Spin coating is the preferred method for application of
thin, uniform films to flat substrates. This process is very simple,
illustrated in Figure 1 at right. An excess amount of polymer solution
is placed on the substrate. The substrate is then rotated at high speed
in order to spread the fluid by centrifugal force. Rotation is continued
for some time, with fluid being spun off the edges of the substrate,
until the desired film thickness is achieved. The solvent is usually
volatile, providing for its simultaneous evaporation.
This process is used extensively in microlithography for the
manufacture of integrated circuits. A photoresist solution is applied to
a wafer using spin coating followed by heating to remove the solvent.
The polymer itself is photochemically active. A positive resist reacts
with radiation to become more soluble. In contrast, a negative resist
reacts with radiation to become less soluble. Thus, the use of masked
radiation followed by selective removal of the
irradiated or nonirradiated polymer provides for the
establishmend of a detailed pattern on the surface.