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<b style="font-family: "Times New Roman", serif; font-size: 12pt; color: inherit; font-style: inherit; font-variant-ligatures: inherit; font-variant-caps: inherit; background-color: ;">NAME:</b><span style="font-family: "Times New Roman", serif; font-size: 12pt;"> Hanyang
Li</span><br>
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<b style="font-size: 12pt; color: inherit; font-style: inherit; font-variant-ligatures: inherit; font-variant-caps: inherit;"><span style="color:#201F1E">ADVISORS: </span></b><span style="font-family: "Times New Roman", serif; font-size: 12pt; color: rgb(32, 31, 30);">Dr.
Vincent Donnelly</span><br>
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<b><span style="color:#201F1E">DATE:</span></b><span style="color:#201F1E"> Monday, June 1st, 2020</span><span style="color:black"></span></p>
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<b><span style="color:#201F1E">TIME</span></b><span style="color:#201F1E">: 10:00 AM</span><span style="color:black"></span></p>
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<b><span style="color:#201F1E">LOCATION:</span></b><span style="color:#201F1E"> Zoom </span></p>
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<b><span style="color:#201F1E"> </span></b><a href="https://uofh.zoom.us/j/91999127721">https://uofh.zoom.us/j/91999127721</a> <span style="color:black"></span></p>
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<b><span style="font-size:14.0pt; color:#201F1E">TITLE:</span><span style="color:#201F1E"> </span><span style="color:black"></span></b></p>
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<b><span style="font-size:14.0pt; font-family:"Calibri",sans-serif">Optical and Mass Spectrometric Measurements of Plasma Diagnostics in Low Frequency, High Density, Remote Source O<sub>2</sub>/Ar, NF<sub>3</sub>/Ar and CH<sub>4</sub>/CO<sub>2</sub> Dry Reforming
Plasmas</span></b><span style="font-size:14.0pt"></span></p>
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<b><span style="font-size:14.0pt; color:#201F1E">ABSTRACT:</span></b><span style="color:black"></span></p>
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<span style="font-family:"Calibri",sans-serif">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/cm<sup>3</sup> with feed gases mixtures
of O<sub>2</sub> or NF<sub>3</sub> 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, O<sub>2</sub> was
about 60% dissociated in dilute O<sub>2</sub> mixtures (10-20%). Dissociation decreased with increasing addition of O<sub>2</sub>, dropping to 10% dissociation for 90 %O<sub>2</sub> in the feed gas. NF<sub>3</sub> was >95% dissociated for all NF<sub>3</sub>/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 O<sub>2</sub> (detected by VUV O<sub>2</sub> absorption). The measured downstream O/O<sub>2</sub> ratio was a strongly increasing function of increasing flow rate, reproduced by a downstream global model with O wall
recombination probability of </span><i><span style="font-family:Symbol">g</span></i><i><sub><span style="font-family:"Calibri",sans-serif">O</span></sub></i><span style="font-family:"Calibri",sans-serif"> of between 0.001 and 0.002. NF<sub>3</sub> does not
reform in the downstream chamber, as verified by VUV absorption and line-of-sight molecular beam mass spectrometry. No NF or NF<sub>2</sub> was detected, and F mostly recombined to form F<sub>2</sub>, detected by molecular beam mass spectrometry, along with
N<sub>2</sub>, at the back of the downstream chamber. <a name="_Hlk7560666">The F<sub>2</sub>, F and N<sub>2</sub> product absolute number densities were consistent with the 3:1 F:N mass balance of the NF<sub>3</sub> feed gas. The gas temperature at the back
downstream chamber was also measured by mass spectrometry, and was found to be 450K for 95% NF<sub>3</sub>/Ar at a flow rates from 200 to 600 sccm and 2 Torr pressure.</a></span></p>
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<span style="font-family:"Calibri",sans-serif">A study of CH<sub>4</sub>-CO<sub>2</sub> 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 ~3x10<sup>12 </sup>cm<sup>-1</sup> 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 CO<sub>2</sub> and CH<sub>4</sub> were found (90%), and the selectivity of CO and H<sub>2</sub>
were around 80% at 300 sccm feed gas flow rate in a 0.5 Torr plasma, with a CO<sub>2</sub>:CH<sub>4</sub> 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
C<sub>2</sub>) and stable products downstream.<span style="color:black"></span></span></p>
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