Multi-scale investigation in the frequency domain of Ar/HMDSO dusty plasma with pulsed injection of HMDSO

Abstract

A combination of time-resolved optical emission spectroscopy measurements and collisionalradiative modeling is used to investigate the phenomena occurring over multiple time scales in the frequency domain of a low-pressure, axially asymmetric, capacitively coupled radiofrequency (RF) argon plasma with pulsed injection of hexamethyldisiloxane (HMDSO, Si 2 O(CH 3) 6). The collisional-radiative model developed here considers the population of argon 1s and all ten 2p levels (in Paschen's notation). The presence of HMDSO in the plasma is accounted for in the model by quenching of the argon 1s states by species generated by plasma processing of HMDSO, including HMDSO-15 (Si 2 O(CH 3) 5), acetylene (C 2 H 2) and methane (CH 4). Detailed analysis of the relative populations of Ar 2p states reveals cyclic evolutions of the electron temperature, electron density and quenching frequency that are shown to be linked to the kinetics of dust formation in Ar/HMDSO plasmas. Penning ionization of HMDSO and its fragments is found to be an important source of electrons for the plasma maintenance. It is at the origin of the cyclic formation/disappearance of the dust cloud, without attenuation of the phenomenon, as long as the pulsed injection of HMDSO is sustained. The multi-scale approach used in this study further reveals the straightforward relation of the frequency of HMDSO pulsed injection, in particular the HMDSO duty cycle, with the frequency of dust formation/ disappearance cycle.

Time‐resolved analysis of the precursor fragmentation kinetics in an hybrid PVD/PECVD dusty plasma with pulsed injection of HMDSO

“…The HMDSO flow is adjusted by a mass flow controller OMICRON switched by a pulse generator AGILENT. It was observed that, under particular conditions, namely an applied power below 60 W and an HMDSO duty cycle above 30%, the pulsed injection of the precursor induces periodic growth of dust, forming a cloud of nanoparticles between the two electrodes …”

“…On one hand, the presence of dust can increase the formation of H 2 by Eley–Rideal reactions on the surface of the nanoparticles. On the other hand, the presence of dust is known to increase the electron temperature which could influence the dissociation degree of HMDSO in the plasma, thus resulting in a rise of atomic and possibly molecular hydrogen . However, since dust formation follows a cyclic behavior, it implies that they grow simultaneously until a critical size is reached, leading to their loss by gravity .…”

“…They were coupled with the predictions of collisional‐radiative modeling describing the population of emitting Ar 2p states to obtain the evolution of the electron temperature (assuming Maxwellian electron energy distribution function) and electron density. Details on this method can be found in our previous works . Because the excitation of emitting Ar 2p levels involves both direct electron‐impact from the ground state (requiring high‐energy electrons >12–13 eV) and stepwise electron‐impact through the Ar 1s metastable levels (requiring low‐energy electrons >2 eV), a broad range of electron energies is considered by the model, including the energy tail.…”

“…Equations were used to calculate the time‐dependant frequency related to the dissociation of HMDSO where ν diss = k dissociation ( T e ) n e is calculated using the time‐resolved values of the electron temperature and electron number density obtained from OES measurements . The main interest in this quantity obtained uniquely from OES data is that it can directly be compared with PSMS data.…”

“…In a recent study, a multiscale evolution of the electron temperature and electron number density in a low‐pressure, axially asymmetric argon rf plasma with a pulsed injection of HMDSO was obtained by OES of Ar 2p‐to‐1s transitions coupled with a collisional‐radiative model describing the populations of emitting Ar 2p states . The results have shown that the injection of HMDSO reduces the electron mean energy since (a) the HMDSO molecule and its fragments decrease the mean ionization threshold of the nominally pure argon plasma and (b) enables Penning ionization through collisions between HMDSO‐related species and argon metastable atoms.…”