Polysilicon Deposition

            

Polysilicon Deposition, or the process of depositing a layer of polycrystalline silicon on a semiconductor wafer, is achieved by pyrolyzing (decomposing thermally) silane, SiH4, inside a low-pressure reactor at a temperature of 580 to 650 deg C. This pyrolysis process involves the following basic reaction:  SiH4 --> Si + 2H2.  

                 

Polysilicon has many applications in VLSI manufacturing.  One of its primary uses is as gate electrode material for MOS devices. A polysilicon gate's electrical conductivity may be increased by depositing a metal (such as tungsten) or a metal silicide (such as tungsten silicide) over the gate.  Polysilicon may also be employed as a resistor, a conductor, or as an ohmic contact for shallow junctions, with the desired electrical conductivity attained by doping the polysilicon material.

    

There are two common low-pressure processes for depositing polysilicon layers: 1) using 100% silane at a pressure of 25-130 Pa (0.2 to 1.0 Torr); and 2) using 20-30% silane (diluted in nitrogen) at the same total pressure.  Both of these processes can deposit polysilicon on 10-200 wafers per run, at a rate of 10-20 nm/min and with thickness uniformities of +/- 5%.

     

The critical process variables for polysilicon deposition include temperature, pressure, silane concentration, and dopant concentration.  Wafer spacing and load size have been shown to have only minor effects on the deposition process. 

  

The rate of polysilicon deposition increases rapidly with temperature, since it follows the Arrhenius equation:

R=A e-qEa/kT where R is the deposition rate, Ea is the activation energy in electron volts, T is the absolute temperature in degrees Kelvin, k is the Boltzmann constant, q is the electron charge, and A is a constant.  The activation energy for polysilicon deposition is about 1.7 eV.

  

Based on this equation, the rate of polysilicon deposition increases as the deposition temperature increases.  There will be a minimum temperature, however, wherein the rate of deposition becomes faster than the rate at which unreacted silane arrives at the surface.  Beyond this temperature, the deposition rate can no longer increase with temperature, since it is now being hampered by lack of silane from which the polysilicon will be generated.  Such a reaction is then said to be 'mass-transport-limited.'  When a polysilicon deposition process becomes mass-transport-limited, the reaction rate becomes dependent primarily on reactant concentration, reactor geometry, and gas flow.

   

When the rate at which polysilicon deposition occurs is slower than the rate at which unreacted silane arrives, then it is said to be surface-reaction-limited.  A deposition process that is surface-reaction-limited is primarily dependent on reactant concentration and reaction temperature. Deposition processes must be surface-reaction-limited because they result in excellent thickness uniformity and step coverage. A plot of the logarithm of the deposition rate against the reciprocal of the absolute temperature in the surface-reaction-limited region results in a straight line whose slope is equal to -qEa/k.

  

At reduced pressure levels for VLSI manufacturing, polysilicon deposition rate below 575 deg C is too slow to be practical.  Above 650 deg C, poor deposition uniformity and excessive roughness will be encountered due to unwanted gas-phase reactions and silane depletion. Pressure can be varied inside a low-pressure reactor either by changing the pumping speed or changing the inlet gas flow into the reactor.  If the inlet gas is composed of both silane and nitrogen, the inlet gas flow, and hence the reactor pressure, may be varied either by changing the nitrogen flow at constant silane flow, or changing both the nitrogen and silane flow to change the total gas flow while keeping the gas ratio constant.

   

Polysilicon doping, if needed, is also done during the deposition process, usually by adding phosphine, arsine, or diborane. Adding phosphine or arsine results in slower deposition, while adding diborane increases the deposition rate. The deposition thickness uniformity usually degrades when dopants are added during deposition.

       

See Also:  PolysiliconPolysilicon Doping Fab Reactions

             

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