Browsing by Author "Overzet, Lawrence J."
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Item A Study in Atmosphere Dielectric Barrier Discharge Plasma Electrode Catalytic Effect Using Oxygen and Nitrogen(2018-05) Gemsheim, Alex Jordan; Overzet, Lawrence J.; Goeckner, Matthew J.Atmospheric dielectric barrier discharge (DBD) plasma creates a gaseous environment conducive to the generation of reactive species. DBD plasmas have a variety of applications in the clinical and industrial fields. During this experiment, an DBD plasma was ignited in controlled gas environments using alumina as a dielectric barrier with gold, copper, or nickel electrodes. The gas environment was formed by varying ratios of oxygen, O2, and nitrogen, N2, with trace amounts of water and carbon dioxide. The plasma transformed these diatomic molecules into reactive species, primarily ozone, O3, and nitric acid, HNO3. A higher initial percentage of oxygen will result in a higher ozone concentration. The selected electrode material also alters the concentrations of ozone and nitric acid. Additionally, the electrode material can modulate the ratio of ozone to nitric acid, providing a specific selectivity between reactive species. The electrode material generating the highest ozone concentration was gold, then copper and nickel. The electrode material with the highest ratio of ozone to nitric acid was nickel at 4:1; then gold at 2:1; and copper at 1.5:1. Utilizing these parameters allows for the generation of reactive species to desired concentrations and specific selectivity.Item Correction of Aspect Ratio Dependent Etch Disparities(A V S Amer Inst Physics) Bates, Robert L.; Goeckner, Matthew J.; Overzet, Lawrence J.; 0000 0000 5396 3610 (Goeckner, MJ); 0000 0003 5379 4329 (Overzet, LJ); 2008008261 (Goeckner, MJ)The etch rate of deep features in silicon, such as trenches and vias, can vary significantly with the feature aspect ratio (AR). Small AR features generally etch faster than large AR features. The reasons for this AR dependence include a slowing of the etch rate with increasing AR due to the necessary transport of molecules into and out of the features as well as ion flux reductions at feature bottom due to the angular spread of the ion flux and ion deflection caused by differential charging of the microstructures. Finding ways to reduce, eliminate, or reverse this AR dependence is both an active subject of research and difficult. In this work, instead of focusing on methods to reduce or prevent AR dependence in an etch process, the authors focus on methods to correct it after the fact. The authors show that an inhibitor film deposition step can be used under some circumstances to allow feature depth disparities to be corrected. This process can be used to correct feature depth disparities whenever the AR dependence of the inhibitor film deposition step is worse (larger) than the AR dependence of the following inhibitor etch step. To test the theory, a plasma process through SF₆/C₄F₈/Ar mixtures was used to both produce trenches of various ARs having significant depth disparities and correct those disparities. The etch depth of small AR features can be held essentially constant while that of larger AR features is increased to match or even exceed.Item Driving Frequency Fluctuations in Pulsed Capacitively Coupled Plasmas(Springer) Poulose, John; Goeckner, Matthew J.; Shannon, Steven; Coumou, David; Overzet, Lawrence J.; 0000 0000 5396 3610 (Goeckner, MJ); 0000 0003 5379 4329 (Overzet, LJ); 2267467 (Goeckner, MJ); Poulose, John; Goeckner, Matthew J.; Overzet, Lawrence J.We report time resolved measurements of the RF current, voltage and complex impedance for pulsed plasmas through electropositive (Ar) and electronegative (CF₄, O₂) gases and gas mixtures. In addition, we report measurements of the effective frequency versus time at various locations within the RF circuitry. The frequency is found to fluctuate away from that sourced by the RF generator when the plasma re-ignites. Plasma re-ignition induces abrupt impedance changes due to the re-formation of the plasma sheath and bulk. These fast changes in the plasma impedance cause the measured changes in the voltage and current frequencies. As a result, the frequency of the RF power at the plasma electrodes was found to be as much as 250 kHz different from that being sourced by the RF generator for short periods of time. These frequency fluctuations are of particular interest to the application of frequency tuned matching networks.Item Gain and Loss Mechanisms for Neutral Species in Low Pressure Fluorocarbon Plasmas by Infrared SpectroscopyNelson, Caleb T.; Overzet, Lawrence J.; Goeckner, Matthew J.; 0000 0000 5396 3610 (Goeckner, MJ); 0000 0003 5379 4329 (Overzet, LJ); 2008008261 (Goeckner, MJ)This article examines the chemical reaction pathways of stable neutral species in fluorocarbon plasmas. Octafluorocyclobutane (c-C4F8) inductively coupled plasma discharges were found to primarily produce stable and metastable products downstream from the discharge, including c-C4F8, C2F4, C2F6, CF4, C3F8, C4F10, C3F6, and CF2. A novel analysis technique allows the estimation of gain and loss rates for neutral species in the steady state as functions of residence time, pressure, and discharge power. The gain and loss rates show that CF4, C2F6, C3F8, and C4F10 share related gain mechanisms, speculated to occur at the surface. Further analysis confirms that CF2 is predominantly produced at the chamber walls through electron impact dissociation of C2F4 and lost through gas-phase addition reactions to form C2F4. Additionally, time-resolved FTIR spectra provide a second-order rate coefficient of 1.8 x 10(-14) cm(3)/s for the gas-phase addition of CF2 to form C2F4. Finally, C2F4, which is much more abundant than CF2 in the discharge, is shown to be dominantly produced through electron impact dissociation of c-C4F8 and lost through either surface or gas-phase addition reactions.Item Phase Resolved Optical Emission Spectroscopic Measurements and Results of Pulsed Capacitively Coupled Radiofrequency Discharges Through Argon and Tetrafluoromethane-Argon(2020-05) Hernandez, Keith; Overzet, Lawrence J.; Goeckner, Matthew J.The experimental results and discussion presented in this dissertation contribute new findings for pulsed capacitively coupled radiofrequency (rf) discharges at low pressures during the re-ignition of the plasma using the phase resolved optical emission spectroscopy, or PROES, technique. The gases used in the experiments were Argon (Ar) and a 50-50 Tetrafluoromethane-Argon (CF₄-Ar). The start of the experiments used optical emission intensity (OEI) measurements to survey the re-ignition process to find and analyze transients with respect to the conditions of the plasma environment. Given the current understanding of the electropositive Ar pulsed plasmas, their OEI results exhibited some expected intensity profiles which have already been measured and modeled given measurements with time resolutions that are larger than the rf period. For instance, the OEI measurements of Ar 10 kHz pulsed plasma, exhibited little transients as the plasma reformed during re-ignition. This was due to the plasma environment not changing substantially for the 50 μs off times and only the emission intensity was seen to increase as the rf voltage amplitude grew. The relative intensity spatial profile not changing during the re-ignition was found to be consistent with the electron heating dynamics also not changing. This relationship was found as a result of the PROES measurements exhibiting the same electron heating profiles within different rf periods throughout the re-ignition. The PROES measurements were compared to PROES measurements made by other groups in continuous wave Ar plasmas. The electron heating mechanism that was found for the Ar 10 kHz PROES measurements was consistent with heating caused by sheath expansion, known as stochastic or α heating. Some profiles like the average sheath width and intensity profiles were also found in the OEI and were seen to translate to the data measured in the PROES. They were seen to be results of the plasma environment reacting to the electrical nature of the charging of the electrodes (rf voltage and DC self-bias) but were not seen to affect the electron heating mechanisms which are of main interest. (These transients and the ones from the other plasmas that do not directly affect the electron heating mechanisms are important and will be discussed in detail in the dissertation.) The next pulsing phenomenon expected to be seen in the OEI surveying measurements was an intensity overshoot as the Ar pulsing frequency was changed to 100 Hz. It was seen that an intensity overshoot occurred but so did other unexpected optical spatial profiles. The intensity overshoot which is consistent with an electron temperature, Tₑ, spike has been modeled and is understood to be a result of an electron density, nₑ, becoming so small that the Tₑ will rapidly rise and will gradually drop as the nₑ grows larger. The current theory does not, however, mention the mechanisms that allows the Tₑ to rise. This is where the PROES results begin to present new findings and further our understanding on what is occurring during re-ignition in pulsed capacitively coupled plasmas. After analyzing the PROES measurements for re-ignition of the Ar 100 Hz plasma, it was found that the OEI and the optical characteristic profiles changing were indications of electron heating-mode changes. The PROES results at the steady state times exhibited heating profiles within the rf period that were consistent with α heating, but when reviewing the PROES results for the intensity overshoot times, more excitations were transpiring for more of the rf period. Not only were there additional excitation events but there was also evidence that the additional excitations were increasing the effectiveness of the dominant α heating mode. The effectiveness of the α heating mode was seen to fade as the presence of the additional heating diminished. Due to this heating mode not being seen in continuous wave (CW) Ar plasmas at this operating conditions (pressure, powers, etc.), literature on other gases were reviewed and a similar heating profile was found. This heating mode is referred to as drift ambipolar (DA) heating. With the Ar pulsed data as a basis, the pulsing frequency effects of CF₄-Ar plasmas were studied next. (Due to the complexity of these plasmas, there were no pulsing models for this gas chemistry to be found to make predictions.) The first pulsing frequency used for CF₄-Ar plasmas was 10 kHz. Though the Ar 10 kHz exhibited no strong transient indications for changes of electron heating dynamics, the CF₄-Ar 10 kHz did with the same intensity overshoot and relative intensity spatial profile changes that were seen in the Ar 100 Hz. It was found that the addition of the CF₄ molecule added electron loss reactions which substantially changed the plasma environment during the 50 μs off times. The PROES measurements of the re-ignition showed that the α heating mode still dominated the electron dynamics but the electronegative nature of the plasma created was conducive for the presence of DA heating mode to appear briefly. This was not too surprising since PROES measurements of pure CW CF₄ plasmas are seen to be dominated by the DA heating mode where most of the excitation occurs near the electrode. This is consistent with the additional heating within the rf period in the PROES measurements during the intensity overshoot time. In order to further explore pulsing frequency effects on the CF₄-Ar plasmas, the pulsing frequency was changed to 1 kHz. Even though the frequency was only lowered by a factor of 10, the effects measured in the OEI survey were unlike anything measured in the previous experiments. The OEI did exhibit an intensity overshoot but with a different optical characteristic profiles occurring earlier in the re-ignition. The OEI also exhibited a weak and nearly instantaneous intensity streak across the viewable discharge gap. The thickness of the maximum intensity was also seen to change throughout the re-ignition; as well as, some intensity “streaks” were seen during the first 5 μs. Unfortunately, the intensity of the emission from the Ar sensing atom dropped substantially for the PROES measurements which made analysis difficult but some claims could still be made off the results. First, the weak intensity signal was an indication of the CF₄ (and its daughter species) reactions dominating the mechanisms by which the electrons were being heated. This is demonstrated by the noisy heating profiles from the PROES results where the excitation rate profiles were unlike the profiles seen in the other three pulsed plasmas. The heating profiles associated with α heating exhibited shallow heating depth (∼ millimeter). This may be indicative of a sheath that is not well formed with a less distinctive edge profile. The PROES also exhibits faint but substantial heating farther into the plasma bulk for the majority of the rf period. This type of heating was typically seen only when the DA mode was occurring in the rf period. This is consistent with electron being energized directly from the rf voltage which is not being effectively screened by the charged population. Therefore, if the sheath is not as well formed it is conceivable that it will not heat and not screen electric fields as effectively. During the process of making experimental measurements, a need was presented to acquire more points with an accurate data representation. Due to the measurement tools limitations already being met, a chapter is also presented for a technique that can temporally deconvolve measurements when the timing resolution of a tool is greater than the time steps between each data point.Item Role of Surface Temperature in Fluorocarbon Plasma-Surface InteractionsNelson, Caleb T.; Overzet, Lawrence J.; Goeckner, Matthew J.; 0000 0000 5396 3610 (Goeckner, MJ); 0000 0003 5379 4329 (Overzet, LJ); 2008008261 (Goeckner, MJ)This article examines plasma-surface reaction channels and the effect of surface temperature on the magnitude of those channels. Neutral species CF4, C2F6, and C3F8 are produced on surfaces. The magnitude of the production channel increases with surface temperature for all species, but favors higher mass species as the temperature is elevated. Additionally, the production rate of CF2 increases by a factor of 5 as the surface temperature is raised from 25 degrees C to 200 degrees C. Fluorine density, on the other hand, does not change as a function of either surface temperature or position outside of the plasma glow. This indicates that fluorine addition in the gas-phase is not a dominant reaction. Heating reactors can result in higher densities of depositing radical species, resulting in increased deposition rates on cooled substrates. Finally, the sticking probability of the depositing free radical species does not change as a function of surface temperature. Instead, the surface temperature acts together with an etchant species (possibly fluorine) to elevate desorption rates on that surface at temperatures lower than those required for unassisted thermal desorption.Item Silicon Etch Using SF₆/C₄F₈/Ar Gas MixturesBates, Robert L.; Thamban, P. L. Stephan; Goeckner, Matthew J.; Overzet, Lawrence J.; 0000 0000 5396 3610 (Goeckner, MJ); 0000 0003 5379 4329 (Overzet, LJ); 0000 0001 2766 4681 (Thamban, PLS); 2008008261 (Goeckner, MJ)While plasmas using mixtures of SF₆, C₄F₈, and Ar are widely used in deep silicon etching, very few studies have linked the discharge parameters to etching results. The authors form such linkages in this report. The authors measured the optical emission intensities of lines from Ar, F, S, SFx, CF₂, C₂, C₃, and CS as a function of the percentage C₄F₈ in the gas flow, the total gas flow rate, and the bias power. In addition, the ion current density and electron temperature were measured using a floating Langmuir probe. For comparison, trenches were etched of various widths and the trench profiles (etch depth, undercut) were measured. The addition of C₄F₈ to an SF₆/Ar plasma acts to reduce the availability of F as well as increase the deposition of passivation film. Sulfur combines with carbon in the plasma efficiently to create a large optical emission of CS and suppress optical emissions from C₂ and C₃. At low fractional flows of C₄F₈, the etch process appears to be controlled by the ion flux more so than by the F density. At large C₄F₈ fractional flows, the etch process appears to be controlled more by the F density than by the ion flux or deposition rate of passivation film. CF₂ and C₂ do not appear to cause deposition from the plasma, but CS and other carbon containing molecules as well as ions do.Item Study and Optimization of PECVD Films Containing Fluorine and Carbon as Ultra Low Dielectric Constant Interlayer Dielectrics in ULSI Devices(A V S Amer Inst Physics) Sundaram, Nandini; Lee, Gil Sik; Goeckner, Matthew J.; Overzet, Lawrence J.; 0000 0001 3865 4673 (Lee, GS); 0000 0000 5396 3610 (Goeckner, MJ); 0000 0003 5379 4329 (Overzet, LJ); Sundaram, Nandini; Lee, Gil Sik; Goeckner, Matthew J.; Overzet, Lawrence J.Fluorinated amorphous carbon films that are thermally stable at 400 ⁰C have been deposited in a plasma enhanced chemical vapor deposition system using tetrafluorocarbon and disilane (5% by volume in helium) as precursors. The bulk dielectric constant (k) of the film has been optimized from 2.0/2.2 to 1.8/1.91 as-deposited and after heat treatment, by varying process parameters including power density, deposition temperature, and wall temperature. Films, failing shrinkage rate requirements, possessing promising k-values have been salvaged by utilizing a novel extended heat treatment scheme. Film properties including chemical bond structure, F/C ratio, refractive index, surface planarity, contact angle, dielectric constant, flatband voltage shift, breakdown field potential and optical energy gap have been evaluated by varying process pressure, power, substrate temperature, and flow rate ratio of processing gases. Both x-ray photoelectron spectroscopy and FTIR results confirm that the stoichiometry of the ultralow k film is close to that of CF₂ with no oxygen. C-V characteristics indicate the presence of negative charges that are either interface trapped charges or bulk charges. Average breakdown field strength was in the range of 2-8 MV/cm while optical energy gap varied between 2.2 and 3.4 eV.Item Temperature Dependence of the Infrared Absorption Cross-Sections of Neutral Species Commonly Found in Fluorocarbon PlasmasNelson, Caleb T.; Overzet, Lawrence J.; Goeckner, Matthew J.; 0000 0000 5396 3610 (Goeckner, MJ); 0000 0003 5379 4329 (Overzet, LJ); 2008008261 (Goeckner, MJ)This article serves as a reference for the analysis of Fourier transform infrared spectroscopy data from processing plasmas. Until now, there has been a lack of accurate reference data for addressing the problems of species identification and density measurements in cases of increasing gas temperatures. Our results show that, while the integrated absorption cross-sections do not change significantly as temperature increases, the temperature of the absorbing species can be estimated from the rotational band maximum in most cases. Integrated absorption cross-sections for c-C3F6, C4F8, C3F8, C2F6, C2F4, and CF4 are presented for all fundamental bands in the 650 cm(-1) to 2000 cm(-1) region. In addition, the binary combination bands up to 4000 cm(-1) are presented for all species. The temperature of each species has been varied to correspond to neutral temperatures commonly found in processing plasmas.