Geometrizzazione con fotolitografia e nanolitografia a raggi X per la fabbricazione di dispositivi a giunzioni Josephson intrinseche in whisker di Bi-2212.

Josephson junctions naturally convert dc voltages into electromagnetic ra- diation with 1 mV corresponding to 0.483 THz. The emission from a sin- gle junction is too weak to be useful for applications, nonetheless stacks of junctions emitting in phase may be used to increase the radiation power. Tightly packed arrays of junctions occur in the structure of the layered high- temperature superconductor, named 𝐵𝑖2𝑆𝑟2𝐶𝑎𝐶𝑢2𝑂8+𝛿 (Bi-2212). Bi-2212 crystals in the form of whiskers have been extensively studied as good candidate for terahertz device fabrication, because of their microscopic size and high crystalline quality. Nanoscale stacks of junctions are usually fabricated by means of Focused Ion Beam (FIB) etching technique, which allows to selectively remove some sections from the whisker in order to force current to flow along the c-axis, where the junctions are piled up. Since FIB technique presents some consid- erable disadvantages, such as ion implantation and mechanical instability of the device, alternative approaches have been investigated. Different experiments highlighted that x-ray photons interaction with inor- ganic solids, like BSCCO whiskers, has an important role in the displacement of interstitial loosely-bound oxygen atoms (∼ 2 eV), responsible for doping level in the material. The nature of synchrotron light interaction with mat- ter is yet to be proved; the main hypothesis is that highly energetic photons produce photoelectrons that are able to knock interstitial atoms out of the material. Another suggested explanation is that oxygen removal could be associated with a local temperature increase in and around the spot of ir- radiation. This hypothesis has been explored by means of a finite element model, which pointed out a raise of 13 K. Whatever the cause for the change in the doping level, this effect could lead to the development of a new mask-less resist-free direct writing process suitable for device fabrication, up to nanometric resolution. For my thesis I joined an experiment at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, whose main purpose was to test the feasibility of the leading-edge technique above-mentioned. It was possible to successfully fabricate intrinsic Josephson junction devices on some Bi-2212 samples. This was achieved by a change of conducting properties of the whisker in the irradiated areas with a 17.5 keV beam, instead of removing them as in FIB method. In relation to this experiment my work has been focused on samples prepara- tion through the making of silver contacts on the whisker with photolithogra- phy and physical vapour deposition, in collaboration with the national insti- tute for metrological research (INRiM - Istituto Nazionale di Ricerca Metro- logica) in Turin. In order to monitor the modification of electrical properties of the sample, I also performed resistance versus temperature measurement and current voltage characterization before and after the exposure to hard x-ray beam. These measurements, together with x-ray diffraction patterns acquisition, allowed us to understand the effect of irradiation on BSCCO. This thesis will be divided into four main parts. First of all I'll introduce the subject of this study, which consists of the structure, properties and possible applications of BSCCO material. The second part constitutes a background of knowledge about different techniques available for the fabrication Joseph- son devices. Moreover I will go through a detailed report of the experimental procedure needed for sample preparation with photolithography, for x-ray nano-lithography and electrical characterization. Finally I will give the main results of this work and summarize it as a conclusion in the last few chapters.