Thermal oxidation mechanism and stress evolution in Ta thin films

Abstract

Oxidation-induced stress evolutions in Ta thin films were investigated using ex situ microstructure analyses and in situ wafer curvature measurements. It was revealed that Ta thin films are oxidized to a crystalline TaO2 layer, which is subsequently oxidized to an amorphous tantalum pentoxide (a-Ta2O5) layer. Initial layered oxidation from Ta to TaO2 phases abruptly induces high compressive stress up to about 3.5 GPa with fast diffusion of oxygen through the Ta layer. Subsequently, it is followed by stress relaxation with the oxidation time, which is related to the slow oxidation from TaO2 to Ta2O5 phases. The initial compressive stress originates from the molar volume expansion during the layered formation of TaO2 from the Ta layer, while the relaxation of the compressive stresses is ascribed to the amorphous character of the a-Ta2O5 layer. According to Kissinger's analysis of the stress evolution during an isochronic heating process, the oxygen diffusion process through the a-Ta2O5 layer is the rate-controlling stage in the layered oxidation process of forming a a-Ta2O5/TaO2/Ta multilayer and has an activation energy of about 190.8 kJ/mol.

A Co‐catalyst‐Loaded Ta3N5 Photoanode with a High Solar Photocurrent for Water Splitting upon Facile Removal of the Surface Layer

“…In addition, the thicker the film, the larger is the stress. [27] When the Ta 2 O 5 film is thicker than the critical thickness, the surface layer will be exfoliated during the transformation from Ta 2 O 5 to Ta 3 N 5 since some large cracks form to release the high stress in the film (Figure 1 g, 1 h, and 1 i). In previous studies, the Ta foils were only oxidized at 550 8C for a shorter time ( 30 minutes), [18,21] the thickness of the Ta 2 O 5 film was less than the critical thickness, therefore, no surface thermal exfoliation was observed by other authors.…”

Fabrication of a Robust Tantalum Nitride Photoanode from a Flame‐Heating‐Derived Compact Oxide Film

“…The Ta 2 O 5 layer stuck to the underlying Ta substrate tightly and can hardly be removed with scratching, indicating the excellent mechanical stability of the films. Generally, due to the large density difference between Ta 2 O 5 and Ta, the stress in Ta 2 O 5 /Ta electrodes will become distinct with the increase of the thickness of the Ta 2 O 5 layers . In our case, when the Ta 2 O 5 /Ta precursor electrode was prepared by means of the traditional method by heating the foil in a muffle furnace, the resulting Ta 2 O 5 /Ta (t) curled and broke into pieces easily (Figure S2).…”

Barrier Properties of Cr/Ta-Coated Zr-1Nb Alloy under High-Temperature Oxidation

“…β-Ta has a tetragonal structure with a large unit cell (a ≈ 10.2 Å and c ≈ 5.3 Å), whereas α-Ta has a cubic lattice with a much smaller unit cell (a ≈ 3.3 Å). High-temperature oxidation leads to β-Ta→α-Ta transformation [38,39]. This transformation results in compressive stresses (strains) in the tantalum layer.…”