. This stage utilizes the high-density plasma and high-ionization copper ion cloud generated by high-power pulsed magnetron sputtering technology to deposit copper atoms on the substrate surface in a highly oriented manner. The process parameters are set as follows: argon partial pressure is controlled in the range of 0.3 to 0.4 Pa, sputtering power is set in the range of 1.8 to 3.5 kW, high-power pulse pulse width is 50 to 200 μs, pulse frequency is 100 to 500 Hz, duty cycle is 5% to 15%, and sputtering time is 1 to 2 minutes. The copper ions generated by high-power pulsed magnetron sputtering technology have high kinetic energy and bombard the substrate surface at high speed under the acceleration of the electric field, forming a fine-grained and extremely dense initial copper layer. The deposition thickness of this initial copper layer is controlled in the range of 30 to 100 nm, and its grain size can be controlled in the range of 10 to 30 nm. The high-density grain structure provides a good physical basis for the uniform growth of subsequent copper layers. This process feature enables it to produce an excellent mechanical interlocking effect at the organic-inorganic composite activation interface, which significantly enhances the bonding force between the copper seed layer and the activation interface.

[0048] The second stage is the DC magnetron sputtering stage. Based on the dense initial copper layer formed in the first stage, DC magnetron sputtering technology is used to provide a stable copper atom flux, allowing the copper layer to thicken uniformly on top of the initial dense layer. The process parameters are set as follows: argon partial pressure controlled within the range of 0.4 to 0.5 Pa, sputtering power set to 1 to 1.5 kW, and sputtering time 2 to 3 minutes. During DC magnetron sputtering, copper atoms are continuously sputtered from the copper target surface under argon ion bombardment. These copper atoms deposit on the surface of the initial copper layer in a low-energy state, primarily in a layered growth mode, forming a continuous copper layer with consistent grain orientation and a smooth surface. The thickness of the copper layer deposited in the second stage is controlled between 70 and 200 nm. After being superimposed with the copper layer from the first stage, the total copper seed layer thickness is 100 to 300 nm.

[0049] The gradient composite sputtering process, through the synergistic combination of high-power pulsed sputtering and DC sputtering, addresses the dual requirements of interfacial adhesion and deposition efficiency. The high-power pulsed sputtering stage provides a dense initial layer with high interfacial adhesion, while the DC sputtering stage provides an efficient and uniform thickening layer. The resulting gradient composite structure avoids the limitations of single sputtering modes in terms of efficiency or interfacial quality. After sputtering, the interfacial roughness Rz of the copper seed layer is no greater than 0.2 μm. This nanoscale surface roughness can significantly reduce the skin effect loss of high-frequency signals from the source. The skin effect causes high-frequency signals to mainly propagate along the conductor surface. The lower the roughness, the smoother the signal transmission path and the smaller the signal attenuation. Especially in the millimeter-wave band, the effect of a low-roughness interface on improving signal integrity is more significant.

[0050] During sputtering, temperature control of the magnetron sputtering chamber is crucial for protecting the heat-sensitive high-frequency substrate. The sputtering carriage is equipped with a cooling circulation system, with the cooling medium temperature controlled between 15 and 25 degrees Celsius. The cooling flow rate is dynamically adjusted according to the chamber's heat load to ensure that the substrate surface temperature does not exceed 100 degrees Celsius throughout the sputtering process. Simultaneously, the sputtering power is initiated using a gradient increase method to avoid thermal shock to the substrate from instantaneous high power.

[0051] Step 5: Low-temperature, low-stress electroplating thickening After magnetron sputtering of the gradient pure copper seed layer, an acidic copper sulfate electroplating system is used to thicken the pure copper seed layer under low temperature, low current density, and sulfur-free brightener conditions to achieve the target copper thickness for the circuit. This electroplating process further increases the copper layer thickness while strictly controlling the surface roughness and internal stress of the copper layer to ensure the signal transmission performance and mechanical reliability of the high-frequency circuit board.

[0052] The electroplating solution for the low-temperature, low-stress electroplating thickening process uses an acidic copper sulfate system. Its basic formulation consists of: copper sulfate concentration of 60-80 g / L, sulfuric acid concentration of 150-200 g / L, and chloride ion concentration of 40-80 mg / L. No sulfur-based brighteners are added to the electroplating solution to avoid increased high-frequency dielectric loss due to the accumulation of brightener decomposition products at copper grain boundaries. The electroplating solution temperature is strictly controlled within the range of 18-22 degrees Celsius. Low-temperature electroplating conditions effectively suppress the tendency for grain coarsening during copper deposition, reduce internal stress in the copper layer, and control surface roughness growth.

[0053] The current density during the electroplating process was set to 0.8 to 1.5 A / dm². Under low current density conditions, the reduction rate of copper ions was moderate, allowing copper atoms sufficient time to migrate to the lowest energy lattice positions during deposition, resulting in a fine-grained, uniformly oriented copper layer structure. Cathode current pulse technology was employed during the electroplating process. Periodic current pulses and intervals further optimized the crystal quality and surface smoothness of the copper layer. The pulse electroplating parameters were set as follows: pulse time 100 to 500 ms, off-time 50 to 200 ms, and peak pulse current density 1.5 to 2 times the DC current density.

[0054] During the electroplating process, the temperature, pH value, and copper ion concentration of the plating solution must be continuously monitored to ensure its stability. Temperature fluctuations should be controlled within ±1 degree Celsius, and the pH value should be adjusted in real time using an online monitoring system. After electroplating to the target copper thickness, the substrate is removed and washed with water to remove residual plating solution, followed by drying. The surface roughness Rz of the electroplated copper layer should not exceed 0.8 μm, and the target copper thickness is set to 10 to 25 μm according to the circuit design requirements.

[0055] The combination of low-temperature, low-current-density acidic copper sulfate electroplating conditions and a dense seed layer formed by high-power pulse sputtering ensures the continuity of the interface quality between the electroplated thick copper layer and the seed layer. The electroplated copper layer achieves the target circuit thickness while maintaining low roughness characteristics. Its fine and dense grain structure, low internal stress, and tight metallurgical bond with the seed layer provide a highly reliable conductor path for stable transmission of high-frequency signals.

[0056] Step 6: Demolding and Shaping After completing the low-temperature, low-stress electroplating thickening, the high-temperature resistant photosensitive dry film on the surface of the substrate is peeled off using an alkaline film-removing solution. After water washing and drying, a low-loss high-frequency circuit board with complete metallized holes and fine lines is obtained.

[0057] The alkaline stripping solution is an aqueous solution of sodium hydroxide and an alkanolamine compound, with an effective mass concentration of 3% to 5%. The alkanolamine compound includes, but is not limited to, triethanolamine, diethanolamine, monoethanolamine, and dimethylethanolamine. Sodium hydroxide provides a strongly alkaline environment, enabling it to saponify the photosensitive components in the dry film, converting them into soluble substances. The alkanolamine compound components have a swelling effect on the high-temperature resistant photosensitive dry film, which can disrupt the adhesion between the dry film and the substrate. The stripping temperature is controlled within the range of 75 to 85 degrees Celsius. Under this temperature condition, the activity of the alkaline stripping solution reaches its optimal balance, ensuring a sufficient chemical reaction rate while avoiding damage to the substrate or copper layer due to excessive temperature. The peeling time is set to 3 to 5 minutes based on the dry film thickness and the stripping solution concentration to ensure complete peeling of the dry film without leaving any residue.

[0058] During the film removal process, the electroplated substrate is immersed in heated film removal solution. A rocking or rolling motion is used to facilitate contact between the film removal solution and the dry film, accelerating the removal process. After film removal, the substrate is removed and subjected to a multi-stage water washing process: the first step is a warm water rinse (40-50 degrees Celsius) to remove residual film removal solution from the substrate surface; the second step is a cold water rinse (tap water) for 2-3 minutes; the third step is a final rinse with pure water (conductivity ≤ 18.2 MΩ·cm) for 1-2 minutes to ensure the substrate surface is clean and free of ion residue.

[0059] After washing, the substrate is dried at a temperature controlled between 80 and 100 degrees Celsius for 20 to 40 minutes to ensure complete drying. The resulting high-frequency circuit board has a complete metallized hole structure and fine conductor lines. The accuracy of the circuit pattern depends on the development accuracy of the photosensitive dry film in the pattern masking process and the level of residual adhesive control in the film removal process.

[0060] In the pattern masking process, the precise pattern masking of the photosensitive dry film combined with the selective deposition of the pure copper layer enables the direct formation of fine circuits, avoiding the side etching problems in traditional subtractive processes. After film removal, the circuit edges are steep, the line width is uniform, and the insulation spacing is clear, with a minimum line width and line spacing of 20μm / 20μm. The organic-inorganic composite interface activation layer between the copper layer and the substrate ensures high peel strength without introducing additional dielectric loss sources, enabling the final product to exhibit excellent signal transmission characteristics in high-frequency application scenarios such as 24GHz and 77GHz automotive radar, 5G millimeter-wave antennas, and 6G communication substrates.

[0061] Specific application example: Fabrication of high-frequency circuit boards for 77GHz automotive radar Based on the above methods and steps, a detailed explanation will be given using a 77GHz automotive radar high-frequency circuit board as a specific application example.

[0062] A 0.5mm thick copper-free polytetrafluoroethylene (PTFE) high-frequency substrate was selected, with a dielectric constant Dk of 3.5 and a dielectric loss Df of 0.0018. Through-holes were fabricated using laser drilling, with the laser pulse energy and repetition frequency precisely set according to the target hole diameter of 0.2mm. After laser drilling, a plasma descaling process was used to remove the carbonized layer on the hole walls. The plasma treatment parameters were: a mixture of oxygen and carbon tetrafluoride gas, with oxygen accounting for 70% by volume and carbon tetrafluoride for 30% by volume; a reaction chamber pressure of 35Pa; a radio frequency power of 1200W; and a treatment time of 3 minutes. Subsequently, multi-stage ultrasonic cleaning was performed, consisting of 5 minutes of alkaline cleaning solution cleaning, 3 minutes of deionized water rinsing, 2 minutes of pure water rinsing, and finally, drying in a vacuum oven at 80 degrees Celsius for 40 minutes.

[0063] The activation parameters for the titanium-free composite interface are as follows: In the argon-oxygen mixed plasma activation treatment, the volume ratio of argon to oxygen is 9:1, the radio frequency power is 600W, the treatment time is 6 minutes, and the substrate surface temperature does not exceed 55 degrees Celsius during the treatment. After the plasma treatment, a vacuum dehumidification pretreatment is performed, with a pretreatment vacuum degree of [missing information]. The treatment time is 2 minutes. Then, a 0.8% perfluorooctyltriethoxysilane ethanol solution is coated. Spin coating is used to ensure uniform coverage of the solution. The spin coating speed is 800 to 1200 r / min and the spin coating time is 30 seconds. After coating, it is dried at 50 degrees Celsius for 4 minutes to form an organic-inorganic composite activated interface.

[0064] The pattern masking process uses a high-temperature resistant photosensitive dry film with a thickness of 25 μm, which is bonded to the activated substrate surface via hot pressing at a pressure of 0.4 MPa, a temperature of 95 degrees Celsius, and a time of 15 seconds. After bonding, LEDs are exposed at a wavelength of 405 nm and an energy density of 80 mJ / cm², with the exposure time set according to the photosensitivity of the dry film. Following exposure, a 1% sodium carbonate aqueous solution is used for development at a temperature of 25 degrees Celsius for 60 seconds. After development, the surface is washed and dried to form a precise 77 GHz radar antenna circuit pattern masking structure.

[0065] The magnetron sputtering process parameters for gradient pure copper seed layers are as follows: Place the masked substrate in the magnetron sputtering cavity and evacuate to the baseline vacuum level. The first step was high-power pulsed magnetron sputtering: argon partial pressure 0.35 Pa, sputtering power 2.5 kW, pulse width 100 μs, pulse frequency 300 Hz, duty cycle 10%, sputtering time 1.5 min, resulting in a copper layer thickness of 60 nm. The second step was DC magnetron sputtering: argon partial pressure 0.45 Pa, sputtering power 1.2 kW, sputtering time 2.5 min, resulting in a copper layer thickness of 110 nm. After sputtering, the total thickness of the copper seed layer was measured to be 170 nm, and the interface roughness Rz was 0.17 μm.

[0066] The low-temperature, low-stress electroplating thickening process uses an acidic copper sulfate electroplating system, with an electroplating bath temperature of 20 degrees Celsius, a current density of 1.0 A / dm², and electroplating to a copper thickness of 18 μm. After electroplating, the surface roughness Rz of the copper layer was measured to be 0.45 μm.

[0067] The film removal process uses an alkaline film removal solution prepared with 4% sodium hydroxide and triethanolamine at a mass concentration of 80 degrees Celsius and a peeling time of 4 minutes. After peeling, the circuit is rinsed with warm water, then rinsed with cold water, and finally rinsed with pure water. It is then dried at 90 degrees Celsius for 30 minutes to obtain the finished 77GHz automotive radar high-frequency circuit board.

[0068] The performance of the 77GHz vehicle radar high-frequency circuit board was tested, and the results are as follows: the peel strength between the copper layer and the substrate is 1.36 N / mm; the insertion loss at 80GHz frequency is reduced by 21.3% compared with the traditional chemical copper plating process; after 1000 cycles of thermal cycling reliability testing in the temperature range of -40 degrees Celsius to 125 degrees Celsius, the product showed no blistering, delamination, or circuit cracking, meeting the IPC-6012 Level 3 military reliability standard. Example 2

[0069] To further verify the technical solution of this invention, a 5G millimeter-wave antenna high-frequency substrate is used as another specific application example for illustration. This embodiment shares the same core process flow as Embodiment 1, but differs in specific parameter configurations to adapt to different substrate types and product specification requirements.

[0070] A 0.3mm thick copper-free hydrocarbon resin substrate was selected as the high-frequency substrate. The dielectric constant Dk of the hydrocarbon resin substrate is 3.2, and the dielectric loss Df is 0.0030. This substrate has better processability and cost advantages compared to polytetrafluoroethylene (PTFE) substrate, and also exhibits excellent signal transmission performance in the 5G millimeter-wave band. Mechanical drilling was used for drilling the substrate, employing a 0.15mm diameter carbide drill bit at a drilling speed of 100,000 rpm, a feed rate of 30mm / min, and a retraction rate of 60mm / min. The drill bit was replaced after every 500 holes to ensure hole wall quality. After drilling, a plasma descaling treatment was performed. The oxygen and carbon tetrafluoride mixture had a volume ratio of 65% oxygen and 35% carbon tetrafluoride, a reaction chamber pressure of 40Pa, an RF power of 1000W, and a treatment time of 4 minutes. Subsequent multi-stage cleaning and drying processes followed.

[0071] The activation parameters for the titanium-free composite interface are configured as follows: In the argon-oxygen mixed plasma activation treatment, the volume ratio of argon to oxygen is adjusted to 8.5:1.5, the RF power is increased to 700W, the treatment time is 5 minutes, and the substrate surface temperature is controlled below 58 degrees Celsius. The vacuum degree of the vacuum dehumidification pretreatment is... The processing time was 1.5 minutes. The fluorinated silane coupling agent solution had a mass concentration of 0.6%, and the solvent was anhydrous ethanol. The drying pretreatment temperature was 55 degrees Celsius, and the drying time was 3 minutes.

[0072] The pattern masking process also uses a high-temperature resistant photosensitive dry film with a thickness of 20 μm to accommodate more delicate circuit requirements. The bonding, exposure, and development process parameters are similar to those in Example 1, but the exposure energy density has been adjusted accordingly based on the dry film thickness.

[0073] The parameters for gradient pure copper seed layer magnetron sputtering process are configured as follows: cavity base vacuum level The first step was high-power pulsed magnetron sputtering: sputtering power 2kW, sputtering time 1 minute, depositing a copper layer thickness of 50nm; the second step was DC magnetron sputtering: sputtering power 1.5kW, sputtering time 3 minutes, depositing a copper layer thickness of 120nm. The total thickness of the copper seed layer was 170nm, and the interface roughness Rz was 0.15μm.

[0074] In the low-temperature, low-stress electroplating thickening process, the electroplating bath temperature was 21 degrees Celsius, the current density was 1.2 A / dm², and the copper thickness was 15 μm. After electroplating, the surface roughness Rz of the copper layer was measured to be 0.52 μm.

[0075] The film removal and forming process uses a 4% (w / w) alkaline film removal solution, a film removal temperature of 82 degrees Celsius, and a peeling time of 3.5 minutes. After film removal, the film is thoroughly cleaned and dried.

[0076] The performance test results of the fabricated 5G millimeter-wave antenna high-frequency substrate are as follows: copper layer peel strength 1.28 N / mm; insertion loss at 28 GHz is reduced by 19.7% compared to traditional chemical copper plating; dimensional deformation of the board surface is no more than 0.05%; no dry film residue remains on the board surface after film removal, and the insulation resistance meets design requirements. This 5G millimeter-wave antenna high-frequency substrate can be applied to 5G base station antenna arrays, millimeter-wave radar sensors, and high-speed wireless communication modules.

[0077] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the scope of protection of the present invention is not limited to the specific parameter configurations of the above embodiments. All equivalent transformations or modifications made based on the spirit of the technical solution of the present invention should be covered within the scope of protection of the present invention. The specific implementation of the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, or improvements made without departing from the spirit and scope of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperatures, characterized in that, Includes the following steps: Step 1: Select a high-frequency, low-loss insulating substrate as the carrier substrate, and perform mechanical drilling or laser perforation on the substrate, followed by cleaning to remove adhesive residue, surface dust, and oil stains; Step 2: Perform argon-oxygen mixed plasma activation treatment on the perforated substrate, then coat it with a fluorinated silane coupling agent solution and perform drying pretreatment to form an organic-inorganic composite activation interface, without depositing titanium, chromium, or other transition metal layers throughout the process; Step 3: Laminate or bond a polyimide-modified high-temperature resistant photosensitive dry film over the entire surface of the substrate, and after exposure and development, only the area to be fabricated and the metallized area are exposed. Step 4: The non-circuit area inside the hole is completely covered by dry film. Step 5: The masked substrate is placed in a vacuum magnetron sputtering chamber and pure copper is deposited in a stepwise gradient using a high-power pulse and DC composite process to form a dense conductive seed layer. Step 6: The pure copper seed layer is electroplated to thicken using an acidic copper sulfate electroplating system under low temperature, low current density, and sulfur-free brightener conditions to achieve the target copper thickness for the circuit. Step 7: The photosensitive dry film on the surface of the substrate is peeled off using an alkaline film stripping solution. After washing and drying, a low-loss high-frequency circuit board with complete metallized holes and fine circuits is obtained.

2. The method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature according to claim 1, characterized in that, The high-frequency, low-loss insulating substrate is a copper-free substrate, selected from polytetrafluoroethylene, hydrocarbon resin, low dielectric high glass transition temperature FR-4 substrate, and liquid crystal polymer substrate.

3. The method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature according to claim 1, characterized in that, The process parameters for the argon-oxygen mixed plasma activation treatment in step 2 are as follows: the volume ratio of argon to oxygen is in the range of 9:1 to 8:2, the radio frequency power is in the range of 500 watts to 800 watts, the treatment time is in the range of 5 to 10 minutes, and the surface temperature of the substrate is below 60 degrees Celsius during the treatment.

4. The method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature according to claim 1, characterized in that, The solvent for the fluorinated silane coupling agent solution in step 2 is anhydrous ethanol. The drying pretreatment temperature is in the range of 45 to 55 degrees Celsius, and the drying time is in the range of 3 to 5 minutes. The silane oxygen in the fluorinated silane coupling agent molecule undergoes a condensation reaction with the active hydroxyl groups on the substrate surface to form covalent bonds, and the fluorinated long-chain groups form a low surface energy interface on the outer layer.

5. The method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature according to claim 1, characterized in that, In step 2, after the argon-oxygen mixed plasma activation treatment is completed and before the fluorinated silane coupling agent solution is coated, a vacuum dehumidification pretreatment step is also included. The pretreatment vacuum degree is no greater than 1 x 10^-2 Pascals, and the treatment time is in the range of 1 to 3 minutes.

6. The method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature according to claim 1, characterized in that, The polyimide-modified high-temperature resistant photosensitive dry film described in step 3 has a temperature resistance rating of not less than 180 degrees Celsius, a coefficient of thermal expansion of not more than 20 ppm per degree Celsius, no small molecule volatilization under vacuum sputtering environment, no edge curling, and no adhesive residue on the board surface after peeling.

7. The method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature according to claim 1, characterized in that, The base vacuum level of the vacuum magnetron sputtering cavity described in step 4 is no greater than 5 x 10^-3 Pascals; the gradient deposition is divided into two steps: the first step is high-power pulsed sputtering, with an argon partial pressure in the range of 0.3 to 0.4 Pascals, a sputtering power in the range of 2 to 3 kW, a pulse width in the range of 50 to 200 microseconds, a pulse frequency in the range of 100 to 500 Hz, and a sputtering time in the range of 1 to 2 minutes; the second step is DC sputtering, with an argon partial pressure in the range of 0.4 to 0.5 Pascals, a sputtering power in the range of 1 to 1.5 kW, and a sputtering time in the range of 2 to 3 minutes; after deposition, the interface roughness Rz of the copper seed crystal layer is no greater than 0.2 micrometers.

8. The method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature according to claim 1, characterized in that, The electroplating process described in step 5 uses an acidic copper sulfate electroplating system. No sulfur-based brighteners are added to the electroplating solution. The temperature of the electroplating solution is in the range of 18 to 22 degrees Celsius, the current density is in the range of 0.8 to 1.5 amps per square decimeter, and the surface roughness Rz of the copper layer of the circuit after electroplating is no greater than 0.8 micrometers.

9. The method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature according to claim 1, characterized in that, The alkaline stripping solution mentioned in step 6 is an aqueous solution of sodium hydroxide and organic alcohol amine, with an effective mass concentration in the range of 3% to 5%, a stripping temperature in the range of 75 to 85 degrees Celsius, and a stripping time in the range of 3 to 5 minutes.

10. The method for fabricating low-loss high-frequency circuit boards by magnetron sputtering with a titanium-free transition layer at low temperature according to claim 1, characterized in that, The highest temperature of all processes in the entire process is below 100 degrees Celsius, and does not include chemical copper plating, blackening or browning, titanium or chromium transition layer deposition, or high-temperature baking processes.

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