Browsing by Author "Wolfe, John C."
Now showing 1 - 20 of 69
- Results Per Page
- Sort Options
Item 3-D Plasmonic Nanoarchitectures: Fabrication, Characterization, and Applications(2017-12) Arnob, Md Masud Parvez; Shih, Wei-Chuan; Wolfe, John C.; Zagozdzon-Wosik, Wanda; Chen, Ji; Chu, Wei-Kan; Chen, Tai-YenPlasmonic nanostructures are known to concentrate incident light to their surfaces by collective electron oscillation, a.k.a., localized surface plasmon resonance (LSPR). Plasmonic hot-spot refers to locations where electromagnetic fields are particularly enhanced relative to the incident field. Traditional plasmonic nanomaterials are 1D (e.g., colloidal nanoparticles) or 2D (lithographically patterned nanostructure arrays) in nature, which typically result in sparse field concentration patterns. To improve efficiency and better utilization of hot-spots, 3D plasmonic nanoarchitectures are desired, where abundant hot-spots are formed in a 3D volumetric fashion, a feature drastically departing from traditional nanostructures. In this dissertation, two novel 3D plasmonic nanostructures are reported. The first one is NPG nanoparticle, a disk shaped nanostructure with 3D pore-ligament bi-continuous network. NPG disks are made by the low-cost nanosphere lithography (NSL) technique, which is capable of wafer scale production. NPG disks possess larger surface area and high density internal plasmonic hot-spots, which are absent in its bulk counterparts. Due to these unique properties, NPG disks can be potentially used in various surface enhanced Raman spectroscopy (SERS), surface enhanced fluorescence (SEF), and photothermal based applications. To optimize the performance of NPG disks in various applications and understand its plasmonics better, two different modeling techniques, Bruggeman effective medium theory (B-EMT) model and Nanoporous (NP) model, are introduced and evaluated against the experimental data obtained by an electron beam lithography (EBL) compatible fabrication technique for NPG disks. The EBL method can provide large area 2D patterns of randomly distributed nanodisks with flexible interdisk (center to center) distance. Such flexibility is essential to obtain quasi-single NPG disk response, which typically peaks in the near infrared (NIR) spectrum beyond 1 μm, from ensemble measurements by common UV/VIS/NIR spectrometers instead of a specialized NIR spectroscopic microscope. After successful fabrication and modeling, the plasmon enhanced catalysis application of NPG disks is reported in details. The effectiveness of NPG disks in various applications depends on its LSPR peak position. Hence, optimization of an application might require the fine tuning of the peak position. A novel laser based rapid thermal annealing technique is reported to fine tune the LSPR peak position of NPG disks. The second 3D plasmonic nanostructure, reported in this dissertation, is based on the chicken egg shell, a day-to-day waste material. The 3-dimensional (3D) submicron features on the outer shell (OS), inner shell (IS), and shell membrane (SM) regions are sputter coated with gold found to have excellent SERS performance. Moreover, the outer shell substrate is found to be capable of detecting single bacterial cell. This facile way of fabricating 3D plasmonic substrates can facilitate the adoption of 3D plasmonic substrates by researchers in less fortunate countries.Item A quantum conversion efficiency study of amorphous selenium and its applications in a linear detector array for computer tomography(1980) Huang, Hae-Tian; Ong, Poen S.; El-Kareh, Auguste B.; Ignatiev, Alex L.; Wolfe, John C.This thesis treats the subject of the quantum efficiency of electron-hole pair formation in selenium when bombarded with X rays and with the use of amorphous selenium as an X ray detector. The quantum efficiency is defined here as the number of electron-hole pairs generated by a photon of unit energy. It is measured here with monochromatic X rays instead of bremsstrahlung as was done by other investigators. We have done so to reduce the uncertainty in the photon energy determination. Conflictin quantum efficiency values have been reported in the literature which could have been caused by errors in calculation of the effective photon energy of a continuous spectrum. The results reported here are in closer agreement with what the theory predicts as compared with those observed by other investigators. Amorphous selenium was also evaluated as an X ray sensitive linear detector array. Several models of such a detector array have been designed, fabricated and tested. These detectors also incorporate means to store and subsequently sequentially read out the detected X rays.Item A Study of Superconductivity in a Series of Strontium Platinum Phosphides(2015-08) Jawdat, BenMaan I. 1988-; Chu, Paul C. W.; Chu, Wei-Kan; Miller, John H.; Ting, Chin-Sen; Wolfe, John C.In this work, the superconductivity in a series of strontium platinum phosphides was investigated. Three compounds, SrPt3P, SrPt6P2, and SrPt10P4, the latter two of which we discovered, were synthesized and characterized. We studied SrPt3P(1-x)Six resistively, magnetically, and calorimetrically, and found an apparent non-scaling of the Tc with the density of states at the Fermi level N(EF) that can be attributed to a significant weakening of the electron-phonon interaction strength. We found superconductivity in the new structure type SrPt6P2 at Tc = 0.6 K and attribute the lower value of Tc to a weak coupling strength as evidenced by our specific heat measurement and analysis. Superconductivity at Tc = 1.4 K was found in another new structure type compound SrPt10P4 with structural building blocks reminiscent of those from both SrPt3P and SrPt6P2. In SrPt10P4, our specific heat and upper critical field Hc2 measurements indicate the opening of two superconducting gaps Delta1 and Delta2 below Tc. A method of comparing the contributions to superconductivity in different compounds by analysis of the specific heat is developed in which the contribution from the density of states at the Fermi level N(EF) is separated from that of the interaction strength V and is applied to gain insight into the factors driving the superconductivity in these compounds.Item A theoretical investigation of eddy current response to subsurface flaws for application in nondestructive evaluation(1981) Zaman, Afroz J. M.; Long, Stuart A.; Richards, William F.; Wolfe, John C.; Salama, Kamel; Cook, Bill D.The change in complex impedance of an ideal one turn coil due to the presence of a conducting test object in its field has been derived analytically. The case of a coil coaxial with a conducting cylinder and a coil above a conducting half space have been analyzed separately. From the exact expression of the change in complex impedance of the coil with a cylindrical core, a power series in the ratio of the skin depth to the radius of the cylinder has been developed. Convergence of the series, and hence, the accuracy of the result will depend critically on the assumption that this ratio remains small with respect to one (the maximum value that it can take, so that the assumptions made in the modeling are met, is a function of the geometry). Results are given for a coil of representative radius to show how the change in resistance and change in reactance vary as a function of the actual parameters which are subject to change (radius of the cylinder and the conductivity). A similar approach was taken for the case of a coil above a conducting half space. The change in complex impedance was developed as a power series in the ratio of the skin depth to the radius of the coil such that this ratio is small with respect to one. Results have been given for a coil of representative radius to show how the change in resistance and the change in reactance vary as a function of the lift off distance of the coil and the conductivity of the half space. Next, the change in impedance of a coil due to the presence of a small flaw at any arbitrary location inside the conducting medium has been derived. The change in impedance has been obtained using a first order approximation for the electric field inside the volume of the flaw. The expression for the change in impedance has been factored into two terms; each factor being a function of either the axial or the radial location of the flaw. (Due to the symmetry of the problem there will be no circumferential variation). For the case of a coil above a conducting half space, the variation in the radial location of the flaw is shown to affect only the magnitude of the change in impedance, whereas the variation in the axial location of the flaw is shown to affect both the magnitude and phase of the change in impedance. Results have been plotted to illustrate the behavior of the change in impedance as a function of the radial position of the flaw and the depth of the flaw. From these plots it can be shown that in an actual eddy current testing situation, the radial location of the flaw can be obtained from the maximum in the magnitude variation,and the depth of the flaw can be obtained from the first zero_±n the-variation of the reactive part of the change in impedance. For the case of a coil coaxial with a conducting cylinder, the variation in the axial location of the flaw is shown to affect only the magnitude of the change in impedance. The variation in the radial location of the flaw is shown to affect both the magnitude and phase of the change in impedance. For any geometry corresponding to a practical eddy current testing situation, a parameter can be obtained which will determine the behavior of the magnitude variation for any axial location of the flaw. Plots of this magnitude variation have been provided for different geometrical ratios and show a sharp maximum at the position of the axial location of the flaw. The radial location of the flaw can be determined from the first zero of the reactive part of the change in impedance.Item AC Losses and Mechanical Properties of Multifilamentary High Temperature Superconductor Tapes and Wires(2019-05) Ben Yahia, Anis; Selvamanickam, Venkat; Ryou, Jae-Hyun; Wolfe, John C.; Kulkarni, Yashashree; Ardebili, HalehWith the ability to operate at high magnetic fields and high temperatures, and with significant progress in critical current density, RE-Ba-Cu-O (REBCO, RE = rare earth) coated conductors (CC) have an immense potential for use in various coil and cable applications. However, the tape form of these CC creates additional problems. The excessively high AC losses and the limited flexibility are major challenges impeding the development and commercialization of these tapes. Recently, there have been efforts to convert flat REBCO tapes into a round wire. The diameter of these wires has been reduced using thinner substrate tapes and by positioning the copper stabilizer mainly on the REBCO film side. In this dissertation, using a combination of experimental and analytical results, the copper thickness on the REBCO side has been optimized to maximize the critical current retention of tapes with various substrate thicknesses at small bend diameters. The presented analytical method accounts for the neutral axis shift caused by the progressive plastic deformations. Using these results, an optimal design for ultra-small diameter symmetric tape round (STAR) wires is also proposed. In addition, an alternate approach to enhance REBCO CC bending properties by using two tapes joined face-to-face is presented. In this structure, the two REBCO layers are positioned closer to the neutral axis. Two methods to fabricate such structures were implemented and their bending performance characterized. To reduce the AC losses in REBCO tapes, a fully-scaled reel-to-reel filamentization process allowing the production of long length multifilamentary tapes has been developed. The process uses laser ablation followed by oxygenation and selective electroplating. The AC losses of reel-to-reel produced multifilamentary tapes was significantly reduced by preventing copper growth on their back side. To address the coupling losses over long length, a new transposition pattern was also proposed and implemented. Finally, the striation process was modified to allow the integration of the multifilamentary tapes into the STAR wires. STAR wires with various number of filaments per tape were fabricated and their AC losses characterized. Results showed exceptional AC losses reduction over a broad range of frequencies and fields compared to a normal REBCO tape.Item Advanced Control of Ion and Electron Energy Distributions and Investigation of in-situ Photo-Assisted Etching(2014-05) Zhu, Weiye 1987-; Donnelly, Vincent M.; Economou, Demetre J.; Stein, Gila E.; Wolfe, John C.; Ruchhoeft, PaulPrecise control of ion energy distribution (IED) is critical to achieve highly selective, low damage etching. A novel approach to control IED using pulsed plasma with synchronously pulsed dc bias on a boundary electrode in Ar gas is first presented. Synchronization of the dc bias applied during the afterglow of a pulsed plasma and the plasma rf power resulted in a double-peaked IED. The mean energies of the two peaks, as well as the peak separation, were controlled by adjusting the applied dc bias and the discharge pressure. Nearly mono-energetic IEDs can be extracted in the afterglow of a pulsed plasma. With precisely controlled IEDs, a new, important phenomenon is reported: photo-assisted etching of p-type Si in chlorine-containing plasmas. This mechanism was first discovered in mostly Ar plasmas with a few percent added Cl2. A substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments were carried out with light and ions from the plasma either reaching the surface or being blocked, showing conclusively that the “sub-threshold” etching was due to photons. Sub-threshold etching rates scaled with the product of surface halogen coverage and Ar emission intensity. Etching rates measured under MgF2, quartz, and opaque windows showed that sub-threshold etching is due to photon-stimulated processes on the surface, with VUV photons being much more effective than longer wavelengths. In an effort to manipulate the electron energy distribution function (EEDF) and plasma density, a plasma reactor incorporating dual tandem plasma sources separated by a grid is presented. As feature sizes shrink to the nanometer scale, tuning the EEDF becomes increasingly important for both plasma etching and deposition. By pulsing the main plasma source, while maintaining the tandem source in continuous wave mode, a low electron temperature of ~1 eV at high plasma density (1011 cm-3) was realized. This was achieved by applying a dc bias to a boundary electrode in the tandem source. The electron temperature in the afterglow period could be controlled by changing that bias voltage.Item Advanced Pulsed Plasma Techniques(2018-08) List, Tyler Lee; Donnelly, Vincent M.; Economou, Demetre J.; Nikolaou, Michael; Ruchhoeft, Paul; Wolfe, John C.A method for controlling ion energies on insulating surfaces using pulsed plasmas is presented. DC pulses are periodically applied to the chuck holding the substrate in the afterglow of a pulsed plasma to attract an electron swarm to the sample surface. Surface potential measurements validated the proposed method and helped investigate the effect of changing pulse width, amplitude, and frequency of the chuck bias on the resulting surface potential waveform. Retarding field energy analyzer measurements were performed and corrected for the non-uniform charge distribution that prevailed at an applied RF frequency less than the ion sheath transit frequency. Etching of quartz discs and 1000nm-thick SiO2 films on Si wafers was also performed. An etching threshold was found at 100 V chuck bias for both types of substrates, beyond which the etching rate increased proportionally with the square root of chuck bias. No clear effect of the boundary bias on etching rate was seen. Time-resolved UV absorption of pulsed electronegative fluorocarbon and chlorine inductively-coupled plasmas is presented. The time-dependent variation of Si-byproducts were verified against previous publications as well as the limits of the current apparatus. The lack of decay of CF2 in 100 Hz and 500 Hz pulsed C4F8 plasmas was also seen. Time-dependent studies of power-modulated chlorine inductively-coupled plasmas are presented. Power at 13.56 MHz applied to the plasma was modulated between high and low power states. Time-resolved optical emission, power delivery, and Langmuir probe measurements revealed at least two periodic steady-state conditions upon switching from high to low power: a “normal” mode in which electron temperature (Te) remains constant, while electron and ion number densities (ne and n+) and optical emission spectroscopic (OES) intensities smoothly drop to a level roughly equal to the fractional drop in power, and an “abnormal” mode in which ne, n+ and OES intensities plummet before rising to values commensurate with the drop in power. Whether the plasma operates in the normal or abnormal mode is sensitive to settings on the matching network and is also a function of pressure and pulsing parameters.Item Advances in Raman and Surface-Enhanced Raman Spectroscopy: Instrumentation, Plasmonic Engineering and Biomolecular Sensing(2014-08) Qi, Ji; Shih, Wei-Chuan; Wolfe, John C.; Han, Zhu; Larin, Kirill V.; Willson, Richard C.; Roysam, BadrinathRaman spectroscopy is a powerful technique for label-free molecular sensing and imaging in various fields. High molecular specificity, non-invasive sampling approach and the need for little or no sample preparation make Raman spectroscopy uniquely advantageous compared to other analytical techniques. However, Raman spectroscopy suffers from the intrinsic limitation of weak signal intensity. Therefore, time-sensitive studies such as diagnosis and clinical applications require improving the throughput of Raman instrumentation. Alternatively, surface-enhanced Raman scattering (SERS) improves the sensitivity by 10^6 to 10^14 times, making the weak Raman intensity no longer a limitation. Nevertheless, it is still a big challenge to engineer plasmonic substrates with high SERS enhancement, good uniformity and reproducibility. This thesis presents advances in: (1) Raman instrumentation towards high-throughput, environmental, biological and biomedical analysis; (2) SERS substrates with high enhancement factor (EF), uniformity and reproducibility; (3) biosensing applications including imaging of cell population and detection of biomolecules towards high time efficiency and sensitivity. In Raman instrumentation, we have built a high-throughput line-scan Raman microscope system and a novel parallel Raman microscope based on multiple-point active-illumination and wide-field hyperspectral data collection. Using the line-scan Raman microscope, we have performed chemical imaging of intact biological cells at the cell population level. We have also demonstrated more flexibility and throughput from the active-illumination Raman microscope in rapid chemical identification and screening of micro and nanoparticles and bacterial spores. Both Raman microscopes have been used to evaluate the large-area SERS uniformity of DC-sputtered gold nanoislands, a low-cost means to fabricate plasmonic substrates. In plasmonic engineering, we have introduced patterned nanoporous gold nanoparticles that feature 3-dimensional mesoporous network with pore size on the order of 10 nm throughput the sub-wavelength nanoparticles. We showed that the plasmonic resonance can be tuned by geometrical engineering of either the external nanoparticle size and shape or the nanoporous network. As an example, we have developed disk-shaped entities, also known as nanoporous gold disks (NPGD) with highly uniform and reproducible SERS EF exceeding 10^8. Label-free, multiplexed molecular sensing and imaging has been demonstrated on NPGD substrates. Using the line-scan Raman microscope and the NPGD substrates, we have successfully developed a label-free DNA hybridization sensor at the single-molecule level in microfluidics. We have observed discrete, individual DNA hybridization events by in situ monitoring the hybridization process using SERS. The advances and promising results presented in this thesis demonstrate potential impact in Raman/SERS imaging and sensing in environmental, biological and biomedical applications.Item Alloy evaporation for VLSI metallization(1986) Hughes, James John, III; Wolfe, John C.; Smither, Miles A.; Pollard, RichardWe set out to build an alloy deposition system. In order to do this, a new type of source holder was developed and a new kind of evaporation crucible was devised. Additionally, a SCR power controller was constructed to enable the deployment of a feedback control system. With this apparatus, a variety of alloy films were deposited. The effect of crosstalk was analyzed and a correction technique was determined. The X-ray analysis of the compositions of the deposited films indicates that the correction technique is accurate.Item Cadmium telluride thin film threshold and memory switches(1979) Hwang, Thomas Jen-Ming; Wolfe, John C.; El-Kareh, Auguste B.; Ong, Poen S.; Wang, Keith Y.; Ting, Chin-Sen; Moss, Simon C.Item Chamber Wall Interactions with HBr/Cl2/O2 Plasmas Studied by the “Spinning Wall” Method(2015-05) Srivastava, Ashutosh K.; Donnelly, Vincent M.; Economou, Demetre J.; Stein, Gila E.; Wolfe, John C.; Ruchhoeft, PaulPlasma etching is a widely used method to pattern materials in the fabrication of microelectronic devices. As the minimum feature sizes, or so-called critical dimensions, shrink beyond 14 nm, plasma etching processes need to be ever more tightly controlled. At low pressures in the range of 1-100 mTorr, typical in current plasma processes, the ~cm mean free path of species ranges are comparable to reactor dimensions. Consequently, gas phase reactions (especially three-body processes) become less likely and heterogeneous reactions on chamber walls become increasingly important. The surface layers formed on the reactor walls become a source of production or loss of species. As a result, shifting plasma composition leads to process drifts, leading to changes in etching rates, profiles, selectivity, and yields. Hence, it is of prime importance to understand the interactions of plasmas with the dynamic chamber wall surfaces. HBr plasmas are used to etch Si, as well as GaN, PZT, InP, indium zinc oxide and other materials. In Si etching, HBr plasmas create better anisotropic profiles than Cl2 plasmas, with better selectivity toward SiO2. Selectivity can be further improved by adding oxygen to the plasma. The feed gas composition of HBr/Cl2/O2 plasmas is optimized to best meet the needs of the particular application. Keeping such a complex process stable over time requires tight control over all plasma parameters, including reactor wall conditions. Here, we have studied the interaction of HBr/Cl2/O2 inductively-coupled plasmas with reactor chamber wall deposits, with and without Si etching, using the “spinning wall” technique. The spinning wall is part of the reactor chamber walls, allowing near-real time analysis of the composition of surface layers via Auger electron spectroscopy, and determination of species desorbing off the walls by mass spectrometry. In HBr plasmas with no bias voltage on the Si substrate, and hence no Si etching, HBr is ~30% dissociated and H2 and Br2 form in the plasma. Layers deposited on the reactor chamber contained little if any Br under these conditions. Adding O2 to an HBr plasma leads to formation of Br2 and H2O products that desorb from the spinning wall. H2O has a very long residence time on the surface. With bias voltage applied to the Si substrate in an HBr plasma, SiBr and SiBr3 are prominent mass spectrometer signals, SiBr2 and SiBr4 appear to be the major gas phase components, and a SiOxBry layer deposits on the spinning wall. Adding 20% O2 to HBr stops etching and eliminates Br from the surface layer, indicating that Br on the reactor walls is a result of SiBrx impingement, and not from bromination by impinging Br. With HBr/Cl2 plasmas and no bias on the stage, a SiOxCly layer deposits; no Br is detected. In addition, the mass spectrum of HBr and Cl2 gas mixture without plasma revealed HCl, Br2 and BrCl species. Further experiments revealed that these products were the result of reactions between HBr and Cl2 on the plasma reactor walls. With plasma and bias on the Si substrate, both Br and Cl incorporate in a depositing layer. Adding 20% O2 to a HBr/Cl2 plasma with substrate bias suppresses Br adsorption, but Cl still adsorbs. In 40% O2/HBr/Cl2 plasmas with stage bias, Cl adsorption also ceases.Item Characterization of Amorphous Carbon Films for Mask Protection during Ion Beam Bombardment(2017-05) Kusko, Rebecca Elizabeth; Wolfe, John C.; Shih, Wei-Chuan; Wood, Lowell T.The field of lithography is in simplest terms the use of a beam, incident on a mask, to transfer the mask pattern onto the substrate. This process is utilized by every semiconductor company to create the microchips, which make modern life possible. Though photons are the industry standard, atom/ion beam lithography is a specific niche wherein atoms or ions are used to transfer a mask pattern. Although this technique grants excellent flexibility as far as depth of field and diffraction limit, the use of massive particles causes mask damage, limiting commercial applications of the method. To address this problem, we refine a process to coat masks with diamond-like carbon (DLC) to protect them from ion bombardment. This continues the work of Wasson, Hudek, Nounu, and Abichandani, who pioneered deposition techniques to create amorphous carbon films with low initial compressive stress, which remains constant to very high ion doses. In particular, Nounu and Abichandani’s coating technique is repeated but more fully characterized, refined to eliminate the effects of mask charging, and improved with respect to radiation resistance. Further stress measurements are taken, with particular emphasis on in-situ, in vacuum measurements of film response to ion bombardment.Item Coadsorption of mixed anionic and cationic surfactants in reversed-phase high performance liquid chromatography(1983) Lin, Way-Yu; Deming, Stanley N.; Wendlandt, Wesley W.; Vestal, Marvin L.; Geanangel, Russell A.; Wolfe, John C.The combined effects of mixed alkylsulfonate and alkylamine surfactants on the liquid chromatographic retention times of aniline, phenylethylamine, benezene- sulfonic acid, and chromotropic acid are described by a quantitative thermodynamic model that assumes electrostatic interaction of charged solutes with the anionic and cationic surfactants in the adsorbed phase. The results of this study indicate (1) the synergistic effect of coadsorption (the total amount of the mixed surfactants adsorbed on the surface is higher than the amount of either individual surfactant adsorbed on the surface at the same total bulk concentration); (2) the competitive nature of the coadsorption processes (at any given bulk liquid concentration of one surfactant, the surface mole fraction of that surfactant decreases with increasing concentration of the other surfactant); and (3) the possibility of using liquid chromatography as a tool for the measurement of adsorption free energy of surfactants at the mobile phase/stationary phase interface.Item Control of Photo-Assisted Etching of Si in Chlorine Containing Plasmas(2021-05) Du, Linfeng; Donnelly, Vincent M.; Economou, Demetre J.; Grabow, Lars C.; Ruchhoeft, Paul; Wolfe, John C.Plasma etching is critical for the fabrication of very large-scale integration (VLSI) microelectronic devices. In-plasma photo-assisted etching (PAE) with ion energy below the ion-assisted etching (IAE) was discovered in our laboratory several years ago. PAE may be advantageous or detrimental, depending on the situation. Thus, it is desirable to be able to control PAE. This thesis reports fundamental studies of PAE suppression using oxygen additions to the plasma and PAE enhancement by extra VUV flux irradiating the Si substrate. O2 addition to a baseline plasma was used to control PAE of p-type Si (100) and poly-Si in a high density, Faraday-shielded, Ar/Cl2 (225/25 sccm) inductively coupled plasma. The etching rate of samples etched under PAE condition was not affected by 0.1 or 0.25 sccm O2 addition to the base case Ar/Cl2 plasma, while etching stopped with more than 0.5 sccm O2 addition. Ion assisted etching, was not affected by up to 2 sccm of oxygen addition, but decreased rapidly when more than 5 sccm O2 was added to the plasma. Thus, there was a window of O2 additions that completely suppressed PAE but allowed IAE to proceed unobstructed. Vacuum transferred X-ray photoelectron spectroscopy (XPS) showed that PAE-treated samples had a thicker surface oxide and lower chlorine concentration (SiCl). In contrast, the surface of IAE samples contained Si dangling bonds and higher silicon chlorides (SiCl2 and SiCl3). The PAE rate of p-type Si was enhanced by extra vacuum ultraviolet (VUV) photon flux. Etching was carried out in the Ar/Cl2 ICP used above (main ICP). Additional photon flux was provided by a compact Ar/He (2.5/47.5 sccm) plasma source (auxiliary ICP), in tandem with the main ICP. The auxiliary ICP was also used as a standalone plasma source to determine (using VUV spectroscopy) the composition of the Ar/He gas mixture that maximized the 106.7 nm line (1s4) of the resonant state of argon. Most PAE-treated samples had a rough surface under the scanning electron microscope (SEM). A significant etching enhancement of 51% was found at a pressure of 15 mTorr, where the PAE rate with only the 60 W main ICP ON was 0.069 μm/min, increasing to 0.104 μm/min when the 200 W auxiliary ICP was also turned ON. XPS survey results showed that PAE with only the main ICP ON resulted in the same [Cl]/[Si] surface concentration ratio of about 0.15, when both ICPs were ON, suggesting that the chemical composition of the silicon surface was not affected by the additional VUV photon flux. High resolution XPS indicated that the extra VUV photon flux from the auxiliary ICP may induce surface conversion of lower chlorides to higher chlorides. Surprisingly large PAE yields 356 and 135 Si/photon were found at neutral-to-photon flux ratios of 19 × 104 and 0.9 × 104, respectively.Item Design and Analysis of Hardware-Based Scheduler and Multi-Photon Quantum Cryptography Protocol(2017-12) Wu, Junchao; Chen, Yuhua; Chen, Jinghong; Wolfe, John C.; Vipulanandan, Cumaraswamy; Cheng, Albert M. K.Quantum cryptography is gaining more attentions in recent years. In this dissertation, a multi-photon quantum cryptography method is analyzed to take into account the fiber loss and collective-rotation noise. Compared with single photon protocols, the multi-photon protocol has longer transmission distance with better noise tolerance. Scheduling overheads in multiprocessor systems are typically ignored. However, as the number of processors in systems increases, they can potentially become the performance bottleneck in practical systems. In this dissertation, a new hardware-based scheduling algorithm is proposed for multiprocessor task scheduling. The proposed algorithm has O(1) runtime complexity and can be scaled to schedule hundreds or thousands of processors. In addition, this dissertation develops analytical models for hardware-based algorithms. The proposed models can be used not only to evaluate existing hardware implementations, but also to guide the design for new hardware acceleration implementations. Two datacenter-oriented hardware schedulers are designed as case studies to demonstrate the effectiveness of the proposed hardware-based analytical models, which help to better understand the hardware algorithm resource cost and further improve hardware design schemes.Item Development of a Prototype Manufacturing Process for Reliable Optical-Fiber Based Neural Probes(2015-12) Awale, Apeksha S.; Wolfe, John C.; Shih, Wei-Chuan; Zagozdzon-Wosik, Wanda; Miller, John H., Jr.; Dragoi, ValentinThe function of a neuron depends on its microcircuitry – the inputs it receives from local and long-range connections and the outputs it sends to other neurons. Mapping these connections is typically done by stimulating a population of neurons chemically, electrically, or optically, and recording the induced extracellular action potentials with implanted metallic probes. The probes may be cylindrical needles or thin planar blades. The needles have an advantage for deep structures since their circular cross-section minimizes friction, hence insertion force, while planar probes provide much greater design flexibility at low cost by leveraging semiconductor manufacturing technology. In this thesis, we explore the possibility of manufacturing cylindrical probes with dense thin film electrode patterns on fine optical fibers, thus, providing the design flexibility of planar probes in the cylindrical format required for deep brain applications. Our group reported the fabrication of cylindrical probes with 4-integrated electrodes on 60 µm optical fibers at EIPBN-2013. These proof-of-concept optrodes were used to detect photo-simulated electrical activity of neurons in the primary visual cortex of Olemur garnettii, a non-prosimian primate. However, processing times were unacceptably long, about 1 month for a batch of 4 probes, and all experienced short-term electrode failure in cerebro-spinal fluid through insulator delamination, which remains a major obstacle to long-term viability of many state-of-the-art probe technologies. In this thesis we report optimized processes that reduce the time and increase batch size for fabricating 4-channel optrodes that result in a projected processing time of 80 minutes for a batch of 16 probes-about 5 minutes/probe. Our most important achievement was the development of a rugged, pin-hole free dielectric coating with stable impedance in phosphate-buffered saline over a period of 10 days under moderate (6mA/cm2) electrical stimulation at frequencies from 200-10,000 Hz. Electrode impedance on a 60 µm fiber was unchanged after 6 repeated insertions to a depth of 3.8 cm in agar gel (Landor Trading Company), which simulates the consistency of brain tissue. Scanning electron microscopy showed that scratching was absent on probes that had been inserted to a depth of 3.8 cm in 75 µm and 438 µm stainless steel canulae.Item Development of Flexible Neural Probes for Stimulation and Recording in the Central Nervous System(2013-08) Gheewala, Mufaddal; Wolfe, John C.; Shih, Wei-Chuan; Sheth, Bhavin R.; Dani, John A.; Randall, John N.; Pang, Stella W.; Purushothaman, GopathyThe functionality of cortical neurons depends on the strength of its local and long-range synapses and the interpretation of the physical and functional anatomy depends on the understanding of these connections. Optogenetics uses genetic manipulations to insert opsin containing ion channels into neurons. Then light can be used to optically gate ion-transport across the plasma membrane to stimulate or silence spiking activity with greater cellular specificity and spatio-temporal resolution than previously possible. While great progress has been made in the genetic methods used in optogenetics, little progress has been made in improving the devices (optrodes) used to simultaneously photostimulate and record neural activity. In this dissertation we describe the development of a new probe concept based on the integration of micrometer-scale thin film electrodes and associated interconnect wiring on the cylindrical surface of fine optical fibers with tight manufacturing tolerances. The use of optical fibers as probe substrates provides high intensity, multi-spectral light delivery with essentially no coupling loss, as well as the strength and stiffness required for deep-brain applications. High resolution permits a very high electrode count on thin fibers, and high dimensional precision enables accurate 3-D localization of neuronal sources. Moreover, the technology is compatible with high throughput manufacturing at very low cost, an important consideration for wide dissemination of the technology, particularly for linear and 2D-array applications. A second crucial development is the design and implementation of a multi-electrode interface between thin-film wiring on the (cylindrical) probe and state-of-the-art neuro-amplifier and signal processing systems. Two-channel prototypes have been fabricated and used in preliminary experiments to 1) record photostimulated neural activity in a group of genetically identified neurons in the primate primary visual cortex at the Vanderbilt University School of Medicine (VUSM), and 2) demonstrate source localization in the rat hippocampus at the Baylor College of Medicine (BCM). The prototypes had 15x15 µm2 gold electrodes on 65 µm optical fibers with lengths up to 3 cm. In future work, we propose to further reduce probe diameter to the ~30 µm range to develop probes with advanced functionality and extend the technology to 1- and 2-D arrays.Item Development of Lithography and Stencil Mask Alignment Processes for Manufacturing High Density Thin Film Electrodes on Optical Fiber Substrates(2021-05) Jonnalagadda, Venu Sushir; Wolfe, John C.; Zagozdzon-Wosik, Wanda; Ruchhoeft, Paul; Wood, Lowell T.; Economou, Demetre J.Neural probes with micron-scale electrodes are capable of recording action potential waveforms from single neurons with spatial and temporal resolution beyond the capability of other technologies (e.g. electroencephalography, electrocorticography). For the isolation of individual neurons, the determination of the frequency and strength of neural synapses is essential. Connectivity between neurons can be inferred by electrical or chemical stimulation of a cluster of neurons. It is however, difficult to determine specific synaptic locations, since a large number of neurons is indiscriminately stimulated. The isolation and stimulation of a specific set of neurons can be accomplished by opto-genetics, where genetically modified neurons are stimulated by light. This requires implantable probes with reliable light delivery for stimulation of neurons and integrated conductor wires for obtaining laminar recordings from different cortices of the brain. Optical fibers make good substrates for neural probe technology, since they have the strength and stiffness required to target deep brain structures and the ability to deliver multi-spectral light, essentially without coupling losses. In this dissertation, we report the development of i) an atom beam lithography (ABL) tool used for proximity printing on cylindrical substrates and ii) a mechanical alignment process for printing high-density (32 electrodes) micron-scale electrodes on optical fiber substrates. This tool features a high-brightness source of 50 keV helium atoms with a 0.5 nm penumbral blur for a 5-micron proximity gap and a flux density of 1.25×10^13 particles/s-cm2. An advanced alignment process is reported where the fibers are held in anisotropically etched silicon v-grooves, which are nearly atomically straight. Longitudinal and lateral alignment of stencil mask patterns was done using high-precision fiber stops and cubic beads, respectively. The longitudinal and transverse overlay accuracies were 0.8 + 0.39 micrometers and 0.1 + 0.05 micrometers, respectively.Item Development of Multi-Electrode Neural Probe on Optical Fiber Substrate for Brain-Machine Interfaces(2018-05) Tisa, Tamanna Afrin; Wolfe, John C.; Shih, Wei-Chuan; Zagozdzon-Wosik, Wanda; Charlson, Earl J.; Wood, Lowell T.; Ardebili, HalehBrain-machine interfaces (BMIs) aim to restore communication and control of prosthetic devices to individuals with neurological injury or disease, by recording the neural activity, and mapping or decoding it in to a motor command. One of the great challenges in this effort is to develop reliable neural probes that are capable of processing the activity of large ensembles of cortical neurons. In this dissertation, we reported a method for fabricating highly reliable neural probes with integrated, thin film conductor and dielectric coatings on the cylindrical surface of fine optical fibers for brain-machine interfacing. The use of optical fibers as probe substrates provide the strength and stiffness required for deep-brain applications, as well as the high intensity, multi-spectral light delivery with essentially no coupling loss is useful for optogenetics application in neuroscience. Early probes were fabricated on 65 µm optical fiber substrates with polyimide jackets. Electrodes were defined over this jacket, and high quality in-vivo recordings were acquired in area V1 of the Greater Northern Galago (Galago garnetti). Microscopic examination of the probes after extraction from the brain, showed that the jacket had cracked and delaminated; the glass itself may have cracked. Invariably this happened near the probe tip, suggesting that micro-cracking of the unprotected fiber end was the cause of the problem. So, a new jacket of cross-linked plasma-deposited styrene was developed. This layer was impervious to water vapor, as well as hot acids and bases. Single channel probes fabricated with this jacket survived a battery of reliability tests, including continuous soaking in PBS for 30 days, multiple insertions in agar gel and cannulas, disinfection, and marinating overnight in a whole mouse brain in the Dragoi lab. Moreover, test-to-test and lot-to-lot variation of the 2 kHz impedance was less than 1 % (3). High quality, in-vitro spike recordings were acquired in a living mouse brain slice at the Dragoi lab. Thus, reliability of the contact fabrication process has been established. In this dissertation, we also reported the extension of the technology that we developed for single channel prototypes to probes with a large number (>30) of micrometer-scale contacts that are needed to map laminar circuits in the brain. For fabricating those multi-electrode neural probes, significant advancement in alignment technology was required. The near-atomic straightness of fiber holder and accurate registration of the mask pattern with the V-grooves ensures that the printed pattern will be centered on the bottom of the fiber. The overlay of patterns on the fiber was ensured, a) longitudinally by using fiber stops, high precision ball bearings which were hold to lithographically defined pits at the tip-end of each V-groove, b) rotationally by using a high precision cubic bead glued to the end of the fiber as the reference. SEM images showed that longitudinal and lateral pattern overlay error was always below 2 µm without any outliers.Item Effect of a magnetic field on the electronic transport of a two dimensional system(1985) Cai, Wei; Ting, Chin-Sen; Wolfe, John C.; Reiter, George F.; Ignatiev, Alex L.; Hu, BambiBy use of nonlinear force and energy balance equations in steady state for an electron-phonon-impurity system, the longitudinal electric field Ex and electron temperature Tg are calculated as functions of current density J [...]