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arXiv:2109.06491 · Applied Physics Letters

Loss mechanisms in TiN high impedance superconducting microwave circuits

Kazi Rafsanjani Amin, Carine Ladner, Guillaume Jourdan, Sébastien Hentz, Nicolas Roch, Julien Renard

Published Apr 18, 2022• 32 citations
arXivHTMLPDF

Abstract

Aluminum-based platforms have allowed to reach major milestones for superconducting quantum circuits. For the next generation of devices, materials that are able to maintain low microwave losses while providing new functionalities, such as large kinetic inductance or compatibility with CMOS platform, are needed. Here, we report on a combined direct current and microwave investigation of titanium nitride films of different thicknesses grown using CMOS compatible methods. For microwave resonators made of 3 nm thick TiN, we measured large kinetic inductance LK ∼240 pH/sq, high mode impedance of ∼4.2 kΩ while maintaining microwave quality factor ∼105 in the single photon limit. We present an in-depth study of the microwave loss mechanisms in these devices that indicates the importance of quasiparticles and provide insight for further improvement.

DOI
10.1063/5.0086019

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Aluminum-based platforms have allowed to reach major milestones for superconducting quantum circuits. For the next generation of devices, materials that are able to maintain low microwave losses while providing new functionalities, such as large kinetic inductance or compatibility with CMOS platform, are needed. Here, we report on a combined direct current and microwave investigation of titanium nitride films of different thicknesses grown using CMOS compatible methods. For microwave resonators made of 3 nm thick TiN, we measured large kinetic inductance LK ∼240 pH/sq, high mode impedance of ∼4.2 kΩ while maintaining microwave quality factor ∼105 in the single photon limit. We present an in-depth study of the microwave loss mechanisms in these devices that indicates the importance of quasiparticles and provide insight for further improvement.
Abstract Low microwave loss and regulatable kinetic inductance are two key characteristics of superconducting films to manufacture high-performance quantum devices. Epitaxial titanium nitride (TiN) thin films have been found to exhibit significant advantages in these two aspects. In this paper, we investigated into the dielectric loss and kinetic inductance of epitaxial TiN films, which were prepared on both silicon and sapphire substrates with different temperatures. Crystal structure, surface morphology, and superconducting properties were characterized systematically. The radio-frequency response of microwave resonators made of TiN films was studied at low temperatures to extract the information of kinetic inductance and quality factor. We obtained the highest internal quality factor up to 1.6 × 10 6 at single photon regime and achieved a regulation of kinetic inductance exceeding 16 times by altering the substrate and the sputtering temperature of the TiN films. This work not only provides an essential foundation for the precise design and preparation of low-loss TiN resonators for superconducting quantum bits, but also demonstrates TiN film as an excellent material platform with both low microwave loss and high kinetic inductance that can be applied in other fields such as microwave kinetic inductance detectors.
Superconducting high kinetic inductance elements constitute a valuable resource for quantum circuit design and millimeter-wave detection. Granular aluminum (grAl) in the superconducting regime is a particularly interesting material since it has already shown a kinetic inductance in the range of nH/□ and its deposition is compatible with conventional Al/AlOx/Al Josephson junction fabrication. We characterize microwave resonators fabricated from grAl with a room temperature resistivity of 4×10^{3} μΩ cm, which is a factor of 3 below the superconductor to insulator transition, showing a kinetic inductance fraction close to unity. The measured internal quality factors are on the order of Q_{i}=10^{5} in the single photon regime, and we demonstrate that nonequilibrium quasiparticles (QPs) constitute the dominant loss mechanism. We extract QP relaxation times in the range of 1 s and we observe QP bursts every ∼20 s. The current level of coherence of grAl resonators makes them attractive for integration in quantum devices, while it also evidences the need to reduce the density of nonequilibrium QPs.
The promise of tantalum for realizing superconducting quantum devices has generated interest in its compound films, particularly nitrides. Among these, cubic-phase tantalum carbonitride (TaCxN1−x) offers reduced susceptibility to oxidation and a high critical temperature, yet its microwave properties remain largely unexplored. Here, we investigate plasma-enhanced atomic layer deposition of cubic-phase TaCxN1−x thin films for superconducting microwave circuits. Structural and transport measurements reveal nanocrystalline morphology with sub-10 nm grains and superconductivity in the dirty limit. Coplanar waveguide resonators exhibit moderately high kinetic inductance (16.8 pH/sq for 30 nm films) with potential for enhancement through dimensional scaling. The films also support high internal quality factors exceeding 105 at 50 mK in the single-photon regime, comparable to granular aluminum. Loss analysis identifies the two-level systems as the dominant limiting mechanism, with potential for further reduction through interface engineering. These results establish atomic layer deposited TaCxN1−x as a promising material for scalable, low-loss, high-inductance superconducting circuits.
Titanium nitride is an attractive material for a range of superconducting quantum-circuit applications owing to its low microwave losses, high surface inductance, and chemical stability. The physical properties and device performance, nevertheless, depend strongly on the quality of the materials. In this paper, we focus on the highly crystalline and epitaxial titanium nitride thin films deposited on sapphire substrates using magnetron sputtering at an intermediate temperature (${300}^{\ensuremath{\circ}}\mathrm{C}$). We performed a set of systematic and comprehensive material characterizations to thoroughly understand the structural, chemical, and transport properties. Microwave losses at low temperatures were studied using patterned microwave resonators, where the best internal quality factor in the single-photon regime is measured to be $3.3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ and that in the high-power regime $>1.0\ifmmode\times\else\texttimes\fi{}{10}^{7}$. Adjusted with the material filling factor of the resonators, the microwave loss-tangent here compares well with the previously reported best values for superconducting resonators. This work lays the foundation of using epitaxial titanium nitride for low-loss superconducting quantum circuits.
Tantalum (Ta) has recently received considerable attention in manufacturing robust superconducting quantum circuits. Ta offers low microwave loss, high kinetic inductance compared to aluminium (Al) and niobium (Nb), and good compatibility with complementary metal-oxide-semiconductor (CMOS) technology, which is essential for quantum computing applications. Here, we demonstrate the fabrication engineering of thickness-dependent high quality factor (high-Q_i) Ta superconducting microwave coplanar waveguide resonators. All films are deposited on high-resistivity silicon substrates at room temperature without additional substrate heating. Before Ta deposition, a niobium (Nb) seed layer is used to ensure a body-centred cubic lattice (α-Ta) formation. We further engineer the kinetic inductance (L_K) resonators by varying Ta film thicknesses. High L_K is a key advantage for applications because it facilitates the realisation of high-impedance, compact quantum circuits with enhanced coupling to qubits. The maximum internal quality factor Q_i of ~ 3.6 * 10^6 is achieved at the high power regime for 100 nm Ta, while the highest kinetic inductance is obtained to be 0.6 pH/sq for the thinnest film, which is 40 nm. This combination of high Q_i and high L_K highlights the potential of Ta microwave circuits for high-fidelity operations of compact quantum circuits.
We present a method to systematically locate and extract capacitive and inductive losses in superconducting resonators at microwave frequencies by use of mixed-material, lumped element devices. In these devices, ultra-low loss titanium nitride was progressively replaced with aluminum in the inter-digitated capacitor and meandered inductor elements. By measuring the power dependent loss at 50 mK as the Al/TiN fraction in each element is increased, we find that at low electric field, i.e., in the single photon limit, the loss is two level system in nature and is correlated with the amount of Al capacitance rather than the Al inductance. In the high electric field limit, the remaining loss is linearly related to the product of the Al area times its inductance and is likely due to quasiparticles generated by stray IR radiation. At elevated temperature, additional loss is correlated with the amount of Al in the inductance, with a power independent TiN-Al interface loss term that exponentially decreases as the temperature is reduced. The TiN-Al interface loss is vanishingly small at the 50 mK base temperature.