Sheath Width in Negative-Ion-Containing Plasma

Sheath width in negative-ion-containing plasma, which is important in plasma etching, is numerically investigated on the basis of Poisson's equation. With the aid of the previously reported Bohm criterion voltage, the negative ion term is consistently incorporated into the sheath equation. One of the important findings is that the sheath width increases with an increase in the negative ion density. This is simply because the positive-ion current decreases with negative-ion density even under the condition of a constant plasma density. The positive-ion density in the sheath front must decrease to keep neutrality with negative charges when the negative ions with low temperature are introduced, provided that the Bohm criterion voltage is not greatly different.