A method for improving the carrier lifetime of a silicon carbide epitaxial wafer

Abstract

The present invention provides a method for improving the carrier lifetime of a silicon carbide epitaxial wafer, comprising the following steps: S1. placing a silicon-face silicon carbide substrate on a graphite pedestal; S2. adjusting the conditions in the reaction chamber and growing a buffer layer; S3. keeping the growth temperature and pressure unchanged, reducing the C / Si ratio, and then increasing the C / Si ratio in a linearly gradient manner for the first growth; S4. keeping the growth temperature and pressure unchanged, and continuing with n times of rate-switching gradient layer growth; S5. adjusting the conditions in the reaction chamber and completing the growth of the epitaxial layer; S6. after completing the growth of the epitaxial layer, closing the growth source and the doping source, and finally filling the pressure in the reaction chamber with argon to atmospheric pressure and then opening the chamber to take out the wafer. By adopting the method of stepwise growth of the coating layer, the present invention optimizes the gradient layer design, achieves the purpose of compensating for the carbon vacancies generated in the buffer layer and suppressing the formation of carbon vacancies in the gradient layer, and greatly improves the carrier lifetime of the silicon carbide epitaxial wafer.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor materials, and in particular relates to a method for improving the carrier lifetime of a silicon carbide epitaxial wafer. Background Art

[0002] Silicon carbide (SiC), as the third-generation wide bandgap semiconductor material, has excellent material properties and has great application potential in high temperature, high frequency, high power, radiation resistance and other fields. In particular, 4H-SiC bipolar devices, due to the characteristics of conductivity modulation, can achieve ultra-high withstand voltage while having ultra-low on-resistance, which is particularly suitable for high voltage and ultra-high voltage applications.

[0003] The regulation of carrier lifetime in ultra-thick low-doped epitaxial materials used in ultra-high voltage power electronic devices (>10kV) is crucial for realizing low-power bipolar devices. Studies have shown that low minority carrier lifetime is very unfavorable for devices. It will significantly increase the on-state resistance of SiC-based IGBT devices, resulting in increased device power consumption. Carbon vacancy defects will be formed during the homoepitaxial growth of silicon carbide, introducing deep energy level defects. Such deep energy level defects will become the recombination center of carriers, resulting in a decrease in the carrier lifetime of the epitaxial layer and an inability to achieve effective conductivity modulation.

[0004] In order to make 4H-SiC high-voltage power electronic devices meet the requirements of engineering applications, the problem of too short carrier lifetime in SiC thick-layer epitaxy must first be solved. At present, the international community mainly adopts the method of post-processing silicon carbide epitaxial wafers. There are two main methods used, namely carbon implantation combined with high-temperature annealing and long-term high-temperature oxidation. Both of these post-processing methods have certain disadvantages. The method of carbon implantation combined with high-temperature annealing is prone to introduce new defects during the high-energy carbon ion implantation process; at the same time, it is limited by the depth of carbon implantation, and the effect is not ideal for thick-layer silicon carbide epitaxy. The high-temperature oxidation method requires a long period of high-temperature oxidation treatment, which greatly increases the time and economic costs, so new processing methods need to be developed.

[0005] Controlling the minority carrier lifetime of SiC materials requires consideration from two aspects: First, pay attention to control during the process so that it does not change. Mainly pay attention to the control of cleanliness and operation process to avoid the introduction of harmful impurities and reduce process-induced secondary defects. The second is to intentionally reduce impurities and defects, because many defects and impurities will constitute recombination centers. Through epitaxial growth process optimization, oxidation / annealing and ion implantation, defects are annihilated to achieve the purpose of regulating the minority carrier lifetime. The current commercial silicon carbide material has an epitaxial layer carrier lifetime level of only 0.5-2μs, which is difficult to meet device requirements.

[0006] Therefore, controlling the SiC epitaxial process and directly reducing carbon vacancies during epitaxy is the simplest and most effective method to improve the carrier lifetime of the SiC epitaxial layer. Existing research work has shown that using process conditions with a high carbon-to-silicon ratio at the gas inlet (C / Si ≥ 1.5) can effectively inhibit the formation of carbon vacancies in the epitaxial layer, thereby improving the carrier lifetime. The breakdown voltage of SiC power electronic devices is directly related to the epitaxial thickness. The epitaxial materials suitable for the development of 4H-SiC ultra-high voltage power electronic devices even reach more than 100 microns. Obviously, too low an epitaxial growth rate cannot be adopted. However, for SiC epitaxy, with the increase in the epitaxial rate, the process window of the C / Si ratio at the gas inlet gradually becomes smaller. Under high-speed epitaxial processes, when the C / Si ratio at the gas inlet is greater than 1.5, obvious step-bunching morphology appears on the surface of the epitaxial wafer, resulting in an increase in surface roughness and introducing additional problems such as an increase in triangular defects. Summary of the Invention

[0007] Aiming at the problem that the carrier lifetime of existing silicon carbide epitaxial wafers is low and effective conductance modulation cannot be achieved, the present invention provides a method for improving the carrier lifetime of silicon carbide epitaxial wafers.

[0008] The present invention adopts the following technical solutions:

[0009] A method for improving the carrier lifetime of silicon carbide epitaxial wafers, comprising the following steps:

[0010] S1. Place the selected silicon-face silicon carbide substrate biased in the <11-20> direction by 4° or 8° on the graphite pedestal in the reaction chamber of the SiC epitaxial system.

[0011] S2. Replace the gas in the reaction chamber with argon, and then introduce hydrogen into the reaction chamber. Introduce silicon source and carbon source into the reaction chamber as growth sources with a C / Si ratio of R0, and introduce n-type or p-type doping source. Gradually heat the reaction chamber to the epitaxial growth temperature, and select hydrogen or argon as the gas lift gas to drive the rotation of the graphite pedestal to grow the buffer layer.

[0012] S3. Keep the growth temperature and pressure unchanged, reduce the C / Si ratio to R 1L , and then increase the C / Si ratio to R 1H in a linearly graded manner to perform the first rate-switching gradient layer growth, where R 1L is less than R0, and R 1H is greater than R0;

[0013] S4. Keep the growth temperature and pressure unchanged, and continue to perform n times of rate-switching gradient layer growth until R nHWhen the C / Si ratio is greater than that for epitaxial layer growth, stop growing the gradient layer, where the growth of the i-th rate-switching gradient layer is as follows: first reduce the C / Si ratio to R iL , and then increase the C / Si ratio to R iH in a linearly varying manner, where the R iL is less than R (i-1)H , and the R iH is greater than R (i-1)H , 2 ≤ i ≤ n;

[0014] S5. Change the doping source flow rate to the set value required for the epitaxial layer in the grown epitaxial structure, and complete the growth of the epitaxial layer under the condition of keeping the C / Si ratio of the epitaxial layer unchanged;

[0015] S6. After completing the growth of the epitaxial layer, turn off the growth source and the doping source, reduce the temperature of the reaction chamber to room temperature in a hydrogen atmosphere, then evacuate the hydrogen, use argon to replace the gas in the reaction chamber multiple times, and finally inflate the pressure of the reaction chamber to atmospheric pressure with argon, and then open the chamber to take out the wafer.

[0016] Furthermore, in step S2, when growing the buffer layer, the hydrogen flow rate is 90 - 150 L / min, and the reaction chamber pressure is 80 - 200 mbar.

[0017] Furthermore, in step S2, the value of the C / Si ratio R0 used for buffer layer epitaxial growth is 0.45 - 0.75, and the growth rate is less than 20 μm / h.

[0018] Furthermore, the thickness of the buffer layer grown in step S2 is 0.5 - 2 μm.

[0019] Furthermore, the value of the C / Si ratio used for epitaxial layer growth in step S4 is 0.85 - 1.15

[0020] Furthermore, in step S5, the growth rate of the epitaxial layer is greater than 30 - 90 μm / h.

[0021] Furthermore, the growth time of the first rate-switching gradient layer in step S3 is equal to that of the i-th rate-switching gradient layer in step S4, where 1 ≤ i ≤ n.

[0022] Furthermore, the total growth time of the gradient layers in steps S3 and S4 is 0.5 - 5 minutes.

[0023] Advantages of the present invention:

[0024] (1) By adopting the method of stepwise growth of the coating layer, the present invention optimizes the gradient layer design, achieving the purpose of compensating for carbon vacancies generated in the buffer layer and suppressing the formation of carbon vacancies in the gradient layer;

[0025] (2)The carrier lifetime of the SiC epitaxial wafer grown by the method provided by the present invention reaches 4.3 μs. Compared with the currently commercial silicon carbide materials, the carrier lifetime level of the epitaxial layer is only 0.5 - 2 μs, which greatly improves the carrier lifetime of the SiC epitaxial wafer. Description of the Drawings

[0026] Figure 1 It is a schematic flow chart of the conventional process method.

[0027] Figure 2 It is a schematic flow chart of the method for improving the carrier lifetime of the SiC epitaxial wafer of the present invention.

[0028] Figure 3 The carrier lifetime of the SiC epitaxial wafer grown by using the process method of the present invention reaches 4.3 μs. Detailed Embodiments

[0029] The present invention will be further described below in conjunction with embodiments. The embodiments are only used to illustrate the present invention and do not constitute a limitation on the scope of the claims. Other alternative means that can be conceived by those skilled in the art are within the scope of the claims of the present invention.

[0030] Conventional technical means for growing epitaxial wafers in the prior art are as Figure 1 shown. After the growth of the buffer layer is completed, the carbon-silicon ratio growth gradient layer is linearly increased. After the carbon-silicon ratio reaches the growth conditions of the epitaxial layer, the growth of the epitaxial layer begins. Its characteristic is that the carbon-silicon ratio continuously increases during the growth of the gradient layer, and the lifetime level of the epitaxial layer grown by this method is poor.

[0031] The present invention provides a method for improving the carrier lifetime of a SiC epitaxial wafer. As Figure 2 shown, after the growth of the buffer layer is completed, when the gradient layer is grown, a step-by-step growth method is used, that is, there will be a time period when the carbon-silicon ratio decreases during the growth process. After the growth of the gradient layer is completed, the growth of the outer research layer is carried out, which specifically includes the following steps:

[0032] S1. Place the selected silicon-face silicon carbide substrate biased at 4° or 8° in the <11 - 20> direction on the graphite pedestal in the reaction chamber of the SiC epitaxial system.

[0033] S2. Replace the gas in the reaction chamber with argon, then introduce hydrogen into the reaction chamber, gradually increase the hydrogen flow rate to 90 - 150 L / min, control the reaction chamber pressure to 80 - 200 mbar, introduce silicon source and carbon source into the reaction chamber as growth sources at a C / Si ratio R0 value of 0.45 - 0.75, and introduce n-type or p-type doping source. Gradually heat the reaction chamber to the epitaxial growth temperature, select hydrogen or argon as the aerostatic gas to drive the graphite pedestal to rotate, and grow a buffer layer with a thickness of 0.5 - 2 μm at a growth rate of less than 20 μm / h;

[0034] S3. As Figure 2 shown, keep the growth temperature and pressure unchanged, reduce the C / Si ratio to R 1L , then increase the C / Si ratio to R 1H in a linearly graded manner to perform the first rate-switching gradient layer growth, where the R 1L is less than R0, and the R 1H is greater than R0;

[0035] S4. As Figure 2 shown, after completing the first rate-switching gradient layer growth, keep the growth temperature and pressure unchanged, and continue to perform n rate-switching gradient layer growths until R nH is greater than the C / Si ratio of the epitaxial layer growth, where the value of the C / Si ratio of the epitaxial layer growth is 0.85 - 1.15. The i-th rate-switching gradient layer growth is as follows: first reduce the C / Si ratio to R iL , then increase the C / Si ratio to R iH in a linearly graded manner, where the R iL is less than R (i-1)H , and the R iH is greater than R (i-1)H , 2 ≤ i ≤ n. The growth time from the first rate-switching gradient layer to the n-th rate-switching gradient layer is equal, and the total growth time is 0.5 - 5 minutes;

[0036] S5. Change the doping source flow rate to the set value required for the epitaxial layer in the grown epitaxial structure, and complete the growth of the epitaxial layer at a growth rate greater than 30 μm / h while keeping the C / Si ratio of the epitaxial layer unchanged;

[0037] S6. After completing the growth of the epitaxial layer, turn off the growth source and doping source, cool the reaction chamber temperature to room temperature in a hydrogen atmosphere, then exhaust the hydrogen, use argon to replace the gas in the reaction chamber multiple times, and finally fill the reaction chamber pressure with argon to atmospheric pressure, and then open the chamber to take out the wafer.

[0038] Example 1

[0039] In the provided SiC chemical vapor deposition epitaxial system, a method for growing epitaxial materials for a 15 kV IGBT structure with improved carrier lifetime of silicon carbide epitaxial wafers by optimizing the gradient layer structure includes the following steps:

[0040] S1. Select a silicon-face silicon carbide substrate biased 4° in the <11-20> direction, and place the cleaned substrate on a graphite pedestal in the reaction chamber of the SiC epitaxial system.

[0041] S2. Replace the gas in the reaction chamber with argon, select argon as the aerostatic gas to drive the rotation of the graphite pedestal, open the hydrogen switch leading to the reaction chamber, control the pressure in the reaction chamber to gradually increase to 100 mbar, and at the same time control the hydrogen flow rate to gradually increase to 100 L / min. Gradually heat the reaction chamber to the growth temperature of 1650 °C, and maintain the reaction chamber temperature for 5 minutes after reaching the temperature to perform in-situ hydrogen (H2) etching on the substrate.

[0042] Then introduce ethylene with a flow rate of 40 sccm, trichlorosilane with a flow rate of 100 sccm, and trimethylaluminum with a hydrogen carrier gas flow rate of 200 sccm as a p-type doping source to grow the p-type highly doped layer of the SiC IGBT structure. The growth time is 60 minutes, and the growth thickness is 15 μm with a doping concentration of 5E18 cm -3 of the highly doped p-type layer of the IGBT structure.

[0043] Then introduce nitrogen with a flow rate of 100 sccm into the reaction chamber, reduce the ethylene flow rate to 30 sccm, make the C / Si ratio R0 0.6, grow for 5 minutes, and slowly grow a 1-μm-thick conventional buffer layer.

[0044] S3. Keep the growth temperature and pressure unchanged, reduce the ethylene flow rate to 25 sccm, make the C / Si ratio decrease to a low value R 1L = 0.5 below the C / Si ratio R0 for buffer layer growth, and then increase the ethylene and trichlorosilane flow rates in a linearly graded manner, that is, increase the C / Si to a high value R 1H = 0.75 for the first rate-switching gradient layer growth.

[0045] S4. Keep the growth temperature and pressure unchanged, and continue to perform n rate-switching gradient layer growths until R nH is greater than the C / Si ratio for epitaxial layer growth, and then stop the growth of the gradient layer. In this embodiment, the C / Si ratio for epitaxial layer growth is 1.0. Therefore, when R nH is greater than 1.0, the growth of the gradient layer is completed. Among them, the i-th rate-switching gradient layer growth is as follows: first reduce the C / Si ratio to R iL , and then increase the C / Si ratio to R iH in a linearly graded manner, and the R iLLess than R (i-1)H , said R iH Greater than R (i-1)H , 2 ≤ i ≤ n. In this embodiment, the C / Si ratio (R nL and R nH ) is shown in Table 1 below: For the 2nd time, R 2L is 0.6, less than 0.75 of R 1H , R 2H is 0.85, greater than 0.75 of R 1H ; For the 3rd time, R 3L is 0.7, less than 0.85 of R 2H , R 3H is 0.95, greater than 0.85 of R 2H ; For the 4th time, R 4L is 0.8, less than 0.95 of R 3H , R 4H is 1.05, greater than 0.95 of R 3H . At this time, the growth rate switching gradient layer growth R 4H for the 4th time is 1.05, greater than the C / Si ratio 1.0 of the epitaxial layer growth, then the gradient layer growth ends. The growth time from the 1st rate switching gradient layer to the 4th rate switching gradient layer is 30 s each, and the total growth time is 2 minutes; the conventional gradient layer growth time under this growth condition is 2 minutes.

[0046] Table 1 C / Si ratios of each growth condition

[0047]

[0048] S5. Change the flow rates of ethylene and trichlorosilane to 150 sccm and 300 sccm respectively, introduce nitrogen gas with the flow rate required for the epitaxial layer in the epitaxial structure, and keep the C / Si ratio of the epitaxial layer equal to 1 unchanged to complete the growth of the epitaxial layer of the device structure. The growth rate is 30 μm / h, the growth time is 150 minutes, and the growth thickness is 150 μm to complete the growth of the entire IGBT structure.

[0049] S6. After completing the growth of the epitaxial structure, turn off the growth source and doping source, cool the reaction chamber temperature to room temperature in a hydrogen atmosphere. After the reaction chamber temperature reaches room temperature, exhaust the hydrogen, perform multiple replacements of the gas in the reaction chamber with argon, and finally fill the reaction chamber pressure with argon to atmospheric pressure, then open the chamber to take out the wafer.

[0050] The test results of the carrier lifetime of the SiC epitaxial wafer in this embodiment are as Figure 3 shown. The carrier lifetime of the epitaxial wafer is 4.3 μs. It can be seen from the data that by using the method provided by the present invention, the carrier lifetime of the silicon carbide epitaxial wafer can be greatly improved.

[0051] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A method for improving the carrier lifetime of a silicon carbide epitaxial wafer, characterized in that, Including the following steps: S1. Place the selected silicon-face silicon carbide substrate with a bias of 4° or 8° in the <11-20> direction on the graphite pedestal in the reaction chamber of the SiC epitaxial system. S2. Replace the gas in the reaction chamber with argon, then introduce hydrogen into the reaction chamber. Use the silicon source and carbon source as the growth sources and introduce the n-type or p-type doping source into the reaction chamber at a C / Si ratio of R0. Gradually heat the reaction chamber to the epitaxial growth temperature, and use hydrogen or argon as the gas-lift gas to drive the rotation of the graphite pedestal to grow the buffer layer. S3. Keep the growth temperature and pressure constant and reduce the C / Si ratio to R 1L , and then increase the C / Si ratio to R in a linearly graded manner 1H , and perform the first rate-switching gradient layer growth, where the R 1L is less than R0, and the R 1H is greater than R0; S4. Keep the growth temperature and pressure constant, and continue to perform n times of rate-switching gradient layer growth until R nH When it is greater than the C / Si ratio of the epitaxial layer growth, stop the growth of the gradient layer. The i-th rate-switching gradient layer growth is as follows: first reduce the C / Si ratio to R iL , and then increase the C / Si ratio to R iH in a linearly varying manner. The R iL is less than R (i-1)H , and the R iH is greater than R (i-1)H , where 2 ≤ i ≤ n; S5. Change the flow rate of the doping source to the set value required for the epitaxial layer in the grown epitaxial structure, and complete the growth of the epitaxial layer under the condition that the C / Si ratio of the epitaxial layer remains unchanged. S6. After completing the growth of the epitaxial layer, turn off the growth source and the doping source, cool the temperature of the reaction chamber to room temperature in a hydrogen atmosphere, then evacuate the hydrogen, use argon to replace the gas in the reaction chamber multiple times, and finally inflate the pressure of the reaction chamber to atmospheric pressure with argon, and then open the chamber to take out the wafer.

2. The method for improving the carrier lifetime of a silicon carbide epitaxial wafer according to claim 1, wherein In step S2, when growing the buffer layer, the hydrogen flow rate is 90~150 L / min, and the reaction chamber pressure is 80~200 mbar.

3. The method for improving the carrier lifetime of a silicon carbide epitaxial wafer according to claim 1, characterized in that, In step S2, the value of the C / Si ratio R0 used for the epitaxial growth of the buffer layer is 0.45~0.75, and the growth rate is less than 20 μm / h.

4. The method for improving the carrier lifetime of a silicon carbide epitaxial wafer according to claim 1, characterized in that, In step S2, the thickness of the grown buffer layer is 0.5~2 μm.

5. The method for improving the carrier lifetime of a silicon carbide epitaxial wafer according to claim 1, wherein In step S4, the value of the C / Si ratio used for the epitaxial growth of the epitaxial layer is 0.85~1.

15.

6. The method for improving the carrier lifetime of a silicon carbide epitaxial wafer according to claim 1, wherein, In step S5, the growth rate of the epitaxial layer is greater than 30~90 μm / h.

7. The method for improving the carrier lifetime of a silicon carbide epitaxial wafer according to claim 1, characterized in that, In step S3, the time for the first rate-switching gradient layer growth and the time for the i-th rate-switching gradient layer growth in step S4 are equal, where 1≤i≤n.

8. The method for improving the carrier lifetime of a silicon carbide epitaxial wafer according to claim 1, characterized in that, The total time for growing the gradient layer in step S3 and step S4 is 0.5~5 minutes.

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