Niobium sputter-coated copper resonators are successfully employed in operating particle accelerators, the most outstanding example being LEP2 at CERN. In this review we present recent progress in the understanding of the basic principles governing their behaviour, based on an extensive R&D programme carried out at CERN on 1.5 GHz resonators operated in the TM010 mode. At the present stage of the s tudy, no fundamental limitation has been found which would prevent the use of this technology for future high-field, high-Q accelerating cavities

Benvenuti, Cristoforo

Calatroni, Sergio

Campisi, I E

Darriulat, Pierre

Durand, C

Peck, M A

Russo, R

Valente, A M

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsCERN   EST/97-02(SM)NIOBIUM SPUTTER-COATED COPPER RESONATORSC. Benvenuti, S. Calatroni, I. E. Campisi1, P. Darriulat, C. Durand2 , M. Peck,R. Russo, A.M. ValenteAbstractNiobium sputter-coated copper resonators are successfully employed in operatingparticle accelerators, the most outstanding example being LEP2 at CERN. In thisreview we present recent progress in the understanding of the basic principlesgoverning their behaviour, based on an extensive R&D programme carried out atCERN on 1.5 GHz resonators operated in the TM010 mode. At the present stage of thestudy, no fundamental limitation has been found which would prevent the use of thistechnology for future high-field, high-Q accelerating cavities.Paper presented at the Eighth Workshop on RF SuperconductivityAbano, Italy, 6-10 October 1997Geneva, SwitzerlandDecember 1997                                                       1 On leave from TJNAF, Newport News, Virginia, USA2 Present address: INFN, Laboratori Nazionali di Legnaro, I-35020 Legnaro, PD, Italy21. IntroductionSuperconducting accelerating cavities are widely employed today in particleaccelerators, both for their ability to provide beams of a quality not attainable with copperstructures, and for the lower overall operating cost [1]. These cavities are manufacturedessentially from bulk niobium, the most notable exception being the 352 MHz cavities forLEP2 at CERN. In this case, the choice has been made to employ niobium-coated coppercavities, following the successful development of the technology of sputter deposition ofniobium films inside copper resonators [2]. This technology is now widely recognised asbeing a valid and in some cases superior alternative to bulk niobium resonators, and isbeing actively pursued in many research laboratories [3]. As is the case for bulk niobium,however, the niobium on copper technology still suffers from some limitations that preventthe exploitation of the superconducting material up to its theoretical limit.An extensive R&D programme is being carried out at CERN, to better understandthe physical mechanism governing the behaviour of thin niobium films in high-intensity RFfield. The research has been focused onto coating a large number of different 1.5 GHzresonators operated in the TM010 mode, and correlating the basic coating and materialparameters to measured RF superconducting properties.2. Experimental methodThe resonators are constructed of Oxygen-Free Electronic grade (OFE) copper,which provides a substrate with an impurity content in the ppm range. The cavityconstruction technique has a profound effect on the structural properties of the coppersubstrate, which in turn may affect the growth and/or the purity of the niobium film. Theneed to avoid welds in the high field region of the cavity in order to reduce any risk due tothe presence of a molten section of the substrate, suggests the use of seamless cavities.These are manufactured by two different methods, namely hydroforming [4] and spinning[5]. Hydroformed cavities are produced by inflating at high pressure a copper tube inside amould, with three intermediate annealing steps at 600°C. The resulting cavity surface hasa grain size of a few hundred microns, and an average roughness of the order of onemicron. Spun cavities are produced by shaping a 3 mm copper sheet around a specialcollapsible mandrel, which can then be removed from the inside. The copper sheet isannealed at 250°C before the spinning process, but no annealing takes place afterwards.The resulting copper grain size is smaller, and so is the average roughness.The inner surface of the cavities is prepared for coating using the same polishingsolution developed for LEP2 cavities [6]. Other methods such as electropolishing arebeing actively investigated. It has been proven by material analysis that after beingshaped, the inner surface of the cavities is damaged and some copper must be removedto obtain a suitable surface. At least 120 Pm of copper must be removed from spuncavities, whereas 40 Pm are generally sufficient for hydroformed ones.The sputtering niobium cathode is assembled in a class 100 clean room, where it isalways kept when not in use. Once deposited, the niobium layer can be etched away, andthe copper substrate can be used again for a subsequent coating after only a light (10 Pmto 20 Pm) chemical polishing. High pressure water rinsing with ultra high purity water at100 bar is performed before each deposition and before each RF measurement.The coating method follows closely the sputtering procedure developed for LEP2,based on a cylindrical magnetron configuration, and adapted to 1.5 GHz cavities [7], asillustrated in Fig. 1. In a noble gas atmosphere, generally argon, at a pressure of31.5x10-3 mbar, a potential difference is established between the central cathode and thegrounded cavity. The electrical current of the glow discharge is stabilised at 3 A, resultingin a voltage of approximately 360 V depending on the magnetic induction of thepermanent magnet. The sputter-coating takes place usually at 150°C, and the coatingthickness is 1.5 Pm obtained after 15 minutes of treatment. The cut-off tubes are coatedfirst, with slightly modified parameters: 10-2 mbar, 1 A, 320 V.Extensive material analyses of samples cut from a spun cavity coated with thesestandard parameters have been carried out. The films have a Residual Resistivity Ratio(RRR) of 11±1, measured after dissolving the copper substrate in nitric acid. The averagegrain size is 100±50 nm, as measured by TEM in plane views close to the film surface.The argon content, due to the sputtering process, has been measured by thermalextraction to be 400 ppm. X-ray diffraction analysis shows that the film growspreferentially in the <1,1,0> direction with an expanded lattice parameter [8], producing anincrease in Tc.Fig. 1: Schematic drawing of the 1.5 GHz single-cell cavity sputtering system.The kind of noble gas used for sputtering, the temperature of the coating and itsthickness are some of the variables that influence all these quantities, and their variationhas been explored in detail throughout this project. In the following, we will howeverconcentrate mainly on the analysis of the results obtained with 28 standard coatings (i.e.produced with the coating parameters described above).4As a reference, bulk niobium cavities produced either by spinning (courtesy of INFN-LNL) or by deep drawing (courtesy of TJNAF) have also been measured to compare ourresults with a known standard. The nominal RRR of the niobium employed for their fabricationis 300.3. RF characterisationThe RF measurements are performed with a mostly standard RF system, in a liquidhelium cryostat. Fixed antennas are used to couple the RF power in and out the cavity.Temperature scans are performed between 4.2 K and 1.7  K, and the surface resistanceis measured as a function of the RF field amplitude. Since the surface resistance at 4.2 Kis much higher than at 1.7 K, the two measurements give almost uncorrelatedmeasurements of the BCS surface resistance and of the residual resistance. Thetemperature dependence of the various contributions to the resistance are then easilyextracted.A particularity of the present study is the systematic measurement of the surfaceresistance when an external magnetic flux is trapped into the film when crossing thesuperconducting transition. This measurement gives hints about the effect of localiseddefects (acting as pinning centres for the trapped fluxoids) on the surface resistance. Filmcavities always show complete trapping, contrary to bulk cavities where trapping isincomplete in cavities fired at high temperature (~1000°C). The superconducting criticaltemperature can also be measured through the observation of the transition from theMeissner state.The penetration depth can be computed by measuring the frequency shift as afunction of temperature, normalised to the frequency shift of bulk niobium used as a clean-limit reference ("/[0 ~ 20). The penetration depth is in turn linked to the electron mean freepath through the formula O(")=OCLEAN(1+S[0/2")½An approximate evaluation of the lower critical field Hc1 is made by using a smallsuperconducting coil placed in the cryostat at the equator of the cavity, and by measuringthe value of the punch-through field at which the surface resistance starts increasing.Since the field lines are distorted by the presence of the cavity it is only possible toperform relative measurements, and the results must be normalised to the valuemeasured for bulk niobium.4. DiscussionIn order to better understand the physical phenomena governing the surfaceresistance at constant frequency Rs of the superconductors under study, it is convenientto separate it in three additive terms:Rs(HRF, Hext, T) = RBCS(HRF, T) + Rres(HRF) + Rfl(HRF, Hext, T)The first term, the BCS surface resistance, is found to depend both on temperature andRF field amplitude. The second term, the residual resistance, which depends only on thefield amplitude and by definition not on temperature, is to some extent out of control of theexperiment as will be shown below. The last term expresses the dependence on externalmagnetic flux.54.1 BCS resistanceThe predictions by the BCS theory of the temperature and frequency dependence ofthe surface resistance have been extensively confirmed by experimental evidence [9], inparticular for bulk niobium. Less data exist for the dependence of the surface resistanceon the electron mean free path. No comprehensive calculations, nor systematic dataexists on the other hand for the dependence of the BCS surface resistance on RF fieldstrength. It is therefore important to assess in this report the behaviour of niobium thinfilms, where the electron mean free path can easily be tuned by varying the depositionparameters.In this work it has been chosen to factorise the BCS resistance as the product of twoterms, one depending only on T and the other only on HRF namely RBCS(HRF, T) =R0BCS(T) G(HRF). The validity of this factorisation is proven in Fig. 2, where isothermal scansof RBCS at various temperatures for a particular cavity are plotted in logarithmic scale. Thefact that the lines are parallel shows indeed that the RF field dependence is the same, thecurves differing only by a multiplicative factor.Fig. 2: Isothermal scans of HRF on a particular film. RBCS(T) is plotted as a function of HRFfor (from top to bottom) T= 4.23 K, 3.90 K, 3.47 K, 3.07 K, 2.59 K, 2.41 K, 2.15 K.The temperature dependent factor R0BCS(T) is consistent with BCS theory. Fig. 3shows its variation as a function of temperature for the 28 standard films, normalised tothe value at 4.2 K. An exponential dependence can be used to fit an approximate value forthe superconducting energy gap, equal to 19.6 K in temperature units. This value togetherwith the average critical temperature of 9.54 K gives a coupling parameter D=2.05.The actual value of RBCS depends strongly on the electron mean free path [10]. Since"/[0 is estimated to be about 0.7 for our films, compared to about 20 for the bulk, it is notsurprising that the BCS surface resistance of bulk niobium is more than double that forfilms (~900 n: compared to ~400 n: at zero RF field). This is illustrated in Fig. 4, fromwhich one can also infer that the RF field dependent factor G(HRF) is approximately equalfor bulk and for film.64.2 Residual resistanceThe measurement of the BCS surface resistance confirms that the basicsuperconducting properties of the films are well understood. On the other hand, the ultimategoal of the study is the understanding of the factors governing the residual resistance. At thepresent stage, the residual resistance is under control only at the level of a few tens ofnanoohms. Many phenomena can have an influence on the residual resistance. One notableexample is the nature of the substrate, as illustrated in Fig. 5. Spun cavities seem to provideon average better results than hydroformed cavities, both in terms of Rres and of its increasewith RF field. The most apparent difference between the two kinds of substrates is surfaceroughness. No clear mechanism relates roughness to residual resistance, although it may bespeculated that this influences the film crystalline structure.00.20.40.60.811.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2T [K]RBCS(T)/RBCS(4.2 K)Fig. 3: Average temperature dependence of RBCS(T)/RBCS(4.2 K) for 28 standard films.The line is the best fit of an exponential temperature dependence for RBCS(T).10010000 10 20 30 40 50R BCS (4.2 K) [nΩ]HRF  [mT]Fig. 4: Dependence of RBCS(4.2 K) on HRF for film (rounds) and bulk cavities, as received (squares)or vacuum fired (diamonds). Error bars show the spread from different cavities.7It must be noted that cavities with very low residual resistance over a broad range ofRF field happen to be, from time to time, produced. This provides evidence against theconjecture that the small grain size would be a fundamental limitation in the use of filmcavities [11]. Fig. 6 shows a few such examples, together with a bulk cavity as reference.All the measurements have been stopped well below the field threshold for electronemission, but in some cases this threshold is quite high. No systematic attempt was madeto optimise the high pressure water rinsing procedure in order to attain high maximumfields in the cavities. However, accelerating fields in excess of 15 MV/m have beenrepeatedly obtained, supporting the hypothesis that films behave not differently from bulkin this respect.0.00.501.01.52.02.53.03.54.00 10 20 30 40 50 60 70∆Rres/ ∆H RF [n Ω/mT]Rres [nΩ]Fig. 5: Slope at 1.7 K plotted as a function of Rres for 28 standard cavities. The slope isarbitrarily calculated by a linear extrapolation of the Rs(HRF) behaviour at low fieldand up to a few tens of mT, varying for each cavity. Hydroformed cavities arerepresented by squares, spun cavities by round symbols.4.3 Fluxon-induced surface resistanceMagnetic fluxes up to a few Gauss are used to measure the fluxon-induced surfaceresistance. It is experimentally proven that this quantity obeys, within reasonable limits, abilinear relation of the form Rfl=(Rfl0+Rfl1HRF)Hext. Although some models exist for thetheoretical dependence on an external magnetic field, few descriptions correlate thisdependence with the RF field amplitude.The two quantities Rfl0 and Rfl1 turn out to be very sensitive to variations in purity. Thedifference between bulk niobium and standard films is quite remarkable, with values ofRfl0of 4.8 n:G-1 for films and 127 n:G-1 for bulk. On the other hand, Rfl1 does not changemuch, being about 1.15 n:G-1mT-1 in both cases.A more detailed analysis of fluxon induced losses, together with a complete report ofall the measurements described herein, will be published in a forthcoming paper.81081091010101110120 5 10 15 20Bulk NbQ0 (1.7 K)Eacc [MV/m]Nb/CuFig. 6: Q vs. HRF curves for three Nb/Cu cavities, and a deep-drawn bulk niobium forcomparison. The Q values have been measured at 1.7 K.4.4 Penetration depth and Hc1Both the penetration depth and the lower critical field Hc1 depend on the electronmean free path ". By changing the mass of the noble gas used for the sputtering process,or using mixtures of two gases in various proportions, one can tune the quantity of sputtergas atoms trapped in the film, and in turn ". Fig. 7 shows the comparison ofmeasurements of the normalised values of Hc1 and O for cavities coated with variousgases, indicating that the mean free path can effectively be changed in this way [12].0.11100.1 1 10 100(Hc1/Hc1,clean)-2(λ/λclean)2Fig. 7: Comparison of penetration depth l and Hc1 measurements, normalised to the values ofthe clean limit (bulk niobium). Higher values correspond to shorter mean free paths.95. ConclusionsThe standard coating procedure gives excellent and reproducible results, in terms ofBCS parameters. New procedures for fine analysis of RF data have been set-up, whichallow the distinction of subtle differences among coatings. On this basis, it will now bepossible to explore the full range of the coating parameters in order to pinpoint thosevariables which are less understood, and which may affect the residual resistance.One possible such candidate is the interface between copper and niobium, that canbe studied either by sputter-etching of the copper substrate, or by the deposition of asuitable interlayer.Further conclusions from these investigations will be available in the near future.AcknowledgementsThe authors would like to thank: V. Palmieri of LNL-INFN for providing spun copper andniobium cavities; P. Kneisel of TJNAF for providing deep-drawn niobium cavities and formeasurements; S. Bauer and S. Forel for chemical treatments; F. Scalambrin for help with filmcoatings; S. Marsh and S. Sgobba for metallurgical analyses; N. Rensing and A. Cassinese forparticipation in the early part of the study; G. Cavallari, E. Haebel, E. Mahner, W. Weingartenfor stimulating discussions; J. M. Rieubland and his staff for support in the CryogenicsLaboratory of CERN.References[1] H. Padamsee et al., Ann. Rev. Nucl. Part. Sci. 43 (1993) 635[2] C. Benvenuti, Part. Accel. 40 (1992) 43[3] Proceedings of the 8th Workshop on RF Superconductivity, Abano Terme, Italy, 1997[4] V. Palmieri et al., paper presented at 4th Topical Symposium on Superconductivity andSuperconducting Materials Technologies, Firenze, Italy, 1994[5] C. Hauviller, paper presented at the IEEE Particle Accelerators Conference, ChicagoIL, USA, 1989[6] J-P. Birabeau and J.M.A. Guérin, Patent Nq 88 09820, Institut National De La PropriétéIndustrielle, 1993[7] G. Orlandi et al., CERN-MT/93-13 (SM), paper presented at the 6th Workshop on RFSuperconductivity, CEBAF, Newport News VA, USA, 1993[8] R. Russo and S. Sgobba, “Influence of coating temperature on niobium films”, paperpresented at the 8th Workshop on RF superconductivity, Abano Terme, Italy, 1997[9] J.P. Turneaure et al., J. Supercond. 4 (1991) 341[10] J. Halbritter, Z. Phys 266 (1974) 209[11] C. Benvenuti et al., Physica B 197 (1994) 72[12] C. Benvenuti et al., “Properties of copper cavities coated with different dischargegases”, paper presented at the 8th Workshop on RF superconductivity, Abano Terme,Italy, 1997

Niobium sputter-coated copper resonators

Niobium sputter-coated copper resonators

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