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arXiv:2011.09446 · Journal of Applied Physics

Superconducting titanium nitride films grown by directional reactive evaporation

Raymond Mencia, Yen‐Hsiang Lin, Vladimir Manucharyan

Published Dec 13, 2021• 4 citations
arXivHTMLPDF

Abstract

We report a novel method of growing strongly disordered superconducting titanium nitride (TiN) thin films by reactive electron-beam deposition. The normal state sheet resistance and superconducting critical temperature (Tc) can be tuned by controlling the deposition pressure in the range of 1.1×10−6–3.1×10−5mbar. For 10nm thick films, the sheet resistance (R□) reaches 1361Ω/□ and Tc=0.77K, which translates into an estimate for the sheet inductance as large as L□=2.4nH/□. Benefiting from the directionality of reactive evaporation, we fabricated RF test devices with micrometer-sized dimensions using a resist mask and a lift-off process, which would be difficult with sputtering or atomic layer deposition methods. The spectroscopic measurements result in consistent sheet inductance values in two different device geometries, and the quality factors ranged from Q = 300 to 2200. The loss is possibly due to the presence of titanium oxynitride (TiNxOy) in the morphological composition of our films. The flexibility of the lift-off process suggests applications of reactively evaporated TiN for making supporting structures around quantum circuits, such as readout resonators or compact on-chip filters.

DOI
10.1063/5.0048819

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Superconductor Science and Technology•Sep 18, 2024•4 citations
We report a novel method of growing strongly disordered superconducting titanium nitride (TiN) thin films by reactive electron-beam deposition. The normal state sheet resistance and superconducting critical temperature (Tc) can be tuned by controlling the deposition pressure in the range of 1.1×10−6–3.1×10−5mbar. For 10nm thick films, the sheet resistance (R□) reaches 1361Ω/□ and Tc=0.77K, which translates into an estimate for the sheet inductance as large as L□=2.4nH/□. Benefiting from the directionality of reactive evaporation, we fabricated RF test devices with micrometer-sized dimensions using a resist mask and a lift-off process, which would be difficult with sputtering or atomic layer deposition methods. The spectroscopic measurements result in consistent sheet inductance values in two different device geometries, and the quality factors ranged from Q = 300 to 2200. The loss is possibly due to the presence of titanium oxynitride (TiNxOy) in the morphological composition of our films. The flexibility of the lift-off process suggests applications of reactively evaporated TiN for making supporting structures around quantum circuits, such as readout resonators or compact on-chip filters.
We have grown superconducting TiN films by atomic layer deposition with thicknesses ranging from 6 to 89 nm. This deposition method allows us to tune the resistivity and critical temperature by controlling the film thickness. The microwave properties are measured, using a coplanar-waveguide resonator, and we find internal quality factors above a million, high sheet inductances (5.2-620 pH), and pulse response times up to 100 μs. The high normal state resistivity of the films (>; 100 μΩ · cm) affects the superconducting state and thereby the electrodynamic response. The microwave response is modeled using a quasiparticle density of states modified with an effective pair-breaker, consistently describing the measured temperature dependence of the quality factor and the resonant frequency.
We report on the fabrication and characterization of superconducting coplanar waveguide (CPW) resonators based on titanium nitride (TiN) thin films deposited on 200\,mm diameter high-resistivity Si(100) substrates. We systematically investigate how deposition conditions, dry-etch power and in-situ resist strip temperature affect morphology, superconducting properties and dielectric losses. By tuning reactive sputtering conditions, three distinct preferred out-of-plane crystal orientations - (111), (200), and a mix of both are achieved. Our results demonstrate that all films exhibit similar minimal two-level system (TLS) losses, with TiN111 exhibiting the lowest median TLS losses δ~TLS\tildeδ_\mathrm{TLS}δ~TLS​, and greater robustness against reoxidation. The applied structuring process, in contrast, has a far greater influence on the TLS loss than the crystal orientation of the TiN film and, consequently, the intrinsic material properties of the superconducting layer. The lowest TLS losses for all TiN depositions were achieved with a low power etch and low temperature resist strip. An additional buffered oxide etch (BOE) treatment could remove high-loss interfacial oxides at the metal-air (MA) and substrate-air (SA) interface and recover the etch-induced TLS losses. Consequently, TiN resonators exhibiting δ~TLS\tildeδ_\mathrm{TLS}δ~TLS​ values as low as 9.67×10−79.67 \times 10^{-7}9.67×10−7 were realized. The corresponding median low-power loss, δ~LP\tildeδ_\mathrm{LP}δ~LP​, amounts to 11.04×10−711.04 \times 10^{-7}11.04×10−7, which translates to an internal quality factor approaching one million. These findings highlight the critical role of process induced oxide formation at the MA and SA interfaces in limiting the performance of TiN resonators and provide a scalable, low-loss process compatible with industry-grade 200\,mm CMOS qubit fabrication workflows.
This is a study of the growth of titanium-nitride (TiN) by plasma assisted molecular beam epitaxy (MBE) for applications in low loss quantum circuits. Titanium nitride is a material known to have low loss at microwave frequencies which sees practical use in many emerging quantum device architectures. The goal of this research is to investigate improvements in TiN thin films utilizing MBE to produce highly pure, highly crystalline films. A review of the theory of superconducting qubits is given and recent developments in superconducting qubit research are summarized to elucidate how loss has been reduced historically and the importance of materials improvements to the future of scalable quantum computation. The dynamics of epitaxial film growth are discussed, focusing on the formation of a minimum energy surface, the thermally driven kinetics of atoms deposited on the surface, and how these lead to the formation of large scale crystalline domains in the film. An overview of the MBE growth chamber is given and the growth procedure for TiN is discussed. Transport and morphology measurement results are discussed for MBE TiN where, for optimized growth conditions, films are highly crystalline with superconducting transition temperatures(Tc) as high as Tc = 6.1K, matching the value for bulk crystals and exceeding all reported thin film values. Microwave loss measurements of transmission line resonators fabricated from these films are shown to have low-power quality factors(Q) as high as 200,000 and an increase to Q = 400,000 at high power, consistent with the saturation of two-level-systems (TLS) at high power. These results confirm a connection between the high quality crystalline materials produced in this work and low loss in quantum circuits.
Titanium nitride (TiNx) films are ideal for use in superconducting microresonator detectors for the following reasons: (a) the critical temperature varies with composition (0<Tc<5 K); (b) the normal-state resistivity is large, ρn∼100 μΩ cm, facilitating efficient photon absorption and providing a large kinetic inductance and detector responsivity; and (c) TiN films are very hard and mechanically robust. Resonators using reactively sputtered TiN films show remarkably low loss (Qi>107) and have noise properties similar to resonators made using other materials, while the quasiparticle lifetimes are reasonably long, 10–200 μs. TiN microresonators should therefore reach sensitivities well below 10−19 W Hz−1/2.
Superconducting thin films with high intrinsic kinetic inductance are of great importance for photon detectors, achieving strong coupling in hybrid systems, and protected qubits. We report on the performance of titanium nitride resonators, patterned on thin films (9–110 nm) grown by atomic layer deposition, with sheet inductances of up to 234 pH/□. For films thicker than 14 nm, quality factors measured in the quantum regime range from 0.2 to 1.0 × 106 and are likely limited by dielectric two-level systems. Additionally, we show characteristic impedances up to 28 kΩ, with no significant degradation of the internal quality factor as the impedance increases. These high impedances correspond to an increased single photon coupling strength of 24 times compared to a 50 Ω resonator, transformative for hybrid quantum systems and quantum sensing.
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.