2, Samsung Electronics, Albany, New York, United States
3, IBM Research, Albany, New York, United States
Advanced logic CMOS devices require aggressive shrinking for performance and cost. Among many parameters to be scaled, it is particularly difficult to achieve the lowest threshold voltage (Vt) for p-FET (Field Effect Transistors) devices because of higher process sensitivity due to Fermi-level pinning and oxygen out-diffusion at high-k metal gate stacks. These challenges demand work function engineering for the future technologies. For reducing P-FET Vt, increasing the effective work function (EWF) can be one solution. There have been reports that oxygen has the property to boost the work function of the metals such as titanium silicon nitride, molybdenum and tantalum carbide [1-3].
In this paper, we demonstrate a method to reduce the p-FET Vt without the penalties associated with typical gate stack processing. We selected a Transition Metal Composite (here after, Metal-A) having a p-band edge work function as the metal gate for p-FET. Metal-A is deposited using Atomic Layer Deposition (ALD) on high-k dielectric layers, and also it is carefully treated using a modified oxidation method. Measurements from fully integrated advanced transistors showed around 80 mv of Vt modulation without degradation of the inversion thickness (Tinv), which means no increase of equivalent oxide thickness (EOT) through the modified oxidation.
For further understanding of the oxidation effect on EWF (Effective Work Function) change, ab-initio investigation was performed using stacks of Metal-A on a High-K layer (here, Hafnium oxide, HfO2 was chosen as a general material). We computed the EWF from the interface dipole and the vacuum work function of the metal, which can eliminate the need to compute the band offsets. Thus, it avoids the errors introduced in the band structure and the valence band offset calculations.
By comparing EWF values computed for a defect-free-reference against EWF values for the same reference containing oxygen defects at or near the HfO2/Metal-A interface, we found that the presence of oxygen vacancies (Vo) and oxygen interstitials (Oi) plays a big role in modulating the interface dipole. Furthermore, when oxygen interstitials are incorporated in the bulk Metal-A, the resulting EWF is substantially higher. Based on the simulation, we explain the physical mechanism of EWF modulation (accordingly, Vt modulation) obtained in the above experiments: (1) the oxygen atoms diffused from Metal-A replace Vo in high-k layer and also modify the dipole configuration and (2) the residual oxygen interstitials in the Metal-A also increase the bulk EWF value. These results clearly demonstrate the possibility of EWF engineering towards the p-FET band edge in advanced FETs through the proposed modified oxidation method.
 H. Luan, et.al., App. Phys. Lett, vol. 88, p. 142113, 2006.
 Z. Li, et. al., J. Electrochem. Soc., vol. 155 pp. H481-H484, 2008.
 W. Mizubayashi, et. al., VLSI Technology, 2008 Symposium on pp. 42-43, 2008.