Process development, material analysis, and electrical characterization of ultra thin hafnium silicate films for alternative gate dielectric application

Gopalan, Sundararaman
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As aggressive scaling of the Metal-Oxide-Semiconductor (MOS) integrated circuit continues, the required equivalent oxide thickness (EOT) for 100nm technology and beyond is expected to be less than 12Å. In this ultra-thin film regime, SiO2 or oxynitrides face serious limitations due to direct tunneling currents, dopant penetration, and reliability concerns. HfO2, ZrO2 and their silicates are being considered as potential candidates for replacing conventional oxides or oxynitrides owing to their thermodynamic stability in contact with silicon, high dielectric constants, chemical stability at high temperatures, etc. Although HfO2 has shown promising electrical characteristics such as low EOT and leakage, low crystallization temperature, poor interface quality, uncontrolled interfacial layer growth, and reliability issues are some of the concerns with it. HfSixOy has been sought as a solution to alleviate some of these problems. viii Hafnium silicate films were deposited by co-sputtering of hafnium and silicon in argon ambient followed by annealing. Hafnium silicates with more than 13% Si remains amorphous even after 1000°C anneals. They also show better thermal stability characteristics than HfO2 at all compositions studied. They have been shown to be compatible with poly and TaN gate electrodes. Through process optimization, Hf-silicate films have been scaled down to 10.3Å, with leakage in the order of 1mA/cm2 at -1.5V. Low dispersion, low stress-induced leakage currents, high effective breakdown fields, and large lifetimes were obtained. Good thermal stability with polysilicon and TaN electrodes were also obtained. MOS transistors have been demonstrated with poly and TaN electrodes, showing excellent subthreshold and drive current characteristics. The effects of NH3 pre-treatment on the electrical and reliability characteristics were also investigated. While thermal stability, EOT, leakage and breakdown were improved significantly, threshold voltage shifts, high hysteresis, poor MOSFET and poor reliability characteristics were obtained for the NH3 pre-treated samples. In conclusion, promising electrical and material characteristics were obtained for Hf-silicates indicating that they are attractive candidates for alternative gate dielectric applications.