Preparation method of high-optical-transmittance CVD polycrystalline diamond thick film

Abstract

The application discloses a preparation method of high-optical-transmittance CVD polycrystal diamond thick film, and the method comprises the following steps: annealing, double-side planarization polishing and cleaning of the prepared CVD polycrystal diamond self-supporting film, and then growing diamond film on the surface of the diamond film again to prepare the optical diamond thick film with the thickness of more than millimeter, so that the problem of serious reduction of diamond grade optical performance caused by crystal orientation out of control and crystal defects in the preparation of polycrystal diamond thick film by the existing process is solved. The diamond thick film prepared by the method has the average grain size of less than 80 microns, the black defect size of less than 5 microns, the black defect density of less than 3 / cm 2 , and the infrared waveband optical transmittance of 70.8% to 71.2%, and can be applied to optical devices such as X-ray windows, infrared guidance windows and high-energy microwave feeding windows.

Description

Technical Field

[0001] This invention belongs to the field of CVD polycrystalline diamond preparation technology, and specifically relates to a method for preparing a CVD polycrystalline diamond thick film with high optical transmittance. Background Technology

[0002] CVD diamond has an ultra-wide spectral transmittance range, excellent optical transmittance, ultra-high hardness and thermal conductivity, and extremely low coefficient of thermal expansion. It also has low dielectric loss, which can meet the application requirements of window materials from X-ray, deep ultraviolet to microwave bands. It has become the first choice for optical devices under extreme conditions such as X-ray windows, infrared guidance windows, high-power CO2 laser windows, and high-energy microwave feed windows.

[0003] Currently, methods for preparing CVD diamond include the hot filament method, the hot cathode method, the DC arc method, and the microwave method. Among these, the microwave method, which uses microwaves to excite plasma, has the advantages of high plasma density, good stability, and no electrode impurities introduced, making it the only method for preparing optical and electronic grade diamond. Compared to the limitations of single-crystal diamond in terms of synthesis size and cost, large-size polycrystalline diamond has achieved low-cost applications in optics, heat sinks, and other fields. MPCVD polycrystalline diamond, as an optical window material, needs to meet certain size and strength requirements in addition to satisfying properties such as transmittance and thermal conductivity. Therefore, large-size MPCVD polycrystalline diamond films with thicknesses exceeding millimeters are required.

[0004] However, limited by existing equipment and manufacturing processes, the deposition instability and stress release during the fabrication of large-size MPCVD polycrystalline diamond films easily lead to the formation of black defects, consisting of non-diamond carbon phases, pores, and microcracks, at the grain boundaries and within the crystals. This is especially true when fabricating millimeter-scale thick diamond films, where the size and density of black defects increase significantly with film thickness due to grain growth, increased grain unevenness, and uncontrolled crystal orientation. The presence of these black defects causes light absorption and refraction, severely reducing the optical transmittance of the diamond film. Therefore, addressing the black defect problem caused by excessively large grain size and uncontrolled orientation in the later stages of MPCVD diamond thick film growth is crucial for fabricating high-optical-transmittance CVD polycrystalline diamond thick films. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art. By annealing the prepared CVD polycrystalline diamond self-supporting film at high temperature, performing double-sided planarization and polishing, and then chemically and ion cleaning, the film is brazed onto the substrate surface for re-growth. This method solves the technical problem of severely reduced optical transmittance of diamond thick films caused by crystal orientation loss and crystal defects due to excessively large grain size in the later stage of growth when preparing diamond thick films using existing growth processes.

[0006] The solution to the technical problem of this invention is: a method for preparing a high optical transmittance CVD polycrystalline diamond thick film, the specific steps of which are as follows:

[0007] Step a: A polycrystalline diamond film is grown on the surface of a single-crystal silicon substrate using MPCVD. The growth parameters are: microwave power 5~60kW, deposition gas pressure 12~18kPa, substrate temperature 800~1000℃, gas flow ratio H2:CH4:O2=100:(3~6):(0.1~0.5), gas purity >99.999%; the thickness of the polycrystalline diamond film is <0.6mm, the average grain size is <80μm, and the self-supporting diamond film is obtained by peeling off the substrate.

[0008] Step b: Perform high-temperature annealing on the diamond self-supporting film to reduce the growth stress of the diamond film and improve the quality of the diamond film.

[0009] Step c: Perform double-sided planarization polishing on the diamond self-supporting film to remove the uneven grain layer and the deformed nucleation layer on the growth surface;

[0010] Step d: The double-sided polished diamond film is chemically cleaned and ion cleaned to remove impurities and polishing damage layers remaining after polishing. After cleaning, the diamond film is brazed onto the surface of the single crystal silicon substrate.

[0011] Step e: A diamond film is regrown on the surface of the diamond film using MPCVD; the growth parameters are: microwave power 5~60kW, deposition gas pressure 12~18kPa, substrate temperature 800~1000℃, gas flow ratio H2:CH4:O2=100:(3~6):(0.1~0.5), gas purity >99.999%; the thickness of the diamond film grown in a single step is <0.6mm, and the average grain size is <80μm;

[0012] Step f: Repeat steps b to e until the polycrystalline diamond film thickness reaches the target thickness;

[0013] Step g: After the diamond thick film grown to the target thickness is peeled off from the substrate, it is annealed and then polished to obtain a CVD polycrystalline diamond thick film with high optical transmittance.

[0014] Furthermore, the annealing in steps b and g is performed in a vacuum furnace or in a microwave CVD device under a hydrogen and oxygen atmosphere. The vacuum degree of the vacuum furnace is <0.5 Pa, and the flow ratio of hydrogen to oxygen is (100~200):1. The annealing temperature is 800~1200℃, and the annealing time is 30~120 min. The annealing treatment can release the growth stress of the diamond film on the one hand, and on the other hand, the hydrogen and oxygen atmosphere helps to hydrogenate and oxidize the non-diamond phase or impurities inside the diamond self-supporting film, thereby improving the optical transmittance of the diamond film.

[0015] Furthermore, the method of double-sided planarization polishing of the diamond film in step c is one or more combined polishing methods such as mechanical polishing, laser polishing, and chemical mechanical polishing.

[0016] Furthermore, in step c, the thickness of the diamond film growth surface removed is >60μm, and the thickness of the nucleation layer removed is >15μm; the surface roughness of the diamond film after double-sided planarization and polishing is <10nm, so as to improve the nucleation density and secondary nucleation rate when the diamond film is regrown on the surface; the flatness and thickness deviation of the diamond film after double-sided planarization and polishing are <10μm, so as to facilitate the consistent control of the growth temperature after the diamond film is brazed to the silicon substrate.

[0017] Furthermore, the chemical cleaning method for the diamond film after double-sided planarization and polishing in step d is as follows: by volume ratio, sulfuric acid + nitric acid = 3:1 is heated to above 80°C and cleaned for 30~60 minutes.

[0018] Furthermore, in step d, the diamond film is cleaned by hydrogen ion etching, with a hydrogen flow rate of 400~1000 sccm, an etching temperature of 600~800℃, and an etching time of 10~30 min.

[0019] Furthermore, in step g, the surface roughness of the polished diamond thick film is <10 nm, the flatness and thickness deviation are <10 μm, the thickness of the obtained CVD diamond thick film is >1 mm, the black defect size is <5 μm, and the black defect density is <3 / cm³. 2 It has an infrared optical transmittance of 70.8%~71.2% and can be used as a window material for optical devices under extreme conditions such as X-ray windows, infrared guidance windows, high-power CO2 laser windows, and high-energy microwave feed windows.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] This invention provides a method for preparing high optical transmittance CVD polycrystalline diamond thick films. Through a growth-intermediate transition treatment-re-growth process, a film with a thickness >1 mm, average grain size <80 μm, black defect size <5 μm, and black defect density <3 / cm³ is prepared.2 The invention produces thick diamond films with infrared transmittance reaching 70.8%~71.2%. The polycrystalline diamond thick films prepared by this invention have grains within a certain size range, avoiding the severe reduction in diamond optical performance caused by uncontrolled crystal orientation and crystal defects in diamond thick films prepared through single growth or multiple growth without intermediate transition treatment. Furthermore, the uniform fine grains contribute to improving the fracture strength of the diamond thick film. The diamond films prepared by this invention overcome the limitations of growth thickness, enabling the fabrication of large-size optical diamond thick films with thicknesses exceeding millimeters. After annealing, polishing, and cleaning, the infrared transmittance of the prepared diamond thick films can reach up to 71.2%, approaching the theoretical upper limit of 71.6% for diamond infrared transmittance. This allows for applications in optical devices such as X-ray windows, infrared guidance windows, high-power CO2 laser windows, and high-energy microwave feed windows. Attached Figure Description

[0022] Figure 1 Flowchart of the steps in the embodiment of the present invention;

[0023] Figure 2 Image of a 50*0.50mm diamond film after double-sided polishing in Example 1;

[0024] Figure 3 The ultraviolet-visible-infrared transmittance spectrum of the 50*0.50mm double-sided polished diamond film in Example 1;

[0025] Figure 4 Image of the diamond film with dimensions of 50*1.09mm in Example 1;

[0026] Figure 5 The ultraviolet-visible-infrared transmittance spectrum of the 50*1.0mm double-sided polished diamond film in Example 1;

[0027] Figure 6 Image of the diamond film with dimensions of 60*2.07mm in Example 2;

[0028] Figure 7 The ultraviolet-visible-infrared transmittance spectrum of the 60*2.0mm double-sided polished diamond film in Example 2;

[0029] Figure 8 Image of a diamond film measuring 60*1.53mm in Comparative Example 1;

[0030] Figure 9 Ultraviolet-visible-infrared transmittance spectrum of the 60*1.45mm double-sided polished diamond film in Comparative Example 1;

[0031] Figure 10Image of a diamond film measuring 60*1.56mm in Comparative Example 2;

[0032] Figure 11 The UV-Vis-IR transmittance spectrum of the 60*1.50mm double-sided polished diamond film in Comparative Example 2. Detailed Implementation

[0033] The technical solution of the present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0034] Example 1

[0035] A method for preparing high optical transmittance CVD polycrystalline diamond thick films, such as... Figure 1 As shown, the specific process is as follows:

[0036] Using a 50*3mm diameter monocrystalline silicon wafer with a polished main surface as a substrate, diamond micropowder with a particle size of 0.5~1.5 micrometers was mechanically ground on the main surface of the monocrystalline silicon for 15 minutes. After grinding, the monocrystalline silicon wafer was ultrasonically cleaned with acetone and alcohol for 10 minutes in sequence, and then wiped dry with a lint-free cloth.

[0037] Polycrystalline diamond films were grown on polished single-crystal silicon substrates using microwave plasma chemical vapor deposition (MPCVD). The growth parameters were set as follows: microwave power 5.5 kW, deposition gas pressure 14 kPa, growth temperature 900 °C, gas flow ratio H2:CH4:O2 = 100:3:0.2, gas purity > 99.999%, and deposition time 180 h. A diamond film with a diameter of 50 mm and a thickness of approximately 0.58 mm was obtained, with black defect size < 5 μm and black defect density < 3 / cm³. 2 The average grain size was approximately 65 μm. After growth, the silicon wafer was dissolved using a 2:1 volume ratio of 40 wt% hydrofluoric acid and 68 wt% nitric acid to obtain a diamond self-supporting film.

[0038] The diamond self-supporting film stripped from the substrate was subjected to high-temperature annealing. The diamond self-supporting film was annealed using an MPCVD device in a hydrogen and oxygen atmosphere. The hydrogen flow rate was set to 400 sccm, the oxygen flow rate to 2 sccm, the annealing temperature to 1000℃, and the annealing time to 60 min.

[0039] The annealed polycrystalline diamond self-supporting film underwent double-sided planarization and polishing. Diamond abrasive and a cast iron disc were used to remove the diamond film's grain layer and nucleation layer. 60 μm of the growth surface and 20 μm of the nucleation layer were removed. After double-sided grinding, mechanical polishing was performed using a ceramic diamond wheel. The surface roughness of the polished diamond film was approximately 2-5 nm, the flatness was 3 μm, and the thickness deviation was 6 μm. (See physical example.) Figure 2 As shown.

[0040] The polished diamond film was cleaned and brazed. It was then acid-washed at 85°C for 30 minutes using a mixture of 98wt% sulfuric acid and 68wt% nitric acid in a 3:1 (volume ratio) solution to remove residual metal powder, graphite carbon, and other impurities from the polished surface. The cleaned diamond film appeared as follows: Figure 2 As shown, the diamond film is approximately 0.50 mm thick, completely transparent with no visible black defects. Spectrophotometer measurements of the transmittance in the ultraviolet-visible-infrared bands are as follows: Figure 3 It is known that the diamond film has a transmittance of up to 71.2% in the infrared band. After chemical cleaning, the diamond film is placed in an MPCVD device for ion cleaning. Hydrogen flow rate of 500 sccm is used to etch at 600℃ for 10 minutes to remove the damaged layer from mechanical polishing. After cleaning, the diamond film is brazed onto the surface of a 50*3mm diameter unpolished silicon substrate for regrowth of the diamond film.

[0041] A diamond film was regrown on the surface of the original diamond film using an MPCVD device. Before growth, the substrate was ultrasonically cleaned with acetone and alcohol for 10 minutes sequentially, then dried with a lint-free cloth. The growth parameters were set exactly the same as those used in the first growth. After a growth time of 180 hours, the substrate was dissolved and peeled off using hydrofluoric acid and nitric acid to obtain the desired result. Figure 4 The diamond thick film shown has a diameter of 50 mm and a thickness of approximately 1.09 mm. The average grain size of the diamond thick film is approximately 66 μm.

[0042] After annealing at 1000℃ for 60 min using an MPCVD machine, the diamond thick film was ground with diamond abrasive and a cast iron disc to remove the grain layer. The film was then polished using a ceramic diamond grinding wheel, resulting in a 1.0 mm thick diamond film with a roughness of 2-5 nm, a flatness of 3 μm, and a thickness deviation of 5 μm. Spectrophotometry measurements showed the following transmittance: UV-Vis-IR. Figure 5 As shown, the transmittance in the infrared band at 2500nm can reach up to 71.0%, and the quality and transmittance of the diamond film do not show a decreasing trend with the increase of film thickness.

[0043] Example 2

[0044] A method for preparing a high optical transmittance CVD polycrystalline diamond thick film, the specific process of which is as follows in this embodiment:

[0045] Using a 60*3mm diameter monocrystalline silicon wafer with a polished main surface as a substrate, diamond micropowder with a particle size of 0.5~1.5 micrometers was mechanically ground on the main surface of the monocrystalline silicon for 15 minutes. After grinding, the monocrystalline silicon wafer was ultrasonically cleaned with acetone and alcohol for 10 minutes in sequence, and then wiped dry with a lint-free cloth.

[0046] Polycrystalline diamond films were grown on the surface of polished monocrystalline silicon substrates using microwave plasma chemical vapor deposition (MPCVD). The growth parameters were set as follows: microwave power 8.5 kW, deposition gas pressure 16 kPa, growth temperature 950 °C, gas flow ratio H2:CH4:O2 = 100:4:0.3, gas purity > 99.999%, and deposition time 160 h. A diamond film with a diameter of 60 mm and a thickness of approximately 0.59 mm was obtained, with an average grain size of approximately 70 μm. After growth, the silicon wafer was dissolved using a mixture of 40 wt% hydrofluoric acid and 68 wt% nitric acid in a volume ratio of 2:1 to obtain a self-supporting diamond film.

[0047] The diamond self-supported film was annealed in a high-temperature furnace under vacuum conditions. The annealing temperature was set at 1200℃ and the annealing time was 90 min. After annealing, the polycrystalline diamond self-supported film was planarized and polished on both sides. The diamond grain layer and nucleation layer were removed by laser grinding. The thickness of the removed layer was 70 μm on the growth surface and 20 μm on the nucleation layer. After double-sided grinding, mechanical polishing was performed with a ceramic diamond wheel to obtain a diamond film with a surface roughness of 2~5 nm, a flatness of 6 μm, and a thickness deviation of 7 μm.

[0048] The double-sided polished diamond film was cleaned by using a mixture of 98wt% sulfuric acid and 68wt% nitric acid in a 3:1 (volume ratio) heated to 80°C for 40 minutes to remove residual metal powder, graphite carbon, and other impurities from the polished surface. The polishing damage layer was then removed by high-temperature hydrogen ion etching at 750°C for 5 minutes with a hydrogen flow rate of 1000 sccm. After cleaning, the diamond film thickness was approximately 0.5 mm. The diamond film was then brazed onto a 60*3 mm diameter silicon substrate for the re-growth of the diamond film.

[0049] A diamond film was regrown on the surface of the original diamond film using an MPCVD device. Before growth, the substrate was ultrasonically cleaned with acetone and alcohol for 10 minutes sequentially, then dried with a lint-free cloth. Growth parameters were set exactly the same as for the first growth. After a growth time of 160 hours, the substrate was dissolved and peeled off using hydrofluoric acid and nitric acid to obtain a thick diamond film. The annealing-double-sided polishing-cleaning-brazing-growth steps were repeated four times to prepare the desired diamond film. Figure 6 The diamond film shown has a diameter of 60 mm and a thickness of 2.07 mm, with black defect size < 5 μm and black defect density < 3 / cm³. 2 The average surface size of the grains is 66 μm. After annealing at 1200℃ for 90 min, mechanical polishing with a ceramic diamond wheel yields a diamond film with a thickness of 2.0 mm, a roughness of 3-6 nm, a flatness of 4 μm, and a thickness deviation of 5 μm. The UV-Vis-IR transmittance was tested as follows: Figure 7 As shown, the transmittance in the infrared band at 2500nm can reach up to 70.8%.

[0050] Comparative Example 1

[0051] A polycrystalline diamond self-supporting film was grown using an MPCVD device. A 60*3mm diameter monocrystalline silicon substrate was used as the substrate material after surface grinding. The polycrystalline diamond film was then grown using microwave plasma chemical vapor deposition (MPCVD). The growth parameters were consistent with those in Example 2: microwave power 8.5kW, deposition gas pressure 16kPa, growth temperature 950℃, gas flow ratio H2:CH4:O2=100:4:0.3, gas purity >99.999%, and deposition time 180h. After growth, the silicon wafer was dissolved using a 2:1 volume ratio of 40wt% hydrofluoric acid + 68wt% nitric acid to obtain a 0.57mm thick diamond self-supporting film.

[0052] The diamond film dissolved from the silicon wafer was planarized and polished on both sides. Laser polishing was used to remove the diamond film grain layer and nucleation layer, removing 60 μm from the growth surface and 20 μm from the nucleation layer. After double-sided polishing, mechanical polishing with a ceramic diamond wheel yielded a diamond film with a surface roughness of 2-5 nm, a flatness of 3 μm, and a thickness deviation of 4 μm. The polished diamond film was then cleaned using a mixture of 98 wt% sulfuric acid and 68 wt% nitric acid in a 3:1 volume ratio, heated to 80°C for 40 minutes to remove residual metal powder, graphitic carbon, and other impurities from the polished surface. A high-temperature hydrogen ion etching process was then used to remove the polishing damage layer, with a hydrogen flow rate of 1000 sccm and etching at 750°C for 5 minutes. After cleaning, the diamond film thickness was approximately 0.5 mm. The cleaned diamond film was then brazed onto the silicon substrate for further diamond film growth.

[0053] Repeated growth - double-sided planarization and polishing - cleaning steps twice, resulting in a three-stage growth process to achieve the desired effect. Figure 8 The diamond self-supporting thick film shown has a diameter of 60 mm and a thickness of approximately 1.53 mm. Figure 8 It can be seen that the diamond film contains a large number of black defects, and the transparency is visibly reduced. The size of the black defects can reach more than 20 μm, and the density of black defects is >100 / cm³. 2 The average grain size is approximately 74 μm. The size and density of black defects inside the diamond film grown without annealing treatment are significantly increased. Mechanical polishing with ceramic diamond wheels on both sides yields a diamond film with a roughness of 2–5 nm, a flatness of 6 μm, a thickness deviation of 4 μm, and a thickness of 1.45 mm. The ultraviolet-visible-infrared transmittance was tested as follows: Figure 9 As shown, by Figure 9 It can be seen that the transmittance of the infrared band at 2500nm is only 50.6%, indicating a significant decrease in the quality and transmittance of the diamond film.

[0054] Comparative Example 2

[0055] A polycrystalline diamond self-supporting film was grown using an MPCVD device. A single-crystal silicon substrate with a diameter of 60*3mm and a polished main surface was used as the substrate material. Then, a polycrystalline diamond film was grown using a microwave plasma chemical vapor deposition device. The growth parameters were set to be consistent with those in Example 2: microwave power 8.5kW, deposition gas pressure 16kPa, growth temperature 950℃, gas flow ratio of H2:CH4:O2=100:4:0.3, gas purity >99.999%, and deposition time 180h. After growth, the diamond film was annealed at high temperature in a high-temperature furnace under vacuum conditions without peeling off the substrate. The annealing temperature of the diamond film was set to 1200℃ and the annealing time was 90min.

[0056] After two repeated growth-annealing steps, the silicon substrate was dissolved and peeled off using a 2:1 volume ratio of 40wt% hydrofluoric acid and 68wt% nitric acid. This process was repeated three times to obtain the desired result. Figure 10 The diamond self-supporting thick film shown has a diameter of 60 mm and a thickness of approximately 1.56 mm. Figure 10 It can be seen that the diamond film contains a large number of black defects, and the transparency is visibly reduced. The size of the black defects can reach more than 60 μm, and the density of black defects is >50 / cm³. 2 The average grain size is approximately 220 μm. The size, density, and grain size of the black spots in the diamond film grown without double-sided planarization and polishing are significantly increased. Double-sided mechanical polishing with ceramic diamond wheels yields a diamond film with a roughness of 3-5 nm, a flatness of 8 μm, a thickness deviation of 6 μm, and a thickness of 1.50 mm. The ultraviolet-visible-infrared transmittance was tested as follows: Figure 11 As shown, by Figure 11 It can be seen that the transmittance of the infrared band at 2500nm is only 38.1%, indicating a significant decrease in the quality and transmittance of the diamond film.

[0057] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a high optical transmittance CVD polycrystalline diamond thick film, characterized in that, Includes the following steps: Step a: A polycrystalline diamond film is grown on the surface of a single-crystal silicon substrate using MPCVD. The growth parameters are: microwave power 5~60kW, deposition gas pressure 12~18kPa, substrate temperature 800~1000℃, and gas flow ratio H2:CH4:O2=100:(3~6):(0.1~0.5). The thickness of the polycrystalline diamond film is <0.6mm, and the average grain size is <80μm. The substrate is then peeled off to obtain a self-supporting diamond film. Step b: Perform high-temperature annealing on the diamond self-supporting film; Step c: Perform double-sided planarization polishing on the diamond self-supporting film to remove the uneven grain layer and the deformed nucleation layer on the growth surface; Step d: The double-sided polished diamond film is chemically cleaned and ion cleaned. After cleaning, the diamond film is brazed onto the surface of the single crystal silicon substrate. Step e: A diamond film is regrown on the surface of the diamond film using MPCVD; the growth parameters are: microwave power 5~60kW, deposition gas pressure 12~18kPa, substrate temperature 800~1000℃, and gas flow ratio H2:CH4:O2=100:(3~6):(0.1~0.5); the thickness of the diamond film grown in a single step is <0.6mm, and the average grain size is <80μm; Step f: Repeat steps b to e until the polycrystalline diamond film thickness reaches the target thickness; Step g: After the diamond thick film grown to the target thickness is peeled off from the substrate, it is annealed and then polished to obtain a CVD polycrystalline diamond thick film with high optical transmittance.

2. The method for preparing high optical transmittance CVD polycrystalline diamond thick films according to claim 1, characterized in that: In steps b and g, the annealing is carried out in a vacuum environment or a protective atmosphere at a temperature of 800~1200℃ for 30~120min.

3. The method for preparing high optical transmittance CVD polycrystalline diamond thick films according to claim 2, characterized in that: The vacuum level in the vacuum environment is <0.5 Pa; the protective atmosphere refers to a mixed atmosphere of hydrogen and oxygen, in which the flow rate ratio of hydrogen to oxygen is (100~200):

1.

4. The method for preparing high optical transmittance CVD polycrystalline diamond thick films according to claim 1, characterized in that: The method for double-sided planarization polishing of the diamond film in step c is one or more of the following combined polishing methods: laser polishing, mechanical polishing, and chemical mechanical polishing.

5. The method for preparing high optical transmittance CVD polycrystalline diamond thick films according to claim 1, characterized in that: In step c, the thickness of the diamond film growth surface is removed >60μm, the thickness of the nucleation layer is removed >15μm, and the surface roughness of the diamond film after double-sided planarization and polishing is <10nm, and the flatness and thickness deviation are <10μm.

6. The method for preparing high optical transmittance CVD polycrystalline diamond thick films according to claim 1, characterized in that: The chemical cleaning method for the diamond film after double-sided planarization and polishing in step d is as follows: by volume ratio, sulfuric acid + nitric acid = 3:1 is heated to above 80°C and cleaned for 30~60 minutes.

7. The method for preparing high optical transmittance CVD polycrystalline diamond thick films according to claim 1, characterized in that: In step d, the diamond film is cleaned by hydrogen ion etching, with a hydrogen flow rate of 400 sccm to 1000 sccm, an etching temperature of 600 to 800°C, and an etching time of 10 to 30 minutes.

8. The method for preparing high optical transmittance CVD polycrystalline diamond thick films according to claim 1, characterized in that: In step g, the surface roughness of the polished diamond thick film is <10 nm, the flatness and thickness deviation are <10 μm, the thickness of the obtained CVD diamond thick film is >1 mm, the black defect size is <5 μm, and the black defect density is <3 / cm³. 2 The optical transmittance in the infrared band is 70.8%~71.2%.

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