Sofie Beyne

We demonstrate high-fidelity single qubit control in a natural Si-MOS quantum dot fabricated in an industrial 300 mm wafer process on a silicon on insulator wafer using electron spin resonance. A relatively high optimal Rabi frequency of 5 MHz is achieved, decoupling the electron spin from its 29Si environment. Tracking the qubit frequency reduces the impact of low-frequency noise in the qubit frequency and improves the TRabi from 7 to 11 μs at a Rabi frequency of 5 MHz, resulting in Q-factors exceeding 50. Randomized benchmarking returns an average single gate control fidelity of 99.5±0.3%. As a result of pulse-area calibration, this fidelity is limited by the Rabi Q-factor. These results show that a fast Rabi frequency and a feedback protocol enable high fidelity in these Si-MOS devices, despite the low-frequency magnetic noise.

Silicon spin qubits in gate-defined quantum dots leverage established semiconductor infrastructure and offer a scalable path toward transformative quantum technologies. Holes spins in silicon offer compact all-electrical control, whilst retaining the salient features of a quantum dot qubit architecture. However, silicon hole spin qubits are not as advanced as electrons, due to increased susceptibility to disorder and more complex spin physics. Here we demonstrate single-qubit gate fidelities up to 99.8% and a two-qubit gate quality factor of 240, indicating a physical fidelity limit of 99.7%. These results represent the highest performance reported in natural silicon to date, made possible by fast qubit control, exchange pulsing, and industrial-grade fabrication. Notably, we achieve these results in a near-identical device as used for highly reproducible, high-fidelity electron spin qubits. With isotopic purification and further device-level optimisations, silicon hole spin qubits are poised to unlock a new operation regime for quantum CMOS architectures.

A model explaining Lorentzian low-frequency noise spectra observed in electronic interconnects is presented. The model is based on the interaction of electrons with vacancies, whose thermodynamic equilibrium properties determine the temperature dependence of the noise. The model, therefore, allows calculating vacancy formation and migration enthalpies. It is experimentally verified on copper and tungsten interconnects; the sum of formation and migration enthalpies matches the electromigration activation energy obtained using standard accelerated tests.

The electromigration (EM) activation energy (EA) of alternative metals, such as Ru and Co, was obtained using low-frequency noise (LFN) measurements. High activation energies were expected, but values of ≈1 eV are found, most likely related to diffusion along with the metal-dielectric interface. Wafer-level accelerated EM tests were carried out to compare the LFN EA to the EM EA in the Ru wires. The calculation of the EM EA is found to be strongly dependent on the assumed temperature profile in the wire due to Joule heating (JH). The temperature profile was calculated analytically, assuming the contacts are at ambient temperature. For a void forming in direct proximity of the contact, the EM EA then matches the LFN EA. For a void at average wire temperature (ambient + JH), EA≈ 2 eV. In addition to demonstrating the application of LFN to study EM in alternative metals, this article also cautions for the impact of JH on the calculation of EA in interconnects.

A new electromigration (EM) test method, based on low-frequency noise (LFN) measurements is demonstrated and validated for advanced nano-interconnects, conformable to the 10 nm semiconductor technology nodes and beyond. For the first time, the methodology is applied to electromigration test structures where different metal layers are connected by vias. Firstly, it is established that LFN can be used to calculate electromigration activation energies in via-containing nano-interconnects. This finding is verified by comparing the activation energy obtained by LFN with the one calculated using standard accelerated EM tests on a wide variety of interconnect types. The large correlation between both values (≥90%) indicates that LFN can indeed be applied to successfully determine EM activation energies. Secondly, LFN is shown to provide an indication of electromigration damage in nano-interconnects, which allows linking the noise magnitude with a defect concentration. The first two findings are then combined to provide qualitative predictions of electromigration lifetimes. Two models are proposed: one for void nucleation (the time to form a critical void that will thereafter start growing) and one for void growth (the time between the formation of the critical void and failure, i.e. when the void spans the entire line thickness). Also in this case very strong correlations between LFN measurements and EM experiments were found (≥90% for single metal lines and ≥80% for via-containing structures). The main advantages of using LFN measurements over standard tests for electromigration characterization are that they are non-destructive (no electromigration damage is induced during the test), much faster (a 300 mm wafer can be characterized in a matter of days, as opposed to the weeks it can take using standard EM tests), and that they can be done at test conditions closer to actual operation of the interconnects, such that inducing different mechanisms due to over-stress is avoided.

1.8-MeV proton irradiation to a fluence of 10¹⁴/cm² does not significantly affect the resistance or low-frequency noise of copper or ruthenium resistors fabricated via modern microelectronic fabrication techniques used to form metal lines. The room-temperature noise of these Cu and Ru resistors is surprisingly similar to that of Cu and Pt metal lines and wires fabricated using late-1970s nanofabrication techniques; however, measurements of the temperature dependence of the noise show that the defect kinetics are quite different among the various materials. A large increase in the noise magnitude observed above 200 K in Cu but not in Ru is consistent with the superior resistance to electromigration that Ru lines have shown, relative to Cu.