Dislocation identification method and dislocation etchant for silicon carbide crystal

Abstract

The application discloses a dislocation recognition method and a dislocation etchant of a silicon carbide crystal, and belongs to the technical field of crystal material testing and characterization. The dislocation recognition method comprises the following steps: (1) KOH, NaOH and MgO after drying are heated to dissolve, and then Na2O2 is added to obtain an etchant, and a silicon carbide crystal is placed in the etchant for etching; (2) the etching pit morphology after etching is observed, and basal plane dislocations, edge dislocations and screw dislocations in the silicon carbide crystal are recognized. Through the method, TSD, TED and BPD of the silicon carbide crystal can be distinguished and recognized, and the method can be widely applied to dislocation recognition of high-nitrogen-doped silicon carbide crystals, light-nitrogen-doped silicon carbide crystals and common high-purity silicon carbide crystals.

Description

Technical Field

[0001] This application relates to a method for dislocation identification in silicon carbide crystals and a dislocation etchant, belonging to the field of crystal material testing and characterization technology. Background Technology

[0002] Dislocations are generated in crystals due to doping or thermal stress. These dislocations have higher strain energy, leading to higher chemical potential energy and molecular activity. This makes them more susceptible to chemical reactions with KOH, thus revealing the dislocation morphology. Based on the crystal structure of 4H-SiC single crystals, the surface morphology of its screw dislocations (TSDs) and edge dislocations (TEDs) is typically hexagonal. Screw dislocations (TSDs) can be inherited by epitaxial wafers, causing adverse effects on power devices such as increased leakage current and reduced breakdown voltage. Therefore, accurately identifying the TSD density can predict defective areas, reducing epitaxial and device risks, and lowering costs.

[0003] Traditional etching methods use KOH etching, which involves two reaction mechanisms: one is a chemical reaction without free charge carriers, which is an anisotropic reaction; the other is an electrochemical reaction with free charge carriers, which is an isotropic reaction. It is precisely this isotropic electrochemical reaction that causes the surface morphology of dislocations to tend towards an n+ type circular shape. Consequently, screw dislocations and edge dislocations appear very similar in morphology during identification, making accurate differentiation impossible.

[0004] CN111238910A mentions that adding Na2O2 to KOH will result in a hexagonal morphology for TSD. MoonkyongNA et al. verified through experiments that the KOH and Na2O2 etching method used in the patent CN111238910A can only be used for ordinary nitrogen-doped silicon carbide crystals, and cannot be used for highly nitrogen-doped silicon carbide crystals. Furthermore, the appearance of dislocations reflects the increase in crystal stress, and the substrate annealing to reduce stress has been widely studied and applied. Marek Skowronski et al. verified through experiments that when the substrate is annealed, free electrons generated by nitrogen ionization are adsorbed around the TSD. The increase in electron concentration will accelerate the rate of electrochemical reaction, thus causing the TSD to show an elliptical shape. Therefore, the etchant and etching method mentioned in CN111238910A are even less applicable to highly nitrogen-doped silicon carbide crystals after high-temperature annealing. Summary of the Invention

[0005] To address the aforementioned issues, a method for dislocation identification in silicon carbide crystals is provided. This method can distinguish and identify TSD, TED, and BPD in silicon carbide crystals and can be widely applied to dislocation identification in highly nitrogen-doped silicon carbide crystals, lightly nitrogen-doped silicon carbide crystals, and ordinary high-purity silicon carbide crystals.

[0006] According to one aspect of this application, a method for dislocation identification in a silicon carbide crystal is provided, comprising the following steps:

[0007] (1) Heat the dried KOH, NaOH and MgO until they dissolve, then add Na2O2 to obtain an etchant, and place the silicon carbide crystal in the etchant for etching;

[0008] (2) Observe the morphology of the corrosion pits after corrosion is completed, and identify the basal plane dislocations, edge dislocations and screw dislocations in the silicon carbide crystal.

[0009] This dislocation identification method enhances the corrosion properties of the etchant by adding NaOH and MgO, causing TSDs to appear hexagonal, TEDs to appear circular, and BPDs to appear teardrop-shaped or strip-shaped. Furthermore, the size of the TSD is larger than that of the TED, thus effectively identifying these three types of dislocations. This dislocation identification method is applicable to special crystals such as highly nitrogen-doped silicon carbide, especially to highly nitrogen-doped silicon carbide crystals after high-temperature annealing, thus possessing universality and suitability for industrial application.

[0010] Because KOH and NaOH have a strong water adsorption property, Na2O2 reacts immediately with water to form NaOH and O2, decomposing completely before corrosion even begins. Once completely melted, KOH and NaOH no longer absorb water. The method involves heating dried KOH, NaOH, and MgO until dissolved, followed by the addition of Na2O2. This ensures good drying of the corrosive agent, allowing Na2O2 to decompose only to produce oxygen. Furthermore, MgO, as a suspension, effectively mitigates the decomposition and volatilization of Na2O2 in the solution, providing an oxygen-rich environment that effectively blocks oxygen volatilization, thus achieving the difference between TSD and TED.

[0011] Optionally, the etchant comprises, by weight, 50-80 parts KOH, 20-50 parts NaOH, 1-6 parts Na2O2 and 5-20 parts MgO.

[0012] The addition of NaOH can form a low-temperature eutectic mixture with KOH, lowering the melting point and reaction rate of KOH. Under the above weight ratio, TSD will appear hexagonal. Too much or too little NaOH will make TSD appear round, but its size will still be larger than TED. Although it is still possible to distinguish between TSD and TED, it will reduce the recognition accuracy. Therefore, the presence or absence of NaOH will affect the distinction between TSD and TED, and the amount of NaOH will affect the recognition accuracy.

[0013] Optionally, the weight ratio of KOH to NaOH is 6-7:3-4. This weight ratio can improve the accuracy of dislocation identification, facilitate operation, and reduce operating costs.

[0014] Optionally, in step (1), Na2O2 is added and the temperature is raised to 460-500℃, and the temperature is maintained for 30 min-24 h to obtain the corrosive agent.

[0015] This temperature and time ensure that the Na2O2 etchant is fully dissolved and can exert its corrosive effect. If the temperature is below 460°C, the Na2O2 will not decompose to produce oxygen, and it will not be able to effectively etch the silicon carbide crystal. If the temperature is above 550°C, the etchant will corrode the crystal too quickly, and the TSD will appear round, with smaller differences in the size of the dislocations. If the time is less than 30 minutes, the Na2O2 will dissolve unevenly, the corrosion effect will be poor, the recognition accuracy will be reduced, and damage to the silicon carbide substrate will be caused. If the time is more than 24 hours, the Na2O2 will decompose completely, and the etchant will not be able to effectively etch the silicon carbide crystal.

[0016] Optionally, the corrosion temperature in step (1) is 460-550℃ and the corrosion time is 3-20min.

[0017] Optionally, the silicon carbide crystal has a nitrogen doping concentration greater than 5*10⁻⁶. 18 cm -3 In step (2) of high nitrogen-doped silicon carbide crystals, hexagonal corrosion pits with an equal-area circle diameter of 35-55 μm correspond to screw dislocations, circular corrosion pits with an equal-area circle diameter of 15-30 μm correspond to edge dislocations, and teardrop-shaped or elongated corrosion pits correspond to basal plane dislocations. This size is applicable to both high nitrogen-doped silicon carbide crystals before and after high-temperature annealing.

[0018] Optionally, when the silicon carbide crystal is a high-purity silicon carbide crystal, in step (2), the hexagonal corrosion pit with an equal-area circle diameter of 70-110μm corresponds to a screw dislocation, the circular corrosion pit with an equal-area circle diameter of 30-60μm corresponds to an edge dislocation, and the teardrop-shaped or elongated corrosion pit corresponds to a base plane dislocation.

[0019] Optionally, the silicon carbide crystal has a nitrogen doping concentration of less than 1*10⁻⁶. 16 cm -3 When lightly nitrogen-doped silicon carbide crystals are used, in step (2), hexagonal corrosion pits with an equal volume circle diameter of 50-85μm correspond to screw dislocations, circular corrosion pits with an equal volume circle diameter of 20-45μm correspond to edge dislocations, and teardrop-shaped or elongated corrosion pits correspond to base plane dislocations.

[0020] Optionally, in step (1), the KOH, NaOH and MgO are heated to the point of dissolution at a temperature of 200-300℃.

[0021] According to another aspect of this application, a dislocation etchant for silicon carbide crystals is provided, the etchant comprising 50-80 parts KOH, 20-50 parts NaOH, 1-6 parts Na2O2 and 5-20 parts MgO;

[0022] Preferably, the preparation method of the etchant is as follows: heating dried KOH, NaOH and MgO until dissolved, then adding Na2O2 and heating to 460-500℃, and holding at that temperature for 30min-24h to obtain the etchant.

[0023] The beneficial effects of this application include, but are not limited to:

[0024] 1. The dislocation identification method for silicon carbide crystals of this application can widely distinguish and identify TSD, TED and BPD of various types of silicon carbide crystals. It has universality and is suitable for industrial testing applications.

[0025] 2. The dislocation identification method for silicon carbide crystals in this application uses NaOH added to the etchant to lower the melting point and reaction rate of KOH, thereby distinguishing between three types of dislocations: TSD, TED, and BPD. The MgO added to the etchant can effectively alleviate the decomposition and volatilization of Na2O2 in the solution, provide an oxygen-rich environment, and block the volatilization of oxygen, thus achieving the difference in size between TSD and TED.

[0026] 3. According to the dislocation identification method for silicon carbide crystals of this application, when the method identifies light nitrogen-doped silicon carbide crystals, the size of TSD and TED dislocations is about 1.5 times that of high nitrogen-doped silicon carbide crystals, and when it identifies high-purity silicon carbide crystals, the size of TSD and TED dislocations is about 2 times that of high nitrogen-doped silicon carbide crystals. Attached Figure Description

[0027] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0028] Figure 1 This is a diagram showing the dislocation distribution of high-nitrogen-doped silicon carbide after high-temperature annealing and etching, as described in Example 1 of this application.

[0029] Figure 2 This is a diagram showing the dislocation distribution of high-nitrogen-doped silicon carbide after etching without high-temperature annealing, as described in Example 2 of this application.

[0030] Figure 3 This is a diagram showing the dislocation distribution of light nitrogen-doped silicon carbide after high-temperature annealing and etching, as described in Example 3 of this application.

[0031] Figure 4This is a diagram showing the dislocation distribution of high-purity silicon carbide after high-temperature annealing and etching, as described in Example 4 of this application.

[0032] Figure 5 This is a diagram showing the dislocation distribution of high-nitrogen-doped silicon carbide after high-temperature annealing and etching, as described in Comparative Example 1 of this application.

[0033] Figure 6 This is a diagram showing the dislocation distribution of high-nitrogen-doped silicon carbide after high-temperature annealing and etching, as described in Comparative Example 2 of this application.

[0034] Figure 7 This is a diagram showing the dislocation distribution of high-nitrogen-doped silicon carbide after high-temperature annealing and etching, as described in Comparative Example 3 of this application. Detailed Implementation

[0035] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0036] Unless otherwise specified, all raw materials used in the embodiments of this application were purchased through commercial channels.

[0037] Unless otherwise specified, the methods described in the embodiments are conventional methods in the art. The substrates used in the following embodiments and comparative examples are all polished substrates.

[0038] Example 1: Dislocation Identification Method for High-Nitrogen-Doped Silicon Carbide Crystals After High-Temperature Annealing

[0039] This embodiment relates to a method for dislocation identification in a high-nitrogen-doped silicon carbide crystal after high-temperature annealing, wherein the nitrogen doping concentration of the high-nitrogen-doped silicon carbide crystal is greater than 5*10⁻⁶. 18 cm -3 It includes the following steps:

[0040] (1) KOH, NaOH, Na2O2 and MgO were placed in an oven and dried at 80°C for 24 hours. 70 parts of dried KOH, 30 parts of NaOH and 15 parts of MgO were placed in a Ni crucible and heated to 300°C to dissolve. Then 2 parts of Na2O2 were added and the temperature was raised to 460°C. After holding the temperature for 3 hours, an etchant was obtained. The high nitrogen doped silicon carbide substrate after high temperature annealing was placed in the etchant and etched at 460°C for 8 minutes.

[0041] (2) The Si surface of the etched silicon carbide crystal was observed using an optical metallographic microscope, and the results are as follows: Figure 1 As shown in the characterization diagram above, in the high nitrogen-doped silicon carbide substrate after high-temperature annealing, the hexagonal corrosion pits with an equal-area circle diameter of 35-55 μm correspond to screw dislocations, the circular corrosion pits with an equal-area circle diameter of 15-30 μm correspond to edge dislocations, and the teardrop-shaped or elongated corrosion pits correspond to base plane dislocations. The densities of the three types of dislocations detected by this method are shown in Table 1.

[0042] Example 2: Dislocation Identification Method for High-Nitrogen-Doped Silicon Carbide Crystals Without High-Temperature Annealing

[0043] This embodiment relates to a method for dislocation identification in a high-nitrogen-doped silicon carbide crystal that has not undergone high-temperature annealing, wherein the nitrogen doping concentration of the high-nitrogen-doped silicon carbide crystal is greater than 5*10⁻⁶. 18 cm -3 It includes the following steps:

[0044] (1) KOH, NaOH, Na2O2 and MgO were placed in an oven and dried at 80°C for 24 hours. 60 parts of dried KOH, 40 parts of NaOH and 15 parts of MgO were placed in a Ni crucible and heated to 300°C to dissolve. Then 2 parts of Na2O2 were added and the temperature was raised to 460°C. After holding the temperature for 5 hours, an etchant was obtained. The high nitrogen doped silicon carbide substrate that had not been annealed at high temperature was placed in the etchant and etched at 460°C for 8 minutes.

[0045] (2) The Si surface of the etched silicon carbide crystal was observed using an optical metallographic microscope, and the results are as follows: Figure 2 As shown, the etchant and etching method can accurately distinguish edge dislocations, screw dislocations and base plane dislocations in highly nitrogen-doped silicon carbide through observation with an optical metallographic microscope. The densities of the three types of dislocations detected by this method are shown in Table 1.

[0046] Example 3: Dislocation identification method for light nitrogen-doped silicon carbide crystals after high-temperature annealing

[0047] This embodiment relates to a method for dislocation identification in a lightly nitrogen-doped silicon carbide crystal after high-temperature annealing, wherein the nitrogen doping concentration of the lightly nitrogen-doped silicon carbide crystal is less than 1*10⁻⁶. 16 cm -3 It includes the following steps:

[0048] (1) KOH, NaOH, Na2O2 and MgO were placed in an oven and dried at 80°C for 24 hours. 50 parts of dried KOH, 50 parts of NaOH and 5 parts of MgO were placed in a Ni crucible and heated to 200°C to dissolve. Then 1 part of Na2O2 was added and the temperature was raised to 460-550°C. After holding the temperature for 30 minutes, an etchant was obtained. The light nitrogen-doped silicon carbide substrate after high-temperature annealing was placed in the etchant and etched at 460°C for 10 minutes.

[0049] (2) The Si surface of the etched silicon carbide crystal was observed using an optical metallographic microscope, and the results are as follows: Figure 3As shown, the morphologies of TSD, TED, and BPD are similar to those in Example 1. However, compared to the highly nitrogen-doped silicon carbide crystal, the size of TSD and TED after etching of the lightly nitrogen-doped silicon carbide crystal after high-temperature annealing is approximately 1.5 times that of the highly nitrogen-doped silicon carbide crystal. The densities of the three dislocations detected by this method are shown in Table 1.

[0050] Example 4: Dislocation Identification Method for High-Purity Silicon Carbide Crystals After High-Temperature Annealing

[0051] This embodiment relates to a method for identifying dislocations in silicon carbide crystals after high-temperature annealing. The method includes the following steps:

[0052] (1) KOH, NaOH, Na2O2 and MgO were placed in an oven and dried at 80°C for 24 hours. 80 parts of dried KOH, 20 parts of NaOH and 20 parts of MgO were placed in a Ni crucible and heated to 300°C to dissolve. Then 6 parts of Na2O2 were added and the temperature was raised to 550°C. After holding the temperature for 24 hours, an etchant was obtained. The high-purity silicon carbide substrate after high-temperature annealing was placed in the etchant and etched at 550°C for 3 minutes.

[0053] (2) The Si surface of the etched silicon carbide crystal was observed using an optical metallographic microscope, and the results are as follows: Figure 4 As shown, the morphologies of TSD, TED, and BPD are similar to those in Example 1. However, compared to the high-nitrogen-doped silicon carbide crystal, the size of TSD and TED after high-temperature annealing and etching of the high-purity silicon carbide crystal is approximately twice that of the high-nitrogen-doped silicon carbide crystal. The densities of the three dislocations detected by this method are shown in Table 1.

[0054] Comparative Example 1

[0055] The difference between this comparative example and Example 1 is that in step (1), KOH and Na2O2 are placed in an oven and dried at 80°C for 24 hours. 70 parts of the dried KOH are placed in a Ni crucible and heated to 300°C to dissolve. Then, 2 parts of Na2O2 are added and the temperature is raised to 460°C. After holding the temperature for 3 hours, an etchant is obtained. The high-nitrogen-doped silicon carbide substrate after high-temperature annealing is placed in the etchant and etched at 460°C for 8 minutes.

[0056] The Si surface of the etched silicon carbide crystal was observed using an optical metallographic microscope, and the results are as follows: Figure 5 As shown in the results, the dislocations etched by this method are all circular and almost the same size. TSD does not appear as hexagonal, and it is impossible to distinguish between TSD and TED. The dislocation density detected by this method is shown in Table 1.

[0057] Comparative Example 2

[0058] The difference between this comparative example and Comparative Example 1 is that 70 parts KOH were replaced with 42 parts KOH and 28 parts NaOH, while the rest were the same as in Comparative Example 1. The etched silicon carbide crystal Si surface was observed using a metallographic microscope, and the results are as follows: Figure 6 As shown in the results, the dislocations etched by this method are also nearly circular and almost the same size. However, compared with Comparative Example 1, under the same corrosion conditions, the dislocations etched by this formula are larger, indicating that NaOH can significantly reduce the corrosion reaction temperature and reduce production costs. The dislocation density detected by this method is shown in Table 1.

[0059] Comparative Example 3

[0060] The difference between this comparative example and Example 1 is that in step (1), KOH, NaOH, Na2O2 and MgO are placed in an oven and dried at 80°C for 24 hours. 70 parts of KOH, 30 parts of NaOH, 15 parts of MgO and 2 parts of Na2O2 after drying are placed in a Ni crucible and heated to 460°C to dissolve. After holding at the temperature for 3 hours, an etchant is obtained. The high-nitrogen-doped silicon carbide substrate after high-temperature annealing is placed in the etchant and etched at 460°C for 8 minutes.

[0061] The Si surface of the etched silicon carbide crystal was observed using an optical metallographic microscope, and the results are as follows: Figure 7 As shown in the results, the addition of MgO caused the dislocation morphology of both TSD and TED to change from circular to hexagonal. This indicates that the MgO suspension has a strong sealing effect on the oxygen produced by the decomposition of Na2O2. The role of oxygen is to enhance the oxidation reaction. However, it is still impossible to distinguish TSD and TED by size and morphology. The dislocation density detected by this method is shown in Table 1.

[0062] Test case

[0063] The dislocation density (TSD) of adjacent silicon carbide substrates from the same crystal rod was measured using X-ray diffraction (XRT) and epitaxial methods. The TSD density of adjacent wafers from the same crystal rod was also measured using the methods described in the examples or comparative examples to verify the accuracy of the dislocation identification methods. Five parallel experiments were performed for each method, and the average value was taken. The dislocation density results are shown in Table 1, with units of (dislocations / cm²). -2 ).

[0064] The specific method for XRT testing the density of TSD is as follows: X-rays are shone onto the substrate surface, and the internal microstructure is analyzed in a non-destructive state based on the changes in diffraction contrast and the de-image law in the substrate, without the need for corrosion intervention.

[0065] The specific method for measuring TSD density using epitaxial growth is as follows: a homogeneous epitaxial layer is grown on the Si surface of a substrate using an epitaxial process, with a nitrogen doping concentration of 1*10⁻⁶.15 Then, by etching in the same way as the substrate, dislocations can be observed under a microscope.

[0066] The test methods in the examples and comparative examples are obtained by using an automatic dislocation scanner to scan and photograph the dislocations with a microscope, and then the program identifies the dislocation type based on the size and shape of the dislocations, or by using a magnifying instrument to magnify the dislocations and then observing them with the naked eye.

[0067] Table 1

[0068]

[0069] Compared with the XRT test method, the epitaxial test method has higher accuracy. According to the data in the table above, the dislocation identification method of this application can not only distinguish between three types of dislocations: base plane dislocations, edge dislocations, and screw dislocations, but also the density of the three types of dislocations identified is basically consistent with that of XRT test and epitaxial test, and is closer to the result of epitaxial test, which means that the dislocation identification method of this application has a high dislocation identification accuracy.

[0070] The above description is merely an embodiment of this application, and the scope of protection of this application is not limited to these specific embodiments, but is determined by the claims of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the technical concept and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for dislocation identification in silicon carbide crystals after high-temperature annealing, characterized in that, Includes the following steps: (1) The dried KOH, NaOH and MgO are heated to 200-300℃ to dissolve, and then Na2O2 is added to obtain an etchant. The silicon carbide crystals after high-temperature annealing are placed in the etchant for etching. By weight, the etchant includes 50-80 parts KOH, 20-50 parts NaOH, 1-6 parts Na2O2 and 5-20 parts MgO. The weight ratio of KOH to NaOH is 6-7:3-4. (2) Observe the morphology of the corrosion pits after corrosion is completed, and accurately identify the basal plane dislocations, edge dislocations and screw dislocations in the silicon carbide crystal; The silicon carbide crystal has a nitrogen doping concentration greater than 5 × 10⁻⁶. 18 cm -3 When using highly nitrogen-doped silicon carbide crystals, in step (2), hexagonal corrosion pits with an equal-area circle diameter of 35-55 μm correspond to screw dislocations, circular corrosion pits with an equal-area circle diameter of 15-30 μm correspond to edge dislocations, and teardrop-shaped or elongated corrosion pits correspond to base plane dislocations. When the silicon carbide crystal is a high-purity silicon carbide crystal, in step (2), the hexagonal corrosion pit with an equal-area circle diameter of 70-110μm corresponds to a screw dislocation, the circular corrosion pit with an equal-area circle diameter of 30-60μm corresponds to an edge dislocation, and the teardrop-shaped or elongated corrosion pit corresponds to a base plane dislocation. The silicon carbide crystal has a nitrogen doping concentration of less than 1×10⁻⁶. 16 cm -3 When light nitrogen-doped silicon carbide crystals are used, in step (2), hexagonal corrosion pits with an equal volume circle diameter of 50-85μm correspond to screw dislocations, circular corrosion pits with an equal volume circle diameter of 20-45μm correspond to edge dislocations, and teardrop-shaped or elongated corrosion pits correspond to base plane dislocations.

2. The method for dislocation identification in silicon carbide crystals after high-temperature annealing according to claim 1, characterized in that, In step (1), Na2O2 is added and the temperature is raised to 460-500℃. After holding the temperature for 30 min-24 h, the corrosive agent is obtained.

3. The method for dislocation identification in silicon carbide crystals after high-temperature annealing according to claim 1, characterized in that, The corrosion temperature in step (1) is 460-550℃ and the corrosion time is 3-20min.

4. A dislocation etchant for silicon carbide crystals after high-temperature annealing, characterized in that, The corrosive agent comprises 50-80 parts KOH, 20-50 parts NaOH, 1-6 parts Na2O2 and 5-20 parts MgO; The preparation method of the etchant is as follows: heat dried KOH, NaOH and MgO until dissolved, then add Na2O2 and heat to 460-500℃, and keep at this temperature for 30min-24h to obtain the etchant.

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