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CHEMICAL VAPOR DEPOSITION

14.6 WAFER PROCESSING

can then be used to estimate the rate coefficients for the rest. For CVD tungsten, this has been done
to varying levels of detail. 4,5,6

Gas-Phase Chemistry. Up until now we have discussed reactions occurring on the wafer surface.
However, in many CVD processes, reactions occur in the gas prior to the reactants reaching the
wafer. This is particularly true for high-pressure processes (for instance, those operating at 1 atm)
and for plasma processes. A classic example of a thermal gas-phase reaction is
SiH → SiH + H
4 2 2
in polysilicon deposition. Coltrin, Kee, and Miller found that this reaction plays a key role in the
2
deposition of Si on the wafer surface. Deposition comes from both SiH directly and from SiH . As
4 2
the temperature increases, the gas-phase reaction becomes increasingly important and a larger frac-
tion of the deposition is from SiH . Interestingly, adding H to the mixture suppresses, to some
2 2
degree, the formation of SiH , making SiH the dominant surface reactant.
2 4
Thermodynamics Versus Kinetics. For a thermal CVD process to move forward, it must be ener-
getically favorable. That is, the energy level of the final state must be lower than the initial state.
This is a problem in thermodynamics and is solved by minimizing Gibbs’ free energy. However,
just because a reaction is thermodynamically favorable, it doesn’t mean it is useful for CVD.
Thermodynamics doesn’t consider the rate of reaction. It only says that eventually the reaction will
occur. It is kinetics that deals with the rates of reactions. In order to be useful for CVD applications,
a reaction must be both thermodynamically and kinetically favorable (i.e., occur at a fast rate). When
investigating a new chemistry, it is common to begin with a thermodynamics study to confirm its via-
7
bility. Middleman and Hochberg provide a detailed example of this type of calculation.

14.2.3 Plasma
Plasma processing is widely used in semiconductor processing, particularly in deposition, etching,
and photoresist stripping. In CVD, the energy in the plasma supplements the thermal energy present,
allowing a much wider range of chemistries to be used while maintaining moderate wafer tempera-
tures. Plasma can also produce ion bombardment of the wafer, which can be beneficial for improv-
ing feature filling.
E-energy Instead of Thermal. To prevent excessive diffusion of dopants and other problems, CVD
processes generally cannot be run at temperatures over 400°C. Yet, some very useful chemistries
don’t have practical deposition rates in this temperature range. For instance, thermal deposition of
SiO from TEOS and N O requires temperatures of 700°C or higher. So how can one take advantage
2 2
of the excellent step coverage of this film without damaging the device being fabricated? The answer
is plasma processing. In typical plasma, the electrons have temperatures of 10,000 K or higher, while
the wafer remains cool.
Allows Lower Temperatures. The energy required to break chemical bonds is typically a few elec-
tron volts (eV). The average electron energy in plasma is typically in this range, and at the high energy
tail of the electron distribution, the energies are much higher (more than 10 eV). These electrons col-
lide with neutral gas molecules and either excite or ionize them. Their tremendous energy allows
reactions that would normally require hundreds of degrees to occur at room temperature. In the
TEOS case mentioned previously, the addition of plasma allows the process to be operated in the
range of 300 to 400°C. This process can take place at room temperature, but it is found that the film
quality is greatly improved by heating the wafer.
Ion Bombardment. If the wafer platen is negative biased, positive ions will be drawn out of the
plasma and will bombard the wafer surface. For low-density plasma, such as typical capacitively
coupled systems, ion fluxes are not high enough to provide significant etching. However, they can

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