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arXiv:1212.4434 · IEEE Transactions on Applied Superconductivity

Microwave Properties of Superconducting Atomic-Layer Deposited TiN Films

P. C. J. J. Coumou, Marc R. Zuiddam, E. F. C. Driessen, Pieter de Visser, J. J. A. Baselmans, T. M. Klapwijk

Published Dec 27, 2012• 35 citations
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Abstract

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.

DOI
10.1109/tasc.2012.2236603

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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 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 fabricated TiN coplanar waveguides using standard lithography techniques followed by ICP etch. In order to achieve high quality factor, we investigated the film growth by choosing different deposition conditions for various substrates. Quality factors of waveguide resonators were measured at 20 mK in both high and low microwave power limits. An inner quality factor of several million was achieved at high power limit for a predominantly (200)-oriented TiN film which was grown on HF cleaned silicon wafer. A quality factor of larger than one million was achieved at high power limit for TiN film grown on sapphire.
We apply the superconducting proximity effect in TiN/Ti multi-layer films to tune the critical temperature, TC, to within 10 mK with high uniformity (less than 15 mK spread) across a 75 mm wafer. Reproducible TC’s are obtained from 0.8 to 2.5 K. These films had high resistivities, >100μΩ cm, and internal quality factors for resonators in the GHz range, on the order of 100 k and higher. Trilayers of both TiN/Ti/TiN and thicker superlattice films were prepared, demonstrating a well controlled process for films over a wide thickness range. Detectors were fabricated and shown to have single photon resolution at 1550 nm. The high uniformity and controllability coupled with the high quality factor, kinetic inductance, and inertness of TiN make these films ideal for use in frequency multiplexed kinetic inductance detectors and potentially other applications such as nanowire detectors, transition edge sensors, and associated quantum information applications.
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.
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Tantalum thin films sputtered on unheated silicon substrates are characterized with microwaves at around 10 GHz in a 10 mK environment. We show that the phase of tantalum with a body-centered cubic lattice (α-Ta) can be grown selectively by depositing a niobium buffer layer prior to a tantalum film. The physical properties of the films, such as superconducting transition temperature and crystallinity, change markedly with the addition of the buffer layer. Coplanar waveguide resonators based on the composite film exhibit significantly enhanced internal quality factors compared with a film without the buffer layer. The internal quality factor approaches 2 × 107 at a large-photon-number limit. While the quality factor decreases at the single-photon level owing to two-level system (TLS) loss, we have deduced that one of the causes of TLS loss is the amorphous silicon layer at the film–substrate interface, which originates from the substrate cleaning before the film deposition rather than the film itself. The temperature dependence of the internal quality factors shows a marked rise below 200 mK, suggesting the presence of TLS–TLS interactions. The present low-loss tantalum films can be deposited without substrate heating and thus have various potential applications in superconducting quantum electronics.