|Ph.D Student||Rothschild Jonathan Avner|
|Subject||The Effect of Alloying on the Effective Work Function at|
Metal/High-K Dielectric Interfaces
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Moshe Eizenberg|
|Full Thesis text|
The continuing downscaling of metal-oxide-semiconductor (MOS) devices has recently brought a change in the gate materials from the classic poly-crystalline-Si/SiO2 stack to metal/high-K dielectric stack due to unacceptable high leakage currents and other related difficulties. One of the biggest challenges, which have surfaced during the replacement of the gate materials, is the control of Fermi level position at metal/high-K interfaces, which has a major effect on the performance of MOS devices.
The goals of this research were to tune the Fermi level position at the metal/high-K interface of MOS devices by means of alloying metals and to correlate between the electrical properties of alloys and their composition, structure and morphology by studying the compositional dependence of the Fermi level position in alloy systems.
The first alloy system studied was the Hf-Ni binary metal alloy system, where a low (3.9eV) and a high (5.3eV) vacuum work-function (FVac) value metals (Hf and Ni, respectfully) were deposited as multi-layers of the pure metals and subsequently intermixed by thermal treatment. The FVac measurements and the FEff measurements were performed separately in order to isolate the effect of the metal itself from the metal/high-K interface and high-K stack effects.
The compositional dependence of FVac and FEff in the Hf-Ni system was non-linear with a strong trend towards the values of pure Hf. This behavior is explained by the enrichment of Hf at the surface (for FVac measurements) or to the interface (for FEff measurements). No Fermi level pinning was observed on HfO2, however the FEff values were shifted above the FVac values and the FEff values on SiO2 due to charges in the HfO2 layer or to dipoles in the oxide stack. In addition, some interesting metallurgic phenomena were observed in the system, such as the formation of Kirkendall voids and SiO2 "scavenging" in capacitors with Hf as the bottom electrode.
The second alloy system studied was the refractory nitride system of HfNx, whose composition was controlled by the Ar/N2 flow ratio during deposition. A nearly linear dependence of the FVac values on the N/Hf ratio from 4.5 eV to 5 eV. However, the FEff values of HfNx are lower, varying from 4.3 eV to 4.7 eV, saturating at N/Hf ratios larger than one. The reason for this difference in behavior is attributed to a chemical reaction at the HfNx/SiO2 interface and the ensuing formation of HfNxOy and SiNxOy at the interface.