Integration of Cobalt Ferromagnetic Control Gates for Electrical and Magnetic Manipulation of Semiconductor Quantum Dots

The rise of electron spin qubit architectures for quantum computing processors has led to a strong interest in designing and integrating ferromagnets to induce stray magnetic fields for electron dipole spin resonance (EDSR). The integration of nanomagnets imposes however strict layout and processing constraints, challenging the arrangement of different gating layers and the control of neighboring qubit frequencies. This work reports a successful integration of nano-sized cobalt control gates into a multi-gate FD-SOI nanowire with nanometer-scale dot-to-magnet pitch, simultaneously exploiting electrical and ferromagnetic properties of the gate stack at nanoscale. The electrical characterization of the multi-gate nanowire exhibits full field effect functionality of all ferromagnetic gates from room temperature to 10 mK, proving quantum dot formation when ferromagnets are operated as barrier gates. The front-end-of-line (FEOL) compatible integration of cobalt is examined by energy dispersive X-ray spectroscopy and high/low frequency capacitance characterization, confirming the quality of interfaces and control over material diffusion. Insights into the magnetic properties of thin films and patterned control-gates are provided by vibrating sample magnetometry and electron holography measurements. Micromagnetic simulations anticipate that this structure fulfills the requirements for EDSR driving for magnetic fields higher than 1 T, where a homogeneous magnetization along the hard magnetic axis of the Co gates is expected. The FDSOI architecture showcased in this study provides a scalable alternative to micromagnets deposited in the back-end-of-line (BEOL) and middle-of-line (MOL) processes, while bringing technological insights for the FEOL-compatible integration of Co nanostructures in spin qubit devices.

💡 Research Summary

This paper presents a fully CMOS‑compatible approach to integrate cobalt (Co) nanogates directly into the front‑end‑of‑line (FEOL) of fully‑depleted silicon‑on‑insulator (FDSOI) nanowire quantum‑dot devices, thereby combining electrical gating and magnetic actuation within the same gate stack. The authors first motivate the need for magnetic field gradients to enable electric‑dipole spin resonance (EDSR) in silicon spin qubits, noting that conventional micromagnets placed in back‑end‑of‑line (BEOL) or middle‑of‑line (MOL) layers add layout complexity, increase the number of metal layers, and limit scalability. By embedding ferromagnetic control gates made of Co, the device can simultaneously serve as a depletion/barrier gate and a source of a local Zeeman field.

Key materials and process innovations are described in detail. A thin chromium (Cr) adhesion layer (2‑3 nm) is deposited before Co evaporation to act as a diffusion barrier and to improve adhesion to the gate oxide. The stack is annealed at ≤ 300 °C in forming gas, a temperature chosen to avoid excessive Co oxidation (Co(OH)₂, CoO, Co₃O₄) and to prevent stress‑induced damage to the atomic‑layer‑deposited (ALD) dielectrics. Transmission electron microscopy with energy‑dispersive X‑ray spectroscopy (EDX‑TEM) confirms that neither Cr/Co nor Ti/Pd diffuses into the oxide, while a thin SiO₂ interlayer effectively blocks the formation of interfacial silicides.

The magnetic properties of the Co films are characterized by vibrating‑sample magnetometry (VSM) and electron holography. VSM shows a sharp increase in coercivity after annealing at 400 °C, attributed to grain growth (from ~15 nm to ~40‑50 nm) and a phase transition from hexagonal close‑packed (hcp) to face‑centered cubic (fcc). Electron holography of patterned gates reveals magnetic field gradients of 30‑50 mT in the vicinity of the nanogates, sufficient to generate Rabi frequencies in the tens of megahertz range for EDSR. Micromagnetic simulations predict that, under an external field > 1 T, the Co gates magnetize uniformly along their hard axis, providing a stable, homogeneous stray field.

Electrical performance is evaluated through MOS capacitance–voltage (C‑V) measurements on test capacitors with Cr/Co and Ti/Pd gates on both thermal SiO₂ and ALD Al₂O₃ dielectrics. Using the 1/C² method, the effective metal work function (ϕ_m,eff) and interface trap density (N_eff) are extracted. Both metal stacks exhibit trap densities on the order of 10¹¹ cm⁻³, which decrease after forming‑gas anneal. The work function is found to be strongly influenced by the dielectric: on SiO₂ it approaches the vacuum value, whereas on high‑k Al₂O₃ it shifts upward due to charge‑neutrality level pinning and the formation of interfacial dipoles. This analysis confirms that Cr/Co gates are electrically viable as high‑quality MOS electrodes.

A proof‑of‑concept quantum‑dot device is fabricated on an ultra‑thin FDSOI nanowire. The device incorporates Ti/Pd plunger gates and Cr/Co barrier gates arranged with nanometer‑scale dot‑to‑magnet pitch. Electrical measurements from room temperature down to 10 mK demonstrate full field‑effect operation of all gates; when the Co gates are biased as barriers, well‑defined quantum dots form, confirming that the ferromagnetic gates can serve the dual role of electrostatic confinement and magnetic field source.

The authors argue that this architecture offers several advantages for scalable silicon spin‑qubit processors: (1) it reduces the number of metal layers by merging magnetic and electrostatic functions, (2) it enables tighter spacing between qubits while maintaining independent frequency control, and (3) it avoids the thermal budget and contamination issues associated with BEOL micromagnets. The paper also outlines remaining challenges: maintaining the 300 °C thermal limit to prevent Co oxidation, optimizing the Co‑Al₂O₃ interface for long‑term reliability, and integrating high‑frequency RF lines for fast EDSR driving.

In summary, the work demonstrates that cobalt nanogates can be integrated into FEOL processes without compromising electrical performance, while providing the strong, localized magnetic gradients required for high‑fidelity electric‑dipole spin resonance. This represents a significant step toward dense, CMOS‑compatible spin‑qubit arrays and opens a pathway for industrial‑scale quantum‑processor fabrication.