Controlling the spin bowl environment to optimize spin coat quality
A variety of factors can affect the coat quality when spin processing. This article will focus on the role that the bowl environment, and in particlar the ability to control the fume exhaust, has on this process.
The drying rate of the resin fluid during the spin process is defined
by the nature of the fluid itself (volatility of the solvent systems
used) as well as by the air surrounding the substrate during the spin
process. Just as a damp cloth will dry faster on a breezy dry day than
during damp weather, the resin will dry depending on the ambient
conditions around it. It is well known that such factors as air
temperature and humidity play a large role in determining coated film
properties. It is also very
important that the airflow and associated turbulence above the
substrate itself be minimized, or at least held constant, during the
To be able to control this environment, there are some destinct advantages to using a "closed bowl" spin coater. While an airtight environment is not required, the exhaust lid
should allow only minimal exhaust during the spin process. Combining with an exhaust port, preferably located beneath the spin chuck, the exhaust lid
becomes part of a system to minimize unwanted random turbulence. The two distinct advantages to this system are:
1. slowed drying of the fluid
2. minimized susceptibility to ambient humidity variations
The slower rate of drying offers the advantage of increased film
thickness uniformity across the coated substrates. Because the fluid dries as it
moves toward the edge of the substrate during the spin process, radial thickness non-uniformities may occur since the fluid viscosity
changes with distance from the center of the substrate. By slowing the
rate of drying, it is possible for the viscosity to remain more
constant across the substrate.
rate and hence final film thickness is also affected by ambient
humidity. Variations of only a few percent relative humidity can result
in large changes in film thickness. By spinning in a closed bowl the
vapors of the solvents in the resin itself are retained in the bowl
environment and tend to overshadow the affects of minor humidity
variations. At the end of the spin process, when the lid is lifted to
remove the substrate, full exhaust is maintained to contain and remove
Another advantage to this "closed bowl" design
is the reduced susceptibility to variations in air flow around the
spinning substrate. In a typical clean room, there is a
constant downward flow of air at about 100 feet per minute (30m/min).
Various factors affect the local properties of this air flow.
Turbulence and eddy currents are common results of this high degree of
air flow. Minor changes in the nature of the environment can create
drastic alteration in the downward flow of air. By closing the bowl
with a smooth lid surface, variations and turbulence caused by the
presence of operators and other equipment are eliminated from the spin
Maintaining spin coat uniformity is critical to the creation of micro devices and using a controlled, closed bowl environment can be a key element to achieving coat uniformity success.