Fourth, Physical vapor transfer method
Physical vapor transport (PVT) method originated from the vapor phase sublimation technology invented by Lely in 1955. The SiC powder is placed in a graphite tube and heated to high temperature to decompose and sublimate the SiC powder, and then the graphite tube is cooled. After the decomposition of the SiC powder, the vapor phase components are deposited and crystallized into SiC crystals around the graphite tube. Although this method is difficult to obtain large size SiC single crystals, and the deposition process in the graphite tube is difficult to control, it provides ideas for subsequent researchers.
Y.m. Terairov et al. in Russia introduced the concept of seed crystals on this basis, and solved the problem of uncontrollable crystal shape and nucleation position of SiC crystals. Subsequent researchers continued to improve and eventually developed the physical gas phase transport (PVT) method in industrial use today.
As the earliest SiC crystal growth method, physical vapor transfer method is the most mainstream growth method for SiC crystal growth. Compared with other methods, the method has low requirements for growth equipment, simple growth process, strong controllability, thorough development and research, and has realized industrial application. The structure of crystal grown by the current mainstream PVT method is shown in the figure.
The axial and radial temperature fields can be controlled by controlling the external thermal insulation conditions of the graphite crucible. The SiC powder is placed at the bottom of the graphite crucible with a higher temperature, and the SiC seed crystal is fixed at the top of the graphite crucible with a lower temperature. The distance between the powder and the seed is generally controlled to be tens of millimeters to avoid contact between the growing single crystal and the powder. The temperature gradient is usually in the range of 15-35℃/cm. An inert gas of 50-5000 Pa is kept in the furnace to increase convection. In this way, after the SiC powder is heated to 2000-2500℃ by induction heating, the SiC powder will sublimate and decompose into Si, Si2C, SiC2 and other vapor components, and be transported to the seed end with gas convection, and the SiC crystal is crystallized on the seed crystal to achieve single crystal growth. Its typical growth rate is 0.1-2mm/h.
PVT process focuses on the control of growth temperature, temperature gradient, growth surface, material surface spacing and growth pressure, its advantage is that its process is relatively mature, raw materials are easy to produce, the cost is low, but the growth process of PVT method is difficult to observe, crystal growth rate of 0.2-0.4mm/h, it is difficult to grow crystals with large thickness (>50mm). After decades of continuous efforts, the current market for SiC substrate wafers grown by PVT method has been very huge, and the annual output of SiC substrate wafers can reach hundreds of thousands of wafers, and its size is gradually changing from 4 inches to 6 inches, and has developed 8 inches of SiC substrate samples.
Fifth, High temperature chemical vapor deposition method
High Temperature Chemical Vapor Deposition (HTCVD) is an improved method based on Chemical Vapor Deposition (CVD). The method was first proposed in 1995 by Kordina et al., Linkoping University, Sweden.
The growth structure diagram is shown in the figure:
The axial and radial temperature fields can be controlled by controlling the external thermal insulation conditions of the graphite crucible. The SiC powder is placed at the bottom of the graphite crucible with a higher temperature, and the SiC seed crystal is fixed at the top of the graphite crucible with a lower temperature. The distance between the powder and the seed is generally controlled to be tens of millimeters to avoid contact between the growing single crystal and the powder. The temperature gradient is usually in the range of 15-35℃/cm. An inert gas of 50-5000 Pa is kept in the furnace to increase convection. In this way, after the SiC powder is heated to 2000-2500℃ by induction heating, the SiC powder will sublimate and decompose into Si, Si2C, SiC2 and other vapor components, and be transported to the seed end with gas convection, and the SiC crystal is crystallized on the seed crystal to achieve single crystal growth. Its typical growth rate is 0.1-2mm/h.
PVT process focuses on the control of growth temperature, temperature gradient, growth surface, material surface spacing and growth pressure, its advantage is that its process is relatively mature, raw materials are easy to produce, the cost is low, but the growth process of PVT method is difficult to observe, crystal growth rate of 0.2-0.4mm/h, it is difficult to grow crystals with large thickness (>50mm). After decades of continuous efforts, the current market for SiC substrate wafers grown by PVT method has been very huge, and the annual output of SiC substrate wafers can reach hundreds of thousands of wafers, and its size is gradually changing from 4 inches to 6 inches, and has developed 8 inches of SiC substrate samples.
Fifth, High temperature chemical vapor deposition method
High Temperature Chemical Vapor Deposition (HTCVD) is an improved method based on Chemical Vapor Deposition (CVD). The method was first proposed in 1995 by Kordina et al., Linkoping University, Sweden.
The growth structure diagram is shown in the figure:
When the SiC crystal is grown by liquid phase method, the temperature and convection distribution inside the auxiliary solution are shown in the figure:
It can be seen that the temperature near the crucible wall in the auxiliary solution is higher, while the temperature at the seed crystal is lower. During the growth process, the graphite crucible provides C source for crystal growth. Because the temperature at the crucible wall is high, the solubility of C is large, and the dissolution rate is fast, a large amount of C will be dissolved at the crucible wall to form a saturated solution of C. These solutions with a large amount of C dissolved will be transported to the lower part of the seed crystals by convection within the auxiliary solution. Due to the low temperature of the seed crystal end, the solubility of the corresponding C decreases correspondingly, and the original C-saturated solution becomes a supersaturated solution of C after being transferred to the low temperature end under this condition. Suprataturated C in solution combined with Si in auxiliary solution can grow SiC crystal epitaxial on seed crystal. When the superforated part of C precipitates out, the solution returns to the high-temperature end of the crucible wall with convection, and dissolves C again to form a saturated solution.
The whole process repeats, and the SiC crystal grows. In the process of liquid phase growth, the dissolution and precipitation of C in solution is a very important index of growth progress. In order to ensure stable crystal growth, it is necessary to maintain a balance between the dissolution of C at the crucible wall and the precipitation at the seed end. If the dissolution of C is greater than the precipitation of C, then the C in the crystal is gradually enriched, and spontaneous nucleation of SiC will occur. If the dissolution of C is less than the precipitation of C, the crystal growth will be difficult to carry out due to the lack of solute.
At the same time, the transport of C by convection also affects the supply of C during growth. In order to grow SiC crystals with good enough crystal quality and sufficient thickness, it is necessary to ensure the balance of the above three elements, which greatly increases the difficulty of SiC liquid phase growth. However, with the gradual improvement and improvement of related theories and technologies, the advantages of liquid phase growth of SiC crystals will gradually show.
At present, the liquid phase growth of 2-inch SiC crystals can be achieved in Japan, and the liquid phase growth of 4-inch crystals is also being developed. At present, the relevant domestic research has not seen good results, and it is necessary to follow up the relevant research work.
Seventh, Physical and chemical properties of SiC crystals
(1) Mechanical properties: SiC crystals have extremely high hardness and good wear resistance. Its Mohs hardness is between 9.2 and 9.3, and its Krit hardness is between 2900 and 3100Kg/mm2, which is second only to diamond crystals among materials that have been discovered. Due to the excellent mechanical properties of SiC, powder SiC is often used in the cutting or grinding industry, with an annual demand of up to millions of tons. The wear-resistant coating on some workpieces will also use SiC coating, for example, the wear-resistant coating on some warships is composed of SiC coating.
(2) Thermal properties: thermal conductivity of SiC can reach 3-5 W/cm·K, which is 3 times that of traditional semiconductor Si and 8 times that of GaAs. The heat production of the device prepared by SiC can be quickly conducted away, so the requirements of the heat dissipation conditions of the SiC device are relatively loose, and it is more suitable for the preparation of high-power devices. SiC has stable thermodynamic properties. Under normal pressure conditions, SiC will be directly decomposed into vapor containing Si and C at higher.
(3) Chemical properties: SiC has stable chemical properties, good corrosion resistance, and does not react with any known acid at room temperature. SiC placed in the air for a long time will slowly form a thin layer of dense SiO2, preventing further oxidation reactions. When the temperature rises to more than 1700℃, the SiO2 thin layer melts and oxidizes rapidly. SiC can undergo a slow oxidation reaction with molten oxidants or bases, and SiC wafers are usually corroded in molten KOH and Na2O2 to characterize the dislocation in SiC crystals.
(4) Electrical properties: SiC as a representative material of wide bandgap semiconductors, 6H-SiC and 4H-SiC bandgap widths are 3.0 eV and 3.2 eV respectively, which is 3 times that of Si and 2 times that of GaAs. Semi-conductor devices made of SiC have smaller leakage current and larger breakdown electric field, so SiC is considered as an ideal material for high-power devices. The saturated electron mobility of SiC is also 2 times higher than that of Si, and it also has obvious advantages in the preparation of high-frequency devices. P-type SiC crystals or N-type SiC crystals can be obtained by doping the impurity atoms in the crystals. At present, P-type SiC crystals are mainly doped by Al, B, Be, O, Ga, Sc and other atoms, and N-type sic crystals are mainly doped by N atoms. The difference of doping concentration and type will have a great impact on the physical and chemical properties of SiC. At the same time, the free carrier can be nailed by the deep level doping such as V, the resistivity can be increased, and the semi-insulating SiC crystal can be obtained.
(5) Optical properties: Due to the relatively wide band gap, the undoped SiC crystal is colorless and transparent. The doped SiC crystals show different colors due to their different properties, for example, 6H-SiC is green after doping N; 4H-SiC is brown. 15R-SiC is yellow. Doped with Al, 4H-SiC appears blue. It is an intuitive method to distinguish SiC crystal type by observing the difference of color. With the continuous research on SiC related fields in the past 20 years, great breakthroughs have been made in related technologies.
Eighth, Introduction of SiC development status
At present, the SiC industry has become increasingly perfect, from substrate wafers, epitaxial wafers to device production, packaging, the entire industrial chain has matured, and it can supply SiC related products to the market.
Cree is a leader in the SiC crystal growth industry with a leading position in both size and quality of SiC substrate wafers. Cree currently produces 300,000 SiC substrate chips per year, accounting for more than 80% of global shipments.
In September 2019, Cree announced that it will build a new facility in New York State, USA, which will use the most advanced technology to grow 200 mm diameter power and RF SiC substrate wafers, indicating that its 200 mm SiC substrate material preparation technology has become more mature.
At present, the mainstream products of SiC substrate chips on the market are mainly 4H-SiC and 6H-SiC conductive and semi-insulated types of 2-6 inches.
In October 2015, Cree was the first to launch 200 mm SiC substrate wafers for N-type and LED, marking the beginning of the 8-inch SiC substrate wafers to the market.
In 2016, Romm started sponsoring the Venturi team and was the first to use the IGBT + SiC SBD combination in the car to replace the IGBT + Si FRD solution in the traditional 200 kW inverter. After the improvement, the weight of the inverter is reduced by 2 kg and the size is reduced by 19% while maintaining the same power.
In 2017, after the further adoption of SiC MOS + SiC SBD, not only the weight is reduced by 6 kg, the size is reduced by 43%, and the inverter power is also increased from 200 kW to 220 kW.
After Tesla adopted SIC-based devices in the main drive inverters of its Model 3 products in 2018, the demonstration effect was rapidly amplified, making the xEV automotive market soon a source of excitement for the SiC market. With the successful application of SiC, its related market output value has also risen rapidly.
Ninth, Conclusion:
With the continuous improvement of SiC related industry technologies, its yield and reliability will be further improved, the price of SiC devices will also be reduced, and the market competitiveness of SiC will be more obvious. In the future, SiC devices will be more widely used in various fields such as automobiles, communications, power grids, and transportation, and the product market will be broader, and the market size will be further expanded, becoming an important support for the national economy.
Post time: Jan-25-2024