1 What is silicon carbide? Characteristic material SIC(4H-) SI GaAs Forbidden band width (ev) 3.3 1.12 1.43 Critical electric field (10-6v/cm) 3.0 0.25 0.50 Thermal conductivity (w/cmk) 5.0 1.50 0.50 Vsat (107 cm/s) 2.0 1.00 1.00 In fact, silicon carbide is not a newly discovered material. Some even argue that it is the great-grandfather of all semiconductors. The first report on silicon carbide came from the hands of the Swedes in 1842. Silicon carbide does not have its own mineral deposits like other minerals. It does not naturally occur in nature, but it needs to be controlled by the smelting technology control process of the refining furnace. Early silicon carbide was only used for grinding and cutting. The development of silicon carbide in the last century was extremely slow and difficult. Table 2 shows the main experiences in the development of SIC. 1905 First discovered silicon carbide in meteorite 1907 The first silicon carbide light-emitting diode was born 1955 A major breakthrough in theory and technology, LELY proposed the concept of growing high-quality carbonization, and since then SIC has been regarded as an important electronic material. 1958 The first World Silicon Carbide Conference in Boston for academic exchanges 1978 In the 1960s and 1970s, silicon carbide was mainly studied by the former Soviet Union. Grain purification growth method using "LELY improvement technology" for the first time in 1978 1987-present The silicon carbide production line was established with the research results of CREE, and suppliers began to provide commercial silicon carbide substrates. It is expected that within ten years (the first decade of the 21st century) = silicon carbide devices will have a breakthrough. Both SIC single crystal materials and SIC device manufacturing processes have undergone significant development, and silicon carbide materials are beginning to mature. Together with silicon materials as the main and future semiconductor materials, it plays an irreplaceable role in some devices and holds the right market. Silicon carbide is a complex of atoms rather than a single crystal. The main difference and performance lies in the relative number of silicon and carbon atoms. And the different structures of the atomic arrangement. The physical properties of silicon carbide depend on the carbon-silicon atomic arrangement of the crystal. The most common and typical structure is the hexagonal system, called 6H, 4H and 3C silicon carbide. [next] Figure 1 Comparison of properties of single crystal SIC and SI materials Material properties SI device SIC device Current density (A/cm 2 ) 30 100 to 300 (up to 500) Maximum working temperature (°C) 180-200 (PN) 600(max)(PIN) Device withstand voltage 1 5 to 10 (times) On-state loss 1 1/4 to 1/10 Switching loss 1 1/10 to 1/100 Working temperature (°C) 180 300～500 3 SIC device manufacturing Figure 3 Planar layout of various sizes of chips on the SIC wafer Figure 4 Progress in microtube density of wafers and optimal wafers produced by SIC Customized Diffusor,Semi-Finished Product Joint,Glass Downstem,Customized Color Smoking Accessories HEBEI DINGSHUO GLASSWARE CO.,LTD , https://www.dsglasspipe.com
Silicon Carbide (SIC) is recognized as a "future material" in the semiconductor industry and is a new type of semiconductor material with broad development potential in the new century. It is expected that there will be rapid development and significant results in the next 5 to 10 years. The main factor driving the development of silicon carbide is that the loading of silicon (SI) materials has reached the limit, and there is no room for breakthrough in the performance and capability limits of semiconductor devices using silicon as a substrate.
Silicon (SI) and silicon carbide (SIC) and other semiconductor materials differ greatly in electrical and physical properties (Table 1), but have well-known similar elements and structural compositions.
Table (1) Comparison of performance of several semiconductor materials
Table (2) History of SIC Materials Development
SIC is a "wide bandgap" semiconductor with very different physical properties than silicon. Single crystal silicon carbide (SIC) has many superior physical properties than single crystal silicon (SI), such as (1) approximately 10 times the electric field strength; (2) approximately 3 times higher thermal conductivity; (3) approximately 3 wide Double the forbidden band width; (4) about twice the saturation drift speed (see Figure 1). 
Theoretically, SIC devices operate at temperatures of 500 ° C or higher, and silicon devices are not possible. The thermal conductivity of silicon carbide exceeds the thermal conductivity of copper , and the heat generated by the device is transferred quickly, which is undoubtedly beneficial to the improvement of the through-flow performance of the device.
SIC has strong radiation resistance, and the fabricated device can be used in electronic equipment near nuclear reactors and in space. Small transmission, high electric field strength and high saturation drift mobility are beneficial to device volume reduction and complex internal structure establishment.
Therefore, it is expected that the SIC materials and device processes will be improved in the near future. Part of the SI field being replaced by SIC is a ready-to-go target.
The history of the development of semiconductor materials shows that “wide bandgap†materials are always in a state of difficulty and slow progress. If you want to achieve rapid development, you must meet the following conditions:
l Applicable and efficient substrate material l Growth of ultra-large area and high-quality single crystal film l Effectively and accurately control the doping of N-type and P-type regions. l Appropriate effective insulation method, for example MIS device l development surface Modeling and Corrosion Processes The success of semiconductor materials development is primarily based on the performance and suitability of the device. SIC devices are ideal for use in the field of power converters and high temperature operation. At the end of the last century, SIC devices have achieved remarkable results. The maximum voltage of PN junction devices, 4.5KV, has been born and has been successfully applied. The sensitivity of SIC photodiodes has been shown to be four orders of magnitude higher than that of similar devices in the SI, and the current characteristics allow for higher power densities. This has a significant improvement in the size, efficiency and performance of power electronics. It can also be used in special fields such as radar, automobiles, airplanes, and communications. With the perfection and maturity of SIC materials and devices, the value is truly realized in the potential field, while other semiconductors are unattainable environmental conditions, especially the harsh conditions of space will provide an excellent application for the advantages of SIC devices. So in any case, SIC is a "material of the future." [next]
2 ideal power switching device
The main characteristics of power semiconductor devices are high voltage, high power, and low on-state loss, that is, power semiconductor devices have small on-state resistance (low on-state voltage drop), fast switching speed (frequency), and small switching loss.
Power switching devices without power loss do not exist, but in recent years there have been several devices that are close to each other, that is, compared with conventional power semiconductor devices, the on-state voltage drop and switching loss are very small, almost close to ideal. Semiconductor device.
The MOSFET of SI material is a power semiconductor device with simple driving, fast switching frequency and speed, and low power loss or switching loss. However, the shortcoming is that the voltage is not high, and the power consumption increases rapidly with the increase of voltage. IGBT is an improved power device for MOSFETs. It also has the characteristics of simple switching speed of MOSEFT device driving circuit. In the 1980s, IGBT replaced bipolar junction transistor, and the new withstand voltage level increased from several hundred volts to more than 2KV. Power semiconductor devices. But higher than 2KV power device system, GTO or IGCT still firmly occupy and control the market, making IGBT impossible. GTO, IGCT as a power switching device, with high voltage, high current, can produce very A device with high power, but needs to be driven by a more complex and powerful control circuit than MOSFETs and IGBTs.
Power electronics design engineers want a device that is as easy to use as a MOSFET and can generate very powerful devices like IGCT and GTO. SIC MOSFET devices can basically achieve the above requirements.
It is clear from Table (1) that the SIC material has a higher critical electric field strength than the SI material; the value of Emax (sic) is about 10 times that of silicon. Therefore, the PN junction withstand voltage is also set, which is required for SIC devices. The thickness of the substrate material will be one tenth of that of the SI device. The relationship between the PN junction withstand voltage and the thickness of the substrate material is described by the triangular electric field distribution of (2), and the maximum blocking voltage is calculated by the formula (1).
Fig. 2 P+n-diode blocking state space electric field distribution Vb is PN withstand voltage; Emax is breakdown electric field strength; W depletion layer width (Fig. 2) depletion layer width W is mainly determined by doping amount (see formula (2), the low doping layer provides most of the depletion layer
Nd is a low doping concentration, ε is a relative dielectric constant, ε0 is a vacuum permittivity, V is an applied voltage, and Vdo is a built-in potential.
The low doped layer provides a wider depletion region (see Figure 2). The breakdown electric field strength of SIC is one parameter level higher than SI, which means that the device with the same blocking voltage needs to be two orders of magnitude lower in the doping concentration of the SI device in order to increase the width of the depletion layer. Therefore, the SI device is effective. The base width is also approximately 10 times that of the SIC.
Taking a 5KV withstand voltage rectifier diode silicon device as an example, according to equation (1), the width of the depletion layer is about 350 μm, and the corresponding doping concentration calculated by equation (2) is about 2.5×10 13 cm −3 . The same voltage-resistant SIC device is as high as 8×10 15 cm -3 . Although the above calculations are similar, the advantages of SIC devices have been clearly shown.
Considering the 5KV rectifier diode, the minority carrier lifetime of the SI device is on the order of 10 to 100 μs, and the SIC device requires that the minority carrier lifetime is 1 to 2 orders of magnitude lower than that of the SI device. Because the long minority life is not conducive to device shutdown.
In addition, thermal stability can ensure the high temperature operation of the device. Because all the power consumption of the device generates heat. It can only be dissipated by the substrate. In order to guarantee the permissible operating temperature, a large cooling device must be configured to dissipate heat. Due to the high thermal conductivity of SIC and the thermal stability of high temperature, the cooling device is significantly smaller than that of SI, and the entire system is also made smaller.
SIC's MOSEFT has low conduction loss. MOSEFT device is a good performance switching device, especially suitable for power electronic devices above 20KC. The breakdown voltage of the device is qualitatively given by equation (3) to give Rds in equation (3). , on is the characteristic resistance of the PN junction (Ω-cm2); Vb is the PN junction blocking withstand voltage; ε is the dielectric constant; ε0 is the vacuum permittivity; Emax maximum critical field μ is the carrier (electron) migration rate.
The physical properties of the semiconductor show that the value of the resistance Rds.on increases as the width of the drift region increases, and decreases as the doping concentration increases, which is due to an increase in the number of carrier flows. [next]
According to the mode (3), it is not difficult to see that the resistance of the MOSFET drift region increases squarely with the increase of the breakdown voltage, and reaches the critical maximum value for the silicon material only at several hundred volts. The resistance is increased as the critical electric field increases. Because the SIC critical electric field strength is 10 times higher than that of silicon, the conduction loss of the SIC MOSFET is much lower than that of the silicon device.
10KV Bipolar SIC Thyristors As discussed earlier, it can be concluded that SIC-made MOSFET devices and Schottky diodes have much higher withstand voltage than SI devices. It can be as high as several thousand volts, so MOSFET devices are expected to be used in many fields.
SIC bipolar devices, such as the thyristor 10KV withstand voltage level, are also easy to manufacture, and the minority carrier lifetime can be maintained at between 1μs and 10μs to achieve good switching characteristics. Bipolar SI thyristors, with a typical breakdown voltage of 6KV-7KV, are the manufacturing cost characteristics of the device. It is also the best compromise between on-state loss and switching loss. The limit condition is that the thickness of the silicon wafer is about 1 mm, and the lifetime of the minority carrier is about 100 μS. This device can only be used in systems under power frequency conditions, and the application range is limited due to excessive switching losses.
Operating Temperature of SIC Devices SI bipolar power semiconductor devices with a suitable operating temperature of less than 125 °C. Unipolar devices, such as MOSFETs, have a maximum operating temperature of 150 °C. The highest temperature tolerated is the extreme temperature of the semiconductor material, ie the density of the current carrying current is no longer determined by doping, but by the forbidden band width of the semiconductor, commonly referred to as the intrinsic temperature. Above this extreme temperature, all current control and voltage blocking capabilities will disappear. For SI, the limit temperature is around 300 °C. The operating temperature of the SIC device is much higher than the operating temperature of the SI device. Since the SIC's PN junction leakage current is extremely small, it can also block at temperatures well above 300 °C, and the limit temperature can reach above 1000 °C.
A silicon carbide MOSFET developed by a research center in the United States has an operating temperature of 650 ° C. This high temperature capability creates many favorable conditions for power electronic system design engineers. The low loss of SIC devices is based on the comparison of silicon devices. The main differences in performance between SIC devices and SI devices are shown in Table 3.
Table 3 SI device and SIC device performance comparison
SIC SIC material and device development work, as Western countries, universities and large companies to the strength of the main body, invest a lot of capital, manpower, and achieved certain results, we have a high level of laboratory samples, only the PN junction withstand voltage Up to 10,000 volts. But there are few power devices that are truly commercially valuable and have a certain amount of production. The main reason is the limitation of the quality of SIC materials.
The components required for power electronics require high voltage, high current, and low switching loss. As far as power electronics is concerned, ABB has invested in the research and development of SIC devices and its level of achievement is at the world's leading level. One of the research results is a breakdown voltage of 4.5KV PIN diode and 2.5KV JBS (junction potential Schottky diode)
At present, the production of SIC unipolar devices has little effect on material quality problems, affecting only the production yield of large-capacity devices, and high-power devices are packaged in parallel with several chips.
The current status of SIC materials, for high-voltage bipolar devices, material quality still has major problems. Reliability indicators must also not meet actual needs, and production yields are low. It is clear that SIC materials are still a respectable and awesome material for power electronics. SIC's high-power devices must solve the material quality problems before they are practical and commercialized. The most important solution is the small holes (between 0.1μm and 5μm in diameter) that penetrate the microtube defects of the substrate. Commercially available substrates have micropipe density of no more than 102 to 103/cm2, followed by thicker layers (50 μm or more) and bipolar devices with better doping concentration (less than 1015 cm-3). [next]
CREE is the most famous company in the world for research and development and production of SIC materials and devices. Among them, the Φ35mm 4H-SIC wafer is grown by hot-disk CVD with an epitaxial layer of 35-45 μm thick, and the doping concentration is below 1015 cm-3. This doping depends on the critical electric field strength used, and the theoretical blocking voltage is 4.5 to 6 kV. ABB uses a special-purpose optical microscope to place 20mm2, 40mm2 area diode square chips in a defect-free wafer position. This instrument can inspect each wafer purchased from CREE, automatically detect it by computer, and identify and record the correct location of each defect on the SIC wafer in the computer. And automatically generate the position of 20mm2, 40mm2 chip, and also automatically form the technical file data, and draw the layout pattern of the chip.
Inspect microtubule defects using a laser probe, computer identification, data analysis and plotting a chip map on a crystal image, and placing small (1 to 5 mm2) detection devices in the remaining areas outside the large-area chip position. (image 3)
ABB's research center focuses on device research, including etching, dielectric deposition, oxidation, photolithography, metallization, and ohmic contact formation. The lithography process uses a Laser Lithagraphic system. This is different from the traditional IC process, the main reason is that the surface of the SIC wafer is rough and uneven, and 9 exposures are required. What is important is that the Laser system is used in conjunction with a computer system for easy automatic positioning.
At present, SIC devices, especially bipolar power devices, are difficult to commercialize and mass produce. The main reasons are:
(1) There are many defects in SIC single crystal materials, and the material quality has not been solved so far;
(2) Design and process control techniques are more difficult;
(3) Special requirements for process equipment, high technical standards, example ion implantation, epitaxial equipment, laser exposure lithography machine, etc.;
(4) Capital investment is large, and operating costs and development costs are expensive. It is generally difficult to carry out research and development work.
Currently, the world's research and development of SIC devices include Cree in the United States, Siemens in Germany, Toshiba in Japan, Mitsubishi Corporation, and Fujifilm. ABB has cooperated with Sweden and invested heavily in the development of diodes for power transmission and transformation projects. Germany's Siemens products are positioned below 1200V low-voltage, low-power devices, which have been commercialized. ABB's products are mainly positioned at 4500V, high voltage and high current devices. [next]
4 Why SIC devices are still not popular
The advantages of silicon carbide devices have been well known since the 1960s. The reason why it has not been popularized is because there are many technical problems including manufacturing. Until now, industrial applications of SIC materials have been used primarily as abrasives (corundum).
The SIC does not melt within a controlled pressure range, but instead directly transitions to a gaseous state at a sublimation point of about 2500 °C. Therefore, the growth of SIC single crystal can only start from the gas phase. This process is much more complicated than the growth of SIC, and SI melts at about 1400 °C. A major obstacle to making SIC technology unsuccessful in commercial success is the lack of a suitable substrate material for industrial production of power semiconductor devices. In the case of SI, a single crystal substrate is often referred to as a wafer, which is a prerequisite and guarantee for production. A method of growing large area SIC substrates was successfully developed in the late 1970s. However, substrates grown using the improved method known as Lely are plagued by a microtube defect.
As long as a microtube passes through the high-voltage PN junction, it will destroy the PN junction blocking voltage. In the past three years, the defect density has dropped from tens of thousands to tens of thousands per square millimeter. In addition to this improvement, when the maximum size of the device is limited to a few square millimeters, the production yield may be greater than a few percent, such that each device has a maximum current rating of a few amps. Therefore, there is a need for greater technical improvements to the substrate material of the SIC before the commercial success of the SIC power device.
Figure 4 shows that the yield of different devices is 40% and 90%. Now that SIC materials and optoelectronic devices have met the requirements, they are not affected by material quality, and the industrial yield and reliability of the devices are also Meet the requirements. The high frequency device mainly includes a unipolar device in the MOSFET SCHOTTKY diode. The micropipe defect density of SIC materials basically meets the requirements, and only has a certain impact on the yield. SIC materials for high-voltage, high-power devices take about two years to further improve material defect density. In short, no matter how difficult the current situation is, how to develop semiconductors, SIC is undoubtedly a promising "material of the future" in the new century.