Method for scalable fabrication of ultraflat polycrystalline diamond membranes

Abstract

This invention provides a method for large-scale manufacturing of ultra-flat polycrystalline diamond films, comprising: (1) growing a polycrystalline diamond film on a growth substrate with diamond seed crystals on its surface using chemical vapor deposition; (2) cutting off at least a portion of the grown substrate to form a boundary line between the polycrystalline diamond film and the growth substrate; (3) applying a release tape; and (4) pulling the release tape to peel the polycrystalline diamond film off the surface of the growth substrate, thereby exposing the nucleation surface of the polycrystalline diamond film, wherein the roughness of the exposed nucleation surface is less than the roughness of the growth surface. This invention, by flipping the nucleation surface of the grown polycrystalline diamond film, develops a novel and simple method for manufacturing affordable, scalable, ultra-flat, and transferable polycrystalline diamond films, which has great potential in practical applications.

Description

A method for large-scale manufacturing of ultra-flat polycrystalline diamond films The present invention belongs to the technical field of the preparation process of polycrystalline diamond films, and particularly relates to a method for large-scale manufacturing of polycrystalline diamond films with ultra-flat surfaces. High-quality synthetic diamond materials have always been considered ideal candidate materials for applications in the mechanical, electronic, and optical fields due to their excellent physical and chemical properties. So far, diamond materials have been successfully synthesized, but it is still quite challenging to obtain large-scale, high-quality, and ultra-thin diamond films. Doped diamond can further expand its applications in electronic devices, such as p-type semiconductors and superconductors doped with boron, as well as quantum photonics materials based on atomic defects such as doped nitrogen and silicon. In particular, those materials called the next-generation diamond substrates, such as films, are expected to play an important role in the effective heat dissipation of circuits and high-power electronic devices, especially for emerging soft / flexible electronic devices. Realizing the above diamond-based functional elements necessarily involves standard nanomanufacturing and seamless integration, all of which pose higher requirements for the surface roughness of diamond materials. In addition, the flat surface of the device is also a prerequisite for many testing / characterization methods, which are widely used in the corresponding fields, such as thermal conductivity measurement and bonding in the semiconductor industry. However, different from other materials such as silicon, the planarization of diamond materials is very difficult, and it requires a large amount of time and energy through traditional methods (such as polishing). Diamond films are mainly prepared by homoepitaxial and heteroepitaxial growth of chemical vapor deposition (CVD) technology. Single-crystal diamond (SCD) films are mainly obtained by homoepitaxial growth on a bulk diamond substrate, which represents the highest quality required for many applications. For example, it has high mobility for electrons and holes and excellent resistance to radiation, and also has high value for detectors in nuclear and high-energy physics experiments. However, the expensive growth process, limited manufacturing scale, and complex processing process of CVD-grown SCD hinder its popularization and application. Although the heteroepitaxial 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. Due to lattice mismatch, it tends to form polycrystalline diamond (PCD) films. Therefore, it is still very challenging to construct large-area SCD films on any substrate. In contrast, polycrystalline diamond (PCD) films that can be easily obtained by large-scale manufacturing, are inexpensive and easy to grow can be obtained on various common substrates, such as Si, SiC, TiC, Co, Pt, Al 2 O 3, on substrates such as Ni and Re. In particular, on the basis of a sufficiently high nuclear density, it can form a continuous film, and the quality of the film is closely related to the directionality of the nuclei. To optimize the PCD film, researchers have proposed the concepts of nanocrystalline and ultrananocrystalline diamond films, forming continuous nuclei by depositing diamond seeds. However, the surface smoothness of the PCD film is not satisfactory, which is one of the biggest bottlenecks in promoting such an excellent platform. In summary, the extremely strict growth conditions of single-crystal diamond (SCD) have greatly hindered the application of diamond materials in large-scale and cost-acceptable fields; while polycrystalline diamond (PCD) has poor surface topography and high roughness due to uneven contours, severely restricting its further construction of high-precision photonic structures at the nano / micron scale. Therefore, the object of the present invention is to overcome the problems of poor surface topography and high roughness existing in polycrystalline diamond (PCD) films, and provide a simple method to prepare large-area, ultra-flat and transferable PCD films in a scalable and controllable manner. The first aspect of the present invention provides a method for large-scale manufacturing of ultra-flat polycrystalline diamond films, the method comprising: (1) Growing a layer of 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 "growth surface") having a first roughness and a nucleation surface (or referred to as "nucleation surface") joined to the growth substrate; (2) Cutting off at least a part of the growth substrate with a polycrystalline diamond film grown on its surface, and this cutting operation causes a demarcation line where the polycrystalline diamond film joins the growth substrate to be formed at the cutting position; (3) Affixing a peeling tape to the growth surface of the polycrystalline diamond film, wherein the peeling tape extends at least beyond the polycrystalline diamond film at the edge of the cutting position to form a peeling operation end; (4) Pulling the peeling operation end to peel the polycrystalline diamond film from the surface of the growth substrate, so that the nucleation surface of the polycrystalline diamond film is exposed, wherein the exposed nucleation surface has a second roughness, and the second roughness is less than the first roughness. According to the method provided by the present invention, wherein the first roughness can be 10 - 200 nm, for example 20 - 50 nm. In a preferred embodiment of the present invention, the second roughness can be 0.5 - 5 nm, preferably 0.5 - 2 nm, more preferably 0.5 - 1 nm. According to the method provided by the present invention, wherein the particle size of the diamond seeds in step (1) can be 2 - 10 nm. In some embodiments of the present invention, the growth substrate can be selected from Si, SiC, TiC, Co, Pt, Al 2 O 3 , Ni, Re, Ir, SiO 2 and one or more of 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, in step (1), a microwave plasma-assisted chemical vapor deposition (MPCVD) apparatus is used for growing the 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. According to the method provided by the present invention, wherein, in step (1), the deposition temperature of the chemical vapor deposition method can be 800 - 1000 °C, preferably 850 - 950 °C. According to the method provided by the present invention, wherein, in step (2), the cutting operation is preferably performed perpendicular to the growth surface of the polycrystalline diamond film. The cutting operation can be performed along a straight line, a curve or an arc. In a preferred embodiment of the present invention, the length of the demarcation line formed by the cutting operation where the polycrystalline diamond film is joined to the growth substrate is at least 1% of the perimeter of the growth surface of the polycrystalline diamond film, for example, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, preferably 5% - 20%, and more preferably 10% - 12%. In a preferred embodiment of the present invention, for facilitating large-scale production, the cutting operation in step (2) can be performed along a straight line. When the growth substrate is circular, the length of the demarcation line where the cut polycrystalline diamond film is joined to the growth substrate is preferably 20% - 50% of the substrate diameter. The inventors of the present invention have found through a large number of experimental studies that through the cutting operation in step (2), the adhesion force generated by the excessive coverage of the polycrystalline diamond film on the side wall of the growth substrate can be reduced. Starting from the cut, the polycrystalline diamond film can be gradually detached from the growth substrate with the help of a peeling tape, so as to obtain a complete polycrystalline diamond film, and the growth substrate after peeling can also be recycled. According to the method provided by the present invention, wherein, in step (3), the peeling tape can be 3M tape, PVC tape, PE tape, etc. In an embodiment of the present invention, the peeling tape completely covers the remaining polycrystalline diamond film after the cutting operation, and at least has a margin at the cutting edge to facilitate subsequent peeling operations. Through a large number of experimental studies, the inventors of the present invention have found that although the peeling performance of PCD films with different thicknesses is related to their mechanical strength, different peeling operations (such as speed, peeling angle) also have an impact on it. Performing reasonable peeling operations within the allowable range of mechanical strength is a prerequisite for obtaining a large-area, complete and less cracked film. In the present invention, the peeling angle refers to the angle between the peeling force that separates the polycrystalline diamond film from the growth substrate by pulling the peeling operation end and the growth surface of the polycrystalline diamond film (defining the starting position of the peeling operation end of the peeling tape as 0°). Among them, the range of the peeling angle for obtaining a complete and less cracked polycrystalline diamond film is called the safe peeling angle. In the present invention, integrity refers to the percentage of the area of the polycrystalline diamond film obtained by peeling in the area of the growth substrate after peeling. According to the method provided by the present invention, wherein, the speed of pulling the peeling operation end to peel the polycrystalline diamond film from the surface of the growth substrate in step (4) can be 1-10 mm / s. In a preferred embodiment of the present invention, the peeling angle of the peeling operation in step (4) can be 10-90°. According to the method provided by the present invention, 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. The levels of these optical parameters of the polycrystalline diamond film prepared by the method of the present invention are comparable to those of SCD grown by the CVD method. According to the method provided by the present invention, wherein, compared with the position of the XRD diffraction peak of the (111) crystal plane of the growth surface, the position of the XRD diffraction peak of the (111) crystal plane of the nucleation surface is closer to the standard XRD diffraction peak of the (111) crystal plane of single crystal diamond; and the full width at half maximum (FWHM) of the Raman spectrum of the (111) crystal plane of the nucleation surface is smaller than the full width at half maximum (FWHM) of the (111) crystal plane of the growth surface. The shift of the XRD diffraction peak indicates the improvement of the crystal quality, and the narrowing of the full width at half maximum of the Raman spectrum means the improvement of the internal crystallization of the film. According to the method provided by the present invention, wherein, the method may further include reducing the viscosity of the peeling tape with a solvent to release the polycrystalline diamond film; or a heat-releasing type peeling tape can be used to release the polycrystalline diamond film from the peeling tape by heating. The second aspect of the present invention also provides a method for regulating the energy band of a diamond film, the method includes 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 realizing the regulation of 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. Although the growth surfaces of polycrystalline diamond films have been extensively reported in the literature, the nucleation surfaces have rarely been mentioned because they are difficult to expose to the public eye. The present invention first proposes a novel and simple method to in-situ obtain the nucleation surface of polycrystalline diamond without introducing any damage. Using this method, large-area, ultra-flat, and transferable polycrystalline diamond films can be fabricated on a large scale and controllably. By characterizing the surface morphology and material properties of the nucleation surface in detail, the effectiveness of the method of the present invention is verified. Compared with other methods of exposing the nucleation surface (such as the method of sacrificial growth substrates 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 peeling method can maintain the original state of the nucleation surface without causing any damage. By characterizing the roughness of the growth surfaces and nucleation surfaces of polycrystalline diamond films with different thicknesses, it can be seen that the roughness of the nucleation surfaces of various thicknesses can be less than 1 nm, while the roughness of the growth surfaces increases with the thickness. Compared with the reported research on optimizing the surface roughness of diamond films, the peeling method of the present invention has obvious advantages both in terms of the thickness range and the size of the film. Therefore, so far, the method of the present invention can be considered the best method for obtaining polycrystalline diamond films with the largest size, the flattest surface, and the widest 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 promising to become the basic components of various high-performance structures and devices in different fields. The present invention will be further described in detail below in conjunction with specific embodiments. The examples given are only for clarifying the present invention and not for limiting the scope of the present invention. Figure 1 is a schematic diagram of the method for preparing a polycrystalline diamond film according to the present invention. The preparation process includes: I. CVD growth of polycrystalline diamond film; II. Cutting a part of the grown substrate and the polycrystalline diamond film to form a new boundary; III. Pasting a peeling tape for mechanical peeling to separate the polycrystalline diamond film from the substrate. Example 1 Manufacture of polycrystalline diamond film (1) Growth of polycrystalline diamond film on a silicon substrate Heteroepitaxial growth of diamond on a silicon substrate includes three steps: pretreatment of the substrate, deposition of diamond seeds, and CVD growth of polycrystalline diamond film. Pretreatment of the substrate: Pretreat the silicon surface with hydrogen plasma at a power of 1300 W, a chamber pressure of 35 Torr, and 300 sccm H 2Under the condition of air flow, a 2-inch silicon wafer was placed in a microwave plasma-assisted chemical vapor deposition (MPCVD) device for 10 minutes. Deposition of diamond seeds: Diamond seeds with a size less than 10 nm (purchased from Tokyo Chemical Industry Co., Ltd.) were mixed with dimethyl sulfoxide (DMSO), absolute ethanol, and acetone at 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 the silicon wafer, and the deposition parameters were: 500 r / min, adding 3 drops within 15 s, and then increasing the rotation speed to 4500 r / min for 110 s. Repeat three times. CVD growth of diamond film: The silicon wafer deposited with diamond seeds 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 with a thickness of about 447 was grown on the silicon substrate. (2) Cutting of polycrystalline diamond film and growth substrate Using a silicon wafer cutting machine or a diamond knife, a 2-inch silicon wafer with a polycrystalline diamond film grown on its surface obtained in step (1) was cut along a straight line at a distance of 2 - 5 mm from the edge on the back of the growth substrate, so as to form a new boundary, and a clear demarcation line where the polycrystalline diamond film is joined to the growth substrate appears at the cutting position. (3) Attachment of peeling tape A peeling tape with a size of 10 cm × 6 cm was attached to the growth surface of the polycrystalline diamond film, so that it completely covered the growth surface of the polycrystalline diamond film and extended beyond the edge at the cutting position of the polycrystalline diamond film to form a peeling operation end (peeling pull handle). (4) Peeling of polycrystalline diamond film Pulling the peeling operation end reserved in step (3) at a peeling angle of 40 - 60 degrees and a speed of 5 mm / s to peel the polycrystalline diamond film from the surface of the silicon wafer, so as to expose the nucleation surface of the polycrystalline diamond film. Figure 2 is a photograph taken during the operation of this embodiment. Among them, Figures 2(a), 2(b), and 2(c) are photographs of a 2-inch silicon wafer with a polycrystalline diamond film grown on its surface, the peeling process, the polycrystalline diamond film after peeling, and the silicon wafer after peeling, respectively. Example 2 - 5 Manufacture of polycrystalline diamond films with different thicknesses Polycrystalline diamond films with thicknesses of 200 nm, 400 nm, 800 nm, and 1000 nm were fabricated respectively according to a method similar to that of Example 1. Figure 3 is a photograph of the nucleation surface of polycrystalline diamond films with different thicknesses after peeling. It can be seen from the figure that polycrystalline diamond films with different thicknesses can almost be completely peeled off from the substrate, demonstrating the effectiveness of the peeling method of the present invention. To effectively evaluate their performance, the number of microcracks (generated during the peeling process) and integrity (the proportion of the growth substrate size) of these polycrystalline diamond films were statistically analyzed. Figure 4 is a statistical chart of the cracks and integrity of polycrystalline diamond films with different thicknesses prepared in Examples 2 - 5. The figures on the right are optical images magnified 50 times of polycrystalline diamond films with thicknesses of 200 nm (upper) and 1000 nm (lower) respectively. It can be seen from the figure that the crack density corresponding to the 200-nm thin film is significantly higher than that of the 1000-nm thin film. From these results, it can be seen that as the thickness decreases, the number of cracks increases, indicating that when the thickness is reduced to 200 nm, the PCD film has peeling properties similar to two-dimensional materials. At the same time, as the thickness is reduced to 200 nm, the integrity rate of the peeled film decreases from 100% to 99.43%, which is mainly related to the different adhesion forces of the growth substrate. Example 6 Fabrication of Optimized Polycrystalline Diamond Films Polycrystalline diamond films with a thickness of 1000 nm were fabricated according to a method similar to that of Example 1, except that before the substrate pretreatment, the surface of the silicon wafer was polished first to reduce its roughness from 1 nm to 0.5 nm. Morphology and Property Characterization of Polycrystalline Diamond Films 1. SEM characterization was performed on the growth surface, nucleation surface, and cross-section of polycrystalline diamond films with different thicknesses prepared in Examples 2 - 6. Figure 5 shows 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 shows SEM images of the growth surface (top) and nucleation surface (bottom) of polycrystalline diamond films with different thicknesses prepared in Examples 2-5. As can be seen from Figure 5, various large grains (hundreds of nanometers) exist on the growth surface, but the nucleation surface exhibits a flat morphology without any visible grains; this gradient change in grain size can also be seen from the cross-section. In addition, the SEM images in Figure 6 also show that as the thickness increases, the grains on the growth surface also increase, which forms a sharp contrast with the nucleation surface with unchanged surface morphology. This obvious difference in the growth surface morphology is mainly related to the growth mode of the PCD film. Specifically, before CVD growth, diamond particles with a size not greater than 10 nm are deposited on the silicon substrate as seeds; during the subsequent growth process, since the silicon substrate hinders the growth of diamond in the bottom direction, the diamond particles can only grow randomly upward and horizontally. 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. 2. The growth surface and nucleation surface of polycrystalline diamond films with different thicknesses prepared in Examples 2-6 were characterized by atomic force microscopy (AFM). Figure 7 shows AFM images of the growth surface (a) and nucleation surface (b) of a 1000-nm-thick polycrystalline diamond film prepared in Example 5 and the nucleation surface (c) of a 1000-nm-thick polycrystalline diamond film prepared in Example 6. From the AFM images in Figure 7, it can be seen that the surface roughness (Ra) of the polycrystalline diamond film in Example 5 decreased from 36.2 nm (growth surface) to 0.952 nm (nucleation surface). When a smoother growth substrate was used in Example 6, the surface roughness continued to decrease to 0.612 nm. This ultra-flat surface can be comparable to polished single-crystal diamond, meeting the requirements of high-precision nanomachining. In addition, compared with other strategies for exposing the nucleation surface, such as sacrificing the growth substrate, the method of the present invention avoids impurities caused by multiple operations. Therefore, this peeling method can maintain the original state of the nucleation surface without causing any damage. Figure 8 shows (a) the roughness statistics of the growth surface and nucleation surface of PCD films with different thicknesses prepared in Example 2-5 (statistics area 20 μm × 20 μm); (b) the comparison of the film thickness and roughness between the method of the present invention and other reported methods; and (c) the comparison of the film size and roughness between the method of the present invention and other methods. By comparing the roughness of the growth surface and nucleation surface of PCD films with different thicknesses, it can be seen that the roughness of the nucleation surface of both thicknesses is less than 1 nm, while the roughness of the growth surface continuously increases. Compared with the reported research on optimizing the surface roughness of diamond films, the peeling method of the present invention has obvious advantages both in terms of the thickness range and the size of the PCD film. Therefore, so far, it can be considered as the best method to obtain PCD films with the largest size, the flattest 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. 3. Use an optical ellipsometer to measure the refractive index of the growth surface and nucleation surface of the polycrystalline diamond film in Example 1. As shown in Figure 9, in the figure, n represents the refractive index and k represents the extinction coefficient. From the measurement results in Figure 9, 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. 4. Use X-ray diffraction (XRD) technology to characterize the crystal properties of the polycrystalline diamond film prepared in Example 1. As shown in Figure 10, obvious diffraction peaks are 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 the properties of diamond. The strong diffraction peak of the (111) crystal plane indicates its dominant position, which is consistent with that in other reports. In addition, the full width at half maximum (FWHM) of the diffraction peak of the (111) crystal plane on the nucleation surface is 0.64° narrower than that on the growth surface. In addition to the change in FWHM, the position of the diffraction peak of the (111) crystal plane shows an obvious shift (Figure 10, inset), from 44.36° (growth surface) to 43.96° (nucleation surface), which is close to the position (43.95°) of the standard (111)-oriented SCD. The shift of the diffraction peak indicates the improvement of the crystal quality, which may be related to the release of internal stress. Usually, due to the lattice mismatch between diamond and the heterogeneous substrate, residual stress will be introduced into the film, which will further induce lattice distortion, resulting in 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. 5. Use Raman spectroscopy to further characterize the crystal properties of the polycrystalline diamond film prepared in Example 1. From the Raman spectrum in Figure 11, at 1332.8 cm -1An obvious diamond scattering peak was observed, which further confirmed the state of the film. In addition, from the inset of Fig. 11, the FWHM of the nucleation surface (7.43 cm -1 )is narrower than that of the growth surface (7.67 cm -1 ), indicating that the internal crystallization of the film measured from the nucleation surface has been improved. It can be seen that in addition to the reduced roughness of the nucleation surface, according to the performance of Raman shift and X-ray diffraction, the crystallization quality has also been optimized compared with the growth surface. In addition, in terms of optical properties, the refractive index of the nucleation surface is lower than that of the growth surface and can be comparable to that of single crystals. Table 1 summarizes the comparison of the polycrystalline diamond films prepared by the method of the present invention with the previously reported polycrystalline diamond films in terms of surface roughness, thickness range and size (area of the film). Table 1 It can be seen from the data in Table 1 that the polycrystalline diamond film obtained by the method of the present invention has very low surface roughness, approaching the level of polished single-crystalline diamond bulk. In addition, the obtained thin films are superior to the previously reported works in terms of size and thickness. Example 7 Manufacture and characterization of a polycrystalline diamond film with a thickness of 4 μm According to a method similar to Example 1, a polycrystalline diamond film with a thickness of 4 μm was manufactured, and then the polycrystalline diamond film was directly attached to a flexible PDMS substrate for bending performance and electrical conductivity tests. Bending performance of the polycrystalline diamond film The polycrystalline diamond film peeled off by the method of the present invention can be deformed arbitrarily without being restricted by the rigid growth substrate. Fig. 12 shows a 360-degree bending mode photo (A) of a 4-μm-thick PCD film and a winding photo (B) on cylinders with different radii. As shown in Fig. 12, for the film with a thickness of about 4 μm prepared in Example 7, it can support a 360° bending mode, and the tolerable radius of curvature can be reduced to 2 mm. And the film remains stable after more than 2500 bending cycles. Fatigue test of the polycrystalline diamond film A polycrystalline diamond film with a thickness of 4 μm manufactured by the method of Example 1 was pasted on a flexible PDMS substrate, and it was subjected to a 2% deformation cycle. Figure 19(a) is a photograph after undergoing > 10,000 deformation cycles under 2% strain; Figure 19(b) is a 5-fold magnified optical image corresponding to Region 1 and Region 2 in Figure 19(a). It can be seen that the polycrystalline diamond film of the present invention can withstand > 10,000 deformation cycles under 2% strain without any damage. Compared with the maximum strain of previously reported polycrystalline diamond films occurring in microscale samples, these incredible diamond bending and fatigue resistance properties were first observed at the macroscale. According to previous theoretical predictions of single-crystalline diamond nanocolumns, the energy band would gradually decrease with the increase of strain. However, due to the complex crystal structure inside diamond, this principle has been rarely verified in theory and experiments. Fortunately, through a simple peeling strategy, the present invention can observe the dynamic characteristics of independent polycrystalline diamond films. Conductive properties of polycrystalline diamond films and their energy band modulation In order to investigate the conductive properties of polycrystalline diamond films under different bending degrees, the inventors adhered a 4-μm-thick polycrystalline diamond film prepared in Example 7 on a 1-mm-thick flexible PDMS (polydimethylsiloxane) substrate, and then successively pasted soft conductive tapes. By controlling the deformation of the PDMS substrate, the bending operation of the polycrystalline diamond film can be realized. 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. According to the calculation results shown in Figure 13, it can be seen that the strain distribution is uneven, and the maximum deformation occurs at the center position whether on the top or bottom surface of the PDMS. Figure 14 shows the conductivity changes 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 was observed that the film has excellent piezoresistive properties. In both bending modes, the resistance gradually decreases with the increase of strain, but the change amplitude of the resistance in the downward pressure mode is greater than that in the upward pull mode. In addition, during the loading-unloading cycle, the resistance can be completely restored. The above results indicate that the energy band structure of polycrystalline diamond films can be modulated by strain. As the strain degree increases, the energy band of diamond gradually decreases, so the conductivity increases. Generally, the modulation of the 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 modulating the diamond energy band by strain proposed in the present invention greatly reduces the preparation difficulty and production cost, and provides convenience for the development of flexible diamond semiconductor devices and diamond-based sensing devices. Example 8 Material properties of polycrystalline diamond film Figure 15 shows the Raman spectrum (a), and the XPS measurement results (b) and (c) of the polycrystalline diamond film obtained in Example 5 above. Figure 16 is a summary of the contact angle experiment photos and data of the growth surface and nucleation surface of PCD films with different thicknesses. Figure 17 shows the friction coefficient of the growth surface and nucleation surface of the polycrystalline diamond film obtained in Example 5 (a), and the relationship curve between the external force acting on the growth surface and nucleation surface and the formed indentation depth (b). Figure 18 shows 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. The above results show that, compared with the growth surface, the nucleation surface has a higher refractive index and a lower extinction coefficient. The refractive index of the nucleation surface is comparable to that of single-crystalline bulk diamond. Moreover, as the thickness increases, the refractive indices of both the growth surface and the nucleation surface decrease, while the extinction coefficient increases. Such diamond films with high refractive index and low absorption coefficient provide possibilities for the development of diamond-based micro-nano optical devices, such as optical superlenses, optical modulators, etc. Example 9 Influence of different peeling angles According to a method similar to that of Example 1, polycrystalline diamond films with different thicknesses were fabricated, and films with different thicknesses were peeled at a constant rate (10 mm / min) and different peeling angles (20° - 90°). In addition, the possible crack propagation was simulated by the phase field method. For the specific introduction of the phase field method, please refer to the following two documents: Miehe, C., Hofacker, M. & Welschinger, F. A phase field model for rate-independent crack propagation: Robust algorithmic implementation based on operator splits. Computer Methods in Applied Mechanics and Engineering 199, 2765 - 2778 (2010). Spatschek, R., Brener, E. & Karma, A. Phase field modeling of crack propagation. Philosophical Magazine 91, 75-95 (2011). By calculating the crack density on the peeled film, it is found that the peeling quality depends on the matching of the peeling angle and the film thickness. For thicker films (800 nm and 1000 nm), crack-free films can be obtained using a wide range of peeling angles (20° - 90°); when the film thickness is 600 nm, the range of peeling angles with the fewest cracks is reduced to 40° - 70°. When the film continues to become thinner, the operating window continues to shrink. Phase field method simulations were carried out for the peeling of 600 nm thick polycrystalline diamond films at different peeling angles of 30°, 60° and 90°. The initial sizes of the cracks were set to be 56 nm (upper surface) and 1.2 nm (lower surface) in the phase field simulations, with 95% of the cracks being smaller than these sizes on the corresponding surfaces. The simulation results show that at a small peeling angle of 30°, the separated film undergoes slight bending deformation, and both the lower and upper surfaces of the film are under tensile stress, ultimately leading to the propagation of large cracks starting from the upper surface. In contrast, when the peeling angle is large (90°), the upper surface (concave side) of the film is under compression, thus preventing possible crack growth. However, on the lower surface (convex side) of the highly deformed film, the elevated tensile stress triggers the propagation of small initial cracks there. Therefore, when peeling at a medium peeling angle (60°), crack propagation does not occur within the 600 nm thick film. For thicker films (700 nm to 1000 nm), simulations show that peeling angles between approximately 30° and 90° can avoid crack propagation. These results are consistent with the observed crack density of different peeled films. Therefore, they can be used to guide the large-scale production of crack-free films. Example 10 Integrity and quality evaluation of polycrystalline diamond films The initial resistance of the growth surface of the polycrystalline diamond film prepared in Example 1 was measured to be approximately 104 Ω. Since the resistance of the diamond film is determined by its integrity and quality, to evaluate the potential impact of the peeling process on the film quality, a wafer array was fabricated on a 2-inch diamond film, and each in the array contains a pair of gold electrodes. The resistance changes before and after peeling were measured. Figure 21 shows the photographs (a) before peeling and (b) after peeling and the resistance changes (c) before and after peeling of the wafer array fabricated on a 2-inch polycrystalline diamond film. As can be seen from Figure (c) of Figure 21, a relatively uniform resistance distribution is observed, indicating the consistency of the film of the present invention over the entire 2-inch diamond film. After peeling, the resistance at each position remains almost the same (the overall resistance decreases, which may be related to the detachment from the silicon substrate). This result shows that the polycrystalline diamond film of the present invention has good integrity and high quality. Example 11 Micro / Nano Fabrication on Polycrystalline Diamond Films Electron beam lithography (EBL) and plasma etching techniques were used to prepare diamond micro / nano structures on the growth surface and nucleation surface of the polycrystalline diamond film obtained in Example 1, respectively. The results are shown in Figure 22. By comparing the micro / nano structures obtained on the growth surface and nucleation surface, it can be seen that the residues on the nucleation surface are significantly less and the structural accuracy is higher. Example 12 Application of Polycrystalline Diamond Films as Quantum Platforms A polycrystalline diamond film was fabricated in a similar manner to Example 1, except that nitrogen gas was introduced during the CVD growth process to create special atomic defects, called nitrogen-vacancy (NV) color centers, on the film. The diamond film was excited with a 532 nm laser (Changchun New Industry Co., MGL-FN-5321W). The laser was focused on the diamond film through a 40× air objective. The excited NV fluorescence was filtered with a 605 nm long-pass filter and imaged on an EMCCD (Teledyne Photometrics, Evolve 512 Delta), as shown in Figure (a) of Figure 23. The inset in the fluorescence image shows the bright spots corresponding to the emission of the NV color centers. The peak at approximately 640 nm represents the zero phonon line of NV-. The 532 nm laser was kept constant throughout the measurement, and optically detected magnetic resonance (ODMR) measurements were performed. Triggered by an external pulse, the MW (microwave) frequency was linearly scanned from 2800 to 2940 MHz. The ODMR spectral results shown in Figure (b) of Figure 23 indicate that under the action of an external magnetic field, the spin states (±1) of the NV color centers can be effectively separated. Therefore, the polycrystalline diamond film provided by the present invention has great application potential in quantum technology. None Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, where: Figure 1 is a schematic diagram of a method for preparing a polycrystalline diamond film according to the present invention. Figures 2(a), 2(b), and 2(c) are photographs of a 2-inch silicon wafer with a polycrystalline diamond film grown on its surface (a), the peeling process (b), the polycrystalline diamond film after peeling and the silicon wafer after peeling (c) in Example 1 in sequence. Figure 3 is a photograph of the nucleation surface of polycrystalline diamond films with different thicknesses prepared in Examples 2-5. Figure 4 is a statistical chart of cracks and integrity of polycrystalline diamond films with different thicknesses prepared in Examples 2-5. Figure 5 is an SEM image of the growth surface (a), nucleation surface (b), and cross-section (c) of a 1000-nm-thick polycrystalline diamond film prepared in Example 5. Figure 6 is an SEM image of the growth surface (upper) and nucleation surface (lower) of polycrystalline diamond films with different thicknesses prepared in Examples 2-5. Figure 7 is an AFM image of the growth surface (a) and nucleation surface (b) of a 1000-nm-thick polycrystalline diamond film prepared in Example 5 and the nucleation surface (c) of a 1000-nm-thick polycrystalline diamond film prepared in Example 6. Figure 8 is (a) a roughness statistic of the growth surface and nucleation surface of PCD films with different thicknesses prepared in Examples 2-5 (statistical area 20 μm × 20 μm); (b) a comparison of the present invention method with other reported methods in terms of film thickness and roughness; and (c) a comparison of the present invention method with other methods in terms of film size and roughness. Figure 9 is a comparison of the refractive indices of the growth surface (a) and nucleation surface (b) of the polycrystalline diamond film prepared in Example 1. Figure 10 is the XRD results of the growth surface (a) and nucleation surface (b) of the polycrystalline diamond film prepared in Example 1. Figure 11 is the Raman spectra of the growth surface (a) and nucleation surface (b) of the polycrystalline diamond film prepared in Example 1. Figure 12 is a photograph of the 360-degree bending mode (a) and a photograph of winding on cylinders with different radii (b) of a 4-μm-thick polycrystalline diamond film prepared in Example 7. Figure 13 is different bending modes (a) of a 4-μm-thick polycrystalline diamond film prepared in Example 7 on a flexible PDMS substrate and the corresponding strain distribution on the surface (b). Figure 14 is the change in conductivity of a 4-μm-thick polycrystalline diamond film prepared in Example 7 under different strains (a) and bending cycles (b). Figure 15 is the Raman spectrum (a) and XPS measurement results (b and c) of the polycrystalline diamond film prepared in Example 5. Figure 16 is a photograph of the contact angle experiment and data summary of the growth surface and nucleation surface of PCD films with different thicknesses. Figure 17 is the friction coefficient of the growth surface and nucleation surface (a) of the polycrystalline diamond film prepared in Example 5 and the relationship curve between the external force acting on the growth surface and nucleation surface and the indentation depth formed (b).FIG. 18 shows 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. FIG. 19 shows the fatigue test results of the polycrystalline diamond film of the present invention. Among them, (a) is a photograph after undergoing > 10,000 deformation cycles at 2% strain; (b) is a 5-fold magnified optical image corresponding to regions 1 and 2 in (a). FIG. 20 shows the comparison between the experimental statistics (a) of the microcrack density on different thickness films peeled at different peeling angles in Example 9 of the present invention and the probability (b) of crack propagation on different thickness films at different peeling angles calculated by simulation. FIG. 21 shows the photographs before (a) and after (b) peeling and the resistance change (c) before and after peeling after fabricating a wafer array on a 2-inch polycrystalline diamond film in Example 10 of the present invention. FIG. 22 shows the scanning electron microscope images of micro / nanofabrication on the growth surface and nucleation surface of a polycrystalline diamond film respectively in Example 11 of the present invention. FIG. 23 shows the fluorescence spectrum (a) of the NV color centers in the polycrystalline diamond film prepared in Example 12 of the present invention; and the results of optically detected magnetic resonance (ODMR) test of the polycrystalline diamond film with NV color centers (b).

Claims

1. A method for large-scale manufacturing of ultra-flat polycrystalline diamond films, the method comprising: (1) A polycrystalline diamond film is grown on a growth substrate with diamond seed crystals 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) At least a portion of the growth substrate on which the polycrystalline diamond film is grown is cut off, the cutting operation forming a boundary line between the polycrystalline diamond film and the growth substrate at the cut point; (3) A release tape is adhered to the growth surface of the polycrystalline diamond film, wherein the release tape extends beyond the polycrystalline diamond film at least at the edge of the cut point to form a release operation end; (4) The release operation end is pulled to peel the polycrystalline diamond film off the surface of the growth substrate to expose the nucleation surface of the polycrystalline diamond film, wherein the exposed nucleation surface has a second roughness, and the second roughness is less than the first roughness.

2. The method as described in request item 1, wherein, The first roughness is 10~200 nm; the second roughness is 0.5~5 nm.

3. The method as described in claim 2, wherein, The first roughness is 20~50 nm; the second roughness is 0.5~2 nm.

4. The method as described in request item 3, wherein, The second roughness is 0.5~1 nm.

5. The method as described in request item 1, wherein, The diamond seed crystals mentioned in step (1) have a particle size of 2~10 nm.

6. The method as described in request item 1, wherein, In step (1), a microwave plasma-assisted chemical vapor deposition apparatus is used to grow polycrystalline diamond films.

7. The method as described in request item 1, wherein, The deposition temperature of the chemical vapor deposition method described in step (1) is 800~1000℃.

8. The method as described in request item 1, wherein, The deposition temperature of the chemical vapor deposition method described in step (1) is 850~950℃.

9. The method as described in claim 1, wherein, The growth substrate is selected from one or more of Si, SiC, TiC, Co, Pt, Al2O3, Ni, Re, Ir, SiO2 and Mo.

10. The method as described in claim 1, wherein, The surface roughness of the growth substrate is less than 2 nm.

11. The method as described in claim 10, wherein, The surface roughness of the growth substrate is less than 1 nm.

12. The method as described in claim 11, wherein, The surface roughness of the growth substrate is less than 0.5 nm.

13. The method as described in claim 1, wherein, The thickness of the polycrystalline diamond film is 200 nm to 100 μm.

14. The method as described in claim 13, wherein, The thickness of the polycrystalline diamond film is 200 nm to 10 μm.

15. The method as described in claim 14, wherein, The thickness of the polycrystalline diamond film is 400 nm to 5 μm.

16. The method as described in claim 1, wherein, The cutting operation described in step (2) is to cut perpendicular to the growth surface of the polycrystalline diamond film.

17. The method as described in claim 1, wherein, The cutting operation in step (2) ensures that the length of the boundary line between the formed polycrystalline diamond film and the growth substrate is at least 1% of the perimeter of the growth surface of the polycrystalline diamond film.

18. The method as described in claim 17, wherein, The cutting operation in step (2) ensures that the length of the boundary line between the formed polycrystalline diamond film and the growth substrate is at least 5% to 20% of the perimeter of the growth surface of the polycrystalline diamond film.

19. The method as described in claim 18, wherein, The cutting operation in step (2) ensures that the length of the boundary line between the formed polycrystalline diamond film and the growth substrate is at least 10% to 12% of the perimeter of the growth surface of the polycrystalline diamond film.

20. The method as described in claim 1, wherein, 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.

21. The method as described in claim 1, wherein, In step (4), the speed at which the polycrystalline diamond film is peeled off the growth substrate surface by pulling the peeling operation end is 1~10 mm / s.

22. The method as described in claim 1, wherein, The peeling angle in step (4) is 10 to 90 degrees.

23. The method as described in claim 1, wherein, The nucleation surface has a higher refractive index than the growth surface.

24. The method as described in claim 1, wherein, The nucleation surface has a lower extinction coefficient than the growth surface.

25. The method as described in claim 1, wherein, Compared to 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 full width at half maximum (FWHM) of the (111) crystal plane XRD diffraction peak of the nucleation surface is smaller than that of the (111) crystal plane XRD diffraction peak of the growth surface.

Images