Progress Towards Merged-Element Transmons Using Crystal Silicon Fins: The Silicon FinMET

A conventional transmon device consists of a small Josephson Junction (JJ) in parallel with a large separate planar capacitor. Parallel plate capacitors are much more compact than planar structures and they confine electric fields within the dielectric region between the superconducting layers. This reduces the mode volume while placing a high demand on the dielectric quality and the superconductor-dielectric interfaces. Further size reduction can be achieved by utilizing the capacitance of the JJ itself rather than a large separate capacitor creating a merged-element transmon (MET), which can improve the scalability by a factor of 10⁴ [1, 2]. We have recently shown that transmons made with Al contacted single crystal Si fin parallel plate capacitors are both low loss and compact [3, 4]. The high aspect ratio single crystal Si fins are fabricated from low-loss high-resistivity Si(110) substrates using electron beam lithography and selective crystallographic etching yielding fins down to 50 nm in thickness suitable for capacitors [4]. Typically, JJs consist of two aluminum superconductors separated by a thin ~1 nm thick amorphous aluminum oxide tunnel barrier which is believed to inherently introduce losses due to its amorphous structure [5]. Theoretical predictions indicate that a single crystal Si barrier must be less than 10 nm thick for tunneling [6]. Utilizing a digital etching process, we have been able to thin the Si fins to 6nm. In this presentation, I will present the progress in Si fin capacitors, JJs and the development of the Si FinMET.

 

1          R. Zhao, et al., Phys. Rev. Appl. 14, 064006 (2020)

2          H. J. Mamin, et al., Phys. Rev. Appl. 16, 024023 (2021)

3          A. Goswami, et al., Appl. Phys. Lett. 121, 064001 (2022)

4          A. P. McFadden, et al., NPJ Quantum Information 11, 11 (2025)

5          S. Oh, et al., Phys. Rev. B 74, 100502 (2006)

6          J. K. Nangoi, et al., Phys. Rev. B 110, 035302 (2024)

 

Co-authors: T.A.J. van Schijndel¹, A.P. McFadden², Y. Wu¹, A. Goswami¹, W.J. Yánez-Parreño¹, J.K. Nangoi¹, T. Zha^2,3, T.F.Q. Larson², S. Sahu², S. Gill², F. Lecocq², R. Simmonds², and C. G. Van de Walle¹,

¹UCSB, ²NIST, ³U. of Colorado, Boulder