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This post was last edited by Ains on 2026-3-2 15:05. With the rapid development of superconducting quantum information systems, tantalum (Ta), thanks to its excellent superconducting properties, has become a key candidate material for low-loss superconducting circuits and superconducting qubits with long coherence times. Recently, the research team has achieved the low-temperature growth of tantalum thin films using the AINS low-temperature tantalum diffusion technique, successfully overcoming the limitations of traditional growth methods and opening up new pathways for improving the performance of superconducting devices and their application in various scenarios. 1. Tantalum: A \"promising candidate\" in the field of superconductivity. The reason why tantalum stands out in this field lies in its unique physical and chemical properties: • Excellent superconducting properties: The α-Ta phase with a body-centered cubic structure (the stable phase) has a superconducting transition temperature (Tc) of 4.5 K, which is much higher than that of the metastable β-Ta phase, which is 0.6–1 K. This high Tc value is crucial for developing high-performance superconducting devices ； • Exceptional chemical stability: Allows surface contaminants to be removed through aggressive chemical cleaning during device manufacturing, thereby preventing energy loss caused by impurities ； • High-quality natural oxides: Their native oxide properties are superior to those of common superconducting materials such as niobium (Nb), aluminum (Al), and titanium nitride (TiN), enabling effective reduction of interfacial losses. Thanks to these advantages, the coherence time of tantalum-based qubits has exceeded 0.5 milliseconds, making them an important material foundation for advancing the practical application of quantum computing. 2. The “bottlenecks” of traditional growth processes: Despite its excellent properties, the traditional method of growing tantalum faces numerous limitations: • During deposition at room temperature, tantalum thin films tend to form the metastable phase β-Ta, which results in a significant reduction in their superconducting properties ； • To obtain α-Ta, the substrate must be heated to above 500°C, which causes interfacial reactions between the substrate such as silicon (Si) and tantalum, resulting in the formation of tantalum silicides and severely affecting the coherence time of the qubit ； • Although growth can be assisted by nucleation layers such as niobium (Nb) and titanium (Ti), additional layers may trigger new interfacial reactions and superconductivity suppression, increasing the risk of energy loss. These issues severely limit the use of tantalum in multi-substrate systems, especially as it is not possible to make full use of established silicon-based technologies for the large-scale integration of superconducting devices. 3. AINS low-temperature tantalum diffusion technology: An innovative solution that overcomes existing limitations. The AINS low-temperature tantalum diffusion technology proposed by the research team relies on precise temperature control and vacuum environment management to enable the orderly deposition and diffusion of tantalum atoms at low temperatures, thereby eliminating the constraints of traditional processes. Key details of the preparation process: 1. A specialized low-temperature molecular beam epitaxy (MBE) reaction chamber is used, equipped with a liquid nitrogen shroud that allows the basic vacuum level to be maintained at 2×10⁻¹¹ mbar; during the deposition process, the vacuum level in the main chamber is kept below 1×10⁻¹⁰ mbar, thus preventing impurities from interfering with atomic diffusion ； 2. Precise control of the substrate temperature is achieved through a low-temperature control panel; the substrate is actively cooled to below 20 K throughout the deposition process (down to as low as 7 K). Silicon diodes are used to monitor the temperature near the substrate in real time, and calibration is carried out using dummy block thermocouples to ensure that the actual substrate temperature deviation does not exceed 10 K, thereby minimizing excessive atomic migration and interfacial reactions ； 3. The titanium source is evaporated through a separate evaporation chamber and precisely introduced into the reaction chamber via a gate valve. A quartz crystal microbalance is used to monitor the growth rate and film thickness in real time, ensuring that the nominal thickness of all films is accurately controlled at 50 nm ； 4. Without the need for an additional nucleation layer, the tantalum atoms spontaneously form a stable α-Ta phase through slow atomic diffusion and deposition in a low-temperature environment, followed by recrystallization upon heating, thereby enabling compatible integration with various substrates. Using this technique, single-phase α-Ta can be stably grown on various substrates (Al₂O₃(0001), Si(001), Si(111), SiNₓ, GaAs(001)). After heating the thin films to room temperature, X-ray θ-2θ diffraction confirmed that they are polycrystalline α-Ta, with no residual β-Ta phase. Key performance breakthrough 1. Electrical properties comparable to those of bulk materials: 50 nm α-Ta films grown on Al₂O₃(0001) substrates at 7 K using the AINS low-temperature tantalum diffusion technique • The resistivity at room temperature (ρ₃₀₀K) is only 13.4 µΩ·cm, which is close to the 13 µΩ·cm value of bulk α-Ta ； • The residual resistivity ratio (RRR) is 17.3, indicating extremely low carrier scattering ； • The superconducting transition temperature (Tc) is 4.14 K, which is comparable to that of bulk materials. Comparison of the structure and electrical properties of tantalum thin films at different growth temperatures (Figure 1): Figure 1: (a) Comparison of X-ray diffraction patterns ； (b) Temperature dependence curve of resistivity ； (c) Normalized resistance near the superconducting transition temperature. 2. The surface roughness was significantly reduced, as shown by AFM tests (Figure 2 in the original text). The root-mean-square (RMS) roughness of the films grown at 7 K using the AINS low-temperature tantalum diffusion technique was only 0.45 nm, which is much lower than the 1.57 nm for films grown at room temperature. A smoother surface helps to reduce interfacial scattering and energy loss, thanks to the advantages of uniform atomic diffusion and denser deposition in a low-temperature environment. Surface morphology of tantalum thin films (Figure 2): Figure 2: (a) Films grown at 292 K (room temperature) ； (b) Films grown by AINS low-temperature tantalum diffusion technique at 7 K. 3. Excellent multi-substrate compatibility: α-Ta films grown on various substrates such as Si(111), Si(001), GaAs(001), and SiNₓ using the AINS low-temperature tantalum diffusion technique all exhibited a sharp superconducting transition, with Tc values ranging from 4.06 to 4.21 K and RRR values between 9.2 and 16.3. X-ray diffraction also revealed typical α-Ta characteristic peaks (Figure 3), indicating that the growth process governed by this technique is not restricted by the crystal structure or orientation of the substrate, and that atomic diffusion and phase formation proceed in an autonomous manner. Electrical and structural properties of tantalum thin films on different substrates (Figure 3 in the original text): Figure 3: (a)–(d) show the normalized resistances of the thin films on Si(111), Si(001), GaAs(001), and SiNₓ substrates, respectively ； (e) X-ray diffraction patterns of thin films on different substrates. 4. Microwave performance verification: Potential for low-loss applications. To assess the practicality of the thin films prepared using AINS low-temperature tantalum diffusion technology in superconducting circuits, the research team converted the tantalum thin films grown at 7 K into coplanar waveguide (CPW) resonators with a gap width of 3 µm, and conducted microwave tests at low temperatures (35 mK). • Process optimization: After patterning 10 mm×10 mm chips using direct-write lithography, the tantalum thin films were etched using Cl₂/BCl₃ inductively coupled plasma (ICP); simultaneously, the silicon substrate samples were etched to create grooves about 100 nm deep, thereby reducing the electric field influence at the substrate surface and the interface ； After 2 minutes of cleaning via 6:1 buffered oxide etch (BOE), rinsing with deionized water, and drying with nitrogen, the aluminum wire-bonded package is formed into a 2-port device ； • Test results: The low-power internal quality factor (Qi,LP) of the resonator on the Si(111) substrate reached 1.9 million, with a loss tangent (δLP) of only 5.3×10⁻⁷ ； The resonator Qi,LP on the Al₂O₃(0001) substrate is 729,000, and δLP is 1.4×10⁻⁶ ； • Performance profile: It reaches the level of the current state-of-the-art superconducting devices, demonstrating that the polycrystalline structure does not have a negative impact on microwave performance when this technology is employed, thereby overturning the traditional belief that epitaxial growth is a necessary condition for high-performance superconducting circuits. Resonator microwave test results (Figures 4, 5): Figure 4: (a) Optical microscope image of the CPW resonator ； (b)–(d) S₂₁ transmission data and fitting results. Figure 5: (a) Variation of Qi with the average number of photons in the cavity ； (b) The average, maximum, and minimum values of Qi,LP on different substrates. 5. Technical principles and predictions regarding future applications: The tantalum film prepared using the AINS low-temperature tantalum diffusion technique forms a metastable amorphous structure during the low-temperature deposition stage due to low atomic mobility; as the temperature rises, the latent heat released drives recrystallization, resulting in the formation of a thermodynamically stable polycrystalline α-Ta phase. This process does not rely on epitaxial guidance from the substrate, thus enabling uniform growth on various substrates and avoiding the formation of the β-Ta phase. The application of this technological breakthrough not only resolves the challenges associated with the growth of tantalum-based superconducting devices but also expands their scope of application: • Scaling up quantum computing: High-performance tantalum thin films can be grown on silicon substrates, enabling the large-scale integration of qubits through established silicon-based manufacturing processes ； • Topological quantum computing: Successful growth of α-Ta on III-V group semiconductors such as GaAs(001) provides a new approach for superconductor-semiconductor heterostructures (the core of topological quantum computing) ； • Multifunctional superconducting devices: Suitable for various applications that require low-loss superconducting materials, such as superconducting microwave detectors and superconducting sensors. 6. Conclusion The tantalum thin films grown using the AINS low-temperature tantalum diffusion technique, with their single-phase α-Ta structure, excellent electrical and microwave properties, and multi-substrate compatibility, make them an ideal material for superconducting circuits. By employing precise vacuum control, low-temperature regulation, and atomic diffusion management, this technology overcomes the limitations of traditional growth processes. Its fabrication process does not require a nucleation layer, it exhibits wide substrate compatibility, and the resulting thin films have excellent quality; the polycrystalline α-Ta thin films produced using this method can achieve low-loss superconducting properties even in the absence of an epitaxial structure. With further technological improvements, tantalum-based superconducting devices are expected to achieve breakthroughs in both performance and scalability, thereby accelerating the practical application of superconducting technology in more fields. Statement: This article was first published on the WeChat official account; the original title is “Tantalum thin films grown at low temperatures: a new option for low-loss superconducting circuits”. The intellectual property rights related to the content of this article belong to this service account or its owners. Without the authorization of this service or its owners, no one is allowed to use such content without permission (including but not limited to copying, distributing, displaying, mirroring, uploading, downloading, reproducing, or summarizing it), nor may they grant others the right to use such intellectual property. 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