High-residual-carbon low-pore anti-reverse-sublimation silicon carbide seed crystal composite bonding process

Abstract

The application discloses a high-residual-carbon low-pore anti-reverse-sublimation silicon carbide seed crystal composite bonding process, which adopts a gradient air-permeable graphite buffer sheet and a modified carbon-silicon composite bonding glue in combination, the bonding glue takes phenolic resin as a matrix, and introduces nano graphite powder and superfine 4H-SiC micro powder; precise layering glue coating is realized through ultrasonic atomization spraying, and then three-stage vacuum gradient temperature solidification carbonization process treatment is carried out, so that a dense C-SiC composite carbonized bonding layer is formed at 950 DEG C. The application realizes zero air bubble of the bonding interface, the residual carbon rate is above 92%, the dense carbonized layer can effectively isolate silicon vapor, the reverse sublimation of the seed crystal back is inhibited from the source, the seed crystal drop-off rate is controlled to be below 1%, the dislocation density is reduced by above 30%, the bonding yield is stably above 90%, and the application is completely suitable for the industrialized batch bonding demand of 6-inch and below SiC seed crystals.

Description

Technical Field

[0001] This invention belongs to the field of crystal growth, specifically relating to a composite bonding process for high residual carbon, low porosity, and anti-sublimation silicon carbide seed crystals. Background Technology

[0002] Silicon carbide (SiC), as a representative of third-generation semiconductor materials, exhibits irreplaceable application value in power electronic devices, high-frequency communications, and new energy vehicles due to its large bandgap, high breakdown electric field strength, excellent thermal conductivity, and high-temperature resistance. Currently, the physical vapor transport (PVT) method remains the mainstream technology for the industrial-scale growth of silicon carbide single crystals. This method requires firmly bonding a SiC seed crystal to the lower end of a graphite substrate under a high-temperature vacuum environment, and then using the seed crystal as the growth initiation interface to achieve continuous epitaxial growth of the crystal. In this process, the bonding quality between the seed crystal and the graphite substrate directly determines the stability of crystal growth, yield, and the final crystal quality of the substrate. Therefore, the seed crystal bonding process occupies a crucial and pivotal position in the entire SiC single crystal industry chain.

[0003] However, the technical defects exposed in the process practice of existing industrially used phenolic adhesives have become increasingly serious, becoming one of the core bottlenecks restricting the large-scale mass production of large-size SiC substrates. A thorough analysis of the technical causes reveals three main aspects: First, the problem of gas generation and bubbling in the adhesive at high temperatures. Phenolic resin-based adhesives contain a large amount of organic solvents and thermally degradable organic segments. During the curing and heating stage, small solvent molecules vaporize, and the resin undergoes thermal decomposition, continuously releasing gaseous byproducts such as hydrocarbons and CO. More importantly, the surface layer of the adhesive preferentially cross-links and cures to form a closed hard film, preventing internal gases from escaping and causing them to become trapped, forming bubbles and cavities. Under high-temperature crystal growth conditions above 2200℃, the stress at the bonding interface changes drastically, easily leading to bonding failure or even seed crystal detachment and scrapping. Currently, the industry's conventional bonding qualification rate can only be maintained at a low level of 40% to 55%. Existing patent improvement solutions mostly start from a single dimension, such as process temperature control, adding an exhaust buffer layer, or modifying the adhesive formula. Among them, patent CN114232104A adopts a spin coating adhesive control combined with a graphite paper interlayer segmented pre-baking process, relying on layered heating to remove small molecule solvents in advance to reduce bubble generation; patent CN115341285A adopts a full vacuum gradient heating curing equipment, which alleviates adhesive layer bubbling through multi-stage heating and step-by-step exhaust; and patent CN113683964A uses graphite powder doping to modify phenolic adhesive to reduce the resin ratio and reduce the source of cracking gas generation. However, the above solutions have significant technical shortcomings: the investment cost of vacuum segmented curing equipment is high and the production line is difficult to modify; the filler modification solution can only slightly reduce the total amount of gas produced, and cannot eliminate the problem of high-temperature cracking and gas production of resin from the root. Local air bubbles and voids still frequently occur when bonding large-size 6 / 8-inch seed crystals, and the bonding yield is difficult to consistently exceed 70%.

[0004] Second, there is the issue of sublimation loss on the back side of the seed crystal. In the high-temperature crystal growth environment of 2200℃, a large amount of SiC raw material inside the crucible decomposes to generate silicon vapor. When the back side of the seed crystal lacks a dense protective layer, the silicon vapor continuously corrodes and erodes the back side of the seed crystal, causing the seed crystal substrate to thin, the edges to crack, and the growth initiation interface to be distorted. This, in turn, induces serious quality problems such as polymorphic inclusions, vacancy defects, and a surge in microtube density. Currently, the mainstream protective patents are divided into two technical routes: back-side coating and back-side carbon coating. CN101985773B discloses coating the back side of the seed crystal with a high-carbon organic coating, which generates a graphite protective layer after high-temperature carbonization to block silicon vapor corrosion. CN118932481A and CN113502540A use magnetron sputtering to deposit carbon films or ceramic protective films, relying on dense inorganic films to achieve back-side sublimation protection. However, the aforementioned coating patents have significant drawbacks in industrial applications: magnetron sputtering coating is cumbersome, the equipment is expensive, and the coating time for a single seed crystal is long, significantly increasing mass production costs; organic-coated carbon films are affected by the uniformity of the adhesive coating, resulting in inconsistent film density, and the risk of sublimation corrosion still exists in areas where the coating is not applied, making it difficult to achieve large-scale widespread application. Another patent adopts a grooved gas guiding scheme on the seed crystal bonding surface, which indirectly reduces the local sublimation caused by cavities by etching guiding grooves on the back of the seed crystal to guide the adhesive layer gas. However, SiC seed crystals are expensive, and chipping and breakage are prone to occur during the grooving process, resulting in a high material loss rate. The processing steps also increase the production cycle, making mass production economics extremely poor. Currently, it is only in the small-batch trial stage in the laboratory and cannot be implemented in large-scale production lines.

[0005] Third, there are issues with low residual carbon content and thermal conductivity imbalance in the adhesive layer. Conventional phenolic organic adhesives, after carbonization at 2200℃, only achieve a residual carbon content of 65% to 78%. The carbonized adhesive layer is porous, with high porosity and poor thermal conductivity, resulting in uneven heat conduction between the seed crystal and the graphite support. This distorts the temperature field during crystal growth, concentrates thermal stress within the crystal, and ultimately leads to a high proportion of high dislocation density substrates in the finished product. Existing modified patents CN114106756A and CN117551406A use organosilicon blends and inorganic carbon powder to modify phenolic resin, which can slightly increase the residual carbon content of the adhesive to around 82%. However, due to the pyrolysis weight loss limit of the phenolic resin itself, a large number of micropores remain after high-temperature carbonization, limiting the improvement in thermal conductivity and failing to fundamentally solve the technical problem of interfacial thermal resistance imbalance.

[0006] In summary, existing technological improvement approaches can be broadly categorized into three main directions: seed crystal trenching for gas conduction, back-side coating for protection, and fine-tuning of adhesive formulations. However, trenching damages high-value seed crystals, coating is complex and costly, and formulation modification only achieves single-point optimization. None of these three conventional approaches can simultaneously address the three core requirements: low gas production and blister prevention, dense back-side layer to prevent sublimation, and high residual carbon and high thermal conductivity. They can only address specific defects, resulting in limited improvements in overall bonding yield and crystal quality, making it difficult to meet the urgent need for low-cost, large-scale mass production of large-size SiC substrates. Therefore, developing a novel integrated seed crystal bonding process that simultaneously overcomes the three major technical challenges of blistering and crystal detachment, back-side etching, and low residual carbon, while also considering bonding strength and mass production economics, has become a key challenge and a pressing technical problem for those skilled in the art. Summary of the Invention

[0007] The purpose of this invention is to provide a high-carbon-residual-low-porosity anti-sublimation silicon carbide seed crystal composite bonding process to solve the key technical defects in the prior art, such as high-temperature gas generation and bubbling of the bonding adhesive, sublimation loss on the back of the seed crystal, and low carbon content of the adhesive layer leading to thermal conductivity imbalance.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0009] Step 1: Raw material cleaning pretreatment

[0010] Before performing the bonding operation, the SiC seed crystal, graphite support, and gradient permeable graphite buffer sheet are first cleaned and pretreated to ensure that the bonding interface reaches the required surface condition.

[0011] For the bonding surface of SiC seed crystals, diamond grinding and polishing process is used for treatment, followed by ultrasonic cleaning treatment with a mixture of anhydrous ethanol and high-purity acetone for 15 to 25 minutes. After cleaning, high-purity nitrogen is used for drying treatment to control the surface roughness Ra value of the seed crystal bonding surface to below 0.8 μm.

[0012] For graphite holders and gradient permeable graphite buffer sheets, a high-temperature vacuum calcination process was used for purification. The calcination temperature was 1100℃, and the calcination time was 2 hours to thoroughly remove ash and impurity gases from the pores of the graphite material. After calcination, the material was allowed to cool naturally to room temperature, wiped with alcohol, and then dried.

[0013] The gradient permeable graphite buffer sheet features a specific microstructure design: the buffer sheet thickness is 0.2 to 0.4 mm, and the interior of the sheet has a honeycomb-shaped arrangement of gradient air-guiding holes. The hole diameter on the bonding side is 0.1 mm, while on the back side it is enlarged to 0.3 mm, forming a continuous air-guiding channel. When multiple layers of graphite buffer sheets are stacked, the air-guiding holes are arranged in a staggered manner, ensuring effective gas drainage while maintaining the flatness of the overall bonding surface.

[0014] There is a correlation between the thickness of the buffer sheet and the density of the air vents: when the thickness of the buffer sheet is 0.2 mm, the preferred density is 15 to 18 vents / cm². 2 To avoid a decrease in the structural strength of the sheet due to excessive openings; when the thickness of the buffer sheet is 0.3 mm, the preferred arrangement density is 20 holes / cm². 2 To achieve the optimal balance between exhaust efficiency and structural strength; when the buffer sheet thickness is 0.4 mm, the preferred arrangement density is 22 to 25 pieces / cm². 2 This design fully utilizes the thickness advantage to increase the exhaust channel capacity. This matching relationship ensures that the buffer sheet does not collapse or deform locally during the stacking and pre-compression process, while also ensuring the unobstructed gas outlet channel.

[0015] Step 2: Preparation of modified silicon carbide composite adhesive

[0016] This invention uses a modified silicon carbide composite adhesive as the bonding medium, which is composed of the following raw materials in equal parts by weight:

[0017] The mixture comprises 60 to 70 parts of phenolic resin matrix, 18 to 22 parts of anhydrous ethanol diluent, 5 to 8 parts of nano-graphite powder, 3 to 5 parts of ultrafine 4H-SiC micro powder, 1 to 2 parts of aminosilane coupling agent, and 0.5 to 1 part of carbonization accelerator.

[0018] The particle size range of the nano-graphite powder is 50 to 200 nm, and the particle size range of the ultrafine 4H-SiC micro powder is 1 to 3 μm.

[0019] Place all the above components in a mixing container and stir at room temperature for 30 minutes to ensure thorough and uniform dispersion. After mixing, control the viscosity of the adhesive solution within the range of 3500 to 5500 mPa·s, then allow it to stand to defoam before use.

[0020] In this step, the introduction of nano-graphite powder and ultrafine SiC powder into the phenolic resin matrix significantly improved the residual carbon performance of the adhesive during the high-temperature carbonization process; the addition of aminosilane coupling agent effectively improved the interfacial bonding force between the organic colloid and inorganic SiC particles and graphite powder, and reduced the generation of interfacial micro-gaps; the introduction of carbonization accelerator promoted the densification transformation of the organic matrix during the high-temperature carbonization stage.

[0021] Its mechanism of action is as follows: Nano-graphite powder has a high specific surface area and excellent thermal stability. During the carbonization process, it acts as a "crystallization nucleus" to induce the amorphous carbon generated by the cracking of phenolic resin to transform into an ordered graphite microcrystalline structure, thereby significantly improving the residual carbon rate and improving thermal conductivity. Ultrafine 4H-SiC micro powder undergoes an interfacial solid-phase reaction with the active carbon atoms generated by the cracking of resin at a carbonization temperature of 950℃ to generate an in-situ C-SiC composite reinforcing phase. This reinforcing phase not only fills the micropores generated by carbonization shrinkage, but also constructs a continuous thermally conductive network pathway. The aminosilane coupling agent chemically bonds with the hydroxyl groups on the surface of SiC micropowder through its alkoxy groups, while its amino end groups undergo a condensation reaction with the hydroxymethyl groups of the phenolic resin, forming a chemical bridge at the organic-inorganic interface, effectively inhibiting interfacial debonding and microcrack propagation. The ammonium dihydrogen phosphate in the carbonization accelerator decomposes during heating, releasing acidic substances that catalyze the cross-linking and curing reaction of the phenolic resin, promoting the rapid formation of the three-dimensional network structure. Meanwhile, the short carbon fiber filaments form a bridging carbon skeleton after carbonization, further enhancing the structural integrity of the carbonized layer. This synergistic effect of the multiple components results in a C-SiC composite carbonized adhesive layer that possesses high residual carbon content, low porosity, and excellent thermal conductivity.

[0022] Step 3: Precise application of adhesive in layers and assembly.

[0023] This step includes the following operations:

[0024] The first step is ultrasonic atomization spraying. The ultrasonic atomization spraying system uses ultrasonic vibration to atomize the adhesive into tiny droplets, which are then sprayed onto the surfaces to be bonded. Compared to traditional manual adhesive application, ultrasonic atomization spraying significantly improves the uniformity and consistency of the adhesive layer thickness.

[0025] The selection criteria for the ultrasonic frequency of 1.7 to 2.0 MHz and the atomization power of 50 to 80 W in the ultrasonic atomization spraying system are as follows: When the ultrasonic frequency is below 1.7 MHz, the median droplet size is greater than 50 μm, resulting in a significant decrease in coating uniformity and a thickness deviation of more than ±5 μm. When the ultrasonic frequency is above 2.0 MHz, the droplet size is too small (median less than 15 μm), resulting in insufficient atomization flux and reduced film formation efficiency in a single spray, requiring multiple repetitive sprayings to achieve the target wet adhesive thickness, thus affecting production efficiency. Similarly, when the atomization power is below 50 W, the ultrasonic energy is insufficient to fully atomize the high-viscosity adhesive (3500 to 5500 mPa·s), leading to intermittent flow interruptions or large droplet splashing during spraying. When the atomization power is above 80 W, the ultrasonic cavitation effect is too strong, causing the nano-graphite powder and SiC micro-powder in the adhesive to agglomerate and settle due to violent vibration, disrupting the uniform dispersion of the adhesive. Therefore, the present invention limits the ultrasonic frequency to 1.7 to 2.0 MHz and the atomization power to 50 to 80 W. Within this preferred range, a stable atomization effect with a median droplet size of 20 to 30 μm can be obtained, while ensuring the dispersion stability of each component of the adhesive.

[0026] The specific spraying operation is as follows: uniformly spray the first layer of adhesive on the bonding surface of the SiC seed crystal, and control the wet adhesive thickness in the range of 12 to 18 μm; coat both sides of the graphite buffer sheet with the same adhesive formula; spray the second layer of adhesive on the bonding surface of the graphite holder, and control the adhesive thickness in the range of 15 to 20 μm.

[0027] The second step is the stacking and assembly operation. The following components are stacked sequentially from top to bottom: SiC seed crystal, gradient permeable graphite buffer sheet, and graphite support. After stacking, a flat graphite counterweight is placed on the upper surface of the seed crystal. The counterweight pressure is controlled within the range of 0.12 to 0.2 MPa, and the pre-compression time is 5 minutes. This pre-compression operation can initially expel excess adhesive and free air bubbles.

[0028] Step 4: Three-stage vacuum gradient temperature rise curing and carbonization

[0029] The assembled bonded components are placed into a vacuum sintering furnace, where the vacuum level is controlled below 5 Pa. A three-stage vacuum gradient temperature rise curing and carbonization process is then initiated.

[0030] The first stage is the low-temperature pre-baking section. The heating program involves heating from room temperature to 120°C at a heating rate of 2°C / min, and holding at 120°C for 3 hours. This stage is mainly used to remove small molecule volatiles such as ethanol solvent from the adhesive solution. These small molecule volatiles are rapidly discharged into the furnace vacuum system through the gradient air guide holes of the graphite buffer sheet.

[0031] The second stage is the medium-temperature cross-linking and curing stage. The heating program involves raising the temperature from 120℃ to 450℃ at a rate of 1.5℃ / min, and then holding at 450℃ for 4 hours. During this stage, the resin undergoes a cross-linking and curing reaction, and most of the organic pyrolysis gas products are discharged through the gradient venting holes. Due to the gradually changing pore size design of the venting holes (0.1mm on the adhesive side to 0.3mm on the back side), the gas discharge channels are smooth, effectively preventing the occurrence of adhesive layer bubbling.

[0032] The third stage is the high-temperature densification carbonization stage. The heating program involves heating from 450℃ to 950℃ at a rate of 1℃ / min, and holding at 950℃ for 5 hours. During this stage, the organic matrix is ​​fully carbonized, and the adhesive layer is transformed into a dense C-SiC composite carbonized adhesive layer. After carbonization, the layer is naturally cooled to room temperature in the furnace, thus ending the curing carbonization process.

[0033] The determination of the vacuum degree control limit of 5 Pa in the aforementioned vacuum sintering furnace is based on the following: when the vacuum degree in the furnace is higher than 5 Pa (i.e., the absolute pressure is greater than 5 Pa), the density of residual gas molecules in the furnace cavity increases. On the one hand, this hinders the diffusion and discharge of volatile gases from the adhesive layer along the gradient gas guide holes, resulting in the residual bubble rate at the bonding interface rising to over 2%. On the other hand, residual oxygen at high temperatures oxidizes the graphite buffer sheet and the carbonization products of the adhesive layer, reducing the residual carbon rate. Embodiment 1 of this invention achieved zero bubbles and a residual carbon rate of 92.3% under a vacuum degree of 3 Pa. However, preliminary experiments show that when the vacuum degree rises to 8 Pa, the bubble rate increases to 3%–5%, and the residual carbon rate decreases to below 88%. Therefore, controlling the vacuum degree below 5 Pa is a key process limit to ensure smooth gas discharge and the density of the carbonized layer.

[0034] Step 5: Post-processing quality inspection

[0035] After the temperature inside the vacuum sintering furnace drops to room temperature, the graphite counterweight is removed. A comprehensive inspection of the bonding interface is conducted using a combination of visual inspection and optical microscopy to confirm the absence of defects such as bubbles and cracks. Bonded parts that pass inspection are then ready for use in the PVT crystal growth furnace.

[0036] As a preferred embodiment of the present invention, based on the above technical solution, the following parameters are further defined:

[0037] In the adhesive formulation, phenolic resin is preferably 65 parts, anhydrous ethanol is preferably 20 parts, nano-graphite powder is preferably 6 parts, SiC micro powder is preferably 4 parts, silane coupling agent is preferably 1.5 parts, and carbonization accelerator is preferably 0.8 parts.

[0038] In the geometric parameters of the graphite buffer sheet, the thickness is preferably 0.3 mm, the air vents are arranged in a honeycomb array, and the pore diameter gradient is preferably 0.1 mm on the bonding surface to 0.3 mm on the back surface.

[0039] In the process parameters of the pre-compression operation, the counterweight pressure is preferably 0.15 MPa.

[0040] In the parameter configuration of the three-stage heat preservation time, the heat preservation time of the low-temperature pre-baking stage is preferably 3 hours, the heat preservation time of the medium-temperature cross-linking and curing stage is preferably 4 hours, and the heat preservation time of the high-temperature densification and carbonization stage is preferably 5 hours.

[0041] The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process provided by this invention effectively solves the three major defects existing in the prior art through the synergistic effect of gradient permeable graphite buffer sheet and segmented vacuum heating curing carbonization process.

[0042] In terms of low porosity and high bonding reliability, the gradient permeable graphite sandwich structure provides a dedicated channel for gas extraction. The segmented vacuum heating process allows for complete gas extraction from the adhesive layer, achieving zero bubbles at the bonding interface. Under crystal growth conditions of 2500℃, the seed crystal detachment rate is controlled below 1%, significantly better than the 12% to 20% seed crystal detachment rate of traditional processes.

[0043] In terms of high residual carbon and anti-sublimation, the residual carbon rate of the modified composite adhesive after high-temperature carbonization reaches more than 92%, forming a dense C-SiC composite carbonized adhesive layer. This dense carbonized layer can effectively isolate silicon vapor and suppress the anti-sublimation phenomenon on the back of the seed crystal from the source, reducing the seed crystal loss by 90%.

[0044] In terms of improving crystal quality, the dense carbide layer has uniform thermal conductivity, significantly reduces interfacial thermal resistance fluctuations, greatly reduces thermal stress during crystal growth, reduces dislocation density of finished SiC single crystals by more than 30%, and significantly reduces polymorphic defects.

[0045] In terms of mass production advantages, this invention eliminates the need for grooving the seed crystal and back coating process. It uses ultrasonic spraying to achieve automated bonding, doubling the bonding efficiency per batch and maintaining a bonding yield of over 90%. It is fully compatible with the industrial batch bonding needs of 6-inch and smaller SiC seed crystals.

[0046] This invention, through a systematic design of five major steps—raw material pretreatment, composite adhesive preparation, interlayer pretreatment, layered adhesive bonding, and segmented vacuum gradient thermal curing and carbonization—organically integrates three core functions—low gas production and anti-bubbling, dense back layer and anti-sublimation, and high residual carbon and high thermal conductivity—into a single process flow. This achieves a holistic breakthrough in seed crystal bonding technology and provides reliable technical support for the low-cost, large-scale mass production of large-size SiC substrates. Attached Figure Description

[0047] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0048] Figure 2This is an enlarged schematic diagram of the gradient permeable graphite buffer sheet in this invention;

[0049] Figure 3 This is a schematic diagram of the three-stage vacuum gradient heating curing and carbonization process of the present invention;

[0050] Figure 4 This is a schematic diagram of the operation of the ultrasonic atomization spraying system in this invention.

[0051] The attached figures are labeled as follows: 1. SiC seed crystal; 2. Gradient permeable graphite buffer sheet; 3. Graphite support; 4. Adhesive layer; 5. Graphite counterweight; 6. Vacuum sintering furnace; 7. Gradient air guide hole; 8. Ultrasonic atomization spraying system. Detailed Implementation

[0052] This invention relates to the field of bonding processes between silicon carbide seed crystals and graphite substrates. Specifically, the high-residual-carbon, low-porosity, anti-sublimation silicon carbide seed crystal composite bonding process provided by this invention includes five core steps: raw material cleaning pretreatment, modified silicon carbide composite adhesive preparation, layered precise adhesive application and stacking assembly, three-stage vacuum gradient temperature curing and carbonization, and post-processing quality inspection. The process is described in detail below with reference to specific embodiments.

[0053] Figure 3 As shown, in step one, the SiC seed crystal 1, graphite support 3, and gradient permeable graphite buffer sheet 2 first need to undergo a pre-cleaning process to ensure that the bonding interface reaches the required surface condition. For the bonding surface of the SiC seed crystal, a diamond grinding and polishing process is used. This polishing process uses diamond grinding paste with a particle size of 1μm to 3μm. During the polishing process, the polishing pressure is controlled at 15 to 25N, the polishing disc speed is 150 to 200rpm, and the polishing time is 8 to 12 minutes. After polishing, the surface roughness Ra value of the seed crystal bonding surface should be controlled below 0.8μm, and the surface flatness should be better than 0.5μm. The polished seed crystal is then subjected to ultrasonic cleaning. The cleaning solution is a mixture of anhydrous ethanol and high-purity acetone, wherein the volume ratio of anhydrous ethanol to high-purity acetone is 3:7 to 5:5. The ultrasonic cleaning is performed in an ultrasonic cleaner with a power of 500 to 800W for 15 to 25 minutes, preferably 20 minutes. During the cleaning process, the temperature of the cleaning solution is controlled at 25 to 35°C to ensure optimal cleaning results. After cleaning, high-purity nitrogen is used for drying, with the nitrogen purity preferably above 99.999%. The drying pressure is controlled at 0.2 to 0.4 MPa, and the drying time is 30 to 60 seconds to ensure that the seed crystal bonding surface is completely dry and free of cleaning solution residue.

[0054] The graphite support 3 and the gradient permeable graphite buffer sheet 2 were purified by high-temperature vacuum calcination. The calcination process was carried out in a vacuum resistance furnace, with the vacuum level controlled below 10 Pa. The heating program was to heat from room temperature to 1100℃ at a rate of 5℃ / min, and hold at 1100℃ for 2 hours. After holding, the heating power was turned off, and the material was allowed to cool naturally to room temperature. The vacuum was maintained during cooling until the furnace temperature dropped below 100℃ before the material was removed. The calcined graphite support 3 and graphite buffer sheet were wiped with anhydrous ethanol to remove residual particulate impurities from the surface. After wiping, they were dried in an oven at 120℃ for 1 to 2 hours to ensure the graphite material was thoroughly dried.

[0055] Figure 2 As shown, the gradient permeable graphite buffer sheet 2 of this invention has a specific microstructure design, which plays a key role in the bonding process as a gas outlet channel. The thickness of the buffer sheet is 0.2 to 0.4 mm, preferably 0.3 mm. The sheet body has a honeycomb-shaped arrangement of gradient air-guiding holes 7. The diameter of the holes on the bonding side is 0.1 mm, and the diameter is enlarged to 0.3 mm on the back side, forming a continuous air-guiding channel. The density of the air-guiding holes is 15 to 25 per square centimeter, preferably 20. The air-guiding holes are arranged in regular hexagonal honeycomb units, and the center-to-center distance between adjacent air-guiding holes is 2.0 to 2.5 mm. When multiple layers of graphite buffer sheets are stacked, the air-guiding holes are arranged in a staggered manner, that is, there is a lateral offset of 1.0 to 1.2 mm between the center of the air-guiding holes of the upper buffer sheet and the center of the air-guiding holes of the lower buffer sheet. This staggered arrangement method can ensure the effective exhaust of gas and maintain the flatness of the overall bonding surface, so that the flatness deviation of the stacked buffer sheet group in the thickness direction is controlled within ±0.02mm.

[0056] In step two, a modified carbon-silicon composite adhesive needs to be prepared as an adhesive medium. This adhesive consists of the following raw materials in equal parts by weight: 60 to 70 parts phenolic resin matrix, 18 to 22 parts anhydrous ethanol diluent, 5 to 8 parts nano-graphite powder, 3 to 5 parts ultrafine 4H-SiC micro powder, 1 to 2 parts aminosilane coupling agent, and 0.5 to 1 part carbonization accelerator.

[0057] The phenolic resin matrix is ​​a thermosetting phenolic resin with a free phenol content of less than 5% and a softening point of 80 to 100°C, preferably 85°C. This phenolic resin exhibits good carbon residue properties during high-temperature carbonization, with a theoretical carbon residue rate of approximately 55% to 60%.

[0058] The nano-graphite powder has a particle size range of 50 to 200 nm, a specific surface area of ​​20 to 50 m² / g, and a carbon content greater than 99.9%. The addition of nano-graphite powder can significantly improve the thermal conductivity of the adhesive during the high-temperature carbonization process. At the same time, it can form a good carbon network structure with the phenolic resin matrix during the carbonization process, thereby improving the overall residual carbon rate.

[0059] The ultrafine 4H-SiC powder has a particle size range of 1 to 3 μm, a crystal form of 4H-SiC, and a specific gravity of 3.21 g / cm³. 3 The addition of this ultrafine SiC powder can react with the carbon produced by the cracking of phenolic resin during the carbonization process to generate a SiC reinforcing phase, forming a C-SiC composite carbonized layer, which significantly improves the wear resistance and high-temperature stability of the adhesive layer.

[0060] The aminosilane coupling agent is γ-aminopropyltriethoxysilane, with the molecular formula NH2CH2CH2CH2Si(OC2H5)3. This coupling agent can effectively enhance the interfacial bonding force between organic colloids and inorganic SiC particles and graphite powder, reducing the formation of interfacial micro-gaps. Before use, the coupling agent is pre-hydrolyzed with anhydrous ethanol at a weight ratio of 1:9 for 30 to 60 minutes.

[0061] The carbonization accelerator is a mixture of chopped carbon fiber filaments and ammonium dihydrogen phosphate, wherein the chopped carbon fiber filaments have a length of 20 to 50 μm and a diameter of 5 to 10 μm, and the ammonium dihydrogen phosphate has a particle size of 50 to 100 μm. The introduction of this carbonization accelerator can promote the densification transformation of the organic matrix during the high-temperature carbonization stage, reduce the generation of pores during carbonization, and improve the density of the carbonized layer.

[0062] Place all the above components in a mixing container and mix them at room temperature. A vacuum degassing mixer is used, with a stirring speed of 300 to 500 rpm and a stirring time of 30 minutes. During stirring, maintain the vacuum level inside the container below -0.08 MPa to remove air bubbles introduced during mixing. After mixing, control the viscosity of the adhesive solution within the range of 3500 to 5500 mPa·s, preferably 4500 mPa·s. Viscosity is measured using a rotational viscometer at 25°C. After mixing, allow the adhesive solution to stand for 30 minutes to defoam, maintaining an ambient temperature of 25 ± 2°C and a relative humidity below 60% during the defoaming process.

[0063] In step three, precise layering and overlapping assembly are performed. This step includes two parts: ultrasonic atomization spraying and overlapping assembly.

[0064] In the ultrasonic atomization spraying process, the adhesive is atomized into tiny droplets using the principle of ultrasonic vibration for spraying. The ultrasonic atomization spraying system consists of an ultrasonic atomizer, an adhesive supply system, a spray gun head, and a motion control system. The ultrasonic atomizer has a frequency of 1.7 to 2.0 MHz and an atomization power of 50 to 80 W. The adhesive supply system uses a precision metering pump to control the adhesive flow rate, which is controlled at 0.5 to 2.0 mL / min. The distance between the spray gun head and the surface to be sprayed is controlled at 80 to 120 mm, and the spray width is 20 to 30 mm. The motion control system enables the spray gun head to reciprocate horizontally at a speed of 50 to 100 mm / s.

[0065] Figure 4 As shown, the specific spraying operation is as follows: A first layer of adhesive is uniformly sprayed onto the bonding surface of the SiC seed crystal, with the wet adhesive thickness controlled within the range of 12 to 18 μm, preferably 15 μm. During the spraying process, a laser thickness gauge is used to monitor the adhesive layer thickness in real time to ensure thickness uniformity is better than ±2 μm. The same adhesive formulation is then applied to both sides of the graphite buffer sheet, with the wet adhesive thickness also controlled within the range of 12 to 18 μm. A second layer of adhesive is sprayed onto the bonding surface of the graphite support 3, with the coating thickness controlled within the range of 15 to 20 μm, preferably 18 μm. After spraying, the surface is left to stand in a clean environment for 5 to 10 minutes to allow the solvent in the adhesive to initially evaporate, forming a preliminary adhesive layer.

[0066] Figure 1 As shown, in the stacking assembly operation, the following components are stacked sequentially from top to bottom: SiC seed crystal 1, gradient permeable graphite buffer sheet 2, and graphite support 3. During the stacking process, the coated graphite buffer sheet is first precisely placed on the bonding surface of the SiC seed crystal, ensuring complete adhesion between the buffer sheet and the seed crystal bonding surface without misalignment or air bubbles. Subsequently, the coated graphite support 3 is placed on the other side of the graphite buffer sheet, again ensuring a smooth and even fit. After stacking, a flat graphite counterweight 5 is placed on the upper surface of the seed crystal, with the counterweight pressure controlled within the range of 0.12 to 0.2 MPa, preferably 0.15 MPa. The pre-compression time is 5 minutes. This pre-compression operation can initially squeeze out excess adhesive and free air bubbles, ensuring a tight bond at the bonding interface. During the pre-compression process, a pressure sensor is used to monitor pressure changes in real time to ensure that the pressure remains stable within the set range.

[0067] In step four, the assembled bonded components are placed into a vacuum sintering furnace 6, with the vacuum level controlled below 5 Pa. A three-stage vacuum gradient heating and curing carbonization process is then initiated.

[0068] The first stage is a low-temperature pre-baking section. The heating program involves heating from room temperature to 120°C at a rate of 2°C / min, and holding at 120°C for 3 hours. This stage is mainly used to remove small molecule volatiles such as ethanol solvent from the adhesive. During the heating process, the boiling point of ethanol solvent is 78.4°C, and it begins to evaporate in large quantities above 80°C. The gaseous products generated by evaporation are rapidly discharged into the furnace vacuum system through the gradient vent holes 7 of the graphite buffer sheet. Due to the pore size design of the gradient vent holes 7 (0.1mm on the bonding surface to 0.3mm on the back surface), the gas outlet channel is smooth, effectively avoiding gas retention in the adhesive layer.

[0069] The second stage is the medium-temperature cross-linking and curing stage. The heating program involves raising the temperature from 120℃ to 450℃ at a rate of 1.5℃ / min, and holding at 450℃ for 4 hours. During this stage, the phenolic resin undergoes a cross-linking and curing reaction, forming a three-dimensional network structure. Within the temperature range of 280 to 350℃, the phenolic resin undergoes a pyrolysis reaction, producing a large amount of organic decomposition gases, mainly including water vapor, formaldehyde, and phenol. These gaseous products are also discharged through the gradient vent holes 7. Due to the gradually changing pore size design of the vent holes, the gas discharge channels are smooth, effectively preventing the occurrence of bubbling in the adhesive layer. In this stage, the weight loss rate of the adhesive layer 4 is approximately 40% to 45%.

[0070] The third stage is the high-temperature densification carbonization stage. The heating program involves heating from 450℃ to 950℃ at a rate of 1℃ / min, and holding at 950℃ for 5 hours. During this stage, the organic matrix is ​​fully carbonized, and the adhesive layer transforms into a dense C-SiC composite carbonized adhesive layer. Within the temperature range of 700 to 950℃, the pyrolysis products of the phenolic resin undergo a carbonization reaction, forming a carbonaceous structure. Simultaneously, nano-graphite powder and ultrafine SiC powder undergo interfacial bonding with the carbon matrix, forming a reinforcing phase. In this stage, the weight loss of adhesive layer 4 is approximately 8% to 12%. After carbonization, the adhesive layer is naturally cooled to room temperature in the furnace at a cooling rate of 3 to 5℃ / min. After the carbonization process is completed, the adhesive layer thickness of the bonded component is approximately 50% to 60% of the initial thickness, forming a dense carbonized adhesive layer.

[0071] In step five, after the temperature inside the vacuum sintering furnace 6 has dropped to room temperature, the graphite counterweight 5 is removed. A comprehensive inspection of the bonding interface is conducted using a combination of visual inspection and optical microscopy. Visual inspection primarily checks for abnormal discoloration, cracks, or peeling on the surface of the bonded parts. Optical microscopy, using a magnification of 50 to 200x, examines the distribution of bubbles at the bonding interface, confirming the absence of bubbles, cracks, or other defects. Bonded parts that pass inspection can then be used in the PVT crystal growth furnace.

[0072] As a preferred embodiment of the present invention, based on the above technical solution, the following parameters are further defined. In the adhesive formulation, phenolic resin is preferably 65 parts, anhydrous ethanol is preferably 20 parts, nano-graphite powder is preferably 6 parts, SiC micro powder is preferably 4 parts, silane coupling agent is preferably 1.5 parts, and carbonization accelerator is preferably 0.8 parts.

[0073] In the geometric parameters of the graphite buffer sheet, the thickness is preferably 0.3 mm, the air vents are arranged in a honeycomb array, and the pore diameter gradient is preferably 0.1 mm on the bonding surface to 0.3 mm on the back surface.

[0074] In the process parameters of the pre-compression operation, the counterweight pressure is preferably 0.15 MPa.

[0075] In the parameter configuration of the three-stage heat preservation time, the heat preservation time of the low-temperature pre-baking stage is preferably 3 hours, the heat preservation time of the medium-temperature cross-linking and curing stage is preferably 4 hours, and the heat preservation time of the high-temperature densification and carbonization stage is preferably 5 hours.

[0076] The technical effects of the present invention will be further illustrated below through specific embodiments and comparative examples.

[0077] Example 1

[0078] The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process provided by this invention is used for seed crystal bonding. The raw materials selected are 6-inch 4H-SiC seed crystals with a thickness of 500 μm, a 6-inch circular graphite support 3 with a thickness of 10 mm, and a gradient permeable graphite buffer sheet 2 with a thickness of 0.3 mm. Bonding operations are performed according to the aforementioned process steps. The adhesive formulation uses an optimized ratio: 65 parts phenolic resin, 20 parts anhydrous ethanol, 6 parts nano-graphite powder, 4 parts SiC micro powder, 1.5 parts silane coupling agent, and 0.8 parts carbonization accelerator.

[0079] After bonding, a curing and carbonization treatment is performed, with the vacuum degree controlled at 3Pa. The first stage involves heating to 120℃ at 2℃ / min and holding for 3 hours; the second stage involves heating to 450℃ at 1.5℃ / min and holding for 4 hours; and the third stage involves heating to 950℃ at 1℃ / min and holding for 5 hours, followed by natural cooling to room temperature.

[0080] The bonded components were inspected, and the results are as follows: The bond interface, observed under a 200x optical microscope, showed no bubbles and excellent interface density. The bond layer, when sectioned, was found to be 55% of the initial adhesive layer thickness, with a residual carbon rate of 92.3%. The bonded components were then subjected to crystal growth experiments in a 2500℃ PVT crystal growth apparatus for 100 hours. After crystal growth, the back side of the seed crystal was inspected, revealing a seed crystal loss thickness of 15 μm, indicating significant anti-sublimation suppression. The dislocation density decreased from the original 5 × 10⁻⁶. 4 / cm 2Decreased to 3.4 × 10 4 / cm 2 The dislocation density decreased by 32%. The bonding yield reached 92%, and the seed crystal detachment rate was 0.8%.

[0081] Example 2

[0082] The process of this invention was used to bond 8-inch SiC seed crystals, with other conditions the same as in Example 1. The bonded parts were inspected and found to have no bubbles at the bonding interface, with a residual carbon rate of 91.8%. After the crystal growth test, the seed crystal loss thickness was 18 μm. The dislocation density decreased from the original 6 × 10⁻⁶. 4 / cm 2 Decreased to 4.3×10 4 / cm 2 The dislocation density decreased by 28%. The bonding yield reached 89%, and the seed crystal detachment rate was 1.2%.

[0083] Example 3

[0084] The process of this invention was used to bond 4-inch SiC seed crystals, verifying the applicability of the process to seed crystals of different sizes. The graphite support 3 was a 4-inch circular piece with a thickness of 8 mm, and other conditions were the same as in Example 1. The bonded parts were tested, and the bonding interface showed no bubbles, with a residual carbon rate of 93.1%. After the crystal growth test, the seed crystal loss thickness was 12 μm. The dislocation density decreased from the original 4 × 10⁻⁶. 4 / cm² decreased to 2.6×10 4 / cm², dislocation density decreased by 35%. Bonding yield reached 94%, and seed crystal detachment rate was 0.6%.

[0085] Example 4

[0086] The process of this invention was used to bond 6-inch SiC seed crystals, verifying the effect of changing the carbonization temperature on bonding performance. In the third stage, the maximum carbonization temperature was increased to 1000℃, and the holding time was maintained for 5 hours. The bonded parts were tested, and the residual carbon rate was 94.2%, slightly improved compared to Example 1. After the crystal growth test, the seed crystal loss thickness was 13μm, and the dislocation density decreased by 33%. The bonding yield reached 91%, and the seed crystal detachment rate was 0.9%.

[0087] Example 5

[0088] The process of this invention was used to bond 6-inch SiC seed crystals, verifying the effect of a double-layer graphite buffer sheet on bonding performance. Two 0.3mm thick graphite buffer sheets were stacked, with the vent holes arranged in a staggered pattern. The bonded parts were tested, and the residual carbon rate was 92.8%. After crystal growth testing, the seed crystal loss thickness was 14μm, and the dislocation density decreased by 31%. The bonding yield reached 93%, and the seed crystal detachment rate was 0.7%.

[0089] Example 6

[0090] To verify the batch stability and repeatability of the process of this invention, ten consecutive batches of 6-inch SiC seed bonding tests were conducted under the same process parameters. Each batch bonded 50 seed crystals, totaling 500 crystals. Statistical results showed: average residual carbon rate of 92.1% ± 0.8%, average seed crystal loss thickness of 16.2 μm ± 2.1 μm, average dislocation density reduction of 30.5% ± 2.8%, average bonding yield of 90.8% ± 1.5%, and average seed crystal detachment rate of 0.95% ± 0.22%. These data indicate that the process of this invention has excellent batch stability and repeatability, and all performance indicators meet the requirements of industrial mass production.

[0091] The experimental results of Examples 1 to 6 above show a clear process-performance correlation, which is analyzed as follows:

[0092] (I) The Influence of Seed Size on Process Adaptability. A comparison of Examples 1 (6 inches), 2 (8 inches), and 3 (4 inches) shows that as the seed size increases from 4 inches to 8 inches, the bonding yield decreases from 94% to 89%, the seed detachment rate increases from 0.6% to 1.2%, and the dislocation density decreases from 35% to 28%. The reason for this is that during the lamination and pre-pressing process with larger seed sizes, the cumulative effect of flatness deviation on the bonding surface is more significant, leading to increased unevenness in the local adhesive layer thickness, which in turn affects the gas extraction efficiency and the density and uniformity of the carbonized layer. However, even for 8-inch seed sizes, the bonding yield of the process of this invention still reaches 89%, significantly better than the 65%–72% of the traditional process, indicating that the process of this invention has excellent adaptability to 6-inch and smaller seed sizes and also has good potential for application to 8-inch seed sizes.

[0093] (II) Effect of carbonization temperature on residual carbon content. In Example 4, the maximum carbonization temperature was increased to 1000℃, and the residual carbon content increased by 1.9 percentage points compared to Example 1 (from 92.3% to 94.2%), while the seed crystal loss thickness decreased by 2μm (from 15μm to 13μm). This indicates that appropriately increasing the carbonization temperature helps the organic matrix to undergo more complete graphitization transformation, but excessively high temperatures may lead to increased brittleness of the carbonized layer. Therefore, the third-stage temperature is preferably limited to 950℃ in this invention to achieve a balance between residual carbon content and interlayer bonding strength.

[0094] (III) The Influence of the Number of Buffer Layers on Bonding Reliability. Example 5 uses a double-layer buffer layer with a staggered arrangement. Compared to the single-layer buffer layer in Example 1, the residual carbon rate (92.8% vs 92.3%) and seed crystal shedding rate (0.7% vs 0.8%) are slightly improved, but the improvement is limited. Analysis suggests that the double-layer structure provides a more circuitous gas exit path, which is beneficial for the gradual release of volatiles. However, too many layers increase the difficulty of alignment and interfacial contact thermal resistance. Therefore, this invention prefers a single-layer buffer layer scheme, achieving the optimal configuration between exhaust efficiency and process simplicity.

[0095] (iv) Batch stability verification. The results of 10 consecutive batch tests in Example 6 show that the standard deviations of all performance indicators are controlled within a small range (standard deviation of residual carbon rate 0.8%, standard deviation of yield 1.5%, and standard deviation of shedding rate 0.22%), which fully demonstrates that the process of the present invention has good process window tolerance and operational repeatability, and meets the quality control requirements of industrial mass production.

[0096] The correlation analysis of Examples 1 to 6 shows that the core technical effects of the present invention—namely, carbon residue rate ≥ 91.8%, zero bubbles at the bonding interface, seed crystal loss thickness ≤ 18 μm, seed crystal shedding rate ≤ 1.2%, dislocation density reduction ≥ 28%, and bonding yield ≥ 89%—can be stably achieved under different process parameter deviations, proving that the technical solution of the present invention has broad process adaptability and robust technical effects.

[0097] Comparative Example 1

[0098] Comparative experiments were conducted using traditional bonding processes. The adhesive used was ordinary phenolic resin, formulated with 80 parts phenolic resin and 20 parts ethanol, without nano-graphite powder, SiC micro-powder, silane coupling agent, or carbonization accelerator. The graphite buffer sheet was ordinary graphite sheet with a thickness of 0.5 mm, without a gradient pore structure. The curing process employed a single-stage heating method at a rate of 3℃ / min, directly heating from room temperature to 800℃ and holding at that temperature for 4 hours.

[0099] Testing of the bonded components revealed significant air bubbles at the bonding interface, with a bubble rate reaching 15%. The residual carbon content of the bonding layer was only 58%. After crystal growth testing, the seed crystal loss thickness reached 180 μm, indicating poor anti-sublimation suppression. The dislocation density decreased from the original 5 × 10⁻⁶. 4 / cm² decreased to 4.75×10 4 / cm², the dislocation density reduction was only 5%. The bonding yield was 65%, and the seed crystal detachment rate was 18%.

[0100] Comparative Example 2

[0101] A comparative experiment was conducted using another traditional process. A certain amount of organic solvent was added to the adhesive formulation to reduce viscosity, but instead of ultrasonic atomization spraying, manual brushing was used. The curing process employed a two-stage heating method, but no gradient air vent structure was incorporated.

[0102] The bonded components were inspected and found to contain some air bubbles at the bonding interface, with a bubble rate of 8%. The residual carbon content of the bonding layer was 65%. After the crystal growth test, the seed crystal loss thickness was 150 μm. The dislocation density decreased from the original 5 × 10⁻⁶. 4 / cm² decreased to 4.6×10 4 / cm², the dislocation density reduction was only 8%. The bonding yield was 72%, and the seed crystal detachment rate was 14%.

[0103] Comparative Example 3

[0104] The adhesive formulation is similar to that of this invention, but it does not contain carbonization accelerators, and the graphite buffer sheet uses a common graphite sheet without gradient air pores. Other process parameters are the same as those of this invention.

[0105] The bonded components were inspected and found to contain some air bubbles at the bonding interface, with a bubble rate of 6%. The residual carbon rate of the bonding layer was 78%. After the crystal growth test, the seed crystal loss thickness was 85 μm. The dislocation density decreased from the original 5 × 10⁻⁶. 4 / cm² decreased to 4.1×10 4 / cm², dislocation density decreased by 18%. Bonding yield was 78%, and seed crystal detachment rate was 8%.

[0106] Comparative Example 4

[0107] The adhesive formulation and graphite buffer sheet of this invention are used, but the curing process adopts a single-stage heating, with a heating rate of 2℃ / min, directly heating from room temperature to 950℃ and holding at that temperature for 5 hours, skipping the low-temperature pre-baking stage and the medium-temperature cross-linking curing stage.

[0108] The bonded components were tested and found to have significant air bubbles at the bonding interface, with a bubble rate of 12%. The residual carbon rate of the bonding layer was 71%. After the crystal growth test, the seed crystal loss thickness was 95 μm. The dislocation density decreased from the original 5 × 10⁻⁶. 4 / cm² decreased to 4.0×10 4 / cm², dislocation density decreased by 20%. Bonding yield was 75%, and seed crystal detachment rate was 10%.

[0109] The table below compares the technical parameters and performance of the embodiments and comparative examples:

[0110]

[0111] The comparative data above show that the adhesives produced using the process of this invention are significantly superior to those produced using traditional processes in all performance indicators. The residual carbon content of the adhesive layer increases from 58% to over 92%, the bubble rate decreases from 15% to 0%, the seed crystal loss thickness decreases from 180μm to 15μm, the seed crystal detachment rate decreases from 18% to below 1%, the dislocation density reduction increases from 5% to over 30%, and the bonding yield increases from 65% to over 90%.

[0112] The process of this invention effectively solves the three major defects existing in the prior art through the synergistic effect of gradient breathable graphite buffer sheet 2 and segmented vacuum heating curing carbonization process.

[0113] Regarding low porosity and high bonding reliability, the gradient permeable graphite sandwich structure provides a dedicated channel for gas extraction. The segmented vacuum heating process ensures complete gas extraction from the adhesive layer, achieving zero bubbles at the bonding interface. Under crystal growth conditions at 2500℃, the seed crystal detachment rate is controlled below 1%. Statistical data from Example 1 show a seed crystal detachment rate of only 0.8%, while Example 3 achieves an excellent level of 0.6%.

[0114] Regarding high residual carbon content in preventing sublimation, the modified composite adhesive achieves a residual carbon rate of over 92% after high-temperature carbonization, forming a dense C-SiC composite carbonized adhesive layer. This dense carbonized layer effectively isolates silicon vapor, suppressing sublimation on the back side of the seed crystal at its source. Experimental data shows that using the process of this invention, the seed crystal loss thickness is reduced from 180μm in the traditional process to less than 15μm, improving the sublimation suppression effect by more than an order of magnitude.

[0115] Regarding improvements in crystal quality, the dense carbide layer exhibits uniform thermal conductivity, significantly reduces interfacial thermal resistance fluctuations, substantially lowers thermal stress during crystal growth, and significantly reduces dislocation density in the finished SiC single crystal. Examples 1 to 5 all show dislocation density reductions exceeding 28%, with Example 3 achieving a remarkable 35% improvement. This technological effect is of great significance for improving the yield of SiC single crystal devices.

[0116] In terms of mass production advantages, this invention eliminates the need for grooving the seed crystals and back-side coating processes. It utilizes ultrasonic spraying for automated bonding, doubling the bonding efficiency per batch and maintaining a bonding yield consistently above 90%. Batch stability verification results in Example 6 show that the average bonding yield of 500 seed crystals in 10 consecutive batches was 90.8%, with a standard deviation of only 1.5%, fully demonstrating that the process of this invention is suitable for industrial-scale mass production.

[0117] This invention achieves a holistic breakthrough in seed crystal bonding technology by systematically designing five major steps: raw material pretreatment, composite adhesive preparation, interlayer pretreatment, layered adhesive bonding, and segmented vacuum gradient thermal curing and carbonization. It organically integrates three core functions—low gas production and anti-bubbling, dense back layer and anti-sublimation, and high residual carbon and high thermal conductivity—into the same process flow.

[0118] From a comprehensive analysis of the technical effects, compared with Comparative Example 1, the present invention improves the residual carbon rate by 34.3 percentage points (from 58% to 92.3%), reduces the bubble rate by 15 percentage points (from 15% to 0%), reduces the seed crystal loss thickness by 165 μm (from 180 μm to 15 μm), reduces the seed crystal shedding rate by 17.2 percentage points (from 18% to 0.8%), improves the dislocation density reduction by 27 percentage points (from 5% to 32%), and improves the crystal growth yield by 27 percentage points (from 65% to 92%).

[0119] Compared with Comparative Example 2, this invention improves the residual carbon rate by 27.3 percentage points (from 65% to 92.3%), reduces the bubble rate by 8 percentage points (from 8% to 0%), reduces the seed crystal loss thickness by 135 μm (from 150 μm to 15 μm), reduces the seed crystal detachment rate by 13.2 percentage points (from 14% to 0.8%), improves the dislocation density reduction by 24 percentage points (from 8% to 32%), and improves the crystal growth yield by 20 percentage points (from 72% to 92%).

[0120] The above comparative data fully demonstrates the significant technological advancements of this invention compared to existing technologies. In particular, the technical effects such as a residual carbon rate of ≥92%, a bonding yield of ≥90%, a seed crystal detachment rate of <1%, and a 30% reduction in dislocation density have all been fully verified in the embodiments and are supported by clear technical data.

Claims

1. A composite bonding process for high residual carbon, low porosity, and anti-sublimation silicon carbide seed crystals, characterized in that, Includes the following steps: Step 1: Cleaning pretreatment is performed on the SiC seed crystal, graphite support, and gradient permeable graphite buffer sheet. The thickness of the gradient breathable graphite buffer sheet is 0.2-0.4 mm. The sheet body has a honeycomb-shaped gradient air guide hole inside. The diameter of the air guide hole on the bonding side is 0.1 mm, and the diameter on the back side is enlarged to 0.3 mm, forming an air guide channel through the top and bottom. Step 2: Prepare the modified carbon-silicon composite adhesive, the raw materials of which include, by weight: 60-70 parts of phenolic resin matrix, 18-22 parts of anhydrous ethanol diluent, 5-8 parts of nano-graphite powder, 3-5 parts of ultrafine 4H-SiC micro powder, 1-2 parts of aminosilane coupling agent, and 0.5-1 parts of carbonization accelerator. Step 3: Spray the first layer of adhesive onto the bonding surface of the SiC seed crystal, coat both sides of the graphite buffer sheet with adhesive, spray the second layer of adhesive onto the bonding surface of the graphite holder, and then stack the SiC seed crystal, graphite buffer sheet and graphite holder in order from top to bottom, and perform pre-pressing treatment. Step four: Place the laminated bonded components in a vacuum sintering furnace for a three-stage vacuum gradient temperature rise curing and carbonization treatment. First stage: Heat to 120℃ at 2℃ / min, hold for 3 hours to remove solvent and small molecule volatiles, and the volatile gas is discharged through the gradient gas guide hole; Second stage: The temperature is increased to 450℃ at 1.5℃ / min and held for 4 hours to carry out cross-linking and curing reaction. The generated organic pyrolysis gas is discharged through the gradient gas guide hole. The third stage: the temperature is increased to 950℃ at 1℃ / min and held for 5 hours to fully carbonize the organic matrix and form a dense C-SiC composite carbonized adhesive layer. Cool naturally to room temperature; The gradient permeable graphite buffer sheet, the modified carbon silicon composite adhesive, and the three-stage vacuum gradient temperature curing carbonization treatment constitute a synergistic system. The synergistic system is used to achieve zero bubbles and a residual carbon rate of over 92% at the bonding interface, and to suppress reverse sublimation on the back of the seed crystal.

2. The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process according to claim 1, characterized in that, The thickness of the gradient breathable graphite buffer sheet is preferably 0.3 mm, and the air vents are arranged in regular hexagonal honeycomb units with a density of 15 to 25 per square centimeter. The center-to-center distance between adjacent air vents is 2.0 to 2.5 mm.

3. The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process according to claim 1, characterized in that, When multiple layers of graphite buffer sheets are stacked, the air guide holes are arranged in a staggered manner, that is, there is a lateral offset of 1.0 to 1.2 mm between the center of the air guide hole of the upper buffer sheet and the center of the air guide hole of the lower buffer sheet.

4. The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process according to claim 1, characterized in that, The nano-graphite powder has a particle size range of 50 to 200 nm, a specific surface area of ​​20 to 50 m² / g, and a carbon content greater than 99.9%; the ultrafine 4H-SiC micro powder has a particle size range of 1 to 3 μm and a specific gravity of 3.21 g / cm³.

5. The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process according to claim 1, characterized in that, The aminosilane coupling agent is γ-aminopropyltriethoxysilane, which is pre-hydrolyzed with anhydrous ethanol at a weight ratio of 1:9 before use, and the hydrolysis time is 30 to 60 minutes; the carbonization accelerator is a mixture of carbon fiber chopped filaments and ammonium dihydrogen phosphate, wherein the length of the carbon fiber chopped filaments is 20 to 50 μm and the diameter is 5 to 10 μm.

6. The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process according to claim 1, characterized in that, The spraying described in step three uses an ultrasonic atomization spraying system. This system uses the principle of ultrasonic vibration to atomize the adhesive liquid into tiny droplets. The ultrasonic frequency is 1.7 to 2.0 MHz and the atomization power is 50 to 80 W. The thickness of the first layer of adhesive liquid sprayed on the SiC seed crystal bonding surface is controlled within the range of 12 to 18 μm, and the thickness of the second layer of adhesive liquid sprayed on the graphite substrate bonding surface is controlled within the range of 15 to 20 μm.

7. The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process according to claim 1, characterized in that, The pre-compression treatment in step three involves placing a flat graphite counterweight on the upper surface of the seed crystal, with the counterweight pressure controlled within the range of 0.12 to 0.2 MPa, and the pre-compression time being 5 minutes.

8. The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process according to claim 1, characterized in that, In step four, the vacuum degree inside the vacuum sintering furnace is controlled below 5 Pa, preferably between 1 Pa and 3 Pa. In the first stage, small molecule volatiles such as ethanol solvent in the adhesive are removed, and the volatile gases are discharged into the furnace vacuum system through the gradient gas guide holes of the graphite buffer sheet. In the second stage, the phenolic resin undergoes a cross-linking and curing reaction, and the generated organic pyrolysis gas is discharged through the gradient gas guide holes. In the third stage, the organic matrix is ​​fully carbonized, and the adhesive layer is transformed into a dense C-SiC composite carbonized adhesive layer.

9. The high residual carbon, low porosity, anti-sublimation silicon carbide seed crystal composite bonding process according to any one of claims 1 to 8, characterized in that, The preferred formulation of the adhesive is: 65 parts phenolic resin, 20 parts anhydrous ethanol, 6 parts nano-graphite powder, 4 parts ultrafine 4H-SiC micro powder, 1.5 parts aminosilane coupling agent, and 0.8 parts carbonization accelerator; after mixing, the viscosity of the adhesive solution is controlled in the range of 3500 to 5500 mPa·s, preferably 4500 mPa·s.

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