The development of novel reactive sputtering methods in the study of strontium bismuth tantalate

Abstract

Contrary to the claims of numerous reports over the last two decades, there does not yet exist a viable plasma sputtering method for complex oxides such as the lead- and bismuth-containing perovskites. In fact, the lack of reproducibility in any vapor deposition means for such materials is the primary reason why the promise of nonvolatile ferroelectric RAM (FeRAM) has delivered so little over the last two decades, with only low-density devices being provided, instead, by solution and mist techniques (via both sol-gel and MOD). In part, the present work provides the first comprehensive treatment of the challenges present in the plasma sputter deposition of ferroelectric strontium bismuth tantalate (SBT), with SBT thin films fabricated under a wide variety of deposition approaches. As such, ferroelectric SBT was successfully formed by the primary plasma sputtering methods used for ferroelectric materials in the past; namely, R.F. sputtering of ceramic targets and D.C. reactive sputtering of metal targets. Capacitors were fabricated to establish the ferroelectric properties of the SBT films. The approach allows for new insights into phase formation in the SBT system. The inadequacies of the previous sputtering approaches used for ferroelectrics (as well as High Tc superconductors) are highlighted in the context of their inherent run-to-run instability. Subsequently, there is developed a new and rather fundamental vapor deposition technique, termed Electron-Assisted Deposition (EAD), which provides uniquely non-equilibrium growth conditions. This terminology is founded in the direct contradistinction of the presently developed process with previously developed Ion-Assisted Deposition (IAD) processes. The developed EAD process provides the first means by which metallic-mode reactive sputtering, wherein the desired compound is formed by surface reactions at the film's growth interface, can be implemented for a ferroelectric or, in fact, the multicomponent perovskites, in general. In addition to other benefits, such metallic-mode sputtering is found to be inherently more stable and reproducible than the previously used ceramic-target and poisoned-target approaches.