Magnetic field strength influence on the reactive magnetron sputter deposition of Ta2O5

TL;DR

This study investigates how magnetic field strength variations during reactive magnetron sputtering affect target poisoning, film stoichiometry, and crystal structure, emphasizing the importance of magnetic field control for high-quality Ta2O5 film deposition.

Contribution

It provides new insights into the influence of magnetic field strength on target poisoning and film properties during reactive sputtering, highlighting the need for optimized magnetic conditions.

Findings

Increasing oxygen flow shifts Ta to TaOx and TaOx to Ta2O5 transitions.

Higher magnetic fields promote earlier transition to Ta2O5, affecting film quality.

Stable, stoichiometric Ta2O5 films require specific magnetic and current conditions.

Abstract

Reactive magnetron sputtering enables the deposition of various thin films to be used for protective as well as optical and electronic applications. However, progressing target erosion during sputtering results in increased magnetic field strengths at the target surface. Consequently, the glow discharge, the target poisoning, and hence the morphology, crystal structure and stoichiometry of the prepared thin films are influenced. Therefore, these effects were investigated by varying the cathode current Im between 0.50 and 1.00 A, the magnetic field strength B between 45 and 90 mT, and the O2/(Ar+O2) flow rate ratio between 0 and 100%. With increasing oxygen flow ratio a sub-stoichiometric TaOx oxide forms at the metallic Ta target surface which further transfers to a non-conductive tantalum pentoxide Ta2O5, impeding a stable DC glow discharge. These two transition zones (from Ta to TaOx and from TaOx to Ta2O5) shift to higher oxygen flow rates for increasing target currents. Contrary, increasing the magnetic field strength (e.g., due to sputter erosion) mainly shifts the TaOx to Ta2O5 transition to lower oxygen flow rates while marginally influencing the Ta to TaOx transition. To allow for a stable DC glow discharge (and to suppress the formation of non-conductive Ta2O5 at the target) even at a flow rate ratio of 100% either a high target current (Im >= 1 A) or a low magnetic field strength (B <= 60 mT) is necessary. These conditions are required to prepare stoichiometric and fully crystalline Ta2O5 films. Our investigations clearly demonstrate the importance of the magnetic field strength, which changes during sputter erosion, on the target poisoning and the resulting film quality.