by hearingearly hearingearly

1. Different material properties

Silicon carbide substrate is the core material of the newly developed wide bandgap semiconductor. Silicon carbide substrate is mainly used in microwave electronics, power electronics and other fields. It is at the front end of the wide bandgap semiconductor industry chain and is a cutting-edge and basic core key material. 4H-SiC has a bandgap of 3.2 (eV), a saturated electron drift rate of 2.00 (107 cm/s), a breakdown electric field strength of 3.5 (MV/cm), and a thermal conductivity of 4.00 (W·cm-1·K-1 ), which has several times the advantages of silicon-based ones.

The forbidden band refers to the energy range in the energy band structure where the density of energy states is zero, often used to represent the energy range between the valence band and the conduction band; the saturation electron drift rate refers to the electron drift rate that after reaching a certain range, it no longer follows the electric field. The limit value that continues to increase due to the action; the electron drift rate refers to the average speed of electrons moving under the action of an electric field; the thermal conductivity refers to the measure of the material’s ability to conduct heat, also known as the thermal conductivity; the breakdown electric field strength refers to the dielectric under the action of a strong enough electric field Will lose its dielectric properties and become a conductor, which is called dielectric breakdown, and the corresponding electric field intensity is called breakdown electric field intensity.

2. Consumables with high disposable prices account for a large proportion

(1) Judging from the overall amount of raw material expenditures, the company’s raw material expenditures increased from 32.5 million yuan in 2018 to 199.269 million yuan in 2021. This shows the market popularity and market development prospects of silicon carbide, from 32.5 million yuan to 199.269 million yuan in 2021. The compound growth rate from RMB 10,000 to 19926.9 is as high as more than 6%, which shows the market’s recognition of silicon carbide and the market popularity of silicon carbide;

(2) Among the raw materials for the preparation of silicon carbide substrates, taking 2021 as an example, graphite parts accounted for 45.21% of the cost, and graphite felt accounted for 41.32%, accounting for 86.53% of the raw material cost. Compared with carbon powder, There is a huge difference between the proportions of silicon powder at 0.97% and 1.99%. Compared with other polishing liquids with 2.01%, polishing pads with 1.75%, diamond powder with 2.34%, and others with proportions of 4.18%, there is also a huge difference. Among them, the largest proportion of raw materials It is graphite parts 45.21%, and the raw material with the lowest proportion is cutting steel wire 0.22%;

(3) Among the raw materials for the preparation of silicon carbide substrates from 2018 to 2021, the raw materials can be divided into three categories according to the fluctuation trend of their cost proportions: The first category is that the proportion shows an upward trend, such as graphite parts rising from 32.98% to 45.21% %, graphite felt increased from 37.06% to 41.32%; the second category is raw materials whose proportion has remained basically stable, such as cutting steel wire, the proportion has been maintained at 0.25 (within ±0.5), and the proportion of polishing fluid has been maintained at 1% (± About 1%) and the proportion of polishing pads has remained at 2% (within ±0.5%); the third category is raw materials that show a downward trend in proportion, such as toner, which dropped from 5.71% in 2018 to 0.97% in 2021. The proportion of silicon powder increased from 5.47% in 2018 to 1.99% in 2021, and the proportion of other products decreased from 8.75% in 2018 to 4.18% in 2021.

Based on the above analysis, it can be seen that in the preparation process of silicon carbide, the excessive proportion of disposable high-priced consumables is one of the reasons for the high production cost of silicon carbide substrates. Crucible (graphite parts) refers to a vessel made of graphite powder of a certain particle size, pressed under high pressure and calcined at high temperature for a long time. It has the characteristics of high temperature resistance, strong thermal conductivity, good corrosion resistance, and long life. It is an important part of the growth process of silicon carbide crystals. One of the consumables, its proportion in the raw materials for silicon carbide substrate production will reach more than 45% by 2021, and its proportion is also showing an upward trend. This is a big reason for the high cost of silicon carbide preparation.

3. The preparation process conditions require high PVT method refers to Physical Vapor Transportation.

Physical vapor phase transfer method, a common silicon carbide crystal growth method, heats silicon carbide powder at high temperatures above 2,300°C and low pressure close to vacuum, causing it to sublime to produce reactions containing different gas phase components such as Si, Si2C, and SiC2. Gas; due to the different gas phase partial pressures of the Si and C components formed by the solid phase sublimation reaction, the Si/C stoichiometric ratio varies with the thermal field distribution. It is necessary to distribute and transport the gas phase components according to the designed thermal field and temperature gradient. The components are transported to the established crystallization position in the growth chamber; in order to avoid disordered gas phase crystallization to form polycrystalline silicon carbide, a silicon carbide seed crystal (seed) is set at the top of the growth chamber and transported to the gas phase at the seed crystal. The components are atomically deposited on the surface of the seed crystal driven by the supersaturation of the gas phase components, and grow into silicon carbide single crystals.

The entire solid-gas-solid reaction process of the above silicon carbide single crystal preparation is in a complete and sealed growth chamber. The various parameters of the reaction system are coupled with each other. Any fluctuation in growth conditions will cause changes in the entire single crystal growth system, affecting The stability of silicon carbide crystal growth; in addition, silicon carbide single crystals have a variety of atomic connection and bonding methods in different close-packed structures in their crystal orientation, thus forming more than 200 crystal forms of silicon carbide isomeric structures, and The energy conversion barrier between different crystal forms is extremely low. Therefore, the transformation of different crystal forms is very easy to occur in the PVT single crystal growth system, leading to serious quality problems such as disorder of the target crystal form and various crystal defects. Therefore, special testing equipment is needed to detect the crystal form and various defects of the crystal ingot.

It can be seen from the above that the process realization conditions for silicon carbide preparation are extremely demanding and include the following points:

(1) There are many environmental impurities in the synthesis process of silicon carbide powder, making it difficult to obtain high-purity powder; incomplete reaction between silicon powder and carbon powder as the reaction source can easily cause an imbalance in the Si/C ratio; after the synthesis of silicon carbide powder, The crystal form and particle size are difficult to control;

(2) The “solid-gas-solid” conversion and recrystallization process is completed in a closed graphite chamber at high temperatures above 2,300°C and close to vacuum. The growth cycle is long, control is difficult, and defects such as microtubes and inclusions are prone to occur;

(3) Silicon carbide includes more than 200 different crystal forms, but production generally requires only one crystal form. Crystal form transformation is prone to occur during the growth process, resulting in multi-type inclusion defects. During the preparation process, a single specific crystal form is difficult to stably control, and different The extremely low energy conversion barrier between crystal forms makes control more difficult. Parameter control and related research during the period require huge R&D costs, which is another major reason for the high cost of compliant silicon carbide.