Polycrystalline diamond window and preparation method thereof

Abstract

The invention discloses a preparation method of a polycrystalline diamond window, which comprises the following steps of: spin-coating diamond powder turbid liquid on the surface of a silicon wafer, and growing a diamond wafer on the silicon wafer for not less than 300 hours by adopting MPCVD (Microwave Plasma Chemical Vapor Deposition) equipment; carrying out grinding, ultrasonic cleaning and blow-drying on the diamond wafer, and then putting the diamond wafer into MPCVD for hydrogen plasma etching; starting a bias power supply at the temperature of 800-900 DEG C by adopting MPCVD equipment, and secondarily growing uniformly-oriented diamonds on the diamond wafer; carrying out grinding, ultrasonic cleaning and blow-drying on the diamond wafer subjected to secondary growth again, and then putting the diamond wafer into MPCVD for hydrogen plasma etching; and adding the diamond wafer into corrosive liquid so as to remove the silicon wafer to obtain the polycrystalline diamond window. The polycrystalline diamond prepared by the preparation method is smooth in surface and less in surface damage.

Description

Technical Field

[0001] The invention belongs to the field of diamond material preparation, and in particular relates to a polycrystalline diamond window and a preparation method thereof. Background Art

[0002] Optical window components are one of the basic optical components in various optical systems. They are used to separate the inside and outside of the instrument to ensure the normal operation of the components inside the system. At the same time, they also serve as a structural component to ensure the normal transmission of infrared rays. Due to their harsh operating environment and high-power energy output, infrared window components require all-round excellent performance: 1) Optical performance: good optical transmittance in the infrared band of operation, that is, small absorption coefficient and low spontaneous radiation rate to ensure the effective transmission of light waves, and at the same time, the stability of optical performance can be guaranteed under extreme environments; 2) Thermal performance: high thermal conductivity to ensure effective heat dissipation of the window, and low thermal expansion coefficient to prevent deformation of the window due to long-term operation or excessive changes in ambient temperature; 3) Mechanical properties: due to their harsh working environment, optical windows have to face impacts such as strong airflow and dust, so the material is required to have sufficient mechanical strength to ensure its normal operation; 4) Chemical properties: acid and alkali corrosion resistance, rain erosion resistance, and not easy to react with corrosive gases in the atmosphere. At the same time, from an application perspective, the larger the size of the window, the more it can meet performance indicators such as high power, high heat dissipation, wide detection field of view, high resolution, and high precision, thereby realizing applications in satellite detection, laser weapons, missile infrared guidance, high-power microwave transmission media and devices, nuclear industry, and other fields.

[0003] The current mainstream infrared window materials are mostly polycrystalline transparent ceramics, which can be divided into medium-wave infrared materials (3μm~5μm) and long-wave infrared materials (8μm~12μm) according to their wavelength. Common medium-wave infrared materials include magnesium fluoride (MgF 2 ), alumina single crystal, magnesium aluminate spinel (MgAl 2 O 4 ), aluminum oxynitride (AlON) and yttrium oxide (Y 2 O 3 ) transparent ceramics. Long-wave infrared materials include transparent ceramics such as sulfide (ZnS), selenide (ZnSe), and cadmium telluride (CdTe). However, although these materials have excellent optical properties, their mechanical properties and thermal stability are poor and they cannot adapt to harsh service conditions. For example, ZnSe is currently the most commonly used infrared window due to its extremely low infrared absorption coefficient, but its thermal conductivity is poor. Even a very small amount of absorption will cause a large temperature rise. This temperature rise will cause deformation of the window material and refraction of the incident light, both of which will cause thermo-optical effects and produce serious phase differences, which will lead to device failure.

[0004] Diamond is a typical atomic crystal, belonging to the cubic crystal system with a face-centered cubic structure. However, different from the conventional face-centered cubic structure, it is composed of two sets of face-centered cubic unit cells that are displaced by 1 / 4 of the body diagonal length. This unique crystal structure of diamond endows it with extremely special physical and chemical properties. For example, the C-C bond in it has a short bond length and high strength, and is very difficult to be broken. Therefore, diamond has extremely high hardness and temperature stability. At the same time, the σ bond formed between C atoms in diamond has high strength and stability, and the bonding electrons are not easily excited. So diamond has an extremely low short-wave absorption limit of 225 nm. At the same time, it is not easy to absorb photons, making it have extremely high transmittance from ultraviolet to far-infrared and even microwave bands. On this basis, diamond also has extremely high thermal conductivity and extremely low thermal expansion coefficient. These excellent properties make diamond have great application potential in the field of optical windows.

[0005] However, due to the limitations of growth technology and cost, it is currently very difficult to obtain large-size (inch-level) single-crystal diamond wafers. Therefore, self-supporting polycrystalline diamond is the preferred material for infrared windows. For infrared window materials, not only its internal crystal structure restricts its performance, but its surface quality also has a crucial impact on performance. Therefore, optical windows require not only extremely high crystal quality of the material but also good surface planarization technology as support. However, due to the extremely complex structure of polycrystalline diamond, it is very difficult to obtain a smooth surface that meets the application requirements of infrared windows through simple processing. Currently, the growth and processing technologies of diamond are separated from each other, without considering the influence of the two on the final performance, and it is difficult to obtain a diamond infrared optical window that can meet the actual application requirements.

[0006] Therefore, developing a high-performance manufacturing method for polycrystalline diamond infrared windows plays a crucial role in improving the application of diamond in the infrared field. Summary of the Invention

[0007] The present invention provides a preparation method for a polycrystalline diamond window. The polycrystalline diamond prepared by this preparation method has a smooth surface and less surface damage.

[0008] The present invention provides a preparation method for a polycrystalline diamond window, including:

[0009] (1) Spin-coating a diamond powder suspension on the surface of a silicon wafer, and growing a diamond wafer on the silicon wafer using an MPCVD device for no less than 300 h;

[0010] (2) Grinding, ultrasonically cleaning, and drying the diamond wafer in step (1), and then placing it into the MPCVD for hydrogen plasma etching;

[0011] (3) Use an MPCVD device to turn on the bias power supply at a temperature of 800 - 900 °C, and grow diamond with a uniform orientation on the diamond wafer obtained in step (2).

[0012] (4) Grind, ultrasonically clean, and dry the diamond wafer after secondary growth again, and then place it into the MPCVD for hydrogen plasma etching.

[0013] (5) Add the diamond wafer obtained in step (4) to the etching solution to remove the silicon wafer to obtain a polycrystalline diamond window.

[0014] In the present invention, by growing a diamond wafer with a uniform orientation through secondary growth, due to the uniform orientation, the surface of the diamond wafer becomes smoother after grinding. Since the orientation anisotropy of diamond is very obvious, that is, there are obvious differences in the material removal rate between grains with different orientations under the same working conditions, the height differences between grains with different orientations on the surface of randomly oriented grown diamond will be very obvious after processing. By performing surface uniform growth, the surface height differences caused by orientation anisotropy can be significantly improved, effectively improving the surface quality after processing. If the same process is used for stable growth within 300 h, the grain size of the diamond will continue to grow, affecting the overall quality of the diamond wafer. At the same time, the uniform orientation growth has high requirements for the stability of process conditions, and long-term growth will lead to increased costs. Therefore, the method of two-stage growth is selected to control the surface orientation uniformity.

[0015] In the present invention, by controlling the temperature during secondary growth, the surface orientation is made uniform. Both too high and too low temperatures will result in non-uniform diamond orientation, and at the same time, the growth of small grains will occur, which is caused by the change in the concentration and activity of active groups in the plasma due to temperature.

[0016] Preferably, grow diamond with (100), (110), or (111) orientation on the diamond wafer obtained in step (2).

[0017] More preferably, grow diamond with (100) orientation on the diamond wafer obtained in step (2). By growing diamond with (100) orientation through secondary growth, not only can the surface roughness of the diamond wafer be controlled, but also the surface damage after processing is the lowest among all orientations.

[0018] Preferably, in step (3), turn on the bias power supply at a temperature of 840 - 860 °C. In the present invention, by further controlling the temperature, the degree of surface orientation uniformity of the diamond grown through secondary growth is better, and the surface damage during the processing can be more effectively controlled and the surface quality can be improved.

[0019] Preferably, in step (3), the time for the secondary growth of diamond with a uniform orientation on the diamond wafer obtained in step (2) is 5 - 10 h, so that the thickness of the secondarily grown diamond layer is between 10 - 20 μm, ensuring the wear thickness requirement during secondary processing while effectively improving the surface quality after processing.

[0020] Preferably, in step (1), during the growth of the diamond wafer on the silicon wafer using an MPCVD device, nitrogen gas is introduced for 0.5 - 1 h every 8 - 10 h. The present invention avoids excessive growth of grain size by periodically adding nitrogen gas.

[0021] Preferably, the grinding method is to use a diamond grinding wheel for grinding. The initial grinding load is 3 - 5 kg, and the load is gradually increased to 7 - 8 kg as the processing time increases. The rotational speed of the grinding wheel is 700 - 1400 rpm. By controlling the grinding process parameters, the surface of the diamond wafer is flattened.

[0022] Preferably, the temperature of the hydrogen plasma etching is 700 - 750 °C, and the etching time is 0.5 - 2 hours. The present invention controls the temperature and etching time of the hydrogen plasma etching to remove the damage layer generated during the grinding process as much as possible.

[0023] Preferably, in step (5), the added etching solution is a mixed solution of nitric acid and hydrofluoric acid, and the mass ratio of nitric acid to hydrofluoric acid is 1:2.5 - 3.5.

[0024] On the other hand, the present invention also provides a polycrystalline diamond window, which is prepared by the preparation method of the polycrystalline diamond window described above.

[0025] Preferably, the roughness of the polycrystalline diamond window is <3 nm (1.91 * 1.19 mm 2 ) and <1 nm (30 * 30 μm 2 ).

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] By secondarily growing diamond with a uniform orientation on a multi - oriented diamond wafer, the present invention can greatly reduce the difference in the height and depression of surface grains caused by surface orientation anisotropy, and thus can greatly improve the surface quality of the finally obtained diamond wafer, that is, reduce the surface roughness and surface loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a photograph of the diamond wafer obtained in the first growth provided by Embodiment 1 of the present invention;

[0029] Figure 2The photograph of the diamond wafer after the first grinding process provided by Embodiment 1 of the present invention;

[0030] Figure 3 The SEM photograph and 3D contour map of the secondary-grown (100)-oriented diamond provided by Embodiment 1 of the present invention;

[0031] Figure 4 The photograph of the diamond wafer after the secondary growth of the (100)-oriented diamond and the corresponding surface roughness map provided by Embodiment 1 of the present invention;

[0032] Figure 5 The SEM photograph of the secondary-grown (100)-oriented diamond provided by Embodiment 2 of the present invention;

[0033] Figure 6 The SEM photograph of the secondary-grown (100)-oriented diamond provided by Embodiment 3 of the present invention;

[0034] Figure 7 The SEM photograph of the secondary-grown (100)-oriented diamond provided by Embodiment 4 of the present invention;

[0035] Figure 8 The SEM photograph of the secondary-grown (100)-oriented diamond provided by Comparative Example 1 of the present invention;

[0036] Figure 9 The SEM photograph of the secondary-grown (100)-oriented diamond provided by Comparative Example 2 of the present invention. Detailed Description of the Invention

[0037] The following describes in detail the specific embodiments of the present invention in conjunction with the technical solutions.

[0038] Embodiment 1

[0039] A 3-inch (diameter 76.2 mm) single-crystal silicon (100) wafer is used as the growth substrate. Before growth, the surface of the substrate is spin-coated with a diamond powder suspension, where the diamond powder suspension is composed of diamond powder with a diameter of 100 nm and an alcohol solution, and the mass ratio of the two is 1:100. The spin-coating speed is 500 rpm, and the spin-coating time is 20 min.

[0040] After spin-coating, the wafer is left to stand and dry, and then placed in an MPCVD device for growth. The growth atmosphere is composed of hydrogen, methane, and an inert gas. During the growth process, the hydrogen flow rate is 500 sccm, the methane concentration is 3%, the growth temperature is controlled at 850 - 900 degrees Celsius, and the chamber pressure is 10 kPa. During the growth process, nitrogen is introduced into the chamber every 7 hours for 1 hour, the nitrogen concentration is 50 ppm, and the total growth time is 300 hours. As Figure 1As shown, the surface of the diamond wafer after the first growth is relatively uniform as a whole, without obvious edge effects and grain growth phenomena occurring.

[0041] After the growth is completed, the diamond wafer with the silicon substrate is taken out for surface planarization processing. Diamond grinding wheels are used for grinding. The initial grinding load is 2 kg, and the load is gradually increased to 10 kg as the processing time increases. The grinding wheel speed is always maintained at 900 rpm, and the processing duration is 100 hours to ensure that the entire surface of the wafer is processed.

[0042] After the processing is completed, the diamond wafer is placed in alcohol for ultrasonic cleaning to remove surface residues. The processed sample is as Figure 2 shown. At this time, there is still a certain degree of processing damage layer on the wafer surface. For the 100 orientation, the damage depth is less than 2 nm and can be basically ignored, while for the 110 orientation, the thickness of the damage layer can reach the order of 10 nm, and for the 111 orientation (the cleavage plane of diamond), it exceeds 15 nm. After the ultra-cleaning is completed, the diamond wafer is placed in the MPCVD equipment again. First, its surface is etched by hydrogen plasma. The ambient atmosphere is hydrogen, its flow rate is 500 sccm, the etching temperature is 750 degrees Celsius, the chamber pressure is 10 kPa, and the etching time is 30 minutes.

[0043] After the etching is completed, the output power of the microwave source is increased, the temperature of the substrate is raised to 850 degrees Celsius, and methane is introduced at the same time. The methane concentration is 5% of that of hydrogen.

[0044] After the atmosphere in the chamber is stabilized, the bias power supply is turned on, the bias source voltage is raised to 100 V, and the growth of (100)-oriented diamond is started. The growth time is 10 hours. After the growth is completed, secondary grinding processing is carried out. The grinding load is 2 kg, and the grinding time is 2 hours. Figure 3 The microscopic SEM photos of the sample surface after the growth is completed and the corresponding 3D contour maps are given, as shown in Figure 3 a and b of. The diamond surface presents square blocks of different sizes, and the height of the overall surface can also be seen in the corresponding 3D contour map measured by a laser confocal microscope.

[0045] The processed diamond wafer is ultrasonically cleaned. After the cleaning is completed, it is dried with nitrogen. After that, it is placed in the MPCVD equipment for plasma etching treatment to remove the surface damage layer and contaminants; the etched diamond wafer is placed in a mixed solution of nitric acid and hydrofluoric acid with a ratio of 3:1 for etching to remove the silicon substrate, and finally a complete self-supporting diamond wafer is obtained. Its optical photo is as shown in Figure 4 a of; the corresponding surface roughness is measured by a white light interferometer, and the results are as shown in Figure 4As shown in Figure b, the surface roughness after processing reaches 2.83 nm within the range of 1.91 * 1.19 mm. 2

[0046] Example 2

[0047] Compared with Example 1, the difference is that after the first etching is completed, the output power of the microwave source is increased, the temperature of the substrate is raised to 800 degrees Celsius, and methane is introduced simultaneously.

[0048] As Figure 5 shown, the diamond surface shows certain (100) orientation characteristics, but the grain edges are irregular and there is a certain degree of stacking, indicating that there is a tendency for (100) orientation growth on the diamond surface at this time, but there is still room for adjustment.

[0049] Example 3

[0050] Compared with Example 1, the difference is that after the first etching is completed, the output power of the microwave source is increased, the temperature of the substrate is raised to 900 degrees Celsius, and methane is introduced simultaneously.

[0051] As Figure 6 shown, the grains are significantly larger, but there is still a certain degree of stacking.

[0052] Example 4

[0053] Compared with Example 1, the difference is that after the first etching is completed, the output power of the microwave source is increased, the temperature of the substrate is raised to 820 degrees Celsius, and methane is introduced simultaneously.

[0054] As Figure 7 shown, at this time, the grains significantly exhibit obvious square characteristics of (100) orientation, indicating that the diamond grown under this condition is more in line with the (100) orientation.

[0055] Comparative Example 1

[0056] Compared with Example 1, the difference is that after the first etching is completed, the output power of the microwave source is increased, the temperature of the substrate is raised to 750 degrees Celsius, and methane is introduced simultaneously.

[0057] As Figure 8 shown, although the surface of a single grain is relatively smooth, the stacking between grains is obvious and the shape around the grains is irregular, indicating that there is a large deviation in the grain orientation at this time.

[0058] Comparative Example 2

[0059] Compared with Example 1, the difference is that after the first etching is completed, the output power of the microwave source is increased, the temperature of the substrate is raised to 950 degrees Celsius, and methane is introduced simultaneously.

[0060] As Figure 9 shown, although the square features of the 100 orientation can still be seen on the diamond surface after growth is completed, many grain fragmentation phenomena appear on the surface, indicating that the quality of the grown diamond is poor at this time.

Claims

1. A method for preparing a polycrystalline diamond window, characterized in that: include: (1) Spin-coating a diamond powder suspension on the surface of a silicon wafer and growing a diamond wafer on the silicon wafer using an MPCVD device for a growth time of not less than 300 hours; (2) grinding, ultrasonically cleaning, and drying the diamond wafer of step (1), and then placing it in MPCVD for hydrogen plasma etching; (3) using an MPCVD device to turn on a bias power supply at a temperature of 800-900° C. to secondary grow uniformly oriented diamonds on the diamond wafer obtained in step (2); (4) Grinding, ultrasonic cleaning, and drying the diamond wafer after secondary growth again, and then placing it in MPCVD for hydrogen plasma etching; (5) Adding the diamond wafer obtained in step (4) into an etching solution to remove the silicon wafer to obtain a polycrystalline diamond window.

2. The method for preparing a polycrystalline diamond window according to claim 1, characterized in that: Secondary growth of diamonds in (100), (110) or (111) orientations is performed on the diamond wafer obtained in step (2).

3. The method for preparing a polycrystalline diamond window according to claim 1 or 2, characterized in that: Secondary growth of (100) oriented diamonds on the diamond wafer obtained in step (2).

4. The method for preparing a polycrystalline diamond window according to claim 1, characterized in that: In step (3), the bias power supply is turned on at a temperature of 840-860°C.

5. The method for preparing a polycrystalline diamond window according to claim 1, characterized in that: In step (1), during the process of growing diamond wafers on silicon wafers using MPCVD equipment, nitrogen is introduced for 0.5-1 hour every 8-10 hours.

6. The method for preparing a polycrystalline diamond window according to claim 1, characterized in that: The grinding method is to use a diamond grinding wheel for grinding, the initial grinding load is 3-5kg, and the load is gradually increased to 10-15kg as the processing time increases, and the rotation speed of the grinding wheel is 700-1400rpm.

7. The method for preparing a polycrystalline diamond window according to claim 1, characterized in that: The temperature of the hydrogen plasma etching is 700-750° C., and the etching time is 0.5-2 h.

8. The method for preparing a polycrystalline diamond window according to claim 1, characterized in that: In step (5), the added etching solution is a mixed solution of nitric acid and hydrofluoric acid, and the mass ratio of the nitric acid to the hydrofluoric acid is 1:2.5-3.

5.

9. A polycrystalline diamond window, characterized in that: The polycrystalline diamond window is prepared by the method for preparing the polycrystalline diamond window according to any one of claims 1 to 8.

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