Elsevier

Thin Solid Films

Volume 428, Issues 1–2, 20 March 2003, Pages 248-252
Thin Solid Films

Interface of tantalum oxide films on silicon by UV annealing at low temperature

https://doi.org/10.1016/S0040-6090(02)01280-4Get rights and content

Abstract

In previous work, we have grown 4–10 nm Ta2O5 films by photo-induced chemical vapour deposition (photo-CVD) using a special precursor injection system, which exhibited leakage currents as low as 2.19×10−7 A/cm2 at 1 MV/cm. However properties of these films are known to deteriorate with decreasing film thickness. UV annealing at low temperatures using an excimer UV source can improve the electrical properties of these films dramatically. In this paper, tantalum pentoxide thin films with thicknesses of approximately 40 nm grown by photo-CVD have been annealed at low temperature using an excimer UV lamp. Film properties have been characterised using ellipsometry, Fourier transform infrared spectroscopy, UV spectrophotometry, capacitance–voltage and current–voltage techniques. After UV annealing, improved leakage current densities as low as 4.0×10−8 A/cm2 at 1 MV/cm, and breakdown fields higher than 3.0 MV/cm can be achieved. Investigation of the interfacial SiOx layer formed during deposition and after UV annealing by X-ray photoelectron spectroscopy and TEM reveals that thickness increases with UV annealing time and that the suboxides in the film and at the interface are converted into stoichiometric oxide, leading to an improvement of the electrical properties.

Introduction

We have previously reported the growth of 4–10 nm Ta2O5 films on c-Si by photo-induced chemical vapour deposition (photo-CVD) using 222 nm excimer lamps and a special precursor injection system, which exhibited leakage currents as low as 2.19×10−7 A/cm2 at 1 MV/cm [1], [2], [3]. These layers were nonstoichiometric and their optical and electrical properties were strongly affected by both the O/Ta ratio and their interfacial properties. More recently we have found that UV annealing at low temperatures can significantly improve the electrical properties of these films [4], [5], [6], [7], [8]. In this paper, we investigate the interface changes in the Ta2O5/SiO2/Si system as a function of UV annealing using X-ray photoelectron spectroscopy (XPS), FTIR and TEM. Improvements in both the interfacial and electrical properties of these layers by UV annealing in pure oxygen and other atmospheres are also discussed.

Section snippets

Experimental details

The experiments were performed on n-type 4-in. Si (1 0 0) substrates (resistivity 2–4 Ω cm (i.e. Nd∼1×1015/cm3), which received a standard FSIB clean. Tantalum tetraethoxy dimethylaminoethoxide (Ta(OEt)4(dmae)) (made by Inorgtech. Ltd, UK), dissolved in an anhydrite cyclohexane solvent (10%), was the precursor used. Nitrous oxide (N2O) was introduced as the oxidising agent at a fixed flow rate of 20 sccm. A 60 sccm argon plus 20 sccm N2O gas flow carried the precursor through a showerhead at 110

Results and discussion

Films with thickness approximately 40 nm deposited at a temperature of 350 °C, were UV annealed in 1000 mbar in pure oxygen at 350 °C for 10, 15 and 40 min, respectively. Fig. 1 shows TEM cross-section images of these layers. As can be seen the Ta2O5 films (top layer) are amorphous in both the as-deposited and UV annealed states. This was also confirmed by XRD [7]. Also the interfacial layer remains constant during 10-min annealing and then increases with annealing time up to 40 min. This

Conclusions

UV annealing of photo deposited Ta2O5 films has been performed in 1000 mbar of pure oxygen under various conditions, leading to an increase in thickness of the interfacial layer with Si and a dramatic improvement of the layer electrical properties. This improvement is due to conversion of suboxides in the layer into more stoichiometric Ta2O5 and the formation of SiO2 at the interface of the Ta2O5 with Si. Reactive oxygen radicals formed by the high photon energy of the 222 nm excimer lamps

Acknowledgements

This work was supported by the European Commission under IST Research project No. 10541 tantalum pentoxide photodeposition on silicon (TOPS-II).

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