Fluid Simulation For Sheath Characteristics In Dual-frequency Capacitively Coupled Discharges

Capacitively coupled plasmas (CCPs) are used extensively in the macro-electronics manufacturing as etching, deposition and other surface treatment devices; for etching processing, plasma density and ion energy bombarding the electrodes are crucial issues because the high plasma density leads to high etch rates while ion bombardment energy is vital for improving selectivity and avoiding dielectric damage. Controlling the plasma density and the ion energy independently can modulate etch rates, etching selectivity and other processing parameters indirectly. Dual frequency CCP has been considered as an effective tool for the independent control of the plasma density and ion bombarding energy, since it can generate large area uniformly plasma. The plasma parameters such as plasma density, ion bombardment energy can be effectively controlled by modulating the discharge parameters of low and high frequency sources. For its simple configuration and low cost, it has been widely used in the next generation plasma etching equipments.Ions must pass through the sheath before reacting with the wafer; hence, plasma etching processing is determined directly by physical parameters and process in the sheath, so the sheath characteristics is extensively studied by many research workers.In Chapter 2 , a one dimensional fluid model is developed for plasma sheaths. Ion density and ion drift velocity are described by cold fluid equations, electron density is given by Boltzmann relation and instantaneous electric field is determined by Poisson's equation; the relation between the sheath thickness and potential is determined self-consistently by the current balance equation which is obtained from the equivalent circuit model.We studied the characteristics and parameters of plasma sheath driven by dual radio frequency and dual pulse respectively, such as plasma density, sheath electron spatiotemporal distribution and charging effect. The results show characteristics of the sheath are modulated by low and high sources; plasma density is modulated by high source but ion energy modulated by low one independently; and the surface charge density on the insulating substrate with the pulse bias is obviously lower than that of the radio frequency bias.In Chapter 3, a two dimensional model is also developed for the case of an electrode with a step or cylindrical hole, physical parameters of plasma sheath is studied under the different discharge pressure; The fluid equations describing the ion were solved by Flux Corrected Transport (FCT) method, and two dimensional Poisson equation is solved by the Successive-Over-Relaxation (SOR) Gauss-Seidel iterative method. The results show the electrode potential and sheath thickness were controlled by both low-frequency and high-frequency sources, the potential at the same position in the sheath decreased as the pressure increased, while the shape of potential-isoline near the electrode is similar with the shape of electrode, which is also referred as "Plasma molding" effect; ion density in sheath has the similar profiles with potential; radial electric field and ion velocity at inner side of hole are strong and changed significantly in space.