DLC coating on a trench-shaped target by bipolar PBII

• The DLC films were coated on trench surfaces by using bipolar PBII.
• The structure and hardness of DLC film on the inside wall is quite different.
• The non-uniformity of film properties were investigated by plasma simulation.

Bipolar-type plasma based ion implantation (bipolar PBII) and deposition is a promising coating technique, especially for the complex-shaped target surface. In this study, diamond-like carbon (DLC) films were prepared by a bipolar PBII technique to a trench-shaped target (20 mm pitch and 10 mm depth), and the uniformity, structural and mechanical properties of DLC films coated on the top, inside wall and bottom surfaces of the trench were evaluated by surface profilometer, Raman spectroscopy and nanoindentation hardness tests. Moreover, plasma simulation was conducted to reveal the DLC coating mechanism for finding the optimum coating conditions. Particle-In-Cell with Monte Carlo Collision (PIC–MCC) method and Direct Simulation Monte Carlo (DSMC) method were used for computing the plasma behavior. As a result, thickness uniformity of DLC films was improved with decreasing negative pulse voltage. On the other hand, the thickness of DLC films on the inside wall of the trench was much smaller than those of the top and bottom surfaces of the trench. This is because the conformal ion sheath cannot be formed around the inside wall, which in turn leads to the decrease of incidence ion flux to the inside wall, and the idea has been verified through the simulation of plasma behavior. From the Raman measurements of DLC coatings on the inside wall, it is observed that the Raman G-peak position shifts to a higher wave number and the Full Width at Half Maximum of G-peak (FWHM(G)) decreases to a smaller value compared to those of the films on the top and bottom surfaces. These indicate that the structure of DLC coatings on the inside wall surface has changed compared to those on top and bottom surfaces, i.e., graphitized, which results in the reduction of hardness.