Growth characterization and properties of diamond-like carbon films by electron cyclotron resonance chemical vapor deposition
Introduction
Diamond-like carbon (DLC) films have been extensively studied in recent years because of their unique properties such as high hardness, low friction, and wear, chemical inertness to acid and alkalis and optical transparency, etc. These properties make them suitable as protective coatings for mechanical tools and infrared optics as well as sensors designed to work in an aggressive environment. The biocompatibility allows them to be used as protective coatings for hip joint replacements and other medical implants. They are good electrical insulators and have a high breakdown voltage, which makes them good candidates for an inter-metal dielectric in modern integrated circuits. Recently, DLC films have been applied in large-area flat panel displays as electron emitting materials due to their field-emission properties [1], [2], [3], [4], [5], [6], [7].
Many low-pressure deposition techniques, such as radio-frequency (RF) discharged plasma enhanced chemical vapor deposition (PE-CVD) [8], ion beam-assisted deposition (IBAD) [4], laser ablation and mass selected ion beam deposition (MS-IBD) or filtered cathodic vacuum arc deposition (FC-VAD) [9], [10], have been successfully used to prepare the hydrogenated (a-C:H) or hydrogen-free (a-C) DLC films. The large-area, uniform films deposited at low temperatures are required for the applications of DLC films in optics, electronics and protective coatings. Electron cyclotron resonance (ECR) discharge can efficiently produce high-density, non-contaminated plasma at low pressure levels. There is an increasing demand for ECR plasma sources with low power consumption and uniformity over large processing areas. It has been shown that ECR microwave plasma can be utilized to deposit thin films such as Si, SiC, diamond, DLC, TiN and Ta2O5, etc. [11], [12], [13], [14], [15], [16].
A numbers of studies on DLC films by ECR-CVD have reported [17], [18], [19], [20], [21] that conditions of a negative bias voltage of more than −100 V and a microwave power of 50–200 W were favorable for the growth of DLC films with a high sp3 content. It was found that ion acceleration by bias voltage between plasma and substrate is significantly effective to produce sp3 hybridized CC in the films, implying that the ions possess a relative high kinetic energy for changing bonding status [9], [18]. However, the increase in the bias voltage will raise the substrate temperature and results in the formation of graphitic carbon in films [9], [17], [20], [21]. In addition, the gas mixtures of methane and hydrogen usually used in the deposition process would probably increase the atomic hydrogen content in the films, resulting in softer DLC films [9], [17], [18].
In this study, DLC films were prepared from CH4-Ar plasma by RF biased ECR microwave plasma chemical vapor deposition. The structure and properties of the films were examined as a function of microwave power, RF bias and the reactant CH4/Ar gas ratio.
Section snippets
Experimental
Depositions were carried out using an ASTeX microwave power supply source. Methane and argon were used as reactant gases and fed through the upper gas inlet into the ECR chamber. The gas flow was measured by mass flow controllers and the pressure was monitored by a capacitance manometer. The ECR plasma, generated by a microwave power supply and two magnets, was extracted by the divergent magnetic field into the reaction vessel below. A three-stub tuner was used to match the reflected microwave
Growth rate
The DLC films synthesized by ECR plasma process were visually very smooth and flat. No visible grain feature was observed from the scanning electron microscopy (SEM). The film growth rate was obviously affected by the microwave power used. Fig. 1 shows the dependence of deposition rates on the microwave power at the conditions of RF bias −200 V and CH4/(CH4+Ar) of 3/7. The deposition rate increased with the increase in microwave power, reaching a maximum at power of 150 W. Since the increase of
Conclusion
High hardness DLC films have been deposited at a high growth rate on low temperature substrates using ECR-MPCVD CH4-Ar plasma processing. The CH4/(CH4+Ar) gas ratio, microwave power and negative RF bias affected the structure and properties of the DLC films. Increasing the CH4/(CH4+Ar) gas ratio changed the films structure from polymer-like to DLC structure and increased the films hardness. The increase in microwave power and RF bias both increased the content of sp3 bond in the amorphous
Acknowledgements
The authors wish to thank the National Science Council of Taiwan for the financial support under the project ‘NSC 89-2218-E-006-017’.
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