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CHEMICAL VAPOR DEPOSITION

14.14 WAFER PROCESSING

(e.g., He) to a heavy one (Ar) that supports the three-body collisions needed for gas-phase reac-
18
tions. In extreme cases, GPN can be seen during processing by shining a bright light into the cham-
ber and observing a gray or black smoke. Another sign—SEM photos show the particles to be
spherical in shape and uniform in size. They may also be agglomerations of spheres. That is a strong
evidence of GPN.
Gas-phase reactions can also occur upstream in the gas box. For instance, an air leak in a silane
line will cause “dusting” of the line.
Flaking. Flaking occurs when the CVD film has become too thick on the walls of the chamber.
CVD films often have high stresses and at sufficient thickness will crack and shed particles. These
particles tend to be irregular in shape with a wide size distribution. Their composition is similar to
the film being deposited. Both thermal and pressure cycling can make the problem worse. More fre-
quent or effective cleans will generally fix the problem.

Abrasion. Misadjustment of moving parts can cause components to scrape in the process chamber.
At the point of contact, particles are shed, and they may end up on the wafer. Like flakes, these par-
ticles are irregular in shape and of varied size. However, their composition is that of the scraping
components, not the film. Also the particle pattern on the wafer may be nonuniform, with higher par-
ticle densities near the point of contact.
Condensation. In moving from the fab environment to the CVD process chamber, the wafers are
pumped down to vacuum in a load lock. Before pump down, this load lock is filled with air or per-
haps N with a residual of air. During a fast pump down the air temperature drops rapidly, and the
2
moisture present in the air can condense. Such condensation occurs preferentially at nucleation sites,
such as dust particles in the air. The result is a small water droplet, surrounding a particle that can
fall onto the wafer. The water then evaporates leaving behind a contaminant. If this is the case, par-
titioning experiments will point to the load lock as the particle source rather than the process cham-
ber. Condensation can be eliminated by slowing the pump down or reducing the temperature drop.
The latter can be done by shrinking the load-lock volume so all of the gas is in close proximity to a
warm wall. 19

14.5.2 Deposition Rate and Uniformity
Fundamentally, the deposition rate depends on just two things—the local magnitude of reactive
species and energy (either thermal or RF). Armed with this information, plus an understanding of the
process parameters affecting the rate, one can design experiments to identify the source of the drift-
ing deposition rate or poor uniformity. Some typical problem areas are as follows:
Gas delivery. If the rate is dropping, it may be due to a clog in the gas delivery system or a prob-
lem with an MFC. Rate of rise tests can determine if you are getting the flow you desire.
Sinks. When the reactant concentration at the wafer drops, it can be due to a delivery problem,
or it can be that the reactants are being consumed elsewhere. For instance, if a powder builds up
on the showerhead over time, it will present an increased surface area in which more material will
deposit, reducing the deposition rate on the wafer.
Poisons. After chamber maintenance, the chamber walls can be left contaminated. Materials in
the chamber can oxidize on air exposure, moisture can adsorb onto the walls, skin oils can be
transferred, and so forth. These compounds can poison the CVD reaction, reducing the deposi-
tion rate or adversely affecting uniformity.
Wafer temperature. The platen temperature can drift due to problems with the temperature con-
troller or measurement. The coupling to the wafer can also fluctuate. For instance, if wafers are
heated up too quickly, especially 300 mm wafers, they can bow, taking on a bowl shape. It can
take a minute or more for them to relax. Or the emissivity of the wafer or platen may change,
altering the radiative heat transfer. Changes in showerhead emissivity or temperature can also

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