Preparation method of optical-grade diamond

Abstract

The invention belongs to the field of diamond crystal materials, and particularly relates to a preparation method of optical-grade diamond. The method comprises the following steps: carrying out epitaxy by adopting an MPCVD method to obtain a diamond crystal; polycrystals on the surface and the side face of the diamond are removed through laser cutting and laser polishing; carrying out high-temperature and high-pressure annealing on the processed crystal in a cubic press; and after annealing is finished, the crystal is subjected to surface treatment such as grinding and polishing. The diamond prepared by the method has the characteristics of small stress birefringence, good optical uniformity and high optical transmittance, and can be used as a Raman laser gain medium.

Description

Technical Field

[0001] The invention belongs to the field of diamond crystal materials, and in particular relates to a method for preparing optical-grade diamond. Background Art

[0002] Raman lasers and Brillouin lasers are important means to achieve high brightness, narrow linewidth, low noise, and laser output in special bands and frequencies. They play an irreplaceable role in space exploration, high-energy physics, lidar, optoelectronic countermeasures, and other fields. Diamond has the highest thermal conductivity among natural materials, a wide light transmission range, and extremely high Raman gain and Raman frequency shift, making it an ideal material for nonlinear optical applications. At present, lasers using diamond as Raman media have excellent performance under pulses and high-power continuous pumping in the ultraviolet to infrared bands. In addition, diamond-based Brillouin lasers have also achieved multi-band free-space Brillouin laser operation, with a power of more than 10 times that of other gain media.

[0003] The development and breakthrough of high-power diamond laser technology has put forward higher requirements on the size and quality of diamond. However, the single crystal diamond currently prepared by CVD technology generally has problems such as poor optical uniformity, stress birefringence, and low light transmittance, which makes it difficult to meet the needs of nonlinear optical applications such as diamond Raman laser.

[0004] In recent years, domestic and foreign researchers have conducted a lot of research to prepare high-quality optical-grade diamonds that can be used for applications such as Raman lasers. For example, patent WO2011076642A1 proposes a method for controlling the dislocation density of diamond by step-by-step growth to improve the optical uniformity of the crystal. This method etches the substrate and accurately controls the nitrogen content at different growth stages. However, the step-by-step growth method is cumbersome and places high demands on the accurate control of the growth atmosphere. Nitrogen impurities are introduced during the growth process, which has an adverse effect on the transmittance of the crystal. Patent CN112647127A proposes a method for changing the arrangement of carbon atoms at the defect by metal co-doping, thereby inhibiting the epitaxy of line defects and reducing the dislocation density. However, this method introduces metal impurities into the diamond, which has an adverse effect on the optical properties of the crystal, such as the transmittance. Patent CN110697704B proposes a method for removing diamond impurities and changing the color of the crystal by high-temperature and high-pressure annealing. However, this method is only for natural diamonds. Natural diamonds are difficult to apply in the optical field due to their poor quality. Summary of the invention

[0005] In view of the above-mentioned deficiencies of the prior art, the present invention provides a method for preparing optical-grade diamond. The present invention can finally obtain an uncoated optical transmittance (@1.2μm) higher than 70% and an optical uniformity better than 1×10 -4 of single crystal diamond.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for preparing optical grade diamond, characterized by comprising the following steps:

[0008] Step 1. Using MPCVD to grow diamond with a lower defect density;

[0009] Step 2. removing the polycrystalline part of the diamond surface and side by laser cutting and laser polishing technology;

[0010] Step 3. Placing the treated diamond in a high temperature and high pressure (HPHT) device for high temperature and high pressure annealing treatment, wherein the temperature is raised to a predetermined high temperature within a predetermined time, and the pressure is quickly raised to a predetermined pressure, and after maintaining the temperature and pressure for a period of time, the temperature is lowered to room temperature through a preset cooling program, and the pressure is maintained at normal pressure;

[0011] Step 4: Perform surface grinding and polishing on the annealed diamond.

[0012] Preferably, the MPCVD growth process described in step 1 is specifically as follows:

[0013] Step 1.1 Place the diamond single crystal substrate into the MPCVD equipment and evacuate to 10 -3 Below Pa;

[0014] Step 1.2: introducing hydrogen gas and adjusting the pressure in the MPCVD device to a predetermined value, while adjusting the microwave power to generate plasma;

[0015] Step 1.3: gradually increase the pressure and microwave power, and after the substrate temperature reaches 750-900°C, introduce oxygen and keep the microwave power and pressure unchanged to perform hydrogen-oxygen plasma etching for 0.5-2h;

[0016] Step 1.4. Stop the oxygen supply and continue etching in the hydrogen plasma environment for 1-2 hours;

[0017] Step 1.5: introducing methane gas and adjusting the substrate temperature to 850-1050°C to grow diamond;

[0018] Step 1.5. After the growth is completed, slowly cool down to room temperature and take out the crystal.

[0019] Preferably, in step 1.1, the diamond single crystal substrate is CVD diamond or HPHT diamond.

[0020] Preferably, in step 1.2, the predetermined value of the pressure is 750-900.

[0021] Preferably, in step 1.3, the concentration of the oxygen introduced is 0.5%-2% of the hydrogen.

[0022] Preferably, in step 1.5, the concentration of the introduced methane gas is 2%-5% of the hydrogen gas.

[0023] Preferably, the predetermined high temperature in step 3 is 2000-2200°C, and the predetermined high pressure is 5-7 GPa, and under this condition, the temperature and pressure are maintained for 1-2 hours.

[0024] Preferably, the preset cooling program in step 3 is to keep the pressure constant, reduce the temperature to 1200° C. within 0.5-1 h and maintain it for 5-10 min;

[0025] Preferably, after the cooling in step 3 is completed, the pressure is reduced to normal pressure.

[0026] The diamond single crystal obtained by the above preparation method is applied in diamond Raman laser.

[0027] Compared with the prior art, the technical effects of the present invention are:

[0028] 1) By removing the polycrystalline part of the diamond surface and side through laser cutting and laser polishing technology, high-precision processing can be achieved, ensuring the flatness and finish of the diamond surface and reducing surface defects caused by processing. It also ensures that there is no direct contact with the diamond during the laser processing process, avoiding scratches and contamination that may be introduced by traditional mechanical processing, which is conducive to maintaining the optical properties of the diamond, improving production efficiency and reducing costs.

[0029] 2) High temperature and high pressure annealing can optimize the internal structure of diamond crystals, reduce residual stress and internal defects, and improve the integrity and optical properties of the crystals. In particular, based on the structure and performance of diamond crystals, the specific high temperature and high pressure conditions (such as 2000-2200℃, 5-7GPa) used make diamond crystals more stable and suitable for optical applications under extreme conditions. Combined with keeping the pressure constant, the temperature is reduced to 1200℃ within 0.5-1h and maintained for a period of time, and then slowly reduced to room temperature. This sophisticated cooling procedure reduces thermal stress and internal defects caused by rapid temperature changes.

[0030] 3) The present invention pre-grows diamond single crystals with low defect density by MPCVD method, which reduces the number of defects that need to be eliminated in the subsequent HPHT annealing process, thereby improving the efficiency and effect of the annealing process. At the same time, the introduction of laser cutting and polishing technology also further improves the smoothness and flatness of the diamond surface, providing a better foundation for the subsequent annealing process.

[0031] 4) The present invention uses the MPCVD method to grow diamond single crystals with low defect density by precisely controlling the reaction conditions. The defect density can reduce the scattering and absorption of light and improve the optical performance. At the same time, during the annealing process, combined with the high temperature and high pressure environment of the HPHT equipment, residual stress is eliminated, the grains are refined, and the integrity and stability of the crystal are improved. Ensuring that the diamond crystal remains stable under extreme conditions is particularly important for the preparation of high-performance optical-grade diamonds.

[0032] 5) The MPCVD method and HPHT work together to comprehensively optimize the internal structure of the diamond crystal. The low defect density diamond grown by the MPCVD method provides a good starting point for the HPHT annealing treatment, while the HPHT annealing treatment further eliminates residual stress and internal defects, improving the integrity and stability of the crystal. The present invention does not introduce additional impurities into the crystal. The optical transmittance (@1.2μm) of the uncoated diamond prepared by the present invention is higher than 70%, and the optical uniformity is better than 1×10 -4 . BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the stress birefringence distribution diagram of the diamond crystal prepared in Example 1 under the polarizing stress meter.

[0034] Figure 2 is the transmission spectrum of the diamond crystal prepared in Example 1. DETAILED DESCRIPTION

[0035] The specific implementation scheme of the present invention will be described in detail below with reference to examples.

[0036] A method for preparing a diamond crystal with high optical uniformity comprises the following steps:

[0037] S1. Place the diamond substrate into the MPCVD device and evacuate it to below a predetermined pressure;

[0038] S2. introducing a predetermined flow rate of hydrogen and adjusting the pressure to a predetermined value;

[0039] S3. Adjust the microwave power to a predetermined value to generate plasma;

[0040] S4. Gradually increase the microwave power and pressure while monitoring the substrate temperature. By precisely controlling the rate of increase of microwave power and pressure, as well as monitoring the substrate temperature, fine control of the diamond growth environment is achieved, providing a stable foundation for subsequent etching and growth steps.

[0041] S5. When the substrate temperature reaches a predetermined value, a predetermined flow of oxygen is introduced to perform hydrogen-oxygen plasma etching. By introducing oxygen and hydrogen to form hydrogen-oxygen plasma, the diamond surface is etched, effectively removing impurities and defects on the surface, providing a purer growth interface for subsequent growth steps.

[0042] S6. Stop the oxygen supply, keep other parameters unchanged, and continue hydrogen plasma etching to further purify the diamond surface;

[0043] S7. A predetermined flow rate of methane is introduced, and the substrate temperature is adjusted to a predetermined growth temperature to grow diamond. By precisely controlling the flow rate of methane and the substrate temperature, efficient and stable growth of diamond is achieved.

[0044] S8. After the growth is completed, slowly cool down to room temperature and take out the crystal;

[0045] S9. Perform surface treatments such as grinding and polishing on the diamond to obtain diamond crystals with high optical uniformity.

[0046] Preferably, the method further comprises the following steps:

[0047] S10. Use laser cutting and laser polishing technology to remove polycrystalline on the surface and sides of the diamond, so as to more accurately control the shape and surface quality of the diamond and further improve the optical performance of the diamond.

[0048] S11. The treated diamond is placed in a six-sided top machine, heated to a predetermined high temperature, and kept at a predetermined high pressure. The six-sided top machine provides a uniform high temperature and high pressure environment to further improve the optical properties and stress distribution of the diamond;

[0049] S12. Keep the pressure constant, cool down to the intermediate temperature and keep it for a period of time, so as to reduce the thermal stress inside the diamond and improve the stability of the crystal;

[0050] S13. Continue to maintain the pressure constant, cool down to room temperature, and then reduce the pressure to normal pressure to obtain a stable diamond crystal.

[0051] Preferably, the predetermined pressure is 10 -3 Pa, the predetermined flow rate of hydrogen is 500-1000sccm, the predetermined pressure value is 800-3000Pa, the predetermined microwave power value is 300-800W, the predetermined substrate temperature is 800-900, the predetermined flow rate of oxygen is 5-10sccm, the predetermined flow rate of methane is 10-50sccm, and the predetermined growth temperature is 850-1050.

[0052] Through these specific process parameters, the present invention realizes precise control over the diamond growth process, thereby obtaining diamond crystals with high optical uniformity and low stress birefringence.

[0053] Example 1

[0054] 1. Place the diamond substrate in the MPCVD equipment and evacuate for 10 -3 Below Pa;

[0055] 2. Introduce hydrogen with a flow rate of 1000 sccm, adjust the pressure to 3000 Pa, adjust the microwave power to 800 W, and generate plasma.

[0056] 3. Gradually increase the microwave power and pressure while monitoring the substrate temperature. When the substrate temperature reaches 900°C, introduce oxygen with a flow rate of 5 sccm, and keep the microwave power and pressure unchanged to allow the diamond to be etched with hydrogen and oxygen plasma under this condition for 0.5 hours.

[0057] 4. After the hydrogen-oxygen plasma etching is completed, stop introducing oxygen and keep other parameters unchanged, allowing the diamond to be etched in the hydrogen plasma environment for 1 hour.

[0058] 5. After the hydrogen plasma etching is completed, methane is introduced with a flow rate set to 50 sccm, and the substrate temperature is adjusted to 1050°C to start diamond growth.

[0059] 6. After the growth is completed, slowly cool down to room temperature and take out the crystal.

[0060] 7. Use laser cutting and laser polishing technology to remove polycrystals on the surface and sides of the diamond.

[0061] 8. Place the treated diamond in a six-sided top machine and heat it to 2200°C within 1 hour. During the heating process, increase the pressure to 7 GPa within 10 seconds and maintain the temperature and pressure for 2 hours under this condition.

[0062] 9. Keeping the pressure constant, reduce the temperature to 1200°C within 1 hour and maintain it for 10 minutes.

[0063] 10. Continue to maintain the pressure unchanged and cool the crystal to room temperature within 1.5 hours. After the cooling is completed, reduce the pressure to normal pressure.

[0064] 11. After annealing, the diamond is subjected to surface treatments such as grinding and polishing to obtain diamond light with good optical uniformity and high transmittance.

[0065] Figure 1This is the stress birefringence distribution diagram of the diamond crystal prepared in Example 1 under the polarization stress meter. As shown in the figure, the colors of area 1 and area 2 are uniform, indicating that the dislocation density is low, the stress birefringence is small, and the optical uniformity is good, which can meet the requirements of nonlinear optical applications such as Raman laser. The optical uniformity is better than 1×10 -4 .

[0066] Figure 2 This is the transmission spectrum of the diamond crystal prepared in Example 1. As shown in the figure, the optical transmittance (@1μm) can reach 70.8%, and the light absorption loss is extremely low.

[0067] Example 2

[0068] 1. Place the diamond substrate in the MPCVD equipment and evacuate for 10 -3 Pa, then introduce 500 sccm of hydrogen and adjust the pressure to 800 Pa, and adjust the microwave power to 300 W to generate plasma.

[0069] 2. Gradually increase the microwave power and pressure. When the substrate temperature reaches 900°C, introduce 10 sccm of oxygen and keep the microwave power and pressure unchanged. Etch the diamond under this condition for 0.5 h.

[0070] 3. After the hydrogen-oxygen plasma etching is completed, stop introducing oxygen and keep other parameters unchanged, allowing the diamond to be etched in the hydrogen plasma environment for 2 hours.

[0071] 4. After the hydrogen plasma etching is completed, 10 sccm of methane is introduced and the substrate temperature is adjusted to 850°C for diamond growth.

[0072] 5. After the growth is completed, slowly cool down to room temperature and take out the crystal.

[0073] 6. Use laser cutting and laser polishing to remove polycrystals on the surface and sides of the diamond.

[0074] 7. Place the diamond in a six-sided top machine and heat it to 2000°C within 0.5h. During the heating process, increase the pressure to 5GPa within 10 seconds. Keep the temperature and pressure under this condition for 1h.

[0075] 8. Maintaining the pressure constant, reduce the temperature to 1200°C within 0.5 h and maintain it for 5 min.

[0076] 9. Continue to maintain the pressure unchanged and cool the crystal to room temperature within 1 hour. After the cooling is completed, reduce the pressure to normal pressure.

[0077] 10. After annealing, the diamond is subjected to surface treatment such as grinding and polishing.

[0078] Embodiment 3

[0079] 1. Place the diamond substrate in the MPCVD equipment and evacuate for 10 -3 Pa, then introduce 600 sccm of hydrogen and adjust the pressure to 2000 Pa, and adjust the microwave power to 500 W to generate plasma.

[0080] 2. Gradually increase the microwave power and pressure. When the substrate temperature reaches 800°C, introduce 6 sccm of oxygen and keep the microwave power and pressure unchanged. Etch the diamond under this condition for 0.8 h.

[0081] 3. After the hydrogen-oxygen plasma etching is completed, stop introducing oxygen and keep other parameters unchanged, allowing the diamond to be etched in the hydrogen plasma environment for 1.5 hours.

[0082] 4. After the hydrogen plasma etching is completed, 24 sccm of methane is introduced and the substrate temperature is adjusted to 900°C for diamond growth.

[0083] 5. After the growth is completed, slowly cool down to room temperature and take out the crystal.

[0084] 6. Use laser cutting and laser polishing to remove polycrystals on the surface and sides of the diamond.

[0085] 7. Place the diamond in a six-sided top machine and raise the temperature to 2100°C within 1 hour. During the heating process, increase the pressure to 6 GPa within 10 seconds. Keep the temperature and pressure under this condition for 0.8 hours.

[0086] 8. Maintaining the pressure constant, reduce the temperature to 1200°C within 0.8h and maintain it for 8min.

[0087] 9. Continue to maintain the pressure unchanged and cool the crystal to room temperature within 1.5 hours. After the cooling is completed, reduce the pressure to normal pressure.

[0088] 10. After annealing, the diamond is subjected to surface treatment such as grinding and polishing.

[0089] The method for preparing optical grade diamond provided by the present invention has significant technical advantages:

[0090] 1) The present invention provides detailed MPCVD diamond growth process parameters, including hydrogen pressure, microwave power, substrate temperature, oxygen and methane concentrations, etc. The growth environment can be precisely controlled to grow diamonds with lower defect density, which helps to achieve efficient and stable growth of diamonds and control their optical properties.

[0091] 2) After the growth is completed, the polycrystals on the surface and sides of the diamond are removed by laser cutting and laser polishing technology, which can ensure the flatness and smoothness of the diamond surface and provide good conditions for subsequent high temperature and high pressure annealing and surface treatment.

[0092] 3) In the high temperature and high pressure annealing process, the present invention accurately controls the changes in temperature and pressure. Accurate control helps to reduce the thermal stress inside the diamond, prevent crystal cracking, and ensure the uniformity and stability of the annealing process.

[0093] 4) Through the preparation method of the present invention, the optical transmittance (@1.2μm) of the uncoated film can be higher than 70%, and the optical uniformity is better than 1×10 -4 Single crystal diamond. High-quality optical grade diamond has potential application value in optical components such as optical windows, lenses, reflectors, and heat sink materials for high-power lasers.

Claims

1. A method for preparing optical grade diamond, characterized in that The following steps are involved: Step 1. Using MPCVD to grow diamond with a lower defect density; Step 2. removing the polycrystalline part of the diamond surface and side by laser cutting and laser polishing technology; Step 3. Place the treated diamond in a HPHT device for high temperature and high pressure annealing treatment, wherein the temperature is raised to a predetermined high temperature within a predetermined time, and the pressure is quickly raised to a predetermined pressure, and after maintaining the temperature and pressure for a period of time, the temperature is lowered to room temperature through a preset cooling program, and the pressure is maintained at normal pressure; Step 4: Perform surface grinding and polishing on the annealed diamond.

2. A method for preparing optical grade diamond according to claim 1, characterized in that The MPCVD growth process described in step 1 is specifically as follows: Step 1.1 Place the diamond single crystal substrate into the MPCVD equipment and evacuate to 10 -3 Below Pa; Step 1.2: introducing hydrogen gas and adjusting the pressure in the MPCVD device to a predetermined value of 750-900, while adjusting the microwave power to generate plasma; Step 1.3: gradually increase the pressure and microwave power, and after the substrate temperature reaches 750-900°C, introduce oxygen and keep the microwave power and pressure unchanged to perform hydrogen-oxygen plasma etching for 0.5-2h, wherein the concentration of oxygen is 0.5%-2% of hydrogen; Step 1.

4. Stop the oxygen supply and continue etching in the hydrogen plasma environment for 1-2 hours; Step 1.5: introducing methane gas and adjusting the substrate temperature to 850-1050°C to grow diamond, wherein the concentration of methane gas is 2%-5% of hydrogen; After the growth in step 1.6 is completed, slowly cool down to room temperature and take out the crystal.

3. The method for preparing optical grade diamond according to claim 1, characterized in that: The predetermined high temperature in step 3 is 2000-2200°C, and the predetermined high pressure is 5-7 GPa. Under this condition, the temperature and pressure are maintained for 1-2 hours.

4. The method for preparing optical grade diamond according to claim 1, characterized in that: The preset cooling program in step 3 is to keep the pressure constant, reduce the temperature to 1200° C. within 0.5-1 hour and maintain it for 5-10 minutes.

5. The method for preparing optical grade diamond according to claim 1, characterized in that: After the cooling in step 3 is completed, the pressure is reduced to normal pressure.

6. Use of the diamond single crystal obtained by the preparation method according to any one of claims 1 to 5 in diamond Raman laser.

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