Diederik Depla

Understanding the discharge voltage behavior during reactive sputtering of oxides

The discharge voltage was measured for 15 different metallic target materials at constant current before and after plasma oxidation in order to understand its behavior during reactive magnetron sputtering. Plasma oxidation of the target surface was achieved by sputtering the target in pure oxygen. The discharge voltage measured in pure argon is characteristic for each kind of metallic target and is related to the ion induced secondary electron emission (ISEE) coefficient of the target material. Based on this relation a value for the ISEE coefficient of the oxidized target surface can be calculated. Two distinct groups can be discerned: for one group the ISEE coefficient of the oxidized target surface is larger than the ISEE coefficient of the metal, while the opposite behavior is noticed for the second group. This difference seems to find its origin in the reduction behavior of the oxides under ion bombardment, since the ISEE coefficient of the oxide can be related to the simulated degree of reduction of ...

Reactive sputter deposition of TiN layers: modelling the growth by characterization of particle fluxes towards the substrate

The growth of reactively sputtered TiN films is discussed. First, an overview of the existing models in the literature that describe the development of the orientation and microstructure is given. Then, these models are critically confronted with the results of experiments published in the literature and performed by the authors. The latter experiments focus especially on the determination of the energy flux and atomic N/Ti flux towards the substrate and lead to the conclusion that these fluxes towards the substrate play a key role in the growth of the TiN films. This relation between these fluxes and the microstructure of the TiN films gives further evidence to the previously published extended structure zone model.

A heterogeneous Ru–hydroxyapatite catalyst for mild racemization of alcohols

Racemization of chirally stable secondary alcohols was performed using a truly heterogeneous catalyst under mild reaction conditions in the absence of any additive or cocatalyst. The catalyst is based on Ru3+ immobilized on calcium hydroxyapatite (Ru–HAP). The preparation of the calcium hydroxyapatite (HAP) support and the Ru content and immobilization were optimized leading to maximal catalytic performance. Via combined XRD, Raman, SEM, XPS, EPR, N2 adsorption, and ICP analysis, information on both the compositional and the structural properties of the material was obtained. The Ru–HAP catalyst is able to racemize benzylic as well as aliphatic secondary alcohols. The catalyst scope was further investigated for racemization of several mono- and polyfunctional secondary alcohols. Performing standard racemization of a reference substrate in the presence of a functional additive demonstrated the inhibitory effect of some of these functions. Finally the heterogeneous nature of the catalyst was verified by filtrate tests.

The metal flux from a rotating cylindrical magnetron: a Monte Carlo simulation

A Monte Carlo simulation of the metal flux from a small scale rotating cylindrical magnetron is presented. The model describes the sputtered particles trajectories through the gas in a user definable 3D set-up. The ejection positions of the sputtered particles are generated according to the simulated ion current density on the target. The thermal motion of the background gas is included, with collisions modelled based on either quantum chemical or screened Coulomb interaction potentials. Experimental characterization of the metal flux was performed for Cu, Al and Ti targets at a range of argon pressures (0.3-1 Pa) by measuring deposition rate distributions. A comparison with preliminary simulations showed the importance of a correct description of the nascent angular distribution of the sputtered particles. Therefore, this distribution was not considered as given by sputter simulation or analytical formula, but was instead reconstructed from the low pressure experimental deposition profiles. The typical heart-like shaped emission observed at low energy sputtering was found and a comparison was made with results from binary collision approximation modelling. The spatial, pressure and material dependence of the metal flux in the chamber was then simulated and found to be in good agreement with the experiment.

Towards a more complete model for reactive magnetron sputtering

Using an analytical model, the effect of reactive ion implantation on the state of the target surface during magnetron sputtering is studied. To describe the substrate condition during reactive sputter deposition, the model uses the well-known Berg model approach. However, the target condition is modelled differently, i.e. besides chemisorption the model includes reactive ion implantation. To test this latter modification, the simulation results are compared with the results of oblique reactive ion beam experiments. The good agreement forms the basis of using the model for reactive magnetron sputtering. It is shown that the well-known hysteresis behaviour of the deposition parameters can be described. Although at first sight the model is comparable to other models, it is shown that the kinetics are clearly different. A good description of the kinetics during reactive magnetron sputtering is essential to model, for example, the reactive sputter process using a rotating cylindrical magnetron.

Correlation between electron and negative O- ion emission during reactive sputtering of oxides

The energy distribution of negative O− ions has been measured during the reactive magnetron sputtering of 13 different target materials by the means of energy resolved mass spectrometry. For the same series of target materials the ion-induced secondary electron emission coefficient was determined in an earlier published research. A correlation between this ion-induced secondary electron emission coefficient and the emission of the high energetic negative O− ions was observed. This correlation should be taken into consideration in the selection of oxides for their high electron emission coefficients as these materials will emit at the same time negative O− ions.

Extracting organic contaminants from water using the metal–organic framework CrIII(OH)·{O2C–C6H4–CO2}

The water-stable metal-organic framework MIL-53(Cr) is able to adsorb phenol and p-cresol from contaminated water as well as the monomeric sugar D-(-)-fructose. Based on the isotherm for phenol uptake from the liquid phase, it is proposed that the framework breathes to maximize the uptake.

Modeling the flux of high energy negative ions during reactive magnetron sputtering

The negative ion flux during reactive sputtering from planar and rotating cylindrical magnetrons has been studied. Energy resolved mass spectrometry was used to measure the energy and mass distribution of the negative ions. Also the angular distribution of the high energy ions was characterized for planar as well as for rotating cylindrical magnetrons. Besides these measurements, a binary collision Monte Carlo simulation code, SiMTRA, was adapted in order to simulate the energy, mass, and angular distribution of the high energy negative ions. All simulated distributions, for both planar and rotating cylindrical magnetrons, were in excellent correspondence with the experimental observations. Also a model for the amount of high energy negative O− ions was proposed. Indeed, the logarithm of the amount of high energy negative O− ions is shown to be related to the secondary electron emission yield of the oxide target, and these two parameters are known to be related to the work function. The SiMTRA simulations...

Mechanisms of target poisoning during magnetron sputtering as investigated by real-time in situ analysis and collisional computer simulation

The evolution of reactive gas uptake at the target surface has been investigated by real-time in situ diagnostics during magnetron sputtering. Using a planar circular dc magnetron for reactive sputter deposition of TiN from a Ti target in an argon/nitrogen gas mixture, the target uptake of nitrogen was determined at varying gas flows of nitrogen using the N14(d,α)C12 nuclear reaction, directly demonstrating the target “poisoning” effect. The expected hysteresis behavior at increasing/decreasing nitrogen gas flow is confirmed. Within the precision of the measurement, the nitrogen content remains unaltered after switching off the magnetron, indicating the absence of a significant mobile fraction of nitrogen in the target. The maximum amount of retained nitrogen significantly exceeds one adsorbed monolayer, which is attributed to nitrogen ion implantation and recoil implantation of adsorbed nitrogen. This is quantitatively reproduced by TRIDYN collisional computer simulations.

TiO2 films prepared by DC magnetron sputtering from ceramic targets

Abstract The deposition of stoichiometric TiO 2 films for V 2 O 5 /TiO 2 anatase catalysts was investigated. DC magnetron sputtering from ceramic oxide targets in an argon/oxygen atmosphere was chosen as deposition technique. The target behaviour upon sputtering was followed by means of the target voltage. The layers were prepared with different oxygen concentrations in the plasma and the influence upon electronic and optical properties of the deposited layers was investigated. Surface XPS measurements showed that stoichiometry is obtained for low oxygen mole fractions, while for higher oxygen contributions, Ti 3+ species are increasingly present. This departure from stoichiometry is ascribed to active resputtering of the deposited layer by O − ions. Due to the higher sputter yield of oxygen as compared to titanium, this resputtering resulted in the observed reduction. From optical transmission measurements, the refractive index was found to decrease with increasing oxygen mole fraction, while the band gap increased considerably. The variation of the optical absorption with the oxygen concentration in the plasma was found to confirm the XPS analysis regarding the stoichiometry of the layers. Polycrystalline anatase layers were obtained at elevated substrate temperatures.