Modeling of Self-Aligned Silicidation in 2D and 3D: Growth Suppression by Oxygen Diffusion
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Modeling of Self-Aligned Silicidation in 2D and 3D: Growth Suppression by Oxygen Diffusion Victor Moroz and Takako Okada* Avant! Corporation, 46871 Bayside Parkway, Fremont, CA 94538, U.S.A. * Toshiba Corporation, Research and Development Center, 1 Komukai-Toshiba-cho, Kawasaki 210, Japan. ABSTRACT Stress-strain effects and physical processes during formation of the self-aligned silicides are analyzed. A new model for predictive simulation of the self-aligned silicidation is suggested. The model is based on suppression of diffusion and reaction rate of the silicon atoms inside silicide in the presence of oxygen atoms, injected into silicide from the neighbor oxide regions such as oxide spacer, TEOS at STI (Shallow Trench Isolation) and pad oxide. The model is demonstrated to explain the experimentally observed silicide shape. INTRODUCTION Self-aligned silicidation is a commonly accepted method of reducing series resistance of the source, drain, and gate terminals of deep submicron MOSFETs. Typical experimentally observed TiSi2 shape near an oxide spacer is shown in Figure 1.
Figure 1. TEM image of self-aligned titanium silicide next to oxide spacer from [1]. Silicidation was performed at 730oC.
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There is virtually 100% suppression of the lateral silicidation under the spacer. This observation did not receive a satisfactory theoretical explanation, yet its understanding is necessary for predictive simulation of the silicide shape, which determines source/drain impurity diffusion and stress distribution around the silicide layer. Simulated TiSi2 shape is shown in Figure 2 using the same process flow as in the experiment, depicted in Figure 1. Lateral silicidation here is comparable to the vertical silicidation due to the assumption of constant diffusivity for the silicon atoms, which are the dominant diffusing species in TiSi2. Similar shape is obtained also if metal atoms are the dominant diffusing species, or if several species are simultaneously contributing to the silicide growth, as long as their diffusivities and reaction rates are constant throughout the silicide. THEORY It has been suggested that mechanical stress can be responsible for suppressing the silicide growth at the spacer corner [1]. However, stress-strain analysis of silicidation shows that stress changes sign near the end of the spacer (see, for example Figure 2). Such a stress pattern in silicide is due to the downward movement of the shrinking metal layer next to the spacer. The entire metal layer is moving down to provide metal atoms for the growing silicide. The movement is affected by the adjacent oxide spacer, which slows down the metal flow along the spacer, but in turn pushes down the silicon substrate below.
Figure 2. Simulated self-aligned titanium silicidation at 730oC. Hydrostatic pressure is shown in MPa at the silicidation temperature. Silicon atoms diffusivity and reaction rate are constant.
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Similar stress distribution has been obtained in [1]. Such a stress pattern with stresses of the opposite sign is inherent
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