Physical vapor transport method for improving resistivity uniformity of silicon carbide crystals

Abstract

The invention discloses a physical vapor transport method for improving resistivity uniformity of a silicon carbide crystal, which is characterized in that a silicon carbide raw material is subjected to nitriding pretreatment, so that the nitrogen content in the raw material is kept uniform, and the resistivity uniformity in the growing silicon carbide crystal and at the head and the tail of the crystal can be greatly improved. Compared with a process without raw material nitriding pretreatment, the method has the advantages that the deviation of the maximum value and the minimum value of resistivity in the crystal wafer is reduced to be less than 2% from about 10%, the uniformity is improved by nearly 5 times, and particularly, the deviation in the tail wafer is reduced to be less than 1%; the deviation of the maximum resistivity and the minimum resistivity of the head piece and the tail piece of the same crystal is reduced from about 20% to about 10%, and the uniformity is nearly doubled.

Description

Technical Field

[0001] The present invention relates to a physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals. Background Art

[0002] With the continuous development of technology, in the face of the higher performance requirements of next-generation power electronic devices, existing widely used semiconductor materials such as silicon (Si) and germanium (Ge) are increasingly difficult to meet these new demands due to the limitations of their own physical and chemical properties. The discovery and development of third-generation wide-bandgap semiconductor materials represented by crystals such as silicon carbide (SiC) and gallium nitride (GaN) have brought new hope for the development of a new generation of power electronic devices.

[0003] Compared with traditional silicon materials, silicon carbide crystals have outstanding advantages such as wide bandgap, high saturated electron mobility, high breakdown field strength, and high thermal conductivity, and their physical and chemical properties are also particularly stable, making them one of the best substrate materials for future power electronic devices. Currently, power electronic devices prepared using silicon carbide materials have been widely applied in important fields related to the national economy and people's livelihood, such as transportation (electric vehicles, high-speed rails, rail transit, airplanes, etc.), new energy, and communications.

[0004] In recent years, due to the urgent demand for energy conservation, environmental protection, and efficient electric energy conversion in electric vehicles, electric bicycles, rail transit, white household appliances, etc., the demand for silicon carbide crystals has become increasingly strong. However, due to the high price of silicon carbide substrates, their application scenarios are severely restricted. Therefore, reducing the cost of silicon carbide crystals is the primary problem faced by the silicon carbide industry and the only way to promote silicon carbide crystals to a broader market.

[0005] In order to reduce the cost of silicon carbide substrates, the industry has adopted means such as increasing the crystal diameter (developing 8-inch substrates), growing thicker crystals (thickness > 30 mm), improving the quality and yield of crystals, developing new processing technologies (laser cutting, rapid polishing, etc.), and researching and developing new crystal growth technologies (liquid phase method) to achieve cost reduction and efficiency improvement of silicon carbide crystals. Among them, improving the resistivity uniformity of crystals is one of the important directions for improving the performance consistency of silicon carbide substrates and increasing the product yield. Because the resistivity magnitude and uniformity of the substrate directly affect the performance of the final device and the consistency of product performance.

[0006] During the growth process of silicon carbide crystals, the resistivity is generally regulated by adjusting the nitrogen content (the higher the nitrogen content, the lower the resistivity of the crystal), and the resistivity uniformity is regulated by adjusting the growth interface shape and growth rate. To obtain a lower resistivity, high doping is generally required. However, high doping easily causes dislocation defects in the crystal, affecting the crystal quality. Patent 202211490608 discloses a method of applying silicon carbide powder with different nitrogen contents filled in a ring-shaped two-layer to the growth process of silicon carbide crystals, and the in-wafer resistivity difference is obtained as 3 mΩ·cm. Patent 202310797984 discloses a method of making the formed silicon carbide crystals and silicon carbide wafers have uniform resistivity by controlling the ratio difference between the axial temperature gradient and the radial temperature gradient of the silicon carbide crystals and adjusting the doping amount of nitrogen concentration. The resistivity is in the range of 15 - 20 mΩ·cm, and the in-wafer resistivity uniformity deviation is less than 0.4%. Patent 202322608500 discloses a silicon carbide crystal growth device, which can improve the problem of excessive temperature gradient during the growth of silicon carbide single crystals, and then keep the convexity consistent during the growth process of silicon carbide crystals. The height difference of the crystal protrusion (growth interface shape) is 0 - 3 mm, and finally the in-wafer resistance difference is less than 0.2 mΩ / cm 2 . Patent 202410530915 discloses a growth structure and method, by supplementing 20 - 50% of nitrogen through an additional supplement port during the growth process, making the difference between the maximum and minimum resistivity less than 2 mΩ·cm.

[0007] The above patents improve the resistivity uniformity of silicon carbide crystals from aspects such as the growth furnace structure, growth process, loading method, and controlling the temperature gradient of crystal growth. Although the above methods improve the resistivity uniformity of the crystal to a certain extent, they also indirectly lead to the complication of the growth device or loading process and reduce the efficiency, or due to the increase in convexity, the crystal stress becomes larger, making the crystal prone to cracking and other adverse factors. Moreover, the above measures only improve the resistivity uniformity within the wafer, and do not explain the resistivity uniformity of the crystal head and tail. The present invention provides a simple and feasible new method for improving the resistivity uniformity of silicon carbide crystals, which can not only greatly improve the resistivity within the silicon carbide crystal wafer, but also greatly improve the resistivity uniformity of the crystal head and tail. Summary of the Invention

[0008] The purpose of the present invention is to provide a physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals to solve the problems raised in the above background technology.

[0009] To achieve the above purpose, the present invention provides the following technical solution: A physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals, comprising the following steps: (1) Use a conventional PVT method crystal growth device and thermal field structure for furnace loading. The furnace loading raw material is silicon carbide powder pretreated by gradient nitriding. (2) Implement dynamic nitriding treatment synchronously during the heating and temperature rising stage: Introduce nitrogen at 30 - 60% of the total carrier gas flow rate, control the growth chamber pressure to be 300 - 700 mbar, and maintain the axial temperature gradient ΔT = 15 - 20 °C / cm. (3) During the crystal seeding and growth stage, adjust the nitrogen flow rate to 40 - 60% of the conventional process flow rate, and synchronously control the growth rate to be 0.2 - 0.25 mm / h. (4) After crystal growth is completed, perform programmed cooling. The cooling rate ≤ 5 °C / min, and maintain the nitrogen partial pressure at 10 - 30% of the total gas pressure throughout the process. (5) Obtain silicon carbide crystals with an axial resistivity gradient < 1.2% / cm and a resistivity difference within a 6-inch wafer < 0.15 mΩ·cm.

[0010] Preferably, the gradient nitriding pretreatment includes: Perform three-stage treatment under a nitrogen partial pressure gradient field: Initial activation stage (1500 - 1600 °C / Ar:N 2 = 4:1), gradient penetration stage (1800 - 190 °C / axial ΔT = 100 °C), steady-state diffusion stage (2000 - 2100 °C / isothermal holding); The uniformity of the nitrogen concentration distribution of the raw material after pretreatment ≥ 98.7%.

[0011] Preferably, in step (2), the carrier gas is a mixed gas of argon and nitrogen, where the argon flow rate is 100 - 400 SCCM, and the nitrogen flow rate is 40 - 50% of the argon flow rate.

[0012] Preferably, in step (2), the growth chamber pressure is controlled to be 400 - 600 mbar, and the nitrogen inlet position is the bottom porous gas distributor of the growth chamber. The aperture gradient of the distributor is 0.5 - 2 mm (increasing from the center to the edge).

[0013] Preferably, in step (3), the nitrogen flow rate is dynamically adjusted through a closed-loop feedback system. The adjustment parameters include: The convexity of the real-time crystal growth interface (control fluctuation < 0.5 mm); The change amount of the axial temperature gradient (Δ(ΔT) ≤ 0.5 °C / cm); The growth rate deviation (≤ ±0.02 mm / h).

[0014] Preferably, the programmed cooling stage adopts a segmented cooling strategy: The first stage (from the growth temperature to 1500 °C): The cooling rate is 3 - 5 °C / min, and the nitrogen partial pressure is maintained at 20 - 30%. Second stage (1500°C to 800°C): The cooling rate is 1 - 3°C / min, and the nitrogen partial pressure is gradually reduced to 5 - 10%; Third stage (<800°C): Natural cooling, and the nitrogen supply is turned off.

[0015] Preferably, in step (1), the loading is carried out by a three-dimensional spiral stacking method, the interlayer rotation angle is 25 - 35°, and the powder packing density gradient is 1.8 - 2.2 g / cm³ (decreasing from the center to the edge).

[0016] Preferably, the axial resistivity gradient of the silicon carbide crystal is <1.2% / cm, the resistivity difference within a 6-inch wafer is <0.15 mΩ·cm, and the resistivity difference between the head and the tail is <8%.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: The traditional method of doping nitrogen through a carrier gas is restricted by the growth temperature gradient and the shape of the growth interface. The gas flow pattern in the growth chamber is fixed, resulting in insufficient mixing of nitrogen atoms in the carrier gas with the gas-phase components of silicon carbide, which will cause uneven nitrogen concentration locally. This is the key reason for the uneven resistivity of silicon carbide crystals; In this invention patent, nitrogen is introduced into the raw materials for nitriding at a flow rate of 30 - 60% of the carrier gas during the heating stage, so that nitrogen atoms can enter the lattice of the silicon carbide raw materials in advance during heating or adsorb on the surface of the raw materials, achieving the purpose of pre-mixing, and making the nitrogen content and distribution in the raw materials more uniform; During the growth process, nitrogen sublimates together with the volatilization of the raw materials and simultaneously condenses and crystallizes at the growth interface, which can greatly reduce the requirement for the mixing uniformity of nitrogen atoms with the gas-phase components of silicon carbide and improve the nitrogen content uniformity in the crystal; During the normal growth process, since the nitrogen gas flow rate can be set to 40 - 60% of the conventional process, the problem that high nitrogen doping easily causes an increase in dislocations and stacking faults is greatly reduced, and the quality of the crystal is improved; Compared with the existing process, on the premise of keeping all process equipment and configurations unchanged, only nitrogen needs to be introduced during the heating process for nitriding treatment of the raw materials. The process operation is convenient, simple, and easy to produce. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 : Schematic diagram of the structure of the gradient nitriding pretreatment device; Figure 2 : Flow chart of the dynamic nitrogen gas flow closed-loop feedback system; Figure 3 : Comparison chart of resistivity uniformity of Examples 1 - 3.

[0019] As shown in the figure: 1. Temperature gradient sensor, 2. Interface convexity detector, 3. Data acquisition and processor, 4. PLC controller, 5. Nitrogen flow regulating valve, 6. Growth rate detector, 7. Crystal growth chamber. Detailed implementation manners

[0020] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0021] Please refer to Figure 1-2 , the present invention provides a technical solution: a physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals, including the following steps: (1) Use a conventional PVT method crystal growth device and a thermal field structure for furnace loading. The furnace loading raw material is silicon carbide powder pretreated by gradient nitriding. (2) Implement dynamic nitriding treatment synchronously during the heating and temperature rising stage: Introduce nitrogen at 30 - 60% of the total carrier gas flow rate, control the growth chamber pressure to be 300 - 700 mbar, and maintain the axial temperature gradient ΔT = 15 - 20 °C / cm. (3) During the crystal seeding and growth stage, adjust the nitrogen flow rate to 40 - 60% of the conventional process flow rate, and synchronously control the growth rate to be 0.2 - 0.25 mm / h. (4) After the crystal growth is completed, perform programmed cooling with a cooling rate ≤ 5 °C / min, and maintain the nitrogen partial pressure at 10 - 30% of the total pressure throughout the process. (5) Obtain silicon carbide crystals with an axial resistivity gradient < 1.2% / cm and a resistivity difference within a 6-inch wafer < 0.15 mΩ·cm. The gradient nitriding pretreatment includes: Perform three-stage treatment under a nitrogen partial pressure gradient field: Initial activation stage (1500 - 1600 °C / Ar:N 2 = 4:1), gradient penetration stage (1800 - 190 °C / axial ΔT = 100 °C), steady-state diffusion stage (2000 - 2100 °C / isothermal holding); The uniformity of the nitrogen concentration distribution of the raw material after pretreatment is ≥ 98.7%.

[0022] In step (2), the carrier gas is a mixed gas of argon and nitrogen, where the argon flow rate is 100 - 400 SCCM, and the nitrogen flow rate is 40 - 50% of the argon flow rate. In step (2), the pressure in the growth chamber is controlled to be 400 - 600 mbar, and the nitrogen is introduced at the bottom of the growth chamber through a porous gas distributor, with the pore size gradient of the distributor being 0.5 - 2 mm (increasing from the center to the edge). In step (3), the nitrogen flow rate is dynamically adjusted through a closed-loop feedback system, and the adjustment parameters include: The convexity of the real-time crystal growth interface (controlled fluctuation < 0.5 mm); The change in the axial temperature gradient (Δ(ΔT) ≤ 0.5 °C / cm); The growth rate deviation (≤ ±0.02 mm / h).

[0023] In the programmed cooling stage, a segmented cooling strategy is adopted: The first stage (from the growth temperature to 1500 °C): the cooling rate is 3 - 5 °C / min, and the nitrogen partial pressure is maintained at 20 - 30%; The second stage (from 1500 °C to 800 °C): the cooling rate is 1 - 3 °C / min, and the nitrogen partial pressure is gradually reduced to 5 - 10%; The third stage (< 800 °C): natural cooling, and the nitrogen supply is turned off.

[0024] In step (1), the loading is carried out by a three-dimensional spiral stacking method, with the interlayer rotation angle being 25 - 35°, and the powder packing density gradient being 1.8 - 2.2 g / cm³ (decreasing from the center to the edge).

[0025] Example 1: Implementation under basic process parameters Step 1: Raw material pretreatment Load the silicon carbide powder into a multi-zone controllable atmosphere reaction furnace and carry out three-stage nitriding: (1) Initial activation: 1550 °C / Ar:N 2 = 4:1, hold for 2 hours; (2) Gradient penetration: 1850 °C / axial ΔT = 100 °C / radial N 2 Gradient difference 20%, hold for 3 hours; (3) Steady-state diffusion: 2050 °C / Ar:N 2 = 1:1, isothermal hold for 4 hours; Programmed cooling (5 °C / min) to room temperature to obtain pretreated raw materials with a nitrogen concentration uniformity ≥ 98.5%.

[0026] Step 2: Crystal growth Loading: Adopt the three-dimensional spiral stacking method (interlayer rotation 30°), packing density gradient 1.9 - 2.1 g / cm³; Dynamic nitriding: During the heating-up stage, argon (300 SCCM) and nitrogen (45% of the argon flow rate) are introduced, and the pressure is 500 mbar; Growth regulation: After inoculation, the nitrogen flow rate is adjusted to 45% of the conventional process, the growth rate is 0.22 mm / h, and the axial ΔT = 18 °C / cm; Program-controlled cooling: Cooling in stages (the first stage at 5 °C / min to 1500 °C, the second stage at 2 °C / min to 800 °C, and the third stage natural cooling).

[0027] Results: The resistivity difference within a 6-inch wafer is 0.12 mΩ·cm; The resistivity difference between the head and the tail is 7.3%; The dislocation density is 180 cm⁻². Example 2: Application of a closed-loop feedback system

[0028] Same as Example 1, adjust the parameters of the gradient infiltration stage to: axial ΔT = 120 °C, radial N 2 Gradient difference 25%.

[0029] Dynamic regulation: Use a closed-loop feedback system to adjust the nitrogen flow rate in real time. The parameters include: The convexity fluctuation of the growth interface < 0.3 mm (monitored by a laser interferometer); The change in axial ΔT ≤ 0.3 °C / cm; The growth rate deviation is ±0.01 mm / h; The position where nitrogen is introduced is the bottom porous distributor (pore size gradient 0.8 - 1.8 mm).

[0030] Results: The resistivity difference within the wafer is 0.09 mΩ·cm; The resistivity difference between the head and the tail is 6.5%; Axial gradient 0.9% / cm. Example 3: Low-cost loading optimization plan

[0031] Simplify the pretreatment stage: Only perform gradient infiltration (1800 °C / ΔT = 80 °C) and steady-state diffusion (2000 °C / 3 h).

[0032] Loading: Use a two-dimensional concentric circle loading method (instead of a three-dimensional helix), and the packing density is 1.8 g / cm³; Dynamic nitriding: Argon flow rate 200 SCCM, nitrogen proportion 40%, pressure 450 mbar; Program-controlled cooling: The cooling rate in the first stage is 4 °C / min, and the nitrogen partial pressure is 25%.

[0033] Results: The in - wafer resistivity range is 0.18 mΩ·cm; The resistivity difference between the head and the tail is 9.8%; The production cost is reduced by 22%.

[0034] Figure 3 As shown, the resistivity uniformity comparison chart of Examples 1 - 3.

[0035] Comparison of technical effects:

[0036] Technical effects: In - wafer uniformity: The in - wafer resistivity range of a 6 - inch wafer is < 0.15 mΩ·cm; Axial uniformity: The resistivity difference between the head and the tail of the crystal is < 8%, and the axial gradient is < 1.2% / cm; Cost optimization: The comprehensive production cost is reduced by about 18%, and the crystal dislocation density is < 200 cm⁻².

[0037] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals, characterized in that: The following steps are involved: (1) A conventional PVT crystal growth device and a thermal field structure are used to load the furnace, and the raw material for the furnace is silicon carbide powder pretreated by gradient nitriding; (2) Dynamic nitriding treatment is carried out simultaneously during the heating stage: nitrogen is introduced at 30-60% of the total carrier gas flow rate, the growth chamber pressure is controlled at 300-700 mbar, and the axial temperature gradient ΔT is maintained at 15-20°C / cm; (3) During the crystal seeding and growth stage, the nitrogen flow rate was adjusted to 40-60% of the conventional process flow rate, and the growth rate was simultaneously regulated to 0.2-0.25 mm / h; (4) After the crystal growth is completed, programmed cooling is performed with a cooling rate of ≤5°C / min, and the nitrogen partial pressure is maintained at 10-30% of the total gas pressure throughout the process; (5) Obtain silicon carbide crystals with an axial resistivity gradient of less than 1.2% / cm and an extreme resistivity difference within a 6-inch wafer of less than 0.15 mΩ·cm.

2. A physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals according to claim 1, characterized in that: The gradient nitriding pretreatment comprises: Three-stage treatment was carried out under nitrogen partial pressure gradient field: initial activation stage (1500-1600°C / Ar:N2=4:1), gradient penetration stage (1800-190°C / axial ΔT=100°C), steady-state diffusion stage (2000-2100°C / isothermal holding); The nitrogen concentration distribution uniformity of the raw material after pretreatment is ≥98.7%.

3. A physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals according to claim 1, characterized in that: The carrier gas in step (2) is a mixed gas of argon and nitrogen, wherein the argon flow rate is 100-400 SCCM and the nitrogen flow rate is 40-50% of the argon flow rate.

4. A physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals according to claim 3, characterized in that: In step (2), the pressure of the growth chamber is controlled to be 400-600 mbar, and the nitrogen is introduced into the porous gas distributor at the bottom of the growth chamber, and the pore size gradient of the distributor is 0.5-2 mm (increasing from the center to the edge).

5. A physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals according to claim 1, characterized in that: In step (3), the nitrogen flow rate is dynamically adjusted through a closed-loop feedback system, and the adjustment parameters include: Real-time crystal growth interface convexity (control fluctuation <0.5 mm); Axial temperature gradient change (Δ(ΔT)≤0.5℃ / cm); Growth rate deviation (≤±0.02 mm / h).

6. A physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals according to claim 1, characterized in that: The programmed cooling stage adopts a segmented cooling strategy: The first stage (growth temperature to 1500°C): cooling rate 3-5°C / min, nitrogen partial pressure maintained at 20-30%; The second stage (1500℃ to 800℃): cooling rate 1-3℃ / min, nitrogen partial pressure gradually reduced to 5-10%; The third stage (<800℃): natural cooling, turn off the nitrogen supply.

7. A physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals according to claim 1, characterized in that: In step (1), the material is loaded by a three-dimensional spiral stacking method, the interlayer rotation angle is 25-35°, and the powder packing density gradient is 1.8-2.2 g / cm³ (decreasing from the center to the edge).

8. A physical vapor transport method for improving the resistivity uniformity of silicon carbide crystals according to any one of claims 1 to 7, characterized in that: The axial resistivity gradient of the silicon carbide crystal is less than 1.2% / cm, the resistivity range within a 6-inch wafer is less than 0.15 mΩ·cm, and the resistivity difference between the head and tail is less than 8%.

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