Optical and Mass Spectrometric Measurements of Plasma Diagnostics in Low Frequency, High Density, Remote Source O2/Ar, NF3/Ar and CH4/CO2 Dry Reforming Plasmas



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Remote plasma sources are widely used in applications such as chamber cleaning and flowable chemical vapor deposition. In these processes, it is desirable that the dissociation rate of the feed gases be as high as possible and stable. Here, we present results on radical densities and gas dissociation fractions for a 400 kHz toroidal transformer-coupled plasma source (MKS Instruments), operating at a power density of 5 – 50 W/cm3 with feed gases mixtures of O2 or NF3 in Ar, and pressures of 0.4 or 2.0 Torr. Radical densities and feed gas dissociation percentages in the plasma were measured by optical emission spectroscopy (OES), combined with Ar actinometry. In the plasma, O2 was about 60% dissociated in dilute O2 mixtures (10-20%). Dissociation decreased with increasing addition of O2, dropping to 10% dissociation for 90 %O2 in the feed gas. NF3 was >95% dissociated for all NF3/Ar mixtures. Little or no dependence on flow rate was found. Plasma products flowed into an anodized Al downstream chamber that is probed by vacuum ultraviolet (VUV) absorption spectroscopy and line-of-sight molecular beam mass spectrometry. In the downstream chamber, O recombined on the walls to form O2 (detected by VUV O2 absorption). The measured downstream O/O2 ratio was a strongly increasing function of increasing flow rate, reproduced by a downstream global model with O wall recombination probability of O of between 0.001 and 0.002. NF3 does not reform in the downstream chamber, as verified by VUV absorption and line-of-sight molecular beam mass spectrometry. No NF or NF2 was detected, and F mostly recombined to form F2, detected by molecular beam mass spectrometry, along with N2, at the back of the downstream chamber. The F2, F and N2 product absolute number densities were consistent with the 3:1 F:N mass balance of the NF3 feed gas. The gas temperature at the back downstream chamber was also measured by mass spectrometry, and was found to be 450K for 95% NF3/Ar at a flow rates from 200 to 600 sccm and 2 Torr pressure. A study of CH4-CO2 plasma reforming also carried out in this high power density, toroidal transformer-coupled plasma, operating at low pressure (0.2-0.7 Torr). Intermediate between a thermal and non-thermal plasma (electron density of ~3x1012 cm-1 and gas temperature of ~6000 K), the low pressure study provides a unique set of conditions to investigate reaction mechanisms, where three-body reactions can be ignored. Reactive species in the plasma were identified by optical emission spectroscopy. End products of the reforming process were measured by mass spectrometry. Quite high conversions of CO2 and CH4 were found (90%), and the selectivity of CO and H2 were around 80% at 300 sccm feed gas flow rate in a 0.5 Torr plasma, with a CO2:CH4 of 1:1. A detailed reaction mechanism is presented, taking into account the combined detection of reactive intermediates in the plasma (H, O, CH, and C2) and stable products downstream.



Plasma Physics, Plasma Chemistry, Vacuum Technology


Portions of this document appear in: Li, Hanyang, Yingliang Zhou, and Vincent M. Donnelly. "Optical and mass spectrometric measurements of dissociation in low frequency, high density, remote source O2/Ar and NF3/Ar plasmas." Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 38.2 (2020): 023011.