Thin films of  polycrystalline silicon, or polysilicon (also known as poly-Si or poly), are widely used as MOS transistor gate electrodes and for interconnection in MOS circuits.  It is also used as resistor, as well as in ensuring ohmic contacts for shallow junctions.  When used as gate electrode, a metal (such as tungsten) or metal silicide (such as tantalum silicide) may be deposited over it to enhance its conductivity.


Poly-Si is known to be compatible with high temperature processing and interfaces very well with thermal SiO2.  As a gate electrode, it has also been proven to be more reliable than Al.  It can also be deposited conformally over steep topography.  Heavily-doped poly thin films can also be used in emitter structures in bipolar circuits.  Lightly-doped poly films can also be used as resistors.


Poly-Si is usually deposited by thermal decomposition or pyrolysis of silane at temperatures from 580-650 degrees C, with the deposition rate exponentially increasing with temperature.  The deposition rate is also affected by the pressure of silane, which translates to silane concentration. Other important variables in polysilicon deposition are pressure and dopant concentration. 


Fig. 1. SEM Photos of Polysilicon Lines


The electrical characteristics of a poly-Si thin film depends on its doping.  As in single-crystal silicon, heavier doping results in lower resistivity. Poly-Si is more resistive than single-crystal silicon for any given level of doping mainly because the grain boundaries in poly-Si hamper carrier mobility. Common dopants for polysilicon include arsenic, phosphorus, and boron. Polysilicon is usually deposited undoped, with the dopants just introduced later on after deposition.


There are three ways to dope polysilicon, namely, diffusion, ion implantation, and in situ doping.  Diffusion doping consists of depositing a very heavily-doped silicon glass over the undoped polysilicon.  This glass will serve as the source of dopant for the poly-Si.  Dopant diffusion takes place at a high temperature, i.e., 900-1000 deg C.  Ion implant is more precise in terms of dopant concentration control and consists of directly bombarding the poly-Si layer with high-energy ions.  In situ doping consists of adding dopant gases to the CVD reactant gases during the epi deposition process.  


Fig. 2. Example of a Low-Pressure CVD (LPCVD) furnace that can be used for polysilicon deposition


Wafer Fab Links:  Incoming Wafers Epitaxy Diffusion Ion Implant Polysilicon

Dielectric Lithography/Etch Thin Films Metallization Glassivation Probe/Trim


See Also:  Polysilicon Deposition; Polysilicon Doping;  IC Manufacturing




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