Hess, Dennis W.
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ItemPlasma Processing of Thin Films [Part I and II]( 2014-09-02) Hess, Dennis W.Fabrication of devices and structures for integrated circuits, sensors, photonics, and MEMS/NEMS requires layers of patterned thin films. For nearly all film materials, patterns are generated by lithographic processes, followed by plasma etching. Plasmas or glow discharges are ionized gases that contain electrons, ions, neutral species and photons that promote chemical reactions and ensure that anisotropic etch profiles can be obtained. This tutorial will discuss the fundamental physics and chemistry of plasmas, plasma reactor configurations, unique properties of plasmas that allow thin film processing for a variety of applications, and approaches to control etch rates, profiles, and etch selectivity.
ItemPhotoelectron spectroscopy studies of plasma-fluorinated epitaxial graphene(Georgia Institute of Technology, 2012-05) Sherpa, Sonam D. ; Paniagua, Sergio A. ; Levitin, Galit ; Marder, Seth R. ; Williams, M. D. ; Hess, Dennis W.Fluorination of graphene has emerged as an attractive approach toward manipulating its physical, chemical, and electronic properties. To this end, we have demonstrated the viability of sulfur hexafluoride plasmas to fluorinate graphene as a safer alternat
ItemThe GT Materials Research Science and Engineering Center (MRSEC) on New Electronic Materials: Research, Education and Outreach(Georgia Institute of Technology, 2010-04-13) Hess, Dennis W.The Georgia Tech Materials Research Science and Engineering Center (MRSEC) was established by NSF in September 2008. This Center is funded for 6 years and is a cross-disciplinary effort involving GT as well as The University of California Berkeley, The University of California Riverside, Alabama A&M, and The University of Michigan. Initially, the Center focus is on graphene, a material with the requisite properties and potential to serve as the successor to silicon in ICs, sensors, and MEMS devices. Although research is a major effort of Center activity, substantial efforts are in place for education and outreach. This presentation will describe the organization of the GT MRSEC, the breadth of research that has been undertaken, the educational activities underway and planned, including a Partnership for Research and Education in Materials (PREM) with The Atlanta University Center (Clark Atlanta, Morehouse and Spelman Universities), and the outreach programs. Future plans and goals will be described.
ItemFunctionalization of high frequency SAW RFID devices for ozone dosimetry(Georgia Institute of Technology, 2009) Westafer, Ryan S. ; Levitin, Galit ; Hess, Dennis W. ; Bergin, Michael H. ; Edmonsonx, Peter J. ; Hunt, William D.In this paper we report new work on the gravimetric detection of ozone at EPA and OSHA relevant concentrations (approximately 100 ppb) in filtered ambient air. We have extended our proof-of-concept work which used both quartz crystal microbalance (QCM) and surface acoustic wave (SAW) resonators. We now enable detection using our high frequency SAW RFID devices. Such surface wave devices are extremely sensitive to the viscosity, thickness, and uniformity of the reactive or sorbent coating. We report laboratory characterization of our polymer-coated SAW sensors operating between 200 and 600 MHz on lithium niobate substrates. Return loss measurements confirm adequate load bearing even at 550 MHz. We compare both the temperature and ozone sensitivity of the RFID devices to conventional resonators. In conclusion, we suggest the design improvements to yield a next generation of SAW RFID ozone sensors with even greater sensitivity.
ItemTransport behavior of atomic layer deposition precursors through polymer masking layers: Influence on area selective atomic layer deposition(Georgia Institute of Technology, 2007-09) Sinha, Ashwini ; Hess, Dennis W. ; Henderson, Clifford L.Sorption and diffusion of precursors through polymer layers were considered as limitations to the successful implementation of a polymer film-based masking approach to area selective atomic layer deposition techniques (ASALDT). Quartz crystal microbalance studies were used to estimate solubility and diffusivity of ALD precursors through supported thin polymer films at elevated temperatures. Specifically, measurements have been performed to estimate the solubility of water in polyhydroxystyrene, polymethylmethacrylate (PMMA), and hexafluoroisopropylalcohol polynorbornene. In addition, diffusion coefficients and solubilities of titanium tetrachloride (TiCl₄) and titanium isopropoxide (Ti(ipr)₄ through PMMAhave also been determined. The results suggest that polymer films exhibit insignificant water uptake at high temperature (~160 °C) and, hence, sorption of water into polymer films does not pose limitations to polymer masking-based ASALDT. Diffusion coefficient measurements of metal precursors account for the role of precursor size in determining the minimum polymer masking layer thickness for a successful ASALDT process.
ItemArea selective atomic layer, deposition of titanium dioxide: Effect of precursor chemistry(Georgia Institute of Technology, 2006-11) Sinha, Ashwini ; Hess, Dennis W. ; Henderson, Clifford L.Area selective atomic layer deposition (ALD) of titanium dioxide using polymer films as masking layers has been investigated. A number of factors which must be considered while designing a successful area selective ALD process have been determined and are briefly discussed. Reactivity of the polymer with the ALD precursor species, diffusion of ALD precursors through the polymer mask, and remnant precursor content in the masking film during ALD cycling are key factors. This article investigates the effect of different precursor chemistries in view of the above mentioned factors. Titanium tetrachloride and titanium isopropoxide have been used as two different metal precursors in conjunction with poly(methyl methacrylate) films as photodefinable masking layers. Processing problems arising from factors such as diffusion of precursors through the masking layer can be solved through careful choice of ALD precursors.
ItemA top surface imaging method using area selective ALD on chemically amplified polymer photoresist films(Georgia Institute of Technology, 2006) Sinha, Ashwini ; Hess, Dennis W. ; Henderson, Clifford L.A method for performing top surface imaging TSI on a single polymeric photoresist film using area selective atomic layer deposition ALD is presented. In this method, exposure of the polymer thin film creates reactive hydroxyl sites on the film surface in the exposed areas that subsequently act as nucleation and growth sites for deposition of metal oxide features using a chemically selective atomic layer deposition process. Specifically, it is shown that titanium isopropoxide and water can be used as ALD precursors in conjunction with a chemically amplified photoresist film, formulated using a protected polymer poly tert-butyl methacrylate and a photoacid generator triphenylsulfonium tris perfluoromethanesulfonyl methide , to successfully perform such an area selective ALD TSI process. Using this material set and methodology, micrometer-scale photoresist features are defined, metal oxide patterns are produced, and these patterns have been transferred through the polymer film via plasma etching. One unique feature of this TSI process is that it has been achieved without requiring a descum etch, which is commonly needed with other TSI methods, due to the highly selective nature of the ALD process.
ItemArea-selective ALD of titanium dioxide using lithographically defined poly (methyl methacrylate) films(Georgia Institute of Technology, 2006) Sinha, A. ; Hess, Dennis W. ; Henderson, Clifford L.An approach to area-selective atomic layer deposition techniques based on the use of a lithographically definable polymeric masking layer has been reported. Successful direct patterned deposition of TiO2 is demonstrated using a poly methyl methacrylate masking layer that has been patterned using deep-UV lithography. A number of factors which must be considered in designing patternable polymeric masking materials and processes have been determined and are briefly discussed, including reactivity of the polymer with the atomic layer deposited ALD precursor species, diffusion of ALD precursors through the polymer mask, and remnant precursor content in the masking film during ALD cycling.
ItemPatterning via surface monolayer initiated polymerization: A study of surface initiator photoreaction kinetics(Georgia Institute of Technology, 2004-11) McCoy, Kendra ; Hess, Dennis W. ; Henderson, Clifford L.Surface monolayer initiated polymerization (SMIP) is a potential method for achieving high resolution patterning of surfaces and materials that could be used as an alternative to conventional lithographic methods based on photoresist thin films. This article reports on the photochemical kinetic rate constants of two candidate azo-type surface bound photoradical initiator molecules. X-ray photoelectron spectroscopy was utilized to monitor the relative concentration of azo initiator on a silicon surface as a function of exposure dose to 248 nm radiation. This photochemical decomposition data showed that the photoreaction for both initiators followed first order kinetics with photoreaction rate constants in the range of 4.5×10⁻³ cm²∕mJ to 9.7×10⁻³ cm²∕mJ. The difference in the observed rate constants for the two azo initiators was attributed primarily to differences in their quantum efficiencies. These differences in quantum efficiency were attributed to the size of the nontethered fragment that would be produced from the initiator, with larger fragments producing slower photochemical decomposition kinetics. Thus, photoradical initiators with small non-surface bound fragments are desirable in terms of increasing the photosensitivity of such SMIP processes. For successful positive tone imaging using a SMIP process with photoradical initiators, it was estimated that approximately 99% of the monolayer must be decomposed in order to produce images in the resulting polymer layers. Using this information in conjunction with the photochemical reaction rate constants for the azo initiators, exposure doses on the order of 400 mJ∕cm² and larger would be required for the present initiators.