Substrate biasing as a tuning knob for superconducting films – the “ALD and ALE for quantum project” celebrates its first year with a publication

In July 2022 we announced the research grant for the ‘Atomic Layer Deposition and Etching for Quantum Computing’ NWO project on AtomicLimits. This project focusing on atomic scale processing for superconducting quantum devices had a flying start in October 2022. One year later we can look back at a productive first year. It was a year in which we gained a lot of insight into connecting the fields of atomic scale processing and quantum technology. We are excited to announce that the first paper of this project has been published as an Editor’s pick in Applied Physics Letters: ‘Ultrathin superconducting TaC_xN_1−x films prepared by plasma-enhanced atomic layer deposition with ion-energy control’ by Peeters et al. This open-access publication is our first step in adapting atomic scale processing techniques developed for nanoelectronics to the quantum technology toolbox: our first nanostep into quantum! The paper explores the preparation of high-quality superconducting films, focusing on the understanding and control needed to tailor these films to quantum technologies. In this blog, we will discuss the takeaways of our paper and announce our blog series ‘our nanosteps into quantum’, where we share our contributions to the quantum technology toolbox and the lessons we learned as a nanoelectronics group entering the field of quantum.

The choice to tailor ALD processes to quantum technology did not come out of the blue: ALD has a great track record in tackling comparable challenges in the field of nanoelectronics. It has been a truly enabling method in recent generations of nanoelectronic devices, pushing the limits on feature size, 3D scaling, and device performance. Similarly, ALD is expected to play a key role in realizing large-scale quantum computing, as reaching this goal requires major advances in materials and fabrication. One of the main priorities is the growth of high-quality superconducting films, where Ta is receiving quite some attention because of the record superconducting quantum bit performance, measured in coherence time, achieved with this material. In the race for the longest coherence time the interfaces are unavoidable hurdles. No matter how you design your qubit, ‘lossy interfaces’ will degrade its performance. For this reason, TaC_xN_1-x, which is more resistant to oxidation than Ta, holds much promise. TaC_xN_1-x can be prepared by plasma-enhanced ALD (PEALD), which is known for the growth of highly-conductive metal-nitride films with control on the atomic level. Even more control is provided by the application of a radiofrequency bias voltage to the substrate during plasma exposure, which can be used to increase the energy of the ions that impact the growing film. This allows us to tune material properties, as elaborated in a recent blog post. The figure below shows various plasma control knobs, which consequently serve as material tuning knobs, that we have at our disposal. While the plasma control knobs of source power, pressure, and substrate bias are only independent in the idealized case, their effects on their corresponding plasma parameters are strong enough to depict them in such as manner. In our current work, we operate at low plasma powers (100 W) and pressures (6 mTorr) to study the effects of highly energetic ions on TaC_xN_1-x films.

In the publication, we demonstrate that ion-energy control in PEALD is not only vital for the growth of highly-conductive, dense material with low impurity contents, but also for achieving superconductivity in ultrathin TaC_xN_1-x films. By optimizing ion energies, we obtain superconducting TaC_xN_1-x films with high critical temperatures (T_c) of superconductivity for film thicknesses down to 7 nm. The effects of the ion-energy tuning knob and the importance of reliable control are illustrated in the figure below.

Let’s summarize the main takeaways from our paper:

• Ultrathin superconducting TaC_xN_1-x films were successfully prepared by PEALD with substrate biasing.
• Ion-energy control through substrate biasing can be used as a tuning knob for material properties, which enables the preparation of highly-conductive metal nitrides.
• Energetic ions enhance conductivity, purity, density, and critical temperature of superconductivity (T_c) in TaC_xN_1-x films prepared by PEALD and in this way enable the growth of superconducting TaC_xN_1-x films down to 7 nm film thickness.
• PEALD with substrate biasing could become an enabling step in tackling quantum materials challenges.

Having demonstrated the high material quality and T_c achievable in ultrathin films through PEALD combined with substrate biasing, the next step is to explore the fabrication of low-loss superconducting quantum devices. Our first avenue is the superconducting resonator, an application which demands a blog post of its own. As has been discussed in our contribution at the AVS 69^th International Symposium & Exhibition, which took place November 5-10 in Portland, we observe substrate biasing works as a tuning knob for superconducting resonator performance too! Hence, we learned that using the technique of substrate biasing and performing room-temperature material characterization can already give strong insight into the expected resonator performance.

Of course, there are many lessons to share, not only on the ins and outs of superconducting resonators and other quantum circuit components, but also on concepts which might seem trivial to seasoned quantum insiders but are completely unknown to the average material scientist. In this way, we hope to provide material scientists looking to venture into the world of quantum with tips and tricks for a flying start. On top of that, we need to bring our tailored toolbox to the attention of the quantum community. To transition to large-scale quantum technologies the worlds of material and quantum scientists must be brought together. The various quantum-focused materials and processing talks at the AVS 69^th International Symposium & Exhibition are a great illustration of this growing effort. So, let’s continue this trend; stay tuned for our first blog in ‘our nanosteps into quantum’ series!