Polysilicon films are used in VLSI
· As a polysilicon gate in a self-aligned structure
· As an intermediate conductors in two-level structures.
Grain-boundary diffusion model
· Impurity diffusion in polysilicon film can be explained qualitatively by a grain-boundary diffusion model.
· The diffusivity of impurity atoms that diffuse along grain boundaries can be up to 100 times larger than the diffusivities in single crystal lattice.
· The polycrystal film is considered to be composed of single crystallites of varying sizes that are separated by grain boundaries and impurity atoms inside each crystalite have diffusivities either comparable to or 10 times larger than those found in the single crystal.
Factors influencing diffusion in polysilicon
· Texture of the film because impurity atoms diffuse along grain boundaries.
· Film deposition temperature
· Rate of deposition
· Thickness of the film
· Composition of the substrate film.