Fabrication of Aluminum-Silicon-Aluminum Junctions on Si Fins—Towards Fin Based Merged Element Transmons – FinMET

Transmon qubits have led to the demonstration of multiple breakthrough experiments in quantum information science [1]. Currently these transmons are fabricated using a thermally oxidized Al/AlO_x/Al Josephson junction in parallel with a shunt capacitor. The size of the current device structures makes it difficult to integrate into a densely integrated scalable technology platform. The shunt capacitors, that are designed to minimize the loss from surfaces and interfaces, can occupy areas of 100s of micrometer squares. Further, lack of precise control of the tunnel barrier thickness leads to spread in frequency allocations of the transmons, limiting reliable fabrication of a large number of these devices [2].<br/>The merged element transmon (mergemon) addresses and solves several of these challenges by merging the Josephson junction and the shunt capacitor into a single superconductor-tunnel barrier-superconductor structure [3]. This results in significant reduction of the device area by ~4 orders of magnitude as well as improved suppression of unwanted radiation and qubit-qubit coupling due to their reduced sizes. However, in these demonstrations, the lossy amorphous tunnel barrier and the inhomogeneous interfaces were identified to be the major limiting factors for qubit relaxation times [3]. A crystalline tunnel barrier with low energy barrier can improve qubit performances. However, such crystalline barriers are challenging to realize in the standard planar junction geometry. In this work, a merged element transmon device concept using Si fins was explored. Using Si (110) wafers, fins were defined parallel to {111} facet. These Si fins after deposition of superconductors on both surfaces can act as perfectly crystalline tunnel barriers. Simultaneously, these fins can keep the area usage to a minimum, allowing dense on-chip integration.<br/>The etching of the fins was performed using a low-pressure chemical vapor deposited SiN_x hard mask and a combination of timed dry and wet etches. The dry etches were done using a CF₄/O₂ chemistry in a plasma etcher. The wet etch was performed using KOH at elevated temperatures. Extreme aspect ratio fins with dimensions of 100µm (length) × 2.5µm (height) × 80nm (thickness) with smooth surfaces were obtained. The surface step bunching was found to depend on the precise orientation of the fins with respect to the {111} facet. The undercut of the Si under the hard mask was subsequently used to shadow-deposit aluminum on both surfaces of the fins creating a superconductor-semiconductor-superconductor junction. The deposition was performed using molecular beam epitaxy in an ultra-high vacuum environment. This self-aligned process can potentially create a merged element transmon, FinMET, without the need for further processing steps. The fin junctions were characterized using scanning electron microscopy, cross-sectional transmission electron microscopy and energy-dispersive spectroscopy revealing damage free barriers and continuous superconductor coverage. Next steps in reaching tunneling thickness regime through further thinning of Si fins, as well as fabrication and testing of the FinMET devices will be presented and discussed.<br/><b>References:</b><br/>[1] J. Koch <i>et.al.</i> Phys. Rev. A 76, 042319, (2007)<br/>[2] J. M. Kreikebaum, <i>et.al.</i> Supercond. Sci. & Technol. 33 06LT02 (2020)<br/>[3] R. Zhao<i> et.al.</i> Phys. Rev. Applied 14, 064006 (2020)