Crystallization behaviour of ALD-Ta₂O₅ thin films: the application of in-situ TEM

Abstract

The microstructure of Ta₂O₅ thin films, deposited onto Si substrates by atomic layer deposition (ALD), was investigated, using in situ transmission electron microscopy (TEM). As-deposited amorphous films crystallize as the orthorhombic phase L-Ta₂O₅ upon heating at 750°C. Two dominant crystallographic orientations are found, one with (0 0 1) and (1 11 0) planes perpendicular to the substrate, while the other has (0 0 1) planes parallel to the substrate. The grains consist of subgrains which are rotated a few degrees with respect to each other. The kinetics of the crystallization were studied by in-situ TEM heating experiments carried out at nominal temperatures of 790°C, 820°C and 850°C. They reveal that the growth and crystallization activation energies are about 4.2 eV and 6.3 eV, respectively. Tilted subgrains keep forming during growth until they come in contact with neighbouring grains. The crystallization behaviour can be approximated by the Kolmogorov–Johnson–Mehl–Avrami (KJMA or Avrami) equation, giving mode parameters of 2.5, 1.9, and 1.7 at 790°C, 820°C and 850°C, respectively. A small value of mode parameters is attributed to decreasing growth and nucleation rates with time.

Acknowledgements

This work was partially supported by the Stanford University Initiative on Nanoscale-Materials and Process, the Stanford Nanocharacterization Laboratory and by Samsung Corporation. We are also grateful to Professor H. Saka who gave us the opportunity to present this work first at the conference on In Situ Microscopy held at Nagoya University in January, 2003.