Importance of Nucleation Phase in Microwave PECVD of Ultra-Nanocrystalline Diamond Films
| Authors | |
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| Year of publication | 2007 |
| Type | Article in Proceedings |
| Conference | New Perspectives of Plasma Science and Technology CD |
| MU Faculty or unit | |
| Citation | |
| Field | Plasma physics |
| Keywords | nanocrystalline diamond; plasma enhanced CVD; hardness; optical constants |
| Description | Microcrystalline diamond finds several applications due to its high hardness but also as electronic and optical devices. However, its roughness makes some applications like tribology, emission cathodes for flat panel displays, optical coatings and emerging Nano/Micro-Electro-Mechanical Systems (N/MEMS) difficult. A major advance was achieved in early 90ties when the crystalline size was decreased from down to nanometers. However, the processes leading to the deposition of small grain-sized diamond films are not yet properly understood and these films exhibit different properties and morphology depending on the method of preparation. Therefore, the “nanocrystalline diamond” (NCD) covers very different materials such as columnary grown films with the grain sizes usually quoted below 100 nm (but 30 nm are nowadays possible)and continuous dense coatings with grain sizes reaching 5-15 nm grown under high re-nucleation rates. The latter were firstly prepared by the group of D. M. Gruen and the term ultra-nanocrystalline diamond (UNCD) was used starting from the year 2000 in order to distinguish them from other films with the grain sizes below 100 nm. Our UNCD films were deposited by microwave (2.45 GHz) PECVD in the ASTeX type reactor combined with the rf capactive discharge providing a negative dc self-bias –125 V at the substrate holder. The deposition mixture consisted of 9 % of CH4 in H2. The applied mw power, pressure and substrate temperature were 850 W, 7.5 kPa and 1090 K, respectively. The continuous re-nucleation rate was achieved by ion bombardment due the self-bias. The films exhibited very low roughness (rms of heights 9 nm) and high hardness and elastic modulus, 70 and 375 GPa, respectively. The nucleation phase turned to be very important for good mechanical properties of the films. High hardness and good toughness were achieved only for an immediate start of the nucleation after mw and rf discharges were ignited whereas much worse films were obtained when the substrate was first heated, and then methane was added. The latter films responded quite non-uniformly to the mechanical tests which resulted in the hardness scattered in the range 30-60 GPa. Moreover, because of frequent cracking, the mechanical properties had to be studied by a continuous stiffness nanoindentation technique only. It is expected that the reason for the bad performace of the latter films lay in an interfacial layer of bad quality. The nucleation and film growth were studied by SEM, AFM and modelling of optical response of the substrate-interfacial layers-diamond film systems in UV/VIS/NIR range. |
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