R. De Gryse

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 ...

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.

Effect of microstructure and crystallinity on the photocatalytic activity of TiO2 thin films deposited by dc magnetron sputtering

TiO2 thin films were deposited by dc magnetron sputtering from sub-stoichiometric TiO2−x targets. The as deposited films were XRD amorphous and their microstructure was varied by changing the Ar-pressure and the applied dc power during sputtering. An anneal treatment at 673 K/1 h in air resulted in the crystallization of a preferentially oriented anatase phase, the degree of which depended on the deposition conditions. The microstructure, optical and photocatalytic properties of the thin films were investigated using SEM, UV–VIS spectroscopy and a custom made photocatalytic test reactor, respectively. The thin film density was estimated from their refractive index. Density and crystallinity were the most important factors determining the photocatalytic activity of the thin films.

Determination of the effective electron emission yields of compound materials

The effective electron emission yield from oxidized and nitrided targets in a magnetron discharge is determined based on the linear relationship between the inverse of the measured discharge voltage and the ion induced emission yield of metals. The emission yield of these compound materials depends on their electronic properties. Indeed, nitrides with a wide band gap have a high yield while small band gap semiconductors and conductors have a low emission yield. The same seems to hold for the oxides. However, for these latter materials the behaviour is complicated by reduction of the oxide under ion bombardment.

Influence of oxygen addition on the target voltage during reactive sputtering of aluminium.

The voltage of an aluminium target changes during magnetron sputtering when oxygen is added to the argon plasma. The variation in target voltage has been ascribed to a target surface modification, which alters the ion-induced secondary electron emission (ISEE) coefficient. As most models assume that the target surface modification was induced by chemisorption of oxygen on the aluminium target, we have measured the influence of chemisorption on the target voltage. At low oxygen exposure an absolute target voltage increase as a function of the exposure was noticed. Extending the oxidation period resulted in an absolute target voltage decrease if the exposure was increased. Comparing these results with the measurements performed regarding reactive sputtering, we came to the conclusion that chemisorption cannot explain the target surface modification during reactive sputtering. Indeed, experiments concerning the stability of the target surface modification induced by reactive sputtering clearly indicated that it is not stable. This shows that the target surface modification during reactive sputtering is not the formation of a stable surface compound by a chemical reaction between oxygen molecules and the aluminium surface, as noticed during chemisorption. The chemical reaction between implanted reactive gas atoms and the target atoms forms the basic idea of the presented approach to describe the target surface changes.

Chemical instability of the target surface during DC-magnetron sputtering of ITO-coatings

Abstract Indium–tin oxide (ITO) coatings on polymer film substrates should have the required properties already in their as-deposited state and should be chemically and thermally stable. The coatings can be produced either by reactive DC-magnetron sputtering of In/Sn (e.g., 80/20, wt.%) metallic alloy targets or by pseudo-reactive sputtering of ITO (In2O3SnO2 90/10, wt.%) targets. In the former process (and to a lesser extent in the latter process), a surface instability of the target arises: black crystals of a dark phase grow on the target surface, changing the target voltage and causing pronounced arcing in the plasma. This harmful phenomenon grows worse during a sputter run and finally necessitates abortion of the run and mechanical cleaning of the target. This phenomenon has never been explained in literature. In this paper, it will be shown that the black crystals are In2O and that they originate from an equilibrium reaction at the target surface in which In2O3 is decomposed into In2O and O2. This reaction is especially annoying with reactive processes in which optical plasma emission monitoring (PEM) is used as a control means to obtain the correct stoichiometry of the coatings at relatively high sputter deposition rates.

Electronic and optical characterisation of TiO2 films deposited from ceramic targets

Abstract The deposition of stoichiometric TiO2 films for abrasion resistant V2O5/TiO2 anatase catalysts was investigated. DC magnetron sputtering from ceramic oxide targets in an argon/oxygen atmosphere was chosen as deposition technique. Smaller amounts of oxygen gas proved necessary to achieve stoichiometry in the deposited layers, than during deposition processes involving metal targets. TiO2 layers were prepared with different oxygen mole fractions and the influence upon electronic and optical properties was investigated. Surface XPS measurements showed that stoichiometry is obtained for low oxygen mole fractions, while for higher oxygen contributions, Ti3+ species are increasingly present. This departure from stoichiometry was 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. Post-deposition annealing in oxygen improved the crystallinity of the layers. The refractive index and optical band gap were both reduced by this treatment and the oxygen deficiency of the layers was in general seen to decrease.

Target voltage behaviour during DC sputtering of silicon in an argon/nitrogen mixture

Abstract On addition of nitrogen to an argon plasma, the silicon target voltage changes. By following the target voltage during the addition and removal of the reactive gas, the influence of the plasma condition was separated from the influence of the target condition on the target voltage. In this way, we were able to investigate the target surface modification during reactive sputtering. The target surface modification seems to be induced by reactive ion implantation. The target surface modification is not stable and N 2 desorption from a silicon target after reactive sputtering is noticed.

Recapture of secondary electrons by the target in a DC planar magnetron discharge

Abstract In this paper we describe a simple two-dimensional model that allows the study of the individual secondary electron orbits in a DC planar magnetron discharge. Emphasis is on the recapture of secondary electrons by the target, which is enabled by their small initial energy, because this reduces the effective secondary electron yield as seen by the discharge. This reduction depends strongly on both the position along the race track and the gas pressure and it can be substantial for typical planar magnetron operating conditions. Our simple model allows to conclude that because of the sensitivity of the discharge on the secondary electron yield, the current–voltage characteristic, the spatial distribution as well as the pressure dependence of the planar magnetron discharge will be influenced by recapture.

Hysteresis behavior during reactive magnetron sputtering of Al2O3 using a rotating cylindrical magnetron

Rotating cylindrical magnetrons are used intensively on industrial scale. A rotating cylindrical magnetron on laboratory scale makes it possible to study this deposition technique in detail and under well controlled conditions. Therefore, a small scale rotating cylindrical magnetron was designed and used to study the influence of the rotation speed on the hysteresis behavior during reactive magnetron sputtering of aluminum in Ar∕O2 in dc mode. This study reveals that the hysteresis shifts towards lower oxygen flows when the rotation speed of the target is increased, i.e., target poisoning occurs more readily when the rotation speed is increased. The shift is more pronounced for the lower branch of the hysteresis loop than for the upper branch of the hysteresis. This behavior can be understood qualitatively. The results also show that the oxidation mechanism inside the race track is different from the oxidation mechanism outside the race track. Indeed, outside the race track the oxidation mechanism is only...