The effects ofheat treatment on the microstructure of Al/Ti metallization multilayers were studied by electron spectroscopic imaging (ESI) in the transmission electron microscope (TEM). The multilayers were deposited on a suite of piezoelectric substrate materials (Quartz, LiTaO3, and LiNbO3) by electron beam evaporation. In the as-deposited state, we did not detect a reaction between the individual Al and Ti layers (Fig. 1(a)). However, after annealing in air at 450 °C for five hours, we found the intermediate Ti layer completely dissolved. In the course of this reaction, the microstructure of the layers transforms from rectangular to globular grains consisting of the intermetallic phase Al3Ti within an Al matrix.
A suitable method to visualize the chemical composition is a detailed analysis of the acquired ESI images by bivariate histogram analysis. The analysis was carried out on both the Al and Ti images obtained on the LiNbO3 substrate. The histogram analysis results in various clusters which correspond to the individual compositions which are shown in the real space image (Fig. 1(b)). Around the green marked Al3Ti grains a yellow border can be seen, which indicates a higher Al content and reveals the edges of the grains. At these edges, there is pure Al above or below the Al3Ti grains. The spatial distribution of the phases can be derived from this histogram analysis method. The combination with X-ray spectroscopy reveals also the composition of the identified phases.
Our results demonstrate the feasibility of forming the intermetallic Al3Ti phase directly from deposited metallic Al and Ti layer stacks under the constraint of preserving a Ti adhesive layer on top of the substrate.
M. Hofmann, T. Gemming, S. Menzel, K. Wetzig: Microstructure of Al/Ti metallization layers, Zeitschrift für Metallkunde 94  (2003) 317-322 (http://www.ijmr.de/directlink.asp?MK030317).
M. Hofmann, T. Gemming, K. Wetzig: Microstructure and composition of annealed Al/Ti-metallization layers, Analytical and Bioanalytical Chemistry 379  (2004) 547-553 (doi:10.1007/s00216-004-2619-9).