Method for manufacturing super-flat polycrystalline diamond film on large scale

Abstract

The invention provides a method for manufacturing a super-flat polycrystalline diamond film on a large scale. The method comprises the following steps: (1) growing a layer of polycrystalline diamond film on a growth substrate with a diamond seed crystal on the surface by adopting a chemical vapor deposition method; (2) cutting off at least one part of the substrate after growth to form a boundary for jointing the polycrystalline diamond film and the growth substrate; (3) sticking a stripping adhesive tape; and (4) the stripping adhesive tape is pulled to strip the polycrystalline diamond film from the surface of the growth substrate, so that the nucleation surface of the polycrystalline diamond film is exposed, and the roughness of the exposed nucleation surface is smaller than that of the growth surface. According to the method, the nucleation surface of the grown polycrystalline diamond film is turned over, the novel and simple method is developed to manufacture the polycrystalline diamond film which is reasonable in price, capable of growing on a large scale, super-flat and transferable, and the method has huge potential in practical application.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation technology of polycrystalline diamond films, and in particular relates to a method for large-scale manufacturing of polycrystalline diamond films with ultra-flat surfaces. Background Art

[0002] High-quality synthetic diamond materials have been considered as ideal candidates for applications in mechanical, electronic and optical fields due to their excellent physical and chemical properties. So far, people have successfully synthesized diamond materials, but it is still quite challenging to obtain large-scale, high-quality and ultra-thin diamond films.

[0003] Doping diamond can further expand its applications in electronic devices, such as boron-doped p-type semiconductors and superconductors, as well as quantum photonics materials based on atomic defects such as doped nitrogen and silicon, especially those materials called next-generation diamond substrates, such as membranes, which are expected to play an important role in the efficient heat dissipation of circuits and high-power electronic devices, especially for emerging soft / flexible electronic devices.

[0004] Realizing the above-mentioned diamond-based functional elements necessarily involves standard nanofabrication and seamless integration, all of which place higher demands on the surface roughness of diamond materials. In addition, a flat surface of the device is also a prerequisite for many testing / characterization methods that are widely used in the corresponding fields, such as thermal conductivity measurement and bonding in the semiconductor industry.

[0005] However, unlike other materials such as silicon, planarization of diamond is very difficult and requires a lot of time and energy through traditional methods such as polishing.

[0006] Diamond films are mainly produced by homogeneous and heterogeneous growth using chemical vapor deposition (CVD) technology. Single crystal diamond (SCD) films are mainly obtained by homogeneous growth on bulk diamond substrates, and they represent the highest quality required for many applications. For example, it has high mobility for electrons and holes and excellent resistance to radiation, and is also highly valuable for detectors in nuclear and high-energy physics experiments. However, the expensive growth process, limited manufacturing scale, and complex processing of CVD-grown SCD have hindered its widespread application.

[0007] Although heterogeneous growth of SCD on iridium has been demonstrated in a limited number of laboratories around the world, it is still difficult to control the quality of the obtained diamond, which tends to form polycrystalline diamond (PCD) films due to lattice mismatch. Therefore, it is still very challenging to build large-area SCD films on arbitrary substrates.

[0008] In contrast, scalable, inexpensive, and easy-to-grow polycrystalline diamond (PCD) films can be readily obtained on a variety of common substrates, such as Si, SiC, TiC, Co, Pt, Al2O3, Ni, and Re. In particular, based on a sufficiently high nucleus density, it can form a continuous film, and the quality of the film is closely related to the directionality of the nuclei. In order to optimize PCD films, researchers have proposed the concept of nanocrystalline and ultra-nanocrystalline diamond films, which form continuous nuclei by depositing diamond seeds. However, the surface smoothness of PCD films is less than satisfactory, which is one of the biggest bottlenecks in promoting such an excellent platform.

[0009] In summary, the extremely strict growth conditions of single crystal diamond (SCD) have greatly hindered the application of diamond materials in large-scale, cost-acceptable fields; and the poor surface morphology and high roughness of polycrystalline diamond (PCD) due to its uneven profile have seriously limited its further construction of high-precision photonic structures at the nano / micrometer scale. Summary of the invention

[0010] Therefore, the purpose of the present invention is to overcome the problems of poor surface morphology and high roughness of polycrystalline diamond (PCD) films and to provide a simple method for preparing large-area, ultra-flat and transferable PCD films in a scalable and controllable manner.

[0011] A first aspect of the present invention provides a method for large-scale manufacturing of ultra-flat polycrystalline diamond films, the method comprising:

[0012] (1) growing a polycrystalline diamond film on a growth substrate having diamond seeds on its surface by chemical vapor deposition, wherein the polycrystalline diamond film comprises: a growth surface (or referred to as a "growth surface") having a first roughness and a nucleation surface (or referred to as a "nucleation surface") bonded to the growth substrate;

[0013] (2) cutting at least a portion of the growth substrate on which the polycrystalline diamond film is grown, wherein the cutting operation forms a boundary line where the polycrystalline diamond film is bonded to the growth substrate at the cutting position;

[0014] (3) sticking a stripping tape to the growth surface of the polycrystalline diamond film, wherein the stripping tape extends beyond the polycrystalline diamond film at least at an edge of the cut portion to form a stripping operation end;

[0015] (4) pulling the stripping operation end to strip the polycrystalline diamond film from the surface of the growth substrate to expose a nucleation surface of the polycrystalline diamond film, wherein the exposed nucleation surface has a second roughness, and the second roughness is smaller than the first roughness.

[0016] According to the method provided by the present invention, the first roughness may be 10-200 nm, for example 20-50 nm. In a preferred embodiment of the present invention, the second roughness may be 0.5-5 nm, preferably 0.5-2 nm, more preferably 0.5-1 nm.

[0017] According to the method provided by the present invention, the particle size of the diamond seed crystal described in step (1) can be 2 to 10 nm. In some embodiments of the present invention, the growth substrate can be selected from one or more of Si, SiC, TiC, Co, Pt, Al2O3, Ni, Re, Ir, SiO2 and Mo. In a preferred embodiment of the present invention, the surface roughness of the growth substrate is less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm. Preferably, a microwave plasma assisted chemical vapor deposition (MPCVD) device is used in step (1) to grow a polycrystalline diamond film. In some embodiments of the present invention, the thickness of the polycrystalline diamond film can be 200 nm to 100 μm, preferably 200 nm to 10 μm, and more preferably 400 nm to 5 μm.

[0018] According to the method provided by the present invention, the trimming operation described in step (2) is preferably performed perpendicular to the growth surface of the polycrystalline diamond film. The trimming operation can be performed along a straight line, a curve or an arc. In a preferred embodiment of the present invention, the trimming operation makes the length of the boundary line where the formed polycrystalline diamond film is bonded to the growth substrate at least 1% of the circumference of the growth surface of the polycrystalline diamond film, for example, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, preferably 5% to 20%, preferably 10% to 12%.

[0019] In a preferred embodiment of the present invention, in order to facilitate mass production, the cutting operation in step (2) can be performed along a straight line. When the growth substrate is circular, the length of the boundary line where the polycrystalline diamond film formed by cutting is bonded to the growth substrate is preferably 20% to 50% of the substrate diameter.

[0020] The inventors of the present invention have found through a large number of experimental studies that the cutting operation described in step (2) can reduce the adhesion force caused by excessive coverage of the polycrystalline diamond film on the side wall of the growth substrate. Starting from the cutting point, the polycrystalline diamond film can be gradually separated from the growth substrate with the help of a stripping tape, thereby obtaining a complete polycrystalline diamond film, and the growth substrate after stripping can also be recycled.

[0021] According to the method provided by the present invention, the stripping tape in step (3) can be 3M tape, PVC tape, PE tape, etc. In an embodiment of the present invention, the stripping tape completely covers the polycrystalline diamond film remaining after the cutting operation, and at least leaves a margin at the cutting edge to facilitate the subsequent stripping operation.

[0022] The inventors of the present invention have found through a large number of experimental studies that although the stripping performance of PCD films of different thicknesses is related to their mechanical strength, different stripping operations (such as speed and stripping angle) will also affect them. Performing reasonable stripping operations within the range allowed by mechanical strength is the prerequisite for obtaining a large-area, complete film with fewer cracks.

[0023] In the present invention, the peeling angle refers to the angle between the peeling force that pulls the peeling operation end to separate the polycrystalline diamond film from the growth substrate and the growth surface of the polycrystalline diamond film (the starting position of the peeling operation end of the peeling tape is defined as 0°). Among them, the peeling angle range for obtaining a complete polycrystalline diamond film with fewer cracks is called the safe peeling angle.

[0024] In the present invention, integrity refers to the percentage of the area of ​​the polycrystalline diamond film obtained by peeling to the area of ​​the growth substrate after peeling.

[0025] According to the method provided by the present invention, the speed of pulling the stripping operation end to strip the polycrystalline diamond film from the growth substrate surface in step (4) can be 1 to 10 mm / s. In a preferred embodiment of the present invention, the stripping angle of the stripping operation in step (4) can be 10 to 90°.

[0026] According to the method provided by the present invention, the nucleation surface has a higher refractive index than the growth surface. Preferably, the nucleation surface has a lower extinction coefficient than the growth surface.

[0027] The levels of these optical parameters of the polycrystalline diamond film produced by the method of the present invention are comparable to those of SCD grown by the CVD method.

[0028] According to the method provided by the present invention, the position of the XRD diffraction peak of the (111) crystal plane of the nucleation surface is closer to the standard (111) crystal plane XRD diffraction peak of single crystal diamond than the position of the (111) crystal plane XRD diffraction peak of the growth surface; and the full width at half maximum (FWHM) of the Raman spectrum of the (111) crystal plane of the nucleation surface is less than the full width at half maximum (FWHM) of the (111) crystal plane of the growth surface. The movement of the XRD diffraction peak indicates the improvement of the crystallization quality, and the narrowing of the full width at half maximum of the Raman spectrum means that the internal crystallization of the film has been improved.

[0029] According to the method provided by the present invention, the method may further include reducing the viscosity of the stripping tape with a solvent to release the polycrystalline diamond film; or a thermal release stripping tape may be used to release the polycrystalline diamond film from the stripping tape by heating.

[0030] The second aspect of the present invention also provides a method for regulating the energy band of a diamond film, the method comprising attaching the polycrystalline diamond film prepared by the present invention to the surface of a flexible substrate, stretching and / or compressing the polycrystalline diamond film by bending the flexible substrate, and regulating the energy band structure of the polycrystalline diamond film through the strain generated by the polycrystalline diamond film during the stretching and / or compression process.

[0031] Although the growth surface of polycrystalline diamond films has been extensively studied, the nucleation surface is rarely mentioned because it is difficult to expose to the public. The present invention proposes for the first time a novel and simple method to obtain the nucleation surface of polycrystalline diamond in situ without introducing any damage. This method can be used to manufacture large-area, ultra-flat and transferable polycrystalline diamond films on a large scale and in a controllable manner. The effectiveness of the method of the present invention is verified by detailed characterization of the surface morphology and material properties of the nucleation surface.

[0032] Compared with other methods of exposing the nucleation surface (such as the sacrificial growth substrate method described in the previous application 202310540694.7), the method of the present invention reduces impurities caused by multiple operations (such as grinding and etching). Therefore, this stripping method can maintain the original state of the nucleation surface without causing any damage. By characterizing the roughness of the growth surface and the nucleation surface of polycrystalline diamond films of different thicknesses, it can be seen that the roughness of the nucleation surface of various thicknesses can be less than 1nm, while the roughness of the growth surface increases with thickness. Compared with the reported studies on optimizing the surface roughness of diamond films, the stripping method of the present invention has obvious advantages both in thickness range and in film size. Therefore, to date, the method of the present invention can be considered to be the best method for obtaining polycrystalline diamond films of the largest size, the smoothest surface and the largest thickness range. The novel, scalable and controllable strategy proposed by the present invention paves the way for the development of high-quality and cost-effective polycrystalline diamond films, making polycrystalline diamond films expected to become basic components of various high-performance structures and devices in different fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The embodiments of the present invention are described in detail below with reference to the accompanying drawings, wherein:

[0034] Figure 1 It is a schematic diagram of the method for preparing a polycrystalline diamond film of the present invention.

[0035] Figure 2 Figures (a), (b) and (c) are photographs of a 2-inch silicon wafer having a polycrystalline diamond film grown on its surface in Example 1 (a), the peeling process (b), the polycrystalline diamond film after peeling, and the silicon wafer after peeling (c).

[0036] Figure 3 The following are photos of the nucleation surfaces of polycrystalline diamond films of different thicknesses prepared in Examples 2-5.

[0037] Figure 4 Statistical diagrams of cracks and integrity of polycrystalline diamond films of different thicknesses prepared in Examples 2-5.

[0038] Figure 5 SEM images of the growth surface (a), nucleation surface (b) and cross section (c) of the polycrystalline diamond film with a thickness of 1000 nm prepared in Example 5.

[0039] Figure 6 SEM images of the growth surface (upper) and nucleation surface (lower) of polycrystalline diamond films of different thicknesses prepared in Examples 2-5.

[0040] Figure 7 AFM images of the growth surface (a) and nucleation surface (b) of the 1000 nm thick polycrystalline diamond film prepared in Example 5 and the nucleation surface (c) of the 1000 nm thick polycrystalline diamond film prepared in Example 6.

[0041] Figure 8 (a) Roughness statistics of growth surface and nucleation surface of PCD films of different thicknesses prepared in Examples 2-5 (statistical area 20 μm×20 μm); (b) Comparison of film thickness and roughness between the method of the present invention and other reported methods; and (c) Comparison of film size and roughness between the method of the present invention and other methods.

[0042] Fig. 9 Comparison of the refractive indices of the growth surface (a) and the nucleation surface (b) of the polycrystalline diamond film prepared in Example 1.

[0043] Fig.10 1 and 2. The XRD results of the growth surface (a) and the nucleation surface (b) of the polycrystalline diamond film prepared in Example 1.

[0044] Fig.11 The Raman spectra of the growth surface (a) and the nucleation surface (b) of the polycrystalline diamond film prepared in Example 1.

[0045] Fig.12 The 360-degree bending photograph (a) of the 4 μm thick polycrystalline diamond film prepared in Example 7 and the winding photograph (b) on cylinders with different radii.

[0046] Fig.13 Different bending modes of the 4 μm thick polycrystalline diamond film prepared in Example 7 on a flexible PDMS substrate (a) and the corresponding strain distribution on the surface (b).

[0047] Fig.14 The conductivity change of the 4 μm thick polycrystalline diamond film prepared in Example 7 under different strains (a) and bending cycles (b).

[0048] Fig.15 The Raman spectrum (a) and XPS measurement results (b and c) of the polycrystalline diamond film prepared in Example 5.

[0049] Fig.16 The following are the experimental photos and data summary of the contact angles of the growth surface and nucleation surface of PCD films with different thicknesses.

[0050] Fig.17 The friction coefficient of the growth surface and the nucleation surface of the polycrystalline diamond film prepared in Example 5 (a) and the relationship curve between the external force applied to the growth surface and the nucleation surface and the depth of the indentation formed (b).

[0051] Fig.18 The refractive index (a) and extinction coefficient (b) of the growth surface and nucleation surface of a polycrystalline diamond film with a thickness of 500 nm; and the refractive index (c) and extinction coefficient (d) of the growth surface and nucleation surface of a polycrystalline diamond film with a thickness of 1000 nm. DETAILED DESCRIPTION

[0052] The present invention is further described in detail below in conjunction with specific embodiments. The given examples are only for illustrating the present invention, rather than for limiting the scope of the present invention.

[0053] Figure 1 Schematic diagram of the method for preparing a polycrystalline diamond film of the present invention, the preparation process includes:

[0054] I. CVD growth of polycrystalline diamond films;

[0055] II. cutting off a portion of the grown substrate and polycrystalline diamond film to form a new boundary;

[0056] III. Paste a peeling tape to perform mechanical peeling to separate the polycrystalline diamond film from the substrate.

[0057] Example 1

[0058] Fabrication of polycrystalline diamond films

[0059] (1) Growth of polycrystalline diamond films on silicon substrates

[0060] Diamond heteroepitaxial growth on silicon substrates includes three steps: substrate pretreatment, deposition of diamond seeds and CVD growth of polycrystalline diamond films.

[0061] Substrate pretreatment: The silicon surface was pretreated with hydrogen plasma at 1300 W power, 35 Torr chamber pressure and 300 sccm H 2 A 2-inch silicon wafer was placed in a microwave plasma assisted chemical vapor deposition (MPCVD) apparatus under air flow conditions for 10 minutes.

[0062] Deposition of diamond seed crystals: Diamond seed crystals with a size of less than 10 nm (purchased from Tokyo Chemical Industry Co., Ltd.) were mixed with dimethyl sulfoxide (DMSO), anhydrous ethanol and acetone in a mass ratio of 1:5000:250:250. The mixture was ultrasonically dispersed for 12 hours and then centrifuged at 1000 r / min for 20 minutes to separate impurities. The suspension was spin-coated on a silicon wafer with deposition parameters: 500 r / min, 3 drops were added within 15 s, and then the speed was increased to 4500 r / min for 110 s. Repeat three times.

[0063] CVD growth of diamond film: A silicon wafer with diamond seeds deposited thereon was placed in an MPCVD device (Seki 6350) to grow a diamond film. The main growth parameters included: 3400 W microwave power, 900 °C temperature, 15 sccm methane flow rate and 40 minutes growth time. A PCD film about 447 thick was grown on the silicon substrate.

[0064] (2) Cutting of polycrystalline diamond film and growth substrate

[0065] A silicon wafer cutter or a diamond knife is used to cut the 2-inch silicon wafer with the polycrystalline diamond film grown on the surface obtained in step (1) along a straight line 2 to 5 mm from the edge of the back side of the growth substrate to form a new boundary, so that a clear boundary line where the polycrystalline diamond film is bonded to the growth substrate appears at the cutting position.

[0066] (3) Sticking of peeling tape

[0067] A stripping tape of 10 cm×6 cm is attached to the growth surface of the polycrystalline diamond film so as to completely cover the growth surface of the polycrystalline diamond film and extend out from the edge of the cut portion of the polycrystalline diamond film to form a stripping operation end (stripping pull handle).

[0068] (4) Stripping of polycrystalline diamond film

[0069] The stripping operation end reserved in step (3) is pulled at a stripping angle of 40 to 60 degrees and a speed of 5 mm / s to strip the polycrystalline diamond film from the surface of the silicon wafer, exposing the nucleation surface of the polycrystalline diamond film.

[0070] Figure 2 is a photo taken during the operation of this embodiment. Figure 2 Figures (a), (b) and (c) are photographs of a 2-inch silicon wafer with a polycrystalline diamond film grown on the surface, the peeling process, the polycrystalline diamond film after peeling, and the silicon wafer after peeling.

[0071] Embodiment 2-5

[0072] Fabrication of polycrystalline diamond films with different thicknesses

[0073] According to a method similar to that of Example 1, polycrystalline diamond films having thicknesses of 200 nm, 400 nm, 800 nm, and 1000 nm were produced, respectively. Figure 3 2 is a photograph of the nucleation surface of polycrystalline diamond films of different thicknesses after peeling. It can be seen from the figure that polycrystalline diamond films of different thicknesses can be almost completely peeled off from the substrate, proving the effectiveness of the peeling method of the present invention.

[0074] In order to effectively evaluate their performance, the number of microcracks (generated during the peeling process) and the integrity (as a proportion of the growth substrate size) of these polycrystalline diamond films were counted. Figure 4 The following are statistical graphs of cracks and integrity of polycrystalline diamond films of different thicknesses prepared in Examples 2-5. The figures on the right are 50 times magnified optical images of polycrystalline diamond films with thicknesses of 200nm (above) and 1000nm (below). It can be seen from the figure that the crack density corresponding to the 200-nanometer film is significantly higher than that of the 1000-nanometer film. From these results, it can be seen that the number of cracks increases with decreasing thickness, indicating that when the thickness is reduced to 200nm, the PCD film has a peeling property similar to that of a two-dimensional material. At the same time, as the thickness is reduced to 200nm, the integrity of the peeling film decreases from 100% to 99.43%, which is mainly related to the different adhesion of the growth substrate.

[0075] Example 6

[0076] Fabrication of optimized polycrystalline diamond films

[0077] A polycrystalline diamond film with a thickness of 1000 nm was manufactured in a manner similar to that of Example 1, except that, before substrate pretreatment, the surface of the silicon wafer was polished to reduce its roughness from 1 nm to 0.5 nm.

[0078] Morphology and Properties of Polycrystalline Diamond Films

[0079] 1. The growth surface, nucleation surface and cross section of the polycrystalline diamond films of different thicknesses prepared in Examples 2-6 were characterized by SEM.

[0080] Figure 5 Shown are typical SEM images of the growth surface, nucleation surface and cross section of a 1000 nm thick polycrystalline diamond film prepared in Example 5. Figure 6 Shown are SEM images of the growth surface (upper) and nucleation surface (lower) of polycrystalline diamond films of different thicknesses prepared in Examples 2-5. Figure 5 It can be seen that there are various large grains (hundreds of nanometers) on the growth surface, but the nucleation surface is flat without any visible grains; this gradient change in grain size can also be seen from the cross section. Figure 6 The SEM images also show that the grains on the growth surface grow larger with increasing thickness, which is in sharp contrast to the nucleation surface where the surface morphology remains unchanged. This obvious difference in growth surface morphology is mainly related to the growth mode of PCD films. Specifically, diamond particles with a size of no more than 10nm are deposited as seeds on a silicon substrate before CVD growth; in the subsequent growth process, the diamond particles can only grow randomly in the upward and horizontal directions because the silicon substrate hinders the growth of diamonds toward the bottom. In particular, the morphology of the nucleation surface depends on the surface of the growth substrate, the size of the seeds, and the nucleation density.

[0081] 2. Atomic force microscopy (AFM) was used to characterize the growth surface and nucleation surface of the polycrystalline diamond films of different thicknesses prepared in Examples 2-6.

[0082] Figure 7 Shown are AFM images of the growth surface (a) and the nucleation surface (b) of the 1000 nm thick polycrystalline diamond film prepared in Example 5 and the nucleation surface (c) of the 1000 nm thick polycrystalline diamond film prepared in Example 6.

[0083] pass Figure 7 From the AFM images, it can be seen that the surface roughness (Ra) of the polycrystalline diamond film of Example 5 is reduced from 36.2nm (growth surface) to 0.952nm (nucleation surface), and when a smoother growth substrate is used in Example 6, the surface roughness is further reduced to 0.612nm. This ultra-flat surface is comparable to polished single-crystal diamond, meeting the requirements of high-precision nano-machining. In addition, compared with other strategies for exposing the nucleation surface, such as sacrificial growth substrates, the method of the present invention avoids impurities caused by multiple operations. Therefore, this stripping method can maintain the original state of the nucleation surface without causing any damage.

[0084] Figure 8The following are (a) the statistics of the roughness of the growth surface and nucleation surface of PCD films of different thicknesses prepared in Examples 2-5 (statistical area 20μm×20μm); (b) the comparison of the method of the present invention with other reported methods in terms of film thickness and roughness; and (c) the comparison of the method of the present invention with other methods in terms of film size and roughness. By comparing the roughness of the growth surface and nucleation surface of PCD films of different thicknesses, it can be seen that the roughness of the nucleation surface of both thicknesses is less than 1nm, while the roughness of the growth surface is increasing. Compared with the reported studies on optimizing the surface roughness of diamond films, the stripping method of the present invention has obvious advantages both in thickness range and in PCD film size. Therefore, so far, it can be considered as the best method to obtain PCD films with the largest size, the smoothest surface and the largest thickness range. In addition, if the growth time is extended and a larger growth substrate is used, thicker and larger PCD films can be obtained.

[0085] 3. The refractive index of the growth surface and the nucleation surface of the polycrystalline diamond film of Example 1 was measured using an optical ellipsometer. Fig. 9 As shown in the figure, n represents the refractive index and k represents the extinction coefficient. Fig. 9 From the measurement results, the nucleation surface has a higher refractive index and a lower extinction coefficient than the growth surface, and it is found that the levels of these optical parameters of the nucleation surface are comparable to those of SCD grown by the CVD method.

[0086] 4. X-ray diffraction (XRD) technology was used to characterize the crystallization characteristics of the polycrystalline diamond film obtained in Example 1. Fig.10 As shown in the figure, obvious diffraction peaks were obtained at 43.96°, 75.14° and 91.52°, corresponding to the (111), (220) and (311) crystal planes, respectively, confirming that the nucleation surface has diamond properties. The strong diffraction peak of the (111) crystal plane indicates its dominance, which is consistent with other reports. In addition, the full width at half maximum (FWHM) of the (111) crystal plane diffraction peak of the nucleation surface is 0.64° narrower than that of the growth surface. In addition to the change in FWHM, the position of the diffraction peak of the (111) crystal plane shows a significant shift ( Fig.10 , inset), from 44.36° (growth surface) to 43.96° (nucleation surface), which is close to the position of the SCD (43.95°) of the standard (111) orientation. The shift of the diffraction peak indicates an improvement in crystal quality, which may be related to the release of internal stress. Normally, due to the lattice mismatch between diamond and the foreign substrate, residual stress will be introduced into the film, which will further induce lattice distortion and lead to abnormal movement of the diffraction peak. By adopting the manufacturing method of the present invention, the residual stress has the opportunity to be released after removing the growth substrate.

[0087] 5. Raman spectroscopy is used to further characterize the crystallization characteristics of the polycrystalline diamond film obtained in Example 1. Fig.11 From the Raman spectrum, at 1332.8cm -1 An obvious diamond scattering peak was observed at , which further proved the state of the film. Fig.11 From the illustration, the FWHM of the nucleation surface (7.43 cm -1 ) than the FWHM of the growth surface (7.67cm -1 ) is narrower than that of the membrane, which indicates that the internal crystallization of the film measured from the nucleation surface is improved.

[0088] It can be seen that in addition to the reduced roughness of the nucleation surface, the crystallization quality is also optimized compared to the growth surface based on the Raman shift and X-ray diffraction properties. In addition, in terms of optical properties, the refractive index of the nucleation surface is lower than that of the growth surface, comparable to that of single crystals.

[0089] Table 1 summarizes the comparison of the polycrystalline diamond films prepared by the method of the present invention with previously reported polycrystalline diamond films in terms of surface roughness, thickness range and size (film area).

[0090] Table 1

[0091]

[0092] It can be seen from the data in Table 1 that the polycrystalline diamond film obtained by the method of the present invention has a very low surface roughness, close to the level of a polished single crystal diamond block. In addition, the obtained film is superior to the previously reported work in terms of size and thickness.

[0093] Example 7

[0094] Fabrication and characterization of 4 μm thick polycrystalline diamond films

[0095] A polycrystalline diamond film with a thickness of 4 μm was manufactured in a similar manner to that of Example 1, and then the polycrystalline diamond film was directly attached to a flexible PDMS substrate for testing of bending properties and conductive properties.

[0096] Bending properties of polycrystalline diamond films

[0097] The polycrystalline diamond film stripped by the method of the present invention can be deformed arbitrarily without being restricted by the rigid growth substrate. Fig.12 Photos of the 360-degree bending of a 4 μm thick PCD film (A) and its wrapping around cylinders of different radii (B).

[0098] like Fig.12As shown, for a film about 4 μm thick prepared in Example 7, it can support a 360° bending mode, and its tolerable radius of curvature can be reduced to 2 mm. And the film remains stable after more than 2500 bending cycles. Compared with the previously reported maximum strain of polycrystalline diamond films occurring on microscale samples, these incredible diamond bending properties are observed for the first time on a macroscopic scale. According to previous theoretical predictions of single-crystal diamond nanopillars, the energy band will gradually decrease with increasing strain. However, due to the complex crystal structure inside the diamond, this principle has rarely been confirmed in both theory and experiment. Fortunately, the present invention can observe the dynamic characteristics of independent polycrystalline diamond films through a simple peeling strategy.

[0099] Conductive properties of polycrystalline diamond films and their bandgap control

[0100] In order to investigate the conductive properties of the polycrystalline diamond film under different bending degrees, the inventor adhered the polycrystalline diamond film with a thickness of 4 μm prepared in Example 7 to a flexible PDMS (polydimethylsiloxane) substrate with a thickness of 1 mm, and then attached soft conductive tapes in turn. By controlling the deformation of the PDMS substrate, the bending operation of the polycrystalline diamond film can be achieved. In addition, by bending the PDMS substrate in different directions, the deformation of the polycrystalline diamond film under tensile and compressive strains can be observed respectively. Fig.13 From the calculation results shown, it can be seen that the strain distribution is uneven. Whether it is the top or bottom surface of PDMS, the largest deformation occurs in the center.

[0101] Fig.14 The conductivity of the 4 μm thick polycrystalline diamond film prepared in Example 7 under different strains (a) and bending cycles (b). By measuring the resistance under dynamic strain, it is observed that the film has a huge piezoresistive property. In both bending modes, the resistance gradually decreases with increasing strain, but the change in resistance in the push-down mode is greater than that in the pull-up mode. In addition, the resistance can be fully recovered during the load-unload cycle.

[0102] The above results show that the energy band structure of polycrystalline diamond film can be regulated by strain. As the degree of strain increases, the energy band of diamond will gradually decrease, so the conductivity will increase. Generally speaking, the regulation of diamond energy band is mainly achieved through element doping, but diamond doping has high requirements for the process and is difficult to complete in ordinary laboratories. The method of strain regulation of diamond energy band proposed in the present invention greatly reduces the difficulty of preparation and production cost, and provides convenience for the development of flexible diamond semiconductor devices and diamond-based sensor devices.

[0103] Material properties of polycrystalline diamond films

[0104] Fig.15The Raman spectrum (A) and XPS measurement results (B and C) of the polycrystalline diamond film prepared in Example 5.

[0105] Fig.16 The following are the experimental photos and data summary of the contact angles of the growth surface and nucleation surface of PCD films with different thicknesses.

[0106] Fig.17 The friction coefficient of the growth surface and the nucleation surface of the polycrystalline diamond film prepared in Example 5 (a) and the relationship curve between the external force applied to the growth surface and the nucleation surface and the depth of the indentation formed (b).

[0107] Fig.18 The refractive index (A) and extinction coefficient (B) of the growth surface and nucleation surface of a polycrystalline diamond film with a thickness of 500 nm; and the refractive index (C) and extinction coefficient (D) of the growth surface and nucleation surface of a polycrystalline diamond film with a thickness of 1000 nm.

[0108] The above results show that the nucleation surface has a higher refractive index and a lower extinction coefficient than the growth surface. The refractive index of the nucleation surface is comparable to that of single crystal bulk diamond. Moreover, as the thickness increases, the refractive index of both the growth surface and the nucleation surface decreases, while the extinction coefficient increases. This diamond film with a high refractive index and low absorption coefficient provides the possibility for the development of diamond-based micro-nano optical devices, such as optical superlenses, optical modulators, etc.

Claims

1. A method for large-scale production of ultra-flat polycrystalline diamond films, the method comprising: (1) growing a polycrystalline diamond film on a growth substrate having diamond seeds on its surface by chemical vapor deposition, wherein the polycrystalline diamond film comprises: a growth surface having a first roughness and a nucleation surface bonded to the growth substrate; (2) cutting at least a portion of the growth substrate on which the polycrystalline diamond film is grown, wherein the cutting operation forms a boundary line where the polycrystalline diamond film is bonded to the growth substrate at the cutting position; (3) sticking a stripping tape to the growth surface of the polycrystalline diamond film, wherein the stripping tape extends beyond the polycrystalline diamond film at least at an edge of the cut portion to form a stripping operation end; (4) pulling the stripping operation end to strip the polycrystalline diamond film from the surface of the growth substrate to expose a nucleation surface of the polycrystalline diamond film, wherein the exposed nucleation surface has a second roughness, and the second roughness is smaller than the first roughness.

2. The method according to claim 1, wherein: The first roughness is 10-200 nm, preferably 20-50 nm; preferably, the second roughness is 0.5-5 nm, preferably 0.5-2 nm, more preferably 0.5-1 nm.

3. The method according to claim 1 or 2, wherein: The particle size of the diamond seed crystal described in step (1) is 2 to 10 nm.

4. The method according to any one of claims 1 to 3, wherein: In step (1), a microwave plasma assisted chemical vapor deposition device is used to grow a polycrystalline diamond film.

5. The method according to any one of claims 1 to 4, wherein: The growth substrate is selected from one or more of Si, SiC, TiC, Co, Pt, Al2O3, Ni, Re, Ir, SiO2 and Mo; preferably, the surface roughness of the growth substrate is less than 2nm, preferably less than 1nm, and more preferably less than 0.5nm.

6. The method according to any one of claims 1 to 5, wherein: The thickness of the polycrystalline diamond film is 200 nm to 100 μm, preferably 200 nm to 10 μm, and more preferably 400 nm to 5 μm.

7. The method according to any one of claims 1 to 6, wherein: The trimming operation described in step (2) is performed perpendicularly to the growth surface of the polycrystalline diamond film; preferably, the trimming operation is performed so that the length of the boundary line where the formed polycrystalline diamond film is bonded to the growth substrate is at least 1% of the circumference of the growth surface of the polycrystalline diamond film, for example, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, preferably 5% to 20%, more preferably 10% to 12%; Preferably, the growth substrate is circular, and the length of the boundary line between the polycrystalline diamond film cut in step (2) and the growth substrate is 20% to 50% of the diameter of the growth substrate.

8. The method according to any one of claims 1 to 7, wherein: In step (4), the speed of pulling the stripping operation end to strip the polycrystalline diamond film from the surface of the growth substrate is 1 to 10 mm / s; preferably, the stripping angle of the stripping operation in step (4) is 10 to 90 degrees.

9. The method according to any one of claims 1 to 8, wherein: The nucleation surface has a higher refractive index than the growth surface; preferably, the nucleation surface has a lower extinction coefficient than the growth surface; Preferably, compared with the position of the (111) crystal plane XRD diffraction peak of the growth surface, the position of the (111) crystal plane XRD diffraction peak of the nucleation surface is closer to the standard (111) crystal plane XRD diffraction peak of single crystal diamond; and the half-maximum full width of the (111) crystal plane XRD diffraction peak of the nucleation surface is smaller than the half-maximum full width of the (111) crystal plane XRD diffraction peak of the growth surface.

10. A method for regulating the energy band of a diamond film, the method comprising attaching a polycrystalline diamond film manufactured by the method of any one of claims 1 to 9 to the surface of a flexible substrate, stretching and / or compressing the polycrystalline diamond film by bending the flexible substrate, and regulating the energy band structure of the polycrystalline diamond film by the strain generated by the polycrystalline diamond film during the stretching and / or compression process.

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