A METHOD OF EVALUATING MULTILAYER FILMS FOR SRFAPPLICATIONS ∗

D. Bowring† , University of Virginia, Charlottesville, VA, U.S.A.L. Phillips, JLAB, Newport News, VA, U.S.A.

Abstract

The lower critical magnetic field of an SRF cavity maybe increased by screening the interior fields from the cav-ity bulk. This screening could be accomplished by coatingthe interior of a cavity with alternating layers of insulatorand superconductor, each of which is thinner than the Lon-don penetration depth of the superconductor. This idea hasbeen proposed by Gurevich [A. Gurevich, Appl. Phys. Lett.88, 012511 (2006)]. We have developed a method for mea-suring the behavior of such a multilayer system. A super-conducting disk resonator is deposited on top of a smallmultilayer sample. The small sample approach allows forflexibility in the evaluation of many different materials andconfigurations. The onset of magnetic flux penetration inthe superconductor can be observed from the change in res-onator Q and the detection of flux flow voltages. Since thedisk resonator applies a known field to the multilayer sys-tem, the lower critical magnetic field of the system may bemeasured. This paper presents design analysis for an ex-perimental program for multilayer film evaluation.

INTRODUCTION

Gurevich [1] has proposed a method for raising the ef-fective lower critical magnetic field Hc1 of a superconduct-ing radio frequency (SRF) cavity by means of a multilayerthin film coating on the inner surface. The coating consistsof alternating layers of insulating and and superconduct-ing films, where the superconducting layer thickness is lessthan the London penetration depth. In principle, this typeof coating on an elliptical cavity could increase the peakmagnetic field (and therefore the accelerating gradient) pastthe ∼180 mT limit for bulk Nb. This paper presents theexperimental design for an evaluation of the multilayer ap-proach.

An overview of [1] is presented in this section. We treatthe example of a multilayer structure composed of layersof Nb-Ti and aluminum oxide. The superconducting layersscreen the cavity fields from the bulk, as shown in Figure 1.The field decays exponentially across the superconductingfilm, establishing a current imbalance between the top andbottom and skewing the free energy per unit length G/L

∗Work supported by the United States Department of Energy and Jef-ferson Science Associates. Poster supported by a Student Travel Grantfrom SRF 2009.

† dbowring@jlab.org

0 100 200 300 400 500 600 700 800 9000

0.2

0.4

0.6

0.8

1

1.2

H/H

0

Multilayer film depth (nm)

Figure 1: As in [1], the normalized field H/H0 as a func-tion of sample depth for alternating layers of Nb-Ti andAl2O3. λ ∼ 250 nm and the layer thickness d ∼ 0.75λ.

for flux vortex motion [1, 2]:

G/L =φ2

0

4πμ0λ2ln

[d

1.07ξcos(

πu

d)]− φ0

∫ d/2

u

J(z)dz.

The first term represents the kinetic energy of a movingvortex, and the second term is the Lorentz force contribu-tion from the net current across the film. u is the depth ofthe vortex in the superconducting layer, ξ is the coherencelength (≈ 0.7 nm for Nb-Ti), and we assume a penetrationdepth λ ∼ 250 nm [2]. This skewing of the free energy isshown in Figure 2.

Figure 2: As in [1], the normalized vortex free energyG/G0 as a function of displacement u from the center ofa superconducting Nb-Ti film. The red line represents thebarrier to flux motion arising from a net current across thefilm.

As the current work is an evaluation of the multilayer

Proceedings of SRF2009, Berlin, Germany TUPPO043

04 Measurement techniques

309

approach, we use Nb-Ti thin films for their relatively lowvalue of Hc1. The objective is to quantify the performanceof the multilayer approach by exceeding Hc1 ≈ 50 −100 Oe in small multilayer samples, rather than to achievefields comparable to those in elliptical high-gradient cavi-ties. This paper presents the experimental design for sucha proof-of-principle study.

EXPERIMENTAL DESIGN

Any RF structure designed to evaluate multilayer filmsmust satisfy several basic requirements. First, the appliedfield must be parallel to the sample surface. Hc1 for athin film is significantly higher for parallel than for obliquefields [3]. Parallel magnetic fields also mimic the field dis-tribution for the TM011 accelerating mode of a typical el-liptical cavity.

In addition, the onset of flux penetration in the sam-ple should be the only factor influencing resonator per-formance. We therefore require the magnetic field to bestrongest on the sample surface, ensuring that any dissipa-tion from RF vortex motion occurs within the multilayerstructure. Equivalently, the sample must have a lower Hc1

than any other part of the experimental apparatus. Samplegeometry should be flat, with minimal surface area to facil-itate film quality control and analysis. And finally, multi-pacting should be minimized so that the onset of flux pen-etration and subsequent vortex motion are relatively strongcontributors to RF losses. The issue of vortex dissipationwill be discussed in more detail below.

To satisfy the above requirements, we have designeda microstrip disk resonator that mounts on top of inter-changeable flat multilayer samples.

Disk Resonator

The disk operates in the TM01 mode, such that the mag-netic field is everywhere parallel to the sample. Figure 3shows the magnetic fields from a simulation of this configu-ration for a 4 cm diameter disk operating in the TM01 modeat 2.9 GHz. Note that the magnetic field vanishes at thedisk edge, as do the corresponding electric currents. Thisminimizes radiation losses from edge effects [4]. Since thedisk is flat, field emission is minimized. Also, since thedisk is immersed directly in liquid helium with no cavityenclosure, multipacting is minimized. The disk is madefrom high-RRR bulk niobium, so that it can reliably pro-duce magnetic fields well above the lower critical field ofthe Nb-Ti multilayers while itself remaining free of fluxpenetration.

Sample Design

Samples are 8 cm square and of varying thickness, de-pending on the penetration depth of the superconductorused. For Nb-Ti (λ ∼ 250 nm) the film and dielectric lay-ers are each 190 nm thick. To obtain a value of Hc1 which

Figure 3: CST Microwave Studio finite element simulationof the magnetic field from a microstrip disk resonator. Thefield is entirely tangential to the disk surface, with no ra-dial or longitudinal components. Stripline for capacitiveRF coupling is seen on either side of the disk.

is significantly lower than that for the bulk Nb disk res-onators, these films are sputtered rather than using the en-ergetic deposition methods available at Jefferson Lab [6].At least initially, film surface roughness and texture are oflittle concern.

Each multilayer sample is, via masking, deposited simul-taneously with a control sample on the same substrate, asshown schematically in Figure 4. The control sample is asingle thin layer of Nb-Ti, identical to that of the multilayersample, but lacking the bulk Nb ground plane. This effec-tively forms a traditional disk resonator, as in [5]. Deposit-ing both multilayer and control samples simultaneously inthe same chamber eliminates variations in film quality andthickness, allowing for a direct comparison of, for example,Q vs. E curves.

Figure 4: Schematic of the experimental apparatus. Mul-tilayer and “control” samples are deposited simultaneouslyon the same substrate.

RF Coupling

To allow the simultaneous evaluation of multilayer andcontrol samples, two high-RRR bulk Nb disk resonatorsare positioned side-by-side on a movable Teflon sheet, as

TUPPO043 Proceedings of SRF2009, Berlin, Germany

04 Measurement techniques

310

in Figure 4. RF power is capacitively coupled to the res-onators via Nb stripline [7]. Variable coupling is achievedby mounting movable Nb plates above the gap between res-onator and stripline, creating a variable gap capacitance.The Teflon sheet is used to help match the relatively largeedge impedance of the disk resonator to the 50Ω input andoutput couplers. In this arrangement, RF power can be sup-plied to one or the other disk resonator without removingthe samples from the liquid helium or disturbing the cou-pling.

The disks, Nb stripline, and Teflon sheet together makeup a movable module. Fixed SMA-type RF connectorsare used to couple power to the resonators. Using thesame fixed resonator and coupler scheme ensures that fieldstrength and coupling constants do not vary between sam-ples.

MEASUREMENTS

The RF breakdown field is characterized by entrant fluxvortices in the thin superconducting layers. The onset offlux penetration in these layers can be measured via tworelated phenomena: flux flow voltage signals propagatingin the thin dielectric layer, and a change in the Q of theresonator.

Flux ow oltage

A transport current density �J on the thin film surfacegenerates a Lorentz force per unit vortex length �F/L =�J×�φ0, where �φ0 is the flux quantum. If the Lorentz force islarger than the vortex pinning force, the fluxoid will exhibitviscous flow across the film surface, perpendicular to �J .This flux motion, in turn, induces a dissipative electric fieldparallel to �J . Essentially, the motion of flux lines across thefilm creates a propagating voltage signal in the dielectric,which may be measured at the sample edge [3, 8].

Resonator Q

Because vortex motion is dissipative, flux flow voltagesignals can be directly correlated with a drop in resonatorQ. Below Hc1 we calculate Q0 ≈ 6× 107. After the onsetof flux penetration, we can use the thermal feedback modelfor vortex dissipation to calculate the extent of Q degrada-tion [1, 2]. This approach requires knowledge of the tem-perature dependence of the thin film surface impedance,as well as the penetration depth. Therefore, measurementsmust be made for each film. The new surface impedancecharacterization (SIC) facility [9] at Jefferson Lab will beuseful in making such measurements.

CURRENT AND FUTURE WORK

Experimental design is complete and construction of theapparatus is underway. We are currently assembling thecryostat insert and the RF couplers. In parallel with this,we have begun sample preparation. Note that the Nb-Ti

films discussed here are a preliminary effort. In principle,any superconductor of any thickness less than λ may beused with this system. We intend to evaluate as many su-perconducting materials as possible.

Calculations of loaded Q and RF breakdown field de-pend heavily on the thin film penetration depth, coherencelength, and thickness. Very rough a priori estimates arepossible, but accurate predictions of these values requireTEM and SIC measurements of prepared samples. Fur-thermore, analytic calculations of the RF breakdown field[1] use a vortex image approach that assumes a microscop-ically flat film surface. We would like to model the effectof film roughness on the multilayer breakdown field.

ACKNOWLEDGEMENTS

The authors would like to thank Jean Delayen and TomGoodman for their helpful discussions.

REFERENCES

[1] A. Gurevich, “Enhancement of rf breakdown field of su-perconductors by multilayer coating”. Appl. Phys. Lett. 88,012511 (2006).

[2] G. Stejic et al. “Effect of geometry on the critical current ofthin films.” Phys. Rev. B 49, 2 (1994).

[3] M. Tinkham, “Introduction to Superconductivity, 2nd Ed.”Dover Publications, Inc. Mineola, NY. 1996.

[4] A.P. Jenkins et al. “Microstrip Disk Resonators For FiltersFabricated From TBCCO Thin FIlms.” IEEE Trans. App. Su-percond. 7, 2 (1997).

[5] A.G. Derneryd, “Analysis of the Microstrip Disk Antenna El-ement.” IEEE. Trans. Ant. Prop. AP-27, 5 (1979).

[6] A.-M. Valente, et al, “Niobium thin film cavity deposition byECR plasma.” Proc. EPAC 2004, Lucerne, Switzerland.

[7] P. Benedek, P. Sylvester, “Equivalent Capacitances for Mi-crostrip Gaps and Steps.” IEEE Trans. Microwave TheoryTech. 20, 11 (1972).

[8] Y.B. Kim, C.F. Hempstead, A.R. Strnad. “Flux-Flow Re-sistance in Type-II Superconductors.” Phys. Rev. 139, 4A(1965).

[9] B. Xiao et al. “RF Surface Impedance Measurement of Poly-crystalline and Large Grain Nb Disk Sample at 7.5 GHz.”Proc. SRF 2009, Berlin, Germany.

Fl V

Proceedings of SRF2009, Berlin, Germany TUPPO043

04 Measurement techniques

311