Achim von Keudell

Thermal conductivity of amorphous carbon thin films

Thermal conductivities Λ of amorphous carbon thin films are measured in the temperatures range 80–400 K using the 3ω method. Sample films range from soft a-C:H prepared by remote-plasma deposition (Λ=0.20 W m−1 K−1 at room temperature) to amorphous diamond with a large fraction of sp3 bonded carbon deposited from a filtered-arc source (Λ=2.2 W m−1 K−1). Effective-medium theory provides a phenomenological description of the variation of conductivity with mass density. The thermal conductivities are in good agreement with the minimum thermal conductivity calculated from the measured atomic density and longitudinal speed of sound.

Formation of polymer-like hydrocarbon films from radical beams of methyl and atomic hydrogen

Abstract Amorphous hydrogenated carbon films (a-C:H) are usually deposited in low-temperature plasmas from a hydrocarbon precursor gas. The feed gas 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, flux, and energy of the film forming species. Depending on process parameters, film properties range from polymer-like and soft films to hard and wear resistant coatings. Despite the great importance of this material for a wide range of applications, detailed knowledge on elementary mechanisms of film formation at the plasma–surface boundary is still lacking. One approach to isolate and to quantify individual growth mechanisms is to study selected surface processes in quantified radical-beam experiments. In recent years, such an experiment employing radical sources for atomic hydrogen (H) and methyl radicals (CH3) has been developed in our group. The interaction of these species with a-C:H films is monitored in real time by ellipsometry and infrared spectroscopy. The formation of polymer-like hydrocarbon films from beams of methyl radicals and atomic hydrogen is considered a model system for a-C:H growth in low-temperature plasmas from a hydrocarbon precursor gas. Ion-induced effects are not considered, because they are of minor importance for the formation of polymer-like C:H films. It is shown that the sticking coefficient of methyl radicals s(CH3) is only 10−4 at room temperature and becomes negative 10−4 at a substrate temperature of 600 K indicating film etching rather than film growth. Above 700 K, s(CH3) is again positive in the range of 10−4 and the formation of a graphite-like C:H layer via CH3 adsorption is observed. The low sticking coefficient of s(CH3)∼10−4 can be enhanced by two orders of magnitude to s(CH3|H)∼10−2, if an additional flux of atomic hydrogen is simultaneously interacting with the surface. This growth synergism is described by a model based on dangling bond creation via abstraction of surface bonded hydrogen by incoming atomic hydrogen, followed by chemisorption and incorporation of CH3. Methyl incorporation is a two-step process consisting of the formation of a tri-hydride terminated surface by CH3 chemisorption followed by hydrogen elimination due to hydrogen abstraction by incident H, which transforms tri- into mono-hydride surface groups. The existence of a partly tri-hydride terminated surface is directly observed via infrared spectroscopy. The process of forming or removing tri-hydrides can explain not only the measured growth rates and mutual interactions between CH3 and H in steady state, but also the dynamic of s(CH3|H) after a step-like change of the CH3 and H fluxes. Based on a modeling of the experimental results using a set of rate equations, a cross-section between 2.4 and 5.4 A2 for CH3 chemisorption at a dangling bond and a cross-section of ∼10−3 A2 for abstraction of surface bonded hydrogen by CH3 are determined. The consequences of these results for the understanding of film formation in low-temperature plasmas are discussed.

Surface processes during thin-film growth

The growth of thin films from low-temperature plasmas plays an important role in many applications such as optical or wear-resistant layers or for the fabrication of electronic devices. Albeit of great importance, the underlying growth mechanisms responsible for film formation from low-temperature plasmas are not well known. The direct identification of a growth mechanism is often hampered by the huge complexity of the bulk plasma processes and the plasma-surface interaction. The distribution of impinging species is very diverse, and ions, radicals and neutrals are interacting simultaneously with the growing film surface. A macroscopic quantity such as the growth rate can be the result of possible synergisms and anti-synergisms among a large variety of growth precursors. Due to the broad range of plasma-deposited materials as well as deposition methods, the objective of this paper is not to review them all, but to present a basic overview on the elementary surface mechanisms of radicals and ions. On the basis of these surface reactions, typical growth models will be discussed. As an example, for surface processes during thin-film growth, the current deposition models for amorphous hydrogenated carbon and silicon films are presented.

Mechanisms of the Deposition of Hydrogenated Carbon Films

The paper reviews the elementary processes during the plasma-enhanced chemical vapour deposition of hydrogenated carbon films from methane, with special emphasis on the surface processes which determine the growth rate and film structure. Corresponding model calculations are critically discussed in comparison to experimental findings. Whereas a simplified plasma modeling can be performed with some reliability, only very limited information is available on the the surface physical and chemical mechanisms determining the growth rate as well as the stoichiometry and the structure of the deposited films. Proposed surface models involving widely different processes yield similar results and are thus indiscernible in comparison to results from deposition experiments. Nevertheless, reasonable fits to growth data can be obtained using a combined plasma-surface model. For the formation of film structure, recent ellipsometric data indicate that hydrogen chemistry might play a decisive role in addition to or rather than ion collisional effects.

Calibration of a miniaturized retarding field analyzer for low-temperature plasmas: geometrical transparency and collisional effects

Retarding field analyzers (RFAs) are important diagnostics to measure fluxes and energies of ions impinging onto the wall of a plasma reactor. Any quantitative use of the data requires a proper calibration, which is here performed for a miniaturized RFA. The calibration accounts for the transparencies of the RFA grids as well as for collisions inside the RFA. An analytical model is derived which covers both geometrical and collisional effects. The model is calibrated and experimentally verified using a Langmuir probe. We find that the transparency of an RFA is a random variable which depends on the individual alignment of the RFA grids. Collisions inside the RFA limit the ion current transfer through the RFA at higher pressures. A simple method is presented which allows one to remove these artefacts from the RFA data and to obtain quantitative ion velocity distributions.

Elimination of biological contaminations from surfaces by plasma discharges: chemical sputtering.

Plasma treatment of surfaces as a sterilisation or decontamination method is a promising approach to overcome limitations of conventional techniques. The precise characterisation of the employed plasma discharges, the application of sensitive surface diagnostic methods and targeted experiments to separate the effects of different agents, have led to rapid progress in the understanding of different relevant elementary processes. This contribution provides an overview of the most relevant and recent results, which reveal the importance of chemical sputtering as one of the most important processes for the elimination of biological residuals. Selected studies on the interaction of plasmas with bacteria, proteins and polypeptides are highlighted, and investigations employing beams of atoms and ions confirming the prominent role of chemical sputtering are presented. With this knowledge, it is possible to optimize the plasma treatment for decontamination/sterilisation purposes in terms of discharge composition, density of active species and UV radiation intensity.

Control of the plasma chemistry of a pulsed inductively coupled methane plasma

The plasma chemistry in a pulsed inductively coupled plasma from methane is investigated. The densities of stable neutrals up to C4H10 are measured using quantitative mass spectrometry. It is demonstrated that the plasma composition for neutral species depends uniquely on the dissipated energy per source gas molecule, Emean. This quantity can most easily be controlled by changing plasma power, gas flow or duty cycle of the pulsed plasma.

Temperature dependence of the sticking coefficient of methyl radicals at hydrocarbon film surfaces

The temperature dependence of the interaction of methyl radicals with the surface of a hard, amorphous hydrogenated carbon film is investigated using in situ real-time ellipsometry and infrared spectroscopy. This interaction is considered as an important process during plasma deposition of polymer-like hydrocarbon films or formation of polycrystalline diamond in methane-containing discharges. At room temperature CH3 adsorbs at sp2-coordinated CC bonds at the physical surface of the hard C:H film and forms a completely sp3-hybridized C:H adsorbate with a thickness of ∼0.17 nm. In the following, steady-state film growth is observed with a sticking coefficient of s(CH3)=10−4. At a substrate temperature of T=570 K, incident CH3 causes net erosion with an etching yield of Y(CH3)=10−4. At temperatures above 650 K the sticking coefficient of CH3 becomes positive again, leading to a graphite-like C:H adsorbate. CH3 adsorption is described by a reaction scheme based on the creation of dangling bonds at the film su...

Ion-enhanced oxidation of aluminum as a fundamental surface process during target poisoning in reactive magnetron sputtering

Plasma deposition of aluminum oxide by reactive magnetron sputtering (RMS) using an aluminum target and argon and oxygen as working gases is an important technological process. The undesired oxidation of the target itself, however, causes the so-called target poisoning, which leads to strong hysteresis effects during RMS operation. The oxidation occurs by chemisorption of oxygen atoms and molecules with a simultaneous ion bombardment being present. This heterogenous surface reaction is studied in a quantified particle beam experiment employing beams of oxygen molecules and argon ions impinging onto an aluminum-coated quartz microbalance. The oxidation and/or sputtering rates are measured with this microbalance and the resulting oxide layers are analyzed by x-ray photoelectron spectroscopy. The sticking coefficient of oxygen molecules is determined to 0.015 in the zero coverage limit. The sputtering yields of pure aluminum by argon ions are determined to 0.4, 0.62, and 0.8 at 200, 300, and 400 eV. The vari...

Particle-beam experiment to study heterogeneous surface reactions relevant to plasma-assisted thin film growth and etching

An ultrahigh-vacuum-based particle-beam experiment to study heterogeneous surface reactions relevant to plasma–surface interaction processes is presented. The experiment comprises two radical beam sources and a source for low energy ions. As diagnostic tools real-time in situ ellipsometry and infrared spectroscopy are implemented. The infrared sensitivity for thin films is enhanced through application of an optical cavity substrate. The fluxes of the radical beam sources are quantified absolutely for the production of hydrogen atoms and methyl radicals. The ion source is also quantified for a wide variety of ionic species, e.g., He+, Ar+, H+, H2+, H3+, and CH3+. Ion energies from above 1 keV down to 1 eV are achievable. The setup allows one to investigate heterogeneous surface processes of one single species or simultaneous interaction of up to three different, individually controllable species with a surface of interest. By running the radical sources to produce hydrogen and methyl radicals and the ion s...