PARTICULATE RELEASE FROM SURFACES EXPOSED TO A PLASMA

Contamination can occur during plasma processing when micrometer-size particulates fall from vacuum vessel walls onto a wafer. In situ light-scattering measurements show how particulates are shed from walls. Using a test surface coated with micron-size particles, we find that when a plasma is turned on, particulates are released rapidly, and when it is turned off, this release stops. This proves that plasma exposure causes particulate shedding. The rate of dust shedding increases with plasma density. The inventory of dust on the surface decays exponentially in time, with a time constant almost-equal-to 10(2) s in our experiment, for plasma densities of almost-equal-to 10(14) m-3. Particulates become negatively charged due to the flux of electrons and ions onto the surface and are then pulled off the surface by the electric field in the plasma sheath. An individual dust grain is shed when its charge Q becomes sufficiently negative.