Integrated seed wafer assembly and method of making same

Abstract

This application relates to the field of crystal growth technology and discloses an integrated seed wafer assembly and its fabrication method. The fabrication method of the integrated seed wafer assembly includes: forming a carbon film on the silicon surface of a seed wafer to obtain a seed wafer containing the carbon film; forming a first adhesive layer on the surface of the carbon film away from the seed wafer; forming a second adhesive layer on any surface of a graphite plate; and bonding graphite paper between the first adhesive layer and the second adhesive layer to obtain an integrated seed wafer intermediate; subjecting the integrated seed wafer intermediate to gradient hot pressing to obtain an integrated seed wafer; wherein the gradient hot pressing includes at least two hot pressing operations with increasing temperature gradients. The integrated seed wafer assembly obtained by this method exhibits high bonding strength between the seed wafer and the graphite component. Furthermore, using this integrated seed wafer assembly during crystal growth helps improve the quality of the wafer.

Description

Technical Field

[0001] This application relates to the field of crystal growth technology, and more specifically, to an integrated seed wafer assembly and its preparation method. Background Technology

[0002] Silicon carbide, as a third-generation wide-bandgap semiconductor material, plays an irreplaceable role in high-end fields such as new energy and aerospace due to its excellent electrical and thermal properties. The preparation of high-quality silicon carbide single crystals depends on the reliable bonding of seed wafers, and the bonding quality of the seed wafers has a significant impact on the yield and performance of crystal growth.

[0003] In most related technologies, silicon carbide seed wafer bonding processes involve directly coating the surface of the silicon carbide seed wafer with adhesive, and then performing a single hot-pressing process with a crucible lid or graphite component. However, this single hot-pressing process provides limited improvement in the bond strength between the seed wafer and the crucible lid, meaning there is still a problem of insufficient bond strength, which can lead to back corrosion and uneven coloring of the final wafer. Therefore, developing a bonding process that can further enhance the bond strength between the seed wafer and the crucible lid or graphite component is one of the current challenges.

[0004] Application content This application aims to at least partially solve one of the technical problems in the related art. Therefore, one objective of this application is to propose an integrated seed wafer assembly and its fabrication method. The fabrication method of the integrated bonding assembly of this application can ensure the bonding strength of the seed wafer, and using the integrated seed wafer assembly of this application for crystal growth can effectively solve problems such as wafer back corrosion and uneven color.

[0005] The first aspect of this application discloses a method for fabricating an integrated seed wafer assembly, comprising: A carbon film is formed on the silicon surface of the seed wafer to obtain a seed wafer containing the carbon film; A first adhesive layer is formed on the surface of the carbon film away from the seed wafer, a second adhesive layer is formed on any one surface of the graphite plate, and graphite paper is bonded between the first adhesive layer and the second adhesive layer to obtain an integrated seed wafer intermediate. The integrated seed wafer intermediate is subjected to gradient hot pressing to obtain an integrated seed wafer assembly; The gradient hot pressing includes at least two hot pressing cycles with an increasing temperature gradient.

[0006] The integrated seed wafer assembly in this application is prepared by gradient hot pressing. Specifically, under a relatively low-temperature hot pressing process, the curing agent in the first and second adhesive layers is slowly activated, and a large number of epoxy groups open the ring to form a linear cross-linked structure, the chemical formula of which can be represented as: [-C2H2O-]. n(Short chain) + m(-C2H3O) → [-C2H4O-C2H4O-] n+m (Linear structure) helps retain the flexibility of the adhesive layer and buffers the thermal expansion difference between the seed wafer and the adhesive; furthermore, under a high-temperature hot-pressing process, the first and second adhesive layers undergo deep curing, and the linear structure further polymerizes to form a three-dimensional network dense structure, the chemical formula of which can be represented as: [-C2H4O-C2H4O-] n+m →[-C2H4O-] x (Three-dimensional network), and simultaneously, the first adhesive layer reacts with the carbon film surface at the -C=O depth to form stable COC bonds, further enhancing the interfacial bonding strength. Therefore, the seed wafer and graphite component in the integrated seed wafer assembly obtained through gradient hot pressing exhibit high bonding strength.

[0007] In addition, the integrated seed wafer assembly according to the above embodiments of this application may also have the following additional technical features: In some embodiments of this application, the gradient hot pressing includes: The integrated seed wafer intermediate is subjected to a first hot pressing to obtain a first integrated seed wafer intermediate; The first integrated seed wafer intermediate is subjected to a first cooling process to obtain a second integrated seed wafer intermediate. The second integrated seed wafer intermediate is subjected to a second hot pressing to obtain the third integrated seed wafer intermediate; The third integrated seed wafer intermediate is subjected to a second cooling process to obtain an integrated seed wafer.

[0008] In some embodiments of this application, at least one of the following conditions is satisfied: The pressure of the first hot press is 200 kgf to 400 kgf; The temperature of the first hot pressing is 170℃~190℃; The heating rate of the first hot press is 4℃ / min~6℃ / min; The first hot pressing time is 35 min to 45 min; The first cooling rate is 2℃ / min to 4℃ / min; The endpoint temperature of the first cooling process is 80℃~120℃; The pressure of the second hot press is 200 kgf to 400 kgf; The temperature of the second hot pressing is 270℃~290℃; The heating rate of the second hot press is 2℃ / min~4℃ / min; The second hot pressing time is 50 min to 70 min; The second cooling rate is 1℃ / min to 3℃ / min; The endpoint temperature of the second cooling process is 20℃~30℃.

[0009] In some embodiments of this application, forming a carbon film on the silicon surface of the seed wafer includes: An organic adhesive layer is formed on the silicon surface of the seed wafer, and the organic adhesive layer is subjected to pre-curing and gradient curing in sequence; wherein the gradient curing includes a first curing, a second curing and a third curing performed in sequence with an increasing curing temperature gradient.

[0010] In some embodiments of this application, at least one of the following conditions is satisfied: The adhesive layer includes a photoresist layer; The pre-curing temperature is 100℃~120℃; The pre-curing time is 15 min to 25 min; The first curing temperature is 200℃~230℃; The first curing time is 25 min to 35 min; The heating rate for the first curing process is 1.5℃ / min to 2.5℃ / min; The second curing temperature is 400℃~450℃; The second curing time is 50 min to 70 min; The heating rate for the second curing process is 0.8℃ / min to 1.2℃ / min; The temperature for the third curing process is 650℃~750℃; The third curing time is 100 min to 140 min; The heating rate for the third curing process is 0.8℃ / min to 1.2℃ / min.

[0011] In some embodiments of this application, at least one of the following conditions is satisfied: The thickness of the carbon film is 0.4 μm to 0.6 μm; The carbon film has a density of ≥92%; The thickness of the graphite paper is 0.1mm to 0.3mm.

[0012] In some embodiments of this application, forming the first adhesive layer includes: coating the surface of the carbon film away from the seed wafer with a high-temperature negative photoresist, and baking it at 90°C to 110°C for 15 to 20 minutes to form a high-temperature negative photoresist layer; The formation of the second adhesive layer includes: forming an epoxy-coated high-temperature resistant photoresist on any one surface of the graphite plate, and baking it at 90°C to 110°C for 15 to 20 minutes to form an epoxy-coated high-temperature resistant photoresist layer.

[0013] In some embodiments of this application, at least one of the following conditions is satisfied: The thickness of the first adhesive layer is 1.5μm~2.5μm; The thickness of the second adhesive layer is 2μm~3μm.

[0014] In some embodiments of this application, after forming a carbon film on the silicon surface of the seed wafer and before forming the first adhesive layer and the second adhesive layer, at least one of the following is further included: The carbon film is activated by plasma to achieve a surface etching depth of ≤0.05μm and a surface roughness of 0.1μm-0.2μm. The graphite plate is activated using plasma, the graphite plate has a purity ≥ 99.9%, and the surface roughness of the graphite plate is ≤ 0.2 μm.

[0015] In a second aspect of this application, an integrated seed wafer assembly is proposed, obtained by the aforementioned integrated seed wafer assembly fabrication method. As a result, the seed wafers in this integrated seed wafer assembly exhibit high bonding strength. Furthermore, using this integrated seed wafer assembly during crystal growth helps improve wafer quality. Attached Figure Description

[0016] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is a flowchart of an integrated seed wafer assembly fabrication method according to an embodiment of this application.

[0017] Figure 2 This is a schematic diagram of the integrated seed wafer assembly during assembly according to an embodiment of this application. Detailed Implementation

[0018] The embodiments of this application are described in detail below. The embodiments described below are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0019] The first aspect of this application proposes a method for fabricating an integrated seed wafer assembly, referring to... Figure 1 ,include: S10: A carbon film is formed on the silicon surface of the seed wafer to obtain a seed wafer containing the carbon film.

[0020] In this step, forming a carbon film on the silicon surface of the seed wafer includes: forming an organic adhesive layer on the silicon surface of the seed wafer, and subjecting the organic adhesive layer to pre-curing and gradient curing sequentially. The gradient curing includes a first curing, a second curing, and a third curing performed sequentially with increasing curing temperatures. This forms a dense and highly adhesive inorganic carbon film, providing a stable interface for subsequent bonding and completely preventing direct contact between the adhesive and SiC, thus fundamentally solving the problem of back corrosion on the wafer.

[0021] For example, the carbon film formation process includes: coating a photoresist onto the silicon surface of a seed wafer, then pre-curing and gradient curing the photoresist under oxygen-free conditions to completely transform the photoresist into an inorganic amorphous carbon film. This carbon film retains the carbon atoms in the photoresist while removing heteroatoms such as hydrogen and oxygen. Specifically, SU-8 2002 / SHR-1000 photoresist can be used, or it can be replaced with domestic BP-212 photoresist or imported AZ4562 photoresist. All photoresists need to be warmed for 25-35 minutes and filtered through a 0.2μm filter before use. During coating, pre-uniform at 500-700 rpm for 6-10 seconds, then form the film at 2000-3000 rpm for 20-30 seconds, ensuring that the thickness of the coated photoresist is approximately 1.5μm-2.5μm.

[0022] During the coating process described above, the hydroxyl groups on the surface of the seed wafer interact weakly with the epoxy groups in the photoresist (such as the ethylene oxide structure in SU-8) through hydrogen bonding, achieving initial adsorption. The chemical formula can be represented as ≡Si-OH+-C2H3O→≡Si-O…H-C2H3 (hydrogen bonding). The polymer chains within the photoresist are tightly arranged through van der Waals forces. The centrifugal force generated by the high-speed rotation of the spin coater during coating ensures the photoresist is evenly spread on the wafer surface. The rotation speed is negatively correlated with the thickness of the photoresist layer (the higher the speed, the thinner the layer). Temperature recovery of the photoresist reduces its viscosity and improves its fluidity (preventing layer delamination), while filtration removes tiny particles from the photoresist, preventing pinholes or protrusions in the carbon film. This contributes to the formation of a carbon film with uniform thickness and no particle defects.

[0023] In some embodiments, pre-curing involves placing the photoresist-coated seed wafer on a hot plate at a temperature of 100°C to 120°C, specifically 100°C, 110°C, or 120°C. The heat conducted by the hot plate causes the surface temperature of the seed wafer to rise uniformly, and the saturated vapor pressure of the solvent increases with increasing temperature, which helps to achieve efficient solvent removal. The holding time is 15 min to 25 min, specifically 15 min, 20 min, or 25 min. After the holding time, the wafer can be allowed to cool naturally to room temperature without the need for light protection. The pre-curing process causes the organic solvent (such as PGMEA) in the photoresist to evaporate, preventing rapid vaporization of the solvent during subsequent carbonization and thus preventing cracking of the carbon film. Simultaneously, it allows the photoresist to form a preliminary cross-linked three-dimensional network, improving its mechanical stability in subsequent processes and preventing detachment or deformation.

[0024] Specifically, solvent molecules in the photoresist gain energy under heating conditions, breaking free from van der Waals forces and evaporating. The curing agent in the photoresist (such as hexamethylenetetramine in SU-8) initiates ring-opening of the epoxy groups at 100℃~120℃, resulting in a partial cross-linking reaction (n(-C2H3O)→[-C2H4O-)). n This forms a network structure with short-chain connections; at the same time, the cross-linking reaction is controllable at the above temperature (avoiding complete curing), and the initially cross-linked structure can retain a certain degree of flexibility, while improving the adhesion between the photoresist layer and the seed wafer.

[0025] In some embodiments, the gradient curing described above includes placing the pre-cured product in a tubular atmosphere furnace, purging it with 99.999% high-purity argon or nitrogen (flow rate of 60 sccm to 100 sccm) for at least 25 minutes, and finally performing a gradient temperature increase starting from room temperature. The first curing temperature is 200℃ to 230℃ (which can be 200℃, 210℃, 220℃, 230℃, etc.), the first curing time is 25 minutes to 35 minutes (which can be 25 minutes, 30 minutes, 35 minutes, etc.), and the first curing heating rate is 1.5℃ / min to 2.5℃ / min (which can be 1.5℃ / min, 2℃ / min, 2.5℃ / min, etc.). The second curing temperature is 400℃~450℃ (can be 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, etc.), the second curing time is 50min~70min (can be 50min, 60min, 70min, etc.), and the heating rate for the second curing is 0.8℃ / min~1.2℃ / min (can be 0.8℃ / min, 1.0℃ / min, 1.2℃ / min, etc.). The third curing temperature is 650℃~750℃ (can be 650℃, 700℃, 750℃, etc.), the third curing time is 100min~140min (can be 100min, 110min, 120min, 130min, 140min, etc.), and the heating rate for the third curing is 0.8℃ / min~1.2℃ / min (can be 0.8℃ / min, 1.0℃ / min, 1.2℃ / min, etc.). After the third curing is complete, allow it to cool naturally to room temperature.

[0026] Specifically, the first curing process further evaporates the residual solvent in the photoresist layer. During the second curing process, the CH and CO bonds in the photoresist break, generating small molecule gases such as H2O, CO, and CO2 that escape, while the polymer chains undergo chain breakage and rearrangement. During the third curing process, the remaining organic groups completely decompose, resulting in a product with the chemical formula C. 18 H 24 O6→18C+12H2O↑+3CO2↑, carbon element passes through sp... 2 Hybridization forms an amorphous carbon network structure, which forms Si-C chemical bonds with Si atoms on the seed wafer surface, with the chemical formula C+≡Si-OH→≡Si-C (strong chemical bond) + H2O↑. Inert gases (argon / nitrogen) can inhibit oxidation of the carbon film and the seed wafer surface. The gradient curing process in this application, through multi-stage slow heating, avoids the concentrated escape of small molecule gases that could lead to porosity in the carbon film, ensuring its density. Simultaneously, the multi-stage migration and recombination of carbon elements forms a stable amorphous structure, and the formation of Si-C bonds significantly enhances the bonding strength between the carbon film and the seed wafer.

[0027] In some embodiments, the thickness of the carbon film is 0.4 μm to 0.6 μm, specifically, it can be 0.4 μm, 0.5 μm, 0.6 μm, etc. The thickness of the carbon film within the above range helps to provide a stable interface for subsequent bonding.

[0028] In some embodiments, the carbon film has a density ≥92%. A carbon film density within the above range helps to provide a stable interface for subsequent bonding, while avoiding the formation of hexagonal void defects during crystal growth.

[0029] In some embodiments, before forming a carbon film on the surface of the seed wafer, the process further includes: pre-treatment of the seed wafer. The pre-treatment includes cleaning the seed wafer with acetone or isopropanol (12-18 min ultrasonic cleaning), rinsing it with deionized water and drying it with nitrogen gas after cleaning, and then treating the seed wafer with 70-90 W oxygen plasma for 6-10 min.

[0030] Specifically, acetone or isopropanol is used to ultrasonically clean the seed wafer, dissolving oil stains on the surface of the seed wafer through the principle of "like dissolves like". The ultrasonic waves utilize the high-frequency vibration to generate microbubbles that burst, forming shock waves that peel off particulate impurities attached to the surface. High-energy particles of oxygen plasma bombard the surface, breaking the Si-C bonds and inert groups adsorbed on the surface of the seed wafer. These particles react with the C and Si elements on the surface of the seed wafer to generate small amounts of volatile CO, CO2, and SiO2 (trace amounts). This introduces active groups such as hydroxyl (-OH) and carboxyl (-COOH) groups onto the surface of the seed wafer, enhancing the hydrogen bonding and chemical adsorption capacity between the photoresist and the SiC surface.

[0031] S20: A first adhesive layer is formed on the surface of the carbon film away from the seed wafer, a second adhesive layer is formed on any surface of the graphite plate, and graphite paper is bonded between the first adhesive layer and the second adhesive layer to obtain an integrated seed wafer intermediate.

[0032] In this step, forming the first adhesive layer includes: coating the surface of the carbon film away from the seed wafer with a high-temperature negative photoresist, and baking it at 90℃~110℃ for 15min~20min to form a high-temperature negative photoresist layer. Forming the second adhesive layer includes: coating any one surface of the graphite plate with an epoxy-based high-temperature resistant photoresist, and baking it at 90℃~110℃ for 15min~20min to form an epoxy-based high-temperature resistant photoresist layer. After forming the adhesive layer, one side of the graphite paper is precisely aligned with the high-temperature negative photoresist layer, and a surface pressure of 5 kg is applied and held for 10 s. Then, the unattached side of the graphite paper is aligned with the epoxy-based high-temperature resistant photoresist, and a surface pressure of 10 kg is applied and held for 15 s to obtain an integrated seed wafer intermediate. Figure 2As can be seen, in this integrated seed wafer assembly, the graphite paper 2 is disposed between the seed wafer 1 and the graphite plate 3, and is fixed by the tooling mold 4. The flatness of the intermediate body of this integrated seed wafer is ≤0.3 μm.

[0033] Specifically, to form the first and second adhesive layers, two identical spin coaters can be used. The adhesive layers are coated on either the surface of the carbon film furthest from the seed wafer or on any surface of the graphite plate. Then, the spin coaters are activated in baking mode (humidity 40%~60%). After baking, a high-temperature negative photoresist layer and an epoxy-based high-temperature resistant photoresist layer are obtained. During baking, the epoxy groups in the adhesive undergo hydrogen bonding and preliminary chemical adsorption with the active groups on the surface of the carbon film or graphite plate, forming an interfacial bond. The polymer chains inside the adhesive layer are tightly packed through van der Waals forces. Simultaneously, the solvent evaporation rate can be controlled during baking, thereby enhancing the interfacial bond between the adhesive layer and the substrate. Under the above baking conditions, problems such as surface crusting of the adhesive layer with residual solvent inside, or ring-opening of some epoxy groups, can be largely avoided.

[0034] For example, when coating a carbon film on a surface away from the seed wafer with a high-temperature negative photoresist, the photoresist needs to be warmed up for 25 to 35 minutes and filtered with a 0.2 μm filter membrane before use. During coating, it is first pre-uniformed at 700 to 900 rpm for 4 to 6 seconds, and then formed into a film at 2500 to 3500 rpm for 15 to 25 seconds.

[0035] For example, when coating any surface of a graphite plate with epoxy-based high-temperature resistant photoresist, the photoresist needs to be warmed up for 25 to 35 minutes and filtered with a 0.2 μm filter membrane before use. During coating, it should first be pre-uniformed at 800 to 1000 rpm for 5 to 7 seconds, and then formed into a film at 2000 to 3000 rpm for 20 to 30 seconds.

[0036] In some embodiments, after forming a carbon film on the silicon surface of the seed wafer and before forming the first adhesive layer, the method further includes: activating the carbon film using plasma, wherein the surface etching depth of the activated carbon film is ≤0.05 μm and the surface roughness is 0.1 μm-0.2 μm. This creates an active interface, eliminates the defect layer on the carbon film of the seed wafer, and improves its adhesion.

[0037] The surface roughness of the carbon film mentioned above can be tested using the AFM method. By detecting the minute deformation of the probe cantilever, the three-dimensional morphology data of the surface is obtained, thus yielding the surface roughness of the carbon film.

[0038] For example, the activation treatment of the carbon film includes maintaining a distance of 5-8 mm between the carbon film and the plasma at a power of 40-60 W and an argon flow rate of 50-80 sccm for 4-6 minutes. During this process, high-energy Ar...+ When the loose layer on the surface of the carbon film is peeled off, the C=C bond reacts with trace amounts of O2 to generate -C=O active groups.

[0039] In some embodiments, the thickness of the graphite paper is 0.1mm to 0.3mm, specifically 0.1mm, 0.2mm, 0.3mm, etc. The carbon film on the seed wafer and the surface of the graphite plate may have varying degrees of warping. If the seed wafer and the graphite plate are directly brought into hard contact, problems such as weak adhesion, uneven glue application, and air bubbles can easily occur. Therefore, a flexible buffer layer, namely graphite paper, is added between the seed wafer and the graphite plate. This can both level the warping of the seed wafer and the graphite plate and buffer the difference in thermal expansion between the seed wafer and the graphite plate, further improving the bonding quality.

[0040] In some embodiments, prior to forming the second adhesive layer, the process further includes: activating the graphite plate using plasma. Exemplarily, this activation process includes: vacuum calcining the graphite plate at 1000 °C for 2 h to remove impurities, followed by cooling and treatment with the same plasma parameters as described above (40-60 W power, 50-80 sccm argon flow rate, maintaining a specific distance of 5-8 mm between the graphite plate and the plasma, treatment for 4-6 min). During this process, Ar… + The graphite powder and inert layer on the surface of the graphite plate are removed by bombardment, and active groups such as -C=O and -OH are introduced. The purity of the graphite plate is ≥99.9%, and the surface roughness of the unactivated graphite plate (i.e., the original graphite plate) is generally ≤0.2μm.

[0041] The surface roughness of the graphite plate mentioned above can be tested using the AFM method.

[0042] Specifically, the advantages of plasma treatment for graphite plates are reflected in three aspects: 1. Overcoming surface inertia: The original surface of the graphite plate is a highly stable sp... 2 1. Hybrid carbon structure lacks active groups, making direct coating prone to "virtual adhesion." Plasma treatment introduces active sites such as -C=O and -OH, providing anchor points for chemical bonding. 2. Optimized interface adaptation: Trace amounts of graphite dust remain after calcination of the graphite plate. Plasma bombardment can completely remove this dust and simultaneously form a micro-nano rough surface (the surface roughness of the graphite plate increases from 0.2μm to 0.5μm), increasing the contact area of ​​the adhesive layer. 3. Ensured process synergy: Using the same parameters as the seed wafer improves the matching density of active groups at the dual interfaces, ensuring balanced strength of the "seed wafer-graphite paper-graphite plate" dual bonding interface. Ultimately, the dual active interfaces increase the bonding strength of both interfaces by more than 50%, preventing stress cracking caused by bonding failure on the graphite plate side during crystal growth from the source.

[0043] S30: The integrated seed wafer intermediate is subjected to gradient hot pressing to obtain an integrated seed wafer, wherein the gradient hot pressing includes at least two hot pressing cycles with increasing hot pressing temperature gradient.

[0044] In this step, for example, the gradient hot pressing includes a first hot pressing and a second hot pressing. Specifically, it includes: performing a first hot pressing on the integrated seed wafer intermediate to obtain a first integrated seed wafer intermediate; performing a first cooling on the first integrated seed wafer intermediate to obtain a second integrated seed wafer intermediate; performing a second hot pressing on the second integrated seed wafer intermediate to obtain a third integrated seed wafer intermediate; and performing a second cooling on the third integrated seed wafer intermediate to obtain an integrated seed wafer.

[0045] In some embodiments, the temperature of the first hot pressing is 170℃~190℃ (specifically, it can be 170℃, 175℃, 180℃, 185℃, 190℃, etc.). The hot pressing process uses a polyimide-based composite flexible indenter (coated with a 0.5mm graphite thermally conductive layer). The temperature is increased to the first hot pressing temperature at a rate of 4℃ / min~6℃ / min before hot pressing begins. The pressure is 200 kgf~400 kgf (specifically, for a 6-inch seed wafer, the first hot pressing pressure is 200~300 kgf; for an 8-inch seed wafer, the first hot pressing pressure is 300~400 kgf), and the pressure is maintained for 35min~45min. After hot pressing, the temperature is reduced to 80~120℃ at a rate of 2℃ / min~4℃ / min. This process allows for controlled cross-linking and interfacial bonding of the adhesive, buffers the thermal expansion difference between the seed wafer and the adhesive, releases initial stress, and ensures a tight, bubble-free interfacial bond.

[0046] The use of a polyimide-based composite flexible indenter in the above process helps to fully fill the gaps at the interface with the adhesive layer. The curing agent is slowly activated, and a large number of epoxy groups open to form a linear cross-linked structure, which can be represented by the following chemical formula: [-C2H2O-] n (Short chain) + m(-C2H3O) → [-C2H4O-C2H4O-] n+m (Linear structure). This helps to maintain the flexibility of the adhesive layer, buffer the difference in thermal expansion between the seed wafer and the adhesive, and ensure that the solvent residue is reduced to below 0.1%.

[0047] In some embodiments, the temperature of the second hot pressing is 270℃~290℃ (specifically, it can be 270℃, 275℃, 280℃, 285℃, 290℃, etc.). A solid 304 stainless steel pressing block is used during the hot pressing process. The temperature is increased to the second hot pressing temperature at a rate of 2℃ / min~4℃ / min, and then hot pressing begins. The pressure is 200 kgf~400 kgf (specifically, for a 6-inch seed wafer, the second hot pressing pressure is 200~300 kgf; for an 8-inch seed wafer, the second hot pressing pressure is 300~400 kgf), and the pressure is held for 50min~70min. After hot pressing, the temperature is reduced to 20~30℃ at a rate of 1℃ / min~3℃ / min. This process provides deep curing, which can increase the overall bond strength to over 20MPa, ensuring the stability of the adhesive layer at the high temperature of crystal growth. The gradient hot pressing process of this application helps to solve the problem of thermal expansion mismatch and ensures uniform heat conduction during crystal growth.

[0048] The second hot-pressing process uses the rigid pressure of a stainless steel pressing block to promote the tight bonding of molecular chains, further polymerizing the linear structure to form a three-dimensional network structure, which can be represented by the chemical formula: [-C2H4O-C2H4O-]. n+m →[-C2H4O-] x (Three-dimensional network), while the first adhesive layer reacts deeply with the carbon film surface to form stable COC bonds, improving the interfacial bonding strength by 20% compared to a single hot-pressing process. The 350kg high pressure controls the adhesive layer thickness deviation to ≤0.08μm, improves the uniformity of heat conduction, and the stainless steel pressure block ensures stable pressure transmission, further improving curing uniformity and bonding strength.

[0049] In some embodiments, after gradient hot pressing, the integrated seed wafer assembly obtained above is further subjected to post-processing. The post-processing includes: stabilizing the integrated seed wafer assembly by baking at 200°C to 240°C, and then cleaning off any excess adhesive.

[0050] Specifically, baking for stabilization involves holding the product in an oven at 200℃~240℃ for 100~140 minutes, followed by natural cooling to room temperature. This process releases interfacial stress and internal stress within the adhesive layer accumulated during gradient hot pressing, helping to reduce wafer warpage that may occur during crystal growth. Simultaneously, this process removes trace amounts of residual small molecules from the adhesive layer, further refining the cross-linking structure and enhancing the long-term stability of the bond.

[0051] Excess adhesive removal involves wiping the adhesive layer on the edges of the integrated seed wafer assembly with a lint-free cloth soaked in isopropyl alcohol. This prevents the overflowing adhesive layer from pyrolyzing during crystal growth, generating impurities that contaminate the back and edges of the wafer, thus ensuring a clean appearance of the grown wafer. The use of isopropyl alcohol to remove the adhesive layer is based on the principle of "like dissolves like," as it is highly volatile, leaves no residue after wiping, and will not corrode or damage the cured adhesive layer or carbon film.

[0052] In a second aspect of this application, an integrated seed wafer assembly is proposed, obtained by the aforementioned integrated seed wafer assembly fabrication method. As a result, the seed wafers in this integrated seed wafer assembly exhibit high bonding strength. Furthermore, using this integrated seed wafer assembly during crystal growth helps improve wafer quality.

[0053] The integrated seed wafer assembly and its fabrication method of this application have at least the following beneficial effects: 1. The value of the carbonization process in forming carbon films: By precisely controlling the carbonization temperature rise curve, the photoresist is completely transformed into a dense amorphous carbon film. The formation of Si-C chemical bonds makes the carbon film firmly bonded to the seed wafer. The isolation barrier completely blocks the penetration of pyrolysis impurities in the adhesive, and the back corrosion rate after crystal growth is reduced from 12% in the traditional process to 0%. 2. The value of interface strengthening in the activation process: The precise etching and activation of argon plasma can improve surface activity without damaging the carbon film and graphite plate, thereby increasing the bonding force of the adhesive by more than 50%. The activation of the graphite plate can break the surface inertia, remove impurities, and increase the contact area, thus avoiding stress cracking caused by interface delamination during crystal growth. 3. Performance control value of hot pressing process: The multi-stage gradient process solves the problem of thermal expansion mismatch between SiC and adhesive, with warpage ≤30 μm. At the same time, the three-dimensional cross-linking structure makes the bonding strength ≥20 MPa, ensuring uniform heat conduction during crystal growth and improving the color uniformity of the wafer. 4. Efficiency advantages of integrated hot pressing: process time is reduced by 50%, solvent residue is reduced by 62.5%, and bubble defect rate is reduced from 3% to 0.5%, balancing efficiency and quality.

[0054] The present application will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present application in any way. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in the art or in accordance with the product manual.

[0055] Example 1 1. Pretreatment of silicon carbide seed wafers: Using 8-inch 4H-SiC seed wafers, sonicate in acetone for 15 minutes, then sonicate in isopropanol for 15 minutes, and finally rinse with deionized water 3 times (2 minutes each time), dry with high-purity nitrogen (99.999%), and treat under 80W oxygen plasma for 8 minutes in a Class 100 cleanroom environment.

[0056] 2. Select SU-8 2002 photoresist, allow it to warm to room temperature for 30 minutes, and then filter it with a 0.2μm filter membrane; use a spin coater to coat the photoresist onto the silicon surface of the seed wafer. The spin coater parameters are: 600rpm for 8s pre-spinning, 2500rpm for 25s film formation, and a photoresist layer thickness of 2.0μm. After holding the film at 110 °C on a hot plate for 20 min, it was allowed to cool naturally to room temperature without needing to avoid light. The seed wafer was then placed in a tube furnace, and argon gas (flow rate 80 sccm) was introduced for purging for 30 min. The heating program was then initiated: the temperature was raised from room temperature to 200 °C (heating rate 2 °C / min, holding for 30 min), then from 200 °C to 400 °C (heating rate 1 °C / min, holding for 60 min), then from 400 °C to 700 °C (heating rate 1 °C / min, holding for 120 min), and allowed to cool naturally to room temperature. The carbon film thickness was 0.55 μm, and the density was 95%.

[0057] 3. The seed wafer carbon film and graphite plate are simultaneously subjected to 50 W argon plasma treatment for 5 min. Before plasma treatment, the graphite plate is calcined at 1000 ℃ for 30 min, and the surface roughness of the graphite plate is 0.15 μm. 4. Forming the first adhesive layer on the carbon film surface: High temperature resistant negative photoresist is selected, and the film is warmed at room temperature for 30 min and then filtered through a 0.2μm filter membrane. Spin coater parameters: 800 rpm for pre-spinning for 5 s and 3000 rpm for film formation for 20 s. A second adhesive layer was formed on one side of the graphite plate: epoxy-based high-temperature resistant photoresist was selected, and the film was cooled to room temperature for 30 min and then filtered through a 0.2 μm filter membrane. The spin coater parameters were: 900 rpm for pre-uniformation for 6 s and 2500 rpm for film formation for 25 s. After applying the adhesive, the baking module of the spin coater is started simultaneously. The ambient humidity is 50%, and the temperature is increased to 100℃ at a rate of 10℃ / min and held for 18 min. The thickness of the first adhesive layer is 2.0 μm, and the thickness of the second adhesive layer is 2.5 μm. 5. Select 0.4 mm thick high-density graphite paper. No pretreatment is required. Align one side of the graphite paper precisely with the coated surface of the baked seed wafer and apply 5 kg surface pressure for 10 seconds. Then align the uncoated side of the graphite paper with the coated surface of the baked graphite plate and apply 10 kg surface pressure for 15 seconds. After pasting, the overall flatness of the integrated seed wafer assembly intermediate is measured to be 0.25μm using a laser flatness meter.

[0058] 6. Use a 500kg hot press with a pressure set to 350kg. In the first stage, use a PI-based flexible press head, heat up to 180℃ at a rate of 5℃ / min (hold pressure for 40 min), and then cool down to 100℃ at a rate of 3℃ / min. In the second stage, replace the press with a 304 stainless steel press block, heat up to 280℃ at a rate of 3℃ / min (hold pressure for 60 min), and then cool down to room temperature at a rate of 2℃ / min.

[0059] Comparative Example 1 Similar to Example 1, the main difference is that there is no carbon film formation process, and the hot pressing is a single hot pressing, only the integrated seed wafer assembly intermediate is hot pressed once. Specifically, the pressure is set to 350 kg and held at 220°C for 60 min.

[0060] Comparative Example 2 Similar to Example 1, the main difference is that the hot pressing is a single hot pressing, and only the integrated seed wafer assembly intermediate is hot pressed once. Specifically, the pressure is set to 350 kg and held at 220°C for 60 min.

[0061] Comparative Example 3 Similar to Example 1, the main difference is that no activation treatment was performed before the second adhesive layer was formed on one side of the graphite plate, and the hot pressing was a single hot pressing, only the intermediate body of the integrated seed wafer assembly was hot pressed once. Specifically, the pressure was set to 350 kg and held at 220°C for 60 min.

[0062] Comparative Example 4 Similar to Example 1, the main difference is that: there is no activation treatment as in Example 1, and no carbon film is formed. That is, the seed wafer is directly bonded to the graphite plate and graphite paper to obtain an integrated seed wafer intermediate. The hot pressing is a single hot pressing, and only the integrated seed wafer assembly intermediate is hot pressed once. Specifically, the pressure is set to 350 kg and held at 220°C for 60 min.

[0063] Test Results The bonding strength of the integrated seed wafer assembly in Example 1 and Comparative Examples 1-4 was tested, and crystal growth was performed using the integrated seed wafer assembly. The back corrosion rate, color unevenness rate, and crystal growth interface delamination rate of the obtained wafers were statistically analyzed. The specific results are shown in Table 1.

[0064] Test methods Bond strength test: The tensile shear method is used. A load perpendicular or parallel to the bonding interface is applied by a mechanical testing machine, and the maximum stress when the interface peels off is measured to characterize the bond strength.

[0065] Back corrosion rate: By visually marking the areas of back corrosion on the crystal surface, the ratio of the area of ​​the back corrosion area to the total area of ​​the crystal surface is calculated, which is the back corrosion rate.

[0066] Color unevenness: By visually marking the areas of uneven color on the crystal surface, the proportion of the area of ​​the uneven color area to the total area of ​​the crystal surface is calculated, which is the color unevenness.

[0067] Crystal growth interface delamination rate: By visually marking the delamination areas at the crystal interface, the ratio of the delamination area to the total interface area is calculated, which is the crystal growth interface delamination rate.

[0068] Table 1

[0069] Conclusion: The integrated seed wafer assembly prepared in this application has high bonding strength between the seed wafer and the graphite plate; at the same time, the processes of forming a carbon film on the surface of the seed wafer and activating the seed wafer and graphite plate will affect the performance of the integrated seed wafer assembly, thereby helping to improve the quality of the crystal.

[0070] In the description of this application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0071] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0072] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for preparing an integrated seed wafer assembly, characterized in that, include: A carbon film is formed on the silicon surface of the seed wafer to obtain a seed wafer containing the carbon film; A first adhesive layer is formed on the surface of the carbon film away from the seed wafer, a second adhesive layer is formed on any one surface of the graphite plate, and graphite paper is bonded between the first adhesive layer and the second adhesive layer to obtain an integrated seed wafer intermediate. The integrated seed wafer intermediate is subjected to gradient hot pressing to obtain an integrated seed wafer; The gradient hot pressing includes at least two hot pressing cycles with an increasing temperature gradient.

2. The preparation method according to claim 1, characterized in that, The gradient hot pressing includes: The integrated seed wafer intermediate is subjected to a first hot pressing to obtain a first integrated seed wafer intermediate; The first integrated seed wafer intermediate is subjected to a first cooling process to obtain a second integrated seed wafer intermediate. The second integrated seed wafer intermediate is subjected to a second hot pressing to obtain the third integrated seed wafer intermediate; The third integrated seed wafer intermediate is subjected to a second cooling process to obtain an integrated seed wafer.

3. The preparation method according to claim 2, characterized in that, At least one of the following conditions must be met: The pressure of the first hot press is 200 kgf to 400 kgf; The temperature of the first hot press is 170℃~190℃; The heating rate of the first hot press is 4℃ / min~6℃ / min; The first hot pressing time is 35 min to 45 min; The first cooling rate is 2℃ / min to 4℃ / min; The endpoint temperature of the first cooling process is 80℃~120℃; The pressure of the second hot press is 200 kgf to 400 kgf; The temperature of the second hot pressing is 270℃~290℃; The heating rate of the second hot press is 2℃ / min~4℃ / min; The second hot pressing time is 50 min to 70 min; The second cooling rate is 1℃ / min to 3℃ / min; The endpoint temperature of the second cooling process is 20℃~30℃.

4. The preparation method according to claim 1, characterized in that, The formation of a carbon film on the silicon surface of the seed wafer includes: An organic adhesive layer is formed on the silicon surface of the seed wafer, and the organic adhesive layer is subjected to pre-curing and gradient curing in sequence; wherein the gradient curing includes a first curing, a second curing and a third curing performed in sequence with an increasing curing temperature gradient.

5. The preparation method according to claim 4, characterized in that, At least one of the following conditions must be met: The adhesive layer includes a photoresist layer; The pre-curing temperature is 100℃~120℃; The pre-curing time is 15 min to 25 min; The first curing temperature is 200℃~230℃; The first curing time is 25 min to 35 min; The heating rate for the first curing process is 1.5℃ / min to 2.5℃ / min; The second curing temperature is 400℃~450℃; The second curing time is 50 min to 70 min; The heating rate for the second curing process is 0.8℃ / min to 1.2℃ / min; The temperature for the third curing process is 650℃~750℃; The third curing time is 100 min to 140 min; The heating rate for the third curing process is 0.8℃ / min to 1.2℃ / min.

6. The preparation method according to claim 1, characterized in that, At least one of the following conditions must be met: The thickness of the carbon film is 0.4 μm to 0.6 μm; The carbon film has a density of ≥92%; The thickness of the graphite paper is 0.1mm to 0.3mm.

7. The preparation method according to claim 1, characterized in that, The formation of the first adhesive layer includes: coating the surface of the carbon film away from the seed wafer with a high-temperature negative photoresist, and baking it at 90°C to 110°C for 15 min to 20 min to form a high-temperature negative photoresist layer; The formation of the second adhesive layer includes: forming an epoxy-coated high-temperature resistant photoresist on any one surface of the graphite plate, and baking it at 90°C to 110°C for 15 to 20 minutes to form an epoxy-coated high-temperature resistant photoresist layer.

8. The preparation method according to claim 7, characterized in that, At least one of the following conditions must be met: The thickness of the first adhesive layer is 1.5μm~2.5μm; The thickness of the second adhesive layer is 2μm~3μm.

9. The preparation method according to claim 1, characterized in that, After forming a carbon film on the silicon surface of the seed wafer, and before forming the first adhesive layer and the second adhesive layer, at least one of the following is also included: The carbon film is activated using plasma to achieve a surface etching depth of ≤0.05 μm and a surface roughness of 0.1 μm-0.2 μm. The graphite plate is activated using plasma, the graphite plate has a purity ≥ 99.9%, and the surface roughness of the graphite plate is ≤ 0.2 μm.

10. An integrated seed wafer assembly, characterized in that, It is obtained by the integrated seed wafer assembly preparation method according to any one of claims 1 to 9.

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