Overzet, Lawrence J.

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Lawrence Overzet is a Professor of Electrical Engineering. His research is mainly focused on the plasmas used in semiconductor device manufacturing. He serves as a fellow of the Alan G. MacDiarmid NanoTech Institute. and is the founder and head of the Plasma Applications Laboratory.

Learn more about Dr. Overzet's work on his home, NanoTech Institute faculty and Research Explorer pages.


Recent Submissions

Now showing 1 - 8 of 8
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    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.
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    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.
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    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.
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    Silicon Etch Using SF₆/C₄F₈/Ar Gas Mixtures
    Bates, 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.
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    Temperature Dependence of the Infrared Absorption Cross-Sections of Neutral Species Commonly Found in Fluorocarbon Plasmas
    Nelson, 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.
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    Role of Surface Temperature in Fluorocarbon Plasma-Surface Interactions
    Nelson, 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.
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    Gain and Loss Mechanisms for Neutral Species in Low Pressure Fluorocarbon Plasmas by Infrared Spectroscopy
    Nelson, 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.
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    Effect of Acetylene Concentration and Thermal Ramping Rate on the Growth of Spin-Capable Carbon Nanotube Forests
    Lee, Kyung Hwan; Jung, Daewoong; Burk, Dorothea; Overzet, Lawrence J.; Lee, Gil Sik; 0000 0001 3865 4673 (Lee, GS); 0000 0003 5379 4329 (Overzet, LJ)
    Spin-capable multiwalled carbon nanotube (MWCNT) forests that can form webs, sheets, and yarns provide a promising means for advancing various technologies. It is necessary to understand the critical factors to grow spin-capable carbon nanotubes (CNTs) in a repeatable fashion. Here we show how both the spinning capability and morphology of MWCNT forests are significantly changed by controlling the C2H2 concentration and ramp rate of temperature. The acetylene gas flow was varied in the range of 0.25-6.94% by volume. The MWCNTs grown at C2H2 concentrations between 1.47-3.37% are well-aligned and become spin-capable. The well-aligned forests have higher areal density and shorter distance between CNTs. The thermal ramp rate was also changed from 30 degrees C/min to 70 degrees C/min. A specific range of thermal ramp rate is also required to have the suitably sized nanoparticles with sufficient density resulting in higher CNT areal density for spinnable MWCNTs. A ramp rate of 50 degrees C/min forms suitable sized nanoparticles with sufficient density to produce CNT forests with a higher areal density and a shorter tube spacing.

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