Investigation On Ion Energy Distributions In Capacitively Coupled Plasmas

Capacitively coupled plasmas(CCPs) are widely used for material processing, like etching and deposition, especially in the fabrication of large-scale integrated circuits. Ion energy distribtutions(IEDs) on substrate surfaces are crucial parameters for the process performance. IEDs are affected by collision processes(like charge transfer and collision-induced dissociation) and the rf electric field in the sheath. It has been known that low-energy multiple-peak structures in IEDs are caused by the combined effect of the rf modulation of the sheath electric field and charge transfer collisions. However, charge transfer in different regions of the rf sheath may have different effects on IEDs, which is not investigated in the literature. For CF4 discharges, reported measurments show smooth continua structures in IEDs while the underlying mechanism is still unknown.Dominant mechanisms affecting IEDs in Ar and CF4 CCPs are investigated by measurement and modeling in this work. Analytical models of IEDs are introduced. New findings in this work are as follows:1. An analytical model of IEDs in collisional rf sheaths(where the ion mean free path is smaller than the sheath thickness) is developed. With this model, charge transfer in the space charge region of the rf sheath is found to be the dominant source of low-energy multiple-peak structures in IEDs. In previous works, charge transfer in the space charge region is ignored and the model introduced is limited in weakly collisional sheaths(the ion mean free path is larger than the sheath thickness). 2. With the analytical model of IEDs, a novel method is introduced for the estimation of the electron density by IEDs measurement. This method is valid when charge transfer is the dominant ion-neutral collisions in the sheath. 3. Collision-induced dissociation of CF3+ is found to be the dominant mechanism affecting IEDs in CF4 discharges. Continua structures in IEDs of CF2+, CF+ and C+ are found to be caused by the kinetic energy released during the dissociation process.4. An analytical model of IEDs in dual-frequency rf sheaths is introduced. Model results are in good agreement with reported simulations results. 5. The transient period at the beginning of the the pulse-on phase in pulsed CF4 discharges is found to play an important role in IEDs. The dependence of IEDs on the pulse repetation frequency, the rf power and the gas pressure in pulsed CF4 CCPs is found.