Preparation and characterization of high temperature superconducting thin films

Thin films of the Bi₂Sr₂Ca₂Cu₃O[sub y] superconductor with T[sub c] in excess of 106K were produced through a two step process involving rf magnetron sputtering of an oxide target followed by annealing in oxygen. The effects of various production parameters on the deposition and quality of the films were studied. During deposition, ion bombardment of the films directly in front of the sputtering target lead to an increase in film density and to resputtering. Films from this area tended to flake off of the substrate during processing. Heating the substrate during deposition reduced the flaking problem, but caused the concentration of bismuth to drop at temperatures above 350° C. Optimal post-deposition processing conditions were dependent on several factors including film composition, lead doping, and oxygen concentration in the annealing atmosphere. After processing, only lead doped films resulted in significant amounts of 2223 and zero resistance T[sub c]'s above 100K. The optimal initial film composition was Pb[sub 1.1]Bi₂Sr₂Ca₂Cu₃O[sub y]. The best annealing conditions for such films were 865-875°C for 16 h in flowing oxygen. During processing, films developed in a sequence of 2201, 2212, and, in lead doped films, 2223. The lead dopant was steadily lost during processing, and no detectable amounts remained in some of the best films after processing. Development of the 2223 phase stopped when the lead loss from the films was essentially complete.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Controlled self-assembly of nanocrystalline arrays studied by 3D kinetic monte carlo modeling

Fabrication of self-assembled arrays of nanocrystals (NCs) by physical vapor deposition (PVD) is a promising technique rated highly for its potential for various electronic, photonic, and sensing applications. However, the self-assembly process is not straightforward to control and direct in a desired way. A detailed understanding of how to control the size, shape, and density of self-assembled NCs by varying the accessible PVD process conditions, such as deposition rate, duration, or temperature, is critical for the potential of self-assembled nanofabrication to be fully realized. In this paper, we report a systematic kinetic Monte Carlo modeling that explicitly represents PVD synthesis of self-assembled metallic NCs on a crystalline substrate. We investigate how varying the duration of deposition, deposition rate, temperature, and substrate wetting conditions may affect the morphologies of arrays of self-assembled metallic islands and compare our results with previously reported experimentally observed surface morphologies generated by PVD and theoretical studies. \ua9 2011 American Chemical Society.Peer reviewed: YesNRC publication: Ye

Nanopatterning of PMMA on insulating surfaces with various anticharging schemes using 30 keV electron beam lithography

As a low cost and high throughput method for nanoscale pattern replication, step and flash imprint lithography (SFIL) with UV transparent masters is gaining prominence for its potential in photonics and integrated-circuit fabrication. However, dielectric materials appropriate for fabricating nanostructured SFIL masters present a challenge when employing electron beam lithography (EBL) because insulator substrates covered by polymeric resists such as PMMA tend to accumulate charge during EBL exposures, thereby degrading the process. In this work we explore the performance of four different EBL anticharging schemes for nanofabrication of dense arrays of dots having diameters 16-30 nm in PMMA on UV transparent fused silica (FS) substrates. These include overlayers of aluminum or a water-soluble conducting polymer, as well as sandwiching of Al or Cr thin films between the substrate and PMMA. The quality of patterns transferred from PMMA into the underlying metallic layers was analyzed, and the grain size of the metal was found to be the limiting factor determining the edge roughness. The best resolution was attained employing the conducting polymer top-coating. This scheme also involves fewer processing steps. The authors have used this technique for lift-off of Cr and Au as well as Cr masked etch transfer of nanosized patterns into glass substrates for UV-transparent master mold fabrication. \ua9 2011 American Vacuum Society.Peer reviewed: YesNRC publication: Ye

SML resist processing for high-aspect-ratio and high-sensitivity electron beam lithography

A detailed process characterization of SML electron beam resist for high-aspect-ratio nanopatterning at high sensitivity is presented. SML contrast curves were generated for methyl isobutyl ketone (MIBK), MIBK/isopropyl alcohol (IPA) (1:3), IPA/water (7:3), n-amyl acetate, xylene, and xylene/methanol (3:1) developers. Using IPA/water developer, the sensitivity of SML was improved considerably and found to be comparable to benchmark polymethylmethacrylate (PMMA) resist without affecting the aspect ratio performance. Employing 30-keV exposures and ultrasonic IPA/water development, an aspect ratio of 9:1 in 50-nm half-pitch dense grating patterns was achieved representing a greater than two times improvement over PMMA. Through demonstration of 25-nm lift-off features, the pattern transfer performance of SML is also addressed. \ua9 2013 Mohammad et al.Peer reviewed: YesNRC publication: Ye

Density multiplication of nanostructures fabricated by ultralow voltage electron beam lithography using PMMA as positive- and negative-tone resist

The authors report a density multiplication process for nanoscale patterns composed of dots and lines using electron beam lithography with low voltage 1 keV exposures and cold development. The density doubling is achieved in a single exposure-development step using polymethylmethacrylate (PMMA) as the resist. PMMA exhibits a dual positive- and negative-tone behavior depending on the electron dose employed in this density multiplication process. Fabricated nanostructures are characterized via scanning electron microscopy and subsequent feature size measurements. After density doubling, the minimum dot diameter of an initially 80 nm pitch array of single pixel dots was measured as approximately 27 nm, and the minimum width in an initially 100 nm pitch array of lines was approximately 21 nm. Methodologies for controlling the dimensions of fabricated structures are discussed. Modeling of the electron beam exposure has been carried out using an original electron beam lithography simulator in order to understand the nominal yields of scission in PMMA required in order to achieve the density multiplication, and the results are discussed. \ua9 2011 American Vacuum Society.Peer reviewed: YesNRC publication: Ye

Surface enhanced raman spectroscopy detection of biomolecules using EBL fabricated nanostructured substrates

Fabrication and characterization of conjugate nano-biological systems interfacing metallic nanostructures on solid supports with immobilized biomolecules is reported. The entire sequence of relevant experimental steps is described, involving the fabrication of nanostructured substrates using electron beam lithography, immobilization of biomolecules on the substrates, and their characterization utilizing surface-enhanced Raman spectroscopy (SERS). Three different designs of nano-biological systems are employed, including protein A, glucose binding protein, and a dopamine binding DNA aptamer. In the latter two cases, the binding of respective ligands, D-glucose and dopamine, is also included. The three kinds of biomolecules are immobilized on nanostructured substrates by different methods, and the results of SERS imaging are reported. The capabilities of SERS to detect vibrational modes from surface-immobilized proteins, as well as to capture the protein-ligand and aptamer-ligand binding are demonstrated. The results also illustrate the influence of the surface nanostructure geometry, biomolecules immobilization strategy, Raman activity of the molecules and presence or absence of the ligand binding on the SERS spectra acquired.Peer reviewed: YesNRC publication: Ye

Study of the interaction of polymethylmethacrylate fragments with methyl isobutyl ketone and isopropyl alcohol

Exposure of polymethylmethacrylate (PMMA) during electron beam lithography (EBL) produces small polymer fragments that dissolve rapidly during the development process. The resist dissolution behavior varies greatly depending on the nature of the developer (solvent) and therefore influences the selection of the EBL parameters, such as dose (sensitivity). A molecular scale examination of the development process is necessary to elucidate the resist-developer interaction mechanisms. In this work, the authors investigate the interaction of short PMMA chains (containing up to 10 MMA units) with common developer components methyl isobutyl ketone (MIBK) and isopropyl alcohol (IPA). For this purpose, the authors conduct molecular dynamics simulations using the Accelrys Materials Studio package. The simulation results were used to characterize the mixtures in the spirit of the Flory-Huggins theory of polymers and also to extract the diffusivities. The authors found that the behavior of PMMA fragments differed considerably in MIBK as compared with IPA. PMMA fragments containing more than three monomers exhibit stronger attractive interaction with MIBK. For all fragment sizes simulated, the diffusivity of PMMA fragments is 60-160 higher in MIBK as well. Similarly, the authors observed differences in the gyration radii. The authors conclude that the kinetic factor seems to be more significant as compared to affinity factor when accounting for differences in exposure sensitivities due to developer selection.Peer reviewed: YesNRC publication: Ye