Researches

Bipolar plasma-based ion implantation and deposition (bipolar PBII&D) has been recognized as a promising technique for coating deposition on complex three-dimensional targets. As the target is fully immersed in the plasma throughout the process, the plasma sheath can be formed with a quite high conformability around the target. In this study, a-C:H film was deposited on a micro-trench pattern by using bipolar PBII&D, and the structure of the a-C:H film across the overall surface region of the trench was examined by making use of their corresponding Raman spectra. The two types of negative high voltage pulses were applied to the targets for comparison: -0.5 and -15 kV. The scale of micro-trench used in the study is much smaller than that of plasma sheath produced under these negative voltages (about 1cm and 14cm for -0.5kV and -15kV, respectively). The plasma behavior (i.e., ion flux, impact angle and energy) in the surrounding of the micro-trench was calculated with Particle-In-Cell Monte Carlo Collision Method (PIC-MCCM). As a result, a-C:H film was successfully coated on the overall surface of the trench. When the applied negative voltage was -0.5 kV, the structure of a-C:H film coated on the sidewall of the trench became more polymer-like carbon (PLC) than those of the top and bottom surfaces did. This, as indicated by the simulation results, is because the ions, which strike the sidewall, tend to have less incident energy. Whereas in the case of -15 kV, the a-C:H film on the sidewall was more graphite-like carbon (GLC) film, despite of its smaller incident ion energy in comparison to those of the top and bottom surfaces. This phenomenon is attributed to the sputtering effect from the bottom surface of the trench, as evidenced by the plasma simulation.

In the present study, a-C:H films were prepared by bipolar plasma based ion implantation and deposition (PBII&D), and the structural and mechanical properties of the a-C:H films deposited on Si substrates were evaluated by Raman spectroscopy. In the PBII&D processing, the positive and negative pulse voltages were varied from 1 to 3 kV and from −1 to −15 kV, respectively. With an increase in the pulse voltages, the Raman G-peak position and I(D) / I(G) ratio increased, and the G-peak full width at half maximum (FWHM(G)) decreased, indicating graphitization of the a-C:H films. In the low wavenumber regime, the FWHM(G) increases when the G-peak shifts to higher wavenumbers, reaching a maximum value at around 1540 cm−1, and then decreases. This behavior was due to the structural changes occurring in the a-C:H films with an increase in the wavenumber. Diamond-like carbon (DLC) to polymer-like carbon (PLC) transition occurred in the low wavenumber regime, and DLC to graphite-like carbon (GLC) transition occurred in the high wavenumber regime. Further, two different trends were observed in the relationship between the mechanical properties (hardness, elastic modulus, and internal stress) of the DLC films and the FWHM(G), originating from the structural change from DLC to GLC and PLC.