Ion kinetic effects in radio-frequency sheaths

The kinetic effects of ions on the radio-frequency collisionless sheath dynamics are explored by solving the ion Vlasov equation with an ion source term. The ion velocity distribution function is obtained numerically. It is found to depend strongly on the source term. The macroscopic variables such as the ion density, velocity, electric field, and sheath voltage predicted by the kinetic model are in good agreement with the fluid model. The predicted period-averaged ion energy distributions at the cathodes, however, are different. In the high frequency regime, multiple peaks can be formed with some process parameters such as bias voltage and rf frequency. The mechanism for this multiple-peak structure can be attributed to the rf modulation of the sheath potential in combination with the creation of cold ions in the sheath as shown by Wild and Koidl for charge-exchange collisional plasma [J. Appl. Phys. 69, 2909 (1991)]. It is also found that the ion source temperature plays an important role in determining the ion velocity distribution function.