Polycrystalline α-tantalum (110) films deposited on the c-plane sapphire substrate by sputtering are used in superconducting qubits nowadays. However, these films always occasionally form other structures, such as α-tantalum (111) grains and β-tantalum grains. To improve the film quality, we investigate the growth of α-tantalum (110) films on the a-plane sapphire substrate under varying conditions by molecular beam epitaxy technology. The optimized α-tantalum (110) film is a single crystal, with a smooth surface and atomically flat metal–substrate interface. The film with thickness of 30 nm shows a Tc of 4.12 K and a high residual resistance ratio of 9.53. The quarter wavelength coplanar waveguide resonators fabricated with the 150 nm optimized α-tantalum (110) film exhibit intrinsic quality factor of over one million under single photon excitation at millikelvin temperature.
Polycrystalline α-tantalum (110) films deposited on the c-plane sapphire substrate by sputtering are used in superconducting qubits nowadays. However, these films always occasionally form other structures, such as α-tantalum (111) grains and β-tantalum grains. To improve the film quality, we investigate the growth of α-tantalum (110) films on the a-plane sapphire substrate under varying conditions by molecular beam epitaxy technology. The optimized α-tantalum (110) film is a single crystal, with a smooth surface and atomically flat metal–substrate interface. The film with thickness of 30 nm shows a Tc of 4.12 K and a high residual resistance ratio of 9.53. The quarter wavelength coplanar waveguide resonators fabricated with the 150 nm optimized α-tantalum (110) film exhibit intrinsic quality factor of over one million under single photon excitation at millikelvin temperature.
Abstract Realization of practical superconducting quantum computing requires many qubits of long coherence time. Compared to the commonly used Ta deposited on c-plane sapphire, which occasionally form α -Ta (111) grains and β -tantalum grains, high quality Ta (110) film can grow epitaxial on a-plane sapphire because of the atomic relationships at the interface. Well-ordered α - Ta (110) film on wafer-scale a-plane sapphire has been prepared. The film exhibits high residual resistance ratio. Transmon qubits fabricated using these film shows relaxation times exceeding 150 μ s. The results suggest Ta film on a-plane sapphire is a promising choice for long coherence time qubit on wafer scale.
Tantalum films incorporated into superconducting circuits have exhibited low surface losses, resulting in long-lived qubit states. The remaining loss pathways originate in microscopic defects that manifest as two level systems (TLSs) at low temperatures. These defects limit performance, so careful attention to tantalum film structures is critical for optimal use in quantum devices. In this work, we investigate the growth of tantalum using magnetron sputtering on sapphire, Si, and photoresist substrates. In the case of sapphire, we present procedures for the growth of fully-oriented films with α-Ta [1 1 1]//Al2O3 [0 0 0 1] and α-Ta [1 −1 0]//Al2O3 [1 0 −1 0] orientational relationships and having residual resistivity ratio (RRR) ∼ 60 for 220 nm thick films. On Si, we find a complex grain texturing with Ta [1 1 0] normal to the substrate and RRR ∼ 30. We further demonstrate airbridge fabrication using Nb to nucleate α-Ta on photoresist surfaces. For the films on sapphire, resonators show TLS-limited quality factors of 1.3 ± 0.3 × 106 at 10 mK (for a waveguide gap and conductor width of 3 and 6 μm, respectively). Structural characterization using scanning electron microscopy, x-ray diffraction, low temperature transport, secondary ion mass spectrometry, and transmission electron microscopy reveal the dependence of residual impurities and screw dislocation density on processing conditions. The results provide practical insights into the fabrication of advanced superconducting devices including qubit arrays and guide future works on crystallographically deterministic qubit fabrication.
Superconducting qubits are a promising platform for large-scale quantum computing. Besides the Josephson junction, most parts of a superconducting qubit are made of a planar, patterned superconducting thin films. In the past, most qubit architectures have relied on niobium (Nb) as the material of choice for the superconducting layer. However, there is also a variety of alternative materials with potentially less losses, which may thereby result in increased qubit performance. One such material is tantalum (Ta), for which high-performance qubit components have already been demonstrated. In this study, we report the sputter-deposition of Ta thin films directly on heated and unheated silicon (Si) substrates as well as onto different, nanometer-thin seed layers from tantalum nitride (TaN), titanium nitride (TiN) or aluminum nitride (AIN) that were deposited first. The thin films are characterized in terms of surface morphology, crystal structure, phase composition, critical temperature, residual resistance ratio (RRR) and RF-performance. We obtain thin films indicative of pure alpha-Ta for high temperature (600°C) sputtering directly on silicon and for Ta deposited on TaN or TiN seed layers. Coplanar waveguide (CPW) resonator measurements show that the Ta deposited directly on the heated silicon substrate performs best with internal quality factors Qi reaching 1xl06in the single-photon regime, measured at T=100 mK.
Alpha-tantalum on silicon is a promising platform for high-coherence superconducting quantum circuits. However, the growth mechanism of alpha-tantalum on silicon remains poorly understood. We present a comprehensive study on alpha-tantalum film growth on various substrates. The decisive role of a substrate material Debye temperature on the phase selection mechanism in tantalum film growth is experimentally confirmed, contradicting the prior assumptions on substrate temperature influence. Crucially, we confirm that alpha-tantalum starts growing only after a 7–10- nm-thick beta-tantalum sublayer. It results in ranging the critical temperature of α-Ta films from 3.77 to 4.39 K for the total thickness from 20 to 150 nm, respectively. Finally, we compared high-quality Al and Ta coplanar resonators on silicon, demonstrating compact tantalum resonators (4/10.5/4 μm) with an internal quality factor exceeding 10 × 106 at single-photon excitation powers.
We acquire tantalum thin film in its α phase (α-Ta) using direct-current magnetron sputtering. According to x-ray diffraction results, 110-Ta is dominant. Quarter-wavelength coplanar waveguide resonators are fabricated with the α-Ta film and characterized at millikelvin in a dilution refrigerator. In the single photon regime, an intrinsic quality factor (Qi) up to 3×106 is obtained in these resonators. At high power, Qi rises to 6×106. Moreover, we also fabricate an array with 7 × 7 lumped element resonators using the α-Ta film. The array shows excellent uniformity. At high power, Qis of all pixels exceed 1×106. In the single photon regime, Qis of over 90% pixels exceed 1×106. Superconducting quantum computing and ultrasensitive electromagnetic wave detectors will benefit a lot from devices based on the α-Ta film.
Quantum coherence in superconducting circuits has increased steadily over the last decades as a result of a growing understanding of the various loss mechanisms. Recently, tantalum (Ta) emerged as a promising material to address microscopic sources of loss found on niobium (Nb) or aluminum (Al) surfaces. However, the effects of film and interface microstructure on low-temperature microwave loss are still not well understood. Here we present a systematic study of the structural and electrical properties of Ta and Nb films sputtered on c-plane sapphire at varying growth temperatures. As growth temperature is increased, our results show that the onset of epitaxial growth of alpha-phase Ta correlates with lower Ta surface roughness, higher critical temperature, and higher residual resistivity ratio, but surprisingly also correlates with a significant increase in loss at microwave frequency. Notably, this high level of loss is not observed in Nb films prepared in the same way and having very similar structure. By experimentally controlling the surface on which the Ta film is nucleated, we determine that the source of loss is only present in samples having an epitaxial Ta/sapphire interface and show that it is apparently mitigated by either growing a thin, epitaxial Nb inter-layer between the Ta film and the substrate or by intentionally treating, and effectively damaging, the sapphire surface with an in-situ argon plasma before Ta growth. In addition to elucidating this interfacial microwave loss, this work provides adequate process details to aid reproducible growth of low-loss Ta films across fabrication facilities.