Multilayer thin films of aluminum oxide and tantalum oxide deposited by pulsed direct current magnetron sputtering for dielectric applications

Drevet, Richard and Souček, Pavel and Mareš, Pavel and Dubau, Martin and Czigány, Zsolt and Balázsi, Katalin (2023) Multilayer thin films of aluminum oxide and tantalum oxide deposited by pulsed direct current magnetron sputtering for dielectric applications. VACUUM, 210. ISSN 0042-207X

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Abstract

This research describes the synthesis of multilayer thin films of aluminum oxide and tantalum oxide for dielectric applications. The multilayer thin films are made of two, four, or eight oxide layers produced by physical vapor deposition (PVD), specifically mid-frequency pulsed direct current magnetron sputtering. The oxide layers are stoichiometric Al2O3 and Ta2O5 with two specific morphologies observed from cross-section images obtained by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Al2O3 layers exhibit a columnar structure whereas the Ta2O5 layers are uniform and dense. However, the morphology of the Ta2O5 layers changes progressively in the four-layer and eight-layer systems under the influence of the morphology of the Al2O3 layer below. This behavior is induced by the morphological continuity of the interface between two oxide layers. X-ray diffraction (XRD) shows the low crystallinity of these oxide layers due to the experimental conditions used during the magnetron sputtering process, particularly the low deposition temperature. The dielectric behavior of the multilayer thin films is studied by dielectric strength measurements. The results are compared to the values obtained for single layers of Al2O3 and Ta2O5 produced under the same experimental conditions. The two-layer system shows an intermediate value compared to the single layers, higher than Al2O3 and lower than Ta2O5. The dielectric strengths of the four-layer and the eight-layer systems are higher than those measured for the single layers of Al2O3 and Ta2O5. Finally, the morphology and the crystallinity of the multilayer thin films are changed by thermal annealing of these samples at 850 °C under vacuum. The thermal annealing induces crystallization of the Ta2O5 layers and the loss of morphological continuity at the interface between the oxide layers. These modifications result however in a lower dielectric strength for all the multilayer thin films.

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