Tantalum oxide thin films for dielectric applications by low-pressure chemical vapor deposition : physical and electrical properties

High quality Ta2O5 films suitable for 64 Mb DRAM use were deposited by low-pressure chemical vapour deposition (LPCVD) from Ta(OC2H5)5 (tantalum pentaethoxide) and oxygen. The films were deposited on silicon, polysilicon and SiO2. Thickness reproducibility, across-the-wafer uniformity, and conformality and step-coverage all are excellent. As-deposited films are amorphous with smooth surfaces. Annealed films are polycrystalline, and their surfaces are characterised by 2 nm high, 1 micron diameter nucleation centres surrounded by circular crystallisation fronts. The films must be annealed to get acceptable leakage currents. Leakage currents for annealed 10 to 40 nm Ta2O5 films are independent of film thickness and are less than or equal to 10 exp(-9) A/cm2 at a gate voltage of 1.5 V. Effective dielectric constants decrease with Ta2O5 film thickness. The smallest observed equivalent SiO2 thickness, t(ox,eff), is 3.5 nm for 7.5 nm Ta2O5/Si. The minimum practical t(ox,eff) for the Ta2O5/Si system is approximately 3 nm. These electrical results are explained by the presence of a thin SiO2 layer at the Ta2O5/Si interface. The SiO2 layer dominates the electrical behaviour of thin annealed Ta2O5 films on Si. Effects of the surface structure and minimum t(ox,eff) on device integration are discussed. 24 refs.