Impact of hexamethyldisiloxane admixtures on the discharge characteristics of a dielectric barrier discharge in argon for thin film deposition

Abstract

Atmospheric‐pressure dielectric barrier discharges (DBDs) in argon with admixtures of small amounts of hexamethyldisiloxane (HMDSO) have been analysed by means of numerical modelling. A time‐dependent, spatially one‐dimensional fluid‐Poisson model has been used, which takes into account the spatial variation of the discharge plasma between the plane‐parallel dielectrics covering the electrodes. Main features of the model, including the reaction kinetics for HMDSO, are given. Good agreement with related experimental studies of the ignition voltage for HMDSO amounts of up to 200 ppm and the temporal course of the discharge current for conditions typical of deposition experiments is obtained by the model calculations when assuming that 30% of the reactions of HMDSO with excited argon atoms, with a rate coefficient of 5.0 × 10−10 cm3/s, lead to the production of electrons due to Penning ionization. The modelling results for constant frequency f = 86.2 kHz and applied voltage Ua = 4 kV show that the electrical energy dissipated in the DBD decreases with an increasing amount of HMDSO and enable the determination of the energy absorbed per HMDSO molecule on the basis of their energy balance. The analysis of the plasma–chemical processes also makes clear that collision processes of HMDSO with excited argon atoms and molecules leading to neutral reaction products are essential for the formation of thin polymer films.

Large‐area atmospheric pressure dielectric barrier discharges in Ar–HMDSO mixtures: Experiments and fluid modelling

“…At such strong degrees of dilution in Ar or He, the excitation, dissociation, and ionisation of monomer molecules by direct electron collisions play a negligible role, when compared with corresponding processes driven by excited noble gas atoms or noble gas ions, transferring energy or charge to a monomer molecule, as shown in a recently published numerical study . Here, DBDs in argon with small admixtures, , of HMDSO were analysed using a time‐dependent, spatially one‐dimensional fluid‐Poisson model.…”

“…A drawback of the small‐scale electrode configuration with a length of only 1 cm in the gas flow direction is its relatively large effect of electrode edges and the resulting fringing electric fields, rendering difficult a direct comparison of the temporal behaviour of electrical discharge properties for operating conditions typical of deposition experiments . For example, an experimental value of 6.6 pF was obtained for the cell capacitance, a factor of 1.74 larger than that calculated using the equation for an ideal parallel‐plate capacitor (3.79 pF).…”

“…On the basis of an established reaction kinetics scheme for argon, which was extended by 21 collision processes of HMDSO () with electrons, different excited states of argon atoms and molecules, and atomic and molecular argon ions, as well as the dissociative recombination of electrons due to collisions with the pentamethyldisiloxanyl ion , the fluid model calculations were able to reproduce several discharge characteristics reasonably well. This concerns, for instance, the temporal evolution of the discharge current for the DBD experiments in an Ar–HMDSO mixture with 300 ppm HMDSO reported by Morent et al as well as the measured substantial decrease of the ignition voltage and the dissipated power at a constant applied voltage with growing . Penning ionisation processes due to energy transfer from excited (metastable and resonance) argon atoms to HMDSO were found to become the predominant source for the production of electrons above about x = 5 ppm in the small‐scale DBD reactor.…”

“…On the other hand, increasing the length of the electrodes, that is, their extension in the gas flow direction, inevitably enhances chemical inhomogeneities due to plasma‐chemical conversion, along with film deposition during the passage of the monomer through the plasma zone. For these reasons, a plane‐parallel discharge configuration with rectangular electrodes having a length of only 1 cm in the gas flow direction, a width of 8 cm, and a discharge gap of 1 mm had been investigated in the previous experiments …”

“…Experimental data from experiments using the DBD reactor described by Nisol et al, which was also applied to studies using HMDSO as a precursor, are of particular interest here, as much effort has been invested in its precise electrical characterisation and process monitoring. Therefore, the results of the corresponding comparative experimental and modelling studies are presented and discussed in the current paper, where the same numerical model as in Loffhagen et al has been employed for the model calculations. The range of HMDSO fractions () studied extends up to 1,600 ppm, in accordance with an experimental work, instead of only 300 ppm as in the previous paper.…”