Method for prolonging service life of minority carriers of SiC epitaxy

Abstract

The invention discloses a method for prolonging the service life of a SiC epitaxial minority carrier, and relates to the field of semiconductor materials. The method comprises the following steps: S1, after a SiC substrate is subjected to standard cleaning, epitaxially growing an epitaxial layer with the thickness of more than 10 microns; s2, carrying out standard cleaning again, carrying out high-temperature oxidation on the 4H-SiC epitaxial wafer in an O2 atmosphere, forming a SiO2 oxide layer and partial interstitial carbon atoms and carbon clusters on the SiC surface, and carrying out NO high-temperature passivation annealing; s3, removing a surface oxide layer of the SiC epitaxial wafer subjected to high-temperature oxidation and surface passivation treatment by using a BOE solution, and performing standard cleaning again; s4, putting the SiC epitaxial wafer into a high-temperature annealing device, raising the temperature, reducing the pressure intensity of a furnace chamber, and performing low-pressure hydrogen annealing on the SiC epitaxial wafer in an H2 atmosphere; and S5, repeating the steps S2 to S4. High-temperature oxidation and low-pressure hydrogen annealing are combined, so that the time required by high-temperature oxidation is shortened, the epitaxial wafer with longer minority carrier lifetime is obtained, and the minority carrier lifetime of N and P type SiC can be effectively prolonged.

Description

Technical Field

[0001] The invention relates to the field of semiconductor materials, and in particular to a method for improving the lifetime of SiC epitaxial minority carriers. Background Art

[0002] The minority carrier lifetime in semiconductors can improve the conductivity modulation characteristics of bipolar semiconductor devices and reduce the on-resistance of the devices, so the minority carrier lifetime plays a vital role in improving and optimizing the performance of semiconductor devices. In order to achieve an effective conductivity modulation effect, the minority carrier lifetime in the drift layer must be high enough. Silicon carbide (SiC), as one of the important materials of the third-generation wide bandgap semiconductors, has excellent properties such as wide bandgap, high carrier saturation rate, and good thermal conductivity, so it can be widely used in high-power electronic power devices that are resistant to high voltage, high temperature, and radiation. With the continuous increase in the on-resistance of SiC power devices, unipolar devices that work with one carrier can no longer meet the requirements of ultra-high voltage applications, and bipolar devices must be used. There are two types of carriers working simultaneously in the bipolar device. When its breakdown voltage increases, the existence of the conductivity modulation effect can effectively reduce the on-resistance. Good conductivity modulation must have a high minority carrier lifetime, so it is hoped that the minority carrier lifetime of SiC materials can reach the microsecond level, so as to effectively modulate the device and achieve the effect of reducing the on-resistance.

[0003] At present, the minority carrier lifetime of commercial SiC epitaxial wafers is only 0.5 to 1μs, which is difficult to meet the needs of high-voltage bipolar devices. The two methods commonly used in the world are ultra-long high-temperature oxidation and high-temperature carbon ion implantation to improve the minority carrier lifetime in SiC, but both methods have great limitations. High-temperature oxidation often requires oxidation at a high temperature of 1400℃ for more than 48 hours to eliminate the carbon vacancy defects of epitaxial thickness of more than 10μm, while carbon ion implantation will cause lattice damage and introduce other deep energy level defects. In addition, these two methods have no obvious effect on improving the minority carrier lifetime of P-type SiC epitaxial layer. Therefore, the market urgently needs a method with higher efficiency, lower cost and the ability to improve the minority carrier lifetime of both N-type and P-type SiC epitaxial layers. Summary of the invention

[0004] The purpose of the present invention is to provide a highly efficient method for improving the minority carrier lifetime of SiC epitaxially, in view of the adverse effects of the short minority carrier lifetime of current bipolar semiconductor devices on device performance, the long time-consuming process for improving the minority carrier lifetime, the high process cost, and the difficulty in improving the minority carrier lifetime of P-type SiC. The method combines high-temperature oxidation with low-pressure hydrogen annealing, which not only greatly reduces the time required for high-temperature oxidation, but also can obtain epitaxial wafers with longer minority carrier lifetimes, and can effectively improve the minority carrier lifetimes of both N-type and P-type SiC.

[0005] In order to achieve the above-mentioned invention object, the specific technical solution provided by the present invention is:

[0006] The present invention comprises the following steps:

[0007] S1. Performing standard cleaning on the SiC substrate, and epitaxially growing an epitaxial layer with a thickness of more than 10 μm on the cleaned SiC substrate;

[0008] S2, after standard cleaning of the sample after epitaxial growth in step S1, high-temperature oxidation of the 4H-SiC epitaxial wafer is performed in an O2 atmosphere to form a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters on the SiC surface, and then NO high-temperature passivation annealing is performed;

[0009] S3, removing the surface oxide layer of the SiC epitaxial wafer subjected to high temperature oxidation and surface passivation treatment using BOE solution, and then performing standard cleaning again;

[0010] S4, placing the SiC epitaxial wafer after the standard cleaning in S3 into a high-temperature annealing device, raising the temperature, lowering the furnace chamber pressure, and performing a low-pressure hydrogen annealing process on the SiC epitaxial wafer in a H2 atmosphere;

[0011] S5. Repeat steps S2, S3, and S4 to further provide excess carbon atoms, eliminate deeper carbon vacancy defects in the SiC epitaxial layer, and increase the minority carrier lifetime of the SiC epitaxial wafer.

[0012] In step S1, the steps of performing standard cleaning on the SiC substrate are:

[0013] a. Ultrasonic cleaning with toluene, acetone and ethanol for 3 to 7 times, then rinse with deionized water;

[0014] b. Soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:20 by volume) for 1 to 30 minutes;

[0015] c. Put the substrate soaked in hydrofluoric acid into the third solution and boil for 10 to 60 minutes; the first solution is concentrated sulfuric acid: hydrogen peroxide = 2 to 10:1 by volume;

[0016] d. Boil the substrate boiled in concentrated sulfuric acid with liquid No. 1 and liquid No. 2 in turn for 10 to 60 minutes, then rinse with deionized water and blow dry with nitrogen. Liquid No. 1 is a mixture of ammonia water, hydrogen peroxide and deionized water, with a volume ratio of ammonia water: hydrogen peroxide: deionized water = 1 to 5: 1 to 8: 2 to 10. Liquid No. 2 is a mixture of hydrochloric acid, hydrogen peroxide and deionized water, with a volume ratio of hydrochloric acid: hydrogen peroxide: deionized water = 1 to 5: 1 to 8: 2 to 10.

[0017] In step S1, the epitaxy is to epitaxially grow a 10-300 um N-type or P-type epitaxial layer on a highly doped n-type 4H-SiC substrate with a 2-8° off-angle and a C / Si ratio of 1-1.5.

[0018] In step S2, the high temperature oxidation temperature is 600-1600°C, the high temperature oxidation time is 1-8h, the high temperature passivation annealing gas can be one of NO, NO2 and POCl3, the high temperature passivation temperature is 600-1400°C, and the passivation time is 1-2h; preferably, POCl3 annealing is performed at an environment above 600-1100°C for more than 10min; POCl3 is selected as the annealing atmosphere, which can effectively reduce the interface state density, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime.

[0019] In step S4, the pressure of the low-pressure hydrogen annealing is 10-800 mabr, the hydrogen annealing temperature is 800-1600° C., and the hydrogen annealing time is 10-360 min.

[0020] In step S5, the number of times of repeating steps S2, S3, and S4 can be 1 to 3 times.

[0021] The basic principle and physical mechanism of the present invention are: under high temperature conditions, the SiC surface is converted into SiO2 through oxidation. This process can not only oxidize the surface, but also produce excess carbon atoms and carbon clusters. The excess carbon atoms can eliminate some carbon vacancies and increase the minority carrier lifetime. Combined with the hydrogen annealing process, hydrogen atoms can further decompose the carbon clusters formed by high temperature oxidation and the carbon clusters they have into carbon atoms that are more easily formed by the vacancy exchange mechanism to eliminate carbon vacancies. On the other hand, hydrogen annealing under low pressure selectively etches Si atoms and a chemical reaction occurs. The chemical reaction equation occurring in this process is: under low pressure (1), Si(s) →Si(g); (2), C(s)+2H2(g)←CH4 (g); This selective etching will further form a SiC surface rich in carbon atoms. The larger concentration gradient makes it easier for carbon atoms to complete the diffusion of the vacancy exchange mechanism in a shorter time. In addition, hydrogen atoms themselves can passivate a few carbon vacancies that are difficult to eliminate. Based on the above principles, the combination and mutual promotion of high-temperature oxidation and hydrogen annealing can greatly eliminate carbon vacancies in SiC, and can eliminate carbon clusters generated by high-temperature oxidation and itself, which is an important reason for greatly improving the minority carrier lifetime and reducing the process time.

[0022] Compared with the prior art, the present invention has the following outstanding advantages:

[0023] 1. The present invention provides a highly efficient process method that can significantly and stably improve the minority carrier lifetime of N and P type SiC. It combines high temperature oxidation with low pressure hydrogen annealing. On the basis of conventional high temperature oxidation to eliminate carbon vacancies, hydrogen annealing can not only eliminate carbon vacancies with its own hydrogen atoms, but also decompose the byproduct carbon clusters produced by high temperature oxidation to further eliminate carbon vacancies. High temperature oxidation can provide carbon clusters, and hydrogen annealing can further eliminate the byproducts of high temperature oxidation. The combination of high temperature oxidation and low pressure hydrogen annealing is not a simple superposition of two methods, but promotes and improves each other in physical mechanism, further significantly improves the minority carrier lifetime of SiC and significantly reduces the time required for high temperature oxidation.

[0024] 2. The present invention provides a highly efficient process method that can significantly and stably improve the minority carrier lifetime of N and P type SiC; hydrogen atoms can passivate carbon vacancies, but the passivation of hydrogen atoms alone is unstable; this method uses hydrogen annealing under low pressure to carbonize the SiC surface, and further promotes the vacancy exchange of carbon atoms to eliminate carbon vacancies on the basis of hydrogen atom passivation of carbon vacancies, thereby significantly improving the minority carrier lifetime of SiC and also improving the stability of passivation. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a preparation process flow chart of the present invention.

[0026] Figure 2 It is a comparison diagram of minority carrier lifetime before and after adopting the process of the present invention. DETAILED DESCRIPTION

[0027] In order to make the technical features and practical effects of the present invention easy to understand, Figure 1 The present invention is further described in the process flow chart in combination with the specific implementation method.

[0028] The present invention utilizes a combination of high-temperature oxidation and low-pressure hydrogen annealing to reduce deep energy level defects in a silicon carbide (SiC) epitaxial layer, thereby increasing the minority carrier lifetime. After standard cleaning, an epitaxial layer having a thickness of more than 10 microns is grown on a highly doped n- or p-type SiC substrate under growth conditions of C / Si of 1 to 1.5, and a method of combining high-temperature oxidation and low-pressure hydrogen annealing for multiple times is used to significantly increase the minority carrier lifetime in the SiC epitaxial layer while improving its stability.

[0029] like Figure 1 , the embodiment of the present invention includes the following steps:

[0030] 1: Standard cleaning of 4H-SiC substrate samples:

[0031] a. Ultrasonic cleaning with toluene, acetone and ethanol for 3 to 7 times, each ultrasonic cleaning for 10-20 minutes, and then rinse with deionized water;

[0032] b. Soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:1-20 by volume) for 1-30 minutes;

[0033] c. Put the substrate soaked in hydrofluoric acid into liquid No. 3 and boil for 10 to 60 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 2 to 10:1 by volume;

[0034] d. The substrate boiled in concentrated sulfuric acid is boiled in liquid No. 1 and liquid No. 2 for 10 to 60 minutes, then rinsed with deionized water and blown dry with nitrogen. Liquid No. 1 is a mixture of ammonia, hydrogen peroxide and deionized water, with a volume ratio of ammonia: hydrogen peroxide: deionized water = 1 to 5: 1 to 8: 2 to 10, and liquid No. 3 is a mixture of hydrochloric acid, hydrogen peroxide and deionized water, with a volume ratio of hydrochloric acid: hydrogen peroxide: deionized water = 1 to 5: 1 to 8: 2 to 10;

[0035] 2: After cleaning, the (0001) Si surface of the highly doped n-type 4H-SiC substrate is 2 to 8 degrees off-axis and the n or p-type doping concentration is 1×10 14 ~1×10 15 cm 3 The thickness of the epitaxial layer is 10 to 150 μm, and the growth C / Si ratio is 1 to 1.5;

[0036] 3: After the sample after epitaxial growth in step 2 is cleaned again in the same standard as in step 1, the 4H-SiC epitaxial wafer is subjected to high-temperature oxidation in an O2 atmosphere. The oxidation temperature can be 600-1600°C and the oxidation time can be 1-8h. After high-temperature oxidation, a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters are formed on the SiC surface. Then, high-temperature annealing with NO and NO2 at above 600-1400°C can effectively reduce the interface state density for surface passivation, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime;

[0037] 4: Use BOE solution with a ratio of NH4F to HF of 1 to 6:1 to remove the oxide layer on the surface of the sample in step 3;

[0038] 5: The sample with the surface oxide layer removed is placed in a high temperature annealing device for hydrogen annealing at 900-1200°C under a pressure of 100-800 mbar. The annealing time can be 10-360 min. During the hydrogen annealing process, hydrogen atoms can directly passivate carbon vacancies on the one hand, and decompose carbon clusters to form more easily diffusible carbon atoms to eliminate carbon vacancies on the other hand;

[0039] 6: Repeat steps 3 to 5. High-temperature oxidation can provide a large number of carbon atoms and carbon clusters, which are all decomposed and diffused under the action of hydrogen annealing, greatly reducing the concentration of carbon vacancies, reducing the recombination centers of carriers, and increasing the minority carrier lifetime. Multiple high-temperature oxidation and hydrogen annealing can enhance this effect.

[0040] 7: Finally, annealing in a POCl3 environment at 600-1100°C for surface passivation can effectively reduce the interface state density, thereby reducing the surface recombination rate and further increasing the surface minority carrier lifetime.

[0041] The invention also discloses silicon carbide, which is obtained by the method as described above.

[0042] Example 1

[0043] Step 1: ultrasonically clean the substrate with toluene, acetone and ethanol for 3 times, each ultrasonic cleaning for 10 minutes, and then rinse it with deionized water; soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:20) for 1 minute; boil the substrate soaked in hydrofluoric acid in liquid No. 3 for 60 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 4:1; boil the substrate boiled in concentrated sulfuric acid in liquid No. 1 and liquid No. 2 for 60 minutes in turn, rinse it with deionized water, and blow it dry with nitrogen for standby use; liquid No. 1 is ammonia water: hydrogen peroxide: deionized water = 4:1:1, and liquid No. 2 is hydrochloric acid: hydrogen peroxide: deionized water = 4:1:1;

[0044] Step 2: After cleaning, the (0001) Si plane of the highly doped n-type 4H-SiC substrate is 4° off-axis and the n-type doping concentration is 1×10 14 cm 3 The epitaxial layer is 150um thick and the growth C / Si ratio is 1.5;

[0045] Step 3: After the epitaxially grown sample is cleaned again in the same standard as in step 1, the 4H-SiC epitaxial wafer is subjected to high-temperature oxidation in an O2 atmosphere at a temperature of 600°C for 8 hours; after high-temperature oxidation, a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters are formed on the SiC surface, and then high-temperature annealing with NO and NO2 at above 600°C is performed to effectively reduce the interface state density, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime;

[0046] Step 4: Use a BOE solution with a ratio of NH4F and HF of 1:1 to remove the oxide layer on the surface of the sample in step 3;

[0047] Step 5: The sample with the surface oxide layer removed is placed in a high temperature annealing device for hydrogen annealing at 900°C under a pressure of 100 mbar for 360 min. During the hydrogen annealing process, hydrogen atoms can directly passivate carbon vacancies on the one hand, and decompose carbon clusters to form carbon atoms that are more easily diffused to eliminate carbon vacancies on the other hand.

[0048] Step 6: Repeat steps 3 to 5. High-temperature oxidation can provide a large number of carbon atoms and carbon clusters, which are all decomposed and diffused under the action of hydrogen annealing to greatly reduce the concentration of carbon vacancies, reduce the recombination centers of carriers, and increase the minority carrier lifetime. Multiple high-temperature oxidation and hydrogen annealing can enhance this effect.

[0049] Step 7: Finally, annealing in a POCl3 environment at 600°C is performed for surface passivation, which effectively reduces the interface state density, thereby reducing the surface recombination rate and further improving the surface minority carrier lifetime.

[0050] Example 2

[0051] Step 1: ultrasonically clean the substrate with toluene, acetone and ethanol for 3 times, each ultrasonic cleaning for 20 minutes, and then rinse it with deionized water; soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:15) for 25 minutes; boil the substrate soaked in hydrofluoric acid in liquid No. 3 for 50 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 10:1; boil the substrate boiled in concentrated sulfuric acid in liquid No. 1 and liquid No. 2 for 50 minutes in turn, rinse it with deionized water, and blow it dry with nitrogen for standby use; liquid No. 1 is ammonia water: hydrogen peroxide: deionized water = 10:1:1, and liquid No. 2 is hydrochloric acid: hydrogen peroxide: deionized water = 10:1:1;

[0052] Step 2: After cleaning, the (0001) Si plane of the highly doped n-type 4H-SiC substrate is 8° off-axis and the n-type doping concentration is 1×10 15 cm 3 The epitaxial layer is 120 um thick and the growth C / Si ratio is 1.4;

[0053] Step 3: After the epitaxially grown sample is cleaned again in the same standard as in step 1, the 4H-SiC epitaxial wafer is subjected to high-temperature oxidation in an O2 atmosphere at a temperature of 800°C for 7 hours; after high-temperature oxidation, a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters are formed on the SiC surface, and then high-temperature annealing with NO and NO2 at above 800°C is performed to effectively reduce the interface state density, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime;

[0054] Step 4: Use BOE solution with a ratio of NH4F and HF of 2:1 to remove the oxide layer on the surface of the sample in step 3;

[0055] Step 5: The sample with the surface oxide layer removed is placed in a high temperature annealing device for hydrogen annealing at 1000°C under a pressure of 200 mbar for 60 minutes. During the hydrogen annealing process, hydrogen atoms can directly passivate carbon vacancies on the one hand, and decompose carbon clusters to form more easily diffusible carbon atoms to eliminate carbon vacancies on the other hand.

[0056] Step 6: Repeat steps 3 to 5. High-temperature oxidation can provide a large number of carbon atoms and carbon clusters, which are all decomposed and diffused under the action of hydrogen annealing to greatly reduce the concentration of carbon vacancies, reduce the recombination centers of carriers, and increase the minority carrier lifetime;

[0057] Combining multiple high-temperature oxidations with hydrogen annealing can enhance this effect;

[0058] Step 7: Finally, annealing in a POCl3 environment at 700°C is performed for surface passivation, which effectively reduces the interface state density, thereby reducing the surface recombination rate and further improving the surface minority carrier lifetime.

[0059] Example 3

[0060] Step 1: ultrasonically clean the substrate with toluene, acetone and ethanol for 5 times, each ultrasonic cleaning for 10 minutes, and then rinse it with deionized water; soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:10) for 20 minutes; boil the substrate soaked in hydrofluoric acid in liquid No. 3 for 40 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 8:1; boil the substrate boiled in concentrated sulfuric acid in liquid No. 1 and liquid No. 2 for 40 minutes in turn, rinse it with deionized water, and blow it dry with nitrogen for standby use; liquid No. 1 is ammonia water: hydrogen peroxide: deionized water = 8:1:1, and liquid No. 2 is hydrochloric acid: hydrogen peroxide: deionized water = 8:1:1;

[0061] Step 2: After cleaning, the (0001) Si plane of the highly doped n-type 4H-SiC substrate is 6° off-axis and the n-type doping concentration is 8×10 14 cm 3 The epitaxial layer is 90um thick and the growth C / Si ratio is 1.3;

[0062] Step 3: After the epitaxially grown sample is cleaned again in the same standard as in step 1, the 4H-SiC epitaxial wafer is subjected to high-temperature oxidation in an O2 atmosphere at a temperature of 1000°C for 6 hours; after high-temperature oxidation, a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters are formed on the SiC surface, and then high-temperature annealing with NO and NO2 at above 1000°C is performed to effectively reduce the interface state density, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime;

[0063] Step 4: Use BOE solution with a ratio of NH4F and HF of 3:1 to remove the oxide layer on the surface of the sample in step 3;

[0064] Step 5: The sample with the surface oxide layer removed is placed in a high temperature annealing device for hydrogen annealing at 1100°C under a pressure of 300 mbar for 120 min. During the hydrogen annealing process, hydrogen atoms can directly passivate carbon vacancies on the one hand, and decompose carbon clusters to form more easily diffusible carbon atoms to eliminate carbon vacancies on the other hand.

[0065] Step 6: Repeat steps 3 to 5. Multiple high-temperature oxidations can provide a large number of carbon atoms and carbon clusters, which are all decomposed and diffused under the action of hydrogen annealing to significantly reduce the carbon vacancy concentration, reduce the recombination center of carriers, and increase the minority carrier lifetime;

[0066] Combining multiple high-temperature oxidations with hydrogen annealing can enhance this effect;

[0067] Step 7: Finally, annealing in a POCl3 environment at 800°C is performed for surface passivation, which effectively reduces the interface state density, thereby reducing the surface recombination rate and further improving the surface minority carrier lifetime.

[0068] Example 4

[0069] Step 1: ultrasonically clean the substrate with toluene, acetone and ethanol for 5 times, each ultrasonic cleaning for 20 minutes, and then rinse it with deionized water; soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:5) for 15 minutes; boil the substrate soaked in hydrofluoric acid in liquid No. 3 for 30 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 6:1; boil the substrate boiled in concentrated sulfuric acid in liquid No. 1 and liquid No. 2 for 30 minutes in turn, rinse it with deionized water, and blow it dry with nitrogen for standby use; liquid No. 1 is ammonia water: hydrogen peroxide: deionized water = 6:1:1, and liquid No. 2 is hydrochloric acid: hydrogen peroxide: deionized water = 6:1:1;

[0070] Step 2: After cleaning, the (0001) Si plane of the highly doped n-type 4H-SiC substrate is 2° off-axis and the n-type doping concentration is 6×10 14 cm 3 The epitaxial layer is 60um thick and the growth C / Si ratio is 1.2;

[0071] Step 3: After the epitaxially grown sample is cleaned again in the same standard as in step 1, the 4H-SiC epitaxial wafer is subjected to high-temperature oxidation in an O2 atmosphere at a temperature of 1200°C for 5 hours; after high-temperature oxidation, a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters are formed on the SiC surface, and then high-temperature annealing with NO and NO2 at above 1200°C is performed to effectively reduce the interface state density, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime;

[0072] Step 4: Use BOE solution with a ratio of NH4F and HF of 4:1 to remove the oxide layer on the surface of the sample in step 3;

[0073] Step 5: The sample with the surface oxide layer removed is placed in a high temperature annealing device for hydrogen annealing at 1200°C under a pressure of 400 mbar for 180 min. During the hydrogen annealing process, hydrogen atoms can directly passivate carbon vacancies on the one hand, and decompose carbon clusters to form more easily diffusible carbon atoms to eliminate carbon vacancies on the other hand.

[0074] Step 6: Repeat steps 3 to 5. High-temperature oxidation can provide a large number of carbon atoms and carbon clusters, which are all decomposed and diffused under the action of hydrogen annealing to greatly reduce the concentration of carbon vacancies, reduce the recombination centers of carriers, and increase the minority carrier lifetime. Multiple high-temperature oxidation and hydrogen annealing can enhance this effect.

[0075] Step 7: Finally, annealing in a POCl3 environment at 900°C is performed for surface passivation, which effectively reduces the interface state density, thereby reducing the surface recombination rate and further improving the surface minority carrier lifetime.

[0076] Example 5

[0077] Step 1: ultrasonically clean the substrate with toluene, acetone and ethanol for 7 times, each ultrasonic cleaning for 10 minutes, and then rinse it with deionized water; soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:5) for 10 minutes; boil the substrate soaked in hydrofluoric acid in liquid No. 3 for 20 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 2:1; boil the substrate boiled in concentrated sulfuric acid in liquid No. 1 and liquid No. 2 for 20 minutes in turn, rinse it with deionized water, and blow it dry with nitrogen for standby use; liquid No. 1 is ammonia water: hydrogen peroxide: deionized water = 2:1:1, and liquid No. 2 is hydrochloric acid: hydrogen peroxide: deionized water = 2:1:1;

[0078] Step 2: After cleaning, the (0001) Si plane of the highly doped n-type 4H-SiC substrate is 8° off-axis and the p-type doping concentration is 4×10 14 cm 3 The epitaxial layer is 30um thick and the growth C / Si ratio is 1.1;

[0079] Step 3: After the epitaxially grown sample is cleaned again in the same standard as in step 1, the 4H-SiC epitaxial wafer is subjected to high-temperature oxidation in an O2 atmosphere at a temperature of 1400°C for 4 hours; after high-temperature oxidation, a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters are formed on the SiC surface, and then high-temperature annealing with NO and NO2 at above 1400°C is performed to effectively reduce the interface state density, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime;

[0080] Step 4: Use BOE solution with a ratio of NH4F and HF of 5:1 to remove the oxide layer on the surface of the sample in step 3;

[0081] Step 5: The sample with the surface oxide layer removed is placed in a high temperature annealing device for hydrogen annealing at 1200°C under a pressure of 500 mbar for 240 min. During the hydrogen annealing process, hydrogen atoms can directly passivate carbon vacancies on the one hand, and decompose carbon clusters to form more easily diffusible carbon atoms to eliminate carbon vacancies on the other hand.

[0082] Step 6: Repeat steps 3 to 5. High-temperature oxidation can provide a large number of carbon atoms and carbon clusters, which are all decomposed and diffused under the action of hydrogen annealing to greatly reduce the concentration of carbon vacancies, reduce the recombination centers of carriers, and increase the minority carrier lifetime;

[0083] Combining multiple high-temperature oxidations with hydrogen annealing can enhance this effect;

[0084] Step 7: Finally, annealing in a POCl3 environment at 1000°C is performed for surface passivation, which effectively reduces the interface state density, thereby reducing the surface recombination rate and further improving the surface minority carrier lifetime.

[0085] Example 6

[0086] Step 1: ultrasonically clean the substrate with toluene, acetone and ethanol for 7 times, each ultrasonic cleaning for 20 minutes, and then rinse it with deionized water; soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:1) for 5 minutes; boil the substrate soaked in hydrofluoric acid in liquid No. 3 for 10 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 1:1; boil the substrate boiled in concentrated sulfuric acid in liquid No. 1 and liquid No. 2 for 10 minutes in turn, rinse it with deionized water, and blow it dry with nitrogen for standby use; liquid No. 1 is ammonia water: hydrogen peroxide: deionized water = 1:1:1, and liquid No. 2 is hydrochloric acid: hydrogen peroxide: deionized water = 1:1:1;

[0087] Step 2: After cleaning, the (0001) Si plane of the highly doped n-type 4H-SiC substrate is 6° off-axis and the p-type doping concentration is 2×10 14 cm 3 The epitaxial layer is 150um thick and the growth C / Si ratio is 1;

[0088] Step 3: After the epitaxially grown sample is cleaned again in the same standard as in step 1, the 4H-SiC epitaxial wafer is subjected to high-temperature oxidation in an O2 atmosphere at a temperature of 1600°C for 3 hours; after high-temperature oxidation, a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters are formed on the SiC surface, and then high-temperature annealing with NO and NO2 at above 1400°C is performed to effectively reduce the interface state density, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime;

[0089] Step 4: Use BOE solution with a ratio of NH4F and HF of 6:1 to remove the oxide layer on the surface of the sample in step 3;

[0090] Step 5: The sample with the surface oxide layer removed was placed in a high temperature annealing device for hydrogen annealing at 1100°C under a pressure of 600 mbar for 300 min. During the hydrogen annealing process, hydrogen atoms can directly passivate carbon vacancies on the one hand, and decompose carbon clusters to form more easily diffusible carbon atoms to eliminate carbon vacancies on the other hand.

[0091] Step 6: Repeat steps 3 to 5. High-temperature oxidation can provide a large number of carbon atoms and carbon clusters, which are all decomposed and diffused under the action of hydrogen annealing to greatly reduce the concentration of carbon vacancies, reduce the recombination centers of carriers, and increase the minority carrier lifetime;

[0092] Combining multiple high-temperature oxidations with hydrogen annealing can enhance this effect;

[0093] Step 7: Annealing in a POCl3 environment at 1100°C for surface passivation effectively reduces the interface state density, thereby reducing the surface recombination rate and further increasing the surface minority carrier lifetime.

[0094] Example 7

[0095] Step 1: ultrasonically clean the substrate with toluene, acetone and ethanol for 3 times, each ultrasonic cleaning for 10 minutes, and then rinse it with deionized water; soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:20) for 5 minutes; boil the substrate soaked in hydrofluoric acid in liquid No. 3 for 20 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 4:1; boil the substrate boiled in concentrated sulfuric acid in liquid No. 1 and liquid No. 2 for 20 minutes in turn, rinse it with deionized water, and blow it dry with nitrogen for standby use; liquid No. 1 is ammonia water: hydrogen peroxide: deionized water = 4:1:1, and liquid No. 2 is hydrochloric acid: hydrogen peroxide: deionized water = 4:1:1;

[0096] Step 2: After cleaning, the (0001) Si surface of the highly doped n-type 4H-SiC substrate is 4° off-axis and the p-type doping concentration is 2×10 14 cm 3The epitaxial layer is 150um thick and the growth C / Si ratio is 1.3;

[0097] Step 3: After the epitaxially grown sample is cleaned again in the same standard as in step 1, the 4H-SiC epitaxial wafer is subjected to high-temperature oxidation in an O2 atmosphere at a temperature of 1400°C for 2 hours; after high-temperature oxidation, a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters are formed on the SiC surface, and then high-temperature annealing with NO and NO2 at above 1200°C is performed to effectively reduce the interface state density, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime;

[0098] Step 4: Use BOE solution with a ratio of NH4F and HF of 3:1 to remove the oxide layer on the surface of the sample in step 3;

[0099] Step 5: The sample with the surface oxide layer removed is placed in a high temperature annealing device for hydrogen annealing at 1000°C under a pressure of 700 mbar for 360 min. During the hydrogen annealing process, hydrogen atoms can directly passivate carbon vacancies on the one hand, and decompose carbon clusters to form more easily diffusible carbon atoms to eliminate carbon vacancies on the other hand.

[0100] Step 6: Repeat steps 3 to 5. High-temperature oxidation can provide a large number of carbon atoms and carbon clusters, which are all decomposed and diffused under the action of hydrogen annealing to greatly reduce the concentration of carbon vacancies, reduce the recombination centers of carriers, and increase the minority carrier lifetime;

[0101] Combining multiple high-temperature oxidations with hydrogen annealing can enhance this effect;

[0102] Step 7: Finally, annealing in a POCl3 environment at 850°C is performed for surface passivation, which effectively reduces the interface state density, thereby reducing the surface recombination rate and further improving the surface minority carrier lifetime.

[0103] Example 8

[0104] Step 1: ultrasonically clean the substrate with toluene, acetone and ethanol for 5 times, each ultrasonic cleaning for 10 minutes, and then rinse it with deionized water; soak the rinsed substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:20) for 10 minutes; boil the substrate soaked in hydrofluoric acid in liquid No. 3 for 20 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 4:1; boil the substrate boiled in concentrated sulfuric acid in liquid No. 1 and liquid No. 2 for 20 minutes in turn, rinse it with deionized water, and blow it dry with nitrogen for standby use; liquid No. 1 is ammonia water: hydrogen peroxide: deionized water = 4:1:1, and liquid No. 2 is hydrochloric acid: hydrogen peroxide: deionized water = 4:1:1;

[0105] Step 2: After cleaning, the (0001) Si surface of the highly doped n-type 4H-SiC substrate is 2° off-axis and the p-type doping concentration is 2×1014 cm 3 The epitaxial layer is 150um thick and the growth C / Si ratio is 1.3;

[0106] Step 3: After the epitaxially grown sample is cleaned again in the same standard as in step 1, the 4H-SiC epitaxial wafer is subjected to high-temperature oxidation in an O2 atmosphere at a temperature of 1400°C for 1 hour; after high-temperature oxidation, a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters are formed on the SiC surface, and then high-temperature annealing with NO and NO2 at above 1400°C is performed to effectively reduce the interface state density, thereby reducing the surface recombination rate and increasing the surface minority carrier lifetime;

[0107] Step 4: Use BOE solution with a ratio of NH4F and HF of 3:1 to remove the oxide layer on the surface of the sample in step 3;

[0108] Step 5: The sample with the surface oxide layer removed is placed in a high temperature annealing device for hydrogen annealing at 900°C under a pressure of 800 mbar for 360 min. During the hydrogen annealing process, hydrogen atoms can directly passivate carbon vacancies on the one hand, and decompose carbon clusters to form carbon atoms that are more easily diffused to eliminate carbon vacancies on the other hand.

[0109] Step 6: Repeat steps 3 to 5. High-temperature oxidation can provide a large number of carbon atoms and carbon clusters, which are all decomposed and diffused under the action of hydrogen annealing to greatly reduce the concentration of carbon vacancies, reduce the recombination centers of carriers, and increase the minority carrier lifetime. Multiple high-temperature oxidation and hydrogen annealing can enhance this effect.

[0110] Step 7: Finally, annealing in a POCl3 environment at 850°C is performed for surface passivation, which effectively reduces the interface state density, thereby reducing the surface recombination rate and further improving the surface minority carrier lifetime.

[0111] In some embodiments 1 to 8 of the present invention, the minority carrier lifetime of the silicon carbide after being treated by the high-temperature oxidation combined with low-pressure hydrogen annealing process provided by the present invention can reach 7 microseconds or greater.

[0112] In an exemplary embodiment, the method of the present invention for improving the minority carrier lifetime of silicon carbide (SiC) epitaxial material combines a high-temperature oxidation method with low-pressure hydrogen annealing to perform high-temperature oxidation on a SiC thick epitaxial layer (150 microns). First, the 4H-SiC thick epitaxial layer is subjected to standard cleaning, and then O2 is introduced to oxidize the sample at a high temperature of 1300°C for 8 hours. Subsequently, the temperature condition is maintained unchanged, and NO gas is introduced for nitrogen oxide oxidation for 60 minutes. Then, the oxide layer on the surface of the epitaxial wafer is removed by soaking in HF buffer (HF:H2O volume ratio is 1:20) for 5 minutes, and finally, H2 atmosphere is introduced and annealed at 1000°C for 180 minutes. The carrier lifetime is tested by methods such as μ-PCD (microwave photoconductivity decay method) or TRPL (time-resolved photoluminescence spectroscopy). The test results are as follows: Figure 2 The SiC epitaxial layer shown in the figure has a body minority carrier lifetime of up to 25.46 μs after the process method improved by the present invention. The present invention can prepare ultra-high carrier lifetime, low deep energy level defect density, low interface state density, and the present invention can improve the minority carrier lifetime of both N-type and P-type epitaxial layers, and can meet the 4H-SiC thick epitaxial material required for the preparation of bipolar devices, which is suitable for semiconductor high-power electronic power devices, can significantly reduce energy consumption, and improve device performance.

[0113] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for improving the lifetime of minority carriers in SiC epitaxy, characterized in that The following steps are involved: S1. Performing standard cleaning on the SiC substrate, and epitaxially growing an epitaxial layer with a thickness of more than 10 μm on the cleaned SiC substrate; S2, after standard cleaning of the sample after epitaxial growth in step S1, high-temperature oxidation of the 4H-SiC epitaxial wafer is performed in an O2 atmosphere to form a SiO2 oxide layer and some interstitial carbon atoms and carbon clusters on the SiC surface, and then NO high-temperature passivation annealing is performed; S3, removing the surface oxide layer of the SiC epitaxial wafer subjected to high temperature oxidation and surface passivation treatment using BOE solution, and then performing standard cleaning again; S4, placing the SiC epitaxial wafer after the standard cleaning in S3 into a high-temperature annealing device, raising the temperature, lowering the furnace chamber pressure, and performing a low-pressure hydrogen annealing process on the SiC epitaxial wafer in a H2 atmosphere; S5. Repeat steps S2, S3, and S4 to provide excess carbon atoms, eliminate deeper carbon vacancy defects in the SiC epitaxial layer, increase the minority carrier lifetime of the SiC epitaxial wafer, and optimize the performance of the SiC epitaxial wafer.

2. A method for improving the minority carrier lifetime in SiC epitaxy as claimed in claim 1, characterized in that In step S1, the steps of performing standard cleaning on the SiC substrate are: a. Ultrasonic cleaning with toluene, acetone and ethanol for 3 to 7 times, then rinse with deionized water; b. Soak the rinsed substrate in diluted hydrofluoric acid for 1 to 30 minutes; the diluted hydrofluoric acid has a volume ratio of hydrogen fluoride to deionized water of 1:20; c. Put the substrate soaked in hydrofluoric acid into liquid No. 3 and boil for 10 to 60 minutes; the liquid No. 3 is concentrated sulfuric acid: hydrogen peroxide = 2 to 10: 1 by volume; d. Boil the substrate boiled in concentrated sulfuric acid with liquid No. 1 and liquid No. 2 in turn for 10 to 60 minutes, then rinse with deionized water and blow dry with nitrogen. Liquid No. 1 is a mixture of ammonia water, hydrogen peroxide and deionized water, with a volume ratio of ammonia water: hydrogen peroxide: deionized water = 1 to 5: 1 to 8: 2 to 10. Liquid No. 2 is a mixture of hydrochloric acid, hydrogen peroxide and deionized water, with a volume ratio of hydrochloric acid: hydrogen peroxide: deionized water = 1 to 5: 1 to 8: 2 to 10.

3. A method for improving the minority carrier lifetime in SiC epitaxy as claimed in claim 1, characterized in that In step S1, the epitaxy is to epitaxially grow a 10-300 um N-type or P-type epitaxial layer on a highly doped n-type 4H-SiC substrate with a 2-8° off-angle and a C / Si ratio of 1-1.

5.

4. A method for improving the lifetime of minority carriers in SiC epitaxy as claimed in claim 1, characterized in that In step S2, the high temperature oxidation temperature is 600-1600°C, the high temperature oxidation time is 1-8h, the high temperature passivation annealing gas is one of NO, NO2 and POCl3, the high temperature passivation temperature is 600-1400°C, and the high temperature passivation time is 1-2h.

5. A method for improving the lifetime of minority carriers in SiC epitaxy as claimed in claim 4, characterized in that Annealing with POCl3 at 600-1100°C for more than 10 min, with POCl3 as the annealing atmosphere, can effectively reduce the interface state density, reduce the surface recombination rate, and increase the surface minority carrier lifetime.

6. A method for improving the lifetime of minority carriers in SiC epitaxy as claimed in claim 1, characterized in that In step S4, the pressure of the low-pressure hydrogen annealing is 10-800 mabr, the hydrogen annealing temperature is 800-1600° C., and the hydrogen annealing time is 10-360 min.

7. A method for improving the lifetime of minority carriers in SiC epitaxy as claimed in claim 1, characterized in that In step S5, steps S2, S3, and S4 are repeated 1 to 3 times.

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