40.1 ELECTROSTATIC CHARGE IN SEMICONDUCTOR CLEANROOMS

Electrostatic charge grows to extremely high levels in the environment of a semiconductor fab. This is due to several properties of the cleanroom. First, since the mechanism for developing static charge is a surface phenomenon related to contact followed by the separation of dissimilar materials, the presence of water absorbed into the surface of the material from the humidity in the air diminishes the effect. The phenomenon, called triboelectrification, involves the transfer of electrons from the conduction band of one material to the conduction band of the other material. The presence of water at the surface decreases the difference between the energy levels of the bands and diminishes the amount of static charge that is transferred.1 ^,2

Second, many of the items in the cleanroom are constructed from plastics, which, in general, are extremely good insulators. In fact, it is necessary to employ these materials because they are inherent to the process. For example, quartz is required for reticles because of its optical properties, and Teflon is required because it is impervious to hydrofluoric acid. Also, the extreme cleanliness of the surfaces of objects in the fab denies static surface charge a conduction path to ground on an insulator.

These issues of increased static charge generation and decreased static charge dissipation result in significantly higher static charge levels in a semiconductor fab than in a conventional room. In fact, it is not uncommon for a...