By Wang Guangzu, Zhengzhou Abrasives & Grinding Research Institute
Diamond films are generally of polycrystalline structure. Due to their high surface energy, they tend to have relatively high surface roughness. This is attributed to the relatively large grain size of diamond films, which typically ranges from 1 μm to several tens of micrometers. Such surface characteristics severely limit their application in optics and electronics. To overcome this limitation, it is necessary to reduce the grain size of diamond films. Although mechanical polishing can lower surface roughness, diamond films are exceptionally hard and difficult to polish. Consequently, fabricating nanoscale diamond films becomes an effective solution.
Extremely High Nucleation Density
To achieve nanoscale diamond films with a grain size below 100 nm, the grain density should be around 1012/cm2. This requires the nucleation density of the diamond film to be no less than 1010/cm2. In practice, to realize the deposition of nanocrystalline diamond films, the nucleation density of diamond should exceed 1010/cm2.
Extremely High Secondary Nucleation Rate
A high rate of secondary nucleation is essential to suppress the growth of diamond grains, thereby achieving nanoscale diamond films. Figure 1 illustrates the differences between MCD (Microcrystalline Diamond Film) and UNCD (Ultrananocrystalline Diamond Film).
Figure 1: Schematic Diagram of the Differences Between Nanocrystalline Diamond Films (NCD) and Ultrananocrystalline Diamond Films (UNCD)
If no secondary nucleation occurs during the growth of diamond films, the grains grow larger over time, eventually forming microcrystalline diamond films (MCD). When the growth process involves a significant secondary nucleation rate, the film's growth is accompanied by the formation of small crystals and new crystals nucleating on the growth facets, resulting in nanocrystalline diamond films (NCD) with grain sizes smaller than 100 nm. If the secondary nucleation rate during the growth process is even higher, the film grows with small grains and new crystals forming through secondary nucleation on the growing facets, yielding ultrananocrystalline diamond films (UNCD) with grain sizes of 3–5 nm. However, most literature does not distinguish between NCD and UNCD.
In the growth process of MCD, excess hydrogen preferentially etches the sp² phase, stabilizing the diamond phase and suppressing secondary nucleation. Therefore, reducing the proportion of hydrogen allows for some sp² carbon to exist on the growth facets, creating new nucleation sites. Researchers in China and abroad often use various methods to increase nucleation density or secondary nucleation rates to achieve the preparation of nanocrystalline diamond films. These methods include different substrate pretreatments, applying negative bias voltage, and adjusting deposition process parameters (e.g., gas composition, temperature, pressure), or combining multiple techniques.
The equipment used for fabricating nanocrystalline diamond films is similar to that for conventional diamond films. The most commonly used methods are Hot Filament Chemical Vapor Deposition (HFCVD) and Microwave Plasma Chemical Vapor Deposition (MPCVD). Their schematic structures are shown in Figures 2 and 3, respectively.
HFCVD relies on the high temperature of filaments to decompose gases. The carbon-containing mixed gas is activated when passing through the hot filaments, forming a plasma region near the filaments. The substrate surface receives thermal radiation and gas convection heat transfer, attaining a certain temperature. The temperature of the substrate surface varies depending on the spacing between the filaments and the substrate. Applying a DC bias between the filaments and the substrate can increase nucleation density.
The advantages of HFCVD include the ability to grow films on substrates with complex shapes or large areas, simple equipment, and low cost, making it suitable for industrial production. However, its disadvantages are the lower quality of the fabricated diamond films, the fragility of the filaments, and their short lifespan.
Figure 2: Schematic Diagram of Hot Filament CVD (HFCVD) Equipment
Figure 3: Schematic Diagram of Microwave Plasma CVD (MPCVD) Equipment
The preparation of nanocrystalline diamond films, similar to microcrystalline diamond films, begins with substrate pretreatment to increase nucleation density. Methods to enhance nucleation density include mechanical polishing, ultrasonic treatment, and ion implantation, among others. Different pretreatment methods significantly affect the structure and properties of the films. In experiments, multiple methods can be combined to further improve nucleation density.
For instance, Yen Chih Lee and colleagues at Tsinghua University used MPCVD to fabricate diamond films under the following conditions: 1500 W power, 20 kPa pressure, reaction gas ratio of 1% CH₄/Ar, 400°C temperature, and 3–6 hours of deposition time. They compared the effects of three pretreatment methods on the structure and properties of diamond films:
The results showed that all three methods yielded nanocrystalline diamond films, but the U-m treatment achieved the highest nucleation density.
Applying bias voltage to the substrate in plasma CVD or hot filament CVD systems is one of the commonly used methods for fabricating nanocrystalline diamond films. It is widely recognized that bias application improves nucleation density.
For example, Tien Syh Yang studied the effect of negative bias voltage on diamond film formation in an MPCVD system using 1% CH₄/H₂. The results indicated that without bias, the films produced were microcrystalline diamond films with grain sizes of 1–3 μm. When a -250 V bias was applied, nanocrystalline diamond films were obtained with significantly reduced surface roughness.
The negative bias on the substrate accelerates the dissociated positive ions in the plasma toward the substrate, increasing their energy. High-energy ions bombard the diamond crystals already formed, distorting their lattice structure and making it difficult for carbon to grow along the original crystal orientation. The bombardment of CHx radicals provides additional energy to generate more active nucleation sites, thereby increasing the rate of secondary nucleation. The balance between diamond grain growth and the secondary nucleation process ultimately determines the grain size of the diamond films.
Reducing the reaction pressure is another commonly adopted measure for producing nanocrystalline diamond films. For instance, in an HFCVD system with 1% CH₄/H₂, diamond films were deposited on mirror-polished Si(100) substrates. When the reaction pressure was reduced from 5 kPa to 0.125 kPa, the grain size decreased by an order of magnitude.
Observations showed that at 0.5 kPa, the diamond film consisted of granular grains with sizes of 50–80 nm, characteristic of nanocrystalline diamond films. At 2.8 kPa, the film consisted of columnar grains with sizes of 200 nm, characteristic of microcrystalline diamond films.
Lower reaction pressure leads to an increase in substrate temperature and a higher dissociation rate of H₂. Additionally, it increases the mean free path of various particles in the reaction chamber. These factors result in a higher quantity and velocity of particles reaching the substrate. The increased particle quantity promotes diamond nucleation, while the higher particle velocity and energy enhance their surface mobility, facilitating aggregation. This results in a high secondary nucleation rate, enabling the transition from microcrystalline diamond films to nanocrystalline diamond films.
Deposition temperature significantly influences the growth of nanocrystalline diamond (NCD) films. Increased temperature facilitates the release of hydrogen from the deposits, enhances the activity of carbon atoms, and affects the etching rate of other carbon forms, which aids in NCD film growth. Experiments have shown that within a specific temperature range, the growth rate of diamond films follows the Arrhenius relationship with temperature. However, experimental reports indicate that the crystalline fraction in NCD films has little correlation with temperature. This suggests that the secondary nucleation rate in the formation of NCD is minimally affected by temperature or that it is not a thermally activated process.
When the temperature exceeds a certain threshold (around 600°C), carbon atoms in the diamond grains migrate to the grain surface, increasing surface roughness and transitioning the deposition process to microcrystalline diamond (MCD) growth. Further temperature increases can result in the transformation of the diamond phase into the graphite phase.
Among all influencing factors, the composition of reactant gases plays a key role as it provides the carbon-containing radicals necessary for growth and secondary nucleation. Therefore, gas composition is one of the primary factors affecting NCD growth. During MCD growth, excess hydrogen preferentially etches the sp² phase, stabilizing the diamond phase and suppressing secondary nucleation. By reducing the hydrogen content, sp² carbon can exist on the growth facets, creating new nucleation sites. This can be achieved through two approaches:
In MCD fabrication, the commonly used reaction gases are CH₄ and H₂, with CH₄ content not exceeding 1%. As the proportion of CH₄ increases (1%–10%), the grain size of the resulting films decreases gradually, from hundreds of nanometers to tens of nanometers. These nanostructured diamond films typically exhibit a cauliflower-like or quasi-spherical morphology, with a smoother surface than MCD but increased grain boundaries. The grain boundaries contain a significant amount of sp²-bonded carbon impurities.
The increased CH₄ proportion alters the chemical composition (e.g., H, CH₃, CH₂, CH, C₂H₂, C₂) and their ratios in the gas phase, thereby influencing the nucleation and growth processes of the films.
Studies have found that when Ar replaces H₂ in the CH₄/H₂ system, the proportions of CH₃ and C₂ radicals change. When Ar content in the reaction gas reaches 70%–90%, CH₃ becomes the primary active radical in the plasma. When H₂ content is 20%–70%, C₂ gradually replaces CH₃ as the main effective active radical, and the resulting films transform into nanocrystalline structures with reduced growth rates.
When the Ar content reaches 90%–99% (typical conditions for NCD film preparation), C₂ becomes the dominant species in the plasma, yielding relatively pure NCD films. It is thus believed that carbon dimers (C₂) play a critical role in secondary nucleation during NCD growth. The secondary nucleation of NCD occurs at carbon atoms on diamond surfaces that are not hydrogen-terminated.
Regardless of whether the transformation from MCD to NCD films is achieved by increasing the CH₄ content in the CH₄/H₂ system or by replacing H₂ with Ar, a significant increase in C₂ concentration in the system is observed, often surpassing that of CH₃ radicals. Consequently, C₂ is widely regarded as the primary radical involved in NCD film preparation.