Chemistry Reference
In-Depth Information
Figure 8.10.
The aggregation of aerosol particles to form (a) fuzzballs (homogeneous
flocculation) and (b) dust layers or surface dust deposits (heterogeneous Flocculation) due to
the overwhelming attractive forces in gaseous media of low dielectric constant.
Such attractive interactions can be particularly important in situations where the
presence of even a few extraneous particles on a surface can be highly detrimental,
as in the production of microchips for the electronics industry (Figure 8.11). The
presence of a single dust particle on the surface of a silicon wafer before coating
with the photoresist resin that will be used to engrave the final circuit will, in all
probability, result in a defective product in that area. When one considers that mod-
ern chips may have circuit line spacings of less than 10
4
cm, a particle of that
diameter or even smaller will represent a veritable monkeywrench in the works.
For that reason, extreme measures must be taken to ensure that aerosol particles
are absent (to the extent technologically possible) in production areas.
In a '' stable'' cloud formation, the water drops lie in a size range in which air
currents and other forces allow them to remain dispersed in a more-or-less stable
way. Such clouds are usually the characteristics white because the small size of the
light scattering particles produces less scattering of the incident sunlight. When
droplets coalesce, as in rain formation in clouds, the identity of the small individual
drops begins to be lost and larger drops formed, producing the darker, heavier
clouds characteristic of rain and thunder showers. The radius of the drop will
greatly affect the free-fall rate. If the drop forms under relatively calm conditions
(e.g., little vertical convection to retard the drops' fall), small, gentle rainfall will
Figure 8.11.
The importance of maintaining a clean, aerosol-free atmosphere is vitally
important in the electronics industry: (a) dust-confaminated semiconductor surface; (b)
photoresist coated over adhering dust particles; (c) developed microcircuit with defects.
