Scalable method for fabricating ultra-flat polycrystalline diamond films

Abstract

A scalable method for producing an ultraflat polycrystalline diamond film, comprising the following steps: (1) growing a polycrystalline diamond film on a growth substrate having diamond seeds on its surface using chemical vapor deposition; (2) cutting a portion of the growth substrate having the grown polycrystalline diamond film to form a boundary at the cut portion where the polycrystalline diamond film is bonded to the growth substrate; (3) applying a release tape; and (4) peeling the polycrystalline diamond film from the surface of the growth substrate and pulling the peeling end to expose the nucleating surface of the polycrystalline diamond film. Here, the exposed nucleating surface has a second roughness, which is less than the first roughness. By inverting the nucleating surface of the grown polycrystalline diamond film, a novel and simple approach is provided for producing inexpensive, large-area, ultraflat, and transmissible polycrystalline diamond films with great practical potential.

Description

Technical Field

[0001] The present invention belongs to the technical field of the manufacturing process of polycrystalline diamond films, and particularly relates to a scalable manufacturing method for polycrystalline diamond films having a super flat surface.

Background Art

[0002] Due to its excellent physical and chemical properties, high-quality synthetic diamond materials have long been considered excellent options for use in the fields of mechanics, electronics, and photonics. To date, although the synthesis of diamond materials has been successful, it is still difficult to obtain large-scale, high-quality ultra-thin diamond films.

[0003] Doped diamond can further expand the use of diamond in electronic devices such as boron-doped p-type semiconductors and superconductors, as well as in quantum photonics based on atomic defects doped with nitrogen, silicon, etc. In particular, the so-called next-generation diamond substrates, for example, the films, are expected to play an important role in the effective heat dissipation of electric circuits and high-power electronic devices, especially in emerging soft / flexible electronics.

[0004] The realization of the above diamond-based functional units necessarily requires standard nanomanufacturing and seamless integration, all of which impose higher requirements on the surface roughness of diamond materials. Furthermore, the flatness of the surface of the fabricated device is also a prerequisite for many test / characteristic evaluation methods widely used in related fields such as thermal conductivity measurement, and bonding in the semiconductor industry.

[0005] However, unlike other materials such as silicon, diamond materials are very difficult to planarize, and the use of conventional methods such as polishing is time-consuming and energy-consuming.

[0006] Diamond films are primarily obtained through uniform and heterogeneous growth using chemical vapor deposition (CVD) technology. Single-crystal diamond (SCD) films are mainly obtained through uniform growth on bulk diamond substrates, which represents the highest quality required for many applications. For example, their high electron and hole mobility, as well as excellent radiation resistance, make them extremely useful as detectors in nuclear and high-energy physics experiments. However, the high cost of growth, limited scalability, and the complexity of processing CVD-grown SCDs have hindered their widespread adoption.

[0007] While heterogeneous growth of SCD on iridium has been demonstrated in a very small number of laboratories worldwide, controlling the quality of the resulting diamond remains difficult, and polycrystalline diamond (PCD) films tend to form due to lattice mismatch. Therefore, establishing large-area SCD films on arbitrary substrates remains extremely challenging.

[0008] On the other hand, scalable, inexpensive, and easily grown polycrystalline diamond films are readily available on a variety of common substrates, such as Si, SiC, TiC, Co, Pt, Al2O3, Ni, and Re. In particular, it is possible to form continuous films based on a sufficiently high density of atomic nuclei, and their quality is closely related to the non-orientation of the atomic nuclei. To optimize polycrystalline diamond films, several researchers have proposed the concepts of nanocrystalline and ultra-nanocrystalline diamond films by depositing diamond seeds to form continuous nuclei. However, the insufficient surface smoothness of polycrystalline diamond films is one of the biggest obstacles to advancing such an excellent platform.

[0009] In summary, on the one hand, the extremely stringent growth conditions for single-crystal diamond (SCD) significantly hinder the use of diamond materials on a large scale and within cost-acceptable limits, while on the other hand, polycrystalline diamond (PCD) is severely limited in its ability to further construct high-precision photonic structures at the nano / micron scale due to insufficient surface morphology resulting from profile non-uniformity and high roughness. [Overview of the project] [Problems that the invention aims to solve]

[0010] Therefore, the object of the present invention is to solve the existing problems of insufficient surface morphology and high roughness of polycrystalline diamond (PCD) films, and to provide a simple approach for fabricating large-area, ultra-flat, and transferable PCD films in a scalable and controllable manner. [Means for solving the problem]

[0011] In a first embodiment, the present invention relates to a method for fabricating ultraflat polycrystalline diamond films on a large scale, (1) A step of growing a polycrystalline diamond film on a growth substrate having diamond seeds on its surface by chemical vapor deposition, wherein the polycrystalline diamond film includes a growth surface having a first roughness (also called the "growth surface") and a nucleation surface bonded to the growth substrate (also called the "nucleation surface"), (2) A step of cutting at least a portion of the growth substrate having a polycrystalline diamond film grown on its surface to form a boundary at the cut portion where the polycrystalline diamond film is bonded to the growth substrate, (3) A step of attaching a release tape to the growth surface of a polycrystalline diamond film, wherein the release tape extends beyond the polycrystalline diamond film at least at the edge of the cut portion to form an end for the release work, (4) A step of peeling a polycrystalline diamond film from the surface of a growth substrate and pulling an end for peeling operation to expose the nucleating surface of the polycrystalline diamond film, wherein the exposed nucleating surface has a second roughness, and the second roughness is less than the first roughness. This provides a method that includes [something].

[0012] According to the method of the present invention, the first roughness is 10 to 200 nm, for example, 20 to 50 nm. In a preferred embodiment of the present invention, the second roughness is 0.5 to 5 nm, preferably 0.5 to 2 nm, more preferably 0.5 to 1 nm.

[0013] According to the method of the present invention, the diamond seed in step (1) may have a particle size of 2 to 10 nm. In some embodiments of the present invention, the growth substrate may be one or more selected from the group consisting 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 chemical vapor deposition (MPCVD) apparatus is used for growing the polycrystalline diamond film in step (1). In some embodiments of the present invention, the polycrystalline diamond film may have a thickness of 200 nm to 100 μm, preferably 200 nm to 10 μm, and more preferably 400 nm to 5 μm.

[0014] According to the method of the present invention, the deposition temperature of the chemical vapor deposition method in step (1) may be 800 to 1000°C, more preferably 850 to 950°C.

[0015] According to the method of the present invention, the cutting operation in step (2) is preferably carried out perpendicular to the growth surface of the polycrystalline diamond film. The cutting operation can be carried out along a straight line, a curve, or an arc. In a preferred embodiment of the present invention, the cutting operation is carried out such that the length of the boundary formed where the polycrystalline diamond film bonds 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%.

[0016] In a preferred embodiment of the present invention, the cutting operation in step (2) may be performed along a straight line to facilitate large-scale fabrication. When the growth substrate is circular, the length of the boundary formed by cutting where the polycrystalline diamond film bonds to the growth substrate is preferably 20% to 50% of the substrate diameter.

[0017] The inventors of this invention have found through numerous experimental studies that the cutting operation in step (2) can reduce the adhesive force caused by excessive coating of the polycrystalline diamond film on the sidewall of the growth substrate. Starting from the cut section, the polycrystalline diamond film can be gradually removed from the growth substrate with the help of release tape, thereby obtaining an intact polycrystalline diamond film, and the growth substrate after peeling can be recycled for further use.

[0018] According to the method of the present invention, the release tape in step (3) may be 3M tape, PVC tape, PE tape, etc. In the embodiment of the present invention, the release tape completely covers the polycrystalline diamond film remaining after the cutting operation, and leaves at least a margin at the cut edge to facilitate subsequent peeling operations.

[0019] Through a number of experimental studies, the inventor of the present invention has found that the peeling performance of PCD films with different thicknesses is related to the mechanical strength, but is also affected by differences in the peeling operation (for example, speed, peeling angle). A reasonable peeling operation within the allowable range of mechanical strength is a prerequisite for obtaining a large-area and intact film with few cracks.

[0020] In the present invention, the peeling angle is the angle between the peeling force that pulls the end for the peeling operation to separate the polycrystalline diamond film from the growth substrate and the growth surface of the polycrystalline diamond film (the starting position of the end for the peeling operation of the peeling tape is defined as 0°). In particular, the range of the peeling angle for obtaining a polycrystalline diamond film that is intact and has few cracks is called the safe peeling angle.

[0021] In the present invention, intactness is defined as the ratio of the area of the polycrystalline diamond film obtained by peeling to the area of the growth substrate after peeling.

[0022] According to the method of the present invention, the speed at which the end for the peeling operation is pulled to peel the polycrystalline diamond film from the surface of the growth substrate in step (4) may be 1 to 10 mm / s. In a preferred embodiment of the present invention, the peeling angle of the peeling operation in step (4) may be 10 to 90 degrees.

[0023] According to the method of the present invention, the nucleation surface has a higher refractive index than the growth surface. Preferably, the nucleation surface has a lower attenuation coefficient than the growth surface.

[0024] The polycrystalline diamond film produced by the method of the present invention has optical parameters at the same level as those of SCD grown by the CVD method.

[0025] According to the method of the present invention, the position of the XRD diffraction peak of the (111) crystal plane of the nucleation surface is closer to the position of the (111) crystal plane of the standard single crystal diamond compared to the position of the (111) crystal plane 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 smaller than the full width at half maximum of the (111) crystal plane of the growth surface. The shift of the XRD diffraction peak indicates an improvement in crystal quality, and the narrowing of the full width at half maximum of the Raman spectrum means an improvement in the internal crystallization of the film.

[0026] According to the method of the present invention, the above method can further include a step of removing the polycrystalline diamond film by reducing the adhesiveness of the peeling tape with a solvent, or a step of removing the polycrystalline diamond film from the peeling tape by heating using a heat releasable peeling tape.

[0027] In a second aspect, the present invention provides a method for modulating the energy band of a diamond film, including a step of attaching a polycrystalline diamond film produced by the present invention to the surface of a flexible substrate, a step of stretching and / or compressing the polycrystalline diamond film by bending the flexible substrate, and a step of realizing modulation of the energy band composition of the polycrystalline diamond film due to the strain generated by the polycrystalline diamond film during the above stretching and / or compression.

Advantages of the Invention

[0028] The growth surface of the PCD film has been reported in many studies, but the state of the nucleation surface has rarely been mentioned because it is difficult to expose to the public vision. The present invention first provides a novel and simple method for obtaining the nucleation surface of the PCD film in situ without causing damage, which can be used to produce large-area, ultra-flat and transferable polycrystalline diamond films in a large-scale and controlled manner. The effectiveness of the method of the present invention has been verified by a detailed characterization of the surface morphology and material properties of the nucleation surface.

[0029] Compared to other methods for exposing the nucleated surface (e.g., the method of sacrificing the growth substrate described in the previous Chinese Patent Application No. 202310540694.7), the method of the present invention reduces impurities resulting from multiple operations (e.g., grinding, etching). Therefore, this exfoliation method can preserve the original state of the nucleated surface without causing damage. Characterization of the roughness of the growth surface and nucleated surface of PCD films of different thicknesses shows that the roughness of the nucleated surface of various thicknesses can be less than 1 nm, and the roughness of the growth surface increases with thickness. Compared to reported studies on the optimization of surface roughness of diamond films, the exfoliation method of the present invention has clear advantages in both the range of film thickness and size. Therefore, to date, the method of the present invention can be considered the best method for obtaining PCD films with the maximum size, the flattest surface, and the widest thickness range. The novel, scalable, and controllable strategy proposed by the present invention opens the way to the development of high-quality, cost-effective polycrystalline diamond films, which are expected to become fundamental components of various high-performance structures and devices in diverse fields.

[0030] Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. [Brief explanation of the drawing]

[0031] [Figure 1] This is a schematic diagram of the method for preparing a polycrystalline diamond film according to the present invention. [Figure 2] The images show, in order, a 2-inch silicon wafer with a polycrystalline diamond film grown on its surface in Example 1 (a), the peeling process (b), and the polycrystalline diamond film and silicon wafer after peeling (c). [Figure 3] These are photographs of the nucleation surfaces of polycrystalline diamond films of different thicknesses prepared in Examples 2-5. [Figure 4] This is a statistical diagram of cracking and integrity of polycrystalline diamond films of different thicknesses prepared in Examples 2-5. [Figure 5]These are SEM images 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] These are SEM images of the growth surface (top) and nucleation surface (bottom) of polycrystalline diamond films of different thicknesses prepared in Examples 2-5. [Figure 7] These are 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. [Figure 8] This figure shows (a) statistics of the roughness of the growth surface and nucleation surface (statistical area: 20 μm × 20 μm) of PCD films of different thicknesses prepared in Examples 2 to 5, (b) a comparison of the method of the present invention with other reported methods regarding film thickness and roughness, and (c) a comparison of the method of the present invention with other reported methods regarding film size and roughness. [Figure 9] This figure shows a comparison of the refractive indices of the growth surface (a) and the nucleation surface (b) of the polycrystalline diamond film prepared in Example 1. [Figure 10] This figure shows a comparison of the XRD of the growth surface and nucleation surface of the polycrystalline diamond film prepared in Example 1. [Figure 11] This figure shows the Raman spectra of the growth surface and nucleation surface of the polycrystalline diamond film prepared in Example 1. [Figure 12] These are photographs of the 4 μm thick polycrystalline diamond film prepared in Example 7, in a 360-degree bent state (a) and in a state where it is wrapped around cylinders of different radii (b). [Figure 13] This figure shows various bending modes (a) and corresponding surface strain distributions (b) of a 4 μm thick polycrystalline diamond film prepared on a flexible PDMS substrate in Example 7. [Figure 14] This figure shows the change in conductivity of the 4 μm thick polycrystalline diamond film prepared in Example 7 under different strain (a) and bending cycles (b). [Figure 15]This figure shows the Raman spectrum (a) and XPS measurement results (b and c) of the polycrystalline diamond film prepared in Example 5. [Figure 16] This figure shows photographs and a summary of data from contact angle experiments on the growth and nucleation surfaces of PCD films of different thicknesses. [Figure 17] This figure shows the coefficient of friction of the growth surface and nucleation surface of the polycrystalline diamond film prepared in Example 5 (a), and a curve (b) showing the relationship between the external force applied to the growth surface and nucleation surface and the depth of the formed indentation. [Figure 18] This figure shows the refractive index (a) and extinction coefficient (b) of the growth surface and nucleation surface of a 500 nm thick polycrystalline diamond film, and the refractive index (c) and extinction coefficient (d) of the growth surface and nucleation surface of a 1000 nm thick polycrystalline diamond film. [Figure 19] This figure shows the fatigue test results of the polycrystalline diamond film of the present invention, where panel (a) is a photograph after undergoing >10,000 deformation cycles with a 2% strain; and panel (b) is a 5x optical image of region 1 and region 2 of panel (a), respectively. [Figure 20] This figure shows experimental statistics (a) of the microcrack density on films of different thicknesses that underwent delamination at different delamination angles, and the probability of crack propagation on films of different thicknesses that underwent delamination at different delamination angles, obtained by simulation calculations, in Example 9 of the present invention. [Figure 21] This figure shows photographs of a chip array fabricated on a 2-inch polycrystalline diamond film in Example 10 of the present invention, before (a) and after (b) peeling, and a diagram showing the change in resistance before and after peeling (c). [Figure 22] These are scanning electron microscope images of micro / nano fabrication performed on the growth surface and nucleation surface of a polycrystalline diamond film, respectively, in Example 11 of the present invention. [Figure 23] This figure shows the fluorescence spectrum of the NV color center of the polycrystalline diamond film prepared in Example 12 of the present invention (a), and the photodetector magnetic resonance (ODMR) test results of the polycrystalline diamond film having the NV center (b). [Modes for carrying out the invention]

[0032] The present invention will be described in further detail below in relation to specific embodiments, but the examples are given for illustrative purposes only and are not intended to limit the scope of the invention.

[0033] Figure 1 is a schematic diagram of the method for preparing the polycrystalline diamond film of the present invention, and the preparation procedure includes the following: I. Growing polycrystalline diamond films by CVD; II. After growth, a portion of the substrate and the polycrystalline diamond film is cut to form a new boundary; III. To separate the polycrystalline diamond film from the substrate, a release tape for mechanical peeling is applied. [Examples]

[0034] Example 1 Fabrication of polycrystalline diamond films (1) Growth of polycrystalline diamond film on a silicon substrate The heteroepitaxial growth of diamond on a silicon substrate involves three steps: substrate pretreatment, diamond seed deposition, and CVD growth of the polycrystalline diamond film. Substrate pretreatment: The silicon surface was pretreated with hydrogen plasma. Specifically, a 2-inch silicon wafer was placed in a microwave plasma chemical vapor deposition (MPCVD) apparatus for 10 minutes under conditions of 1300 W output, a cavity pressure of 35 Torr, and an H2 gas flow of 300 sccm. Diamond Seed Deposition: Diamond seeds smaller 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 sonicated for 12 hours to disperse, and then centrifuged at a rotation speed of 1000 rpm for 20 minutes to separate impurities. The suspension was spin-coated onto a silicon wafer (deposition parameters: 500 rpm, 3 drops added within 15 seconds, then rotation speed increased to 4500 rpm for 110 seconds). This procedure was repeated three times. CVD growth of diamond films: A silicon wafer with a diamond seed deposited was placed in an MPCVD apparatus (Seki 6350) to grow a diamond film. The main growth parameters were microwave power of 3400W, temperature of 900°C, methane flow rate of 15 sccm, and growth time of 40 minutes, resulting in the growth of a polycrystalline diamond film with a thickness of approximately 447 nm on a silicon substrate.

[0035] (2) Cutting of polycrystalline diamond film and growth substrate A 2-inch silicon wafer having a polycrystalline diamond film grown on its surface, prepared in step (1), was cut along a straight line at a distance of 2 to 5 mm from the edge of the back surface of the growth substrate using a silicon wafer cutter or diamond knife, thereby creating a new boundary and producing a clear separation line at the cut portion where the polycrystalline diamond film is bonded to the growth substrate.

[0036] (3) Application of release tape A release tape measuring 10 cm x 6 cm was attached to the growth surface of the polycrystalline diamond film, so that the tape completely covered the growth surface of the polycrystalline diamond film and extended beyond the edge of the polycrystalline diamond film at the cut end, forming a peeling end (a tab to pull during peeling).

[0037] (4) Peeling off of polycrystalline diamond film The peeling end prepared in step (3) was pulled at a peeling angle of 40 to 60 degrees and a speed of 5 mm / s to peel the polycrystalline diamond film from the silicon wafer surface and expose the nucleation surface of the polycrystalline diamond film.

[0038] Figure 2 shows photographs taken during the process of this embodiment. In Figure 2, panels (a), (b), and (c) are, in order, photographs of a 2-inch silicon wafer with a polycrystalline diamond film grown on its surface, the delamination process, and the polycrystalline diamond film and silicon wafer after delamination.

[0039] Examples 2-5 Fabrication of polycrystalline diamond films of different thicknesses Polycrystalline diamond films with thicknesses of 200 nm, 400 nm, 800 nm, and 1000 nm were fabricated using the same method as in Example 1. Figure 3 shows photographs of the nucleation surfaces of the polycrystalline diamond films of different thicknesses after peeling. As can be seen from the figure, the polycrystalline diamond films of different thicknesses can be peeled almost completely from the substrate, demonstrating the effectiveness of the peeling method of the present invention.

[0040] To effectively evaluate the performance of these polycrystalline diamond films, statistics were performed on the number of microcracks (generated during the delamination process) and their integrity (as a percentage of the size of the growing substrate). Figure 4 shows statistical data on cracks (cracks / square inch) and integrity of polycrystalline diamond films of different thicknesses prepared in Examples 2-5. The right panel shows 50x optical images of polycrystalline diamond films with thicknesses of 200 nm (top) and 1000 nm (bottom). As can be seen from the figure, the crack density corresponding to the 200 nm film was significantly higher than that corresponding to the 1000 nm film. These results show that the number of cracks increases as the thickness decreases. This indicates that the delamination performance of the PCD film is similar to that of the two-dimensional material when the thickness is reduced to 200 nm. At the same time, when the thickness is reduced to 200 nm, the integrity of the delaminate film decreases from 100% to 99.43%. This is mainly related to the difference in adhesion strength with the growing substrate.

[0041] Example 6 Fabrication of optimized polycrystalline diamond films A 1000 nm thick polycrystalline diamond film was fabricated in the same manner as in Example 1, except that the surface of the silicon wafer was polished before substrate pretreatment to reduce the roughness from 1 nm to 0.5 nm.

[0042] Evaluation of the morphology and properties of polycrystalline diamond films 1. The growth surface, nucleation surface, and cross-section of polycrystalline diamond films of different thicknesses prepared in Examples 2-6 were characterized by SEM.

[0043] Figure 5 shows SEM images of the growth surface (a), nucleation surface (b), and cross-section (c) of a 1000 nm thick polycrystalline diamond film fabricated in Example 5. Figure 6 shows SEM images of the growth surface (top) and nucleation surface (bottom) of polycrystalline diamond films of different thicknesses fabricated in Examples 2-5. As can be seen from Figure 5, the growth surface has various large crystal grains (on a 100 nanometer scale), while the nucleation surface shows a flat morphology with no visible crystal grains. The gradual change in crystal grain size along the thickness direction can also be seen in the cross-sectional view. Furthermore, from the SEM image in Figure 6, it can be seen that the crystal grains on the growth surface increase in size as the thickness increases, in contrast to the nucleation surface where the surface morphology does not change. This clear difference in surface morphology is mainly due to the PCD film growth method. Specifically, diamond particles smaller than 10 nm in size, used as seeds, are deposited on the silicon substrate before CVD growth. In subsequent growth stages, the silicon substrate inhibits downward growth of the diamond, so 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 seed size, and the nucleation density.

[0044] 2. The growth surface and nucleation surface of polycrystalline diamond films of different thicknesses, fabricated in Examples 2-6, were characterized using an atomic force microscope (AFM).

[0045] Figure 7 shows 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.

[0046] As can be seen from the AFM image in Figure 7, the surface roughness (Ra) of the polycrystalline diamond film fabricated in Example 5 was reduced from 36.2 nm (growth surface) to 0.952 nm (nucleation surface), and in Example 6, using a smoother growth substrate, it was further reduced to 0.612 nm. This ultra-flat surface is comparable to polished single-crystal bulk diamond and meets the requirements of high-precision nanofabrication. Furthermore, compared to other strategies for exposing the nucleation surface, such as sacrificing the growth substrate, the method of the present invention avoids contamination caused by multiple operations. Thus, this exfoliation method allows the original state of the nucleation surface to be preserved without causing damage.

[0047] Figure 8 shows (a) statistics of the roughness of the growth surface and nucleation surface of PCD films of different thicknesses (statistical area 20 μm × 20 μm) prepared in Examples 2-5; (b) a comparison of the method of the present invention with other reported methods regarding film thickness and roughness; and (c) a comparison of the method of the present invention with other reported methods regarding 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 is less than 1 nm at all thicknesses, while the roughness of the growth surface increases. Compared with reported studies on the optimization of surface roughness of diamond films, the peeling method of the present invention has clear advantages in either the range of PCD film thickness or size. Therefore, to date, the method of the present invention can be considered the best method for obtaining PCD films with the maximum size, flattest surface, and widest thickness range. Furthermore, thicker and larger PCD films can be obtained by extending the growth time and using a larger growth substrate.

[0048] 3. The refractive indices of the growth surface and nucleation surface of the polycrystalline diamond film in Example 1 were measured using an optical ellipsometer. As shown in Figure 9, n represents the refractive index and k represents the extinction coefficient. From the measurement results in Figure 9, it was confirmed that the nucleation surface had a higher refractive index and a lower extinction coefficient than the growth surface, and that the levels of these optical parameters of the nucleation surface were equivalent to those of SCD grown by the CVD method.

[0049] 4. The crystallinity of the polycrystalline diamond film prepared in Example 1 was evaluated using X-ray diffraction (XRD). As shown in Figure 10, separate diffraction peaks were obtained at 43.96°, 75.14°, and 91.52°, which correspond to the (111), (220), and (311) crystal planes, respectively, confirming the diamond properties of the nucleation surface. The strong diffraction peak of the (111) crystal plane indicates its dominance and is consistent with other reports. Furthermore, 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 of the growth surface. In addition to this change in FWHM, the position of the diffraction peak of the (111) crystal plane shows a significant shift from 44.36° (growth surface) to 43.96° (nucleation surface) (Figure 10, inset), which is close to the position of a standard (111) oriented SCD (43.95°). A shift in diffraction peaks indicates an improvement in crystal quality and may be related to the release of internal stress. Typically, lattice mismatch between the diamond and the non-uniform substrate introduces residual stress into the film, which further causes lattice distortion and leads to an abnormal shift in diffraction peaks. By using the fabrication method of the present invention, residual stress can be released after the growth substrate is removed.

[0050] 5. The crystallinity of the polycrystalline diamond film prepared in Example 1 was further evaluated using Raman spectroscopy. From the Raman spectrum plot in Figure 11, a clear scattering peak for diamond was observed at 1332.8 cm⁻¹. -1 This was observed, further demonstrating the state of this membrane. Furthermore, from the inset in Figure 11, the FWHM of the nucleation surface (7.43 cm) -1 ) is the FWHM of the growth surface (7.67cm -1 This indicates a narrower range than the previous range, and shows an improvement in the internal crystallinity of the film as measured from the nucleation surface.

[0051] Based on Raman shift and X-ray diffraction characteristics, it can be seen that, in addition to a reduction in the roughness of the nucleated surface, the crystal quality of the nucleated surface is optimized compared to the grown surface. Furthermore, regarding optical properties, the refractive index of the nucleated surface is lower than that of the grown surface and is comparable to that of a single crystal.

[0052] Table 1 summarizes a comparison between polycrystalline diamond films prepared by the present invention and previously reported polycrystalline diamond films in terms of surface roughness, thickness range, and size (film area).

[0053] [Table 1]

[0054] As can be seen from the data in Table 1, the polycrystalline diamond films obtained by the method of the present invention have very low surface roughness, close to that of polished single-crystal diamond bulk. Furthermore, the obtained films are superior to those in previously reported studies in terms of both size and thickness.

[0055] Example 7 Preparation and characterization of a 4 μm thick polycrystalline diamond film. A polycrystalline diamond film with a thickness of 4 μm was fabricated using the same method as in Example 1, and then this polycrystalline diamond film was directly attached to a flexible PDMS substrate for bending performance and conductivity testing.

[0056] Bending performance of polycrystalline diamond film The polycrystalline diamond film exfoliated by the method of the present invention can undergo any deformation without being limited to a rigid growth substrate. Figure 12 shows photographs of a 4 μm thick polycrystalline diamond film bent 360 degrees (a) and wrapped around cylinders of different radii (b).

[0057] As shown in Figure 12, the approximately 4 μm thick film fabricated in Example 7 can withstand a 360° bending mode, and the allowable radius of curvature can be reduced to 2 mm. Furthermore, the film remains stable after more than 2500 bending cycles.

[0058] Fatigue testing of polycrystalline diamond films A 4 μm thick polycrystalline diamond film prepared by the method of Example 1 was mounted on a flexible PDMS substrate, and a 2% deformation cycle was performed. Panel (a) of Figure 19 shows a photograph after >10,000 deformation cycles at 2% strain; panel (b) shows 5x optical images corresponding to regions 1 and 2 in panel (a), respectively. It can be seen that the polycrystalline diamond film of the present invention can withstand more than 10,000 deformation cycles at 2% strain without causing damage.

[0059] Compared to the maximum strain of PCD films obtained from microscale samples reported to date, the incredible bending performance and fatigue resistance of the diamond described above have been observed for the first time on a macroscale. Theoretical predictions from previous studies on single-crystal diamond nanopillars suggest that the energy band gradually decreases with increasing strain. However, the principle of this PCD method has been largely unproven both theoretically and experimentally due to its complex internal crystal structure. Fortunately, in this invention, the dynamic properties of a self-supporting PCD film can be observed using a simple exfoliation strategy.

[0060] Conductivity and energy band modulation of polycrystalline diamond films To investigate the electrical conductivity of the PCD film under different bending levels, the inventors attached a 4 μm thick PCD film prepared in Example 7 to a 1 mm thick flexible PDMS (polydimethylsiloxane) substrate, and then sequentially attached a soft conductive tape. By controlling the deformation of the PDMS substrate, the bending operation of the PCD film can be achieved. Furthermore, by bending the PDMS substrate in different directions, the deformed PCD film under tensile strain and compressive strain can be observed separately. From the calculation results shown in Figure 13, it can be seen that the strain distribution is non-uniform, and that the maximum deformation occurs at either the upper or lower center of the PDMS.

[0061] Figure 14 shows the change in conductivity of a 4 μm thick polycrystalline diamond film prepared in Example 7 under different strain (a) and bending cycles (b). By measuring the resistance under dynamic strain, a large piezoresistive behavior is observed for this film. In both bending modes, the resistance gradually decreases as the strain increases. However, the magnitude of the change in resistance under compression mode was greater than under tension mode. Furthermore, the resistance can be fully recovered during load / unload cycles.

[0062] The above results indicate that the energy band structure of polycrystalline diamond films can be modulated by strain, and that as the degree of strain increases, the energy band of diamond gradually decreases, resulting in an increase in conductivity. Generally, modulation of diamond energy bands is mainly achieved by elemental doping. However, diamond doping is process-intensive and difficult to perform in a typical laboratory. The strain modulation method for diamond energy bands proposed in this invention significantly reduces fabrication difficulty and production costs, facilitating the development of flexible diamond semiconductor devices and diamond-based sensor devices.

[0063] Example 8 Properties of polycrystalline diamond films

[0064] Figure 15 shows the Raman spectrum (a) and XPS measurement results (b) and (c) of the polycrystalline diamond film prepared in Example 5.

[0065] Figure 16 shows experimental photographs and data summaries of the contact angles of the growth and nucleation surfaces of PCD films of different thicknesses.

[0066] Figure 17 shows the coefficient of friction of the growth surface and nucleation surface of the polycrystalline diamond film fabricated in Example 5 (a), and a curve (b) showing the relationship between the external force applied to the growth surface and nucleation surface and the depth of the formed indentation.

[0067] Figure 18 shows the refractive index (a) and extinction coefficient (b) of the growth surface and nucleation surface of a 500 nm thick polycrystalline diamond film, and the refractive index (c) and extinction coefficient (d) of the growth surface and nucleation surface of a 1000 nm thick polycrystalline diamond film. The results above show that the nucleated surface exhibits a higher refractive index and a lower extinction coefficient compared to the grown surface. The refractive index of the nucleated surface is comparable to that of single-crystal bulk diamond. Furthermore, both the grown surface and the nucleated surface show a decrease in refractive index and an increase in extinction coefficient with increasing thickness. The diamond film of this disclosure is characterized by a high refractive index and a low absorption coefficient, and is expected to lead to the development of diamond-based micro-optics and nano-optical devices, such as optical superlenses and optical modulators.

[0068] Example 9 Effects of different peeling angles Polycrystalline diamond films of different thicknesses were fabricated using the same method as in Example 1. The films of different thicknesses were peeled at a constant speed (10 mm / min) and at different peeling angles (20° to 90°). Furthermore, possible crack propagation was simulated using the phase field method. Details of the phase field method are described in the following two papers: Miehe, C., Hofacker, M. & Welschinger, F. Aphase 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)

[0069] Calculating the crack density of the exfoliated film reveals that the exfoliation performance depends on the exfoliation angle and film thickness. For thicker films (800 nm and 1000 nm), a wide range of exfoliation angles (20° to 90°) can be used to obtain crack-free films. When the film thickness reaches 600 nm, the effective range of exfoliation angles at which cracks are minimized narrows to 40° to 70°, and this working window continues to narrow as the film thickness decreases.

[0070] Phase field simulations were performed on a 600 nm thick polycrystalline diamond film, with delamination carried out at different delamination angles of 30°, 60°, and 90°. In the phase field simulations, the initial crack sizes were set to 56 nm (upper side) and 1.2 nm (lower side), and 95% of the cracks on each surface were smaller than these dimensions. The simulation results showed that when the delaminated film was delaminated at a small angle of 30°, it experienced gentle bending deformation, resulting in tensile stress on both the lower and upper sides, which ultimately led to the propagation of large cracks originating from the upper surface. In contrast, the large delamination angle of 90° resulted in compression on the upper (concave) side of the film, inhibiting crack growth, but the increased tensile stress on the significantly deformed lower (convex) side of the film was strong enough to initiate the propagation of small initial cracks. As a result, the 600 nm thick film did not experience any excessive tensile or compressive stress that would promote crack initiation during delamination performed at an intermediate delamination angle of 60°.

[0071] For thicker films of 700–1000 nm, simulations suggested that delamination angles within the range of approximately 30–90° effectively prevent crack propagation. These results are consistent with experimental observations of crack density in various delaminated films, indicating their potential application as guidelines for efficiently scaling up the production of crack-free films.

[0072] Example 10 Flawlessness and quality of polycrystalline diamond films The initial resistance of the growth surface of the polycrystalline diamond film fabricated in Example 1 was measured to be approximately 10⁴ Ω.

[0073] Since the resistance of a diamond film is determined by its integrity and quality, a chip array containing a gold electrode pair was fabricated on a 2-inch diamond film to evaluate the potential impact of the exfoliation process on film quality, and the change in resistance before and after exfoliation was measured. Figure 21 shows photographs of the chip array fabricated on a 2-inch polycrystalline diamond film before (a) and after (b) exfoliation, as well as the change in resistance before and after exfoliation (c).

[0074] A relatively uniform resistance distribution can be seen in panel (c) of Figure 21, indicating that the film of the present invention is uniform throughout the entire 2-inch diamond film, and that the resistance at each point after peeling is nearly constant (the overall resistance is reduced, which may be related to desorption from the silicon substrate). This result demonstrates that the polycrystalline diamond film of the present invention has good scratch resistance and high quality.

[0075] Example 11 Micro / nano processing of polycrystalline diamond films Diamond micro / nanostructures were fabricated on the growth surface and nucleation surface of the polycrystalline diamond film prepared in Example 1 using electron beam lithography (EBL) and plasma etching techniques, respectively. The results are shown in Figure 22. Comparing micro / nano structures fabricated on the growth surface and the nucleation surface reveals that the nucleation surface produces significantly less residue and exhibits higher structural precision.

[0076] Example 12 Applications of polycrystalline diamond films as quantum platforms A polycrystalline diamond film was prepared using the same method as in Example 1, except that nitrogen gas was introduced during CVD growth to create special atomic defects known as nitrogen vacancy (NV) centers in the film.

[0077] A 532nm laser (Changchun New Industries, MGL-FN-532-1W) is used to excite the diamond film. The laser passes through a 40× air objective lens and is focused onto the diamond film. The excited NV fluorescence is filtered through a 605nm long-pass filter and imaged using an EMCD (Teledyne Photometrics, EVOLVE 512 Delta) as shown in panel (a) of Figure 23. The fluorescence image (inset) shows a bright spot corresponding to the emission from the NV center. The peak at approximately 640nm in the spectrum indicates the NV- zero phonon line.

[0078] In the photodetector magnetic resonance (ODMR) measurement, the 532 nm laser is kept constant throughout the measurement. The microwave (MW) frequency is linearly swept from 2800 MHz to 2940 MHz and triggered by an external pulse. The ODMR spectrum shown in panel (b) of Figure 23 demonstrates the effective separation of the spin states (±1) of the NV centers under external magnetic field application. This demonstrates the great potential of the polycrystalline diamond film offered by this invention for quantum technology applications.

Claims

1. A scalable method for fabricating ultraflat polycrystalline diamond films, (1) A step of growing a polycrystalline diamond film on a growth substrate having diamond seeds on its surface using chemical vapor deposition, wherein the polycrystalline diamond film includes a growth surface having a first roughness and a nucleating surface bonded to the growth substrate. (2) A step of cutting at least a portion of the growth substrate having the polycrystalline diamond film grown on its surface to form a boundary in the cut portion where the polycrystalline diamond film is bonded to the growth substrate, (3) A step of attaching a release tape 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 portion to form an end for the release work, (4) A step of peeling the polycrystalline diamond film from the surface of the growth substrate and pulling the peeling end to expose the nucleating surface of the polycrystalline diamond film, wherein the exposed nucleating surface has a second roughness, and the second roughness is less than the first roughness. Methods that include...

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

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

4. A microwave plasma chemical vapor deposition apparatus is used in step (1) for the growth of the polycrystalline diamond film. Preferably, the deposition temperature of the chemical vapor deposition method in step (1) is 800 to 1000°C, more preferably 850 to 950°C. The method according to any one of claims 1 to 3.

5. The growth substrate is made of Si, SiC, TiC, Co, Pt, or Al. ２ O ３ , Ni, Re, Ir, SiO ２ The method according to any one of claims 1 to 4, wherein one or more are selected from the group consisting of and Mo, and preferably the surface roughness of the growth substrate is less than 2 nm, preferably less than 1 nm, more preferably less than 0.5 nm.

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

7. The cutting operation in step (2) is performed perpendicular to the growth surface of the polycrystalline diamond film, and preferably the cutting operation is performed such that the length of the boundary formed where the polycrystalline diamond film bonds 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 formed by cutting the location where the polycrystalline diamond film bonds to the growth substrate in step (2) is 20% to 50% of the diameter of the growth substrate. The method according to any one of claims 1 to 6.

8. The method according to any one of claims 1 to 7, wherein in step (4), the speed at which the peeling end is pulled to peel the polycrystalline diamond film from the surface of the growth substrate is 1 to 10 mm / s, and preferably the peeling angle of the peeling operation in step (4) is 10 to 90 degrees.

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

10. A method for modulating the energy band of a diamond film, comprising the steps of: attaching a polycrystalline diamond film prepared 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 realizing modulation of the energy band composition of the polycrystalline diamond film by strain generated by the polycrystalline diamond film during the stretching and / or compression.

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