1 Introduction
Diamond possesses excellent mechanical, optical, thermal, and electrical properties, making it a typical multifunctional material with broad application prospects in many high-tech fields such as aerospace, energy, intelligent sensors, and precision machining [1]. With the increasing demand for diamond, natural diamond reserves are limited and extremely expensive, which cannot meet the growing social needs. Therefore, researchers have been continuously exploring new methods for synthesizing diamond artificially.
The main methods for preparing synthetic diamond are the high-pressure high-temperature (HPHT) method and chemical vapor deposition (CVD). Diamond particles synthesized by the HPHT method are mostly small in size and often contain a large amount of catalyst impurities. Hence, their application scope is limited, primarily to abrasives and tools (e.g., polycrystalline diamond, PCD) [2]. In recent years, chemical vapor deposition (CVD) technology has emerged as a highly promising research hotspot. Diamond crystals prepared by CVD have grain sizes and crystal shapes closest to those of natural diamond and exhibit excellent properties [3].
2 Preparation of Diamond Films by CVD
The CVD method is a key technology for preparing thin films by chemical means. It has been widely used in the preparation of various thin film materials and is also an important method for producing diamond films. In recent years, research on diamond film synthesis by CVD has progressed rapidly, and large-scale production and application capabilities have now been achieved.
2.1 Growth Mechanism [4]
CVD synthesis of diamond generally involves decomposing carbon-containing precursors using energy in a reducing atmosphere to generate atomic hydrogen, which facilitates diamond growth. The growth process specifically includes the following basic steps: (1) The precursor (exemplified by CH₄) is introduced into the CVD reactor; (2) The gaseous precursor is decomposed by energy (plasma or thermal energy) into free radicals (CₓHᵧ, etc.); (3) Free radicals collide with each other until they reach the substrate surface; (4) Free radicals adsorb on the substrate surface; (5) The adsorbed free radicals decompose to form active carbon species and diffuse on the surface; (6) Active species form the diamond lattice; (7) Non-active species (such as hydrogen) desorb from the surface, forming hydrogen molecules or other by-products; (8) By-products diffuse away from the surface through the boundary layer and are eventually removed by the bulk gas flow.
2.2 Preparation Techniques
Since the emergence of CVD diamond preparation in the 1980s, researchers have conducted sustained in-depth studies, leading to the development of various CVD methods [5]. Currently, mature diamond film preparation techniques include hot filament CVD (HFCVD), microwave plasma CVD (MPCVD), and direct current arc plasma jet CVD (DCPJCVD).
2.2.1 Hot Filament CVD (HFCVD)
In the HFCVD method, hydrocarbon gases such as methane (CH₄) and acetylene, together with hydrogen (H₂), are introduced into the reaction chamber. The filament temperature in the chamber is above 2000 °C, and the mixed gas is decomposed at high temperature to produce sp³ hybridized carbon radicals necessary for diamond synthesis, which then form a diamond film on the substrate surface [6].
The HFCVD method has the advantages of simple equipment, high film deposition rate, easy operation, low cost, and mature technology. It has been widely used in industrial production and is also one of the main methods for preparing microcrystalline diamond coatings, nanocrystalline diamond coatings, and diamond-like carbon coatings [2]. Its disadvantages include: the hot filament does not excite the gas to a high degree, and the filament itself may contaminate the diamond film; the filament is prone to deformation during heating, which degrades the uniformity of the deposited film; and the filament has a short lifetime, making it unsuitable for long-term deposition of thick film samples [7].
Currently, research on diamond film preparation using HFCVD is relatively mature both domestically and internationally. However, further exploration is still needed on how to adjust process parameters to improve diamond film quality [2]. Key process parameters include gas flow rate and ratio, filament temperature, substrate type and temperature, and reaction chamber pressure.
2.2.2 Direct Current Arc Plasma Jet CVD (DCPJCVD)
The DCPJCVD method is a unique CVD diamond growth technique developed by Chinese researchers [4]. Its principle is to use high-power DC arc discharge to excite a reaction gas mainly composed of CH₄-H₂-Ar into a plasma, which is then ejected at high speed onto the substrate surface, depositing a diamond film [7].
Compared with other CVD deposition techniques, DCPJCVD has the advantages of the highest deposition rate, low discharge voltage, high discharge current, high ionization degree, high plasma density, and large deposition area. Its maximum deposition rate can reach 1000 μm/h, making it suitable for the production of large-area diamond films. It is currently the fastest method for synthesizing diamond films, but it requires significant equipment investment, complex processing, and the film uniformity still needs improvement [4].
In China, the University of Science and Technology Beijing and the Hebei Academy of Sciences jointly developed and refined this technology [8]. At present, diamond films prepared by DC arc plasma jet technology in China are at an advanced level in terms of quality and area compared to those produced by the same method abroad, and related diamond film products have already entered the international market in bulk as superhard tool materials.
2.2.3 Microwave Plasma CVD (MPCVD)
In the MPCVD method for diamond film preparation, the high-frequency electromagnetic field generated by a microwave magnetron causes electrons to oscillate vigorously in a confined space. These electrons collide intensely with gas molecules and atoms in the space, ionizing the gas and generating active radicals such as -CH₃ and H. These active species undergo physicochemical reactions, move toward the substrate, and then adsorb, diffuse, nucleate, and grow on the substrate surface, ultimately yielding a diamond film [9].
A notable feature of MPCVD is that the microwave plasma can achieve stable discharge without electrodes via the high-frequency electric field, thereby reducing sample contamination. In addition, the microwave causes vigorous gas oscillation, effectively activating the gas and generating high-density plasma, which enables the growth of high-quality diamond films [10]. The difficulty lies in forming a large and uniform deposition area, which is affected by many factors, including the electric field distribution in the cavity, substrate temperature variation, microwave power, and distribution of active radicals. Moreover, the diamond film deposition rate is generally lower than that of DC arc plasma jet CVD [4].
To compensate for the low deposition rate of MPCVD, foreign researchers have continuously increased the input power of the equipment to enhance the deposition rate. Over decades of development, the power has risen from a few hundred watts to nearly 100 kW, significantly improving the deposition area, deposition rate, and quality of diamond films. Domestic research on MPCVD diamond preparation started relatively late, but through continuous research and development, China has largely kept pace with international progress, though gaps remain in quality and commercial applications. In the past 20 years, domestic research teams have focused on resonator design to pursue large-scale commercial diamond development [11].
3 Applications of Diamond Films
3.1 Optical Applications
Diamond films possess excellent optical properties, including high transmittance, low refractive index, and low scattering. They are often used to fabricate optical windows, optical lenses, and other components, with outstanding application prospects in high-tech fields such as military, communications, and satellite technology. Diamond films have excellent physicochemical properties, particularly good transmittance from the ultraviolet to the far-infrared and microwave ranges. When used for optical windows, they can effectively avoid thermal lens effects, rain erosion, sand erosion, and optical distortion. Additionally, diamond films are often used in X-ray windows and masks, X-ray transmission targets, optical lenses, and other devices [12].
3.2 Thermal Applications
In the past, high density, high power, and small volume have become the ongoing trends in the development of electronic devices in the microelectronics field. However, with this trend, the heat generated by components will increase substantially. CVD diamond films have properties comparable to those of natural diamond, especially an extremely low coefficient of thermal expansion, ultra-high thermal conductivity, and electrical insulation. They are ideal heat sink materials and packaging materials, widely used in high-power laser devices, large-scale integrated circuits, RF power transistors, high-power optical windows, and other devices.
3.3 Acoustic Applications
With the widespread adoption of audio equipment, consumer demands for speaker quality have steadily increased. Compared with traditional diaphragm materials such as carbon fiber, ceramic, and beryllium, diamond films have lower density and higher elastic modulus. At the same time, the grain boundaries in polycrystalline diamond films can provide ideal internal friction, giving them comprehensive properties far superior to those of traditional materials, making diamond an excellent choice for speaker diaphragms. In addition, using high acoustic velocity media is a key means to increase the operating frequency of devices. Diamond is currently the material with the highest known acoustic propagation velocity, which can greatly improve the operating frequency of surface acoustic wave (SAW) devices, enabling the interconversion of RF signals and mechanical vibrations, and offering broad application prospects in satellite, mobile communications, and other fields [10].
3.4 Electrical Applications
Due to diamond's wide bandgap, high electron and hole mobility, high breakdown electric field, low dielectric constant, high resistivity, and high thermal conductivity, it is highly suitable for highly integrated semiconductor devices operating under high temperature, high bias, high power, and high radiation conditions. Therefore, diamond is expected to replace silicon as an ideal material for electronic devices that must operate reliably under harsh conditions such as high temperature and radiation resistance.
3.5 Biomedical Applications
Diamond films combine good biocompatibility, excellent mechanical properties, and chemical stability, making them highly promising next-generation biomaterials. Their superior mechanical properties and chemical stability can greatly reduce wear and electrochemical corrosion of diamond-coated implants. The surface modifiability, biocompatibility, and excellent electrical conductivity after doping of CVD diamond films also make them excellent supports for biosensors. Compared with other supports, biosensors based on diamond film substrates offer higher stability, reliability, and sensitivity.