Nucleation of Superconductivity in a Mesoscopic Loop of Finite Width

The normal/superconducting phase boundary Tc has been calculated for mesoscopic loops, as a function of an applied perpendicular magnetic field H. While for thin-wire loops and filled disks the Tc(H) curves are well known, the intermediate case, namely mesoscopic loops of finite wire width, have been studied much less. The linearized first Ginzburg-Landau equation is solved with the proper normal/vacuum boundary conditions both at the internal and at the external loop radius. For thin-wire loops the Tc(H) oscillations are perfectly periodic, and the Tc(H) background is parabolic (this is the usual Little-Parks effect). For loops of thicker wire width, there is a crossover magnetic field above which Tc(H) becomes quasi-linear, with the period identical to the Tc(H) of a filled disk (i.e. pseudoperiodic oscillations). This dimensional transition is similar to the 2D-3D transition for thin films in a parallel field, where vortices start penetrating the material as soon as the film thickness exceeds the temperature dependent coherence length by a factor 1.8. For the presently studied loops, the crossover point is controlled by a similar condition. In the high field '3D' regime, a giant vortex state establishes, where only a surface superconducting sheath near the sample's outer radius is present.Comment: 7 pages text, 2 EPS figures, uses LaTeX's elsart.sty, proceedings of the First Euroconference on "Vortex Matter in Superconductors", held in Crete (18-24 september 1999

Flux pinning by regular arrays of ferromagnetic dots

The pinning of flux lines by two different types of regular arrays of submicron magnetic dots is studied in superconducting Pb films; rectangular Co dots with in-plane magnetization are used as pinning centers to investigate the influence of the magnetic stray field of the dots on the pinning phenomena, whereas multilayered Co/Pt dots with out-of-plane magnetization are used to study the magnetic interaction between the flux lines and the magnetic moment of the dots. For both types of pinning arrays, matching anomalies are observed in the magnetization curves versus perpendicular applied field at integer and rational multiples of the first matching field, which correspond to stable flux configurations in the artificially created pinning potential. By varying the magnetic domain structure of the Co dots with in-plane magnetization, a clear influence of the stray field of the dots on the pinning efficiency is found. For the Co/Pt dots with out-of-plane magnetization, a pronounced field asymmetry is observed in the magnetization curves when the dots are magnetized in a perpendicular field prior to the measurement. This asymmetry can be attributed to the interaction of the out-of-plane magnetic moment of the Co/Pt dots with the local field of the flux lines and indicates that flux pinning is stronger when the magnetic moment of the dot and the field of the flux line have the same polarity.Comment: 7 pages including figures; submitted for publication in Physica C (Proceedings ESF-Vortex Conference, 18-24 Sept. 1999, Crete, Greece

Early learning on hyper-NA lithography using two-beam immersion interference

Two-beam interference of 193nm laser light can print dense line-space patterns in photoresist, down to a resolution that can only be obtained using hyper-NA scanners, and allows for early learning on hyper-NA imaging and process development. For this purpose, a dedicated two-beam interference immersion printer, operating at 193nm wavelength, was installed in the IMEC cleanroom. The interference printer consistently generates L/S patterns at 130nm, 90nm, and 72nm pitch with exposure latitudes in the 12-26% range (when using TE-polarized light). At these pitches, process and imaging issues have been Studied that are of direct interest for hyper-NA lithography. On the imaging side, we discuss the flexibility of the printer towards working with various polarizations. We show how reflection reduction strategies at the high incidence angles of hyper-NA imaging can be tested in the interference printer. On the processing side, we have screened a number of resists at 90nm pitch. A methodology to study static and dynamic leaching was developed. Several liquids with refractive index > 1.6 are Currently being developed as potential candidates to replace water for optical lithography at 38nm half-pitch. We have used the interference printer at 72nm pitch, with both water and liquids of refractive index 1.65