Electron dynamics in planar radio frequency magnetron plasmas: II. Heating and energization mechanisms studied via a 2d3v particle-in-cell/Monte Carlo code

Abstract

The present work investigates electron transport and heating mechanisms using an (r, z) particle-in-cell (PIC) simulation of a typical rf-driven axisymmetric magnetron discharge with a conducting target. Due to a strong geometric asymmetry and a blocking capacitor, the discharge features a large negative self-bias conducive to sputtering applications. Employing decomposition of the electron transport parallel and perpendicular to the magnetic field lines, it is shown that for the considered magnetic field topology the electron current flows through different channels in the (r, z) plane: a “transverse” one, which involves current flow through the electrons’ magnetic confinement region (EMCR) above the racetrack, and two “longitudinal” ones, where electrons’ guiding centers move along the magnetic field lines. Electrons gain energy from the electric field along these channels following various mechanisms, which are rather distinct from those sustaining dc-powered magnetrons. The longitudinal power absorption involves mirror-effect heating (MEH), nonlinear electron resonance heating (NERH), magnetized bounce heating (MBH), and the heating by the ambipolar field at the sheath-presheath interface. The MEH and MBH represent two new mechanisms missing from the previous literature. The MEH is caused by a reversed electric field needed to overcome the mirror force generated in a nonuniform magnetic field to ensure sufficient flux of electrons to the powered electrode, and the MBH is related to a possibility for an electron to undergo multiple reflections from the expanding sheath in the longitudinal channels connected by the arc-like magnetic field. The electron heating in the transverse channel is caused mostly by the essentially collisionless Hall heating in the EMCR above the racetrack, generating a strong E × B azimuthal drift velocity. The latter mechanism results in an efficient electron energization, i.e., energy transfer from the electric field to electrons in the inelastic range. Since the main electron population energized by this mechanism remains confined within the discharge for a long time, its contribution to the ionization processes is dominant.

Electron dynamics in planar radio frequency magnetron plasmas: III. Comparison of experimental investigations of power absorption dynamics to simulation results

“…Here, only a brief description is given. For a more detailed account of the numerical techniques used we refer to [61].…”

“…In the framework of the companion papers the electron power absorption dynamics in a geometrically asymmetric magnetized CCRF discharge operated in argon are investigated in the presence of a conducting target. While the companion papers [60,61] focus on the fundamental heating mechanisms based on 1d3V and 2d3v particle-in-cell (PIC)/Monte Carlo collisions (MCC) simulations of a single set of typical conditions, in this work the plasma discharge is studied as a function of the neutral gas pressure and the magnetic flux density and based on experiments and 2d3v-PIC simulations. The applied magnetic field has a magnetron-like torus shape that provides closed field lines at the powered electrode.…”

Spatial distributions of the ion flux in a capacitive hydrogen RF discharge using a hollow cathode with double toroidal grooves enclosed by magnets

“…For comparison with the conventional planar magnetron cathode, [21][22][23][24][25][26][27] the hollow cathode with double toroidal grooves enclosed by magnets has some advantages such as closed E × B drift motion trajectories of electrons near the bottom and the side walls inside the toroidal grooves and electron confinement by the cuspate profile of the magnetic field lines inside the toroidal grooves as shown in Fig. 4.…”

“…It is well-known that the conventional RF magnetron plasma [21][22][23][24][25][26][27] with a normal planar magnetron cathode is an attractive high-density plasma with plasma densities of up to 10 10 cm −3 for pure argon plasma. It has been reported that the plasma density is decreased from 10 10 to 10 9 cm −3 by increasing the H 2 gas flow rate in mixture gases of argon and hydrogen.…”

Frequency-dependent electron power absorption mode transitions in capacitively coupled argon-oxygen plasmas

“…The computational investigation of the spatio-temporal dynamics of the electron power absorption is based on the Boltzmann term analysis, a computational diagnostic tool, which is capable of providing a complete description of the electron power absorption in CCPs [55,56]. This method, first proposed in [57] and later revisited in [58], has recently been applied to CCPs under various conditions: in inert gases [8,55,56,[59][60][61][62][63], in electronegative gases [36,54,64,65], in CCPs at atmospheric pressure [66], and in magnetized CCPs as well [46,[67][68][69].…”