Fabbricazione e caratterizzazione di dispositivi a giunzioni Josephson intrinseche di Bi-2212

Nowadays arrays of superconducting Josephson junctions are routinely used in many National Metrology Laboratories to reproduce the unit of voltage. From a physical viewpoint, the array behavior is described by the Josephson equation V = ?nh=(2e), expressing the linear dependence of array DC voltage V on the frequency ? of a microwave signal radiated on the array. What is most important for Metrology is the value of the proportionality factor, nh=(2e), which can be calculated in terms of an integer number n and two fundamental constants: the electron charge e and the Planck constant h. Fundamental constants can be determined with high accuracy and, being "fundamental", their value is independent of measurement conditions. This is essential for metrological applications: the fundamental constant proportionality between voltage and frequency is an "intrinsic guarantee" of the accuracy of the generated voltage, once the frequency is known. With Josephson arrays then we can exploit the accuracy of frequency standards in voltage measurements, making it possible to measure DC voltages, with relative un certainties as low as 10-11. High-temperature superconductors are ?t for such applications because they can be described as a sequence of superconductor/insulating interfaces at a nanometric level, which is commonly referred to as their Intrinsic Josephson Junctions (IJJs) structure. However, stacks of hundreds of in-series junctions are required to reach a device with \high" Voltage, which can be achieved by exploiting the crystal structure of the Bi-2212 superconductors if the current is forced to flow along the crystal c-axis. In this framework, Bi-2212 microcrystals (whiskers), thanks to their micrometric size, are good starting points for the fabrication of devices with a high degree of miniaturization. NIS produce microcrystals suitable for the IJJ stacks fabrication. I have been working on the implementation of the devices using whiskers fabricated by NIS, the manufacturing took place both at INRIM (National Institute of Metrological Research) and at the University of Turin. The following paragraphs explain how I fabricated and mesured the device: ˆ I began by selecting a Bi-2212 whisker according to its surface regularity and size (800x10?m). Then I ?xed it to a silicon substrate with a STYCAST resin. ˆ I used photolithography to de?ne the contact area. I spread a thick resist layer (more than 3?m) by spinner, as the whisker is about 1 micro thick and I need to apply at least 1 micro silver ?lm for contacts ˆ I evaporated 1 micrometer of silver on the device, and then I lifted it o? by bathing it with acetone. So far the device was not yet working, as there wasn't any electrical contact between the whisker and the silver ?lm. Only after annealing at 450o for 50 minutes the device was ready for use. ˆ I used the FIB (Focused Ion Beam) to etch the whisker and create the c-axis channel. ˆ I measured this device at cryogenic temperature with INRIM instru- ment. Finally, I sent two di?erent devices to the Grenoble synchrotron for an X-ray analysis, aimed at assessing the extent of lattice Bi-2212 modi?cations, owing to Gallium ions implanted into the device by FIB during etching step.