Methods to reduce stress in large-size DC arc plasma CVD diamond thick films

Abstract

A method for reducing stress in large-size DC arc plasma CVD diamond thick films belongs to the field of diamond material preparation. After DC arc plasma CVD deposition of a diamond thick film, an AC coil is used to heat and anneal the Mo substrate to reduce growth stress. After annealing, the temperature of the Mo substrate is slowly reduced, allowing the thermal stress of the diamond thick film to be released gradually, resulting in a low-stress, crack-free large-size diamond thick film. This invention utilizes AC coils for heating and annealing the Mo substrate and for slow cooling, which can reduce the growth stress of the diamond thick film and slowly release the thermal stress, reducing the possibility of the diamond thick film cracking due to excessive stress. The application process is relatively simple and has excellent results. The prepared large-size diamond thick film can be used as a heat sink material or optical window for integrated semiconductor devices.

Description

Technical Field

[0001] This invention belongs to the field of diamond semiconductor material preparation technology, and in particular relates to a method for reducing stress in large-size DC arc plasma CVD diamond thick films. Technical Background

[0002] Diamond is an emerging ultrawide bandgap semiconductor material, characterized by a large bandgap (-5.5 eV) and high breakdown voltage (>10 MV•cm). -1 High carrier mobility (holes: 3800 cm⁻¹) 2 •V -1 •s -1 Electron: 4500 cm 2 •V -1 •s -1 ), extremely high thermal conductivity (22 W•K) -1 •cm -1 Diamond possesses advantages such as radiation resistance. It has immense application potential in heat sinks, high-power, high-frequency devices, optical windows, and quantum information. As various electronic devices gradually develop towards miniaturization, integration, and high power, heat dissipation becomes increasingly important, becoming one of the key factors restricting further development in these fields. Due to its ultra-high thermal conductivity, diamond is one of the best choices for ultra-wide bandgap semiconductor heat sink materials. Chemical vapor deposition (CVD) is one of the main technologies for preparing high-quality diamond films. Currently, the main CVD diamond film preparation methods include hot-wire CVD (HFCVD), DC-jet CVD, DC-PACVD, and microwave plasma CVD (MPCVD). DC-jet CVD, with its high plasma density and temperature, theoretically allows for the efficient preparation of high-quality diamond films. Research on the preparation of large-size CVD thick diamond films has significant practical implications. Large-size CVD diamond thick films are mainly used in heat sink materials and optical windows to solve the heat dissipation problem of high-power devices and improve device performance and lifespan. However, in practical applications, we always want to obtain large-size thick diamond films. During the preparation of large-size CVD diamond thick films, due to drastic temperature changes and the continuous accumulation of growth defects, huge stresses are generated in the diamond film, causing warping or even fracture. This will seriously affect the acquisition of intact large-size CVD diamond thick films, thus affecting subsequent processing and applications. For example, large warping will limit the bonding of diamond thick films with semiconductor materials (such as GaN), which not only increases production costs but also greatly limits the application of diamond thick films in the semiconductor field.

[0003] To prepare intact, crack-free diamond films, several methods have been proposed to reduce the stress in CVD diamond films. For example, Li Qingli et al. proposed a method (CN113684466B) to separate the substrate from the water-cooling stage after growth to reduce the cooling rate and thus decrease thermal stress. However, this method involves performing the separation operation after the equipment is shut down, causing the substrate temperature to drop by about 200°C in a very short time, resulting in significant thermal stress. Liu Duo et al. proposed a method for preparing low-stress diamond films by synthesizing a metallic Ga buffer layer using ultrasonic cavitation technology (CN117026201A), but this method only reduces thermal stress. Peng Yan et al. proposed a multi-coil adjustable resistance-assisted heating MPCVD device and a method for growing diamond thin films (CN117026201A), which also only reduces thermal stress.

[0004] In summary, it is meaningful to propose a method that can simultaneously reduce thermal stress and growth stress and avoid temperature abrupt changes in diamond films in order to achieve the goal of depositing large-area diamond thick films with low stress and no cracks. Summary of the Invention

[0005] This invention provides a method for reducing stress in large-size DC arc plasma CVD diamond thick films, solving the technical problem in the prior art where excessive stress in diamond thick films is caused by excessively rapid cooling rate and accumulation of growth defects, especially for large-size thick films, which easily leads to warping or even breakage.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A method for reducing stress in large-size DC arc plasma CVD diamond thick films, characterized by the following steps: after growth is completed, the AC coil is raised to the same height as the Mo substrate, and while cooling, the AC coil is used to heat the Mo substrate and keep it at the growth temperature until the arc is extinguished and the power is turned off; after the vacuum pump draws the chamber pressure down to below 1 Pa, the AC coil is used to continue heating the Mo substrate to 100-400°C above the growth temperature for annealing; after the annealing is completed, the temperature of the Mo substrate is slowly reduced to room temperature.

[0008] Furthermore, the step of heating the Mo substrate with an AC coil includes:

[0009] 1) Grow a diamond thick film on a Mo substrate at a growth temperature of 800-950℃. Before cooling down and shutting down after growth is complete, control the AC coil to rise from the initial position to the same height as the substrate.

[0010] 2) Gradually reduce the current of the DC arc plasma CVD equipment while turning on the AC coil power system to generate induced eddy currents in the Mo substrate;

[0011] 3) While reducing the current, gradually increase the current of the AC coil, and keep the Mo substrate temperature at the growth temperature.

[0012] 4) After the arc is extinguished, quickly shut off the gas and wait for the vacuum pump to reduce the chamber pressure to below 1 Pa. Then, continue to increase the AC coil current and increase the output power. Heat the Mo substrate to 100-400°C above the growth temperature and hold for annealing for 1.5-2.5 hours.

[0013] 5) After the annealing process is completed, slowly reduce the current of the AC coil until the temperature of the Mo substrate slowly decreases to room temperature;

[0014] By heating and slowly cooling the Mo substrate using the AC coil, stress in the diamond thick film can be effectively reduced.

[0015] Furthermore, the AC coil can be freely raised and lowered during the diamond thick film growth process, and its diameter is 2 cm larger than that of the water-cooling stage.

[0016] Furthermore, the diameter of the AC coil is 100-260mm, the lifting range is 0-240mm, and the AC coil is made of hollow copper tube with an outer diameter of 4-5mm and an inner diameter of 2-3mm, with flowing cooling water inside.

[0017] Furthermore, the temperature variation of the Mo substrate should be kept below 8-10℃ / h.

[0018] The annealing temperature should be maintained at 100-400℃ above the growth temperature.

[0019] Furthermore, the AC coil has 4-10 turns. Due to the large size of the Mo substrate, and the need for a relatively slow heating rate and uniform temperature, a low-frequency coil with a frequency of 50-500Hz is selected.

[0020] The key to the implementation of this invention lies in:

[0021] (1) To ensure that the magnetic field generated by the AC coil has as little influence on the arc shape as possible, a coil with a low number of turns should be used.

[0022] (2) To ensure that the temperature does not change abruptly during the process of reducing the input power of the DC arc plasma CVD equipment, the infrared thermometer reading should be kept at ±10℃ of the growth temperature.

[0023] (3) To ensure that the AC coil is not melted by the high temperature arc, cooling water must be circulated inside the coil, with the temperature maintained below 30-35℃ and the flow rate maintained at 5-15L / M.

[0024] (4) To ensure that thermal stress is released slowly, the temperature change of the Mo substrate observed by the infrared thermometer is kept at 1℃ / min during the cooling process.

[0025] (5) To ensure that the diamond thick film is not oxidized during the annealing process, the temperature must be increased only when the pressure in the chamber is less than 1 Pa.

[0026] (6) To ensure the quality of the prepared diamond thick film, the ultimate vacuum of the vacuum chamber must be less than 0.7 Pa and the leakage rate must be less than 1.10. -3 Pa·m 3 / s, cavity pressure between 2.8-3.0kPa.

[0027] The beneficial effects of this invention and the prior art are as follows:

[0028] (1) The heating and annealing process after growth can effectively reduce growth stress.

[0029] (2) It avoids the rapid cooling of the Mo substrate, allowing thermal stress to be released slowly and fully.

[0030] (3) This invention is particularly suitable for large-size diamond thick films, which is beneficial for the future preparation of large-size, crack-free diamond thick films.

[0031] (4) This invention is an in-situ processing method that does not require additional equipment and is relatively simple to operate. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the reduced power operation state of a CVD device according to an embodiment of the present invention;

[0033] Figure 2 This is a schematic diagram of the working state during growth according to an embodiment of the present invention.

[0034] Figure 3 This is a curve showing the temperature change of a Mo substrate over time according to an embodiment of the present invention.

[0035] In the picture:

[0036] 1. Upper magnetic field coil; 2. Lower magnetic field coil; 3. AC coil; 4. Mo substrate; 5. Water-cooled stage; 6. Infrared thermometer; 7. Vacuum chamber; 8. Anode. Detailed Implementation

[0037] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0038] Example 1:

[0039] Check the ultimate vacuum and leakage rate of vacuum chamber 7. The ultimate vacuum is 0.5 Pa, and the calculated leakage rate is 9.7 × 10⁻⁶ Pa. -4 Pa·m 3 / s. Place the treated Mo substrate 4 on the water-cooled stage 5, ensuring that the centers of both are on the same axis and in good contact, and perform a vacuum process until the pressure reaches 0.5 Pa. Adjust the AC coil 3 to its initial position, such as... Figure 2 As shown. First, 5.0 slm argon gas is introduced, and the cavity pressure is adjusted to 2.5 kPa. The initial current of the upper magnetic field coil 1 is 2.5A. The DC power supply current of the arc plasma generator is set to 160A, and the power switch is turned on to allow the arc to break down the gas. At the same time, hydrogen gas is introduced in a ratio (H2:Ar=1.5:1) to control the cavity pressure at 2.8 kPa. After the arc stabilizes, the DC power supply current of the arc plasma generator is gradually increased to 230A. The upper and lower magnetic field coils 1 and 2 are adjusted to adjust the arc shape. The substrate surface temperature reaches 870℃, and the diamond film begins to grow stably. After 800 hours of stable growth, the power of the CVD equipment is slowly reduced. The AC coil 3 is raised to the same height as the Mo substrate 4, as shown. Figure 1 As shown. Reduce the power of the CVD equipment while simultaneously turning on the AC coil 3 power supply to heat the Mo substrate 4, stabilizing it at the growth temperature of 870℃. After arc extinguishing, quickly shut off all gas supply lines, causing the chamber pressure to drop to 0.9 Pa. Continue increasing the current of AC coil 3, slowly increasing the temperature of the Mo substrate 4 to 1250℃, and hold for annealing for 2 hours. After annealing, slowly reduce the current of AC coil 3, allowing the temperature of the Mo substrate 4 to slowly decrease to room temperature. Open the venting valve to slowly release the gas.

[0040] The diamond film was removed; it was intact with a uniform surface thickness and no cracks were observed.

[0041] Example 2:

[0042] Check the ultimate vacuum and leakage rate of vacuum chamber 7. The ultimate vacuum is 0.43 Pa, and the calculated leakage rate is 8.4 × 10⁻⁶ Pa. - 4 Pa·m 3 / s. Place the treated Mo substrate 4 on the water-cooled stage 5, ensuring that the centers of both are on the same axis and in good contact, and perform vacuum treatment until the pressure reaches 0.57 Pa. Adjust the AC coil 3 to the initial position, such as... Figure 2As shown. First, 5.0 slm argon gas is introduced, and the cavity pressure is adjusted to 2.6 kPa. The initial current of the upper magnetic field coil 1 is 2.5A. The DC power supply current of the arc plasma generator is set to 160A, and the power switch is turned on to allow the arc to break down the gas. At the same time, hydrogen gas is introduced in proportion (H2:Ar=1.5:1) to control the cavity pressure at 2.9 kPa. After the arc stabilizes, the DC power supply current of the arc plasma generator is gradually increased to 230A. The upper and lower magnetic field coils 1 and 2 are adjusted to adjust the arc shape. The substrate surface temperature reaches 870℃, and the diamond film begins to grow stably. After 800 hours of stable growth, the power of the CVD equipment is slowly reduced. The AC coil 3 is raised to the same height as the Mo substrate 4, as shown. Figure 1 As shown. Reduce the power of the CVD equipment while simultaneously turning on the AC coil 3 power supply to heat the Mo substrate 4, stabilizing it at the growth temperature of 860℃. After arc extinguishing, quickly shut off all gas supply lines, causing the chamber pressure to drop to 0.9 Pa. Continue increasing the current of AC coil 3, slowly increasing the temperature of the Mo substrate 4 to 1200℃, and hold for annealing for 2 hours. After annealing, slowly reduce the current of AC coil 3, allowing the temperature of the Mo substrate 4 to slowly decrease to room temperature. Open the venting valve to slowly release the gas.

[0043] The diamond film was removed; it was intact with a uniform surface thickness and no cracks were observed.

[0044] Implementation Case 3:

[0045] Check the ultimate vacuum and leakage rate of vacuum chamber 7. The ultimate vacuum is 0.53 Pa, and the calculated leakage rate is 7.8 × 10⁻⁶ Pa. - 4 Pa·m 3 / s. Place the treated Mo substrate 4 on the water-cooled stage 5, ensuring that the centers of both are on the same axis and in good contact, and perform vacuum treatment until the pressure reaches 0.83 Pa. Adjust the AC coil 3 to the initial position, such as... Figure 2 As shown. First, 5.0 slm argon gas is introduced, and the cavity pressure is adjusted to 2.95 kPa. The initial current of the upper magnetic field coil 1 is 2.5A. The DC power supply current of the arc plasma generator is set to 160A, and the power switch is turned on to allow the arc to break down the gas. At the same time, hydrogen gas is introduced in a ratio (H2:Ar=1.5:1) to control the cavity pressure at 2.95 kPa. After the arc stabilizes, the DC power supply current of the arc plasma generator is gradually increased to 240A. The upper and lower magnetic field coils 1 and 2 are adjusted to adjust the arc shape. The substrate surface temperature reaches 870℃, and the diamond film begins to grow stably. After 800 hours of stable growth, the power of the CVD equipment is slowly reduced. The AC coil 3 is raised to the same height as the Mo substrate 4, as shown. Figure 1As shown. Reduce the power of the CVD equipment while simultaneously turning on the AC coil 3 power supply to heat the Mo substrate 4, stabilizing it at the growth temperature of 880℃. After arc extinguishing, quickly shut off all gas supply lines, causing the chamber pressure to drop to 0.9 Pa. Continue increasing the current of AC coil 3, slowly increasing the temperature of the Mo substrate 4 to 1100℃, and hold for annealing for 2 hours. After annealing, slowly reduce the current of AC coil 3, allowing the temperature of the Mo substrate 4 to slowly decrease to room temperature. Open the venting valve to slowly release the gas.

[0046] The diamond film was removed; it was intact with a uniform surface thickness and no cracks were observed.

Claims

1. A method for reducing stress in large-size DC arc plasma CVD diamond thick films, characterized in that the steps include... include: 1) Grow a diamond thick film on a Mo substrate at a growth temperature of 800-950℃; before cooling down and shutting down after growth is complete, control the AC coil to rise from the initial position to the same height as the substrate. 2) Gradually reduce the current of the DC arc plasma CVD equipment while turning on the AC coil power system to generate induced eddy currents in the Mo substrate. 3) While reducing the current, gradually increase the current of the AC coil, and maintain the temperature of the Mo substrate at the growth temperature; 4) After the arc is extinguished, quickly shut off the gas and wait for the vacuum pump to reduce the chamber pressure to below 1 Pa. Then, continue to increase the AC coil current and increase the output power. Heat the Mo substrate to 100-400°C above the growth temperature and hold for annealing for 1.5-2.5 hours. 5) After the annealing process is completed, the current of the AC coil is slowly reduced until the temperature of the Mo substrate slowly drops to room temperature; to ensure that the thermal stress is released slowly, the temperature change of the Mo substrate observed by the infrared thermometer is kept at 1℃ / min during the cooling process. By heating and slowly cooling the Mo substrate using the AC coil, stress in the diamond thick film can be effectively reduced.

2. The method for reducing stress in large-size DC arc plasma CVD diamond thick films according to claim 1, characterized in that, The AC coil can move freely up and down during the diamond thick film growth process, and its diameter is 2 cm larger than that of the water-cooled stage.

3. The method for reducing stress in large-size DC arc plasma CVD diamond thick films according to claim 1, characterized in that, The diameter of the AC coil is 100-260mm, the lifting range is 0-240mm, and the AC coil is made of hollow copper tube with an outer diameter of 4-5mm and an inner diameter of 2-3mm, with flowing cooling water inside.

4. The method for reducing stress in large-size DC arc plasma CVD diamond thick films according to claim 1, wherein the AC coil has 4-10 turns; since the Mo substrate is large in size, and the heating rate cannot be too fast and the temperature must be uniform, a low-frequency coil with a frequency of 50-500Hz is selected.

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