A Dual Frequency Plasma Sheath Model

A model which predicts the voltage distributions in the sheaths of a dual‐frequency parallel‐plate plasma reactor is presented. Both time‐dependent and time‐averaged Poisson's equations are solved to predict the sheath thicknesses and potential distributions as a function of time in the two sheaths. The model was developed for both collisional and collisionless sheaths by using a hydrodynamic model to predict the ion velocity in the sheaths. The assumptions used in the formulation of the model as well as the method of solution are discussed. The model predictions in single‐frequency mode agree with predictions of other single‐frequency sheath models and available experimental data in both the high frequency (13.56 MHz) and low frequency (100–500 kHz) regimes. The model predictions in dual‐frequency mode indicate that the variation in time of the sheath thicknesses and the potential distributions in the sheaths can be characterized by the ratio of low frequency potential to total applied potential (low plus high frequency). The use of dual‐frequency power in plasma‐enhanced chemical‐vapor‐deposition equipment reportedly allows more control over critical film properties such as stress, composition, and step‐coverage than single frequency units and are widely used for depositing silicon dioxide films. This work is the basis for understanding this additional control over film properties.