C. Hopf

Growth mechanism of amorphous hydrogenated carbon

Amorphous hydrogenated carbon (a-C:H) films offer a wide range of applications due to their extraordinary material properties like high hardness, chemical inertness and infrared transparency. The films are usually deposited in low temperature plasmas from a hydrocarbon precursor gas, which is dissociated and ionized in the plasma and radicals and ions impinging onto the surface leading to film growth. Final stoichiometry and material properties depend strongly on composition, fluxes and energy of the film forming species. The underlying growth mechanisms are investigated by means of quantified particle beam experiments employing radical sources for atomic hydrogen (H) and methyl (CH ) radicals as well as an argon ion beam. The interaction of these species 3 with amorphous hydrogenated carbon films is investigated in real time by ellipsometry and infrared spectroscopy. The formation of hydrocarbon films from beams of CH , H and Ar is considered a model system for growth of amorphous hydrogenated q 3 carbon films in low temperature plasmas from a hydrocarbon precursor gas. The growing film surface can be separated in a chemistry-dominated growth zone with a thickness of 2 nm on top of an ion-dominated growth zone. In the chemistry-dominated growth zone incident atomic hydrogen governs the surface activation as well as film stoichiometry. In the ion-dominated growth zone, especially hydrogen ions, displace bonded hydrogen in the film. Displaced hydrogen recombine and form H molecules, 2

Chemical sputtering of hydrocarbon films

Erosion of hydrocarbon films at room temperature due to argon ions and thermal atomic hydrogen is investigated in a particle-beam experiment. Physical sputtering by the ions is observed at energies ⩾200 eV and reaches a yield of 0.5 at an ion energy of 800 eV. The measured yields are in agreement with TRIM.SP computer simulations, and a threshold energy of ≃58 eV is derived for physical sputtering. Erosion by simultaneous fluxes of argon ions and thermal hydrogen atoms is observed at all energies investigated down to 20 eV and reaches a yield of about 3 at an ion energy of 800 eV and a hydrogen-atom-to-argon-ion-flux ratio of 400. It is proposed that the significant decrease of the threshold energy as well as the increase of the absolute yields is due to the process of chemical sputtering: Within a collision cascade caused by the incident ions, bonds are broken and instantaneously passivated by the abundant flux of atomic hydrogen. This leads to the formation of hydrocarbon molecules within the common range of ions and hydrogen atoms. Finally, the molecules diffuse to the surface and desorb. The threshold energy of chemical sputtering is on the order of typical carbon–carbon bond energies in organic compounds of several eV. Based on this mechanism a model for the energy dependence of the chemical sputtering yield is presented, which leads to good agreement with the data.

Surface loss probabilities of hydrocarbon radicals on amorphous hydrogenated carbon film surfaces: Consequences for the formation of re-deposited layers in fusion experiments

The success of present-day fusion experiments relies on the use of a divertor which efficiently pumps impurities generated by erosion of the first wall. In most fusion experiments the divertor surface consists of graphite tiles or carbon fiber composites. They are bombarded by ions from the scrape-off layer which are guided into the divertor by the magnetic field. This impinging ion flux leads to sputtering of the divertor tiles releasing carbon and hydrocarbon compounds into the gas phase. This emitted carbon flux is excited in the divertor plasma and dissipates the plasma power via radiation, leading to a reduction of the heat flux onto the divertor surface. Most emitted carbon and hydrocarbon species re-deposit promptly in or in proximity to the divertor. This balance between deposition and erosion is crucial for the performance of a divertor in a next-step device, since the total lifetime before replacement strongly depends on the ability to control this re-deposition.

Surface loss probabilities of hydrocarbon radicals on amorphous hydrogenated carbon film surfaces

The surface loss probabilities of hydrocarbon radicals on the surface of amorphous hydrogenated carbon (C:H) films are investigated by depositing films inside a cavity with walls made from silicon substrates. This cavity is exposed to a discharge using different hydrocarbon source gases. Particles from the plasma can enter the cavity through a slit. The surface loss probability β is determined by analysis of the deposition profile inside the cavity. This surface loss probability corresponds to the sum of the probabilities of effective sticking on the surface and formation of a nonreactive volatile product via surface reactions. By comparing the deposition profiles measured in CH4, C2H2, C2H4, C2H6 discharges one obtains for C2H radicals β=0.80±0.05, for C2H3 radicals β=0.35±0.1, and for C2H5 radicals β<10−3. The growth rate of C:H films is, therefore, very sensitive to any contribution of undersaturated C2Hx species in the impinging flux from a hydrocarbon discharge.

Chemical sputtering of hydrocarbon films by low-energy Ar+ ion and H atom impact

Erosion of hydrocarbon films is investigated in a particle-beam experiment employing sources for argon ions and hydrogen atoms. Sputtering by argon ions sets in above a threshold of 58 eV and reaches a yield of 0.5 at an ion energy of 800 eV. Sputtering by argon ions with an additional flux of thermal hydrogen atoms towards the surface occurs above a threshold of 1.3 eV and reaches a yield of about three at an ion energy of 800 eV and a hydrogen atom to argon ion flux ratio of 400. A pronounced dependence of the yield on this flux density ratio is observed. It is proposed that the shift of the threshold energy as well as the change in the absolute yields is due to the process of chemical sputtering: within a collision cascade of the incident ions, broken bonds are instantaneously passivated by the abundant flux of atomic hydrogen. This leads to the formation of hydrocarbon molecules within the ion penetration range, which diffuse to the surface and desorb. This has important implications for the lifetime of plasma wall components in the divertor region of next step nuclear fusion devices.

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 ∼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...

Ion-induced surface activation, chemical sputtering, and hydrogen release during plasma-assisted hydrocarbon film growth

Synergisms between different species emerging from hydrocarbon plasmas can enhance the chemisorption of radicals at the surface of a growing film. It has been shown that the rate of CH3 chemisorption can be increased by a simultaneously incident flux of ions or H atoms; the latter species cause the formation of surface dangling bonds, which serve as preferred adsorption sites for incoming CH3. These synergisms can, however, be counteracted by erosion processes due to the same species. The interplay between the enhancement of film growth by ion/H atom assisted chemisorption and simultaneous erosion processes is investigated in a particle-beam experiment. An a-C:H film is exposed to three individually controllable quantified particle beams—ions, CH3, and atomic hydrogen. The data can be consistently explained if we include two effects counteracting ion- or H-induced chemisorption: (i) recombination of neighboring dangling bonds and (ii) the occurrence of chemical sputtering in case of an intense H flux. Fin...

Chemical sputtering of carbon by nitrogen ions

Chemical sputtering of amorphous hydrogenated carbon layers by nitrogen molecular ions was studied as a function of the ion energy in the range from 30to900eV. The sputtering yield shows only a very weak variation with energy in the range from 900 down to 50eV. For lower energies it decreases significantly. This behavior is interpreted as an indication of chemical sputtering.

Chemical sputtering of carbon films by simultaneous irradiation with argon ions and molecular oxygen

The erosion of hard amorphous hydrocarbon films by bombardment with argon ions and simultaneous exposure to thermal molecular oxygen is studied as a function of oxygenflux density (0-11400 times the ionflux density), ion energy (20-800eV), and surface temperature (110-875K). While erosion due to Ar + ions only is dominated by physical sputtering, the additional presence of molecular oxygen leads to a marked increase of erosion, indicating chemical sputtering. The erosion yield increases with both ion energy and oxygen flux density. Starting from about 700K thermal chemical erosion (combustion) by O2 is observed even without ion bombardment. Additional ion bombardment in this temperature range causes an increase of the erosion rate over the sum of thermal chemical erosion and the rate observed at room temperature. Below 300K, the rate increases with decreasing temperature. We explain the latter behavior by the ion-induced reaction of adsorbed oxygen which constitutes a significant surface coverage only at low temperatures. A rate equation model is presented, which incorporates the mechanisms of physical sputtering, chemical reaction of O2 at reactive sites created by ion bombardment, the ion-induced reaction of adsorbed oxygen and ion-enhanced thermal chemical erosion. The models nine free parameters are optimized by fitting 68 experimental data points. The model yields good agreement in all investigated dependences.

The influence of hydrogen ion bombardment on plasma-assisted hydrocarbon film growth

The influence of hydrogen ion bombardment on plasma deposition of amorphous hydrogenated carbon films is investigated in a particle beam experiment employing sources for methyl radicals as carbon carrying precursor, for atomic hydrogen, and for low energy ions. Elementary plasma-surface processes such as ion-induced formation of surface dangling bonds (dbs) and ion-induced formation of molecules in the subsurface region are studied. It is shown that the absolute growth rate can be dominated by the flux of atomic hydrogen towards the growing film surface since the ion-induced formation of surface dbs may be largely compensated by hydrogen addition. The film properties are dominated by subsurface hydrogen depletion caused by the ion bombardment.