Aluminum nitride substrate cleaning method

Abstract

The invention discloses an aluminum nitride substrate cleaning method. The cleaning method comprises an ectopic chemical pretreatment process and an in-situ Al auxiliary cleaning process. Wherein the ectopic chemical pretreatment comprises the following steps of: removing oil stain on the surface of the AlN substrate by using a cleaning agent, then removing hydroxide on the surface of the substrate by using an acid solution mixed solution, and finally treating by using hydrofluoric acid to expose atomic steps on the substrate. And then an in-situ Al auxiliary cleaning process is executed, specifically, the AlN substrate is heated in a high-vacuum environment through molecular beam epitaxy, aluminum steam adsorption-desorption cleaning is carried out, and a smooth atom step edge is formed on the surface of the substrate after multiple times of circulation. According to the method, oil stains, oxides, hydroxides, Si, O, C and other impurities on the surface of the aluminum nitride substrate can be effectively eliminated, so that the surface of the substrate presents atomic steps, the quality of an epitaxial crystal ingot is remarkably optimized, and the method is very suitable for cleaning treatment of the high-quality aluminum nitride substrate.

Description

Technical Field

[0001] This invention relates to the field of semiconductor material preparation technology, specifically to a method for cleaning aluminum nitride substrates, and more particularly to a cleaning treatment for high-quality aluminum nitride substrates. Background Technology

[0002] Aluminum nitride (AlN) is an ultra-wide bandgap semiconductor material with great application potential, boasting a bandgap of ~6.2 eV. It also exhibits high breakdown field strength, high saturated electron drift velocity, and excellent thermal conductivity and radiation resistance, making it the optimal substrate material for ultraviolet / deep ultraviolet LEDs and an ideal substrate material for GaN power devices. The substrate surface quality is closely related to the quality of the epitaxial film and directly affects the final device performance. In the process of homoepitaxial growth of high-quality AlN single crystals, substrate surface quality is a key factor determining the quality of the epitaxial ingot; therefore, the cleaning treatment of the substrate surface is particularly important.

[0003] Traditional AlN substrate cleaning methods mainly include wet chemical treatment and high-temperature annealing, but these methods have obvious drawbacks.

[0004] Traditional wet chemical etching processes use acidic or alkaline solutions, such as HCl, HF, KOH, and NH4OH. Both acidic and alkaline solutions exhibit varying degrees of anisotropic chemical etching of AlN. NH4OH and KOH, in particular, cause significant etching of the AlN surface, forming etch pits and deteriorating surface roughness. This rough surface becomes a source of defects in the epitaxial layer (such as dislocations and stacking faults), reducing device performance. Furthermore, wet chemical processing can alter surface stoichiometry and leave residues. For example, HF, when removing surface oxides, may cause N-richness or Al-F bonding, and leave residual fluorides such as Al3F. These changes alter the surface chemical state and band structure. Changes in surface states also alter the initial interface of epitaxial growth, potentially introducing defect states and affecting the heterojunction interface quality. Additionally, wet cleaning easily introduces impurities and contaminants that are re-adsorbed onto the substrate surface. These contaminants become deep-level centers, acting as carrier traps or scattering centers, severely reducing device breakdown voltage and reliability.

[0005] High-temperature annealing (heating to 1000℃) is energy-intensive, and AlN undergoes thermal decomposition at high temperatures, especially in vacuum or non-nitrogen-rich atmospheres. Therefore, high-temperature annealing can also trigger AlN surface decomposition, leading to the formation of Al droplets and nitrogen vacancies, severely damaging surface smoothness and stoichiometry. The defects generated by decomposition can extend into the epitaxial layer, becoming leakage channels and rendering the device unusable. High-temperature annealing primarily targets volatile contaminants. For some non-volatile contaminants, high temperatures may cause them to diffuse into the substrate or agglomerate, making them even more difficult to remove. For example, while high-temperature ammonia annealing can partially convert surface hydroxides, it cannot completely remove deep oxides. Furthermore, high temperatures can easily introduce lattice damage into the substrate.

[0006] In summary, existing cleaning processes are insufficient to achieve substrate surface cleaning at the atomic scale, leading to problems such as high dislocation density and excessive impurity concentration during subsequent epitaxial growth (e.g., the concentration of impurities such as Si, O, and C is typically higher than 1×10⁻⁶). 17 atoms / cm 3 This severely affects the performance of devices formed on the substrate. Summary of the Invention

[0007] Based on the problems of insufficient surface cleanliness, easy residue of impurities, and introduction of damage in the existing aluminum nitride substrate cleaning process, this invention aims to provide an efficient aluminum nitride substrate cleaning process. By designing the cleaning process and optimizing parameters, the surface quality of aluminum nitride substrates is improved, thus ensuring high-quality epitaxial growth and the performance of the final device.

[0008] The present invention is achieved through the following technical solution.

[0009] The present invention provides a method for cleaning aluminum nitride substrates, the method comprising an ex-situ chemical pretreatment process in step S1 and an in-situ Al-assisted cleaning process in step S2, performed sequentially.

[0010] The S1 step of the ex-situ chemical pretreatment process includes the following steps: S11: Clean the aluminum nitride substrate with a cleaning agent to remove surface oil and dirt. The cleaning agent used in this step is an organic cleaning agent such as acetone or isopropanol. More preferably, the aluminum nitride single crystal substrate has a C-oriented crystal plane with an A-axis deviation of ±10°; the thickness of the aluminum nitride substrate is 0.3-2 mm, and the diameter is 10-150 mm. S12: The aluminum nitride substrate cleaned in S11 is treated with a mixture of H3PO4 and H2SO4 at a temperature T1 for a period of time t1 to remove hydroxides from the substrate surface. The volume ratio of H3PO4 to H2SO4 is 1:2 to 1:4; the holding time t1 is 5-20 min; and the temperature T1 is 60-80℃.

[0011] S13: The aluminum nitride substrate treated in S12 is treated with hydrofluoric acid solution at a temperature T2 for a period of time t2 to remove residual oxides, and then rinsed with deionized water to expose atomic steps. The concentration of hydrofluoric acid is 5-15%. Preferably, the time t2 is 5-20 min, and the temperature of the hydrofluoric acid solution T2 is 20-40℃.

[0012] The S2 step of the in-situ Al-assisted cleaning process includes the following steps: The aluminum nitride substrate, after being pretreated by ex-situ chemical treatment, was loaded into a molecular beam epitaxy (MBE) system and heated to a temperature T3 under a high vacuum environment. Aluminum vapor was introduced, and adsorption-desorption cleaning was performed for a period of time t3 each time. The cleaning cycle was repeated n times. During the cleaning process, the surface cleanliness was monitored by reflection high-energy electron diffraction. The number of cycles was adjusted according to the monitoring results until a smooth atomic step edge was formed on the substrate surface.

[0013] Preferably, the temperature T3 is 1050-1200℃.

[0014] Preferably, the Al flux of aluminum vapor introduced is 5-10 nm / min.

[0015] Preferably, the cleaning time t3 is 0.1-2 min, and the number of cycles is 5-20.

[0016] The present invention achieves the following beneficial effects: 1. After ex-situ cleaning, the surface roughness of the substrate is reduced to the atomic level; then, in-situ Al-assisted cleaning is used to further remove oxide and hydroxide contaminants, forming clean atomic level step edges, effectively avoiding the impact of lattice damage layers on the growth quality of subsequent epitaxial layers.

[0017] 2. In-situ Al-assisted cleaning can specifically remove impurities such as Si, O, and C from the crystal surface, reducing the impurity concentration to 1×10⁻⁶. 17 atoms / cm 3 The following measures are taken to avoid impurities from degrading the quality of epitaxial ingots.

[0018] 3. This process is highly adaptable to substrate size and can be applied to various AlN substrates with diameters of 10~150 mm and thicknesses of 0.3~2 mm, making it widely applicable.

[0019] 4. The introduction of high-energy electron diffraction monitoring during the cleaning process can strictly control the surface quality of the AlN substrate after cleaning, ensuring the formation of atomic-level step edges on the surface, and further guaranteeing the stability of epitaxial ingot quality.

[0020] 5. The apparatus and assembly process used in this invention are simple and have strong process repeatability. They can be directly applied to large-scale industrial production, effectively improving the mass production quality and yield of aluminum nitride ingots and their substrates, and providing high-quality substrate surfaces for epitaxial growth processes required in fields such as ultraviolet / deep ultraviolet LEDs and GaN power devices. Attached Figure Description

[0021] Figure 1 The image shows the AFM image of the surface morphology of the AlN substrate after cleaning by the method of Embodiment 2 of the present invention.

[0022] Figure 2 The image shows the surface morphology (AFM) of the AlN substrate after cleaning using only chemical cleaning method in Comparative Example 1 of this invention. Detailed Implementation

[0023] The aluminum nitride substrate cleaning process of the present invention will be described in detail below with reference to specific embodiments. However, the embodiments do not limit the present invention. Any structural, methodological or functional modifications made by those skilled in the art based on the embodiments are included within the protection scope of the present invention.

[0024] The aluminum nitride substrate cleaning process of the present invention will be further described below through specific embodiments.

[0025] Example 1 The specific process of cleaning the aluminum nitride substrate in this embodiment is as follows.

[0026] First, perform the S1 ex-situ chemical pretreatment process, following these steps in sequence: S11. Select an AlN single crystal substrate with a diameter of 50.8 mm. After chemical polishing, its thickness is 0.5 mm. Its crystal orientation is positive C and deviated from A by -0.5°. First, use room temperature acetone for ultrasonic cleaning for 2 min to remove oil stains from the surface of the aluminum nitride single crystal substrate.

[0027] S12, the aluminum nitride single crystal substrate cleaned with acetone is then treated with a mixture of H3PO4 and H2SO4 in a volume ratio of 1:3 at 60°C for 5 min to remove hydroxides from the surface of the aluminum nitride substrate.

[0028] S13. The acid-washed aluminum nitride single crystal substrate was treated with a 15% hydrofluoric acid solution at 20°C for 10 min to remove residual oxides. After treatment, it was rinsed with deionized water to expose atomic steps. Secondary ion mass spectrometry (SIMS) analysis showed that the concentrations of Si, O, and C impurities on the crystal surface were 2.5 × 10⁻⁶. 17 atoms / cm 3 3.8×10 17 atoms / cm 38.5×10 17 atoms / cm 3 .

[0029] Then, the S2 in-situ Al-assisted cleaning process is performed, following these steps in sequence: An aluminum nitride single-crystal substrate, after in-situ chemical pretreatment, was loaded into a molecular beam epitaxy system and heated to 1100℃ under high vacuum. The Al flux was 6 nm / min, and adsorption-desorption cleaning was performed for 1 min each time, repeated 5 times. Surface cleanliness was monitored by high-energy reflection electron diffraction, revealing the formation of smooth atomic steps on the aluminum nitride substrate surface. Secondary ion mass spectrometry (SIMS) analysis showed that the concentrations of Si, O, and C impurities on the crystal surface were 0.2 × 10⁻⁶. 17 atoms / cm 3 0.8×10 17 atoms / cm 3 0.9×10 17 atoms / cm 3 .

[0030] Example 2 The specific process of cleaning the aluminum nitride substrate in this embodiment is as follows.

[0031] First, perform the S1 ex-situ chemical pretreatment process, following these steps: S11. Select an unpolished AlN single crystal substrate with a diameter of 100 mm and a thickness of 0.8 mm. The crystal orientation is positive C-axis biased towards A-axis by +0.5°. The aluminum nitride substrate is first ultrasonically cleaned with room temperature isopropanol for 2 min to remove surface oil.

[0032] S12, the aluminum nitride substrate that has been cleaned with acetone is then treated with a mixture of H3PO4 and H2SO4 in a volume ratio of 1:4 at 40°C for 10 min to remove hydroxides from the surface of the aluminum nitride substrate.

[0033] S13. The acid-washed aluminum nitride single crystal substrate was treated with an 8% hydrofluoric acid solution at 20°C for 10 min to remove residual oxides. After treatment, it was rinsed with deionized water to expose atomic steps. Secondary ion mass spectrometry (SIMS) analysis showed that the concentrations of Si, O, and C impurities on the crystal surface were 3.2 × 10⁻⁶. 17 atoms / cm 3 4.8×10 17 atoms / cm 3 1.0×10 18 atoms / cm 3 .

[0034] Next, the S2 in-situ Al-assisted cleaning process is performed, following these steps in sequence: An aluminum nitride substrate pretreated in situ was loaded into a molecular beam epitaxy (MBE) system and heated to 1200°C under high vacuum. The Al flux was 10 nm / min, and adsorption-desorption cleaning was performed for 1 min each time, repeated 10 times. Surface cleanliness was monitored by high-energy reflection electron diffraction, revealing smooth atomic steps on the aluminum nitride substrate surface. Figure 1 The AFM image of the aluminum nitride substrate after the cleaning process in this embodiment shows that the surface roughness Ra of the aluminum nitride substrate after the cleaning process is less than about 0.08 nm. The image also shows that the AlN substrate surface after the method of this invention exhibits a clear stepped flow morphology, which is beneficial for subsequent epitaxial growth. Secondary ion mass spectrometry (SIMS) analysis revealed that the concentrations of Si, O, and C impurities on the crystal surface are 0.3 × 10⁻⁶. 17 atoms / cm 3 0.7×10 17 atoms / cm 3 0.7×10 17 atoms / cm 3 .

[0035] Comparative Example 1 This comparative example only implements the initial acid cleaning process and does not perform the subsequent in-situ Al-assisted cleaning process.

[0036] The same aluminum nitride single crystal substrate as in Example 2 was selected, and the specific cleaning process was as follows: (1) The aluminum nitride wafer was first cleaned with acetone solution by ultrasonic vibration at room temperature for 5 min to remove surface organic matter; (2) It was then immersed in H2SO4: H2O2=3:1 (volume ratio) solution at 80℃ for 10 min to remove residual organic contaminants and some metals on the wafer surface; (3) The acid-washed aluminum nitride single crystal substrate was treated with 8% hydrofluoric acid solution at room temperature for 10 min to remove residual oxides; (4) Finally, the wafer was rinsed with flowing deionized EDI water for 10 min to remove residual chemical reagents on the wafer surface.

[0037] The surface of the aluminum nitride substrate after cleaning in this comparative example was tested using AFM. Figure 2 As can be seen, the surface roughness Ra of the aluminum nitride substrate obtained by the comparative chemical cleaning method is approximately 0.15 nm, which is nearly twice that of the method in Example 2. Secondary ion mass spectrometry (SIMS) analysis showed that the concentrations of Si, O, and C impurities on the surface of the comparative crystal were 2 × 10⁻⁶. 17 atoms / cm 3 3.5×10 17 atoms / cm 3 8×10 17 atoms / cm 3.

[0038] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for cleaning an aluminum nitride substrate, characterized in that: The method includes a sequential ex-situ chemical pretreatment process and an in-situ Al-assisted cleaning process. The ex-situ chemical pretreatment process includes the following steps: S11: Clean the aluminum nitride substrate with an organic cleaning agent first; S12: Next, use a mixture of H3PO4 and H2SO4 to clean the aluminum nitride substrate to remove surface hydroxides, and clean at temperature T1 for time t1. S13: Use hydrofluoric acid solution to treat for time t2 at temperature T2 to remove residual oxides, and then rinse with deionized water to expose atomic steps; The in-situ Al-assisted cleaning process is as follows: the S13-treated aluminum nitride substrate is subjected to molecular beam epitaxy and heated to temperature T3 in a high vacuum environment. Aluminum vapor is introduced for adsorption-desorption cleaning and the process is repeated n times, with each cycle lasting t3. The surface cleanliness is monitored by high-energy electron diffraction, and the number of cycles is adjusted according to the monitoring results until a smooth atomic step edge is formed on the substrate surface.

2. The method for cleaning an aluminum nitride substrate according to claim 1, characterized in that: In step S11, the aluminum nitride substrate is C-oriented and its orientation is ±10° off the A-direction; the thickness of the aluminum nitride single crystal substrate is 0.3-2 mm and its diameter is 10-150 mm.

3. The method for cleaning an aluminum nitride substrate according to claim 1, characterized in that: In step S11, the organic cleaning agent is selected from acetone and isopropanol.

4. The method for cleaning an aluminum nitride substrate according to claim 1, characterized in that: In step S12, the volume ratio of H3PO4 to H2SO4 is 1:2 to 1:

4.

5. The method for cleaning an aluminum nitride substrate according to claim 1, characterized in that: In step S12, the heat preservation time t1 is 5-20 min, and the temperature T1 is 60℃-80℃.

6. The method for cleaning an aluminum nitride substrate according to claim 1, characterized in that: The concentration of hydrofluoric acid in step S13 is 5%-15%.

7. The method for cleaning an aluminum nitride substrate according to claim 1, characterized in that: In step S13, the time t2 is 5-20 min, and the temperature T2 is 20℃-40℃.

8. The method for cleaning an aluminum nitride substrate according to claim 1, characterized in that: The temperature T3 is 1050℃-1200℃.

9. The method for cleaning an aluminum nitride substrate according to claim 1, characterized in that: The Al flux of the aluminum vapor introduced is 5-10 nm / min.

10. The method for cleaning an aluminum nitride substrate according to claim 1, characterized in that: The time t3 is 0.1-2 min, and the number of cycles n refers to 5-20 cycles.

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