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Jan 17, 2013 - Superconducting tungsten silicide alloys have a tunable. TC with silicon content, a high normal state resistivity, and a high density. In this work ...
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013

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Tungsten Silicide Alloys for Microwave Kinetic Inductance Detectors Orlando Quaranta, Thomas W. Cecil, and Antonino Miceli

Abstract—Microwave kinetic inductance detectors are used to detect photons over a large range of wavelengths from submillimeter to X-ray. The common material requirements for this application are: high internal quality factor (Qi ), high kinetic inductance fraction, long quasiparticle lifetime, and in the case of X-ray photons stopping power (i.e., dense, high atomic number materials). Superconducting tungsten silicide alloys have a tunable TC with silicon content, a high normal state resistivity, and a high density. In this work, we investigate the properties of thin films of tungsten silicide made of two different stoichiometry: WSi2 and W5 Si3 with particular attention to their application to microwave kinetic inductance detectors. We present a study of the structural and transport properties of films deposited under different conditions for both stoichiometry. Quarter wavelength microwave coplanar waveguide resonators have been fabricated from films of both stoichiometry and we present measurements of the microwave properties of these films as well as quasiparticle lifetimes using X-ray photons. Index Terms—Microwave devices, radiation detectors, silicides, superconducting materials, thin film sensors, X-ray detectors.

I. I NTRODUCTION

T

HE INTEREST in microwave kinetic inductance detectors (MKID) has steadily grown in the ten years since the first devices were realized [1]. They have been used in application ranging from radio astronomy [2] to communications wavelengths [3] to x-ray [4] and gamma ray detection [5]. In its simplest form, an MKID is an LC resonator that converts a change in quasiparticle density due to photon absorption into a change in the phase and amplitude of a microwave signal coupled to the resonator. Numerous factors determine the sensitivity of an MKID, including the kinetic inductance fraction, α, the superconductors critical temperature, TC , the single spin density of states, N0 , quasiparticle lifetime, τ , the resonator quality factor, QR , and the resonators volume, V . α and QR serve as scaling factors to amplify the small change in kinetic inductance into a measurable change in phase and amplitude. TC , N0 , and V determine the number of quasiparticles that are generated for a given photon energy, and τ affects the noise

Manuscript received October 8, 2012; accepted November 28, 2012. Date of publication December 12, 2012; date of current version January 17, 2013. The research carried out at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. The authors are with the Argonne National Laboratory, Argonne, IL 60439 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2012.2232963

properties—the longer the excess of quasiparticle is present the more accurately it can be measured. To decrease the noise equivalent power (or increase the energy resolution) an MKID should have a large α, QR , and τ , with a small TC , N0 and V . In 2010 LeDuc et al. [6] introduced MKIDs made of TiN which met all of these characteristics. For our application of x-ray detection, we sought a material with similar qualities, but the addition of a high density to provide a high absorption efficiency to x-ray photons. We began looking at tungsten silicide (W-Si) alloys, which have been used in x-ray optics and superconducting nanowire single photon detectors. The superconducting properties of tungsten silicide alloys were studied in detail by Kondo in the early 1990s [7]. Although pure tungsten offers a high stopping power due to its high density and Z-number, it has a TC of ∼0.4 K making it impractical for MKIDs that typically operate at T /TC < 0.1. Kondo found that adding silicon to the tungsten created a rapid enhancement of the TC with a maximum TC of 5 K at 25% silicon. The enhanced TC remained above 1.9 K (the lower limit of Kondo’s measurement range) for Si concentrations between 5% and 60%. As expected for an alloy superconductor, the resistivity rapidly increased with Si content to ∼100 μΩ · cm, remaining largely constant in the enhanced TC range, and then increasing again with increasing Si content. Similar results were seen with the residual resistance ratio (RRR), which is slightly less than 1 in the enhanced TC range, increased below 5% Si, and decreased above 60% Si. The enhanced TC was found to coincide with a change in the film structure from crystalline to amorphous. Tungsten silicide is a disordered metallic system with a metal-to-insulator transition. The enhancement in the superconductive transition temperature can be explained by the interplay between the electronic screening length and density of states which are altered by the conductivity [8]. The conductivity of tungsten silicide is controlled by the silicon content. Recently W-Si films have been used as superconducting nanowire single photon detectors because of their structural homogeneity [9] and we first reported on its use for kinetic inductance detectors [10]. Here we provide greater detail on the material properties and characterization. II. T HIN F ILM FABRICATION AND C HARACTERIZATION We have examined tungsten silicide alloys of two stoichiometry: W5 Si3 and WSi2 . The films were deposited using DC magnetron sputtering from a single compound target onto both silicon and sapphire wafers. The W5 Si3 alloy with a Si content of 37.5% is expected to show a strongly enhanced TC near

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Fig. 1. (a) XRD plot showing the θ–2θ spectrum of 100 nm W5 Si3 films deposited on Si with argon pressure of 4, 6, 10, 12, and 14 mTorr from bottom to top respectively. The curves have been offset for clarity. (b) XRD plot showing the θ–2θ spectrum of 100 nm W5 Si3 (red) and WSi2 (black) films deposited on R-plane sapphire at 10 mTorr and 8 mTorr, respectively. Other than the sapphire peak the weak and broad features from the W5 Si3 indicate an amorphous film. (c) Transition temperature, TC , of W5 Si3 as a function of argon pressure during sputter deposition. (d) Resistivity of W5 Si3 (blue circle) and WSi2 (red square) as a function of temperature. Both materials show a sharp transition at TC with a TC of 1.8 K for WSi2 and 4 K for W5 Si3 .

5 K and resistivity of ∼100 μΩ · cm, while the TC for the WSi2 with Si content of 66% is expected to fall below 1.9 K with a resistivity of several hundreds of μΩ · cm. We deposited 100 nm thick films with argon pressures ranging from 4 mTorr to 14 mTorr to examine the effect on TC , resistivity, and film structure. The normal state resistivity of the films was measured using a four point probe and the critical temperature was determined from the change in resistance with temperature. Film structure was determined by θ–2θ XRD measurements. Fig. 1(a) shows the XRD spectrum for W5 Si3 films deposited on Si with argon pressures of 4, 6, 10, 12, and 14 mTorr from bottom to top, respectively. The film deposited at 10 mTorr shows the least features indicating the relatively amorphous nature of the film. Similar results were seen with the WSi2 films with the least features seen at 8 mTorr. Fig. 1(b) shows the XRD spectrum of a W5 Si3 and a WSi2 film deposited on R-plane sapphire at 10 mTorr and 8 mTorr respectively. The scans show the peaks from the sapphire wafer and weak and broad feature in the W5 Si3 , as seen in the film on Si. The W5 Si3 film shows a TC of 4 K and a normal state resistivity of ∼ 200 μΩ · cm while the WSi2 film shows a TC of 1.8 K and a resistivity of ∼ 450 μΩ · cm. For both materials the transition is sharp as shown in Fig. 1(d) and the materials have a RRR of slightly less than one, as can be expected from an amorphous material. The

Fig. 2. Kinetic inductance fraction of CPW resonator on sapphire as a function of film thickness calculated using measured TC and normal state resistivities; open symbols are calculated values, filled symbols are measured data, line is a guide to the eye. Upper inset shows the transmission, |S21 | (red circles), past a WSi2 CPW resonator with 100 nm nominal thicknesses, fr = 1.766 GHz and Qi = 7.1 × 105 as determined from Lorentzian fit (blue line). Lower inset shows |S21 | for a 365 nm W5 Si3 CPW resonator with fr = 4.705 GHz and Qi = 1.7 × 105 . Arrows indicate location of resonance on kinetic inductance fraction curve.

TC of the W5 Si3 films shows a strong variation with pressure, increasing from 3 K at a pressure of 4 mTorr to 4 K for films deposited at pressures above 10 mTorr as shown in Fig. 1(d). We have yet to study the TC variation of WSi2 with pressure. The resistivity of both materials was mostly constant over the range of pressures examined. The TC and normal state resistivity of these films allows the calculation of both the surface inductance and penetration depth of the films. According to the Mattis-Bardeen theory the surface inductance for film thicknesses less than the penetration depth can be approximated with [11]   Rs (1) Ls ≈ πΔ while the penetration depth in the local limit is given by   λ= πΔμ0 σ20 K

(2)

where LS is the surface inductance, RS is the surface resistance, 2 Δ is the energy gap determined from TC , σ20 K is the surface conductivity at 20 K in units of μΩ · cm and λ is the penetration depth in nm. LS is an important parameter when determining the kinetic inductance fraction of a resonator. We can calculate the kinetic inductance fraction as a function of thickness for the W-Si alloys from simulations using the planar E&M software Sonnet. As shown in Fig. 2, the kinetic inductance fraction remains high up to film thickness of a micron or more, opening the possibility of direct detection of x-rays in lumped-element MKIDs as we previously reported.

QUARANTA et al.: TUNGSTEN SILICIDE ALLOYS FOR MKIDs

Fig. 3. Fractional frequency shift of 100 nm (square) and 1 μm (circle) thick WSi2 CPW resonators on sapphire indicating a high degree of film uniformity (i.e., TC and α), which we believe is due to the uniform composition of the films. The lines are calculated using the Mattis-Bardeen Theory with TC = 1.75 K, α = 0.92, and γ = 1 for the 100 nm film (blue line) and TC = 1.80 K, α = 0.53, and γ = 1/3 for the 1 μm film (black line).

III. MKIDS OF T UNGSTEN S ILICIDE We have fabricated co-planar waveguide (CPW) superconducting resonators with films ranging from 80 nm to 2 μm in thickness. The upper inset in Fig. 2 shows the transmission, |S21 |, past a WSi2 CPW resonator with 100 nm nominal thicknesses. The resonator has a resonance frequency, fr , of 1.766 GHz with an internal quality factor, Qi , of 7.1 × 105 . The lower inset in Fig. 2 shows |S21 | for a 365 nm W5 Si3 CPW resonator with fr = 4.705 GHz and Qi = 1.7 × 105 . The geometric resonance frequency for these resonators is 6.2 GHz. The resonance frequency for both materials has been significantly shifted down in frequency due to the large kinetic inductance fraction of the alloys. The kinetic inductance fraction can be calculated from experimental data using  2 fm (3) α=1− fg where fm is the measured resonance frequency and fg is the geometric resonance frequency. The 100 nm WSi2 film shows a kinetic inductance fraction of 92% and the 365 nm W5 Si3 has a kinetic inductance factor of 42%. These values are shown with the filled symbols in Fig. 2 and agree well with the simulated values based on LS and TC . These α values are comparable to those for TiN and much higher than those of elementals superconductors such as Al and Nb. Because both the alloys we have chosen to examine are stoichiometric we expect to see a uniform film quality (i.e. TC and α) over the surface of our wafer. Non-uniformity in TC has been an issue for sub-stoichiometric TiN films (TC < 4 K) due to the sensitivity of the TC on nitrogen content [12]. In Fig. 3 we show the fractional frequency shift of nine 100 nm and eight 1 μm thick WSi2 resonators as a function of temperature. The symbols are measured data and the lines are calculated using the Mattis-Bardeen Theory, which depends on the material TC and α, with TC = 1.75 K, α = 0.92 and γ = 1 for the 100 nm film and TC = 1.80 K, α = 0.53 and γ = 1/3 for the 1 μm film. The curves are a good match to the data, although we have yet to investigate deviations from the theory as have been seen

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Fig. 4. Phase pulses from Fe-55 x-ray in WSi2 (dashed line) and W5 Si3 (solid line) CPW resonators on sapphire normalized to the same size to facilitate the comparison. Both resonators show a pulse decay time of < 5 μs, which is near the resonator lifetime, suggesting that the quasiparticle lifetime is actually shorter.

with TiN [13]. The resonators show a high degree of uniformity across the 16 mm chips and from one chip to another. Photon absorption in MKIDs causes a non-equilibrium population of quasiparticles which decays over some period of time (i.e., the quasiparticle lifetime, τqp ). The energy resolution of −1/2 an MKID scales as τqp . The quasiparticle lifetime can be influence by a number of parameters such as TC , with which it should scale inversely with the square. We have measured the quasiparticle lifetime of W5 Si3 and WSi2 by illuminating MKIDs with an Fe-55 x-ray source (the devices tested were used only to examine the quasiparticle lifetime and are not designed for optimal signal to noise ratio). We observed a decay time of < 5 μs for both stoichiometry as shown in Fig. 4, where the pulses where normalized to the same size to facilitate the comparison. This lifetime is on the order of the resonator lifetime, τ = QR /2πfr , and with the lack of variation with TC indicates that the quasiparticle lifetime is less than 5 μs. We suspect that the disordered nature of the film limits the quasiparticle diffusion and self-recombination is strongly enhanced. It may be possible to increase the quasiparticle lifetime by placing the resonators on a SiN membrane, creating a bottleneck to limit phonon loss [14]. We plan to fabricate tungsten silicide MKIDs on SiN membranes to examine this effect. However, it is also conceivable to use MKIDs as an equilibrium x-ray detector where the MKID is used as a thermometer to measure the temperature rise from a thermalized x-ray photon (i.e., an MKID microcalorimeter). In such a detector, the short quasiparticle lifetime should not be a limiting factor. Finally, the frequency noise is a way to compare the noise of resonators with differing quality factors. It can be thought of as the frequency perturbation needed to create the associated phase perturbation. In Fig. 5 we show the frequency noise of CPW resonators of 350 nm films of W5 Si3 , WSi2 , and Al at similar internal resonator powers. The noise in the W-Si alloys is comparable to that of the Al. IV. C ONCLUSION We have studied two alloys of W-Si (W5 Si3 and WSi2 ) for use as kinetic inductance detectors. The alloys have

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[3]

[4]

[5]

[6]

Fig. 5. Frequency noise of 350 nm CPW resonators of Al (solid blue), W5 Si3 (dotted red), and WSi2 (dashed green) on sapphire at an internal power of ∼ −40 dBm.

superconducting transition temperatures well suited to use as MKIDs along with a high normal state resistivity. Resonators fabricated from these alloys have shown large kinetic inductance fractions along with high quality factors and low frequency noise. We plan to further investigate methods to increase the quasiparticle lifetime along with using W-Si MKIDs as microcalorimeters.

[7] [8]

[9]

[10] [11]

ACKNOWLEDGMENT The authors thank L. Gades, C. Liu, S. Miller, B. Fisher, and Y. Feng.

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