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

Abstract

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 ∼0.65±0.15 and in an acetylene discharge ∼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.

Redeposition of amorphous hydrogenated carbon films during thermal decomposition

“…The properties of a-C:H films deposited from CH 4 is by and large identical to those of a-C:D films deposited from CD 4 . The most important deposition parameter is the energy of ions impinging on the growing film surface during deposition [17][18][19][20].…”

“…6). So we have to conclude that CD 4 measured in the secondary peaks is not deposited as a CD 4 molecule, but is a reaction product generated during the following desorption step. This indicates that the released species spectrum is not identical to the adsorbed species spectrum.…”

“…A typical hard film with a hydrogen content of about 30% and a soft film with 50%. Soft a-C:D films exhibit a desorption maximum at T≈740 K while hard films have their maximum at T≈870 K. For hard films the dominantly released species are D 2 and CD 4 . On the contrary, the product spectrum of soft films is dominated by long chain hydrocarbons.…”

“…II C. Figure 6 shows the CD 4 signal measured with the mass spectrometer and the pressure gauge reading in the main chamber as a function of time during two separate experiments in the oven using a hard and a soft film, respectively. In each experiment first a normal TE run was performed and then the oven was shifted in steps of 2 cm.…”

“…A number of investigations conducted in fusion plasma devices as well as in laboratory experiments have been devoted to study the deposition of such redeposited layers [3][4][5][6][7][8][9][10][11]. Redeposited layers growing in remote areas without direct plasma contact are soft hydrocarbon films with high hydrogen content and it seems that hydrocarbon species with a relatively high sticking coefficient contribute dominantly to deposition [12].…”