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CHEMICAL VAPOR DEPOSITION

CHEMICAL VAPOR DEPOSITION 14.15

affect the wafer temperature. For unclamped wafers, changes in pressure, especially in the 1 to
10-torr range, can have a large effect on the wafer temperature. If the platen roughens over time,
it can reduce heat transfer.

14.5.3 Film Properties
Diagnosing problems with film properties such as film stress, refractive index, or dielectric constant
is difficult because of our weak theoretical understanding of these things. The most effective way to
treat these problems is to use design of experiments (DOE) methodology to empirically determine
the relationships between the film properties of interest and the process parameters (e.g., flows, RF
20
power, pressure, and temperature. Once this mapping is complete, the cause of a property drift can
be assigned to the parameters that affect it. For instance, if refractive index has been found, through
a DOE study, to increase with increasing RF power and decreasing pressure, one can then look for
problems in these two areas. Perhaps the matching network is not working properly, leading to high
reflected power levels. Or perhaps some reactant by-products have found their way into the pressure
sensor, leading to stray readings.

14.6 FUTURE TRENDS

The transition from aluminum metalization to copper, combined with the ongoing need to reduce
feature sizes, is continuing to drive CVD development. In addition, there is a strong economic
push to keep each layer of the device economical. Atomic layer deposition has attracted a lot of
interest for its ability to deposit extremely conformal films. In this method, a reactant (gas A) is
first flowed, causing a single monolayer to adsorb onto the wafer. This is followed by gas B that
reacts with the monolayer to form an atomic layer of film. This is repeated until the desired thick-
ness is achieved. ALD produces highly conformal films, even into deep features. However, its one
layer at a time approach results in a slow deposition rate. Thus its applications have tended to be
in thinner films, like nucleation and barrier layers. A review of ALD is found in Ref. 21. Recently,
a hybrid ALD-CVD method has been developed by Hausmann et al., which promises CVD-like
22
deposition rates combined with the perfect conformity of ALD. It relies on balancing two com-
peting reactions to produce a self-limiting layer tens of atomic layers thick per cycle. This may
extend ALD processing to thicker film applications. Another approach to filling device features is
superconformal filling in which the feature bottom has a higher deposition rate than the top. This
superfilling technique, originally developed for electroplating, has been applied to CVD copper,
23
using an iodine catalyst. See Josell et al. for details. Another method of achieving excellent film
conformality is to operate the process chamber under extremely high process conditions. At pres-
sures of typically 200 atm, the CO ambient becomes a supercritical fluid. It then has a density
2
similar to a liquid while maintaining a diffusion coefficient similar to a gas. This allows reactants
to be very efficiently transported into vias and trenches. Watkins et al. have shown excellent film
conformity using this technique. 24

REFERENCES

1. Mattox, D. M., “The History of Vacuum Coating Technology: Part V,” Vac. Technol. Coat, 2002, pp. 32–37.
2. Coltrin, M. E., R. J. Kee, and J. A. Miller, “A Mathematical Model of Silicon Chemical Vapor Deposition,”
J Electrochem Soc., Vol. 131, pp. 1206–1213, 1986.
3. Hierlemann, M., C. Werner, and A. Spizer, “Equipment Simulation of SiGe Heteroepitaxy: Model Validation
by Ab Initio Calculations of Surface Diffusion Processes,” J. Vac. Sci. Technol. B, Vol. 15, pp. 945–941, 1997.

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