Date of Award
Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)
Committee for the Interdisciplinary Program in Materials Science and Engineering
J. K. Hirvonen
N. M. Ravindra
The purpose of this research was to investigate the effects of ion bombardment on the crystallographic phases of tantalum films during their deposition by magnetron sputtering process, and to gain understanding of the mechanism of the ion-solid interactions during the thin film growth. Tantalum (Ta) exists in two distinct crystallographic phases: a stable α-phase with a body centered cubic (bcc) lattice structure and a metastable β-phase with a tetragonal lattice structure. The tough and ductile α-phase Ta is desired in most industrial applications, such as diffusion barrier layers in integrated circuits, metallic corrosion protective layers, and in biomedical devices. The β-phase Ta is hard and brittle, and its presence may compromise the film performance. Bulk Ta metal has the α-phase structure but the β-phase appears in thin films, unless special means are used to avoid its growth.
In this work a DC magnetron sputtering system was modified for RF operation along with a provision for DC biasing the substrate to accelerate inert gas ions towards the tantalum thin film during its growth process. The experiments demonstrated that the ion bombardment energy, controlled by varying the bias voltages, has a strong effect on the crystallographic phase of tantalum films as well as their surface morphology. High quality bcc (α-phase) tantalum thin films were deposited under -150 V substrate bias at an ion current density of approximately ~0.4 mA/cm2, both on silicon and aluminum substrates. These ion bombardment conditions for bcc α-phase Ta growth are quite different from those previous reported, however, it was found that the total energy delivered by ions per deposited atom in this and in previous work is approximately the same.
Ion bombardments by plasma Ar ions accelerated to the biased substrate also sputtered away the deposited Ta during the film growth, resulting in thinner films at higher ion energies, which was measured by Rutherford backscattering spectroscopy. The sputtering yield derived from these data was compared with previously published data and theoretical predictions. This ion assisted deposition process was further studied by molecular dynamic simulations and statistical analysis of ion impacts and atom impingements on the film surface. It was concluded that these two events are independent, and that the film crystallographic structure transformation is induced by ion impacts rearranging the deposited atoms after their arrival on the growing film.
The results for this work have demonstrated that bcc α-phase tantalum thin films can be grown on both silicon and aluminum substrates at room temperature by RF magnetron sputter with ion bombardment conditions very different than those previous reported. The advantage of the new process is that it can be carried out in a standard magnetron deposition system with a provision of substrate biasing to control the ion bombardment. Such a system is easier to scale to industrial operation than those described earlier, which requires a more complex apparatus.
Ren, Hua, "Ion assisted magnetron sputtering of tantalum thin film deposition and characterization" (2007). Dissertations. 832.