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CHEMICAL VAPOR DEPOSITION

CHEMICAL VAPOR DEPOSITION 14.9

14.2.5 Particles
Like any piece of semiconductor equipment, CVD systems can suffer from particle contamination.
In addition to the typical sources, like flaking or friction, CVD systems can create particles from the
chemistry itself. To get a handle on the particles, it is important to understand both how they are cre-
ated and how they are transported to the wafer.

Creation Mechanisms. Particles can be generated in a chamber volume through gas-phase reactions,
or they can form at surfaces through flaking and abrasion. In addition, moisture droplets can nucle-
ate on particles in the load lock during pumpdown. Of these mechanisms, only gas-phase nucleation
(GPN) is unique to CVD. In the ideal CVD process, the gaseous precursors don’t react in the gas-
phase, or do so very slowly. However, at high pressures—where there are many gas collisions—and
high temperatures, many processes will proceed in the gas phase. The chemistry of GPN is complex
and has only been studied in depth for silicon deposition. 10,11 Once created, particles can be trans-
ported to the wafer surface, resulting in a defect.

Transport Mechanisms. There are five mechanisms for transporting particles in CVD reactors.
Gravitational setting. Large particles can fall onto the wafer surface. The settling time is a func-
tion of particle size and density, as well as the chamber pressure. Figure 14.6 shows settling times
3
for various size particles of density 1 g/cm .
Convection. Like species transport, the gas flows can carry the particles to the wafer surface.
Brownian diffusion. Small particles (less than 1 µm) can be moved around by collisions with
the gas molecules in the chamber. This motion is random and results in a diffusive-type behavior
where the particles tend to diffuse from regions of high concentration to low.
Thermophoresis. Just as with thermal diffusion, temperature gradients can move particles away
from hot surfaces toward cold. This is beneficial in cold-wall reactors where the wafer is much
hotter than its surroundings. Interestingly, even very small temperature differences, like a few
degrees, can push particles away from the wafer surface. 7
Electric fields. In a plasma environment, particles tend to pick up charge. They are then transported
by the local electric fields. These fields can form regions that serve as particle collection areas or
traps. Once the plasma is shut off, these clouds of particles can migrate to the wafer surface. 12

1000

100
10 1
Terminal velocity (cm/s) 0.01 1 torr
0.01 torr
0.1
0.1 torr

0.001
10 torr
0.0001
100 torr
0.00001 760 torr
0.01 0.1 1 10
0.000001
Particle diameter (µm)
FIGURE 14.6 Particle settling time at different pressures.

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