Highly compact and highly heat-conductive hot-pressed aluminum nitride substrate and method for manufacturing the same

Abstract

This invention discloses a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate and its preparation method, belonging to the field of ceramic thermally conductive materials technology. The aluminum nitride substrate is composed of aluminum nitride powder and a composite sintering aid, the composite sintering aid being a fluoride, with a total addition amount of 2%-8% of the aluminum nitride powder mass. The aluminum nitride substrate has a multi-layer composite structure, including an upper dense layer, a middle stress buffer layer, and a lower dense layer. The porosity of the upper and lower dense layers is ≤0.3%, and the porosity of the middle stress buffer layer is 1%-5%. The preparation method includes steps such as raw material mixing, multi-layer molding, hot-pressing sintering, and diamond wire saw cutting. This invention achieves grain boundary purification through a pure fluoride composite aid system, combined with a multi-layer structure design to absorb cutting stress and thermal stress, resulting in a substrate density ≥99.5%, thermal conductivity ≥240W / (mK), and an ultra-thin substrate edge chipping rate ≤3%, while simultaneously reducing the sintering temperature and shortening the production cycle.

Description

Technical Field

[0001] This invention relates to the field of ceramic thermally conductive materials technology, and more specifically, to a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate and its preparation method. Background Technology

[0002] Aluminum nitride ceramics, due to their high theoretical thermal conductivity, matching coefficient of thermal expansion with silicon, and excellent electrical insulation properties, have become a core heat dissipation substrate material in fields such as semiconductor power device packaging and new energy vehicle electronic control modules. As electronic devices develop towards higher power density and ultra-thin designs, higher requirements are placed on the high density, high thermal conductivity, and ultra-thin processing precision of aluminum nitride substrates.

[0003] Existing aluminum nitride substrate fabrication technologies mainly include three pathways: pressureless sintering, tape casting, and hot pressing sintering. Pressureless sintering, as disclosed in patent CN115010499A, reduces sintering difficulty but struggles to achieve high density and limits thermal conductivity. Tape casting combined with atmosphere sintering, as disclosed in patent CN113021660A, is suitable for fabricating large-area thin sheets, but uneven sintering shrinkage easily leads to warping, and subsequent cutting causes significant edge chipping. Hot pressing sintering can improve density, but existing technologies often use oxides or boride additives, which can easily introduce oxygen impurities or form brittle phases, such as the fluoride and boride composite system disclosed in patent CN119390454A, which suffers from reduced mechanical strength. Furthermore, in existing technologies, the sintering and cutting processes are independent, lacking a systematic and coordinated design, resulting in the failure to effectively solve the challenges of ultra-thin substrate processing.

[0004] To address the aforementioned shortcomings, this invention proposes a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate and its preparation method. Grain boundary purification is achieved through a pure fluoride composite additive system, and the cutting stress is absorbed by a multi-layer composite structure design. Furthermore, the diamond wire saw cutting process is optimized to achieve integrated synergy between sintering and processing, thus meeting the dual requirements of high-power devices for thermal conductivity and processing precision. Summary of the Invention

[0005] To overcome the aforementioned deficiencies of the prior art, the present invention provides a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate and its preparation method, thereby addressing the problems mentioned in the background art.

[0006] In a first aspect, embodiments of this application provide a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate, comprising: aluminum nitride powder and composite sintering aids; Aluminum nitride powder is α-AlN powder; The α-AlN powder has a purity ≥99.8%, an average particle size of 1–3 μm, and an oxygen impurity content ≤0.3%. The composite sintering aid is at least two of yttrium fluoride, lanthanum fluoride, cerium fluoride, and neodymium fluoride; The mass of the composite sintering aid is 2%–8% of the mass of the aluminum nitride powder; The aluminum nitride substrate has a multilayer composite structure; The multilayer composite structure includes an upper dense layer, an intermediate stress buffer layer, and a lower dense layer; The porosity of the upper and lower dense layers is ≤0.3%; The porosity of the intermediate stress buffer layer is 1%–5%.

[0007] In some embodiments of this application, the mass ratio of each component in the composite sintering aid is YF3:LaF3:CeF3:NdF3=1—3:1—2:0—2:0—1, and the content of at least two components is not zero.

[0008] In some embodiments of this application, the content of composite sintering aids in the upper dense layer, the intermediate stress buffer layer and the lower dense layer is distributed in a gradient.

[0009] In some embodiments of this application, the intermediate stress buffer layer comprises a second phase material; The second phase material is selected from at least one of boron nitride, silicon carbide whiskers, graphene, and carbon nanotubes; The amount of the second phase material added is 1%–10% of the mass of aluminum nitride powder.

[0010] Secondly, embodiments of this application provide a method for preparing a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate, comprising the following steps: S1. Aluminum nitride powder is mixed with composite sintering aid, and after ball milling, drying and granulation, a green body with a multi-layer composite structure is formed by multi-layer casting, gradient feeding or step-by-step pressing process. S2. Place the green blank in a hot pressing sintering furnace for hot pressing sintering, and keep it at temperature and pressure for 4-8 hours to obtain the sintered body; S3. The sintered body is cut using a diamond wire saw to obtain aluminum nitride substrate sheets with a thickness of 0.3mm-1.0mm.

[0011] In some embodiments of this application, the ball-to-material ratio in the ball mill is 8-12:1, and the ball milling time is 12-18 hours; The drying method is spray drying, with an inlet temperature of 180℃-200℃ and an outlet temperature of 85℃-95℃.

[0012] In some embodiments of this application, the specific method of hot pressing sintering is as follows: nitrogen gas is introduced into the hot pressing sintering furnace, the purity of the nitrogen gas is ≥99.9%, and the furnace pressure of the hot pressing sintering furnace is 0.2MPa-0.3MPa; The heating rate of hot pressing sintering is 4℃ / min-6℃ / min. When the temperature reaches 600℃, it is held for 50-70 minutes for degreasing treatment. The cooling rate of hot pressing sintering is 2℃ / min-3℃ / min, and it is allowed to cool naturally to room temperature after the temperature reaches below 200℃.

[0013] In some embodiments of this application, the diamond wire saw has a diamond mesh count of 200-300 and a wire diameter of 0.12mm-0.18mm.

[0014] In some embodiments of this application, the specific cutting process parameters are: linear speed of 28m / s-32m / s, workpiece feed speed of 0.1mm / min-0.15mm / min, and tension of 25N-35N; Coolant is continuously sprayed during the cutting process. The coolant is an extreme pressure emulsion with a pressure of 0.3 MPa to 0.5 MPa.

[0015] In some embodiments of this application, the aluminum nitride substrate sheet also needs to be ground and polished to make its surface roughness Ra≤0.1μm, and then cleaned and dried.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention employs two or more fluoride composite additives to promote densification by forming a low-melting-point eutectic liquid phase, while simultaneously achieving grain boundary purification through reaction with oxygen impurities on the AlN surface. This additive system avoids lattice oxygen defects introduced by oxides and brittle phases formed by borides, ensuring that only an extremely thin continuous phase remains at the grain boundaries, resulting in direct bonding between grains. Grain boundary purification reduces phonon scattering centers, and high density eliminates phonon scattering by pores. The synergistic effect of these two factors brings the substrate's thermal conductivity close to the theoretical upper limit of AlN polycrystalline materials, exhibiting extremely low thermal conductivity degradation after high-temperature service.

[0017] 2. This invention employs a three-layer composite structure consisting of an upper dense layer, a middle stress buffer layer, and a lower dense layer. The middle buffer layer, through its controllable porosity and the introduction of a second-phase material, effectively absorbs cutting stress and thermal cycling stress, suppressing crack propagation. This structural design, combined with an optimized diamond wire saw cutting process, enables the ultra-thin substrate to maintain high bending strength while achieving extremely low edge chipping and warpage, with a surface roughness reaching mirror-level, meeting the stringent processing requirements of semiconductor packaging for ultra-thin substrates.

[0018] 3. This invention reduces the hot-pressing sintering temperature to 1500-1800℃, a significant decrease compared to existing technologies. Simultaneously, by integrating one-step hot-pressing with diamond wire saw cutting, the production process is simplified and the production cycle shortened. The multi-layer composite structure is formed in one step using gradient fabrication or step-by-step pressing, eliminating the need for additional processes. These process improvements reduce equipment energy consumption and mold wear, decrease subsequent processing steps, and effectively lower overall production costs, demonstrating good industrial economics and adaptability to mass production. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0020] Figure 1 A schematic diagram of the three-layer composite structure provided by the present invention; Figure 2 This is a gradient distribution diagram of fluoride auxiliaries provided by the present invention; Figure 3 The preparation process flow diagram provided by the present invention; Figure 4 A comparative data chart of key performance indicators provided for this invention. Detailed Implementation

[0021] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] To enable those skilled in the art to better understand the present application, the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] Example 1 Please see Figure 1 and Figure 4 This invention provides a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate. To achieve the above objectives, this invention employs the following technical solutions: The high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate provided in this embodiment is composed of aluminum nitride powder and composite sintering aids. The aluminum nitride powder is α-AlN powder with a purity ≥99.8%, an average particle size of 1-3 μm, and an oxygen impurity content ≤0.3%, ensuring high thermal conductivity and inhibiting AlN hydrolysis. Through extensive experimental research, the inventors discovered that when the oxygen impurity content of the AlN powder exceeds 0.5%, even under optimal sintering processes, the final substrate's thermal conductivity is difficult to exceed 200 W / (m²). This is because oxygen impurities dissolve into the AlN lattice, forming aluminum vacancy defects and severely scattering phonons. Therefore, in this embodiment, the oxygen impurity content of the AlN powder is strictly controlled to be ≤0.3%, ensuring the foundation for achieving high thermal conductivity from the source.

[0024] The composite sintering aid is at least two of yttrium fluoride, lanthanum fluoride, cerium fluoride, and neodymium fluoride. In this embodiment, a combination of three fluorides, YF3, LaF3, and CeF3, is specifically selected, with a total mass of 4.2% of the aluminum nitride powder mass. Through extensive comparative experiments, the inventors discovered that using composite aids of two or more fluorides has a significant synergistic effect compared to using a single fluoride: the multi-component fluorides form a low-melting-point eutectic phase at high temperatures (eutectic point can be as low as 1400-1550℃), resulting in a lower liquid phase emergence temperature, a more abundant liquid phase, and a more uniform liquid phase distribution. This allows for more thorough wetting of AlN particles, promoting particle rearrangement and material migration, thereby achieving densification at lower temperatures. Simultaneously, the multi-component fluorides react with oxygen impurities on the AlN surface to generate volatile aluminum fluoride oxides, which can more thoroughly purify grain boundaries and prevent the formation of a low thermal conductivity grain boundary glass phase. In this embodiment, the mass ratio of each component is YF3:LaF3:CeF3=4:3:1. This ratio has been optimized to form the optimal amount and viscosity of liquid phase at a sintering temperature of 1700℃, which ensures sufficient densification while avoiding excessive liquid phase leading to abnormal grain coarsening.

[0025] Please see Figure 1In this embodiment, the aluminum nitride substrate has a multilayer composite structure, specifically including an upper dense layer, an intermediate stress buffer layer, and a lower dense layer. The porosity of the upper and lower dense layers is ≤0.3%, ensuring high thermal conductivity; the porosity of the intermediate stress buffer layer is 1-5%, used to absorb cutting stress and thermal stress, and inhibit crack propagation. In this embodiment, the porosity of the intermediate stress buffer layer is controlled at 2.5%, achieved by adding 3% polymethyl methacrylate (PMMA) microspheres as a pore-forming agent. The PMMA microspheres have a particle size of 3μm and completely decompose during high-temperature sintering to form uniformly distributed micropores. The inventors have found that when the porosity of the intermediate layer is less than 1%, the stress buffering effect is not significant; when the porosity is greater than 5%, although it can further reduce stress, it will have a significant impact on the overall thermal conductivity and may reduce the mechanical strength of the substrate. Therefore, controlling the porosity of the intermediate layer within the range of 1-5% can achieve the best stress buffering effect without significantly sacrificing thermal conductivity.

[0026] Specifically, the preparation method of this embodiment includes the following steps: S1. Raw Material Pretreatment: Weigh 95g of α-AlN powder with a purity of 99.8% and an average particle size of 2μm, along with composite fluoride additives (YF32.0g, LaF31.5g, CeF30.5g, total addition 4.2%), add 6g of anhydrous ethanol as a dispersant and 1.2g of polyvinyl alcohol as a binder, and ball mill for 15 hours using silicon nitride ceramic balls as the grinding medium (ball-to-material ratio 10:1) to ensure uniform dispersion of the additives (particle spacing ≤5μm). The inventors have found that ball milling time has a significant impact on the uniformity of additive dispersion: when the ball milling time is less than 12 hours, the fluoride additives are prone to agglomeration, leading to uneven distribution of the grain boundary phase after sintering. Excessively thick grain boundaries in some areas affect thermal conductivity, while excessively thin grain boundaries in others result in insufficient densification. When the ball milling time is longer than 18 hours, the AlN particles may be over-refined, increasing the powder surface energy and causing abnormal grain growth during sintering. Therefore, controlling the ball milling time within the range of 12-18 hours is crucial to ensuring uniform dispersion of the additives and controllable grain size. The ball-milled slurry is then spray-dried at an inlet temperature of 190℃ and an outlet temperature of 90℃, controlling the powder moisture content to ≤0.1%. After granulation, the powder particle size is 20-50μm, improving molding fluidity. Controlling the spray drying outlet temperature is critical: excessively high temperatures (>95℃) will cause premature volatilization of the binder, reduced interparticle bonding, and insufficient green body strength; excessively low temperatures (<85℃) will result in insufficient drying, and residual moisture can easily lead to cracking of the green body during subsequent degreasing.

[0027] S2. Multi-layer molding: Powders for the upper dense layer, intermediate stress buffer layer, and lower dense layer are prepared separately. The upper and lower dense layers use the basic formula described above; the intermediate stress buffer layer has an additional 3% PMMA microspheres added as a pore-forming agent. A multi-layer casting process is used, sequentially laying the powders for the lower dense layer, intermediate buffer layer, and upper dense layer, followed by cold isostatic pressing at 70 MPa for 30 minutes, resulting in a green body density of 2.7 g / cm³. 3 The inventors have discovered that the density of the green body has a significant impact on the densification degree of the final sintered body: when the green body density is below 2.6 g / cm³... 3 At this stage, the particles are loosely packed, resulting in a large sintering shrinkage rate, which easily leads to deformation or cracking of the sintered body; the density of the green body is higher than 2.8 g / cm³. 3 While this process facilitates sintering and densification, it also increases the difficulty of pre-pressing and can lead to poor interlayer bonding. Therefore, the pre-pressing process was optimized to control the green body density at 2.6-2.8 g / cm³. 3 Within a certain range, this is a prerequisite for ensuring the quality of the sintered body.

[0028] S3. Hot pressing sintering: The green body is placed in a hot pressing sintering furnace, and high-purity nitrogen gas (purity ≥99.999%, furnace pressure 0.25MPa) is introduced. The temperature is raised to 600℃ at a heating rate of 5℃ / min and held for 60 minutes to remove the binder and moisture. The temperature is then raised to 1700℃, and a pressure of 30MPa is applied. The temperature and pressure are held for 6 hours. Then, the temperature is slowly cooled to below 200℃ at a cooling rate of 2.5℃ / min and allowed to cool naturally to room temperature to obtain the sintered body.

[0029] The hot-pressing sintering process of this invention has the following key technical points: the heating rate is controlled at 4-6℃ / min to avoid excessively rapid heating that could lead to excessive temperature difference between the inside and outside of the blank, causing thermal stress cracking, while ensuring that the binder is fully decomposed and removed during the degreasing stage; degreasing is performed at 600℃ for 50-70 minutes to ensure complete decomposition of the binder and avoid residual carbon affecting thermal conductivity; the sintering temperature is 1500-1800℃, which is 100-300℃ lower than the prior art (usually ≥1800℃), significantly reducing energy consumption, while avoiding AlN decomposition and abnormal grain growth at excessively high temperatures; a pressure of 20-40MPa is applied to promote particle rearrangement and plastic flow, accelerating the densification process; the cooling rate is 2-3℃ / min, slow cooling reduces thermal stress and prevents substrate warping and deformation. Through extensive process optimization experiments, the inventors have found that the best densification effect and microstructure can be obtained within the above parameter range.

[0030] S4. Diamond Wire Saw Cutting: An electroplated diamond wire saw is used to cut the sintered body. The diamond mesh size is 250 mesh, the wire diameter is 0.15 mm, the wire speed is 30 m / s, the workpiece feed speed is 0.12 mm / min, and the tension is 30 N. Extreme pressure emulsion coolant is continuously sprayed during the cutting process at a cooling pressure of 0.4 MPa. The sintered body is cut into 0.5 mm thick aluminum nitride substrate sheets. The inventors have found that the diamond wire saw cutting parameters have a decisive influence on the chipping rate and warpage of the ultra-thin AlN substrate: when the wire speed is too high (>32 m / s), heat accumulation during cutting leads to thermal damage, increasing the risk of chipping; when the wire speed is too low (<28 m / s), the cutting efficiency decreases, and cutting marks are easily generated, affecting surface quality. When the feed rate is too high (>0.15 mm / min), the cutting load increases significantly, and the chipping rate rises substantially; when the feed rate is too low (<0.1 mm / min), the cutting efficiency is too low, making it uneconomical. Controlling the tension within the range of 25-35 N ensures cutting accuracy while preventing wire saw breakage. A coolant pressure of 0.3-0.5 MPa creates a continuous and stable coolant flow, effectively removing cutting heat and preventing thermal damage. The synergistic matching of these parameters is crucial for achieving high-precision cutting of ultra-thin substrates.

[0031] S5. Post-processing: The cut substrate sheet is polished on both sides using diamond polishing slurry to achieve a surface roughness Ra≤0.08μm; then, it is ultrasonically cleaned with anhydrous ethanol for 15 minutes to remove surface impurities, and dried at 80℃ to obtain the finished product. The grinding and polishing process not only affects the surface roughness but also relates to the bonding strength between the substrate and the subsequent metallization layer. By optimizing the particle size of the polishing slurry and the polishing time, the inventors obtained a high-gloss surface with Ra≤0.1μm, which is beneficial to improving the adhesion of the metallization layer.

[0032] In this embodiment, the performance of the prepared aluminum nitride substrate was tested, and the results are as follows: Figure 4 As shown: density 99.7%, porosity 0.22%, thermal conductivity 265 W / (m²). K); surface roughness Ra=0.08μm, warpage 0.15%, edge chipping rate 2.1%; bending strength 380MPa, no cracks or deformation after 1000 cycles of thermal cycling (-50℃~800℃), service life 8500 hours. These performance indicators demonstrate that the aluminum nitride substrate prepared in this embodiment possesses excellent comprehensive performance, particularly in the three key indicators of thermal conductivity, density, and edge chipping rate, which are significantly superior to existing technologies.

[0033] Example 2 Please see Figure 2 and Figure 4 This invention provides a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate. To achieve the above objectives, this invention employs the following technical solutions: The main difference between this embodiment and Embodiment 1 lies in the gradient distribution design of the composite sintering aid. Please refer to... Figure 2 In this embodiment, the aluminum nitride substrate also adopts a three-layer composite structure of upper dense layer, middle stress buffer layer and lower dense layer, but the content of composite sintering aid in each layer is distributed in a gradient. Figure 2 The gradient distribution design of the fluoride auxiliaries of this invention is demonstrated, which is one of the important innovations of this invention.

[0034] Specifically, the raw material formulation in this embodiment is as follows: The aluminum nitride powder used is α-AlN powder with a purity ≥99.8% and an average particle size of 1-3 μm, with a total dosage of 93 g. The composite sintering aid is a combination of three fluorides: YF3, LaF3, and NdF3. The total addition amount of the composite sintering aid in the upper and lower dense layers is 3.5%, specifically composed of 2.0 g of YF3 and 1.5 g of LaF3 (based on 100 g of AlN); the total addition amount of the composite sintering aid in the intermediate stress buffer layer is 7.0%, specifically composed of 2.5 g of YF3, 2.0 g of LaF3, and 0.5 g of NdF3 (based on 100 g of AlN). That is, the intermediate layer uses a low-melting-point eutectic combination of YF3 + LaF3 + NdF3, and the total addition amount is significantly higher than that in the upper and lower dense layers.

[0035] Please see Figure 2 The gradient distribution design of fluoride additives in this invention has the following technical mechanism: the intermediate buffer layer is enriched with a low-melting-point fluoride combination (YF3+CeF3+NdF3 or YF3+LaF3+NdF3), which forms a low-melting-point eutectic liquid phase (eutectic point can be as low as 1400-1550℃) during high-temperature sintering, and forms a continuous grain boundary phase after cooling. The elastic modulus of this grain boundary phase (about 50-80 GPa) is lower than that of AlN grains (about 310 GPa), forming a "soft-encapsulated" structure that can effectively absorb thermal and cutting stresses. When cracks propagate to the intermediate layer, they will deflect, bifurcate, or even terminate upon encountering the soft grain boundary phase, thereby significantly improving the fracture resistance of the substrate. At the same time, the higher fluoride content in the intermediate layer can react more fully with oxygen impurities on the AlN surface to generate volatile aluminum fluoride oxides or stable fluorine-containing compounds, further purifying the crystal lattice and improving thermal conductivity. The lower fluoride content in the upper / lower dense layers avoids excessive grain boundary phases from affecting thermal conductivity, thus maintaining the high thermal conductivity of the high-density layers.

[0036] Through thermodynamic calculations and experimental verification, the inventors discovered significant differences in the eutectic temperature and liquid phase characteristics of different fluoride combinations: the eutectic point of the YF3-LaF3 binary system is approximately 1520℃, that of the YF3-CeF3 binary system is approximately 1480℃, that of the YF3-NdF3 binary system is approximately 1450℃, while the eutectic point of the YF3-CeF3-NdF3 ternary system can be as low as below 1400℃. Therefore, by selecting a low-melting-point fluoride combination in the intermediate layer, a sufficient liquid phase can be formed at a lower temperature, which is beneficial for densification and reduces the sintering temperature. Simultaneously, the difference in ionic radii of different fluorides can adjust the thermal expansion coefficient of the grain boundary phase, allowing it to better match the AlN grains and reduce thermal stress.

[0037] Specifically, the preparation method of this embodiment includes the following steps: S1. Raw Material Pretreatment: Prepare powders for the upper dense layer, intermediate stress buffer layer, and lower dense layer separately. Upper dense layer powder: 100g AlN powder (standard), 32.0g YF, 31.5g LaF; Intermediate buffer layer powder: 100g AlN powder, 32.5g YF, 32.0g LaF, 30.5g NdF; Lower dense layer powder: same as the upper layer. Add anhydrous ethanol (6g / 100g AlN) and polyvinyl alcohol (1.2g / 100g AlN) to each layer of powder, and ball mill for 12 hours using silicon nitride ceramic balls as the grinding medium (ball-to-powder ratio 10:1). Spray dry separately at an inlet temperature of 185℃ and an outlet temperature of 88℃.

[0038] It is particularly important to emphasize that when preparing powder in layers, the ball milling time and spray drying parameters of each layer should be kept consistent to ensure that the particle size distribution and flowability of the powder in each layer are similar, thus avoiding poor interlayer bonding due to differences in powder properties during subsequent multi-layer molding. At the same time, the surface properties of the powder in the intermediate buffer layer may change due to the addition of a higher content of fluoride, requiring appropriate adjustment of process parameters to ensure molding quality.

[0039] S2. Multi-layer molding: A gradient material distribution process is used, where a dense layer of powder, an intermediate buffer layer of powder, and an upper dense layer of powder are sequentially laid in a graphite mold, with a thickness ratio of 4:2:4. Then, cold isostatic pressing is performed at a pressure of 60 MPa for 30 minutes, resulting in a green compact with a density of 2.65 g / cm³. 3 The gradient fabric application process is one of the key technologies of this invention. By controlling the thickness and uniformity of each powder layer, a gradient transition of components between layers can be achieved, avoiding interfacial stress caused by abrupt changes in composition. The inventors have found that when the intermediate layer thickness is too low (<10%), the stress buffering effect is not significant; when the intermediate layer thickness is too high (>30%), although it can further improve impact resistance, it will reduce the overall thermal conductivity. Therefore, controlling the intermediate layer thickness within the range of 10-30% of the total thickness is the optimal choice.

[0040] S3. Hot Press Sintering: The green compact is placed in a hot press sintering furnace, and high-purity nitrogen gas (purity ≥99.999%, furnace pressure 0.25 MPa) is introduced. The temperature is raised to 600°C at a rate of 5°C / min and held for 60 minutes for degreasing. The temperature is then raised to 1650°C, a pressure of 25 MPa is applied, and the temperature and pressure are maintained for 7 hours. The temperature is then cooled to below 200°C at a rate of 2.5°C / min and allowed to cool naturally to room temperature. This embodiment uses a sintering temperature of 1650°C, which is lower than the 1700°C in Example 1. This is because the intermediate buffer layer uses a low-melting-point fluoride combination, which can form a sufficient liquid phase at a lower temperature to promote densification. Through comparative experiments, the inventors found that when a low-melting-point fluoride combination is used in the intermediate layer, the sintering temperature can be reduced by 50-100°C without affecting the densification effect. This discovery is of great significance for reducing energy consumption.

[0041] S4. Diamond Wire Saw Cutting: Electroplated diamond wire saw cutting is used with a diamond mesh count of 200, a wire diameter of 0.15 mm, a wire speed of 28 m / s, a workpiece feed rate of 0.1 mm / min, a tension of 30 N, and extreme pressure emulsion spraying for cooling (pressure 0.4 MPa). The cut is to a 0.3 mm thin sheet. In this embodiment, the cutting thickness is 0.3 mm, which is the extreme specification for ultra-thin substrates, placing higher demands on the cutting process. By optimizing the cutting parameters, the inventors achieved satisfactory cutting quality at lower wire speeds and feed rates, verifying the applicability of the invention's process to extreme specifications.

[0042] S5. Post-treatment: Polish with diamond polishing slurry to Ra=0.09μm, ultrasonically clean with anhydrous ethanol for 15 minutes, and dry at 80℃.

[0043] In this embodiment, the performance of the prepared aluminum nitride substrate was tested, and the results are as follows: Figure 4 As shown: density 99.6%, porosity 0.28%, thermal conductivity 252 W / (m²). K); Surface roughness Ra=0.09μm, warpage 0.18%, chipping rate 2.8%; flexural strength 360MPa, service life 8200 hours. Compared with Example 1, this example, through the gradient distribution design of fluoride additives, further improves the stress absorption capacity of the intermediate buffer layer while maintaining high thermal conductivity, achieving a flexural strength of 360MPa and controlling the chipping rate to within 3%. Especially for the ultra-thin specification of 0.3mm, the 2.8% chipping rate is at an industry-leading level.

[0044] Example 3 Please see Figure 3 and Figure 4This invention provides a method for preparing a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate. To achieve the above objectives, this invention is implemented through the following technical solutions: This embodiment focuses on demonstrating the complete process flow and parameter optimization of the preparation method of the present invention, while verifying the impact of different fluoride combinations and process parameters on the final performance. Please refer to... Figure 3 The preparation process of the present invention mainly includes raw material preparation, ball milling and mixing, spray drying, multi-layer molding, hot pressing and sintering, diamond wire saw cutting, grinding and polishing, and cleaning and drying. The process parameters of each step are strictly implemented within the scope defined in the claims.

[0045] Specifically, the raw material formulation in this embodiment is as follows: 92g of α-AlN powder with a purity ≥99.8% and an average particle size of 1-3μm is used. The composite sintering aid is a combination of three fluorides: YF3, CeF3, and NdF3, with a total addition of 8.0% and a mass ratio of YF3:CeF3:NdF3 = 3:3:2. This embodiment does not employ a multilayer composite structure; it serves as a comparative example to verify the effect of the fluoride composite aid and to demonstrate complete process parameters. The inventors chose to combine CeF3 and NdF3 with YF3 because CeF3 and NdF3 have lower melting points (CeF3 melting point approximately 1460℃, NdF3 melting point approximately 1410℃), allowing them to form a liquid phase at lower temperatures, which is beneficial for reducing the sintering temperature. Simultaneously, CeF3... 3+ and Nd 3+ Its large ionic radius allows for adjustment of the composition and structure of the grain boundary phase, thereby optimizing thermal conductivity.

[0046] Specifically, the preparation method of this embodiment includes the following steps: S1. Raw Material Pretreatment: Mix 92g AlN powder, 33.0g YF, 33.0g CeF, and 32.0g NdF, add 8g anhydrous ethanol and 1.8g polyvinyl alcohol, and ball mill for 18 hours using silicon nitride ceramic balls as the grinding medium (ball-to-powder ratio 10:1). Spray dry the ball-milled slurry at an inlet temperature of 195℃ and an outlet temperature of 92℃, controlling the powder moisture content to ≤0.1%. The granulated powder has a particle size of 20-50μm. Load the dried powder into a graphite mold and cold isostatically press it at 80MPa for 30 minutes to obtain a green body with a density of 2.75g / cm³. 3 .

[0047] This embodiment employs a relatively high fluoride addition (8.0%) and a relatively high pre-compression pressure (80 MPa) to investigate the material properties and process feasibility under extreme conditions. The inventors discovered through thermogravimetric-differential thermal analysis that when the fluoride addition reaches 8%, the amount of eutectic liquid phase increases significantly, which is beneficial for obtaining higher density. However, it is necessary to carefully control the thickness of the grain boundary phase to avoid excessively thick grain boundary phase affecting thermal conductivity. Simultaneously, a higher pre-compression pressure can achieve higher green density, which is beneficial for sintering densification; however, excessively high pressure may lead to poor interlayer bonding and requires careful control.

[0048] S2. Hot Press Sintering: The green body is placed in a hot press sintering furnace, and high-purity nitrogen gas (purity ≥99999%, furnace pressure 0.3MPa) is introduced. The following heating regime is followed: heating rate 5℃ / min, holding at 600℃ for 60 minutes to remove binder and moisture; continue heating at 5℃ / min to 1800℃, apply pressure of 40MPa, and hold at that temperature and pressure for 4 hours. After holding, cool slowly to below 200℃ at a cooling rate of 2.5℃ / min, and then allow to cool naturally to room temperature to obtain the sintered body.

[0049] Please see Figure 3 The hot-pressing sintering process of this invention has the following technical features: the heating rate is controlled at 4-6℃ / min to avoid cracking or uneven temperature caused by excessive heating; degreasing is performed at 600℃ for 50-70 minutes to ensure complete decomposition and removal of the binder; the sintering temperature is controlled at 1500-1800℃, which is 100-300℃ lower than the prior art (usually ≥1800℃), significantly reducing energy consumption; a pressure of 20-40MPa is applied to promote densification while avoiding excessive pressure that could damage the mold; and the cooling rate is controlled at 2-3℃ / min to slowly reduce thermal stress.

[0050] This embodiment uses a sintering temperature of 1800℃ and a pressure of 40MPa, which is at the upper limit of the parameter range of this invention. Through comparative experiments, the inventors found that under these conditions, the AlN grain size is approximately 3-5μm, the grain boundary phase thickness is approximately 50-100nm, and the density can reach over 99.8%. However, excessively high sintering temperatures can lead to abnormal grain growth, which in turn reduces thermal conductivity and significantly increases energy consumption. Therefore, 1800℃ is the recommended upper temperature limit of this invention; for general applications, a temperature of around 1700℃ is recommended to balance performance and cost.

[0051] S3. Diamond wire saw cutting: The sintered body is cut using an electroplated diamond wire saw with a diamond mesh of 300 mesh, a wire diameter of 0.12 mm, a wire speed of 32 m / s, a workpiece feed speed of 0.15 mm / min, and a tension of 35 N. Extreme pressure emulsion coolant is continuously sprayed during the cutting process with a cooling pressure of 0.5 MPa. The sintered body is cut into aluminum nitride substrate sheets with a thickness of 1.0 mm.

[0052] Please see Figure 3 The diamond wire saw cutting process of the present invention has the following technical features: diamond mesh size of 200-300 mesh, balancing cutting efficiency and surface quality; linear speed of 28-32 m / s and feed speed of 0.1-0.15 mm / min, the two working together to ensure stable cutting; tension of 25-35 N, to prevent the wire saw from loosening or breaking; extreme pressure emulsion spraying for cooling, pressure of 0.3-0.5 MPa, effectively removing cutting heat and preventing thermal damage.

[0053] This embodiment employs a relatively high linear speed (32 m / s) and feed rate (0.15 mm / min), as well as a relatively fine diamond wire (0.12 mm), to verify the applicability of the cutting process of this invention under high-efficiency processing conditions. Experimental results show that even under these conditions, the chipping rate is still controlled at an ultra-low level of 1.8%, indicating that the process of this invention has a wide process window and good industrial applicability.

[0054] S4. Post-processing: The cut substrate sheet is polished on both sides with diamond polishing slurry to make the surface roughness Ra≤0.07μm; then it is ultrasonically cleaned with anhydrous ethanol for 15 minutes to remove surface impurities, and then dried at 80℃ to obtain the finished product.

[0055] In this embodiment, the performance of the prepared aluminum nitride substrate was tested, and the results are as follows: Figure 4 As shown: density 99.8%, porosity 0.18%, thermal conductivity 278 W / (m²). K); surface roughness Ra=0.07μm, warpage 0.12%, edge chipping rate 1.8%; flexural strength 400MPa, service life 9000 hours. This embodiment uses a higher fluoride addition (8.0%) and a higher sintering temperature (1800℃) to obtain a higher thermal conductivity (278W / m). The material exhibits high K content and density (99.8%), while the chipping rate is controlled to an ultra-low level of 1.8% through optimized cutting processes. This result fully demonstrates the excellent performance of the technical solution of this invention under extreme conditions.

[0056] Example 4 Please see Figure 1 , Figure 2 , Figure 3 and Figure 4 This invention provides a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate. To achieve the above objectives, this invention employs the following technical solutions: This embodiment integrates the three-layer composite structure of Embodiment 1, the fluoride gradient distribution of Embodiment 2, and the optimized process parameters of Embodiment 3. Furthermore, it introduces a second-phase material to enhance the stress absorption capacity of the intermediate buffer layer, forming a complete integrated "material-process-structure" technical solution to verify the comprehensive technical effect of the present invention. This is a preferred embodiment of the present invention and represents the best practice of the technical solution.

[0057] Specifically, the raw material formulation in this embodiment is as follows: The aluminum nitride powder used is α-AlN powder with a purity ≥99.8% and an average particle size of 1-3 μm. The composite sintering aid is a combination of four fluorides: YF3, LaF3, CeF3, and NdF3, arranged in a gradient distribution design. The total amount of composite sintering aid added in the upper and lower dense layers is 4%, with a mass ratio of YF3:LaF3:CeF3 = 2:1.5:0.5; the total amount of composite sintering aid added in the intermediate stress buffer layer is 7%, with a mass ratio of YF3:LaF3:CeF3:NdF3 = 2:1.5:1:1.5. The intermediate layer also incorporates 5% h-BN as a second phase material and 2% PMMA microspheres as a pore-forming agent to form a composite stress buffer layer with a porosity of approximately 3.5%.

[0058] Through extensive experimental research, the inventors discovered that introducing lamellar h-BN as a second phase into the intermediate buffer layer has multiple benefits: h-BN has a layered structure, which can effectively deflect crack propagation paths; h-BN itself has high thermal conductivity (up to 200-400 W / (m²) in-plane). The addition of h-BN (K) has a relatively small impact on overall thermal conductivity; its coefficient of thermal expansion is close to that of AlN, resulting in good thermal matching; h-BN also has good lubricity, which can reduce frictional resistance during cutting and further reduce edge chipping. However, the amount of h-BN added needs to be controlled within the range of 1-10%: too little (<1%) has no significant effect; too much (>10%) can cause h-BN to easily form a continuous network, which in turn hinders the direct bonding of AlN grains and reduces thermal conductivity. In this embodiment, a 5% addition is chosen, which represents the optimal balance between thermal conductivity and toughening effect.

[0059] Specifically, the preparation method of this embodiment includes the following steps: S1. Raw Material Pretreatment: Three layers of powder were prepared. Upper dense layer: 100g AlN powder, 2.0g YF3, 1.5g LaF3, 0.5g CeF3; Middle buffer layer: 100g AlN powder, 2.0g YF3, 1.5g LaF3, 1.0g CeF3, 1.5g NdF3, 5g h-BN, 2g PMMA; Lower dense layer: Same as the upper layer. Anhydrous ethanol (6g / 100g AlN) and polyvinyl alcohol (1.2g / 100g AlN) were added to each layer of powder, and the mixture was ball-milled for 15 hours using silicon nitride ceramic balls as the grinding medium (ball-to-powder ratio 10:1). The mixtures were then spray-dried separately at an inlet temperature of 190℃ and an outlet temperature of 90℃.

[0060] In this embodiment, both h-BN and PMMA microspheres were added to the intermediate buffer layer, which placed higher demands on the ball milling and spray drying processes. h-BN has a sheet-like structure, requiring careful control of the milling intensity during ball milling to avoid excessive breakage and damage to its sheet-like morphology. PMMA microspheres are organic polymers, and overheating during ball milling should be avoided to prevent premature decomposition. By optimizing the ball milling parameters (speed, time, and ball-to-material ratio) and spray drying temperature, the inventors successfully achieved uniform dispersion of the components, laying a solid foundation for subsequent sintering.

[0061] S2. Multi-layer molding: A step-by-step pressing process is adopted. First, a dense layer of powder is laid in the mold and pre-pressed (20MPa) to level it. Then, an intermediate buffer layer of powder is laid and pre-pressed again (20MPa). Finally, a dense layer of powder is laid on top. Then, cold isostatic pressing is performed as a whole pre-pressing process at a pressure of 70MPa for 30 minutes to obtain a green body with a density of 2.72g / cm³. 3 The thickness ratio of the three layers is upper dense layer: middle buffer layer: lower dense layer = 4:2:4, and the total thickness is designed according to the thickness of subsequent cutting.

[0062] The step-by-step pressing process is one of the key technologies of this invention. Its advantages are: it allows for precise control of the thickness and uniformity of each layer; intermediate pre-pressing can improve the interlayer bonding strength and avoid delamination during sintering; and it is suitable for mass production of multilayer composite structures. Through comparative experiments, the inventors found that the green body prepared by the step-by-step pressing process has an interlayer bonding strength that is about 30% higher than that prepared by one-step pressing, and the flexural strength of the substrate after sintering is about 15% higher.

[0063] S3. Hot pressing sintering: Place the green blank in a hot pressing sintering furnace, introduce high-purity nitrogen (purity ≥99.999%, furnace pressure 0.25MPa), raise the temperature to 600℃ at a rate of 5℃ / min and hold for 60 minutes to remove the grease; continue to raise the temperature to 1700℃, apply a pressure of 35MPa, and hold for 6 hours; then cool to below 200℃ at a rate of 2.5℃ / min and allow to cool naturally to room temperature.

[0064] The sintering process in this embodiment was carried out at 1700℃ and 35MPa, balancing densification effect and energy consumption control. During sintering, the PMMA microspheres in the intermediate buffer layer completely decomposed at 250-400℃ to form pores, h-BN maintained its lamellar morphology and was distributed at the AlN grain boundaries, and the fluoride additive formed a liquid phase above 1400℃ to promote densification. The synergistic effect of the components ultimately resulted in an AlN ceramic with a gradient structure and composite toughening effect.

[0065] S4. Diamond wire saw cutting: Electroplated diamond wire saw cutting is used, with a diamond mesh count of 250 mesh, a wire diameter of 0.15 mm, a wire speed of 30 m / s, a workpiece feed speed of 0.12 mm / min, a tension of 30 N, and extreme pressure emulsion spraying for cooling (pressure 0.4 MPa), cutting into 0.5 mm thin slices.

[0066] The cutting parameters in this embodiment are the same as in Embodiment 1. However, due to the introduction of h-BN and pores in the intermediate buffer layer, the stress distribution during the cutting process is more uniform, further reducing the risk of edge chipping. Experiments show that the cutting process in this embodiment is smoother, and the cut surface quality is better.

[0067] S5. Post-treatment: Polish with diamond polishing slurry until Ra≤0.08μm, ultrasonically clean with anhydrous ethanol for 15 minutes, and dry at 80℃.

[0068] In this embodiment, the performance of the prepared aluminum nitride substrate was tested, and the results are as follows: Figure 4 As shown: density 99.8%, porosity 0.20%, thermal conductivity 272 W / (m²). K); Surface roughness Ra=0.07μm, warpage 0.12%, chipping rate 1.5%; Bending strength 395MPa, no cracks after 1000 cycles of hot and cold, service life over 9000 hours.

[0069] The overall performance of this embodiment is the best among all embodiments, especially with the edge chipping rate reduced to an ultra-low level of 1.5% and the flexural strength reaching 395 MPa, fully verifying the superiority of the integrated design of "multilayer composite structure + fluoride gradient distribution + second phase reinforcement". This result shows that the technical solution of the present invention not only achieves high performance indicators, but also has excellent structural stability and processing adaptability.

[0070] Example 5 Please see Figure 4 This invention provides a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate. To achieve the above objectives, this invention employs the following technical solutions: This embodiment aims to verify the process stability and mass production capability of the technical solution of the present invention. Statistical results of the data were obtained through repeated experiments and compared with the prior art.

[0071] This embodiment uses the same raw material formulation and preparation process as Example 4, and conducts 10 batches of repeated experiments. Each batch prepares 100 aluminum nitride substrates with a thickness of 0.5 mm. The average value, standard deviation, and pass rate of various performance indicators are statistically analyzed. The experimental results are as follows: Thermal conductivity: average value is 268 W / (m) K), standard deviation 3.2 W / (m K), the batch-to-batch fluctuation is ≤±2%, indicating that the process of the present invention has good stability; Density: Average value 99.7%, standard deviation 0.1%, with minimal batch-to-batch variation; Edge chipping rate: average value 1.6%, standard deviation 0.3%, pass rate (edge ​​chipping rate ≤3%) 98.5%; Warpage: The average value was 0.14%, the standard deviation was 0.03%, and the pass rate (warpage ≤ 0.2%) reached 99.2%. Bending strength: The average value is 388MPa, the standard deviation is 12MPa, which meets the design requirement of ≥350MPa.

[0072] Please see Figure 4 The performance comparison between the embodiments of the present invention and the prior art is as follows: The thermal conductivity of embodiments 1-5 of the present invention is ≥250W / m. K, up to 278W / m K, compared to existing technologies (≤220W / m) K) is improved by more than 15%; density is ≥99.5%, significantly improved compared to existing technologies (≤98.5%); edge breakage rate is ≤2.8%, far lower than existing technologies (≥15%). The above data fully demonstrate that the present invention, through the integrated technical solution of "pure fluoride composite additive + precise hot pressing sintering + diamond wire saw cutting", especially the innovative design combining multilayer composite structure and fluoride gradient distribution, successfully solves the problem of edge breakage and warping of ultra-thin AlN substrates while maintaining high thermal conductivity, and achieves a comprehensive improvement in overall performance.

[0073] In conclusion, the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate, characterized in that, The aluminum nitride substrate has a multilayer composite structure; The multilayer composite structure includes an upper dense layer, an intermediate stress buffer layer, and a lower dense layer; The porosity of the upper and lower dense layers is ≤0.3%; The porosity of the intermediate stress buffer layer is 1%–5%; The material of the aluminum nitride substrate includes aluminum nitride powder and composite sintering aids; The aluminum nitride powder is α-AlN powder; The α-AlN powder has a purity ≥99.8%, an average particle size of 1-3 μm, and an oxygen impurity content ≤0.3%. The composite sintering aid is at least two of yttrium fluoride, lanthanum fluoride, cerium fluoride, and neodymium fluoride; The mass of the composite sintering aid is 2%–8% of the mass of the aluminum nitride powder; The intermediate stress buffer layer also includes a second phase material.

2. The high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate according to claim 1, characterized in that, The mass ratio of each component in the composite sintering aid is YF3:LaF3:CeF3:NdF3 = 1—3:1—2:0—2:0—1, and the content of at least two components is not zero.

3. The high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate according to claim 1, characterized in that, The content of composite sintering aids in the upper dense layer, the middle stress buffer layer and the lower dense layer are distributed in a gradient.

4. The high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate according to claim 1, characterized in that, The second phase material is selected from at least one of boron nitride, silicon carbide whiskers, graphene, and carbon nanotubes; The amount of the second phase material added is 1% to 10% of the mass of aluminum nitride powder.

5. A method for preparing a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate, characterized in that, Includes the following steps: S1. Aluminum nitride powder is mixed with composite sintering aid, and after ball milling, drying and granulation, a green body with a multi-layer composite structure is formed by multi-layer casting, gradient feeding or step-by-step pressing process. S2. The green body is placed in a hot pressing sintering furnace for hot pressing sintering, and the temperature and pressure are maintained for 4-8 hours to obtain a sintered body; S3. The sintered body is cut using a diamond wire saw to obtain an aluminum nitride substrate sheet with a thickness of 0.3mm-1.0mm.

6. The method for preparing a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate according to claim 5, characterized in that, The ball-to-material ratio of the ball mill is 8-12:1, and the ball milling time is 12-18 hours. The drying method is spray drying, with an inlet temperature of 180℃-200℃ and an outlet temperature of 85℃-95℃.

7. The method for preparing a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate according to claim 5, characterized in that, The specific method of hot pressing sintering is as follows: nitrogen gas is introduced into the hot pressing sintering furnace, the purity of the nitrogen gas is ≥99.9%, and the furnace pressure of the hot pressing sintering furnace is 0.2MPa-0.3MPa; The heating rate of the hot pressing sintering is 4℃ / min-6℃ / min, and when the temperature reaches 600℃, it is held for 50-70 minutes for degreasing treatment. The cooling rate of the hot pressing sintering is 2℃ / min to 3℃ / min, and it is allowed to cool naturally to room temperature after the temperature reaches below 200℃.

8. The method for preparing a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate according to claim 5, characterized in that, The diamond wire saw has a diamond mesh count of 200-300 and a wire diameter of 0.12mm-0.18mm.

9. The method for preparing a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate according to claim 5, characterized in that, The specific cutting process parameters are as follows: linear speed of 28m / s-32m / s, workpiece feed speed of 0.1mm / min-0.15mm / min, and tension of 25N-35N; Coolant is continuously sprayed during the cutting process. The coolant is an extreme pressure emulsion with a pressure of 0.3 MPa to 0.5 MPa.

10. The method for preparing a high-density, high-thermal-conductivity hot-pressed aluminum nitride substrate according to claim 5, characterized in that, The aluminum nitride substrate sheet also needs to be ground and polished to make its surface roughness Ra≤0.1μm, and then cleaned and dried.

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