Application of Direct Laser Writing for the Fabrication of Superconducting Tunnel Junctions and Phononic Crystal Structures

Abstract

Phononic crystals (PnC) are periodic structures analogous to the more common photonic crystals. Instead of a periodic dielectric constant they have a periodic elasticity and density, and thus they alter the flow of vibrational energy (heat and/or sound) through a material. This thesis focuses on the fabrication of three-dimensional (3D) PnC structures, integration of tunnel junction devices with them and studying their thermal properties. The 3D PnC structures, which in this case are 3D square lattices of spheres, were fabricated with 3D lithography using established methods. However, getting the exact wanted geometry required a lot of work and thus part of the thesis revolves around the optimization of the design. Then we had to develop a method for the fabrication of measurement electronics on these 3D structures as no conventional methods really allow such fabrication. Next we wanted to prove that the developed method can produce the necessary measurement devices. In this case the devices are superconductor-insulator-normalmetal-insulator-superconductor (SINIS) junction pairs. We show that the method can produce good quality SINIS junctions, both on a flat substrate and on a 3D structure, using low-temperature measurements. The last part of the thesis focuses on the thermal conductance measurements of the fabricated PnC structures. These measurements were made for two different PnC structures with sphere diameters of 3.1 μm and 5.0 μm, and also for a control bulk structure with the same geometry. The results of these measurements are compared to finite element method simulations made with a ballistic model. Keywords: Phononic crystal, 3D lithography, thermal conductance, low temperature