W. Möller

Tridyn — A TRIM simulation code including dynamic composition changes

Abstract Based on the sputtering version of the TRIM program for multicomponent targets, a Monte Carlo code has been developed which computes range profiles of implanted ions, composition profiles of the target and sputtering rates for a dynamically varying target composition. It takes into account compositional changes both due to the spatial distribution of target atoms deposited in collision cascades, and due to the presence of the implanted ions. The local density of the target is allowed to relax according to a given function of the densities of the individual components. The applications of the program cover a wide range of problems like the collisional atomic mixing of multilayered targets, dynamic implantation profiles at large fluences, and the fluence-dependent preferential sputtering of multicomponent materials. The present paper provides a description of the program and a critical comparison to similar Monte Carlo codes. As an application, the behaviour of the Ta-C system under He bombardment is studied with respect to sputtering yields, surface composition and composition profiles. Satisfactory agreement is obtained with experimental results given in the literature.

On the structure of thin hydrocarbon films

We compare reported compositions of a‐C:H films in a ternary phase diagram. It is assumed that the films comprised three phases: sp3 hybridized carbon, sp2 hybridized carbon and hydrogen. The data are found to split into two well‐separated groups. This separation depends on the method used to measure the sp3/sp2 ratio. We conclude from the comparison of NMR and infrared data that infrared analysis does not provide a quantitative measure of the sp3/sp2 ratio.

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.

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.

Mechanism of enhanced sputtering of carbon at temperatures above 1200°C

Abstract At temperatures above 1300 K, graphite shows an increasing sputtering yield with increasing temperature. At a given temperature, the enhancement scales with the energy deposited into nuclear collisions in the surface layer for different ions and energies. A new model is applied for the enhancement of the sputtering yield, which was originally developed for the explanation of the volume swelling of graphite under neutron bombardment, and has been modified for near surface ion implantation. It assumes the formation of interstitial atoms and vacancies, which diffuse with different activation energies and recombine or annihilate according to various mechanisms. The flux of interstitials arriving at the surface is assumed to evaporate at the elevated temperature. This model is discussed and compared to experimental sputtering data. The predictions of the model are found to be in good quantitative agreement with the experimental data. The results are extrapolated to fusion-relevant conditions predicting the enhanced sputtering of carbon during low-energy (10 to 1000 eV) deuterium implantation.

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.

Properties of carbonization layers relevant to plasma-surface-interactions

Abstract Thin carbonaceous films have been prepared on samples of stainless steel, Inconel 600, Inconel 625 and silicon by carbonization from CH 4 -H 2 mixtures. They were investigated by transmission electron microscopy, nuclear reaction and backscattering techniques and thermal desorption. The films are semi-transparent, amorphous and homogeneous down to ~10 A. Their composition is H/C = 0.4 ± 0.1 and independent of the substrate. The density is 1.4 g cm −3 , the average C-C atomic distance ~2.5 A. The films release H 2 and about 3–10% CH 4 upon heating. The desorption of CH 4 occurs in a well defined peak around 500°C, whereas H 2 is relased up to 1100°C. The films turn black in colour after the CH 4 has been released, possibly due to a transformation from the initial amorphous into a graphitic structure.

Modelling and computer simulation of ion-beam- and plasma-assisted film growth

Abstract Computer simulations of the binary collision approximation type have been applied to problems of the ion-assisted deposition of thin films. Calculations can be performed using simplifying rate equation models, into which yields obtained from static Trim simulations are inserted. Alternatively, the dynamic-composition code Tridyn allows direct and complete simulations of the time-dependent processes. Results are shown for different processes of ion-beam-assisted deposition and plasma-enhanced chemical vapour deposition. Simulations of the formation of boron nitride films deposited from evaporated boron and energetic nitrogen show excellent agreement with experimental results for nitrogen concentrations below the stoichiometric limit. For high N-to-B flux ratios, non-collisional mechanisms (ion-induced outdiffusion and surface trapping of outdiffusing nitrogen) have been included in the simulations, again producing good agreement with the experimental results. Under appropriate conditions, the ion-assisted evaporation of titanium is subject to a delicate balance of deposition and erosion, which results in selective deposition on different substrates and a self-controlled stationary film thickness. Finally, a simple model is evaluated for the composition of hydrogenated carbon films grown in methane plasmas. The hydrogen content of the films decreases with increasing ion energy owing to ion-induced release of hydrogen.

Nitrogen implantation into metals: a numerical model to explain the high temperature shape of the nitrogen depth profile

Abstract The depth profile of nitrogen implanted in pure iron shows a surface peak whose area increases with the temperature of the sample during the implantation. This surface peak has already been observed by other workers. The nitrogen depth profiles have been measured on a set of iron samples (99.5% purity) implanted with 100 keV N 2 + ions at different doses ( 2 × 10 16 −5 × 10 17 ions cm −2 ) and at different temperatures (20–200 °C). In order to explain the shape of the depth profiles and in particular the existence of the surface peak and its dependence on implantation temperature, a numerical model has been developed. This model takes into account preferential sputtering, ion beam mixing, thermal diffusion, radiation-enhanced diffusion (RED), radiation-induced segregation (RIS), secondary phase precipitation and radiolytic decomposition of the precipitates. The surface peak cannot be explained by RED or RIS alone but is the consequence of an enhanced precipitation near the surface of the sample. Samples of copper, molybdenum and nickel have also been investigated in order to confirm that hypothesis and to show that there is a correlation between the presence of the surface peak and the formation energy of the corresponding nitrides. The ion deposition profiles, sputtering yields, ion beam mixing and radiolytic decomposition of the precipitates were calculated with the TRIDYN program.

Hydrogen-Ion-Induced Detrapping of Implanted Deuterium in Graphite

Abstract The release of saturated 300 eV D + implants in graphite by 6–30 keV H + bombardment has been measured at temperatures between 295 and 473 K and fluences up to more than 3 × 10 18 H + -ions/cm 2 . The decrease of retained D is found to be slower than exponential. Release rates decrease with increasing energy indicating that detrapping is primarily due to atomic collisons. A faster release is observed at higher target temperatures during H + bombardment and some enhancement persists if the target is annealed after deuterium implantation. The release of low energy implants by high energy ions cannot be described by the local mixing model. Instead, a model accounting for local molecular recombination and detrapping gives qualitative agreement with experimental dependence on H + energy. It cannot, however, explain the temperature dependence.