Highly reliable MIS capacitors with plasma nitridation and doubled dielectric-constant tantalum pentoxide

We studied effective thinning of metal-insulator-semiconductor tantalum pentoxide capacitors experimentally for DRAM application. First, we investigated the dielectric constant of a tantalum pentoxide film deposited and crystallized on an oxidation-resistant thick silicon-nitride film. Dependence of electrically equivalent thickness on physical thickness of tantalum pentoxide revealed an increased dielectric constant of 60, whereas the films on a silicon-dioxide film had a dielectric constant of no more than 40. To apply this increased dielectric constant to DRAM capacitors, we applied novel plasma nitridation on the surface of polysilicon. The plasma-nitrided surface showed fair oxidation resistance up to 800°C, at which a tantalum pentoxide film fully crystallizes. The temperature was 100°C higher than that of a conventional treatment using rapid thermal nitridation (RTN). The improved oxidation resistance enabled the increased dielectric constant as well as suppression of silicon oxide between the film and polysilicon. Consequently, effective thinning by 10% was demonstrated even on rugged polysilicon without increase of leakage current. Time-dependent dielectric-breakdown measurements revealed that the tantalum pentoxide capacitors fabricated using plasma nitridation are expected to have a lifetime three orders of magnitude longer than that of those fabricated using RTN.