Influence of the temporal variations of plasma composition on the cyclic formation of dust in hexamethyldisiloxane-argon radiofrequency discharges: Analysis by time-resolved mass spectrometry

B. Despax,H. Caquineau,F. Gaboriau,K. Makasheva

doi:10.1063/1.4966254

Abstract

Cyclic formation of dust nanoparticles in hexamethyldisiloxane (HMDSO, Si2O(CH3)6)-argon RF discharge with pulsed injection of HMDSO was studied using time-resolved mass spectrometry (MS) and optical emission spectroscopy (OES). A large amount of C2H2, considered as promoter of dust nucleation in hydrocarbon plasmas, was found as a by-product of HMDSO fragmentation. Although no negative ions were detected the presence of C2H2 in the HMDSO-Ar discharge supports the hypothesis of a dust growth mechanism based on negative ions being trapped in the plasma. It was found that at the beginning of each cycle of dust formation during α–γ′ transition, the discharge sustaining process is accompanied by a strong consumption of the HMDSO main by-products: HMDSO-15, CH4 and C2H2. At the end of the cycle, corresponding to the progressive disappearance of dust, the discharge switches back to its initial conditions. The beginning of the inverse transition, i.e. γ′–α, is most likely correlated to the growing void in the dust cloud and dust disappearance. In presence of dust nanoparticles, Ar+ and ArH+ dominate the ion population and consequently the discharge maintenance. When the dust particles gradually disappear, the discharge is rather controlled by Si2O(CH3)5+ ions. Moreover, the increased amount of such heavy ions reveals clearly their important income in the dust repelling process due to the drag force on the large sized dust even at short time scale during the injection time of HMDSO. Atomic-H production increases during the transition from dusty plasma to HMDSO-rich plasma with no dust and its role is associated to a delay in the dust nucleation stage.

Highlights

The scientific and technological interests in physical phenomena appearing in dusty plasmas have been maintained for more than two decades, with different purposes
The various processes put in play are revealed in this work by progressive in time mass spectrometry in correlation with the discharge glow evolution analyzed by optical emission spectroscopy
The α–γ transition in the discharge maintenance is observed for each cycle at the stage of dust formation whereas the reverse transition, i.e. γ –α transition, occurs at the end of the cycle when the dust disappears from the discharge

Summary

INTRODUCTION

The scientific and technological interests in physical phenomena appearing in dusty plasmas have been maintained for more than two decades, with different purposes. An important increase of the electron temperature and a strong decrease of the electron density were measured when dust particles became detectable in the plasma which confirms the transition character of the discharge sustaining.[18,19] At the final stage of the dusty plasma period, when dust particles gradually disappear from the discharge a large asymmetric void was observed in our discharge.[4,19] According to Samsonov and Goree,[14] the instabilities of dusty plasma and the subsequent creation of a void in the dust cloud are due to the following mechanisms: (i) the electrons are depleted in the dust cloud; (ii) the onset of the variation of plasma parameters is triggered by a specific particle size; (iii) the ionization in the plasma bulk is enhanced, visually represented by an enhanced plasma glow; (iv) the ion drag force is partly responsible for the void formation. Instabilities of the dust cloud induced by the pulsed injection of the reactive HMDSO precursor are observed and commented

EXPERIMENTAL PART

RESULTS

Molecular hydrogen formation – a specific behavior

CONSISTENT DESCRIPTION OF THE OBSERVED PROCESSES - ATTEMPT

CONCLUSIONS