Thin Solid Films-Elsevier Sequoia S.A., Lausanne-Printed in Switzerland 319 THE MECHANICAL PROPERTIES OF ANODIC TANTALUM OXIDE FILMS * D. ELIEZER AND D. G. BRANDON Department of Materials Engineering, Technion--Israel Institute of Technology, Haifa (Israel) (Received May 16, 1972) The mechanical properties of tantalum oxide films formed by anodizing were determined using a bulge test. The correlations between electrolytic break- down and microstructure and between the microstructure and the observed mechanical properties are discussed. It is concluded that a direct relation exists between the incidence of crystalline film defects and the reduction in mechanical properties above a critical film thickness. The mechanical properties were found to be sensitive to the holding time at maximum cell voltage (anodic annealing time). INTRODUCTION Previous work ~has shown that the bulge test is a convenient and reproducible technique for characterizing anodic aluminium oxide t~lms in the thickness range 500-3000 A. This work has now been extended to tantalum oxide and preliminary results for this material are reported here. In view of the importance of these films as thin windows and as elements in solid state devices, and in view of their role as a barrier conferring corrosion resistance on the substrate, it is perhaps surprising that more attention has not been paid to their mechanical properties. EXPERIMENTAL Anodic oxide films were grown on chemically polished tantalum strip of 99.95% purity (Table I) in 0.1 M aqueous ammonium tartrate at a constant current density of 25 mAcm -2, to a thickness of between 1000 and 3000 A. The thickness was assumed to be linearly proportional to the final cell voltage with a constant of proportionality of 17 A V- 1 at the bath temperature used (15 °C) 2. TABLE I CONCENTRATION OF IMPURITIESIN 9 9 . 9 5 ~ TANTALUM Impurity Nb Fe C O N CI Concentration (ppm) 500 50 30 50 30 100 * Paper presented at the International Conference on Thin Films, " Application of Thin Films ", Venice, Italy, May 15-19, 1972; Paper 6.7. Thin Solid Films, 12 (1972) 319-323