A. J. Bourdillon

Stabilisation of 110 K superconducting phase in BiSrCaCuO Pb substitution

Abstract The effect of Pb substitution for Bi in the Bi-Sr-Ca-Cu-O systems, together with heat treatment, on the superconducting transition was investigated. Up to 40% Pb substitution, the crystal structure remains essentially unchanged, but the sub-unit cell parameters decrease with an increase of Pb substitution; some extra peaks from the X-ray diffraction patterns could be indexed assuming c=37 A . The Pb substitution tends to stabilise the high- T c phase transition at 110 K and 95 K even for a wide range of heat treatment conditions due to the increase of Ca and Cu concentration in the superconducting phase. Ag addition sharpens all transitions due to the improvement of the connectivity between grains. Higher temperature treatment promotes the 95 K phase transition and raises the zero resistance temperature. Quenching treatment results in the enhancement of the 95 K phase transition for specimens without Pb substitutions; it also causes semiconductor behaviour for Pb-substituted specimens depending upon the optimisation of the levels of Cu 3+ concentration and oxygen deficiency.

High quality YBa2Cu3O7 Josephson junctions made by direct electron beam writing

High‐Tc Josephson junctions have been fabricated by direct electron beam writing over YBa2Cu3O7 thin‐film microbridges, using scanning transmission electron microscope (STEM) with an accelerating voltage of 80–120 kV. Annealing at 330–380 K increases Tc and Ic of the junctions and makes them more stable. In the operating range of a few degrees below Tc, the junctions show 100% magnetic field modulation of the critical current, microwave‐induced Shapiro steps oscillating according to the resistively shunted junction (RSJ) model, and RSJ current‐voltage characteristics with IcRn product up to 0.5–0.6 mV at 75 K and 0.3 mV at 77 K.

EPR and NMR measurements on high-temperature superconductors

Some EPR and NMR experiments at the Cu sites in the mixed-phase materials Ba0.6Y0.4CuOy and Ba0.2Y0.8CuOy, and the pure-phase compounds YBa2Cu3Oy (1:2:3), BaCuO2 (0:1:1), Y2Cu2O5 (2:0:2), Y2BaCuO8 (2:1:1), YBa3Cu2Oy (1:3:2) and CuO are presented and discussed. In particular, it is shown that any EPR signal in nominally pure 1:2:3 superconducting material is probably due to a small amount of impurity 2:1:1 (greenphase) compound. For the 2:1:1 and 1:3:2 compounds the authors find g/sub /// approximately=2.23 and gperpendicular to approximately=2.09 with a linewidth of approximately=0.025 T, whereas for the 2:0:2 compound g=2.12(2) with a linewidth of 0.047(3)T. Some suggestions are also made concerning the absence of both NMR and EPR signals in the 1:2:3 compound, in terms of (i) Andersons resonating bond model and (ii) recent FLAPW band-structure calculations. In particular, it is argued that the delocalised nature of the Cu d electrons probably gives rise to exceptional spin-spin dipolar broadening of both the NMR and EPR transitions.

Demagnification in proximity x-ray lithography and extensibility to 25 nm by optimizing Fresnel diffraction

This new understanding and demonstration of features printed by proximity x-ray lithography allows a revolutionary extension and simplification of otherwise established processes for microfabrication. The ability to produce fine features is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature bias. In the classical approach the bias is minimized. Bias optimization in terms of mask/wafer gap and resist processing allows the formation, on a wafer, of features smaller than those on the mask: thus producing local demagnification. This demagnification ( ? 3- ? 6) is achieved without lenses or mirrors, but it offers the same advantages as projection optical lithography in terms of critical dimension control. The photoelectron blur is more or less pronounced depending on exposure dose and development conditions. Resist exposure and process can be optimized to utilize a ~ 50% photoelectron energy loss range. In consequence proximity x-ray lithography is extensible to feature sizes below 25 nm, taking advantage of comparatively large mask features (> 100 nm) and large gaps (30-15 ? m). The method is demonstrated for demagnification values down to ? 3.5. To produce DRAM half-pitch fine features, techniques such as multiple exposures with a single development step are proposed.

A comparison of the stability of Bi2Sr2CaCu2O8+y with YBa2Cu3O6.5+y in various solutions

The chemical behaviour of both YBa2Cu3O6.5+y (123) and Bi2Sr2CaCu2O8+y (2212) in various solvents was studied in a comparative manner. In both compounds superconductivity apparently depends on a high-oxidation state of Cu. Electrochemical corrosion rates were measured by a variety of techniques and are related to the concentration of labile Cu3+ ions in the specimens. Generally (2212) is the more stable compound, except in strongly basic solutions owing to the amphoteric nature of Bi. In the presence of acidic or basic solutions Bi3+ is oxidised to Bi4+ or Bi5+ by the high-oxidation state of Cu in (2212) (similar to oxidation by Na2O2), demonstrating a common feature of cuprate superconductors. The concentration of labile ions was determined by the volumetric measurement of evolved oxygen from acid solution, and the technique was extended to measure the concentration of undecomposed or re-formed carbonates after sintering.

Labile Cu3+ ions correlated with superconducting properties in YBa2Cu3O7−x

Abstract The volumetric measurement technique was used for the direct determination of Cu 3+ in pure YBa 2 Cu 3 O 7−x and in doped materials. The results are in excellent agreement with those obtained by other measurement techniques. Cu 3+ ions are found in both the orthorhombic and tetragonal phases of {123}. The Cu 3+ concentration correlates with, and is crucial to the superconducting properties of the materials. These results demonstrate the importance of chemical models in determining the precise pairing mechanism in these high temperature superconducting ceramics.

Current applications of electron energy loss spectroscopy

Abstract It is undoubtedly true that the advent of efficient energy loss spectrometers for transmission microscopes over the last few years has been of considerable assistance, at least qualitatively, for the analysis of light elements and, to a more limited extent, in structure interpretation. Rather frustratingly, given the potentially better spatial resolution of EELS over EDX, realistic quantitative analysis remains difficult, and similarly - while fascinating effects are seen in, for example, the crystallographic orientation dependence of the signal - these are currently only broadly interpretable in relation to those observed in EDX. The reasons for this are discussed, as are the relative advantages of large and small collection angles for different types of experiment.

Study of acid diffusion in a positive tone chemically amplified resist using an on-wafer imaging technique

In this article, the acid diffusion coefficient of Shipley positive tone resist, UV-III, in an as-spin coated film is determined by using an on-wafer imaging technique. This technique involves incorporation of a pH-dependent fluorescence material, 5-aminofluorescence, into the resist. This fluorescence material images the acid distribution by generating fluorescence intensity contrast under a confocal microscope. Using a laser beam of 488 nm wavelength and an objective lens of 100× (with oil immersion), the nominal feature sizes of 0.20 μm can be resolved with a high signal-to-noise ratio. The acid diffusion coefficient is determined from 1 μm lines and spaces (1:9), which are generated by e-beam writing under different post exposure baking (PEB) times. The value obtained during the PEB is smaller than 3.5×10−13 cm2/s. The resist properties (e.g., sensitivity, resolution, and thermal stability) are not changed significantly by the low loading of the fluorophore because the dose to size and the printed lin...

Electron microscopy and microanalysis of a YBa2Cu3Ox superconducting oxide

The onset of superconductivity at 140 K has been observed in the high Tc Y‐Ba‐Cu‐O system in samples consisting of 90% by volume of a single‐phase oxide of average cation ratio Y:Ba:Cu≂1:2:3. A sharp superconducting transition in the resistivity has been measured, where Tc0=140 K, Tc=93.2 K, ΔTc=0.5 K, and ‘‘zero resistance’’ was observed at 92.0 K. Transmission electron microscopy and energy dispersive spectroscopy measurements revealed that the superconducting phase consisted of a large number of microcrystallites of ≂0.5 μm size and that these contained a very high defect microtwinned/faulted structure.

Near field x-ray lithography simulations for printing fine bridges

By using the near field in proximity x-ray lithography (PXL), a technique is demonstrated that extends beyond a resolution of 25 nm print feature size for dense lines. ‘Demagnification by bias’ of clear mask features is positively used in Fresnel diffraction together with multiple exposures of sharp peaks. Exposures are performed without lenses or mirrors between the mask and wafer, and ‘demagnification’ is achieved in a selectable range, 1×–9×. The pitch is kept small by multiple stepped exposures of sharp, intense image peaks followed by single development. Low pitch nested lines are demonstrated. The optical field is kept compact at the mask. Since the mask–wafer gap scales as the square of the mask feature size, the mask feature sizes and mask–wafer gaps are comparatively large. Because the features are themselves larger, the masks are more easily manufactured. Meanwhile, exposure times for development levels high on sharp peaks are short, and there are further benefits including defect reduction, virtual elimination of sidebands, etc. A critical condition (CC) has been identified that is typically used for the highest resolution. Many devices, including batches of microprocessors, have been demonstrated previously by traditional 1× PXL, which is the only next generation lithography developed and which is now further extended. For two-dimensional near field patterning, temporal and spatial incoherence at the CC have been used to show not only that peculiarities in the aerial pattern, such as ‘ripple’ and ‘bright spots’, can be virtually eliminated but also that there is an optimum demagnification, around 3×, in the Fresnel diffraction, where the contrast and, therefore, critical dimension control are highest. In the simulation of a bridge pattern, ‘ripple’ is likewise controlled. Blur and run-out are compared for various sources. Magnification corrections can be applied by various means. Extension to 15 nm printed features is predicted.