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.