Energy-saving single crystal furnace thermal field system based on gradient control and its reconstruction process

Abstract

This invention discloses an energy-saving single-crystal furnace thermal field system based on gradient control and its modification process, belonging to the field of photovoltaic and semiconductor material preparation technology. It includes a bottom insulation layer composed of integral or large-block cured felt spliced ​​together; a side insulation cylinder composed of integral or segmented cured felt, with the segments seamlessly spliced ​​using a labyrinth-style seal, and the gap between its outer wall and the inner wall of the furnace designed to maximize insulation thickness; and a top insulation cover plate composed of integral or large-block cured felt, with a reinforcing ring at the center observation hole. The cured felt is a rigid carbon fiber composite material treated with a densification process, with a lower thermal conductivity than soft felt of the same density. The bottom insulation layer can adopt a multi-layer density gradient composite structure, and a high-reflectivity graphite layer can be set on the inner wall of the side insulation cylinder. This invention eliminates the heat leakage caused by the gaps in the multi-layer splicing of traditional soft felt, achieving reduced heating power, extended thermal field life, long-term stable insulation performance, and reduced energy consumption and operating costs.

Description

Technical Field

[0001] This invention belongs to the field of photovoltaic and semiconductor material preparation technology, specifically an energy-saving single crystal furnace thermal field system based on gradient control and its modification process. Background Technology

[0002] The Czochralski method is currently the mainstream technology for manufacturing single-crystal silicon rods for large-scale integrated circuits and photovoltaic single-crystal silicon rods. In the Czochralski single-crystal furnace for growing single-crystal silicon, the thermal field system is a complex temperature field control core composed of graphite heaters, quartz crucibles, graphite crucibles, flow guide tubes, and various insulation materials. The structural design and material selection of the thermal field directly determine key quality indicators such as temperature gradient distribution, energy consumption level, crystallization rate, and oxygen content of the single-crystal silicon rod during the crystal pulling process. With the photovoltaic and semiconductor industries' continuous pursuit of reducing the cost per kilowatt-hour and chip manufacturing cost, the size of single-crystal silicon wafers is constantly evolving towards larger sizes, which directly drives the development of single-crystal silicon furnace thermal fields from the early 16-inch and 18-inch sizes to larger diameters.

[0003] In recent years, to improve the production efficiency of existing equipment and reduce retrofit costs, converting older, fixed-size 85-type monocrystalline silicon furnaces into furnaces with 18-inch large-diameter hot zones for crystal pulling of larger monocrystalline silicon rods has become a common technological upgrade path in the industry. However, this method of cramming a large hot zone into a small furnace body makes the radial and axial space inside the furnace extremely compact, placing near-limit demands on the insulation efficiency of the heat preservation system to ensure the stability and low power consumption of the crystal pulling hot zone. How to construct a sufficiently efficient insulation barrier within the limited furnace space to prevent the radiative loss of high-temperature heat to the water-cooled furnace walls has become a core technical challenge determining the energy consumption level and operational economy of the retrofitted equipment during monocrystalline silicon crystal pulling.

[0004] Currently, the most common insulation solution for adapting an 18-inch hot zone to a Type 85 monocrystalline silicon furnace is the all-soft carbon felt solution. This solution uses multiple layers of soft needle-punched carbon felt with a density of approximately 0.10-0.18 g / cm³, which are wrapped around the graphite inner cylinder layer by layer from the inside out by 7 to 8 layers of arc-shaped carbon felt sheets and tied and fixed with molybdenum wire; at the bottom, multiple layers of soft felt pads are stacked under the reflector; the top cover plate is also made of multiple ring-shaped soft felt pieces spliced ​​together. While this solution leverages the advantages of soft felt—low ​​cost, easy processing, and good thermal shock resistance—its inherent flaws become apparent when addressing the extreme space requirements of monocrystalline silicon pulling under an 85-furnace + 18-inch hot zone: First, the flexible nature of the soft felt necessitates a multi-layered and multi-block construction method, inevitably introducing numerous interlayer gaps and inter-block seams. These gaps become major channels for heat radiation leakage in a high-temperature vacuum environment, preventing effective reduction of heating power and increasing the cost of monocrystalline silicon pulling. Second, under long-term high-temperature thermal cycling at temperatures above 1500℃, the carbon fibers of the soft felt are prone to embrittlement, pulverization, and irreversible shrinkage, leading to a loosening of the insulation layer structure, widening of gaps, and a sharp decline in insulation performance with each pulling furnace cycle. This not only forces frequent replacement of the hot zone and increases operating costs but also severely affects the consistency of crystal quality during the pulling process due to continuous temperature field drift, ultimately reducing the quality of the monocrystalline silicon rod. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide an energy-saving single crystal furnace hot zone system based on gradient control and its modification process, so as to solve the problems of low insulation efficiency, high energy consumption and poor stability caused by the use of traditional soft carbon felt insulation material when adapting the existing 85-type single crystal furnace to an 18-inch hot zone.

[0006] To achieve the above-mentioned objectives, the technical solution of the present invention will be described as follows:

[0007] A gradient-controlled energy-saving single-crystal furnace hot zone system is applied to an 85-type single-crystal furnace adapted to an 18-inch hot zone. The system includes: The bottom insulation layer, located at the bottom of the furnace body, is made of rigid cured felt obtained through a densification process. The bottom insulation layer is an integral structure or is composed of 1 to 3 large pieces of cured felt spliced ​​together. Its shape is adapted to the furnace bottom tray, and its thickness is adapted to the height of the furnace bottom space. It is used to prevent heat from radiating and dissipating to the furnace bottom water-cooling structure. The side insulation cylinder is coaxially located above the bottom insulation layer, surrounding the graphite heater and crucible. It is made of rigid cured felt obtained through a densification process. The side insulation cylinder is an integral cylindrical structure or is composed of 2 or 3 segmented structures. When a segmented structure is used, the segments are spliced ​​together using a labyrinth-type sealing structure. The height of the side insulation cylinder matches the height of the graphite heater in the 18-inch hot zone. A uniform radiative gap is formed between its inner wall and the outer wall of the heater. The radial gap between its outer wall and the inner wall of the 85-type single crystal furnace body is less than or equal to 10 mm. The top insulation cover plate, which covers the top of the hot zone, is made of rigid curing felt obtained through a densification process. The top insulation cover plate is an integral ring structure or is spliced ​​together from four or fewer large pieces of curing felt. An observation hole for crystal growth observation is provided at the center of the top insulation cover plate. The observation hole is surrounded by a curing felt reinforcement ring to prevent deformation under long-term high-temperature use. The cured felt is a rigid carbon fiber composite material obtained by densification treatment through chemical vapor deposition, resin impregnation carbonization, or a combination of both. Its internal carbon fiber skeleton is firmly bonded by pyrolytic carbon or resin carbon bonding phase to form a three-dimensional integral network structure. The density of the cured felt is 0.15 to 0.30 g / cm³.

[0008] The bottom insulation layer, side insulation cylinders, and top insulation cover plate form a rigid insulation cavity that integrates with the internal space of the 85-type single crystal furnace. This invention, through innovation in insulation materials—from flexible felt to rigid cured felt—and a reconstructive design of the insulation structure—from multi-layered stacking to a monolithic, large-scale construction—achieves a near-seamless rigid insulation cavity for the first time within the extreme space constraints of an 18-inch hot zone in an 85-type single crystal furnace. This solution solves the problem of heat radiation leakage caused by numerous seams in traditional soft felt solutions, reducing heating power and shortening material curing time. Simultaneously, the excellent high-temperature deformation resistance of the cured felt material extends the lifespan of the hot zone and improves long-term temperature stability, providing a reliable guarantee for the stable production of high-quality single crystal silicon.

[0009] According to the above-mentioned optimization scheme of the energy-saving single crystal furnace hot field system, the labyrinth sealing structure in the side insulation cylinder segmented structure is selected from one of the stepped mating surface, concave-convex mating surface or tenon-mortise mating surface; the uniform radiation gap between the inner wall of the side insulation cylinder and the outer wall of the heater is 5-15mm. This solution optimizes and limits the key structural features of the side insulation cylinder. First, the introduction of the labyrinth-type sealing structure ensures convenient installation while achieving a seamless seal at the joints between the segments. Even after long-term high-temperature use and slight material shrinkage, the labyrinth path effectively blocks the straight-line penetration of radiant heat, reducing the rate of insulation performance degradation. Second, the radiative gap between the inner wall of the side insulation cylinder and the outer wall of the heater is optimized to 5-15mm. This range is determined based on extensive thermal field simulations and experimental verification: too small a gap can easily lead to installation interference or short circuits due to thermal expansion; too large a gap will reduce the efficiency of radiative heat exchange and increase the transfer of heat to the insulation cylinder. The 5-15mm gap range achieves the optimal balance between the efficient transfer of radiant energy from the heater to the silicon melt and the minimization of heat loss from the insulation layer.

[0010] According to the optimized scheme of the above-mentioned energy-saving single crystal furnace hot zone system, the bottom insulation layer is composed of multiple layers of cured felt with different densities. The layer density near the center of the hot zone is higher than that away from the center, forming a radially distributed gradient insulation structure. This scheme achieves optimized matching of thermal performance by designing the bottom insulation layer as a density gradient structure with higher density on the inside and lower density on the outside: the high-density layer near the center of the hot zone has higher strength and a smoother surface, which can withstand high-temperature radiation and possible mechanical contact, while reducing carbon dust generation; the low-density layer away from the center of the hot zone has a lower thermal conductivity and better insulation performance, maximizing the prevention of heat loss to the furnace bottom. Compared with single-density cured felt, this gradient composite structure can further improve the insulation efficiency under the same thickness conditions, while reducing the amount of expensive high-density materials used, achieving comprehensive optimization of performance and cost. This design reflects a profound understanding of the temperature distribution law inside the hot zone and a refined utilization of material properties.

[0011] According to the optimized scheme of the above-mentioned energy-saving single crystal furnace hot zone system, the inner wall of the side insulation cylinder is also provided with a layer of highly reflective graphite paper or graphite foil to reflect some of the radiant heat back to the central area of ​​the hot zone. Through the provision of the highly reflective graphite paper or graphite foil layer, some of the radiant heat that would otherwise be absorbed by the inner wall of the insulation cylinder and conducted outwards is reflected back to the heater and crucible area, forming a secondary utilization of heat. This design is equivalent to constructing a heat reflection barrier between the insulation layer and the heater, further reducing the heat load and heat loss of the insulation layer; at the same time, the smooth surface of the graphite layer effectively prevents carbon fiber dust from contaminating the silicon melt, which has a positive effect on improving crystal quality. This feature achieves energy saving and efficiency improvement at a relatively low cost.

[0012] According to the optimized scheme of the energy-saving single crystal furnace hot zone system described above, the reinforcing ring of the cured felt around the observation hole of the top insulation cover plate has a thickness greater than the thickness of the top insulation cover plate itself, and is either integrally formed or separately fixedly connected to the top insulation cover plate. The observation hole, as an essential channel for observing crystal growth, is surrounded by an area prone to thermal stress concentration and high-temperature deformation due to structural discontinuity. By setting a thicker reinforcing ring, the structural strength loss caused by the opening is effectively compensated, preventing warping, cracking, or pulverization of the observation hole edge under long-term high-temperature use, thus ensuring the long-term stability of the airtightness and insulation performance of the top insulation layer. The integrally formed or separately fixedly connected design of the reinforcing ring and the cover plate ensures both structural reliability and manufacturing convenience. This detailed optimization reflects the comprehensive consideration of the long-term operational reliability of the hot zone system in this invention, extending the overall service life of the hot zone from a subtle perspective.

[0013] This invention also introduces a modification process for an energy-saving single-crystal furnace thermal field system based on gradient control, as described above, comprising the following steps:

[0014] Step 1: Remove all soft carbon felt insulation material from the original thermal field system and clean up any residue inside the furnace;

[0015] Step 2: Clean and inspect the graphite electrodes, heaters and support structures inside the furnace to ensure they are intact and in normal working order;

[0016] Step 3: Install the bottom insulation layer, side insulation cylinder, and top insulation cover plate in the predetermined positions inside the furnace in sequence. If the side insulation cylinder adopts a segmented structure, insert each segment in sequence according to the predetermined order and apply high-temperature carbon adhesive to the splicing surface to ensure a tight fit between each component and between the component and the furnace body, eliminating installation gaps.

[0017] Step 4: Perform matching and debugging of crystal pulling process parameters. The matching and debugging specifically includes: heating at a power 5%-15% lower than the original process during the material preparation stage; growing stably at a power 10-15 kW lower than the original process during the constant diameter growth stage; adjusting the pulling speed accordingly to maintain the stability of the crystal diameter; and adjusting the crucible rotation speed and crystal rotation speed during the crystal pulling and shoulder formation stages to adapt to the convection characteristics of the new thermal field.

[0018] The method described in this solution has the following advantages: First, the steps are clear and the operation is simple, and ordinary technical workers can master it after a short training period, which facilitates its large-scale promotion and application in the industry; Second, the controllable and reproducible quality of the renovation is ensured through the full-process management of dismantling, cleaning, inspection, installation, and commissioning; Third, the use of high-temperature carbon adhesive and the elimination of splicing gaps during the installation process maximize the seamless advantages of the curing felt components; Fourth, the setting of the commissioning stage enables the new thermal field system to achieve optimal matching with the original equipment, ensuring the effective implementation of energy-saving effects.

[0019] In this invention, the material preparation stage refers to the initial heating stage in the single crystal growth process. Its goal is to completely melt the solid polycrystalline silicon raw material in the quartz crucible into a molten silicon. During this stage, a graphite heater is energized to raise the temperature, transferring heat to the crucible and the internal solid silicon material via radiation. This raises the temperature above the melting point of silicon and maintains it for a sufficient time to ensure all solid silicon material is completely melted. Subsequently, a short-term temperature stabilization stage is entered, ensuring uniform silicon melt temperature and stable thermal convection, facilitating subsequent crystal pulling operations. In the energy-saving single crystal furnace thermal field system described in this invention, the use of a high-density rigid cured felt bottom insulation layer, side insulation cylinders, and top insulation cover provides superior insulation performance compared to traditional all-soft felt thermal field systems. Heat is not easily dissipated, allowing for heating with 5%-15% less power than the original process during the material preparation stage. This achieves energy saving, equipment protection, and improved melt stability.

[0020] This invention also introduces a process for single crystal pulling using the aforementioned gradient-controlled energy-saving single crystal furnace thermal field system, comprising: controlling the heating power during the material preparation and constant diameter growth stages using a power lower than that of the full-soft felt thermal field system, wherein the heating power is 10%-20% lower than that of the full-soft felt thermal field system. The power reduction in this solution directly reflects the fundamental improvement in the system's thermal insulation performance and is the most direct economic benefit obtainable by the user. Taking a workshop with an annual production capacity of 1000 tons of single crystal silicon as an example, this process can save millions of yuan in electricity costs annually. Furthermore, the increased production capacity due to the shortened material preparation time can create additional economic benefits.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. This invention eliminates the massive gap network formed by stacking dozens of soft felt sheets in traditional solutions by using a rigid, hardened felt obtained through a densification process. Whether it's a monolithic or minimally segmented side insulation cylinder, bottom insulation layer, or top cover, a near-seamless, dense thermal insulation barrier is constructed, sealing the main channels for high-temperature radiant heat leakage to the water-cooled furnace wall. This qualitative leap in thermal storage and insulation capacity directly translates into a reduction in heating power during monocrystalline silicon ingot pulling. Throughout the entire process of pulling monocrystalline silicon rods of the same specifications, especially during high-energy-consuming stages such as material preparation and constant-diameter growth, the reduced heating power results in a decrease in the power consumption per unit output of monocrystalline silicon. Simultaneously, the improved thermal efficiency shortens the material preparation time, effectively increasing the ingot pulling capacity per unit time for a single piece of equipment, creating direct economic value for downstream users.

[0023] 2. Cured felt, due to its internal carbon fiber skeleton being firmly bonded by pyrolytic carbon or resin carbon, forms a stable overall network structure, possessing superior high-temperature deformation resistance and thermal shock resistance compared to soft felt. Throughout its 2-3 year lifespan, cured felt components do not experience the fiber pulverization, shrinkage deformation, and interlayer delamination common in soft felt. Its geometric dimensions and insulation performance remain highly stable. The direct benefits of this characteristic are twofold: firstly, the long-term stability of the temperature field within the monocrystalline silicon thermal zone is fundamentally guaranteed, and temperature gradient fluctuations during monocrystalline silicon crystal growth are effectively suppressed, thereby improving the success rate of crystal pulling, crystal formation rate, and the consistency of crystal quality; secondly, the replacement cycle of the core insulation components of the thermal zone is extended to more than three times that of traditional soft felt solutions, reducing capacity losses and spare parts procurement costs caused by furnace shutdowns for thermal zone replacement, effectively controlling the overall operating cost of monocrystalline silicon crystal pulling.

[0024] 3. The cured felt material used in this invention has a smooth and dense surface after densification treatment. Compared to loose, porous, and easily powdery soft carbon felt, it produces almost no carbon particle dust under high-temperature airflow and thermal shock. This clean internal thermal environment effectively avoids carbon dust contamination of the silicon melt, reducing the risk of crystal dislocations or other micro-defects induced by such impurities. This advantage is crucial for the pulling of high-quality single-crystal silicon, especially semiconductor-grade single-crystal silicon with extremely stringent impurity content requirements. It not only improves the yield of qualified crystal rods but also provides a high-quality substrate guarantee for improving the yield of subsequent chip manufacturing processes, supporting the improvement of quality and efficiency in the industry chain from the source. Attached Figure Description

[0025] Fig. 1 This is a schematic diagram of the energy-saving single crystal furnace thermal field system based on gradient control and its modification process according to the present invention.

[0026] Fig. 2 This is a flowchart of the energy-saving single crystal furnace thermal field system modification method in this invention;

[0027] Fig. 3 This is a flowchart of the single crystal pulling process using an energy-saving single crystal furnace thermal field system in this invention.

[0028] In the diagram, 1 is the bottom insulation layer; 2 is the side insulation cylinder; and 3 is the top insulation cover. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] like Figs. 1-3 As shown, this invention discloses an energy-saving single-crystal furnace thermal field system based on gradient control and its modification process, specifically applied to the upgrade and modification of an 85-type single-crystal furnace to adapt to an 18-inch thermal field. This system is a complete replacement for the original insulation system using a multi-layer soft carbon felt stacked and spliced ​​structure. By constructing a rigid, dense, and seamless thermal insulation barrier, it fundamentally eliminates heat radiation leakage channels. Specifically, the system consists of three core components:

[0031] 1. Bottom Insulation Layer 1: Bottom insulation layer 1 is located at the bottom of the furnace body and is made of rigid cured felt obtained through a densification process. To accommodate the limited furnace bottom space of the 85-type single crystal furnace and achieve optimal insulation performance, the bottom insulation layer is designed as an integral structure or is composed of no more than three large pieces of cured felt spliced ​​together. Its shape is precisely matched to the furnace bottom tray, and its thickness is designed to maximize the available space at the furnace bottom, aiming to maximally block heat radiation loss to the furnace bottom water-cooling structure. As a preferred embodiment, the bottom insulation layer can be composed of multiple layers of cured felt with different densities, where the layer density near the center of the hot zone is higher than that far from the center of the hot zone, forming a radially distributed gradient insulation structure. This design ensures structural strength and erosion resistance in high-temperature areas while achieving better thermal insulation performance in low-temperature areas, embodying the core innovative concept of "gradient control".

[0032] 2. Side Insulation Cylinder 2: Side insulation cylinder 2 is coaxially positioned above the bottom insulation layer, surrounding the graphite heater and crucible. It is also made of rigid cured felt obtained through a densification process. Considering the size limitations of the 85-type single crystal furnace door and the convenience of installation and operation, the side insulation cylinder can be designed as an integral cylindrical structure or composed of 2-3 segmented structures. When using a segmented structure, the segments are seamlessly joined using a labyrinth-type sealing structure. This labyrinth-type sealing structure can specifically use one of the following: stepped mating surface, concave-convex mating surface, or tenon-and-mortise mating surface, to ensure that even after long-term high-temperature use, even if the material shrinks slightly, the joint can still effectively block the straight-line penetration of radiant heat.

[0033] The geometry of the side insulation cylinder 2 is precisely designed: its height is precisely matched to the height of the graphite heater in the 18-inch hot zone, ensuring complete coverage of the heating area; a uniform radiative gap is formed between its inner wall and the outer wall of the heater, preferably 5-15mm, which avoids installation interference or short circuits due to thermal expansion, and ensures efficient radiative heat exchange; the radial gap between its outer wall and the inner wall of the 85-type single crystal furnace does not exceed 10mm, maximizing the effective insulation thickness and making full use of every inch of space inside the furnace. As a further preferred option, the inner wall of the side insulation cylinder is also provided with a layer of highly reflective graphite paper or graphite foil. This reflective layer can reflect some of the radiant heat back to the center area of ​​the hot zone, forming a secondary utilization of heat, which can further reduce the heating power by 3%-5%.

[0034] 3. Top Insulation Cover 3: The top insulation cover 3 covers the top of the thermal field and is made of rigid cured felt obtained through a densification process. For ease of installation and maintenance, the top insulation cover is designed as an integral ring structure or is spliced ​​from no more than 4 large pieces of cured felt. An observation hole for crystal growth observation is set in the center. The observation hole is locally reinforced by a cured felt reinforcing ring to prevent deformation under long-term high-temperature use. The thickness of the reinforcing ring is greater than the thickness of the top insulation cover body and is an integrally formed structure or a separate fixed connection structure with the top insulation cover. This effectively compensates for the structural strength loss caused by the opening and ensures the long-term stability of the airtightness and insulation performance of the top insulation layer.

[0035] The bottom insulation layer 1, the side insulation cylinder 2, and the top insulation cover plate 3 together form a nearly seamless rigid insulation cavity that precisely matches the internal space of the 85-type single crystal furnace. Compared with the traditional insulation structure made of dozens of soft carbon felts stacked and spliced ​​together, this cavity eliminates more than 90% of the internal gaps, thereby completely blocking the main channels for high-temperature radiant heat to leak to the water-cooled furnace wall.

[0036] The cured felt described in this invention is a rigid carbon fiber composite material obtained by densification treatment using chemical vapor deposition, resin impregnation carbonization, or a combination of both. After the above processes, its internal carbon fiber skeleton is firmly bonded by pyrolytic carbon or resin-carbon binding phases, forming a three-dimensional integral network structure, thereby possessing mechanical strength, high-temperature deformation resistance, and self-supporting properties far superior to soft carbon felt.

[0037] In terms of physical properties, the density of the cured felt is preferably controlled within the range of 0.15–0.30 g / cm³. Within this density range, the cured felt maintains both good thermal insulation performance and sufficient strength required for structural components. More importantly, after densification treatment, the high-temperature thermal conductivity of the cured felt is lower than that of soft carbon felt of the same density at the same temperature. This means that under the same thickness conditions, the cured felt can provide better thermal insulation. At the same time, the surface of the cured felt is dense and smooth, and it hardly produces carbon particle dust under the scouring of high-temperature airflow and thermal shock, effectively avoiding carbon dust contamination of the silicon melt and playing a positive role in improving crystal quality.

[0038] In this invention, the thermal field refers to the entire heating and insulation system within the furnace used to melt polycrystalline silicon raw materials and maintain the temperature distribution required for crystal growth. It typically includes heaters (such as graphite heaters), insulation materials (such as bottom insulation layers, side insulation cylinders, and top insulation covers), crucibles, and their supporting structures. The structural design of the thermal field determines the temperature gradient, heat loss path, and stability of the crystal growth interface within the furnace, making it a core technical area for single crystal growth.

[0039] The furnace bottom water-cooling structure is located at the bottom of the single crystal furnace body. It is a metal structure with circulating cooling water inside, used to absorb heat leaking from the bottom of the hot zone, prevent the furnace bottom temperature from getting too high, and protect the furnace body and seals. In the energy-saving design, the bottom insulation layer is used to block heat from radiating and dissipating to this water-cooling structure, thereby reducing heat waste.

[0040] The furnace bottom tray is a load-bearing component installed above the furnace bottom water-cooling structure. It is usually made of graphite or carbon-based composite material and is used to support the crucible and the silicon material inside. The shape of the tray is adapted to the bottom insulation layer. The two are usually tightly fitted or maintain a small gap to ensure the insulation effect and stably transfer the mechanical load.

[0041] The graphite heater is located on the side of the hot zone and is made of high-purity graphite. When energized, it generates radiant heat to melt the silicon material and maintain the temperature of the molten silicon in the crucible. A uniform radiant gap is left between its outer wall and the inner wall of the side insulation cylinder to ensure heating efficiency and prevent short circuits.

[0042] The outer periphery of the crucible refers to the space surrounding the outside of the crucible, including the gap between the crucible sidewall and the heater, the area between the bottom of the crucible and the bottom insulation layer, etc. The side insulation cylinder surrounds the graphite heater and the outer periphery of the crucible, and plays the role of reflecting heat and reducing radial heat loss.

[0043] To enable the engineering application of the aforementioned thermal field system, this invention also provides a complete modification method, including the following steps:

[0044] Step 1: Removal and Cleaning: Remove all soft carbon felt insulation materials from the original thermal field system, including multi-layer curved side soft felt, bottom soft felt pad, top spliced ​​soft felt, etc., and thoroughly clean the residual carbon dust and debris inside the furnace.

[0045] Step 2: Inspection and Preparation: Clean and inspect the graphite electrodes, heaters, and support structures inside the furnace to ensure they are intact and in normal working order. If any parts are found to be damaged or misaligned, they should be repaired or adjusted first.

[0046] Step 3: Install the new system: Install the bottom insulation layer 1, side insulation cylinder 2, and top insulation cover plate 3 in the predetermined positions inside the furnace. If the side insulation cylinder has a segmented structure, insert each segment in the predetermined order and apply high-temperature carbon adhesive to the joint surfaces to ensure a tight fit between the segments. During installation, ensure a tight fit between all components and between the components and the furnace body to minimize installation gaps.

[0047] Step 4: Debugging and Optimization: Adjust the crystal pulling process parameters for optimal matching. Specifically, during the material preparation stage, increase the temperature with a power 5%-15% lower than the original process; during the constant diameter growth stage, maintain stable growth with a power 10-15 kW lower, while adjusting the pulling speed accordingly to maintain a stable crystal diameter; and during the crystal pulling and shoulder formation stages, adaptively optimize the crucible rotation speed and crystal rotation speed to match the thermal convection characteristics under the new thermal field. Through the above adjustments, while ensuring crystal quality, gradually reduce the heating power, observe the crystal growth morphology, until the lowest stable operating power window is found, and solidify to form a new optimized process formula.

[0048] In this invention, the fine-tuning of the pulling speed refers to making small, precise adjustments to the crystal rod lifting speed during the constant-diameter stage of single crystal growth, based on the characteristics of the new thermal field. Because the new thermal field system has better insulation performance than the traditional all-soft felt thermal field, and the heat is more concentrated, while gradually reducing the heating power in steps of 0.3-0.8 kW during the constant-diameter growth stage, it is necessary to fine-tune the pulling speed to match the power changes in order to maintain the stability of the crystal diameter. The principle is that by changing the pulling speed by a small margin (e.g., adjusting by a fraction of a millimeter per minute each time), the latent heat released during crystallization can be precisely balanced with the heat provided by the heater, thereby offsetting the thermal field fluctuations caused by the power reduction and ensuring that the crystal diameter is always controlled within the target range.

[0049] When using the thermal field system described in this invention for single-crystal pulling, the key feature is that during the material preparation and constant-diameter growth stages, a lower heating power than that of the traditional all-soft felt thermal field system is used for control. This lower heating power can be 10%-20% lower than that of the traditional all-soft felt thermal field system. Taking a workshop with an annual production capacity of 1000 tons of single-crystal silicon as an example, this process can save millions of yuan in electricity costs annually. Furthermore, the increased production capacity due to the 10%-15% reduction in material preparation time can create additional economic benefits.

[0050] The cured felt component described in this invention can be widely used in the 18-inch thermal insulation system of a Type 85 single crystal furnace, specifically as a side insulation cylinder 1, a bottom insulation layer 2, or a top insulation cover 3, to replace the original multi-layer soft carbon felt stacked and spliced ​​structure. This application solution can fundamentally eliminate gaps inside the insulation layer, reduce heat radiation leakage, extend the service life of the thermal field, and provide an economical, efficient, and reliable solution for the energy-saving upgrade and transformation of existing Type 85 single crystal furnaces.

[0051] Example 1:

[0052] Photovoltaic material manufacturers possess multiple Type 85 monocrystalline furnaces. The original solution used an 18-inch hot zone combined with fully soft carbon felt insulation, which resulted in high power consumption per unit output, frequent replacement of the soft felt insulation layer causing downtime losses, and significant fluctuations in crystal quality. To conduct a pilot technical upgrade, this embodiment employs a double-layer composite structure formed by chemical vapor deposition and the addition of a low-density cured felt pad underneath. Considering the furnace door size limitations of the Type 85 monocrystalline furnace, the side insulation cylinder is designed with a three-lobed structure. A stepped labyrinth sealing structure is used between the lobes to ensure a radiant heat blocking path. After assembly, the total height of the insulation cylinder matches the height of the graphite heater. A uniform radiant gap is maintained between the inner wall and the outer wall of the heater, while the gap between the outer wall and the inner wall of the furnace is designed to maximize the effective insulation thickness. The top insulation cover is designed as a splicing of multiple fan-shaped cured felt pieces, with a cured felt reinforcement ring, thicker than the cover body, around the central observation hole and secured with graphite screws. During the renovation, the original soft felt was removed sequentially, the furnace body was cleaned, problematic heaters were inspected and replaced, and the crucible support shaft was calibrated. The bottom insulation layer, side insulation cylinders, and top cover were then installed from bottom to top, ensuring minimal seams after installation. During the process debugging phase, the melting power was reduced compared to the original process, and the melting time was correspondingly shortened. During the crystal pulling and shoulder formation phase, the crucible rotation speed and crystal rotation speed were adjusted to match the thermal convection characteristics of the new thermal field. During the constant diameter growth phase, the power was gradually reduced in a step-by-step manner, and the pulling speed was stabilized within the process requirements. Statistics after continuous operation showed reduced power consumption per unit output, a narrower power fluctuation range, and improved crystal quality consistency. After long-term operation and furnace shutdown inspection, the stepped sealing structure at the side insulation cylinder seams was intact with no straight gaps, and there were no obvious traces of powdering on the inner wall of the insulation cylinder. The insulation performance degradation rate was far superior to the original soft felt solution, and the service life was extended.

[0053] Example 2:

[0054] Semiconductor-grade monocrystalline silicon manufacturers have extremely stringent requirements for crystal quality. The original 85-type monocrystalline furnace all-soft felt solution suffers from dislocation problems caused by carbon dust contamination and the impact of oxygen content fluctuations on subsequent epitaxial processes. This embodiment uses a combined process to densify ultra-high purity cured felt to construct the thermal field system, and introduces high-reflectivity graphite paper reflective layer technology to improve heat preservation efficiency: the bottom insulation layer adopts an integral structure and is designed as a multi-layer density gradient composite to form a gradient insulation structure along the axial or radial direction; the side insulation cylinder is designed as an integral cylindrical structure to eliminate splicing gaps due to the allowable furnace door diameter, and is treated with a densification process, with high-purity graphite paper attached to the inner wall to reflect radiant heat back to the center of the thermal field; the top insulation cover is designed as an integral ring structure, with a reinforcing ring integrally formed with the cover around the central observation hole, and the lower surface is also attached with high-purity graphite paper. During the renovation, after removing the original soft felt, the furnace inner wall was thoroughly cleaned. After confirming the heater was in good condition and the crucible support shaft coaxiality met requirements, the bottom insulation layer was installed and coated with a thin layer of graphite lubricant. Using specialized hoisting clamps, the integral side insulation cylinder was vertically lowered from the furnace top and its coaxiality was adjusted and fixed. Finally, the top insulation cover was installed, with no gaps between the joints. During the process debugging phase, the material processing power was reduced compared to the original process. Thanks to the heat reflection effect of the graphite paper reflective layer, the material processing time was shortened. During the crystal pulling and shoulder formation stage, the crucible rotation speed and crystal rotation speed were adjusted to match a stable thermal field. During the constant diameter growth stage, the power was adjusted with precise step sizes, and the pulling speed remained stable within the process requirements, with minimal power fluctuations. After continuous operation, tests showed reduced power consumption per unit output, lower carbon content, narrower oxygen content fluctuations, lower dislocation density, and improved crystal formation rate. After a long period of operation, the furnace was shut down for inspection. The integral insulation cylinder structure was intact without deformation or cracking. The surface of the graphite paper reflective layer on the inner wall was smooth without obvious oxidation or peeling. The bottom gradient insulation layer was tightly bonded with no delamination. The insulation performance degradation rate was extremely low. The cleanliness inside the furnace was extremely high with almost no visible carbon dust deposits. The total service life was improved compared to the original soft felt solution. The company has decided to upgrade and renovate all similar furnaces according to this solution.

[0055] In summary, this invention, through the organic combination of material innovation and structural reconstruction, achieves a substantial improvement in heat preservation performance and a reduction in energy consumption in the specific scenario of an 85-type single crystal furnace adapted to an 18-inch hot zone. Simultaneously, it extends the lifespan of the hot zone and improves the crystal growth environment, demonstrating industrial application value and economic and social benefits. The above embodiments are merely preferred examples of this invention; any equivalent transformations or modifications based on the technical concept of this invention fall within the protection scope of this invention.

[0056] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Although the applicant has described the present invention in detail with reference to preferred embodiments, those skilled in the art should understand that any modifications or equivalent substitutions made to the technical solutions of the present invention cannot depart from the spirit and scope of the present invention and should be covered within the scope of the claims of the present invention.

Claims

1. An energy-saving single-crystal furnace hot zone system based on gradient control, applied to an 85-type single-crystal furnace adapted to an 18-inch hot zone, characterized in that, The system includes: The bottom insulation layer (1) is set at the bottom of the furnace body and is made of rigid curing felt obtained by densification process. The bottom insulation layer (1) is an integral structure or is spliced ​​from 1 to 3 large pieces of curing felt. Its shape is adapted to the furnace bottom tray. The thickness of the bottom insulation layer (1) is adapted to the height of the furnace bottom space and is used to block the heat from radiating to the furnace bottom water cooling structure. The side insulation cylinder (2) is coaxially arranged above the bottom insulation layer (1) and surrounds the graphite heater and crucible. It is made of rigid curing felt obtained by densification process. The side insulation cylinder (2) is an integral cylindrical structure or a combination of 2 or 3 segmented structures. When the segmented structure is used, the segments are spliced ​​together by a labyrinth sealing structure. The height of the side insulation cylinder (2) matches the height of the graphite heater in the 18-inch hot field. A uniform radiation gap is formed between its inner wall and the outer wall of the heater. The radial gap between its outer wall and the inner wall of the 85-type single crystal furnace is less than or equal to 10 mm. The top insulation cover (3) covers the top of the hot field and is made of rigid curing felt obtained by densification process. The top insulation cover (3) is an integral ring structure or is spliced ​​from 4 large pieces of curing felt. The top insulation cover (3) has an observation hole for crystal growth observation at the center. The observation hole is surrounded by a curing felt reinforcement ring to prevent deformation under long-term high temperature use. The cured felt is a rigid carbon fiber composite material obtained by densification treatment through chemical vapor deposition, resin impregnation carbonization, or a combination of both. Its internal carbon fiber skeleton is firmly bonded by pyrolytic carbon or resin carbon bonding phase to form a three-dimensional integral network structure. The density of the cured felt is 0.15 to 0.30 g / cm³. The bottom insulation layer (1), the side insulation cylinder (2) and the top insulation cover plate (3) form a rigid insulation cavity that is integrated with the internal space of the 85-type single crystal furnace body.

2. The energy-saving single-crystal furnace thermal field system based on gradient control according to claim 1, characterized in that, The labyrinth sealing structure in the split structure of the side insulation cylinder (2) is selected from one of the stepped mating surface, the concave-convex mating surface or the tenon-and-mortise mating surface; the uniform radiation gap between the inner wall of the side insulation cylinder (2) and the outer wall of the heater is 5-15mm.

3. The energy-saving single-crystal furnace thermal field system based on gradient control according to claim 1, characterized in that, The bottom insulation layer (1) is composed of multiple layers of cured felt with different densities. The layer density near the center of the thermal field is higher than that far from the center of the thermal field, forming a gradient insulation structure distributed radially.

4. The energy-saving single-crystal furnace thermal field system based on gradient control according to claim 1, characterized in that, The inner wall of the side insulation cylinder (2) is also provided with a layer of graphite paper or graphite foil with high reflectivity, which is used to reflect part of the radiant heat back to the center area of ​​the thermal field.

5. The energy-saving single-crystal furnace thermal field system based on gradient control according to claim 1, characterized in that, The observation hole of the top insulation cover (3) is surrounded by a curing felt reinforcing ring, the thickness of which is greater than the thickness of the top insulation cover (3) body, and it is an integrally formed structure or a separate fixed connection structure with the top insulation cover (3).

6. A process for retrofitting the energy-saving single-crystal furnace thermal field system based on gradient control as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Step 1: Remove all soft carbon felt insulation material from the original thermal field system and clean up any residue inside the furnace; Step 2: Clean and inspect the graphite electrodes, heaters, and support structures inside the furnace, and ensure they are in normal working position; Step 3: Install the bottom insulation layer (1), side insulation cylinder (2), and top insulation cover plate (3) in the predetermined positions inside the furnace in sequence. If the side insulation cylinder adopts a segmented structure, put each segment in in the predetermined order and apply high-temperature carbon adhesive to the splicing surface. Step 4: Perform matching and debugging of crystal pulling process parameters. The matching and debugging specifically includes: heating at a power 5%-15% lower than the original process during the material preparation stage; growing stably at a power 10-15 kW lower than the original process during the constant diameter growth stage; adjusting the pulling speed accordingly to maintain the stability of the crystal diameter; and adjusting the crucible rotation speed and crystal rotation speed during the crystal pulling and shoulder formation stages to adapt to the convection characteristics of the new thermal field.

7. The modification process according to claim 6, characterized in that, include: During the material preparation and constant diameter growth stages, the heating power is controlled by a lower power than that of the full soft felt thermal field system, which is 10%-20% lower.

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