An Investigation of Structures of Thermal and Anodic Tantalum Oxide Films

Abstract

Tantalum oxide films produced from anodic oxidation and thermal oxidation at 400 and 800°C have been investigated with respect to long-and short-range structure by X-ray diffraction and X-ray absorption spectroscopy, respectively. Films formed at 800°C show orthorhombic structure (␤-Ta 2 O 5 ) with greater disorder compared to the ␤-Ta 2 O 5 powder reference, where six O atoms were observed at an average radial distances of 1.957 Ϯ 0.014 (Ta 2 O 5 powder͒ and 1.971 Ϯ 0.007 Å ͑800°C Ta oxide film͒. Lower temperature films consist primarily of metastable TaO x (Ta 2 O) and appear to be relatively stable for extended times of treatment with a first shell not unlike that of ␤-Ta 2 O 5 , based on a linear combination analysis. Furthermore, using a linear combination of ␤-Ta 2 O 5 and ␣-Ta metal, the amorphous anodic oxide films studied as a function of thickness were observed to exhibit a relatively ordered local structure consistent with the high-temperature ͑i.e., 800°C͒ phase. This work demonstrates that the nanoscale oxide films formed on tantalum exhibit an ordered local structure, reflecting the very compact nature that enhances its corrosion resistance.High-temperature and anodic tantalum oxide films have been studied extensively in the field of corrosion. 1,2 Ta possesses a high melting point ͑2996°C͒, is highly ductile, and is corrosion resistant in aggressive environments. 3 The low thermal expansion and the predictability of tantalum oxidation are additional characteristics that make tantalum attractive for high-temperature applications. In addition, with a dielectric constant greater than 20, tantalum pentoxide (Ta 2 O 5 ) has received significant attention in the semiconductor industry. 4 Although Ta itself is highly reactive, the stable, dense nanometer oxide film formed provides an effective barrier to corrosive environments.Thermodynamically, tantalum pentoxide (Ta 2 O 5 ) is the only stable high-temperature oxidation product. 1 However, thin films of Ta 2 O 5 have several different structures depending on the system temperature. 5-10 The principal structures of crystalline Ta 2 O 5 are orthorhombic ͑␣ and ␤ phases͒ 5-7 and hexagonal ͑␦ phase͒. 9,10 Fukumoto and Miwa 10 reported that the hexagonal structure of Ta 2 O 5 has the space group P6/mmm with lattice constants of a ϭ 7.191 Å and c ϭ 3.831 Å and eight oxygen atoms in the first shell. The orthorhombic structure occurs in two forms with a reversible transition reported at approximately 1360°C: 5-7 the high-temperature ␣-phase (H-Ta 2 O 5 ) and the low-temperature ␤-phase (L-Ta 2 O 5 ). A recent study 7 on the low-temperature oxide showed that ␤-Ta 2 O 5 has unit-cell parameters of a ϭ 6.217 Å, b ϭ 3.677 Å, and c ϭ 7.794 Å with the space group Pccm. Voltovich et al. 11 reported that over the temperature range of 600-1000°C ␤-Ta 2 O 5 was produced. However, the formation of metastable tantalum oxide phases occurs between 300 and 800°C. 12 Steeb and Renner 13 found that at approximately 400°C, platelets of metastable metallic oxide phases are nuclea...

Measurement and Modeling of Short and Medium Range Order in Amorphous Ta2O5 Thin Films

“…Recently published Ta-centered local structure from Ta L 3 edge EXAFS data3031 indicate that the Ta ions are surrounded by between 5 and 6 oxygens. There are different ways in which these Ta centered polyhedra connect to each other to give rise to different polymorphs.…”

“…Extracting the bond distances and average coordination number constrains (shown in Fig. 2b), from Ta-centered information from Ta L 3 edge EXAFS data3031 and O-centered information from NMR spectroscopy32, we construct the backbone of the topological PSU as a kinked Ta-O-Ta chain, with a Ta-O bond of ca. 2.05 Å and a Ta-O-Ta angle of approximately 135–150 degrees.…”

Effect of Composition on the Photoelectrochemical Behavior of Anodic Oxides on Binary Aluminum Alloys

“…Moreover by comparing the bandgap value of thermal Ta 2 O 5 with the E g opt values in Table VII we suggest that amorphous mixed oxide is formed at any thickness during anodic oxidation of Al-34 atom % Ta alloy while a quasi-crystalline mixed oxide is formed by thermal oxidation in analogy with similar finding on pure Ta metal. 40 As for the estimate of flatband potential, it seems evident that the inversion photocurrent potential is more anodic than any expected value if we consider that both Ta 2 O 5 and Al 2 O 3 display more ca- thodic V FB values than the inversion photocurrent potential of the mixed oxide. A rationale for this discrepancy could be traced to the fact that under illumination a surface trapping of photogenerated hole could occur in localized states near the valence band mobility edge with a consequent shift of the bandedges.…”

Evolution of epitaxial Ta2O5 and Ta2O films during thermal oxidation of epitaxial tantalum films on sapphire substrates

“…[6][7][8][9][10][11] The Ta 2 O has a cubic structure with a lattice constant of 0.668 nm whereas Ta 2 O 5 reported to have structures which include orthorhombic ͑␤͒ crystal structure with a = 0.620 nm, b = 0.366 nm, and c = 0.389 nm and hexagonal ͑␦͒ crystal structure with a = 0.724 nm and c = 1.161 nm. [6][7][8][9][10][11] The Ta 2 O has a cubic structure with a lattice constant of 0.668 nm whereas Ta 2 O 5 reported to have structures which include orthorhombic ͑␤͒ crystal structure with a = 0.620 nm, b = 0.366 nm, and c = 0.389 nm and hexagonal ͑␦͒ crystal structure with a = 0.724 nm and c = 1.161 nm.…”