Driving frequency effect on discharge \nparameters and higher harmonic \ngeneration in capacitive discharges at \nconstant power densities
Sarveshwar Sharma, Nishant Sirse, Animesh Kuley, Abhijit Sen, M. M. Turner
Abstract
Very high frequency (VHF) driven capacitive discharges are now being increasingly adopted for \nplasma-based materials processing due to their high processing rates and lower substrate damage. Past \nstudies related to complex plasma dynamics and higher harmonics generation in such systems were \nlimited to constant voltage/current conditions, whereas, industrial systems are mostly driven by \nconstant power density sources. In the present study, using particle-in-cell (PIC) simulation, we \nexplore the dynamics of collisionless symmetric capacitive discharges that is operated at constant \npower densities. Our focus is on the effect of the driving frequency on the discharge parameters like \nthe electron density/temperature, the electron energy distribution function (EEDF), the ion energy \ndistribution function (IEDF), and the generation of higher harmonics in the device. The simulations \nare performed for a driving frequency from 27.12-100 MHz in argon plasma at a gas pressure of 1 Pa \nand for two values of the power density, namely, 2 kW/m3 and 20 kW/m3. It is observed that the \nrequired discharge voltage for maintaining constant power density decreases and discharge current \nincreases with an increase in the driving frequency. A transition frequency is observed at both power \ndensities. The density decreases (electron temperature increases) before the transition frequency and \nthe trend is reversed after crossing the transition frequency. The EEDF shows an enhancement in the \npopulation of the mid-energy range of electrons as the driving frequency increases up to the transition \nfrequency thereby changing the shape of EEDF from bi-Maxwellian to nearly Maxwellian, and then \ntransforms into a nearly bi-Maxwellian at higher driving frequencies. The IEDF at the electrode \nsurface shows bimodal behaviour at a lower driving frequency, becoming more pronounced at a power density of 20 kW/m3, and then turning into a single energy peak. The corresponding maximum \nion energy is found to decrease with driving frequency.