Nanofabbricazione di dispositivi TEM per la misura del Momento Angolare Orbitale di fasci elettronici

In this thesis I present my M. Sc. final work that I performed in the group of Prof. Stefano Frabboni and Vincenzo Grillo at the University of Modena and Reggio Emilia (UNIMORE). The field of research is Electron Holography and in particular the nanofabrication of Transmission Electron Microscope (TEM) devices designed to modify the phase of electron beams in order to manipulate and measure the Orbital Angular Momentum (OAM). The capability to study the change in the phase distribution of an electron beam after it has travelled through a specimen, for example with interferometric techniques, provides information on the distribution of local electromagnetic potentials. This is particularly interesting to detect magnetic properties of nanoparticles and molecules. In particular the OAM, which is strictly related to the wave front of the beam, carries high degree of information. The excitation of a sample with a phase-structured beam allows the direct study of particular and exotic effect such as transition radiation and Landau states dynamics in real space. Shaping the phase of an electron beam in the correct way is a very critical point and requires nanometric features that are typically produced by a Focused Ion Beam and Electron Beam Litography. Such techniques are fundamental to fabricate devices able to detect and measure the phase of that beams. A challenging issue is the measuring of the OAM since it is not straightforwardly amenable to an intensity measure. In fact, such quantity lies in the wavefunction in the form of exp(ilp) where l and p are the OAM quantum number and the azimuthal angle respectively: such exponential form when squared (to give intensity) becomes one for any value of p and any information is lost. This is the well-known phase problem common to many techniques. Up to now several methods have been proposed to measure the OAM of an electron beam, including for example magnetic “monopole” able to impart the beam an l OAM value: electrons carrying an opposite -l become Gaussian-shaped while the others acquire a phase. Measuring the fraction of the Gaussian component provides the initial percentage of structured vortex beam. This method presents many disadvantages including the necessary presence of a number of devices equal to the components of l that have to be measured and the unsuitableness for large values of OAM. Following the idea of the optical counterpart the concept of sorter has been introduced for electron beams: the basic idea is to change the wavefront in such a way to separate efficiently the different components of OAM so that an intensity peak is recorded for each of them. The aim of this thesis is the improvement of such method My work was focused on the optimization of the fabrication process exploiting characterisation techniques such as Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Electron Energy Loss Spectroscopy (EELS). Besides, the Electron Beam Litography process involved the use of a Plasma chamber for the etching of the sample and all the steps typical of litography such as spinning, developing and removing the resist. The choice of the resists and the control of the parameters provided by the datasheets was another important part of the work in order to optimize the whole process. With the FIB I fabricated both the two component of the measuring system, the '' Sorter 1'' and the Sorter 2. I exploited EBL to optimize the second component, the Sorter 2. The main progress with respect to the state-of-the-art results is a new configuration based on a Sorter 1called ''Fan-out'' which is aimed to separate efficiently the various diffractive orders and, in this way, to increase the resolution of the system. Our group is planning to publish the results related to this device which has already proven to be the most efficient at the moment.