An evaluation of the electrical, material, and reliability characteristics and process viability of ZrO₂ and ZrOxNy for future generation MOS gate dielectric
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Over the past decades, continuing advancements in processes and tools and the introduction of new materials have facilitated the rapid downscaling of metaloxide-semiconductor (MOS) technology. The 90 nm technology node and beyond will face one of the toughest challenges of the semiconductor industry – the replacement of conventional silicon dioxide (SiO2) gate dielectric with a high-k dielectric material. SiO2 has been used as the gate oxide since the inception of the MOSFET, but is reaching its physical scaling limits (~10-15Å) due to excessive gate leakage current and reliability issues. High-k materials are required to reduce leakage current while maintaining a low equivalent oxide thickness (EOT). However, the integration of high-k dielectrics into the MOS process will be a serious challenge. One promising high-k candidate is zirconium oxide (ZrO2) since it is has demonstrated thermal stability on Si, a dielectric constant ~20, a bandgap of 5.8 eV, low EOT (= 10Å), and low gate leakage. In this research, sputter-deposited ZrO2 and nitrogen-incorporated ZrO2 (ZrOxNy) were evaluated in terms of electrical, material, and reliability characteristics to determine their viability as high-k gate dielectrics. Initially, platinum-gated MOS capacitors were studied to optimize the ZrO2 deposition process and demonstrate low EOT (8.2Å) and low leakage. Unfortunately, both ZrO2 and ZrOxNy were found to be incompatible with the polysilicon gate electrode process due to the formation of Zr-silicide and consequently, high leakage. However, dual metal gate electrodes will eventually replace polysilicon because of the polysilicon depletion effect. Tantalum nitride (TaN) is a promising NMOS metal gate candidate due to its thermal stability and low resistivity. Both TaN-gated MOS capacitors and selfaligned transistors using ZrO2 and ZrOxNy were fabricated to demonstrate process viability and characterized to yield low EOT (9.5Å), low leakage, negligible frequency dispersion, low C-V hysteresis, good thermal stability, and well behaved transistor characteristics. In addition, a high temperature (500-600ºC) forming gas anneal was found to improve mobility, subthreshold swing, and transconductance. Overall, ZrO2 and ZrOxNy have demonstrated extremely promising electrical, material, and reliability characteristics as well as process viability and require further investigation as potential high-k gate dielectrics.