The high degree of vibrational excitation of O 2 ground state molecules recently observed in inductively coupled plasma discharges is investigated experimentally in more detail and interpreted using a detailed self-consistent 0D global kinetic model for oxygen plasmas. Additional experimental results are presented and used to validate the model. The vibrational kinetics considers vibrational levels up to v=41 and accounts for electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) in collisions with O 2 molecules and O atoms, vibration-to-vibration energy exchanges (V-V), excitation of electronically excited states, dissociative electron attachment, and electron impact dissociation. Measurements were performed at pressures of 10-80 mTorr (1.33 and 10.67 Pa) and radio frequency (13.56 MHz) powers up to 500 W. The simulation results are compared with the absolute densities in each O 2 vibrational level obtained by high sensitivity absorption spectroscopy measurements of the Schumann-Runge bands for O 2 (X, v=4-18), O( 3 P) atom density measurements by two-photon absorption laser induced fluorescence (TALIF) calibrated against Xe, and laser photodetachment measurements of the O − negative ions. The highly excited O 2 (X, v) distribution exhibits a shape similar to a Treanor-Gordiets distribution, but its origin lies in electron impact e-V collisions and not in V-V up-pumping, in contrast to what happens in all other molecular gases known to date. The relaxation of vibrational quanta is mainly due to V-T energy-transfer collisions with O atoms and to electron impact dissociation of vibrationally excited molecules, e+O 2 (X, v)→O( 3 P)+O( 3 P).

By using a self-consistent particle-in-cell simulation, we investigated the effect of driving frequency (27.12–70 MHz) on the electron energy distribution function (EEDF) and electron-sheath interaction in a low pressure (5 mTorr) capacitively coupled Ar discharge for a fixed discharge voltage. We observed a mode transition with driving frequency, changing the shape of EEDF from a strongly bi-Maxwellian at a driving frequency of 27.12 MHz to a convex type distribution at an intermediate frequency, 50 MHz, and finally becomes a weak bi-Maxwellian at a higher driving frequency, i.e., above 50 MHz. The transition is caused by the electric field transients, which is of the order of electron plasma frequency caused by the energetic “beams” of electrons ejected from near the sheath edge. Below the transition frequency, 50 MHz, these high energy electrons redistribute their energy with low energy electrons, thereby increasing the effective electron temperature in the plasma, whereas the plasma density remains nearly constant. Above the transition frequency, high-energy electrons are confined between opposite sheaths, which increase the ionization probability and therefore the plasma density increases drastically.

One-dimensional particle-in-cell simulation is used to simulate the capacitively coupled argon plasma for a range of driving frequency from 13.56 MHz to 100 MHz. The argon chemistry set can, selectively, include two metastable levels enabling multi-step ionization and metastable pooling. The results show that the plasma density decreases when metastable atoms are included with higher discrepancy at higher excitation frequency. The contribution of multistep ionization to overall density increases with excitation frequency. The electron temperature increases with the inclusion of metastable atoms and decreases with excitation frequency. At lower excitation frequency, the density of Ar ** (3p 5 4p, 13.1 eV) is higher than Ar * (3p 5 4s, 11.6 eV), whereas, at higher excitation frequencies the Ar * (3p 5 4s, 11.6 eV) is the dominant metastable atom. The metastable and electron temperature profile evolve from a parabolic profile at lower excitation frequency to a saddle type profile at higher excitation frequency. With metastable, the electron energy distribution function (EEDF) changes its shape from Druyvesteyn type, at low excitation frequency, to bi-Maxwellian, at high frequency plasma excitation, however a three-temperature EEDF is observed without metastable atoms. I. INTRODUCTIONCapacitively coupled plasma (CCP) discharge driven at 13.56 MHz remains industrial standard tool for plasma processing applications including thin film depositions and reactive ion etching (RIE) 1 . A recent trend towards increase in the processing rates of CCP discharges Page 2 of 14 is utilizing plasma excitation in very high frequency (VHF) range, i.e. from 30 -300 MHz.Further benefits of VHF CCP discharges are low temperature processing, lower substrate damage and unique gas-phase chemistry 2-7 . Most recent example is the deposition of thin film of silicon nitride (Si 3 N 4 ) on lithesome substrate for flexible organic electronic devices, where using 162 MHz plasma excitation a thin film of high optical transmittance and low WVTR is deposited at a substrate temperature of 100 0 C 8-9 . In etching of S-MAP and organic low-k film having an inorganic film etch mask, the 100 MHz radio-frequency (rf) RIE plasma process have shown great improvement of the carbon film etch profile, high selectivity, less erosion of the SiO 2 mask edge, and straight sidewall profile in comparison to 13.56 MHz plasma excitation 10 .Besides process improvements, the VHF plasma excitation showed significant difference in the discharge characteristics including electron heating mechanism when compared to 13.56 MHz CCP. For instance, the VHF excited CCP discharges produce high plasma density and low self-bias 11 . This is due to increase in the discharge current and plasma density at VHF plasma excitation for a constant discharge power. The VHF plasma excitations also exhibit a bi-Maxwellian type electron energy distribution function (EEDF), irrespective of gas pressure or gas type [12][13][14][15] . The transformation of EEDF to bi-Maxwellian at VHF was also pr...

The dynamical characteristics of a single frequency low pressure capacitively coupled plasma (CCP) device under varying applied RF voltages and driving frequencies are studied using particle-in-cell/Monte Carlo collision simulations. An operational regime is identified where for a given voltage the plasma density is found to remain constant over a range of driving frequencies and to then increase rapidly as a function of the driving frequency. The threshold frequency for this mode transition as well as the value of the constant density is found to increase with an increase in the applied voltage. Over the constant density range, for a given voltage, the sheath width is seen to increase as a function of the increasing driving frequency, thereby changing the ion energy without affecting the ion density. Our parametric study thus indicates that the twin knobs of the applied voltage and driving frequency offer a means of independently controlling the density and the ion energy in a low pressure CCP device that may be usefully exploited for plasma processing applications.Single frequency capacitively coupled (CCP) plasma devices have been extensively employed in the plasma processing industry for applications like the etching and deposition of thin films 1 . A CCP device consists of two parallel electrodes between which a plasma discharge is struck by biasing the electrodes with a radio frequency power supply that is typically operated at a single frequency of 13.56 MHz. The bulk plasma is separated from the electrodes by the formation of space charge sheaths whose thicknesses oscillate at the driving frequency. The sheath and bulk plasma properties of the discharge are influenced by a number of factors such as the driving frequency, the applied voltage, the background neutral pressure, the gap between the electrodes and the relative areas of the electrodes. For material processing applications the plasma density and the ion energy are important parameters that have a significant impact on the quality and rate of etching/deposition processes. In low pressure discharges the ions gain energy mainly through their acceleration from the DC bias field within the sheath while the electrons gain energy through a stochastic heating process through their interaction with the high-voltage oscillating sheath 2-17 . The plasma density is controlled both by the applied voltage and the driving frequency. Typically it increases linearly with the applied voltage and quadratically with the driving frequency. In the traditional single frequency CCP device the plasma density and the ion energy cannot be changed independently since an increase in the RF power changes both the density of the plasma and the sheath voltage which affects the energy of the ions 1 . Such a limitation of the single frequency CCP has led to searches for alternative configurations as well as explorations of different parametric regimes. The dual frequency CCP has been a major development in this direca) e-mail: sarvesh@ipr.res.in tion and is now quite widely used ...

The effects of the discharge voltage on the formation and nature of electric field transients in a symmetric, collisionless, very high frequency, capacitively coupled plasma are studied using a self-consistent particle-in-cell (PIC) simulation code. At a driving frequency of 60 MHz and 5 mTorr of argon gas pressure, the discharge voltage is varied from 10V to 150 V for a fixed discharge gap. It is observed that an increase in the discharge voltage causes filamentation in the electric field transients and to create multiple higher harmonics in the bulk plasma. Correspondingly, higher harmonics, up to 7 th harmonic, in the discharge current are also observed. The power in the higher harmonics increases with a rise in the discharge voltage. The plasma density continues to increase with the discharge voltage but in a non-linear manner, whereas, the bulk electron temperature decreases. Meanwhile, the electron energy distribution function (EEDF) evolves from a Maxwellian at lower discharge voltages to a bi-Maxwellian at higher discharge voltages.