Willinger and Colleagues Fabricates Tantalum Films for High-Coherence Superconducting Qubits

Moritz Singer and colleagues at Technical University of Munich show that the performance of tantalum superconducting waveguide resonators is sharply affected by strain and structural defects at the interface between the metal film and its substrate. They fabricated 200nm thick tantalum films on various seed layers and silicon substrates, achieving internal quality factors of up to 1.5 million at 100 mK. A key link between microstrain, interfacial disorder, and resonator quality is now clear, highlighting the vital role of interface engineering to improve the performance of superconducting quantum circuits.

Microstrain reduction unlocks record quality factors in tantalum thin films

Internal quality factors of up to 1.5 million were achieved in tantalum films, representing a substantial leap beyond previously attainable levels. Historically, limitations in minimising defects within superconducting materials have presented a significant obstacle to the fabrication of resonators with such high Q-factors. These high Q-factors are crucial because they directly relate to the coherence time of superconducting qubits, the fundamental building blocks of quantum computers. Longer coherence times allow for more complex and reliable quantum computations. The team at Technical University of Munich demonstrated that strain and structural imperfections at the interface between the film and its substrate heavily influence the radio frequency performance of alpha-tantalum thin films, even when bulk material properties remain consistent. This finding is particularly important as it suggests that optimising performance doesn’t necessarily require altering the tantalum itself, but rather refining the conditions under which it is deposited and interfaces with other materials.

Williamson-Hall analysis and cross-sectional HR-TEM revealed a direct correlation between reduced microstrain and improved quality factor, confirming the importance of precise interface control for optimising superconducting thin films used in quantum computing. The Williamson-Hall method, a refinement of X-ray diffraction analysis, allows researchers to separate the contributions of crystallite size and microstrain to peak broadening in diffraction patterns. By analysing these broadened peaks, the team could quantify the level of strain within the tantalum films. Cross-sectional high-resolution transmission electron microscopy (HR-TEM) provided direct, atomic-scale imaging of the interface, revealing the presence of dislocations and other defects contributing to the observed strain. Films deposited at temperatures exceeding 500°C, and utilising niobium, titanium nitride, or tantalum nitride as seed layers, consistently formed the desired alpha-tantalum crystalline phase. Alpha-tantalum is favoured due to its superior superconducting properties compared to other tantalum allotropes. Residual resistance ratios were also characterised to assess film quality, providing a comprehensive evaluation of material properties. The residual resistance ratio, calculated as the resistance at 4.2K divided by the resistance at 300K, serves as an indicator of the film’s purity and the density of scattering centres that impede electron flow.

Direct visualisation of disorder at the interfaces between the tantalum film and the substrate was achieved using cross-sectional high-resolution transmission electron microscopy, further supporting the role of strain. The HR-TEM images revealed that lattice mismatch between the tantalum film and the silicon substrate, or the seed layer, introduced significant strain. This strain manifested as dislocations and other defects, which act as energy dissipation centres, reducing the Q-factor. Varying radio frequency performance was exhibited by films on different seed layers, highlighting the importance of interface control and suggesting that seed layer composition influences the degree of microstrain. The choice of seed layer appears to affect the nucleation and growth of the tantalum film, influencing the density and type of interfacial defects. Thorough assessment of film quality was also conducted via characterisation of residual resistance ratios, confirming the correlation between low defect density and high superconducting performance.

Seed layer influence dictates radio frequency superconducting performance

Tantalum films are emerging as a strong contender for materials capable of maintaining delicate superconducting states, a necessity for viable quantum computers. Superconducting qubits rely on the ability of electrons to flow without resistance, but this state is easily disrupted by environmental noise and material imperfections. Tantalum’s relatively high critical temperature and potential for low loss make it an attractive material for building these qubits. However, researchers at Technical University of Munich discovered that achieving the correct crystal structure, alpha-tantalum, is not sufficient to guarantee optimal performance. A hidden complexity was revealed by significant variations in radio frequency performance arising from the underlying seed layer used during deposition. The seed layer acts as a template for the growth of the tantalum film, influencing its crystalline structure and interfacial properties.

Achieving high internal quality factors, reaching 1.5 million in tantalum films, hinges on controlling the material’s structure at its interface with the substrate, as demonstrated by the team at Technical University of Munich. This represents a significant improvement over previous results, which typically achieved Q-factors in the tens or hundreds of thousands. The deposition process involved sputtering 200nm thick tantalum films onto high-resistivity silicon (100) substrates, with varying deposition temperatures ranging from 20°C to 600°C. Despite similar bulk properties of the resulting tantalum, subtle variations in the seed layer used during deposition sharply alter radio frequency performance. The use of different seed layers, niobium, titanium nitride, and tantalum nitride, allowed the researchers to systematically investigate the impact of interfacial properties on the superconducting performance. The technique of Williamson-Hall analysis revealed a direct link between reduced microstrain and improved resonator quality by determining the amount of distortion within a material’s crystal structure. This analysis confirmed that minimising strain at the interface is critical for achieving high Q-factors.

The implications of this research extend beyond simply improving the performance of tantalum-based qubits. The findings highlight the general importance of interface engineering in superconducting materials’ science. By carefully controlling the composition and structure of the interface between a superconducting film and its substrate, it may be possible to significantly enhance the performance of a wide range of superconducting devices. Future work could focus on exploring novel seed layer materials and deposition techniques to further reduce microstrain and improve the coherence times of superconducting qubits, paving the way for more powerful and reliable quantum computers.

The team demonstrated that strain and structural imperfections at the interface between the film and its substrate heavily influence the radio frequency performance of alpha-tantalum thin films, even when bulk material properties remain consistent. This finding is particularly important as it suggests that optimising performance doesn’t necessarily require altering the tantalum itself, but rather refining the conditions under which it is deposited and interfaces with other materials. The deposition process involved sputtering 200nm thick tantalum films onto high-resistivity silicon (100) substrates, with varying deposition temperatures ranging from 20°C to 600°C. Furthermore, the use of different seed layers, niobium, titanium nitride, and tantalum nitride, allowed the researchers to systematically investigate the impact of interfacial properties on the superconducting performance. This represents a significant improvement over previous results, which typically achieved Q-factors in the tens or hundreds of thousands.

Researchers demonstrated that optimising the interface between tantalum films and their substrates is crucial for achieving high-performance superconducting quantum circuits. They found a correlation between decreasing microstrain and increasing internal quality factors, reaching up to 1.5 million at 100 mK. This suggests that refining deposition conditions and seed layers, such as niobium, titanium nitride, and tantalum nitride, is more impactful than altering the tantalum material itself. The authors intend to explore novel seed layer materials and deposition techniques to further reduce microstrain and improve qubit coherence times.