At present, the SiC industry is transforming from 150mm (6 inches) to 200mm (8 inches). In order to meet the urgent demand for large-sized and high-quality SiC homogeneous epitaxial wafers in the industry, self-developed 200mm SiC epitaxial growth equipment has been used to successfully prepare 150mm and 200mm 4H SiC homogeneous epitaxial wafers on domestic substrates, and a homogeneous epitaxial process suitable for 150mm and 200mm has been developed. The epitaxial growth rate can be greater than 60 μ m/h. While meeting high-speed epitaxy, the quality of epitaxial wafers is excellent. The thickness uniformity of 150mm and 200mm SiC epitaxial wafers can be controlled within 1.5%, the concentration uniformity is less than 3%, the fatal defect density is less than 0.3 defects/cm2, and the root mean square Ra of epitaxial surface roughness is less than 0.15 nm. All core process indicators are at the advanced level of the industry.
Silicon carbide (SiC) is one of the representatives of third-generation semiconductor materials, which has the characteristics of high breakdown field strength, excellent thermal conductivity performance, high electron saturation drift speed, and strong radiation resistance. It greatly expands the energy processing capability of power devices and can meet the service requirements of the next generation of power electronic equipment under extreme conditions such as high power, small size, high temperature, and high radiation. It can reduce space, power consumption, and cooling requirements, and has brought revolutionary changes to fields such as new energy vehicles, rail transit, and smart grids. Therefore, silicon carbide semiconductor has become a recognized ideal material that will lead the next generation of high-power power electronic devices. In recent years, thanks to the policy support from the government for the development of the third-generation semiconductor industry, the research and construction of the 150mm SiC device industry system has been basically completed in China, and the security of the industry chain has been basically guaranteed. Therefore, the industry's focus is gradually shifting towards cost control and efficiency improvement. According to Table 1, compared to 150mm, 200mm silicon carbide has a higher edge utilization rate, and the output of a single wafer chip can be increased by about 1.8 times. After the technology matures, the manufacturing cost of a single chip can be reduced by 30%. The breakthrough in 200mm technology is a direct means of "cost reduction and efficiency improvement", and it is also the key to China's semiconductor industry moving towards "parallel running" or even "leading".
1.1 Principle of SiC Epitaxial Process
The 4H SiC homoepitaxial growth process mainly includes two key steps, namely high-temperature in-situ etching of the 4H SiC substrate and homochemical vapor deposition process. The main purpose of substrate in-situ etching is to remove subsurface damage, residual polishing solution, particles, and oxide layers on the substrate after wafer polishing, and to form regular atomic step structures on the substrate surface through etching. In situ etching is usually carried out under a hydrogen atmosphere, and a small amount of auxiliary gases such as hydrogen chloride, propane, ethylene, or silane can be added according to actual process requirements. The temperature of in-situ hydrogen etching is generally above 1600 ℃, and the pressure in the reaction chamber is generally controlled below 2 × 104 Pa during the etching process.
The 4H SiC homoepitaxial growth process mainly includes two key steps, namely high-temperature in-situ etching of the 4H SiC substrate and homochemical vapor deposition process. The main purpose of substrate in-situ etching is to remove subsurface damage, residual polishing solution, particles, and oxide layers on the substrate after wafer polishing, and to form regular atomic step structures on the substrate surface through etching. In situ etching is usually carried out under a hydrogen atmosphere, and a small amount of auxiliary gases such as hydrogen chloride, propane, ethylene, or silane can be added according to actual process requirements. The temperature of in-situ hydrogen etching is generally above 1600 ℃, and the pressure in the reaction chamber is generally controlled below 2 × 104 Pa during the etching process.
1.2 200 mm (8 inches) SiC epitaxial equipment and process conditions
The experiments described in this article were conducted on a 150/200 mm (6/8 inch) compatible single-chip horizontal hot wall SiC epitaxial device independently developed by the 48th Research Institute of China Electronics Technology Group Corporation. The epitaxial furnace supports fully automatic wafer loading and unloading. Figure 1 shows the schematic diagram of the internal structure of the epitaxial equipment reaction chamber. As shown in Figure 1, the outer wall of the reaction chamber is a quartz bell jar with a water-cooled interlayer, and the inside of the bell jar is a high-temperature reaction chamber. The reaction chamber is composed of insulated carbon felt, high-purity special graphite chamber, graphite based air floating rotating base, etc. The entire quartz bell jar is covered with a cylindrical induction coil on the outside, and the reaction chamber inside the bell jar is heated by electromagnetic induction through an intermediate frequency induction power supply. As shown in Figure 1 (b), the carrier gas, reaction gas, and doping gas all flow through the wafer surface in a horizontal laminar flow upstream of the reaction chamber to reach the downstream of the reaction chamber, and are discharged from the exhaust end. To ensure intra wafer consistency, the wafer carried by the air floating base always rotates during the process.
The experiments described in this article were conducted on a 150/200 mm (6/8 inch) compatible single-chip horizontal hot wall SiC epitaxial device independently developed by the 48th Research Institute of China Electronics Technology Group Corporation. The epitaxial furnace supports fully automatic wafer loading and unloading. Figure 1 shows the schematic diagram of the internal structure of the epitaxial equipment reaction chamber. As shown in Figure 1, the outer wall of the reaction chamber is a quartz bell jar with a water-cooled interlayer, and the inside of the bell jar is a high-temperature reaction chamber. The reaction chamber is composed of insulated carbon felt, high-purity special graphite chamber, graphite based air floating rotating base, etc. The entire quartz bell jar is covered with a cylindrical induction coil on the outside, and the reaction chamber inside the bell jar is heated by electromagnetic induction through an intermediate frequency induction power supply. As shown in Figure 1 (b), the carrier gas, reaction gas, and doping gas all flow through the wafer surface in a horizontal laminar flow upstream of the reaction chamber to reach the downstream of the reaction chamber, and are discharged from the exhaust end. To ensure intra wafer consistency, the wafer carried by the air floating base always rotates during the process.
1.3 Testing and characterization of epitaxial wafers
Fourier transform infrared spectrometer (equipment manufacturer ThermalFisher, model iS50) and mercury probe concentration tester (equipment manufacturer Semilab, model 530L) were used to characterize the mean and distribution of epitaxial layer thickness and doping concentration, respectively; The thickness and doping concentration of each point inside the epitaxial layer are determined by removing 5 mm from the edge and taking a diameter line that intersects with the normal of the main reference edge at a 45 ° angle at the center of the wafer. For a single diameter line on a 150 mm wafer, 9 points are taken (the two diameters are perpendicular to each other), and for a 200 mm wafer, 21 points are taken, as shown in Figure 2. Using an atomic force microscope (equipment manufacturer Bruker, model Dimension Icon), a 30 μ m x 30 μ m area was selected to test the surface roughness of the epitaxial layer in the center and edge regions (with a 5 mm edge removal) of the epitaxial wafer; Epitaxial layer defects are characterized using a surface defect tester (manufactured by China Electronics Technology Fenghua, model Mars 4410 pro).
2.1 Epitaxial layer thickness and uniformity
The thickness, doping concentration, and uniformity of the epitaxial layer are one of the core indicators for judging the quality of epitaxial wafers. Accurate and controllable thickness, doping concentration, and on-chip uniformity are key to ensuring the performance and consistency of SiC power devices. The thickness and doping concentration uniformity of the epitaxial layer are also important criteria for measuring the process capability of epitaxial equipment.
Figure 3 shows the thickness uniformity and distribution curves of 150mm and 200mm SiC epitaxial wafers. As shown in the figure, the thickness distribution curve of the epitaxial layer is symmetrical to the center point of the wafer. The epitaxial process time is 600 seconds, and the average thickness of the epitaxial layer on a 150 mm epitaxial wafer is 10.89 μ m, with a thickness uniformity of 1.05%. Through calculation, the epitaxial growth rate is 65.3 μ m/h, which is a typical level of fast epitaxial process. Under the same epitaxial process time, the thickness of the epitaxial layer on a 200 mm epitaxial wafer is 10.10 μ m, with a thickness uniformity of less than 1.36%. The overall growth rate is 60.60 μ m/h, slightly lower than the growth rate of a 150 mm epitaxial wafer. This is because there is significant loss along the way when the silicon source and carbon source flow from the upstream of the reaction chamber through the wafer surface to the downstream of the reaction chamber. However, the area of a 200 mm wafer is larger than that of a 150 mm wafer, and the gas flowing through the surface of a 200 mm wafer has a longer distance, consuming more source gas along the way. Under the condition of the wafer maintaining rotation, the overall thickness of the epitaxial layer is thinner, resulting in a slower growth rate. Overall, the thickness uniformity of 150mm and 200mm epitaxial wafers is excellent, and the process capability of the equipment can meet the requirements of high-quality devices.
2.2 Doping concentration and uniformity of epitaxial layer
Figure 4 shows the uniformity of doping concentration and curve distribution of 150mm and 200mm SiC epitaxial wafers. As shown in the figure, the concentration distribution curve on the epitaxial wafer has obvious symmetry relative to the center of the wafer. The doping concentration uniformity of the 150mm and 200mm epitaxial layers is 2.80% and 2.66%, respectively, both of which can be controlled within 3%, which is an excellent level for similar international equipment. The distribution of doping concentration curve in the epitaxial layer follows a "W" shape along the diameter direction, which is mainly determined by the flow field of the horizontal hot wall epitaxial growth furnace. This is because the airflow direction of the horizontal airflow epitaxial growth furnace flows in from the inlet end (upstream) and flows out from the downstream end in a laminar manner through the wafer surface; Due to the higher depletion rate of the carbon source (C2H4) compared to the silicon source (TCS), the actual C/Si on the wafer surface gradually decreases from the edge to the center during wafer rotation (with fewer carbon sources at the center). According to the "site competition theory" of C and N, the doping concentration at the center of the wafer gradually decreases towards the edge. To achieve excellent concentration uniformity, additional edge N2 is added as compensation during the epitaxial process to slow down the decrease in doping concentration from the center to the edge, resulting in a "W" - shaped doping concentration curve in the end.
2.3 Epitaxial layer defects
In addition to thickness and doping concentration, the level of defect control in epitaxial layers is also a core parameter for measuring the quality of epitaxial wafers and an important indicator of the process capability of epitaxial equipment. Although SBD and MOSFET have different requirements for defects, more obvious surface morphology defects such as drop defects, triangle defects, carrot defects, comet defects, etc. are also defined as the killer defects of SBD and MOSFET devices. Chips containing these defects have a high probability of failure, so controlling the number of fatal defects is extremely important for improving chip yield and reducing costs. Figure 5 shows the distribution of fatal defects in 150mm and 200mm SiC epitaxial wafers. Under the condition that there is no significant imbalance in the C/Si ratio, carrot defects and comet defects can be basically eliminated, while falling object defects, triangle defects, and cleanliness control during the operation of the epitaxial equipment, impurity levels in the graphite parts of the reaction chamber, and substrate quality are related. According to Table 2, the fatal defect density of 150mm and 200mm epitaxial wafers can be controlled within 0.3 pieces/cm2, which is an excellent level for devices of the same type. The fatal defect density control level of 150mm epitaxial wafers is better than that of 200mm epitaxial wafers, because the preparation process of 150mm substrates is more mature than that of 200mm substrates, the substrate quality is better, and the impurity control level of 150mm graphite reaction chambers is better, resulting in a phenomenon.
2.4 Surface roughness of epitaxial wafers
Figure 6 shows AFM images of the surface of 150 mm and 200 mm SiC epitaxial wafers, respectively. From the figure, it can be seen that the root mean square roughness Ra of the surface of the 150 mm and 200 mm epitaxial wafers is 0.129 nm and 0.113 nm, respectively, and the surface of the epitaxial layer is smooth without obvious macro step aggregation phenomenon. This phenomenon indicates that the growth of the epitaxial layer throughout the epitaxial process always maintains a step flow growth mode and no step aggregation occurs. From this, it can be seen that using the optimized epitaxial growth process, smooth surface epitaxial layers can be obtained on both 150mm and 200mm low angle substrates.
Figure 6 shows AFM images of the surface of 150 mm and 200 mm SiC epitaxial wafers, respectively. From the figure, it can be seen that the root mean square roughness Ra of the surface of the 150 mm and 200 mm epitaxial wafers is 0.129 nm and 0.113 nm, respectively, and the surface of the epitaxial layer is smooth without obvious macro step aggregation phenomenon. This phenomenon indicates that the growth of the epitaxial layer throughout the epitaxial process always maintains a step flow growth mode and no step aggregation occurs. From this, it can be seen that using the optimized epitaxial growth process, smooth surface epitaxial layers can be obtained on both 150mm and 200mm low angle substrates.
Source: Electronic Industry Specialized Equipment
Authors: Xie Tianle, Li Ping, Yang Yu, Gong Xiaoliang, Ba Sai, Chen Guoqin, Wan Shengqiang
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