or the process of depositing a layer of polycrystalline silicon on a
semiconductor wafer, is achieved by pyrolyzing (decomposing thermally)
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
There are two common
low-pressure processes for depositing polysilicon layers: 1) using
silane at a pressure of 25-130 Pa (0.2 to 1.0 Torr);
and 2) using
(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%.
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
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
polysilicon deposition process becomes mass-transport-limited, the
reaction rate becomes dependent primarily on reactant concentration,
reactor geometry, and gas flow.
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When the rate
at which polysilicon deposition occurs is slower than the rate at which
unreacted silane arrives, then it is said to be
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
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.
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.
low-pressure reactor either by changing the
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.
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.
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