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arXiv:2402.11720 · IEEE Transactions on Applied Superconductivity

Characterization of NbTiN Films With Thicknesses Below 20 nm for Low Power Kinetic Inductance Amplifiers

Andrea Giachero, Michael R. Vissers, Jordan Wheeler, M. Malnou, Jason E. Austermann, Johannes Hubmayr, Angelo Nucciotti, Joel N. Ullom, J. Gao

Published Mar 6, 2023• 15 citations
arXivHTMLPDF

Abstract

A quantum-limited amplification chain is a fundamental advantage for any application that may benefit from the detection of very faint signals. Reading out arrays of superconducting detectors (TESs or MKIDs), resonant cavities, or qubits, calls for large bandwidth amplifiers in addition to having the lowest possible noise. At millikelvin temperatures, Kinetic Inductance Traveling-Wave Parametric Amplifiers (KI-TWPAs) working in 3-wave-mixing (3WM) and fabricated from a 20 nm thick NbTiN film have shown promising noise performances, as they can operate close to the quantum limit (Malnou et al., 2021). However, they still require fairly high pump power. Devices that would require lower pump power would be easier to implement in readout chains, could reach the quantum limit and they would be compatible with qubit readout. A possible solution for obtaining this optimal configuration is to use a thinner superconducting film. In this work we explore the properties of NbTiN films with a thickness less than 20 nm and we report the obtained experimental characterizations in terms of critical temperature, normal resistivity, and kinetic inductance. A new design for a 3WM KI-TWPA amplifier, based on these developed superconducting films, is introduced and discussed.

DOI
10.1109/tasc.2023.3253069

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A quantum-limited amplification chain is a fundamental advantage for any application that may benefit from the detection of very faint signals. Reading out arrays of superconducting detectors (TESs or MKIDs), resonant cavities, or qubits, calls for large bandwidth amplifiers in addition to having the lowest possible noise. At millikelvin temperatures, Kinetic Inductance Traveling-Wave Parametric Amplifiers (KI-TWPAs) working in 3-wave-mixing (3WM) and fabricated from a 20 nm thick NbTiN film have shown promising noise performances, as they can operate close to the quantum limit (Malnou et al., 2021). However, they still require fairly high pump power. Devices that would require lower pump power would be easier to implement in readout chains, could reach the quantum limit and they would be compatible with qubit readout. A possible solution for obtaining this optimal configuration is to use a thinner superconducting film. In this work we explore the properties of NbTiN films with a thickness less than 20 nm and we report the obtained experimental characterizations in terms of critical temperature, normal resistivity, and kinetic inductance. A new design for a 3WM KI-TWPA amplifier, based on these developed superconducting films, is introduced and discussed.
Abstract A wide-bandwidth and low-noise amplification chain in the microwave regime is crucial for the efficient read-out of quantum systems based on superconducting detectors, such as Microwave Kinetic Inductance Detectors (MKIDs), Transition Edge Sensors (TESs), Magnetic Microcalorimeters (MMCs), and RF cavities, as well as qubits. Kinetic Inductance Travelling Wave Parametric Amplifiers (KI-TWPAs) operated in a three-wave mixing fashion have demonstrated exceptional dynamic range and low-noise performance, approaching the quantum limit. These amplifiers can be fabricated using a single layer of a high kinetic inductance film as weakly dispersive artificial transmission lines, with the ability to control the phase-matched bandwidth through dispersion engineering. In this study, we present the optimisation of the rf sputter-deposition process of NbTiN films using a Nb80%Ti20% target, with the goal of achieving precise control over film characteristics, resulting in high kinetic inductance while maintaining a high transition temperature. The parameter landscape related to the different sputtering conditions, such as pressure, power, and nitrogen flow, has been explored and the film thickness has been used as a fine-tuning parameter to adjust the properties of the final NbTiN films used for the fabrication of KI-TWPAs. As a final result, we have obtained a NbTiN film with a kinetic inductance of 8.5 pH/sq which we have exploited to fabricate KI-TWPA prototype devices, showing promising amplification performance.
Kinetic inductance traveling-wave parametric amplifiers (KI-TWPAs) have a wide instantaneous bandwidth with a near quantum-limited noise performance and a relatively high dynamic range. Because of this, they are suitable readout devices for cryogenic detectors and superconducting qubits and have a variety of applications in quantum sensing. This work discusses the design, fabrication, and performance of a KI-TWPA based on four-wave mixing in a NbTiN microstrip transmission line. This device amplifies a signal band from 4 to 8 GHz without contamination from image tones, which are produced in a separate higher frequency band. The 4–8 GHz band is commonly used to read out cryogenic detectors, such as microwave kinetic inductance detectors and Josephson junction-based qubits. We report a measured maximum gain of over 20 dB using four-wave mixing with a 1 dB gain compression point of −58 dBm at 15 dB of gain over that band. The bandwidth and peak gain are tunable by adjusting the pump-tone frequency and power. Using a Y-factor method, we measure an amplifier-added noise of 0.5 ≤ Nadded ≤ 1.5 photons from 4.5 to 8 GHz.
Ultralow-noise microwave amplification and detection play a central role in different applications, going from fundamental physics experiments to the deployment of quantum technologies. In many applications the necessity of reading multiple detectors, or cavities or qubits, calls for large bandwidth amplifiers with the lowest possible noise. Current technologies are based on High Electron Mobility Transistors and Josephson Parametric Amplifiers. Both have limitations, the former in terms of the minimum noise, the latter in terms of bandwidth. Superconducting Traveling Wave Parametric Amplifiers (TWPAs) have the potential of offering quantum limited noise and large bandwidth. These amplifiers are based on the parametric amplification of microwaves traveling along a transmission line with embedded nonlinear elements. We are developing superconducting TWPAs based both on Josephson junction arrays (Traveling Wave Josephson Parametric Amplifiers) and on nonlinear kinetic inductance (Dispersion Engineered Traveling Wave Kinetic Inductance Amplifiers). Our goal is to achieve large bandwidth (in the 5 to 10 GHz range), large gain (more than 20 dB), large saturation power (more than −50 dBm), and near quantum limited noise (noise temperature less than 600 mK). Current achievements in the design and development of the high performance TWPAs are here reported and discussed, together with current limitations and possible future developments.
Noise at the quantum limit over a broad bandwidth is a fundamental requirement for future cryogenic experiments for neutrino mass measurements, dark matter searches and Cosmic Microwave Background (CMB) measurements as well as for fast high-fidelity read-out of superconducting qubits. In the last years, Josephson Parametric Amplifiers (JPA) have demonstrated noise levels close to the quantum limit, but due to their narrow bandwidth, only few detectors or qubits per line can be read out in parallel. An alternative and innovative solution is based on superconducting parametric amplification exploiting the travelling-wave concept. Within the DARTWARS (Detector Array Readout with Travelling Wave AmplifieRS) project, we develop Kinetic Inductance Travelling-Wave Parametric Amplifiers (KI-TWPAs) for low temperature detectors and qubit read-out. KI-TWPAs are typically operated in a threewave mixing (3WM) mode and are characterised by a high gain, a high saturation power, a large amplification bandwidth and nearly quantum limited noise performance. The goal of the DARTWARS project is to optimise the KI-TWPA design, explore new materials, and investigate alternative fabrication processes in order to enhance the overall performance of the amplifier. In this contribution we present the advancements made by the DARTWARS collaboration to produce a working prototype of a KI-TWPA, from the fabrication to the characterisation.
Background Kinetic inductance travelling-wave parametric amplifiers (KITWPAs) have proven effective in achieving high gain over broad bandwidths whilst achieving near quantum-limited noise performance, properties which are extremely important for many ultra-sensitive experiments such as radio/millimetre/sub-millimetre astronomy, neutrino mass experiments, and axion dark matter searches, as well as commercial applications like quantum computational and communication systems. Methods This paper introduces the preliminary experimental exploration of an alternative KITWPA utilising thin titanium nitride (TiN) films as opposed to the conventionally used niobium titanium nitride (NbTiN) films. TiN films exhibit properties that could be advantageous for KITWPA operation, owing to its controllable critical temperature (T c) and physical robustness, hence KITWPAs fabricated from TiN films have the potential to replace NbTiN-based TWPAs for various applications, such as reducing the high pump power required to drive a NbTiN TWPA. Our TiN-based KITWPA devices were designed and fabricated with a 42 cm long coplanar waveguide transmission line patterned into a 100 nm thick TiN film, wound in a double spiral fashion to fit into a 2.2×2.2 cm chip. Results We successfully achieved broadband performance, with the best device displaying a peak averaged gain of ~5 dB with a bandwidth of ~3–13 GHz, making this one of the first broadband gain measurements of a TiN KITWPA reported in the literature. Despite this, the measured gain in all batches was lower than simulated due unexpected additional loss mechanisms. Conclusions The preliminary results presented here suggest that TiN films are suitable for KITWPA operation, although further investigation is required to improve the gain performance.
We present a detailed simulation and design framework for realizing traveling wave parametric amplifiers (TWPAs) using the nonlinear kinetic inductance of disordered superconductors -- in our case niobium-titanium-nitride (NbTiN). These kinetic inductance TWPAs (KITs) operate via three-wave mixing (3WM) to achieve high broadband gain and near-quantum-limited (nQL) noise. Representative fabricated devices -- realized using an inverted microstrip (IMS), dispersion-engineered, artificial transmission line -- demonstrate power gains above 25 dB, bandwidths beyond 3 GHz, and achieve ultimate system noise levels of 1.1 quanta even when operated with no magnetic shielding. These performance metrics are competitive with state-of-the-art Josephson-junction-based TWPAs but involve simpler fabrication and able to providing three orders of magnitude higher dynamic range (IIP1=−68IIP_1 = -68IIP1​=−68 dBm, IIP3=−55IIP_3 = -55IIP3​=−55 dBm), and high magnetic field resilience -- making KITs an attractive technology for highly multiplexed readout of quantum information and superconducting detector systems.