A. von Keudell

Plasma chemical vapor deposition of hydrocarbon films: The influence of hydrocarbon source gas on the film properties

Hydrocarbon films were prepared by electron cyclotron resonance plasma deposition from different hydrocarbon source gases at varying ion energies. The source gases used were the saturated hydrocarbons CH4, C2H6, C3H8, C4H10 (n- and iso-) and the unsaturated hydrocarbons C2H4 and C2H2 as well as mixtures of these gases with hydrogen. Film deposition was analyzed in situ by real-time ellipsometry, and the resulting films ex situ by ion-beam analysis. On the basis of the large range of deposition parameters investigated, the correlation between hydrocarbon source gas, deposition parameters, and film properties was determined. The film properties are found to be influenced over a wide range not only by the energy of the impinging ions, but also by the choice of source gas. This is in contrast to a widely accepted study where no dependence of the film properties on the source gas was observed, this being ascribed to a “lost-memory effect.” A strong correlation was found between the hydrogen content of the film...

Growth and erosion of hydrocarbon films investigated by in situ ellipsometry

The growth of hydrocarbon films (C:H films) from a methane plasma and their erosion by a hydrogen plasma are investigated by means of in situ ellipsometry. The kinetic energy of the ions impinging on the surface during deposition and erosion is varied by applying a rf bias resulting in a dc self‐bias ranging from floating potential up to 100 V. In addition, the substrate temperature is varied from room temperature up to 600 K. The direct comparison between the growth and erosion indicates that the temperature dependence of the growth rate during deposition from a methane plasma is caused by the temperature‐dependent erosion due to reactions with the abundant atomic hydrogen. Furthermore, the synergistic effects between hydrogen ions and atomic hydrogen on the etch rate of C:H films are investigated. The underlying surface reactions during the erosion show up in the optical response of the deposited films as measured by ellipsometry. These results are compared with findings in the literature on the element...

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

A combined plasma‐surface model for the deposition of C:H films from a methane plasma

The deposition of C:H layers by an electron‐cyclotron‐resonance plasma from methane was investigated. C:H was deposited at a methane pressure of 1.6 Pa and a substrate temperature between room temperature and 700 K. The film composition, morphology, and structure were investigated by high‐energy ion beam analysis and scanning electron microscopy. A combined plasma‐surface model for thin‐film deposition is proposed, which includes the electron‐induced dissociation of methane in the plasma and a growth model. The dominant reactions for film growth are the adsorption of the radical CH3, the direct incorporation of the ions, and the etching reactions with atomic hydrogen from the plasma. A consistent description for the deposition of hydrocarbon layers emerges. It compares favorably with measurements on the temperature dependence of the film growth and the influence of variable gas flow through the reactor on the growth rate and the film morphology.

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.

Direct Identification of the Synergism between Methyl Radicals and Atomic Hydrogen during Growth of Amorphous Hydrogenated Carbon Films

The simultaneous interaction of methyl radicals (CH3) and atomic hydrogen (H) with the surface of amorphous hydrogenated carbon (a-C:H) film is investigated. Two identical quantified beam sources for H and CH3 are used. The growth and/or erosion during the simultaneous interaction of the two beams with an amorphous hydrogenated carbon film is monitored by using in situ real-time ellipsometry at a substrate temperature of 320 K. Interaction with the CH3 beam alone causes slow growth, corresponding to a sticking coefficient for CH3 of ∼3×10−5. Simultaneous interaction of the atomic hydrogen beam and the CH3 radical beam yields a sticking coefficient for CH3 of 3×10−3, which is two orders of magnitude larger than for CH3 alone. From a microscopic modeling of this synergistic growth, the reaction probability for CH3 adsorbing at an adsorption site, which is created by atomic hydrogen at the surface, is derived to be 0.14.

Erosion of thin hydrogenated carbon films in oxygen, oxygen/hydrogen and water plasmas

Abstract The erosion of amorphous hydrogenated carbon films in oxygen, oxygen/hydrogen and water electron cyclotron resonance plasmas was investigated by in situ ellipsometry. The erosion was measured as a function of the energy of the impinging ions and the substrate temperature. Erosion is most effective in pure oxygen plasmas. The erosion rate rises with increasing ion energy and substrate temperature, in the latter case however only at low ion energies. The reaction layer at the surface of the eroded film is further analyzed by X-ray photoelectron spectroscopy (XPS). The C 1s peak of the XPS spectra shows, that oxygen is implanted in the films and forms double and single bonds to the carbon atoms. This modification, however, is limited to a few atomic layers.

Deposition of dense hydrocarbon films from a nonbiased microwave plasma

An electron cyclotron resonance plasma was used to prepare C:H layers from methane. The temperature dependence of the deposition rate was investigated at substrate temperatures ranging from room temperature to 700 K, at a gas pressure of 1.6 Pa. Despite low ion energies corresponding to the plasma potential, transparent hard coatings were obtained at elevated temperature with a density up to 2 g cm−3. A deposition model is proposed which describes the growth from an adsorbed layer, including surface reactions with radicals and atomic hydrogen as well as the direct incorporation of ions. Two different deposition processes can be identified, yielding polymerlike films in the temperature range up to 450 K and dense hydrocarbons above this temperature. The observed temperature dependence of the film properties such as H/C ratio, index of refraction, and density is consistent with the predictions of the model.