Effect of Al Doping on the Reliability of ALD HfO2

CMOS device scaling below the 22 nm technology node requires gate dielectric materials with superior properties to those of conventional high-k materials. It was found that, Al incorporation into HfO₂ results in an increase in the transition temperature from amorphous to polycrystalline state (1-2). Improved thermal stability (2) and enhanced permittivity (3) by Al doping into ALD HfO₂ was also reported. As a result of the improvement in film properties, improved equivalent oxide thickness (EOT) values, reduced gate leakage current, lower hysteresis and improved interface quality were also observed for Al doped HfO₂ (4-6). However, intermixing of HfO₂ and Al₂O₃ depends significantly on the stack structure, which results in variations in the stoichiometry in the film (7).  The relative position of HfO₂ and Al₂O₃ in HfO₂/Al₂O₃bi-layer has a significant influence on positive charge formation in the dielectrics and also on the quality of the interfacial layer (IL) (8-10).

In this work, we deposited and studied the reliability of 40 cycles ALD HfO₂ doped with Al in the deposition process. The ALD thin film composition was measured by X-ray photoelectron spectroscopy (XPS). Thin films with Al/(Al+Hf)% composition ranging 0% to 8% were deposited and annealed at 800^oC in N₂ ambient. MOS capacitors were then fabricated with ALD TiN as the metal gate. The electrical measurements show that, devices with ~2% Al/(Hf+Al)% have an optimized performance with 17% lower EOT than undoped HfO₂. The amount of positive charge and gate leakage current density were also evaluated for all devices.

The reliability of these devices was monitored by subjecting them to a constant field stress at E= 27.5 MV/cm in the gate injection mode, where the applied voltage to the gate was modified according to the EOT to have equal field across all the dielectrics. Stress-induced trap generation in the dielectrics was observed due to bulk and interface defects. Trap formation followed a power law function with stress time.  Observed stress induced flat-band voltage shift (Fig. 1) shows that devices with Al/(Hf+Al)% =2%  have 36% reduction in the rate of trap generation as compared to HfO₂, whereas degraded flat-band voltage shift was observed for devices with Al/(Hf+Al)% =7%. It can be inferred that an optimized Al concentration in HfO₂can lead to an improvement of the device performance. In order to further understand the reliability characteristics we plan to evaluate the change in interface state density with stress and their time dependent dielectric breakdown (TDDB) characteristics.

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