Effect of O2post-deposition anneals on the properties of ultra-thin SiOx/ZrO2gate dielectric stacks

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Abstract
The effect of post-deposition anneal in O2 at different temperatures (500-700 °C) on the microstructure and electrical properties of SiOx/ZrO2 gate dielectric stacks is investigated. It is shown that the as-deposited ZrO2 layers are partly amorphous and crystallize after post-deposition anneal. From the analysis of high-frequency capacitance-voltage (C-V) characteristics, positive (negative) fixed charge is found in the as-deposited layer when Al (Au) electrodes are used. Furthermore, positive charge is generated in the gate stack and the density of interface states is increased during post-deposition anneal in O2. Both positive charge and interface state density can be greatly reduced by an additional post-metallization anneal in H2 at low temperature (400 °C). The dielectric constant of the gate stack presents a large increase after O2 post-deposition anneal at 700 °C, which may be attributed to the partial transformation of the silicon oxide interfacial layer into a Zr silicate. The leakage current through the gate stack is reduced by several orders of magnitude after O2 annealing, consistent with the increase in SiOx layer thickness, as well as the reduction in bulk trap density revealed by the reduced hysteresis effect observed in the C-V characteristics. The temperature dependence of the current density through the gate stack is also reduced after the post-deposition anneal in O2, which is attributed to the reduction in the trap-assisted tunnelling contribution to the leakage current.