Surface loss probabilities of the dominant neutral precursors for film growth in methane and acetylene discharges

The surface loss probabilities of the dominant neutral growth species emanating from methane and acetylene discharges are investigated by depositing thin films inside a cavity. The walls of this cavity are made from silicon substrates. Particles from the plasma can enter the cavity through a slit. The surface loss probability is determined by analysis of the deposition profile inside the cavity. This surface loss probability corresponds to the sum of the probability for effective sticking on the surface and the probability for the formation of a nonreactive volatile product via surface reactions. In a methane discharge the surface loss probability is similar to 0.65 +/- 0.15 and in an acetylene discharge similar to 0.92 +/- 0.05, respectively. The dominant contribution in the neutral radical flux emanating from a methane discharge towards the surface consists of CH3 radicals, as known from experiments using mass spectrometry. Furthermore, it is known from literature that the upper limit for the reaction probability for CH3 is in the range of 10(-2). This leads us to the conclusion that the CH3 radical is not the dominant neutral growth precursor in a methane discharge. This result refutes the widely accepted picture that CH3 radicals are the dominant neutral growth precursor in a-C:H growth from methane. From a comparison of the composition of the neutral flux towards the surface from methane and acetylene discharges, we conclude that in both cases C2Hy species are the dominant neutral precursors for film growth. (C) 1999 American Institute of Physics. [S0003-6951(99)03425-7].