Characteristics of Ni/Gd FUSI for NMOS Gate Electrode Applications
ABSTRACT This letter investigates the work function tuning of nickel/gadolinium (Ni/Gd) fully silicided (FUSI) gate electrodes on HfSiOx dielectrics. It was found that as the percentage of Gd in the Ni/Gd increased from 10% to 30%, the effective work function value after a one-step 450-degC FUSI anneal decreased from 4.75 to 4.35 eV. In addition, the presence of Gd also resulted in lowering of equivalent oxide thickness (EOT) values. The mechanism for a decreased EOT is attributed to the reduction of low-kappa interfacial layers by the presence of Gd in the gate stack. The decrease in work function is attributed to the creation of oxygen vacancies within the high-kappa layer created by the presence of Gd layer.
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ABSTRACT: Work function is experimentally known to be different for different faces of a crystal by amounts ranging from one-tenth to half a volt. For tungsten the faces can be arranged according to decreasing work function as follows: 110, 211, 100 and finally 111. The explanations so far suggested for the differences of the work function are discussed and shown to give either an incorrect sequence or a wrong order of magnitude of the observed differences. The author uses the picture of Wigner and Bardeen according to which the work function is a sum of a volume contribution and a contribution due to a double layer on the surface of the metal. The origin of the latter can be described in the following manner. With every atom one can associate a polyhedron ("s-polyhedron") with the atom at its center, such that it contains all points nearer to the atom under consideration than to any other atom. If the distribution of the electron density within these polyhedra of the surface atoms was the same as for the inside atoms then there would be no double layer on the surface. However, this is not the case since the total energy is lowered by a redistribution of the electron cloud on the surface. There are two effects: the first is a partial spread of the charge out of the s-polyhedra and the second is a tendency to smooth out the surface of the polyhedra. In consequence of the second effect the surfaces of equal charge density are more nearly plane than in the original picture. The two effects have opposite influences and since they are comparable in magnitude, it is not possible to predict the sign of the total double layer without numerical computations. Some general formulae for the double layers are derived and discussed more fully in the case of a simple cubic and a body-centered cubic lattice. The minimum problem of the surface energy is solved for four faces of a body-centered crystal and the results are applied to the case of tungsten. One obtains the differences between the work functions for different directions. The results agree satisfactorily with the experimental data: assuming a reasonable density of the free electrons, one obtains the correct sequence of faces and the correct differences of the work function. The surface energies are calculated an d found in agreement with the observed stability of certain crystal faces.Physical Review - PHYS REV X. 01/1941; 60(9):661-674.
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ABSTRACT: The full silicidation of silicon gate electrodes using Ni–Tb alloy was investigated for metal oxide semiconductor field effect transistor (MOSFET) applications. Results showed that a dual-layer silicide gate consisting of a top NiTb silicide layer and a bottom NiSi layer was formed. Full silicidation using resulted in a gate work function of 4.41 eV. This is lower than a work function of 4.68 eV obtained from full Ni silicidation. The work function lowering was linked to structural changes in the underlying NiSi layer. No physical or electrical degradation was observed after forming gas annealing at 420°C for 30 min. NiTb-silicided gate electrodes have work functions suitable for application in advanced transistor structures.Journal of The Electrochemical Society. 03/2006; 153(4):G337-G340.
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ABSTRACT: In this letter, an n-type near-band edge fully silicided (FUSI) material-Yb-doped Ni FUSI is demonstrated for the first time. By doping Yb into Ni FUSI, it is shown that while maintaining the same equivalent oxide thickness and the similar device reliability, the work function of Ni FUSI (on SiON dielectrics) could be tuned from 4.72 to 4.22 eV. Yb-doped Ni FUSI is promising for the gate electrode application in n-MOSFETs.IEEE Electron Device Letters 04/2006; · 2.79 Impact Factor