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 emana ting from methane and acetylene discharges are investigated by depositing t hin films inside a cavity. The walls of this cavity are made from silicon s ubstrates. Particles from the plasma can enter the cavity through a slit. T he surface loss probability is determined by analysis of the deposition pro file inside the cavity. This surface loss probability corresponds to the su m of the probability for effective sticking on the surface and the probabil ity for the formation of a nonreactive volatile product via surface reactio ns. 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, respective ly. The dominant contribution in the neutral radical flux emanating from a methane discharge towards the surface consists of CH3 radicals, as known fr om experiments using mass spectrometry. Furthermore, it is known from liter ature that the upper limit for the reaction probability for CH3 is in the r ange 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 neut ral 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 acety lene discharges, we conclude that in both cases C2Hy species are the domina nt neutral precursors for film growth. (C) 1999 American Institute of Physi cs. [S0003-6951(99)03425-7].