Electrical properties of ultra thin Al2O3 and HfO2 films as gate dielectrics in MOS technology

The rapidly evolving silicon industry demands devices with high-speed and low power consumption. This has led to aggressive scaling of the dimensions in metal oxide semiconductor field effect transistors (MOSFETs). The channel length has been reduced as a result of this scaling. The industry favorite, SlO2, has reached limitations in the thickness regime of 1-1.5 nm as a gate dielectric. High-κ gate dielectrics such as Al203 and HfO2 and their silicates are some of the materials that may, probably, replace SlO2, as gate dielectric in the next four to five years. The present study is an attempt to understand the electrical characteristics of these exciting materials grown by atomic layer deposition (ALD) technique. The flat band voltages (VFB) were determined from C-V measurements on circularly patterned MOS capacitors. For phosphorous doped polysilicon electrodes and Al-oxide based dielectrics, positive shifts in VFB were observed, relative to a pure SlO2 control, ranging from 0.2 to 0.8V. It is believed that this is caused by fixed charges. Rapid thermal annealing at 1000°C tends to decrease VFB relative to a 800°C anneal. Changes in VFB UP to 0.35 V are observed for films deposited over SlO2 underlayers, while smaller changes, up to 0.05 V, are observed for films deposited directly on Si. Spike annealing is also observed to reduce oxide leakage. HfO2 showed large amount of leakage resulting in difficulty in performing capacitance measurements. ZrO2 was found to be reacting with polycrystalline silicon and thus high leakage current was observed.