Cryogenic Front-to-Back Coupling in FD-SOI for Tunable Qubits by Localized Metallic Back-Gate

In this study, we develop and experimentally validate the integration of a localized metallic back-gate into a multi-gate silicon-on-insulator (SOI) field-effect transistor (FET), aimed at enabling front-gate coupling efficiency from room to cryogenic (mK) temperatures. The fabrication was enabled by the so-called Nanomole process, via controlled and localized vapor-phase etching of either the BOX (with vapor-HF) or the silicon substrate (with XeF^2 ) followed by ALD metallization. We report the electrical characterization of front and back gate transistor operation and resulting threshold voltage control at cryogenic temperatures. Additional cryogenic simulations support our data, showing quantum dot formation via front-back coupling. The localized dopant-free multi-gate coupling proposed here unlocks novel possibilities for quantum dots and spin qubit applications including charge and spin noise reduction, volume inversion and tunable spin-valley mixing.