Reduction of Sodium Ion Density in Molybdenum‐Gate MOS Devices with the Addition of Tantalum to Gate Electrodes

Abstract

The reduction mechanism of mobile ion contamination in Ta-Mo alloy gate MOS structures has been examined by varying the deposition and annealing conditions of gate metals. Mobile ion density in the gate SiO2 for the Ta-Mo gate structures decreases drastically after forming gas annealing at 1000~ whereas the density increases during nitrogen gas annealing. The structure and composition analysis for the Ta-Mo gate observed by x-ray diffraction, SIMS, and AES indicates that nitriding of Ta-Mo films occurs during the forming gas annealing, and the nitride, MoTaN (tetragonal, CrNbN-type structure), is stable even at 1000~ during annealing. These facts suggest that MoTaN formation at the boundaries of Mo grains, which is similar to the stuffing effect observed in TiN films, is responsible for immobilizing the sodium atoms within the gate electrodes. As a result, the diffusion of sodium from the gate electrode into the SiO2 layers is suppressed.With regard to the development of highly conductive and self-aligned gate electrodes and interconnection lines, molybdenum (Mo) is under active investigation as an alternative to the widely used polycrystalline silicon gate in MOS LSI's (1). In the self-aligned gate process, high temperature annealing above 900~ is necessary to activate implanted ions in the source and drain regions following gate electrode formation. This process causes the impurities sodium (Na) and potassium (K), contained in the Mo electrodes and having a concentration less than 1 ppm (2), to diffuse into the gate SiO2 layers. This alkali ion contamination is known to result in flatband voltage shifts in MOS structures (3). It is essential to prevent this ionic Na and K contamination in the thermally grown SiO2 layers in order to stabilize the Mo-gate MOS characteristics.A phosphosilicate glass stabilization process (4) and an oxidation technique for silicon in the presence of chlorine and chlorine compounds (5) have been developed as methods to obtain passivated oxides. Another possible solution, high temperature annealing of Mo gate electrodes in a forming gas, has been reported to be effective in reducing this contamination (6). We have recently shown that the effect of forming gas annealing is enhanced by hydrogen doping and the addition of tantalum to Mo films (2). The Ta addition is intended to increase the amount of doped hydrogen in the films as a result of the fact that Ta has a higher hydrogen absorption property than Mo (7). It has also been shown that the Na diffusion from these specially designed gate electrodes into the gate oxides is almost entirely prevented when high temperature annealing is performed in a forming gas, although the physical origin of this effect remains unclear.In this work, a reduction mechanism of mobile ion contamination in the Ta-Mo alloy gate MOS structures is examined by varying the deposition and annealing conditions of gate metals. Structural and compositional changes in the Ta-Mo films as a function of process conditions are measured using an x-ray diffractometer, SI...