Ar/CF₄ capacitively coupled plasma generated using 40 MHz sinusoidal and 800 kHz rectangular waveform voltages

Abstract This article discusses the characteristics of an Ar/CF 4 capacitively coupled plasma (CCP) excited using 40 MHz sinusoidal and 800 kHz rectangular voltage waveforms. The simulations focus on the effect of the low frequency (LF) rectangular wave duty cycle (defined as the period at negative voltage) on the plasma properties and uniformity for constant 100 W power at 40 MHz and 20 mTorr gas pressure. Given the importance of kinetic effects in low pressure CCPs, a hybrid plasma model is used. This model treats electrons as particles using the particle-in-cell formalism while ions and neutral species are represented as fluids. By incorporating electron kinetic effects, this approach allows for the accurate modeling of low-pressure CCPs with complex plasma chemistries. Results show that, at 80% duty cycle, the peak in the density of all species is near the edge of the electrodes. As the LF rectangular wave duty cycle is decreased while keeping the 40 MHz power fixed, the species’ densities increase, the 40 MHz radio-frequency voltage decreases, and the peak in species’ densities shifts towards the chamber center. These trends can be explained based on how the LF voltage modulates the coupling of 40 MHz power to the electrons. Under the conditions considered, the plasma is mostly produced through electron stochastic heating at the sheath edge by the 40 MHz voltage. The 40 MHz couples to the electrons more efficiently when the LF voltage at the powered electrode sheath is small and the sheath is thin. The plasma is produced relatively uniformly in the inter-electrode region during this phase. Therefore, at small duty cycles when the powered electrode sheath is thin for a long time, the plasma is uniform and requires a smaller 40 MHz voltage to deposit 100 W at 40 MHz in the plasma. When the LF voltage in the powered electrode sheath is large and negative, plasma production is weak and occurs at the edge of the powered electrode where the sheath is thinner. At large duty cycles, the plasma is efficiently produced for only a short period, necessitating a larger 40 MHz voltage. The plasma density also peaks near the electrode edge at large duty cycles.