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arXiv:2004.00736 · Superconductor Science and Technology

Characterization of sputtered hafnium thin films for high quality factor microwave kinetic inductance detectors

Grégoire Coiffard, M. Daal, Nicholas Zobrist, Noah J. Swimmer, Sarah Steiger, B. Bumble, Benjamin A. Mazin

Published Apr 27, 2020• 18 citations
arXivHTMLPDF

Abstract

Abstract Hafnium is an elemental superconductor which crystallizes in a hexagonal close packed structure, has a transition temperature T C ≃ 400 mK, and has a high normal state resistivity around 90 µ ω cm. In Microwave Kinetic Inductance Detectors (MKIDs), these properties are advantageous since they allow for creating detectors sensitive to optical and near infra-red radiation. In this work, we study how sputter conditions and especially the power applied to the target during the deposition, affect the hafnium T C , resistivity, stress, texture and preferred crystal orientation. We find that the position of the target with respect to the substrate strongly affects the orientation of the crystallites in the films and the internal quality factor, Q i , of MKIDs fabricated from the films. In particular, we demonstrate that a DC magnetron sputter deposition at a normal angle of incidence, low pressure, and low plasma power promotes the growth of compressive (002)-oriented films and that such films can be used to make high quality factor MKIDs with Q i up to 600,000.

DOI
10.1088/1361-6668/ab8d99

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Abstract Hafnium is an elemental superconductor which crystallizes in a hexagonal close packed structure, has a transition temperature T C ≃ 400 mK, and has a high normal state resistivity around 90 µ ω cm. In Microwave Kinetic Inductance Detectors (MKIDs), these properties are advantageous since they allow for creating detectors sensitive to optical and near infra-red radiation. In this work, we study how sputter conditions and especially the power applied to the target during the deposition, affect the hafnium T C , resistivity, stress, texture and preferred crystal orientation. We find that the position of the target with respect to the substrate strongly affects the orientation of the crystallites in the films and the internal quality factor, Q i , of MKIDs fabricated from the films. In particular, we demonstrate that a DC magnetron sputter deposition at a normal angle of incidence, low pressure, and low plasma power promotes the growth of compressive (002)-oriented films and that such films can be used to make high quality factor MKIDs with Q i up to 600,000.
We report on the design and performance of microwave kinetic inductance detectors (MKIDs) sensitive to single photons in the optical to near-infrared range using hafnium as the sensor material. Our test device had a superconducting transition temperature of 395 mK and a room temperature normal state resistivity of 97 μΩ cm with RRR=1.6. Resonators on the device displayed internal quality factors of around 200 000. Similar to the analysis of MKIDs made from other highly resistive superconductors, we find that modeling the temperature response of the detector requires an extra broadening parameter in the superconducting density of states. Finally, we show that this material and design is compatible with a full-array fabrication process, which resulted in pixels with decay times of about 40 μs and resolving powers of ∼9 at 800 nm.
Hafnium (Hf), as one of the few currently available elemental metals with low critical temperature (Tc ≤ 1 K) and high kinetic inductance (Ls ≥ 10 pH/□) among superconducting materials, retained potential for the development of detectors which were sensitive to near-infrared photons, and possessed high quality factor (Qi). In this paper, Hf thin film were prepared using magnetron sputtering. To characterize the low-temperature properties, a 1/4 wavelength coplanar waveguide resonator based on Hf thin films was fabricated. Detailed experimental tests and studies were conducted on the surface characteristics of the thin films, as well as the correlation and dynamics among the resonator's resonant frequency, power, and temperature. Based on the test results, the Tc of the Hf thin films prepared was 356 mK, with Ls of 20.5 pH/□ and Qi reaching 2.6×103.
We describe measurements of the properties, at dc, gigahertz, and terahertz frequencies, of thin (10 nm) aluminum films with 10 Omega/square normal state sheet resistance. Such films can be applied to construct microwave kinetic inductance detector arrays for submillimeter and far-infrared astronomical applications in which incident power excites quasiparticles directly in a superconducting resonator that is configured to present a matched-impedance to the high frequency radiation being detected. For films 10 nm thick, we report normal state sheet resistance, resistance-temperature curves for the superconducting transition, quality factor and kinetic inductance fraction for microwave resonators made from patterned films, and terahertz measurements of sheet impedance measured with a Fourier Transform Spectrometer. We compare properties with similar resonators made from niobium 600 nm thick.
High energy gap Nb-based superconducting alloys with low normal state resistivity are fundamentally important for realization of low-loss high frequency circuits operating above the typical ~670 GHz gap frequency of elemental Nb. NbTiN has been shown to have a superconducting energy gap nearly twice that of elemental Nb, and is emerging as an important material for various detector and superconducting computing technologies. In this paper, we report on the growth of high-quality face-centered cubic (FCC) NbTiN films through the use of an alternative materials deposition technology-reactive bias target ion beam deposition. This novel technique offers more degrees of freedom compared to conventional sputtering, notably through decoupling the plasma generation and target bias, through use of a hollow-cathode ion source and independently pulse biased sputtering targets. The reported films are of high quality, with increased transition temperatures, reduced normal state resistivity, and surface roughness, compared to our previously reported reactive magnetron sputtered NbTiN films. In particular, at UVA, we are investigating these NbTiN films for wide-bandgap all-NbTiN superconducting-insulating-superconducting mixers, kinetic inductance detectors, and traveling-wave kinetic inductance parametric amplifier devices.
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.
Abstract We investigate thin films of conducting aluminium-oxide, also known as granular aluminium, as a material for superconducting high quality, high kinetic inductance circuits. The films are deposited by an optimised reactive DC magnetron sputter process and characterised using microwave measurement techniques at milli-Kelvin temperatures. We show that, by precise control of the reactive sputter conditions, a high room temperature sheet resistance and therefore high kinetic inductance at low temperatures can be obtained. For a coplanar waveguide resonator with 1.5 kΩ sheet resistance and a kinetic inductance fraction close to unity, we measure a quality factor in the order of 700 000 at 20 mK. Furthermore, we observe a sheet resistance reduction by gentle heat treatment in air. This behaviour is exploited to study the kinetic inductance change using the microwave response of a coplanar wave guide resonator. We find the correlation between the kinetic inductance and the sheet resistance to be in good agreement with theoretical expectations.