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

摘要

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 degrees 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 degrees C, 820 degrees C and 850 degrees 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 degrees C, 820 degrees C and 850 degrees C, respectively. A small value of mode parameters is attributed to decreasing growth and nucleation rates with time.