Low pressure capacitively coupled He–O2/Ar mixture plasma is investigated using optical emission spectroscopy and Langmuir probe (LP) techniques and the effects of discharge parameters i.e. radio frequency (RF) power, filling gas pressure and oxygen concentration on electron density (ne), electron temperature (Te), excitation temperature (Texc), plasma potential (Vp) and electron energy probability function (EEPF) are monitored. It is noted that increases with increase in RF power and filling gas pressure, while it decreases with increasing O2 concentration. The LP technique and Boltzmann plot method are employed to determine and and estimated by both methods shows similar decreasing trend with increasing RF power and filling gas pressure. A sudden increase in and decrease in is noted with RF power, indicating a mode transition i.e. from alpha to the gamma mode. The threshold RF power for the mode shifts from higher to lower value (150–120 W) with increasing gas pressure (0.3–0.5 mbar). This trend reverses and shifts from lower to higher RF value i.e. from 110 W (pure helium) to 150 W (8% O2) with increase in O2 concentration. Investigation of EEPF profile states evolution from Druyvesteyn-like to bi-Maxwellian distribution with increase in RF power and filling gas pressure; due to mode transition and decrease in the height of high energy tail of EEPF. Similarly, the effect of O2 mixing on the shape of EEPF is also investigated. It is noted that in pure helium discharge the EEPF is bi-Maxwellian in nature, while addition of O2 in the mixture results in the broadening of the EEPF. Moreover, the height of high energy tail of EEPF also increases. Finally, an increasing trend in atomic oxygen density is noted with increase in RF power, pressure and O2 concentration.

No abstract

No abstract

No abstract

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.