Unexpectedly large electric forces on dust particles in a temporal afterglow plasma at high gas pressures

Experiments were performed to test a theoretical expectation that late in a temporal afterglow, the electric force FE acting on dust particles should diminish at a higher gas pressure. This electric force is the product of a residual charge Q that forms in the first few milliseconds of the afterglow when electrons and ions are present, and a dc electric field E that can persist into the late afterglow, after electrons and ions are gone. As the argon gas pressure was increased from 8 to 90 mTorr, we found that FE actually increased, instead of diminishing considerably as was expected due to the pressure-dependence of the mobility-limited velocities of ions and electrons during the early afterglow. We suggest that this finding can be explained by the evolution of the electric field in the afterglow, which will develop more slowly at a higher gas pressure. This experimental finding of a large electric force offers encouragement that in semiconductor manufacturing, controlling the charge and forces acting on particles in the afterglow can help mitigate contamination of substrates, even at higher pressures than previously expected.