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Special Seminar

The Shadow of a Carbon Nanotube

Dr. John C. Wolfe

by: Dr. John C. Wolfe

Date: Thursday August 18, 2005

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The features of ion beam proximity lithography include sub-nanometer limits for diffraction, penumbra, and resist scattering. Further, by using atoms, which have essentially the same interaction with resist and mask materials as ions, the technology becomes immune to fixed and mobile charge, either in the mask or on the wafer. Thus, atom beam lithography (ABL) seems ideal for prototyping and later manufacturing nanoscale integrated circuits. The fundamental challenge is to fabricate a stencil mask with atomically smooth sidewalls in a membrane that is thick enough (e.g. 0.5 μm) to stop the atoms. We have shown that a scattering mask can bypass this issue. A thin evaporated film was used to shrink the openings of a conventional stencil mask and, while not thick enough to actually stop the particles, enough were scattered into the walls of the mask to generate very good exposure contrast. This approach gives the possibility of using very thin, potentially self-assembled, scattering layer structures to form complex, high density masks. An important open question is how thick these features need to be for faithful atom beam replication. In this talk, we report the ability of a carbon nanotube, 18 nm in diameter, to cast a well-defined shadow in a broad beam of energetic (30 keV) helium atoms. When imaged in resist and engraved into silicon dioxide, the projected replica retains the natural smoothness of the nanotube and shows, for the first time that ultra-thin, self-assembled structures can be used as masks in nanoscale printing.

Experiments were carried out using a 30 keV atom beam proximity lithography system. Briefly, a beam of helium atoms, generated by charge transfer scattering in the extraction region of a duoplasmatron ion source, drifts through a 10 M long tube, and impinges on a stencil mask where the transmitted beamlets transfer the mask pattern to the substrate. The mask was prepared by sprinkling a dry nanotube powder onto a 3 μm thick silicon stencil mask with 1 μm wide openings. The mask was clamped, with 5 μm thick cleaved mica spacers, to a silicon substrate coated with 44 nm thick thermal SiO2 and 50 nm thick PMMA resist. After exposure, the PMMA was developed and the resist pattern transferred into the oxide to a depth of 37 nm by CHF3-RIE. Atoms incident upon the thick regions of the mask are absorbed while scattering generates image contrast for the nanotube. Experimental data will be presented, showing a 20 nm wide nanotube suspended over an opening in the stencil mask, with its image after resist removal, engraved into oxide. Also shown is linewidth versus exposure dose, normalized to the critical dose, of a different tube, 18 nm in diameter. Since printed and nominal linewidth are generally equal at twice the critical dose, the difference between these values, about 4.6 nm, is a measure of pattern bias, perhaps an artifact of metrology, resist development, and/or etching. After subtracting the bias from the measured data, a threshold development model with a blur of 5 nm (FWHM) describes the experiment reasonably well. Thus, the ultimate resolution may be near 4 nm. This result shows that the edges of a nanotube, just a few atomic layers thick, generate enough scattering to be printed. I will report experiments to better understand the pattern bias issue and to determine the resolution limit.

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Special Seminar

Room Temperature Superconductivity: The “Hole” Story

by: Prof. Manuel de Llano

Date: Friday July 22, 2005

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

By recognizing the vital importance of two-hole Cooper pairs in addition to the standard two-electron ones in a many-electron system, the concept of pairing has been re-examined with striking conclusions. Based on this, Bose-Einstein condensation (BEC) theory is generalized to include not boson-boson interactions (also neglected in BCS theory) but rather boson-fermion interaction vertices reminiscent of the Fröhlich electron-phonon interaction. Instead of phonons, the bosons in the generalized BEC theory are now both particle and hole Cooper pairs, and it reduces to all the old known statistical theories as special cases–including the so-called “BCS-Bose crossover” picture, which in turn generalizes BCS theory. With no adjustable parameters, the generalized BEC theory yields substantially higher transtion temperatures (including room-temperature superconductivity) without invoking non-phonon dynamics.

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Special Seminar

The Mixed State of BSCCO Visualized in Real Space with Single Vortex Resolution: Solid-Liquid Phase Transition and Magnetization Reversal

by: Alexander Schwarz

Date: Friday May 27, 2005

Time: 1:00 pm – 2:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

It will be shown that vortex states in BSCCO can be visualized in real space with single vortex resolution using magnetic force microscopy. This technique is than applied to investigate two phenomena: vortex lattice melting and magnetization reversal. The former can be triggered by increasing either temperature or magnetic field. It turns out that the way how the solid-liquid phase transition occurs, appears to be quite different. In the latter case, a flux-antiflux boundary propagates through the sample. Particularly, it is possible to observe the annihilation of individual vortex-antivortex pairs.

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Special Seminar

The New Generation of Superconductor Electric Power Equipment

by: Alexis Malozemoff

Date: Monday May 23, 2005

Time: 4:30 pm – 5:30 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Rising energy demands, driven by population and economic growth, face an increasing clash with resource, land use and other environmental limits. Amidst the fierce debate over general energy policy priorities, there is broad consensus on the urgent need to modernize and strengthen the electric power grid. High Temperature Superconductor (HTS) wire is one of the keys to achieving these goals. Superconductivity is the amazing property of certain materials to conduct electricity with no resistive loss and high current density, enabling a new generation of electrical power equipment that is efficient, compact and very low in environmental impact. This vision has been enabled by the successful development and commercialization of robust, long-length, high performance HTS wires.

Examples of HTS applications, all in an advanced prototype stage, include:

  • High-capacity, controllable HTS cables, which offer increased delivery capacity, essentially zero local environmental impact and the ability to offload overburdened sections of the grid;
  • Dynamic HTS synchronous condensers offer large amounts of rapidly adjusted reactive power to improve grid stability and efficiency;
  • Utility generators that produce more electricity for every unit of fuel consumed; and
  • Fault current limiters and transformers that enable more reliable, lower cost operation of the grid.

This presentation will describe these applications, along with the superconductor wire that underlies them, and will assess their potential impact on the major grid challenges our society faces today.

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Special Seminar

Second-generation HTS Conductors

Dr. Venkat  Selvamanickam

by: Dr. Venkat Selvamanickam

Date: Monday May 02, 2005

Time: 3:00 pm – 4:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

High temperature superconductors (HTS) are nearing their commercial viability with the projected roll out of second-generation conductors within one year. Second-generation HTS conductors promise to meet the price-performance characteristics needed for widespread use of HTS. SuperPower has been working on the scale up of second-generation HTS since its formation in 2000. This presentation would discuss the R&D over the last 5 years at SuperPower that has resulted in successful scale up of high-throughput processes to produce 100 m lengths of second-generation HTS conductors. The R&D has been an integration of basic materials science, equipment engineering, and process development. Such an integration was applied to all eight processing steps involved in fabrication of second-generation HTS conductors that include substrate polishing, buffer deposition, superconductor deposition, slitting, and copper stabilizer application. In addition, novel characterization techniques were applied to develop off-line and on-line quality control tools. The presentation would provide the latest development in the scale up R&D of second-generation HTS conductors as well as detail the remaining challenges for successful use of HTS in commercial applications

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