Temporal evolution of plasma parameters in a pulse-modulated capacitively coupled Ar/O2 mixture discharge

The time-resolved measurements of the plasma parameters are carried out in a pulse-modulated low pressure capacitively coupled discharge operated in argon and argon–oxygen mixtures. Measurements are performed at a constant radio-frequency (RF) power of 130 W at 13.56 MHz, with the pulse repetition frequency of 1 kHz and 50% duty cycle. A single RF compensated Langmuir probe, synchronized with the modulated signal, is used to investigate the effects of filling gas pressure and O2 content in the discharge on the temporal evolution of electron density ne, electron temperature Te, plasma potential Vp, and electron energy probability function (EEPF). ne shows a rapid increase with time during the start of the pulse-on phase, followed by a gradual increase to attain a steady state value. During the pulse-off phase, ne decreases gradually. ne drops at a higher pressure and with increasing O2 content in the discharge. The temporal variations in Te and Vp, on the other hand, are more abrupt than the temporal variation in ne during the rise and the fall of the pulse. Both Te and Vp show an increase with rising pressure and with increasing O2 content in the discharge. The investigation of the temporal evolution of the EEPF reveals that the discharge is in a highly non-equilibrium state at the start of the pulse and evolves from bi-Maxwellian to Maxwellian distribution with the passage of time. The density of low energy electrons decreases, while the density of high energy electrons increases with increasing pressure and O2 content in the discharge. Moreover, several peaks and broadening in the EEPF profile are also noted with increasing O2 content.