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APL Quantum

1/f frequency fluctuations in high kinetic inductance CoSi2 microwave cavities

Weijun Zeng, Ilari Lilja, Ekaterina Mukhanova, Elica Anne Heredia, Chun-Wei Wu, Tosson Elalaily, Juhn‐Jong Lin, Sheng-Shiuan Yeh, Pertti Juhani Hakonen

Published Jun 1, 2025• 1 citation
Publisher

Abstract

Cobalt disilicide provides a promising nearly epitaxial superconducting material on silicon, which is compatible with high-density integrated circuit technology. We have characterized CoSi2 superconducting microwave cavities around 5.5 GHz for resonance frequency fluctuations at temperatures of 10–200 mK. Comparing with materials used in kinetic inductance detectors, we found relatively weak fluctuations (δfr/f0)2 following the spectral density A/fγ, with A ≃ 6 × 10−16 and γ slightly below 1 at an average number of photons of 30; the noise decreased with measurement power as 1/P1/2. The connection of the frequency fluctuations to two-level states and kinetic inductance fluctuations due to resistance noise is discussed.

DOI
10.1063/5.0251425

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jun 14, 2006•9 citations
Cobalt disilicide provides a promising nearly epitaxial superconducting material on silicon, which is compatible with high-density integrated circuit technology. We have characterized CoSi2 superconducting microwave cavities around 5.5 GHz for resonance frequency fluctuations at temperatures of 10–200 mK. Comparing with materials used in kinetic inductance detectors, we found relatively weak fluctuations (δfr/f0)2 following the spectral density A/fγ, with A ≃ 6 × 10−16 and γ slightly below 1 at an average number of photons of 30; the noise decreased with measurement power as 1/P1/2. The connection of the frequency fluctuations to two-level states and kinetic inductance fluctuations due to resistance noise is discussed.
1/f frequency fluctuations due to kinetic inductance in CoSi2_22​ microwave cavities
Cobalt disilicide provides a promising nearly-epitaxial superconducting material on silicon, which is compatible with high-density integrated circuit technology. We have characterized CoSi2_{2}2​ superconducting microwave cavities around 5.5 GHz for resonance frequency fluctuations at temperatures 10 - 200 mK. We found relatively weak fluctuations (δf/f)2(δf/f)^2(δf/f)2 following the spectral density $A/f^γ $, with A≃6×10−16A \simeq 6 \times 10^{-16}A≃6×10−16 and γγγ slightly below 1 at an average number of photons of 10410^4104; the noise decreased with measurement power as 1/P1/21/P^{1/2}1/P1/2. We identify the noise as arising from kinetic inductance fluctuations and discuss possible origins of such fluctuations.
We have looked into cobalt disilicide (CoSi2) as a potential building block for superconducting quantum circuits. In order to achieve this, we annealed a thin layer of Co to create microwave cavities with thickness of d = 10–105 nm from CoSi2 embedded in the silicon substrate. The cavity properties were measured as a function of temperature and power. In the films measuring 10 and 25 nm, we find a significant kinetic inductance LK with a non-BCS power-law variation δLK ∝ T4.3±0.2 at low temperatures. The quality factor of the studied microwave resonances varied from 3 × 103 (d = 10 nm) to ∼5 × 104 (d = 105 nm) and increased as d(A − log d) with thickness, with two-level systems having very little effect. The power dependence of kinetic inductance was analyzed in terms of heat flow due to electron–phonon coupling, which was found to be stronger than estimated for heat relaxation by regular quasiparticles.
As a newly developed pair-breaking superconducting detector, microwave kinetic inductance detectors are simple to integrate in the frequency domain and have already been used in astronomical detection and array imaging at the (sub)millimeter and optical wavelengths. For these applications, the dark noise level of kinetic inductance detector is one of the key performance indicators. Herein a noise power spectrum analysis method is introduced in detail, which can accurately and effectively analyze the noise spectrum of kinetic inductance detector in a wide frequency range. This method can well balance the noise spectrum resolution and variance performance, by taking the noise data at the resonance frequency with two sampling rates and setting the appropriate frequency resolutions for different frequency bands. This method is used to characterize and compare the noise of aluminum (Al) kinetic inductance detectors made from two different micromachining processes. We deposite a 25-nm-thick aluminum film on high-resistivity silicon substrate for one device, while place one silicon nitride (SiNx) film on the top and one on the bottom of the aluminum film for another device. It is found that the frequency noise of the device with two silicon nitride films is about 25% to 50% of the bare aluminum device. Using this double silicon nitride film fabrication technique, we further fabricate a few groups of lumped-element aluminum kinetic inductance detectors with various inductor and interdigitated capacitor designs. We investigate the noise properties of these devices at different microwave driven power and bath temperatures, and the experimental results show typical two-level system noise behaviors. Our work provides a standard method to characterize the noise power spectrum of kinetic inductor detector, and also paves the way to developing low-noise aluminum kinetic inductance detectors for terahertz imaging, photon-counting and energy-resolving applications.
Abstract A microwave kinetic inductance detector (MKID) is a cutting-edge superconducting detector. It comprises a resonator circuit constructed with a superconducting film on a dielectric substrate. To expand its field of application, it is important to establish a method to suppress the two-level system (TLS) noise that is caused by the electric fluctuations between the two energy states at the surface of the substrate. The electric field density can be decreased by expanding the strip width (S) and gap width from the ground plane (W) in the MKID circuit, allowing the suppression of TLS noise. However, this effect has not yet been confirmed for MKIDs made with niobium films on silicon substrates. In this study, we demonstrate its effectiveness for such MKIDs. We expanded the dimension of the circuit from (S, W) = (3.00 μm, 4.00 μm) to (S, W) = (5.00 μm, 23.7 μm), and achieved an increased suppression of 5.5 dB in TLS noise.
We report on the development of microwave kinetic inductance detectors (MKIDs) using platinum silicide as the sensor material. MKIDs are an emerging superconducting detector technology, capable of measuring the arrival times of single photons to better than two microseconds and their energies to around ten percent. Previously, MKIDs have been fabricated using either sub-stoichiometric titanium nitride or aluminum, but TiN suffers from the spatial inhomogeneities in the superconducting critical temperature and Al has a low kinetic inductance fraction, causing low detector sensitivity. To address these issues, we have instead fabricated the PtSi microresonators with the superconducting critical temperatures of 944 ± 12 mK and high internal quality factors (Qi≳106). These devices show typical quasiparticle lifetimes of τqp≈30–40 μs and spectral resolution, R=λ/Δλ, of 8 at 406.6 nm. We compare PtSi MKIDs to those fabricated with TiN and detail the substantial advantages that PtSi MKIDs have to offer.
Excess phase noise has been observed in microwave kinetic inductance detectors (MKIDs) which prevents the noise-equivalent power (NEP) of current detectors from reaching theoretical limits. One characteristic of this excess noise is its dependence on the power of the readout signal: the phase noise decreases as the readout power increases. We investigated this power dependence in a variety of devices, varying the substrate (silicon and sapphire), superconductor (aluminum and niobium) and resonator parameters (resonant frequency, quality factor and resonator geometry). We find that the phase noise has a power law dependence on the readout power, and that the exponent is -1/2 in all our devices. We suggest that this phase noise is caused by coupling between the high-Q microwave resonator that forms the sensitive element of the MKID and two-level systems associated with disorder in the dielectric material of the resonator. The physical situation is analogous to the resonance fluorescence in quantum optics, and we are investigating the application of resonance fluorescence theory to MKID phase noise.