Invited paper reports 3-step characterization chain and resulting methodologies and metrics that accelerate learning, provide data on device performance expectations at cryogenic temperatures
HONOLULU, Hawaii — June 16, 2022 — Silicon spin qubits represent one of the most promising avenues to achieving practical quantum computing, and new research into room-temperature parametric test procedures supports both their strong performance potential and the opportunities to ease their transition into manufacturing by leveraging well-characterized processes and materials from the semiconductor sector.
In an invited paper, “Specificities of FDSOI QD Arrays Integration and Characterization”, presented at the 2022 IEEE VLSI Symposium on Technology & Circuits, researchers from CEA-Leti, Université Grenoble Alpes, CNRS Institut Néel, and CEA-Irig shared a new three-step characterization chain for linear silicon quantum dot (QD) arrays fabricated on fully depleted silicon-on-insulator (FDSOI) material. The team also offered several proposals for methodologies and metrics that can accelerate learning cycles at 300K (equivalent to 80 degrees F or 27 degrees C), while generating statistical data on expected device performance at cryogenic temperatures.
The three-step approach maximizes the effectiveness of higher-temperature testing as part of the broader push towards making silicon QD devices manufacturable, by enabling developers to detect and analyze issues at the earliest and simplest point. “It’s a powerful step towards industrialization,” noted Dr. Maud Vinet, quantum computing program director at CEA-Leti.
The initial wafer-level room temperature characterization step, which uses transistor-like testing protocols to gather data in a matter of hours, is followed by a more time-consuming wafer-level QD characterization step at less than 2 degrees K, and a die-level qubit manipulation step (which can take days per device) at under 100mK.
The research team used the process to assess several considerations related to production of integrated QD arrays and make recommendations for addressing them.
One such consideration relates to the proposed use of linear arrays of floating-gate QDs to operate in a manner similar to a single-gate standard transistor. The researchers found that inner gates in these arrays offer consistent state-of-the-art performance on threshold voltage (Vth) and subthreshold slope (SS), but outer gates exhibit more variability. The paper proposes dealing with these edge effect peculiarities (which can be caused by factors like random dopant fluctuation) by using the outer gates as access gates rather than for confinement of QDs.
Additionally, the paper notes that while the split-gate design being explored for linear QD arrays offers several functional advantages, its successful implementation will require very strict overlay control on one specific lithography step to achieve good symmetry, which is needed for consistent performance.
A third recommendation is focused on the issue of spurious dots within the qubit layer — a major source of yield loss in silicon QD arrays. Spurious dots can be detected during cryogenic testing, but revealing the inter-gate defectivity that causes them earlier in the characterization chain (such as during 300K tests) would greatly accelerate the learning cycle. Although standard transistor parametric tests are unsuitable for the task, the researchers developed a 300K voltage-sweep technique capable of monitoring the screening effect inter-gate defectivity has on exchange gate polarization.
One key strength of the FDSOI material used in the group’s efforts, said Vinet, is the fact that back gates can be used to draw charges away from the interfaces. Back gates are typically fabricated using dopant implantation, which has the potential to introduce defects or parasitic dopants in the qubit layer. An alternative fabrication approach, using a TSV-like metallic back gate electrode, would be a way to alleviate this drawback while also enabling back-biasing, she added.
“These research results represent a significant step towards addressing the broader silicon spin qubit integration challenges we discussed at last December’s IEDM conference,” said Heimanu Niebojewski, CEA-Leti lead device engineer. “It’s a very encouraging sign of the technology’s maturation.”
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CEA has a key role in transferring scientific knowledge and innovation from research to industry. This high-level technological research is carried out in particular in electronic and integrated systems, from microscale to nanoscale. It has a wide range of industrial applications in the fields of transport, health, safety and telecommunications, contributing to the creation of high-quality and competitive products.
Leti, a technology research institute at CEA, is a global leader in miniaturization technologies enabling smart, energy-efficient and secure solutions for industry. Founded in 1967, CEA-Leti pioneers micro-& nanotechnologies, tailoring differentiating applicative solutions for global companies, SMEs and startups. CEA-Leti tackles critical challenges in healthcare, energy and digital migration. From sensors to data processing and computing solutions, CEA-Leti’s multidisciplinary teams deliver solid expertise, leveraging world-class pre-industrialization facilities. With a staff of more than 1,900, a portfolio of 3,100 patents, 11,000 sq. meters of cleanroom space and a clear IP policy, the institute is based in Grenoble, France, and has offices in Silicon Valley and Tokyo. CEA-Leti has launched 65 startups and is a member of the Carnot Institutes network. Follow us on www.leti-cea.com and @CEA_Leti.
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