Abstract
In this work, ZrO2 thin films were studied as a replacement of SiO2 for gate dielectric beyond the 100-nm CMOS technology. In the first part, microstructural and electrical characteristics of as-grown ZrO2 thin films having different thicknesses of 1.2 to 10 nm were investigated. The films were grown on a p-Si substrate by chemical-vapor deposition (CVD) at 275o C. The ZrO2 films had a microstructure that changed from amorphous to polycrystalline with increasing film thickness. From the capacitance-voltage (C-V) relation of the Al/ZrO2/SiOx/p-Si capacitors, the density of the oxide-trapped charge drastically increased from 2.22 x 1010 to 3.54 x 1012 cm-2. Furthermore, an increase of interface-state density was also found from the increase of turn-around voltage in the current-voltage (I-V) relation. In addition, the leakage current from gate injection followed the direct tunneling of holes from substrate to gate before hard breakdown. However, for the thicker films, the leakage current changed to Fowler-Nordheim tunneling. Second, we investigated the effects of post-annealing on the bulk and interfacial characteristics of ultrathin ZrO2 films. The interfacial layer (IL) is mainly composed of Zr-silicate for annealing in N2, but it is mostly SiO2 for annealing in O2. The annealing also effectively reduces the oxide trapped-charge density in ZrO2, as demonstrated by the reduction of hysteresis in the capacitance-voltage relation. Lower leakage current from substrate injection in association with the reduction of depletion layer, was found due to the growth of Zr-silicate IL in N2 annealing, but the leakage from gate injection increased in conjunction with the crystallization of ZrO2 layer. In contrast, the relatively thick SiO2 IL formed in O2 annealing reduces the leakage for both substrate and gate injection. There is also a significant shift of the turn-around voltage from inversion to accumulation, but not with voltage swept back. Finally, we have reported the temperature dependence of current density characteristics measured from 298 to 423 K. For substrate injection, the increase of leakage current well agrees with the temperature dependence of electron concentration in p-type Si. For gate injection, the leakage current is nearly T-independent (D-T conduction) at low voltage, while the leakage behavior at higher voltage changes from field-dependence (P-F conduction) for the as-deposited ZrO2 to either strong T-dependence (S-K conduction) for N2–annealed specimens or insensitive to temperature (F-N conduction) for O2–annealed specimens. Different models in energy-band diagram are proposed to illustrate the conduction mechanisms from direct tunneling, P-F hopping, Schottky thermal emission, to F-N tunneling.