Comparative experimental analysis of the a-C:H deposition processes using CH4 and C2H2 as precursors

Abstract

The plasma enhanced chemical vapor deposition of a-C:H films using methane and acetylene as precursors was studied. Noninvasive in situ techniques were used to analyze the plasma processes with respect to the self-bias voltage, the displacement currents to the grounded electrode, the neutral gas composition, the optical sheath thickness as well as current and energy of the ions hitting the powered electrode. The a-C:H films were characterized for their deposition rate, surface roughness, hardness, mass density, and hydrogen content. Ion mean free paths, suitable for low-pressure rf sheaths, have been quantified for both precursors. The film with the highest hardness of 25GPa was formed in the C2H2 discharge when the mean energy per deposited carbon atom was approximately 50eV. The hardness obtained with the CH4 discharge was lower at 17GPa and less sensitive to changes in the process parameters. It was found that the creation of hard (hardness >15GPa) a-C:H films from both precursors is possible if the mean energy per deposited carbon atom exceeds only ∼15eV. Further film characteristics such as surface roughness and hydrogen content show the interplay of ion flux and deposition from radicals to form the a-C:H structure and properties.

Plasma parameters of pulsed-dc discharges in methane used to deposit diamondlike carbon films

“…The ion temperature T i was close to room temperature. The dominant heating mechanism for rf discharges in CH 4 at this p was observed by Peter et al 42 to be Ohmic heating. Processes heated by such a mechanism fulfill…”

“…According to Peter et al, 42 the value of i for CH 4 at a p of 10 Pa is around 1.2 mm, which is of the order of or lower than s. The value of n at the sheath edge is assumed to be n s Ϸ 0.63n e . In this case, V 0 equals V p minus the cathode voltage.…”

“…Hydrocarbons are the main precursors for the synthesis of DLC films: propane ͑C 3 H 8 ͒, 39 benzene ͑C 6 H 6 ͒, 40 acetylene ͑C 2 H 2 ͒, and CH 4 are of particular importance. 21,[41][42][43] Although C 2 H 2 produces harder films and faster deposition, CH 4 is commonly used to coat electronic devices and is less harmful. 21 The precursor can be mixed with other gases such as argon ͑Ar͒ and hydrogen ͑H 2 ͒ in order to modify the plasma chemistry and the surface morphology.…”

Growth Mechanism of Oxygen-Containing Functional Plasma Polymers

“…The nearly V 0 % W 0.5 relation indicates Ohmic heating. [22] The sheath voltage can be calculated to be 0.39 V 0 for symmetric discharges, [39,40] and the mean ion energies E i impinging on the substrate to be roughly 15% of the sheath voltage at a pressure of 10 Pa due to collisions in the plasma sheath. [41] Hence, the mean ion energies reached during the a-C:H:O film growth were in the range of a few tens of eV.…”

“…The latter, however, requires high ion energies per deposited atom. [22] Therefore, the knowledge of electron density beside sheath potential is important to obtain the energy dissipated in the growing film.…”