Influence of excitation frequency on the metastable atoms and electron energy distribution function in a capacitively coupled argon discharge

Abstract

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...

Non‐self‐sustained electron beam RF‐generated plasma in application for functional surface pretreatment

“…Our simulation code implements the classic particle‐in‐cell Monte Carlo collision algorithm. [ 52–54 ] The direct verification of our PIC‐MC code was recently carried out by Kovalev et al [ 55 ] by means of comparison with the results of other codes obtained in studies of Lafleur et al [ 56 ] and Sharma et al [ 57 ] under exactly the same conditions in 100% argon CCP. Both referred PIC codes also participated in the global verification for He plasma published by Turner et al [ 58 ]…”

Theoretical and experimental aspects of non-equilibrium plasmas in different regimes: fundamentals and selected applications

“…Data from Ref. [107] ference between the two cases is quantitative, with the ground model predicting higher values than the selfconsistent, even if both models show growing trend with respect to the field frequency. Different behaviours are obtained for the electron temperature.…”

“…As an example, Sharma et al [107] discuss the effects of metastable state kinetics in a high-frequency parallelplate argon discharges, modelling the system with 1D Particle In Cell (PIC) Monte Carlo Collisions (MCC) code. In particular the paper is focused on the role of processes involving metastable excited states in affecting the plasma properties as a function of the applied field frequency.…”

“…The discharge is obtained with a parallel plate with a gap of 3.2 cm, peak voltage drop of 100 V and gas pressure of 100 mTorr. Data from Ref [107].…”

Non-equilibrium plasma for ignition and combustion enhancement

“…Nevertheless, it has been proved that the EEDF can be strongly affected by processes starting from excited states such as ionization from metastable states in noble gases [34], superelastic collisions [35] from vibrational and electronically excited states and excitation from vibrationally excited states [36]. For electron molar fraction χ e > 10 −3 , also electron-electron collisions become effective [37,38] and must be included in the Boltzmann solver.…”