Axial uniformity of diamond-like carbon film deposited on metal rod by using microwave–sheath voltage combination plasma

• DLC film was deposited at high deposition rate (> 100 μm/h) on long rod substrate.
• High resistivity DLC leads to the plasma shrinkage and film non-uniformity.
• The uniformity was improved by increasing the duty and power of bias voltage.
• DLC film with good uniformity and high hardness was deposited on long rod.

We developed a novel method for the deposition of diamond-like carbon (DLC) films at high deposition rates (over 100 μm/h) using microwave–sheath voltage combination plasma. This method was applied for depositing a DLC film on a three-dimensional metal substrate, but we found that the film thickness was not uniform along the direction of microwave propagation. Hence, we investigated the factors affecting the axial uniformity of DLC film deposited on long metal rods (1 cm in diameter). Since the axial distribution of plasma was observed to affect the uniformity of DLC, we evaluated the axial distribution of ion density using a Langmuir Probe in Ar plasma that was generated along both DLC-coated and non-coated rods. The results showed that the ion density along the axis of the DLC-coated rod decayed at a shorter distance than that along the axis of the uncoated one. The voltage of the ion sheath presumably decreased owing to the voltage drop in the DLC film with high resistivity, in accordance with the short microwave decay. The uniformity considerably improved by using a duty ratio of 50% for the pulsed operation of both the microwaves and the substrate bias at − 200 V. When the substrate voltage was increased to − 500 V, the hardness of the DLC film measured by the nano-indenter was about 25 GPa, uniformly distributed along the rod axis. Raman results show that the film is typical DLC structure, and the structure shows good uniformity along the rod axis.