The existence of a double S-shaped process curve during reactive magnetron sputtering

The four dimensional parameter space (discharge voltage and current and reactive gas flow and pressure) related to a reactive Ar/O2 DC magnetron discharge with an aluminum target and constant pumping speed was acquired by measuring current-voltage characteristics at different oxygen flows. The projection onto the pressure-flow plane allows us to study the well-known S-shaped process curve. This experimental procedure guarantees no time dependent effects on the result. The obtained process curve appears not to be unique but rather two significantly different S-shaped curves are noticed which depend on the history of the steady state target condition. As such, this result has not only an important impact on the fundamental description of the reactive sputtering process but it can also have its consequences on typical feedback control systems for the operation in the transition regime of the hysteresis during reactive magnetron sputtering

Anomalous effects in the aluminum oxide sputtering yield

The sputtering yield of aluminum oxide during reactive magnetron sputtering has been quantified by a new and fast method. The method is based on the meticulous determination of the reactive gas consumption during reactive DC magnetron sputtering and has been deployed to determine the sputtering yield of aluminum oxide. The accuracy of the proposed method is demonstrated by comparing its results to the common weight loss method excluding secondary effects such as redeposition. Both methods exhibit a decrease in sputtering yield with increasing discharge current. This feature of the aluminum oxide sputtering yield is described for the first time. It resembles the discrepancy between published high sputtering yield values determined by low current ion beams and the low deposition rate in the poisoned mode during reactive magnetron sputtering. Moreover, the usefulness of the new method arises from its time-resolved capabilities. The evolution of the alumina sputtering yield can now be measured up to a resolution of seconds. This reveals the complex dynamical behavior of the sputtering yield. A plausible explanation of the observed anomalies seems to originate from the balance between retention and out-diffusion of implanted gas atoms, while other possible causes are commented

Modeling reactive magnetron sputtering : opportunities and challenges

The complexity of the reactive magnetron sputtering process is demonstrated by four simulation examples. The examples, commonly encountered during the application of this process for thin film deposition, are described by a numerical model for reactive sputter deposition. A short description of the current model precedes these case studies. In the first example, redeposition of sputtered atoms on the target is studied by its effect on the hysteresis behavior often observed during reactive sputtering. Secondly, the complexity of current-voltage characteristics during reactive magnetron sputtering is treated. The influence of substrate rotation and the pulsing of the discharge current illustrate the time dependence of the reactive sputtering process. As a conclusion, the two main challenges for a further improvement of the model are discussed

On the target surface cleanness during magnetron sputtering

The magnetron sputter deposition of metallic thin films usually requires high vacuum sputtering conditions to avoid the contamination of impurities on the substrate. The presence of these impurities highly influence the thin film texture and microstructure. The impurities in the ambient gas not only directly affect the substrate condition, they will also adsorb onto the target surface. As they get sputtered, they will be directed towards the substrate with energies much higher than the thermal ambient gas particles, which has a huge impact on the thin film characteristics. In this way, the target surface cleanness can play an essential role for thin film growth. To quantify the target cleanness, the compound formation on a tantalum target was studied by poisoning the target as a function of the oxygen exposure and subsequently sputter cleaning the formed compound layer in pure argon. The discharge voltage during this sputter cleaning stage contains a lot of information from which the compound layer thickness can be deduced. By studying the compound formation rate, the target surface cleanness during different sputtering conditions can be estimate

An X-ray photoelectron spectroscopy study of the target surface composition after reactive magnetron sputtering

The target surface stoichiometry after reactive DC magnetron sputtering has been studied by X-ray Photoelectron Spectroscopy (XPS). The target was transferred in-vacuo from the deposition chamber to the XPS analysis chamber to avoid any influence by ambient exposure. The study was performed on an aluminum and a tantalum target, both reactively sputtered in different mixtures of oxygen and argon. XPS analysis showed that for the aluminum target only stoichiometric aluminum oxide (Al2O3) is formed but for the tantalum target different suboxides (Ta2O, TaO, Ta2O3, TaO2 and Ta2O3) are present. The formation of a stoichiometric or substoichiometric oxide on the target surface is related to the observed respectively decreasing or increasing discharge voltage behavior upon oxygen addition. This change in discharge voltage behavior is generally linked to a change in electron emission from the target. The material dependency of both the potential and kinetic electron emission mechanisms results in disparate electron yield values between oxides and suboxides residing on the target surface

Sputter yield measurements to evaluate the target state during reactive magnetron sputtering

The sputter yield and discharge voltage of fourteen target materials (Al, Cr, Cu, Mg, Mo, Nb, Pb, Ta, Ti, V, W, Y, Zn, and Zr) have been measured during reactive sputtering in argon/oxygen mixtures. The obtained oxide sputter yields strongly differ from the published data based on ion beam experiments. A second observation is that based on the discharge voltage behavior observed during target oxidation, the materials can be subdivided into two groups. For the first group, the discharge voltage increases on the target oxidation, while for the second group the opposite behavior is observed. Both observations are explained based on a model that accounts for oxygen implantation into the target, preferential oxygen sputtering, and additional oxygen loss mechanisms such as outdiffusion. The difference between both groups can be explained from the oxygen fraction in the gas discharge required to fully oxidize the target surface. This required fraction is lower for the first group, and higher for the second group, than the oxygen fraction when the reactive sputter process switches into poisoned mode. The required fraction is mainly defined by the oxide sputter yield. The lower sputter yield as compared to literature values can be attributed to implanted oxygen that dilutes the formed oxide and/or continuously replaces sputtered oxygen