Hydrogen elimination as a key step for the formation of polymerlike hydrocarbon films

Abstract

Recent experiments using H and CH3 radical beams as a model system for plasma deposition of C:H films revealed that CH3 adsorption at dangling bonds at the film surface is an important step for film formation. CH3 adsorption onto dangling bonds, which are created by hydrogen abstraction from incoming H, implies a net incorporation of two hydrogen atoms per carbon atom during steady state growth, although the H/C ratio of the deposited layers is only ∼1. Therefore, a reaction step of hydrogen elimination is essential to describe C:H film formation consistently. This hydrogen elimination is investigated by exposing polymerlike C:H films to quantified radical beams of CH3 and H. The resulting C:H film formation is monitored by in situ ellipsometry and infrared reflection spectroscopy. Based on experimental data, a model is developed to describe hydrogen elimination via a two step process: (i) first abstraction of surface bonded hydrogen from incoming H, followed by (ii) the recombination of dangling bonds via a local rearrangement of the C:H network. The latter process is self-limiting leading to a characteristic H/C ratio of 1 as a stoichiometric limit. Therefore, hydrogen elimination by atomic hydrogen corresponds to a key step for C:H film growth.