Progettazione e sviluppo di un processo bridge-free per la realizzazione di giunzioni Josephson per applicazioni di quantum computing

Abstract

The present thesis focuses on the laboratory fabrication of Al/AlOₓ/Al Josephson junctions using a bridge-free shadow evaporation technique, developed within the broader context of superconducting technologies for quantum computing. The interest in such devices arises from the central role played by the Josephson junction in superconducting qubits: unlike a purely harmonic LC oscillator, characterized by equally spaced energy levels, the presence of the junction introduces a nonlinearity that leads to an anharmonic energy spectrum. This allows for the selective addressing of the fundamental transition, which is a necessary condition for implementing a circuit-based qubit. In this framework, the quality of the AlOₓ tunnel barrier and of the metal interfaces is of crucial importance, as it directly affects not only the static electrical parameters of the junction but also dielectric losses and, more generally, the performance of the device in quantum applications. Following a theoretical overview of superconducting qubits and the physics of Al/AlOₓ/Al Josephson junctions, the work focuses on the experimental implementation and optimization of a bridge-free fabrication approach, adopted as an alternative to conventional suspended-bridge techniques. This method preserves the advantages of in-situ shadow evaporation for barrier formation while replacing the mechanical fragility of the suspended bridge with a geometrical control of the resist undercut. The operating principle relies on engineering an asymmetric profile in a bilayer resist system such that, during the two angled evaporations, only one deposition reaches the substrate along the connection lines, while the other is deposited on the resist sidewalls and subsequently removed during lift-off. In the central region, the overlap of the two evaporated aluminum layers, separated by an intermediate oxidation step, defines the Al/AlOₓ/Al superconductor–insulator–superconductor structure. This approach results in a more robust masking scheme compared to suspended-bridge processes, albeit at the cost of increased sensitivity to the precise definition and reproducibility of the undercut. The experimental activity was structured in several stages. First, the positive resists PMMA and MMA, used to form the bilayer lithographic stack, were characterized in order to determine their response as a function of electron-beam exposure dose and development conditions. A calibration mask consisting of an array of square patterns exposed at varying doses was employed to extract contrast curves and identify suitable exposure parameters. The development depth was quantitatively measured using atomic force microscopy (AFM), enabling a direct correlation between exposure dose and resulting resist profile. This step proved essential for selecting the most appropriate resist configuration capable of producing a controlled undercut compatible with the requirements of the bridge-free process. Subsequently, the lithographic process and evaporation geometry were calibrated through iterative refinement of the mask design, exposure parameters, and development conditions. Cross-sectional analysis, obtained via controlled sample cleaving followed by scanning electron microscopy (SEM), played a key role in assessing the three-dimensional profile of the resist, verifying the presence and quality of the undercut, and identifying potential structural instabilities such as resist collapse. At the same time, modifications were introduced to the evaporation system to enable deposition at the desired angles. This required the design of a new sample holder and a complete recalibration of the quartz crystal microbalance, as the altered geometry significantly affected the relationship between the nominal thickness reading and the actual film thickness deposited on the sample. A further important aspect of the work involved the progressive design and optimization of the full device mask, including contact pads, alignment markers, and the active junction region. The evolution of the layout, from initial test structures to the final configuration, was guided by experimental observations of pattern distortions and dose non-uniformities. It was found that the writing order of the electron-beam lithography pattern generator could significantly influence the final pattern quality, introducing systematic variations in feature definition. To mitigate this effect, a constrained writing sequence was implemented, ensuring improved reproducibility of the exposure process. Once all process parameters were stabilized, a comprehensive Standard Operating Procedure (SOP) was established, covering both the fabrication of Cr/Au contact structures and the subsequent lithography, angled evaporation, oxidation, and lift-off steps required for the realization of the Josephson junction. Finally, prior to fabricating fully oxidized junctions, a preliminary electrical validation of the process was carried out by producing test structures without the intermediate oxidation step. This allowed for the verification of electrical continuity in the aluminum structures. Measurements performed using a probe station confirmed ohmic behavior and the absence of macroscopic discontinuities in the conduction path. The measured resistance values were higher than those predicted using bulk aluminum resistivity; however, this discrepancy is consistent with the well-known behavior of thin evaporated films, in which surface scattering, grain boundaries, and interface effects lead to an increased effective resistivity. Although these measurements do not yet constitute a full characterization of the Josephson junction in the quantum regime, they provide an essential validation of the fabrication process and the structural integrity of the devices. In conclusion, this work enabled the development and calibration of a bridge-free fabrication process for Al/AlOₓ/Al Josephson junctions, highlighting the strong interplay between lithographic control, evaporation geometry, barrier formation, and electrical response. The main contribution of the thesis lies in transforming a conceptual fabrication approach into a structured and reproducible experimental procedure, based on resist characterization, cross-sectional analysis, instrumental recalibration, and preliminary electrical validation. The results obtained provide a solid foundation for further optimization of the tunnel barrier, cryogenic characterization, and integration of these junctions into superconducting circuits for quantum computing applications.