The Key Factor of Single Crystal Silicon Growth Quality - Graphite Thermal Field

The growth process of monocrystalline silicon is entirely carried out in a thermal field. A good thermal field is beneficial for improving crystal quality and has high 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. The difference in materials used in the thermal field directly determines the service life of the thermal field. However, an improperly designed thermal field not only makes it difficult to grow crystals that meet quality requirements, but also cannot grow complete single crystals under certain process requirements. This is also why the Czochralski 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 will only provide a brief introduction to 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. Thermal field materials refer to the structure and insulation of the vacuum furnace chamber for crystal growth, which is essential for creating an appropriate temperature distribution around the semiconductor melt and crystal.

1. Thermal field structural materials

The basic supporting material for the growth of monocrystalline silicon by Czochralski method is high-purity graphite. Graphite materials play a very important role in modern industry, and can be used as thermal field structural components such as heaters, guide tubes, crucibles, insulation tubes, crucible trays, etc. in the preparation of monocrystalline silicon by Czochralski method.

The reason for choosing graphite material is because it is easy to prepare large volume, machinability, and high-temperature resistance. Carbon in the form of diamond or graphite has a higher melting point than any element or compound. Graphite materials are quite sturdy, especially at high temperatures, and their conductivity and thermal conductivity are also quite good. Its conductivity makes it suitable as a heater material, and it has a satisfactory thermal conductivity, which can evenly distribute the heat generated by the heater to other parts of the crucible and thermal field. However, at high temperatures, especially over long distances, radiation is the main heat transfer method.

Graphite components are initially formed by mixing fine carbon particles with adhesive and forming them through extrusion or isostatic pressing. High quality graphite parts are usually formed by isostatic pressing. The whole block is first carbonized and then graphitized at very high temperatures, approaching 3000 ℃. The components processed from these whole blocks are usually purified in a chlorine containing atmosphere at high temperatures to remove metal contamination and meet the requirements of the semiconductor industry. However, even with appropriate purification, the level of metal contamination is several orders of magnitude higher than allowed for silicon single crystal materials. Therefore, in thermal field design, attention must be paid to preventing contamination of these components from entering the melt or crystal surface.

Graphite materials have slight permeability, which makes it easy for the remaining metal inside to reach the surface. In addition, silicon monoxide present around the graphite surface in the purified gas can penetrate into most materials and undergo reactions.

Early single crystal silicon furnace heaters were made of refractory metals such as tungsten and molybdenum. With the increasing maturity of graphite processing technology, the electrical performance of the connections between graphite components has become stable, and single crystal silicon furnace heaters have completely replaced materials such as tungsten and molybdenum heaters. The most widely used graphite material currently is isostatic pressing graphite.

In Czochralski single crystal silicon furnaces, sometimes C/C composite materials are also used, and they have now been used to manufacture components such as bolts, nuts, crucibles, and bearing plates. Carbon/carbon (c/c) composite material is a carbon fiber reinforced carbon based composite material, which has a series of excellent properties such as high specific strength, high specific modulus, low coefficient of thermal expansion, good conductivity, high fracture toughness, low specific gravity, thermal shock resistance, corrosion resistance, and high temperature resistance. Currently, it is widely used as a new type of high-temperature resistant structural material in aerospace, racing, biomaterials, and other fields.

There are many other materials used to create thermal fields. Carbon fiber reinforced graphite has better mechanical properties; But the price is higher and there are other requirements for the design. Silicon carbide (SiC) is a better material than graphite in many aspects, but its cost is much higher and it is difficult to prepare large volume components. However, SiC is often used as a CVD coating to improve the lifespan of graphite components exposed to aggressive silicon monoxide gas and reduce contamination from graphite. The dense CVD silicon carbide coating effectively prevents pollutants inside the microporous graphite material from reaching the surface.

Another type is CVD carbon, which can also form a dense layer above graphite components. Other high-temperature resistant materials, such as molybdenum or ceramic materials that can coexist with the environment, can be used in areas where there is no risk of contaminating the melt. However, the applicability of oxide ceramics in direct contact with graphite materials at high temperatures is limited, and if insulation is required, there are almost no other options. One type is hexagonal boron nitride (sometimes referred to as white graphite due to its similar properties), but its mechanical properties are poor. Molybdenum is usually reasonably used in high-temperature situations because it has a moderate cost, low diffusion rate in silicon crystals, and a very low segregation coefficient of about 5 × 108, which allows for a certain amount of molybdenum contamination before damaging the crystal structure.

2. Thermal insulation materials for thermal fields

The most commonly used insulation material is carbon felt in different forms. Carbon felt is made of thin fibers, which act as insulation because they repeatedly block thermal radiation over short distances. Soft carbon felt is woven into relatively thin sheet like materials, then cut into the desired shape and tightly bent to a reasonable radius. Cured felt is composed of similar fiber materials and uses carbon containing binders to connect dispersed fibers into more solid and shaped objects. The use of carbon chemical vapor deposition as a substitute for binders can improve the mechanical properties of materials.

Usually, the outer surface of the insulation cured felt is coated with a continuous graphite coating or foil to reduce erosion, wear, and particle contamination. Other types of carbon based insulation materials also exist, such as carbon foam. Generally speaking, graphitized materials are clearly the preferred choice, as graphitization greatly reduces the surface area of fibers. The gas output of these high surface area materials is greatly reduced, and the time required to vacuum the furnace to the appropriate level is shorter. Another type is C/C composite material, which has outstanding characteristics of light weight, high damage tolerance, and high strength. Used to replace graphite components in the thermal field, it significantly reduces the frequency of graphite component replacement, improves single crystal quality and production stability.

According to the classification of raw materials, carbon felt can be divided into polyacrylonitrile based carbon felt, adhesive based carbon felt, and asphalt based carbon felt.

Polyacrylonitrile based carbon felt has a high ash content, and after high-temperature treatment, the single filament becomes brittle. During operation, it is easy to produce dust pollution to the furnace environment. At the same time, the fibers are easy to enter human pores and respiratory tract, posing a threat to human health; The adhesive based carbon felt has good thermal insulation performance, is relatively soft after heat treatment, and is not easy to generate dust. However, the cross-section of the adhesive based precursor is irregular, and there are many grooves on the fiber surface. In the presence of an oxidizing atmosphere in the Czochralski furnace, CO2 and other gases are easily generated, causing the deposition of oxygen and carbon elements in the monocrystalline silicon material. At present, the most commonly used material in the semiconductor single crystal industry is asphalt based carbon felt, which has poorer insulation performance than adhesive based carbon felt. However, asphalt based carbon felt has higher purity and lower dust generation.

Due to the unstable shape of the charcoal felt, it is inconvenient to operate. Nowadays, many companies have developed a new type of thermal insulation material, solidified carbon felt, based on carbon felt. Cured carbon felt, also known as hard felt, is a type of carbon felt that has a certain shape and self-sustaining properties after being laminated, cured, and carbonized by impregnating soft felt with resin.

The growth quality of monocrystalline silicon is directly affected by the thermal field environment, and carbon fiber insulation materials play a crucial role in this environment. Carbon fiber insulation soft felt still holds significant advantages in the photovoltaic semiconductor industry due to its cost advantage, excellent insulation effect, flexible design, and customizable shape. In addition, carbon fiber hard insulation felt will have greater development space in the thermal field material market due to its certain strength and higher operability.