The growth process of monocrystalline silicon is completely carried out in the thermal field. A good thermal field is conducive to improving the quality of crystals and has a higher crystallization efficiency. The design of the thermal field largely determines the changes in temperature gradients in the dynamic thermal field and the flow of gas in the furnace chamber. The difference in the materials used in the thermal field directly determines the service life of the thermal field. An unreasonable thermal field is not only difficult to grow crystals that meet quality requirements, but also cannot grow complete monocrystalline under certain process requirements. This is why the direct-pull monocrystalline silicon industry regards thermal field design as the most core technology and invests huge manpower and material resources in thermal field research and development.
The thermal system is composed of various thermal field materials. We only briefly introduce the materials used in the thermal field. As for the temperature distribution in the thermal field and its impact on crystal pulling, we will not analyze it here. The thermal field material refers to the structure and thermal insulation part in the vacuum furnace chamber of crystal growth, which is essential for creating an appropriate temperature distribution around the semiconductor melt and crystal.
1. Thermal field structure material
The basic supporting material for the direct-pull method to grow monocrystalline silicon is high-purity graphite. Graphite materials play a very important role in modern industry. They can be used as heat field structural components such as heaters, guide tubes, crucibles, insulation tubes, crucible trays, etc. in the preparation of monocrystalline silicon by the Czochralski method.
Graphite materials are selected because they are easy to prepare in large volumes, can be processed and are resistant to high temperatures. Carbon in the form of diamond or graphite has a higher melting point than any element or compound. Graphite materials are quite strong, especially at high temperatures, and their electrical and thermal conductivity is also quite good. Its electrical conductivity makes it suitable as a heater material. It has a satisfactory thermal conductivity coefficient, which allows the heat generated by the heater to be evenly distributed to the crucible and other parts of the heat field. However, at high temperatures, especially over long distances, the main heat transfer mode is radiation.
Graphite parts are initially made of fine carbonaceous particles mixed with a binder and formed by extrusion or isostatic pressing. High-quality graphite parts are usually isostatically pressed. The whole piece is first carbonized and then graphitized at very high temperatures, close to 3000°C. The parts processed from these whole pieces are usually purified in a chlorine-containing atmosphere at high temperatures to remove metal contamination to meet the requirements of the semiconductor industry. However, even after proper purification, the level of metal contamination is several orders of magnitude higher than that allowed for silicon monocrystalline materials. Therefore, care must be taken in the thermal field design to prevent contamination of these components from entering the melt or crystal surface.
Graphite materials are slightly permeable, which makes it easy for the remaining metal inside to reach the surface. In addition, the silicon monoxide present in the purge gas around the graphite surface can penetrate into most materials and react.
Early monocrystalline silicon furnace heaters were made of refractory metals such as tungsten and molybdenum. With the increasing maturity of graphite processing technology, the electrical properties of the connection between graphite components have become stable, and monocrystalline silicon furnace heaters have completely replaced tungsten, molybdenum and other material heaters. At present, the most widely used graphite material is isostatic graphite. my country’s isostatic graphite preparation technology is relatively backward, and most of the graphite materials used in the domestic photovoltaic industry are imported from abroad. Foreign isostatic graphite manufacturers mainly include Germany’s SGL, Japan’s Tokai Carbon, Japan’s Toyo Tanso, etc. In Czochralski monocrystalline silicon furnaces, C/C composite materials are sometimes used, and they have begun to be used to manufacture bolts, nuts, crucibles, load plates and other components. Carbon/carbon (C/C) composites are carbon fiber reinforced carbon-based composites with a series of excellent properties such as high specific strength, high specific modulus, low thermal expansion coefficient, good electrical conductivity, high fracture toughness, low specific gravity, thermal shock resistance, corrosion resistance, and high temperature resistance. At present, they are widely used in aerospace, racing, biomaterials and other fields as new high-temperature resistant structural materials. At present, the main bottlenecks encountered by domestic C/C composites are still cost and industrialization issues.
There are many other materials used to make thermal fields. Carbon fiber reinforced graphite has better mechanical properties; but it is more expensive and has other requirements for design. Silicon carbide (SiC) is a better material than graphite in many aspects, but it is much more expensive and difficult to prepare large-volume parts. However, SiC is often used as a CVD coating to increase the life of graphite parts exposed to corrosive silicon monoxide gas, and can also reduce contamination from graphite. The dense CVD silicon carbide coating effectively prevents contaminants inside the microporous graphite material from reaching the surface.
Another is CVD carbon, which can also form a dense layer above the graphite part. Other high temperature resistant materials, such as molybdenum or ceramic materials that can coexist with the environment, can be used where there is no risk of contaminating the melt. However, oxide ceramics are generally limited in their applicability to graphite materials at high temperatures, and there are few other options if insulation is required. One is hexagonal boron nitride (sometimes called white graphite due to similar properties), but the mechanical properties are poor. Molybdenum is generally used reasonably for high temperature situations because of its moderate cost, low diffusion rate in silicon crystals, and a very low segregation coefficient of about 5×108, which allows a certain amount of molybdenum contamination before destroying the crystal structure.
2. Thermal insulation materials
The most commonly used insulation material is carbon felt in various forms. Carbon felt is made of thin fibers, which act as insulation because they block thermal radiation multiple times over a short distance. The soft carbon felt is woven into relatively thin sheets of material, which are then cut into the desired shape and tightly bent into a reasonable radius. Cured felts are composed of similar fiber materials, and a carbon-containing binder is used to connect the dispersed fibers into a more solid and shaped object. The use of chemical vapor deposition of carbon instead of a binder can improve the mechanical properties of the material.
Typically, the outer surface of the thermal insulation curing felt is coated with a continuous graphite coating or foil to reduce erosion and wear as well as particulate contamination. Other types of carbon-based thermal insulation materials also exist, such as carbon foam. In general, graphitized materials are obviously preferred because graphitization greatly reduces the surface area of the fiber. The outgassing of these high-surface-area materials is greatly reduced, and it takes less time to pump the furnace to a suitable vacuum. Another is C/C composite material, which has outstanding characteristics such as light weight, high damage tolerance and high strength. Used in thermal fields to replace graphite parts significantly reduces the frequency of graphite parts replacement, improves monocrystalline quality and production stability.
According to the raw material classification, carbon felt can be divided into polyacrylonitrile-based carbon felt, viscose-based carbon felt, and pitch-based carbon felt.
Polyacrylonitrile-based carbon felt has a large ash content. After high-temperature treatment, the single fiber becomes brittle. During operation, it is easy to generate dust to pollute the furnace environment. At the same time, the fiber can easily enter the pores and respiratory tract of the human body, which is harmful to human health. Viscose-based carbon felt has good thermal insulation performance. It is relatively soft after heat treatment and is not easy to generate dust. However, the cross-section of the viscose-based raw fiber is irregular, and there are many grooves on the fiber surface. It is easy to generate gases such as C02 under the oxidizing atmosphere of the CZ silicon furnace, causing the precipitation of oxygen and carbon elements in the monocrystalline silicon material. The main manufacturers include German SGL and other companies. At present, the most widely used in the semiconductor monocrystalline industry is pitch-based carbon felt, which has worse thermal insulation performance than viscose-based carbon felt, but pitch-based carbon felt has a higher purity and a lower dust emission. Manufacturers include Japan’s Kureha Chemical and Osaka Gas.
Because the shape of carbon felt is not fixed, it is inconvenient to operate. Now many companies have developed a new thermal insulation material based on carbon felt-cured carbon felt. Cured carbon felt, also called hard felt, is a carbon felt with a certain shape and self-sustaining property after soft felt is impregnated with resin, laminated, cured and carbonized.
The growth quality of monocrystalline silicon is directly affected by the thermal environment, and carbon fiber thermal insulation materials play a key role in this environment. Carbon fiber thermal insulation soft felt still has a significant advantage in the photovoltaic semiconductor industry due to its cost advantage, excellent thermal insulation effect, flexible design and customizable shape. In addition, carbon fiber hard thermal insulation felt will have greater development space in the thermal field material market due to its certain strength and higher operability. We are committed to research and development in the field of thermal insulation materials, and continuously optimize product performance to promote the prosperity and development of the photovoltaic semiconductor industry.
Post time: Jun-12-2024