How to achieve high quality crystal growth? - SiC crystal growth furnace

1.What is the basic principle of silicon carbide crystal growth furnace?

The working principle of the silicon carbide crystal growth furnace is physical sublimation (PVT). The PVT method is one of the most efficient methods for growing high-purity SiC single crystals. Through precise control of the thermal field, atmosphere and growth parameters, the silicon carbide crystal growth furnace can operate stably at high temperatures to complete the sublimation, gas phase transmission and condensation crystallization process of SiC powder.

1.1 Working principle of the growth furnace

● PVT method

The core of the PVT method is to sublimate silicon carbide powder into gaseous components at high temperatures, and condense on the seed crystal through gas phase transmission to form a single crystal structure. This method has significant advantages in preparing high-purity, large-size crystals.

● Basic process of crystal growth

✔ Sublimation: SiC powder in the crucible is sublimated into gaseous components such as Si, C2 and SiC2 at a high temperature above 2000℃.

✔ Transport: Under the action of thermal gradient, the gaseous components are transmitted from the high temperature zone (powder zone) to the low temperature zone (seed crystal surface).

✔ Condensation crystallization: Volatile components precipitate on the seed crystal surface and grow along the lattice direction to form a single crystal.

1.2 Specific principles of crystal growth

The growth process of silicon carbide crystals is divided into three stages, which are closely linked to each other and affect the final quality of the crystal.

✔ SiC powder sublimation: Under high temperature conditions, solid SiC (silicon carbide) will sublimate into gaseous silicon (Si) and gaseous carbon (C), and the reaction is as follows:

SiC (s) → Si (g) + C (g)

And more complex secondary reactions to generate volatile gaseous components (such as SiC2). High temperature is a necessary condition to promote sublimation reactions.

✔ Gas phase transport: The gaseous components are transported from the sublimation zone of the crucible to the seed zone under the drive of the temperature gradient. The stability of the gas flow determines the uniformity of the deposition.

✔ Condensation crystallization: At lower temperatures, volatile gaseous components combine with the surface of the seed crystal to form solid crystals. This process involves complex mechanisms of thermodynamics and crystallography.

1.3 Key parameters for silicon carbide crystal growth

High-quality SiC crystals require precise control of the following parameters:

✔ Temperature: The sublimation zone needs to be kept above 2000℃ to ensure complete decomposition of the powder.The temperature of the seed zone is controlled at 1600-1800℃ to ensure a moderate deposition rate.

✔ Pressure: PVT growth is usually carried out in a low-pressure environment of 10-20 Torr to maintain the stability of gas phase transport.Too high or too low pressure will lead to too fast crystal growth rate or increased defects.

✔ Atmosphere: Use high-purity argon as a carrier gas to avoid impurity contamination during the reaction process.The purity of the atmosphere is crucial to the suppression of crystal defects.

✔ Time: The crystal growth time is usually up to tens of hours to achieve uniform growth and appropriate thickness.

2.What is the structure of the silicon carbide crystal growth furnace?

The optimization of the structure of the silicon carbide crystal growth furnace mainly focuses on high-temperature heating, atmosphere control, temperature field design and monitoring system.

2.1 Main components of the growth furnace

● High-temperature heating system

✔ Resistance heating: Use high-temperature resistance wire (such as molybdenum, tungsten) to directly provide heat energy. The advantage is high temperature control accuracy, but the life is limited at high temperature.

✔ Induction heating: Eddy current heating is generated in the crucible through an induction coil. It has the advantages of high efficiency and non-contact, but the equipment cost is relatively high.

● Graphite crucible and substrate seed station

✔ High-purity graphite crucible ensures high-temperature stability.

✔ The design of the seed station must take into account both airflow uniformity and thermal conductivity.

● Atmosphere control device

✔ Equipped with a high-purity gas delivery system and a pressure regulating valve to ensure the purity and stability of the reaction environment.

● Temperature field uniformity design

✔ By optimizing the crucible wall thickness, heating element distribution and heat shield structure, the uniform distribution of the temperature field is achieved, reducing the impact of thermal stress on the crystal.

2.2 Temperature field and thermal gradient design

✔ Importance of temperature field uniformity: Uneven temperature field will lead to different local growth rates and defects inside the crystal. The uniformity of temperature field can be greatly improved through annular symmetry design and heat shield optimization.

✔ Precise control of thermal gradient: Adjust the power distribution of heaters and use heat shields to separate different areas to reduce temperature differences. Because thermal gradients have a direct impact on crystal thickness and surface quality.

2.3 Monitoring system for crystal growth process

✔ Temperature monitoring: Use fiber optic temperature sensors to monitor the real-time temperature of the sublimation zone and seed zone. The data feedback system can automatically adjust the heating power.

✔ Growth rate monitoring: Use laser interferometry to measure the growth rate of the crystal surface. Combine monitoring data with modeling algorithms to dynamically optimize the process.

3.What are the technical difficulties of silicon carbide crystal growth furnace?

The technical bottlenecks of silicon carbide crystal growth furnace are mainly concentrated in high-temperature materials, temperature field control, defect suppression and size expansion.

3.1 Selection and challenges of high-temperature materials

Graphite is easily oxidized at extremely high temperatures, and SiC coating needs to be added to improve oxidation resistance. The quality of the coating directly affects the life of the furnace.

Heating element life and temperature limit. High-temperature resistance wires need to have high fatigue resistance. Induction heating equipment needs to optimize the coil heat dissipation design.

3.2 Precise control of temperature and thermal field

The influence of non-uniform thermal field will lead to an increase in stacking faults and dislocations. The furnace thermal field simulation model needs to be optimized to detect problems in advance.

Reliability of high-temperature monitoring equipment. High-temperature sensors need to be resistant to radiation and thermal shock.

3.3 Control of crystal defects

Stacking faults, dislocations and polymorphic hybrids are the main defect types. Optimizing the thermal field and atmosphere helps to reduce the defect density.

Control of impurity sources. The use of high-purity materials and the sealing of the furnace are crucial to impurity suppression.

3.4 Challenges of large-size crystal growth

The requirements of thermal field uniformity for size expansion. When the crystal size is expanded from 4 inches to 8 inches, the temperature field uniformity design needs to be fully upgraded.

Solution to crack and warping problems. Reduce crystal deformation by reducing thermal stress gradient.

4.What are the raw materials for growing high-quality SiC crystals?

VeTek Semiconductor has developed a new SiC single crystal raw material - High purity CVD SiC raw material. This product fills the domestic gap and is also at the leading level globally, and will be in a long-term leading position in the competition. Traditional silicon carbide raw materials are produced by the reaction of high-purity silicon and graphite, which are high in cost, low in purity and small in size.

VeTek Semiconductor's fluidized bed technology uses methyltrichlorosilane to generate silicon carbide raw materials through chemical vapor deposition, and the main by-product is hydrochloric acid. Hydrochloric acid can form salts by neutralizing with alkali, and will not cause any pollution to the environment. 

At the same time, methyltrichlorosilane is a widely used industrial gas with low cost and wide sources, especially China is the main producer of methyltrichlorosilane. Therefore, VeTek Semiconductor's High purity CVD SiC raw material has international leading competitiveness in terms of cost and quality.The purity of High purity CVD SiC raw material is higher than 99.9995%.

✔ Large size and high density: The average particle size is about 4-10mm, and the particle size of domestic Acheson raw materials is <2.5mm. The same volume crucible can hold more than 1.5kg of raw materials, which is conducive to solving the problem of insufficient supply of large-size crystal growth materials, alleviating the graphitization of raw materials, reducing carbon wrapping and improving crystal quality.

✔ Low Si/C ratio: It is closer to 1:1 than the Acheson raw materials of the self-propagating method, which can reduce the defects induced by the increase of Si partial pressure.

✔ High output value: The grown raw materials still maintain the prototype, reduce recrystallization, reduce the graphitization of raw materials, reduce carbon wrapping defects, and improve the quality of crystals.

✔ Higher purity: The purity of raw materials produced by the CVD method is higher than that of the Acheson raw materials of the self-propagating method. The nitrogen content has reached 0.09ppm without additional purification. This raw material can also play an important role in the semi-insulating field.

✔ Lower cost: The uniform evaporation rate facilitates process and product quality control, while improving the utilization rate of raw materials (utilization rate>50%, 4.5kg raw materials produce 3.5kg ingots), reducing costs.

✔ Low human error rate: Chemical vapor deposition avoids impurities introduced by human operation.