Doped tantalum oxide high K dielectric thin films

Abstract

To support the next generation of sub 100nm ULSI devices, a new high dielectric constant (K) thin film material must be developed. To reach these new smaller dimensions, thermally grown silicon dioxide would need to eventually be scaled to a thickness of around 1.2nm. At this thickness silicon dioxide fails due to direct tunneling current. In addition, such a thin layer is an ineffective barrier against boron diffusion. Tantalum oxide is a high K dielectric material that is currently being used in thin film transistors. Tantalum oxide has a dielectric constant of 20 to 25, but it typically has high leakage current due to the crystalline structure of the film or defects. To strengthen the electrical characteristics of tantalum oxide, it was doped with varying amounts of titanium oxide, aluminum oxide and silicon dioxide. The composite oxide films were deposited by reactive radio frequency (RF) cc-sputtering of two targets in a variety of oxygen and argon feed gas mixtures. The targets used were tantalum, aluminum, silicon and titanium. The films were also post-sputter annealed in oxygen at 600⁰C for 60 minutes, 700⁰C for 10 minutes or 700⁰C for 30 minutes. For tantalum oxide films doped with titanium oxide, the most important finding was the large increase in the dielectric constant of these films. Films deposited by co-sputtering titanium at or above 140W in 50 or 90% oxygen feed gases and subsequently annealed in oxygen at 700⁰C for 10 minutes had dielectric constants which ranged from 37 to 85. For tantalum oxide films doped with aluminum oxide, the most important finding was that the leakage current decreased and the breakdown voltage increased as the aluminum cc-sputter power was increased. Possible explanations for the changes in the electrical properties of the doped tantalum oxide films are given.